Lithium-oxygen batteries (Li-O2) can provide higher energy than traditional lithium-ion batteries through the reaction of metallic lithium with oxygen, which will improve important conditions for the realization of heavy-duty vehicles and aviation electrification. However, considering that in organic electrolytes, oxygen reduction products such as Li2O2 and Li2O do not dissolve to a large extent, computational studies also show that the thermodynamic barrier for oxygen reduction and precipitation on the surface of Li2O2 is small, indicating its rapid oxygen reduction and precipitation dynamic properties. In practical studies, it was found that Li-O2 batteries using aprotic electrolytes have large voltage hysteresis and low Coulombic efficiency between discharge and charge (<>
A solution to the instability of traditional electrolytes is to use molten salt electrolytes. Studies have shown that Li-O2 batteries can be cycled in molten nitrate (LiNO3/KNO3) with reversible two-electron oxygen reduction with only 0.1 V lag. Subsequently, the reversible four-electron oxygen reduction process was investigated with Ni and Fe3O4 electrodes, and these Li-O2 batteries with LiNO3/KNO3 could deliver an energy density of 220 Wh/kg and could be cycled for 150 times. However, there are still many unanswered questions regarding the oxygen reduction and evolution mechanism of such Li-O2 batteries with molten nitrate electrolytes, and in-depth studies are needed to explore the reasons for their increased energy and power densities.
Recently, molten nitrate electrolytes have been demonstrated to be redox active in Li-Ar batteries, in which nitrate can be reversibly reduced to produce nitrite and Li2O (LiNO3+2e-+2Li+→LiNO2+Li2O, 2.6 V), due to This nitrate/nitrite redox reaction can occur at a discharge voltage similar to that in Li-O2 batteries, and nitrate/nitrite redox may be involved in oxygen reduction and precipitation in Li-O2 batteries, so this work is very important for Research this in depth.
Recently, Professor Shao Yang of MIT and Dr. Graham Leverick (co-communication) published an article entitled “Nitrate-mediated four-electron oxygen reduction on metal oxides for lithium-oxygen batteries” in Joule. The work mainly found in Li In the -O2 battery, in the molten salt system, nitrate is electrochemically reduced to nitrite, and then nitrite is chemically oxidized to nitrate by oxygen molecules, thereby promoting the 4e-oxygen reduction process. Through isotope labeling experiments and theoretical calculations, a series of transition metal catalysts were studied to understand the NO3-redox reaction mechanism, which provided guidance for the application of new catalysts in lithium molten salt batteries and 4e-/O2 molten salt Li-O2 batteries. At the same time, a new type of molten salt lithium-oxygen battery system has also been developed, which provides a new research idea for high-performance lithium metal battery system.
Figure 1. Discharge behavior study in O2 and Ar in Li molten salt battery. (a) Schematic diagram of molten salt battery structure; (b) discharge voltage curve; (c) determination of NO2- and Li2O content; (d) DEMS monitoring of 32O2, 34O2 and 36O2 oxygen evolution rate; (e) Raman spectrum.
The authors’ study shows that the redox of nitrate anions in molten salt electrolytes is crucial to achieve the four-electron reduction of O2. The 18O isotope labeling experiments show that the Li2O formed during discharge obtains O2 from the reduction of nitrate anions in the molten salt electrolyte, and at 423 K, when discharged in 36O2 with LiN16O3/KN16O3, Li-O2 batteries show similar discharge and The charge voltage, and the capacities are comparable, and the 4e-/O2 reduction process is shown by trace measurements of the oxygen pressures consumed by 32O2 and 36O2 during discharge. Notably, oxygen evolution DEMS measurements on electrodes discharged in 36O2 did not produce 36O2, but 32O2, an observation indicating that the oxygen molecules were not directly reduced to form Li2O. Conversely, the detection of 32O2 during charging indicates that Li216O is formed by oxygen transfer in the nitrate during discharge. This result is also confirmed in the Raman spectrum of Li-O. A series of isotopic observations suggest that the reduction of nitrate to nitrite plays a key role in the formation of Li2O through the four-electron reduction of O2 for the high capacity of Li-O2 batteries.
Figure 2. Molten salt Li-O2 battery performance. (a) The discharge curves at different rates; (b) The charge-discharge curves of 1 mA/cm2 with different cycles; (c) The charge-discharge curves of 0.2 mA/cm2 with different cycles.
The experimental results show that the addition of nano-sized Ni particles (80-150 nm, 4.84 m2/g) to the Ni325 electrode can improve the power density of Li-O2 batteries, and maintain a similar discharge voltage when the current density is 2.0 mA/cm2. At the same time, it was found that at low current density, the discharge voltage dropped significantly without the addition of nano-Ni particles. The study showed that the Li-O2 battery with nano-Ni/Ni325 electrode can cycle stably at 2.0 mA/cm2 with high Coulombic efficiency. The mass capacity of this Li-O2 battery can be greatly improved by replacing the nano-Ni/Ni325 electrode with nickel-plated Vulcan carbon (XC72) supported by carbon paper, and the Ni/VC/CP electrode is stably cycled at 0.2 mA/cm2 after activation Over 80 cycles, the reversible capacity is 0.35 mAh/cm2. Based on this, a high-quality specific energy Ni/VC electrode (2,900 Wh/g) was designed, and the above findings provide a new strategy for developing practical Li-O2 batteries with power and energy density comparable to Li-ion batteries.
