Lithium cobalt oxide positive electrode material and preparation method and application thereof

Abstract

The invention discloses a lithium cobalt oxide positive electrode material as well as a preparation method and application thereof. The positive electrode material comprises a main body material as well as a metal oxide coating layer and a carbon layer which sequentially coat the main body material from inside to outside, the general chemical formula of the main body material is Li < x > Na < 1-x > CoO2; and the crystal phase of the main body material is an O2 phase. The positive electrode material can be used for preparing a lithium ion battery, and is prepared by the following steps: S1, mixing and sintering a sodium source and a cobalt source to obtain a P2-phase precursor; s2, preparing O2-phase lithium cobalt oxide from the P2-phase precursor and a lithium source; and S3, mixing the O2-phase lithium cobalt oxide with organic metal salt, and sintering at low temperature. According to the method, O2-phase lithium cobalt oxide is coated in a low-temperature sintering mode, compared with an existing coating mode, the coating effect is good, and coating elements cannot be diffused into the main body material.

Description

Technical Field

[0001] The present invention relates to a cathode material for lithium-ion batteries, and particularly to a lithium cobalt oxide cathode material with good coating uniformity and excellent electrical performance, and a preparation method and application thereof. Background Art

[0002] The theoretical capacity of lithium cobalt oxide material is as high as 274 mAh / g, and the tap density is usually 4.1 - 4.2 g / cm 3 , and it has always dominated the 3C field by virtue of its volume energy density advantage. Among them, the O2-phase lithium cobalt oxide material was reported as early as 1982. Experimental verification shows that its capacity performance is equivalent to that of the O3-phase material. In addition, due to the structural differences, its stability and rate performance are better than those of the O3-phase material. However, uncoated O2-phase lithium cobalt oxide will cause rapid capacity decay during cycling due to factors such as the reaction between particles and electrolyte and surface structure phase transformation. Therefore, coating treatment is required to improve surface stability.

[0003] Metal oxides are the most commonly used coating substances for lithium cobalt oxide materials. The metal elements include Al, Ni, Mn, etc. However, the melting points of the metal oxides of these elements are very high, while the phase transition temperature of O2-phase lithium cobalt oxide is relatively low. It will transform into the O3 phase above 230°C and completely transform into the O3 phase above 280°C. Therefore, it is difficult to achieve the requirement of coating metal oxides on O2-phase lithium cobalt oxide through the high-temperature solid-phase coating process of traditional O3-phase lithium cobalt oxide. At present, the existing technologies generally adopt the method of advancing the coating to the P2 precursor or the method of physical mixing coating without sintering to solve the above problems. For example, in the patent application document CN119601631A, the P2-phase precursor of sodium cobaltate is mixed with the coating material and then subjected to coating solid-phase sintering to obtain a P2-phase precursor with a coating layer, and then mixed with a lithium source for ion exchange to obtain an O2-phase lithium cobalt oxide material with a coating layer. However, advancing the coating to the P2 precursor stage has many potential hazards. For example, the coating process will promote the further growth of P2 grains, thus affecting subsequent lithium-sodium replacement. The coating elements may diffuse into the bulk phase, resulting in uneven surface coating or even insufficient coating, and the coating layer itself may affect subsequent lithium-sodium replacement. Therefore, it ultimately leads to performance losses such as reduced purity of O2-phase lithium cobalt oxide, Na residue, and capacity loss; another example is the patent application with the publication number CN119650618A, which mixes the O2-phase lithium cobalt oxide material doped with M element with the coating material for fusion coating treatment (physical mixing) to obtain an O2-phase lithium cobalt oxide material with an oxide coating layer. However, the coating effect and coating uniformity of this method are difficult to guarantee. Summary of the Invention

[0004] The present invention provides a lithium cobalt oxide cathode material, a preparation method and an application thereof, so as to solve the technical problems of poor coating effect and poor electrochemical performance in the existing coating methods mentioned in the background art.

[0005] To solve the above technical problems, the technical solution proposed by the present invention is as follows: A lithium cobalt oxide cathode material, comprising a main material and a metal oxide coating layer and a carbon layer sequentially coated on the outside of the main material from the inside to the outside; the chemical general formula of the main material is Li x Na 1-x CoO 2 , where 0.9 ≤ x ≤ 1; the crystal phase of the main material is the O2 phase; the metal oxide coating layer and the carbon layer are formed by low-temperature sintering of the main material and an organometallic salt at 140~180°C.

