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Iron fluoride vertical nanosheets array modified with graphene quantum dots as long-life cathode for lithium ion batteries
Chemical Engineering Journal 371 (2019) 245–251
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Iron fluoride vertical nanosheets array modified with graphene quantum dots as long-life cathode for lithium ion batteries ⁎
Qi Zhanga, Chengzhi Suna, Lishuang Fanc, Naiqing Zhangb,c, , Kening Sunb,c,
T
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a
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China c Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China b
H I GH L IG H T S
·0.33H O nanosheets array electrode was constructed for the first time. • FeF graphene quantum dots were used to surface modification of FeF ·0.33H O. • Firstly • GQDs surface modification can greatly enhance the cycle performance of the electrode. 3
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A R T I C LE I N FO
A B S T R A C T
Keywords: Iron fluoride Graphene quantum dots Array electrode LIBs Super long-life
We successfully built a unique architecture of FeF3·0.33H2O, a self-supported, binder-free vertical nanosheets array directly growing on Ti foil by a simple solvothermal approach and firstly use graphene quantum dots to surface modification. The high specific surface area of 2D nanosheets and high conductivity of graphene quantum dots provide an expressway for charge transfer and ion transport. This unique structural design endows a capacity of 161.5 mAh g−1 at 1 C and a capability of 117 mAh g−1 at a high current density of 4 Ag−1 (20 C). Graphene quantum dots surface modification can stabilize the structure and reduce possible side reactions, making the cycle performance greatly improved. Even cycled over 1000 times at 2 C the capacity attenuation rate is as low as 0.03% per cycle.
1. Introduction Like the heart means to humans, energy supply device is crucial to electronic product. Lithium ion batteries (LIBs) have been successfully applied in electronics and electric vehicles because of their outstanding high capacity and long cycle life [1,2]. However, existing LIBs cannot meet the consumers pursuing for superior performance. Developing high specific capacity and high rate battery is now the focus of research. Low capacity of conventional cathode materials (140 mAh g−1 for LiCoO2 and 170 mAh g−1 for LiFePO4) which are limited by the intercalation reaction mechanism has become a bottleneck to the development of LIBs. Therefore, exploring new cathode materials becomes the top priority of research for LIBs [3,4]. In recent years, transition metal fluorides (MxFy, M = Fe, Mn, Co, Cu, etc.) gradually attract particular attention, especially the iron fluoride (FeF3) with merits of high-capacity, high-voltage, and low-cost becomes a favorable cathode material [5–9]. A discharge plateau 2.73 V vs. Li+/Li, specific capacity 237 mAh g−1 can reach benefit from the ⁎
high ionic bonding characteristics and small molecular weight. Among various crystal form of iron fluoride, FeF3·0.33H2O has unique advantages [10,11]. Hexagonal-tungsten-bronze-type FeF3·0.33H2O has unusual tunnel structure and better electrochemical activity compare with the common ReO3-type FeF3 without crystal water. Despite many advantages, there are still some problems for practical application of FeF3·0.33H2O such as poor conductivity and limited electrode reversibility. Generally, there are two effective ways to address these issues. Firstly, constructing unique nanostructures like nanoparticles [12–14], porous nanospheres [15–17], and nanowires [18,19] can increase the surface area and the reactive sites, which are in favor of reversible electrode reaction. Another solution is combine FeF3·0.33H2O with high conductive materials such as carbon nanotube [10,11,20], graphene [21–24], and conductive polymers [25]. These high conductive materials can provide highways for rapid electron transfer which would facilitate a rapid reaction. Using these two methods can indeed improve the electrochemical performance of iron fluoride to some extent. However, the performance improvement is limited, and cannot meet
Corresponding authors. E-mail addresses: [email protected] (N. Zhang), [email protected] (K. Sun).
https://doi.org/10.1016/j.cej.2019.04.073 Received 14 January 2019; Received in revised form 4 March 2019; Accepted 10 April 2019 Available online 11 April 2019 1385-8947/ © 2019 Published by Elsevier B.V.
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bronze-type FeF3·0.33H2O, JCPDS card no. 76-1262). The XRD result of the GQDs@FFNA can also correspond to the standard peaks after the peaks of the titanium are neglected. And there is no distinctly peaks of GQDs were detected. The X-ray photoelectron spectroscopy (XPS) was used to analyze the state of the surface element of GQDs@FFNA. It can be determined that the peaks of Fe, F, and C appear on the spectrum (Fig. 2b). The peak of Fe 2p3/2 located at 710.8 eV (Fig. 2c) suggests the +3 oxidation state of Fe element [34]. The C 1s peak of carbon can be divided into three peaks at 284.6 eV, 286.24 eV and 287.88 eV, corresponding to CeC, CeO and C]O, respectively [34] (Fig. 2d). The fitting result of C 1s is similar to the typical peaks of graphene, for which CeC peak is strong, CeO and C]O peaks are weak. This indicating that the carbon element on the surface is mainly graphene [35–37]. The surface morphology of the electrodes were observed by scanning electron microscope (SEM). The FeF3·0.33H2O nanosheets formed by solvothermal (Fig. 3a) have a thickness of 20–30 nm in size and these nanosheets grown vertically on the substrate to form an array. After modified by the GQDs, the surface of the nanosheets (Fig. 3b) are no longer smooth and as if covered with a layer of gauze. A appropriate electrophoresis time were carried out based on the concentration of GQDs solution to prevent excessive deposition of GQDs (Fig. S3). Atomic force microscopy (AFM) shows that the GQDs have a average size of 5.65 nm and a thickness of 1.5 nm (Fig. 3c,d). It indicates that there are about 3–5 layers of carbon atoms in GQDs. The prepared GQDs have