Beyond the Norm: Synthesis and Electrochemical Study of High Concentrated NaPF₆ Electrolytes

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. Figure 1