Preparation method of lithium iron phosphate

Abstract

The invention provides a preparation method of lithium iron phosphate, and relates to the technical field of battery positive electrode materials. The preparation method comprises the following steps: providing a lithium iron phosphate precursor material; and sintering the lithium iron phosphate precursor material in a protective atmosphere to obtain the lithium iron phosphate material, the sintering treatment sequentially comprises heating sintering, constant-temperature sintering and cooling sintering, a heating sintering process section is provided with a plurality of heating intervals, the content of reducing gas in residual gas in the plurality of heating intervals is 0.65-6% in the material routing direction, and the reducing gas is selected from at least one of CO, H2, C2H2 and C2H4. According to the invention, the retention time of the reducing gas generated in the sintering process in the temperature rise interval is reduced to a proper range, so that the content of magnetisable metal particles in the prepared lithium iron phosphate material is remarkably reduced, and the lithium iron phosphate material contains extremely little iron phosphide.

Description

Technical Field

[0001] The invention relates to the technical field of battery positive electrode materials, and in particular to a method for preparing lithium iron phosphate. Background Art

[0002] Lithium iron phosphate (LiFePO4) is a commonly used cathode material for lithium-ion batteries (such as batteries used in the field of new energy vehicles). It is widely used for its high safety, long cycle life and stable electrochemical performance. During the production process of lithium iron phosphate, impurities such as magnetic metal foreign matter, magnetizable metal particles and iron phosphide are generated. If not removed in time, it may affect the performance and safety of the battery.

[0003] At present, the commonly used demagnetization method in the production process of lithium iron phosphate is to use the high gradient magnetic field generated by the electromagnetic iron remover to adsorb the magnetic particles in the powder, thereby efficiently removing the strong magnetic particles. However, when the magnetic force of the electromagnetic iron remover is disturbed or weakened, this method has a poor demagnetization effect on weakly magnetic metal particles, and cannot reduce the generation of iron phosphide or other weakly magnetic magnetizable metal particles, resulting in excessive impurities such as iron phosphide or other weakly magnetic magnetizable metal particles in the prepared lithium iron phosphate, affecting the quality of the prepared lithium iron phosphate.

[0004] Therefore, it is urgent to propose a new method for preparing lithium iron phosphate to solve the problem of excessive impurity content such as iron phosphide or other weakly magnetic magnetizable metal particles in the existing lithium iron phosphate production process. Summary of the invention

[0005] In view of the technical problems existing in the background technology, the present application provides a method for preparing lithium iron phosphate, aiming to solve the technical problem of excessive impurity content such as iron phosphide or other weakly magnetic magnetizable metal particles in the existing lithium iron phosphate production process.

[0006] In a first aspect, an embodiment of the present application provides a method for preparing lithium iron phosphate, comprising the following steps: providing a lithium iron phosphate precursor material; sintering the lithium iron phosphate precursor material under a protective atmosphere to obtain a lithium iron phosphate material; the sintering treatment sequentially comprises temperature rising sintering, constant temperature sintering and temperature falling sintering, wherein the temperature rising sintering process section is provided with a plurality of temperature rising intervals, and the temperatures of the plurality of temperature rising intervals increase sequentially along the material routing direction, and the content of reducing gas in the residual gas in the plurality of temperature rising intervals is 0.65-6% (V / V), and the reducing gas is selected from at least one of CO, H2, C2H2, and C2H4.

[0007] In the technical solution of the embodiment of the present application, the lithium iron phosphate precursor material is subjected to temperature rising sintering, constant temperature sintering and temperature falling sintering, and further a number of temperature rising intervals are provided in the temperature rising sintering process section, and the temperatures of the several temperature rising intervals are controlled to increase successively, and the content of reducing gas in the residual gas is 0.65-6%, so that the content of reducing gas generated during the sintering process in the temperature rising interval is reduced to a suitable range, thereby significantly reducing the content of magnetizable metal particles in the prepared lithium iron phosphate material, and containing very little iron phosphide.

[0008] In some embodiments, along the material routing direction, the temperature rising sintering process section is provided with n temperature rising intervals, and the temperature of the nth temperature rising interval is T n , the input flow rate of the protective gas in the nth heating interval is X n And the exhaust gas flow rate is Y n , satisfying the following relationship: 0<T n -T n-1 ≤100; 3≤X n -Y n ≤6; n is a positive integer greater than 1, the temperature unit is ℃, and the flow unit is m 3 / h.

[0009] In this embodiment, the temperature of each heating interval is controlled within the range of the present application, so that the lithium iron phosphate precursor material is sintered more fully, and at the same time, the input flow rate of the protective gas and the exhaust flow rate of the exhaust gas in each heating interval are controlled within the range of the present application, so that the content of reducing gas in the residual gas in each heating interval is within the specified range of the present application, thereby reducing the content of magnetizable metal particles and reducing the generation of impurity iron phosphide.

[0010] In some embodiments, the temperature of the n heating intervals is 110-530°C, and the input flow rate of the protective gas is 7-11m 3 / h, the exhaust gas flow rate is 3.1-5.3m 3 / h, the heating and sintering time is 3-9h.

[0011] In this embodiment, the input flow rate of the protective gas is controlled to be 7-11m 3 / h, the exhaust gas flow rate is 3.1-5.3m 3 / h, the content of reducing gas in the residual gas in each heating interval can be controlled to be 0.65-6%, thereby reducing the over-reduction reaction between the reducing gas and the material, and further reducing the formation of magnetizable metal particles and iron phosphide.

[0012] In some embodiments, the temperature of the constant temperature sintering is 700-800°C, and the input flow rate of the protective gas is 5-7m 3 / h, sintering time is 9-11h; the cooling sintering process section is equipped with several cooling zones, along the material routing direction, the temperature of several cooling zones decreases successively, and the input flow rate of protective gas is maintained at 6-8m 3 / h, sintering time is 5-7h.

[0013] In this embodiment, by controlling the constant temperature sintering temperature to 700-800°C and the sintering time to 9-11h, and the cooling sintering time to 5-7h, the sintering can be more complete, and the prepared lithium iron phosphate particles can be uniform with better performance; at the same time, the introduction of protective gas can further prevent the generation of impurities during the sintering process.

[0014] In some embodiments, the preparation method of lithium iron phosphate precursor material includes the following steps: dispersing an iron source, a lithium source, and a carbon source in water to obtain a first slurry; grinding and demagnetizing the first slurry in sequence to obtain a second slurry; spray drying the second slurry to obtain a lithium iron phosphate precursor material.

[0015] In this embodiment, the raw materials such as the iron source, lithium source, and carbon source used are cheap and easy to obtain, and the preparation process is simple, thus greatly saving production costs. Among them, by demagnetizing the first slurry, the magnetic particles in the raw materials can be removed, and the content of magnetizable metal particles in the subsequently prepared lithium iron phosphate can be further reduced.

[0016] In some embodiments, during the preparation of the first slurry, the iron source includes anhydrous ferric phosphate, and the molar ratio of iron element to phosphorus element in the anhydrous ferric phosphate is (0.95-0.99):1, the molar ratio of lithium element in the lithium source to iron element in the iron source is (1.028-1.041):1, and the added mass of the carbon source accounts for 8-13% of the mass of the iron source; the solid content of the first slurry is 30-45%.

