Preparation method of gas-phase carbon layer coated lithium iron phosphate

Abstract

The invention relates to the technical field of lithium ion batteries, and particularly discloses a preparation method of gas-phase carbon layer coated lithium iron phosphate, which comprises the following steps: preparing a second carbon source solution, and introducing inert gas into a sintering furnace through the second carbon source solution for multi-stage temperature sintering. A second carbon source solution can be brought in while inert gas is introduced into the sintering furnace, so that the lithium iron phosphate intermediate is subjected to multi-stage temperature sintering under the atmosphere condition containing the carbon source, and the carbon source is deposited on the surfaces of lithium iron phosphate particles in the sintering process and coats the surfaces of the lithium iron phosphate particles in a gas form; the gas-phase carbon layer is coated on the surface of the primary particle, so that the carbon coating layer of the primary particle is more uniform, the electron conduction among the particles is faster, and the structure stability of the primary particle is better as the carbon layer is uniformly coated on the surface of the primary particle, so that the compaction density and the large rate capability of the prepared gas-phase carbon layer coated lithium iron phosphate material are better improved, and the performance of the gas-phase carbon layer coated lithium iron phosphate material is improved. The problem of non-uniform carbon coating in the prior art is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, and particularly relates to a preparation method of lithium iron phosphate coated with a gas-phase carbon layer. BACKGROUND

[0002] As a positive material of lithium ion batteries, lithium iron phosphate has the advantages of high discharge platform voltage, long cycle life, excellent thermal stability, low cost and the like, and its theoretical capacity can reach 170 mAh / g. At present, the methods for preparing lithium iron phosphate mainly include high-temperature solid-phase method, carbon thermal reduction method, hydrothermal method, sol-gel method and the like. However, the traditional carbon coating method of lithium iron phosphate positive material has the defects of uneven carbon coating and the like, which leads to low conductivity of the material, poor conductivity between particles, and further slow lithium ion diffusion rate and low diffusion coefficient, thereby seriously restricting the application of the material in high-power power sources and limiting the electrochemical performance of the material at high rates.

[0003] For example, in the prior art, a carbon source is deposited by gas phase or a hydrocarbon gas such as methane or ethylene is used for carbon coating of the lithium iron phosphate positive material. However, in the operation process, two main problems are found. On the one hand, it is difficult to complete the gas phase deposition on the surface of the material, and the carbon coating is uneven. On the other hand, there is a great safety risk.

[0004] In view of this, the present application provides a preparation method of lithium iron phosphate coated with a gas-phase carbon layer. SUMMARY

[0005] The present application aims to provide a preparation method of lithium iron phosphate coated with a gas-phase carbon layer. The second carbon source solution is prepared, and the inert gas is introduced into the sintering furnace through the second carbon source solution and subjected to multi-stage temperature sintering. The lithium iron phosphate intermediate is subjected to multi-stage temperature sintering in the atmosphere containing the carbon source. In the sintering process, the carbon source is deposited on the surface of the lithium iron phosphate particles and coated on the surface of the lithium iron phosphate particles in the form of gas. As a result, the carbon coating layer of the primary particles is more uniform, the electron conduction between the particles is faster, and the structure stability of the primary particles is better due to the uniform coating of the carbon layer on the surface of the primary particles. Therefore, the compaction density and the high-rate performance of the prepared lithium iron phosphate coated with a gas-phase carbon layer are better improved.

