metals Article
Synthesis of Core-Shell Carbon Encapsulated Fe2O3 Composite through a Facile Hydrothermal Approach and Their Application as Anode Materials for Sodium-Ion Batteries Yongguang Zhang 1 , Zhumabay Bakenov 2 1 2
*
ID
, Taizhe Tan 1, * and Jin Huang 1, *
School of Materials and Energy, Synergy Innovation Institute of GDUT, Guangdong University of Technology, Guangzhou 510006, China;
[email protected] Institute of Batteries LLC, School of Engineering, National Laboratory Astana, Nazarbayev University, 53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan;
[email protected] Correspondence:
[email protected] (T.T.);
[email protected] (J.H.)
Received: 24 May 2018; Accepted: 15 June 2018; Published: 18 June 2018
Abstract: Carbon encapsulated Fe2 O3 nanoparticles (C@Fe2 O3 ) were successfully synthesized via a facile and environmentally friendly hydrothermal method and prototyped in anode materials for sodium ion batteries (SIBs). High-resolution transmission and scanning electronic microscopy observations exhibited the formation of a highly core-shelled C@Fe2 O3 composite consisting of carbon layers coated onto uniform Fe2 O3 nanoparticles with a median diameter of 46.1 nm. This core-shell structure can repress the aggregation of Fe2 O3 nanoparticles, preventing the harsh volume change of the electrode, enhancing the electric conductivity of the active materials, and promoting Na-ion transformation during cycling. The electrochemical performances of the C@Fe2 O3 composite, as anodes for SIBs, retained a reversible capacity of 305 mAh g−1 after 100 cycles at 50 mA g−1 and exhibited an excellent cyclability at various current densities due to the synergistic effect between the carbon layers and Fe2 O3 . These results suggest that C@Fe2 O3 composites present much potential as anode materials for rechargeable SIBs. Keywords: C@Fe2 O3 composite; anode; sodium ion battery
1. Introduction Recently, sodium-ion batteries (SIBs) have attracted intensive attention as an alternative to lithium-ion batteries (LIBs). This interest can be attributed to the fact that sodium is abundantly available and is economically cheaper than lithium [1,2]. Graphite was established as the gold-standard anode material for lithium ion batteries (LIBs) due to their modest reversible capacity (372 mAh g−1 ), superior cycling behavior, high Coulombic efficiency, low cost, and low and flat potential plateaus, which provide a decent voltage window [3]. However, conventional graphite is not suitable for SIB anodes because the larger atomic radius of sodium requires intercalation hosts, with larger diffusion channels [4]. In recent years, the researched anode materials for SIBs transition metal oxides, such as TiO2 , ZnO, and Co3 O4 , were found to hold the greatest potential for anode materials in SIBs due to their exceptional theoretical capacities, good versatility, and cost advantages [5–10]. In general, these anodes face severe challenges in poor electronic conductivity and huge volume changes during cycling. Therefore, the development of anode materials with excellent electrochemical performances is crucial. In particular, Fe2 O3 , which possess a lofty theoretical specific capacity (1007 mAh g−1 ) [11], has garnered much attention in recent years [8–11]. However, the cycling stability of the Fe2 O3 anode is significantly hindered by the harsh volume expansion/contraction during the Na alloying/dealloying Metals 2018, 8, 461; doi:10.3390/met8060461
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processes. To boost the stability of the Fe2 O3 anode and better the capacity retention and cycle life, an effective strategy, which has been previously investigated, is to combine Fe2 O3 nanoparticles with carbonaceous substances. For example, Wang’s group reported the preparation of α-Fe2 O3 /RGO composites via microwave autoclave, with the resulting nanocomposites delivering a capacity of 310 mAh g−1 after 150 cycles at 100 mA g−1 [12]. In lithium-ion batteries, the carbon encapsulated Fe2 O3 nanoparticles (C@Fe2 O3 ) as anode materials attract special interest of researchers since this core-shell structured nanocomposite exhibits enhanced physical and chemical properties [13]. Coating layers of carbon onto a Fe2 O3 core will simultaneously improve the electric conductivity and act as a buffer layer to mitigate the mechanical strain and improve the mechanical integrity due to the ductile and elastic nature of carbonaceous materials [14,15]. All these merits are beneficial for the development of electrochemical performance. Encouraged by these promising and interesting works, herein, we prepared core-shell structure C@Fe2 O3 composites by a one-step hydrothermal procedure. Carbon layers were homogenously coated on the Fe2 O3 nanoparticles, with a median diameter of 46.1 nm. From this unique design, the C@Fe2 O3 composites exhibit an impressive Na-ion storage performance with a high capacity and good cycles. 