[0017] In this embodiment, by controlling the molar ratio of iron element to phosphorus element in anhydrous ferric phosphate to be (0.95-0.99):1, and the molar ratio of lithium element in the lithium source to iron element in the iron source to be (1.028-1.041):1, each element is fully reacted to obtain a lithium iron phosphate precursor material with higher purity, and the generation of impurities is avoided; by controlling the added mass of the carbon source to account for 8-13% of the mass of the iron source, the performance of the lithium iron phosphate precursor material can be improved; by controlling the solid content of the first slurry to be 30-45%, the raw materials can be fully dispersed to facilitate subsequent grinding.

[0018] In some embodiments, the lithium source includes at least one of lithium carbonate and lithium hydroxide; and / or the carbon source includes at least one of glucose monohydrate, citric acid monohydrate and sucrose.

[0019] In this embodiment, the lithium source and carbon source used are both low-priced and easily available materials, which are conducive to industrial mass production.

[0020] In some embodiments, during the preparation of the second slurry, the grinding step specifically includes: performing multi-stage grinding on the first slurry, and controlling the particle size D50 to be 0.28-0.39 μm.

[0021] In this embodiment, the particle size D50 of the slurry after grinding is controlled to be 0.28-0.39 μm, so that the magnetic particles adsorbed on the surface of the slurry particles or wrapped in the slurry particles are fully separated, which facilitates the subsequent removal of the magnetic particles in the slurry.

[0022] In some embodiments, during the preparation of the second slurry, the demagnetization treatment includes: demagnetization under a magnetic field strength of 6000-12000 GS.

[0023] In this embodiment, the magnetic particles in the second slurry can be fully and efficiently removed by controlling the magnetic field strength to 6000-12000 GS.

[0024] In some embodiments, the process conditions of spray drying are: inlet air temperature is 100-300° C., outlet air temperature is 70-90° C., and spray particle size D50 ≤ 40 μm.

[0025] In this embodiment, by controlling the air inlet temperature to 100-300°C, the air outlet temperature to 70-90°C and the spray particle size D50≤40μm, it is beneficial to obtain a fully dry lithium iron phosphate precursor powder with a moderate particle size, which is beneficial for subsequent better sintering.

[0026] In the second aspect, an embodiment of the present application provides a kiln for sintering lithium iron phosphate, wherein the kiln includes a furnace body, an exhaust duct and an exhaust fan, wherein a first exhaust valve is provided at a connection point between one end of the exhaust duct and the furnace body, the other end of the exhaust duct is connected to the exhaust fan, and a second exhaust valve is provided on the outlet duct of the exhaust fan.

[0027] In the technical solution of the embodiment of the present application, through the joint action of a plurality of first exhaust valves, an exhaust fan and a second exhaust valve, the reducing gas generated during the sintering process of lithium iron phosphate is discharged in time, and the reducing gas in the furnace body is reduced to a suitable range, thereby significantly reducing the content of magnetizable metal particles in the prepared lithium iron phosphate material and containing very little iron phosphide.

[0028] In some embodiments, the interior of the furnace body includes a temperature rising sintering process section, a constant temperature sintering process section and a temperature falling sintering process section, wherein the temperature rising sintering process section includes a first temperature rising interval, a second temperature rising interval, a third temperature rising interval, a fourth temperature rising interval, a fifth temperature rising interval, and a sixth temperature rising interval which are sequentially connected, and an air inlet is provided at the bottom of each chamber corresponding to the first temperature rising interval, the second temperature rising interval, the third temperature rising interval, the fourth temperature rising interval, the fifth temperature rising interval, the sixth temperature rising interval, the constant temperature sintering process section and the temperature falling sintering process section, and an air outlet is provided at the top of each chamber, and a first exhaust valve is provided at the air outlet of each chamber for independently controlling the gas discharge amount of each chamber.

[0029] In this embodiment, by controlling the input amount of inert gas and the exhaust amount of gas in each heating interval in the heating sintering process section, the content of reducing gas in the residual gas in each heating interval is controlled within the specified range of the present application, thereby reducing the content of magnetizable metal particles and reducing the generation of impurity iron phosphide.

[0030] In some embodiments, the outlet pipe of the exhaust fan is connected to an acid mist absorption tower and a tail gas absorption tower in sequence, wherein the pressure of the acid mist absorption tower is -0.01 to -0.3 kPa; and the fan frequency of the tail gas absorption tower is 5 to 12 Hz.

[0031] In this embodiment, by controlling the pressure of the acid mist absorption tower and the fan frequency of the tail gas absorption tower within a specific range, the generated reducing gas can be further quickly discharged from the furnace body, and it is ensured that the gas discharged from the kiln is fully treated to avoid polluting the environment.

[0032] The beneficial effects of the present invention are as follows: different from the prior art, in the present application, the lithium iron phosphate precursor material is subjected to temperature rising sintering, constant temperature sintering and temperature falling sintering, and further a plurality of temperature rising intervals are provided in the temperature rising sintering process section, and the temperatures of the plurality of temperature rising intervals are controlled to increase successively, and the content of reducing gas in the residual gas is 0.65-6%, so that the content of reducing gas generated during the sintering process in the temperature rising interval is reduced to a suitable range, thereby significantly reducing the content of magnetizable metal particles in the prepared lithium iron phosphate material, and containing very little iron phosphide.

[0033] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0035] Figure 1 This is the XRD pattern of the lithium iron phosphate material prepared in Example 2 of the present application; Figure 2A This is a morphology diagram of magnetizable particles separated from the lithium iron phosphate material prepared in Example 1 of the present application; Figure 2B This is a SEM image of the iron phosphide separated from the lithium iron phosphate material prepared in Example 1 of the present application; Figure 3A This is a morphology diagram of magnetizable particles separated from the lithium iron phosphate material prepared in Comparative Example 1 of the present application; Figure 3B This is a SEM image of the iron phosphide separated from the lithium iron phosphate material prepared in Comparative Example 1 of the present application; Figure 4 This is a schematic diagram of a kiln structure in a specific implementation of sintering lithium iron phosphate material in this application; Among them: 1: furnace body; 11: temperature rising sintering process section; 111: first temperature rising interval; 112: second temperature rising interval; 113: third temperature rising interval; 114: fourth temperature rising interval; 115: fifth temperature rising interval; 116: sixth temperature rising interval; 12: constant temperature sintering process section; 13: temperature falling sintering process section; 2: exhaust duct; 21: first exhaust valve; 3: exhaust fan; 31: second exhaust valve. DETAILED DESCRIPTION

[0036] The following embodiments of the technical solution of the present application are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0038] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0039] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0040] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0041] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0042] At present, in the preparation process of lithium iron phosphate, a high-gradient magnetic field generated by an electromagnetic iron remover is often used to adsorb magnetic particles in the powder, thereby efficiently removing strongly magnetic particles. However, this method has a poor demagnetizing effect on weakly magnetic metal particles, resulting in excessive impurities such as iron phosphide or other weakly magnetic magnetizable metal particles in the prepared lithium iron phosphate.

[0043] In order to solve the technical problem that the existing method for preparing lithium iron phosphate has excessive impurities such as iron phosphide or other weakly magnetic magnetizable metal particles in the prepared lithium iron phosphate, the present application provides a method for preparing lithium iron phosphate. In the present application, a plurality of heating intervals are provided in a heating sintering process section, and the temperatures of the plurality of heating intervals are controlled to increase successively, and the content of reducing gas in the residual gas is 0.65-6%, so that the content of reducing gas generated in the sintering process in the heating interval is reduced to a suitable range, thereby significantly reducing the content of magnetizable metal particles in the prepared lithium iron phosphate material, and containing very little iron phosphide.

