Three-dimensional composite Li metal anode by simple mechanical modification for high-energy batteries

Abstract

Lithium (Li) metal is believed to be the “Holy Grail” among all anode materials for next-generation Li-based batteries due to its high theoretical specific capacity (3860 mAh/g) and lowest redox potential (−3.04 V). Disappointingly, uncontrolled dendrite formation and “hostless” deposition impede its further development. It is well accepted that the construction of three-dimensional (3D) composite Li metal anode could tackle the above problems to some extent by reducing local current density and maintaining electrode volume during cycling. However, most strategies to build 3D composite Li metal anode require either electrodeposition or melt-infusion process. In spite of their effectiveness, these procedures bring multiple complex processing steps, high temperature, and harsh experimental conditions which cannot meet the actual production demand in consideration of cost and safety. Under this condition, a novel method to construct 3D composite anode via simple mechanical modification has been recently proposed which does not involve harsh conditions, fussy procedures, or fancy equipment. In this mini review, a systematic and in-depth investigation of this mechanical deformation technique to build 3D composite Li metal anode is provided. First, by summarizing a number of recent studies, different mechanical modification approaches are classified clearly according to their specific procedures. Then, the effect of each individual mechanical modification approach and its working mechanisms is reviewed. Afterwards, the merits and limits of different approaches are compared. Finally, a general summary and perspective on construction strategies for next-generation 3D composite Li anode are presented.

Graphical abstract

Keywords

lithium (Li)-ion battery (LIB) / Li metal battery / three-dimensional (3D) composite Li metal anode / mechanical modification / reducing local current density

1 Introduction

The increasing fossil energy crisis and conjoint environment pollution problem have been forcing the development of renewable energy, such as solar energy, windy energy, tidy energy, and thermal energy [1–3]. In addition, the storage and application of the collected electricity further push the energy-storage devices forward, especially for the secondary batteries [4,5]. Recently, lithium (Li)-ion batteries (LIBs) have been applied in electric vehicles and portable electronics, promoting the revolutionary progress of green transportation and low-carbon productivity [6–10]. However, the short-range problem of electric vehicle and unsatisfactory cell phone battery life restricted by limited LIB energy density is urgently to be resolved [11–13]. Therefore, advanced batteries with higher energy density are keenly required for long-range service life. Compared with traditional graphite or newly-emerged Si-based electrode, Li metal is recognized as one of the most promising anode materials for next generation high energy density batteries on account of its ultrahigh theoretical capacity and lowest redox potential [14–18].

In spite of various advantages brought by Li metal anodes for batteries, however, the notorious dendrite issue and “hostless” deposition which cause safety risks and capacity fading severely hindered its practical application [19–24]. Dendrite problem of Li metal anode has been first coming into researchers’ sight, and multiple strategies have been developed to suppress Li dendrite including separator engineering, current collector modification, artificial solid electrolyte interphase (SEI), electrolyte additives, solvation engineering and utilization of solid-state electrolyte, etc. [25–30]. But the “hostless” deposition problem lacks focus and remains unsolved. Fortunately, a series of composite Li metal anodes with a three-dimensional (3D) current collector has been proposed to tackle the “hostless” deposition problem by maintaining a stable electrode volume. Moreover, this design has been shown to be also effective on dendrite suppression since the increased surface area lowers local current density [31–35]. Based on the fact that the Sand’s time is inversely proportional to the current density [36], 3D composite Li metal anode could reduce dendrite risk effectively compared with that of the conventional planar Li metal anode [37]. At the same time, the high structural stability reassures the stable electrode shape/volume of the 3D composite Li metal anode [38–40].

