rGO composites with a novel interfacial characteristic and enhanced ultrastable lithium storage performance

Applied Surface Science 507 (2020) 145051

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FeC2O4@Fe2O3/rGO composites with a novel interfacial characteristic and enhanced ultrastable lithium storage performance

T

Keyu Zhanga,b,c, Da Zhanga,b, Yin Lia,b, Li Wanga,b, Feng Lianga,b,c, Yongnian Daia, ⁎ Yaochun Yaoa,b,c, a

National Local Joint Engineering Laboratory of Lithium Ion Battery and Material Preparation Technology, Kunming University of Science and Technology, Kunming 650093, China b National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, Yun-nan, China c State Key Laboratory of Complex Nonferrous Metal Resources Clear Utilization, Kunming University of Science and Technology, Kunming 650093, Yun-nan, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Iron oxalate Lithium-ion batteries In situ synthesis Reduced graphene oxide Synergistic effect

Iron oxalate (FeC2O4) is considered as a potential new energy storage material because of its high reversible capacity and attractive cost performance platform. However, the slow chemical reaction kinetics and unstable structures of FeC2O4 make it difficult to achieve high-rate and long-cycling lithium storage performance. Herein, a facile and scalable approach is developed to synthesize multilayer FeC2O4 coated with Fe2O3 nanodots and adsorbed on flexible conductive rGO matrix (FeC2O4@Fe2O3/rGO), which exhibits great potential as an anode material for high-rate and stable-cycling performance. Furthermore, encapsulation of Fe2O3 nanodots on surface of FeC2O4 particle promotes the formation of a stable core-shell structure with an irreversible “organic” layer as the shell, and inhibits the disintegration of repeated volume expansion of FeC2O4 particles at high current density. The improvement of cycling performance is attributed to the existence of flexible rGO which can work like a spider web and anchor of the FeC2O4@Fe2O3 composites onto its one-dimensional scaffold, providing an enhanced electrical pathway of high conductivity for the electrochemical reaction between Li and Fe. This work offers a promising material architecture for obtaining the stable and reliable anode materials with high energy density.

1. Introduction Electrochemically active transition-metal oxides (TMOs), such as FeOx [1–3], CoOx [4,5], and MnOx [6,7], have been widely applied on the energy storage field due to their high capacity. Besides these metal oxides, as early as 1968, Aoki and Hiroi have reported transition metal oxalates (TMOxs) as satisfactory electrode materials for primary batteries [8]. However, these materials have received less attention than their equivalent oxides due to early studies demonstrating the ability of oxides to give high theoretical capacities and voltages when reacted with lithium (Li) [9–12]. Until to 2008, Aragón et al. discovered and demonstrated a high-capacity and improved rate performance for TMOxs which is also regarded as promising candidates for lithium storage materials [13,14]. Compared with the equivalent metal oxides, these novel electrode materials represented by FeC2O4 and CoC2O4 have been shown to possess comparable, and at times higher electrochemical reactivity, better capacity retention, and lower cost [15–17].

Larger capacity can be attributed to their special electrochemical reaction and lithium storage mechanism, which can be described as the decomposition of electrolyte, the capacitive effect, and the conversion reaction between lithium and metal oxalate (the theoretical expectation of ~370 mAh g−1, as shown in Eq. (1)) [18]. MC2O4 + 2Li+ + 2e− → Li2C2O4 + M

(1)

Therefore, much attention has been once again paid on exploring their potential applications in lithium-ion batteries (LIBs) [19]. Despite of these attractive features, FeC2O4 also has the characteristics of slow diffusion of lithium ions and low intrinsic conductivity, forming a large number of inactivated composite materials to result in high irreversible capacity and poor cyclability during charge-discharge process [20,21]. Furthermore, unstable solid electrolyte interface (SEI) films accelerate the pulverization and deterioration of active materials and thus limit its energy storage performance, especially at high current densities [22]. Coating carbonaceous substances has been widely considered as an

⁎ Corresponding author at: National Engineering Laboratory for Vacuum Metallurgy, Kunming University of Science and Technology, Kunming 650093, Yun-nan, China. E-mail address: [email protected] (Y. Yao).

https://doi.org/10.1016/j.apsusc.2019.145051 Received 22 October 2019; Received in revised form 27 November 2019; Accepted 11 December 2019 Available online 13 December 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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improve the mobility of Li+ ions in channels between the FeC2O4@ Fe2O3 particles and flexible rGO. Based on above novelties, the FeC2O4@Fe2O3 /rGO composite will be a promising material for highperformance metal oxalate-based LIBs.

