Low-impurity phosphate-based positive electrode material, preparation method thereof, positive electrode, and lithium secondary battery

Abstract

The application provides a low-impurity phosphate-based positive electrode material, a preparation method of the low-impurity phosphate-based positive electrode material, a positive electrode and a lithium secondary battery. x M 1‑x PO4, and the preparation method comprises the following steps: filling raw materials required for synthesizing LiFe x M 1‑x PO4 into a sagger to form a raw material layer; spraying a protective solution on a surface layer of the raw material layer, and then performing sintering under a protective gas condition to obtain the low-impurity phosphate-based positive electrode material LiFe x M 1‑x PO4; wherein M is selected from transition metal elements, and 0

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium battery materials, in particular to a low-impurity phosphate-based positive electrode material, a preparation method thereof, a positive electrode and a lithium secondary battery. BACKGROUND

[0002] With the wide application of lithium ion batteries in electric vehicles and energy storage fields, high energy density and high stability have become the goal pursued in the industry. At present, phosphate-based positive electrode materials with high compaction and high energy density have gradually become mainstream battery materials. How to ensure that the positive electrode material has high compaction density characteristics while taking into account high capacity and low impurity characteristics has become the focus of people's research. At present, material manufacturers mainly start from the following two aspects to synthesize high compaction phosphate-based positive electrode materials: one is to improve the sintering temperature, and the other is to adjust the raw material formula. Although the above two means can improve the compaction density of the material, lithium iron phosphate and lithium manganese iron phosphate and other phosphate-based positive electrode materials will cause element segregation and floating during the sintering process. During high-temperature sintering, iron magnetic substances (such as elemental iron and iron phosphide) impurities will be generated due to uneven distribution of elements (especially on the surface). The generation of these impurities will cause part of the capacity loss, and will also seriously affect the high-temperature storage, cycle life and safety performance of the battery. Therefore, how to synthesize high compaction phosphate-based materials while ensuring high capacity and low impurity of the material has become a problem to be solved. SUMMARY

[0003] Therefore, it is necessary to provide a preparation method of a phosphate-based positive electrode material with high capacity and low impurity. Further, a low-impurity phosphate-based positive electrode material, a positive electrode and a lithium secondary battery are provided.

[0004] The first aspect of the present application provides a preparation method of a low-impurity phosphate-based positive electrode material LiFe x M 1-x PO4, comprising the following steps:

[0005] Filling raw materials required for synthesizing LiFe x M 1-x PO4 into a sagger to form a raw material layer;

[0006] Spraying a protective solution on the surface layer of the raw material layer, and then sintering under the condition of a protective gas to obtain a low-impurity phosphate-based positive electrode material LiFe x M 1-x PO4;

[0007] Wherein, M is selected from transition metal elements, and 0 < x ≤ 1;

[0008] The protective solution comprises at least one of n-butyl titanate and titanium dioxide, and a titanium phosphate ester compound;

[0009] The sintering temperature is 700-1000℃, and the time is 5-15h.

[0010] Preferably, M is selected from at least one of Mn, Co, Ni, Ti, Zr, V and Nb.

[0011] Preferably, x is 0.1-1, and further preferably x is 0.4-1.

[0012] In some embodiments, the titanium phosphate ester compound is isopropyl tri(dioctylphosphato) titanate.

[0013] In some embodiments, the molar ratio of the total amount of n-butyl titanate and titanium dioxide in the protective solution to the isopropyl tri(dioctylphosphato) titanate is 1:(0.2-5). It can be understood that when the protective solution contains only n-butyl titanate and does not contain titanium dioxide, the molar ratio of the total amount of n-butyl titanate and titanium dioxide to the isopropyl tri(dioctylphosphato) titanate is the molar ratio of n-butyl titanate to the isopropyl tri(dioctylphosphato) titanate. Similarly, when the protective solution contains titanium dioxide and does not contain n-butyl titanate, the molar ratio refers to the molar ratio of titanium dioxide to the isopropyl tri(dioctylphosphato) titanate.

[0014] In some embodiments, the total mass fraction of n-butyl titanate, titanium dioxide and the titanium phosphate ester compound in the protective solution is 10%-50%.

[0015] In some embodiments, the solvent in the protective solution is isopropyl alcohol or ethanol.

[0016] In some embodiments, when spraying the protective solution on the surface layer of the raw material layer, the spraying speed is 5-50mL / min, and the spraying time is 5-100s. It can be understood that the spraying is realized by an atomizing nozzle, the protective solution is pumped into the atomizing nozzle while being stirred, and then atomized into small droplets, uniformly sprayed on the surface layer of the raw material layer, and the spraying time is controlled to realize the surface layer coverage and infiltration of the material.

