Composite lithium iron phosphate material, positive electrode using same, and lithium-ion battery

Abstract

A composite lithium iron phosphate material, a positive electrode using same, and a lithium-ion battery. Raw materials for preparing the composite lithium iron phosphate material comprise an iron phosphate precursor, a lithium source and a carbon source. The carbon source coats the iron phosphate precursor and the lithium source to form the composite lithium iron phosphate material. The carbon source comprises a synthetic polymer carbon source and a biomass carbon source; and the biomass carbon source comprises carbon fibers.

Description

A composite lithium iron phosphate material and positive electrode and lithium ion battery using the same

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on December 14, 2023, with application number 202311725784X. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application belongs to the technical field of lithium-ion batteries, and in particular relates to a composite lithium iron phosphate material and a positive electrode and a lithium-ion battery using the same. Background Art

[0003] The booming new energy vehicle industry demonstrates the enormous commercial value of lithium-ion batteries, while also placing higher demands on them. Lithium iron phosphate (LiFePO4) has garnered widespread attention due to its advantageous properties. After years of research and development, my country's lithium iron phosphate industry has matured and matured, becoming the only country in the world to integrate large-scale R&D and commercial applications. Researchers have used various methods to functionalize lithium iron phosphate, enabling its successful application in new energy vehicle power batteries, starting power supplies, energy storage systems, and other fields.

[0004] However, the current commercial carbon-coated lithium iron phosphate (LFP) has a low tap density. This low powder tap / compact density severely hinders its widespread application. Furthermore, since increasing the tap / compact density of LFP materials is beneficial for the development of high-energy-density cathode materials for lithium-ion batteries, which is an inevitable requirement for the development of lithium-ion battery materials, improving the tap / compact density of LFP materials has become an urgent need in the field. Technical issues

[0005] In order to improve the compaction density of lithium iron phosphate materials and increase the specific capacity of lithium iron phosphate materials, the present application provides a composite lithium iron phosphate material and a positive electrode and a lithium ion battery using the same.

[0006] Technical Solution

[0007] In the first aspect, the present application provides a composite lithium iron phosphate material. The raw materials for preparing the composite lithium iron phosphate material include an iron phosphate precursor, a lithium source, and a carbon source. The carbon source coats the iron phosphate precursor and the lithium source to obtain an orthorhombic composite lithium iron phosphate material; wherein the carbon source includes a synthetic polymer carbon source and a biomass carbon source; the biomass carbon source includes carbon fiber.

[0008] In a second aspect, the present application provides a positive electrode, comprising a current collector and a positive electrode active coating disposed on the surface of the current collector, wherein the positive electrode active coating contains the composite lithium iron phosphate material as described above.

[0009] In a third aspect, the present application provides a lithium-ion battery comprising the positive electrode as described above. Beneficial effects

[0010] This application, by carbon coating the lithium iron phosphate precursor, can not only form a porous carbon film on the surface of the lithium iron phosphate particles to enhance the conductivity of the lithium iron phosphate; it can also improve the solid phase interface between the lithium iron phosphate material and the electrolyte, enhance the electrochemical adsorption performance; it can also promote the formation and growth of lithium iron phosphate particles. In addition, by combining synthetic polymer carbon sources with biomass carbon sources, the compaction density and processing performance of lithium iron phosphate can be significantly improved. Among them, the introduction of synthetic polymer carbon sources can effectively reduce free carbon, increase the proportion of bound carbon, and thus increase the compaction density; and the introduction of biomass carbon sources can improve the problem of rapid increase in processing viscosity caused by a large number of -OH functional groups in polymer carbon, and improve the processing performance of lithium iron phosphate. Among them, the introduction of carbon fiber helps to improve the conductivity of lithium iron phosphate and increase the compaction density. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG1 is an XRD pattern of the lithium iron phosphate materials prepared in Example 1 and Comparative Example 1;

[0012] FIG2 is an SEM image of the lithium iron phosphate materials prepared in Example 1 and Comparative Example 1. Modes for Carrying Out the Invention

[0013] In one embodiment, raw materials for preparing the lithium iron phosphate precursor include an iron source and a phosphorus source.

