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 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 <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 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.

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Chemical components of surface films of deposited lithium on nickel substrates in electrolytes with LiN (SO 2 CF 3 ) 2 ) ͑LiTFSI͒, LiN (SO 2 C 2 F 5 ) 2 ͑LiBETI͒, LiPF 6 solutes, and tetrahydrofuran solvents were characterized by Fourier-transform infrared, twodimensional nuclear magnetic resonance ͑2D NMR͒, X-ray photoelectron spectroscopy, evolved gas analysis, and ion chromatograph in order to understand the electrochemical performance of lithium imide/cyclic ether-based electrolytes. The top layers of the surface film were ROCO 2 Li, Li 2 CO 3 , polymer constituents, and LiF. The inner layers of the surface film consisted of Li 2 O and carbide species. In imide/cyclic ether-based electrolytes, Li 2 S 2 O 4 and Li 2 SO 3 as outer layers, and Li 2 S as the inner layer were formed on a nickel substrate as reductive constituents of imide solute. We found that organic surface layers consisted of lithium etoxides, lithium ethylene dicarbonate (CH 2 OCO 2 Li) 2 , polyethylene oxide, and lithium ethylene dicarbonate containing an oxyethylene unit by 1 H, 13 C, and 2D NMR. Li cycling efficiency affects not only the deposited lithium morphology but also chemical components.

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In order to improve the cycling performance of lithium-ion batteries with nonflammable trimethyl phosphate ͑TMP͒-based electrolytes, amorphous carbon ͑AC͒ was used as the anode material. It was found that the reduction decomposition of TMP solvent, which occurred without limit on a natural graphite anode and concomitantly generated a large amount of methane (CH 4 ) and ethylene (C 2 H 4 ) gases, was considerably suppressed on amorphous carbon anode. This improvement was attributed to the disordered structure of amorphous carbon, which hindered the cointercalation of TMP solvent. The charge/discharge result and cyclic voltammetry further disclosed that a highly stable and passivating surface film, called the solid electrolyte interphase film, was formed on the AC surface at the potential near 1 V. As a result, an AC/LiCoO 2 ion cell with 1 mol/dm 3 LiPF 6 /ethylene carbonate ͑EC͒ ϩ propylene carbonate ͑PC͒ ϩ diethylcarbonate ͑DEC͒ ϩ TMP ͑30:30:20:20͒ nonflammable electrolyte exhibited promising cycling performance. Furthermore, this electrolyte was also found to have good low-temperature performance with the freezing point of ϽϪ40°C. Thermal test results disclosed that a lithium-ion cell with 1 mol/dm 3 LiPF 6 /EC ϩ PC ϩ DEC ϩ TMP ͑30:30:20:20͒ exhibited good thermal stability.