Layered manganese oxide cathode boosting high-capacity and long-term cyclability in aqueous Zinc-Ion batteries

Development of aqueous zinc-ion batteries (ZIBs) promises low-cost and safe energy storage systems. From the existing natural resources manganese-based compounds are desirable cathodes materials for aqueous ZIBs. We present a layered birnessite-type δ–K_0.32MnO₂·0·15H₂O (MnO₂) as a candidate cathode material. By adding reduced graphene oxide (rGO) to enhance electron transport, we present the electrode performance in Al pouch cells (3.2 × 3.4 cm²) achieving a high-capacity of 373 mAh g¹⁻ at 0.1C which retained over 99 % for 120 cycles. Additionally, performance is highlighted at 5C and 10C, retaining 89 % for 500 cycles and 35 % for 2,000 cycles, respectively. The main redox process involves the Mn⁴⁺/Mn³⁺ redox couple, accompanied by a conversion reaction through the de/protonation process. Operando XRD, operando pH measurement, and time-of-flight secondary-ion mass spectroscopy prove that the de/protonation process of solvated zinc ions [Zn(H₂O)₆]²⁺, followed by protonation of the active material leads to the formation of K_xMnOOH_{(1- x)} during discharge and vice versa during charging. The conversion reaction resulting from the de/protonation processes leads to the amorphization of the active material after the prolonged cycles. Operando pH analysis shows the influence of the pH on de/protonation of [Zn(H₂O)₆]²⁺ complex, thus implicating it as a determinant of the capacity.