Method for improving surface coating effect of lithium cobalt oxide positive electrode material

Abstract

The application discloses a method for improving surface coating effect of lithium cobalt oxide positive electrode material and belongs to the technical field of lithium ion battery electrode material. The method for improving surface coating effect of lithium cobalt oxide positive electrode material comprises the following steps: mixing large-particle lithium cobalt oxide and cobalt hydroxide to obtain a mixed system S1, mixing small-particle lithium cobalt oxide with the mixed system S1, and then performing low-temperature sintering for coating. The large-particle lithium cobalt oxide has a particle size of 15.5 microns <= D50 <= 19.5 microns, and the small-particle lithium cobalt oxide has a particle size of 2.0 microns <= D50 <= 6.0 microns. The application adjusts the material mixing sequence of the coating process, adopts the mode of "mixing large-particle lithium cobalt oxide and cobalt hydroxide for a period of time, and then adding small-particle lithium cobalt oxide for mixing", solves the problem of low effective coating amount of large-particle lithium cobalt oxide, improves the mass concentration percentage of surface element Al (hereinafter referred to as surface element Al concentration) of lithium cobalt oxide, and further improves the cycle performance of lithium cobalt oxide positive electrode material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery electrode materials, and particularly relates to a method for improving surface coating effect of lithium cobalt oxide positive electrode material. BACKGROUND

[0002] With the upgrading of lithium cobalt oxide positive electrode material, people have higher and higher requirements for its electrical performance, especially in high-temperature and high-voltage environments. Lithium cobalt oxide material will gradually have problems such as irreversible structural phase change, decline in surface interface stability, and decline in safety performance, which seriously limits the development and application of lithium cobalt oxide positive electrode material. Foreign customers such as Samsung and Murata have extremely strict requirements on the indicators of 4.45V and subsequent 4.48V products. During the development of 4.45V, customers proposed the index of completing the mass concentration percentage of surface-added element aluminum (hereinafter referred to as surface element Al concentration) of lithium cobalt oxide. This is because the surface element Al concentration mainly affects the high-temperature cycle performance of the battery, and the index has been officially included in the product specification. However, in the actual production process, due to the limitation of raw materials and the influence of production process quality, the index is basically unable to meet the requirements of customers. Therefore, without changing other indicators, in order to improve the competitiveness of the product, it is urgent to develop a method for improving the high-temperature cycle performance of lithium cobalt oxide positive electrode material.

[0003] At present, the method of coating is mostly used to improve the stability of structure and surface interface under high voltage, and the surface coating effect of lithium cobalt oxide positive electrode material will affect its high-temperature cycle performance. Generally speaking, the better the surface coating effect, the higher the high-temperature cycle performance of the positive electrode material, and vice versa.

[0004] The existing technology shows that the residual lithium content of lithium cobalt oxide after primary sintering can affect the coating effect of lithium cobalt oxide. The higher the residual lithium content, the lower the surface element Al concentration, and vice versa. However, too low residual lithium after primary sintering will affect the specific capacity, float charge and impedance performance of the positive electrode material. Therefore, only by controlling the residual lithium within a certain range can the coating effect of lithium cobalt oxide positive electrode material be improved without changing other performances. However, in actual production, factors such as large fluctuation of main content of raw materials (tricobalt tetroxide and lithium carbonate), insufficient measurement accuracy of mixing equipment increase the difficulty of controlling the residual lithium content. In addition, the uneven distribution of atmosphere in the sintering furnace caused by gas flow limits the sintering reaction, resulting in uneven distribution of residual lithium in lithium cobalt oxide particles. Therefore, it is crucial to develop a simple and efficient method for improving the surface coating effect of lithium cobalt oxide positive electrode material. SUMMARY

[0005] Therefore, the present application aims to solve the technical problem of providing a method for improving the surface coating effect of lithium cobalt oxide positive electrode material. The method improves the effective coating amount of large-particle lithium cobalt oxide, and further improves the surface element Al concentration content of lithium cobalt oxide positive electrode material.

[0006] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0007] The present application provides a method for improving the surface coating effect of lithium cobalt oxide cathode material, comprising the following steps: mixing large particle lithium cobalt oxide with cobalt hydroxide to obtain a mixed system S1, then mixing small particle lithium cobalt oxide with the mixed system S1, and then low-temperature sintering for coating.

[0008] Preferably, the particle size of the large particle lithium cobalt oxide is 15.5 μm≤D50≤19.5 μm.

[0009] Preferably, the particle size of the small particle lithium cobalt oxide is 2.0 μm≤D50≤6.0 μm.

