Na⁺ Mobility in PEO-Based Composite Solid-State Electrolytes by NMR

Charge transfer and mobility are essential for electrochemical processes in batteries, which need to be understood in detail for optimization, especially in the case of all-solid-state batteries. Wide line NMR is well-known in solid-state NMR and allows the quantification of ion mobility in ordered crystalline and amorphous structures. Temperature-dependent ²³Na-NMR is sensitive to ion mobility via longitudinal relaxation, but also via line analysis and transverse relaxation. As ²³Na is a spin 3/2 nucleus, ²³Na-NMR is also susceptible to electric field gradients caused by their nearest neighbor environment and, therefore, reflects not only the mobility of ²³Na⁺ but also the molecular dynamics in the neighborhood, which are investigated in this paper. The named NMR methods were explored to study ²³Na⁺ mobility in the solid electrolytes NaSICON (sodium (Na) Super Ionic CONductor, here Na_3.4Zr₂Si_2.4P_0.6O₁₂), the salt NaTFSI (sodium bis(trifluoromethyl sulfonyl)imide), as well as in the polymer-based electrolytes PEO-NaSICON, PEO-NaTFSI, and PEO-NaTFSI-NaSICON.