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

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 (Ea ∼0.75 eV, 7.6 × 10-12 S cm-1 for NaMgCl3; Ea ∼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): Ea ∼0.06 eV, σ ∼1.24 × 10-1 S cm-1 and NaMgBr3 (100): Ea ∼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.