Chromium(II) Hexacyanoferrate-Based Thin Films as a Material for Aqueous Alkali Metal Cation Batteries

Identification and characterization of novel battery electrode materials are key factors in transitioning the grids to renewable energy provision. Given the scale of the challenge, special attention should be paid to safety and availability of resources. This paper presents a new electrode material for aqueous batteries and supercapacitors based on highly available resources: chromium(II) hexacyanoferrate (CrHCF) thin films. Electrodeposited CrHCF exhibited "half-charge" potentials (E_1/2) of ∼0.69 and ∼0.72 V versus silver/silver chloride (reference electrode) for Na and K intercalation, respectively, a high specific capacity of ∼88 mA h/g (10 C), and a good rate performance at fast C-rate (360 C). The electrolyte composition significantly influences the long-term cycling stability of the CrHCF electrodes and the choice of the intercalating alkali metal cations significantly impacts the E_1/2 potentials. Finally, a CrHCF-based symmetric cell (quasi-supercapacitor) was constructed and showed high specific energy of ∼4.6 W h/kg at 100 C.