Lithium cobalt oxide positive electrode material and lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium cobalt oxide positive electrode material and a lithium ion secondary battery comprising the lithium cobalt oxide positive electrode material. The lithium cobalt oxide positive electrode material comprises lithium cobalt oxide particles, the lithium cobalt oxide particles comprise a matrix, and the matrix is provided with a first area and a second area located on the outer surface of the first area; the mass content of the Ni element in the lithium cobalt oxide positive electrode material is less than that of the Mn element; the mass content of the Mn element in the second region is greater than that in the first region; when the lithium cobalt oxide positive electrode material takes lithium metal as a counter electrode, the lithium cobalt oxide positive electrode material is charged to 4.5 V, and the lithium cobalt oxide positive electrode material has a first diffraction peak at 38-38.2 degrees in an X-ray diffraction pattern. The lithium cobalt oxide positive electrode material disclosed by the invention has relatively high gram volume exertion and structural stability. Comprising the lithium cobalt oxide positive electrode material can give consideration to better energy density, rate capability and high-voltage cycle stability.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a lithium cobalt oxide cathode material and a lithium-ion secondary battery including the lithium cobalt oxide cathode material. Background Technology

[0002] With the rapid development of electric vehicles and portable electronic devices, the requirements for energy density, cycle life, and rate performance of lithium-ion batteries are becoming increasingly stringent. Lithium cobalt oxide, as an important cathode material for lithium-ion batteries, possesses high theoretical specific capacity, excellent cycle performance, and mature preparation technology, and has now become the mainstream cathode material for consumer electronics batteries.

[0003] However, when the charging cutoff voltage is ≥4.5V, the lithium cobalt oxide undergoes a deep delithiation, resulting in an irreversible phase transition, causing crystalline structure distortion. This is accompanied by volume changes and stress concentration, which leads to the formation of a phase transition layer with high impedance on the surface. This directly affects the structural stability of lithium cobalt oxide and significantly increases the interfacial impedance of the cathode, resulting in a severe decline in the battery's energy density, cycle performance, and rate performance.

[0004] Therefore, it is necessary to improve the specific capacity and structural stability of lithium cobalt oxide under high voltage conditions. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of poor specific capacity and structural stability of lithium cobalt oxide under high voltage in the prior art, and to provide a lithium cobalt oxide cathode material and a lithium-ion secondary battery including this lithium cobalt oxide cathode material. This invention modifies lithium cobalt oxide to improve its specific capacity while suppressing phase transitions in its bulk and surface phases. The lithium-ion secondary battery (hereinafter referred to as the battery) including the lithium cobalt oxide cathode material of this invention can achieve a better balance of energy density, rate performance, and high-voltage cycle stability.

[0006] To further improve the specific capacity of lithium cobalt oxide, related technologies often require higher operating voltages (e.g., charging cutoff voltage ≥ 4.5V) to extract more lithium, but this leads to a decrease in the structural stability of lithium cobalt oxide. Research has found that the reason for this problem lies in the fact that Co contributes electrons in lithium cobalt oxide (Co... 3+ →Co 4+ Therefore, an increase in delithiation is accompanied by Co 4+ With the increase in the proportion of lithium cobalt oxide, especially under conditions of high polarization, lithium cobalt oxide particles exhibit uneven delithiation, with the delithiation on the surface being far greater than that in the bulk phase. Therefore, cycling under high pressure exacerbates the dissolution of Co. The dissolution of Co also leads to the release of O, such as O-containing free radicals and O2. Simultaneously, the drastic change in the c-axis of lithium cobalt oxide causes lattice distortion, stress concentration, particle cracking and breakage, resulting in severe capacity decay in the battery.

[0007] For the reasons stated above, the inventors of this invention have discovered that the problems of lithium cobalt oxide in the related art can be improved by the following methods: First, introducing Ni can improve the specific capacity of lithium cobalt oxide at low voltages. Specifically: Ni 2+ Capable of contributing 2 electrons (Ni) 2+ →Ni 4+ ), Ni 3+ Able to contribute 1 electron (Ni) 3+ →Ni 4+ However, simply introducing Ni may cause Li / Ni mixing, further affecting the structural stability of lithium cobalt oxide. Therefore, to reduce the adverse effects of Li / Ni mixing on lithium cobalt oxide, Mn is also introduced. Specifically, Mn exists stably in the +4 valence state in the lithium cobalt oxide lattice. Its ionic radius is more reasonably different from the size of cobalt and lithium ions, allowing it to accurately occupy the transition metal layer and avoid lattice distortion caused by ion size mismatch. In addition, Ni is prone to +2 / +3 valence state fluctuations during discharge. The ionic radius of +2 Ni is similar to that of lithium ions, making it easy to embed in the lithium layer and cause mixing. The high valence state of Mn (+4 valence) can maintain the overall positive charge balance of the transition metal layer, reducing the valence state fluctuation of Ni and indirectly reducing the probability of Ni occupying lithium sites. Furthermore, when the mass content of Mn in lithium cobalt oxide is greater than that of Ni, the inhibitory effect of Mn on Li / Ni mixing can be guaranteed.

[0008] Secondly, as the voltage increases, the degree of lithium insertion / extraction in the cathode material intensifies, leading to increased lattice stress and potentially causing structural distortion or even collapse, thus affecting cycle stability. This invention, however, simultaneously introduces Mn and Ni elements to modify lithium cobalt oxide. When lithium cobalt oxide is charged to 4.5V with lithium metal as the counter electrode, its X-ray diffraction (XRD) spectrum still exhibits a new diffraction peak at 38°-38.2°. It is speculated that this diffraction peak corresponds to a new structure resembling a nickel-manganese spinel phase. This new phase can serve as additional lithium insertion / extraction active sites in the delithiation process, maintaining its crystal structure stability even under highly delithiated conditions, further improving the specific capacity of the cathode material and the cycle stability of the battery at high voltages.

[0009] Third, the ionic bonds on the surface of lithium cobalt oxide are weakened, making it easier for Li / Ni mixing and phase transitions to occur. Mn exists stably in lithium cobalt oxide with a +4 valence, and its bonding strength with oxygen is higher than that of Co and Ni. Therefore, when the mass content of Mn in the second region is greater than that in the first region, the high Mn content can form a stable transition metal-oxygen framework in the second region, thereby suppressing Li / Ni mixing and phase transitions and preventing surface structure collapse. Furthermore, during battery charging and discharging, the difference in lattice expansion / contraction coefficients between the first and second regions can easily lead to cracks at the interface. The high Mn content in the second region can buffer the stress between the first and second regions through its own elastic deformation, preventing crack formation.

[0010] Based on this, the following solution is proposed: The first aspect of this invention provides a lithium cobalt oxide cathode material containing Ni, Mn, and Al elements; the lithium cobalt oxide cathode material comprises lithium cobalt oxide particles, each lithium cobalt oxide particle comprising a matrix, the matrix having a first region and a second region located on the outer surface of the first region; extending from the surface of the matrix towards the center, the second region is a region with a depth of 80 nm from the surface of the matrix, and the first region is the region remaining in the matrix excluding the second region; the first region contains Ni, Mn, and Al elements; the second region contains Ni, Mn, and Al elements; the first region has an R-3m layered structure; the mass content of Ni element in the lithium cobalt oxide cathode material is less than the mass content of Mn element; the mass content of Mn element in the second region is greater than the mass content in the first region; when lithium metal is used as the counter electrode and charged to 4.5V, the lithium cobalt oxide cathode material has a first diffraction peak located at 38°-38.2° in an X-ray diffraction pattern.

[0011] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising the lithium cobalt oxide positive electrode material described in the first aspect of the present invention.

[0012] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) The lithium cobalt oxide cathode material of the present invention has superior specific capacity and structural stability; (2) The lithium-ion secondary battery of the present invention can take into account superior energy density, rate performance and high voltage cycle stability.

[0013] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0014] Figure 1 The diagram shown is a schematic representation of the structure of lithium cobalt oxide particles in an example of the present invention.

[0015] Figure 2 The above are X-ray diffraction patterns of lithium cobalt oxide cathode material charged to different voltage states in an example of the present invention.

[0016] Figure 3 The image shows the XRD pattern of lithium cobalt oxide cathode material without the introduction of Ni and Mn elements.

[0017] Figure 4 The figure shows the atomic percentage curve of the elements in the second region of the lithium cobalt oxide cathode material in an example of the present invention, along the radial direction from the surface to the center.

[0018] Figure 5 The figure shows the curve of the c-axis value of the lithium cobalt oxide cathode material as a function of charging voltage in an example of the present invention.

[0019] Figure 6 The image shows X-ray diffraction patterns of the lithium cobalt oxide cathode material charged to different voltage states in an example of the present invention.

[0020] Figure 7 The diagram shown is a schematic representation of the structure of lithium cobalt oxide particles in an example of the present invention. Detailed Implementation

[0021] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0022] The first aspect of the present invention provides a lithium cobalt oxide cathode material containing Ni, Mn, and Al elements. The lithium cobalt oxide cathode material comprises lithium cobalt oxide particles, each lithium cobalt oxide particle comprising a matrix, the matrix having a first region and a second region located on the outer surface of the first region; extending towards the center along the surface of the matrix, the second region is a region with a depth of 80 nm from the surface of the matrix, and the first region is the region remaining in the matrix excluding the second region.

[0023] like Figure 1The figure shows a schematic diagram of the structure of lithium cobalt oxide particles in an example of the present invention. As can be seen from the figure, lithium cobalt oxide has a first region 1 and a second region 2 located on the outer surface of the first region 1.

[0024] The first region contains Ni, Mn, and Al elements. The second region contains Ni, Mn, and Al elements. The first region contains an R-3m layered structure. The R-3m layered structure refers to Li... + Co 3+ and O 2- Occupying positions 3a, 3b, and 6c respectively, Co 3 + and Li + It is situated within octahedral voids composed of oxygen atoms, exhibiting a layer of Co along the c-axis. 3+ One layer of Li + Alternating layered structures.

[0025] In this invention, whether the first region and the second region contain Ni, Mn and Al elements, and whether the first region contains an R-3m layered structure, can be determined by conventional methods in the art, such as by high-resolution transmission electron microscopy (HRTEM).

[0026] The mass content of Ni in the lithium cobalt oxide is less than the mass content of Mn. The mass contents of Ni and Mn in the lithium cobalt oxide can be determined by conventional methods in the art, such as inductively coupled plasma optical emission spectrometry (ICP-OES), with specific operating procedures referring to GB / T 30902-2014.

[0027] The mass content of Mn element in the second region is greater than that in the first region. The mass content of Mn element in the first and second regions can be tested using conventional methods in the art. For example, after polishing the cross-section of lithium cobalt oxide with an argon ion mill, it can be observed in a transmission electron microscope (TEM) at maximum magnification. Using an energy dispersive spectroscopy (EDS) spectrometer, the first and second regions can be scanned separately, with at least 10 points taken in each region, to test and calculate the mass content, and the average value is taken.

