I think this lack of demand for storage has prevented innovation. Why bother with grid storage, when day and night prices aren't much different? Well now there are times with negative prices, so there is going to be incentive to get paid to store electricity and sell it later. This incentive should grow as renewable penetration grows. If we have deep solar penetration, then there is the big problem that demand is very high in the early evening and the PV suddenly ramps down production as the sun sets. You can cycle your storage every buying dirt cheap solar and selling to the peak price a few hours later.

Yes, there is a market for stationary energy storage for electrical grids. It's small so far, and pretty fragmented, though growing quickly from a small base. It's anyone's guess whether the stationary storage market will sustain distinct storage chemistries that aren't suitable for mobile applications (various kinds of flow batteries, sodium ion batteries, sodium-sulfur, liquid metal...) or whether lithium ion will end up everywhere due to its early lead and continuing cost/technical improvements. Even lithium ion ends up everywhere, there could still be chemistry differences between cells built for stationary grid backup and cars or phones, but they would be more subtle.