Nickel-containing ternary positive electrode material, preparation method thereof and lithium ion secondary battery

Abstract

The invention discloses a nickel-containing ternary positive electrode material, a preparation method thereof and a lithium ion secondary battery. The nickel-containing ternary positive electrode material comprises a compound with the chemical formula of LiNi < 1-x-y > Co < x > M < y > O < 2 >, and M is Mn or Al, xgt; 0, ygt; x + y is less than or equal to 0.2, and the nickel-containing ternary positive electrode material has a surface hydroxyl group. The surface of the nickel-containing ternary positive electrode material is rich in a hydroxyl structure, so that the stability of the surface of the nickel-containing ternary positive electrode material is improved, the reaction between the material and an electrolyte is reduced, and the processability and the cycling stability of the positive electrode material are improved.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion battery electrode materials, and more particularly, to a nickel-containing ternary cathode material, a preparation method thereof, and a lithium-ion secondary battery. Background Art

[0002] Lithium-ion batteries have the advantages of high working voltage, high specific energy, long cycle life, light weight, low self-discharge, no memory effect, and high performance-price ratio, and have become the main choice for rechargeable power sources in high-power electric vehicles, artificial satellites, aerospace and other fields. Therefore, lithium-ion batteries and their related materials have become a research hotspot for scientific researchers. The cathode material is one of the key materials of lithium-ion batteries and determines the performance of lithium-ion batteries. Currently, the biggest bottleneck restricting the energy density, power density, cycle life and safety of lithium-ion power batteries lies in the cathode material.

[0003] Among the current cathode materials for power lithium-ion batteries, the nickel-cobalt-manganese lithium ternary layered cathode material LiNi 1-x- y Co x Mn y O2 simultaneously has the advantages of Co-based, Ni-based and Mn-based cathode materials: high specific capacity, good cycle performance, good thermal stability and environmental friendliness, the preparation conditions are relatively mild, and the material cost is low, becoming a cathode material with a large increase in the application field of power lithium-ion batteries in the global market in recent years.

[0004] However, the nickel-cobalt-manganese lithium ternary layered cathode material also has defects. It is easily corroded during the cycling process, resulting in a reduced cycle life, which affects the electrochemical performance such as cycle performance, thus restricting the large-scale commercial application of this type of material; moreover, due to the high nickel content of the nickel-containing ternary cathode material, when the material is exposed to air, it is extremely easy to react with moisture in the air, resulting in a relatively high content of surface residual alkali (the main component of which is lithium carbonate). Therefore, strict control of the moisture content in the environment is required during the production and packaging of the nickel-cobalt-manganese lithium ternary layered cathode material, which seriously affects the processing performance of the cathode material and thus becomes a shackle for the further development of the nickel-containing ternary cathode material.

[0005] The solution of the prior art to solve the above problems is to remove the residual alkali and form a surface coating layer. However, although the introduction of the surface coating layer can improve the cycle performance of the lithium-ion battery, the presence of the coating layer reduces the proportion of the active material, which sacrifices more active material in order to improve the surface stability of the cathode material.

[0006] Therefore, how to obtain a nickel-containing ternary cathode material with good cycle performance and at the same time improve the processability of the nickel-containing ternary cathode material has become an urgent technical problem in the field. Summary of the Invention

[0007] The main object of the present invention is to provide a nickel-containing ternary cathode material, a preparation method thereof, and a lithium ion secondary battery, so as to solve the problems of poor processing performance and cycling performance in the prior art.

[0008] To achieve the above object, according to one aspect of the present invention, there is provided a nickel-containing ternary cathode material, comprising a compound having the chemical formula LiNi 1-x-y Co x M y O2, wherein M is Mn or Al, x > 0, y > 0, x + y ≤ 0.2, and the nickel-containing ternary cathode material has surface hydroxyl groups.

[0009] Further, in the above nickel-containing ternary cathode material, the surface hydroxyl groups are the hydroxyl groups in the [AO5OH] structural unit, wherein A represents a transition metal ion in the lattice, and the transition metal ion is selected from Ni, Co, Mn or Al.

[0010] Further, in the above nickel-containing ternary cathode material, A has a six-coordinate structure, wherein oxygen ions are at lattice sites and hydroxyl groups are on the crystal surface.

[0011] Further, in the above nickel-containing ternary cathode material, the nickel-containing ternary cathode material contains 100 to 600 ppm of Cr.

[0012] According to another aspect of the present invention, there is provided a method for preparing the above nickel-containing ternary cathode material of the present invention, the method comprising the following steps:

[0013] - Step S1: Mix a nickel-containing ternary cathode material precursor with a lithium source, and then fire in an oxygen atmosphere to obtain a nickel-containing ternary cathode material matrix;

[0014] - Step S2: Mix the nickel-containing ternary cathode material matrix with a chromium-containing compound, and fire in an oxygen atmosphere to obtain a nickel-containing ternary cathode material having surface chromate;

[0015] - Step S3: Immerse the nickel-containing ternary cathode material having surface chromate in an aqueous LiOH solution to obtain a nickel-containing ternary cathode material having surface hydroxyl groups.

