NaM2(PO4)(SO4)2 (M= Fe, V, Cr) As Electroactive Materials for Na-Ion Batteries

To date, research on sodium-ion batteries started to gain more interest. It is mainly motivated by the large abundance of sodium and the hope to produce batteries that are cheaper compared to LIBs and less prone to resource issues and safety concerns [1-3]. NASICON compounds have been well investigated as possible electroactive materials for sodium-, lithium-, and magnesium-ion batteries. In this paper, we report on the structural and electrochemical properties of the NASICON phases NaFe 2-x M x (PO 4 )(SO 4 ) 2 (M=V, Cr and 0 < x < 1) [4] . The crystal structures were solved by the Rietveld method using powder X-ray diffraction (PXRD) data. The electrochemical performances were examined by galvanometric cycling and cyclic voltammetry. The stability of the NASICON structure during cycling was also followed by ex-situ PXRD experiments. The NaFe 2-x M x (PO 4 )(SO 4 ) 2 powders (0 < x < 1) were synthesized via a solid state synthesis route from stoichiometric mixtures of NaNO 3 (Aldrich, ≥99 %), Fe(NO₃)₃·9H₂O (Aldrich, ≥98%), Cr(NO₃)₃·9H₂O (Aldrich, ≥98%), NH 4 VO₃ (Aldrich, ≥99%), (NH 4 ) 2 SO 4 (Aldrich, ≥99 %), NH 4 H 2 PO 4 (Merk, ≥99%) and citric acid C 6 H 8 O 7 (Riedel-deHaën) (CA). The starting raw materials, dissolved in an aqueous medium, were stirred at 80 o C until evaporation of water. The resulting powders were calcined at 500 o C for several hours under argon to obtain pure NaFe 2-x V x (PO 4 )(SO 4 ) 2 powders. TGA experiments performed on all the samples resulted in a 30% weight loss which corresponds to the departure of two SO 2 molecules, and hence confirm the chemical compositions of NaFe 2-x V x (PO 4 )(SO 4 ) 2 . This was also confirmed by EDS analyses. The crystal structure of the NaFe 2-x V x (PO 4 )(SO 4 ) 2 were solved using X-ray powder diffraction data. A phase transition from R-3c (x = 0) to R-3 (x > 0) was observed during the replacement of iron by vanadium. The structure can be described as a covalent skeleton [(Fe/V) 2 P 3 O 12 ] - built of Fe/VO 6 octahedra and XO 4 tetrahedra (X= S, P), which forms 3D interconnected channels with several types of interstitial positions where sodium and vacancies are distributed. The cycling performances of NaFe 2-x V x (PO 4 )(SO 4 ) 2 were studied in cells embedding (NaPF 6 :EC:PC) electrolyte and Na-metal (Figure 1). Additional details will be provided during the conference. Figure 1. Charge/discharge curves of NaFe 2-x V x (PO 4 )(SO 4 ) 2 at the rate of C/10 vs. Na + /Na (a) and capacity retention for x = 0.4 (b) and rate capability for x = 0.4 (c). Reference: [1] Makino, K.; Katayama, Y.; Miura, T.; Kishi, T., J. Power Sources 2002, 112 , 85−89. [2] Gaubicher, J.; Wurm, C.; Goward, G.; Masquelier, C., Nazar, L., Chem. Mater. 2000, 12 , 3240−3242. [3] Jian, Z.; Zhao, L.; Pan, H.; Hu, Y.-S.; Li, H.; Chen, W., Chen, L., Electrochem. Commun. 2012, 14 , 86−89. [4] H. Ben Yahia, R. Essehli, R. Amin, K. Boulahya, T. Okumura, I. Belharouak, Journal of Power Sources 382 (2018) 144-151. Figure 1