CN115939356A - A LiFePO4-Li5FeO4 in-situ composite cathode material and its preparation and application - Google Patents

Abstract

The invention belongs to the field of battery anode materials, and particularly relates to LiFePO ₄ ‑Li ₅ FeO ₄ The preparation method of the in-situ composite cathode material comprises the following steps of ²⁺ Source, fe ³⁺ Carrying out hydrothermal treatment on raw material solutions of a source, an ionic surfactant, a lithium source and a phosphorus source to obtain a precursor, and then roasting the precursor to prepare the LiFePO ₄ ‑Li ₅ FeO ₄ Compounding the positive electrode material in situ; in the raw material solution, fe ³⁺ 、Fe ²⁺ The element ratio of (A) is 0.02 to 0.1; the element ratio of Li to Fe is 1.15-1.6; fe ²⁺ : the molar ratio of P is 1:0.95 to 1.1. The invention also comprises the material prepared by the preparation method and the application of the material in lithium ion batteries. According to the scheme, the active material with excellent electrochemical performance can be obtained.

Description

A LiFePO4-Li5FeO4 in-situ composite cathode material and its preparation and application

technical field

The invention relates to the field of positive electrode materials for lithium batteries, in particular to the field of modification of lithium iron phosphate materials.

Background technique

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. An SEI film will be formed at the interface of the negative electrode material, and the formation of SEI is an irreversible process. The Li ⁺ used to form SEI can no longer be embedded in the positive electrode material during the discharge process, resulting in a loss of battery capacity.

Studies have found that the formation of SEI film will consume a part of Li ⁺ in the cathode material, which in turn leads to irreversible capacity loss of the electrode material. Therefore, this part of capacity loss can be compensated by pre-supplementing lithium. Pre-supplementary lithium technology is mainly divided into two types, one is negative electrode material lithium supplement technology, which has high requirements on the operating environment and is difficult to commercialize; the other is positive electrode material lithium supplement technology, which has relatively low technical requirements. It is simple, and the lithium replenishing agent generally uses Li _X MO ₄ (M=Fe, Co, Mn), a lithium-rich positive electrode material with an inverse fluorite structure. Among them, the lithium ferrite (Li ₅ FeO ₄ ) lithium supplement has the advantages of simple synthesis process, low material price, and high safety of lithium supplement, and is the preferred choice of lithium supplement.

The current conventional method is to add a positive electrode lithium supplement to the positive electrode slurry. This method will occupy the usage of the positive electrode active material, which will affect the energy density of the battery, resulting in the problem that the effect of improving the battery capacity is not obvious, and the surface of the positive electrode will be increased later. impedance.

Contents of the invention

The purpose of the present invention is to provide a method for preparing a LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material, aiming at preparing an active material of LiFePO ₄ -Li ₅ FeO ₄ in-situ composite.

The second purpose of the present invention is to provide the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material prepared by the preparation method and its application in lithium ion batteries.

The third object of the present invention is to provide a lithium ion battery comprising the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite cathode material.

Li ₅ FeO ₄ has a high first effect and is often used as a lithium supplement. However, it is often slurry-composited or coated with active materials in existing methods, so it is difficult to effectively exert its performance. For this reason, the inventors tried for the first time in the industry a one-pot synthesis method to realize LiFePO ₄ -Li ₅ FeO ₄ in-situ recombination. However, early research found that the one-pot in-situ recombination process is likely to cause Fe ³⁺ to LiFePO4. Lattice doping, so not only does not improve the performance, but limits the performance of the material. Therefore, to solve the problem that it is difficult to obtain LiFePO ₄ -Li ₅ FeO ₄ in-situ composite phase with high selectivity in a one-pot synthesis process, the present invention provides the following improvement scheme:

A preparation method of LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material, the raw material solution containing Fe ²⁺ source, Fe ³⁺ source, ionic surfactant, lithium source, phosphorus source is subjected to hydrothermal treatment to obtain precursor body, and then roasting the precursor to prepare the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material;

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.

The present invention provides the idea of one-pot in-situ synthesis for the first time, and further solves the problem of difficult formation of LiFePO ₄ -Li ₅ FeO ₄ dual phases and difficulties at the lattice level in the process of one-pot in-situ synthesis through the combined control of ionic surfactants and various parameters. Combination problem, can successfully obtain LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material, and the material has excellent electrochemical performance.