[0006] The lithium cobalt oxide cathode material of the present invention, while maintaining the O2 crystal phase, has a metal oxide layer coated on the surface of the material, and has both high-rate performance and high surface stability. At the same time, the presence of the carbon layer can further form a conductive network and further protection outside the metal oxide layer, thereby enhancing the electrochemical performance and stability of the lithium cobalt oxide cathode material.

[0007] As a further preference of the above technical solution, the metal elements of the metal oxide coating layer include at least one of Ni, Mn, Nb, Al, La, Y, Ce, and Ti; the mass ratio of the metal elements in the metal oxide coating layer in the lithium cobalt oxide cathode material is 1500~2000 ppm. Al, Ni, Mn, and Ti are the main metal elements of the metal oxide coating layer, while metal ions such as La, Y, Ce, and Nb have the characteristics of high valence states and large radii. Therefore, it is difficult to incorporate into the crystal lattice and is easily enriched at the surface and interface of lithium cobalt oxide to form a coating structure. In addition, since these elements do not have electrochemical activity, they are not only chemically stable on the surface, but also have very stable high-voltage properties and can withstand working conditions above 4.6V without decomposition, thereby effectively improving the high-voltage stability of lithium cobalt oxide.

[0008] As a further preference of the above technical solution, the thickness of the metal oxide coating layer is 1~2 nm, and the thickness of the carbon layer is 0.5~1 nm.

[0009] As a further preference of the above technical solution, the space group of the lithium cobalt oxide cathode material is P63mc, and the main peak of the XRD of the lithium cobalt oxide cathode material is located at 18.6 ± 0.1°.

[0010] Based on the same technical concept, the present invention also provides a preparation method of the above lithium cobalt oxide cathode material, comprising the following steps: S1. Mix a sodium source and a cobalt source and then sinter to obtain a P2-phase precursor; S2. Use the P2-phase precursor and a lithium source as raw materials to prepare O2-phase lithium cobalt oxide; S3. Mix the O2-phase lithium cobaltate with an organometallic salt and sinter at a low temperature to melt or decompose the organometallic salt, forming a metal oxide coating layer on the surface of the O2-phase lithium cobaltate, thereby obtaining the lithium cobaltate cathode material; the melting point or decomposition temperature of the organometallic salt is lower than the transformation temperature of the O2-phase lithium cobaltate to the O3-phase lithium cobaltate.

[0011] The present invention selects an organometallic salt as the coating raw material for the lithium cobaltate cathode material, and utilizes the characteristic of the low melting point or decomposition temperature of the organometallic salt to achieve coating at a low temperature of the metal oxide, thereby effectively avoiding the transformation of the O2-phase lithium cobaltate cathode material to the O3-phase due to too high a coating temperature and retaining the advantages of the O2-phase lithium cobaltate. At the same time, carbonaceous matter will remain on the material surface after sintering of the organometallic salt. On the one hand, it forms a conductive network layer, further improving the rate performance of the cathode material. On the other hand, the carbon layer strengthens and fills the morphology of the metal oxide coating layer, further improving the coating effect. Compared with the existing physical coating process, the method of the present invention has a more uniform coating effect. Compared with the existing process of pre-coating on the precursor and then performing ion exchange, the method of the present invention can effectively avoid the adverse effects of the coating elements diffusing into the bulk phase and the coating layer on the subsequent lithium-sodium replacement, ensuring the purity and performance of the lithium cobaltate material. In addition, the inventor also found that the above process of coating on the P2 precursor will cause coarsening of the metal oxide particles in the coating layer, resulting in further phase separation and affecting the interface stability. The method of the present invention also perfectly avoids this problem and ensures the interface mechanical stability.

[0012] As a further preference of the above technical solution, in S1, the molar ratio of sodium to cobalt in the sodium source and the cobalt source is (0.6~1):1; the sintering temperature is 780~820 °C, and the sintering time is 16~30 h.

[0013] As a further preference of the above technical solution, the sodium source includes at least one of sodium carbonate, sodium hydroxide, and sodium bicarbonate; the cobalt source includes at least one of cobalt tetroxide, cobalt hydroxide, cobaltous oxide, cobalt nitrate, cobalt sulfate, and cobalt oxyhydroxide.

