A modified positive electrode material and a preparation method thereof

Abstract

The application discloses a modified positive electrode material and a preparation method thereof, and belongs to the technical field of lithium batteries. The modified positive electrode material comprises a lithium iron manganese phosphate base material and an organic coating attached to the surface of the lithium iron manganese phosphate base material; wherein the material of the organic coating is a carboxyl-containing organic compound. The carboxyl functional group of the organic compound reacts with the lithium compound remaining on the surface of the lithium iron manganese phosphate base material, thereby not only reducing the surface alkalinity of the positive electrode material, but also preventing the material from directly contacting H2O / CO2 in the air. Compared with the original material, the modified material has less surface impurities, a more stable near-surface structure, a higher capacity retention rate after storage under the same conditions, and smaller changes in morphology and structure.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery, in particular to a modified positive electrode material and a preparation method thereof. BACKGROUND

[0002] LiMnPO4 is also attracting more and more attention, and the biggest advantage of the material is its high working potential (4.1V vs. Li + / Li), and the theoretical energy density (701Wh / kg) is about 20% higher than that of LiFePO4 (586Wh / kg), and it is considered to be the most potential material to achieve the maximum actual energy density. However, the actual electrochemical performance of LiMnPO4 is not as good as that of LiFePO4, and the energy density utilization is also low, and it is difficult to obtain good electrochemical performance. In recent years, researchers have combined lithium iron phosphate and lithium manganese phosphate to form a mixed solid solution: lithium manganese iron phosphate, which has the advantages of both. Lithium manganese iron phosphate has high energy density of lithium manganese phosphate and good cycle performance of lithium iron phosphate, and is a very promising lithium ion battery positive electrode material.

[0003] In order to solve the two problems of low electronic conductivity and low one-dimensional lithium ion diffusion rate of LiMPO4 (M=Fe, Mn) material, and fully exert the electrochemical performance of the material, researchers have carried out a lot of research on LiMPO4 (M=Fe, Mn) material. The most commonly used methods include particle refinement and carbon coating modification. Carbon coating can improve the conductivity of manganese iron lithium, reduce particle growth, and inhibit the dissolution of manganese. Nanocrystallization can shorten the lithium ion migration path, and the larger specific surface area provides more interface area for the full contact of the material and the electrolyte.

[0004] However, the small particles and high carbon content of lithium manganese iron phosphate result in high specific surface area, and the surface exposed to humid air can absorb a large amount of H2O / CO2, which can react with residual lithium compounds to generate surface impurities such as LiOH and Li2CO3, resulting in difficult processing, high slurry viscosity during slurry mixing, and easy gelation. In addition, the water in the cell is also difficult to dry out, which is a major pain point in the current application of lithium manganese iron phosphate.

[0005] Therefore, it is necessary and important to find an effective method to remove residual lithium compounds and improve the storage stability of lithium manganese iron phosphate material. SUMMARY

[0006] The main purpose of the present application is to provide a modified positive electrode material and a preparation method thereof, so as to solve the problem that the residual lithium compounds on the surface of lithium manganese iron phosphate can absorb H2O / CO2 in the air to generate surface impurities, resulting in unstable storage of lithium manganese iron phosphate.

[0007] To achieve the above object, according to one aspect of the present application, there is provided a modified positive electrode material comprising a lithium iron manganese phosphate base material and an organic coating layer attached to the surface of the lithium iron manganese phosphate base material.

[0008] In the present application, the material of the organic coating layer is an organic compound containing a carboxyl functional group.

[0009] Further, the carboxyl-containing organic compound is selected from one or more of polyacrylic acid, 9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid, carboxymethyl chitosan, sodium alginate, carboxymethyl cellulose, polyamide acid, and carboxyl styrene-butadiene latex.

[0010] Further, the content of the carboxyl group in the organic compound is 0.5wt% to 8wt%.

[0011] The selection of the above-mentioned carboxyl-containing organic compound in the present application can better react with the residual lithium compounds on the surface of the lithium iron manganese phosphate, reduce the pH value of the lithium iron manganese phosphate base material, and form a protective film on the surface of the base material to isolate water and carbon dioxide in the air, etc.

[0012] Further, the mass percentage content of the organic coating layer in the lithium iron manganese phosphate base material is 0.6% to 1.9%, preferably 0.8% to 1.8%.

[0013] Further, the thickness of the organic coating layer is 2 to 30 nm, preferably 7 to 10 nm.

[0014] The thickness of the organic coating layer in the present application can ensure the conductivity of the lithium iron manganese phosphate and fully form a protective film on the surface of the base material within the thickness range.

[0015] Further, the pH value of the modified positive electrode material is 7 to 9, preferably the pH value is 7 to 8.

[0016] The modified positive electrode material of the present application has the above-mentioned pH value, which is easier to process during the slurry process.

[0017] Further, the water content of the modified positive electrode material is 500 to 1000 ppm, preferably 500 to 800 ppm, and further preferably 500 to 700 ppm.

[0018] The modified positive electrode material of the present application has a low water content, which is beneficial to improve the electrochemical performance of the base material.

[0019] Further, the residual alkali content of the modified positive electrode material is 1300 to 1800 ppm.

[0020] The modified positive electrode material of the present application has a low residual alkali content, which is beneficial to the flowability and processability of the slurry, can reduce the absorption of moisture in the environment, and improve the structural stability and storage stability of the base material.

