It's not so much that it's hard technologically, as the Tesla massive battery system in Australia has recently proven, but to do on a REALLY huge scale at an economical $ per kWh cost, and the lifecycle cost of the batteries. When compared to current non-carbon-producing energy sources like big hydroelectric dams, big pumped-storage hydroelectric.

https://reneweconomy.com.au/tesla-big-battery-turns-one-cele...

Every few months I see some big press release and tech press media hype about liquid metal batteries and flow batteries, I've yet to see something useful that has a market price beyond "contact us for more details".

It seems to mostly be all test and evaluation installs on a small scale so far.

https://www.seattletimes.com/seattle-news/pullman-project-te...