Negative-plate Additives

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 

The final reason for the renewed interest in expanders — and indeed in all types of additive — is the desire to improve the coulombic output of negative plates, primarily in order to improve the specific power and specific energy of lead-acid batteries. This objective has assumed more importance recently given the need to develop batteries for EV and HEV applications. 

Early studies on the effects of carbon addition to negative pastes [21,25] showed that carbon functions primarily as a conductor, and thereby enhances formation efficiency and reduces the level of residual sulfate. It may also improve recharge, particularly under deep-discharge conditions. It was further concluded that, when combined with barium sulfate and an organic component, a typical carbon addition of 0.2wt.% has little influence on the discharge performance and cycle-life of batteries. 

Carbon content (wt.%) Free-lead content (wt.%) Porosity (%) Mean pore size (nm) Specific surface-area 

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. 

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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. 

In addition to carbon-based systems, other intercalation compounds are also currently being proposed as alternative lithium ion cell negative plates. Examples include LiJtTiS2, Li/TiC, L /H-sO and, more recently, a family of Li SnOv compounds. However, the applicability of these materials in practical batteries has not yet been established, and coke and graphite are still the materials used in all commercial lithium ion cells. 

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.)... 

Resolution of the above problems is made more difficult by the sensitivity of negative plates to various secondary reactions, such as those caused by impurities and additives. These may affect the self-discharge rate, as well as the charge-acceptance and the efficiency at which the charge input is subsequently used. The transient nature of these effects increases the difficulty of understanding their influence. Several studies are now underway to increase this understanding and... 

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.%. 

The eoneern over the performance of negative plates in VRLA batteries has resulted in renewed interest in the influence and mechanisms of organic additives and extensive research programmes have been carried out under the auspices of the ALABC. This work has included an assessment of 34 materials, five of which were synthetie organie compounds that were identified to have the potential to act as effective expander components in lead-acid batteries [32]. Preliminary screening tests for stability in acid, impurities and thermal stability, followed by studies of potentiostatic transients, impedance plots, and cyclic voltammograms [33], have... 

All the organic additives increased the BET specific surface-area of the negative plate. In the absence of an additive, the surface area was 0.2m g Vanisperse A — probably the organic material which is most commonly used by the battery industry... 

Table 5.3. Range of mean active-material utilization (mAhg ) of negative plates with varying amounts of additive. ...

Fig. 5.12. Effect of t5 pe and amount of organic additive on utilization of negative-plate material after 100 cycles at 25 and 40°C.

Fig. 5.13. Change in cathodic peak potential E p — E) of negative plates with (E exp) and without organic additive ( ).

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