A highly conductive carbon-coated lithium iron phosphate cathode material, its preparation method and application

Abstract

This invention discloses a highly conductive carbon-coated lithium iron phosphate cathode material, its preparation method, and its applications, comprising the following steps: dividing Ketjen black into several portions, adding the Ketjen black portion by portion to a dispersion containing a dispersant under stirring and ultrasonication to obtain a carbon nanotube conductive network slurry; subsequently adding polyethylene glycol to the slurry and stirring to form a conductive colloid; preparing a precursor solution by dissolving iron phosphate, lithium carbonate, and glucose; adding the conductive colloid to the precursor solution under stirring, and continuously stirring to obtain a composite slurry; ball milling the composite slurry and then spray granulating it to obtain precursor powder; and heat-treating the precursor powder. The process of this invention not only significantly reduces the volume resistivity of the material but also maintains a reasonable compaction density. Compared with conventional simple physical mixing or single carbon coating processes, this composite material exhibits significant advantages in conductivity, cycle stability, and process controllability.

Description

Technical Field

[0001] This invention belongs to the field of battery materials, and particularly relates to a highly conductive carbon-coated lithium iron phosphate, its preparation method, and its application. Background Technology

[0002] With the widespread application of lithium-ion batteries in electric vehicles, energy storage systems, and other fields, lithium iron phosphate (LiFePO4) cathode materials have attracted much attention due to their high safety, long cycle life, and environmentally friendly properties. However, this material has a low intrinsic electronic conductivity (approximately 10⁻⁶). -9 Problems such as S / cm and slow lithium-ion diffusion severely restrict its application performance in high-power batteries. In particular, during high-current charging and discharging, the internal resistance of the material leads to increased battery polarization, limited capacity utilization, and decreased cycle performance.

[0003] Currently, carbon material composite modification is commonly used to improve the conductivity of lithium iron phosphate (LFP) batteries by constructing conductive networks on their surfaces to enhance electron transport capabilities. Commonly used carbon materials include graphene, carbon nanotubes, and conductive carbon black, which, due to their high conductivity and unique morphology, facilitate the formation of effective conductive pathways. However, existing carbon composite processes still face a series of prominent problems: carbon materials are prone to agglomeration, making it difficult to disperse uniformly in the matrix and resulting in discontinuous conductive networks; the interfacial bonding between carbon materials and LFP particles is weak, increasing interfacial contact resistance and affecting charge transport efficiency; the introduction of high carbon content often reduces the tap density and compaction density of the material, thus affecting the volumetric energy density of the battery; in addition, some composite processes, such as vapor deposition, have high equipment requirements and high energy consumption, making them difficult to adapt to the needs of large-scale production, and cost control faces challenges.

[0004] While existing technologies such as carbon coating and carbothermal reduction can partially improve conductivity, they often suffer from problems such as uneven coating layers and imprecise control of graphitization, making it difficult to stably reduce the powder resistivity to below 10 Ω·cm. Furthermore, it is challenging to simultaneously achieve optimal material processing performance and compaction density. At the same time, systematic and in-depth research is lacking on the composite effects of carbon materials of different dimensions and their interfacial interaction mechanisms with lithium iron phosphate, hindering breakthroughs in composite material performance and industrial applications.

[0005] Therefore, there is an urgent need to develop a new lithium iron phosphate / carbon composite process that can achieve uniform dispersion of carbon materials, excellent conductivity, and good processing characteristics. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the background art above, and to provide a carbon-coated lithium iron phosphate cathode material, its preparation method and application, so as to improve the conductivity of lithium iron phosphate.

[0007] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0008] A method for preparing a highly conductive carbon-coated lithium iron phosphate cathode material includes the following steps:

[0009] S1. Ketjenblack is divided into several portions, and under stirring and ultrasonication, the Ketjenblack is added portion by portion to a dispersion containing a dispersant to obtain a carbon nanotube conductive network slurry. Then, polyethylene glycol is added to the slurry, and the mixture is stirred at 45-60°C to form a conductive colloid. The amount of polyethylene glycol added is 8-18% of the total mass of Ketjenblack.

