Negative plates expander

Non-sulfonated lignins find utility as emulsifiers and stabilizers in water-based asphalt emulsions, as coreactants in phenolic binder applications, as negative plate expanders in lead acid storage batteries, as protein coagulants in fat rendering, and as flocculants in waste water systems. 

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. 

Constructional details of a modem lead-acid cell for locomotives in coal mines are given in Fig. 6.67. A copper expanded metal lattice parallel with the negative plate considerably lowers the internal resistance of the cell. The total view of a 540V 80 Ah lead-acid accumulator for locomotives in coal mines is given in Fig. 6.68. 

The secondary lead structure of the completely formed plates with or without expander is shown in Fig. 3.57. In NAM without expander, the secondary lead structure covers the skeleton in the form of a smooth layer, whereas the secondary lead structure in the NAM of expander-containing plates comprises individual Pb crystals which are located over the skeleton structure. Hence, organic expanders regulate the processes involved in the formation of both types of structure in the lead active-mass during the formation of negative plates. 

In VRLA batteries, oxygen reaches the negative plates and is reduced to form water. It also oxidizes the expander and thereby produces carbon dioxide. Consequently, the morphology of the secondary Pb crystals is changed and they become dendrite-like, similar to those shown in Fig. 3.57(a). This results in capacity... 

Fig. 3.55. Polarization curves during second hour (first stage — I) and seventh hour (second stage — II) of formation of negative plates with or without expander [66].

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... 

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. 

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... 

Battery practice has proved that for batteries produced with 3BS pastes, tbe maximum allowable content of orthorhomb-PbO is 10 wt%. If 4BS pastes are used, however, this upper limit is increased to 20 wt%. The above limits hold for positive battery plates. Tbe phase composition of the pastes for negative plates is practically of less importance, because no 4BS crystals are formed in the presence of expander. Here, the content of orthorhomb-PbO should not exceed 15 wt%. 

On adding the expander, formation of 4BS stops, orthorhombic PbO is probably converted into tet-PbO, which is associated with the formation of a certain amount of 3BS, too. As a result of these processes the paste contains no orthorhombic PbO. And, as the latter is one of the basic initial compounds needed for 4BS nucleation, this latter phase is no longer formed in the paste. Formation of 4BS is blocked as a result of lack of orthorhombic PbO in the paste. Hence, no 4BS crystals are formed in the pastes for negative plates containing expander. 

Pastes for positive and negative plates are prepared in separate mixers so as to avoid contamination of the positive paste with BaS04 and expander from the negative paste. Such impurities would impair the performance of positive plates. 

Experiments were performed using plates with expanders Quebraco and EZE-Skitan. The resulting capacity curves, as a function of the time of oxidation or reduction, are presented in Fig. 7.7. Besides the plates subjected to oxidation or reduction, reference negative plates with the same expanders were left in the test cells with no hydrogen or oxygen attack. 

Quebraco disintegrates when attacked by hydrogen and even more rapidly under oxygen attack. EZE-Skitan reacts with H2 and O2, but the resulting changes in expander structme improve its activity and thus the capacity of the negative plate increases. 

Figure 7.8 presents the capacity curves for cells assembled with negative plates with different expanders. It is evident from the data in the figure that expanders Mimosa and Velex have but a weak effect on battery cycle life. SNK and especially EZE-Skitan and Quebraco, improve... 

Changes in capacity during cycling of cells with negative plates with five different expanders or... 

The influence of temperature on the cycle life of negative plates containing the above discussed expanders is illustrated in Fig. 7.10 [26]. As all above tests were performed with VRLA cells, the effect of oxygen (during operation of the oxygen cycle) on the expander should also be added to the temperature effects. 

When the battery is cycled at 60 °C and is of the VRLA type, expanders containing lignin and its derivatives disintegrate, as a result of which the battery cycle life is reduced almost twice. In order to improve the cycle life performance of the negative plates, the battery temperamre should be kept equal to about 40 °C. 

The cycle life within one cycle-set depends strongly on the nature and properties of the carbon or graphite additives used. These materials differ in particle size, structure and affinity to lead and to the expander. Of special importance is the interface between carbon and lead particles, and its area as it determines the resistance that electrons have to overcome when transferred between these two phases and thus affects the potential and the rate of the electrochemical reactions at the carbon/solution interface. Only a limited number of carbon and graphite materials have optimum structural characteristics and may improve substantially the cycle life performance of the cells. It is of crucial importance to identify the most effective carbon (graphite) additives, i.e. with most beneficial effect on the parallel mechanism of charge of the negative plates. 

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