A flower‐like VO2(B)/V2CTx heterojunction as high kinetic rechargeable anode for sodium‐ion batteries

VO2(B) is considered as a promising anode material for the next‐generation sodium‐ion batteries (SIBs) due to its accessible raw materials and considerable theoretical capacity. However, the VO2(B) electrode has inherent defects such as low conductivity and serious volume expansion, which hinder their practical application. Herein, a flower‐like VO2(B)/V2CTx (VO@VC) heterojunction was prepared by a simple hydrothermal synthesis method with in situ growth. The flower‐like structure composed of thin nanosheets alleviates the volume expansion, as well as the rapid Na+ transport pathways are built by the heterojunction structure, resulting in long‐term cycling stability and superior rate performance. At a current density of 100 mA g−1, VO@VC anode can maintain a specific capacity of 276 mAh g−1 with an average coulombic efficiency of 98.7% after 100 cycles. Additionally, even at a current density of 2 A g−1, the VO@VC anode still exhibited a capacity of 132.9 mAh g−1 for 1000 cycles. The enhanced reaction kinetics can be attributed to the fast Na+ adsorption and storage at interfaces, which has been confirmed by the experimental and theoretical methods. These results demonstrate that the tailored nanoarchitecture design and additional surface engineering are effective strategies for optimizing vanadium‐based anode.


| INTRODUCTION
On the path toward the goal of carbon neutrality, a well-defind direction for green energy has attracted widespread attention.To achieve high efficiency and stability, the advanced energy storage device needs to be developed with low-cost and be environmentally friendly.Sodium-ion batteries (SIBs), as a promising candidate for lithium-ion batteries (LIBs), have drawn extensive research due to the rich resources, low cost, etc.As an important component of high-performance SIBs, high activity and superior kinetics are required for the anode materials.Recently, carbonaceous materials, 1 alloys, 2 and metal chalcogenides [3][4][5] have been extensively investigated for SIBs.Among them, the low potential of carbonaceous materials may increase the safety hazards in SIBs, which can be ascribed to the sodium evolution risk.Although the alloys usually exhibit ultrahigh capacity and good conductivity, the severe volume expansion, low interfacial stability, and high-cost lead to limited practicality in the large-scale energy storage field.
In comparison of the carbonaceous and alloy materials, metal chalcogenides usually possess moderate voltage range, high specific capacity, and controllable volume deformation, which can be regarded as feasible anodes for SIBs.Among numerous metal chalcogenides, vanadium oxides with abundant resources and variable electrochemical activity are used as promising host for highly reversible sodium storage.It is noted that V 2 O 3 and V 2 O 5 with large lattice spacing have been recently synthesized for sodium ion (Na + ) storage. 6In particular, the VO 2 (B) has a unique two-dimensional layered structure formed from edge-sharing VO 6 octahedra, which provides smooth paths for Na + migration. 7Hence, the rate performance and cycling life of VO 2 (B) electrode show a significant advantage.However, the VO 2 (B) suffers from low electronic conductivity, limited volume space, and continuous agglomeration, resulting in a huge room for improvement of capacity fading and kinetic sluggishness, which hinder the practical application. 8herefore, simple synthesis method, tailored nanoarchitecture design and additional surface engineering may facilitate industrialization. 9o far, various nanostructures of VO 2 (B), such as nanorods, 10 nanoneedles, 11 nanobelts, 8 nanospheres, 12,13 nanofibers, 14 nanosheets, 15 and 3D grid structure 16 have been synthesized to obtain advanced electrochemical properties.These novel structures not only can provide adequate spaces for relieving volume deformation, but also construct rich interfaces to enhance ion transport.For the strategy of building a composite system, various carbonaceous materials have been introduced into VO 2 (B) to form a rapid electron and ion conductive network.For example, He et al. demonstrated a one-step microwave-assisted solvothermal process to prepare VO 2 /rGO nanocomposite, delivering an acceptable rate capability (~100 mAh g −1 at 800 mA g −1 ). 10 Compared to common carbonaceous matrices, MXenes have multiple functional groups on the surfaces that can serve as active sites for Na + adsorption, as well as possess outstanding mechanical properties that can reinforce the sodium storage host. 17They also have unique layered structures with superior electrical conductivity, remarkable chemical durability, and highly specific surface areas.Therefore, the MXenes are considered as feasible choices for building composite.In previous works, the MXenes can exhibit physical confinement effects and strong chemical interfacial interactions to suppress volume expansion and promote conductivity. 189 V 2 CT x , as a novel MXenes material with similar chemical composition and layered structure of VO 2 (B), can form heterojunctions in composition to generate synergistic effects for improving sodium storage performance. 20erein, a flower-like VO 2 (B)/V 2 CT x sample (named as VO@VC) has been successfully synthesized using a facile one-step hydrothermal method, achieving coordination modulation and in situ growing simultaneously.Due to the similar chemical structure between VO 2 (B) (named as VO) and V 2 CT x (named as VC), the VO@VC heterojunctions with special laminated structure construct a fastly conductive network, as well as expose abundant active sites to increase the ion diffusion pathways.As a result, the as-prepared VO@VC anode displays high capacity, excellent rate capability, and high cycling stability.In this study, the designed composite materials maintain outstanding structural stability and fast reaction kinetics during long-term sodium storage process.This novel in-situ self-assembly method combined with tailored nanoarchitecture design and additional surface engineering offers a new direction for synthesizing highly efficient anode materials for SIBs.

