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N-Doped Dual Carbon-Confined 3D Architecture rGO/Fe3O4/ AC Nanocomposite for High-Performance Lithium-Ion Batteries Ranran Ding, Jie Zhang, Jie Qi, Zhenhua Li, Chengyang Wang, and Mingming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00353 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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N-Doped Dual Carbon-Confined 3D Architecture rGO/Fe3O4/AC Nanocomposite for High-Performance Lithium-Ion Batteries Ranran Dingab, Jie Zhangab, Jie Qiab, Zhenhua Liab, Chengyang Wangab, Mingming Chenab∗ a

Key Laboratory for Green Chemical Technology of MOE, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin University, Tianjin 300072, P. R. China *E-mail: [email protected]

ABSTRACT: To address the issues of low electrical conductivity, sluggish lithiation kinetics and dramatic volume variation in Fe3O4 anodes of Lithium ion battery, herein, a double carbon-confined three-dimensional (3D) nanocomposite architecture was synthesized by an electrostatically assisted self-assembly strategy. In the constructed architecture, the ultrafine Fe3O4 subunits (~10 nm) self-organize to form nanospheres (NSs) that are fully coated by amorphous carbon (AC), formatting core-shell structural Fe3O4/AC NSs. By further encapsulation by reduced graphene oxide (rGO) layers, a constructed 3D architecture was built as dual carbon-confined rGO/Fe3O4/AC. Such structure restrains the adverse reaction of the electrolyte, improves the electronic conductivity and buffers the mechanical stress of the entire electrode, thus performing excellent long-term cycling stability (99.4% capacity retention after 465 cycles relevant to the second cycle at 5 A g-1). Kinetic analysis reveals that a dual lithium storage mechanism including a diffusion reaction mechanism and a surface capacitive behavior mechanism coexists in the composites. Consequently, the resulting rGO/Fe3O4/AC nanocomposite delivers a high reversible capacity (835.8 mA h g-1 for 300 cycles at 1 A g-1) as well as remarkable rate capability (436.7 mA h g-1 at 10 A g-1). KEYWORDS: dual lithium storage mechanism; 3D architecture; double carbon confined; electrostatically assisted self-assembly; Li-ion batteries 1. INTRODUCTION Lithium-ion batteries (LIBs) currently face a specific energy density limitation. As an anode material, conventional graphite continues to be disputed due to its low theoretical capacity (372 mA h g-1) and inferior performance at high rates.1,2 Some competitive anode materials are appealing. During the past several years, Fe3O4 has been extensively researched as an anode material for LIBs due to its high theoretical capacity (926 mA h g-1), relatively low-voltage plateau (approximately 0.8 V) and natural abundance.3 However, bare Fe3O4 as an anode material is limited by several intractable obstacles including (1) inferior intrinsic electrical conductivity and

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sluggish ion transport kinetics, which hamper the fast transfer of electrons/ions inside the active materials and restrict their electrochemical reactions;4,5 and (2) dramatic volume variations during the charging/discharging processes, leading to severe electrode material pulverization and the formation of a unstable solid-electrolyte interphase (SEI) film.6 These obstacles eventually result in low coulombic efficiency, inferior rate capability and significant capacity fading. Therefore, it is imperative to seek effective solutions to circumvent these issues and realize high-efficiency energy storage. Nano sized Fe3O4 particles coated with a carbon layer are commonly suggested to solve the addressed issues described above. Nanostructured Fe3O4 exhibits favorable kinetics relative to their micrometer-sized counterparts because they can effectively increase active sites, lower the absolute volume change, and shorten the diffusion pathway of lithium ions within the particles.7,8 Furthermore, previous reports have shown that decreasing the active particle size into the nanometer range can also contribute to surface capacitive effects that obviously improve the reversibility and high rate performance in LIBs.9 The protective carbon shell can act as a buffering layer for the dramatic volume variation, improve the electronic conductivity, and prevent Fe3O4 nanoparticles from contacting directly with the electrolyte, thus restraining the cyclical formation and rupture of the SEI film, which preserves interfacial stabilization.10,11 Lu and coworkers reported that Fe3O4 nanoparticles covered with an amorphous carbon layer performed at a capacity up to 800 mA h g-1, corresponding to a capacity retention rate of 99% after 200 cycles at 500 mA g-1.12 Chen and coworkers synthesized carbon-encapsulated Fe3O4, which exhibited a high discharge capacity of 832 mA h g-1 after 150 cycles.13 Even though the carbon coating strategy more or less effectively enhances the electrochemical performance, it is still insufficient because single carbon-coated Fe3O4 still has a large contact resistance among nanoparticles, sluggish lithium-ion diffusion and unavoidable aggregation during cycling14. The incorporation of Fe3O4 into an interconnected three-dimensional (3D) hierarchical carbonaceous network has proven to be an effective strategy to address the above-described challenges and further improve the electrode structural stability and electrical conductivity.15,16 For example, Luo and coworkers have reported that 3D Fe3O4/NC/NG nanohybrids can attain a high reversible capacity of 952 mA h g-1 at 200 mA g-1 after 100 cycles.6 The 3D net-like FeOx/C composite exhibited long-term cycling stability and a high rate capability of 714.7 mA h g-1 after

