Natural Lignin: A Sustainable and Cost‐Effective Electrode Material for High‐Temperature Na‐Ion Battery

Rechargeable sodium‐ion batteries usually suffer from accelerated electrode destruction at high temperatures and high synthesis costs of electrode materials. Therefore, it is highly desirable to explore novel organic electrodes considering their cost‐effectiveness and large adaptability to volume changes. Herein, natural biomass, pristine lignin, is employed as the sodium‐ion battery anodes, and their sodium storage performance is investigated at room temperature and 60 °C. The lignin anodes exhibit excellent high‐temperature sodium‐ion battery performance. This mainly results from the generation of abundant reactive sites (C=O) due to the high temperature‐induced homogeneous cleavage of the Cβ–O bond in the lignin macromolecule. This work can inspire researchers to explore other natural organic materials for large‐scale applications and high‐value utilization in advanced energy storage devices.


Introduction
Nowadays, Li-ion batteries have been used as the dominant energy storage media for portable electronic devices and electric vehicles.[3] Owing to the abundant resources and wide distribution of sodium, sodium-ion batteries (SIBs) have aroused significant research interest as cost-effective energy storage devices for potential large-scale applications. [4,5]o promote the development of SIBs, many electrode materials have been investigated.In particular, significant progress has been made in high-performance cathode materials for SIBs, such as sodium layer oxides and sodium polyionic compounds. [6,7]In contrast, anode materials for SIBs remain less developed, and many challenges remain to be addressed. [8]For instance, hard carbon is currently the most commercially promising SIB anode, but its synthesis process often involves high-temperature calcination and emissions of hazardous substances.Typical inorganic metal compounds (i.e., oxides, sulfides, etc.) as SIB anodes usually exhibit large volume changes during cycling and sluggish ion diffusion kinetics due to the larger radius of sodium than lithium (1.02A vs 0.76 A).This can easily cause pulverization of electrode materials and thereby poor cycling stability, hindering their development for practical applications. [9]Furthermore, the practical operation of rechargeable batteries often involves complex service conditions and wide working temperature ranges, such as in tropical countries (hot climates).A few studies on SIBs have demonstrated that more intense electrolyte convection and faster Na + intercalation/de-intercalation above 55 °C can damage the surface electrode structure. [10,11]Therefore, it is urgent to develop high-performance SIB anodes via green and low-cost methods for high-temperature operation.
An extremely promising strategy is to explore new organic compounds with specific functional groups, such as -OH, -C=O, and -C=N. [12]ompared to conventional inorganic materials, organic electrode materials offer the following advantages. [13]First, organic materials mainly contain natural elements (including C, H, O, and N) without heavy metals such as Co and Ni.They can be obtained from biomass resources, thus making organic SIBs (OSIBs) cost-effective and environmentally friendly throughout the battery life cycle (i.e., generation, utilization, and recycling).In addition, organic compounds demonstrate tremendous structural diversity, which enables the adaption of their physicochemical properties and offers further possibilities for OSIBs; for example, their specific capacities can be increased by introducing many functional groups.Moreover, Liang's team reported a surface reaction mechanism for Na + storage in organic anode materials, different from the conventional conversion and de-intercalation mechanism.Surface functional groups of organic materials react with Na + to realize the intercalation/de-intercalation process, thus suppressing the volume change of electrodes and maintaining their integrity (even under high-temperature conditions). [14]OSIBs tend to exhibit stable electrochemical performance at high temperatures based on this mechanism.Therefore, given the above advantages, organic compounds have been considered the most promising anode candidates for high-temperature SIBs.
Lignin is the second most abundant terrestrial renewable resource in nature after cellulose.It is a promising biomass resource because of its competitive price and abundant functional groups.It has attracted extensive research interest from academia and industry. [15]To achieve value-added applications for lignin, many lignin-derived materials have been developed and applied as different components in rechargeable batteries, including electrodes, binders, and separators. [16]Among them, lignin carbon-based electrode materials have become one of the current research hotspots.Unfortunately, the carbonization process is high-energy-consumption, and leaves the inherent redox properties of the lignin molecular structure unavailable for exploitation.For this reason, Casado et al. [17] firstly prepared poly(3,4-ethylenedioxythiophene) (PEDOT)/lignin composite as cathode material for SIBs.However, it delivered a maximum capacity value of only 70 mAh g À1 , and the corresponding mechanism has not been systematically investigated.
