State-of-Health Diagnostics in Lithium-Ion Batteries: A Methodological Framework for Scalable Mechanical Ageing Assessments

Pressure-related quantities such as internal pressure, cell expansion, and housing deformation are promising mechanical indicators of lithium-ion battery ageing because they are directly linked to degradation processes such as gas formation, lithium plating, and interphase growth. However, existing studies are largely limited to single-cell experiments under controlled laboratory conditions and do not provide a systematic framework for larger battery systems. This work reviews pressure- and expansion-based diagnostic approaches and derives a structured methodology for their practical application. The framework includes representative operating conditions, format-dependent assignment of measurement principles, sensor placement, sensor selection, and cycle-wise data processing. The results show that pressure-based diagnostics must be understood as a format-dependent methodological family, with dilatometric or force-based approaches being most suitable for pouch cells and strain-based methods for rigid housings. Overall, the study provides a scalable basis for integrating mechanical ageing diagnostics into future battery monitoring systems. The framework further describes the mechanical coupling of cells within a module, which determines both the magnitude of the measurable signal and the extent to which the state of individual cells can be resolved.