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

摘要

Thermal stability and electrochemical behavior of fluorine compound-mixed electrolyte solutions have been investigated using 1 molL~(-1) LiClO_4-EC/DEC/PC, 1 molL~(-1) LiPF_6-EC/DMC and 1 molL~(-1) LiPF_6-EC/EMQ 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 ℃, respectively. In LiClO_4-containing electrolyte solution, lithiated graphite easily reacts with surface film and electrolyte solutions with and without fluoro-ethers, giving exothermic peaks at 153-162 ℃. However, exothermic peaks at 111-137 ℃ caused by the reaction of lithiated graphite with surface film and electrolyte solutions are weak in LiPF_6-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 dein-tercalated Li than fluoro-ether-mixed ones. Electrochemical oxidation currents are decreased by mixing of fluoro-ethers in both LiClO_4- and LiPF_6-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.