Page 225 - Handbook of Battery Materials

194 6 Lead Oxides

When specimens of pure lead and a 5% antimony alloy were periodically oxidized
and reduced, lead oxide layers were observed with different structures:
• coarse and insulating with antimony-free electrode,
• fine and low resistance with the antimony alloy [38].

The origin of such insulating layers may explain the high resistance also
established within the active material when antimony is not present.
The ‘Kugelhaufen’ model mentioned in the preceding section explains the
beneficial influence of antimony by improved conductivity of the zones that
connect the spheres. According to the gel model, antimony decreases the
crystallinity of PbO 2 and so increases the conductivity by the gel zones [39], and
especially influences the structure of the corrosion layer intermediate between the
grid and active material [40].
Addition of tin to the positive-grid alloy also has a capacity-stabilizing effect, but
this apparently concerns only the boundary between the grid and active material.
Phosphoric acid (H 3 PO 4 ) is added in small amounts to the electrolyte. A beneficial
effect on cycle stability has long been known for this acid, which has been used
as an additive in conventional lead–acid batteries for many years to improve cycle
stability, although the disadvantage of a slightly reduced capacity had to be accepted
[41]. Addition of phosphoric acid to the electrolyte improves long-term capacity
and reduces the formation of sulfate layers around the grid [42]. The addition of
20–35 g dm −3 phosphoric acid was protected by patent for valve-regulated lead–acid
batteries with gelled electrolyte [43]. Extensive experiments [44] showed that, at low
H 3 PO 4 concentrations, Pb 3 (PO 4 ) 2 acts as an intermediary in the corrosion of Pb to
PbO 2 . Clearly, the phosphoric acid influences the formation of lead dioxide (PbO 2 )
on account of its strong adsorption and leads to a fine grain structure of the positive
active material [45]. However, in spite of the repeated use of phosphoric acid in
lead–acid batteries, some questions on its interaction are still to be elucidated [46].

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