[0010] S2. Prepare a precursor solution by mixing iron phosphate, lithium carbonate and glucose. Add the conductive colloid obtained in S1 to the precursor solution while stirring. Continue stirring to obtain a composite slurry.

[0011] S3. The composite slurry obtained in S2 is ball-milled and then spray-granulated to obtain precursor powder;

[0012] S4. The precursor powder obtained in S3 is subjected to heat treatment to obtain the highly conductive carbon-coated lithium iron phosphate cathode material;

[0013] The heat treatment is divided into the following three stages:

[0014] In the first stage, the temperature is raised to 120~180℃ and held in an inert atmosphere, and then raised to 280~350℃ and held.

[0015] In the second stage, the temperature is raised to 400~500℃ and held in an inert or weakly reducing atmosphere.

[0016] The third stage involves heating the material to 650-750℃ in a reducing atmosphere and then holding it at that temperature.

[0017] As a further improvement, the dispersion containing the dispersant in S1 is prepared by the following steps: dissolving the dispersant in a solvent and then subjecting it to mechanical stirring and ultrasonic treatment to form a dispersion;

[0018] The dispersant is at least one of lignin sulfonate, polyvinylpyrrolidone, or sodium dodecyl sulfate, and its addition amount is 0.3%-0.8% of the theoretical mass of lithium iron phosphate.

[0019] As a further improvement, in S1, the Ketjen black conductive agent is divided into 4-6 parts. The first part of the conductive agent is added to the dispersion under shear conditions of 800-1200 rpm and sheared and dispersed. Then, the remaining Ketjen black is added part by part under stirring conditions of 200-400 rpm and ultrasonic synergy. After all the addition is completed, the carbon nanotube conductive network slurry is obtained under shear conditions of 800-1200 rpm and ultrasonic synergy.

[0020] As a further improvement, polyethylene glycol was added to S1 and stirred to form a conductive colloid with a viscosity of 800-2500 cP.

[0021] The molecular weight of the polyethylene glycol is 200-600.

[0022] As a further improvement, the amount of Ketjen black added is 5.0%-10.0% of the theoretical mass of lithium iron phosphate, and the amount of glucose added is 20%-35% of the theoretical mass of lithium iron phosphate. The total carbon content in the final highly conductive carbon-coated lithium iron phosphate cathode material is 1.5-2.5 wt%.

[0023] As a further improvement, S2 adds the conductive colloid to the precursor solution and stirs and composites it at 45-65°C.

[0024] As a further improvement, the heat treatment described in S4 includes:

[0025] In the first stage, in an inert atmosphere, the temperature is increased to 120~180℃ at a heating rate of 1-3℃ / min and held for 0.5-1.5 hours, and then increased to 280~350℃ at the same rate and held for 1-3 hours.

[0026] In the second stage, under an inert or weakly reducing atmosphere, the temperature is increased to 400-500℃ at a rate of 3-5℃ / min and held for 1.5-3 hours; the weakly reducing atmosphere is an argon-hydrogen mixture, with hydrogen accounting for 3%-8% of the volume.

[0027] In the third stage, the temperature is increased to 650~750℃ in a reducing atmosphere at a heating rate of 2-5℃ / min and held for 8-15 hours; the reducing atmosphere is an argon-hydrogen mixture with hydrogen accounting for 5%-10% of the volume.

[0028] The present invention also provides a highly conductive carbon-coated lithium iron phosphate cathode material, which is prepared by the preparation method described above.

[0029] The present invention also provides a lithium-ion battery positive electrode sheet, which comprises the aforementioned highly conductive carbon-coated lithium iron phosphate positive electrode material.

