Regulating Interfacial Na-Ion Flux via Artificial Layers with Fast Ionic Conductivity for Stable and High-Rate Na Metal Batteries

Metallic Na electrodes are promising anodes for low-cost and high-energy density batteries due to their natural abundance and high specific capacity. Unfortunately, they are extremely reactive and spontaneously form unstable solid-electrolyte interphases, which lead to critical challenges including growth of dendritic/mossy Na structures and fast degradation. We report here the design of artificial interphase films that have intrinsic high Na+-ion conductivity, which enable protected Na electrodes with simultaneously improved surface stability and redox kinetics. They were prepared from Mo6S8 films, which transform to NaxMo6S8 (x ≈ 16) through an in-situ sodiation process when pressed onto Na metal. The protected Na electrodes were stable in dry air for days and exhibited 2.5 times higher exchange current density compared with pristine Na electrodes. They enabled symmetric batteries with stable cycling for 1200 h at 0.5 mA cm-2 and fast Na metal batteries with substantially improved high-rate performance and robust durability for 1000 cycles.