The answer is likely going to have to be hydrogen, but there's a pretty difficult catch with hydrogen: it makes zero sense to invest in any hydrogen electricity storage infrastructure until the grid is already like 80+% renewable.

There's simply no sense in turning electricity into hydrogen so that it could be used in 6 months (losing 50+% of the energy along the way as heat!) when you could just sell that electricity right now, or stick it in a battery so you can use it 6 hours from now.

There will be an economic case for hydrogen energy storage in Europe in 10 years, but unfortunately the technology is basically sitting at a standstill right now with no attention and no investment because it's not needed yet.