Ultrahigh‐Rate and Ultralong‐Duration Sodium Storage Enabled by Sodiation‐Driven Reconfiguration

Despite their variable valence and favorable sodiation/desodiation potential, vanadium sulfides (VSx) used as anode materials of sodium‐ion batteries (SIBs) have been held back by their capacity decline and low cycling capability, associated with the structure distortion volume expansion and pulverization. This study reports an accessible process to tackle these challenges via fabricating a 3D‐VSx anode for SIBs with ultrahigh‐rate and ultralong‐duration stable sodium storage. The sodiation‐driven reactivation of micro‐nano 3D‐VSx activates the reconfiguration effect, effectively maintaining structural integrity. Interestingly, the mechanical degradation of 3D‐VSx over the sodiation process can be controlled by fine‐tuning the operating voltage. The self‐reconfigured open nanostructures with large void space not only effectively withstand repetitive volume changes and mitigate the damaging mechanical stresses, but also in turn construct a self‐optimized shortened ion diffusion pathway. Moreover, the sodiation‐driven reconfiguration excites many active sites and optimizes a stable solid‐electrolyte interface, thereby delivering a reversible capacity of 961.4 mA h g−1 after 1500 cycles at a high rate of 2 A g−1. This work provides new insight into the rational design of electrodes toward long‐lived SIBs through sodiation‐driven reconfiguration.


DOI: 10.1002/aenm.202204324
(LIBs), due to the abundance and wide distribution of sodium in earth crust comparing to the scarce and nonuniform distributed lithium. [1] However, in practical applications, SIBs suffer from low capacity and poor rate performance owning to the large ionic radius of Na-ion. [2,3] Typically, to improve the electrochemical performance of SIBs during charge/discharge processes, different materials have been developed as anode electrodes for SIBs, including various carbonaceous materials, transition metal compounds and so on. [4,5] Among these, carbonaceous materials have limited capacities due to their poor storage capability for Na-ion, while transition metal compounds (TMDs) can make up these shortages and exhibit ideal theoretical specific capacities. [6,7] Vanadium sulfides (VS x ), with various crystal structures, such as VS 2 , V 2 S 3 , V 3 S 4 , V 5 S 8 and VS 4 have attracted increasing attention because they could offer proper interlayer spacing to accommodate Na + ions and have both intercalation/deintercalation and conversion energy storage processes. [8][9][10][11][12][13][14][15] For example, VS 2 with a large interlayer spacing of 0.575 nm delivered a higher theoretical specific capacity of about 800 mAh g −1 (comparing to 372 mAh g −1 of graphite anode in commercialized LIBs), suggesting that vanadiumbased sulfides could be a good candidate for SIBs. [11] Nevertheless, such vanadium-based sulfides frequently suffer significant mechanical pulverization during long-term cycling as a result of volume expansion caused by sodization/desodization, which results in severe irreversible capacity degradation and unsatisfactory cycling performance. [16,17] Up to now, the best (and one of the very few) cycling performance of all vanadium sulfides is V 5 S 8 /C electrode with 4000 cycles but only with a low specific capacity of 340 mAh g −1 (at 2 A g −1 ). [14] The best capacity performance so far of all vanadium sulfides is VS 4 -CN-Hs with 863 mA h g −1 at 0.1 A g −1 but only for 30 cycles. [15] Therefore, developing high-performance vanadium sulfide anode with both high capacity (energy density) and good cycling performance presents a key challenge for commercializing this vanadium sulfide-anode SIBs.
3D micro/nanostructuring of vanadium sulfide anode might provide a clue to address the above challenge. Such 3D micro/nanostructures have been approved for overcoming Despite their variable valence and favorable sodiation/desodiation potential, vanadium sulfides (VS x ) used as anode materials of sodium-ion batteries (SIBs) have been held back by their capacity decline and low cycling capability, associated with the structure distortion volume expansion and pulverization. This study reports an accessible process to tackle these challenges via fabricating a 3D-VS x anode for SIBs with ultrahigh-rate and ultralong-duration stable sodium storage. The sodiation-driven reactivation of micro-nano 3D-VS x activates the reconfiguration effect, effectively maintaining structural integrity. Interestingly, the mechanical degradation of 3D-VS x over the sodiation process can be controlled by fine-tuning the operating voltage. The self-reconfigured open nanostructures with large void space not only effectively withstand repetitive volume changes and mitigate the damaging mechanical stresses, but also in turn construct a self-optimized shortened ion diffusion pathway. Moreover, the sodiationdriven reconfiguration excites many active sites and optimizes a stable solid-electrolyte interface, thereby delivering a reversible capacity of 961.4 mA h g −1 after 1500 cycles at a high rate of 2 A g −1 . This work provides new insight into the rational design of electrodes toward long-lived SIBs through sodiation-driven reconfiguration.

