(Invited) Analytical Methods for Understanding Multiscale Thermal Transport in Li-Ion Batteries Towards Improved Safety and Performance
Authors: Jain Ankur 
Published: February 2020

Li-ion cells are widely used for electrochemical energy storage and conversion in applications such as consumer electronics, transportation, grid storage, etc. Multiple transport processes such as ionic, electronic and thermal transport occur simultaneously in a Li-ion cell, and interact with each other through a number of mechanisms at multiple length scales. This considerable complexity makes it imperative to carry out a careful, analytical investigation of these transport processes for optimizing performance and ensuring safety. This presentation will summarize ongoing research on analytical modeling of heat transfer in Li-ion cells with the overarching of improving safety and performance of electrochemical devices and systems. The governing energy conservation equation in a Li-ion cell is solved in a variety of scenarios to help better understand the nature of heat transfer in the cell. Specifically the effect of temperature-dependent, exothermic decomposition processes is examined. It is shown that such processes can lead to a thermal runaway situation wherein the cell temperature becomes unbounded. It is shown whether thermal runaway occurs or not depends on the value of a specific non-dimensional parameter called the thermal runaway number which captures the contributions of three different thermal processes that combine to determine the thermal fate of the cell. This treatment offers a potential mechanism to monitor the health of a cell and proactively take measures to prevent thermal runaway before it is too late. Further, analytical modeling of temperature distribution with a cell is carried out in order to develop a method for non-invasively determining the core temperature of a cell. This is important because a direct measurement of the core temperature is not possible in most cases due to lack of physical access to the cell core. An analytical model for the temperature distribution in the cell is developed using the method of undetermined parameters. This shows that the core temperature of a cell undergoing any arbitrary process can be determined through appropriate time and space integrals of the outside temperature of the cell, combined with certain other thermal parameters. This has enabled an experimental method for non-invasive core temperature measurement of a Li-ion cell based on infrared thermography of its outside surface. These results indicate, for example, that the core temperature of the cell may be hundreds of degrees Celsius greater than the measured outside temperature during a thermal runaway event. Given that the thermal fate of the cell is ultimately governed by its peak temperature, which occurs at its core, this is an important insight into the thermal state of a cell undergoing, or close to thermal runaway. In addition to improving the understanding of fundamental heat transfer processes in a Li-ion cell, the analytical modeling work described here also forms the basis for novel techniques for measurement of critical processes and properties, thereby contributing towards improved overall safety and performance of the cell.