[0044] In a first aspect, an embodiment of the present application provides a method for preparing lithium iron phosphate, comprising the following steps: providing a lithium iron phosphate precursor material; sintering the lithium iron phosphate precursor material under a protective atmosphere to obtain a lithium iron phosphate material; the sintering treatment sequentially comprises temperature rising sintering, constant temperature sintering and temperature falling sintering, wherein the temperature rising sintering process section is provided with a plurality of temperature rising intervals, and the temperatures of the plurality of temperature rising intervals increase sequentially along the material routing direction, and the content of reducing gas in the residual gas in the plurality of temperature rising intervals is 0.65-6% (V / V), and the reducing gas is selected from at least one of CO, H2, C2H2, and C2H4.

[0045] In this embodiment, the lithium iron phosphate precursor material is sintered by a three-stage sintering process of temperature rising sintering, constant temperature sintering and temperature falling sintering to make the sintering more complete. At the same time, a number of temperature rising intervals are provided in the temperature rising sintering process section, and the temperatures of the several temperature rising intervals are controlled to increase successively. The content of reducing gas in the residual gas is 0.65-6%, so that the content of reducing gas generated during the sintering process in the temperature rising interval is reduced to a suitable range, thereby significantly reducing the content of magnetizable metal particles in the prepared lithium iron phosphate material, and containing very little iron phosphide.

[0046] It can be understood that, in this embodiment, the protective gas can be conventionally selected according to actual use needs, for example, it can be argon and / or nitrogen. In this application, the protective gas is preferably nitrogen.

[0047] Furthermore, in some embodiments, along the material routing direction, the temperature-raising sintering process section is provided with n temperature-raising intervals, and the temperature of the nth temperature-raising interval is T n , the input flow rate of the protective gas in the nth heating interval is X n And the exhaust gas flow rate is Y n , satisfying the following relationship: 0<T n -T n-1 ≤100; 3≤X n -Y n ≤6; n is a positive integer greater than 1, the temperature unit is ℃, and the flow unit is m 3 / h.

[0048] In the technical solution of the embodiment of the present application, the sintering of the lithium iron phosphate precursor material is more complete by controlling the temperature of each heating interval within the scope of the present application, and at the same time controlling the input flow rate of the protective gas and the flow rate of the exhaust gas in each heating interval within the scope of the present application, so that the content of reducing gas in the residual gas in each heating interval is within the specified range of the present application, thereby reducing the content of magnetizable metal particles and reducing the generation of impurity iron phosphide.

[0049] Specifically, 0<T n -T n-1≤100, for example, it can be 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 or other values ​​within the range; 3≤X n -Y n ≤6, for example, it can be 3, 4, 5, 6 or other values ​​within this range.

[0050] Furthermore, in some embodiments, the temperature of the n heating intervals is 110-530°C, and the input flow rate of the protective gas is 7-11m 3 / h, the exhaust gas flow rate is 3.1-5.3m 3 / h, the heating and sintering time is 3-9h.

[0051] In the technical solution of the embodiment of the present application, in the chamber of each heating interval, the amount of reducing gas generated by the reaction is adjusted by controlling the input flow rate of the protective gas within the scope of the present application; further, by controlling the flow rate of the exhaust gas within the scope of the present application, the content of the reducing gas in the residual gas is adjusted within the specified range of the present application, thereby reducing the over-reduction reaction between the reducing gas and the material, and further reducing the generation of magnetizable metal particles and iron phosphide.

[0052] Furthermore, in some preferred embodiments, the temperature-raising sintering process section is provided with 6 temperature-raising intervals, including a first temperature-raising interval, a second temperature-raising interval, a third temperature-raising interval, a fourth temperature-raising interval, a fifth temperature-raising interval and a sixth temperature-raising interval, wherein the temperature of the first temperature-raising interval is 110-130°C, and the input flow rate of the protective gas is 9-11m / s. 3 / h, the exhaust gas flow rate is 5.1-5.3m 3 / h, the content of reducing gas in the residual gas is 0.65-4% (V / V), and the sintering time is 0.5-1.5h; the temperature in the second heating interval is 190-210℃, and the input flow rate of protective gas is 9-11m 3 / h, the exhaust gas flow rate is 5.1-5.3m 3 / h, the content of reducing gas in the residual gas is 0.65-4% (V / V), and the sintering time is 0.5-1.5h; the temperature of the third heating interval is 270-290℃, and the input flow rate of protective gas is 9-11m 3 / h, the exhaust gas flow rate is 4.7-4.9m 3 / h, the content of reducing gas in the residual gas is 0.65-4% (V / V), and the sintering time is 0.5-1.5h; the temperature in the fourth heating interval is 350-370℃, and the input flow rate of protective gas is 8-10m 3 / h, the exhaust gas flow rate is 4.7-4.9m 3 / h, the content of reducing gas in the residual gas is 1.4-6% (V / V), and the sintering time is 0.5-1.5h; the temperature of the fifth heating interval is 430-450℃, and the input flow rate of protective gas is 7-9m 3 / h, the exhaust gas flow rate is 3.5-3.7m 3 / h, the content of reducing gas in the residual gas is 1.4-6% (V / V), and the sintering time is 0.5-1.5h; the temperature of the sixth heating interval is 510-530℃, and the input flow rate of protective gas is 8-10m 3 / h, the exhaust gas flow rate is 3.1-3.3m 3 / h, the content of reducing gas in the residual gas is 1.4-6% (V / V), and the sintering time is 0.5-1.5h.

[0053] It is understandable that, in a preferred embodiment, the number of temperature-raising intervals set in the temperature-raising sintering process section can be conventionally adjusted according to actual use needs, for example, it can be 3, 6, 9 or more. In the present application, the number of temperature-raising intervals set in the temperature-raising sintering process section is preferably 6. The content of reducing gas in the residual gas can be detected by conventional gas detection methods, for example, gas chromatography analysis can be used to detect the gas.

