Tuning the Electronic and Ionic Thermoelectric Transport Properties in Lithium-and Sodium-Based Polymer Electrolytes by Carbon Based Additive

Hybrid-organic energy storage media utilize the electrochemical transport of ionic species, at present, preferentially originating from lithium compounds. As such, much work on the device performance is devoted to cyclability, energy density and conductivity. Vice versa, the ionic transport characteristics offer also intriguing possibilities for the implementation of this material class in thermoelectrics (TE), the latter considered essential for recovering waste heat into electrical power. At the same time, the electrolytes’ sustainability and alternatives to lithium are becoming increasingly important.<br/>In this work, we present a comparative study on the electrical and thermoelectrical properties of a methacrylate-based solution processable solid polymer electrolyte containing lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) [1] and sodium bis(trifluoromethanesulfonyl)imide salt (NaTFSI), respectively. By means of impedance spectroscopy over a frequency range from 10^-1 Hz up to 5×10⁵ Hz and in a technologically relevant temperature range between 263 K and 363 K, we investigate the transport and dynamics of charge carriers in the solidified electrolytes. The observed high ionic conductivity of about 10^-3 S m^-1 at room temperature distinguishes this material for polymer battery applications [2]. In combination with highly sensitive thermovoltage measurements, we demonstrate that the electronic and ionic transport properties can be efficiently tuned by the content of lithium- and sodium-salt, respectively. Complementary multinuclear solid-state NMR studies further indicate interactions between ions and the polymer backbone. Additionally, a significant influence of the cations on the crosslinking of the resulting polymer network can be observed. These results showcase the potential for sustainable sodium-based ionic thermoelectric applications.<br/>By further varying the concentration of suspended carbon-based additives, we investigate the influence of the respective dimensionality on the charge carrier transport. Different transport regimes merge and can be related to a Vogel-Fulcher-Tammann and Arrhenius activated conductivity. For these composite materials, we were able to increase the power factor by several orders of magnitude. Even more, we can reversibly switch the sign of the occurring thermovoltage and thus the respective operational mode by tuning the ambient temperature, which promises new applications in autonomous TE units.<br/>Together with the high electrical conductivity achieved on macroscopic length scales, thermovoltages of about 2 mV K^-1 allow for high output powers while the polymeric matrix maintains the temperature gradient which in turn is a prerequisite for application in thermoelectric generators (TEG). As a proof-of-concept, an all organic TEG verifies the functionality of our approach and, thereby, substantiates the potential of mixed ionic and electronic materials for future TE applications.<br/><br/>MF and JP thank the Bavarian Ministry of Science and the Arts for the generous support by the research program <i>Solar Technologies Go Hybrid</i>.<br/><br/>[1] M. Frank, J. Pflaum, <i>Adv. </i><i>Funct. Mater.</i>, <b>2022</b>, 32, 2203277.<br/>[2] J. R. Nair et al., <i>React. Funct. Polym., </i><b>2011,</b> <i>71</i>, pp. 409-416.