Changes in Heat Generation during Degradation of Commercial Lithium-Ion Batteries

Abstract

The performance, lifetime and safety issues of lithium ion batteries (LIB) are strongly linked to temperature and many studies in the past three decades have been devoted to improving the understanding of thermal behaviour of LIB at cell and pack level1. We present ageing and diagnostic data of a commercial pouch LIB and present a thermal battery model. The thermal battery model is used to discuss changes in different heat generation sources alongside battery ageing measurements. It further allows for a differentiation between temperature induced and C-rate accelerated ageing.A selection of LCO-graphite cells was aged and cycled in the full state-of-charge window at 5, 15, 25, 35 and 45°C. We compare constant current cycling with charge and discharge rates of 1 C, 2 C and 3 C, and constant power discharging. The cells were monitored in detail during the ageing including surface. The performance at lower C-rates, internal resistances and the changes in cell thickness were obtained as functions of changes in state-of-health. Thermal conductivity measurements were performed for the electrode material at end and beginning of life.A 1D analytical thermal model is used for a discussion about the impact of the heat sources, the effect of a change in heat transport with ageing due to decreased thermal conductivities, and the impact of internal temperature gradients on ageing. By utilizing the thermal battery model, it is possible to analyze the effect of the different heat sources on the overall battery temperature as well as degradation dependent changes in heat generation. The heat sources included in the model are the ohmic heat produced, heat production due to overpotential, and reversible entropic heat due to the electrode reactions, where the entropic heat can both act as a heat source and heat sink. The transfer of heat inside the battery is described using Fourier’s second law. These heat and temperature effects allow further to differentiate between temperature induced and C-rate accelerated ageing. Together with evaluating the impact of temperature gradients over the cell compared to the maximum cell temperature, this allows to estimate the impact of an improved thermal management system. References 1 Spitthoff, L.; Burheim, O.S.; Shearing, P.R. Temperature, Ageing and Thermal Management of Lithium-Ion Batteries. Energies 2021, 14, 1248. https://doi.org/10.3390/en14051248