Non-uniform lithium-ion migration on micrometre scale for garnet- and NASICON-type solid electrolytes studied by 7Li PGSE-NMR diffusion spectroscopy

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

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+2xP_1-xAl_xS₄ (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 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 Pnma space group, a = 12.9572(13) Å, b = 8.0861(8) Å, c = 6.1466(6) Å, and V = 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.

NMR Investigations of Crystalline and Glassy Solid Electrolytes for Lithium Batteries: A Brief Review

The widespread use of energy storage for commercial products and services have led to great advancements in the field of lithium-based battery research. In particular, solid state lithium batteries show great promise for future commercial use, as solid electrolytes safely allow for the use of lithium-metal anodes, which can significantly increase the total energy density. Of the solid electrolytes, inorganic glass-ceramics and Li-based garnet electrolytes have received much attention in the past few years due to the high ionic conductivity achieved compared to polymer-based electrolytes. This review covers recent work on novel glassy and crystalline electrolyte materials, with a particular focus on the use of solid-state nuclear magnetic resonance spectroscopy for structural characterization and transport measurements.

Relationship between Li+ diffusion and ion conduction for single-crystal and powder garnet-type electrolytes studied by 7Li PGSE NMR spectroscopy

Ion-conducting garnets are important candidates for use in all-solid Li batteries and numerous materials have been synthesized with high ionic conductivities. For understanding ion conduction mechanisms, knowledge on Li⁺ diffusion behaviour is essential. The proposed nano-scale lithium pathways are composed of tortuous and narrow Li⁺ channels. The pulsed gradient spin-echo (PGSE) NMR method provides time-dependent ⁷Li diffusion on the micrometre space. For powder samples, collision-diffraction echo-attenuation plots were observed in a short observation time, which had not been fully explained. The diffraction patterns were reduced or disappeared for single-crystal garnet samples of Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZO-Ta) and Li_6.5La₃Zr_1.5Nb_0.5O₁₂ (LLZO-Nb). The inner morphology and grain boundaries affect importantly the collision-diffraction behaviours which is inherent to powder samples. The ⁷Li diffusion observed by PGSE-NMR depends on the observation time (Δ) and the pulsed field gradient (PFG) strength (g) in both powder and single-crystal samples, and the anomalous effects were reduced in the single-crystal samples. The scattered Li diffusion constants converged to a unique value (D_Li) with a long Δ and a large g, which is eventually the smallest value. The D_Li activation energy was close to that of the ionic conductivity (σ). The D_Li values are plotted versus the σ values measured for four powder and two single-crystal garnet samples. Assuming the Nernst-Einstein (NE) relation which was derived for isolated ions in solution, the carrier numbers (N_NE) were estimated from the experimental values of D_Li and σ. The N_NE values of metal-containing garnets were large (<10²³ cm^-3) and insensitive to temperature. They were larger than Li atomic numbers in cm³ calculated from the density, molecular formula and Avogadro number for LLZOs except for cubic LLZO (Li₇La₃Zr₂O₁₂, N_NE∼ 10²⁰ cm^-3).