High‐Stability of Heterostructured Bi2S3/VS4/rGO Anode Enabled by Electrolyte Optimization for Fast‐Charging Sodium‐Ion Batteries

Sodium‐ion batteries are attracting great attention as an alternative to lithium‐ion batteries due to the lower cost and better sustainability of sodium. Although the metal sulfide‐based anodes demonstrate much higher theoretical capacity than the hard carbon anodes, the severe capacity degradation and inferior rate capability caused by poor electrical conductivity and sluggish kinetics hinder their applications. Herein, a novel bimetallic sulfide‐based anode wrapped by reduced graphene oxide (i.e., Bi2S3/VS4/rGO) is presented, in which the heterointerfaces between Bi2S3 and VS4 are well distributed among the composite, leading to the promoted charge transfer and the improved Na+ transport kinetics. Combined with electrolyte optimization, the Bi2S3/VS4/rGO demonstrates excellent electrochemical performance, including excellent rate capabilities over 10 A g−1, and a long lifespan over 1000 cycles. This work indicates the significance of the synergistic effect of structure regulation and electrolyte optimization for achieving fast‐charging performance.


Introduction
Sodium-ion batteries (SIBs) have drawn widespread concern due to the rich reserves, low price, and good sustainability of sodium, which are emerging as the alternatives to lithium-ion batteries (LIBs) as the next-generation energy storage devices. [1]owever, it is challenging for SIBs to be satisfied with the requirements for high energy density and longer recharge mileage due to the commercial low-capacity hard carbon anode (i.e., <300 mAh g À1 ). [2]Thus, metal sulfide materials, such as Bi 2 S 3 , [3] CoS x , [4] MoS 2 , [5] Sb 2 S 3 , [6] SnS, [7] and VS x , [8] have been widely developed for replacing the hard carbon anodes due to their higher capacities (i.e., >400 mAh g À1 ), strong redox reversibility, and abundant resources.Among various metal sulfides, VS 4 and Bi 2 S 3 are preferred as potential anode candidates for SIBs because of their abundant resource, high theoretical capacity (e.g., 1196 mAh g À1 for VS 4 , 625 mAh g À1 for Bi 2 S 3 ), and the unique chain-like stacking structure.Nevertheless, VS 4 and Bi 2 S 3 -based materials always suffer from severe capacity decay and poor rate capability, which is caused by the drastic volumetric variation, sluggish ion transport kinetics, and sulfur shuttle effect during the sodiation/desodiation process. [9,10]o date, structure design in diverse dimensions (e.g., nanograins, [11] nanorods, [12] nanospheres, [13] 3D hierarchical structured materials) [14] followed by carbon modification [15] has been the most popular strategy to address the intrinsic issues of the metal sulfide anodes, which can promote the electrical conductivity and mitigate the volume change of metal sulfide anodes.A hierarchically structured Bi 2 S 3 /graphene aerogel composite was proposed for SIBs, demonstrating a stable capacity of 348 mAh g À1 over 120 cycles at the current density of 1.0 A g À1 . [16]Yu et al. [17] prepared the flower-like VS 4 @C anode, which achieved a stable capacity of 283.3 mAh g À1 over 300 cycles at the current density of 0.5 A g À1 .More recent studies demonstrate that constructing heterostructure in the bimetallic sulfides with different bandgaps can form built-in electric fields at the heterointerface, which can boost the sodium storage capability through the abundant active sites, and enhance electronic/ionic conductivity. [18]For example, a stable capacity of 427.7 mAh g À1 over 100 cycles at the current density of 0.5 A g À1 for the heterojunction-type Bi 2 S 3 /MoS 2 anode can be delivered, which can be attributed to the enhancements of reaction dynamics. [19]esides, electrolyte engineering is also an efficient approach to form a stabilized solid electrolyte interface (SEI) on the electrode surface and then restrain the side reactions, giving rise to an improved performance of metal sulfide-based anodes. [20]or instance, Qin et al. [21] designed an ether-based electrolyte (i.e., 1.0 M NaCF 3 SO 3 in diethylene glycol dimethyl ether), in which a thin and stable SEI layer was formed on the electrode, suppressing the continuous electrolyte decomposition, and then endowing an excellent sodium storage capability (i.e., 341.7 mAh g À1 over 250 cycles at the current density of 0.2 A g À1 ).However, the cycle life and rate performances of these reported metal sulfide anodes are still unsatisfactory to meet the demands in practical application.Therefore, it is still necessary to rationally regulate structure/component and optimize electrolyte system to boost the performances of metal sulfide-based anodes.
Herein, we present a novel bimetallic sulfide wrapped by reduced graphene oxide (i.e., Bi 2 S 3 /VS 4 /rGO) as the anode for SIBs via a one-pot hydrothermal strategy, in which the well-distributed heterostructures between Bi 2 S 3 and VS 4 can provide abundant active interfaces and phase boundaries to enhance the charge transfer and Na þ transport kinetics.With an electrolyte optimization strategy, the Bi 2 S 3 /VS 4 /rGO anode demonstrates fascinating rate capability and long lifespan (i.e., 780 mAh g À1 over 1000 cycles at the current density of 10 A g À1 ) in the electrolyte of 1.0 M NaPF 6 in dimethyl ether (DME), which is better than those of most reported before.These results indicate that the rational designed architecture and the optimized electrolyte are vital to stabilize the electrode simultaneously.

