Ion Transport at Polymer–Argyrodite Interfaces

Solid-state electrolytes,
particularly polymer/ceramic composite
electrolytes, are emerging as promising candidates for lithium-ion
batteries due to their high ionic conductivity and mechanical flexibility.
The interfaces that arise between the inorganic and organic materials
in these composites play a crucial role in ion transport mechanisms.
While lithium ions are proposed to diffuse across or parallel to the
interface, few studies have directly examined the quantitative impact
of these pathways on ion transport and little is known about how they
affect the overall conductivity. Here, we present an atomistic study
of lithium-ion (Li+) transport across well-defined polymer–argyrodite
interfaces. We present a force field for polymer–argyrodite
interfacial systems, and we carry out molecular dynamics and enhanced
sampling simulations of several composite systems, including poly(ethylene
oxide) (PEO)/Li6PS5Cl, hydrogenated nitrile
butadiene rubber (HNBR)/Li6PS5Cl, and poly(vinylidene
fluoride-co-hexafluoropropylene) (PVDF-HFP)/Li6PS5Cl. For the materials considered here, Li-ion
exhibits a preference for the ceramic material, as revealed by free
energy differences for Li-ion between the inorganic and the organic
polymer phase in excess of 13 kBT. The relative free energy profiles of Li-ion for different
polymeric materials exhibit similar shapes, but their magnitude depends
on the strength of interaction between the polymers and Li-ion: the
greater the interaction between the polymer and Li-ions, the smaller
the free energy difference between the inorganic and organic materials.
The influence of the interface is felt over a range of approximately
1.5 nm, after which the behavior of Li-ion in the polymer is comparable
to that in the bulk. Near the interface, Li-ion transport primarily
occurs parallel to the interfacial plane, and ion mobility is considerably
slower near the interface itself, consistent with the reduced segmental
mobility of the polymer in the vicinity of the ceramic material. These
findings provide insights into ionic complexation and transport mechanisms
in composite systems, and will help improve design of improved solid
electrolyte systems.