Preparation method of lithium iron oxide for supplementing lithium of positive electrode material and application thereof

Abstract

The application discloses a preparation method of lithium ferrite for anode material lithium supplement and application thereof. The lithium ferrite is prepared by the following steps: mixing lithium carbonate and a carbon source, sintering, obtaining conductive carbon coated lithium oxide, mixing the lithium oxide with nano Fe2O3, pressing, sintering, and cooling to obtain Li5FeO4. After a small amount of the lithium ferrite prepared by the application is added into a commercial lithium iron phosphate anode, the first circle of charge and discharge shows excellent lithium supplement performance, and the capacity attenuation problem of the lithium iron phosphate battery anode material is improved. In addition, the preparation process of the anode lithium supplement material is simple, the cost is low, the prepared Li5FeO4 has a proper particle size, and the lithium ferrite has great commercialization potential.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of lithium ion batteries, and relates to preparation and application of a lithium ion battery positive electrode lithium supplement material, in particular to a preparation method and application of lithium ferrite for supplementing a positive electrode material. BACKGROUND

[0002] Irreversible capacity loss of a lithium ion battery in the first cycle is a major problem that limits the energy density and cycle performance of the battery, which is mainly caused by consumption of active lithium due to formation of a solid electrolyte interface (SEI) film on the negative electrode surface during the first charging process. For example, a lithium iron phosphate positive electrode material has excellent electrochemical stability and good cycle performance due to the strong phosphorus-oxygen bond in the olivine structure, but the ion conductivity of the positive electrode material electrode prepared therefrom is poor, and there is a large irreversible loss in the first cycle charge and discharge capacity, so the application of pre-lithiation treatment is particularly important. The current common pre-lithiation technology is mainly divided into negative electrode pretreatment and positive electrode additive lithium supplement. The positive electrode additive lithium supplement technology refers to adding a new lithium source to the positive electrode, which is simple to operate and can be compatible with the current lithium ion battery process production. Common positive electrode lithium supplement additives include Li2NiO2, Li5FeO4 ternary and Li3N, Li2O and Li2O2 binary lithium-rich compounds, which have high capacity and reasonable charge and discharge potential, so as to provide a large amount of irreversible lithium ions in the first cycle charging and discharging, which can effectively compensate for the irreversible capacity loss of the lithium ion battery in the first cycle. Among them, Li5FeO4 as a lithium-rich compound has a theoretical specific capacity of 867 mAh / g, which has great potential for lithium supplement application. In addition, due to the advantages of high lithium supplement safety and low material cost, it has been preliminarily applied in the market.

[0003] At present, the preparation method of Li5FeO4 is mainly through high-temperature sintering or liquid phase method. However, the existing preparation method has a complex process flow, often needs multiple high-temperature sintering, and the sintering cost is high. In addition, the residual organic solvent added may reduce the purity of the product, thereby affecting the electrochemical performance of the battery. SUMMARY

[0004] The purpose of the present application is to provide a preparation method and application of lithium ferrite (Li5FeO4) for supplementing a positive electrode material, which is simple to operate and overcomes the problems in the prior art.

[0005] To achieve the above purpose, the present application provides the following technical solutions:

[0006] The preparation method of lithium ferrite for supplementing a positive electrode material provided by the present application comprises the following steps:

[0007] (1) mixing lithium carbonate and carbon source uniformly, then ball milling, and then sintering in inert gas atmosphere, and then cooling to obtain carbon-coated lithium oxide;

[0008] (2) mixing the carbon-coated lithium oxide obtained in step (1) and nano-sized Fe2O3 uniformly, then pressing and sintering to obtain the lithium iron oxide.

[0009] Preferably, in step (1), the carbon source is at least one of conductive carbon black, activated carbon powder and sucrose, and the molar ratio of lithium carbonate to carbon source is (1-12):1.

[0010] Preferably, in step (1), the ball milling device is a planetary ball mill; the ball milling is dry milling or wet milling, and when wet milling, anhydrous ethanol is added, the ball milling medium is zirconia, and the ball-to-material ratio is 1:(20-30); the self-rotation speed of the ball mill is 400-600 rpm, and the ball milling time is 4-6 h.

[0011] Preferably, in step (1), the sintering temperature is 800-950℃, the heating rate is 5-10℃ / min, and the sintering time is 3-8 h; the inert gas is argon or nitrogen.

