Enhancing Ionic Conductivity in Lithium Tetrahaloaluminates via a Mixed-Halide Strategy.

Area of Science:

• Materials Science
• Electrochemistry
• Solid-state ionics

Background:

• Halide-based solid electrolytes offer wide electrochemical windows and moderate ionic conductivity.
• Lithium tetrahaloaluminates (LiAlX4) are promising candidates for solid electrolytes.
• Enhancing ionic conductivity is crucial for next-generation solid-state batteries.

Purpose of the Study:

• To investigate a mixed-halide strategy for improving ionic conductivity in LiAlX4.
• To explore the relationship between compositional complexity, entropy, and ionic transport.
• To demonstrate entropy-driven design for advanced solid electrolytes.

Main Methods:

• Synthesis of twenty LiAlX4 compositions (single-, binary-, ternary-halide) via mechanochemical route.
• Measurement of ionic conductivities using electrochemical impedance spectroscopy.
• Characterization of local environments using solid-state Nuclear Magnetic Resonance (NMR) and X-ray Diffraction (XRD).

Main Results:

• Highest ionic conductivity observed in ternary compositions near the center of the Cl-Br-I triangle, correlating with maximized configurational entropy.
• Solid-state NMR confirmed random anion mixing and motional narrowing of 7Li, linked to enhanced conductivity.
• Arrhenius plots followed the Meyer-Neldel rule, indicating Li+ migration via multiphonon excitations.

Conclusions:

• Mixed-halide strategies and increased compositional complexity enhance ionic conductivity in LiAlX4 solid electrolytes.
• Entropy-driven design is a viable approach for developing high-performance solid-state batteries.
• Understanding Li+ migration mechanisms provides insights for further material optimization.