good dispersibility in water and the dispersion exhibits typical fluorescence characteristics (Fig. S4). These colloid-like hydrophilic groups formed by GQDs in water can be moved and adsorbed on the surface of the FFNA electrode during electrophoresis. In order to further observe the fine structure of the FFNA and GQDs@FFNA electrodes, the morphologies of the samples were observed by transmission electron microscopy (TEM). As shown in Fig. 4a,b, the FeF3·0.33H2O nanosheets are consist of nanoparticles under 10 nm. These nanoparticles agglomerated into a porous nanosheet structure which would facilitate the entry of electrolyte into the interior of the material. Fig. 4c shows the high resolution TEM image of FeF3·0.33H2O nanosheet. The lattice fringe spacing marked in the figure is 0.19 nm and 0.37 nm, corresponding to the (0 0 4) and (2 0 0) plane of FeF3·0.33H2O [38]. Fig. 4d,e shows the TEM pictures of GQDs@ FFNA. At low magnification (Fig. 4d), some flakes can be seen on the surface of nanosheet and distorted lattice structure can be seen after magnification (Fig. 4e). The twisted lattice structure (marked by red circle) are similar to the lattice spacing of the graphene oxide [39]. Because the GQDs were made through the oxidation of carbon cloth [30], properties of some large flakes in GQDs certainly show a similarity to graphene oxide. In order to verify the uniformity coverage of GQDs, EDS mapping of Fe, F, and C elements were conducted and shown in Fig. 4f. It can be seen that the C element is evenly distributed on the surface of the material and consistent with the Fe and F elements. It proves that GQDs are evenly coated on the surface of FeF3·0.33H2O nanosheets. The GQDs on the surface of the nanosheets can avoid the direct contact with the electrolyte to reduce the formation of excessive solid electrolyte interface (SEI) film and the dissolution of nanosheets [40–42]. In addition, the GQDs on surface can also act as current collectors to increase the transfer speed of electrons generated by the reaction during charge-discharge process. The charge/discharge process of GQDs@FFNA electrodes were performed by assembling electrodes into coin cells use the metal lithium as counter electrode. The galvanostatic charge/discharge operation was carried out at a voltage range of 1.7–4.5 V. The battery for the rate test will perform a few cycles low-rate charge/discharge (Such as 0.1 C for 5 cycles) before starting the high-rate test to stabilize the battery and the cycle performance is directly tested after the battery is assembled. For comparison, pure FeF3·0.33H2O powder electrode and FFNA electrode were also tested in same condition. The charge/discharge curves of GQDs@FFNA electrode at different rates were
the practical application yet. Directly constructing the array electrode can avoid the use of conductive agent and binder. Furthermore, nanostructured array electrode can reduce the ion migration path and provide more reaction sites [26–29]. However, the weak bonding between the active material and the conductive matrix is a fatal problem which would cause the active material peeling off during long cycles. In this work, self-supported, binder-free electrode, iron fluoride vertical nanosheets array was synthesized by directly hydrothermally grown iron fluoride vertical nanosheets array on treated titanium foil, followed by electrophoresis graphene quantum dots (GQDs) to surface modification. Compared with the previous structure, the surface of the treated titanium foil has a rough morphology, and the vertical nanosheets array grown are more closely integrated with the titanium substrate. GQDs with special physical and chemical properties have successfully used in solar cells, bioanalysis, sensors and supercapacitors [30–33]. The GQDs modification can not only improve the overall electrical conductivity of the electrode while also possess the protective effect on the nanosheets structure. For the first time, GQDs was introduced to FeF3·0.33H2O material in this work and greatly improve its electrochemical performance. Through the synergistic effect of the vertical nanosheets array and GQDs modification, the as-prepared GQDs modified FeF3·0.33H2O nanosheets array electrode (GQDs@ FFNA) achieves superior high rate performance and ultra-long cycle performance. A capacity of 117 mAh g−1 can reach at a high rate of 20 C and the capacity attenuation rate is as low as 0.03% per cycle during 1000 cycles charge and discharge at 2 C (1 C = 200 mAh g−1).
2. Results and discussion The synthetic strategy for GQDs@FFNA is schematically depicted in Fig. 1. In a typical procedure, Ti foil is cut into the appropriate size and ultrasonic treatment with ethanol and deionized water for 30 min, respectively. Then H2O2 is used for etching to form a rough surface. After that FeF3·0.33H2O nanosheets array (FFNA) were grown by solvothermal use ionic liquid [Bmim][BF4] as fluorine source. The rough surface constructed would favor the nanosheets tightly integrated with the matrix (Fig. S1). As revealed in Fig. S1, after the etching treatment of H2O2, the surface of originally flattened Ti foil becomes a honeycomb-like multi-stage structure. The honeycomb-like surface can limit the agglomeration and growth of FeF3·0.33H2O nanocrystals during solvothermal process, while the FeF3·0.33H2O nanocrystals grown on the smooth surface would agglomerated into large flowers. The GQDs are introduced to the surface of the nanosheets by electrophoresis. The GQDs@FFNA electrode can be obtained after a heat treatment to improve the binding force between the QGDs and the nanosheets. Digital photos of FFNA electrode and GQDs@FFNA electrode are shown in Fig. S2. Fig. 2a shows the X-ray diffraction (XRD) patterns of pure FeF3·0.33H2O and the GQDs@FFNA electrode respectively. All the diffraction peaks of pure FeF3·0.33H2O collected from the bottom of reactor correspond exactly to the standard peaks (hexagonal-tungsten-
Fig. 1. Schematic illustration of the synthesis of GQDs@FFNA. 246
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Fig. 2. (a) XRD patterns of FeF3·0.33H2O powder and the GQDs@FFNA (b) XPS spectra of GQDs@FFNA and (c) Fe 2p (d) C 1s.
