Perovskite fluoride coated lithium iron phosphate composite material and preparation method thereof, positive plate and lithium battery

Abstract

The invention relates to the technical field of battery materials, and discloses a perovskite fluoride coated lithium iron phosphate composite material, a preparation method thereof, a positive plate and a lithium battery. The preparation method of the perovskite fluoride coated lithium iron phosphate composite material comprises the following steps: mixing a solution in which divalent metal chloride and sodium salt are dissolved and lithium iron phosphate is dispersed with a solution containing fluoride, and reacting to generate the perovskite fluoride coated lithium iron phosphate composite material; and centrifuging the product of the hydrothermal reaction, and then carrying out vacuum drying on the product obtained by centrifuging to obtain the perovskite fluoride coated lithium iron phosphate composite material. The perovskite fluoride coated lithium iron phosphate composite material provided by the invention has excellent conductivity, rate capability and cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a perovskite fluoride-coated lithium iron phosphate composite material, a preparation method thereof, a positive electrode sheet, and a lithium battery. Background Art

[0002] Lithium iron phosphate (LiFePO4, abbreviated as LFP) is a common cathode material for lithium-ion batteries and belongs to the olivine structure. The theoretical specific capacity is 170 mAh g -1 . It has the advantages of high safety, long cycle life, and low cost. Due to its stable three-dimensional framework structure and small volume change during charge and discharge, the theoretical cycle life can reach 2000-5000 times, far exceeding that of ternary materials, and it is widely used in fields such as electric vehicles, energy storage systems, and consumer electronics. However, due to the low intrinsic electronic conductivity and slow lithium-ion diffusion rate of lithium iron phosphate, its rate performance is affected. To improve its conductivity, researchers have improved its conductivity through modification methods such as material nanosizing, compounding with conductive materials, and surface coating with metal compounds. For example, sol-gel method, hydrothermal method, etc. are used to reduce the particle size, shorten the lithium-ion diffusion path, and improve the reaction kinetics; compound carbon materials (such as graphene, carbon nanotubes) are used to enhance its electron conduction ability; surface coating with metal oxides (such as Al2O3, TiO2) is used to reduce side reactions of the electrolyte and extend the cycle life. Summary of the Invention

[0003] The purpose of the present invention is to provide a perovskite fluoride-coated lithium iron phosphate composite material, a preparation method thereof, a positive electrode sheet, and a lithium battery. The preparation method of the present invention can enhance the conductivity of lithium iron phosphate, thereby providing a lithium iron phosphate composite material with excellent electrochemical performance. The method of the present invention has low cost and simple steps.

[0004] To achieve the above object, the technical solution of the present invention is as follows:

[0005] In the first aspect of the present invention, there is provided a perovskite fluoride-coated lithium iron phosphate composite material, which includes mixing reaction liquid 1 and reaction liquid 2 for hydrothermal reaction, and obtaining a solid-liquid mixture after the reaction ends. A divalent metal chloride and a sodium salt are dissolved in the reaction liquid 1, and a fluoride is dissolved in the reaction liquid 2. Lithium iron phosphate is dispersed in at least one of the reaction liquid 1 and the reaction liquid 2; centrifuging and vacuum drying the above solid-liquid mixture to obtain a lithium iron phosphate material coated with a perovskite fluoride shell.

[0006] Wherein, it further includes at least one of the following features (1)-(13):

[0007] (1) In the perovskite fluoride-coated lithium iron phosphate material, the mass ratio of perovskite fluoride to lithium iron phosphate is 0.05 - 0.25:1, preferably 0.2:1;

[0008] (2) In the reaction solution 1, the concentration of the divalent metal chloride is 0.05 - 0.2 mol / L, preferably 0.1 mol / L;

[0009] (3) In the reaction solution 1, the concentration of the sodium salt is 0.2 - 0.4 mol / L, preferably 0.32 mol / L;

[0010] (4) In the reaction solution 2, the concentration of the fluoride salt is 0.15 - 0.3 mol / L, preferably 0.2 mol / L;

[0011] (5) The lithium iron phosphate is dispersed in at least one of the reaction solution 1 and the reaction solution 2. The way of mixing and reacting the reaction solution 1 and the reaction solution 2 is: adding the reaction solution 2 to the reaction solution 1 under vigorous stirring;

