An investigation into the effect of carbon type addictives on the negative electrode during the partial state of charge capacity cycling of lead acid batteries

It is well known that a conventional lead acid cell that is exposed to a partial state of charge capacity cycling (PSoCCC) would experience a build-up of irreversible PbSO4 on the negative electrode. This results into a damaged negative electrode due to excessive PbSO4 formation by the typical visual “Venetian Blinds” effect of the active material. This displays the loss of adhesion of the active material with the electrode’s grids thereby making large sections of the material ineffective and reducing the cells useful capacity during high current applications. The addition of certain graphites to the negative paste mix had proven to be successful to reduce this effect. In the first part of the study, the physical and chemical properties of the various additives that are added to the negative electrode paste mix were comparatively studied. This was done to investigate any significant differences between various suppliers that could possibly influence the electrochemical characteristics of the Pb-acid battery performance. This comparative study was done by using the following analytical techniques; BET surface area, laser diffraction particle size, PXRD, TGA-MS and SEM. The study showed that there were no significant differences between the additives supplied from different suppliers except for some anomalies in the usefulness of techniques such as N2 adsorption to study the BET surface area of BaSO4. In order to reduce the sulphation effect from occurring within the Pb-acid battery a number of adjustments are made to the electrode active material. For example, Pb-acid battery manufacturers make use of an inert polymer based material, known as Polymat, to cover the electrode surfaces as part of their continuous electrode pasting process. It is made from a non woven polyester fiber that is applied to the pasted electrodes during the continuous pasting process. In this study the Polymat pasted electrodes has demonstrated a better physical adhesion of the active material to the grid support thereby maintaining the active material’s physical integrity. This however did not reduce the sulphation effect due to the high rate partial state of capacity cycling (HRPSoCCC) test but reduced the physical damage due to the irreversible active material blistering effect. The study investigated what effect the Polymat on the electrodes has on the III battery’s Cold Cranking Ability (CCA) at -18 degree C, the HRPSoCCC cycling and its active material utilization. The study showed that there was little or no differences in the CCA and HRPSoCCC capabilities of cells made with the Polymat when compared to cells without the Polymat, with significant improvement in active material’s adhesion and integrity to the grid wire. This was confirmed by PXRD and SEM analysis. Negative electrodes were made with four types of graphites (natural, flake, expanded and nano fibre) added to the negative paste mixture in order to reduce the effect of sulphation. The study looked at using statistical design of experiment (DoE) principles to investigate the variables (additives) such as different graphites, BaSO4 and Vanisperse to the negative electrode paste mixture where upon measuring the responses (electrochemical tests) a set of controlled experiments were done to study the extent of the variables interaction, dependency and independency on the cells electrochemical properties. This was especially in relation to the improvement of the battery’s ability to work under HRPSoCCC. The statistical analysis showed that there was a notable significant influence of the amounts of vanisperse, BaSO4 and their respective interactions on a number of electrochemical responses, such as the Peukert constant (n), CCA discharge time, material utilization at different discharge rates and the ability to capacity cycle under the simulated HRPSoCCC testing. The study did not suggest an optimized concentration of the additives, but did give an indication that there was a statistical significant trend in certain electrochemical responses with an interaction between the amounts of the additives BaSO4 and Vanisperse. The study also showed that the addition of a small amount of Nano carbon can significantly change the observed crystal morphology of the negative active material and that an improvement in the number of capacity cycles can be achieved during the HRPSoCCC test when compared to the other types of graphite additives