From Powder to Sheets – a Comparative Study for Solution-Cast Solid Electrolyte/Binder-Sheets As Separators in All-Solid-State Batteries

Abstract

Sulfidic all-solid-state batteries (ASSBs) are considered as one of the next-generation energy storage systems.[1,2] Most of the previous research studies have investigated ASSBs that are based on a pellet-type separators, i.e., a highly compressed solid electrolyte (SE) powder, with thicknesses of several hundreds of microns.[3] However, their applicability is limited to small electrode area single-layer lab-scale cells, as large-format cells would require separator pellets with a large area that are difficult to produce. Furthermore, in order to be competitive or superior to state-of-the-art lithium-ion batteries based on liquid electrolytes, the thickness of the solid electrolyte separator in ASSBs should be less than 100 μm.[4] Consequently, a key for the industrialization of large-format and high energy density ASSBs is the development of sheet-type cell components. Up to date, only a few publications focus on sheet-type SE separators, investigating appropriate solvents and binders for wet-processed SE/binder composites.[4-7] In the present study, we report on the preparation of composite sheet-type separators for ASSBs by a slurry-based process, investigating three different sulfidic solid electrolyte systems (Li6PS5Cl, Li7P3S11 and Li10SnP2S12) in combination with different volume fractions of hydrogenated nitrile butadiene rubber (HNBR, φHNBR = 1.7 – 16.0 vol.-%) binder.By means of the analysis of top-view and cross-sectional scanning electron microscopy (SEM) images of the prepared sheet-type separators, combined with energy dispersive X-ray spectroscopy (EDX), we characterize the influence of the slurry-coating process conditions as well as of the binder volume fraction on their morphology. Densification of the obtained separator sheets is easily possible by uniaxial compression at room temperature, resulting in residual pore volume fractions of ≈ 3 – 12 %, depending on the respective solid electrolyte. While increasing fabrication pressures lead to a reduction of void volume and an increasing contacting between the solid electrolyte particles, very high fabrication pressures (> 200 MPa) lead to a fusing of the SE particles and to nearly pore-free separator sheets.Besides the micromorphology and porosity of the SE/HNBR separator sheets, we investigate the dependence of binder content and processing conditions on their ionic conductivity by means of potentiostatic electrochemical impedance spectroscopy (PEIS) measurements that are performed using a novel, in-house designed cell setup. As one would expect, the Li+-ion conductivity of the SE/HNBR separator sheets decreases with increasing binder volume fraction (see Figure 1a). Interestingly, however, the relationship between Li+-ion conductivity and binder volume fraction (φ HNBR) is essentially identical for the three different sulfidic electrolytes that are examined in our study (see Figure 1b). To obtain mechanically stable and free-standing SE/HNBR separator sheets, an HNBR volume fraction of approximately 8 vol.-% is required for all three SEs, yielding in a separator sheet conductivity of ≈ 0.5 mS/cm at 70 MPa, which is ≈ 3-fold lower than that of the pure, binder-free SE powder conductivity measured at the same conditions. Nevertheless, by considering the currently achievable thicknesses of these solution-cast separator sheets of ≈ 50 µm, their overall areal resistance (in unit of Ω·cm2) is equal or even lower than that of the typically substantially thicker pellet-type separators, so that these separator-sheets can be used for the assembly of large-format cells with competitive separator resistances.[1] J. Janek and W. G. Zeier, Nature Energy, 1, 16141 (2016).[2] W. Zhang, D. A. Weber, H. Weigand, T. Arlt, I. Manke, D. Schroder, R. Koerver, T. Leichtweiss, P. Hartmann, W. G. Zeier and J. Janek, ACS Appl Mater Interfaces, 9, 17835 (2017).[3] M. R. Busche, D. A. Weber, Y. Schneider, C. Dietrich, S. Wenzel, T. Leichtweiss, D. Schröder, W. Zhang, H. Weigand, D. Walter, S. J. Sedlmaier, D. Houtarde, L. F. Nazar and J. Janek, Chemistry of Materials, 28, 6152 (2016).[4] N. Riphaus, P. Strobl, B. Stiaszny, T. Zinkevich, M. Yavuz, J. Schnell, S. Indris, H. A. Gasteiger and S. J. Sedlmaier, Journal of the Electrochemical Society, 165, A3993 (2018).[5] K. Lee, S. Kim, J. Park, S. H. Park, A. Coskun, D. S. Jung, W. Cho and J. W. Choi, Journal of the Electrochemical Society, 164, A2075 (2017).[6] A. Sakuda, K. Kuratani, M. Yamamoto, M. Takahashi, T. Takeuchi and H. Kobayashi, Journal of the Electrochemical Society, 164, A2474 (2017).[7] F. Shen, M. B. Dixit, W. Zaman, N. Hortance, B. Rogers and K. B. Hatzell, Journal of the Electrochemical Society, 166, A3182 (2019). Acknowledgements: This work was carried out as part of the research project “Industrialisierbarkeit Festkörperelektrolyte”, funded by the Bavarian Ministry of Economic Affairs, Regional Development and Energy. This work was also supported by the BMW Group. Figure 1