[0030] The present invention also provides a lithium-ion battery comprising the aforementioned lithium-ion battery positive electrode.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] This invention employs a stepwise addition of Ketjenblack and a mechanical-ultrasonic synergistic dispersion technique, along with in-situ curing of polyethylene glycol, to construct a viscoelastic conductive colloid, thus resolving the subsequent ball milling stratification issue. A precise carbon content balancing strategy is implemented, controlling the addition of Ketjenblack and glucose to achieve a final carbon content of 1.5-2.5 wt%. A precisely controlled three-stage programmed heat treatment process is designed, with staged heating in an inert and reducing atmosphere to complete the decomposition, carbonization, and crystallization of organic matter. These three technologies work synergistically to successfully construct a continuous and stable three-dimensional conductive network within a lithium iron phosphate matrix, resulting in a lithium iron phosphate cathode material with a uniform conductive network, uniform crystal size (1-3 μm), and excellent conductivity.

[0033] The process of this invention not only significantly reduces the volume resistivity of the material and maintains a reasonable compaction density, but also provides an effective technical path for preparing high-performance lithium-ion battery cathode materials.

[0034] Compared with conventional simple physical mixing or single carbon coating processes, this composite material exhibits significant advantages in conductivity, cycle stability, and process controllability, providing a reliable material solution for the development of next-generation high-power lithium-ion batteries. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a TEM image of the lithium iron phosphate cathode material of Example 1.

[0037] Figure 2 This is a photograph of the lithium iron phosphate cathode material from Example 1.

[0038] Figure 3 The button battery made of lithium iron phosphate cathode material in Example 1, after the first activation, shows the battery capacity versus charge cycle curve at 0.1C.

[0039] Figure 4 The button battery made of lithium iron phosphate cathode material in Example 1, after the first activation, shows the battery capacity versus discharge cycles at 0.1C.

[0040] Figure 5 The graph shows the capacity and number of cycles of a button battery made from the lithium iron phosphate cathode material of Example 1, after 1C fast charging and then 0.5C charging.

[0041] Figure 6The graph shows the capacity and number of cycles of a button battery made from the lithium iron phosphate cathode material of Example 1, after 1C fast charging and 1C discharge.

[0042] Figure 7 The graph shows the capacity and number of cycles of a button battery made from the lithium iron phosphate cathode material of Example 1, after 5C fast charging and then 0.5C charging.

[0043] Figure 8 The graph shows the capacity and number of cycles of a button battery made from the lithium iron phosphate cathode material of Example 1, after 5C fast charging and then 5C discharge.

[0044] Figure 9 The button batteries made of lithium iron phosphate cathode material (Comparative Examples 1-3) are shown in the capacity versus charge cycle curves at 0.1C after the first activation cycle. Detailed Implementation

[0045] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0046] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0047] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0048] In some specific embodiments, the preparation method of the highly conductive carbon-coated lithium iron phosphate cathode material of the present invention includes the following steps:

[0049] (1) Construction of conductive carbon network: dispersant selection and synergistic dispersion

[0050] At least one of lignin sulfonate, polyvinylpyrrolidone (PVP) or sodium dodecyl sulfate (SDS) is used as a dispersant.

[0051] Weigh out 0.3%-0.8% of the theoretical mass of lithium iron phosphate (calculated based on the stoichiometric ratio of the masses of iron phosphate and lithium carbonate according to the chemical reaction), and dissolve it in anhydrous ethanol. The mass ratio of dispersant to solvent is 1:(80-120). Combine mechanical stirring (400-600 rpm) and ultrasonic treatment (frequency 35-45 kHz, power 250-350 W) for 20-40 minutes to obtain a homogeneous dispersion. The treatment process is as follows: first, mechanically stir for 10-20 minutes for initial mixing, then turn on the ultrasonic treatment and maintain stirring for 10-30 minutes to obtain a highly homogeneous and stable dispersion.

