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

Abstract

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 (storage 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 (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.