Fe-based hybrid electrocatalysts for nonaqueous lithium-oxygen batteries

Lithium–oxygen batteries promise high energy densities, but are confronted with challenges, such as high overpotentials and sudden death during discharge–charge cycling, because the oxygen electrode is covered with the insulating discharge product, Li2O2. Here, we synthesized low–cost Fe–based nanocomposites via an electrical wire pulse process, as a hybrid electrocatalyst for the oxygen electrode of Li–O2 batteries. Fe3O4-Fe nanohybrids–containing electrodes exhibited a high discharge capacity (13,890 mA h gc −1 at a current density of 500 mA gc −1), long cycle stability (100 cycles at a current rate of 500 mA gc −1 and fixed capacity regime of 1,000 mA h gc −1), and low overpotential (1.39 V at 40 cycles). This superior performance resulted from the good electrical conductivity of the Fe metal nanoparticles during discharge–charge cycling, which could enhance the oxygen reduction reaction and oxygen evolution reaction activities. We have demonstrated the increased electrical conductivity of the Fe3O4-Fe nanohybrids using electrochemical impedance spectroscopy.

g c −1 ), low overpotential (1.39 V at 500 mA g c −1 ), and excellent high rate stability (150 cycles at 2,000 mA g c −1 with a fixed capacity regime of 1,000 mA h g c −1 ). Figure 1 shows a schematic of the selection process and typical field emission scanning electron microscopy (FESEM) images of the products obtained using the electrical wire pulse method. After the selection step, in which the suspension was allowed to settle, two different morphologies were obtained ( Fig. 1b- The crystal structures of the Fe 3 O 4 nanospheres and the Fe 3 O 4 -Fe nanohybrids were recorded using XRD, as shown in Fig. 2a. The X-ray diffraction (XRD) pattern of the Fe 3 O 4 nanospheres can be indexed to cubic Fe 3 O 4 (PDF card No. 88-0315). No signals from other impurities could be clearly detected, indicating that a high purity magnetite sample was prepared from the upper-colloid solution after settling. The XRD pattern of the Fe 3 O 4 -Fe nanohybrids had diffraction peaks that were similar to those of the Fe 3 O 4 nanospheres. In addition, new diffraction peaks at 44.7° and 65° were observed, which could be attributed to cubic Fe (PDF card No. 06-0696).

Results and Discussion
To further confirm the morphology and structure of the Fe 3 O 4 nanospheres and the Fe 3 O 4 -Fe nanohybrids, TEM images were obtained, as shown in Fig. 2b-f. Figure 2b and c show typical transmission electron microscopy (TEM) images of the Fe 3 O 4 nanospheres, with a similar morphology to that observed by SEM. The Fe 3 O 4 nanospheres had diameters of less than 50 nm and were aggregated with an approximately spherical shape. The selected area electron diffraction (SAED) pattern demonstrates the highly crystalline nature of the nanoparticles (inset of Fig. 2b). The lattice spacing, calculated based on the electron diffraction patterns, are in agreement with that of cubic Fe 3 O 4 (PDF card No. 88-0315). The high-resolution TEM (HRTEM) image in Fig. 2c shows that the inter-planar distance in the Fe 3 O 4 nanospheres is 0.25 nm, which is consistent with the (311) plane of cubic Fe 3 O 4 .
The TEM images of the Fe 3 O 4 -Fe nanohybrids reveal 0D spheres deposited on 2D flake composites (Fig. 2d). The SAED pattern of the nanocomposites can be indexed to cubic Fe 3 O 4 (PDF card No. 88-0315) and cubic Fe (PDF card No. 06-0696), which is consistent with the XRD results. Figure 2e shows the HRTEM image of a selected nanosphere in Fig. 2d (indicated by the red circle), and reveals the presence of two different types of nanospheres. The inter-planar distance of 0.25 nm corresponds to the (311) plane of Fe 3 O 4 , whereas the other region with an inter-planar distance of 0.20 nm is indexed to the (110) plane of Fe. The energy-dispersive X-ray spectroscopy (EDS) mapping profile of the Fe 3 O 4 -Fe nanohybrids, as shown in Supplementary Fig. S1, indicating that Fe and O elements are respectively populated in the Fe 3 O 4 -Fe nanohybrids. Supplementary Fig. S1b show the  Table S1). Figure 2f shows the HRTEM image of a selected nanoflake in Fig. 2d 26 . Oxygen vacancy generated the charge of the defect state, which can expected vigorous electrocatalytic activity.
