Notice the NCR18650PD cell has the Tesla patented vent design and is NCA chemistry. This is the only cell on the market with the C-shaped vent. Tesla buys these cells direct from Panasonic with no cover and only gets grade AAA matched cells. This vent design is designed to work with the wiring bonded pack in the Tesla. When the vent pops open it breaks the wire bond. The wire bond itself also acts as a fusible link and as a bonus is makes it super easy to automate assembly. The patented pack design is the best in the world.