Understanding Li-Ion Battery Thermal Runaway through Small, Slow and in Situ Sensing Nail Penetration

Abstract

Thermal runaway is a critical safety challenge for Li-ion batteries, yet it is still not well understood [1]. Here we propose understanding thermal runaway behaviors and mechanisms through small, slow and in situ sensing (3S) nail penetration.As schematically shown in Figure 1, the 3S nail penetration is different from conventional nail penetration testing [2] in three aspects. First, the nail is small in diameter, reducing influences of nail on thermal runaway behaviors [3]. Second, the nail penetrates Li-ion cell very slowly, enabling precise control of penetration depth or even single layer internal short circuit. Third, a micro temperature sensor is embedded in the nail tip as inspired by previous reports [4-7], providing in situ sensing of temperature at internal short circuit spot. Voltage between the nail and Li-ion cell tabs is also monitored as inspired by a previous report [8], allowing detection of nail tip location.Figure 2 shows comparison of temperature results for 3 Ah Li-ion pouch cells using 3S nail penetration and conventional nail penetration. It can be seen that 3S nail penetration provides much more details of thermal runaway than conventional nail penetration. Most interestingly, three internal temperature peaks were observed during a period of more than 100 seconds, with the third peak over 500 °C, but the temperature quickly decreased after these peaks. Thermal runaway did not occur until the fourth temperature peak which reached 800 °C.Based on further investigation, the initial internal temperature peaks occurred when the nail tip reached aluminum foil current collector and caused internal short circuit between aluminum foil and anode. The quick decrease of internal temperature after each peak could be attributed to stop of internal short circuit current. The stop of internal short circuit current could be further attributed to rupture of aluminum foil by nail penetration and significant increase of contact resistance between the nail and aluminum foil. These detailed understanding could help development of fundamentally safer Li-ion batteries.