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

Area of Science:

• Materials Science
• Computational Chemistry
• Solid-State Physics

Background:

• Lithium transition metal oxides are crucial cathode materials for rechargeable batteries.
• Accurate thermodynamic data is essential for understanding material stability and performance.
• Experimental data for some oxides, like LiNiO2, are scarce, necessitating theoretical investigations.

Purpose of the Study:

• To compute formation energies and enthalpies of LiCoO2, LiNiO2, and LiMnO2 using DFT.
• To assess the accuracy of various exchange-correlation functionals for these materials.
• To investigate the influence of crystallographic distortions and vibrational contributions on thermodynamic properties.

Main Methods:

• Density Functional Theory (DFT) computations were employed.
• Hess cycles were utilized in conjunction with DFT for energy calculations.
• The PBE for solids (PBEsol) functional was identified as the most accurate.
• Harmonic and quasi-harmonic approximations were used to include vibrational effects.

Main Results:

• Enthalpies of formation at 0 K were determined for LiCoO2, LiNiO2, orthorhombic LiMnO2, and rhombohedral LiMnO2.
• Calculated formation energies showed good agreement with experimental data for LiCoO2 (1.6 kJ/mol difference) and LiMnO2 (0.01 kJ/mol difference).
• A significant discrepancy (approx. 24 kJ/mol) was observed for LiNiO2, prompting further investigation.
• Heat capacity calculations for LiCoO2 closely matched experimental values (3.3% difference at 300 K), while LiNiO2 showed a larger difference (17% at 300 K).
• New heat capacity data for o-LiMnO2 were computed.

Conclusions:

• The PBEsol functional provides accurate formation energies for LiCoO2 and LiMnO2.
• The study highlights the need for experimental measurements of thermodynamic properties for LiNiO2 due to significant discrepancies.
• Vibrational contributions and crystallographic distortions play a role in the energetic properties of these materials.
• The computed thermodynamic and structural data provide valuable insights for battery material development.