Department of Physics

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    First principle study of Nax[TiyZnzMnw]O2 as a cathode material for sodium-ion batteries
    (2025-09-05) Ranwaha, Tshifhiwa Steven; Maluta, N. E.; Maphanga, R. R.
    The fast-growing energy generation from renewable sources such as solar, wind, and waves is calling for reliable energy storage technologies with high energy density, high power, and low cost, because the energy harvested from these renewable energy sources is intermittent. Currently, the leading technology in energy storage is the lithium-ion battery (LIB), While lithium possesses numerous electrochemical advantages that make it a critical component in modern energy storage technologies, its continued viability is increasingly challenged by the rapid depletion of accessible lithium reserves and its uneven geographical distribution, which pose significant constraints on sustainable and equitable resource utilization. The development of electric vehicles and plug-in hybrid electric vehicles has raised everybody’s expectations as well as requirements for the materials employed. That is why there is an urgency to find alternative technologies which would replace LIBs. In search of alternative technology, sodium-ion batteries are a promising solution for large-scale electrochemical energy storage, owing to their low cost, materials abundance, good reversibility, and decent energy density. For sodium-ion batteries to achieve comparable performance to current lithium-ion batteries, significant improvements are still required in cathode, anode, and electrolyte materials. In this study, first- principles method based on the density functional theory was used to investigate the structural, electronic, mechanical and thermodynamics properties of Na intercalated electrode material NaxMnO2 electrode materials doped with Titanium (Ti) and Zinc (Zn) using random substitution doping method. The investigation was based on the effect of Na atom de-intercalation on the 2 X 1 X 1 NaxMn0.5Ti0.5O2 and 2 X 1 X 1 NaxMn 0.5Zn 0.5O2 supercells. The effects of dopants Ti and Zn on the NaXMnO2 stretch the lattice v parameters, resulting in volume expansion, this is because the atomic radii of the dopants are not the same as those of the host Mn. The electronic properties of the two doped systems show that the band gap is reduced by the effect of the dopants. The calculated elastic constants for the NaxMn0.5Ti0.5O2 and NaxMn0.5Zn0.5O2 bulk structures, as well as the NaxMn0.5Ti0.5O2 and NaxMn0.5Zn0.5O2 supercells, indicate mechanical stability for this compound as they meet the monoclinic structure mechanical stability criterion. In further investigation, the voltage window for the Ti-doped system was found to be between 3.410 V and 4.132 V. We found the voltage window for the Zn-doped system to lie between 2.221 V and 4.337 V. The calculated formation energies are negative, indicating that the material is thermodynamically stable and potentially amenable to synthesis under standard conditions. This inherent stability, coupled with favorable electrochemical characteristics such as appropriate voltage profiles, sufficient capacity, and adequate ionic conductivity, positions the material as a promising candidate for cathode applications in sodium-ion batteries. Furthermore, the cluster expansion formalism was used to investigate the NaxMnTiO2 and NaxMnZnO2 phase stabilities. The method determines stable multi-component crystal structures and ranks metastable structures by the enthalpy of formation while maintaining the predictive power and accuracy of first-principles density functional methods. The findings predict that all nickel-doped LMO structures on the ground state line are most likely stable. Relevant structures are NaMnO2, NaTiO2, NaTiMn2O2 and NaTi2MnO2 for NaxMnTiO2 CE-predicted structures and NaMnO2, NaZnO2, Na3Mn2ZnO6, Na6MnZn5O12, Na6Mn2Zn4O12, Na2MnZnO4 and Na5Mn4ZnO10 for NaxMnZnO2 CE-predicted structures. They were selected based on how well they weighed the cross-validation score (CVs) of 1.7 meV for NaxMnTiO2 CE-predicted structures and 1.9 meV NaxMnZnO2 CE-predicted structures, which is a statistical way of describing how good the cluster expansion is at predicting the energy of each stable structure. Although the structures have different symmetries and space groups, they were further investigated by calculating the structural, electronic, mechanical, and thermodynamical properties. The results show that all CE-predicted structures have a wide diffusion compared to the parent structure (NaMnO2). The reduction of band gap was also observed which give evidence that the structures are becoming metals and have an improved conductivity. The results showed that all the predicted structures met the stability requirements for monoclinic structures and were stable in terms of thermodynamics. For Ti-doped systems, the ductility was only observed on NaTiMn2O2 CE-predicted structure NaMnO2 doped with Zn found to be ductile which implies that these materials can bend without deformation, resulting in fewer cracks during battery operation. This study enhances the fundamental understanding of dopant-induced effects on NaMnO₂-based cathode materials, providing a prospective option to improve Na⁺ mobility, electrical conductivity, and structural stability. It presents a comprehensive analysis of the beneficial effects of Ti and Zn doping in the enhancement of sodium-ion battery performance. It provides the theoretical framework that underpins the development of advanced, cost-effective, economical, and thermally stable cathode materials which are crucial large-scale energy storage applications.