(518k) Reducing Zinc Redistribution and Extending Cycle Life with Electrochemically Synthesized Zinc Oxide Anodes in Rechargeable Alkaline Batteries

Zinc has found its way into a host of battery systems such as zinc- air, zinc- bromine, silver-zinc, zinc- manganese dioxide and zinc-nickel batteries on account of its low toxicity, low cost, high theoretical specific capacity (820 mAh g^-1) and ease of handling. A preliminary economic analysis done on a large scale prismatic 1030 Wh Zn-MnO₂ battery unit has shown that if the utilization on the zinc electrode is increased (e.g., to 25% of its theoretical capacity of 820 mAh g^-1) then the total fabrication cost of the battery can be brought down well below $100 per KWh. In the last one year where the cost of Zn has steadily declined to about $2.5 kg^-1, the cost target of a under $100 per KWh battery can be achieved even with a Zn utilization of 12 - 15 %. However, due to the unpredictable behavior of metallic zinc anodes in concentrated alkaline electrolyte, its use has been greatly limited. Principal causes of cell failure have been ascribed to zinc dendrite formation leading to shorting, and to loss of capacity due to anode shape change. The problem of undesirable zinc morphology has been addressed to a great extent with the use of additives in the electrode and the electrolyte. The problem of zinc electrode shape change, however has not been fully overcome due to the movement of zincate ions in the electrolyte accompanied by electrodeposition of metallic zinc in disparate locations when cycled in concentrated alkaline electrolyte. Zinc electrode shape change is the redistribution of active material over the electrode surface as a result of cycling. This phenomenon gradually develops over several cycles resulting in reduced active electrode surface area as the zinc active material accumulates towards the bottom and other electrolyte rich regions, while severely denuding the upper regions of the electrode. To potentially mitigate this phenomenon, we investigate the cycling of an in-situ formed zinc-based anode comprising a highly porous electrochemically synthesized zinc oxide matrix with electrodeposited zinc particles. These anodes are formed by fully discharging ‘conventional’ anodes and then charging them up partly to result in anodes consisting of an electrochemically formed ZnO matrix in which the Zn particles are quite uniformly distributed and embedded. These ZnO /Zn anodes have a porosity of ~70 % which is twice that of the conventionally made metallic zinc electrode. Compared to cycling of the conventional zinc anodes at the same depth of discharge (utilization of the theoretical capacity of Zn), we find a ~70 % improvement in cycle life performance at 12 % utilization and at a rate of 10 mA cm^-2, without any additive. A quantitative chemical analysis of the bulk electrolyte revealed undersaturated zincate ion concentration in the electrochemically synthesized ZnO cells indicating reduced zincate movement. To gain more insight, extensive ‘post mortem’ measurements of Zn and ZnO masses and their distribution on both the electrochemically synthesized ZnO anodes and the conventionally made zinc anodes (the controls) were made as functions of cycle life. The experiments conducted on the electrochemically synthesized ZnO anodes showed higher retention of Zn and ZnO on the electrode surface, especially in the upper region of vertically aligned Zn anodes signifying reduced redistribution of active material compared to the controls. An Image analysis conducted on the Zn particles also revealed a narrower particle size distribution in the range of 62 - 79 μm for the electrochemically synthesized ZnO anodes compared to that of the control anodes which showed a wider particle size distribution of 51 - 96 μm. The results suggest that the electrochemically synthesized ZnO anodes provide a matrix in which the zinc particles retain their localized distribution on cycling better than the control electrodes conventionally made by binding zinc particles together, which migrate more freely on cycling.