[0010] The present application improves the surface coating effect of lithium cobalt oxide material by adjusting the mixing order of materials in the coating process, using the method of "mixing large particle lithium cobalt oxide and cobalt hydroxide for a certain time, then adding small particle lithium cobalt oxide for mixing", thereby improving the problem of low cobalt hydroxide coating caused by the small specific surface area of large particle lithium cobalt oxide.

[0011] The reason for using the above mixing method for coating is that the large particle lithium cobalt oxide is first mixed uniformly with cobalt hydroxide, and then small particle lithium cobalt oxide is added. Small particle lithium cobalt oxide is difficult to steal cobalt hydroxide from the surface of large particle lithium cobalt oxide, thereby increasing the effective coating amount of cobalt hydroxide on the surface of large particle lithium cobalt oxide, and further improving the surface element Al concentration of lithium cobalt oxide material.

[0012] Preferably, in the present application, the large particle lithium cobalt oxide and the small particle lithium cobalt oxide are both generated by the reaction of tricobalt tetraoxide and lithium carbonate under high temperature and oxygen conditions.

[0013] In the present application, large particle tricobalt tetraoxide and small particle tricobalt tetraoxide are mixed with lithium carbonate respectively, sintered, and then subjected to airflow crushing to preliminarily form large particle lithium cobalt oxide and small particle lithium cobalt oxide. Then the above mixing method is used for coating, so that the coating effect of large particle lithium cobalt oxide is improved, thereby improving the overall surface element Al concentration of lithium cobalt oxide material.

[0014] The surface element Al in the present application comes from the reactant tricobalt tetraoxide. Both large and small particle tricobalt tetraoxide are doped with Al element during the precipitation production process.

[0015] During the above lithium cobalt oxide particle formation process, Al in tricobalt tetraoxide is uniformly doped inside the lithium cobalt oxide particles.

[0016] After the surface coating process, the cobalt atoms of the cobalt hydroxide enter the surface layer position of the lithium cobalt oxide particles, and the Al atoms in the surface layer position of the lithium cobalt oxide particles are replaced to the surface of the lithium cobalt oxide particles. The more cobalt atoms of the cobalt hydroxide enter the surface layer position of the lithium cobalt oxide particles, the more Al atoms are replaced, indicating that the surface coating effect is better. Therefore, the higher the content of Al elements on the surface, the better the surface coating effect of the lithium cobalt oxide positive electrode material.

[0017] Preferably, the mass ratio of the large particle lithium cobalt oxide to the cobalt hydroxide is (15-30):1; more preferably (15-28):1; further preferably (17-25):1; and more preferably (20-22):1.

[0018] If the content of the cobalt hydroxide is too high, the surface coating amount of the lithium cobalt oxide material will be too large, which can easily cause the battery capacity to decay, the floating performance to decrease, the impedance to increase, and other adverse risks.

[0019] Preferably, the mixing time of the large particle lithium cobalt oxide and the cobalt hydroxide is 1-3 min; more preferably 1.5-2.5 min. In some embodiments of the present application, the mixing time of the large particle lithium cobalt oxide and the cobalt hydroxide is 2 min.

[0020] Preferably, the mass ratio of the large particle lithium cobalt oxide to the small particle lithium cobalt oxide is (4-6):1; more preferably (4.0-5.5):1; and further preferably (4.7-5.3):1.

[0021] Preferably, the mixing time of the small particle lithium cobalt oxide and the mixing system S1 is 6-10 min; more preferably 7-9 min. In some embodiments of the present application, the mixing time of the large particle lithium cobalt oxide and the cobalt hydroxide is 8 min.

[0022] Preferably, the temperature of the low-temperature sintering is 800-900℃; more preferably 830-880℃.

[0023] Preferably, the time of the low-temperature sintering is 250-350 min; more preferably 280-320 min.

[0024] The low-temperature sintering is preferably carried out in a natural air gas atmosphere.

[0025] When the coating is carried out by the above mixing method, the added cobalt hydroxide not only forms a coating layer on the surface of the lithium cobalt oxide material, but also consumes residual lithium under low-temperature conditions, thereby improving the electrical properties of the lithium cobalt oxide material.

[0026] The present application also provides a lithium cobalt oxide electrode prepared from the lithium cobalt oxide material coated by the cobalt hydroxide.

[0027] Preferably, the cobalt hydroxide coated lithium cobalt oxide material is prepared by the above preparation method.

[0028] The present application improves the structural stability and thermal stability of the lithium cobalt oxide material by coating a thin and stable cobalt hydroxide coating layer on the surface of the lithium cobalt oxide material, reduces the side reaction between the lithium cobalt oxide material and the electrolyte, prevents the corrosion of the material by the harmful gas generated by the decomposition of the electrolyte, and optimizes the safety performance and service life of the lithium cobalt oxide electrode.

