  The coupled transport of charge and heat provide fundamental insights into
the microscopic thermodynamics and kinetics of materials. We describe a
sensitive ac differential resistance bridge that enables measurements of the
temperature difference on two sides of a coin cell with a resolution of better
than 10 uK. We use this temperature difference metrology to determine the ionic
Peltier coefficients of symmetric Li-ion electrochemical cells as a function of
Li salt concentration, solvent composition, electrode material, and
temperature. The Peltier coefficients {\Pi} are negative, i.e., heat flows in
the direction opposite to the drift of Li ions in the applied electric field,
large, 30 kJ mol-1, and increase with increasing temperature at T > 300 K. The
Peltier coefficient is approximately constant on time scales that span the
characteristic time for mass diffusion across the thickness of the electrolyte,
suggesting that heat of transport plays a minor role in comparison to the
changes in partial molar entropy of Li at the interface between the electrode
and electrolyte. Our work demonstrates a new platform for studying the
non-equilibrium thermodynamics of electrochemical cells and provides a window
into the transport properties of electrochemical materials through measurements
of temperature differences and heat currents that complement traditional
measurements of voltages and charge currents.
