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Empa, UNIGE team develop prototype solid-state sodium battery; focus on improving the solid-solid interface

Researchers at Empa and the University of Geneva (UNIGE) have developed a prototype of a novel solid-state sodium battery with the potential to store extra energy and with improved safety.

With a NaCrO2 cathode, closo-borate solid electrolyte and metallic sodium anode, the cell demonstrated reversible and stable cycling with a capacity of 85 mAh g-1 at C/20 and 80 mAh g-1 at C/5 with more than 90% capacity retention after 20 cycles at C/20 and 85% after 250 cycles at C/5. A paper on their work is published in the RSC journal Energy & Environmental Science.

Rechargeable all-solid-state batteries promise higher energy density and improved operational safety. While several classes of solid-state electrolytes with high ionic conductivity have been identified over recent years, compatibility issues between electrodes and the solid-state electrolyte render their integration into a full battery cell challenging.

… Here we report on the first successful realization of a stable 3V all-solid-state sodium-ion battery using a closo-borate based electrolyte, in combination with a sodium metal anode and a NaCrO2 based cathode. We show that by improving the solid-solid interface between the electrolyte and the cathode, Na2(B12H12)0.5(B10H10)0. can sustain stable and efficient cycling up to a voltage of 3.25 V with a reversible capacity of 85 mAh g-1 at a rate of C/20. We also demonstrate excellent capacity retention of 85% after 250 cycles at C/5 and further discuss the stability of the material towards higher voltages.

—Duchêne et al.

Duchêne et al. Click to enlarge.

The closo-borate sodium superionic conductor—Na2(B12H12)0.5(B10H10)0.5—combines high Na+ conductivity of 0.9 mS cm-1 at 20 °C with an excellent thermal stability up to at least 300 °C, and a large electrochemical stability window of 3 V. Furthermore, since the closo-borate is an inorganic conductor, it removes the risk of the battery catching fire while recharging.

The difficulty was establishing close contact between the battery’s three layers.

—Léo Duchêne, first author

The researchers dissolved part of the battery electrolyte in a solvent before adding the sodium chromium oxide powder. Once the solvent had evaporated, they stacked the cathode powder composite with the electrolyte and anode, compressing the various layers to form the battery.

The team then tested the battery. The closo-borate solid electrolyte withstood three volts, whereas many solid electrolytes previously studied are damaged at the same voltage, said Arndt Remhof, a re­searcher at Empa and leader of the project, which is supported by the Swiss National Science Foundation (SNSF) and the Swiss Competence Center for Energy Research on Heat and Electricity Storage (SCCER-HaE).

The cells showed faster fading when cycled to voltages outside the stability window of the electrolyte due to electrolyte decomposition. Room temperature cycling was not possible because of dendrite formation.

Our results thus confirm the change of paradigm in designing solid-state batteries that is the shift in focus from conductivity improvement to interface optimization, and demonstrate that closo-borate electrolytes can play a significant role in this new development strategy for high performance all-solid-state batteries.

Duchêne et al.


  • L Duchêne, RS Kühnel, E Stilp, E Cuervo-Reyes, A Remhof, H Hagemann, C Battaglia (2017) “A stable 3 V all-solid-state sodium-ion battery based on a closo-borate electrolyte,” Energy & Environmental Science doi: 10.1039/C7EE02420G


And Bri

Years ago, this battery article would have generated many hopes but now bloggers learned that they are late and would not improve batteries. I said to sell your big gasguzzlers and buy a small more efficient car and drive it slow. This seam out of reach simple science for many here that always recommend to buy a limp tesla model x at more that 100 000$ paid in part by taxpayers where gasoline cars are paid by normal consumers and are money making for car manufacturers.


If room-temperature cycling wasn't possible due to dendrites, it begs the question what temperature it WAS cycled at.  Given the mention of 300°C, this suggests that it was above the melting point of sodium.  This makes it not a strictly solid-state battery.

Dendrite formation would only be an issue on charging, so if the battery can be discharged in the solid state it can self-heat or at least provide the energy to get into the liquid state for charging.  This makes it somewhat like the Zebra battery but not requiring such high temperatures.



Years ago this article would not have been posted on this board. In the good ol' days everything was about Atkinson cycles, compression ratios, planetary gears, and CVTs.
The mere fact that so many of the articles posted today are about EVs is a sign of the impact of those government dollars invested years ago.

By the way a disproportionate share of those government dollars are provided by me and I'm happy to see them invested in my future rather than my past though the dollars my government is investing in FFs is still dramatically higher than what is invested in our future.

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