Study On Some Kinds Of Electrolytes And Their Additives Performance For Lithium-ion Batteries

Use of some non-storing energy materials is one of the most economic and effective methods for the improvement of Li-ion battery performance, which is called electrolyte additive. Organic electrolyte additives have remarkable characteristics as fewer doses, but more efficiency. With amount of an additive in the electrolyte is no more than 5% by weight or by volume while its presence significantly improves the cells capacity and cycle life of Li-ion batteries as well as improves some macroscopic performances without increasing costs. By electrochemical mechanism of non-aqueous electrolyte additives, electrolyte additives are divided as film-forming additive, conductive additive, flame-retarding (FR) additive, overcharge-protecting additive and so on.In this paper, we introduced three new additives tetraethyloxysilane (TEOS), tetrapropyloxysilane (TPOS) and tetramethyloxysilane (TMOS) which have not been reported before, and studied the influences on the different lithium-ion batteries (LiMn2O4/Li, LiCoO2/Li and LiFePO4/Li). Besides, we compared electrochemical performance with two different electrolytes.This paper including four parts:1. Research on the influence on film-forming additive TEOS and LiMn2O4, LiCoO2 and LiFePO4 battery. Many kinds of analyses methods, such as the charge/discharge test, CV, EIS and SEM have been used to study the influence of the different electrolytes on the battery performance. The result indicated because informed the suitable SEI film, when 5% TEOS was mixed into the electrolyte, the discharge efficiency and cycleability LiMn2O4 of LiCoO2 and have been improved obviously. Especially with LiMn2O4 battery, the initial discharge capacity reached to 115.9 mAh/g, and it lost only 3.01% after 50 cycles. Besides, we found that TEOS can improve the electrochemical performance of LiMn2O4 at low temperature.2. Research on the influence of FR additive TPOS and LiMn2O4, LiCoO2 and LiFePO4 battery. We found that mixed 5% TPOS, the decomposed temperature of electrolyte was lifted from 151.8℃to 165.8℃by DSC, which showed that the thermal stability with TPOS was better than that without it. The charge/discharge test, CV, EIS and SEM have been used to study the influence of the different electrolytes on the battery performance. The result indicated that when 5% TPOS mixed into the electrolyte, the discharge efficiency and cycleability of LiMn2O4 and LiCoO2 batteries have not been improved. However, the performance of LiFePO4 was been improved with TPOS. The initial capacity was 128.9 mAh/g and charge-discharge 50 times it was 139.3 mAh/g, the initial capacity, the cycleability and cycle life were improved. The ruslut shows that TPOS fitted for LiFePO4 battery.3. Research on the compatibility of FR additive TMOS and LiMn2O4, LiCoO2 and LiFePO4 battery. The decomposed temperature of electrolyte was lifted from 151.8℃to 199.8℃with TMOS by DSC, which indicated that TMOS is an effective FR additive as the thermal stability with TPOS was better than that without it. The charge/discharge test, CV and EIS have been used to study the influence of the different electrolyte on the battery performance. The result indicated that TMOS has better FR effectiveness but it can not improve the electrochemistry performance.4. The comparison of electrochemical performance between LiPF6-EC/DEC and LiBF4–EC/DEC. Made use of the conductivity meter, charge/discharge test, CV and EIS in room temperature (RT) we discovered that because of the high conductivity in RT, the cycleability and cycle life of LiPF6 were both better than LiBF4. However, we found that LiBF4 is more stable in high temperature than LiPF6 by DSC. When two lithium salts were dissolved into the EC/DEC, the low temperature test indicated that the curve of LiBF4 was smoother and the discharge voltage was higher than LiPF6. All of them meant that the LiBF4 was more suitable than LiPF6 to be used in extreme environment.