Long cycle life lithium iron phosphate composite material and preparation method thereof

Abstract

The application relates to the technical field of lithium ion battery positive electrode materials, in particular to a long-cycle-life lithium iron phosphate composite material and a preparation method thereof. The preparation method comprises the following steps: (1) preparing a uniform mixture; (2) preparing a uniform dispersion liquid; (3) preparing a first intermediate product; (4) preparing a second intermediate product; and (5) preparing the long-cycle-life lithium iron phosphate composite material. The carbon nanotubes form a stable three-dimensional space conductive network, the conductive capacity of the material is improved, meanwhile, the 'cage' structure formed by the lithium supplementing agent and the carbon nanotubes continuously supplements the loss of active lithium in the positive electrode material in the circulation process, so that the battery capacity and the cycle performance are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery cathode materials, and particularly relates to a long-cycle-life lithium iron phosphate composite material and a preparation method thereof. BACKGROUND

[0002] Among known lithium intercalation compounds, olivine-type lithium iron phosphate is considered as one of the best cathode materials for lithium ion batteries due to its high working voltage, large energy density, low self-discharge rate, no memory effect and high safety.

[0003] Although lithium iron phosphate batteries have many advantages, they also have fundamental defects that cannot be ignored, among which the short cycle life limits the application of lithium iron phosphate batteries in the industrial large energy storage field. Existing research shows that more than 80% of the cycle life attenuation of lithium iron phosphate batteries is caused by the loss of active lithium ions in the system.

[0004] Therefore, how to maintain a high content of active lithium ions in the system, and provide a long-cycle-life lithium iron phosphate composite material and a preparation method thereof, is of great significance to the stable development of China's energy storage battery industry. SUMMARY

[0005] In order to solve the problems in the prior art, the application provides a long-cycle-life lithium iron phosphate composite material and a preparation method thereof, which aims to solve the capacity attenuation problem of lithium iron phosphate batteries during the cycle process and improve the service life thereof.

[0006] In order to achieve the above-mentioned purpose, the application adopts the following technical scheme:

[0007] A preparation method of a long-cycle-life lithium iron phosphate composite material, characterized in that the method comprises the following steps:

[0008] (1) dry-mixing carbon nanotubes and a lithium supplement to obtain a uniform mixture;

[0009] (2) sand-mixing an iron source, a phosphorus source, a lithium source, a carbon source, a metal dopant and water to obtain a uniform dispersion liquid;

[0010] (3) spray-drying the uniform dispersion liquid obtained in step (2) to obtain a first intermediate product;

[0011] (4) uniformly mixing the uniform mixture obtained in step (1) and the first intermediate product obtained in step (3) to obtain a second intermediate product;

[0012] (5) sintering the second intermediate product obtained in step (4), and then crushing to obtain the long-cycle-life lithium iron phosphate composite material.

[0013] Preferably, the hollow inner cavity diameter of the carbon nanotube in step (1) is 300-600 nm, and the length-diameter ratio is 3-8; the lithium supplementing agent is one or more selected from Li2O, Li2O2, LiF, Li2S, Li3N, LiFeO5, Li6CoO4 and Li2C2O4; the mass ratio of the carbon nanotube to the lithium supplementing agent is (0.5-1.0):(0.5-1.0); and the dry mixing is performed in a micro mixer, and the dry mixing time is 20-40 min.

[0014] Preferably, the iron source and the phosphorus source in step (2) are iron phosphate; the molar ratio of Fe / P of the iron phosphate is 0.960-0.975, and the mass ratio of the iron phosphate to the carbon nanotube is (98-99):(0.5-1.0); the lithium source is one or more selected from lithium carbonate, lithium oxalate, lithium phosphate and lithium hydroxide; the molar ratio of Li / Fe of the lithium source to the iron phosphate is 1.01-1.07; the carbon source is a mixture of an organic carbon source and an inorganic carbon source, the organic carbon source is one or more selected from glucose, sucrose, fructose, maltose, PEG, PVA and citric acid, and the inorganic carbon source is one or more selected from acetylene black, graphite aqueous solution, carbon nanotube and graphene; and the mass ratio of the carbon source to the iron phosphate is 1:(10-99).

[0015] Preferably, the metal dopant in step (2) is a mixture of titanium dioxide and magnesium acetate tetrahydrate; the mass ratio of the titanium dioxide to the iron phosphate is (14-25):10000, and the mass ratio of the magnesium acetate tetrahydrate to the iron phosphate is (8-14):1000; the mass ratio of the water to the iron phosphate is 2:(2-5); the sand milling temperature is 30-45 ℃, the sand milling particle size D50 is controlled to be 0.4-0.6 μm, and the solid content is 30-45 wt%.

