Amorphous FeP2 and its preparation method and application

Abstract

The present invention belongs to the technical field of lithium battery electrode materials and discloses an amorphous FeP2, a preparation method, and an application thereof. The preparation method is as follows: an iron salt, urea, and water are subjected to a hydrothermal reaction to obtain an iron source precursor; and the iron source precursor and a phosphorus source are subjected to a phosphating reaction in a mixed gas to obtain amorphous FeP2. The resulting amorphous FeP2 electrode has abundant unsaturated coordination bonds, providing more active sites and ion diffusion channels, which are conducive to the diffusion and charge transfer of lithium ions, and exhibits superior cycle stability and rate performance. When charged and discharged at a high current density of 2A / g, the reversible specific capacity after 1000 cycles is 640.5mAh / g, and the capacity loss rate is only 18.4%.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium battery electrode materials, and in particular to amorphous FeP2 and a preparation method and application thereof. Background Art

[0002] In recent years, China has vigorously developed green energy to replace oil and other traditional energy sources. Clean, renewable resources such as solar, wind, tidal, and nuclear energy have been vigorously developed to reduce carbon emissions and address the global warming crisis. Rechargeable lithium-ion batteries (LIBs), as the driving force of new energy vehicles, are one of the effective measures to reduce carbon emissions due to their zero CO2 tailpipe emissions. Lithium-ion batteries are large-capacity, stable rechargeable batteries with excellent properties such as low pollution, high energy density, long cycle life, and high rate capability. They have become a major focus of global energy development.

[0003] Compared to intercalation-type carbon anode materials, metal phosphides can store energy through intercalation and conversion reactions, resulting in excellent specific capacity. Furthermore, iron and phosphorus are abundant and easily mined. Therefore, economical iron phosphide is a highly promising anode material for high-performance lithium-ion batteries. Currently, six types of iron phosphides are known: Fe4P, Fe3P, Fe2P, FeP, FeP2, and FeP4, each with distinct electronic and physicochemical properties. The theoretical specific capacitance of iron phosphide increases with increasing phosphorus content, while the electrical conductivity decreases. To date, Fe2P and FeP have been extensively studied in LIBs. FeP2 has a theoretical specific capacity of up to 1365 mAh / g, exceeding that of Fe2P (563 mAh / g) and FeP (926 mAh / g), and significantly exceeding that of traditional graphite anodes (372 mAh / g). However, FeP2 exhibits significant volume changes during lithiation / delithiation, which can lead to irreversible side reactions such as material pulverization, resulting in poor battery cycling stability.

[0004] Amorphous materials have the characteristics of long-range disorder, short-range order, and abundant structural defects, and are a new strategy to effectively improve the structural stability of materials. During the lithium extraction / insertion process, the expansion rate of amorphous materials in all directions is consistent, which can maintain the high strain of the material and improve the structural stability of the amorphous material. The amorphous state leads to an increase in the MP bond length and a decrease in the coordination number, and produces a large number of active sites and ion diffusion channels, accelerating ion diffusion and charge transfer. In addition, the Gibbs free energy change between amorphous materials and Li is lower than that of their crystalline state, and the reversibility of the conversion reaction of amorphous materials is stronger. Therefore, amorphous materials exhibit better performance in lithium battery electrodes than crystalline materials.

[0005] Compared with the amorphous phase synthesis of iron oxide or iron sulfide, the synthesis of amorphous FeP2 nanomaterials is more difficult, and its main challenge is the low-temperature phosphating process. First, the high temperature of the FeP2 phosphating process is inconsistent with the low temperature of the preparation of amorphous materials. Phosphorus vapor can be used to prepare FeP2 nanomaterials, but because phosphorus molecules are inert in the gas phase, the phosphating reaction requires high temperature (>500°C). However, amorphous FeP2 materials are also not suitable for high-temperature preparation. Higher preparation temperatures will cause amorphous FeP2 materials to be converted into crystalline FeP2 materials. Secondly, low-temperature phosphating requires a phosphorus source with high reactivity (such as PH3), but PH3 is a highly toxic gas that can seriously endanger the life and health of scientific researchers. For the above reasons, there is currently no low-temperature preparation method for amorphous FeP2 materials.

