Some groups like LightSail think that compressed air storage is going to have the lowest lifetime costs. Others think that flow batteries will win. Others think

Some groups like LightSail think that compressed air storage is going to have the lowest lifetime costs. Others think that flow batteries will win. Others think that thermal storage will win. Others think that big banks of lithium ion batteries will win. I don't know who's going to win. All of the above approaches are technically viable. The winners and losers are going to be shaped by things other than simple technical viability: business plan execution, manufacturing scalability, path dependency... I say that storage is not "solved" because there's no default good-enough solution proven yet. We have to wait for some years after plans leave PowerPoint and inhabit the real world to see what works well in practice.