Cavity-containing Fe2O3 / C composite material as well as preparation method and application thereof

Abstract

The invention provides a cavity-containing Fe2O3 / C composite material as well as a preparation method and application thereof, and belongs to the technical field of material preparation and environmental protection, the cavity-containing Fe2O3 / C composite material is of a core-shell structure, a shell layer is a carbon shell, an inner core is Fe2O3 nanoparticles, the size of the Fe2O3 nanoparticles is 8-60nm, the Fe2O3 nanoparticles are wrapped in the carbon shell layer, the carbon shell layer is of an amorphous control structure, and the Fe2O3 nanoparticles are uniformly dispersed in the carbon shell layer. The cavity-containing Fe2O3 / C composite material is prepared by adopting a one-pot hydrothermal method, that is, the preparation of a Fe2O3 precursor and the coating of a SiO2 template are successfully completed in a hydrothermal process, the preparation process is greatly simplified, the material as a negative electrode material can be applied to a lithium ion battery, a sodium ion battery or a potassium ion battery, and since the cavity is reserved between the Fe2O3 nanoparticles and the carbon shell, the material can be applied to a lithium ion battery, a sodium ion battery or a potassium ion battery. The volume change caused by ion deintercalation can be effectively buffered, and the battery cycle performance and rate capability of the material can be effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of electrochemical materials of carbon and metal oxides, and specifically refers to a Fe2O3 / C composite material containing voids, a preparation method and an application thereof. Background Art

[0002] In order to find a balance between economic development and living environment, the development and utilization of sustainable energy has received more and more attention. At the same time, storing the generated energy in the form of electricity is a key part of sustainable energy development and utilization. Lithium-ion batteries are considered to be one of the most promising energy storage devices due to their high energy capacity, long cycle life, and fast charge and discharge speed. In order to meet the rapid iteration of electronic devices such as electric vehicles, LIBs with higher energy density are urgently needed. However, the currently commercialized graphite anode has a limited theoretical specific capacity (372mAh g -1 ) and it is difficult to meet the demand for higher energy density of LIBs. Therefore, many efforts have been made to manufacture alternative anode materials with good stability and high specific capacity. This is also the only way for the development of LIBs.

[0003] Among many alternative anode materials, Fe2O3 materials have rich electrochemical properties, higher theoretical capacity (1007 mAh g -1 ), abundant mineral reserves, affordable cost, non-toxicity, high corrosion resistance and safety, and thus is considered a promising anode for high-performance LIBs. Despite its many advantages, Fe2O3 faces the problems of poor electronic conductivity and particularly large volume fluctuations during the charge / discharge process, which leads to structural failure of the electrode and rapid capacity decay of the battery.

[0004] It is of great significance to develop new iron oxide and carbon composite materials and preparation processes. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a void-containing Fe2O3 / C composite material and a preparation method and application thereof. The void-containing Fe2O3 / C composite material is prepared by a one-pot hydrothermal method, that is, the preparation of Fe2O3 / precursor and the coating of SiO2 template are successfully completed in one hydrothermal process, which greatly simplifies the preparation process and can effectively overcome the above problems. The conditions are mild, the operation is convenient, the repeatability is high, and large-scale production is possible. In addition, the voids of Fe2O3 / C can be easily controlled by adjusting the amount of tetraethyl orthosilicate in the preparation process, and the appropriate void size plays a key role in improving the electrochemical properties of Fe2O3 / C materials.

[0006] To achieve the above-mentioned purpose, the present scheme first provides a void-containing Fe2O3 / C composite material, wherein the void-containing Fe2O3 / C composite material comprises a carbon layer shell and a Fe2O3 nanoparticle core, wherein there is a space gap between the carbon layer shell and the Fe2O3 nanoparticle core, and the Fe2O3 nanoparticle core is wrapped in the carbon layer shell.

[0007] Preferably, the carbon layer shell is an amorphous controlled structure with a thickness of 2-5 nm, and the carbon layer shell has a mesoporous structure with a pore size of 2-6 nm; the Fe2O3 nanoparticle size is 8-100 nm.

[0008] Preferably, the specific surface area of ​​the void-containing Fe2O3 / C composite material is 150-360m 2 / g, and the content of the carbon layer shell in the void-containing Fe2O3 / C composite material is 20%-40%.

[0009] Based on a general inventive concept, the present invention also provides a method for preparing a void-containing Fe2O3 / C composite material, comprising the following steps:

[0010] S1. Disperse the iron source compound in deionized water, stir evenly to obtain solution A, add alkali source and silicon source to solution A to obtain solution B; transfer solution B to a hydrothermal kettle to perform hydrothermal reaction, centrifuge the product after the reaction is completed, wash, and dry to obtain Fe2O3@SiO2;

[0011] S2, dispersing the Fe2O3@SiO2 obtained in step S1 in a mixed solution of deionized water and alcohol, adding hexadecyltrimethylammonium bromide and ammonia water, and then adding resorcinol and formaldehyde respectively, the resulting solution is sealed and stirred at room temperature for 1-48 hours and then dried in an oven at 80°C;

[0012] S3, calcining the system obtained in step S2 at 400-800° C. for 1-10 hours in an argon atmosphere to obtain

[0013] Fe2O3@SiO2@C;

[0014] S4. Immerse the Fe2O3@SiO2@C obtained in step S3 in a NaOH solution and maintain it at 25°C-100°C for 1-24 hours. After natural cooling, wash the final product with deionized water until the pH of the supernatant is close to 7 to obtain the void-containing Fe2O3 / C composite material.

