A niobium-doped and carbon nanotube-coated lithium nickel iron phosphate material and a preparation method thereof

Abstract

The application discloses a kind of niobium doped and carbon nanotube continuous coated lithium nickel iron phosphate materials and preparation method thereof, belong to lithium ion battery cathode material technical field.The application realizes the concentration gradient distribution of niobium element in material interior by step-by-step addition of niobium source and phosphorus source, and by three-stage temperature control sintering, niobium is preferentially doped in [100] crystal direction, thereby significantly improving bulk phase conductivity and lithium ion transference rate, while avoiding capacity loss caused by excessive doping by stabilizing lattice structure.In addition, carbon nanotube and nitrogen-containing carbon source synergistic effect, form Li-N-C covalent bond on the surface of material, enhance interface stability, effectively inhibit interface side reaction, improve surface conductivity.The application method is successfully solved by doping and coating double modification the problem of low conductivity and poor cycle stability of lithium nickel iron phosphate material, and the final product shows excellent rate performance and cycle life.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, specifically a niobium-doped and carbon nanotube-coated nickel iron phosphate material and its preparation method. Background Technology

[0002] In recent years, with the booming development of the new energy vehicle industry, energy storage and conversion have become an important issue that urgently needs to be addressed.

[0003] Lithium-ion batteries (LIBs), as one of the most mature clean energy storage devices currently available, are widely used in various electronic devices and products. Among them, the mainstream olivine-structured LiFePO4 (LFP) cathode material exhibits good cycle stability, high safety, and high discharge capacity (170 mAh·g). -1 While LiFePO4 boasts advantages such as low density and low cost, its low discharge plateau (3.4V vs Li / Li+), low energy density, low conductivity, and poor rate performance limit its further development. Compared to the traditional olivine-structured cathode material LiFePO4, LiNiFePO4 offers numerous advantages. It combines the strengths of both LiFePO4 and LiNiPO4, exhibiting the same good safety profile as LiFePO4. Furthermore, the introduction of Ni significantly improves the theoretical energy density, meeting the higher requirements of power battery applications and giving it a competitive edge and promising market prospects. However, LiNiFePO4 still suffers from drawbacks such as low compaction density and low electronic conductivity, limiting its application in power batteries. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a niobium-doped and carbon nanotube-coated nickel iron phosphate cathode material, its preparation method, and its application.

[0005] The inventors discovered that niobium ions (Nb) 5+Doping can replace lithium sites or transition metal sites in the crystal lattice, generating additional free electrons, effectively reducing the resistivity of the material, and improving the charge transport efficiency during charging and discharging. The core innovation of this invention lies in the stepwise addition of niobium source and phosphorus source, which realizes the concentration gradient distribution of niobium element in the material, and promotes niobium preferential doping in the

[100] crystal orientation through three-stage temperature-controlled sintering, thereby significantly improving the bulk conductivity and lithium ion mobility, and avoiding capacity loss caused by excessive doping while stabilizing the crystal structure. When combined with carbon nanotube coating, niobium doping can further enhance the interface stability and form a dual protection mechanism of "bulk doping-surface coating". Adding nitrogen-containing carbon source, carbonizing with carbon nanotube on the material surface to form Li-NC covalent bonds, and the Li-NC bond can form a stable chemical bridge, significantly improving the interface adhesion between the carbon layer and the electrode material, enhancing interface stability, effectively suppressing interface side reactions, improving surface conductivity, and jointly solving the problems of poor conductivity and cycle decay. The technical solution of the present invention is as follows: A method for preparing a niobium-doped and carbon nanotube-coated nickel-iron-phosphate material includes the following steps: (a) Add nickel source, iron source, carbon source, first part of phosphorus source and first part of niobium source and deionized water into a nano-grinding mill for cyclic reaction; after grinding until the particles are nano-sized and uniformly dispersed, add lithium source to continue the reaction until the system is strongly alkaline.

[0006] (b) Add a second phosphorus source to adjust the pH to a weakly alkaline range, add a second niobium source, react to the control endpoint to form a niobium gradient distribution precursor slurry, spray to obtain the precursor, and perform gradient sintering in an inert atmosphere to obtain... Sample sintered in one pass.

[0007] (c) The sintered sample is ground and reacted with a nitrogen-containing carbon source, carbon nanotubes and deionized water until the endpoint is reached to achieve uniform coating of carbon nanotubes on the material surface. Spray sintering is then performed to finally obtain niobium-doped and carbon nanotube composite coated nickel iron phosphate material.

