Positive electrodes grids

Highly corrosion-resistant, antimony-free, lead alloys have been used successfully in the positive electrode grids of both single-plate and spiral-wound cells. Spiral-wound cells almost exclusively feature binary lead-tin alloys [36,41] whereas flat-plate electrodes use either lead-calcium alloys, with or without silver additive [26], or lead-tin alloys [48]. Binary lead-tin is known to be highly corrosion-resistant, but rather soft, which is a handicap for plate stacking in prismatic cells. 

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. 

Corrosion of the current-conducting elements in the positive electrode, as of the plate support (grid), bus bars, and termi-... 

In Sec. 4.3.3 it has been shown that corrosion is one or the reactions that cause selfdischarge of the positive electrode. In connection with Fig. 8 it has been mentioned that an anodic current, the corrosion current, must flow continuously to stabilize the lead dioxide layer at the grid surface. Then the PbOA layer remains thin because PbOr is always converted into Pb02 by... 

Typical pore size distributions result in mean pore diameters of around 15 //m. Even long and intensive efforts did not succeed in decreasing this value decisively in order to enable production of micropo-rous pocketing material resistant to penetration [65, 66], In practice PVC separators prove themselves in starter batteries in climatically warmer areas, where the battery life is however noticeably reduced because of increased corrosion rates at elevated temperature and vibration due to the road condition. The failure modes are similar for all leaf separator versions shedding of positive active mass fills the mud room at the bottom of the container and leads to bottom shorts there, unless — which is the normal case — the grids of the positive electrodes are totally corroded beforehand. 

The oxidative stability is excellent. Direct contact between the glass mat and the positive electrode effects a far lower tendency to shed active mass thus as a general rule the failure mode is positive grid coiTosion. 

These equations are based on the thermodynamically stable species. Further research is needed to clarify the actual intermediate formed during overcharge. In reality, the oxygen cycle can not be fully balanced because of other side reactions, that include grid corrosion, formation of residual lead oxides in the positive electrode, and oxidation of organic materials in the cell. As a result, some gases, primarily hydrogen and carbon dioxide (53), are vented. 

FM2. Water loss is reduced to the extent that periodic replenishment, as required by flooded cells, is no longer necessary. Nevertheless, both hydrogen evolution at the negative electrode and grid corrosion at the positive electrode do cause some water loss. 

Figure 9 0 Components of a lead acid battery, (a) Cast lead alloy grid, (b) separator, (c) positive electrode, (d) negative plate, (e) negative plate group, (f) positive plate group, (g) plate block with separator, and (h) the complete battery. (Reproduced with permission from Ref. [25], 1985, Wiley-VCH.)...

Flat plate and tubular positive plate cells are produced for stationary duty, but where reliability is a prime consideration, Plante cells are used. In a Plants cell, the positive electrodes are manufactured by a quite different process. The oxide is formed by electrochemical oxidation (say, 10 mA cm for 20 h) of a lead baseplate or grid, often shaped to increase its surface area, in an electrolyte which contains sulphuric acid and an anion (perchlorate or nitrate) wliich forms a soluble Pb " salt. This leads to a layer of thick porous oxide the nitrate or perchlorate is present to prevent total passivation of the lead surface. The resulting plate, thickness 6—12 mm, is then reduced to form spongy lead metal, is washed thoroughly, and is recharged when in a fabricated cell. The active material formed in this way adheres to the base lead better than pasted materials and therefore cycles more reUably. Against this, there is less active material on each plate and, inevitably, the energy density of the battery will suffer 7—12 Wh kg is typical. 

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