Lithium ion diffusion in solid electrolyte (Li2S)7(P2S5)3 measured by pulsed-gradient spin-echo 7Li NMR spectroscopy

The sulfide based solid electrolyte (Li2S)7(P2S5)3 has large ionic conductivity, and thus its application in solid-state lithium rechargeable batteries as an electrolyte has been developed. We observed the 7Li and 31P nuclear magnetic resonance (NMR) spectra of crystallized (Li2S)7(P2S5)3 in the steady state and found that narrow and broad components coexist in wide temperature range of the 7Li resonance. The measurement of 7Li ion diffusion was performed for the narrow component between 303 and 353 K by the pulsed-gradient spin-echo (PGSE) NMR method. Lithium ion diffusion scattered in a polydispersive manner and the apparent diffusion constants of the Li+ (DLi) varied depending on measurement conditions at a certain temperature. The temperature-dependent ionic conductivity was observed from 263 to 353 K and the activation energy was 31.4 kJ mol− 1 for the crystallized (Li2S)7(P2S5)3. The DLi evaluated from ionic conductivity using the Nernst-Einstein relation was consistent with the scattering apparent DLi observed by the PGSE NMR method. The translational movement of Li+ observed by the PGSE NMR is extremely complex. Within a short distance and a short period, Li+ ions move quickly and slow down with time and distance. This is the first trial to measure the lithium diffusion directly for continuous space in the solid electrolyte.

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• A crystalline solid electrolyte (Li2S)7(P2S5)3 was prepared by high-energy ball milling.
• Ionic conductivity was measured between 263 and 353 K.
• 7Li migration behaviors were measured by pulsed-gradient spin-echo (PGSE) NMR method.
• Lithium ion translational movement scatters in various manners.