Page 37 - Handbook of Battery Materials

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1
Thermodynamics and Mechanistics

Karsten Pinkwart and Jens T¨ ubke

1.1
Electrochemical Power Sources

Electrochemical power sources convert chemical energy into electrical energy (see
Figure 1.1). At least two reaction partners undergo a chemical process during this
operation. The energy of this reaction is available as electric current at a defined
voltage and time [1].
Electrochemical power sources differ from others such as thermal power plants
in the fact that the energy conversion occurs without any intermediate steps;
for example, in the case of thermal power plants, fuel is first converted into
thermal energy (in furnaces or combustion chambers), then into mechanical
energy, and finally into electric power by means of generators. In the case of
electrochemical power sources, this multistep process is replaced by one step only.
As a consequence, electrochemical systems show some advantages such as high
energy efficiency.
The existing types of electrochemical storage systems vary according to the nature
of the chemical reaction, structural features, and design. This reflects the large
number of possible applications.
The simplest system consists of one electrochemical cell – the so-called galvanic
element [1]. This supplies a comparatively low cell voltage of 0.5–5 V. To obtain
a higher voltage the cell can be connected in series with others, and for a higher
capacity it is necessary to link them in parallel. In both cases the resulting ensemble
is called a battery.
Depending on the principle of operation, cells are classified as follows:

1) Primary cells are nonrechargeable cells in which the electrochemical reaction
is irreversible. They contain only a fixed amount of the reacting compounds
and can be discharged only once. The reacting compounds are consumed by
discharging, and the cell cannot be used again. A well-known example of a
primary cell is the Daniell element (Figure 1.2), consisting of zinc and copper
as the electrode materials.

Handbook of Battery Materials, Second Edition. Edited by Claus Daniel and J¨ urgen O. Besenhard.
 2011 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2011 by Wiley-VCH Verlag GmbH & Co. KGaA.