Effect of Lewis Acids on Graphite-Electrode Properties in EC-Based Electrolyte Solutions with Organophosphorus Compounds

Abstract

Compatibility of self-extinguishing property and electrochemical stability of electrolyte solutions containing organophosphorus compounds as flame retardants has been required for practical application of electrolyte solutions to lithium-ion batteries. By adding a Lewis acid, Ca2+, Mg2+, or Na+, to ethylene carbonate (EC)-based electrolyte solutions containing 50 vol.% trimethylphosphate or dimethyl methylphosphonate, the electrolyte solutions are able to suppress the co-intercalation of the organophosphorus compounds with Li+ into graphite. However, the coulombic efficiency in the first charge-discharge in a graphite/Li metal half-cell with an electrolyte solution depends on the species of the added Lewis acid and organophosphorus compound. The difference between the chemical-shift changes of the oxygen of the C=O bond in EC and the oxygen of the P=O bond in organophosphorus compounds in the oxygen-17 nuclear magnetic resonance (17O NMR) spectra of the electrolyte solutions was investigated. The investigation revealed that the difference in the acidities of the Lewis acids, electron-donating abilities of the organophosphorus compounds, and solvated structures with a Lewis acid affect the electrochemical stability of self-extinguishing electrolyte solutions.