Study On Electrochemical Performance Of FeP Anode And R-MnO₂ Cathode Materials For Alkali Metal Ion Batteries

High capacity,stable cycle performance and high rate performance of cathode and anode materials as important components for the alkali metal ion batterie have been widely researched.Therefore,the paper selects R-MnO₂ as an cathode material and FeP as an anode material.Using a first-principles method combined with experiment structural changes of R-MnO₂during the discharge process and the electrochemical performance during the charge/discharge processes are studied,and migration paths and migration energy barrier of K and Na in the FeP are investigated to clarify their diffusion mechanism and different rate performances of FeP as anode material for Na-and K-ion batteries.For R-MnO₂ cathode materials,the most stable position of Na in R-MnO₂ is 4c wyckoff site.The insertion of Na in R-MnO₂ makes it gradually change from the channel R-MnO₂stucture to the layer NaMnO₂ stucture.As C_Naa is 0.04 mol%,the material has a channel structure;as C_Na=0.04 mol%-0.42 mol%,the material is a mixed phase wiht the channel R-MnO₂ stucture and the layer NaMnO₂ stucture and as C_Na?0.42 mol%,the material has a layer NaMnO₂ structure,consistent with our XRD experimental results.An experimental phenomenon that the first-discharge voltage platform of R-MnO₂ is lower than that of the second-discharge is contrary to the general situation.Therefore,the Fermi level and the highest occupied orbital of R-MnO₂,Na_0.33MnO₂ and NaMnO₂ systems using the first-principles method are calculated and find that the Fermi energy difference between R-MnO₂ and NaMnO₂is lower than that between Na_0.33MnO₂ and NaMnO₂,which makes the transformation voltage platform of R-MnO₂ into NaMnO₂ lower than that of Na_0.33MnO₂ into NaMnO₂.For the FeP/C anode materials,the rate performance of the K-ion batteries is better than that of the Na-ion batteries.To explain this experimental phenomenon,we calculate the diffusion of Na/K in FeP using a first-principles method.The potential energy surface is obtained throngh considering seven different binding sites（noted as S1,S2-S7）of Na/K in FeP and calculations find that S1 is the most stable position.Therefore,S1 is used as the reaction site and production site for the diffusion of Na/K,then across the secondary stable site,finally back to the next most stable S1 site.Four diffusion paths P0?P4 among which P0 is a direct one-step diffusion path and the other are a multi-step diffusion are explored.The calculations reveals that the beneficial migration path of Na/K in FeP is a multi-step diffusion one using a CI-NEB method.The migration energy barriers mainly affect diffusion activation energies and are related to the binding energies of Na and K and FeP.In the easiest diffusion path P1,the maximum diffusion activation energy（2.89 eV）of K in FeP is significantly smaller than that（3.99 eV）of Na in FeP.Therefore,the rate performance of K-ion batteries is better than that of Na-ion batteries when FeP as the anode material.