Atomistic And Engineering Insights Into Layered Oxide Cathodes For Sibs: A Combined DFT, Molecular Dynamics, And Data-Driven Design Study

Abstract

Examining Na₀.₉₅Mn₀.₄Fe₀.₂₅Ni₀.₃₅O₂. It is charge-balanced as Na⁺₀.₉₅ + Mn⁴⁺₀.₄ + Fe³⁺₀.₂₅ + Ni²⁺₀.₃₅ + O²⁻₂ = 0, by deploying a first-principles and machine-learning workflow to link atomic-scale structure to electrochemical behavior. DFT+U calculations were performed in Quantum ESPRESSO. Utilizing the PBE+U functional with Hubbard U values of Mn = 3.9 eV, Fe = 5.3 eV, and Ni = 6.2 eV. These calculations gave a relaxed unit-cell volume of 515.32 ų and a lattice parameter a = 6.056 Å. The electronic band gap is 1.11 eV. Moreover, the magnetic moments reflect the underlying exchange interactions. The ground state is ferrimagnetic, with moments of Mn⁴⁺ = 2.95 μB, Fe³⁺ = 3.94 μB, and Ni²⁺ = 1.58 μB. Bader charge analysis gave the following atomic charges. Na was +0.85 e, Mn +1.83 e, Fe +1.80 e, Ni +1.23 e, and O −1.20 e. It shows that the oxygen carries a noticeable negative charge. By refining the MACE V5 potential. Starting from the MACE-MP-0 foundation model, we fine-tuned it on 88 DFT frames, achieving a force RMSE of 43.8 meV/Å and an energy RMSE of 5.0 meV/atom. CHGNet v0.3.0 was used for NEB and MD simulations. For the stoichiometric composition (x = 0.95), CI-NEB calculations across three pathways gave migration barriers of 325, 340, and 442 meV, yielding a mean of 369 meV. It's worth noting that the force RMSE remains below 50 meV/Å. Transition-state theory then gave D_self(300 K) = 7.2×10⁻¹¹ cm²/s at Ea = 369 meV. Kinetic Monte Carlo simulations produced D_KMC(300 K) = 8.0×10⁻¹⁰ cm²/s with an activation energy of 349 meV. The Nernst Einstein relation gave σ(300 K) = 9.6×10⁻⁶ S/cm. Na-depleted simulations showed enhanced ion mobility. At x = 0.67 the mean-squared-displacement was 61 times larger than in the stoichiometric material. At x = 0.50 the enhancement reached 187-fold. I suspect the observed boost in mobility at low Na content stems from easier hopping pathways. This came with a drop in activation energy to 312 meV, signaling a highly mobile transport regime. We built a DFT+U voltage profile from 11 of 17 ordered structures. The O3 phase lies between 2.68 V and 3.38 V, while the P3 phase spans 2.26 V to 3.48 V. A single O'3 point appears at 4.26 V for x = 0.17→0.33. The Ni²⁺→Ni³⁺LS couple drives the main redox activity throughout desodiation, while Fe³⁺ remains largely stable. A weak oxygen-redox signal shows up at low Na content. This is indicated by a decrease in the O Löwdin charge from 5.25 e to 4.99 e at x = 0.17. Together, these results link composition, structure, ion transport, and thermodynamics. They establish a direct atomic-scale connection for this sodium-ion cathode material.