Low‐LUMO Orotic Acid Enables Li₃N‐Embeded Solid Electrolyte Interphase for Stable All‐Solid‐State Lithium Metal Batteries

Biomacromolecules hold promise as sustainable fillers to address the challenges of low ionic conductivity and interfacial instability in polymer electrolytes, yet their structural complexity obscures in-depth mechanistic understanding. Herein, small biomolecules are strategically incorporated, which retain the key functional groups of biomacromolecules, as highly effective fillers for polymer electrolytes. Orotic acid (OA), a small-molecule building block of ribonucleic acid, enhances lithium (Li) ion conductivity by facilitating the dissociation of lithium bis(trifluoromethanesulfonyl)imide without compromising the crystallinity of the polyethylene oxide (PEO) matrix. The amide-rich conjugated structure endows OA with a low lowest unoccupied molecular orbital energy level, promoting its preferential decomposition to form a lithium nitride (Li₃N) embedded solid electrolyte interphase. This dual-function mechanism enables the OA-optimized PEO electrolyte to achieve 95.2% capacity retention after 350 cycles (0.5 C, initial capacity: 148 mAh g^-1) and >80% retention after 600 cycles (1.0 C, initial capacity: 128.6 mAh g^-1) in Li||LiFePO₄ cells.