• • Li|Li cells achieve >2000 h of stable cycling, demonstrating a substantial improvement in dendrite suppression and interfacial stability, critical for extending the operational lifetime of solid-state lithium metal batteries.
• • Li|LiNi0.8Co0.1Mn0.1O2 full cells surpass 600 cycles, indicating excellent compatibility with high-nickel cathodes and potential for high-energy-density applications.
• • The heterostructured SEI integrates LiF domains (dendrite-suppressing) with Li3N ion channels (ultra-fast ion transport), resolving the trade-off between mechanical robustness and ionic conductivity that plagues conventional LiF-rich SEIs.
• • The design is guided by quantifiable descriptors (ionic potential and donor number), providing a rational, transferable framework for engineering SEIs in solid-state polymer electrolytes, moving beyond empirical trial-and-error.