Utilization of Ca K-Edge X-ray Absorption Near Edge Structure to Identify Intercalation in Potential Multivalent Battery Materials

Abstract

Multivalent-ion batteries have been under investigation as a technology that can provide energy densities beyond that provided by lithium ion batteries. An emerging cation of interest is calcium, as it has similar electrochemical properties and size to sodium, and systems incorporating it have high theoretical density. Common techniques used to investigate intercalation, such as NMR, are currently unable to distinguish calcium's role in energy storage materials. In this work, we investigated the ability of calcium XANES to provide fingerprint identification of intercalated species using Ca ion exchanged NaxCoO2 and a Ca(PF6)2-based electrolyte as a case study. Ca XANES was able to detect intercalation down to concentrations of ∼1 Ca per 950 Å3, even with residual electrolyte on the surface. The ability to observe the structure of the intercalated ion will enable the discovery of new electrolytes and intercalation cathodes by helping to confirm theoretical predictions, allowing multivalent batteries to become a viable high energy density storage technology.