The aging of lithium-ion batteries has a significant impact on the internal structure due to phenomena such as the growth of the solid electrolyte interface (SEI) and the particles cracking thus altering the internal properties of the materials, such as: thermal conductivity and specific heat capacity.
Despite the importance of these changes, the relationship between the evolution of thermal properties and aging is not widely covered in the literature.
Therefore, an experimental campaign based on the study of both beginning-of-life and real-world aged LFP cylindrical battery cells from 8-10 years automotive aging, with down to 50% residual capacity, mostly due to loss of lithium inventory, was conducted.
Two methodologies were employed: one using external heating in an adiabatic configuration, and another based on internal heat generation due to the real battery operation. Temperature measurements were performed with advanced thermal sensors, Fiber Bragg Gratings (FBG), which are highly sensitive to rapid temperature fluctuations, placed both externally and internally to the cell after precise drilling.
These methods provide detailed information on the evolution of thermal properties as cells age. Tests performed at different states of charge (SOC) reveal that when the state of health (SOH) decreases up to 40%, the thermal conductivity, differentiated into radial and longitudinal, decreases about 15-20%, while the specific heat increases about 10-15%.