Page 203 - Handbook of Battery Materials

172 6 Lead Oxides

hydrated and forms a gel structure at the phase boundary between the solid material
and the electrolyte (cf. Ref. [6]).
In battery electrodes, the stoichiometric composition is usually not accomplished,
and oxidation ends at a composition of about PbO 1.98 [7].
α-PbO 2 , is formed in an alkaline environment, whereas β-PbO 2 is produced
in an acidic medium. Both modifications can be prepared by chemical and
electrochemical methods (for detailed descriptions of preparations methods and
structure see Ref. [5], p. 19, and Ref. [8]). In the positive electrodes of lead–acid
batteries, a certain proportion of α-PbO 2 is formed during the electrochemical
conversion of the electrode (cf. Sections 6.4.2.2 and 6.4.2.3). When the battery
has been discharged and is charged again, only β-PbO 2 is formed on account
of the acidic environment. For this reason, the content of α-PbO 2 decreases
with the number of discharge–charge cycles, especially for a high initial α-PbO 2
content (cf. Ref. [5], p. 267, especially Figures 3.27 and 3.28). Plates containing
much α-PbO 2 show a reduced initial capacity, which increases gradually on
account of the conversion of α-PbO 2 to β-PbO 2 . There was some indication
that plates with high α-PbO 2 , content would outperform other plates in standby
applications. Therefore, the α/β-PbO 2 ratio has occasionally been specified for
stationary lead–acid batteries, but these observations have not been confirmed
in general [6]. Other experiments indicate that the structure of lead dioxide
agglomerates, which can be influenced by the formation process, is important for
cycle stability [9] (cf. also the description of ‘paste mixing’ in Section 6.4.2.1).
Under normal conditions β-PbO 2 , is the more stable modification. Under a high
pressure (>8500 bar) the β modification can be transformed to α-PbO 2 .
6.2.4
Nonstoichiometric PbO x Phases

PbO x phases with x between 1.42 and 1.58 can be formed by oxidation of PbO (cf.
Ref. [5], p. 18). It is assumed that such compounds are formed underneath the
protecting PbO 2 layer at a corroding lead surface (cf. Figure 6.8).
6.2.5
Basic Sulfates
Basic sulfates are intermediate compounds that contain lead oxide and lead
sulfate and to some extent also water (Table 6.1). They are stable only in alkaline
environment.
Basic sulfates are important intermediates during the manufacturing process,
since they determine the structure of the active material in the positive electrode,
which again is decisive with respect to the performance data and service life of the
battery (cf. Ref. [9]).

6.2.6
Physical and Chemical Properties

Some physical and chemical properties of the lead oxides are compiled in Table 6.2.