LiFePO4-Li5FeO4 in-situ composite cathode material and preparation and application thereof

Abstract

The present invention belongs to the field of battery positive electrode materials, and specifically relates to a method for preparing a LiFePO4-Li5FeO4 in-situ composite positive electrode material. 2+ Source, Fe 3+ The raw material solution of the lithium source, ionic surfactant, lithium source and phosphorus source is hydrothermally treated to obtain a precursor, and then the precursor is calcined to prepare the LiFePO4-Li5FeO4 in-situ composite positive electrode material; in the raw material solution, Fe 3+ 、Fe 2+ The element ratio of Li:Fe is 0.02~0.1:1; the element ratio of Li:Fe is 1.15~1.6:1; 2+ The molar ratio of :P is 1:0.95-1.1. The present invention also includes materials prepared by the preparation method and their application in lithium-ion batteries. The solution of the present invention can obtain active materials with excellent electrochemical properties.

Description

Technical Field

[0001] The present invention relates to the field of lithium battery positive electrode materials, and in particular to the field of modification of lithium iron phosphate materials. Background Art

[0002] Lithium-ion batteries are currently the most promising and fastest-growing high-efficiency secondary batteries. They have many advantages, such as high specific energy, low self-discharge, good cycle performance, and no memory effect. However, during the first charge and discharge process of lithium-ion batteries, a SEI film will form at the interface of the negative electrode material. The formation of SEI is an irreversible process. The Lithium used to form SEI + During the discharge process, it can no longer be embedded in the positive electrode material, resulting in a loss of battery capacity.

[0003] The study found that the formation of SEI film consumes part of the Li in the positive electrode material. + , which in turn leads to irreversible capacity loss of the electrode material. Therefore, this capacity loss can be compensated by pre-replenishing lithium. There are two main types of pre-replenishing lithium technologies: one is the negative electrode material lithium replenishment technology, which has high requirements for the operating environment and is difficult to commercialize; the other is the positive electrode material lithium replenishment technology, which has relatively low requirements and a simple method, and the lithium replenishment agent generally uses an inverse fluorite structure lithium-rich positive electrode material Li X Among them, lithium ferrite (Li5FeO4) has the advantages of simple synthesis process, low material price and high safety of lithium supplementation, and is the preferred choice of lithium supplementation agent.

[0004] The current conventional practice is to add positive electrode lithium supplements to the positive electrode slurry. This method will occupy the usage of positive electrode active materials, thereby affecting the battery energy density, resulting in the problem of insignificant battery capacity improvement effect, and will also increase the impedance of the positive electrode surface in the later stage. Summary of the Invention

[0005] The present invention aims to provide a method for preparing a LiFePO4-Li5FeO4 in-situ composite positive electrode material, aiming to prepare a LiFePO4-Li5FeO4 in-situ composite active material.

[0006] The second object of the present invention is to provide a LiFePO4-Li5FeO4 in-situ composite positive electrode material prepared by the preparation method and its application in lithium ion batteries.

[0007] The third object of the present invention is to provide a lithium ion battery comprising the LiFePO4-Li5FeO4 in-situ composite positive electrode material.

[0008] Li5FeO4 has high initial efficiency, and is often used as a lithium supplement, however, existing means often slurry composite or coating with active material, so it is difficult to effectively exert its performance. Therefore, the inventors first attempt to realize LiFePO4-Li5FeO4 in-situ composite in one pot synthesis, however, early research found that one pot in-situ composite process is easy to cause Fe 3+ Doping of lithium iron phosphate lattice, so not only will not improve performance, but also limit the performance of the material. Therefore, in view of the problem that it is difficult to obtain LiFePO4-Li5FeO4 in-situ composite phase with high selectivity in one pot synthesis process, the present application provides the following improvement scheme:

[0009] A preparation method of LiFePO4-Li5FeO4 in-situ composite positive electrode material, the raw material solution containing Fe 2+ source, Fe 3+ source, ionic surfactant, lithium source and phosphorus source is subjected to hydrothermal treatment to obtain a precursor, and then the precursor is subjected to calcination treatment to prepare the LiFePO4-Li5FeO4 in-situ composite positive electrode material;

[0010] In the raw material solution, the element ratio of Fe 3+ and Fe 2+ is 0.02-0.1:1; the element ratio of Li:Fe is 1.15-1.6:1; the molar ratio of Fe 2+ :P is 1:0.95-1.1.

