Assessing degradation modes in cylindrical lithium-ion batteries: A non-invasive method using the entropic heat flow or surface temperature

Degradation mode analysis for lithium-ion batteries is typically conducted based on a comparison of potential measurements (PM) between a pristine and aged cell. This publication introduces two methods, either based on heat flow in an isothermal micro-calorimeter (CM) or cell surface temperature (TM). Their feasibility for degradation mode analysis was tested using commercial 18650 LFP/graphite and 21700 NCA/silicon–graphite cells. To reconstruct the entropic coefficient (EnCo) of the pristine and aged full-cell, pristine half-cell EnCos were measured in a micro-calorimeter. These half-cell EnCos were then superimposed to reconstruct the full-cell EnCo from either of the two new methods, CM and TM. The conventional PM-method serves as a validation. The degradation mode results indicate that the proposed methods are possibly more effective for LFP, while providing equivalent results for NCA and graphite electrodes. For silicon–graphite electrodes, an electrode composite model is potentially required. Regarding the measurement of the EnCo itself, the introduced methods significantly reduce measurement time compared to conventional methods, such as potentiometric tracking of entropy change in aging markers. The proposed TM-method might be of interest in applications, as the change in EnCo can be observed by a PT100 temperature sensor, facilitating a non-invasive rapid degradation mode estimation method. • Estimating degradation modes non-invasively via heat flow or surface temperature. • Continuous full-cell entropy reconstruction from half-cell entropy over aging. • Entropy-based estimation benefits cells with LFP-cathode. • In future, estimation of parallel-connected cell degradation using full-cell entropy.