The Earth itself is a giant fission reactor and molten metal thermal battery, but for some reason nobody thinks about it.
This would be far easier than fusion or even next generation full cycle fission but it’s barely funded.
For instance sodium can be used in its metal form directly to produce electricity in a fuel cell e.g. as described in US3730776A (https://patents.google.com/patent/US3730776A/en). The fuel cell does not require any special materials, however it still is a challenge to construct from the engineering point of view. Also, the proposed design can be greatly improved with additional knowledge about the reactions occurring. The resulting sodium hydroxide solution can be recycled using the well known Castner process, the "waste" hydrogen resulting from both reactions can be used as fuel or for other industrial processes.
Btw. the energy density of sodium metal is 3694 Wh/kg and 3555 Wh/litre - so an order of magnitude more than a typical Li-Ion battery. Also, the advantage of decoupling capacity from power is dramatic. Of course, you can also split the process of storing and releasing of energy which can be an advantage. Last but not least, sodium in its metal form is not hard to store or to transport. At ~100°C it becomes liquid and therefore even easier to transfer using regular steel pipes.
That sodium battery sounds great for grid scale storage and maybe aviation. The major problem with lithium there is weight per kWh. Wouldn’t use it in anything consumer because liquid sodium loves to party.
Liquid sodium is totally fine unless you pour water over it. You have problems with other substances too and it depends what use case and limits you have whether you are willing to go with sodium. For example, filling a big tank with normal gasoline is dangerous without protective atmosphere and other precautions like good grounding because it tends to produce flammable vapors that catch fire easily. Not a problem with sodium. Also, if you happen to spill sodium into water, it neutralizes quickly doing damage only for a short period of time. That cannot be said about any kind of mineral oil, gasoline that has a tendency to destroy ecosystems etc. Last but not least, you can produce sodium metal without begging mostly totalitarian/ authoritarian regimes for natural resources which is the case with the majority of fossil fuels today.
Because of electronics and fine machines we learned a lot about how to make stuff water tight. Sodium is in my opinion mostly much easier to handle than hydrogen, which leaks even through steel, which you have to keep under high pressure to have any kind of decent volumetric energy density or cool to very low temperatures if liquid.
If you used sodium for grid scale storage than you would more or less solve the electricity storage problem and most likely quite cheaply. The great thing about it is, you can store and transport sodium metal easily. Storing power for the winter becomes practical. For stationary stuff in homes or businesses you could probably use sodium too. You could replace old central furnaces with it and instead use the fuel cell and a heat pump perhaps with a small battery and PV to smooth out the peaks. That way you probably wouldn't need to upgrade the last mile of the grid in many places. You could power boats with it. For aviation it doesn't seem that great as you would still have to handle the resulting sodium hydroxide.