Introduction
For large-scale energy storage, sodium-ion batteries (SIBs) are considered as a promising supplement to lithium-ion batteries severe self-aggregation of low-dimensional nanostructured electrodes (e.g., 1D nanowires, 2D nanosheets), [18][19][20] allowing large surface area and ultra-short diffusion paths for ionic/electronic transport (Figure 1a). Moreover, it enables good integrity of the whole electrode by relieving mechanical stress/strain and pulverization, giving the electrode material both good structural stability and sufficient active sites (Figure 1b). These features of 3D micro/nanostructured electrode shall be capable of avoiding pulverization and facilitating insertion/extraction of sodium ions, by which much improved battery performances could be achieved especially at high charge/discharge rates.
In this work, we demonstrate a 3D electrode of vanadium sulfides (VS x ) material for SIBs, delivering a reversible capacity of 961.4 mA h g −1 after 1500 cycles at a high rate of 2 A g −1 , which makes this work the best combined performance of capacity/cycling ever reported so far. The mechanism and importance of sodiation-driven reconfiguration of 3D-VSx were elucidated by surface phenomena and electrochemical reaction investigations, which have not been reported previously in vanadium sulfide anodes. It is found that the insertion/extraction behavior of Na + is partially irreversible, which is the main reason for sodiation-driven reconfiguration. 3D-VS x via sodiation-driven reconfiguration to increase active sites and mitigate volume changes shows unique characteristics of sodium ion storage: i) induction of a large number of Na + storage active sites, promoting stable final high capacity; ii) self-reconfiguring open nanostructures (micro-nanoflake nanosheets to micro-nanoparticles) with larger void space to withstand repetitive volume changes and nanoscale diffusion length of 3D-VS x allowing ultra-fast Na + and electron transport; and iii) optimized and stable solid-electrolyte interfaces that can accommodate long-term cycling. Moreover, the Na + adsorption energy and diffusion energy barriers were calculated by density functional theory (DFT) calculations, suggesting that the

Structural Characterization of 3D-VS x
The 3D-VS x was fabricated by a carbon-free and in situ hydrothermal approach. Figure 1 and Figure S1, Supporting Information, show the scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) images of the 3D-VS x sample prepared by the hydrothermal reaction of NaVO 4 and C 2 H 5 NS in deionized (DI) water. It is observed that the samples with different reaction times of 16, 24, and 32 h show different morphologies in Figure 1c and Figure S1, Supporting Information. The 3D-VS x sample with reaction time of 16 h is a lot of hollow microspheres with diameters of 1-3 µm and rough surfaces ( Figure S1, Supporting Information). As the growth time is extended to 24 h, nanoplates with thicknesses of 20-80 nm were grown from the rough surface of hollow microspheres, and subsequently formed into micron-sized 3D-VS x flowers (24 h) with the diameter about 2.5-7 µm (Figure 1c,d). When the time is further prolonged to 32 h ( Figure S1, Supporting Information), the gaps between the nanosheets of 3D-VS x will be gradually blocked, which is not beneficial for ion transfer and storage. Based on these results, a schematic diagram of the growth of 3D-VS x is presented in Figure S1, Supporting Information. We also note that the 3D-VS x samples with different reaction times present the same crystal structure, as evidenced by the corresponding XRD patterns in Figure S2, Supporting Information.