[0014] As a further preference of the above technical solution, in S2, the P2-phase precursor is placed in a lithium source solution for ion exchange to obtain O2-phase lithium cobaltate.

[0015] As a further preference of the above technical solution, the lithium ion concentration in the lithium source solution is 5 M, the solid-liquid ratio of the P2-phase precursor to the lithium source solution is 1:(25~35), and the molar ratio of lithium in the lithium source solution to sodium in the P2-phase precursor is (1~20):1; the reaction temperature during the ion exchange process is 75~90 °C, and the reaction time is 10~24 h.

[0016] As a further preference of the above technical solution, the lithium source includes at least one of lithium nitrate, lithium chloride, lithium carbonate, lithium acetate and lithium hydroxide.

[0017] As a further preference of the above technical solution, after the O2-phase lithium cobaltate is prepared, the O2-phase lithium cobaltate is washed with water and dried, and the drying temperature is 80-150 °C, and the drying time is ≥12 h.

[0018] As a further preference of the above technical solution, in S3, the organometallic salt includes at least one of aluminum isopropoxide, ammonium niobium oxalate, manganese formate, nickel acetate, lanthanum isopropoxide, and yttrium acetylacetonate; the mass of the organometallic salt is 1%-10% of the total mass of the organometallic salt and the O2-phase lithium cobaltate.

[0019] As a further preference of the above technical solution, in S3, the time for low-temperature sintering is 4-6 h.

[0020] Based on the same inventive concept, the present invention also provides an application of the above lithium cobaltate cathode material in a lithium-ion battery.

[0021] The present invention has the following beneficial effects: By selecting an organometallic salt with a melting point or decomposition temperature lower than the phase transition temperature (230 °C) of the O2 lithium cobaltate phase, the present invention realizes the low-temperature sintering method to coat the O2-phase lithium cobaltate, avoiding the transformation of the O2-phase lithium cobaltate into the O3 phase, and retaining the advantages of the O2-phase lithium cobaltate material in terms of stability and electrochemical performance. Compared with the existing coating methods, the method of the present invention can form a stable and uniform coating layer on the surface of the O2-phase lithium cobaltate material, with good coating effect, and the coating elements will not diffuse into the main body material, further ensuring the electrochemical performance of the lithium cobaltate cathode material. At the same time, after the low-temperature coating of the organometallic salt, carbonaceous matter will remain on the material surface, thus in-situ forming a conductive network on the surface of the lithium cobaltate, further improving the kinetic conditions of the cathode material and enhancing the rate performance. Description of the Drawings

[0022] Figure 1 XRD test result of the lithium cobaltate cathode material in Example 1; Figure 2 TEM test result of the lithium cobaltate cathode material in Example 5; Figure 3 SEM test result of the lithium cobaltate cathode material in Example 2; Figure 4 SEM test result of the lithium cobaltate cathode material in Comparative Example 2; Figure 5 EPMA test result of the lithium cobaltate cathode material in Example 5; Figure 6EPMA test results of the lithium cobalt oxide cathode material for Comparative Example 3; Figure 7 Rate performance test results of Example 5 and Comparative Example 3. Detailed implementation mode

[0023] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention can be implemented in many different ways defined and covered by the claims.

[0024] Example 1: The lithium cobalt oxide cathode material of this example can be used to make lithium-ion batteries, including a main material, a manganese oxide coating layer and a carbon layer coated outside the main material (in each embodiment of the present invention, the thickness of the metal oxide coating layer and the carbon layer is obtained through TEM testing. In the TEM image, the carbon layer has a lower contrast due to its amorphous shape, while the metal oxide coating layer has a higher contrast); the thickness of the manganese oxide coating layer is 2 nm, and the mass ratio of manganese element in the lithium cobalt oxide cathode material is 1500 ppm; the carbon layer thickness is 0.5 - 1 nm; the chemical general formula of the main material is Li 0.97 Na 0.03 CoO 2 ; the space group of the lithium cobalt oxide cathode material is P63mc, and the main peak of the XRD of the lithium cobalt oxide cathode material is located at 18.6 ± 0.1°. Its XRD test results are as Figure 1 shown, and the result shows a pure-phase O2 structure lithium cobalt oxide.

