Effect of organo-fluorine compounds on the thermal stability and electrochemical properties of electrolyte solutions for lithium ion batteries

Abstract

Thermal stability and electrochemical behavior of fluorine compound-mixed electrolyte solutions have been investigated using 1 molL−1 LiClO4-EC/DEC/PC, 1 molL−1 LiPF6-EC/DMC and 1 molL−1 LiPF6-EC/EMC/PC. DSC study indicates that no exothermic reaction occurs between metallic Li and fluoro-ethers while EC/DMC mixture and fluoro-carbonate start to react with Li at 180 and 226 °C, respectively. In LiClO4-containing electrolyte solution, lithiated graphite easily reacts with surface film and electrolyte solutions with and without fluoro-ethers, giving exothermic peaks at 153–162 °C. However, exothermic peaks at 111–137 °C caused by the reaction of lithiated graphite with surface film and electrolyte solutions are weak in LiPF6-containing solutions probably because LiF contained in the surface film prevents the reaction with deintercalated Li. Fluoro-carbonate-mixed solution gives the lower reactivity with deintercalated Li than fluoro-ether-mixed ones. Electrochemical oxidation currents are decreased by mixing of fluoro-ethers in both LiClO4- and LiPF6-containing electrolyte solutions. Charge/discharge experiments indicate that first coulombic efficiencies of natural graphite are mostly high in fluoro-ether-mixed electrolyte solutions, and fluoro-ethers facilitate the formation of surface film on graphite in PC-containing solutions.