Determination of Radial Thermal Conductivity and Specific Heat in Externally-Heated LiCoO₂ cells

Abstract

In this presentation, we will investigate the thermophysical properties of LiCoO2 18650 battery cells. Due to their versatility and high energy density, lithium-ion batteries have become a major power source in a number of consumer and industrial products over the last decade. As their popularity and number of applications increase, so do instances of catastrophic battery failure. Excessive external heating, which in turn leads to thermal runaway, is a principle cause of cell failure and failure events continue to occur despite the installation of safety features, such as vents and thermal fuses, within the cells. Models of cell failure do exist and they have been used to improve upon battery safety features; however, failure models require prior knowledge of the cell’s thermophysical properties. For many common, commercial lithium-ion batteries, LiCoO2 18650 cells included, knowledge of their thermophysical properties is often either incomplete or absent entirely. To determine the radial thermal conductivity and specific heat of a LiCoO2 18650 cell, we conducted external heating experiments and created a numerical heat transfer model. In our experiments, a thermocouple and a heat flux sensor were mounted to the battery surface and, without damaging the cell, external heat was provided using NiCr wire, wrapped helically around the cell’s exterior. Using the time-dependent heat flux data measured at the battery’s surface, our computational model solved the heat conduction equation for the cell’s radial temperature profile. The radial thermal conductivity and specific heat of the LiCoO2 18650 cell were then found by minimizing the difference between the experimental surface temperature data and the model’s predicted surface temperature over time. Matching the model’s results with the experimental data was achieved with a parametric exploration of specific heat values and the Secant Method, which iterated over the radial thermal conductivity. Over the course of our presentation, the experimental setup, the numerical model, and the results we obtained will be explored in detail. We will also discuss how this method can be used to study the radial thermal conductivity and specific heat of any cylindrical cell.