A pseudocapacitive molecule-induced strategy to construct flexible high-performance asymmetric supercapacitors

Abstract

The combination of high-voltage windows and bending stability remains a challenge for supercapacitors. Here, we present an “advantage-complementary strategy” using sodium lignosulfonate as a pseudocapacitive molecule to regulate the spatial stacking pattern of graphene oxide and the interfacial architectures of graphene oxide and polyaniline. Flexible and sustainable sodium lignosulfonate-based electrodes are successfully developed, showing perfect bending stability and high electronic conductivity and specific capacitance (521 F·g⁻¹ at 0.5 A·g^–1). Due to the resulting rational interfacial structure and stable ion-electron transport, the asymmetric supercapacitors provide a wide voltage window reaching 1.7 V, outstanding bending stability and high energy-power density of 83.87 Wh·kg^–1 at 3.4 kW·kg^–1. These properties are superior to other reported cases of asymmetric energy enrichment. The synergistic strategy of sodium lignosulfonate on graphene oxide and polyaniline is undoubtedly beneficial to advance the process for the construction of green flexible supercapacitors with remarkably wide voltage windows and excellent bending stability.

Key words： molecular synergy; pseudocapacitive lignosulfonate; flexible electronic devices; asymmetric supercapacitor; wide voltage windows

1 Introduction

Accompanied by the urgency of high-quality pursuit in miniaturized and form-adaptive electronic devices, flexible all-solid-state supercapacitors (ASSSCs) featuring high energy enrichment and excellent long-term operational stability have stimulated research enthusiasm and possess promising potential as power sources [1–3]. However, the dual presentation of high energy enrichment and flexible bendability has been the key limitation to the application of ASSSC-based electronic devices and represents the bottleneck to the compositional and structural optimization of electrode materials [4–6]. In principle, the two key metrics most directly related to the energy enrichment of ASSSCs are the energy-power density and the voltage window [7]. The improvement of energy enrichment can be satisfied based on adjusting the structures of electrodes and designing different asymmetric components of ASSSCs [8,9].

The structural design of functional materials and developing asymmetric ASSSCs based on derivative electrode materials are promising choices, especially for biomass-based supercapacitors (nanocellulose-enabled HPC/NiCo₂O₄, all-wood-structured asymmetric supercapacitor, etc.) [10–13]. However, for current asymmetric ASSSCs, poor component matching leads to progressively thicker electrodes, which severely affects the energy-delivery key performance indicators of ASSSCs toward miniaturization and shape adaptation [14–17]. With regards to the electrodes, an effective approach is to perfectly integrate the different windows of both electrodes, i.e., capacitive and pseudocapacitive, to boost the device voltage and flexibility. Therefore, it is crucial to find a pair of electrodes with matching capacitance and reasonable structural design.

Although graphene oxide (GO) and polyaniline (PANI) are commonly used as charge-rich substrates for supercapacitors due to their distinct advantages, they each have their own shortcomings. Since the strength of the bonding interaction is not enough to solve the bottleneck problem of overlapping between GO layers, resulting in a reduction in the effective contact area and an impact on the shuttling behavior of ions inside electrode and electrolyte [18–20]. Furthermore, GO, as a supercapacitor-like carbon material electrode, suffers from the physical adsorption energy storage process, resulting in its specific capacitance not meeting the needs of high-capacitance and flexible energy storage [21]. For PANI, as a conductive material, its characteristics mean that multiple cycles often lead to structural degradation, thereby affecting the cycling performance of the device [22,23]. Therefore, blocking the overlap of graphene, regulating the energy storage mechanism, and solving the structural degradation of PANI are pivotal factors for breakthroughs in flexible high-performance energy enrichment electrodes.

Sodium lignosulfonate (SL), a biomass material with enriched phenolic hydroxyl side chains, can be reversibly converted between phenol and phenoquinone by gaining and losing electrons, resulting in theoretical capacitances far exceeding many organic conducting components [24,25]. However, current research on SL is mainly focused on introducing it into other conductive substrates to increase the conductivity of electrode materials, such as PEDOT/SL composite [26] and SL/SWCNT-HNO₃ hydrogel electrodes [27]. For this pseudocapacitive feature of SL, inducing the electrode composition structures with controllability, compatibility and the intrinsic lifting mechanism of pseudocapacitance in an integrated system needs to be further studied.