[0006] To solve the above technical problems, the present application provides a preparation method of lithium iron phosphate coated with a gas-phase carbon layer, which comprises the following steps: S1, dissolving a lithium source in water to form a solution, then adding an iron source, a phosphorus source and a first carbon source, uniformly mixing and stirring, then adding an additive and continuously stirring and ball milling, and obtaining a lithium iron phosphate precursor slurry solution after the solutes in the slurry are uniformly dispersed and the particle size reaches the standard; S2, spray drying the lithium iron phosphate precursor slurry solution to obtain a lithium iron phosphate intermediate; S3, a second carbon source solution is prepared by dissolving a second carbon source in a dispersion solution; S4, the lithium iron phosphate intermediate obtained in S2 is placed in a sintering furnace, and then an inert gas is introduced into the sintering furnace through the second carbon source solution in S3, so that the lithium iron phosphate intermediate is sintered at multiple temperatures in an atmosphere containing the carbon source, and the second carbon source is coated on the surface of the lithium iron phosphate particles in the form of gas during the sintering process; and after cooling, the lithium iron phosphate with a gas-phase carbon layer is obtained; S5, the lithium iron phosphate with a gas-phase carbon layer is subjected to airflow crushing, and the particle size is controlled to be 0.7-1.5 μm, so that the lithium iron phosphate material coated with a gas-phase carbon layer is obtained.

[0007] Preferably, in S1, the molar ratio of lithium / iron is controlled to be 1.02-1.08:1, the molar ratio of lithium / phosphorus is controlled to be 1.001-1.009:1, the first carbon source accounts for 2%-30% of the total mass of the lithium source, the iron source and the phosphorus source, the additive accounts for 0.1%-3.6% of the total mass of the lithium source, the iron source and the phosphorus source, the solid content of the slurry solution is controlled to be 30-50%, and the grinding particle size is controlled to be 0.35-0.65 μm.

[0008] Preferably, in S1, the lithium source is one or more of lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium hydroxide.

[0009] Preferably, in S1, the iron source is one or more of iron phosphate, ferrous oxalate, diiron trioxide and triiron tetroxide.

[0010] Preferably, in S1, the phosphorus source is one or more of iron phosphate, lithium dihydrogen phosphate, phosphoric acid, lithium phosphate and ammonium dihydrogen phosphate.

[0011] Preferably, in S1, the additive is one or more of titanium dioxide, tetrabutyl titanate, magnesium oxide, manganese carbonate, vanadium pentoxide and ammonium metavanadate.

[0012] Preferably, in S1, the first carbon source and the second carbon source in S3 are one or more of glucose, polyethylene glycol, phenolic resin, alginic acid, sucrose, starch, polyvinylpyrrolidone, carbon nanotube and graphite powder.

[0013] Preferably, in S2, the atomization temperature is 210-270 °C, the exhaust temperature is 70-90 °C, and the feeding speed is 20-50 rpm / min.

[0014] Preferably, in S3, the dispersion solution is a mixed solution of one or more of water, methanol, ethanol and ethylene glycol; the mass of the second carbon source in S3 accounts for 1%-20% of the total mass of the lithium source, the iron source and the phosphorus source, and the solid content in the second carbon source solution is 5-50%.

[0015] Preferably, the sintering furnace in S4 adopts a rotary kiln, the inert gas is one of nitrogen and argon, the multi-stage temperature sintering includes a temperature rising stage and a temperature maintaining stage, the temperature rising stage is 300-500 DEG C, the temperature maintaining stage is 650-850 DEG C, and the total time length of the two stages of temperature maintaining is 4-12h.

