First Principles Study On Energy Storage Performance Of V-based MXenes

Electrochemical energy storage devices have become one of the most potential energy storage systems because of high efficiency and excellent energy storage capacity.Among them,lithium-ion batteries(LIBs)are currently energizing portable devices,electric vehicles,state grid and other emerging fields due to light weight,long service life,high energy density and good cycle stability.At the same time,zinc-ion batteries(ZIBs)have attracted wide attention due to high abundance,low cost,high theoretical capacity(5854 m Ah·cm^-3)and low redox potential(-0.76 V relative to the standard hydrogen electrode).MXenes have an excellent application prospect and its unique 2D structure due to its excellent metal conductivity and high hydrophilic surface in the field of electrochemical energy storage.In the MXenes family,V_n+1C_n(n=1,2,3)is considered as one of the promising candidates for electrode materials,which has a certain scientific value for the study of the energy storage performance of the system.In this paper,the structural characteristics and electronic structures of the V_n+1C_n(n=1,2,3)and V₂N monolayers,and the adsorption and transport properties of Li atoms on V_n+1C_n(n=1,2,3)monolayer are investigated.Using the Bader charge analyze,we determine the adsorption characteristics of the single Li atom on the V_n+1C_n(n=1,2,3)monolayer,and analyze the effects of functional groups on the surface of the V_n+1C_n(n=1,2,3)monolayer.In addition,the stable functional V₂C and V₂N monolayers are constructed,and analyze the adsorption and transport ability of Zn atoms on their surface.Finally,we also calculated the adsorption energy,open-circuit voltage and theoretical capacity of Zn atoms on the surface of our system,and discuss the mechanism of storage Zn atoms.The specific research contents as following as:1.By calculating the mechanical properties of the V_n+1Al C_n and V₂Al N phases,the V_n+1C_n(n=1,2,3)and V₂N monolayers can theoretically be obtained by selective etching of the Al layers.The density of states near the V_n+1C_n(n=1,2,3)and V₂N Fermi levels are mainly contributed to the V-3d orbit,both showing metal-like properties.2.The adsorption and diffusion behavior of Li atoms on the surfaces of V_n+1C_n(n=1,2,3)and V_n+1C_nT₂(n=1,2,3;T=O,F,OH)are systematically studied.The theoretical adsorption capacity of lithium in the V_n+1C_n monolayer is up to 1412.39m Ah·g^-1(V₂C),909.62 m Ah·g^-1(V₃C₂)and 670.82 m Ah·g^-1(V₄C₃),by comparing the adsorption behavior of Li atoms on the V_n+1C_n(n=1,2,3)and V_n+1C_nT₂(n=1,2,3;T=O,F,OH),we found that the presence of Li⁺could adjust the V_n+1C_n(n=1,2,3).3.The adsorption configurations of Zn atoms on V₂X and V₂XO₂(X=C,N)monolayer surfaces are established.It is confirmed by calculation that Zn atoms can bond to V₂X and V₂XO₂,and it is found that V₂X and V₂XO₂ have high storage capacity,low diffusion barrier and suitable operating voltage.In addition,V₂NO₂ monolayer can provide higher theoretical capacity(2175.55 m Ah·g^-1),smaller diffusion barrier(0.12e V)and lower OCV(0.51 V),is a very excellent ZIBs electrode material.