Here, we use SL as a pseudocapacitive material acting on PANI to increase the speed of the reversible redox reaction by doping and de-doping. Furthermore, utilizing the SL molecular-induced behavior, we enable controllable design of the spatial packing patterns of GO. This enables the full use of the characteristics of SL, GO and PANI, and solves the problems associated with their individual applications. Based on this novel integration strategy, various self-supporting electrode materials, such as LG-150 and LGP-150, with flexibility and designability are easily prepared by vacuum filtration, in situ polymerization and hydrothermal reduction. We also develop symmetric and asymmetric bendable-angle all-solid-state supercapacitors using these flexible electrodes, which show remarkably wide voltage windows (1.7 V), ideal cycling stability and excellent bending stability.

2 Experimental

2.1 Materials and chemicals

SL (M_r = 20000) was received from Yates International Trade Co., Ltd. (China). Natural graphite flakes (325 mesh) were purchased from Henglide Graphite Co., Ltd. (China). Ascorbic acid (VC) was obtained from Damao Chemical Reagent Fctory (China). Dealkaline lignin (DL), poly(ethylene glycol) diglycidyl ether (M_n = 500), poly(vinyl alcohol), aniline and ammonium persulfate (APS) were provided by Aladdin Biochemical Technology (China). H₂SO₄ (98%), KOH, HCl (37%), and ethanol were provided by Sinopharm Chemical Reagent Co., Ltd. (China). The chemicals mentioned above were used without further purification.

2.2 Pseudocapacitive molecule-induced strategy for bendable-angle electrodes

Based on pseudocapacitive molecule-induced strategy, various self-supporting electrode materials with flexibility and designability are prepared in three simple steps based on vacuum filtration [28]. First, 14 mg GO/deionised water mixed system was sonicated for 15 min to obtain a 2 mg·mL⁻¹ GO colloidal dispersion, then 14 mg of SL was added and kept stirring at 150 °C for 3 h, and then the LG-150 film was obtained by vacuum filtration.

Mix 0.45 g of aniline and 50 mL of 1 mol∙L^–1 HCl with rapid stirring for 30 min to obtain a well-dispersed solution. The above aniline solution was applied to the LG-150 film to achieve full contact penetration, kept at 0 °C for 3 h, then 1.37 g APS was added to the aniline solution at 0 °C, stirred slowly until the APS dissolved, placed at 0 °C for 12 h for polymerization, and then clamped out with tweezers, washed to obtain the LGP film. A VC solution of 4 mg∙mL^–1 was obtained by dissolving 28 mg VC in deionised water. The aniline-polymerized LGP film and VC solutions were fully contacted and mixed for hydrothermal (150 °C) for 12 h to obtain the VC-reduced LGP-150 film with a thickness of 0.1 mm.

Comparative samples were obtained by adjusting the components (GO, PANI, SL), hydrothermal temperature (80–180 °C) and polymerization time (3–15 h). This series of samples were collectively referred to as LGP-x (x is the hydrothermal temperature), named as LGP-80, LGP-100, LGP-120 and LGP-180 films, respectively. In addition, GO-150 without SL and PANI were also obtained.

2.3 Structural design of the symmetric and asymmetric bendable-angle ASSSCs

The DLG electrolyte was constructed based on the reported method [6]. Two pieces of 1 cm × 1 cm LGP-150 were fully contact-dipped in 1 mol∙L^–1 H₂SO₄ electrolyte for 12 h. Then, two pieces of above LGP-150 films were pressed on carbon cloth (size of 1 cm × 3 cm) as the current collector under 15 MPa for 15 min and separated by a 1.2 cm × 1.2 cm piece of DLG electrolyte. After that, the excess water was removed under 2 MPa at 50 °C for 2 h, resulting in the symmetric bendable-angle ASSSCs of LGP-150//DLG//LGP-150 structures. The comparison device is symmetric bendable-angle ASSSCs with LG-150//DLG//LG-150 sandwich structure. The asymmetric ASSSCs were prepared by referring to the method for preparing the symmetric ASSSCs. The only difference is that the composition of asymmetric ASSSCs is based on LGP-150 as the positive electrode and LG-150 as the negative electrode to fabricate.

2.4 Characterization and measurements

Fourier transform infrared (FTIR) spectra were tested on a FTIR spectrometer (Thermo Fisher Nicolet iS 50). Raman spectra were obtained using an inVia Reflex laser Raman spectrometer (Renishaw, UK) in the range of 500–3000 cm⁻¹. X-ray diffraction (XRD) patterns were acquired using a Rigaku D/MAX 2500V instrument over 5° to 80°. XPS spectra were obtained by a Thermo ESCALAB 250X1 + instrument (Thermo Fisher Scientific, USA). A field emission scanning electron microscopy (FE-SEM, Zeiss Stgima 300) equipped with an energy dispersive X-ray spectroscopy (EDS) accessory was employed. The electrochemical measurements of electrodes and ASSSCs were performed on an electrochemical workstation (CS350, Wuhan Corrtest Instruments Corp., Ltd., China) to obtain cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) curves in three-electrode or two-electrode systems.

