Slurry-Based Processing of Solid Electrolytes – a Comparative Binder Study
Authors: Riphaus Nathalie , Strobl Philipp , Sedlmaier Stefan J. , Schnell Joscha , Gasteiger Hubert A. , Stiaszny Barbara 
Published: February 2020

Battery electric vehicles (BEV) are constantly gaining popularity. Effective penetration of the mass market, how­ever, requires a significant improvement in energy density whilst keeping costs reasonable. Volumetric energy den­sities exceeding 800 Wh∙l-1 are targeted at cell level. In contrast, only up to 400 Wh∙l-1 are reached in current pris­matic cells of BEVs, based on established Li-ion battery (LIB) technologies.1

The all-solid-state battery (ASSB) has attracted growing interest as a promising battery system to achieve higher energy density.2 This is based on the assumption that solid electrolytes (SE) can enable the use of lithium metal anodes and high-voltage cathode materials.3 Apart from that, though, most bulk-type ASSBs reported to date are based on thick SE layers and cathodes with low cathode active material loadings, resulting in energy densities below 50 Wh∙l-1.4,5

Indeed, for being competitive with conventional LIBs, the SE layer thickness has to be lower than 100 µm.6 In addi­tion, a certain mechanical stability is essential for scalable processing.7 Evolution from pellet-type ASSBs, prepared by powder compression, to sheet-type ASSBs, which are based on slurry-coating processes, is therefore essential.4

To date, literature related to solution-based processing of ASSBs is scarce, and even less is reported on fabrication of thin, free-standing SE layers. Nam et al. applied a poly­mer scaffold to obtain bendable SE layers with a thickness of ~70 µm.8 They tested two sulfide-based SEs, crystalline Li10GeP2S12 (LGPS) and glass-ceramic Li3PS4 (LPS). The latter has also been used in a study by Lee at al.,9 who investigated different solvents and binders.
In general, slurry-based processing of SEs demands for considered choice of solvent and binder. Additional chal­lenges arise from possible shrinkage and warpage during drying and densification of the SE sheet.6 Requirements on the binder include solubility in the solvent, non-reactivity with the SE, good adhesive strength and minimal effect on the total resistivity of the SE layer.6,9 Polymeric binders with polar functional groups, which can interact with the SE, have been reported as favorable.9 This however con­trasts with the requirements on the solvent. Due to the high reactivity of sulfide-based SEs with polar protic solvents, less polar solvents such as toluene have to be used, limi­ting the number of applicable binders.10

In the present study, we applied a slurry-coating process to fabricate bendable thin and free-standing SE sheets. The thiostannate analogue of LGPS, Li10SnP2S12 (LSPS), that shows a comparably high ionic conductivity of 2 mS/cm,11 and toluene were selected as the model electrolyte and sol­vent, respectively. We systematically investigated a wide range of binders with regard to their impact on processabi­lity, flexibility, density and resistivity of the resulting SE layer. We will show clear differences between the binders. For those binders performing well, homogeneous distribu­tion between the SE particles, chemical compatibility with LSPS as well as excellent flexibility of the compressed SE sheets will be shown. Compared to recent studies,7–10 this contribution demonstrates a significant expansion of tested binders and analytical tests, addressing requirements for scalable roll-to-roll processing of ASSBs.

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Figure 1