Page 201 - Handbook of Battery Materials

170 6 Lead Oxides

are many features that favor this system, which to a large extent are due to the
substances that form the reacting components in the electrodes, for example:
• The same chemical element forms the active material in both electrodes: lead as
metal (Pb) in the negative and as lead dioxide (PbO 2 ) in the positive electrode.
• The reactants are solids of low solubility, and the reactions are highly reversible.
• The reactants compounds lead (Pb), lead sulfate (PbSO 4 ), and lead dioxide (PbO 2 )
are well-defined chemical compounds, and there are no intermediate states of
oxidation. As a consequence, any voltage above the open circuit voltage results
in complete charge, and equalizing charges are not required when the battery is
used under continual charging in standby applications.
• The electronic conductivity of lead dioxide is comparatively high; thus there is no
need for conducting additives.
• Due to the high potential of the PbO 2 /PbSO 4 electrode, the cell voltage of 2 V is
fairly high, and a comparatively small number of cells is sufficient for a certain
battery voltage.
• The high potential of the positive electrode, on the other hand, does not allow
the use of conducting metals like copper within the positive electrode. Lead can
be used instead due to its passive properties caused by a (PbO 2 ) layer that largely
protects the underlying material but conducts the electronic current and so allows
electrochemical reactions at its surface.

This list could be extended (cf. Ref. [4]). It simultaneously indicates the large
number of parameters that influence the properties of a battery.

6.2
Lead/Oxygen Compounds

Lead forms two types of chemical compounds: lead (II) and lead (IV) compounds
based on Pb 2+ and Pb 4+ ions, where those based on Pb 2+ ions are the more
stable. The metal is oxidized even at room temperature to lead oxide (PbO) and
also by water that contains oxygen and forms lead hydroxide (Pb(OH) 2 ). In the
lead–acid battery, the (less stable) lead (IV) oxide (lead dioxide, PbO 2 ), is of greatest
importance. Besides these two, a number of oxides that are mostly mixtures are
observed in the battery. A brief survey will now be given of those compounds that
are of interest for lead–acid batteries.

6.2.1
Lead Oxide (PbO)

Lead oxide is formed by oxidation of a lead surface according to Equation 6.1.
2Pb + O 2 → 2PbO (6.1)
One technical process involves blowing air above the surface of molten lead (cf. the
Barton process in Section 6.4.2.1), but at room temperature reaction 6.1 also soon