Computational design for fluorophosphate cathode materials for Na-Based Batteries | NHR-Verein: Wir sind Hochleistungsrechnen

Sodium-ion batteries (NIBs) have emerged as a sustainable and economic alternative to Li-ion batteries, addressing critical supply chain and resource challenges. Among the most promising cathode materials is the fluorophosphate Na3V2(PO4)2F3, which demonstrates remarkable capacity retention and rate capability. However, Vanadium creates supply chain issues, driving the exploration of transition metal (M) replacements according to Na3M2(PO4)2F3. Leveraging advanced computational methods such as DFT with r2SCAN functional and NEB calculations, we explore the stability, operating voltage, and crystal structure of such novel materials. Additionally, we explore the thermodynamic stability of different innovative synthesis routes.

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Method

The computational investigations were performed using density functional theory (DFT) with the r2SCAN functional[4], Hubbard correction and D4 dispersion corrections for selected transition metals. The NaₓM₂(PO₄)₂F₃ (0 ≤ x ≤ 4) structure was investigated with varying sodium contents, where M represents different first and second row of transition metals, i.e. Ti, V, Cr, Mn, Ni, Fe, Co, Mo, Nb, and Zr. These transition metal-based compounds were investigated based on the NVPF results and were analysed as potential replacements for vanadium. These simulations were conducted using the Vienna Ab initio Simulation Package (VASP), with augmented plane-wave pseudo-potentials. The structural optimization was carried out by allowing atomic positions and lattice parameters to relax until specific forces and energy convergence criteria. For each optimization, the maximum number of ionic relaxation steps was set to 100 per run but depending on the specific transition metal and structural configuration, multiple calculations were required to achieve full convergence. To evaluate the electronic properties, density of states (DOS) calculations was performed using a self-consistent field (SCF) approach. Sodium diffusion pathways and energy barriers were analysed using the Nudged Elastic Band (NEB) method with the climbing image (CI-NEB) approach[5], applied for N₀VPF, N₁VPF and N₃VPF for three distinct migration paths. The initial and final configurations were derived from fully optimized structures, with a total of 8 intermediate images, including the initial and final states using the VTST tools package. Additionally, thermodynamic feasibility of potential synthesis routes was evaluated.

Results

Formation energy calculations were performed across multiple NVPF configurations. Based on these results, we computed voltage profile and performed DOS analysis for all materials (Figure 1).

Variations in cell parameters provided additional insights into the structural effects of the M incorporation (not shown here). Additionally, the thermodynamic assessment of alternative synthesis routes ravelled promising results, suggesting the thermodynamic feasibility using alternative transition metals (Mn, Fe, Mo). Our results allowed us to identify transition metals exhibiting promising properties, which we are now targeting experimentally. Finally NEB calculations were conducted for N₀VPF, N₁VPF and N₃VPF, as well as for the most promising metals M, identifying Na migration pathways and corresponding activation energy barriers.

Ongoing Work/Outlook

Future research will further extend NEB calculations to the most promising transition metals. In the long term, we aim to make further investigate this promising structural framework for different applications in Na-based batteries.

References

[1] Mariyappan et al. J. Electrochem. Soc., 2018, 165(16), A3714–A3722.

[2] Broux et. al. Small, 2018., 3(4).

[3] Akhtar et al. J. Mat. Chem. A, 2023, 11(46), 25650–25661.

[4] Ehlert et al. J. Chem. Phys., 2021, 154(6).

[5] Sheppard et al. J. Chem. Phys., 2012, 136, 074103.

[6] Nowagiel et al. Coatings, 2023, 13, 482