The Study On Functional Electrolyte For Lithium-Ion Batteries At A High Voltage

The Lithium-ion batteries(LIBs)are widely used in consumer electronics and will likely dominate large-scale applications for electric vehicles and energy storage systems.However,there is still greater demand for the higher energy density LIBs.One method to increase the energy density of LIBs is to increase the operating voltage of cathodes such as LiNi0.5Co0.2Mn0.3O2 and LiCoO2.Unfortunately,the electrolyte decomposition and transition metal dissolution at high voltage(? 4.4 V vs.Li/Li+)have substantially limited its further application.To solve these problems,from the two aspects of electrolyte additives and solvents,an ideal high-voltage additive and a high-voltage mixed solvent have been found through structural design and theoretical calculation,thus significantly improving the electrochemical performance of LiNi0.5Co0.2Mn0.3O2 and LiCoO2 materials at high voltages.The research content mainly includes the following two aspects:In the chapter 3,a novel strategy for selecting an additive based on understanding the influence of different functional groups on electrochemical characteristics is adopted.N,N-diethylamino trimethylsilane(DEATMS),which is functionalized with a Si-N bond,trimethylsilyl group and diethyl group,is tested as a bifunctional electrolyte additive to scavenge H2O and neutralize HF in the electrolyte and enhance the behaviour of graphite/LiNi0.5Co0.2Mn0.3O2 cells operating at a high voltage(4.5 V vs.Li/Li+)or elevated temperature(55 ?),With 2%DEATMS,no LiPF6 hydrolysis species is found after storage at 55 ? for 6 days,even with a moisture content of 2000 ppm in the electrolyte.In addition,the graphite/LiNi0.5C0O.2Mn0.3O2 cells containing DEATMS show cycling performances superior to those of the cells without DEATMS upon cycling for 100 cycles at a high voltage(85.5%vs.72.0%)or high temperature(81.2%vs.70.6%).The sacrificial oxidation of DEATMS on the cathode surface restricts direct contact between the electrolyte and cathode,improving the electrochemical performance.The exsitu surface analysis of the electrodes after cycling indicates that both the electrolyte decomposition and Mn dissolution are inhibited by the deposition of DEATMS oxidation products.In the chapter 4,a novel high-voltage electrolyte based on the guidance of theoretical calculations combined with the advantages of different solvents is prepared.1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether(HFE)which is resistant to oxidation and reduction,fluoroethylene carbonate(FEC)which is resistant to oxidation and facilitates the formation of stable SEI layer,dimethyl sulfone(MSM)which is preferentially oxidized on the cathode surface and participates in the formation of CEI layer and low-viscosity dimethyl carbonate(DMC)are chosen as solvents.It is found that the electrolyte derived from these solvents not only greatly improves the cycling performance of Li/LiCoO2 cells at 4.45 V and 4.55 V,but also is perfectly compatible with graphite anodes.At the same time,it also has a certain increase in the safety performance of the battery,not only delaying the peak temperature of main exothermic peak,but also significantly reducing maximum heat flow value.