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Uniform FexNiy Nanospheres: Cost-Effective Electrocatalysts for Nonaqueous Rechargeable Li–O2 Batteries

Uniform FexNiy nanospheres were synthesized via a simple solvothermal method and used as electrocatalysts for Li–O2 batteries. Fe7Ni3 nanospheres exhibited relatively high catalytic activities in the electrochemical tests. They delivered a reversible capacity of more than 7000 mAh/gKB and gave a discharge–charge voltage gap reduction of 250 mV compared with Ketjen Black.


INTRODUCTION
To meet the requirements of energy storage and conversion, research efforts have been directed toward rechargeable nonaqueous Li−O 2 batteries because of their extremely high theoretical specific energy of 11 400 Wh kg −1 . The energy density is 5−10 times higher than that of the intercalation cathode in the conventional lithium-ion batteries, and it is also competitive to that of gasoline or diesel oil. 1,2 However, lots of challenges still need to be overcome before Li−O 2 batteries can find practical applications in electric vehicles, hybrid vehicles, and stationary energy storage.
Among various issues, the sluggish kinetics of the oxygen reaction is an essential problem resulting in high overpotentials and affecting the performance of Li−O 2 batteries, especially for the oxygen evolution reaction (OER). 3,4 The high overpotential leads to poor round-trip efficiency and cyclability. One promising way of reducing the overpotentials is to add catalysts for the oxygen reduction reaction (ORR) and OER. 5−7 Attention has been paid to explore effective catalysts to improve the energy efficiency of Li−O 2 batteries. Cathode materials have been explored as effective catalysts in Li−O 2 batteries, including porous carbon materials, 8−10 noble metals, 11−16 noble metal oxides, 17,18 transitional metal oxides, 19−24 and perovskite-based 25,26 and pyrochlore-based 27 compounds. Among those mentioned above, carbon materials, such as graphene, 28,29 carbon nanotubes, 30,31 and mesoporous carbon, 9 show both high specific capacity and high ORR activity due to their large specific area and superior electronic conductivity. However, the OER activity is insufficient. The noble metal-based materials have been attracting much attention in this field because of their high activity for both ORR and OER, but they are too expensive for general use. Thus, the transitional metal oxides have been developed into promising electrocatalysts for ORR and OER process because of their low cost and high activity. However, there is still a huge gap in comparison to noble metals, so a lower overpotential needs to be achieved by further study. Therefore, exploring new catalysts to reduce the overpotential and increase the round-trip efficiency is still an important challenge for Li−O 2 batteries. The transitional metals and their alloys in nanoscale have good electronic conductivity, which is beneficial for the catalyst synergistic effect to possess high electrocatalytic activity, and various active sites. Previous studies have shown that Co 32 and Ni 33 nanoparticles have a higher electrocatalytic activity for OER or ORR. It has been proved that the transition metals could accelerate the reaction process. The current research also demonstrates that FeCo/CNT displays excellent electrocatalytic activity during the OER and ORR processes, which is more superior than that of the CNTs. 34 Similarly, in comparison with the single metal, FeNi 3 @C presents superior electrocatalytic activity in the OER process with water system due to the optimized surface electronic structure as a result of the introduction of Fe, which also contributed to the synergistic effect of the alloy. 35 All of these above reports suggest that the transition metal alloys in nanoscale are promising electrocatalysts in the Li−O 2 batteries, as well as other electrocatalytic field.
Moreover, the earth-enriched element of Fe and Ni were studied as transition metal alloys to prepare cost-effective electrocatalysts. Therefore, we have selected two stable structures Fe 7 Ni 3 and FeNi 3 , which have a higher content of Fe atoms and synergistic effect. In this work, uniform Fe x Ni y nanospheres, including the body-centered cubic Fe 7 Ni 3 and the face-centered cubic FeNi 3 , were synthesized via a simple solvothermal method and employed in the Li−O 2 battery for the first time. The Fe x Ni y nanospheres enhanced the performance of the Li−O 2 batteries efficiently. In particular, the Fe 7 Ni 3 nanosphere presents a superior performance with a higher initial discharge capacity of 7266 mAh/g KB and a lower overpotential during the charge−discharge process, wherein the voltage gap is reduced by 250 mV compared with that of the Ketjen Blank (KB) cathode.

