Page 437 - Handbook of Battery Materials

14.5 Thermodynamic Basis for Electrode Potentials and Capacities 409

of the local dimensional changes is proportional to the particle size, smaller
particles lead to fewer problems with the decrepitation, or ‘crumbling’, of electrode
microstructure that often leads to loss of electrical contact, and thus capacity loss,
as well as macroscopic dimensional problems.

14.4
Alloys Formed In situ from Convertible Oxides

A renewed interest in noncarbonaceous lithium alloy electrodes arose recently
as the result of the announcement by Fuji Photo Film Co. of the development
of a new generation of lithium batteries based upon the use of an amorphous
tin-based composite oxide in the negative electrode [13]. It is claimed that
−1
these electrodes have a volumetric capacity of 3200 Ah L , which is four times
that commonly achieved with carbonaceous negative electrodes, and a specific
−1
capacity of 800 mAh g , twice that generally found in carbon-containing negative
electrodes.
According to the public announcement, a new company, Fujifilm Celltec Co.,
has been formed to produce products based upon this approach. It was reported
that some 200 patents have been applied for in this connection. Unfortunately,
there is little yet available in the standard literature concerning these matters.
To date, there are only references to some of the patents [14–17]. However, what
must be happening seems rather obvious.
If, as an example, we make the assumption that the electrode initially has the
composition SnO, if we introduce lithium into it there will be a displacement
reaction in which Li 2 O will be formed at the expense of the SnO due to the
difference in the values of their Gibbs free energies of formation (−562.1 kJ mol −1
for Li 2 O and −256.8 kJ mol −1 in the case of SnO). This is equivalent to a driving
force of 1.58 V. The other product will be elemental Sn, and as additional Li is
brought into the electrode this will react further to form the various Li-Sn alloys
that are discussed in some detail later in this section. This simplified picture
is consistent with what has been found in experiments of this general type [18].

14.5
Thermodynamic Basis for Electrode Potentials and Capacities under Conditions in
which Complete Equilibrium can be Assumed

The general thermodynamic treatment of binary systems which involve the incor-
poration of an electroactive species into a solid alloy electrode under the assumption
of complete equilibrium was presented by Weppner and Huggins [19–21]. Under
these conditions the Gibbs Phase Rule specifies that the electrochemical potential
varies with composition in the single-phase regions of a binary phase diagram
and is composition-independent in two-phase regions if the temperature and total
pressure are kept constant.