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

Abstract

The invention provides a low-impurity phosphate positive electrode material and a preparation method thereof, a positive electrode and a lithium secondary battery. The preparation method of the low-impurity phosphate positive electrode material LiFexM1-xPO4 comprises the following steps: filling a sagger with raw materials required for synthesizing the LiFexM1-xPO4 to form a raw material layer; spraying protective liquid on the surface layer of the raw material layer, and then sintering under the condition of protective gas to obtain the low-impurity phosphate positive electrode material LiFexM1-xPO4, wherein M is selected from transition metal elements, and 0 lt; x < = 1; the protection liquid comprises at least one of tetrabutyl titanate and titanium dioxide, and a titanium phosphate compound; the sintering temperature ranges from 700 DEG C to 1000 DEG C, and the sintering time ranges from 5 h to 15 h. According to the preparation method, the protection liquid with specific components is sprayed on the surface layer of the raw material layer, so that the impurity content of the surface of the positive electrode material is effectively reduced, and the capacity of the positive electrode material is improved.

Description

Technical Field

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

[0002] With the widespread adoption of lithium-ion batteries in electric vehicles and energy storage, high energy density and high stability have become industry-wide goals. Currently, high-density, high-energy-density phosphate cathode materials are becoming mainstream battery materials. How to achieve high density while simultaneously maintaining high capacity and low impurities has become a key research focus. Currently, material manufacturers primarily synthesize high-density phosphate cathode materials through two approaches: increasing the sintering temperature and adjusting the raw material formulation. While both approaches can improve the material's density, phosphate-based cathode materials, such as lithium iron phosphate and lithium manganese iron phosphate, can experience element segregation and flotation during sintering. During high-temperature sintering, uneven distribution of elements (especially on the surface) can lead to the formation of ferromagnetic impurities (such as elemental iron and iron phosphide). These impurities can lead to partial capacity loss and severely impact the battery's high-temperature storage, cycle life, and safety. Therefore, how to synthesize high-density phosphate-based materials while maintaining high capacity and low impurities remains a pressing challenge. Summary of the Invention

[0003] Therefore, it is necessary to provide a method for preparing a phosphate-based cathode material with high capacity and low impurity content, and further provide a low-impurity phosphate-based cathode material, a cathode, and a lithium secondary battery.

[0004] The present application provides a low-impurity phosphate-based positive electrode material LiFe x M 1-x The preparation method of PO4 comprises the following steps: The synthesis of LiFe x M 1-x The raw materials required for PO4 are filled into the sagger to form a raw material layer; Spray the protective liquid on the surface of the raw material layer, and then sinter it under protective gas conditions 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 <x≤1; The protective liquid includes at least one of n-butyl titanate and titanium dioxide, and a titanium phosphate compound; The sintering temperature is 700°C to 1000°C, and the sintering time is 5h to 15h.

[0005] Preferably, M is at least one element selected from Mn, Co, Ni, Ti, Zr, V and Nb.

[0006] Preferably, x is 0.1-1, and more preferably, x is 0.4-1.

[0007] In some embodiments, the titanium phosphate ester compound is isopropyl tris(dioctylphosphoyloxy)titanate.

[0008] In some embodiments, the molar ratio of the total amount of n-butyl titanate and titanium dioxide to isopropyl tris(dioctyl phosphate) titanate in the protective solution is 1:(0.2-5). It should be understood that when the protective solution contains only n-butyl titanate but no titanium dioxide, the molar ratio of the total amount of n-butyl titanate and titanium dioxide to isopropyl tris(dioctyl phosphate) titanate is the molar ratio of n-butyl titanate to isopropyl tris(dioctyl phosphate) titanate. Similarly, when the protective solution contains titanium dioxide but no n-butyl titanate, the molar ratio refers to the molar ratio of titanium dioxide to isopropyl tris(dioctyl phosphate) titanate.

[0009] In some embodiments, the total mass fraction of n-butyl titanate, titanium dioxide, and titanium phosphate compounds in the protective solution is 10% to 50%.

[0010] In some embodiments, the solvent in the protective solution is isopropanol or ethanol.