[0015] Preferably, the iron source compound in step S1 is selected from one or more of ferrous ion chloride, ferrous ion nitrate, and ferrous ion sulfate; the alkali source is selected from one or more of sodium hydroxide, ammonia water, sodium carbonate, or urea; and the silicon source is selected from one or more of tetraethyl orthosilicate, silicate, and silicon dioxide.

[0016] Preferably, the molar ratio of the iron source compound, the silicon source and resorcinol is 30:5-20:9.

[0017] Preferably, the temperature of the hydrothermal reaction in step S1 is 120-210° C., and the time is 1-48 h.

[0018] Based on a general inventive concept, the present invention also provides an application of a void-containing Fe2O3 / C composite material in the preparation of a primary or secondary electrochemical generator, a high-energy generator and an electrochemical luminescence modulation system.

[0019] Preferably, the secondary battery is selected from a lithium ion battery, a sodium ion battery or a potassium ion battery.

[0020] The mechanism of preparation of this scheme is as follows:

[0021] The Fe2O3 / C composite material containing voids was prepared by a one-pot hydrothermal method, that is, the preparation of Fe2O3 precursor and coating of SiO2 template were successfully completed in one hydrothermal process. The voids of Fe2O3 / C can also be easily controlled by adjusting the dosage of tetraethyl orthosilicate in the preparation process. The appropriate void size plays a key role in improving the electrochemical properties of Fe2O3 / C materials.

[0022] The composite material prepared by this scheme has a large specific surface area (150-360m 2 / g) and rich porous structure, with shorter ion and electron transfer distance and increased contact area between electrolyte and active material, which is beneficial to reduce the transmission distance of embedded and deintercalated ions in the material during charge and discharge and improve the storage capacity, thereby improving the cycle performance and rate performance of the battery. The composite material has a core-shell structure, the outer shell is a carbon shell, the inner core is Fe2O3 nanoparticles, and there is a space gap between the carbon shell and the Fe2O3 nanoparticles. This appropriate space gap and carbon coating synergistically assist to improve the permeability of the active material electrolyte, realize rapid charge and ion transfer, and alleviate the volume effect, and finally obtain high electrochemical reaction kinetics and excellent structural stability.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The preparation process of metal oxide / carbon composite materials in the prior art involves multiple steps, usually preparing metal oxide first, then coating the silica template, and then coating the carbon layer. This leads to uneven coating on the surface of the metal oxide, poor particle size control, poor particle uniformity, and increased material preparation costs. The present invention adopts a one-pot hydrothermal method to prepare Fe2O3 / C composite materials containing voids, that is, the preparation of Fe2O3 precursor and the coating of SiO2 template are successfully completed in one hydrothermal process, which greatly simplifies the preparation process and can effectively overcome the above problems. In addition, the conditions are mild, the operation is convenient, the repeatability is high, and large-scale production is possible. In addition, the voids of Fe2O3 / C can be easily controlled by adjusting the amount of tetraethyl orthosilicate in the preparation process. Experimental results show that appropriate void size plays a key role in improving the electrochemical properties of Fe2O3 / C materials.

[0025] (2) The Fe2O3 / C composite material containing voids prepared in this scheme has a large specific surface area and a rich porous structure, which is beneficial to reducing the transmission distance of embedded and deintercalated ions in the material during charging and discharging and improving the storage capacity, thereby improving the cycle performance and rate performance of the battery. The composite material has a core-shell structure, the outer shell is a carbon shell, and the inner core is Fe2O3 nanoparticles. There is a space gap between the carbon shell and the Fe2O3 nanoparticles. The appropriate space gap and carbon coating synergistically assist to improve the permeability of the active material electrolyte, realize rapid charge and ion transport, and alleviate the volume effect, ultimately obtaining high electrochemical reaction kinetics and excellent structural stability.