3 Results and discussion

Sustainable SL-based flexible electrode materials were constructed using SL as a pseudocapacitive molecule to regulate the spatial stacking pattern of GO and the interfacial architectures of GO and PANI through hydrogen bonding, π–π and electrostatic interactions (Fig.1(a)). The absorption peaks at 3480, 1640, 1580, 1380 and 1200–1000 cm^–1 for GO belong to the –OH and C=C vibrational absorption peaks, the deformation vibration of adsorbed water molecules, –OH deformation absorption and C–O–C stretching vibration, respectively (Fig.1(b)) [29]. After the hydrothermal reduction of VC, the oxygen-containing functional groups in the spectrum of GO-150, such as –OH vibrations and deformation absorption at ~3480 and 1380 cm⁻¹, respectively, the deformation vibration of adsorbed water molecules at 1580 cm^–1 and the C–O–C stretching vibration at 1200–1000 cm^–1, appeared significantly weakened or disappeared, which proves that GO was reduced after hydrothermal reduction [30]. In comparison to GO-150, the characteristic broad peaks of LG-150 and LGP-150 at 1200–1000 cm^–1 become broader and more intense, representing C–S bond formation and demonstrating the successful intervention of SL [31].

Fig.1 Chemical-molecular interactions and microtopographic effects of GO and PANI using pseudocapacitive molecules of SL: (a) schematic illustration of interactions between components, (b) FTIR spectra, (c) XRD patterns, (d) Raman spectra, (e−l) FE-SEM images of GO-150, LG-150, LGP, LGP-80, LGP-100, LGP-120, LGP-150, LGP-180 films, (m) Cross-section and (n) EDS mappings of LGP-150 films.

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For GO, representative (002) and (100) crystal planes are detected at 10.14° and 20.38° (Fig.1(c)). The (002) plane represents the characteristic diffraction peak of GO and its disappearance in GO-150, LG-150 and LGP-150 proves that GO is fully reduced [31]. The (100) plane qualitatively reflects the stacking structure in the sheet from the patterns. The high (100) peak exhibited by LGP-150 may be due to the tight binding between the GO and PANI segments. The broader and weaker (100) peak in LG-150 illustrates that the addition of SL in the GO sheet increases the interplanar spacing.

Raman spectra are also critical to reveal the effects of GO, SL and PANI on the π-electron structure of LGP-150 (Fig.1(d)). The D and G bands (1343, 1594 cm^–1) can reflect the disordered carbon and in-plane vibration of the sp²-hybridized carbon, respectively [9]. The bands at 1583 (C–C of the benzoid ring), 1481 (C=N of the quinoid), 1336 (C–N⁺), 767 (ring deformation of the benzene/quinoid rings), 516 (phenazine-like segment) and 406 cm^–1 (out-of-plane C–H wagging), respectively, reflecting the network structure of LGP-150 [32]. The calculated I_D/I_G values of the D to G band intensity for GO, GO-150, LG-150 and LGP-150 are 0.99, 1.0, 0.99 and 0.96, respectively. Notably, compared with GO, GO-150 and LG-150, the I_D/I_G of LGP-150 is significantly decreased, suggesting that the degree of conjugation and electron delocalization may be significantly enhanced [33]. This result stems from the efficient π–π interaction and chemical reaction between GO and PANI components, which give rise to a large strong π–π conjugated system and reduce the defects or vacancies in the GO-150 sheets.

