Open Access

The development of post-lithium-ion batteries has sparked significant interest in alkali-metal anodes, particularly sodium (Na), potassium (K), and sodium−potassium (Na−K) alloys. Na−K alloys are promising for partially liquid anodes due to their unique low melting points. A critical factor influencing Na−K-based anode performance is wetting behavior, which governs electrical conductivity, mechanical contact, and long-term stability. At the heart of wetting lies surface tension, a fundamental property of solid−liquid− gas interactions. However, the surface tension of alkali metals and their alloys, particularly Na−K systems, remains poorly understood due to experimental and theoretical challenges. This study bridged these gaps by employing Du Noüy ring tensiometry for the first time in alkali-metal systems to measure the surface tension of Na, K, and Na−K alloys across temperatures from ambient to 180 °C. A key innovation in this work is the development of the push-in Du Noüy method, which provided significantly higher precision and reliability compared to the traditional pull-out technique, without requiring a correction factor. The measured surface tension decreased with increasing temperature for the studied Na−K alloys. For instance, for a eutectic Na−K mixture, the surface tension decreases from 121.7 mN m−1 to 112.2 mN m−1 when increasing the temperature from ambient to 180 °C. Additionally, this study presented the first use of Gibbs free energy minimization to model the surface tension of the Na−K system. The robust method significantly enhanced the predictive accuracy compared to the previous simplified model, reducing deviations from 25% to 2%. Our findings reveal that surface tension increases with sodium mole fraction in the bulk phase, yet the surface monolayer remains potassium-rich, indicating non-ideal surface behavior. This study deepens the understanding of alkali-metal wetting behavior, providing valuable insights for designing optimized interfaces in next-generation semi-solid alkali-metal batteries.

AbstractSorbents are promising materials for applications in atmospheric water harvesting, thermal energy storage, and passive cooling, thereby addressing central challenges related to water scarcity and the global energy transition. Recently, hygroscopic hydrogel composites have emerged as high‐performance sorbents. However, many of these systems are fabricated with unsustainable and costly sorbent materials, which hinders their wide deployment. Here, the synthesis of high‐performance, cost‐efficient polyacrylamide hydrogels loaded with unprecedented amounts of calcium chloride is demonstrated. To this end, the swelling procedure of polyacrylamide hydrogels in aqueous calcium chloride solutions is optimized. The achievable salt loading in the hydrogel is characterized as a function of temperature, calcium chloride concentration in the swelling solution, and the hydrogel preparation conditions. The obtained hydrogel‐salt composites are shown to be stable under repeated sorption‐desorption cycling and enable water uptakes of 0.92 and 2.38 grams of water per gram of dry materials at 30% and 70% relative humidity, respectively. The resulting cost‐performance ratio substantially exceeds lithium chloride‐based systems. Further, the mechanistic insights on hydrogel salt interactions can guide the design of sustainable and low‐cost sorbent materials for future applications in water and energy.

Potassium–sulfur (K–S) batteries represent a promising class of next‐generation energy storage systems, leveraging the abundance of potassium and the high theoretical capacity of sulfur. However, their development is limited by an incomplete understanding of polysulfide chemistry. Here, we combine spectroscopic techniques with electrochemical analysis to elucidate the formation, speciation, and redox behavior of potassium polysulfides in hybrid‐electrolyte K–S cells operating at 60°C. UV–vis spectroscopy reveals that sulfur radical anions, especially S3•−, act as key intermediates whose evolution reflects redox equilibria and state of charge. Modulating the catholyte composition shows that incorporating a eutectic ε‐caprolactam:acetamide (CPL:Am, 1:1) mixture into tetraglyme (G4) enhances polysulfide solubility and capacity. Cell architecture plays a critical role: glass cells used to couple spectroscopy and electrochemistry exhibit rapid capacity loss and Coulombic efficiencies of ∼80%, while compact piston‐type cells boost performance. Electrolyte choice within piston cells further differentiates behavior, with tetraglyme yielding 561 mAh g−1 and gradual fading, whereas CPL:Am–G4 electrolytes initially reach 1354 mAh g−1 with more pronounced decay. These insights highlight the intertwined effects of solvent chemistry and cell design, offering guidance for developing stable, high‐performance K–S batteries.