Local Cation Coordination Motifs in Polyphosphazene Based Composite Electrolytes

Solid electrolytes in the system BMEAP/LiTf/Al 2 O 3 (BMEAP = poly[(bis(2-methoxy-ethyl)-amino)(2-x)(propylamino) x ]polyphosphazene; x = 0.4, Tf = CF 3 SO 3 ) were studied employing a range of advanced solid state NMR methodologies including 7 Li-{ 1 H ) CPMAS, REDOR, and 7 Li-{ 1 H)-CPMAS-{ 27 Al} REAPDOR NMR spectroscopy. In the binary system BMEAP/LiTf, three different 7 Li signals corresponding to different Li environments could be identified. Approximately 10% of the Li in the samples proved to be rather mobile with the mobility ultimately linked to the polymer dynamics, whereas 30% of the total Li content contributes to a signal which could be assigned to a Li species located within the pockets opened by the BMEA side chains as evidenced from various 7 Li-{ 1 H} double resonance NMR experiments such as CPMAS and REDOR NMR. The majority of the Li cations, however (ca. 62% of the Li), was found to be completely immobile, even at elevated temperatures (365 K). However, the results of 19 F MAS NMR experiments rule out the existence of any extended precipitates of crystalline LiTf. In the Al 2 O 3 -containing samples, a further Li signal could be identified by 7 Li-{ 1 H} CPMAS spectroscopy. This signal intensity is increasing with decreasing particle size of the added alumina. Employing 7 Li-{ 1 H}-CPMAS-{ 27 Al} REAPDOR NMR spectroscopy, this signal could be safely assigned to Li cations in ultimate proximity to the alumina surface. To our knowledge, this is the first direct experimental proof for such an interaction between Li cations and the alumina surface based on dipolar recoupling NMR techniques, which has been suggested as an important step in the mechanism of ion conduction in composite systems.