Publisher Summary This chapter deals with the explanation of sodium-sulfur secondary batteries, which relies on the recently discovered properties of β-alumina; namely, that it combines very low electronic conductivity with an unusually high ionic conductivity which is specific to sodium ions as charge carriers. In the sodium-sulphur battery, instead of solid electrodes separated by a liquid electrolyte, sodium β-alumina is used as a solid electrolyte, specifically conducting sodium ions between liquid electrodes of sodium metal and sulfur. An alternative design is developed by Chloride Silent Power (UK). This cell is unusual because it embodies liquid electrodes and a solid electrolyte. This cell has a theoretical e.m.f, of 2.076 V at the cell operating temperature of 350°C for 70% discharge. Reaction products are molten at this temperature. Discharge is normally terminated between 1.76 and 1.9 V. During cell discharge sodium ions migrate through the solid electrolyte phase into the positive compartment, to combine with the S2- ions produced there. The sodium ions are pumped through the β-alumina to the negative compartment during recharge. The role of β-alumina is critical. β-Alumina is a member of a class of solid materials with high ionic conductivities that are variously called optimized ionic conductors, superionic conductors and, more usually, fast-ion conductors.