Positive plate and lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a positive plate and a lithium ion secondary battery comprising the same. The positive plate comprises the positive active material and the conductive agent which are highly matched, so that the compaction density and the conductivity of the positive plate are improved at the same time. Comprising the positive plate disclosed by the invention can give consideration to both energy density and rate capability.

Description

Technical Field

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

[0002] In recent years, lithium-ion batteries have become the primary power source for modern electronic devices and electric vehicles, drawing significant attention to their energy density and cycle life. High-nickel cathode materials are favored in the lithium-ion battery market due to their low cost and high specific capacity; however, they also suffer from low compaction density and poor rate performance. Current technologies typically increase the nickel content in high-nickel cathode materials to over 80% to improve energy density, but this reduces the rate performance of such batteries. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned problems in the prior art and to provide a positive electrode sheet and a lithium-ion secondary battery including the positive electrode sheet. The positive electrode sheet of this invention includes a highly compatible positive electrode active material and a conductive agent, thereby simultaneously improving the compaction density and conductivity of the positive electrode sheet. The lithium-ion secondary battery including the positive electrode sheet of this invention (hereinafter referred to as the battery) can achieve both high energy density and rate performance.

[0004] In related technologies, high-nickel cathode materials suffer from low compaction density and poor conductivity. The reasons for these problems are: First, as the nickel content increases, the Li... + / Ni 2+ The mixing degree deepens, and high temperature will exacerbate Li + / Ni 2 + Mixed arrangement, therefore, in order to reduce the impact of high temperature on Li + / Ni 2+ First, the mixing process has an impact. The synthesis temperature of high-nickel cathode materials is relatively low, which makes it impossible to form large particle sizes and results in low compaction density. Second, nickel itself has poor electrical conductivity. When the nickel content in the cathode active material increases, the conductivity of high-nickel materials will deteriorate, thereby affecting the transport of lithium ions and causing a decrease in the rate performance of the battery.

[0005] Carbon nanotubes have attracted much attention due to their high conductivity, high specific surface area, and excellent electrocatalytic performance. Therefore, carbon nanotubes are frequently used as conductive agents in lithium-ion batteries. However, the inventors of this invention found that simply combining high-nickel cathode materials with carbon nanotubes only slightly improves the conductivity of the cathode, but the improvement is not significant. To address this problem, the inventors conducted extensive targeted research and discovered that because carbon nanotubes are very small in size and have a large specific surface area, they are difficult to disperse and prone to aggregation, resulting in low conductivity. Based on these findings, the inventors of this invention, after extensive research, proposed the following solution:

[0006] The first aspect of this invention provides a positive electrode sheet, the positive electrode sheet comprising a positive electrode active material and a conductive agent; the positive electrode active material comprises a first particle and a second particle; the first particle comprises a single crystal particle, and the second particle comprises a polycrystalline particle; the first particle comprises a material with the chemical formula Li x1 Ni a1 Co b1 Mn c1 M 1 d1 For O2, 0.9 ≤ x1 ≤ 1.1, 0.8 ≤ a1 ≤ 0.99, 0.01 ≤ b1 ≤ 0.1, 0.01 ≤ c1 ≤ 0.1, 0 ≤ d1 ≤ 0.05, M 1 Including at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb; the second particle includes the chemical formula Li x2 Ni a2 Co b2 Mn c2 M 2 d2 For O2, the following conditions apply: 0.9 ≤ x² ≤ 1.1, 0.8 ≤ a² ≤ 0.95, 0.01 ≤ b² ≤ 0.2, 0.01 ≤ c² ≤ 0.1, 0 ≤ d² ≤ 0.05, M 2 The material includes at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb; in the positive electrode active material, the ratio of the number of moles of element Ni to the total number of moles of element Ni, element Co, and element Mn is x; the conductive agent includes carbon nanotubes with a diameter of y in nm; x and y satisfy: y / x is 1-75.

[0007] The diameter of carbon nanotubes is crucial for improving the conductivity of the cathode. When the ratio x of the molar number of Ni to the total molar number of Ni, Co, and Mn, and the diameter y of the carbon nanotubes satisfy a specific relationship, the carbon nanotubes can be uniformly dispersed around the cathode active material and form better electrical connections with it. This improves the lithium-ion transport capacity and enhances the capacity utilization of the cathode active material, enabling the battery to achieve both energy density and rate performance. Furthermore, carbon nanotubes can fill the pores between the cathode active materials, which can improve the compaction density of the cathode to some extent. As x increases, the compaction density and conductivity of the cathode decrease. Therefore, it is necessary to increase the diameter of the carbon nanotubes to control the adverse effects of changes in x on the compaction density and conductivity of the cathode. When y / x is large (e.g., greater than 75), y is too large relative to x, which reduces the dispersion of carbon nanotubes, making it difficult for them to insert into the primary particles of the positive electrode active material, resulting in reduced conductivity. When y / x is small (e.g., less than 1), y is too small relative to x, resulting in relatively thin carbon nanotubes with insufficient thickness and toughness, leading to poor dispersion. Furthermore, a large x is mismatched with the carbon nanotubes, resulting in reduced conductivity.

[0008] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising the positive electrode sheet described in the first aspect of the present invention.

[0009] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0010] (1) The positive electrode sheet of the present invention has a better compaction density, which can improve the energy density of the battery;

[0011] (2) The positive electrode of the present invention has a good conductive network, which can improve the charge transmission efficiency, thereby improving the capacity of the battery and realizing the increase of the battery's energy density;

[0012] (3) The battery of the present invention has good rate performance and high temperature 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 image shown is a scanning electron microscope (SEM) image of the positive electrode sheet in an example of the present invention. Detailed Implementation

[0015] 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.