2. Materials and Methods 2.1. Materials Preparation The C@Fe2 O3 composites with a core-shell structure were synthesized by the hydro-thermal method. Glucose (0.3 g) and iron nitrate hydrate (0.4 g) were dispersed in distilled water (40 mL) using a thermostatic magnetic mixer (Zhengzhou Teer Instruments, Zhengzhou, China) for 0.5 h to form a uniform suspension. Subsequently, the mixed solution was displaced to reaction still after 0.25 h sonication using an ultrasonic cell disruptor (Ningbo Scientz Biotechnology Co., Ningbo, China), then heated in a 40 mL Teflon-lined stainless-steel autoclave for 9 h at 190 ◦ C, and then left in ambient conditions. After cooling to room temperature, the products were washed thoroughly via centrifugal with deionized water. Finally, the washed products were dried under vacuum at 80 ◦ C for 24 h to prepare carbon encapsulated Fe2 O3 nanoparticles in a core-shell structure. 2.2. Materials Characterization The crystal phase of C@Fe2 O3 composites was analyzed by Rigaku D/Max 2500V/pc X-ray diffractometer (XRD) (Rigaku-TTRIII, Tokyo, Japan) with Cu Kα (λ = 1.5406 Å) in scanning step of 0.02◦ . The morphology was observed by a JEOL JSM-6700F scanning electron microscope (SEM, equipped with EDX elemental analysis, JEOL, Tokyo, Japan) and high-resolution transmission electron microscopy (HR-TEM, JEOL, Tokyo, Japan) from a JEOL JEM-2100F instrument with 200 kV acceleration voltage. Brunauer Emmett Teller (BET) (V-Sorb 2800P) tests were conducted to investigate the specific surface area of the composites. X-ray photoelectron spectroscopy (XPS) data was obtained with a PHI 5000 Versa Probe system (Ulvac-Phi, Kanagawa, Japan). The carbon content was estimated by thermo gravimetric analysis (TGA, SDTQ600, TA Instruments-Waters LLC, Newcastle, PA, USA). Raman spectroscopy analysis was examined on Raman spectroscopy (LabRAM Hr 800, HORIBA Jobin Yvon, Paris, France) with the laser wavelength of 514 nm. 2.3. Electrochemical Measurements The electrochemical measurements were conducted at room temperature using coin cells (CR2025), with a sodium foil as the counter and reference electrode. The working electrode consisted of a C@Fe2 O3 slurry, acetylene black, and sodium alginate binder combined at a weight ratio, 7:2:1, and evenly applied onto copper foil with 15 mm in diameter. The as-prepared working electrodes were dried in a vacuum oven at 80 ◦ C for over 12 h to remove any residual moisture. The electrolyte was comprised of 1 M NaClO4 dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 by volume),
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were probed between 0.01 and 3.0 V (vs. Na/Na+) with a battery testing system (BTS-5V5 mA, +) with Neware, Shenzhen, China). capacity was calculated in reference the mass of the total were probed between 0.01 The andspecific 3.0 V (vs. Na/Na a battery testing to system (BTS-5V5 mA, and the separator used was a glass fiber film. The electrochemical performances were probed between active material. Neware, Shenzhen, China). The specific capacity was calculated in reference to the mass of the total 0.01 and 3.0 V (vs. Na/Na+ ) with a battery testing system (BTS-5V5 mA, Neware, Shenzhen, China). active material. The specific capacity was calculated in reference to the mass of the total active material. 3. Results and Discussion 3. Results and Discussion The crystalline structure of the C@Fe2O3 composite was characterized by XRD. Figure 1 shows 3. Results and Discussion that the typical diffraction peaks at 30.3°, 57.4°, andwas 63.1°, which correspond the reflections The crystalline structure of the
[email protected]°, 2O3 composite characterized by XRD.toFigure 1 shows The crystalline structure of the C@Fe2 O3 composite was characterized by XRD. Figure 1 shows of (2 the 0 6),typical (1 1 9),diffraction (1 1 15), and (4 0at12) planes of the Fe 2and O 3 structure (JPDS No. 25-1402), respectively. that peaks 30.3°, 35.7°, 57.4°, 63.1°, which correspond to the reflections that the typical diffraction peaks at 30.3◦ , 35.7◦ , 57.4◦ , and 63.1◦ , which correspond to the reflections There no(1evident peak toplanes the presence of2amorphous of (2 0is6), 1 9), (1carbon 1 15), and (4due 0 12) of the Fe O3 structurecarbon. (JPDS No. 25-1402), respectively. of (2 0 6), (1 1 9), (1 1 15), and (4 0 12) planes of the Fe2 O3 structure (JPDS No. 25-1402), respectively. There is no evident carbon peak due to the presence of amorphous carbon. There is no evident carbon peak due to the presence of amorphous carbon.