[0028] When lithium metal is used as the counter electrode, and the cathode material is charged to 4.5V, the X-ray diffraction pattern shows a first diffraction peak at 38°-38.2°; at 4.37V, it shows a second and third diffraction peak at 37.6°-37.85°; and at 4.44V, it shows a fourth diffraction peak at 37.7°-37.9°. Figure 2The above are X-ray diffraction patterns of the lithium cobalt oxide cathode material charged to different voltage states in an example of the present invention. As can be seen from the figures, at a charging state of 4.37V, there are second and third diffraction peaks located at 37.64° and 37.73° respectively, indicating the participation of a new phase in delithiation. As the voltage further increases, these new diffraction peaks gradually shift to higher angles and merge. At a charging state of 4.44V, they shift to 37.8° and merge into a new diffraction peak, with a fourth diffraction peak at 37.8°; at a charging state of 4.5V, they shift to 38.1°, with a first diffraction peak at 38.1°; at a charging state of 4.56V, the above diffraction peaks disappear, meaning that the main range for the specific capacity contribution of the new phase is between 4.3V and 4.56V. For comparison... Figure 3 The image shows the XRD pattern of lithium cobalt oxide cathode material without the introduction of Ni and Mn elements, compared with... Figure 3 and Figure 2 It is known that if Ni and Mn elements are not introduced, the first, second, third, and fourth diffraction peaks will not appear at a specific charging voltage, and therefore, its specific capacity will decrease.

[0029] In this invention, introducing Ni into lithium cobalt oxide can improve its specific capacity. 2+ It loses electrons and produces Ni. 3+ And / or Ni 4+ Meanwhile, to mitigate the adverse effects of Li / Ni mixing, Mn is introduced. This invention introduces both Mn and Ni elements, which may cause lithium cobalt oxide to form a novel nickel-manganese spinel-like structure, participating in the delithiation process and further improving the specific capacity of the cathode material. When lithium metal is used as the counter electrode in the lithium cobalt oxide cathode material, at a charging voltage of 4.37V, Ni... 2+ It loses electrons and produces Ni. 3+ or Ni 4+ The new phase begins to participate in delithiation, thus contributing specific capacity. When charged to 4.44V, the crystal structure of the new phase is adjusted during the continuous delithiation process, which is conducive to its stable contribution of capacity over a wider voltage range. When charged to 4.5V, the delithiation activity of the new phase is still at a high level, which helps to maintain capacity output at a higher voltage.

[0030] In this invention, the "first diffraction peak and second diffraction peak located at 37.6°-37.85°" refers to the position corresponding to the peak values ​​of the first diffraction peak and the second diffraction peak being located in the range of 37.6°-37.85°.

[0031] In this invention, the phrase "the lithium cobalt oxide cathode material, with lithium metal as the counter electrode, charged to 4.37V, exhibits a second and third diffraction peak at 37.6°-37.85° in the X-ray diffraction pattern" refers to fabricating the lithium cobalt oxide cathode material into a coin cell using lithium metal as the counter electrode, charging the coin cell to 4.37V (charging rate of 0.2C), disassembling the coin cell, and then performing XRD testing on the lithium cobalt oxide cathode material. The XRD testing conditions are as follows: in-situ XRD measurement is performed using a Cu-Kα X-ray (λ=1.5406 Å) diffractometer with a scanning rate of 3° / minute and a scanning range (2θ) of 15-47°.

[0032] In this invention, the coin cell is prepared as follows: At 25°C, the lithium cobalt oxide cathode material, polyvinylidene fluoride, and conductive carbon black are mixed uniformly in N-methylpyrrolidone at a mass ratio of 92:3:5 to prepare a slurry; the slurry is then uniformly coated onto both sides of an 8μm thick aluminum foil, with a coating density of approximately 9-32 mg / cm². 2 Then, the aluminum foil coated with the slurry was placed in a vacuum drying oven and dried at 80°C for 12 hours. The dried electrode was cut into round pieces with a diameter of 20 mm to make the positive electrode. Under conditions of 25°C, using a lithium metal sheet as the counter electrode, the above-obtained positive electrode as the working electrode, a polyethylene membrane as the battery separator, and a 1 mol / L LiPF6 solution and a 1:1 volume ratio mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) as the electrolyte, a CR2430 button cell was assembled in a glove box under an argon atmosphere.

[0033] In the second region, along the radial direction from the surface of the lithium cobalt oxide to the center, in the linear scanning energy dispersive spectroscopy (ESI) curve of transmission electron microscopy (TEM), the atomic percentage curve of Al has a first peak, and the atomic percentage curve of Mn has a second peak. The peak position of the second peak is closer to the substrate surface than the peak position of the first peak. Figure 4 The figure shows the atomic percentage curve of elements in the second region of the lithium cobalt oxide cathode material in an embodiment of the present invention, along the radial direction from the surface to the center. The horizontal axis represents the dimension of the surface away from the first region from the second region, and the vertical axis represents the atomic percentage. Figure 4 It can be seen that in the second region, at a distance of 21.9 nm from the surface of the second region away from the first region, the percentage of Mn atoms is 0.25%; at a distance of 44.4 nm from the surface of the second region away from the first region, the percentage of Al atoms is 3.71%.

[0034] After Al occupies a Co site, it can participate in the formation of strong "Al-O" bonds, suppressing lattice distortion of lithium cobalt oxide and improving its crystal structure stability; furthermore, Al 3+ionic radius and Co 3+ The ionic radii are similar, and the steric hindrance is relatively small, ensuring that it does not affect Li. + The transport of Mn and Al was further improved by modifying lithium cobalt oxide. In the second region, along the radial direction from the surface to the center, the second peak of Mn in the linear scanning electron microscopy (LSI) spectrum was closer to the matrix surface than the first peak of Al. This further enhances the specific capacity and structural stability of lithium cobalt oxide. The reason is that the peak position in the atomic percentage curve reflects the maximum concentration of the element. When Mn occupies the more superficial metal sites in the second region, it does not significantly hinder the transport of Li. + The insertion / extraction channels maximize the preservation of electrochemical active sites on the lithium cobalt oxide surface, preventing capacity from being occupied by inert elements. However, if Al, as an inert element, is present in excessive amounts on the surface of the second region, it will occupy active sites, reduce electron conduction efficiency, and lead to a decrease in specific capacity. Setting its maximum concentration below that of Mn allows for a smaller impact on specific capacity while enhancing the overall lattice rigidity of the second region through strong bonding, buffering volume expansion / contraction during battery charging and discharging, and preventing cracks at the interface between the first and second regions. This arrangement better protects the surface, reduces lattice stress during deep delithiation, suppresses lattice oxygen evolution, and makes the lithium cobalt oxide structure more stable during high-voltage cycling.

[0035] In this invention, the atomic percentage of Co in the first region fluctuates by ≤50%. The atomic percentage of Al in the first region fluctuates by ≤5%, for example, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%. The atomic percentage of Mn in the first region fluctuates by ≤0.5%, for example, 0.5%, 0.4%, 0.3%, 0.2%, or 0.1%. The atomic percentage of Ni in the first region fluctuates by ≤0.3%, for example, 0.3%, 0.2%, or 0.1%. The atomic percentage fluctuations of each element in the first region can be obtained by transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS), specifically as follows: The battery is discharged to 0% SOC (e.g., discharged to 3V), the positive electrode is disassembled and removed. The cross-section of the positive electrode is polished using an argon-ion polisher. The obtained cross-section is imaged in the TEM equipment to locate the cross-section of the lithium cobalt oxide particle. In the TEM-EDS software, an arbitrary straight path passing through the center of the cross-section of the lithium cobalt oxide particle is drawn (avoiding the protrusions on the outer surface of the particle). The line scan mode is activated, and the electron beam moves point by point along the path from the surface to the center, collecting energy spectra and generating elemental distribution curves. After data analysis, the distribution information of each element is obtained. Through data processing, the cross-sectional TEM-EDS analysis curve of the lithium cobalt oxide particle, i.e., the distribution curve of each element, can be obtained. The horizontal axis represents the distance from the start to the end of the line scan, and the vertical axis represents the atomic percentage. For the lithium cobalt oxide particle, the difference between the maximum and minimum atomic percentages of each element along the line scan path is calculated. The average of these differences for the corresponding elements in five particles is taken as the atomic percentage fluctuation value of that element in the first region. It is worth noting that when the abscissa is 80 nm, that is, within a region 80 nm deep from the substrate surface, both the first peak of Al and the second peak of Mn exist simultaneously. As the abscissa continues to increase, the atomic percentage fluctuations of each element in the remaining region of the substrate satisfy the aforementioned range. Therefore, this invention uses 80 nm as the abscissa as the boundary between the first and second regions, with the region near the center of the lithium cobalt oxide particle forming the first region and the region near the surface of the lithium cobalt oxide particle forming the second region.

[0036] The small fluctuation in the atomic percentage of at least one of Co, Al, Mn, and Ni in the first region indicates that at least one of these elements is uniformly distributed within the first region, preventing the formation of localized enrichment or deficiency areas. Furthermore, the presence of diffraction peaks in the X-ray diffraction pattern of lithium cobalt oxide when charged to 4.5V with lithium metal as the counter electrode indicates a stable crystal structure without lattice distortion, allowing for the formation of a continuous layered structure in the second region. + The embedding / de-embedding provides a superior channel environment, avoids local channel blockage, and ensures Li +Smooth migration within the first region facilitates the utilization of specific capacity and further enhances the structural stability of the cathode material.

[0037] In this invention, the atomic percentage fluctuation of Ni in the second region is ≤0.3%, for example, 0.3%, 0.2%, or 0.1%. The atomic percentage fluctuation of Ni in the second region can be referenced to the atomic percentage fluctuation of each element in the first region, which will not be elaborated here.

[0038] Compared to the significant fluctuations in the atomic percentages of Al and Mn elements in the second region, the atomic percentage fluctuation of Ni element in the second region is ≤0.3%, indicating that Al and Mn elements are distributed in a gradient manner in the second region, while Ni element is distributed uniformly. The reason for this arrangement is that, as mentioned earlier, the higher atomic percentages of Al and Mn elements near the surface of the second region are beneficial for stabilizing the structural stability of lithium cobalt oxide. Ni element is a key active element in the second region; its stable content ensures uniform electrochemical activity throughout the entire second region, avoiding insufficient activity due to local Ni deficiency or Li / Ni mixing caused by local Ni enrichment, thereby further guaranteeing the stable performance of the specific capacity of lithium cobalt oxide.

[0039] In this invention, in the second region, the atomic percentage of Co gradually increases in the radial direction from the surface of the lithium cobalt oxide to the center.

[0040] In this invention, the first peak appears at a depth range of 5nm-70nm from the surface of the substrate, for example, 5nm, 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, or 70nm. The second peak appears at a depth range of 1nm-50nm from the surface of the substrate, for example, 1nm, 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm, or 50nm.

[0041] In one instance, the first peak appeared at a depth of 10 nm to 50 nm from the surface of the substrate.

[0042] In one instance, the second peak appeared at a depth of 5 nm to 30 nm from the surface of the substrate.