[0016] Preferably, in step (3), the spray drying controls the feeding port temperature to be 180-240 ℃, and the discharging port temperature to be 90-120 ℃; and the particle size D50 of the obtained first intermediate product is controlled to be 20-45 μm, and the moisture content is ≤1.5 wt%.

[0017] Preferably, in step (4), the mixing is performed in a micro mixer, and the mixing time is 30-60 min.

[0018] Preferably, in step (5), the sintering is performed in a nitrogen atmosphere, the sintering temperature is 760-780 ℃, and the sintering time is 8-12 h; the crushing controls the particle size D50 to be 0.6-1.6 μm, and the average value of the primary particle size is 180-250 nm.

[0019] The application also claims a long-cycle-life lithium iron phosphate composite material prepared by the above method.

[0020] The application also claims a kind of application of the above long cycle life lithium iron phosphate positive electrode material in the technical field of lithium ion battery positive electrode material.

[0021] Compared with the prior art, the application has the following beneficial effects:

[0022] (1) The application uses self-produced iron phosphate as iron source and phosphorus source, which not only reduces the cost, but also ensures the stability of the iron-phosphorus ratio during feeding and improves the feasibility of industrialization.

[0023] (2) The application combines in-situ doping with organic-inorganic carbon sources, so that lithium iron phosphate agglomerated particles are less, polarization and impedance are smaller during electrochemical reaction, and thus the material has better conductivity, higher specific capacity and long cycle performance.

[0024] (3) The application co-dopes Mg and Ti metals, which provides good steric hindrance effect during sintering, inhibits the growth of lithium iron phosphate primary particles, shortens the lithium ion diffusion path, and improves the specific capacity.

[0025] (4) The carbon nanotubes can form a stable three-dimensional space conductive network, which not only maintains the stability of the system structure, but also greatly improves the conductivity of the material; the lithium supplement agent has a very large irreversible capacity, and a small amount of the "cage" structure formed by the lithium supplement agent and the carbon nanotubes continuously supplements the loss of active lithium in the positive electrode material during the cycle process, thereby improving the battery capacity and cycle performance. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a schematic diagram of the "cage" structure formed by the lithium supplement agent and the carbon nanotubes of the application;

[0027] Figure 2 is a cycle number-capacity retention rate curve of the sample prepared in Example 1 of the application. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solution and advantages of the application clearer, the application will be further described in detail below with reference to the examples. Of course, the specific examples described herein are only used to explain the application and do not limit the application.

[0029] The steps in the application are arranged with labels, but are not used to limit the order of the steps, unless the order of the steps is explicitly stated or the execution of a step needs other steps as a basis, otherwise the relative order of the steps can be adjusted. It can be understood that the term "and / or" used herein involves and covers any and all possible combinations of one or more of the associated listed items.

[0030] Unless otherwise defined, all chemical reagents and materials used in the present application are commercially available or synthesized from commercially available materials.

[0031] Example 1

[0032] A long cycle life lithium iron phosphate composite material and a preparation method thereof, comprising the following steps:

[0033] (1) 100 g of carbon nanotubes with a hollow cavity diameter of 400 nm and a length of 2000 nm and 100 g of Li2O are put into a micro-mixer and mixed for 30 min to obtain a uniform mixture;

[0034] (2) 10000 g of iron phosphate and 1603.7 g of lithium hydroxide are mixed, then 580 g of glucose and 100 g of a graphite aqueous solution with a solid content of 13 wt% are added as a carbon source, 80.0 g of magnesium acetate tetrahydrate and 25.0 g of titanium dioxide are added, and 15000 g of water is added, and sand milling is carried out at 40℃, to control the particle size D50 to be 0.5 μm, and the solid content to be 40 wt% of a uniform dispersion;

[0035] (3) The dispersion obtained in step (2) is spray dried, the inlet temperature is controlled to be 200℃, and the outlet temperature is controlled to be 100℃, to obtain a first intermediate product with a particle size D50 of 30 μm and a moisture content of ≤1.5 wt%;

[0036] (4) The uniform mixture obtained in step (1) and the first intermediate product obtained in step (3) are mixed for 45 min to obtain a second intermediate product;

[0037] (5) The second intermediate product obtained in step (4) is sintered under a nitrogen atmosphere, the sintering temperature is controlled to be 770℃, and the sintering time is controlled to be 10 h; then it is crushed, the particle size D50 is controlled to be 1.0 μm, and the average value of the primary particle size is 210 nm, to finally obtain a long cycle life lithium iron phosphate positive electrode material with a carbon content of 1.0%.

