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.

References

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