Sodium-ion batteries could be a cheaper and more sustainable alternative to the commonly used lithium-ion batteries. All-solid-state batteries could solve another problem of standard Li-ion batteries: the hazards caused by flammable liquid electrolytes.

Arndt Remhof, Empa, Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, and colleagues have combined these two approaches towards safer high-energy-density batteries and have developed all-solid-state sodium-ion batteries. The team used an inorganic, closo-borate-based electrolyte with a high sodium ion conductivity and thermal stability up to 300 °C. They combined this electrolyte with a solid metallic sodium anode and a mixed sodium chromium oxide cathode.

To ensure good contact between the electrodes and the electrolyte, the team dissolved part of the electrolyte in methanol, mixed it directly with the cathode material, and evaporated the solvent. The resulting composite was then stacked with the rest of the electrolyte, pressed together at 900 MPa, and combined with the sodium anode to form the finished cell.

The electrolyte is electrochemically stable at up to 3.25 V, and the battery retains 85 % of its capacity after 250 cycles. According to the researchers, the results demonstrate that interface optimization can be just as important for solid-state batteries as improving the electrolyte’s conductivity, and show that closo-borate-based electrolytes are promising for high-performance batteries.

• A stable 3 V all-solid-state sodium-ion battery based on a closo-borate electrolyte,
  Léo Duchêne, Ruben-Simon Kühnel, Evelyn Stilp, Eduardo Cuervo-Reyes, Arndt Remhof, Hans Hagemann, Corsin Battaglia,
  Energy Environ. Sci. 2017.
  DOI: 10.1039/c7ee02420g