Technology

Mechanical energy storage, like pumped hydro or flywheel. Thermal energy storage, like molten salt.

Electrochemical isn't entirely off the table either: less-volatile chemistries are available, and better containment methods can reduce risks.

Non-electrical chemical storage methods are available: electrical energy can be used for hydrogen electrolysis, or Fischer-Tropsch hydrocarbon fuels. Fuel cells, and traditional ICE generators can recover the energy put into those (relatively) stable fuels, or we can export it from the electrical generation industry to the transportation industry.

There's also avoiding (or minimizing) the need for storage at all, with "demand shaping". Basically, we radically overbuild solar, wind, wave, tidal, etc. Normally, that would tank energy prices and be unprofitable, but we also build out some massive, flexible demand to buy this excess power. Because they are extremely overbuilt, the minimal output from these sources during suboptimal conditions is more than enough to meet normal demands; we just shut off the flexible additional demand we added. We "shape" our "demand" to match what we are able to supply.

A really strong elastic band.

Lifting your mom with a pulley.

I believe there is battery tech that is newer but being deployed into production that is iron based. It is heavier and less energy dense than lithium. But for power grid level deployment that should be fine and iron is a bit harder to catch on fire.

Pumped hydro

Also, are those battery fires more frequent // important than petrol ones ?

Petrol fires use oxygen from the air. They can be extinguished by removing the oxygen: covering it in firefighting foam, or displacing it with CO2, for example.

Batteries contain both their fuel and their oxidizer together in one case. You can't remove the oxygen. So long as they are hot enough, they keep burning, even if they are underwater. The only way to extinguish them is to remove the heat. Which is practically impossible.

No. And the petrol fires are many and ongoing in everyone's cars. Also large petrol fires are not always reported in the US. I can think of one specific instance that tho' a major fire, producing a wall of smoke, yet I could only find one news report of it's existence.

How is a Megawatt not a measure of energy?