| Morphology and structure characterization
The environmental-friendly and simple hydrothermal method has been recognized for synthetizing uniform nanometer materials with high purity and controllable morphology.A schematic illustration of the detailed preparation process for the VO@VC sample is shown in Figure 1.First, the sodium borohydride was added into the deionized water to form Metaborate ions (BO 2

−
), which can combine with VO 2+ ions to promote the dissolution of vanadium acetylacetonate oxide.The various surface terminations with negative charges on the surfaces of V 2 CTx (such as −F, −O, and −OH) promote VO 2+ to adsorb continuously on its surface.Then, the thiourea was introduced, which complexes with VO 2+ to form a stable complex on the V 2 CTx surface.Finally, the stable complex converts to VO@VC heterojunctions material under hydrothermal reaction.
The morphologies and micro-structures of the asprepared samples, including pure VO, VC, and VO@VC, were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively.The SEM image (Supporting Information: Figure S1) of the VC sample shows a monolayer structure, which provides a broad space for the in situ growth of VO nanosheets.An ultrasonic dispersion process was carried out to attain dispersed VC nanosheets without significant agglomeration.As shown in Figure 2A,B and Supporting Information: Figure S2, both VO and VO@VC samples exhibit flower-like hybrid architecture assembled via laminated nanosheets with a uniform thickness of 80 nm.To be sure, the existence of VC did not influence the microstructure of the VO nanosheets.The VO@VC composite exhibits a three-dimensional (3D) conductive network and the built-in electric field at the heterojunction interface for ions/electrons, which can provide sufficient spaces to suffer volume expansion and form excellent sodium transmission pathways between electrodes/electrolytes, further enhancing the electrochemical performance via a synergistic effect.The elemental mappings are employed to investigate the element distribution in the VO@VC composite.Figure 2C and Supporting Information: Figure S3 shows the scanning TEM-HAADF images of the VO@VC samples, suggesting the uniform distribution of V, O, and C elements throughout the flower-like structure.
Furthermore, the typical TEM image of the VO@VC sample (Figure 2D) shows that the nanoflakes are laminated structures, which is consistent with the SEM images.The high-resolution TEM (HRTEM) image (Figure 2E) of the VO@VC indicates the excellent crystallinity of VO@VC.It can be seen that the apparent lattice fringes with a spacing of 0.353 nm are attributed to the (110) crystal plane of VO 2 (B).Furthermore, Supporting Information: Figure S4 illustrates the presence of interleaving between (002) and (200) crystal planes, along with lattice micro-defects.Moreover, the irregular lattice fringes at the interfaces indicate a strong interaction between V 2 CTx and VO 2 (B).Similarly, the selected-area electron diffraction pattern (SAED) of Figure 2F with multiple diffraction rings indicates the polycrystalline nature of the monoclinic phase, corresponding to the (110) and (003) planes of VO 2 (B).
The X-ray diffraction (XRD) patterns of VC, VO, and VO@VC samples are shown in Figure 3A.First, the peaks located in the range of 5-10°can be assigned to V 2 CT x in composite.In addition, other diffraction peaks of VO@VC can be well indexed to the pure monoclinic VO 2 (B) (with theoretical lattice parameters of a = 12.093 Å, b = 3.7021 Å, c = 6.433Å, in the C 2 /m space group, JCPDS NO. 81-2392), indicating that the introduction of V 2 CT x did not cause significant changes of the VO 2 (B).
Further X-ray photoelectron spectroscopy (XPS) analysis was used to characterize the chemical bonds and valence states of VC, VO, and VO@VC.Supporting Information: Figure S5   V 2p peaks clearly appearing, which is consistent with the EDS results.The C percentage in VO@VC is apparently higher than that in VO, which can be attributed to the existence of VC.The high-resolution XPS spectrums for V 2p are presented in Supporting Information: Figure S6, Figure 3C,D.The V 2p can be fitted with two spin-orbit splitting of V 2p 1/2 and V 2p 3/2 in VO@VC.The two peaks centered at 524.27 and 516.89 eV are assigned to V 4+ in VO 2 (B), and the peaks centered at 523.08 eV and 515.58 are referred to V 2+ in V 2 CT x . 21As shown in Supporting Information: Table S1, the V 2p 1/2 and V 2p 3/2 spectra of VO@VC are positioned between VO and VC, which indicate that the electrons were transferred from VC to VO driven by the built-in electric field formed in the heterojunction. 22In addition, the characteristic peak of O 1s (Figure 3F) is deconvoluted into three peaks with the binding energy of 284.8, 285.7, and 288.9 eV, owing to the contributions of the C-C, C-O, and C═O bonds, respectively, which formed by the composite between VO 2 (B) and V 2 CT x . 23

