Multifunctional solid-state materials are crucial for enabling safer and simpler lithium battery architectures. Here we report a scalable, solvent-free mechanochemical strategy to overcome the chemical inertness of hexagonal boron nitride and produce Li₂(BN)₆C₂O₄ (LBNCO), a lithium-rich boron nitride nanostructure. High-energy milling simultaneously exfoliates hBN and induces robust covalent oxalate functionalization, yielding a nanocrystalline lamellar compound with intrinsically incorporated lithium. LBNCO exhibits Li⁺ ionic conductivity, electronic insulation, and thermal stability exceeding 350 ^∘C. Additional mechanochemical processing enables further lithium enrichment without compromising chemical stability, allowing tunable ionic conductivity. By integrating ion transport, mechanical robustness, and thermal stability within a single material, LBNCO establishes a new platform for solid-state lithium batteries and related technologies.