Understanding the Second Electron Reaction Mechanism of γ-MnO2 and the Rechargeability of Spinel Phases in Alkaline Zn|MnO2 Batteries

Abstract

The alkaline zinc manganese dioxide (Zn|γ-MnO2) chemistry has become the dominant chemistry in the primary battery industry since its introduction in the early 1960s. This is mainly due to its inexpensive basis materials, safety characteristics, and high theoretical energy density (>400 Wh/L). Currently, there is also a growing interest in making this chemistry reversible for grid-scale energy storage 1,2. However, a long cycle-life is usually obtained by limiting the depth of discharge (DOD) of a γ-MnO2 cathode. It is widely agreed that high DOD usually leads to detrimental phase transformations of γ-MnO2, such as the formation of spinel phase materials like hetaerolite (ZnMn2O4) and hausmannite (Mn3O4). Hetaerolite and hausmannite are usually considered as highly resistive and electrochemically inactive materials. They are known to affect γ-MnO2 discharge, especially in its second electron reduction process, which leads to a significant capacity loss. However, the above understanding is true only in certain circumstances. Despite being studied extensively, the reaction pathway of γ-MnO2 reduction still remains unclear, especially in the second electron region. Factors such as electrode conductivity, porosity, accessibility to electrolyte, and product solubility can significantly affect its discharge performance. In this presentation, the cycling performance of a γ-MnO2 electrode is studied both galvanostatically and potentiostatically. Its phase change during cycling is characterized by X-ray diffraction. It is found that with a good conductive carbon matrix and a highly porous structure, hausmannite is not formed during the first discharge cycle. In the absence of zincate ions, which is realized by applying a Zn-blocking separator 3,4, the achievable capacity of a Zn| γ-MnO2 primary battery is almost doubled, with a potential plateau around 0.95V, which is characteristic of the second electron reduction of γ-MnO2. The energy density achieved above 0.8V is increased by more than 50% (Figure 1). It is also found that the spinel phase materials are rechargeable with this highly conductive and porous electrode structure. A capacity close to 300 mAh/g-MnO2 can be delivered from both materials, suggesting that hetaerolite and hausmannite are not necessarily irreversible materials in a battery, which further provides more pathways for accessing the second electron capacity of MnO2.