High-capacity retention ratio composite electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to a composite electrode material with a high capacity retention rate. According to the technical scheme, the composite electrode material with the high capacity retention rate sequentially comprises porous carbon, nanometer lithium iron phosphate and a carbon coating layer from inside to outside, and the nanometer lithium iron phosphate is embedded into the surface of the porous carbon. The preparation method comprises the following steps: (1) stirring and mixing nano lithium iron phosphate and porous carbon, and pressing to obtain a nano lithium iron phosphate / porous carbon composite material; and (2) mixing the crushed nano lithium iron phosphate / porous carbon composite material and the crushed carbon source, heating and stirring, and carbonizing to obtain the high-capacity retention ratio composite electrode material. Nanometer lithium iron phosphate and porous carbon are mixed, pressed and then subjected to carbon coating, and the high-capacity retention ratio composite electrode material with good electrochemical performance is obtained.

Description

[0001] This application is a divisional application. The application number of the parent application is 202211553314.5. The application date is December 6, 2022. The name of the invention is "A carbon-coated nano lithium iron phosphate / porous carbon composite material and its preparation method and application". Technical Field

[0002] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a composite electrode material with high capacity retention rate, a preparation method and an application thereof. Background Art

[0003] Energy storage has become the fourth major element of the new power system, mainly solving the problems of output stabilization, frequency / voltage regulation and peak-to-valley filling for power generation, peak-to-valley filling, frequency regulation, reactive power regulation, power system standby and black start for power grid, and peak-to-valley filling, new energy consumption and capacity management for users. As an energy storage device with the advantages of high specific capacity, good high-temperature stability, wide operating temperature range and relative safety, lithium iron phosphate batteries have been rapidly developed and applied in new power systems. However, the existing lithium iron phosphate positive electrode materials cannot meet the requirements of high rate and long cycle life in new power systems due to their poor bulk conductivity and slow lithium ion conduction rate, as well as the shortcomings of low rate performance (less than 20C) and poor high-rate charge and discharge cycle life caused by lithium ion intercalation chemical reactions. Therefore, the modification of lithium iron phosphate positive electrode materials has become a hot topic of research.

[0004] Chinese patent application document (CN101630742A) discloses a lithium iron phosphate / carbon nanocomposite and its preparation method, using polymer gelling agent, phosphate, lithium salt and iron salt as raw materials, through sol gelation and heat treatment, to obtain a lithium iron phosphate / carbon nanocomposite composed of lithium iron phosphate nanoparticles and carbon nanoparticles in situ composite, 0.1C discharge capacity reaches 145mAh / g, 60C discharge capacity reaches 45mAh / g, lithium iron phosphate and carbon material only form a simple composite structure, the material chemical stability is poor, and the capacity is small during high rate charge and discharge. Chinese patent application document (CN110277541A) discloses a lithium iron phosphate composite positive electrode material and its preparation method, using gelatin as carbon source and nitrogen source, combining graphene to obtain nano lithium iron phosphate particles coated with nitrogen-rich carbon on the surface by gel method, and uniformly growing in the three-dimensional nitrogen-rich graphene network structure, 50C discharge capacity remains 83mAh / g, the preparation method is relatively complicated, and the high rate charge and discharge capacity is not significantly improved. A Chinese patent application document (CN112397698A) discloses a composite conductive agent-coated lithium iron phosphate material and a preparation method thereof. An anion exchange resin is used to adsorb potassium ferrate, and then porous carbon / iron oxide is prepared by wrapping and sintering with activated carbon. A portion of the carbon source is added to the iron source in advance, and a liquid phase mixing method is adopted to prepare a lithium iron phosphate precursor. At the same time, a high molecular conductive compound polyaniline is added. The preparation process is relatively complicated, and polyaniline is not easy to disperse evenly in the composite material, and it is difficult to form a complete and effective three-dimensional conductive network structure. The cycle performance at high rates is poor. Summary of the invention

[0005] The purpose of the present invention is to provide a carbon-coated nano lithium iron phosphate / porous carbon composite material in response to the above technical problems, which has both "double-layer" physical energy storage and lithium ion de- / embedding chemical energy storage, and can improve the cycle life and high-rate performance of lithium-ion batteries.

