Fabrication And Electrochemical Properties Of V-MXene Nanocomposites

MXenes,as a unique two-dimensional（2D）transition metal carbide/nitride layered material that exhibits exceptional electrical conductivity in the nucleus,low energy barriers to metal ion diffusion;excellent interlayer spacing for easy ion embedding;and a large number of surface end groups（e.g.-F,-OH,-O）,giving them tunable chemical properties.Such a wide range of outstanding properties offers the possibility of preparing high performance MXenes-based nanocomposites,making them highly promising for energy storage applications.Although the chemical diversity of the MXenes family is very rich and more than 30 MXenes can already be synthesized experimentally,most research has focused on Ti₃C₂T_x MXene.In this paper,V-MXene（V₂CT_x）with multiple chemical valence states was investigated.Various V-MXene nanocomposites were prepared using acid/alkali etching,hydrothermal methods,electrochemical synthesis and vacuum filtration,and their electrochemical properties were investigated for application in lithium-ion capacitors（LICs）and aqueous zinc-ion batteries（AZIBs）.The main work can be summarized as follows:（1）An intralayer and interlayer co-modification strategy was proposed to modulate the surface chemistry of V₂CT_x MXene,while single-walled carbon nanotubes（SWCNT）were used to composite with it to successfully prepare flexible self-supporting thin film electrodes（VCT-K@C）with a three-dimensional conductive network and apply them as anode materials in lithium-ion capacitors.The introduction of K⁺into V₂CT_x MXene can effectively stabilize the interlayer structure and prevent the aggregation of MXene nanosheets;meanwhile,the controlled modification of-O groups on the surface of V₂CT_x MXene can greatly improve the Li⁺storage capacity;finally,SWCNT is used as a bridge between V₂CT_x MXene nanosheets to open up a channel for ion/electron transport in the longitudinal direction.Benefiting from the synergistic effect of VCT-K and SWCNT,VCT-K@C exhibits a high capacity of 671.8m Ah g^-1 at a small current density of 0.1 A g^-1 and can also exhibit an excellent reversible capacity of 318 m Ah g^-1 at 1.0 A g^-1.More importantly,the LIC with VCT-K@C as the anode and commercial activated carbon（AC）as the cathode provides an ultra-high energy/power density(≈140.5 Wh kg^-1/19.0 k W kg^-1)that exceeds most of the reported LICs with typical MXenes-based intercalated electrodes.In addition,the VCT-K@C//AC demonstrates 6000 cycles of stability（86%capacity retention）at a high current density of 10 A g^-1.Thus,this work presents a new MXenes-based LICs that combines excellent high energy/power density and long cycle stability.（2）In order to improve the electrochemical oxidation efficiency of V₂CT_x MXene in aqueous Zn-ion batteries,the conducting polymer polyaniline（PANI）was used as the catalytic material and grown in situ on V₂CT_x MXene by chemical polymerization,and used as the cathode material for aqueous Zn-ion batteries.The experimental results demonstrate that the introduction of the conducting polymer PANI can significantly improve the oxidation efficiency,shorten the energy consumption time,and effectively alleviate the oxidation instability of V₂CT_x MXene in the first circle.Moreover,PANI can expand the channel of V₂CT_x MXene to facilitate the diffusion of Zn²⁺ions and act as an interlayer"pillar"to inhibit the inevitable structural collapse of V₂CT_x MXene during the cycling process.Electrochemical results indicate that the nanocomposite exhibits a reversible specific capacity of up to 361.9 m Ah g^-1(0.1 A g^-1)and can maintain67.7%of the initial capacity(5.0 A g^-1)after 4000 charge/discharge cycles.（3）A three-dimensional nano-flower-like ammonium vanadate hydrate(NH₄V₈O₂₀·n H₂O,NHVO)was prepared by a simple hydrothermal method using V₂CT_xMXene as the substrate and ammonium chloride as the ammonia source,and further compounded with SWCNT by vacuum filtration to obtain a flexible cathode material（NHVO@C）for aqueous zinc-ion batteries.The three-dimensional nano-flower-like morphology presented by the NHVO provides sufficient contact area between the electrolyte and the electrode material and ensures a short Zn²⁺transport pathway.At the same time,the SWCNT act as a conductive medium,improving the overall electrical conductivity of the cathode.Notably,the use of V₂CT_x MXene as a substrate induces the formation of low-valent vanadium and the creation of oxygen vacancies,which effectively mitigates electrostatic effects,leads to more electron transfer to the surrounding vanadium atoms and increases the stability of the structure.