Effect of Plasticizers on Performance in Single-Ion Conducting Polymer Electrolytes: Implications for Lithium-Ion Batteries

Plasticizers play a crucial role in enhancing cation
transport
in single-ion conducting polymer electrolytes. However, a fundamental
understanding of how different plasticizers facilitate ionic conductivity
remains incomplete. To elucidate the molecular mechanisms by which
plasticizers promote lithium-ion transport, equilibrium molecular
dynamics simulations were performed to investigate lithium-ion transport
behavior in modified polyethylene terephthalate (mPET) electrolytes
plasticized with varying concentrations of fluoroethylene carbonate
(FEC) and propylene carbonate (PC). Our simulation results show that
both systems exhibit comparable lithium-ion diffusion coefficients
and ionic conductivities at plasticizer concentrations below 40 wt
%. In contrast, at concentrations above 40 wt %, the PC-plasticized
system displays higher lithium-ion diffusion coefficients and ionic
conductivities than the FEC-plasticized system. These observations
can be attributed to the synergistic effects of plasticizer electrostatic
properties and polymer chain flexibility. Specifically, quantitative
comparisons of electrostatic surface potentials among FEC and PC indicate
that the electron distribution of plasticizers governs their ability
to compete with the polymer matrix for lithium-ion coordination, thereby
determining the local coordination environment. In addition, radius
of gyration analysis reveals that mPET chains in PC-plasticized systems
exhibit greater flexibility, providing more continuous pathways for
lithium-ion hopping between polymer chains. This enhanced flexibility
is consistent with the reduced mean lifetimes of Li–O coordination
pairs observed at plasticizer concentrations above 40 wt %. Overall,
this work provides molecular-level insights that can guide the rational
design of plasticizers to improve ionic conductivity in single-ion
conducting polymer electrolytes.