The surface microstructure of the electrodes plays an important role in improving the contact, reducing contact resistance and facilitating electron transport (Fig.1(e)–Fig.1(l)). The rough surface of the GO-150 sample has ridges, wavy textures and bulges within the stacked sheets. The root of the lower electrochemical performance of GO-150 may originate from the unresolved re-stacking problem between graphene sheets [34]. The LG-150 sample shows a smooth surface, which proves that lignin can enhance the stacking level of GO. Before the hydrothermal reduction, the surface of the LGP sample appears as a staggered and interwoven nanowire structure, and the hydrothermal reduction treatment at 80–180 °C resulted in the morphology of the polyaniline nanowires showing irregularly shaped hollow nanoflowers. This structural transformation is beneficial to the electrochemical performance of LGP-150 electrode due to the increased active sites. Among them, the most closely bound PANI morphology was found in the morphological structure of LGP-150. The intrinsic reason for this change is based on the fact that the acceleration of molecular motion often shows a positive correlation trend with increasing temperature, thus mapping in terms of the degree of GO reduction and the stability of the structure. As the temperature increases, the bonding between the PANI and LG film becomes tighter, and at 150 °C, it is tightly adhered to the GO sheet, suggesting that it has the most robust conductive structure. When the temperature is raised to 180 °C, the bonding between PANI and LG becomes worse and the electrochemical performance is drastically reduced. Fig.1(m) shows a cross section of LGP-150 exhibiting a dense wavy lamellar structure with a width ranging from ~4.6–5.4 μm. The EDS mapping of the LGP-150 electrode (Fig.1(n)) confirms that the composite is homogeneous, with a uniform distribution of C, O, N and S and their contents decreased sequentially.

According to the above analysis, the design, construction, and molecular synergies of the LG-150 and LGP-150 electrodes are shown in detail in Fig.2. First, the self-supporting matrix was prepared via molecular strategies (hydrogen bonding and molecular interactions) between GO and SL. SL, as a dispersion medium, effectively improves the overlap of GO and expands the layer spacing of GO, thereby making the LG composite an ideal substrate for compounding key components (Fig.2(a)). The PANI nanoflowers form tight bonds with SL and GO utilizing strong π–π and electrostatic interactions with the pseudoelectric-contributing groups of SL and are uniformly embedded in the LG layers, which is positive for the rational intercalation and uniform complexation of PANI (Fig.2(b)).

Fig.2 Pseudocapacitive molecule-induced strategy for flexible SL-based electrode: (a, b) design, construction, and molecular synergy of LG-150 and LGP-150 electrodes, (c) XPS survey spectra, (d) XPS peak fitting results for the C1s region, (e–h) N1s and C1s spectra.

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Moreover, XPS characterization of GO, GO-150, LG-150 and LGP-150 and the relevant chemical compositions are shown in Fig.2(c)–Fig.2(h). GO-150 exhibits a lower oxygen content (22.45 wt %) than GO, which is attributed to the reduction of hydroxyl groups during the hydrothermal reduction of VC (Fig.2(c) and Fig.2(d)). Compared with GO-150, the higher elemental oxygen content of LG-150 (27.8 wt %) can be attributed to the introduction of SO₃^– [27]. Additional S 2p peaks are observed at 168 eV for LG-150 and LGP-150, in comparison with GO-150 and GO, providing stronger support for the successful coupling of SL, GO and PANI. In particular, an N 1s peak (~399 eV) is found in LGP-150 because of the introduction of PANI into SL and GO [30]. As shown in Fig.2(e), the N 1s peaks of LGP-150 are fitted into four peaks at 396.7 eV (=N–, 4.9 at %), 398.9 eV (–NH–, 58.9 at %), 399.8 eV (N⁺, 26.0 at %) and 401.3 eV (azane type, 10.2 at %). The fitting of the C 1s peaks of GO-150 and LG-150 yielded three main peaks of C=C/C–C, C–O/C–S and C=O at 283.9, 284.3–284.4 and 286.9 eV (Fig.2(f) and Fig.2(g)) [35,36]. The C 1s peaks were fitted to the five peaks of C=C/C–C (283.9 eV), C–N (284.1 eV), C–O/C–S (286.1 eV), C=O (287.0 eV) and C=N (287.9 eV) of LGP-150 (Fig.2(h)). The C–O/C–S (285.3–285.7 eV) ratios are 18 at % for GO-150, 11 at % for LG-150 and 15 at % for LGP-150. Additionally, the higher content of C=O bonds in LGP-150 (3.5 at %) than that in LG-150 (2.2 at %), implying that more quinone functional groups are present in LGP-150 [37].

