A positive electrode active material, a method for preparing the same, and an application thereof

Abstract

The application provides a positive electrode active material, a preparation method and application thereof. The positive electrode active material comprises a ternary positive electrode active material and a coating layer coated on at least part of the surface of the ternary positive electrode active material; the coating layer comprises a lithium oxide containing an M element, wherein the M element comprises at least one of Nb, Sr, Ce and Mo; in a DSC test of the positive electrode active material, a first characteristic peak and a second characteristic peak exist, the temperature of the first characteristic peak is not lower than 200 DEG C, and the temperature of the second characteristic peak is not lower than 210 DEG C; the temperature of the first characteristic peak is lower than the temperature of the second characteristic peak, and the temperature difference between the first characteristic peak and the second characteristic peak is greater than 10 DEG C. The positive electrode active material has high structural stability and thermal stability, and when the positive electrode active material is applied to a lithium ion battery, the safety and high-temperature cycle stability of the battery can be effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion batteries, and relates to a positive electrode active material, and more particularly to a positive electrode active material, its preparation method and application. Background Technology

[0002] Lithium-ion batteries are widely used in popular fields such as electric vehicles, portable electronic devices, energy storage, and aerospace due to their advantages such as high energy density, low self-discharge, no memory effect, wide operating temperature range, and environmental friendliness. However, with the continuous development of science and technology, higher requirements have been placed on improving the energy density of lithium-ion batteries.

[0003] Currently used cathode materials mainly include lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, and ternary cathode active materials. Among them, ternary cathode active materials lithium nickel cobalt manganese oxide (NCM) and lithium nickel cobalt aluminum oxide (NCA) have attracted widespread attention due to their higher energy density, specific capacity, and power density. However, NCM and NCA have poor thermal and structural stability, leading to a decline in the high-temperature cycle performance and safety performance of batteries. Currently, ternary cathode active materials are usually modified by doping or coating to improve their thermal and structural stability; however, doping processes are complex and the doping effect is generally limited, while coating methods generally employ dry or wet methods, but both dry and wet coating methods suffer from unsatisfactory coating effects, thus failing to effectively improve the thermal and structural stability of cathode materials.

[0004] Therefore, it is urgent to develop a positive electrode active material with high thermal and structural stability to improve battery safety and high-temperature cycling performance in order to address the above problems. Summary of the Invention

[0005] To address the aforementioned shortcomings, this invention provides a positive electrode active material that exhibits high thermal stability and structural stability.

[0006] This invention provides a method for preparing a positive electrode active material, which has high thermal stability and structural stability.

[0007] The present invention provides a positive electrode sheet, which includes the above-mentioned positive electrode active material or the positive electrode active material prepared by the above-mentioned preparation method. Since the positive electrode active material has high thermal stability and structural stability, the application of the positive electrode sheet in lithium-ion batteries can significantly improve the safety and high-temperature cycle stability of the battery.

[0008] The present invention provides a lithium-ion battery comprising the above-mentioned positive electrode active material, or the positive electrode active material prepared by the above-mentioned preparation method, or the above-mentioned positive electrode sheet, and the lithium-ion battery has high safety and high temperature cycle stability.

[0009] This invention provides a positive electrode active material, the positive electrode active material comprising a ternary positive electrode active material and a coating layer covering at least a portion of the surface of the ternary positive electrode active material;

[0010] The coating layer comprises a lithium oxide containing element M, wherein M includes at least one of Nb, Sr, Ce, and Mo;

[0011] In the DSC test of the positive electrode active material, there are a first characteristic peak and a second characteristic peak. The temperature of the first characteristic peak is not lower than 200℃ and the temperature of the second characteristic peak is not lower than 210℃.

[0012] The temperature of the first characteristic peak is lower than the temperature of the second characteristic peak, and the temperature difference between the first characteristic peak and the second characteristic peak is greater than 10°C.

[0013] Furthermore, the core ternary cathode active material comprises the chemical composition of Formula 1.

[0014] Li 1+a [Ni x Co y A z ]O 2±c Formula 1

[0015] In Equation 1, 0.6≤x<1, 0<y<0.3, 0<z<0.3, 0<a<0.2, c<0.02, x+y+z=1, and A is Al or Mn.

[0016] Furthermore, the intensity of the first characteristic peak is 0.5–5 W / g, and / or the intensity of the second characteristic peak is 10–90 W / g.

[0017] Furthermore, the full width at half maximum (FWHM) of the first characteristic peak is greater than that of the second characteristic peak.

[0018] The full width at half maximum (FWHM) of the first characteristic peak is 1 to 15, and / or the full width at half maximum (FWHM) of the second characteristic peak is 0.2 to 2.

[0019] Furthermore, the mass percentage content of element M in the coating layer in the positive electrode active material is 1600-8500 ppm;

[0020] And / or, the thickness of the coating layer is 10nm to 100nm.

[0021] Furthermore, the specific surface area of ​​the positive electrode active material is >1m². 2 / g.

[0022] This invention provides a method for preparing a positive electrode active material, comprising the following steps:

[0023] The ternary cathode active material with a water content of 1-10 wt% was dispersed and continuously stirred at a stirring speed of 300-1000 rpm. During the stirring process, 500-2000 ppm of coating salt was added every 5-30 minutes until the addition was complete. After stirring for 1-3 hours, a mixture was obtained. Three samples were extracted from the mixture, and the mass content of element M in the three samples was tested. When the mass content deviation of element M in the three samples was <10%, the mixture was sintered at a sintering temperature of 620-720℃ for 6-15 hours to obtain the cathode active material. The solubility of the coating salt in water was 0.01-1 g / 100 g.

