Speaker
Description
We investigate battery relevant mechanically rigid M1I0 polymer and viscolelastic M1I3 polymer, with and without Li, to understand the interplay between dynamics/mechanics and columbic efficiency in batteries. The polymer network consists of a PFPE backbone interconnected by urethane units containing H-bonds, with H atoms involved in N-H bond being responsible for polymer rigidity. When Li salt is added, Li cations bind to the oxygen atoms in the urethane units. In fact, FTIR revealed a lengthening of the C=O bond on addition of Li salt, which was attributed to weakening of H-bond, as Li cation formed Lewis acid complexation with the basic carbonyl oxygen site. This interaction is known also to increase barrier heights to rotation.
At the highest energy resolution QENS (at LET) revealed localized dynamics in all polymers. Amongst other fast motions, these could be a due to constrained motions of the hydrogen-bonded units. While the extracted correlation times for M1I0, M1I3 and Li_M1I0 are similar, the correlation time for Li_M1I3 was twice as long. A similar trend is observed at the intermediate resolution. Interestingly, at the lower energy resolution which probes slightly faster dynamics, the pristine polymers still show localized motions, whereas polymers with Li salt show diffusive motions. At these temporal scales, H-bond breaking is possible which may result in decoupling of Li-ions, facilitating thereby Li-ion migration, also across the hydrogen-bond connected chains (intrachain migration).
The QENS signal unfortunately contains significant coherent contribution which limits the accessible Q-range for incoherent dynamics. At Q values below 1 Å-1, we account for the coherent contribution with a linear background. However, at higher Q values, we require characterizing the ratio of incoherent to coherent via polarisation methods. We plan polarized-QENS measurement in the future to widen the Q-range for our analysis and confirm our assumptions in the lower Q-range.
