A lithium iron phosphate composite electrode material and its preparation method

Abstract

This invention relates to a lithium iron phosphate composite electrode material and its preparation method, belonging to the field of electrode material technology. The preparation method of this invention significantly improves the electrochemical performance of the material through a wet-mixing precursor, modified carbon nanotube composite, and double-layer carbon coating process. The modified carbon nanotubes are treated with melamine, potassium hydroxide, and boric acid to achieve nitrogen / boron co-doping and surface functionalization, constructing efficient electron transport channels and shortening the lithium-ion diffusion path through a microporous structure; their high aspect ratio and improved graphitization effectively alleviate charge-discharge volume expansion and ensure high-temperature structural stability. The double-layer carbon coating consists of a polydopamine inner layer and a glucose-derived carbon outer layer. The inner layer strengthens the interface bonding through chemical bonding and guides uniform lithium-ion diffusion, while the highly conductive outer carbon layer provides a fast electron transport channel and mechanical protection, synergistically optimizing ion / electron transport efficiency.

Description

Technical Field

[0001] This invention belongs to the field of electrode material technology, and relates to a lithium iron phosphate composite electrode material and its preparation method. Background Technology

[0002] Lithium iron phosphate (LiFePO4) has become a research hotspot for lithium-ion battery cathode materials due to its high safety, low cost, and environmental friendliness. However, its inherent defects, such as low conductivity, slow lithium-ion migration rate, and insufficient cycle stability, limit its application in high-current scenarios. Existing technologies mostly involve carbon coating, metal ion doping, or nano-modification, but these suffer from problems such as large carbon source consumption, limited conductivity improvement, and complex processes. Therefore, there is an urgent need to develop a lithium iron phosphate composite electrode material and its preparation method. Summary of the Invention

[0003] The purpose of this invention is to provide a lithium iron phosphate composite electrode material and its preparation method, which has good electrochemical performance.

[0004] The objective of this invention can be achieved through the following technical solutions:

[0005] A method for preparing a lithium iron phosphate composite electrode material, the specific steps of which are as follows:

[0006] S1-1: Lithium carbonate, iron oxide and phosphoric acid are mixed and added to deionized water to make the solid-liquid ratio 1:3. The mixture is stirred at 90~100 ℃ for 4~6 h to obtain mixture A;

[0007] S1-2: Add dopamine hydrochloride and modified carbon nanotubes to mixture A, and ball mill them at a ball-to-material ratio of (8~10):1. Use zirconia milling balls and ball mill for 2~3 h. Then place them in a vacuum drying oven at 80~90 ℃ for 12 h to obtain powder B.

[0008] S1-3: Transfer powder B to a muffle furnace for calcination. First, raise the temperature to 600~700 ℃ at a rate of 5 ℃ / min and hold for 3~4 h. Then, raise the temperature to 800~850 ℃ at a rate of 3 ℃ / min and hold for 5~6 h to obtain powder C.

[0009] S1-4: Mix powder C with polyvinylpyrrolidone and glucose, place in a ball mill and ball mill for 1-2 h at a speed of 150-250 rpm, then heat at 350-400 ℃ for 1-2 h to obtain the lithium iron phosphate composite electrode material.

[0010] As a preferred embodiment of the present invention, in S1-1, Li:Fe:PO4 is mixed in a molar ratio of (1~1.1):1:1.

[0011] As a preferred embodiment of the present invention, the method for preparing the modified carbon nanotubes in S1-2 is as follows:

[0012] S3-1: Mix carbon nanotubes, melamine and potassium hydroxide, and stir at 60~80 ℃ for 3~5 h;

[0013] S3-2: Add boric acid and continue stirring for 1-2 h. Then transfer to a muffle furnace for calcination. Heat to 800-820 °C under a high-purity nitrogen atmosphere and hold for 2 h. Finally, wash with ethanol and deionized water and dry under vacuum at 60 °C for 12 h to obtain the modified carbon nanotubes.

[0014] As a preferred embodiment of the present invention, the amount of dopamine hydrochloride added in S1-2 is 3-5% of the mass of mixture A, and the amount of modified carbon nanotubes added is 3-5% of the mass of mixture A.