effective method to improve the conductivity of electrode materials and obtain better electrochemical performance [15,23]. In particular, the new classes of carbon-based nanomaterials (CBN) like reduce graphene oxide (rGO), carbon nanodots (CNDs) and carbon nanotubes (CNTs) are desirable for modifying the kinetics of electrochemical reaction [24–26]. By a simply solvothermal method to synthesize cupric-cobaltous oxalate/functionalized graphene oxide (Cu1/3Co2/3C2O4·xH2O/ FGO) composite, the reversible capacity of this composite increased from 508.0 mAh g−1 to 935.6 mAh g−1 at current rate 2000 mA g−1 after 100 cycles [27]. However, compared with its discharge capacity in first cycle, the capacity retention of this material after 200 cycles decreases to 50%, which is due to the instability interaction between the productions during charge-discharge and low electrochemical activity of metal nanoparticles (Li and Cu) [20,28,29]. In addition, the micronano structure design of active materials is another useful strategy by shortening lithium-ion diffusion distance to improve their electrochemical performance. For instance, anhydrous FeC2O4 rods (906 mAh g−1, at 1C) [18], CoC2O4 nanosheets (824 mAh g−1, at 1C) [30] and hydrate CuC2O4·xH2O cylinder-like nanostructure (970 mAh g−1, at 200 mA g−1) [29] all exhibit higher capacity after 100 cycles. Therefore, the performance can be further improved by optimizing the combined mode and morphological structure between TMOxs particles and CBN [31]. Utilizing electrostatic adsorption technology, NiC2O4·2H2O nanorods attached onto rGO sheets by a self-assembly layerby-layer (SA-LBL) process offered a superior high reversible capacity of 933mAh g−1 at 500 mA g−1 and 87.5% efficiency in the first cycle [32]. Obviously, the key to modify the defects of TMOxs is to combine strategies of shortening lithium-ion diffusion distance and improving electronic conductivity. Surface modification can alleviate the unstabilized SEI films caused by rapid Li insertion and extraction and avoid serious particles cracking, relying on a judicious design of micro-nano structure and synergistic hybridization of two or three active materials [33]. Metal oxides, such as Al2O3 [34], Fe2O3 [35], ZnO [36], SiOx [37], TiO2 [38] and so on, have been widely coated onto the surface of electrode materials to modify their structural stability. For instance, Al2O3-coated LiNi0.5Co0.2Mn0.3O2 was reported to deliver higher capacity retention of 85% after 100 cycles, in comparison with the pristine electrode (only 75%) [39]. And the TiO2–x-coated silicon nanoparticles revealed a high capacity of 939 mAh g−1 at a significantly high current density of 12 A g−1, that is, 89% of the initial capacity at 0.2 A g−1 [40]. Among these metal oxides, especially Fe2O3 can effectively prevent the contact between the electrode surface and organic electrolyte and reduce the corrosion of electrolyte [41]. In addition, as a potential anode material, Fe2O3 can also provide higher capacity, which makes it more excellent than other metal oxides. However, there are few reports on the application of Fe2O3 in surface modification of anode materials and its mechanism in charge-discharge process. Herein, we employ a facile and scalable approach to synthesize multilayer FeC2O4 coated with Fe2O3 nanodots and adsorbed on flexible conductive rGO matrix. This unique nanostructure and synergistic effect of Fe2O3 nanodots and rGO matrix have a series of advantages, making it possible for anode material with high rate and stable cycling performance: (i) The multilayer iron oxalate with a mesoporous nanostructure ensures unobstructed and stable channels for Li+ ions and electron diffusion, improving the reaction kinetics and utilization of the active material [42]. (ii) Encapsulation of Fe2O3 nanodots on the surface of multilayer FeC2O4 can facilitate the formation of stable SEI films and uniform dense amorphous organic layers during cycling. These shell-like components can inhibit the disintegration of FeC2O4 particles undergoing repeated volume expand, especially at high current density. (iv) Utilizing electrostatic self-assembly technology, flexible rGO can fix FeC2O4@Fe2O3 composite on its one-dimensional scaffold like a spider web, providing a high conductivity to enhance electrical path for the electrochemical reaction between Li and Fe, thus improving its cycling performance. Furthermore, the unique wrapping structure also can

