Page 253 - Handbook of Battery Materials

8.3 Battery Anodes (‘Negatives’) 223

The overall reaction is:

3Fe + 8NiO(OH) + 4H 2 O ←→ 8Ni(OH) + Fe 3 O 4 (8.8)
2
The electrochemical equivalent of iron (if only the first reduction step is taken into
−1
account) is 960 Ah kg , and the OCV of the ‘nickel/iron’ cell is 1.4 V.
8.3.4
Lead (Pb)

The lead electrode used as the anode in the well-known lead–acid battery is a rather
complex structure consisting of a metallic grid (lead–antimony, lead–calcium,
or other alloys [43–53]) filled with a paste made from particles of the active
mass, sulfuric acid, and various additives (e.g., expanders, gelling agents, coatings
[54–67], inert materials [68–75], or conductive particles [76–79]). The active mass
originally consists of oxidized lead grains with a residual metal content of about
30%. These grains come either from a Barton reactor or from a ball mill. Their
particular properties are described in another chapter (Part II, Chapter 9).
The pasted and cured anode plates have to pass through the formation process,
which is nothing else but an external electrochemical reduction done in sulfuric
acid (AB2C1) [80].
The regular discharge reaction follows a dissolution/precipitation mechanism
simplified as follows:

Pb ←→ Pb 2+ + 2e − (8.9)
Pb 2+ + SO 4 2− ←→ PbSO 4 (8.10)
Reaction mechanisms have been described in general [81–83] or in detail with
regard to passivation [84], oxygen recombination [85], or the applied mode of
charging [86], particularly pulse charging [87]. Current and potential distributions
were studied by Li and co-authors [88].
Many concepts were developed to overcome one of the main drawbacks of the
lead–acid system: the heavy supporting lead structures (grids, connectors, etc.).
Lead foam [89], lead-plated carbon rods [90], electroplated vitreous carbon [91],
flexible-graphite grids [92], or graphite foams [93] were tested, also lead-plated
materials like titanium [94], Ebonex [95], copper mesh [96], polymeric structures
[97], polymer foam [98], or glass fiber mesh [99]. Warlimont and Hofmann [100]
describe the development of multilayer composite grids.
Some attempts have been made to transform the conventional lead accumulator
into a ‘dissolution’ accumulator by replacing sulfuric acid of any particular concen-
tration [101] with tetrafluoroboric acid (HBF 4 ), but this highly corrosive and toxic
acid was not finally accepted [102]. More recently, methanesulfonic acid was tried
[103]. The electrochemical equivalent of lead is the lowest of all metallic anodes
−1
(260 Ah kg ), but in many applications the high OCV of 2.1 V per cell compensates
for this disadvantage.