Figure 3. Discharge behavior of different electrodes in different gases. (a) Discharge curves of Ti/LiNO3/KNO3/Li molten salt battery under different conditions; (b) Ni/LiNO3/KNO3/Li molten salt battery in different gases; (c) Ni/LiNO3/KNO3/Li molten salt Battery determination of NO2- and Li2O content in different gases; (d) Raman spectrum of molten salt battery: Ni-O.
The Ti(Ni)/LiNO3-KNO3/SS electrode only appeared a small voltage plateau at the end, which corresponds to a distinct characteristic peak at 500 cm-1 in the Raman spectrum. This peak intensity is stronger in Ar than in Ar. This peak feature can be attributed to the Ni-O stretching vibration peak in NiO. There is no Raman signal of Li2O after discharge to the end of the second plateau, which is consistent with the negligible amount of Li2O detected by the titration method. Therefore, the authors believe that the small plateau at 3.0 V during discharge originates from the formation of NiO-like species, rather than the formation of Li2O through reactions such as 2Li++1⁄2O2+Ni(NO3)2+2e-=NiO+2LiNO3. Therefore, the formation of NiO-like species on the oxygen electrode surface in place of Li2O during initial discharge is crucial for realizing Li-O2 batteries with small overpotentials and fast four-electron kinetics.
Figure 4. Performance comparison of different metal nitrate molten salt air batteries. (a) Discharge curves of Ti electrode with different nitrates in O2; (b) Discharge curves of Ti electrode with different nitrates in Ar; (c) Raman spectra of molten salt cells under different nitrate conditions; (d) Different nitrate conditions The yields of Li2O and NO2 were calculated under.
The effect of adding other transition metal nitrates other than Ni2+ on the performance of molten salt batteries was further studied, and it was found that other metal nitrates could also improve the discharge platform of the battery. Interestingly, the dominant discharge plateau size in O2 was found to be Mn(NO3)2<><>
Similarly, Mn3O4, Cu2O, Fe3O4 and Co3O4 species are formed on the surface of the Ti/LiNO3/KNO3/SS electrode from the corresponding transition metal nitrates, and the initial small plateaus during discharge in O2, due to the addition of the same amount of metal nitrates and their similar Morphology, the metal oxides deposited on the Ti electrodes are expected to have similar total surface area, and the increased discharge voltage indicates that the kinetics of four-electron oxygen reduction to Li2O is increased from Mn3O4, Cu2O, Fe3O4 and Co3O4 to NiO. Meanwhile, the discharge voltages of the main plateaus in Ar were found to increase in the order from Fe3O4, Mn3O4, Cu2O, and Co3O4 to NiO, resulting from the corresponding M(NO3)x added to the Ti/LiNO3/KNO3/SS electrode, indicating that NO3- is reduced to NO2- Kinetic increase.
Figure 5. Theoretical calculations for different metal nitrates. (a) Free energy calculations and reaction intermediates; (b) volcano plots between NO3- reduction potentials; (c) NO2-/O2 generation rates normalized to the specific surface area of metal oxides.
Finally, the authors investigated the molecular dynamics origin of the reduction of NO3- to NO2- on the surface of the above metal oxides by DFT calculations:
(Step 1) NO3- and Li+ are adsorbed on the oxide surface to form -NO3–Li+;
(Step 2) -NO2–Li+ is formed by the reaction of 2Li+ and 2e-;
(Step 3) -NO2-Li+ is desorbed and dissolved into the electrolyte.
Although the Gibbs free energy of -Li+-NO3- adsorption was found to increase from Mn3O4, NiO to Cu2O, the Gibbs free energy of -Li+-NO2- desorption (step 3) decreased greatly. The calculated trends of kinetics of NO3 reduction to NO2 are in good agreement with the discharge voltage of the dominant plateau in Ar, with NiO showing the lowest Gibbs free energy in all three steps. The calculations found that the main discharge voltage showed a volcanic trend and was related to the binding energy of the surface adsorbates.
The oxidation kinetics of O2 on NO2- was further investigated and found to be surface-dependent, with NiO being the strongest, although Cu-based electrodes were found to exhibit similar discharge voltages and cycling stability at low rates as Ni, but fast kinetics were expected The chemical NiO-like surface for electrochemical NO3-reduction to NO2- has the highest discharge rate capability for Li-O2 batteries.
In this work, the authors reveal a new mechanism for the 4e-/O2 oxygen reduction reaction in molten salt Li-O2 batteries and find that nitrate plays a key role in the formation of Li2O2. Raman spectroscopy and DEMS analysis of electrodes discharged in 36O2 indicated that Li2O was mainly derived from NO3-reduction, rather than O2. By changing the surface of metal oxides (Mn3O4, Fe3O4, Co3O4, NiO, and Cu2O), it was found that the discharge voltage of NO3- reduction to NO2- exhibited a volcano-shaped dependence on the center of the O 2p band of metal oxides. Understanding the NO3-redox reaction steps through experiments and theoretical calculations provides design principles for the application of novel catalysts in lithium molten salt batteries and 4e-/O2 molten salt Li-O2 batteries. Based on the above design principles, molten-salt Li-O2 batteries with high rate, good cycle stability, and mass specific capacity up to 1100 mAh have been developed, providing new guidance for high-performance lithium metal battery systems.
原文：Nitrate-mediated four-electron oxygen reduction on metal oxides for lithium-oxygen batteries.
Joule, 2022, 6, 1-17. (DOI: 10.1016/j.joule.2022.06.032)
Reviewing Editor: Li Qian