A Study On Fluorinated Carbonates As Electrolyte Solvent For High-Voltage Lithium-ion Batteries

To prolong the using life of mobile devices and extend the cruising range of electric vehicles,lithium-ion batteries are moving towards higher energy density.In high energy density batteries,cathode materials should have high charge-discharge voltage and high specific capacity.The application of new materials will bring up a electrolyte-matching problem.Working at high voltage,conventional carbonate electrolyte can continually decompose so that large quantities of gas and heat release,which could cause some security problems.Therefore,the development of new.type electrolyte systems is getting increasingly urgent.Herein,a fluoride,Ethyl-(2,2,2-trifluoroethyl)carbonate(ETFEC),was selected as the research object ETFEC is synthesized independently,then it is used as a co-solvent to study the effect of fluoride solvent content on the electrochemical properties of different electrolyte systems.In addition,we explore the reasons why the single fluoride solvent system has poor performance,and optimize the electrolyte by using additives.Finally,combined with the component of interphase,the reaction mechanism of linear carbonates was discussed.ETFEC was prepared by the reaction between fluorinated ethanol and ethyl chloroformate.Theoretical calculation and electrochemical test results show that mixing ETFEC in EC/DEC system will improve the oxidation potential of the electrolyte and alleviate the self-discharge phenomenon in cells.Electrochemical behavior of ETFEC as co-solvent in Li/LNMO half cells has been studied,it is found that adding too much ETFEC does nct improve the cycle stability,but tends to accelerate performance degradation.Therefore,the ETFEC addition was controlled at 10%in follow-up experiments.High temperature can exacerbate the oxidation reaction of electrolyte,so the benefits of ETFEC are becoming more obvious.The v high temperature cycle performance of the electrode material with 10%ETFEC electrolyte is enhanced significantly,with the capacity retention improved from 65.3%to 77.1%after 200 cycles.Electrochemical impedance spectroscopy(EIS)and X-ray photoelectron spectroscopy(XPS)show that base electrolyte is oxidized to produced much LiZCO3,and then increase electrochemical impedance of interfacial film.The interfacial morphology is observed by scanning electron microscope(SEM)and transmission electron microscope(TEM).In terms of the interfacial film thickness,we conclude that more electrolyte is involved in the oxidation reactions in baseline group.To explore the possibility of perfluorinated system applications,we used chain ETFEC as a single solvent to prepare electrolyte,but the test results showed that the electrolyte performance was not even as good as the reference electrolyte.When EC or DTD 1s added into base electrolyte(pure ETFEC)as additives,the experimental results show that the cycle performance of LNMO material can be greatly improved in single ETFEC-solvent system with 2 wt%ethylene carbonate(EC)or 2 wt%ethylene sulfate(DTD).i.e.The capacity retention of the materials is 93%after 300 cycles.EIS,TEM and XPS results showed that ETFEC can not form a stable cathode/electrolyte interphase(CEI)film on the surface of LNMO cathode.In contrast,these additives can be oxidized on the surface of LNMO particles and help to the form an interfacial film.This composite CEI film plays a crucial role in suppressing the serious decomposition of the electrolyte at high voltage.Finally,we discuss the oxidation decomposition mechanism of chain organic carbonate:Oxidation reaction falls into two steps,accompanied by an "asymmetric"rupture process.In Li/LNMO half cells,DEC is easily completely oxidized and the rupture of DEC molecule is in the nature of asymmetry.ETFEC tends to occur incomplete oxidation in the presence of the interfacial fllm,and the chemical bond only ruptures on the side lacking-CF3.