Reverse Dual-Ion Battery Enabled by Reversing the Cation/Anion Storage Mechanism in an Aqueous ZnCl2 Water-in-Salt Electrolyte.

Dual ion batteries (DIBs) have garnered significant attention from researchers due to their unique ability to store charges using electrolyte-born ions, making them promising candidates for grid storage applications. However, despite extensive efforts to explore DIBs with various electrolytes, such as organic, aqueous, gel polymer etc., challenges such as electrolyte decomposition and poor stability of anode materials in aqueous electrolytes remain unresolved. To address these issues, we report a novel approach utilizing a flip-cum-reverse sequence of anion/cation storage chemistry in a ZnCl2 water-in-salt electrolyte (ZnCl2 -WiSE)-based reverse dual ion battery (RDIB), employing Zn-based Prussian blue analogue i. e., Zn3 [Fe(CN)6 ]2 and ferrocene-carbon composite (FcC) as cathode and anode electrodes, respectively. The RDIB operates in the opposite direction compared to conventional DIBs, offering a fresh perspective. Through our investigations, we discovered that increasing the concentration of ZnCl2 -WiSE [ZnCl2 -WiSE] resulted in a positive shift of 270 mV in the redox potential for cation/anion (de)insertion at the cathode, and a negative shift of 70 mV at the anode, indicating enhanced performance. Remarkably, the RDIB operate in 10 m ZnCl2 -WiSE exhibited an impressive energy density of 23 Wh kg-1 , showcasing the potential of this approach for high-performance energy storage.