Preparation method and application of ferrous hexafluorosilicate and iron-silicon composite material

Abstract

The application discloses a preparation method and application of ferrous hexafluorosilicate and iron-silicon composite material, and adopts hydrofluoric acid and ferrosilicon alloy to prepare ferrous hexafluorosilicate in one step; the difficulty of the application lies in controlling the content of the hydrofluoric acid and the weight of the ferrosilicon alloy, and regulating a certain proportion to achieve balance; although the hydrofluoric acid is highly toxic, the method can achieve good effects by using a small amount of the hydrofluoric acid, and can adopt the excessive ferrosilicon alloy to minimize environmental pollution and be more friendly to the environment; when the prepared ferrous hexafluorosilicate is used as a negative material of a lithium ion battery, irreversible lithium fluorosilicate can be effectively relieved, thereby being favorable to lithium ion embedding and discharging, better relieving volume expansion caused in the charging and discharging process, maintaining the stability of a pole piece structure, greatly improving the electrochemical performance of the lithium ion battery, improving the specific capacity of the battery, and being capable of well reducing the production cost of the lithium ion.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium battery negative electrode materials, and particularly relates to a preparation method and application of ferrous hexafluorosilicate and iron-silicon composite material. BACKGROUND

[0002] With the continuous progress and development of science and technology, society has been in a long-term energy shortage situation, and in addition, the overuse of fossil energy has led to serious pollution and damage, making energy and environmental crises increasingly serious, thereby causing people's concern about the climate and environment, and the attention to human health problems. For this reason, human beings have begun to turn to clean energy to reduce dependence on fossil energy. This has accelerated the development and storage of new energy, making it one of the important directions of human development and progress today. Due to the portability and ion transfer mechanism of adjustable energy collection and release possessed by lithium ion batteries (LIB), it has become an important energy storage tool for people and is widely used in various electric vehicles. The lithium ion battery (LIB) has the following advantages: higher environmental protection than traditional storage batteries; almost no self-discharge and memory effect; has a longer working life, which can reach more than 500 times; in addition, LIB also has high safety and high power advantages, so it is widely used in various industries. Compared with dry batteries and lead-acid batteries, LIB has many advantages in performance, so it is widely used in the market, especially in the fields of 3C digital products and electric vehicles. LIB has become an important driving force for promoting human scientific and cultural progress, and has become a popular topic for research by various enterprises, colleges and universities, and research institutions. In order to meet the social demand for new energy, LIB is continuously improving the energy density, improving the cycle performance and improving the safety performance.

[0003] LIB is an important energy storage device widely used in electronic devices, electric vehicles and new energy reserves. Its working principle is based on the diffusion of lithium ions in electrode materials. Electrode materials are generally divided into two parts: anode and cathode. The cathode material is usually a compound such as LiCoO2, LiMn2O4, and the anode material is usually graphite. The reason for choosing graphite as the anode material is that it not only has low cost, but also has a suitable lithium intercalation potential, thereby improving the safety performance of the battery. Therefore, the graphite anode has become the mainstream product of lithium ion batteries.

[0004] However, the theoretical specific capacity of graphite-based anodes in lithium batteries is only 372 mAh / g. The actual specific capacity of commercial high-end graphite materials has approached its theoretical specific capacity, about 360-365 mAh / g. Due to the limited improvement of graphite anode materials on the energy density of LIB, it cannot meet the further development needs. Therefore, it is necessary to find other materials or technologies to improve the energy density of LIB.

[0005] Silicon materials have been widely studied as a potential alternative to traditional lithium-ion battery anodes due to their high theoretical specific capacity, abundant reserves, and low cost. However, the volume expansion and contraction of silicon materials during charging and discharging can cause severe volume changes in the electrode, leading to electrode material fracture and battery performance degradation. Therefore, researchers are working to find modification and optimization methods for silicon-based anode materials to overcome these problems.

[0006] In recent years, many studies have focused on developing new silicon-based anode materials, such as silicon nanoparticles, silicon nanowires, and silicon thin films. These materials have smaller sizes and special structures that can alleviate the volume change problem of silicon materials and improve the stability and cycle life of the electrode. However, they often have defects such as complex preparation methods and high production costs, making them unsuitable for large-scale production. Therefore, more researchers are focusing on the composite of silicon-based anode materials with other materials. Through the composite of silicon materials with other materials, the stability of silicon materials can be improved, and the volume change can be reduced, thereby improving the cycle life and stability of the battery.

