Investigating the Protective Function of the β-Li3N Interlayer for Halide-Based All-Solid-State Li Metal Batteries

Halide solid electrolytes are promising candidates for all-solid-state batteries owing to their high ionic conductivity at room temperature and excellent oxidative stability, yet they are chemically unstable against Li metal, forming a mixed ionic/electronic conductor that necessitates a protective interlayer. Here, we evaluate the use of β-Li3N as an interlayer for Li3YBr2Cl4. We found that introducing a thick β-Li3N interlayer into the full cell reduced early polarization and delayed failure, though it did not prevent it. By combining porosity measurement and electrochemical testing, we demonstrate that the β-Li3N interlayer has high porosity, allowing Li to penetrate under stack pressure and during electrodeposition. Moreover, through synchrotron X-ray nano-holo-tomography and scanning microdiffraction, we provide direct visualization of Li penetration into the layers of β-Li3N and LYBC, explaining the significant voltage spikes and the cell polarization observed during electrochemical cycling. This work highlights the need to develop thinner and denser interlayers to overcome the key limitation of the porous microstructure in β-Li3N, which hinders the development of durable halide-based all-solid-state Li-metal batteries.