The characteristic of 3D-VS x was further characterized with the HRTEM and high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM), as shown in Figure 1e-i. Clearly, the well-defined lattice interlayer spacing with 0.577 and 0.252 nm was observed, corresponding to the (100) and (204) plane of the hexagonal V 2 S 3 phase, respectively. [8] Furthermore, elemental mapping analysis displayed the corresponding V, S and O element of 3D-VS x nanoplates (Figure 1g-i), which means the three elements are homogeneously distributed on 3D-VS x nanoplate. The oxygen element of the TEM sample may come from oxidation in the exposed air, because vanadium element is a very oxyphilic substance and is readily oxidized during sample preparation. [20,21] The micronsize VS x flowers provide a very stable structure to avoid agglomeration of 2D nanoplates on the surface of 3D sphere structure, which can significantly decrease the sodium diffusion length within the active nanoplates and can be also promised to mitigate the issues related to conversion reactions. [22] We used the X-ray diffraction (XRD) technique to analyze the crystallographic structure of 3D-VS x sample (24 h). There are four prominent peaks located at 2θ = 15.  Figure 2b. The spectrum presents characteristic peaks at 189.26 cm −1 which is assigned to the stretching mode (A g ) of the VV bonds in 3D-VS x . The peaks at 285.52 and 408.59 cm −1 could be ascribed to the in-plane bending (E 1g ) and out-of-plane stretching (A 1g ) vibration mode of VS in V 2 S 3 , respectively. [23,24] The peak at 989.2 cm −1 is attributed to the stretching VS mode of the vanadyl moiety. [25,26] To evaluate the surface area of 3D-VS x , N 2 adsorption/desorption isotherm curves were measured and are presented in Figure 2c. The results indicated that a surface area of 4.0120 m 2 g −1 for the fresh sample of 3D-VS x , which is attributed to the micronano size of 3D-VS x obtained by the hydrothermal method. The surface composition and the chemical states of the elements of 3D-VS x were also revealed by X-ray photoelectron spectroscopy (XPS) measurements (Figure 2d-f). As shown in Figure 2d, the full XPS survey of 3D-VS x exhibits the existence of vanadium, sulfur, and oxygen elements. The V 2p signal of 3D-VS x can be deconvoluted into four peaks (Figure 2e), where the peaks at 516.7 and 524.1 eV correspond to the 2 p 3/2 and 2 p 1/2 , respectively. They can be ascribed to the components of V 4+ for this sample. [27,28] The rest two peaks appearing at 513.7 and 521.5 eV belonged to the binding energy of 2 p 3/2 and 2 p 1/2 for V 3+ . The existence of V 3+ indicated the self-insertion of V atom in V 2 S 3 . [21,29] The core-level S 2p spectrum of 3D-VS x in Figure 2f, the peaks located at 162.68 and 163.96 eV can be ascribed to the metal-sulfur bonds. [30,31] Among which the peak at 160.88 eV should be attributed to the sulfur element. [32] We further analyze the XPS spectra of O 1s as shown in Figure S4, Supporting Information, the O 1s doublet peaks at 531.48 and 530.18 eV are correlated with adsorbed oxygen and vacancies adsorbed oxygen, respectively, which can be attributed to the adsorption of oxygen in the air via the strong chemical polarity of 3D-VS x . [28,33] Additionally, we obtained a precise and accurate atomic ratio of S/V around 1.74:1 by the inductively coupled plasma-optical emission spectrometer (ICP-OES) technique and energy dispersive spectroscopy (EDS) elemental analysis, as shown in Tables S1 and S2 and Figure S5, Supporting Information, further confirming the rich sulfur element. As a result, non-stoichiometric 3D-VS x compounds have been successfully obtained by a hydrothermal process.

Electrochemical Performance of 3D-VS x
The electrochemical performance of 3D-VS x electrode was evaluated by assembling half-cell SIBs. As displayed in Figure 3a,b, the initial five cyclic voltammetry (CV) curves of 3D-VS x electrode at a scan rate of 0.1 mV s −1 within the potential window of 0.01-3.0 and 0.2-3 V to reveal the sodiation/desodiation reaction mechanism. The shapes of the CV curves and current intensity are well maintained after the first cycle, which demonstrated the excellent reversibility for sodiation/desodiation of the 3D-VS x electrode in both voltage ranges. In the CV curves of 3D-VS x electrode in the potential window of 0.2-3 V, there are several reduction peaks at 2.1, 1.56 and 1.17 V in the first reduction scan, which corresponding the oxidation peak at about 1.45, 1.95 and 2.3 V, respectively. These redox peaks are attributed to the multiple-step intercalation of Na + into 3D-VS x and subsequent conversion reactions. [27,34,35] What's more these peaks are shift in the subsequent cycles, apparently involving a reconfiguration process of 3D-VS x electrode (the following chapter will discuss). In the CV curves of the 3D-VS x electrode within the potential window of 0.01-3 V by contrast, several main reduction peaks at 2.15, 1.5 and 0.17 V are observed for the initial discharge, which corresponds to the oxidation peak at about 0.07, 1.47 and 2.3 V, respectively ( Figure 3b). These peaks in the subsequent cycles slightly shift, indicating the chemical reaction is fully reacted and the valence state is relatively stable in the first few cycles. [11] A couple of peaks at 0.17 and 0.07 V refers to Na + storage in the conductive additive supper P with the conforming in Figure S6, Supporting Information, indicating that supper P reacts at the low potential. It is worth mentioning that, a similar redox couple can be observed in the CV curves of the pure VS 2 electrode ( Figure S7, Supporting Infor-mation), indicating this is a common electrochemical side reaction in V-based sulfide.