[0007] In addition, material composite has the following characteristics: 1. Enhancing mechanical stability: The volume change of silicon materials can easily cause the fracture of electrode materials, thereby reducing the mechanical stability of the battery. Through the composite of silicon materials with other materials, the mechanical stability of the electrode can be enhanced, the fracture of the material can be reduced, and the service life of the battery can be prolonged; 2. Improving electrical conductivity: Certain composite materials can improve the electrical conductivity of silicon-based anode materials, thereby improving the power output and charging and discharging efficiency of the battery; 3. Increasing surface reactivity: The surface reactivity of silicon materials is low, which leads to a slow reaction rate of the battery. Through the composite of silicon materials with other materials, the surface reactivity of silicon materials can be increased, and the reaction rate of the battery can be accelerated; 4. Improving energy density: Silicon materials have a high theoretical specific capacity. Through the composite of silicon materials with other materials, the energy density of the battery can be further improved to meet the higher energy demand applications; 5. Optimizing interface structure: The composite of silicon materials with other materials can optimize the interface structure of the battery, improve the electron and ion transmission rate of the battery, and thereby improve the performance of the battery. These advantages make silicon-based composite anode materials a hot spot for research and development, providing new possibilities for the application of the next generation of high-performance batteries.

[0008] In summary, the study of silicon-based anode materials is to improve the energy density and cycle life of lithium-ion batteries. Through the composite of silicon materials with other materials, researchers hope to achieve higher performance lithium-ion batteries to meet the growing energy demand and environmental sustainability

[0009] Ferrous hexafluorosilicate is a colorless light green powder, which is currently mainly used in the fields of electroplating, dyes, catalysts and the like in industry. At present, the preparation method of ferrous hexafluorosilicate is as follows: ferrous bromide and hexafluorosilicic acid are mixed in a certain proportion, heated to a certain temperature to react, and then the product ferrous hexafluorosilicate is obtained by filtration. This method is relatively expensive in raw materials, and the preparation process needs to be heated, which requires a certain energy consumption. In addition, if the feeding ratio of the reaction is wrong, too many impurities will be present in the prepared ferrous hexafluorosilicate, so this preparation method has certain defects. And so far, there is no literature report on the use of ferrous hexafluorosilicate for preparing lithium ion battery negative electrode materials. SUMMARY

[0010] In order to solve the problems existing in the prior art, the application provides a preparation method and application of ferrous hexafluorosilicate and iron-silicon composite material. The ferrous hexafluorosilicate is prepared by one step of hydrofluoric acid and silicon-iron alloy. This method is extremely simple, the prepared ferrous hexafluorosilicate has extremely high purity, no other by-products, and no other material residues, which saves raw materials to a certain extent.

[0011] In addition, the ferrous hexafluorosilicate prepared by this method has excellent electrochemical performance, which is used as an additive of silicon negative electrode material, so as to relieve the volume expansion problem of the silicon negative electrode and enhance the mechanical stability.

[0012] The technical scheme adopted to achieve the above-mentioned purposes of the application is as follows:

[0013] A preparation method of ferrous hexafluorosilicate, comprising the following steps:

[0014] The silicon-iron alloy Fe x Si y is etched with hydrofluoric acid, after the reaction is completed, filtration is performed, the filtrate is dried, and the light green crystal ferrous hexafluorosilicate is obtained.

[0015] Application of ferrous hexafluorosilicate in preparation of lithium ion battery negative electrode material.

[0016] A preparation method of iron-silicon composite material, comprising the following steps:

[0017] The silicon-iron alloy Fe x Si y is etched with hydrofluoric acid, after the reaction is completed, filtration is performed, the filtrate is dried, and the light green crystal ferrous hexafluorosilicate is obtained, the ferrous hexafluorosilicate and graphite are ball milled, the obtained powder is washed with ethanol, filtration is performed, the filter cake is dried, and the iron-silicon composite material is obtained.

[0018] Or the silicon-iron alloy Fe x Si yThe obtained powder is washed with ethanol, filtered, and the filter cake is dried to obtain a carbon-doped ferrosilicon alloy, and the carbon-doped ferrosilicon alloy is subjected to etching reaction with hydrofluoric acid, after the reaction is completed, filtering is performed, and the filtrate is dried to obtain the iron-silicon composite material.

