Vanadium-doped lithium iron phosphate positive electrode material, preparation method and application thereof

Abstract

This invention belongs to the field of lithium-ion battery technology, and relates to a vanadium-doped lithium iron phosphate cathode material, its preparation method, and its application. The cathode material has the chemical formula LiFePV. x O4, wherein 0.0001≤x≤0.01, is doped with metallic vanadium to replace lithium sites. The cathode material has a D50 of 0.9~2μm and a specific surface area of ​​10-10.5m². 2 / g. The preparation method includes the following steps: (1) Lithium source, iron phosphate, carbon source, vanadium source, surfactant and lithium iron phosphate waste are added to water to prepare a slurry; (2) The slurry is put into a sand mill for sand milling; (3) The sand-milled slurry is spray-dried; (4) The spray-dried powder is sintered; (5) The sintered powder is pulverized by steel mill to obtain lithium iron phosphate cathode material. This invention improves the conductivity of lithium iron phosphate products, reduces battery polarization and improves its rate performance by doping lithium iron phosphate cathode material with vanadium.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a vanadium-doped lithium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] The development of lithium-ion battery technology has accelerated the portability of electronic devices. Lithium iron phosphate (LiFePO4) can be used as the positive electrode in lithium-ion batteries. Safety, lifespan, and cost are the main advantages of LiFePO4. However, the disadvantage of LiFePO4 is its relatively poor electronic conductivity, which is 10⁻⁶. -9 The S / cm range has an activation energy of 0.3–0.5 eV for lithium ions and an apparent diffusion coefficient of 10. -15 ~10 -10 cm 2 The low rate capability ( / s) results in poor rate performance of the material. Therefore, the key to achieving high-speed charge and discharge of LiFePO4 batteries is to improve the electronic conductivity and lithium-ion diffusion rate of the LiFePO4 cathode material.

[0003] The modification and synthesis of LiFePO4 materials are based on leveraging their intrinsic performance advantages and addressing their shortcomings. Currently, there are three main approaches to improving the electrochemical performance of LiFePO4: (1) Reducing the grain size can shorten the LiFePO4 production time. + (1) By improving the diffusion path and shortening the diffusion time within the grains, the rate performance of the battery can be improved; (2) By coating the surface of LiFePO4 grains with a conductive layer, its electronic conductivity and Li can be enhanced. + (2) The diffusion coefficient can be increased, thereby improving the electrochemical performance of the battery; (3) By doping LiFePO4 with different elements, its conductivity can also be increased.

[0004] Elemental doping is considered an important method to improve the internal electronic conductivity and ion diffusivity of LiFePO4 materials. Doping with small amounts of heteroions / elements at Li sites, Fe sites, or other sites is expected to improve the charge-discharge performance of LiFePO4 materials at high current densities. Doping with metallic elements can increase the lattice defects in LiFePO4, which is beneficial for improving the Li-ion conductivity. + The diffusion rate and internal conductivity of the particles are influenced by the following underlying mechanisms: ① Appropriate ion doping increases the diffusion rate of Li. + ① A one-dimensional diffusion channel along the b-axis; ② Doping causes lattice distortion, reducing Li-O bond energy and increasing lithium-ion transport rate; ③ Doping increases Li vacancy concentration, which is beneficial to Li + ④ Doping reduces the band gap between LiFePO4 and FePO4 phases, increasing electronic conductivity; ⑤ Doping suppresses the formation of antisite defects, reducing the impact of defects on Li + Diffusion is hindered. The choice of dopant element will vary depending on the doping site.

[0005] Therefore, how to dope LiFePO4 to improve the electrochemical performance of lithium iron phosphate cathode materials is an urgent problem to be solved. Summary of the Invention

[0006] This invention addresses the problems of poor electronic conductivity and weak ion diffusion in lithium iron phosphate cathode materials by providing a vanadium-doped lithium iron phosphate cathode material, its preparation method, and its application. By doping the lithium iron phosphate cathode material with vanadium, the conductivity of lithium iron phosphate products is improved, battery polarization is reduced, and its rate performance is enhanced.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A vanadium-doped lithium iron phosphate cathode material with the chemical formula LiFePV x O4, wherein 0.0001≤x≤0.01, is doped with vanadium to replace lithium sites. The cathode material has a D50 of 0.9~2μm and a specific surface area of ​​10-10.5m². 2 / g.