[0017] In some embodiments, the raw materials required for synthesizing LiFe x M 1-x The average particle size of the raw materials required for synthesizing LiFe

[0018] In some embodiments, before the raw materials required for synthesizing LiFe x M 1-x The raw materials are sequentially ground and mixed and spray dried before being filled into the die. Through grinding and mixing, uniform mixing of the raw materials can be achieved, and the average particle size of the raw materials can be further controlled.

[0019] Preferably, the mixing is carried out by liquid phase mixing. Further, the solvent used in the liquid phase mixing is one of deionized water, methanol and ethanol.

[0020] In some embodiments, the LiFe x M 1-x PO4is synthesized by using raw materials including a Li source, an Fe source, an M source and a phosphorus source.

[0021] Preferably, the Li source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate and lithium dihydrogen phosphate.

[0022] Preferably, the Fe source is selected from one or more of iron phosphate, ferrous phosphate, ferrous oxalate, iron oxide, manganese iron phosphate and manganese iron oxalate.

[0023] Preferably, the M source is selected from one or more of oxides, carbonates, phosphates, acetates, nitrates and sulfates of the M metal.

[0024] Preferably, the phosphorus source is selected from one or more of iron phosphate, phosphoric acid, ammonium dihydrogen phosphate and di-ammonium hydrogen phosphate.

[0025] In some embodiments, the LiFe x M 1-x PO4is synthesized by using raw materials including a Li source, an Fe source, an M source and a phosphorus source. x M 1-x PO4sto a stoichiometric ratio of LiFe

[0026] In some embodiments, the LiFe x M 1-x PO4is synthesized by using raw materials including a Li source, an Fe source, an M source and a phosphorus source.

[0027] Preferably, the carbon source is selected from one or more of glucose, sucrose, polyethylene glycol and carbon nanotubes. The addition of the carbon source to the raw materials used in the synthesis of LiFe x M 1-x PO4can control the growth rate of the positive electrode material at high temperatures and form a C-coated layer on the surface of the material after sintering, thereby improving the conductivity and electrical performance of the positive electrode material.

[0028] In some embodiments, the protective gas is selected from argon or nitrogen.

[0029] The second aspect of the present application provides a low-impurity phosphate-based positive electrode material LiFe x M 1-x PO4. The low-impurity phosphate-based positive electrode material LiFe x M 1-x PO4is prepared according to the preparation method provided in the first aspect of the present application.

[0030] In some embodiments, the low-impurity phosphate-based positive electrode material LiFe x M 1-x PO4 contains ≤1 ppm of ferromagnetic substances; further, the content of ferromagnetic substances is 0.001 ppm to 1 ppm. Preferably, the low-impurity phosphate-based positive electrode material LiFe x M 1-x PO4 contains ≤0.3 ppm of ferromagnetic substances. It can be understood that the low-impurity phosphate-based positive electrode material LiFe x M 1- x The ferromagnetic substances in the low-impurity phosphate-based positive electrode material LiFe

[0031] The third aspect of the present application provides a positive electrode containing the low-impurity phosphate-based positive electrode material LiFe x M 1-x PO4 prepared by the preparation method provided in the first aspect of the present application.

[0032] The positive electrode in the present application can further contain a conductive agent and a binder. The conductive agent and the binder can be components known in the industry, such as conductive carbon black and PVDF, etc. The content of the conductive agent and the binder can be adjusted as needed, for example, can be 5 wt.% to 15 wt.% of the positive electrode material.

[0033] The fourth aspect of the present application provides a lithium secondary battery comprising the positive electrode provided in the third aspect of the present application.

[0034] Advantages:

[0035] The conventional preparation method of LiFe x M 1-x PO4 causes the decomposition of Li source and C source in the material and the generation of gas volatilization due to the increase of temperature during sintering, wherein Li and C will float up with the gas volatilization, resulting in excessive reduction of the surface layer at high temperature and the generation of ferromagnetic substance impurities (mainly elemental Fe and iron phosphide), and the content of impurities will increase with the increase of temperature, which seriously affects the safety performance and stability of the battery. The preparation method of the present application reduces the problem of local Fe excess caused by excessive consumption of P to produce lithium phosphate due to excessive Li, and the generation of ferromagnetic substances (elemental Fe and iron phosphide) under the condition of reduced C enrichment, by spraying a specific component protective solution with phosphorus and titanium elements on the surface layer of the raw material, and reacting with the floating Li at high temperature.

