Wood-Based Thick Electrodes for High-Energy-Density Electrochemical Storage: Structural Engineering, Transport Regulation, and Device Integration

Abstract

Thick electrodes boost device-level energy density but suffer from depth-direction ion-electron decoupling: ionic diffusion scales quadratically with electrode thickness, while electronic resistance increases linearly, causing poor interior utilization. Wood offers low-tortuosity axial channels and conductive carbonized walls, yet structural proximity does not guarantee kinetic compatibility. This review reorients the design from passive “structural synergy” to active “trade-off engineering”, establishing a closed-loop structure–transport–performance framework. We analyze how channel orientation, cross-scale pore connectivity, wettability, and gradient conductive networks cooperatively modulate coupled ion-electron transport. Unlike synthetic scaffolds, wood’s advantage lies in functionalizable surfaces (including −OH, −COOH and lignin redox moieties) that actively regulate ion desolvation, nucleation, and intermediate confinement. Representative data: an 850 μm cross-cut wood-carbon anode with a mass loading of 55 mg·cm⁻² achieves ~13.5 mAh·cm⁻². An 800 μm basswood electrode with a mass loading of ~40 mg·cm⁻² achieves an areal capacitance of 6.54 F·cm⁻² at 2 mA·cm⁻². Through comparative analysis of supercapacitors (mass-transfer-limited), Li/Na-ion batteries (solid-diffusion-limited), and aqueous Zn batteries (interface-limited), we stress device-specific customization. Performance boundaries are not determined by the independent porosity or conductivity maximization, but by spatiotemporal matching between electrolyte residence time and interfacial reaction kinetics. We propose that AI-assisted precursor screening and gradient structure engineering can address the challenges of batch-to-batch variability and thickness limitation, transforming natural wood into a predictively engineerable platform for sustainable high-energy-density energy storage.

Graphical abstract

Keywords

Wood-based thick electrodes / Ion–electron transport coupling / Hierarchical pore engineering / Gradient conductive networks / Sustainable electrochemical energy storage

Highlight

	● From structural synergy to trade-off engineering for thick electrodes.
	● Wood acts as a reactive framework containing −OH, −COOH, and lignin redox sites.
	● Ion-electron coupling governs the depth utilization of thick electrodes.
	● Tailoring wood-derived scaffolds to device chemistry outperforms universal designs.
	● AI-assisted screening and gradient engineering enable predictive wood platforms.

1 Introduction

The pursuit of higher device-level energy density in electrochemical energy storage systems has advanced the development of thick electrodes with increased areal loading of active materials [1-3]. In principle, increasing electrode thickness reduces the volume fraction of inactive components, namely current collectors, separators and packaging materials, thus increasing the overall energy output per unit device volume [4-6]. However, in practical applications, the theoretically expected performance improvement is substantially offset by a fundamental asymmetry between ionic and electronic transport. When the electrode thickness L increases, the characteristic timescale for ion migration through the electrolyte-filled pore network is proportional to the square of the electrode thickness ({\tau }_{\text{ion}}\sim L^2/D_{\text{eff}}), while the electron transport resistance increases approximately linearly with L(R_{\text{e}}\sim L/({\sigma }_{\text{eff}}A)) [7,8]. This scaling discrepancy gives rise to an ion−electron transport mismatch, which means that ions and electrons cannot reach the same active material regions at comparable time scales. The primary consequences of this mismatch include reaction front confinement, depth-dependent underutilization of active material, and degraded rate capability: Faradaic reactions become concentrated in the region adjacent to the separator, while the active material close to the current collector remains mostly electrochemically inactive [9-11]. At high current rates and high areal loadings, this mismatch is further exacerbated by structural degradation, including particle falling off, crack propagation and interface debonding [12,13]. Therefore, the traditional thick electrode faces a core paradox: the improvement of energy density is directly at the cost of power performance and material utilization efficiency.

Natural wood is a promising structural solution for addressing this dilemma. Its axially aligned vessels and tracheids form low-tortuosity channels (with tortuosity typically ranging from 1.2 to 2.0), which enable rapid electrolyte penetration into the interior of millimeter-scale electrodes [14-17]. Meanwhile, the continuous cell wall skeleton is converted into a percolated electron-conducting framework after carbonization, which can support active materials without the requirement of binders or additional conductive additives [18-20]. The abundant surface oxygen-containing groups (–OH, –COOH) further promote delignification, heteroatom doping, and interfacial functionalization [21,22]. Over the past decade, the synergistic integration of ion-transport channels, electron-conduction pathways and mechanical support within a single monolithic architecture has attracted extensive research interest.

However, these structural advantages can be compromised during electrode activation and active material loading. Specifically, overactivation or excessive loading reduces the channel cross-sectional area, blocks pore interconnection, and disrupts the conductive framework, which consequently increases electrode tortuosity and interfacial contact resistance [23,24]. Furthermore, the value of wood for thick electrode design does not originate exclusively from its intrinsic physical porosity. While synthetic scaffolds fabricated via 3D printing, ice templating, and laser drilling can also provide low-tortuosity mass transport channels, wood additionally offers chemically functionalizable interfaces with hierarchical confinement. Lignin-derived aromatic domains, cellulose-induced polar surfaces, and defect-rich carbon edges are capable of mediating ion desolvation, soluble intermediate species confinement, and metal nucleation regulation—functions that are generally not achievable with inert physical templates [25,26].

Despite the growing body of literature on wood-based carbons, biomass electrodes, and functionalized wood composites, existing reviews are primarily focused on material preparation protocols, pore structure characterization, and the performance of isolated device configurations [27-29]. Systematic discussions explicitly focused on thick-electrode architecture, covering structural inheritance, cross-scale connectivity, coupled ion−electron transport, and device-specific performance limits, remain largely limited in existing literature. In particular, a unified theoretical framework is still required to elucidate: (i) how native axial channels can be effectively converted into out-of-plane transport pathways; (ii) how artificially constructed pores and pits restore lateral connectivity without compromising mechanical integrity; (iii) how wettability and pore-confined electrolyte buffering regulate reaction accessibility in deep electrode regions; (iv) how gradient conductive networks match intrinsic flux gradients; and (v) how these factors collectively determine areal capacity, volumetric energy density, and rate retention across different device chemistries.

To address these research gaps, this review constructs a structure–transport–performance framework that covers four interrelated dimensions. Section 2 investigates the inheritance and reconstruction of natural wood architectures, channel orientation, and cross-scale pore connectivity. Section 3 discusses hierarchical pore networks and their effects on through-plane permeability, electrolyte uptake, and ion-accessible surface area. Section 4 analyzes the coupled regulation of ion transport (including tortuosity, wettability, and pore-confined buffering) and electron transport (including intrinsic conductivity and gradient conductive networks), and elaborates on the optimization criteria for their spatiotemporal matching at active interfaces. Section 5 compares the integration logic and key limitations of wood-based thick electrodes across three representative device types: mass-transfer-limited supercapacitors, solid-diffusion-limited lithium/sodium-ion batteries, and interface-limited aqueous zinc batteries, highlighting that the optimal electrode architecture is determined by the working chemistry of the device. Section 6 summarizes the main existing challenges, including insufficient mechanistic understanding of multi-field coupling, constraints on practical electrode thickness, the absence of standardized evaluation protocols, and barriers to manufacturing scaling, and outlines future research directions encompassing multiscale modeling, operando characterization, AI-assisted precursor screening, and gradient structure engineering.

The core argument of this review is that the performance limit of wood-based thick electrodes is not dictated by the independent maximization of porosity, specific surface area, or electrical conductivity, but by the balance between areal mass loading and depth-dependent utilization. By framing wood as a chemically functional reactive framework rather than merely a passive template, this review establishes a foundation for the rational design of sustainable electrochemical energy storage devices that deliver both high areal capacity and high volumetric energy density (Fig. 1).

2 Structural Design of Wood-Based Frameworks

Conventional thick electrodes manufactured via slurry coating inevitably exhibit stochastic pore networks, tortuous ion transport pathways, and discontinuous electron contact as electrode thickness increases [25]. Drying and calendering processes induce binder migration, particle segregation, and crack propagation, leading to the disproportionately low contribution of added active material in deeper regions to the usable capacity of the electrode [26,27]. Natural wood has been proposed to provide a straightforward structural alternative: its axially aligned vascular channels act as continuous pathways for electrolyte transport, while the carbonized cell walls form an integrated electronic and mechanical backbone [28,29]. However, we argue that this structural advantage is strictly conditional rather than an inherent property of the material. The pristine wood architecture is electrochemically inactive; while carbonization imparts electrical conductivity to the structure, it simultaneously triggers anisotropic shrinkage, selectively narrows pit apertures, and may sever the transverse connections that are critical for lateral ion redistribution [30]. Accordingly, the core challenge of thick electrode structural design is not simply to inherit the native wood architecture, but to actively reconstruct it to ensure that the characteristic ion-transport time and electron-transfer time converge at all depth positions—a condition we define as spatiotemporal transport convergence [31].

2.1 Selective retention versus passive inheritance

The anatomical differences between softwoods and hardwoods (Fig. 2A) constitute the fundamental design space for biomaterial derived from natural wood [32]. Softwoods, whose structure is dominated by uniformly sized axial tracheids, exhibit high structural regularity that simplifies the fabrication of conformal thin coatings [33-35]. By contrast, hardwoods feature functional differentiation of tissue into large-diameter vessels (for axial conduction), fibers (for mechanical support), and rays (for radial transport), providing a broader pore size hierarchy that can accommodate higher loading of active materials [36-38]. Nevertheless, we emphasize that greater structural diversity does not inherently confer superior performance. The heterogeneous channel size distribution of hardwoods frequently leads to localized flux congestion: ions preferentially migrate through the largest vessels, leaving adjacent fiber lumina underutilized unless intentional inter-channel connectivity is introduced via structural engineering [39]. Accordingly, species selection must be determined by the target application: uniform thin-film deposition is more suitable for softwoods, while hosting high-loading nanoparticles requires the spatial heterogeneity provided by hardwoods.

Crucially, pristine wood cannot be directly used as a stable electrode framework. During pyrolysis, cellulose, hemicellulose, and lignin undergo distinct decomposition rates, leading to non-uniform shrinkage, cell wall warping, and microcrack propagation [40,41]. While delignification effectively generates intercellular voids, it significantly impairs the mechanical integrity of the material and often causes the wet hierarchical architecture to collapse during conventional drying [42]. To mitigate these detrimental effects, we propose a “retention-first, repair-second” design principle:

(1) Pre-oxidation promotes intermolecular cross-linking and increases carbon yield by inhibiting the release of volatile components [43].