In the present invention, the idea of one-pot in-situ synthesis and the joint control of the ionic surfactant, the element ratio of Fe ³⁺ and Fe ²⁺ , Li:Fe, Fe ²⁺ :P are synergistically improved to obtain the LiFePO ₄ -Li ₅ FeO ₄ double complex phase, the key to reducing hybridization and improving performance.

In the present invention, Fe ²⁺ source, Fe ³⁺ source are water-soluble salts of respective metal ions; more preferably at least one of sulfate, acetate, chloride;

Preferably, the molar ratio of Fe ³⁺ : Fe ²⁺ is 0.03˜0.08:1.

In the present invention, the ionic surfactant is one or more of sodium lauryl sulfate, sodium glycocholate, sodium dodecylbenzenesulfonate, benzalkonium chloride and benzalkonium bromide ;

Preferably, the ionic surfactant is 1-10 wt% of the total iron source mass of Fe ²⁺ source and Fe ³⁺ source, more preferably 3-6 wt%.

As preferably, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium acetate;

Preferably, the element ratio of Li:Fe (referring to Li/(Fe ²⁺ +Fe ³⁺ ) molar ratio) is 1.2˜1.45:1.

Preferably, the phosphorus source is 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 at least one of

Preferably, the molar ratio of Fe ²⁺ : P is 1:1˜1.1.

Preferably, the solvent in the raw material solution is water, or a mixed solvent of water-organic solvent.

Preferably, the raw material solution is bubbled with nitrogen or an inert gas to replace the air in the raw material solution and the air in the hydrothermal reaction system.

In the present invention, Fe2 ⁺ source, Fe3 ⁺ source, ionic surfactant can be dissolved in water to form solution A, lithium source and phosphorus source can be dissolved in water to obtain solution B, and solution A and solution B are mixed to form the solution A. raw material solution. During the solubilization process, it is preferable to perform bubbling deoxygenation treatment on the solution, especially the raw material solution.

In the raw material solution, the concentration of the iron source is not particularly required, for example, it may be 50-150 g/L, and further may be 90-100 g/L.

Preferably, the hydrothermal temperature is 120-260°C, preferably 150-230°C;

Preferably, the hydrothermal treatment time is 6-24 hours, more preferably 15-20 hours.

In the present invention, after the hydrothermal treatment, the solid is washed with water;

Preferably, washing with water until the conductivity of the washing water is lower than 20 μS/cm;

Preferably, vacuum drying or spray drying is performed after washing to obtain the precursor;

Preferably, the firing process is carried out under a protective atmosphere;

Preferably, the roasting process is a two-stage gradient roasting process, which includes a T1-stage heat preservation process and a T2-stage heat preservation process, wherein the temperature of T1 is 720-780°C, more preferably 750-780°C; the temperature of T2 is 800-880°C °C, more preferably 830-860 °C;

Preferably, the holding time under the T1 section is 5-10 hours;

The holding time under T2 section is 5~10h;

Preferably, the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material is obtained by jet crushing and sieving after calcination.

A preferred preparation method of LiFePO ₄ -Li ₅ FeO ₄ in situ composite cathode material of the present invention comprises the following steps:

(1) Mix ferrous iron source, ferric iron source, and ionic surfactant in an aqueous solvent to obtain 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 lauryl sulfate, sodium glycocholate, sodium dodecylbenzenesulfonate, benzalkonium chloride and benzalkonium bromide; the ionic surfactant It is 1-10 wt% of the mass of iron source (including divalent and trivalent iron source).

(2) Lithium source and phosphorus source are mixed in an aqueous solvent to obtain mixed solution B; the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium acetate; Li:(Fe ^{2 +} +Fe ³⁺ ) in a molar ratio of 1.2 to 1.45:1; the source of phosphorus is phosphoric acid, ammonium phosphate, sodium phosphate, potassium phosphate, ammonium monohydrogen phosphate, sodium monohydrogen phosphate, potassium monohydrogen phosphate, dihydrogen phosphate At least one of ammonium hydrogen, sodium dihydrogen phosphate and potassium dihydrogen phosphate; the molar ratio of Fe ²⁺ : P is 1:1～1.1;

(3) Add mixed solution A and mixed solution B together into the autoclave, pass inert gas into the autoclave to exhaust the solution and the air in the autoclave, under stirring conditions, hydrothermal reaction, hydrothermal reaction temperature is 120 React at ~260°C and a pressure of 0.3~3MPa for 6~24h, then release the pressure and cool, pour out the slurry, wash with water, dry, sinter, pulverize and sieve to obtain LiFePO ₄ ·Li ₅ FeO ₄ composite cathode material.

