Solid-state lithium batteries are regarded as a promising alternative to traditional lithium-ion batteries due to their high energy density and safety. The development of solid electrolytes with high ionic conductivity is key to their application. The lithium-ion transport mechanisms in solid electrolytes can be categorized into single-ion migration and multi-ion correlated migration. Generally, multi-ion correlated migration can significantly reduce energy barriers compared to single-ion migration, making the promotion of correlated migration a crucial principle in designing solid electrolyte materials with high ionic conductivity. Titanite-type LiTaSiO₅ is a new oxide solid electrolyte, in which Zr⁴⁺ doping at Ta⁵⁺ site introduces excess lithium ions, facilitating correlated migration and significantly improving ionic conductivity. However, the relationship between lithium-ion distribution, correlated migration, and ionic transport properties still require in-depth investigation. In this study, ab initio molecular dynamics (AIMD) simulations were employed to investigate the possible Li sites and migration channels in the monoclinic titanite-type Li_1+xTa_1-xZr_xSiO₅(x = 0, 0.125) systems. The relationships between lithium-ion distribution, the degree of correlated migration, and ionic transport properties were also elucidated.

AIMD simulations were performed using the Vienna ab initio simulation package (VASP) with the plane wave projector augmented (PAW) method and the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. For the pristine LiTaSiO₅ (ICSD No. 39648, space group P2₁/c), a 2×2×2 supercell was constructed, containing 32 formula units (256 atoms total). For the Zr-doped system, four Ta⁵⁺ were substituted with Zr⁴⁺, with four additional Li⁺ ions incorporated to maintain charge neutrality. AIMD simulations were carried out for the canonical (NVT) ensemble using a Nosé-Hoover thermostat at four elevated temperatures (800, 1000, 1200, and 1400 K) with a time step of 2 fs. To enhance computational efficiency, the plane-wave basis set was determined with a cutoff energy of 400 eV and integration in reciprocal space was performed at the Γ-point only. All the structures were heated from 100 K to targeted temperatures by velocity scaling over 2 ps, and then equilibrated at the desired temperature for 20 ps. The analysis of possible Li sites, jump events, and diffusion properties based on AIMD data was performed using our in-house developed code, which is integrated into our group's computational platform for electrochemical energy storage materials design (bmaterials.cn).

The framework structure of the monoclinic titanite-type LiTaSiO₅ consists of SiO₄ tetrahedra and TaO₆ octahedra. Through crystal structure analysis, we identified 11 distinct interstitial sites (It1-Itl1), where the It1 site corresponds to the Li1 lattice site while It2 and It3 sites align with two previously reported potential interstitial sites. Lithium ion migration exhibits strong preference for specific channels. At 800 K, 81.8% of jumps occur along the It2-Li1/It4-It7-It3-It7-Li1/It4-It2 channel ([101] direction) through elementary channels (Li1-It2, Li1-It7, It2-It4, It3-It7 and It4-It7), clearly identifying this as the optimal long-range migration channel. The non-unity occupation probabilities at all sites indicate Li⁺ disorder, with primary Li⁺ distribution at Li1, It2, It3, It4, and It7 sites. Previous studies confirm that Li1 is the lowest-energy stable site, while It2 and It3 serve as saddle points in the energy landscape. Zr doping significantly alters the Li⁺ distribution by increasing the Li⁺ concentration and promoting occupation of higher-energy sites. At 800 K, Li1 occupancy decreases from 41.8% to 33.1%, while It2 and It3 occupancies increase dramatically from 7.2% to 27.9% and from 7.4% to 24.3%, respectively. This redistribution, accompanied by increased configurational entropy, enhanced Li⁺ disorder and promotes low-energy-barrier correlated migration. As a result, the correlated migration percentage increases, the overall activation energy decreases, and the Li⁺ diffusion coefficient improves significantly.

The main conclusions of this study are summarized as following. The LiTaSiO₅ unit cell contains 11 distinct types of interstitial sites and shows anisotropic lithium-ion transport along the [101] direction through the It2-Li1/It4-It7-It3-It7-Li1/It4-It2 channel. After Zr doping, the lithium-ion concentration increases, leading to higher occupancy of lithium ions at high-energy sites (It2/It3), more disordered lithium-ion distribution, and an increase configurational entropy. This Li distribution enhances the probability of low-energy-barrier correlated migration, resulting in a higher correlated migration percentage, reduced overall activation energy, and significantly improved ionic transport performance.

The methodologies employed in this study are applicable to inorganic crystalline solid electrolytes with stable framework structures, which have been successfully applied in previous studies to several representative lithium/sodium solid electrolytes, such as LiTa₂PO₈ and Na₃Zr₂Si₂PO₁₂. However, these methods rely on the framework ions to identify the positions of lithium ions during the migration processes, making them unsuitable for certain specific solid electrolyte systems, such as amorphous solid electrolytes or those exhibiting the polyanion rotation effect.