[0021] Further, the chemical formula of the lithium iron manganese phosphate substrate is LiFe 1-x Mn x PO4 / C, 0.5≤x≤0.9, C is a carbon coating material, and the mass percentage of the carbon coating material in the lithium iron manganese phosphate substrate is 1.5% to 2.5%.

[0022] Further, the specific surface area of the lithium iron manganese phosphate substrate is 15 to 30 m 2 / g.

[0023] The large specific surface area of the present application can make the electrolyte and the positive electrode material fully contact, thereby facilitating the electrochemical performance of the material.

[0024] Further, the pH value of the lithium iron manganese phosphate substrate is 9 to 11.

[0025] Further, the moisture content of the lithium iron manganese phosphate substrate is 1000 to 2000 ppm.

[0026] Further, the residual alkali content of the lithium iron manganese phosphate substrate is 5500 to 7500 ppm.

[0027] Further, the primary particle size of the lithium iron manganese phosphate substrate is 100 to 300 nm.

[0028] The large specific surface area and small particle size of the present application are more conducive to maintaining the electrical performance of the lithium iron manganese phosphate material.

[0029] According to a second aspect of the present application, a preparation method of the modified positive electrode material is provided, comprising the following steps:

[0030] Step S1: preparing each raw material according to the component allocation ratio;

[0031] Step S2: mixing the carboxyl-containing organic compound and the solvent to form an organic solution;

[0032] Step S3: mixing the lithium iron manganese phosphate substrate and the organic solution to form a slurry;

[0033] Step S4: performing desolventizing treatment on the slurry, and drying and crushing to obtain the modified positive electrode material.

[0034] Further, the mass ratio of the lithium iron manganese phosphate substrate to the organic compound is 100:(0.5 to 2).

[0035] The mass ratio of the present application can make the carboxyl-containing organic compound and the residual lithium compound on the surface of the substrate fully react without excess.

[0036] Further, in step S2, the carboxyl-containing organic compound is selected from one or more of polyacrylic acid, 9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid, carboxymethyl chitosan, sodium alginate, carboxymethyl cellulose, polyamide acid, and carboxylated styrene-butadiene latex, and the content of carboxyl groups in the organic compound is 0.5-8 wt%.

[0037] The carboxyl-containing organic compound selected in the present application can form strong hydrogen bonds with the active material having hydroxyl groups on the surface, so as to promote the formation of a more uniform film on the surface of the electrode material and protect the substrate from moisture in the environment.

[0038] Further, in step S2, the solvent is selected from one or more of water, ethanol, methanol, N-methylpyrrolidone, acetone, dimethylformamide, toluene, triethanolamine, styrene, perchloroethylene, and trichloroethylene.

[0039] Further, the ratio of the carboxyl-containing organic compound to the solvent is (0.1-2) g / 100 mL.

[0040] Further, in step S3, the lithium iron manganese phosphate substrate and the organic solution are stirred at a temperature of 50-65℃ for 1.5-3 h to form a slurry.

[0041] Further, in step S4, the drying process includes drying in a vacuum oven at 90-110℃ for 22-25 h.

[0042] Further, the desolventization treatment is selected from one or more of centrifugation, suction filtration, air blowing drying, freeze drying, and rotary evaporation.

[0043] Further, the crushing is selected from one or more of jaw crushing, roller crushing, air flow crushing, and ball milling.

[0044] According to a third aspect of the present application, there is provided a positive electrode sheet, comprising a foil material and a positive electrode active material attached to the surface of the foil material; wherein the positive electrode active material is the modified positive electrode material described above.

[0045] According to a fourth aspect of the present application, there is provided a lithium battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; wherein the positive electrode sheet is the positive electrode sheet described above.

[0046] The technical scheme of the present application provides a modified positive electrode material lithium iron manganese phosphate, through the reaction of the carboxyl functional groups on the surface of the polymer with the residual lithium compounds on the surface of the lithium iron manganese phosphate, the alkalinity of the lithium iron manganese phosphate material is reduced, the elimination reaction of C-H and C-F of the binder PVDF caused by the attack of the alkali base is avoided, the chemical cross-linking is formed by the C=C double bond, the continuous HF elimination reaction occurs, the gel-like slurry is avoided, and the slurry processability is improved; the polymer forms a film on the surface of the lithium iron manganese phosphate to build a protective layer, the degradation of the electrochemical performance and the change of the structure are effectively alleviated, the sensitivity to the humid air is reduced, and the storage performance is improved; the preparation process can adopt conventional means, the process flow is shorter, the equipment investment is smaller, and the scheme is simple and easy to operate. BRIEF DESCRIPTION OF DRAWINGS

[0047] The drawings accompanying the specification of the present application form a part thereof, serve to provide further understanding of the present application, and together with the description of the exemplary embodiments of the present application and the explanation thereof serve to explain the present application, and do not constitute an improper limitation of the present application.