2. Experimental 2.1. Preparation of nanosized FeC2O4·2H2O with multilayer structure All of the chemical reagents in experiments were of analytical grade (Sinopharm, Shanghai, China). The common-FeC2O4·2H2O was prepared by a low-temperature solvothermal method using the mixed of alcohol and water as solvent. In this typical experiment, a total of 1 mmol of iron (II) sulfate heptahydrate (FeSO4·7H2O) and 0.2 mmol ascorbic acid (C6H8O6) were dissolved in 45 mL deionized water to form a homogeneous and clear solution at room temperature. An equal molar ratio of oxalic acid dihydrate (H2C2O4·2H2O) was prepared in 12 mL of alcohol and added to the FeSO4 solution slowly under normal stirring for 1 h. Afterwards, the mixture was then transferred to 80 mL Teflon-lined stainless steel autoclave and heated at 50 °C for 12 h. Finally, the yellow precipitates (dihydrate iron oxalate) were separated by centrifuge (6000 rpm, 5 min), washed with ethanol absolute for 3 times until all soluble materials were removed, and dried in a vacuum desiccator at 80 °C for 12 h. 2.2. Preparation of the composites of FeC2O4·2H2O adhered with PDDA The as-synthesized FeC2O4·2H2O powders (2.0 g) were sonicated in deionized water (80 ml) for half an hour to disperse them uniformly. 1 ml Poly (diallyldimethylammonium chloride) (PDDA, MW 200 000–350 000 Da, 20% wt) solution was then added into the suspension and vigorously stirred for 2 h. Finally, the FeC2O4·2H2O@PDDA composite was collected by centrifuge at 6000 rpm for 5 min and further dried in Ar atmosphere at 50 °C for 6 h. 2.3. Preparation of FeC2O4@Fe2O3/rGO composites The synthesis graphene oxide (GO) was based on the Hummers method [43–44]. To fabricate the aqueous rGO [45], 80 ml of the prepared GO suspension (0.2 ml mg−1) and 10 ml of NaOH solution (8 M) were loaded into a Teflon-lined stainless steel autoclave by heating at 80 °C for 30 min. After cooling to room temperature, the rGO was obtained by intensive centrifugation (20000 rpm, 1 h) of the final suspension, followed by an washing with deionized water to pH = 7 and freeze-drying before using. The FeC2O4@Fe2O3 and FeC2O4@ Fe2O3/rGO were prepared by in situ growth process and self-assembly technique utilized electrostatic adsorption. 0.1 g dried rGO was added in 80 ml mixed solvent of alcohol and water (volume ratio 1:1) and sonicated for 1 h. Then, the rGO dispersion was mixed with FeC2O4·2H2O@PDDA (2.0 g) and stirred for 2 h at 45 °C. The precipitates were filtered and washed thoroughly four times with deionized water and absolute alcohol and dried at 80 °C under vacuum. The final products were collected for characterization and electrochemical measurement after sintering in a tubular furnace at 300 °C for 3 h under Ar protection. The FeC2O4@Fe2O3 was fabricated as mentioned above without rGO. 2.4. Materials characterization Crystallographic characterization of these materials were identified by X-ray diffraction (XRD, Rigaku, D/Mac-3c) employing Cu Kα radiation with a scan speed of 0.02° (2θ) per second range from 10° to 70°. Laser Raman Spectroscopy (LRS) was measured to confirm the presence of rGO in the FeC2O4@Fe2O3/rGO composite using labRAM (Horiba, HR800) at a wavelength of 532 nm within the wavenumber range of 500 to 3000 cm−1. The elemental states in the composite were revealed 2

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because the ionization of iron oxalate can hardly penetrate submicrometer-sized particles, especially in deionized water system. In the process of electrostatic adsorption, the FeC2O4·2H2O/PDDA@Fe(C2O4)n composite with positive charge was dipped into rGO dispersion, and graphene sheets with negative charge were self-assembled on the surface of this composite to yield FeC2O4·2H2O/PDDA@Fe(C2O4)n/rGO. Final, in the annealing step, both crystallized water molecules and PDDA are evaporated to form a complex porous structure. Meantime, the Fe(C2O4)n is decomposed to Fe2O3 nanodots matrix wrapping on outside of the multilayer structured FeC2O4 nanorods, namely FeC2O4@ Fe2O3/rGO composite. Details of experimental methods are described in the experimental parts. The crystalline microstructure of FeC2O4@Fe2O3/rGO composite was tested by X-ray diffraction (XRD) and Raman, as shown in Fig. 2a–b XRD spectra confirms the presence of FeC2O4·2H2O in both FeC2O4·2H2O/PDDA@Fe(C2O4)n/rGO and FeC2O4·2H2O/PDDA@ Fe (C2O4)n composite with larger crystallite size and pure β orthorhombic phase [47]. The peaks correspond to the planes of (2 0 2), (0 0 4), (4 0 0), (0 2 2), (2 2 4), (6 0 2) and (0 2 6), referring to PDF No. 022–0635. By comparison to these dihydrates, these samples exhibit wider and weaker peak widths after annealing treatment, indicating poor crystallinity, which is associated with losing the supporting function of water molecules (Fig. 2c). However, there is almost no rGO signal in FeC2O4@Fe2O3/rGO composite, indicating effective deposition of active substances on surface of rGO [27]. Raman spectrum of б(C-O) mode for iron oxalate powder presents a dominant peak around 1497 cm−1 [48–49]. Meanwhile, a medium-strong band around at 916 cm−1 is assigned to be б(C-C) mode, showing the maintenance of multilayer structure of FeC2O4 [50]. After complexing with rGO, all peaks of FeC2O4 still exist, and FeC2O4@Fe2O3/rGO complex form new peaks. For FeC2O4@Fe2O3/rGO composite, the main peaks at 1353 and 2700 cm−1 correspond to the D- and G- band of flexible rGO [51]. While, the Fe2O3-related peaks are not found in both of XRD pattern and Raman spectrum, which is the reason for the low content of Fe2O3. To clearly reveal the internal morphology and structure of FeC2O4@ Fe2O3/rGO composite, the synthesized samples were confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and fast Fourier transform (FFT). As shown in Fig. 3a–c, the common FeC2O4 shows large number of two-dimensional rod-like particles with lengths of ~600 nm and widths of ~200 nm, and good monodispersity. After adhering with PDDA and coating by rGO, these submicrometer-sized rods also have uniform morphology, which shows that the electrostatic self-assembly technique and in situ growth process did not damage the crystal structure and morphology of multilayer FeC2O4. In addition, the flexible rGO layers liking a spider web wrap around the FeC2O4@Fe2O3 composites onto its one-dimensional scaffold (Fig. 3c). TEM and HRTEM images further describe the special shape of FeC2O4@Fe2O3/rGO composite and existence of Fe2O3. The multilayer and complex porous structure can be observed for common FeC2O4, which is corresponded

