Lithium iron phosphate material as well as preparation method and application thereof

Abstract

The invention relates to a lithium iron phosphate material as well as a preparation method and application thereof. The preparation method comprises the following steps: (1) ball-milling and mixing a lithium source, an iron source, a phosphorus source, a doping element and a carbon source to obtain a mixed material; (2) presintering the mixed material, cooling, crushing, and sieving to obtain a first intermediate; (3) performing high-temperature sintering on the first intermediate, cooling, crushing, and sieving to obtain a second intermediate; and (4) carrying out atomic deposition reaction on the second intermediate under the condition that the gas source flow rate is not less than 70sccm to obtain the lithium iron phosphate material, the atomic deposition reaction comprises the following steps: firstly introducing an aluminum oxide precursor for reaction, then introducing water vapor for reaction, and sequentially carrying out 7-15 times of circular reaction. When the lithium iron phosphate material prepared by the method is applied to preparation of a lithium ion battery as a battery positive electrode material, the battery capacity and the long cycle performance can be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of inorganic compounds and materials, relates to cathode materials for lithium-ion batteries, and specifically relates to a lithium iron phosphate material, a preparation method thereof, and an application thereof. Background Art

[0002] Lithium batteries mainly consist of a cathode material, an anode material, a separator, an electrolyte, and a battery case, etc. As an important part of lithium-ion batteries, the cathode material directly determines the performance of lithium batteries such as energy density, safety, service life, charging time, and temperature adaptability, and is a decisive factor for the electrochemical performance of the battery. In addition, the cost of the cathode material also has a crucial impact on the overall cost of lithium batteries. Therefore, the cathode material plays a guiding role in the development of the lithium battery industry. As the preferred material for current domestic lithium-ion power batteries, lithium iron phosphate has the following advantages: First, the safety requirements for power batteries are high, and lithium iron phosphate has good safety performance; Second, from the perspective of service life, lithium iron phosphate batteries can achieve a long life equivalent to the vehicle operation life cycle; Third, in terms of charging speed, speed, efficiency, and safety can be balanced. Therefore, lithium iron phosphate power batteries still best meet the safety requirements of domestic new energy buses currently.

[0003] However, currently, lithium iron phosphate materials cannot simultaneously meet the requirements of batteries for high capacity and long cycle performance. Summary of the Invention

[0004] Based on this, the purpose of the present invention is to improve the capacity and long cycle performance of lithium iron phosphate cathode materials.

[0005] In order to achieve the above invention purpose, the present invention includes the following technical solutions.

[0006] In the first aspect, the present invention provides a preparation method of a lithium iron phosphate material, comprising the following steps:

[0007] (1) Ball-milling and mixing a lithium source, an iron source, a phosphorus source, a doping element, and a carbon source to obtain a mixed material;

[0008] (2) Pre-sintering the mixed material, cooling, pulverizing, and sieving to obtain a first intermediate;

[0009] (3) High-temperature sintering the first intermediate, cooling, pulverizing, and sieving to obtain a second intermediate;

[0010] (4) Performing an atomic deposition reaction on the second intermediate under the condition that the gas source flow rate is not less than 70 sccm to obtain the lithium iron phosphate material;

[0011] The atomic deposition reaction includes: first introducing an alumina precursor for reaction, then introducing water vapor for reaction, and performing 7 to 15 cycles of reaction in sequence.

[0012] In a second aspect, the present invention provides a lithium iron phosphate material prepared by the above preparation method.

[0013] In a third aspect, the present invention provides the use of the lithium iron phosphate material as a cathode material of a battery in the preparation of a lithium ion battery.

[0014] The present invention has the following beneficial effects:

[0015] In the preparation process of lithium iron phosphate of the present invention, doping elements are introduced to prepare a cathode material with high capacity. Then, after the powder is in a fluidized state in a fluidized container, an atomic deposition technique is used to form a dense and uniform coating layer with a certain thickness on its surface, reducing side reactions during cycling of the particles and maintaining the stability of the material, and reducing fragmentation in the later stage of cycling; the two cooperate with each other to prepare a lithium iron phosphate cathode material with high discharge capacity and excellent cycling performance. Applying the lithium iron phosphate material prepared by the present invention as a cathode material of a battery in the preparation of a lithium ion battery can effectively improve the battery capacity and long cycling performance.

[0016] The present invention introduces an atomic deposition technique and uses a fluidization method to make the powder particles exist in the powder holder in a continuous motion manner. Subsequently, the atomic deposition technique is used to achieve surface coating in a dispersed state of the powder. This method can improve the coating uniformity and overall coating rate, and this method can complete the coating without high-temperature sintering again, and a uniform and dense coating layer is formed on the particle surface, maintaining the stability of the material, reducing side reactions during cycling of the particles, reducing fragmentation in the later stage of cycling, and improving the cycling capacity retention rate of the cathode material. Description of the Drawings

[0017] Figure 1 SEM image of the lithium iron phosphate material prepared in Example 1.

[0018] Figure 2 SEM image of the uncoated lithium iron phosphate material prepared in Comparative Example 1 Detailed Embodiments

[0019] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, these embodiments are provided to make the understanding of the disclosed content of the present invention more thorough and comprehensive.

[0020] In the following examples, the experimental methods without specific conditions are generally carried out under conventional conditions or according to the conditions recommended by the manufacturer. All common chemical reagents used in the examples are commercially available products.

[0021] Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used in the present invention includes any and all combinations of one or more of the related listed items.

[0022] In addition, as used in the present invention, the term "or" is an inclusive "or" symbol and is equivalent to the term "and / or", unless the context clearly dictates otherwise. The term "based on" is not exclusive and allows for other factors not described, unless the context clearly dictates otherwise. Further, throughout the specification, the meanings of "a", "an", and "the" include plural referents. The meaning of "in..." includes "in..." and "on...".