The Fe 3 O 4 nanoflakes can be achieved by oriented attachment growth. The oriented attachment growth is contributed to reduce the overall energy of the formed Fe 3 O 4 nanocrystals. In other words, when the growth of Fe 3 O 4 seeds through a kinetically controlled process ceases, the 2D hexagonal flakes grows into more thermodynamically favored shape according to different surface facets of different surface energy 27,28 . Fe 3 O 4 has a cubic inverse spinel structure which consists of a face-centered-cubic (FCC) close-packed structure with oxygen anions. As a FCC close-packed structure, the surface energies corresponding to different surface facets usually increase in the order of γ {111} < γ {100} < γ {110} . In this crystal structure, the Fe 3 O 4 crystals usually exist with {111} planes as the basal surfaces.
In the electrical wire pulse method, when a high rate of energy is injected by a pulse with a high-density current, the fine metal wire is heated in a liquid solvent, and the generated energy leads to evaporation and condensation of the metal as a highly dispersed nanocolloids. The nanocolloids quickly grow through vapor cooling in the solvent 29,30 . In case of our work, the oriented attachment growth would occurs to form hexagonal Fe 3 O 4 nanoflakes at early growth stage by rapid hot-injection provided from a pulse with a high-density current. At high temperature, high energy facets will lead to a fast growth rate compared to low energy facets. During rapid pulsed explosion process, the Fe 3 O 4 nanospheres and the Fe 3 O 4 -Fe nanohybrids are produced because of repeated energy injection. As can be seen Supplementary Fig. S4, typical TEM images of the Fe 3 O 4 -Fe nanohybrids are observed the attachments between aggregated particles, even larger ones. A hexagonal shaped aggregate can be seen being formed by the aggregation step ( Supplementary Fig. S4a). This aggregated particles will continue to undergo oriented attachment growth to form a polycrystalline structure and subsequent recrystallization to a single crystal since the low surface energy of the {111} facet can no longer compensate for excessive strain energy ( Supplementary Fig. S4b). Actually, both the oriented attachment growth and Ostwald ripening occur simultaneously 31,32 . In the early growth stages of hot-injection, the strong surface adsorption lead to the oriented attachment growth, and the Ostwald ripening is thermodynamically disturbed. Consequently, in later growth stage, large Fe 3 O 4 nanospheres and Fe 3 O 4 nanoflakes are formed by the Ostwald ripening.
The particle size distribution of the Fe 3 O 4 -Fe nanohybrids and Fe 3 O 4 nanospheres can be obtained by dynamic light scattering analysis after adequate ultrasonic treatment. We observed three peaks for the Fe 3 O 4 -Fe nanohybrids around 60 nm, 310 nm, and 9 μm (Fig. 3b). However, the hydrodynamic diameters of the Fe 3 O 4 nanospheres were concentrated at 300 nm and 12 μm, with no peak around 60 nm (Fig. 3c) Supplementary Fig. S5. After discharge process, ORR aggregates (Li 2 O 2 ) were covered fully on surface of the Fe 3 O 4 -Fe NH electrode (Supplementary Fig. S5a). The ORR aggregates disappeared at the end of the charge process ( Supplementary Fig. S5b), indicating the high reversibility of the Fe 3 O 4 -Fe NH electrode. However, the Fe 3 O 4 NS electrode after discharge process are formed toroidal shaped ORR aggregates and were covered incompletely ( Supplementary Fig. S5c). At the end of the charge process, the ORR aggregates were not fully decomposed ( Supplementary Fig. S5d). The Fe 3 O 4 NS electrode indicates to operate the irreversible discharge-charge process.
The cyclic voltammograms (CV) for the Fe 3 O 4 -Fe NH and Fe 3 O 4 NS electrodes are shown in Supplementary  Fig S6. Both the anodic and cathodic peaks for the Fe 3 O 4 -Fe NH electrode were significantly larger than those for the Fe 3 O 4 NS electrode over five cycles. These results demonstrate the promising ORR/OER activity of the   (Fig. 4b). Indeed, the Fe 3 O 4 -Fe NH electrode could effectively enhance the electrocatalyst kinetics over 100 cycles (Fig. 4d).