[0019] The ferrosilicon alloy Fe x Si y , ferrous hexafluorosilicate and graphite are subjected to ball milling, the obtained powder is washed with ethanol, filtered, and the filter cake is dried to obtain the iron-silicon composite material.

[0020] Further, the ferrosilicon alloy Fe x Si y , wherein x+y=100.

[0021] Further, the concentration of the hydrofluoric acid is 1M.

[0022] Further, the ferrosilicon alloy Fe x Si y , and hydrogen fluoride is 5:1-4.

[0023] Further, the etching reaction is carried out at a temperature of 40℃ for 24h.

[0024] Further, the etching reaction is carried out under a closed condition.

[0025] Further, the filtrate is vacuum dried at 80℃.

[0026] Further, the ball milling time is 30min.

[0027] An iron-silicon composite material in the preparation of a silicon negative electrode material for a lithium ion battery.

[0028] Compared with the prior art, the advantages and beneficial effects of the present application are:

[0029] 1. The present application uses hydrofluoric acid and ferrosilicon alloy to prepare ferrous hexafluorosilicate in one step, and the difficulty lies in the content of hydrofluoric acid and the weight control of ferrosilicon alloy. A certain proportion is adjusted to achieve balance. Although hydrofluoric acid is highly toxic, this method can achieve good results with very small amount of hydrofluoric acid, and the environmental pollution can be minimized by using excess ferrosilicon alloy, which is friendly to the environment.

[0030] 2. The present application uses hydrofluoric acid and ferrosilicon alloy to prepare ferrous hexafluorosilicate in one step, and the raw material price is low, the preparation method is simple, the preparation condition is simple and easy to control, so the preparation cost is low, the selection of raw materials is flexible, and the purity of the prepared ferrous hexafluorosilicate is extremely high, which is very beneficial to industrialized mass production.

[0031] 3、The prepared ferrous hexafluorosilicate as the negative material of the lithium ion battery has excellent electrochemical performance, high cycle stability, high capacity, reversible reaction, and greatly improved cycle performance.

[0032] 4、The prepared ferrous hexafluorosilicate as the negative material of the lithium ion battery can effectively alleviate the formation of irreversible lithium fluorosilicate, thereby facilitating the embedding and extraction of lithium ions, better alleviating the volume expansion caused in the charging and discharging process, maintaining the stability of the pole piece structure, greatly improving the electrochemical performance of the lithium ion battery, and improving the specific capacity of the battery due to the synergistic effect of multiple metals, and can well reduce the production cost of lithium ions.

[0033] 5、The prepared ferrous hexafluorosilicate can be used as an additive of silicon negative material, thereby alleviating the volume expansion problem of the silicon negative electrode, and also has the advantages of enhancing the mechanical stability, increasing the surface reactivity, improving the energy density, improving the electrical conductivity, and optimizing the interface structure. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 The reaction process diagram for preparing ferrous hexafluorosilicate in Example 1.

[0035] Figure 2 The XRD diagram of the ferrous hexafluorosilicate prepared in Example 1 and Fe 25 Si 75 alloy powder.

[0036] Figure 3 The SEM diagram of the ferrous hexafluorosilicate prepared in Example 1.

[0037] Figure 4 The SEM diagram of the ferrous hexafluorosilicate prepared in Example 1 and Fe 25 Si 75 alloy powder.

[0038] Figure 5 The TEM diagram of the ferrous hexafluorosilicate prepared in Example 1.

[0039] Figure 6 The HRTEM diagram of the ferrous hexafluorosilicate prepared in Example 1.

[0040] Figure 7 The element mapping diagram of the ferrous hexafluorosilicate prepared in Example 1, wherein, Figure 6 (a) is the element mapping diagram of F, Fe, and Si elements; Figure 6 (b) is the element mapping diagram of F element; Figure 6 (c) is the element mapping diagram of Fe element; Figure 6 (d) is the element mapping diagram of Si element.

[0041] Figure 8 Cyclic voltammogram of ferrous hexafluorosilicate prepared in Example 1.

[0042] Figure 9 Cycling stability plot of ferrous hexafluorosilicate prepared in Example 1.

[0043] Figure 10 SEM image of silicon-iron composite prepared for Example 6.

[0044] Figure 11 TEM image of silicon-iron composite prepared for Example 6.

