Lithium iron phosphate positive electrode material with low specific surface area as well as preparation method and application of lithium iron phosphate positive electrode material

Abstract

The invention belongs to the technical field of lithium ion batteries, and provides a low-specific-surface-area lithium iron phosphate positive electrode material and a preparation method and application thereof.The raw materials of the lithium iron phosphate positive electrode material comprise a phosphorus source, a lithium source, an iron source and a carbon source, and the lithium iron phosphate positive electrode material is prepared through two carbon thermal reduction reactions; the lithium iron phosphate positive electrode material is characterized in that the specific surface area of the lithium iron phosphate positive electrode material is 7.20 m < 2 > / g to 7.30 m < 2 > / g; the carbon content of the lithium iron phosphate positive electrode material is 1.07%-1.09%; and the resistivity of the lithium iron phosphate positive electrode material is 15.70 omega.cm to 16.70 omega.cm. The lithium iron phosphate positive electrode material disclosed by the invention is low in specific surface area, good in electrical properties such as resistivity and the like, and beneficial to improving the solid content, reducing the glue consumption, accelerating the production rhythm, improving the production efficiency and saving the cost during later battery manufacturing and coating.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, and in particular to a low specific surface area lithium iron phosphate cathode material, its preparation method and application. Background Technology

[0002] Lithium iron phosphate (LFP) batteries possess advantages such as good safety performance, long cycle life, and abundant raw material resources. The use of LFP cathode materials significantly improves the capacity, cycle performance, safety performance, and rate performance of lithium-ion batteries. Compared with lithium cobalt oxide and ternary materials, LFP cathode materials offer over 20% better cycle performance, over 70% lower raw material costs, and superior safety performance. Therefore, LFP cathode materials are currently the most commonly used cathode material for large-scale power lithium batteries worldwide. Specific surface area is one of the important factors affecting the performance of LFP cathode materials, but excessively high specific surface area has the following disadvantages:

[0003] ① Increased self-discharge rate: A high specific surface area means that more active sites are exposed in the electrolyte, which may accelerate the decomposition reaction of the electrolyte on the surface of the positive / negative electrode materials, leading to an increase in self-discharge rate and affecting the battery's storage performance;

[0004] ② Reduced energy density: Although a high specific surface area can theoretically provide more reaction sites, which is beneficial to improving the charge and discharge rate of the battery, it also increases the proportion of inactive materials (such as binders and conductive agents) and the volume of the material itself, thereby reducing the energy density of the battery to some extent.

[0005] ③Stability issues: An excessively high specific surface area may lead to unstable structure on the material surface, making it prone to side reactions with the electrolyte and the formation of a solid electrolyte interphase (SEI) film. This not only consumes lithium ions in the electrolyte but may also increase the internal resistance of the battery, affecting the cycle stability and lifespan of the battery.

[0006] ④ Increased difficulty in process control: Preparing lithium iron phosphate cathode materials with high specific surface area requires more refined synthesis techniques and process control, which undoubtedly increases production costs and technical difficulty. Moreover, cathode materials with high specific surface area are more prone to agglomeration during actual processing, affecting their dispersion uniformity in the electrode and thus affecting the overall performance of the battery;

[0007] ⑤ Safety risks: An excessively high specific surface area of ​​the cathode material may lead to more intense internal reactions in the battery, especially under high temperature or abuse conditions, which may increase the risk of thermal runaway and affect the battery's safety performance.

[0008] Therefore, when designing and selecting lithium iron phosphate cathode materials, it is necessary to comprehensively consider the impact of specific surface area on lithium battery performance. In order to find the optimal specific surface area range through reasonable material design and optimization, so as to balance the performance requirements of lithium batteries in all aspects.