have a large specific surface area which facilitate the contact with electrolyte and reduce the migration distance of Li+. And the GQDs uniformly distributed on the surface can improve the electronic conductivity which is beneficial to high-rate reactions. To further understand the reasons for the excellent rate performance of GQDs@FFNA electrode, electrochemical impedance spectroscopy (EIS) measurements were carried out (Fig. 5c). In general, the semicircle at high-frequency corresponds to the charge transfer resistance (Rct); the inclined line corresponds to the lithium-diffusion process within the electrodes. As shown, the GQDs@FFNA electrode possesses a smaller semicircle diameter than the FFNA electrode and the powder electrode indicate that the GQDs@FFNA electrode has a smaller Rct. The small Rct implying that the GQDs@FFNA electrode possesses a high electron conductivity and a rapid charge transfer reaction for lithium ion insertion and extraction. The results are consistent with the rate performance. Fig. 5d shows the cycle performance of FeF3·0.33H2O powder electrode, FFNA electrode and GQDs@FFNA electrode. After 1000 cycles charge/discharge at 2 C, the GQDs@FFNA electrode still can deliver a capacity of 96 mAh g−1. The attenuation rate is as low as 0.03%/ cycle for GQDs@FFNA electrode. For FFNA electrode just 39 mAh g−1 remained after 500 cycles at 2 C. And there was almost no capacity reserved after 300 cycles charge/discharge at 2 C for FeF3·0.33H2O powder electrode. The GQDs@FFNA electrode has greatly enhanced long cycle stability maybe due to the improved electrical conductivity and the shielding effect of GQDs in preventing the dissolution of FeF3·0.33H2O in electrolyte and the increasing of internal resistance. EIS measurements were carried out after different cycles for FFNA and GQDs@FFNA cells to illustrate the role of GQDs in suppressing the increase of internal resistance of the battery. Fig. 6 shows the Nyquist plots of FFNA and GQDs@FFNA cells for fresh and after 50, 100, and 500 cycles, respectively. The semicircle corresponds to the charge
investigated and shown in Fig. 5a. There is a slash within the discharge voltage range and no strict platform exist. Because Li+ enters the lattice of FeF3·0.33H2O through a solid solution reaction instead of intercalation reaction. And the shape of the charge/discharge curves remains almost unchanged at higher current densities only with the voltage hysteresis becomes larger. There is no platform for the conversion reaction in the test voltage range and the CV curves (Fig. S5) show only one pair of broad peak origin from solid solution reaction, there is no peaks for conversion reaction in the test voltage range 1.7–4.5 V. The charge/discharge test shows that the GQDs@FFNA electrode delivers a capacity of 161.5 mAhg−1, 155.2 mAhg−1, 142 mAhg−1, 128 mAhg−1, and 113 mAhg−1 at 1 C, 2 C, 5 C, 10 C, and 20 C, respectively. To the best of our knowledge, this is the best rate performance reported for FeF3·0.33H2O electrode. For easy comparison, Fig. 5b shows the rate performance of FeF3·0.33H2O powder electrode, FFNA electrode, and GQDs@FFNA electrode. It shows that although the FeF3·0.33H2O powder electrode and the FFNA electrode can release similar capacity to GQDs@FFNA electrode at the low rate and the FFNA electrode even have a higher capacity at first two cycles maybe due to the larger specific surface area lead to more SEI films formed. But when the current density increases, the discharge capacity decrease rapidly. Due to the poor conductivity of the FFNA electrode and limited Li+ diffusion coefficient, Li+ cannot completely migrate into the interior of FeF3⋅0.33H2O particles. And the battery is heavily polarized under high current charge/discharge conditions, the charge/discharge cut-off voltage is quickly reached. So, when the current density increases, the discharge capacity of FFNA electrode decrease rapidly. The FFNA electrode only release a capacity of 81 mAh g−1 at 10 C and 60 mAh g−1 at 20 C. The FeF3·0.33H2O powder electrode can't keep stable at high rate, release a capacity lower than 40 mAh g−1 at 10 C and shows nearly no capacity at 20 C. The reason for the excellent rate performance of GQDs@FFNA is that the structure of the nanosheets 247
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Fig. 3. (a) SEM picture of FFNA (b) SEM picture of GQDs@FFNA (c) AFM image of GQDs (d) thickness of GQDs.
for a coin cell. However, the charge transfer resistance of cell assembled by GQDs@FFNA was only about 150 Ω after 500 cycles. The result is consistent with the cycle performance test. The GQDs surface modification can reduce the continuous formation of non-conductive SEI film which would resulting in difficult to charge transfer. For the reasons of the capacity attenuation of the iron fluoride
transfer resistance is the main component of the internal resistance of the battery. Obviously can be seen that the charge transfer resistance of cells assembled by FFNA electrode increases rapidly with the number of cycles. After 500 cycles charge/discharge the charge transfer resistance of cells assembled by FFNA electrode has exceeded 1500 Ω. It is impossible to work properly with such a large charge transfer resistance
Fig. 4. (a,b,c) The TEM picture of FFNA (d,e) TEM picture of GQDs@FFNA (f) TEM image of GQDs@FFNA and the corresponding EDS mapping of Fe, F, and C elements. 248
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Fig. 5. (a) Charge and discharge profiles at different current rates for GQDs@FFNA electrode; (b) The Rate performance of FeF3·0.33H2O powder electrode, FFNA electrode and GQDs@FFNA electrode; (c) Nyquist plots of three electrodes; (d) The cycle performance of three electrodes at 2 C rate.
electrode, we speculate that it may be due to the presence of side reactions. The equation is as follows:
Li+ + FeF3·0.33H2 O+ e− → Li1 − x FeF3 − x ·0.33H2 O+ x LiF Side reaction may occur when the Li+ enter into the interior of the FeF3·0.33H2O crystal through the solid solution reaction. The side reaction would result the formation of LiF and LiF is an important component of the SEI film. This may partial explain why the iron fluoride electrode with a thick SEI film after cycled. XPS spectum of the Fe element on the surface of FFNA and GQDs@FFNA electrode after 200 cycles were used to analysis the valence state. For the GQDs@FFNA electrode, the Fe 2p peaks of surface Fe element are located at 710.8 eV and 725.1 eV (Fig. 7). It is almost unchanged after 200 cycles when compared with fresh electrode (Fig. 2c). Compared with GQDs@FFNA electrode, the Fe 2p peaks of surface Fe element on FFNA electrode after 200 cycles exist a significant shift. This shift to low binding energy indicates that the composition of electrode surface contain FeF2 [43,44]. The presence of this side reaction may be one of the most important reasons for the capacity decay of iron fluoride electrodes. However, the presence of GQDs can reduce the sites where this side reaction can occur and improve the cycle performance of the electrode. Fig. 8 is the schematic diagram of GQDs surface modification to inhibit the formation of excessive SEI film. For FFNA electrode, the FeF3·0.33H2O
Fig. 7. XPS spectrum for Fe 2p of FFNA and GQDs@FFNA electrode after 200 cycles charge/discharge.
