Sodium-ion batteries are batteries that store energy using sodium ions instead of the lithium ions in most batteries today. Sodium is far more plentiful than lithium and cheaper, since common salt contains it. So sodium-ion cells are being built for grid storage, where weight and size matter less than cost. India has found some lithium deposits but does not mine any lithium yet. So a battery that avoids lithium is of interest here.
Inside a cell, charged sodium ions move through a liquid between two electrodes. When the cell charges, the ions move to one electrode. When it discharges, they move back, and electrons travel through the outside circuit as electric current. Lithium-ion cells work the same way, only with lithium ions.
A sodium-ion cell holds less energy for its weight than a lithium cell. Say one kilogram of a common lithium cell holds about 160 watt-hours, and one kilogram of a sodium-ion cell holds about 120. To store the same energy, the sodium-ion system needs about a third more cells. That is a problem in a phone or an electric car, but not in a grid battery, which sits on the ground and can be as large as needed. For a storage system the important numbers are cost per unit of energy stored, safety and number of charge cycles.
The cell chemistry depends on advanced materials. The same cells could also be used in battery swapping for vehicles, where cost matters. The page on cells, modules and packs explains how the cells are grouped. The diagram shows one such storage system at work. The maker named below, Peak Energy, is a company in the United States, where it sells sodium-ion storage.