Lithium Superionic Behavior and Defect Robustness in LiNbOCl₄: A First-Principles Molecular Dynamics Study

Abstract

Lithium-ion conductors that exhibit high ionic conductivity, thermal robustness, and mechanical compliance are essential for advancing all-solid-state battery technologies. In this study, we systematically investigate Li-ion transport in pristine and LiCl Schottky-defected LiNbOCl₄ using density functional theory molecular dynamics (AIMD). Pristine LiNbOCl₄ demonstrates robust superionic behavior with low activation energy (0.236 eV) and high room-temperature conductivity (9.57 × 10⁻³ S cm⁻¹), facilitated by a rigid Nb–O–Cl framework and a disordered Li sublattice. Introducing LiCl Schottky defects slightly increases the activation energy to 0.241 eV and slightly reduces conductivity to 8.20 × 10⁻³ S cm-1. While defects preserve global percolation networks and mechanical softness, they introduce localized structural disruptions at vacancy-adjacent polyhedra. Notably, the dynamic gating mechanism where coherent anion rotations transiently expand diffusion bottlenecks is impaired. Unlike the classical paddle-wheel mechanism involving rotating polyanion clusters, this mechanism describes a distinct mode of transient bottleneck expansion driven by coordinated motion of individual halide and oxide anions within an oxyhalide lattice. The disruption of this mechanism is reflected in the emergence of rotational incoherence, rapid bond angle decorrelation, attenuation of high-frequency O-based phonon modes (~80 meV), and a strong reduction and spatial localization of anion reorientation events that are associated with enhanced Li-ion motion, as identified by event-triggered ensemble. Together, these effects suppress bottleneck breathing, limiting the transient widening of diffusion pathways and effectively increasing the migration barrier. Li space-time correlation analysis further reveals diminished temporal coherence and transport cooperativity in the defected structure. These findings underscore the importance of cooperative lattice dynamics in enabling low-barrier Li-ion transport and demonstrate that LiNbOCl₄ retains high conductivity even under defect-induced perturbations, establishing it as a defect-tolerant candidate for next-generation solid electrolytes.