Solid-state lithium-ion batteries are a promising type of batteries for future applications. While replacing flammable liquid electrolytes with solid alternatives gives advantage to improved safety, the higher density of these solids compared to the currently used liquids is detrimental to the gravimetric energy density. Therefore, for sufficient battery cell performance thin and dense electrolyte layers of highly Li⁺ conductive material are needed. Currently, materials investigated for solid electrolyte application include ceramics, glasses and polymers.

Ceramic Li₇La₃Zr₂O₁₂ (LLZO) with cubic garnet like crystal structure has emerged as one of the most promising materials as electrolyte. Its advantages include high Li⁺ conductivity and chemical stability vs. lithium metal anodes. The material can be synthesised by solid state reactions as well as various wet chemistry routes. The product usually obtained from all methods is a powder. Processing this powder into a thin electrolyte layer that is suitable for battery production proofs to be difficult. Physical vapour deposition methods (e.g. pulsed laser deposition or sputtering) have been used to create thin film layers of LLZO, but these methods are limited and not suitable for large scale production. Easier processing techniques (e.g. tape casting and extrusion) are found for solid polymer electrolytes, but so far ionic conductivities of solid polymer electrolytes fall short of ceramic and glass electrolytes.

The objective of this study is to combine the advantages of ceramic and polymer electrolytes by fabrication of a hybrid solid state electrolyte. The hybrid mainly consists of LLZO and employs a polymer electrolyte as a binder. The aim is to produce a hybrid electrolyte for easier processing of solid-state batteries. Therefore, LLZO powder obtained from a facile wet chemistry synthesis route is mixed with solid polymer electrolyte. Thin hybrid electrolyte membranes are obtained using the tape casting method. Ceramic content of the hybrid electrolyte is varied up to 80-wt %.

Both LLZO powders and the hybrid electrolytes are characterized in regard to structure and electrochemical performance. Phase composition and crystal structure of the prepared powder are checked using X-ray diffraction (XRD). Morphology of powders and microstructure of the hybrid electrolytes are investigated with scanning electron microscopy (SEM). Thermal behavior of the hybrid electrolytes is examined using differential scanning calorimetry (DSC). Electrochemical impedance spectroscopy (EIS) is used to evaluate the ionic conductivity and special attention is paid to the ceramic polymer interaction. Its influence on lithium ion transport is discussed in detail.

We show that the combination of LLZO with polymer greatly simplifies the production of thin solid state electrolytes for lithium-ion batteries with only small losses in conductivity.