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

Abstract Solid polymer electrolytes in the system PEO n LiNTf 2 (NTf 2  =  N (CF 3 SO 2 ) 2 ; 3 ≤  n  ≤ 20) were synthesised and characterised employing DSC, X-ray diffraction and a range of solid state NMR methodologies including 7 Li- and 19 F (MAS) NMR allowing to distinguish and quantify mobile and immobile cationic and anionic species, temperature dependent 13 C CPMAS NMR to identify amorphous and crystalline regions and 13 C–{ 7 Li} 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 3 LiNTf 2 and PEO 6 LiNTf 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 6 LiNTf 2 and amorphous PEO 11 LiNTf 2 coexist. The combined results from temperature dependent 13 C-CPMAS and REDOR NMR experiments performed at temperatures below the glass transition temperature indicate that PEO 10 LiNTf 2 reversibly separates into crystalline PEO and PEO 6 LiNTf 2 upon cooling.