[0045] Figure 12 Cyclic voltammogram of silicon-iron composite prepared for Example 4.

[0046] Figure 13 Cyclic voltammogram of silicon-iron composite prepared for Example 5.

[0047] Figure 14 Cyclic voltammogram of silicon-iron composite prepared for Example 6.

[0048] Figure 15 Cyclic voltammogram of silicon-iron alloy Fe 25 Si 75 Comparison of electrode sheet cross-section before and after 200 cycles of charge.

[0049] Figure 16 Cyclic voltammogram of silicon-iron alloy Fe 25 Si 75 Comparison of electrode sheet cross-section before and after 200 cycles of charge.

[0050] Figure 17 Cyclic voltammogram of silicon-iron alloy Fe 25 Si 75 XPS spectra at different cycle numbers. Wherein, Figure 17 (a) is the C 1s high-resolution XPS spectrum; Figure 17 (b) is the Si 1s high-resolution XPS spectrum; Figure 17 (c) is the Li 1s high-resolution XPS spectrum.

[0051] Figure 18 XPS spectra of silicon-iron composite prepared for Example 6 at different cycle numbers. Wherein, Figure 18 (a) is the C 1s high-resolution XPS spectrum; Figure 18 (b) is the Si 1s high-resolution XPS spectrum; Figure 18 (c) is the Li 1s high-resolution XPS spectrum.

[0052] Figure 19Cyclic stability performance diagrams of the silicon-iron composite materials prepared in Examples 4-6. Detailed Implementation

[0053] The present invention will now be described in detail with reference to specific embodiments.

[0054] Example 1

[0055] Take Fe 25 Si 75 1g of alloy powder was placed in a plastic beaker, and 30mL of 1M hydrofluoric acid was added to the beaker. The beaker was sealed with plastic wrap and placed at 40℃ for 24h to allow the reaction to be as complete as possible. After the reaction was completed, the excess impurities were removed by filtration. The filtrate was poured into a petri dish and placed in an 80℃ oven for vacuum drying for 24h to obtain 1.45g of light green crystals of FeSiF6.

[0056] The process for preparing FeSiF6 in this embodiment is as follows: Figure 1 As shown, from Figure 1 It can be observed that a small amount of bubbles are generated in the beaker after the addition of hydrofluoric acid. After 12 hours of soaking reaction, Fe can be clearly observed. 25 Si 75 The particles almost disappeared, and the solution changed from clear and transparent to a light green liquid, which then formed light green crystals after drying.

[0057] For Fe 25 Si 75 The alloy powder and the FeSiF6 crystal prepared in this embodiment were subjected to X-ray diffraction analysis, and the obtained XRD patterns are shown below. Figure 2 As shown. By Figure 2 (a) It can be seen that Fe 45 Si 55 It consists of FeSi and FeSi2 phases. (Comparison) Figure 2 (a) and Figure 2 (b) It was found that Fe 25 Si 75 After the alloy powder was soaked in HF, the FeSi and FeSi2 phases almost completely disappeared and were completely converted into FeSiF6. The characteristic peaks of the synthesized FeSiF6 index almost matched the standard data (PDF#26-0799) of the original FeSiF6·6H2O.

[0058] according to Figure 2 The results of the test analysis and Figure 1 Based on the phenomena observed during the reaction process, it can be inferred that the following reactions occurred during the soaking process:

[0059] FeSi + 6HF = FeSiF6 + H2↑

[0060] FeSi2+6HF=FeSiF6+FeSiF6+2H2↑

[0061] Therefore, this preparation method has fewer limitations on ferrosilicon alloys, resulting in a wider variety of alloys. This method significantly reduces the preparation cost of ferrous hexafluorosilicate, simplifies the preparation steps, and produces samples with extremely high purity, offering substantial advantages.

[0062] The FeSiF6 crystal prepared in this embodiment was scanned using a scanning electron microscope, and the resulting SEM image is shown below. Figure 3 As shown, from Figure 3 As can be seen from (a) and 3(b), the prepared FeSiF6 structures are all hexagonal prisms with a width of approximately 2 μm and relatively smooth sides. High-resolution imaging further confirms their superior quality. Figure 3 (c) As can be seen, a layered structure exists at the top, presumably formed during the crystallization process. Furthermore, transmission electron microscopy image 3(c) also shows the columnar structure of FeSiF6, with an outer layer encapsulated by a transparent layer of crystal water. This suggests that FeSiF6 undergoes continuous growth during the evaporation and crystallization process. (FeSiF6 is a silicon-iron alloy.) 25 Si 75 SEM image as follows Figure 4 As shown, by Figure 4 It can be seen that the silicon-iron alloy Fe 25 Si 75 The particles are irregular and their distribution is not uniform. Therefore, FeSiF6 prepared by this method has a uniform morphology and is not affected by the original morphology of the ferrosilicon alloy.