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300 cycles at 1 A g-1.17 However, although various 3D Fe3O4/carbon composites have been synthesized, aggregation, uncontrollable distribution, and even the detachment of the active particles from the 3D carbonaceous network are still unavoidable because of the poor physical interaction between the Fe3O4 nanoparticles and carbon matrix, which cannot endure large volume changes in the electrode upon cycling18. Therefore, the development of a more effective strategy to design smarter composite architectures that simultaneously realize high capacity, fast kinetics and long-term cycling stability are still urgently needed but remain immensely challenging. In this work, we introduce an electrostatically assisted self-assembly strategy to design a robust double carbon-confined nanocomposite to address the aforementioned problems. In the constructed nanocomposite (rGO/Fe3O4/AC), Fe3O4 nanospheres (NSs) composed of numerous ultrafine subunits are fully coated by a thin amorphous carbon (AC) shell and are further encapsulated in the continuous 3D reduced graphene oxide (rGO) framework, integrating the multiple merits of nanostructures, carbon coatings and interconnected 3D conductive networks. Benefiting from the unique structural features and composition, the resulting rGO/Fe3O4/AC nanocomposite delivers remarkable electrochemical lithium-storage performance. Detailed studies reveal that the impressive electrochemical performance of rGO/Fe3O4/AC is attributed to the uniform distribution and plane-to-point intimate interface contacts between the rGO layer and the active particles, which increase the electrical conductivity and structural stability of the composite. Moreover, an electrochemical impedance spectroscopy study reveals that the amorphous carbon shell enables a stable interface between the electrolyte and composite, and the kinetic analysis indicates that a dual lithium storage mechanism with a diffusion reaction and surface capacitive behavior coexists in the composites, finally resulting in superior long-life stability and high rate performance in lithium storage. 2. RESULTS AND DISCUSSION The synthetic process for rGO/Fe3O4/AC is illustrated in Scheme 1. First, Fe2O3 NSs were simply obtained by calcination of the prepared Prussian blue (PB), as shown in Figure S1b in the Supporting Information. Then, the obtained Fe2O3 NSs were coated with polydopamine (PDA) through an oxidization and cyclization reaction.12 Second, the obtained core-shell structure Fe2O3/PDA was grafted from the cationic polyelectrolyte poly(diallyldimethylammonium chloride) (PDDA) to create a positively charged surface (denoted as Fe2O3/PDA-PDDA).19 This surface charging was verified by a zeta potential measurement, as shown in Figure S1g (Supporting Information). Third, driven by electrostatic interactions, the positively charged

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Fe2O3/PDA-PDDA was assembled with negatively charged graphene oxide (GO) layers into a 3D nanocomposite architecture (denoted as GO/Fe2O3/PDA). Their assembly processes (with/without PDDA) were comparably demonstrated in the control experiments (Figure S1j, k in Supporting Information). Finally, the rGO/Fe3O4/AC composite can be obtained by thermal-induced reduction precursor carbonization at 500 °C for 4 h under N2. Scheme 1. Schematic illustration of the multiple-step fabrication procedure of the rGO/Fe3O4/AC nanocomposite.