[0016] Further, in the above method, the nickel-containing ternary cathode material precursor has the chemical formula Ni 1-x-y Co x M y (OH)2, wherein M is Mn or Al, x > 0, y > 0, x + y ≤ 0.2.

[0017] Further, in the above method, in steps S1 and S2, the firing temperature is 680°C to 800°C, and the firing time is 10 hours to 30 hours.

[0018] Further, in the above method, in step S2, the chromium-containing compound is chromium oxide, lithium chromate or lithium dichromate.

[0019] Further, in the above method, in step S3, the concentration of the LiOH aqueous solution is 1.0 mol / L to 2.5 mol / L.

[0020] Further, in the above method, the molar ratio of Cr in the chromium-containing compound to metal NCM in the nickel-containing ternary cathode material precursor is 0 < Cr / NCM ≤ 0.02.

[0021] Further, the method further includes step S4: drying the nickel-containing ternary cathode material with surface hydroxyl groups in an oxygen atmosphere, preferably in an oxygen furnace, at a temperature of 100°C to 300°C, and preferably the drying time is 2 hours to 10 hours.

[0022] Further, in the above method, in step S3, the soaking time is 10 - 60 min.

[0023] According to another aspect of the present invention, there is provided a lithium-ion secondary battery, including: a positive electrode sheet, a negative electrode sheet, and a separator, wherein the positive electrode sheet includes the nickel-containing ternary cathode material of the present invention as described above or prepared by the method of the present invention as described above.

[0024] In the nickel-containing ternary cathode material, its preparation method and the lithium-ion secondary battery of the present invention, by making the surface of the nickel-containing ternary cathode material rich in hydroxyl structures, it is beneficial to improve the surface stability of the nickel-containing ternary cathode material, reduce the reaction between the material and the electrolyte, and thus improve the processing performance and cycle stability of the cathode material. In the ternary cathode material, the surface hydroxyl structure is similar to the function of the coating layer, but occupies a smaller mass ratio than the coating layer, which is beneficial to increasing the proportion of the active material and will not sacrifice too much active material due to improving the surface stability of the cathode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Shows a schematic diagram of the surface group reconstruction of the present invention.

[0026] Figure 2 Shows the cycle performance graph of the coin cell containing the cathode material prepared in Example 1.

[0027] Figure 3 Shows the cycle performance graph of the coin cell containing the cathode material prepared in Comparative Example 1.

[0028] Figure 4The figure shows the cycling performance graph of a coin cell containing the cathode material prepared in Comparative Example 2.

[0029] Figure 5 The figure shows the cycling performance graph of a coin cell containing the cathode material prepared in Comparative Example 3.

[0030] Figure 6 The figure shows a comparative graph of the cycling performance of coin cells containing the cathode materials prepared in Example 1 and Comparative Examples 1-3.

[0031] Figure 7 The figure shows the charge-discharge curves of coin cells containing the cathode materials prepared in Examples 1-3 and Comparative Example 4.

[0032] Figure 8 The figure shows a comparative graph of the cycling performance of coin cells containing the cathode materials prepared in Examples 1, 4, and 5.

[0033] Figure 9 The figure shows the charge-discharge curves of coin cells containing the cathode materials prepared in Examples 6 and 7.

[0034] Figure 10 A and B in [reference] respectively show the surface morphologies of the cathode materials prepared in Example 1 and Comparative Example 5 observed by high-resolution transmission electron microscopy. Detailed Description of the Invention

[0035] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments and the drawings.

[0036] As described in the background art, in the prior art, only the solutions of removing residual alkali and forming a surface coating layer have been proposed to improve the cycling performance and processing performance of the cathode material. However, although the introduction of the surface coating layer can improve the cycling performance of the material, the presence of the coating layer reduces the proportion of the active material, which sacrifices a relatively large amount of active material in order to improve the surface stability of the material, and these methods do not involve the regulation of surface groups, especially surface hydroxyl groups. If only the surface groups are regulated, it will hardly affect the proportion of the active material, so that the surface stability of the cathode material can be improved without sacrificing the active material. Therefore, there is still a need to further improve the existing cathode materials. In view of the problems in the prior art, according to a typical embodiment of the present invention, a nickel-containing ternary cathode material is provided, which comprises a compound with the chemical formula LiNi 1-x-y Co x M y O2, where M is Mn or Al, x > 0, y > 0, x + y ≤ 0.2, and the nickel-containing ternary cathode material has surface hydroxyl groups.