Synthetic Strategy
The synthetic procedure for Bi 2 S 3 /VS 4 /rGO composite is illustrated in Figure 1a.First, VO 3 À and Bi 3þ , dissociated from NH 4 VO 3 and Bi(NO 3 ) 3 •5H 2 O, could be absorbed on the rGO surface through electronegativity and rich oxygen-containing functional groups, where the added thioacetamide (TAA) served as the sulfur source and reducing agent.Then, in the acidic solution environment ) during the hydrothermal process.By this way, Bi 2 S 3 nanorods formed on the rGO substrate preferentially and then the VS 4 was coated on the surface of Bi 2 S 3 .Finally, the heterostructure Bi 2 S 3 /VS 4 /rGO can be formed by magically self-assembly.

Architecture Features
The morphology and structural characteristics of the Bi 2 S 3 /VS 4 /rGO are first characterized.It can be found that the Bi 2 S 3 nanorods and VS 4 nanospheres cross-linked and were wrapped by the rGO (Figure 1b-d), which is different from the stacked Bi 2 S 3 /rGO and VS 4 /rGO composites (Figure S1 and S2, Supporting Information).In addition, the heterostructure of the Bi 2 S 3 /VS 4 /rGO can be confirmed by the selected area electron diffraction (SAED), in which the concentric diffraction rings match the multi phases of Bi 2 S 3 and VS 4 (Figure 1e).Besides, the lattice spacing of 2.70 and 3.76 Å matching with the (410) and (211) lattice plane of Bi 2 S 3 and VS 4 , respectively, can be observed in Figure 1f, further confirming the existence of Bi 2 S 3 /VS 4 heterojunction.The fine distribution of Bi, V, S, and C elements detected by EDXA mapping confirms the main components of Bi 2 S 3 /VS 4 /rGO (Figure 1g).
The crystalline characteristics of the Bi 2 S 3 /VS 4 /rGO were further identified by the X-ray powder diffraction (XRD), in which the diffraction peaks accord well with the different crystal planes in the orthorhombic Bi 2 S 3 (JCPDS No. 84-0729) and monoclinic VS 4 (JCPDS No. 72-1294), respectively (Figure 2a).Two main characteristic peaks located at 1353 and 1593 cm À1 can be observed in Raman spectroscopy, which agrees to the graphite sp 2 carbon (G-band) and disordered sp 3 carbon/structural defects (D-band) of the rGO, respectively, confirming the existence of rGO with much defect sites (Figure 2b).In addition, the characteristic peaks of Bi (orange square) and V (blue square) alternately can be observed by the elemental line scanning of the Bi 2 S 3 /VS 4 /rGO, which indicates the presence of two different phase in the composite, confirming the existence of heterostructure between Bi 2 S 3 and VS 4 (Figure 2c). [22]Besides, the chemical compositions and valence of elements were further determined by X-ray photoelectron spectrometry (XPS) as shown in Figure 2d,e.3a,8] We find that a strong electron interaction is established between VS 4 and rGO through the V─C bonds, which can be confirmed by the binding energy of V─C bond at 521.V 2p spectrum.In addition, the existence of heterostructure between Bi 2 S 3 and VS 4 can be further judged by the opposite shift of V 2p and Bi 4f binding energy in the Bi 2 S 3 /VS 4 /rGO. [23]This is because the electron cloud can flow from Bi 2 S 3 to VS 4 when the two materials with different band gaps are in contact, triggering the peak shifts of the V 2p and Bi 4f spectrum.This result can be confirmed by DFT calculation as shown in Figure S3, Supporting Information.Moreover, the Bi 2 S 3 /VS 4 /rGO possess an increased surface area of 13.52 m 2 g À1 (Figure 2f ) compared to 7.49 m 2 g À1 of Bi 2 S 3 /VS 4 (Figure S4, Supporting Information), which can be ascribed to the introduction of rGO.

