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 MnIV to MnII, 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. Cycleability of Bi-modified, layered δ-MnO2 (Bi-birnessite) has been noted, but it is stable only when mass loading is low. At high areal capacity of 21 mAh/cm2 Bi-birnessite still degrades after a few cycles, shown in Fig 1a. However, it has been discovered that addition of Cu to the cathode results in full rechargeability, with all cycles after the first showing the potential profile of birnessite. The mechanism by which Cu allows this stability is not understood. Operando experiments during initial cycling of a Cu-containing cathode were performed at beamline 5-ID of the NSLS-II, using X-ray fluorescence (XRF) mapping with a one micron beam size. Individual spatial mapping of the elements Mn, Bi, and Cu (Figs 1c and 1d) revealed that at the end of charge 2, Cu had spread to an area generally overlapping the Mn particles, with a Pearson correlation of 0.64 (Fig 1e). μ-XANES was performed on individual MnO2 particles and showed transitions from CuII to Cu0, confirming Cu was electrochemically active. Since there was no corresponding fcc Cu diffraction signal, this suggests zerovalent Cu present in the birnessite interlayer and provides greater mechanistic understanding.

Operando XRF mapping and μ-XANES of a Cu-containing Bi-birnessite cathode for high density, low-cost aqueous batteries

256th National Meeting (2018)

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