[0037] In the technical solution of the present invention, by enriching the surface of the nickel-containing ternary cathode material with a hydroxyl structure, it is beneficial to improve the surface stability of the nickel-containing ternary cathode material, reduce the reaction between the material and the electrolyte, and thus improve the processing performance and cycle stability of the cathode material. In the ternary cathode material, the surface hydroxyl structure is similar to the function of the coating layer, but occupies a smaller mass ratio than the coating layer, which is beneficial to increasing the proportion of active materials and will not sacrifice too much active material due to improving the surface stability of the cathode material.

[0038] In one example, the surface hydroxyl group is the hydroxyl group in the [AO5OH] structural unit, where A represents a transition metal ion in the lattice, and the transition metal ion is selected from Ni, Co, Mn, or Al. The hydroxyl group in the [AO5OH] structural unit is not easily dissociated and dissolved, so as to better improve the surface stability of the cathode material.

[0039] In one example, A has a six-coordinate structure, where oxygen ions are at the lattice sites and hydroxyl groups are on the crystal surface. The hydroxyl group in the [AO5OH] structural unit is not easily dissociated and dissolved, so as to better improve the surface stability of the cathode material.

[0040] In one example, the nickel-containing ternary cathode material contains 100 to 600 ppm of Cr. For example, the nickel-containing ternary cathode material may contain 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, 350 ppm, 400 ppm, 450 ppm, 500 ppm, 550 ppm, or 600 ppm of Cr.

[0041] According to another typical embodiment of the present invention, a method for preparing the above-mentioned nickel-containing ternary cathode material of the present invention is provided. The method includes the following steps: - Step S1: Mix the nickel-containing ternary cathode material precursor with a lithium source, and then sinter in an oxygen atmosphere to obtain a nickel-containing ternary cathode material matrix; - Step S2: Mix the nickel-containing ternary cathode material matrix with a chromium-containing compound, and sinter in an oxygen atmosphere to obtain a nickel-containing ternary cathode material with surface chromate; - Step S3: Immerse the nickel-containing ternary cathode material with surface chromate in an aqueous LiOH solution to obtain a nickel-containing ternary cathode material with surface hydroxyl groups.

[0042] Due to the characteristics of crystal periodicity, there are always some ions at the edge of the nickel-containing ternary cathode material in an incomplete coordination state. This incomplete state will result in strong ion reaction activity on the material surface and is prone to react with the electrolyte. This situation is particularly obvious in high-nickel materials. The trivalent nickel ions on the surface are extremely oxidizing in an incomplete coordination state and are prone to oxidize the electrolyte and cause side reactions. If a hydroxyl ligand is introduced on the surface of the cathode material, the coordination integrity of the surface nickel ions will be improved, thereby reducing their reaction activity.

[0043] In the technical solution of the present invention, chromate is introduced during the firing stage of the nickel-containing ternary cathode material matrix to promote the formation of chromate groups on the surface. Then, the nickel-containing ternary cathode material with surface chromate is soaked in an aqueous lithium hydroxide solution. Through the displacement reaction between chromate and hydroxide, a surface hydroxyl structure is constructed (see Figure 1 ), and finally, a nickel-containing ternary cathode material rich in hydroxyl groups on the surface is obtained. By making the surface of the nickel-containing ternary cathode material rich in hydroxyl structure, it is beneficial to improve the surface stability of the nickel-containing ternary cathode material, reduce the reaction between the material and the electrolyte, and thus improve the processing performance and cycle stability of the cathode material. In the ternary cathode material, the surface hydroxyl structure is similar to the function of the coating layer, but occupies a smaller mass ratio than the coating layer, which is beneficial to increasing the proportion of active materials and will not sacrifice too much active material due to improving the surface stability of the cathode material.

[0044] Principle of constructing surface groups

[0045] The transition metal ions in the lattice of the nickel-containing ternary cathode material are in a six-coordinate structure [AO6]. At the crystal surface, due to the termination of the periodic lattice structure, it is easy to form a situation where the transition metal coordination is incomplete, forming an incomplete coordination structure [AO5], which is not conducive to the stability of the nickel-containing ternary cathode material. During the preparation process of the nickel-containing ternary cathode material, chromate groups that are stable at high temperatures are introduced, which can form a coordination structure with the [AO5] coordination sites to improve the coordination integrity of the surface transition metal ions, thereby improving the stability of the nickel-containing ternary cathode material. However, since chromate is an easily dissociable and soluble group, in order to further improve the surface stability of the nickel-containing ternary cathode material, the surface groups can be changed into hydroxyl groups that are not easily dissociable and soluble through the displacement of hydroxide with chromate, forming a [AO5OH] structure.

[0046] Soaking the nickel-containing ternary cathode material matrix directly in an alkaline solution (for example, an aqueous lithium hydroxide solution) is difficult to construct a surface hydroxyl structure because it does not conform to electrical neutrality. As in the present invention, chromate that is stable at high temperatures is introduced during the firing stage to form chromate groups on the surface of the nickel-containing ternary cathode material matrix, and then the nickel-containing ternary cathode material with surface chromate is soaked in an aqueous lithium hydroxide solution. Through the displacement reaction between chromate and hydroxide, a surface structure rich in hydroxyl groups can be formed on the nickel-containing ternary cathode material.