by an X-ray photoelectron spectroscopy (XPS, ULVAC, PHI-5000) measurement with a monochromatic Al X-ray source (1486.6 eV). The surface morphology and structure of samples were carried out by scanning electron microscopy (SEM, Hitachi SU8010) with field emission SEM operated at 10 kV. The interior structure and surface states of the morphology was measured by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM, Tecnai-TF30) with an acceleration voltage of 200 kV. Energy-dispersive spectrometer (EDS) tomography in an aberration-corrected scanning transmission electron microscopy (STEM) was used to analyse the atomic structure and quantify the elements (Fe, C and O) distribution within FeC2O4@Fe2O3 nanoparticles. 2.5. Electrochemical measurements Electrochemical performance was tested in CR2025 coin-type cells with a metallic lithium foil as the counter electrode and a Celgard 2400 microporous membrane as the separator. The working electrode was prepared by mixing 60 wt% as-prepared active material, 30 wt% conductive additive (Super P), and 10 wt% polyvinylidene fluoride (PVDF) as binder in N-methyl pyrrolidinone (NMP) to form a viscous slurry. Then, the slurry was uniformly coated onto a copper current collector, and further dried at 80 °C under vacuum for 24 h. The dried electrode was cut into a wafer with the diameter of 14 mm. Then, the cells were assembled in a glove box (H2O, O2 < 1 ppm) filled with Ar gas using an electrolyte composed of 1 mol L−1 LiPF6 in ethylene carbonate and diethyl carbonate (EC/DEC, 1:1 by volume). Galvanostatic charge–discharge cycle tests were carried out on an electrochemical test instrument (XWJ Neware Tech. Co., BTS3000, China) at current densities of 0.1, 0.2, 0.5, 1, 2, 3, and 5 A g−1 between 0.01 and 3 V (vs. Li/Li+) under 25 °C. Cyclic voltammetry (CV) was performed using electrochemical workstation (Ametek, PMC-1000dc, America) at a scan rate of 0.2 mV s−1. Electrochemical impedance spectra (EIS) were measured in the frequency range from 100 kHz to 10 mHz with an alternatingcurrent perturbation of 10 mV at an open-circuit potential. 3. Results and discussion The FeC2O4@Fe2O3/rGO composite was prepared by electrostatic self-assembly technique and in situ growth process of multilayer FeC2O4 with the assistance of poly diallyldimethylammonium chloride (PDDA) via three main steps, including surface modification, electrostatic adsorption and annealing, as shown in Fig. 1. First, multilayer FeC2O4·2H2O and positively charged PDDA are fully dispersed in deionized water and uniformly mixed to form FeC2O4·2H2O/PDDA. In surface modification step, a small amount of FeC2O4·2H2O could reversibly ionize into oxalate complexes, Fe(C2O4)n−2(n−1), which adhere with partial positively charged PDDA and form PDDA@Fe(C2O4)n composite adhering on outside of the particles [42,46]. It is noted that Fe(C2O4)n−2(n−1) has little effect on particles’ multilayer structure,

Fig. 1. Schematic of the synthesis procedures for the composite of multilayer FeC2O4 coated with Fe2O3 nanodots and adsorbed on flexible rGO. 3

