Thermodynamic properties of LiNiO₂, LiCoO₂, and LiMnO₂ using density-functional theory

The formation energies of LiCoO₂, LiNiO₂ and LiMnO₂ were calculated using a combination of adequately selected Hess cycles and DFT computations. Several exchange-correlation functionals were tested and PBE for solids (PBEsol) turned out to be the most accurate. The enthalpies of formation at 0 K are -168.0 kJ mol at^-1 for LiCoO₂, -173.2 kJ mol at^-1 for LiNiO₂, -209.9 kJ mol at^-1 for o-LiMnO₂ and -208.8 kJ mol at^-1 for r-LiMnO₂. In comparison to experimental formation energy data, a difference of 1.6 and 0.01 kJ mol at^-1 was obtained for LiCoO₂ and LiMnO₂, respectively. By contrast, a much larger discrepancy, around 24 kJ mol at^-1, was obtained for LiNiO₂ and confirmed by using an additional and independent Hess cycle. The influence of slight crystallographic distortions associated with magnetism and/or the Jahn-Teller effect on energy was carefully searched for and taken into account, as well as corrections arising from vibrational contributions. Hence, these results should motivate future measurements of the thermodynamic properties of LiNiO₂, which are currently scarce. Vibrational contributions to the structural and energetic properties were computed within the harmonic and the quasi-harmonic approximations. The LiCoO₂ heat capacity at constant pressure is in excellent agreement with experimental data, with a difference of only 3.3% at 300 K. In the case of LiNiO₂ the difference reaches 17% at 300 K, which could also motivate further investigation. The C_p(T) value for the orthorhombic phase o-LiMnO₂, for which no previous data were available, was computed. Structural properties such as specific mass, bulk modulus and coefficient of thermal expansion are presented.