[0048] Figure 1 A modification schematic diagram of the lithium iron manganese phosphate modified positive electrode material provided by the embodiment of the present application;

[0049] Figure 2 A scanning electron microscope image of the LMFP-N1 substrate used in the embodiment 1 of the present application;

[0050] Figure 3 A transmission electron microscope image of the selected region of the modified positive electrode material prepared in the embodiment 1 of the present application;

[0051] Figure 4 A physical picture of the modified positive electrode material prepared in the embodiment 5 of the present application after the screening of the slurry;

[0052] Figure 5 A physical picture of the slurry of the LMFP-N3 used in the comparative example 3 of the present application after the screening;

[0053] Figure 6 A cycle performance comparison diagram of the modified positive electrode material (a) of the embodiment 1 and the storage for 3 days (b), the substrate (c) of the comparative example 1 and the storage for 3 days (d). DETAILED DESCRIPTION

[0054] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0055] In the process of lithium-mixed sintering of the positive electrode material, an excess of lithium salt is often added to make up for the loss of lithium salt due to volatilization during high-temperature sintering, prevent the material from being lithium-deficient, and fully ensure that the lithium iron manganese phosphate positive electrode material is formed in stoichiometric ratio. However, the use of excess lithium carbonate will cause an increase in the residual lithium compound on the surface of the synthesized lithium iron manganese phosphate positive electrode, and the unreacted lithium exists in the form of Li2O on the surface of the material after calcination; with the decrease of temperature and contact with air, Li2O is converted into LiOH, LiHCO3 and Li2CO3 on the surface of the material after contact with CO2 and H2O, which will cause the instability of the lithium iron manganese phosphate.

[0056] The equations of the reaction process are mainly:

[0057] (1) Li2O + H2O → 2LiOH; (2) LiOH + CO2 → LiHCO3; (3) Li2O + CO2 → Li2CO3.

[0058] In addition, compared with other positive electrode materials, the electronic conductivity of the phosphate polyanion material is relatively low. The periodic and interval atomic arrangement of MO6 octahedron and PO4 tetrahedron leads to the transfer of electrons in the mode of M-O-P-O-M, which is much slower than the mode of M-O-M. The electronic conductivity of the phosphate polyanion material can be improved by means of conductive carbon coating and particle refinement; however, an excess of carbon and too small particle size distribution will increase the specific surface area of the material, resulting in a high moisture content of the material. Finally, the attack of the base on the surface of the material on the C-H and C-F of PVDF during homogenization will occur elimination reaction, forming C=C double bond and chemical crosslinking. Since the regularity of the PVDF chain is very high, continuous HF elimination reaction is easy to occur, which finally leads to gelation, increasing the difficulty of slurry processing.

[0059] To solve the above problems, the present application provides a modified positive electrode material, which comprises a lithium iron manganese phosphate base material and an organic coating attached to the surface of the lithium iron manganese phosphate base material; wherein the material of the organic coating is a carboxyl (-COOH) containing organic compound.

[0060] Compared with the traditional inorganic carbon source coating, the -COOH group on the surface of the polymer can consume the residual Li2CO3 and LiOH on the surface of the lithium iron manganese phosphate by dissociation, to generate -COOLi, H2O and CO2, reduce the residual alkali on the surface of the material, improve the fluidity of the slurry and avoid the occurrence of slurry gelation. Meanwhile, the polymer forms a protective layer on the surface of the lithium iron manganese phosphate to protect the substrate from direct contact with air, effectively alleviate the degradation of electrochemical performance and the change of structure, reduce the sensitivity to humid air and improve the storage performance. In addition, the coating formed by the polymer reaction does not need to be carbonized, which reduces the production cost. The modified material has fewer impurities on the surface and a more stable near-surface structure, and has better electrochemical performance.

[0061] In order to better utilize the carboxyl group to react with the residual lithium compound on the surface of the lithium iron manganese phosphate, thereby reducing the residual alkali on the surface of the material, improving the fluidity of the slurry, avoiding the slurry gelation and better constructing the organic protective film layer on the surface of the substrate, the application selects the carboxyl-containing organic compound from one or more of polyacrylic acid, 9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid, carboxymethyl chitosan, sodium alginate, carboxymethyl cellulose, polyamide acid and carboxyl styrene-butadiene latex. The content of the carboxyl group in the organic compound is 0.5-8wt%. For example, the content of the carboxyl group is selected from any one of 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7% and 8% or a range value between any two of them. The carboxyl-containing organic compound selected by the application hardly swells in the electrolyte carbonate solvent, does not dissolve with the organic solvent and has a certain viscosity, without other side effects, which can effectively maintain the structural stability. By controlling the content of the carboxyl group in the organic compound, the carboxyl-containing organic compound can better form a strong hydrogen bond with the active material containing hydroxyl groups on the surface, to promote the formation of a more uniform film coating on the surface of the electrode material. In addition, the above-mentioned organic compound can also improve the electronic conductivity of the material, for example, 9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid can improve the electronic conductivity of the material while playing a coating function.

[0062] In the structure of the modified positive electrode material of the application, the mass percentage of the organic compound coating in the lithium iron manganese phosphate substrate is 0.6%-1.9%, preferably 0.8%-1.8%. For example, 0.6%, 0.75%, 1%, 1.25%, 1.5%, 1.75% and 1.9%. If the content of the above-mentioned organic coating is too high, it will affect the conductivity and specific capacity of the lithium iron manganese phosphate substrate. If the content is too low, the carboxylic acid-containing organic compound may not react sufficiently with the residual lithium compound on the surface of the substrate, and the modification effect is poor.