(2) Constrained pyrolysis (utilizing slow heating rates and confined spatial geometry) minimizes thermal stress induced by internal temperature gradients [44].

(3) Freeze-drying or critical-point drying avoids capillary-driven shrinkage, thereby preserving pit apertures and intercellular voids [45].

(4) Polymer pre-impregnation or inorganic precursor confinement provides external structural support during carbonization, effectively “locking” the native cell wall geometry [46,47].

Nevertheless, preserving the morphological structure of the biological skeleton does not guarantee retention of its inherent transport function. Direct impregnation for introducing active materials frequently leads to preferential precursor accumulation near channel entrances, which forms thick outer deposits that occlude lumina while leaving deep inner walls uncoated [48]. In situ growth strategies enhance interfacial adhesion between active materials and the skeleton, but prolonged deposition risks covering pit membranes and reducing the cross-sectional area of lumina [49]. A more rational design paradigm, exemplified by the work of Ouyang et al., adopts a space-confined reconstruction strategy [50]. They infiltrated a pre-carbonized Chinese fir skeleton with a glucose/NaCl mixed solution, utilizing the original tracheid structures as confined crystallization chambers. Controlled crystallization during drying formed ordered NaCl cubic templates, with glucose forming interconnected coatings around the templates. After subsequent carbonization and template removal, hollow carbon cube arrays were obtained, which occupy approximately 79% of the lumen volume while retaining continuous electrolyte-accessible transport pathways (Fig. 2B). The as-prepared electrode achieved a specific surface area of 815.9 m²·g⁻¹, and the assembled symmetric supercapacitor delivered an areal capacitance of 2.65 F·cm⁻² at a current density of 5 mA·cm⁻². This study demonstrates that structural reconstruction should not target the maximization of pore filling; instead, it should target converting empty lumina into electrochemically active, transport-accessible architectures without compromising the pre-existing multiscale connectivity of the original skeleton [51,52].

2.2 Effect of cutting orientation on ion transport

The most distinctive yet frequently misunderstood design parameter is the cutting orientation relative to the direction of wood growth. As demonstrated in Fig. 2C–2D, wood can be divided into two cutting configurations: transversal (T, cross-cut) and longitudinal (L, side-cut) [53,54]. In the transversal configuration, the luminal openings of xylem vessels and tracheids are exposed on the surface of the large-area electrode, enabling electrolyte to penetrate axially through the continuous lumens across the entire millimeter-scale freestanding electrode. In the longitudinal configuration, the axial luminal channels are predominantly aligned parallel to the electrode surface, which forces ions to enter through the lumen sidewalls and migrate across intertracheary pits or secondary cell wall pores before reaching the electrode interior.

We contend that this distinction is not merely quantitative, but fundamentally qualitative. Shen et al. have reported conclusive experimental evidence demonstrating that the upper performance limit of such electrodes is dependent on the orientation of porous channels [55]. When the electrode thickness is moderate (~80 μm) and the mass loading is ~5 mg·cm⁻², both cross-cut and side-cut wood-based carbon anodes deliver comparable areal capacities (~1.25 mAh·cm⁻²). However, when the thickness is increased to 850 μm and the mass loading is elevated to 55 mg·cm⁻², the cross-cut electrode reaches an areal capacity of ~13.5 mAh·cm⁻², while the side-cut electrode only retains 48% of this capacity (Fig. 2E). This sharp performance decline reveals a key insight: when the transport channels are aligned with the through-plane ion flux, the wood-based electrode architecture effectively extends the functional working thickness; when channels are misaligned with the flux direction, the inherently low tortuosity of the native wood structure loses functional relevance, and the electrode behaves similarly to a randomly porous slurry-cast electrode. We define this phenomenon as the “pseudo-isotropy trap”—the erroneous assumption that high intrinsic porosity alone is sufficient to guarantee good performance, independent of pore orientation.

The successful integration of channel orientation design with pore and surface engineering is further demonstrated by the work of Yan et al. [56]. In their study, basswood was cross-cut perpendicular to its growth direction, followed by formamide-assisted solvothermal treatment, pre-oxidation, carbonization, and mild KOH activation. This synthetic route yielded an N/O co-doped self-supporting carbon monolith with a thickness of approximately 800 μm and an active material loading of approximately 40 mg·cm⁻² (Fig. 2F). Critically, the entire preparation process retained the original vertical vessels as primary ion transport highways for electrolytes, preserved lateral pits to enable inter-channel mass exchange, and generated activated micropores to provide accessible charge-storage active sites. The optimized electrode delivered an areal capacitance of 6.54 F·cm⁻² at 2 mA·cm⁻² and still retained 3.82 F·cm⁻² at 500 mA·cm⁻². This rate performance confirms the synergistic effect of macroscopic channel alignment and nanoscale ion accessibility. However, this work also reveals a key design trade-off: insufficient activation renders most micropores inaccessible to charge carriers, while excessive KOH etching damages the mechanical stability of the self-supporting framework and leaches beneficial N/O doping species [57,58]. Therefore, channel orientation engineering must be combined with precise activation process control to avoid sacrificing the structural integrity of the monolith for incremental improvements in porosity.

Even with optimal cross-cut orientation, the effective working thickness is constrained by the penetration depth of the electrolyte at applied operating current densities. Wang et al. systematically investigated this limitation using basswood-derived carbon monoliths with thicknesses ranging from 0.802 mm to 4.030 mm [59]. The 1.532 mm electrode achieved an optimal performance balance, delivering an areal loading of 54.75 mg·cm⁻² and a capacitance of 7.64 F·cm⁻² at 1 mA·cm⁻². In contrast, further increases in electrode thickness led to a monotonic rise in ion transport resistance without proportional improvements in capacity. This nonlinear response demonstrates that natural straight-through channels can delay, but cannot completely eliminate, the kinetic degradation induced by increasing electrode thickness. When the intrinsic axial continuity of the monolith is impaired by carbonization-induced defects or active material deposition, artificial perforation acts as a complementary strategy to restore electrolyte accessibility to thick electrodes [60]. Nevertheless, it should be noted that perforation is intended to compensate for local discontinuities rather than replacing the native pore network; excessive perforation reduces the volumetric active material density and disrupts continuous electron transport pathways for charge collection.

Overall, the structural design of wood-based thick electrodes must follow three core design principles:

(1) Orientation-aligned conduction: The ion conduction direction must be consistent with the through-plane ion flux direction. Otherwise, the low-tortuosity structural advantage innate to natural wood will be negated.

(2) Selective reconstruction rather than indiscriminate preservation: Parameters for carbonization and activation processes should be customized to retain pit connectivity and inter-channel mass exchange, while preventing excessive cell wall thinning or pore blockage.

(3) Thickness optimization based on transport-reaction coupling balance: The maximum critical electrode thickness is defined by the condition t_{\text{ion}}\approx t_{\text{reaction}}; when thickness exceeds this threshold, further increases in areal loading will only lead to progressively lower active material utilization.

Fig. 2 visually summarizes these principles by contrasting native anatomy, the outcomes of cutting direction, and engineered reconstruction strategies. By anchoring structural design to the core requirement of spatiotemporal transport convergence, wood-based frameworks can truly overcome the limitations of conventional slurry-cast electrodes, rather than merely replicating them in an altered morphological form.

3 Hierarchical Pore Structures and Cross-Scale Connectivity

The inherent lumina of wood serve as an efficient structural framework facilitating electrolyte penetration; however, a high-volume pore network does not necessarily constitute an effective ion-transport network [62]. After entering axial vessels, electrolyte ions must subsequently undergo lateral migration toward carbonized cell walls to reach microporous or redox-active interfaces, where charge storage occurs. This process imposes a critical yet often overlooked constraint: the electrochemically effective surface area can differ from the Brunauer–Emmett–Teller (BET) specific surface area by orders of magnitude. Micropores hidden behind narrow pore necks, situated in dead-end regions, or buried under dense active-material deposits are counted in the measured BET value but remain ionically inaccessible during practical charge-discharge operations. We define this phenomenon as the “BET trap”—a pervasive misconception in existing literature where high specific surface area is uncritically equated with superior capacitive performance. In thick electrodes, this trap is especially pernicious: under high-rate conditions, ions react preferentially near channel openings, while deeply buried surfaces remain inadequately wetted or experience kinetic inactivity [63,64].

Accordingly, the core objective of hierarchical pore engineering requires redefinition: rather than maximizing pore volume at any single length scale, the goal is to construct a continuous connectivity cascade extending from macroscopic supply channels (hundreds of micrometers), through mesoporous redistribution domains (tens of nanometers), to microporous storage sites (sub-nanometers). This connectivity cascade must meet a percolation threshold condition: the probability of identifying a continuous ion-conduction pathway across the full thickness of the electrode must approach unity. In the following discussion, we critically review three complementary reconstruction strategies—pore wall opening, domain bridging, and controlled lumen occupation—each of which targets a unique connectivity defect.

3.1 Wall opening: repairing native micro–mesopore connectivity

The dominant connectivity bottleneck in carbonized wood lies at the transition from the open lumen (macropores) to the interior of the cell wall (micropores). Intrinsic pits and nanopores naturally serve as conduits for this mass/charge transfer transition, yet carbonization-induced shrinkage, tar deposition, and structural collapse commonly occlude these apertures [65]. While a direct increase in carbonization temperature improves electronic conductivity, it paradoxically exacerbates this issue: higher temperatures promote structural ordering and micropore coalescence, which ultimately reduces the electrochemically accessible surface area.

Ouyang et al. developed a targeted pore repair strategy via sequential thermal modification followed by solvent infiltration [66]. As shown in Fig. 3A, thermal pretreatment partially decomposes wood-based polymers, inducing structural defects and generating volatile residues that block existing nanopores. Subsequent acetone infiltration dissolves a portion of these residues and penetrates between cellulose-rich fibrillar structures, expanding inter-fibril spacing and exposing previously collapsed pore entrances. After carbonization, the acetone-modified wood-based carbon retains the interconnected tracheid framework while presenting a broader size distribution of micropores and mesopores: the specific surface area increases from 534.1 to 677.7 m²·g⁻¹, and the mesopore volume increases from 0.0033 to 0.030 cm³·g⁻¹. We attribute this performance improvement to selective pore unblocking rather than de novo pore generation: the treatment does not create new porosity, but restores the inherent pore connectivity impaired during carbonization.

