Modified Li7P3S11 Glass-Ceramic Electrolyte and Its Characterization

Li₇P₃S₁₁ glass ceramics have high conductivities competitive with liquid electrolytes, making them good candidates as solid-state electrolytes for all-solid-state lithium-ion batteries. However, the metastable nature and performance of Li₇P₃S₁₁ glass ceramics remain mysterious. Herein, modified Li₇P₃S₁₁ glass ceramics with compositions of 70Li₂S-30P₂S₅ were prepared via two-step mechanical milling and thermal annealing. Li₇P₃S₁₁ glass ceramics synthesized using the conventional method (mechanical milling and thermal annealing) were again ball-milled to obtain amorphous 70Li₂S-30P₂S₅ with a peculiar glass structure. Further thermal annealing was carried out to crystallize the glass. The obtained crystalline phase was analogous to the original Li₇P₃S₁₁ phase, but the conductivity was enhanced by a factor of 1.7. Based on ³¹P solid-state nuclear magnetic resonance (NMR) spectroscopy, the Li₇P₃S₁₁ phase contained an additional PS₄^3- unit. A rational deconvolution procedure for the ³¹P solid-state NMR spectra based on crystalline Li₇P₃S₁₁ was developed and applied to the samples. The analysis can resolve the additional crystalline PS₄^3- unit in the Li₇P₃S₁₁ structure. Based on two-dimensional double-quantum ³¹P NMR spectroscopy, the additional PS₄^3- unit is located adjacent to the P₂S₇^4- unit, suggesting that P₂S₇^4- is divided into two PS₄^3- units in the Li₇P₃S₁₁ phase. The flip motion of Li⁺ was also investigated based on the ⁷Li spin-lattice relaxation time. The independent activation energy of spin-lattice relaxation with respect to temperature in the Li₇P₃S₁₁ phase was attributed to a conduction path between the two PS₄^3- units. The findings provide a synthetic route that can be used to develop metastable solid-state electrolytes.