> cheap as dirt
There are lots of things left to make grid battery storage cheap.
As well as the cells needing to be engineered to be cheaper, there are lots of changes to the battery packs that can be made, together with changes to inverters.
For batteries, I'd like to see research into less consistent manufacturing and higher failure rates. Current packs the entire pack is unusable if just one cell fails in a way that leads to lots of heat production. If pack balancing circuits had the ability to take a cell or a parallel group of cells 'out of circuit' while still using the rest of the pack, then battery lifespans could be dramatically increased and it would be possible to manufacture cells far cheaper.
For inverters, we should go for a direct-to-10kV inverter process. No transformers. At 10kV, currents are far lower and therefore wires can be far thinner (and cheaper).
Consider making batteries ~15kV too - that reduces by ~30% the amount of expensive silicon needed, together with big reductions in copper costs, at the expense of extra design effort for much higher voltage batteries. At these higher voltages, you'd either use oil cooling, or you'd have ~10 separate coolant loops, one at each ~1500 volts of potential.