Fig. 6. Nyquist plots of electrodes at different cycles. (a) FFNA, (b) GQDs@FFNA. 249
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was transferred to an oven at 120 °C for 12 h and naturally cooling to room temperature. The obtained FeF3·0.33H2O nanosheets array (FFNA) on titanium foil was washed by alcohol and deionized water respectively for three times after reaction and dried at 80 °C under vacuum for 12 h. The yellow precipitation in the bottom of autoclave was also collected. The Ti foil is attached to the inner wall of the reactor, and FeF3⋅0.33H2O only grown on one side. A small amount of FeF3⋅0.33H2O grown on the other side was also removed by wiping with ethanol. The mass loading of FeF3·0.33H2O is calculated based on one side. 4.2. Synthesis of GQDs@FFNA GQDs were synthesized through the method reported previously. Simply, 0.3 g carbon fibers mixed with 60 ml H2SO4 and 20 ml HNO3, then the mixture refluxed for 12 h at 120 °C. After reaction and cooling to room temperature, diluted with deionized water and adjust the pH to 8 with Na2CO3, then the final solution was further dialyzed for at least 3 days. GQDs solution obtained can be diluted when used according to the actual situation. GQDs@FFNA was prepared by facile electrophoresis process. The as-prepared GQDs solution was used as electrolyte, FeF3·0.33H2O nanosheets array on titanium foil prepared previously was used as electrode and Pt plate was used as counter electrode. Then a positive potential of 6 V for different time in the condition of ice-bath was applied to deposit functional GQDs onto the surface of FeF3·0.33H2O nanosheets array. Finally, the GQDs@FFNA was heated at 150 °C in Ar atmosphere for 2 h to improve the adhesion of GQDs.
Fig. 8. Schematic diagram of GQDs surface modification to inhibit the formation of excessive SEI film.
nanosheets are directly exposed to the electrolyte. A thick SEI film will form after long cycle and FeF3·0.33H2O will change to FeF3-x·0.33H2O. The non-conductive thick SEI film limits electron transfer during the reaction process, resulting in increased battery internal resistance and rapid capacity decay. A GQDs layer on the surface of FeF3·0.33H2O nanosheets can avoid its directly contact with the electrolyte. Only a small amount of SEI film will form on the surface of GQDs. Therefore, even undergo a long cycle, the overall conductivity of the electrode remains well, the capacity decay is slow, and the electrode structure remains intact. Even after 1000 cycles, the GQDs@FFNA electrode can keep the structure basically unchanged (Fig. S6a). But for FFNA electrode, due to lack of protection, the nanosheets structure cannot be maintained after 500 cycles charge/discharge (Fig. S6b).
4.3. Material characterizations and cell fabrication The crystal structures of the samples were identified by X-ray diffraction (XRD, Rigaku D/Max-γB with Cu Kα radiation). The structures of the samples were investigated by transmission electron microscopy (HRTEM, JEOL JEM-2010F at 200 kV) and Scanning electron microscope (SEM, Hitachi, SU8010). An atomic force microscopy (Dimension Icon, Bruker) was used to measure the size and thickness of GQDs. The mass loading of active materials was measured by electronic balance (Mettler Toledo MS105DU) and the mass loading of GQDs@FFNA is about 0.8 mg cm−2. The as-prepared GQDs@FFNA was directly cut into pieces and used as cathode. As a comparison, the FeF3·0.33H2O powder collected from the bottom of reactor was also assembled into batteries. The electrode was fabricated by mixing active materials with polyvinylidene fluoride and carbon black with a weight ratio of 8:1:1 and Al foil as collector. The coin type (CR- 2025) cells were assembled in an argon-filled glovebox using 1 M LiPF6 in diethyl carbonate, ethylene carbonate and ethylmethyl carbonate (DC/EC/EMC, 1:1:1 in vol) as an electrolyte. Metal lithium was used as the counter electrode. The discharge/charge measurements were carried out on a Neware (Shenzhen Neware Electronic Co., China) battery test system. Electrical impedance spectroscopy (EIS) measurements were carried out on an electrochemical workstation (CHI 660 d) over the frequency range between 0.01 Hz and 100000 Hz.
3. Conclusions In conclusion, we developed a unique synthetic strategy to fabricate self-supported, binder free iron fluoride vertical nanosheets array by a solvothermal approach and followed by surface modification of GQDs. The electrode constructed displays excellent cycle performance while experiencing high charge/discharge rates. It can be attributed to the advantages of the vertical nanosheets array structure and the quantum dots modification, which provide high specific surface area, porous structure for electrolyte penetration and charge transfer expressway. And the presence of GQDs can inhibit the formation of excessive SEI film and improve the cycle performance of the electrode. Even at a high rate of 20 C, a capacity of 113 mAh g−1 can still obtained and a capacity of 96 mAh g−1 can be maintained after 1000 cycles at 2 C. The superior electrochemical performance combined with the unique structure of GQDs@FFNA electrode demonstrates its great potential of being utilized as the cathode in high rate battery application. And the method of preparing the array electrodes of this unique structure can be applied to various types of energy storage devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21646012), the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (no. 2016DX08). China Postdoctoral Science Foundation (no. 2016M600253, 2017T100246), and the Postdoctoral Foundation of Heilongjiang Province. The Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 201836).