[0006] Therefore, it is of great significance to study and obtain an amorphous FeP2 with high energy density, high stability, high conductivity and small volume expansion during charging and discharging, as well as its preparation method and application. Summary of the Invention

[0007] In view of this, the present invention provides an amorphous FeP2 and a preparation method and application thereof, the purpose of which is to solve the problems of high preparation temperature and use of highly toxic PH3 gas in the prior art.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for preparing amorphous FeP2, comprising the following steps:

[0010] (1) hydrothermally reacting iron salt, urea and water to obtain an iron source precursor;

[0011] (2) The iron source precursor and the phosphorus source are subjected to a phosphating reaction in a mixed gas to obtain amorphous FeP2.

[0012] Preferably, the iron salt is ferric chloride hexahydrate.

[0013] Preferably, the usage ratio of the iron salt, urea and water is 1 mol: 1-3 mol: 5-10 L.

[0014] Preferably, in step (1), the temperature of the hydrothermal reaction is 100-120° C., and the time of the hydrothermal reaction is 6-8 hours.

[0015] Preferably, in step (2), the phosphorus source is sodium hypophosphite; and the mass ratio of the iron source precursor to the phosphorus source is 1:10-25.

[0016] Preferably, in step (2), the phosphating reaction is carried out in a tubular furnace; the iron source precursor is located in the middle and downstream of the tubular furnace, the phosphorus source is located in the upstream of the tubular furnace, and the distance between the iron source precursor and the phosphorus source is 1 to 5 cm.

[0017] Preferably, in step (2), the mixed gas is a mixed gas of hydrogen and argon, the flow rate of the mixed gas is 400-500 cc / min, and the volume ratio of the hydrogen to argon is 10-20:80-90.

[0018] Preferably, in step (2), the heating rate of the phosphating reaction is 2-4°C / min, the temperature of the phosphating reaction is 200-300°C, and the time of the phosphating reaction is 4-8h.

[0019] The present invention also provides amorphous FeP2 prepared by the method for preparing amorphous FeP2.

[0020] The present invention also provides the application of the amorphous FeP2 in lithium ion batteries.

[0021] It can be seen from the above technical solutions that compared with the prior art, the present invention has the following beneficial effects:

[0022] (1) The present invention comprehensively regulates parameters such as the phosphorus source supply, hydrogen-argon mixed atmosphere, phosphating temperature, and phosphating time. The hydrogen content in the mixed gas increases the activity of P in the phosphorus source, avoids the use of highly toxic PH3 gas, and thus realizes the regulation of the phosphating reaction energy barrier; the phosphorus source supply is used to increase the P reaction amount, and the gas flow rate and the distance between the two porcelain boats are combined to regulate the supply efficiency of P atoms in the phosphating process; and then a lower heating rate, a lower phosphating temperature, and a longer phosphating time are used to perform low-crystallinity phosphating, thereby avoiding the formation of crystalline materials due to excessively high phosphating temperatures, thereby realizing low-temperature phosphating to prepare amorphous FeP2 materials.

[0023] (2) Compared with crystalline FeP2 material, the amorphous FeP2 material prepared by the present invention has the following advantages:

[0024] Amorphous FeP2 materials have lower potential barriers, and the Gibbs free energy change of the reaction with Li is lower than that of crystalline FeP2 materials. The reversibility of the conversion reaction of amorphous FeP2 materials is stronger.

[0025] Amorphous FeP2 materials have abundant unsaturated coordination bonds, which provide more active sites and ion diffusion channels, which are beneficial to the diffusion and charge transfer of lithium ions.

[0026] The isotropic properties of amorphous FeP2 materials are conducive to maintaining high volume strain in all directions, inhibiting the volume deformation of amorphous FeP2 materials and improving structural stability and cycle stability;

[0027] The amorphous FeP2 material has extremely low crystallinity, exhibits amorphous characteristics, and has excellent electrochemical properties. It has a high reversible specific capacity and stable cycle rate performance, providing more solutions for the application of amorphous FeP2 as a negative electrode material in lithium-ion batteries.

