What Happens to MnO₂ When It Comes in Contact with Zn²⁺? An Electrochemical Study in Aid of Zn/MnO₂‐Based Rechargeable Batteries

Abstract

In the science and technology of electrochemical energy storage, different allotropes of MnO2, fabricated with a variety of methods, are assembled into electrodes, playing the role of cathode or oxygen reduction reaction (ORR) electrocatalyst. Often, MnO2‐based cathodes are combined with Zn anodes into different types of batteries, resulting in contact between MnO2 and its electrochemical reaction products, and Zn2+. Awareness is growing that this interaction adversely affects the functional performance of MnO2, but no definitive understanding has been reached for this issue. This study contributes, through electrochemical measurements accompanied by microscopy and Raman spectroscopy, to a better understanding of the way the electrochemical behavior of two technologically representative types of manganese dioxide ‐ hydrothermally grown α‐MnO2 and electrodeposited γ‐MnO2 (EDM) ‐ is degraded when these materials are exposed to neutral and alkaline aqueous solutions, containing Zn2+. Specifically, we highlighted different types of irreversible changes in electrochemical response, which can be interpreted with phase‐formation processes. Such changes result in the deactivation of α‐MnO2 as ORR electrocatalyst, and of both α‐MnO2 and EDM as zinc‐ion battery (ZIB) cathodes. The electroactivity of EDM for ZIB operation can be restored if Mn2+ is added to the neutral electrolyte, because a phase, active in discharge, is electrodeposited during charging.

Potassium-Ion-Doped Manganese Oxides and Kaolinite Electrolyte Additives for Aqueous Zinc-Ion Batteries

“…Figure b exhibits the cyclic voltammetry (CV) curves of KKM and α-MnO 2 within the voltage range from 0.8 to 1.8 V at a scan rate of 0.2 mV s –1 in the KL additive electrolyte. Two couples’ redox peaks of KKM can be visualized for electrodes, which are associated with the intercalation/deintercalation process of H + and Zn 2+ in a complete CV cycle, accompanied by changes in the valence state of manganese. , Meanwhile, the valence of Mn decreases from +4 to +3 for the reduction peak, leading to the consecutive formation of MnOOH and ZnMn 2 O 4 , which promotes the H + insertion followed by Zn 2+ insertion . In contrast to MnO 2 , KKM exhibits a larger area under the CV curves, signifying a greater capacity than MnO 2 , demonstrating a faster ion transport rate with a larger peak current.…”

“…51,52 Meanwhile, the valence of Mn decreases from +4 to +3 for the reduction peak, leading to the consecutive formation of MnOOH and ZnMn 2 O 4 , which promotes the H + insertion followed by Zn 2+ insertion. 53 In contrast to MnO 2 , KKM exhibits a larger area under the CV curves, signifying a greater capacity than MnO 2 , demonstrating a faster ion transport rate with a larger peak current. Figure S5a shows the CV curves of KKM in different electrolytes.…”

Insight on the Energy Storage Mechanism and Kinetic Dynamic of Manganese Oxide‐Based Aqueous Zinc‐Ion Batteries

“…Zn has the characteristics of ideal high theoretical capacity (820 mAh g −1 ), extremely low redox potential (−0.76 V vs SHE), smaller bare ion radius (≈0.74 Å), smaller hydrated ion radius (≈4.6 Å for Zn(H 2 O) 6 2+ ), and low even nontoxic as the anode of AZIB. [23][24][25][26][27][28][29][30] Zn also has the characteristics of lower cost (Zn-2600 USD/ton, Li-130000 USD/ton, Na-3000 USD/ton, K-13000 USD/ton, Cu-6510 USD/ton) which is the most critical step for AZIB to be applied in practice. [31] In addition, zinc metal anodes can be used directly in water electrolytes, whereas Li, Na, and K metal anodes require organic electrolytes.…”