Facile Synthesis of Multi-channel Surface Modied Amorphous Iron Oxide Nanospheres as a High-performance Anode Material for Lithium-ion Batteries

The application of iron oxide as anode of lithium-ion batteries is hindered by its poor cycle stability, low rate performance and large voltage hysteresis. To address these problems, multi-channel surface modied amorphous Fe 2 O 3 nanospheres were synthesized by using a facile hydrothermal method, which exhibited outstanding electrochemical performances. According to crystalline state and microstructure, it was found that surface structure of the amorphous Fe 2 O 3 nanospheres can be controlled by adjusting the reaction time, due to the synergistic effect of ripening and hydrogen ion etching. Owing to the isotropic nature and the absence of grain boundaries, the amorphous Fe 2 O 3 nanospheres could withstand high strains during the intercalation of lithium ions. Meanwhile, the multi-channel surface structure can not only increase the contact area between Fe 2 O 3 nanospheres and electrolyte, but also reserve space for volume expansion after lithium storage, thereby effectively alleviating the volume change during the intercalation-deintercalation of lithium ions. As conrmed by the Galvanostatic intermittent titration analysis results, the amorphous Fe 2 O 3 electrode had higher Li + diffusion coecient than the crystalline counterpart. As a result, the multi-channel surface modied amorphous Fe 2 O 3 electrode exhibited higher specic capacity, more stable cycle performance and narrower voltage hysteresis. It is believed that amorphous metal oxides have great potential as high-performance anode of next-generation lithium-ion batteries.


Introduction
Due to their high energy/power densities and long cycle life, lithium ion batteries (LIBs) have been widely used in portable electronic devices and electric vehicles (EV) [1]. However, the commercial graphite electrodes with low theoretical capacity of 372 mAh/g cannot meet the requirements of high energy density [2,3]. Therefore, various anode materials with higher speci c capacity than graphite have been explored, such as metal [4], silicon-based materials [5,6], MXene [7], nitrides [8,9], sul des [10] and transition metal oxides (TMOs) [11]. Iron oxide (Fe 2 O 3 ) has received widespread attention, due to its natural abundance, low price, non-toxicity and high theoretical speci c capacity (1007 mAh/g) [12,13]. Unfortunately, similar to other TMOs, Fe 2 O 3 also has poor electronic conductivity, low Coulomb e ciency, high potential hysteresis and large volume change, during the charge/discharge process, leading to rapid fading in capacity [14,15].
In order to tackle these issues of Fe 2 O 3 anodes, various strategies have been proposed recently. For instance, one strategy is to build 1D-3D nanostructures, such as nanowires [16], nanorods [17], nanotubes [18], nanosheets [19], nanoboxes [20], ower-like [21] and nanospheres [22]. Gu  nanorods as a sacri cial template to synthesize hollow Fe 2 O 3 nanotubes, the Fe 2 O 3 electrode had a speci c capacity of 764.2 mAh/g after 500 cycles at a current density of 0.5 A/g [18]. With these Fe 2 O 3 nanostructures, the diffusion path of lithium ions can be effectively shortened and the drastic volume changes during the lithiation/delithiation process can be alleviated. Another way is to hybridize Fe 2 O 3 with conductive carbonaceous materials or metals, thereby increasing the conductivity and the diffusion rate of lithium ions. Yu et al. reported Fe 2 O 3 /graphene hybrid-based electrodes with largely enhanced conductivity, a high reversible capacity of 658.5 mA h/g was achieved after 200 cycles at 1 A/g [23]. Finally, Fe 2 O 3 has also been compounded with other metal oxides to achieve high electrochemical performances. In a binary hybrid, the two active materials reacted with lithium at different voltages, respectively. The synergistic effect between them can not only improve the storage performance of lithium, but also inhibit the expansion of electrode materials [24]. Zhang et al. prepared Fe 2 O 3 /SnSSe hexagonal nanoplates from hot-inject process in oil phase, the hybrid anode can maintain a capacity of 755 mAh/g after 100 cycles at a current density of 200 mA/g [25]. However, these methods are either complicated operations or involve organic solvents, leading to electrode materials with high costs.