Though the design of 3D composite Li metal anode tackles the two intrinsic problems of Li anode and could bring additional benefits, its fabrication poses a great challenge. Most of the fabrication strategies require harsh experimental conditions and complex procedures, which greatly impede the practical application of this 3D composite anode design. Taking temperature as an example, a major fabrication approach called “thermal infusion” requires at least about 180 °C or above for the fabrication to take place since Li needs to be melted first before infusion. The low melting point of 170 °C of Li makes it easier for Li metal to be melting infused into various porous frameworks to form the 3D composite anode. There are enormous works which verify the validation of the as-derived 3D composite Li metal anode [41–47]. Despite their effectiveness, drawback still remains for this thermal infusion method in terms of the high-temperature condition and extra lithophilic treatment which may involve additional steps [48,49]. In addition to these harsh conditions and additional steps, the traditional way to fabricate 3D composite Li anode may also require delicate instruments and high-profile equipment, which again lowers the potential of this design in speaking of large-scale industrial implications.

Under this circumstance, novel methods to build 3D composite Li anode are highly demanded. It is noteworthy that, apart from the low melting point, another intrinsic characteristic of Li brought by its weak metallic bonding is the softness [50]. Fortunately, various types of 3D (and/or) composite Li anode have been developed successfully based on the softness and ductility of Li metal using simple mechanical modifications, which get rid of harsh experimental conditions, high temperatures, complex procedures, and the necessity of delicate instruments. At the very beginning, an intuitively 3D structured Li metal was fabricated by the simple mechanical surface modification technique, such as the mechanical micro-needle or the mechanical roll-press technique [51,52]. The as-fabricated patterned Li metal demonstrates an increased active surface area which leads to a reduced local current density during galvanostatic cycling, eventually contributing to a better cycling performance. At the same time, researchers found that the mechanical surface modification technique also led to reduced resistance thanks to the newly created fresh Li on pristine passivated surface [51]. Subsequently, the relationship between current density distribution and pattern dimension was also studied systematically [53,54]. Additionally, the optimization of pattern geometric and dimension was proposed, and the suitable pattern width, depth, and interval were screened out for stable Li metal deposition. Moreover, mold selection was also optimized by researchers to get a less roughness surface of the patterns and pave the road for industrial scalable production [55]. Recently, the 3D geometrical design technique was also successfully adopted for structural optimization of Li metal based solid-state battery [56]. With the performance improvement resulted from mechanical surface modification on Li alone reaching its limit, researchers gradually committed to the development of 3D composite Li metal anode by introducing another component, which is usually the porous matrix or 3D current collector. For instance, the parallelly aligned two-dimensional material (MXene) layers were introduced on the Li metal surface by the single-step mechanical calendaring technique [57]. This kind of bilayer structured MXene/Li composite anode is believed to homogenize Li-ion distribution and restrain dendrites growth [58]. In addition, researchers fabricated bulkly homogenized composite MXene/Li, reduced graphene oxide (rGO)/Li, and graphene/Li electrodes by the repeated mechanical calendaring and folding technique [59–61]. The carbon or MXene based 3D frameworks could act as a mechanical supporter, an electrical conductor, as well as an ionic distributor simultaneously in the bulk composite anode [61]. Moreover, a trilayer structured composite Li metal anode was developed to guide stable Li deposition into the porous space sandwiched between an upper electronic insulating layer and a lithophilic sublayer [62]. More interestingly, an upgraded mechanical rolling and cutting technique emerged to regulate the direction of Li deposition and reduce local current density [63]. In general, the mechanical modification technique was simple but effective, facile, eco-friend, cost-effective in building 3D composite Li anode with lower local current density and mechanical support during cycling, which gets rid of complex instruments and harsh experimental conditions and is also easy for large-scale practical implications.

In the following sections of this mini-review, we summarized recently published works relating to this mechanical strategy. We hope this mini-review may help to highlight the significance and effectiveness of this mechanical technique, and promise its future success in other battery chemistries based on metal anodes.