[0036] Further, the Ti-containing compound is enriched on the surface layer, which can form active components such as lithium titanium phosphate on the surface of the cathode material, and has good stability at high temperature, can form a surface isolation layer, can effectively reduce the high-temperature sensitivity of the material, is conducive to the synthesis of high-compactness material, and can further improve the capacity and electrical conductivity, and can further inhibit metal dissolution, prevent self-discharge of the battery, and improve the safety of the battery.

[0037] In addition, a special organic P source + Ti source is selected, which contains a strong adhesion C chain, which can be used as a C source, and after spraying, it penetrates to the surface of the raw material particles to form a wrapping layer, which can improve the uniformity of the particle surface coating and fluidity, reduce floating carbon, and improve the compaction and electrical properties. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the drawings.

[0039] Figure 1 SEM image of the low-impurity phosphate-based cathode material LiFePO4 prepared in Example 1. DETAILED DESCRIPTION

[0040] The embodiments described in the specification are only for the purpose of explaining the present application, and are not intended to limit the present application.

[0041] For the sake of simplicity, only some numerical ranges are explicitly disclosed herein. However, any lower limit can be combined with any upper limit to form a range that is not explicitly recited; and any lower limit can be combined with other lower limits to form a range that is not explicitly recited, and likewise any upper limit can be combined with any other upper limit to form a range that is not explicitly recited. In addition, although not explicitly recited, every point or individual number between the range endpoints is included in the range. Thus, each point or individual number can serve as its own lower limit or upper limit to combine with any other point or individual number or to combine with other lower limits or upper limits to form a range that is not explicitly recited.

[0042] In the description herein, it should be noted that, unless otherwise specified, "above", "below" include the number, and the meaning of "multiple" in "one or more" is two or more.

[0043] In the present application, if not specifically stated, all steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, it is mentioned that the method can further comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0044] The present application is further illustrated below with reference to examples. It should be understood that these examples are only for illustrative purposes, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and used directly without further treatment, and the instruments used in the examples are commercially available.

[0045] Example 1.

[0046] S1, preparation of spray material: according to the stoichiometric ratio of LiFePO4, the corresponding mass of iron phosphate, lithium phosphate, glucose and polyethylene glycol is weighed and dispersed in deionized water, the solid content of the slurry is controlled to be 38%, the slurry is subjected to sand milling treatment until the particle size is 0.33 μm, and then spray drying treatment is carried out to obtain the spray material and load it into the sagger.

[0047] S2, spraying treatment: according to the molar ratio of n-butyl titanate to isopropyl tri(dioctyl phosphoric acyl oxygen) titanate of 1:0.5, isopropyl tri(dioctyl phosphoric acyl oxygen) titanate and n-butyl titanate are weighed; according to the total mass fraction of isopropyl tri(dioctyl phosphoric acyl oxygen) titanate and n-butyl titanate of 30%, ethanol solvent is mixed and dispersed uniformly to obtain a protective liquid; then the protective liquid is pumped into the atomizing nozzle while stirring, the spray cover is covered on the sagger loaded with the spray material obtained in step S1, the spraying is started, the atomizing speed is controlled, the spraying is carried out for 10 seconds, and the protective liquid is uniformly covered on the surface layer of the spray material.

[0048] S3, sintering: the material obtained in step S2 is placed in an atmosphere furnace and sintered, heated to 810°C under nitrogen atmosphere protection for 10 hours, and cooled with the furnace to obtain a low-impurity phosphate-based positive electrode material LiFePO4.

[0049] Example 2.

[0050] S1, preparation of spray material: according to the stoichiometric ratio of LiFePO4, the corresponding mass of iron phosphate, lithium phosphate, glucose and polyethylene glycol is weighed and dispersed in deionized water, the solid content of the slurry is controlled to be 38%, the slurry is subjected to sand milling treatment until the particle size is 0.33 μm, and then spray drying treatment is carried out to obtain the spray material and load it into the sagger. 0.4 Mn0.6 PO4 stoichiometric ratio of the corresponding mass of iron phosphate, trimanganese tetroxide, lithium carbonate, glucose, polyethylene glycol is dispersed in deionized water, the solid content of the slurry is controlled to be 35%, the slurry is sand ground to a particle size of 0.25μm, and then spray drying treatment is performed to obtain a spray material and load into a sagger.