[0063] The FeSiF6 crystal prepared in this embodiment was scanned using a transmission electron microscope, and the resulting TEM image is shown below. Figure 5 As shown, by Figure 5 (b) shows the hollow structure and non-uniform distribution of the prepared FeSiF6 crystals. The presence of the hollow structure may indicate that Li is present in the electrode material. + It provides an embedded space, which makes its performance more stable.

[0064] The FeSiF6 crystal prepared in this embodiment was scanned using a high-resolution transmission electron microscope (HRTEM), and the obtained HRTEM images are shown below. Figure 6 As shown, Figure 6 The particles show clear lattice fringes. The interplanar spacing at three locations in the image was measured, and the measured lattice fringes were 0.42 nm, 0.38 nm, and 0.33 nm, respectively. The lattice fringes are assigned to FeSiF6 (space group). Space Group Space Group (012), (021), (202) crystal faces of FeSiF6.

[0065] Element mapping analysis was performed on the FeSiF6 crystal prepared in the example, and the obtained element mapping diagram is shown in Figure 7 From 7(a), it can be seen that F, Fe and Si elements are uniformly distributed in the columnar structure of FeSiF6. By comparing Figure 7 (b), Figure 7 (c) and Figure 7 (d), it is found that the point position of Si element is less, which may be related to its molecular structure. Si is in the inside, and the signal is relatively weak.

[0066] The FeSiF6 crystal prepared in the example was subjected to cyclic voltammetry test by using two electrode system: FeSiF6 as working electrode, lithium sheet as counter electrode and LB-1016 as electrolyte, and the scanning rate was 0.5 mV / s. The obtained cyclic voltammetry curve is shown in Figure 8 From Figure 8 it can be seen that the FeSiF6 prepared in the example has two pairs of reversible redox peaks, and in the repeated scanning cycle, the redox peak does not shift obviously, which indicates that the FeSiF6 prepared in the example has good stability and reversibility.

[0067] The FeSiF6 crystal prepared in the example was subjected to cyclic stability test by using two electrode system in the voltage range of 0-3V and the current density of 1A / g (200 cycles). The obtained cyclic stability performance diagram is shown in Figure 9 From Figure 9 it can be seen that the capacitance of FeSiF6 remains basically unchanged after 200 cycles, which indicates that the FeSiF6 prepared in the example has excellent cyclic stability.

[0068] Example 2

[0069] Fe 45 Si 55 powder 1.5g was taken in a plastic beaker, 30mL of 1M hydrofluoric acid was taken in the beaker, the beaker was sealed with plastic wrap, and the beaker was placed in a 40℃ reaction for 24h to make the reaction as complete as possible. After the reaction was completed, the excess impurities were removed by filtration, the filtrate was poured into a culture dish, and the culture dish was placed in an 80℃ oven for vacuum drying for 24h to obtain light green crystals FeSiF6.

[0070] Example 3

[0071] Fe 30 Si 701.5 g powder in a plastic beaker, take 30 mL of 1M hydrofluoric acid in the beaker, seal the beaker with plastic wrap, place the beaker in a 40°C oven for 24 h, let the reaction be as complete as possible, after the reaction is completed, remove the excess impurities by filtration, pour the filtrate into a culture dish, put the culture dish into an 80°C oven, vacuum dry for 24 h, get light green crystals FeSiF6.

[0072] Example 4

[0073] S1, take 0.6 g of Fe 25 Si 75 1.5 g powder in a plastic beaker, take 30 mL of 1M hydrofluoric acid in the beaker, seal the beaker with plastic wrap, place the beaker in a 40°C oven for 24 h, let the reaction be as complete as possible, after the reaction is completed, pour the resulting mixed product into a culture dish, put the culture dish into an 80°C oven, vacuum dry for 24 h, get light green solid.