[0006] The technical solution of the present invention is a carbon-coated nano-lithium iron phosphate / porous carbon composite material, which includes porous carbon, nano-lithium iron phosphate and a carbon coating layer from the inside to the outside, and the nano-lithium iron phosphate is embedded in the porous carbon surface.

[0007] By setting porous carbon inside the lithium iron phosphate and forming a carbon coating layer outside, a composite material with "double-layer" physical energy storage is obtained, which provides more lithium storage sites for the charging and discharging process of the lithium iron phosphate, improves the cycle life and rate performance of the composite material. The double-layer carbon material also increases the conductivity of the composite material, further improving the electrochemical performance of the composite material.

[0008] Furthermore, the porous carbon is one or more of activated carbon, carbon aerogel, and graphene, and the particle size of the porous carbon is 5 to 8 μm.

[0009] Furthermore, the particle size of the nano lithium iron phosphate is 5 to 200 nm.

[0010] The method for preparing the carbon-coated nano-lithium iron phosphate / porous carbon composite material comprises the following steps:

[0011] (1) mixing nano-lithium iron phosphate and porous carbon and then pressing to obtain a nano-lithium iron phosphate / porous carbon composite material;

[0012] (2) The crushed nano lithium iron phosphate / porous carbon composite material and the crushed carbon source are mixed, heated and stirred, and carbonized to obtain a carbon-coated nano lithium iron phosphate / porous carbon composite material.

[0013] By simply mixing and then pressing, the nano lithium iron phosphate particles are embedded in the porous carbon surface, forming a uniform porous carbon coating layer on the surface of the lithium iron phosphate. The adsorption of the porous carbon coating on the electrolyte is beneficial to the infiltration of the electrolyte, reducing the interface impedance between the electrolyte and the lithium iron phosphate, and improving the charge and discharge capacity of the composite material.

[0014] Furthermore, in step (1), the mass ratio of nano lithium iron phosphate to porous carbon is (95-99): (5-1).

[0015] Furthermore, in step (1), the stirring speed is 20 to 90 rpm, and the stirring time is 1 to 900 min.

[0016] Furthermore, in step (1), the pressing pressure is 300 to 500 tons and the pressing time is 1 to 10 minutes.

[0017] Furthermore, the carbon source is one or more of asphalt, epoxy resin, phenolic resin, polyvinyl alcohol, polymethyl methacrylate, styrene-butadiene rubber, polyvinylidene fluoride, and polytetrafluoroethylene.

[0018] Furthermore, in step (2), the nano lithium iron phosphate / porous carbon composite material and the carbon source can be crushed by ball milling, with a ball milling speed of 30 to 50 rpm and a ball milling time of 1 to 3 min.

[0019] Furthermore, in step (2), the mass ratio of the nano lithium iron phosphate / porous carbon composite material to the carbon source is (95-99): (5-1).

[0020] Furthermore, in step (2), the heating and stirring temperature is 300-400° C., the stirring speed is 10-100 rpm, and the stirring time is 3-5 h.

[0021] Furthermore, in step (2), the carbonization temperature is 600-700° C., and the carbonization time is 1-3 h.

[0022] The present invention also provides a lithium ion battery, wherein the positive electrode material is the carbon-coated nano lithium iron phosphate / porous carbon composite material.

[0023] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0024] (1) A carbon-coated nano-lithium iron phosphate / porous carbon composite material with good electrochemical performance is obtained by mixing nano-lithium iron phosphate and porous carbon, pressing and then carbon coating.