The morphology and porous feature of the resulting rGO/Fe3O4/AC nanocomposite was investigated via a series of measurements. As shown in Figure 1a and

S1j (Supporting Information), the rGO/Fe3O4/AC precursor with an

interconnected 3D-GO framework structure is composed of highly dispersed NSs, and a significant fraction of the NSs are encapsulated by GO layers with close interface contact, revealing an effective self-assembly between the modified Fe3O4/PDA and negatively charged GO. The GO layer is rather thin, and the surface is very smooth with a highly porous structure. After the thermally induced reaction, the rGO/Fe3O4/AC composite maintains the structure of the precursor, except the rGO-wrapped NSs shrunk and their surfaces are very rough (Figure 1b). TEM characterization further validated that the Fe3O4/AC NSs are captured by the twisted rGO layer and are distributed homogeneously in the continuous 3D rGO framework (Figure 1d). The energy dispersive X-ray (EDX) spectrum (Figure S1I in Supporting Information) and elemental mapping disclosed that the coexistence of elemental Fe, C, O, and N in the composite and the Fe3O4/AC NSs are fully incorporated into the rGO framework (Figure 1g). Such a geometric confinement of Fe3O4/AC NSs within highly defected (porous, wrinkled and/or twisted) rGO network can enhance interface contact and suppress Fe3O4/AC agglomeration and detachment during cycling. The

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interpenetrating electron/ion transport channels and integrated 3D conductive network ensure rapid electron and ion transport. To better understand their internal structure, the Fe3O4/AC NSs were also characterized by TEM. Figure 1c shows that the Fe3O4/AC NSs display a typical core-shell structure, and the core comprises numerous ultrasmall Fe3O4 subunits with a size of approximately 10 nm and exhibits a highly porous structure. An energy-dispersive X-ray (EDX) line profile reveals that the carbon layer has a thickness of ~15 nm and the size of the Fe3O4/AC is approximately 100 nm (inset in Figure 1c). A high-magnification TEM (HRTEM) image of the region marked with a rectangle in Figure 1d shows that the carbon shell is amorphous, and the interplanar spacing of the core is 0.25 nm, ascribed to the (311) plane of Fe3O4 (Figure 1e). The nitrogen adsorption/desorption isotherms of rGO/Fe3O4/AC display typical type IV behavior, implying an abundance of mesopores (inset of Figure 1f) and a specific surface area up to 68.3 m2 g-1. The pore distribution of rGO/Fe3O4/AC shows an interconnected multimodal distribution ranging from 3 to 37 nm. Such a favorable specific surface area and suitable pore distribution may be conducive to accelerating mass diffusion of the electrolyte and the buffering volume changes in rGO/Fe3O4/AC during the electrochemical reaction.20 The compositions and surface properties of the resulting samples were characterized via X-ray diffraction (XRD), Raman spectroscopy, thermogravimetric (TG) analysis, and X-ray photoelectron spectroscopy (XPS). As exhibited in Figure 1i, the characteristic peaks of α-Fe2O3 (JCPDS no.89-8103) can be observed for the sintered PB sample at 450 °C. However, a clear transformation of the crystalline phase from α-Fe2O3 to Fe3O4 (JCPDS no. 65-3107) is detected in the composites, which may be caused by the carbothermic reduction of α-Fe2O3 after an annealing treatment in a N2 atmosphere. In the Fe3O4/rGO patterns, a diffraction hump in the 24-28° range was observed, originating from the stacking diffraction peaks of rGO, which is consistent with the SEM image in Figure S1k in Supporting Information. However, the absence of this peak in the patterns of rGO/Fe3O4/AC indicates that the Fe3O4/AC NSs are efficiently fixed and cover the rGO surface to restrain the restacking of the rGO layers. Additionally, no apparent graphite diffraction peaks are observed in the Fe3O4/AC sample, suggesting that the carbon coating layer is amorphous, which is in good accordance with the TEM observations. To demonstrate the existence of rGO in the rGO/Fe3O4/AC composite, Raman spectra were obtained. As shown in Figure S1 h (Supporting Information), there are two fundamental peaks

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at 1350 and 1590 cm-1 associated with the D- and G-bands of rGO, respectively, indicating the existence of rGO in the rGO/Fe3O4/AC.21 Compared to GO, the ratio of