[0074] S2, take 0.6 g of light green solid and 0.4 g of graphite (high energy density) in a ball mill jar, and put 40 g of 3 mm stainless steel balls and 10 g of 1.5 mm stainless steel balls, ball mill in high-energy ball mill SPEX 8000M for 30 min, wash the resulting powder with ethanol, filter, vacuum dry the filter cake in an 80 oven for 6 h, get iron-silicon composite Fe-Si-T@C.

[0075] The iron-silicon composite Fe-Si-T@C prepared in this example is subjected to cyclic voltammetry test using a two-electrode system, and the resulting cyclic voltammogram is as shown in Figure 12 From the figure, it can be seen that in addition to the silicon negative electrode located at 0.34V and 0.53V lithium alloying peak and 0.15V delithiation peak, there is also a reaction peak of FeSiF6, which is located at 1.52 and 2.09 lithium peak and 1.49 delithiation peak, and it can be seen that the peak value is stable, without obvious change.

[0076] Example 5

[0077] S1, take 0.6 g of Fe 25 Si 75 alloy powder, 0.4 g of graphite (high energy density) in a ball mill jar, and put 40 g of 3 mm stainless steel balls and 10 g of 1.5 mm stainless steel balls, ball mill in high-energy ball mill SPEX 8000M for 30 min, wash the resulting powder with ethanol, filter, vacuum dry the filter cake in an 80 oven for 6 h, get carbon-doped silicon-iron alloy.

[0078] S2, carbon-doped Fe 25 Si 75The powder is added into a plastic beaker, 30 mL of 1 M hydrofluoric acid is taken in the beaker, the beaker is sealed with a plastic wrap, and the beaker is placed in a 40 °C oven for 24 h to allow the reaction to be as complete as possible. After the reaction is completed, the excess impurities are removed by filtration, the filtrate is poured into a culture dish, and the culture dish is placed in an 80 °C oven for vacuum drying for 24 h to obtain the iron-silicon composite Fe-Si@C-T.

[0079] The iron-silicon composite Fe-Si@C-T prepared in this example is subjected to cyclic voltammetry test by using a two-electrode system. The obtained cyclic voltammogram is shown in Figure 13 The silicon reaction peak position is unchanged and more obvious, and compared with Figure 12 , there is a slight shift in the peak value, which is mainly caused by the different diffusion rates of ions (such as lithium ions) at a higher current density, but there is also a more obvious peak value.

[0080] Example 6

[0081] 0.3 g of Fe 25 Si 75 alloy powder, 0.3 g of FeSiF6 powder and 0.4 g of graphite (high energy density) are taken in a ball milling jar, and 40 g of 3 mm stainless steel balls and 10 g of 1.5 mm stainless steel balls are put in. Ball milling is carried out in a high-energy ball mill SPEX 8000M for 30 min. The obtained powder is washed with ethanol, filtered, and the filter cake is vacuum dried in an 80 °C oven for 6 h to obtain the iron-silicon composite Mbmc.

[0082] The iron-silicon composite Mbmc prepared in this example is scanned by a scanning electron microscope. The obtained TEM image is shown in Figure 10 From the figure, it can be seen that FeSiF6 exists, and in addition, it can be seen that the graphite mainly exists in a laminar structure and sandwiches the raw material Fe 25 Si 75 . After ball milling, part of the FeSiF6 is broken and mixed in the graphite sandwich.

[0083] The iron-silicon composite Mbmc prepared in this example is scanned by a transmission electron microscope. The obtained SEM image is shown in Figure 11 The graphite in the figure is more obvious and exists in a laminar structure, covering other substances.

[0084] The iron-silicon composite Mbmc prepared in this example is subjected to cyclic voltammetry test by using a two-electrode system: the iron-silicon composite Mbmc is used as the working electrode, lithium is used as the counter electrode, and LB-1016 is used as the electrolyte, and the scanning rate is 0.5 mV / s. The obtained cyclic voltammogram is shown in Figure 14 The silicon reaction peak position is unchanged and more obvious, and compared with Figure 14It can be seen that there is a reversible peak at 1.47 V, and according to the pure phase, there is also a peak near 0.92 V, and the corresponding oxidation peak in the oxidation process appears at 2.02 V and 1.40 V. Through the study of FeOF by Chevrier et al., it is speculated that the two pairs of reversible peaks correspond to the following two electrochemical processes:

[0085]

[0086]

[0087] In the first process, Li + inserts into the structure of FeSiF6, without destroying the structure of FeSiF6, part of Fe 2+ in FeSiF6 is oxidized to Fe 3+ . In the second process, when the number of Li + ions embedded in the structure of FeSiF6 continues to increase, Li + continues to embed in the structure of FeSiF6 to convert more Fe 3+ to Fe 2+ , until all Fe 3+ is completely converted to Fe 2+ . And in this curve, no other element is found to have a significant change in valence, which may be that other elements are relatively stable in this voltage range.