Electrochemical Performance
The remarkable sodium storage capability of Bi 2 S 3 /VS 4 /rGO was first confirmed in a sodium-ion half-cell, in which the 1.0 M NaPF 6 in DME electrolyte was employed.The high reversibility and stability of sodium storage for Bi 2 S 3 /VS 4 /rGO were confirmed by the measurement of cyclic voltammetry (CV) as shown in Figure 3a, in which the CV curves can be overlapped well after the first cycle.Meanwhile, the cathodic peaks at 1.95 and 1.55 V represent the insertion of Na þ into the interlayer space of Bi 2 S 3 and VS 4 to form Na x Bi 2 S 3 and Na , respectively.The sharp cathodic peak located at 1.12 V should be attributed to the formation of SEI layer and the conversion reaction from Na x Bi 2 S 3 to the Bi 0 -Na 2 S composites (i.e., Na . Subsequently, the multistep Na-Bi alloying reactions occur at 0.62 and 0.42 V (i.e., Bi 0 þ xNa þ þ xe À ↔ Na x Bi).And then followed the conversion reaction of Na x VS 4 operates at %0.25 V (i.e., Na . Afterward, during the first anodic scan, the two peaks located at 0.59 and 0.77 V represent reversible dealloying reactions from Na x Bi to Bi (i.e., Na x Bi ↔ xNa þ þ Bi 0 þ xe À ), while the peaks at 1.87 and 2.10 V correspond to the reversible conversion reaction to rebuild Bi 2 S 3 and VS 4 (i.e., 2Bi Note that in the following scans, the peak at 1.87 V is integrated to the peak at 2.10 V, which can be attributed to structure evolution in the electrode during the discharge/charge procedure. [24]he typical (dis-)charge curves of Bi 2 S 3 /VS 4 /rGO are shown in Figure 3b, where the Bi 2 S 3 /VS 4 /rGO show a higher initial coulombic efficiency (ICE) of 81.7% than that of Bi 2 S 3 /rGO (i.e., 57.6%) and VS 4 /rGO (i.e., 79.9%).The improved ICE of Bi 2 S 3 /VS 4 /rGO is attributed to that: 1) the formed heterointerface can improve the electron transfer between Bi 2 S 3 and VS 4 , thereby inducing electron redistribution to form built-in electric fields and improve the electronic structure of the active centers to achieve a higher reversibility of the conversion reactions and 2) the rGO can improve the electrical conductivity of Bi 2 S 3 /VS 4 and lessen the contact between the metal sulfide and the electrolyte, giving rise to the mitigated decomposition of the electrolyte.Moreover, the long cycle performance of Bi 2 S 3 /VS 4 /rGO is demonstrated at the current density of 10.0 A g À1 , where the high capacity of 746.5 mAh g À1 over 1000 cycles was obtained (Figure 3c).In stark contrast, Bi 2 S 3 /rGO, VS 4 /rGO, and Bi 2 S 3 /VS 4 cannot work anymore after less than 425 cycles at the current density of 10 A g À1 (Figure 3c and S5, Supporting Information), which may be caused by the dissolution of polysulfides (i.e., NaPSs) and the followed shuttle effect. [25]In addition, another great feature of the Bi 2 S 3 /VS 4 /rGO electrode is the excellent rate capability, in which high capacities of 941.9, 893.3, 848.2, 808.6, and 778.7 mAh g À1 at the current densities of 0.5, 1, 2, 5, and 10 A g À1 can be delivered after activation, respectively (Figure 3d).This result of rate capability is superior to those of most Bi 2 S 3 or VS 4 -based anodes reported previously (Figure 3e).