To further investigate the enhanced electrocatalyst kinetics, we employed ex-situ FESEM after dischargecharge cycling and EIS analysis. The morphological changes after discharge and charge at a current rate of 500 mA g c −1 for the Fe 3 O 4 -Fe NH electrodes are shown in Fig. 5a and b. After cell discharge, large amounts of ORR aggregates (Li 2 O 2 ) were formed on the surface of the electrode (Fig. 5a). During the subsequent charge process, the ORR aggregates disappeared at a charge capacity of 1,000 mA h g c −1 (Fig. 5b), indicating the high reversibility of the Fe 3 O 4 -Fe NH electrode.
The impedance behavior of both the Fe 3 O 4 -Fe NH and Fe 3 O 4 NS electrodes in fresh cells is almost the same, as shown in Fig. 5c-e. After the discharge process, the resistances of both the Fe 3 O 4 -Fe NH and Fe 3 O 4 NS electrodes increase significantly, which is due to the generation of insulating ORR products (Li 2 O 2 ). Interestingly, after the charge process, the resistances of both electrodes are remarkably reduced (Fig. 5e), indicating that the formed insulating ORR products can be completely decomposed during the charge process, which is consistent with the SEM images of the Fe 3 O 4 -Fe NH electrode after the charge process (Fig. 5b).
A fitting model using equivalent circuits and the corresponding fitting values are depicted in Supplementary  Table S3. In the Nyquist plots, the semicircle observed in the high-frequency region appears to be associated with film formation, mainly the accumulation of discharge products such as Li 2 O 2 and the formation of a solid electrolyte interphase (SEI) layer owing to electrolyte decomposition on the electrodes. The intercept of this semicircle with the Z real axis in the high-frequency region represents the total resistance of the electrolyte, separator, and electrical contacts (R e ), whereas the diameter of the semicircle represents the interfacial resistance (R i ), which is related to the coverage of Li 2 O 2 and SEI layers. The semicircle in the medium-frequency region is associated with the time constant for charge-transfer resistance (R ct ) at the electrode/electrolyte interface 36   decrease the electrode resistance. Similarly, the Fe 3 O 4 -Fe NH electrode has a significantly smaller R ct than the Fe 3 O 4 NS electrode after both the discharge and charge processes (see Supplementary Fig. S7). R ct is inversely proportional to the rate coefficient of the chemical reaction, the porosity and the O 2 concentration in the oxygen electrode. In Li-O 2 battery, electrolytes should diffuse safely reduced oxygen species (O 2 − or O 2 2− by ORR: O 2 + ne − → O 2 n− , n = 1 or 2). As higher O 2 concentration, R CT is lower [37][38][39] . Decrease of R CT indicates facilitated oxygen diffusion pathway and less agglomeration of the oxygen electrode. This means that the electrolyte and the electrode is activated. Therefore, the R CT is remarkably reduced that the Fe 3 O 4 -Fe NH electrode is relatively stable cycling, and provide evidences for the superior performance of the Fe 3 O 4 -Fe NH electrode due to intimate contact and effective lithium ions and oxygen diffusion. Interestingly, R i of the two electrodes show a similar tendency after discharge, and R i of both electrodes disappear after the charging process of both electrodes due to sufficient OER activity, except for R i after the first charge of the Fe 3 O 4 NS electrode. This contributes to higher electronic conductivity, indicating the high ORR/OER activity of the Fe 3 O 4 -Fe NH electrode. We have evaluated the stability of Fe nanospheres through XPS. As shown in Supplementary Fig. S8, the Fe 2p XPS spectra of pristine Fe 3 O 4 -Fe NH electrode obviously indicate Fe 0 . The Fe 2p peaks at approximately 707 eV and 720 eV are associated, with the Fe 0 2p3/2 and 2p1/2 states of the Fe metal, respectively 40 . The Fe 2p peaks disappear after discharge process because it is covered by ORR products. After charge process, the Fe 2p peaks appear again. Therefore, it seems that even after discharge-charge cycling, relatively stable Fe remains rather than complete oxidation.  Additionally, another considerable improvement of the Fe 3 O 4 -Fe NH electrode is a high rate performance in a Li-O 2 battery, Fig. 6 show the galvanostatic discharge-charge cycling performance of the Fe 3 O 4 -Fe NH and Fe 3 O 4 NS electrodes obtained in the fixed capacity regime of 1,000 mA h g c −1 at a rate of 2,000 mA g c −1 . The Fe 3 O 4 -Fe NH electrode exhibited more stable cycling performance over 150 cycles compared to the Fe 3 O 4 NS electrode. The Fe 3 O 4 NS electrode exhibited a sudden deterioration of discharge capacity because its active sites were blocked by intrinsically electronically insulating ORR products, which could be confirmed again for electrode resistance problems (Fig. 5c-e). We note that the high catalytic activity of Fe 3 O 4 -Fe NH electrode, its result in an enhancement of the electronic conductivity, and it promote the decomposition of the insulating products.

