for Lithium Ion Batteries facile synthesis using L-Arginine as the hydrolysis-controlling agent for the formation of

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1 High Crystallized Fe 2 O 3 /Graphene Composites as High-performance Anode Materials for Lithium Ion Batteries By Hongwei Zhang, Liang Zhou and Chengzhou Yu* Abstract Fe 2 O 3 with high crystallinity anchored on graphene sheets has been synthesized by and onestep facile synthesis using L-Arginine as the hydrolysis-controlling agent for the formation of Fe 2 O 3 nanoparticles and reducing agent for graphene oxide (denoted as A-Fe 2 O 3 /G). When applied as an anode material, A-Fe 2 O 3 /G shows excellent cycle stability and good rate performance. A reversible capacity as high as 862 ma h/g can be retained even after 100 cycles with the capacity retention of nearly 90%. A reversible capacity of 500 ma h/g is retained when the current increased to 2000 ma/g, which is still much higher than the theoretical capacity of graphite Introduction Lithium ion batteries (LIBs) have been widely used in portable electronic devices such as smart phones, tablets, and laptop computers. They are also seriously considered as the power sources of choice for upcoming hybrid, plug-in hybrid, and all electronic vehicles. Graphite is the most widely employed anode materials in commercially available LIBs. However, the limited capacity (theoretical capacity: 372 ma h/g) as well as the safety hazards arising from an operating voltage close to that of Li/Li + avoid its large-scale applications. To fully satisfy the increasing demands of the newly emerging applications, advanced LIBs with better performance in terms of energy density, power density, calendar life, and safety are required. Transition metal oxides have been considered as promising anode materials for LIBs with high theoretical capacity compared to that of graphite. 1 Among them, nanostructured Fe 2 O 3 has been widely studied as anode materials for LIBs applications due to its high theoretical 25 specific capacity, low cost, abundance, and environmental friendliness. 2-4 However, the

2 practical application of Fe 2 O 3 for LIBs is still hindered by the huge volume expansion during charge/discharge as well as low electrical conductivity which result in fast capacity fading and poor rate performance. It has been proved that the introduction of carbon matrix such as graphene to form Fe 2 O 3 /graphene composite can significantly enhance the electrochemical performance because graphene can not only buffer the huge volume expansion but also highly improve the electrical conductivity. 5,6 On the other hand, the properties of crystals are determined by serious parameters such as morphology, particle size and crystallinity Although several work about Fe 2 O 3 /graphene composites as anode materials have been reported, most of them focus on the design of Fe 2 O 3 shapes and composite architecture The role of crystallinity of Fe 2 O 3 in Fe 2 O 3 /graphene composite on the electrochemical performance has rarely been investigated. Moreover, hydrazine, a toxic chemical, is usually necessary to reduce graphene oxide to graphene. Herein, we report an one-step facile synthesis of Fe 2 O 3 with high crystallinity anchored on graphene sheets using L-Arginine as the hydrolysis-controlling agent for the formation of Fe 2 O 3 and reducing agent for graphene oxide (denoted as A-Fe 2 O 3 /G). Fe 2 O 3 /graphene composite is also prepared using urea as the alkaline agent (denoted as U-Fe 2 O 3 /G) for comparison. X-ray diffraction (XRD) patterns demonstrate that A-Fe 2 O 3 /G shows much higher crystallinity compared to U-Fe 2 O 3 /G. A-Fe 2 O 3 /G exhibited excellent cycle stability when used as an anode material. A high reversible capacity (862 ma h/g) with the capacity retention of nearly 90% can be obtained even after 100 cycles. The A-Fe 2 O 3 /G shows a good rate performance as well. A reversible capacity of 500 ma h/g is retained when the current increased to 2000 ma/g, which is still much higher than the theoretical capacity of graphite. Our work may present a new guidance to the synthesis of other metal oxide-based composite with highly enhanced electrochemical performance for LIBs applications. Experimental