[0024] Furthermore, the water content of the ternary cathode active material is 3-6 wt%.

[0025] And / or, the median particle size of the ternary cathode active material is >10 μm;

[0026] And / or, the coated salt includes at least one of niobium oxalate, strontium oxalate, cerium oxalate, and molybdate.

[0027] The present invention provides a positive electrode sheet, comprising the positive electrode active material described in any one of the above-mentioned methods or the positive electrode active material prepared by the above-mentioned preparation method.

[0028] The present invention provides a lithium-ion battery, comprising the above-described positive electrode active material, or the positive electrode active material prepared by the above-described preparation method, or the above-described positive electrode sheet.

[0029] The positive electrode active material provided by the present invention has a core-shell structure, including a core ternary positive electrode active material and a coating layer. The coating layer includes lithium oxide containing element M. The positive electrode active material has high structural stability and thermal stability, which can effectively improve the safety and high-temperature cycle stability of the battery. Based on this phenomenon, the inventors analyzed the DSC test results of the positive electrode active material and concluded that: during the DSC test, as the temperature gradually increases, the positive electrode active material undergoes two phase transitions. The first phase transition is from a layered structure to a spinel structure, corresponding to the first characteristic peak. The second phase transition is from a spinel structure to a rock salt phase structure, corresponding to the second characteristic peak. When the peak temperature corresponding to the first characteristic peak is not lower than 200℃, the temperature at which the positive electrode active material undergoes the first phase transition is relatively high, its structure is more stable, its thermal stability is better, and the corresponding full-charge thermal runaway initiation temperature is higher. When the peak temperature corresponding to the second characteristic peak is not lower than 210℃, and the temperature difference between the first and second characteristic peaks is greater than 10℃, the second phase transition occurs later, which can prolong the phase transition time, thereby giving the positive electrode active material higher structural stability and thermal stability. Correspondingly, the maximum full-charge thermal runaway temperature is also lower, thus effectively improving the battery's safety and high-temperature cycle stability. Attached Figure Description

[0030] Figure 1 This is a SEM image of the positive electrode active material prepared in Example 1 of the present invention;

[0031] Figure 2 This is a SEM image of the positive electrode active material prepared in Comparative Example 2 of the present invention;

[0032] Figure 3 The image shows the DSC test result of the positive electrode active material prepared in Example 1 of this invention.

[0033] Figure 4 The image shows the DSC test result of the positive electrode active material prepared in Comparative Example 2 of this invention. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0035] The first aspect of the present invention provides a positive electrode active material, the positive electrode active material comprising a ternary positive electrode active material and a coating layer covering at least a portion of the surface of the ternary positive electrode active material;

[0036] The coating layer comprises a lithium oxide containing element M, wherein M includes at least one of Nb, Sr, Ce, and Mo;

[0037] In the DSC test of the positive electrode active material, there are a first characteristic peak and a second characteristic peak. The temperature of the first characteristic peak is not lower than 200℃ and the temperature of the second characteristic peak is not lower than 210℃.

[0038] The temperature of the first characteristic peak is lower than the temperature of the second characteristic peak, and the temperature difference between the first and second characteristic peaks is greater than 10℃.

[0039] It can be understood that when the element M is Nb, the lithium oxide containing the element M is lithium niobate; when the element M is Sr, the lithium oxide containing the element M is lithium strontium oxide; when the element M is Ce, the lithium oxide containing the element M is lithium cerium oxide; and when the element M is Mo, the lithium oxide containing the element M is lithium molybdate.

[0040] The first and second characteristic peaks in this invention can be two independent characteristic peaks or two characteristic peaks that partially overlap.

[0041] The ternary cathode active material in this invention includes lithium nickel cobalt aluminum oxide or lithium nickel cobalt manganese oxide, preferably a polycrystalline material. This invention does not specifically limit the source of the ternary cathode active material; for example, it can be a commercially available product well-known to those skilled in the art or a product prepared using conventional methods.

[0042] The DSC test in this invention includes: Preparation of a coin cell: The positive electrode active material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) of this invention are thoroughly mixed with N-methylpyrrolidone at a mass ratio of 95:3:24 to obtain a uniform positive electrode slurry; this positive electrode slurry is coated onto both functional surfaces of an aluminum foil, vacuum dried at 110°C, and cold-pressed to obtain a compaction density of 3.4 g / cm³. 3 The positive electrode sheet has a total active layer thickness of 100 μm. At 25 °C and normal pressure (0.1 MPa), lithium metal sheets, positive electrode sheets, and electrolytes are assembled into a CR2026 coin cell lithium-ion battery in a coin cell box. The electrolyte includes lithium salt LiPF6 and anhydrous organic solvents ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), and ethylene carbonate (VC). The concentration of lithium salt is 1 mol / L, and the volume ratio of EC, DEC, PC, and VC is 30:48:20:2.

[0043] When the Ni content in the positive electrode active material is 60%–85%, the coin cell prepared above is charged at a rate of 0.33C to a cutoff voltage of 4.4V, and then the positive electrode is disassembled and subjected to DSC testing. When the Ni content in the positive electrode active material is 85%–100%, the coin cell prepared above is charged at a rate of 0.33C to a cutoff voltage of 4.3V, and then the positive electrode is disassembled and subjected to DSC testing. It should be noted that the Ni content here refers to the molar content. The DSC test conditions are as follows: the disassembled positive electrode is cleaned with DMC and dried, a certain amount of 0.3mL of electrolyte is added, and the thermal decomposition curve is tested using a differential scanning calorimeter to obtain the temperatures corresponding to the first and second characteristic peaks. The heating rate is set to 10℃ / min, the temperature range is 50–350℃, the purging gas is N2, and the flow rate is 10mL / min.