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Fig. 2. XRD spectra of samples: (a) FeC2O4·2H2O, PDDA-adsorbed FeC2O4·2H2O nanorods and rGO-assisted composite of FeC2O4·2H2O/PDDA; (b) the dehydrated samples; (c) Raman spectra of FeC2O4, FeC2O4@Fe2O3, and FeC2O4@Fe2O3/rGO.

rGO between FeC2O4 particles, the average interlayer distance of which is 3.64 Å, similar to that of the d-spacing reported by Moon [52]. Fig. 4 shows CV profiles of as prepared FeC2O4 electrodes at initial and after steady cycles. The peaks of initial cycle curves are nearly overlapped without much distinction for each electrode, indicating little influence of Fe2O3 nanodots and flexible rGO on electrochemical behaviour. Three obvious reduction peaks at ~1.8, ~1.1 and ~0.7 V along with one shoulder near ~0.25 V, and two broad oxidation peaks around at ~1.25 and ~1.5 V are observed, which can be ascribed to the complicated decomposition of electrolyte, reversible conversion reaction and existence of reversible capacitive effect (Fig. 4a) [13,53,54]. All the peaks and shoulder are also in consistence with initial discharge–charge profiles of as-prepared FeC2O4 electrodes at 0.1 A g−1 (Fig. 5a). While, with the later scans from 2nd to 8th (in Fig. 4b–d), curves are perfectly different with that of in first cycle, suggesting

to that reported in literatures [18,42]. As shown in Fig. 3g and j, FeC2O4@Fe2O3 and FeC2O4@Fe2O3/rGO composites exhibit an intriguing structure similar to the morphology of speckle, as shown in Fig. S1. HR-TEM images in Fig. 3h and l clearly reveal the nanodots wrapped on the surface of FeC2O4 particles and the continuous lattice fringes which indicate single crystals differing from amorphous multilayer FeC2O4. The lattice fringes show interlayer spacing of 0.267 and 0.249 nm, corresponding to the d-spacing of Fe2O3 (1 0 4) and (1 1 0) planes, respectively. Fast Fourier transform (FFT) spot diagram (Fig. 3i) also clearly verifies the single crystal nature of the particles. This demonstrates that surface regions of FeC2O4 particles are uniformly rich in Fe2O3 nanodots matrix, which is further cinfirmed by the energy dispersive spectroscopy (EDS) line scan result and X-ray photoelectron spectroscopy (XPS) analysis as presented in Fig. S2. Furthermore, Fig. 3k shows the lattice fringes (11 layers in a ~ 4 nm stack) of flexible

Fig. 3. SEM images of the composite samples: (a) FeC2O4, (b) FeC2O4@Fe2O3 and (c) FeC2O4@Fe2O3/rGO. TEM micrographs of the common-FeC2O4 at (d) low and (e) high magnifications. (f) fast Fourier transform (FFT) images. (g and h) TEM/HR-TEM images of ferric oxide nanodots assisted composite of FeC2O4. (i) FFT for metal oxide nanodots. (j and k) TEM/HR-TEM images of FeC2O4@Fe2O3/rGO. (l) HR-TEM images of Fe2O3 nanodots. 4

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Fig. 4. Cyclic voltammetry (CV) curves of the first cycle for all as-prepared FeC2O4 composite electrodes (a) and the later scans from 2nd to 6th of (b) FeC2O4, (c) FeC2O4@Fe2O3, and (d) FeC2O4@Fe2O3/rGO. All scans were performed at a rate of 0.2 mV s−1 in the range of 0.01–3.0 V.

electrolyte. It is worth mentioning that a significant shoulder of all samples is observed near 0.25 V in reducing process, demonstrating the existence of reversible capacitive effect. The initial discharge-charge profiles of FeC2O4, FeC2O4@Fe2O3 and FeC2O4@Fe2O3/rGO at various current densities in a voltage range of 0.01–3 V are presented inFig. 5. All three electrodes reveal a similar Li+ insertion/extraction tendency upon discharge-charge process at 0.1 A g−1, obtaining charge capacity of 1093.43, 942.81 and 1054.69 mAh g−1 for common FeC2O4, FeC2O4@Fe2O3 and FeC2O4@Fe2O3/rGO composite with their reversible capacity retentions of 69.54%, 63.11% and 69.32%, respectively (Fig. 5a). A lack of distinct differences suggests that doping process does not hinder electrochemical behaviour at low current. Whereas, FeC2O4@Fe2O3/rGO electrode exhibits the largest irreversible capacity retention of 40.24% and 48.02% at current