| Electrochemical performance analysis
The electrochemical performances of the VO@VC anode were described in half-cells using sodium metal as a counter electrode.Cyclic voltammetry (CV) of VO@VC at a sweep rate of 0.2 mV s −1 was further investigated in Figure 4A.During the first discharge process, an irreversible reduction peak (denoted as peak 1) was centered at 1.03 V (vs.Na + /Na), which can be ascribed to the formation of solid electrode interphase (SEI) generated on the anode surface.The two pairs of redox peaks F I G U R E 4 (A) CV curves of VO@VC at a sweep rate of 0.2 mV s −1 ; (B-E) cycling performance (C) and rate performance (D) of as-prepared anodes, as well as the corresponding galvanostatic discharge/charge profiles (B, E); (F) Nyquist plots of pre-and postcycling of VO and VO@VC; (G) long-term cycling performance at 2 A g −1 for VO and VO@VC anodes.that appeared at 0.61/0.64V and 1.34/1.54V, respectively, were caused by the insertion and extraction behaviors of Na + .The CV curves almost overlap, except for the first cycle, indicating the good cycling stability and reversibility of the VO@VC anode.The related reaction mechanism is described as follows 24 : ⇌ e NaVO Na VO + xNa + x .
The galvanostatic discharge/charge (GDC) profiles are presented in Figure 4B, Supporting Information: Figures S7 and S8.It can be noted that the VO@VC, VO, and VC anodes exhibited acceptable initial discharge capacities, but the subsequent irreversible capacity loss can be mainly attributed to the electrolyte decomposition and the formation of SEI.Nevertheless, the VC anode displayed an obvious capacity fade, while VO and VO@VC indicated considerable reversibility corresponding to the overlap of charge-discharge profiles.Figure 4C further shows the cycling properties of VO@VC, VO, and VC anodes at 100 mA g −1 in the voltage range of 0.01-3 V.The VC showed a low initial coulombic efficiency (CE) and a poor cycling stability (less than 50 mAh g −1 after 100 cycles).The VO displayed an acceptable initial charge capacity of 206.5 mAh g −1 and good cycling stability compared to VC, but its kinetics still needed improvement.Nevertheless, the VO@VC anode exhibited a high initial charge capacity of 279 mAh g −1 , accompanied by a considerable initial CE of 60.82%.After 100 cycles, the VO@VC anode still displayed a high discharge capacity of 276 mAh g −1 corresponding to a capacity retention (C 100 / C 2 ) of 97.5% and an average CE of 99.8% (from 2nd cycle to 100th cycle), which is much higher than that of VO anode with a capacity retention (C 100 /C 2 ) of 78% and an average CE of 98.7%.The improved reversibility and cycling stability of VO@VC anode are benefit from the fast Na + transportation resulted by synergistic effect between V 2 CT x and VO 2 (B). 4 The obviously low capacity provided by VC anode demonstrates that the capacity of VO@VC mainly originates from the VO 2 (B), while the V 2 CT x plays a critical role in improving the structure stability, electronic, and ionic conductivity.
The rate performances of VO and VO@VC anodes were tested at increased current densities from 0.1 to 3.2 A g −1 (Figure 4D).The VO@VC anode exhibited the ultrahigh rate performance than that of VO under different current densities.The average discharge specific capacities of VO@VC were 278, 233, 209, 185, 152, and 102 mAh g −1 at current densities of 0.1, 0.2, 0.4, 0.8, 1.6, and 3.2 A g −1 , respectively.When the current density was recovered to 0.1 A g −1 , the specific capacity returned to 96.04% of the initial value, indicating that the VO@VC anodes maintained excellent structural stability after cycling at a high rate.The GDC profiles of different current densities are shown in Figure 4E and Supporting Information: Figure S9 to further study the intensification behaviors of each anode material.It is noticed that VO@VC anode exhibited a lower voltage polarization of 1.03 V than 1.44 V of VO at 3.2 A g −1 , which is ascribed to the lower charge-transfer resistance of VO@VC.The enhanced interfacial compatibility and ion transfer kinetics between the VO@VC anode and electrolyte were further confirmed by electrochemical impedance spectroscopy (EIS) of pre-and postcycling (Figure 4F).Before cycling, the charge-transfer resistance (R ct ) of VO@VC (245 Ω) was lower than that of VO (468 Ω), indicating the higher diffusion kinetics in heterojunctions structure.After 10 cycles, the charge-transfer resistance (R ct ) of VO (1563 Ω) was increased significantly than VO@VC (791 Ω).The lower change of R ct is another reason for excellent cycling stability and rate capability.Furthermore, the long-term cycling performance (Figure 4G) of VO@VC was measured at 2 A g −1 .The specific capacity of VO@VC remained at 132.9 mAh g −1 even after 1000 cycles, exhibiting superior structural stability compared to VO.As shown in Supporting Information: Figure S10, When matched with Prussian blue (PB) cathode, the full cell can deliver 69 mAh g −1 after 50 cycles at 500 mA g −1 in the voltage range from 0.5 to 2.5 V, which shows the potential for practical applications of VO@VC anode.Compared with many previously reported VO 2 anodes (Supporting Information: Table S2), the VO@VC in this work shows excellent performance.

