The Research On Construction And Electrochemical Performance Of High-voltage Electrolyte System For Lithium Ion Battery

With the rapid development of high-voltage systems,research and development of high-voltage electrolyte system have become increasingly critical.Currently,commercial conventional high voltage electrolyte is mainly composed of LiPF₆ and carbonate-based solvent with oxidative decomposition voltage around 4.5 V（vs.Li/Li⁺）.However,when LiPF₆ is employ as lithium salts for lithium ion battery,it will occur self-catalytic reaction with acidic substances,and the positive electrode material will suffer from dissolution.The transition metal ion from positive electrode will be further deposited on the negative electrode layer of graphite,causing rapid capacity decay.LiDFOB,which combines the advantages of LiBOB and LiBF₄,is selected as the electrolyte lithium salt.The high-voltage electrolyte system applied in the LiNi_0.5Mn_1.5O₄ cathode system has been modified by fully taking advantage of its excellent film-forming properties either in positive material or in negative material.First,by building LiDFOB-DEC/EC/ADN electrolyte system gives in-depth study on the mechanism of the high-voltage electrolyte ADN.Compared to the additive-free systems,the capacity retention rate of the electrolyte system containing additive is up to 81.2%,the cycle efficiency is increased to 72.8%.Two conjectures about the reaction mechanism of ADN on the surface of LMNO cathode are put forward to analyze the experimental data.According to the data from XPS test,it can be conclused that ADN will have steric hindrance to reduce the decomposition of the electrolyte by enriching on the surface of the positive.The analysis of ADN decomposition products by the quantum chemistry calculation draws the conclusion that the product is so stable that hinders high valent Ni⁴⁺ to oxide the electrolyte.After that,LiDFOB-DEC/EC/ADN electrolyte system is optimized to overcome the drawback of low discharge capacity at room temperature under the effect mechanism of ADN.The introduction of sulfur-containing solvents in the system is to optimize the properties of interfacial film both on the positive and negative.Meanwhile,we also study the mechanism of the improved interfical film overcoming the easily decomposition of ADN on the negative.It also analyses the effect on the performance of sulfur-containing solvents ES,DMS and SL on the positive and negative electrodes.Finally,we conclude that the LiDFOB-ES and LiDFOB-SL are suitable system.LiDFOB-ES improve the discharge capacity of the battery on the positive electrode,which is associated with the formation of a stable interfacial film.LiDFOB-SL increases the capacity retention（72.5%）by preferential deposition on the negative electrode surface to reduce the decomposition of ADN on the negative electrode.The study proposes the ideas for the design and development of future high-voltage electrolyte.