Electrolyte Dependency on Ca2+ Insertion/Extraction Properties of V2O5
Authors: Murata Yoshiaki , Obata Tomohiro , Hamasaki Masashi , Inada Ryoji , Sakurai Yoji 
Published: February 2020

Recently, calcium ion batteries (CIBs) have been attracting attention as next-generation batteries because of their higher energy density, higher safety, lower costs and more abundant elements than lithium ion batteries (LIBs). On the other hand, CIBs have several problems impeding their progress. One critical issue is the development of a suitable organic electrolyte without a negative impact on the electrochemical properties of the electrode material. In our past study, we reported that the overvoltage decreased when water was added to the electrolyte1. One reason for the decrease of overvoltage is a change in the electrolyte structure such as the solvation structure and the degree of salt dissociation. In this study, we investigated the electrolyte structure with varied organic solvent species and salt concentration, and examined the electrolyte dependency on Ca2+ insertion and extraction properties of V2O5.
We used Ca(TFSI)2 as the electrolyte salt, which was vacuum dried at 150℃ for 12h before use. For the organic solvents, EC: DMC (1:1 in vol.), EC: PC (1:1 in vol.), triglyme (G3) and acetonitrile (AN) were used. The concentration of electrolyte was adjusted to 0.2-0.5 molL-1.V2O5 nano-sheet synthesized by hydrothermal-assisted method2 was used as the working electrode. The electrode was prepared by mixing the synthesized V2O5, acetylene black and polyvinylidene difluoride dissolved in N-methyl-2pyrrolidinone with a weight ratio of 70:20:10, then coated onto a carbon coated titanium foil and vacuum dried at 120℃ overnight. A constant current discharge-charge experiment was performed at 0.05C-1C.
The results of electrolyte structure analysis showed that the contact ion pair (CIP) decreased as the dipole moment of the organic solvent decreased except for G3. When G3 was used, it is considered that the solvent-separated ion pair (SSIP) was formed because of the small dipole moment of G3. Interestingly, a decrease in the amount of CIP due to the decrease of concentration was observed when using EC: DMC or EC: PC, but not when using G3 or AN.
The results of the constant current discharge-charge experiment were strongly related to the amount of CIP. The coulombic efficiency tended to improve as the amount of CIP decreased except for using G3, in particular, a high coulombic efficiency of 95% was shown when using 0.3M Ca(TFSI)2/EC: DMC. As a factor of the instability of electrolyte when G3 was used, Ca2+(G3)TFSI- SSIP may also be reductively unstable as Ca2+TFSI- CIP. On the other hand, G3-based electrolyte showed the lowest overvoltage and the best rate performance. According to the surface analysis of the electrode after the test, the concentration of calcium in the surface film was less than that for other solvents. It suggests that the nature of the surface film may be related to a decrease in the overvoltage. When a solvent other than G3 was used, the overvoltage decreased as the amount of CIP decreased, and a relatively high rate performance was obtained for 0.3 molL-1 Ca(TFSI)2/EC: DMC, similar to the tendency of coulombic efficiency.

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[1] Y. Murata et al., Electrochim. Acta 294 (2019) 210-216.
[2] H. Song et al., J. Power Sources 294 (2015) 1-7.