[0012] It should be noted that when the carbon source is conductive carbon black, it has the following effects: 1) it can increase the electrical conductivity of the material; 2) it can improve the efficiency of the subsequent ball milling process, further reduce the size of the powder material, and increase the specific surface area of the material to avoid the powder particles from being too large due to long-time sintering.

[0013] Preferably, in step (2), the sintering process is divided into two stages: the first stage is sintering at a temperature of 100-600℃ for 1-12 h; and the second stage is sintering at a temperature of 800-900℃ for 20-48 h.

[0014] Preferably, in step (2), the molar ratio of carbon-coated lithium oxide to nano-sized Fe2O3 is (5-5.5):1.

[0015] The lithium iron oxide prepared according to the above preparation method.

[0016] As a general technical concept, the present application also provides a preparation method of the lithium iron oxide pre-lithiated positive electrode sheet, which comprises the following steps: adding the Li5FeO4 as a positive electrode material lithium supplement additive to a positive electrode active material, then uniformly mixing the Li5FeO4 with a conductive agent and a binder, and then obtaining a pre-lithiated positive electrode sheet after subsequent processing.

[0017] Preferably, the addition amount of the Li5FeO4 is less than or equal to 7% of the total weight of the Li5FeO4 and the positive electrode active material.

[0018] The pre-lithiated positive electrode sheet is prepared according to the preparation method.

[0019] The application further provides a lithium ion battery comprising the pre-lithiated positive electrode sheet.

[0020] Preferably, the lithium ion battery is assembled from the pre-lithiated positive electrode sheet, an electrolyte, a separator and a negative electrode sheet.

[0021] The application has the following beneficial effects:

[0022] (1) The preparation method is simple, easy to operate, and the raw materials are easy to obtain and have low cost. Moreover, if the carbon source is conductive carbon black, it can also be used as part of the electrode conductive material, and no other impurities are introduced.

[0023] (2) The Li5FeO4 prepared by the application has a suitable particle size, and can be directly used after sintering without subsequent treatment.

[0024] (3) In the application, Li5FeO4 can be coated together with the positive electrode material, which is simple to operate, and the amount of lithium supplement added is small, which can be realized under the existing lithium battery manufacturing conditions, and can greatly reduce the production cost. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 X-ray diffraction (XRD) pattern of Li5FeO4 prepared in Example 1 of the application;

[0026] Figure 2 Scanning electron microscope (SEM) pattern of Li5FeO4 prepared in Example 1 of the application;

[0027] Figure 3 First circle and second circle charge-discharge curve diagrams of the button cell prepared in Application Example 1 and Comparative Example 2 of the application;

[0028] Figure 4 First 100 circle cycle diagrams of the button cell prepared in Application Example 1 and Comparative Examples 2-3 of the application. DETAILED DESCRIPTION

[0029] In order to facilitate the understanding of the application, the application will be described in more detail and in a more comprehensive and detailed manner in combination with the drawings and preferred embodiments of the specification, but the protection scope of the application is not limited to the following specific embodiments.

[0030] Unless otherwise defined, all the professional terms used below have the same meaning as generally understood by those skilled in the art. The professional terms used below are only for the purpose of describing the specific embodiments, and are not intended to limit the protection scope of the application.

[0031] Unless otherwise specifically indicated, various materials, reagents, instruments and equipment and so on used in the present application can be purchased from the market or can be prepared by the existing method.

[0032] Example 1

[0033] A lithium ferrite for supplementing lithium of a positive electrode material is prepared, and the specific steps are as follows:

[0034] 1. Preparation of carbon-coated lithium oxide:

[0035] (1) Li2CO3 and conductive carbon black are weighed according to a molar ratio of 1:1, mixed uniformly after grinding for 10 min, and then ball milled for 6 h at a rotation speed of 500 r / min. Then the mixed material is taken out and placed in a vacuum drying box for vacuum drying, with the temperature in the drying box being 70℃, and the drying time being 10 h.

[0036] (2) The dried mixed material is placed in a graphite crucible and transferred to a tube furnace for sintering, protected by Ar gas atmosphere, heated to 900℃ at a rate of 5℃ / min, calcined for 4 h, and then taken out after natural cooling, quickly transferred to a glove box for simple grinding, to obtain a carbon-coated Li2O sample.