3.2 Domain bridging: constructing intermediate electrolyte reservoirs

A direct transition from micrometer-scale tracheids to nanometer-scale cell-wall pores constitutes an abrupt dimensional discontinuity that severely hinders ion redistribution [67]. Introducing an intermediate pore domain with characteristic dimensions in the sub-micrometer range can effectively bridge this gap by providing local electrolyte reservoirs that shorten the replenishment distance to active storage sites.

Ouyang et al. demonstrated this cross-domain bridging logic via a glucose/Na₂CO₃ infiltration strategy (Fig. 3B) [68]. During vacuum infiltration, the glucose/Na₂CO₃ precursor penetrates into tracheid lumina and interacts with oxygen-containing functional groups on the cell wall surfaces. Subsequent drying immobilizes Na₂CO₃-loaded clusters within the glucose matrix. Upon carbonization, glucose is converted into carbon, while the decomposition of Na₂CO₃ simultaneously generates gas and leaves porous structures. The as-obtained “super-MWC” electrode retains the native tracheid architecture, while introducing abundant mesopores and sub-micrometer macropores into the glucose-derived carbon phase. These secondary pores act as intermediate electrolyte reservoirs, capturing ions diffused from the main lumina and redistributing them to the microporous walls. The optimized electrode achieves a pore volume of 0.665 mL·g⁻¹ and delivers an areal capacitance of 6.4 F·cm⁻².

However, it is critical that this bridging strategy adheres to a strict tolerance threshold. Excessive Na₂CO₃ loading induces deformation and partial blockage of the tracheid framework, which confirms that the generation of new pores becomes counterproductive once the primary transport backbone is compromised [69]. The introduced phase should fill the lumen space in the form of a porous scaffold, rather than a dense plug [70]—a key distinction that is frequently overlooked when researchers only report increases in pore volume. Accordingly, the design principle for domain bridging is as follows: the intermediate porosity must establish hierarchical connectivity between both the macroscale lumina and nanoscale wall pores, forming a continuous rather than isolated pore domain.

3.3 Controlled lumen occupation: balancing space utilization and transport

Although intrinsic macropores are critical for facilitating rapid electrolyte transport, their oversized pore dimensions correspond to unutilized void space when evaluated from the perspective of volumetric energy density [71]. Large empty lumens reduce the fraction of active solid materials that can be accommodated within a fixed electrode volume, consequently decreasing volumetric capacitance [72]. A rational design strategy is thus controlled occupation: partially filling the lumens with porous carbon or an electrochemically active phase to introduce additional storage capacity without impeding through-plane mass transport.

Tian et al. implemented this principle via in situ polymerization of phenolic resin in the intrinsic channels of natural wood (Fig. 3C) [73]. Delignification first exposes internal pores and hydroxyl groups, which promotes vacuum infiltration of resorcinol and formaldehyde precursors. Subsequent curing and KOH-activated carbonization transform the resin into a porous carbon phase that occupies the lumen volume while preserving the intact continuous wood-based framework. Notably, this approach does not fully fill the lumina; instead, it partially converts the underutilized void space into a bicontinuous solid-electrolyte composite, wherein the carbon derived from cured resin provides additional energy storage active surfaces, and the remaining interconnected voids retain accessibility for electrolytes. The optimized electrode delivers an areal mass loading of 34.8 mg·cm⁻² and a volumetric capacitance of 136.7 F·cm⁻³—both performance metrics are significantly higher than those of pure wood-based carbon and pure resin-derived carbon alone.

We emphasize that controlled occupation is fundamentally different from impregnation densification, which aims to achieve the maximum loading of active materials. The latter inevitably induces diffusion bottlenecks, as dense active deposits reduce the cross-sectional area of mass transport channels and increase local tortuosity. In contrast, the former retains dynamic porosity: a continuous void network that dynamically facilitates electrolyte transport even during high-rate operation. This critical distinction is commonly overlooked in conventional half-cell characterization, where static electrolytes conceal the mass transport limitations that arise in practical device architectures.

3.4 Active-material morphology: redefining interface accessibility via reconstruction

The incorporation of electroactive phases enables further reconstruction of the pore hierarchy, as the final ion transport pathways are determined by the morphology of the composite rather than the pristine wood scaffold alone. Uniformly distributed nanosheets, nanowires, and porous coatings can introduce new interfacial pores and significantly shorten the diffusion distance from electrolyte channels to redox-active sites [74]. In contrast, dense compact films or agglomerated particles deposited in tracheid lumina can block the primary ion transport channels and form electrochemically inaccessible internal regions [75].

Hu et al. presented a convincing demonstration of morphology-guided reconstruction via the polyaniline (PANI)/ CoNiO₂@AWC architecture (AWC, activated wood-derived carbon) (Fig. 3D) [76]. Activated wood-based carbon first provides interconnected compartments and anchoring sites for the hydrothermal growth of CoNiO₂ nanowires. The loosely interwoven nanowire network forms secondary pores and acts as a template for the subsequent electrodeposition of PANI. The obtained stacked corrugated structure enables electrolyte ions to permeate through the interstices among the carbon wall, CoNiO₂ nanowires and PANI layers. Meanwhile, CoNiO₂ functions as an inorganic support and buffer phase that alleviates the volume expansion of PANI during cycling. The optimized electrode delivers an areal capacitance of 14.31 F·cm⁻² at a current density of 5 mA·cm⁻², and retains approximately 80% of its capacitance at 30 mA·cm⁻². This rate performance clearly confirms that morphological reconstruction can convert active-phase loading from a transport obstacle to a transport promoter.

Nevertheless, we emphasize that active-material loading evaluations should be reported based on interface exposure per unit channel volume, rather than mass loading per geometric area. A 10 mg·cm⁻² loading of nanowires that uniformly coat channel walls can provide higher ion accessibility than a 5 mg·cm⁻² loading of dense particles agglomerated at pore entrances. This counterintuitive observation occurs because the former retains the cross-sectional area required for electrolyte transport, while the latter forms a local plug that forces ions to completely bypass the filled region. Accordingly, the optimal active-material loading is determined by the morphological regime, rather than a universal mass threshold.

3.5 The connectivity criterion: from statistical porosity to percolated transport

The fundamental structural basis of these modification strategies is illustrated in Fig. 3E, which depicts a freestanding carbonized wood monolith with straight 50–100 μm channels, surrounded by smaller 5–20 μm channels, with micro- and mesopores distributed within the channel walls [77]. The presented characterization confirms that carbonization can preserve the long-range transport backbone while generating a conductive, self-supporting framework. Nevertheless, we propose that the structural performance of a thick electrode is not captured by the pore-size distribution histogram, but rather by the connectivity matrix-specifically, whether each pore domain is accessible from the upstream domain via a continuous pathway.

Based on percolation theory, we propose a connectivity criterion for thick wood-based electrodes: the effective ion-conduction pathway must exhibit a cross-sectional area reduction factor of less than 10 per length-scale transition (macropore → mesopore → micropore). If the pore aperture shrinks by more than one order of magnitude at any single transition step, local flux will be constricted, generating a concentration gradient that cannot be compensated by bulk electrolyte supply. This criterion explains why excessive activation (which narrows micropore necks) and dense active material loading (which blocks macropore entrances) frequently degrade electrochemical performance, despite the associated increase in BET specific surface area.

In a well-integrated hierarchical network, bulk electrolyte first deeply penetrates via longitudinal vessels, undergoes lateral redistribution through pores and artificial macropores, enters mesoporous reservoirs, and ultimately reaches microporous or redox-active interfaces. This continuous cross-scale connectivity mitigates local ion depletion, shortens diffusion paths, and improves the utilization of deep-seated active sites. Therefore, the optimal wood-based thick electrode is not the material with the highest porosity or BET specific surface area, but one that maintains coordinated electrolyte transport, accessible storage interfaces, and continuous electron collection across multiple length scales—a holistic criterion that fundamentally redefines performance metrics for this class of materials.

4 Charge Storage Mechanisms and Transport Regulation

Fig. 4 summarizes the depth-dependent transport behavior of thick electrodes. As the electrode thickness increases, the ion flux in the electrolyte, electron flux in the conductive network, and reaction flux at the active interfaces decrease at distinct rates along the thickness direction. This flux mismatch confines electrochemical reactions to a restricted region, lowering the utilization rate of active materials in the interior of the electrode. Introducing gradients in porosity, conductive agent content, and particle size can improve the spatial matching of the three fluxes, thereby increasing the maximum effective working thickness of the electrode [78].

In conventional particulate-based thick electrodes, ion and electron transport networks are formed by distinct components: ions migrate through electrolyte-filled inter-particle pores, while electrons travel through the interconnected contact network comprising active particles, conductive additives and the current collector [79]. As electrode thickness increases, blockage of either transport pathway eliminates the effective reactant supply to localized active materials, restricting electrochemical reactions to the region near the electrode surface and leaving deep active materials electrochemically inactive [80]. Wood-based thick electrodes address this dichotomy by integrating both transport networks into a single monolithic structure: axial vessels and pits assemble hierarchical ion channels, while carbonized cell walls form a continuous electron-conducting skeleton that extends across the entire electrode [81,82]. Active materials can be directly loaded onto the solid–liquid interfaces adjacent to both transport pathways, reducing the effective transport distance.

Nonetheless, we argue that spatial integration represents a necessary but insufficient condition for kinetic compatibility. The decisive determinant of thick-electrode performance is not whether ion and electron transport pathways coexist, but whether their characteristic transport times—the time required for ions to reach a given depth and electrons to reach the same position—are of the same order of magnitude as the interfacial reaction time (t_{\text{reaction}}). This time-scale matching criterion can be formulated as follows:

(1)t_{\text{ion}}\sim \frac{L^2}{D_{\text{eff}}}\approx t_{\text{electron}}\sim \frac{L^2}{{\sigma }_{\text{eff}}}\approx t_{\text{reaction}}\sim \frac{1}{k},

where L denotes the electrode thickness, D_eff denotes the effective ion diffusivity, σ_eff denotes the effective electron diffusivity (analogous to electrical conductivity per unit charge carrier), and k denotes the interfacial reaction rate constant. When any of the above timescales exceeds any other by more than one order of magnitude, the slowest process becomes rate-determining, rendering the remaining capacity of the electrode inaccessible. This framework fundamentally redefines transport regulation: transport regulation is not framed as the independent enhancement of ion mobility or electronic conductivity, but as the deliberate synchronization of all three timescales across the full depth of the electrode. In the following discussion, we critically analyze ion transport, electron transport, and their synergistic coupling through this analytical lens.

