Lithium iron phosphate composite positive electrode material, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate composite positive electrode material and a preparation method thereof and a lithium ion battery, the lithium iron phosphate composite positive electrode material comprises a core and a carbon coating layer coating the core, the core comprises a lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure, and the carbon coating layer is coated on the lithium iron phosphate-carbon composite material skeleton. The conductive agent is loaded on the framework of the lithium iron phosphate-carbon composite material with the three-dimensional porous structure. When the lithium iron phosphate composite positive electrode material provided by the invention is used for the lithium ion battery, the discharge gram capacity under high rate is relatively high, so that the lithium iron phosphate composite positive electrode material has good rate capability.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium - ion batteries, and in particular to a lithium iron phosphate composite cathode material, a preparation method thereof, and a lithium - ion battery. Background Art

[0002] With the continuous growth of energy demand and the pursuit of sustainable development, lithium - ion batteries, as an efficient energy storage device, have been widely used in various fields. As an important component of the cathode material for lithium - ion batteries, lithium iron phosphate has attracted much attention due to its many advantages. The lithium iron phosphate material has the advantages of high safety, good thermal stability, long cycle life, rich raw materials, relatively low price, environmental friendliness without heavy metals, etc., but its conductivity is poor: the electronic conductivity is low, which affects the charge - discharge rate of the battery.

[0003] The above - mentioned disadvantages are related to the formation of a two - phase structure of iron phosphate after lithium deintercalation from the material. Since both the electronic and ionic conductivities of iron phosphate are low, the central lithium iron phosphate cannot be fully utilized. During the discharge process, iron phosphate is continuously converted into lithium iron phosphate, and the diffusion interface decreases until the amount of diffused lithium ions is insufficient to maintain the current, so that iron phosphate cannot be converted into the lithium iron phosphate phase either. Therefore, the actual utilization efficiency of the lithium iron phosphate material at high rates is significantly reduced, and the specific capacity decreases significantly. How to improve the rate performance of the lithium iron phosphate cathode material has become the focus of research in this technical field.

[0004] The invention application with the publication number of CN117326539A discloses a preparation method of carbon - coated lithium iron phosphate. The preparation method includes: mixing iron phosphate and a first carbon source, ball - milling, spray - drying, pre - sintering, and then mixing with lithium carbonate and a second carbon source, ball - milling, spray - drying, and multi - stage temperature sintering to finally obtain carbon - coated lithium iron phosphate. Although this method can improve the conductivity of the lithium iron phosphate cathode material and thus improve its rate performance, the discharge specific capacity of the obtained material at high rates is still low, and the rate performance is still poor. Summary of the Invention

[0005] Aiming at the above deficiencies in the prior art, the purpose of the present invention is to provide a lithium iron phosphate composite cathode material, a preparation method thereof, and a lithium - ion battery. When the lithium iron phosphate composite cathode material provided by the present invention is used in a lithium - ion battery, the discharge specific capacity at high rates is high, so that it has good rate performance.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a lithium iron phosphate composite cathode material, which comprises a core and a carbon coating layer covering the core. The core comprises a lithium iron phosphate-carbon composite material framework with a three-dimensional porous structure, and a conductive agent loaded on the lithium iron phosphate-carbon composite material framework with the three-dimensional porous structure.

[0008] When the lithium iron phosphate composite cathode material provided by the present invention is used in a lithium-ion battery, the discharge specific capacity at high rates is relatively high, so that it has good rate performance.

[0009] Furthermore, the conductive agent comprises at least one of a one-dimensional conductive agent and a two-dimensional conductive agent. Among them, the one-dimensional conductive agent comprises conductive carbon black, and the two-dimensional conductive agent comprises at least one of carbon nanotubes, graphene oxide, reduced graphene oxide and graphene;

[0010] and / or, the mass ratio of the conductive agent to the lithium iron phosphate-carbon composite material framework with the three-dimensional porous structure is (10-20):100;

[0011] and / or, the conductive agent comprises conductive carbon black and carbon nanotubes, and the mass ratio of the conductive carbon black to the carbon nanotubes is (1-2):(1-2).

[0012] Furthermore, the conductive agent is loaded on the lithium iron phosphate-carbon composite material framework with the three-dimensional porous structure by means of electrostatic spraying.

[0013] Furthermore, the carbon coating layer is prepared by heat-treating a second carbon source, and the mass ratio of the second carbon source to the core is (0.1-0.4):1;

[0014] and / or, the second carbon source comprises at least one of carbon nanotubes, graphene, carbon fibers and fullerenes.

[0015] In a second aspect, the present invention provides a preparation method of the lithium iron phosphate composite cathode material as described in the first aspect. The preparation method comprises the following steps:

[0016] S1. Mix a phosphorus source, an iron source, a lithium source with a first carbon source and a first dispersant to obtain a mixed slurry;

[0017] S2. Add the mixed slurry into water, stir it, and then perform freeze-drying;

[0018] S3. Sinter the material obtained after drying in step S2 to obtain a lithium iron phosphate-carbon composite material framework with a three-dimensional porous structure;

[0019] S4. Disperse the conductive agent and the second dispersant in an organic solvent to obtain a conductive agent dispersion liquid, and then deposit the conductive agent on the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure by means of electrostatic spraying, followed by drying and heat treatment under a protective gas atmosphere to obtain a lithium iron phosphate intermediate product with a three-dimensional cooperative conductive network formed on the surface and inside;

[0020] S5. Mix the lithium iron phosphate intermediate product obtained in step S4 with a second carbon source and then sinter them to obtain the lithium iron phosphate composite cathode material.