[0044] In this invention, the temperature of the first characteristic peak refers to the temperature value corresponding to the highest point of the first characteristic peak, and the temperature of the second characteristic peak refers to the temperature value corresponding to the highest point of the second characteristic peak.

[0045] The positive electrode active material of the present invention includes a ternary positive electrode active material and a coating layer covering at least a portion of the surface of the ternary positive electrode active material, wherein the coating layer includes lithium oxide containing element M; the positive electrode active material has high structural stability and thermal stability, and applying the positive electrode active material to lithium-ion batteries can effectively improve the safety and high-temperature cycle stability of the batteries. Based on this phenomenon, the inventors analyzed the DSC test results of the positive electrode active material and concluded that: during the DSC test, as the temperature gradually increases, the positive electrode active material first transforms from a layered structure to a spinel structure, and then from a spinel structure to a rock salt phase structure. The transformation from a layered structure to a spinel structure corresponds to the first characteristic peak in the DSC test results, while the transformation from a spinel structure to a rock salt phase structure corresponds to the second characteristic peak. When the temperature corresponding to the first characteristic peak is not lower than 200℃, the temperature at which the layered structure transforms to the spinel structure can be increased, giving the positive electrode active material higher structural stability, improving its thermal battery stability, and thus increasing the full-charge thermal runaway initiation temperature. When the temperature corresponding to the second characteristic peak is not lower than 210℃, and the temperature difference between the first and second characteristic peaks is not lower than 10℃, the time for the transformation from the spinel structure to the rock salt phase structure can be delayed, further improving the structural and thermal stability of the positive electrode active material, reducing the maximum full-charge thermal runaway temperature, and thus comprehensively improving the battery's safety and high-temperature cycle performance.

[0046] In one specific embodiment, the ternary cathode active material comprises the chemical composition of Formula 1.

[0047] Li1+a [Ni x Co y A z ]O 2±c Formula 1

[0048] In Equation 1, 0.6≤x<1, 0<y<0.3, 0<z<0.3, 0<a<0.2, c<0.02, x+y+z=1, and A is Al or Mn.

[0049] When the positive electrode active material includes the chemical composition of Formula 1, the positive electrode active material has high structural stability and thermal stability.

[0050] Furthermore, when 0.85≤x<1, the Ni content in the positive electrode active material is relatively high. At this time, not only can higher structural stability and thermal stability be achieved, effectively improving the safety and high-temperature cycle performance of the battery, but the battery can also have higher capacity and energy density.

[0051] This invention does not limit the preparation method of the ternary cathode active material. For example, it can be prepared by including the following steps:

[0052] 1) Ni, the precursor of the positive electrode active material x Co y Mn z (OH)2 or Ni x Co y Al z (OH)2 and lithium hydroxide (LiOH) are added to a high-speed mixer and mixed for 1 hour to obtain a mixture. The molar ratio of Me in the positive electrode active material precursor to Li in the lithium hydroxide is 1:(1~1.2), where Me represents the total molar amount of Ni, Co, Mn or Ni, Co, Al in the positive electrode active material precursor.

[0053] 2) The above mixture is placed in an atmosphere sintering furnace for sintering at a temperature of 800-920℃ for 10-20 hours to obtain a sintered product. The sintered product is then crushed, sieved, washed, filtered, and dried to obtain a ternary positive electrode active material.

[0054] In one specific embodiment, the intensity of the first characteristic peak is 0.5–5 W / g. Within this range, the positive electrode active material is less likely to transform from a layered structure to a spinel structure, exhibiting higher structural and thermal stability, which can further improve the battery's cycle performance and increase the full-charge thermal runaway initiation temperature.

[0055] In one specific embodiment, the intensity of the second characteristic peak is 10–90 W / g. When the peak intensity of the second characteristic peak is within the aforementioned range, the positive electrode active material is less likely to undergo a transformation from a spinel structure to a rock salt phase structure at high temperatures, which can further improve the structural stability and thermal stability of the positive electrode active material. Correspondingly, it can further reduce the maximum temperature of the battery's full-charge thermal runaway and improve the battery's safety.

[0056] It is understood that since the DSC test curve is a scanning curve with temperature on the x-axis and the heat required to achieve zero temperature difference between the sample and the reference on the y-axis, the heat in the DSC curve does not start from 0. Therefore, the peak intensity of the first characteristic peak and the peak intensity of the second characteristic peak in this invention refer to the peak intensity values ​​starting from the baseline. Figure 3 and Figure 4 As shown in the figure, the dashed line is the baseline.

[0057] In one specific embodiment, the half-width at half-maximum (WHM) of the first characteristic peak is greater than the half-width at half-maximum (WHM) of the second characteristic peak; the WHM of the first characteristic peak is 1 to 15. When the WHM of the first characteristic peak is greater than the WHM of the second characteristic peak, and the WHM of the first characteristic peak is within the aforementioned range, the time for the transition from a layered structure to a spinel structure can be further extended, and the severity of the transition from a layered structure to a spinel structure can be reduced, thereby increasing the full-charge thermal runaway initiation temperature and improving the cycle performance of the battery.

[0058] In one specific embodiment, the half-width at half-maximum (WHM) of the first characteristic peak is greater than that of the second characteristic peak; the WHM of the second characteristic peak is 0.2–2. This not only prolongs the transition time from spinel structure to rock salt phase structure but also slows down the intensity of the reaction during the transition, thereby reducing the maximum temperature of the battery's full-charge thermal runaway and thus improving battery safety.

