First-principles Study On Energy Storage Performance Of Cr₂C-based MXene

Rechargeable ion batteries（IBs）are developed rapidly in recent years.Among them,lithium-ion batteries are widely used in electronic devices because of their advantages of high energy density,long service life and short charging time.However,the development of lithium-ion batteries has been seriously hindered,due to the limitation of lithium resources in nature and high cost.Therefore,there is an urgent need to seek other types of metal-ion batteries.Sodium,potassium,and magnesium are more abundant in the earth’s crust and cost less than lithium.Thus,they could be the next generation of IBs.In addition,it is crucial to seek corresponding high-performance electrode materials.With excellent mechanical properties,high electronic conductivity and hydrophilicity,MXenes have attracted much attention in the field of energy storage.In particular,O or S-functionalized MXenes have good electrochemical performance as electrode materials.In this paper,four different Cr₂CT₂ monolayers are constructed,namely,Cr₂CO₂,Cr₂CS₂,Cr₂CSO-Ⅰand Cr₂CSO-Ⅱmonolayers.The structure and electronic structure of these four materials are explored by first-principles calculation,and their energy storage performance for Li,Na,K and Mg is systematically analyzed.The results show that Cr₂CT₂ have excellent performance which can be used as electrode materials of metal-ion batteries.The content and results are as follows:1.All Cr₂CT₂ configurations are established.The most stable configurations are confirmed by principle of the lowest energy.The thermodynamic stabilities of the structures are verified by phonon spectrum and molecular dynamics.Besides,Through the analysis of the DOS of the materials,it is found that they all show metal-like characteristics.At the same time,the mechanical properties of configurations are also calculated,and the stabilities of the materials are analyzed from another perspective.According to the above research results,Cr₂CT₂ have the basic conditions as good electrode materials.2.The adsorption properties of Cr₂CO₂ and Cr₂CS₂ monolayers with four different metal atoms are analyzed.Firstly,the adsorption properties of a single metal atom are studied:By calculating the adsorption energies,charge transfer and diffusion performance of metal atoms on Cr₂CO₂ and Cr₂CS₂ surfaces,it is found that metal atoms can be adsorbed on the surface of monolayers and have a low diffusion barrier,which guarantees the transition ability when Cr₂CT₂ monolayers used as electrode materials.In addition,the adsorption properties of Cr₂CO₂ and Cr₂CS₂ monolayers are analyzed:The layer adsorption energies,OCV and maximum capacities are calculated.The results shows that the capacities of Cr₂CO₂ to Mg(878.721 m Ah·g^-1)and Cr₂CS₂ to Li(724.351 m Ah·g^-1)are prominent and the corresponding OCV is 0.164 V and 0.117 V,respectively.Low diffusion barrier and high capacity indicate that Cr₂CT₂ can be used as good electrode materials.3.The energy storage performance of Cr₂CSO-Ⅰand Cr₂CSO-Ⅱmonolayers are explored.It is calculated that the capacity of Cr₂CSO-Ⅰto Mg is high(824.646 m Ah·g^-1),but lower than that of Cr₂CO₂ to Mg.Cr₂CSO-Ⅱhas a prominent capacity to Li(558.354 m Ah·g^-1),but is lower than Cr₂CS₂ to Li.The reason for these phenomena is that the molar mass of Cr₂CSO-Ⅰis larger than that of Cr₂CO₂,and Cr₂CSO-Ⅱhas only two adsorption sites for metal-ion.These indicate that the bifunctional Cr₂C monolayers reduce the adsorption capacities of specific metal atoms.