[0011] The present application first provides a one-pot in-situ synthesis idea, further through the joint control of ionic surfactant and various parameters, solves the problem of difficult formation of LiFePO4-Li5FeO4 double phase and difficult lattice level composite in one-pot in-situ synthesis process, can successfully obtain LiFePO4-Li5FeO4 in-situ composite positive electrode material, and the material has excellent electrochemical performance.

[0012] In the present application, the one-pot in-situ synthesis idea and the joint control of ionic surfactant, the element ratio of Fe 3+ and Fe 2+ , Li:Fe, Fe 2+ :P are the key to synergistically improve the LiFePO4-Li5FeO4 double composite phase, reduce hybridization and improve performance.

[0013] In the present application, the Fe 2+ source and the Fe 3+ source are water-soluble salts of respective metal ions; further preferably at least one of sulfate, acetate and chloride;

[0014] Preferably, the Fe 3+: Fe 2+ The molar ratio of Fe

[0015] In the present application, the ionic surfactant is one or more of sodium dodecyl sulfate, sodium glycocholate, sodium dodecyl benzene sulfonate, benzalkonium chloride and benzalkonium bromide.

[0016] Preferably, the ionic surfactant is Fe 2+ source, and the total mass of Fe 3+ source is 1-10wt%, and further preferably 3-6wt%.

[0017] Preferably, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium nitrate and lithium acetate.

[0018] Preferably, the elemental ratio of Li:Fe (referring to Li / (Fe 2+ + Fe 3+ ) molar ratio) is 1.2-1.45:1.

[0019] Preferably, the phosphorus source is at least one of phosphoric acid, ammonium phosphate, sodium phosphate, potassium phosphate, monobasic ammonium phosphate, monobasic sodium phosphate, monobasic potassium phosphate, dibasic ammonium phosphate, dibasic sodium phosphate and dibasic potassium phosphate.

[0020] Preferably, the molar ratio of Fe 2+ :P is 1:1-1.1.

[0021] Preferably, the solvent in the raw material solution is water or a mixture of water and an organic solvent.

[0022] Preferably, the raw material solution is subjected to bubbling treatment with nitrogen or inert gas to replace the air in the raw material solution and the air in the hydrothermal reaction system.

[0023] In the present application, the Fe 2+ source, the Fe 3+ source and the ionic surfactant can be dissolved in water to form solution A, and the lithium source and the phosphorus source can be dissolved in water to form solution B, and then solution A and solution B are mixed to form the raw material solution.

[0024] The concentration of the iron source in the raw material solution is not particularly limited, and for example, can be 50-150g / L, and further can be 90-100g / L.

[0025] Preferably, the temperature of the hydrothermal treatment is 120-260℃, and preferably 150-230℃.

[0026] Preferably, the time of the hydrothermal treatment is 6-24h, and further preferably 15-20h.

[0027] In the present application, after the hydrothermal treatment, the solid is washed with water;

[0028] Preferably, the water washing is until the conductivity of the washing water is less than 20 μS / cm;

[0029] Preferably, the precursor is prepared by vacuum drying or spray drying after the washing;

[0030] Preferably, the calcination process is carried out in a protective atmosphere;

[0031] Preferably, the calcination process is a two-stage gradient calcination process, which comprises a T1 stage holding process and a T2 stage holding process, wherein the temperature of T1 is 720-780°C, and further preferably 750-780°C; the temperature of T2 is 800-880°C, and further preferably 830-860°C;

[0032] Preferably, the holding time at T1 stage is 5-10 h;

[0033] The holding time at T2 stage is 5-10 h;

[0034] Preferably, the LiFePO4-Li5FeO4 in-situ composite cathode material is prepared by airflow crushing and sieving after the calcination.

[0035] A preferred method for preparing the LiFePO4-Li5FeO4 in-situ composite cathode material of the present application comprises the following steps:

[0036] (1) mixing divalent iron source, trivalent iron source and ionic surfactant in an aqueous solvent to obtain a mixed solution A; the molar ratio of Fe 3+ : Fe 2+ is 0.03-0.08:1; the ionic surfactant comprises one or more of sodium dodecyl sulfate, sodium glycocholate, sodium dodecyl benzene sulfonate, benzalkonium chloride and benzalkonium bromide; the ionic surfactant is 1-10 wt% of the mass of the iron source (including divalent and trivalent iron source).