[0043] In this invention, the positions of the first peak and the second peak can be obtained by the atomic percentage fluctuation curves of Al and Mn elements in the second region, which will not be elaborated here.

[0044] In this invention, in the first region, the atomic percentage of Al is w1, the atomic percentage of Mn is w2, and the atomic percentage of Ni is w3, wherein w1 > w2 > w3. 3+ With a relatively small radius, Al exhibits the strongest bonding strength with oxygen. The relatively high Al content in the first region allows it to fully occupy transition metal layer sites, enhancing lattice rigidity and bonding stability, reducing the generation and propagation of cracks within the first region, and preventing the dissolution of active components due to electrolyte intrusion, thus providing a stable structural framework. Mn exists stably in the +4 valence state, and its ionic radius is similar to that of Co. 3+ With a high degree of matching, the moderate content can fill the remaining transition metal layer space after Al occupies the sites, optimize the orderliness of the layered structure, and provide a suitable environment for Li. + The insertion and extraction of Ni provide a smooth channel, improving capacity utilization. The relatively low Ni content can supplement the capacity utilization of the first region without causing structural problems.

[0045] In this invention, the atomic percentages of Al, Mn, and Ni elements in the first region can be obtained by methods conventional in the art, such as TEM-EDS testing.

[0046] In this invention, the atomic percentage of Mn is greater than that of Ni in the second region. The method for testing the atomic percentages of Mn and Ni in the second region is the same as that used in the first region, and will not be repeated here.

[0047] In this invention, the atomic percentage of Co in the second region is less than that in the first region. The atomic percentages of Co in the first and second regions are determined with reference to the atomic percentages of Mn and Ni in the first region, and will not be elaborated further here.

[0048] In this invention, the atomic percentage of Al or Mn elements in the first region is based on the atomic percentage of Mn and Ni elements in the first region, which will not be repeated here.

[0049] In this invention, when lithium metal is used as the counter electrode, the c-axis shrinkage rate of the lithium cobalt oxide cathode material, based on the c-axis value at a charging state of 4.2V, is 2.5%-3.5% at a charging state of 4.6V, for example, 2.5%, 2.8%, 3%, 3.1%, 3.2%, 3.3%, 3.4%, or 3.5%. The c-axis value can be calculated from the position of peak 003 in the XRD spectrum of the lithium cobalt oxide cathode material. Figure 5The figure shows the curve of the c-axis value of the lithium cobalt oxide cathode material as a function of charging voltage in an example of the present invention. As can be seen from the figure, compared with the highest value of the c-axis, the shrinkage rate of the c-axis is only 3.47% when charged to 4.6V. The low shrinkage rate of the c-axis indicates that lithium cobalt oxide can still maintain crystal structure stability during deep delithiation at a high voltage of 4.6V, effectively suppressing high-voltage-induced interlayer collapse and lattice distortion, thereby ensuring the stability of the Li-cobalt oxide cathode. + The insertion / extraction channels are smooth, reducing capacity decay caused by structural damage and significantly improving the cycling stability of the material under high voltage.

[0050] The lithium cobalt oxide cathode material of the present invention exhibits a low c-axis shrinkage rate, and this is also reflected in the small shift of the 003 crystal plane during charging in the XRD pattern of the lithium cobalt oxide cathode material. When lithium metal is used as the counter electrode and the material is charged to 4.6V, the 003 crystal plane peak is located at x1 in the X-ray diffraction pattern of the lithium cobalt oxide cathode material; when lithium metal is used as the counter electrode and the material is charged to 4.7V, the 003 crystal plane peak is located at x2 in the X-ray diffraction pattern of the lithium cobalt oxide cathode material. Figure 6 The figure shows the X-ray diffraction patterns of the lithium cobalt oxide cathode material charged to different voltage states in an example of the present invention. As can be seen from the figure, at 4.6V, the 003 crystal plane peak is located at 19.13° (x1), and at 4.7V, the 003 crystal plane peak is located at 19.47° (x2). The 003 crystal plane shift is relatively small under the high-voltage charging state of 4.6V-4.7V, indicating that the cathode material can still maintain the stability of its layered lattice structure under high voltage conditions, ensuring smooth lithium-ion insertion / extraction channels, reducing capacity decay, and further improving the cycle stability of the material under high voltage.

[0051] In this invention, x1 is 19.13° ± 0.15°. x2 is 19.47° ± 0.15°.

[0052] In one instance, x1 is 19.1° ± 0.1°.

[0053] In one instance, x2 is 19.47° ± 0.1°.

[0054] In this invention, Mn occupies Co sites. Ni occupies Co sites. Ni also occupies Li sites. Because Ni... 2+ The radius is 0.69 Å, which is similar to that of Li. + Since the radii are close, there is a partial Ni. 2+ Occupy the Li position.

[0055] In this invention, the lithium cobalt oxide contains Ni 2+ Ni 3+ Mn 4+ and Co 3+In lithium cobalt oxide, Ni exists primarily in the +2 valence state. In the aforementioned lithium cobalt oxide, Co... 3+ The quantity is n1, Mn 4+ The quantity n2, Ni 2+ The quantity is n3, Ni 3+ The quantity is n⁴; n² / n¹ > n³ / n¹ > n⁴ / n¹. Mn 4+ The high content of Mn ensures the structural stability of lithium cobalt oxide cathode materials. 4+ A higher proportion of Ni can effectively suppress Li / Ni mixing and enhance lattice stability; 2+ The dominant capacity contribution can enhance the specific capacity utilization of cathode materials.

[0056] In this invention, Ar sputtering XPS can be used to analyze the valence states of each element in lithium cobalt oxide.

[0057] In this invention, in the lithium cobalt oxide, Co 3+ The quantity is greater than Co 2+ The quantity; and the Co on the surface of the lithium cobalt oxide 3+ The quantity is less than Co 2+ The quantity of Co was measured. After cutting open the discharged lithium cobalt oxide particles, electron energy loss spectroscopy (EELS) was used in TEM to scan the electron energy loss spectra (1nm-200nm) from the surface to the bulk phase. The L3 and L2 sideband absorptions of Co showed an initial shift to higher wavenumbers before remaining constant. The shift range for L3 was 0.8eV-1.2eV, and for L2, it was also 0.8eV-1.2eV. The surface L3 was at 778.3eV ± 0.2eV, and the bulk L3 was at 779.1eV ± 0.2eV. The surface L2 was at 792.5eV ± 0.3eV, and the bulk L2 was at 793.2eV ± 0.3eV. This indicates that the average valence state of Co on the lithium cobalt oxide surface is less than +3, and more biased towards +2. The Co on the lithium cobalt oxide surface... 2+ It can synergistically optimize the effect with the higher concentrations of Al and Mn elements in the second region, and to a certain extent change the lattice structure and surface properties of the material, making Li + The energy barrier is relatively low during the insertion and extraction processes, which helps to accelerate Li... + This increases the diffusion rate, thereby improving the battery's charging and discharging efficiency, enhancing its rate performance, and enabling the battery to maintain good performance even under high-current charging and discharging conditions.

[0058] In this invention, the lithium cobalt oxide particles also have protrusions located on the outer surface of the lithium cobalt oxide particles, partially covering the second region. Protrusions on the outer surface of the lithium cobalt oxide particles can suppress side reactions between the surface and the electrolyte, further improving the structural stability of the lithium cobalt oxide cathode material. Figure 7The figure shows a schematic diagram of the structure of lithium cobalt oxide particles in an embodiment of the present invention. As can be seen from the figure, the lithium cobalt oxide has a first region 1, a second region 2 located on the outer surface of the first region 1, and a protrusion 3 located on the outer surface of the lithium cobalt oxide; wherein the protrusion 3 partially covers the second region 2.

[0059] In this invention, the protrusion contains at least one of the elements Al, Ti, La, Y, and Zr.

[0060] In one example, the protrusions include Al2O3, TiO2, and Li4Ti5O. 12 , Li2CoTiO4, Li2CoTi3O8, Li7La3Zr2O 12 Li₂ZrO₃, Co₃O₄, CoO and Li 3x La （2 / 3-x） At least one of TiO3, wherein 0 <x<2 / 3。

[0061] In one example, the protrusion contains Al. The Al content in the protrusion in the lithium cobalt oxide is 700ppm-1200ppm by mass, for example, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm or 1200ppm.

[0062] In one example, the protrusion contains Ti. The Ti content in the lithium cobalt oxide is 450ppm-1050ppm by mass, for example, 450ppm, 600ppm, 800ppm or 1050ppm.

[0063] In one example, the protrusion contains the element La. The mass content of the element La in the lithium cobalt oxide is 300ppm-800ppm, for example, 300ppm, 500ppm, 700ppm or 800ppm.

[0064] In one example, the protrusion contains element Y. The mass content of element Y in the lithium cobalt oxide is 200ppm-1200ppm, for example, 200ppm, 600ppm, 1000ppm or 1200ppm.

[0065] In one example, the protrusion contains Zr. The Zr content in the lithium cobalt oxide is 200ppm-1000ppm by mass, for example, 200ppm, 400ppm, 800ppm or 1000ppm.

[0066] In this invention, the mass content of elements Y, La, Zr, and Ti in the lithium cobalt oxide can be obtained by methods conventional in the art, such as using inductively coupled plasma-emission spectrometry (ICP-OES).

[0067] In this invention, the boundary between the protrusion and the second region has at least one of the following: a layered structure with space group R-3m, a spinel phase with space group Fd3m, and a rock salt phase with space group R-3m. When the above-mentioned crystalline phases are present, the lithium-ion conductivity of the lithium cobalt oxide particles can be further improved, the structural stability at the boundary can be enhanced, the volumetric strain of the material during charging and discharging can be buffered, and the structural stability of the cathode material can be improved.

[0068] In this invention, the height of the protrusion is 10nm-300nm, for example, 10nm, 50nm, 100nm, 150nm, 200nm, 250nm, or 300nm. The width of the orthogonal projection of the protrusion onto the lithium cobalt oxide surface is 10nm-300nm, for example, 10nm, 50nm, 100nm, 150nm, 200nm, 250nm, or 300nm. The appropriate height of the protrusions on the lithium cobalt oxide particle surface reduces direct contact between the material and the electrolyte, optimizes lithium-ion transport channels, reduces surface side reactions, enhances the capacity utilization of lithium cobalt oxide, and maintains structural stability.

[0069] In this invention, the height of the protrusion and the width of the orthographic projection of the protrusion onto the lithium cobalt oxide surface have conventional meanings in the art. Generally, the height of the protrusion is considered to refer to the vertical distance from the highest point of the protrusion to the outer surface of the second region; this can be measured using conventional methods in the art, such as measuring the height of at least 10 protrusions using an atomic force microscope and taking the average value. The width of the orthographic projection of the protrusion onto the lithium cobalt oxide surface refers to the width being the diameter of a regular circle when the orthographic projection is a regular circle; and the width being the equivalent diameter of a regular circle with the same area as the irregular circle when the orthographic projection is not a regular circle; this can also be measured using conventional methods in the art, such as using TEM to measure the width of the orthographic projection of at least 10 protrusions onto the lithium cobalt oxide surface and taking the average value.

