Nonflammable trimethyl phosphate solvent-containing electrolytes for lithium-ion batteries - I. Fundamental properties

To develop nonflammable electrolytes for lithium-ion batteries. the fundame ntal properties of trimethyl phosphate (TMP)-based electrolytes with LiPF6 as solute were investigated for natural graphite anode and LiCoO2 cathodes. It was found that the TMP had good oxidation stability and or reduction st ability, which led to TMP reduction decomposition on the natural graphite e lectrode at the negative potential of 1.2 V. To solve this problem, ethylen e carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) cos olvents were mixed with TMP solvent. As a result, the reduction decompositi on of the TMP solvent was considerably suppressed in <10% TMP containing EC + PC + TMP and <25% TMP containing EC + DEC + TMP electrolytes due to the formation of good solid electrolyte interphase film on natural graphite ele ctrode in these two mixed electrolytes. The nonflammability of the TMP elec trolyte declined with mixing flammable cosolvents, which was explained by a flame retarding mechanism involving a hydrogen radical trap in the gas pha se. According to this mechanism, it was deduced that the cosolvents with hi -fi boiling point and fewer hydrogen atoms were promising for nonflammabili ty of mixed electrolytes. Furthermore, a thermal test disclosed that the th ermal stability of lithium-ion cells may be improved by using TMP-containin g electrolytes. (C) 2001 The Electrochemical Society.