A123 Systems Inc., a Waltham-based company that makes electric-car batteries, said it has developed an improved lithium-ion cell that can cut costs of rechargeable and hybrid vehicles. The “nanophosphate EXT” cells will be produced next year and can be used longer and run more efficiently in high and low temperatures than competing lithium-ion packs. That reduces the need for systems in electric cars to maintain a consistent battery temperature, said chief executive David Vieau.