[0009] Currently, existing technologies are continuously developing processes for preparing low specific surface area lithium iron phosphate cathode materials. For example, patent application CN108550826A discloses a method for preparing high-carbon-content, low-specific-surface-area lithium iron phosphate and its application, solving the problem of excessive specific surface area caused by the high carbon content of materials in existing manufacturing methods. This method includes the following steps: a) mixing lithium source compound, iron source compound, and phosphorus source compound in a stoichiometric ratio with a solvent, then adding a doped metal oxide and a primary carbon source, pre-dispersing and ball-milling until uniform, followed by ultrafine sand milling to obtain a precursor slurry; b) centrifugally spray-drying the precursor slurry from step a to obtain precursor powder; c) calcining the precursor powder from step b at a constant temperature in an inert atmosphere to obtain lithium iron phosphate material; d) pulverizing, sieving, removing iron, and packaging the lithium iron phosphate material obtained in step c through a closed-loop airflow system to obtain the finished product. This invention prepares lithium iron phosphate with high carbon content and low specific surface area by optimizing the carbon source ratio. However, the high carbon content of this method will limit the transport of lithium ions to a certain extent, which may lead to a reduction in the capacity of the material. Summary of the Invention

[0010] In view of the shortcomings of the prior art, the present invention provides a low specific surface area lithium iron phosphate cathode material, its preparation method and application, so as to solve the problem that the high specific surface area of ​​existing lithium iron phosphate cathode materials affects the safety, stability and other performance of lithium batteries.

[0011] To achieve the above and related objectives, the present invention adopts the following technical solution:

[0012] The first aspect of this invention provides a low specific surface area lithium iron phosphate cathode material, wherein the raw materials for the lithium iron phosphate cathode material include a phosphorus source, a lithium source, an iron source, and a carbon source, and are prepared by two carbothermal reduction reactions; and the lithium iron phosphate cathode material satisfies the following characteristics (a) to (c):

[0013] (a) The specific surface area of ​​the lithium iron phosphate cathode material is 7.20 m². 2 / g~7.30m 2 / g;

[0014] (b) The carbon content of the lithium iron phosphate cathode material is 1.07% to 1.09%;

[0015] (c) The resistivity of the lithium iron phosphate cathode material is 15.70 Ω·cm to 16.70 Ω·cm.

[0016] Furthermore, the lithium iron phosphate cathode material is obtained through secondary pulverization. After the second pulverization, the particle size of the lithium iron phosphate cathode material satisfies the following relationship:

[0017] 1.00μm≦DV50≦1.10μm.

[0018] Furthermore, the raw materials also include dopants, such as polyethylene glycol and titanium dioxide.

[0019] A second aspect of the present invention provides a method for preparing a low specific surface area lithium iron phosphate cathode material, the method comprising the following steps:

[0020] (1) A precursor is obtained by performing a carbothermic reduction reaction on a phosphorus source, a lithium source, an iron source and a primary carbon source;

[0021] (2) A secondary carbon source is incorporated into the precursor to conduct a secondary carbothermic reduction reaction to obtain lithium iron phosphate cathode material. This secondary carbothermic reduction reaction includes the steps of fine grinding, sintering, and pulverization.

[0022] The amount of secondary carbon source incorporated is 4.3% of the precursor mass; the fine grinding particle size is 0.8μm≦D50≦0.9μm, or 1.65μm≦D90≦1.85μm; the secondary grinding particle size of the lithium iron phosphate cathode material is 1.00μm≦DV50≦1.10μm.

[0023] Based on the aforementioned technical means, this invention employs a secondary carbothermal reduction process. By controlling the fine grinding particle size and the secondary crushing particle size, the proportion of secondary carbon source incorporated is optimized to reduce the specific surface area of ​​the lithium iron phosphate cathode material. The secondary carbon doping in this invention primarily serves as the coating carbon for the lithium iron phosphate precursor, thereby reducing the resistivity of the lithium iron phosphate cathode material and enabling the battery performance to meet the requirements for cycle stability and lifespan.