[0015] As a preferred embodiment of the present invention, the ball milling speed in S1-2 is 300~360 rpm.

[0016] As a preferred embodiment of the present invention, the calcination atmosphere in S1-3 is high-purity argon.

[0017] As a preferred embodiment of the present invention, the amount of polyvinylpyrrolidone added in S1-4 is 1-3% of the mass of powder C, and the amount of glucose added is 5-8% of the mass of powder C.

[0018] As a preferred embodiment of the present invention, carbon nanotubes, melamine and potassium hydroxide are mixed in S3-1 at a mass ratio of 1:(3~5):(15~20).

[0019] As a preferred embodiment of the present invention, the amount of boric acid added in S3-2 is 5-10% of the mass of carbon nanotubes.

[0020] As a preferred embodiment of the present invention, the heating rate in S3-2 is 10 ℃ / min.

[0021] First, this invention uses a wet process to mix lithium carbonate, iron oxide, and phosphoric acid to form a uniform precursor solution, promoting the initial nucleation of lithium iron phosphate crystals. Subsequently, a three-dimensional conductive network is constructed using modified carbon nanotubes to reduce the impact of the intrinsic low conductivity of lithium iron phosphate. Polydopamine and glucose-derived carbon form a bilayer carbon coating, further shortening the lithium-ion diffusion path and significantly improving the electrochemical performance of lithium iron phosphate.

[0022] During ball milling, dopamine hydrochloride self-polymerizes to form polydopamine, which tightly encapsulates lithium iron phosphate particles. The catechol groups in the polydopamine molecule form hydrogen bonds or chemical bonds with the Fe-O bonds on the surface of lithium iron phosphate, ensuring a tight bond between the carbon layer and the active material and preventing the carbon layer from falling off during charging and discharging. Polydopamine itself has a certain degree of conductivity and can act as a bridge for electron transport, connecting lithium iron phosphate particles with the outer carbon nanotube conductive network. In addition, polydopamine can fill the microcracks on the surface of lithium iron phosphate particles, reducing direct contact between the electrolyte and the active material and inhibiting side reactions.

[0023] Glucose forms an amorphous carbon layer after pyrolysis. The electrical conductivity of amorphous carbon is significantly higher than that of lithium iron phosphate, which can construct a fast electron transport channel. At the same time, the pyrolysis of glucose releases gases such as H2O and CO2, forming micropores and mesopores, which provide diffusion channels for Li⁺. In addition, the outer carbon shell can buffer the volume change of lithium iron phosphate during charging and discharging, preventing particle pulverization.

[0024] The double-layer carbon coating exhibits a significant synergistic effect. The inner polydopamine layer tightly connects the outer highly conductive carbon layer to the lithium iron phosphate particles, forming a three-dimensional conductive network. This transforms the electron transport path from inter-particle jumps to a continuous conductive network, significantly improving electronic conductivity. The amino and hydroxyl groups of the inner polydopamine layer can weakly coordinate with Li⁺, guiding Li⁺ to diffuse uniformly to the lithium iron phosphate surface. The porous structure of the outer glucose-derived carbon provides low-resistance diffusion channels, further optimizing the lithium-ion diffusion path. Furthermore, the double-layer carbon coating stabilizes and enhances interfacial stability. The polydopamine layer blocks acidic substances in the electrolyte from corroding the lithium iron phosphate, while the outer carbon shell suppresses side reactions at the electrode / electrolyte interface. The double-layer carbon coating structure buffers volume changes caused by Li⁺ insertion / extraction. Simultaneously, it reduces the contact between lithium iron phosphate and the electrolyte at high temperatures.

[0025] In the preparation of modified carbon nanotubes, nitrogen and boron elements are introduced onto the surface of the carbon nanotubes by blending melamine, potassium hydroxide, and boric acid, thereby improving their conductivity and structural stability. Melamine decomposes during high-temperature calcination, releasing nitrogen atoms which are then doped into the carbon nanotube lattice. The electronegativity difference between nitrogen and carbon atoms induces charge redistribution, forming defects and active sites on the carbon nanotube surface, significantly improving electron mobility. Boric acid generates boron oxide during calcination, which further reacts with carbon to form boron doping. The electron-deficient properties of boron atoms can adjust the band structure of carbon materials, constructing more efficient electron transport channels.