[0059] In one specific embodiment, the mass percentage of element M in the coating layer within the positive electrode active material is 1600–8500 ppm. Within this range, the content of element M coated on the surface of the positive electrode active material is moderate and will not negatively affect the cycle performance of the battery. This allows the battery to exhibit both low maximum temperature and low onset temperature of full-charge thermal runaway while maintaining high cycle performance. Specifically, the mass percentage of element M in the coating layer within the positive electrode active material can be further controlled by adjusting the amount of coating salt added during the preparation process, keeping it between 1600 and 8500 ppm.

[0060] The mass percentage of element M in the coating layer of the present invention in the positive electrode active material was obtained by ICP testing.

[0061] In one specific embodiment, the thickness of the coating layer is 10 nm to 100 nm. When the thickness of the coating layer is within this range, the temperature difference between the first characteristic peak and the second characteristic peak can be further increased, thereby further prolonging the time for the spinel structure to transform into the rock salt phase structure, improving the structural stability and thermal stability of the positive electrode active material, reducing the maximum temperature of the battery's full-charge thermal runaway, and giving the battery higher safety and high-temperature cycle stability.

[0062] In one specific embodiment, the specific surface area of ​​the positive electrode active material is >1m². 2 / g. Within this range, the positive electrode active material has a high specific surface area, which can effectively improve the wettability of the electrolyte to the electrode, facilitate lithium-ion diffusion, ensure capacity utilization, and improve the rate performance and cycle performance of the battery.

[0063] A second aspect of the present invention provides a method for preparing a positive electrode active material according to the first aspect, comprising the following steps:

[0064] The ternary cathode active material with a water content of 1-10 wt% was dispersed and continuously stirred at a stirring speed of 300-1000 rpm. During the stirring process, 500-2000 ppm of coating salt was added every 5-30 minutes until the addition was complete. After stirring for 1-3 hours, a mixture was obtained. Three samples were extracted from the mixture, and the mass content of element M in the three samples was tested. When the mass content of element M in the three samples differed by less than 10%, the mixture was sintered at a temperature of 620-720℃ for 6-15 hours to obtain the cathode active material. The solubility of the coating salt in water was 0.01-1 g / 100 g.

[0065] The solubility of the coated salt in water in this invention refers to its solubility at room temperature (25°C).

[0066] The present invention does not specifically limit the type of coated salt, as long as the coated salt includes element M (at least one of Nb, Sr, Ce, and Mo) and has a solubility of 0.01 to 1 g / 100 g in water at room temperature (25°C).

[0067] The present invention does not specify the method for testing the mass content of element M in the sample; for example, it can be tested by ICP.

[0068] This invention does not limit the method for controlling the water content of ternary cathode active materials. For example, after washing the ternary cathode active material, the water content in the ternary cathode active material can be further controlled by controlling the filtration time, so that the water content of the ternary cathode active material is between 1 and 10 wt%. Alternatively, the ternary cathode active material with a water content of 1 to 10 wt% can be obtained by uniformly mixing the completely dried ternary cathode active material with deionized water.

[0069] The present invention does not specifically limit the amount of coating salt added. Furthermore, the mass percentage of element M in the final prepared positive electrode active material can be adjusted to be between 1600 and 8500 ppm by adjusting the amount of coating salt added.

[0070] The method for preparing the positive electrode active material in this invention involves controlling the water content of the ternary positive electrode active material and using a slightly soluble coating salt. Specifically, the solubility of the coating salt in water is controlled to be 0.01–1 g / 100 g. A special mixing method is employed, and a core-shell structured positive electrode active material is prepared at a specific sintering temperature and time. In the DSC test of this positive electrode active material, a first characteristic peak at a temperature not lower than 200°C and a second characteristic peak at a temperature not lower than 210°C are observed, with a temperature difference of not less than 10°C between the two peaks. The first characteristic peak corresponds to the transformation of the positive electrode active material from a layered structure to a spinel structure, and the second characteristic peak corresponds to the transformation of the positive electrode active material from a spinel structure to a rock salt phase structure. At this point, the positive electrode active material exhibits high structural stability and thermal stability. Consequently, the lithium-ion battery including this positive electrode active material has a high full-charge thermal runaway initiation temperature and a low full-charge thermal runaway maximum temperature, thus exhibiting high safety performance and high-temperature cycling performance.

[0071] In one specific embodiment, the water content of the ternary cathode active material is 3-6 wt%. Within this range, the ternary cathode active material can be better mixed with the coating salt, improving the uniformity of the coating layer on the surface of the ternary cathode active material, thereby further improving the structural stability and thermal stability of the material, and giving the battery higher safety and high-temperature cycle performance.

[0072] In one specific embodiment, the median particle size of the ternary cathode active material is 10–20 μm. When the median particle size of the ternary cathode active material is within the aforementioned range, agglomeration caused by excessively small particles can be avoided, improving the uniformity of the coating layer; at the same time, it also has high strength and is not easily broken during the coating process.

[0073] In one specific embodiment, the coating salt includes at least one of niobium oxalate, strontium oxalate, cerium oxalate, and molybdic acid.

[0074] A third aspect of this invention provides a positive electrode sheet, comprising the positive electrode active material of the first aspect or the positive electrode active material prepared by the preparation method of the second aspect. Because this positive electrode active material has a core-shell structure and exhibits a first characteristic peak at a temperature not lower than 200°C and a second characteristic peak at a temperature not lower than 210°C in DSC testing, with a temperature difference between the first and second characteristic peaks greater than 10°C, this positive electrode active material possesses high structural stability and thermal stability. Its application in lithium-ion batteries can effectively improve battery safety and high-temperature cycle performance.

