Recently, a great deal of attention has been paid to multivalent ion batteries as post lithium ion batteries (LIBs) [1]. Among them, calcium ion batteries (CIBs) could exhibit a high cell voltage and energy density due to the low standard electrode potential of Ca/Ca²⁺, -2.87 V vs. NHE, which is comparable to that of Li/Li⁺ (-3.05V vs. NHE). In addition, CIBs have attractive advantages such as the higher safety and lower cost compared with LIBs. However, there are only a few reports in which electrodes for CIBs show a reversible calcium ion insertion/extraction. Almost a decade ago, we found that some vanadium oxides, e.g. crystalline V₂O₅ with layer structure, undergo reversible reaction with Ca²⁺[2,3].

However, a V₂O₅ cathode exhibits the large overvoltage as well as the poor rate performances due to its low electronic conductivity. In this study, we studied the effect of substituting Mo⁶⁺ for a part of V⁵⁺ in V₂O₅ in order to improve the electronic conductivity of V₂O₅. The carrier electrons are expected to be generated by the substitution of Mo in terms of the so-called valence control expressed as V⁵⁺/V⁴⁺. We synthesized a series of Mo-substituted V₂O₅, i.e. V_2-xMo_xO₅ (x=0.02, 0.06, 0.1), by solid state reaction, and evaluated their electrochemical properties in comparison with a parent V₂O₅.

XRD patterns of the synthesized V_2-xMo_xO₅ (x=0.02, 0.06, 0.1) powder all exhibited main diffraction peaks similar to those of V₂O₅ without impurity phases, indicating the solid solution formation. In addition, XPS measurements of V2p and Mo3d orbital showed main peaks attributed to V⁴⁺ and Mo⁶⁺ by Mo substitution in V₂O₅. Therefore, the V sites in V₂O₅ were suggested to be partially substituted to Mo. Galvanostatic cycling in LIBs revealed that a V_1.94Mo_0.06O₅ electrode showed the lowest overvoltage than that of other compositions. Therefore, we focused on V_1.94Mo_0.06O₅ in order to evaluate electrochemical characteristics for CIBs. The V_1.94Mo_0.06O₅ cathode evaluated in CIBs showed reversible discharge and charge profiles with less overvoltage as well as the improvement in cycle performance compared to V₂O₅. XRD and EDX analyses also supported the reversible insertion/extraction reaction of V_1.94Mo_0.06O₅ with Ca²⁺ ions. Therefore, it is suggested that V_1.94Mo_0.06O₅is feasible as one of cathode materials for CIBs.

References

[1] J. Muldoon, C.B. Bucur and T. Gregory, Chem. Rev., 114 (2014) 11683.

[2] M. Hayashi, H. Arai, H. Ohtsuka and Y. Sakurai, J. Power Sources, 119-121 (2003) 617.

[3] M. Hayashi, H. Arai, H. Ohtsuka and Y. Sakurai, Electrochem. Solid-State Lett., 7 (2004) A119.