[0009] Preferably, the chemical formula of the above-mentioned vanadium-doped lithium iron phosphate cathode material is LiFePV. x O4, wherein 0.0030≤x≤0.0094, the D50 of the positive electrode material is 1.0~1.2μm, and the specific surface area is 10.42m². 2 / g.

[0010] A method for preparing vanadium-doped lithium iron phosphate cathode material includes the following steps:

[0011] (1) Add lithium source, iron phosphate, carbon source, vanadium source, surfactant and lithium iron phosphate waste into water to prepare a slurry;

[0012] (2) Put the slurry into a sand mill and grind it;

[0013] (3) Spray dry the slurry after sand milling;

[0014] (4) Sinter the spray-dried powder;

[0015] (5) The sintered powder is pulverized by steel mill to obtain lithium iron phosphate cathode material.

[0016] Preferably, in step (1), the lithium source is lithium carbonate; the carbon source is sucrose; the vanadium source is ammonium metavanadate; the surfactant is polyethylene glycol; and the lithium iron phosphate waste is lithium iron phosphate powder with a compaction density of less than 2.4 and a magnetic material content of more than 1 ppm.

[0017] The lithium source, iron phosphate, and vanadium source are proportioned according to the chemical formula of the vanadium-doped lithium iron phosphate cathode material; the amount of carbon source added is in the range of 3% to 5% of the mass of iron phosphate; the amount of surfactant added is in the range of 8% to 10% of the mass of iron phosphate; and the amount of lithium iron phosphate waste added is 8% to 12% of the mass of iron phosphate.

[0018] Preferably, in step (2), the slurry is sand-milled at a speed of 800-1000 rpm for 48-50 min; during sand-milling, the median particle size D50 is controlled to be 0.4-0.45 μm.

[0019] Preferably, in step (3), the inlet temperature of the spray dryer is 160-260°C, the outlet temperature is 80-120°C, and the median particle size D50 of the powder is controlled to be 30-45 μm.

[0020] Preferably, in step (3), the powder after spray drying is sintered at 780-790°C for 8-10 hours.

[0021] Beneficial technical effects of the present invention:

[0022] (1) The raw materials used in this invention are inexpensive and easy to obtain, and the preparation method provided is simple and quick, and can be used for large-scale mass production.

[0023] (2) The material prepared after element doping still retains the olivine structure of lithium iron phosphate and has good crystallinity without changing the structural characteristics of the original material.

[0024] (3) By doping vanadium with trace amounts, the finished powder particles can be effectively refined, thereby improving the conductivity and rate performance of lithium iron phosphate cathode materials.

[0025] (4) This invention effectively utilizes unqualified low-density lithium iron phosphate waste from the production process, and improves the material morphology and particle size distribution by mixing and re-firing. Attached Figure Description

[0026] Figure 1 These are the XRD patterns and magnified images of the (101) and (111) peaks of the cathode materials prepared in Examples 1-6 and Comparative Example 1;

[0027] Figure 2 These are the XRD patterns and Rietveld refinement results of the cathode materials prepared in Examples 1, 3, 6 and Comparative Example 1;

[0028] Figure 3 These are scanning electron microscope images of the cathode materials prepared in Examples 1-6 and Comparative Example 1, magnified 10,000 times.

[0029] Figure 4These are scanning electron microscope images of the cathode materials prepared in Examples 1-6 and Comparative Example 1, magnified 20,000 times.

[0030] Figure 5 These are the cycling performance graphs of the cathode materials prepared in Examples 1-6 and Comparative Example 1 at 1C;

[0031] Figure 6 These are the charge-discharge curves of the cathode materials prepared in Examples 1-6 and Comparative Example 1 at a rate of 0.1C;

[0032] Figure 7 The discharge specific capacity curves (a) and charge-discharge efficiency curves (b) of the cathode materials prepared in Examples 1-6 and Comparative Example 1 under 0.1C and 0.5C rate conditions are shown.

[0033] Figure 8 These are the capacity retention curves of the cathode materials prepared in Examples 2, 4, 6 and Comparative Example 1 under multiple cycles;

[0034] Figure 9 These are graphs showing the trends in compaction density, specific surface area, and resistivity of the cathode materials prepared in Examples 1-6 and Comparative Example 1. Detailed Implementation

[0035] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Comparative Example 1

[0037] The preparation steps of lithium iron phosphate cathode material are as follows:

[0038] (1) Prepare a slurry by mixing 10 kg of ferric phosphate, 2.501 kg of lithium carbonate, 0.375 kg of sucrose, 0.9 kg of polyethylene glycol (PEG) and 19.5 kg of water;

[0039] (2) The slurry from step (1) was placed into a 30L Langling sand mill for sand milling. The sand milling time was 50 minutes and the sand milling speed was 900 rpm. The median particle size D50 of the slurry after sand milling was 0.426 μm.

