Carbon-doped coating layer modified lithium iron phosphate cathode material and preparation method and application thereof

Abstract

The application discloses a lithium iron phosphate positive electrode material doped with a carbon-coated layer and a preparation method and application thereof, and modifies the carbon-coated layer of lithium iron phosphate by doping phosphorus elements and fluorine elements, so as to improve the electronic conductivity and ion diffusion rate of lithium iron phosphate. The low-temperature and rate performance of lithium iron phosphate is improved. The carbon layer doped with phosphorus elements can increase the graphitization degree and accelerate the electronic transmission rate, and greatly reduce the electronic polarization of lithium iron phosphate. Due to the high electronegativity difference of C-F bonds, the C-F bond groups in the carbon layer doped with fluorine can enhance the chemical and thermal stability of the coated layer, and can promote the adsorption of lithium ions dissolved in the amorphous carbon layer, and improve the lithium ion diffusion rate. Under the synergistic action of the two factors, the electronic conductivity and ion diffusion rate of lithium iron phosphate can be greatly improved, and the stability of the carbon-coated layer is further improved, which is beneficial to improving the electrochemical performance and cycle stability of the battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and relates to a lithium iron phosphate positive electrode material, in particular to a phosphorus and fluorine doped carbon coating layer modified lithium iron phosphate positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] Lithium iron phosphate is a main positive electrode material of lithium ion batteries, has a wide raw material source, is low in cost and long in cycle life. However, the low electronic conductivity and slow ion diffusion rate of lithium iron phosphate limit its further development.

[0003] At present, the main measures for modification and optimization of lithium iron phosphate in the industry are surface coating, ion doping, particle morphology control and particle nanocrystallization. Among them, carbon coating on the surface of lithium iron phosphate is the most widely used, low-cost and effective technical means. The carbon coating layer can enhance the electronic conduction and ion migration rate between lithium iron phosphate particles, reduce the polarization loss of the battery, and also inhibit the growth of the crystal grains to some extent, shorten the lithium ion deintercalation path and improve the rate performance.

[0004] At present, various carbon sources are generally used to form a composite carbon coating layer to coat lithium iron phosphate. However, the decomposition temperatures of different carbon sources are different, and the control requirements for sintering temperature are high. In addition, the uniformity of the carbon coating layer also has a significant impact on the conductivity of lithium iron phosphate. Therefore, how to ensure the uniform distribution of various carbon sources and form a uniform coating layer on the surface of lithium iron phosphate particles is particularly important, which has high requirements for production equipment and synthesis process. At the same time, the capacity decay of lithium iron phosphate batteries at low temperature and the poor rate performance are still difficult to solve, which is mainly caused by the low electronic conductivity and ion diffusion rate of lithium iron phosphate. Therefore, it is particularly important to find a technical method to further improve the conductivity of lithium iron phosphate for the development of lithium iron phosphate batteries. SUMMARY

[0005] In order to solve the above problems, the application provides a phosphorus and fluorine doped carbon coating layer modified lithium iron phosphate positive electrode material and a preparation method and application thereof. The carbon coating layer of lithium iron phosphate is modified by doping phosphorus (P) and fluorine (F) elements to improve the electronic conductivity and ion diffusion rate of lithium iron phosphate. The low temperature and rate performance of lithium iron phosphate are improved.

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

[0007] The application first provides a preparation method of a doped carbon coating layer modified lithium iron phosphate positive electrode material, which comprises the following steps:

[0008] 1) ball-milling a lithium source, an iron source, a phosphorus source and a carbon source to obtain a mixture;

[0009] 2) drying the mixture obtained in step 1) and heat-treating it in an inert atmosphere; after the heat-treatment, grinding to obtain carbon-coated lithium iron phosphate;

[0010] 3) ball-milling the carbon-coated lithium iron phosphate obtained in step 2), triphenyl phosphate and polyvinylidene fluoride, using NMP as the ball-milling medium; after the ball-milling, rotary drying to obtain a mixture;

[0011] 4) calcining the mixture obtained in step 3) in an inert atmosphere; after the calcination, grinding and sieving to obtain a phosphorus and fluorine doped carbon-coated layer modified lithium iron phosphate positive electrode material.

[0012] In the present application, the carbon-coated layer of lithium iron phosphate is modified by doping with phosphorus (P) and fluorine (F) elements, which improves the electronic conductivity and ion diffusion rate of the lithium iron phosphate, thereby further improving the electrical performance of the lithium iron phosphate. On the one hand, the P element doped carbon layer can increase the graphitization degree and accelerate the electronic transmission rate, greatly reducing the electronic polarization of the lithium iron phosphate; on the other hand, due to the high electronegativity difference of the C-F bond, the C-F bond groups in the F doped carbon layer can enhance the chemical and thermal stability of the coating layer, and at the same time can promote the adsorption of lithium ions dissolved in the amorphous carbon layer, improving the lithium ion diffusion rate. Under the synergistic action of the two factors, the electronic conductivity and ion diffusion rate of the lithium iron phosphate can be greatly improved, and the stability of the carbon-coated layer is further improved, which is beneficial to improving the electrochemical performance and cycle stability of the battery.