[0016] The present application has the following advantages: 1. The preparation method of the gas phase carbon layer coated lithium iron phosphate of the present application can make the inert gas enter the sintering furnace while bringing the second carbon source solution into the sintering furnace by the method of making the inert gas pass through the second carbon source solution into the sintering furnace for multi-stage temperature sintering, compared with the prior art of directly adding hydrocarbon gas carbon sources such as methane and ethylene, the present application makes the lithium iron phosphate intermediate perform multi-stage temperature sintering in the atmosphere containing carbon sources, the carbon source deposits on the surface of the lithium iron phosphate particles and coats the surface of the lithium iron phosphate particles in the form of gas in the sintering process, so that the carbon coating layer of the primary particles is more uniform, the electron conduction between the particles is faster, and the structure stability of the primary particles is better due to the uniform coating of the carbon layer on the surface of the primary particles, so that the compaction density and the large rate performance of the prepared gas phase carbon layer coated lithium iron phosphate material are better improved, and the safety factor is high in the operation process, the problem of uneven carbon coating in the prior art is solved, the preparation method is simple and efficient, the process is simple, and it is conducive to large-scale production. 2. The powder compaction density of the gas phase carbon layer coated lithium iron phosphate material prepared by the present application can reach 2.55g / cm 3 , the first charge specific capacity can reach 161.58mAh / g, the first discharge specific capacity can reach 158.83mAh / g, the first efficiency can reach 98.30%, the 1C discharge specific capacity can reach 146.38mAh / g, and the conductivity can be improved to 4*10 -2 S / cm, which greatly improves the conductivity and large rate discharge performance of the lithium iron phosphate positive electrode material. 3. The present application can use different types of first carbon source and second carbon source combination, on the basis of the first carbon source coating, further make the second carbon source uniformly coated on the surface of the lithium iron phosphate particles in the form of gas in the sintering process under the condition of containing carbon source in the atmosphere, greatly make up the lattice defects caused by high temperature in the coating process of the first carbon source, so that the carbon layer structure has higher order, the carbon layer thickness is more uniform, and the conductivity is further improved; in addition, the carbon deposition effectively reduces the porosity of the surface of the lithium iron phosphate particles, so as to realize higher powder compaction density. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the present application, the drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative effort based on these drawings.

[0018] Figure 1 is an SEM image of the gas-phase carbon-coated lithium iron phosphate material of Example 1 of the present application; Figure 2 is an XRD image of the gas-phase carbon-coated lithium iron phosphate material of Example 1 of the present application. DETAILED DESCRIPTION

[0019] The technical solutions in the embodiments of the present application will be described clearly and completely in the following with reference to the drawings in the present application specification. Obviously, the described embodiments are only some embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative effort belong to the scope of protection of the present application. Example 1

[0020] A preparation method of a gas-phase carbon-coated lithium iron phosphate, comprising the following steps: S1, first dissolve lithium carbonate in water to form a solution, then add iron phosphate (molar ratio of lithium to iron is 1.04:1), phosphoric acid (molar ratio of lithium to phosphorus is 1.003:1), glucose and sucrose (9% of the total mass of lithium carbonate and iron phosphate solid powder), mix and stir uniformly, then add vanadium pentoxide (0.35% of the total mass of lithium carbonate and iron phosphate solid powder), continue to stir and ball mill, control the grinding particle size to be 0.45 μm, and the solid content to be 35%, to obtain a lithium iron phosphate precursor slurry solution; S2, spray dry the above lithium iron phosphate precursor slurry solution, set the atomization temperature to be 225°C, the exhaust temperature to be 75°C, and the feeding speed to be 40 mL / min, to obtain a lithium iron phosphate intermediate; S3, weigh polyethylene glycol according to 3% of the total mass of the solid powder in S1, and disperse it in a methanol solution to prepare a second carbon source solution (solid content is 15%); S4, the lithium iron phosphate intermediate obtained in S2 is placed in a sintering furnace, nitrogen gas is passed into the sintering furnace through the second carbon source solution in S3, the lithium iron phosphate intermediate is heated to 400℃ at a rate of 2℃ / min under an atmosphere containing the carbon source, and the temperature is maintained for 2h, then the temperature is increased to 760℃ at a rate of 3℃ / min, and the temperature is maintained for 8h, in the sintering process, the second carbon source is coated on the surface of the lithium iron phosphate particles in the form of gas, and after cooling, the lithium iron phosphate modified by the gas-phase carbon layer is prepared; S5, the material obtained after sintering is subjected to air flow crushing, the crushing particle size D50 is controlled to be 0.95μm, and the lithium iron phosphate material A coated with the gas-phase carbon layer is prepared.

