Phase separation and local cation coordination in PEO nLiNTf 2 polymer electrolytes: Lessons from solid state NMR

Abstract

Solid polymer electrolytes in the system PEO nLiNTf 2 (NTf 2 = N(CF 3SO 2) 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 NTf 2 anions being more mobile than lithium cations. While PEO 3LiNTf 2 and PEO 6LiNTf 2 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 PEO 6LiNTf 2 and amorphous PEO 11LiNTf 2 coexist. The combined results from temperature dependent 13C-CPMAS and REDOR NMR experiments performed at temperatures below the glass transition temperature indicate that PEO 10LiNTf 2 reversibly separates into crystalline PEO and PEO 6LiNTf 2 upon cooling.