[0025] The preparation method of the lithium cobalt oxide cathode material of this example includes the following steps: S1. Mix a sodium source (sodium carbonate with D50 = 6 μm) and a cobalt source (cobalt tetroxide with D50 = 3.5 μm). The molar ratio of sodium to cobalt in the sodium source and the cobalt source is 0.7:1. Sinter in an oxygen atmosphere at 800 °C for 20 h (heating rate is 10 °C / min). After sintering, cool naturally to obtain a P2-phase precursor. The chemical formula of the P2-phase precursor is Na 0.7 CoO 2 ; S2. Place the P2-phase precursor in a mixed solution of lithium chloride and lithium hydroxide (in the mixed solution, the molar ratio of LiCl to LiOH is 37:63, and the lithium ion concentration is 5 M) for ion exchange. The molar ratio of lithium element in the mixed solution to sodium element in the P2-phase precursor is 15:1. The solid-liquid ratio of the P2-phase precursor and the mixed solution is 15:1 (the mass of the P2-phase precursor is 2 kg, and the volume of the mixed solution is 30 L). During the ion exchange process, the reaction temperature is controlled at 90 °C, and the reaction time is 12 h to obtain O2-phase lithium cobalt oxide; wash and dry the O2-phase lithium cobalt oxide. The drying temperature is 100 °C, and the drying time is 12 h; S3. Mix lithium cobaltate in the O2 phase with manganese formate (the mass of manganese formate is 2% of the total mass of lithium cobaltate in the O2 phase and manganese formate), and sinter at a low temperature of 180 °C in an air atmosphere for 5 h to form a manganese oxide coating layer and a carbon layer on the surface of lithium cobaltate in the O2 phase, thus obtaining the lithium cobaltate cathode material of this example.

[0026] Example 2: The lithium cobaltate cathode material of this example can be used to fabricate a lithium-ion battery, including a main material and a lanthanum oxide coating layer coated outside the main material; the thickness of the lanthanum oxide coating layer is 2 nm, and the mass proportion of lanthanum element in the lithium cobaltate cathode material is 1500 ppm; the carbon layer thickness is 0.5 - 1 nm; the chemical general formula of the main material is Li 0.97 Na 0.03 CoO 2 ; the space group of the lithium cobaltate cathode material is P63mc, and the main peak of the XRD of the lithium cobaltate cathode material is located at 18.6 ± 0.1°. Its SEM image is as shown in Figure 3 shown.

[0027] The preparation method of the lithium cobaltate cathode material of this example includes the following steps: S1. Mix a sodium source and a cobalt source (the sodium source and cobalt source are the same as those selected in Example 1), and the molar ratio of sodium to cobalt in the sodium source and cobalt source is 0.7:1. Sinter in an oxygen atmosphere at 800 °C for 20 h (the heating rate is 10 °C / min), and naturally cool after sintering to obtain a P2-phase precursor. The chemical formula of the P2-phase precursor is Na 0.7 CoO 2 ; S2. Place the P2-phase precursor in a mixed solution of lithium chloride and lithium hydroxide (in the mixed solution, the mass ratio of LiCl to LiOH is 37:63, and the lithium ion concentration is 5 M) for ion exchange. The molar ratio of lithium element in the mixed solution to sodium element in the P2-phase precursor is 15:1, and the solid-liquid ratio of the P2-phase precursor and the mixed solution is 15:1 (the mass of the P2-phase precursor is 2 kg, and the volume of the mixed solution is 30 L). During the ion exchange process, the reaction temperature is controlled at 90 °C, and the reaction time is 12 h to obtain lithium cobaltate in the O2 phase; wash and dry the lithium cobaltate in the O2 phase, the drying temperature is 80 °C, and the drying time is 14 h; S3. Mix lithium cobaltate in the O2 phase with lanthanum isopropoxide (the mass of lanthanum isopropoxide is 2% of the total mass of lithium cobaltate in the O2 phase and lanthanum isopropoxide), and sinter at a low temperature of 170 °C in an air atmosphere for 5 h to form a lanthanum oxide coating layer and a carbon layer on the surface of lithium cobaltate in the O2 phase, thus obtaining the lithium cobaltate cathode material of this example.

