Rechargeable Alkaline MnO₂ Cathodes for Grid Storage—Mechanism of Bi Modifier

The rechargeable Zn-MnO₂ alkaline battery is one of the few capable of meeting the extreme cost requirements needed for global integration of battery storage in the power grid. Modifying the MnO₂ cathode with Bi allows it to cycle reversibly between layered Mn oxides -MnO₂ and Mn(OH)₂ without detrimental formation of Mn₃O₄. This provides a cathode capacity of 617 mAh/g, which modeling has shown to enable energy storage costs below $50/kWh. However, commercial adoption requires a more complete understanding of the variables that affect the performance of the alkaline MnO₂ cathode. This talk will present an updated understanding of the mechanism of the Bi-modified alkaline MnO₂ cathode.

The alkaline MnO₂ cathode achieves a full two-electron cycling reaction. This is a 2-step electrochemical process: the Mn⁴⁺/Mn³⁺ step occurs via proton insertion/extraction while the Mn³⁺/Mn²⁺ step occurs via dissolution/precipitation. While Bi modification prevents the rapid failure of the alkaline MnO₂ cathode due to Mn₃O₄ formation, a gradual decline in capacity is still observed (~25% over 100 cycles). To better understand where this capacity loss comes from, greater understanding of the electrochemical mechanism is needed. Because the cycling materials are poorly crystalline, techniques other than diffraction were needed. We used operando extended X-ray absorption fine structure (EXAFS) to characterize the first cycle of alkaline MnO₂ electrodes with and without Bi additive. Data shows that when Bi is not included Mn₃O₄ forms during the Mn²⁺/Mn³⁺ step of the first charge, suggesting it forms via aqueous complexing of which is blocked by Bi additive. This means Bi is not structurally incorporated in the material during cycling, which was further confirmed by comparison to synthesized Bi-doped -MnO₂ which was found to have a local coordination unique and distinct from a rechargeable cathode. Post-mortem TEM of particle cross-sections confirmed that Bi is concentrated around particle surfaces and its effect on cycling is largely interfacial.