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The reversible heat in lithium-ion batteries (LIBs) due to entropy change isfundamentally important for understanding the chemical reactions in LIBs anddeveloping proper thermal management strategies. However, the directmeasurements of reversible heat are challenging due to the limited temperatureresolution of applied thermometry. In this work, by developing anultra-sensitive thermometry with a differential AC bridge using twothermistors, the noise-equivalent temperature resolution we achieve (10 uK) isseveral orders of magnitude higher than previous thermometry applied on LIBs.We directly observe reversible heat absorption of a LIR2032 coin cell duringcharging with negligible irreversible heat generation and a linear relationbetween heat generations and discharging currents. The cell entropy changesdetermined from the reversible heat agree excellently with those measured fromtemperature dependent open circuit voltage. Moreover, it is found that thelarge reversible entropy change can cancel out the irreversible entropygeneration at a charging rate as large as C/3.7 and produce azero-heat-dissipation LIB during charging. Our work significantly contributesto fundamental understanding of the entropy changes and heat generations of thechemical reactions in LIBs, and reveals that reversible heat absorption can bean effective way to cool LIBs during charging.

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