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

Abstract

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.