[0026] (3) The composite material prepared in this scheme exhibits excellent cycle performance as a negative electrode material for lithium-ion batteries. -1 Under the current density, the capacity of the Fe2O3 / C composite material containing voids in the present invention still shows more than 500mAg after 250 cycles. -1 , CES is more than 99%; at the same time, the charge and discharge current is from 100mAg -1 Increased to 3000mAg -1 The rate performance test was carried out at 100 and 3000mAh g -1 In the 10th cycle, the discharge capacities were 704.0 and 294.2 mAh g -1 When the current density returned to 100 mAh g -1 When the discharge capacity is restored to 563.9 mAh g -1 , confirming the good rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0028] Figure 1 are the SEM and TEM images of the void-containing Fe2O3 / C in Experimental Example 1, a is the SEM image of Fe2O3@C-0.25, with a scale of 500 nm, b is the SEM image of Fe2O3@C-0.25, with a scale of 200 nm, c is the distribution diagram of C, O and Fe elements in Fe2O3@C-0.25, df are TEM images of Fe2O3@C-0.25 at different magnifications, g is a high-resolution transmission electron microscopy (HRTEM) image of Fe2O3@C-0.25, h is a fast Fourier transform (FFT) spectrum of Fe2O3@C-0.25, i is an inverse fast Fourier transform (IFFT) image of Fe2O3@C-0.25, and j is a linear scan image;

[0029] Figure 2 TEM images of different samples in Experimental Example 2: (a, d) are TEM images of the iron oxide precursor Fe2O3@SiO2, (b, e) are TEM images of Fe2O3, and (c, f) are TEM images of Fe2O3 / C containing voids prepared in Example 2;

[0030] Figure 3 are XRD patterns, Raman spectra, TG curves and N2 adsorption-desorption isotherms of different samples in Experimental Example 2; a is the XRD spectrum, b is the Raman spectrum, c is the thermogravimetric curve, and d is the nitrogen desorption isotherm;

[0031] Figure 4 is the high-resolution XPS spectrum of Fe2O3@C-0.25 in Experimental Example 3; a is the high-resolution XPS spectrum of Fe 2p, b is the high-resolution XPS spectrum of C1s, and c is the high-resolution XPS spectrum of O1s;

[0032] Figure 5 CVs, constant current charge / discharge curves, cycle performance and rate performance of different samples in Experimental Example 4: (a, d) are CVs and constant current charge / discharge curves of Fe2O3; (b, e) are CVs and constant current charge / discharge curves of Fe2O3 / C containing voids prepared in Example 1; (c, f) are CVs and constant current charge / discharge curves of Fe2O3 / C containing voids prepared in Example 2; (g) is the cycle performance of different samples, and (h) is the rate performance of different samples;

[0033] Figure 6 is the log(I) to log(V) and capacitance contribution (at 0.5 mV s) of the electrode materials prepared from different samples in Experimental Example 4 -1 (bottom), a is the variable speed cyclic voltammetry curve of Fe2O3, b is the linear graph of Fe2O3log(i) vs.log(v), c is the percentage graph of Fe2O3 pseudocapacitance, d is the variable speed cyclic voltammetry curve of Fe2O3@C-0.25, e is the linear graph of Fe2O3@C-0.25log(i) vs.log(v), f is the percentage graph of Fe2O3@C-0.25 pseudocapacitance, g is the variable speed cyclic voltammetry curve of Fe2O3@C-0.5, h is the linear graph of Fe2O3@C-0.5log(i) vs.log(v), and i is the percentage graph of Fe2O3@C-0.5 pseudocapacitance. DETAILED DESCRIPTION

[0034] In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0035] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention. Without departing from the spirit and substance of the present invention, modifications or substitutions made to the methods, steps or conditions of the present invention are within the scope of the present invention.

[0036] Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art; unless otherwise specified, the reagents used in the examples are commercially available.

[0037] Example 1

[0038] Preparation of Fe2O3 / C composites containing voids

[0039] S1. Add 0.8g of FeSO4·7H2O to 60ml of deionized water and dissolve it by magnetic stirring. Then, add 6ml of ammonia (NH3·H2O, 25wt.%) to the above solution under stirring to form a suspension. After 30min, add 0.25ml of tetraethyl orthosilicate (TEOS) dropwise. After stirring for 5min, transfer the suspension to a 100ml Teflon-lined stainless steel autoclave and seal it. Keep it in an oven at 180°C for 24h. Collect the resulting precursor by centrifugation and wash it with deionized water (the first three times) and ethanol (the last time). Then, dry it at 70°C to obtain Fe2O3@SiO2.

[0040] S2. 0.4 g of Fe2O3@SiO2 prepared in S1 was dispersed in a mixture of 20 mL of deionized water and 10 mL of ethanol by ultrasonication for 30 minutes to form a suspension. Then, 0.2 g of hexadecyltrimethylammonium bromide (CTAB) and 0.2 mL of NH3·H2O were added to the suspension under stirring. 0.1 g of resorcinol and 0.1 mL of formaldehyde were added at intervals of 30 minutes. The suspension was sealed and stirred at room temperature for 24 hours, and finally kept in an oven at 80°C for 24 hours.

[0041] S3. The product obtained in step S2 was calcined at 600°C for 3 hours under an argon atmosphere to obtain Fe2O3@SiO2@C-0.25 (0.25 represents the amount of TEOS).

[0042] S4. Immerse Fe2O3@SiO2@C-0.25 in 3M NaOH solution and keep it in an oven at 70°C for 24 h. After natural cooling, wash the target Fe2O3@C-0.25 repeatedly with deionized water until the pH of the supernatant is close to 7.