[0028] Example 3: The lithium cobaltate cathode material of this embodiment can be used to fabricate lithium-ion batteries, and it includes a main material and a yttrium oxide coating layer coated outside the main material; the thickness of the yttrium oxide coating layer is 2 nm, and the mass ratio of yttrium element in the lithium cobaltate cathode material is 1500 ppm; the thickness of the carbon layer is 0.5 - 1 nm; the chemical general formula of the main material is Li 0.97 Na 0.03 CoO 2 ; the space group of the lithium cobaltate cathode material is P63mc, and the main peak of the XRD of the lithium cobaltate cathode material is located at 18.6 ± 0.1°.

[0029] The preparation method of the lithium cobaltate cathode material of this embodiment includes the following steps: S1. Mix a sodium source and a cobalt source (the sodium source and cobalt source are the same as those in Example 1), the molar ratio of sodium to cobalt in the sodium source and cobalt source is 0.7:1, sinter in an oxygen atmosphere at 800 °C for 20 h (the heating rate is 10 °C / min), and naturally cool down after sintering to obtain a P2-phase precursor. The chemical formula of the P2-phase precursor is Na 0.7 CoO 2 ; S2. Place the P2-phase precursor in a mixed solution of lithium chloride and lithium hydroxide (in the mixed solution, the mass ratio of LiCl to LiOH is 37:63, and the lithium ion concentration is 5 M) for ion exchange. The molar ratio of lithium element in the mixed solution to sodium element in the P2-phase precursor is 15:1, and the solid-liquid ratio of the P2-phase precursor to the mixed solution is 15:1 (the mass of the P2-phase precursor is 2 kg, and the volume of the mixed solution is 30 L). During the ion exchange process, the reaction temperature is controlled at 90 °C, and the reaction time is 12 h to obtain O2-phase lithium cobaltate; wash and dry the O2-phase lithium cobaltate, the drying temperature is 150 °C, and the drying time is 12 h; S3. Mix the O2-phase lithium cobaltate with yttrium acetylacetonate (the mass of yttrium acetylacetonate is 2% of the total mass of the O2-phase lithium cobaltate and yttrium acetylacetonate), and sinter at a low temperature of 180 °C in an air atmosphere for 5 h to form a yttrium oxide coating layer and a carbon layer on the surface of the O2-phase lithium cobaltate, thus obtaining the lithium cobaltate cathode material of this embodiment.

[0030] Example 4: The lithium cobaltate cathode material of this embodiment can be used to fabricate lithium-ion batteries, and it includes a main material and a nickel oxide coating layer coated outside the main material; the thickness of the nickel oxide coating layer is 2 nm, and the mass ratio of nickel element in the lithium cobaltate cathode material is 1500 ppm; the thickness of the carbon layer is 0.5 - 1 nm; the chemical general formula of the main material is Li 0.97 Na 0.03 CoO 2 ; the space group of the lithium cobaltate cathode material is P63mc, and the main peak of the XRD of the lithium cobaltate cathode material is located at 18.6 ± 0.1°.

[0031] The preparation method of the lithium cobalt oxide cathode material of this embodiment includes the following steps: S1. Mix a sodium source and a cobalt source (the sodium source and cobalt source are the same as those in Example 1). The molar ratio of sodium to cobalt in the sodium source and cobalt source is 0.7:1. Sinter at 800 °C for 20 h (heating rate is 10 °C / min) in an oxygen atmosphere. After sintering, cool naturally to obtain a P2-phase precursor. The chemical formula of the P2-phase precursor is Na 0.7 CoO 2 ; S2. Place the P2-phase precursor in a mixed solution of lithium chloride and lithium hydroxide (in the mixed solution, the mass ratio of LiCl to LiOH is 37:63, and the lithium ion concentration is 5 M) for ion exchange. The molar ratio of lithium element in the mixed solution to sodium element in the P2-phase precursor is 15:1. The solid-liquid ratio of the P2-phase precursor to the mixed solution is 15:1 (the mass of the P2-phase precursor is 2 kg, and the volume of the mixed solution is 30 L). During the ion exchange process, control the reaction temperature at 90 °C and the reaction time at 12 h to obtain O2-phase lithium cobalt oxide. Wash and dry the O2-phase lithium cobalt oxide. The drying temperature is 100 °C and the drying time is 12 h; S3. Mix the O2-phase lithium cobalt oxide with nickel acetate (the mass of nickel acetate is 2% of the total mass of the O2-phase lithium cobalt oxide and nickel acetate). Sinter at 180 °C for 5 h in an air atmosphere to form a nickel oxide coating layer and a carbon layer on the surface of the O2-phase lithium cobalt oxide, that is, obtain the lithium cobalt oxide cathode material of this embodiment.

