Aging diagnostics in lithium-ion batteries with differential mechanical measurements

This work investigates how the mechanical response of lithium-ion batteries evolves with aging and demonstrates how mechanical measurements can be used to estimate degradation mechanisms, presenting interesting advantages over traditional voltage-based methods. A lithium cobalt oxide-graphite battery was cycled over 1,000 times to a state of health of 70%, with periodic performance tests measuring capacity, resistance, voltage, temperature, and deformation during the charge/discharge cycles. The deformation measurements can be distinguished into a reversible component (expansion during charge recovered with the shrinkage during discharge) and an irreversible component (increase of the battery thickness through aging). From these measurements, differential deformation and incremental deformation analyses are performed to assess degradation mechanisms, innovatively demonstrating that the loss of active material and the loss of lithium inventory estimated from deformation measurements are perfectly aligned with those estimated with traditional voltage measurements at low current. In contrast to differential voltage, differential deformation offers the significant advantage of being applicable even at high current rates, which are typical of real-world charging profiles. Finally, the calculated degradation mechanisms are correlated with the physical phenomena occurring within the battery, e.g. the growth of the solid electrolyte interface and particle cracking, finding a satisfactory agreement with the trend of the measured capacity, resistance and reversible, and irreversible deformation.