| Kinetic performance analysis
To reveal the energy storage and kinetic optimization mechanism in depth, the CV curves at different scan rates (0.2-5 mV s −1 ) and galvanostatic intermittent titration technique (GITT) were employed (Figure 5).As shown in Figure 5A, the redox peaks became stronger and were accompanied by a slight shift in peak position as the increased scan rates, suggesting a weak polarization effect and high reversibility of the VO@VC anode.The detailed information of CV curves can be described by the power-law relationship between current (i) and scan rate (v) by the following equation.
where a and b are adjustable parameters.Among, the two extreme values for b, 0.5 and 1, correspond to diffusioncontrolled (Faraday process) and capacitive-controlled processes (non-Faraday process), respectively. 26The calculated b values of VO@VC anode based on Equation (4) for the cathodic and anodic processes are 0.91 and 0.82, respectively, which are higher than that of VO anode (Figure 5B and Supporting Information: Figure S11), suggesting substantial improvement of sodium storage reaction kinetics via increased capacitive-controlled process caused by the large specific surface areas and rich reactive sites of VO@VC heterojunctions.Meanwhile, the capacitive-controlled process displays a quasi-linear dependence of potential on capacity, with an inconspicuous voltage plateau and a small voltage polarization hysteresis, which is consistent with the analysis results in Figure 4. 27 Additionally, the below equation is used to quantify the pseudocapacitive contributions: where k 1 and k 2 are constants, and we can separate the current from capacitive contribution and diffusion contribution (pseudocapacitive contribution), respectively.For instance, the pseudocapacitive contribution to the total current of VO@VC is 54.16% at 0.2 mV s −1 (Figure 5C).As the scan rate increased, the pseudocapacitive contributions rose to 55.96%, 62.21%, 68.91%, and 83.79% at 0.5, 1, 2, and 5 mV s −1 , respectively (Figure 5D,E).For comparison, the pseudocapacitive contribution ratio of VO anode is only 68.2% even at 5 mV s −1 (Supporting Information: Figure S12).The increased pseudocapacitive effect of VO@VC can be regarded as the rich phase boundaries of V 2 CT x (with rich surface groups) and built-in electric field at the heterojunction interfaces.Furthermore, the C-O and C═O bonds in VO@VC can also greatly promote the pseudocapacitive effect. 28The above results confirm rapid electron and ion transport for prominent reaction kinetics and rate performance of VO@VC.
GITT was employed to further calculate the diffusion kinetic properties of Na + in VO@VC and VO anodes.The diffusion coefficient of Na + (D Na + ) can be calculated using the below equation as follows: where τ is relaxation time (s); m is the active mass (g); V M is the molar volume (cm 3 mol −1 ); M B is the molar mass (g mol −1 ); S is the total surface area of the electrode (cm 2 ); ΔE s is the stable potential difference between two pulses (V); ΔE t is the potential difference caused by constant current charging (discharging) in a single pulse (V).In this work, the relaxation time τ is 3600 s and the total surface area of the electrode S is 0.95 cm 2 .