[0012] (6) The lithium iron phosphate is dispersed in at least one of the reaction solution 1 and the reaction solution 2. The preparation method includes: adding lithium iron phosphate to the reaction solution, mixing, and then placing it in an ultrasonic cell disruptor for comminution. The comminution power is set to 50 - 100%, preferably 80%; the comminution time is 2 - 3 h, preferably 3 h;

[0013] (7) The divalent metal chloride in the reaction solution 1 is at least one of magnesium chloride, manganese chloride, ferrous chloride, cobalt chloride, nickel chloride, copper chloride, and zinc chloride, preferably nickel chloride;

[0014] (8) The sodium salt in the reaction solution 1 is at least one of sodium carbonate, sodium sulfate, sodium acetate, sodium nitrate, and sodium chloride, preferably sodium acetate;

[0015] (9) The fluoride in the reaction solution 2 is at least one of hydrogen fluoride, ammonium fluoride, ammonium bifluoride, and sodium fluoride, preferably ammonium fluoride;

[0016] (10) The temperature of the hydrothermal reaction is 160 - 240 °C, preferably 200 °C; the time is 10 - 20 h, preferably 16 h;

[0017] (11) The rotation speed of the centrifugation operation is 5000 - 10000 rpm, preferably 8000 rpm;

[0018] (12) The temperature of the vacuum drying is 60 - 100 °C, preferably 80 °C; the time is 10 - 15 h, preferably 12 h;

[0019] (13) The particle size of lithium iron phosphate dispersed in the reaction solution 1 or the reaction solution 2 is 300 nm to 1.5 μm, preferably 500 nm.

[0020] Among them, a surfactant is also dispersed in the reaction solution 1 or the reaction solution 2.

[0021] Among them, the surfactant is selected from at least one of polyvinylpyrrolidone, sodium citrate, oleic acid, and stearic acid, preferably sodium citrate.

[0022] Among them, the solvent in the reaction solution 1 or the reaction solution 2 is at least one of water, methanol, ethanol, ethylene glycol, glycerol, and N,N-dimethylformamide, preferably ethylene glycol and water.

[0023] In the second aspect of the present invention, a perovskite fluoride-coated lithium iron phosphate composite material is provided, which is prepared by the preparation method according to any one of the foregoing embodiments.

[0024] In the third aspect of the present invention, a positive electrode sheet is provided, and its active material includes the perovskite fluoride-coated lithium iron phosphate composite material prepared according to the foregoing embodiments.

[0025] In the fourth aspect of the present invention, a lithium battery is provided, including the positive electrode sheet prepared according to the foregoing embodiments.

[0026] Compared with the existing technology, the beneficial effects of the present invention are as follows:

[0027] (1) The present invention prepares a perovskite fluoride-coated lithium iron phosphate composite material. The preparation process of this material is simple, which can improve the conductivity of lithium iron phosphate, thereby improving its cycle and rate performance.

[0028] (2) The composite material prepared by the present invention contains nickel elements, which can improve the capacity of lithium iron phosphate, extend the battery life and stabilize the battery performance.

[0029] (3) The positive electrode sheet provided by the present invention can be prepared by a simple coating process without the protection of a special atmosphere. Brief Description of the Drawings

[0030] Figure 1 It is a schematic structural diagram of the perovskite fluoride-coated lithium iron phosphate composite material prepared in Example 1 of the present invention;

[0031] Figure 2 It is the rate (a) and cycle performance (b) of the battery assembled with the perovskite fluoride-coated lithium iron phosphate composite material and commercial lithium iron phosphate in Example 2 of the present invention. Detailed Embodiments

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments and the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Usually, the descriptions and the embodiments shown in the accompanying drawings here can be implemented through various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claims to be protected, but merely represents the selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the scope of protection of the present invention.

[0033] The present invention places no special restrictions on the sources of all raw materials used. They can be commercially available or self-made, and there are no special restrictions on their purity.

[0034] Example 1:

[0035] This example provides a method for preparing a perovskite fluoride-coated lithium iron phosphate composite material.