2 Discussion

2.1 Mechanical surface modification

Stepping back from the study to fabricate Li metal anode with a smooth surface, Li-metal with patterned surface was successfully developed for guidance of Li deposition. Peter Bieker is well known as the pioneer who come up with the idea of the mechanical surface modification technique for electrochemical performance optimization. In his work reported previously, inspired by skin needling for transdermal drug delivery, a simple micro-needle technique was proposed to produce rough surface on Li metal [51]. In Fig.1, specifically, a polylactide micro-needle roller containing 20 lines of micro-arrays with a length of 200 μm was adopted to print micro-patterns by repeated rolling processing on the Li metal surface. This process is facile, ecofriendly, costless, and easy to scale up. The as-fabricated micro-needle patterned Li metal anode was expected to demonstrate an increased active surface area which led to obviously reduced current density during galvanostatic cycling, consequently retarding dendrite growth and triggering stable cycling of Li metal batteries using this modified anode compared with that using bare Li anode. Apart from the alleviated current density, the micro-needle technique could also locally direct Li plating. For one reason, due to the tip size effect, electrons tend to concentrate in the vicinity of the pits of the patterned Li metal. Therefore, the deposition process will initiate inside the holes preferentially. For another, as shown in Fig.1(a), different from the intrinsic Li metal surface covered by Li₂O or Li₂CO₃ passivation layer in the region A and B, the fresh Li metal on the vertical wall surface newly created inside the pits in region C shows a higher electrical conductivity, and thus Li deposition will occur preferentially at the wall surface rather than on the unmodified surface (A) or at the bottom of the pit (B). Benefiting from the patterns produced by the micro-needle technique, the deposited Li is mainly confined inside the pits, without forming obvious dendrites. Besides, the electrochemical performance of the LiFePO₄ (LFP)/Li cell in Fig.1(b) and Fig.1(d) also verifies the effectiveness of the micro-needle technique. Accompanying with above research, Peter Bieker also found that the roughness of the native passivation layer on Li metal has been playing a crucial role in determining the electrochemical performance of the Li-metal battery. The rough passivation surface of the as-received Li metal will result in locally aggregated charges and further geometric perturbations in the vicinity of the Li metal surface during the galvanostatic cycling, eventually foster the propagation of the dendrites and further deteriorate the roughness of the Li metal.

In view of the above analysis, in order to achieve a uniform deposition morphology, Peter Bieker et al., developed a simple, fast, and reproducible mechanical press technique to smoothen the roughness and decrease the thickness of the native passivation layer on the Li metal surface [52]. Fig.1(e) shows that after employing a uniaxial roll-pressure on the as-received Li metal, a smoother and thinner passivation layer is obtained on the Li metal surface. The height profiles in Fig.1(f) imply that the average roughness of the roll-pressed Li metal is 37.3 nm, one third of that for pristine counterpart. In Fig.1(g), the initial interfacial resistance of the symmetric cell decreases from 1600 to 850 Ω for the pristine and the roll-pressed Li metal, respectively. In addition, the subsequent galvanostatic cycling test in Fig.1(h) indicates that when cycled at 0.1 mA/cm², the roll-pressed sample shows an alleviated initial deposition-dissolution overpotential of 45 mV compared with that of 85 mV of the as-received one due to the decreased interfacial impedance after reducing the thickness of the passivation layer. The overpotential of the roll-pressed Li metal stabilizes at 20 mV during the rest cycling, and a more homogeneous deposition-dissolution morphology is observed benefiting from the thinner passivation layer. The X-ray photoelectron spectrometer (XPS) result also verifies that the flat and thin passivation layer could facilitate the decomposition of Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to LiF on the Li metal surface, which is in favor of reducing impedance and extending cell lifetime.

These two findings mentioned above illustrates the effectiveness of optimizing the electrochemical performance of Li metal anode by a simple mechanically processing process. The patterned Li metal or the flat passivation layer in the vicinity of the Li surface produced by using the micro-needle technique and roll-press technique play a significant role in reducing the current densities and the interfacial impedance respectively, eventually rendering an enhanced electrochemical performance of Li anode.

Even though, being limited by the lower sophistication of the instruments at that time, the coarse rolling/pressing process with a simple roller hardly meets the precision demand of Li metal deposition in micrometer domain.

