Zonal processes

Pb + PbS04 zones, the latter will progress towards the interior of the paste. These zonal proeesses are illustrated in Fig. 3.45. 

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Zonal processes are due to the reactions that proceed between the ions of the solution and the solid porous mass as a result of which a new solid porous phase is formed [4]. The latter occupies a certain zone in the volume of the solid porous mass, see Fig. 3.3. 

The reaction between the ions of the solution and the initial solid porous phases proceeds in a reaction layer, which is located between the zone of the initial phases and the zone occupied by the reaction products. The reaction layer shifts in space and penetrates the volume of the initial phase zone. Thus, the volume of the zone occupied by the reaction products grows at the expense of the initial phase zone. The direction in which the reaction product zone grows depends on the transport hindrances experienced by the ions on their way from the solution volume to the reaction layer and/or on the transport difficulties met by the ions formed in the reaction layer on their way to the solution volume. On soaking and formation of the positive and negative plates, some zonal processes occur. During soaking, only chemical reactions proceed in the reaction layer (i.e., chemical zonal processes). 

During plate formation, electrochemical reactions proceed in the reaction layer (i.e., electrochemical zonal processes). 

In H2SO4 solution of low rel. dens., the zonal processes should be carefully controlled as they create an inhomogeneity in the phase composition of the paste throughout the plate cross-section, which leads to inhomogeneity of the PAM structure. 

Zonal processes during formation of PAM from 3BS-cured pastes... 

Zonal processes are slow processes. They are responsible for the long duration of plate formation. Hence, it is important to find methods to by-pass the zonal processes. In order to accelerate the formation of positive plates, some conductive additives have been added to the positive pastes [28]. These additives increase the conductivity of the cured paste and formation proceeds almost uniformly throughout the whole plate volume as the electric current flows along the conductive additive network. The additives should be chemically stable in H2SO4 solution. Data have been reported about successful attempts to reduce the duration of the formation procedure to 8 h. 

Each of the above parameters exerts an influence on the formation of the structure of the active masses and on the nature of their interfaces with the grids. Thus, the parameters affect the performance of the battery. The algorithm of the formation should take into account the zonal processes that occur on both types of plate so as to ensure the formation of appropriate active-mass structures, which would guarantee high battery performance characteristics. 

Positive-plate formation. As discussed earlier (Figs. 3.16 and 3.17), zonal processes occur during formation of the positive plates. In the case of the positive-plate... 

Negative-plate formation. The stages in the formation of negative plates have been determined in terms of the zonal processes as presented in Fig. 3.45. The reactions involved in each of the formation stages are presented schematically in Figs. 3.43 and 3.44, and are summarized in Fig. 3.59. The formation can be divided into the following stages. 

In view of the potential-time curve profile, as well as the reaction schemes and zonal processes presented in Figs. 3.13 to 3.17, and the structure of the plate as shown in Fig. 3.58, formation can be divided into four stages. 

Zonal Processes DuringPlate Soaking Forming Different Sub-Layers Across the Plate Thickness [4]... 

Zonal processes are slow processes. They are responsible for the long duration of plate formation. Hence, it is important to find methods to by-pass the zonal processes. In order to accelerate the formation of positive plates, some conductive additives have been added to the... 

Pb02, when present in sufficient amounts, may suppress the zonal processes and thus improve substantially the efficiency of the formation process. This can be achieved, however, only if the leady oxide does not contain much lead. In the opposite case, the following reaction proceeds ... 

The distribution of the a-Pb02 and (8-Pb02 phases through the cross-section of the plate depends on the local pH of the solution that fills die pores of the reaction layer. It has been established that j8-Pb02 predominates in the outer layers of the plate, whereas a-Pb02 is found mostly in the plate core [16—21]. This phase distribution is determined greatly by the zonal processes that occur in the plate. 

S04 ions must move in the reverse direction. The rates of these flows control the rate of the electrochemical reactions in the reaction layer. Since H" " ions are much more mobile than 804 ions, they will maintain the electric neutrality of the reaction layer. The FT ions experience the least transport difficulties when passing along the shortest paths between the surface plate layers and the bulk of electrolyte. Therefore, the (Pb + PbS04) zones will first grow in the plate surface layers and, after the plates are fully covered by the (Pb + PbS04) zones, the latter will progress towards the interior of the paste [4]. These zonal processes are illustrated in Fig. 11.4. 

Having made this overview of the processes that occur in VRLA cells during charge, we can distinguish four groups of zonal processes that deserve to be discussed in more detail. These are (a) processes on the positive plates (b) processes on the negative plates (c) transport processes through the separator and (d) thermal processes in VRLAB. 

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