There's a catch: Lithium-ion battery technology is approaching some very real limits imposed by the physical world, according to researchers. While battery performance has improved markedly and costs have fallen to around $150 per kilowatt hour, that's still more than the $100 per kWh goal set by the US Department of Energy. Costs are also soaring for rare metals used in battery electrodes. High demand has led to shocking abuses in Africa, where some cobalt mines exploit child labor, and to environmental violations in China, where mining dust has polluted villages, according to recent reporting in the science journal Nature. In any case, Mother Earth isn't making any more cobalt or nickel: Demand will outstrip production within 20 years, researchers predict. Although crucial, current battery technology is neither clean nor renewable.