[0021] Example 2

[0022] A preparation method of lithium iron phosphate coated with a gas-phase carbon layer, the preparation method comprising the following steps: S1, lithium carbonate is first dissolved in water to form a solution, then iron phosphate, diiron trioxide (molar ratio of lithium to iron is 1.07), phosphoric acid, and lithium dihydrogen phosphate (molar ratio of lithium to phosphorus is 1.01) are added, polyvinylpyrrolidone and carbon nanotubes (accounting for 9% of the total mass of solid powders of lithium carbonate, iron phosphate, diiron trioxide, and lithium dihydrogen phosphate) are added, the mixture is uniformly stirred, titanium dioxide (accounting for 0.34% of the total mass of solid powders) and ammonium metavanadate (accounting for 0.34% of the total mass of solid powders of lithium carbonate, iron phosphate, diiron trioxide, and lithium dihydrogen phosphate) are continuously stirred and ball milled, the grinding particle size is controlled to be 0.4μm, and the solutes are uniformly dispersed to obtain a lithium iron phosphate precursor slurry solution with a solid content of 41%; S2, the lithium iron phosphate precursor slurry solution is subjected to spray drying, the atomization temperature is set to 245℃, the exhaust temperature is set to 80℃, and the feeding speed is set to 35mL / min, and the lithium iron phosphate intermediate is obtained; S3, starch is weighed according to 4% of the total mass of solid powders in S1, and is dissolved in an ethanol dispersion solution to prepare a second carbon source solution (solid content is 20%); S4, the lithium iron phosphate intermediate obtained in S2 is placed in a sintering furnace, nitrogen gas is passed into the sintering furnace through the second carbon source solution in S3, the lithium iron phosphate intermediate is heated to 500℃ at a rate of 2℃ / min under an atmosphere containing the carbon source, and the temperature is maintained for 2h, then the temperature is increased to 790℃ at a rate of 3℃ / min, and the temperature is maintained for 10h, in the sintering process, the second carbon source is coated on the surface of the lithium iron phosphate particles in the form of gas, and after cooling, the lithium iron phosphate modified by the gas-phase carbon layer is prepared; S5, the material obtained after sintering is subjected to air flow crushing, the crushing particle size D50 is controlled to be 1.1μm, and the lithium iron phosphate material B coated with the gas-phase carbon layer is prepared. Example 3

[0023] A preparation method of a lithium iron phosphate coated with a gas-phase carbon layer, comprising the following steps: S1, dissolving lithium carbonate in water to form a solution, then adding iron phosphate and ferrous oxalate (lithium / iron molar ratio of 1.06:1), lithium dihydrogen phosphate (lithium / phosphorus molar ratio of 1.005:1), polyvinylpyrrolidone and sucrose (10% of the total mass of lithium carbonate, iron phosphate, ferrous oxalate, lithium dihydrogen phosphate solid powder), mixing and stirring uniformly, then adding ammonium metavanadate (0.23% of the total mass of lithium carbonate, iron phosphate, ferrous oxalate, lithium dihydrogen phosphate solid powder) and tetrabutyl titanate (0.25% of the total mass of lithium carbonate, iron phosphate, ferrous oxalate, lithium dihydrogen phosphate solid powder) to continue stirring and ball milling, until the solutes in the slurry are uniformly dispersed and the particle size reaches 0.38 μm to obtain a lithium iron phosphate precursor slurry solution, with a solid content of 45%; S2, spray drying the lithium iron phosphate precursor slurry solution, setting the atomization temperature to 255℃, the exhaust temperature to 85℃, and the feeding speed to 30 mL / min to obtain a lithium iron phosphate intermediate; S3, taking 4% of the total mass of the solid powder in S1 to prepare a second carbon source solution by dissolving glucose and polyethylene glycol in an ethanol dispersion solution (solid content of 18%); S4, placing the lithium iron phosphate intermediate obtained in S2 in a sintering furnace, passing nitrogen gas through the second carbon source solution into the sintering furnace, and allowing the lithium iron phosphate intermediate to be coated with the second carbon source in the form of gas under the condition of an atmosphere containing a carbon source, by setting a program gradient temperature rise, heating to 450℃ at a rate of 2℃ / min, maintaining the temperature for 2 h, then heating to 780℃ at a rate of 3℃ / min, maintaining the temperature for 8 h, and allowing the second carbon source to be coated on the surface of the lithium iron phosphate particles in the form of gas during the sintering process, and preparing a lithium iron phosphate modified with a gas-phase carbon layer after cooling; S5, performing airflow crushing on the material obtained after sintering, controlling the crushing particle size D50 to be 1.05 μm, and preparing a lithium iron phosphate material C coated with a gas-phase carbon layer.

