Anisotropic Ion Diffusion and Electrochemically Driven Transport in Nanostructured Block Copolymer Electrolytes

Nanostructured block copolymer electrolytes have the potential to enable solid-state batteries with lithium metal anodes. Improving the performance of these electrolytes requires understanding the molecular mechanisms that govern ion motion. We present complete characterization of ion transport in lamellar block copolymers with lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI). Individual self-diffusion coefficients of the Li⁺ and TFSI^- ions are measured by pulsed-field gradient NMR. The data indicate that salt diffusion in nanostructured block copolymers is locally anisotropic, and this enables determination of a diffusion coefficient parallel to the lamellae, D_||, and a diffusion coefficient through defects in the lamellae, D_⊥. This is the first direct measurement of anisotropic ion diffusion in polymer electrolytes. We find agreement between the diffusion coefficient D_⊥ and electrochemically determined diffusion coefficients, indicating that the performance of nanostructured block copolymer electrolytes in batteries is limited by ion transport through defects. This finding motivated the design of block copolymers where the diffusion pathways for the Li⁺ and TFSI^- are fundamentally different: defects only limit the transport of TFSI^- while Li⁺ diffuses isotropically.