unstable electrochemical reversibility. The reducing peak near 0.75 V is gradually enhanced, it confirms that the complex nanocomposites fabricated in first discharge-charge cycle need to activate again [19]. However, the disappearance of latter two peaks around 1.4 V and 0.9 V after 4 cycles indicates the formation of SEI films. Compared with FeC2O4 and FeC2O4@Fe2O3, FeC2O4@Fe2O3/rGO exhibits a smaller difference in reduction peak at 0.7 V in final three cycles (4th to 8th cycles), implying that electrochemical activity of reaction outcomes (such as metal nanoparticles and oxalate matrices) is enhanced in FeC2O4@Fe2O3/rGO. Furthermore, the final anodic peaks of FeC2O4 and FeC2O4@Fe2O3 corresponding to the Fe2+/Fe redox reaction around ~0.75/1.5 V are broader and more asymmetric than that of FeC2O4@Fe2O3/rGO. This phenomenon can be attributed to the simultaneous occurrence of conversion reaction and decomposition of

Fig. 5. Comparison of the first discharge-charge curves for all types of electrodes measured by applying current densities of (a) 0.1 A g−1, (b) 0.5 A g−1 and (c) 5 A g−1 at 25 °C. 5

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Fig. 6. TEM/HR-TEM measurement of common-FeC2O4 (a–b and e–g) and FeC2O4@Fe2O3/rGO (c-d and h-i) electrodes after first cycle (at the charged state) at current rate of 0.1 A−1 (a–d) and 5 A g−1 (e–i).

densities of 0.5 and 5 A g−1, as compared with those of bare FeC2O4 for 29.79% and 36.69%. In addition, with current densities increasing, the second voltage plateau region around 1.1 V gradually decreases to 0.9 V, which may be ascribed to the voltage hysteresis during decomposition process of electrolyte. To further understand the electrochemical mechanism and low reversible capacity retention of FeC2O4@Fe2O3/rGO electrode at high current density, common FeC2O4 and FeC2O4@Fe2O3/rGO electrodes after first cycle at 0.1 and 5 A g−1, were dispersed in N-methyl pyrrolidinone by ultrasonication. Then, a droplet of the dispersion liquid was placed onto a piece of carbon substrate and copper grid for HRTEM characterization. As shown in Fig. 6a–d, after the electrodes charged to 3 V at low current density (0.1 A g−1), although a slight of dense SEI film is observed on particle surface of both common FeC2O4 (17.39 nm) and FeC2O4@Fe2O3/rGO (15.59 nm), original multilayer microrod morphology has been well maintained. On the contrary, when the electrodes charged at high current density (5 A g−1), a plenty of loose electrochemical products accumulated by spherical materials are uniformly adhered on the edges of common FeC2O4 particles with thickness of ~29 nm (Fig. 6e and f). Higher magnification TEM images (Fig. 6g and S3) further details the composition of spherical substance with the clear and continuous lattice fringes. The nanocrystals with different lattice channels are obviously detected with characteristic inter-planar distances close to those of 0.192 nm (LiF), 0.257 nm (Li2O) and 0.278 nm (Li2CO3), respectively. LiF is an inevitable product from the lithium salt (LiPF6) in electrolyte via electrochemical process, and both of Li2O and Li2CO3 arise from the continuous decomposition of lithium ethylene dicarbonate (LEDC) [55]. However, the particles of FeC2O4@Fe2O3/rGO are encapsulated by a thicker colloidal and amorphous “organic” layer originated from the decomposition of electrolyte during discharge and charge process different with SEI film (nearly ~53 nm, in Fig. 6h and i), forming a consolidated core-shell structure and thus producing a large irreversible capacity in the initial discharge-charge process [42]. The formation of irreversible “organic” layer is greatly ascribe to the electrochemical behaviour of Fe2O3 nanodots. According to the recent reports [2,56], Fe2O3 can consume a part of lithium to form two Li-intercalated phases (LiFe2O3 and Li2Fe2O3) around 1.2 V during the charge process. At the same time, the FeC2O4 electrode exhibits decomposition of electrolyte, which results in the interaction between Li-intercalated phases and “organic” productions. In addition, the rough surface of FeC2O4@Fe2O3/rGO also creates more chance for the firm attachment of this irreversible layer. Therefore, it is of great significance to study the mechanism of interfacial