[0063] The above carboxyl-containing organic matter screened by the present application can better reduce the pH value of the surface of the lithium iron manganese phosphate substrate. For example, the pH value of the lithium iron manganese phosphate substrate before modification is generally 9-11, and the pH value of the modified material is 7-9 or 7-8. The pH value of the modified material is reduced, the alkalinity is neutralized, indicating that the surface alkaline impurities are reduced, and the above specific organic matter selected by the present application plays an important role in reducing the pH value after modification, thereby avoiding the occurrence of gel phenomenon, otherwise it may eventually lead to a significant decrease in battery performance.

[0064] In order to better form a protective coating film on the surface of the lithium iron manganese phosphate and not affect the electrochemical performance of the lithium iron manganese phosphate itself, the thickness of the organic coating is 2-30 nm, preferably 5-20 nm, and further preferably 7-10 nm. The thickness of the organic coating in the modified positive electrode material of the present application is more than 30 nm, which will affect the conductivity of the lithium iron manganese phosphate substrate. Less than 2 nm indicates that the reaction is not sufficient, the residual lithium compound on the surface of the substrate cannot be removed sufficiently, the alkalinity cannot be reduced, and a suitable protective film cannot be constructed to isolate the moisture in the air.

[0065] The moisture content of the modified positive electrode material of the present application is 500-1000 ppm, preferably 500-800 ppm, and further preferably 500-700 ppm. After modification by the present application, the water content of the lithium iron manganese phosphate substrate can be significantly reduced, and the electrochemical performance can be improved. For example, the water content of the modified positive electrode material after 72 hours of storage is 1.5-2.0 times the water content before storage; wherein the storage conditions include: temperature 45-55℃, relative humidity 75%-85% RH, indicating that the modified positive electrode material has good storage stability.

[0066] The residual alkali content of the lithium iron manganese phosphate substrate before modification of the present application is 5500-7500 ppm; the residual alkali content of the modified positive electrode material is 1300-1800 ppm. The small residual alkali content can improve the fluidity of the slurry and avoid the occurrence of slurry gel phenomenon. The modified positive electrode material will not absorb more moisture from the environment, and will not affect the electrochemical performance of the material, which is beneficial to the storage stability.

[0067] The present application forms a carboxyl-containing coating protective film on the surface of the substrate, which is more suitable for lithium iron manganese phosphate substrate, and the chemical formula is LiFe 1-x Mn x PO4 / C, 0.5≤x≤0.9, C is a carbon coating layer, and the mass percentage of the carbon coating layer material in the lithium iron manganese phosphate substrate is 1.5%-2.5%. The preparation method of the carbon-modified coated lithium iron manganese phosphate substrate is a prior art, for example, mixing lithium source, iron source, manganese source, phosphorus source and carbon source (glucose, sucrose, etc.), sintering, and obtaining a carbon-coated lithium iron manganese phosphate material.

[0068] The specific surface area of the lithium iron manganese phosphate substrate of the present application is 15-30 m 2 / g, a larger specific surface area can provide more interface area for the material and electrolyte to fully contact, but at the same time, it can increase the reaction with moisture in the air, thus resulting in a higher moisture content of the substrate, when the water-containing positive electrode material contacts the electrolyte, it can react with the fluorine-containing lithium salt to produce HF, which can cause corrosion damage and affect the electrochemical performance of the material; therefore, after the above modification, the organic coating film is constructed on the surface of the lithium iron manganese phosphate, which can avoid the reaction of the substrate surface with H2O / CO2 in the humid air, and improve the processability of the lithium iron manganese phosphate material, the slurry is easy to process during the slurry preparation process, and the storage stability of the lithium iron manganese phosphate material is further improved, and the advantages brought by the larger specific surface area are retained, for example, the substrate and the electrolyte can fully contact to improve the electrochemical performance.

[0069] The particle size of the lithium iron manganese phosphate substrate of the present application is 100-300 nm, which is smaller than the conventional lithium iron phosphate particle of 300-400 nm, and small particles can bring higher capacity and better rate, but small particles can also increase the specific surface area of the substrate, increase the absorption of moisture in the air, and thus result in a higher moisture content of the substrate, which can also affect the processability of the substrate; therefore, after the above modification, the organic coating film is constructed on the surface of the lithium iron manganese phosphate, the processability and electrochemical performance of the small particles are improved, and the advantages brought by the increased specific surface area of the small particles are retained, for example, the substrate and the electrolyte can fully contact to improve the electrochemical performance.

[0070] The moisture of the lithium iron manganese phosphate substrate before modification of the present application is 1000-2000 ppm, and the moisture of the positive electrode material after modification is 500-1000 ppm, preferably 500-800 ppm, and further preferably 500-700 ppm. After the modification of the present application, the water content of the lithium iron manganese phosphate substrate can be significantly reduced, and the electrochemical performance can be improved.

[0071] According to another aspect of the present application, a preparation method of the modified positive electrode material is provided, comprising the following steps:

[0072] Step S1: prepare each raw material according to the component allocation ratio;

[0073] Step S2: mix the carboxyl-containing organic compound and the solvent to form an organic solution;

[0074] Step S3: mix the lithium iron manganese phosphate substrate and the organic solution to form a slurry;

[0075] Step S4: perform desolventizing treatment on the slurry, and after drying and crushing, the modified positive electrode material is obtained.

