Formation of Li₃O₄nano particles in the discharge products of non-aqueous lithium–oxygen batteries leads to lower charge overvoltage

Abstract

Density functional theory calculations are made for bulk thermodynamic properties and surface energies of Li2O2, a primary discharge product, and Li3O4, a possible byproduct in the discharge products, of the non-aqueous lithium-oxygen batteries. Results show that the standard formation Gibbs free energy of bulk Li3O4 is marginally higher than that of Li2O2, but the surface energy of Li3O4 is much lower. Low surface energy results in both lowered nucleation energy and formation Gibbs free energy in the nanometer regime, allowing the Li3O4 nano particles to nucleate ahead of Li2O2 during the discharge process and to exist stably when particle sizes are smaller than about 40 nm. The scanning transmission electron microscopy (STEM) image of Li3O4 crystals is simulated and compared with the measured STEM image of the discharge product particles. The consistency between the simulated and measured STEM images suggests that the Li3O4 phase can exist stably as a discharge product. The energy profile of the oxygen evolution reaction (OER) occurring on the most abundant surfaces of Li3O4 is also calculated. The predicted overpotential for the OER on the {0001} surface (0.30 V) shows a good agreement with experimental data. The presence of more electronically conductive Li3O4 nano particles in the primary discharge product Li2O2 tends to decrease the charge overvoltage of the batteries, explaining why the lower voltage area (<3.5 V) was widely observed during the charging of the batteries. An increase in the oxygen pressure or a decrease in temperature enhances the stability of the Li3O4 phase and increase the proportion of the Li3O4 phase in the discharge products, consequently leading to a lower overall charge overvoltage.

Oxygen‐Rich Lithium Oxide Phases Formed at High Pressure for Potential Lithium–Air Battery Electrode

“…Yao et al investigated the thermal stabilities of Li 2 O 2 and Li 2 O at the battery working temperature 5. Shi et al reported density functional theory (DFT) calculations and experimental confirmation of Li 3 O 4 nanoparticles as a discharge byproduct 6. Also, the recent discovery of lithium superoxide as a discharge product has attracted much attention, and it is considered as a highly promising candidate for the next‐generation lithium battery 7, 8, 9, 10, 11, 13…”

Anomalous Enhancement of Li‐O2 Battery Performance with Li2O2 Films Assisted by NiFeOx Nanofiber Catalysts: Insights into Morphology Control

“…The discharge products in the NiFeO x /CNT electrode were stable and could not be further reduced in Ar (Figure b; Figure S4, Supporting Information), indicating unlikely a mixture of Li 2 O 2 and LiO 2 . The lattice parameters (Table ) and XRD patterns (Figure a; Figure S5, Supporting Information) indicate that the discharge products were not P‐6m2 or P‐3m1 Li 3 O 4 , instead, more likely Li 2 O 2 with Li 2a vacancies. The DFT calculations reveal that the formation of Li 2a vacancies is thermodynamically unfavorable except under LiO 2 ‐rich conditions (Figure c).…”

“…The supersaturation can be calculated according to the relationship where is the free energy for the formation of Li 2 O 2 near the nucleation sites at temperature T and 1 atm, Z is the number of electrons involved in the reactions, F is the Faraday constant, ϕ is the electrochemical potential, k B is the Boltzmann constant, and is the oxygen pressure.…”

Strategies Toward Stable Nonaqueous Alkali Metal–O2 Batteries

“…Zhao's group confirmed the existence of Li 3 O 4 crystal in the discharge product and predicted a low charge overpotential of 0.30 V on (0001) surface of Li 3 O 4 by density functional theory calculation. [44] Later, a series of stable oxygen-rich lithium oxide phases were experimentally synthesized under high pressure and high temperature, including Li 2 O 3 and LiO 4 . [45] The discovery of these new lithium oxides enriches lithium oxide family and may provide alternative choices for stable Li-O 2 battery chemistry.…”