[0075] The fourth aspect of the present invention provides a lithium-ion battery, which, since it includes the positive electrode active material of the first aspect, or the positive electrode active material prepared by the preparation method of the second aspect, or the positive electrode sheet of the third aspect, has high safety and high temperature cycle performance.

[0076] The high-nickel cathode active material of the present invention will be described in detail below through specific embodiments.

[0077] Example 1

[0078] 1) Ni, the precursor of the positive electrode active material 0.8 Co 0.1 Mn 0.1 (OH)2 and lithium hydroxide (LiOH) are added to a high-speed mixer and mixed for 1 hour to obtain a mixture. The molar ratio of Me in the positive electrode active material precursor to Li in the lithium hydroxide is 1:1.05, where Me represents the total molar amount of Ni, Co, and Mn in the positive electrode active material precursor.

[0079] 2) The mixture was placed in an atmosphere sintering furnace for sintering at a temperature of 830℃ for 18 hours to obtain the sintered product. The sintered product was crushed, sieved, and washed with water. The mass ratio of water to the sieved material was 1:1 during the washing process. The washing time was 1 minute, followed by filtration for 18 minutes to obtain a ternary positive electrode active material with a water content of 5.3 wt% and a median particle size of 12.59 μm.

[0080] 3) The above-mentioned moist ternary cathode active material was dispersed and continuously stirred at a stirring speed of 400 rpm. During the stirring process, 1000 ppm of the coated niobium oxalate (the total mass of Nb in the coated salt is 6000 ppm by mass) was added every 10 minutes until the addition was complete. After stirring for 3 hours, a mixture was obtained. Three samples were extracted from the mixture and ICP tests were performed on the three samples. The Nb content deviation was found to be <10%. The mixture was sintered at a temperature of 700°C for 10 hours to obtain the cathode active material of this embodiment. ICP testing showed that the mass percentage of Nb in the coating layer in the cathode active material was 4852 ppm.

[0081] Example 2

[0082] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 10 min, and the water content of the ternary positive electrode active material becomes 8.9 wt%.

[0083] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb element content of 4875 ppm in the coating layer.

[0084] Example 3

[0085] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 19 min and the water content of the ternary positive electrode active material becomes 4.8 wt%.

[0086] In step (3), 1000 ppm of coated niobium oxalate is added every 5 minutes, and the total mass percentage of Nb in the coated salt in the ternary cathode active material is adjusted to 10000 ppm.

[0087] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 8125 ppm in the coating layer.

[0088] Example 4

[0089] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 16 min and the water content of the ternary positive electrode active material becomes 6.4 wt%.

[0090] In step (3), 500 ppm of coated niobium oxalate is added every 20 min, and the total mass percentage of Nb in the coated salt in the ternary cathode active material is adjusted to 2000 ppm.

[0091] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 1689 ppm in the coating layer.

[0092] Example 5

[0093] 1) Ni, the precursor of the positive electrode active material 0.8 Co 0.18 Al 0.02 (OH)2 and lithium hydroxide (LiOH) are added to a high-speed mixer and mixed for 1 hour to obtain a mixture. The molar ratio of Me in the positive electrode active material precursor to Li in the lithium hydroxide is 1:1.05, where Me represents the total molar amount of Ni, Co and Al in the positive electrode active material precursor.

[0094] 2) The mixture was placed in an atmosphere sintering furnace for sintering at a temperature of 825℃ for 18 hours to obtain the sintered product. The sintered product was crushed, sieved, and washed with water. The mass ratio of water to the sieved material was 1:1 during the washing process. The washing time was 1 minute and the material was filtered for 20 minutes to obtain a ternary positive electrode active material with a water content of 4.4 wt% and a median particle size of 12.59 μm.

[0095] 3) Disperse the above-mentioned moist ternary cathode active material and stir continuously at a stirring speed of 400 rpm. During the stirring process, add 1500 ppm of coated niobium oxalate (the total mass of Nb element in the coated salt is 6000 ppm by mass) every 10 min until the addition is complete. Continue stirring for 3 h to obtain a mixture. Take three samples from the mixture and perform ICP testing on the three samples respectively. The Nb element content deviation is <10%. Sinter the mixture at a temperature of 700℃ for 10 h to obtain the cathode active material of this embodiment.

[0096] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 4845 ppm in the coating layer.

[0097] Example 6

[0098] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 40 min, and the water content of the ternary positive electrode active material becomes 1.3 wt%.

[0099] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 4825 ppm in the coating layer.

[0100] Example 7

[0101] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 15 min to obtain a ternary positive electrode active material with a water content of 6.7 wt%.

[0102] In step (3), the total mass percentage of Nb in the coated salt in the ternary cathode active material is adjusted to 2000 ppm, the sintering temperature is adjusted to 720℃, and the sintering time is adjusted to 12h.

[0103] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 1652 ppm in the coating layer.

[0104] Example 8

[0105] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 40 min to obtain a ternary positive electrode active material with a water content of 1.3 wt%.

[0106] In step (3), 2000 ppm of coated niobium oxalate is added every 30 min. The total mass percentage of Nb in the coated salt in the ternary cathode active material is adjusted to 10000 ppm. The sintering temperature is adjusted to 680℃ and the sintering time is adjusted to 6h.

[0107] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb element content of 8210 ppm in the coating layer.

[0108] Example 9

[0109] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 20 min to obtain a ternary positive electrode active material with a water content of 4.4 wt%.

