The accelerating consumption of the limited lithium resources may affect future prices of lithium-ion batteries. In contrast, sodium-ion battery is considered to be a cheaper alternative to lithium-ion battery on account of the globally abundant sodium resources¹. In order to become commercially viable, the sodium-ion battery needs to deliver long cycling life with good capacity and energy density while still ensuring safety. In this study, the interactions between selected sodium-ion anodes and electrolytes were carefully investigated. A newly designed ether-based non-flammable electrolyte, 1M NaBF₄ in tetraglyme² was tested with three types of anodes (Na₂Ti₃O₇/C, Graphite and Hard Carbon), and the results were compared with those obtained with the popularly used carbonate-based electrolyte, 1M NaClO₄ in EC:PC (v:v=1:1).

With 1M NaBF₄ in tetraglyme electrolyte, stable half-cell cycling performance was achieved for all three anodes. Being glyme-based, this electrolyte also enabled excellent cycling of graphite anode through solvent co-intercalation reaction mechanism (with conventional carbonate-based electrolytes, graphite cannot store sodium)^2,3. Compared with 1M NaClO₄ in EC:PC, 1M NaBF₄ in tetraglyme showed much higher first cycle coloumbic efficiencies for Na₂Ti₃O₇/C and Hard Carbon half-cells, indicating less solid -electrolyte interphase (SEI) formation on the surface of the electrodes. Furthermore, the cycling stability and capacity of Na₂Ti₃O₇/C were similar with both of the two electrolytes while better performance was achieved with 1M NaBF₄ in tetraglyme for Hard Carbon.

Thermal stability studies were conducted for all the electrodes at the states of pristine, fully sodiated and fully desodiated using Differential Scanning Calorimetry (DSC). Furthermore, investigations on the SEI formation on the various anodes were also performed using the techniques of Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR), Field Emission Scanning Electron Microscope (FESEM), Energy-Dispersive X-ray Spectroscopy (EDX), Electrochemical Impedance Spectroscopy (EIS) and DSC. The DSC analysis showed that the anodes cycled with 1M NaBF₄ in tetraglyme were more thermally stable than their counterparts cycled with 1M NaClO₄ in EC:PC. The SEI investigations corroborated the cycling and DSC results, indicating that the SEI formed on the various anodes using 1M NaBF₄ in tetraglyme was not only thinner, but also thermally more stable than the SEI formed using 1M NaClO₄ in EC:PC. Such details from the various studies, which will be revealed in the presentation, unambiguously proved that 1M NaBF₄ in tetraglyme was a much safer choice as an electrolyte for sodium-ion batteries compared with 1M NaClO₄ in EC:PC especially for low voltage anodes.

[1] Kubota, K., & Komaba, S. (2015). Review—Practical Issues and Future Perspective for Na-ion Batteries. Journal of The Electrochemical Society, 162(14), A2538–A2550.

[2] Rudola, A., Du, K., & Balaya, P. (2017). Monoclinic Sodium Iron Hexacyanoferrate Cathode and Non-Flammable Glyme-Based Electrolyte for Inexpensive Sodium-ion Batteries, Journal of The Electrochemical Society, 164 (6), A1098-A1109.

[3] Jache, B.; Adelhelm, P. (2014) Use of Graphite as a Highly Reversible Electrode with Superior Cycle Life for Sodium-Ion Batteries by Making Use of Co-Intercalation Phenomena. Angew. Chemie - Int. Ed., 53 (38), 10169–10173.