[0054] More specifically, the temperature in the first heating interval is 110-130°C, for example, 110°C, 120°C, 130°C or other values ​​within the range; the input flow rate of the protective gas is 9-11m 3 / h, for example, 9m 3 / h、10m 3 / h、11m 3 / h or other values ​​within this range; the exhaust gas flow rate is 5.1-5.3m 3 / h, for example, 5.1m 3 / h、5.2m 3 / h、5.3m 3 / h or other values ​​within the range; the content of reducing gas in the residual gas is 0.65-4% (V / V), for example, it can be 0.65%, 2%, 4% or other values ​​within the range; the sintering time is 0.5-1.5h, for example, it can be 0.5h, 1h, 1.5h or other values ​​within the range; the temperature in the second heating interval is 190-210℃, for example, it can be 190℃, 200℃, 210℃ or other values ​​within the range; the input flow rate of the protective gas is 9-11m 3 / h, for example, 9m 3 / h、10m 3 / h、11m 3 / h or other values ​​within this range; the exhaust gas flow rate is 5.1-5.3m 3 / h, for example, 5.1m 3 / h、5.2m 3 / h、5.3m 3 / h or other values ​​within the range; the content of reducing gas in the residual gas is 0.65-4% (V / V), for example, it can be 0.65%, 2%, 4% or other values ​​within the range; the sintering time is 0.5-1.5h, for example, it can be 0.5h, 1h, 1.5h or other values ​​within the range; the temperature in the third heating interval is 270-290℃, for example, it can be 270℃, 280℃, 290℃ or other values ​​within the range; the input flow rate of the protective gas is 9-11m 3 / h, for example, 9m 3 / h、10m 3 / h、11m 3 / h or other values ​​within this range; the exhaust gas flow rate is 4.7-4.9m 3 / h, for example, 4.7m 3 / h、4.8m 3 / h、4.9m 3 / h or other values ​​within the range; the content of reducing gas in the residual gas is 0.65-4% (V / V), for example, it can be 0.65%, 2%, 4% or other values ​​within the range; the sintering time is 0.5-1.5h, for example, it can be 0.5h, 1h, 1.5h or other values ​​within the range; the temperature in the fourth heating interval is 350-370℃, for example, it can be 350℃, 360℃, 370℃ or other values ​​within the range; the input flow rate of the protective gas is 8-10m 3 / h, for example, 8m 3 / h、9m 3 / h、10m 3 / h or other values ​​within this range; the exhaust gas flow rate is 4.7-4.9m 3 / h, for example, 4.7m 3 / h、4.8m 3 / h、4.9m 3 / h or other values ​​within the range; the content of reducing gas in the residual gas is 1.4-6% (V / V), for example, it can be 1.4%, 3%, 6% or other values ​​within the range; the sintering time is 0.5-1.5h, for example, it can be 0.5h, 1h, 1.5h or other values ​​within the range; the temperature in the fifth heating interval is 430-450℃, for example, it can be 430℃, 440℃, 450℃ or other values ​​within the range; the input flow rate of the protective gas is 7-9m 3 / h, for example, 7m 3 / h、8m 3 / h、9m 3 / h or other values ​​within this range; the exhaust gas flow rate is 3.5-3.7m 3 / h, for example, 3.5m 3 / h、3.6m 3 / h、3.7m 3 / h or other values ​​within the range; the content of reducing gas in the residual gas is 1.4-6% (V / V), for example, it can be 1.4%, 3%, 6% or other values ​​within the range; the sintering time is 0.5-1.5h, for example, it can be 0.5h, 1h, 1.5h or other values ​​within the range; the temperature in the sixth heating interval is 510-530℃, for example, it can be 510℃, 520℃, 530℃ or other values ​​within the range; the input flow rate of the protective gas is 8-10m 3 / h, for example, 8m 3 / h、9m 3 / h、10m 3 / h or other values ​​within this range; the exhaust gas flow rate is 3.1-3.3m 3 / h, for example, 3.1m 3 / h、3.2m 3 / h、3.3m 3 / h or other values ​​within this range; the content of reducing gas in the residual gas is 1.4-6% (V / V), for example, it can be 1.4%, 3%, 6% or other values ​​within this range; the sintering time is 0.5-1.5h, for example, it can be 0.5h, 1h, 1.5h or other values ​​within this range.

[0055] Furthermore, in some embodiments, the temperature of the constant temperature sintering is 700-800°C, and the input flow rate of the protective gas is 5-7m 3 / h, sintering time is 9-11h; the cooling sintering process section is equipped with several cooling zones, along the material routing direction, the temperature of several cooling zones decreases successively, and the input flow rate of the protective gas is maintained at 6-8m 3 / h, sintering time is 5-7h.

[0056] In the technical solution of the embodiment of the present application, by controlling the temperature and sintering time of constant temperature sintering and the time of cooling sintering within the scope of the present application, the sintering can be made more complete, and the prepared lithium iron phosphate particles can be made uniform and have better performance. If the sintering temperature is low or the sintering time is short, the sintering is incomplete. If the sintering temperature is high or the sintering time is long, the prepared lithium iron phosphate particles will agglomerate and the performance will decrease. At the same time, the introduction of protective gas can further prevent the generation of impurities during the sintering process.

[0057] Specifically, the temperature of the constant temperature sintering is 700-800°C, for example, 700°C, 720°C, 750°C, 780°C, 800°C or other values ​​within this range; the input flow rate of the protective gas is 5-7m 3 / h, for example, 5m 3 / h、6m 3 / h、7m 3 / h or other values ​​within this range; the sintering time is 9-11h, for example, 9h, 10h, 11h or other values ​​within this range; during the cooling sintering process, the input flow rate of the protective gas is 6-8m 3 / h, for example, 6m 3 / h、7m 3 / h、8m 3 / h or other values ​​within this range; the sintering time is 5-7h, for example, it can be 5h, 6h, 7h or other values ​​within this range.

[0058] Furthermore, in some preferred embodiments, there are 6 cooling intervals, namely the first cooling interval, the second cooling interval, the third cooling interval, the fourth cooling interval, the fifth cooling interval and the sixth cooling interval, and along the material routing direction, the temperature of the 6 cooling intervals decreases from 690-710°C in the first cooling interval to 170-180°C in the sixth cooling interval with a temperature difference of 105°C.

[0059] It is understandable that, in a preferred embodiment, the number of cooling intervals can be conventionally adjusted according to actual use needs, for example, it can be 3, 6, 9 or more. In the present application, the number of cooling intervals is preferably 6.

[0060] Furthermore, in some embodiments, the preparation method of the lithium iron phosphate precursor material includes the following steps: dispersing an iron source, a lithium source, and a carbon source in water to obtain a first slurry; grinding and demagnetizing the first slurry in sequence to obtain a second slurry; spray drying the second slurry to obtain a lithium iron phosphate precursor material.

[0061] In the technical solution of the embodiment of the present application, the raw materials such as iron source, lithium source, carbon source, etc. are cheap and easy to obtain, and the preparation process is simple, thus greatly saving production costs and facilitating large-scale industrial production. Among them, by demagnetizing the first slurry, the magnetic particles in the raw materials can be removed, and the content of magnetizable metal particles in the subsequently prepared lithium iron phosphate can be further reduced.

[0062] Furthermore, in some embodiments, during the preparation of the first slurry, the iron source includes anhydrous ferric phosphate, and the molar ratio of iron element to phosphorus element in the anhydrous ferric phosphate is (0.95-0.99):1, preferably (0.97-0.98):1, the molar ratio of lithium element in the lithium source to iron element in the iron source is (1.028-1.041):1, preferably (1.03-1.035):1, the added mass of the carbon source accounts for 8-13% of the mass of the iron source, preferably 9-10%; the solid content of the first slurry is 30-45%, preferably 38-42%.

[0063] In the technical scheme of the embodiment of the present application, by controlling the molar ratio of iron element to phosphorus element in anhydrous ferric phosphate and the molar ratio of lithium element in lithium source to iron element in iron source within the scope of the present application, each element is fully reacted to obtain a lithium iron phosphate precursor material with higher purity and avoid the generation of impurities. If the molar ratio of the elements is too low or too high, impurities will be generated, reducing the purity of the lithium iron phosphate precursor material; by controlling the added mass of the carbon source within the scope of the present application, the performance of the lithium iron phosphate precursor material can be improved. If the added mass of the carbon source is too low, the carbon coating is incomplete, which significantly reduces the performance of the lithium iron phosphate precursor material. If the added mass of the carbon source is too high, the carbon coating is too thick, which also reduces the performance of the lithium iron phosphate precursor material; by controlling the solid content of the first slurry within the scope of the present application, the raw materials can be fully dispersed to facilitate subsequent grinding.

[0064] Specifically, the molar ratio of iron to phosphorus in anhydrous ferric phosphate is (0.95-0.99):1, for example, it can be 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1 or other ratios within this range; the molar ratio of lithium in the lithium source to iron in the iron source is (1.028-1.041):1, for example, it can be 1.028:1, 1.03:1, 1.033:1, 1.035:1, 1.037:1, 1.039:1, 1.041:1 or other ratios within this range; the added mass of the carbon source accounts for 8-13% of the mass of the iron source, for example, it can be 8%, 9%, 10%, 11%, 12%, 13% or other values ​​within this range; the solid content of the first slurry is 30-45%, for example, it can be 30%, 35%, 40%, 45% or other values ​​within this range.

