Computational Design and Experimental Realization of Substituted Fluorophosphate Cathode Materials for Na-ion Batteries

Significant scientific and economic challenges are presented by developing battery systems with increased energy storage capacity. Geopolitical issues might pose supply chain risks for lithium (Li)-ion batteries, which are frequently used in portable electronics. As an alternative, sodium-ion batteries (NIBs) have emerged as a promising candidate due to their potential for economic viability and reduced supply chain vulnerabilities.
One of the most examined cathode materials is the fluorophosphate family Na₃V₂(PO₄)₂F_3−2yO_2y (0 < y < 1), which has demonstrated superior performance to most layered oxides and is being developed as a competitive sodium-ion cathode material in academia and in the start-up environment [1]. Current research is directed towards improving the performance of Na₃V₂(PO₄)₂F₃, that experimentally has shown impressive stable capacity and commendable rate capability over 4000 cycles [2]. Recently, we reported new computational and experimental findings regarding the solid-state synthesis of NVPF_3−2yO_2y compounds, notably Na₃V₂(PO₄)₂F₂O and Na₃V₂(PO₄)₂FO₂, where we have identified favorable thermodynamics for an innovative synthesis route, particularly when utilizing (VO)₂P₂O₇ as starting material [3]. This will be a first topic of our presentation.
Nonetheless, V is not an abundant element (roughly with the same content as Ni in the Earth crust) and more importantly does not have a developed supply chain at large scale. Therefore, the use of alternative elements to replace V in the NVPF structural framework would be highly welcome. Our work is directed towards enhancing the sustainability and performance of Na₃V₂(PO₄)₂F₃ by substituting Vanadium with other transition metal elements. While some of these have been recently investigated [4], we propose an ample selection of substitutional elements and investigate them via Density Functional Theory (DFT) with VASP, using the accurate r2SCAN functional with D4 dispersion corrections [5], as well as the nudged elastic band (NEB) method to compute activation barriers for Na migration. Finally, we computationally explore the feasibility of various synthesis routes of Na₃V₂(PO₄)₂F₃ with these alternative transition metals ^[6] and we report on the experimental realization of the family of mixed Fe-V compounds Na₃V_2-xFe_x(PO₄)₂F₃ for 0 < x < 2. The structure and electrochemical performances of these materials as a function of Fe content will be elucidated^[7].

References
[1] Mariyappan, S., Wang, Q., & Tarascon, J. M. (2018). Journal of the Electrochemical Society, 165(16), A3714–A3722.
[2] Broux, T., Fauth, F., Hall, N., Chatillon, Y., Bianchini, M., Bamine, T., Leriche, J., Suard, E., Carlier, D., Reynier, Y., Simonin, L., Masquelier, C., & Croguennec, L. (2018). Small Methods, 3(4).
[3]Akhtar, M., Arraghraghi, H., Kunz, S., Wang, Q., & Bianchini, M. (2023). Journal of Materials Chemistry. A, 11(46), 25650–25661.
[4] Van Der Lubbe, S. C. C., Wang, Z., Lee, D. K. J., & Canepa, P. (2023). Chemistry of Materials, 35(13), 5116–5126.
[5]Ehlert, S., Huniar, U., Ning, J., Furness, J. W., Sun, J., Kaplan, A. D., Perdew, J. P., & Brandenburg, J. G. (2021). The Journal of Chemical Physics, 154(6).
[6] Arraghraghi, H., … Bianchini, M. (2025). submitted.
[7] Knies, S., … Bianchini, M. (2025). Submitted.