THE EFFECT OF TIN ON THE PERFORMANCE OF POSITIVE PLATES IN LEAD-ACID-BATTERIES

There are many reports that the use of non- or low-antimonial grids in lead/acid batteries can give rise to the development of a high-impedance 'passivation' layer at the grid/active-material interface. It is generally agreed that the layer has a duplex structure that comprises alpha-PbO deposited directly on the grid surface beneath a compact covering of PbSO4; basic sulfates and alpha-PbO2 may also be present. The development of this structure hinders recovery from prolonged deep discharge or self-discharge. Similar phenomena can be observed with dry-charged positive plates. Passivation can also occur during cycling and float operations but, in these duties, the formation of a non-conductive layer of PbSO4 is thought to be the prime cause of the degradation in plate performance. The incorporation of tin in the positive grid (either in the alloy itself or as a surface layer) is found to reduce the level of alpha-PbO and greatly alleviate passivation problems relating to charge acceptance. Various mechanisms have been proposed for this tin effect and range from a semiconductor-type doping of alpha-PbO to changes in the porosity of the PbSO4 layer and/or the reactivity of alpha-PbO towards oxidation. Thc benefits of tin in cycling and float duties are less obvious and it is probable that other features of cell design are more important determinants of battery performance. The action of tin when incorporated in the positive active material requireS further exploration.