ID/IG in rGO/Fe3O4/AC is lower, confirming a higher graphitization degree, which is crucial to improving the electrical conductivity. An XPS survey spectrum shows that the surface of the rGO/Fe3O4/AC mainly consists of elemental O, C and N (Figure S2b in Supporting Information). Compared to Fe3O4 and Fe3O4/rGO, the negligible Fe peak implies that Fe3O4 appears to be fully encapsulated in the PDA-derived carbon shells, confirming the conformal coating nature of PDA.22 The high-resolution C1s spectrum shows four types of carbon: sp2-C at 284.6 eV, sp3-C at 285.4 eV, N-sp3C at 287.8 eV, and C-C=C at 288.6 eV (Figure S2f in Supporting Information).23,24 The N content in rGO/Fe3O4/AC was 3.6 at%, which was deconvoluted into pyridinic N (398.7 eV), pyrrolic N (399.9 eV), graphitic N (401.2 eV) and pyridine N (402.1 eV), as displayed Figure 1h.25,26 The N-sp3C peak was identified, verifying the successful doping of nitrogen in the rGO/Fe3O4/AC composite, which is critical for enhancing the electrical conductivity and ion permeability of the carbon layer. The Fe 2p spectrum is presented in Figure S2e (Supporting Information). This spectrum can be fitted with four Gaussian peaks: the peaks at 708.8 eV and 721.8 eV are assigned to Fe2+ 2p3/2 and Fe2+ 2p1/2 in Fe2+, and those at 710.8 and 724.1 eV are attributed to Fe3+ 2p3/2 and Fe3+ 2p1/2 in Fe3+. The absence of the satellite peak characteristic of γ-Fe2O3 also indicates the presence of only Fe3O4 in the rGO/Fe3O4/AC composites.27 To evaluate the mass ratio of Fe3O4 in rGO/Fe3O4/AC, the TGA results are presented in Figure S1f (Supporting Information). On the basis of the TG results, the AC and rGO content in the rGO/Fe3O4/AC are determined to be 23 wt% and 20.8 wt%, respectively, and the weight fractions of Fe3O4 are calculated to be ~56.2 wt%. These results demonstrate the successful fabrication of the rGO/Fe3O4/AC nanocomposite, in which the Fe3O4 NSs with an amorphous carbon coating layer were encapsulated in the 3D-rGO network. These integrated components and structure merits will be beneficial for the electrochemical performance in LIBs.

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Figure 1. Typical characterizations of the resulting samples. SEM images of the (a) rGO/Fe3O4/AC precursor and (b) rGO/Fe3O4/AC. TEM image of (c) Fe3O4/AC, inset shows the corresponding EDX line scanning spectra and (d) rGO/Fe3O4/AC. (e) HR-TEM image, (f) Nitrogen adsorption/ desorption isotherms and the pore-size distribution plots, and (g) elemental mapping images of rGO/Fe3O4/AC. (h) High-resolution XPS spectra of N 1s in rGO/Fe3O4/AC. (i) XRD patterns of the synthesized samples.

All features, including the interconnected multimodal pores, excellent 3D conductivity network and well-distributed Fe3O4/AC NSs, endow the rGO/Fe3O4/AC nanocomposite with great potential as LIB anodes. The electrochemical performance characteristics of the rGO/Fe3O4/AC composite are investigated using coin cells. To reveal the detailed reaction processes, the cyclic voltammetry (CV) curves of the rGO/Fe3O4/AC electrode during the first five scans are exhibited in Figure 2a. During the initial cycle, a small peak is recorded at ~ 0.96 V in the discharge branch but vanished in following cycles, which is assigned to the lithiation reaction of Fe3O4 (Fe3O4 + 2Li+ + 2e- → Li2Fe3O4) via the insertion mechanism; meanwhile, another well-defined peak appeared at ~ 0.6 V, corresponding to the conversion reaction of Li2Fe3O4 to Fe0 (Li2(Fe3O4) + 6Li+ + 6e- → 3Fe0 + 4Li2O) along with the irreversible formation of an SEI film.12,

