Modification And Electrochemical Performance Of Lithium-rich Manganese-based Layered Cathode Materials

Lithium-ion batteries are widely used in 3C electronic products and new energy vehicles because of their reliable energy density and cycle life.With the development of society,people have put forward higher requirements for the battery life of electric vehicles.In the lithium-ion battery system,the energy density is mainly determined by the electrode material.Therefore,improving the battery life of electric vehicles requires further improvement of the lithium battery cathode electrode.Layered lithium-rich manganese-based oxides Li1.2Mn0.54Ni0.26O2(LMNO)with high energy density,low cost,and reliable safety are considered one of the most promising cathode materials.However,commercial application of LMNO is hindered by unstable structure.In order to improve the structural stability of LMNO,this paper adopted anion and cation codoping method to improve its electrochemical performance such as initial Coulombic efficiency,cycle stability,rate performance and energy density.The main research contents are as follows:(1)Firstly,LMNO was modified by Sn and Cl co-doping.Cl doping can enhance the redox activity and reversibility of lattice oxygen,which can stabilize the transition metal layer(TM)-oxygen bond of the material.At the same time,the bonding energy of Sn-O is higher than that of Mn-O,thereby inhibiting the crystallization.Therefore,the Coulombic efficiency of Sn and Cl co-doped LMNSnClO at first cycle is increased to 83.43%.O is replaced by Cl to form TM-Cl bonds with higher energy,and the electrochemically inert Sn4+ can act as a supporting structure.Sn,Cl co-doped LMNSnClO can hinder TM migration during charge and discharge and reduce harmful phase transitions.The capacity retention rate is up to 86%after 400 cycles at 1 C,and the cycle stability has been significantly improved.Sn and Cl doping with large ionic radii in the lattice can widen the interlayer spacing and promote the transport of lithium ions,thus improving the rate performance of the material.(2)On the basis of retaining the original cycle stability,the anion is replaced by F to improve the discharge specific capacity of the material.Meanwhile,Sn was still used as the cation.Since the electronegativity of F is higher than that of O,F-doping can increase the charge compensation of the transition metal layer,which enables TM to donate extra electrons before activation and the capacity is enhanced.The formed spinel structured surface layer due to F doping can provide a 3D diffusion path for lithium ions,and the stable spinel structure on the surface can also improve the interface between the electrode and the electrolyte,thereby reducing the charge transfer resistance.The discharge specific capacity of Sn,F co-doped LMNSFO is 20%higher than that of unmodified LMNO.At the same time,the specific capacity at high current density is increased to 134.7mAh g-1,and the capacity retention rate is 90.6%after 100 cycles at 1C,maintaining a high cycle stability.In summary,this paper proposes two methods of anion-cation co-doping modification to improve the electrochemical performance of layered lithium-rich manganese-based cathode materials.The modified materials have improved initial Coulombic efficiency,discharge specific capacity,cycle stability and rate performance.This significant improvement provides a new design idea for the commercial application of layered lithium-rich manganese-based cathode materials.