[0026] Potassium hydroxide, acting as an activator, etches oxygen-containing functional groups onto the surface of carbon nanotubes. These functional groups can form hydrogen bonds or chemical bonds with phosphate groups on the surface of lithium iron phosphate, reducing interfacial resistance. Simultaneously, potassium hydroxide activation etches micropores into the carbon nanotube surface, forming a three-dimensional porous network. These channels shorten the diffusion path of lithium ions. Boron oxides generated from the decomposition of boric acid can undergo coordination reactions with lithium iron phosphate, forming a bridging structure that further strengthens interfacial bonding. Boron atoms possess empty p orbitals, which can interact with Li... + Weak coordination occurs, guiding Li + Uniform diffusion on the surface of carbon nanotubes avoids localized concentration polarization.

[0027] The high aspect ratio and excellent mechanical properties of modified carbon nanotubes can form a rigid framework between lithium iron phosphate particles, mitigating the Li-induced stress during charging and discharging. + Volume changes caused by insertion / extraction. Nitrogen and boron co-doping modification improves the graphitization degree of carbon nanotubes, making them less prone to structural collapse at high temperatures and ensuring the integrity of the electrode structure. The oxygen-containing functional groups on the surface of modified carbon nanotubes can bond with polydopamine formed by dopamine self-polymerization through hydrogen bonds or covalent bonds to construct a carbon nanotube-polydopamine bilayer conductive network, further improving electron transport efficiency.

[0028] The beneficial effects of this invention are:

[0029] (1) This invention constructs a nitrogen / boron co-doped three-dimensional conductive framework by modifying carbon nanotubes with melamine, potassium hydroxide, and boric acid. Melamine decomposes at high temperature to release nitrogen atoms, which form defects and active sites on the surface of carbon nanotubes by utilizing the difference in electronegativity, significantly improving electron mobility. Boron oxide generated by boric acid adjusts the band structure of carbon materials and constructs efficient electron transport channels. Potassium hydroxide acts as an activator, both etching the surface of carbon nanotubes to generate oxygen-containing functional groups and reduce interfacial resistance, and forming a microporous structure to shorten the lithium-ion diffusion path. In addition, the bridging effect between boron oxide and lithium iron phosphate strengthens the interfacial bonding, and the empty p orbitals of boron atoms can guide the uniform diffusion of lithium ions and avoid local polarization. The high aspect ratio of carbon nanotubes and the graphitization degree improved by nitrogen-boron co-doping make them form a rigid framework in the electrode, alleviate the volume expansion during charging and discharging, and maintain structural stability at high temperatures, effectively solving the problems of poor intrinsic conductivity and easy structural damage of lithium iron phosphate.

[0030] (2) During ball milling, dopamine hydrochloride self-polymerizes to form a polydopamine inner coating layer. Its catechol groups are tightly bonded to the Fe-O bonds on the surface of lithium iron phosphate, which not only prevents the carbon layer from falling off but also fills microcracks in the particles and inhibits electrolyte corrosion. The conductivity of polydopamine itself acts as an electron transport bridge, connecting lithium iron phosphate particles with the outer carbon nanotube network, while the weak coordination of amino and hydroxyl groups with lithium ions guides their uniform diffusion. The amorphous carbon shell formed by the pyrolysis of glucose in the outer layer further constructs a fast electron transport channel. Its porous structure provides a low-resistance diffusion path for lithium ions, while mechanically buffering the volume change during charge and discharge to prevent particle pulverization. The synergistic effect of the double coating further optimizes ion / electron transport and improves the electrochemical performance of the material. Detailed Implementation

[0031] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with embodiments, is provided below.

[0032] Example 1

[0033] A method for preparing a lithium iron phosphate composite electrode material, the specific steps of which are as follows:

[0034] S1-1: Lithium carbonate, iron oxide and phosphoric acid are mixed in a molar ratio of 1.05:1:1 and added to deionized water to make a solid-liquid ratio of 1:3. The mixture is stirred at 95 °C for 5 h to obtain mixture A.

