The global drive towards decarbonisation means there is increasing need for large-scale energy storage, which has focussed research efforts into sodium-ion batteries (SIBs).¹ SIBs are well suited to grid-storage applications due to the wide abundance, even distribution and low cost of sodium deposits.² This contrasts to the better known and understood lithium-ion batteries (LIBs), where lithium is much less abundant. Additionally, the use of sodium has other sustainability implications, which are desirable for grid-scale applications, as it allows cobalt-free cathodes to be used and the copper current collectors at the anode to be replaced by aluminium.

Currently SIBs use NaPF₆ as the chosen electrolyte salt,³ however commercial supplies of this are often of low-grade and contain NaF (a hydrolysis product of NaPF₆), making it unsuitable for battery use.⁴ Alternatively, pre-made electrolyte solutions of NaPF₆ may be purchased, but the cost can be prohibitive and limits solvent exploration.

Herein, this talk will detail the synthesis of high-grade NaPF₆ from the addition of NH₄PF₆ with sodium metal in THF solvent. By performing the reaction under anhydrous conditions, NaPF₆ can be prepared in the absence of hydrolysis products (NaF), as confirmed by solid-state NMR spectroscopy and powder X-ray diffraction. With high-grade NaPF₆ in hand, we have looked at the effects of using higher concentration electrolyte solutions of NaPF₆ (>1 M) in ethylene carbonate:diethyl carbonate (EC:DEC 1:1 v/v) solvent. This showed the degradation dynamics of sodium metal-electrolyte interface are different for more concentrated (>1 M) electrolytes, although there is a trade-off with bulk conductivity compared to 1 M solutions. Lastly, the performance of the synthesized NaPF₆ has been tested in commercial 2- and 3-electrode pouch cells by Faradion Ltd, UK.⁵

References:

1 D. Kundu, E. Talaie, V. Duffort and L. F. Nazar, Angew. Chem. Int. Ed., 2015, 54, 3431–3448.

2 C. Vaalma, D. Buchholz, M. Weil and S. Passerini, Nat. Rev. Mater., 2018, 3, 18013.

3 A. Ponrouch, D. Monti, A. Boschin, B. Steen, P. Johansson and M. R. Palacín, J. Mater. Chem. A, 2015, 3, 22–42.

4 A. Bhide, J. Hofmann, A. Katharina Dürr, J. Janek and P. Adelhelm, Phys. Chem. Chem. Phys., 2014, 16, 1987–1998.

5 D. M. C. Ould, S. Menkin, C. A. O'Keefe, F. Coowar, J. Barker, Clare P. Grey and D. S. Wright, Angew. Chem. Int. Ed., 2021, 60, 24882–24887.