Comprehensive analysis of thermal runaway and rupture of lithium-ion batteries under mechanical abuse conditions

Sidewall rupture of lithium-ion batteries plays an important role in thermal runaway (TR) propagation because flame burst from the side of cell can directly heat adjacent cells. However, the understanding of sidewall rupture in high specific energy cells under mechanical abuse conditions remains limited. In this work, nail penetration is adopted as a trigger method of TR of 21700-format cylindrical cells with high specific energy (257.0 W∙h/kg). The effects of test parameters including nail diameter, nail speed, penetrating location, penetrating depth, and state of charge on likelihood and severity of thermal runaway and sidewall rupture behaviour were investigated. A series of equipment including high-definition cameras, thermal imaging camera, X-ray computed tomography (CT), cycler and electronic balance were adopted to reveal the behaviour and the mechanism of TR and sidewall rupture. Discussion on CT scan and fire behaviour provides new perspectives for understanding sidewall rupture and TR mechanisms in high specific energy cells. The results show that the mean mass loss ratio of the cell with 100% SoC is greater than 45% under each test condition, and the maximum of them is as high as 62.5% when penetrating off-centre from the cell bottom and with a penetrating depth of 10 mm. The likelihood of sidewall rupture increases with the increasing nail speed, nail diameter, penetrating depth and state of charge when penetrating from the top cover of the cell, but it is little affected by the penetrating depth and nail diameter for penetrating from the bottom of the cell. For the first time such a relationship is presented. The root-cause analysis for the sidewall rupture of the cell has been discussed, which highlights the three key factors, including the casing strength, the internal pressure, and the opening area of the venting disk.