Ionic Peltier Effect in Li-Ion Electrolytes

TL;DR

This study measures the ionic Peltier effect in Li-ion electrolytes, revealing how heat transport relates to ion movement and material properties, advancing understanding of electrochemical thermodynamics.

Contribution

It introduces a sensitive ac resistance bridge for measuring ionic Peltier coefficients in Li-ion cells, providing new insights into heat and charge transport mechanisms.

Findings

Peltier coefficients are negative and large (~30 kJ/mol).

Heat flow opposes Li ion drift and increases with temperature above 300 K.

Heat of transport is minor compared to entropy changes at interfaces.

Abstract

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.