Negative plate sulfation

Paste Mixing. The active materials for both positive and negative plates are made from the identical base materials. Lead oxide, fibers, water, and a dilute solution of sulfuric acid are combined in an agitated batch mixer or reactor to form a pastelike mixture of lead sulfates, the normal, tribasic, and tetrabasic sulfates, plus PbO, water, and free lead. The positive and negative pastes differ only in additives to the base mixture. Organic expanders, barium sulfate [7727-43-7] BaSO carbon, and occasionally mineral oil are added to the negative paste. Red lead [1314-41 -6] or minium, Pb O, is sometimes added to the positive mix. The paste for both electrodes is characterized by cube weight or density, penetration, and raw plate density. 

FIGURE 11.4 An aqueous solution of potassium sulfate conducts electricity. When metallic plates (electrodes) charged by a battery are put in the solution, positive ions (K ) migrate toward the negative plate and negative ions (SOl ) migrate toward the positive plate. 

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.)... 

Traditionally, negative plates in lead-acid batteries contain a combination of carbon black, barium sulfate, and an organic additive which is usually a wood extract. These additives are collectively called an expander , although this term is often used purely for the organic component of the mix. The presence of the expander helps to... 

Both Shiomi et al. [26] and Saez et al. [27] have suggested that high levels of carbon improve the rechargeabihty of negative plates by providing eonduetive networks around the peripheries of lead sulfate crystals and, thereby, extend battery cycle-life. 

The influence of barium sulfate additions on the performance of negative plates has been studied extensively [21,25]. Barium sulfate is isomorphous with lead sulfate and therefore functions as a nucleation centre ( seed ) for the precipitation of the discharge product and favourably restricts its crystal size (see Section 4.5.1, Chapter 4). Strontium sulfate behaves similarly. It has been suggested that the effectiveness of barium sulfate is directly attributable to the number of nucleii present rather than to the amount added. The optimum amount of barium sulfate when combined with other additives for use in automotive batteries has been found to be between 0.3 and 0.5wt.%. 

In a different battery test with a simulated EV load pattern, a SWP-7 cell with an assembly pressure of 60 kPa achieved 450 cycles versus 270 cycles for an AGM cell with 73 kPa. The failure mode was found not to be the expansion of positive plate but, rather, sulfation of the negative plate. This led to the conclusion that the favourable mechanical properties of SWP-type separators suppress degradation of the positive active-material. 

There have been a small number of reports published recently that describe the development of purpose-built batteries for RAPS systems. A VRLA battery for use in PV power systems has been described by Shiomi et al. [27]. The negative plates contain a high level of carbon, but the particle size and concentration of the additive are not described. The recommended level of carbon is simply given as ten times normal levels . The type of VRLA battery used in the experiments is also unclear. Batteries with and without the additional carbon were operated under simulated PV duty for extended periods. Increasing the carbon by ten-fold was found to extend the cycle-life of the batteries from 400 to 1000 cycles. This improvement was attributed to the formation of a conductive network of carbon around the peripheries of lead sulfate crystals. The subsequent increase in conductivity was claimed to improve the rechargeability of the negative plates and, thereby, to suppress the accumulation of lead sulfate. 

Failure modes Corrosion, shedding PCL 1, PCL 2 (see Chapter 9) Under-charge and sulfation of negative plates... 

Recent investigations conducted by CSIRO [7,8] have provided the following explanation for the mechanism of lead sulfate accumulation in negative plates during HRPSoC duty. 

Fig. 17.6. Schematic representation of the distribution of lead sulfate in a negative plate subjected to (a) low-rate discharge or (b) high-rate discharge.

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