Interestingly, the discharge specific capacity of 3D-VS x electrode increases slowly to a very high capacity and then can remain stable after many cycles. As shown in Figure 3c,d, the charge curves of each cycle nearly overlap in the section of 0.01-1.45 V, but the curves gradually become a wider plateau at about 1.5 and 2.1 V with the increase of cycle number, indicating an ever-increasing capacity and Na + reconfiguration. It is worth mentioning that the vanadium sulfides, such as VS 2 , V 3 S 4 and V 5 S 8 , often undergo the reversible conversion reaction between vanadium sulfides and metallic vanadium. [11,24,25] Therefore, the high-oxidation end product after recharge to 3.0 V indicates the enhanced kinetics of conversion reactions for the as-prepared electrodes. [36] In fact, the sodiation-driven reconfiguration phenomena well agreed with the galvanostatic  charge-discharge curves. Compared with the increasing cycle of discharge profiles, two sloping plateaus gradually emerge around 1.55 and 0.89 V. What's more the charge and discharge curves have the same shape in the range 0.2-3 V, indicating that 3D-VS x did not react completely within 0.2-3 V at the beginning, but they provide reversible sodiation reconfiguration capacity in both voltage ranges.
In addition, the typical charge/discharge curves of the 3D-VS x electrode for the first cycles at 100 mA g −1 exhibited a high initial discharge/charge capacity of 943.21/819.88 mA h g −1 with a remarkable initial Coulombic efficiency of 86.92% within voltage range of 0.2-3 V, and the irreversible capacity loss is due to the formation of solid electrolyte interphase (SEI) layer. [19,24] The reversible capacity of the 3D-VS x electrode can be maintained at 982.27 mA h g −1 with the ultra-high Coulombic efficiency of around 100% even after 100 cycles (Figure 3e). Due to the low current density, electrons and ions can enter the active material efficiently, ensuring fast and complete reactions of sodiation and desodiation process, thus reaching a stable high capacity quickly (Figure 3e). In contrast, the electrochemical performance of 3D-VS x electrode within 0.01-3 V delivered a high initial discharge/charge capacity of 1008.9/873.2 mA h g −1 with a Coulombic efficiency of 86.54% and could still be maintained at a high capacity after 90 cycles (Figure 3f). Noticeably, the reversible capacity of 3D-VS x is higher than that of the theoretical capacity of 811 mA h g −1 with 6 Na + storing. [8] The excess reversible capacity might be ascribed to synergistic effects with the S impurities (1672 mA h g −1 ) and non-conversion reaction driven by sodiation reconfiguration. [37,38] At the same time, considering the influence of the supper P ( Figures S8 and S9, Supporting Information), the reversible capacity contribution of conductive additive supper P are 164.8 and 105.6 mA h g −1 within 0.01-3 and 0.2-3 V, respectively. This can be negligible compared to the high reversible capacity of 3D-VS x . In addition, considering the electrochemical influence of possible surface oxygen products, batteries with VO x as anode electrodes were also tested to indicate the behavior of electrochemical performance within 0.01-3 and 0.2-3 V. As shown in Figures S10 and S11, Supporting Information, at the beginning of the discharge, the specific capacity of VO x both decreased rapidly, and then maintained a low specific capacity of 192.5 and 170.77 mA h g −1 , respectively, which illustrated that the capacity contribution of possible surface oxygen products in the overall electrochemical performance of 3D-VS x can be neglected.
