Lithium-ion batteries are appear to be the battery of choice when it comes to most things, from smart phones to electric cars. However when moving to larger scale electrical energy storage, the cost becomes a very limiting factor, this is in part due to the expense of raw materials. Therefore research is being conducted into using gravity as a way of providing electricity during peak periods.
Essentially, a large mass is dropped at a time when there is a high demand for electricity, powering turbines for example, and then lifted at a time when there is not. The benefit of these types of systems are in the potential lower cost to power production ratio, compared to that of typical lithium-ion batteries.