To prepare flexible ASSSCs devices with optimal performance, electrode materials with different compositions, hydrothermal temperatures and polymerization times were preferentially selected (Fig. S3, cf. Electronic Supplementary Material, ESM, Fig.3(a)). The CV curves (Fig.3(b)) of the LGP-150 electrode (5 to 200 mV∙s^–1) have a rectangular-like shape and contain characteristic redox peaks, originating from the transition of the emerald green imine morphology between the oxidized and reduced states due to the proton doping/de-doping of PANI and the reversible conversion of the Q and QH₂ groups contained in SL [27,38]. The increase in scan rate did not result in a large change in the shape of the CV curve, revealing its good rate capability. However, the redox peaks became inconspicuous at high scan rates, with plausible explanations lying in the damage of PANI under high current shocks and the inability of the green imine form of PANI to switch rapidly between the oxidized and reduced states. All GCD curves (Fig.3(c)) exhibit nonlinear isosceles triangle-like shapes, reflecting the pseudocapacitance behavior induced by PANI and SL. The specific capacitance of the LGP-150 showed a decreasing trend (530 to 410 F∙g^–1) with the increase of 0.5 to 5 A∙g^–1, and the capacity retention was 77%. In addition, the LGP-150 electrode exhibits comparable electrochemical performance to PANI-, lignin- or graphene-based electrode materials, with more details shown in Table S1 (cf. ESM). Therefore, the analysis of Figs. S(1–3) and the above electrochemical analysis indicate that the LGP-150 electrode has the optimal capacitance performance.

Fig.3 Electrochemical index and bending stability of LGP-150 electrode: (a) three-electrode testing model, (b, c) CV and GCD curves, (d, e) CV curves and specific capacitance at different bending angles at 5 mV∙s^–1, (f, g) CV curves and specific capacitance at 5 mV∙s^–1 for different folding cycles.

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To construct flexible ASSSCs, the preparation of the electrodes is particularly critical, especially for portable electronic devices that need to meet requirements such as lightweight and flexibility. Based on these considerations, the electrochemical properties of the LGP-150 electrode were further explored after different bending degrees and folding cycles (Fig.3(d)). Under the curved schematic diagram of the LGP-150 electrode shown in the inset, the CV curves nearly overlap, with no obvious shape deviation observed. The mass specific capacitances (Fig.3(e)) are 239, 236, 236, 235 and 239 F∙g^–1 at different bending angles with outstanding retention of 98%, which was retained after bending, quantitatively supporting the flexibility and adaptability of the LGP-150. Fig.3(f) shows the electrochemical cycling stability of the LGP-150 electrode against folding. The inset shows a schematic of one folding (~180°) of the LGP-150 electrode. It is shown that the area of the CV curve gradually decreases after folding but does not become smaller after 100 cycles of folding, showing a gradually increasing trend. In addition, the CV curve after 350 cycles of folding is nearly coincident with that of 150 cycles of folding. As the number of folding increases from 0 to 350 cycles (Fig.3(g)), the specific capacitance was calculated to be 235, 218, 203, 214, 188, 181, 203 and 201 F∙g^–1 and the specific capacitance retention rate was not less than 77%, thereby suggesting the good flexibility adaptability of LGP-150. The variation of specific capacitance at different folding cycles may be derived from the change of the active site of the PANI active material sliding during the folding process, as well as the difference in electrochemical properties due to the change in the mass of the active species.

The electrochemical performance, pseudocapacitor intrinsic boosting mechanism and bending stability of LGP-150-based symmetric ASSSCs are shown in Fig.4. The hierarchical composition design of the symmetric ASSSCs is presented in Fig.4(a) and Fig.4(b). It is noteworthy that the symmetric ASSSCs were obtained by overlapping layers with LGP-150 as the optimal electrode and LG-150 as the control, DLG gel as the electrolyte. The CV curve (Fig.4(c)) of the ASSSCs shows two distinct Faraday peaks (0.1–0.6 V) ascribed to the redox reaction between the Q/QH₂ group of SL and the redox state of the green imine of PANI. Charge–discharge stimulates the reversible redox change of the Q/QH₂ structure in SL (QH₂ = Q + 2e^– + 2H⁺), resulting in a large amount of pseudocapacitance. The GCD curves (Fig.4(d)) are nonlinear isosceles triangle shapes, indicating the existence of a pseudocapacitive energy storage mechanism, which contributes to the increase in energy enrichment of ASSSCs devices. The calculations show that the LGP-150-based ASSSC has a higher value at 1 A∙g^–1 (185 F∙g^–1) than the LG-150-based ASSSC (124 F∙g^–1), the former being 1.5 times higher than the latter. The LGP-150-based ASSSC still maintained high value of 104 F∙g^–1 (Fig.4(e)) tested up to 7 A∙g^–1. The excellent rate capability of LGP-150-based ASSSCs is closely related to the tight adhesion of PANI on GO nanosheets and the improved effect of SL on the GO laminar structure, which positively contributes to enhancing the rate of ion diffusion and charge transport even at high current densities. The optimization index of capacitor performance is often determined by how well it behaves with resistors. As shown in the impedance fitting plot in Fig.4(f), compared with the symmetric ASSSCs based on the LG-150 electrode, the symmetric ASSSCs based on the LGP-150 electrode have a smaller semicircle diameter and a higher slope. The fitting results show that the R_s of symmetric ASSSCs based on the LGP-150 and LG-150 electrodes are 2.1 and 2.5 Ω, and the R_ct is 0.1 and 1.2 Ω. This indicates that the symmetric ASSSCs based on the LGP-150 electrode have a faster ion diffusion ability and smaller equivalent series resistance.

