In-situ carbon-coated positive electrode lithium supplement agent, preparation method and application

Abstract

The invention discloses an in-situ carbon-coated positive electrode lithium supplement agent, a preparation method and application, and belongs to the field of lithium ion batteries. The preparation method of the positive electrode lithium supplement agent comprises the following steps: S1, mixing lithium oxalate with a carbon source, and grinding to prepare mixed powder; s2, the mixed powder is placed in a reaction kettle, sealing and vacuumizing are conducted, carbon dioxide gas is introduced, and the pressure is controlled to be 10-20 Mpa; keeping the temperature at 30-60 DEG C and reacting for 10-24 hours; and S3, exhausting, washing with alcohol, centrifuging and drying to obtain the in-situ carbon-coated positive electrode lithium supplement agent. The invention provides a preparation method of an in-situ carbon-coated positive electrode lithium supplement agent, which comprises the following steps: placing lithium oxalate and a carbon source in a supercritical carbon dioxide fluid, stirring and dissolving, controlling the reaction temperature and pressure to promote the polymerization reaction of carbon source micromolecules, and depositing a layer of compact and uniform carbon material on the surface of lithium oxalate particles in situ, thereby obtaining the in-situ carbon-coated positive electrode lithium supplement agent. The conductivity and the ion mobility of the material are improved, so that the electrochemical activity of lithium oxalate is improved.

Description

Technical Field

[0001] The present invention relates to an in-situ, and in particular to an in-situ carbon-coated positive electrode lithium supplement agent, a preparation method and application thereof. Background Art

[0002] With the rapid development of new energy and power storage industries, the market demand for high specific energy and long life lithium-ion batteries has become more urgent. In recent years, lithium replenishment technology has attracted widespread attention as one of the important means to solve the loss of active lithium in lithium-ion batteries and improve battery energy density and cycle life.

[0003] Lithium replenishment is to introduce additional lithium sources into the positive or negative electrode materials to compensate for the irreversible capacity loss of lithium-ion batteries during formation and subsequent cycles. Currently, there are two main directions: negative electrode pre-lithiation and positive electrode lithium replenishment. Compared with negative electrode pre-lithiation, positive electrode lithium replenishment technology is favored by the industry because of its advantages such as high safety, simple process and wide application range.

[0004] At present, the cathode lithium supplements that are being studied more are mainly lithium nitride, lithium sulfide, lithium-rich nickel acid lithium and lithium-rich ferrite. However, lithium sulfide and lithium-rich ferrite have the problem of residual inactive substances after delithiation, which will not only reduce the energy density of lithium-ion batteries, but also may affect the electrical properties of lithium-ion batteries; in addition, materials such as lithium nitride, lithium sulfide, lithium-rich nickel acid lithium and lithium ferrite have poor water and air stability, strict environmental requirements, high processing difficulty, and high price. Therefore, some organic lithium salts with high environmental stability and low cost have gradually attracted people's research interest, such as lithium oxalate, lithium squarate and dilithium rose palmate.

[0005] Among them, lithium oxalate has the highest theoretical specific capacity, which can reach 525mAh / g. In addition, the carbon dioxide gas produced by decomposition during the first charging process can be discharged during the production process of lithium-ion batteries, and will not reduce the energy density of lithium-ion batteries. It is considered to be a positive electrode lithium supplement with great application prospects. However, as a lithium supplement, the capacity of lithium oxalate in actual applications is far lower than the theoretical value. The delithiation potential is usually higher than 4.7V. It is easy to cause side reactions such as electrolyte decomposition at high potentials, and cannot match most commercial positive electrode materials. In order to improve the electrochemical activity of lithium oxalate, the existing technology is to reduce the particle size of lithium oxalate, optimize the type of conductive agent, introduce catalysts, etc. However, the pretreatment of lithium oxalate and the preparation process of the catalyst are relatively cumbersome, which is not conducive to large-scale application. Summary of the invention

[0006] The object of the present invention is to provide an in-situ carbon-coated positive electrode lithium replenisher with high delithiation capacity and low delithiation potential; another object of the present invention is to provide a method for preparing an in-situ carbon-coated positive electrode lithium replenisher with high delithiation capacity and low delithiation potential.

