NaMgX₃ (X = Cl, Br) for solid electrolyte interphases: atomistic insights into defects, surfaces and doping...

Progress in metal-ion batteries depends on engineered solid electrolyte interphases with tunable defect chemistry and robust mechanics. In this work, atomistic simulations provide a quantitative comparison of the properties of NaMgCl₃ and NaMgBr₃ with direct relevance to solid electrolyte interphase design. The results confirm wide band gaps of 5.0 eV (NaMgCl₃) and 3.7 eV (NaMgBr₃), establishing both compounds as electronically insulating and mechanically stable and suitable for the electrolyte interphase. Br substitution expands the lattice and octahedral volumes and lengthens Mg-X bonds, producing a softer, more deformable bromide framework that opens conduction channels. Defect chemistry is governed by Na-X Schottky formation, Na⁺ and Li⁺ Frenkel disorder, and aliovalent substitution: divalent substitution at Na⁺-sites is the most practical Na⁺ vacancy source (in particular the Zn²⁺ dopant) and trivalent dopants bind strongly but require non-equilibrium incorporation. Bulk metrics show that the Br^- anion lowers migration barriers and raises the conductivity (E_a ∼0.75 eV, 7.6 × 10^-12 S cm^-1 for NaMgCl₃; E_a ∼0.41 eV, 2.1 × 10^-6 S cm^-1 for NaMgBr₃). Surface energetics are facet dependent: lowest-energy facets are NaMgCl₃ [(001), γ ∼0.29 J m^-2] and NaMgBr₃ [(111, γ ∼0.15 J m^-2], while (100) is high-energy (∼4.0-5.0 J m^-2); high-energy facets give the lowest surface barriers (e.g., NaMgCl₃ (100): E_a ∼0.06 eV, σ ∼1.24 × 10^-1 S cm^-1 and NaMgBr₃ (100): E_a ∼0.06 eV, σ ∼1.19 × 10^-3 S cm^-1) whereas stable facets are far less conductive. These computed results provide predictions for experimental synthesis, facet engineering, and controlled doping to tune interphase performance and harness defect-driven reorganization for conductive SEI formation.