In this work, pristine lignin, for the first time, was utilized as the anode material for high-temperature SIBs.Its sodium storage mechanism was revealed by the experimental analysis and theoretical calculation.When operated at 60 °C, the lignin anode exhibits high specific capacity (255 mAh g À1 at 200 mA g À1 ), great rate capability, and long cycling stability (capacities gradually enhance and tend to be stable after 200 cycles at 1000 mA g À1 ).This is mainly ascribed to the collective effects of the unique reaction mechanism for Na + storage, fast transport at high temperatures, and the formation of dense SEI film.To the best of our knowledge, the excellent high-temperature Na + storage performance is unprecedented and superior to literature reports.This study provides novel ideas to develop biomass-derived sustainable organic electrodes for rechargeable batteries operated at high temperatures.

Characterizations of Lignin
The raw material of dealkaline lignin has Na 2 SO 4 impurity, as revealed by the XRD pattern in Figure S1a, Supporting Information.The SEM image shows regular-shaped Na 2 SO 4 micro-particles that adhere to the surface of the dealkaline lignin (Figure S1b, Supporting Information).The Na 2 SO 4 is the byproduct produced during the extraction of dealkaline lignin by the kraft pulping. [18]To exclude the Na 2 SO 4 impurity, the dealkaline lignin was washed thoroughly with distilled water and dried in a vacuum to obtain the lignin.The surface morphology of the lignin is presented in Figure S2, Supporting Information.It can be observed that lignin is a large granular structure with a smooth surface and several dispersed microscopic cavities.This morphology is due to the p-p conjugated aromatic structures as well as van der Waals forces and hydrogen bonding between the lignin chains. [19]Such microstructure morphology facilitates the diffusion of sodium ions in the anode. [20,21]Additionally, the XRD pattern of lignin does not indicate any sharp peaks (Figure 1a).SEM and XRD data work together to illustrate the removal of Na 2 SO 4 , suggesting the successful formation of purely amorphous lignin.Subsequently, N 2 adsorption/desorption studies have been carried out on the lignin, as shown in Figure S3, Supporting Information, which has a BET surface area of 6.2 m2 g À1 and an average pore size of 18.3 nm.It should be mentioned that such a porous structure facilitates faster electrolyte infiltration.The TG-DTG analytical spectrum of lignin at a heating rate of 10 °C min À1 is shown in Figure S4, Supporting Information.The result reveals that lignin has an onset decomposition temperature of approximately 200 °C in a nitrogen atmosphere, and therefore it exhibits good thermal stability. [22]To get more information about the structure of the lignin molecule, lignin was also examined using the XPS and FTIR techniques.As shown in Figure 1b, the C1s spectrum can be deconvoluted into three peaks.In detail, the peak at 284.7 eV is assigned to the C-C/C-H bond, and the peaks located at binding energies of 288.7 and 286.2 eV correspond to C=O carbonyl and C-O phenol (alcohol or ether) functional groups, respectively.As can be seen in Figure 1c, a wide absorption band in the range of 3700-3000 cm À1 is mainly associated with the stretching vibration of the phenol hydroxyl group (Ar-OH) and the alcohol hydroxyl group (R-OH).The characteristic absorption bands of -C=O are found at 1690 cm À1 .Furthermore, the multi peaks at 1125, 1085, and 1037 cm À1 should be assigned to the absorption of ether bonding C-O-C. [23]In combination with FTIR, XPS indicates that lignin is abundant in functional groups, such as carbonyls, phenol, and aromatic groups.It is clear from the above that lignin also has a unique molecular structure besides its environmental friendliness, high stability, and low cost.
The specific molecular structure fragment of lignin is presented in Figure 1d.It is a 3D amorphous polymer consisting of three canonical monolignols by C-O bonds and C-C bonds links: p-coumaryl alcohol (corresponding to H unit, red), coniferyl alcohol (corresponding to G unit, green), and sinapyl alcohol (corresponding to S unit, blue).Therefore, we sought to explore the sodium storage potential of lignin as an anode for SIBs at room temperature and high temperature.

