Phase separation and local cation coordination in PEOnLiNTf2 polymer electrolytes: Lessons from solid state NMR

Solid polymer electrolytes in the system PEOnLiNTf2 (NTf2 = N(CF3SO2)2; 3 ≤ n ≤ 20) were synthesised and characterised employing DSC, X-ray diffraction and a range of solid state NMR methodologies including 7Li- and 19F (MAS) NMR allowing to distinguish and quantify mobile and immobile cationic and anionic species, temperature dependent 13C CPMAS NMR to identify amorphous and crystalline regions and 13C–{7Li} REDOR NMR to study the interaction between the Li cations and the polyether chains in the various identified phases. Significant ion mobility was found for compositions with EO:Li ≥ 10 only, with NTf2 anions being more mobile than lithium cations. While PEO3LiNTf2 and PEO6LiNTf2 mainly consist of the stoichiometric crystalline complexes, the dilute compositions show a complex phase behaviour. The results suggest that in this regime, pristine PEO, crystalline PEO6LiNTf2 and amorphous PEO11LiNTf2 coexist. The combined results from temperature dependent 13C-CPMAS and REDOR NMR experiments performed at temperatures below the glass transition temperature indicate that PEO10LiNTf2 reversibly separates into crystalline PEO and PEO6LiNTf2 upon cooling.