[0014] In one embodiment, the iron source and the phosphorus source are provided by ferric phosphate (FePO 4 ), and in the ferric phosphate (FePO 4 ), the molar ratio of Fe:P is 0.96-0.99:1.

[0015] In one embodiment, the lithium source includes lithium carbonate (Li 2 CO 3 ).

[0016] In one embodiment, a method for preparing carbon fibers includes the following steps: S1. dissolving plant powder in an organic solvent and producing nanofibers by electrospinning; S2. subsequently subjecting the nanofibers to stabilization, pre-oxidation, and carbonization to obtain carbon fibers. Using electrospinning to produce nanofibers helps improve particle size and enhance carbon coating performance.

[0017] In one embodiment, the plant powder includes at least one of rice husk powder and wheat straw powder.

[0018] In one embodiment, in S1 , the particle size of the nanofibers is 50-200 nm.

[0019] In one embodiment, in S2, the stabilization treatment includes a crushing treatment and a dissolving treatment, and the stabilization treatment is performed at 20-30°C.

[0020] In one embodiment, in S2 , the reaction temperature of the pre-oxidation treatment is 500-600° C.

[0021] In one embodiment, the carbonization treatment is performed by heating to 700° C. and reacting for 5 to 10 hours.

[0022] In one embodiment, the biomass carbon source also includes a carbohydrate carbon source, and the mass ratio of synthetic polymer carbon source: carbohydrate carbon source: carbon fiber is 4-6:3-5:1.

[0023] In one embodiment, the synthetic polymer carbon source includes polyethylene glycol.

[0024] In one embodiment, the carbohydrate carbon source includes glucose.

[0025] In one embodiment, the raw materials used to prepare the composite lithium iron phosphate material also include additives, including titanium and vanadium. The introduction of the titanium and vanadium additives effectively refines the grain size, reduces the size of small particles, and increases the proportion of small particles, significantly improving the low-temperature and rate performance of the lithium iron phosphate material. Furthermore, a gradation of large and small particles is achieved, thereby increasing the compaction density of the lithium iron phosphate material.

[0026] In one embodiment, in the raw materials for preparing the composite lithium iron phosphate material, the content of the titanium additive is 2000-5000 ppm; and / or the content of the vanadium additive is 2000-5000 ppm.

[0027] In one embodiment, the titanium additive includes titanium dioxide (TiO 2 ).

[0028] In one embodiment, the vanadium additive includes ammonium metavanadate (NH 4 VO 3 ).

[0029] In one embodiment, calculated by mass percentage, the composite lithium iron phosphate material has a carbon content of 1.1-1.6%, a titanium content of 1000-2000 ppm, and a vanadium content of 2000-3000 ppm.

[0030] In one embodiment, a method for preparing a composite lithium iron phosphate material includes the following steps: mixing and dispersing a lithium iron phosphate precursor and a carbon source, and then transferring the reaction system to an inert gas atmosphere and calcining to obtain the composite lithium iron phosphate material.

[0031] In one embodiment, the mixing and dispersion is performed by ball milling, and the ball milling time is 4 to 6 hours.

[0032] In one embodiment, the inert gas includes nitrogen.

[0033] In one embodiment, the calcination temperature is 650-750°C.

[0034] In one embodiment, the particle size of the composite lithium iron phosphate material is 120-1500 nm.

[0035] Example 1

[0036] 1. Preparation method of carbon fiber

[0037] Rice husks were mechanically ground to produce rice husk powder, which was then dissolved in an ethanol solution and stirred for 6 hours to produce an 8% rice husk powder solution. The rice husk powder solution was then placed into a syringe and injected into an electrospinning machine to produce nanofibers with a particle size of 100 nm. The nanofibers were then stabilized in an oven at 25°C for 24 hours, followed by a pre-oxidation treatment at 550°C for 1 hour, and then carbonized at 700°C for 6 hours to produce carbon fibers. The stabilization treatment included crushing and dissolution.