3 Synthesis of A-Fe 2 O 3 /G and U-Fe 2 O 3 /G Graphene oxide (GO) suspension (2 mg/l) was first prepared according to previous report. 17 For the synthesis of A-Fe 2 O 3 /G, 0.8 g of Fe(NO 3 ) 3 9H 2 O and 0.4 g of L-Arginine was first dissolve in 20 ml of water and then mixed together with 20 ml of GO suspension. The mixture was ultrasonicated for 1 hour before hydrothermal treatment at 180 C for 12 hours. The resulting precipitation was centrifuged and washed repeatedly with deionized water and dried at 50 C for 12 h. Bare Fe 2 O 3 nanoparticles were also prepared using the same procedure mentioned above except the addition of graphene oxide. U-Fe 2 O 3 /G was prepared using the similar method mentioned above. The only difference was that L-Arginine was replaced by urea in the synthesis of U-Fe 2 O 3 /G. Materials characterizations The morphology of the samples was investigated by field emission scanning electron microscope (FESEM, JEOL 7001F) operated at 15 kv and transmission electron microscope (TEM, Tecnai F20) at 200 kv. Thermo gravimetric analysis (TGA) was carried out on a TGA/DSC1 STAR e System under air flow ( ºC, 5 ºC/min). X-ray diffraction (XRD) patterns were collected on a Rigaku Miniflex X-ray Diffractometer with Co Kα radiation with λ=0.179 nm. X-ray photoelectron spectra (XPS) were collected on a Kratos Axis ULTRA X- ray photoelectron spectrometer using a monochromatic Al KR ( ev) X-ray source and a 165 mm hemispherical electron energy analyzer. Electrochemical measurements The electrochemical measurements were carried out using CR2032 coin-type cells on a MTI 8 Channels Battery Analyser at room temperature. The lithium metal chips were used as both the counter electrode and reference electrode. The working electrode is consisted of active material, conductive acetylene black, and polyvinylidene fluoride (PVDF) binder in a weight ratio of 80:10:10. The electrolyte is composed of 1 M LiPF 6 in a mixture of ethylene

4 carbonate, dimethyl carbonate, and diethyl carbonate (1:1:1 in volume). Cells were fabricated in an Ar-filled glovebox with moisture and oxygen concentrations below 0.1 ppm and sealed on a MSK-110 Compact Hydraulic Crimping Machine. Results and discussion The preparation strategy of A-Fe 2 O 3 /G composites is schematically illustrated in Scheme Firstly, Fe 3+ species and L-Arginine were dissolved in the GO suspension followed by ultrasonication for 1 hour. The mixture was then transferred into Teflon-line autoclave and heated at 180 C for 24 hours. A similar strategy in which L-Arginine was replaced by urea was used for the synthesis of U-Fe 2 O 3 /G. The morphology of the samples was investigated by TEM and SEM. Figure 1a and 1b show that A-Fe 2 O 3 /G exhibited typical 2D sheet-like structure with polyhedral Fe 2 O 3 nanoparticles homogenously distributing on the surface of graphene sheets. The Fe 2 O 3 particle size of A- Fe 2 O 3 /G is around 50 nm. The high resolution TEM shows the interlay distance of can be clearly seen in Figure 1c. The well-resolved lattice fringes with interplane distances of 0.37 nm and 0.27 nm come from the (012) plane and (104) plane, respectively. Figure 1d shows a selected area electron diffraction (SAED) pattern and the serious of concentric rings can be clearly seen. Figure 1e and 1f show the morphology of U-Fe 2 O 3 /G which is prepared with the presence of urea. The structure U-Fe 2 O 3 /G is similar with that of A-Fe 2 O 3 /G in term of particle sizes and shapes of Fe 2 O 3. The XPS spectrum of C 1s for GO, A-Fe 2 O 3 /G and U- Fe 2 O 3 /G are shown in Figure 2. The significant intensity enhancement of sp 2 C=C bonds and the decrease of epoxy C-O bonds demonstrate the efficient reduction of GO during hydrothermal treatment at 180 C. To quantify the weight ratio of Fe 2 O 3 to graphene in the composites, thermogravimetric analysis (TGA) was carried out in air from 50 C to 800 C at a heating rate of 5 C/min. the results are shown in Figure 3a. The little weight loss below 100 C could probably be