RESULTS AND DISCUSSION
The Fe x Ni y nanospheres were prepared via a facile solvothermal method. During the slow processes of nucleation and growth, some critical factors needed to be controlled. First, before the reaction, the mixture must be continuously stirred to be homogeneous. Simultaneously, the oxygen of the system was removed by a swift Ar flow for about 5 min. In addition, the octadecylamine served as a surfactant and was significant for the reaction. It tended to form a polymer−metal complex prescursor in the presence of the long-chain organic compounds and restricted the nucleation and the growth of the crystal nucleus. 36−38 The hydrazine hydrate acted as a reductant to reduce Fe 3+ and Ni 2+ and then formed an alloy in the alkaline system. The Fe x Ni y nanospheres served as electrocatalysts for Li−O 2 batteries, and the Li−O 2 batteries performance were investigated, as shown in Figure 1. The Li− O 2 batteries were assembled with a Li foil serving as an anode and Fe x Ni y nanospheres mixed with KB was employed as the air cathode; the two ends were separated by a glass fiber filter separator. The X-ray diffraction (XRD) patterns of the asprepared samples are presented in Figure 2. As shown in Figure  2, the XRD results show the diffraction peaks that matched well with the body-centered cubic Fe 7 Ni 3 phase (JCPDS 65-7251) and the face-centered cubic FeNi 3 phase (JCPDS 38-0419). Through inductively coupled plasma-atomic emission spectrum (ICP-AES) measurement, the atomic ratios of the two samples were determined to be 7:3 and 1:3 (Table 1), corresponding to the compositions of Fe 7 Ni 3 and FeNi 3 , consistent with the XRD results.  Figure 3a. The TEM and HRTEM images of Fe 7 Ni 3 nanospheres shown in Figures 3b and S1a give further evidence that the as-prepared secondary particles are composed of Fe 7 Ni 3 primary nanoparticles with the size of less than 10 nm. The composition of FeNi 3 was similar to that of Fe 7 Ni 3 , but the range of particle size was different. The HRTEM image for Fe 7 Ni 3 nanospheres shown in Figure 3c confirms the unidirectional fringe pattern and the measured interlayer spacing of 0.203 nm, corresponding to the (110) plane. The corresponding selected area electron diffraction (SAED) pattern in this region is shown in the inset of Figure 3c, which shows that the clear diffraction ring can be indexed to the    X-ray photoelectron spectroscopy (XPS) was employed to further distinguish the formation of Fe 7 Ni 3 and FeNi 3 alloys, whose spectra are illustrated in  Figure 4c,f, respectively, are assigned to Ni 2+ . 39 These results suggest a large amount of Fe  and Ni on the surface of the as-prepared Fe x Ni y nanospheres. The oxides were formed due to exposure of the active metal nanoparticles to air, which caused the surface oxidation. Combining the XPS and XRD results, we can prove that Fe x Ni y alloys were formed rather than the individual metals of Fe and Ni. 44 The Fe x Ni y nanospoheres generally reveal a higher electrical conductivity than metal oxides, which could decrease the resistance and facilitate the transfer of electrons in the oxygen cathodes. Besides, the size of the as-prepared samples is tiny which is endowed with large surface area, resulting in abundant active sites in catalytic reactions. Therefore, these Fe x Ni y nanospheres are promising electrocatalysts for Li−O 2 batteries, and their performance is being further studied. To prove the electrocatalytic activity of the Fe x Ni y nanospheres, they were assembled as the oxygen electrode in Li−O 2 batteries. For a comparison, the pure Ketjen Blank (KB) electrode was used in the same conditions. Figure 5a displays the discharge−charge curves of the electrodes of KB, Fe 7 Ni 3 (KB + Fe 7 Ni 3 nanospheres), and FeNi 3 (KB + FeNi 3 nanospheres) at the current density of 150 mA/g KB . Fe 7 Ni 3 nanospheres showed the best electrochemical property. The