[0011] In some embodiments, when spraying the protective liquid on the surface of the raw material layer, the spray rate is 5 mL / min to 50 mL / min, and the spray time is 5 to 100 seconds. It will be understood that spraying is achieved through an atomizing nozzle. The protective liquid is pumped into the atomizing nozzle while stirring, and then atomized into small droplets, which are evenly sprayed on the surface of the raw material layer. The spray time is controlled to achieve surface coverage and infiltration of the material.

[0012] In some embodiments, the synthesis of LiFe x M 1-x The average particle size of the raw materials required for PO4 is 0.2μm~0.4μm.

[0013] In some embodiments, the synthesis of LiFe x M 1-x Before the raw materials required for PO4 are filled into the sagger, they are ground, mixed and spray-dried in sequence. Through grinding and mixing, the raw materials can be evenly mixed and the average particle size of the raw materials can be further controlled.

[0014] Preferably, the grinding and mixing is carried out by liquid phase grinding and mixing. Further, the solvent used in the liquid phase grinding and mixing is one of deionized water, methanol and ethanol.

[0015] In some embodiments, the synthesis of LiFe x M 1-x The raw materials required for PO4 include Li source, Fe source, M source and phosphorus source; Preferably, the Li source is selected from one or more of lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate and lithium dihydrogen phosphate; Preferably, the Fe source is selected from one or more of ferric phosphate, ferrous phosphate, ferrous oxalate, ferric oxide, ferromanganese phosphate and ferromanganese oxalate; Preferably, the M source is selected from one or more of the oxides, carbonates, phosphates, acetates, nitrates and sulfates of the M metal; Preferably, the phosphorus source is selected from one or more of ferric phosphate, phosphoric acid, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0016] In some embodiments, the synthesis of LiFe x M 1-x Among the raw materials required for PO4, LiFe x M 1-x The PO4 is prepared in a stoichiometric ratio, and the excess coefficient of the Li source is 1~1.15.

[0017] In some embodiments, the synthesis of LiFe x M 1-x The raw materials required for PO4 also include a carbon source.

[0018] Preferably, the carbon source is selected from one or more of glucose, sucrose, polyethylene glycol and carbon nanotubes. x M 1-x Adding a carbon source to the raw materials required for PO4 can control the growth rate of the positive electrode material at high temperature, and form a C coating layer on the surface of the material after sintering, thereby improving the conductivity and electrical properties of the positive electrode material.

[0019] In some embodiments, the protective gas is selected from argon or nitrogen.

[0020] 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 cathode material LiFe x M 1-x PO4 is prepared according to the preparation method provided in the first aspect of this application.

[0021] In some embodiments, the low-impurity phosphate-based cathode material LiFe x M 1-x The content of ferromagnetic substances in PO4 is ≤1ppm; further, the content of ferromagnetic substances is 0.001ppm~1ppm. Preferably, the low-impurity phosphate-based positive electrode material LiFe xM 1-x The content of ferromagnetic substances in PO4 is ≤0.3ppm. It can be understood that the low-impurity phosphate-based positive electrode material LiFe x M 1- x The ferromagnetic substances in PO4 include free iron and iron phosphide.

[0022] The third aspect of the present application provides a positive electrode, which contains a low-impurity phosphate-based positive electrode material LiFe prepared according to the preparation method provided in the first aspect of the present application. x M 1-x PO4.

[0023] The positive electrode in this application may further include a conductive agent and a binder. These agents may be known components in the industry, such as conductive carbon black and PVDF. The content of the conductive agent and binder can be adjusted as needed, for example, 5 wt.% to 15 wt.% of the positive electrode material.

[0024] A fourth aspect of the present application provides a lithium secondary battery, comprising the positive electrode provided in the third aspect of the present application.