[0023] In some of the embodiments, a method for preparing a lithium iron phosphate material is involved, including the following steps:

[0024] (1) Ball-milling and mixing a lithium source, an iron source, a phosphorus source, a doping element, and a carbon source to obtain a mixed material;

[0025] (2) Presintering the mixed material, pulverizing it after cooling, and sieving it to obtain a first intermediate;

[0026] (3) High-temperature sintering the first intermediate, pulverizing it after cooling, and sieving it to obtain a second intermediate;

[0027] (4) Performing an atomic deposition reaction on the second intermediate under the condition that the gas source flow rate is not less than 70 sccm to obtain the lithium iron phosphate material;

[0028] The atomic deposition reaction includes: first introducing an alumina precursor for reaction, then introducing water vapor for reaction, and performing 7 to 15 cycles of reaction in sequence.

[0029] In the preparation process of lithium iron phosphate in the present invention, introducing a doping element can prepare a cathode material with high capacity. Then, after making the powder present a fluidized state in a fluidized container, an atomic deposition technique is used to form a dense and uniform coating layer with a certain thickness on its surface, reducing side reactions during the cycling process and maintaining the stability of the material and reducing fragmentation in the later stage of cycling; the two cooperate synergistically to prepare a lithium iron phosphate cathode material with excellent high capacity and long cycling performance.

[0030] The present invention introduces an atomic deposition technique. By using a fluidization method, powder particles exist in a powder holder in a continuous motion manner. Subsequently, the atomic deposition technique is used to achieve surface coating in a dispersed state of the powder. This method can improve the coating uniformity and the overall coating rate. Moreover, this method can complete the coating without the need for high-temperature sintering again, and a uniform and dense coating layer is formed on the particle surface, maintaining the stability of the material, reducing side reactions during the cycling process of the particles, reducing fragmentation in the later stage of cycling, and enhancing the cycling capacity retention rate of the cathode material.

[0031] In some of these embodiments, step (4) specifically includes: loading the second intermediate into a fluidization container, evacuating the air and then introducing an inert gas (such as nitrogen and / or argon), setting the gas source flow rate to be not less than 70 sccm. After the pressure in the fluidization container is stable, heat up. After the temperature is stable, first introduce an alumina precursor for reaction, and then introduce water vapor for reaction. Perform 7 - 15 cycles of reaction in sequence, and then cool to obtain the lithium iron phosphate material.

[0032] In some of these preferred embodiments, the gas source flow rate is 70 sccm - 200 sccm, and more preferably 90 sccm - 150 sccm. The gas source flow rate affects the fluidization state of the materials in the fluidization container. When the gas flow rate is too low, the particles cannot exhibit a fluidization state inside the container, resulting in uneven powder coating and thus an unsatisfactory cycling capacity retention rate of the material. When the gas source flow rate is within the range of 90 sccm - 150 sccm, a more ideal cycling capacity retention rate can be obtained.

[0033] In some of these preferred embodiments, the number of cycles of the cyclic reaction is 8 - 12 times. The number of cycles of the cyclic reaction determines the thickness of the alumina coating layer. The conductivity of alumina is relatively poor. When the thickness of the coating layer is insufficient, its protective effect on the particles is not good, resulting in an unsatisfactory cycling capacity retention rate of the material. When the thickness of the coating layer is too high, it will reduce the conductive effect of the material and limit the diffusion of lithium ions, thereby inhibiting the capacity performance of the material and reducing the energy efficiency. When the number of cycles of the cyclic reaction is 8 - 12 times, the most ideal capacity and cycling capacity retention rate can be obtained.

[0034] In some of these preferred embodiments, the alumina precursor is selected from at least one of trimethylaluminum, triethylaluminum, and triisobutylaluminum, and more preferably trimethylaluminum.

[0035] In some of these preferred embodiments, the temperature of the pipeline in step (4) is 70°C - 150°C, and the reaction temperature in the fluidization container is 150°C - 260°C.

[0036] A suitable pipeline temperature can ensure that there is no residual alumina precursor in the pipeline, preventing pipeline blockage; a suitable reaction temperature in the fluidization container can enable the alumina precursor to better react on the particle surface to form a coating layer.

[0037] In some preferred embodiments, in step (4), the pipeline temperature is 80°C to 120°C, and the reaction temperature in the fluidization container is 175°C to 250°C.

[0038] In some preferred embodiments, in step (4), the pipeline temperature is 80°C to 90°C, and the reaction temperature in the fluidization container is 240°C to 250°C.

[0039] In some preferred embodiments, the lithium source is selected from at least one of lithium carbonate, lithium acetate, lithium oxalate, lithium hydroxide, and lithium oxide; for example, it can be a combination of lithium hydroxide and lithium oxide, and the molar ratio of lithium hydroxide to lithium oxide is preferably 1:0.8 to 1.2.

[0040] In some preferred embodiments, the iron source is selected from at least one of iron oxalate, iron nitrate, iron hydroxide, iron acetate, ferrous oxalate, and ferrous nitrate.

[0041] In some preferred embodiments, the phosphorus source is selected from at least one of ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid.

[0042] In some preferred embodiments, the carbon source is selected from at least one of glucose, sucrose, citric acid, maltose, and polyethylene glycol. For example, it can be a mixed carbon source of maltose and polyethylene glycol, and the mass ratio of maltose to polyethylene glycol is preferably 1:2 to 4, more preferably 1:3.

[0043] The amount of carbon source has an impact on the performance of the obtained material particles. If the amount of carbon source is too low, an effective and uniform carbon coating network cannot be formed and the growth of particles cannot be inhibited, which will lead to the growth and enlargement of particles, resulting in an increase in the lithium ion deintercalation path, an increase in polarization, a decrease in discharge capacity, and poor rate performance; if the amount of carbon source is too high, it will affect the increase in the specific surface area of the particles, and there will be more contact sites between the material and the particles, which will further increase side reactions and reduce active lithium, leading to a faster cycle decay. Considering the above effects, the amount of carbon source is preferably 5wt%-15wt% of the weight of the lithium iron phosphate material, preferably 6wt%-8wt%, and more preferably 7%.

