*3.3. Sodium–Sulfur Battery*

The anodes of sodium–sulfur (Na-S) batteries are viscous liquid sodium and sulfur, and they run at hot temperatures, around 300 and 350 ◦C, to maintain the electrode's liquid and to provide strong ionic conductivity in the electrolyte, which is a ceramic material [32,33]. At processing temperature, the electrolyte is beta-alumina (b-Al2O3), which transmits sodium ions. When sodium and sulfur are released, sodium polysulfide is produced [34]. They have a far superior energy density and durability over lead–acid batteries. To preclude cell defects from proliferating, security is essential, and careful design is essential. The chemistry and principal components of a sodium–sulfur battery are shown in Figure 8.

**Figure 8.** Chemistry and principal components of a sodium–sulfur battery.

Although Na-S batteries are made from abundant and inexpensive raw items, the production processes, as well as the necessity for insulation, cooling, and temperature control, make them fairly pricey. They are more cost-effective in large units since the thermal management of smaller batteries contributes to the cost relative to the battery's volume. As a consequence, they are primarily employed for utility load levelling in big substations.

## *3.4. Sodium–Nickel Chloride Battery*

In Na-NiCl2 batteries, a beta-alumina electrolyte is employed; however, the cathode is nickel chloride submerged in sodium aluminum chloride (NaAlCl4), a molten salt that conducts sodium ions, as opposed to a sulfur electrode [35]. Nickel metal and sodium chloride are created when sodium reacts with nickel chloride during discharge [35,36]. The system still needs heating, insulation, and temperature control even though it operates at a lower temperature than Na-S batteries (300 ◦C) [37]. Compared to Na-S batteries, the energy density is higher, but the battery life is longer. Figure 9 depicts the chemistry and main parts of a sodium–nickel chloride battery.

**Figure 9.** Chemistry and principal components of a sodium–nickel chloride battery.
