Critical Role of pH Evolution of Electrolyte in the Reaction Mechanism for Rechargeable Zinc Batteries

Abstract

The reaction mechanism of α-MnO having 2×2 tunnel structure with zinc ions in a zinc rechargeable battery, employing an aqueous zinc sulfate electrolyte, was investigated by in situ monitoring structural changes and water chemistry alterations during the reaction. Contrary to the conventional belief that zinc ions intercalate into the tunnels of α-MnO , we reveal that they actually precipitate in the form of layered zinc hydroxide sulfate (Zn (OH) (SO )⋅5 H O) on the α-MnO surface. This precipitation occurs because unstable trivalent manganese disproportionates and is dissolved in the electrolyte during the discharge process, resulting in a gradual increase in the pH value of the electrolyte. This causes zinc hydroxide sulfate to crystallize from the electrolyte on the electrode surface. During the charge process, the pH value of the electrolyte decreases due to recombination of manganese on the cathode, leading to dissolution of zinc hydroxide sulfate back into the electrolyte. An analogous phenomenon is also observed in todorokite, a manganese dioxide polymorph with 3×3 tunnel structure that is an indication for the critical role of pH changes of the electrolyte in the reaction mechanism of this battery system.

Recent Progress in the Electrolytes of Aqueous Zinc‐Ion Batteries

“…[3a] However,t he mechanisms of aqueous ZIBs with ZnSO 4 electrolytes are still being debated, which are caused by the formation of Zn 4 (OH) 6 SO 4 ·n H 2 O(ZHS). [32] ZHS was generally regulated by the pH changes of the electrolytes during the discharge process,w hich could be formulized as [Eq. (8)]:…”

Rational Development of Neutral Aqueous Electrolytes for Zinc–Air Batteries

“…where b is the Bruggeman coefficient, D i is the diffusion coefficient, z i is the charge number,a nd t i is the transference number of the solute. The convective flux of quasiparticle q is defined by Equation (18):…”

“…Experimental studies show that zinc batteries based on these electrolytes can be reliably operated over hundreds or even thousands of cycles. [13,14,[16][17][18] However,t ok eep the pH value of the electrolyte in the near-neutral regime, ab uffer solution is required to counteract the effects of the oxygen-reductiona nd oxygen-evolution reactions. In the case of ZnCl 2 , this is accomplished by the addition of NH 4 Cl and NH 4 OH.…”

Challenges and perspectives for manganese‐based oxides for advanced aqueous zinc‐ion batteries

“…Apart from the above reaction mechanisms mentioned, a chemical conversion reaction mechanism based on reversible precipitation/dissolution of Zn 4 (OH) 6 SO 4 · n H 2 O (ZHS) has also been proposed (Figure A) . For the first time, Oh et al found that the ZHS is obtained after fully discharging, and disappears after fully charging (Figure B).…”

“…Reproduced with permission from Ref. , Copyright 2016, Wiley‐VCH. C‐H, TEM/high resolution TEM images of MnO 2 electrodes during electrochemical process.…”