[0024] Furthermore, the primary carbon source and the secondary carbon source are selected from at least one of glucose, sucrose, starch, citric acid, and ascorbic acid.

[0025] Furthermore, in step (1), the molar ratio of lithium source, iron source and phosphorus source is (1.04~1.08):(0.92~0.98):(1.01~1.09).

[0026] Furthermore, the iron source is selected from at least one of ferric oxide, magnetite, ferrous oxide, ferrous chloride, ferrous sulfate, and ferrous oxalate;

[0027] And / or, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate;

[0028] And / or, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium chloride, lithium nitrate, and lithium phosphate.

[0029] Furthermore, the carbothermic reduction reaction in step (1) includes a ball milling step, wherein the ball milling particle size is 0.30 μm ≤ D50 ≤ 0.60 μm, or the ball milling particle size is 0.85 μm ≤ D90 ≤ 1.95 μm.

[0030] A third aspect of the present invention provides a cathode material comprising the above-described lithium iron phosphate cathode material, or comprising a low specific surface area lithium iron phosphate cathode material prepared according to the above-described preparation method.

[0031] A fourth aspect of the present invention provides a lithium battery comprising the above-described positive electrode.

[0032] The beneficial technical effects of this invention are as follows:

[0033] This invention optimizes the iron oxide red process, using a two-stage carbothermal reduction method to prepare lithium iron phosphate cathode materials. By controlling the particle size of the secondary fine grinding and secondary crushing, and optimizing the amount of secondary carbon source incorporated, the specific surface area of ​​the lithium iron phosphate cathode material is reduced while ensuring excellent battery performance.

[0034] The lithium iron phosphate cathode material of this invention has a significantly lower specific surface area than existing cathode materials, and its resistivity, 0.1C specific capacity and other electrical properties meet the performance requirements of lithium batteries. The low specific surface area lithium iron phosphate cathode material of this invention is beneficial for increasing the solid content, reducing the amount of glue used, accelerating the production pace, improving production efficiency and saving costs during the later stage of battery manufacturing and coating.

[0035] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Attached Figure Description

[0036] The accompanying drawings, incorporated in and forming part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. It is obvious that the drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other drawings based on these drawings without inventive effort. In the drawings:

[0037] Figure 1 This is a flowchart illustrating a method for preparing a low specific surface area lithium iron phosphate cathode material according to an embodiment of this application. Detailed Implementation

[0038] Unless otherwise defined, all technical and / or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should be understood that certain features of the invention (described in the context of separate embodiments for clarity) may also be provided in combination in a single embodiment. Conversely, multiple features of the invention (described in the context of a single embodiment for brevity) may also be provided separately or in any suitable combination or, where appropriate, in any other described embodiment of the invention. Certain features described in the context of various embodiments will not be considered essential features of those embodiments unless the embodiment is inoperable without those elements. The invention is further illustrated below by specific examples; however, it should be noted that the specific process conditions and results described in the embodiments of the invention are merely illustrative and should not be construed as limiting the scope of protection of the invention. All equivalent changes or modifications made in accordance with the spirit and essence of the invention should be covered within the scope of protection of the invention.

[0039] First, it should be noted that the materials used in the embodiments and comparative examples of this application are all commercially available.

[0040] This invention provides a method for preparing a low specific surface area lithium iron phosphate cathode material, the method comprising the following steps:

[0041] (1) A phosphorus source, lithium source, iron source and primary carbon source are mixed and then ball-milled, spray-dried, sintered and pulverized to complete a primary carbothermic reduction reaction to obtain a precursor.

[0042] (2) A secondary carbon source is incorporated into the precursor, and the secondary carbothermic reduction reaction is completed by sequential coarse grinding, fine grinding, spray drying, sintering, and pulverization to obtain lithium iron phosphate cathode material.

