Decoding electrochemo-mechanical degradation in solid-state batteries: A phase-field study toward robust cathode design

The commercialization of all-solid-state batteries (ASSBs) is hindered by complex electro-chemo-mechanical degradation processes in composite cathodes, particularly particle fracture and solid electrolyte (SE)–particle interfacial debonding. To uncover the underlying mechanisms, we develop a fully coupled three-dimensional electro-chemo-mechanical phase-field model incorporating electrochemical reaction kinetics, mechanical deformation, interfacial decohesion, particle fracture, and realistic polycrystalline microstructures. Simulations reveal that interfacial debonding stems from mismatched contraction between particles and the SE during delithiation, while anisotropic volume changes within primary crystallites induce localized GPa-level stresses, initiating cracks at particle junctions and impacting over 18 % of the secondary particle volume. Although softer SEs delay damage onset, they cannot prevent fracture or debonding. Strengthening the particle–SE interface reduces interfacial separation but accelerates internal fracture. In contrast, enhancing active material fracture toughness effectively suppresses crack initiation. Based on these insights, we propose a cathode design strategy combining microstructural homogenization, strong interfacial bonding, compliant SEs (e.g., sulfides), and improved fracture toughness. This work provides a critical mechanistic understanding of degradation pathways in ASSB cathodes and offers actionable guidelines for the design of durable, high-performance solid-state batteries.