Can an electrolyte for lithium-ion batteries be too stable?

A new differential scanning calorimeter (DSC) sample cell has allowed for the analysis of lithium-ion battery electrolytes at high temperatures. These welded stainless-steel sample tubes contain any decomposition gases that typically leak from conventional hermetically sealed DSC sample pans. Electrolytes of LiPF6 in ethylene carbonate, diethyl carbonate at various concentrations were found to be less thermally stable than electrolytes based on lithium imide-type salts. When these electrolytes were added to portions of Li0.5CoO2, the samples containing the lithium imide [including (CF3SO2)(2)N- and (C2F5SO2)(2)N-2]electrolytes were surprisingly less thermally stable than those with the LiPF6 electrolyte. In the LiPF6 electrolyte, polyethylene-oxide-like polymer forms from the reaction between the solvent and HF impurities, which are generated by the reaction of water (from the combustion of solvent by Li0.5CoO2) with LiPF6. Polymer is not produced in the imide electrolytes. The polymer deposits on the surface of the Li0.5CoO2 particles and hinders the release of oxygen, resulting in a more controlled decomposition and a more thermally stable sample. Therefore, even though the imide electrolytes are more thermally stable than LiPF6 electrolytes, in the presence of Li0.5CoO2, they react more strongly, because the polymer is absent. (C) 2002 The Electrochemical Society.