Element, elementary cell, electrolyte

A battery element or elementary cell comprises two electrodes immersed in an electrolyte.4- 5 For instance, in a very common lithium-ion secondary battery, these two electrodes are made of a lithiated metal oxide and carbon. 

Usually, the electrodes are in the solid state and the electrolyte is in the liquid or gelled state. Conversely, in sodium-sulfur secondary batteries 

2 The term energy covers two physically distinct notions  

Confusion often arises, because these two entities are expressed in terms of the same unit the Joule (or, more commonly, the kWh). 

3 The term immersed is used for historical reasons because, in the first secondary battery (lead secondary battery introduced by Gaston Plante in 1859) and all those for almost a century afterward, the electrodes were immersed in a liquid electrolyte (dilute sulfuric acid or potassium, for instance). Nowadays, the electrolyte can also impregnate a solid matrix, or be 

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Elementary Na/S battery science The basic science of the Na/S cell is identical to that of the fuel cell. In both cases the essential is an electrolyte. On one side of the electrolyte membrane there is a source of sodium ions at a particular chemical potential, and on the other side a sink for the ions at a relatively lower chemical potential. The elements of the process are illustrated in Fig. 4.28. 

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