5 attributed to the evaporation of residual moisture in the samples. The weight loss between 250 C and 450 C was ascribed to the combustion of graphene in air. 6 Therefore, the TGA results revealed the weight percentages of GO in both A-Fe 2 O 3 /G and U-Fe 2 O 3 /G are around 13%. The components of both A-Fe 2 O 3 /G and U-Fe 2 O 3 /G are also confirmed by XRD patterns in Figure 3b. All the diffraction peaks from A-Fe 2 O 3 /G and U-Fe 2 O 3 /G are consistent with the standard values for α-fe 2 O 3 (Hematite, JCPDS NO ). When comparing the XRD patterns between A-Fe 2 O 3 /G and U-Fe 2 O 3 /G, one can find that the intensity of A-Fe 2 O 3 /G is much higher than that of U-Fe 2 O 3 /G, which means A-Fe 2 O 3 /G has much better crystallinity. The high crystallinity of A-Fe 2 O 3 /G may attribute to the weak basic condition due to the slow hydrolysis of L-Arginine to release OH- group during the hydrothermal process. 18 reaction process can be described as following: The 113 H 2 NCH(NH)NH(CH 2 ) 3 CH(NH 2 )COOH + H 2 O [H 2 NCH(NH 2 )NH(CH 2 ) 3 CH(NH 2 )COOH] + + OH Fe OH - Fe 2 O 3 + 3H 2 O Coin-type cells were fabricated to investigate the electrochemical behaviour of A-Fe 2 O 3 /G and U-Fe 2 O 3 /G. Figure 4a shows the first three cyclic voltammogram (CV) curves of A- Fe 2 O 3 /G at room temperature at the scan rate of 1 mv/s between 0.0 V to 3.0 V. Four distinct peaks can be clearly observed at 0.62 V, 1.41 V, 1.62 V and 1.97 V from the first cathodic scanning, which can be ascribed to the conversion of Fe 2 O 3 to Fe 0 and the formation of solid electrolyte interphase (SEI) layer on the surface of electrode. Meanwhile, two broad peaks are present at 1.61 V and 1.89 V in the following anodic process, corresponding to the reverse oxidation of Fe 0 to Fe 2 O 3. In the subsequent cathodic processes, the peak initially presented at 0.62 V shifts right, while other three peaks cannot be observed any more. No peak shift is observed unless there is a little decrease of the peaks intensity, exhibiting a good reproducibility in the anodic processes.

6 Figure 4b shows the charge-discharge voltage profiles of A-Fe 2 O 3 /G at the current of 0.2 C (1 C= 1000 ma/g) between 0.05 and 3 V. Three distinct voltage plateaus could be clearly identified at about 1.62 V, 1.1 V and 0.8 V, respectively. The voltage plateau at 1.6 V is reported in hematite nanoparticles and can be ascribed to the insertion of lithium ion into Fe 2 O 3 crystal structure. 19 The plateaus at 1.1 V and 0.8 V can be attributed to the reduction of Fe 3+ to Fe 2+ and the reduction of Fe 2+ to Fe 0, respectively. This conversion reaction (Fe 3+ to Fe 0 ) dominantly contributes to the lithium-storage capacity of the material and gives a high initial discharge capacity of 1268 ma h/g. A reversible charge capacity of 856 ma h/g can be delivered, which leads to an irreversible capacity loss of 32.5% for the first cycle (the capacity value is based on the total mass of A-Fe 2 O 3 /G). The initial capacity loss is caused by the formation of a solid electrolyte interphase (SEI) layer on the electrode surface during the discharge-charge process. The discharge capacities of A-Fe 2 O 3 /G in 2 nd, 20 th, 50 th and 100 th cycles are 945, 932, 930 and 862 ma h/g, respectively. And the charge capacities in 2 nd, 20 th, 50 th and 100 th cycles are 867, 921, 917 and 843 ma h/g, respectively. Figure 4c shows the cycle performance of A-Fe 2 O 3 /G, U-Fe 2 O 3 /G and bare Fe 2 O 3 at the current of 0.2 C between 0.05 and 3 V. It can be seen that the capacity of A-Fe 2 O 3 /G is not so stable in first several cycles which might be resulted from the activation process of nanostructured metal oxide particles. 20 After activation, the A-Fe 2 O 3 /G composite exhibits a high reversible capacity (~862 ma h/g) even after 100 cycles, leading to good capacity retention of more than 90%. Also the Coulombic efficiency is as high as 98%, showing excellent reversible property of A- Fe 2 O 3 /G as anode materials for LIBs. In contrast, bare Fe 2 O 3 shows poor cycle stability due to the absence of graphene. The specific capacity rapidly drops to 320 ma h/g after 50 cycles. U-Fe 2 O 3 /G also shows poor cycle performance compared to A-Fe 2 O 3 /G. The capacity continuously decreases and only 372 ma h/g is retained after 50 cycles.

