Material design and stability of All-Solid-State Lithium batteries

All-solid-state lithium batteries (ASSBs) are considered a promising future battery technology due to their high safety and energy density. However, they are suffering from mechanical fatigue during cycling, caused by induced mechanical stresses by volume changes in the electrode active materials which is constrained by the solid electrolyte. This research is based on computer-aided material design (Geodict), to reconstruct an experimental microstructure of composite cathode consisting of LiCoO2/Li7La3Zr2O12 (LCO/LLZO), followed by regenerating this structure while varying its microstructural design parameters (solid volume fraction of LCO – SVFLCO, relative density – 𝜌̅ and grain sizes of LCO and LLZO – dLCO, dLLZO respectively). Where each variation occurred while fixing the other parameters. We calculated the electrochemical stresses during cycle and relative conductivities (electronic and ionic) for each variation. This methodology enables systematic investigation of the impact of microstructural design parameters on mechanical stress distribution and relative conductivities, including configurations not easily accessible experimentally. We found that mechanical stresses and conductivities exhibit a linear relationship with the variation of SVFLCO and a progressive relation with variation of 𝜌̅, while the grain sizes of LCO and LLZO show negligible influence. A new dimensionless parameter, Kn, defined as the ratio of relative interface area between solid contents of composite cathode to the volume ratio of these contents (n = LCO or LLZO), was introduced as a governing factor of the induced mechanical stress. On the other hand, the electronic and ionic conductivities are primarily governed by the volume fractions of LCO and LLZO, respectively. To further enhance stress prediction in ASSBs, we developed a "chemo-thermal stress" model that integrates residual thermal stress after sintering with chemical stress induced during (de)lithiation. This holistic approach demonstrates that thermal stress has a significant impact on the net stress values (chemo-thermal) in active materials. For LCO, thermal stress mitigates the chemical stress during delithiation, reducing total chemo-thermal stress by approximately 43%. In contrast, residual stress amplifies chemical stress in Li0.5NCM955 and Li0.1NCM955 by around 42% and 15%, respectively. Moreover, our results revealed that among the studied cathode active materials (CAMs), LCO exhibits superior mechanical behavior due to lower overall stress and a predominance of compressive stress, reducing failure risk in brittle oxide materials. Finally, we investigated the influence of cathode grain morphology on induced chemo-thermalmechanical stress, focusing on systems combining spherical or hexagonal LCO with spherical or fiber-shaped LLZO. Modeled microstructures of four configurations—spherical LCO–spherical LLZO, spherical LCO–fiber LLZO, hexagonal LCO–spherical LLZO, and hexagonal LCO–fiber LLZO—were analyzed. Results show that fiber-shaped LLZO combined with textured hexagonal LCO reduces mechanical stress by 11.0% in LCO and 9.0% in LLZO. while, randomly oriented spherical LCO increases induced stresses in both phases. The alignment of fiber LLZO critically affects the stress distribution through facet-specific contact interfaces, emphasizing the importance of microstructural engineering to enhance the mechanical reliability of ASSBs.