Mn-Fe Prussian blue analogue as low-cost robust cathode for non-aqueous Zn-ion batteries

Zn-ion batteries (ZIBs) are one of the most promising alternatives for large-scale energy storage systems due to two-electron transfer capability, safety, low toxicity, and low cost. However, most of the cathode materials reported for non-aqueous ZIBs still show unsatisfactory electrochemical performance. To overcome this, factors such as type of crystal structure, transition metal-ion coordination, and nature of the d orbitals being filled must be taken into consideration in the design of suitable host materials. Herein, we report the use of a Prussian blue analogue, sodium manganese hexacyanoferrate (NaMnFe-PB), as cathode in ZIBs. Its ease of synthesis, rigid open framework, and compositional and electrochemical tunability make this low-cost Mn-Fe-based compound highly attractive. The strong interactions among the two redox centers Mn III /Mn II, Fe III /Fe II, and C N - ligand allow two close voltage plateaus at ∼1.50 V vs Zn/Zn 2+, delivering a capacity of 89.5 mAh g -1. Ex-situ X-ray absorption and diffraction techniques confirm the high redox reversibility and structural stability of NaMnFe-PB upon divalent guest insertion. The Fe(CN) 6 vacancies and coordinated water in the host lattice are believed to facilitate cation diffusion. The insight gained in this work may pave the way for the design of low-cost cathode materials for next-generation large-scale energy storage systems.