Plasma Enabled Fe2O3/Fe3O4 Nano-aggregates Anchored on Nitrogen-doped Graphene as Anode for Sodium-Ion Batteries.

Low electrical conductivity severely limits the application of Fe2O3 in lithium- and sodium-ion batteries. In respect of this, we design and fabricate Fe2O3/Fe3O4 nano-aggregates anchored on nitrogen-doped graphene as an anode for sodium-ion batteries with the assistance of microwave plasma. The highly conductive Fe3O4 in the composite can function as a highway of electron transport, and the voids and phase boundaries in the Fe2O3/Fe3O4 heterostructure facilitate Na+ ion diffusion into the nano-aggregates. Furthermore, the Fe-O-C bonds between the nano-aggregates and graphene not only stabilize the structural integrity, but also enhance the charge transfer. Consequently, the Fe2O3/Fe3O4/NG anode exhibits specific capacity up to 362 mAh g-1 at 100 mA g-1, excellent rate capability, and stable long-term cycling performance. This multi-component-based heterostructure design can be used in anode materials for lithium- and sodium-ion batteries, and potential opens a new path for energy storage electrodes.

1. Introduction

Insufficient pan class="Chemical">lithium resources will seriously threaten the availability of future pan class="Chemical">lithium-ion batteries (LIB). Owipan class="Chemical">ng to a similar working mechanism to LIBs, abundance, lower price, and environmental friendliness, sodium-ion batteries (SIBs) have gradually become a hotspot for energy storage [1,2,3,4]. Advances in SIB technology rely on the selection of suitable electrode materials to accommodate, insert, and extract Na+ ions with a larger radius than Li+ [5,6].

Amopan class="Chemical">ngn> many pan class="Chemical">metal oxides, the theoretical capacity of pan class="Chemical">Fe2O3 is as high as ~1007 mAh g−1, and its environmental friendliness, low cost, and abundant resources make it an ideal anode material for SIBs [7]. However, Fe2O3 undergoes a large volume expansion (200%) during charge-discharge, resulting in electrode pulverization and deteriorated electrical contact problems [8,9]. On the other hand, the low electrical conductivity of Fe2O3 (10−14 S cm−1) and low ionic diffusivity limits the rate performance of batteries [10,11]. Common strategies are to design nanosized iron oxide, and to use conductive additives, such as graphene [9,12,13]. Another iron-based oxide, Fe3O4, has a slightly lower specific capacity (926 mAh g−1) and much higher electrical conductivity (102~103 S cm−1) than Fe2O3 [14]. Rationally compounding Fe2O3 and Fe3O4 may improve the conductivity, facilitate the redox charge transfer, and thus the rate performance. As has been demonstrated in supercapacitors, the conductive Fe3O4 and capacitive Fe2O3 synergistically produced excellent rate capability and cycling stability [15,16,17]. Recently, α-Fe2O3/Fe3O4 composite, Fe2O3/Fe3O4/FeCO3 composite, and porous Fe2O3/Fe3O4@carbon have also been applied in LIBs and exhibited improved electrochemical performance in comparison with the Fe2O3 electrodes [18,19,20]. In these studies, the Fe3O4 plays the role of electron transport pathway, while the porous structure can facilitate fast ion transport. It is even claimed that the hetero-interfaces between the oxide components may provide an enhanced inner electric field which can assist the electron transfer and Li+ diffusion [19].

In this work, we designed pan class="Chemical">Fe2O3/pan class="Chemical">Fe3O4 nano-aggregates anchored on nitrogen-doped graphene (Fe2O3/Fe3O4/NG) as an anode of SIBs. Upon a microwave plasma process, the Fe2O3/Fe3O4/NG composite exhibited rich phase boundaries and voids. When working as an SIB anode material, the Fe2O3/Fe3O4/NG shows high specific capacity (362 mAh g−1 at 100 mA g−1), excellent cycling stability (84% capacity retention after 100 cycles at 1 A g−1), and superior high-rate capability. The improved electrochemical performances are due to fast electron transport through Fe3O4, accelerated Na+ ion transport through Fe2O3/Fe3O4 phase boundaries, and voids in the nano-aggregates.