[0038] 2. Preparation method of composite lithium iron phosphate

[0039] Iron phosphate and lithium carbonate were accurately weighed in a stoichiometric ratio of 2:1 and placed in a ball mill. Six percent carbon source (polyethylene glycol: glucose: carbon fiber = 5:4:1, calculated by weight), 3000 ppm TiO2, and 3000 ppm NH4VO3 were added, followed by 40 mL of anhydrous ethanol and ball milling for 5 hours. The slurry was then dried in an 80°C oven and the resulting solid was sintered in a nitrogen atmosphere resistance furnace at 710°C for 10 hours to produce a composite lithium iron phosphate material.

[0040] 3. Preparation of positive electrode and lithium-ion battery

[0041] According to the mass ratio, the prepared composite lithium iron phosphate material: Super P: PVDF = 95:2.5:2.5 is mixed with an appropriate amount of NMP solvent to prepare an electrode slurry, which is then evenly coated on the surface of aluminum foil and dried to obtain the positive electrode sheet; the electrolyte used is a mixture of ethylene carbonate and dimethyl carbonate dissolved in 1 mol / L LiPF6, wherein, according to the volume ratio, ethylene carbonate: dimethyl carbonate = 1:1.

[0042] The prepared positive electrode sheet, negative electrode sheet (lithium sheet) and electrolyte are combined into a lithium-ion battery.

[0043] Example 2

[0044] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the composite lithium iron phosphate material in this embodiment, the ratio of polyethylene glycol: glucose: carbon fiber is 6:3:1. Apart from the above differences, the operating steps for preparing the composite lithium iron phosphate material, the positive electrode and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0045] Example 3

[0046] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the composite lithium iron phosphate material in this embodiment, the ratio of polyethylene glycol: glucose: carbon fiber is 4:5:1. Apart from the above differences, the operating steps for preparing the composite lithium iron phosphate material, the positive electrode and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0047] Example 4

[0048] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the composite lithium iron phosphate material in this embodiment, the ratio of polyethylene glycol: glucose: carbon fiber is 3:6:1. Apart from the above differences, the operating steps for preparing the composite lithium iron phosphate material, the positive electrode and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0049] Example 5

[0050] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that the content of the titanium additive in the preparation of the composite lithium iron phosphate material in this embodiment is 2000 ppm, and the content of the vanadium additive is 5000 ppm. Apart from the above differences, the operating steps for preparing the composite lithium iron phosphate material, the positive electrode and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0051] Example 6

[0052] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that the content of the titanium additive in the preparation of the composite lithium iron phosphate material in this embodiment is 5000 ppm, and the content of the vanadium additive is 2000 ppm. Except for the above differences, the operating steps for preparing the composite lithium iron phosphate material, the positive electrode and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0053] Example 7

[0054] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that this embodiment replaces the vanadium additive with an equal mass fraction of a titanium additive when preparing the composite lithium iron phosphate material. Apart from the above differences, the operating steps for preparing the positive electrode and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0055] Example 8

[0056] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that this embodiment replaces the titanium additive with an equal mass fraction of a vanadium additive when preparing the composite lithium iron phosphate material. Apart from the above differences, the operating steps for preparing the positive electrode and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0057] Example 9

[0058] This example prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that, when preparing the nanofibers, this example uses wheat straw powder in equal parts by mass instead of rice husk powder. Apart from the above differences, the operating steps for preparing the composite lithium iron phosphate material, the positive electrode and the lithium-ion battery in this example are strictly consistent with those in Example 1.

[0059] Example 10

[0060] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that this embodiment does not perform stabilization treatment when preparing carbon fibers. Apart from the above differences, the operating steps for preparing the composite lithium iron phosphate material, the positive electrode and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0061] Example 11

[0062] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that this embodiment does not perform pre-oxidation treatment when preparing carbon fiber. Apart from the above differences, the operating steps for preparing the composite lithium iron phosphate material, positive electrode and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0063] Example 12

[0064] This embodiment prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that when preparing the composite lithium iron phosphate material in this embodiment, the calcination temperature is 600°C. Apart from the above differences, the operating steps for preparing the composite lithium iron phosphate material, the positive electrode and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0065] Example 13

[0066] This example prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that this example performs calcination in an air atmosphere when preparing the composite lithium iron phosphate material. Apart from the above differences, the operating steps for preparing the positive electrode and lithium-ion battery in this example are strictly consistent with those in Example 1.

