Energy storage helps overcome barriers for intermittent renewable energy, and is therefore an important aspect of a sustainable energy system. The most commonly used storage method is pumped-storage hydroelectricity, which requires locations with large differences in height and access to water. Battery storage power stations and home energy storage are being deployed widely. Some lithium-ion batteries contain cobalt, now largely mined in Congo some unsustainably. Responsible sourcing of cobalt and more diverse geographical sourcing may ensure a more stable supply-chain. Environmental impacts can be reduced by downcycling and recycling. Batteries typically have capacity to store enough electricity to last for short periods; research is ongoing into technology with sufficient capacity to last through seasons. Pumped hydro storage and power-to-gas with capacity for multi-month usage has been implemented in some locations.