[0025] (2) The composite material’s “double-layer” physical energy storage provides more lithium storage sites during the charge and discharge process of lithium iron phosphate, improving the composite material’s cycle life and rate performance;

[0026] (3) The double-layer carbon material also increases the conductivity of the composite material, further improving the electrochemical performance of the composite material;

[0027] (4) Pressing to embed nano lithium iron phosphate particles into the porous carbon surface, forming a uniform porous carbon coating layer on the surface of lithium iron phosphate, and the adsorption of the porous carbon coating layer on the electrolyte is beneficial to the infiltration of the electrolyte, reducing the interface impedance between the electrolyte and lithium iron phosphate, and improving the charge and discharge capacity of the composite material;

[0028] (5) The preparation method in the technical solution of the present invention is simple and efficient, and is suitable for industrial production applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0030] Figure 1 The present invention is a flow chart for preparing the carbon-coated nano-lithium iron phosphate / porous carbon composite material;

[0031] Figure 2 This is a rate performance diagram of the carbon-coated nano-lithium iron phosphate / porous carbon composite material obtained in Example 1;

[0032] Figure 3 This is a 50C charge and discharge cycle test chart of the carbon-coated nano-lithium iron phosphate / porous carbon composite material obtained in Example 1. DETAILED DESCRIPTION

[0033] The technical scheme of the present invention is further described below by specific examples and accompanying drawings. It should be understood that the specific examples described herein are only used to help understand the present invention and are not intended for specific limitations of the present invention. The accompanying drawings used herein are only for better illustrating the disclosure of the present invention and do not have a limiting effect on the scope of protection. If not otherwise specified, the raw materials used in the embodiments of the present invention are all commonly used raw materials in the art, and the methods used in the embodiments are all conventional methods in the art.

[0034] Example 1

[0035] The preparation method of the carbon-coated nano-lithium iron phosphate / porous carbon composite material of this embodiment is as follows:

[0036] (1) putting nano lithium iron phosphate (10 nm) and porous carbon (6 μm) into a V-type mixer at a mass ratio of 98:2, and mixing them at a stirring speed of 23 rpm for 10 min, so that the materials are tumbled back and forth to evenly mix to obtain a mixture;

[0037] (2) placing the mixture into a square steel cavity and pressing it with a hydraulic press at a pressure of 400 tons for 3 minutes to press the nano-lithium iron phosphate into the surface of the porous carbon to form a square brick-shaped nano-lithium iron phosphate / porous carbon composite material;

[0038] (3) The nano lithium iron phosphate / porous carbon composite material is ball-milled for 2 min to obtain nano lithium iron phosphate / porous carbon composite powder, which is mixed with the ball-milled asphalt in a mass ratio of 95:5 in a V-type mixer, and the mixture is placed in a drum furnace and stirred at 320°C for 4 h with an air flow rate of 2 m 3 / h, to obtain asphalt-coated nano-lithium iron phosphate / porous carbon composite material;

[0039] (4) The asphalt-coated nano-lithium iron phosphate / porous carbon composite material is carbonized in a push-plate kiln at 650°C for 2 h to obtain a carbon-coated nano-lithium iron phosphate / porous carbon composite material.

[0040] Example 2

[0041] The preparation method of the carbon-coated nano-lithium iron phosphate / porous carbon composite material of this embodiment is as follows:

[0042] (1) putting nano lithium iron phosphate (30 nm) and porous carbon (7 μm) into a V-type mixer at a mass ratio of 97:3, and mixing at a stirring speed of 70 rpm for 2 min, so that the materials are tumbled back and forth and evenly mixed to obtain a mixture;

[0043] (2) placing the mixture into a square steel cavity and pressing it with a hydraulic press at a pressure of 500 tons for 1 minute to press the nano-lithium iron phosphate into the surface of the porous carbon to form a square brick-shaped nano-lithium iron phosphate / porous carbon composite material;

[0044] (3) The nano lithium iron phosphate / porous carbon composite material was ball-milled for 1 min to obtain nano lithium iron phosphate / porous carbon composite powder, which was mixed with the ball-milled styrene butadiene rubber in a V-type mixer at a mass ratio of 96:4. The mixture was placed in a drum furnace and stirred at 350° C. for 5 h with an air flow rate of 1 m 3 / h, to obtain asphalt-coated nano-lithium iron phosphate / porous carbon composite material;

[0045] (4) The asphalt-coated nano-lithium iron phosphate / porous carbon composite material is carbonized in a push-plate kiln at 600°C for 1 h to obtain a carbon-coated nano-lithium iron phosphate / porous carbon composite material.