[0047] The present invention places no particular restrictions on the precursor of the nickel-containing ternary cathode material, and any precursor of the nickel-containing ternary cathode material well-known to those skilled in the art can be used. The present invention preferably uses nickel cobalt manganese hydroxide. More preferably, in the above method, the precursor of the nickel-containing ternary cathode material has the chemical formula Ni 1-x-y Co x M y(OH)2, where M is Mn or Al, x > 0, y > 0, x + y ≤ 0.2, preferably x + y ≤ 0.1.

[0048] The present invention has no particular limitation on the lithium source, and any lithium source well-known to those skilled in the art that can be used to prepare the nickel-containing ternary cathode material is acceptable. Preferably, in the above method, the lithium source is preferably one or more of lithium carbonate, lithium hydroxide, lithium nitrate, and lithium acetate, more preferably lithium carbonate, lithium hydroxide, lithium nitrate, or lithium acetate, and most preferably lithium hydroxide monohydrate. The molar ratio Li / NCM of the lithium source to the nickel-containing ternary cathode material precursor is preferably 1 - 1.1, preferably 1.01 - 1.1, preferably 1.01 - 1.06, where the number of moles of the above nickel-containing ternary cathode material precursor is the total number of moles calculated based on the moles of nickel, cobalt, manganese, or nickel, cobalt, aluminum (NCM) metals, and the number of moles of the above lithium source is the number of moles calculated based on lithium metal.

[0049] The present invention has no particular limitation on the firing conditions in step S1 and step S2. Preferably, in step S1 and step S2, the firing temperature is 680 °C to 800 °C, and the firing time is 10 hours to 30 hours. For example, the firing temperature can be 680 °C, 690 °C, 700 °C, 710 °C, 720 °C, 730 °C, 740 °C, 750 °C, 760 °C, 770 °C, 780 °C, 790 °C, or 800 °C, and the firing time can be 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, 25 hours, 26 hours, 27 hours, 28 hours, 29 hours, or 30 hours.

[0050] The present invention has no particular limitation on the chromium-containing compound used in step S2. Preferably, in step S2, the chromium-containing compound is chromium oxide, lithium chromate, or lithium dichromate, more preferably Cr2O3, lithium chromate, or lithium dichromate.

[0051] Preferably, in step S3, the concentration of the LiOH aqueous solution is 0.5 mol / L to 2.5 mol / L, more preferably 1.0 mol / L to 2.5 mol / L. Water has a dissociation equilibrium, and the concentration of hydrogen ions is relatively low in an alkaline environment. Reducing the hydrogen ion concentration is beneficial to inhibiting the lithium-hydrogen exchange that occurs when the nickel-containing ternary cathode material contacts the aqueous solution, and reducing the damage to the body of the nickel-containing ternary cathode material during the soaking process. This kind of damage to the nickel-containing ternary cathode material by water or hydrogen ions will deteriorate the kinetics of the material, and the polarization of the charging curve will increase significantly. A high concentration of hydroxide is beneficial to the full replacement of chromate on the surface and is also beneficial to inhibiting the lithium-hydrogen exchange in the body of the material during the soaking process. When the concentration of the LiOH aqueous solution is above 1.0 mol / L, the lithium-hydrogen exchange can be well inhibited. In addition, considering that the solubility of LiOH in water at room temperature is 2.5 mol / L, the upper limit concentration of the LiOH aqueous solution is limited to 2.5 mol / L to ensure that all LiOH is dissolved in water.

[0052] For example, in step S3, the concentration of the LiOH aqueous solution can be 1.0 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, 2.0 mol / L, 2.1 mol / L, 2.2 mol / L, 2.3 mol / L, 2.4 mol / L or 2.5 mol / L.

[0053] In addition, the present application does not particularly limit the soaking conditions (time and temperature) in step S3. The temperature is preferably room temperature, such as 20 - 35 °C. For the soaking time, if the time is too short, it is not easy to control in the experiment, and the time control error for each time is relatively large; when the soaking time is above 10 min, it can not only ensure that the soaking process of the LiOH aqueous solution is easy to operate, but also result in less time error. Therefore, the soaking time is preferably above 10 min, more preferably 10 - 60 min. For example, the soaking time can be 10 min, 20 min, 30 min, 40 min, 50 min or 60 min. Surface hydroxyl groups belong to structural groups that are easily decomposed at high temperatures. Therefore, the LiOH aqueous solution is usually not directly added at high temperatures to construct, but is constructed at a relatively mild temperature (such as 20 - 35 °C).