[0044] In some preferred embodiments, the doping element is selected from at least one of V, Ti, Nb, Mg, Mn, and Zr. For example, it can be a combination of V and Zr, and the molar ratio of V to Zr is preferably 1 to 2:1. The combination of two doping elements is more conducive to improving the capacity performance of the lithium iron phosphate cathode material.

[0045] In some of the preferred embodiments, the molar ratio of the doping element to the iron element is 0.4:99.6 to 1:99. The added doping element is used to replace the iron site, and the added substance can be an oxide or acid salt of the element. For example, it can be magnesium nitrate, vanadium pentoxide, titanium dioxide, vanadium pentoxide, zirconium carbonate, etc. When replacing the raw material element with the doping element, the amount of the raw material element needs to be correspondingly reduced. The amount of the doping element has a great influence on the specific capacity of the cathode material. If the amount is too small, the capacity of the material cannot reach the ideal effect; if the amount is too high, it will lead to an increase in crystal nucleus defects, poor structural stability, and a risk of cation dissolution. When the molar ratio of the doping element to the iron element is 0.4:99.6 to 1:99, the obtained cathode material has a relatively high capacity.

[0046] In some of the preferred embodiments, the Li:Fe:P molar ratio of the lithium source, iron source, and phosphorus source is 1.01 to 1.05:0.96 to 1.01:1, preferably 1.03:0.99:1. This preferred ratio can make the crystal nucleus structure of the obtained material stable, with sufficient active lithium and no excessive formation of impurity phases.

[0047] In some of the preferred embodiments, the ball milling in step (1) can be dry or wet ball milling. For wet ball milling, ethanol is used as the medium, and then the sample is taken and dried before the next sintering.

[0048] In some of the preferred embodiments, the process conditions for dry ball milling include: the mass ratio of zirconium beads to the total raw materials is 0.8 to 1.2:1, the ball milling time is 20 min to 40 min, and the rotation speed is 100 rpm to 300 rpm.

[0049] In some of the preferred embodiments, the heating rate for pre-sintering in step (2) is 5°C / min to 15°C / min, the pre-sintering temperature is 350°C to 480°C, and the holding time is 3 hours to 8 hours.

[0050] In some of the preferred embodiments, the sieving in step (3) is carried out using a 200-mesh sieve.

[0051] In some of the preferred embodiments, the heating rate for high-temperature sintering in step (3) is 5°C / min to 15°C / min, the high-temperature sintering temperature is 700°C to 780°C, and the holding time is 4 hours to 10 hours.

[0052] In some of the preferred embodiments, the sieving in step (3) is carried out using a 200-mesh sieve.

[0053] Among them, the pre-sintering in step (2) and the high-temperature sintering in step (3) are carried out in a sintering furnace under an inert gas atmosphere. The sintering furnace can be equipment such as a tube furnace or a roller hearth kiln.

[0054] In some embodiments, the present invention also provides the application of the lithium iron phosphate material prepared by the preparation method of the method of the present invention as a cathode material of a battery in the preparation of a lithium ion battery.

[0055] Applying the lithium iron phosphate material prepared by the present invention as a cathode material of a battery in the preparation of a lithium ion battery can effectively improve the battery capacity and long cycle performance.

[0056] The following further describes the present invention in detail with reference to specific embodiments.

[0057] Example 1

[0058] The preparation method of the lithium iron phosphate material provided in this example includes the following steps:

[0059] (1) Lithium carbonate, iron nitrate, ammonium dihydrogen phosphate (Li:Fe:P molar ratio is 1.03:0.99:1), as well as maltose and polyethylene glycol with a mass ratio of 1:3 (as a carbon source, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material) and magnesium nitrate (providing doped element Mg to replace the iron position, and its molar amount accounts for 0.5% of the iron element) are subjected to dry ball milling and mixing (zircon beads: total raw material mass ratio is 1:1, ball milling for 30 min, rotation speed 200 rpm) to obtain a mixed material.

[0060] (2) Transfer the above mixed material to a sintering furnace using a firing boat and perform pre-sintering in a nitrogen inert atmosphere (heating rate is 10 °C / min, pre-sintering temperature is 450 °C, heat preservation time is 4 hours). After naturally cooling to room temperature, take out the material and crush it with a wall breaker, and then sieve it through a 200-mesh sieve to obtain a first intermediate.

[0061] (3) Transfer the above intermediate to a sintering furnace using a firing boat and perform high-temperature sintering in an inert atmosphere (heating rate is 10 °C / min, sintering temperature is 760 °C, heat preservation time is 5 hours). After naturally cooling to room temperature, take out the material and crush it with a wall breaker, and then sieve it through a 200-mesh sieve to obtain a second intermediate.

[0062] (4) Load the second intermediate into a 500 mL fluidized container, assemble the pipeline for atomic deposition reaction. After evacuating the air in the fluidized container, introduce the inert gas argon with a gas flow rate of 90 sccm. After the pressure in the fluidized container is stabilized (determined by the rheological curve, with the pressure curve fluctuating up and down within 0.5 Pa), raise the temperature. The temperature of the pipeline is 120 °C and the reaction temperature of the container is 215 °C. After the temperature is stabilized, first introduce trimethylaluminum for reaction. After the reaction is complete, then introduce water vapor for reaction (an atomic mass spectrometer for residual gas analysis is connected to the end of the atomic deposition exhaust gas. When the peak value of trimethylaluminum or water content surges, it indicates that the reaction is complete); perform 10 cycle reactions in sequence (that is, react with trimethylaluminum and water vapor in sequence for 10 cycles). After the reaction times are over, cool to room temperature, take out the material, and obtain the final product lithium iron phosphate material.