2. Materials and Methods

2.1. Materials Preparation

The pan class="Chemical">graphene oxide (GO) used in this work was produced from natural pan class="Chemical">graphite flakes (Acros, Geel, Belgium) by a modified Hummer’s method [21]. The aqueous GO dispersion was centrifuged at 11,000 rpm for 30 min and was redispersed in de-ionized (DI) pan class="Chemical">water more than 3 times. By discarding the supernatant, GO suspension in N, N-dimethyl formamide (DMF, 98%, Sigma-Aldrich, St. Louis, MO, USA) with 2 mg mL−1 was made.

pan class="Chemical">Fe2O3/pan class="Chemical">NG was synthesized by a solvothermal method. Fe(NO3)3·9H2O (840 mg, 99.9%, Sigma-Aldrich) was dissolved in the as-obtained GO suspension (100 mL). After stirring for 60 min, the mix was transferred into a Teflon-lined autoclave and was kept at 180 °C for 6 h. The as-obtained gel was rinsed in DI water for 24 h, then freeze-dried at −60 °C, 20 Pa for 24 h to generate Fe2O3/NG. The as-prepared bulk Fe2O3/NG was ground to powder with a mortar and pestle. After treating in a home-made microwave plasma fluidized bed (power of microwave = 400 W, Ar 40 sccm, H2 10 sccm, 20 min), Fe2O3/Fe3O4/NG was obtained.

2.2. Materials Characterization

The crystal structure and phases of the material were characterized by X-ray diffraction (XRD, Bruker AXS GmbH, Karlsruhe, Germany) usipan class="Chemical">ngn> Cu-Kα radiation. Morphology was analyzed by a field emission scannipan class="Chemical">ng electron microscope (SEM, Hitachi S4800, Tokyo, Japan). The transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) were performed on a Hitachi HT7700 (Tokyo, Japan) and Talos F200X (Waltham, MA, USA), respectively. Thermogravimetric analysis was performed on PE STA8000 (Waltham, MA, USA). X-ray photoelectron spectroscopy (XPS) was performed on KRATOS Axis Supra XPS system (Kratos, Manchester, UK). The measured binding energies were calibrated to the reference energy by C1s = 284.6 eV. Raman spectroscopy was measured using the RM2000 system (Renishaw, London, UK), using a laser source with 17 mW at 532 nm.

2.3. Electrochemical Measurements

The active material, pan class="Chemical">carbonn> black, and pan class="Chemical">polyvinylidene fluoride (pan class="Chemical">PVDF, Sigma-Aldrich) were added to N-methyl-2-pyrrolidone (NMP, 98%, Sigma-Aldrich) at a mass ratio of 8: 1: 1, and fully stirred to obtain a slurry. The resulting slurry was applied to copper foil, and dried in a vacuum oven at 100 °C for 12 h to obtain negative electrodes. The total mass loadings on a copper foil (20 mm in diameter, 99.99%, Sigma-Aldrich) are 1.13 and 0.75 mg for the Fe2O3/Fe3O4/NG and Fe2O3/NG electrodes, respectively. They were electrochemically characterized in CR2016-type coin cells, using Na metal as the counter electrode and reference electrode, and glass microfiber (Whatman, Little Chalfont, UK) as the separator. Dissolving 1 M NaClO4 (98%, Acros) in a mixed solvent of ethylene carbonate (EC, Acros) and diethyl carbonate (DEC, Acros) with 1:1 volume ratio, and adding 5 vol.% of fluoroethylene carbonate (FEC, Acros) additive, the electrolyte was prepared. Galvanostatic charge-discharge were performed on a LAND battery tester (Wuhan, China) in range of 0.01 to 3.0 V (vs. Na/Na+). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were performed using the CHI660E electrochemical workstation (Shanghai, China). All as-assembled half cells stood in a glove box for 12 h EIS tests without cycling. All capacities were calculated based on the mass of the active materials.

3. Results

The pan class="Chemical">Fe2O3/pan class="Chemical">Fe3O4/NG nanocomposite was obtained by a solvothermal synthesis of Fe2O3/NG followed by a microwave plasma treatment. Taking advantage of the activation ability of plasma, the Fe2O3 was partially reduced to the Fe3O4, and Fe2O3/Fe3O4 nanoaggregates were formed, as illustrated in Scheme 1 [22].