The GITT curves of VO and VO@VC are shown in Figure 5F with the schematic diagram (Supporting Information: Figure S13).Both VO and VO@VC show higher D Na + (maintained at ~10 −9 ) during the discharge and charging process (Figure 5G).The layered crystal structure facilitates rapid Na + transmission, while the additional surface engineering further enhances its transport properties.Nevertheless, the D Na + of VO@VC is slightly higher than that of VO, indicating the rapid Na + diffusion in VO@VC.Moreover, the D Na + shows a relatively slow downward trend with the increase of discharge and charge depth.During the discharge process, the coulombic repulsion forces increased with continuous insertion of Na + , which hinders the further diffusion of Na + .In the charge process, the concentration gradient of Na + in electrolyte gradually decreases, leading to the D Na + decline.Based on the above test results, the excellent electrochemical performance of VO@VC can be ascribed to the delicate microscopic morphology and heterojunctions structure design composited with V 2 CT X , which provides abundant surface active sites and high conductivity.
To further investigate the accelerated charge transfer kinetics of VO@VC, the interfacial electronic structure and sodium adsorption were illuminated by density functional theory (DFT) calculations.The constructed calculation models are shown in Supporting Information: Figure S14.According to density of states (DOS) shown in Figure 6A,B, the VO exhibited semiconductor behavior with a narrow bandgap of 0.53 eV, while the VC showed a typical metallic characteristic without obvious bandgap.Hence, at the heterojunction interface, electrons will spontaneously transfer from VC to VO until their Fermi levels reach thermodynamic equilibrium, resulting in higher electron density near the Fermi level at the VO@VC heterojunction interface (Figure 6C) compared to that of bare VO. 29 The differential charge density of VO@VC heterostructure can further prove this conclusion.As shown in Figure 6D, the charge density at the VO@VC heterojunction interface exhibits a significant charge transfer, indicating the strong interaction between VO and VC.The electrons' behaviors mentioned above align with the XPS results because of the built-in electric field.The Motty-Schottky effect generated by the metal-semiconductor heterojunction forms built-in electric field, which offers the driving force for rapid transport of ions and electrons. 22In addition, the adsorption behaviors of Na + on VC, VO, and VO@VC were investigated.The adsorption sites are shown in Supporting Information: Figure S15 and the calculated results are displayed in Figure 6E.The VO@VC heterostructure exhibited the highest adsorption energy for Na + (−7.31 eV) than VC (−3.93 eV), and VO (−3.67 eV), indicating that the fast transport and stable storage of Na + are attained at the interfaces of VO@VC heterojunction.In brief, based on the results of DFT calculations, the heterojunction structure enhances the transport properties of Na + , contributing to the excellent electrochemical cycling performance and rate performance.

