Oxide vs. Non-oxide Cathode Materials for Aqueous Zn Batteries: An Insight into the Charge Storage Mechanism and Consequences Thereof.

Aqueous Zn-ion batteries, which are being proposed as large scale energy storage solutions due to their unparalleled safety and cost advantage, are comprised of a positive host (cathode) material, a metallic zinc anode, and a mildly acidic aqueous electrolyte (pH ~ 3 - 7). Typically, the charge storage mechanism is believed to be reversible Zn2+ (de)intercalation in the cathode host, with the exception of α-MnO2, for which multiple vastly different and contradicting mechanisms have been proposed. However, our present study, combining electrochemical, operando X-ray diffraction (XRD), electron microscopy in conjunction with energy dispersive X-ray spectroscopy (EDX), and in situ pH evolution analyses on two oxide hosts - tunneled α-MnO2 and layered V3O7·H2O vis-à-vis two non-oxide hosts - layered VS2 and tunneled Zn3[Fe(CN)6]2, suggests that oxides and non-oxides follow two dissimilar charge storage mechanisms. While the oxides behave as dominant proton intercalation materials, the non-oxides undergo exclusive zinc intercalation. Stabilization of the H+ on the hydroxyl terminated oxide surface is revealed to facilitate the proton intercalation by a preliminary molecular dynamics simulation study. Proton intercalation for both oxides leads to the precipitation of layered double hydroxide (LDH) - Zn4SO4(OH)6·5H2O with ZnSO4/H2O electrolyte and a triflate anion (CF3SO3-) based LDH with Zn(SO3CF3)2/H2O electrolyte - on the electrode surface. The LDH precipitation buffers the pH of the electrolytes to a mildly acidic value, sustaining the proton intercalation to deliver large specific capacities for the oxides. Moreover, we also show that the stability of the LDH precipitate is crucial for the rechargeability of the oxide cathodes, revealing a critical link between the charge storage mechanism and the performance of the oxide hosts in aqueous zinc batteries.