[0038] Example 2

[0039] A long cycle life lithium iron phosphate composite material and a preparation method thereof, comprising the following steps:

[0040] (1) 100 g of carbon nanotubes with a hollow cavity diameter of 400 nm and a length of 2000 nm and 100 g of Li2S are put into a micro-mixer and mixed for 20 min to obtain a uniform mixture;

[0041] (2) mixing 10000 g of iron phosphate and 1603.7 g of lithium oxalate, then adding 580 g of maltose and 13 g of acetylene black with a solid content of 13 wt% as a carbon source, while adding 80.0 g of magnesium acetate tetrahydrate and 25.0 g of titanium dioxide, and 15000 g of water, sand grinding at 30℃, controlling the particle size D50 to be 0.4μm, and the solid content of the uniform dispersion liquid to be 30wt%;

[0042] (3) spray drying the dispersion liquid obtained in step (2), controlling the inlet temperature to be 180℃, and the outlet temperature to be 90℃, to obtain a first intermediate product with a particle size D50 of 20μm and a moisture content of ≤1.5wt%;

[0043] (4) mixing the uniform mixture obtained in step (1) and the first intermediate product obtained in step (3) for 30min to obtain a second intermediate product;

[0044] (5) sintering the second intermediate product obtained in step (4) under a nitrogen atmosphere, controlling the sintering temperature to be 760℃, and the sintering time to be 12h; then crushing, controlling the particle size D50 to be 0.6μm, and the average value of primary particle size to be 180nm, to finally obtain a long cycle life lithium iron phosphate positive electrode material with a carbon content of 1.00%.

[0045] Example 3

[0046] A long cycle life lithium iron phosphate composite material and a preparation method thereof, comprising the following steps:

[0047] (1) putting 100 g of carbon nanotubes with a hollow inner cavity diameter of 400 nm and a length of 2000 nm, 40 g of Li2O2 and 60 Li3N into a micro mixer and mixing for 40min to obtain a uniform mixture;

[0048] (2) mixing 10000 g of iron phosphate and a mixture of 903.4 g of lithium hydroxide and 700.3 g of lithium carbonate, then adding 350 g of glucose, 230 g of fructose, and 5 g of acetylene black and 8 g of graphene as a carbon source, while adding 80.0 g of magnesium acetate tetrahydrate and 25.0 g of titanium dioxide, and 15000 g of water, sand grinding at 45℃, controlling the particle size D50 to be 0.6μm, and the solid content of the uniform dispersion liquid to be 45wt%;

[0049] (3) spray drying the dispersion liquid obtained in step (2), controlling the inlet temperature to be 240℃, and the outlet temperature to be 120℃, to obtain a first intermediate product with a particle size D50 of 45μm and a moisture content of ≤1.5wt%;

[0050] (4) mixing the uniform mixture obtained in step (1) and the first intermediate product obtained in step (3) for 60min to obtain a second intermediate product;

[0051] (5) The second intermediate product obtained in step (4) is sintered under a nitrogen atmosphere, the sintering temperature is controlled at 780°C, the sintering time is 8h; then it is crushed, the particle size D50 is controlled at 1.6μm, the average value of primary particle size is 250nm, and finally a long cycle life lithium iron phosphate positive electrode material with a carbon content of 1.00% is prepared.

[0052] Comparative Example 1

[0053] A lithium iron phosphate composite material and a preparation method thereof, comprising the following steps:

[0054] (1) 10000g of iron phosphate and 1603.7g of lithium hydroxide are mixed, then 580g of glucose and 100g of a graphite aqueous solution with a solid content of 13wt% are added as a carbon source, 80.0g of magnesium acetate tetrahydrate and 25.0g of titanium dioxide are added, and 15000g of water is added, sand milling is carried out at 40°C, and a uniform dispersion liquid with a particle size D50 of 0.5μm and a solid content of 40wt% is obtained;

[0055] (2) The dispersion liquid obtained in step (1) is spray dried, the inlet temperature is controlled at 200°C, the outlet temperature is controlled at 100°C, a first intermediate product with a particle size D50 of 30μm and a moisture content of ≤1.5wt% is prepared;

[0056] (3) The first intermediate product obtained in step (2) is sintered under a nitrogen atmosphere, the sintering temperature is controlled at 770°C, the sintering time is 10h; then it is crushed, the particle size D50 is controlled at 1.0μm, the average value of primary particle size is 210nm, and finally a lithium iron phosphate positive electrode material is prepared.