[0043] The amount of secondary carbon source incorporated is 4.3% of the precursor mass; the fine grinding particle size is 0.8μm≦D50≦0.9μm or 1.65μm≦D90≦1.85μm; the secondary grinding particle size of the lithium iron phosphate cathode material is 1.00μm≦DV50≦1.10μm.

[0044] In one embodiment of this application, the iron source is selected from at least one of ferric oxide, magnetite, ferrous oxide, ferrous chloride, ferrous sulfate, and ferrous oxalate; the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; and the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium chloride, lithium nitrate, and lithium phosphate. The primary and secondary carbon sources are selected from at least one of glucose, sucrose, starch, citric acid, and ascorbic acid.

[0045] In one embodiment of this application, the mixture in step (1) further includes a dopant and a medium water, and the dopant includes polyethylene glycol and titanium dioxide.

[0046] In one embodiment of this application, step (1) is specifically as follows:

[0047] (1.1) Add medium water and lithium source to the reactor in sequence, stir for 1 hour, then add phosphorus source and stir for 1 hour, then add iron source and stir for 0.5 hours. After stirring, add dopant and carbon source in sequence and continue stirring for 0.5 hours. During the above process, add medium water each time material is added to prepare a slurry with a solid content of 40% to 60%.

[0048] The above slurry was fed into a sand mill equipped with zirconia balls for ball milling. After 1 hour of ball milling, samples were taken for particle size analysis to control the final ball-milled particle size to be 0.30 μm ≤ D50 ≤ 0.60 μm or 0.85 μm ≤ D90 ≤ 1.95 μm.

[0049] The mixer used in the above mixing process was set to a frequency of 23Hz; the sand mill used 1.0μm to 1.2μm zirconia balls and the rotation speed was set to 680rpm.

[0050] The molar ratio of lithium source, iron source and phosphorus source is (1.04~1.08):(0.92~0.98):(1.01~1.09).

[0051] (1.2) The ball-milled material is fed into an atomizing tower for the first spray drying. The inlet air temperature is set to 240±5℃, the outlet air temperature is set to 110±5℃, the atomizer frequency is 35Hz, and the atomization flow rate is 0.6±0.1m³. 3 / h, the bag temperature is 92±2℃.

[0052] (1.3) The material after the first spray drying is fed into a roller kiln for the first sintering. During sintering, the loading rate is 10±1 kg, the roller speed is 3562 mm / h, and the kiln temperature zones 1-14 are set to 280℃, 380℃, 500℃, 580℃, 620℃, 650℃, 680℃, 715℃, 715℃, 715℃, 715℃, 715℃, and 715℃. The nitrogen flow rate is set to 480 m³ / h. 3 / h, nitrogen and oxygen content <50ppm, furnace pressure <50Pa.

[0053] (1.4) The material after the first sintering is crushed using a pulverizer to obtain the precursor. The feeding screw frequency is set to 50Hz, the crushing frequency is set to 25Hz, the crushing current is set to 35±5A, the grading frequency is set to 35Hz, and the induced draft frequency is set to 50Hz.

[0054] In one embodiment of this application, step (2) is specifically as follows:

[0055] (2.1) A secondary carbon source and medium water are added to the precursor and stirred in a reactor. The amount of secondary carbon source added is 4.3% of the precursor mass. The stirring motor frequency is set to 23Hz to obtain a slurry with a solid content of 45% to 55%.

[0056] The above slurry was fed into a sand mill equipped with zirconia balls for coarse grinding. The coarse grinding speed was set to 1000 rpm, and the size of the zirconia balls was 1.4 μm to 1.6 μm.

[0057] After coarse grinding, the rotation speed is set to 1200 rpm and the size of the zirconia balls is 0.4μm to 0.5μm for fine grinding. The final fine grinding particle size is controlled to be 0.8μm≦D50≦0.9μm or 1.65μm≦D90≦1.85μm.

