Ion\nTransport at Polymer–Argyrodite Interfaces

Solid-state electrolytes,\nparticularly polymer/ceramic composite\nelectrolytes, are emerging as promising candidates for lithium-ion\nbatteries due to their high ionic conductivity and mechanical flexibility.\nThe interfaces that arise between the inorganic and organic materials\nin these composites play a crucial role in ion transport mechanisms.\nWhile lithium ions are proposed to diffuse across or parallel to the\ninterface, few studies have directly examined the quantitative impact\nof these pathways on ion transport and little is known about how they\naffect the overall conductivity. Here, we present an atomistic study\nof lithium-ion (Li<sup>+</sup>) transport across well-defined polymer–argyrodite\ninterfaces. We present a force field for polymer–argyrodite\ninterfacial systems, and we carry out molecular dynamics and enhanced\nsampling simulations of several composite systems, including poly(ethylene\noxide) (PEO)/Li<sub>6</sub>PS<sub>5</sub>Cl, hydrogenated nitrile\nbutadiene rubber (HNBR)/Li<sub>6</sub>PS<sub>5</sub>Cl, and poly(vinylidene\nfluoride-<i>co</i>-hexafluoropropylene) (PVDF-HFP)/Li<sub>6</sub>PS<sub>5</sub>Cl. For the materials considered here, Li-ion\nexhibits a preference for the ceramic material, as revealed by free\nenergy differences for Li-ion between the inorganic and the organic\npolymer phase in excess of 13 <i>k</i><sub>B</sub><i>T</i>. The relative free energy profiles of Li-ion for different\npolymeric materials exhibit similar shapes, but their magnitude depends\non the strength of interaction between the polymers and Li-ion: the\ngreater the interaction between the polymer and Li-ions, the smaller\nthe free energy difference between the inorganic and organic materials.\nThe influence of the interface is felt over a range of approximately\n1.5 nm, after which the behavior of Li-ion in the polymer is comparable\nto that in the bulk. Near the interface, Li-ion transport primarily\noccurs parallel to the interfacial plane, and ion mobility is considerably\nslower near the interface itself, consistent with the reduced segmental\nmobility of the polymer in the vicinity of the ceramic material. These\nfindings provide insights into ionic complexation and transport mechanisms\nin composite systems, and will help improve design of improved solid\nelectrolyte systems.