The Compensation Effect Mechanism of Fe–Ni Mixed Prussian Blue Analogues in Aqueous Rechargeable Aluminum-Ion Batteries

An aluminum-ion battery was assembled with potassium nickel hexacyanoferrate (KNHCF) as a cathode and Al foil as an anode in aqueous electrolyte for the first time, based on Al³⁺ intercalation and deintercalation. A combination of ex situ XRD, X-ray photoelectron spectroscopy (XPS), galvanostatic intermittent titration technique (GITT), and differential capacity analysis was used to unveil the crystal structure changes and the insertion/extraction mechanism of Al³⁺. Al³⁺ could reversibly insert/extract into/from KNHCF nanoparticles through a single-phase reaction with reduction/oxidation of Fe and Ni. Over long-term cycling, it was Fe rather than Ni that contributed to more capacity owing to the dissolution of Ni from the KNHCF structure, which could be expressed as a compensation effect of mixed redox centers in KNHCF. KNHCF delivered an initial discharge capacity of 46.5 mAh g⁻¹. The capacity decay could be attributed to the unstable interface between Al foil and the aqueous electrolyte owing to the catalytic activity of the Ni transferring from Ni dissolution of KNHCF to the Al foil anode, rather than KNHCF structure collapse; KNHCF maintained its 3 D framework structure for 500 cycles. This work is expected to inspire more exhaustive investigations of the mechanisms that occur in aluminum-ion batteries.