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 Cp(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.