Interface Modification And Mechanism Study Of Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ Cathode Material

To meet the requirements of high energy/power density and safety cathode materials for lithium-ion batteries（LIBs）in portable electronic products and electric vehicles,the nickel-rich layered transition metal oxide(Li Ni_0.8Co_0.1Mn_0.1O₂,x≥0.6,x+y+z=1)has become one of the key cathode materials in recent years.For Ni-rich layered cathodes,elevating the operating voltage and Ni content is undoubtedly an effective method to obtain higher specific capacity as well as achieve the goal of high energy density（capacity×voltage）.However,Li⁺/Ni²⁺cation mixing,internal stress accumulation,transition metal（TM）dissolution,and the oxidation decomposition of electrolyte under high voltage seriously hinder the further commercialization for LIBs.Many studies have shown that the physicochemical properties,structure and electrochemical stability for cathode materials from surface to bulk are closely related to the cathode-electrolyte interface.Therefore,constructing high quality cathode-electrolyte interface is an effectively way to realize the long lifespan cycle for nickel-rich cathode materials.Ideally,the electrode-electrolyte interface structure should meet the following conditions:1)In terms of morphology,it is dense,continuous,uniform and thin;2)In terms of performance,it can accelerate the conductivity of lithium ions,inhibit the dissolution of transition metal and lattice oxygen escape,and separate the side reactions between cathode and electrolyte;3)Most importantly,it could balance flexibility and mechanical strength to withstand volume changes for the cathode materials during cycling.Considering the regulating effect of nickel-rich cathode-electrolyte interface layer on Li⁺diffusion,charge transfer and parasitic reaction,this paper adopts surface coating and electrolyte design to construct high-performance cathode-electrolyte interface respectively to improve the long cycle characteristics for nickel-rich LIBs.Related research contents and achievements mainly include the following two parts:1.To solve the problem of volume expansion for nickel-rich layered oxide cathode materials induced by internal stress accumulation during repeated cycling.Li Al Si O₄（LASO）,a fast ion conductor with unique negative thermal expansion（NTE）effect,was selected as the coating layer for surface modification.The experimental results show that the unique negative thermal expansion characteristic of LASO not only inhibits the degradation of the nickel-rich cathode due to phase transition and volume expansion at repeated cycles and high temperatures,but also acts as a physical barrier to protect the nickel-rich layered cathode from electrolyte erosion.Besides,it’s fast ion conductor property provide a robust migration path for Li⁺at the cathode-electrolyte interface.Therefore,compared with the pristine material,the nickel-rich layered cathode optimized by LASO has better cycle stability,rate performance and thermal stability.The results fully demonstrate the importance synergistic effect for multifunctional coating materials,especially potential benefits of negative thermal expansion compounds on improving the microstructure stability of long-life nickel-rich LIBs cathode materials.2.In view of the bottleneck problem that traditional commercial carbonate electrolytes are prone to oxidation under high voltage conditions（>4.3V）and further form an unstable electrode-electrolyte interface,which accelerates the deterioration of the electrochemical performance of LIBs,from the perspective of promoting Li⁺transmission at electrode-electrolyte interface,we constructed a Li F/Li₂CO₃heterostructured electrode-electrolyte interface by using pentafluorostyrene（PFBE）as electrolyte additive.Experimental results show that Li F/Li₂CO₃-rich EEIs could effectively improve Li⁺transport compared with Li F and Li₂CO₃.And the interphase could inhibit the generation of microcracks caused by large volume changes in nickel-rich cathode materials.In addition,Li F exhibits a favorable passivation ability to protect both cathode and anode as a good insulator due to its large band gap.Consequently,Li F/Li₂CO₃-rich EEIs provide both mechanical and chemical protection,meanwhile control Li⁺transport.As expected,4.5 V Li||NCM811 batteries achieved superior cycling performance with the help of Li F/Li₂CO₃-rich EEIs.Also,the Li||NCM811 pouch cells with such electrolytes maintained a stable energy density of～485 Wh kg^-1 with an actual capacity of～6.69 Ah.This provides a new direction for the development and mass production of future high energy density LIBs.