A first-principles Quantum Monte Carlo study of Li-air battery cathode materials*

Li-air batteries are poised to revolutionize energy storage. With an extremely high theoretical energy density from the strong Li-O bonds in the cathode, Li-air cells outperform intercalation chemistries. Accurate cohesive and formation energies are crucial to understand reaction dynamics and stability of Li-air cathodes. Previous computational studies modeled the electronic and thermodynamic properties of Li₂O₂ using Density Functional Theory (DFT) with PBE, HSE and GW methods. However, reported values are not consistent due to functional dependences of DFT. In this study, we have used Quantum Monte Carlo (QMC), an approach to solving the many-body Schrödinger equation that is near chemical accuracy and less dependent on accurate DFT orbitals than GW. After carrying out convergence test for QMC simulation, we concluded that nonmagnetic Li₂O₂ is the mid-step in charging and discharging of LiO₂ (nonmagnetic) and Li₂O (antiferromagnetic). This mid-step has been highly debated as the energetic ordering depends on the DFT functional and U correction (Föppl and Féher phases). The QMC results provide not only a useful prediction related to the understanding of Li-air batteries, but also a crucial benchmarking data for the future use of DFT applied to these systems.