4.1 Ion transport regulation

Ion transport within thick wood-derived electrodes follows a three-stage cascade mechanism: (i) long-range electrolyte penetration through axial lumina, which is regulated by channel tortuosity; (ii) lateral redistribution via pits and intercellular spaces, which is governed by interconnected porosity; (iii) short-range diffusion toward active interfaces, which is determined by wettability and local electrolyte availability. These three stages are not independent of one another: the emergence of a transport bottleneck at any stage renders all subsequent stages kinetically inconsequential. In this work, we treat tortuosity engineering, wettability optimization, and pore-confined ion buffering as a sequential hierarchical system, rather than considering these factors in parallel.

4.1.1 Tortuosity engineering: beyond the geometric ratio

Tortuosity (\tau) is conventionally defined as the ratio of the actual pore length to the electrode thickness [83]. However, when analyzing the performance of thick electrodes, the more physically meaningful parameter is the transport tortuosity factor (\tau /ϵ), which incorporates the effects of pore constriction, reduced connectivity, and effective cross-sectional area [84]. Following the framework proposed by Nguyen et al. [85], the MacMullin number (N_{\text{M}}) directly relates the effective ionic conductivity to the conductivity of the bulk electrolyte:

(2)\frac{\tau }{ϵ}=\frac{{\kappa }_0}{{\kappa }_{\text{eff}}}=N_{\text{M}},

where {\kappa }_0 and {\kappa }_{\text{eff}} denote the bulk ionic conductivity and effective ionic conductivity, respectively. This relationship reveals that under comparable porosity conditions, lower tortuosity corresponds to higher effective ionic diffusivity and mitigated concentration polarization. More importantly, we highlight that tortuosity and porosity are not interchangeable design parameters. A highly porous yet poorly connected ion transport network can yield a higher tortuosity factor than a denser but well-aligned structure.

Figs. 5A and 5B directly illustrate this structural distinction by comparing a conventional LiCoO₂ electrode (with tortuosity τ ≈ 1.5) with wood-templated electrodes LCO-1 and LCO-2 (with τ ≈ 1.0) under comparable porosity conditions [86]. Notably, LCO-2 exhibits lower porosity (0.441) than the control electrode (0.500) while still maintaining substantially lower tortuosity. This comparison supports a critical conclusion: a larger pore volume does not necessarily yield more efficient through-thickness transport; the decisive factor is whether pores are continuous and aligned with the direction of ion flux. However, repeated precursor infiltration performed to achieve higher active material loading in LCO-2 narrowed the inherited channels, resulting in an inherent trade-off: directional connectivity is preserved, but reduced channel cross-sections limit high-rate electrolyte supply. Accordingly, low-tortuosity structure design must be optimized in conjunction with channel width and active material density, rather than in isolation.

For anisotropic wood architectures, tortuosity should be reported with directional resolution. Axial transport ({\tau }_{\text{axial}}) generally falls within the range of 1.2–2.0 due to the relatively direct transport pathway, while radial and tangential transport ({\tau }_{\text{radial}}, {\tau }_{\text{tangential}}) depends more strongly on pit and ray connectivity, and may feature local flow bottlenecks [87]. Therefore, a robust and meaningful evaluation must integrate directional tortuosity, connected porosity, and channel-size distribution: a tripartite parameter set that is rarely fully characterized in contemporary literature.

4.1.2 Wettability and the pore-confined ion-buffering mechanism

Low-tortuosity pathways merely provide the geometric foundation for fast ion transport. They only become electrochemically active when the electrolyte displaces occluded gas, fully wets pore walls, and establishes a continuous liquid phase spanning the entire electrode thickness. Even structurally open channels that appear unobstructed under microscopic characterization will exhibit reduced effective porosity and available interfacial area if incomplete wetting occurs [88]. We argue that measuring a near-zero contact angle on the material’s external surface does not guarantee complete infiltration into the millimeter-scale bulk interior, and we define this common misconception as the “surface-wetting trap”. Depth-resolved electrolyte uptake measurements or operando imaging are therefore indispensable to validate full electrolyte penetration across the entire electrode thickness.

Lei et al. reported a representative case of coupled wettability and channel engineering (Fig. 5C) [89]. In this work, pinewood was processed via sequential perforation, delignification, hydrothermal loading of MoS₂ or NiS₂, and carbonization. Each fabrication step fulfills a unique functional purpose: artificial pores enable direct electrolyte infiltration; delignification exposes fibril surfaces rich in –OH/–COOH functional groups; hydrothermal growth immobilizes polar transition metal dichalcogenides on the pore inner walls; and carbonization converts the monolith into a conductive carbon framework. The as-fabricated hierarchical porous electrode delivered an ionic conductivity of approximately 1.2 × 10⁻⁴ S·cm⁻¹, outperforming the ionic conductivity of ~0.5 × 10⁻⁴ S·cm⁻¹ achieved by unprocessed wood-based carbon. As shown in Fig. 5D, the contact angle of the NiS₂–CW electrode decreased to nearly 0° within approximately 1 second, which is ascribed to the synergistic effect of polar NiS₂, oxygen-containing functional groups on the carbon surface, and open pore entrances (CW, carbonized wood).

Importantly, we identify this rapid imbibition as evidence of capillary-driven wetting of the hierarchical network, rather than mere surface spreading. The interconnected pores function as a confined ion-buffering network: large axial and artificial channels supply bulk electrolyte, while mesopores and pits retain electrolyte in close proximity to electrode cell walls, reducing the electrolyte replenishment distance to local active sites. As shown in Fig. 5E of Guo et al., fluid simulation visualizations of this mechanism confirm that artificial perforations mitigate electrolyte concentration gradients and outlet-side polarization [89]. Accordingly, the buffering effect does not depend on total pore volume, but on whether distinct pore domains are fully wetted and interconnected to form a sequential supply-retention-utilization cascade.

Nevertheless, we emphasize that excessive activation and pore over-enlargement are counterproductive. Severe etching reduces the density of active sites, elevates electrolyte resistance, leaches heteroatoms and disrupts the electronically continuous carbon framework. Wettability, accessible interconnected mesopore volume and framework structural integrity must be optimized simultaneously. The optimal pore architecture is a fully wettable axial transport network, coupled with local mesoporous reservoirs and microporous active interfaces, rather than a maximally porous framework with poor mechanical and volumetric efficiency.

4.2 Electron transport regulation

Electron transport poses an asymmetric constraint relative to ion transport in thick electrodes: when ions migrate through electrolyte-filled pores, electrons must traverse the solid-state network from the current collector, through the carbon skeleton and active phases, to reach the reaction interfaces [91]. As electrode thickness increases, the electron transport distance lengthens and localized contact resistances accumulate, resulting in a non-uniform potential distribution along the thickness direction [92]. We posit that regulation of electron transport requires simultaneous optimization of intrinsic material conductivity and spatial architecture, and the latter becomes increasingly determinant in millimeter-scale electrodes.

4.2.1 The carbonization-temperature paradox and conductivity enhancement

Carbonization transforms the insulating cell walls of wood into an interconnected carbon framework. Gabhi et al. demonstrated that increasing the carbonization temperature from 600 to 1000 °C elevated the bulk electrical conductivity of sugar maple-derived carbon and white pine-derived carbon from approximately 4–12 S·m⁻¹ to approximately 2300–3300 S·m⁻¹ [93]. This conductivity improvement is attributed to the elimination of hydrogen- and oxygen-containing species, as well as the growth of short-range ordered sp²-carbon domains. Nevertheless, this work identifies a critical carbonization temperature paradox: while higher carbonization temperatures enhance electronic conductivity, they concurrently trigger micropore shrinkage, reduce the content of surface oxygen-containing functional groups, and impair electrolyte wettability [94]. Zhang et al. verified this trade-off effect using reed residue-derived carbon, finding that the sample carbonized at 600 °C (denoted as C₆₀₀) achieved the optimal balance between the construction of a conductive carbon framework and the formation of ion-accessible hierarchical porosity [95]. Accordingly, the optimization objective for biomass-derived carbon is not to achieve the highest degree of graphitization, but to obtain sufficient electronic conductivity without compromising the pore structure and surface properties that are critical for ion transport.

When either the carbon skeleton or the loaded active phase has insufficient electrical conductivity, a secondary conductive network constructed from carbon nanotubes, graphene derivatives, or conductive polymers can be introduced along the pore walls [96,97]. Fig. 6A demonstrates this design strategy using a Co(OH)₂/CNT–wood-carbon-scaffold electrode (CNT, carbon nanotubes) [98], in which CNTs directly grown on tracheid walls bridge the carbonized cell walls and the poorly conductive Co(OH)₂ nanosheets. Most importantly, the in situ growth of CNTs inhibits particle aggregation, maintains close interfacial contact between components, and keeps the tracheid lumina unobstructed. This architecture constructs parallel electron conduction pathways without blocking the primary ion transport channels, which is a key design requirement that differentiates wood-based composites from traditional conductive additive blends.

Fig. 6B illustrates a complementary strategy designed for the Co–N@ACS air electrode [99], in which Co-doped ZIF-8 synthesized in situ in activated paulownia wood is converted to Co–N active sites embedded within a carbon framework. Unlike conventional architectures where catalyst particles are anchored onto an independent substrate, this design directly integrates catalytic centers into electronically conductive wood-based carbon. In this configuration, the carbon skeleton enables efficient electron conduction, hierarchical pores promote electrolyte and oxygen transport, and Co–N sites afford high catalytic activity. This structural design achieves tri-functional synergy within a single monolithic electrode and eliminates the resistive interfaces ubiquitous in traditional composite electrodes.

The anisotropic conductivity of wood-based carbon must also be matched to the geometry of the current collector. Fig. 6C resolves the electron and electrolyte transport pathways in wood-based solid-state supercapacitors [100]. The aligned carbon microchannels enable preferential electron conduction along the direction of wood growth, while electrolyte penetration can proceed from both the longitudinal and transverse surfaces. By orienting the interdigitated fingers parallel to the axial carbon walls and collecting current from both sides, the current collector can intercept a greater number of continuous conductive paths. This configuration minimizes electron transfer via discontinuous radial connections and eliminates unnecessary bypassing around pore lumina.

Accordingly, enhancing the intrinsic conductivity of thick wood-based electrodes requires coordinated regulation of carbonization processes, secondary conductive phases, active material interfaces and current collection directions. High material conductivity alone is insufficient when the inherited axial pathways exhibit poor interfacial contact; in contrast, a moderately ordered carbon skeleton can support efficient charge transport at the electrode scale when its conductive interfaces and device geometry are properly integrated.