Fig.4 The electrochemical performance, pseudocapacitor intrinsic boosting mechanism and bending stability of LGP-150-based symmetric ASSSCs: (a, b) assembly of LG-150 and LGP-150 based symmetric ASSSCs, (c–f) CV curves, GCD curves, specific capacitance changes, and EIS fitting results, (g) the capacitively-controlled oxidation-reduction process of LGP-150 based symmetric ASSSCs, (h) CV curves at 50 mV∙s^–1 at different bending angles (The inset shows the digital of the flexible ASSSCs), (i) power-law relationships, (j) plot of capacitive contribution to the total current at 10 mV∙s^–1, (k) 83% of the total current is capacitive, capacitance contribution at different scan rates.

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For the symmetric ASSSCs, even at different tortuosities, it is still realized as a capacitively-controlled oxidation–reduction process and is not affected by external bending forces attributed to the pseudocapacitor intrinsic boosting mechanism, which is achieved through the synergistic effect of component molecules, electrodes and electrolytes (Fig.4(g)). The bending-resistant capacitive properties of the symmetric ASSSC were tested (the inset of Fig.4(h) illustrates the bending of the ASSSC). The ASSSCs were found to have approximately the same capacitance by varying the bend angle, even at 135° higher. A power-law equation (i = aV^b, where a and b are constants) was employed to quantify the capacitive contribution of the redox peaks in the CV curve [39]. Fig.4(i) shows the power-law relationship between the peak currents and the corresponding current densities, extrapolated from the slopes of the logI–logv plot for all two redox peaks with b values of 0.90 and 0.91, respectively, revealing a redox-controlled capacitive process. The energy storage process of the LGP-150 electrode is diffusion controlled, but the conclusion obtained based on the symmetric ASSSCs of the LGP-150 electrode is capacitance controlled, and this change is closely related to the synergistic effect among the internal components of the LGP-150 electrode and DLG electrolyte. The presence of DL in the DLG electrolyte contributes to the pseudocapacitance of the whole device and the DL is also rich in phenoquinone groups, enabling fast and reversible conversion of Q and QH₂. After assembling the electrode and electrolyte in a tight fit, there are more active sites inside the ASSSCs than inside the electrode, which greatly contributes to the pseudocapacitance response. Further, the total capacitive contribution was obtained by Dunn’s formula, i(V) = K₁v + K₂v^1/2 (K₁v represents the capacitive contribution and K₂v^1/2 implies the diffusion contribution) [40]. The total capacitive contribution of the LGP-150-based ASSSCs was represented in Fig.4(j), where 83% of the total current is the capacitive contribution. The capacitive current is higher around peak 1 and lower around peak 2, which can be judged and supported from the b values of these two peaks. In addition, the overall trend (Fig.4(k)) of the contribution of the capacitive current to the total current in the ASSSCs increases with the increased scan rate and stabilizes at ~90% after 30 mV∙s^–1.

High-voltage window and bending stability of LGP-150-based symmetric ASSSCs are shown in Fig.5. A settable window is necessary to explore the potential possibilities of symmetric ASSSCs based on LGP-150 electrodes in the field of flexible electronic devices (Fig.5(a)). Fig.5(b) shows a set of CV curves of the ASSSCs, which exhibits a broad potentials gradually extending from 0.0–0.8 V to 0.0–1.5 V. The rectangular-shaped curve has no significant deviation and the low potential curve is contained within the curve at the 0.0–1.5 V potential window as a whole, suggesting that the potential window is adjustable, stable and has a wide range of applications. Tests with different operating windows showed (Fig.5(c)) that the narrow potential window was wrapped within the wide potential window during the step-by-step change from 0.0–0.8 V to 0.0–1.5 V. Combined with CV analysis, this further demonstrates that the symmetric ASSSCs with LGP-150 as the electrode have a flexible and scalable potential window.

