Towards a Molecular Understanding of the Interfacial Li-Ion Dynamics in Composite Solid-State Electrolytes Under High Polymer Confinement—the Case of Polyethylene Oxide—Li₇La₃Zr₂O₁₂ Composites

Composite solid-state electrolytes (CSSEs) comprising a conductive, flexible polymer matrix embedding ceramic filler particles have emerged as a promising option for all-solid-state Li-ion batteries. Understanding the interfacial diffusion of Li⁺ as a function of filler content is crucial to formulating high-performance and cost-efficient CSSE. By considering as a case study a CSSE constituted by ion-conducting Ga³⁺ doped-Li₇La₃Zr₂O₁₂ (LLZO) garnet fillers embedded within a poly (ethylene oxide) and lithium bis(trifluoromethanesulfonyl) imide polymer matrix (PEO(LiTFSI)), we study the interfacial distribution and dynamics of Li⁺ at conditions of high polymer phase confinement in fully amorphous PEO above the melting point, <i>T</i>_<i>m</i>. Such confinement scenario aims to simulate the interstitial space between filler particles in close proximity, which is tantamount to the percolation conditions at which conductivity enhancement has been experimentally observed. By combining classical umbrella sampling calculations, molecular dynamics simulations, and hybrid Monte Carlo methods, we show that the free energy profile associated with the extraction of a Li⁺ ion from the garnet surface is site-dependent and highly asymmetric, suggesting a strong thermodynamic drive towards Li⁺ adsorption onto the garnet phase. Adsorbed Li⁺ is stabilized by exposed oxygens on the garnet surface and oxygens from bound PEO. Only beyond 0.35 nm from the garnet surface, the polymer coordination shell reaches its typical bulk density of 5 oxygens per Li⁺, leading to a slight decrease in the free energy. We also find that this thermodynamic drive towards Li⁺ uptake is counteracted by a kinetic barrier: free polymer chains in the bulk are significantly more mobile than garnet–bound chains. Hence, they can rapidly adapt to coordinate and stabilize incoming Li⁺ ions, dramatically reducing the probability that a Li⁺ion approaches within 0.35 nm of the garnet surface. Under high polymer confinement (i.e., high filler volume fraction), the overlapping of bound polymer shells from opposite surfaces reduces the availability of free chains, leading to a significant decrease in Li⁺ diffusivity within the interstitial space.<br/><br/>We put forward that understanding Li⁺ diffusion in LLZO:PEO (LiTFSI) CSSEs above <i>T</i>_<i>m</i>, and at conditions of high polymer confinement provides valuable insights to interpret the variation of ionic conductivity with volume fraction <i>below</i> <i>T</i>_<i>m</i><i> </i>(at typical operating conditions). This is because filler particles are surrounded by a layer of amorphous polymer even at room temperature (RT), as filler particles disrupt structural regularity. Based on this premise, we are able to qualitatively describe the experimental variation of RT conductivity with filler content reported in recent experimental work.