[0016] A first aspect of the present invention provides a positive electrode sheet, which may include a positive electrode active material and a conductive agent. The positive electrode active material may include a first particle and a second particle. The first particle may include a single-crystal particle, and the second particle may include a polycrystalline particle. The first particle may contain a material with the chemical formula Li. x1 Ni a1 Co b1 Mn c1 M 1 d1 For O2, 0.9 ≤ x1 ≤ 1.1 (e.g., 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1, 1.05, or 1.1), 0.8 ≤ a1 ≤ 0.99 (e.g., 0.8, 0.85, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, or 0.99), 0.01 ≤ b1 ≤ 0.1 (e.g., 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1), 0.01≤c1≤0.1 (e.g., 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.1), 0≤d1≤0.05 (e.g., 0, 0.01, 0.02, 0.03, 0.04, or 0.05), M 1 It may include at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb. The second particle may include the chemical formula Li. x2 Ni a2 Co b2 Mn c2 M 2 d2For substances containing O2, 0.9 ≤ x² ≤ 1.1 (e.g., 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1, 1.05, or 1.1), 0.8 ≤ a² ≤ 0.95 (e.g., 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, or 0.95), and 0.01 ≤ b² ≤ 0.2 (e.g., 0.01, 0.0...). 2, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19 or 0.2), 0.01≤c2≤0.1 (e.g., 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1), 0≤d2≤0.05 (e.g., 0, 0.01, 0.02, 0.03, 0.04 or 0.05), M 2 It may include at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb. In the positive electrode active material, the ratio of the molar number of Ni to the total molar number of Ni, Co, and Mn is x. The conductive agent may include carbon nanotubes with a diameter of y in nm. x and y satisfy: y / x is 1-75, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or 75.

[0017] In one instance, y / x is 2-75.

[0018] In one instance, y / x is 5-25.5.

[0019] In one instance, y / x is 5.3-24.

[0020] like Figure 1 The image shown is a scanning electron microscope (SEM) image of the positive electrode sheet in an example of the present invention. As can be seen from the image, the positive electrode sheet includes a positive electrode active material and a conductive agent, wherein the conductive agent includes carbon nanotubes.

[0021] In one example, the carbon nanotubes are arranged in clusters. "Arranged in clusters" means that several carbon nanotubes are arranged in bundles.

[0022] In this invention, the ratio x of the number of moles of element Ni to the total number of moles of element Ni, element Co and element Mn can be 0.87-0.97, for example 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96 or 0.97.

[0023] In one instance, x is between 0.91 and 0.96.

[0024] When x is too small (e.g., less than 0.87), the specific capacity of the positive electrode active material is difficult to increase further due to the low nickel content, and the energy density of the battery is also limited. When x is too large (e.g., greater than 0.97), the conductivity of the positive electrode is reduced due to the limited cobalt content, and the stability of the positive electrode active material is greatly reduced due to the high oxidizing properties of nickel.

[0025] In this invention, the ratio x of the number of moles of element Ni to the total number of moles of element Ni, element Co and element Mn can be obtained by conventional methods in the art, such as inductively coupled plasma optical emission spectrometry (ICP-OES) or ethylenediaminetetraacetic acid (EDTA) potentiometric titration.

[0026] In this invention, y can be 0.5-80, and the unit is nm, for example, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75 or 80.

[0027] In one example, y is 2-70, and the unit is nm.

[0028] In one example, y is 5-25, and the unit is nm.

[0029] In one example, y is 0.5-10, and the unit is nm.

[0030] In one example, y is 2-80, and the unit is nm.

[0031] In one example, y is 1-5, and the unit is nm.

[0032] In one example, y is 5-30, and the unit is nm.

[0033] When y is too small (e.g., less than 0.5 nm), carbon nanotubes are prone to breakage due to poor toughness, thus failing to improve the electron transport rate in the positive electrode. When y is too large (e.g., greater than 80 nm), it acts as a barrier to other conductive agents in the positive electrode (e.g., carbon black particles adsorbed by van der Waals forces), making it difficult to form a relatively uniform conductive network and failing to provide more conductive paths.

[0034] In this invention, the diameter y of the carbon nanotube can be obtained by conventional methods in the art, such as using a transmission electron microscope (TEM) to randomly select at least 10 carbon nanotubes within a field of view, measure the outer diameter of each carbon nanotube, and take the average value.

[0035] In one instance, the first particle is a single-crystal particle.

[0036] In one instance, the second particle is a polycrystalline particle.

[0037] In this invention, the carbon nanotubes include at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0038] Positive electrode active materials include single-crystal particles and polycrystalline particles. Single-crystal particles have better structural stability than polycrystalline particles, but the lithium-ion transport efficiency in single-crystal particles is lower than that in polycrystalline particles. Single-walled carbon nanotubes (SUVs) have better conductivity than multi-walled carbon nanotubes (MWCs); however, SUVs have a larger specific surface area, making them more prone to catalyzing electrolytes and causing gas generation (especially in high-temperature environments). Therefore, to further improve the structural stability and ionic conductivity of the positive electrode, it is necessary to combine SUVs and MWCs to work with positive electrode active materials that include both single-crystal and polycrystalline particles. This approach can improve the ionic conductivity of the positive electrode while ensuring high-temperature cycling stability.

[0039] In one example, the carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0040] In this invention, the mass ratio of the single-walled carbon nanotubes to the multi-walled carbon nanotubes can be 1:(3-20), for example, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19 or 1:20.

