The structure of a polymer electrolyte, $\mathrm{P}\left(\mathrm{EO}{)}_{7.5}\mathrm{LiN}\right(\mathrm{SO}{}_2\mathrm{CF}{}_3{)}_2$, has been determined by neutron diffraction with isotropic substitution. The Li ions are bonded on average to five ether oxygens belonging to pairs of PEO coils. These are arranged with a considerable degree of extended-range order providing pathways for the Li ion conduction. The lack of ion pairing in this system below $4.8Å$ is reminiscent of that observed in the remarkable structure of $\mathrm{P}(\mathrm{EO}{)}_6\mathrm{LiAsF}{}_6$ and implies that anions and cations are free to migrate independently.

• Received 8 December 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.5536