[0021] Further, the phosphorus source includes at least one of phosphoric acid and monoammonium phosphate;

[0022] And / or, the iron source includes at least one of ferrous oxalate, iron oxide red, iron phosphate, iron powder, and ferrous sulfate;

[0023] And / or, the lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate;

[0024] And / or, the first carbon source includes at least one of glucose, sucrose, starch, polystyrene microspheres, polyethylene glycol, carbon black, graphite, carbon nanotubes, and graphene;

[0025] And / or, the first dispersant includes at least one of polyvinylpyrrolidone, polyacrylic acid, sodium polystyrene sulfonate, Tween series, and sodium carboxymethyl cellulose;

[0026] And / or, in step S1, among the iron source, phosphorus source, and lithium source, the molar ratio of elements Fe, P, and Li is 1:1:(1.02 - 1.12);

[0027] And / or, in step S1, the mass ratio of the first carbon source to the total mass of the phosphorus source, iron source, and lithium source is (1 - 5):100, and the mass ratio of the first dispersant to the total mass of the phosphorus source, iron source, and lithium source is (0.5 - 3):100;

[0028] And / or, in step S1, the mixing is carried out in a ball mill at 200 - 800 r / min for ball milling for 4 - 10 h, where the material-to-ball ratio is (5 - 7):(0.75 - 1).

[0029] Further, in step S2, the water includes deionized water, and the solid content of the mixed liquid obtained by adding the mixed slurry to the water is 30 - 75%;

[0030] And / or, in step S2, the freeze-drying includes placing it in a freezing device, freezing it into a solid, and then placing it in a vacuum freeze-dryer for vacuum drying;

[0031] And / or, in step S2, use a double planetary stirring device to stir at a linear velocity of 6 - 12 m / s for 1 - 6 h. In the freezing device, use liquid nitrogen at -196°C for freezing, with a freezing time of 20 - 50 min. The vacuum drying time is 24 - 72 h, and the vacuum drying temperature is 60 - 120°C;

[0032] And / or, in step S3, the sintering atmosphere is a mixed gas atmosphere of argon and hydrogen, the sintering temperature is 700 - 900°C, and the sintering time is 6 - 24 h.

[0033] Further, in step S4, the conductive agent includes at least one of a one-dimensional conductive agent and a two-dimensional conductive agent. Among them, the one-dimensional conductive agent includes conductive carbon black, and the two-dimensional conductive agent includes at least one of carbon nanotubes, graphene oxide, reduced graphene oxide, and graphene;

[0034] And / or, in step S4, the second dispersant includes at least one of polyvinylpyrrolidone, polyacrylic acid, and sodium polystyrene sulfonate;

[0035] And / or, in step S4, the mass ratio of the conductive agent to the second dispersant is (10 - 20):(0.1 - 0.5);

[0036] And / or, in step S4, the organic solvent includes absolute ethanol;

[0037] And / or, in step S4, the mass ratio of the conductive agent to the organic solvent is (10 - 20):(50 - 100);

[0038] And / or, in step S4, the electrostatic spraying includes loading the conductive agent dispersion liquid into an electrostatic spray gun, connecting the spray gun to a high-voltage electrostatic generator to make the conductive agent dispersion liquid carry a negative charge. At the same time, ground the three-dimensional porous structure lithium iron phosphate-carbon composite material skeleton to be sprayed to make it a positive electrode. After turning on the spray gun, the conductive agent dispersion liquid is adsorbed onto the three-dimensional porous structure lithium iron phosphate-carbon composite material skeleton under the action of the electrostatic field to form a uniform coating. Among them, the spraying voltage is 5 - 15 kV, the spraying current is 50 - 100 μA, the spraying distance is 5 - 25 cm, the flow rate of the conductive agent dispersion liquid is 5 - 30 mL / min, the atomization pressure is 0.1 - 0.5 MPa, and the atomization angle is 30 - 90°;

[0039] And / or, in step S4, the drying is vacuum drying, and the drying temperature is 60 - 120°C;

[0040] And / or, in step S4, the protective gas includes argon;

[0041] And / or, in step S4, the temperature of the heat treatment is 400 - 600 °C, and the time of the heat treatment is 60 - 90 min.

[0042] Further, in step S5, the second carbon source includes at least one of glucose, sucrose, starch, polystyrene microspheres, polyethylene glycol, carbon black, graphite, carbon nanotubes, and graphene;

[0043] And / or, in step S5, the mass ratio of the lithium iron phosphate intermediate product to the second carbon source is 1:(0.1 - 0.4);

[0044] And / or, in step S5, the mixing includes placing the lithium iron phosphate intermediate product obtained in step S4 and the second carbon source in a ball milling tank, adding water, and performing mixing and stirring. The rotation speed of the mixing and stirring is 800 - 1200 rpm, and the time of the mixing and stirring is 4 - 8 h;

[0045] And / or, in step S5, the sintering includes calcining at 700 - 900 °C for 6 - 9 h by heating at 3 - 6 °C / min under nitrogen protection.

[0046] In a third aspect, the present invention provides a lithium - ion battery, including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The active material of the positive electrode sheet includes the lithium iron phosphate composite positive electrode material described in the first aspect or the lithium iron phosphate composite positive electrode material prepared by the preparation method described in the second aspect.

[0047] Compared with the prior art, the beneficial effects of the present invention at least include one of the following:

[0048] (1) When the lithium iron phosphate composite positive electrode material provided by the present invention is used in a lithium - ion battery, the discharge specific capacity at high rates is relatively high, so it has good rate performance.