Electrochemical Properties of Lignin Anode
As shown in Figure 2, the electrochemical properties of the lignin anode were carried out in sodium-ion half-cell devices at RT and 60 °C, which has rarely been reported before.A small broad CV peak appears at around 0.8 V during the first sodiation process under both experimental temperatures, which is not visible in the following scans (Figure S5, Supporting Information).This phenomenon is probably owed to both the irreversible reaction of the organic anode with the liquid electrolyte to generate the solid electrolyte interphase (SEI) and the irreversible binding of the phenol group to the sodium ion. [24,25]urthermore, we note that the area of the CV curve under room temperature increases markedly with scanning times, while the CV curves at 60 °C overlap to a better extent.This indicates that the redox reversibility of lignin is improved at high temperatures (60 °C) in comparison to room temperature.More than that, the peaks are almost in the same position.The intensity of the peaks becomes stronger as the reaction proceeds, which also indicates good reversibility of the association/disassociation of the Na atom with the lignin anode. [26]igure 2a demonstrates the second scan CV curves of the lignin anode at two different operating temperatures.A wave with a small, broad cathodic peak at %1.8 V is present on both CV curves.[29] In comparison, these two peaks become weaker and are shifted towards higher potentials during the room-temperature desodiation process.The different operating temperatures lead to a difference in their electrochemical behaviors.The first five galvanostatic charge/discharge curves (GCD curves) of lignin anode collected at the current density of 200 mA g À1 under 60 °C were recorded in Fig- ure 2b.As can be seen from the GCD curve, the first fifth sodiation capacity of the lignin anode is 174, 188, 200, 209, and 214 mAh g À1 , respectively.The reasons for the gradual increase in capacity with the number of the charge/discharge cycling will be investigated and discussed in the later part.The GCD profiles inform the discharging Energy Environ.Mater.2024, 7, e12538 platform at 1.8 V and two charging platforms at respective 1.9 and 2.7 V, which is in close agreement with the CV curves of the lignin anode in Figure 2a.The sodium storage capacities with corresponding Coulombic efficiencies (CE) operated under RT, and 60 °C were investigated at 200 mA g À1 (Figure 2c).Under different cycling temperatures, both show a similar increasing trend of capacity with the number of the charge/discharge cycling, which indicates that the structure of the lignin molecule is gradually broken and engaged in the reaction, which is called the activation process of lignin electrode. [30][33] At 60 °C, the initial sodiation capacity of the lignin anode at 200 mA g À1 is 175 mAh g À1 .After 60 cycles, the battery can achieve a reversible capacity of 262 mAh g À1 , and the CE is close to 100% for each cycle.Figure 2c demonstrates that the lignin anode exhibits relatively poor capacity and Coulombic efficiency at ambient temperature compared to those at high temperature.The sodiation capacity is 19 mAh g À1 at the first cycle, and the CE is only 87.2%.The reversible capacity slowly increases from 19 to 77 mAh g À1 after 60 cycles.Outstandingly, the reversible capacities at 60 °C are higher than those at RT, which we infer may be due to the greater number of reactive sites and the faster reaction kinetics. [14]In addition, the acceleration of reaction kinetics at higher temperatures can be further verified by the electrochemical impedance spectroscopy (EIS) spectrum (Figure S6, Supporting Information).The EIS spectrum of the lignin anode reveals that the overall impedance at high temperatures is much lower than that at room temperature.Moreover, the remarkable reduction in overall impedance at high temperatures ensures a better rate capability than the SIBs operating at RT.As summarized in Figure 2d, the reversible capacities of lignin anode slowly decrease from 182 to 171, 154, and 127 mAh g À1 at conditions of different rates of 200, 500, 1000, and 2000 mA g À1 .Even if Energy Environ.Mater.2024, 7, e12538 the current density is further increased to 5000 mA g À1 , the lignin anode can still maintain a specific capacity of 94 mAh g À1 .Additionally, the lignin anode delivers reversible capacities over 260 mAh g À1 when returned to low current density (100 mA g À1 ), suggesting no electrode destruction in the high-rate cycle.While at room temperature, the lignin only delivers a reversible capacity of about 32 mAh g À1 at 200 mA g À1 , and this value is almost retained when the rate increases to 5000 mA g À1 .The capacity retention at high current densities is presumed to be due to the slow activation of the lignin anode at room temperature to compensate for the loss of capacity.By the way, the contribution of the conductive agent to the capacity is negligible. [34]igure 2e shows the corresponding charge/discharge profiles at various current densities.It can be noticed that, at 200-1000 mA g À1 , the typical charging/discharging plateau exists in the curves.At 2000 mA g À1 , the cell's overall impedance causes an increase in polarization.When the current density is 5000 mA g À1 , the potential rises rapidly to the cutoff potential due to the limitation of sodium ion diffusion in the electrode. [35]The cycling stability of the lignin anode at high temperatures was further investigated in this work (Figure 2f).The lignin anode exhibits an initial capacity of about 170 mAh g À1 (1000 mA g À1 ).