Figure 1. X-ray diffractometer (XRD) patterns of the C@Fe2O3 composite. Figure O33 composite. composite. Figure 1. 1. X-ray X-ray diffractometer diffractometer (XRD) (XRD) patterns patterns of of the the C@Fe C@Fe22O
Raman spectra further confirmed the presence of carbon in the composites. As shown in Figure −1 are assigned to typical D and G bands of carbon, 2, two obvious bands at 1350 and 1586 cm Raman further confirmed thepresence presence carbonininthe thecomposites. composites.As Asshown shownininFigure Figure spectra further confirmed the ofofcarbon 2, −1 −1 are assigned − 1 respectively. A 2D band of 2693 cm is also observed, confirming the presence of carbon from the 2, two obvious bands at 1350 and 1586 cm to typical D and G bands of carbon, two obvious bands at 1350 and 1586 cm are assigned to typical D and G bands of carbon, respectively. −1 iscomposites. −of 1 is hydro-thermal process glucose in the respectively. 2D cm band of 2693observed, cm also observed, presence carbon from the A 2D band ofA 2693 also confirming theconfirming presence ofthe carbon fromof the hydro-thermal hydro-thermal process glucose in the composites. process of glucose in theofcomposites.
Figure Figure 2. 2. Raman Raman spectrum spectrum of of the the C@Fe C@Fe22O O33composite. composite. Figure 2. Raman spectrum of the C@Fe2O3 composite.
The morphology and particle sizes of C@Fe2O3 composites were characterized by SEM The morphology and particle sizes of C@Fe2 O3 composites were characterized by SEM measurements. As shown inparticle Figure sizes 3a,b, the C@Fe22O O33 composites are spheres,by with a The morphology and of C@Fe wereuniform characterized SEM measurements. As shown in Figure 3a,b, the C@Fe 2 O3 composites are uniform spheres, with a median median diameterAs of approximately 50 nm. inset in2O Figure 3b depicts theuniform average diameterwith of the measurements. shown 50 innm. Figure 3a,b,The the C@Fe 3 composites are diameter of approximately The inset in Figure 3b depicts the average diameterspheres, of the C@Fe2 Oa3 C@Fe 2 O 3 composites was 46.1 nm. The structure of the C@Fe 2 O 3 composite was further investigated median diameter of approximately 50 nm. TheC@Fe inset in Figure 3b depicts the average diameter of the composites was 46.1 nm. The structure of the 2 O3 composite was further investigated by TEM. by TEM. 3c illustrates carbon wrapping of the Fe2O 3 nanoparticles, which further possess an average C@Fe 2O3 Figure composites was 46.1 nm. The structure C@Fe 2O3 composite Figure 3c illustrates carbon wrapping of Fe2 O3 nanoparticles, which possesswas an averageinvestigated diameter of diameter of ~10 nm, confirming a core-shell structure. The HRTEM analysis, in an Figure 3d, by TEM. Figure 3c aillustrates wrapping of Feanalysis, 2O 3 nanoparticles, whichshown possess average ~10 nm, confirming core-shell carbon structure. The HRTEM shown in Figure 3d, reveals the lattice reveals the lattice fringe, with a distance spacing of 0.238 nm, corresponding to the (2 2 6) plane of diameter of a~10 nm, confirming a core-shell structure. The HRTEM analysis, shown in Figure 3d, fringe, with distance spacing of 0.238 nm, corresponding to the (2 2 6) plane of Fe 2 O3 . The image also Fe 2O3. The illustrates that the thickness the carbon layer was approximately nm. reveals theimage lattice also fringe, with a distance spacing ofof0.238 nm, corresponding to the (2 2 6) 0.83 plane of Fe2O3. The image also illustrates that the thickness of the carbon layer was approximately 0.83 nm.