[0052] (2) Stepwise dispersion of conductive agent and solidification of slurry

[0053] Ketjen Black conductive agent (total addition amount of 5.0%-10.0% of the theoretical mass of lithium iron phosphate) was precisely and evenly divided into 4-6 portions. Under continuous high-speed shear dispersion (800-1200 rpm), the first portion of conductive agent was added to the dispersion and treated for 5-15 minutes to initially break up agglomerates. Subsequently, while maintaining low-speed stirring (200-400 rpm), ultrasound (frequency 35-45 kHz) was activated, and the remaining conductive agent was added in portions every 15-25 minutes, utilizing ultrasonic cavitation to achieve nanoscale dispersion. After all the conductive agent was added, high-speed shear (800-1200 rpm) and ultrasound synergistic treatment continued for 0.5-1.5 hours to ensure the formation of a stable carbon nanotube conductive network slurry.

[0054] Subsequently, polyethylene glycol (PEG, molecular weight 200-600, added at 8-18% of the total solids mass of the conductive colloid (i.e., the total mass of Ketjen Black)) is added as a curing agent and pore-forming agent. The mixture is stirred for 1.5-2.5 hours at 45-60℃ and 500-700 rpm, during which the solvent slowly evaporates, ultimately forming a homogeneous conductive colloid with high viscosity and suitable flowability (800-2500 cP).

[0055] (3) Preparation and compounding of precursor solutions

[0056] Accurately weigh iron phosphate and lithium carbonate according to the stoichiometric ratio Li:Fe = 1.03-1.08:1. Add them together with glucose (20%-35% of the theoretical mass of lithium iron phosphate) to deionized water, and stir magnetically in a water bath at 50-70℃ until completely dissolved to prepare a clear precursor solution with a solid content (mass percentage of iron phosphate + lithium carbonate) of 18-25 wt%.

[0057] All the conductive colloids obtained in step (2) (the Ketjen black in the conductive colloids accounts for about 5%-10% of the total mass of iron phosphate and lithium carbonate in the precursor) are slowly added to the precursor solution under high-speed stirring, and stirred continuously at 45-65℃ for 1.5-2.5 hours to achieve preliminary molecular-level composite and homogenization of the conductive colloids and the precursor ionic solution, thus obtaining the composite slurry.

[0058] (4) Mechanical ball milling: efficient wet mixing and refining

[0059] Transfer the composite slurry into a vacuum ball mill jar, and add zirconia grinding balls of different diameters at a ball-to-material ratio of (8:1) to (12:1) (recommended ratio: Φ5mm:Φ3mm = 1:(1.5-2.5)). Perform wet ball milling at 300-400 rpm for 3-5 hours until the slurry D50 reaches less than 1.0 μm (preferably 0.5-1.0 μm).

[0060] The ball milling process is carried out under vacuum or inert gas protection to prevent material oxidation and eliminate bubbles, so that the material can be fully refined, mixed and activated at the nanoscale.

[0061] (5) Spray granulation: precise parameter control and precursor forming

[0062] The ball-milled slurry is filtered through a 150-250 mesh sieve to remove residual grinding ball fragments and a very small amount of large particles. Granulation is performed using a centrifugal spray dryer, with the feed rate and atomizing disc speed controlled to match the slurry viscosity. The inlet temperature is set at 210-230℃, and the outlet temperature is controlled at 85-105℃. By optimizing the parameters, spherical precursor powder with good sphericity, narrow particle size distribution, and excellent flowability is obtained, with a target particle size D50 of 1-3 μm. The powder should be loose and free of lumps, with a moisture content of less than 1.5%.

[0063] (6) Staged heat treatment and carbon coating: programmed temperature-controlled atmosphere sintering

[0064] This process takes place in a programmable temperature-controlled tube furnace and is divided into three stages, designed to precisely control the decomposition, carbonization, and crystallization growth of organic matter.