[0043] Example 2

[0044] Preparation of Fe2O3 / C composites containing voids

[0045] S1. Add 0.8g of FeSO4·7H2O to 60ml of deionized water, then dissolve by magnetic stirring, then add 6ml of ammonia (NH3·H2O, 28wt.%) to the above solution under stirring to form a suspension, after 30min, add 0.5ml of TEOS dropwise, after stirring for 5min, transfer the suspension to a 100ml Teflon-lined stainless steel autoclave and seal it, keep it in an oven at 180°C for 24h, collect the precursor by centrifugation, wash with deionized water (the first three times) and ethanol (the last time), and then dry it at 70°C to obtain Fe2O3@SiO2.

[0046] S2. 0.4 g of Fe2O3@SiO2 prepared in S1 was dispersed in a mixture of 20 mL of deionized water and 10 mL of ethanol by ultrasonication for 30 minutes to form a suspension. Then, 0.2 g of CTAB and 0.2 mL of NH3·H2O were added to the suspension under stirring. 0.1 g of resorcinol and 0.1 mL of formaldehyde were added at intervals of 30 minutes. The suspension was sealed and stirred at room temperature for 24 hours, and finally kept in an oven at 80°C for 24 hours.

[0047] S3. The product obtained in step S2 is calcined at 600°C for 3 hours in an argon atmosphere to obtain Fe2O3@SiO2@C-0.5 (0.5 represents the amount of TEOS).

[0048] S4. Fe2O3@SiO2@C-0.5 was immersed in 3M NaOH solution and kept in an oven at 70°C for 24 h. After natural cooling, the target Fe2O3@C-0.5 was repeatedly washed with deionized water until the pH of the supernatant was close to 7.

[0049] Comparative Example 1

[0050] Preparation of pure Fe2O3 nanoparticles

[0051] S1. Add 0.8 g of FeSO4·7H2O to 60 ml of deionized water and dissolve it by magnetic stirring. Then, add 6 ml of aqueous ammonia (NH3·H2O, 25 wt.%) to the above solution under stirring to form a suspension. Transfer the suspension to a 100 ml Teflon-lined stainless steel autoclave and seal it. Keep it in an oven at 180°C for 24 hours. Collect the resulting precursor by centrifugation and wash it with deionized water (the first three times) and ethanol (the last time), and then dry it at 70°C to obtain Fe2O3@SiO2.

[0052] S2, calcining the product obtained in step S1 at 600°C in an argon atmosphere for 3 hours to obtain Fe2O3@SiO2;

[0053] S3. Fe2O3@SiO2 was immersed in 3M NaOH solution and kept in an oven at 70°C for 24 h. After natural cooling, it was repeatedly washed with deionized water to obtain pure Fe2O3 particles.

[0054] Experimental Example 1

[0055] Investigation of the morphological characteristics of Fe2O3 / C composites containing voids

[0056] The composite material prepared in Example 1 was taken and its morphology was studied using scanning electron microscopy (SEM) analysis and transmission electron microscopy (TEM) analysis. Figure 1 Figures a and b in the figure show SEM images of Fe2O3@C-0.25 prepared in Example 1 with different magnifications.

[0057] The results show that Fe2O3@C-0.25 is composed of interconnected nanoparticles. It is worth noting that cavities can be found on some nanoparticles, indicating the presence of hollow or void-related structures in Fe2O3@C-0.25. Based on SEM observation, the surface of Fe2O3@C-0.25 was found to be smooth and dense, which is consistent with the carbon coating structure. Figure 1 As shown in Figure (c), C, O, and Fe elements are well distributed in Fe2O3@C-0.25. O and Fe elements come from Fe2O3NPs while C element comes from the carbon coating, which is confirmed in the material characterization of the system.

[0058] The results show that in Fe2O3@C-0.25, dark Fe2O3NPs with a size of 8 to 60nm are encapsulated in a lightweight interconnected carbon shell. The carbon shell presents an amorphous-controlled structure with a thickness of 2 to 5nm, and there are a large number of voids of different sizes between the Fe2O3NPs and the carbon shell.

[0059] Figure 1 Figures h and j in the figure describe the analysis results of FFT, IFFT and line intensity distribution, respectively, which confirm that these lattice bars have a lattice spacing of 0.59 nm, corresponding to the (110) plane of cubic phase Fe2O3 (according to JCPDS no.39-1346, for the (110) plane, d spacing = 5.9180 Å). Therefore, the void-containing composite of Fe2O3@C-0.25 was successfully prepared by a relatively simple method.

[0060] Experimental Example 2

[0061] Investigation of the structural differences between pure Fe2O3 nanoparticles and Fe2O3 / C composites containing voids

[0062] The microstructures of Fe2O3@c-0.25 prepared in Example 1, Fe2O3@C-0.5 prepared in Example 2 and pure Fe2O3 nanoparticles prepared in Comparative Example 1 were compared.

