Nonflammable Trimethyl Phosphate Solvent-Containing Electrolytes for Lithium-Ion Batteries: I. Fundamental Properties

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Abstract
To develop nonflammable electrolytes for lithium-ion batteries, the fundamental properties of trimethyl phosphate (TMP)-based electrolytes with LiPF6LiPF6 as solute were investigated for natural graphite anode and LiCoO2LiCoO2 cathodes. It was found that the TMP solvent had good oxidation stability and poor reduction stability, which led to TMP reduction decomposition on the natural graphite electrode at the negative potential of 1.2 V. To solve this problem, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) cosolvents were mixed with TMP solvent. As a result, the reduction decomposition of the TMP solvent was considerably suppressed in EC+PC+TMPEC+PC+TMP and EC+DEC+TMPEC+DEC+TMP electrolytes due to the formation of good solid electrolyte interphase film on natural graphite electrode in these two mixed electrolytes. The nonflammability of the TMP electrolyte declined with mixing flammable cosolvents, which was explained by a flame retarding mechanism involving a hydrogen radical trap in the gas phase. According to this mechanism, it was deduced that the cosolvents with high boiling point and fewer hydrogen atoms were promising for nonflammability of mixed electrolytes. Furthermore, a thermal test disclosed that the thermal stability of lithium-ion cells may be improved by using TMP-containing electrolytes. © 2001 The Electrochemical Society. All rights reserved.