Inspired by Peter Bieker’s works, more researchers have been committed to the study of mechanical surface modification technique on Li metal anode, e.g., structural design of patterns to reduce the current densities on the Li metal surface, the influence of size-dimensions of patterns to the interfacial resistance of Li metal, the flexible large-scale stamps for roll-to-roll industrialization. To figure out the influence of dimensions including the height, width, and ridge length of patterns on the electrochemical performance of the mechanical surface modified Li metal anode, Park et al. simulated the current density distribution on the surface of inverted pyramid patterned Li metal anodes assisted by the COMSOL multiphysics [53]. As shown in Fig.2(a), the patterns with unit dimensions of height (50 μm), width (50 μm), and ridge length (40 μm) were found to be the ideal size to get the minimal current densities on Li metal surface. Under the guidance of finite elemental simulation, a stainless-steel stamp with optimal pyramid reliefs (height: 50 μm, width: 50 μm, ridge length: 40 μm) was designed and the surface-patterned Li metal was prepared by using the stamping process. With decreased effective current density, the surface-patterned Li metal anode could confine the deposition/dissolution process inside the patterns. They also found that the morphological variation of the patterns during the galvanostatic cycling was significantly affected by the current densities. A smooth filling of Li metal at a moderate rate and rough granular filling at an increased rate were verified by using scanning electron microscope (SEM) characterization in Fig.2(b)–Fig.2(e). In Fig.2(f) and Fig.2(g), the symmetric cell assembled with surface-patterned Li metal exhibits a smaller impedance and reduced polarization compared with the bare Li metal counterpart. In Fig.2(h), the LiMn₂O₄(LMO)/Li cells based on surface-patterned Li metal anode exhibit a retention of 88.7% after 450 cycles compared with that of 43.9% after 250 cycles for the bare Li metal. Kim et al. investigated the impact of size change of the square micro-patterns on the Li metal surface with the deposition behavior of Li metal anode during galvanostatic cycling [54]. They found that Li deposited preferentially on the top edge of the sub-micron pattern and on the bottom of the larger pattern. The square patterned Li metal with a width of 40 μm and a thickness of 12 μm was prepared by using a polymer mold as shown in Fig.2(i). In addition, a smooth deposition was observed in Fig.2(j) at current densities ranging from 0.1 to 0.5 mA/cm². They insisted that the larger pattern could help to reduce the resistance at the bottom of the patterns, thus leading to a densely-packed Li deposition. Moreover, the comparison of deposition mechanism between two different patterns was intuitively illustrated in Fig.2(k) and Fig.2(l). In Fig.2(m)−Fig.2(o), the mechanism was further verified by the electrochemical performance comparison of the symmetric cells and the Li/LFP cells, respectively. Bae et al. analyzed the influence of stamp material on the electrochemical performance of surface patterned Li metal anode and found that a flexible epoxy resin-based stamp was superior to the stainless-steel based stamp as the former produces less roughness on pattern surface, without dimensional restrictions for industrialization, and avoids physical damage during the mechanical processing [55]. The digital photo comparison between stainless-steel based stamp (Fig.2(p)) and epoxy resin-based stamp (Fig.2(t)) shows the practicability for scale-up of the latter one. Additionally, a smoother pattern surface (Fig.2(u)) could be produced by the epoxy resin-based stamp compared to that for stainless-steel counterpart (Fig.2(q)). Eventually, the Li ion flux was guided effectively inside the surface patterns produced by the epoxy resin-based stamp (Fig.2(v)), leading to a superior electrochemistry performance of full LMO/Li cell (Fig.2(s) and Fig.2(w)).

The practicality of the mechanical surface modification technique has already been verified in conventional batteries with liquid electrolyte. Benefited from the reduced current densities and guided Li ion flux during galvanostatic cycling, the surface-patterned Li metal anode exhibits enhanced dendrite resistibility. But it is unknown whether this technique can be used in solid-state batteries until a relevant work reported recently by Xu et al. [56]. Inspired by the mechanical surface modification technique successfully applied in the Li metal, they considered the replication of the technique in solid-state electrolyte. Specifically, as illustrated in Fig.3(a), the surface-patterned Li₇La₃Zr₂O₁₂ (LLZO) was fabricated by laser machining. They found the reduced current density and amplified stress at the Li/3D-LLZO interface could balance the Li stripping and replenishment at the interface as shown in Fig.3(b), thus preventing the void formation and dendrite progression at higher cycling rate. Finally, in Fig.3(c), the critical current density at which the short circuit occurs increases from 0.3 to 0.7 mA/cm² when the planar LLZO was replaced with 3D-LLZO. In Fig.3(d), the Li/3D-LLZO/Li cell presents a superior cyclic performance over 500 h under 0.5 mA/cm² compared with that of 2 h for the planar LLZO counterpart. The mechanical surface modification technique indeed contributes a lot to the improved interfacial stability of solid-state batteries on basis of the synergistic effect in electrochemical and mechanical aspects.