[0046] Example 3

[0047] The preparation method of the carbon-coated nano-lithium iron phosphate / porous carbon composite material of this embodiment is as follows:

[0048] (1) putting nano lithium iron phosphate (100 nm) and porous carbon (5 μm) into a V-type mixer at a mass ratio of 96:4, and mixing them at a stirring speed of 50 rpm for 700 min, so that the materials are tumbled back and forth and evenly mixed to obtain a mixture;

[0049] (2) placing the mixture into a square steel cavity and pressing it with a hydraulic press at a pressure of 400 tons for 6 minutes to press the nano-lithium iron phosphate into the surface of the porous carbon to form a square brick-shaped nano-lithium iron phosphate / porous carbon composite material;

[0050] (3) The nano lithium iron phosphate / porous carbon composite material is ball-milled for 2 min to obtain nano lithium iron phosphate / porous carbon composite powder, which is mixed with the ball-milled phenolic resin in a V-type mixer at a mass ratio of 95:5. The mixture is placed in a drum furnace and stirred at 400°C for 4 h with an air flow rate of 2 m 3 / h, to obtain asphalt-coated nano-lithium iron phosphate / porous carbon composite material;

[0051] (4) The asphalt-coated nano-lithium iron phosphate / porous carbon composite material is carbonized at 670°C in a push-plate kiln for 2.5 h to obtain a carbon-coated nano-lithium iron phosphate / porous carbon composite material.

[0052] Example 4

[0053] The preparation method of the carbon-coated nano-lithium iron phosphate / porous carbon composite material of this embodiment is as follows:

[0054] (1) putting nano lithium iron phosphate (180 nm) and porous carbon (6 μm) into a V-type mixer at a mass ratio of 99:1, mixing at a stirring speed of 90 rpm for 300 min, and making the materials tumble back and forth to evenly mix to obtain a mixture;

[0055] (2) placing the mixture into a square steel cavity and pressing it with a hydraulic press at a pressure of 300 tons for 10 minutes to press the nano-lithium iron phosphate into the surface of the porous carbon to form a square brick-shaped nano-lithium iron phosphate / porous carbon composite material;

[0056] (3) The nano lithium iron phosphate / porous carbon composite material was ball-milled for 2 min to obtain nano lithium iron phosphate / porous carbon composite powder, which was mixed with the ball-milled polyvinyl alcohol in a V-type mixer at a mass ratio of 98:2. The mixture was placed in a drum furnace and stirred at 330° C. for 4.3 h with an air flow rate of 3 m / s. 3 / h, to obtain asphalt-coated nano-lithium iron phosphate / porous carbon composite material;

[0057] (4) The asphalt-coated nano-lithium iron phosphate / porous carbon composite material is carbonized in a push-plate kiln at 600°C for 1.5 h to obtain a carbon-coated nano-lithium iron phosphate / porous carbon composite material.

[0058] Comparative Example 1

[0059] The preparation method of the composite material of this comparative example is as follows:

[0060] (1) putting nano lithium iron phosphate (10 nm) and porous carbon (6 μm) into a V-type mixer at a mass ratio of 98:2, and mixing them at a stirring speed of 23 rpm for 10 min, so that the materials are tumbled back and forth to evenly mix to obtain a mixture;

[0061] (2) The mixture was ball-milled for 2 min to obtain nano-lithium iron phosphate / porous carbon composite powder, which was mixed with the ball-milled asphalt in a mass ratio of 95:5 in a V-type mixer. The mixture was placed in a drum furnace and stirred at 320° C. for 4 h with an air flow rate of 2 m / s. 3 / h, to obtain asphalt-coated nano-lithium iron phosphate / porous carbon composite material;

[0062] (3) The asphalt-coated nano-lithium iron phosphate / porous carbon composite material is carbonized in a push-plate kiln at 650°C for 2 h to obtain a carbon-coated nano-lithium iron phosphate / porous carbon composite material.

