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.