Highly Stable Garnet Fe2Mo3O12 Cathode Boosts the Lithium–Air Battery Performance Featuring a Polyhedral Framework and Cationic Vacancy Concentrated Surface

Abstract Lithium–air batteries (LABs), owing to their ultrahigh theoretical energy density, are recognized as one of the next‐generation energy storage techniques. However, it remains a tricky problem to find highly active cathode catalyst operating within ambient air. In this contribution, a highly active Fe2Mo3O12 (FeMoO) garnet cathode catalyst for LABs is reported. The experimental and theoretical analysis demonstrate that the highly stable polyhedral framework, composed of Fe—O octahedrons and M—O tetrahedrons, provides a highly effective air catalytic activity and long‐term stability, and meanwhile keeps good structural stability. The FeMoO electrode delivers a cycle life of over 1800 h by applying a simple half‐sealed condition in ambient air. It is found that surface‐rich Fe vacancy can act as an O2 pump to accelerate the catalytic reaction. Furthermore, the FeMoO catalyst exhibits a superior catalytic capability for the decomposition of Li2CO3. H2O in the air can be regarded as the main contribution to the anode corrosion and the deterioration of LAB cells could be attributed to the formation of LiOH·H2O at the end of cycling. The present work provides in‐depth insights to understand the catalytic mechanism in air and constitutes a conceptual breakthrough in catalyst design for efficient cell structure in practical LABs.

As shown in Figure 1d and S2ab, all the FeMoO nanomaterials display an irregular morphology in which the particle size gradually increases along with temperature rising, ca.150 nm for the FeMoO-400 sample ( Figure S2a), ca.250 nm for the FeMoO-500 sample ( Figure S2b), and ca. 500 nm for FeMoO-600 sample ( Figure  1d). TEM images further reveal the morphology of FeMoO nanoparticles (Figure 1e and S2cd). HRTEM images display the microstructure of FeMoO (Figure 1f and S2ef) and the lattice spacing of 0.386 nm corresponds to the (202) plane of Fe2(MoO4)3 (PDF No. 83-1701), confirming the high crystalline nature of nanoparticles.   respectively, [2] demonstrating the Mo 6+ oxidation state. Figure S4c shows the O 1s XPS spectra of 530.5 and 531.4 eV, [3] attributed to the lattice oxygen and vacancy oxygen in the samples, respectively.      upper-limited specific capacity of 600 mAh g −1 at a current density of 500 mA g −1 in O2 atmosphere.
As a comparison, the cycle stability of the FeMoO electrode was also tested in a pure oxygen environment ( Figure S9ad). With an upper capacity of 600 mAh g 1 and a current density of 500 mA g 1 , the FeMoO-600 electrode was able to maintain its capacity for 500 cycles and the discharge voltage was still maintained at 2.5 V in the 400th cycle. FeMoO-500 and FeMoO-400 electrodes can work stably for 380 and 295 cycles, respectively.  .      Li2xO2 is gradually converted into Li2O2. Throughout the discharge process, the low crystallinity mixture Li2xO2 is always the main product in the discharge process, which is conducive to the improvement of electron transfer and ion migration, and the cycle stability of Li-air batteries. [5] After discharge to 2000 mAh g 1 , the peak of Li2CO3 is identified and gradually increases with further discharge. After recharging ( Figure   S13b), the discharge products almost decomposed completely. As shown in Figure 3c, 21 the C 1s spectrum of the cathode is similar during the discharge process. [6] Among them, the binding energies at 287.1 eV and 293.2 eV can be attributed to the polytetrafluoroethylene (PTFE) binder. The discharge product Li2CO3 (290.2 eV) gradually becomes obvious with the discharge process, which is consistent with the Li 1s XPS results ( Figure 3b). As shown in Figure S13b, the Fe 2p spectrum is almost unchanged, and the binding energy near the 709 and 714 eV can be attributed to Fe 2+ and Fe 3+ . The binding energy near 719.6 eV can be assigned to the satellite peak. [7] As shown in Figure S12c, Mo 3d core level has two peaks at binding energies of 233.4eV and 234.7eV, corresponding with the Mo 6+ valence state indicating the stable surface condition of FeMoO catalyst in LABs. [8] In addition, similar results of FeMoO-600 electrode were also obtained testing in open air and oxygen atmosphere ( Figure S1415). Meanwhile, the discharge product was mainly Li2xO2 for the FeMoO electrode when tested in O2 and no Li2CO3 was detected.  cm 1 (νCO) can be attributed to low-crystalline Li2CO3, which results from the side reaction of carbon and the existence of CO2 in the air. [9] The LiTFSI/TEGDME electrolyte typical peaks were also detected at 1040 to 1350 cm (νSO= 1057 cm 1 , νSO2=1134 cm 1 , CF = 1186 cm 1 , CF = 1226 cm 1 , νasSO2 =1334 cm 1 , and νasSO2= 1350 cm 1 ). [10] The peak belonging to Li2CO3 became stronger during the discharge process in the open air and was not obviously in O2. It can be concluded that there existed few Li2CO3 as the discharge product in the O2 atmosphere, and Li2CO3 was part of the discharge product in the open air.

Theoretical calculations
The adsorption energies (Ead) were calculated using the formula: where Esub-ads, Esub, and Emol are the total energy of the adsorption system, slab model, and adsorbed molecules, respectively.
The reaction free energy of each reaction coordinate was calculated by the equation:     Potential-dependent electrochemical phase diagrams of surface products are of great significance for studying the formation, transformation, and decomposition of products. As shown in Figure S23, the electrochemical phase diagrams of the products for eight different surfaces were calculated. LiO2 before forms at a high potential for all planes compared to Li2O2. However, it is difficult for LiO2 to exist stably, especially on (22-2) and (40-2)@VFe surfaces at high potential.
(Li2O2)2 as a nucleation product has a strong transformation tendency on (012), (220), , (40-2), (202) surfaces, and has a specific transformation potential on other surfaces, which implies 31 inhomogeneity of nucleation and polymorphism/amorphism of the products. On the surface of (202), (Li2O2)2 has a strong tendency to generate below 2.68V. At the (202) surface with Fe defects, the potential is raised to 2.77 V, but Li2O2 is formed as the main product above this voltage. Under the difference between Fe defect and defect-free surface, this difference will lead to a limited or weakened surface nucleation process and the generation of polycrystalline or even amorphous products.    [24] graphene-Co3O4 0.325 LiPF6/EC/DEC Ambient air 160 50/200 (125h) [25] NiCo2O4@Ni LiPF6/EC/DEC Ambient air 0.5 mA/cm 2 100/1 mAh/cm 2 (400h) [26] Fe@La0.