[0063] Comparative Example 2

[0064] The preparation method of the composite material of this comparative example is as follows:

[0065] (1) putting nano lithium iron phosphate (10 nm) and porous carbon (6 μm) into a V-type mixer at a mass ratio of 98:2, and mixing them at a stirring speed of 23 rpm for 10 min, so that the materials are tumbled back and forth to evenly mix to obtain a mixture;

[0066] (2) placing the mixture into a square steel cavity and pressing it with a hydraulic press at a pressure of 400 tons for 3 minutes to press the nano-lithium iron phosphate into the surface of the porous carbon to form a square brick-shaped nano-lithium iron phosphate / porous carbon composite material;

[0067] (3) The square brick-shaped nano-lithium iron phosphate / porous carbon composite material was ball-milled for 2 minutes to obtain nano-lithium iron phosphate / porous carbon composite powder, and the nano-lithium iron phosphate / porous carbon composite material was carbonized at 650°C for 2 hours in a push plate kiln to obtain the nano-lithium iron phosphate / porous carbon composite material.

[0068] Comparative Example 3

[0069] The only difference between this comparative example and Example 1 is that in step (3), the mass ratio of nano lithium iron phosphate / porous carbon composite powder to asphalt is 99.5:0.5.

[0070] Comparative Example 4

[0071] The only difference between this comparative example and Example 1 is that in step (3), the mass ratio of nano lithium iron phosphate / porous carbon composite powder to asphalt is 90:10.

[0072] Comparative Example 5

[0073] The preparation method of the composite material of this comparative example is as follows:

[0074] (1) Nano-lithium iron phosphate (10 nm) and ball-milled asphalt were mixed in a V-type mixer at a mass ratio of 95:5. The mixture was placed in a drum furnace and stirred at 320°C for 4 h with an air flow rate of 2 m / s. 3 / h, to obtain asphalt-coated nano-lithium iron phosphate composite material;

[0075] (2) The asphalt-coated nano-lithium iron phosphate composite material is carbonized in a push-plate kiln at 650°C for 2 h to obtain a carbon-coated nano-lithium iron phosphate composite material.

[0076] Comparative Example 6

[0077] The difference between this comparative example and Example 1 is that in step (1), the mass ratio of nano-lithium iron phosphate to porous carbon is 99.5:0.5.

[0078] Comparative Example 7

[0079] The difference between this comparative example and Example 1 is that the mass ratio of nano-lithium iron phosphate to porous carbon in step (1) is 90:10.

[0080] The composite materials obtained in the above examples and comparative examples were used as positive electrode materials for lithium-ion batteries, mixed evenly with conductive agents and PVDF / NMP, and then coated on copper foil, dried in vacuum, compacted by a roller machine, and punched into circular pole pieces. Metal lithium negative electrode, PC as a separator, LiPF6 / (EC+DMC+DEC) with a concentration of 1 mol / L (volume ratio 1:1:1) as an electrolyte, button-type half-cells were assembled in a vacuum glove box, and constant current charge and discharge and rate performance tests were performed.

[0081] Table 1 Lithium-ion battery performance data table

[0082] 0.1C discharge capacity 50C discharge capacity 80C discharge capacity 50C capacity retention rate Example 1 150mAh / g 100mAh / g 84mAh / g 95.3％ Example 2 152mAh / g 101mAh / g 87mAh / g 96.1％ Example 3 150mAh / g 101mAh / g 89mAh / g 96.4％ Example 4 151mAh / g 100mAh / g 85mAh / g 95.7％ Comparative Example 1 151mAh / g 92mAh / g 73mAh / g 89.3％ Comparative Example 2 145mAh / g 85mAh / g 67mAh / g 81.2％ Comparative Example 3 148mAh / g 91mAh / g 73mAh / g 83.3％ Comparative Example 4 137mAh / g 81mAh / g 54mAh / g 86.6％ Comparative Example 5 157mAh / g 72mAh / g 42mAh / g 48.3％ Comparative Example 6 145mAh / g 79mAh / g 48mAh / g 51.6％ Comparative Example 7 125mAh / g 81mAh / g 69mAh / g 95.3％