4. Experimental section 4.1. Synthesis of FFNA on titanium foil Titanium foil was pretreated by H2O2 (10 wt%) etching for 24 h at 70 °C to form a honeycomb-like roughened surface. Subsequently put into a 100 ml Teflon autoclave against the inner wall. Then 50 ml anhydrous alcohol, 0.5 g Fe(NO3)3·9H2O, 5 ml ionic liquids [Bmim][BF4] stir well and transferred to the autoclave. Then the sealed autoclave
Conflicts of interest There are no conflicts to declare. 250
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Appendix A. Supplementary data
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Iron fluoride vertical nanosheets array modified with graphene quantum dots as long-life cathode for lithium ion batteries ⁎
Qi Zhanga, Chengzhi Suna, Lishuang Fanc, Naiqing Zhangb,c, , Kening Sunb,c,
T
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a
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China c Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, 92 Xidazhi Street, Harbin, China b
H I GH L IG H T S
·0.33H O nanosheets array electrode was constructed for the first time. • FeF graphene quantum dots were used to surface modification of FeF ·0.33H O. • Firstly • GQDs surface modification can greatly enhance the cycle performance of the electrode. 3
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A R T I C LE I N FO
A B S T R A C T
Keywords: Iron fluoride Graphene quantum dots Array electrode LIBs Super long-life
We successfully built a unique architecture of FeF3·0.33H2O, a self-supported, binder-free vertical nanosheets array directly growing on Ti foil by a simple solvothermal approach and firstly use graphene quantum dots to surface modification. The high specific surface area of 2D nanosheets and high conductivity of graphene quantum dots provide an expressway for charge transfer and ion transport. This unique structural design endows a capacity of 161.5 mAh g−1 at 1 C and a capability of 117 mAh g−1 at a high current density of 4 Ag−1 (20 C). Graphene quantum dots surface modification can stabilize the structure and reduce possible side reactions, making the cycle performance greatly improved. Even cycled over 1000 times at 2 C the capacity attenuation rate is as low as 0.03% per cycle.
1. Introduction Like the heart means to humans, energy supply device is crucial to electronic product. Lithium ion batteries (LIBs) have been successfully applied in electronics and electric vehicles because of their outstanding high capacity and long cycle life [1,2]. However, existing LIBs cannot meet the consumers pursuing for superior performance. Developing high specific capacity and high rate battery is now the focus of research. Low capacity of conventional cathode materials (140 mAh g−1 for LiCoO2 and 170 mAh g−1 for LiFePO4) which are limited by the intercalation reaction mechanism has become a bottleneck to the development of LIBs. Therefore, exploring new cathode materials becomes the top priority of research for LIBs [3,4]. In recent years, transition metal fluorides (MxFy, M = Fe, Mn, Co, Cu, etc.) gradually attract particular attention, especially the iron fluoride (FeF3) with merits of high-capacity, high-voltage, and low-cost becomes a favorable cathode material [5–9]. A discharge plateau 2.73 V vs. Li+/Li, specific capacity 237 mAh g−1 can reach benefit from the ⁎
high ionic bonding characteristics and small molecular weight. Among various crystal form of iron fluoride, FeF3·0.33H2O has unique advantages [10,11]. Hexagonal-tungsten-bronze-type FeF3·0.33H2O has unusual tunnel structure and better electrochemical activity compare with the common ReO3-type FeF3 without crystal water. Despite many advantages, there are still some problems for practical application of FeF3·0.33H2O such as poor conductivity and limited electrode reversibility. Generally, there are two effective ways to address these issues. Firstly, constructing unique nanostructures like nanoparticles [12–14], porous nanospheres [15–17], and nanowires [18,19] can increase the surface area and the reactive sites, which are in favor of reversible electrode reaction. Another solution is combine FeF3·0.33H2O with high conductive materials such as carbon nanotube [10,11,20], graphene [21–24], and conductive polymers [25]. These high conductive materials can provide highways for rapid electron transfer which would facilitate a rapid reaction. Using these two methods can indeed improve the electrochemical performance of iron fluoride to some extent. However, the performance improvement is limited, and cannot meet
Corresponding authors. E-mail addresses: [email protected] (N. Zhang), [email protected] (K. Sun).
https://doi.org/10.1016/j.cej.2019.04.073 Received 14 January 2019; Received in revised form 4 March 2019; Accepted 10 April 2019 Available online 11 April 2019 1385-8947/ © 2019 Published by Elsevier B.V.
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bronze-type FeF3·0.33H2O, JCPDS card no. 76-1262). The XRD result of the GQDs@FFNA can also correspond to the standard peaks after the peaks of the titanium are neglected. And there is no distinctly peaks of GQDs were detected. The X-ray photoelectron spectroscopy (XPS) was used to analyze the state of the surface element of GQDs@FFNA. It can be determined that the peaks of Fe, F, and C appear on the spectrum (Fig. 2b). The peak of Fe 2p3/2 located at 710.8 eV (Fig. 2c) suggests the +3 oxidation state of Fe element [34]. The C 1s peak of carbon can be divided into three peaks at 284.6 eV, 286.24 eV and 287.88 eV, corresponding to CeC, CeO and C]O, respectively [34] (Fig. 2d). The fitting result of C 1s is similar to the typical peaks of graphene, for which CeC peak is strong, CeO and C]O peaks are weak. This indicating that the carbon element on the surface is mainly graphene [35–37]. The surface morphology of the electrodes were observed by scanning electron microscope (SEM). The FeF3·0.33H2O nanosheets formed by solvothermal (Fig. 3a) have a thickness of 20–30 nm in size and these nanosheets grown vertically on the substrate to form an array. After modified by the GQDs, the surface of the nanosheets (Fig. 3b) are no longer smooth and as if covered with a layer of gauze. A appropriate electrophoresis time were carried out based on the concentration of GQDs solution to prevent excessive deposition of GQDs (Fig. S3). Atomic force microscopy (AFM) shows