[Image above] Cross-sectional morphology of heat-treated glass-ceramic Na3PS4 and glass Na3PS4−xOx solid electrolytes. Scale bar 10 μm. Credit: Chi et al., Nature Communications (CC BY 4.0)
By Laurel Sheppard
With cost, supply, and environmental issues plaguing the lithium-ion battery industry, a viable alternative is needed, especially for long-duration grid-scale energy storage.
All-solid-state sodium batteries are a potential solution. These batteries use nonflammable solid-state electrolytes and earth-abundant sodium metal anodes, giving them an edge in both safety and cost compared to lithium-ion batteries.
Currently, the only successfully commercialized battery featuring a sodium metal anode is the well-known, high-temperature sodium–sulfur battery. However, with a required working temperature above 300°C, both the sodium anode and sulfur cathode are liquids, which significantly increases the operational cost and decreases safety due to potential catastrophic failure of the thin ceramic solid-state electrolyte.
A sodium battery that operates at ambient temperatures (under 100°C) with a solid sodium metal anode would allow for safer usage in a broader range of applications. However, when the sodium metal anode is in a solid state, the solid electrolyte now must be not only resistant to direct chemical and electrochemical reactions with sodium but also resistant to solid metallic sodium dendrite penetration.
Researchers have investigated different materials as solid electrolytes for ambient-temperature all-solid-state sodium batteries, including ceramics and glass-ceramics. However, these inorganic electrolytes come with pros and cons.
- Ceramics. Although mechanical properties are attractive, long sintering times and high temperatures above 1,500°C are required. Poor wettability with sodium metal and short circuiting are other limitations.
- Glass-ceramics. These electrolytes can mitigate dendrite formation and growth. However, they become unstable when they encounter sodium metal. As such, sodium alloys like sodium-tin are often used as the anode, which increases anode voltage and decreases the energy density.
With these challenges, researchers are still searching for a solid electrolyte that fulfills the requirements of being mechanically and chemically stable, low cost, and easily fabricated.
In a recent open-access paper, researchers led by University of Houston and Iowa State University propose a new solution: a homogeneous oxysulfide glass electrolyte based on Na3PS4−xOx doped with a small amount of oxygen, up to 15 mol.% (x = 0.60).
The genesis for this study traces to an earlier paper published in 2020 by the Iowa State co-authors, ACerS Fellow Steve Martin and ACerS member Steven Kmiec. In that paper, they explain that Na3PS4 is a promising solid-state electrolyte due to its high ionic conductivity, but it is known to be unstable against sodium metal. However, through experimentation, they discovered that adding oxygen to Na3PS4 to create Na3PS3O will significantly slow the reaction with sodium metal.
In the new paper, they along with the University of Houston researchers and collaborators at Rice University, Purdue University, and University of California, Irvine explore further the potential of this oxysulfide glass system.
They prepared the glass electrolytes via high-energy planetary ball milling of simple precursors. The resulting electrolytes were fully dense and homogeneous with robust mechanical properties. Adding oxygen boosted the mechanical strength.
For Na3PS3.4O0.6 electrolytes, the Young’s modulus and hardness were approximately 21 GPa and 1.0 GPa, respectively. These values are the highest among the Na3PS4−xOx series and are even higher than those reported for hot-pressed sulfide-based lithium-ion and sodium-ion electrolytes.
In addition to these impressive properties, the researchers state the critical current density—up to 2.3 mA cm−2—is equivalent to the state-of-the-art critical current density value of lithium-based sulfide solid electrolytes.
The Na3PS4−xOx composition also enabled reversible sodium plating and stripping, which is difficult to achieve using a pure sulfide electrolyte. The researchers attribute this ability to a self-passivating interphase at the interface between the sodium metal anode and electrolyte.
When the researchers designed and tested batteries featuring tri-layer electrolytes composed of these oxysulfide glasses (Na3PS3.85O0.15 as the inside layer and Na3PS3.4O0.6 as the outside layers), they were able to cycle the batteries for up to 500 hours without short circuiting. Plus, the coulombic efficiency and capacity retention were much higher than those for sodium–sulfur batteries using oxide or polymer solid-state electrolytes.
“These new oxysulfide [solid-state electrolytes], as well as the tri-layer composite [solid-state electrolytes] that they enable, could pave the way for the development of new glass electrolytes for high-energy, safe, low-cost, and long-cycle-life solid-state batteries in general, and all-solid-state [sodium–sulfur] batteries for energy storage devices in particular,” they conclude.
The open-access paper, published in Nature Communications, is “An electrochemically stable homogeneous glassy electrolyte formed at room temperature for all-solid-state sodium batteries” (DOI: 10.1038/s41467-022-30517-y).