Ionic Peltier Effect in Li-Ion Electrolytes

The insertion of an ion from an electrode to an electrolyte involves changes in the thermodynamics properties and the transport of charge and heat. The coupled transport of charge and heat provides fundamental insights on the microscopic thermodynamics and kinetics of this process. We describe a sensitive ac differential resistance bridge that enables measurements of the temperature difference on two sides of electrochemical cell with a resolution of better than 10 μK. 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 Π are negative, i.e., heat is absorbed at the cathode and emitted at the anode, large in absolute magnitude, Π >30 kJ/mol, become less negative with increasing concentration, and becomes slightly more negative with increasing temperature. 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 the heat of transport plays a minor role in comparison to the changes in entropy 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 compliment traditional measurements of voltages and charge currents.