In a transport context, a lithium-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. Environmental concerns exist over lithium batteries, as the lithium has to be mined and the production process uses large amounts of water. Lithium batteries contain toxic chemicals, making recycling very difficult and they are also potentially flammable. There are also concerns about continuity of lithium supplies, as over half the global reserves are in China.