The reversible capacities of the lignin anode are gradually increased with the number of the charge/discharge cycling and reach the maximum value of 210 mAh g À1 after 100 cycles.After 100 cycles, the electrode reaches the stable cycling stage, and after 200 cycles, it shows a reversible capacity of 193 mAh g À1 with a slight capacity fading rate of 0.08% per cycle.The slight capacity decay may be ascribed to the dissolution of the active material in the organic electrolyte. [36]All the electrochemical test data together illustrate the superior performance of the battery at 60 °C compared to room temperature.

Sodium Storage Mechanism of Lignin Anode
The sodium storage mechanisms of the lignin anode under RT and 60 °C were carried out via ex-situ XPS analysis of the electrodes at different states during the initial charge/discharge cycle.As Figure 3a clearly shows, the corresponding spectrum of C1s and O1s gathered from the electrodes at different states vary considerably, indicating that the structure of the lignin changes during the first cycle.The Na 1s spectrum in Figure 3b shows that hardly any Na 1s signal is found at the pristine state electrode.The intensity of the Na 1s signal increases with the sodiation process and decreases with the desodiation process at either RT or 60 °C.However, even in the fully desodiated state (3.0 V), the Na 1s peak on the XPS spectrum is still clearly visible.This indicates that not all sodium can be extracted from the lignin anode during charging due to the inevitable and irreversible reactions during the first charge/discharge test.To investigate the reaction mechanism of the lignin anode, we further explored the high-resolution O 1s XPS spectrum, and the results are as presented in Figure 3c.For pristine state electrodes, the O 1s peaks of 531.6 and 533.2 eV binding energies are ascribed to C=O and C-O groups in the lignin, respectively.When first fully sodiated state at room temperature, the C-O peak almost disappears, with the intensity of the C=O peak significantly reduced.At the same time, there are three new peaks at 530.8, 531.3, and 535.9 eV, respectively.The peak of 530.8 eV is ascribed to C-O-Na production, which indicates that C-O (containing C-OH and C-O-C bonding) and C=O binding react with the dissolved sodium ion, resulting in the formation of a C-O-Na group. [36]The two peaks at 531.3 and 535.9 eV are assigned as the peaks of Na 2 CO 3 and Na Auger, which are caused by the side reaction of the electrolyte at the Energy Environ.Mater.2024, 7, e12538 electrode surface. [37]It is noted that the C=O peak remains despite the electrode being in the fully sodiated state.This may be in response to the small number of carboxyl groups in the CNT.During the electrode desodiation process, the intensity of the C=O and C-O peaks increases remarkably, suggesting that the C=O and C-O bonding in the lignin structure are the main active sites for reversible storage of sodium.However, the intensity of the C-O peak fails to return to its initial state compared to the C=O peak, indicating that the coordination of the C-OH group with Na + is irreversible, a common phenomenon reported in organic electrodes. [25]Notably, the XPS spectrum at RT and 60 °C almost overlap in the fully sodiated state.We hypothesize that the mechanism of sodium insertion in the first cycle at these two temperatures is similar (Figure 3a).On the other hand, the O 1s XPS spectrum of the lignin electrode shows a great difference when in the fully desodiated state at 60 °C.As disclosed in Figure 3c, the peak of C-O almost disappears.In contrast, the integral area of the C=O peak becomes larger.To avoid this difference being influenced by the SEI at different temperatures, we characterized the XPS spectra of the lignin electrode in the fully desodiated state at different etch depths.As can be seen from Figure S7, Supporting Information, the ratio of C=O to C-O is approximately 