A company called ARES is working on a system that basically consists of automated electric trains carrying bricks uphill to store and then coasting back downhill with them to retrieve power. Basically pumped hydro but without a need for large quantities of water (and really ideal for installation in the desert, where there's a lot of open land with gentle slopes). They say efficiency near 80% with a first installation going in in Nevada in 2017.

This is comparable efficiency to batteries but less expensive and with significant reuse of well-established technologies and industries (e.g. most of the construction work can be done by existing rail contractors).

Although battery technology continues to advance, my suspicion is that large-scale electricity storage is going to tend to be mechanical for cost reasons.