First-Principles Calculations of Li₂MgSO Oxysulfide as a Solid-State Electrolyte

As pivotal components in modern energy storage systems, lithium-ion batteries remain constrained by inherent limitations in safety and energy density associated with conventional liquid electrolytes. All-solid-state lithium-ion batteries represent a promising technological pathway to synergistically enhance both energy density and intrinsic safety through the implementation of non-flammable solid-state electrolytes. However, the fundamental trade-off between ionic conductivity and chemical stability persists as a critical challenge in solid-state electrolyte development. The existing material systems exhibit some distinct limitations, i.e., sulfide-based solid-state electrolytes (e.g., Li₁₀GeP₂S₁₂) demonstrate a superior ionic conductivity (~10^-2 S/cm), but suffer from pronounced hygroscopic and oxidative instability, and oxide-based counterparts (e.g., Li₇La₃Zr₃O₁₂) display exceptional environmental stability with inadequate ionic conduction performance. This challenge leads to extensive research into oxysulfide electrolyte systems that combine advantageous characteristics of both material classes through chemical composition. Recent researches on LiAlSO-based materials reveal that enhanced lithium-ion concentration and isoelectronic substitution of Al with Li–Be groups can significantly improve an ionic conductivity. However, the inherent biotoxicity of beryllium substantially elevates synthesis costs and complicates manufacturing processes. To address this limitation, this work was to propose a novel Li₂MgSO electrolyte system for replacing beryllium with the homologous element magnesium. This study also investigated the structural stability, electronic characteristics, and lithium-ion transport mechanisms via utilizing the crystal structure prediction program (CALYPSO) and density functional theory (DFT) calculations. The complementary anion/cation substitution experiments could elucidate composition-property relationships, providing a theoretical foundation for developing cost-effective, high-performance solid-state electrolytes.

The crystal structure of Li₂MgSO was predicted by the CALYPSO method with the particle swarm optimization algorithm.The total energy calculations and geometry optimizations were performed using the Vienna Ab initio Simulation Package (VASP) within the framework of density functional theory (DFT). The projector augmented wave (PAW) approach was used to describe the core-valence electron interaction. The generalized gradient approximation in the parameterization of Perdew, Burke, and Ernzerhof was used to describe the exchange-correlation potential. The plane-wave energy cut-off was set to 500 eV. The geometric structures were fully relaxed until the residual force on each atom was less than 0.02 eV·Å^–1, and the energy converged to less than 10^–5 eV. The hybrid density functional calculation, which was more accurate for electronic band structure calculations, was used to calculate the electronic density of states. The phonon spectrum calculations were carried out based on density-functional perturbation theory, and the phonon dispersion relations and thermodynamic properties were derived by a software named PHONOPY. Bond Valence Site Energy (BVSE) was used to model the possible diffusion channels of Li ions in Li₂MgSO. The migration barriers of Li ions were obtained by the climbing image nudged elastic band (CI-NEB) method. Ab initio molecular dynamics (AIMD) simulations were further employed to investigate the ion transport properties of Li₂MgSO. The simulations were conducted in the canonical ensemble (NVT) using a Nosé-Hoover thermostat at different spanning temperatures (i.e., 800–1600 K). Each simulation lasted for 40000 steps with one time step of 1 fs.

The application of CALYPSO structural prediction program and first-principles calculations indicates that the ground-state configuration of Li₂MgSO can crystallize in a P42₁m space group symmetry. The phonon dispersion analysis reveals a complete absence of imaginary frequencies, confirming the lattice dynamic stability. Electronic structure characterization discloses a substantial band gap of 4.76 eV, satisfying a critical requirement for solid-state electrolytes via ensuring electrochemical stability against redox reactions. The mechanical property evaluation demonstrates the Young modulus of 66.81 GPa and the Pugh ratio of 1.93, indicative of superior crack propagation resistance and favorable ductile behavior, respectively. The collective computational evidence indicates that Li₂MgSO exhibits a promising potential as a high-performance solid-state electrolyte, achieving an optimal balance in structural stability, wide electrochemical window, and mechanical properties.

The BVSE and CI-NEB calculations reveal that there is a three-dimensional ion transport network in Li₂MgSO, comprising two distinct pathways, i.e.,1) a 2D fast diffusion channel within the ab-plane of LiMgSO layers exhibiting a migration barrier of approximately 0.34 eV, and 2) the interlayers form three-dimensional interconnected channels with a low energy barrier (0.17 eV) through vacancy migration. This unique structure substantially enhances a lithium-ion mobility, thus leading to exceptional ionic transport performance.

The AIMD simulations are used to investigate the ion migration characteristics of Li₂MgSO, Li₂MgSeO, and Li₂ZnSO, yielding the activation energies of 0.68, 0.82 eV, and 0.35 eV for lithium diffusion, respectively. The mean square displacement (MSD) analysis reveals the comparative dynamics, i.e., lithium ions in Li₂MgSeO displayed marginally reduce mobility relative to Li₂MgSO, whereas Li₂ZnSO exhibits a twofold increase in MSD values, indicating a superior ionic migration capability. These findings indicate cation-induced effects as the primary governing mechanism for ion transport in Li₂MgSO systems, providing a strategic guidance for optimizing cationic species to engineer expanded transport channels and enhance ionic conductivity.

The theoretical calculations indicated that Li₂MgSO had the excellent kinetic properties, mechanical stability, and a wide bandgap (4.76 eV). The ion transport kinetics analysis unveiled a unique three-dimensional ionic transport network in Li₂MgSO, comprising interlayer migration channels and two-dimensional in-plane migration pathways, with low migration barriers favorable for rapid lithium-ion transport. From the systematic substitution studies of anions (Se) and cations (Zn), Li₂ZnSO demonstrated the superior lithium-ion migration kinetics, compared to Li₂MgSO and Li₂MgSeO, indicating that lithium-ion migration in Li₂MgSO system could be primarily regulated by cation-induced effects.