Tunning The Interfacial Structure Of Lithium-Rich Manganese-Based Oxides Cathode Materials For Lithium-ion Batteries

Developing lithium-ion batteries（LIBs）with high energy density and high power density is one of the significant goals in the field of electrochemical energy storage.Compared with anode material,the energy density of cathode material is the main factor that restricts the energy density of LIBs.Among many cathode materials for LIBs,lithium-rich manganese-based oxides（x Li₂Mn O₃·（1-x）Li TMO₂,TM=Ni,Mn,Co and other transition metals,LRMO for short）have a high reversible specific capacity(250 m Ah·g^-1),making it one of the most attractive cathode materials for LIBs.However,large initial capacity loss,poor rate performance,and rapid attenuation of voltage plateau seriously hinder its commercial application.Starting from the electrode/electrolyte interface,this dissertation explores the influence of the structure and components of the electrode/electrolyte interface on the electrochemical performance of LRMO cathode materials by studying different electrolytes and functional additives,sequentially improving the cycling stability of LRMO cathode materials.The main research work and results are as follows:（1）By comparing the influence on electrochemical performance of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂in different electrolytes,we explore the mechanism of action.The results show that the electrolyte（LB-372）with more fluorinated solvents can alter the composition of electrode-electrolyte interface,which means the increase of Li F content promotes the stability of electrode-electrolyte interface.Under the current density of 100 m A·g^-1,LRMO have a discharge specific capacity of 216 m Ah·g^-1after100 cycles,and the capacity retention rate increases to 90%,which is significantly higher than 80%of traditional electrolyte.Obviously,the reversible specific capacity of LRMO in LB-372 electrolyte is also higher than that in conventional electrolyte at higher current density.The above results illustrate the stability of electrode/electrolyte interphase can be regulated by fluorinated solvents.（2）In order to further improve the cycling stability of LRMO,the analysis find that HF in the electrolyte will constantly corrode LRMO,the electrode-electrolyte interface and the current collector,which will result in a rapid decline in electrochemical performance.Therefore,in this dissertation,trimethoxy（3,3,3-trifluoropropyl）silane（TMTFS）is selected as electrolyte additive and study its effect on the electrochemical performance of LRMO.The results display that TMTFS can effectively remove HF in the electrolyte.In addition,TMTFS decreases the interaction between solvent and lithium-ion,resulting in accelerating lithium ions transport in electrolyte.Not only that,a thinner and Li F-dominated cathode-electrolyte interphase（CEI）is formed,which provide more stable interface for LRMO and isolate direct contact between electrode and electrolyte,further leading to the occurrence of less side reactions.After 200 cycles,LRMO electrode still maintains a capacity retention rate of 91%with a reversible specific capacity of 228m Ah·g^-1(100 m A·g^-1).In addition,LRMO in this electrolyte can still cycle stably in a wide voltage range of 2.0-5.0 V.The innovations of this dissertation are as follows:trimethoxy（3,3,3-trifluoropropyl）silane（TMTFS）is used as electrolyte additive for LRMO.A thin and dense Li F-dominated CEI is constructed in situ.At the same time,HF in the electrolyte is effectively removed and the transference number of lithium-ion is increased.This study provides a novel idea for development of new electrolytes.