M o d u L e 2 : a p p L i c a t I o n s a n d I m p L i c a t I o n s

Figure 10). The problem is that 
for the process to be efficient, the thermoelectric materials need to be good electrical conductors but 
poor thermal conductors, so that the temperature difference inside the material remains. More specif-
ically, thermoelectric materials are ranked by their figure of merit, ZT, which is defined as ZT = S2σT/k, 
where S is the thermopower (or Seebeck coefficient), σ is the electrical conductivity, k is the thermal 
conductivity and T is the absolute temperature. To be competitive with conventional refrigerators and 
generators, thermoelectric materials with ZT > 3 must be developed. Yet in five decades of research, 
the room temperature ZT of bulk semiconductors has increased only marginally, from 0.6 to about 1.0. 
The challenge lies in the fact that S, T and k are interdependent: changing one alters the others, making 
optimisation extremely difficult.
However, in the last years, there have been reports of dramatic increases in the properties of thermo-
electric materials. In all these cases, the material has been found to be nanostructured. Researchers 
are studying these nanomaterials in detail to understand whether this is a result of quantum confine-
ment or photon dynamics and transport. What is clear is that to have optimum properties, the material 
needs to have high symmetry at the nanoscale level, and needs to incorporate heavy elements because 
of itslow thermal conductivity.
Some examples of nanostructured thermoelectric materials are half-heuster alloys (ZrNiSn), Zn
4
Sb
3
, 
skutterudines, and novel PbTematerials (e.g. LAST-18, SALT-20 and LASTT).

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M O D U L E 2 : A P P L I C A T I O N S A N D I M P L I C A T I O N S
Rechargeable
batteries
Rechargeable batteries are energy storage devices used to power small electronic equipment such as 
mobile phones and personal computers, where high power and light weight are important. These same 
attributes are required for electric vehicles, hybrid electric vehicles, power tool and backup power sub-
systems. In simple terms, a battery is an electrochemical device that generates direct current through 
a coupled set of reduction-oxidation (redox) reactions. The positive electrode is reduced (captures 
electrons) and the negative electrode is oxidised (releases electrons). The battery consists of a positive 
electrode facing a negative electrode divided by a porous separator that prevents the electrodes from 
touching, and an ionic electrolyte, which is a conducting medium that ensures movement of the ions 
from one electrode to the other. Intercalation-based batteries using the small lithium (Li+) ion are the 
most used (