This paper investigated the electrochemical behavior and thermal properties of vinyl ethylene carbonate ͑VEC͒ and biphenyl ͑BP͒ additives with triphenyl phosphate ͑TPP͒-based, nonflammable electrolytes for Li-ion batteries. Mesocarbon microbeads and LiCoO 2 were used as the anode and cathode materials, respectively. The main analysis tools were cyclic voltammetry, differential scanning calorimetry, electrochemical impedance spectroscopy, and scanning electron microscopy. The results showed that the oxidizing potential of VEC and BP is about 4.9 and 4.6 V vs Li/Li + , respectively, in TPP-containing electrolytes. Consequently, we found that the addition of 0.1 wt % BP to TPP-based electrolytes improved the cell performance and thermal stability of electrolytes for Li-ion batteries.Li-ion batteries are widely used as power sources for various electronic devices because of their high energy density, long cycle life, and low self-discharge properties compared to the other rechargeable batteries. Today, small-capacity Li-ion batteries have been applied to cellular phones, digital cameras, and notebook computers. At the same time, the application of Li-ion batteries is expanding to vehicles and other large-scale power systems. However, large-size Li-ion batteries are as yet commercially unavailable, primarily due to the associated safety problem. 1-4 Electrolytes for Liion batteries typically are carbonate-based and flammable. When the internal pressure of the battery increases and the battery is mechanically destroyed, the electrolyte could cause dangerous situations such as leakage, fire, and explosion. 5 Many researchers have recently been focusing on the development of flame-retardant additives to lower the flammability of the liquid electrolytes, with some success. 6-8 However, the reduction in flammability with flameretardant additions has only been achieved at the expense of other indexes of cell performances such as ionic conductivity of the electrolyte and reversibility of the cell. 9 To improve the performance of Li-ion cells, various functional additives, such as vinyl ethylene carbonates ͑VEC͒, 10-13 biphenyl ͑BP͒, 4,14 vinylene carbonate ͑VC͒, 15,16 vinyl acetate ͑VA͒, 17,18 ethylene sulfite ͑ES͒, 19,20 and propylene sulfite ͑PS͒, 21 have been investigated. Wang et al. 10 reported that the addition of VEC into propylene carbonate-based electrolyte helped to form a compact solid electrolyte interface ͑SEI͒ layer on the graphite anode during the first discharge process. Hu et al. 11 reported that the addition of 5 vol % VEC produced good cycleability because of the formation of a stable SEI film. Abe et al. 14 reported that BP additive leads to the formation of a thin surface layer ͑electro-conducting membrane͒ which protects further electrolyte decomposition on the cathode active sites.In the present work, we studied the influence of two functional electrolyte additives, VEC and BP, on Li-ion cell performance in a electrolyte of 1.1 M LiPF 6 dissolved in ethylene carbonate and ethyl-methyl carbonate ͑EC/EM...