[0054] Preferably, in the above method, the molar ratio of Cr in the chromium-containing compound to the metal NCM in the nickel-containing ternary cathode material precursor is 0 < Cr / NCM ≤ 0.02. Excessive chromium may affect the performance of the base material (mainly the capacity decrease). For example, the molar ratio Cr / NCM can be 0.01 or 0.02.

[0055] In a further embodiment, the method further includes step S4: drying the nickel-containing ternary cathode material having surface hydroxyl groups in an oxygen atmosphere, preferably in an oxygen furnace, at a temperature of 100°C to 300°C. Preferably, the drying time is 2 hours to 10 hours. This drying temperature and time range can ensure that the nickel-containing ternary cathode material with surface hydroxyl groups is dried while better ensuring that the hydroxyl groups on the surface of the nickel-containing ternary cathode material are not damaged.

[0056] For example, in step S4, the drying time can be 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours, and the drying temperature can be 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C or 300°C.

[0057] According to another typical embodiment of the present invention, a lithium-ion secondary battery is provided, including: a positive electrode sheet, a negative electrode sheet, and a separator, wherein the positive electrode sheet includes the nickel-containing ternary cathode material of the present invention described above or prepared by the method of the present invention described above.

[0058] In the technical solution of the present invention, by making the surface of the nickel-containing ternary cathode material rich in hydroxyl structures, it is beneficial to improve the surface stability of the nickel-containing ternary cathode material, reduce the reaction between the material and the electrolyte, and thus improve the processing performance and cycle stability of the cathode material. In the ternary cathode material, the surface hydroxyl structure is similar to the function of the coating layer, but occupies a smaller mass ratio than the coating layer, which is beneficial to increasing the proportion of active materials and will not sacrifice too much active material due to improving the surface stability of the cathode material.

[0059] Examples

[0060] Preparation of Cathode Material

[0061] Example 1

[0062] 9.2 grams of nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02(OH)2 was mixed evenly with 4.45 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.06), and calcined at 680 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material matrix; the obtained nickel-containing ternary cathode material matrix was mixed evenly with 0.076 g of Cr2O3 (NCM / Cr = 1 / 0.01), and calcined at 740 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material with surface chromate. The obtained nickel-containing ternary cathode material with surface chromate was soaked in 20 ml of 2 mol / L LiOH aqueous solution for 10 min, filtered and transferred to an oxygen furnace, and dried at 180 °C to obtain a nickel-containing ternary cathode material with surface hydroxyl groups.

[0063] Example 2

[0064] 9.2 g of nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02 (OH)2 was mixed evenly with 4.45 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.06), and calcined at 680 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material matrix; the obtained nickel-containing ternary cathode material matrix was mixed evenly with 0.076 g of Cr2O3 (NCM / Cr = 1 / 0.01), and calcined at 740 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material with surface chromate. The obtained nickel-containing ternary cathode material with surface chromate was soaked in 20 ml of 0.5 mol / L LiOH aqueous solution for 10 min, filtered and transferred to an oxygen furnace, and dried at 180 °C to obtain a nickel-containing ternary cathode material with surface hydroxyl groups.

[0065] Example 3

[0066] 9.2 g of nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02 (OH)2 was mixed evenly with 4.45 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.06), and calcined at 680 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material matrix; the obtained nickel-containing ternary cathode material matrix was mixed evenly with 0.076 g of Cr2O3 (NCM / Cr = 1 / 0.01), and calcined at 740 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material with surface chromate. The obtained nickel-containing ternary cathode material with surface chromate was soaked in 20 ml of 1 mol / L LiOH aqueous solution for 10 min, filtered and transferred to an oxygen furnace, and dried at 180 °C to obtain a nickel-containing ternary cathode material with surface hydroxyl groups.

[0067] Example 4

[0068] Mix 9.2 g of the nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02 (OH)₂ evenly with 4.45 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.06), and sinter at 680 °C in an oxygen atmosphere for 10 h to obtain the nickel-containing ternary cathode material matrix; mix the obtained nickel-containing ternary cathode material matrix evenly with 0.152 g of Cr₂O₃ (NCM / Cr = 1 / 0.02), and sinter at 740 °C in an oxygen atmosphere for 10 h to obtain the nickel-containing ternary cathode material with surface chromate. Immerse the obtained nickel-containing ternary cathode material with surface chromate in 20 ml of 2 mol / L LiOH aqueous solution for 10 min, transfer it to an oxygen furnace after suction filtration, and dry it at 180 °C to obtain the nickel-containing ternary cathode material with surface hydroxyl groups.