[0063] The SEM image of the final product lithium iron phosphate material is as Figure 1 shown, and it is spherical or quasi-spherical. The SEM image of the second intermediate (lithium iron phosphate material not coated by atomic deposition technology) is as Figure 2 shown, and it can be seen that the particles not coated by atomic deposition technology are smoother. It shows that the coating layer has been successfully covered on the surface of the material particles, and the coating layer makes its surface rough.

[0064] Example 2

[0065] (1) Mix lithium hydroxide, lithium oxide, iron oxalate, and phosphoric acid with a molar ratio of 1:1 (Li:Fe:P molar ratio is 1.03:0.99:1), as well as maltose and polyethylene glycol with a mass ratio of 1:3 (as the carbon source, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material), vanadium pentoxide (providing the doping element V to replace the iron site, and its molar amount accounts for 0.3% of the iron element), and zirconium carbonate (providing the doping element Zr to replace the iron site, and its molar amount accounts for 0.2% of the iron element) by dry ball milling (the mass ratio of zirconium beads to the total raw material is 1:1, ball milling for 30 min at a rotation speed of 200 rpm) to obtain a mixed material.

[0066] (2) Transfer the above mixed material to a sintering furnace using a firing boat and perform pre-sintering in a nitrogen inert atmosphere (heating rate is 10 °C / min, pre-sintering temperature is 450 °C, and holding time is 4 hours). After naturally cooling to room temperature, take out the material and crush it using a blender, and then screen it through a 200-mesh sieve to obtain the first intermediate.

[0067] (3) Transfer the above intermediate to a sintering furnace using a firing boat and perform high-temperature sintering in an inert atmosphere (heating rate is 10 °C / min, sintering temperature is 760 °C, and holding time is 5 hours). After naturally cooling to room temperature, take out the material and crush it using a blender, and then screen it through a 200-mesh sieve to obtain the second intermediate.

[0068] (4) Load the second intermediate into a 500 mL fluidized container, assemble the pipeline for atomic deposition reaction. After evacuating the air in the fluidized container, introduce the inert gas argon with a gas flow rate of 150 sccm. After the pressure in the fluidized container is stabilized (determined by the rheological curve, with the pressure curve fluctuating within 0.5 Pa up and down), heat up. Set the pipeline temperature to 80 °C and the container reaction temperature to 250 °C. After the temperature is stabilized, first introduce trimethylaluminum for reaction. After the reaction is complete, then introduce water vapor for reaction (an atomic mass spectrometer for residual gas analysis is connected to the end of the atomic deposition tail gas. When the peak value of the trimethylaluminum or water content surges, it indicates that the reaction is complete); perform 10 cycles of reaction in sequence (that is, react with trimethylaluminum and water vapor in sequence for 10 cycles). After the number of reaction times is over, cool to room temperature, take out the material, and obtain the final product lithium iron phosphate material.

[0069] Example 3

[0070] (1) Dry ball-mill and mix lithium acetate, iron hydroxide and iron acetate with a molar ratio of 2:5, hydrogen phosphate (with a Li:Fe:P molar ratio of 1.03:0.99:1), as well as maltose and polyethylene glycol with a mass ratio of 1:3 (as the carbon source, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material) and titanium dioxide (providing doped element Ti to dope and replace the iron site, and its molar amount accounts for 0.5% of the iron element) (zirconium beads: the total raw material mass ratio is 1:1, ball-mill for 30 min, rotation speed 200 rpm) to obtain a mixed material.

[0071] (2) Transfer the above mixed material to a sintering furnace using a boat and perform pre-sintering in a nitrogen inert atmosphere (heating rate is 10 °C / min, pre-sintering temperature is 450 °C, holding time is 4 hours). After naturally cooling to room temperature, take out the material and crush it with a blender, and then sieve it through a 200-mesh sieve to obtain the first intermediate.

[0072] (3) Transfer the above intermediate to a sintering furnace using a boat and perform high-temperature sintering in an inert atmosphere (heating rate is 10 °C / min, sintering temperature is 760 °C, holding time is 5 hours). After naturally cooling to room temperature, take out the material and crush it with a blender, and then sieve it through a 200-mesh sieve to obtain the second intermediate.

[0073] (4) Load the second intermediate into a 500 mL fluidized container, assemble the pipeline for atomic deposition reaction. After evacuating the vacuum in the fluidized container, introduce the inert gas argon with a gas flow rate of 100 sccm. After the pressure in the fluidized container is stable (determined by the rheological curve, with the pressure curve fluctuating within 0.5 Pa up and down), raise the temperature. Set the pipeline temperature to 120 °C and the container reaction temperature to 175 °C. After the temperature is stable, first introduce trimethylaluminum for reaction. After the reaction is complete, then introduce water vapor for reaction (an atomic mass spectrometer for residual gas analysis is connected to the tail end of the atomic deposition exhaust gas. When the peak value of the trimethylaluminum or water content surges, it indicates that the reaction is complete); perform 10 cycle reactions in sequence (that is, react with trimethylaluminum and water vapor in sequence, cycle 10 times). After the reaction times are over, cool to room temperature, take out the material, and obtain the final product lithium iron phosphate material.

[0074] Example 4

[0075] (1) Dry ball-mill and mix lithium acetate, iron hydroxide and iron acetate with a molar ratio of 2:5, ammonium hydrogen phosphate (Li:Fe:P molar ratio is 1.03:0.99:1), maltose and polyethylene glycol with a mass ratio of 1:3 (as the carbon source, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material), and vanadium pentoxide (providing the doping element V to dope and replace the iron site, and its molar amount accounts for 0.5% of the iron element) (zirconium beads: total raw material mass ratio is 1:1, ball-mill for 30 min, rotation speed 200 rpm) to obtain a mixed material.

