Direct Observation of Reversible Heat Absorption in Li-ion Battery Enabled by Ultra-Sensitive Thermometry

The reversible heat in lithium-ion batteries (LIBs) due to entropy change is fundamentally important for understanding the chemical reactions in LIBs and developing proper thermal management strategies. However, the direct measurements of reversible heat are challenging due to the limited temperature resolution of applied thermometry. In this work, by developing an ultra-sensitive thermometry with a differential AC bridge using two thermistors, the noise-equivalent temperature resolution we achieve (10 uK) is several orders of magnitude higher than previous thermometry applied on LIBs. We directly observe reversible heat absorption of a LIR2032 coin cell during charging with negligible irreversible heat generation and a linear relation between heat generations and discharging currents. The cell entropy changes determined from the reversible heat agree excellently with those measured from temperature dependent open circuit voltage. Moreover, it is found that the large reversible entropy change can cancel out the irreversible entropy generation at a charging rate as large as C/3.7 and produce a zero-heat-dissipation LIB during charging. Our work significantly contributes to fundamental understanding of the entropy changes and heat generations of the chemical reactions in LIBs, and reveals that reversible heat absorption can be an effective way to cool LIBs during charging.

Good Solid-State Electrolytes Have Low, Glass-like Thermal Conductivity

Management of heat during charging and discharging of Li-ion batteries is critical for their safety, reliability, and performance. Understanding the thermal conductivity of the materials comprising batteries is crucial for controlling the temperature and temperature distribution in batteries. This work provides systemic quantitative measurements of the thermal conductivity of three important classes of solid electrolytes (oxides, sulfides, and halides) over the temperature range 150-350 K. Studies include the oxides Li1.5Al0.5Ge1.5(PO4)3 and Li6.4La3Zr1.4Ta0.6O12, sulfides Li2S-P2S5, Li6PS5Cl, and Na3PS4, and halides Li3InCl6 and Li3YCl6. Thermal conductivities of sulfide and halide solid electrolytes are in the range 0.45-0.70 W m-1 K-1; thermal conductivities of Li6.4La3Zr1.4Ta0.6O12 and Li1.5Al0.5Ge1.5(PO4)3 are 1.4 W m-1 K-1 and 2.2 W m-1 K-1, respectively. For most of the solid electrolytes studied in this work, the thermal conductivity increases with increasing temperature; i.e., the thermal conductivity has a glass-like temperature dependence. The measured room-temperature thermal conductivities agree well with the calculated minimum thermal conductivities indicating the phonon mean-free-paths in these solid electrolytes are close to an atomic spacing. We attribute the low, glass-like thermal conductivity of the solid electrolytes investigated to the combination of their complex crystal structures and the atomic-scale disorder induced by the materials processing methods that are typically needed to produce high ionic conductivities.

Thermal Visualization of Buried Interfaces by Transient and Steady-State Responses of Time-Domain Thermoreflectance

Thermal resistances from interfaces impede heat dissipation in micro/nanoscale electronics, especially for high-power electronics. Despite the growing importance of understanding interfacial thermal transport, advanced thermal characterization techniques which can visualize thermal conductance across buried interfaces, especially for nonmetal-nonmetal interfaces, are still under development. This work reports a dual-modulation-frequency TDTR mapping technique to visualize the thermal conduction across buried semiconductor interfaces for beta-Ga2O3-SiC samples. Both the beta-Ga2O3 thermal conductivity and the buried beta-Ga2O3-SiC thermal boundary conductance (TBC) are visualized for an area of 200 um x 200 um. Areas with low TBC values ( smaller than 20 MW/m2-K) are successfully identified on the TBC map, which correspond to weakly bonded interfaces caused by high-temperature annealing. The steady-state temperature rise (detector voltage), usually ignored in TDTR measurements, is found to be able to probe TBC variations of the buried interfaces without the limit of thermal penetration depth. This technique can be applied to detect defects/voids in deeply buried heterogeneous interfaces non-destructively, and also opens a door for the visualization of thermal conductance in nanoscale nonhomogeneous structures.

Thermal Conductivity Mapping of Oxidized SiC SiC Composites by Time Domain Thermoreflectance with Heterodyne Detection

Silicon carbide silicon carbide (SiC SiC) composites are often used in oxidizing environments at high temperatures. Measurements of the thermal conductance of the oxide layer provide a way to better understand the oxidation process with high spatial resolution. We use time domain thermoreflectance (TDTR) to map the thermal conductance of the oxide layer and the thermal conductivity of the SiC SiC composite with a spatial resolution of 3 {mu}m. Heterodyne detection using a 50 kHz modulated probe beam and a 10 MHz modulated pump suppresses the coherent pick-up and enables faster data acquisition than what has previously been possible using sequential demodulation. By analyzing the noise of the measured signals, we find that in the limit of small integration time constants or low laser powers, the dominant source of noise is the input noise of the preamplifier. The thermal conductance of the oxide that forms on the fiber region is lower than the oxide on the matrix due to small differences in thickness and thermal conductivity.