[0070] In this invention, the lithium cobalt oxide cathode material has an Al content of w4, a Mn content of w5, and a Ni content of w6, wherein w4 > w5 > w6. The high Al content acts as a "structural pillar," stabilizing the crystal lattice and suppressing oxygen evolution and irreversible phase transitions under high voltage. An appropriate amount of Mn further stabilizes the structure and suppresses Li / Ni mixing. The relatively low Ni content, while ensuring capacity contribution, avoids severe Li / Ni mixing, cation disorder, and structural stress problems caused by excessive Ni.

[0071] In one example, the mass ratio of Mn to Ni in the lithium cobalt oxide cathode material is 1.2-5, for example, 1.2, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5. This invention, by controlling the mass ratio of Mn to Ni in the lithium cobalt oxide cathode material within a suitable range, ensures that Ni fully participates in the charge compensation process, improving the specific capacity of the lithium cobalt oxide cathode material. Simultaneously, the doping of Mn stabilizes the crystal structure, suppressing irreversible phase transitions under high voltage, further optimizing the specific capacity and structural stability of the cathode material, and effectively improving the cycle stability of the battery under high voltage conditions.

[0072] In this invention, the Al element in the lithium cobalt oxide cathode material has a mass content of 6000ppm-10000ppm, for example, 6000ppm, 8000ppm, or 10000ppm. The Mn element in the lithium cobalt oxide cathode material has a mass content of 600ppm-3500ppm, for example, 600ppm, 1000ppm, 2000ppm, 3000ppm, or 3500ppm. The Ni element in the lithium cobalt oxide cathode material has a mass content of 400ppm-2000ppm, for example, 400ppm, 500ppm, 1000ppm, 1500ppm, or 2000ppm.

[0073] In one example, the Al content in the lithium cobalt oxide is 7500ppm-9000ppm by mass.

[0074] In one example, the mass content of Mn in the lithium cobalt oxide is 1500ppm-2500ppm.

[0075] In one example, the Ni element in the lithium cobalt oxide has a mass content of 700ppm-1500ppm.

[0076] In one example, the lithium cobalt oxide further comprises Mg. The mass content of Mg in the lithium cobalt oxide is 600ppm-2000ppm, for example, 600ppm, 1000ppm, 1500ppm or 2000ppm.

[0077] In one example, the Mg element in the lithium cobalt oxide has a mass content of 1000ppm-1500ppm.

[0078] When the mass content of sulfur (S) and calcium (Ca) in lithium cobalt oxide is within a specific range, it is beneficial to improve the specific capacity and structural stability of the lithium cobalt oxide cathode material, enabling it to maintain good stability under high voltage, suppressing the dissolution of transition metal ions, reducing interfacial side reactions, and ensuring rapid lithium-ion transport. The mass content of sulfur in the lithium cobalt oxide is less than 200 ppm, for example, 199 ppm, 150 ppm, 100 ppm, 50 ppm, or 0 ppm. The mass content of calcium (Ca) in the lithium cobalt oxide is less than 200 ppm, for example, 199 ppm, 150 ppm, 100 ppm, 50 ppm, or 0 ppm.

[0079] In addition to improving the lithium cobalt oxide cathode material itself, particles with different sizes can be blended together. This allows for better particle packing, filling the gaps between large particles with smaller ones, thereby increasing the overall compaction density of the cathode active coating. Furthermore, particles of different sizes can create a richer and more rational pore structure. The larger pores formed by larger particles facilitate rapid electrolyte penetration and storage, while the relatively smaller pores around smaller particles provide more transport channels for lithium ions. This combination ensures sufficient contact between the electrolyte and the lithium cobalt oxide cathode material, and facilitates smoother lithium ion diffusion within the cathode sheet, improving the battery's rate performance and charge / discharge efficiency.

[0080] In this invention, the lithium cobalt oxide comprises a first particle and a second particle. The median particle size Dv50 of the first particle is 9.58 μm-20.5 μm, for example, 9.5 μm, 15 μm, 18.5 μm, or 20.5 μm. The median particle size Dv50 of the second particle is D, where 1 μm ≤ D < 9.5 μm, for example, 1 μm, 5 μm, 8.5 μm, 9 μm, or 9.4 μm. The median particle size Dv50 of the first and second particles can be obtained by methods conventional in the art, such as using a laser particle size analyzer.

[0081] In one example, the median particle size Dv50 of the first particle is 12.5 μm-20.5 μm. The median particle size Dv50 of the second particle is 1 μm-5.5 μm.

[0082] In this invention, the average particle size of the first particle is 12 μm-22 μm, for example, 12 μm, 18 μm, or 22 μm. The average particle size of the second particle is 0.5 μm-6.5 μm, for example, 0.5 μm, 3 μm, 5 μm, or 6.5 μm. The average particle sizes of the first and second particles can be obtained by methods conventional in the art, for example, by selecting at least 10 first and second particles respectively from a scanning electron microscope (SEM) image of the lithium cobalt oxide cathode material, measuring the particle size of each particle, and taking the average value.

[0083] The present invention also provides a method for preparing the lithium cobalt oxide cathode material, the method comprising at least the following steps: (1) Preparation of the first particle precursor: The first Co source, the first Li source, and the first Al source, each with specific Mg, Mn, and Ni contents in a specific region, are mixed uniformly and then subjected to the first sintering in a dry air atmosphere. (2) Preparation of the second particle precursor: The second Co source, the second Li source, and the second Al source, each with specific Mg, Mn, and Ni contents in a specific region, are mixed uniformly, and then subjected to a second sintering in a dry air atmosphere. (3) The first particulate precursor prepared in step (1) and the second particulate precursor prepared in step (2) are mixed evenly, and a third Al source and at least one of the following optional metal sources are added: Ti source, La source, Y source and Zr source; a second sintering is performed in a dry air atmosphere; wherein, "optional" means that it may include at least one of the Ti source, the La source, the Y source and the Zr source, or may not include at least one of the Ti source, the La source, the Y source and the Zr source.

[0084] In this invention, the specific amounts of the materials used are as described above and will not be repeated here.

[0085] In this invention, the temperature of the first sintering is 900℃-1050℃, for example, 900℃, 920℃, 940℃, 960℃, 980℃, 1000℃, or 1050℃. The time of the first sintering is 8h-12h, for example, 8h, 9h, 10h, 11h, or 12h. The heating rate of the first sintering is 1℃ / min-5℃ / min, for example, 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, or 5℃ / min.

[0086] In this invention, step (1) further includes naturally cooling to room temperature after the first sintering.

[0087] In this invention, the second sintering temperature is 900℃-1050℃, for example, 900℃, 920℃, 940℃, 960℃, 980℃, 1000℃, or 1050℃. The second sintering time is 5h-10h, for example, 5h, 6h, 8h, or 10h. The heating rate of the second sintering is 1℃ / min-5℃ / min, for example, 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, or 5℃ / min.

[0088] In this invention, step (2) further includes naturally cooling to room temperature after the second sintering.

[0089] In this invention, in step (3), when the first lithium cobalt oxide particle precursor and the second lithium cobalt oxide particle precursor are simultaneously subjected to the second sintering, the first lithium cobalt oxide particle precursor and the second lithium cobalt oxide particle precursor can be mixed in any mass ratio.

[0090] When the temperatures of the first and second sintering processes are in the range of 900℃-1050℃, Al and Mn will segregate to the second region.

[0091] In this invention, the temperature of the third sintering is 700℃-950℃, for example, 700℃, 750℃, 800℃, 850℃ or 900℃. The time of the third sintering is 1h-5h, for example, 1h, 2h, 3h, 4h or 5h.

[0092] In this invention, step (3) further includes naturally cooling to room temperature after the third sintering.

[0093] In this invention, the first Co source and the second Co source each independently comprise at least one of cobalt tetroxide, cobalt hydroxide, and cobalt carbonate. The first Li source and the second Li source each independently comprise lithium carbonate and / or lithium fluoride. The Zr source is zirconium oxide and / or zirconium hydroxide. The first Al source, the second Al source, and the third Al source comprise at least one of Al₂O₃, Al(OH)₃, Al₂(SO₄)₃, Al₂(CO₃)₃, and Al(NO₃)₃. The Ti source comprises at least one of TiO₂, TiF₄, TiCl₃, and LaTiO₃. The La source comprises at least one of La₂O₃, La(OH)₃, La(CO₃)₃, La₂(SO₄)₃, LaTiO₃, and LaZrO₃. The Y source comprises at least one of Y₂O₃, Y(OH)₃, Y₂(CO₃)₃, Y₂(SO₄)₃, and Y(NO₃)₃.

[0094] In one instance, the Ti source comprises TiO2.

[0095] In one instance, the La source comprises La2O3.

[0096] In this invention, the median particle size Dv50 of the first Co source is 9.5 μm-20.5 μm, for example, 9.5 μm, 15.5 μm, or 20.5 μm. The median particle size Dv50 of the second Co source is 1 μm-9 μm, for example, 1 μm, 3 μm, 6 μm, or 9 μm.

[0097] The present invention also provides a positive electrode sheet, the positive electrode sheet comprising the lithium cobalt oxide positive electrode material described in the first aspect of the present invention. The positive electrode sheet includes a positive current collector and a positive active coating located on at least one side surface of the positive current collector, the positive active coating comprising the lithium cobalt oxide positive electrode material.

[0098] In one example, the compaction density of the positive electrode is 3.2 g / cm³. 3 -4.5g / cm 3 .

[0099] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery including the positive electrode plate.

[0100] In this invention, the lithium-ion secondary battery further includes an electrolyte.

[0101] In one example, the electrolyte comprises a carbonate solvent. The carbonate solvent includes, for example, at least one selected from ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The carbonate solvent does not include fluoroethylene carbonate (FEC). Carbonate solvents have good solubility for lithium salts, enabling them to fully dissolve and dissociate into lithium ions, ensuring a sufficiently high concentration of freely moving lithium ions in the electrolyte. This plays a fundamental role in the smooth transport of lithium ions in the electrolyte, thereby improving the overall kinetic performance of the battery system.

[0102] In one example, the electrolyte includes FEC. FEC decomposes on the negative electrode side to produce fluorinated organic compounds and other components, which can construct a dense and stable protective film (SEI film), effectively preventing further side reactions between the electrolyte and the electrode active materials. Especially for silicon-carbon negative electrodes, the SEI film effectively buffers volume expansion during cycling, maintaining interface stability. Simultaneously, because the SEI film formed with the participation of FEC has good lithium-ion conductivity, lithium ions can shuttle more quickly between the electrode and electrolyte during high-rate charge and discharge, reducing polarization during electrode reactions. This helps the battery maintain a high charge and discharge capacity under high-current charge and discharge conditions, improving the battery's rate performance and meeting the needs of applications with high charge and discharge speed requirements.

[0103] In this invention, the FEC content is 5%-20% based on the total mass of the electrolyte, for example, 5%, 10%, 15% or 20%.