[0076] The above method of the present application first mixes the carboxyl-containing organic compound and the solvent, and then stirs to form a uniform mixed solution; then the lithium iron manganese phosphate substrate is added to the mixed solution, and continues to be stirred to form a slurry; finally, the slurry is subjected to solvent removal treatment, and then dried and crushed to obtain the modified lithium iron manganese phosphate positive electrode material; the preparation process can be carried out by conventional means, the process flow is shorter, the equipment investment is smaller, the scheme is simple and easy to operate, and the modified positive electrode material prepared has a carboxyl-containing organic compound coating on the surface, which can eliminate the residual lithium compounds on the surface of the substrate, reduce the residual alkali, and the organic film can protect the substrate from the influence of moisture in the air, thereby improving the structural stability, the electrochemical performance and the storage stability of the modified positive electrode material.

[0077] In order to better utilize the carboxyl groups to react with the residual lithium compounds on the surface of the lithium iron manganese phosphate, thereby reducing the residual alkali on the surface of the material, improving the flowability of the slurry, avoiding the gelation of the slurry, and better constructing an organic protective film layer on the surface of the substrate, the carboxyl-containing organic compound selected in step S2 of the present application is selected from one or more of polyacrylic acid, 9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid, carboxymethyl chitosan, sodium alginate, carboxymethyl cellulose, polyamide acid, and carboxylated butadiene styrene latex, and the content of carboxyl groups in the organic compound is 0.5-8wt%. The carboxyl-containing organic compounds selected in the present application hardly swell in the carbonate solvent of the electrolyte, and can effectively maintain the structural stability; the carboxyl groups can form strong hydrogen bonds or covalent bonds with active material containing hydroxyl groups and other groups on the surface, promoting the formation of a more uniform coating on the surface of the electrode material; and the above-mentioned organic compounds can also improve the electronic conductivity of the material, for example, 9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid can improve the electronic conductivity of the material while playing a coating function.

[0078] As a preferred embodiment, the mass ratio of the lithium iron manganese phosphate substrate and the organic compound in step S3 of the present application is 100:(0.5-2); for example, 100:0.5, 100:0.75, 100:1.0, 100:1.25, 100:1.5, 100:1.75, 100:2.0. If the content of the organic coating is too high, it will affect the conductivity and specific capacity of the lithium iron manganese phosphate substrate, and if the content is too low, the reaction between the carboxylic acid-containing organic compound and the residual lithium compounds on the surface of the substrate may not be sufficient, and the modification effect is poor.

[0079] As a preferred embodiment, in step S2, the solvent is selected from one or more of water, ethanol, methanol, N-methyl pyrrolidone, acetone, dimethylformamide, toluene, triethanolamine, styrene, perchloroethylene and trichloroethylene. The above-mentioned solvents are better soluble in the carboxyl-containing organic compounds.

[0080] As a preferred embodiment, the ratio of the carboxyl-containing organic compound and the solvent is (0.1-2) g / 100 mL. The above ratio can further improve the solubility of the carboxyl-containing organic compound, facilitate mixing with the lithium iron manganese phosphate substrate to form a uniform slurry, and facilitate the electrochemical performance of the positive electrode material.

[0081] As a preferred embodiment, in step S3, the lithium iron manganese phosphate substrate and the organic solution are stirred at a temperature of 50-65℃ for 1.5-3 h to form a slurry. The lithium iron manganese phosphate substrate and the carboxyl-containing organic compound in the present application can be fully reacted at the above reaction temperature, fully remove the residual lithium compounds on the surface of the substrate, reduce the residual alkali, construct a protective film to protect the substrate from the influence of environmental moisture, improve the structural stability of the positive electrode material, promote the storage stability, and improve the electrochemical performance.

[0082] As a preferred embodiment, the desolventization treatment is selected from one or more of centrifugation, suction filtration, air blowing drying, freeze drying, and rotary evaporation. The above desolventization treatment methods in the present application can be selected according to actual needs, and the above desolventization methods can better remove the solvent in the prepared slurry without affecting the active material itself.

[0083] As a preferred embodiment, in step S4, the drying process includes drying in a vacuum oven at 90-110℃ for 22-25 h. The above drying method can also be selected from other suitable methods as long as it does not affect the structure and properties of the positive electrode material and can achieve the purpose of drying.

[0084] As a preferred embodiment, the crushing is selected from one or more of jaw crushing, roller crushing, air flow crushing, and ball milling. The above crushing methods in the present application can better crush the dried positive electrode material into smaller particles, and the specific method can be selected according to actual needs.

[0085] The modified positive electrode material is coated on the surface of a foil material (carbon-coated aluminum foil) to form a positive electrode sheet with stable storage performance and electrochemical performance.

[0086] The prepared new type of positive electrode sheet, negative electrode sheet, separator, and electrolyte can be used to prepare a lithium battery with stable storage performance and good electrochemical performance.

[0087] The present application will be further described in detail below in combination with specific examples, which cannot be understood as limiting the scope of the present application.

[0088] Example 1

[0089] Preparation of the modified positive electrode material: according to Figure 1The modification schematic diagram is shown. 20 g of polyacrylic acid is dissolved in 3000 mL of methanol, and a mixed solution is obtained after ultrasonic stirring. 2 kg of purchased LMFP-N1 substrate is dispersed into the mixed solution, and after stirring at 60°C for 2 h, the sample is centrifuged for several times and dried in a 100°C vacuum oven for 24 h.