[0049] (2) The present invention provides a high - rate lithium iron phosphate positive electrode material. The high - rate lithium iron phosphate positive electrode material manufactures a three - dimensional porous structure from a lithium iron phosphate precursor through a low - temperature freezing technique, sprays a conductive agent dispersion on the surface and inside of the material through an electrostatic spraying technique to form a three - dimensional synergistic conductive network, and then obtains a final lithium iron phosphate porous composite material after high - temperature sintering carbon coating. The synergistic effect of the three - dimensional porous structure formed by the low - temperature freezing technique, the conductive network constructed by electrostatic spraying, and the high - temperature sintering carbon coating significantly improves the conductivity, ion diffusion rate, and structural stability of the material. When this composite material is used as the positive electrode of a lithium - ion battery, it can effectively enhance the rate performance and cycle stability of the battery. Description of the Drawings

[0050] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0051] Figure 1 It is a schematic structural diagram of the lithium iron phosphate composite cathode material provided in Embodiment 1 of the present invention.

[0052] Icon: 1 - Lithium iron phosphate precursor with a three-dimensional porous structure; 2 - Pores on the surface of the precursor; 3 - Conductive carbon black; 4 - Carbon nanotubes; 5 - Carbon coating; 6 - Graphene carbon coating layer. Specific Embodiments

[0053] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in combination with the embodiments of the present invention. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. The process parameters without specific conditions noted in the following embodiments are usually in accordance with conventional conditions.

[0054] In the ranges disclosed in the present invention, the endpoints and any values of the ranges are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in the present invention.

[0055] In the first aspect, the present invention provides a lithium iron phosphate composite cathode material. The lithium iron phosphate composite cathode material includes a core and a carbon coating layer covering the core. The core includes a lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure and a conductive agent loaded on the lithium iron phosphate-carbon composite material skeleton with the three-dimensional porous structure.

[0056] When the lithium iron phosphate composite cathode material provided by the present invention is used in a lithium ion battery, the discharge specific capacity at high rates is relatively high, so it has good rate performance.

[0057] In the above lithium iron phosphate composite cathode material, as an alternative embodiment, the conductive agent includes at least one of one-dimensional conductive agents and two-dimensional conductive agents. Among them, the one-dimensional conductive agent includes conductive carbon black. For example, the conductive carbon black can be at least one of conductive furnace black (CF), super conductive furnace black (SCF), extra conductive furnace black (XCF), and acetylene black. The conductive carbon black has a small particle size, a large specific surface area and is hard, and has a low resistivity, which can increase the electronic conductivity of the lithium iron phosphate skeleton; the two-dimensional conductive agent includes at least one of carbon nanotubes, graphene oxide, reduced graphene oxide and graphene.

[0058] In the above lithium iron phosphate composite cathode material, as an alternative embodiment, the mass ratio of the conductive agent to the lithium iron phosphate-carbon composite material skeleton having a three-dimensional porous structure is (10-20):100. For example, it can be 10:100, 12:100, 14:100, 16:100, 18:100 or 20:100.

[0059] In the above lithium iron phosphate composite cathode material, as an alternative embodiment, the conductive agent includes conductive carbon black and carbon nanotubes, and the mass ratio of the conductive carbon black to the carbon nanotubes is (1-2):(1-2). Carbon nanotubes have high conductivity due to their large aspect ratio. When added to the material, they can improve the charging speed and capacity of the battery. Their high conductivity and large specific surface area contribute to the rapid transmission and storage of charges, thus improving the performance of the battery. Therefore, a larger addition amount can improve the electrical performance but increase the cost accordingly.

[0060] In the above lithium iron phosphate composite cathode material, as an alternative embodiment, the conductive agent is loaded on the lithium iron phosphate-carbon composite material skeleton having a three-dimensional porous structure by means of electrostatic spraying.

[0061] In the above lithium iron phosphate composite cathode material, as an alternative embodiment, the carbon coating layer is prepared by heat-treating a second carbon source, and the mass ratio of the second carbon source to the core is (0.1-0.4):1. For example, it can be 0.1:1, 0.2:1, 0.3:1 or 0.4:1.

[0062] In the above lithium iron phosphate composite cathode material, as an alternative embodiment, the second carbon source includes at least one of carbon nanotubes, graphene, carbon fibers, and fullerenes.

[0063] In a second aspect, the present invention provides a preparation method of the lithium iron phosphate composite cathode material as described in the first aspect. The preparation method includes the following steps:

[0064] S1. Mix a phosphorus source, an iron source, a lithium source with a first carbon source and a first dispersant to obtain a mixed slurry;

[0065] S2. Add the mixed slurry into water, stir it, and then conduct freeze-drying;

[0066] S3. Sinter the material obtained after drying in step S2 to obtain a lithium iron phosphate-carbon composite material framework with a three-dimensional porous structure (precursor of lithium iron phosphate composite cathode material);

[0067] S4. Disperse the conductive agent and the second dispersant in an organic solvent to obtain a conductive agent dispersion liquid, and then deposit the conductive agent on the lithium iron phosphate-carbon composite material framework with a three-dimensional porous structure by means of electrostatic spraying, followed by drying and heat treatment in a protective gas atmosphere to prepare a lithium iron phosphate intermediate product with a three-dimensional synergistic conductive network formed on the surface and inside;

[0068] S5. Mix the lithium iron phosphate intermediate product obtained in step S4 with a second carbon source and then conduct sintering to obtain the lithium iron phosphate composite cathode material (high-rate lithium iron phosphate composite cathode material).

[0069] In the above preparation method of the lithium iron phosphate composite cathode material, as an optional implementation manner, the phosphorus source includes at least one of phosphoric acid and monoammonium phosphate.