Conclusion
In this work, we fabricated the Fe 3 O 4 -Fe nanohybrid and the Fe 3 O 4 nanosphere electrocatalysts for Li-O 2 batteries via the electrical wire pulse process. The obtained the Fe 3 O 4 -Fe nanohybrids featured 0D spheres deposited on 2D flake composites, good dispersibility, and electronic conductivity. In Li−O 2 battery tests at a current rate of 500 mA g c , and stable cycling over 100 cycles at a fixed capacity of 1,000 mA h g c −1 . Importantly, a comparison of the EIS results for the Fe 3 O 4 -Fe NH and Fe 3 O 4 NS electrodes demonstrates the origin of the good ORR/OER activity.

Method
Materials and synthesis. Commercial Fe wire (0.2 mm in diameter) was purchased Nano Tech (Korea), and electrical pulse equipment (NTi-mini P, Nano Tech, Korea) was used to fabricate the Fe 3 O 4 -Fe nanohybrids and the Fe 3 O 4 nanospheres. As a similar process reported in previous works 25,26 . Fe-based aqueous nanocolloidal suspension could be successfully obtained. After completing the electrical wire pulse process with Fe wire, the obtained nanocolloidal suspension was allowed to settle for 3 days and divided into two classes of colloidal suspensions. Then, the selected nanocolloidal suspension was sonicated and filtered through a nylon membrane (Durapore, 0.22 mm, Millipore) several times, and subsequently dried at 120 °C for 8 hr. , and X-ray photo-electron spectroscopy (XPS; PHI X-tool, ULVAC-PHI, Japan). The phase and crystal structure were characterized by X-ray diffraction (XRD; Ultima III, Rigaku). The particle size distribution was determined from dynamic light scattering (DLS) analyses using a particle size analyzer (PSA; ELSZ-1000, Otsuka Electronics Korea Co. Ltd.).

Electrochemical performance of Li−O 2 cells.
The electrochemical performance of the Fe 3 O 4 nanospheres and the Fe 3 O 4 -Fe nanohybrids was evaluated using Swagelok-type cells. The electrode was prepared by mixing each Fe nanopowder (45%) with Super P carbon black (45%) and carboxymethyl cellulose (10%; CMC, Aldrich, Average Mw ~700,000). The obtained slurry was spread onto nickel foam, and the loading weight of the electrode was adjusted to above 0.2 mg of super P carbon black per cm 2 The Li-O 2 cells were assembled in an Ar-filled glove box. The cells consisted of a lithium foil as the anode, a glass microfiber filter (Celgard 2400, Wellcos) as the separator, 1 M LiNO 3 (Alfa Aesar, anhydrous, ≥99.999%) in N,N-dimethylacetamide (DMAc; Alfa Aesar, anhydrous, ≥99.8%) as the electrolyte, the electrode, and carbon cloth (W0S1002, CeTech) as a gas diffusion layer. All measurements were conducted in 1.5 atm dry oxygen to avoid any negative effects of humidity and CO 2 . The assembled cells were tested with an automatic battery cycler (WBCS 3000, WonAtech) in a voltage window of 2.0-4.8 V. Electrochemical impedance spectroscopy (EIS) was performed with an electrochemical workstation (Ivium-n-Stat electrochemical analyzer, Ivium Technologies B. V.). The impedance response was collected by applying AC voltages of 10 mV while maintaining a constant DC voltage in the frequency range of 0.01 Hz to 100 kHz. All the above measurements were conducted at room temperature.