Electrochemical Analysis
The reasons for the outstanding performance of Bi 2 S 3 /VS 4 /rGO were first investigated by electrochemical impedance spectroscopy (EIS).It can be found that the charge transfer resistance (R ct ) of the fresh Bi 2 S 3 /VS 4 /rGO electrode (i.e., 17.08 Ω) is much lower than 27.87 and 19.46 Ω of the Bi 2 S 3 /rGO and VS 4 /rGO electrode, respectively (Figure 4a and Table S1, Supporting Information).This should be attributed to the formation of a stable SEI and the suppressed decomposition of electrolyte.Meanwhile, the rGO can improve the electrical conductivity of Bi 2 S 3 /VS 4 as well as maintain the electrode integrity.The dynamic process was further studied by galvanostatic intermittent titration technique (GITT) with a pulse current density of 0.1 A g À1 for 10 min between 30 min rest intervals as shown in Figure 4b,c.The diffusion coefficient of Na þ (D Na þ) was acquired via the following equation [27] D ¼ 4L 2 πτ where L is the sodium-ion diffusion length, τ is the relaxation time, and t, ΔE S , and ΔE t are the duration of the current pulse, potential variation between steps, and voltage variation during the pulse process, respectively.It can be found that the calculated D Na þ of Bi 2 S 3 /VS 4 /rGO in the sodiation processes is higher than those of Bi 2 S 3 /rGO (Figure 4d).This should be attributed to the formed heterostructures between Bi 2 S 3 and VS 4 as well as the solid rGO skeleton with effective electronic conducting network, which can enhance the electrical conductivity and then boost the electron transfer and Na þ transport kinetics.
The reasons for high capacity, especially at high current densities, can be further investigated by CV (Figure 4e).Generally, a relationship between the peak current (i) and scan rate (v) in a scan voltammetry measurement can be calculated as follows i ¼ av b (2) where a and b are regulable parameters.The b values for the Bi 2 S 3 /VS 4 /rGO electrode at the peaks 1, 2, and 3 are 0.82, 1.0, and 0.92, respectively (Figure 4f ), by plotting the log (i) and log (v).Thus, it can be confirmed that the pseudocapacitive process is mainly responsible for the Bi 2 S 3 /VS 4 /rGO electrode based on the classification of b-value (i.e., 0.5 < b < 1).Then, the relative contribution of the pseudocapacitive process and the diffusion process can be quantitatively calculated by the following equation [28] i where the k 1 and k 2 can be calculated from the slope of the iv À1/2 versus v 1/2 plot (Figure 4g).According to the above analysis, we conclude that 1) the pseudocapacitive process dominated the sodium storage mechanism in Bi 2 S 3 /VS 4 /rGO, particularly at the high scans; 2) the pseudocapacitive contribution can boost the capacity markedly, which is responsible for the high capacities at high current densities; and 3) the pseudocapacitive contribution of Bi 2 S 3 /VS 4 /rGO (i.e., 99.12%) is the largest compared to those of Bi 2 S 3 /rGO (i.e., 90%) and VS 4 /rGO (i.e., 98%, as shown in Figure S6 and S7, Supporting Information) at 2 mV s À1 , which is consistent with the best rate capability of Bi 2 S 3 /VS 4 /rGO.The pseudocapacitive charge storage of Bi 2 S 3 /VS 4 /rGO can be attributed to the rich boundaries during the self-assembly process as well as the much defect sites of the rGO.