[0041] In related technologies, when using high-nickel materials as positive electrode active materials, there is a problem of severe gas generation during battery cycling due to their poor structural stability. The reason for this severe gas generation is that the surface activity of the positive electrode active material increases with increasing nickel content, which increases the risk of side reactions with the electrolyte, leading to gas generation. Therefore, a coating layer can be applied to the surface of the positive electrode active material, which can not only increase the surface stability of the positive electrode active material but also suppress the risk of side reactions with the electrolyte. However, simply applying a coating layer to the surface of the positive electrode active material without control will affect the transport of lithium ions in the battery, resulting in a deterioration in the battery's rate performance. Therefore, the inventors of this invention, through extensive research, discovered that by controlling the thickness of the coating layer on the surface of the positive electrode active material and the diameter of the carbon nanotubes, the battery can generate less gas during cycling, exhibiting better high-temperature cycling stability and furnace temperature safety performance, and also achieving better lithium ion transport efficiency.

[0042] In this invention, the positive electrode active material may include a coating layer, which may be located on the surface of the positive electrode active material, and the thickness of the coating layer is T. The diameter y and T of the carbon nanotubes satisfy the following: y / T can be 0.05-2, for example, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or 2.

[0043] In one instance, y / T is 0.145-0.8.

[0044] When y / T is small (e.g., less than 0.05), the diameter of the carbon nanotubes is relatively small compared to the thickness of the coating layer, which cannot effectively compensate for the adverse effects on lithium-ion transport caused by the excessive thickness of the coating layer; while when y / T is large (e.g., greater than 2), the diameter of the carbon nanotubes is relatively large compared to the thickness of the coating layer, making it difficult for the carbon nanotubes to be inserted into the primary particles of the positive electrode active material, thus weakening the conductivity.

[0045] In this invention, the thickness T of the coating layer can be 5-50 nm; for example, 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50.

[0046] In one example, T is 10⁻⁴⁵, and the unit is nm.

[0047] In one instance, T is 25-40, and the unit is nm.

[0048] In this invention, the thickness T of the coating layer can be obtained by conventional methods in the art, such as by TEM, randomly selecting at least 20 positive electrode active material particles in the field of view, randomly selecting at least 10 sites on the surface of each particle, measuring the coating layer thickness corresponding to each site, and taking the average value.

[0049] In this invention, the specific surface area of ​​the positive electrode active material is B, with units of m². 2 / g; the length of the carbon nanotube is L, in μm; B and L satisfy: B / L can be 0.05-0.5, for example 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4 or 0.5.

[0050] In one instance, the B / L ratio is 0.07-0.25.

[0051] To further improve the connectivity and stability of the conductive network between the positive electrode active material and carbon nanotubes, it is necessary to further control the relationship between the specific surface area of ​​the positive electrode active material and the length of the carbon nanotubes. The number of active sites on the surface of the positive electrode active material increases with the increase of its specific surface area. These active sites can directly contact the carbon nanotubes, ensuring sufficient dispersion of the carbon nanotubes and contributing to the formation of a stable and highly interconnected conductive network. When the B / L ratio is small (e.g., less than 0.05), the specific surface area of ​​the positive electrode active material is too small, and the length of the carbon nanotubes is too large, resulting in a mismatch and poor dispersion of both materials. The carbon nanotubes are prone to aggregation, failing to form an effective conductive network. Conversely, when the B / L ratio is large (e.g., greater than 0.5), the specific surface area of ​​the positive electrode active material is too large, and the length of the carbon nanotubes is too small, again resulting in a mismatch and poor dispersion of both materials. Furthermore, the carbon nanotubes struggle to fully connect with the positive electrode active material, leading to decreased conductivity of the positive electrode sheet.

[0052] In this invention, the specific surface area B of the positive electrode active material can be 0.5-1.5, in m². 2 / g, for example, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4 or 1.5.

[0053] In one example, the specific surface area B of the positive electrode active material is 0.7-1.1, in m². 2 / g.

[0054] When B is small (e.g., less than 0.5m) 2 When B is large (e.g., greater than 1.5m), the dispersion of the positive electrode active material in the slurry is poor, resulting in empty foils during the coating process, thus reducing the conductivity of the positive electrode sheet; while when B is large (e.g., greater than 1.5m), the dispersion of the positive electrode active material in the slurry is poor, resulting in empty foils during the coating process, thus reducing the conductivity of the positive electrode sheet. 2When the concentration is / g), the positive electrode active material is prone to agglomeration in the slurry, which leads to the formation of a gel in the slurry and is not conducive to coating.

[0055] In this invention, the specific surface area B of the positive electrode active material can be obtained by conventional methods in the art, such as nitrogen adsorption.

[0056] In this invention, the length L of the carbon nanotube can be 0.5-12.5 μm, for example, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 or 12.5.

[0057] In one example, the length L of the carbon nanotube is 2-11, in μm.

[0058] In one example, the length L of the carbon nanotube is 4-10 μm.

[0059] In one example, the length L of the carbon nanotube is 0.5-3.5 μm.

[0060] In one example, the length L of the carbon nanotube is 3.5-12.5 μm.

[0061] In one example, the length L of the carbon nanotube is 1-3 μm.

[0062] In one example, the length L of the carbon nanotube is 5-10 μm.

[0063] When L is small (e.g., less than 0.5 μm), there is less storage space and transport channels provided for lithium ions, which will cause greater polarization in the battery and is not conducive to cycle stability; while when L is large (e.g., greater than 12.5 μm), carbon nanotubes are prone to agglomeration, which weakens the conductivity.