[0065] Further, in some embodiments, the lithium source includes at least one of lithium carbonate and lithium hydroxide; and / or the carbon source includes at least one of glucose monohydrate, citric acid monohydrate and sucrose.

[0066] In the technical solution of the embodiment of the present application, the lithium source and carbon source used are both low-priced and easily available materials, which are conducive to industrial mass production.

[0067] Furthermore, in some embodiments, during the preparation of the second slurry, the grinding step specifically includes: subjecting the first slurry to multi-stage grinding, and controlling the particle size D50 to 0.28-0.39 μm, preferably 0.30-0.33 μm.

[0068] In the technical solution of the embodiment of the present application, by controlling the particle size of the slurry after grinding within the scope of the present application, the magnetic particles adsorbed on the surface of the slurry particles or wrapped in the slurry particles are fully separated, which facilitates the subsequent removal of the magnetic particles in the slurry; if the particle size is too small, the magnetic particles have been completely separated at this time, increasing the grinding cost; if the particle size is too large, the magnetic particles are not completely separated, resulting in impurities in the subsequently prepared products.

[0069] Specifically, the particle size D50 is 0.28-0.39 μm, for example, 0.28 μm, 0.30 μm, 0.33 μm, 0.35 μm, 0.37 μm, 0.39 μm or other values ​​within this range.

[0070] Further, in some embodiments, during the preparation of the second slurry, the demagnetization treatment includes: demagnetization under the condition of a magnetic field strength of 6000-12000 GS (preferably 8000 GS).

[0071] In the technical solution of the embodiment of the present application, by controlling the magnetic field strength within the scope of the present application, the magnetic particles in the second slurry can be fully and efficiently removed; if the magnetic field strength is low, the magnetic particles are not completely removed; if the magnetic field strength is high, a permanent magnetic iron remover needs to be installed, resulting in waste.

[0072] Specifically, the magnetic field strength is 6000-12000 GS, for example, it can be 6000 GS, 8000 GS, 10000 GS, 12000 GS or other values ​​within this range.

[0073] Furthermore, in some embodiments, the process conditions of spray drying are: inlet air temperature is 100-300°C, preferably 300°C, outlet air temperature is 70-90°C, preferably 80-85°C, and spray particle size D50≤40μm, preferably 10-30μm.

[0074] In the technical solution of the embodiment of the present application, by controlling the inlet air temperature, outlet air temperature and spray particle size within the scope of the present application, it is beneficial to obtain a fully dry lithium iron phosphate precursor powder with a moderate particle size, which is beneficial for subsequent better sintering.

[0075] Specifically, the inlet air temperature is 100-300°C, for example, it can be 100°C, 150°C, 200°C, 250°C, 300°C or other values ​​within this range; the outlet air temperature is 70-90°C, for example, it can be 70°C, 75°C, 80°C, 85°C, 90°C or other values ​​within this range; the spray particle size D50≤40μm, for example, it can be 5μm, 10μm, 20μm, 30μm, 40μm or other values ​​within this range.

[0076] Second, see Figure 4 , which is a kiln for sintering lithium iron phosphate provided in an embodiment of the present application, the kiln comprises a furnace body 1, an exhaust duct 2 and an exhaust fan 3, wherein a first exhaust valve 21 is provided at the connection point between one end of the exhaust duct 2 and the furnace body 1, the other end of the exhaust duct 2 is connected to the exhaust fan 3, and a second exhaust valve 31 is provided on the outlet pipe of the exhaust fan 3.

[0077] In the technical solution of the embodiment of the present application, through the joint action of a plurality of first exhaust valves 21, the exhaust fan 3 and the second exhaust valve 31, the reducing gas generated during the sintering process of lithium iron phosphate is discharged in time, and the reducing gas in the furnace body 1 is reduced to a suitable range, thereby significantly reducing the content of magnetizable metal particles in the prepared lithium iron phosphate material and containing very little iron phosphide.

[0078] Specifically, the gas discharge flow rate is determined by adjusting the opening of the first exhaust valves 21, thereby reducing the reducing gas content in the furnace body 1 to a suitable range; further, according to the gas discharge flow rate, the frequency of the exhaust fan 3 and the opening of the second exhaust valve 31 are adjusted to discharge the gas discharged from the exhaust pipe 2 as quickly as possible.

[0079] Furthermore, in some embodiments, the furnace body 1 includes a temperature rising sintering process section 11, a constant temperature sintering process section 12 and a temperature falling sintering process section 13, wherein the temperature rising sintering process section includes a first temperature rising interval 111, a second temperature rising interval 112, a third temperature rising interval 113, a fourth temperature rising interval 114, a fifth temperature rising interval 115, and a sixth temperature rising interval 116 which are sequentially connected, and an air inlet is provided at the bottom of each chamber corresponding to the first temperature rising interval 111, the second temperature rising interval 112, the third temperature rising interval 113, the fourth temperature rising interval 114, the fifth temperature rising interval 115, the sixth temperature rising interval 116, the constant temperature sintering process section 117 and the temperature falling sintering process section 118, and an air outlet is provided at the top of each chamber, and a first exhaust valve 21 is provided at the air outlet of each chamber for independently controlling the gas discharge amount of each chamber.

[0080] In the technical solution of the embodiment of the present application, by controlling the input amount of inert gas and the exhaust amount of gas in each heating interval, the content of reducing gas in the residual gas in each heating interval is controlled within the specified range of the present application, thereby reducing the content of magnetizable metal particles and reducing the generation of impurity iron phosphide.

[0081] Furthermore, in some embodiments, the outlet pipe of the exhaust fan 3 is connected to an acid mist absorption tower and a tail gas absorption tower (not shown in the figure) in sequence, wherein the pressure of the acid mist absorption tower is -0.01 to -0.3 kPa; and the fan frequency of the tail gas absorption tower is 5 to 12 Hz.

[0082] In the technical solution of the embodiment of the present application, by controlling the pressure of the acid mist absorption tower and the fan frequency of the exhaust gas absorption tower within a specific range, the generated reducing gas can be further quickly discharged from the furnace body, and the gas discharged from the kiln can be fully treated to avoid polluting the environment.

[0083] Specifically, the pressure of the acid mist absorption tower can be, for example, -0.01 kpa, -0.05 kpa, -0.1 kpa, -0.2 kpa, -0.3 kpa or other values ​​within this range; the fan frequency of the tail gas absorption tower can be, for example, 5 Hz, 7 Hz, 9 Hz, 12 Hz or other values ​​within this range.

[0084] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used without specifying the manufacturer are all conventional products that can be obtained commercially.