[0025] Beneficial effects:

[0026] Traditional LiFe x M 1-x In the preparation method of PO4, the temperature rises during sintering, causing the Li source and C source in the material to decompose and produce gas volatilization. Both Li and C will float up along with the gas volatilization, resulting in excessive reduction of the surface at high temperature, and the generation of ferromagnetic impurities (mainly elemental Fe and iron phosphide). The impurity content will increase with the temperature, seriously affecting the safety performance and stability of the battery. The preparation method of the present application sprays a protective liquid with specific components of phosphorus and titanium elements on the surface of the raw material, which reacts with the floating Li at high temperature, thereby reducing the problem of local Fe excess caused by excessive Li consuming P to produce lithium phosphate, and generating ferromagnetic substances (elemental Fe and iron phosphide) under conditions of reduced C enrichment.

[0027] Furthermore, Ti-containing compounds are enriched in the surface layer and can form active ingredients such as lithium titanium phosphate on the surface of the positive electrode material. They have good stability at high temperatures and can form a surface isolation layer, which can effectively reduce the high-temperature sensitivity of the material and is conducive to the synthesis of high-density materials; at the same time, it can improve the capacity and conductivity; further, it can also inhibit metal dissolution, prevent the battery from self-discharge, and improve battery safety.

[0028] In addition, a special organic P source + Ti source is selected, which contains a C chain with strong adhesion and can be used as a C source. After spraying, it penetrates into the surface of the raw material particles to form a coating layer, which can improve the uniformity and fluidity of the particle surface coating, reduce floating carbon, and improve compaction and electrical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. Obviously, the drawings described below are only some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on the drawings without paying any creative work.

[0030] Figure 1 This is the SEM image of the low-impurity phosphate-based positive electrode material LiFePO4 prepared in Example 1. DETAILED DESCRIPTION

[0031] The embodiments described in this specification are only for explaining the present application and are not intended to limit the present application.

[0032] For simplicity, only some numerical ranges are explicitly disclosed herein. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, and similarly, any upper limit can be combined with any other upper limit to form an unspecified range. In addition, although not explicitly stated, each point or individual value between the endpoints of a range is included in the range. Thus, each point or individual value can serve as its own lower limit or upper limit and be combined with any other point or individual value, or with other lower limits or upper limits, to form an unspecified range.

[0033] In the description of this article, it should be noted that, unless otherwise specified, “above” and “below” are inclusive of the number itself, and the “multiple” in “one or more” and “one or more” means more than two.

[0034] In this application, unless otherwise specified, all steps of the application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, mentioning that the method may also include step (c) indicates that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0035] The present application will be further described below in conjunction with the examples. It should be understood that these examples are intended for illustrative purposes only, 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 by mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing, and the instruments used in the examples are commercially available.

[0036] Example 1.

[0037] S1. Preparation of spray material: According to the stoichiometric ratio of LiFePO4, corresponding masses of iron phosphate, lithium phosphate, glucose, and polyethylene glycol are weighed and dispersed in deionized water to control the solid content of the slurry to 38%. The slurry is sand-milled to a particle size of 0.33 μm and then spray-dried to obtain a spray material, which is then loaded into a sagger.

[0038] S2. Spraying treatment: Weigh isopropyl tri(dioctyl phosphate) titanate and n-butyl titanate at a molar ratio of 1:0.5; mix ethanol solvent at a total mass fraction of 30% of isopropyl tri(dioctyl phosphate) titanate and n-butyl titanate, and disperse evenly to obtain a protective liquid; then pump the protective liquid into the atomizing nozzle while stirring, cover the sagger filled with the spray material obtained in step S1 with a spray hood, start spraying, control the atomizing speed, and spray for 10 seconds to ensure that the protective liquid evenly covers the surface of the spray material.

[0039] S3. Sintering: The material obtained in step S2 is placed in an atmosphere furnace for sintering. The temperature is raised to 810° C. under nitrogen atmosphere and sintered for 10 hours. The furnace is cooled to obtain a low-impurity phosphate-based positive electrode material LiFePO 4 .

[0040] Example 2.

[0041] S1. Prepare spray material: press LiFe 0.4 Mn 0.6 The corresponding mass of iron phosphate, manganese tetraoxide, lithium carbonate, glucose and polyethylene glycol were weighed and dispersed in deionized water in the stoichiometric ratio of PO4. The solid content of the slurry was controlled to be 35%. The slurry was sand-milled to a particle size of 0.25 μm and then spray-dried to obtain a spray material which was loaded into a sagger.