[0040] (3) The slurry after sand milling in step (2) was spray dried using a Pioneer spray dryer. The inlet temperature of the spray dryer was set to 260℃ and the outlet temperature was 100℃. The resulting dried powder had a D50 of 38.2μm.

[0041] (4) The powder after spray drying in step (3) is placed into the Zhongzhou kiln for sintering. The heating rate is 8℃ / min, the sintering temperature is 780℃, the sintering time is 9h, and the powder is cooled with the furnace after the sintering is completed.

[0042] (5) The powder from step (4) is pulverized in a steel mill to obtain the finished product of Comparative Example 1.

[0043] Example 1

[0044] Similar to Comparative Example 1, the only difference is:

[0045] In step (1), 0.01 kg of ammonium metavanadate and 1 kg of low-compacted density (compacted density less than 2.4, magnetic material content higher than 1 ppm) lithium iron phosphate waste (LiFePO4) are added to the slurry preparation.

[0046] In step (2), the grinding time is 48 minutes.

[0047] Example 2

[0048] Same as Example 1, except that the mass of ammonium metavanadate doped in the slurry in step (1) is 0.015 kg.

[0049] Example 3

[0050] Same as Example 1, except that the ammonium metavanadate doping mass in the slurry in step (1) is 0.02 kg.

[0051] Example 4

[0052] Same as Example 1, except that the ammonium metavanadate doping mass in the slurry in step (1) is 0.025 kg.

[0053] Example 5

[0054] Same as Example 1, except that the ammonium metavanadate doping mass in the slurry in step (1) is 0.03 kg.

[0055] Example 6

[0056] Same as Example 1, except that the ammonium metavanadate doping mass in the slurry in step (1) is 0.035 kg.

[0057] The raw material formulations for Examples 1-6 and Comparative Example 1 are shown in Table 1.

[0058] Table 1

[0059]

[0060]

[0061] Characterization of basic physicochemical properties:

[0062] The particle size distribution of the material was tested using a laser particle size analyzer; the compaction density of the material was measured using a powder compaction density test method; the carbon content of the material was tested using a thermal analysis method; the specific surface area of ​​the material was tested using a dynamic method; the resistivity of the material was tested using a powder pressing method; and the pH value of the finished powder was tested using a buffer solution method.

[0063] The test results of the finished powders prepared in Examples 1-6 and Comparative Example 1 are shown in Table 2.

[0064] Table 2

[0065]

[0066] ICP test:

[0067] The elemental content in the cathode material was measured using inductively coupled plasma optical emission spectrometry (ICP-OES). The mass percentage test results of each element in the finished powders prepared in Examples 1-6 and Comparative Example 1 are shown in Table 3.

[0068] Table 3

[0069]

[0070]

[0071] Material structure characterization and analysis:

[0072] Material structure characterization was performed using XRD testing, with a scanning range of 10°–80°. Figure 1 For the XRD patterns of Comparative Example 1 and Examples 1, 2, 3, 4, 5, and 6, from... Figure 1 As can be seen from the data, the vanadium-doped sample showed no obvious impurity peaks. Combined with Table 3, it can be concluded that vanadium was successfully doped into the crystal lattice of the lithium iron phosphate material. To further understand the influence of vanadium on the crystal structure of lithium iron phosphate, the XRD patterns of Comparative Example 1 and Examples 1, 3, and 6 were subjected to Rietveld refinement to obtain changes in their crystal structure parameters. The refinement results show... Figure 2 And in Table 4.

[0073] Table 4

[0074]

[0075] Morphological characteristics:

[0076] The particle morphology of Comparative Example 1 and Examples 1, 2, 3, 4, 5, and 6 was observed using scanning electron microscopy (SEM). Figure 3 and Figure 4As shown, Examples 1, 2, 3, 4, 5, and 6 still maintain the morphology of mixed distribution of large and small particles. Combined with the particle size analysis of the materials in Table 2, as the proportion of vanadium doping increases, the particles tend to become finer. However, a small number of powder particles in Examples 1 (D99: 17.2 μm) and Examples 2 (D99: 23.3 μm) show coarsening.