[0090] LMFP-N1 physical property parameters: chemical formula LiFe 0.4 Mn 0.6 PO4 / C, specific surface area 18.5 m 2 / g, carbon content 1.71%, moisture 1115 ppm, pH 9.8, and average primary particle size 220 nm.

[0091] Figure 2 The SEM image of the LMFP-N1 substrate shows that the secondary agglomerates are composed of spherical primary particles with a particle size of 220 nm, and the primary particles are uniform and closely connected.

[0092] Figure 3 The TEM image of the modified material of Example 1 shows that the surface of each region of the single particle is covered with a uniform layer of polyacrylic acid with a thickness of 7-10 nm.

[0093] Example 2

[0094] Example 2 differs from Example 1 in that the LMFP-N1 substrate is replaced by LMFP-N2.

[0095] LMFP-N2 physical property parameters: chemical formula LiFe 0.3 Mn 0.7 PO4 / C, specific surface area 21.3 m 2 / g, carbon content 1.84%, moisture 1240 ppm, pH 10.1, and average primary particle size 190 nm.

[0096] Example 3

[0097] Example 3 differs from Example 1 in that the LMFP-N1 substrate is replaced by LMFP-N3.

[0098] LMFP-N3 physical property parameters: chemical formula LiFe 0.2 Mn 0.8 PO4 / C, specific surface area 25.4 m 2 / g, carbon content 2.01%, moisture 1850 ppm, pH 10.5, and average primary particle size 150 nm.

[0099] Example 4

[0100] Example 4 and Example 2 differ in that the mass of polyacrylic acid is increased to 25 g.

[0101] Example 5

[0102] Example 5 and Example 3 differ in that the mass of polyacrylic acid is increased to 30 g.

[0103] Example 6

[0104] Example 6 and Example 1 differ in that the mass of polyacrylic acid is increased to 40 g.

[0105] Example 7

[0106] Example 7 and Example 1 differ in that 20 g of polyacrylic acid is replaced by 25 g of carboxymethyl chitosan.

[0107] Example 8

[0108] Example 8 and Example 1 differ in that 20 g of polyacrylic acid is replaced by 18 g of 9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid.

[0109] Example 9

[0110] Example 9 and Example 1 differ in that 20 g of polyacrylic acid is replaced by 24 g of carboxylated styrene-butadiene latex.

[0111] Example 10

[0112] Example 10 and Example 1 differ in that 20 g of polyacrylic acid is replaced by 25 g of sodium alginate.

[0113] Example 11

[0114] Example 11 and Example 1 differ in that 20 g of polyacrylic acid is replaced by 23 g of carboxymethyl cellulose.

[0115] Comparative Example 1

[0116] Comparative Example 1 and Example 1 differ in that LMFP-N1 is not modified.

[0117] Comparative Example 2

[0118] Comparative Example 2 and Example 2 differ in that LMFP-N2 is not modified.

[0119] Comparative Example 3

[0120] Comparative Example 3 and Example 3 differ in that LMFP-N3 is not modified.

[0121] The modified positive electrode materials of Examples 1-11 and the unmodified LMFP of Comparative Examples 1-3 were subjected to pH testing, moisture testing, residual base testing and material storage testing, and the test results are shown in Table 1.

[0122] (1) pH testing: 10 g of the sample was weighed, stirred with 90 g of deionized water for 30 min, and then the pH meter was calibrated to test the pH value of the solution.

[0123] (2) Moisture testing: Karl Fischer method with high sensitivity was used for testing: I2, SO2, pyridine, anhydrous CH3OH were used to prepare reagents, which reacted with water in the sample, and the water content in the sample was calculated.

[0124] (3) Residual base testing: 5 g of the positive electrode powder was dispersed in 100 g of pure water, stirred, filtered, and the filtrate was placed in a potentiometric titrator. 0.1 mol / L standard hydrochloric acid solution was used for titration, and the volumes of standard hydrochloric acid used at pH≈8.5 and pH≈4.5 were recorded.

[0125] (4) Material storage: the 11 positive electrode material samples prepared in Examples 1-11 were placed in a humidity chamber, the temperature was controlled at 50°C, and the relative humidity was 80% RH. After three days of storage, the positive electrode material samples were taken out for testing.

[0126] Preparation of battery positive electrode sheet: the dried material was dry mixed for 20 min after being weighed in a mass ratio of modified positive electrode material (modified positive electrode material of Examples 1-11 and unmodified LMFP of Comparative Examples 1-3, respectively, in turn): conductive carbon black (Super-P): polyvinylidene fluoride (PVDF) = 96:1.5:2, then N-methyl pyrrolidone (NMP) solvent and 0.5 wt.% of carbon nanotubes based on the raw material were added, and the slurry was mixed at a speed of 1950 r / min for 15 min. The slurry was uniformly coated on the surface of the carbon-coated aluminum foil and placed in a 90°C vacuum drying oven for 12 h. The dried positive electrode coating was rolled twice by a manual roller machine and cut into 8 mm diameter discs by a manual slicing machine. The obtained positive electrode sheet was placed in an 80°C air drying oven for 10 h. Finally, the dried positive electrode sheet was selected, weighed, and numbered with a suitable number of positive electrode sheets before use, and 11 modified positive electrode sheets and 3 unmodified electrode sheets were obtained.