[0032] Example 5: The lithium cobalt oxide cathode material of this embodiment can be used to manufacture a lithium-ion battery, including a main material and an aluminum oxide coating layer coated outside the main material; the thickness of the aluminum oxide coating layer is 2 nm, and the mass proportion of aluminum element in the lithium cobalt oxide cathode material is 1500 ppm; the carbon layer thickness is 0.5 - 1 nm; the chemical general formula of the main material is Li 0.97 Na 0.03 CoO 2 ; The space group of the lithium cobalt oxide cathode material is P63mc, and the main peak of the XRD of the lithium cobalt oxide cathode material is located at 18.6 ± 0.1°. Its TEM image is as shown in Figure 2 It can be seen the aluminum oxide coating layer coated outside the main material and the residual carbon layer on the outermost layer. Its EPMA image is as shown in Figure 5 shown.

[0033] The preparation method of the lithium cobalt oxide cathode material of this embodiment includes the following steps: S1. Mix the sodium source and cobalt source (the same sodium source and cobalt source as in Example 1). The molar ratio of sodium to cobalt in the sodium source and cobalt source is 0.7:1. Sinter at 800 °C for 20 h in an oxygen atmosphere (heating rate is 10 °C / min). After sintering, cool naturally to obtain the P2-phase precursor. The chemical formula of the P2-phase precursor is Na 0.7 CoO 2 ; S2. Place the P2-phase precursor in a mixed solution of lithium chloride and lithium hydroxide (in the mixed solution, the mass ratio of LiCl to LiOH is 37:63, and the lithium ion concentration is 5 M) for ion exchange. The molar ratio of lithium element in the mixed solution to sodium element in the P2-phase precursor is 15:1. The solid-liquid ratio of the P2-phase precursor to the mixed solution is 15:1. During the ion exchange process, control the reaction temperature at 90 °C and the reaction time at 12 h to obtain lithium cobaltate of the O2 phase; wash and dry the lithium cobaltate of the O2 phase. The drying temperature is 100 °C and the drying time is 12 h; S3. Mix the lithium cobaltate of the O2 phase with aluminum isopropoxide (the mass of aluminum isopropoxide is 2% of the total mass of the lithium cobaltate of the O2 phase and aluminum isopropoxide). Sinter at a low temperature of 140 °C for 5 h in an air atmosphere to form an alumina coating layer and a carbon layer on the surface of the lithium cobaltate of the O2 phase, thus obtaining the lithium cobaltate cathode material of this example.

[0034] Comparative Example 1: The lithium cobaltate cathode material of this comparative example is prepared by the following method: S1. Mix the sodium source and cobalt source (the same sodium source and cobalt source as in Example 1). The molar ratio of sodium to cobalt in the sodium source and cobalt source is 0.7:1. Sinter at 800 °C for 20 h in an oxygen atmosphere (heating rate is 10 °C / min). After sintering, cool naturally to obtain the P2-phase precursor. The chemical formula of the P2-phase precursor is Na 0.7 CoO 2 ; S2. Place the P2-phase precursor in a mixed solution of lithium chloride and lithium hydroxide (in the mixed solution, the mass ratio of LiCl to LiOH is 37:63, and the lithium ion concentration is 5 M) for ion exchange. The molar ratio of lithium element in the mixed solution to sodium element in the P2-phase precursor is 15:1. The solid-liquid ratio of the P2-phase precursor to the mixed solution is 15:1. During the ion exchange process, control the reaction temperature at 90 °C and the reaction time at 12 h to obtain lithium cobaltate of the O2 phase; wash and dry the lithium cobaltate of the O2 phase. The drying temperature is 80 - 150 °C and the drying time is ≥12 h, thus obtaining the lithium cobaltate cathode material of this comparative example.