In contrast to the intercalation reaction of graphite electrodes, transition metal oxide (TMO) can interact with lithium through phase conversion reaction (MO x + 2xLi ↔ M + xLi 2 O). Therefore, the feasibility and reversibility of the reaction process are dependent on the thermodynamic and kinetic parameters of the conversion reaction. The Gibbs free energy change (ΔG) of the reaction between amorphous Fe 2 O 3 and lithium is 0.27 eV lower than that of its crystalline counterpart. The lower the Gibbs free energy change, the stronger the reversibility of the conversion reaction [26]. Shi and Zhu combined amorphous Fe 2 O 3 with graphene or nitrogen-doped carbon, respectively, and the corresponding electrodes were used in lithium-ion batteries or sodium-ion batteries to achieve high cycle stability [27,28]. In addition, amorphous materials have higher capability to withstand strains and shorter pathway for the diffusion of lithium ions [29][30][31]. It has been con rmed that anodes based on amorphous phases Si, Fe 2 O 3 , TiO 2 and SnO 2 could buffer the volume change and hence exhibited improved cycle performance [32][33][34][35][36].
Therefore, it is expected that amorphous materials are promising candidates as electrode, due to their fast reaction kinetics, strong reversibility and narrowe potential hysteresis [37,38].
In this work, a simple and low-cost hydrothermal method was used to synthesize multi-channel surface modi ed amorphous Fe 2 O 3 nanospheres as LIBs anodes. Besides the isotropic nature and the lack of grain boundaries, the multi-channel structure is more favorable for the intercalation and deintercalation of lithium ions, and ensures intimate contact between the active materials and electrolyte, which is very bene cial to the cycle stability of the electrode. Compared with its crystalline counterpart, the multichannel surface modi ed amorphous Fe 2 O 3 electrode exhibits higher speci c capacity, more stable cycle performance and narrower voltage hysteresis.

Characterization
Crystallinity of the samples was examined by using X-ray diffraction (XRD) with a Bruker-D8 X-ray diffractometer with nickel ltered copper K radiation (λ = 1.5406 Å). Morphologies of the samples were observed by using eld emission scanning electron microscopy (FESEM, JEOL JSM-7500F) and transmission electron microscopy (TEM, JEM-2100P). Elemental compositions and chemical states were analyzed by using X-ray photoelectron spectroscopy (XPS, Thermo K-Alpha). Speci c surface area of the samples was measured by using the Brutern-Emmett-Teller (BET) method with a Micrometrics ASAP 2420 surface analyzer.

Electrochemical characterization
The active materials (80 wt.%) were mixed with conductive carbon black (10 wt.%) and CMC (10 wt.%) in deionized water to form slurries. The slurries were coated on nickel foam (99.5%, Alfa Aesar) as current collector, followed by drying in a vacuum oven at 80°C for 12 h to obtain electrodes. Coin cells (CR 2032) with lithium foil as the counter electrode were assembled in a glove box lled with high-purity Ar gas (> 99.999%). Electrolyte consisting of 1 M LiPF 6 in a mixture of vinyl acetate (EC), ethylene carbonate (DEC) and dimethyl carbonate (DMC) (1:1:1 by volume) was used with a micro-porous polymer membrane separator (Celgard 2400). The cells were charged and discharged between 0.005 V and 3 V (versus Li/Li + ) using blue electric test system (CT2001A). Galvanostatic intermittent titration technique (GITT) was employed by charging/discharging the cells at a current of 100 mA/g for 20 min and it took about 4 h until the cut-off voltage limits were reached. Prior to post-cycling characterization, the cells were charged at 3 V for 48 h to ensure full extraction of Li. CHI660E electrochemical analyzer was used to record cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The voltage range of CV measurement was 0.01-3 V and the scan rate was 0.1 mV/s. The EIS measurement was conducted from 100 kHz to 10 MHz. Figure 1 shows schematic diagrams describing formation