[0057] The difference between this comparative example and Example 1 is that there is no carbon nanotube and lithium supplementing agent in the system.

[0058] Comparative Example 2

[0059] A lithium iron phosphate composite material and a preparation method thereof, comprising the following steps:

[0060] (1) 100g of carbon nanotubes with a hollow inner cavity diameter of 400nm and a length of 2000nm and 100g of Li2O are put into a micro-mixer and mixed for 30min to obtain a uniform mixture;

[0061] (2) 10000g of iron phosphate and 1603.7g of lithium hydroxide are mixed, then 580g of glucose is added, 80.0g of magnesium acetate tetrahydrate and 25.0g of titanium dioxide are added, and 15000g of water is added, sand milling is carried out at 40°C, and a uniform dispersion liquid with a particle size D50 of 0.5μm and a solid content of 40wt% is obtained;

[0062] (3) the dispersion liquid obtained in step (2) is spray dried, the inlet temperature is controlled at 200°C, and the outlet temperature is controlled at 100°C, to obtain a first intermediate product with a particle size D50 of 30 μm and a moisture content of ≤1.5 wt%;

[0063] (4) the uniform mixture obtained in step (1) and the first intermediate product obtained in step (3) are mixed for 45 min to obtain a second intermediate product;

[0064] (5) the second intermediate product obtained in step (4) is sintered under a nitrogen atmosphere, the sintering temperature is controlled at 770°C, and the sintering time is controlled at 10 h; then, the product is crushed, the particle size D50 is controlled at 1.0 μm, and the average value of the primary particle size is controlled at 210 nm, to finally obtain a lithium iron phosphate positive electrode material.

[0065] The difference between the present comparative example and example 1 is that there is no inorganic carbon source in the system.

[0066] Comparative Example 3

[0067] A lithium iron phosphate composite material and a preparation method thereof, comprising the following steps:

[0068] (1) 100 g of carbon nanotubes with a hollow inner cavity diameter of 400 nm and a length of 2000 nm and 100 g of Li2O are put into a micro-mixer and mixed for 30 min to obtain a uniform mixture;

[0069] (2) 10000 g of iron phosphate and 1603.7 g of lithium hydroxide are mixed, then 580 g of glucose and 100 g of a graphite aqueous solution with a solid content of 13 wt% are added as a carbon source, 80.0 g of magnesium acetate tetrahydrate is added, and 15000 g of water is added, and the mixture is sand-milled at 40°C to obtain a uniform dispersion liquid with a particle size D50 of 0.5 μm and a solid content of 40 wt%;

[0070] (3) the dispersion liquid obtained in step (2) is spray dried, the inlet temperature is controlled at 200°C, and the outlet temperature is controlled at 100°C, to obtain a first intermediate product with a particle size D50 of 30 μm and a moisture content of ≤1.5 wt%;

[0071] (4) the uniform mixture obtained in step (1) and the first intermediate product obtained in step (3) are mixed for 45 min to obtain a second intermediate product;

[0072] (5) the second intermediate product obtained in step (4) is sintered under a nitrogen atmosphere, the sintering temperature is controlled at 770°C, and the sintering time is controlled at 10 h; then, the product is crushed, the particle size D50 is controlled at 1.0 μm, and the average value of the primary particle size is controlled at 210 nm, to finally obtain a long-cycle-life lithium iron phosphate positive electrode material with a carbon content of 1.0%.

[0073] The present comparative example differs from example 1 in that only Mg metal is doped in the system.

[0074] The lithium iron phosphate composite materials prepared in examples 1-3 and comparative examples 1-3 are dispersed in NMP in a mass ratio of 90:5:5 together with Super-P and PVDF, and then coated on an aluminum foil after being uniformly ball-milled, vacuum dried to obtain positive electrode sheets, and electrolyte is 1 mol / L of LiPF6, wherein the volume ratio of solvent is EC:DMC:EMC=1:1:1 (volume ratio), the separator is Celgard polypropylene film, and the metal lithium sheet is the negative electrode, and then assembled into a coin cell. The test voltage range is 2.0V-3.75V, charged to 3.75V in a constant current and constant voltage charging mode, discharged to 2.0V in a constant current discharging mode, and the charge and discharge current is 0.1C for 1 cycle; and then cycled for 500 cycles at a 1C charge and discharge current, and the cutoff voltage condition is the same as that of 0.1C. The test results are shown in Table 1.