| CONCLUSION
In summary, a flower-like VO 2 (B)/V 2 CT x heterojunction has been successfully prepared by a facile one-step hydrothermal method.The novel microstructure maintains structural stability and exhibits Na + storage potential.Meanwhile, the synergistic coupling effect of interlayer confinement caused by heterojunction structure enhances the charge transfer kinetics of VO@VC.Due to above excellent designed nanoarchitecture and the rapid Na + transport achieved by the built-in electric field in heterojunction interface, the VO@VC anode displays high specific capacity, outstanding cycling performance, and excellent rate stability for SIBs.After 100 cycles, the VO@VC anode still exhibited a high discharge capacity of 276 mAh g −1 at 0.1 A g −1 .Notably, the VO@VC anode delivered a considerable specific capacity of 132.9 mAh g −1 at a high current density of 2 A g −1 even after 1000 cycles.In conclusion, the strategy, which combines a simple synthesis method, tailored nanoarchitecture design and additional surface engineering, provides unprecedented potential for fabricating exceptional anode materials for SIBs.

| Synthesis of VO 2 (B)
The VO 2 (B) nanosheets were synthesized by hydrothermal method.First, add 1.0 g of polyvinylpyrrolidone (PVP) to 60 mL of deionized water and stir to form a transparent solution.Then, add 0.159 g VO(acca) 2 and 37 mg NaBH 4 into the solution and stir them until VO(acca) 2 dissolved.Quickly, add 0.75 g CH 4 N 2 S to the above solution and stir at high speed for 15 min, forming a uniform suspension.Then, the aqueous solution was transferred to a sealed Teflon-lined stainless steel autoclave and heated at 150°C for 24 h.Subsequently, the autoclave was naturally quenched under the room temperature.After washing several times by distilled water and ethanol by a centrifuge, the black precipitate was formed.

| Synthesis of VO 2 (B)/V 2 CT x heterojunctions
The VO 2 (B)/V 2 CT x Heterojunctions were synthesized by the hydrothermal method.First, add 1.0 g of PVP to 60 mL of deionized water and stir to form a transparent solution.Then, 30 mg V 2 CT x was added to the solution and sonicated for 30 min to form a uniform black dispersion solution.after that, add 0.159 g VO(acca) 2 and 37 mg NaBH 4 into the solution and stir them until VO(acca) 2 dissolved.Quickly, add 0.75 g CH 4 N 2 S to the above solution and stir at high speed for 15 min, forming a uniform suspension.Then, the aqueous solution was transferred to a sealed Teflon-lined stainless steel autoclave and heated at 150°C for 24 h.Subsequently, the autoclave was naturally quenched under the room temperature.After washing several times by distilled water and ethanol by a centrifuge, the black precipitate was formed.

| Synthesis of PB cathode
To form solution A, dissolve 3.04 g of sodium ferricyanide in 160 mL of deionized water and stir.For solution B, dissolve 0.3 g of sodium chloride in 40 mL of deionized water.Add solution A drop by drop to solution B and then add 1.92 g of concentrated hydrochloric acid to the beaker.Cover the beaker with plastic wrap and stir the mixture for 30 min.Then, the aqueous solution was transferred to be heated at 80°C for 24 h.Subsequently, the autoclave was naturally quenched under the room temperature.After washing several times by distilled water and ethanol by a centrifuge, the black precipitate was formed.