[0085] 1. Preparation method Example 1 A method for preparing lithium iron phosphate comprises the following steps: S1. Preparation of lithium iron phosphate precursor material: anhydrous iron phosphate, lithium carbonate and glucose monohydrate are dispersed in water, wherein the molar ratio of iron element to phosphorus element in anhydrous iron phosphate is 0.95:1, and the molar ratio of lithium element in lithium carbonate to iron element in anhydrous iron phosphate is 1.028:1, to obtain a first slurry with a solid content of 30%; the first slurry is subjected to multi-stage grinding, and the particle size D50 is controlled to be 0.28 μm, and the second slurry is obtained after demagnetization under the condition of a magnetic field strength of 6000 GS; the second slurry is spray-dried, wherein the process conditions of spray drying are: the inlet air temperature is 100°C, the outlet air temperature is 70°C, and the spray particle size D50 is 10 μm, to obtain a lithium iron phosphate precursor material; S2. Preparation of lithium iron phosphate material: sintering the lithium iron phosphate precursor material obtained in step S1 under a nitrogen atmosphere; Specifically, along the material routing direction, the lithium iron phosphate precursor material first enters the first heating interval 111, the second heating interval 112, the third heating interval 113, the fourth heating interval 114, the fifth heating interval 115 and the sixth heating interval 116 in the heating sintering process section 11 in the furnace body 1, and the temperature of the first heating interval 111 is controlled to be 110°C, and the input flow rate of nitrogen is 9m 3 / h, the exhaust gas flow rate is 5.1m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.2%, 0.05%, 0.2%, and 0.2% (V / V), respectively, and the sintering time is 0.5h; the temperature of the second heating interval 112 is 190°C, and the input flow rate of nitrogen is 9m 3 / h, the exhaust gas flow rate is 5.1m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.2%, 0.05%, 0.2%, and 0.2% (V / V), respectively, and the sintering time is 0.5h; the temperature of the third heating interval 113 is 270°C, and the input flow rate of nitrogen is 9m 3 / h, the exhaust gas flow rate is 4.7m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.2%, 0.05%, 0.2%, and 0.2% (V / V), respectively, and the sintering time is 0.5h; the temperature of the fourth heating interval 114 is 350°C, and the input flow rate of nitrogen is 8m 3 / h, the exhaust gas flow rate is 4.7m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.4%, 0.2%, 0.4%, and 0.4% (V / V), respectively, and the sintering time is 0.5h; the temperature of the fifth heating interval 115 is 430℃, and the input flow rate of nitrogen is 7m 3 / h, the exhaust gas flow rate is 3.5m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.4%, 0.2%, 0.4%, and 0.4% (V / V), respectively, and the sintering time is 0.5h; the temperature of the sixth heating interval 116 is 510°C, and the input flow rate of nitrogen is 8m 3 / h, the exhaust gas flow rate is 3.1m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.4%, 0.2%, 0.4%, and 0.4% (V / V), respectively, and the sintering time is 0.5h; Then enter the constant temperature sintering process section 12, control the constant temperature sintering temperature to 750℃, and the nitrogen input flow rate to 6m 3 / h, sintering time is 10h; Continue to enter the first cooling interval, the second cooling interval, the third cooling interval, the fourth cooling interval, the fifth cooling interval and the sixth cooling interval in the cooling sintering process section 13, respectively control the temperature of the first cooling interval to 700°C, the input flow rate of nitrogen to 7m 3 / h, sintering time is 1h; the temperature of the second cooling zone is 595℃, and the nitrogen input flow rate is 7m 3 / h, the sintering time is 1h; the temperature of the third cooling zone is 490℃, and the nitrogen input flow rate is 7m 3 / h, the sintering time is 1h; the temperature of the fourth cooling zone is 385℃, and the input flow rate of nitrogen is 7m 3 / h, the sintering time is 1h; the temperature of the fifth cooling zone is 280℃, and the nitrogen input flow rate is 7m 3 / h, sintering time is 1h; the temperature of the sixth cooling zone is 175℃, and the input flow rate of nitrogen is 7m 3 / h, sintering time is 1h; Finally, lithium iron phosphate material is obtained.

[0086] Example 2 A method for preparing lithium iron phosphate comprises the following steps: S1. Preparation of lithium iron phosphate precursor material: anhydrous iron phosphate, lithium carbonate and glucose monohydrate are dispersed in water, wherein the molar ratio of iron element to phosphorus element in anhydrous iron phosphate is 0.96:1, and the molar ratio of lithium element in lithium carbonate to iron element in anhydrous iron phosphate is 1.029:1, to obtain a first slurry with a solid content of 35%; the first slurry is subjected to multi-stage grinding, and the particle size D50 is controlled to be 0.29 μm, and the second slurry is obtained after demagnetization under the condition of a magnetic field strength of 7000 GS; the second slurry is spray-dried, wherein the process conditions of spray drying are: the inlet air temperature is 150°C, the outlet air temperature is 75°C, and the spray particle size D50 is 20 μm, to obtain a lithium iron phosphate precursor material; S2. Preparation of lithium iron phosphate material: sintering the lithium iron phosphate precursor material obtained in step S1 under a nitrogen atmosphere; Specifically, along the material routing direction, the lithium iron phosphate precursor material first enters the first heating interval 111, the second heating interval 112, the third heating interval 113, the fourth heating interval 114, the fifth heating interval 115 and the sixth heating interval 116 in the heating sintering process section 11 in the furnace body 1, and the temperature of the first heating interval 111 is controlled to be 120°C, and the input flow rate of nitrogen is 10m 3 / h, the exhaust gas flow rate is 5.2m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.5%, 0.5%, 0.5%, and 0.5% (V / V), respectively, and the sintering time is 1h; the temperature of the second heating interval 112 is 200℃, and the input flow rate of nitrogen is 10m 3 / h, the exhaust gas flow rate is 5.2m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.5%, 0.5%, 0.5%, and 0.5% (V / V), respectively, and the sintering time is 1h; the temperature of the third heating interval 113 is 280°C, and the input flow rate of nitrogen is 10m 3 / h, the exhaust gas flow rate is 4.8m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.5%, 0.5%, 0.5%, and 0.5% (V / V), respectively, and the sintering time is 1h; the temperature of the fourth heating interval 114 is 360°C, and the input flow rate of nitrogen is 9m 3 / h, the exhaust gas flow rate is 4.8m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.5%, 1.5%, 0.5%, and 0.5% (V / V), respectively, and the sintering time is 1h; the temperature of the fifth heating interval 115 is 440℃, and the input flow rate of nitrogen is 8m 3 / h, the exhaust gas flow rate is 3.6m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 0.5%, 1.5%, 0.5%, and 0.5% (V / V), respectively, and the sintering time is 1h; the temperature of the sixth heating interval 116 is 520°C, and the input flow rate of nitrogen is 9m 3 / h, the exhaust gas flow rate is 3.2m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.5%, 1.5%, 0.5%, and 0.5% (V / V), respectively, and the sintering time is 1h; Then enter the constant temperature sintering process section 12, control the constant temperature sintering temperature to 750℃, and the nitrogen input flow rate to 6m 3 / h, sintering time is 10h; Continue to enter the first cooling interval, the second cooling interval, the third cooling interval, the fourth cooling interval, the fifth cooling interval and the sixth cooling interval in the cooling sintering process section 13, respectively control the temperature of the first cooling interval to 700°C, the input flow rate of nitrogen to 7m 3 / h, sintering time is 1h; the temperature of the second cooling zone is 595℃, and the nitrogen input flow rate is 7m 3 / h, the sintering time is 1h; the temperature of the third cooling zone is 490℃, and the nitrogen input flow rate is 7m 3 / h, the sintering time is 1h; the temperature of the fourth cooling zone is 385℃, and the input flow rate of nitrogen is 7m 3 / h, the sintering time is 1h; the temperature of the fifth cooling zone is 280℃, and the nitrogen input flow rate is 7m 3 / h, sintering time is 1h; the temperature of the sixth cooling zone is 175℃, and the input flow rate of nitrogen is 7m 3 / h, sintering time is 1h; Finally, lithium iron phosphate material is obtained.