[0083] The process of preparing the carbon-coated nano lithium iron phosphate / porous carbon composite material of the present invention is as follows: Figure 1 As shown, according to Figure 2 , Figure 3 It can be seen from the experimental data in Table 1 that the obtained carbon-coated nano-lithium iron phosphate / porous carbon composite material has good cycle life and rate performance. Comparative Example 1 is not pressed, which will cause uneven carbon source asphalt coating, separation of nano-lithium iron phosphate and porous carbon, poor ion transmission at the interface of nano-lithium iron phosphate and porous carbon, reduced rate performance, and reduced cycle life; Comparative Example 2 has no carbon coating layer and the carbon coating layer of Comparative Example 3 has too little carbon content, resulting in lithium iron phosphate exposure and reduced cycle performance; Comparative Example 4 has too much carbon content coated by the carbon coating layer, resulting in a low relative content of nano-lithium iron phosphate and a reduced capacity; Comparative Example 5 has no porous carbon and Comparative Example 6 has too little porous carbon content, which reduces the cycle life and rate characteristics; Comparative Example 7 has too much porous carbon content, resulting in a low relative content of nano-lithium iron phosphate and a reduced capacity.

[0084] Finally, it should be noted that the specific embodiments described herein are merely examples of the spirit of the present invention, and are not intended to limit the implementation methods of the present invention. Those skilled in the art of the present invention may make various modifications or supplements to the described embodiments or replace them in similar ways. It is not necessary and impossible to provide all examples of all implementation methods here. However, these obvious changes or modifications derived from the essential spirit of the present invention still fall within the scope of protection of the present invention, and interpreting them as any additional limitation is contrary to the spirit of the present invention.

Claims

1. A composite electrode material with high capacity retention, characterized in that: From the inside to the outside, it includes porous carbon, nano lithium iron phosphate and carbon coating layer, and the nano lithium iron phosphate is embedded in the porous carbon surface; The particle size of nano lithium iron phosphate is 100nm; The method for preparing the composite electrode material with high capacity retention rate comprises the following steps: (1) mixing nano-lithium iron phosphate and porous carbon and then pressing to obtain a nano-lithium iron phosphate / porous carbon composite material; (2) mixing the crushed nano lithium iron phosphate / porous carbon composite material and the crushed carbon source, heating and stirring, and carbonizing to obtain a composite electrode material with a high capacity retention rate; In the step (1), the mass ratio of nano-lithium iron phosphate to porous carbon is 96:4; The pressing pressure in step (1) is 400 tons and the pressing time is 6 minutes; In the step (2), the mass ratio of the nano-lithium iron phosphate / porous carbon composite material to the carbon source is 95:5; In step (2), the carbonization temperature is 670° C. and the carbonization time is 2.5 h; The heating and stirring temperature in step (2) is 400° C., and the heating and stirring time is 4 hours; The heating and stirring in step (2) is performed by a V-type mixer, and the air flow rate of the V-type mixer is 2m 3 / h; The carbon source is phenolic resin.

2. The high capacity retention composite electrode material according to claim 1, characterized in that: The porous carbon is one or more of activated carbon, carbon aerogel and graphene, and the particle size of the porous carbon is 5 μm.

3. A method for preparing a composite electrode material with a high capacity retention rate as claimed in claim 1, characterized in that: The following steps are involved: (1) mixing nano-lithium iron phosphate and porous carbon and then pressing to obtain a nano-lithium iron phosphate / porous carbon composite material; (2) mixing the crushed nano lithium iron phosphate / porous carbon composite material and the crushed carbon source, heating and stirring, and carbonizing to obtain a composite electrode material with a high capacity retention rate; In the step (1), the mass ratio of nano-lithium iron phosphate to porous carbon is 96:4; The pressing pressure in step (1) is 400 tons and the pressing time is 6 minutes; In the step (2), the mass ratio of the nano-lithium iron phosphate / porous carbon composite material to the carbon source is 95:5; In step (2), the carbonization temperature is 670° C. and the carbonization time is 2.5 h; The heating and stirring temperature in step (2) is 400° C., and the heating and stirring time is 4 hours; The heating and stirring in step (2) is performed by a V-type mixer, and the air flow rate of the V-type mixer is 2m 3 / h.

4. A lithium ion battery, characterized in that: The positive electrode material of the lithium-ion battery comprises the high capacity retention composite electrode material according to claim 1.

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