[0029] Compared with the prior art, the method for improving the surface coating effect of the lithium cobalt oxide positive electrode material provided by the present application comprises the following steps: mixing large particle lithium cobalt oxide and cobalt hydroxide to obtain a mixed system S1, then mixing small particle lithium cobalt oxide with the mixed system S1, and then performing low-temperature sintering for coating; the particle size of the large particle lithium cobalt oxide is 15.5 μm≤D50≤19.5 μm, and the particle size of the small particle lithium cobalt oxide is 2.0 μm≤D50≤6.0 μm. By adjusting the material mixing order of the coating process and adopting the mode of “mixing the large particle lithium cobalt oxide and the cobalt hydroxide for a period of time first, and then adding the small particle lithium cobalt oxide for mixing”, the problem of low effective coating amount of the large particle lithium cobalt oxide is solved, the mass concentration percentage of the surface element Al of the lithium cobalt oxide is improved, and thus the high-temperature cycle performance of the lithium cobalt oxide positive electrode material can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 The original process flow chart for the surface coating of the lithium cobalt oxide positive electrode material;

[0031] Figure 2 The improved process flow chart for the surface coating of the lithium cobalt oxide positive electrode material of the present application;

[0032] Figure 3 The particle size distribution (PSD) graph of the lithium cobalt oxide particles in Example 1;

[0033] Figure 4 The scanning electron microscope (SEM) graph of the large particle lithium cobalt oxide in Example 1;

[0034] Figure 5 The scanning electron microscope (SEM) graph of the small particle lithium cobalt oxide in Example 1;

[0035] Figure 6 The improvement principle graph for improving the surface coating effect of the lithium cobalt oxide positive electrode material, in which the additive is cobalt hydroxide;

[0036] Figure 7 The charge-discharge rate curve graph of samples 1, 3 and 4;

[0037] Figure 8 The 50-week cycle performance curve graph of samples 1-6. DETAILED DESCRIPTION

[0038] To further illustrate the present application, the method for coating the surface of the lithium cobalt oxide cathode material provided by the present application is described in detail below in conjunction with examples.

[0039] The level of the surface coating effect of the cathode material is measured by the surface element Al concentration of the cathode material. The principle of testing the surface element Al content is as follows: a certain amount of lithium cobalt oxide is weighed, and a certain amount of dilute acid is dissolved for a certain time. The dilute acid can be dilute hydrochloric acid (concentration of 0.02 mol / L), and the ratio of lithium cobalt oxide to dilute hydrochloric acid is 0.5 g:10 mL, and the dissolution time is 10 min. Since the dilute hydrochloric acid is weakly acidic and the amount of acid is small, only the surface layer of the lithium cobalt oxide can be dissolved. After filtering the above mixture, the mass concentration percentage of element Al in the filtrate is tested by inductively coupled plasma instrument, and the specific formula is: surface element Al concentration = Al% / (Al%+Co%) * 100%. Taking the current mainstream products of 4.45V and 4.48V as examples, the surface element Al concentration is between 0.3 and 0.35, and the overall level is relatively low.

[0040] Example 1

[0041] Three groups of samples, numbered as sample 1, sample 2 and sample 3, are prepared. The three groups of samples have the same process, but different batches.

[0042] Step 1: 100g of large particle lithium cobalt oxide and 4.8g of cobalt hydroxide are first added to a high-speed mixer and mixed for 2min;

[0043] Step 2: 20g of small particle lithium cobalt oxide is added and mixed for 8min. After mixing, the material is loaded into a crucible, and then the above three groups of mixed samples are placed in a muffle furnace with a constant temperature platform temperature of 850℃±5℃ for 300min. After the reaction is completed, the surface element Al concentration of the lithium cobalt oxide product and the product yield are tested.

[0044] Table 1 Experimental parameters for preparing samples 1-3

[0045]

[0046] Table 2 Product yield results of samples 1-3

[0047]

[0048] The mass concentration percentage of surface element Al of samples 1-3 is between 0.406 and 0.422, which is about 25.2% higher than the current mainstream products of 4.45V and 4.48V, and the improvement is significant.

[0049] Comparative Example 1

[0050] Three groups of samples, numbered as sample 4, sample 5 and sample 6, were prepared, and the processes of the three groups of samples were completely same, except that the batches were different.

