The influence of different negative expanders on the performance of VRLA single cells

In the first part of the present project, electrochemical characterisation techniques for several expander materials were developed and applied to a total of 19 potential expander candidates, selected from various different sources: conventional and novel expanders, new synthetic products, and natural substances. From the obtained results, seven most effective candidates were selected for further testing in real VRLA single cells. In these tests, all the candidates have been tested at the 0.2% concentration level in the paste, and for three selected candidates, alternative levels of the expander, carbon black and barium sulphate have been additionally tested. After the cell formation, initially a series of preliminary discharges at different discharge rates were carried out in order to characterise the initial performance of each formulation. Then, the single cells were submitted to the initial tests previous to the EUCAR ECE15L cycling specification for electric vehicles and then to the ECE15L cycle life test at 80% depth-of-discharge (DoD). The results of the preliminary discharges showed that differences up to 10–13% can be found in the initial capacity delivered by the different formulations at the test rates. The results of the cycle life tests showed that all the tested variables (type/concentration of the expander, concentration of carbon black and concentration of barium sulphate) usually caused a variation in the cycle life behaviour, although the amount of this variation was strongly dependent on the considered expander. The most significant effect was observed when the carbon black level was increased, producing remarkable improvements both in the initial discharge capacities and in the cycle life performance.