Thermal Behavior of Charged Graphite and Li x CoO2 in Electrolytes Containing Alkyl Phosphate for Lithium-Ion Cells

The thermal behavior of charged graphite anode and Li x CoO 2 cathode was investigated focusing on nonflammable electrolytes containing alkyl phosphates as a flame retardant by differential scanning calorimetry. Trimethyl phosphate improved the thermal stability on the cathode side, while a violent exothermic reaction occurred on the anode side. It was demonstrated that the additive technique provides important improvements electrochemically and thermally even in a unique LiBF4/γ-butyrolactone-based electrolyte containing a reductive phosphate solvent. The flame retardant concept of liquid electrolytes is intrinsically ineffective inside a cell, because the excellent flame retardant, tris(2,2,2-trifluoroethyl) phosphate did not make the anode and cathode more thermally stable.

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