Thermal and electrochemical properties of nonflammable electrolyte solutions containing fluorinated alkylphosphates for lithium-ion batteries

• Published in: Journal of power sources Vol. 332; pp. 322 - 329
• Main Authors: Todorov, Yanko Marinov, Aoki, Masahiro, Mimura, Hideyuki, Fujii, Kenta, Yoshimoto, Nobuko, Morita, Masayuki
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.11.2016
• Subjects: Fluorinated alkylphosphates; Fluoroethylene carbonate; Lithium-ion battery; Nonflammable electrolyte; Thermal safety; Lithium-ion battery; Nonflammable electrolyte; Fluorinated alkylphosphates; Fluoroethylene carbonate; Thermal safety
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2016.09.119

Nonflammable organic electrolyte solutions containing fluorinated alkylphosphates (FAP) have been examined as safer electrolytes for lithium-ion batteries (LIB). Although the ionic conductivity of LiPF6 in neat tris(2,2,2-trifluoroethyl)phosphate (TFEP) solvent is very low, it increases upon the addition of alkyl carbonates such as ethylene carbonate (EC) and fluoroethylene carbonate (4-fluoro-2-oxo-1,3-dioxolane, FEC). A specific conductivity of 1 mS cm−1 or higher was obtained at room temperature for the system containing proper amounts of the carbonates and 0.5 M (mol dm−3) LiPF6. A conventional mixed alkylcarbonate-based solution containing LiPF6 showed a sign of considerable exothermic reactions on the differential scanning calorimetry (DSC) response below 300 °C. However, the LiPF6/TFEP solution showed no significant exothermic response up to 400 °C, even in the presence of charged LiCoO2 (LCO) positive electrode. The addition of an alkylcarbonate to the LiPF6/TFEP solution produced an exothermic response as a result of the thermal decomposition of the carbonate over the charged LCO. However, the temperature at which the exothermic reaction starts was significantly higher in the system containing FEC than that containing EC. The thermal analysis results suggested that the LiPF6/FEC + TFEP combination could work as a safer electrolyte system in LIB under severe conditions. •Electrolyte solutions containing fluorinated alkylphosphates were investigated.•Ionic conductivity of LiPF6 solution was improved by addition of alkylcarbonates.•Solutions containing fluorinated alkylphosphates showed low exothermic heat.•Proper solvent compositions were proposed as safer LIB electrolyte systems.