Application Research Of Fluoroethylene Carbonate In The New Type Lithium Ion Battery Materials

According to China's battery technology roadmap,the development of high capacity Li-rich manganese based solid solution/silicon carbon(OLOIISi/C)system battery is an important direction to meet the demands of battery with high specific energy.Because of the volume effect of silicon during charge/discharge process,the toughness of SEI film formed by traditional carbonate-based electrolyte on the silicon anode surface is not strong enough,and this has lead to a serious decline in the performance of the electrode.Li-rich manganese based solid solution have a high capacity advantage under high voltage(?4.5V),but traditional carbonate solvent can be severely oxidized under this voltage conditions,leading to sever electrochemical performance degradation of Li-rich manganese based electrode and battery.Therefore,the development of high voltage electrolyte matching with the OLOIISi/C system is the key to promote the development of the battery.In this paper,in order to improve the cycle stability of OLOIISi/C battery system,the influence of fluoroethylene carbonate(FEC)as the electrolyte solvent component to the Li-rich manganese based solid solution materials,silicon carbon composite materials,and the OLOISi/C battery system has been studied and the functional electrolyte for silicon carbon materials and high voltage cathode materials has been developed.The following is the main research contents and results:1M LiPF6/EC-DEC-DMC(1:1:1 wt%)was used as the fundamental electrolyte component,and EC was gradually replaced by FEC to prepare a series of electrolytes.(1)Effect of electrolyte substituted by different amount FEC on the electrochemical performance and electrode characteristics of silicon carbon materials was studied.The experimental results show FEC can greatly improve the cycle performance of Si/C.And for the impact on specific capacity of Si/C anode,the first specific capacity of Si/C anode firstly increased and then decreased with the increasing amount of FEC substitution.Therefore there is a threshold for the amount of FEC substitution,when the proportion of FEC was 13.89%,the Si/C anode specific capacity reached the maximum value,which is 738.9mAh/g,and simulataneously the cycle performance is better.The capacity retention rate is 57.8%and has been improved 27.8%compared with the baseline electrolyte after 150 cylces.The SEM and XPS results suggested that FEC is beneficial to the formation of a thin and stable SEI film on silicon based anode,restraining the decomposition of carbonate solvents.On the basis of the 13.89%FEC substitution amount,the effect of electrochemical performance of silicon based anode was tested by adding SA,EMITFSI,TMSB and LiBOB these four kinds of additives.The results showed that the first specific capacity of silicon based anode declined with the addition of LiBOB but improved when adding SA,EMITFSI and TMSB.In addition,the cycle stability is best by adding SA for the Si/C anode and the capacity retention rate is 61.39%after 100 cylces.Therefore,the effect is the best for the Si/C anode by adding SA in the 13.89%FEC electrolyte.(2)Effect of electrolyte substituted by different amount FEC on the electrochemical performance and electrode characteristics of Li-rich manganese based solid solution materials was studied.Research shows that electrolyte oxidation decomposition voltage is higher with the increasing addition of FEC.And the electrolyte oxidation potential was improved to 4.7V,when the proportion of FEC is 33.33%.At the same time,the electrochemical performance of OLO electrode has a positive correlation with the FEC substituted amount.As the FEC ratio increases,the cycle stability of the OLO cathode is better,and the discharge medium voltage is higher,and the polarization is smaller.Therefore,when the propotion of FEC is 33.33%(EC was totally replaced by FEC),the OLO has the best electrochemical performance.The dischanrge capacity and capacity retention rate are 200.5 mAh/g and 85.72%respectively after 100 cyclces and the discharge capacity decay down to 80 mAh/g in baseline electrolyte after 60 cyclces.However,the substitution by FEC has little effect on the first specific capacity and coulombic efficiency of the OLO cathode.XRD results display that the addition of FEC can mitigate the changes of OLO structure during cycling,and can reduce the formation of spinel phase.The effect is remarkable when the proportion of FEC is 33.33%.Therefore,FEC can be used as the high voltage electrolyte solvent to improve the cycle performance of Li-rich manganese based solid solution.On the basis of the 33.33%FEC substitution amount,PS,TMSB,EMITFSI and LiDFOB this four kinds of additives was added and the results show the first discharge specific capacity of the OLO cathode have been improved,and EMITFSI has the most comprehensive effect.And the capacity retention rate of the OLO anthode is 97.09%after 100 cycles.(3)The above experimental results show that the best proportion of FEC is 33.33%for OLO materialsthe,and 13.89%for Si/C materials.Therefore the proportion of FEC through the range of 13.89%?33.33%is studied on the effect of the electrochemical performance of OLOIISi/C battery.With the increase of the substitution amount of FEC,the cycle stability has been improved,and the decay of discharge medium voltage has been slowed down.The cycle stability of full cell has been increased most significantly when the proportion of FEC is 33.33%.The capacity retention rate is 66.38%and which has been improved 25.96%campared with the electrolyte that the proportion of FEC is 13.89%after 100 cylces.