[0069] Example 5

[0070] Mix 9.2 g of the nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02 (OH)₂ evenly with 4.45 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.06), and sinter at 680 °C in an oxygen atmosphere for 10 h to obtain the nickel-containing ternary cathode material matrix; mix the obtained nickel-containing ternary cathode material matrix evenly with 0.228 g of Cr₂O₃ (NCM / Cr = 1 / 0.03), and sinter at 740 °C in an oxygen atmosphere for 10 h to obtain the nickel-containing ternary cathode material with surface chromate. Immerse the obtained nickel-containing ternary cathode material with surface chromate in 20 ml of 2 mol / L LiOH aqueous solution for 10 min, transfer it to an oxygen furnace after suction filtration, and dry it at 180 °C to obtain the nickel-containing ternary cathode material with surface hydroxyl groups.

[0071] Example 6

[0072] Mix 9.2 g of the nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02(OH)₂ was mixed evenly with 4.45 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.06), and fired at 680 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material matrix; the obtained nickel-containing ternary cathode material matrix was mixed evenly with 0.076 g of Cr₂O₃ (NCM / Cr = 1 / 0.01), and fired at 740 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material with surface chromate. The obtained nickel-containing ternary cathode material with surface chromate was soaked in 20 ml of 1 mol / L LiOH aqueous solution for 30 min, filtered and transferred to an oxygen furnace, and dried at 180 °C to obtain a nickel-containing ternary cathode material with surface hydroxyl groups.

[0073] Example 7

[0074] 9.2 g of nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02 (OH)₂ was mixed evenly with 4.45 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.06), and fired at 680 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material matrix; the obtained nickel-containing ternary cathode material matrix was mixed evenly with 0.076 g of Cr₂O₃ (NCM / Cr = 1 / 0.01), and fired at 740 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material with surface chromate. The obtained nickel-containing ternary cathode material with surface chromate was soaked in 20 ml of 1 mol / L LiOH aqueous solution for 60 min, filtered and transferred to an oxygen furnace, and dried at 180 °C to obtain a nickel-containing ternary cathode material with surface hydroxyl groups.

[0075] Comparative Example 1

[0076] 9.2 g of nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02 (OH)₂ was mixed evenly with 4.45 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.06), and fired at 680 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material matrix; the obtained nickel-containing ternary cathode material matrix was mixed evenly with 0.076 g of Cr₂O₃ (NCM / Cr = 1 / 0.01), and fired at 740 °C in an oxygen atmosphere for 10 h to obtain a nickel-containing ternary cathode material with surface chromate.

[0077] Comparative Example 2

[0078] 9.2 g of nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02(OH)2 was mixed evenly with 4.2 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.0), and fired at 680 °C for 10 h in an oxygen atmosphere to obtain a nickel-containing ternary cathode material matrix; after the obtained nickel-containing ternary cathode material matrix was ground evenly, it was fired for the second time at 740 °C for 10 h in an oxygen atmosphere to obtain the cathode material.

[0079] Comparative Example 3

[0080] 9.2 g of a nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02 (OH)2 and 0.08 g of TiO2 were mixed evenly in ethanol, dried and then mixed evenly with 4.33 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.03), and fired at 680 °C for 10 h in an oxygen atmosphere to obtain a nickel-containing ternary cathode material matrix; after the obtained nickel-containing ternary cathode material matrix was ground evenly, it was fired for the second time at 740 °C for 10 h in an oxygen atmosphere to obtain the cathode material.

[0081] Comparative Example 4

[0082] 9.2 g of a nickel-containing ternary cathode material precursor Ni 0.96 Co 0.02 Mn 0.02 (OH)2 was mixed evenly with 4.45 g of lithium hydroxide monohydrate (NCM / Li = 1 / 1.06), and fired at 680 °C for 10 h in an oxygen atmosphere to obtain a nickel-containing ternary cathode material matrix; the obtained nickel-containing ternary cathode material matrix was mixed evenly with 0.076 g of Cr2O3 (NCM / Cr = 1 / 0.01), and fired at 740 °C for 10 h in an oxygen atmosphere to obtain a nickel-containing ternary cathode material with surface chromate. The obtained nickel-containing ternary cathode material with surface chromate was soaked in 20 ml of deionized water for 10 min, filtered by suction and transferred to an oxygen furnace, and dried at 180 °C to obtain the cathode material.

[0083] Comparative Example 5

[0084] 0.375 g of Al(NO3)3·9H2O was dissolved in 10 ml of deionized water to prepare an aluminum nitrate solution, and 0.126 g of LiOH·H2O was dissolved in 10 ml of deionized water to prepare a lithium hydroxide solution. 8 g of LiNi 0.96 Co 0.02 Mn 0.02 O2 cathode material was added to the above aluminum nitrate solution and stirred into a suspension. Under stirring, the above lithium hydroxide solution was completely dropped into the suspension, and stirring was continued for 10 min. After suction filtration and washing with deionized water 3 times, it was then dried in a vacuum oven at 100 °C to obtain an aluminum hydroxide-coated cathode material.