[0104] In one example, the electrolyte comprises a fluorocarboxylic acid ester. The fluorocarboxylic acid ester includes ethyl fluorocarbonate. Ethyl fluorocarbonate refers to a substance in which a fluorine atom is substituted at any position in ethyl acetate.

[0105] In one example, the fluorocarboxylic acid ester includes 2,2-difluoroethyl acetate (DFEA) and / or ethyl 2,2-difluoroacetate.

[0106] In one example, the fluorocarboxylic acid ester includes DFEA. The fluorine atoms in the DFEA molecule give it a low HOMO energy level, effectively reducing further reactions between the electrolyte and the positive electrode, suppressing the oxidative decomposition of the electrolyte by metals such as Ni and Mn under high voltage, and improving the stability of the electrolyte in high-voltage environments. This ensures stable battery operation under high voltage and helps improve the battery's energy density. Furthermore, the Y and La elements contained in the protrusions of lithium cobalt oxide can coordinate with DFEA to form a denser CEI film, reducing further electrolyte decomposition. Both factors synergistically improve the battery's cycle stability.

[0107] In this invention, based on the total mass of the electrolyte, the content of the fluorocarboxylic acid ester is 10%-50%, for example, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45% or 50%.

[0108] In one example, the electrolyte comprises 1,3,6-hexanetrionitrile (HTCN). Nitrile additives can form a stable CEI film at the high-voltage cathode interface, preventing the dissolution of metal ions such as Ni and Mn, electrolyte decomposition, and the corrosion of the cathode by hydrogen fluoride, thus protecting the crystal structure of the lithium cobalt oxide cathode material and significantly improving the electrochemical performance of the battery at high rates. Furthermore, the C≡N functional groups in its molecules can complex with Co ions in the lithium cobalt oxide cathode material, thereby inhibiting the decomposition of the lithium cobalt oxide cathode material by the electrolyte; it can also pre-oxidize to alleviate electrolyte decomposition and reduce electrode surface polarization. In particular, HTCN contains three cyano groups, exhibiting high polarity, strong coordination ability, and antioxidant properties, as well as low viscosity, which can improve ion transport efficiency and contribute to improving the high-current charge-discharge performance of the battery. Additionally, the molecular volume effect of HTCN can prevent electrolyte solvent molecules from approaching the Co active sites, thereby further inhibiting the dissolution of transition metals (especially the high concentration of Mn in the second region) on the surface.

[0109] In this invention, the HTCN content is 0.5%-8% based on the total mass of the electrolyte, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7% or 8%.

[0110] In this invention, the mass content of the carbonate solvent, FEC, the fluorocarboxylic acid ester and HTCN in the electrolyte can be obtained by methods conventional in the art, such as gas chromatography-mass spectrometry (GC-MS).

[0111] In this invention, the battery further includes a negative electrode sheet. The negative electrode sheet comprises a negative electrode material, which includes a silicon-carbon material.

[0112] In one example, the silicon-carbon material comprises a porous carbon matrix and silicon material located in the internal channels of the porous carbon matrix.

[0113] In this invention, the silicon-carbon material includes first silicon-carbon particles and / or second silicon-carbon particles.

[0114] In one example, the sphericity of the first silicon carbide particle is S1, where 0.7 ≤ S1 ≤ 1, for example, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, or 1. The sphericity of the second silicon carbide particle is S2, where 0.3 ≤ S2 < 0.7, for example, 0.3, 0.4, 0.5, 0.6, or 0.69.

[0115] In one example, the average particle size of the first silicon carbide particles is 1 μm-15 μm, for example, 1 μm, 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm or 15 μm.

[0116] In one example, the average particle size of the first silicon carbide particles is 1 μm-10 μm.

[0117] In one example, the average particle size of the second silicon carbide particle is 6 μm-15 μm, for example, 6 μm, 8 μm, 10 μm, 12 μm, 13 μm or 15 μm.

[0118] In one example, the mass content of elemental Si in the first silicon-carbon particle is 25%-70%, for example, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%. The mass content of elemental Si in the second silicon-carbon particle is 20%-60%, for example, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or 60%.

[0119] The first type of silicon-carbon particles has a high sphericity, which provides uniform dispersion, reduces stress concentration, alleviates silicon volume expansion, and lowers the risk of separator puncture. The second type of silicon-carbon particles has a lower sphericity, which increases the compaction density of the negative electrode. When these two types of silicon-carbon particles are used synergistically, on the one hand, the volume expansion of the silicon-carbon material can be effectively reduced, while optimizing the material's packing density and further increasing the compaction density of the negative electrode. This allows the negative electrode to accommodate more active material per unit volume, contributing to an increase in the battery's volumetric energy density. On the other hand, it can improve the diffusion kinetics of lithium ions within the negative electrode, improving the battery's rate performance. If the second type of silicon-carbon particles are used alone, due to their larger particle size and irregular shape, the drastic volume expansion (up to 300%) can cause the negative electrode active material to pulverize and detach, while the sharp edges can puncture the separator, causing a short circuit in the battery. In addition, the relatively poor conductivity of the second silicon-carbon particles hinders the transport of lithium ions and electrons at the negative electrode, resulting in poor battery rate performance. If the first silicon-carbon particles are used alone, their high sphericity leads to poor adhesion to the binder, making them prone to detaching from the conductive network and becoming inactive in the later stages of cycling. This results in uneven lithium insertion / extraction of silicon particles, accelerated degradation of active materials, and consequently affects battery cycle life.

[0120] In this invention, the sphericity S1 of the first silicon-carbon particle and the sphericity S2 of the second silicon-carbon particle can be tested using conventional methods in the art. For example, after disassembling the battery, the negative electrode sheet is removed, cleaned with dimethyl carbonate (DMC), and dried. The cross-section of the negative electrode sheet is polished using an argon ion mill, and then observed using backscatter imaging mode in an SEM device. A first silicon-carbon particle and a second silicon-carbon particle with a continuous and smooth outline are found. Any two points on the edge of the particle are connected to form a straight line segment inside the particle. The longest straight line segment inside the particle is selected, and its length is denoted as Z1. The midpoint of this longest straight line segment is taken, and a straight line is drawn through this midpoint to form a straight line segment with both ends at the edge of the particle. The shortest straight line segment is selected, and its length is denoted as Z2. The sphericity of the particle is then Z2 / Z1. At least 10 first silicon-carbon particles and 10 second silicon-carbon particles are selected respectively, and the sphericity is measured and the average value is taken.

[0121] In this invention, the average particle size of the first and second silicon-carbon particles can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. Alternatively, the negative electrode sheet before soaking in electrolyte can be directly taken, and the negative electrode sheet can be cut using an argon-ion milling machine with a CP laser. Then, it can be observed using a scanning electron microscope (SEM) (using backscatter imaging mode). In this mode, the contrast of the silicon-carbon material is brighter (which can be used to distinguish the carbon-based material and conductive agent in the negative electrode active layer). At a magnification of 5K, at least 20 first and second silicon-carbon particles are randomly selected, and the particle size of each silicon-carbon particle is measured and the average value is taken. If the number of particles is less than 20 at 5K magnification, another microscopic image is taken until 20 particles are measured. When the particles in the mirror image are regular circles, the particle diameter is the diameter of the regular circle; when the particles in the mirror image are not "regular circles", connect any two points on the edge of the particle to form a straight line segment inside the particle, and select the longest straight line segment inside the particle as the particle diameter.

[0122] In this invention, the mass content of element Si in the first / second silicon-carbon particles can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode is disassembled and removed, or the negative electrode is taken directly before immersion in electrolyte. The cross-section of the negative electrode is polished using an argon ion mill, and the silicon-carbon material is observed in SEM equipment using backscatter imaging mode to maximize magnification. The cross-sections of the first and second silicon-carbon particles are scanned using an energy dispersive spectroscopy (EDS) instrument, with the scanned area not less than 50% of the particle cross-section and the scanning range completely within the particle cross-section. The mass content of element Si is then calculated. At least 10 particles are selected for measurement, and the average value is taken.

[0123] In this invention, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side of the surface of the negative electrode current collector. The negative electrode active layer includes the silicon-carbon material; the negative electrode active layer also includes single-walled carbon nanotubes. By constructing a continuous conductive network using single-walled carbon nanotubes, not only can the volume change caused by the continuous expansion and contraction of the negative electrode Si during charging and discharging be suppressed, but the poor conductivity of the negative electrode material (such as graphite) itself can also be effectively improved, reducing the transmission impedance of electrons and lithium ions, providing more fast transmission channels for lithium ions, and improving the rate performance of the battery.

[0124] In one example, the diameter of the single-walled carbon nanotube is 1 nm to 10 nm (e.g., 1 nm, 2 nm, 4 nm, 6 nm, 8 nm or 10 nm).

[0125] In one example, the diameter of the single-walled carbon nanotube is 1 nm to 5 nm.

[0126] In one instance, the length of the single-walled carbon nanotube is greater than 3 nm (e.g., 3 nm, 5 nm, 10 nm, 100 nm, 500 nm or 1000 nm).

[0127] In one instance, the length of the single-walled carbon nanotube is greater than 5 nm (e.g., 5 nm, 10 nm, 100 nm, 500 nm or 1000 nm).

[0128] The batteries can all be assembled in accordance with conventional methods in the field.

[0129] The charging cutoff voltage of the battery (lithium-ion secondary battery) is ≥4.5V.

[0130] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0131] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0132] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0133] The following preparation examples are used to prepare the lithium cobalt oxide cathode material of the present invention.

[0134] Preparation Example 1 Lithium cobalt oxide cathode material was prepared according to the following method: (1) Preparation of the first particle precursor: Co3O4 (median particle size Dv50 of 16.5μm) and Li2CO3 containing elements Mg, Mn and Ni in a specific region and Li2CO3 were weighed and mixed evenly according to the molar ratio of Co to Li of 1:1.03. Al2O3 was then added and mixed evenly. The mixture was placed in a muffle furnace and heated to 1000℃ at a rate of 5℃ / min in a dry air atmosphere. After high-temperature calcination for 10h, it was naturally cooled to room temperature to obtain the first particle precursor. (2) Preparation of the second particle precursor: Co3O4 (median particle size Dv50 of 4.5 μm) containing elements Mg, Mn and Ni in a specific region and Li2CO3 were weighed and mixed evenly according to the molar ratio of Co to Li of 1:1.03. Al2O3 was then added and mixed evenly. The mixture was placed in a muffle furnace and heated to 950℃ at a rate of 5℃ / min in a dry air atmosphere. After high-temperature calcination for 10h, it was naturally cooled to room temperature to obtain the second particle precursor. (3) After mixing the first particle precursor and the second particle precursor evenly at a mass ratio of 80:20, Al2O3, TiO2, La2O3, Y2O3 and ZrO2 are added and mixed evenly. The mixture is placed in a muffle furnace and heated to 850°C at a rate of 5°C / min in a dry air atmosphere. After high-temperature calcination for 3 hours, it is naturally cooled to room temperature to prepare the first particle and the second particle. The lithium cobalt oxide exhibits a first region, a second region, and protrusions. The atomic percentage fluctuation of Co in the first region is ≤50%, Al in the first region is ≤5%, Mn in the first region is ≤0.5%, Ni in the first region is ≤0.3%, and Ni in the second region is ≤0.3%. The first peak appears at a distance of 25 nm from the substrate surface, and the second peak appears at a distance of 15 nm from the substrate surface. In the first region, w1 > w2 > w3. In the second region… In the first region, the atomic percentage of Mn is greater than that of Ni; in the second region, the atomic percentage of Co is less than that of Co in the first region; in lithium cobalt oxide, w4 > w5 > w6; in lithium cobalt oxide, the mass content of Mn to Ni is 2.55; the mass content of Al in lithium cobalt oxide is 8235 ppm, the mass content of Mn in lithium cobalt oxide is 2312 ppm, the mass content of Ni in lithium cobalt oxide is 906 ppm, and the mass content of Mg in lithium cobalt oxide is 1256 ppm. When lithium metal is used as the counter electrode, and the lithium cobalt oxide is charged to 4.6V, the c-axis shrinkage rate is 3.25%; x1 is 19.1°, x2 is 19.47°; the protrusions contain Al, Ti, La, Y and Zr, the height of the protrusions is 154 nm, and the width of the orthographic projection of the protrusions on the surface of the lithium cobalt oxide is 156 nm. The mass content of Ti in the lithium cobalt oxide is 654 ppm, the mass content of La in the lithium cobalt oxide is 415 ppm, the mass content of Y in the lithium cobalt oxide is 1014 ppm, and the mass content of Zr in the lithium cobalt oxide is 421 ppm.

