63010 2025 eng 25985 25995 11 17 17 article American Chemical Society (ACS) Washington, DC ACS 1 -- -- -- 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. ACS Applied Materials & Interfaces 10.1021/acsami.5c02183 1944-8252 urn:nbn:de:kobv:b43-630102 true journal-article publish false true 30.04.2025 Creative Commons - CC BY - Namensnennung 4.0 International Naiyu Qi Rachana Somaskandan Gustav Graeber eng uncontrolled Sodium−potassium alloy eng uncontrolled Liquid-metal anodes eng uncontrolled Alkali metal eng uncontrolled Batteries eng uncontrolled Surface tension Angewandte Physik 3 Gefahrgutumschließungen; Energiespeicher Energie Elektrische Energiespeicher und -umwandlung Verlagsliteratur Datei für die Öffentlichkeit verfügbar ("Open Access") Wissenschaftliche Artikel der BAM 3.0 Abteilungsleitung und andere Bundesanstalt für Materialforschung und -prüfung (BAM) https://opus4.kobv.de/opus4-bam/files/63010/qi-et-al-2025-high-precision-surface-tension.pdf