Elucidating the Effect of Dry-Room Atmospheres on the Performance of Sulfidic Sheet-Type Separators for All-Solid-State Batteries
Authors: Kutsch Tobias , Schuster Robin , Töpper Hans-Christoph , Gasteiger Hubert A. 
Published: September 2023

Electrifying the automotive sector requires energy storage systems with high energy and power density, intrinsic safety, and long durability.[1] Lithium-based all-solid-state batteries (ASSBs) are considered promising next-generation energy storage devices, as they possibly meet these requirements by replacing organic liquid electrolytes with non-flammable solid electrolytes (SEs) and by enabling the application of novel high energy density anode concepts.[2, 3]

Among the SE material classes being pursued for use in ASSBs, sulfidic systems are considered particularly advantageous, as they possess high ionic conductivities and low Young’s moduli, enabling easier processing and densification compared to oxidic systems.[4] However, a rarely addressed key bottleneck to commercializing ASSBs with sulfidic SEs is the requirement on the environmental conditions during handling, processing, and assembly of the battery components, which will be dictated by the instability of sulfidic SEs towards humidity.[5] On the laboratory scale, this issue has been avoided by handling the sulfidic ASSB materials, components, and cells in argon-filled gloveboxes, but for the commercial production of ASSBs their manufacturing would preferably be done in a dry-room atmosphere.[6] However, very few studies exist that examine whether the dew points of typical dry-rooms are sufficient to handle and/or store ASSB components based on sulfidic SEs.[7,8]

In this contribution, we will examine the dry-room compatibility of scalable sulfidic composite (Li6PS5Cl/HNBR) sheet-type separators,[9] investigating the influence of the exposure time to a dry-room atmosphere with a dew point of -48°C on the stability of the SE and on its electrochemical properties. Using X-ray diffraction (XRD) combined with Rietveld refinement and scanning electron microscopy (SEM), the influence on the morphology and the formation of degradation products upon reaction with the small concentration of water in a dry-room will be investigated. The results show that only long exposure times (> 96 h) lead to a significant hydrolysis-induced change in the XRD diffractograms, which indicates the formation of degradation products such as Li2S, LiCl, and Li3PO4.

By means of potential electrochemical impedance spectroscopy (PEIS), the impact of different exposure times on the Li+-ion conductivity of the sheet-type Li6PS5Cl/HNBR separators and the activation energy for the Li+-ion transport are investigated (cf. figure 1). While short exposure times (< 12 h) have a minor influence on the Li+-ion conductivity (lowering it from 0.3 mS/cm to 0.25 mS/cm) and a negligible effect on the activation energy, longer exposure times (> 48 h) lead to a ≈ 10-fold loss of ionic conductivity (to ≈ 0.03 mS/cm) and a drastic increase in activation energy.

Besides measuring the loss of ionic conductivity, the separator sheets are also used for assembling LiNi0.6Co0.2Mn0.2O2|LiIn half-cells, which are investigated using galvanostatic cycling. An analysis of the capacity profiles shows that an increased exposure of the separators to the dry-room atmosphere leads to a significant IR-drop, to lower initial discharge capacity, and enhanced capacity fading over several charge/discharge cycles. Further insights into these degradation phenomena will be gained by analyzing the dQ/dV plots, the mean (dis)charge voltage, and the full-cell impedance of the half-cells.

Based on our findings, it can be concluded that moderate exposure times in a dry-room of up to 12 h have only a negligible effect on the chemical integrity and performance of sulfidic Li6PS5Cl/HNBR separators sheets. This opens the route of defining viable storage and processing times of sulfidic ASSBs cell components in industrial dry-room environments.

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Figure 1