[0051] Step 1: 100 g of large-particle lithium cobalt oxide, 20 g of small-particle lithium cobalt oxide and 4.8 g of cobalt hydroxide were added into a high-speed mixer at one time, and mixed for 10 min;

[0052] Step 2: After mixing, the material was loaded into a crucible, and then the three groups of mixed samples were placed in a muffle furnace, with a constant temperature platform temperature of 850℃±5℃, and a constant temperature time of 300 min. After the reaction was completed, the mass concentration percentage of surface element Al of the lithium cobalt oxide product and the product yield were tested.

[0053] Table 3: Experimental parameters for preparing samples 4-6

[0054]

[0055] Table 4: Product yield results of samples 4-6

[0056]

[0057] From the results, under the process condition of one-time mixing of large-particle lithium cobalt oxide, small-particle lithium cobalt oxide and cobalt hydroxide, the surface element Al content of samples 4-6 was between 0.319 and 0.341, which was relatively low as a whole. The product yield levels of the three groups of experiments were basically consistent, and were all 99.2%.

[0058] The above results show that, compared with samples 4-6 obtained by using the traditional coating method, the surface element Al content of samples 1-3 obtained by using the coating method of the present application is improved by about 28.1%, and the coating effect on the surface of lithium cobalt oxide is obviously improved.

[0059] Comparative example 2

[0060] Large-particle lithium cobalt oxide and small-particle lithium cobalt oxide were respectively coated on the surface to verify the difference in surface coating effect between the two.

[0061] Step 1: 100 g of large-particle lithium cobalt oxide and 4.0 g of cobalt hydroxide were added into a high-speed mixer, and mixed for 10 min, and then the material was discharged, and the material was numbered as sample 7;

[0062] Step 2: 20 g of small-particle lithium cobalt oxide and 0.8 g of cobalt hydroxide were added into a high-speed mixer, and mixed for 10 min, and then the material was numbered as sample 8;

[0063] Step 3: After mixing, the material was loaded into a crucible, and then the two groups of mixed samples were placed in a muffle furnace, with a constant temperature platform temperature of 850℃±5℃, and a constant temperature time of 300 min. After the reaction was completed, the surface element Al of the lithium cobalt oxide product and the product yield were tested.

[0064] Table 5 Experimental parameters for preparing samples 7-8

[0065]

[0066] Table 6 Product yield results for preparing samples 7-8

[0067]

[0068] From the results, the content of surface element Al in the large particles alone coated was 0.281, and the content of surface element Al in the small particles alone coated was 0.520, indicating that the small particle alone coating effect was obviously higher than that of the large particles. However, since the current lithium cobaltate production method is to mix large and small particles of lithium cobaltate according to a proportion and then coat, the purpose is to improve the compaction and energy density of the product, and the capacity and platform voltage of the large or small particles of lithium cobaltate alone coated are lower than those of the mixed coated particles, so the large and small particles of lithium cobaltate alone coated process cannot be used to improve the surface coating effect.

[0069] The product yield of the large and small particles alone coated was basically consistent, both being 99.2%.

[0070] The large and small particles of lithium cobaltate in Example 1 were compared and analyzed from the aspects of particle size distribution and specific surface area, and the results were as follows:

[0071] Table 7 Comparison of results of particle size distribution and specific surface area of large and small particles of lithium cobaltate

[0072]

[0073] From the particle size distribution results, the particle size of the large particles was much larger than that of the small particles; from the specific surface area and SEM results, the specific surface area of the small particles was much larger than that of the large particles, and in the mixing process, more cobalt hydroxide adhered to the surface of the small particles, so the small particle alone coating effect was obviously higher than that of the large particles.

[0074] Comparative Example 3

[0075] Compared with Comparative Example 2, the other processes were unchanged, the content of cobalt hydroxide was increased, and the large and small particles of lithium cobaltate were separately coated, in order to verify the influence degree of the amount of cobalt hydroxide on the surface coating of the large and small particles of lithium cobaltate.

[0076] Step 1: 100 g of large particles of lithium cobaltate and 4.5 g of cobalt hydroxide were added into a high-speed mixer, and the materials were discharged after mixing for 10 min, and the material number was sample 9;

[0077] Step 2: 20 g of small particles of lithium cobaltate and 0.9 g of cobalt hydroxide were added into a high-speed mixer, and the materials were mixed for 10 min, and the material number was sample 10;

[0078] Step 3: After mixing, the material is loaded into a crucible, and then the above-mentioned two groups of mixed samples are placed in a muffle furnace, with a constant temperature platform temperature of 850℃±5℃ and a constant temperature time of 300min. After the reaction is completed, the surface elements Al of the lithium cobalt oxide product and the product yield are tested.

