The influence of lithium salt on the interfacial reactions controlling the thermal stability of graphite anodes

The thermal stability of graphite anodes used in Li-ion batteries has been investigated, with the influence of electrolyte salt under special scrutiny. LiPF6, LiBF4, LiCF3SO3 and LiN(SO2CF3)(2) in an ethylene carbonate (EC)/dimethyl carbonate (DMC) solvent mixture. Differential scanning calorimetry (DSC) showed exothermic reactions in the temperature range 60-200 degreesC for all electrolyte systems. The reactions were coupled to decomposition of the solid electrolyte interphase (SEI) and reactions involving intercalated lithium. The onset temperature of the exothermic reactions increased with type of salt in the order: LiBF4 < LiPF6 < LiCF3SO3 < LiN(SO2CF3)(2). X-ray photoelectron spectroscopy (XPS) was used to identify surface species formed prior to and after the exothermic reactions, to clarify different thermal behaviour for different salts. The decomposed SEI's in LiCF3SO3 and LiN(SO2CF3)(2) electrolytes were found to be mainly solvent-based. including lithium alkyl carbonate decomposition to stable Li2CO3 and the formation of poly(ethylene oxide) (PEO)-type polymers. In the LiBF4 and LiPF6 systems, decomposition was governed by salt reactions, which decomposed the salts and resulted in the main product LiF. (C) 2002 Elsevier Science Ltd. All rights reserved.