MetMat 2022: Poster presentation by Dr. Violeta Koleva

Mixed Phosphate-Sulfate NASICON phase as Electrode Material in Sodium-Ion Batteries

Violeta Koleva *, Silva Stanchovska, Sonya Harizanova and Radostina Stoyanova *

Abstract: The rising demand for electronic equipment, electric vehicles, and smart grids, makes the development of new high-performance energy storage systems a major concern. Sodium-ion battery (SIB) technology is one of the best alternative candidates to the lithium analogue due to the low cost and the abundance of sodium which are critical issues for large-scale application. One of the challenges facing SIBs is the exploration of new cheap electrode materials with efficient electrochemical performances suitable for practical applications. Among the sodium intercalation compounds, phosphate framework materials have attracted increasing attention due to the high structural stability, facile reaction mechanism and rich structural diversity. To address the above challenges, different types of mixed polyanion materials containing PO4 units have been designed as potential cathode candidates for SIBs: Na4Fe3(PO4)2(P2O7) (M= Fe, Mn, Co,Ni), Na3Mn(PO4)(CO3), Na2MPO4F (M = Fe, Mn, Co and Ni), Na3V2(PO4)2F3. On the other hand, the presence of highly electronegative SO42- ions in the mixed polyanion frameworks has been found to increase the operational potential that is beneficial for achievement of high energy density.
The present study is focused on the synthesis, physicochemical and electrochemical characterization of a mixed phosphate-sulfate compound with the following composition: NaFeV(PO4)(SO4)2. To date only one report is available in the literature for this compound synthesized by a sol-gel method [1].
For the preparation of this material we have applied a low-temperature method, where the precursor is obtained by freeze-drying of a solution containing NaH2PO4, Fe(NO3)3, (NH4)2SO4 and NH4VO3 dissolved in the presence of citric acid, all taken in the needed amounts. The thermal decomposition and annealing at 400 °C of the precursor yields a very well crystalline phase of pure NaFeV(PO4)(SO4)2 (NFVPS). The compound crystallizes in the trigonal space group R-3 with lattice parameters: a = 8.4056(4) Å and c = 22.0575(6) Å. It has a NASICON-type structure, where Fe3+ and V3+ ions are randomly distributed over the two 6c crystallographic sites, while SO42- and PO43- ions share one 18f site with an occupancy ratio of 2:1 (Figure 1a). The thermal behavior is followed by using TG-DTA technique combined with mass spectrometry analysis. The vibrational characteristics are examined by IR spectroscopy. The intercalation properties are studied by cyclic voltammetry using NaPF6 and LiPF6 electrolytes. In order to increase the electron conductivity of NFVPS two types of composites have been obtained and tested: (1) NFVPS/C prepared by ball-milling of NFVPS with carbon black; (2) NFVPS/rGO, where rGO (reduced graphene oxide) has been introduced during the freeze-drying process.

Figure 1. Crystal structure (a) and CV curves of NFVPS/C (b) and NFVPS/rGO (c) in Na-half cells.

The redox peaks observed in the CV curves confirm that NFVPS is electrochemically active as cathode material in both Na and Li ion cells (Figure 1b,c). The intercalation of Na+ and Li+ ions take place owing to the Fe2+/Fe3+ and V3+/V4+ redox pairs but with different mechanism for NFVPS/C and NFVPS/rGO (Figure 1b,c).
In conclusion: A little known mixed phosphate-sulfate intercalation material has been successfully prepared by low-temperature precursor method. NaFeV(PO4)(SO4)2 is electrochemically active as cathode material in both Na and Li ion cells. The studies on the cycling stability, rate capability and intercalation mechanism of NaFeV(PO4)(SO4)2 are in progress.

1. R. Essehli, A. Alkhateeb, A. Mahmoud, F. Boschini, H. Ben Yahia, R. Amin and I. Belharouak, J.
Power Sources 469 (2020) 228417.
The authors from CARiM’s Research Team are noted with *

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