For the nail penetration test, the cell was fully charged (1C CCCV charge with a cutoff voltage of 4.2 V and a cutoff current of C/20). Thermocouples were placed at 10 mm to the geometry center of the cell and at the negative tab of the cell. The nail diameter is 5 mm, made of heat-resisting steel (point angle of the nail is 60°; nail surface is clean, without rust or oil). Rate of penetration was at 30 mm/s; the nail penetrated through the geometrical center of the electrode plane perpendicularly and stayed inside the cell. The observation time was 1 hour until the cell cooled down and the cell voltage dropped to nearly zero.