This talk will discuss deep cycling of MnO₂ as an aqueous, low-cost battery cathode material. Deep cycling is defined as reduction through nearly all of the theoretical capacity from Mn^IV to Mn^II, or 617 mAh/g-MnO₂, while remaining reversible. MnO₂ is one of the safest and most inexpensive battery materials ever discovered, and is well known in consumer primary batteries. In these cells MnO₂ operates by proton (H⁺) insertion, but is not generally considered rechargeable due to a panoply of irreversible phase changes triggered during discharge.[1] Cycleability of Bi-modified δ-MnO₂ was researched in the 1980s as a candidate cathode for electric vehicle batteries.[2-4] However, while this material can achieve thousands of cycles under certain conditions, performance is generally a strong function of several factors such as cycling protocol (e.g. galvanostatic vs. CV sweeps), active material mass loading, and electrode thickness. These last two, loading and thickness, can be quantified together by an areal capacity. Modeling indicates that areal capacities of >20 mAh/cm²have potential to result in high energy density aqueous, rechargeable batteries when paired with zinc anodes. These characteristics meet the requirements for a collection of emerging applications, namely grid-scale electrical storage.[5]

We have recently reported that high cycleability of Bi-modified MnO₂ was achieved by further doping the material with other compounds.[6] Furthermore, this cycleability extends to high areal capacity electrodes. Figure 1 shows C/3 cycling of Bi-modified MnO₂ cathodes, both with and without doping. Both electrodes had areal capacities of 21 mAh/cm², produced from a cathode mix of 45% γ-MnO₂, 9% Bi₂O₃, and 46% CNTs. While the undoped electrode lost significant capacity by the third cycle, the doped electrode achieved steady-state cycling after an initial conversion from γ-MnO₂ to δ-MnO₂during cycle 1. To establish the difference in mechanism between the two electrodes, a study was undertaken to observe the material conditions within the cathodes in operando.

Energy dispersive X-ray diffraction (EDXRD) data were collected at beamline 6-BM-A at the Advanced Photon Source. Figure 2 shows diffraction data within a doped Bi-modified cathode during the second cycle as a function of location within the cathode. Both δ-MnO₂ and a modified bismuth material resembling the structure of Bi₁₂MnO₂₀ sillenite were observed.[7] The discharge products Mn(OH)₂ and metallic Bi were observed in the ranges indicated. Reversible formation of spinel Mn₃O₄ was also observed, apparent from the (103) and (211) reflections. This finding was significant, as spinel formation is generally considered to be irreversible in alkaline batteries, and the source of non-rechargeability. This spinel was confined to the region near the separator, while Mn(OH)₂and Bi were located throughout. Further experiments using localized XANES and X-ray fluorescence to track the dopant will also be discussed. These were performed at beamline 5-ID (SRX) at the National Synchrotron Light Source II.

Acknowledgements

This work was supported by the U.S. Department of Energy, ARPA-E under award number DE AR0000150. This research used beamline 5-ID (SRX) 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. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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