Rechargeable batteries - High-temperature batteries

The advantages of lithium as an anode material were discussed in dossier [D 3 354] "Lithium batteries". These considerations also apply to sodium, with its low standard electrode potential (– 2.714 V/ENH, Table 1 in dossier [D 3 351] "Theoretical considerations") and low density (0.97 $g\cdot c\text{}{m}^{-3}$ ). Finally, the melting temperature of sodium (98 ˚C) is lower than that of lithium (180.5 ˚C). It therefore appears that these two metals are, a priori, of roughly comparable interest. The promotion of sodium is the result of the discovery of ceramics that are inert to this alkali, allowing the sodium ion to circulate. These ceramics are commonly known as alumina $\beta$ .

Alkali metals react violently with water, so the use of non-aqueous electrolytes is essential. Two solutions are currently used: either a liquid medium consisting of molten salts or, in the case of sodium, a solid medium such as β-alumina. In all circumstances, the temperature of the accumulator must be maintained well above ambient temperature, typically in the 300 to 400 ˚C range, whether this is to reach the melting zone of the salts or to give the ceramic sufficient ionic conductivity. In the latter case, contact between the electrode material and the ceramic requires the presence of a liquid. This requires either a liquid electrode material, or a liquid "secondary" electrolyte sandwiched between the ceramic and the solid electrode. Accumulators based on these principles, known as "high-temperature" accumulators, are a recent development. Work on some of them has not been pursued, given the scale of the difficulties encountered.