[0085] Preparation of Button Battery

[0086] Preparation of the positive electrode sheet

[0087] Disperse 0.425 g of the positive electrode material prepared in the above-mentioned examples or comparative examples, 0.035 g of acetylene black conductive agent, and 0.040 g of polyvinylidene fluoride in 0.6 g of N-methylpyrrolidone, and mix evenly to obtain a positive electrode mixture slurry. Subsequently, coat the positive electrode mixture slurry onto a pre-weighed aluminum foil, dry it, and press it into a sheet to obtain a positive electrode sheet, and weigh the positive electrode sheet for later use.

[0088] Preparation of the electrolyte

[0089] Mix 15.0 g of ethylene carbonate, 70.0 g of dimethyl carbonate, and 15.0 g of lithium hexafluorophosphate to prepare an electrolyte.

[0090] Assembly of the battery

[0091] Assemble a CR2025 button battery in a glove box. Use the positive electrode sheet obtained in the above steps as the positive electrode and the lithium sheet as the negative electrode. Assemble the positive electrode, negative electrode, separator, and the battery case of the button battery and inject the electrolyte, and seal it with a mold to obtain a button battery to be tested.

[0092] Testing of Battery Cycling Performance

[0093] 1. Perform charge and discharge tests on the button battery containing the positive electrode material prepared in Example 1 or Comparative Examples 1-3 under the following conditions:

[0094] Charging: Charge the positive electrode at a constant current density of 200 mAg -1 to 4.25 V, and then charge at a constant voltage of 4.25 V until the current density ≤ 5 mAg -1 , and the charging is completed; Discharging: Discharge the positive electrode at a constant current density of 1000 mAg -1 to 2.5 V; Perform continuous charge and discharge cycles 100 times as above.

[0095] Test temperature: 60 °C;

[0096] The test results of the button battery containing the positive electrode material prepared in Example 1 or Comparative Examples 1-3 are shown separately in Figures 2 - 5 ;

[0097] For the convenience of comparing the test results of the button battery containing the positive electrode material prepared in Example 1 or Comparative Examples 1-3, integrate the results in Figures 2 - 5 into Figure 6 .

[0098] From Figures 2 - 6It can be seen that the button-type battery containing the positive electrode material prepared in Example 1 has a significantly improved capacity compared to the button-type batteries containing the positive electrode materials prepared in Comparative Examples 1-3.

[0099] 2. Charge and discharge tests were performed on button cells containing the positive electrode materials of Comparative Example 4 or Examples 1-3 under the following conditions:

[0100] Charging: Charge the positive electrode at 200mAg -1 Charge at a constant current of 4.25V and a constant voltage of 4.25V to a current density of ≤5mAg -1 , charging is completed; discharge: the positive electrode is charged at 20mAg -1 The current density is constant current discharge to 2.0V;

[0101] Test temperature: 30℃;

[0102] The test results of button cells containing the positive electrode materials of Comparative Example 4 or Examples 1-3 are Figure 7 From Figure 7 It can be seen from the charging curve in that immersing the nickel-containing ternary positive electrode material with surface chromate in a LiOH aqueous solution with a concentration of more than 1 mol / L does not lead to an increase in the initial polarization.

[0103] 3. Charge and discharge tests were performed on button cells containing the positive electrode materials of Examples 1, 4-5 under the following conditions:

[0104] Charging: Charge the positive electrode at 200mAg -1 Charge at a constant current of 4.25V and a constant voltage of 4.25V to a current density of ≤5mAg -1 , charging is completed; discharge: the positive electrode is charged at 1000mAg -1 The battery was discharged at a constant current density of 100 V to 2.5 V; the charge and discharge cycles were repeated 100 times.

[0105] Test temperature: 60℃;

[0106] The test results of the button cells containing the positive electrode materials of Examples 1, 4-5 are Figure 8 From Figure 8 As can be seen from the charging curve in , an increase in the Cr content is detrimental to the material capacity, so it is preferred to control Cr / NCM below 0.02.

[0107] 4. Charge and discharge tests were performed on button cells containing the positive electrode materials of Examples 6-7 under the following conditions:

[0108] Charging: Charge the positive electrode at 200mAg -1 Charge at a constant current of 4.25V and a constant voltage of 4.25V to a current density of ≤5mAg-1 When charging is completed; discharging: the positive electrode is discharged at a constant current density of 20 mAg -1 to 2.0 V;

[0109] Test temperature: 30 °C;

[0110] The test results of the coin cells containing the cathode materials of Examples 6-7 are shown in Figure 9 . It can be seen from Figure 9 the charging curve in that soaking the nickel-containing ternary cathode material with surface chromate in an aqueous LiOH solution with a concentration of more than 1 mol / L will not cause an increase in initial polarization, but the capacity will slightly decrease with the extension of time.

