Preparation And High Efficiency Lithium Storage Performance Of 2D V₂C-based Layered Materials

Among all kinds of energy storage systems,lithium-ion batteries(LIBs),as one of the most successful clean energy storage devices,have attracted wide attention because of their excellent reversible capacity,high energy density and long cycle life.Recently,MXenes have been extensively studied because of their unique 2D structure,excellent conductivity,excellent electrochemical stability,and ability to accommodate inserts.However,the layered structure of MXenes is prone to collapse or stacking during the long-term cycle of the battery,resulting in reduced specific capacity and rate performance.In this paper,V₂Al C was used as the MAX phase,different etching agents were selected and etching conditions were optimized to prepare2D V₂C with high specific capacity.2D V₂C was compounded with V₅S₈ and NiCo-LDH by in-situ growth method to prevent the collapse or stacking of layered structures and improve its electrochemical performance.Through selective etching of aluminum from V₂Al C using different etching agents to prepare two-dimensional layered V₂C.Lithium storage performance of V₂C MXenes was optimized by controlling etching agent and etching time.Secondly,two-dimensional V₂C/V₅S₈composites were obtained by in-situ sulfuration method.In addition,two-dimensional V₂C was compounded with NiCo-LDH by reflux method,and then the P-doped V₂C/NiCo-LDH composites were obtained by phosphorization.2D V₂C-based layered composites materials with excellent electrochemical properties were obtained.The main research contents are as follows:(1)Different etching agents(HF+HNO₃,HF+H₂O₂,HF or HCl+Li F)were used to selectively etch V₂Al C to obtain two-dimensional layered V₂C.The study found that when HCl+Li F is used as etching agent,the interlayer spacing and the integrity of the layered structure of V₂C can be effectively controlled by adjusting the etching time.With the prolonging of etching time,the interlayer spacing becomes wider,but,if the etching time is too long,the layered structure will be destroyed,which has an adverse effect on ion diffusion and electron transfer in the electrochemical reaction process.The V₂C obtained at 90 ^oC for 3 days exhibits a complete layered structure with an interlayer distance of 12.13(?).When the current density is 200 m A g^-1,the specific discharge capacity can still be maintained at 446 m Ah g^-1 after 250cycles.In the rate performance test,high specific capacities of 152-339 m Ah g^-1 can be retained by V₂C electrodes with varied current densities of 100-4000 m A g^-1.Even at high current density of 8000 m A g^-1,a high capacity of 141 m Ah g^-1 can still be delivered.A capacity of384 m Ah g^-1 can be recovered when the current density returns back to 100 m A g^-1.(2)The optimal prepared V₂C was used as the substrate material,different sulfur loading methods(melt diffusion method,CTAB treating-melt diffusion method,CS₂ sulfur loading method and CS₂+NMP sulfur loading method)were used to obtain V₂C/S composites.Subsequently,V₂C/V₅S₈ composites were obtained by in-situ sulfuration at different temperatures.Electrochemical tests show that the V₂C/V₅S₈ composite(V₂CS-CTAB NMP-450)prepared by CS₂+NMP as sulfur loading agent with sulfurizing temperature of 450 ^oC displays the best electrochemical properties,and the discharge specific capacity can reach 796m Ah g^-1 after 250 cycles at a current density of 200 m Ah g^-1.In the EIS performance test,the charge-transfer resistance of the V₂CS-CTAB NMP-450 electrode is only 49 ohm,which presents a great improvement compared to the charge-transfer resistance of V₂C(171 ohm),indicating the fast electron/Li⁺transfer.Further kinetic analysis confirmed that the capacitive control process plays a dominant role in the electrochemical reaction,which is also the reason for its excellent rate performance.(3)The as-synthesized V₂C is electronegative due to the presence of surface functional groups,while Ni²⁺,Co²⁺cations are electropositive.Ni²⁺,Co²⁺cations can embed in the interlayer of V₂C by electrostatic attraction method.Then,two-dimensional V₂C/NiCo-LDH(VNC)composites were in-situ grown by reflux method,and further phosphoriation was carried out to obtain P-doped V₂C/NiCo-LDH(P-VNC)composites.Among them,NiCo-LDH nanosheets grow evenly on the surface and interlayer of V₂C,effectively inhibiting the collapse or restack of V₂C layered structures.By controlling the molar ratio of V₂C and NiCo-LDH in the composites,the P-VNC-1 composite presents the optimal electrochemical performance,and the discharge specific capacity of P-VNC-1 electrode is 1226 m Ah g^-1 after 200 cycles at the current density of 200 m A g^-1.Intereatingly,the discharge specific capacity of P-VNC-1electrode is 1077 m Ah g^-1 after 700 cycles at the current density of 500 m A g^-1,exhibiting excellent cycle stability and good rate performance.