[0028] The amorphous FeP2 electrode exhibits better cycle stability and rate performance. When charged and discharged at a high current density of 2A / g, the reversible specific capacity can reach 640.5mAh / g after 1000 cycles, and the capacity loss rate is as low as 18.4%. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0030] Figure 1 XRD patterns of the amorphous FeP2 material obtained in Examples 1 and 2 and the crystalline FeP2 material obtained in Comparative Example 1;

[0031] Figure 2 The SEM images of the amorphous FeP2 material obtained in Example 1 at different magnifications are shown;

[0032] Figure 3 CV curve of the amorphous FeP2 electrode obtained in Example 1;

[0033] Figure 4 Graph showing the cycling performance of the amorphous FeP2 electrode obtained in Examples 1 and 2 and the crystalline FeP2 electrode obtained in Comparative Example 1;

[0034] Figure 5 Graph showing the rate performance of the amorphous FeP2 electrode obtained in Example 1 and the crystalline FeP2 electrode obtained in Comparative Example 1;

[0035] Figure 6 Electrochemical impedance spectroscopy (EIS) of the amorphous FeP2 electrode obtained in Example 1 and the crystalline FeP2 electrode obtained in Comparative Example 1.

[0036] Figure 7 CV curve of the amorphous FeP2 electrode obtained in Example 1 at a scanning rate of 0.1 to 2 mV / s;

[0037] Figure 8 The corresponding log(i) vs. log(v) diagram of the amorphous FeP2 electrode obtained in Example 1;

[0038] Figure 9This is an estimated graph of the pseudocapacitive contribution of the amorphous FeP2 electrode obtained in Example 1 at a scan rate of 2.0 mV / s;

[0039] Figure 10 This is a graph showing the contribution of the pseudocapacitance and diffusion control effect of the amorphous FeP2 electrode obtained in Example 1 to the total charge storage. DETAILED DESCRIPTION

[0040] The present invention provides a method for preparing amorphous FeP2, comprising the following steps:

[0041] (1) hydrothermally reacting iron salt, urea and water to obtain an iron source precursor;

[0042] (2) The iron source precursor and the phosphorus source are subjected to a phosphating reaction in a mixed gas to obtain amorphous FeP2.

[0043] In the present invention, the iron salt is preferably ferric chloride hexahydrate.

[0044] In the present invention, the usage ratio of the iron salt, urea and water is preferably 1 mol: 1-3 mol: 5-10 L, more preferably 1 mol: 1.5-2.5 mol: 6-8.5 L, and more preferably 1 mol: 1.8-2 mol: 6.5-7.5 L.

[0045] In the present invention, in step (1), the iron salt, urea and water are mixed before the hydrothermal reaction. The mixing time is preferably 20 to 40 minutes, more preferably 25 to 35 minutes, and more preferably 30 minutes. The mixing speed is preferably 450 to 550 rpm, more preferably 460 to 520 rpm, and more preferably 480 to 500 rpm.

[0046] In the present invention, in step (1), the temperature of the hydrothermal reaction is preferably 100-120°C, more preferably 105-118°C, more preferably 110-116°C, and the time of the hydrothermal reaction is preferably 6-8h, more preferably 6.5-7.5h, more preferably 7h.

[0047] In the present invention, in the step (1), cooling, washing and drying are carried out in sequence after the hydrothermal reaction; the cooling temperature is preferably 20-30°C, more preferably 22-28°C, and more preferably 24-26°C; the washing is performed by alternating deionized water and anhydrous ethanol as one wash, the number of washes is preferably 5-7 times, more preferably 6 times, the washing is preferably centrifugal washing, the rotation speed of the centrifugal washing is preferably 4500-5500rpm, more preferably 4600-5200rpm, more preferably 4800-5000rpm, the time for each washing with deionized water and anhydrous ethanol is preferably 8-12min, more preferably 9-11min, and more preferably 10min; the drying temperature is preferably 100-120°C, more preferably 105-115°C, more preferably 110-112°C, and the drying time is preferably 10-14h, more preferably 11-13h, and more preferably 12h.

