Reversible Calcium Ion Batteries Using a Dehydrated Prussian Blue Analogue Cathode

Prussian blue analogue (KMFe-PBA), KxMFe(CN)6·nH2O, was investigated as a new cathode material to insert/extract Ca2+ reversibly using a Ca-based organic electrolyte. A dehydrated KNiFe-PBA (i.e. KxNiFe(CN)6) electrode with a high electro-conductive additive demonstrated reversible capacities of ∼50 mAh/g with Coulombic efficiencies of ∼92% even though the decomposed capacity of the electrolyte was indicated at the discharge. During the 1st discharge and discharge-charge, a detailed structural characterization was performed using an ex-situ X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). XRD patterns and XPS spectra on the discharged and discharged-charged cathodes indicate that Ca2+ is inserted and extracted into and from the interstitial sites of KNiFe-PBA without destruction of the open framework structure.