[0088] The cross-section of the electrode sheet of the ferrosilicon alloy Fe 25 Si 75 before and after 200 cycles of charge and discharge in a two-electrode system at a voltage range of 0-3 V and a current density of 1 A / g is shown in Figure 15 From Figure 15 it can be seen that the ferrosilicon alloy Fe 25 Si 75 has a serious volume expansion after 200 cycles, with a volume expansion of 3.57 times.

[0089] The cross-section of the electrode sheet of the ferrosilicon composite material Mbmc prepared in this embodiment before and after 200 cycles of charge and discharge in a two-electrode system at a voltage range of 0-3 V and a current density of 1 A / g is shown in Figure 16 From Figure 16 it can be seen that the volume of the ferrosilicon composite material Mbmc has hardly changed.

[0090] To detect whether the effect of FeSiF6 on the silicon-based negative electrode is directly related to the improved SEI chemical properties, the ferrosilicon alloy Fe 25 Si 75XPS analysis was performed on both the iron-silicon composite material Mbmc and the silicon-iron alloy before and after different cycles of charge-discharge. 25 Si 75 XPS spectra as follows Figure 17 As shown, the XPS spectrum of the iron-silicon composite material Mbmc is as follows. Figure 18 As shown. Figure 17 a and 18a show the C1s spectra of both materials after different cycles. Before cycling, the iron-silicon composite material Mbmc showed dominant peaks in the C1s spectrum belonging to CO, C=O, CH, and CC, exhibiting typical characteristics of organic solvent molecules, mainly graphite, carbon black, and various C bonds in the CMC binder. The presence of CF bonds was also observed. Figure 18 As shown in Figure a, after 50 cycles, the CC / CH bond at 284.8 eV increased, mainly due to the decomposition of organic components in the electrolyte during SEI film formation, leading to enhanced peak intensity. Furthermore, after 50 cycles, ROCO2Li (where R represents an organic group, i.e., a residue or group of an organic molecule; this R can represent a long carbon chain, cyclic structure, or other carbon-based structure) and LiCO3 (at 289.6 eV) appeared in C1s, which are among the main components of the SEI film. Meanwhile, Fe... 25 Si 75 The high content of ROCO2Li and LiCO3 in the material indicates a thicker SEI film, while the thinner SEI film in the Mbmc material contributes to its more stable performance. After 200 cycles, Fe... 25 Si 75 The peak intensities of ROCO2Li and LiCO3 in the composite material decreased, while the Mbmc of the composite material was enhanced. It is speculated that this is because Fe... 25 Si 75 The peak intensity fluctuates because the SEI film breaks down and regenerates due to volume expansion. The SEI film toughness of Mbmc is enhanced by the addition of FeSiF6. During cycling, the volume expansion caused by silicon material promotes the gradual accumulation of its SEI film thickness.

[0091] The detection results for Si 1s are as follows Figure 17 As shown in b and 18b, both materials exhibit Si-Si, Si-Fe, Si-C, and SiO2 bonds at 99.4 eV, 99.8 eV, 102 eV, and 103.8 eV in the initial stage. However, after cycling, these bonds all transform into SiO2 bonds at ~101 eV. x And Li at 102 eV x SiO y Li x SiO yThe formation of Fe is mainly due to the reaction of Si-O bonds on the surface participating in the SEI film, while during cycling, Fe was found to... 25 Si 75 Peaks representing Si-Si / Si-Fe bonds were detected in the electrode at both 150 and 200 cycles, while the peak values ​​for FeSiF6 and Mbmc were more stable, presumably mainly due to the presence of Fe. 25 Si 75 The SEI film of the electrode was damaged due to volume expansion, which led to the detection inside the material, a result that is consistent with previous results.