[0075] Table 1

[0076]

[0077] As shown in Table 1, the long cycle life lithium iron phosphate positive material has a more uniform primary particle size distribution, higher 1C capacity and better cycle performance.

[0078] The above only describes the preferred embodiments of the present application, and it should be noted that for those skilled in the art, without departing from the concept of the present application, several improvements and changes can be made, which are all within the protection scope of the present application.

Claims

1. A method for preparing a long-cycle-life lithium iron phosphate composite material, characterized in that, Includes the following steps: (1) The carbon nanotubes and lithium supplement were dry-mixed to obtain a homogeneous mixture; (2) The iron source, phosphorus source, lithium source, carbon source, metal dopant and water are mixed by sand milling to obtain a uniform dispersion; (3) The uniformly dispersed liquid obtained in step (2) is spray-dried to obtain the first intermediate product; (4) Mix the homogeneous mixture obtained in step (1) and the first intermediate product obtained in step (3) until homogeneous to obtain the second intermediate product; (5) The second intermediate product obtained in step (4) is sintered and then crushed to obtain the long cycle life lithium iron phosphate composite material. The hollow inner diameter of the carbon nanotubes in step (1) is 300-600 nm, and the aspect ratio is 3-8; the lithium supplement is selected from one or more of Li2O, Li2O2, LiF, Li2S, Li3N, LiFeO5, Li6CoO4 and Li2C2O4; the mass ratio of the carbon nanotubes to the lithium supplement is (0.5-1.0):(0.5-1.0); the dry mixing is carried out in a micro mixer for 20-40 min; the carbon source is a mixture of organic and inorganic carbon sources; the iron and phosphorus sources in step (2) are iron phosphate.

2. The method for preparing a long-cycle-life lithium iron phosphate composite material according to claim 1, characterized in that, The molar ratio of iron phosphate (Fe / P) is 0.960–0.975, and the mass ratio of iron phosphate to carbon nanotubes is (98–99):(0.5–1.0); the lithium source is one or more selected from lithium carbonate, lithium oxalate, lithium phosphate, and lithium hydroxide; the molar ratio of lithium source to iron phosphate (Li / Fe) is 1.01–1.07; the organic carbon source is one or more selected from glucose, sucrose, fructose, maltose, PEG, PVA, and citric acid; the inorganic carbon source is one or more selected from acetylene black, graphite ink solution, carbon nanotubes, and graphene; the mass ratio of carbon source to iron phosphate is 1:(10–99).

3. The method for preparing a long-cycle-life lithium iron phosphate composite material according to claim 1, characterized in that, The metal dopant in step (2) is a mixture of titanium dioxide and magnesium acetate tetrahydrate; the mass ratio of titanium dioxide to ferric phosphate is (14-25):10000, the mass ratio of magnesium acetate tetrahydrate to ferric phosphate is (8-14):1000; the mass ratio of water to ferric phosphate is 2:(2-5); the milling temperature is 30-45℃, the milling particle size D50 is controlled to be 0.4-0.6μm, and the solid content is 30-45wt%.

4. The method for preparing a long-cycle-life lithium iron phosphate composite material according to claim 1, characterized in that, In step (3), the spray drying process controls the inlet temperature to be 180-240°C and the outlet temperature to be 90-120°C; the particle size D50 of the obtained first intermediate product is controlled to be 20-45 μm and the moisture content to be ≤1.5 wt%.

5. The method for preparing a long-cycle-life lithium iron phosphate composite material according to claim 1, characterized in that, The mixing in step (4) is carried out in a micro mixer for 30 to 60 minutes.

6. The method for preparing a long-cycle-life lithium iron phosphate composite material according to claim 1, characterized in that, The sintering in step (5) is carried out in a nitrogen atmosphere, the sintering temperature is 760-780℃, and the sintering time is 8-12h; the particle size D50 of the pulverization is 0.6-1.6μm, and the average particle size of the primary particles is 180-250nm.

7. A long-cycle-life lithium iron phosphate composite material prepared by the method according to any one of claims 1-6.

8. The application of the long cycle life lithium iron phosphate composite material of claim 7 in the field of lithium-ion battery cathode material technology.

Images