that the GQDs have a average size of 5.65 nm and a thickness of 1.5 nm (Fig. 3c,d). It indicates that there are about 3–5 layers of carbon atoms in GQDs. The prepared GQDs have good dispersibility in water and the dispersion exhibits typical fluorescence characteristics (Fig. S4). These colloid-like hydrophilic groups formed by GQDs in water can be moved and adsorbed on the surface of the FFNA electrode during electrophoresis. In order to further observe the fine structure of the FFNA and GQDs@FFNA electrodes, the morphologies of the samples were observed by transmission electron microscopy (TEM). As shown in Fig. 4a,b, the FeF3·0.33H2O nanosheets are consist of nanoparticles under 10 nm. These nanoparticles agglomerated into a porous nanosheet structure which would facilitate the entry of electrolyte into the interior of the material. Fig. 4c shows the high resolution TEM image of FeF3·0.33H2O nanosheet. The lattice fringe spacing marked in the figure is 0.19 nm and 0.37 nm, corresponding to the (0 0 4) and (2 0 0) plane of FeF3·0.33H2O [38]. Fig. 4d,e shows the TEM pictures of GQDs@ FFNA. At low magnification (Fig. 4d), some flakes can be seen on the surface of nanosheet and distorted lattice structure can be seen after magnification (Fig. 4e). The twisted lattice structure (marked by red circle) are similar to the lattice spacing of the graphene oxide [39]. Because the GQDs were made through the oxidation of carbon cloth [30], properties of some large flakes in GQDs certainly show a similarity to graphene oxide. In order to verify the uniformity coverage of GQDs, EDS mapping of Fe, F, and C elements were conducted and shown in Fig. 4f. It can be seen that the C element is evenly distributed on the surface of the material and consistent with the Fe and F elements. It proves that GQDs are evenly coated on the surface of FeF3·0.33H2O nanosheets. The GQDs on the surface of the nanosheets can avoid the direct contact with the electrolyte to reduce the formation of excessive solid electrolyte interface (SEI) film and the dissolution of nanosheets [40–42]. In addition, the GQDs on surface can also act as current collectors to increase the transfer speed of electrons generated by the reaction during charge-discharge process. The charge/discharge process of GQDs@FFNA electrodes were performed by assembling electrodes into coin cells use the metal lithium as counter electrode. The galvanostatic charge/discharge operation was carried out at a voltage range of 1.7–4.5 V. The battery for the rate test will perform a few cycles low-rate charge/discharge (Such as 0.1 C for 5 cycles) before starting the high-rate test to stabilize the battery and the cycle performance is directly tested after the battery is assembled. For comparison, pure FeF3·0.33H2O powder electrode and FFNA electrode were also tested in same condition. The charge/discharge curves of GQDs@FFNA electrode at different rates were
the practical application yet. Directly constructing the array electrode can avoid the use of conductive agent and binder. Furthermore, nanostructured array electrode can reduce the ion migration path and provide more reaction sites [26–29]. However, the weak bonding between the active material and the conductive matrix is a fatal problem which would cause the active material peeling off during long cycles. In this work, self-supported, binder-free electrode, iron fluoride vertical nanosheets array was synthesized by directly hydrothermally grown iron fluoride vertical nanosheets array on treated titanium foil, followed by electrophoresis graphene quantum dots (GQDs) to surface modification. Compared with the previous structure, the surface of the treated titanium foil has a rough morphology, and the vertical nanosheets array grown are more closely integrated with the titanium substrate. GQDs with special physical and chemical properties have successfully used in solar cells, bioanalysis, sensors and supercapacitors [30–33]. The GQDs modification can not only improve the overall electrical conductivity of the electrode while also possess the protective effect on the nanosheets structure. For the first time, GQDs was introduced to FeF3·0.33H2O material in this work and greatly improve its electrochemical performance. Through the synergistic effect of the vertical nanosheets array and GQDs modification, the as-prepared GQDs modified FeF3·0.33H2O nanosheets array electrode (GQDs@ FFNA) achieves superior high rate performance and ultra-long cycle performance. A capacity of 117 mAh g−1 can reach at a high rate of 20 C and the capacity attenuation rate is as low as 0.03% per cycle during 1000 cycles charge and discharge at 2 C (1 C = 200 mAh g−1).
2. Results and discussion The synthetic strategy for GQDs@FFNA is schematically depicted in Fig. 1. In a typical procedure, Ti foil is cut into the appropriate size and ultrasonic treatment with ethanol and deionized water for 30 min, respectively. Then H2O2 is used for etching to form a rough surface. After that FeF3·0.33H2O nanosheets array (FFNA) were grown by solvothermal use ionic liquid [Bmim][BF4] as fluorine source. The rough surface constructed would favor the nanosheets tightly integrated with the matrix (Fig. S1). As revealed in Fig. S1, after the etching treatment of H2O2, the surface of originally flattened Ti foil becomes a honeycomb-like multi-stage structure. The honeycomb-like surface can limit the agglomeration and growth of FeF3·0.33H2O nanocrystals during solvothermal process, while the FeF3·0.33H2O nanocrystals grown on the smooth surface would agglomerated into large flowers. The GQDs are introduced to the surface of the nanosheets by electrophoresis. The GQDs@FFNA electrode can be obtained after a heat treatment to improve the binding force between the QGDs and the nanosheets. Digital photos of FFNA electrode and GQDs@FFNA electrode are shown in Fig. S2. Fig. 2a shows the X-ray diffraction (XRD) patterns of pure FeF3·0.33H2O and the GQDs@FFNA electrode respectively. All the diffraction peaks of pure FeF3·0.33H2O collected from the bottom of reactor correspond exactly to the standard peaks (hexagonal-tungsten-
Fig. 1. Schematic illustration of the synthesis of GQDs@FFNA. 246
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Fig. 2. (a) XRD patterns of FeF3·0.33H2O powder and the GQDs@FFNA (b) XPS spectra of GQDs@FFNA and (c) Fe 2p (d) C 1s.