[0058] In this application, the molar ratio of carbon source to iron source is 1:4, whereby the carbon source includes a primary carbon source and a secondary carbon source. The secondary carbon source is primarily used as a coating carbon for lithium iron phosphate to reduce the resistivity of the lithium iron phosphate cathode material. To ensure the battery performance meets requirements, the amount of coating carbon should be appropriately reduced. Therefore, this application optimizes the amount of secondary carbon source incorporated and limits the molar ratio of carbon source to iron source to avoid excessively high carbon content in the lithium iron phosphate cathode material, which could limit lithium-ion transport and reduce the cathode material's capacity.

[0059] (2.2) The finely ground material is fed into an atomizing tower for a second spray drying. The inlet air temperature is set to 200℃, the outlet air temperature is set to 70℃, the atomizer frequency is set to 35Hz, and the atomization flow rate is set to 0.82m³ / h. 3 / h, bag temperature set to 66℃, atomizer cooling temperature 55℃, tower negative pressure -220Pa, air supply frequency set to 29Hz, induced draft frequency set to 34Hz, and potting weight 11±1kg.

[0060] (2.3) The material after the second spray drying is fed into a roller kiln for the second sintering. During the sintering process, the loading roller speed is 2083 mm / h, the kiln temperature zones 1-10 are set to 540℃, 630℃, 690℃, 780℃, 780℃, 780℃, 780℃, 780℃, 780℃, and 780℃, and the nitrogen flow rate is set to 230 m³ / h. 3 / h, nitrogen and oxygen content <50ppm, furnace pressure <50Pa.

[0061] (2.4) The material after the second sintering is crushed using a pulverizer to obtain lithium iron phosphate cathode material. The feeding screw frequency is set to 30Hz, the crushing frequency is set to 48Hz, the crushing current is set to 30A~45A, the grading frequency is set to 40Hz, the induced draft frequency is set to 42Hz, and the secondary crushing particle size is 1.00μm≦DV50≦1.10μm.

[0062] This invention also provides a low specific surface area lithium iron phosphate cathode material, which is prepared by means of a phosphorus source, a lithium source, an iron source and a carbon source through two carbothermal reduction reactions; and the lithium iron phosphate cathode material satisfies the following characteristics (a) to (c):

[0063] (a) The specific surface area of ​​the lithium iron phosphate cathode material is 7.20 m². 2 / g~7.30m 2 / g;

[0064] (b) The carbon content of the lithium iron phosphate cathode material is 1.07% to 1.09%;

[0065] (c) The resistivity of the lithium iron phosphate cathode material is 15.70 Ω·cm to 16.70 Ω·cm.

[0066] In one embodiment of this application, the raw materials also include dopants and water as a medium.

[0067] In one embodiment of this application, the lithium iron phosphate cathode material of this application is based on as follows: Figure 1 The process shown is as follows: The raw materials are mixed and then subjected to a first ball milling, a first centrifugal spray drying, a first high-temperature sintering, and a first pulverization to obtain a precursor. The precursor is then mixed with a secondary carbon source and subjected to coarse and fine milling, a second centrifugal spray drying, a second high-temperature sintering, and a second pulverization to obtain a lithium iron phosphate cathode material with a secondary pulverization particle size of 1.00 μm ≤ DV50 ≤ 1.10 μm. After mixing and packaging, the product is obtained.

[0068] In one embodiment of this application, the specific surface area of ​​the lithium iron phosphate cathode material of this application is significantly reduced compared to conventional processes (the specific surface area of ​​existing low specific surface area lithium iron phosphate cathode materials is 12m²). 2 / g~14m 2 / g), which is beneficial to downstream battery manufacturing, as well as improving the cycle stability, safety and lifespan of batteries.

[0069] The present invention also provides a cathode, comprising the above-described lithium iron phosphate cathode material, or comprising a low specific surface area lithium iron phosphate cathode material prepared according to the above preparation method.

[0070] The present invention also provides a lithium battery comprising the above-described positive electrode.

