Since 2010, the cost of lithium-ion battery packs for electric vehicles has dropped 60%, from $1000 to $400 per kilowatt-hour (kWh) according to analysis by BNEF. At the same time, concerns regarding urban air pollution and our need to deploy zero emission vehicles in these cities are increasing. Melissa C. Lott takes a closer look at how EVs could help – and what the limitations might be.