[0007] The present invention discloses a method for preparing an in-situ carbon-coated positive electrode lithium supplement agent, comprising the following steps:

[0008] S1: mixing lithium oxalate and a carbon source, grinding, and preparing a mixed powder;

[0009] S2: placing the mixed powder in a reaction kettle, sealing, evacuating, introducing carbon dioxide gas, and controlling the pressure to be 10-20 MPa; maintaining the temperature at 30-60° C. for reaction for 10-24 hours;

[0010] S3: exhaust, alcohol washing, centrifugation, drying, and obtaining an in-situ carbon-coated positive electrode lithium supplement.

[0011] At present, the commonly used coating modification methods mainly include solid phase and liquid phase sintering, both of which require carbon coating at high temperature (>700°C), but the decomposition temperature of lithium oxalate is relatively low (300°C) and it is unstable at high temperature. The present invention adopts a preparation process of supercritical carbon dioxide coating, which can greatly reduce the reaction temperature and prevent the decomposition of lithium oxalate; at the same time, it does not involve high-energy ball milling, sintering and other high-energy consumption processes required by transition metal oxides or traditional carbon coating, and has the advantages of mild reaction conditions, simple and convenient operation, rapid and efficient, low cost, no "three wastes", etc., and is suitable for large-scale batch production.

[0012] The preparation method of the in-situ carbon-coated positive electrode lithium supplement disclosed in the present invention, by introducing organic small molecules, graphene, polymers and other carbon sources, a uniform and thickness-controllable carbon material is grown in-situ on the surface of lithium oxalate by polymerization in supercritical carbon dioxide fluid, which can not only limit the growth of lithium oxalate, reduce the particle size of lithium oxalate, and promote the full use of the capacity of the lithium supplement, but also the carbon coating layer acts as a catalyst to improve the electrochemical activity of lithium oxalate and reduce the decomposition potential, so that it can be used as a positive electrode lithium supplement for existing positive electrode systems to compensate for the loss of active lithium during the formation and circulation of lithium ion batteries, and improve the capacity and cycle life of the battery. In addition, the carbon material with excellent conductivity can also avoid the problem of increased battery polarization caused by the addition of the positive electrode lithium supplement, and further improve the low temperature performance and rate performance of the battery.

[0013] The present invention uses supercritical carbon dioxide as a solvent and a reaction medium, and has the advantages of being environmentally friendly, non-toxic, safe and recyclable.

[0014] Furthermore, in step S1, the mass ratio of the lithium oxalate to the carbon source is (20-6):1.

[0015] Furthermore, in step S1, the carbon source includes one or more of glucose, sucrose, fructose, citric acid, carbon nanotubes, graphene, polyaniline, and polyacrylonitrile.

[0016] Furthermore, in step S1, the grinding time is 30-90 min.

[0017] Furthermore, in step S3, the control parameters of drying are: drying at 50-80° C. for 18-24 hours.

[0018] The present invention also discloses an in-situ carbon-coated positive electrode lithium supplement agent, which is prepared by the preparation method described above, and the carbon element is coated on the surface of lithium oxalate particles.

[0019] Furthermore, the particle size of the positive electrode lithium replenisher particles is 100-800 nm; the mass fraction of carbon element in the positive electrode lithium replenisher is 5-15%.

[0020] The particle size of lithium oxalate particles is reduced to nanometer scale in supercritical carbon dioxide fluid.

[0021] Furthermore, the lithium desorption potential of the positive electrode lithium supplement agent is ≤4.4V; and the lithium desorption capacity is 390-450mAh / g.

[0022] The present invention also discloses a lithium-supplemented positive electrode, comprising the positive electrode lithium supplement agent as described above, lithium iron phosphate or lithium nickel cobalt manganese oxide; the mass ratio of the positive electrode lithium supplement agent to the lithium iron phosphate or lithium nickel cobalt manganese oxide is (0.005-0.04):1.

[0023] The lithium-replenishing positive electrode, negative electrode, separator and electrolyte are assembled into a lithium-ion battery according to the existing lithium-ion battery production process. During the first charge, the battery is charged to the decomposition voltage of the positive electrode lithium replenisher or above to promote the full decomposition of the positive electrode lithium replenisher, release active lithium, and compensate for the irreversible capacity loss during formation and subsequent cycles.

