Reversible Electrochemical Energy Storage Based on Zinc-Halide Chemistry

The development of rechargeable Zinc-ion batteries (ZIBs) has been hindered by the lack of efficient cathode materials due to the strong binding of divalent zinc ions with the host lattice. Herein, we report a strategy that eliminates the participation of Zn²⁺ within the cathode chemistry. The approach involves the use of composite cathode materials that contain Zn halides (ZnCl₂, ZnBr₂, and ZnI₂) and carbon (graphite or activated carbon), where the halide ions act both as charge carriers and redox centers while using a Zn²⁺-conducting water-in-salt gel electrolyte. The use of graphite in the composite electrode produced batterylike behavior, where the voltage plateau was related to the standard potential of the halogen species. When activated carbon was used in the composite, however, the cell acted as a hybrid Zn-ion capacitor due to the fast, reversible halide ion electrosorption/desorption in the carbon pores. The ZnX₂-activated carbon composite delivers a capacity of over 400 mAh g^-1 and cell energy density of 140 Wh kg^-1 while retaining over 95% of its capacity after 500 cycles. The halogen reaction mechanism has been elucidated using combinations of electrochemical and in situ spectroscopic techniques.