mixedalkali

The mixed alkali effect (MAE) refers to a non-linear change in ionic transport properties that occurs when a solid electrolyte or glass contains more than one type of alkali ion, such as Na+, K+, Li+, or Cs+. In many alkali-containing glasses, introducing a second alkali ion reduces ionic conductivity compared with a single-alkali system. The conductivity versus composition often shows a pronounced minimum at intermediate concentrations of the two alkalis, though conductivity can partially recover at higher concentrations of one alkali. The phenomenon is most extensively studied in alkali silicate glasses but also appears in other oxide glasses and some polymer electrolytes.

Mechanisms underlying the MAE are the subject of ongoing research. The effect is attributed to structural and dynamic frustration of mobile ions: different alkali ions prefer distinct sites in the glass network, leading to less favorable mixed-ion hopping and the creation of energetically unfavorable regions. Trapping, clustering, and site blocking of certain ions, along with changes in the network structure (such as nonbridging oxygens), can hinder ion transport. While several models exist, no single explanation fully accounts for all observations; the effect depends on temperature, composition, and the specific material system.

Applications and implications include the design of solid-state ion-conducting materials for batteries, sensors, and electrochromic devices. Understanding the mixed alkali effect helps in optimizing conductivity and stability, guiding material selection and composition to balance transport properties with structural integrity.