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illustrates that thedispersive thickness spectrometer of the carbon(EDS) layer was approximately 0.83 nm. energy dispersive The energy element mapping, shown in The Figure 4, clearly The energy dispersive spectrometer (EDS) element mapping, shown in Figure 4, clearly spectrometer (EDS) element mapping, shown in Figure 4, clearly demonstrates that the C@Fe2 O3 demonstrates that the C@Fe2O3 composite mainly consist of C, Fe, and O, with a uniform demonstrates that the C@Fe2O3 composite mainly consist of C, Fe, and O, with a uniform composite mainly consist of C, Fe, and O, with a uniform distribution. distribution. distribution.
Figure 3. SEM images (a,b) and correspondingmedian median diameter and TEM images (c,d)(c,d) of theof the Figure 3. SEM images (a,b) and corresponding diameter(inset) (inset) and TEM images Figure 3. SEM images (a,b) and corresponding median diameter (inset) and TEM images (c,d) of the C@Fe 2O3 composites. C@FeC@Fe O composites. 2 3 2O3 composites.
Figure 4. SEM image (a) of the C@Fe2O3 composite and corresponding carbon (b), iron (c), and Figure 4. SEM image (a) of the C@Fe2O3 composite and corresponding carbon (b), iron (c), and
Figure 4. SEM (a)mapping. of the C@Fe2 O3 composite and corresponding carbon (b), iron (c), and oxygen oxygen (d)image element oxygen (d) element mapping. (d) element mapping. The chemical composition and chemical bonding were further investigated by XPS, shown in The chemical composition and chemical bonding were further investigated by XPS, shown in Figure 5. As shown in Figure 5a, the peaks of Fe 2p, C 1s, and O 1s were detected in the C@Fe 2O3 Figure 5. As shown in Figure and 5a, the peaks ofbonding Fe 2p, C were 1s, and O 1s were detected inbythe C@Fe 2O3 The chemical composition chemical further investigated XPS, shown in composite. Figure 5b delivers two peaks located at 724.7 and 711.1 eV in the high-resolution Fe 2p composite. Figure 5b delivers two peaks located at 724.7 and 711.1 eV in the high-resolution Fe 2p Figure 5. As shown in Figure 5a, the peaks of Fe 2p, C 1s, and O 1s were detected in the C@Fe2 O3
composite. Figure 5b delivers two peaks located at 724.7 and 711.1 eV in the high-resolution Fe 2p spectra, corresponding to Fe 2p1/2 and Fe 2p3/2, respectively. In addition, two weak satellite peaks at 733.3 and 719.5 eV can be attributed to the Fe 2p1/2 and Fe 2p3/2, respectively. This phenomenon
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spectra, to Fe 2p1/2 Metals 2018,corresponding 8, x FOR PEER REVIEW
and Fe 2p3/2, respectively. In addition, two weak satellite 5peaks of 11 at 733.3 and 719.5 eV can be attributed to the Fe 2p1/2 and Fe 2p3/2, respectively. This phenomenon Metals 2018, 8, 461 5 of 10 3+ only. Figure spectra, corresponding to Fe 2p1/2 and Fe 2p3/2, In5caddition, two weak indicates that the composite was composed of Ferespectively. shows three peaks satellite at 288.6,peaks 286.6, at 733.3 and eVCcan be attributed to the Fe to 2p1/2 and Fe 2p3/2, and respectively. This phenomenon and 284.8 eV719.5 in the 1s spectra, corresponding the C=O, C–O–C, C–C, respectively. Figure 5d 3+ only. Figure 5c shows three peaks at 288.6, 286.6, indicates that thecomposite composite wascomposed composed ofFe Fe shows two peaks located at was 531.8 and 529.8 of eV in3+ the O 1sFigure spectra, attributed to 288.6, Fe–O–H and indicates that the only. 5c which shows is three peaks at 286.6, and 284.8 eVin inthe theCCOverall, 1sspectra, spectra, corresponding tothe theC=O, C=O, C–O–C, and C–C, C–C, respectively. respectively. Figure 5d Fe–O, respectively. thecorresponding results indicate successful