Study Of Electrochemical Energy Storage Behavior Of Two-Dimensional Vanadium Carbides

Energy and environment crisis have become hot topics in the world today.Limited fossil energy reserves and increasingly serious environmental pollution problems lead to more focus on sustainable and clean energy sources.While these energy sources are characterized by regional differences and intermittency.To meet the needs of electric vehicles,large-scale power grids and portable electronic devices,it is crucial to develop more reliable energy storage devices with high-energy,high-power,and great cycle stability.Electrochemical energy storage devices,including rechargeable batteries,and supercapacitors,have been widely studied during these years due to the low-cost,safety,and great capacity.Among many energy storage methods,electrochemical energy storage devices,especially lithium-ion batteries and supercapacitors,have attracted wide attention in the scientific community for their high efficiency in converting chemical energy into electrical energy and the high energy density and power density provided by solid-state electrodes.Supercapacitor is an important part of electrochemical energy storage system which stores charge by forming a double electric layer on the electrode/electrolyte interface with great rate performance and long life cycles.So,it is suitable for the storage of fast power supply devices and backup energy.Lithium-ion battery,also known as rocking chair battery,mainly relies on the intercalation/extraction of lithium ions to the working electrodes to store energy,accompanied by redox reaction on the electrode,with high energy density and low self-discharge rate.However,the low rate performance of lithium-ion batteries and the low energy density of supercapacitors still cannot meet the needs of today’s technological development.Further technological innovations need to improve all relevant performance indicators,especially the search for new electrode materials and new energy storage mechanisms.MXene,a family of two-dimensional transition metal carbides/nitrides synthesized by selectively removing the A element from precursor phase M_n+1AX_n（M is an early transition metal,A stands for element from IIIA and IVA,X is C and/or N,and n=1,2,or 3）,has great potential in the field of energy storage due to the versatile composition,good conductivity,unique two-dimensional layered structure and hydrophilic surface.The high electrical conductivity ensures the fast electron transport,and the two-dimensional layered structure provides sufficient reactive sites and short ion transport paths,thus ensuring the electrochemical capacity and rate performance.In addition,the surface of MXenes are chemically terminated with O,OH,or F atoms during etching process and the surface termination varies according to etchants and synthesis condition.The surface termination of MXenes obtained by various methods is essential and vital to the property of MXenes including electronic structure,energy storage performance,catalytic performance and other physicochemical properties.The main etching methods include hydrofluoric acid etching method,in-situ hydrofluoric acid etching method,melting fluorine salt etching,alkali etching,electrochemical etching,CVD etching and melting Lewis acid salt etching.In fact,most of the MXene materials that have been synthesized by experiments can be by controlling the reaction time and the concentration of hydrofluoric acid.At present,there are more than 30 known MXene materials,such as Ti₃C₂T_x,Ti₂CT_x,Nb₄C₃T_x,Nb₂CT_x,V₂CT_x.However,due to the complexity of the synthesis of MXene materials,most of the current research work are still focused on Ti₃C₂T_x.As a member of the MXene family,two-dimensional Vanadium-based Carbides-V₂CT_xhave lower mass and higher electrochemical activity compared with Ti₃C₂T_x,and have achieved better electrochemical performance in lithium ion batteries,sodium ion capacitors,aluminum ion batteries and zinc ion capacitors.The research focus of this paper is the preparation of V₂CT_xand the study of electrochemical energy storage behavior,the specific content is as follows:First of all,precursor V₂Al C was synthesized by high temperature sintering the commercial vanadium powder,aluminum powder and graphite powder.By adjusting the reaction time and concentration of etching agent,we using in-situ hydrofluoric acid etching method by mixing Li F and HCl to synthesize multilayer V₂CT_xmaterial.Then we explore the electrochemical performance of V₂CT_xin lithium ion battery through the cyclic voltammograms and GCPL method and good cycle stability and rate performance are shown.At the current density of 50 m A g^-1,V₂CT_xelectrode shows the first-cycle discharge capacity of 595 m Ah g^-1,and the specific capacity can still reach 182 m Ah g^-1at the high current density of 5 A g^-1.Since the surface V atoms are quite active,V₂CT_xis unstable in either acidic or alkaline conditions.In addition,neutral electrolytes are more mild,safe and environmentally friendly.Then,we synthesize free-standing V₂CT_xflexible film（d-V₂C）by filtrating TMA⁺intercalated V₂CT_xpowder and investigate the electrochemical energy storage behavior of d-V₂C in different aqueous electrolytes（Li₂SO₄,Na₂SO₄,K₂SO₄and Mg SO₄）by Cyclic Voltammograms,GCPL,Electrochemical Impedance Spectroscopy and in situ X-ray diffraction techniques.Good specific capacitances are achieved,specifically 208 F g^-1in 0.5 M Li₂SO₄,225F g^-1in 1 M Mg SO₄,120 F g^-1in 1M Na₂SO₄and 104 F g^-1in 0.5 M K₂SO₄.Through the in situ XRD,we found that during the cycling process,A clear shift of（002）was visible during cycling in in Mg SO₄and Li₂SO₄electrolytes and the c-lattice parameter of d-V₂C will change with applying potential in the same trend:A slight shrinkage of c value is observed with increasing voltage,which proved the intercalation/de-intercalation phenomenon of Li⁺and Mg²⁺into d-V₂C.We can deduce that the charge storage mechanism of V₂C may be highly related to the size of cations,cations with smaller ion radius like Mg²⁺and Li⁺could intercalate into the interlayer space and interact with the surface V atom and termination groups,while larger ion like Na⁺and K⁺could only access into some shallow sites and thus contribute to better rate and cycling performance.In conclusion,practical application of V₂C material in aqueous electrolytes can be realized in neutral environment and reasonable choice of electrolyte with different cations should be considered for different demand.