Electrolyte Dependency on Ca<sup>2+</sup> Insertion and Extraction Properties of V<sub>2</sub>O<sub>5</sub>
Yoshiaki Murata, Ryoji Inada, Yoji Sakurai
Abstract
The influence of the electrolyte structure on the Ca 2+ ion insertion and extraction properties of V 2 O 5 was studied by changing the electrolyte concentration or solvent. The electrolyte structure was analyzed by Raman spectroscopy. The most significant changes were found in the molar ratio of the contact ion pair (CIP) in the total ionic species. Among the various electrolytes, 0.3 M Ca(TFSI) 2 dissolved in ethylene carbonate and dimethyl carbonate (0.3 M EC:DMC) and 0.5 M Ca(TFSI) 2 dissolved in triglyme (0.5 M G3) have relatively small molar ratios of CIPs. The electrochemical performance was strongly related to the molar ratio of the CIPs. A high coulombic efficiency and high capacity were observed when using 0.3 M EC–DMC. Moreover, 0.5 M G3 showed the highest capacity despite its low coulombic efficiency. This could be related to the formation of solvent-separated ion pairs (SSIPs) due to the low polarity of G3 and its solvation form that encapsulates Ca 2+ . SSIPs had a reductively unstable character as that of the CIPs. Surface analysis revealed that the thinner the surface film produced, the lower the CIP content. This was deemed responsible for the rate performance enhancements, given the potential electrochemical instability of the Ca-containing CIPs.