Electrode Evolution and Analysis
To further figure out the reasons for the excellent electrochemical performance of Bi 2 S 3 /VS 4 /rGO, the structural evolution analysis was conducted by examining the Bi 2 S 3 /VS 4 /rGO electrode after 200 cycles.First, the SEM images of the Bi 2 S 3 /VS 4 /rGO electrode before and after 200 cycles are shown in Figure 5a-d.It is can be observed that the whole electrode is well preserved and the Bi 2 S 3 /VS 4 nanoparticles were still closely anchored on the rGO during the cycling, indicating good contact between Bi 2 S 3 /VS 4 and rGO (Figure 5c,d).Then, we find that the sizes of Bi 2 S 3 /VS 4 /rGO were reduced compared to the pristine (Figure 5a,b), which may be derived from the pulverization in the continuous discharge/charge process.This phenomenon is consistent with the observed trend of capacity increase during the long cycling, in which more active sites of Bi 2 S 3 /VS 4 /rGO are exposed, giving rise to an increased capacity.
To better investigate the evolution of the crystalline phase, the HR-TEM of the Bi 2 S 3 /VS 4 /rGO electrode at full discharged/ charged state after 200 cycles was executed.We can find that some lattice fringe with the interlayer spacing of 2.18, 3.26, 2.19, and 2.73 Å can correspond well to the (111), ( 200), (111), and (110) planes of V 0 , Na 2 S, NaBi, and Na 3 Bi phases at the full discharged state, respectively.However, the interlayer spacing of 2.71, 3.15, and 2.44 Å can correspond well to the (301), (À202), and (004) planes of Bi 2 S 3 , VS 4 , and Na 3 Bi phases at the full charged state, respectively (Figure 5e,f ).When compared to the pristine structure, the heterostructures between Bi 2 S 3 and VS 4 and new phases formed heterostructures in the regenerated and recrystallized composites can be observed (Figure 5e,f and S8, Supporting Information), which can offer more active sites and abundant phase boundaries to store Na þ , leading to the increased capacity and faster kinetics of the conversion reactions (Figure 5g).

Electrolyte Analysis
Interestingly, we find that the electrochemical performance of Bi 2 S 3 /VS 4 /rGO can be significantly influenced by the electrolyte.In detail, when the solvent was changed from DME to ethylene carbonate/dimethyl carbonate/fluoroethylene carbonate (EC/DMC/FEC), a fast capacity decay can be observed (Figure 6a), demonstrating the importance of the solvents.Thus, the solvent difference in the different electrolytes was analyzed by Raman spectrogram, in which the changed stretching vibrations of P-F vibration at 768.8 cm À1 can be applied to comprehend the interaction between PF 6 À and Na þ in the Na þ solvation structure (Figure 6b).Note that the peak of PF 6 À shows different redshift degrees when the NaPF 6 was dissolved by different solvents.It can be found that the largest redshift degree (i.e., 740.8 cm À1 ) can be observed in the 1.0 M NaPF 6 in DME electrolyte compared to that in the 1.0 M NaPF 6 in EC/DMC/FEC electrolyte (i.e., 741.8 cm À1 ).These comparative results indicate that the Na þ -PF 6 À interaction in DME-based electrolyte is weaker than that in the EC-based electrolyte.This should be attributed to the higher steric hindrance of the Na þ [DME] 2 bidentate chelation when compared to that in that of the Na þ [EC] 4 monodentate chelation (Figure 6c).
Based on the above analysis results, the Na þ solvation structures and corresponding interfacial models derived from the Na þ desolvation process on the surface of Bi 2 S 3 /VS 4 /rGO anodes are presented in Figure 6d,e, in which the Na þ [solvent] x [anion] (i.e., x, calculated by the molar ratio of electrolyte compositions) was used to describe the electrolyte. [29]Moreover, the thermodynamic properties of the solvation coordination were analyzed to interpret the electrochemical stability of the electrolytes based on the lowest-unoccupied molecular orbital (LUMO) energy level.In the 1.0 M NaPF 6 in DME electrolyte (i.e., Na þ [DME] 9.61 [PF 6 À ], Figure 6d), the first solvation shell was composed of Na þ -DME pairs, while the PF 6 À anion can keep farther from the surface of the Bi 2 S 3 /VS 4 /rGO anode due to the stronger Na þ -DME interaction (i.e., À1.38 eV vs À0.44 eV of Na þ -EC, Figure 6f ), leading to the improved stability of PF 6 À .During the desolvation process, a stable SEI can be formed after the Na þ -DME-PF 6 À (i.e., À0.044 Hartree, Figure S9, Supporting Information) accept one electron, and then further suppress the electrolyte decomposition, the dissolution of NaPSs, and the followed shuttle effect.As a result, a superior long-term cycle of the Bi 2 S 3 /VS 4 /rGO anode can be observed in the DME electrolyte.
When the DME solvent was replaced by EC/DMC/FEC (i.e., Na þ [EC] 7.5 [DMC] 5.9 [FEC] 0.7 [PF 6 À ], Figure 6e), the first solvation shell was mainly composed of Na þ -EC (i.e., À0.44 eV), while the FEC additive can participate in regulating the solvation shell through the interaction between Na þ and FEC (i.e., À0.37 eV, Figure 6f ).PF 6 À can appear more frequently on the surface of the Bi 2 S 3 /VS 4 /rGO anode due to the stronger interaction between Na þ and PF 6 À in the Na þ [EC] 7.5 [DMC] 5.9 [FEC] 0.7 [PF 6 À ] electrolyte.As a result, when the anode accepts one electron, the Na þ -FEC-PF 6 À could be preferentially reduced to form a SEI on the Bi 2 S 3 /VS 4 /rGO anode surface due to the low stability of Na þ -FEC-PF 6 À (i.e., À0.098 Hartree vs À0.073 Hartree of Na þ -EC-PF 6 À , Figure S9, Supporting Information).However, the formed SEI is insufficient to effectively mitigate the electrolyte decomposition because of the low LUMO energy level of Na þ -EC-PF 6 À .Thus, a fast capacity decay of the Bi 2 S 3 /VS 4 /rGO anode is observed in the electrolyte of 1.0 M NaPF 6 in EC/DMC/ FEC.All above results demonstrate that the electrolyte compatibility is of great significance in stabilizing the electrode performance.