Scheme 1

Schematics of synthesis of Fe2O3/NG and Fe2O3/Fe3O4/NG composites.

As shown in the SEM and TEM images (Figure 1a,b), pan class="Chemical">n class="Chemical">Fe2O3 nanopan>rticles with size around 50 nm are well enveloped in pan class="Chemical">graphene in the pan class="Chemical">Fe2O3/NG composite. After plasma radiation, the morphology of the composite and the particle size distribution showed no significant change (Figure 1c and the inset). Under TEM observation, however, the previous spherical or ellipsoidal particles turned out to be in irregular shape. Meanwhile, dark and bright areas can be differentiated, of which the bright area is perhaps voids in the particles (Figure 1d). HRTEM reveals that the inhomogeneity of the particles stems from phase segregation of Fe2O3 and Fe3O4. In a close view of a particle, fringe spacing of 0.37 nm (corresponding to (012) plane of Fe2O3) and 0.29 nm (corresponding to the (220) plane of Fe3O4) can be found simultaneously (Figure 1e). The results confirmed that the Fe2O3 was partially reduced to Fe3O4 by energetic species in the plasma. Driven by phase segregation, Fe2O3/Fe3O4 nano-aggregates were eventually formed. The mass fraction of active materials ise about 87 and 80 wt.% for Fe2O3/Fe3O4/NG and Fe2O3/NG, respectively, as calculated from the thermal-gravimetric analysis and differential thermal analysis (TGA/DTA) results (Figure 1f).

Figure 1

(a,b) SEM, TEM images of Fe2O3/NG composites. (c,d) SEM, TEM images of Fe2O3/Fe3O4/NG composites. The inset shows particle size distribution of Fe2O3/Fe3O4. (e) An HRTEM image of a Fe2O3/Fe3O4 particle, the right two panels denote interplane spacings of Fe2O3 and Fe3O4. (f) TGA curves of Fe2O3/NG and Fe2O3/Fe3O4/NG composites.

XRD analysis shows diffraction peaks at 24.13, 33.15, 35.61, 40.85, 49.48, 54.09, 62.45, and 63.9° for the pan class="Chemical">Fe2O3/pan class="Chemical">NG (Figure 2a), which are corresponding to the (012), (104), (110), (113), (024), (116), (214), and (300) planes of Fe2O3 (PDF No.33-0664), respectively. For the Fe2O3/Fe3O4/NG, additional peaks at 30.07, 37.05, and 47.13° can be found, which are corresponding to the (220), (222), and (331) planes of Fe3O4 (PDF No 99-0073), respectively, confirming coexistence of Fe2O3 and Fe3O4 phases (Figure 2a). The Raman D peak of RGO are upshifted from 1340 (RGO) to 1343 cm−1 (NG) and G peak from 1573 to 1576 cm−1 after N doping (Figure 2b). This may be due to the strain of the graphene basal plane caused by C–N bond, indicating the successful insertion of nitrogen heteroatoms [23]. Additionally, the unchanged Raman spectrum after microwave plasma process indicates stable N doping upon plasma (red curve in Figure 2b).

Figure 2

(a) XRD patterns of Fe2O3/Fe3O4/NG and Fe2O3/NG composites. (b) Raman spectra of microwave processed NG (MW-NG), NG, and RGO in the range of 1000–1800 cm−1.

XPS survey scans discovered pan class="Chemical">Fe 2p, O 1s, N 1s, and C 1s states in both pan class="Chemical">Fe2O3/Fe3O4/NG and Fe2O3/NG (Figure 3a). In Figure 3b, the high-resolution Fe 2p spectra of the composites have two main peaks at 711.2 and 724.7 eV, corresponding to Fe 2p3/2 and Fe 2p1/2, respectively. In the Fe2O3/NG, the Fe 2p spectrum consists of a series of characteristic peaks of Fe3+ in Fe2O3, which are two spin energy separated components (Fe3+ (oh) at 711.1 and 725.1 eV), Fe 2p3/2 satellite (719.7 eV), and Fe 2p1/2 satellite (733.8 eV) signals, but no F0e or Fe2+ signal is observed. The Fe2O3/Fe3O4/NG has not only the same components with Fe2O3/NG, but also Fe2+ (oh), Fe3+(td), Fe2+(oh), and Fe3+(td) signals of Fe3O4 at 710.0, 713.0, 723.6, and 727.3 eV, respectively [17]. The XPS analysis clearly evidences the coexistence of Fe2O3 and Fe3O4 in the hybrid material. With the component area ratio of Fe2+ to Fe3+, the atomic ratio of Fe2+ to Fe3+ can be estimated to be 0.102. Combined with the TGA results, the mass loading of Fe2O3 and Fe3O4 are calculated to be 0.61 and 0.23 mg, respectively, as described in the Supporting Information for detail.