[0064] In this invention, the length L of the carbon nanotube can be obtained by conventional methods in the art, such as TEM, where at least 10 carbon nanotubes are randomly selected within a field of view, the length of each carbon nanotube is measured, and the average value is taken.

[0065] In one instance, 0.9 ≤ a1 ≤ 0.99.

[0066] In this invention, based on the total mass of the positive electrode active material, the content of the first particle is C1, 50% ≤ C1 < 100%, for example, C1 is 50%, 60%, 70%, 80%, 90%, 99%, or 99.9%.

[0067] In one instance, 70% ≤ C1 ≤ 90%.

[0068] The inventors of this invention have discovered that a specific content of the first particle, matched with a specific y / x ratio, enables the first particle, the second particle, and the carbon nanotubes to bond more tightly, which is beneficial to Li + This improves the rate performance of the battery; it also helps to increase the compaction density of the positive electrode active material, thereby increasing the energy density of the battery; and it can delay the occurrence of side reactions between the positive electrode active material and the electrolyte, thus improving the high-temperature cycle performance of the battery.

[0069] In this invention, the particle size Dv of the first particle 1 10 can be 0.5μm-4μm, for example, 0.5μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, or 4μm. The median particle size Dv of the first particle... 1 The particle size Dv of the first particle can be 1.2μm-6μm, for example, 1.2μm, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, or 6μm. 1 99 ≤ 10 μm, for example, 10 μm, 9 μm, 8 μm, 7 μm, 6 μm or 5 μm.

[0070] In this invention, the particle size Dv of the second particle 2 10 can be 3μm-16μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or 16μm. The median particle size Dv of the second particle... 2 50 can be 4μm-21μm, for example, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, or 21μm. The particle size Dv of the second particle... 2 99 ≤ 25 μm, for example, 25 μm, 24 μm, 23 μm, 22 μm, 21 μm, 20 μm, 19 μm, 18 μm, 17 μm, 16 μm or 15 μm.

[0071] The inventors of this invention discovered that when the particle size Dv of the first particle... 1 10. Median particle size Dv 1 50 and particle size Dv 1 99 and the particle size Dv of the second particle 2 10. Median particle size Dv 2 50 and particle size Dv 2Within a specific range, the first and second particles can maintain relatively stable bulk and surface structures during battery charge-discharge cycles, thereby improving the high-temperature cycle stability of the battery.

[0072] In this invention, the second particle may comprise a particle composed of a plurality of primary particles. The particle size of the primary particles may be 100nm-600nm, for example, 100nm, 200nm, 300nm, 400nm, 500nm, or 600nm. The term "a plurality of" refers to the number of the primary particles being greater than or equal to 2.

[0073] In one example, the primary particle has a particle size of 300 nm to 500 nm.

[0074] In this invention, the particle size of the primary particles can be obtained by conventional methods in the art, such as by SEM. Specifically, take a positive electrode sheet and test the particle size of the primary particles of all second particles within the field of view at 7.3 mm * 10 kX. Take the average value to obtain the particle size of the primary particles.

[0075] In this invention, M 2 It may include Al. The mass content of element Al in the second particle may be 1000ppm-3500ppm, for example, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm or 3500ppm.

[0076] Al can form strong Al-O bonds with oxygen in the positive electrode active material, effectively suppressing the extraction of lattice oxygen. Furthermore, it can slow down the transformation of the positive electrode active material from a layered phase to a spinel phase during cycling, suppressing phase transitions and reducing volume changes during battery charge-discharge cycles. Therefore, Al can stabilize the bulk structure. The second particle has relatively poor structural stability compared to the first particle. Including Al in the second particle can suppress crack formation and stabilize its structure, thus improving the battery's cycle stability. However, the addition of Al can negatively impact the conductivity of the positive electrode active material. Specific conductive agents in the positive electrode can mitigate the degradation of conductivity caused by added Al, allowing the positive electrode to balance stability and conductivity.

[0077] In this invention, the mass content of element Al in the second particle can be obtained by methods conventional in the art, such as ICP.

[0078] In this invention, M 1Element B may be included. The mass content of element B in the first particle may be 500ppm-1300ppm, for example, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm or 1300ppm.

[0079] In one example, the mass content of element B in the first particle is 700ppm-1000ppm.

[0080] Element B can improve the conductivity of the positive electrode active material and inhibit electron transitions at the positive electrode / electrolyte interface, giving the positive electrode active material good resistance to electrolyte decomposition. This improves the conductivity and surface stability of the positive electrode active material, thereby enhancing the battery's rate performance and cycle performance.

[0081] In this invention, the mass content of element B in the first particle can be obtained by methods conventional in the art, such as ICP.

[0082] In this invention, the ratio D2 / D1 of the average particle size of the second particle to the average particle size of the first particle satisfies 1≤D2 / D1≤10, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

[0083] In one instance, 2 ≤ D2 / D1 ≤ 6.5.

[0084] In one instance, 3 ≤ D2 / D1 ≤ 4.5.

[0085] In this invention, 1.5μm≤D1≤5μm, for example, 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm or 5μm.

[0086] In one instance, 2μm≤D1≤4μm.

[0087] In this invention, 5μm≤D2≤20μm, for example, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm or 20μm.

[0088] In one instance, 9μm≤D2≤13μm.

[0089] In this invention, the average particle size D1 of the first particle and the average particle size D2 of the second particle can be obtained by conventional methods in the art, such as by SEM. Specifically: take a positive electrode sheet, test the particle size of all first particles within the field of view at 7.3mm*10kX, and take the average value to obtain the particle size D1 of the first particle; test the particle size of all second particles within the field of view, and take the average value to obtain the particle size D2 of the second particle.