1.8 at room temperature and up to 3.4 at high temperature (60 °C), indicating that the larger integrated area of the C=O peak at high temperature is from the reaction of lignin rather than SEI.Accordingly, we supposed that the C-O bonding may undergo a homogeneous cleavage reaction at 60 °C and the homogeneous cleavage products contain C=O groups, which may be the main reason for the excellent performance at high temperatures. [38]Furthermore, when in the fully desodiated state, the intensity of the Na 2 CO 3 peak at 531.3 eV remains at 60 °C.Meanwhile, it becomes significantly weaker at room temperature, indicating different SEI growth at the two temperatures. [39]Combined with the electrochemical performance, we speculate that high temperature facilitates the formation of dense SEI on the anode, which could enhance the cycling stability of the lignin anode.The C 1s spectral analysis results (Figure S8, Supporting Information) are consistent with the electrochemical processes indicated by the O 1s spectrum.
To further investigate the sodium storage mechanism of the lignin anode and to confirm the C=O and C-O-C bonding transitions, the ex-situ ATR-IR analysis was also applied during the initial charge/discharge cycle (Figure 3d). [40]For pristine state electrodes, the peak at 1690 cm À1 is derived from the C=O bonding stretching vibration, and the group of bands at 1125, 1085, and 1037 cm À1 are ascribed to the stretching vibrations ether bonding C-O-C.The peak of C=O bonds gradually diminishes with the sodiation process and gradually strengthens with the desodiation process, indicating the reaction between C=O bonding and Na + .Meanwhile, the trend in the C-O-C bond also confirms the interaction between Na + and the etheric oxygen of lignin. [41,42]ompared to the pristine state, the C=O peak is enhanced while the C-O-C is diminished when in the fully desodiated state at 60 °C.Thus, the ex-situ ART-IR results further confirm the reaction site of the lignin.
Due to the complex structure of real lignin, the reaction mechanism is often explored through its model compounds.Here, density functional theory (DFT) calculations were performed, and the b-O-4-linked lignin substructure (corresponding to structure A in Figure 4) was employed as a model to investigate further the interaction between lignin molecules and the deposited Na during the charge/discharge processes.The analysis of the calculation results is presented below.The binding site of Na + to structure A was investigated by the natural bond orbital (NBO) charge distribution (Figure S9, Supporting Information).When structure A gets two electrons to be reduced to structure A 2À , the electronegativity of these four oxygen atoms decreases significantly (O7 from À0.68207 to À0.78255, O10 from À0.52557 to À0.55402, O12 from À0.53625 to À0.64191, O13 from À0.68207 to À0.78255, as shown in Table S1, Supporting Information), confirming the tendency of the hydroxyl oxygen, carbonyl oxygen and ether oxygen (in the C-O-C) to bind with Na + , which is in great agreement with the attenuation of the C=O and C-O signals in XPS and ATR-IR spectrum.In addition, energy variation curves of the optimized structure A at different sodiated states are shown in Figure 4a, which is calculated from the energies of the different structures in Table S2, Supporting Information.The DE denotes the binding energy of Na in each step, with DE of the reactions A ? B and B ? C defined as DE 1 and DE 2 , respectively.The calculated values are DE 1 = À13.6271eV and DE 2 = À13.0904eV, and the negative value of DE indicates that Na + is well bound to structure A. Taking the un-substituted structure A as a reference, the order of the structural energy is A > B > C (Figure 4a), indicating that the binding to sodium generates a more stable structure, confirming the possibility of structure to accept 4Na + (Figure 4b).5b), which are capable of further sodium storage.