[0008] Further, the nickel source mentioned in step (a) is one or more of nickel carbonate, nickel tetroxide, and nickel trioxide; The iron source is one or more of ferrous oxalate, ferric phosphate, and ferric oxide. The phosphorus source is one or more of phosphoric acid, monoammonium phosphate, and diammonium phosphate; The carbon source is one or more of glucose, sucrose, fructose, starch, PEG, lactose, and citric acid.

[0009] Further, the niobium source in step (a) is one or more of an aqueous salt of niobium ions, a nano-sized oxide, or an alcohol; preferably, it is one or more of niobium ammonium oxalate, niobium pentoxide, or niobium ethanol.

[0010] Further, the lithium source mentioned in step (a) is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate.

[0011] Furthermore, the amount of the first part of phosphorus source is 80-90% of the total weight of the first part of phosphoric acid and the second part of phosphorus source; the amount of the first part of niobium source is 50-80% of the total weight of the first part of niobium source and the second part of niobium source.

[0012] Furthermore, before adding the lithium source in step (a), the reaction is cyclically carried out until the slurry pH > 3 and D99 < 300nm; after adding the lithium source, the reaction ends at pH > 11. The alkaline environment reduces the ion diffusion barrier, making it easier for niobium to migrate into the particle interior and achieving preferential doping of the core.

[0013] Furthermore, in step (a), the target carbon content of the calcined sample obtained in step (b) is 0.2-1.2%.

[0014] Further, in step (b), the amount of the second part of the phosphorus source is 10-20% of the total weight of the first part of phosphoric acid and the second part of phosphorus source. After adding the second part of the phosphorus source, the pH of the slurry is 10.0-10.5. Appropriately reducing the pH value can slow down the precipitation reaction rate, so that the niobium element mainly grows epitaxially on the surface of the formed crystal nucleus to avoid the niobium element being deposited too quickly, prevent the shell doping concentration from being too high and affecting the lithium ion migration channel, and suppress excessive doping. The amount of the second part of the niobium source is 20-50% of the total weight of the first part of the niobium source and the second part of the niobium source. The reaction control endpoint is D99 < 600 nm.

[0015] Further, the steps described in step (b) In a single sintering sample, the molar ratio of Li, Ni, Fe, Nb and P is (1.00-1.05): x:y:z:1, where 0.2 < x < 0.8, 0.2 < y < 0.8, 0.005 < z < 0.1, and x + y + z = 0.95-0.99.

[0016] Furthermore, the gradient sintering described in step (b) consists of three stages: pretreatment, main reaction, and annealing. The sintering temperature in the pretreatment stage is 400-500℃, and the holding time is 2-3h, which drives niobium to diffuse inward and form a core aggregation. The sintering temperature in the main reaction stage is 550-650℃, and the holding time is 4-6h, which realizes the stable solid solution of niobium in the crystal lattice structure. The sintering temperature in the annealing stage is 700-750℃, and the holding time is 0.5-1h, which can optimize the grain boundary structure and complete the gradient distribution of niobium. The sintering atmosphere is nitrogen or argon.

[0017] Further, in step (c), the weight ratio of the first sintered sample, the nitrogen-containing carbon source, and the carbon nanotubes added is 1:0.03-0.08:0.001-0.01.

[0018] Further, the nitrogen-containing carbon source mentioned in step (c) is one or more of urea, melamine, polyacrylonitrile, polyaniline, and polyvinylpyrrolidone.

[0019] Further, the grinding endpoint described in step (c) is D99 < 500 nm.

[0020] Furthermore, the sintering temperature in step (c) is 650-780℃, the sintering time is 2-10h, and the sintering atmosphere is nitrogen or argon.

[0021] The beneficial effects of this invention are: (1) The core innovation of this invention is that by adding niobium source and phosphorus source in steps, the concentration gradient distribution of niobium element in the material is realized, and the three-stage temperature-controlled sintering promotes niobium to be preferentially doped in the

[100] crystal orientation, thereby significantly improving the bulk conductivity and lithium ion mobility, and avoiding capacity loss caused by excessive doping while stabilizing the crystal structure.

[0022] (2) Carbon nanotubes and nitrogen-containing carbon sources work together to form Li-NC covalent bonds on the material surface, which enhances the interface stability, effectively suppresses interface side reactions, and improves the surface conductivity.

[0023] (3) The preparation process of the present invention is simple, no side reactions occur during the preparation process, the yield is high, it meets the requirements of power battery application, and it is suitable for industrial production. Detailed Implementation

[0024] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments.

[0025] Example 1: This example provides a method for preparing niobium-doped and carbon nanotube-coated nickel iron phosphate material.

