| The key to improve the energy density of lithium ion batteries is to use high specific capacity anode and cathode materials and to increase the working voltage as much as possible.However,the cathode material LiNi0.8Co0.1Mn0.1O2（NCM811）,which is widely concerned,has many problems such as poor structure stability and easy catalytic electrolyte decomposition at high voltage.In this paper,two electrolyte additives were designed and developed for Li/NCM811 half-cells operating at 2.8-4.5 V high voltage,aiming to improve the comprehensive performance of nickel-rich ternary lithium batteries by solving the interface problem of cathode/electrolyte.The main research contents and results are as follows:（1）Firstly,a multi-functional electrolyte additive,methyl 2-（sulfamoylmethyl）benzoate（MDSB）,was selected through structural design and theoretical calculation.Molecular orbital energy calculations and linear sweep voltammetry measurements show that MDSB has the property of preferential oxidation and can broaden the electrochemical window of the electrolyte.The electrochemical test results showed that the Li/NCM811 cells containing 1%MDSB had excellent high voltage stability,with the capacity retention rate of up to 85%after150 cycles.Further mechanism studies showed that MDSB could form a uniform and dense CEI film on the cathode surface,and effectively inhibit the dissolution of transition metals in the cathode and the oxidative decomposition of electrolyte.Nuclear magnetic resonance spectroscopy analysis and theoretical calculation showed that the additive MDSB could effectively remove HF from the electrolyte,thus inhibiting the corrosion of CEI membrane.Therefore,MDSB is a multifunctional additive for film formation and acid removal,and suitable for nickel-rich materials.（2）Another special additive,methyl diphenylphosphine（MDPO）,was screened out based on theoretical calculation and electrochemical performance test.Unlike traditional film-forming electrolyte additives,MDPO does not undergo oxidation reaction within the working voltage range.It forms a stable pyramid configuration with the transition metal in NCM811 through strong physical adsorption energy,thus achieving longer term stability.MDPO has excellent high voltage performance,which significantly improves the comprehensive performance of nickel-rich ternary cells.The physical characterization results also proved that MDPO could effectively inhibit the transition metal dissolution in NCM811materials and the oxidative decomposition of electrolyte.In addition,the introduction of MDPO can also improve the thermal stability and flame retardancy of the electrolyte,thus improving the cells safety performance.In general,MDPO stabilizes the cathode/electrolyte interface in a non-sacrificial manner,improving battery performance and avoiding the failure mechanism of traditional additives to oxidize into films by themselves.This design idea provides a new way to search for new electrolyte additives. |
