Unveiling the Unexpected: Electrolyte Interface Interactions in Potassium Batteries

The energy transition relies on efficient storage solutions, with Li-ion batteries (LIBs) dominating research and industry. However, lithium scarcity limits their large-scale adoption, driving interest in potassium-ion batteries (KIBs) as a cost-effective alternative, especially for stationary storage. Potassium is 900 times more abundant than lithium, has a similar redox potential (-2.93 V vs. SHE), and is compatible with aluminum current collectors, even for anodes. However, their electrochemical behavior challenges many established paradigms derived from lithium systems, particularly at the electrode-electrolyte interface. A key factor in these differences is the distinct ionic nature of potassium. While K⁺ has a larger ionic radius than Li⁺, its smaller Stokes radius in common solvents influences solvation dynamics and transport properties in ways that diverge from lithium-ion batteries. The resulting higher diffusion rate of K-ions allows the use of thicker electrolytes, which can effectively suppress dendrite growth while maintaining high ionic conductivity, leading to enhanced cycling stability in potassium-based cell. To harness these properties, we have developed tailored gel polymer electrolytes (GPEs) for KIBs. One formulation prioritizes mechanical strength crosslinking pre-oxidized lignin with PEGDGE, achieving high ionic conductivity despite increased thickness and enabling long-term cycling stability. Another employs lignin as filler, increasing the conductivity (approaching 10⁻¹ S cm⁻¹) and fostering a more favorable GPE/K-metal interface. Beyond electrolyte engineering, a fundamental understanding of interfacial chemistry in potassium batteries is mandatory to unlock their potential and ensure their safety. Unlike lithium systems, where SEI formation and composition are relatively well understood, potassium batteries exhibit a highly dynamic and evolving SEI. Our investigations reveal that the charge transfer resistance at open circuit voltage (OCV) is initially extremely high but drops dramatically below 100 Ω within the first few cycles. This behavior suggests a progressive transformation of the SEI, leading to a thinner, more conductive layer over time - contrary to what is commonly expected in lithium-based systems. To systematically investigate these interfacial transformations, a combination of X-ray photoelectron spectroscopy (XPS), operando gas chromatography, and atomic force microscopy (AFM) is needed. Our findings demonstrate that the decomposition potential, the SEI composition in potassium batteries are highly dependent on electrolyte chemistry and evolves significantly during cycling. This work contributes to the characterization of potassium battery interfaces, an area that remains relatively unexplored. By revealing unexpected interfacial processes, we offer new insights that can support the future development of stable and high-performance potassium-based energy storage systems.