[0026] 68.37g NiCO3, 193.35g FeC2O4, 195.99g H3PO4, 1.60g Nb2O5, 10g glucose, and 500g deionized water were added to a nano-mill and circulated until pH > 3 and D99 < 300nm. Then, 75.37g Li2CO3 was added and the reaction continued until pH > 11. 34.59g H3PO4 was added to adjust the pH to 10-10.5, and 1.06g niobium source was slowly added, reacting fully until D99 < 600nm. The precursor slurry was spray-dried to obtain the precursor, which was then gradient-sintered in a nitrogen atmosphere at a heating rate of 5℃ / min, held at 450℃ for 2h, 600℃ for 5h, and 750℃ for 1h, respectively, to obtain the desired precursor. Sample 1 was obtained by sintering a single sample. 100g of sample 1, 3g of PVP, 1g of carbon nanotubes and 300g of deionized water were mixed and ground until D99 < 500nm. The mixture was sprayed and heated to 700℃ at a heating rate of 5℃ / min under a nitrogen atmosphere and held for 4h to obtain sample 1.

[0027] Example 2: This example provides a method for preparing niobium-doped and carbon nanotube-coated nickel iron phosphate material.

[0028] The only difference from Example 1 is that the sintering process of the precursor is a single-stage sintering process. In a nitrogen atmosphere, the temperature is raised to 600°C at a heating rate of 5°C / min and held for 5 hours. All other processes are the same to obtain Sample 2.

[0029] Example 3: This example provides a method for preparing niobium-doped and carbon nanotube-coated nickel iron phosphate material.

[0030] The only difference from Example 1 is that 3g PVP was replaced with 3g glucose, resulting in Sample 3.

[0031] Example 4: This example provides a method for preparing niobium-doped and carbon nanotube-coated nickel iron phosphate material.

[0032] 68.37g NiCO3, 193.35g FeC2O4, 230.58g H3PO4, 2.66g Nb2O5, 75.37g Li2CO3, 10g glucose, and 500g deionized water were added to a nano-mill and circulated until D99 < 600. The precursor was obtained by spraying and then gradient sintered in a nitrogen atmosphere at a heating rate of 5℃ / min, holding at 450℃ for 2h, 600℃ for 5h, and 750℃ for 1h, respectively, to obtain... Sample 4 was obtained by first sintering. 100g of sample 4, 3g of PVP, 1g of carbon nanotubes and 300g of deionized water were mixed and ground until D99 < 500nm. The mixture was sprayed and heated to 700℃ at a heating rate of 5℃ / min under a nitrogen atmosphere and held for 4h to obtain sample 4.

[0033] Example 5: This example provides a method for preparing a carbon nanotube-coated nickel iron phosphate material.

[0034] The difference from Example 4 is that 2.66g of Nb2O5 was not added during the reaction stage.

[0035] Comparative Example 1: This comparative example provides a method for preparing niobium-doped nickel iron phosphate material.

[0036] 68.37g NiCO3, 193.35g FeC2O4, 230.58g H3PO4, 2.66g Nb2O5, 75.37g Li2CO3, 10g glucose, 3g PVP, 1g carbon nanotubes, and 500g deionized water were added to a nano-mill and circulated until D99 < 600nm. The precursor was obtained by spraying and then gradient sintered in a nitrogen atmosphere. The samples were heated at a rate of 5℃ / min and held at 450℃ for 2h, 600℃ for 5h, and 750℃ for 1h, respectively, to obtain sample 5.

[0037] Physicochemical and electrochemical performance tests: The lithium nickel iron phosphate cathode materials prepared in the examples and comparative examples were tested for conductivity, BET, and carbon content using a powder conductivity meter, a specific surface area analyzer, and a carbon-sulfur analyzer. Furthermore, the lithium nickel iron phosphate cathode materials were assembled into coin cell half-cells and charged using constant current and constant voltage methods. The physicochemical and electrochemical performance results are shown in Table 1.

[0038] Table 1 Test Results As can be seen from the experimental results in Table 1, (1) Compared with Example 2, the three-stage temperature-controlled sintering in Example 1 promotes niobium preferential doping in the

[100] crystal orientation, thereby significantly improving the bulk conductivity by an order of magnitude.

[0039] (2) Compared with Example 3, the synergistic effect of carbon nanotubes and nitrogen-containing carbon source in Example 1 forms Li-NC covalent bonds on the material surface, enhances interface stability, increases the 0.1C discharge capacity by 5 mAh / g, and increases the retention rate of 1C cycle 1000 cycles from 88.7% to 93.5%.

[0040] (3) Compared with Example 4, Example 1 has a niobium-rich core and niobium-poor shell structure after Nb doping treatment. The 0.1C discharge capacity is increased by 9 mAh / g, and the capacity retention rate is increased from 75.8% to 93.5%. The gradient doping of niobium stabilizes the lattice structure, improves the capacity retention rate, and avoids capacity loss caused by excessive doping.

