Ionic Peltier Effect in Liquid Electrolytes

The Seebeck effect in ionic thermoelectric (i-TE) materials has been extensively studied recently because Seebeck coefficients in i-TE materials are found to be orders of magnitude higher than their counterparts in electronic thermoelectric materials. However, the reverse phenomenon of the Seebeck effect, namely the Peltier effect, remains unexplored in i-TE materials partly due to the experimental difficulty in thermal measurements. Here, we report the first experimental observation of the ionic Peltier effect at the lithium metal-liquid electrolyte interfaces (LiPF₆ and LiTFSI) by developing an ultra-sensitive differential temperature metrology (10 uK temperature resolution). By sandwiching liquid electrolytes between two lithium metal plates to form a coin cell, we can measure the Peltier heat generated in the ionic systems quantitatively by measuring the temperature difference of the two sides of the coin cell with an applied electrical current. To make the effect of the parasitic Joule-heating negligible in the measurements, small electrical currents (mA-scale) are applied because Peltier heat is proportional to current while Joule-heating is proportional to the square of current. The results show that the observed Peltier heat strongly depends on the types and concentrations of electrolytes dissolved in the supporting solvents. Our work sheds light on the thermoelectric effects with ions as the energy carriers, and will impact energy conversion and storage applications such as ionic thermoelectric materials and batteries.