[0111] In addition, the surface morphologies of the cathode materials prepared in Example 1 and Comparative Example 5 were observed by high-resolution transmission electron microscopy (model: FEI F20), see Figure 10 , it can be seen that for the nickel-containing ternary cathode material with surface hydroxyl groups prepared in Example 1, no surface attachment ( Figure 10 A in ) can be observed under the electron microscope, while for the aluminum hydroxide-coated cathode material prepared in Comparative Example 5, surface attachments (coating layers) ( Figure 10 B in ) can be observed under the electron microscope.

[0112] Determination of chromium content

[0113] The chromium contents of the cathode materials prepared in Example 1, Example 4 and Example 5 were determined: 0.050 g of the cathode material was taken, dissolved with 10 ml of concentrated hydrochloric acid (concentration: 35 wt%), and then made up to 100 ml with water. The Cr content in the solution was detected by ICP-MS (model: Agilent ICP-MS 7700), and then the chromium content in the material was calculated (chromium content in the material = solution volume × chromium concentration in the solution ÷ material mass).

[0114] Sample Example 1 Example 4 Example 5 Cr content 200 ppm 360 ppm 500 ppm

[0115] It can be seen from the above experimental results that the above embodiments of the present invention have achieved the following technical effects:

[0116] In the nickel-containing ternary cathode material, its preparation method and the lithium-ion secondary battery of the present invention, by making the surface of the nickel-containing ternary cathode material rich in hydroxyl structures, it is beneficial to improve the surface stability of the nickel-containing ternary cathode material, reduce the reaction between the material and the electrolyte, and thus improve the processing performance and cycle stability of the cathode material. In the ternary cathode material, the surface hydroxyl structure has a similar effect to the coating layer, but occupies a smaller mass ratio than the coating layer, which is beneficial to increasing the proportion of active materials and will not sacrifice too much active materials due to improving the surface stability of the cathode material.

[0117] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, various modifications and variations can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A nickel-containing ternary cathode material, comprising a compound with the chemical formula LiNi 1-x-y Co x M y O2, where M is Mn or Al, x > 0, y > 0, x + y ≤ 0.2, and is characterized in that The nickel-containing ternary cathode material has surface hydroxyl groups.

2. The nickel-containing ternary cathode material according to claim 1, wherein The surface hydroxyl groups are the hydroxyl groups in the [AO5OH] structural unit, where A represents a transition metal ion in the lattice, and the transition metal ion is selected from Ni, Co, Mn, or Al.

3. The nickel-containing ternary cathode material according to claim 2, wherein, A has a six-coordinate structure, where oxygen ions are at the lattice sites and the hydroxyl groups are on the crystal surface.

4. The nickel-containing ternary cathode material according to any one of claims 1 to 3, characterized in that, The nickel-containing ternary cathode material contains 100 to 600 ppm of Cr.

5. A method for preparing the nickel-containing ternary cathode material according to any one of claims 1 to 4, characterized in that, It includes the following steps: - Step S1: Mix the nickel-containing ternary cathode material precursor with a lithium source, and then calcine it in an oxygen atmosphere to obtain a nickel-containing ternary cathode material matrix. - Step S2: Mix the nickel-containing ternary cathode material matrix with a chromium-containing compound, and calcine it in an oxygen atmosphere to obtain a nickel-containing ternary cathode material with surface chromate. - Step S3: Immerse the nickel-containing ternary cathode material with surface chromate in an aqueous LiOH solution to obtain a nickel-containing ternary cathode material with surface hydroxyl groups.

6. The method according to claim 5, wherein The nickel-containing ternary cathode material precursor has the chemical formula Ni 1-x-y Co x M y (OH)2, where M is Mn or Al, x > 0, y > 0, x + y ≤ 0.

2.

7. The method according to claim 5 or 6, characterized in that, In Step S1 and Step S2, the calcination temperature is 680°C to 800°C, and the calcination time is 10 hours to 30 hours.

8. The method according to claim 5 or 6, characterized in that, In Step S2, the chromium-containing compound is chromium oxide, lithium chromate, or lithium dichromate.

9. The method according to claim 5 or 6, characterized in that, In Step S3, the concentration of the aqueous LiOH solution is 1.0 mol / L to 2.5 mol / L.

10. The method according to claim 6, characterized in that, The molar ratio of Cr in the chromium-containing compound to the metal NCM in the nickel-containing ternary cathode material precursor is 0 < Cr / NCM ≤ 0.

02.

11. The method according to claim 5 or 6, characterized in that, The method further includes Step S4: Dry the nickel-containing ternary cathode material with surface hydroxyl groups in an oxygen atmosphere, preferably in an oxygen furnace, at a temperature of 100°C to 300°C, and preferably the drying time is 2 hours to 10 hours.

12. The method according to claim 9, wherein In Step S3, the immersion time is 10 - 60 min.

13. A lithium-ion secondary battery, comprising: a positive electrode sheet, a negative electrode sheet, and a separator, characterized in that the positive electrode sheet includes the nickel-containing ternary cathode material according to any one of claims 1 - 4 or prepared by the method according to any one of claims 5 - 12.

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