[0013] As a preferred scheme of the present application, in step 1), the lithium source includes one or more of lithium hydroxide, lithium carbonate and lithium bicarbonate; the iron source includes one or more of ferrous oxalate, diiron trioxide and iron chloride; the phosphorus source includes one or more of ammonium dihydrogen phosphate and diammonium hydrogen phosphate; the carbon source includes one or more of glucose, sucrose, polyethylene glycol and carbon nanotubes; and the molar ratio of the lithium source, the iron source, the phosphorus source and the carbon source is 1:1:1:0.5.

[0014] As a preferred scheme of the present application, in step 1), the ball-milling medium is anhydrous ethanol, and the ball-milling time is 4-10 h.

[0015] As a preferred scheme of the present application, in step 2), the heating method for the heat-treatment is as follows: increasing the temperature to 350-450℃ at a rate of 10℃ / min, maintaining the temperature for 2-4 hours, then increasing the temperature to 600-800℃ at a rate of 10℃ / min, and maintaining the temperature for 4-10 hours.

[0016] As a preferred scheme of the present application, in step 3), the mass ratio of the carbon-coated lithium iron phosphate, triphenyl phosphate and polyvinylidene fluoride is 10:1:1.

[0017] As a preferred scheme of the present application, in step 3), the ball milling time is 4-10h.

[0018] As a preferred scheme of the present application, in step 4), the calcination temperature is 500-600℃, and the calcination time is 2-4h.

[0019] As a preferred scheme of the present application, in step 2), the inert atmosphere is argon atmosphere; in step 4), the inert atmosphere is argon atmosphere.

[0020] The present application further provides the carbon-doped coated layer modified lithium iron phosphate cathode material prepared by the above preparation method.

[0021] The present application finally provides the application of the above carbon-doped coated layer modified lithium iron phosphate cathode material in lithium ion.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] 1) The present application modifies the carbon-coated layer of lithium iron phosphate by doping phosphorus (P) element and fluorine (F) element, improves the electronic conductivity and ion diffusion rate, so that the electrical performance of lithium iron phosphate is further improved.

[0024] 2) The present application reduces the internal resistance of lithium iron phosphate powder and improves the rate performance of lithium iron phosphate.

[0025] 3) The present application greatly reduces the electronic polarization of lithium iron phosphate, and the stability of the carbon-coated layer is further improved, which is beneficial to improve the electrochemical performance and cycle stability of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is the XRD pattern of lithium iron phosphate.

[0027] Figure 2 is the SEM photo of lithium iron phosphate.

[0028] Figure 3 is the discharge rate curve of lithium iron phosphate.

[0029] Figure 4 is the impedance spectrum of lithium iron phosphate. DETAILED DESCRIPTION

[0030] In order to make the technical means, creative features, purposes and effects of the present application easy to understand, the present application is further described below in combination with specific embodiments, but the following embodiments are only preferred embodiments of the present application, not all. Based on the embodiments in the embodiments, other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application. In the following examples, the experimental methods are conventional methods, and the materials and reagents used in the following examples are commercially available unless otherwise specified.

[0031] Example 1

[0032] Into the ball mill tank, 3.1021 g of lithium carbonate, 3.0211 g of ferrous oxalate, 9.6625 g of ammonium dihydrogen phosphate and 2.1078 g of glucose were added respectively, and finally 25 mL of anhydrous ethanol was added, and the mixture was mixed uniformly by ball milling for 6 h. After ball milling, the mixture was dried and ground, and then put into a tube furnace for high temperature sintering, with the temperature rising system being: the temperature was raised to 400℃ at a temperature rising speed of 10℃ / min, and kept for 4 hours, and then the temperature was raised to 700℃ at a temperature rising speed of 10℃ / min, and kept for 8 hours.

[0033] 20 g of LFPO / C and 2.5 g of TPP and 2 g of PVDF were taken into a ball mill tank containing 25 mL of NMP, and the mixture was mixed uniformly by ball milling for 6 h. The mixture was put into a rotary evaporator for rotary drying, and finally the dried mixture was put into a tube furnace, sintered at 500℃ for 3 h, and then ground and sieved to obtain phosphorus and fluorine doped carbon coated layer modified lithium iron phosphate.

