In situ formation of stable solid electrolyte interphase with high ionic conductivity for long lifespan all-solid-state lithium metal batteries

•A novel solid polymer electrolyte is synthesized for lithium metal batteries.•Dendrite-free Li deposition is observed by scanning electron microscopy.•Interface-rich Li3N, Li3P/Li3PO4 nanocrystal revealed by cryogenic electron microscopy.•Density functional theory calculations indicate favorable grain boundaries.•Around 80% capacity retention after 1000 cycles at 2C and >99% CE, at 20 °C.