The development of zinc electrodes for alkaline rechargeable batteries : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemistry at Massey University, Palmerston North, New Zealand
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Date
2002
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Massey University
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Abstract
The cycle life of nickel zinc cells is determined by a number of parameters, several of which have been investigated. The results of these investigations have led to the development of experimental nickel zinc (Ni-Zn) cells with cycle life in excess of 1200 cycles at high rates of charge/discharge. These are superior to any other nickel zinc cells reported in the open literature.
It was found that gross zinc active mass diffusion was the most significant problem in NiĀ Zn cells and is associated with zinc electrode shape change. The ready access of the electrolyte to the electrode assembly was the main cause for gross diffusion. It was proposed that the access of the electrolyte to the electrode assembly should be restricted.
It was identified that the reduction of zincate was a diffusion-controlled process and led to the growth of zinc dendrites. The addition of quaternary ammonium hydroxide to the electrolyte can efficiently inhibit the reduction of zincate. Among the quaternary ammonium hydroxides, tetrabutylammonium hydroxide saturated in the electrolyte was found to be the best for retarding the reduction of zincate.
Stainless steel wire was successfully adapted to replace the traditional Hg/HgO reference electrode as a simple pseudo-reference electrode for indicating the potentials of both nickel and zinc electrodes in a cell during cycling.
Sponge nickel is an important material used as the current collector to replace the conventional sintered nickel as the current collector used for Ni-Cd, Ni-MH and Ni-Zn cells. Using polymer sponge as a template, sponge nickel was successfully prepared in the laboratory with optimized procedures.
The cell structure was optimized with restriction of electrolyte to the electrode assembly. The electrode assembly was wound with nylon thread and then tightly enclosed in a plastic bag open at the top edges with minimal volume for the electrolyte. The bulk electrolyte was separated from the electrode assembly. The discharge/charge efficiency of a cell with this optimized structure was over 70% for 500 cycles (Cell #5.4).
Addition of zinc stearate or calcium stearate to the zinc active mass to make it hydrophobic was a further effort to restrict the access and diffusion of the electrolyte to the zinc active mass. With the optimized cell structure, 18.9% (w/w) zinc stearate resulted in much prolonged cycle life (Cell #6.3). Over the first 400 cycles the capacity for Cell #6.3 only declined from 94% to 91%. The efficiency remained over 80% for 620 cycles.
Further modifications were made by using 18.9% (w/w) calcium stearate in place of zinc stearate, adding solid KOH into the zinc active mass and electroplating the brass mesh current collector with zinc prior to pasting further prolonged the cycle life of the cells (duplicate cells, Cells #6.4 and #6.5). Cell #6.4 showed a high, and even slightly increasing discharge capacity from 92% to 94% over the first 560 cycles. The discharge/charge efficiency remained over 84.9% for 720 cycles. During these 720 cycles there were 4 periods of overcharging of the cell due to equipment failure, but no long lasting effects were discernible. The result of Cell #6.5 is similar in nature to those for Cell #6.4. The discharge/charge efficiency of Cell #6.4 remained over 70.7 % for 1221 cycles while the discharge/charge efficiency of Cell #6.5 remained over 70.22 % for 1112 cycles. Cells #6.4 and #6.5 exhibit the most prolonged cycle life performance of any other nickel zinc battery described in the open literature.
This new capability was successfully scaled-up by the use of two units in a cell, each identical to Cells #6.4 and #6.5 except for their size and capacity. One such cell (Cell #7.4) had a cycle life approaching 550 cycles with efficiency over 70%, indicating the potential for further scale-up.
Some of these findings are embodied in NZ Provisional Patent Application (No: 510554). This application has progressed to an international PCT (Patent Cooperation Treaty) application.
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Keywords
Electrodes, Zinc, Storage batteries