[0076] (2) Transfer the above mixed material to a sintering furnace using a firing boat and perform pre-sintering in a nitrogen inert atmosphere (heating rate is 10 °C / min, pre-sintering temperature is 450 °C, holding time is 4 hours). After naturally cooling to room temperature, take out the material and crush it using a blender, and then screen it through a 200-mesh sieve to obtain the first intermediate.

[0077] (3) Transfer the above intermediate to a sintering furnace using a firing boat and perform high-temperature sintering in an inert atmosphere (heating rate is 10 °C / min, sintering temperature is 760 °C, holding time is 5 hours). After naturally cooling to room temperature, take out the material and crush it using a blender, and then screen it through a 200-mesh sieve to obtain the second intermediate.

[0078] (4) Load the second intermediate into a 500 mL fluidized container, assemble the pipeline for atomic deposition reaction. After evacuating the air in the fluidized container, introduce inert gas argon with a gas flow rate of 100 sccm. After the pressure in the fluidized container is stable (determined by the rheological curve, with the pressure curve fluctuating within 0.5 Pa), heat up. Set the pipeline temperature to 120 °C and the container reaction temperature to 175 °C. After the temperature is stable, first introduce trimethylaluminum for reaction. After the reaction is complete, then introduce water vapor for reaction (an atomic mass spectrometer for residual gas analysis is connected to the end of the atomic deposition tail gas. When the peak value of trimethylaluminum or water content surges, it indicates that the reaction is complete); perform 10 cycle reactions in sequence (that is, react with trimethylaluminum and water vapor in sequence for 10 cycles). After the reaction times are over, cool to room temperature, take out the material, and obtain the final product lithium iron phosphate material.

[0079] Comparative Example 1 (different from Example 1 in that there is no coating)

[0080] (1) Mix lithium carbonate, iron nitrate, ammonium dihydrogen phosphate (molar ratio of Li:Fe:P is 1.03:0.99:1), maltose and polyethylene glycol with a mass ratio of 1:3 (as a carbon source, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material), and magnesium nitrate (providing doped element Mg to replace the iron site, and its molar amount accounts for 0.5% of the iron element) by dry ball milling (the mass ratio of zirconium beads to the total raw material is 1:1, ball milling for 30 min, rotation speed 200 rpm) to obtain a mixed material.

[0081] (2) Transfer the above mixed material to a sintering furnace using a firing boat and perform pre-sintering in a nitrogen inert atmosphere (heating rate is 10 °C / min, pre-sintering temperature is 450 °C, holding time is 4 hours). After naturally cooling to room temperature, take out the material and crush it with a blender, and then screen it through a 200-mesh sieve to obtain the first intermediate.

[0082] (3) Transfer the above intermediate to a sintering furnace using a firing boat and perform high-temperature sintering in an inert atmosphere (heating rate is 10 °C / min, sintering temperature is 760 °C, holding time is 5 hours). After naturally cooling to room temperature, take out the material and crush it with a blender, and then screen it through a 200-mesh sieve to obtain the uncoated lithium iron phosphate material.

[0083] Comparative Example 2 (different from Example 1 in that there is no formation of a fluidized state)

[0084] (1) Lithium carbonate, iron nitrate, ammonium dihydrogen phosphate (Li:Fe:P molar ratio is 1.03:0.99:1), as well as maltose and polyethylene glycol with a mass ratio of 1:3 (as carbon sources, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material) and magnesium nitrate (providing doped element Mg to dope and replace the iron site, and its molar amount accounts for 0.5% of the iron element) are dry ball-milled and mixed (zirconium beads: the total raw material mass ratio is 1:1, ball-milled for 30 min, rotation speed 200 rpm) to obtain a mixed material.

[0085] (2) The above-mentioned mixed material is transferred to a sintering furnace using a firing boat and pre-sintered in a nitrogen inert atmosphere (heating rate is 10 °C / min, pre-sintering temperature is 450 °C, heat preservation time is 4 hours). After naturally cooling to room temperature, the material is taken out and crushed using a wall breaker, and then sieved through a 200-mesh sieve to obtain a first intermediate.

[0086] (3) The above intermediate is transferred to a sintering furnace using a firing boat and high-temperature sintered in an inert atmosphere (heating rate is 10 °C / min, sintering temperature is 760 °C, heat preservation time is 5 hours). After naturally cooling to room temperature, the material is taken out and crushed using a wall breaker, and then sieved through a 200-mesh sieve to obtain a second intermediate.

[0087] (4) The second intermediate is loaded into a 500 mL fluidization container, and the pipeline is assembled for atomic deposition reaction. After evacuating the vacuum in the fluidization container, inert gas argon is introduced, and the gas flow rate is 50 sccm. After the pressure in the fluidization container is stable (determined by the rheological curve, the pressure curve fluctuates up and down within 0.5 Pa), the temperature is raised. The pipeline temperature is 120 °C and the container reaction temperature is 215 °C. After the temperature is stable, trimethylaluminum is first introduced for reaction. After the reaction is complete, water vapor is introduced for reaction (an atomic mass spectrometer for residual gas analysis is connected to the atomic deposition tail gas. When the peak value of trimethylaluminum or water content surges, it indicates that the reaction is complete); 10 cycles of reaction are carried out in sequence (that is, reacting with trimethylaluminum and water vapor in sequence, cycling 10 times). After the reaction times are over, after cooling to room temperature, the material is taken out to obtain the final lithium iron phosphate material.

