Page 40 - Handbook of Battery Materials

6 1 Thermodynamics and Mechanistics

Apart from the improvement and scaling up of known systems such as the
lead–acid battery, the nickel–cadmium, and the nickel–metal hydride batteries,
new types of cells have been developed, such as the lithium-ion system. The latter
seems to be the most promising system, as will be apparent from the following
sections [3].
To judge which battery systems are likely to be suitable for a given potential
application, a good understanding of the principles of functioning and of the
various materials utilized is necessary (see Table 1.1).
The development of high-performance primary and secondary batteries for
different applications has proved to be an extremely challenging task because of
the need to simultaneously meet multiple battery performance requirements such
as high energy (watt-hours per unit battery mass or volume), high power (watts
per unit battery mass or volume), long life (5–10 years and some hundreds of
charge-discharge cycles), low cost (measured per unit battery capacity), resistance
to abuse and operating temperature extremes, near-perfect safety, and minimal
environmental impact (see Table 1.2 and Table 1.3). Despite years of intensive
worldwide R&D, no battery can meet all of these goals.
The following sections therefore present a short introduction to this topic and
to the basic mechanisms of batteries [4]. Finally, a first overview of the important
criteria used in comparing different systems is given.

1.2
Electrochemical Fundamentals

1.2.1
Electrochemical Cell

The characteristic feature of an electrochemical cell is that the electronic current,
which is the movement of electrons in the external circuit, is generated by the
electrochemical processes at the electrodes. In contrast to the electric current in
the external system, the transportation of the charge between the positive and the
negative electrode within the electrolyte is performed by ions. Generally the current
in the electrolyte consists of the movement of negative and positive ions.
A simplified picture of the electrode processes is shown in Figure 1.3. Starting
with an open circuit, metal A is dipped in the solution, whereupon it partly
dissolves. Electrons remain at the electrode until a characteristic electron density
is built up. For metal B, which is more noble than A (see Section 1.2.2), the same
process takes place, but the amount of dissolution and therefore the resulting
electron density is lower.
If these two electrodes are connected by an electrical conductor, an electron
flow starts from the negative electrode with the higher electron density to the
positive electrode. The system electrode/electrolyte tries to keep the electron
density constant. As a consequence additional metal A dissolves at the negative
−
+
electrode forming A in solution and electrons e , which are located at the surface