  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.
