Doping And Interface Effects In Graphene/LiFePO₄ Cathode Material For Li Ion Battery

Olivine phosphate LiFePO₄ has been widely used as cathodes materials for lithium ion batteries.Nanocrystallization,graphene modification and doping are the efficient methods to improve the rate capacity of Li FePO₄.Therefore,it is of great significance to study the interface properties of graphene/LiFePO₄ and the mechanism of doping effect on the electrochemical performance in microscale.Using the first-principles calculations,the interfacial Li storage behavior at graphene/Li FePO₄ interface,the Mn doping effect,and the electrochemical properties of the graphene/LiFePO₄ interface in the presence of the interfacial Li storage have been comprehensively investigated.It is revealed that the interfacial binding between graphene and LiFePO₄ in parallel orientation is stable and improved through Mn doping.In the presence of interfacial Li storage,the interfacial binding becomes stronger with the increasing amount of Li adatoms.The adsorption sites at the interface,Mn dopant,and graphene modification affects the Li adsorption energy,which can be understood through the edge effect,doping stability,and interfacialbinding strength,respectively.The Li adsorption energies at different sites elucidate thecore-shell model in Li extraction/insertion process and indicate the anomalous Li storage in the interface between graphene and LiFePO₄.The contribution of interfacial Li storage could be 0.36 mAh/m².It is revealed that the graphene modification and the coexistence of graphene modification and Mn dopant improve the Li ion diffusion along the surface.The surface electronic conductivity is improved in the presence of graphene or Mn dopant with respect to the bandlike electron transport.Thereby,the rate capability is expected to be enhanced.During the interfacial Li adsorption,the surface electronic conductivity of the undoped interface is weakened,while the Mn doping maintains the good conductivity in the early stage of the Li interfacial storage.Last but not least,the graphene modification and Mn doping exert positive effect on the surface oxygen stability.