That’s the big, trillion-dollar question, right? Pumped hydro or air is the solution that definitely works, but air is extra lossy and hydro requires a huge reservoir. A battery system would be great, but Li-ion is expensive, so you need a new chemistry. For grid storage, density, hazard and even operating temperature and pressure are not strict requirements, only cost, so it seems likely they exist, and at least sodium is in the early stages of commercialisation already.