28

In the charge branch, a broad oxidation peak at

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1.62-1.91 V results from the overlaying of two oxidation peaks, representing the oxidation of Fe0 to Fe2+ and Fe3+.29 From the second cycle onward, the reduction peaks shift to 0.81 V during discharge, and the oxidation peaks appeared at 1.65 and 1.90 V during charge, which is attributed solely to the electrochemical redox reaction (Fe3O4 ↔ Fe) reactions, as observed for the carbon-encapsulated Fe3O4 nanoparticles14 and proved by the XRD results of the electrodes after the cycling processes (Figure S3 in Supporting Information). Notably, the peak intensity drops sharply during the second cycle, and the CV curves are nearly overlapping in subsequent scans, demonstrating a good structural stability and high electrochemical reversibility after the activation process occurred in the first cycle.30 The galvanostatic discharge-charge results show that the initial discharge curve displays a sharp decrease and then remains relatively horizontal with voltage. This result is associated with two possible reasons: (1) the relatively high electrochemical polarization; and (2) the deactivated surface during the initial lithiation process (Figure S4e in Supporting Information).31 The cycle performance of the rGO/Fe3O4/AC anode is depicted in Figure 2b, and the curves of the pristine Fe3O4, AC, rGO, Fe3O4/AC and Fe3O4/rGO composites are used for comparison. Figures 2b and S4 (Supporting Information) display the results. Strikingly, the rGO/Fe3O4/AC electrode exhibits much better cycling stability than dose the other three samples at a current density of 0.5 and 1 A g-1. At 1 A g-1, the initial discharging and charging capacities of the rGO/Fe3O4/AC electrode are 1011 and 704 mA h g-1, respectively, with a corresponding coulombic efficiency of 69.4%. The huge irreversible capacity primarily arises from the inevitable formation of the SEI film on the electrode surface. Notably, the coulombic efficiency increased from 69.4% to approximately 98% after three cycles and maintains nearly 100% thereafter, suggesting that the lithium insertion/extraction reactions are highly reversible and are related to facile transport of ions and electrons in the electrode. Importantly, the battery delivers a long lifespan up to 300 cycles without an apparent decay. After 300 cycles, the rGO/Fe3O4/AC electrode exhibits a reversible capacity of 835.8 mA h g-1 that is much higher than 96.7 mA h g-1 of bare Fe3O4, 255.2 mA h g-1 of Fe3O4/AC and 553.9 mA h g-1 of the Fe3O4/rGO composites, demonstrating that the construction of coating carbon and encapsulated rGO could progressively optimize the capability of the Fe3O4 NSs. By calculation, 23% of AC and 20.3% of rGO have a capacity contribution of about 98 mA h g-1 in the rGO/Fe3O4/AC composite electrode. Rate

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performance is another important figure of merit for battery application, particularly for power tools that require high power.32 The rate capability of rGO/Fe3O4/AC was evaluated at a series of current densities ranging from 0.1 to 10 A g-1. Figure 2c shows that high specific capacities of 527.6 and 436.7 mA h g-1 can be obtained at large current densities of 5 and 10 A g-1, which are 60.7% and 50.2% of the capacity at 0.1 A g-1, even though the current densities amplified 50-fold and 100-fold, respectively. More importantly, after undergoing an ultrahigh rate up to 10 A, nearly 100% of the capacity can be restored when the current density switches back to 0.1 A g-1, demonstrating good electrochemical stability. To evaluate the high-rate and long-term cycling stability, the cycle performance of the rGO/Fe3O4/AC electrode was examined at a large current density of 5 A g-1 after the rate performance test. Notably, the battery can withstand up to 435 cycles with no obvious capacity fading observed from the second cycle onward. These results strongly demonstrate that the rGO/Fe3O4/AC nanocomposite shows encouraging advantages in terms of both superior electrochemical stability and high rate capability and is a promising electrode material. Figure 2d and Table S1 (Supporting Information) present the difference in the rate performance of rGO/Fe3O4/AC and the previously reported Fe3O4/C-based electrode systems. The result indicate that our prepared material delivers competitive or comparable rate performance compared to the most reported Fe3O4/C-based anode materials.14, 17, 26, 29, 33-39 This excellent rate performance is ascribed to the ultrafine reaction subunits and interconnected electron and ion transport pathways, which ensure highly efficient electrochemical reaction kinetics in the well-distributed Fe3O4 NSs.

Figure 2. (a) CV curves of rGO/Fe3O4/AC at the sweep rate of 0.1 mV s-1. (b) Cycling

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performance of the resulting electrodes at 1 A g-1. (c) Rate capability performance of the rGO/Fe3O4/AC electrode. (d) Comparison of rate capability performance for rGO/Fe3O4/AC with that of the reported Fe3O4/C-based electrodes. (e) Cycling performance of the rGO/Fe3O4/AC at rates of 5 A g-1 after the rate performance test.

