Most conventional batteries today employ organic liquid electrolytes (LEs) that are not only flammable, but also serve as a medium for irreversible side reactions at the electrode interfaces, especially when metal is used as electrode. In post-lithium systems, such as potassium batteries, this issue is even more pronounced due to a higher reactivity of the metal as compared to lithium, and typically results in electrochemical instability leading to a rapid capacity fade of the battery.^[1] When switching from LEs to solid polymer electrolytes (SPEs) that typically show better electrochemical stability at low (< 0.5 V vs. K⁺/K) and high (> 4 V vs. K⁺/K) potentials due to polymers inherent inertness, enhanced cycle life of the battery is expected.^[2] Moreover, well-known disadvantage of SPEs in Li-based batteries, i.e., poor ionic conductivity at ambient temperature, could be overcome in systems with larger cation size, e.g. K^+[3,4], potentially removing some of the bottlenecks previously encountered in the case of Li-transport.

In this presentation, a series of poly(ethylene oxide) - potassium bis(trifluoromethane sulfonyl)imide (PEO-KTFSI) compositions with different salt concentration was investigated for their potential application as SPEs in potassium metal batteries. To identify the most promising candidate in terms of ion transport and mechanical integrity, the effect of KTFSI concentration on thermal, rheological and electrochemical properties was studied. Several electrolyte compositions were examined in solid-state potassium batteries with a potassium metal negative electrode, and a positive electrode from Prussian blue analogue family. Our results reveal the advantages of solid-state systems with respect to improved capacity retention and Coulombic efficiency as compared to the reference system with carbonate-based LE, as demonstrated in Figure 1, thus paving the way for a new generation of potassium batteries with significantly improved key performance parameters.

Figure 1. Comparison of potassium half-cells employing different electrolyte systems: carbonate-based liquid electrolyte (LE) vs. PEO-based solid polymer electrolyte (SPE) (a) capacity retention and (b) corresponding Coulombic efficiencies.

[1] H. Wang, D. Zhai, F. Kang, Energy Environ. Sci. 2020, 13, 4583–4608.

[2] J. Mindemark, M. J. Lacey, T. Bowden, D. Brandell, Prog. Polym. Sci. 2018, 81, 114–143.

[3] M. Perrier, S. Besner, C. Paquette, A. Vallée, S. Lascaud, J. Prud’homme, Electrochim. Acta 1995, 40, 2123–2129.

[4] U. Oteo, M. Martinez-Ibañez, I. Aldalur, E. Sanchez-Diez, J. Carrasco, M. Armand, H. Zhang, ChemElectroChem 2019, 6, 1019–1022.