Research On The Electrolyte Designed For Lithium-ion Batteries With Wide Operating Temperature Range

Today the application of lithium-ion battery is significantly limited by the operating temperature range. In EV and military fields, based on GJB, batteries could work stably at-40~55 ℃, but current lithium-ion batteries could not meet this requirement. In this paper, the wide temperature performance means in the range of-55~60 ℃, the battery could show a capacity and voltage platform close to the room temperature, while maintaining an excellent cycling performance.Electrolyte optimization is the most feasible and economical way to broaden the operating temperature range. In this paper, electrolyte bulk, compatibilities of electrolyte/ graphite anode and electrolyte/ lithium cobalt oxide cathode are studied. The electrochemical stability, solvated structure, conductive ability of electrolyte, the surface chemistry on negative electrode and positive electrode are focused on. By analyzing the relationship between chemical structure and the electrochemical properties, the mechanism of each added component is discussed, and then the optimized chemical structure and addition are observed. Finally, the physical properties and electrochemical properties of new optimized electrolyte, consisting of carbonated-based electrolyte and additional components, is analyzed over a wide temperature range(-55~ 60 ℃). Its charge-discharge performances are tested by lithium cobalt oxide- graphite batteries.In lithium hexafluorophosphate/carbonate-based electrolyte, lithium salt tends to be dissociated to free ions in effect of ethylene carbonate. If the addition of fluorinated ester was below 25 vol%, this co-solvent would only exist in the electrolyte bulk and not participate in the solvation process of the lithium ions. The ionic conductivity of the electrolyte depends on the viscosity. Fluorinated ester with long carbon chain, branched chain, and fluoro-substitution in acyl groups is not available to reduce viscosity, improve conductivity. Fluoroalkyl esters with the number of carbon atoms about 6~8 have suitable physical properties, and these co-solvents could significantly improve low-temperature conductivity of the electrolyte. Electrolyte with 25 vol% co-solvent shows the smallest loss of conductivity at low temperature.Due to the higher reduction potential than that of carbonate solvents, fluorinated ester is prior to be reducted to lithium carboxylate. This reduction product can be arranged on the graphite sheet surface and form a thin and uniform layer, which could prevent the continuous reduction of electrolyte. This plays an important role on the low temperature performance and cycling stability of lithium-ion batteries because of the full protection and high conductivity. Fluorinated ester with longer carbon chain provides better low-temperature modifying effect. In electrolyte containing 25 vol% of trifluoroethyl n-caproate, delithiated capacity retention maintains at 92 %, which is the highest value reported so far as we know. During the temperature range of 25~-55 ℃, activation energy of lithium ion transmission in this modified electrolyte is reduced by about 8 k J·mol-1, compared to the data in base-line electrolyte.The positive electrode and the electrolyte have good compatibility at room temperature; but the compatibility at elevated temperature significantly reduced for the accelerated electrochemical oxidation reaction of carbonates. The cathode/electrolyte compatibility at elevated temperature could be enhanced by adding a tiny amount of grafting polysilane additives. These additives could be exhausted and removed from electrolyte at the end of the film-forming process on positive electrode, to avoid the conflicting effect between low temperature and high temperature components. After adding grafting polysilane additive to electrolyte, cycling stability at a high temperature of the positive electrode could be markedly increased. When low-substituted polysilane is used, oxidation crosslinking of silicon hydrogen bond is the main film-formation mechanism, which has a significant negative effect on rate performance; however, grafting polysilane with high substitution forms a layer on positive electrode mainly by oxidative polymerization in carbonate substituents. With the addition of 0.5 wt% to the base electrolyte, lithium cobalt oxide cathode shows an outstanding cycling stability and high-rate performance. At 60 ℃, nearly 90 % of the capacity retention is delivered after 60 cycles; while there is little compromise on the rate performance at 60 ℃. The capacity retention at 1 C rate can reach over 90 % of that at 0.2 C rate.Lithium cobalt oxide- graphite cells using electrolyte containing 17 vol% trifluoroethyl n-caproate and 0.5 wt% grafting polysilane additive are evaluated comprehensively over wide temperature range. In this modified electrolyte, there is no component with boiling point below 110 ℃ or flashpoint below 25 ℃, which are available to safe use at high temperatures. At-55 ℃, the discharge capacity retention can reach 58 % of that at room temperature, and at 60 ℃, nearly 75 % of the capacity retention is delivered after 30 charge-discharge cycles. The wide temperature performance is superior to the commercial power-type electrolyte and low temperature electrolyte. These results basically achieve the aim that broadening the operating temperature of electrolyte. The properties of the electrolyte bulk(such as melting point, boiling point, oxidation/reduction stability, and ionic conductivity) are necessary conditions for wide operating temperature, the electrolyte/anode compatibility is the major factor in low temperature performance of the battery, and electrolyte/cathode is the key point in elevated temperature performance(≤60 ℃).