[0063] Figure 2 Figures a and d show TEM images of the iron oxide precursor Fe2O3@SiO2 intermediate prepared with 0.25 ml TEOS. Obviously, the iron oxide precursor Fe2O3@SiO2 intermediate is composed of interconnected nanoparticles. The enlarged TEM image ( Figure 2 Figure d shows that the surface of the dark iron oxide precursor nanoparticles is coated with a layer of light-colored SiO2. This indicates that the preparation of Fe2O3 precursor and the coating of SiO2 template are usually achieved in steps.

[0064] Figure 2 b and e show the TEM images of the prepared pure Fe2O3 nanoparticles. No light and dense coating layer was found on the surface of the pure Fe2O3 nanoparticles and the surface became rough, indicating that SiO2 was successfully removed after etching. Some fine nanoparticles appeared on the surface of the pure Fe2O3 nanoparticles, which may be Fe2SiO4, such as Figure 2 As shown in Figures c and f, Fe2O3@C-0.5 shows a similar structure to Fe2O3@C-0.25, such as particle size and carbon shell thickness, but it has a much larger void space. The results confirm that the void size of Fe2O3@C can be easily controlled by adjusting the amount of TEOS.

[0065] The structures of the prepared samples were further investigated by XRD and Raman spectroscopy. Figure 3 As shown in a, these samples show similar diffraction peaks, indicating the presence of the same crystalline phase composition. The XRD spectrum shows that except for two weak diffraction peaks at 25.0° and 51.8° pointing to the (111) and (321) planes of orthorhombic Fe2SiO4 (JCPDS no.34-0178), all diffraction peaks belong to cubic Fe2O3 (JCPDS no.39-1346). The diffraction peak of carbon is mainly due to its amorphous control structure and no diffraction peak of carbon is observed. The Raman spectra of these samples are at 1334 and 1590 cm -1 Two broad peaks are shown at the center, corresponding to the D band and G band of carbon materials, respectively. The D band and G band are related to the in-plane vibration of sp2-bonded carbon atoms of disordered carbon and defective carbon and graphitic carbon. Accordingly, the peak intensity ratio of the D band to the G band (ID / IG) is used to reflect the structural characteristics of carbon defects, disordered structure and degree of graphitization. The ID / IG values ​​of Fe2O3, Fe2O3@C-0.25 and Fe2O3@C-0.5 are 0.94, 0.79 and 0.85, respectively. The carbon in Fe2O3 should come from the carbonization product of the residual organic part of TEOS, so the content is low. The low content may lead to a thinner carbon layer or smaller carbon particles. Fe2O3@C-0.5 has a higher ID / IG value than Fe2O3@C-0.25, which may be due to the larger inner surface area of ​​the carbon shell with more defects caused by the larger voids of Fe2O3@C-0.5. Fe2O3@C-0.25 has the lowest ID / IG value, and carbon with high graphitized structure content (high electrical conductivity and structural strength) is beneficial to the electronic conductivity and structural stability of Fe2O3@C-0.25 composites, such as Figure 3 As shown in Figure c, the carbon decomposition temperature of the Fe2O3 sample is lower than that of Fe2O3@C-0.25 and Fe2O3@C-0.5 due to its lower carbon content and more defects or disordered structure. The weight loss between 300℃ and 500℃ should be attributed to the decomposition temperature of carbon. The Fe2O3 sample shows a weight loss of 7.5%, so the residual carbon content is estimated to be 7.5%. Similarly, the carbon contents of Fe2O3@C-0.25 and Fe2O3@C-0.5 are estimated to be 33.4% and 25.7%. Fe2O3@C-0.5 has a larger void space and therefore has a lower tap density, which leads to a lower carbon content.

[0066] It is well known that the specific surface area can significantly affect the properties of materials. In addition, a large specific surface area is conducive to the formation of capacitance, which is known because Li + Absorption and diffusion on the surface of active materials accelerates Li +The results show that the Brunauer-Emmett-Teller (BET) specific surface areas of Fe2O3, Fe2O3@C-0.25 and Fe2O3@C-0.5 are 133.1, 247.0 and 180.9 m2g, respectively. -1 , and the N2 adsorption-desorption isotherms of Fe2O3@C-0.25 and Fe2O3@C-0.5 show type IV adsorption isotherms in the relative pressure range of 0.4-0.99, indicating that Fe2O3@C-0.25 and Fe2O3@C-0.5 have mesoporous structures. The pore sizes of Fe2O3@C-0.25 and Fe2O3@C-0.5 are mainly concentrated at 4.0nm, indicating that Fe2O3@C-0.25 has excellent surface area and highly developed porous structure. Fe2O3@C composites with large specific surface area and abundant nanopores will have shorter ion and electron transfer distances and increased contact area between electrolyte and active materials, ultimately obtaining enhanced electrochemical kinetics.