[0048] In the present invention, in step (2), the phosphorus source is preferably sodium hypophosphite; the mass ratio of the iron source precursor to the phosphorus source is preferably 1:10-25, more preferably 1:14-20, and even more preferably 1:16-20.

[0049] In the present invention, in step (2), the phosphating reaction is carried out in a tubular furnace; the iron source precursor and the phosphorus source are respectively placed in two porcelain boats, the iron source precursor is located in the middle and downstream of the tubular furnace, and the phosphorus source is located in the upstream of the tubular furnace. The distance between the two porcelain boats is preferably 1 to 5 cm, more preferably 2 to 4 cm, and more preferably 3 cm.

[0050] In the present invention, in step (2), the mixed gas is preferably a mixed gas of hydrogen and argon, and the flow rate of the mixed gas is preferably 400-500cc / min, more preferably 420-480cc / min, and more preferably 450-460cc / min. The volume ratio of hydrogen to argon is preferably 10-20:80-90, more preferably 12-18:82-88, and more preferably 15-16:84-85.

[0051] In the present invention, in the step (2), the heating rate of the phosphating reaction is preferably 2-4°C / min, more preferably 2.2-3.5°C / min, more preferably 2.5-3.0°C / min, the temperature of the phosphating reaction is preferably 200-300°C, more preferably 230-290°C, more preferably 260-280°C, and the time of the phosphating reaction is preferably 4-8h, more preferably 5-7h, more preferably 5.5-6h.

[0052] The present invention also provides amorphous FeP2 prepared by the method for preparing amorphous FeP2.

[0053] The present invention also provides the application of the amorphous FeP2 in lithium ion batteries.

[0054] In the present invention, the amorphous FeP2 is preferably used as an electrode in a lithium-ion battery. The amorphous FeP2 is used in a lithium-ion half-cell comprising the following steps:

[0055] 1) mixing amorphous FeP2, acetylene black, carboxymethyl cellulose and water to obtain an electrode slurry;

[0056] 2) coating the obtained electrode slurry on the surface of nickel foam and drying it to obtain an amorphous FeP2 electrode;

[0057] 3) A CR2032 button cell was assembled in an argon-filled glove box using an amorphous FeP2 electrode as the positive electrode, a lithium sheet as the negative electrode, a Gelgard 2400 membrane, and a 1 mol / L LiPF6 solution as the electrolyte (the solvent was a mixed solution of ethyl carbonate, dimethyl carbonate, and ethyl carbonate in a volume ratio of 1:1:1).

[0058] In the present invention, the mass ratio of the amorphous FeP2, acetylene black, carboxymethyl cellulose and water is preferably 8:1:1:100.

[0059] In the present invention, the loading amount of the electrode slurry on the surface of the nickel foam is preferably 0.6 to 1.4 mg / 115 cm 2 , more preferably 0.8 to 1.2 mg / 115 cm 2 , more preferably 0.9 to 1.0 mg / 115 cm 2 The drying temperature is preferably 80°C, and the drying time is preferably 12 hours.

[0060] The technical solutions provided by the present invention are described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0061] Example 1

[0062] 0.004 mol of ferric chloride hexahydrate, 0.012 mol of urea and 40 mL of deionized water were mixed at a speed of 500 rpm for 0.5 h to form a clear solution. The clear solution was transferred to a 100 mL polytetrafluoroethylene-lined reactor and heated to 120° C. in a blower for 6 h. The mixture was cooled to 25° C. and washed alternately by centrifugation with deionized water and anhydrous ethanol. The deionized water and anhydrous ethanol were washed 6 times each (the speed of the centrifugal washing was 5000 rpm and the time of each centrifugation was 10 min). The mixture was dried in a blower oven at 110° C. for 12 h to obtain an iron source precursor.

