Probing Li-ion conduction in glassy Li₂S-P₂S₅ electrolytes using NMR

Understanding Lithium-ion conduction mechanisms is key to develop new solid-state electrolytes. However, Li-ion dynamics in glassy electrolytes are intricate and remain unclear due to the challenges in understanding the local structures of glasses. In this work, we probe the Li-ion conduction mechanism in Li₂S-P₂S₅ binary glassy system using the combination of variable-temperature ⁷Li and ³¹P NMR relaxometry. The temperature-dependent ³¹P NMR spin-lattice relaxation measurements show that dynamics of thiophosphate dimers are sluggish in 67Li₂S-33P₂S₅ and 70Li₂S-30P₂S₅ glasses while in 75Li₂S-25P₂S₅ glass, the anionic dynamics are enhanced significantly due to the increased amounts of isolated PS₄^3- units. The energy barrier for ³¹P rotation is decreased by a factor of ∼2, indicating drastic enhancement in ³¹P dynamics. We find that the cation and anion motion is highly cooperative in the glassy electrolyte system. The long-range Li-ion diffusion barrier decreases from 0.30 eV in 67Li₂S-33P₂S₅ glass to 0.14 eV in 75Li₂S-25P₂S₅ glass. It is clear that the rapid PS₄^3- rotation can speed up Li-ion migration. Moreover, a large amount of PS₄^3- ions break the rigid networks formed by thiophosphate dimers. This may loosen the glass matrix, expanding Li-ion percolation channels in the 75Li₂S-25P₂S₅ glass.