[0065] Phase 1: Dehydration and initial reaction with organic matter (inert atmosphere)

[0066] The temperature is increased to 120-180℃ at a rate of 1-3℃ / min and held for 0.5-1.5 hours to completely remove residual physically adsorbed water. Then, the temperature is increased to 280-350℃ at the same rate and held for 1-3 hours to promote the full evaporation or initial decomposition of organic matter such as PEG and lignin sulfonate, forming a stable intermediate framework, while glucose undergoes a caramelization reaction.

[0067] Phase Two: Carbonization and Fe3+ Partial reduction (inert or weakly reducing atmosphere)

[0068] Switch the atmosphere to an inert gas (such as argon or nitrogen) or a weakly reducing atmosphere (such as an argon-hydrogen mixture with hydrogen accounting for 3%-8% by volume), and heat to 400-500℃ at a relatively rapid rate of 3-5℃ / min, holding at that temperature for 1.5-3 hours. During this stage, the pre-oxidized organic matter undergoes stable carbonization in an oxygen-deficient environment, generating a uniform amorphous carbon precursor, and completing the Fe... 3+ Partial reduction leads to the formation of lithium iron phosphate crystal nuclei.

[0069] Third stage: Crystallization and carbon coating (reducing atmosphere)

[0070] The atmosphere is adjusted to a stronger reducing atmosphere (such as an argon-hydrogen mixture, with hydrogen accounting for 5%-10% by volume), and the temperature is increased to the target crystallization temperature of 650-750℃ at a heating rate of 2-5℃ / min, and held at this temperature for an extended period of 8-15 hours. This is the core crystallization stage, where crystal nuclei grow fully under the reducing atmosphere and constant high temperature to form complete and regular olivine-type LiFePO4 crystals. Simultaneously, amorphous carbon undergoes high-temperature graphitization (short-range ordering), forming a continuous, dense, and highly conductive carbon coating layer on the crystal surface. The prolonged holding time ensures complete reaction, uniform crystal size, and avoids overgrowth.

[0071] After the entire process is completed, the temperature is programmed to decrease to room temperature at a rate of ≤5℃ / min under a protective atmosphere, and carbon-coated lithium iron phosphate composite material with carbon content of 1.5-2.5 wt% and particle size of 1-3 μm is finally obtained.

[0072] During implementation, the following three key conditions need to be precisely controlled: First, a stepwise dispersion of Ketjenblack and in-situ curing of PEG are employed. The conductive agent is dispersed in 4-6 parts sequentially through high-speed shearing and ultrasonic synergy, and PEG is used to form a viscoelastic colloid (viscosity 800-2500 cP), effectively anchoring the Ketjenblack and fundamentally solving the stratification problem caused by density differences during subsequent ball milling, ensuring the uniform construction of the three-dimensional conductive network. Second, a precise carbon content control strategy is implemented, controlling the amount of Ketjenblack added within the range of this invention, while also taking into account the loss of glucose and other organic components during heat treatment, so that the final carbon content precisely reaches 1.5-2.5 wt%, thus constructing a continuous conductive pathway while avoiding hindering lithium-ion diffusion. Third, a three-stage programmed heat treatment is designed. In an argon atmosphere, segmented heat treatment at 120-350℃ achieves gentle pre-decomposition of organic matter, and carbonization and Fe are completed in an inert atmosphere at 400-500℃. 3+Pre-reduction followed by crystallization at 650-750℃ for 8-15 hours in a reducing atmosphere. Precise matching of the heating rate (1-5℃ / min) with the holding time ensures complete reaction, uniform crystal size (1-3μm), and avoids overgrowth, while simultaneously forming a dense graphitized carbon coating. These three innovations form a closed-loop technology, jointly guaranteeing the material's excellent electronic conductivity and structural stability.

