High-Voltage Manganese Oxide Cathode with Two-Electron Transfer Enabled by a Phosphate Proton Reservoir for Aqueous Zinc Batteries

Abstract

MnO2 cathodes typically undergo one-electron transfer in aqueous zinc batteries. The two-electron MnO2/Mn2+ reaction provides double capacity and higher voltage. However, this requires a highly acidic environment, which challenges the Zn metal anode. Herein, we present a proton reservoir for the MnO2/Mn2+ reaction. Zinc hydrophosphate is codeposited with MnO2 at the cathode. The former deprotonates to release protons and enhances the reduction of MnO2 to Mn2+. The resulting zinc phosphate further interacts with MnO2 and realizes spontaneous water desorption from the MnO2 surface as revealed by theoretical calculations, which facilitates the dissolution process. The hydrophosphate species is reversibly generated upon charge. Based on this reaction mechanism, the cathode achieves a high discharge voltage of 1.75 V. It also delivers 0.99 mAh cm–2 capacity with 99% Coulombic efficiency. Stable capacity retention is realized for over 3000 cycles. This work demonstrates an effective strategy to access the two-electron process of MnO2 cathode materials in aqueous zinc batteries.

Dynamically Interfacial pH‐Buffering Effect Enabled by N ‐Methylimidazole Molecules as Spontaneous Proton Pumps toward Highly Reversible Zinc‐Metal Anodes

“…[ 46c ] Since the conversion of MnO 2 /Mn 2+ is ruled by the Nernst equation, potentials of dissolution or deposition process are determined by the concentration of protons. [ 48 ] Besides, benefitting from the inhibited accumulation of ZSH on Zn anode toward the reduced Zn platting/stripping overpotential, the two cathodic peaks shift positively (1.24 V to 1.28 V and 1.37 V to 1.38 V) with the addition of NMI, and the anodic peak located at 1.57 V shifts negatively to 1.55 V, suggesting the full cell with NZM electrolyte has a higher discharge voltage and smaller polarization. As demonstrated in Figure S33a, Supporting Information, cathodic peak at 1.28 V disappears when ZnSO 4 is removed from the NZM electrolyte, indicating that there is only the dissolution/deposition process in MnSO 4 + NMI electrolyte.…”

Zincophilic Electrode Interphase with Appended Proton Reservoir Ability Stabilizes Zn Metal Anodes

“…In the regard of improving the resistance against HER, except for the high electric resistivity of ZP layer that blocks electron migration, a deeper mechanism takes effect. With the feature of acid‐base amphoteric ion, the phosphate group shows the proton self‐dissociation ability, [18a, 35] which can stock protons reversibly by the formation and breakage of dynamic hydrogen bond as the pH varies, represented as follows: [18b] $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {\rm PO}{_{4}}{^{3- }}+{\rm H}{^{+}}\ {\stackrel{ {\leftarrow} } { {\rightarrow} } } \ {\rm HPO}{_{4}}{^{2- }}\hfill\cr}}$$$ $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {\rm HPO}{_{4}}{^{2- }}+{\rm H}{^{+}}\ {\stackrel{ {\leftarrow} } { {\rightarrow} } } \ {\rm H}{_{2}}{\rm PO}{_{4}}{^{- }}\hfill\cr}}$$$ …”

Zincophilic Electrode Interphase with Appended Proton Reservoir Ability Stabilizes Zn Metal Anodes

“…In the regard of improving the resistance against HER, except for the high electric resistivity of ZP layer that blocks electron migration, a deeper mechanism takes effect. With the feature of acid‐base amphoteric ion, the phosphate group shows the proton self‐dissociation ability, [18a, 35] which can stock protons reversibly by the formation and breakage of dynamic hydrogen bond as the pH varies, represented as follows: [18b] $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {\rm PO}{_{4}}{^{3- }}+{\rm H}{^{+}}\ {\stackrel{ {\leftarrow} } { {\rightarrow} } } \ {\rm HPO}{_{4}}{^{2- }}\hfill\cr}}$$$ $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {\rm HPO}{_{4}}{^{2- }}+{\rm H}{^{+}}\ {\stackrel{ {\leftarrow} } { {\rightarrow} } } \ {\rm H}{_{2}}{\rm PO}{_{4}}{^{- }}\hfill\cr}}$$$ …”