Lithium-ion batteries are currently the most widely used secondary batteries, extensively applied in electric vehicles, laptops, and smartphones. However, commercial lithium-ion batteries generally use flammable organic solvents, which pose safety issues such as leakage and self-ignition. As the demand for higher energy density grows in fields like aerospace and electric vehicles, the existing commercial lithium-ion batteries cannot available. All-solid-state lithium-ion batteries (ASSLBs) with a high energy density and a good safety become one of the ideal alternatives to address the issues of the existing lithium-ion batteries. However, the bottleneck for ASSLB applications is the lack of solid electrolyte (SE) materials with a high ionic conductivity and a good stability simultaneously. Recently, a novel class of revived ternary halide SEs represented by Li₃InCl₆ (LInC) has attracted much attention due to their high room temperature ionic conductivity, high oxidative stability, compatibility with high voltage cathodes, and particularly high humidity stability. However, there are still some application-related issues regarding LIC that remain unclear, such as mechanical and thermal properties, especially the Li-ion migration mechanisms. First-principles calculations can reveal the underlying mechanisms in an atomic scale, compared to experimental methods. This paper was to evaluate the fundamental physical properties required for LInC as a SE material and reveal the main factors for high ionic conductivity, thus providing methods for the improvement and application of LIC SE. The geometric and electric structure, mechanical and thermal properties, defect chemistry, and Li-ion migration mechanism of LIC were investigated, and the findings indicated that LIC could be used as a promising SE material for ASSLBs.

In this work, all the calculations were performed by first-principles methods based on density functional theory (DFT) implementing in a software named VASP (Vienna Ab initio Simulation Package). The interaction between ion cores and valence electrons was described by Projector-Augmented Wave (PAW) with a plane wave cutoff energy of 640 eV. The exchange-correlation functional was described by the Perdew-Burke-Ernzerhof (PBE) functional within the Generalized Gradient Approximation (GGA). During geometry relaxation, the numerical integration over the Brillouin zone was performed by the Monkhorst-Pack method with a k-point grid of 3×2×3; for electronic structure calculations, the grid is 5×3×5, and a Gaussian smearing was applied to the Fermi level with a width of 0.2 eV. The electronic structure and band structure of the system were calculated by hybrid density functionals based on the Heyd-Scuseria-Ernzerhof (HSE06) method. All atoms within the unit cell were fully relaxed with a convergence criterion of 10^-5 eV/atom, and the interatomic forces were less than 0.01 eV/Å. In addition, for the calculation of defects and Li ion migration barriers, a supercell of 2×1×2 was also constructed, containing 24 Li, 8 In, and 48 Cl atoms. The Climb Image Nudged Elastic Band (CI-NEB) method was used to search for the migration path of Li⁺ and to calculate the migration barrier.

We modeled LInC crystal based on the XRD experimental data. LInC unit cell with C2/m space group exhibits the lattice constants (i.e., a = 6.63 Å, b = 11.34 Å, c = 6.46 Å, α = γ = 90°, and β = 112.0°), which are in good agreement with the experimental data. All Cl atoms fully occupy the 4i and 8j Wyckoff sites, In atoms partially occupy the 4g site within the plane (001), and Li atoms fully occupy the 4h and 2d sites within the plane (002). The band structure illustrates LIC with a bandgap of 4.71 eV, indicating a great theoretical electrochemical window. Furthermore, DFT calculations demonstrate that LInC possesses low elastic moduli with B=14.71 GPa, G=7.05 GPa, and E=18.24 GPa, which is smaller than those of Li₁₀GeP₂S₁₂ (LGPS) and Li₃YCl₆ (LYC), and exhibits a high Pugh ratio (i.e., B/G=2.09), indicating its favorable mechanical properties and ductility. LInC can be suitable as a solid electrolyte for all-solid-state batteries. The defect chemistry and Li-ion migration of LInC show that although isolated with a formation energy of -1.43 eV can form spontaneously, migration barriers as high as 0.48 eV indicates that the contribution of to the conductivity of LInC is negligible. Similarly, despite presenting a low migration barrier of 0.17 eV in ab plane, isolated has a high formation energy of 4.75 eV, indicating a negligible contribution to conductivity of bulk LInC. For four considered neutral defect pairs, the defect formation energies and Li-ion migration barriers follow an order of and , respectively. The results indicate that Frenkel defect pair is the dominant defect type, playing a critical role in the high ionic conductivity of LInC. The contribution of and defect pairs to the ionic conductivity of LInC bulk is also not negligible. This work provides valuable insights for evaluating LInC as SE material, and elucidates some dominant factors for a high ionic conductivity of LInC.

This work systematically investigated the basic properties and Li-ion migration mechanisms of LInC by first-principles calculation methods. The results showed that LInC had high theoretical electrochemical window as well as good mechanical property and ductility. Furthermore, the Frenkel defect pair with the low formation energy of 0.24 eV presented a low migration barrier of 0.26 eV, confirming that it could be the dominant defect type in LInC, and Li vacancies were the predominant carriers in LInC bulk. The findings of this work could provide a theoretical basis for the modification and application of LInC materials.