Design of Trigonal Halide Superionic Conductor by Regulating Cation Order-Disorder

Lithium-metal-halides have been recently revisited as promising solid electrolyte (SE) candidates that can provide solutions to the interfacial instability issues found in sulfide- and oxide-based SEs. Although early studies on halide-based SEs indicated that poor ionic conductivities (typically &lt; ~10⁻⁶ S cm⁻¹) constituted a major bottleneck, recent breakthroughs revealed that the ionic conductivity can be boosted by identifying appropriate synthetic routes, introducing a wide range of halide-based Li₃MX₆ (M = Y, Er, or In, X = Cl, Br, or I)¹. Notably, moving from the classical ampule synthesis to mechanochemical synthesis significantly improved room-temperature ionic conductivity up to ~10³ order of magnitude for the Li₃MCl₆(M = Y, Er)^2,3. It was speculated that the higher degree of cation disordering from the mechanochemical synthesis could open up the bottleneck transition triangular area for lithium diffusion. Although the factors leading to the hidden superionic conductivity of halide SEs remain unclear, these previous findings strongly suggest a close interplay between superionic conduction and cation arrangements/occupancies attainable from various synthetic methods^4,5. Hence, understanding the fundamental relationship between the superionic conduction mechanism and structural factors in trigonal Li₃MCl₆ halides is critical for realizing significant advancements in their ionic conductivities.<br/>Herein, we employ first-principles calculations and experiments to investigate the conduction mechanism, propose, and validate our design strategies for enhancing ionic conductivity, using Li₃YCl₆ (LYC) as a model system. Consistent with previous reports, we find that the lithium diffusion occurs three-dimensionally in LYC, which exhibits anisotropic behavior with faster ionic conduction along the <i>c</i>-axis channel than in the ab-plane⁶. However, it is revealed that the lithium diffusion in the ab-plane leverages the superionic conduction in LYC, and, more importantly, it is primarily determined by the partial occupancies of the yttrium (Y) ion in the structure. Our theoretical calculations demonstrate that the presence of Y in the ab-plane interrupts the lithium diffusion with the electrostatic repulsion associated with the tetrahedral site hopping mechanism of lithium ions, resembling that of conventional layered lithium transition-metal oxides. Furthermore, the partial occupancy of Y over a certain ratio entirely disconnects the percolation diffusion pathways of lithium ions in the ab-plane. On the other hand, it is conversely observed that the lack of Y in the ab-plane leads to the collapse of the interlayer space, indicating that Y cations serve as a pillar of the layered lithium diffusion framework. This contradicting effect of Y in the structure signifies the importance of the composition/ordering of M cations in the trigonal Li₃MCl₆. Accordingly, we propose a target region for M partial occupancy that can lead to cation arrangements with percolating and non-collapsed lithium diffusion pathways. Our design rules for the target cation arrangement are experimentally confirmed by an M-deficient Li₃M_0.8Cl₆ (i.e., Li₃Y_0.2Zr_0.6Cl₆), a new halide superionic conductor that exhibits the highest reported room-temperature ionic conductivity (1.19 × 10⁻³ S cm⁻¹) among trigonal halide superionic conductors. This study illuminates the detailed superionic conduction mechanism in halides for the first time and suggests that searching for a new compositional space that balances the lithium percolation and stacking slab distance is an unexplored and efficient pathway for the design of superionic halide SEs.<br/><br/>Reference<br/>1. <i>Energy Environ. Sci</i>. <b>13</b>, 1429-1461 (2020).<br/>2. <i>Z. Anorg. Allg. Chem.</i> <b>613</b>, 26-30 (1992).<br/>3. <i>Adv. Mater.</i> 30, 1803075 (2018).<br/>4. <i>Adv. Energy Mater.</i> 10, 1903719 (2020).<br/>5. <i>Chem. Mater.</i> 33, 327-337 (2021).<br/>6. Angew. Chem., Int. Ed. 58, 8039-8043 (2019).