Nanocrystals with Aggregate Anionic Structure Enable Ion Transport Decoupling of Chain Segment Movement in Poly(ethylene oxide) Electrolytes

All‐solid‐state polymer electrolytes are promising for lithium batteries, but Li+ transport in these electrolytes relies on amorphous chain segment movement, leading to low Li+ mobility and poor mechanical strength. Here we propose a novel Li+ transport mechanism mediated by PEO3:LiBF4 nanocrystals (NCPB) with the aggregate (AGG) anionic structure, which enables a change from amorphous to crystalline phase dominated ion transport in all‐solid‐state PEO/LiBF4 electrolyte. Experiments and simulations reveal that the interaction between Li+ and F in NCPB with AGG anionic structure simultaneously restricts anion transport and reorients anions within the free volume of NCPB, resulting in a three‐coordination intermediate to facilitate Li+ transport. The unique Li+ transport mechanism through NCPB makes the PEO/LiBF4 electrolyte with a high Li+ transference number (0.73) and remarkably increased mechanical strength (Young's modulus > 100 MPa) at 45 °C. As a result, the Li|LiFePO4 batteries with the ultrathin self‐supported PEO/LiBF4 electrolyte (10 μm) exhibit significantly improved cycle life (92% @ 500 cycles) compared to those with PEO/LiTFSI electrolyte (failed @ 68 cycles). This work demonstrates a novel ion transport mechanism for achieving selective and rapid Li+ transport.