Preferably, in step (3), the washing is stopped until the conductivity of the washing water is lower than 20 μS/cm;

The drying described is vacuum drying or spray drying, and the spray drying adopts nitrogen or argon as the gas source;

According to the complaint, sintering is carried out in stages under a nitrogen or argon atmosphere. The temperature of the first stage of sintering is 720-780°C, and the sintering time is 5-10h; the temperature of the second stage of sintering is 800-880°C, and the sintering time is For 5 ~ 10h. In step (3), in the airflow pulverization process, the D50 of the pulverized material is maintained at 0.5-2.5 μm; the sieving adopts an ultrasonic vibrating sieve, and the sieve mesh has an aperture of 100-300 mesh.

The invention also provides the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite cathode material prepared by the preparation method.

The present invention 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.

The present invention also provides a lithium ion battery, comprising LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material; more preferably, the positive electrode includes the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material.

The beneficial effects of the present invention are:

The present invention provides an idea of LiFePO ₄ ·Li ₅ FeO ₄ in-situ compounding, and based on the joint control of the preparation process and parameters, it can solve many preparation problems faced by the one-pot in-situ compounding process, and can obtain excellent electrochemical properties of in situ composite new materials.

Description of drawings

Fig. 1 is the X-ray diffraction diagram of the LiFePO ₄ ·Li ₅ FeO ₄ composite cathode material obtained in Example 1 of the present invention;

Detailed ways

The technical solutions of the present invention will be further described below through specific embodiments. It should be clear to those skilled in the art that the embodiments are only for helping to understand the present invention, and should not be regarded as specific limitations on the present invention.

Example 1

S1 Weigh iron sulfate (10g) and ferrous sulfate (154g) according to the mole of Fe ³⁺ : Fe ²⁺ =0.05:1, dissolve it and sodium lauryl sulfate 5g in 1000g deionized water to obtain mixed solution A ;

S2 Weigh 32.68g of lithium hydroxide according to the molar ratio of Li:(Fe ²⁺ +Fe ³⁺ )=1.3:1, weigh 115g of ammonium dihydrogen phosphate according to the molar ratio of Fe ²⁺ :P=1:1 and dissolve it in 800g to remove Obtain mixed solution B in deionized water;

S3 Add the mixed solution A and the mixed solution B together into the autoclave, and pass nitrogen into the autoclave to exhaust the solution and the air in the autoclave. Under the condition of stirring, the hydrothermal reaction is carried out, and the hydrothermal reaction temperature is 200°C. 15h, then release the pressure and cool, pour out the slurry, wash it with water until the conductivity is 12μS/cm, stop, vacuum dry the material, put it in a nitrogen-protected atmosphere furnace, sinter at 760°C for 7h, then heat up to 860°C for 6h ;

S4 Airflow pulverization of the sintered material, the pulverization gas source is nitrogen, the pulverized D50=1.2μm is controlled, and the pulverized material is passed through a 200-mesh sieve to obtain LiFePO ₄ ·Li ₅ FeO ₄ composite positive electrode material, the XRD of the product is shown in the figure 1, it can be seen that a LiFePO ₄ ·Li ₅ FeO ₄ composite phase is formed without doping.

Example 2

S1 weighs 6 g of ferric sulfate and 174 g of ferrous acetate according to the mole of Fe ³⁺ :Fe ²⁺ =0.03:1, and simultaneously weighs 10 g of sodium dodecylbenzenesulfonate and dissolves it in 1100 g of deionized water to obtain a mixed solution A;

S2 Weigh 30.17g of lithium hydroxide according to the mole of Li:(Fe ²⁺ +Fe ³⁺ )=1.2:1, weigh 116.15g of ammonium dihydrogen phosphate according to the mole of Fe ²⁺ :P=1:1.01 and dissolve it in 800g Obtain mixed solution B in deionized water;

S3 Add mixed solution A and mixed solution B together into the autoclave, and nitrogen gas is introduced into the autoclave to exhaust the solution and the air in the autoclave. Under stirring conditions, hydrothermal reaction is carried out, and the hydrothermal reaction temperature is 230°C. 18h, then release the pressure and cool, pour out the slurry, wash with water until the conductivity is 18μS/cm, stop, vacuum dry the material, put it in a nitrogen-protected atmosphere furnace for sintering at 750°C for 6h, then raise the temperature to 850°C for 8h;

S4 Airflow pulverization of the sintered material, the pulverization gas source is nitrogen, the pulverized D50=1.5μm is controlled, and the pulverized material is passed through a 200-mesh sieve to obtain LiFePO ₄ ·Li ₅ FeO ₄ composite positive electrode material.

