Page 212 - Handbook of Battery Materials

6.4 PbO 2 as Active Material in Lead–Acid Batteries 181

6.4
PbO 2 as Active Material in Lead–Acid Batteries
The active material comprises the substances that constitute the charge–discharge
reaction. In the positive electrode of lead–acid batteries, the active material in the
charged state is lead dioxide (PbO 2 ), which is converted into lead sulfate (PbSO 4 )
when the electrode is discharged. The active material is the most essential part
of a battery, and battery technology has to aim at optimum constitution and
performance for the expected application. This concerns not only the chemical
composition but also the physical structure and its stability. Specialized methods
have been developed to fulfill these requirements, and the primary products as
well as the manufacturing process are usually specified by the individual battery
manufacturer.
It is characteristic for battery manufacture that lead dioxide (PbO 2 )asthe charged
state of the active material is always generated by electrochemical oxidation. Thus,
electron-conducting bridges are established between the fine particles, and a matrix
is formed of comparatively low electronic resistance. Three general types of positive
electrodes are mainly used today: Plant´ e, pasted, and tubular plates, which vary not
only in their design but also in the way they are manufactured.
The charge–discharge reactions occur at the phase boundary between the active
material and the electrolyte. To make sure that a sufficient rate of reaction is
achieved, the surface of the reacting materials has to be large. Otherwise, the
kinetic parameters would reduce the reaction rate too much. Table 6.5 shows the
surface areas of the active materials in the positive and the negative electrode.
Figure 6.3 shows the typical microscopic appearance in the charged and dis-
charged states. Although certain features are characteristic, microscopic pictures
of this kind vary considerably, because of the different parameters that influence
the formation of the crystals when a substance is precipitated. Furthermore, the
charge–discharge conditions and the age of the battery influence the morphology
of the active material (cf., e.g., Refs [8, 13]). The ‘lump’ structure is typical of the
charged active material of the positive electrode, at least for a fairly new electrode.
These ‘lumps’ are porous agglomerates. About 50% of their volume is occupied by
lead dioxide; the other 50% is pores. A large share of micropores produces the high
surface area shown in Table 6.5.
In the lead–acid battery, the reactions at both electrodes include the dissolved
state, which means that the reacting species are dissolved in the course of the

Table 6.5 Surface areas of the active materials in
lead–acid batteries.

−1
2
Substance BET surface (m g )
Lead dioxide, PbO 2 4–6
Lead, Pb 0.3–0.6