[0071] The present invention will be described in detail below through specific examples and embodiments. It should also be understood that the following embodiments are only for specific illustration of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the appropriate range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below.

[0072] Example 1

[0073] (1) Add water and lithium carbonate to the reactor in sequence, stir for 1 hour, then add ammonium dihydrogen phosphate and stir for 1 hour, then add iron oxide and stir for 0.5 hours. After stirring, add PEG-800, titanium dioxide and glucose in sequence and continue stirring for 0.5 hours. During the above process, add water once each time material is added to prepare a slurry with a solid content of 40% to 60%.

[0074] The above slurry was fed into a sand mill equipped with zirconia balls for ball milling. After 1 hour of ball milling, samples were taken for particle size analysis to control the final ball-milled particle size to be 0.30 μm ≤ D50 ≤ 0.60 μm or 0.85 μm ≤ D90 ≤ 1.95 μm.

[0075] The mixer used in the above mixing process was set to a frequency of 23Hz; the sand mill used 1.0μm to 1.2μm zirconia balls and the rotation speed was set to 680rpm.

[0076] The molar ratio of lithium carbonate, iron oxide and ammonium dihydrogen phosphate is 1.04:0.92:1.01.

[0077] (2) The ball-milled material is fed into an atomizing tower for the first spray drying. The inlet air temperature is set to 240℃, the outlet air temperature is set to 110℃, the atomizer frequency is 35Hz, and the atomization flow rate is 0.6m³ / h. 3 / h, the bag temperature is 92℃.

[0078] (3) The material after the first spray drying is fed into a roller kiln for the first sintering. During sintering, the loading rate is 10 kg, the roller speed is 3562 mm / h, and the kiln temperature zones 1-14 are set to 280℃, 380℃, 500℃, 580℃, 620℃, 650℃, 680℃, 715℃, 715℃, 715℃, 715℃, and 715℃. The nitrogen flow rate is set to 480 m³ / h. 3 / h, nitrogen and oxygen content <50ppm, furnace pressure <50Pa.

[0079] (4) The material after the first sintering is crushed using a pulverizer to obtain the precursor. The feeding screw frequency is set to 50Hz, the crushing frequency is set to 25Hz, the crushing current is set to 35A, the grading frequency is set to 35Hz, and the induced draft frequency is set to 50Hz.

[0080] (5) Add glucose and water to the precursor and stir in a reactor. The amount of glucose added is 4.3% of the precursor mass. The stirring motor frequency is set to 23Hz to obtain a slurry with a solid content of 45% to 55%.

[0081] The above slurry was fed into a sand mill equipped with zirconia balls for coarse grinding. The coarse grinding speed was set to 1000 rpm, and the size of the zirconia balls was 1.4 μm to 1.6 μm.

[0082] After coarse grinding, the rotation speed is set to 1200 rpm and the size of the zirconia balls is 0.4μm to 0.5μm for fine grinding. The final fine grinding particle size D50 is controlled to be 0.83μm or the fine grinding particle size D90 is 1.65μm.

[0083] In this embodiment, the molar ratio of total glucose to iron oxide is 1:4.

[0084] (6) The finely ground material is fed into an atomizing tower for a second spray drying. The inlet air temperature is set to 200℃, the outlet air temperature is set to 70℃, the atomizer frequency is set to 35Hz, and the atomization flow rate is set to 0.82m³ / h. 3 / h, bag temperature set to 66℃, atomizer cooling temperature 55℃, tower negative pressure -220Pa, air supply frequency set to 29Hz, induced draft frequency set to 34Hz, and potting capacity 11kg.

[0085] (7) The material after the second spray drying is fed into a roller kiln for the second sintering. During the sintering process, the loading roller speed is 2083 mm / h, the kiln temperature zones 1-10 are set to 540℃, 630℃, 690℃, 780℃, 780℃, 780℃, 780℃, 780℃, and 780℃, and the nitrogen flow rate is set to 230 m³ / h. 3 / h, nitrogen and oxygen content <50ppm, furnace pressure <50Pa.

