Completely Activated and Phase-Transformed KFeMnHCF for Zn/K Hybrid Batteries with 14 500 Cycles by an OH-Rich Hydrogel Electrolyte

Abstract

Metal hexacyanoferrates are recognized as superior cathode materials for zinc and zinc hybrid batteries, particularly the Prussian blue analog (PBA). However, PBA development is hindered by several limitations, including small capacities (<70 mAh g⁻¹) and short lifespans (<1000 cycles). These limitations generally arise due to incomplete activation of redox sites and structure collapse during intercalation/deintercalation of metal ions in PBAs. According to this study, the adoption of a hydroxyl-rich (OH-rich) hydrogel electrolyte with extended electrochemical stability windows (ESWs) can effectively activate the redox site of low-spin Fe of the K<inf>x</inf>Fe<inf>y</inf>Mn<inf>1−y</inf>[Fe(CN)<inf>6</inf>]<inf>w</inf>·zH<inf>2</inf>O (KFeMnHCF) cathode while tuning its structure. Additionally, the strong adhesion of the hydrogel electrolyte inhibits KFeMnHCF particles from falling off the cathode and dissolving. The easy desolvation of metal ions in the developed OH-rich hydrogel electrolytes can lead to a fast and reversible intercalation/deintercalation of metal ions in the PBA cathode. As a result, the Zn||KFeMnHCF hybrid batteries achieve the unprecedented characteristics of 14 500 cycles, a 1.7 V discharge plateau, and a 100 mAh g⁻¹ discharge capacity. The results of this study provide a new understanding of the development of zinc hybrid batteries with PBA cathode materials and present a promising new electrolyte material for this application.