process of the multi-channel surface modi ed amorphous Fe 2 O 3 nanospheres during the hydrothermal treatment and the lattice evolution of Fe 2 O 3 due to corrosion. Fig. 2 shows XRD patterns of the samples A1h and C1h. The diffraction peaks of C1h are consistent with those of α-Fe 2 O 3 (No.33-0664), indicating that it is crystalline α-Fe 2 O 3 . In contrast, A1h has no obvious diffraction peaks, suggesting that it is amorphous. As the hydrothermal treatment time was extended from 1 h to 6 h, the XRD pattern of the samples remained unchanged (Fig. S1). [IMAGE-C:\Workspace\ACDC\ImageHandler\4e Figure 3 shows wide-scan XPS spectrum of A1h, con rming the presence of Fe and O in the amorphous sample. Fe 2p XPS spectrum is shown as an inset in Figure 3. The two peaks at 710.7 eV and 724.5 eV correspond to the binding energies of Fe 2p 3/2 and Fe 2p 1/2, respectively. The two peaks are consistent with the peaks of Fe 3+ [39,40]. In addition, the satellite peak at 719 eV further con rms that the cation is Fe 3+ [26]. Meanwhile, there is an obvious characteristic peak of lattice oxygen (O 1s) at 530 eV, as shown in Fig. S2 [41,42]. Based on the XRD pattern in Fig. 2, it is concluded that amorphous Fe 2 O 3 was formed in the samples (A1h) prepared by using the hydrothermal reaction method with the precursor solution of  There are two main effects of H + ions on the structure and morphology of the synthesized Fe 2 O 3 . On the one hand, H + ions corrode crystal nuclei to form amorphous; On the other hand, H + ions corrode the surface of nanospheres into multiple staggered channels. In high temperature and high pressure environment in the hydrothermal kettle, hematite nuclei formed due to the hydrolysis of the Fe 3+ ions in 6 ] were easily etched by H + ions from NH 4 H 2 PO 4 [43]. The nuclei lose the periodicity of their original crystal structure. In other words, the nuclei became amorphous, as shown in Fig. 1 (b), which then grew into amorphous nanoparticles. As the hydrothermal reaction continues, the amorphous nanoparticles converged into spheres to reduce the total surface energy. The surface of the spheres was smooth after reaction for 1 h, as shown in Fig. 4 (a). Combined the XRD pattern (Fig. S1) and the electron diffraction (SAED) pattern ( Fig. 4(g)) of the sample A1h, it could be determined that the sample A1h was amorphous. In addition, the high concentration of H + ions in the solution continues to corrode Fe 2 O 3 nanospheres. A few holes appeared on the surface of nanoparticles after 3 h of hydrothermal reaction ( Fig. 4 (b)). As the reaction time was increased to 6 h, the pores on the surface of the nanospheres continue to be corroded and grow to form irregular interconnected channels (Fig. 4 (c)). To reveal surface characteristics of the nanospheres, N 2 adsorption-desorption measurement were performed to obtain the BET surface area and the Barrett-Joyner-Halenda (BJH) pore size distribution pro le, with the results to be shown in Fig. S3. The sample A3h has a pore size distribution in the range of 2-10 nm, while the surface of the A6h sample has multiple small holes that are connected to form channels (30-60 nm). The results are consistent with the SEM observation. Representative TEM images of the samples reacted for different times are shown in Fig. 4 (d-f), indicating hollow structure of the Fe 2 O 3 nanospheres. With increasing hydrothermal treatment time, the wall thickness of the hollow nanospheres is decreased. This observation can be understood according to the Ostwald ripening process, because the inner particles have higher surface energy than the outer ones [43].
Electrochemical performances of the multi-channel surface modi ed amorphous Fe 2 O 3 nanospheres and the crystalline Fe 2 O 3 nanospheres were comparatively studied, in terms of cyclic voltammetry (CV) and galvanostatic charge/discharge curves. Fig. 5 shows CV curves of the two samples. CV curves of the crystalline α-Fe 2 O 3 nanospheres are similar to those previously reported in the literature [44,45].
