Li-ion batteries are currently used in portable electronic and power tools, electric vehicles, and grid energy applications due to their high energy density. They have already been commercialized. However the development of higher power and capacity as well as longer life batteries becomes more pressing. One method to attain a longer life cells is to adopt electrolyte additives such as vinylene carbonate (VC) exhibiting the significant increase in lifetime [1].

Boron-based compounds have possibilities to develop advanced liquid electrolytes [2]. Xu and Angell discovered lithium bis(oxalate)borate (LiBOB), which furnished superior anodic stability up to 4.5V vs Li/Li+ [3]. The LiBOB also provided the other interactive properties for Li-ion batteries such as high stability at high temperature [4].

　In this report the effects of tri-isopropoxy-boroxine (TiPBx) as an electrolyte additive were evaluated with composition analysis. Thus 18650-type battery cells equipped with electrolytes (1.0 mol/L LiPF6 in ethylmethyl carbonate (EMC) and ethylene carbonate (EC)) with or without TiPBx were prepared and the charge-discharge tests were conducted. In fact the cell with TiPBx showed the long-life time as compared with no TiPBx where the decrease in discharged capacity was constricted. Therefore the electrolyte solutions were extracted from cycled cells and their solution compositions were detected by nuclear magnetic resonance (NMR) techniques so as to examine the impact of charge-discharge cycles and the presence of TiPBx. The electrolyte compositions analyses with 1H- and 13C-NMR indicated that composition transformations were observed after 1000 cycles without TiPBx. In particular the ester exchange reaction of EMC took place to form diethyl and dimethyl carbonates respectively. 2D HMQC NMR indicated that the ester exchange reaction of EC was also detected to afford linear polycarbonates. On the other hand, the electrolyte composition after 1000 cycles with TiPBx has almost been kept as compared with the initial composition since the ester exchange reactions were suppressed. These indicated that TiPBx prevented the ester exchange reactions giving rise to constricting the decrease in discharge capacity. We will discuss relations of the reaction mechanism of the electrolyte with the degradation of the battery performance.

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[2] H.Horino, H. Tamada, A. Kishimoto, J.Kaneko, Y. Iriyama, Y. Tanaka and T. Fujinami, J. Electrochem., Soc., 157 (6) A677 (2010).
[3] K.Xu and C.A.Angell, Electrochem. Solid-State Lett., 4, E1 (2001).
[4] K. Amine, J.Liu, S.Kang, I.Belharouak, Y, Hyung, D. Vissers and G.Henriksen, J. Power Sources., 129, 14 (2004).