Figure 3

(a) XPS spectra of Fe2O3/Fe3O4/NG and Fe2O3/NG. The corresponding (b) Fe 2p spectra, (c) O 1s spectra, and (d) C 1s spectra.

As shown in Figure 3c, the pan class="Chemical">deconvolution of the O 1s profiles reveals three components, Fe–O (530.2 eV), Fe–O–C (531.2 eV), and C–O (533.8 eV). The areal percentage of the Fe–O–C bonds component increases from 53.5% for the pan class="Chemical">Fe2O3/pan class="Chemical">NG to 58% for the Fe2O3/Fe3O4/NG. The additional Fe–O–C bonds can not only promote the electron transport in the Fe2O3/Fe3O4/NG, but also stabilize the Fe2O3/Fe3O4 on the graphene sheet during charge/discharge, thereby improving the electrochemical kinetics and stability. [24] In Figure 3d, deconvolution of the C 1s profiles of the nanostructured hybrid material reveals four components including C–C (284.7 eV), C–N (285.2 eV), C–O(286.5 eV), and O–C=O (289.35 eV) [25,26], which remain unchanged before and after the plasma process, except for a slight areal ratio decrease of the O–C=O bonds from 22.78% for the Fe2O3/NG to 16.64% for the Fe2O3/Fe3O4/NG.

The CV curves from the 2nd to the 4th cycle of the elpan class="Chemical">ecn>trodes are shown in Figure 4a,b. In the cathodic sweeps, a peak around 1.2 V for the pan class="Chemical">Fe2O3/pan class="Chemical">Fe3O4/NG in Figure 4a can be assigned to the reduction of Fe2+/Fe3+ to Fe0 (Equations (1) and (2)), while the peak around 0.9 V for the Fe2O3/NG in Figure 4b corresponds to the reduction of Fe3+ to Fe0 (Equation (1)). In the anodic sweeps, broad anodic peaks around 1.5 V are ascribed to the oxidation of Fe0 to Fe3+ for both electrodes (insets in Figure 4a,b) [24,27]. Only the Fe2O3/Fe3O4/NG electrode has a small shoulder peak at 0.8 V (inset in Figure 4a), which is an indication of the oxidization of Fe0 to Fe2+ [19]. The lower peak-to-peak separation between the redox peaks and better coincidence of the sweeps of the Fe2O3/Fe3O4/NG suggest faster charge transfer kinetics and better reversibility. Fe Fe

Figure 4

CV curves of (a) Fe2O3/Fe3O4/NG and (b) Fe2O3/NG anodes from the 2nd to 4th cycle at a scan rate of 1.0 mV s−1 between 0.05 and 3 V vs. Na/Na+. Galvanostatic charge-discharge curves of (c) Fe2O3/Fe3O4/NG and (d) Fe2O3/NG anodes at a current density of 100 mA g−1.

Figure 4c,d shows charge–discharge curves of the two types of elpan class="Chemical">ecn>trodes at a current density of 100 mA g−1 in the voltage window of 0.05–3.0 V (vs. Na/Na+). The initial sppan class="Chemical">ecific discharge and charge capacities of the pan class="Chemical">Fe2O3/NG electrode are 931 and 334 mAh g−1, respectively, with an initial coulombic efficiency of 36%. After the microwave plasma process, the obtained Fe2O3/Fe3O4/NG composite exhibits initial specific discharge capacity of 1004 mAh g−1, charge capacity of 363 mAh g−1, and an almost identical initial coulombic efficiency. The lost capacity mainly stems from the formation of SEI and probably degradation of the electrolyte. In the subsequent discharge–charge cycles, the Fe2O3/Fe3O4/NG electrode has significantly higher capacities than the Fe2O3/NG.