[0087] Example 3 A method for preparing lithium iron phosphate comprises the following steps: S1. Preparation of lithium iron phosphate precursor material: anhydrous iron phosphate, lithium carbonate and glucose monohydrate are dispersed in water, wherein the molar ratio of iron element to phosphorus element in anhydrous iron phosphate is 0.99:1, and the molar ratio of lithium element in lithium carbonate to iron element in anhydrous iron phosphate is 1.041:1, to obtain a first slurry with a solid content of 45%; the first slurry is subjected to multi-stage grinding, and the particle size D50 is controlled to be 0.39 μm, and the second slurry is obtained after demagnetization under the condition of a magnetic field strength of 12000 GS; the second slurry is spray-dried, wherein the process conditions of spray drying are: the inlet air temperature is 300°C, the outlet air temperature is 90°C, and the spray particle size D50 is 30 μm, to obtain a lithium iron phosphate precursor material; S2. Preparation of lithium iron phosphate material: sintering the lithium iron phosphate precursor material obtained in step S1 under a nitrogen atmosphere; Specifically, along the material routing direction, the lithium iron phosphate precursor material first enters the first heating interval 111, the second heating interval 112, the third heating interval 113, the fourth heating interval 114, the fifth heating interval 115 and the sixth heating interval 116 in the heating sintering process section 11 in the furnace body 1, and the temperature of the first heating interval 111 is controlled to be 130°C, and the input flow rate of nitrogen is 11m 3 / h, the exhaust gas flow rate is 5.3m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 1%, 1%, 1%, and 1% (V / V), respectively, and the sintering time is 1.5h; the temperature of the second heating interval 112 is 210℃, and the input flow rate of nitrogen is 11m 3 / h, the exhaust gas flow rate is 5.3m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 1%, 1%, 1%, and 1% (V / V), respectively, and the sintering time is 1.5h; the temperature of the third heating interval 113 is 290°C, and the input flow rate of nitrogen is 11m 3 / h, the exhaust gas flow rate is 4.9m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 1%, 1%, 1%, and 1% (V / V), respectively, and the sintering time is 1.5h; the temperature of the fourth heating interval 114 is 370°C, and the input flow rate of nitrogen is 10m 3 / h, the exhaust gas flow rate is 4.9m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 1%, 3%, 1%, and 1% (V / V), respectively, and the sintering time is 1.5h; the temperature of the fifth heating interval 115 is 450℃, and the input flow rate of nitrogen is 9m 3 / h, the exhaust gas flow rate is 3.7m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the residual gas are 1%, 3%, 3%, and 3% (V / V), respectively, and the sintering time is 1.5h; the temperature of the sixth heating interval 116 is 530°C, and the input flow rate of nitrogen is 10m 3 / h, the exhaust gas flow rate is 3.3m 3 / h, sintering time is 1.5h; Then enter the constant temperature sintering process section 12, control the constant temperature sintering temperature to 750℃, and the nitrogen input flow rate to 6m 3 / h, sintering time is 10h; Continue to enter the first cooling interval, the second cooling interval, the third cooling interval, the fourth cooling interval, the fifth cooling interval and the sixth cooling interval in the cooling sintering process section 13, respectively control the temperature of the first cooling interval to 700°C, the input flow rate of nitrogen to 7m 3 / h, sintering time is 1h; the temperature of the second cooling zone is 595℃, and the nitrogen input flow rate is 7m 3 / h, the sintering time is 1h; the temperature of the third cooling zone is 490℃, and the nitrogen input flow rate is 7m 3 / h, the sintering time is 1h; the temperature of the fourth cooling zone is 385℃, and the input flow rate of nitrogen is 7m 3 / h, the sintering time is 1h; the temperature of the fifth cooling zone is 280℃, and the nitrogen input flow rate is 7m 3 / h, sintering time is 1h; the temperature of the sixth cooling zone is 175℃, and the input flow rate of nitrogen is 7m 3 / h, sintering time is 1h; Finally, lithium iron phosphate material is obtained.

[0088] Comparative Example 1 The preparation method of lithium iron phosphate in this comparative example is basically the same as that in Example 2, except that in step S2, the flow rate of the exhaust gas in the first temperature rising interval 111 is controlled to be 2.0 m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.81%, 0.81%, 0.81%, and 0.81% (V / V), respectively, and the flow rate of the exhaust gas in the second heating interval 112 is 2.0m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.81%, 0.81%, 0.81%, and 0.81% (V / V), respectively, and the flow rate of the exhaust gas in the third heating interval 113 is 1.0m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.9%, 0.9%, 0.9%, and 0.9% (V / V), respectively, and the flow rate of the exhaust gas in the fourth heating interval 114 is 1.0m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.9%, 2.69%, 0.9%, and 0.9% (V / V), respectively, and the flow rate of the exhaust gas in the fifth heating interval 115 is 1.0m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.86%, 2.58%, 0.86%, and 0.86% (V / V), respectively, and the flow rate of the exhaust gas in the sixth heating interval 116 is 1.0m 3 / h, at this time, the contents of CO, H2, C2H2 and C2H4 in the remaining gas are 0.84%, 2.53%, 0.84% ​​and 0.84% ​​(V / V), respectively.

[0089] Comparative Example 2 The preparation method of lithium iron phosphate in this comparative example is basically the same as that in Example 2, except that in step S2, the flow rate of the exhaust gas in the first heating interval 111 is controlled to be 8.0 m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.23%, 0.23%, 0.23%, and 0.23% (V / V), respectively, and the flow rate of the exhaust gas in the second heating interval 112 is 8.0m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.23%, 0.23%, 0.23%, and 0.23% (V / V), respectively, and the flow rate of the exhaust gas in the third heating interval 113 is 7.0m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.27%, 0.27%, 0.27%, and 0.27% (V / V), respectively, and the flow rate of the exhaust gas in the fourth heating interval 114 is 7.0m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.27%, 0.81%, 0.27%, and 0.27% (V / V), respectively, and the flow rate of the exhaust gas in the fifth heating interval 115 is 5.0m 3 / h, at this time, the contents of CO, H2, C2H2, and C2H4 in the remaining gas are 0.31%, 0.92%, 0.31%, and 0.31% (V / V), respectively, and the flow rate of the exhaust gas in the sixth heating interval 116 is 5.0m 3 / h, at this time, the contents of CO, H2, C2H2 and C2H4 in the remaining gas are 0.22%, 0.66%, 0.22% and 0.22% (V / V), respectively.

[0090] 2. Test Method 1. XRD test of lithium iron phosphate material The lithium iron phosphate material prepared in Example 2 was subjected to XRD test, and the results were as follows: Figure 1 shown.

[0091] 2. Detection of magnetizable metal particles and iron phosphide content in lithium iron phosphate materials The magnetizable metal particles and the iron phosphide content in the lithium iron phosphate materials prepared in Examples 1-3 and Comparative Example 1 were detected, wherein the magnetizable metal particle detection method specifically includes the following steps: 1) A layer of polymer sleeve is coated on the outside of the 6000GS magnetic rod to obtain a magnetic rod containing a coated sleeve; 2) Stir the emulsifier in a container containing magnetic water until it dissolves; 3) 1 kg of each of the lithium iron phosphate materials prepared in Examples 1-3 and Comparative Example 1 and the magnetic rod with the coated sleeve treated in Step 1) were added to the emulsified dispersion obtained in Step 2) and then stirred; 4) transferring the mixture of magnetic metal particles adsorbed on the outside of the magnetic rod containing the coating sleeve and a small amount of slurry in step 3) to a container containing magnetic water, stirring and cleaning the magnetic rod containing the coating sleeve until the slurry on the magnetic rod is as little as possible; 5) Take out the magnetic bar after stirring and cleaning and put it into a beaker. Rinse the outer sleeve of the magnetic bar with magnetic water. Cut the sleeve and rinse the sleeve with magnetic water until there are no particles left on the surface of the sleeve. Ensure that all adsorbed magnetic metal particles are collected in the beaker (the magnetic water used for rinsing is less than 200mL). Rinse twice with 100mL~150mL magnetic water. 6) Use acid solution to decompose the oxides on the surface of the magnetic metal particles after rinsing, rinse the magnetic metal particles again, filter them onto the filter membrane, place them on the slide after drying, and detect the magnetic particles in the automatic cleanliness analysis system on the cleanliness instrument.