Fig.5 High-voltage window and bending stability of LGP-150-based symmetric ASSSCs: (a) crossover from narrow to wide potential window, (b, c) CV at 50 mV∙s^–1 and GCD curves of LGP-150 based symmetric ASSSCs at 1 A∙g^–1 over potential windows of 0–0.8 to 0–1.5 V, (d) construction of series or parallel symmetric ASSSCs, (e, f) CV and GCD curves of LG-150 and LGP-150-based symmetric ASSSCs in series or parallel, (g, h) capacitance retention and coulomb efficiency under 10000 cycles of GCD tests, (i) energy-power density curves of LG-150 and LGP-150-based symmetric ASSSCs.

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To further explore the large-scale applicability of the assembled supercapacitors, series and parallel are accepted as an assembly form (Fig.5(d)). The CV and GCD curves (Fig.5(e) and 5(f)) of two symmetric ASSSCs in series and parallel indicate that the operating window of the series device will increase and the capacitance of the parallel device will increase without changing the current density or scan rate, as also evidenced by the closed area (CV) and the discharge duration (GCD). In addition, the capacitance retention and coulomb efficiency of LG-150 and LGP-150-based symmetric ASSSCs devices were tested at 5 A∙g^–1, as shown in Fig.5(g) and Fig.5(h). It can be seen that the LG-150 device retains up to 100% specific capacitance and 88% coulombic efficiency in the initial cycle. In contrast, the specific capacitance of the LGP-150-based symmetric ASSSCs decreased slightly to 81.4% after 10000 cycles, but its coulombic efficiency was as high as 100%. It is demonstrated that the LGP-150-based ASSSCs have a more desirable charge/discharge behavior and retain a higher specific capacitance value than the LG-150-based symmetric ASSSCs, despite the specific capacitance loss inevitably occurs by the volume expansion of PANI at high cycle counts. As shown in Fig.5(i), the LGP-150 device has a high value of 54.58 Wh∙kg^–1 at 3 kW∙kg^–1 as calculated from the GCD curve, surpassing the LG-150 symmetric device (3 kW∙kg^–1 and 40.92 Wh∙kg^–1) and state-of-the-art symmetry-based ASSSC devices based on lignin, PANI and graphene, such as modified PANI@OGH films (8.12 kW∙kg^–1 and 19.71 Wh∙kg^–1) [4], LR-60 (40 kW∙kg^–1 and 10 Wh∙kg^–1) [41], Lig/SWCNT-HNO₃ aerogels (0.324 kW∙kg^–1 and 17.1 Wh∙kg^–1) [27], PANI/CNT (2.2 kW∙kg^–1 and 7.1 Wh∙kg^–1) [42] and FrGO/PANI (0.3 kW∙kg^–1 and 16.3 Wh∙kg^–1) [43], with more details shown in Table S2 (cf. ESM). In summary, overall comprehensive performance of ASSSCs mentioned above are significantly improved due to the effective improvement of the doping level of GO interlayer stacking and PANI by SL, as well as the high synergy of strong π–π bonding and electrostatic interactions among the components.

Asymmetric ASSSCs based on the SL-based electrodes are expected to offer a wide voltage window, uncompromised dynamics, enhanced energy density and excellent stability due to the controllable interfacial structures and stable ion-electron transport (derived from the inducibility of the SL molecule) (Fig.6). The asymmetric ASSSCs were constructed using LGP-150 (positive) and LG-150 (negative) electrodes and DLG as the electrolyte (Fig.6(a)). Fig.6(b) represents the CV curves of the asymmetric ASSSCs tested at 10–500 mV∙s^–1 within –0.7–1.0 V. Strikingly, the device also exhibits a well-symmetric rectangular-like shape and capacitance over a wide potential window, and the range covered by the CV curve is not changed by the increase in scan rate. For the asymmetric ASSSC (Fig.6(c)), the GCD curves all exhibit a similar nonlinear isosceles shape without an obvious IR drop as the scan rate gradually increases from 1 to 15 A∙g^–1, which proves its excellent capacitive performance. The corresponding specific capacitances values were 209 (1 A∙g^–1), 134 (5 A∙g^–1), 87 (10 A∙g^–1) and 60 F∙g^–1 (15 A∙g^–1), respectively, showing a high specific capacitance. Fig.6(d) shows the EIS fit of the asymmetric ASSSCs (the inset shows the local amplification and equivalent circuit diagram of its high-frequency region). The equivalent circuit (the red line) obtained by fitting the EIS parameters in the high frequency region of the asymmetric ASSSC shows that the R_s is 2.2 Ω and the R_ct is 2.4 Ω, which proves its low internal resistance and fast charge transfer capability. The fitting results of low-frequency region with a sloping straight line show that the asymmetric ASSSCs have an ideal impedance curve. The LG-150 and LGP-150-based asymmetric ASSSCs were found to have approximately the same capacitance when bent at different degrees, even at 135° (Fig.6(e)). Fig.6(f) shows a diagram of three asymmetric ASSSCs devices connected in series to form LEDs, which can easily light up different colors, including yellow, red, white, and blue by charging.