[0086] (8) The material after the second sintering is crushed using a pulverizer to obtain lithium iron phosphate cathode material. The feeding screw frequency is set to 30Hz, the crushing frequency is set to 48Hz, the crushing current is set to 30A, the grading frequency is set to 40Hz, the induced draft frequency is set to 42Hz, and the secondary crushing particle size DV50 is 1.06μm.

[0087] Examples 2 to 3, Comparative Examples 1 to 4

[0088] The difference between the above embodiments and comparative examples and Example 1 is that the lithium iron phosphate cathode material was prepared according to the method of Example 1 and the parameters shown in Table 1 below.

[0089] Table 1

[0090] Group Fine grinding particle size / μm Secondary carbon source incorporation amount / % Secondary grinding particle size / μm Example 2 D50:0.85 / D90:1.75 4.3 DV50:1.09 Example 3 D50:0.82 / D90:1.85 4.3 DV50:1.03 Comparative Example 1 D50:0.84 / D90:1.65 4.0 DV50:1.08 Comparative Example 2 D50:0.83 / D90:1.85 4.5 DV50:1.02 Comparative Example 3 D50:0.76 / D90:1.55 4.3 DV50:0.95 Comparative Example 4 D50:0.93 / D90:1.95 4.3 DV50:1.23

[0091] Performance testing

[0092] Carbon content: The carbon content of the finished lithium iron phosphate cathode materials in Examples 1-3 and Comparative Examples 1-4 was determined based on the "Chemical Analysis Methods for Lithium Iron Phosphate Part 4: Determination of Carbon Content". The results are shown in Table 2.

[0093] Specific surface area: The specific surface area of ​​the finished lithium iron phosphate cathode materials of Examples 1-3 and Comparative Examples 1-4 was determined by the BET (Brunauer-Emmett-Teller) method. The results are shown in Table 2.

[0094] Resistivity: In accordance with the provisions of GB / T30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Materials for Lithium-ion Batteries", the resistivity of the finished lithium iron phosphate cathode materials of Examples 1-3 and Comparative Examples 1-4 was determined by the four-probe method. The results are shown in Table 2.

[0095] Specific capacity: Electrochemical testing technology was used to measure the first-charge specific capacity, 0.1C specific capacity, 1C specific capacity and 0.1C efficiency of the finished lithium iron phosphate cathode materials in Examples 1-3 and Comparative Examples 1-4, respectively. The measurement results are shown in Table 2.

[0096] The experimental data and analysis are as follows:

[0097] Table 2

[0098]

[0099]

[0100] As shown in Table 2, when the secondary grinding particle size is too high, the specific capacity of the lithium iron phosphate cathode material does not meet the requirements; when the secondary grinding particle size is too low, the 0.1C efficiency of the lithium iron phosphate cathode material is low, and the slurry is prone to agglomeration, affecting the performance of downstream batteries. Therefore, this application controls the secondary grinding particle size range to be 0.8μm≦D50≦0.9μm or the grinding particle size to be 1.65μm≦D90≦1.85μm.

[0101] As shown by the data from Examples 1-3 and Comparative Examples 1-2, the secondary carbon doping in this application mainly serves as coating carbon for the lithium iron phosphate precursor, thereby reducing the resistivity of the lithium iron phosphate cathode material. Lower resistivity helps reduce the battery's internal resistance, improves the battery's charge and discharge efficiency, reduces heat generation, and thus enhances the battery's stability and lifespan. Furthermore, when the cathode material has a high carbon content, it can limit lithium-ion transport to some extent, potentially leading to a reduction in the material's capacity. Therefore, to ensure the battery performance meets requirements, the carbon content should be appropriately reduced. Consequently, this application limits the amount of secondary carbon source incorporated to 4.3% of the precursor mass.