[0042] S2. Spraying treatment: n-butyl titanate and isopropyl tri(dioctyl phosphate) titanate are weighed at a molar ratio of 1:1.5, and an ethanol solvent is mixed so that the total mass fraction of isopropyl tri(dioctyl phosphate) titanate and n-butyl titanate is 30%, and the mixture is evenly dispersed to obtain a protective liquid; then, the protective liquid is pumped into the atomizing nozzle while stirring, and the spray hood is covered over the sagger filled 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 until the protective liquid evenly covers the surface of the spray material.

[0043] S3, sintering: the material obtained in step S2 is placed in an atmosphere furnace for sintering, the temperature is raised to 800 ° C under nitrogen atmosphere protection, sintered for 10 hours, and cooled in the furnace to obtain low-impurity phosphate-based positive electrode material LiFe 0.4 Mn 0.6 PO4.

[0044] Example 3.

[0045] The preparation method of this embodiment is basically the same as that of Example 1, except that in step S2 of this embodiment, n-butyl titanate and isopropyl tri(dioctylphosphoyl) titanate are weighed in a molar ratio of 1:1.5, and other operations and parameters are the same as those of Example 1.

[0046] Example 4.

[0047] The preparation method of this embodiment is basically the same as that of Example 1, with the only difference being that, in step S2 of this embodiment, 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.

[0048] Comparative Example 1.

[0049] S1. Preparation of spray material: According to the stoichiometric ratio of LiFePO4, corresponding masses of iron phosphate, lithium phosphate, glucose, and polyethylene glycol are weighed and dispersed in deionized water to control the solid content of the slurry to 38%. The slurry is sand-milled to a particle size of 0.33 μm and then spray-dried to obtain a spray material, which is then loaded into a sagger.

[0050] S2. Sintering: The material obtained in step S1 is placed in an atmosphere furnace for sintering. The temperature is raised to 810° C. under nitrogen atmosphere and sintered for 10 hours. The furnace is cooled to obtain a low-impurity phosphate-based positive electrode material LiFePO 4 .

[0051] Comparative Example 2.

[0052] The preparation method of this comparative example is basically the same as that of Example 1, except that isopropyl tri(dioctylphosphoyloxy) titanate is not added to the protective solution prepared in step S2 of this comparative example, and only n-butyl titanate is added. Other operations and parameters are the same as those in Example 1.

[0053] Comparative Example 3.

[0054] The preparation method of this comparative example is basically the same as that of Example 1, except that n-butyl titanate is not added to the protective solution prepared in step S2 of this comparative example, and only isopropyl tri(dioctylphosphoyl) titanate is added; other operations and parameters are the same as those in Example 1.

[0055] Product performance test items and methods: Test item 1: Micromorphology test.

[0056] The microstructure of the low-impurity phosphate-based cathode material LiFePO4 prepared in Example 1 was observed by scanning electron microscopy. Figure 1 shown.

[0057] Test item 2: Powder compaction test.

[0058] Use an automatic compaction density meter to perform powder compaction test, accurately weigh 1g of sample, fill the sample into the test mold, adjust the test pressure to 3000kg, press the start button to start the test, maintain the pressure for 10s to complete the test, and read the compaction density data.

[0059] Test Item 3: Ferromagnetic material test.

[0060] Weigh 150g of cathode material into a polyester can. Add 1000mL of pure water and a magnetic bar with a magnetic field strength greater than 5000 gauss. After sealing, place the can on a rolling mill and stir at 90 rpm for 30 minutes. After stirring, remove the magnetic bar. Place the magnetic bar in a clean beaker, add 15mL of HCl and 50mL of deionized water, and heat to dissolve any metal adhering to the magnetic bar. Then, measure the ferromagnetic content using an ICP5800 instrument.

[0061] Test Item 4: Electrical performance test.