[0051] S2, spraying treatment: according to the molar ratio of n-butyl titanate to isopropyl tri(dioctyl phosphoric acyl oxygen) titanate of 1:1.5, n-butyl titanate and isopropyl tri(dioctyl phosphoric acyl oxygen) titanate are weighed, and ethanol solvent is mixed according to the total mass fraction of isopropyl tri(dioctyl phosphoric acyl oxygen) titanate and n-butyl titanate of 30% to disperse uniformly to obtain a protective liquid; then the protective liquid is pumped into an atomizing nozzle while stirring, the spray cover is covered on the sagger loaded with the spray material obtained in step S1, the spraying is started, the atomizing speed is controlled, and the spraying is performed for 15 seconds, and the protective liquid is uniformly covered on the surface layer of the spray material.

[0052] S3, sintering: the material obtained in step S2 is placed in an atmosphere furnace for sintering, heated to 800℃ under nitrogen atmosphere protection for 10 hours, and cooled with the furnace to obtain a low-impurity phosphate-based positive electrode material LiFePO4. 0.4 Mn 0.6 PO4.

[0053] Example 3.

[0054] The preparation method of this example is basically the same as that of example 1, the difference is that in step S2 of this example, n-butyl titanate and isopropyl tri(dioctyl phosphoric acyl oxygen) titanate are weighed according to the molar ratio of 1:1.5, and the other operations and parameters are the same as those of example 1.

[0055] Example 4.

[0056] The preparation method of this example is basically the same as that of example 1, the difference is that in step S2 of this example, the spraying time of the protective liquid is controlled to be 30 seconds, and the other operations and parameters are the same as those of example 1.

[0057] Comparative example 1.

[0058] S1, preparation of spray material: according to the stoichiometric ratio of LiFePO4, the corresponding mass of iron phosphate, lithium phosphate, glucose, polyethylene glycol is dispersed in deionized water, the solid content of the slurry is controlled to be 38%, the slurry is sand ground to a particle size of 0.33μm, and then spray drying treatment is performed to obtain a spray material and load into a sagger.

[0059] S2, sintering: the material obtained in step S1 is placed in an atmosphere furnace for sintering, heated to 810℃ under nitrogen atmosphere protection for 10 hours, and cooled with the furnace to obtain a low-impurity phosphate-based positive electrode material LiFePO4.

[0060] Comparative Example 2.

[0061] The preparation method of the present comparative example is basically the same as that of Example 1, except that no isopropyl tri(dioctyl phosphato) titanate is added to the protective solution prepared in step S2 of the present comparative example, only n-butyl titanate is added, and other operations and parameters are the same as those of Example 1.

[0062] Comparative Example 3.

[0063] The preparation method of the present comparative example is basically the same as that of Example 1, except that no n-butyl titanate is added to the protective solution prepared in step S2 of the present comparative example, only isopropyl tri(dioctyl phosphato) titanate is added; and other operations and parameters are the same as those of Example 1.

[0064] Product performance test items and methods:

[0065] Test item 1: Microstructure test.

[0066] The microstructure of the low-impurity phosphate-based positive electrode material LiFePO4 prepared in Example 1 was observed by scanning electron microscopy. The results are shown in FIG. 1. Figure 1

[0067] Test item 2: Powder compaction test.

[0068] An automatic compaction density instrument was used for powder compaction test. 1 g of sample was accurately weighed and filled into the test mold. The test pressure was adjusted to 3000 kg, the start button was pressed to start the test, and the test was completed after 10 s of pressure holding. The compaction density data was read.

[0069] Test item 3: Ferromagnetic substance test.

[0070] 150 g of positive electrode material was placed in a polyester tank, 1000 mL of pure water was added, a magnetic bar with a magnetic field strength greater than 5000 Gauss was placed in the tank, and the tank was sealed. After sealing, the tank was placed on a tumbler and tumbled at a speed of 90 rpm for 30 min. After the end of the test, the magnetic bar was removed. The magnetic bar was placed in a clean beaker, 15 ml of HCL and 50 mL of deionized water were added, and the adhering metal on the magnetic bar was dissolved by heating. Then, the ferromagnetic substance content was tested by ICP5800 instrument.

[0071] Test item 4: Electrical performance test.

[0072] (1) Preparation of positive electrode sheet:

[0073] ​The positive electrode material prepared in Examples 1-4 and Comparative Examples 1-3 by the above method was used as a positive electrode active material, and the positive electrode active material, superconducting carbon black (SP), and polyvinylidene fluoride (PVDF) were uniformly mixed at a mass ratio of 90:5:5, coated on a 20 μm-thick aluminum foil, and a positive electrode sheet having a surface density of 8 mg / cm2was prepared. 2 Then, by drying, rolling, die cutting, and punching, a positive electrode sheet was prepared.