[0036] (1) Take a 100 mL beaker, add 4 mmol of nickel chloride hexahydrate, 15 mmol of anhydrous sodium acetate, and 40 mL of ethylene glycol to the beaker. Stir to completely dissolve nickel chloride hexahydrate and anhydrous sodium acetate. Weigh 200 mg of commercial lithium iron phosphate and add it to the above mixed solution. After stirring evenly, transfer it to an ultrasonic cell disruptor, set its power to 80%, add 0.1 g of sodium citrate dihydrate during the disruption process, and end the disruption after 3 hours to obtain reaction solution 1;

[0037] (32) Take another 5 mL beaker, add 10 mmol of ammonium fluoride and 2 mL of deionized water to the beaker. Stir to completely dissolve ammonium fluoride, and then add 0.1 g of sodium citrate dihydrate. After stirring and dissolving, obtain reaction solution 2 for standby;

[0038] (3) Slowly add the prepared reaction solution 2 to reaction solution 1 under vigorous stirring. After complete dissolution, transfer the mixture to a 50 mL reaction kettle liner and react evenly at 200 °C for 16 h.

[0039] (4) Centrifuge the obtained solid-liquid mixture after the reaction with absolute ethanol at a rotation speed of 8000 rpm for 5 times;

[0040] (5) Vacuum-dry the above centrifuged material at a drying temperature of 80 °C for 12 h.

[0041] Figure 1Schematic diagram of the structure of the perovskite fluoride-coated lithium iron phosphate composite prepared in Example 1 of the present invention. The results show that the structure of the prepared perovskite fluoride-coated lithium iron phosphate composite is a core-shell structure composed of perovskite fluoride and lithium iron phosphate. Among them, lithium iron phosphate is wrapped inside the perovskite fluoride shell.

[0042] Example 2:

[0043] In this example, the rate and cycle performance of a lithium battery assembled with a positive electrode sheet prepared from a perovskite fluoride-coated lithium iron phosphate composite were tested.

[0044] Using the positive electrode sheet as the working electrode, lithium metal as the counter electrode, 1M LiPF6 in DMC / EC (volume ratio 1:1) with 2% mass fraction of FEC as the electrolyte, and commercial polypropylene as the separator, lithium batteries were assembled separately in a glove box filled with argon using the perovskite fluoride-coated lithium iron phosphate composite and commercial lithium iron phosphate materials as the positive electrode.

[0045] Among them, the preparation method of the positive electrode sheet is as follows: an appropriate amount of N-methylpyrrolidone is added to a mixture of lithium iron phosphate powder, superconducting carbon black, and polyvinylidene fluoride with a mass ratio of 8:1:1. The above mixture is magnetically stirred to obtain a slurry with a moderate viscosity. The slurry is evenly coated on carbon-coated aluminum foil and vacuum dried overnight at 80°C. After drying, the electrode is cut into circular pieces with a diameter of 12 mm, the mass of each positive electrode sheet is weighed, and the mass of the active material is calculated according to the ratio.

[0046] Among them, the preparation method of the lithium battery is as follows: in the glove box, a CR2032 button battery is assembled in the order of the negative electrode shell, the positive electrode sheet, the separator, the electrolyte, the lithium metal, the gasket, the shrapnel, and the positive electrode shell, and then the electrochemical performance is tested on a battery test system.

[0047] Figure 2 Rate (a) and cycle performance (b) of the lithium batteries assembled with the perovskite fluoride-coated lithium iron phosphate composite and commercial lithium iron phosphate materials in Example 2 of the present invention. The results show that the lithium battery assembled with the perovskite fluoride-coated lithium iron phosphate composite has a capacity of approximately 104 mAh g at a current density of 2C (1C = 170 mAh g -1 )), while the lithium battery assembled with commercial lithium iron phosphate material has a capacity of only approximately 60 mAh g at a current density of 2C (a). The lithium battery assembled with the perovskite fluoride-coated lithium iron phosphate composite still has a capacity of 114 mAh g after 200 cycles at a current density of 1C -1 ) -1 ), while the lithium battery assembled with commercial lithium iron phosphate material has a capacity of only approximately 60 mAh g at a current density of 2C (a). The lithium battery assembled with the perovskite fluoride-coated lithium iron phosphate composite still has a capacity of 114 mAh g after 200 cycles at a current density of 1C -1capacity, while the lithium battery assembled with commercial lithium iron phosphate material only has a capacity of 91 mAh g after 200 cycles at a current density of 1C. -1 The cycle stability of the lithium iron phosphate composite coated with perovskite fluoride is significantly higher than that of the lithium battery assembled with commercial lithium iron phosphate material (b).

