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 PEO:LiBF 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/LiBF 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/LiBF electrolyte with a high Li transference number (0.73) and remarkably increased mechanical strength (storage modulus >100 MPa) at 45 °C. As a result, the Li|LiFePO batteries with the ultrathin self-supported PEO/LiBF electrolyte (10 μm) exhibit significantly improved cycle life (97 % @ 468 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.