Improvement in battery technology is relative. You do not need superlight lithium-something batteries for general energy storage. The only questions for such systems are price per kWh, life-time and somewhat good input/output ratios. But especially the last may become unimportant as energy becomes really abundant. It is the same reason why power2gas may become interesting: only 40% energy efficency are sufficient when you produce 5 time your domestic need in a winter storm.