| Morphology and structural characterization
The SEM was performed using a Hitachi S-4800 instrument to capture crucial morphology images.High-resolution transmission electron microscopy (HRTEM; Talos F200S G2 S, 200 kV, USA) was employed to investigate the nanostructures in greater detail and to obtain elemental mappings.In addition to this, XRD was carried out with monochromatic Cu kα radiation (1.5405 Å) on a Hitachi Rigaku-SmartLab SE instrument.The element valences were confirmed using PHI Quantera-II equipment, specifically for vanadium, carbon, and oxygen.

| Electrochemical characterization
The electrochemical measurements carried out in this study involved the assembly of 2032-coin-type half cells in an argon-filled glove box.The working electrodes were first prepared by casting a slurry containing 70 wt% active material, 20 wt% Super P, and 10 wt% Carboxymethylcellulose sodium (CMC-Na) onto a copper foil using a doctor blade.Subsequently, the working electrodes were punched into 11-mmdiameter discs after drying them at room temperature.The counter electrode used in the experiment was sodium foil while the electrolyte comprised 1 M NaClO 4 dissolved in ethylene carbonate (EC): dimethyl carbonate (DMC) (1:1 in volume) containing 5% fluoroethylene carbonate (FEC).A Neware Battery Tester from Neware Technology Limited, China was used to record galvanostatic electrochemical test data at room temperature.Additionally, CV and electrochemical impedance spectroscopy were performed on a CHI 660d electrochemical workstation from ChenHua Instruments Co., with test conditions for CV curves being within the voltage range of 0.01-3.0V (vs.Na + /Na).

| Calculation method
All density DFT calculations were performed in the generalized gradient approximation using the Perdew-Burke-Ernzerhof function with the Vienna ab initio Simulation Package.The adsorption energy (Ea) is calculated by the equation: Ea = E(total) -E(slab) -E(Na).where E(total) and E(slab) correspond to the total energy of the surface slab with and without Na adsorption, respectively, and E(Na) is the energy of a single Na.
Dong et al. construct a Ni 0.5 Co 0.5 Se 2 /Ti 3 C 2 T x composites, and the anchored Ni 0.5 Co 0.5 Se 2 in the Ti 3 C 2 T x shows long cycling stability over 1000 cycles at 2.0 A g −1 .
and Figure 3B present the XPS survey spectra of VO and VO@VC with C 1s, O 1s, and F I G U R E 1 Schematic illustration of the detailed preparation process for the VO@VC sample.F I G U R E 3 (A) X-ray diffraction (XRD) patterns; (B) X-ray photoelectron spectroscopy (XPS) full spectra of VO@VC; (C-F) V 2p and C 1s spectrum of VO (C, D) and VO@VC (E, F).

F I G U R E 2
Morphology characterization of VO@VC: (A, B) Scanning electron microscope (SEM) images; (C) EDX images of V, O, and C elements; (D) transmission electron microscopy (TEM) images; (E) high-resolution TEM (HRTEM) images; (F) selected-area electron diffraction pattern (SAED) images of localized regions at the edges.

i = av . b ( 3 )
Take the logarithm on both sides of Equation (3):

F
I G U R E 5 (A) CV curves at different sweep rates; (B) logi-logv fitting image; (C) calculated percentage of pseudocapacitance at 0. 2 mV s −1 ; (D, E) pseudocapacitance contribution percentage at different sweep rates of VO@VC; (F) galvanostatic intermittent titration technique (GITT) profiles and (G) the corresponding calculated diffusion coefficient.

F
I G U R E 6 (A-C) Density of states (DOS) plots (fermi levels are set as zero and indicated with dashed lines) of VO (A), VC (B), and VO@VC (C); (D) charge density diferences of VO@VC heterointerface, corresponding to its schematic diagram; (E) sodium adsorption energy at hollow site of VC, VO, and VO@VC.