[0127] The above prepared material was used as a positive electrode, a metal lithium sheet was used as a negative electrode, a Celgard 2400 was used as a separator, and a high-pressure-resistant electrolyte of LiPF6+EC / DEC (volume ratio 1:1) was used as an electrolyte. From bottom to top, the order of assembly was a positive electrode shell, a positive electrode active sheet, dropwise addition of electrolyte, a 19 mm separator, dropwise addition of electrolyte, a 12 mm lithium sheet, a 12 mm double-layer foam nickel gasket, and a negative electrode shell. After the battery was assembled, it was sealed with a battery sealing machine. The installed battery was placed in a desiccator for 12 h of storage, and a blue electricity test system was used for testing.

[0128] The sample physical and chemical performance test results of the examples and comparative examples are shown in Table 1; the electrical performance test results are shown in Table 2. Figure 6 .

[0129] Table 1

[0130]

[0131]

[0132] From the data in Table 1, it can be seen that although the lithium iron manganese phosphate material combines the good cycle performance of lithium iron phosphate and the high-voltage platform of lithium manganese phosphate, the low electronic conductivity and lithium ion diffusion rate are still a major disadvantage that limits the development of lithium iron manganese phosphate materials. Therefore, researchers use various modification methods to improve the performance of lithium iron manganese phosphate positive electrode materials, which is the reason for the difference in the physical property parameters of the LMFP-N1 / N2 / N3 base materials. The higher the manganese content, the more necessary it is to use modification methods such as increasing the carbon content and reducing the primary particle size (increasing the specific surface area) to maintain the electrical performance of lithium iron manganese phosphate, which also indirectly leads to excessive moisture and residual alkali content in the material.

[0133] By comparing the physical and chemical properties of the modified lithium iron manganese phosphate base material in the comparative example and the unmodified lithium iron manganese phosphate base material in the comparative example, Table 1 shows that the pH value of the unmodified lithium iron manganese phosphate base material is 9.8-10.5, and the pH value of the carboxyl-modified lithium iron manganese phosphate material is 7.1-8.6. The residual alkali content of the unmodified lithium iron manganese phosphate base material in Comparative Examples 1-3 is 5500-7500 ppm; the residual alkali content of the modified positive electrode material in Examples 1-11 of the present application is 1300-1800 ppm, which is reduced to 1 / 4 of the original. It is obvious that the modification method of the present application significantly reduces the residual alkali content of the modified lithium iron manganese phosphate base material, which can avoid the elimination reaction of C-H and C-F of the binder PVDF attacked by the alkali group on the surface of the base material during the slurry mixing process, form C=C double bonds to produce chemical crosslinking, and further avoid the occurrence of gel phenomenon.

[0134] Table 1 shows that the moisture of the unmodified lithium manganese iron phosphate substrate is 1115-1840 ppm, and the moisture content of the modified lithium manganese iron phosphate substrate is 504-706 ppm. The moisture of the modified substrate is significantly reduced, reduced to 1 / 2 of the original, and after storage in a high temperature and high humidity environment, the moisture content of the modified material increases by about 1.5-1.95 times, while the moisture content of the unmodified lithium manganese iron phosphate substrate increases by 2.5-3 times. The moisture storage increase is reduced by half by modification. The carboxyl modified substrate forms an organic protective film on the surface of the substrate, reducing the adsorption of H2O in the air by the lithium manganese iron phosphate substrate, and the effect is excellent.

[0135] Figure 4 and Figure 5 The pictures of the modified LMFP-N3 of Example 5 and the unmodified LMFP-N3 of Comparative Example 3 after sieving are shown in Figures 1 and 2, respectively. It can be observed that before modification, the slurry is obviously jelly-like and cannot pass through the sieve, while after modification, the material flowability is enhanced and can pass through the sieve smoothly. The carboxyl-containing organic material used in the present application can reduce the viscosity of the slurry and avoid gelation, thereby improving the processing performance.

[0136] Figure 6 The cycle performance of the modified sample of Example 1 (a) and the sample stored for 3 days (b), and the sample of Comparative Example 1 (c) and the sample stored for 3 days (d) are compared. Before cycling at 0.5C rate, all samples are subjected to 50 cycles at 0.2C low rate to activate the active samples completely. The retention rates of the modified sample of Example 1 and the sample stored for 3 days are 88.7% and 80.1%, respectively, while the retention rates of the substrate LMFP-N1 and the sample stored for 3 days without modification are 82.8% and 66.3%, respectively. The capacity retention rate of the modified positive electrode material of the present application is improved by 5.9% compared with the substrate, and the capacity retention rate is improved by 13.8% after storage in a high temperature and high humidity environment. The modification method of the present application can improve the electrochemical performance of the positive electrode material itself and improve the storage stability.

[0137] The above experimental results show that the organic protective film formed on the surface of the lithium manganese iron phosphate substrate can reduce the basicity of the material to some extent, and can prevent the material from directly contacting with H2O / CO2 in the air. Compared with the original material, the modified material has less surface impurities and more stable near-surface structure. When the material is stored under the same conditions (50°C, 80% RH), the stable modified coating can reduce the sensitivity of the positive electrode material to humid air, thereby improving the storage performance of the positive electrode material.