[0067] Experimental Example 3

[0068] Investigation of XPS characterization and surface chemical valence state of Fe2O3@C

[0069] The Fe2O3@c-0.25 prepared in Example 1 was characterized by XPS to explore its surface chemical valence state. Figure 4 Figure a reveals the high-resolution XPS spectrum of Fe 2p orbitals, which is further fitted into six peaks. The two peaks with binding energies of 711.4 and 724.8 eV can be attributed to the Fe 3+ 2p 3 / 2 and Fe 3+ 2p 1 / 2 Two satellite peaks were observed at 734.6 and 718.3 eV, respectively. Then, Fe 2+ 2p 3 / 2 (709.9eV) and Fe 2+ 2p 1 / 2 (723.3eV) species can be attributed to Fe2SiO4. This is consistent with the above XRD analysis. The C 1s high-resolution XPS spectrum is divided into three peaks centered at 284.8, 286.1, and 289.2eV, corresponding to CC, CO, and C=O bonds, respectively. The O 1s high-resolution XPS spectrum shows three main peaks at 529.5, 531.1, and 532.6eV, belonging to lattice O (Fe-O bond), CO bond, and adsorbed H2O, respectively. Obviously, these XPS analysis results are consistent with the chemical composition of the Fe2O3@C-0.25 composite material.

[0070] Experimental Example 4

[0071] Investigating the performance of Fe2O3@C as a negative electrode for lithium-ion batteries

[0072] The Li+ charge storage properties of the prepared samples were investigated in 2016-type button cells using Li foil as counter / reference electrode. First, cyclic voltammetry curves (CVs) of the samples obtained between 0.01 and 3.00 V (vs. Li / Li+) at a scan rate of 0.3 mV s-1 were used to examine their lithium storage properties. Figure 5 As shown in Figure a, the CVs of the Fe2O3 sample show two cathodic peaks during the first cycle. The former (at 1.56 V) is probably related to the Li + Insertion of the crystal structure of Fe2O3 (Fe2O3+2Li+2e - →Li2Fe2O3) and Li + Insert Fe2SiO4(Fe2SiO4+4Li++4e - →2Fe+Li4SiO4, capacity is 526mAh g -1 ). The formation of Li4SiO4 can be used as a solid electrolyte to enhance ionic conductivity. This will contribute to the lithium storage kinetics of Fe2O3@C composites. The latter corresponds to the further reversible lithiation of Li2Fe2O3 (Li2Fe2O3+4Li + +4e-α2Fe+3Li2O) and the irreversible formation of the solid electrolyte interphase (SEI) layer. Therefore, the peak integrated area shows a significant decrease in the subsequent cycles, which can be attributed to the irreversible formation of the SEI layer. Afterwards, there are two broad anodic peaks at 1.78 and 2.11 V, which are attributed to the conversion of FeO to Fe 2+ and from Fe 2+ To Fe 3+ The anodic peak at 2.11 V is Li + It is worth noting that the position and area of ​​the cathodic peak with a lower potential (at 0.42 V) shifted constantly to a higher potential and decreased in subsequent cycles, respectively, which may be caused by the first Li + Caused by changes in material structure after insertion and removal cycles. Figure 5 Figure b and Figure 5 Figure c shows the CVs of Fe2O3@C-0.25 and Fe2O3@C-0.5 obtained under the same conditions as Fe2O3, respectively. The CVs of Fe2O3@C-0.25 and Fe2O3@C-0.5 are similar to those of Fe2O3 because they have the same active material composition (Fe2O3).

[0073] However, the CVs of Fe2O3@C-0.25 and Fe2O3@C-0.5 show lower polarization voltages and more stable shapes than those of Fe2O3, indicating that the introduction of carbon not only enhances the conductivity but also improves the structural stability of Fe2O3@C-0.25 and Fe2O3@C-0.5. The obvious voltage plateaus observed in the galvanostatic charge-discharge curves of Fe2O3@C-0.25 and Fe2O3@C-0.5 electrodes obtained between 0.01 and 3.00 V (vs. Li / Li+) at 200 mA g-1 correspond to the redox peaks in the CVs. The galvanostatic charge-discharge curves of the Fe2O3 electrode shrink rapidly with increasing cycle number, indicating a rapid capacity decay during cycling. This is due to the well-known structural failure of Fe2O3 caused by the huge volume change during Li+ insertion / extraction. However, the curves of Fe2O3@C-0.25 and Fe2O3@C-0.5 show a much smaller shrinkage than that of Fe2O3. The initial discharge / change capacities of Fe2O3, Fe2O3@C-0.25, and Fe2O3@C-0.5 electrodes are 1397.5 / 1037.5, 1121.1 / 706.8, and 1048.5 / 588.8 mAh g -1 , the initial Coulombic efficiencies (ICE) are 74.2%, 63.0% and 56.2% respectively. The irreversible formation of SEI layer in the first cycle is the main reason for the low ICE of the electrode.

[0074] Therefore, the initial capacity loss of Fe2O3 is mainly caused by the irreversible formation of the SEI layer. However, amorphous carbon facilitates the formation of more SEI in the first cycle due to defects, large surface area, and more active sites. As a result, Fe2O3@C-0.25 and Fe2O3@C-0.5 electrodes show lower ICE than Fe2O3 electrodes, and the ICE of Fe2O3@C-0.5 electrode is lower than that of Fe2O3@C-0.25, which may be due to the fact that the carbon of Fe2O3@C-0.5 has more defects (higher ID / IG value) and larger internal surface area, which may increase the electrolyte decomposition reaction on the surface. This is conducive to the formation of capacitance process through Li+ absorption and transfer on the surface of active materials, thereby generating additional lithium storage capacity.