[0037] (2) mixing lithium source and phosphorus source in an aqueous solvent to obtain a mixed solution B; the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium nitrate and lithium acetate; the molar ratio of Li:(Fe 2+ + Fe 3+ ) is 1.2-1.45:1; the phosphorus source is at least one of phosphoric acid, ammonium phosphate, sodium phosphate, potassium phosphate, monobasic ammonium phosphate, monobasic sodium phosphate, monobasic potassium phosphate, dibasic ammonium phosphate, dibasic sodium phosphate and dibasic potassium phosphate; the molar ratio of Fe 2+ :P is 1:1-1.1;

[0038] (3) the mixed solution A and the mixed solution B are added into a high-pressure reaction kettle, inert gas is introduced into the kettle to exhaust the solution and air in the kettle, and then the hydrothermal reaction is carried out under stirring, the hydrothermal reaction temperature is 120-260 DEG C, the pressure is 0.3-3 MPa, and the reaction is carried out for 6-24 hours, then the pressure is released and cooled, the slurry is poured out, and after water washing, drying, sintering and sieving, the LiFePO4-Li5FeO4 composite positive electrode material is obtained.

[0039] Preferably, in step (3), the washing is stopped when the conductivity of the washing water is less than 20 μS / cm.

[0040] The drying is vacuum drying or spray drying, and the spray drying uses nitrogen or argon as the gas source.

[0041] The sintering is carried out in a nitrogen or argon atmosphere, the first-stage sintering temperature is 720-780 DEG C, the sintering time is 5-10 hours, the second-stage sintering temperature is 800-880 DEG C, and the sintering time is 5-10 hours.

[0042] The application also provides the LiFePO4-Li5FeO4 in-situ composite positive electrode material prepared by the preparation method.

[0043] The application benefits from the innovative one-pot preparation method, which can endow the material with special microstructure characteristics, and the preparation method can obtain better performance.

[0044] The application also provides a lithium ion battery containing the LiFePO4-Li5FeO4 in-situ composite positive electrode material.

[0045] The application has the following beneficial effects:

[0046] The application provides a LiFePO4-Li5FeO4 in-situ composite idea, and based on the joint control of the preparation process and parameters, many preparation problems in the one-pot in-situ composite process can be solved, and a new in-situ composite material with excellent electrochemical performance can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 The X-ray diffraction pattern of the LiFePO4-Li5FeO4 composite positive electrode material obtained in Example 1 of the application; DETAILED DESCRIPTION

[0048] The technical solutions of the present application are further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations on the present application.

[0049] Example 1

[0050] S1 according to the mole of Fe 3+ : Fe 2+ of 0.05:1, iron sulfate (10 g) and ferrous sulfate (154 g) are weighed, and they and 5 g of sodium dodecyl sulfate are dissolved in 1000 g of deionized water to obtain a mixed solution A;

[0051] S2 according to the mole of Li:(Fe 2+ + Fe 3+ ) = 1.3:1, lithium hydroxide 32.68 g is weighed, and according to the mole of Fe 2+ : P = 1:1, 115 g of ammonium dihydrogen phosphate is dissolved in 800 g of deionized water to obtain a mixed solution B;

[0052] S3 The mixed solution A and the mixed solution B are added together into a high-pressure reaction kettle, nitrogen is introduced into the kettle to exhaust the solution and the air in the kettle, and under stirring, hydrothermal reaction is carried out at a hydrothermal reaction temperature of 200℃ for 15 h, and then pressure relief cooling is carried out, the slurry is poured out, and water washing is carried out until the conductivity is 12 μS / cm, then the material is vacuum dried and placed in a nitrogen-protected atmosphere furnace to sinter at 760℃ for 7 h, and then the temperature is increased to 860℃ for sintering for 6 h;

[0053] S4 The sintered material is subjected to airflow crushing, the crushing gas source is nitrogen, the crushing D50 is controlled to be 1.2 μm, and the crushed material is passed through a 200 mesh screen to obtain a LiFePO4·Li5FeO4 composite positive electrode material, and the XRD of the product is shown in Figure 1 , it can be seen that the LiFePO4·Li5FeO4 composite phase is formed, and no doping is formed.