[0088] Comparative Example 3 (different from Example 1 in that the coating layer is too thick)

[0089] (1) Lithium carbonate, iron nitrate, ammonium dihydrogen phosphate (Li:Fe:P molar ratio is 1.03:0.99:1), as well as maltose and polyethylene glycol with a mass ratio of 1:3 (as carbon sources, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material) and magnesium nitrate (providing doped element Mg to dope and replace the iron site, and its molar amount accounts for 0.5% of the iron element) are dry ball-milled and mixed (zirconium beads: the total raw material mass ratio is 1:1, ball-milled for 30 min, rotation speed 200 rpm) to obtain a mixed material.

[0090] (2) Transfer the above mixed materials to a sintering furnace using a boat and perform pre-sintering in a nitrogen inert atmosphere (heating rate: 10 °C / min, pre-sintering temperature: 450 °C, holding time: 4 hours). After naturally cooling to room temperature, take out the materials, crush them using a blender, and then sieve them through a 200-mesh sieve to obtain the first intermediate.

[0091] (3) Transfer the above intermediate to a sintering furnace using a boat and perform high-temperature sintering in an inert atmosphere (heating rate: 10 °C / min, sintering temperature: 760 °C, holding time: 5 hours). After naturally cooling to room temperature, take out the materials, crush them using a blender, and then sieve them through a 200-mesh sieve to obtain the second intermediate.

[0092] (4) Load the second intermediate into a 500 mL fluidization container, assemble the pipeline for atomic deposition reaction. After evacuating the air in the fluidization container, introduce the inert gas argon, with a gas flow rate of 90 sccm. After the pressure in the fluidization container stabilizes (determined by the rheological curve, the pressure curve fluctuates up and down within 0.5 Pa), heat up. The pipeline temperature is 120 °C and the container reaction temperature is 215 °C. After the temperature stabilizes, first introduce trimethylaluminum for reaction. After the reaction is complete, then introduce water vapor for reaction (an atomic mass spectrometer for residual gas analysis is connected to the end of the atomic deposition exhaust gas. When the peak value of the trimethylaluminum or water content surges, it indicates that the reaction is complete); perform 25 cycles of reaction in sequence (that is, react with trimethylaluminum and water vapor in sequence, cycle 25 times). After the reaction times are over, cool to room temperature, take out the materials to obtain the final product, lithium iron phosphate material.

[0093] Comparative Example 4 (different from Example 1 in that there is no doping element)

[0094] (1) Mix lithium carbonate, iron nitrate, ammonium dihydrogen phosphate (Li:Fe:P molar ratio is 1.03:0.99:1), and maltose and polyethylene glycol with a mass ratio of 1:3 (as the carbon source, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material) by dry ball milling (zircon beads: total raw material mass ratio is 1:1, ball milling for 30 min, rotation speed 200 rpm) to obtain the mixed materials.

[0095] (2) Transfer the above mixed materials to a sintering furnace using a boat and perform pre-sintering in a nitrogen inert atmosphere (heating rate: 10 °C / min, pre-sintering temperature: 450 °C, holding time: 4 hours). After naturally cooling to room temperature, take out the materials, crush them using a blender, and then sieve them through a 200-mesh sieve to obtain the first intermediate.

[0096] (3) Transfer the above intermediate to a sintering furnace using a firing boat and conduct high-temperature sintering under an inert atmosphere (heating rate: 10 °C / min, sintering temperature: 760 °C, holding time: 5 hours). After naturally cooling to room temperature, take out the material, crush it using a wall breaker, and then sieve it through a 200-mesh sieve to obtain the second intermediate.

[0097] (4) Load the second intermediate into a 500 mL fluidization container, assemble the pipeline for atomic deposition reaction. After evacuating the air in the fluidization container, introduce the inert gas argon, with a gas flow rate of 90 sccm. After the pressure in the fluidization container is stable (determined by the rheological curve, the pressure curve fluctuates within 0.5 Pa), heat up. The pipeline temperature is 120 °C and the container reaction temperature is 215 °C. After the temperature is stable, first introduce trimethylaluminum for reaction. After the reaction is complete, then introduce water vapor for reaction (an atomic mass spectrometer for residual gas analysis is connected to the tail end of the atomic deposition exhaust gas. When the peak value of trimethylaluminum or water content surges, it indicates that the reaction is complete); conduct 10 cycles of reaction in sequence (that is, react with trimethylaluminum and water vapor in sequence, for 10 cycles). After the reaction times are over, cool to room temperature, take out the material, and obtain the final product lithium iron phosphate material.

[0098] Comparative Example 5 (different from Example 1 in that the doping amount is reduced)

[0099] (1) Mix lithium carbonate, iron nitrate, ammonium dihydrogen phosphate (Li:Fe:P molar ratio is 1.03:0.99:1), maltose and polyethylene glycol with a mass ratio of 1:3 (as the carbon source, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material), and magnesium nitrate (providing the doping element Mg to replace the iron site, and its molar amount accounts for 0.1% of the iron element) by dry ball milling (zirconium beads: total raw material mass ratio is 1:1, ball milling for 30 min, rotation speed 200 rpm) to obtain a mixed material.

[0100] (2) Transfer the above mixed material to a sintering furnace using a firing boat and conduct pre-sintering under a nitrogen inert atmosphere (heating rate: 10 °C / min, pre-sintering temperature: 450 °C, holding time: 4 hours). After naturally cooling to room temperature, take out the material, crush it using a wall breaker, and then sieve it through a 200-mesh sieve to obtain the first intermediate.

[0101] (3) Transfer the above intermediate to a sintering furnace using a firing boat and conduct high-temperature sintering under an inert atmosphere (heating rate: 10 °C / min, sintering temperature: 760 °C, holding time: 5 hours). After naturally cooling to room temperature, take out the material, crush it using a wall breaker, and then sieve it through a 200-mesh sieve to obtain the second intermediate.