2.2 Mechanical calendaring

Even though the 3D Li metal electrode can be easily fabricated using the mechanical surface modification technique mentioned above, the as obtained shape will be destroyed during subsequent handling or assembling due to the intrinsic softness of the Li metal. Moreover, once the 3D patterns were filled with deposited Li during the first plating process, the superiority of the modified surface will disappear during the plating/stripping process that follows. Furthermore, the above mechanical surface modification technique can only play around Li only, unable to introduce additional components to it, while for some time another component plays a more important role.

By introducing heterogeneous material onto the surface or into bulk of the Li metal, a variety of characteristics could be imposed to the Li metal by forming a composite electrode. To enhance the adhesion force between the Li metal and the functional layer, Zhang et al. developed a mechanical calendaring technique to fabricate 3D composite Li metal electrode [57]. MXene was chosen as the heterogeneous material on account of its excellent electrical conductivity and expanded interlayer space for Li-ion nucleation and horizontally intercalation. They first prepared the parallelly aligned MXene layer on copper foil based on the Langmuir-Blodgett method. Then, they adopted a simple mechanical calendaring process to transfer the parallel MXene layer to the surface of the Li metal. The coordination effect of the MXene layer is shown in Fig.4(a), which demonstrates that the MXene layer not only benefits the nucleation of lithium on its surface but also facilitates the horizontal growth of lithium along its interlayers. Specifically, in Fig.4(b), even at a deep stripping-plating capacity of 35 mAh/cm², a dendrite-free Li anode still remains after 900 h. Surprisingly, with abundant fluorine termination groups presenting on the MXene nanosheets, a LiF-rich SEI layer was obtained (Fig.4(c)) upon cycling, which contributes to the reduced deposition overpotential of the MXene layer coated Cu anode (Fig.4(d)). Eventually, the symmetric cell assembled by the MXene-coated Li metal exhibits a superior galvanostatic cycling stability in Fig.4(e). Despite the extraordinary dendrite inhibiting ability of the composite Li metal anode that endowed by the aligned MXene layer, the tedious multiple-step procedure significantly impedes the scale-up of the MXene based composite Li metal anode. Hence, Chen et al. proposed an upgraded two-step mechanical calendaring technique to coat MXene stacks (15 μm) onto a thin Li metal (30 μm) [58]. Specifically, the non-delaminated Ti₃C₂T_x MXene stacks were prepared and pressed on a copper foil to reach a designed thickness. Then, the inverted MXene stacks were transferred onto the Li metal by applying a gentle force (Fig.4(f)). This novel two-step mechanical calendaring technique enables the high-efficiency processing, a controllable thickness, and an eco-friendly scale-up for the composite Li metal anode. They found that the MXene layer decorated composite Li metal plays a significant part in reducing the thickness of the inorganic-rich SEI from 132 to 126.4 nm (Fig.4(g) and Fig.4(h)). The uniform and thinner SEI was believed to be beneficial for the alleviated polarization, reduced mass transfer barrier, and restricted nonactive Li formation during cycling. The superior electrochemical stability of the composite Li metal anode was further verified in full cell with a low N/P ratio of 1.52 versus LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532) cathode, reaching an energy density of 366 Wh/kg (Fig.4(i)). They demonstrated the effectiveness of the as-developed mechanical calendaring technique in large-scale preparation of the MXene decorated composite Li metal anode.

