Positive active materials

Lead oxide (PbO) (also called litharge) is formed when the lead surface is exposed to oxygen. Furthermore, it is important as a primary product in the manufacturing process of the active material for the positive and negative electrodes. It is not stable in acidic solution but it is formed as an intermediate layer between lead and lead dioxide at the surface of the corroding grid in the positive electrode. It is also observed underneath lead sulfate layers at the surface of the positive active material. 

Finally, one development results from returning to a basic idea from the dawn of the lead-acid battery, wherein the functions of support for the positive active material and of the separator are combined into one component the gauntlet separator [84] consisting of a coarsely porous, flexible support structure coated with micropo-rous polyethylene material for separation. The future has to show whether this approach will be able to meet all demands. 

In acidic electrolytes only lead, because it forms passive layers on the active surfaces, has proven sufficiently chemically stable to produce durable storage batteries. In contrast, in alkaline medium there are several substances basically suitable as electrode materials nickel hydroxide, silver oxide, and manganese dioxide as positive active materials may be combined with zinc, cadmium, iron, or metal hydrides. In each case potassium hydroxide is the electrolyte, at a concentration — depending on battery systems and application — in the range of 1.15 - 1,45 gem"3. Several elec-... 

This chapter reviews the effects of additives in the positive active-material of the lead acid battery. Common materials found in the oxide and the positive-plate paste, such as lead oxides, basie lead sulfates, and lead earbonate, are not included. Additives and impurities that derive from grid eorrosion or from the reaction of the plate with the electrolyte are also beyond the seope of this chapter. 

In an attempt to reach a better understanding of lead-acid batteries and the physical processes that limit capacity, computer programs were developed that simulate the conductivity of the positive active-material and the diffusion of sulfate ions [2]. 

Attempts to assist the supply of acid to the positive active-material have focused on increasing the porosity of the paste to enhance diffusion or by providing fine local reservoirs of acid within porous particles throughout the plate [4]. One such... 

Silica gel has also been studied as a porosity-increasing additive [5]. The addition of 0.2wt.% resulted in improved capacity at both low and high rates. Although the addition of silica gel was not observed to cause significant physiochemical changes to the positive active-material, it is possible that silica gel promotes nucleation which consequently leads to a finer pore structure in the active material. Since this form of silica is porous, the additive may act as an acid reservoir. Further study of silica gel as an additive, especially at high discharge rates, has been recommended [6]. 

The addition of sulfates to the positive plate was evaluated by Lorenz (as described in Ref. 58). Results showed that 0.5wt.% barium sulfate or strontium sulfate added to the positive active-material reduced the cycle-life from 100 cycles without the additive to 30-50 cycles with the additive under the same conditions. The end-of-life was taken as a 40% decline in the initial capacity. Lorenz further reported that calcium sulfate is not isomorphous with lead sulfate and therefore has no effect on battery life. (Note, calcium sulfate also does not act as an inorganic expander for negative plates.)... 

Tubular plates have been made with chemically prepared lead dioxide and 2.2 or 3.8wt.% sodium sulfate in the positive active-material [44]. Test results on cells cycled at constant-current are shown in Table 4.7. It was concluded that sodium sulfate increases the utilization of material by dissolving to create a more porous structure in the positive plate. It was also noted that graphite has an even greater effect in the same concentration range (see Section 4.4.7). 

The effect of the addition of calcium sulfate to positive pastes has also been investigated [67]. Pasted electrodes were cycled three times as well as to the end of the battery cycle-life. The positive active-material was washed with water, dried, pulverized, and used to pack tubular electrodes. Additions of 0, 0.1, 0.2,... 

Voss [69] has published a comprehensive review of the effects of phosphoric acid on the performance of lead-acid batteries. This review included previously unpublished tests by Kugel, Rabl, and Woost conducted between 1926 and 1935, in which phosphoric acid was added to the positive paste at the 0.9, 2.0,2.9, or 4.0wt.% level, and at 0.9wt.% in the electrolyte. The results showed that, during cycling, the phosphoric acid concentration increases in the electrolyte but decreases in the phosphated positive active-material. It is also higher in the electrolyte when the cell is discharged. Tables 4.8-4.10 show the results of these studies. 

Table 4.8. Content of phosphoric acid in electrolyte and charged positive active-material of...

Although the influence of the above materials over extended battery service has yet to be quantified, experimental trials at 25 and 40°C with three-plate cells which had an excess of positive active-material have provided an indication of then-effectiveness during cycling [9]. The cells were discharged at 1.50 A (16.3 mA cm ) to... 

To restrict movement of positive active-material into separator. 

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