[0074] (2) Preparation of the battery: a button battery shell of R2032 was used for the assembly of the button battery, a lithium sheet was used as the negative electrode, a separator made of PE was used, and 80 μL of electrolyte was added dropwise. The test temperature of the electrical performance was 25°C, the test voltage range was 2.0-4.3 V, the charging was performed in a constant current and constant voltage mode to 4.3 V, and the discharging was performed in a constant current mode to 2.0 V; the first four cycles of the charging and discharging were performed at 0.1 C, 0.2 C, 0.5 C, and 1 C, respectively; and then the cycles were performed at 1 C for 200 times.

[0075] The specific test results of the above performance tests are shown in Table 1.

[0076]

[0077] As can be seen from the test results in Table 1, the positive electrode material prepared in Examples 1-4 using the technical solution of the present application has a high compaction density, and the content of the ferromagnetic substance on the surface of the positive electrode material is significantly reduced. The lithium ion battery prepared using the positive electrode material prepared in Examples 1-3 has a high discharge capacity; and after 200 cycles, the capacity retention rate still reaches more than 93%. It can be seen that the protective layer formed on the surface of the raw material by the protective solution with the specific components in the present application can effectively inhibit the side reactions easily generated in the high-temperature sintering process, reduce the generation of impurities, and improve the electrochemical activity and structural stability of the positive electrode material.

[0078] The above merely describes specific embodiments of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art can easily think of various equivalent modifications or replacements within the technical scope disclosed in the present application, and these modifications or replacements should be encompassed in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A low-impurity phosphate-based cathode material LiFe x M 1-x The preparation method of PO4 is characterized in that: The steps include: Fill the raw materials required for synthesizing LiFe x M 1-x PO4 into the sagger to form a raw material layer; wherein, M is selected from transition metal elements, and 0 < x ≤ 1; the raw materials required for synthesizing LiFe x M 1-x PO4 include Li source, Fe source, M source, phosphorus source and carbon source; the Li source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate and lithium dihydrogen phosphate; the Fe source is selected from one or more of iron phosphate, ferrous phosphate, ferrous oxalate, iron oxide, manganese iron phosphate and manganese iron oxalate; the M source is selected from one or more of oxides, carbonates, phosphates, acetates, nitrates and sulfates of M metal; the phosphorus source is selected from one or more of iron phosphate, phosphoric acid, ammonium dihydrogen phosphate and diammonium hydrogen phosphate; the carbon source is selected from one or more of glucose, sucrose, polyethylene glycol and carbon nanotubes; The surface of the raw material layer is sprayed with a protective liquid, and then sintered under protective gas conditions to obtain the low-impurity phosphate-based positive electrode material LiFe x M 1-x PO4; the protective liquid includes at least one of n-butyl titanate and titanium dioxide, and isopropyl tris (dioctyl phosphate) titanate, and the molar ratio of the total amount of n-butyl titanate and the titanium dioxide in the protective liquid to the isopropyl tris (dioctyl phosphate) titanate is 1: (0.2~5); the sintering temperature is 700℃~1000℃, and the time is 5h~15h.

2. The preparation method according to claim 1, characterized in that The total mass fraction of n-butyl titanate, titanium dioxide and titanium phosphate compounds in the protective solution is 10% to 50%.

3. The preparation method according to claim 1, characterized in that The solvent in the protective solution is isopropanol or ethanol.

4. The preparation method according to claim 1, characterized in that When the protective liquid is sprayed on the surface of the raw material layer, the spraying speed is 5 mL / min to 50 mL / min, and the spraying time is 5 s to 100 s.

5. The preparation method according to any one of claims 1 to 4, characterized in that The synthetic LiFe x M 1-x The average particle size of the raw materials required for PO4 is 0.2μm~0.4μm.

6. A low-impurity phosphate-based cathode material LiFe x M 1-x PO4, characterized in that The low-impurity phosphate-based positive electrode material LiFe x M 1-x PO4 is prepared according to the preparation method according to any one of claims 1 to 5.

7. A positive electrode, characterized in that Contains a low-impurity phosphate-based positive electrode material LiFe prepared by the preparation method according to any one of claims 1 to 5 x M 1-x PO4.

8. A lithium secondary battery, characterized in that: Comprising the positive electrode as claimed in claim 7.

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