First Principles Studies On Structure Design And Energy Storage Performance Of Novel Two-dimensional Materials

Lithium-ion batteries（LIB）are widely used as energy storage devices in portable electronic devices and electronic devices.However,commercial LIBs have many drawbacks,such as medieum capacity and poor cycle rate.Therefore,the search and design of the anode materials for high-performance LIBs has become a research hotspot.As the anode material of LIBs,two-dimensional materials have larger specific surface area,faster ion migration rate,better rate performance and good mechanical properties than three-dimensional materials,which are widely concerned by researchers.In this paper,a special phase transition metal carbide（MXenes）was designed and two transition metal borides（MBenes）were designed by CALYPSO crystal structure prediction software based on particle swarm optimization.The structural stability,electronic structure,mechanical properties and Li-ion storage performance were calculated by using VASP software package.The specific achievements of the research are as follows:1.The 2H phase W₂C monolayer and W₂CO₂ monolayer were designed,and their thermal stability,dynamic stability,thermodynamic stability,mechanical stability,electronic structures and electrochemical performance as anode materials for LIBs were calculated.The calculation results show that the 2H phase W₂C and W₂CO₂ belonging to the P–6m2 space group are stable and have lower energy than the 1T phase belonging to the P–3m1 symmetry group.At the same time,the 2H phase W₂CO₂ monolayer has excellent mechanical properties,and its Young’s modulus reaches 351 N m^–1.After oxygen functionalization,a metal-semiconductor transition occurs in the 2H phase W₂C.As anode materials for Li-ion batteries,the theoretical capacities of2H phase W₂C and W₂CO₂ monolayers are 423.6 m A h g^–1 and 362.5m A h g^–1,respectively,which are higher than those of 1T phase.The diffusion barrier of Li-ion on the surface of W₂C is small,which is 0.046 e V,and increases to 0.377 e V after the oxygen-functionalization.Therefore,similar to other MXenes,surface functionalization should be avoided in practical applications.2.Based on CALYPSO crystal structure prediction software,a two-dimensional ScTi B₂monolayer with P6mm symmetry group was predicted with fixed components,and its stability,electronic structure,mechanical properties,single Li-ion diffusion rates,theoretical capacity and open-circuit voltage as the anode material for LIB were studied,and compared with the hexagonal Ti₂B₂ monolayer.The internal mechanism of high capacity of ScTi B₂ monolayer was analyzed.The results show that ScTi B₂ monolayer has good stability,excellent mechanical properties and exhibits metal characteristic.As the LIB anode material,the ScTi B₂ monolayer exhibit extremely low Li-ion diffusion barriers（0.014 e V at Sc surface and 0.022 e V at Ti surface）,high theoretical capacity(937 m A h g^–1),and suitable open-circuit voltage（0.15 e V～0.48 e V）.Compared with Ti₂B₂ monolayer,the coulomb repulsive force between adsorbed lithium ions is reduced due to the breaking of out-of-plane symmetry,thus enhancing the theoretical capacity.3.Based on CALYPSO crystal structure prediction software,two-dimensional VB₃monolayers with symmetry group Pmmn are predicted with fixed components,and Nb B₃ and Ta B₃monolayers are designed with substitute atoms.Their stability,electronic structure,mechanical properties and electrochemical properties as anode materials for LIBs were investigated by first-principles calculation.The results show that MB₃ monolayer exhibits excellent stability and high Young’s modulus.Electronic structure calculations show that MB₃ monolayers have metal-like conductivity.As the anode material for LIBs,the exposed surface of boron directly adsorbed Li atoms.The theoretical capacities of MB₃ monolayers are high(VB₃:1286 m A h g^–1;Nb B₃:856m A h g^–1;Ta B₃:502 m A h g^–1),the diffusion barrieres are low（VB₃:0.590 e V;Nb B₃:0.202 e V;Ta B₃:0.399 e V）,the open-circuit voltages are maintained between 0-1 e V after the adsorption of multilayer lithium ions.In addition,in the AIMD simulation,the Li-ions remain in an orderly arrangement at room temperature,and the MB₃ structures is not destroyed due to the adsorption of lithium ions,which ensures a good cycle performance during the battery operation as the anode materials.