discharge plateau of Fe 7 Ni 3 electrode was 2.72 V, 50 mV than that of KB (2.67 V) and 50 mV more than that of FeNi 3 (2.67 V) electrodes, which indicated that Fe 7 Ni 3 nanospheres can efficiently facilitate the ORR activity in the Li−O 2 batteries. As for the charge process, the plateau of Fe 7 Ni 3 electrode was about 200 mV lower than that of pure KB electrode. In addition, Fe 7 Ni 3 electrode showed about 180 mV lower charge plateau than FeNi 3 electrode. Fe 7 Ni 3 electrode exhibited a high round-trip efficiency of 63.8%, which was higher than that of FeNi 3 electrode (60.0%) and KB electrode (59.8%), indicating a higher energy density than what could be reached by Fe 7 Ni 3 electrode. 45 It suggested that Fe 7 Ni 3 had more efficient active sites than FeNi 3 for the OER process. As seen from Figure 5a, the initial specific discharge capacity of Fe 7 Ni 3 , FeNi 3 , and KB electrode is 7608, 5507, and 5487 mAh/g KB , respectively. The corresponding coulombic efficiency of Fe 7 Ni 3 , FeNi 3 , and KB electrodes was 95.5, 80.6, and 44.6%, which indicated a high reversibility of Fe 7 Ni 3 electrode. 38 All of these results demonstrated that Fe 7 Ni 3 nanospheres can reduce the discharge−charge overpotential and increase the specific capacity of the Li−O 2 batteries to a great extent. To avoid serious polarization of the electrode during the discharge process, the cycle stability of the Fe x Ni y electrodes was tested under a capacity-limited strategy of 1000 mAh/g KB and a current density of 150 mA/ g KB . As shown in Figure 5b, Fe 7 Ni 3 electrode can remain discharged with no decay after 15 cycles, whereas FeNi 3 electrode only operated for 10 cycles during the capacitylimited cycle test. This result proved that Fe 7 Ni 3 electrode exhibited better performance with more number of cycles. The details of the discharge−charge curves for the Fe x Ni y nanospheres are illustrated in Figure 6. The correlation between the discharge-medium voltage and the cycle numbers is presented in Figure 5b. The discharge-medium voltage was reduced with increasing cycle numbers, indicating the overpotential was generally enhanced, which could be caused by the increased impedance of the Li−O 2 batteries after cycling Figure  7. 45 In rechargeable Li−O 2 batteries, the formation and decomposition of Li 2 O 2 were the dominant reactions during discharge and charge processes. Simultaneously, some byproducts were formed due to the reaction of electrolyte or carbon materials. Undesirable byproducts, such as Li 2 CO 3 , cannot be decomposed completely during the charge process and were accumulated on the air electrode after cycling and finally resulted in an increase in the overpotential. 45,46 In addition, the electrocatalytic activity of OER was the significant factor to affect the overpotential and the charge capacity. The linear sweep voltammetry (LSV) of OER and chronoamperometry profiles were measured after the discharge capacity depth of 1000 mAh g −1 (Figure 8) and in the Li−O 2 battery with the Li 2 O 2 cathode ( Figure S2), respectively. As shown in Figure 8a, it is obvious that the current density of Fe 7 Ni 3 is higher than that of FeNi 3 electrode, which corresponded with Figure S2a (a lower potential of oxidation and a higher current density), indicating a higher electrocatalytic acitivty of Fe 7 Ni 3 electrode. 38 Figure 8b presents the chronoamperometry profiles of Fe 7 Ni 3 and FeNi 3 , which was used to investigate the OER kinetics. The current density of FeNi 3 electrode was lower than that of Fe 7 Ni 3 electrode at the potential of 4.0 V, corresponding with the result of Figure S2b (a lower current density of 0.05 mA cm −2 ), implying a fast kinetics process for OER on Fe 7 Ni 3 electrode. 47 On the basis of these results, the possibility of the capacity decade was related with the OER activity, which corresponded with the discharge−charge profiles (Figure 5a).
