TOYOTA -- A Toyota executive overseeing research said Thursday a battery breakthrough is needed for electric vehicles to become mainstream, and hybrids will remain the best "green" car choice for some time, according to The Associated Press.

His comments came just hours after the Japan Automobile Dealers Association said Toyota's Prius hybrid was the No. 1 selling vehicle in Japan for May, clinching the top spot for the first time - even though the latest model had been on sale for only half the month.

Toyota Executive Vice President Masatami Takimoto said the hybrid system like the one in the Prius is the "most important of our energy-saving technologies," and reiterated that his company plans to have a hybrid version of its entire lineup of products by 2020.