electrochemical reaction in order to reduce the thickness of the adhesive layer and obtain a larger reversible capacity. The rate capabilities of FeC2O4, FeC2O4@Fe2O3 and FeC2O4@ Fe2O3/rGO electrodes are presented at various rates between 0.01 and 3 V (vs. Li/Li+) in Fig. 7a. The cells are first discharged and charged at 0.2 A g−1 for ten cycles, followed by cycling at 0.5, 1, 2, 3, and 5 A g−1 for every five cycles, respectively. The specific discharge capacity of common FeC2O4 electrodes significantly decreases with the increasing current density, finally reaching approximately at 38.17 mAh g−1 at 5 A g−1. Additionally, the discharge capacity decreases obviously at 0.2 A g−1 after high-rate cycles, which may be due to the irreversible defects in crystal structure caused by rapid transport of Li+ ions. Meanwhile, FeC2O4@Fe2O3/rGO composite exhibits excellent discharge capacities of 908.37, 737.99, 656.53, 552.33 and 476.43 mAh g−1 at 0.2, 0.5, 1, 2, 3, and 5 A g−1, respectively. The capacity almost can recover to its initial capacity after high-rate cycles when later discharged at 0.2 A g−1, suggesting that the high current charge/discharge process could not break down the integrity of the electrode. It may be related with the function of “organic” layer. The cyclic stability of all electrodes was further investigated at different current rates (Fig. 7b). It can be clearly seen that the reversible capacity of FeC2O4@Fe2O3/rGO electrode are 873.45, 755.96, 589.54 and 471.43 mAh g−1 at 0.5, 1, 3 and 5 A g−1 after 100 cycles, respectively. The existence of rGO can effectively improve the cyclic stability and reversible capacity at low current density by enhancing the electrical pathway of the electrochemical reaction between Li and Fe. The improvement in rate performance is mainly ascribed to the enhanced structure stability of FeC2O4@Fe2O3/ rGO composite, in which Fe2O3 nanodots play a significant role in formation of the amorphous “organic” layer. The layer prevents volume expansion and allows Li+ ions to penetrate the active materials more easily. In related literatures, the synergistic effects of core–shell structure (constituted by metal oxides, polysulfide and polymer) and carbonaceous substances significantly enhanced the strength of electrode and effectively improved electrons and Li-transport in the electrode [57–59]. These functions of Fe2O3 nanodots and rGO on rate property are further confirmed by long-term cyclability measured at a rate of 0.5 A g−1 for 500 cycles (Fig. 7c). Considering the SEI films and “organic” layer, the reversible capacity in first thirty cycles of FeC2O4@Fe2O3/ rGO is obviously not ideal. However, after several dozen cycles of activation, its reversible capacity and capacity retention are close to 922 mAh g−1 and 61%, then keep stable in subsequent cycle. Moreover, the coulombic efficiency is higher than 100% at first ten cycles, and then reaches almost 100%, which is possibly due to unstable electrochemical 6

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Fig. 7. Galvanostatic cycles at various C rates for all electrodes. (a) Rate capabilities of all electrodes at current densities of 0.2, 0.5, 1, 2, 3 and 5 A g−1. (b) The typical galvanostatic discharge/charge curves after 100 cycles obtained at 0.5–5 A g−1. (c) Long-term cycling performance and coulombic efficiency of FeC2O4, FeC2O4@Fe2O3, and FeC2O4@Fe2O3/rGO at a constant of 0.5A g−1.

cm2 s−1) after 100 cycles, illustrating that the unique wrapping structure constituted by Fe2O3 nanodots and flexible rGO also can provide more stable diffusion channels and reduce the constraint of “organic” layer on the mobility of Li+ ions in intercalation and deintercalation process. To characterize the anode morphology after 500 cycles at 0.5 A g−1, SEM images were taken for both of common FeC2O4 (Fig. 9a and b) and FeC2O4@Fe2O3/rGO composite (Fig. 9c and d). The original morphology of FeC2O4@Fe2O3/rGO composite is well maintained, but a small amount of consolidated sediments formed by electrochemical reaction are accumulated on surface of active materials (Fig. 9c and d). By contrast, for common FeC2O4 electrode, big chunks of FeC2O4 are blended with incompact reaction outcomes, and could hardly be recognized after cycling. The detailed change of morphology and structure for single-particle can also be demonstrated by TEM images in Fig. 9e–h. The transverse cracking of multilayer microrods and exfoliated SEI films through FeC2O4 particle are clearly observed, which may be caused by serious volume expansion and strong structural stress change during long cycling. For FeC2O4@Fe2O3/rGO composite, the nanorods are easily identified everywhere within the “organic” layer, and their sizes are well preserved as indicated in SEM and TEM images before (mostly ~600 nm) and after cycling (mostly 500–650 nm). Meanwhile, the stabilized core-shell structure has been well maintained, and the thickness of irreversible “organic” layer after 500 cycles (Fig. 9h, ~ 49 nm) is similar with that of in first cycle (Fig. 6i, ~53 nm). The support from flexible rGO and “organic” layer architecture also prevent the collapse of microstructure and maintain the stability of Li+ mobility channels, thus improving the reversibility of conversion reaction. Hence, multilayer FeC2O4@Fe2O3/rGO composite with unique wrapping structure promotes the formation of stable core-shell structure with irreversible “organic” layer as the shell, which makes it possible for anode material with high rate and stable cycle