[0067] Comparative Example 1

[0068] This comparative example uses commercial olivine-type lithium iron phosphate to prepare the positive electrode and the lithium-ion battery. Except for the above differences, the operating steps for preparing the positive electrode and the lithium-ion battery in this comparative example are strictly consistent with those in Example 1.

[0069] Comparative Example 2

[0070] This comparative example prepares a composite lithium iron phosphate material with reference to the formula and method provided in Example 1. The difference from Example 1 is that this comparative example replaces the biomass carbon source with an equal mass fraction of a synthetic polymer carbon source when preparing the composite lithium iron phosphate material. Apart from the above differences, the operating steps for preparing the positive electrode and lithium-ion battery in this comparative example are strictly consistent with those in Example 1.

[0071] Comparative Example 3

[0072] This comparative example prepares a composite lithium iron phosphate material by referring to the formula and method provided in Example 1. The difference from Example 1 is that this comparative example replaces carbon fiber with an equal mass fraction of glucose when preparing the composite lithium iron phosphate material. Apart from the above differences, the operating steps for preparing the positive electrode and lithium-ion battery in this comparative example are strictly consistent with those in Example 1.

[0073] Comparative Example 4

[0074] This comparative example prepared a composite lithium iron phosphate material using the formula and method provided in Example 1. Unlike Example 1, this comparative example did not prepare carbon fiber. Instead, the biomass carbon material was prepared by directly calcining rice husk powder. Aside from these differences, the procedures for preparing the composite lithium iron phosphate material, positive electrode, and lithium-ion battery in this comparative example were strictly consistent with those in Example 1. Specifically, the biomass carbon material was prepared by calcining rice husk powder at 700°C for 6 hours to obtain the biomass carbon material.

[0075] Test Example 1

[0076] 1. Test subjects

[0077] The composite lithium iron phosphate materials and corresponding lithium ion batteries prepared in Examples 1 to 13 and Comparative Examples 1 to 4.

[0078] 2. Test Method

[0079] (1) Compacted density: Weigh 1g of sample and test it using an electronic pressure testing machine at a measuring tonnage of 30kN;

[0080] (2) Particle size: Weigh 0.1-0.2 g of sample and disperse it in 500 ml of deionized water. Ultrasonic dispersion is performed for 3 min. The particle size is then measured using a laser particle size analyzer at a detection angle of 0-144°.

[0081] (3) Morphology test: SEM test is performed by using a scanning electron microscope at an acceleration voltage of 10KV and a magnification of 20K;

[0082] (4) XRD test: The test was performed using an X-ray diffractometer at a test speed of 4° / min and a range of 10-80°;

[0083] (4) Gram capacity test: After the assembled battery is charged and discharged for 300 cycles at a current of 500 mA, the discharge capacity of the 300 cycles is recorded. The gram capacity is calculated as shown in formula ①:

[0084] ①

[0085] 3. Test results and analysis

[0086] The test results of this test example are shown in Table 1. Among them, the XRD tests of Comparative Example 1 and Example 1 are shown in Figure 1. By comparing the XRD results, it can be seen that the main phases of the composite lithium iron phosphate prepared in Example 1 are all olivine-shaped and have a good crystal structure. By comparing the SEM test results, as shown in Figure 2, it can be observed that under the same 20k rate, it can be concluded that the small particles of the lithium iron phosphate material prepared in Example 1 are smaller, and the graded matching of large and small particles can be achieved. By comparing the test results of Example 1 and Comparative Example 1, it can be confirmed that the preparation method provided by the present application can effectively improve the compaction density and specific capacity. By comparing Example 1 and Comparative Example 2, it can be confirmed that the addition of a biomass carbon source can reduce the free carbon content and further improve the compaction density and specific capacity. In Comparative Example 3, due to the lack of introduction of carbon fiber, the performance of the obtained lithium iron phosphate material and lithium-ion battery decreased significantly. In Comparative Example 4, biomass carbon is prepared by directly calcining rice husks. Compared with the method of obtaining carbon fibers by electrospinning in Example 1, the biomass carbon used in Comparative Example 4 does not include carbon fibers, resulting in a decrease in the compaction density of the corresponding composite lithium iron phosphate material and the specific capacity performance of the lithium-ion battery.

