Zinc electrode performance in modified electrolyte

The high degree of dissolution that zinc undergoes in alkaline electrolytes is widely acknowledged as being a major cause of the problems encountered in secondary battery systems viz. shape change and dendrite growth. Attempts to alleviate these problems have usually centered on modifications to the electrode, electrolyte and/or separator. This thesis describes the effects of modifying the electrolyte on the electrochemical properties of the electrode. Preliminary evaluation of a number of different electrolyte additives was performed using the classical electrochemical techniques of galvanostatic polarisation, cyclic voltammetry and rotating disc experiments. These methods provided both a quick screening test for assessing a variety of additives, as well as yielding fundamental electrochemical information of the system. Long term cycling experiments were performed on actual battery electrode pastes in the modified electrolytes which exhibited the most promise in initial screening. These trials revealed that a trade-off in utilisation, cycle-life and dendrite growth prevention has to be made in order to optimise the system. Addition of fillers, causing an increase in the surface area of the electrode, was found to improve results, P.V.A. proving to give most benefit. Typical cyclelife performance of a zinc electrode with a 10% P.V.A. addition, in a borate modified electrolyte, revealed that even after seventy C/5 charge-discharge cycles, retention of over 50% of initial capacity was obtained, with no dendrite growth. One electrode additive which did not prove to be succesful was that of graphite, due to excessive hydrogen evolution on recharge. Further investigation of this problem indicated that when zinc can undergo dissolution into solution, and eventually deposit onto the graphite surface, no problem should arise. However in the case of the modified electrolytes, soluble zinc species is minimised, leaving exposed graphite from which hydrogen evolution can occur.