Li/Na Mixed Metal Nitride as a Protective Layer for Na Metal Anodes

Abstract Artificial interphases are essential for stabilizing the highly reactive alkali-metal anode/solid-electrolyte interface in solid-state batteries, yet identifying materials that simultaneously possess high ionic conductivity and interfacial stability remains a major challenge. Here, we propose mixed alkali-metal nitride Li2NaN as a protective interlayer for sodium–metal solid-state batteries. Density functional theory optimization and 10 ps ab initio molecular dynamics simulations show that the Li2NaN interlayer suppresses the decomposition of Na6PS5Cl over the simulated time window, preventing the formation of detrimental phosphorus sulfide species at the protected interface. To further investigate ion transport, molecular dynamics simulations based on a modified universal force field were performed for pristine and vacancy-engineered Li2NaN. While the pristine structure exhibits poor ionic diffusion, introducing Li/Na/N vacancies dramatically enhances alkali-ion transport. The optimized 2.5% vacancy structure achieves Na-ion and Li-ion conductivities of 10.78 and 1.50 mS/cm, respectively, exceeding those of most reported inorganic interphase materials. In particular, Na-ion transport is intrinsically faster than Li-ion transport in the mixed alkali-metal nitride framework. These results identify vacancy-engineered Li2NaN as a fast ion-conducting candidate interphase that is dynamically stable in the simulated interface, providing a rational strategy for designing protective layers for next-generation sodium–metal solid-state batteries.