[0090] In this invention, the median particle size Dv of the second particle 2 50 and the median particle size Dv of the first particle 1 The ratio of 50 to Dv 2 50 / Dv 1 50 satisfies 1≤Dv 2 50 / Dv 1 50≤10, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

[0091] In this invention, the specific surface area of ​​the first particle can be 0.8 m². 2 / g-1.3m 2 / g, for example, 0.8m 2 / g, 0.9m 2 / g, 1m 2 / g, 1.1m 2 / g, 1.2m 2 / g or 1.3m 2 / g.

[0092] In this invention, the specific surface area of ​​the second particle can be 0.35 m². 2 / g-0.6m 2 / g, for example, 0.35m 2 / g, 0.4m 2 / g, 0.45m 2 / g, 0.5m 2 / g, 0.55m 2 / g or 0.6m 2 / g.

[0093] In this invention, the interior of the second particle may have pores. The size of the pores may be 5nm-25nm, for example, 5nm, 10nm, 15nm, 20nm or 25nm.

[0094] The inventors of this invention have discovered that when the size of the pores inside the second particle is within a specific range, it is beneficial for Li + The transmission of the second particle is improved, and the risk of side reactions between the second particle and the electrolyte is significantly reduced, which can effectively improve the cycle stability of the battery.

[0095] In this invention, the particle size Dv of the first particle 1 10. Median particle size Dv 1 50 and particle size Dv 1 90 and the particle size Dv of the second particle 2 10. Median particle size Dv 2 50 and particle size Dv 2 90 can be obtained by methods conventional in the art, such as laser particle size analyzers.

[0096] In this invention, the compaction density of the positive electrode sheet can be 3.3 g / cm³. 3 -3.5g / cm 3 For example, 3.3 g / cm³ 3 3.4g / cm 3 Or 3.5g / cm 3 .

[0097] In this invention, the areal density of the positive electrode can be 10 mg / cm³. 2 -14mg / cm 2 For example, 10 mg / cm 2 11mg / cm 2 12mg / cm 2 13mg / cm 2 Or 14mg / cm 2 .

[0098] In this invention, the porosity of the positive electrode sheet can be 20%-35%, for example, 20%, 25%, 30% or 35%.

[0099] The compaction density, areal density, and porosity of the positive electrode directly affect the overall wetting performance of the electrolyte on the positive electrode; and the wetting performance of the electrolyte directly affects the conductivity and structural stability of the first and second particles. Therefore, a specific compaction density and / or areal density and / or porosity is beneficial for balancing the conductivity and structural stability of the first and second particles, thereby enabling the battery to possess high energy density, cycle stability, and rate performance.

[0100] In this invention, the porosity of the positive electrode can be obtained by methods conventional in the art, such as gas adsorption.

[0101] In this invention, the conductive agent may further include conductive carbon black. The ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black can be (0.01-2):1, for example, 0.01:1, 0.02:1, 0.03:1, 0.04:1, 0.05:1, 0.06:1, 0.07:1, 0.08:1, 0.09:1, 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, or 2:1.

[0102] In one example, the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black is (0.05-0.25):1.

[0103] When the ratio of the carbon nanotube diameter to the average particle size of the conductive carbon black is within a specific range, the carbon nanotubes and conductive carbon black are well-matched, ensuring that the carbon nanotubes have sufficient thickness and toughness to prevent breakage due to poor toughness during contact with the conductive carbon black, thereby improving the electron transport rate. Simultaneously, the carbon nanotubes are not so coarse that they reduce the conductivity of the positive electrode. When the ratio is large (e.g., greater than 2:1), the diameter of the carbon nanotubes is too large relative to the average particle size of the conductive carbon black, which hinders the conductive carbon black particles from adsorbing each other through van der Waals forces, making it difficult to form a relatively uniform conductive network and providing insufficient conductive paths.

[0104] In this invention, the average particle size of the conductive carbon black can be 10nm-800nm, for example, 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm or 800nm.

[0105] In one example, the conductive carbon black has an average particle size of 20 nm to 300 nm.

[0106] In this invention, the average particle size of the conductive carbon black can be obtained by conventional methods in the art, such as SEM, randomly selecting at least 20 conductive carbon black particles in the field of view, measuring the particle size of each conductive carbon black particle, and taking the average value.

[0107] In this invention, the positive electrode sheet may include a positive current collector and a positive active material layer located on at least one side surface of the positive current collector. The positive active material layer may include the positive active material and the conductive agent. The positive active material layer may also include a binder. The binder includes binders conventionally used in the art, such as at least one selected from polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene-butadiene rubber, polytetrafluoroethylene, and polyethylene oxide.

[0108] In this invention, based on the total mass of the positive electrode active material layer, the content of the positive electrode active material can be 80%-99.8% (e.g., 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, 98%, 99%, or 99.8%), the content of the conductive agent can be 0.1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%), and the content of the binder can be 0.1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%).

[0109] In this invention, the mass content of the single-walled carbon nanotubes in the positive electrode active material layer is ≤0.2%. The mass content of the multi-walled carbon nanotubes in the positive electrode active material layer is ≤1%.

[0110] In one example, the mass content of the single-walled carbon nanotubes in the positive electrode active material layer is 0.05%-0.1%.

[0111] In one example, the multi-walled carbon nanotubes in the cathode active material layer have a mass content of 0.5%-0.9%.

