Heat effects and temperature are known to be of great importance to a Li-ion battery’s performance and lifetime expectancy. In this context the reversible heat effects, i.e. effects that are associated with charge transfer and electrode reaction entropy, play an important role [1, 2]. These effects are however scarcely investigated, especially with respect to the single electrode reaction entropy. These values can be obtained when measuring a symmetric cell under a temperature gradient, i.e. two similar electrodes at zero current and at different temperatures [3-4]. We report values of the thermoelectric potential of the symmetric cell consisting of two identical LiCoO₂electrodes separated by an electrolyte:

Al_(s,T) | Li_(1-x) CoO_2(s,T) | EC, DEC, LiPF_6(diss) | Lix CO_{2(s,T + ΔT)} | Al_{(s,T+ ΔT)}

The electrodes are commercial LiCoO2 electrodes on an aluminium current collector, each kept at temperatures; T and T + ΔT. The electrolyte is 1 M lithium hexafluorophosphate (LiPF₆) in a EC DEC mixture (1:1, volume wise) soaked in a separator. The cell is assembled as a pouch cell, and the electrodes are thermostatted to the specified temperatures.

We show that there are two significant contributions of opposite sign to the thermoelectric signal relative to the temperature difference; commonly known as the Seebeck coefficient. The contribution from the Peltier heat is obtained from the steady state value after 3-5 days. The second contribution, which manifests itself within hours, is a Soret effect. From these results, we report the transported entropy of Li⁺ and estimate the heats of transfer for the electrolyte components.

The attached figure shows a cross section of the cell used in these measurements, where one side is heated by hot water and the other cooled by cold water. The pouch cell is sandwiched between these two sides and layers of copper containing thermocouples, so that the actual temperature difference on the surfaces of the pouch cell is known and uniform.

References;

1. Takano, K., et al., Entropy change in lithium ion cells on charge and discharge. Journal of Applied Electrochemistry, 2002. 32(3): p. 251-258.
2. Viswanathan, V.V., et al., Effect of entropy change of lithium intercalation in cathodes and anodes on Li-ion battery thermal management. Journal of Power Sources, 2010. 195(11): p. 3720-3729.
3. S. K. Ratkje, M. Ottøy, R. Halseid, M. Strømgård, Thermoelectric power relevant for the solid-polymer-electrolyte fuel cell, Journal of membrane science 107 (1995) 219–228.
4. S. Kjelstrup, P. J. S. Vie, L. Akyalcin, P. Zefaniya, J. Pharoah, O. S. Burheim, The seebeck coefficient and the peltier effect in a polymer electrolyte membrane cell with two hydrogen electrodes, Electrochimica Acta 99 (2013) 166-175.