[0092] To verify whether there are any errors in the above detection results, Fe... 25 Si 75 The Li 1s of both Mbmc and other materials were also tested, and the results are as follows: Figure 17 As shown in c and 18c, both materials exhibit the same peak positions, indicating that the SEI films of the two materials are nearly identical in terms of Li content, mainly composed of Li₂O at 54.3 eV, ROCO₂Li and LiCO₃ at 54.8 eV, and LiF at 55.7 eV. Furthermore, it can be seen that the initial Fe... 25 Si 75 The content of ROCO2Li and LiCO3 in the electrode is relatively high, and the peak value fluctuates in subsequent cycles. It is speculated that this may be due to uneven SEI film thickness or SEI film rupture. Based on the previous test results, the most likely result is that the volume expansion of the silicon anode caused the SEI film to rupture.

[0093] The iron-silicon composite materials prepared in Examples 4-6 were subjected to cyclic stability tests (200 cycles) using a two-electrode system with a voltage range of 0-3V and a current density of 1A / g. The obtained cyclic stability performance diagrams are shown below. Figure 19 As shown, by Figure 19 It can be seen that the cycling performance of the three iron-silicon composite materials becomes very stable. Fe-Si@CT and Mbmc show almost no capacity decay, and at 1A / g, they still retain discharge specific capacities of 685.7 mAh / g and 974.9 mAh / g respectively after 200 cycles, with capacity retention rates both above 94%, which is consistent with the tested rate performance. Fe-Si-T@C is relatively worse, but it still retains nearly 52% capacity retention rate after 200 cycles.

Claims

1. A process for the preparation of ferrous hexafluorosilicate, characterized in that It comprises the following steps: Silicon-iron alloy Fe x Si y Etching reaction with hydrofluoric acid, after the reaction is completed, filter, dry the filtrate, get light green crystal ferrous hexafluorosilicate.

2. The use of ferrous hexafluorosilicate prepared by the method of claim 1 in the preparation of a negative material for lithium ion batteries.

3. A method of producing a ferrosilicon composite material, characterized by It comprises the following steps: Ferrosilicon alloy Fe x Si y The mixture is etched with hydrofluoric acid. After the reaction is complete, it is filtered and the filtrate is dried to obtain light green crystalline ferrous hexafluorosilicate. Ferrous hexafluorosilicate and graphite are ball-milled, and the resulting powder is washed with ethanol, filtered, and the filter cake is dried to obtain the iron-silicon composite material. or a silicon-iron alloy Fe x Si y and graphite are ball milled, the obtained powder is washed with ethanol, filtered, and the filter cake is dried to obtain a carbon-doped silicon-iron alloy, the carbon-doped silicon-iron alloy is subjected to etching reaction with hydrofluoric acid, after the reaction is completed, filtration is performed, and the filtrate is dried to obtain the iron-silicon composite material; or a ferrosilicon alloy Fe x Si y , ferrous hexafluorosilicate and graphite are ball-milled, the obtained powder is washed with ethanol, filtered, and the filter cake is dried to obtain the ferrosilicon composite material.

4. The method for producing ferrous hexafluorosilicate according to claim 1 or the method for producing iron-silicon composite material according to claim 3, characterized by: The silicon-iron alloy Fe x Si y wherein x+y=100.

5. The method for producing ferrous hexafluorosilicate according to claim 1 or the method for producing iron-silicon composite material according to claim 3, characterized by: The concentration of the hydrofluoric acid is 1-5M.

6. The method for producing ferrous hexafluorosilicate according to claim 1 or the method for producing iron-silicon composite material according to claim 3, characterized by: The silicon-iron alloy Fe x Si y The mass ratio of hydrogen fluoride is 5:1-4.

7. The method for producing ferrous hexafluorosilicate according to claim 1 or the method for producing iron-silicon composite material according to claim 3, characterized by: The temperature of the etching reaction is 20-40℃, and the reaction time is 24-36h.

8. The method for producing ferrous hexafluorosilicate according to claim 1 or the method for producing iron-silicon composite material according to claim 3, characterized by: The etching reaction is carried out in a closed condition.

9. The method for producing ferrous hexafluorosilicate according to claim 1 or the method for producing iron-silicon composite material according to claim 3, characterized by: The filtrate is vacuum dried at 60-80℃.

10. The use of the iron-silicon composite material prepared by the method of claim 3 in the preparation of a silicon negative material for lithium ion batteries.

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