[0054] Example 2

[0055] S1 according to the mole of Fe 3+ : Fe 2+ = 0.03:1, iron sulfate 6 g and ferrous acetate 174 g are weighed, and at the same time, 10 g of sodium dodecylbenzenesulfonate is dissolved in 1100 g of deionized water to obtain a mixed solution A;

[0056] S2 according to the mole of Li:(Fe 2+ + Fe 3+ ) = 1.2:1, lithium hydroxide 30.17 g is weighed, and according to the mole of Fe 2+ : P = 1:1.01, 116.15 g of ammonium dihydrogen phosphate is dissolved in 800 g of deionized water to obtain a mixed solution B;

[0057] S3mixing solution A and mixing solution B are added into a high-pressure reaction kettle, nitrogen is introduced into the kettle to exhaust the solution and air in the kettle, hydrothermal reaction is carried out under stirring, the hydrothermal reaction temperature is 230℃, reaction is carried out for 18h, then pressure relief cooling is carried out, the slurry is poured out, water washing is carried out until the conductivity is 18μS / cm, then the process is stopped, the material is vacuum dried, then it is placed in a nitrogen-protected atmosphere furnace, sintering is carried out at 750℃ for 6h, then the temperature is increased to 850℃, sintering is carried out for 8h;

[0058] S4the sintered material is subjected to air flow crushing, the crushing gas source is nitrogen, the crushing D50 is controlled to be 1.5μm, the crushed material is passed through a 200-mesh screen to obtain the LiFePO4·Li5FeO4 composite positive electrode material.

[0059] Example 3

[0060] S1Fe 3+ :Fe 2+ of 0.08:1, 16g of iron sulfate and 174g of ferrous acetate are weighed, 10g of benzalkonium chloride is dissolved in 1200g of deionized water to obtain a mixed solution A;

[0061] S2Li:(Fe 2+ +Fe 3+ ) of 1.45:1, 36.46g of lithium hydroxide is weighed, ammonium dihydrogen phosphate of Fe 2+ :P of 1:1.1 is weighed, 115g of ammonium dihydrogen phosphate is dissolved in 800g of deionized water to obtain a mixed solution B;

[0062] S3mixing solution A and mixing solution B are added into a high-pressure reaction kettle, nitrogen is introduced into the kettle to exhaust the solution and air in the kettle, hydrothermal reaction is carried out under stirring, the hydrothermal reaction temperature is 150℃, reaction is carried out for 20h, then pressure relief cooling is carried out, the slurry is poured out, water washing is carried out until the conductivity is 15μS / cm, then the process is stopped, the material is subjected to spray drying (nitrogen is used as the gas source), then it is placed in a nitrogen-protected atmosphere furnace, sintering is carried out at 780℃ for 9h, then the temperature is increased to 830℃, sintering is carried out for 7h;

[0063] S4the sintered material is subjected to air flow crushing, the crushing gas source is nitrogen, the crushing D50 is controlled to be 0.8μm, the crushed material is passed through a 150-mesh screen to obtain the LiFePO4·Li5FeO4 composite positive electrode material.

[0064] Comparative Example 1

[0065] Compared with Example 1, the only difference is that in step 1, no trivalent iron source is added, lithium iron phosphate is obtained in step 3, then it is physically mixed with equimolar Li5FeO4 to obtain the composite active material.

[0066] Comparative Example 2

[0067] Comparative Example 1

[0068] Comparative Example 3

[0069] Comparative Example 1

[0070] Comparative Example 4

[0071] Comparative Example 1 2+ +Fe 3+ ) is 1.08:1. Other operations and parameters are the same as those in Example 1.

[0072] Electrical performance test

[0073] (1) Preparation of the positive electrode sheet:

[0074] The positive electrode materials prepared by the above method in Examples 1-3 and Comparative Examples 1-4 are used as the positive electrode active material. The positive electrode active material: SP (superconducting carbon black): PVDF (polyvinylidene fluoride) is uniformly mixed in a mass ratio of 90:5:5, coated on a 20 μm thick aluminum foil, and then dried, rolled, die-cut, and punched into a positive electrode sheet with a surface density of 8 mg / cm 2

[0075] (2) Preparation of the battery: A button cell shell of R2032 is used for the button cell assembly, a lithium sheet is used as the negative electrode, a separator made of PE is used, and 85 μL of electrolyte is added dropwise. The test temperature is 25°C, the test voltage range is 2.0V-3.9V, the charging is performed in a constant current and constant voltage manner to 3.9V, the discharging is performed in a constant current manner to 2.0V, the first four cycles of charging and discharging currents are 0.1C, 0.2C, 0.5C, and 1C, respectively, and the subsequent cycles are performed at a 1C charging and discharging current, and the cycle is 200 times. The test results are shown in Table 1.