[0075] In order to conveniently compare the cycling behavior of the samples, Figure 5 The g graph in Figure 2 reveals that the Fe2O3, Fe2O3@C-0.25 and Fe2O3@C-0.5 electrodes have a high conductivity at 200 mAg -1The first thing that caught our attention is that Fe2O3 exhibits a continuous capacity decay during the entire cycle (the capacity decreases from 1397.5 mAh g in the first cycle to -1 It decreased to 270.4 mAh g at the 210th cycle. -1 ), especially the faster capacity decay after 80 cycles, confirming the poor cycle performance. In contrast, the cycle stability of Fe2O3@C-0.25 and Fe2O3@C-0.5 electrodes performed well. By comparing the long-cycle performance of Fe2O3@C-0.25 and Fe2O3@C-0.5, it was found that Fe2O3@C-0.25 had better cycle stability, which was due to the faster capacity decay of Fe2O3@C-0.5 after 150 cycles. The results show that Fe2O3@C-0.25 and Fe2O3@C-0.5 exhibit 548.9 and 500.5 mAh g-1 over 250 cycles with CES of 99.9% and 99.4%, respectively. The cycling stability of Fe2O3@C-0.5 is lower than that of Fe2O3@C-0.25, which may be attributed to its excessively large void space, which causes the void structure to collapse under external pressure and lithium ion insertion, thus potentially leading to reduced cycling durability. Larger voids have the potential to aggravate stress-induced structural deformation. Only an appropriate void space between Fe2O3 and the carbon substrate can optimize the electrochemical performance of the Fe2O3 / C composite.

[0076] As a result, the Fe2O3@C-0.25 electrode with appropriate voids matching a certain amount of Fe2O3 NPs obtains optimized lithium storage performance. Figure 5 Figure h in Figure 3 further shows that the three electrodes have different -1 The capacity of the Fe2O3 electrode decays rapidly with increasing current density due to its poor electronic conductivity. The Fe2O3@C-0.25 and Fe2O3@C-0.5 electrodes have better rate performance than the Fe2O3 electrode, especially Fe2O3@C-0.25 has the best rate performance, with a capacity of 100, 300, 500, 1000, 2000 and 3000 mAh g -1 In the 10th cycle, the discharge capacities were 704.0, 543.2, 474.2, 399.0, 336.2 and 294.2 mAh g -1 When the current density returns to 100 mAhg -1 When the discharge capacity is restored to 563.9 mAhg -1, confirming the good rate capability. Apparently, the construction of the void space between the Fe2O3 NP core and the carbon coating effectively addresses the structural and interfacial issues of the Fe2O3 anode during cycling. The presence of a large void space between the Fe2O3 NP core and the carbon coating firstly delays the rapid transport of electrons and ions from / to Fe2O3, and secondly leads to the collapse of the void structure during external pressure and lithium ion insertion, ultimately resulting in suboptimal lithium storage performance. Considering that Fe2O3@C-0.25 has a satisfactory void space between the Fe2O3 NP core and the carbon coating, it has reliably obtainable good kinetics and improved cycling stability.

[0077] In order to better understand the lithium storage mechanism of Fe2O3@C-0.25, the -1 The reaction kinetics of the electrode were analyzed by recording CVs at different scan rates within a wide range. Figure 6 Figures a, d, and g show the recorded CVs of Fe2O3, Fe2O3@C-0.25, and Fe2O3@C-0.5 electrodes, respectively. In the CVs of all three samples, the redox peaks gradually upgraded with increasing scan rate, and their profiles remained slightly displaced, indicating that there was a capacitive control process during the charge and discharge of the three anodes. This is a common phenomenon in the lithium storage process of nanostructured materials. The capacitive effect can be determined using the relationship between the peak current (I) and the scan rate (V), which is due to the following equation:

[0078] i=a·vb(1)

[0079] log(i)=b log(v)+log(a)(2)

[0080] Figure 6 The log(I) versus log(V) plots and fitting lines of the Fe2O3, Fe2O3@C-0.25, and Fe2O3@C-0.5 electrodes are given in b, e, and h, respectively. Based on these fitting lines, the b-values ​​of the cathodic / anodic peaks of the Fe2O3, Fe2O33@C-0.25, and Fe2O3@C-0.5 electrodes were calculated to be 0.52 / 0.73, 0.80 / 0.79, and 0.64 / 0.71. This result indicates that the electrochemical reactions of these electrodes are affected by a mixed mechanism of diffusion and capacitance during lithium storage. However, the b-value of Fe2O3 is close to 0.5 during the discharge process, suggesting a diffusion-controlled electrochemical process. The b-value of the Fe2O3@C-0.25 electrode is close to 0.8, indicating the presence of capacitance-dominated behavior during the lithium storage process. The method reported in previous work has been used to quantitatively analyze the contribution of capacitance to the current response.