[0024] The present invention also discloses a lithium-ion battery, comprising the lithium-supplemented positive electrode as described above.

[0025] The present invention provides a method for preparing an in-situ carbon-coated positive electrode lithium supplement, wherein lithium oxalate and a carbon source are placed in a supercritical carbon dioxide fluid for stirring and dissolution, and the reaction temperature and pressure are controlled to promote the polymerization reaction of small molecules of the carbon source, and a layer of dense and uniform carbon material is deposited in situ on the surface of lithium oxalate particles to improve the conductivity and ion mobility of the material, thereby improving the electrochemical activity of lithium oxalate. By carbon-coating and modifying lithium oxalate, not only can the electrochemical activity of lithium oxalate be improved, but also the decomposition of lithium oxalate can be promoted, and a higher capacity and a lower delithiation potential can be achieved, so that it can be used as a positive electrode lithium supplement for positive electrode systems such as lithium iron phosphate and nickel cobalt manganese oxide ternary; in addition, carbon dioxide can also be used as a reaction medium to participate in the polymerization reaction, improve the coating integrity and conductivity of the material, and reduce battery polarization. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1This is a scanning electron microscope image of the in-situ carbon-coated positive electrode lithium supplement prepared in Example 1 of the present invention;

[0027] Figure 2 This is a constant current charge and discharge curve of the in-situ carbon-coated positive electrode lithium supplement prepared in Example 1 of the present invention;

[0028] Figure 3 This is a cycle performance test curve of the in-situ carbon-coated positive electrode lithium supplement agent prepared in Example 1 of the present invention;

[0029] Figure 4 It is a cycle performance test curve diagram of the lithium supplement group battery cells and the control group battery cells prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0030] In order to make the technical solution of the present invention clearer, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] Example 1

[0032] Preparation of in-situ carbon-coated positive electrode lithium supplement:

[0033] S1: Weigh 2 g of lithium oxalate and 0.1 g of fructose powder into an agate mortar, grind for 30 min to fully mix them, and obtain a mixed powder of lithium oxalate and fructose.

[0034] S2: Place the mixed powder in a reactor, seal it, evacuate it, and introduce carbon dioxide gas until the pressure reaches 15 MPa. Then place the reactor in a 50°C water bath with magnetic stirring for 12 hours to fully react.

[0035] S3: After exhausting, the powder was taken out, washed with anhydrous ethanol, centrifuged 5 times to remove the unreacted carbon source, placed in a 60°C oven to dry for 24 hours, and ground after drying to obtain an in-situ carbon-coated positive electrode lithium supplement with a carbon content of 5.5%.

[0036] The obtained in-situ carbon-coated positive electrode lithium supplement powder was tested by scanning electron microscopy. The SEM photo is shown in Figure 1 As shown in the figure, it can be seen that after lithium oxalate is treated with carbon coating, the sample morphology is still lamellar structure, the particle size is significantly reduced to about 100-500nm, and the surface is smooth and the outline is clear.

[0037] In order to verify the lithium replenishing effect of the in-situ carbon-coated positive electrode lithium replenisher, lithium iron phosphate and graphite were used as the positive and negative electrode active materials respectively, and 3wt% of the positive electrode lithium replenisher prepared in Example 1 was added during the positive electrode slurrying process, and a 4Ah soft-pack lithium replenisher battery cell was prepared according to the existing process. At the same time, for the convenience of comparison, a 4Ah soft-pack battery cell without lithium replenisher was prepared according to the same process as a control group. During the first charge, the two groups of batteries were charged to 4.3V at a constant current of 0.05C to promote the full decomposition of lithium oxalate and release active lithium, and then a room temperature 0.5C charge and discharge cycle test was carried out. The results are as follows Figure 4 And as shown in Table 1.

[0038] Table 1 Electrochemical performance of the lithium supplement group and the control group in Example 1

[0039] Group No. First discharge capacity (Ah) 500-week capacity retention rate (%) Example 1 4.20 100 Control group 4.39 95.5

[0040] like Figure 4 As shown in Table 1, after the in-situ carbon-coated positive electrode lithium replenisher was added to the positive electrode active material in the lithium replenisher group of Example 1, the capacity of the battery cell did not decay after 500 cycles, which was 4.5% higher than that of the control group, indicating that the active lithium released by the positive electrode lithium replenisher can be used to compensate for the irreversible capacity loss during formation and subsequent cycles.

