(Reuters) – Electric car maker Tesla Motors Inc has started mass production of lithium-ion battery cells at its gigafactory in Nevada along with Japan's Panasonic Corp, the company said on Wednesday. The cylindrical "2170 cells", which will be used to power Tesla's energy storage products and the new Model 3 sedan, have been jointly designed by Tesla and Panasonic, its longstanding battery partner. The gigafactory will initially produce battery cells for the company's Powerwall 2 and Powerpack 2 energy products, Tesla said.