Development of Flexible Hybrid Electrolyte and 3D Structure for Lithium Batteries

Abstract

As part of the next generation of batteries towards efficiency and safety, all-solid-state batteries (ASSBs) are fundamental. In this context, composite solid electrolytes (CSE) have emerged as a key player towards safety, high stability, and excellent electrochemical performance. In the first part of this work, composite membranes based on PEO-LiTFSI/LLZTO system were prepared by tape-casting, where LLZTO content in the polymer matrix was varied and investigated. It could be observed that increasing the ceramic content in the composites led to a decrease in the melting point, a high agglomeration of the ceramic active filler, and an increase in ionic conductivity at room temperature. Particularly, the ionic conductivity reached a maximum in the order of 10-5 S/cm at room temperature, and 10-4 S/cm at 50 °C for a ceramic content of 50 wt.%. For lower and higher ceramic loads, however, the composite ionic conductivity is decreased, showing a composition dependence in terms of ionic conductivity. In addition, symmetrical cells with Li-metal were built for the CSEs and were electrochemically tested, where a composition dependence could also be seen. For composite membranes without ceramic addition, the interfacial resistance measured by EIS was lower as samples with higher ceramic loads. However, ceramic-free CSE resulted to be lectrochemically unstable after galvanostatic cycling. These results led to the conclusion that a compromise must be made between high ionic conductivity, electrochemical stability, and low interfacial resistance when varying ceramic content in CSE. The use of 3D structures in LLZTO CSE-based batteries can provide enhanced electrodeelectrolyte contact, improved interfacial stability, and increased ion transport efficiency. Thus, in the second part of this work, a porous-dense-porous structure based on LLZTO was fabricated by tape casting. To observe the corresponding interfaces between layers, SEM imaging was conducted, showing a marked interface between layers with high porosity. Later, the 3D structure was infused with a conductive PEO/LiTFSI solution and electrochemically tested. As a result, an ionic conductivity in order of 10-6 S/cm was obtained, showing a good correlation with CSE of similar composition. However, the fabrication of mechanically stable porous-dense-porous 3D structures remains a real challenge. This study provides insights into the fabrication of 3D structures and composite membranes based on LLZTO and LLZTO/PEO systems, respectively. It also provides a good electrochemical understanding of CSE when in contact with Li-metal, adding value to safer and high performance for the future next-generation ASSBs.