Safety, specifically flammability, is still an important concern in lithium-ion batteries, especially as they are now widely used in higher-power and energy applications, such as electric-load leveling and electric vehicle systems. Organophosphorus compounds, such as phosphates, phosphonates, and phosphazenes, are widely known as excellent flame retardants. In our previous research, an ethylene carbonate (EC)-based solution including 50vol% trimethyl phosphate (TMP) self-extinguished after ignition in air. However, TMP has a higher donor number than EC and tends to co-intercalate with lithium ions into the graphite negative electrode, resulting in continuous decomposition and large irreversible capacity loss during the initial cycle [1]. On the other hand, Takeuchi et al. have improved the compatibility of TMP with graphite by introducing a salt composed of a stronger Lewis acid than Li+, such as Ca2+[2].
In this study, we verified the electrochemical stability in a self-extinguishing electrolyte solutions containing TMP or dimethyl methylphosphonate (DMMP) with Lewis acids toward practical use of electrolyte solutions in lithium-ion batteries. The simple addition of Lewis acids is a unique method to improve electrochemical properties of graphite negative electrode. Under the addition of stronger Lewis acids than Li+, the interactions between Li+ and EC or Li+ and TMP should be varied. However, there has been no systematically studied. Therefore, we have conducted NMR studies to evaluate the interactions in the electrolyte solutions containing flame retardant solvents with stronger Lewis acids of Mg2+, Ca2+ and weaker Lewis acids of Na+, K+ than Li+. Here we report firstly how interactions between Li+and EC affect the first coulombic efficiencies of graphite electrode and also show the electrochemical properties of graphite electrode in the above electrolyte solutions.
Results and Discussion
Figure 1 shows the relationship between the coulombic efficiency in charge-discharge at constant current of 0.1 mA cm-2 in a graphite/Li metal half cell and the chemical shifts of 17O nucleus of carbonyl group C=O in EC in 1 mol dm-3 LiPF6 dissolved EC+EMC (1:2, v/v%) and EC+EMC+TMP (1:2:3, v/v/v%) containing no additives, 0.5 mol dm-3 Ca(TFSA)2, Mg(TFSA)2, NaTFSA, and KTFSA. The values of the chemical shift in Fig. 1 are the amount of the change from the chemical shift of 1 mol dm-3 LiPF6 EC+EMC (1:2, v/v%) (triangle). TMP has a higher donor number than EC. Therefore, when TMP was added in the EC+EMC electrolyte solution, the strength of EC-Li+ interaction decreased due to selective solvation of TMP with Li+. As a result, the chemical shift in the electrolyte solution containing TMP “No add.” (cubic) is 11.5 ppm larger than that not containing TMP (triangle).
By adding 0.5 mol dm-3 Ca(TFSA)2, Mg(TFSA)2, and NaTFSA, however, the chemical shift in EC moves in the direction approaching the EC+EMC electrolyte solution (triangle). Therefore, the coulombic efficiencies are improved due to suppression of the co-intercalation of TMP with Li+ into the graphite anode because interaction between EC and Li+ is re-enhanced by the added additives. The less effectiveness to coulombic efficiency by adding of NaTFSA derived from the lower Lewis acidity of Na+ than Li+. Based on these results, the electrolyte solution added KTFSA consisting of K+ which is a weaker Lewis acid than Na+ was not expected to improve the coulombic efficiency. Although, the chemical shift of KTFSA moved less than that of NaTFSA in the direction approaching the EC+EMC electrolyte solution (triangle) as we expected, the coulombic efficiency was more improved than that of Ca(TFSA)2 or Mg(TFSA)2. In Fig. 1, we considered that this improvement in the electrolyte solution added KTFSA was not derived from the solvation of Li+, which was altered by the Lewis acid K+. Therefore, the factor derived from this improvement was elucidated by analysis of the electrode after charging or discharging in an electrolyte solution. Furthermore, we will reveal a roll of each Lewis acid to electrochemical stability of self-extinguishing electrolyte solutions in this presentation
References
[1] H. Nakagawa, M. Ochida, Y. Domi, T. Doi., S. Tsubouchi, T. Yamanaka., T. Abe, and Z. Ogumi, J. Power Sources., 212,148 (2012).
[2] S. Takeuchi, S. Yano, T. Fukutsuka, K. Miyazaki, and T. Abe, J. Electrochem. Soc., 159, A2089 (2012).