transformation of glucose and iron and 284.8 eV 1s to C–O–C, and Figure 5d shows two peaks located at 531.8 and 529.8 eV in the O 1s spectra, which is attributed to Fe–O–H and nitrate hydrate into C@Fe 2 O 3 composite. shows two peaks located at 531.8 and 529.8 eV in the O 1s spectra, which is attributed to Fe–O–H Fe–O, respectively. Overall, thethe results indicate and iron iron and Fe–O, respectively. Overall, results indicatethe thesuccessful successfultransformation transformation of of glucose glucose and nitratehydrate hydrateinto intoC@Fe C@Fe22O O33 composite. composite. nitrate
Figure 5. XPS full spectra (a), Fe 2p (b), C 1s (c), and (d) O 1s of the C@Fe2O3 composite. XPS spectra (a), (b), and ofofthe The Figure carbon content of the C@Fe 2Fe O2p 32p composites tested thermogravimetric (TG) analysis. Figure5.5. XPSfull full spectra (a),Fe (b),CC1s1s(c), (c),was and(d) (d)OO1s1sby theC@Fe C@Fe2 2OO33 composite. composite. From the TG curve in Figure 6, the weight fraction of the carbon was recorded to be 46%. The Thecarbon carbon content ofthe the C@Fe O3distribution 3 composites of by2Othermogravimetric thermogravimetric (TG) analysis. analysis. specific surfacecontent area and pore size C@Fe 3 composite was obtained from The of C@Fe wasthe tested by (TG) 22O From the TG curve in Figure 6, the weight fraction of the carbon was recorded to be 46%. nitrogen adsorption-desorption From Figurewas 7a, recorded the isotherms indicate typeThe IV From the TG curve in Figure 6, the measurements. weight fraction of the carbon to be 46%. The specific specific surface pore size distribution the C@Fe2Obranch 3 composite was obtained from hysteresis loops, withand a marked hysteretic loopC@Fe inofa 2desorption relative pressures surface area and area pore size distribution of the O3 composite wasbetween obtained from nitrogen nitrogen adsorption-desorption measurements. From Figure 7a,the the isotherms indicate P/P0 of 0.8 and 0.95, corresponding to the mesoporous of sample. In type general, thetype typeIV IV adsorption-desorption measurements. From Figure 7a,nature the isotherms indicate IV hysteresis hysteresis loops, with a marked hysteretic loop in a desorption branch between relative pressures hysteresis typically occurs due the aggregated nanoparticle structure. The BET area loops, with aloop marked hysteretic loop into a desorption branch between relative pressures P/P0surface of 0.8 and 2 sample. P/P ofthe 0.8 and 0.95, corresponding to the mesoporous the In general, the type IV for0 corresponding C@Fe 2O3to composite was calculated atnature 85.56 m g−1the . Figure shows the 0.95, the mesoporous nature of the sample. Inofgeneral, type IV 7b hysteresis loop hysteresis loop typically occurs due to the aggregated nanoparticle structure. The BET surface area Barrett-Joyner-Halenda (BJH) pore size adsorption plot for C@Fe 2 O 3 sample, highlighting typically occurs due to the aggregated nanoparticle structure. The BET surface area for the C@Fe2 O3 a 2 2 g−nm 1 .calculated for the was C@Fe 2O3 centered composite atnm.85.56 m g−1. confirms Figure 7b shows the bimodal-pore size atmwas 3.7 and 7b 49.9 This further the mesoporous composite calculated at 85.56 Figure shows the Barrett-Joyner-Halenda (BJH) pore size Barrett-Joyner-Halenda (BJH) porehighlighting size adsorption plot for size C@Fe 2O3 sample, highlighting morphology C@Fe 2O23 O composites. adsorption plotoffor C@Fe a bimodal-pore centered at 3.7 nm and 49.9 nm.a 3 sample, bimodal-pore size centered at 3.7 nm and 49.9 nm. This further confirms the mesoporous This further confirms the mesoporous morphology of C@Fe2 O3 composites. morphology of C@Fe2O3 composites.