[0062] (1) Preparation of positive electrode: The positive electrode materials prepared by the above method in Examples 1 to 4 and Comparative Examples 1 to 3 were used as positive electrode active materials. The positive electrode active materials, superconducting carbon black (SP), and polyvinylidene fluoride (PVDF) were homogenized in a mass ratio of 90:5:5 and coated on a 20 μm thick aluminum foil to produce a positive electrode sheet with an area density of 8 mg / cm 2 , and then through drying, rolling, die cutting, punching into positive electrode sheets.

[0063] (2) Battery preparation: R2032 button cell battery shells were used for button cell assembly. A lithium sheet was used as the negative electrode, a PE separator was used, and 80 μmL of electrolyte was added. The electrical performance test temperature was 25°C, the test voltage range was 2.0V to 4.3V, and the battery was charged to 4.3V using a constant current constant voltage charge mode and discharged to 2.0V using a constant current discharge mode. The charge and discharge currents for the first four cycles were 0.1C, 0.2C, 0.5C, and 1C, respectively. The battery was then charged and discharged at a current of 1C for 200 cycles.

[0064] The specific test results of the above performance test are shown in Table 1.

[0065]

[0066] From the test results in Table 1, it can be seen that the positive electrode materials prepared in Examples 1 to 4 using the technical solution of the present application have a higher compaction density, and the content of ferromagnetic substances on the surface of the positive electrode materials is significantly reduced. The lithium-ion batteries prepared using the positive electrode materials prepared in Examples 1 to 3 have a higher discharge capacity; after 200 cycles, the capacity retention rate still reaches more than 93%. It can be seen that the protective liquid with specific components in the present application forms a protective layer on the surface of the raw material, which can effectively inhibit the side reactions that are easily generated during high-temperature sintering, reduce the generation of impurities, and improve the electrochemical activity and structural stability of the positive electrode material.

[0067] The above description is merely a specific embodiment of the present application, but the scope of protection of the present application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present application, and such modifications or substitutions should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

Claims

1. In a low-impurity phosphate-based cathode material LiFe x M 1-x The preparation method of PO4 is characterized in that: The steps include: The synthesis of LiFe x M 1-x The raw materials required for PO4 are filled into the sagger to form a raw material layer; wherein M is selected from transition metal elements, and 0 <x≤1; 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; Wherein, M is selected from transition metal elements, and 0 <x≤1; The protective liquid includes at least one of n-butyl titanate and titanium dioxide, and a titanium phosphate compound; The sintering temperature is 700° C. to 1000° C., and the sintering time is 5 hours to 15 hours.

2. The preparation method according to claim 1, characterized in that The titanium phosphate compound is isopropyl tris (dioctyl phosphate acyloxy) titanate.

3. The preparation method according to claim 2, characterized in that The molar ratio of the total amount of n-butyl titanate and titanium dioxide to isopropyl tris(dioctylphosphooxy) titanate in the protective solution is 1:(0.2-5).

4. The preparation method according to any one of claims 1 to 3, characterized in that The preparation method satisfies at least one of the following conditions: (1) The total mass fraction of n-butyl titanate, titanium dioxide and titanium phosphate compounds in the protective solution is 10% to 50%; (2) The solvent in the protective solution is isopropanol or ethanol; (3) When spraying the protective liquid on the surface of the raw material layer, the spraying speed is 5 mL / min~50 mL / min, and the spraying time is 5 s~100 s.

5. The preparation method according to any one of claims 1 to 3, 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. The preparation method according to any one of claims 1 to 3, characterized in that The synthetic LiFe x M 1-x The raw materials required for PO4 include Li source, Fe source, M source and phosphorus source; among them, 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 ferric phosphate, ferrous phosphate, ferrous oxalate, ferric oxide, ferromanganese phosphate and ferromanganese oxalate; The M source is selected from one or more of the oxides, carbonates, phosphates, acetates, nitrates and sulfates of the M metal; The phosphorus source is selected from one or more of ferric phosphate, phosphoric acid, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

7. The preparation method according to any one of claims 1 to 3, characterized in that The raw materials also include a carbon source; the carbon source is selected from one or more of glucose, sucrose, polyethylene glycol and carbon nanotubes.

8. 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 7.

9. 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 7 x M 1-x PO4.

10. A lithium secondary battery, characterized in that: Comprising the positive electrode as claimed in claim 9.

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