Based on the above analytical data from XPS, ATR-IR, and DFT, we propose the sodium storage mechanism of the lignin anode at room temperature and 60 °C.As shown in Figure 4, the hydrogen atoms of the two hydroxyl groups in structure A are first irreversibly substituted by two sodium ions to form structure B, evidenced by the electronegativity of oxygen.Subsequently, the carbonyl oxygen and ether oxygen atoms of structure B reversibly combine with the two sodium ions to form structure C. In particular, structure B undergoes further C b -O bond breakage when the cell is cycled at 60 °C, resulting in the generation of C=O functional groups that provide the cell with higher capacity (Figure 5).Therefore, the reasons for the excellent performance of lignin anodes at high temperatures include the greater number of reactive sites, the harder the SEI membrane, and the faster reaction kinetics.

Conclusion
In summary, a new eco-friendly, costeffective organic material was reported in this study.All experimental results demonstrate that when assessed as the anode materials for high-temperature SIBs, lignin organic polymers without further modification exhibit excellent cycling stability and rate performance.The unique sodium-ion storage behavior and the richer reactive sites explain its excellent cycling performance in hightemperature operating environments based on the analysis of ex-situ ATR-IR and XPS as well as theoretical calculations.More importantly, this work may stimulate the exploration of more organic molecules as low-cost but suitable anode materials for high-temperature environments in organic secondary batteries.

Experimental Section
Material characterizations: The sample morphologies were collected by spotlight ion beam field emission scanning electron microscopy (Tescan; LYRA 3 XMU).The XRD patterns of samples were performed on an X-ray diffractometer (Bruker D8 Advance) between 10°and 80°at a speed of 10°min À1 .The specific surface area (measured via the Brunauer-Emmett-Teller method) and the distributions of pore size (analyzed using the Barrett-Joyner-Halenda model) were evaluated by an Automatic rapid specific surface and porosity analyzer (Micromeritics Instrument; ASAP 2460).The thermograms of the lignin were characterized using a Mettler Toledo (TGA/DSC3+) instrument under a nitrogen atmosphere in the temperature range of 30-800 °C (with a heating rate of 10 °C min À1 ).The X-ray photoelectron spectroscopy (XPS) analysis was conducted with a Thermo Fisher Escalab 250Xi spectrometer using an Al Ka source (1486.6 eV).XPS data were analyzed by CASA XPS application with Shirley-type background.Fourier transform infrared spectrum of lignin was performed by using KBr disk from 4000 to 400 cm À1 .
To explore the sodium storage mechanism of lignin, electrodes after the cycling tests were collected by disassembling the cells and immersing the electrodes into dimethoxyethane (DME) for 2 h to remove the residing electrolyte and completely drying before testing.The cycled electrodes were tightly sealed in a container filled with argon gas atmosphere and quickly subject to ATR-IR and XPS testing.An ATR-IR spectrometer equipped with a diamond/ZnSe attenuated total reflectance (ATR) was employed to analyze the structural changes in the lignin anode in the fully charge/discharge states.The electronic states of the lignin electrodes were carried out using ex-situ XPS.To exclude the effect of SEI, we also etched the electrodes at different depths using 3000 eV Ar ions with an etching rate of approximately 25 nm min À1 .All characterization tests were performed at ambient temperature.Preparation of lignin electrodes: The dealkaline lignin power (CAS no.9005-53-2 from Macklin Company) was initially washed with deionized water, and then the Energy Environ.Mater.2024, 7, e12538 lignin active material was achieved after drying.Subsequently, the lignin (60 wt%) was fully mixed with carbon nanotube (CNT, 30 wt%, as a conducting agent) and polyvinylidene fluoride (PVDF, 10 wt%, as a binder) and suspended in N-methyl-2-pyrrolidone (NMP).The homogeneous slurry slurries were formed after 48 h of magnetic stirring.The working electrode was prepared by coating the homogeneous slurry on clean copper foil and further drying at 60 °C under vacuum conditions for 12 h.The working electrodes were then cut into 12 mm diameter disks.The mass loading of active material for each electrode is approximately 0.5 mg.Electrochemical measurements: Sodium foil was used as the counter electrode.The electrolyte adopted 1 M NaPF 6 in DME.Afterward, the CR2023 coin cells were hermetically assembled using the glass microfiber separator (GF/D) with a diameter of 16 mm in the glovebox (oxygen content and humidity below 0.1 ppm).Neware testing system (Neware Co., Ltd., Shenzhen, China) was employed to test the galvanostatic charge/discharge behaviors, where the voltage range was 0.01-3.0V, and the current rates were 100, 200, 500, 1000, 2000, and 5000 mA g À1 .Before the testing, all working electrodes were cycled 10 times at 100 mA g À1 to activate the electrodes.The cyclic voltammetry (CV) was carried out on a CHI 660E electrochemical workstation within the voltage window of 0.01-3.0V at a scan rate of 0.1 mV s À1 .Electrochemical impedance spectroscopy (EIS) measurements were also performed on the CHI 600D electrochemical workstation in the frequency range of 100 kHz-10 mHz.Notably, all battery tests were conducted at room temperature (RT) and 60 °C.