activity of complex nanocomposites and formation of SEI films. The capacities of FeC2O4 and FeC2O4@Fe2O3 (414.36 and 459.46 mAh g−1 after 500 cycles) are lower than that of FeC2O4@Fe2O3/rGO composite. The results show that flexible rGO can anchor FeC2O4@Fe2O3 composite material on its one-dimensional scaffold like a spider web, then improve cyclic stability [60]. Thus, taking all of these benefits together including the multilayer structures of FeC2O4 particles, role of Fe2O3 nanodots, and effective matrix of rGO, the FeC2O4@Fe2O3 composite yields a core-shell structure for obtaining the stable and reliable anode materials with high energy density. Electrochemical impedance spectroscopy (EIS) analysis was performed to ascertain the lithium ion diffusion channels before and after cycling, the results shown in Fig. 8 and Table S1, respectively. An equivalent circuit (insert of Fig. 8c) is also constructed to evaluate the formation of surface film and charge-transfer process [61]. It can be noted that common FeC2O4 has the lowest surface and charge-transfer resistance (Rst+ct) values at 65.7 Ω in Nyquist. However, the resistance increases after 100 and 200 cycles (225.6 and 284.7 Ω), which may be related to the loosened and unstable SEI films on anode surface. In contrast, for FeC2O4@Fe2O3 and FeC2O4@Fe2O3/rGO, the values of Rst+ct at 100th cycle show smaller impedance compared with that of before cycling, possibly due to the initial stabilization process of “organic” layer at first charge. While, a visible impedance decrease is observed from the 100th to the 200th cycle for FeC2O4@Fe2O3/rGO electrode, indicating that flexible rGO can signally modify the particles’ interface characteristics during cycling. To further consider the influence on Li+ ion diffusion coefficient (DLi+), Warburg impedance is fitted according to the proportional relationship between Z’ (Zre) and ω−0.5 (Fig. 8d, e and f) [62]. As shown in Table S1, little difference for the calculated DLi+ of all electrodes is observed for the fresh cells, whereas the DLi+ of FeC2O4@Fe2O3/rGO composite (5.54 × 10−13 cm2 s−1) is nearly thrice over than that of common FeC2O4 (1.67 × 10−13 7

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Fig. 8. Nyquist plots of all FeC2O4 electrodes: (a) FeC2O4, (b) FeC2O4@Fe2O3, and (c) FeC2O4@Fe2O3/rGO. Inset: the corresponding equivalent circuit. Relationship between Zre and ω−0.5 at low frequencies for the fresh cells (d), after 100 (e) and 200 cycles (f) at current density of 0.5 A g−1.

wrapping structure can provide a promising material architecture for the application of TMOxs anode materials with high energy density in the field of energy storage.

performance.

4. Conclusions

Author contributions

In summary, we have successfully synthesized the composite of multilayer FeC2O4 coated with Fe2O3 nanodots and absorbed on flexible conductive rGO matrix via electrostatic self-assembly technique and in situ growth process. We discovered that FeC2O4@Fe2O3/rGO composite could significantly improve the electrochemical reactivity, leading to high reversible capacity retention, excellent rate capability and good long-term cycling stability. Moreover, upon electrochemical cycling, the wrapping of Fe2O3 nanodots promotes the formation of stable coreshell structure with irreversible “organic” layer as the shell. Although it shows larger irreversible capacity retention in initial discharge-charge process, the shell-like components can suppress the disintegration of FeC2O4 particles undergoing repeated volume expand, especially at high current densities, thus maintaining stability of Li+ mobility channels and improving reversibility of conversion reaction. The multilayer FeC2O4@Fe2O3/rGO nanostructured composite with unique

K. Zhang and Y. Yao contributed the central idea and designed experiments; K. Zhang and D. Zhang carried out experiments; K. Zhang analyzed experimental results. Y. Li and L. Wang carried out additional analyses. F. Liang and Y. Dai contributed to refining the ideas. K. Zhang, Y. Li and Y. Yao wrote the manuscript. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgments This work was financially supported by National Natural Science

Fig. 9. SEM images of FeC2O4 (a–b) and FeC2O4@Fe2O3/rGO (c–d) electrodes after 500 cycles at 0.5A g−1 and as well as TEM micrographs ((e–f) for FeC2O4 and (g–h) for FeC2O4@Fe2O3/rGO). 8

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Foundation of China (Grant No. 51364021), the project of Natural Science Foundation of Yunnan Province (Grant No. 2018HB012), the Program for Innovative Research Team in University of Ministry of Education of China (No. IRT_17R48).

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Appendix A. Supplementary material [29]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.145051.

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