[0110] In step (3), the total mass percentage of Nb in the coated salt in the ternary cathode active material is adjusted to 2000 ppm, the sintering temperature is adjusted to 710℃, and the sintering time is adjusted to 10h.

[0111] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 1671 ppm in the coating layer.

[0112] Example 10

[0113] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 18 min to obtain a ternary positive electrode active material with a water content of 5.3 wt%.

[0114] In step (3), the total mass percentage of Nb in the coated salt in the ternary cathode active material is adjusted to 10,000 ppm, the sintering temperature is adjusted to 690℃, and the sintering time is adjusted to 10h.

[0115] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 8115 ppm in the coating layer.

[0116] Example 11

[0117] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 25 min to obtain a ternary positive electrode active material with a water content of 3.2 wt%.

[0118] In step (3), the total mass percentage of Nb in the coated salt in the ternary cathode active material is adjusted to 2000 ppm, the sintering temperature is adjusted to 720℃, and the sintering time is adjusted to 10h.

[0119] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 1751 ppm in the coating layer.

[0120] Example 12

[0121] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 15 min to obtain a ternary positive electrode active material with a water content of 6.7 wt%.

[0122] In step (3), the total mass percentage of Nb in the coated salt in the ternary cathode active material is adjusted to 1800 ppm, the sintering temperature is adjusted to 720℃, and the sintering time is adjusted to 15h.

[0123] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 1498 ppm in the coating layer.

[0124] Example 13

[0125] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 13 min to obtain a ternary positive electrode active material with a water content of 7.1 wt%.

[0126] In step (3), the total mass percentage of Nb in the coated salt in the ternary cathode active material is adjusted to 11,000 ppm, the sintering temperature is adjusted to 680℃, and the sintering time is adjusted to 9h.

[0127] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 8895 ppm in the coating layer.

[0128] Example 14

[0129] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (2), the particle size of the ternary positive electrode active material is adjusted to 8.18 μm and the filtration time is adjusted to 19 min to obtain a ternary positive electrode active material with a water content of 4.8 wt%.

[0130] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 4875 ppm in the coating layer.

[0131] Example 15

[0132] 1) Ni, the precursor of the positive electrode active material 0.9 Co 0.05 Mn 0.05 (OH)2 and lithium hydroxide (LiOH) are added to a high-speed mixer and mixed for 1 hour to obtain a mixture. The molar ratio of Me in the positive electrode active material precursor to Li in the lithium hydroxide is 1:1.05, where Me represents the total molar amount of Ni, Co, and Mn in the positive electrode active material precursor.

[0133] 2) The mixture was placed in an atmosphere sintering furnace for sintering at a temperature of 780℃ for 16 hours to obtain the sintered product. The sintered product was crushed, sieved, and washed with water. The mass ratio of water to the sieved material was 1:1 during the washing process. The washing was carried out for 1 minute and the material was filtered for 20 minutes to obtain a ternary positive electrode active material with a water content of 5.1 wt%.

[0134] 3) The above-mentioned moist ternary cathode active material was broken up and continuously stirred at a stirring speed of 400 rpm. During the stirring process, 1000 ppm of coated niobium oxalate (the total mass of Nb element in the coated salt is 6000 ppm by mass) was added every 10 min until the addition was complete. After stirring for 3 h, a mixture was obtained. Three samples were extracted from the mixture and ICP tests were performed on the three samples respectively. The Nb element content deviation was found to be <10%. The mixture was sintered at a temperature of 700℃ for 10 h to obtain the cathode active material of this embodiment.

[0135] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Nb content of 4869 ppm in the coating layer.

[0136] Example 16

[0137] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (3), the coating salt is replaced with strontium oxalate (SrC2O4).

[0138] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Sr element content of 4892 ppm in the coating layer.

[0139] Example 17

[0140] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (3), the coating salt is replaced with cerium oxalate Ce2(C2O4)3 (the total mass of Ce element in the coating salt is 6000ppm in the ternary positive electrode active material).

[0141] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Ce element content of 4895 ppm in the coating layer.

[0142] Example 18

[0143] The preparation method of the positive electrode active material in this embodiment is basically the same as that in Example 1. The difference is that in step (3), the coating salt is replaced with molybdic acid (MoO3·H2O) (the total mass of Mo element in the coating salt is 6000ppm in the ternary positive electrode active material).

[0144] The positive electrode active material prepared in this embodiment was found by ICP testing to have a Mo content of 4882 ppm in the coating layer.

[0145] Comparative Example 1

[0146] The preparation method of the positive electrode active material in this comparative example is basically the same as that in Example 1. The difference is that in step (3), the filtration time is adjusted to 56 min and the water content of the ternary positive electrode active material is 0.4 wt%. Three samples are extracted from the mixture and tested by ICP respectively. The Nb content deviation is >10%.

[0147] The positive electrode active material prepared in this comparative example was found by ICP testing to have a Nb content of 4765 ppm in the coating layer.

[0148] Comparative Example 2

[0149] The preparation method of the positive electrode active material in this comparative example is basically the same as that in Example 1. The difference is that in step (2), the filtration time is adjusted to 7 min, and the water content of the ternary positive electrode active material becomes 11.7 wt%. Three samples are extracted from the mixture, and the three samples are tested by ICP respectively, and the Nb content deviation is >10%.

[0150] The positive electrode active material prepared in this comparison was found by ICP testing to have a Nb content of 4723 ppm in the coating layer.

[0151] Comparative Example 3

[0152] The preparation method of the positive electrode active material in this comparative example is basically the same as that in Example 1. The difference is that in step (3), the sintering temperature is adjusted to 600℃ and the sintering time is adjusted to 5℃.