[0041] (4) Compared to Example 5, the Nb doping in Example 4 increased the 0.1C discharge capacity by 3 mAh / g.

[0042] (5) Compared with Comparative Example 1, the two-firing process in Examples 1-5 has significantly improved performance compared with the one-firing process.

[0043] Finally, it should be noted that the above embodiments are merely illustrative of several implementations of the present invention and are not intended to limit the scope of the invention. For those skilled in the art, any modifications, equivalent substitutions, or improvements made without departing from the concept of the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A method for preparing a niobium-doped and carbon nanotube-coated nickel-iron-phosphate material, characterized in that, Includes the following steps: (a) Add nickel source, iron source, carbon source, first part of phosphorus source and first part of niobium source to deionized water and circulate them in a nano-grinding mill; circulate until the slurry pH > 3 and D99 < 300 nm, and after grinding until the particles are nano-sized and uniformly dispersed, add lithium source and continue the reaction until the system pH > 11; wherein, the amount of the first part of phosphorus source is 80-90% of the total weight of the first part of phosphorus source and the second part of phosphorus source; the amount of the first part of niobium source is 50-80% of the total weight of the first part of niobium source and the second part of niobium source; (b) Add the second part of the phosphorus source to adjust the pH to 10.0-10.5, add the second part of the niobium source, and react until the control endpoint D99 < 600 nm, forming a niobium gradient distribution precursor slurry. Spray the slurry to obtain the precursor, and perform gradient sintering in an inert atmosphere to obtain the desired product. A single sintering sample is prepared; wherein the amount of phosphorus source in the second part is 10-20% of the total weight of phosphorus source in the first and second parts; the amount of niobium source in the second part is 20-50% of the total weight of niobium source in the first and second parts; the gradient sintering consists of three stages: pretreatment, main reaction, and annealing. The sintering temperature in the pretreatment stage is 400-500℃, and the holding time is 2-3h; the sintering temperature in the main reaction stage is 550-650℃, and the holding time is 4-6h; the sintering temperature in the annealing stage is 700-750℃, and the holding time is 0.5-1h; the sintering atmosphere is nitrogen or argon. (c) The sintered sample is ground together with a nitrogen-containing carbon source, carbon nanotubes and deionized water to achieve uniform coating of carbon nanotubes on the material surface. Spray sintering is then performed to finally obtain niobium-doped and carbon nanotube composite coated nickel iron phosphate material.

2. The method for preparing a niobium-doped and carbon nanotube-coated nickel-iron-phosphate material according to claim 1, characterized in that, The nickel source is one or more of nickel carbonate, nickel tetroxide, and nickel trioxide; The iron source is one or more of ferrous oxalate, ferric phosphate, and ferric oxide. The phosphorus source is one or more of phosphoric acid, monoammonium phosphate, and diammonium phosphate; The carbon source is one or more of glucose, sucrose, fructose, starch, PEG, lactose, and citric acid; The lithium source is one or more of lithium carbonate, lithium hydroxide, lithium phosphate, and lithium dihydrogen phosphate; The niobium source is one or more of niobium ammonium oxalate, niobium pentoxide, and niobium ethanol.

3. The method for preparing a niobium-doped and carbon nanotube-coated nickel-iron-phosphate material according to claim 1, characterized in that, The amount of carbon source added in step (a) is such that the target carbon content of the calcined sample obtained in step (b) is 0.2-1.2%.

4. The method for preparing a niobium-doped and carbon nanotube-coated nickel-iron-phosphate material according to claim 1, characterized in that, The result obtained in step (b) In a single sintering sample, the molar ratio of Li, Ni, Fe, Nb and P is (1.00-1.05): x:y:z:1, where 0.2 < x < 0.8, 0.2 < y < 0.8, 0.005 < z < 0.1, and x + y + z = 0.95-0.

99.

5. The method for preparing a niobium-doped and carbon nanotube-coated nickel-iron-phosphate material according to claim 1, characterized in that, The weight ratio of the primary sintered sample, nitrogen-containing carbon source, and carbon nanotubes added in step (c) is 1:0.03-0.08:0.001-0.

01.

6. The method for preparing a niobium-doped and carbon nanotube-coated nickel-iron-phosphate material according to claim 1, characterized in that, The nitrogen-containing carbon source mentioned in step (c) is one or more of urea, melamine, polyacrylonitrile, polyaniline, and polyvinylpyrrolidone.

7. The method for preparing a niobium-doped and carbon nanotube-coated nickel-iron-phosphate material according to claim 1, characterized in that, The grinding endpoint in step (c) is D99 < 500 nm, the sintering temperature is 650-780℃, the sintering time is 2-10 h, and the sintering atmosphere is nitrogen or argon.

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