Structure of the Solid-State Electrolyte Li_3+2<i>x</i>P_1–<i>x</i>Al<i>_x</i>S₄: Lithium-Ion Transport Properties in Crystalline vs Glassy Phases

Abstract

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_3.3Al_0.15P_0.85S₄ (i.e., Li_9.9Al_0.45P_2.55S₁₂), found in the compositional range Li_3+2<i>x</i>P_1-<i>x</i>Al<i>_x</i>S₄ (<i>x</i> = 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 <i>x</i> (>0.15), crystalline Li₅AlS₄ and a glassy amorphous component exsolve to yield a multiphase mixture. The crystal structure of Li_3.3Al_0.15P_0.85S₄ was elucidated by single-crystal X-ray diffraction and powder neutron diffraction, demonstrating that it belongs to the thio-LISICON family with the <i><i>Pnma</i></i> space group, <i>a</i> = 12.9572(13) Å, <i>b =</i> 8.0861(8) Å, <i>c</i> = 6.1466(6) Å, and <i>V</i> = 644.00(11) ų. 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 Li_3.3Al_0.15P_0.85S₄ 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 Li_3.3Al_0.15P_0.85S₄ 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 Li_3.3Al_0.15P_0.85S₄) and assess the influence of the macroscopic state on microscopic ion transport, as supported by NMR relaxation measurements.