[0153] The positive electrode active material prepared in this comparison was found by ICP testing to have a Nb content of 4715 ppm in the coating layer.

[0154] Comparative Example 4

[0155] The preparation method of the positive electrode active material in this comparative example is basically the same as that in Example 1. The difference is that in step (3), the moist ternary positive electrode active material is broken up, and all the coated niobium oxalate (the total mass of Nb element in the coated salt is 6000ppm in the ternary positive electrode active material) is mixed with the moist ternary positive electrode active material at one time. The mixture is stirred continuously at a stirring speed of 400rpm. After 3h, the mixture is obtained. Three samples are extracted from the mixture and tested by ICP. The Nb element content deviation is >10%. The mixture is sintered at a temperature of 700℃ for 10h to obtain the positive electrode active material of this comparative example.

[0156] The positive electrode active material prepared in this comparison was found by ICP testing to have a Nb content of 4687 ppm in the coating layer.

[0157] Comparative Example 5

[0158] The preparation method of the positive electrode active material in this comparative example is basically the same as that in Example 1. The difference is that in step (3), the above-mentioned moist ternary positive electrode active material is broken up and continuously stirred at a stirring speed of 250 rpm. During the stirring process, 3000 ppm of coated niobium oxalate (the total mass of Nb element in the coated salt is 6000 ppm by mass) is added every 10 min until it is completely added. After stirring for 0.5 h, a mixture is obtained. Three samples are extracted from the mixture and tested by ICP. The Nb element content deviation is >10%. The mixture is sintered at a temperature of 700℃ for 10 h to obtain the positive electrode active material of this comparative example.

[0159] The positive electrode active material prepared in this comparison was found by ICP testing to have a Nb content of 4695 ppm in the coating layer.

[0160] Test case

[0161] 1. The positive electrode active materials prepared in Example 1 and Comparative Example 1 were subjected to SEM testing. The test results are shown in [Figure 1]. Figure 1 and Figure 2 .

[0162] Figure 1 and Figure 2 SEM images of the positive electrode active materials prepared in Example 1 and Comparative Example 1, respectively. Figure 1 and Figure 2 It can be seen that the positive electrode active material prepared in Example 1 has a uniform coating layer on its surface, while the positive electrode active material prepared in Comparative Example 1 has a smaller coating layer on its surface.

[0163] 2. The positive electrode active materials prepared in the above embodiments and comparative examples are applied to coin cells. The specific steps are as follows:

[0164] Preparation of coin cell: The positive electrode active material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) of this invention are thoroughly mixed with N-methylpyrrolidone at a mass ratio of 95:3:24 to obtain a uniform positive electrode slurry; the positive electrode slurry is coated onto both functional surfaces of an aluminum foil, vacuum dried at 110°C, and cold-pressed to obtain a compaction density of 3.4 g / cm³. 3The positive electrode sheet was modified to have a total thickness of 100 μm for the positive active layer. At 25 °C and normal pressure (0.1 MPa), lithium metal sheets, positive electrode sheets, and electrolyte were assembled into a CR2026 coin cell lithium-ion battery in a coin cell box. The electrolyte included lithium salt LiPF6 and anhydrous organic solvents ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), and ethylene carbonate (VC). The concentration of lithium salt was 1 mol / L, and the volume ratio of EC, DEC, PC, and VC was 30:48:20:2.

[0165] (1) DSC test

[0166] When the Ni content in the positive electrode active material is 60%–85%, the coin cell prepared above will be charged at a rate of 0.33C to a cutoff voltage of 4.4V, and then the positive electrode will be disassembled and subjected to DSC testing. When the Ni content in the positive electrode active material is 85%–100%, the coin cell prepared above will be charged at a rate of 0.33C to a cutoff voltage of 4.3V, and then the positive electrode will be disassembled and subjected to DSC testing. It should be noted that the Ni content here refers to the molar content.

[0167] The DSC test conditions are as follows: the disassembled positive electrode is cleaned and dried with DMC, a certain amount of 0.3 mL of electrolyte is added, and the thermal decomposition curve is obtained by differential scanning calorimetry. The temperatures corresponding to the first and second characteristic peaks are obtained. The heating rate is set to 10℃ / min, the temperature range is 50~350℃, the purging gas is N2, and the flow rate is 10 mL / min.

[0168] The test results are shown in Table 1 and Figure 3 and Figure 4 .

[0169] Figure 3 and Figure 4 The DSC test results are shown for the positive electrode active materials prepared in Example 1 and Comparative Example 1, respectively. Figure 3 and Figure 4 It can be seen that the positive electrode active material prepared in Example 1 has a first characteristic peak at a temperature of 229.45℃ and a second characteristic peak at a temperature of 245.74℃. The peak intensity of the first characteristic peak is 2.74 and the half-maximum width is 10.52, while the peak intensity of the second characteristic peak is 58.79 and the half-maximum width is 0.95. However, the positive electrode active material prepared in Comparative Example 1 does not have the first characteristic peak.

[0170] Table 1

[0171]

[0172] As shown in Table 1:

[0173] In the DSC tests of the positive electrode active materials in Examples 1-18, a first characteristic peak and a second characteristic peak were all present, with the temperature of the first characteristic peak not lower than 200℃ and the temperature of the second characteristic peak not lower than 210℃, and the temperature difference between the first and second characteristic peaks was greater than 10℃. However, the positive electrode active materials in Comparative Examples 1-5 did not meet the aforementioned conditions.