[0102] (4) Load the second intermediate into a 500 mL fluidized container, assemble the pipeline for atomic deposition reaction. After evacuating the air in the fluidized container, introduce the inert gas argon with a gas flow rate of 90 sccm. After the pressure in the fluidized container stabilizes (determined by the rheological curve, with the pressure curve fluctuating within 0.5 Pa up and down), raise the temperature. The pipeline temperature is 120 °C and the container reaction temperature is 215 °C. After the temperature stabilizes, first introduce trimethylaluminum for reaction. After the reaction is complete, then introduce water vapor for reaction (an atomic mass spectrometer of a residual gas analyzer is connected to the end of the atomic deposition tail gas. When the peak value of the trimethylaluminum or water content surges, it indicates that the reaction is complete); perform 10 cycles of reaction in sequence (that is, react with trimethylaluminum and water vapor in sequence for 10 cycles). After the number of reaction times is over, cool to room temperature, take out the material, and obtain the final product lithium iron phosphate material.

[0103] Comparative Example 6 (different from Example 1 in that the doping amount is excessive)

[0104] (1) Mix lithium carbonate, iron nitrate, ammonium dihydrogen phosphate (Li:Fe:P molar ratio is 1.03:0.99:1), maltose and polyethylene glycol with a mass ratio of 1:3 (as a carbon source, the addition amount is 10% of the theoretical weight of the lithium iron phosphate material), and magnesium nitrate (providing the doping element Mg to replace the iron site, and its molar amount accounts for 1.1% of the iron element) by dry ball milling (the mass ratio of zirconium beads to the total raw material is 1:1, ball milling for 30 min, rotation speed 200 rpm) to obtain a mixed material.

[0105] (2) Transfer the above mixed material to a sintering furnace using a firing boat and perform pre-sintering in a nitrogen inert atmosphere (heating rate is 10 °C / min, pre-sintering temperature is 450 °C, holding time is 4 hours). After naturally cooling to room temperature, take out the material and crush it with a blender, and then screen it through a 200-mesh sieve to obtain the first intermediate.

[0106] (3) Transfer the above intermediate to a sintering furnace using a firing boat and perform high-temperature sintering in an inert atmosphere (heating rate is 10 °C / min, sintering temperature is 760 °C, holding time is 5 hours). After naturally cooling to room temperature, take out the material and crush it with a blender, and then screen it through a 200-mesh sieve to obtain the second intermediate.

[0107] (4) Load the second intermediate into a 500 mL fluidized container, assemble the pipeline for atomic deposition reaction. After evacuating the air in the fluidized container, introduce inert gas argon with a gas flow rate of 90 sccm. After the pressure in the fluidized container is stable (determined by the rheological curve, the pressure curve fluctuates up and down within 0.5 Pa), heat up. The pipeline temperature is 120 °C and the container reaction temperature is 215 °C. After the temperature is stable, first introduce trimethylaluminum for reaction. After the reaction is complete, then introduce water vapor for reaction (an atomic mass spectrometer for residual gas analysis is connected to the end of the atomic deposition exhaust gas. When the peak value of trimethylaluminum or water content surges, it indicates that the reaction is complete); perform 10 cycle reactions in sequence (that is, react with trimethylaluminum and water vapor in sequence, cycle 10 times). After the number of reaction times is over, cool to room temperature, take out the material, and obtain the final product lithium iron phosphate material.

[0108] Performance Test of Example 5

[0109] 1. Specific surface area: Use a specific surface area analyzer to test the specific surface area of the lithium iron phosphate materials prepared in Examples 1-4 and Comparative Examples 1-5. It is measured by the low-temperature static volumetric method. At the liquid nitrogen temperature, nitrogen is used as the adsorption gas. The specific operation is as follows: Weigh 2 g of the sample and load it into the sample tube. After the sample is loaded, fix the sample tube on the specific surface area analyzer, put on the heating jacket, open the inlet valve at the instrument end (helium: nitrogen = 80:20), with a pressure of 0.3 mpa, and purge for 30 min and heat to 150 °C; after purging, add 2 / 3 of liquid nitrogen to the liquid nitrogen cup for sample testing, and 1 / 2 to the liquid nitrogen cup of the side cold well, and then start the test. After the test is completed, read the data.

[0110] 2. Resistivity: Use a four-probe resistance testing instrument to test the resistance of the lithium iron phosphate powder materials prepared in Examples 1-4 and Comparative Examples 1-5. The test is carried out in a constant-temperature and dry room at a temperature of 25 °C and a humidity below 50% RH. Weigh 1 g of the material into the four-probe resistance testing instrument, click the start button to start the test, and read the data from the display screen.

[0111] 3. Cycle test: Assemble coin cells using the lithium iron phosphate materials prepared in Examples 1-4 and Comparative Examples 1-5 as the positive electrode active material and graphite as the negative electrode active material respectively, and perform charge-discharge cycle tests on the battery with a constant power step of 0.5P. The charge-discharge voltage range is 2.50 V to 3.65 V, and a total of 200 cycles of tests are carried out. The test needs to be carried out in a 25 °C constant-temperature oven. Divide the discharge gram capacity of each cycle by the discharge gram capacity of the first cycle to obtain the capacity retention rate.

[0112] The test results are shown in Table 1. The 0.5P discharge gram capacity in the table is the discharge gram capacity data of the first cycle of the cycle test.

[0113] Table 1 Summary Table of Cycle Test Data

[0114]

[0115] As can be seen from the data in Table 1, in Comparative Example 1, atomic deposition coating was not carried out, and the cycle retention rate decreased significantly, with the retention rate being approximately 5% lower than that of Example 1; in Comparative Example 3, the number of cycle reactions was too large, resulting in an excessive coating layer. The 200-cycle capacity retention rate was basically equivalent to that of Example 1, but the discharge specific capacity decreased significantly, indicating that the lithium-ion diffusion was restricted. The above data results show that coating a passivation film with an appropriate thickness using atomic deposition technology can effectively improve the cycle capacity retention rate. When not coated or coated too little, the cycle performance of the particles cannot reach an optimal level, while excessive coating will affect lithium-ion diffusion and cause a significant decrease in the discharge specific capacity. Due to the poor conductivity of alumina, the resistivity will also increase significantly.

