First-principle Study Of 2D MXene As Anode Materials For Metal-ion Batteries

The scale of manufacturing and consumption in today’s world reaches an unprecedented level,and the humanity is also confronted with serious energy crisis.Therefore,the ion-battery technology has been rapidly developed,but those typical disadventages,such as the scarcity of lithium resources,and the potential safety problems of lithium-ion batteries,haven’t been solved well yet.Potassium-ion batteries（KIBs）are potential substitutions of lithium-ion batteries in the future,considering the superiority of K in abundance and safety compared with lithium.However,the large radius of potassium ion will cause severe capacity degradation and other problems,hence there is an urgent need for new anode materials that can support the intercalation/de-intercalationof potassium ions.Herein we investigated the performances of two S-functionalized MXenes（S-MXenes）Ti₂CS₂and Ti₂NS₂as anodes for KIBs from first-principle calculations.The structural optimization results show that both S-MXenes are hexagonal lattices,with lattice constants of 3.195（?）and 3.176（?）,respectively.Since the radius of nitrogen atom is slightly smaller than that of carbon atom,correspondingly,the lattice constant of Ti₂NS₂is slightly smaller than that of Ti₂CS₂.From the density of states,both S-MXenes remain to be metallic as their pure states.Furthermore,through electronic orbit resolved density of states,the metallicity of the two S-MXenes is derived from the3d orbital electrons of the transition metal titanium atom.Then,by constructing a supercell,we carried out the calculation of adsorption energy of single metal-ion including Li、Na、K and Mg,among which the results of K-ion was the best.Potassium ion on both S-MXenes prefers to adsorbe at the T_Tisite and the T_C/T_Nsite with extremely low adsorption energies of-2.72 e V and-2.37 e V,respectively.Meanwhile,the charge density difference calculation and Bader charge analysis results also confirmed a significant charge transfer between the adsorbed potassium ion and the surface sulfur atoms.These calculation results prove that potassium ion possess a strong chemical adsorption on both S-MXenes.In addition,potassium ion has extremely low migration barriers on the surfaces,which are 0.046 e V and 0.048 e V for Ti₂CS₂and Ti₂NS₂,respectively,which represents an ultrafast kinetics.Finally,by forming a stable potassium ion adsorption layer,the theoretical capacities are as high as 312 m Ah/g and 309 m Ah/g for Ti₂CS₂and Ti₂NS₂,respectively,far higher than traditional graphite and some modified carbon based materials.By forming incomplete double-layer adsorption,the theoretical capacity will be further improved.All the calculation results strongly proved that Ti₂CS₂and Ti₂NS₂are both potential anode materials for potassium ion batteries.