[0034] Control group one

[0035] Into the ball mill tank, 3.1021 g of lithium carbonate, 3.0211 g of ferrous oxalate, 9.6625 g of ammonium dihydrogen phosphate and 2.1078 g of glucose were added respectively, and finally 25 mL of anhydrous ethanol was added, and the mixture was mixed uniformly by ball milling for 6 h. After ball milling, the mixture was dried and ground, and then put into a tube furnace for high temperature sintering, with the temperature rising system being: the temperature was raised to 400℃ at a temperature rising speed of 10℃ / min, and kept for 4 hours, and then the temperature was raised to 700℃ at a temperature rising speed of 10℃ / min, and kept for 8 hours, and then ground and sieved to obtain carbon coated lithium iron phosphate.

[0036] Control group two

[0037] Into the ball mill tank, 3.1021 g of lithium carbonate, 3.0211 g of ferrous oxalate, 9.6625 g of ammonium dihydrogen phosphate and 2.1078 g of glucose were added respectively, and finally 25 mL of anhydrous ethanol was added, and the mixture was mixed uniformly for 6 h. After the ball milling was completed, the mixture was dried and ground, and then was placed in a tube furnace for high-temperature sintering, with the temperature rising program being: the temperature was raised to 400℃ at a temperature rising rate of 10℃ / min, and then was kept for 4 hours, and then the temperature was raised to 700℃ at a temperature rising rate of 10℃ / min, and then was kept for 8 hours.

[0038] Into the ball mill tank, 20 g of LFPO / C and 4 g of TPP were placed, and 25 mL of NMP was added, and the mixture was mixed uniformly for 6 h. The mixture was placed in a rotary evaporator for rotary drying, and finally the dried mixture was placed in a tube furnace, and was sintered at 500℃ for 3 h, and then was ground and sieved to obtain phosphorus and fluorine doped carbon-coated layer modified lithium iron phosphate.

[0039] Control group three

[0040] Into the ball mill tank, 3.1021 g of lithium carbonate, 3.0211 g of ferrous oxalate, 9.6625 g of ammonium dihydrogen phosphate and 2.1078 g of glucose were added respectively, and finally 25 mL of anhydrous ethanol was added, and the mixture was mixed uniformly for 6 h. After the ball milling was completed, the mixture was dried and ground, and then was placed in a tube furnace for high-temperature sintering, with the temperature rising program being: the temperature was raised to 400℃ at a temperature rising rate of 10℃ / min, and then was kept for 4 hours, and then the temperature was raised to 700℃ at a temperature rising rate of 10℃ / min, and then was kept for 8 hours.

[0041] Into the ball mill tank, 20 g of LFPO / C and 4 g of N-methyl pyrrolidone (NMP) were placed, and 25 mL of NMP was added, and the mixture was mixed uniformly for 6 h. The mixture was placed in a rotary evaporator for rotary drying, and finally the dried mixture was placed in a tube furnace, and was sintered at 500℃ for 3 h, and then was ground and sieved to obtain phosphorus and fluorine doped carbon-coated layer modified lithium iron phosphate.

[0042] The lithium iron phosphate prepared in Example 1 and the lithium iron phosphate prepared in the control group one were subjected to X-ray diffraction (XRD) analysis, and the results are shown in Figure 1 After comparing the diffraction curves of the two groups of lithium iron phosphate with the standard PDF card of lithium iron phosphate, no impurity peak was found, indicating that the doping of P and F did not change the olivine structure of lithium iron phosphate.

[0043] Meanwhile, the two groups of samples were subjected to scanning electron microscopy (SEM) analysis, and the results are shown in Figure 2The SEM photos show that the particle size of the lithium iron phosphate primary particles is about 300-500 nm, the particles are uniformly distributed and spherical, indicating that the doping of P and F does not change the micro-morphology and particle size of the lithium iron phosphate. The powder internal resistance of the lithium iron phosphate prepared in the embodiment is tested by using a powder internal resistance tester, and the results are shown in Table 1. The powder internal resistance of the samples of the control group one, the control group two and the control group three is 320 Ω·mm, 252 Ω·mm and 277 Ω·mm respectively under a pressure of 12 MPa, and the powder internal resistance of the sample of the embodiment 1 is 170 Ω·mm, indicating that the electronic conductivity of the lithium iron phosphate is significantly improved after the doping of P and F in the carbon layer.

[0044] Table 1. Powder internal resistance of lithium iron phosphate

[0045]

[0046] Subsequently, the lithium iron phosphate materials prepared in the embodiment 1, the control group one, the control group two and the control group three are added into NMP according to a mass ratio of 8:1:1 together with SP carbon black and PVDF. The slurry is mixed to a suitable viscosity and fineness by using a homogenizer, and then the slurry is uniformly coated on an aluminum foil by using a coating machine and dried in an oven under vacuum to obtain a positive electrode sheet. Finally, the positive electrode sheet, a metal lithium negative electrode sheet, a separator and an electrolyte are assembled into a button cell, and the button cell is tested for 0.1C charge-discharge, rate performance (as shown in FIG. 5) and impedance spectrum (as shown in FIG. 6). Figure 3 Figure 4