[0081] i(V)v1 / 2=k1v1 / 2+k2 (3)

[0082] As a result, the capacitance contributions of Fe2O3, Fe2O3@C-0.25 and Fe2O3@C-0.5 electrodes at 0.5 mV s-1 were 31.48%, 53.91% and 35.36%, respectively. Figure 6 As shown in c, f and i in Figure 2, their capacitance contribution increases with the increase of scan rate. At 1.3 mV s-1, the capacitance contribution of Fe2O3, Fe2O3@C-0.25 and Fe2O3@C-0.5 electrodes increases to 44.22%, 67.21% and 48.78%. The results show that the Fe2O3@C-0.25 electrode exhibits a larger capacitance contribution than the Fe2O3 and Fe2O3@C-0.5 electrodes at each scan rate, which is mainly due to its larger surface area. It has been demonstrated that the capacitance process is carried out through the absorption and transport of ions on the surface of the active material, which helps to obtain additional capacity, accelerate the reaction of Li+ with the active material and reduce the volume effect, ultimately leading to excellent electrochemical kinetics, enhanced structural stability and high capacity of the electrode during cycling. Therefore, a larger capacitance contribution can provide the electrode with a more stable structure, higher capacity and better rate performance. The Fe2O3@C-0.25 electrode has a relatively larger capacitance contribution than the Fe2O3 and Fe2O3@C-0.5 electrodes, thus showing better performance in terms of capacity, cycle stability, and rate capability.

[0083] The above is only a preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments. For those skilled in the art, improvements and changes obtained without departing from the technical concept of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A void-containing Fe2O3 / C composite material, characterized in that: The void-containing Fe2O3 / C composite material comprises a carbon layer shell and a Fe2O3 nanoparticle core, a space gap exists between the carbon layer shell and the Fe2O3 nanoparticle core, and the Fe2O3 nanoparticle core is wrapped in the carbon layer shell.

2. The void-containing Fe2O3 / C composite material according to claim 1, characterized in that: The carbon layer shell is an amorphous controlled structure with a thickness of 2-5nm, and the carbon layer shell has a mesoporous structure with a pore size of 2-6nm; the Fe2O3 nanoparticle size is 8-100nm.

3. The void-containing Fe2O3 / C composite material according to claim 1, characterized in that: The specific surface area of ​​the void-containing Fe2O3 / C composite material is 150-360m 2 / g, and the content of the carbon layer shell in the void-containing Fe2O3 / C composite material is 20%-40%.

4. A method for preparing a void-containing Fe2O3 / C composite material according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. Dispersing an iron source compound in deionized water, stirring evenly to obtain a solution A, and adding an alkali source and a silicon source to the solution A to obtain a solution B; Solution B was transferred to a hydrothermal reactor for hydrothermal reaction. After the reaction, the product was centrifuged, washed, and dried to obtain Fe2O3@SiO2. S2, dispersing the Fe2O3@SiO2 obtained in step S1 in a mixed solution of deionized water and alcohol, adding hexadecyltrimethylammonium bromide and ammonia water, and then adding resorcinol and formaldehyde respectively, the resulting solution is sealed and stirred at room temperature for 1-48 hours and then dried in an oven at 80°C; S3, calcining the system obtained in step S2 at 400-800° C. for 1-10 hours in an argon atmosphere to obtain Fe2O3@SiO2@C; S4. Immerse the Fe2O3@SiO2@C obtained in step S3 in a NaOH solution and maintain it at 25°C-100°C for 1-24 hours. After natural cooling, wash the final product with deionized water until the pH of the supernatant is close to 7 to obtain the void-containing Fe2O3 / C composite material.

5. The preparation method according to claim 4, characterized in that: In step S1, the iron source compound is selected from one or more of ferrous ion chloride, ferrous ion nitrate, and ferrous ion sulfate; the alkali source is selected from one or more of sodium hydroxide, ammonia water, sodium carbonate, or urea; and the silicon source is selected from one or more of tetraethyl orthosilicate, silicate, and silicon dioxide.

6. The preparation method according to claim 4, characterized in that: The molar ratio of the iron source compound, the silicon source and resorcinol is 30:5-20:

9.

7. The preparation method according to claim 4, characterized in that: The temperature of the hydrothermal reaction in step S1 is 120-210° C. and the time is 1-48 hours.

8. Use of the void-containing Fe2O3 / C composite material as described in any one of claims 1 to 3 or the void-containing Fe2O3 / C composite material prepared by the preparation method as described in any one of claims 4 to 7 in the preparation of primary or secondary electrochemical generators, high-energy generators and electrochemical luminescence modulation systems.

9. The use according to claim 8, characterized in that: The secondary electrochemical generator comprises a secondary battery negative electrode material, and the secondary battery is selected from a lithium ion battery, a sodium ion battery or a potassium ion battery.

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