Furthermore, the cycling stability performance is also compared. The long-term cycling life of the 3D-VS x electrode was further detected at high current rates of 500 and 2000 mA g −1 within the voltage of 0.2-3 V (Figure 3h, and Figures S12 and S13, Supporting Information). It is worth noting that the longcycle curve in Figure 3h shows a clear initial capacity drop, which occurs in the first 5 cycles at a low current density of 100 mA g −1 and then evaluates at a high current density of 2000 mA g −1 . The electrons and ions tend to occur on the surface and near the surface, and the bulk phase reaction is incomplete at the high current density of 2000 mA g −1 . That is the main reason for the capacity of the 3D-VS x electrode was only 377.85 mA h g −1 at the 20th cycle. Unlike other batteries, the 3D-VS x half-cell shows an outstanding increasing reversible capacity, achieved by sodiation-driven reconfiguration. [39,40] The 3D-VS x electrode can be completely activated and reconfiguration into a stable structure after about 800 cycles. Then, the increasing reversible capacity can be up to 961.9 mA h g −1 with a Coulombic efficiency of 99.99% in the range of 0.2-3 V after 1500th cycle. It implies that the 3D-VS x electrode has a high desodiation ability, without consuming excessive sodium due to the generation of dendrites or dead sodium, indicating the excellent reversible electrochemical performance of 3D-VS x electrode. Encouragingly, the surprising electrochemical performance of 3D-VS x is advantageous compared with many other anodes such as V-based materials (VS 2 , V 3 S 4 , V 5 S 8 , and VS 4 ), transition-metal sulfides (MoS 2 , SnS, and FeS 2 ) (Table S3, Supporting Information). [8,10,14,15,24,[41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] On the contrary, the electrochemical performance in the voltage 0.01-3 V demonstrates the similar sodiation reconfiguration phenomenon ( Figure S12, Supporting Information). The reversible capacity increases from 617.3 to 985.7 mA h g −1 after the 160th cycle. The poor cycling stability may be related to the side reactions and structure pulverization of the 3D-VS x electrode under low potential 0.01-0.2 V. In fact, some side reactions occur within 0.01-0.2 V, such as the competitive reaction of copper current collectors with sulfides and the co-intercalation of conductive additives with ether-based electrolytes, which are the main reasons for battery failure. To verify this possibility, we synthesized the control half-cells with conductive additives and sodium plate. The SEM images of the dissembled battery of conductive additive discharged to 0.01 V ( Figure S14, Supporting Information) displayed obvious changes, which may be due to the previously mentioned side reactions. Compared, the dissembled battery of conductive additive discharged to 0.2 V showed no significant changes, as shown in Figure S15, Supporting Information. The contrast results implied that the 3D-VS x electrode with the welldesigned structure at 0.2-3 V can refrain from the side reactions between the conductive additives and electrolytes within 0.01-0.2 V during repeated cycling (Figure 3i).
To further investigate the rate performance of 3D-VS x , they were conducted at various current densities and the corresponding 5th discharge capacities of the electrode were 837.5, 803.6, 772.9, 746.7, 716, 663.8 and 613.8 mA h g −1 with the current densities gradually increased to 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g −1 within the voltage range of 0.2-3 V, respectively (Figure 3g). It is worth mentioning that an unexpected specific capacity 613.8 mA h g −1 can be achieved at a high current density of 10 A g −1 . It is especially worthy of mentioning that the high-rate response at 10 A g −1 means an ultrafast charge and discharge time less than 5 min, highlighting the high-power application of the 3D-VS x electrode in SIBs. Furthermore, the current density returned to 100 mA g −1 , the capacity of 3D-VS x also can reach 870.5 mA h g −1 , and then it was enabled by sodiation reconfiguration in the subsequence cycling. For the 3D-VS x within the voltage range 0.01-3 V, it could deliver 5th discharge capacity of 815.5, 811.3, 801.7, 811.8, 804.3, 777.2 and 741.4 mA h g −1 at the same current density. When switched to 100 mA g −1 again, discharge capacities can be recovered to above 959.8 mA h g −1 , which demonstrated excellent reversibility and superior stability of the 3D-VS x anode electrode toward SIBs. It is clear that the rate performance of the 3D-VS x electrode is the best among the reported TMDs anode materials in SIBs (Figure 3j).
In order to reveal the Na + storage mechanism and sodiation reconfiguration process of 3D-VS x , the post-treatment electrode was investigated based on ex situ XRD, SEM, Raman and HRTEM at various depth of discharge/charge of the 1st and after different cycles at 100 mA g −1 . The selected XRD patterns of the 3D-VS x at various potential states (including open circuit voltage [OCV]) were performed during the first cycle at 100 mA g −1 , as depicted in Figure 4a,b (selected XRD) and Figure S16, Supporting Information (full XRD patterns). During the first discharge process (OCV-I-II-III), the XRD patterns of 3D-VS x electrode reveal the phase transformation from V 2 S 3 to Na y V 2 S 3 , implying the step-wise intercalation of Na + into the 3D-VS x , which is consistent with the results of CV curves. [8] Upon further discharge to 0.2 V (III) and 0.01 V (IV), the characteristic peaks of NaVS gradually weaken but always exists. [57,58] And the peaks located at around 27.26, 28.23, 29.5, 32.7, 36.6 and 39.16 appear, which can be attributed to the partial conversion of NaVS to NaS x (NaS 2 , Na 2 S, and Na 2 S 3 ). [38,59] The ex situ SEM (Figure 4c,d) unveil the morphological change from micro-nano hierarchical with nanosheets to micro-nano sea urchin with nanowires of 3D-VS x electrode at the stage IV. In addition, the change of the 3D-VS x structure after the Na + intercalation was observed by the HRTEM measurement. The clear lattice fringe with spacings of 0.735 and 0.324 nm can be seen in the ex situ HRTEM (Figure 4e) image, which may correspond to the (100) plane of Na y V 2 S 3 and the (100) plane of NaS 2 , respectively. The interlayer spacing of Na + -intercalated V 2 S 3 significantly increased from 0.577 (pristine V 2 S 3 ) to 0.735 nm, while, for the charging process, the peaks of Na y V 2 S 3 are well preserved and have not disappeared. At the same time, it can be seen by SEM (Figure 4f,g) that the morphology of 3D-VS x has also returned to the original micro-nano flower. However, the HRTEM image (Figure 4h) demonstrates that the (100) plane of V 2 S 3 still exists. And many dislocations are revealed, due to defects and partial amorphization after sodiation/desodiation, which can provide additional active sites for ions. Thus, it can significantly increase storage of Na + .