[0035] Comparative Example 2: The lithium cobaltate cathode material of this comparative example is prepared by the following method: S1. Mix the sodium source and cobalt source (the sodium source and cobalt source are the same as those in Example 1). The molar ratio of sodium to cobalt in the sodium source and cobalt source is 0.7:1. Sinter in an oxygen atmosphere at 800 °C for 20 h (heating rate is 10 °C / min). After sintering, cool naturally to obtain the P2-phase precursor. The chemical formula of the P2-phase precursor is Na 0.7 CoO 2 ; S2. Mix the P2-phase precursor and lanthanum oxide evenly and sinter at 850 °C to obtain the precursor material coated with lanthanum oxide; S3. Mix the precursor material coated with lanthanum oxide with the lithium source for ion exchange to obtain the lithium cobalt oxide cathode material of this comparative example. Its SEM image is as shown in Figure 4 . By comparing Figure 3 and Figure 4 , it can be seen that there is no phase separation phenomenon on the surface coating layer of the cathode material in Example 2, while phase separation occurs on the surface of the cathode material in this comparative example after high-temperature coating.

[0036] Comparative Example 3: The lithium cobalt oxide cathode material of this comparative example is prepared by the following method: S1. Mix the sodium source and cobalt source (the sodium source and cobalt source are the same as those in Example 1). The molar ratio of sodium to cobalt in the sodium source and cobalt source is 0.7:1. Sinter in an oxygen atmosphere at 600 °C for 20 h (heating rate is 10 °C / min). After sintering, cool naturally to obtain the P2-phase precursor. The chemical formula of the P2-phase precursor is Na 0.7 CoO 2 ; S2. Mix the P2-phase precursor and aluminum oxide evenly and sinter at 600 °C to obtain the precursor material coated with lanthanum oxide; S3. Mix the precursor material coated with lanthanum oxide with the lithium source for ion exchange to obtain the lithium cobalt oxide cathode material of this comparative example.

[0037] Perform EMPA test on the cathode material of this comparative example. The results are as shown in Figure 6 . Compared with Figure 5 , it can be seen that the coated metal element (Al) in the cathode material of this comparative example diffuses significantly, while the coated metal elements in Example 5 are concentrated on the surface.

[0038] Perform rate performance test on the cathode materials of Example 5 and this comparative example. The results are as shown in Figure 7 .

[0039] Assemble the lithium cobalt oxide cathode materials of each example and comparative example into coin cells for lithium-ion battery electrochemical performance test. The results are shown in Table 1 below.

[0040] Among them, the method for fabricating the battery is as follows: Using N-methylpyrrolidone as the solvent, lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride are mixed into a uniform slurry according to a mass ratio of 96:2:2. Then, the slurry is coated on an aluminum foil with a thickness of 10 μm, and the areal loading is 14 mg / cm 2 , and after drying, it is cut into circular positive electrode plates with a diameter of 12 mm. Using the obtained circular electrode plates as the positive electrode, lithium metal as the negative electrode, and a PE separator double-sided coated with alumina as the separator, an electrolyte of 1 mol / L LiPF 6 dissolved in an EC:DMC = 1:1 solvent is selected, and an R2032 coin-type half-cell is assembled with an injection volume of 60 μL.

[0041] Among them, the test conditions are as follows: (1) Capacity test at 25°C: Place the coin-type half-cell in an incubator at 25°C. Set a current density of 27.4 mA / g (0.1C), a charging cut-off voltage of 4.6 V (vs Li+ / Li), a discharging cut-off voltage of 3 V (vs Li+ / Li), and cycle for two weeks. (2) Cycle performance test at 25°C: Place the coin-type half-cell in an incubator at 25°C. After cycling for two weeks in step (1), reset the current to 37 mA / g (0.5C), a charging cut-off voltage of 4.6 V (vs Li+ / Li), a discharging cut-off voltage of 3 V (vs Li+ / Li), and cycle for 100 weeks. (3) Rate charge-discharge performance test at 25°C: Place the coin-type half-cell in an incubator at 25°C. The charging cut-off voltage is 4.6 V (vs Li+ / Li), and the discharging cut-off voltage is 3 V (vs Li+ / Li). Constant current charge and discharge are carried out at 27.4 mA / g (0.1C), 137 mA / g (0.5C), 274 mA / g (1C), 548 mAh / g (2C), and 822 mAh / g (3C) respectively, and cycle for 4 weeks at each current density, and the capacity of the first cycle is taken for comparison.

[0042] Table 1: Statistical table of product performance test results for each example and comparative example

[0043] The above is only the preferred embodiment of the present invention, and the protection scope of the present invention is not limited to the above embodiments. For those skilled in the art of this technology, the improvements and transformations obtained without departing from the technical concept of the present invention should also be regarded as the protection scope of the present invention.