have a large specific surface area which facilitate the contact with electrolyte and reduce the migration distance of Li+. And the GQDs uniformly distributed on the surface can improve the electronic conductivity which is beneficial to high-rate reactions. To further understand the reasons for the excellent rate performance of GQDs@FFNA electrode, electrochemical impedance spectroscopy (EIS) measurements were carried out (Fig. 5c). In general, the semicircle at high-frequency corresponds to the charge transfer resistance (Rct); the inclined line corresponds to the lithium-diffusion process within the electrodes. As shown, the GQDs@FFNA electrode possesses a smaller semicircle diameter than the FFNA electrode and the powder electrode indicate that the GQDs@FFNA electrode has a smaller Rct. The small Rct implying that the GQDs@FFNA electrode possesses a high electron conductivity and a rapid charge transfer reaction for lithium ion insertion and extraction. The results are consistent with the rate performance. Fig. 5d shows the cycle performance of FeF3·0.33H2O powder electrode, FFNA electrode and GQDs@FFNA electrode. After 1000 cycles charge/discharge at 2 C, the GQDs@FFNA electrode still can deliver a capacity of 96 mAh g−1. The attenuation rate is as low as 0.03%/ cycle for GQDs@FFNA electrode. For FFNA electrode just 39 mAh g−1 remained after 500 cycles at 2 C. And there was almost no capacity reserved after 300 cycles charge/discharge at 2 C for FeF3·0.33H2O powder electrode. The GQDs@FFNA electrode has greatly enhanced long cycle stability maybe due to the improved electrical conductivity and the shielding effect of GQDs in preventing the dissolution of FeF3·0.33H2O in electrolyte and the increasing of internal resistance. EIS measurements were carried out after different cycles for FFNA and GQDs@FFNA cells to illustrate the role of GQDs in suppressing the increase of internal resistance of the battery. Fig. 6 shows the Nyquist plots of FFNA and GQDs@FFNA cells for fresh and after 50, 100, and 500 cycles, respectively. The semicircle corresponds to the charge
investigated and shown in Fig. 5a. There is a slash within the discharge voltage range and no strict platform exist. Because Li+ enters the lattice of FeF3·0.33H2O through a solid solution reaction instead of intercalation reaction. And the shape of the charge/discharge curves remains almost unchanged at higher current densities only with the voltage hysteresis becomes larger. There is no platform for the conversion reaction in the test voltage range and the CV curves (Fig. S5) show only one pair of broad peak origin from solid solution reaction, there is no peaks for conversion reaction in the test voltage range 1.7–4.5 V. The charge/discharge test shows that the GQDs@FFNA electrode delivers a capacity of 161.5 mAhg−1, 155.2 mAhg−1, 142 mAhg−1, 128 mAhg−1, and 113 mAhg−1 at 1 C, 2 C, 5 C, 10 C, and 20 C, respectively. To the best of our knowledge, this is the best rate performance reported for FeF3·0.33H2O electrode. For easy comparison, Fig. 5b shows the rate performance of FeF3·0.33H2O powder electrode, FFNA electrode, and GQDs@FFNA electrode. It shows that although the FeF3·0.33H2O powder electrode and the FFNA electrode can release similar capacity to GQDs@FFNA electrode at the low rate and the FFNA electrode even have a higher capacity at first two cycles maybe due to the larger specific surface area lead to more SEI films formed. But when the current density increases, the discharge capacity decrease rapidly. Due to the poor conductivity of the FFNA electrode and limited Li+ diffusion coefficient, Li+ cannot completely migrate into the interior of FeF3⋅0.33H2O particles. And the battery is heavily polarized under high current charge/discharge conditions, the charge/discharge cut-off voltage is quickly reached. So, when the current density increases, the discharge capacity of FFNA electrode decrease rapidly. The FFNA electrode only release a capacity of 81 mAh g−1 at 10 C and 60 mAh g−1 at 20 C. The FeF3·0.33H2O powder electrode can't keep stable at high rate, release a capacity lower than 40 mAh g−1 at 10 C and shows nearly no capacity at 20 C. The reason for the excellent rate performance of GQDs@FFNA is that the structure of the nanosheets 247
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Fig. 3. (a) SEM picture of FFNA (b) SEM picture of GQDs@FFNA (c) AFM image of GQDs (d) thickness of GQDs.
for a coin cell. However, the charge transfer resistance of cell assembled by GQDs@FFNA was only about 150 Ω after 500 cycles. The result is consistent with the cycle performance test. The GQDs surface modification can reduce the continuous formation of non-conductive SEI film which would resulting in difficult to charge transfer. For the reasons of the capacity attenuation of the iron fluoride
transfer resistance is the main component of the internal resistance of the battery. Obviously can be seen that the charge transfer resistance of cells assembled by FFNA electrode increases rapidly with the number of cycles. After 500 cycles charge/discharge the charge transfer resistance of cells assembled by FFNA electrode has exceeded 1500 Ω. It is impossible to work properly with such a large charge transfer resistance
Fig. 4. (a,b,c) The TEM picture of FFNA (d,e) TEM picture of GQDs@FFNA (f) TEM image of GQDs@FFNA and the corresponding EDS mapping of Fe, F, and C elements. 248
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Fig. 5. (a) Charge and discharge profiles at different current rates for GQDs@FFNA electrode; (b) The Rate performance of FeF3·0.33H2O powder electrode, FFNA electrode and GQDs@FFNA electrode; (c) Nyquist plots of three electrodes; (d) The cycle performance of three electrodes at 2 C rate.
electrode, we speculate that it may be due to the presence of side reactions. The equation is as follows:
Li+ + FeF3·0.33H2 O+ e− → Li1 − x FeF3 − x ·0.33H2 O+ x LiF Side reaction may occur when the Li+ enter into the interior of the FeF3·0.33H2O crystal through the solid solution reaction. The side reaction would result the formation of LiF and LiF is an important component of the SEI film. This may partial explain why the iron fluoride electrode with a thick SEI film after cycled. XPS spectum of the Fe element on the surface of FFNA and GQDs@FFNA electrode after 200 cycles were used to analysis the valence state. For the GQDs@FFNA electrode, the Fe 2p peaks of surface Fe element are located at 710.8 eV and 725.1 eV (Fig. 7). It is almost unchanged after 200 cycles when compared with fresh electrode (Fig. 2c). Compared with GQDs@FFNA electrode, the Fe 2p peaks of surface Fe element on FFNA electrode after 200 cycles exist a significant shift. This shift to low binding energy indicates that the composition of electrode surface contain FeF2 [43,44]. The presence of this side reaction may be one of the most important reasons for the capacity decay of iron fluoride electrodes. However, the presence of GQDs can reduce the sites where this side reaction can occur and improve the cycle performance of the electrode. Fig. 8 is the schematic diagram of GQDs surface modification to inhibit the formation of excessive SEI film. For FFNA electrode, the FeF3·0.33H2O
Fig. 7. XPS spectrum for Fe 2p of FFNA and GQDs@FFNA electrode after 200 cycles charge/discharge.