Example 3

S1 weighs 16 g of ferric sulfate and 174 g of ferrous acetate according to the mole of Fe ³⁺ : Fe ²⁺ = 0.08:1, while 10 g of benzalkonium chloride is dissolved in 1200 g of deionized water to obtain mixed solution A;

S2 Weigh 36.46g of lithium hydroxide according to the molar ratio of Li:(Fe ²⁺ +Fe ³⁺ )=1.45:1, weigh 115g of ammonium dihydrogen phosphate according to the molar ratio of Fe ²⁺ :P=1:1.1 and dissolve it in 800g to remove Obtain mixed solution B in deionized water;

S3 Add mixed solution A and mixed solution B together into the autoclave, and nitrogen gas is introduced into the autoclave to exhaust the solution and the air in the autoclave. Under stirring conditions, hydrothermal reaction is carried out, and the hydrothermal reaction temperature is 150°C. 20h, then release the pressure and cool down, pour out the slurry, wash with water until the conductivity is 15μS/cm, then stop, spray dry the material (nitrogen is used as the gas source for spray drying), put it in a nitrogen-protected atmosphere furnace for sintering at 780°C After 9 hours, heat up to 830°C and sinter for 7 hours;

S4 Airflow pulverization of the sintered material, the pulverization gas source is nitrogen, the pulverized D50=0.8μm is controlled, and the pulverized material is passed through a 150-mesh sieve to obtain LiFePO ₄ ·Li ₅ FeO ₄ composite positive electrode material.

Comparative example 1

Compared with Example 1, the only difference is that in step 1, no ferric source is added, and step 3 obtains lithium iron phosphate, which is then physically mixed with equimolar Li ₅ FeO ₄ to obtain a composite active material.

Comparative example 2

Compared with Example 1, the difference is only that the same weight of PVP is used to replace the sodium lauryl sulfate, and other operations and parameters are the same as in Example 1.

Comparative example 3

Compared with Example 1, the only difference is that in step 1, ferric iron source is not added, and step 3 obtains lithium iron phosphate, which is added to an equimolar ferric hydroxide colloid for wet coating and then sintered at 860°C for 6 hours to obtain Li ₅ FeO ₄ coated LiFePO ₄ composite cathode material;

Comparative example 4

Compared with Example 1, the only difference is that in Step 2, the molar ratio of Li:(Fe ²⁺ +Fe ³⁺ ) is 1.08:1. Other operations and parameters are the same as in Example 1.

Electrical performance test

(1) Preparation of positive pole piece:

Using the positive electrode material prepared by the above method in Examples 1-3 and Comparative Examples 1-4 as the positive electrode active material, the positive electrode active material: SP (superconducting carbon black): PVDF (polyvinylidene fluoride) in a mass ratio of 90: Homogenize at a ratio of 5:5, coat on a 20μm thick aluminum foil to make a positive electrode sheet with a surface density of 8mg/cm ² , and then dry, roll, die-cut, and punch into positive electrode sheets.

(2) Preparation of battery: use R2032 button battery case for button battery assembly, use lithium sheet as negative electrode, use PE diaphragm, and add 85 μmL of electrolyte dropwise. The test temperature is 25°C, the test voltage range is 2.0V ~ 3.9V, charge to 3.9V by constant current and constant voltage charging, and discharge to 2.0V by constant current discharge. The charging and discharging currents of the first four cycles are 0.1C respectively , 0.2C, 0.5C, 1C; then charge and discharge current at 1C, cycle 200 times. The test results are shown in Table 1.

Table 1 Chemical performance results of each case

It can be seen from the results in Table 1 that after the LiFePO ₄ ·Li ₅ FeO ₄ composite positive electrode material prepared by the method of the present invention is prepared into a lithium-ion battery, the specific capacity of the first discharge at 1C and the capacity retention of 200 cycles of 1C charge-discharge cycle rates have increased.

