Material Design Strategy for Halide Solid Electrolytes Li₃MX₆ (X = Cl, Br, and I) for All-Solid-State High-Voltage Li-Ion Batteries

Abstract

Although several solid electrolyte (SE) candidates have been explored, achieving the necessary combination of performance, stability, and processability has been challenging. Recently, several lithium ternary halides have attracted increasing attention for SEs because of their favorable combination of high ionic conductivity and wide electrochemical window. This study aims to provide a material design strategy for lithium halides Li3MX6 (X = Cl, Br, and I) for high-voltage all-solid-state Li-ion batteries, achieved by the systematic investigation of crystal structures, phase and electrochemical stabilities, electronic and mechanical properties, and ionic conductivities. Calculation results reveal that the electronegativity difference between M and X affects structural properties and stabilities. Weak Coulomb interactions in Li3MX6 result in the preference of the monoclinic phase, and the oxidation potential and chemical stability against the cathode materials of Li3MX6 increase for relatively small X. Chlorides exhibit the highest oxidation potential (∼4.3 V) among Li3MX6, suggesting that chlorides are appropriate SEs for high-voltage cathodes. The band gap and elastic moduli increase for relatively small X, suggesting the relatively low electronic conductivity and elastic deformability of chlorides. Chlorides with transition metals typically exhibit trigonal phases, a wider electrochemical stability window, a larger band gap, and higher elastic moduli compared to other types of halides. Additionally, chloride Li3MCl6 is expected to have relatively high ionic conductivities with the aliovalent substitution of M3+ to Zr4+ and the anion mixing of Cl with Br. The findings of this study will provide fundamental guidelines for the development of lithium halide SEs for high-voltage all-solid-state Li-ion batteries.

Stacking Faults Assist Lithium-Ion Conduction in a Halide-Based Superionic Conductor

“…Furthermore, Wang et al 18 showed that introducing one and three anti-site defects between Li + and Y 3+ yields an average activation energy of 0.28±0.03 eV and 0.31±0.03 eV, respectively. Similarly, Kim et al, 71 who included 32 explicit van der Waals corrections in their AIMD simulation, obtained a larger average Li + migration activation energy of ≈0.370 eV. While the existing data is in agreement with our results of Figure 8d, by explicitly considering the occurrence of stacking faults in the LYC structure, our simulations go beyond the analysis of Li + migration in pristine bulk structures, which cannot reflect the defect-rich mechanochemically synthesized structures.…”

“…In general, our computed migration barriers agree well with previous computational data. 18,71 Using AIMD on a M1 -M2-bulk configuration, Wang et al 18 derived an average migration barrier of ≈0.19±0.03 eV, which is in agreement with our findings for out-of-plane diffusion. In contrast, they reported a much smaller barrier (≈0.23±0.06 eV) (all confidence intervals listed are 1σ) for the in-plane Li + migration pathway (≈0.358 eV in this work).…”

“…Among the Li 3 MX 6 family of compounds, sparser M 3+ distributions have been reported to improve Li + conduction and are a critical parameter for designing better halide Li-ion solid electrolytes. 71 While intra-particle conduction is facilitated by defects, grain boundaries still hinder long-range Li + conduction, as evidenced by both the decreasing Li + self-diffusion constants at longer ∆ values from PFG-NMR, and the high activation energy from EIS for bulk migration. Previous computational investigations of Li + migration in LYC have mostly relied on ab initio molecular dynamic simulations (AIMD), 18 and are often limited to a single Li/Y ordering in the bulk structure and specific direction for Li + migration.…”

Highly Stable Halide‐Electrolyte‐Based All‐Solid‐State Li–Se Batteries

“…[ 26 ] Halide SSEs can even be synthesized by a facile wet‐chemistry method using water as a solvent. [ 27,28 ] Furthermore, halide SSEs, especially chlorides and fluorides, exhibit wide electrochemical windows among all types of SSEs, which have been proven by theoretical [ 12,29 ] and experimental [ 30,31 ] studies. Until now, all the solid‐state batteries in the halide SSEs system are still focused on traditional oxide cathodes.…”

In Search of the Best Solid Electrolyte-Layered Oxide Pairing for Assembling Practical All-Solid-State Batteries

“…Different electrolyte 4 chemistries indeed differ in their thermodynamic stability against oxidation and reduction, in their electronic and ionic conductivity, in their softness (which is higher for halides than for argyrodite and thiophosphates) and lastly in their thermal stability. 22 Moreover, their chemical-physical properties were also shown to depend on their synthesis method, either via solvent assisted reactions involving precipitation/crystallization in organic solvents, high temperature solid-state reactions (>500 °C) or mechanical ball milling followed by annealing steps. [23][24][25][26] Among them, the solvent-based routes appear to be particularly attractive for solid electrolyte (SE) -CAM composite preparation in a core-shell like structure (namely SE-coated oxide) by maximizing the energy density at electrode level while enhancing ionic percolation and boosting the cell performance.…”