4.2.2 Gradient conductive networks: redefining reaction-zone localization

Gradient conductive networks do not merely serve to increase the average conductivity; instead, their core function is to redistribute electron supply and thereby achieve spatial regulation of the reaction front. In a uniformly conductive framework, electrons preferentially migrate to regions closest to the current collector or electrolyte inlet, leading to reaction localization at the electrode surface and underutilization of the internal active volume. In contrast, a spatially distributed conductivity gradient can shift the reaction zone toward the interior of the electrode and improve the utilization rate of the electrode along the thickness direction.

Zhu et al. validated this principle via a capillary-assisted silver infiltration approach into a basswood-derived carbon framework (Figs. 6D–6E) [101]. Inspired by the water transport mechanism in tree transpiration, a heated carbon framework was brought into contact with a controlled volume of silver precursor solution. The solution permeated upward through the longitudinal channels via capillary action, while rapid solvent evaporation progressively immobilized silver species along the transport direction. This process yielded a gradient distribution of lithiophilic silver nucleation sites, with density increasing from the top to the bottom of the framework, without clogging the intrinsically preserved axial channels. Although this structure is primarily classified as a nucleation site gradient rather than a directly characterized conductivity gradient, the high intrinsic conductivity of silver also improves local electronic accessibility in the silver-rich region. The reduced nucleation barrier and enhanced charge transfer kinetics at the bottom of the framework redirect lithium deposition from the separator-adjacent surface to a bottom-up growth mode, resulting in dense lithium filling and suppressed lithium dendrite growth.

Fig. 6F from Yang et al. further distinguishes lithiophilic gradients from intrinsic conductivity gradients [102]. A lithiophilic gradient can facilitate initial internal lithium nucleation, but its regulatory effect attenuates once active sites are covered by deposited lithium. In contrast, a continuous conductivity gradient maintains an inward-oriented electron transfer pathway throughout the entire deposition process, enabling electrons and Li⁺ to converge within the channels even at high areal capacities. In their liana-stem-derived carbon current collector, unilateral conductive slurry infiltration induced a gradient conductivity that increased from approximately 7.4 S·cm⁻¹ at the top of the structure to approximately 12.5 S·cm⁻¹ at the bottom. Furthermore, an ion-permeable but electronically insulating surface layer suppressed current crowding at pore openings, enabling sustained internal lithium deposition.

We conclude that the optimal gradient direction must be determined by the relative positions of the current collector, electrolyte inlet surface, and target reaction zone. For wood-based thick electrodes, unilateral infiltration, directional metallization, and depth-controlled deposition of reduced graphene oxide (RGO), CNTs, conductive polymers, or metal particles can successfully construct such gradients while retaining intact axial ion channels. Nevertheless, the fabrication precision required to achieve these gradients remains a major challenge, which we will revisit in Section 6.

4.3 Ion–electron coupling: the time-scale matching criterion

Local charge storage occurs exclusively when ions and electrons reach the identical active interface within comparable time scales. In thick electrodes, delayed ion replenishment restricts electrochemical reactions to the electrolyte-entry region of the electrode surface; meanwhile, insufficient electron conductivity deactivates active phases located far from the current collector. Wood offers a unique structural foundation to address this transport mismatch, as its lumina, cell walls, and load-bearing interfaces are spatially integrated within a single hierarchical framework [103]. Nevertheless, it should be emphasized that spatial proximity is a necessary but insufficient condition for kinetic coupling—the determining factor is whether the transport timescales of ions and electrons are synchronized.

Figs. 7A–7E consolidate the coupling concept across multiple wood-based composite systems. Fig. 7A summarizes two fabrication routes: direct carbonization converts cell walls into a continuous electron-conducting network while preserving lumina and pits to facilitate ion migration; alternatively, partially delignified wood is conformally coated with CNTs, conductive polymers, or metal particles, in which the cellulose framework retains liquid-transport channels and the coating provides electron conduction [61]. Fig. 7B illustrates the interfacial arrangement of redox-active wood electrodes: electrolyte stored in large-diameter lumina acts as the primary ion reservoir, interconnected micropores enable proton migration toward redox-active lignin, and a conformal CNT network constructs a continuous electron pathway adjacent to these ion-accessible regions [104]. The two networks are physically independent and separated only by the scale of the cell wall surface, resulting in a local coupling distance of less than 1 μm.

Fig. 7C illustrates this coupling effect in Ni/Co-MOF@CW electrodes (MOF, metal-organic framework) [105], where the continuous wood-based channels facilitate electrolyte transport through the thick electrode framework, while the carbonized cell walls collect electrons and provide structural support for Ni/Co-MOF nanoflowers. The active phase is directly positioned between the electrolyte-filled channels and the conductive carbon substrate, which shortens both ion and electron transfer distances and eliminates the repeated inter-particle contact that introduces interfacial resistance in conventional slurry-cast electrodes.

A comparable principle is demonstrated in the ~1.4 mm-thick NiMoN@WC electrode (Fig. 7D). Preserved longitudinal channels and artificial perforations construct through-thickness electrolyte transport pathways, while transverse pits enable ion redistribution between adjacent pores. NiMoN nanoclusters anchored to channel walls form hydrophilic interfaces with high ion accessibility, and establish direct electronic coupling with the wood-based carbon skeleton. The carbon skeleton concurrently enables electron conduction and immobilizes active particles, while the open multiscale channel architecture prevents conductive modification from blocking electrolyte transport. An approximate capacitance retention of 86% after 10,000 cycles further verifies that stable long-term coupling requires the structural integrity of the pore network, conductive framework and active-phase interface throughout repeated cycling [106].

Fig. 7E extends this concept to FCW@MXene electrodes reported in Ref. [107] (FCW, freeze-thaw/LiCl salt template carbonized wood). The combination of freeze–thaw treatment and LiCl templating constructs interconnected pore structures that enable electrolyte storage and ion redistribution, while the MXene nanosheets improve electrolyte wettability, electronic conductivity, and active site density. The inherent axial wood channels facilitate long-range mass transport, smaller pores promote uniform ion distribution toward the electrode’s internal surfaces, and MXene provides short-range electron transfer pathways along the channel interfaces. Nevertheless, excessive MXene loading induces nanosheet restacking and pore blockage, which demonstrates that improvements in electrical conductivity become counterproductive when accompanied by a reduction in ion-accessible active space.

We propose a quantitative coupling criterion based on the ratio of ion arrival time to electron arrival time at an arbitrary depth x:

(3)R_{\text{coupling}}\left(x\right)=\frac{t_{\text{ion}}\left(x\right)}{t_{\text{electron}}\left(x\right)}=\frac{x^2/D_{\text{eff}}}{x^2/{\sigma }_{\text{eff}}}=\frac{{\sigma }_{\text{eff}}}{D_{\text{eff}}},

where {\sigma }_{\text{eff}} and D_{\text{eff}} denote the effective electron diffusivity (which is directly proportional to electronic conductivity) and effective ion diffusivity, respectively. If R_{\text{coupling}} deviates from unity by more than one order of magnitude at any depth, the transport processes become decoupled, and the reaction front cannot propagate uniformly. This depth-dependent coupling condition clarifies the underlying mechanism for the inevitable formation of reaction gradients in uniform electrodes: the local value of R_{\text{coupling}} varies with x unless both D_{\text{eff}} and {\sigma }_{\text{eff}} are spatially graded.

Notably, in this coupling scenario, the contribution of wood originates from both its inherent physical structure and its chemically functionalizable interface. Lignin-derived aromatic domains, surface oxygen-containing groups and defect-rich carbon edges can actively participate in ion desolvation, charge transfer and nucleation regulation, thereby reducing the interfacial reaction time t_{\text{reaction}} through catalysis and binding effects [108]. Synthetic low-tortuosity scaffolds (3D printed, ice templated, laser drilled) can replicate physical pathways but lack this chemical reactive field functionality. Therefore, the real value of wood-based thick electrodes lies in the integrated physical and chemical transport platform, not just low curvature.

4.4 Design criteria for transport regulation

The preceding analysis derives four operational criteria for transport regulation in wood-based thick electrodes, as outlined below:

(1) Preservation priority: Structural integrity preservation must take precedence over functional performance enhancement. The natural axial channels and continuous cell wall skeleton form the fundamental architecture for both ion and electron transport. All modification strategies—including activation, coating, and active material loading—must retain the continuity of these primary transport pathways. In thick electrode configurations, strategies that significantly compromise channel openness or cell wall integrity are counterproductive, even if they substantially increase specific surface area or electrical conductivity.

(2) Device-specific priority: Design priorities must be determined based on the characteristic time scales of the target device. For mass-transfer-limited systems (e.g., supercapacitors), ion accessibility governed by tortuosity, wettability, and mesopore connectivity is the primary consideration. For solid-diffusion-limited systems (e.g., lithium-ion and sodium-ion batteries), the synergy between ion transport channels, electron conduction networks, and shortened solid-state diffusion paths is equally critical. For interface-limited systems (e.g., aqueous zinc batteries), additional constraints imposed by interfacial stability, dissolution inhibition, and dendrite regulation must also be incorporated into the design.

(3) Gradient utilization: Gradient structural design should be adopted to match the intrinsic flux gradients within thick electrodes. Electron flux in the conductive network, ion flux in the electrolyte, and reaction flux at active material surfaces all decay along the thickness direction, but exhibit distinct decay lengths. Uniform electrode structures inevitably lead to mismatch among these fluxes, resulting in localized reaction zones and underutilized active material regions. Gradient distribution of porosity, electrical conductivity, and active material loading can align the three types of flux, thereby extending the effective working thickness of the electrode and improving overall material utilization.

(4) Thickness-aware and direction-resolved evaluation: Conventional performance metrics such as gravimetric capacity and specific surface area are insufficient for evaluating thick electrodes. Areal capacity, volumetric capacity, direction-dependent tortuosity, thickness-direction potential distribution, and electrolyte infiltration kinetics should be reported in conjunction with conventional metrics, to enable meaningful cross-study comparisons and identify intrinsic transport limitations.

These criteria establish a conceptual framework for the device-specific analysis presented in Section 5. As elaborated in subsequent sections, different device types (including supercapacitors, lithium/sodium-ion batteries, and aqueous zinc batteries) require different priorities to be assigned to each criterion.

5 Device Integration

No single universally optimal wood-based thick-electrode architecture is applicable to all electrochemical energy storage systems [109]. With increases in electrode thickness and areal loading, ion transport through electrolyte-filled pores, electron conduction through the solid framework, solid-state diffusion within active materials, and interfacial reaction kinetics become simultaneously coupled processes [110]. The resultant concentration polarization, ohmic losses, and non-uniform reaction fronts ultimately lead to incomplete utilization of active materials in the deep electrode region [110]. Nevertheless, we argue that the device-dependent performance boundaries do not merely arise from the need to “optimize” a universal scaffold; instead, they fundamentally reflect distinct rate-determining processes across different electrochemical systems.

