Impact of safety valves on thermal runaway characteristics of 21 700-size lithium-ion cells

To demonstrate the impact of safety valves on the thermal runaway characteristics of 21 700-size lithium-ion cells, this work carries out a series of abusive tests including over-heating tests, accelerating rate calorimetry (ARC) tests and overcharge tests; in the meantime, the impact of safety valves on cells with various states of charge (SOC) and states of health (SOH) is unveiled accordingly. Safety valves have a great impact on the thermal runaway behavior of 21 700-size cells, which effectively restrains the thermal runaway risks and hazards of cells under the over-heating conditions. The presence of a safety valve could even prevent a cell from the thermal runaway induced by overcharge. A cell with a larger SOC and/or a lower SOH is prone to the thermal runaway occurring, with an earlier thermal runaway, a lower temperature to thermal runaway and a lower mass loss demonstrated. Besides that, it is found that the advancement ratio of the time to thermal runaway between cells with and without a safety valve was maintained at around 15%; that is, the existence of a safety valve is helpful to delay the encounter of thermal runaway by ∼15%.