The rate performances of pan class="Chemical">Fe2O3/pan class="Chemical">NG and Fe2O3/Fe3O4/NG electrodes are compared in Figure 5a. With progressively growing current density from 100 to 1200 mA g−1, the Fe2O3/Fe3O4/NG composite constantly exhibits higher discharge capacities than the Fe2O3/NG. After a series of cycles, a reversible discharge capacity of 305 mAh g−1 at 100 mA g−1 is reached for the Fe2O3/Fe3O4/NG, comparing with 239 mAh g−1 for the Fe2O3/NG. The specific capacity contributed by Fe2O3 in the Fe2O3/Fe3O4/NG can be estimated by the XPS and TGA results. For example, the reversible capacity contributed by Fe2O3 after the rate performance testing at 100 mA g−1 can be 307–358 mAh g−1, which is higher than the 239 mAh g−1 for the Fe2O3/NG. This evidently suggests a synergetic effect of Fe2O3 and Fe3O4 in the electrode. The calculation of capacity contributed by Fe2O3 is detailed in the Supporting Information.

Figure 5

(a) Rate performance of Fe2O3/NG and Fe2O3/Fe3O4/NG tested at current densities from 100 to 1200 mA g−1. (b) Cycling performances of Fe2O3/NG and Fe2O3/Fe3O4/NG at current density of 100 mA g−1. (c) Cycling performance of Fe2O3/Fe3O4/NG at current density of 1000 mA g−1. (d) Nyquist plots of electrochemical impedance spectra (EIS) of Fe2O3/NG and Fe2O3/Fe3O4/NG.

In Figure 5b, the cyclipan class="Chemical">ngn> performance of pan class="Chemical">Fe2O3/pan class="Chemical">NG and Fe2O3/Fe3O4/NG electrodes in 100 cycles are compared at a current density of 100 mA g−1. After 100 cycles, the capacity of Fe2O3/Fe3O4/NG is 291 mAh g−1, which is much higher than that of Fe2O3/NG 218 mAh g−1. At a higher current density (1000 mA g−1), Fe2O3/Fe3O4/NG can still discharge 158 mAh g−1 after 100 cycles (capacity retention rate 84%, Figure 5c).

In Figure 5d, the elpan class="Chemical">ectrochemical impedance spectra (EIS) Nyquist plots of pan class="Chemical">Fe2O3/NG and Fe2O3/Fe3O4/NG electrodes present a depressed semicircle in the high-frequency region and a straight line in the low-frequency region. To extract the EIS parameters, an equivalent circuit is proposed, as displayed in Supplementary Figure S1. The EIS tests were performed before cycling of the electrodes, with open circuity voltages of 1.65 and 1.47 V for the Fe2O3/NG and Fe2O3/Fe3O4/NG, respectively. As listed in Table 1, both cells present similar values of electrolyte resistance (Rs). The Fe2O3/Fe3O4/NG electrode has a charge transfer resistance (Rct: 134.6 Ω) that is only half of the Fe2O3/NG (210.7 Ω). The reduction of the semicircle (or Rct) is due to more Fe-O-C bonds and the appearance of Fe3O4 in the electrode, indicating that the charge transfer kinetics in Fe2O3/Fe3O4/NG is faster. Moreover, the one order of magnitude higher diffusion coefficient of Na+ (DNa, obtained by fitting the EIS, see Supporting Information for detail) in the Fe2O3/Fe3O4/NG can be attributed to the fast Na+ diffusion through the phase boundaries and voids in the Fe2O3/Fe3O4 heterostructure.

Table 1

The fitted EIS parameters and diffusion coefficients of Fe2O3/NG and Fe2O3/Fe3O4/NG.

