Mechanochemical Synthesis: A Tool to Tune Cation Site Disorder and Ionic Transport Properties of Li₃MCl₆ (M = Y, Er) Superionic Conductors

Abstract

The lithium‐conducting, rare‐earth halides, Li3MX6 (M = Y, Er; X = Cl, Br), have garnered significantly rising interest recently, as they have been reported to have oxidative stability and high ionic conductivities. However, while a multitude of materials exhibit a superionic conductivity close to 1 mS cm−1, the exact design strategies to further improve the ionic transport properties have not been established yet. Here, the influence of the employed synthesis method of mechanochemical milling, compared to subsequent crystallization routines as well as classic solid‐state syntheses on the structure and resulting transport behavior of Li3ErCl6 and Li3YCl6 are explored. Using a combination of X‐ray diffraction, pair distribution function analysis, density functional theory, and impedance spectroscopy, insights into the average and local structural features that influence the underlying transport are provided. The existence of a cation defect within the structure in which Er/Y are disordered to a new position strongly benefits the transport properties. A synthetically tuned, increasing degree of this disordering leads to a decreasing activation energy and increasing ionic conductivity. This work sheds light on the possible synthesis strategies and helps to systematically understand and further improve the properties of this class of materials.

Tailoring the Cation Lattice for Chloride Lithium‐Ion Conductors

“…Our calculated Li-ion conductivity of Li 3 InCl 6 (C2/m) is 1.2 mS cm −1 , in good agreement with the experimental reported value of 1.49 mS cm −1 . [18,[21][22][23] A recent experiment study reports Li-ion conductivity of Li 3 YCl 6 to be 0.51 mS cm −1 , which is slightly lower than our calculated Li-ion conductivity of 14 mS cm −1 , with error bounds in the range of 5 to 47 mS cm −1 and the difference may be attributed to the existence of grain boundary [18] and blocking defects. [19] In addition, a wide range of Zr-doped Li (Figure 2e).…”

“…Our results further confirm recent computation and experimental studies that Li 3 MCl 6 are promising Li SICs for a wide range of cation and substitutions. [18][19][20][21][22][23][24][25] Most Li x MCl 4 and Li x M 1/2 Cl 4 materials even with substantial levels of substitution have Li-ion conductivities much lower than 200 mS cm −1 at 600 K, and 24 of 51 materials exhibit a negligible amount of Li-ion hopping to obtain a statistically reliable ionic diffusivity. There are a few Li-ion conductors as the exceptions in the Li x MCl 4 category, as a result of their distinct local cation coordination, which are further discussed in Section 3.…”

“…In addition, doping and substitutions on these chloride SICs to tune Li concentration and cation concentration were demonstrated to further improve ionic conductivities. [22][23][24][25] It is not clear why certain doping strategies lead to improved Li-ion conductivity. To rationally guide future materials design of new halide Li-ion conductors, a scientific understanding about the effects of cation concentration, cation configuration, and Li concentration on Li-ion conduction is needed.…”

A Database of Ionic Transport Characteristics for Over 29 000 Inorganic Compounds

“…Thus, it is imperative to find a precise and efficient computational method to establish a database that can integrate the data as well as unify their retrievable formats. The First-principles calculation is the preferred method to evaluate transport channel and migration energy of systems, [18][19][20] however, much more time-consuming density functional theory (DFT)-based methods are obviously not an effective solution for screening solid electrolytes. Efforts aimed at developing a method combining geometric configuration analysis based on Voronoi decomposition and bond valence site energy (BVSE) calculations to identify the interstitial positions and transport channels have been undertaken in our previous work.…”

Na2Fe2OS2, a new earth abundant oxysulphide cathode material for Na-ion batteries

“…Mechanochemical synthesis has been shown to be a powerful tool for the synthesis of metastable and so materials such as chalcogenides, [19][20][21] while avoiding severe limitations on the use of toxic sulphurizing agents and therefore gaining substantial interest from industry. 22,23 It additionally has the benet of producing sub-micron particles with appreciable amount of structural disorder and improved ionic conductivity, [24][25][26] leading scientists to adopt this method for battery material processing. [27][28][29][30] Mechanochemical synthesis was thus used for the preparation of Na 2 Fe 2 OS 2 .…”