Page 223 - Handbook of Battery Materials

192 6 Lead Oxides

to stabilize the lead dioxide layer at the grid surface. Then the PbO x layer remains
thin because PbO x is always converted into PbO 2 by further oxidation.
At open-circuit voltage, no anodic current flow through the positive electrode
occurs that can oxidize the PbO (or PbO x ) layer, but the corrosion reaction
Pb + PbO 2 → 2PbO (6.33)
continues between grid and passivating layer. Consequently the PbO (or PbO x )
layer grows between the grid and PbO 2 .
However, as mentioned in Section 6.3.3, PbO and PbO x are not stable against
sulfuric acid, and react very fast according to
PbO + H 2 SO 4 → PbSO 4 + H 2 O (6.34)
as soon as these substances come in contact with it. The protecting lead dioxide layer,
shown in Figure 6.8, would be destroyed by this reaction, and severe grid corrosion
is one of the problems that occur when the battery stands for prolonged periods
without any charging. Battery manufacturers therefore recommend recharging a
lead–acid battery filled with electrolyte within regular periods, which must be
shortened when the battery is stored at elevated temperatures.

6.5.2
Charge Preservation in Negative Electrodes by a PbO Layer

The drying of negative plates is not possible without precautions, because of the
tendency to spontaneous oxidation. This oxidation reaction is much accelerated
by water, and the active material of a moist negative electrode is spontaneously
converted into lead oxide when exposed to air. When, on the other hand, the
charged plate is dry, a thin layer of oxide covers the surface of the active material,
and prevents further oxidation. So, prevention of access of oxygen as long as the
plates are wet is a common feature of various methods to achieve dry charged
negative plates. As a result of the superficial oxidation, a loss of about 10% of
capacity is always incurred with the dry charge process, regardless of the method
applied.
The dried plates can be stored for a practically unlimited time without losing
capacity or ageing. This is true also for complete batteries that are assembled but
not yet filled with electrolyte.

6.6
Ageing Effects

The active material of the positive electrode is prone to lose its mechanical strength
when repeated discharge/charge cycles occur, because the alternating dissolution
and precipitation processes convert the agglomerate structure into an accumulation
of fine crystals [31]. So, the active material suffers degradation, and part of it may
fall off the plate as fine particles. This process is called ‘shedding.’ Shedding