[0092] The detection of the iron phosphide content in the lithium iron phosphate materials prepared in Examples 1-3 and Comparative Example 1 is basically the same as the detection method of the magnetizable metal particles, except that after step 6), the dried sample is directly weighed.

[0093] The above test results are shown in Table 1 below.

[0094] The morphology test of the magnetic particles on the slides in Example 1 and Comparative Example 1 and the SEM test of the iron phosphide on the slides in Example 1 and Comparative Example 1 were respectively performed, and the results were as follows: Figure 2A-2B , as shown in 3A-3B.

[0095] 3. Purity test of lithium iron phosphate in lithium iron phosphate material The purity of lithium iron phosphate in the lithium iron phosphate materials prepared in Examples 1-3 and Comparative Example 2 was tested, wherein the purity detection method of lithium iron phosphate is: purity = (MIN (Li test value / Li mole fraction, Fe test value / Fe mole fraction, P test value / P mole fraction) * (Li, Fe, P mole fraction sum)) / SUM (Li, Fe, P test value sum). The results are shown in Table 2 below.

[0096] III. Analysis of test results of various embodiments and comparative examples from Figure 1 It can be seen from the figure that the XRD pattern of the lithium iron phosphate material prepared in Example 2 has fewer impurity peaks. Figure 2A-2B It can be seen that in the lithium iron phosphate material prepared in Example 1, the number of magnetizable metal particles is small, the particle size is small, and the number of iron phosphides is extremely small. Figure 3A-3B It can be seen that the lithium iron phosphate material prepared in Comparative Example 1 has a large number of magnetizable metal particles, a large particle size, and a very large number of iron phosphides.

[0097] The above results show that, in the present application, by controlling the input flow rate of the protective gas and the flow rate of the exhaust gas within the scope of the present application, the content of the reducing gas in the residual gas in each heating interval can be controlled within the specified range of the present application, thereby reducing the content of the magnetizable metal particles.

[0098] Table 1 Results of magnetizable metal particles and iron phosphide content in lithium iron phosphate materials

[0099] It can be seen from Table 1 that the magnetized metal particles in the lithium iron phosphate materials prepared in Examples 1-3 are ≤50 pcs / kg, the maximum diameter is ≤400 μm, and contain very little iron phosphide; while the magnetized metal particles in the lithium iron phosphate material prepared in Comparative Example 1 are significantly increased, and contain more iron phosphide. The results further show that in the present application, by controlling the input flow rate of the protective gas and the flow rate of the exhaust gas within the scope of the present application, the content of the reducing gas in the residual gas in each heating interval can be controlled within the specified range of the present application, thereby reducing the over-reduction reaction between the reducing gas and the material, thereby reducing the content of the magnetizable metal particles and reducing the generation of impurity iron phosphide.

[0100] Table 2 Purity of lithium iron phosphate in lithium iron phosphate materials

[0101] It can be seen from Table 2 that the purity of lithium iron phosphate in the lithium iron phosphate materials prepared in Examples 1-3 is relatively high, while the purity of lithium iron phosphate in the lithium iron phosphate material prepared in Comparative Example 2 is significantly reduced. The results show that by controlling the content of reducing gas in the residual gas in each heating interval within the specified range of the present application, a sufficient reduction reaction can be carried out to avoid incomplete reaction, which would cause a decrease in the purity of lithium iron phosphate.

[0102] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the subject matter of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A method for preparing lithium iron phosphate, characterized in that: The steps include: Provide lithium iron phosphate precursor materials; Under a protective atmosphere, the lithium iron phosphate precursor material is sintered to obtain a lithium iron phosphate material; the sintering process includes temperature-raising sintering, constant temperature sintering and temperature-lowering sintering in sequence, wherein: The temperature-raising sintering process section is provided with a plurality of temperature-raising intervals. The temperatures of the plurality of temperature-raising intervals increase successively along the material routing direction. The content of reducing gas in the residual gas in the plurality of temperature-raising intervals is 0.65-6%, and the reducing gas is selected from at least one of CO, H2, C2H2, and C2H4.

2. The method for preparing lithium iron phosphate according to claim 1, characterized in that: Along the material routing direction, the temperature rising sintering process section is provided with n temperature rising intervals, and the temperature of the nth temperature rising interval is T n The input flow rate of the protective gas in the nth heating interval is X n And the exhaust gas flow rate is Y n , satisfying the following relationship: 0<T n -T n-1 ≤100; 3≤X n -AND n ≤6; Where n is a positive integer greater than 1, the temperature unit is °C, and the flow unit is m 3 / h.

3. The method for preparing lithium iron phosphate according to claim 2, characterized in that: The temperature of the n heating intervals is 110-530°C, and the input flow rate of the protective gas is 7-11m 3 / h, the exhaust gas flow rate is 3.1-5.3m 3 / h, the heating and sintering time is 3-9h.

4. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The constant temperature sintering temperature is 700-800°C, and the input flow rate of the protective gas is 5-7m 3 / h, sintering time is 9-11h; The cooling sintering process section is provided with several cooling zones. Along the material routing direction, the temperatures of the several cooling zones decrease in sequence, and the input flow rate of the protective gas is maintained at 6-8m 3 / h, sintering time is 5-7h.

5. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The method for preparing the lithium iron phosphate precursor material comprises the following steps: Dispersing an iron source, a lithium source, and a carbon source in water to obtain a first slurry; The first slurry is subjected to grinding treatment and demagnetization treatment in sequence to obtain a second slurry; The second slurry is spray-dried to obtain a lithium iron phosphate precursor material.

6. The method for preparing lithium iron phosphate according to claim 5, characterized in that: In the preparation process of the first slurry, the iron source includes anhydrous iron phosphate, and the molar ratio of the iron element to the phosphorus element in the anhydrous iron phosphate is (0.95-0.99):1, the molar ratio of the lithium element in the lithium source to the iron element in the iron source is (1.028-1.041):1, and the added mass of the carbon source accounts for 8-13% of the mass of the iron source; The solid content of the first slurry is 30-45%.

7. The method for preparing lithium iron phosphate according to claim 6, characterized in that: The lithium source includes at least one of lithium carbonate and lithium hydroxide; and / or, The carbon source includes at least one of glucose monohydrate, citric acid monohydrate and sucrose.

8. The method for preparing lithium iron phosphate according to claim 5, characterized in that: In the preparation process of the second slurry, the grinding treatment step specifically includes: performing multi-stage grinding on the first slurry and controlling the particle size D50 to be 0.28-0.39 μm.

9. The method for preparing lithium iron phosphate according to claim 5, characterized in that: In the preparation process of the second slurry, the demagnetization treatment includes: demagnetization under the condition of a magnetic field strength of 6000-12000GS.

10. The method for preparing lithium iron phosphate according to claim 4, characterized in that: The process conditions of the spray drying are: air inlet temperature is 100-300°C, air outlet temperature is 70-90°C, and spray particle size D50 is ≤40 μm.

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