Based on the characteristic time framework developed in Section 4, we define the dominant time-scale spectrum for three representative device categories as follows:

(1) Supercapacitors: Energy is stored via electric double-layer adsorption or fast surface redox reactions (t_{\text{reaction}}\ll 1 s). The dominant rate-limiting factor is ion transport within the electrolyte phase (t_{\text{ion}}\sim L^2/D_{\text{eff}}). Electron transport (t_{\text{electron}}) is generally faster and does not limit the overall reaction rate when the carbon skeleton maintains structural continuity.

(2) Lithium/Sodium-ion batteries: Energy is stored via solid-state intercalation, alloying, or conversion reactions (t_{\text{solid}}\sim r_{\text{p}}^2/D_{\text{solid}}, typically 1–10³ s). The upper performance limit is governed by the synergistic competition among t_{\text{ion}}, t_{\text{solid}}, and t_{\text{electron}}, where the slowest process among the three dictates the realizable effective capacity.

(3) Aqueous zinc batteries: Energy storage is achieved via Zn²⁺ intercalation or conversion reactions, and the process is complicated by multiple secondary effects including strongly hydrated Zn²⁺ species, cathode dissolution, zinc dendrite growth, hydrogen evolution, and electrode corrosion. In addition to ion transport processes (t_{\text{ion}}), the interfacial stability time t_{\text{interface}}—defined as the time elapsed before the onset of side reactions or performance degradation—acts as an additional critical constraint for device performance.

Therefore, the integration of wood-based thick electrodes should not be interpreted as simply transferring identical wood-based porous carbon scaffolds to different energy storage systems. Instead, it requires device-specific structural and interfacial engineering tailored to the dominant kinetic limitations of the target device. More importantly, the irreplaceable value of wood does not solely derive from its low-tortuosity channels: synthetic architectures including 3D-printed, ice-templated, and laser-drilled structures can also provide directional ion transport pathways [111]. Wood’s unique advantage stems from its inherent simultaneous integration of oriented transport channels, chemically heterogeneous cell walls, native multiscale interfacial chemistry, and mechanically adaptive hierarchical frameworks [112]. After delignification, carbonization, activation, or heteroatom doping, structures derived from cellulose, hemicellulose, and lignin can be converted into polar functional groups, defect-rich carbon domains, aromatic carbon frameworks, and confined nanopores [113]. These chemically active interfaces regulate electrolyte wettability, ion desolvation, active material anchoring, soluble intermediate adsorption, and metal ion nucleation, all of which are functions that pure physical templates cannot replicate [114].

5.1 Mass-transfer-dominated devices: supercapacitors

Supercapacitors store energy through electric double-layer adsorption and rapid surface redox reactions, inherently exhibiting faster reaction kinetics than battery-type electrodes [115-117]. Nevertheless, as electrode thickness and areal mass loading increase, the dominant limiting factor shifts from charge-transfer kinetics to mass transport [118]. The electrolyte is required to penetrate deeper into the porous electrode framework, which inevitably leads to the formation of ion concentration gradients along the thickness direction [119]. As a result, regions near the electrolyte interface preferentially participate in charge storage, while the internal active surfaces remain partially inaccessible to electrolyte ions.

Wood-based carbon offers a unique structural solution, as its naturally aligned vessels and tracheids can be converted into low-tortuosity ion-transport channels [120,121]. As demonstrated in Fig. 8A, natural wood can be converted into vertically aligned carbon architectures via carbonization and activation: the native vascular channels are retained, while additional microporous structures are generated [122]. These hierarchical channels shorten electrolyte diffusion pathways and promote rapid ion penetration into thick electrodes. In contrast to randomly assembled particulate carbon electrodes [123], the continuous wood-based carbon framework eliminates convoluted ion migration pathways and provides a more efficient transport network for high-mass-loading energy storage applications [124,125].

The effectiveness of hierarchical pore regulation is further verified in Fig. 8B, where pore-size distribution analysis confirms the coexistence of micropores and mesopores after the activation treatment [122]. In thick wood-based supercapacitors, pore structures of different scales fulfill distinct functions [126]: macropores act as transport highways for electrolytes, mesopores function as ion buffering reservoirs, and micropores provide abundant surface sites for electric double-layer formation [127]. Nevertheless, it should be noted that simply maximizing microporosity is not an effective strategy for thick electrodes, since isolated micropores with poor connectivity cannot be rapidly accessed during high-rate operation. The core design principle is to construct an interconnected macropore–mesopore–micropore network, rather than merely increasing the BET specific surface area.

Beyond the pore architecture, the orientation of channel and the surface wettability strongly influence the accessibility of electrolytes. Fig. 8C compares the structures with different channel orientations, and demonstrates that vertically aligned channels in the direction of growth exhibit ultrafast, nearly instantaneous wetting. [106]. The inherent anisotropic structure of wood enables electrolyte transport along continuous cell vessel pathways, which reduces through-plane tortuosity. Meanwhile, the hydrophilic functional groups introduced during activation improve the affinity for electrolyte and reduce non-wetting inside the excessive oxidation or excessive surface functionalization. Nevertheless, we stress that excessive oxidation or excessive surface functionalization excessive oxidation or can sacrifice the carbon conductivity and introduces unwanted side reactions, so the improvement of wettability requires a balance between electronic transport and [128].

To further increase the energy density, pseudocapacitive active materials can be introduced onto carbonized structures of wood frames, as shown in [129,130]. However, the loading of active materials must be carefully controlled to preserve ion accessibility, since this sacrifice is an impediment. Fig. 8D illustrates an ultra-thin, wood-structured hydrophilic membrane decorated with nanoscale active materials, which retains its vertically aligned channels after functionalization, as shown in [106]. The conformal growth of nanostructured on the active phases on the pore walls increases the number of redox, which maintains the electrolyte pathways, as shown in [131]. This strategy outperforms the dense filling of wood lumens, which can block ion channels and increase diffusion resistance.

The significance of constructing well-maintained electron/ion coupled transport pathways is further illustrated in Fig. 8E, where the integration of conductive polymers into wood cell walls constructs continuous electronic networks while retaining the native porous structure of the material [132]. In contrast to conventional composite electrodes, in which active materials are randomly distributed, the wood-based framework facilitates directional electron transport and uniform utilization of active materials across the full thickness of the electrode [133,134]. This hierarchical structural integration offers a viable approach to simultaneously enhancing areal capacitance and rate capability [135].

Finally, we contend that the practical value of wood-based thick supercapacitors should be determined by device-level performance, rather than gravimetric metrics [136]. As illustrated in Fig. 8F, when ion transport pathways are rationally designed, increasing electrode mass loading can significantly improve areal energy storage capacity while maintaining a high capacitance retention rate [137]. However, excessive electrode thickness without sufficient ion accessibility will lead to inactive internal regions and decreased volumetric efficiency [138]. Accordingly, future performance evaluation of this type of supercapacitor should comprehensively take into account multiple key parameters: electrode thickness, areal mass loading, areal capacitance, volumetric capacitance, energy density, rate retention and electrolyte consumption.

Key design takeaways for supercapacitors: priority should be ordered as low tortuosity > wettability > mesopore connectivity > accessible surface area. Electronic conductivity only needs to reach the percolation threshold; beyond this point, further improvements deliver diminishing returns relative to optimizations of ion accessibility.

5.2 Phase-reaction-dominated devices: lithium-ion and sodium-ion batteries

In contrast to capacitive energy storage, lithium-ion and sodium-ion batteries involve complex faradaic phase reactions, namely ion intercalation/deintercalation, alloying, and conversion processes [139,140]. These reactions require the coordinated transport of electrolyte-phase ions, interfacial desolvation and charge transfer, solid-state diffusion within active materials, and continuous electron conduction throughout the electrode framework [141,142]. Accordingly, while the intrinsic oriented channels of wood enable efficient electrolyte infiltration, they cannot independently mitigate the kinetic limitations of thick electrodes, including insufficient electronic connectivity, sluggish intraparticle diffusion, and non-uniform interfacial reactions [143,144].

Increasing electrode thickness represents an effective strategy to enhance areal capacity via increasing active material loading [145,146]; nevertheless, this approach inevitably exacerbates reaction heterogeneity [147]. As demonstrated in Fig. 9A, conventional thick electrodes with randomly distributed pores suffer from prolonged Li⁺ transport pathways and poor electrolyte accessibility, leading to insufficient utilization of internal active materials [148]. In comparison, vertically aligned channels constructed from wood-based architectures can substantially reduce electrolyte diffusion distances and form low-tortuosity pathways for ion migration.

In addition to efficient ion transport, high-performance thick electrodes demand the construction of integrated continuous electron transport networks [149]. Fig. 9B presents a vertically patterned channeled electrode architecture integrated with a bicontinuous ion/electron transport network [148]. The aligned channels promote electrolyte infiltration and ion migration, while the interconnected conductive framework facilitates rapid electron transfer between active materials and current collectors [150]. This structural synergy underscores a core design principle: carbonized cell walls should not act solely as mechanical supports, but also function as continuous conductive skeletons to sustain electrochemical activity in the deep regions of thick electrodes.

The significance of hierarchical pore engineering is further illustrated in Fig. 9C, where electrolyte reservoirs and interconnected transport pathways are constructed to optimize the Li⁺ flux distribution within thick electrodes [151]. Macropores serve as electrolyte storage domains, while smaller interconnected pores provide shortened diffusion paths toward active sites. This hierarchical architecture effectively balances electrolyte accessibility and electrode density, inhibiting excessive inactive volume while sustaining fast reaction kinetics [152].

In addition to acting as conductive scaffolds for cathode materials, wood-based carbon can directly serve as hard-carbon anodes, especially for sodium-ion batteries. As illustrated in Fig. 9D, different wood precursors display distinct X-ray diffraction (XRD) profiles, which reflect variations in cellulose crystallinity and the structural ordering of carbon [153]. A higher content of crystalline cellulose promotes the formation of longer turbostratic carbon layers during pyrolysis, which in turn facilitates the generation of closed pores within the carbon matrix [154]. These structural characteristics are critical for sodium storage, as closed pores can provide additional Na⁺ accommodation sites and contribute to the low-voltage plateau capacity [155]. The chemical evolution of wood precursors during carbonization is further elaborated in Fig. 9E, where Fourier transform infrared spectroscopy (FTIR) spectra demonstrate the conversion of cellulose, hemicellulose, and lignin components into carbon frameworks [153]. The removal and reconstruction of oxygen-containing functional groups modulate defect density, surface chemistry, and interfacial reactions [156].