[0073] Example 1

[0074] This embodiment provides a method for preparing a highly conductive carbon-coated lithium iron phosphate cathode material, the specific steps of which are as follows:

[0075] (1) Weigh 1.25 g of polyvinylpyrrolidone (PVP) and dissolve it in 125 g of anhydrous ethanol. Use 500 rpm mechanical stirring and 300 W, 40 kHz ultrasound to treat for 30 minutes.

[0076] (2) Weigh 15 grams of Ketjen Black EC-600JD and divide it into 4 equal portions (3.75 grams each). Add the first portion under high-speed shear at 1000 rpm and treat for 10 minutes. Then, add one portion of Ketjen Black every 20 minutes under stirring at 300 rpm and sonication at 40 kHz. After all portions have been added, continue high-speed shearing at 1000 rpm and sonication for 1 hour. Add about 1.6 grams of PEG-400 and stir at 55°C and 600 rpm for 2 hours to obtain a conductive colloid with a viscosity of about 1500 cP.

[0077] (3) Weigh 200g of iron phosphate and lithium carbonate according to Li:Fe = 1.05:1, and dissolve them together with 50g of glucose in deionized water to prepare a 20 wt% solution. Add all the conductive colloid obtained in step (2) and stir at 55℃ for 2 hours.

[0078] (4) Transfer the composite slurry into a ball mill jar, add zirconia grinding balls at a ball-to-material ratio of 10:1, and ball mill at 350 rpm for 4 hours. After filtration, the D50 is measured to be 0.6 μm.

[0079] (5) Spray drying: inlet temperature 220℃, outlet temperature 95℃, to obtain precursor powder with D50 of 2.1 μm and moisture content of less than 1.5%.

[0080] (6) Heat treatment: First stage: Argon atmosphere, heat up to 150℃ at 2℃ / min and hold for 1 hour, then heat up to 320℃ at the same rate and hold for 2 hours. Second stage: Argon atmosphere, heat up to 450℃ at 5℃ / min and hold for 2 hours. Third stage: Ar + 5% H2 atmosphere, heat up to 700℃ at 3℃ / min and hold for 12 hours, then cool down at a programmed rate of 3℃ / min to obtain highly conductive carbon-coated lithium iron phosphate cathode material.

[0081] Figure 1 The image shows a TEM image of the lithium iron phosphate cathode material from Example 1, illustrating that the lithium iron phosphate is granular with a particle size range of 0.1-1.5 micrometers. Figure 2 The image shows a physical picture of the lithium iron phosphate cathode material from Example 1. The product is in the form of a black powder.

[0082] Example 2

[0083] The only difference between this embodiment and embodiment 1 is that the amount of Ketjen Black EC-600JD added in step (2) is changed to 10 grams (divided into 4 portions, each portion being 2.5 grams). All other steps and parameters are exactly the same as in embodiment 1.

[0084] Example 3

[0085] The only difference between this embodiment and embodiment 1 is that the amount of glucose added in step (3) is changed to 40 grams, and all other steps and parameters are exactly the same as in embodiment 1.

[0086] Example 4

[0087] The only difference between this embodiment and Example 1 is that the dispersant in step (1) is replaced with an equal mass (1.25 g) of lignin sulfonate, and all other steps and parameters are exactly the same as in Example 1.

[0088] Comparative Example 1

[0089] A dry mixing process is employed. Specifically:

[0090] Iron phosphate, lithium carbonate (Li:Fe = 1.05:1), 50 g of glucose solid powder, 15 g of Ketjen black and 1.25 g of PVP were dry ball-milled for 2 hours (ball-to-powder ratio 10:1, 350 rpm). The mixed powder was then subjected to the same heat treatment as in Example 1, step (6).

[0091] Comparative Example 2

[0092] A one-step heat treatment without segmentation is adopted. The only difference between its preparation process and that of Example 1 is that the original three-stage programmed heating process in step (6) is omitted. Instead, the precursor powder is directly heated to 700°C at a rate of 5°C / min and held for 12 hours in a single Ar + 5% H2 reducing atmosphere, followed by programmed cooling.

