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

Abstract

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 MnIII/MnII, FeIII/FeII, and C[tbnd]N- ligand allow two close voltage plateaus at ∼1.50 V vs Zn/Zn2+, 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.