[0037] 2. Preparation of Li5FeO4: Carbon-coated Li2O and nano Fe2O3 are weighed according to a molar ratio of 5.5:1, mixed uniformly, pressed, and then sintered using a muffle furnace in a glove box, specifically: under argon atmosphere, 600℃ for 12 h, then taken out, simply ground for 10 min, and then heated to 800℃, calcined for 48 h, to obtain a positive electrode material lithium supplement additive Li5FeO4. Detection shows that the D50 is 7.660 μm, the D90 is 35.288 μm, and the average grain size is 4.132 μm.

[0038] Example 2

[0039] A lithium ferrite for supplementing lithium of a positive electrode material is prepared, and the specific steps are as follows:

[0040] 1. Preparation of carbon-coated lithium oxide:

[0041] (1) Li2CO3 and conductive carbon black are weighed according to a molar ratio of 6:1, mixed uniformly after grinding for 10 min, and then ball milled for 5 h at a rotation speed of 400 r / min. Then the mixed material is taken out and placed in a vacuum drying box for drying, with the temperature in the drying box being 60℃, and the drying time being 12 h.

[0042] (2) The dried mixed material is placed in a graphite crucible and transferred to a tube furnace for sintering, protected by Ar gas atmosphere, heated to 950℃ at a rate of 10℃ / min, and then kept at 950℃ for 3 h, taken out after natural cooling, quickly transferred to a glove box for simple grinding, to obtain a carbon-coated Li2O sample.

[0043] 2. Preparation of Li5FeO4: carbon-coated Li2O and nano-Fe2O3 were weighed according to a molar ratio of 5:1, mixed uniformly, then the mixture was pressed and transferred to a tube furnace for sintering, specifically: 100℃ for 6h under argon atmosphere, then continued to heat to 850℃, calcined for 36h, to obtain the positive electrode material lithium supplement additive Li5FeO4.

[0044] Example 3

[0045] Preparation of a lithium ferrite for positive electrode material lithium supplement, the specific steps are as follows:

[0046] 1. Preparation of carbon-coated lithium oxide:

[0047] (1) Li2CO3 and sucrose were weighed according to a molar ratio of 12:1, ground for 10min, mixed uniformly, then ball milled for 4h at a speed of 600r / min. The mixture was taken out and dried in a vacuum drying oven, the temperature in the drying oven was 60℃, and the drying time was 12h.

[0048] (2) The dried mixture was placed in a graphite crucible and transferred to a tube furnace for sintering, protected by Ar gas atmosphere, heated to 800℃ at a rate of 8℃ / min, calcined for 8h, then taken out after natural cooling, quickly transferred to a glove box for simple grinding, to obtain the carbon-coated Li2O sample.

[0049] 2. Preparation of Li5FeO4: carbon-coated Li2O and nano-Fe2O3 were weighed according to a molar ratio of 5.2:1, mixed uniformly, then pressed and transferred to a tube furnace for sintering, specifically: 300℃ for 1h under argon atmosphere, then continued to heat to 900℃, calcined for 20h, to obtain the positive electrode material lithium supplement additive Li5FeO4.

[0050] Comparative Example 1

[0051] Li5FeO4 was prepared by traditional secondary sintering method, which was: Li2O and nano-Fe2O3 were weighed according to a molar ratio of 5.5:1, mixed uniformly after grinding, then transferred to a tube furnace for sintering, 600℃ for 12h under argon atmosphere. After taking out and simple grinding for 10min, continued to heat to 800℃, calcined for 48h, to obtain the positive electrode material lithium supplement additive Li5FeO4. The detection showed that the D50 was 26.543μm, the D90 was 103.745μm, the average grain size was 12.769μm, and the particle was relatively coarse.

[0052] Application Example 1

[0053] (1) Preparation of positive electrode: The Li5FeO4 prepared in Example 1 is added to the LiFePO4 positive electrode active material, wherein the mass of the Li5FeO4 additive accounts for 5% of the positive electrode material (LiFePO4 + Li5FeO4). The positive electrode material, conductive carbon black (SuperP) and polyvinylidene fluoride (PVDF) are stirred in an NMP solvent at a mass ratio of 8:1:1 to form a uniformly mixed slurry, which is evenly coated on an aluminum foil with a thickness of 16 μm using a coater. The aluminum foil coated with the slurry is placed on a 100°C baking machine for 4 hours to remove the NMP organic solvent, and then transferred to a vacuum drying oven and vacuum dried at 120°C for 4 hours to remove the trace moisture remaining in the electrode.

