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, Li3.3Al0.15P0.85S4 (i.e., Li9.9Al0.45P2.55S12), found in the compositional range Li3+2xP1-xAlxS4 (x = 0.15, 0.20, and 0.33). 31P 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 Li5AlS4 and a glassy amorphous component exsolve to yield a multiphase mixture. The crystal structure of Li3.3Al0.15P0.85S4 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) Å3. 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 7Li pulsed-field gradient nuclear magnetic resonance spectroscopy (PFG-NMR). The total ionic conductivity of Li3.3Al0.15P0.85S4 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 Li3.3Al0.15P0.85S4 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 Li3.3Al0.15P0.85S4) and assess the influence of the macroscopic state on microscopic ion transport, as supported by NMR relaxation measurements.