New Publication in the HIPOBAT project

Title
Impurities in Na₂S Precursor and Their Effect on the Synthesis of W-Substituted Na₃PS₄: Enabling 20 mS cm⁻¹ Thiophosphate Electrolytes for Sodium Solid-State Batteries

Link
https://doi.org/10.1002/aenm.202503047

Summary

Sodium solid-state batteries are intensively researched, expecting a resource-uncritical alternative to their lithium counterparts. As in the case of lithium, sulfide-type electrolytes show promising ionic conductivities 𝝈, and Na₃PS₄-type solid electrolytes are intensively investigated. Aliovalent substitution of P⁵⁺ by W⁶⁺ is shown to achieve a sodium ion conductivity 𝝈(Na⁺) beyond 10 mS cm⁻¹, rendering them good candidates for cathode composites.

Yet, incorporating WS₄²⁻ into the crystal lattice of Na₃PS₄ is deemed challenging, and Na_3−xP_1−xW_xS₄ electrolytes suffer from WS₂ residue after synthesis. In this work, impurities in the precursor Na₂S are identified and the detrimental influence of SO_x groups in Na₂S on the synthesis of Na₃PS₄ and Na_3−xP_1−xW_xS₄ is demonstrated. The behavior of oxygen as impurity during synthesis is pinpointed, and complete incorporation of tungsten up to x ≈ 0.25 in Na_3−xP_1−xW_xS₄ by purified Na₂S is achieved, realizing up to 𝝈(Na⁺) = 26.4 mS cm⁻¹ at room temperature.

Interview with the lead author and researcher in the HIPOBAT project, Felix Schnaubelt, JLU Giessen

1. Your work comprehensively describes the purification of commercially available Na₂S precursor for the synthesis of thiophosphate electrolytes. What was the initial motivation to look into this in more detail?

The goal of HiPoBat is to investigate and develop high power sodium batteries and Na_3−xP_1−xW_xS₄ fulfills the σ_ion >10 mS cm⁻¹ requirement for high power solid-state batteries with sufficiently thick cathode composites. Thus, I attempted to synthesize Na_2.9P_0.9W_0.1S₄ but could not obtain the desired phase. The presence of oxygen-containing phases in the product mixture quickly led to the assumption of contaminations in the precursor(s) since the synthesis was conducted in an inert atmosphere. Analyzing the precursors revealed significant Na₂SO_x contamination in the only commercially available anhydrous Na₂S at that time. This was the reason I investigated purification and synthesis methods to obtain clean Na₂S. I quickly decided that the H₂ purification route was most suitable for our synthesis capabilities in our laboratory.

2. What was the most difficult experimental obstacle to overcome to receive highly pure Na₂S (e.g., recontamination)?

The most difficult obstacle was to find the right H₂ concentration and reaction temperature. I started with a relatively low H₂ content of 5 % (95 % Ar) and a low temperature of 450 °C, like the literature suggested, but found that these conditions do not work in our oven setup. I gradually increased the temperature and H₂ content until pure Na₂S was obtained. In our setup, I settled with a pure H₂ atmosphere and a temperature of 650 °C. Luckly, recontamination does not seem to occur when the purified Na₂S has very brief atmosphere contact. We could not detect any Na₂SO_x species with XPS, a relatively sensitive method for surface contaminations.

3. On what scale (weight) did you carry out the purification steps? Did you have to construct special sealed containers to move the material or special accessories for the furnace?

I conduct the H₂ treatment with a batch size of 10-12 g Na₂S. I did not design any special containers and found creative use of existing equipment. Any air-tight container that can be evacuated is sufficient to transfer the purified Na₂S to a glovebox.

4. In the second part of your publication, it is described how the high purity of Na₂S enables the synthesis phase pure P⁵⁺ and W⁶⁺ tungsten substituted Na₃PS₄-type solid electrolytes with very high Na⁺ conductivity of 20 mS cm^-1. Have you tested this electrolyte already in half or full electrochemical cells? What are the remaining challenges in terms of electrochemical stability and compatibility?

Anodic and cathodic stability remain a great challenge. We conducted preliminary coulometric titration time analysis (CTTA) measurements and observed that tungsten substitution decreases stability against Na metal drastically. Prior work from our group demonstrated severe degradation in cathode composites containing sulfide-based electrolytes as well. Increasing the stability of Na_3−xP_1−xW_xS₄ electrolytes against active materials is something we are actively looking into.