[0035] S1-2: Dopamine hydrochloride and modified carbon nanotubes were added to mixture A and ball-milled. The amount of dopamine hydrochloride added was 4% of the mass of mixture A, and the amount of modified carbon nanotubes added was 4% of the mass of mixture A. The ball-to-material ratio was 9:1. Zirconia grinding balls were used. The ball milling speed was 330 rpm and the ball milling time was 2.5 h. Then, the mixture was placed in a vacuum drying oven and dried at 85°C for 12 h to obtain powder B.

[0036] S1-3: Powder B was transferred to a muffle furnace for calcination. The temperature was first increased to 650 ℃ at a rate of 5 ℃ / min and held for 3.5 h. Then the temperature was increased to 820 ℃ at a rate of 3 ℃ / min and held for 5.5 h. The calcination atmosphere was high-purity argon, and powder C was obtained.

[0037] S1-4: Mix powder C with polyvinylpyrrolidone and glucose, wherein the amount of polyvinylpyrrolidone added is 2% of the mass of powder C and the amount of glucose added is 6% of the mass of powder C. Place the mixture in a ball mill and ball mill for 1.5 h at a speed of 200 rpm. Then heat it at 380 ℃ for 1.5 h to obtain the lithium iron phosphate composite electrode material.

[0038] The preparation method of the modified carbon nanotubes in S1-2 is as follows:

[0039] S3-1: Mix carbon nanotubes, melamine and potassium hydroxide in a mass ratio of 1:4:18 and stir at 70 °C for 4 hours;

[0040] S3-2: Add boric acid at 8% of the mass of carbon nanotubes, continue stirring for 1.5 h, then transfer to a muffle furnace for calcination, heat to 810 ℃ at a rate of 10 ℃ / min under a high-purity nitrogen atmosphere and hold for 2 h, finally wash with ethanol and deionized water, and place in a vacuum dryer at 60 ℃ for 12 h to obtain the modified carbon nanotubes.

[0041] Example 2

[0042] A method for preparing a lithium iron phosphate composite electrode material, the specific steps of which are as follows:

[0043] S1-1: Lithium carbonate, iron oxide and phosphoric acid are mixed in a molar ratio of 1:1:1 and added to deionized water to make a solid-liquid ratio of 1:3. The mixture is stirred at 90 °C for 4 h to obtain mixture A.

[0044] S1-2: Dopamine hydrochloride and modified carbon nanotubes were added to mixture A and ball-milled. The amount of dopamine hydrochloride added was 3% of the mass of mixture A, and the amount of modified carbon nanotubes added was 3% of the mass of mixture A. The ball-to-material ratio was 8:1. Zirconia grinding balls were used. The ball milling speed was 300 rpm and the ball milling time was 2 h. Then, the mixture was placed in a vacuum drying oven and dried at 80℃ for 12 h to obtain powder B.

[0045] S1-3: Powder B is transferred to a muffle furnace for calcination. The temperature is first increased to 600 ℃ at a rate of 5 ℃ / min and held for 3 h. Then the temperature is increased to 800 ℃ at a rate of 3 ℃ / min and held for 5 h. The calcination atmosphere is high-purity argon, and powder C is obtained.

[0046] S1-4: Mix powder C with polyvinylpyrrolidone and glucose, wherein the amount of polyvinylpyrrolidone added is 1% of the mass of powder C and the amount of glucose added is 5% of the mass of powder C. Place the mixture in a ball mill and ball mill for 1 h at a speed of 150 rpm. Then heat it at 350 ℃ for 1 h to obtain the lithium iron phosphate composite electrode material.

[0047] The preparation method of the modified carbon nanotubes in S1-2 is as follows:

[0048] S3-1: Mix carbon nanotubes, melamine and potassium hydroxide in a mass ratio of 1:3:16 and stir at 60 °C for 3 h.