[0112] The positive electrode of this invention has strong ionic conductivity, which can reduce the ionic polarization of the battery. It can not only effectively improve the terminal voltage of high-rate discharge under low temperature conditions and improve the cold start performance of the battery, but also enable the battery to maintain a low internal resistance under high current density, thereby improving the power density.

[0113] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising the positive electrode sheet described in the first aspect of the present invention.

[0114] In this invention, the battery may further include a negative electrode sheet. The negative electrode sheet may include a negative electrode active material. The negative electrode active material may include silicon-based particles. The silicon-based particles may include at least one of silicon, silicon-oxygen, silicon-carbon, and silicon alloys. Silicon-oxygen refers to a material comprising elemental silicon and elemental oxygen. Silicon-carbon refers to a material comprising elemental silicon and elemental carbon.

[0115] In one example, the silicon-carbon comprises a material formed by filling (including partially or completely filling) the pores of porous amorphous carbon or porous crystalline carbon with silicon or partially oxidized silicon.

[0116] The inventors of this invention have discovered that specific silicon-carbon materials have high compatibility with the positive electrode sheet of this invention. Due to the presence of specific positive electrode active materials and conductive agents, the positive electrode sheet of this invention has good conductivity, which can improve the adverse effects of silicon-based particles on battery conductivity, thereby enabling the battery to have better rate performance.

[0117] In this invention, the negative electrode sheet may include a negative electrode current collector and a negative electrode active material layer on at least one side surface of the negative electrode current collector. The negative electrode active material layer may include the negative electrode active material. The negative electrode active material may also include a carbon-based material. The carbon-based material may include at least one of artificial graphite, natural graphite, mesophase carbon microspheres, soft carbon, and hard carbon.

[0118] In this invention, based on the total mass of the negative electrode active material, the content of the silicon-based particles can be 1%-50%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 20%, 30%, 40% or 50%.

[0119] In one example, the content of silicon-based particles is 4%-30% based on the total mass of the negative electrode active material.

[0120] In this invention, the negative electrode active material layer may further include a negative electrode conductive agent. The negative electrode conductive agent may include single-walled carbon nanotubes.

[0121] The inventors of this invention discovered that when the negative electrode conductive agent includes single-walled carbon nanotubes, the excellent conductivity of the single-walled carbon nanotubes can, to some extent, compensate for the insufficient conductivity of silicon-based particles. Furthermore, because the single-walled carbon nanotubes can form a conductive network around the silicon-based particles, this network possesses a certain degree of toughness and strength, which can limit the volume expansion of the silicon-based particles. Moreover, the negative electrode sheet including both silicon-based particles and single-walled carbon nanotubes has a higher compatibility with the positive electrode sheet of this invention, achieving a better balance between their electronic and ionic conductivity. On one hand, the negative electrode sheet can effectively absorb the capacity of the positive electrode sheet, enabling the battery to achieve a further increase in energy density; on the other hand, the matching electron conduction rates in the negative and positive electrodes are beneficial for further improving the battery's low-temperature performance.

[0122] In this invention, the negative electrode conductive agent may further include at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, and multi-walled carbon nanotubes.

[0123] In this invention, the negative electrode active material layer may further include a negative electrode binder. The negative electrode binder may include binders conventionally used in the art, such as at least one selected from polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene-butadiene rubber, polytetrafluoroethylene, and polyethylene oxide.

[0124] In this invention, based on the total mass of the negative electrode active material layer, the content of the negative electrode active material can be 80%-99.8% (e.g., 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, 98%, 99%, or 99.8%), the content of the negative electrode conductive agent can be 0.1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%), and the content of the negative electrode binder can be 0.1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.1%).

[0125] In this invention, the battery may further include a separator and an electrolyte. The separator may include separators conventionally used in the art. The electrolyte may include electrolytes conventionally used in the art.

[0126] In this invention, the application voltage of the battery can be 2.8V-4.3V.

[0127] In this invention, the battery can be a multi-tab core structure. The multi-tab core structure refers to a battery with a total number of positive and negative tabs greater than two. When the total number of positive and negative tabs is greater than two, the energy density and high-temperature cycle performance of the battery can be further improved. This is because a larger current-carrying area of ​​the tabs is beneficial for increasing heat dissipation and improving electron transport rate, effectively reducing the internal resistance of the positive / negative electrode plates. Furthermore, the multi-tab structure can reduce heat concentration at the tabs, lowering the risk of thermal runaway under abnormal conditions such as overcharging and over-discharging, and improving battery safety.

[0128] 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.

[0129] 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.

[0130] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0131] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0132] Example 1

[0133] The battery is prepared according to the following method:

[0134] (1) Preparation of positive electrode sheet

[0135] A positive electrode active material (mass ratio of the first particle to the second particle is 9:1), polyvinylidene fluoride, single-walled carbon nanotubes (length 2 μm, diameter 3 nm), multi-walled carbon nanotubes (length 8 μm, diameter 20 nm), and conductive carbon black (average particle size 150 nm) were mixed uniformly in a mass ratio of 97.4:1.2:0.1:0.7:0.6. N-methylpyrrolidone (NMP) was added to obtain a positive electrode slurry with a solid content of 65%. This positive electrode slurry was uniformly coated onto a 10 μm aluminum foil using a coating machine. The foil was then dried, rolled, and die-cut (to obtain positive electrode tabs, wherein the number of positive electrode tabs is ≥2), and finally formed into a positive electrode sheet. The compacted density of the positive electrode sheet was 3.45 g / cm³. 3 The surface density is 13 mg / cm³. 2 The porosity is 25%; the first particle is a single crystal particle with the chemical formula LiNi. 0.93 Co 0.05 Mn 0.01 M 1 0.01 O2, M 1 The first particle contains B, Zr, and Y, with a density (D1) of 3 μm. The mass content of element B in the first particle is 800 ppm, the coating thickness in the first particle is 35 nm, and the specific surface area of ​​the first particle is 1.02 m². 2 / g; the second particle is a polycrystalline particle, and its chemical formula is LiNi. 0.9 Co 0.04 Mn 0.03 M 2 0.03 O2, M 2 The second particle contains Al, Zr, and Ti, with a density (D2) of 11 μm. The mass content of Al in the second particle is 2000 ppm. The second particle comprises several primary particles with an average particle size of 400 nm. The specific surface area of ​​the second particle is 0.45 m². 2 / g, the coating thickness in the second particle is 38nm; D2 / D1 is 3.67; in the positive electrode active material, the ratio x of the number of moles of element Ni to the total number of moles of elements Ni, Co and Mn is approximately 0.938, the coating thickness T is approximately 35.2nm, and the specific surface area B is 0.963m². 2 / g; the diameter y of the carbon nanotube is approximately 18 nm, and the length L is approximately 7.3 μm; therefore, y / x is approximately 19.18, y / T is approximately 0.511, B / L is approximately 0.132, and the ratio of the diameter of the carbon nanotube to the average particle size of the conductive carbon black is 0.12:1.

[0136] (2) Preparation of negative electrode sheet

[0137] Artificial graphite, silicon carbide, single-walled carbon nanotubes, multi-walled carbon nanotubes, styrene-butadiene rubber, and sodium carboxymethyl cellulose were mixed evenly in a mass ratio of 81:15.1:0.25:0.15:2.9:0.6 to obtain a material. Ethylene carbonate (1% by mass of total material) was added, along with deionized water, to obtain a negative electrode slurry (solid content of 45%). The negative electrode slurry was uniformly coated onto a 4μm high-strength carbon-coated copper foil, dried, and then rolled, die-cut (to obtain negative electrode tabs, wherein the number of negative electrode tabs is ≥2), and finally formed into a negative electrode sheet.

[0138] (3) Preparation of electrolyte

[0139] In a glove box filled with inert gas (argon) (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, and propyl propionate are mixed evenly in a mass ratio of 15:10:10:65. Then, 1.25 mol / L of fully dried lithium hexafluorophosphate is quickly added and stirred evenly. Finally, 0.5% succinate based on the total mass of the electrolyte is added. After passing the tests for moisture and free acid, the desired electrolyte is obtained.

[0140] (4) Battery fabrication

[0141] The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (including a 5μm thick polyethylene substrate, a 1μm thick ceramic layer on one side of the substrate, and a 1.5μm thick polyvinylidene fluoride adhesive layer on the other side of the substrate) are wound by a winding machine to obtain a battery core in which the positive and negative electrode sheets are separated by the separator. Then, the lithium-ion secondary battery is obtained through welding, encapsulation, liquid injection, formation, gas bag cutting, sorting and other processes.

[0142] Example 2

[0143] The procedure is the same as in Example 1, except that step (1) involves preparing the positive electrode sheet, as follows:

[0144] A positive electrode active material (mass ratio of the first particle to the second particle is 8:2), polyvinylidene fluoride, single-walled carbon nanotubes (length 3μm, diameter 1nm), multi-walled carbon nanotubes (length 5μm, diameter 30nm), and conductive carbon black (average particle size 300nm) were mixed uniformly in a mass ratio of 97.4:1.2:0.15:0.5:0.75. N-methylpyrrolidone (NMP) was added to obtain a positive electrode slurry with a solid content of 65%. This positive electrode slurry was uniformly coated onto a 10μm aluminum foil using a coating machine. The foil was then dried, rolled, and die-cut (to obtain positive electrode tabs, wherein the number of positive electrode tabs is ≥2), and finally formed into a positive electrode sheet. The compacted density of the positive electrode sheet was 3.4 g / cm³.3 The surface density is 13 mg / cm³. 2 The porosity is 27%; the first particle is a single crystal particle with the chemical formula LiNi. 0.95 Co 0.03 Mn 0.01 M 1 0.01 O2, M 1 The first particle contains B, Zr, and Y, with a D1 value of 2 μm. The mass content of element B in the first particle is 700 ppm, the coating thickness of the first particle is 25 nm, and the specific surface area of ​​the first particle is 1.18 m². 2 / g; the second particle is a polycrystalline particle, and its chemical formula is LiNi. 0.92 Co 0.02 Mn 0.02 M 2 0.04 O2, M 2 The second particle contains Al, Zr, and Ti, with a density (D2) of 9 μm. The mass content of Al in the second particle is 2100 ppm. The second particle comprises several primary particles with an average particle size of 300 nm. The specific surface area of ​​the second particle is 0.55 m². 2 / g, the coating thickness in the second particle is 45nm; D2 / D1 is 4.5; in the positive electrode active material, the ratio x of the number of moles of element Ni to the total number of moles of elements Ni, Co and Mn is approximately 0.959, the coating thickness T is approximately 29, and the specific surface area B is 1.054m². 2 / g; the diameter y of the carbon nanotube is approximately 23 nm, and the length L is approximately 4.5 μm; therefore, y / x is approximately 23.97, y / T is approximately 0.793, B / L is approximately 0.234, and the ratio of the diameter of the carbon nanotube to the average particle size of the conductive carbon black is 0.077:1.