Figure 6. Thermogravimetric (TG) curve of the C@Fe2 O3 composite.
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Figure 6. Thermogravimetric (TG) curve of the C@Fe2O3 composite.
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Pore Diameter(nm) Relative Pressure (P/P0) Figure 7. (a) 7.Nitrogen adsorption/desorption isotherms (b)pore the size pore size distributions of Figure (a) Nitrogen adsorption/desorption isotherms andand (b) the distributions of C@Fe2 OC@Fe composites. 2 O 3 com posites. 3
The electrochemical behavior of the C@Fe2O3 composite with Na-ion insertion-extraction was
The electrochemical behavior of the C@Fe2 O3 composite with Na-ion insertion-extraction was tested by cyclic voltammetry (CV) analysis as shown in Figure 8. In the cathodic scan, an irreversible tested by cyclic voltammetry (CV) as shown in Figure 8.toInthe the cathodicofscan, anelectrolyte irreversible +), corresponding peak is observed at around 0.75analysis V (vs. Na/Na formation a solid + ), corresponding to the formation of a solid electrolyte peak isinterface observed(SEI) at around 0.75 V (vs. Na/Na layer [16]. The pronounced reduction peak occurs near 0.08 V, which results in the interface (SEI) layer peak oxidation occurs near 0.08 whichat results in the reduction of the[16]. Fe2O3The [17].pronounced In the anodicreduction scan, two broad peaks areV, located 0.46 V and reduction Fe2 O3 [17].toInthe thereversible anodic scan, two broad oxidation are located at 0.46 V and 1.71of V,the corresponding oxidation of Fe 0 to Fe3+ [18]. peaks The reversible electrochemical can be described as [19]: 1.71 V, reaction corresponding to the reversible oxidation of Fe0 to Fe3+ [18]. The reversible electrochemical reaction can be described as [19]: (1) Fe2O3 + 6Na ↔ 2Fe + 3Na2O Metals 2018, 8, x FOR PEER REVIEW
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Fe2 O3 + 6Na ↔ 2Fe + 3Na2 O
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Figure 9 illustrates the charge–discharge profiles for the initial three cycles of the C@Fe2O3 composites at a current density of 50 mA g−1 and at a potential window between 0.01 and 3.0 V. As observed, the C@Fe2O3 composite delivers an initial discharge capacity of 709 mAh g−1 and a charge capacity of 415 mAh g−1, which mainly originates in the first cycle, partially from the inevitable
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Figure 8. Cyclic voltammetry (CV) curve of the C@Fe2O3 composite at the initial cycle.
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Figure cycles of of the the C@Fe C@Fe2O 3 Figure 99 illustrates illustrates the the charge–discharge charge–discharge profiles profiles for for the the initial initial three three cycles 2 O3 −1 and at a potential window between 0.01 and 3.0 V. As composites at a current density of 50 mA g − 1 composites at a current density of 50 mA g and at a potential window between 0.01 and 3.0 V. observed, the C@Fe 2O3 composite delivers an initial discharge capacity of 709 mAh g−1 and a charge −1 and As observed, the C@Fe 2 O3 composite delivers an initial discharge capacity of 709 mAh g −1, which mainly originates in the first cycle, partially from the inevitable capacity of 415 mAh g − 1 a charge capacity of 415 mAh g , which mainly originates in the first cycle, partially from the inevitable formation formation of of the the SEI SEI film film on on the the surface surface of of the the C@Fe C@Fe22OO3 3electrode electrodeand andthe theunique uniquecore-shell core-shellstructure, structure, which possesses a high specific surface area [20]. SEI films are generated from the decomposition which possesses a high specific surface area [20]. SEI films are generated from the decompositionof of electrolyte, which produces producesa alarge large amount of sodium irreversibly intercalate into the electrolyte, which amount of sodium ionsions and and irreversibly intercalate into the crystal crystal lattice In addition, no obvious voltage plateaus can be in observed inwhich Figureis 9, which is lattice [21]. In [21]. addition, no obvious voltage plateaus can be observed Figure 9, accepted of accepted Fe2O3 asinisthe reported in the literature. Lastly, capacity of the C@Fe2O3 composite Fe2 O3 as of is reported literature. Lastly, the capacity of the the C@Fe 2 O3 composite quickly faded in quickly faded incycles the subsequent cycles because of the drastic change and theOaggregation of the subsequent because of the drastic volume change andvolume the aggregation of Fe 2 3 nanoparticles Fe 2O3 nanoparticles during charge-discharge processes [22]. during charge-discharge processes [22].
