There is a good solution for long term energy storage: use solar energy to make synthetic hydrocarbons. This is a solution that has been proven for billions of years.

We can already capture solar energy at a much better energy efficiency than living beings. Making hydrocarbons with hydrogen extracted from water by electrolysis and concentrated carbon dioxide has acceptable efficiency and already almost one century ago it was possible to do this at a large scale where fossil oil was not available.

The step that has the least efficiency for now is concentrating the dilute carbon dioxide from air, which plants do much better.

There is no doubt that the global efficiency of such a process could have been greatly improved if only a small fraction of the resources allocated to much more frivolous goals had been allocated to this purpose.

While other alternatives are speculative, it is enough to look outside to see plenty of PoCs that this is feasible.