Impact of Structure and Interfaces on Li Ion Transport in Composite Solid Electrolytes

Increasing the Li ion battery energy density is a major challenge, which needs to be addressed in order to reduce their size and weight. Switching to metallic Li anodes would provide such a significant increase in energy density. However, this comes alongside many challenges, among them stability towards the electrolyte or Li dendrite growth. Solid electrolytes (SEs) are a promising replacement for state-of-the-art liquid electrolytes as they are electrochemically more stable and better at preventing Li dendrite growth. For high performance all-solid-state-batteries (ASSBs) the electrode-electrolyte interfaces and electrolyte ionic conductivity are crucial to reach high C-rates.<br/>There are three major classes of SEs, consisting of inorganic ceramics, polymers, or composites of both. Ceramics generally feature high electrochemical stability, high voltage operation, and good ionic conductivity but they are brittle, difficult to manufacture and expensive. Polymers are cheap, easy to manufacture and compatible with slurry-based processes, and form and maintain SE-electrode interfaces well. Yet, their ionic conductivity at room temperature is limited. By combining both polymers and ceramics in composite-solid-electrolytes (CSEs), it is possible to take advantage of both material classes while maintaining the scalability of the fabrication process. However, there is a lack of understanding of lithium ion transport in CSEs.<br/>We investigate the impact of the polymer ceramic structure and the properties of the polymers, ceramic, and their interfaces on transport in CSEs using 3D simulations on realistic CSE structures. We obtain relastic 3D x-ray tomography data on different CSEs made from Al-doped LLZO (LLZO) particles dispersed in a PEO (LiTFSI) matrix. We validate finite element simulations of the ionic conducitivity of these 3D structures with experimentally obtained ionic conductivity data. With the simulation tools and the realistic structures we can gain insight into the relative importance of structure and material properties and their impact on heterogeneity of ionic transport with respect to the properties of the CSE heterostructure. This understanding is crucial as it shows that there can be quite significant local differences in ionic current that will affect cycle life or lithium plating behavior. Our findings emphasize the importance of interface treatments to minimize interface resistances in CSE heterostructures. We also highlight that properties such as mechanical stability and integrity or ionic current heterogeneity need to be considered for performance assessment besides the ionic conductivity.