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 thermophysical 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 (LLI), was conducted.
Two distinct methodologies were implemented to accurately assess the thermal behaviour of the aged battery cells. The first method involved external heating applied in an adiabatic configuration, designed to minimize external heat exchange and provide precise information about heat conduction paths. The second approach, instead, relied on internal heat generation under actual battery operating conditions, thus simulating real-world usage and capturing heat dissipation dynamics of the cell
Temperature measurements were conducted with advanced thermal sensors, Fiber Bragg Grating (FBG), which are highly sensitive to the rapid thermal dynamics occurring within the cell. These were placed both externally and internally in the battery cells, after meticulously drilling holes to avoid compromising the cell’s integrity. The dual placement of FBG sensors allowed for detailed analysis of temperature gradients and thermal anisotropy, providing a comprehensive view of how thermal properties evolve under different operating stresses and aging effects.
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%.