[0063] The iron source precursor and NaH2PO2 with a mass ratio of 1:20 are placed in two porcelain boats respectively. The porcelain boat where the iron source precursor is located is located in the downstream of the tube furnace, and the porcelain boat where the NaH2PO2 is located is located in the upstream of the tube furnace. The two porcelain boats are 2 cm apart. A mixed gas (the volume ratio of hydrogen to argon is 20:80) is introduced at a gas flow rate of 500 cc / min, and the temperature is raised to 260°C at a heating rate of 2.5°C / min. After phosphating at 260°C for 4 hours, amorphous FeP2 material can be obtained.

[0064] Example 2

[0065] The preparation steps of the iron source precursor are the same as those in Example 1;

[0066] The iron source precursor and NaH2PO2 with a mass ratio of 1:15 are placed in two porcelain boats respectively. The porcelain boat where the iron source precursor is located is located in the downstream of the tubular furnace, and the porcelain boat where the NaH2PO2 is located is located in the upstream of the tubular furnace. The two porcelain boats are 3 cm apart. A mixed gas (the volume ratio of hydrogen to argon is 15:85) is introduced at a gas flow rate of 400 cc / min, and the temperature is raised to 280°C at a heating rate of 3°C / min. After phosphating at 280°C for 4 hours, amorphous FeP2 material can be obtained.

[0067] Comparative Example 1

[0068] The preparation steps of the iron source precursor are the same as those in Example 1;

[0069] An iron source precursor and NaH2PO2 with a mass ratio of 1:10 are placed in two porcelain boats respectively. The porcelain boat where the iron source precursor is located is located in the downstream of the tubular furnace, and the porcelain boat where the NaH2PO2 is located is located in the upstream of the tubular furnace. The two porcelain boats are 2 cm apart. A mixed gas (the volume ratio of hydrogen to argon is 5:95) is introduced at a gas flow rate of 400 cc / min, and the temperature is raised to 500°C at a heating rate of 2.5°C / min. After phosphating at 500°C for 4 hours, crystalline FeP2 material can be obtained.

[0070] The amorphous FeP2 materials of Examples 1 and 2 and the crystalline FeP2 material of Comparative Example 1 were respectively used as FeP2 materials to assemble batteries in the following steps and perform performance tests:

[0071] FeP2 material, acetylene black, carboxymethyl cellulose and water were mixed in a mass ratio of 8:1:1:100 to obtain an electrode slurry;

[0072] The obtained electrode slurry was coated on the surface of nickel foam (the loading amount of the electrode slurry was 1 mg / 115 cm 2 ) and then dried at 80 ° C for 12 h to obtain a FeP2 electrode;

[0073] The obtained FeP2 electrode was used as the positive electrode, the lithium sheet as the negative electrode, Gelgard2400 as the separator, and 1 mol / L LiPF6 solution as the electrolyte (the solvent was a mixed solution of ethyl carbonate, dimethyl carbonate and ethyl carbonate in a volume ratio of 1:1:1). CR2032 button batteries were assembled in an argon-filled glove box.

[0074] The XRD patterns of the amorphous FeP2 materials obtained in Examples 1 and 2 and the crystalline FeP2 materials obtained in Comparative Example 1 are shown in FIG. Figure 1 As shown. Figure 1 It can be seen that the amorphous FeP2 materials obtained in Examples 1 to 2 do not show obvious characteristic diffraction peaks, while the crystalline FeP2 material obtained in Comparative Example 1 shows obvious characteristic diffraction peaks.

[0075] The CV curve of the amorphous FeP2 electrode obtained in Example 1 is as follows: Figure 3 As shown. Figure 3 It can be seen that the first three cyclic voltammetry curves of the amorphous FeP2 electrode prepared in Example 1, combined with Figure 1 The XRD test results can explain the lithiation / delithiation behavior of amorphous FeP2 electrode materials.

[0076] The cycle performance diagram of the amorphous FeP2 electrode obtained in Examples 1 and 2 and the crystalline FeP2 electrode obtained in Comparative Example 1 is shown in FIG. Figure 4 As shown. Figure 4 It can be seen that after 1000 cycles of charge and discharge at a high current density of 2A / g, the reversible specific capacity of the FeP2 electrode obtained in Example 1 is 640.5mAh / g, and the capacity loss rate is only 18.4%; the reversible specific capacity of the FeP2 electrode obtained in Example 2 is 574mAh / g, and the capacity loss rate is only 21.6%; the reversible specific capacity of the FeP2 electrode obtained in Comparative Example 1 is only 359mAh / g.