[0054] (2) The lithium iron phosphate half-cell is assembled in a glove box with a high-purity Ar gas atmosphere to ensure that H2O < 0.01ppm and O2 < 0.01ppm. This study uses a CR2032 button battery shell, the negative electrode is a lithium sheet, and the electrolyte composition is LiPF6 / FEC (fluoroethylene carbonate) + DEC (diethyl carbonate). When assembling the battery, start from the positive electrode and assemble in the order of positive electrode shell-positive electrode sheet-diaphragm-lithium sheet-gasket-negative electrode shell. Then use plastic tweezers to place the battery into the battery sealing machine. When the pressure reaches 50MPa, apply pressure 2-3 times. After the battery is sealed, let it stand at room temperature for 8 hours before performing electrochemical testing.

[0055] (3) The assembled button battery was tested with a constant current charge and discharge test system using the Blue Electric test system. The charge and discharge tests were conducted at a rate of 0.02C, and the cycle test was conducted at a rate of 0.05C for the first week and 0.2C for the subsequent weeks. The voltage range was 2-4.3V. The test results are shown in Figure 3 as well as Figure 4 shown.

[0056] Comparative Example 2

[0057] The only difference between this comparative example and application example 1 is that no lithium supplement additive is added to the positive electrode material, and the other steps and conditions remain unchanged. The button cell assembled in this comparative example is subjected to electrochemical testing according to the method of application example 1. The results are shown in FIG. Figure 3 as well as Figure 4 shown.

[0058] Figure 3 The results show that, compared to batteries without a positive electrode lithium replenisher, using the Li₅FeO₄ prepared in Example 1 as a lithium replenisher improves conductivity, subsequent coulombic efficiency, and charge / discharge capacity. This suggests that the pre-lithiation treatment provides additional lithium ions to the positive electrode, compensating for the active lithium consumed during the first cycle of charging.

[0059] Comparative Example 3

[0060] The difference between this comparative example and application example 1 is that the positive electrode material lithium supplement additive is changed to Li5FeO4 prepared in comparative example 1, and the other conditions remain unchanged. The button cell assembled in this comparative example is electrochemically tested according to the method of application example 1, and the results are shown in FIG. Figure 4 shown.

[0061] from Figure 4 As can be seen, using the Li5FeO4 prepared in Comparative Example 1 as a lithium-supplementing material does not improve the battery charge and discharge capacity as much as using the lithium-supplementing additive prepared in Example 1 of the present invention. Furthermore, after 100 cycles, the battery pre-lithiated with the Li5FeO4 prepared in Example 1 of the present invention exhibits significantly improved discharge capacity and cycling stability compared to batteries without the addition of the lithium-supplementing additive and batteries with the Li5FeO4 prepared in Comparative Example 1.

Claims

1. A method for preparing lithium iron oxide for lithium supplement of a cathode material, characterized in that, The method comprises the following steps: (1) mixing lithium carbonate and carbon source uniformly, then ball milling, and then sintering in inert gas atmosphere, and then cooling to obtain carbon-coated lithium oxide; (2) mixing the carbon-coated lithium oxide obtained in step (1) and nano Fe2O3 uniformly, then pressing and sintering to obtain the lithium iron phosphate; In step (1), the carbon source is conductive carbon black; the sintering temperature is 800-950℃, the heating rate is 5-10℃ / min, and the sintering time is 3-8h; In step (2), the sintering process is divided into two stages: the first stage is sintering at a temperature of 100-600℃ for 1-12h; the second stage is sintering at a temperature of 800-900℃ for 20-48h.

2. The production method according to claim 1, characterized by, In step (1), the molar ratio of lithium carbonate to carbon source is (1-12):

1.

3. The preparation method according to claim 1, characterized in that In step (1), the ball milling equipment is a planetary ball mill; the ball milling is dry milling or wet milling, and anhydrous ethanol is added when wet milling; the ball milling medium is zirconium oxide, and the ball-to-material ratio is 1:(20-30); the self-rotation speed of the ball mill is 400-600rpm, the ball milling time is 4-6h, and the inert gas is argon or nitrogen.

4. The method of claim 1, wherein, In step (2), the molar ratio of carbon-coated lithium oxide to nano Fe2O3 is (5-5.5):1.

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