Influence of Functional Groups on Li-Ion Transport in Dual-Ion vs Single-Ion Conducting Comb-Branched Polymer Electrolytes

Solid polymer electrolytes, SPEs, are a safer choice for Li-ion batteries compared with highly volatile and flammable organic solvents. However, poor ionic conductivity and transference number are the biggest hurdles for their commercialization. In a quest to enhance these properties, we employed atomistic molecular dynamics (MD) simulations to investigate dual-ion and single-ion conducting (SIC) comb-branched polymers with different functional groups. The variations in the functional group structure instigate differences in segmental polymer dynamics, the ability to dissociate lithium salt, and dynamic coupling between ions and polymer, all of which collectively impact the ionic conductivity and transference number. We investigate correlations among these parameters to reveal the Li-ion transport mechanisms and examine the impact of these molecular scale characteristics on the ionic conductivity. In SICs, we found that the Li-ion dynamics is slow due to multichain ion coordination, which is absent when anions are not covalently attached to the polymer. Even with the transference number close to unity in SIC electrolytes, the sluggish ion dynamics results in lower Li-ion conductivity compared to the dual-ion conducting electrolytes. Such a trade-off behavior in SICs encourages ideas to maintain the transference number while improving the Li-ion dynamics.