Besides, the 3D-VS x electrode after more prolonged cycles (2nd, 20th, 40th, 60th, and 200th) were also tested by ex situ XRD and SEM at 200 mA g −1 in the voltage window of 0.2-3 V, as shown in Figure 4i-q and Figure S17, Supporting Information. A few conclusions can be drawn from the selected XRD patterns of 3D-VS x electrode (Figure 4i). First, the XRD patterns remain almost unchanged during the cycle, and sharp peaks (204) and (214) are characteristic peaks of the Na y V 2 S 3 , and a slighter shift to lower angles can be seen with the increased cycling number. This confirms the interlayer spacing remains expanded with sodiation-driven reconfiguration. Second, both (204) and (214) peaks gradually enhance during the initial decay phase (first 10 cycles), as previously shown in the discharge XRD at I state. Similar XRD patterns were obtained at 40th cycle and 60th cycle. Thus, these observations confirm that sodiation-driven configuration is the main reason for enhanced Na + storage in 3D-VS x . Third, the SEM images at 10th (Figure 4j,k) exhibit well-define micro-nano hierarchical with nanosheets. Compared to the morphology of the 10th, the SEM images (Figure 4l,m) at 40th display the same feature as those shown at the 10th, but the surface of nanosheets showed a protrusion of nanoparticles. Therefore, it is speculated that these protruding nanoparticles consist of sodiation-driven reconfiguration products. As displayed in Figure 4n,o, although the nanosheets presented some degree of pulverizations after 60 cycles, the whole self-assembled micro-nano hierarchical with nanosheets is well preserved. These results indicate that the 3D-VS x electrode with great structure can effectively avoid the self-aggregation of active nanosheets during long cycling process. These conclusions were also confirmed by ex situ Raman spectra ( Figure S18, Supporting Information). The peaks centered at 322 and 670 cm −1 belong to the vibration modes of Na y V 2 S 3 . [57,58] At the 200th cycle (testing around 3 months), due to the failure and degradation of the 3D-VS x electrode, the diffraction peaks of Na y V 2 S 3 could not be clearly observed on the XRD pattern (Figure 4i and Figure S17, Supporting Information), and some nanoparticles composed of amorphous products encapsulated by the SEI film could be clearly distinguished in the SEM images (Figure 4p,q). Apparently, a thin SEI film can be seen (Figure 4n,o) after 60 cycles. In contrast, after 200 cycles, a thicker SEI film was coated on reconfiguration nanoparticles during the sodiation-driven reconfiguration process. Uncontrolled growth of an SEI film after long-term cycling leads to loss of charge carriers and slower Na + transport, which is also a non-negligible factor for battery failure. Therefore, the Na + storage process of the 3D-VS x is speculated as the following process (Figure 4r): on the first sodiation process, the surface of 3D-VS x nanosheets is converted to form mixed products (Na x S), and the core part of 3D-VS x spheres is inserted and converted to form mixed products (Na y V 2 S 3 and NaS x ). During the charging process, a part of the sodium product regenerated crystalline Na y V 2 S 3 by attaching to the incompletely reacted Na y V 2 S 3 crystal face, and the other part formed amorphous/crystalline V and other compounds. As the number of cycles increased, a slight loss of specific capacity was observed in the first 10 cycles, which may be related to incompletely reacted Na y V 2 S 3 sodiation-driven mechanical degradation and the formation of SEI. With the subsequent cycle number increase, the reconfiguration degree of 3D-VS x electrode deepens, and the Na storage capacity was also increasing, which makes the widening plateau of the charging curve represent the desodiation process. The 3D-VS x electrode then shows remarkable cycling performance without signs of capacity fading up to 1500 cycles. The stabilization process of 3D-VS x may be closely related to the sodiation-driven reconfiguration. Despite this, on the one hand, the micro-nanostructure of 3D-VS x facilitates the insertion and extraction of Na + and buffers the volume change during the electrochemical cycling. On the other hand, the incompletely reacted 3D-VS x core can become a site for attaching various reaction products, which is beneficial to avoid the aggregation and failure of active materials. Both of these aspects give rise to the ultralong stability of 3D-VS x for Na storage.
