From Cation Order to Disorder: Unlocking Ion Transport Pathways in Li-Zn-Zr-Cl Halospinels

Area of Science:

• Materials Science
• Solid-State Chemistry
• Electrochemistry

Background:

• Lithium metal chloride halospinels (Li₂MCl₄) are earth-abundant ion conductors with potential for all-solid-state batteries.
• Poor room-temperature ionic conductivity has hindered their practical application.
• Understanding cation disorder is crucial for optimizing ion transport.

Purpose of the Study:

• To investigate the impact of aliovalent Zr⁴⁺ substitution in Li₂ZnCl₄ on ionic conductivity.
• To elucidate the relationship between cation disorder, vacancy tuning, and ion transport mechanisms.
• To develop a new family of earth-abundant halospinels with enhanced electrochemical properties.

Main Methods:

• Synthesis of Li_2-2x/3Zn_1-x Zr_2x/3Cl₄ series with varying Zr content (x = 0 to 1.0).
• Synchrotron X-ray diffraction (SXRD) for average crystal structure analysis.
• Neutron pair distribution function (nPDF) analysis for local structure characterization.

Main Results:

• Ionic conductivity increased by nearly five orders of magnitude, reaching 6.74(1)× 10^-5 S cm^-1 at x = 0.6.
• Zr⁴⁺ substitution induced cation redistribution into interstitial sites, creating new low-energy ion hopping pathways.
• Structural analysis revealed a rationalization of cation rearrangement based on coordination preferences and electrostatic interactions.

Conclusions:

• Aliovalent substitution of Zr⁴⁺ is an effective strategy to dramatically enhance ionic conductivity in halospinels.
• Cation disorder and vacancy tuning play critical roles in facilitating ion transport.
• This study provides an atomistic understanding for designing high-performance, earth-abundant solid electrolytes.