Conclusion
In summary, a novel heterostructured Bi 2 S 3 /VS 4 /rGO anode was successfully synthesized via a one-pot hydrothermal strategy, in which the heterojunction between Bi 2 S 3 and VS 4 can promote the charge transfer and improve the Na þ transport kinetics by the formed internal electric field.Combined with electrolyte engineering, the as-prepared Bi 2 S 3 /VS 4 /rGO demonstrates an excellent high-rate capability and long lifespan, which is superior to most of the metal sulfide-based anode reported previously.In addition, Na þ solvation structure and interfacial models are established to understand the relationship between electrolytes and electrode performance.All these confirm the significance of electrode material design and electrolyte optimization for improving the performance.We believe that the strategies in this work can be appropriate for synthesizing more electrode materials with greater fast-charging performance while improving the electrode performance from multidimensions.

Figure 4 .
Figure 4. Electrochemical analysis.a) Nyquist plots of Bi 2 S 3 /VS 4 /rGO, Bi 2 S 3 /rGO, and VS 4 /rGO electrode.b) GITT curves and c) voltage versus time curves of Bi 2 S 3 /VS 4 /rGO, Bi 2 S 3 /rGO electrode for a single GITT during the discharge process.d) Na þ diffusion coefficient of Bi 2 S 3 /VS 4 /rGO and Bi 2 S 3 /rGO at different sodiation state.e) CV curves of Bi 2 S 3 /VS 4 /rGO electrode at different scan rate.f ) The corresponding linear relationship between log i and log v. g) Contribution ratio of capacitive capacities at different scan rates.

Figure 5 .
Figure 5. Electrode evolution.SEM, TEM images of the Bi 2 S 3 /VS 4 /rGO electrode a,b) before and c,d) after 200 cycles.e,f ) HR-TEM images of the full discharged and charged state after 200 cycles.g) Schematic illustration of the structural evolution of the Bi 2 S 3 /VS 4 /rGO electrode.

Figure 6 .
Figure 6.Electrolyte analysis.a) Cycling performance of Bi 2 S 3 /VS 4 /rGO anode in the electrolyte of 1.0 M NaPF 6 in DME and 1.0 M NaPF 6 in EC/DMC/FEC.b) Raman spectra of PF 6 À anions in different kinds of solvents.c) Comparative interactions between Na þ and different solvents.Solvation structure and interfacial model of the electrolyte of d) 1.0 M NaPF 6 in DME and e) 1.0 M NaPF 6 in EC/DMC/FEC.f ) Calculated binding energy of Na þ -solvents.