Analysis of the aging effects on the thermal runaway characteristics of Lithium-Ion cells through stepwise reactions

The current political vision to drastically reduce carbon emissions pushes the electrified powertrain into an increasingly important role in the transport sector. However, concerns related to the battery's effectiveness as an energy source need to be overcome to make this technology widespread. One such concern is safety, catching attention as development races towards greater battery energy density. In this way, cathodic chemistry is important since exothermic reactions are unleashed from the components that originally formed the active material. Furthermore, the aging process reduces the battery capacity, reducing the amount of active material and thickening the solid electrolyte interface, which increases the joule effect. For these reasons, thermal runaway under aging conditions must be investigated to assess potential safety issues. Using an accelerating rate colorimeter, the heat-induced thermal runaway tests were performed with two-cathode chemistry (NMC and LFP) under pristine and aged battery conditions. For aging the batteries, the ARC was coupled with a bidirectional source. Two ambient temperatures, 20 °C and 50 °C, were used for the aging tests, being the batteries cycled up to 250 cycles with a determined protocol for charge and discharge. A numerical model was fed with experimental tests, targeting optimizing the battery output parameters and obtaining geometric aspects that are difficult to measure. Unlike the single step model, a stepwise reactions model was created to assess the heat release from different battery components for pristine and aged conditions. The higher endothermic behavior from cathode decomposition and less oxygen released during this reaction make the LFP battery safer than the NMC. For aged batteries, the SEI growth consumes lithium and electrolyte, decreasing the quantity of both components in the anode. Thus, the anode and electrolyte reaction after SEI decomposition is lower, improving battery safety.