[0076] Table 1: Chemical performance results of each case

[0077]

[0078] As can be seen from the results in Table 1, the LiFePO4·Li5FeO4 composite positive electrode material prepared by the method described in the present application has improved 1C initial discharge specific capacity and 1C charge-discharge cycle capacity retention rate after being prepared into a lithium ion battery. ​

[0079] Obviously, the above-mentioned embodiments are only examples for clearly illustrating the present application, and are not intended to limit the present application. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, it is not necessary and also impossible to enumerate all the embodiments. The obvious changes or variations derived from the above description are still within the protection scope of the present application.

Claims

1. A method for preparing a LiFePO4-Li5FeO4 in-situ composite positive electrode material, characterized in that: Will contain Fe 2+ Source, Fe 3+ The raw material solution of the lithium source, ionic surfactant, lithium source and phosphorus source is hydrothermally treated to obtain a precursor, and then the precursor is calcined to prepare the LiFePO4-Li5FeO4 in-situ composite positive electrode material; In the raw material solution, Fe 3+ 、Fe 2+ The element ratio of Li:Fe is 0.02~0.1:1; the element ratio of Li:Fe is 1.2~1.45:1; 2+ The molar ratio of :P is 1:0.95~1.1; Fe 2+ Source, Fe 3+ The sources are water-soluble salts of the respective metal ions; The ionic surfactant is one or more of sodium lauryl sulfate, sodium glycocholate, benzalkonium chloride and benzalkonium bromide; Ionic surfactant is Fe 2+ Source, Fe 3+ 1 to 10 wt% of the total iron source mass; The raw material solution is bubbled with nitrogen or inert gas to replace the air in the raw material solution and the air in the hydrothermal reaction system; The roasting process is a two-stage gradient roasting process, which includes a T1 heat preservation process and a T2 heat preservation process, wherein the temperature of T1 is 720-780°C; the temperature of T2 is 800-880°C.

2. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, characterized in that: Fe 2 + Source, Fe 3+ The source is at least one of sulfate, acetate, and chloride of the respective metal ions.

3. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, characterized in that: Fe 3 + :Fe 2+ The molar ratio is 0.03-0.08:

1.

4. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, wherein: The lithium source is at least one of lithium carbonate, lithium hydroxide, lithium nitrate and lithium acetate.

5. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, wherein: The phosphorus source is at least one of phosphoric acid, ammonium phosphate, sodium phosphate, potassium phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, potassium monohydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate and potassium dihydrogen phosphate.

6. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, wherein: Fe 2 + The molar ratio of :P is 1:1~1.

1.

7. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, wherein: The solvent in the raw material solution is water or a mixed solvent of water and an organic solvent.

8. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, wherein: The hydrothermal temperature is 120-260°C.

9. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 8, characterized in that: The hydrothermal temperature is 150~230℃.

10. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, characterized in that: The hydrothermal treatment time is 6~24h.

11. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, wherein: After the hydrothermal treatment, the solid is washed with water; Wash with water until the conductivity of the washing water is less than 20μS / cm; After washing, the precursor is obtained by vacuum drying or spray drying.

12. The method for preparing the LiFePO4-Li5FeO4 in-situ composite positive electrode material according to claim 1, wherein: The calcination process is carried out under a protective atmosphere; The holding time under section T1 is 5 to 10 hours; The holding time under T2 stage is 5 to 10 hours; After calcination, the mixture is air flow crushed and sieved to obtain the LiFePO4-Li5FeO4 in-situ composite positive electrode material.

13. A LiFePO4-Li5FeO4 in-situ composite positive electrode material prepared by the preparation method according to any one of claims 1 to 12.

14. An application of a LiFePO4-Li5FeO4 in-situ composite cathode material prepared by the preparation method according to any one of claims 1 to 12, characterized in that: It is used as a positive electrode active material to prepare lithium-ion batteries.

15. The use according to claim 14, characterized in that It is used as an active material, combined with a conductive agent and a binder, to prepare the positive electrode of a lithium-ion battery.

16. A lithium ion battery, characterized in that: A LiFePO4-Li5FeO4 in-situ composite positive electrode material prepared by the preparation method according to any one of claims 1 to 12.

17. The lithium ion battery according to claim 16, wherein: The positive electrode contains the LiFePO4-Li5FeO4 in-situ composite positive electrode material.

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