Electrodes calcium-zincate

Cell construction for the Ni-Zn cell is similar to that for the sealed spiral wrap cylindrical and prismatic Ni-Cd and Ni-MH cells for portable electronic applications but with some differences. The Ni-Zn cell incorporates a multilayer micro-porous separator with wicking components used to maintain uniform electrolyte distribution, as well as acting as a buffer/barrier to prevent shorting from zinc dendrite formation. The barrier also slows oxygen gas diffusion and lowers the recombination rate that can lead to pressure buildup inside the cell. The KOH electrolyte contains CaO additions to calcium zincate that can control the free zinc ions in solution. In the cell electrolyte, the CaO reacts with zinc ions to form calcium zincate. Other improvements center on the use of fiber (steel wool) current collectors for the zinc electrode. The Ni-Zn cells have excellent high rate and low temperature performance. 

Zinc electrodes can also be manufactured by the plastic-bonded method, similar to that of the nickel electrode described above. The zinc oxide dry powder, PTFE binder and other additives are blended with an organic solvent and then the mixture is passed through a calendaring process similar to that in Fig. 31.1. The PTFE fibriUates into a nano-structured three-dimensional fiber matrix. This electrode structure for the calcium zincate electrode is shown in Fig. 31.3 for a freshly prepared electrode. The active materials are locked into the electrode structural matrix which helps to reduce the tendency towards shape change and dendritic growth. 

Roll-bonded calcium zincate electrode after 500 cycles. (Courtesy of Ev-... 

Cells containing calcium zincate electrodes can be manufactured in at least two different ways. Calcium hydroxide can be added to the zinc oxide electrode mixture. In this case, the calcium zincate is formed in situ as the electrode is cycled in the ceU during the electrochemical formation process. Another method is to form calcium zincate in a separate step and then use this material in the electrode fabrication process. Calcium zincate can be prepared, purified and identified by its X-ray diffraction pattern, shown in Fig. 31.6. This process produces electrodes with a more uniform distribution of calcium zincate and overcomes the problem of zinc dissolution that occurs during the first few cycles before the calcium-zincate has fully formed in the in situ method. This serves to increase the overall cycle Ufe and performance of the battery. In either case, an important advantage of the plastic-bonded calcium zincate stmcture is the reduction of the tendency to form zinc dendrites. The zinc active materials are embedded in the three-dimensional stmcture of the PTFE nanofibers. This increases the stability of the electrode. Although it is possible for some zinc dissolution and migration to occur in the cell, dendritic deposits which penetrate the separator are rarely seen. 

FIGURE 31.6 X-ray diffraction pattern showing calcium zincate in zinc electrode after cycling. Courtesy... 

More recently it has been shown that electrolyte formulations in the potassium hydroxide concentration range of 20% to 25% can increase the cycle life obtained from the battery. This is particularly hue when used in conjunction with the reduced solubility calcium zincate electrode. The solubility of zinc is substantially reduced by decreasing the concentration of the electrolyte below 25%. This reduces zinc issolution and therefore zinc migration, stabilizing the zinc electrode and increasing the cycle life of the battery. 

Even with the use of reduced solubility calcium-zincate technology, the zinc electrode still has some finite solubility in the alkaline electrolyte. Zinc can form complex zincate anions in the electrolyte and diffuse throughout the battery. Some of this zincate is deposited within the pores of the nickel electrode. This may adversely affect the performance of the nickel electrode and thus the performance of the battery. It is possible that the gradual decrease in capacity observed is partially for this reason. 

---