[0070] In the above preparation method of the lithium iron phosphate composite cathode material, as an optional implementation manner, the iron source includes at least one of ferrous oxalate, iron oxide red (iron(III) oxide), iron phosphate, iron powder, and ferrous sulfate.

[0071] In the above preparation method of the lithium iron phosphate composite cathode material, as an optional implementation manner, the lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate.

[0072] In the above preparation method of the lithium iron phosphate composite cathode material, as an optional implementation manner, the first carbon source includes at least one of glucose, sucrose, starch, polystyrene microspheres, polyethylene glycol, carbon black, graphite, carbon nanotubes, and graphene.

[0073] In the above preparation method of the lithium iron phosphate composite cathode material, as an optional implementation manner, the first dispersant includes at least one of polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), sodium polystyrene sulfonate (PSS), Tween series (such as Tween-60), and sodium carboxymethyl cellulose (CMC).

[0074] In the above preparation method of the lithium iron phosphate composite cathode material, as an optional implementation manner, in step S1, among the iron source, phosphorus source, and lithium source, the molar ratio of elements Fe, P, and Li is 1:1:(1.02 - 1.12).

[0075] In the method for preparing the above-mentioned lithium iron phosphate composite cathode material, as an alternative embodiment, in step S1, the ratio of the mass of the first carbon source to the total mass of the phosphorus source, iron source, and lithium source is (1-5):100 (for example, it can be 1:100, 3:100, or 5:100), and the ratio of the mass of the first dispersant to the total mass of the phosphorus source, iron source, and lithium source is (0.5-3):100. For example, it can be 0.5:100, 1:100, 2:100, or 3:100.

[0076] In the method for preparing the above-mentioned lithium iron phosphate composite cathode material, as an alternative embodiment, in step S1, the mixing is carried out by ball milling at 200-800 r / min (for example, it can be 200 r / min, 400 r / min, 600 r / min, or 800 r / min) for 4-10 h (for example, it can be 4 h, 6 h, 8 h, or 10 h), and the material-to-ball ratio is (5-7):(0.75-1).

[0077] In the method for preparing the above-mentioned lithium iron phosphate composite cathode material, as an alternative embodiment, in step S2, the water includes deionized water, and the solid content of the mixed liquid obtained by adding the mixed slurry to the water is 30-75% (for example, it can be 30%, 40%, 50%, 60%, or 75%).

[0078] In the method for preparing the above-mentioned lithium iron phosphate composite cathode material, as an alternative embodiment, in step S2, the freeze-drying includes placing it in a freezing device, freezing it into a solid, and then placing it in a vacuum freeze-dryer for vacuum drying.

[0079] In the method for preparing the above-mentioned lithium iron phosphate composite cathode material, as an alternative embodiment, in step S2, a double planetary stirring device is used to stir at a linear speed of 6-12 m / s (for example, it can be 6 m / s, 8 m / s, 10 m / s, or 12 m / s) for 1-6 h (for example, it can be 1 h, 3 h, 5 h, or 6 h). In the freezing device, liquid nitrogen at -196°C is used for freezing, and the freezing time is 20-50 min (for example, it can be 20 min, 30 min, 40 min, or 50 min). The time for the vacuum drying is 24-72 h (for example, it can be 24 h, 36 h, 48 h, 60 h, or 72 h), and the temperature for the vacuum drying is 60-120°C (for example, it can be 60°C, 80°C, 100°C, or 120°C).

[0080] In the preparation method of the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S3, the sintering atmosphere is a mixed gas atmosphere of argon and hydrogen, the sintering temperature is 700-900 °C (for example, it can be 700 °C, 800 °C or 900 °C), and the sintering time is 6-24 h (for example, it can be 6 h, 10 h, 14 h, 18 h, 22 h or 24 h).

[0081] In the preparation method of the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S4, the conductive agent includes at least one of one-dimensional conductive agents and two-dimensional conductive agents. Among them, the one-dimensional conductive agent includes conductive carbon black. For example, the conductive carbon black can be at least one of conductive furnace black (CF), super conductive furnace black (SCF), extra conductive furnace black (XCF), and acetylene black. The conductive carbon black has a small particle size, a large specific surface area and is hard, and has a low resistivity, which can increase the electronic conductivity of the lithium iron phosphate skeleton; the two-dimensional conductive agent includes at least one of carbon nanotubes, graphene oxide, reduced graphene oxide and graphene.

[0082] In the preparation method of the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S4, the second dispersant includes at least one of polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), and sodium polystyrene sulfonate (PSS).

[0083] In the preparation method of the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S4, the mass ratio of the conductive agent to the second dispersant is (10-20):(0.1-0.5), for example, it can be 10:0.1, 15:0.2, 18:0.3 or 20:0.5.

[0084] In the preparation method of the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S4, the organic solvent includes absolute ethanol.

[0085] In the preparation method of the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S4, the mass ratio of the conductive agent to the organic solvent is (10-20):(50-100), for example, it can be 10:50, 15:70, 20:90.

