We have recently reported that high cycleability of Bi-modified MnO2 was achieved by further doping the material with other compounds. 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. 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.
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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