[0041] Example 2

[0042] Preparation of in-situ carbon-coated positive electrode lithium supplement:

[0043] S1: 4 g of lithium oxalate and 0.4 g of carbon nanotube powder were weighed into an agate mortar and ground for 60 min to fully mix them to obtain a mixed powder of lithium oxalate and carbon nanotubes.

[0044] S2: Place the above mixed powder in a reactor, seal and evacuate it, and then introduce carbon dioxide gas into it until the pressure reaches 10 MPa. Then place the reactor in a 40°C water bath with magnetic stirring for 18 hours to fully react.

[0045] S3: After exhausting, the powder is taken out, washed with anhydrous ethanol, centrifuged 5 times to remove the unreacted carbon source, placed in a 70°C oven to dry for 18 hours, and ground after drying to obtain an in-situ carbon-coated positive electrode lithium supplement with a carbon content of 10%.

[0046] In order to verify the lithium replenishing effect of the in-situ carbon-coated positive electrode lithium replenisher, lithium iron phosphate and graphite were used as the positive and negative electrode active materials respectively, and 1wt% of the positive electrode lithium replenisher prepared in Example 2 was added during the positive electrode slurrying process, and a 6Ah soft-pack lithium replenisher battery cell was prepared according to the existing process. At the same time, for the convenience of comparison, a 6Ah soft-pack battery cell without lithium replenisher was prepared according to the same process as a control group. During the first charge, the two groups of batteries were charged to 4.3V at a constant current of 0.1C to promote the full decomposition of lithium oxalate and release active lithium, and then a room temperature 0.5C charge and discharge cycle test was carried out. The test results are shown in Table 2.

[0047] Table 2 Electrochemical performance of the lithium supplement group and the control group in Example 2

[0048] Group No. First discharge capacity (Ah) 100-week capacity retention rate (%) Example 2 5.94 100 Control group 6.07 98.5

[0049] As shown in Table 2, compared with the control group, the first discharge capacity of the lithium supplemented group battery of Example 2 increased by 2.1%. After adding positive lithium supplement, the battery capacity of the lithium supplemented group battery did not decay after 100 cycles at 0.5C at room temperature, which was 1.5% higher than that of the control group.

[0050] Example 3

[0051] Preparation of in-situ carbon-coated positive electrode lithium supplement:

[0052] S1: 8 g lithium oxalate, 0.6 g polyaniline and 0.6 g glucose powder were weighed into an agate mortar and ground for 90 min to fully mix them to obtain a mixed powder of lithium oxalate, polyaniline and glucose.

[0053] S2: Place the above mixed powder in a reactor, seal and evacuate it, and then introduce carbon dioxide gas into it until the pressure reaches 20 MPa. Then place the reactor in a 60°C water bath with magnetic stirring for 24 hours to fully react.

[0054] S3: After exhausting, the powder was taken out, washed with dimethyl sulfoxide, centrifuged 5 times to remove the unreacted carbon source, placed in a 60°C oven to dry for 24 hours, and ground after drying to obtain an in-situ carbon-coated positive electrode lithium supplement with a carbon content of 12.5%.

[0055] In order to verify the lithium replenishing effect of the in-situ carbon-coated positive electrode lithium replenisher, lithium iron phosphate and graphite were used as the positive and negative electrode active materials respectively. During the positive electrode slurrying process, 4wt% of the positive electrode lithium replenisher prepared in Example 3 was added, and a 10Ah soft-pack lithium replenisher battery cell was prepared according to the existing process. At the same time, for the convenience of comparison, a 10Ah soft-pack battery cell without lithium replenisher was prepared according to the same process as a control group. During the first charge, the two groups of batteries were charged to 4.3V at a constant current of 0.1C to promote the full decomposition of lithium oxalate and release active lithium, and then a room temperature 0.5C charge and discharge cycle test was carried out. The test results are shown in Table 3.

