Computational modelling study of layered LiNiO2 Surface, surface doping and interaction with the electrolyte in LI-ION batteries

Abstract

Lithium-ion batteries have garnered significant attention due to the growing demand for renewable energy sources, with monoclinic LiNiO₂ emerging as a promising cathode material. This is attributed to its high specific capacity (275 mAh/g) and energy density (629 Wh/kg). However, its practical application is limited by challenges such as low cycling stability and voltage fading. In this study, the bulk structural properties of LiNiO₂ were investigated using first-principles density functional theory, while its low Miller index surfaces were modeled with the METADISE code. The calculated lattice parameters align well with reported data, showing a deviation of less than 2.4%, and the system exhibits a heat of formation of -624.37 kJ/mol, confirming thermodynamic stability. Elastic constant calculations indicate mechanical stability, consistent with monoclinic stability criteria. However, phonon dispersion curves reveal imaginary vibrations in the gamma region, suggesting structural instability. Electronic structure analysis shows that LiNiO₂ has an indirect band gap of 0.708 eV near the Fermi level, indicating magnetic metal characteristics. Additionally, various Miller index surfaces ((110), (100), (010), (001), (111), and (101)) were examined, with the (101) facet identified as the most stable surface. The Nb/Mn doping is found to improve the crystal lattice of LiNiO2 and decrease the volume change. We found that after Nb/Mn surface doping, we observed that the surface free energies are lower compared to the surface energy of pure surfaces, indicating that the surface stabilizes upon doping. That the surface free energies of Nb-doped are lower when they are in the second layer compared to when they are in the first layer it implies that the second layer stabilizes the surface more effectively. Whereas the surface free energies of Mn-doped are higher when they are in the second layer compared to when they are in the first layer. The Bader charge of Nb and Mn are lower in the first layer and the work function in the first layer is higher, which implies that the second layer doped surface is more reactive in the first layer. These findings demonstrate that Nb and Mn doping significantly enhances the surface stability and reactivity of LiNiO2, offering valuable insights for improving its performance as a cathode material in lithium-ion batteries