Testing and Modeling of an In Situ Shear Exfoliated 2D Nanocomposite Coating Casing Material for the Suppression of Li-Ion Battery Fires in Electric Vehicles

The requirements of high energy density Li-ion battery along with its flammable electrolyte in electric and hybrid electric vehicles makes it susceptible to abusive operating conditions leading to thermal runaway, gas venting, fires, and explosions. Though there are external cooling and in-situ measures (e.g., coating of cathode with inert materials) for the prevention of thermal runaway, initiation of thermal runaway in one cell can easily propagate to adjacent cells, the entire pack assembly, and the vehicle. The commercial application generally focuses on water mist for the suppression of fire, the effectiveness of which depends on the types of fire encountered or the particular event scenarios. In this study, a 2D nanocomposite coating material, a mixture of dragon skin® (flame-resistant silicone elastomer from Smooth on, Inc.) and hexagonal boron nitride (DS/hBN), was applied on the traditional polycarbonate casing material to study its effect on flame retardancy. Dragon skin polymer precursor (dragon skin part A) and curing agent (dragon skin part B) with 5% low-cost layered hBN were mixed separately in a Hauschild planetary mixer to get homogenous mixing (at 2500 rpm for 2 minutes). A micro batch mixer was used to simultaneously exfoliate hBN nanosheets from bulk layered hBN raw materials and disperse these nanolayers into both dragon skin polymer precursor and curing agent separately (at 100 rpm for 10 minutes). Dragon skin polymer precursor and curing agent with exfoliated hBN were mixed in a 1:1 weight ratio to coat the polycarbonate and dried for 24 hours. Scanning electron microscopy (SEM) images confirmed the uniform dispersibility of these nanosheets. Standard 'UL94 Flammability Test' protocol was followed to measure the flame retardancy of the prepared sample. Here, Tests were conducted in both the horizontal and vertical testing arrangement specified in the standard. Besides, for the control experiment, another sample containing a coating of only dragon skin (DS) was prepared and tested. In the horizontal test, uncoated polycarbonate was found to burn extensively (labeled as "Failed"), whereas DS-coated and DS/hBN-coated polycarbonate slowed the burning rate by 33 mm/min and 9 mm/min, respectively. The burning rate of DS-coated and DS/hBN-coated samples is lower than 75 mm/min, so these can be categorized as "HB". In the vertical test, after the initial flame exposure, the flame was extinguished within 5 sec and 15 sec for the DS/hBN-coated and the DS-coated sample, respectively. During the burning, no dripping was observed for the DS/hBN-coated sample, whereas the DS-coated sample showed severe dripping and crack propagation in the coating layer. Moreover, during the vertical test of the DS-coated sample, the fire spread quickly and reached the clamp before extinguishment. So, DS-coated and DS/hBN-coated sample was rated as 'Failed' and 'V-1', respectively. These promising findings show the potential hBN nanosheets-based nanocomposites prepared from low-cost materials by in-situ shear exfoliation, which can form three-dimensional networks to rapidly disperse heat and build an efficient char layer for insulating the exposed polymer and forming a thermal barrier. Additionally, A multiphysics model is developed and applied to study the effect of butane fire on a traditional casing material and the 2D nanocomposite-coated material.