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
