Lithium-ion batteries are good for short bursts.

They will hold power for a few hours, which covers an evening at the most, but storing a full day of solar output to release overnight is a different problem, and lithium is expensive at that scale.

The materials also come from supply chains India does not control.

But iron-air batteries work on a cheaper premise.

The cell charges and discharges by oxidising and reducing iron, which is to say by rusting it and then reversing the rust.