Rumors are swirling. Tesla has sent out battery day invites with a teaser image that appears to show 'silicon nanowires' ... Amprius Technologies, a silicon anode/nanowire startup just relocated right next to Tesla's top secret Project Roadrunner Battery facility ... and Elon Musk is replying to Amprius tweets about reaching an energy density of 400Wh/kg, confirming it's only 3-4 years away from mass production. This is huge!!