It is known that the relationship between the peak current intensity and sweep rates contributes to a thorough understanding of the electrode redox kinetics. Therefore, to elucidate the character of the kinetics of the rGO/Fe3O4/AC electrodes, CV measurements with sweep rates in range of 0.1-1.0 mV s-1 were performed (Figure 3a). The current response and sweep rate follow the formula I = aνb, where a and b are adjustable parameters, and b determines the lithium storage types.40 A typical diffusion-controlled process corresponds to b = 0.5, whereas b = 1 indicates a surface capacitive behavior (including the pseudocapacitance contribution from the surface/subsurface charge-transfer process and the double layer capacitive effect).7 In Figure 3b, the calculated b values for both the lithiation and delithiation processes at different potentials are in the range of 0.75~1.0, and the b value is comparatively lower at peak potentials. This b value sheds light on the dual lithium storage mechanism for both the diffusion-controlled redox process and surface capacitive behavior simultaneously attained for the rGO/Fe3O4/AC electrode. When the sweep rate is further increased to 10 mV s-1, a slight deviation in the slope is observed at 2 mV s-1, which causes the b value to decrease and indicates a limitation for the rate capability (Figure 3c and S5c). This finding may arise from numerous reasons, including a rise in the ohmic contribution (the instinctive resistance of the active material and the resistance of the SEI film) and/or diffusion limitations.41 Compared to the pure Fe3O4 electrode, the CV curves of rGO/Fe3O4/AC at different sweep rates display well-defined redox reaction peaks with similar shapes, demonstrating the good response of the electrochemical behavior to various current rates (Figure S5a and b in Supporting Information).2 Another obvious characteristic of the rGO/Fe3O4/AC electrode is that the potential difference shift is significantly lower than that of the bare Fe3O4 electrode when increasing the sweep rate from 0.1 to 10 mV s-1, as displayed in Figure 3d. The large potential difference shift suggests a remarkable polarization of the Fe3O4 electrode. Therefore, the energy requirements to completely charge the material increased by contrast with the energy available for discharge at high current density.41 The polarization of the rGO/Fe3O4/AC electrode

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obviously decreased, signifying that the electrochemical reaction kinetics is enhanced, and the reversibility is higher. The contribution of the surface capacitive capacity is further estimated quantitatively according to 42

i (v) = k1 · v + k2 · v1/2

(1)

where i (v) is the current value at a fixed potential V, v represents the sweep rate, and both k1 and k2 are adjustable parameters. By calculating the constants k1 and k2 (Figure S5d in Supporting Information), the ratios of the surface capacitive contribution at the peak voltages can be quantitatively determined, and the results are shown in Figure 3e and f. The results indicate that the surface capacitive contribution increased as the sweep rates increased. High ratios of 64.1% and 67.8% can be achieved at 1.0 mV s-1 for the reduction and oxidation peaks, respectively. This in turn is a good indication that the surface capacitive effects played the dominant role in the rapid charge and discharge process in the rGO/Fe3O4/AC anode, which is possible due to the ultrafine active subunits. According to the literature, surface capacitive charge storage process is not controlled by diffusion, and a large fraction of lithium-ion storage sites are located on the surface or in the near-surface region, which can effectively improve the reversibility and rate capability in lithium-ion batteries.43 Figure S6 (Supporting Information) displays the galvanostatic discharge-charge profile of the rGO/Fe3O4/AC electrode at different current densities. A sloping region from 2.7 to 0.8 V and one plateau region (~0.8 V) are observed in the profile and are associated with the surface interfacial reaction of the lithium ions and the further conversion reaction to form Fe0, respectively. As seen, the sloping regions are in high coincidence at different current densities, suggesting that a stable surface interfacial lithium-ion storage process is maintained in the rGO/Fe3O4/AC electrode at various reaction rates, contributing to its remarkable rate performance.

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Figure 3. Kinetic analysis of the electrochemical behavior of rGO/Fe3O4/AC. (a) Cyclic voltammograms recorded at sweep rates ranging from 0.1 to 1.0 mV s-1 performed with one cell; (b) b-value determination at different voltages; (c) current response plotted against sweep rates at different voltages; (d) variation in the cathodic peak voltage with the sweep rate; (e) and (f) contribution ratio of the capacitive- and diffusion-controlled capacities at different sweep rates.

