Page 41 - Handbook of Battery Materials

1.2 Electrochemical Fundamentals 7

electron flow

− +

A + e - A + B + B +
A + e - e - e - A + e - e -
A + e - e - e - A + separator + e - e - +
A + e - e - e - A + B e - B
A + A + B +
electrolyte solution
Figure 1.3 Electrochemical cell with negative and positive electrodes.

of metal A.
+
A → A + e −
At the positive electrode the electronic current results in an increasing elec-
tron density. The system electrode B/electrolyte compensates this process by the
consumption of electrons for the deposition of B -ions:
+
B + e → B
−
+
The electronic current stops if one of the following conditions is fulfilled:
• the base metal A is completely dissolved
• all B -ions are precipitated.
+
As a consequence, it is necessary to add a soluble salt to the positive electrode
compartment to maintain the current for a longer period. This salt consists of
+
B -ions and corresponding negative ions. The two electrode compartments are
divided by an appropriate separator to avoid the migration and the deposition of
B -ions at the negative electrode A. Since this separator blocks the exchange of
+
positive ions, only the negative ions are responsible for the charge transport in the
cell. This means that for each electron flowing in the outer circuit from the negative
to the positive electrode, a negative ion in the electrolyte diffuses to the negative
electrode compartment.
Generally, the limiting factor for the electronic current flow is the transport of
these ions. Therefore the electrolyte solution should have a low resistance.
An electrolyte is characterized by its specific resistance ρ( cm), which is defined
as the resistance of the solution between two electrodes each with an area of 1 cm 2
and at a distance apart of 1 cm. The reciprocal of this value is known as the specific
−1
conductivity κ ( −1 cm ) [5]. For comparison, the values for different materials
are given in Figure 1.4.
The conductivity of different electrolyte solutions varies widely. The selection
of a suitable, highly conductive electrolyte solution for an electrochemical cell