7 The rate performance of A-Fe 2 O 3 was investigated at the current of 0.2 C to 2 C (1 C= 1000 ma/g). Figure 4d shows that a reversible capacity of around 600 ma h/g is still retained at a high rate of 2000 ma/g, which is about 67% of the initial capacity. The capacity can restore to 929 ma h/g (around 98% of the initial capacity) while the current decreases from 2 C to 0.2 C, showing excellent rate performance of A-Fe 2 O 3 /G as anode materials for LIBs application. Both A-Fe 2 O 3 /G and U-Fe 2 O 3 /G show similar morphology and structure in term of Fe 2 O 3 particle sizes and shape (from TEM results) and graphene contents (from TGA results). XPS results shows similar reduction degree of graphene oxide in both A-Fe 2 O 3 /G and U-Fe 2 O 3 /G were also obtained after hydrothermal treatment at 180 ºC. The distinct difference between A-Fe 2 O 3 /G and U-Fe 2 O 3 /G one can find is that A-Fe 2 O 3 /G exhibits much higher crystallinity than U-Fe 2 O 3 /G from XRD results. The high crystallinity of Fe 2 O 3 nanoparticles might dominantly contribute to the remarkable cycle stability of A-Fe 2 O 3 /G when compared with U-Fe 2 O 3 /G. In conclusion, we have synthesized Fe 2 O 3 nanoparticles with high crystallinity homogenously anchored on the surface of graphene sheets using L-Arginine as the hydrolysis-controlling agent for the formation of Fe 2 O 3. This A-Fe 2 O 3 composite demonstrated superior cycle performance compared to U-Fe 2 O 3, which might be attributed to the high crystallinity of A- Fe 2 O 3 as the morphology and structure of A-Fe 2 O 3 /G and U-Fe 2 O 3 /G were similar. The A- Fe 2 O 3 composite also showed excellent rate performance as anode materials. Our finding may present as a new guidance to synthesize other metal oxide-based anode materials with highly improved electrochemical performance for LIBs application. 172

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9 Scheme 1. Schematic illustration for the formation of A-Fe 2 O 3 /graphene Figure 1. TEM (a), SEM (b) and HRTEM (c) images of A-Fe 2 O 3 /G, SAED pattern (d) of A- Fe 2 O 3 /G, TEM (e) and SEM (f) images of U- Fe 2 O 3 /G.

10 Figure 2. XPS of graphene oxide (a), U-Fe 2 O 3 /G (b) and A-Fe 2 O 3 /G (c).

11 Figure 3. TGA (a) and XRD patterns (b) of U-Fe 2 O 3 /G and A-Fe 2 O 3 /G.

12 Figure 4. cyclic voltammetric (CV) curves (a) of A-Fe 2 O 3 /G in the 1 st, 2 nd and 3 rd cycles, charge-discharge curves (b) of A-Fe2O3/G, cycle stability of A-Fe 2 O 3, U-Fe 2 O 3 /G and A- Fe 2 O 3 /G, rate performance (d) of A-Fe 2 O 3 /G.