Structure of the Solid-State Electrolyte Li3+2xP1–xAlxS4: Lithium-Ion Transport Properties in Crystalline vs Glassy Phases

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The search for new solid electrolyte materials and an understanding of fast-ion conductivity are crucial for the development of safe and high-power all-solid-state battery technology. Herein, we present the synthesis, structure, and properties of a crystalline lithium-ion conductor, Li 3.3 Al 0.15 P 0.85 S 4 (i.e., Li 9.9 Al 0.45 P 2.55 S 12 ), found in the compositional range Li 3+2 x P 1– x Al x S 4 ( x = 0.15, 0.20, and 0.33). 31 P magic-angle spinning nuclear magnetic resonance (MAS-NMR) aided in identifying the successful introduction of Al into the lattice. At high values of x (>0.15), crystalline Li 5 AlS 4 and a glassy amorphous component exsolve to yield a multiphase mixture. The crystal structure of Li 3.3 Al 0.15 P 0.85 S 4 was elucidated by single-crystal X-ray diffraction and powder neutron diffraction, demonstrating that it belongs to the thio-LISICON family with the Pnma space group, a = 12.9572(13) Å, b = 8.0861(8) Å, c = 6.1466(6) Å, and V = 644.00(11) Å 3 . The Li + -ion conductivity and diffusivity in this bulk material (which contains about 10 wt % of an amorphous phase, as prepared) were studied by electrochemical impedance spectroscopy and 7 Li pulsed-field gradient nuclear magnetic resonance spectroscopy (PFG-NMR). The total ionic conductivity of Li 3.3 Al 0.15 P 0.85 S 4 is 0.22(2) mS·cm –1 at room temperature with an activation energy of 0.30(1) eV. A two-component analysis method based on the Kärger equations was developed to analyze the diffusive exchange between the bulk and amorphous phases of Li 3.3 Al 0.15 P 0.85 S 4 detected via the PFG-NMR signal attenuation curves. This approach was employed to quantitatively compare different sample morphologies (glass powder, crystalline powder, and crystalline pellets of Li 3.3 Al 0.15 P 0.85 S 4 ) and assess the influence of the macroscopic state on microscopic ion transport, as supported by NMR relaxation measurements.