(201d) First-Principles Study of Lithium-Air Batteries Based on Tri-Molybdenum Phosphide (Mo3P) Nanoparticles

Lithium-air batteries are promising competitors to replace traditional lithium ion batteries due to the extremely high theoretical energy density. Despite extensive experimental research for decades, it is still a challenge to find a stable cathode material for both discharge (ORR) and charge (OER) reactions with a low overpotential. Here, we employ density-functional-theory (DFT) calculations to examine the mechanism, thermodynamic overpotential (η) and cyclability of a novel Mo₃P-based Lithium-air battery. We also compare and rationalize our findings with direct comparison with the recent experimental results of our collaborators. In this type of battery, Li₂O₂ is found as the only discharge product up to 1000 cycles due to the two-electron transfer reaction between Li⁺ and O_2. Our DFT results show that the Mo-terminated Mo₃P (110) surface will be spontaneously oxidized in the oxygen-rich environment, thus forming a kinetically stable MoO layer to bring about the initial Li⁺/e^- transfer from aprotic electrolyte to cathode. Furthermore, we find that the initial Li⁺/e^- transfer reaction is the potential determining step (PDS) during ORR with a quite low DFT-based overpotential (η = 0.06 V), in agreement with our experimental detection (η = 0.08 V). In OER, the DFT-based overpotential was determined as 0.33V coupled with the desorption reaction of LiO₂ on the MoO layer covered catalyst, which is in agreement with experimental value (η = 0.27 V) but larger than that (η = 0.12 V)^[1] on the kink sites of the crystal majority Li₂O₂ (0001) facet.^[2,3] Therefore, we reveal that the MoO layer terminated Mo₃P cathode gives rise to a low-overpotential discharge reaction. However, the starting of charge process happens on the surface of agglomerated Li₂O₂ discharge product, while Mo₃P cathode becomes more and more important after most of the Li₂O₂ dissolved. Our results help shed fundamental insight on the role of ultrathin oxide monolayers on phosphide nanoparticles, and how this can potentially lead to a new nanomaterials design dimension.

1. Hummelshøj, J. S., Luntz, A. C. & Nørskov, J. K. Theoretical evidence for low kinetic overpotentials in Li-O₂Journal of Chemical Physics 138, 34703 (2013).

2. Radin, M. D., Tian, F. & Siegel, D. J. Electronic structure of Li₂O₂ (0001) surfaces. Journal of Materials Science 47, 7564–7570 (2012).

3. Radin, M. D., Rodriguez, J. F., Tian, F. & Siegel, D. J. Lithium peroxide surfaces are metallic, while lithium oxide surfaces are not. Journal of the American Chemical Society 134, 1093–1103 (2012).