[0077] The rate performance diagram of the amorphous FeP2 electrode obtained in Example 1 and the crystalline FeP2 electrode obtained in Comparative Example 1 is shown in FIG. Figure 5 As shown. Figure 5 It can be seen that the amorphous FeP2 electrode obtained in Example 1 exhibits excellent cycle stability and rate performance compared to the crystalline FeP2 electrode obtained in Comparative Example 1.

[0078] The electrochemical impedance test diagram of the amorphous FeP2 electrode obtained in Example 1 and the crystalline FeP2 electrode obtained in Comparative Example 1 is as follows: Figure 6 As shown. Figure 6 It can be seen that the semicircle diameter in the high-frequency region of the Nyquist plot of the amorphous FeP2 electrode obtained in Example 1 is smaller, indicating that the amorphous FeP2 electrode obtained in Example 1 has a very small charge transfer resistance, which indicates that amorphous FeP2 improves the conductive properties of the electrode material.

[0079] The CV curve of the amorphous FeP2 electrode obtained in Example 1 at a scanning rate of 0.1 to 2 mV / s is as follows: Figure 7 As shown, the corresponding log(i) vs.log(v) diagram of the amorphous FeP2 electrode obtained in Example 1 is shown in FIG. Figure 8 As shown. Figure 7 Fitting and analysis show that the b values ​​of the cathode and anode are 0.76 and 0.96, respectively, indicating that capacitance dominates the lithium ion storage in the amorphous FeP2 electrode.

[0080] The estimated pseudocapacitive contribution of the amorphous FeP2 electrode obtained in Example 1 at a scan rate of 2.0 mV / s is shown in FIG. Figure 9 As shown. Figure 9 It can be seen that with the increase of scan rate, the pseudocapacitance contribution rate in the amorphous FeP2 electrode increases from 49% (scan rate 0.1 mV / s) to 82% (scan rate 2.0 mV / s).

[0081] Figure 10 The contributions of pseudocapacitance and diffusion-controlled effects to the total charge storage of amorphous FeP2 electrodes at different scan rates are demonstrated.

[0082] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing amorphous FeP2, characterized in that: The steps include: (1) hydrothermally reacting iron salt, urea and water to obtain an iron source precursor; (2) The iron source precursor and the phosphorus source are subjected to a phosphating reaction in a mixed gas to obtain amorphous FeP2; In the step (2), the mass ratio of the iron source precursor to the phosphorus source is 1:15-20; In step (2), the mixed gas is a mixed gas of hydrogen and argon, the flow rate of the mixed gas is 400-500 cc / min, and the volume ratio of the hydrogen to argon is 10-20:80-90; In the step (2), the temperature of the phosphating reaction is 260-280°C; In the step (2), the phosphorus source is sodium hypophosphite; In the step (2), the phosphating reaction is carried out in a tubular furnace; the iron source precursor is located in the middle and downstream of the tubular furnace, the phosphorus source is located in the upstream of the tubular furnace, and the distance between the iron source precursor and the phosphorus source is 1 to 5 cm; In the step (2), the heating rate of the phosphating reaction is 2.5-3°C / min, and the time of the phosphating reaction is 4 hours.

2. The method for preparing amorphous FeP2 according to claim 1, wherein: The iron salt is ferric chloride hexahydrate.

3. The method for preparing amorphous FeP2 according to claim 2, wherein: The usage ratio of the iron salt, urea and water is 1 mol: 1-3 mol: 5-10 L.

4. The method for preparing amorphous FeP2 according to claim 1, wherein: In the step (1), the temperature of the hydrothermal reaction is 100-120° C., and the time of the hydrothermal reaction is 6-8 hours.

5. Amorphous FeP2 obtained by the method for preparing amorphous FeP2 according to any one of claims 1 to 4.

6. Use of the amorphous FeP2 according to claim 5 in lithium-ion batteries.

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