[0138] It should be noted that the terms "first", "second", and the like in the description and in the claims of the present application are used for distinguishing between similar objects and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the present application described herein are, for example, capable of orderly execution or performance.

[0139] The preferred embodiments of the application are described above in detail. The application is not limited to the embodiments described above, but can vary and be modified in various ways. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the application should be included in the scope of the protection of the application.

Claims

1. A modified cathode material, characterized in that, The modified positive electrode material comprises a lithium iron manganese phosphate base material and an organic coating attached to the surface of the lithium iron manganese phosphate base material; the material of the organic coating is a carboxyl-containing organic compound; The carboxyl-containing organic compound is selected from at least one of polyacrylic acid, 9, 9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid, carboxymethyl chitosan, sodium alginate, carboxymethyl cellulose, polyamide acid and carboxyl styrene-butadiene latex; The mass percentage of the organic coating in the lithium iron manganese phosphate base material is 0.6% to 1.9%; The content of carboxyl in the carboxyl-containing organic compound is 0.5wt% to 8wt%; The layer thickness of the organic coating is 2 to 30nm; The residual alkali content of the modified positive electrode material is 1300 to 1800 ppm.

2. The modified cathode material of claim 1, wherein, The mass percentage of the organic coating in the lithium iron manganese phosphate base material is 0.8% to 1.8%; And / or, the layer thickness of the organic coating is 7 to 10nm; And / or, the pH value of the modified positive electrode material is 7 to 9.

3. The modified cathode material of claim 1, wherein, The water content of the modified positive electrode material is 500 to 1000 ppm.

4. The modified cathode material of claim 1, wherein, The pH value of the modified positive electrode material is 7 to 8; And / or, the water content of the modified positive electrode material is 500 to 800 ppm.

5. The modified cathode material of claim 1, wherein, The water content of the modified positive electrode material is 500 to 700 ppm.

6. The modified cathode material according to any one of claims 1 to 5, characterized in that, The chemical formula of the lithium iron manganese phosphate base material is LiFe 1-x Mn x PO4 / C, 0.5≤x≤0.9, C is a carbon coating material, and the mass percentage of the carbon coating material in the lithium iron manganese phosphate base material is 1.5%~2.5%. and / or the specific surface area of the lithium iron manganese phosphate substrate is 15 to 30 m 2 / g; And / or, the primary particle size of the lithium iron manganese phosphate base material is 100 to 300nm; And / or, the pH value of the lithium iron manganese phosphate base material is 9 to 11; And / or, the moisture content of the lithium iron manganese phosphate base material is 1000 to 2000 ppm; And / or, the residual alkali content of the lithium iron manganese phosphate base material is 5500 to 7500 ppm.

7. A method for producing the modified cathode material according to any one of claims 1 to 6, characterized in that, The method comprises the following steps: Step S1: allocate the ratio of each raw material according to the component; Step S2: mix the carboxyl-containing organic compound and the solvent to form an organic solution; Step S3: mix the lithium iron manganese phosphate base material and the organic solution to form a slurry; Step S4: perform desolventizing treatment on the slurry, and after drying and crushing, obtain the modified positive electrode material.

8. The method of claim 7, wherein the modified cathode material is prepared by a process comprising: mixing a lithium transition metal oxide with a lithium source and a lithium ion source to form a mixture; and heating the mixture to form the modified cathode material. The mass ratio of the lithium iron manganese phosphate base material to the carboxyl-containing organic compound is 100: (0.5 to 2).

9. The method of producing a modified cathode material according to claim 7 or 8, characterized in that, In step S2, the carboxyl-containing organic compound is selected from one or more of polyacrylic acid, 9, 9-dioctylfluorene-co-fluorenone-co-methylbenzoic acid, carboxymethyl chitosan, sodium alginate, carboxymethyl cellulose, polyamide acid, and carboxyl styrene-butadiene latex; the content of carboxyl in the carboxyl-containing organic compound is 0.5 to 8wt%; And / or, the mass ratio of the lithium iron manganese phosphate base material to the carboxyl-containing organic compound is 100: (0.9 to 2).

10. The method of producing a modified cathode material according to claim 7 or 8, characterized in that, In step S2, the solvent is selected from one or more of water, ethanol, methanol, N-methyl pyrrolidone, acetone, dimethylformamide, toluene, triethanolamine, styrene, perchloroethylene and trichloroethylene.

11. The method of producing a modified cathode material according to claim 7 or 8, characterized in that, The ratio of the carboxyl-containing organic compound to the solvent is (0.1 to 2) g / 100mL; And / or, in step S3, the lithium iron manganese phosphate substrate and the organic solution are stirred at a temperature of 50-65°C for 1.5-3h to form a slurry; And / or, in step S4, the drying process comprises drying in a vacuum oven at 90-110°C for 22-25h; And / or, the desolventizing treatment is selected from one or more of centrifugation, suction filtration, air blast drying, freeze drying, and rotary evaporation; And / or, the crushing is selected from one or more of jaw crushing, roller crushing, air jet milling, and ball milling.

12. A positive electrode sheet comprising a foil material and a positive electrode active material attached to a surface of the foil material; characterized in that, The positive electrode active material is the modified positive electrode material of any one of claims 1-6 or the modified positive electrode material prepared by the preparation method of any one of claims 7-11.

13. A lithium battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte; characterized in that, The positive electrode plate is the positive electrode plate of claim 12.

Images