[0047] Table 2 is the 0.1C charge-discharge data of the lithium iron phosphate button cell, and it can be seen that the 0.1C discharge gram capacity of the lithium iron phosphate materials prepared in the control group one, the control group two and the control group three is 156.1 mAh / g, 158.7 mAh / g and 157.4 mAh / g respectively, and the 0.1C discharge gram capacity of the lithium iron phosphate material prepared in the embodiment 1 is 162.1 mAh / g, which is higher than that of the lithium iron samples doped with P or F alone, indicating that the doping of P and F in the carbon layer can effectively improve the electronic transmission rate of the carbon coating layer and the diffusion rate of Li + , thereby improving the electronic conductivity of the lithium iron phosphate and increasing the discharge capacity thereof.

[0048] Table 2. 0.1C charge-discharge of lithium iron phosphate button cell

[0049]

[0050] Figure 3 The rate curve of the lithium iron phosphate battery shows that the discharge specific capacity of the lithium iron phosphate of the embodiment 1 at different rates is better than that of each control group, and the lithium iron phosphate of the embodiment 1 exhibits more excellent rate performance.

[0051] ​​Impedance spectrum test was performed on the lithium iron phosphate batteries of each group, and the test results are shown in Figure 4 It can be seen that the impedance of the lithium iron phosphate battery prepared in Example 1 is significantly smaller than that of the control group, indicating that the charge transfer rate of Example 1 is faster, and further indicating that compared with doping P or F alone, doping P and F elements in the carbon layer has a synergistic effect, and greatly improves the electronic and ionic conductivity of lithium iron phosphate.

[0052] The above is only the preferred embodiment of the present application, not any form and substantial limitation of the present application. It should be noted that for ordinary skilled in the art, without departing from the method of the present application, several improvements and supplements can also be made, which should be considered as the protection scope of the present application. For those skilled in the art, without departing from the spirit and scope of the present application, some changes, modifications and equivalent changes of the above disclosed technical content can be made, which are equivalent embodiments of the present application; at the same time, any equivalent change, modification and evolution of the above-mentioned embodiments according to the essential technology of the present application are still within the scope of the technical scheme of the present application.

Claims

1. A method for preparing a carbon-doped coating layer modified lithium iron phosphate cathode material, characterized in that, The preparation method comprises the following steps: 1) ball-milling a lithium source, an iron source, a phosphorus source and a carbon source to obtain a mixture; the lithium source comprises one or more of lithium hydroxide, lithium carbonate and lithium bicarbonate, the iron source comprises one or more of ferrous oxalate, diiron trioxide and iron chloride, the phosphorus source comprises one or more of ammonium dihydrogen phosphate and diammonium hydrogen phosphate, and the carbon source comprises one or more of glucose, sucrose, polyethylene glycol and carbon nanotubes; the molar ratio of the lithium source, the iron source, the phosphorus source and the carbon source is 1:1:1:0.5; 2) drying the mixture obtained in step 1) and heat-treating it in an inert atmosphere; after the heat-treatment, grinding is performed to obtain carbon-coated lithium iron phosphate; the temperature rising method for the heat-treatment is as follows: the temperature is raised to 350-450℃ at a temperature rising rate of 10℃ / min, and then the temperature is kept constant for 2-4 hours; then the temperature is raised to 600-800℃ at a temperature rising rate of 10℃ / min, and then the temperature is kept constant for 4-10 hours; 3) ball-milling the carbon-coated lithium iron phosphate obtained in step 2), triphenyl phosphate and polyvinylidene fluoride in a ball-milling medium of NMP, and then performing spin-drying to obtain a mixture; the mass ratio of the carbon-coated lithium iron phosphate, the triphenyl phosphate and the polyvinylidene fluoride is 10:1:1; 4) calcining the mixture obtained in step 3) in an inert atmosphere, and then performing grinding and screening to obtain a phosphorus and fluorine doped carbon-coated layer modified lithium iron phosphate positive electrode material; the calcination temperature is 500-600℃, and the calcination time is 2-4 hours; In step 1), the ball-milling medium is anhydrous ethanol, and the ball-milling time is 4-10 hours; In step 3), the ball-milling time is 4-10 hours.

2. The method for preparing the carbon-coated modified lithium iron phosphate cathode material according to claim 1, characterized in that, In step 2), the inert atmosphere is an argon atmosphere; in step 4), the inert atmosphere is an argon atmosphere.

3. A carbon-doped coated layer modified lithium iron phosphate cathode material, characterized in that, The preparation method is prepared by any one of claims 1-2.

4. The use of a carbon-doped coating layer modified lithium iron phosphate cathode material according to claim 3, characterized in that, The application of the doped carbon-coated layer modified lithium iron phosphate positive electrode material in lithium ions.

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