2.3 Repeated mechanical calendaring and folding

The surface decoration of MXene on the Li metal using the mechanical calendaring technique was verified to be effective according to the above-mentioned analysis. The only concern is that the improvement of the composite Li anode is mostly localized on the surface of the Li metal anode, which has a negligible effect on the bulk Li, which does not take bulk conductivities such as the bulk mass transfer, volume/shape stability into consideration. In consequence, a bulk homogenized 3D hybrid Li metal anode with the decoration of heterogeneous material both at the surface and inside needs to be urgently developed and utilized. As a representative example, Li et al. again explored an effective mechanical calendaring and folding technique to incorporate Ti₃C₂ MXene into the Li-metal anode, forming a typical homogenized 3D hybrid Li/MXene anode [59]. Specifically, as illustrated in Fig.5(a), the Ti₃C₂ MXene nanosheets were rolled with the Li metal to form an elongated plate and then a subsequent repeated rolling and folding process was executed for the homogenization of the introduced materials. The as-fabricated 3D hybrid anode was regarded as a regulator to guide Li deposition and growth along horizontally oriented interlayers, and the volume change of the Li-metal anode was also suppressed effectively at the mean time (Fig.5(b) and Fig.5(c)). In addition, the symmetric cell using 3D hybrid anode exhibited an excellent cycle stability with a low overpotential of 32 mV at 1 mA/cm², much lower than 200 mV of the bare Li counterpart (Fig.5(d) and Fig.5(e)). The confined Li volume change and regulated Li growth in the homogenized 3D hybrid anode together contribute to its excellent electrochemical performance. In a similar way, Shu et al. demonstrated the universality and generality of the mechanical calendaring technique for carbon materials [60]. They successfully fabricated the homogenized 3D Li/rGO hybrid electrode by the repeated calendaring and folding process (Fig.5(f)). The optical image in Fig.5(g) shows the 3D Li/rGO hybrid anode with a size of 6 cm× 12 cm and a thickness of 100 μm, demonstrating the potential scalability and cost-effectiveness of the mechanical calendaring technique. Besides, the symmetric cell of 3D Li/rGO anode displays a stable cycling stability with a low overpotential of 200 mV for 1800 h as shown in Fig.5(h), which also is supported by the electrochemical impedance analysis in Fig.5(i). The high pore volume and moderate pore size of the heterogeneous rGO could facilitate the migration of Li and enhance the tolerance to volume change of the 3D Li/rGO hybrid anode during cycling.

Benefiting from the mechanical calendaring technique mentioned above, Zhou et al. reported a 3D Li/graphene hybrid anode, achieving a conformal shape-adaptive SEI on the Li metal [61]. They claimed that the repeated calendaring/cutting/stacking process could realize the exfoliation of graphite and parallel alignment of the few-layer graphene in Li metal simultaneously (Fig.6(a)). Moreover, the desired stretchable SEI consisting of parallelly-aligned few-layer graphene could in situ form on 3D Li/graphene hybrid anode after electrochemical extraction of Li (Fig.6(b)), which would play a vital role in restraining dendrites and confining volume change. The electrochemical impedance in Fig.6(c) and the galvanostatic cycling stability of the symmetric cell in Fig.6(d) confirmed the practicality of the 3D Li/graphene hybrid anode for the purpose of suppression of dead Li and dendrites, eventually achieving the high-performance full cell when paired with commercial LiNi_xCo_yMn_zO(NCM) cathode (Fig.6(e)). Inspired by the roll-to-roll technique, a trilayer Li metal anode was constructed by Luo et al. to eliminate the volume expansion of the Li metal during repeat deposition/stripping [62]. They demonstrated a middle porous accommodation layer for stable Li deposition that was sandwiched between an upper electronic insulating layer and a bottom lithiophilic layer as shown in Fig.6(f). Both the Li stripping/plating test in Fig.6(g), the cell pressure change in Fig.6(h), and the full cell test in Fig.6(i) verified the superior electrochemical stability of the trilayer Li metal anode.

The above-mentioned analysis can help a lot when choosing the hetero-materials for composite Li anode. Mostly used materials for Li compositing are graphite, graphene, rGO, and MXene, both of which have an excellent electrical conductivity, a high mechanical strength, and a good Li affinity ability. Besides, the thickness of these 2D layered materials should be kept at nanoscale to avoid energy density decrease. Another key point, the hetero-atoms doped on these materials should be favorable to the formation of inorganic SEI after reaction with liquid electrolyte.