As we all know, the generally accepted mechanism of the Li− O 2 battery during the discharge process is that O 2 undergoes a one-electron reduction to form LiO 2 as an intermediate, which is adsorbed on the electrocatalyst. Due to the different adsorption energy, the LiO 2 displays evident selective adsorption on special crystal planes of electrocatalysts. LiO 2 with an oxygen-rich surface is beneficial to the formation of Li 2 O 2 . Simultaneously, after continuously charging to 4.5 V, the discharge product is decomposed completely. 48,49 However, in this work, the battery operated through a different process in which LiO 2 was the main product of the discharge. To investigate the mechanism of the reaction process, the Raman spectra were employed to detect the intermediate product. Figure 7 illustrates the Raman spectra of Fe 7 Ni 3 and FeNi 3    Figure 7a,b demonstrates the presence of peaks at 1123 and 1505 cm −1 , which is associated with the formation of LiO 2 . The peak at 1123 cm −1 was a signature of the LiO 2 -like component. The peak at 1505 cm −1 was attributed to the distortion of the graphite ring stretching model, which is a consequence of the strong interaction between LiO 2 and graphite carbon surface. 15,50−52 There was a weak peak at about 820 cm −1 in Figure 7b, which could be ascribed to Li 2 O 2 . The typical peak of the commercial Li 2 O 2 was appropriately 790 cm −1 , which was lower than that given in this work. We and other authors suspected that the electrochemically formed Li 2 O 2 differed from the commercial Li 2 O 2 , resulting from the interactions with the Fe x Ni y and graphite carbon surface. 49,53,54 The relatively intensity of LiO 2 was much higher than that of Li 2 O 2 , which indicated that LiO 2 was the dominant product. After continuously charging at 4.5 V, the peaks of Li 2 O 2 and LiO 2 disappeared, indicating the decomposition of Li 2 O 2 and LiO 2 . The similar results were observed for FeNi 3 cathode in the discharge−charge process. Thus, the Fe x Ni y nanospheres tended to catalyze the formation of LiO 2 , which could increase the battery life.
The morphology of the Fe x Ni y electrodes at different states was characterized by ex situ SEM. As shown in Figure 7b, the first discharged Fe 7 Ni 3 electrode seemed like a ball-like composite with a diameter of approximately 1 μm packed on the electrode. The product disappeared and the electrode surface recovered the same as the fresh one (Figure 7a) after the battery was recharged (Figure 7c), indicating Fe 7 Ni 3 electrode has good reversibility over cycling. The discharged product was further investigated by ex situ XRD. The weak diffraction peaks at 32.9, 34.9, and 58.7°(magnified in the inset in Figure 9g) corresponded to Li 2 O 2 (JCPDS 74-0115) and little Li 2 CO 3 that appeared after the first discharge process. In addition, no obvious diffraction peaks of LiO 2 were detected by XRD measurement due to the amorphous nature of the discharge products. 51 As for FeNi 3 electrode, there was no Figure 9. SEM images of the fresh (a), first discharged (b), and first recharged (c) Fe 7 Ni 3 electrodes and the fresh (d), first discharged (e), and first recharged (f) FeNi 3 electrodes. XRD patterns of Fe 7 Ni 3 electrodes (g) and FeNi 3 electrodes (h) at different states, and the inset shows the partial magnified patterns of the discharged electrodes. evident difference at different states in comparison to Fe 7 Ni 3 electrode at different states. The first charged electrode of FeNi 3 seemed blurry. It suggested that some product was decomposed incompletely or some byproducts cannot be decomposed on the surface of the electrode. This result was coincident with the different performance, including the reversibility and overpotentials, between Fe 7 Ni 3 and FeNi 3 electrodes in discharge−charge curves. In this work, Fe 7 Ni 3 cathode presents a higher specific capacity and a lower overpotential, especially in the OER process. Previous researches reported that increasing the mass of Fe in alloy enhanced the OER process, which was attributed to the optimized electronic structure due to the introduction of Fe. 35,42,55 It endowed the active sites with appropriate adsorption energy, which benefited LiO 2 adsorption in the ORR process and improved LiO 2 desorption in the OER process. Fe 7 Ni 3 and FeNi 3 electrodes exhibited big difference in the performances. Therefore, this result may be caused by the mass of Fe, which could optimize the electron structure of the material surface. 35

CONCLUSIONS
In summary, body-centered cubic Fe 7 Ni 3 and face-centered cubic FeNi 3 nanospheres were successfully synthesized by a simple solvothermal method. Fe 7 Ni 3 nanospheres exhibited a much higher activity and durability than FeNi 3 nanospheres and served as electrocatalysts for nonaqueous rechargeable Li−O 2 batteries. Significantly, the Fe x Ni y nanospheres employed as electrocatalysts tend to change the discharge products in nature, differing from other transition metal electrocatalysts. This work would provide a new perspective for transition metals electrocatalysts in Li−O 2 batteries and broaden the scope for future research.