[0049] S3-2: Add 5% boric acid by mass of carbon nanotubes, continue stirring for 1 h, then transfer to a muffle furnace for calcination, heat to 800 ℃ at a rate of 10 ℃ / min under a high-purity nitrogen atmosphere and hold for 2 h, finally wash with ethanol and deionized water, and place in a vacuum dryer at 60 ℃ for 12 h to obtain the modified carbon nanotubes.

[0050] Example 3

[0051] A method for preparing a lithium iron phosphate composite electrode material, the specific steps of which are as follows:

[0052] S1-1: Lithium carbonate, iron oxide and phosphoric acid are mixed in a molar ratio of 1.1:1:1 and added to deionized water to make a solid-liquid ratio of 1:3. The mixture is stirred at 100 °C for 6 h to obtain mixture A.

[0053] S1-2: Dopamine hydrochloride and modified carbon nanotubes were added to mixture A and ball-milled. The amount of dopamine hydrochloride added was 5% of the mass of mixture A, and the amount of modified carbon nanotubes added was 5% of the mass of mixture A. The ball-to-material ratio was 10:1. Zirconia grinding balls were used. The ball milling speed was 360 rpm and the ball milling time was 3 h. Then, the mixture was placed in a vacuum drying oven and dried at 90℃ for 12 h to obtain powder B.

[0054] S1-3: Powder B was transferred to a muffle furnace for calcination. The temperature was first increased to 700 ℃ at a rate of 5 ℃ / min and held for 4 h. Then the temperature was increased to 850 ℃ at a rate of 3 ℃ / min and held for 6 h. The calcination atmosphere was high-purity argon, and powder C was obtained.

[0055] S1-4: Mix powder C with polyvinylpyrrolidone and glucose, wherein the amount of polyvinylpyrrolidone added is 3% of the mass of powder C and the amount of glucose added is 8% of the mass of powder C. Place the mixture in a ball mill and ball mill for 2 hours at a speed of 250 rpm. Then heat it at 400 °C for 2 hours to obtain the lithium iron phosphate composite electrode material.

[0056] The preparation method of the modified carbon nanotubes in S1-2 is as follows:

[0057] S3-1: Mix carbon nanotubes, melamine and potassium hydroxide in a mass ratio of 1:5:20 and stir at 80 ℃ for 5 h.

[0058] S3-2: Add 10% boric acid by mass of carbon nanotubes, continue stirring for 2 h, then transfer to a muffle furnace for calcination, heat to 820 ℃ at a rate of 10 ℃ / min under a high-purity nitrogen atmosphere and hold for 2 h, finally wash with ethanol and deionized water, and place in a vacuum dryer at 60 ℃ for 12 h to obtain the modified carbon nanotubes.

[0059] Comparative Example 1

[0060] Dopamine hydrochloride is not added in S1-2, and the remaining steps are the same as in Example 1.

[0061] Comparative Example 2

[0062] Glucose is not added in S1-4, and the remaining steps are the same as in Example 1.

[0063] Comparative Example 3

[0064] No modification was made to the carbon nanotubes; the remaining steps were the same as in Example 1.

[0065] Comparative Example 4

[0066] Boric acid was not added during the preparation of carbon nanotubes, and the remaining steps were the same as in Example 1.

[0067] Comparative Example 5

[0068] Melamine was not added during the preparation of carbon nanotubes, and the remaining steps were the same as in Example 1.

[0069] Comparative Example 6

[0070] Potassium hydroxide was not added during the preparation of carbon nanotubes, and the remaining steps were the same as in Example 1.

[0071] Long-cycle stability test

[0072] The materials prepared in the examples and comparative examples were mixed with a conductive agent (Super P) and a binder (PVDF) at a mass ratio of 8:1:1, coated onto aluminum foil, dried, and cut into electrodes with a diameter of 12 mm. Using lithium metal as the counter electrode, Celgard 2400 as the separator, and 1M LiPF6 / EC:DMC (volume ratio 1:1) as the electrolyte, CR2032 coin cells were assembled in an argon glove box. Charge-discharge cycles were performed at 1C rate (170 mA / g) on ​​a Blue Battery testing system, with a voltage range of 2.5–4.2 V, and the discharge capacity of each cycle was recorded. Capacity retention (%) = (500th cycle discharge capacity / 1st cycle discharge capacity) × 100%.

