There are numerous viable storage technologies, just competing for which will be cheapest. Until that is settled and the variable share reaches a practical limit, the money is better spent on panels.

Iron-air battery tech is proven. Now it just needs industrial-scale build-out, which takes time: the factories need to be built before the battery farms.

Renewables can, in fact, meet our needs with already mature technology. All that is uncertain is which choices will turn out cheapest. If it were not those batteries, it would be others, or underground compressed air, or molten salt, or liquified air.