Development of lithium-ion batteries has been the focus for researchers working to commercialize a viable power generating source for applications such as electric vehicles. This column has discussed work that is in progress to improve the efficiency and safety of lithium-ion batteries.

But lithium-ion batteries are not the only option under evaluation. Sodium-ion batteries are under consideration because sodium is a lower cost, more abundant option than lithium. Performance limitations with sodium-ion batteries have hindered their commercialization. Dr. Ji-Guang Zhang, laboratory Fellow, Energy Processes & Materials Division, at Pacific Northwest National Laboratory (PNNL) in Richland, Wash., says, “The biggest challenge in developing sodium-ion batteries is to figure out a way to combine high-energy density with long cycle life. Sodium-ion batteries that have been prepared either have high-energy density or long cycle life. The problem is finding a way for sodium-ion batteries to exhibit both characteristics.”

The electrolyte is an important factor in producing a high performance sodium-ion battery. Zhang says, “Newer electrolytes are needed to enable sodium-ion batteries to operate at high capacity under high voltage conditions. Two of the issues are figuring out how to prevent dissolution of the solid-electrolyte interface (SEI) that forms between the electrodes and the electrolyte and minimize degradation of the cathode when high voltage is applied.”

Zhang believes that one of the new cathode materials under development will eventually be able to enable the sodium-ion battery to operate with good capacity under high voltage conditions. The hard carbon anodes used in sodium-ion batteries represent an additional challenge because the components of the SEI layer formed on the anode surface eventually dissolve in the sodium electrolyte currently used in these batteries.

In a previous TLT article,¹ researchers determined that a new electrolyte known as sodium bis (fluorosulfonyl) imide and a nickel-rich O₃ layered, transition metal oxide (that also contains manganese and cobalt) together improve the performance of a sodium-ion battery. The researchers indicated that battery performance improved due in part to the suppression of the buildup of inactive sodium crystals on the surface of the cathode.

A new electrolyte is needed to minimize SEI dissolution and be more compatible with the materials used in the anode and the cathode. Such an electrolyte has now been developed.

NaFSI/DMC:TFP
The researchers developed and determined that an electrolyte containing sodium bis(-fluorosulfonyl) imide (NaFSI), at a concentration of 1.5M, in a mixture of dimethyl carbonate (DMC) and tris (2,2,2-trifluoroethyl) phosphate (TFP), used at a ratio of 1.6:8.4 by weight, enables a sodium-ion battery to achieve long cycling performance under high voltage conditions. The electrolyte is known as NaFSI/DMC:TFP (see Figure 2).



Figure 2. A new electrolyte known as NaFSI/DMC:TFP has been found to improve the cycling performance of sodium-ion batteries under high voltage conditions. Figure courtesy of Pacific Northwest National Laboratory.

Zhang says, “We use three strategies in identifying a suitable electrolyte. The first consideration is to find a solvent that exhibited minimal solvation of the SEI layer. Such a solvent must have a low dielectric constant. The second factor is to regulate the concentration of the salt species (NaFSI) in the solvent. By preparing an electrolyte with a high concentration of the salt, the solvent is less likely to negatively affect the SEI layer. Modification of the solvation structure is the third strategy because it can lead to the formation of more insoluble electrochemical decomposition components in the SEI layer. Reducing the concentration of decomposition products has been found to stabilize the SEI layer. An added benefit is that if the proper salt is used, lower solubility of decomposition products also is found in the electrolyte.”

The researchers decided to work with NaFSI because the lithium version of this salt has been used successfully in lithium-ion batteries. Zhang says, “We felt that NaFSI will be a more stable agent than the currently accepted salt used in the evaluation of sodium-ion batteries, sodium hexafluorophosphate.”

Performance testing used the same anode and cathode in parallel cell orientations. Longer term cycling showed that the NaFSI/ DMC:TFP electrolyte demonstrated greater than 90% retention after more than 300 cycles. A second series of tests evaluated the electrolyte at different charge/discharge rates. Zhang says, “In testing the battery at fast and slow rates, we found NaFSI/DMC:TFP displays good performance at various rates because of improved ionic conductivity and low interfacial resistance.”

The researchers used X-ray photoelectron spectroscopy (XPS) to better understand the composition of the SEI layer during operation of the battery. Zhang says, “Analysis of the SEI layer yielded more inorganic than organic components. This means that the SEI layer is less prone to solubilizing in the organic-rich electrolyte and will be more stable as the battery cycles.”

A comparable analysis was done for the cathode electrolyte interphase (CEI) surrounding a sodium nickel manganese cobalt (NaNMC) oxide cathode. Zhang says, “XPS shows that the CEI is much thinner and more uniform with NaFSI/DMC:TFP. This means that fewer side reactions and less battery capacity loss is obtained.”

Future work will focus on improving the electrolyte. Zhang says, “We will be trying to further improve the stability of the electrolyte at higher voltages. A second objective is to collaborate with another research group at PNNL that is working to develop improved cathodes where the concentration of cobalt is reduced.”

Additional information on this research can be found in a recent article² or by contacting Karyn Hede, media relations advisor and science communicator at PNNL, at Karyn.hede@pnnl.gov.

REFERENCES
1. Canter, N. (2020), “Sodium-ion battery,” TLT, 76 (10), pp. 14-15. Available here.
2. Jin, Y., Le, P., Gao, P., Xu, Y., Xiao, B., Engelhard, M., Cao, X., Vo, T., Hu, J., Zhong, L., Matthews, B., Yi, R., Wang, C., Li, X. Liu, J. and Zhang, J. (2022), “Low-solvation electrolytes for high-voltage sodium-ion batteries,” Nature Energy, 7, pp. 718-725.