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.
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.
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