Date of Award
Zinc Manganese dioxide Batteries, Aqueous Electrolyte, High Zn Depth of Discharge, Zinc shape change quantification, Zinc dendrites, Birnessite, Failure mechanism of MnO2, Impedance Spectroscopy, Stationary Detector Electrode, X-ray Photoelectron Spectroscopy, Cyclic Voltammetry of MnO2, MnO2 Rechargeability in NaOH
Achieving a highly cyclable, high energy density battery with MnO2 cathodes encounters many obstacles. Chief among these is the inability of the widely-used γ-MnO2 polymorph to retain its structural integrity when cycled to near full one-electron discharge capacity, which is about 280mAh/g for commercially available electrolytic manganese dioxide (EMD). In this one-electron range, discharge occurs by proton insertion producing Mn+3 which then reverts to γ-MnO2 on charging. In the first part of the thesis, we investigate the root cause of failure of MnO2 cathodes under deep cycling in the one-electron discharge range and establish a strong link between capacity fade and the amount of birnessite (another polymorph of MnO2) that appears to be formed. We uncover the underlying cause of cathode capacity fade by cycling industrially produced EMD cathodes at various levels of theoretical one-electron capacity, termed depth of discharge (DOD), of 100%, 50% and 30%. As well, the KOH electrolyte concentration is varied to 37, 25, and 10 wt. %, to investigate concentration effects. Based on the findings, we propose that one major cause for loss of capacity on cycling of MnO2 cathodes stems from the solubility of Mn+3 formed during discharge, with higher solubility being found in higher concentration KOH. This solubilization process effectively results in destruction of the γ- MnO2 phase and amorphization of the cathode. On charging the Mn+3 in solution appears to deposit on the cathode surface as birnessite, which contributes little capacity in the first electron discharge region for the cathode.. The results show that the bulk of the γ-MnO2 phase is preserved only in ~10 wt. % KOH, which indicates the attractive range of KOH concentration for cycling of rechargeable γ-MnO2 cathodes.
In the second part of the thesis, we address the problem of redistribution of zinc over the electrode surface, also known as shape change, which is a major cause of failure in alkaline zinc anode batteries. To mitigate this phenomenon, we propose a scalable approach based on an in-situ formed, highly porous electrochemically synthesized ZnO matrix with uniformly electrodeposited zinc particles. The formation approach results in electrochemically synthesized ZnO/Zn anodes providing a stable ZnO matrix in which Zn particles retain their localized distribution on cycling better than control electrodes conventionally made by pasting zinc particles together with a binder resulting in ~70 % improvement in cycle life at 10 mA/cm2 rate.
Kolhekar, Snehal H., "Investigating the Effect of Electrolyte Concentration and Charging Protocol on Materials Utilization in Rechargeable Zinc Manganese Dioxide Batteries" (2021). CUNY Academic Works.