[0024] The lithium iron phosphate material A / B / C coated with a gas-phase carbon layer prepared above was subjected to performance testing, and the specific data are shown in Table 1: Table 1

[0025] According to the data in Table 1, the powder tap density of the lithium iron phosphate material coated with a gas-phase carbon layer prepared in Example 1 is 2.55 g / cm 3 , the first charge specific capacity is 161.58 mAh / g, the first discharge specific capacity is 158.83 mAh / g, the first efficiency is 98.30%, the 1C discharge specific capacity is 146.38 mAh / g, and the conductivity can be increased to 4*10 -2S / cm, greatly improving the conductivity and rate discharge performance of the lithium iron phosphate positive electrode material; The scanning electron microscope image of the lithium iron phosphate material A coated with a gas-phase carbon layer prepared in Embodiment 1 is shown in Figure 1 It can be seen that the material has a round grain morphology and is composed of large and small particles, with the large particles being 1-3 μm and the small particles being 300-500 nm. The round and size-graded particles are beneficial to improving the compaction and the electrical performance. Figure 2 As shown in the XRD, the X diffraction characteristic peaks of the material are completely consistent with the standard card of lithium iron, without phosphorized iron impurity peaks, indicating that the material has high purity, which is beneficial to the structural stability during the charging and discharging process of the material and has a certain effect on improving the cycle performance. The powder compaction density of the lithium iron phosphate material coated with a gas-phase carbon layer prepared in Embodiment 2 is 2.54 g / cm 3 The first charge specific capacity is 158.93 mAh / g, the first discharge specific capacity is 156.97 mAh / g, and the first efficiency is 98.77%. The 1C discharge specific capacity is 144.76 mAh / g. The powder compaction density of the lithium iron phosphate material coated with a gas-phase carbon layer prepared in Embodiment 3 is 2.58 g / cm 3 The first charge specific capacity is 160.49 mAh / g, the first discharge specific capacity is 158.57 mAh / g, and the first efficiency is 98.80%. The 1C discharge specific capacity is 142.62 mAh / g. As can be seen from the comparison between Embodiments 1-3, the first carbon source and the second carbon source are combined in different types in the present application. On the basis of the coating of the first carbon source, the second carbon source is uniformly coated on the surface of the lithium iron phosphate particles in the form of gas during the sintering process under the condition of the carbon source-containing atmosphere, greatly making up for the lattice defects caused by the high temperature during the coating process of the first carbon source, so that the carbon layer structure has higher order, the carbon layer thickness is more uniform, and the conductivity is further improved. In addition, the carbon deposition effectively reduces the porosity of the surface of the lithium iron phosphate particles, so that a higher powder compaction density is achieved. For example, in Embodiment 1, a high discharge specific capacity of 146.38 mAh / g is still achieved under the condition of a powder compaction density of 2.55 g / cm 3

[0026] As can be seen from the comparison between Embodiments 1 and 2, under the condition that the molar ratio of the iron source is constant, the specific surface area of the mixed iron source is slightly increased, and the compaction density and the electrical performance of the prepared lithium iron phosphate positive electrode material are both reduced, indicating that the smaller specific surface area of the phosphoric acid iron mixture in Embodiment 1 is more beneficial to preparing a lithium iron phosphate positive electrode material with better comprehensive performance.

[0027] ​The above merely provides the preferred embodiment of the present application, and cannot allude the protection scope of the present application, therefore, any equivalent changes made according to the claims of the present application shall be within the scope of the present application.

