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6.6 Ageing Effects 193
of the positive active material is a characteristic feature of ageing conventional
lead–acid batteries when they are charged and discharged frequently. It is likewise
also described as ‘soft positives.’ Shedding is only the outward appearance of a
more general ageing process which means that the active material is prone to
disintegration of its electronic conductivity and mechanical strength. This causes
the so-called ‘premature capacity-loss’ [32] (a survey with references is given in Ref.
[33]). It becomes evident as a decreasing utilization factor with increasing cycles.
In a model that considers the active material as an aggregate of spheres (‘Kugel-
haufen’) it is explained by a gradual increase in the ohmic resistance, mainly in the
connecting region of the individual particles of the active material. The connecting
regions establish the electrical contact between the individual particles of the active
material. They are decisive for the ohmic resistance because of the minimized
cross-sectional areas in these bridging zones. The structure of the connecting re-
gions between the particles of active material is largely influenced by the conditions
when these regions are reestablished during the charging process. For this reason,
it is understandable that the charging conditions are important for the stability of
the active material and that in many cases, after a premature decay, full capacity
can be regained with suitable charge/discharge procedures [34]. For this reason,
the premature capacity loss sometimes is called ‘reversible capacity decay.’
Another model assumes that gel zones are formed by hydrated lead dioxide
(PbO(OH) 2 ) and act as bridging elements between the crystallite particles. Electrons
can move along the polymer chains of this gel and so cause electronic conductivity
between the crystalline zones [35].
Quite often, simultaneously with the capacity decay, the formation of a barrier
layer of lead sulfate (PbSO 4 ) is observed between the grid and the active material
[36]. In view of the explanation given above, this layer may be the final stage of
the process. When the ohmic resistance of the active material is increased, the
charge/discharge reaction is restricted to the area close to the grid surface. Then,
deep discharge must happen to this part of the active material, causing a high
concentration of sulfate.
6.6.1
The Influence of Antimony, Tin, and Phosphoric Acid
Antimony (Sb) and tin (Sn) are usually not added to the active material, but both
are alloying components of the grid. They are gradually released from the grid by
corrosion, and permeate the active material by dissolution and diffusion.
The ‘premature capacity loss’ described above, that is, a decay of the utilization
factor, became especially evident when antimony-free alloys were introduced and
such batteries were operated in charge/discharge cycle regimes. For this reason,
this effect is likewise called the ‘antimony-free effect,’ although it is also observed
with grids containing antimony. The mechanism of this effect has not yet been
explained in detail, but antimony has a strong influence on the stability of the
active material that cannot be compensated by special pretreatment or design of
the electrodes [37].