To develop nonflammable electrolytes for lithium-ion batteries. the fundamental 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 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 <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 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 hi-fi 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.