[0073] Conductivity test

[0074] The samples prepared in the examples and comparative examples were cold-pressed into discs with a diameter of 10 mm and a thickness of 1 mm. The resistivity (ρ, Ω·cm) of the discs was measured at room temperature using a four-probe tester. The conductivity was calculated using the formula: conductivity σ (S / cm) = 1 / ρ.

[0075] The specific experimental results are summarized in the table below.

[0076]

[0077] As can be seen from the examples and comparative data, the composite electrode material of the present invention has good electrochemical performance.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention are still within the scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate composite electrode material, characterized in that, The specific steps of the preparation method are as follows: S1-1: Lithium carbonate, iron oxide and phosphoric acid are mixed and added to deionized water to make the solid-liquid ratio 1:

3. The mixture is stirred at 90~100℃ for 4~6 h to obtain mixture A; S1-2: Add dopamine hydrochloride and modified carbon nanotubes to mixture A, and ball mill them at a ball-to-material ratio of (8~10):

1. Use zirconia milling balls and ball mill for 2~3 h. Then place them in a vacuum drying oven at 80~90 ℃ for 12 h to obtain powder B. S1-3: Transfer powder B to a muffle furnace for calcination. First, raise the temperature to 600~700 ℃ at a rate of 5 ℃ / min and hold for 3~4 h. Then, raise the temperature to 800~850 ℃ at a rate of 3 ℃ / min and hold for 5~6 h to obtain powder C. S1-4: Mix powder C with polyvinylpyrrolidone and glucose, place in a ball mill and ball mill for 1-2 h at a speed of 150-250 rpm, then heat at 350-400 ℃ for 1-2 h to obtain the lithium iron phosphate composite electrode material.

2. The method for preparing a lithium iron phosphate composite electrode material according to claim 1, characterized in that, In S1-1, Li:Fe:PO4 is mixed in a molar ratio of (1~1.1):1:

1.

3. The method for preparing a lithium iron phosphate composite electrode material according to claim 1, characterized in that, The preparation method of the modified carbon nanotubes in S1-2 is as follows: S3-1: Mix carbon nanotubes, melamine and potassium hydroxide, and stir at 60~80 ℃ for 3~5 h; S3-2: Add boric acid and continue stirring for 1-2 h. Then transfer to a muffle furnace for calcination. Heat to 800-820 °C under a high-purity nitrogen atmosphere and hold for 2 h. Finally, wash with ethanol and deionized water and dry under vacuum at 60 °C for 12 h to obtain the modified carbon nanotubes.

4. The method for preparing a lithium iron phosphate composite electrode material according to claim 1, characterized in that, The amount of dopamine hydrochloride added in S1-2 is 3-5% of the mass of mixture A, and the amount of modified carbon nanotubes added is 3-5% of the mass of mixture A.

5. The method for preparing a lithium iron phosphate composite electrode material according to claim 1, characterized in that, The ball milling speed in S1-2 is 300~360 rpm.

6. The method for preparing a lithium iron phosphate composite electrode material according to claim 1, characterized in that, The calcination atmosphere in S1-3 is high-purity argon.

7. The method for preparing a lithium iron phosphate composite electrode material according to claim 1, characterized in that, The amount of polyvinylpyrrolidone added in S1-4 is 1-3% of the mass of powder C, and the amount of glucose added is 5-8% of the mass of powder C.

8. The method for preparing a lithium iron phosphate composite electrode material according to claim 3, characterized in that, In S3-1, carbon nanotubes, melamine, and potassium hydroxide are mixed in a mass ratio of 1:(3~5):(15~20).

9. The method for preparing a lithium iron phosphate composite electrode material according to claim 3, characterized in that, The amount of boric acid added in S3-2 is 5-10% of the mass of the carbon nanotubes.

10. The method for preparing a lithium iron phosphate composite electrode material according to claim 3, characterized in that, The heating rate in S3-2 is 10 °C / min.

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