Early-Stage Thermal Safety Evaluation of the NMC811/LLZO/Li Solid-State Battery Chemistry Using Calorimetry and Characterization Methods

With the emergence of various battery chemistries including lithium metal solid-state batteries, early-stage safety evaluations can provide insights into designs for safe material sets, cells, and packs. Differential scanning calorimetry (DSC) can be performed to determine the heat flow of small (<10 mg) samples. We construct hermetically sealed Li/LLZO/NMC811 anode–cathode–electrolyte (ACE) solid-state samples for DSC tests and measure the onset temperature of exothermic reactions, the rate of heat flow, and the total heat released by 500 °C. Proposed chemical reaction pathways are constructed using a mass–energy balance from DSC and TGA, with additional input from other characterization methods. By adding an isothermal hold at the end of the constant rate heating in DSC tests (500 °C in our protocol), we obtain additional heat flow important for determining the reaction pathways. We find that our NMC811/LLZO/Li ACE samples have a total heat release of ∼15 J/mAh at 500 °C with an onset temperature of around 220 °C. We compare solid-state material sets with a Li metal anode and LLZO separator and either NMC811 or lithium cobalt oxide cathode and find that thermal metrics such as the total heat release and the onset temperatures are broadly similar, reflecting a common exothermic reaction path involving oxygen release from layered oxide cathode active materials and HF release from the PVDF binder and their reaction with reductants including lithium metal and conductive carbon.