Cell-Level Mapping of The Thermal Conductivity of Electrode Composites in Lithium-Ion Batteries

In Lithium-ion batteries (LIB), the surface of the active material serves as the primary site for reactions and heat generation. Also, it is the place where the hotspots trend to emerge. Localized high temperatures within the micrometer range can trigger a series of chain reactions, leading to uncontrollable thermal runaway in batteries. Therefore, a detailed understanding of the thermal properties of LIB electrodes is of paramount importance. Previous research has used various methods, for example, laser flash and steady-state measurement, to measure the thermal properties of electrode composite or active materials. However, the previous research typically treats the thermal properties of LIB electrodes as a macroscopic characteristic, failing to provide detailed information about real electrodes. The objective of this study is to produce micrometer-scale resolution mapping for the thermal conductivities of the electrodes.<br/><br/>We adapted TDTR technology, which is a well-established optical pump-probe technique, to perform experiments. In TDTR, the pump laser beam heats the sample surface, and the reflected probe beam is detected to analyze the thermal response of the sample. Before conducting the experiment, we took multiple steps to ensure that the sample would yield a sufficient reflected probe signal. We utilized ion milling techniques to polish the electrode's cross-sectional surface until its Root Mean Square (RMS) roughness within a 10um x 10um area was reduced to less than 50nm. We selected graphite and lithium cobalt oxide (LCO) electrodes, both made from active material particles of approximately 20um in size for the experiment. The local measurement was first conducted under three different conditions: 1) 6um spot size laser at 18.9MHz heating frequency, 2) 3um spot size laser at 9.8MHz heating frequency, and 3)3um spot size at 5.4MHz heating frequency. We fitted the obtained thermal response signal with a thermal model. From this, we derived an isotropic thermal conductivity for LCO, and an anisotropic thermal conductivity for graphite. Finally, we conducted scanning experiments on the electrode within a 30μm x 30μm area. Data was collected at 1 um intervals during the scan. The scanning results clearly indicate the thermal conductivity of each component, with distinct boundaries that align well with the SEM images.<br/><br/>Our study provides detailed thermal properties at high spatial resolution, enabling the calculation of the temperature distribution during battery operation and the prediction of hotspot emergency conditions. This study facilitates the development of thermal management strategies at the electrode level in LIBs, addressing the safety concerns at their root.