Stabilized Co3+/Co4+ Redox Pair in In Situ Produced CoSe2−x‐Derived Cobalt Oxides for Alkaline Zn Batteries with 10 000‐Cycle Lifespan and 1.9‐V Voltage Plateau

In aqueous alkaline Zn batteries (AZBs), the Co3+/Co4+ redox pair offers a higher voltage plateau than its Co2+/Co3+ counterpart. However, related studies are scarce, due to two challenges: the Co3+/Co4+ redox pair is more difficult to activate than Co2+/Co3+; once activated, the Co3+/Co4+ redox pair is unstable, owing to the rapid reduction of surplus Co3+ to Co2+. Herein, CoSe2−x is employed as a cathode material in AZBs. Electrochemical analysis recognizes the principal contributions of the Co3+/Co4+ redox pair to the capacity and voltage plateau. Mechanistic studies reveal that CoSe2−x initially undergoes a phase transformation to derived CoxOySez, which has not been observed in other Zn//cobalt oxide batteries. The Se doping effect is conducive to sustaining abundant and stable Co3+ species in CoxOySez. As a result, the battery achieves a 10 000‐cycle ultralong lifespan with 0.02% cycle−1 capacity decay, a 1.9‐V voltage plateau, and an immense areal specific capacity compared to its low‐valence oxide counterparts. When used in a quasi‐solid‐state electrolyte, as‐assembled AZB delivers 4200 cycles and excellent tailorability, a promising result for wearable applications. The presented effective strategy for obtaining long‐cyclability cathodes via a phase transformation‐induced heteroatom doping effect may promote high‐valence metal species mediation toward highly stable electrodes. CoSe2−x is herein studied as cathode for alkaline Zn batteries, where a Co3+/Co4+ redox pair in in situ produced Se‐doped cobalt oxides are activated and stabilized, demonstrating a 10 000‐cycle ultralong lifespan and 1.9‐V high voltage plateau.