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−11for At aa current current density density of 50 mA g− for100 100cycles, cycles,the thecycling cyclingperformances performances of of C@Fe22O O33composite composite At are shown 10.10. In the first first cycle,cycle, the C@Fe O3 composite manifests a high irreversible discharge are shown ininFigure Figure In the the 2C@Fe 2O3 composite manifests a high irreversible −1 and a relatively −1 capacity of 709 mAh g low initial coulombic efficiency of less than 60%. Subsequently, discharge capacity of 709 mAh g and a relatively low initial coulombic efficiency of less than 60%. 1 for excellent cycleexcellent performance is maintained delivering delivering a reversible capacity capacity of 305 mAh g−mAh Subsequently, cycle performance is maintained a reversible of 305 −1 for 2018, x FOR PEER REVIEW 8 of 11 100 cycles. Furthermore, alongalong with awith high reversible discharge capacity, excellent cyclability was gMetals 100 8,cycles. Furthermore, a high reversible discharge capacity, excellent cyclability demonstrated as well, and the capacity decay was only 0.22% in per cycle. Complimentarily, was demonstrated as well, and the capacity was only 0.22% in per cycle. Complimentarily, the C@Fe 2O3 electrode delivers a coulombic efficiency of over 99%. In summary, these results imply an the C@Fe 2 O3 electrode delivers a coulombic efficiency of over 99%. In summary, these results imply excellent reversibility of sodium insertion/deinsertion in the C@Fe 2O3 composites. an excellent reversibility of sodium insertion/deinsertion in the C@Fe 2 O3 composites.
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Figure 10. Cycling performance and coulombic efficiency of the C@Fe2 O3 composites. Figure 10. Cycling performance and coulombic efficiency of the C@Fe2O3 composites.
Figure 11 exhibits the rate capabilities of the C@Fe2O3 composite electrode at different current densities, spanning from 50 mA g−1 to 1000 mA g−1. The C@Fe2O3 composite electrode demonstrates excellent rate performance and reversible capacities of 364, 291, 245, 206, and 150 mAh g −1 at current densities of 50, 100, 200, 500, and 1000 mA g−1, respectively. Lastly, the C@Fe2O3 composite delivered a reversible capacity of 302 mAh g−1 when returning the current density back to the initial 50 mA g−1.
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Figure 10. Cycling performance and coulombic efficiency of the C@Fe2O3 composites.
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Figure rate capabilities capabilities of of the the C@Fe C@Fe2O 3 composite electrode at different current Figure 11 11 exhibits exhibits the the rate 2 O3 composite electrode at different current −1 −11. The C@Fe2O3 composite electrode demonstrates densities, spanning from 50 mA g to 1000 mA g − 1 − densities, spanning from 50 mA g to 1000 mA g . The C@Fe2 O3 composite electrode demonstrates excellent rate performance performance and and reversible reversible capacities capacities of of 364, 364, 291, 291,245, 245,206, 206,and and150 150mAh mAhgg−−11 at excellent rate at current current −1 densities of 50, 100, 200, 500, and 1000 mA g , respectively. Lastly, the C@Fe 2O3 composite delivered − 1 densities of 50, 100, 200, 500, and 1000 mA g , respectively. Lastly, the C@Fe2 O3 composite delivered aa reversible capacity of of 302 302 mAh mAh gg−−11 when density back back to to the the initial initial 50 50 mA mAgg−−11.. reversible capacity when returning returning the the current current density These results resultsreveal reveal the stable core-shell of the carbon encapsulated Fe2O3 These thatthat the stable core-shell structurestructure of the carbon encapsulated Fe2 O3 nanoparticles nanoparticles can effectively sustain the large volume change, alleviate the aggregation of Feand 2O3 can effectively sustain the large volume change, alleviate the aggregation of Fe2 O3 nanoparticles, nanoparticles, and improve electronic transports during[16,23,24]. sodium insertion [16,23,24]. improve electronic transports during sodium insertion
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Figure 11. Rate performances of the C@Fe2 O3 composites at various current densities. Figure 11. Rate performances of the C@Fe2O3 composites at various current densities.
