Protective NaSICON Interlayer between a Sodium–Tin Alloy Anode and Sulfide-Based Solid Electrolytes for All-Solid-State Sodium Batteries

Abstract

This paper presents a suitable combination of different sodium solid electrolytes to surpass the challenge of highly reactive cell components in sodium batteries. The focus is laid on the introduction of ceramic Na3.4Zr2Si2.4P0.6O12 serving as a protective layer for sulfide-based separator electrolytes to avoid the high reactivity with the sodium metal anode. The chemical instability of the anode|sulfide solid electrolyte interface is demonstrated by impedance spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The Na3.4Zr2Si2.4P0.6O12 disk shows chemical stability with the sodium metal anode as well as the sulfide solid electrolyte. Impedance analysis suggests an electrochemically stable interface. Electron microscopy points to a reaction at the Na3.4Zr2Si2.4P0.6O12 surface toward the sulfide solid electrolyte, which does not seem to affect the performance negatively. The results presented prove the chemical stabilization of the anode-separator interface using a Na3.4Zr2Si2.4P0.6O12 interlayer, which is an important step toward a sodium all-solid-state battery. Due to the applied pressure that is mandatory for battery cells with sulfide-based cathode composite, the use of a brittle ceramic in such cells remains challenging.

Halide and Sulfide Electrolytes in Cathode Composites for Sodium All-Solid-State Batteries and their Stability

“…NFMM has previously been studied in cells with liquid electrolyte and analyzed for its structure and electrochemical properties using XRD and impedance spectroscopy. 17 The theoretical capacities are q th = 173 mAh g −1 for NFM and q th = 178 mAh g −1 for NFMM. Electrochemical cycling as well as impedance spectroscopy measurements after cell assembly (uncycled) and after the first charge and discharge step for the cells with each CAM are presented in Figure 2.…”

“…The Na−Sn alloy electrode is a two-phase Na 3 Sn|Na 15 Sn 4 alloy. 17,21,22 The necessity to apply external pressure in all-solid-state batteries to primarily compact the cathode composite becomes critical at the anode 23 side because of the ductility of sodium metal.…”

“…Figure 7B shows the ASSB full cell (Na−Sn|Na 3 SbS 4 | NFMM:NEZC:C65) cycling performance. 17 The full cell is composed of the same sodium−tin alloy anode (Na−Sn) in contact with the Na 3 SbS 4 solid electrolyte separator as used above. The cathode composite consisted of the sodium transition metal oxide NFMM mixed with NEZC as catholyte and carbon (C65) as an electronic conductor.…”

“…Na−Sn anodes were prepared as reported under argon atmosphere in batches of 2 g by ball milling. 17 Tin powder (100 mesh, 99.999% from Fisher Scientific GmbH) was alloyed with elemental sodium (BASF SE) in a stoichiometric ratio with a 1.05 wt % excess of sodium. The media-to-sample ratio was 40:1, the media being 5 mm ZrO 2 balls in a 45 mL ball milling cup.…”

On the influence of the coherence length on the ionic conductivity in mechanochemically synthesized sodium-conducting halides, Na3−xIn1−xZrxCl6

“…16,17 However, the narrow electrochemical stability window of these sulfide solid electrolytes may result in an incompatibility against the high-voltage cathode active materials or sodium metal anode. 18–21 In contrast, oxide-based Na + conductors seem to show wider electrochemical stability. For example, the sodium solid-state battery employing Sc-substituted NASICON Na 3.4 Sc 0.4 Zr 1.6 (SiO 4 ) 2 (PO 4 ) against the cathode Na x CoO 2 can be operated at a voltage up to 4.2 V vs. Na + /Na.…”