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Why Is Everyone Trying to Build a Solid-State Battery?
(www.construction-physics.com)
Why not go the opposite direction entirely?
Grid scale batteries would be better served by Nickel-Iron batteries with pumped electrolyte. They don't have a fire risk like Li-ion and can be discharged to 0% without damage. The downside is the electrolyte needs changing frequently (daily to weekly) but for a grid scale battery bank, that isn't an issue, just trickle through new electrolyte at a constant rate.
It's not a solution for vehicles or portable devices, but for big batteries we shouldn't be looking at solid state.