[0174] (2) Safety and high-temperature cycle performance tests were conducted on the coin cells prepared above:

[0175] Safety: The button cell prepared in (1) above was fully charged to the cutoff voltage at 25°C at a rate of 1C. It was then charged at a constant voltage at the cutoff voltage until the current was 0.05C, which was considered to be 100% SOC. A heating film was added to the surface of the battery and ensured to be tightly sealed to the battery. The heating rate was set to 5°C / min. A sensor was attached to the surface of the battery, and the starting temperature and the highest temperature of thermal runaway during full charge were tested.

[0176] High-temperature cycling performance: At 45℃, the lithium-ion battery is charged to 4.25V and then charged at a constant voltage to the cutoff current of 0.05C, and then left to rest for 20 minutes; it is then discharged at 1C to 2.8V and left to rest for 20 minutes to obtain the initial capacity Q1 of the battery; after 300 cycles, the capacity Q2 of the battery is obtained. The battery's high-temperature cycle retention rate after 300 cycles is the ratio of Q2 to Q1.

[0177] The test results are shown in Table 2.

[0178] Table 2

[0179]

[0180]

[0181] As shown in Table 2:

[0182] The button cells in Examples 1-18 exhibit better safety and high-temperature cycle performance compared to Comparative Examples 1-5. In Example 13, the highest thermal runaway initiation temperature at full charge reached 242°C, and correspondingly, the highest thermal runaway temperature at full charge was 663°C, with a high-temperature cycle capacity retention rate of up to 93.4%. This demonstrates that the positive electrode active material of the present invention possesses high thermal and structural stability. Therefore, applying this positive electrode sheet to lithium-ion batteries can significantly improve battery safety and high-temperature cycle stability.

[0183] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes a ternary positive electrode active material and a coating layer covering at least a portion of the surface of the ternary positive electrode active material; The coating layer comprises a lithium oxide containing element M, wherein M includes at least one of Nb, Sr, Ce, and Mo; In the DSC test of the positive electrode active material, there are a first characteristic peak and a second characteristic peak. The temperature of the first characteristic peak is not lower than 200℃ and the temperature of the second characteristic peak is not lower than 210℃. The temperature of the first characteristic peak is lower than the temperature of the second characteristic peak, and the temperature difference between the first characteristic peak and the second characteristic peak is greater than 10°C. The DSC test conditions for the positive electrode active material are as follows: when the Ni content in the positive electrode active material is 60%~85%, the battery containing the positive electrode active material is charged at a rate of 0.33C to a cutoff voltage of 4.4V, and then the positive electrode sheet disassembled from the battery is subjected to DSC testing; when the Ni content in the positive electrode active material is 85%~100%, the battery containing the positive electrode active material is charged at a rate of 0.33C to a cutoff voltage of 4.3V, and then the positive electrode sheet disassembled from the battery is subjected to DSC testing; wherein, the Ni content refers to molar content; During the DSC test, as the temperature gradually increases, the positive electrode active material undergoes two phase transitions. The first phase transition is from a layered structure to a spinel structure, corresponding to the first characteristic peak. The second phase transition is from a spinel structure to a rock salt phase structure, corresponding to the second characteristic peak.

2. The positive electrode active material according to claim 1, characterized in that, The ternary cathode active material comprises the chemical composition of Formula 1. Li 1+a [Ni x Co y A z O 2±c Formula 1 In Equation 1, 0.6≤x<1, 0<y<0.3, 0<z<0.3, 0<a<0.2, c<0.02, x+y+z=1, and A is Al or Mn.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The intensity of the first characteristic peak is 0.5–5 W / g, and / or the intensity of the second characteristic peak is 10–90 W / g.

4. The positive electrode active material according to any one of claims 1-3, characterized in that, The half-width at half maximum (WHM) of the first characteristic peak is greater than that of the second characteristic peak. The full width at half maximum (FWHM) of the first characteristic peak is 1 to 15, and / or the full width at half maximum (FWHM) of the second characteristic peak is 0.2 to 2.

5. The positive electrode active material according to any one of claims 1-4, characterized in that, The mass percentage of element M in the coating layer in the positive electrode active material is 1600–8500 ppm; And / or, the thickness of the coating layer is 10nm to 100nm.

6. The positive electrode active material according to any one of claims 1-5, characterized in that, The specific surface area of ​​the positive electrode active material is >1m². 2 / g.

7. A method for preparing the positive electrode active material according to any one of claims 1-6, characterized in that, Includes the following steps: The ternary cathode active material with a water content of 1-10 wt% was dispersed and continuously stirred at a stirring speed of 300-1000 rpm. During the stirring process, 500-2000 ppm of coating salt was added every 5-30 minutes until the addition was complete. After stirring for 1-3 hours, a mixture was obtained. Three samples were extracted from the mixture, and the mass content of element M in the three samples was tested. When the mass content deviation of element M in the three samples was <10%, the mixture was sintered at a sintering temperature of 620-720℃ for 6-15 hours to obtain the cathode active material. The solubility of the coating salt in water was 0.01-1 g / 100 g.

8. The method for preparing the positive electrode active material according to claim 7, characterized in that, The water content of the ternary cathode active material is 3-6 wt%; And / or, the median particle size of the ternary cathode active material is greater than 10 μm; And / or, the coated salt includes at least one of niobium oxalate, strontium oxalate, cerium oxalate, and molybdate.

9. A positive electrode plate, characterized in that, This includes the positive electrode active material as described in any one of claims 1-6 or the positive electrode active material prepared by the preparation method described in claim 7 or 8.

10. A lithium-ion battery, characterized in that, It includes the positive electrode active material according to any one of claims 1-6, or the positive electrode active material prepared by the preparation method according to claim 7 or 8, or the positive electrode sheet according to claim 9.

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