Self‐Assembled 2D VS2/Ti3C2Tx MXene Nanostructures with Ultrafast Kinetics for Superior Electrochemical Sodium‐Ion Storage

Abstract Constructing nanostructures with high structural stability and ultrafast electrochemical reaction kinetics as anodes for sodium‐ion batteries (SIBs) is a big challenge. Herein, the robust 2D VS2/ Ti3C2Tx MXene nanostructures with the strong Ti─S covalent bond synthesized by a one‐pot self‐assembly approach are developed. The strong interfacial interaction renders the material of good structural durability and enhanced reaction kinetics. Meanwhile, the enlarged and few‐layered MXene nanosheets can be easily obtained according to this interaction, providing a conductive network for sufficient electrolyte penetration and rapid charge transfer. As predicted, the VS2/MXene nanostructures exhibit an extremely low sodium diffusion barrier confirmed by DFT calculations and small charge transfer impedance evidenced by electrochemical impedance spectroscopy (EIS) analysis. Therefore, the SIBs based on the VS2/MXene electrode present first‐class electrochemical performance with the ultrahigh average initial columbic efficiency of 95.08% and excellent sodium‐ion storage capacity of 424.6 mAh g−1 even at 10 A g−1. It also shows an outstanding sodium‐ion storage capacity of 514.2 mAh g−1 at 1 A g−1 with a capacity retention of nearly 100% within 500 times high‐rate cycling. Such impressive performance demonstrates the successful synthesis strategy and the great potential of interfacial interactions for high‐performance energy storage devices.

Experimental details.

Materials synthesis (1) Preparation of MXene
Typically, 1 g MAX (Ti 3 AlC 2 , 400 mesh) was added stepwise into 37.5 mL 9 M HCl and also contained 2.4 g LiF, stirring for half an hour at room temperature.The addition time is controlled at about half an hour in case the temperature is too high to cause oxidation.The etching process was then continued to be stirred for 24 h at 35 o C and washed thoroughly with deionized water for several times (4500 rpm, 5 min) until the upper layer of solution appeared the color of dark green.After that, 20 mL deionized water was added into the centrifugal tube followed by manually shaking for 1-2 h and centrifuging at 4500 rpm for 20 min with additional 20 mL deionized water.Finally, the upper solution was collected and stored in the refrigerator.1 mL MXene solution was taken out and freezedried to measure its concentration. (

2) Preparation of VS 2 nanosheets
The preparation of VS 2 nanosheets is according to the literature 1 .Typically, 1 g PVP K-30 was first dissolved in the mixture of 30 mL deionized water and 2 mL ammonium hydroxide.Then 0.234 g NH 4 VO 3 was dissolved in sequence with continuous stirring.Then 1.5026 g C 2 H 5 NS (TAA) was added.The solution kept stirring 1 h at room temperature.After that, the solution was loaded into a Teflon-lined sealed autoclave and maintained at 180 °C for 20 h.The obtained suspension was centrifuged and the product was washed with deionized water and ethanol several times and then dried at 60 °C in a vacuum oven for overnight.Finally, the products were annealed at 300 °C for 2 h to obtain VS 2 naosheets.
(3) Synthesis of VS 2 /MXene 40 mg VS 2 was firstly dispersed in the 40 mL deionized water to get a homogenous solution A. 8 mL MXene aqueous solution with concentration of 1 mg mL -1 was denoted as solution B. Solution B was then slowly added into solution A followed by stirrring for 4 h.The resultant dispersion was centrifuged for 10 min at 11000 rpm.Finally, the product was collected after freeze-drying.

Characterization
The morphology of the prepared materials was observed using a scanning electron microscope (SEM, Hitachi S-4800) together with associated energy-dispersive X-ray spectroscopy (EDX) and a transmission electron microscopy (TEM, JEOL JEM-2100F).X-ray diffraction (XRD) measurements were obtained by using a Bruker D8 Advance X-ray diffractometer with a Cu Kα radiation source over a 2θ range of 5-80 o .The Raman spectra was perfomed with a confocal Raman system (WITec, α300R).An X-ray photoelectron spectroscopy (XPS, PHI Quantera II) with Al Kα source was used to analyze the surface electronic states of the powders.Nitrogen adsorption-desorption isotherms were carried out on a Quantachrome Autosorb IQ analyzer.
Then the slurris were casted onto the Cu foils and dried in the vacuum oven at 70 °C for overnight.
The assembled CR-2032 type coin cells were used the anode electrodes as the working electrodes and sodium foil as the counter and reference electrodes.The electrolyte was 1 M NaPF 6 dissolved into DME solution (DuoDuo-NP-035).Glass fibres (Advantec) were used as separators.The galvanostatic charge/discharge measurements, electrochemical impedance spectroscopic (EIS) and cyclic voltammetry (CV) were performed on the multichannel battery measurement system (Neware) and electrochemical workstation (IM6e) in the voltage of 0.01-3 V.

DFT calculations
All first-principles calculations were performed using the density functional theory (DFT) based on the Vienna ab initio simulation package (VASP) and projector augmented wave (PAW) method.The generalized gradient approximation (GGA) with the scheme of Perdew˗Burke˗Ernzerhof (PBE) was considered for the correlation energy and electron exchange.The supercells were set as Monkhorst˗Pack 3×3×1 k mesh for the Brillouin zone sampling.The climbing image nudged elastic band (NEB) method was utilized to evaluate the diffusion/migration energy barrier.The Bader charge analysis was performed using the Bader Charge Analysis Code.Table S2.The Bader charges Figure 6f of V, S, Ti, C and F of the VS 2 /Ti 3 C 2 F 2 structure.

Figure S2 .
Figure S2.The corresponding EDS images of VS 2 /MXene nanostructures shown in Figure 2c.

Figure
Figure S3.High-resolution V 2p and S 2p spectrum of pristine VS 2 .

Figure
Figure S5.(a) Nitrogen adsorption-desorption isotherms and (b) pore size distribution of three samples.

Figure S7 .
Figure S7.The galvanostatic charge-discharge curves of (a) VS 2 and (b) MXene electrode at various current densities from 0.1 to 10 A g -1 .

Figure S8 .
Figure S8.The galvanostatic charge-discharge curves of (a) VS 2 and (b) MXene electrode at a current density of 0.1 A g -1 .

Figure S10 .
Figure S10.The galvanostatic charge-discharge curves of (a) VS 2 and (b) MXene electrode at a current density of 0.1 A g -1 .Relationship between log (peak current) vs. log (sweep rate) for (c) VS 2 and (d) MXene electrode.Capacitive-controlled and diffusion-controlled contributions for (e) VS 2 and (f) MXene electrode at 5.0 mV s -1 .

Figure S11 .
Figure S11.Diffusion path of one Na atom diffusion on the surface of (a) VS 2 , (b) Ti 3 C 2 F 2 and (c) Ti 3 C 2 (OH) 2 .

Figure
Figure S13.a) Side view and the graphical display of charge differences for VS 2 /Ti 3 C 2 (OH) 2 structure.

Table S1
The comparison of ICEs and cycling performance among the MXene-based materials for SIBs anodes (NM means not mentioned).

Table S3 .
The Bader charges Figure S13a of V, S, Ti, C and O of the VS 2 /Ti 3 C 2 (OH) 2 structure.