Operando EXAFS to Determine the Mechanism of Bi Dopant at Low Concentration in Rechargeable MnO2 Cathodes
Authors: Zimmerer Eric , Guida Dominick P. , Gallaway Joshua W. 
Published: September 2023

The rechargeable Zn-MnO2 battery 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.

Previously, the formation of 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 at points of interest throughout cycling (pristine, intermediate, discharge, intermediate, charged). Specifically, the EXAFS region of the data was used to visualize the dynamic first shell (Mn-O) and second shell (Mn-Mn) interatomic distances during the first cycle. The values throughout cycling were compared with interatomic distances of Mn oxides that have been reported in literature, allowing the data to be used both for “fingerprinting” as a method of identifying the same pristine, intermediate, and discharged phases in both cases. Rapid EXAFS data collection allows for precise analysis of the potentials and states of charge at which these compounds form in each cathode. This portion of the analysis allows for observation of the faster kinetics of changes in Mn coordination in the Bi-modified MnO2 cathode. Furthmore, analysis of the first charge shows that deformation of the Mn(OH)2 discharge product is observed at the same potential energy in both cases, demonstrating that the thermodynamic stability of Mn(OH)2 is not improved in the first cycle of Bi-modified MnO2 cathodes, despite improved capacity. These findings suggest that Bi dopant may primarily act to improve kinetics within the MnO2 electrode rather than enhancing the thermodynamic stability to prevent degradation into Mn3O4.

References

Andrea M. Bruck et al 2020 Electrochem. Soc.
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Acknowledgements

We acknowledge financial support from the National Science Foundation under Award Number CBET-ES-2044602. This research used resources of beamline 7-BM (QAS) of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.