Electrode	Rs (Ω)	Rct (Ω)	DNa+ (cm2 s−1)
Fe2O3/NG	6	210.6	1.65 × 10−12
Fe2O3/Fe3O4/NG	8.9	134.7	1.34 × 10−11

The lopan class="Chemical">ng-term cyclipan class="Chemical">ng performance is elucidated by post-mortem observation, as shown in Figure 6. Serious cracks and particle pan class="Disease">swelling can be observed for the Fe2O3/NG electrode after 100 discharge-charge cycles at 1000 mA g−1 (Figure 6a,b). By contrast, in the same condition, the Fe2O3/Fe3O4/NG electrode remained almost unchanged in terms of particle shape and size (Figure 6c,d). In a word, the comparison clearly demonstrates the stabilizing effect of the Fe2O3/Fe3O4/NG electrodes structure. Even after 300 cycles, the Fe2O3/Fe3O4/NG electrode could retain its particle size, although cracks occurred (Figure 6e,f).

Figure 6

SEM images of the (a,b) Fe2O3/NG and (c,d) Fe2O3/Fe3O4/NG electrodes after 100 discharge-charge cycles at 1000 mA g−1 at different magnifications. SEM images of the (e,f) Fe2O3/Fe3O4/NG electrodes after 300 discharge–charge cycles at 1000 mA g−1 at different magnifications.

4. Discussion

The outstandipan class="Chemical">ng pan class="Chemical">sodium storage properties of the pan class="Chemical">Fe2O3/Fe3O4/NG can be attributed to its unique structural features, as illustrated in Scheme 2. First, the highly conductive Fe3O4 improves electron transport in the hybrid Fe2O3/Fe3O4 nano-aggregates. Second, the phase boundaries and voids in the Fe2O3/Fe3O4 heterostructure provide fast diffusion channels for Na+ ions. Third, the robust interfacial interaction reinforced by Fe–O–C bonds can not only maintain the integrity of the electrode during the long-term cycles, but also provide a highway for electron transfer between the graphene and the Fe2O3/Fe3O4 nano-aggregates. Moreover, N-doping also assists the sodium storage, increasing the reversible capacity of materials. All these structural features endowed by the microwave plasma process lead to outstanding reversible capacity, good rate performance, and long-term cycling stability. As compared in Table 2, the electrochemical performance of the Fe2O3/Fe3O4/NG is among the top-ranking of the reported iron oxide/graphene-based SIB anodes.

Scheme 2

Schematics of structures, electron transfer and transport, and ion diffusion in Fe2O3/NG (upper) and Fe2O3/Fe3O4/NG (lower) composites.

Table 2

Comparison of the specific capacities of iron oxide based anodes in literature and Fe2O3/Fe3O4/NG anode in this work.

Anodes	Current Density (mA g−1)				References
	Capacity (mAh g−1)
Fe2O3/Fe3O4/NG	100	200	1000	1200	This work
	362	300	185	174
Fe2O3/NG	50	100	200	1000	Meng 2017 [28]
	343	285	230	132
Fe2O3/C	50	100	200	1000	Zhang 2018 [29]
	364	291	245	150
Fe3O4/G	100	200	500	1000	Fu 2016 [30]
	310	225	180	140
Fe3O4@C/G	100	200	500	1000	Qi 2019 [31]
	375	300	254	200
Fe2O3@NC	200	500	1000	4000	Guo 2018 [32]
	289	253.7	221.5	167.8
Fe3O4/G/QD	100	200	500	1000	Liu 2016 [33]
	316	273	216	113
Fe3O4/C	100	200	500	1000	Wang 2017 [34]
	293	262	223	195
Fe3O4@N–C	80	240	400	800	Zhao 2019 [35]
	386	315	277	248

5. Conclusions

In summary, the pan class="Chemical">n class="Chemical">Fe2O3/pan>n class="Chemical">Fe3O4 nano-aggregates anchored on nitrogen-doped graphene as an anode for sodium-ion batteries were successfully prepared with the assistance of the microwave plasma process. The highly conductive Fe3O4 in the composite improves electron transport, and the voids and phase boundaries facilitate Na+ ion diffusion into the Fe2O3/Fe3O4 heterostructure. Moreover, the Fe–O–C bonds not only strength the structural robustness, but also electrically bridge the graphene and the Fe2O3/Fe3O4 nano-aggregates. Consequently, the Fe2O3/Fe3O4/NG anode exhibits outstanding electrochemical performance, i.e., high specific capacity, excellent rate capability, and stable long-term cycling performance. The design concept of this unique heterostructure can be extended to other energy storage applications based on metal oxides.