Based on the above analysis, the ultra-high specific capacity and superior rate performance of 3D-VS x may be derived from the following aspects; i) the non-stoichiometric ratio of vanadium and sulfur, leading to edge defects, provides extra active sites for Na + storage; ii) this remarking specific capacity of 3D-VS x is inextricably related to better charge transfer kinetics due to the innate ideas of vanadium's switchable valence states and sodiation-driven reconfiguration; and iii) the 3D micronano hierarchal structure can well prevent the accumulation of nanosheets during electrochemical cycles and ensure full contact between the active material and the electrolyte.
We further explored the origin of the superior rate performance in the 3D-VS x electrode and investigated the kinetics characterization by electrochemical characterization techniques ( Figure 5). First, CV measurements of 3D-VS x electrode are performed at scan rates from 0.2 to 1 mV s −1 . All the CV curves ( Figure 5a) show similar shapes and extremely small polarization at various scan rates before Na-driven reconfiguration. Interestingly, after 10 cycles of pre-electrochemistry and subsequent testing with the same parameters, a sharply enhanced peak at 1.85v (Figure 5d) can be clearly observed with increasing scan rates, which agrees well with the Na-driven reconfiguration of the galvanostatic charge/discharge profile in Figure 3c, with an obvious distinct plateau area. As we know, according to the relationship between peak currents (i) and scan rates (v) (Equations (S1) and (S2), Supporting Information), we can obtain the b value. [27,37,38] As depicted in Figure 5b,e, both display the fitted slope b-values of ≈1 for 3D-VS x electrodes, suggesting that the electrochemical process is dominated by the surfacedcontrolled behavior for Na-ion storage. Moreover, quantification of the capacity contribution ratio can be further calculated from Equation (S3), Supporting Information. Thus, the capacitive contribution ratio of 3D-VS x electrode gradually increases from 91% to 98% (Figure 5c) for electrode under a sodiation configuration process and from 65% to 85% (Figure 5f) for electrode with sodiation reconfiguration. The 3D-VS x electrode with sodiation reconfiguration shows the larger diffusion contribution ratio at the same scan rate as compared to 3D-VS x electrode in the sodiation process. Note that the electrochemical performance of anode materials in SIBs depended on the competition between adsorption and diffusion of ions on the host. Therefore, with the deepening of sodiation-driven reconfiguration, the corresponding contribution of diffusion-controlled behavior increases positively. Given the enhanced diffusion-controlled behavior in the reconfiguration electrode material due to the expended layer space by sodiation-insertion, it is reasonable to propose that sodiation-driven reconfiguration will boost the electrochemical performance for sodium ion storage.
To better understand the effect of the sodiation-driven reconfiguration of 3D-VS x on its kinetic properties, electrochemical impedance spectroscopy (EIS) (Figure 5g,h) was performed to reveal the reaction resistance changes and the diffusion coefficient of Na + before and after sodiation-driven reconfiguration. [35,36] The Nyquist plots of the 3D-VS x electrodes at different states (fresh, after 1 cycle, 2 cycles, 10 cycles, 40 cycles, 60 cycles and 100 cycles) were recorded in the frequency range of 100 KHz to 100 mHz. As can be seen in Figure 5g,h and Table S4, Supporting Information, the total impedance (R total) of the fresh 3D-VS x electrode is as high as 6537.79 Ω, which may be attributed to the fact that the active material does not undergo the electrochemical activation of Na + intercalation and deintercalation, the transport channel of Na + inside the active material is narrow and the SEI layer may be formed by a slight self-discharge during the stationary of the 3D-VS x electrode. The charge transfer resistance (R ct ) of 3D-VS x electrode Figure 5. CV curves of 3D-VS x electrode at various scan rates for a) before and d) after sodiation-driven reconfiguration, Liner fitted b-value according to the relationship of the log(scan rate) and log(peak current) for b) before and e) after sodiation-driven reconfiguration. Capacity contributions at different scan rates with the purple part describing the capacitive contribution and the gray area standing for the diffusion-controlled capacity for c) before and f) after sodiation-driven reconfiguration, Nyquist plots of 3D-VS x electrode for g) fresh and h) electrochemical test at different cycles. i) the charge transfer resistance at different cycles.
