There are already solid batteries sold now to store energy from solar panels. They're mostly used in single homes, however. As solar facilities get larger, solid batteries get more expensive and less efficient, compared to how much energy the whole facility makes. The MIT team thinks an all-liquid battery, filled with searingly hot, molten metals, might be a good alternative. Liquid batteries may be easier (and thus cheaper) to manufacture in larger sizes, and they're expected to last longer than solid ones.