Operando EXAFS to Determine the Mechanism of Bi Dopant at Low Concentration in Rechargeable MnO₂ Cathodes

The rechargeable Zn-MnO₂battery system is one few systems in development capable of meeting the extreme cost requirements needed for global integration of battery storage in the power grid. Doping the MnO₂ cathode with Bi allows it to cycle reversibly between layered Mn oxides δ-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 costs below $50/kWh. However, to achieve commercial adoption, cycling stability must be improved. The mechanism through which the Bi dopant improves rechargeability remains largely unknown, and this must be clarified to rationally improve the system. Our previous work has established that the mechanism involves structurally disordered intermediates, and thus characterization techniques based on short-range order need to be used. This talk will demonstrate rapidly collected, operando extended x-ray absorption fine-structure (EXAFS) data collected on MnO₂ cathodes with and without Bi dopant. Operando techniques are scientifically powerful as they reduce experimental uncertainty resulting from destructive preparation of samples for ex-situ analysis. In addition, rapid EXAFS collection allows for analysis of the kinetics of Mn coordination changes.<br/><br/>The Bi-modified alkaline MnO₂ cathode has been of much interest in literature as only a small amount of Bi₂O₃ additive (Bi:Mn molar ratio 0.075) incorporated by high-energy ball-milling enables rechargeable behavior. Previously, a disordered δ-MnO₂ phase with no diffraction signal was identified during the first charge of Bi-modified MnO₂ cathodes.¹ In this work, operando X-Ray Absorption Spectroscopy (XAS) was employed to further compare Mn coordination environments in standard and Bi-modified cathodes throughout cycling. Specifically, EXAFS analysis was used to visualize the dynamic first shell (Mn-O) and second shell (Mn-Mn) interatomic distances during the first cycle. Comparison with theoretical EXAFS modeled from CIF allows for “fingerprinting” of the charge/discharge products, while rapid EXAFS collection time allows for identification of intermediates and precise analysis of the potentials and states of charge at which materials deform. In the first discharge an earlier conversion to Mn(OH)₂ is observed in the Bi-modified MnO₂ cathode. Upon charge, deformation of Mn(OH)₂ is observed at the same potential energy in both cases, demonstrating that the thermodynamic stability of Mn(OH)₂ is not affect by Bi-modification. Finally, the formation of Mn₃O₄is identified in the standard MnO₂ cathode during the conversion reaction that occurs upon charge. These findings suggest that Bi dopant blocks the pathway leading to the formation of Mn₃O₄ through interfacial effects rather than bulk incorporation.