[0093] Comparative Example 3

[0094] The only difference between this comparative example and Example 1 is that the heating rate and target heating temperature in step (6) of the heat treatment are changed, and it only includes two stages. Specifically:

[0095] First stage: Argon atmosphere, slowly heat to 450℃ at a rate of 10℃ / min and hold for 4 hours.

[0096] Second stage: Ar + 5% H2 atmosphere, heat to 700℃ at a rate of 10℃ / min and hold for 12 hours, then cool down at the same rate.

[0097] The volume resistivity and compaction density of the lithium iron phosphate cathode materials in the examples and comparative examples are shown in Table 1:

[0098] Table 1

[0099]

[0100] Battery performance test

[0101] Fabrication of the button cell: First, place the positive electrode shell, opening upwards, into the lower mold groove of the encapsulation mold. Next, place the lithium iron phosphate electrode sheet (active material side up, the active material being the lithium iron phosphate positive electrode material used in the examples and comparative examples) centered in the positive electrode shell, ensuring it is flat and that the aluminum foil current collector makes good contact with the bottom of the shell. Then, add 50-100 µL of electrolyte to fully wet the electrode sheet. Next, cover with a separator disc, ensuring it completely covers the electrodes to prevent short circuits, and then add another 20-50 µL of electrolyte to wet the separator. Then, lay the lithium metal sheet as the counter electrode flat in the center of the separator, and add pads as needed to ensure tight stacking. Place a wave spring sheet (convex side up) to provide continuous pressure. Finally, cover with the negative electrode shell (opening downwards), close the upper mold, and transfer to a hydraulic encapsulation machine. Slowly apply 800-1000 kgf of pressure for encapsulation. When a "click" sound is heard or significant resistance is felt, the sealing is complete. After removing the battery, check whether its appearance is flat, whether there is any leakage, and whether the curling is even. Then, let it stand in the glove box for 4-12 hours to complete the "aging" process of electrolyte wetting.

[0102] The results of the performance tests are shown in the attached figure.

[0103] Figure 3 The button battery made of lithium iron phosphate cathode material in Example 1, after the first activation, showed a capacity of 159.4-161.5 mAh / g in the 0.1C charging cycle curve.

[0104] Figure 4 The button battery made of lithium iron phosphate cathode material in Example 1, after the first activation, has a capacity of 159.2-161.1 mAh / g in the 0.1C discharge cycle curve.

[0105] Figure 5The button battery made of lithium iron phosphate cathode material in Example 1, with 1C fast charging and then 0.5C charging, shows the battery capacity versus charging cycles. The battery capacity is 160.6-163.2 mAh / g.

[0106] Figure 6 The button battery made of the lithium iron phosphate cathode material of Example 1 is shown in the battery capacity and number of cycles after 1C fast charging and 1C discharge. The battery capacity is 144.6-149.4 mAh / g.

[0107] Figure 7 The button battery made of lithium iron phosphate cathode material in Example 1, with 5C fast charging and then 0.5C charging, shows the battery capacity versus charging cycles. The battery capacity is 143.6-152.2 mAh / g.

[0108] Figure 8 The button battery made of lithium iron phosphate cathode material in Example 1 is shown in the battery capacity and number of cycles after 5C fast charging and 5C discharge. The battery capacity is 120.6-132.2 mAh / g.

[0109] Figure 9 The button batteries made of lithium iron phosphate cathode material (comparative examples 1-3) showed a capacity of 115.3-125.2 mAh / g after the first activation cycle, as shown in the 0.1C charging cycle curve.

[0110] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Therefore, any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention should fall within the protection scope of the present invention.