[0044] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A lithium cobalt oxide positive electrode material, characterized in that: It comprises a main material and a metal oxide coating layer and a carbon layer sequentially coated on the outside of the main material from the inside to the outside; the chemical formula of the main material is Li x Na 1-x CoO2, wherein 0.9≤x≤1; the crystal phase of the main material is O2 phase; the metal oxide coating layer and the carbon layer are formed by mixing the main material with an organic metal salt and sintering at a low temperature of 140-180°C.

2. The lithium cobalt oxide positive electrode material according to claim 1, characterized in that The metal element of the metal oxide coating layer includes at least one of Ni, Mn, Nb, Al, La, Y, Ce and Ti; the mass proportion of the metal element in the metal oxide coating layer in the lithium cobalt oxide positive electrode material is 1500~2000ppm.

3. The lithium cobalt oxide positive electrode material according to claim 1, characterized in that The thickness of the metal oxide coating layer is 1-2 nm, and the thickness of the carbon layer is 0.5-1 nm.

4. The lithium cobalt oxide positive electrode material according to any one of claims 1 to 3, characterized in that: The space group of the lithium cobalt oxide positive electrode material is P63mc, and the XRD main peak of the lithium cobalt oxide positive electrode material is located at 18.6±0.1°.

5. A method for preparing the lithium cobalt oxide positive electrode material according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1, mixing a sodium source and a cobalt source and sintering them to obtain a P2 phase precursor; S2, using the P2 phase precursor and a lithium source as raw materials to prepare O2 phase lithium cobalt oxide; S3. After mixing the O2-phase lithium cobalt oxide and an organic metal salt, sintering at a low temperature of 140-180° C. to melt or decompose the organic metal salt and form a metal oxide coating layer on the surface of the O2-phase lithium cobalt oxide, thereby obtaining the lithium cobalt oxide positive electrode material; the melting point or decomposition temperature of the organic metal salt is lower than the transition temperature of the O2-phase lithium cobalt oxide to the O3-phase lithium cobalt oxide.

6. The method for preparing the lithium cobalt oxide positive electrode material according to claim 5, characterized in that: In S1, the molar ratio of sodium to cobalt in the sodium source and the cobalt source is (0.6-1):1; the sintering temperature is 780-820°C, and the sintering time is 16-30h; the sodium source includes at least one of sodium carbonate, sodium hydroxide and sodium bicarbonate; the cobalt source includes at least one of cobalt tetroxide, cobalt hydroxide, cobaltous oxide, cobalt nitrate, cobalt sulfate and cobalt oxyhydroxide.

7. The method for preparing a lithium cobalt oxide positive electrode material according to claim 5, characterized in that: In S2, the P2 phase precursor is placed in a lithium source solution for ion replacement to obtain O2 phase lithium cobalt oxide; the lithium ion concentration in the lithium source solution is 5M, the solid-liquid ratio of the P2 phase precursor to the lithium source solution is 1: (25-35), and the molar ratio of lithium in the lithium source solution to sodium in the P2 phase precursor is (1-20): 1; the reaction temperature during the ion replacement process is 75-90°C, and the reaction time is 10-24h; the lithium source includes at least one of lithium nitrate, lithium chloride, lithium carbonate, lithium acetate and lithium hydroxide.

8. The method for preparing a lithium cobalt oxide positive electrode material according to claim 5, characterized in that: After the O2 phase lithium cobalt oxide is obtained, the O2 phase lithium cobalt oxide is washed with water and dried at a drying temperature of 80-150° C. for a drying time of ≥12 h.

9. The method for preparing a lithium cobalt oxide positive electrode material according to any one of claims 5 to 8, characterized in that: In S3, the organic metal salt includes at least one of aluminum isopropoxide, ammonium niobium oxalate, manganese formate, nickel acetate, lanthanum isopropoxide, and yttrium acetylacetonate; the mass of the organic metal salt is 1% to 10% of the total mass of the organic metal salt and O2 phase lithium cobalt oxide; and the time of the low-temperature sintering is 4 to 6 hours.

10. Use of the lithium cobalt oxide positive electrode material according to any one of claims 1 to 4 or the lithium cobalt oxide positive electrode material prepared by the preparation method according to any one of claims 5 to 9 in a lithium ion battery.

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