[0087] The composite lithium iron phosphate materials prepared in Examples 1-4 differ only in the mass ratios of polyethylene glycol, glucose, and carbon fiber. The data show that as the content of the synthetic polymer carbon source increases, the compacted density and specific capacity of the composite lithium iron phosphate materials fluctuate. The composite lithium iron phosphate material prepared in Example 1 exhibits the best performance.

[0088] Furthermore, the results obtained from Examples 1 and 5-6 show that the content of the titanium and vanadium additives affects the compacted density. Comparing Example 1 with Examples 7-8 reveals that the titanium and vanadium additives have a synergistic effect, effectively refining the grains and achieving a graded and optimally matched distribution of large and small particles, further increasing the compacted density.

[0089] Comparing Examples 1 and 9, it can be seen that the use of different crop powders or fibers to prepare carbon fibers can affect the performance of the composite lithium iron phosphate material and the lithium-ion battery using it. Furthermore, as shown in Examples 10 and 11, the preparation method of carbon fibers can also affect their performance.

[0090] Furthermore, compared to Example 1, the calcination temperature of the composite lithium iron phosphate material obtained in Example 12 was adjusted to 600°C, resulting in a decrease in the compacted density of the composite lithium iron phosphate material and the specific capacity performance of the lithium-ion battery. In Example 13, the calcination was carried out in air, resulting in a decrease in both the compacted density and specific capacity of the composite lithium iron phosphate material.

[0091] Table 1. Test results of this test case

[0092]

Claims

1. A composite lithium iron phosphate material, wherein the raw materials for preparing the composite lithium iron phosphate material include an iron phosphate precursor, a lithium source, and a carbon source, and the carbon source is coated with the iron phosphate precursor and the lithium source to obtain the composite lithium iron phosphate material; wherein, The carbon source includes a synthetic polymer carbon source and a biomass carbon source; the biomass carbon source includes carbon fiber.

2. The composite lithium iron phosphate material according to claim 1, wherein: The method for preparing the carbon fiber comprises the following steps: S1. dissolving plant powder in an organic solvent and preparing nanofibers by electrospinning; S2. The nanofibers are then subjected to stabilization treatment, pre-oxidation treatment, and carbonization treatment in sequence to obtain the carbon fibers.

3. The composite lithium iron phosphate material according to claim 2, wherein: In S1, the particle size of the nanofibers is 120-1500 nm.

4. The composite lithium iron phosphate material according to claim 2, wherein: In S2, the stabilization treatment includes a crushing treatment and a dissolving treatment, and the stabilization treatment is performed at 20-30°C.

5. The composite lithium iron phosphate material according to claim 2, wherein: In S2, the reaction temperature of the pre-oxidation treatment is 500-600°C.

6. The composite lithium iron phosphate material according to claim 1 or 2, wherein: The biomass carbon source also includes a sugar carbon source. Calculated by mass ratio, the synthetic polymer carbon source: the sugar carbon source: the carbon fiber = 4-6:3-5:

1.

7. The composite lithium iron phosphate material according to claim 1, wherein: The raw materials for preparing the composite lithium iron phosphate material also include additives, and the additives include titanium additives and vanadium additives.

8. The composite lithium iron phosphate material according to claim 1 or 7, wherein: Calculated by mass percentage, the carbon content of the composite lithium iron phosphate material is 1.1-1.6%, the titanium content is 1000-2000 ppm, and the vanadium content is 2000-3000 ppm.

9. The composite lithium iron phosphate material according to claim 1, wherein: The particle size of the composite lithium iron phosphate material is 120-1500 nm.

10. A positive electrode, comprising a current collector and a positive electrode active coating disposed on the surface of the current collector, wherein the positive electrode active coating contains the composite lithium iron phosphate material according to claims 1 to 9.

11. A lithium ion battery, comprising the positive electrode as claimed in claim 10.

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