The Effect of Morphology on Ion Transport and Electrochemical Performance in Polymer-Based Composite Electrolytes*

Solid polymer electrolytes are promising next generation energy storage materials and they are compatible with traditional roll-to-roll manufacturing methods and stack pressure requirements. Inorganic solid electrolytes such as oxides, sulfides, and halides, while exhibiting much higher room temperature ionic conductivity than common polymer electrolytes, all require high stack pressure to maintain contact at the electrode interfaces. Thus, composite electrolytes combining the high ionic conductivity of inorganic electrolytes and the processibility and ability to form conformal contact at the interfaces are promising to fulfill all the requirements for advanced battery technologies. In this presentation, I discuss the development of composite electrolytes with different morphologies and their effect on ion transport and performance.

In the first part, we develop a composite electrolyte with a three-dimensionally (3D) interconnected lithium aluminum titanium phosphate (LATP) ceramic electrolyte framework, and a backfilled polymer electrolyte serving as a processibility phase to enhance mechanical flexibility and ensure conformal contact at the electrode-electrolyte interfaces. We demonstrate that the composite exhibits improved transference number compared to neat polymer and enhanced dendrite resistance compared to dense ceramics. We also discuss the interface resistance within the 3D composite. By replacing the dual-ion-conducting polymer phase with a single-ion-conducting polymer phase, we elucidate factors that control the limiting current density of the composite electrolyte.

In the second part, we discuss the effect of particle size and spatial distribution on the ion transport barrier in a model composite electrolyte consisting of Li_0.34La_0.56TiO₃ (LLTO) ceramic and a single-ion-conducting polymer electrolyte. Broadband dielectric spectroscopy (BDS) reveals that the total ionic conductivity in these polymer-ceramic composites increases with the addition of LLTO nanorods, compared with neat polymer electrolyte. For comparison, composites made from a commercial source of LLTO with micrometer size particles do not show enhancement in conductivity. The spatial distribution of LLTO particles within the polymer matrix also plays a role in ion transport of the composite. These investigations shed light on how to design composite electrolytes to maximize favorable ion transport paths and minimize barriers.