Potentioresistive polymer layer to protect lithium-ion battery from internal short circuit driven thermal runaway

Recently, a new approach to protect Li-ion batteries against electric shock has been reported that relies on voltage-driven switchable-resistance protective layer. When the cell voltage goes beyond operating limits of the cathode material, the conductivity of the potentioresistive layer drops drastically, which prevents thermal runaway of the battery and consequent fire and explosion. While the protection scheme is quite obvious in case of external short circuit or accidentally applied overvoltage, the protection mechanics in case of internal short circuit (ISC) is unclear. In the present work, a computational model is proposed which emulates self-heating and discharge of a protected lithium-ion cell in a nail penetration test. In the model, a wide range of protective layer resistances is screened and cooling conditions are determined that are sufficient to prevent thermal runaway for a given resistance value. For example, an industrial LiCoO₂ 3.2 Ah pouch would require protective layer of at least 0.1 Ω∙m2 to secure safe ISC discharge under free convection conditions. Analysis of the heat and charge transfer processes simulated using COMSOL Multiphysics software shows that both high resistance of the layer and medium resistance of the cathode composite material itself play crucial role in reducing heat generation rate of the protected cell, thus providing unexpected safety characteristics. The theoretical predictions of the model are verified through a nail penetration experiment on the industrial form-factor Li-ion battery with LiCoO₂ standard composite cathode material protected by a potentioresistive layer of polyNiMeOSalen.