However, it should be emphasized that excessive structural defects and open pores may accelerate electrolyte decomposition and the growth of solid-electrolyte interphase, which consequently reduces the initial Coulombic efficiency [157]. Conversely, excessive graphitization will decrease the accessibility of ions to active sites. Accordingly, the carbonization temperature and pore structure of carbon-based electrodes must be elaborately optimized to balance electronic conductivity, ion storage capacity and interfacial stability.

Compared with supercapacitors, lithium-ion and sodium-ion batteries impose more stringent requirements on electrode thickness, since solid-state diffusion and interfacial stability become the dominant limiting factors when the electrode thickness is large. As illustrated in Fig. 9F, increasing electrode thickness can raise the areal active material loading, but maintaining high-capacity retention necessitates effective regulation of ion/electron transport pathways and electrochemical reaction uniformity [148]. Therefore, the critical electrode thickness is not a fixed constant, but is jointly determined by electrolyte conductivity, pore tortuosity, active material particle size, electronic conductivity, electrode porosity and operating current rate.

From a practical perspective, the evaluation of wood-based lithium/sodium battery electrodes should not be limited to half-cell performance. High areal capacity does not necessarily translate to high cell-level energy density when it is accompanied by excessive electrolyte consumption, unbalanced electrode capacities, oversized counter electrodes, or excessive inactive components [158]. Future research should focus on realistic full-cell configurations, including N/P ratio, electrolyte dosage, separator thickness, active material fraction, and electrode density [159].

Key design takeaways for lithium/sodium-ion batteries are summarized as: Prioritization of ion/electron synergy > solid diffusion control > continuous conductive network > SEI regulation. The wood scaffold must simultaneously serve as an ion transport pathway, an electron conductive backbone, and an active material support—a tripartite requirement that necessitates more rigorous structural optimization than that for supercapacitors.

5.3 Interface-restricted devices: aqueous zinc batteries

Aqueous zinc batteries (AZBs) are a promising class of sustainable energy storage systems, benefiting from their high safety, low cost, and the abundant global reserves of zinc resources [160]. However, unlike supercapacitors and lithium-ion/sodium-ion batteries, the performance of AZBs is primarily limited by interfacial reaction kinetics, rather than bulk ion transport alone [161]. Strongly hydrated Zn²⁺ ions feature a large solvation shell and a high desolvation energy barrier. Meanwhile, cathode dissolution, intermediate migration, zinc dendrite growth, hydrogen evolution, and interfacial corrosion continuously degrade the cycling reversibility of aqueous zinc-ion batteries [162,163]. We propose that wood-based thick electrodes for AZBs must be engineered with multifunctional interfaces that concurrently regulate ion transport, confine active species, and stabilize zinc deposition behavior.

The inherent hierarchical structure of wood serves as an ideal structural platform for the fabrication of high-performance zinc electrodes. As shown in Fig. 10A, the freestanding wood-based electrode leverages the naturally aligned vascular channels of wood to construct continuous electrolyte pathways [164]. These vertically aligned channels reduce ion transport tortuosity and promote electrolyte penetration throughout thick electrodes, thereby improving the accessibility of hydrated Zn²⁺ ions [165]. Nevertheless, it should be noted that efficient ion transport in aqueous zinc-ion systems requires not only interconnected open channels but also optimized interfacial chemistry, because the large radius of hydrated Zn²⁺ ions introduces additional desolvation constraints.

To address the key challenges of high-areal-loading electrodes, hierarchical pore engineering and targeted chemical functionalization are indispensable strategies [166]. Fig. 10B illustrates the fabrication of a wood-based zinc host via structural pretreatment and in-situ growth of active components within the pristine wood framework [164]. The well-preserved three-dimensional porous network provides abundant nucleation and deposition sites, and homogenizes Zn²⁺ flux distribution, converting the traditional planar Zn/electrolyte interface into a spatially homogeneous deposition platform [167]. The interconnected wood channels reduce localized current concentration, while the oxygen-containing functional groups naturally existing in the cellulose framework interact with hydrated Zn²⁺ species, lowering the nucleation energy barrier and facilitating uniform zinc deposition and growth.

For the regulation of cathode-side reactions, Fig. 10C presents a representative wood-based hierarchical carbon electrode fabricated via delignification, resin infiltration, in-situ polymerization and heteroatom doping [168]. The as-obtained carbon framework integrates macroporous electrolyte reservoirs, mesoporous ion-buffering regions and microporous active sites, which achieves a balance between rapid ion transport and high-density charge storage [169]. Furthermore, N/O co-heteroatom doping modulates the surface electronic structure and enhances the adsorption/desorption kinetics of Zn²⁺, thereby improving the interfacial reaction activity under high mass loading conditions [94].

The significance of rational pore-size regulation is further emphasized in Fig. 10D [168]. Since Zn²⁺ transport is dominated by hydrated ion species rather than bare ions, excessive micropore confinement may impede ion accessibility and increase transport resistance [170]. Accordingly, pore architecture design for wood-based zinc electrodes should prioritize the size and interaction of hydrated ions over the simple maximization of specific surface area.

In addition to cathode engineering, wood-based architectures offer unique advantages for stabilizing zinc metal anodes. As illustrated in Fig. 10E, a flexible wood@Ni@Zn electrode can be fabricated via delignification of natural wood, electroless Ni plating, and subsequent Zn electrodeposition [171]. The Ni coating converts the inherently electrically insulating wood substrate into a conductive three-dimensional current-distribution framework, while the hierarchical cellulose structure retains favorable electrolyte accessibility. The wood scaffold increases the effective Zn deposition area, homogenizes the electric field distribution, and inhibits heterogeneous localized nucleation. The efficacy of this design strategy is verified in Fig. 10F, where the wood-based Zn host maintains uniform Zn deposition after prolonged cycling. In contrast to planar Zn substrates, which suffer from severe dendritic accumulation and uneven growth, the three-dimensional wood framework enables stable Zn plating/stripping by regulating ion flux and providing uniformly distributed nucleation sites [172].

Nevertheless, it should be noted that excessive porosity and excessive electrolyte uptake should be avoided, since an over-enlarged electrode/electrolyte contact area may accelerate the hydrogen evolution reaction and anode corrosion [173]. Therefore, practical wood-based zinc electrodes require balanced optimization of pore architecture, electrolyte retention capacity, and interfacial chemistry.

Key design takeaway for aqueous zinc batteries: priority should be given to the synergistic optimization of mass transport, zinc deposition confinement and interfacial chemistry. The intrinsic chemical functional groups of wood (−OH, −COOH, and lignin-derived quinone) are not just wettability promoters, but also active participants in Zn²⁺ desolvation, nucleation regulation, and soluble intermediate species confinement—a unique functionality that cannot be replicated by synthetic physical templates.

5.4 Performance boundaries and design principles across device platforms

The preceding discussion leads to a critical conclusion: no universal optimal wood-based thick-electrode architecture exists. The structural requirements for supercapacitors, lithium/sodium-ion batteries, and aqueous zinc batteries differ fundamentally, as their dominant rate-determining processes are distinct:

(1) Supercapacitors (t_{\text{ion}}-limited): Require low tortuosity, excellent wettability, and mesoporous electrolyte reservoirs. Electronic conductivity only needs to reach the percolation threshold, and further improvements deliver diminishing marginal returns.

(2) Lithium/Sodium-ion batteries (t_{\text{ion}}–t_{\text{solid}}–t_{\text{electron}} synergistic): Require continuous ionic transport channels, integrated electronic conduction networks, and shortened solid-state diffusion paths simultaneously. The wood scaffold must fulfill three core functions: acting as an ionic highway, an electron conduction backbone, and an active material support. This tripartite requirement calls for more stringent structural optimization.

(3) Aqueous zinc batteries (t_{\text{ion}}–t_{\text{interface}} coupled): In addition to optimized transport performance, these devices require physical confinement of soluble active species, zincophilic interface regulation, and suppression of parasitic reactions. The chemical functional groups naturally present on wood act as active interface regulators, rather than merely serving as passive supports.

Table 1 summarizes these divergent design principles by comparing the dominant performance limitations, preferred channel orientation, key pore characteristics, conductive network requirements, critical interface properties, and core design priorities across the three energy storage device platforms.

Crucially, we emphasize that the universal advantage of wood-based thick electrodes lies in their ability to simultaneously provide ion transport channels, conductive electronic frameworks, sufficient loading space for active materials, and functionalizable interfaces. However, the device compatibility of such electrodes depends on which of these structural units plays a dominant role in a given specific system. For supercapacitors, wood-based structures are primarily leveraged to enhance the interfacial accessibility of ions. For lithium-ion and sodium-ion batteries, wood structures are mainly utilized to coordinate liquid-phase mass transport, electron conduction, and solid-state diffusion. For aqueous zinc-ion batteries, the hierarchical confinement spaces and tunable interfaces of wood structures are further exploited to suppress active material dissolution, zinc dendrite growth, and undesirable side reactions.

We further posit that the advantages of wood are more pronounced in aqueous zinc-based batteries, and to a lesser degree, in battery systems where interfacial chemistry plays a critical role. For supercapacitors, synthetic low-tortuosity scaffolds (including 3D-printed and ice-templated variants) can compete more directly with the physical structure of wood-based materials. However, for battery systems that require interfacial regulation (e.g., Zn²⁺ desolvation, solid electrolyte interphase (SEI) stabilization, and dendrite suppression), the chemically active interfaces of wood—including oxygen-containing functional groups, defect sites, and lignin-derived redox active moieties—offer functional advantages that cannot be replicated by pure physical templates. Accordingly, the strategic positioning of wood-based thick electrodes should prioritize this chemophysical dual functionality, rather than competing solely on physical porosity or electrical conductivity.

Only by adhering to this device-oriented design principle can wood-based thick electrodes truly transition from natural porous scaffolds to structured electrode platforms for high-areal-energy-density energy storage devices.

6 Challenges and Perspectives

6.1 Challenges

Despite remarkable advances in the structural engineering and transport regulation of wood-based thick electrodes, their practical deployment remains restricted by a series of inherent trade-offs between structural optimization and electrochemical performance. Instead of categorizing these challenges as discrete technical barriers, we reframe them as a quadruple trade-off matrix that requires systematic resolution:

(1) Ion transport vs. mechanical integrity: Artificial perforation, deep delignification, and aggressive activation can effectively reduce transport resistance, yet these approaches inevitably compromise cell wall continuity and structural strength [174]. For aqueous electrolytes, low-tortuosity transport channels can push the critical electrode thickness to the millimeter scale [78]. Nevertheless, in organic electrolyte systems where ionic conductivity is roughly one order of magnitude lower, the theoretical practical thickness limit is projected to decrease significantly, typically falling to the range of hundreds of micrometers. This electrolyte-dependent thickness constraint is rarely addressed in existing studies that exclusively adopt aqueous electrolyte systems.

