A novel electrolyte solvent for rechargeable lithium and lithium-ion batteries

We describe a new type of electrolyte solvent which provides remarkable improvement in properties of electrolyte systems containing it. These solvents contain acidic boron atoms, in most cases linked into heterocycles by reaction of boric acid, or oxide, with glycols. Nicknamed BEG solvents (for boric acid esters of glycol), these show great propensity for dissolving salts, stabilizing alkali metals against corrosion, and in some cases, stabilizing other solvents (particularly alkene carbonates), against anodic decomposition. In the most favorable case so far, the 1,3 propylene glycol boric acid ester (BEG-1) which contains two linked borate groups, the mixed solvent formed by mixing 1 part BEG-1 with two parts ethylene carbonate provides an electrochemical stability window in excess of 5.8 V (cf. 4.5 V for ethylene carbonate alone with the same salt), an ambient temperature conductivity of 1.7 × 10^-3 S cm^-1 with 1 m LiClO₄, and enduring stability against metallic Li, which remains shiny on prolonged (days) immersion at 100°C. We present data on various electrolyte solutions containing these components and then show their utility in devices by rubberizing them with polymers and constructing voltaic cells Li/LiMn₂O₄ and C/LiMn₂O₄, which show excellent charge/discharge characteristics and cyclability.