Aluminium ion doping mechanism of lithium thiophosphate based solid electrolytes revealed with solid-state NMR

We investigate the impact of Al incorporation on the structure and dynamics of Al-doped lithium thiophosphates (Li_3-3AlPS₄) based on β-Li₃PS₄. ²⁷Al and ⁶Li magic-angle spinning NMR spectra confirm that Al³⁺ ions occupy octahedral sites in the structure. Quantitative analyses of ²⁷Al NMR spectra show that the maximum Al incorporation is x = 0.06 in Li_3-3AlPS₄. The ionic conductivity of β-Li₃PS₄ is enhanced by over a factor 3 due to Al incorporation. Further increase of the Al doping level leads to the formation of a more complicated material consisting of multiple crystalline and distorted phases as indicated by ³¹P NMR spectra and powder X-ray diffraction. Consequently, novel Li ion diffusion pathways develop leading to a very high ionic conductivity at room temperature. NMR relaxometry shows that the activation barrier for long-range Li ion diffusion in β-Li₃PS₄ hardly changes upon Al incorporation, but the onset temperature for motional narrowing comes down significantly due to Al doping. The activation barrier in the subsequently formed multiphase material decreases significantly, however, indicating a different more efficient Li ion conduction pathway.