[0048] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a perovskite fluoride-coated lithium iron phosphate composite material, characterized in that, Mix reaction solution 1 and reaction solution 2 to carry out a hydrothermal reaction. After the reaction is completed, a solid-liquid mixture is obtained. In the reaction solution 1, a divalent metal chloride and a sodium salt are dissolved. In the reaction solution 2, a fluoride is dissolved. Lithium iron phosphate is dispersed in at least one of the reaction solution 1 and the reaction solution 2. Carry out centrifugation and vacuum drying operations on the above solid-liquid mixture to obtain a lithium iron phosphate material coated with a perovskite fluoride shell.

2. The perovskite fluoride modified diaphragm according to claim 1, wherein It also includes at least one of the following features (1) to (13): (1) In the perovskite fluoride-coated lithium iron phosphate material, the mass ratio of perovskite fluoride to lithium iron phosphate is 0.05 to 0.25:1; (2) The concentration of the divalent metal chloride in the reaction solution 1 is 0.05 to 0.2 mol / L; (3) The concentration of the sodium salt in the reaction solution 1 is 0.2 to 0.4 mol / L; (4) The concentration of the fluoride salt in the reaction solution 2 is 0.15 to 0.3 mol / L; (5) The lithium iron phosphate is dispersed in at least one of the reaction solution 1 and the reaction solution 2. The way of mixing and reacting the reaction solution 1 and the reaction solution 2 is: under vigorous stirring, add the reaction solution 2 to the reaction solution 1; (6) The lithium iron phosphate is dispersed in at least one of the reaction solution 1 and the reaction solution 2. The preparation method includes: adding lithium iron phosphate to the reaction solution, mixing, and then placing it in an ultrasonic cell disruptor for crushing. The crushing power is set to 50 to 100%, and the crushing time is 2 to 3 h; (7) The divalent metal chloride in the reaction solution 1 is at least one of magnesium chloride, manganese chloride, ferrous chloride, cobalt chloride, nickel chloride, copper chloride, and zinc chloride; (8) The sodium salt in the reaction solution 1 is at least one of sodium carbonate, sodium sulfate, sodium acetate, sodium nitrate, and sodium chloride; (9) The fluoride in the reaction solution 2 is at least one of hydrogen fluoride, ammonium fluoride, ammonium bifluoride, and sodium fluoride; (10) The temperature of the hydrothermal reaction is 160 to 240 °C, and the time is 10 to 20 h; (11) The rotation speed of the centrifugation operation is 5000 to 10000 rpm; (12) The temperature of the vacuum drying is 60 to 100 °C; the time is 10 to 15 h; (13) The particle size of the lithium iron phosphate dispersed in the reaction solution 1 or the reaction solution 2 is 300 nm to 1.5 μm.

3. The preparation method according to claim 1, characterized in that, A surfactant is also dispersed in the reaction solution 1 or the reaction solution 2.

4. The preparation method according to claim 3, wherein, The surfactant is selected from at least one of polyvinylpyrrolidone, sodium citrate, oleic acid, and stearic acid.

5. The preparation method according to claim 1, characterized in that, The solvent in the reaction solution 1 or the reaction solution 2 is at least one of water, methanol, ethanol, ethylene glycol, glycerol, and N,N-dimethylformamide.

6. A lithium iron phosphate composite material coated with perovskite fluoride, characterized in that, Prepared by the preparation method according to any one of claims 1 to 5.

7. A positive electrode sheet, characterized in that, Its active material includes the perovskite fluoride-coated lithium iron phosphate composite material according to claim 6.

8. A lithium battery, characterized in that, Including the positive electrode sheet according to claim 7.

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