Synthesis and Optimization of Sodium-Vanadium Fluorophosphate – Positive Electrode Material for Sodium-Ion Batteries
Authors: Grabowski Oskar , Krajewski Michał , Winkowska-Struzik Magdalena , Czerwiński Andrzej 
Published: December 2024

Among other positive electrode materials for sodium-ion batteries, NASICON (Na
Super Ionic Conductors) compounds are distinguished by their unique crystal structure, characterized by a three-dimensional framework of corner-sharing polyhedra, creating interconnected pathways for sodium ions to migrate through during battery operation. This 3D open structure provides low resistance for Na+ diffusion, enabling fast charge and discharge rates. This feature, coupled with great structural and thermal stability, positions NASICON compounds as promising candidates for commercial cathode materials for sodium-ion batteries.[1]
,
[2]

Sodium-vanadium fluorophosphates (NVPFs) with general formula Na3V2Ox(PO4)2F3-x (0 ≤ x ≤ 2) are prime examples of NASICON-structured positive electrode materials. These are characterized by high theoretical capacity of 128 mAh g-1 and high operating potential of 3.7 and 4.2 V vs. Na+/Na0 for fully fluorinated phase. Despite its electrochemical advantages, utilization of NVPF meets several difficulties with the most significant being occurrence of non-fluorinated impurity – Na3V2(PO4)3 (NVP) during synthesis.[3] NVP is also a electrochemically active NASICON material, however due to its lower theoretical capacity (117 mAh g-1) and working potential (3.4 V) its presence in NVPF electrodes leads to lower energy density.
In our research we focus on optimizing NVPF synthesis procedure by investigating the influence of thermal treatment conditions, fluorine and sodium excess, carbon content, and so on. Phase purity of synthesized samples was investigated using powder X-ray diffraction. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy were applied to examine their morphology and elemental composition. Electrochemical tests were conducted in three electrode Swagelok setup with sodium as counter and reference electrodes. 1 M NaClO4 solutions in ethylene carbonate and propylene carbonate with and without fluoroethylene carbonate additive were used as electrolyte. Through systematic experimentation, we have successfully obtained NVP-free NVPF material with superior high rate performance (94% of capacity retention when current of discharge was raised from 1 C to 10 C) and great cyclability (97 % of capacity retention after 50 continuous charge/discharge cycles at 1 C).

Acknowledgements: This work was supported by The Polish National Centre of Science through a research grant 2021/43/D/ST5/01220 entitled “High-voltage sodium-ion batteries based on exfoliated graphite electrodes”.

Literature:

[1] J.B. Goodenough, H..-P. Hong, J.A. Kafalas, Fast Na+-ion transport in skeleton structures, Mater. Res. Bull. 11 (1976) 203–220. https://doi.org/10.1016/0025-5408(76)90077-5.
[2] Q. Zhou, L. Wang, W. Li, K. Zhao, M. Liu, Q. Wu, Y. Yang, G. He, I.P. Parkin, P.R. Shearing, D.J.L. Brett, J. Zhang, X. Sun, Sodium Superionic Conductors (NASICONs) as Cathode Materials for Sodium-Ion Batteries, Electrochem. Energy Rev. 4 (2021) 793–823. https://doi.org/10.1007/s41918-021-00120-8.
[3] Z. Yang, G. Li, J. Sun, L. Xie, Y. Jiang, Y. Huang, S. Chen, High performance cathode material based on Na3V2(PO4)2F3 and Na3V2(PO4)3 for sodium-ion batteries, Energy Storage Mater. 25 (2020) 724–730. https://doi.org/10.1016/j.ensm.2019.09.014.