(2) Areal loading vs. depth utilization: The core impetus for developing thick electrodes is to elevate areal loading (above 20 mg·cm⁻²) to ultimately improve areal energy density. However, an increase in active material volume inevitably occupies pore space, reduces the effective volume of electrolyte-filled pores, and lowers effective ionic diffusivity. Experimental data demonstrates that when areal loading increases from approximately 10 mg·cm⁻² to 30 mg·cm⁻², active material utilization generally decreases from over 85% to below 50%. We define this phenomenon as the “loading-utilization inversion”: once the critical loading threshold is exceeded, further increases in active material mass lead to diminishing capacity returns, while simultaneously driving a proportional growth in the demand for inactive electrolyte.

(3) Electronic conductivity vs. ionic accessibility: Enhancing the graphitization degree of carbon-based materials or introducing conductive fillers can effectively boost electronic conductivity. However, these strategies often reduce material porosity, constrict ion transport pathways, and even block interconnected ion channels [175]. While gradient conductive networks exhibit promising potential for regulating reaction distribution, their preparation requires complex processing procedures, which poses a substantial challenge to manufacturing reproducibility. Fundamentally, this trade-off is asymmetric: even a minor reduction in ionic accessibility can offset the considerable performance gains obtained from improved electronic conductivity, given that ions act as the primary reactant carriers in electrochemical systems.

(4) Laboratory demonstration vs. industrial translation: The inherent heterogeneity of wood precursors—including density, annual ring width, chemical composition, and anatomical structure, which vary across different species and growth conditions, and even differ within a single trunk—forms a major obstacle to the consistency of large-scale manufacturing [176]. Most existing studies rely on carefully screened small-area wood samples (less than 1 cm²) paired with optimized laboratory-level carbonization protocols. To date, scaling the preparation of wood-based electrodes to the meter scale while maintaining uniform pore architecture and stable electrochemical performance remains an unresolved challenge. Furthermore, the absence of standardized raw material selection criteria and quality assessment protocols further hinders the industrial adoption of wood-based electrode materials.

Beyond these trade-offs, we identify a critical knowledge gap: the fundamental understanding of multi-physics field coupling within thick electrodes remains severely limited. Current electrochemical characterization techniques, including electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge-discharge (GCD), only yield spatially averaged responses and are unable to resolve the depth-dependent distributions of ion concentration, electric potential, and local reaction rates. Operando techniques such as X-ray diffraction and Raman spectroscopy are constrained by insufficient penetration depth for millimeter-scale electrodes [177]. While X-ray computed tomography enables three-dimensional structural reconstruction, it cannot simultaneously visualize the dynamic evolution of electrolyte concentration and solid-phase reactions during electrochemical operation. This characterization bottleneck directly impedes the validation of theoretical models and the rational design of gradient electrode architectures.

6.2 Perspectives

Future research on wood-based thick electrodes should advance synergistically across three complementary dimensions. Only through coordinated progress in these domains can wood-based thick electrodes transition from laboratory-scale demonstrations to practical high-energy-density electrochemical energy storage systems.

Perspective 1: From empirical optimization to predictive multi-scale modeling.

Developing multiscale transport-reaction models that can describe coupled physicochemical processes within thick electrodes should be the top priority of future research. Future theoretical frameworks need to integrate density functional theory (DFT) calculations at the atomic scale, molecular dynamics (MD) simulations at the molecular scale, and phase-field modeling at the mesoscale into a unified computational platform. Such a multiscale framework will facilitate the transition from empirical trial-and-error optimization to mechanism-guided rational design.

More importantly, these models must be validated via advanced operando characterization techniques. Emerging methods including micro-reference electrode arrays, neutron depth profiling, and operando electrochemical nuclear magnetic resonance (NMR) are expected to provide spatially resolved data on ion concentration, potential distribution, and local reaction kinetics across the entire electrode thickness. This experimental feedback will deliver essential support for model validation and parameter calibration, addressing the current lack of direct insights into the internal physicochemical evolution of electrodes during operation.

Perspective 2: From physical templates to chemically active reaction fields.

Future research should go beyond the use of natural wood as the sole precursor, and explore multifunctional hybrid architectures by integrating wood-based frameworks with synthetic polymers, nanomaterials, and other functional components [176,178]. In particular, we propose that wood-based electrodes will evolve from passive structural templates into active chemical reaction fields. The oxygen-containing functional groups, defect-rich carbon domains, and lignin-derived aromatic structures on wood surfaces are not simply wettability modifiers—they are active participants in ion desolvation, charge transfer acceleration, soluble species confinement, and nucleation regulation. This intrinsic chemical functionality endows wood-based materials with a unique competitive advantage over synthetic low-tortuosity scaffolds (including 3D-printed, ice-templated, and laser-drilled variants).

Gradient structural engineering remains a viable practical approach for coordinating ion transport, electron conduction, and reaction distribution in wood-based thick electrodes. Future research should prioritize the development of reproducible fabrication methods to regulate porosity, electrical conductivity, and active material loading uniformly across the full electrode thickness [179].

Perspective 3: From laboratory demonstration to predictively engineerable platforms.

Machine learning demonstrates favorable applicability when deployed on well-defined processing datasets. A database that correlates wood species, density, annual ring width, and chemical composition with carbonization temperature, heating rate, dimensional shrinkage, and pore structure can support wood species screening and the prediction of dimensional shrinkage during carbonization [180]. Processing parameters, including precursor concentration, viscosity, infiltration pressure, and treatment time, can be adjusted to select processing conditions that enhance biomass penetration while mitigating pore blockage. When coupled with depth-resolved characterization measurements, these machine learning models can also guide the regulation of active material distribution from the electrode surface to its interior, improving the balance between areal loading and utilization across the entire electrode thickness.

From a practical application standpoint, the commercialization of wood-based thick electrodes is expected to proceed in a phased manner. In the short to medium term, their intrinsic advantages—low cost, sustainability, and self-supported architecture—make them especially attractive for large-scale aqueous energy storage systems, low-frequency supercapacitors, and flexible or wearable electronic devices, where ultra-high rate capability is not the core performance requirement [181,182]. In the long run, sustained progress in conductivity enhancement, interface engineering, and structural optimization is anticipated to enable their application in high-energy lithium-ion and sodium-ion batteries with organic electrolytes.

The overarching vision is to establish a closed-loop design-manufacture- evaluation paradigm (Fig. 11): multi-scale modeling predicts optimal gradient architectures, AI-assisted screening identifies suitable wood precursors and processing conditions, advanced operando characterization validates transport-reaction coupling mechanisms, and performance feedback refines the model framework. This iterative loop will gradually advance wood-based thick electrodes from natural structural templates to engineered electrode platforms for next-generation sustainable energy storage.

7 Conclusions

This review establishes a systematic structure–transport–performance framework for wood-based thick electrodes, whose core argument extends far beyond conventional structural synergy analysis. We conclude that the rational design of wood-based thick electrodes must be centered on addressing the inherent trade-off between areal loading and depth utilization.

From the structural perspective, the natural axial channels, hierarchical pore networks, and continuous cell wall skeletons of natural wood provide a unique foundation for the fabrication of millimeter-scale self-supporting thick electrodes. Retaining these inherent structural properties during carbonization and functionalization is of critical importance: any damage to the primary transport pathways or mechanical integrity will inevitably weaken the inherent advantages of wood over traditional slurry-cast electrodes. The orientation of channels relative to the transport direction of the device, the connectivity among macropores, mesopores and micropores, as well as the balance between pore accessibility and active material loading, collectively determine the structural performance of wood-based frameworks.

From the transport perspective, ion conduction and electron conduction do not contribute to device performance independently. They are mutually coupled processes that require matching at active interfaces. Merely achieving low tortuosity is insufficient when electrolyte wettability is poor or pore connectivity is inadequate. Similarly, high electronic conductivity alone cannot guarantee satisfying performance if the conductive network is discontinuous or excessive active material filling blocks ion transport pathways. The effective coupling of ion and electron transport is reflected in uniform reaction distribution along the electrode thickness direction and improved utilization of active materials in deep electrode regions. Gradient conductive networks and spatially customized porosity distribution are promising strategies to accommodate the intrinsic flux gradient in thick electrodes.

From the device integration perspective, different electrochemical energy storage systems impose fundamentally different requirements on the structural design of wood-based thick electrodes. Mass-transfer-limited supercapacitors prioritize low tortuosity, sufficient wettability, and mesoporous electrolyte reservoirs. Solid-diffusion-limited lithium-ion/sodium-ion batteries require simultaneous synergy among ion channels, continuous electronic networks and shortened diffusion paths. Interface-limited aqueous zinc batteries additionally require physical confinement of soluble species and zincophilic interface regulation. Therefore, the optimal wood-based structure is device-specific, and design principles should be formulated based on the dominant limiting factor of the target device, rather than adopting a one-size-fits-all universal structural template.

Three paradigm shifts define the future trajectory of this field:

(1) From passive templates to active reactive domains: Oxygenated surfaces, defect-rich structures and lignin-derived redox moieties in wood are not merely structural appendages, but active participants in transport regulation and interfacial electrochemistry. This intrinsic chemical functionality, rather than low tortuosity alone, constitutes wood’s unique competitive advantage over synthetic scaffolds.

(2) From isolated optimization to coordinated engineering: Porosity, conductivity, wettability and active material distribution cannot be optimized independently. These properties must be co-engineered as an integrated system, where gradient architectures align the inherent flux gradients of ions, electrons and reactions across the full thickness of the electrode.

(3) From laboratory demonstration to predictively engineerable platforms: AI-assisted precursor screening, multiscale modeling and standardized manufacturing protocols will transform wood from a heterogeneous natural material into a consistent, scalable electrode platform for sustainable energy storage.

In summary, wood-based thick electrodes offer a feasible pathway toward the development of sustainable, high-energy-density electrochemical energy storage devices. By reframing wood not as a passive structural template, but as an actively engineered, chemically functionalized transport-reaction platform, and by adopting the paradigm shifts outlined above, this field can overcome the long-standing trade-off between high areal loading and active material utilization, ultimately delivering next-generation energy storage solutions that are both high-performance and environmentally sustainable.

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