Apparently, the above-mentioned embodiments are only examples for clear description, rather than limiting the implementation. For those of ordinary skill in the art, other changes or changes in different forms can be made on the basis of the above description. It is not necessary and impossible to exhaustively list all the implementation manners here. And the obvious changes or changes derived therefrom are still within the scope of protection of the present invention.

Claims (10)

1. LiFePO ₄ -Li ₅ FeO ₄ The preparation method of in situ composite positive electrode material, it is characterized in that, will comprise Fe ²⁺ source, Fe ³⁺ source, ionic surfactant, lithium source, the raw material solution of phosphorus source performing hydrothermal treatment to obtain a precursor, and then roasting the precursor to prepare the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material; 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. 2. LiFePO as claimed in claim 1 ₄ -Li _{5 FeO} The preparation method of in situ composite positive electrode material, it is characterized in that, Fe ²⁺ source, Fe ³⁺ source are water-soluble salts of respective metal ions ^; more preferably At least one of sulfate, acetate, chloride; Preferably, the molar ratio of Fe ³⁺ : Fe ²⁺ is 0.03˜0.08:1. 3. The preparation method of LiFePO ₄ -Li ₅ FeO ₄ in-situ composite cathode material as claimed in claim 1, wherein the ionic surfactant is sodium lauryl sulfate, sodium glycocholate, One or more of sodium dodecylbenzenesulfonate, benzalkonium chloride and benzalkonium bromide; Preferably, the ionic surfactant is 1 to 10 wt% of the total iron source mass of Fe ²⁺ source and Fe ³⁺ source; Preferably, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium acetate; Preferably, the element ratio of Li:Fe is 1.2˜1.45:1. 4. The preparation method of LiFePO ₄ -Li ₅ FeO ₄ in situ composite cathode material as claimed in claim 1, characterized in that, the phosphorus source is phosphoric acid, ammonium phosphate, sodium phosphate, potassium phosphate, ammonium monohydrogen phosphate , at least one of sodium monohydrogen phosphate, potassium monohydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate and potassium dihydrogen phosphate; Preferably, the molar ratio of Fe ²⁺ : P is 1:1˜1.1. 5. The method for preparing LiFePO ₄ -Li ₅ FeO ₄ in-situ composite cathode material as claimed in claim 1, wherein the solvent in the raw material solution is water, or a mixed solvent of water-organic solvent; Preferably, the raw material solution is bubbled with nitrogen or an inert gas to replace the air in the raw material solution and the air in the hydrothermal reaction system. 6. The preparation method of LiFePO ₄ -Li ₅ FeO ₄ in situ composite cathode material according to claim 1, characterized in that the temperature of hydrothermal is 120-260°C, preferably 150-230°C; Preferably, the hydrothermal treatment time is 6-24 hours. 7. The preparation method of LiFePO ₄ -Li ₅ FeO ₄ in situ composite cathode material as claimed in claim 1, characterized in that, after the hydrothermal treatment, the solid is washed with water; Preferably, washing with water until the conductivity of the washing water is lower than 20 μS/cm; Preferably, vacuum drying or spray drying is performed after washing to obtain the precursor; Preferably, the firing process is carried out under a protective atmosphere; Preferably, the calcination process is a two-stage gradient calcination process, which includes a T1 stage heat preservation process and a T2 stage heat preservation process, wherein the temperature of T1 is 720-780°C; the temperature of T2 is 800-880°C; Preferably, the holding time under the T1 section is 5-10 hours; The holding time under T2 section is 5~10h; Preferably, the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material is obtained by jet crushing and sieving after calcination. 8. A LiFePO ₄ -Li ₅ FeO ₄ in-situ composite cathode material prepared by the preparation method according to any one of claims 1-7. 9. The application of LiFePO ₄ -Li ₅ FeO ₄ in situ composite positive electrode material prepared by the preparation method according to any one of claims 1 to 7, characterized in that it is used as a positive electrode active material for the preparation of lithium ion battery; Preferably, it is used as an active material, combined with a conductive agent and a binder, to prepare a positive electrode of a lithium-ion battery. 10. A lithium-ion battery, characterized in that it comprises the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material prepared by the preparation method described in any one of claims 1 to 7; Preferably, the positive electrode includes the LiFePO ₄ -Li ₅ FeO ₄ in-situ composite positive electrode material.

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