[0135] Preparation Example 2 Lithium cobalt oxide cathode material was prepared according to the following method: (1) Preparation of the first particle precursor: Co3O4 (median particle size Dv50 of 9.5 μm) containing elements Mg, Mn and Ni in a specific region and Li2CO3 were weighed and mixed evenly according to the molar ratio of Co to Li of 1:1.03. Al2O3 was then added and mixed evenly. The mixture was placed in a muffle furnace and heated to 950℃ at a rate of 5℃ / min in a dry air atmosphere. After high-temperature calcination for 12h, it was naturally cooled to room temperature to obtain the first particle precursor. (2) Preparation of the second particle precursor: Co3O4 (median particle size Dv50 of 1.2 μm) containing elements Mg, Mn and Ni in a specific region and Li2CO3 were weighed and mixed evenly according to the molar ratio of Co to Li of 1:1.03. Al2O3 was then added and mixed evenly. The mixture was placed in a muffle furnace and heated to 900℃ at a rate of 5℃ / min in a dry air atmosphere. After high-temperature calcination for 12h, it was naturally cooled to room temperature to obtain the second particle precursor. (3) After mixing the first particle precursor and the second particle precursor evenly at a mass ratio of 80:20, Al2O3, TiO2, La2O3, Y2O3 and ZrO2 are added and mixed evenly. The mixture is placed in a muffle furnace and heated to 700°C at a rate of 5°C / min in a dry air atmosphere. After high-temperature calcination for 5 hours, it is naturally cooled to room temperature to prepare the first particle and the second particle. The lithium cobalt oxide exhibits a first region, a second region, and protrusions. The atomic percentage fluctuation of Co in the first region is ≤50%, Al in the first region is ≤5%, Mn in the first region is ≤0.5%, Ni in the first region is ≤0.3%, and Ni in the second region is ≤0.3%. The first peak appears at a distance of 11 nm from the substrate surface, and the second peak appears at a distance of 5 nm from the substrate surface. In the first region, w1 > w2 > w3. In the second region… In the first region, the atomic percentage of Mn is greater than that of Ni; the atomic percentage of Co in the second region is less than that in the first region; in lithium cobalt oxide, w4 > w5 > w6; the mass ratio of Mn to Ni in lithium cobalt oxide is 1.21; the mass content of Al in lithium cobalt oxide is 7518 ppm, the mass content of Mn in lithium cobalt oxide is 1651 ppm, the mass content of Ni in lithium cobalt oxide is 1360 ppm, and the mass content of Mg in lithium cobalt oxide is 1498 ppm. When lithium metal is used as the counter electrode, and the lithium cobalt oxide is charged to 4.6V, the c-axis shrinkage rate of lithium cobalt oxide is 3.08%; x1 is 19°, x2 is 19.37°; the protrusions contain Al, Ti, La, Y and Zr, the height of the protrusions is 12nm, and the width of the orthographic projection of the protrusions on the surface of the lithium cobalt oxide is 14nm. The mass content of Ti in lithium cobalt oxide is 456ppm, the mass content of La in lithium cobalt oxide is 304ppm, the mass content of Y in lithium cobalt oxide is 306ppm, and the mass content of Zr in lithium cobalt oxide is 212ppm.

[0136] Preparation Example 3 Lithium cobalt oxide cathode material was prepared according to the following method: (1) Preparation of the first particle precursor: Co3O4 (median particle size Dv50 of 20.5 μm) and Li2CO3 containing elements Mg, Mn and Ni in a specific region and Li2CO3 were weighed and mixed evenly according to the molar ratio of Co to Li of 1:1.03. Al2O3 was then added and mixed evenly. The mixture was placed in a muffle furnace and heated to 1050℃ at a rate of 5℃ / min in a dry air atmosphere. After high-temperature calcination for 8h, it was naturally cooled to room temperature to obtain the first particle precursor. (2) Preparation of the second particle precursor: Co3O4 (median particle size Dv50 of 5.5 μm) containing elements Mg, Mn and Ni in a specific region and Li2CO3 were weighed and mixed evenly according to the molar ratio of Co to Li of 1:1.03. Al2O3 was then added and mixed evenly. The mixture was placed in a muffle furnace and heated to 950℃ at a rate of 5℃ / min in a dry air atmosphere. After high-temperature calcination for 8 hours, it was naturally cooled to room temperature to obtain the second particle precursor. (3) After mixing the first particle precursor and the second particle precursor evenly at a mass ratio of 80:20, Al2O3, TiO2, La2O3, Y2O3 and ZrO2 are added and mixed evenly. The mixture is placed in a muffle furnace and heated to 950°C at a rate of 5°C / min in a dry air atmosphere. After high-temperature calcination for 1.5h, it is naturally cooled to room temperature to prepare the first particle and the second particle. The lithium cobalt oxide exhibits a first region, a second region, and protrusions. The atomic percentage fluctuation of Co in the first region is ≤50%, Al in the first region is ≤5%, Mn in the first region is ≤0.5%, Ni in the first region is ≤0.3%, and Ni in the second region is ≤0.3%. The first peak appears at a distance of 49 nm from the substrate surface, and the second peak appears at a distance of 30 nm from the substrate surface. In the first region, w1 > w2 > w3. In the second region… In the first region, the atomic percentage of Mn is greater than that of Ni; in the second region, the atomic percentage of Co is less than that of Co in the first region; in lithium cobalt oxide, w4 > w5 > w6; in lithium cobalt oxide, the mass content of Mn to Ni is 3.56; the mass content of Al in lithium cobalt oxide is 8896 ppm, the mass content of Mn in lithium cobalt oxide is 2496 ppm, the mass content of Ni in lithium cobalt oxide is 702 ppm, and the mass content of Mg in lithium cobalt oxide is 1021 ppm. When lithium metal is used as the counter electrode, and the lithium cobalt oxide is charged to 4.6V, the c-axis shrinkage rate is 3.47%; x1 is 19.2°, x2 is 19.57°; the protrusions contain Al, Ti, La, Y and Zr, the height of the protrusions is 286 nm, and the width of the orthographic projection of the protrusions on the surface of the lithium cobalt oxide is 294 nm. The mass content of Ti in the lithium cobalt oxide is 946 ppm, the mass content of La in the lithium cobalt oxide is 789 ppm, the mass content of Y in the lithium cobalt oxide is 1187 ppm, and the mass content of Zr in the lithium cobalt oxide is 796 ppm.

[0137] Preparation Example 4 The preparation examples in this group were carried out in accordance with Preparation Example 1. The difference was that the mass ratio of Mn to Ni in lithium cobalt oxide was controlled by changing the mass content of Mn and Ni in Co3O4 in steps (1) and (2), as follows: In preparation example 4a, the mass content of Mn in lithium cobalt oxide was 2815 ppm, the mass content of Ni in lithium cobalt oxide was 512 ppm, and the mass ratio of Mn to Ni in lithium cobalt oxide was 5.63; the atomic percentage fluctuation of Ni in the first region was ≤0.2%. In preparation example 4b, the mass content of Mn in lithium cobalt oxide was 3487 ppm, the mass content of Ni in lithium cobalt oxide was 405 ppm, and the mass ratio of Mn to Ni in lithium cobalt oxide was 8.61; the atomic percentage fluctuation of Ni in the first region was ≤0.2%. In preparation example 4c, the mass content of Mn in lithium cobalt oxide was 1501 ppm, the mass content of Ni in lithium cobalt oxide was 1503 ppm, and the mass ratio of Mn to Ni in lithium cobalt oxide was 1.

[0138] Preparation Example 5 The preparation examples in this group were carried out in accordance with Preparation Example 1. The difference was that the mass content of Al element in lithium cobalt oxide was controlled by changing the amount of Al2O3 added in steps (1) and (2), as follows: Preparation Example 5a: The mass content of Al in lithium cobalt oxide is 6014 ppm; Preparation Example 5b: The mass content of Al in lithium cobalt oxide was 9978 ppm.

[0139] Preparation Example 6 The preparation examples in this group were carried out in accordance with Preparation Example 1. The difference was that the mass content of Mg in lithium cobalt oxide was controlled by changing the mass content of Mg in Co3O4 in steps (1) and (2), as follows: Preparation Example 6a: The mass content of Mg in lithium cobalt oxide is 0 ppm; Preparation Example 6b: The mass content of Mg in lithium cobalt oxide is 603 ppm; Preparation Example 6c: The mass content of Mg in lithium cobalt oxide was 1985 ppm.

[0140] Preparation Example 7 This preparation example is carried out with reference to Preparation Example 1. The difference is that Al2O3 is not added in step (3), and the mass content of Al in Co3O4 used in steps (1) and (2) is reduced so that: the mass content of Al element in lithium cobalt oxide is 898ppm; w2>w3>w1; w5>w6>w4; and the protrusion does not contain Al element.

[0141] Preparation Example 8 The preparation examples in this group were carried out in accordance with Preparation Example 1. The difference was that the types of elements in the protrusions were controlled by changing the amount of Al2O3, TiO2, La2O3, Y2O3 or ZrO2 added in step (3), as follows: Preparation Example 8a: The protrusion does not contain Al. Preparation Example 8b: The protrusion does not contain Ti. Preparation Example 8c: The protrusions do not contain La. Preparation example 8d, the protrusion does not contain Y element; Preparation Example 8e: The protrusions do not contain Zr.