[0086] In the method for preparing the above-mentioned lithium iron phosphate composite cathode material, as an alternative embodiment, in step S4, the electrostatic spraying includes loading the conductive agent dispersion liquid into an electrostatic spray gun, connecting the spray gun to a high-voltage electrostatic generator to make the conductive agent dispersion liquid carry a negative charge. At the same time, the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure to be sprayed is grounded (placing the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure into the electrostatic spraying equipment and connecting the metal part of the electrostatic spraying equipment to the ground through a wire) to make it a positive electrode. After the spray gun is turned on, the conductive agent dispersion liquid is adsorbed onto the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure under the action of the electrostatic field to form a uniform coating. Among them, the spraying voltage is 5-15 kV (for example, it can be 5 kV, 10 kV or 15 kV), the spraying current is 50-100 μA (for example, it can be 50 μA, 60 μA, 70 μA, 80 μA, 90 μA or 100 μA), the spraying distance is 5-25 cm (for example, it can be 5 cm, 10 cm, 15 cm, 20 cm or 25 cm), the flow rate of the conductive agent dispersion liquid is 5-30 mL / min (for example, it can be 5 mL / min, 10 mL / min, 15 mL / min, 20 mL / min, 25 mL / min or 30 mL / min), the atomization pressure is 0.1-0.5 MPa, and the atomization angle is 30-90° (for example, it can be 30°, 40°, 50°, 60°, 70°, 80° or 90°).

[0087] In the method for preparing the above-mentioned lithium iron phosphate composite cathode material, as an alternative embodiment, in step S4, the drying is vacuum drying, and the drying temperature is 60-120 °C, for example, it can be 60 °C, 80 °C, 100 °C or 120 °C.

[0088] In the method for preparing the above-mentioned lithium iron phosphate composite cathode material, as an alternative embodiment, in step S4, the protective gas includes argon.

[0089] In the method for preparing the above-mentioned lithium iron phosphate composite cathode material, as an alternative embodiment, in step S4, the temperature of the heat treatment is 400-600 °C (for example, it can be 400 °C, 450 °C, 500 °C, 550 °C or 600 °C), and the time of the heat treatment is 60-90 min (for example, it can be 60 min, 70 min, 80 min or 90 min). If the temperature is too high: the material structure changes, components volatilize, and chemical reactions are abnormal; if the temperature is too low: the sintering is incomplete and the performance does not reach the optimal state.

[0090] In the method for preparing the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S5, the second carbon source includes at least one of glucose, sucrose, starch, polystyrene microspheres, polyethylene glycol, carbon black, graphite, carbon nanotubes, and graphene.

[0091] In the method for preparing the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S5, the mass ratio of the lithium iron phosphate intermediate product to the second carbon source is 1:(0.1 - 0.4), for example, it can be 1:0.1, 1:0.2, 1:0.3, or 1:0.4.

[0092] In the method for preparing the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S5, the mixing includes placing the lithium iron phosphate intermediate product obtained in step S4 and the second carbon source in a ball milling tank, adding water, and performing mixing and stirring. The rotation speed of the mixing and stirring is 800 - 1200 rpm (for example, it can be 800 rpm, 900 rpm, 1000 rpm, 1100 rpm, or 1200 rpm), and the time of the mixing and stirring is 4 - 8 h (for example, it can be 4 h, 5 h, 6 h, 7 h, or 8 h).

[0093] In the method for preparing the above lithium iron phosphate composite cathode material, as an alternative embodiment, in step S5, the sintering includes calcining at 700 - 900 °C (for example, it can be 700 °C, 750 °C, 800 °C, 850 °C, or 900 °C) for 6 - 9 h under nitrogen protection conditions at a heating rate of 3 - 6 °C / min.

[0094] In the third aspect, the present invention provides a lithium ion battery, including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The active material of the positive electrode sheet includes the lithium iron phosphate composite cathode material described in the first aspect or the lithium iron phosphate composite cathode material prepared by the preparation method described in the second aspect.

[0095] The present invention will be further described in detail below with specific examples and comparative examples.

[0096] Example 1

[0097] As Figure 1 described, this example provides a lithium iron phosphate composite cathode material. The lithium iron phosphate composite cathode material includes a core and a graphene carbon coating layer 6 covering the core. The core includes a lithium iron phosphate-carbon composite material framework with a three-dimensional porous structure, as well as conductive carbon black 3 and carbon nanotubes 4 loaded on the lithium iron phosphate-carbon composite material framework with the three-dimensional porous structure. The lithium iron phosphate-carbon composite material framework is a lithium iron phosphate precursor 1 with a three-dimensional porous structure coated with carbon 5, and the conductive carbon black 3 and carbon nanotubes 4 are loaded in the surface pores 2 of the precursor.

[0098] The preparation method of the lithium iron phosphate composite cathode material provided by this embodiment includes the following steps:

[0099] S1. Industrial phosphoric acid, ferrous oxalate, lithium carbonate, carbon black (acetylene black), and PVP are ball-milled and mixed in a ball mill at 500 r / min for 6 h to obtain a mixed slurry. Among them, in ferrous oxalate, industrial phosphoric acid, and lithium carbonate, the molar ratio of elements Fe, P, and Li is 1:1:1.08. The dosage of carbon black is 2% of the total mass of ferrous oxalate, industrial phosphoric acid, and lithium carbonate, the dosage of PVP is 0.5% of the total mass of ferrous oxalate, industrial phosphoric acid, and lithium carbonate, and the material-to-ball ratio is 6:1.

[0100] S2. The mixed slurry is added to deionized water to ensure a solid content of 45%. It is stirred with a double planetary stirring device at a linear speed of 6 m / s for 2 h, then placed in liquid nitrogen at -196 °C and frozen for 30 min to become a solid, and then placed in a vacuum freeze dryer for vacuum drying at 80 °C for 48 h.

[0101] S3. The material obtained after freeze-drying in step S2 is sintered at 750 °C for 6 h in a mixed gas atmosphere of argon and hydrogen (volume ratio 1:1) to obtain a lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure (precursor of the lithium iron phosphate composite cathode material).