Electrochemical impedance spectroscopy (EIS) measurements were measured to compare the Electrochemical impedance spectroscopy (EIS) measurements were measured to compare the Nyquist plots of C@Fe2 O3 when fresh and after 100 cycles, as shown in Figure 12. The C@Fe2 O3 Nyquist plots of C@Fe2O3 when fresh and after 100 cycles, as shown in Figure 12. The C@Fe2O3 composite, after 100 cycles, had much lower charge transfer resistance, corresponding to the smaller composite, after 100PEER cycles, had much lower charge transfer resistance, corresponding to the smaller Metals 2018,at 8, xhigh FOR REVIEW 9 of 11 semicircle to middle frequency. As a result, the C@Fe2 O3 composite exhibits a favorable semicircle at high to middle frequency. As a result, the C@Fe2O3 composite exhibits a favorable electrochemical performance during cycling processes. electrochemical performance during cycling processes.
Figure 12.12. Impedance plots of of thethe C@Fe when fresh and after 100 cycles. 2 O23Ocomposite Figure Impedance plots C@Fe 3 composite when fresh and after 100 cycles.
Theelectrochemical electrochemicalperformance performanceofof C@Fe 2O compositesobtained obtainedininthis thispaper paperisisbetter betterthan than The C@Fe 2O 3 3composites those previously reported results with 2 O 3 (Table 1) [12,25]. Compared with Fe 2 O 3 , the cycling those ofof previously reported results with FeFe O (Table 1) [12,25]. Compared with Fe O , the cycling 2 3 2 3 performanceofof the C@Fe 2O compositesisismuch muchbetter, better,showing showingthat thatthe thecarbon carboncontributes contributestotothe the performance the C@Fe 2O 3 3composites capacity the C@Fe 2 O 3 composites and the carbon is taking a positive effect. capacity ofof the C@Fe O composites and the carbon is taking a positive effect. 2 3 Table 1. Comparison of the electrochemical performances of Fe2O3 and C@Fe2O3 composite for sodium-ion batteries. Materials
Discharge Capacity (mAh g−1)
Cycle Number
Current Density (mAh g−1)
References
Fe2O3 α-Fe2O3 C@Fe2O3
16 10 305
10 50 100
50 100 50
[25] [12] This study
Metals 2018, 8, 461
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Table 1. Comparison of the electrochemical performances of Fe2 O3 and C@Fe2 O3 composite for sodium-ion batteries. Materials
Discharge Capacity (mAh g−1 )
Cycle Number
Current Density (mAh g−1 )
References
Fe2 O3 α-Fe2 O3 C@Fe2 O3
16 10 305
10 50 100
50 100 50
[25] [12] This study
4. Conclusions In summary, we reported a facile hydrothermal approach to fabricate a stable core-shell structure composed of carbon encapsulated Fe2 O3 nanoparticles and investigated its application as anode materials for SIBs. A reversible capacity of 305 mAh g−1 can be obtained at 50 mA g−1 for up to 100 cycles for the C@Fe2 O3 composites and it delivered a reversible capacity of 150 mAh g−1 at a current density of 1000 mA g−1 . When the current rate returned to 50 mA g−1 , the reversible capacity could recover up to 302 mAh g−1 , indicating high rate capabilities even at very high current densities. The electrochemical results demonstrated that the C@Fe2 O3 composites feature excellent cycling performance and good rate capability due to the unique core-shell structure of the SIB anode. Author Contributions: Formal Analysis, Y.Z.; Investigation, Y.Z.; Writing-Original Draft Preparation, T.T. and Y.Z.; Writing-Review & Editing, Z.B. and J.H.; Supervision, J.H.; Project Administration, T.T.; Funding Acquisition, Y.Z. Funding: This research was funded by the China Postdoctoral Science Foundation (2018M630924); Program for the Outstanding Talents of Guangdong sailing plan, Human Resources and Social Security of Guangdong Province (201634007). ZB acknowledges support from a research project by the Ministry of Education and Science “Zinc based Rechargeable Aqueous Battery: A green, safe and economic battery for Space Applications” (AP05136016). Conflicts of Interest: The authors declare no conflict of interest.
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