Chemo-thermal stress in all-solid-state batteries: Impact of cathode active materials and microstructure

The transition from conventional lithium-ion to all-solid-state lithium batteries (ASSBs) promises enhanced safety and higher energy density but also gives rise to new challenges, like capacity degradation due to enhanced mechanical stresses. This study addresses the often-overlooked residual (thermal) mechanical stress arising during manufacturing, which can significantly contribute to the overall mechanical stress. While stress evolution during battery operation is often only associated with the de-/lithiation-induced stresses from the active material, we introduce a “chemo-thermal stress” description. By this integration of thermal and chemical stresses, we developed a more accurate level to simulate real-life conditions, especially for all-solid-state batteries. This holistic approach demonstrated for the first time, that thermal stresses from manufacturing can reduce the induced mechanical stress in LiCoO₂ (LCO) during delithiation, resulting in the total chemo-thermal stress being approximately 43 % lower. In contrast, residual thermal stress exacerbates chemical stress in Li_0.5NCM955 and Li_0.1NCM955, leading to a principal stress increases of approximately 42 % and 15 %, respectively. We also examine the impact of microstructural design parameters, particularly the solid volume fraction of the cathode active material (CAM) and relative density, on the induced mechanical stresses within CAM and the solid electrolyte (SE). Our investigation reveals that the volume change in cathode active materials, a primary contributor to induced mechanical stress in ASSBs, is not a reliable factor for predicting final stresses in actual full battery cells. Additionally, our findings highlight LCO's superior mechanical behavior compared to Li_0.5NCM955 and Li_0.1NCM955, attributed to lower overall stress and prevalent compressive stress, which mitigates failure risks in oxide materials.