[0056] Table 3 Electrochemical performance of the lithium supplement group and the control group in Example 3

[0057] Group No. First discharge capacity (Ah) 800-week capacity retention rate (%) Example 3 10.05 97 Control group 10.47 92.5

[0058] As shown in Table 3, compared with the control group, the first discharge capacity of the lithium supplement group of Example 3 increased by 4.2%. Then, a charge and discharge cycle test was performed at room temperature and 0.5C. After adding the positive electrode lithium supplement, the capacity retention rate of the battery cell of the lithium supplement group of Example 3 after 800 cycles was increased by 4.5% compared with the control group.

[0059] The positive electrode lithium replenishing agent provided by the present invention can normally exert the lithium replenishing capacity under the constant current charging current of 0.05C and 0.1C rate.

[0060] Comparative Example 1

[0061] Commercially available lithium oxalate.

[0062] Testing the electrochemical performance of lithium supplements:

[0063] The lithium supplement prepared in Examples 1-3 and the lithium supplement prepared in Comparative Example 1 were used as positive electrode materials, and positive electrode plates were prepared according to the mass ratio of positive electrode material: conductive agent carbon black: binder PVDF (polyvinylidene fluoride) = 7:2:1. The above-mentioned plates were used as working electrodes, and the lithium plates were used as counter electrodes. They were assembled into button-type half-cells with a diaphragm and an electrolyte in a glove box. The assembled half-cells were then subjected to a 0.025C charge-discharge cycle performance test with a voltage range of 2.5-4.5V. The results are as follows Figure 2 As shown and Table 4.

[0064] Table 4 Lithium supplement performance test results

[0065]

[0066] As shown in Table 4, the electrochemical performance of Examples 1-3 is significantly better than that of the comparative example. Figure 3 As shown, in the subsequent cycle process, Example 1 has almost no charging capacity, which means that the lithium oxalate has been basically decomposed completely and the lithium release process is irreversible, and can be used as a positive electrode lithium supplement.

[0067] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for preparing an in-situ carbon-coated positive electrode lithium supplement agent, characterized in that: The following steps are involved: S1: mixing lithium oxalate and a carbon source, grinding, and preparing a mixed powder; S2: placing the mixed powder in a reaction kettle, sealing, evacuating, introducing carbon dioxide gas, and controlling the pressure to be 10-20 MPa; maintaining the temperature at 30-60° C. for reaction for 10-24 hours; S3: exhaust, alcohol washing, centrifugation, drying, and obtaining an in-situ carbon-coated positive electrode lithium supplement.

2. The method for preparing the in-situ carbon-coated positive electrode lithium supplement according to claim 1, characterized in that: In step S1, the mass ratio of the lithium oxalate to the carbon source is 20-6:

1.

3. The method for preparing the in-situ carbon-coated positive electrode lithium supplement according to claim 2, characterized in that: In step S1, the carbon source includes one or more of glucose, sucrose, fructose, citric acid, carbon nanotubes, graphene, polyaniline, and polyacrylonitrile.

4. The method for preparing the in-situ carbon-coated positive electrode lithium supplement according to claim 1, characterized in that: In step S1, the grinding time is 30-90 min.

5. The method for preparing the in-situ carbon-coated positive electrode lithium supplement according to claim 1, characterized in that: In step S3, the control parameters of drying are: drying at 50-80° C. for 18-24 hours.

6. An in-situ carbon-coated positive electrode lithium supplement, characterized in that: The lithium oxalate particles are prepared by the preparation method described in any one of claims 1 to 5, wherein the carbon element is coated on the surface of the lithium oxalate particles.

7. The positive electrode lithium supplement according to claim 6, characterized in that: The particle size of the positive electrode lithium replenisher particles is 100-800nm; the mass fraction of carbon element in the positive electrode lithium replenisher is 5-15%.

8. The positive electrode lithium supplement according to claim 7, characterized in that: The lithium desorption potential of the positive electrode lithium supplement agent is ≤4.4V; and the lithium desorption capacity is 390-450mAh / g.

9. A lithium-supplemented positive electrode, characterized in that: It comprises the positive electrode lithium supplement agent as described in any one of claims 7-8, lithium iron phosphate or lithium nickel cobalt manganese oxide; the mass ratio of the positive electrode lithium supplement agent to the lithium iron phosphate or lithium nickel cobalt manganese oxide is 0.005-0.04:

1.

10. A lithium ion battery, characterized in that: It comprises the lithium-supplemented positive electrode as claimed in claim 9.

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