[0102] S4. Conductive carbon black (acetylene black), carbon nanotubes, and polyvinylpyrrolidone (PVP) are added to an anhydrous ethanol solution and dispersed evenly to obtain a conductive agent dispersion liquid. Then, the conductive carbon black and carbon nanotubes are deposited on the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure obtained in step S3 by means of electrostatic spraying, vacuum dried at 80 °C for 48 h, and then calcined at 500 °C for 60 min under argon to obtain a lithium iron phosphate intermediate product with a three-dimensional cooperative conductive network formed on the surface and inside. Among them, the mass ratio of the conductive carbon black, carbon nanotubes, polyvinylpyrrolidone (PVP), anhydrous ethanol, and the lithium iron phosphate-carbon composite material skeleton is 8:8:0.2:90:100. The electrostatic spraying includes loading the conductive agent dispersion liquid into an electrostatic spray gun, connecting the spray gun to a high-voltage electrostatic generator to make the conductive agent dispersion liquid carry a negative charge. At the same time, the lithium iron phosphate-carbon composite material skeleton to be sprayed is grounded (placing the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure into an electrostatic spraying device and connecting the metal part of the electrostatic spraying device to the ground) to make it a positive electrode. After the spray gun is opened, the conductive agent dispersion liquid is adsorbed onto the lithium iron phosphate-carbon composite material skeleton under the action of the electrostatic field to form a uniform coating. Among them, the spraying voltage is 8 kV, the spraying current is 60 μA, the spraying distance is 13 cm, the flow rate of the conductive agent dispersion liquid is 10 mL / min, the atomization pressure is 0.2 MPa, and the atomization angle is 60°.

[0104] S5. Preparation of carbon-coated composite porous material: The lithium iron phosphate intermediate product and graphene are uniformly mixed at a mass ratio of 1:0.3 and placed in a ball milling tank. Then, deionized water is added to ensure that the solid content is 80%. The mixture is stirred at 1200 rpm for 6 h, and then fully dried at 80 °C and placed in a sintering furnace. Under the protection of nitrogen, it is heated to 800 °C at a rate of 5 °C / min and calcined for 8 h. Subsequently, it is cooled to room temperature to obtain a carbon-coated composite porous material (lithium iron phosphate composite cathode material).

[0105] Example 2

[0106] The preparation method of the lithium iron phosphate composite cathode material provided in this example is basically the same as that in Example 1, except that in step S1, the lithium source is lithium hydroxide, the carbon source is glucose, and the dispersant is Tween-60; in step S1, the mixing is carried out by ball milling at 200 r / min in a ball mill for 10 h; in step S2, a double planetary stirring device is used to stir at a linear speed of 10 m / s for 1 h. In the freezing device, liquid nitrogen at -196 °C is used for freezing, and the freezing time is 40 min. The vacuum drying time is 36 h, and the vacuum drying temperature is 100 °C; in step S3, the sintering atmosphere is a mixed gas atmosphere of argon and hydrogen, the sintering temperature is 800 °C, and the sintering time is 10 h.

[0107] Example 3

[0108] The preparation method of the lithium iron phosphate composite cathode material provided in this example is basically the same as that in Example 1, except that in step S1, the dispersant is sodium carboxymethyl cellulose.

[0109] In step S1, the mixing is carried out by ball milling at 800 r / min in a ball mill for 4 h; in step S4, the spraying voltage is 10 kV, the spraying current is 80 μA, the spraying distance is 20 cm, the flow rate of the conductive agent dispersion liquid is 15 mL / min, the atomization pressure is 0.4 MPa, and the atomization angle is 90°.

[0110] Example 4

[0111] The preparation method of the lithium iron phosphate composite cathode material provided in this example is basically the same as that in Example 1, except that in step S2, the water includes deionized water, and the solid content of the mixed liquid obtained by adding the mixed slurry to the water is 65%; in step S4, the dispersant is sodium polystyrene sulfonate (PSS); in step S4, the spraying voltage is 15 kV, the spraying current is 100 μA, the spraying distance is 25 cm, the flow rate of the conductive agent dispersion liquid is 25 mL / min, the atomization pressure is 0.4 MPa, and the atomization angle is 90°.

[0112] Comparative Example 1

[0113] The preparation method of the lithium iron phosphate composite cathode material provided in this comparative example is basically the same as that of Example 1, except that step S5 is not included.

[0114] Comparative Example 2

[0115] The preparation method of the lithium iron phosphate composite cathode material provided in this comparative example is basically the same as that of Example 1, except that step S4 and step S5 are not included.

[0116] Comparative Example 3

[0117] The preparation method of the lithium iron phosphate composite cathode material provided in this comparative example is basically the same as that of Example 1, except that in step S2, vacuum drying is directly carried out without freezing treatment.

[0118] Comparative Example 4

[0119] The preparation method of the lithium iron phosphate composite cathode material provided in this comparative example is basically the same as that of Example 1, except that in step S4, instead of using the electrostatic spraying method, the conductive agent dispersion liquid and the lithium iron phosphate-carbon composite material framework with a three-dimensional porous structure are placed in a ball milling tank and mixed and stirred at 1200 rpm for 10 h, then vacuum dried at 80 °C for 48 h, and then calcined at 500 °C for 60 min under argon to obtain a lithium iron phosphate intermediate product with a three-dimensional synergistic conductive network formed on the surface and inside.