[0079] Table 8 Experimental parameters for preparing samples 9-10

[0080]

[0081] Table 9 Product yield results of samples 9-10

[0082]

[0083] Compared with sample 7 and sample 8, the surface element Al concentration of the large particle lithium cobalt oxide alone coating increases significantly, and the surface element Al concentration of the small particle lithium cobalt oxide does not change significantly, indicating that increasing the amount of added cobalt hydroxide cannot significantly increase the surface element Al concentration of the small particle, but can significantly increase the surface element Al concentration of the large particle. However, increasing the amount of added cobalt hydroxide can easily cause the residual cobalt level of the product to increase, which can worsen the specific capacity and impedance performance of the positive electrode material.

[0084] By comparing the electrical performance data of the above samples, the charge-discharge rate curve shown in FIG. 1 is obtained. Figure 7 The results show that there is no significant difference in the 0.1C charge-discharge rate curve between sample 4 before improvement and samples 1 and 3 after improvement.

[0085] In addition, by testing the 50-week cycle performance of the above samples, the cycle curve shown in FIG. 2 is obtained. Figure 8 The results show that the capacity retention rate of samples 4, 5 and 6 is 60% when the cycle number is 33-38 weeks, and the capacity retention rate of samples 1, 2 and 3 is 60% when the cycle number is 43-50 weeks, and the cycle performance is obviously improved.

[0086] The present application adopts the method of "mixing large particle lithium cobalt oxide + cobalt hydroxide for a certain time, and then adding small particle lithium cobalt oxide for mixing", so that more cobalt hydroxide is coated on the surface of large particle lithium cobalt oxide. The principle is that the large particle lithium cobalt oxide is first mixed uniformly with cobalt hydroxide, and then small particle lithium cobalt oxide is added, which is difficult to snatch cobalt hydroxide from the surface of large particle lithium cobalt oxide, so as to increase the effective coating amount of large particle lithium cobalt oxide, and further improve the surface element Al content of lithium cobalt oxide.

[0087] To sum up, the application greatly improves the coating effect of lithium cobalt oxide material by changing the mixing order of large and small particle lithium cobalt oxide and cobalt hydroxide, and improves the surface element Al concentration content of lithium cobalt oxide product. Under the premise of not changing the amount of cobalt hydroxide, that is, without affecting other electrical properties of lithium cobalt oxide (such as capacity, floating performance, impedance, etc.), by improving the mixing order of the coating process, the surface element Al concentration content of lithium cobalt oxide is improved, and the high-temperature cycle performance of lithium cobalt oxide positive electrode material is improved.

[0088] The above description of the embodiments is only used to help understand the method of the application and its core idea. It should be pointed out that for those skilled in the art, without departing from the principles of the application, some improvements and modifications can be made to the application, and these improvements and modifications also fall within the protection scope of the claims of the application.

Claims

1. A method for improving the surface coating effect of lithium cobalt oxide cathode material, characterized in that, Includes the following steps: Large-particle lithium cobalt oxide is mixed with cobalt hydroxide to obtain a mixed system S1. Then, small-particle lithium cobalt oxide is mixed with the mixed system S1 and then coated by low-temperature sintering. The large-particle lithium cobalt oxide has a particle size of 15.5 μm ≤ D50 ≤ 19.5 μm; The particle size of the small lithium cobalt oxide particles is 2.0 μm ≤ D50 ≤ 6.0 μm; The temperature for the low-temperature sintering is 800℃~900℃; the time for the low-temperature sintering is 250~350 min; The mass ratio of the large-particle lithium cobalt oxide to cobalt hydroxide is (15~30):1 The mixing time of the large-particle lithium cobalt oxide and cobalt hydroxide is 1~3 min; Both the large-particle lithium cobalt oxide and the small-particle lithium cobalt oxide particles are uniformly doped with Al.

2. The method for improving the surface coating effect of lithium cobalt oxide cathode material according to claim 1, characterized in that, Both the large-particle lithium cobalt oxide and the small-particle lithium cobalt oxide are produced by the reaction of cobalt tetroxide and lithium carbonate under high temperature and oxygen conditions.

3. The method for improving the surface coating effect of lithium cobalt oxide cathode material according to claim 1, characterized in that, The mass ratio of large-particle lithium cobalt oxide to small-particle lithium cobalt oxide is (4~6):

1.

4. The method for improving the surface coating effect of lithium cobalt oxide cathode material according to claim 1, characterized in that, The mixing time between the small lithium cobalt oxide particles and the mixed system S1 is 6~10 min.

5. A lithium cobalt oxide electrode, characterized in that, It was prepared from coated lithium cobalt oxide material; The coated lithium cobalt oxide material is prepared by the method described in any one of claims 1 to 4.

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