Novel Insights into Energy Storage Mechanism of Aqueous Rechargeable Zn/MnO2 Batteries with Participation of Mn2+

Abstract

HIGHLIGHTS• Pourbaix diagram of Mn-Zn-H 2 O system was used to analyze the charge-discharge processes of Zn/MnO 2 batteries.• Electrochemical reactions with the participation of various ions inside Zn/MnO 2 batteries were revealed.• A detailed explanation of phase evolution inside Zn/MnO 2 batteries was provided.ABSTRACT Aqueous rechargeable Zn/MnO 2 zinc-ion batteries (ZIBs) are reviving recently due to their low cost, non-toxicity, and natural abundance.However, their energy storage mechanism remains controversial due to their complicated electrochemical reactions. Meanwhile, to achieve satisfactory cyclic stability and rate performance of the Zn/MnO 2 ZIBs, Mn 2+ is introduced in the electrolyte (e.g., ZnSO 4 solution), which leads to more complicated reactions inside the ZIBs systems. Herein, based on comprehensive analysis methods including electrochemical analysis and Pourbaix diagram, we provide novel insights into the energy storage mechanism of Zn/MnO 2 batteries in the presence of Mn 2+ . A complex series of electrochemical reactions with the coparticipation of Zn 2+ , H + , Mn 2+ , SO 4 2− , and OH − were revealed. During the first discharge process, co-insertion of Zn 2+ and H + promotes the transformation of MnO 2 into Zn x MnO 4 , MnOOH, and Mn 2 O 3 , accompanying with increased electrolyte pH and the formation of ZnSO 4 ·3Zn(OH) 2 ·5H 2 O. During the subsequent charge process, Zn x MnO 4 , MnOOH, and Mn 2 O 3 revert to α-MnO 2 with the extraction of Zn 2+ and H + , while ZnSO 4 ·3Zn(OH) 2 ·5H 2 O reacts with Mn 2+ to form ZnMn 3 O 7 ·3H 2 O. In the following charge/discharge processes, besides aforementioned electrochemical reactions, Zn 2+ reversibly insert into/extract from α-MnO 2 , Zn x MnO 4 , and ZnMn 3 O 7 ·3H 2 O hosts; ZnSO 4 ·3Zn(OH) 2 ·5H 2 O, Zn 2 Mn 3 O 8 , and ZnMn 2 O 4 convert mutually with the participation of Mn 2+ . This work is believed to provide theoretical guidance for further research on high-performance ZIBs.

Electrodeposited Manganese Oxide on Carbon Paper for Zinc‐Ion Battery Cathodes

“…Large plate‐like formations, not present in the as deposited sample or the sample from Figure a, are present. The plates resemble ZSH that has been reported in the literature to form during discharge of Mn oxide electrodes . Interestingly, these plates tend to initially form in the areas between the Mn 3 O 4 islands on the CP and so the Mn 3 O 4 surface is only partially covered.…”

“…On subsequent cycling, two reduction peaks are present at 1.26 V and 1.37 V on the cathodic scan. Two oxidation peaks are also present during the anodic scan at 1.57 V and a small shoulder at ∼1.60 V. The CV results show relatively good overlap upon cycling and the CV behaviour is similar to that of other Mn oxide‐based ZIBs …”

“…Interestingly, these plates tend to initially form in the areas between the Mn 3 O 4 islands on the CP and so the Mn 3 O 4 surface is only partially covered. In the literature, ZSH generally completely covers the surface of the cathode . The edges of the Mn 3 O 4 deposits (within the gaps) likely act as preferential nucleation sites (larger surface area per unit volume) for ZSH formation, resulting in ZSH formation between the Mn 3 O 4 islands, rather than forming directly on top of the Mn 3 O 4 layer.…”

“…Of the different materials explored for ZIB cathodes, the most commonly studied material is Mn oxide. This is in large part due to Mn oxide's large theoretical capacity, non‐toxicity, abundance, numerous stable valence states, and low cost . Of the different Mn oxide phases that have been studied, there have only been a handful of instances where Mn 3 O 4 has been used as a cathode material for ZIBs .…”

“…These issues have generally been associated with cathode material phase change upon cycling, Mn dissolution, poor Mn oxide conductivity, or the formation of reaction side products . Dissolution of Mn is generally addressed by the addition of Mn into the electrolyte, while poor conductivity is minimized by the use of carbon additives or conductive substrates . Recently, Fang et al.…”