[0120] Performance Test

[0121] The composite materials prepared in the examples and comparative examples were applied to the cathode to assemble a coin-type half cell, and the specific process was as follows: according to the mass ratio of lithium iron phosphate composite cathode material: SP: PVDF = 97:1:2, the lithium iron phosphate composite cathode material, SP and PVDF were added to an agate mortar, and then an appropriate amount of NMP solvent was dropped in and ground evenly. Finally, the mixture was evenly coated on the surface of the aluminum foil and vacuum dried at 110 °C for 12 h; the dried electrode sheet was rolled and cut into a circular sheet with a diameter of 12 mm, and a lithium sheet was used as the counter electrode to assemble a coin-type half cell. The specific capacity performance, rate performance and cycle performance of the coin-type half cell were determined. Among them, the specific capacity performance study mainly tested the discharge specific capacity in the first cycle at 0.1C at 25 °C, the rate performance study mainly tested the discharge specific capacity under constant current discharge at 25 °C, 3C and 5C, and the cycle performance study mainly tested the capacity retention rate after 500 cycles at 3C at 25 °C. The test results are shown in Table 1.

[0122] The method for measuring the 0.1C capacity is as follows: The prepared button cell is tested using a 5V 10mA LAND battery test system in an environment of 25°C ± 2°C. The test process is to charge at a constant current and voltage of 0.1C until 3.65V, with a cut-off current of 0.05C. After standing for 5 minutes, it is discharged at a constant current of 0.1C to the lower limit voltage of 2.0V and then stands for 5 minutes. The discharge specific capacity (mAh / g) of the positive electrode at 0.1C = 0.1C discharge capacity / ((electrode sheet weight - foil weight) * active material ratio).

[0123] The method for testing the 3C specific capacity: The constant current charge-discharge test method is adopted. The voltage test range is 2.0 - 3.65V. After first performing the 0.1C capacity test on the button cell, the charge-discharge current value is changed to complete the 3C current value test, and the discharge specific capacity of the positive electrode material at the 5th time of 3C is calculated.

[0124] The method for testing the 5C specific capacity: The constant current charge-discharge test method is adopted. The voltage test range is 2.0 - 3.65V. After first performing the 0.1C capacity test on the button cell, the charge-discharge current value is changed to complete the 5C current value test, and the discharge specific capacity of the positive electrode material at the 5th time of 5C is calculated.

[0125] The method for measuring the capacity retention rate at 3C after 500 cycles at 25°C is as follows: After first performing the 0.1C capacity test on the button cell, the 3C capacity test step is cycled 500 times; The capacity retention rate after 500 cycles = discharge capacity at the 500th cycle / discharge capacity at the 2nd cycle * 100%.

[0126] Table 1

[0127]

[0128]

[0129] It can be seen at least the following points from Table 1:

[0130] (1) By comparing Example 1 with Comparative Examples 1 - 2, it can be known that by gradually loading the conductive agent on the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure and performing secondary coating on the lithium iron phosphate material, the discharge specific capacity and cycle stability of the lithium iron phosphate composite positive electrode material at high rates are gradually improved. By comparing Comparative Example 2 with Comparative Example 1, it can be seen that forming a three-dimensional synergistic conductive network on the surface and inside of the lithium iron phosphate-carbon composite material skeleton can further improve the conductivity and cycle performance of the material.

[0131] (2) By comparing Example 1 and Comparative Example 3, it can be seen that when the core of the lithium iron phosphate composite cathode material does not have a three-dimensional porous structure, both the discharge specific capacity and the cycle stability at high rates decrease. (3) By comparing Example 1 and Comparative Example 4, it can be seen that compared with the conventional wet mixing method, by using the electrostatic spraying method to load the conductive agent on the three-dimensional porous structure of the lithium iron phosphate-carbon composite material framework in the present invention, the discharge specific capacity and the cycle stability of the lithium iron phosphate composite cathode material at high rates can be further improved.

[0132] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A lithium iron phosphate composite positive electrode material, characterized in that: The lithium iron phosphate composite positive electrode material includes a core and a carbon coating layer coating the core, wherein the core includes a lithium iron phosphate-carbon composite material skeleton having a three-dimensional porous structure, and a conductive agent loaded on the lithium iron phosphate-carbon composite material skeleton having a three-dimensional porous structure.

2. The lithium iron phosphate composite positive electrode material according to claim 1, characterized in that: The conductive agent includes at least one of a one-dimensional conductive agent and a two-dimensional conductive agent, wherein the one-dimensional conductive agent includes conductive carbon black, and the two-dimensional conductive agent includes at least one of carbon nanotubes, graphene oxide, reduced graphene oxide and graphene; And / or, the mass ratio of the conductive agent to the lithium iron phosphate-carbon composite material skeleton having a three-dimensional porous structure is (10-20):100; And / or, the conductive agent includes conductive carbon black and carbon nanotubes, and the mass ratio of the conductive carbon black to the carbon nanotubes is (1-2): (1-2).

3. The lithium iron phosphate composite positive electrode material according to claim 1, characterized in that: The conductive agent is loaded on the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure by electrostatic spraying.

4. The lithium iron phosphate composite positive electrode material according to claim 1, characterized in that: The carbon coating layer is prepared by heat treatment of a second carbon source, and the mass ratio of the second carbon source to the core is (0.1-0.4):1; And / or, the second carbon source includes at least one of carbon nanotubes, graphene, carbon fiber, and fullerene.

