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S C I E N T I F I C H I G H L I G H T S

9 2 H I G H L I G H T S 2 0 2 3 I

Making bilayer LLZO membranes for solid-state batteries: exploring the sintering process with X-ray diffraction

Dense porous bilayer membranes were manufactured as a separator for high-performance lithium-garnet solid-state batteries. In-situ X-ray diffraction revealed the chemical reactions that occur during high-temperature sintering, emphasising the importance of the sintering step in manufacturing solid-state electrolytes.

In battery research, the replacement of liquid lithium-ion (Li-ion) electrolytes with their non-flammable and non- toxic solid counterparts based on Li7La3Zr2O12 (LLZO) with the garnet-type structure is pursued as a compelling approach to improve the energy density, cycling stability and safety of Li-ion batteries [1]. However, commercial Li-garnet solid-state batteries still face performance challenges, especially with the Li/LLZO interface, leading to low cycling stability and Li dendrite formation (projections of metal that can build up on the lithium surface and penetrate into the solid electrolyte, eventually crossing from one electrode to the other and shorting the battery cell). To address Li dendrite formation, an innovative dense porous LLZO microstructure design has

been proposed [2]. It offers solutions to mitigate dendrite- related issues by enabling Li storage in the scaffold during deposition, reducing volume changes and minimising void formation. Additionally, a denser layer can act as a protective barrier against Li dendrites.

This work introduces an innovative tape-casting technique for LLZO membrane manufacture, involving the sequential casting of dense (8-10 µm) and porous (32-35 µm) layers. In short, bilayer dense porous LLZO membranes were prepared via the following steps: i) Preparation of slurries for porous and dense LLZO layers, ii) their sequential tape-casting, iii) pilling off the LLZO tape from the substrate, iv) de-binding, and v) sintering. As shown in Figure 70a-c, no delamination was observed between the two layers along the entire interface of the prepared membranes after sintering, showing good adhesion between the two layers. X-ray tomography images showed that the porous layer contained only open-pore channels with a porosity above 50 vol%, while the dense layer showed no porosity, was pinhole-free and had a continuous dense structure.

Preliminary experiments revealed that the addition of Li2CO3 as an additive to compensate Li losses during de-binding and sintering is paramount for obtaining an impurity-free cubic LLZO phase. Thus, it was identified

Fig. 70: a-c) Cross-section scanning electron microscopy

images of LLZO membranes after sintering. d) XRD patterns of LLZO

membrane at different stages during the manufacturing process.

e) Measured XRD map of LLZO membrane during sintering. Right

side: Temperature profile of the heat-treatment.