2.4 Mechanical rolling and cutting

Although the 3D hybrid anode homogenized by the repeated calendaring and folding process is valid in regulating Li deposition and inhibiting dendrites, the hybrid anode is a disordered mixture at a microscopic scale. Moreover, the advantage of the layered structure is weakened which is supposed to have a directed Li ion transport parallelly. Delightedly, Liang et al. solved this concern effectively by inventing a so-called rolling-cutting technique [63]. They presented a precisely designed 3D structured anode with a controllable electrode thickness, which could decouple the direction of Li ion conduction and Li deposition into perpendicular and horizontal respectively. Therefore, the worries of dendrites propagation toward cathode could be ignored radically. The eco-friend process is shown in Fig.7(a) and 7(b), while the zoom-in microstructure in Fig.7(c) illustrates the orderly stacked spiral columns of the 3D anode. They emphasized that the key of the 3D anode lay in converting the orientation of Li stripping and plating parallel to the cathode part, which was clearly shown in Fig.7(d). Therefore, the as-fabricated 3D anode exhibits an excellent electrochemical stability both in the symmetric cells in Fig.7(e) and the full cells paired with the LCO cathode in Fig.7(f). They demonstrated the potential industrialization of the optimized 3D anode with a highly controllable and reproducible microstructure accomplished by an upgraded mechanical rolling and cutting technique. Similarly, almost at the same time, Chen et al. also developed a coiled Li anode with an upright structure using a very similar rolling-cutting technique [64]. They successfully introduced a more inner reaction interface for Li deposition through constructing a Swiss-roll liked glass fiber film/Li foil composite anode (Fig.7(g)). The Swiss-roll liked electrode was prepared by coiling of the glass fiber film (thickness of 270 μm) and Li foil (thickness of 180 μm). The high porosity and large space of the glass fiber can provide a high-areal-capacity Li accommodation without inter-space clogging. The volume change and dendrites growth were inhibited effectively thanks to the inner growth behavior (Fig.7(h)). Li deposition occurs initially on the glass fiber in the vicinity of the electronic-conducting upright Li plate while the high electrolyte uptake ability of glass fiber can sustain a durable ionic transport, leading to the lateral Li deposition at a relative low polarization. The upright Li anode paired with the Li₄Ti₅O₁₂ cathode remains stable over 2500 cycles at 1C with a reversible capacity of 141 mAh/g and a Coulombic efficiency of almost 100% (Fig.7(i)).

3 Conclusions and future prospect

Be forced by the ever-increasing long-range demand of electrical vehicles, this paper reviewed recent 3D design solutions for “hostless” Li anode toward dendrite-free high-energy density Li metal batteries. Along this master line and combining the historic and up-to-date reports, it systematically studied the function mechanism of 3D composite Li and summarized the advantages of mechanical modification technique among different strategies in preparation of 3D Li metal anodes. Detailed processing procedures of different mechanical modification techniques were discussed and compared, and characterization as well as the volume change constrain ability of the as-derived 3D anodes were summarized in depth. The dendrites growth of mechanically modified 3D Li anode was influenced by various factors, including the surface roughness, pattern morphology, pattern size, composite layer contents, multi-layered structures, and Li deposition redirection. Correspondingly, further optimization tactics of the mechanical technique could be extracted from the above strategies in stabilizing the 3D or 3D composite Li anodes. This paper could provide a thorough quick view and a helpful guidance for the design and industrialization of 3D Li anodes for high energy density batteries.

For the future, several significant concerns still need to be resolved for the actual implementation of mechanically modified 3D Li anode in industrial area. Further attempts are envisioned to introduce roll-to-roll technique into the mechanical modification process for large scale production of 3D composite Li anode, to combine computational simulation with experimental testing to get deeper understanding of Li deposition mechanism and precise prediction of performance, and to expand the application of 3D composite Li anode from coin cell to high-capacity pouch cell. Inspired by the facile mechanical modification technique and driven by the continual efforts from worldwide researchers, the pursue of real application of 3D composite Li anode will be expected to be a reality very soon.

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