Tesla is now producing lithium-ion battery cells at its “Gigafactory,” the massive plant just outside Reno, Nevada, that the company claims will become the largest factory of its kind in the world. The cells will be used for Powerwall 2 and Powerpack 2 energy-storage battery packs, at first. The company held a grand opening for the Gigafactory last year, but until now the factory has only assembled battery packs, and not the cells that go into them. Those cells have been imported by Panasonic, which partnered with Tesla on the Gigafactory and has been the sole supplier of battery cells…