Fig. 6. Nyquist plots of electrodes at different cycles. (a) FFNA, (b) GQDs@FFNA. 249
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was transferred to an oven at 120 °C for 12 h and naturally cooling to room temperature. The obtained FeF3·0.33H2O nanosheets array (FFNA) on titanium foil was washed by alcohol and deionized water respectively for three times after reaction and dried at 80 °C under vacuum for 12 h. The yellow precipitation in the bottom of autoclave was also collected. The Ti foil is attached to the inner wall of the reactor, and FeF3⋅0.33H2O only grown on one side. A small amount of FeF3⋅0.33H2O grown on the other side was also removed by wiping with ethanol. The mass loading of FeF3·0.33H2O is calculated based on one side. 4.2. Synthesis of GQDs@FFNA GQDs were synthesized through the method reported previously. Simply, 0.3 g carbon fibers mixed with 60 ml H2SO4 and 20 ml HNO3, then the mixture refluxed for 12 h at 120 °C. After reaction and cooling to room temperature, diluted with deionized water and adjust the pH to 8 with Na2CO3, then the final solution was further dialyzed for at least 3 days. GQDs solution obtained can be diluted when used according to the actual situation. GQDs@FFNA was prepared by facile electrophoresis process. The as-prepared GQDs solution was used as electrolyte, FeF3·0.33H2O nanosheets array on titanium foil prepared previously was used as electrode and Pt plate was used as counter electrode. Then a positive potential of 6 V for different time in the condition of ice-bath was applied to deposit functional GQDs onto the surface of FeF3·0.33H2O nanosheets array. Finally, the GQDs@FFNA was heated at 150 °C in Ar atmosphere for 2 h to improve the adhesion of GQDs.
Fig. 8. Schematic diagram of GQDs surface modification to inhibit the formation of excessive SEI film.
nanosheets are directly exposed to the electrolyte. A thick SEI film will form after long cycle and FeF3·0.33H2O will change to FeF3-x·0.33H2O. The non-conductive thick SEI film limits electron transfer during the reaction process, resulting in increased battery internal resistance and rapid capacity decay. A GQDs layer on the surface of FeF3·0.33H2O nanosheets can avoid its directly contact with the electrolyte. Only a small amount of SEI film will form on the surface of GQDs. Therefore, even undergo a long cycle, the overall conductivity of the electrode remains well, the capacity decay is slow, and the electrode structure remains intact. Even after 1000 cycles, the GQDs@FFNA electrode can keep the structure basically unchanged (Fig. S6a). But for FFNA electrode, due to lack of protection, the nanosheets structure cannot be maintained after 500 cycles charge/discharge (Fig. S6b).
4.3. Material characterizations and cell fabrication The crystal structures of the samples were identified by X-ray diffraction (XRD, Rigaku D/Max-γB with Cu Kα radiation). The structures of the samples were investigated by transmission electron microscopy (HRTEM, JEOL JEM-2010F at 200 kV) and Scanning electron microscope (SEM, Hitachi, SU8010). An atomic force microscopy (Dimension Icon, Bruker) was used to measure the size and thickness of GQDs. The mass loading of active materials was measured by electronic balance (Mettler Toledo MS105DU) and the mass loading of GQDs@FFNA is about 0.8 mg cm−2. The as-prepared GQDs@FFNA was directly cut into pieces and used as cathode. As a comparison, the FeF3·0.33H2O powder collected from the bottom of reactor was also assembled into batteries. The electrode was fabricated by mixing active materials with polyvinylidene fluoride and carbon black with a weight ratio of 8:1:1 and Al foil as collector. The coin type (CR- 2025) cells were assembled in an argon-filled glovebox using 1 M LiPF6 in diethyl carbonate, ethylene carbonate and ethylmethyl carbonate (DC/EC/EMC, 1:1:1 in vol) as an electrolyte. Metal lithium was used as the counter electrode. The discharge/charge measurements were carried out on a Neware (Shenzhen Neware Electronic Co., China) battery test system. Electrical impedance spectroscopy (EIS) measurements were carried out on an electrochemical workstation (CHI 660 d) over the frequency range between 0.01 Hz and 100000 Hz.
3. Conclusions In conclusion, we developed a unique synthetic strategy to fabricate self-supported, binder free iron fluoride vertical nanosheets array by a solvothermal approach and followed by surface modification of GQDs. The electrode constructed displays excellent cycle performance while experiencing high charge/discharge rates. It can be attributed to the advantages of the vertical nanosheets array structure and the quantum dots modification, which provide high specific surface area, porous structure for electrolyte penetration and charge transfer expressway. And the presence of GQDs can inhibit the formation of excessive SEI film and improve the cycle performance of the electrode. Even at a high rate of 20 C, a capacity of 113 mAh g−1 can still obtained and a capacity of 96 mAh g−1 can be maintained after 1000 cycles at 2 C. The superior electrochemical performance combined with the unique structure of GQDs@FFNA electrode demonstrates its great potential of being utilized as the cathode in high rate battery application. And the method of preparing the array electrodes of this unique structure can be applied to various types of energy storage devices.
Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 21646012), the State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (no. 2016DX08). China Postdoctoral Science Foundation (no. 2016M600253, 2017T100246), and the Postdoctoral Foundation of Heilongjiang Province. The Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 201836).
4. Experimental section 4.1. Synthesis of FFNA on titanium foil Titanium foil was pretreated by H2O2 (10 wt%) etching for 24 h at 70 °C to form a honeycomb-like roughened surface. Subsequently put into a 100 ml Teflon autoclave against the inner wall. Then 50 ml anhydrous alcohol, 0.5 g Fe(NO3)3·9H2O, 5 ml ionic liquids [Bmim][BF4] stir well and transferred to the autoclave. Then the sealed autoclave
Conflicts of interest There are no conflicts to declare. 250
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Appendix A. Supplementary data
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