[0077] Electrochemical performance measurement:

[0078] The lithium iron phosphate cathode material powder prepared in Comparative Example 1 and Examples 1-6 was mixed with carbon black and polyvinylidene fluoride (PVDF) at a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) was added as a dispersant. The mixture was then centrifuged and ball-milled to prepare a uniform slurry, which was then uniformly coated onto aluminum foil. The aluminum foil was then transferred to a blower dryer and dried at 80°C for 4 hours. After drying, it was cut into 12mm diameter electrode sheets using a die-cutting machine, rolled, and vacuum-dried at 110°C for 12 hours. Using lithium metal sheets as the negative electrode, a Calgard 2400 polypropylene membrane as the separator, and 1.0M LiTFSI DOL / DME (volume ratio 1:1) and 2.0% LiNO3 (mass percentage) as the electrolyte, the mixture was assembled into a CR2016 button cell in an argon-filled glove box. The assembled battery was subjected to charge-discharge cycle testing using the Xinwei Battery Testing System. The test voltage range was 2–3.75V, the test temperature was 25℃, and the charge-discharge cycle was performed under test conditions of 0.1C and 0.5C rates. Figure 5 Table 5 shows the cycling performance of the cathode material at 0.5C for 150 cycles. The first-cycle discharge capacity and coulombic efficiency of the cathode material are also shown in Table 5. It can be seen that vanadium doping effectively improves the initial capacity and coulombic efficiency of the cathode material. Example 6 exhibits the best electrochemical performance, with first-cycle discharge capacities of 158.87 mAh·g at 0.1C and 0.5C, respectively. -1 and 152.07mAh·g -1 The corresponding coulomb efficiencies are 99.37% and 94.94%, respectively.

[0079] The charge-discharge curves of the cathode materials prepared in Comparative Example 1 and Examples 1-6 at a 0.1C rate are shown below. Figure 6 ,according to Figure 6 It can be seen that with the increase of vanadium additive, the polarization decay phenomenon of the charge and discharge curve is gradually improved and weakened, especially the spontaneous discharge process without external electrical excitation.

[0080] The fluctuations in discharge specific capacity and charge / discharge efficiency of the cathode materials prepared in Comparative Example 1 and Examples 1-6 under 0.1C and 0.5C rates are shown in the figure. Figure 7 Combining Figure 7It is evident that the vanadium content can improve the discharge specific capacity and charge / discharge efficiency, and there are more significant differences in the 0.5C rate discharge specific capacity and discharge efficiency among different samples. Specifically, compared to LFP-0 (without vanadium), LFP-6 achieved a 0.1C discharge specific capacity of 160.5 mAh / g (an increase of 3.7%) and a 0.5C discharge specific capacity of 155.4 mAh / g (an increase of 5.12%), indicating that vanadium content can improve the high-rate characteristics of LFP cathode materials.

[0081] In addition, we conducted multi-cycle capacity retention tests using a 1.0C / 1.0C charge / discharge rate, and the trend is shown in [see figure]. Figure 8 Combining Figure 8 It can be seen that the 1.0C rate discharge specific capacity of the LFP cathode material is improved overall with the addition of vanadium. Simultaneously, the discharge specific capacity of all samples increases with the increase in the number of cycles, indicating that the lithium-ion insertion and extraction channels of the vanadium-modified LFP cathode material are activated during multiple charge-discharge cycles. Furthermore, the discharge specific capacity of all samples gradually stabilizes after more than 15 cycles. Among them, the sample with the highest vanadium content (LFP-6) has a significantly better discharge specific capacity than the other samples. Compared with the vanadium-free sample (K0), the LFP-6 sample achieved a discharge specific capacity of 154.1 mAh / g after 50 discharge cycles, which is 3.2 mAh / g higher.

[0082] In conjunction with the ion compensation mechanism, when a cation in a substance is replaced by a higher-valence cation, vacancies are generated in the crystal structure to maintain charge balance, forming crystal defects. This reduces the long-range order of the crystal lattice, increases the degree of lattice disorder, and reduces the interaction force between Li-O bonds. V5 + After addition, the LFP cathode material particles were rounded and refined, and the Li content was broadened. + One-dimensional transmission channel reduces Li + The resistance to insertion and extraction is reduced, thereby increasing the specific capacity of LFP cathode materials. Simultaneously, based on coordination chemistry theory, oxygen atoms in LiO6 octahedrons, FeO6 octahedrons, and PO4 tetrahedra possess lone pairs of electrons, while V5 atoms with a coordination number of 6... + It can form a covalent complex with oxygen atoms that have lone pairs of electrons. Under the influence of Coulomb force, it "clamps" the oxygen atom. Due to the attraction between opposite charges, it indirectly conducts and "clamps" the multivalent Fe2+ in the FeO6 octahedron. + / Fe3 + And P5, which has a tetrahedral multi-charge of PO4 + However, for ions with small ionic radii (ionic radius of 1000 ppm), ... ), monovalent Li + The weak Coulomb attraction of the olivine weakens the influence of the olivine structure on Li. +The constraint of the material improves the rate performance and cycle capacity retention of LFP cathode materials to a certain extent.