Comparatively, there are three differences in the CV curve between the amorphous and the crystalline Fe 2 O 3 nanospheres. Firstly, the intensity of the reduction peak of the amorphous Fe 2 O 3 during the rst two cycles is lower than that of the crystalline Fe 2 O 3 . This may be attributed to the long-range disorder of the amorphous state and its low reaction Gibbs free energy change (ΔG), i.e., lithium can be intercalated/deintercalated more easily in amorphous Fe 2 O 3 . The volume change of the amorphous electrode is a gradual process, different from the sudden change of the crystalline electrode, which is bene cial to the integrity and cycle stability of the electrode [26,46]. Secondly, the main cathode peak shifted by 0.05 V in the second cathodic scan and the magnitude of the peak shift is much smaller than that of the crystalline Fe 2 O 3 , which may be caused by the better reversibility of the amorphous Fe 2 O 3 electrode. The peak near 1.2-1.4 V may be related to the formation of solid solution compounds, owing to the insertion of Li + ions into the amorphous Fe 2 O 3 [26,47]. Finally, the rst anodic scan of the amorphous electrode has two cathodic peaks at 1.47 V and 2.0 V, corresponding to the oxidation of Fe(0) to Fe 2+ and further to Fe 3+ , respectively [47,48].  Galvanostatic charge/discharge measurements were conducted at a current density of 100 mA/g in the voltage range of 0.005-3.0 V. As shown in Fig. 6 (a, b), the rst discharge speci c capacity of the amorphous electrode (A6h) is 1187.3 mAh/g. The value of the crystalline electrode (C1h) is slightly higher (1305 mAh/g). The large irreversible capacity of the two samples in the rst cycle is a common phenomenon, which is related to the decomposition of electrolyte and the formation of SEI layer [48]. The speci c capacity of the crystalline sample decreased rapidly, while the value of the amorphous one is much stable. The values are 817 mAh/g, 815.5 mAh/g, 818.8 mAh/g, and 822.2 mAh/g in the four cycles. Fig. 6 (c) shows cycle performances of the amorphous and the crystalline Fe 2 O 3 electrodes.
Comparatively, the amorphous electrode has higher cycle stability, with the speci c capacity remaining at 875.2 mAh/g after 70 charge-discharge cycles. At the same time, the Coulombic e ciency is close to 100%. Also, the cycle speci c capacity is increased slightly, corresponding to a growth rate of 7.12%. The increase in speci c capacity of the amorphous electrode can be ascribed to the reversible formation of polymer gel-like lm and the larger electrochemically active surface area of the Fe 2 O 3 shell [2,49,50].
With further cycling, the capacitive-like storage effect is strengthened, which is advantageous for high power applications [51]. In addition, the amorphous Fe 2 O 3 electrode (A6h) with multi-channel microstructure provided more active sites and space for lithium intercalation, thus promoting the interfacial lithium storage of the active materials, which also contributed to the high speci c capacity [11,52]. In order to further identify the difference in lithium ion storage performance between the amorphous and the crystalline Fe 2 O 3 electrodes, rate performance tests were performed, with the results to be shown in Fig. 6 (d). When the current density is increased from 100 to 2000 mA/g, the average discharge capacities of the amorphous electrode (A6h) and the crystalline one (C1h) are decreased from 849.7 to 478.0 mAh/g and from 829.7 to 44.6 mAh/g, respectively. As the current density is restored to 100 mA/g, the speci c capacity of the amorphous electrode quickly recovered to 783.3 mAh/g, which is much higher than that (544.2 mAh/g) of the crystalline electrode. The corresponding charge/discharge curves at different current densities are shown in Fig. 6 (e) and (f), respectively. As the current density is increased from 100 to 2000 mA/g, the discharge voltage plateau of the amorphous electrode (A6h) only slightly decreased, indicating that it has a relatively low polarization [53]. The outstanding rate performance is attributed to the well-distributed multi-channel structure, which offered a large electrode/electrolyte interface area and shortened the transport path of electrons and ions.
Notably, the discharge/charge voltage curves of amorphous and crystalline Fe 2 O 3 electrodes during different cycles are shown in Fig. S4. During the cycle, the discharge platforms of the amorphous Fe 2 O 3 electrodes are stable, indicating their low polarization and potential hysteresis [47]. The low potential hysteresis is linked with its faster kinetics and higher energy e ciency, which is an important factor for the practical applications [54]. In order to con rm this result and understand the ion diffusion kinetics, galvanostatic intermittent titration (GITT) measurement was utilized to analyze the lithium ion transport kinetics of the electrode. Fig. S5 shows potential change of the sample as a function of time. The cells were repeatedly subject to a current pulse of 100 mA/g for 20 min and then relaxed for 240 min. The long relaxation time was used to full relaxation of lithium diffusion to reach equilibrium potential and minimize the self-discharge of Fe 2 O 3 during the test. The discharge/charge curves of the amorphous and crystalline electrodes have similar trends. Current pulse step polarization curves of the amorphous and the crystalline Fe 2 O 3 electrodes at different potentials are shown in Fig. 7 (a) and (b), respectively.