[0142] Unless otherwise stated, the lithium cobalt oxide cathode materials obtained in Preparation Examples 1 to 8 all satisfy the following: The lithium cobalt oxide particles comprise a first particle (Dv50 of 9.5μm-20.5μm, average particle size of 12μm-22μm) and a second particle (1μm≤D<9.5μm, average particle size of 0.5μm-6.5μm); the lithium cobalt oxide contains Ni, Mn, and Al elements; the lithium cobalt oxide has a first region and a second region located on the outer surface of the first region; the first region contains Ni, Mn, and Al elements; the second region contains Ni, Mn, and Al elements; the first region has an R-3m layered structure; the mass content of Ni element in the lithium cobalt oxide is less than the mass content of Mn element; the mass content of Mn element in the second region is greater than the mass content in the first region; In the second region, along the radial direction from the surface of lithium cobalt oxide to the center, in the linear scanning energy dispersive spectroscopy curve of transmission electron microscopy, the atomic percentage curve of Al element has a first peak, and the atomic percentage curve of Mn element has a second peak. The peak position of the second peak is closer to the matrix surface than the peak position of the first peak. Lithium cobalt oxide satisfies: n2 / n1 > n3 / n1 > n4 / n1, Co 3+ The quantity is greater than Co2+ The quantity, and on the surface of lithium cobalt oxide Co 3 + The quantity is less than Co 2+ The quantity; the boundary between the protrusion and the second region has at least one of the following: a layered structure with space group R-3m, a spinel phase with space group Fd3m, and a rock salt phase with space group R-3m; the mass content of sulfur in lithium cobalt oxide is less than 200 ppm, and the mass content of calcium in lithium cobalt oxide is less than 200 ppm.

[0143] Comparative preparation group 1 The comparative preparation examples in this group were prepared in accordance with Preparation Example 1. The difference was that the elemental composition of lithium cobalt oxide was controlled by changing the mass content of Mn, the mass content of Ni, and the amount of Al2O3 added in Co3O4 in steps (1) and (2), as well as the amount of Al2O3 added in step (3). The specific details are as follows: Compared to preparation example 1a, lithium cobalt oxide does not contain element Ni; Compared to preparation example 1b, lithium cobalt oxide does not contain the element Mn; Compared to preparation example 1c, lithium cobalt oxide does not contain elemental Al.

[0144] Comparative Preparation Example 2 This comparative preparation example is prepared in accordance with preparation example 1, except that the mass content of Mn and the mass content of Ni in Co3O4 in steps (1) and (2) are changed so that: the mass content of Ni in lithium cobalt oxide is greater than the mass content of Mn; in the first region, w1 > w3 > w2; in the second region, the atomic percentage of Ni is greater than the atomic percentage of Mn; n2 / n1 > n4 / n1 > n3 / n1.

[0145] Comparative preparation example 3 This comparative preparation example is prepared in accordance with preparation example 1. The difference is that in steps (1) and (2), Co3O4 contains uniformly distributed Al, Mg, Mn and Ni, that is, the atomic percentage of Al and Mn elements does not have a peak in the second region.

[0146] Comparative preparation example 4 This comparative preparation example is prepared in accordance with preparation example 1, except that the amount of Al2O3 added in step (3) is increased and the amount of Al2O3 added in steps (1) and (2) is decreased, so that: the atomic percentage curve of Al element has a first peak and the atomic percentage curve of Mn element has a second peak, and the peak position of the first peak is closer to the matrix surface than the peak position of the second peak.

[0147] Table 1 shows the specific diffraction peak positions of the lithium cobalt oxide cathode materials obtained in Preparation Examples 1-8 when lithium metal is used as the counter electrode and charged to different voltages. When lithium cobalt oxide cathode materials obtained in Comparative Preparation Example 1 and Comparative Preparation Example 3 were used with lithium metal as the counter electrode, no diffraction peaks appeared in the ranges of 4.5V (38°-38.2°), 4.37V (37.6°-37.85°), and 4.44V (37.7°-37.9°). When lithium metal was used as the counter electrode in the lithium cobalt oxide cathode material obtained in Comparative Preparation Example 2, the positions of the first diffraction peak were 38.15°, the second diffraction peak were 37.64°, the third diffraction peak were 37.75°, and the fourth diffraction peak was 37.7°. When lithium metal was used as the counter electrode in the lithium cobalt oxide cathode material obtained in Comparative Preparation Example 4, the positions of the first diffraction peak were 38.15°, the second diffraction peak were 37.61°, the third diffraction peak were 37.76°, and the fourth diffraction peak was 37.7°.

[0148] The following examples illustrate the battery of the present invention.

[0149] Example 1 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet The lithium cobalt oxide cathode material prepared in Example 1, conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 97:1.5:1.5. N-methylpyrrolidone was added, and the mixture was stirred under vacuum until it was uniformly mixed to obtain a cathode slurry with a solid content of 95%. The cathode slurry was uniformly coated on both sides of an aluminum foil and baked in an oven at 100°C for 12 hours. After rolling and slitting, the cathode sheet was obtained. (2) Preparation of negative electrode sheet Artificial graphite, silicon-carbon materials (a mixture of first and second silicon-carbon materials at a mass ratio of 5:1), carbon nanotubes, lithium carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylic acid were mixed at a mass ratio of 72:25:0.5:0.8:0.5:1.2, and deionized water was added to prepare a negative electrode slurry. This negative electrode slurry was coated onto both sides of a copper foil, baked, rolled, and die-cut to obtain the negative electrode sheet. The silicon-carbon material comprises a porous carbon matrix and silicon material located within the pores of the porous carbon matrix; S1 is 0.9, and S2 is 0.4; the average particle size of the first silicon-carbon particles is 4.5 μm, and the average particle size of the second silicon-carbon particles is 10.5 μm; the mass content of elemental Si in the first silicon-carbon particles is 40%, and the mass content of elemental Si in the second silicon-carbon particles is 35%; the single-walled carbon nanotubes have a diameter of 3.5 nm and a length of 15.6 μm. (3) Battery preparation The positive electrode sheet and separator (polyethylene film) prepared in step (1) and the negative electrode sheet prepared in step (2) are stacked in sequence to ensure that the separator is between the positive and negative electrodes to provide isolation. Then, a core is formed by stacking them in sequence. The core is placed in a steel shell with an inner cavity, and the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DMC / DEC with a volume ratio of 1:1:1, with the addition of FEC, DFEA and HTCN) is injected into the dried bare battery. After vacuum sealing, settling, formation, shaping and sorting, a lithium-ion battery is obtained. The electrolyte contains 13% FEC, 30% DFEA, 4% HTCN, and 1.2 mol / L lithium hexafluorophosphate.

[0150] Example 2 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet The lithium cobalt oxide cathode material prepared in Example 2, conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 97:1.5:1.5. N-methylpyrrolidone was added, and the mixture was stirred under vacuum until it was uniformly mixed to obtain a cathode slurry with a solid content of 95%. The cathode slurry was uniformly coated on both sides of an aluminum foil and baked in an oven at 100°C for 12 hours. After rolling and slitting, the cathode sheet was obtained. (2) Preparation of negative electrode sheet Artificial graphite, silicon-carbon materials (a mixture of first and second silicon-carbon materials at a mass ratio of 5:1), carbon nanotubes, lithium carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylic acid were mixed at a mass ratio of 72:25:0.5:0.8:0.5:1.2, and deionized water was added to prepare a negative electrode slurry. This negative electrode slurry was coated onto both sides of a copper foil, baked, rolled, and die-cut to obtain the negative electrode sheet. The silicon-carbon material comprises a porous carbon matrix and silicon material located within the pores of the porous carbon matrix; S1 is 0.7, and S2 is 0.3; the average particle size of the first silicon-carbon particles is 1.5 μm, and the average particle size of the second silicon-carbon particles is 6.2 μm; the mass content of elemental Si in the first silicon-carbon particles is 25%, and the mass content of elemental Si in the second silicon-carbon particles is 20%; the single-walled carbon nanotubes have a diameter of 1.3 nm and a length of 5.2 μm. (3) Battery preparation The positive electrode sheet and separator (polyethylene film) prepared in step (1) and the negative electrode sheet prepared in step (2) are stacked in sequence to ensure that the separator is between the positive and negative electrodes to provide isolation. Then, a core is formed by stacking them in sequence. The core is placed in a steel shell with an inner cavity, and the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DMC / DEC with a volume ratio of 1:1:1, with the addition of FEC, DFEA and HTCN) is injected into the dried bare battery. After vacuum sealing, settling, formation, shaping and sorting, a lithium-ion battery is obtained. The electrolyte contains 20% FEC, 35% DFEA, 1% HTCN, and 1.2 mol / L lithium hexafluorophosphate.

[0151] Example 3 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet The lithium cobalt oxide cathode material prepared in Example 3, conductive carbon black, and polyvinylidene fluoride were mixed in a mass ratio of 97:1.5:1.5. N-methylpyrrolidone was added, and the mixture was stirred under vacuum until it was uniformly mixed to obtain a cathode slurry with a solid content of 95%. The cathode slurry was uniformly coated on both sides of an aluminum foil and baked in an oven at 100°C for 12 hours. After rolling and slitting, the cathode sheet was obtained. (2) Preparation of negative electrode sheet Artificial graphite, silicon-carbon materials (a mixture of first and second silicon-carbon materials at a mass ratio of 5:1), carbon nanotubes, lithium carboxymethyl cellulose, styrene-butadiene rubber, and polyacrylic acid were mixed at a mass ratio of 72:25:0.5:0.8:0.5:1.2, and deionized water was added to prepare a negative electrode slurry. This negative electrode slurry was coated onto both sides of a copper foil, baked, rolled, and die-cut to obtain the negative electrode sheet. The silicon-carbon material comprises a porous carbon matrix and silicon material located within the pores of the porous carbon matrix; S1 is 1, and S2 is 0.6; the average particle size of the first silicon-carbon particles is 9.5 μm, and the average particle size of the second silicon-carbon particles is 14.8 μm; the mass content of elemental Si in the first silicon-carbon particles is 70%, and the mass content of elemental Si in the second silicon-carbon particles is 60%; the single-walled carbon nanotubes have a diameter of 4.6 nm and a length of 20 μm. (3) Battery preparation The positive electrode sheet and separator (polyethylene film) prepared in step (1) and the negative electrode sheet prepared in step (2) are stacked in sequence to ensure that the separator is between the positive and negative electrodes to provide isolation. Then, a core is formed by stacking them in sequence. The core is placed in a steel shell with an inner cavity, and the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DMC / DEC with a volume ratio of 1:1:1, with the addition of FEC, DFEA and HTCN) is injected into the dried bare battery. After vacuum sealing, settling, formation, shaping and sorting, a lithium-ion battery is obtained. The electrolyte contains 5% FEC, 20% DFEA, 8% HTCN, and 1.2 mol / L lithium hexafluorophosphate.

[0152] Examples 4-8 and Comparative Examples 1-3 were carried out in accordance with Example 1, except that the lithium cobalt oxide cathode material prepared in Preparation Example 1 was replaced with the same mass, as shown in Table 2.

[0153] Example 9 group This set of embodiments is based on Embodiment 1, except that the composition of the electrolyte is changed, as follows: Example 9a: The FEC content in the electrolyte is 1% by mass. Example 9b: The electrolyte does not contain DFEA; In Example 9c, the mass content of HTCN in the electrolyte was 0.5%.

