Surface Structure Design And Mechanism Exploration Of Layered Cathode Materials For Lithium Ion Batteries

Layered cathode materials are considered to be the mainstream cathode materials for next-generation lithium-ion batteries（LIBs）due to their high capacity and voltage,superior rate performance.However,there are still some problems need to be solved urgently to meet the requirements of modern society for higher energy density,higher rate capability and longer cycle life of LIBs,including inherent cation mixing in the layered structure,lattice oxygen release,structural degradation,transition metal ions dissolution and electrode-electrolyte interface side reactions during cycling,etc,.To address the above challenges,this paper explores the effects of modification strategies,such as element doping and/or surface coating,on the surface structure and electrochemical performance of layered cathode materials by starting from the surface structure design.In addition,the regulation mechanism of the surface structure,the evolutionary law of the surface chemical properties and the design principle of the electronic structure were deeply studied,and the relationship between the chemical composition,surface structure and electrochemical performance of the material was revealed.The details are as follows:（1）The high-performance Ni-rich layered material（NCMLB）was obtained by controlling the surface structure and chemical properties of the Ni-rich LiNi_0.6Co_0.2Mn_0.2O₂（NCM）material through the synergistic effect of boron doping and excess lithium ratio.Detailed studies show that the interaction of boron doping and excess lithium ratio can effectively suppress the Li⁺/Ni²⁺cation mixing,and synergistically promotes the electronic and ionic conductivities of NCM,thus inhibiting the structural degradation,and suppressing the migration of TM cations and electrode-electrolyte interface side reactions during cycling,thereby alleviating the capacity/voltage decay.（2）A Ceria@Rocksalt@Layered heteroepitaxial interface with oxygen anions buffer effect was constructed on the surface of Li-rich layered cathode Li_1.2Mn_0.53Ni_0.27O₂ by ceria treatment.The ceria coating with oxygen vacancies can be used as"oxygen reservoirs",thereby suppressing the irreversible lattice oxygen release and electrode-electrolyte interface side reactions during cycling.Subsequently,Ce ion doping can reduce the TM-O covalency and band gap,and improve the cycling performance and rate capability of the material.Moreover,the induced rocksalt structure ensures a stable interface between the ceria coating and the layered structure,effectively suppresses the TM ions dissolution and structural degradation,and enhances the ionic and electronic conductivities.These synergistic effects enable the modified materials to exhibit higher reversible capacity,excellent cycling performance and rate capability.（3）A La/Al co-doping strategy was proposed to tune the fine electronic structure and surface structure of Li_1.2Mn_0.53Ni_0.27O₂ layered cathode.The strong La-O and Al-O bonds induced by La/Al co-doping weaken the covalency of TM-O bonds and promote oxygen covalent electron localization,thereby facilitating charge transfer and Li⁺migration,enhancing O^n-redox and inhibiting lattice oxygen release.Simultaneously,the induced Layered@Rocksalt heteroepitaxial interface can suppress the structure degradation and side reactions during cycling.As a result,the co-doped material exhibit a capacity retention of 93.6% for 200 cycles,and can release a capacity of 156.8 mAh g^-1 at 10 C.（4）A layered@Rocksalt@Li-Co-PO₄ heterointerface was constructed on the surface of Li-rich materials by La/Al co-doping and Li-Co-PO₄coating.The results show that La/Al co-doping and the induced rocksalt structure reduce TM-O covalent and promote oxygen covalent electron localization,thereby promoting Li⁺migration and stabilizing the oxygen lattice framework.The ionic conductor Li-Co-PO₄ coating formed by reacting cobalt source and phosphorus source with residual lithium on the surface of the La/Al co-doped material during the material synthesis,ensuring the rapid de-intercalation of Li⁺ and the stability at high voltage,and reduce the electrode-electrolyte interface side reactions by eliminating the residual lithium on the surface.Under the synergistic effect of the three-phase heterointerface,the modified material exhibits a high capacity retention rate of 80.06% after 500 cycles at 1C,and 87.78%at after 150 cycles at 1C within 2-4.9 V.（5）Synergistically utilizing the spontaneous polarization and piezoelectricity of Bi_0.5Na_0.5TiO₃（BNT）ferroelectric material and La/Al co-doping strategy,a heterocoherent ferroelectric interface with lithium-conduction and oxygen-barrier was constructed on the surface of Li-rich layered material.The introduction of La/Al improves the ionic and electronic conductivities of the material and enhances the redox reversibility and reactivity of lattice oxygen.Subsequently,the spontaneous polarization and piezoelectric effect of the BNT coating promote the migration of Li⁺during charge-discharge and hinder the continuous migration of O^n-into the electrolyte.The coherent interface ensures the integrity and stability of the layered structure during cycling,thereby inhibiting the volume change of the material and suppress the structural degradation and side reactions at the electrode-electrolyte interface during cycling.