Density functional theory calculations: All structures were optimized using density functional theory (DFT) at B3LYP hybrid functional with 6-31G(d) basis set with Gaussian 16 package. [43]Then, a frequency calculation was carried out to ensure that the optimized structure is the local minimal without imaginary frequency.A single point energy calculation was performed at the same hybrid functional with the def2-TZVP basis set. [44][47] The energy of each structure was the sum of zero-point energy and electronic energy.Enthalpy was calculated by the Shermo package. [48]Molecular orbitals were analyzed by the Multiwfn package. [49]

Figure 1 .
Figure 1.a) X-ray diffraction pattern of lignin.b) The high-resolution C 1s spectrum of lignin.c) Fourier-transform infrared (FTIR) spectrum of lignin.d) The schematic diagram of using lignin as the organic anode of SIBs.The three canonical monolignols: p-coumaryl alcohol (corresponding to H unit, red), coniferyl alcohol (corresponding to G unit, green), and sinapyl alcohol (corresponding to S unit, blue).

Figure 2 .
Figure 2. The electrochemical properties of lignin anode in Na-ion battery at RT and 60 °C.a) The 2nd scan CV curves of the lignin anode with a scanning rate of 0.1 mV s À1 in 0.01-3.0V (vs Na + /Na).b) The first five galvanostatic charge/discharge profiles at the current density of 200 mA g À1 under 60 °C.c) The sodium storage capacities with corresponding Coulombic Efficiencies (CE) operated under RT and 60 °C at 200 mA g À1 .d) The rate capability at various current densities, and e) the corresponding charge/discharge profiles.f) Cycling stability at a current density of 1000 mA g À1 .

Figure 3 .
Figure 3.The corresponding test profiles of the lignin electrode at the pristine state, fully sodiated state (0.01 V), and fully desodiated state (3.0 V) during the initial charge/discharge cycling test under RT and 60 °C: a) C 1s, O 1s spectrum, b) Na 1s spectrum, c) The high-resolution O 1s spectrum, and d) ATR-IR spectrum.
Furthermore, Figure 5a reveals the homogeneous cleavage pathway of structure B and its corresponding bond dissociation enthalpy (DH).The order of DH for the structure B is as follows: C b -O (DH 4 ) < O-CH 3 (DH 1 ) < O-CH 3 (DH 6 ) < O-C aromatic4 (DH 5 ) < C a -C b (DH 3 ) < C aromatic -C a (DH 2 ).Combined with the analysis of the results of the XPS and ATR-IR data in the previous section, it was further confirmed the main reason for the excellent performance of the lignin anode at high temperatures is that a large number of C b -O bonds undergo a homogeneous cleavage reaction at 60 °C during charge process due to the smallest value of DH 4 and that the homogeneous cleavage products contain C=O groups (corresponding to structure D in Figure

Figure 4 .
Figure 4. a) Energy variation curves of the optimized structure A (b-O-4-linked lignin) at different sodiated states and b) the proposed sodium intercalation mechanism of lignin anode according to the energy variation curves.DE denotes the binding energy to the sodium atom in each step.