It is firmly believed that the electrochemical properties of LIBs greatly depend on electron and lithium-ion transport and structural stability. Thus, to explore the origin of the outstanding electrochemical performance of the rGO/Fe3O4/AC electrode, both the electrochemical impedance spectra (EIS) and the microstructure of the electrodes before and after the cycling were examined. As displayed in Figure 4a, the Nyquist plots of the four fresh cells show similar shapes of a semicircle in the high-to-middle frequency domain and a linear Warburg tail in the low-frequency region. The semicircle in the high-frequency region ascribed to the SEI film (Rsf) resistance and in the medium-frequency region represents the charge transfer impedance (Rct), whereas the sloped line in the low frequency region is attributed to the Warburg impedance (Zw)44-46. It can be clearly observed that the rGO/Fe3O4/AC and Fe3O4/rGO electrodes show much lower Rct values than the bare Fe3O4 and Fe3O4/AC at a fresh cycle, demonstrating that the proper incorporation of rGO can greatly improve the conductivity. Moreover, the more-vertical line in the low frequency region suggests a strengthened capacitive-like behavior in the rGO/Fe3O4/AC electrode. Importantly, both the Rsf and Rct values after the 100th and 300th cycle differs only slightly from those for 3rd cycle, suggesting that the rGO/Fe3O4/AC composite maintains a stable electrode structure after the multiple

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discharge and charge processes (Figure 4b). This structure was also analyzed by SEM. As shown in Figure 4e and f, even after 300 cycles, the rGO/Fe3O4/AC electrode exhibits an integrated and relatively smooth surface, confirming exceptional structural stability. The excellent structural and interface stability can be attributed to the protection effect of the amorphous carbon shell and the encapsuling effect of the cloth-like graphene layers that effectively prevent the continual reformation of the SEI film and active NSs detachment during cycling. In addition, the continuous 3D network and interconnected multimodal porous structure can elastically buffer the mechanical stress of the entire electrode, thus ensuring the structural integrity of the electrode. In contrast, as exhibited in Figure 4c, the values of Rsf and Rct changed obviously after 300 cycles and varied with the applied voltage during the 3rd discharge cycle for the Fe3O4/rGO electrode, confirming that the formed SEI layer on the Fe3O4/rGO surface is unstable. From the SEM images (Figure 4 g and h), the Fe3O4/rGO electrode shows a rough surface with several cracks, which has peeled off from the copper foil after 300 cycles. This peeling may be caused by the absence of the coating effect of the AC and rGO layers, which lead to the Fe3O4 NSs pulverization and aggregation that destroy the electrical connections during prolonged cycling. The Li ion transport properties of the rGO/Fe3O4/AC and Fe3O4/rGO electrodes after 300 cycles were studied based on the Warburg coefficient (σ) associated with the slope of the linear fits in the low-frequency region.47,48 As depicted in Figure 4d, the σ value of rGO/Fe3O4/AC (57.4 Ω rad1/2 s-1/2) is much smaller than that of the Fe3O4/rGO (192.7 Ω rad1/2 s-1/2) after 300 cycles. Therefore, the rGO/Fe3O4/AC electrode has better Li-ion transport kinetics.

Figure 4. Nyquist plots of (a) four lithium half-cells before cycling; (b) rGO/Fe3O4/AC half-cell

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after the 3rd, 100th and 300th cycle at discharged state of 0.1 V; (c) Fe3O4/rGO half-cell after the 300th cycle, and the insert presents selected voltages in during the 3rd cycle; (d) Linear fits of the relationship between Z ́ and ω-0.5 in the low-frequency region referring to Fe3O4/rGO and rGO/Fe3O4/AC after 300 cycles; (e-f) Camera photos and SEM images of rGO/Fe3O4/AC, (g-h) Fe3O4/rGO after 300 cycles at 1 A g -1, and (i) Illustration of the electrolyte transportation in the Fe3O4/AC NSs.

