Fundamental Investigations on the Ionic Transport and Thermodynamic Properties of Potassium-Ion Electrolytes
Authors: Jagger Ben , Dhir Shobhan , Zhao Junyi , Pasta Mauro 
Published: December 2024

Potassium-ion batteries (KIBs) represent a promising complementary technology to lithium-ion batteries (LIBs) due to the availability and low cost of potassium. KIBs could also be produced with graphite (G) anodes and Prussian blue analog (PBA) cathodes, reducing the demand for rare, costly elements necessary in LIBs.1 However, there is currently no electrolyte capable of providing practical coulombic efficiencies in high-voltage G||PBA cells, requiring further electrolyte development.2

Current research primarily focuses on developing electrolytes able to simultaneously passivate Al current collectors, resist oxidation at the high voltage of the cathode, and form a stable solid electrolyte interphase (SEI) on the anode,2 but frequently neglects electrolyte transport properties. Ionic transport in the electrolyte influences cell rate capability, low-temperature performance, and degradation, as the formation of concentration gradients introduces additional overpotentials and promotes irreversible side reactions.3 Accurate electrolyte transport property characterization is therefore critical to understand and optimize electrolyte performance.
Through optimization of potassium metal electrode preparation, we conducted the first full characterization of a non-aqueous K-ion electrolyte, providing the concentration-dependent salt diffusivity, transference number, ionic conductivity, and thermodynamic factor of the potassium bis(fluorosulfonyl)imide (KFSI) in 1,2-dimethoxyethane (DME) system. We further compare these properties with those from the equivalent Li-ion electrolyte (LiFSI in DME), demonstrating that the K-ion electrolyte displays superior transport properties due to the lower charge density of K+ compared to Li+.4

However, full electrolyte characterization is traditionally a slow and laborious process, requiring large volumes of electrolyte and separate experimental setups to measure each property. An alternative approach is to utilize techniques capable of electrolyte concentration gradient visualization during polarization, which, when combined with measurement of the concentration overpotential, enable the full suite of transport and thermodynamic properties to be determined in a single experiment.5 This approach was implemented in a technique called operando Raman gradient analysis (ORGA) to characterize KFSI in triethyl phosphate (TEP), another promising K-ion electrolyte. ORGA gives results in agreement with the conventional state-of-the-art methods, while proving to be more electrolyte- and time-efficient.6

Full-cell modeling with these two K-ion electrolytes reveals the significant impact of transport properties on accessible capacity at high rates,7 demonstrating the need to consider electrolyte transport when designing future K-ion electrolytes, and highlighting the importance of fast and accurate techniques to measure these properties, such as ORGA.
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Zhao, J., Jagger, B., Olbrich, L. F., Ihli, J., Dhir, S., Zyskin, M., Ma, X. & Pasta, M. Transport and Thermodynamic Properties of KFSI in TEP by Operando Raman Gradient Analysis. ACS Energy Lett.
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Dhir, S., Cattermull, J., Jagger, B., Schart, M., Olbrich, L. F., Chen. Y., Sada, K., Goodwin, A. & Pasta, M. Characterisation and Modelling of Potassium-Ion Batteries. Preprint at Research Square: https://doi.org/10.21203/rs.3.rs-3734005/v1

Figure 1