Lithium-ion (Li-ion) cells are increasingly used in high-voltage and high-capacity modules. The Li-ion chemistry has the highest energy density of all rechargeable battery chemistries, but associated with that energy is the issue of catastrophic thermal runaway with a fire. With recent incidents in the commercial aerospace and electronics sectors, methods are required to prevent cell-to-cell thermal runaway propagation. The goal of this work was to achieve a common method for triggering a single cell in a Li-ion battery module into thermal runaway, determine if one can consistently obtain this thermal runaway event, and design mitigation measures to address propagation of the thermal runaway to other cells in the module.
A standardized method was used to trigger thermal runaway of a single cell in a Li-ion battery module. Two cell designs were chosen for testing based on different physical dimensions and venting characteristics. A heater tape common to both designs was chosen. The intent was to take a single cell in the worst-case location in a module into thermal runaway, and determine if the thermal runaway propagates to the other cells in the battery module.
Cells in series and parallel configurations were tested. The heat was provided to the cell at a constant heater power of 20 W (20 V and 1 A), which was half the maximum power the heater can handle. This heating rate was chosen because it would provide steady heating of the cell. If the heating rate was too high (greater than 10 °C per minute), the cell would undergo a sudden thermal runaway. If the rate was too low (1 to 2 °C per minute), the heat dissipation from the cell would be higher than the heating rate and the cell would not enter thermal runaway.
Different cell designs of various Li-ion chemistries, as well as physical formats, were studied. Testing consisted of designing the cell modules with spacing between the cells, introduction of a radiant barrier, and placing the cells in a module manufactured using intumescent materials. It was determined that at least 2-mm spacing was required for cylindrical cell designs. For cell formats that had vents on the side as with the prismatic cell design tested under this program, a physical separation between neighboring cells was required. This was achieved by using intumescent materials as well as the radiant barrier.