Effect And Mechanism Of Fluorinated Solvents On Electrochemical Performance Of 4.5 V LiNi_0.5Co_0.2Mn_0.3O₂/ Artificial Graphite Cells

Traditional carbonate electrolyte is prone to oxidation and decomposition under high voltage,which leads to the rapid decay of battery capacity and even results in the thermal runaway of battery,which seriously restricts the practical application of high voltage lithium ion battery.Therefore,it is necessary to develop electrolyte matching with high voltage cathode materials to improve the energy density of lithium ion batteries.In this paper,4.5 V Li Ni_0.5Co_0.2Mn_0.3O₂/artificial graphite pouch battery is taken as the research object.the effects of fluorinated cyclic carbonate,fluorinated linear carbonate and fluorinated linear ether solvents on the electrochemical performance of the battery under high voltage conditions are systematically studied.Combined with electrochemical impedance spectroscopy（EIS）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,X-ray diffraction（XRD）,liquid/gas chromatogrphy mass spectrometry（LC/GC-MS）,inductively coupled plasma emission spectrometry（ICP-OES）and other characterization methods,the mechanisms of fluorinated solvents were analyzed.It provides reference for the development of high voltage electrolyte.This work used 4-fluoro-1,3-dioxolan-2-one（FEC）and di-fluoro ethylene carbonate（DFEC）as co-solvent to replace ethylene carbonate（EC）and the effects of FEC and DFEC on the rate performance and high temperature cycle performance of Li Ni_0.5Co_0.2Mn_0.3O₂/artificial graphite pouch batteries were investigated.Theoretical calculation and linear sweep voltammetry（LSV）show that FEC and DFEC can obviously improve the oxidation stability of electrolyte after replacing EC.The electrochemical test results show that after 500 cycles between 3.0～4.5 V at 1 C at 45℃,the discharge capacity and capacity retention rate of Li Ni_0.5Co_0.2Mn_0.3O₂/artificial graphite pouch battery in EC electrolyte are 558.2 m Ah and31.6%,respectively,which are 1395.6 m Ah,77.8%and 1431.7 m Ah,82.8%with FEC and DFEC electrolyte,respectively.Then,the mechanisms of FEC and DFEC in the battery were analyzed,and the passivation films formed by FEC and DFEC on the surfaces of anode and cathode materials could effectively inhibit the side reaction of electrolyte,reduce the damage of electrolyte to the structure and interface of cathode and anode materials,and then improve the high temperature cycling performance of the battery.In order to further improve the performance of 4.5 V Li Ni_0.5Co_0.2Mn_0.3O₂/artificial graphite pouch battery,the effects of substituting diethyl carbonate（DEC）for bis（2,2,2-trifluoroethyl）carbonate（TFEC）and methyl（2,2,2-trifluoroethyl）carbonate（MTFEC）on battery performance were investigated based on FEC electrolyte.Through theoretical calculation and LSV results,it is found that TFEC and MTFEC can further improve the oxidation stability of the electrolyte after replacing DEC.The test results show that after 700cycles at 45℃at 1 C,the discharge capacity and capacity retention rate of Li Ni_0.5Co_0.2Mn_0.3O₂/artificial graphite pouch battery in DEC electrolyte are 798.9 m Ah and45.5%,which are are 1262.3 m Ah,72.5%and 1442.8 m Ah,81.6%in TFEC and MTFEC electrolyte respectively.The results show that TFEC and MTFEC can participate in the formation of passivation films on anode and cathode surfaces,inhibit the decomposition of electrolyte at anode and cathode interfaces and increase of battery impedance,and effectively protect the stability of electrode-electrolyte interfaces and the stability of anode and cathode material structure.Thus improving the high temperature cycle stability of the battery under high voltage.Based on the advantages of low viscosity of ether solvents,this paper also discusses that the effects of partial replacement of DEC by tetrafluoroethyl tetrafluoropropyl ether（TFETFPE）or trifluoroethyl tetrafluoroethyl ether（TFETFEE）on the performance of 4.5 V Li Ni_0.5Co_0.2Mn_0.3O₂/artificial graphite pouch battery on the basis of FEC by cycle testing,EIS and non-in-situ physical characterization.The results show that the discharge capacity and capacity retention rate of battery in DEC electrolyte are 357.2 m Ah and 21.2%,respectively,after 900 cycles at 45℃at 1 C,while the discharge capacity and capacity retention rate in TFETFPE and TFETFEE electrolyte are increased to 1230.1 m Ah,72.2%and 1300.1 m Ah and77.4%.At the same time,SEM,XRD,ICP and other analysis methods confirm that the introduction of TFETFPE and TFETFEE can form an interface film with better thermal stability on the cathode surface,and effectively inhibit the decomposition of electrolyte and damage to the structure of anode and cathode materials under high voltage,then increasing the electrochemical performance of high voltage Li Ni_0.5Co_0.2Mn_0.3O₂/artificial graphite pouch battery.