[0102] In this application, the particle size of the secondary grinding is affected by the particle size of the secondary fine grinding. If the particle size of the secondary grinding is too small, the carbon layer of the cathode material will peel off. If the particle size of the secondary grinding is too large, the sintered agglomerated material will not be opened up, resulting in more pores, which will affect the specific surface area of ​​the final cathode material and thus affect the coating of the downstream battery manufacturing. Therefore, this application limits the particle size of the secondary grinding to 1.00μm≦DV50≦1.10μm.

[0103] As can be seen from the above data, the lithium iron phosphate cathode material prepared in this application embodiment has low resistivity and high specific capacity on the basis of low specific surface area. The specific capacity at 0.1C is greater than 154mAh / g. The cathode material performance meets the requirements for battery use, and has higher safety and better stability.

[0104] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A low specific surface area lithium iron phosphate cathode material, characterized in that, The raw materials for the lithium iron phosphate cathode material include a phosphorus source, a lithium source, an iron source, and a carbon source, and are prepared through two carbothermal reduction reactions; and the lithium iron phosphate cathode material satisfies the following characteristics (a) to (c): (a) The specific surface area of ​​the lithium iron phosphate cathode material is 7.20 m². 2 / g~7.30m 2 / g; (b) The carbon content of the lithium iron phosphate cathode material is 1.07% to 1.09%; (c) The resistivity of the lithium iron phosphate cathode material is 15.70 Ω·cm to 16.70 Ω·cm.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The lithium iron phosphate cathode material is obtained through a second grinding process. After the second grinding, the particle size of the lithium iron phosphate cathode material satisfies the following relationship: 1.00μm≦DV50≦1.10μm.

3. The lithium iron phosphate cathode material according to claim 1, characterized in that, The raw materials also include dopants, including polyethylene glycol and titanium dioxide.

4. A method for preparing a low specific surface area lithium iron phosphate cathode material, characterized in that, The method comprises the following steps: (1) A precursor is obtained by performing a carbothermic reduction reaction on a phosphorus source, a lithium source, an iron source and a primary carbon source; (2) A secondary carbon source is incorporated into the precursor to perform a secondary carbothermal reduction reaction to obtain the lithium iron phosphate cathode material. The secondary carbothermal reduction reaction includes the steps of fine grinding, sintering, and pulverization. The amount of secondary carbon source incorporated is 4.3% of the precursor mass; the fine grinding particle size is 0.8μm≦D50≦0.9μm, or the fine grinding particle size is 1.65μm≦D90≦1.85μm; the secondary grinding particle size of the lithium iron phosphate cathode material is 1.00μm≦DV50≦1.10μm.

5. The preparation method according to claim 4, characterized in that, The primary carbon source and the secondary carbon source are selected from at least one of glucose, sucrose, starch, citric acid, and ascorbic acid.

6. The preparation method according to claim 4, characterized in that, The molar ratio of the lithium source, the iron source and the phosphorus source in step (1) is (1.04~1.08):(0.92~0.98):(1.01~1.09).

7. The preparation method according to claim 6, characterized in that, The iron source is selected from at least one of ferric oxide, ferric oxide, ferrous oxide, ferrous chloride, ferrous sulfate, and ferrous oxalate. And / or, the phosphorus source is selected from at least one of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; And / or, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide, lithium chloride, lithium nitrate, and lithium phosphate.

8. The preparation method according to claim 4, characterized in that, The carbothermic reduction reaction in step (1) includes a ball milling step, wherein the ball milling particle size is 0.30μm≦D50≦0.60μm, or the ball milling particle size is 0.85μm≦D90≦1.95μm.

9. A positive electrode, characterized in that, Includes the lithium iron phosphate cathode material according to any one of claims 1 to 3, or includes the low specific surface area lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 4 to 8.

10. A lithium battery, characterized in that, Includes the positive electrode as described in claim 9.

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