Page 306 - Handbook of Battery Materials

276 10 Carbons

and (iv) only low contents of impurities. Graphite has higher electrical conductivity
than acetylene black but it is not capable of retaining the same amount of electrolyte
or demonstrating the same mechanical properties in the cell. Acetylene black has
a well-developed chain structure, and it is this characteristic which provides the
capability to retain a significant amount of electrolyte. In addition, acetylene black
is produced with a low ash content and it does not contain surface groups. The
results obtained by Bregazzi [21] indicate that acetylene black is capable of retaining
over three times as much electrolyte (cubic centimeter electrolyte/gram carbon)
as graphite. The capacity of Leclanch´ e cells is dependent on the amount and type
of carbon black that is used. Generally about 55 vol% carbon black mixed with
MnO 2 yields the maximum capacity [22]. This composition agrees closely with the
minimum in the electrical resistivity of the electrode mixture.
A carbon rod is used as a current collector for the positive electrode in dry cells.
It is made by heating an extruded mixture of carbon (petroleum coke, graphite)
and pitch which serves as a binder. A heat treatment at temperatures of about
◦
1100 C is used to carbonize the pitch and to produce a solid structure with low
resistance. For example, Takahashi [23] reported that heat treatment reduced the
specific resistance from 1 to 3.6 × 10 −3 cm and the density increased from
−3
1.7 to 2.02 g cm . Fischer and Wissler [24] derived an experimental relationship
(Equation 10.1) between the electrical conductivity, compaction pressure, and
properties of graphite powder:

log ρ = K − 0.45 log L c − 0.43 log d 50 − 0.54 log p (10.1)

where ρ is the electrical resistivity, K is a constant, L c is the crystallite size in
the direction perpendicular to the basal plane, d 50 is the mean particle diameter,
and p is the compaction pressure. This relationship indicates that the electrical
resistivity decreases as the crystallite size increases, and with a given average
particle size and compaction pressure. When graphite is mixed with MnO 2 in an
electrode structure, the conductivity increases with a decrease in the particle size
of graphite. In addition, the conductivity increases dramatically when the graphite
concentration increases above about 10%.
Another example of the use of a graphite as an additive to improve the electronic
conductivity of an electrode can be found in the discussion of the Fe/NiOOH cell
developed by Edison in the early 1900s [25]. The positive electrode which contained
graphite (20–30% graphite flake) degraded rapidly during charge because of
oxidation and swelling. This experience led to the development of electrolytic nickel
flakes and eventually to the porous nickel plaque for use in NiOOH electrodes.
Composite structures that consist of carbon particles and a polymer or plastic
material are useful for bipolar separators or electrode substrates in aqueous batter-
ies. These structures must be impermeable to the electrolyte and electrochemical
reactants or products. Furthermore, they must have acceptable electronic conduc-
tivity and mechanical properties. The physicochemical properties of carbon blacks,
which are commonly used, have a major effect on the desirable properties of the
conductive composite structures. Physicochemical properties such as the surface