Predicting the Onset of Vent Valve Opening in an 18650 Cell during Thermal Runaway 2022-28-0417

Oven exposure testing is a standard benchmark (where Li-ion cells are exposed to higher temperature) that Li-ion cells must pass to get approval for sale by the regulating bodies. These tests are designed to ensure the safety of battery user as the Li-ion batteries are vulnerable to abuse conditions. However, these tests can be both costly and time consuming. Hence, development of simulation capabilities which can replace the physical test to a certain extent helps both battery manufacturers and OEMs not only in the cost cutting but also to optimize the critical parameters which can directly influence the safety criterions.

In this paper, a numerical model of 18650 NCM Li-ion cell in an oven test condition is developed to study the thermal runaway, cell venting, internal pressure buildup, and gas flow behavior using ANSYS FLUENT commercial software. Thermal abuse reactions are described using by a four-equation lumped reaction model which considers the reaction kinetics of Solid Electrolyte Interface (SEI) decomposition reactions, anode-electrolyte reactions, cathode-electrolyte reactions & electrolyte decomposition. Vent gas generation is modelled using a user defined function (UDF) and Vent Gas release mechanism is modelled using macro code in C language. The model developed is validated against the experimental results and found to be in good agreement. The thermal runaway initiation time, corresponding onset temperature and vent valve opening (VVO) timing has been found out and the thermal runaway and vent flow characteristics has been studied. Further, a series of computational fluid dynamics (CFD) simulations have been conducted on this setup with various external oven temperature to find out its effects on thermal runaway events and thereby define its abuse tolerance limits. It is found that the thermal runaway initiation condition, the pressure buildup, and the valve opening are highly sensitive to external heating temperature. No thermal runaway is observed when the external oven temperature is below 120 ̊C and no vent gas flow is observed below 160 ̊C. As the vent valve opening is a critical safety parameter in avoiding catastrophic events following the thermal runaway, this study will be helpful in choosing the right working condition for Li-ion battery cell and are insightful in vent valve designing.