The electric vehicle battery start-up hit one of its biggest milestones yet, giving its languishing stock a much-needed fillip.It's been a monster week for QuantumScape (QS 15.72%). The stock, which was down 22% this year until July 10, is now up 15% year to date as of this writing thanks to a massive development that sent it skyrocketing on Thursday. Shares of the electric vehicle (EV) battery start-up were trading 57% higher for the week through 11 a.m. ET Friday.QuantumScape's deal with Volkswagen is a h ...