Claims

1. A method for preparing a highly conductive carbon-coated lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1. Ketjenblack is divided into several portions, and under stirring and ultrasonication, the Ketjenblack is added portion by portion to a dispersion containing a dispersant to obtain a carbon nanotube conductive network slurry. Then, polyethylene glycol is added to the slurry, and the mixture is stirred at 45-60°C to form a conductive colloid. The amount of polyethylene glycol added is 8-18% of the total mass of Ketjenblack. S2. Prepare a precursor solution by mixing iron phosphate, lithium carbonate and glucose. Add the conductive colloid obtained in S1 to the precursor solution while stirring. Continue stirring to obtain a composite slurry. S3. The composite slurry obtained in S2 is ball-milled and then spray-granulated to obtain precursor powder; S4. The precursor powder obtained in S3 is subjected to heat treatment to obtain the highly conductive carbon-coated lithium iron phosphate cathode material; The heat treatment is divided into the following three stages: In the first stage, the temperature is raised to 120~180℃ and held in an inert atmosphere, and then raised to 280~350℃ and held. In the second stage, the temperature is raised to 400~500℃ and held in an inert or weakly reducing atmosphere. The third stage involves heating the material to 650-750℃ in a reducing atmosphere and then holding it at that temperature.

2. The method for preparing the highly conductive carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The dispersion containing the dispersant described in S1 is prepared by the following steps: dissolving the dispersant in a solvent and then subjecting it to mechanical stirring and ultrasonic treatment to form a dispersion; The dispersant is at least one of lignin sulfonate, polyvinylpyrrolidone, or sodium dodecyl sulfate, and its addition amount is 0.3%-0.8% of the theoretical mass of lithium iron phosphate.

3. The method for preparing the highly conductive carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, In S1, the Ketjen black conductive agent is divided into 4-6 parts. The first part of the conductive agent is added to the dispersion under shearing conditions of 800-1200 rpm and sheared and dispersed. Then, the remaining Ketjen black is added part by part under stirring conditions of 200-400 rpm and ultrasonic synergy. After all the addition is completed, the carbon nanotube conductive network slurry is obtained by continuing to treat under shearing conditions of 800-1200 rpm and ultrasonic synergy.

4. The method for preparing the highly conductive carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, After adding polyethylene glycol to S1, stirring forms a conductive colloid with a viscosity of 800-2500 cP; The molecular weight of the polyethylene glycol is 200-600.

5. The method for preparing the highly conductive carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The amount of Ketjen black added is 5.0%-10.0% of the theoretical mass of lithium iron phosphate, and the amount of glucose added is 20%-35% of the theoretical mass of lithium iron phosphate. The total carbon content in the final highly conductive carbon-coated lithium iron phosphate cathode material is 1.5-2.5 wt%.

6. The method for preparing the highly conductive carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, S2 adds the conductive colloid to the precursor solution and stirs and composites it at 45-65℃.

7. The method for preparing the highly conductive carbon-coated lithium iron phosphate cathode material according to claim 1, characterized in that, The heat treatment described in S4 includes: In the first stage, in an inert atmosphere, the temperature is increased to 120~180℃ at a heating rate of 1-3℃ / min and held for 0.5-1.5 hours, and then increased to 280~350℃ at the same rate and held for 1-3 hours. In the second stage, under an inert or weakly reducing atmosphere, the temperature is increased to 400-500℃ at a rate of 3-5℃ / min and held for 1.5-3 hours; the weakly reducing atmosphere is an argon-hydrogen mixture, with hydrogen accounting for 3%-8% of the volume. In the third stage, the temperature is increased to 650~750℃ in a reducing atmosphere at a heating rate of 2-5℃ / min and held for 8-15 hours; the reducing atmosphere is an argon-hydrogen mixture with hydrogen accounting for 5%-10% of the volume.

8. A highly conductive carbon-coated lithium iron phosphate cathode material, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 7.

9. A positive electrode sheet for a lithium-ion battery, characterized in that, It comprises the highly conductive carbon-coated lithium iron phosphate cathode material as described in claim 8.

10. A lithium-ion battery, characterized in that, It comprises the lithium-ion battery positive electrode sheet as described in claim 9.

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