EXPERIMENTAL SECTION
4.1. Preparation of Fe x Ni y Nanospheres. KOH (1 g) and octadecylamine (2 g) were added into 35 mL N-methylpyrrolidone, and then the certain amounts of iron(III) acetylacetonate and nickel(II) acetylacetonate were added successively with continuous stirring. When the solution became homogeneous, the mixture was transferred to a Teflon-lined autoclave followed by adding 5 mL hydrazine hydrate and removing the air by Ar flow for about 5 min. Then, the autoclave was treated at a temperature of 180°C in an electric oven for 2 h to produce Fe x Ni y nanospheres. Finally, the products were obtained by centrifuging at 12 000 rpm. The as-prepared samples were washed alternately several times with n-hexane, acetone, and dried at 40°C under vacuum.
4.2. Characterization. The crystalline structures of the assynthesized samples were analyzed by X-ray powder diffraction (XRD) using a Phillips X'pert Pro MPD diffractometer (Cu Kα, λ = 1.54056 Å). The size distribution and morphology of the products were characterized by a field emission scanning electron microscopy (FESEM, acceleration voltage of 5 kV, S-4800, Hitachi) and a high-resolution transmission electron microscopy (HRTEM, an acceleration voltage of 200 kV, JEM-2010, JEOL). The compositions of the as-prepared samples were investigated by inductively coupled plasma-atomic emission spectrum (ICP-AES, Jarrel-ASH, ICAP-9000). The surface composition of the samples were characterized by X-ray photoelectron spectroscopy (XPS) using ESCALAB 250Xi spectrometer (Thermo Fisher) with Al Kα radiation as the Xray source for excitation.
4.3. Electrochemical Measurements. The air electrode was assembled in the composition of Fe x Ni y nanospheres 45 wt %, Ketjen Black (KB) 45 wt %, and poly(vinylidene fluoride) (PVDF) binder 10 wt %. The pure KB electrode consisted of KB 90 wt % and PVDF 10 wt %. N-Methyl-pyrrolidone was added to the powder mixture to make a uniform slurry. Then, the slurry was pasted on the carbon papers and dried at 80°C overnight in a vacuum oven to form air electrodes. Each of the composite electrode paste was ca. 0.8 mg. In comparison, the KB electrode was ca. 0.4 mg. Electrochemical measurements were carried out in CR2032 coin cells with holes to allow O 2 diffusion. The coin cells consisted of a Li foil anode, a glass fiber filter separator, an electrolyte (1 M LiTFSI-TEGDME) and air electrode, were assembled in an Argon-filled glove box with water and oxygen levels less than 0.5 ppm. Galvanostatic discharge−charge measurements were carried out on a Neware battery test system, and the current density and specific capacity was calculated based on the amount of KB. The linear sweep voltammetry (LSV) and chronoamperometry were measured on an electrochemical workstation (Zennium IM6 station) after discharge at the depth of 1000 mAh g −1 .

Author Contributions
The manuscript was written through contributions of all of the authors. All of the authors approved the final version of the manuscript.

Notes
The authors declare no competing financial interest.