[0116] From the comparison between Comparative Example 2 and Example 1, it can be seen that when the gas flow rate is low, the particles do not show a fluidized state inside the container, and the powder coating is uneven, resulting in a lower cycle capacity retention rate than that of the lithium iron phosphate cathode material coated under a fluidized state.

[0117] In Comparative Example 4, doping elements were not used, and there was an obvious difference in the discharge specific capacity compared with Examples 1 to 4, which was approximately 9 mAh / g lower. This shows that doping elements are also beneficial to the performance of the specific capacity, and high-capacity lithium iron phosphate cathode materials cannot be obtained without introducing doping elements. From Examples 1 to 4, it was observed that different doping elements would result in different first-cycle specific capacities. When the total amount of doping elements remained unchanged, introducing two doping elements could make the lithium iron phosphate cathode material have a higher first-cycle specific capacity, and the performance difference of doping a single element was smaller. Comparing Comparative Examples 5 and 6 with Example 1, it can be seen that after reducing the doping amount, the specific capacity of the cathode material decreased, and the coating only maintained the cycle performance; while excessive doping would lead to an increase in crystal nucleus defects, poor structural stability, and accelerated cycle decay.

[0118] The above-described embodiments only represent several implementation manners of the present invention, and the description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A method for preparing a lithium iron phosphate material, characterized in that: The following steps are involved: (1) ball-milling a lithium source, an iron source, a phosphorus source, a doping element, and a carbon source to obtain a mixed material; (2) pre-sintering the mixed material, crushing it after cooling, and sieving it to obtain a first intermediate; (3) sintering the first intermediate at high temperature, crushing it after cooling, and sieving it to obtain a second intermediate; (4) subjecting the second intermediate to an atomic deposition reaction at a gas source flow rate of not less than 70 sccm to obtain the lithium iron phosphate material; The atomic deposition reaction comprises: firstly introducing an aluminum oxide precursor for reaction, then introducing water vapor for reaction, and performing 7 to 15 cycles of reaction in sequence.

2. The method for preparing the lithium iron phosphate material according to claim 1, characterized in that: Step (4) comprises: loading the second intermediate into a fluidized container, introducing an inert gas after evacuation, setting the gas source flow rate to be not less than 70 sccm, raising the temperature after the air pressure in the fluidized container is stabilized, and after the temperature is stabilized, first introducing an alumina precursor for reaction, and then introducing water vapor for reaction, and performing 7 to 15 cycles of reaction in sequence, and cooling to obtain the lithium iron phosphate material.

3. The method for preparing the lithium iron phosphate material according to claim 2, characterized in that: The gas source flow rate is 70 sccm to 200 sccm; and / or, The number of cyclic reactions is 8 to 12 times; and / or, The aluminum oxide precursor is selected from at least one of trimethylaluminum, triethylaluminum and triisobutylaluminum; and / or, In step (4), the pipeline temperature is 70°C to 150°C, and the reaction temperature in the fluidized bed container is 150°C to 260°C; and / or, The inert gas is nitrogen and / or argon.

4. The method for preparing the lithium iron phosphate material according to any one of claims 1 to 3, characterized in that: The lithium source is selected from at least one of lithium carbonate, lithium acetate, lithium oxalate, lithium hydroxide and lithium oxide; and / or, The iron source is selected from at least one of ferric oxalate, ferric nitrate, ferric hydroxide, ferric acetate, ferrous oxalate and ferrous nitrate; and / or, The phosphorus source is selected from at least one of ammonium hydrogen phosphate, ammonium dihydrogen phosphate and phosphoric acid; and / or, The carbon source is selected from at least one of glucose, sucrose, citric acid, maltose and polyethylene glycol; and / or, The doping element is selected from at least one of V, Ti, Nb, Mg, Mn and Zr.

5. The method for preparing the lithium iron phosphate material according to claim 4, characterized in that: The lithium source is lithium hydroxide and lithium oxide in a molar ratio of 1:0.8-1.2; and / or, The carbon source is maltose and polyethylene glycol in a mass ratio of 1:2-4; and / or, The doping elements are V and Zr in a molar ratio of 1 to 2:

1.

6. The method for preparing the lithium iron phosphate material according to any one of claims 1 to 3, characterized in that: The molar ratio of Li:Fe:P of the lithium source, iron source and phosphorus source is 1.01-1.05:0.96-1.01:1; and / or, The molar ratio of the doping element to the iron element is 0.4:99.6 to 1:99; and / or, The carbon source is 5wt% to 15wt% of the weight of the lithium iron phosphate material.

7. The method for preparing the lithium iron phosphate material according to any one of claims 1 to 3, characterized in that: The heating rate of the pre-sintering in step (2) is 5°C / min to 15°C / min, the pre-sintering temperature is 350°C to 480°C, and the holding time is 3 hours to 8 hours; and / or, The pre-sintering in step (2) is carried out in an inert gas atmosphere; and / or, The sieving in step (2) is performed using a 200-mesh sieve.

8. The method for preparing the lithium iron phosphate material according to any one of claims 1 to 3, characterized in that: In step (3), the heating rate of high temperature sintering is 5°C / min to 15°C / min, the temperature of high temperature sintering is 700°C to 780°C, and the holding time is 4 hours to 10 hours; and / or, The pre-sintering in step (3) is carried out in an inert gas atmosphere; and / or, The sieving in step (3) is performed using a 200-mesh sieve.

9. A lithium iron phosphate material prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the lithium iron phosphate material according to claim 9 as a positive electrode material for a battery in the preparation of a lithium ion battery.

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