Claims

1. A method for producing a lithium iron phosphate coated with a gas-phase carbon layer, characterized by, The preparation method comprises the following steps: S1, dissolving a lithium source in water to form a solution, then adding an iron source, a phosphorus source and a first carbon source, uniformly mixing and stirring, then adding an additive and continuing to stir and ball mill, and obtaining a lithium iron phosphate precursor slurry solution after the solutes in the slurry are uniformly dispersed and the particle size reaches a standard; S2, spray drying the lithium iron phosphate precursor slurry solution to obtain a lithium iron phosphate intermediate; S3, dissolving a second carbon source in a dispersion solution to obtain a second carbon source solution; S4, placing the lithium iron phosphate intermediate obtained in S2 in a sintering furnace, then making an inert gas pass through the second carbon source solution in S3 into the sintering furnace, and sintering the lithium iron phosphate intermediate in an atmosphere containing the carbon source in multiple temperature stages, so that the second carbon source is coated on the surface of the lithium iron phosphate particles in the form of a gas during the sintering process, and the gas-phase carbon layer modified lithium iron phosphate is obtained after cooling and taking out; S5, airflow crushing the gas-phase carbon layer modified lithium iron phosphate, and controlling the particle size to 0.7-1.5 microns to obtain a gas-phase carbon layer coated lithium iron phosphate material.

2. The method for preparing the lithium iron phosphate coated with the gas-phase carbon layer according to claim 1, characterized in that, In S1, the molar ratio of lithium / iron is 1.02-1.08:1, the molar ratio of lithium / phosphorus is 1.001-1.009:1, the mass of the first carbon source accounts for 2%-30% of the total mass of the lithium source, the iron source and the phosphorus source mixed solid powder, the additive accounts for 0.1%-3.6% of the total mass of the lithium source, the iron source and the phosphorus source mixed solid powder, the solid content of the slurry solution is controlled to 30-50%, and the grinding particle size is controlled to 0.35-0.65 microns.

3. The method for preparing gas-phase carbon-coated lithium iron phosphate according to claim 1, characterized in that, The lithium source in S1 is one or more of lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium hydroxide.

4. The method for preparing gas-phase carbon-coated lithium iron phosphate according to claim 1, characterized in that, The iron source in S1 is one or more of iron phosphate, ferrous oxalate, diiron trioxide and triiron tetroxide.

5. The method for preparing gas-phase carbon-coated lithium iron phosphate according to claim 1, characterized in that, The phosphorus source in S1 is one or more of iron phosphate, lithium dihydrogen phosphate, phosphoric acid, lithium phosphate and ammonium dihydrogen phosphate.

6. The method for preparing gas-phase carbon-coated lithium iron phosphate according to claim 1, characterized in that, The additive in S1 is one or more of titanium dioxide, tetrabutyl titanate, magnesium oxide, manganese carbonate, vanadium pentoxide and ammonium metavanadate.

7. The method for preparing gas-phase carbon-coated lithium iron phosphate according to claim 1, characterized in that, The first carbon source in S1 and the second carbon source in S3 are one or more of glucose, polyethylene glycol, phenolic resin, alginic acid, sucrose, starch, polyvinylpyrrolidone, carbon nanotube and graphite powder.

8. The method for preparing gas-phase carbon-coated lithium iron phosphate according to claim 1, characterized in that, The atomization temperature in S2 is 210-270 DEG C, the exhaust temperature is 70-90 DEG C, and the feeding speed is 20-50 rpm / min.

9. The method for preparing gas-phase carbon-coated lithium iron phosphate according to claim 1, characterized in that, The dispersion solution in S3 is a mixed solution of one or more of water, methanol, ethanol and ethylene glycol; the mass of the second carbon source in S3 accounts for 1%-20% of the total mass of the lithium source, the iron source and the phosphorus source mixed solid powder, and the solid content in the second carbon source solution is 5-50%.

10. The method for preparing gas-phase carbon-coated lithium iron phosphate according to claim 1, characterized in that, The sintering furnace in S4 is a rotary kiln, the inert gas is one of nitrogen and argon, the multiple temperature sintering includes a heating stage and a holding stage, the heating stage is 300-500 DEG C, the holding stage is 650-850 DEG C, and the total time length of the two stages is 4-12 hours.

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