[0083] Comparison of other physical and chemical indicators:

[0084] The trends of compaction density, specific surface area, and resistivity of different LFP cathode materials are shown in the figure. Figure 9 .according to Figure 9 It can be seen that with the increase of vanadium content in the additive, the compaction density of the LFP cathode material increases slightly, and remains stable at 2.32–2.37 g / cm³. 3 Between; the specific surface area remained stable at 9.35–9.45 m². 2 The resistivity decreased from 30 Ω·cm to 16 Ω·cm, a reduction of 53.3%, which is beneficial for improving thermal management during the charging and discharging process of lithium iron phosphate (LFP) lithium-ion batteries. The reason for this improved resistivity is that the vanadium additive can round and refine the particles of the LFP cathode material, forming spherical or near-spherical particles with a well-proportioned size distribution, thus weakening the influence of single-charge Li... + The "control" ability improved Li + The embedding and extraction channels. This is consistent with the changes in electrical properties.

[0085] Table 5

[0086]

[0087] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A vanadium-doped lithium iron phosphate cathode material, characterized in that, Its chemical formula is LiFePV x O4, wherein 0.0001≤x≤0.01, is doped with vanadium to replace lithium sites. The cathode material has a D50 of 0.9~2μm and a specific surface area of ​​10-10.5m². 2 / g.

2. The vanadium-doped lithium iron phosphate cathode material according to claim 1, characterized in that, The chemical formula is LiFePV x O4, wherein 0.0030≤x≤0.0094, the D50 of the positive electrode material is 1.0~1.2μm, and the specific surface area is 10.42m². 2 / g.

3. The method for preparing a vanadium-doped lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, Includes the following steps: (1) Add lithium source, iron phosphate, carbon source, vanadium source, surfactant and lithium iron phosphate waste into water to prepare a slurry; (2) Put the slurry into a sand mill and grind it; (3) Spray dry the slurry after sand milling; (4) Sinter the spray-dried powder; (5) The sintered powder is pulverized by steel mill to obtain lithium iron phosphate cathode material.

4. The method for preparing a vanadium-doped lithium iron phosphate cathode material according to claim 3, characterized in that, In step (1), the lithium source is lithium carbonate; the carbon source is sucrose; the vanadium source is ammonium metavanadate; the surfactant is polyethylene glycol; and the lithium iron phosphate waste is lithium iron phosphate powder with a compaction density of less than 2.4 and a magnetic material content of more than 1 ppm. The lithium source, iron phosphate, and vanadium source are proportioned according to the chemical formula of the vanadium-doped lithium iron phosphate cathode material; the amount of carbon source added is in the range of 3% to 5% of the mass of iron phosphate; the amount of surfactant added is in the range of 8% to 10% of the mass of iron phosphate; and the amount of lithium iron phosphate waste added is 8% to 12% of the mass of iron phosphate.

5. The method for preparing a vanadium-doped lithium iron phosphate cathode material according to claim 3, characterized in that, In step (2), the slurry is sand-milled at a speed of 800-1000 rpm for 48-50 min; during sand milling, the median particle size D50 is controlled to be 0.4-0.45 μm.

6. The method for preparing a vanadium-doped lithium iron phosphate cathode material according to claim 3, characterized in that, In step (3), the inlet temperature of the spray dryer is 160-260℃ and the outlet temperature is 80-120℃. The median particle size D50 of the powder is controlled to be 30-45μm during spray drying.

7. The method for preparing a vanadium-doped lithium iron phosphate cathode material according to claim 3, characterized in that, In step (3), the spray-dried powder is sintered at 780-790°C for 8-10 hours.

8. The application of the vanadium-doped lithium iron phosphate cathode material according to claims 1-2 or the vanadium-doped lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 3-7 in the field of lithium-ion batteries.

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