Non-Invasive Estimation of Lithium-Ion Cell Thermo-Physical Properties

Abstract

Cylindrical Li-ion cells have one of the highest energy density and power density of all Li-ion cell types and typically employ a spiral electrode assembly. This spiral assembly leads to a large anisotropy leading to a drastic difference in the thermo-physical properties in the axial and the radial direction. The radial direction has multiple layers of electrodes and separators leading to a high thermal impedance in this direction, whereas in the axial direction, not many obstacles are present and hence the thermal conductivity is on the higher side. This research describes a novel experimental technique to measure the anisotropic thermal conductivity and heat capacity of Li-ion cells using thermal impedance spectroscopy (T.I.S). It is paramount to experimentally measure the radial and axial thermal conductivities of a cylindrical Li-ion cell because the assumption of isotropic thermal transport properties in Li-ion cell design would lead us to either under predict the value or over predict the value of the temperature field - both of which would lead to highly undesirable results.

The experimental measurements indicate that radial thermal conductivity is two orders of magnitude lower than axial thermal conductivity for cylindrical 18650 cells which is in sync to what we intuited. Moreover, the work presented here also establishes a trend of the change in thermos-physical properties with varying the state of charge of the cell. This is extremely helpful in order to develop an efficient cooling system for any device that needs to continuously charge and discharge over thousands of cycles. The data helps to account for the change in the thermal conductivity and heat capacity over a period of cycling of the cell and thus encouraging an update in the cooling system for the device also in order to avoid hazardous situations such as thermal runaway.

Lastly, the technique presented in this research is a non-invasive, robust, quick and extremely economical way to determine the thermos-physical properties of an 18650 lithium-ion cell. It also determines the change in thermos-physical properties with the changing state of health of the cell.