Fig.6 High-voltage window, bending and cycling stability of LG-150 and LGP-150-based asymmetric ASSSCs: (a) assembly of LG-150 and LGP-150 based asymmetric ASSSCs, (b–d) CV, GCD, EIS fitting curve, (e) CV curves at 10 mV∙s^–1 at different bending angles, (f) optical images of lighted diodes, (g, h) cyclic stability of 5000 cycles GCD tests at 5 A∙g^–1, (i) energy-power density curves.

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The asymmetric ASSSCs device represents a high capacitance retention rate of 101% (Fig.6(g) and Fig.6(h)), which is higher than that of the LG-150 (100%) and LGP-150 (81.4%) symmetric devices. The coulombic efficiency is stable at 92%, suggesting that the asymmetric ASSSCs have high cycling stability. The energy and power densities (Fig.6(i)) of the asymmetric ASSSCs device reach 83.87 and 3.40 kW∙kg^–1, which are superior to the LG-150- and LGP-150-based symmetric ASSSCs, indicating that the asymmetric device design is effective for the energy density enhancement. The construction of the asymmetric ASSSC device is based on the fact that the LGP-150 positive and negative terminals have a synergistic potential window and specific capacitance, resulting in an asymmetric ASSSC exhibiting a wide potential window from 0.0 to 1.7 V and a high value of 209 F∙g^–1 at 1 A∙g^–1. Besides, the asymmetric ASSSC devices are comparable to many advanced PANI-, graphene- or lignin-based asymmetric ASSSCs, such as GO@Zn-Co-Ni//AC (64.91 Wh∙kg^–1 and 0.8 kW∙kg^–1) [44], AC/lig-MnO₂//AC (14.11 Wh∙kg^–1 and 1 kW∙kg^–1) [45], e-CMG//MnO₂/e-CMG (44 Wh∙kg^–1 and 11.2 kW∙kg^–1) [46], rGO@Mn₃O₄//rGO@VO₂ (42.7 Wh∙kg^–1 and 0.3 kW∙kg^–1) [15] and PANI@Mn₃O₄//AC (40.2 Wh∙kg^–1 and 0.34 kW∙kg^–1) [47], with more details given in Table S3 (cf. ESM). These results show that symmetric and asymmetric ASSSCs based on LGP-150 and LG-150 electrodes present wide voltage windows, uncompromised dynamics, enhanced energy density and excellent bending and cycling stability within the field of flexible electronic devices.

4 Conclusions

Herein, bendable ASSSCs based on pseudocapacitive lignosulfonate with remarkably wide voltage windows and high energy output are realized by flexible electrodes and a quasi-solid electrolyte. Flexible LG-150 and LGP-150 electrodes were prepared by a three-step process of vacuum filtration, in situ polymerization and hydrothermal reduction through molecular synergies to achieve versatile applications of SL, first as dispersants to improve GO stacking, secondly as powerful nucleation sites for PANI and finally as a natural source of pseudocapacitance. Two flexible electrodes (LGP-150 and LG-150) were assembled with lignin gel electrolyte for both symmetric and asymmetric ASSSCs. Symmetric ASSSCs exhibited high flexibility, excellent cycling stability (81.4% at 10000 cycles), high coulombic efficiency (100% at 10000 cycles), capacitive contribution of 83% and a high value of 54.58 Wh∙kg^–1 at 3 kW∙kg^–1. Compared with symmetric ASSSCs, the LGP-150//LG-150 asymmetrical ASSSCs based on LGP-150 and LG-150 electrodes exhibits a significant extended window (–0.7–1.0 V) and a high value of 83.87 Wh∙kg^–1 at 3.4 kW∙kg^–1. Three asymmetrical ASSSCs in series can light up red, yellow, white or blue LEDs of different powers as desired. This work, based on exploiting the pseudocapacitive properties of lignin, opens a new avenue for flexible energy storage ASSSCs with very wide voltage windows and high energy and power densities.

Acknowledgements

This work was supported by the Natural Science Foundation of Guangxi (Grant No. 2018GXNSFBA138025) and the National Natural Science Foundation of China (Grant No. 32171720).

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11705-023-2304-1 and is accessible for authorized users.

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