In general, all of the above results strongly support the conclusion that the as-prepared double carbon-confined rGO/Fe3O4/AC electrode shows high-capacity, fast kinetics and long-term cycle stability. These excellent electrochemical properties are ascribed to the designed structural merits as illustrated in Scheme 2, and the most likely explanation is as follows. First, the Fe3O4 NSs that are composed of numerous ultrafine Fe3O4 subunits endow the electrodes with non-negligible porous interfaces. These interfaces increase the likelihood of the utilization of the inner active materials and significantly promote the extent of surface capacitive lithium-ion storage, which are critical for enhancing the reaction kinetics. Second, the highly porous amorphous carbon shell can act as a shield that effectively relieves the volume change in the active particles and restrains the generated Fe nanocrystals from catalyzing the decomposition of the formed SEI film, thus preserving its structural and interfacial stability. Third, the 3D architecture provides plane-to-point intimate interface contacts between the rGO layer and the active particles, effectively preventing active particle detachment and aggregation, which ensures the structural integrity of the electrode. Finally, the abundance of the multimodal porous structure with favorable ion diffusion channels and numerous conductive linkages between the active particles provides great electrolyte wettability to ensure a lithium-ion supply for the electrochemistry reaction and facilitates ion/electron transport within the overall electrode, thus guaranteeing outstanding electron/ion conductivity. As expected, rGO/Fe3O4/AC shows excellent structural stability and electron/lithium ion conductivity, finally resulting in superior long-life stability and rate performance. Scheme 2. Illustration of the rGO/Fe3O4/AC electrode under the charge/discharge process.

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3. CONCLUSIONS In summary, a double carbon-confined rGO/Fe3O4/AC nanocomposite was fabricated by a simple electrostatically assisted self-assembly process. Benefiting from the ultrafine active subunits, a continuous conductive network, interpenetrating ion transport channels, and a stable electrode structure, the as-prepared nanocomposite demonstrated outstanding lithium-storage performance in terms of a remarkable reversible capacity and impressive rate capability. The characteristic of containing both a diffusion-controlled redox process and surface capacitive behavior in this electrode also contributes to the exceptional electrochemical performance. These encouraging research findings may pave the way for the constructing high power and energy densities and the long-term stability of electrode materials. 4. EXPERIMENTAL METHODS 4.1. Synthesis of the Fe3O4 and Fe2O3 nanospheres: A total of 3.8 g of polyvineypirrolydone (PVP, K58, Tianjin Guangfu Fine Chemical Industry Research Institute) and 0.44 g K4Fe(CN)6·3H2O (Tianjin Jiangtian Chemical Tech Co., Ltd.) were added into 50 mL of HCl (Tianjin Jiangtian Chemical Tech Co., Ltd.) solution (0.1 M) via magnetic stirring. The obtained clear solution was transferred into an Erlenmeyer flask and heated at 80 °C for 24 h in an oven. Finally, the obtained blue powder was collected, washed with ethanol and dried at 80 °C for 24 h. Next, the powder was sintered at 350 and 450 °C in air for 3 h to obtain the Fe3O4 and Fe2O3 NSs, respectively. 4.2. Synthesis of the core-shell structure of the Fe3O4/AC nanospheres: First, 0.1 g Fe2O3 NSs were dispersed in tris-buffer (50 mL, pH: 8.5, Shanghai Aladdin Reagent Co., Ltd.) to form a suspension assisted with ultrasonication.11 Then, 50 mg dopamine (Shanghai Aladdin Reagent Co., Ltd.) was added into the above suspension and underwent a reaction for 24 h at 30 °C under continuous magnetic stirring. Subsequently, polydopamine (PDA)-coated Fe2O3 (Fe2O3/PDA) NSs were collected, washed with ethanol, and vacuum dried at 60 °C in an electric oven for 24 h. Finally, the Fe2O3/PDA was transformed into the Fe3O4/AC core-shell nanocomposite by thermal annealing at 450 °C for 3 h under a N2 flow. For comparison, by adjusting the coating time from 12 to 36 h, the samples of Fe3O4/AC-6 (coating time 6 h), Fe3O4/AC-12 (coating time 12 h) and Fe3O4/AC-36 (coating time 36 h) were prepared. 4.3. Fabrication of 3D rGO/Fe3O4/AC architecture: rGO/Fe3O4/AC was synthesized by a

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well-designed self-assembly method. Briefly, Fe2O3/PDA was first decorated with poly(diallyldimethylammonium chloride) (PDDA, Mw