The idea has been kicking around for a while because it's superficially appealing.

Benefits:

- you can store huge amounts of energy in relatively cheap tanks

- because the + and - storage are physically separate, self discharge can be very low

- those two make it appealing for long term storage

Downsides:

- reagents end up being some combination of expensive (vanadium), poisonous, corrosive, etc. resulting in safety overhead

- requires specialized plumbing. This is very expensive. Ultimately this is the issue with nuclear as well, every joint requires careful inspection

- does not scale down nicely to house sizes

- ultimately likely to get economically lapped by incremental improvements to lithium or sodium battery chemistry

- the charge/discharge cell requires a liquid/liquid membrane separator which can exchange charge; these tend to have lifetime issues.

Flow batteries specifically are liquid, or more accurately fluid, there can be gas flow batteries, where the chemical sub-products are pumped in and out of the cells.

They tend to be large utility scale things that look like a chemical plant, correction, are a chemical plant with electrical storage as the product.

Liquid electrolyte batteries have been a thing from when batteries have been invented. But not much work has been done to really scale it up.

It's not a new idea, they already exist.

A huge redox-flow battery is currently under construction in Switzerland https://www.swissinfo.ch/eng/climate-solutions/switzerland-b...

You might find the home made batteries of this guy interesting as well https://www.youtube.com/watch?v=eq7fR9ISuCw