MODELING AND MEASUREMENTS OF THERMAL TRANSPORT IN LI-ION BASED ENERGY CONVERSION AND STORAGE DEVICES

Abstract

Heat transfer is of significant importance in energy conversion and storage devices such as Lithium-ion batteries for its safe operation and performance. Li-ion batteries are considered to be the state of the art among the energy storage devices due to their very high energy density and high power density. However, the safety of Li-ion batteries has become a concern in light of recent incidents, where there have been catastrophic events reported due to overheating of large battery packs. It is imperative to fully understand the nature of thermal transport in Li-ion cells. However, this has not been explored in detail yet. Recent findings have suggested very large thermal conductivity anisotropy in cylindrical Li-ion batteries. In order to make accurate predictions of thermal behavior of the Li-ion cell, it is very important for thermal models to account for such a high degree of anisotropy. In the present work, analytical steady state and transient thermal models have been developed for a cylindrical Li-ion cell. These models can be used as a tool to optimize design parameters and provide directions for further research in improving heat transfer inside a cell for improved safety. A key conclusion from this thermal modeling work is the presence of a very large temperature gradient inside the battery, which indicates poor heat transfer from the core to the surface of a cell, even if cooled aggressively at the surface. This can be explained by a thermal resistor model where the radial conduction resistance is shown to be the rate limiting factor in heat dissipation. From the thermal modeling work, it appears that the heat from the core of the cell isn’t being effectively transferred to the surface. This can be due to the large thermal resistance offered by the battery material which the heat has to propagate through. If an axial fluidic channel is provided through the core of the cell, it can significantly improve heat removal from the core of the cell. Fluid flow through an annular channel is a promising strategy for cooling a Li-ion cell. A simplified analytical model is developed to understand this in detail. The model predicts temperature rise inside a cell as a function of average convective heat transfer coefficient over the channel surface and the size of the channel. Gain in terms of higher charge/discharge rate due to the effective cooling is also estimated. Reduction in temperature rise or increase in power density is also compared against the reduction in energy density as a function of channel size. A related fundamental problem of conjugate heat transfer is solved. A classical example of such a problem is flow in a thick tube, which is similar to the problem of the proposed design of a Li-ion battery with an axial fluidic channel. A framework is proposed to solve conjugate heat transfer problem and is demonstrated by solving a couple of commonly occurring conjugate heat transfer problems in reality. The method is validated with a well validated past model and finite element solver. Experimental demonstration of the conceptual design of a Li-ion cell with an annular channel has been done on a thermal test cell. Various cooling strategies, active as well as passive, have been implemented, evaluated and compared. Active cooling is demonstrated by passing air through the channel in the thermal test cell. For passive cooling, a heat pipe/copper rod is inserted in the channel with the tip protruding outside the cell. The results from this work show effectiveness of internal cooling of a Li-ion cell over convectional external cooling . Thermal runaway leads to catastrophic events in energy storage devices with high energy density, such as a pack of Li-ion cells. An experimentally validated thermal model is developed to capture the nonlinear nature of heat generation in a Li-ion cell due to the temperature dependent behavior of exothermic electrochemical reactions. The thermal modeling effort lead to the discovery of a non-dimensional number named as Thermal Runaway Number (TRN) which can help predict onset of thermal runaway and determine thermal stability of a Li-ion cell. This analysis can prove to be crucial to better understand thermal runaway phenomena in Li-ion batteries. Further, thermal modeling to compute temperature in Li-ion cell with temperature dependent heat generation has also been developed. The model has been experimentally validated by simulating temperature dependent heat generation for different values of heat generation parameters. Effect of heat transfer parameters on temperature has also been analyzed.