According to the polarization curves, when the cells went to a higher voltage upon charging, it took more time to relax the pulse to reach a stable state (Fig. 7 (a-2) and Fig. 7 (b-6)). Similarly, when the cells were discharged at a lower voltage, the pulse relaxation would be delayed ( Fig. 7 (a-3) and Fig. 7 (b-7)). These phenomena indicate that the lithium diffusion coe cient will change with the change of potential.
Overpotential refers to the voltage difference between the equilibrium potential at the end of relaxation and that at the end of the current pulse (≈ ΔE τ -ΔE s , ignoring IR drop) [55]. The overpotential during charging is greater than that during discharging. As clearly seen in Fig. 7 (a) and (b), the voltage hysteresis of the amorphous electrode is signi cantly less pronounced than that of the crystalline electrode. The difference in polarization may be attributed to their difference in kinetics [56].
In order to better understand diffusion kinetics of Li + , galvanostatic intermittent titration (GITT) data were used to derive the diffusion coe cients (D Li+ , cm 2 /s) of the Fe 2 O 3 electrodes, which can be estimated by using the Fick ' s law [51]:  Fig. 7 (c) and (d), respectively. It can be seen that the D Li+ values in the two electrodes range from 10 13 to 10 10 cm 2 /s, in agreement with the reported data of α-Fe 2 O 3 nanoparticles (10 14 -10 11 cm 2 /s) [57] and -Fe 2 O 3 electrode (9.96 × 10 13 cm 2 /s) [58]. It is worth noting that the amorphous electrode showed higher D Li+ , because mainly of the lack of grain boundaries, thus shortening the diffusion pathways and reducing the diffusion resistance. Reaction kinetics of the amorphous and the crystalline Fe 2 O 3 electrodes were also evaluated by using electrochemical impedance spectroscopy (EIS). All Nyquist plots exhibited a recessed semicircle in the high-frequency region and a sloping line in the low-frequency region, which correspond to the charge transfer resistance and the diffusion impedance of Li + , respectively. As shown in Fig. 8 diameter of the amorphous Fe 2 O 3 electrode in the high frequency region is much smaller than that of the crystalline Fe 2 O 3 electrode. Therefore, due to the noncrystalline nature, amorphous Fe 2 O 3 greatly ensured a rapid charge transfer, thus facilitating faster lithiation/delithiation kinetics [38]. The EIS results are consistent with the cycle performances ( Fig. 6 (c)) and the rate performances ( Fig. 6 (d)).

Conclusions
In summary, multi-channel surface modi ed amorphous Fe 2 O 3 nanospheres have been successfully prepared by using a facile hydrothermal method, due to the synergistic effect of ripening and hydrogen ion etching. Owing to the isotropic nature and the lack of grain boundaries, the amorphous Fe 2 O 3 facilitated high lithium ion insertion and withstood high strains. The multi-channel surface modi ed structure of the amorphous Fe 2 O 3 not only ensured close contact between the internal active materials and the electrolyte, but also effectively alleviated the volume change during the intercalation/deintercalation of lithium ions. The multi-channel surface modi ed amorphous Fe 2 O 3 nanospheres electrode exhibited excellent cycle stability (875.2 mAh/g after 70 cycles at 100 mA/g) and superior rate performance (56.3% capacity retention from 0.1 to 2.0 A/g) and narrow voltage hysteresis.
The amorphous Fe 2 O 3 electrode exhibited faster electrochemical reaction kinetics, higher Li + diffusion coe cient and lower overpotential, as compared with its crystalline counterpart. The results in our present study can be used as a reference for the synthesis of amorphous transitional metal oxides        Nyquist plots of the amorphous and the crystalline Fe2O3 electrodes.