Compared with organic lithium-ion batteries, all-solid-state batteries are expected to improve battery safety and energy density simultaneously. They have attracted extensive attention. The ideal solid electrolyte material should have the basic properties of electronic insulation, wide electrochemical window, good interface compatibility and high ionic conductivity. Many types of solid electrolyte materials are reported, including oxides, sulfides, halides, borohydrides and phosphates, each of which has advantages and disadvantages. For instance, lithium-based halide and sulfide solid electrolytes have a high ionic conductivity but a narrow electrochemical window, and they are unstable to lithium metal negatives. The interface compatibility between oxide solid electrolyte and electrode is poor, and lithium dendrites grow rapidly along the grain boundary in oxide solid electrolyte. To further develop all-solid-state batteries with a higher energy density, a longer cycle life and a higher safety, solid electrolyte materials with excellent comprehensive performance must be designed. Anti-perovskite superionic conductors based on cluster anions have attracted much attention due to their potential applications in solid electrolytes for rechargeable batteries. However, little theoretical studies on the phase stability, electrochemical stability and interface compatibility of anti-perovskite X₃OBH₄(X=Li, Na) materials have been reported yet. In this work, the electronic structure, phase stability, electrochemical stability, interface compatibility, mechanical properties and ion transport properties of anti-perovskite X₃OBH₄(X=Li, Na) materials were systematically investigated via first-principles calculation.

All the calculations were performed based on density functional theory (DFT) by a projector augmented wave method, as implemented in the Vienna ab initio Simulation Package (VASP). The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) was applied to treat the electronic exchange-correlation interactions. The cutoff energy was set to 520 eV. The crystal structure was fully relaxed until the convergence criteria for each atomic force and energy were less than 0.02 eV/Å and 10^-5 eV, respectively. Based on electrochemical energy storage materials design platform (bmaterials. cn), the phase stability and interfacial stability (including electrochemical and chemical stability) of X₃OBH₄(X=Li, Na) were evaluated.

The results show that X₃OBH₄(X=Li, Na) is a thermodynamically metastable and wide-band insulator at 0 K, which is unstable at a high pressure. Based on the energy calculated by DFT, the phase diagrams of Na-NaBH₄-O₂ and Li-LiBH₄-O₂ are constructed, respectively, and the calculated E_hull of Li₃OBH₄ and Na₃OBH₄ is 52.4 meV/atom and 110.7 meV/atom, respectively. X₃OBH₄(X=Li, Na) is thermodynamically unstable at 0 K. Since the E_hull value is relatively small, it is possible to stabilize the compound through the regulation of external conditions such as high temperature, high pressure and high entropy. Based on the lithium (sodium) giant potential phase diagram of the constructed X-O-B-H quaternary system, the voltage distribution and phase equilibrium of X₃OBH₄(X=Li, Na) in the process of lithiation/delithiation are calculated by DFT. The electrochemical window range of X₃OBH₄(X=Li, Na) is 0.53–0.93 V and 0–0.41 V, respectively. The corresponding decomposition product XBH₄(X=Li, Na) has a wide electrochemical stability window, which can protect the solid electrolyte. The calculated moduli of B, E and G of X₃OBH₄ (X=Li, Na) are greater than those of lithium (sodium) metal or even Li₃PS₄ electrolyte, indicating that X₃OBH₄ (X=Li, Na) can effectively block the growth of lithium (sodium) dendrites and has a good mechanical contact at the electrode/solid electrolyte interface. In addition, the low migration barriers of X₃OBH₄(X=Li, Na) are 0.34 eV and 0.35 eV, respectively, and the ionic conductivity at room temperature can reach 10^-4 S/cm. The rotation of the superhalogen promotes the movement of the lithium/sodium ions, thereby increasing their ionic conductivity.

The electronic properties, phase stability, electrochemical stability, chemical stability, mechanical properties and ion transport mechanism of the anti-perovskite type X₃OBH₄(X=Li, Na) were systematically investigated via first-principles calculation. The results showed that the crystal structure of X₃OBH₄(X=Li, Na) could be a metastable electronic insulator with a wide band gap. Under electrochemical oxidation conditions, X₃OBH₄(X=Li, Na) could be thermodynamically unstable and easily oxidized at relatively high voltages. However, the decomposition products could form a protective layer at the interface, preventing the electrolyte from further reacting and providing an improved electrochemical stability. In addition, X₃OBH₄(X=Li, Na) also had a good interface compatibility with typical cathode materials. The calculated mechanical properties indicated that X₃OBH₄(X=Li, Na) was brittle. However, their relatively large shear modulus indicated that they could be stable for lithium/sodium metal dendrites growth. By CI-NEB calculation, X₃OBH₄(X=Li, Na) showed a low migration barrier. In summary, these theoretical results could favor to better understand the thermodynamic and kinetic processes of X₃OBH₄(X=Li, Na), and provide a theoretical guidance for the development of high-performance solid electrolytes.