at the 1st, 2nd, 10th, 40th, 60th and 100th were 192.4, 65.1, 24.01, 10.49, 17.1 and 17.4 Ω, respectively (Figure 5i). It is shown that the electron transfer kinetics can be effectively improved with the sodiation-insertion configuration for the electrode material. Furthermore, the R ct value of 3D-VS x electrode at the 40th cycle is lower, showing the faster charge-transfer for Na + with the increase of capacity. It may be ascribed to the optimized SEI film during the sodiation-driven reconfiguration process. After the 40th cycle, the fragmentation of the electrode material and the regeneration of SEI film led to an increase of R ct value, which is detrimental to the electrochemical kinetics for this period. The value of R total has been decreasing, indicating that the polarization effect of the electrochemical reaction gradually weakens after continuous cycling. This may be due to the larger crystals being converted into amorphous/nanocrystalline particles with the reaction depth of the 3D-VS x electrode, which is conducive to accelerating the electrochemical reaction. In short, the low charge transfer resistance mainly stems from multiple factors: i) the opening of charge transport channels after sodiation-driven reconfiguration; ii) the infiltration of electrolyte through the subsequently cycling; and iii) the formation of stable SEI film reduces the energy barrier for charge transfer, thus ensuring high-rate performance. [36,60] An investigation based on DFT is further conducted to gain insight into the sodiation mechanism of Na + in 3D-VS x . As demonstrated in the inset of Figure 6a,b, both V 2 S 3 and Na y V 2 S 3 exhibits no gap near the Fermi level, suggesting enhanced electron conductivity. [12,[61][62][63] Compared with total partial density of states (DOS) of pure V 2 S 3 , the Na y V 2 S 3 displays a higher total DOS. It may be ascribed to the s orbitals of Na atoms overlapping with that of p orbitals of S atoms and d orbitals of V atoms, suggesting the interaction among Na, V and S. In addition, after Na-inserted into the V 2 S 3 , the DOS of NaV 2 S 3 displays a slight delocalization. These also identify the conductivity of active material is further improved after Nainsertion. To the best of our knowledge, the excellent Na ion Figure 6. The total and partial density of states for a) V 2 S 3 and b) NaV 2 S 3 ; c) the DFT calculation of energy barrier and structural evolution for a sodium atom in V 2 S 3 with Na during sodiation; inset shows the calculation models of Na + adsorption on the structure (Na-ads), inserting into the structure lattices for transition states (TS1), inserting into translate states (Na-in), diffusion into the channel for transition states (TS2) and the final states (Na-tra), respectively. storage performance of the 3D-VS x is significantly related to the localized interfacial behavior and migration energy barriers of Na + . The adsorption and diffusion paths for Na + are evaluated by the solid-state dimer method and corresponding migration energy barriers are obtained. [61,62] The periodic structure of V 2 S 3 is shown in Figure 6c and Figure S19, Supporting Information. The migration pathway is from an adsorption surface into the structure lattices for TS1 and then diffusion into the channel for TS2, and the corresponding diffusion energy barrier of Na + in V 2 S 3 are 0.42 and 0.23 eV, respectively, suggesting the highest mobility of Na + in the channel (along a) of V 2 S 3 . To better understand the sodiation-driven reconfiguration, we also analyzed and compared the diffusion energy barrier of Na + in model of NaV 2 S 3 , as shown in Figure S20, Supporting Information. The migration pathway is easy to diffuse from the adsorption surface of Na-2-ads to the channel of Na-2-in, and the corresponding diffusion energy barrier of Na + in NaV 2 S 3 is 0.396 eV, suggesting the higher mobility after sodiation-driven reconfiguration and results in the excellent electrochemical performance of the 3D-VS x electrode.

Conclusion
In summary, 3D-VS x without carbon was investigated as an anode electrode for SIBs in an ether-based electrolyte. Benefiting from the structural advantages, the high pseudocapacitance contribution and the intermediates of sodiation-driven reconfiguration, as-prepared 3D-VS x endow to achieve a remarking rate capability of 613.8 mA h g −1 at 10 A g −1 and cycling stability without capacity fading 961.4 mA h g −1 after 1500 cycles at a high rate of 2 A g −1 . We also used ex situ XRD and SEM methods to analyze the intercalation and conversion reaction mechanism of sodium storage and proposed the sodiation-driven reconfiguration based on post-cycle surface and crystal structure changes in 3D-VS x . The DFT calculation results confirmed that the 3D-VS x electrode is more favorable for Na + ions to diffuse within the lattice layer. All these effects work together to obtain stable final high capacity. In addition, the micrometer-sized structure, asprepared 3D-VS x presented a relatively high tapped density, which can be packed together densely and is hopeful for the practical applications. This work provides a promising anode material that may pave the way for the development of highcapacity insertion and conversion materials with stable cycling for prospective energy storage applications.