[0154] Example 10 group This set of embodiments is based on Embodiment 1, except that the composition of the silicon-carbon material is changed, as follows: Example 10a: The silicon-carbon material contains only first silicon-carbon particles; Example 10b: The silicon-carbon material contains only second silicon-carbon particles.

[0155] Example 11 This embodiment is based on Embodiment 1, except that the negative electrode active layer does not contain single-walled carbon nanotubes.

[0156] Test case (1) Gram capacity test The specific capacity of the cathode materials prepared in the preparation examples and the comparative preparation examples was tested. The specific testing methods are as follows: The positive electrode material, acetylene black, and polyvinylidene fluoride were uniformly mixed at a mass ratio of 94:3:3 and dispersed in NMP solvent to form a slurry. The slurry was uniformly coated on an aluminum foil sheet and dried at 80°C for 12 hours. The dried electrode sheet was cut into round pieces and placed in a glove box for later use. The electrode sheet prepared above was used as the positive electrode sheet, the lithium metal sheet as the negative electrode, the microporous polypropylene membrane as the separator, and 1 mol / L LiPF6 (EC:DMC:EMC=1:1:1) as the electrolyte to assemble a button cell. The button cells prepared above were left to stand for 10 minutes at 25℃±2℃; discharged to 3V at 0.2C and left to stand for 10 minutes; charged to 4.55V at 0.2C in a constant temperature room at 25℃, cut off at 0.05C and left to stand for 10 minutes; discharged to 3V at 0.2C and the discharge capacity was calculated. The results are recorded in Table 2.

[0157] (2) Loop testing The batteries prepared in the examples and comparative examples were subjected to cycle tests, and the specific test methods are as follows: The battery was left to stand at 45℃±2℃ for 10 minutes, then discharged at 0.2C to 3.0V and left to stand for 10 minutes. It was then charged at 0.7C to the upper cutoff voltage limit of 4.55V, cut off at 0.05C, and left to stand for 10 minutes. Finally, it was discharged at 0.5C to 3V and left to stand for 10 minutes, yielding the initial capacity C0. This charging and discharging process was repeated until the 200th constant-voltage charging cycle ended, followed by a 10-minute stand. Then, the battery was discharged again at a current density of 0.2C to 3.0V and left to stand for 10 minutes. The discharge capacity at this point was recorded as the post-cycle capacity C1. The cycle capacity retention rate is calculated as C1×100% / C0, and the results are shown in Table 2.

[0158] (3) Discharge rate test The batteries prepared in the examples and comparative examples were subjected to discharge rate tests. The specific test methods are as follows: At 25℃±2℃, the battery was left to stand for 10 minutes; then discharged at 0.2C to 3V, and left to stand for 10 minutes; fully charged at 1.5C in a 25℃ constant temperature chamber, with a cutoff voltage of 4.55V and a cutoff current of 0.025C, and left to stand for 10 minutes; then discharged at 2.5C to 3V in a 25℃ constant temperature chamber or incubator environment, and left to stand for 10 minutes; this charging and discharging process was repeated until the 200th cycle of constant voltage charging was completed, followed by a 10-minute stand, and then discharged again at a current density of 0.2C to 3.0V, and left to stand for 10 minutes. The discharge capacity of the battery at this point was recorded as the capacity C1 after the cycle. The cycle capacity retention rate is calculated as C1×100% / C0, and the results are recorded in Table 2.

[0159] Table 1 Table 2 As can be seen from Table 2, the present invention is beneficial to improving the specific capacity of the cathode material compared with the comparative example. The present invention significantly improves the rate performance and cycle stability of the battery.

[0160] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium cobalt oxide cathode material, characterized in that, The lithium cobalt oxide cathode material contains Ni, Mn and Al elements; the lithium cobalt oxide cathode material includes lithium cobalt oxide particles, the lithium cobalt oxide particles include a matrix, the matrix has a first region and a second region located on the outer surface of the first region; extending from the surface of the matrix towards the center, the second region is a region with a depth of 80 nm from the surface of the matrix, and the first region is the region remaining in the matrix excluding the second region; The first region contains Ni, Mn, and Al elements; the second region contains Ni, Mn, and Al elements; the first region contains an R-3m layered structure; The mass content of Ni in the lithium cobalt oxide cathode material is less than the mass content of Mn. The mass content of Mn element in the second region is greater than that in the first region; When the lithium cobalt oxide cathode material is charged to 4.5V with lithium metal as the counter electrode, the lithium cobalt oxide cathode material has a first diffraction peak located at 38°-38.2° in the X-ray diffraction pattern.

2. The lithium cobalt oxide cathode material according to claim 1, wherein, The atomic percentage of Co in the first region fluctuates by ≤50%; And / or, the atomic percentage of Al in the first region fluctuates by ≤5%; And / or, the atomic percentage of Mn in the first region fluctuates by ≤0.5%; And / or, the atomic percentage of Ni in the first region fluctuates by ≤0.3%; And / or, the atomic percentage of Ni in the second region fluctuates by ≤0.3%; And / or, in the second region, along the radial direction from the surface to the center of the lithium cobalt oxide particle, in the linear scanning energy dispersive spectroscopy curve of the transmission electron microscope, the atomic percentage curve of Al element has a first peak and the atomic percentage curve of Mn element has a second peak, and the peak position of the second peak is closer to the matrix surface than the peak position of the first peak. Preferably, the first peak appears in the depth range of 5nm-70nm from the surface of the substrate; more preferably, the first peak appears in the depth range of 10nm-50nm from the surface of the substrate. Preferably, the second peak appears in the depth range of 1nm-50nm from the surface of the substrate; more preferably, the second peak appears in the depth range of 5nm-30nm from the surface of the substrate.

3. The lithium cobalt oxide cathode material according to claim 1 or 2, wherein, In the first region, the atomic percentage of Al is w1, the atomic percentage of Mn is w2, and the atomic percentage of Ni is w3, where w1 > w2 > w3; And / or, in the second region, the atomic percentage of Mn is greater than the atomic percentage of Ni; And / or, the atomic percentage of Co in the second region is less than the atomic percentage of Co in the first region; And / or, when the lithium cobalt oxide cathode material uses lithium metal as the counter electrode, when charged to 4.37V, the lithium cobalt oxide cathode material has a second diffraction peak and a third diffraction peak located at 37.6°-37.85° in the X-ray diffraction pattern; when charged to 4.44V, the lithium cobalt oxide cathode material has a fourth diffraction peak located at 37.7°-37.9° in the X-ray diffraction pattern.

4. The lithium cobalt oxide cathode material according to claim 1 or 2, wherein, When lithium metal is used as the counter electrode, the c-axis shrinkage rate of the lithium cobalt oxide cathode material is 2.5%-3.5% when charged to 4.6V, based on the c-axis value at 4.2V. And / or, when the lithium cobalt oxide cathode material uses lithium metal as the counter electrode, in the X-ray diffraction pattern of the lithium cobalt oxide cathode material charged to 4.6V, the 003 crystal plane peak is located at x1; preferably, x1 is 19.1°±0.15°; more preferably, it is 19.1°±0.1°; And / or, when the lithium cobalt oxide cathode material uses lithium metal as the counter electrode, in the X-ray diffraction pattern of the lithium cobalt oxide cathode material charged to 4.7V, the 003 crystal plane peak is located at x2; preferably, x2 is 19.47°±0.15°; more preferably, it is 19.47°±0.1°.

5. The lithium cobalt oxide cathode material according to claim 1 or 2, wherein, The lithium cobalt oxide cathode material contains Ni 2+ and Ni 3+ In the lithium cobalt oxide cathode material, Co 3+ The quantity is n1, Mn 4+ The quantity n2, Ni 2+ The quantity is n3, Ni 3+ The quantity is n4; n2 / n1 > n3 / n1 > n4 / n1; And / or, in the lithium cobalt oxide cathode material, Co 3+ The quantity is greater than Co 2+ The quantity; and the Co on the surface of the lithium cobalt oxide cathode material 3+ The quantity is less than Co 2+ The quantity.

6. The lithium cobalt oxide cathode material according to claim 1 or 2, wherein, In the lithium cobalt oxide cathode material, the mass content of Al is w4, the mass content of Mn is w5, and the mass content of Ni is w6, wherein w4 > w5 > w6; Preferably, in the lithium cobalt oxide cathode material, the mass content of Mn element to the mass content of Ni element is 1.2-5; Preferably, the mass content of Al element in the lithium cobalt oxide cathode material is 6000ppm-12000ppm; more preferably, it is 7000ppm-10000ppm. Preferably, the mass content of Mn element in the lithium cobalt oxide cathode material is 600ppm-3500ppm; more preferably, it is 1000ppm-2500ppm. Preferably, the mass content of Ni in the lithium cobalt oxide cathode material is 400ppm-2000ppm; more preferably, it is 500ppm-1500ppm. Preferably, the lithium cobalt oxide further comprises Mg; more preferably, the mass content of Mg in the lithium cobalt oxide cathode material is 400ppm-2000ppm; even more preferably 600ppm-1500ppm; Preferably, the mass content of sulfur in the lithium cobalt oxide cathode material is less than 200 ppm; Preferably, the mass content of Ca in the lithium cobalt oxide cathode material is less than 200 ppm.

7. The lithium cobalt oxide cathode material according to claim 1 or 2, wherein, The lithium cobalt oxide particles also have protrusions located on the outer surface of the lithium cobalt oxide particles, partially covering the second region; Preferably, the protrusion contains at least one of the elements Al, Ti, La, Y, and Zr; Preferably, the height of the protrusion is 10nm-300nm, and the width of the protrusion's orthogonal projection on the lithium cobalt oxide surface is 10nm-300nm.

8. The lithium cobalt oxide cathode material according to claim 7, wherein, The mass content of element Ti in the lithium cobalt oxide cathode material is 450ppm-1050ppm; And / or, the mass content of element La in the lithium cobalt oxide cathode material is 300ppm-800ppm; And / or, the mass content of element Y in the lithium cobalt oxide cathode material is 200ppm-1200ppm; And / or, the mass content of element Zr in the lithium cobalt oxide cathode material is 200ppm-1000ppm; And / or, the boundary between the protrusion and the second region has at least one of the following: a layered structure with space group R-3m, a spinel phase with space group Fd3m, and a rock salt phase with space group R-3m.

9. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes a positive electrode sheet, which includes the lithium cobalt oxide positive electrode material according to any one of claims 1-8.

10. The lithium-ion secondary battery according to claim 9, wherein, The lithium-ion secondary battery further includes an electrolyte, which comprises fluoroethylene carbonate; preferably, the mass content of fluoroethylene carbonate in the electrolyte is 5%-20%. And / or, the electrolyte comprises a fluorocarboxylic acid ester; preferably, the fluorocarboxylic acid ester in the electrolyte contains 10%-50% by mass; And / or, the electrolyte comprises 1,3,6-hexanetrionitrile, preferably, the mass content of 1,3,6-hexanetrionitrile in the electrolyte is 0.5%-8%.

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