Deciphering the cycling mechanism of manganese-oxide cathodes in zinc-ion batter-ies in near-neutral electrolytes: A theory based approach

Abstract

Zinc-ion batteries (ZIB) rely on a lithium-ion-like Zn2+-shuttle, which allows higher roundtrip efficiencies and better cycle life than most other zinc-based chemistries. The most promising and best-studied candidates for zinc-ion batteries are manganese-oxide (MnO2) cathodes. ZIBs with aqueous neutral zinc sulfate (ZnSO4) electrolytes exhibit two distinct phases during charge and discharge. Experiments have revealed that the second discharge phase goes hand-in-hand with the precipitation of zinc-hydroxide sulfate (ZHS) at the cathode. Besides the most desired zinc insertion, the proposed working mechanisms of the MnO2 cathodes are the (co-)-insertion of protons into the cathode and the reversible dissolution of the MnO2 itself, both having similar effects on the local pH evolution eventually leading to ZHS precipitation. Recently, the close interaction of cathode dissolution and the formation of ZHS was shown experimentally.Empirically, higher cycling stability was achieved by electrolyte additives, such as MnSO4, but most of the work does not focus on the close interaction of the electrolytes pH and the cathodic reaction dynamics. Most importantly, a complete and consistent under-standing of the two-phase charge and discharge mechanisms of such ZIBs is still lacking.

Our work uses a continuum full-cell model supported by DFT calculation to investigate the implications and details of the experimentally observed properties and deduced claims. We integrate the complex formation reactions in near-neutral aqueous electrolytes into a contin-uum 1D+1D battery model. We combine this approach with DFT calculations of the cathode structure with inserted zinc and proton to give a detailed picture of ZIB's cycling behaviour. We investigate the interaction of the cathodic proton reactions and the electrolyte's stability and precipitation characteristics in full-cell simulations. This helps us to identify the details of the charge-storage mechanism. We validate our model with experiments by comparing it with cell voltages during galvanostatic discharge and cyclometric voltammograms. Subsequently, we use this model to identify how the cycling behaviour is impacted by cell design and electrolyte composition and discuss optimizations of the interplay of cathodic dissolution and precipitation of ZHS.