5. A method for preparing a lithium iron phosphate composite positive electrode material according to any one of claims 1 to 4, characterized in that: The preparation method comprises the following steps: S1, mixing a phosphorus source, an iron source, a lithium source, a first carbon source, and a first dispersant to obtain a mixed slurry; S2, adding the mixed slurry into water, stirring, and freeze-drying; S3, sintering the material obtained after drying in step S2 to obtain a lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure; S4, dispersing the conductive agent and the second dispersant in an organic solvent to obtain a conductive agent dispersion, and then depositing the conductive agent on the lithium iron phosphate-carbon composite material skeleton having a three-dimensional porous structure by electrostatic spraying, followed by drying and heat treatment under a protective gas atmosphere to obtain a lithium iron phosphate intermediate product having a three-dimensional synergistic conductive network formed on the surface and inside; S5. The lithium iron phosphate intermediate product obtained in step S4 and the second carbon source are mixed and sintered to obtain the lithium iron phosphate composite positive electrode material.

6. The method for preparing a lithium iron phosphate composite positive electrode material according to claim 5, characterized in that: The phosphorus source includes at least one of phosphoric acid and monoammonium phosphate; And / or, the iron source includes at least one of ferrous oxalate, red iron oxide, ferric phosphate, iron powder, and ferrous sulfate; And / or, the lithium source includes at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; and / or, the first carbon source comprises at least one of glucose, sucrose, starch, polystyrene microspheres, polyethylene glycol, carbon black, graphite, carbon nanotubes, and graphene; And / or, the first dispersant includes at least one of polyvinyl pyrrolidone, polyacrylic acid, sodium polystyrene sulfonate, Tween series, and sodium carboxymethyl cellulose; and / or, in step S1, the molar ratio of the elements Fe, P and Li in the iron source, phosphorus source and lithium source is 1:1:(1.02-1.12); And / or, in step S1, the ratio of the mass of the first carbon source to the total mass of the phosphorus source, the iron source, and the lithium source is (1-5):100, and the ratio of the mass of the first dispersant to the total mass of the phosphorus source, the iron source, and the lithium source is (0.5-3):100; And / or, in step S1, the mixing is performed by ball milling in a ball mill at 200-800 r / min for 4-10 h, wherein the material-ball ratio is (5-7): (0.75-1).

7. The method for preparing a lithium iron phosphate composite positive electrode material according to claim 5, characterized in that: In step S2, the water includes deionized water, and the solid content of the mixed solution obtained by adding the mixed slurry into water is 30-75%; And / or, in step S2, the freeze drying includes placing in a freezing device, freezing to become a solid, and then placing in a vacuum freeze dryer for vacuum drying; And / or, in step S2, a double planetary stirring device is used to stir at a linear speed of 6-12 m / s for 1-6 h, and in the freezing device, liquid nitrogen at -196° C. is used for freezing, the freezing time is 20-50 min, the vacuum drying time is 24-72 h, and the vacuum drying temperature is 60-120° C.; And / or, in step S3, the sintering atmosphere is a mixed gas atmosphere of argon and hydrogen, the sintering temperature is 700-900° C., and the sintering time is 6-24 hours.

8. The method for preparing a lithium iron phosphate composite positive electrode material according to claim 5, characterized in that: In step S4, the conductive agent includes at least one of a one-dimensional conductive agent and a two-dimensional conductive agent, wherein the one-dimensional conductive agent includes conductive carbon black, and the two-dimensional conductive agent includes at least one of carbon nanotubes, graphene oxide, reduced graphene oxide, and graphene; and / or, in step S4, the second dispersant comprises at least one of polyvinyl pyrrolidone, polyacrylic acid, and sodium polystyrene sulfonate; and / or, in step S4, the mass ratio of the conductive agent to the second dispersant is (10-20): (0.1-0.5); and / or, in step S4, the organic solvent comprises anhydrous ethanol; And / or, in step S4, the mass ratio of the conductive agent to the organic solvent is (10-20): (50-100); And / or, in step S4, the electrostatic spraying includes loading the conductive agent dispersion into an electrostatic spray gun, connecting the spray gun to a high-voltage electrostatic generator, so that the conductive agent dispersion is negatively charged, and at the same time, the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure to be sprayed is grounded to make it a positive electrode, and after turning on the spray gun, the conductive agent dispersion is adsorbed onto the lithium iron phosphate-carbon composite material skeleton with a three-dimensional porous structure under the action of the electrostatic field to form a uniform coating, wherein the spraying voltage is 5-15kV, the spraying current is 50-100μA, the spraying distance is 5-25cm, the conductive agent dispersion flow rate is 5-30mL / min, the atomization pressure is 0.1-0.5MPa, and the atomization angle is 30-90°; And / or, in step S4, the drying is vacuum drying, and the drying temperature is 60-120° C.; And / or, in step S4, the shielding gas includes argon; And / or, in step S4, the temperature of the heating treatment is 400-600° C., and the time of the heating treatment is 60-90 min.

9. The method for preparing a lithium iron phosphate composite positive electrode material according to claim 5, characterized in that: In step S5, the second carbon source includes at least one of glucose, sucrose, starch, polystyrene microspheres, polyethylene glycol, carbon black, graphite, carbon nanotubes, and graphene; and / or, in step S5, the mass ratio of the lithium iron phosphate intermediate product to the second carbon source is 1:(0.1-0.4); And / or, in step S5, the mixing includes mixing the lithium iron phosphate intermediate product obtained in step S4 and the second carbon source and placing them in a ball mill, then adding water, and mixing and stirring, the mixing and stirring speed is 800-1200 rpm, and the mixing and stirring time is 4-8 hours; And / or, in step S5, the sintering includes heating to 700-900°C at 3-6°C / min and calcining for 6-9h under nitrogen protection conditions.

10. A lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, characterized in that: The active material of the positive electrode sheet comprises the lithium iron phosphate composite positive electrode material according to any one of claims 1 to 4 or the lithium iron phosphate composite positive electrode material prepared by the preparation method according to any one of claims 5 to 9.

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