Understanding the mechanism by which bismuth improves lead-acid battery capacity

To elucidate the mechanism by which bismuth enhances the capacity of valve- regulated lead-acid (VRLA) batteries, model experiments are performed on pu lverized positive electrodes produced either from leady oxide, which contai ns virtually no bismuth (termed 'Bi-free oxide'), or from Pasminco VRLA Ref ined(TM) oxide, which is of high purity and contains a specified amount (0. 05 wt.%) of bismuth. The electrodes are compressed under a range of pressur es (1.4 to 60 kPa). Below 40 kPa, the presence of bismuth increases the ini tial capacity. At all pressures, bismuth enhances the rate at which the cap acity develops during cycling. Reconnection of the separated agglomerates o f lead dioxide is the key factor in restoring the capacity of the pulverize d electrode. Electron micrographs reveal that there are two essential types of contact in the positive material: (i) 'micro-contact' between individua l irregular-shaped or individual needle-like crystals, to form the agglomer ates; (ii) 'macro-contact' between individual agglomerates, to form the ske leton of the positive mass. Bismuth encourages the growth of fine needle-li ke crystals on the surface of the agglomerates. These crystals spread out a nd inter-weld to form 'bridges' between the agglomerates and, thereby, cons olidate the porous mass of the electrode. This influence of bismuth on morp hology is considered to be responsible for the demonstrated improvements in capacity performance. (C) 2000 Elsevier Science S.A. All rights reserved.