Computational Design of Fluorophosphate Cathode Materials for Na-Based 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.<br/>One of the most examined cathode materials is the fluorophosphate family Na₃V₂(PO₄)₂F_3−2yO_2y (0 &lt; y &lt; 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].<br/>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 more 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 are exploring the feasibility of various synthesis routes of Na₃V₂(PO₄)₂F₃ with these alternative transition metals.<br/>References :<br/>[1] <b>Mariyappan, S., Wang, Q., & Tarascon, J. M. (2018). Will Sodium Layered Oxides Ever Be Competitive for Sodium Ion Battery Applications? </b><b><i>Journal of the Electrochemical Society</i></b><b>, </b><b><i>165</i>(16), A3714−A3722.</b><br/>[2]<b> 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). High Rate Performance for Carbon−Coated Na3V2(PO4)2F3 in Na−Ion Batteries. <i>Small Methods</i>, </b><b><i>3</i></b><b>(4).</b><br/>[3]<b>Akhtar, M., Arraghraghi, H., Kunz, S., Wang, Q., & Bianchini, M. (2023). A novel solid-state synthesis route for high voltage Na3V2(PO4)2F3−2yO2y cathode materials for Na−ion batteries. <i>Journal of Materials Chemistry. A</i>, </b><b><i>11</i>(46), 25650−25661.</b><br/>[4] <b>Van Der Lubbe, S. C. C., Wang, Z., Lee, D. K. J., & Canepa, P. (2023). Unlocking the Inaccessible Energy Density of Sodium Vanadium Fluorophosphate Electrode Materials by Transition Metal Mixing. </b><b><i>Chemistry of Materials</i></b><b>, </b><b><i>35</i>(13), 5116−5126.</b><br/><b>[5]Ehlert, S., Huniar, U., Ning, J., Furness, J. W., Sun, J., Kaplan, A. D., Perdew, J. P., & Brandenburg, J. G. (2021). r2SCAN−D4: Dispersion corrected meta−generalized gradient approximation for general chemical applications. <i>The Journal of Chemical Physics, 154(6).</i></b>