[0145] Example 3

[0146] The procedure is the same as in Example 1, except that step (1) involves preparing the positive electrode sheet, as follows:

[0147] The positive electrode active material (mass ratio of the first particle to the second particle is 7:3), polyvinylidene fluoride, single-walled carbon nanotubes (length 1 μm, diameter 5 nm), multi-walled carbon nanotubes (length 10 μm, diameter 5 nm), and conductive carbon black (average particle size 20 nm) were mixed in a mass ratio of 97.4:1.2:0.05:

[0148] Mix 0.9:0.45 ratio thoroughly, add N-methylpyrrolidone (NMP) to obtain a positive electrode slurry with a solid content of 65%; coat the positive electrode slurry evenly onto a 10μm aluminum foil using a coating machine, dry, roll, die-cut (to obtain positive electrode tabs, wherein the number of positive electrode tabs is ≥2), and then form a positive electrode sheet; wherein the compacted density of the positive electrode sheet is 3.35 g / cm³. 3 The surface density is 13 mg / cm³. 2 The porosity is 29%; the first particle is a single crystal particle with the chemical formula LiNi. 0.91 Co 0.04 Mn 0.02 M 1 0.03 O2, M 1 The first particle contains B, Zr, and Y, with a density (D1) of 4 μm. The mass content of element B in the first particle is 1000 ppm. The coating thickness of the first particle is 41 nm, and the specific surface area of ​​the first particle is 0.97 m². 2 / g; the second particle is a polycrystalline particle, and its chemical formula is LiNi. 0.83 Co 0.1 Mn 0.05 M 2 0.02 O2, M 2 The second particle contains Al, Zr, and Ti, with a density (D2) of 13 μm. The mass content of Al in the second particle is 1800 ppm. The second particle comprises several primary particles with an average particle size of 500 nm. The specific surface area of ​​the second particle is 0.35 m². 2 / g, the coating thickness in the second particle is 18nm; D2 / D1 is 3.25; in the positive electrode active material, the ratio x of the number of moles of element Ni to the total number of moles of elements Ni, Co and Mn is approximately 0.911, the coating thickness T is approximately 34, and the specific surface area B is 0.784m². 2 / g; the diameter y of the carbon nanotube is about 5nm and the length L is about 9.5μm; then y / x is about 5.49, y / T is about 0.147, B / L is about 0.083, and the ratio of the diameter of the carbon nanotube to the average particle size of the conductive carbon black is 0.25:1.

[0149] Example 4 group

[0150] This set of examples is used to verify the impact of changes in "y / x".

[0151] This set of embodiments refers to Embodiments 2 and 3, the difference being that y / x is changed by changing the conductive agent, as detailed below:

[0152] Example 4a was carried out with reference to Example 2, except that the mass contents of single-walled carbon nanotubes (length 1 μm, diameter 5 nm), multi-walled carbon nanotubes (length 10 μm, diameter 5 nm), and conductive carbon black (average particle size 20 nm) in the negative electrode active material layer were 0.05%, 0.9%, and 0.45%, respectively; wherein the diameter y of the carbon nanotubes was approximately 5 nm, and the length L was approximately 9.5 μm; therefore, y / x was approximately 5.21, y / T was approximately 0.172, B / L was approximately 0.111, and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black was 0.25:1;

[0153] Example 4b was carried out with reference to Example 3, except that the mass contents of single-walled carbon nanotubes (3 μm in length and 1 nm in diameter), multi-walled carbon nanotubes (5 μm in length and 30 nm in diameter), and conductive carbon black (300 nm in average particle size) in the negative electrode active material layer were 0.15%, 0.5%, and 0.75%, respectively; the diameter y of the carbon nanotubes was approximately 23 nm, and the length L was approximately 4.5 μm; therefore, y / x was approximately 25.25, y / T was approximately 0.676, B / L was approximately 0.174, and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black was 0.077:1.

[0154] Example 5 group

[0155] This set of embodiments is used to verify the effect of changing the "ratio x of the number of moles of element Ni to the total number of moles of elements Ni, Co, and Mn". In this embodiment, only the material selection of the first and second particles is changed, without changing their average particle size, doping elements, etc.

[0156] This set of embodiments is based on Embodiment 1, except that x is changed by altering the mass ratio of the first particle to the second particle or the mass ratio of the first particle to the second particle, as detailed below:

[0157] Example 5a, the first particle has the chemical formula LiNi 0.9 Co 0.07 Mn 0.015 M 1 0.015 O2, the second particle has the chemical formula LiNi 0.8 Co 0.07 Mn 0.08 M 2 0.05 O2; x is approximately 0.907, y / x is 19.86;

[0158] Example 5b, the first particle has the chemical formula LiNi 0.96 Co 0.02 Mn 0.01 M1 0.01 O2, the second particle has the chemical formula LiNi 0.94 Co 0.02 Mn 0.02 M 2 0.02 O2; x is approximately 0.969, y / x is 18.58;

[0159] Example 5c, the first particle has the chemical formula LiNi 0.9 Co 0.07 Mn 0.015 M 1 0.015 O2, the second particle has the chemical formula LiNi 0.8 Co 0.07 Mn 0.08 M 2 0.05 O2, the mass ratio of the first particle to the second particle is 5:5; x is approximately 0.878, y / x is 20.5; the thickness T of the coating layer is approximately 36.5 nm, and the specific surface area B is 0.735 m². 2 / g; the compaction density of the positive electrode is 3.32 g / cm³. 3 The porosity is 29.5%; y / T is approximately 0.493, and B / L is approximately 0.101.

[0160] Example 6 group

[0161] This set of examples is used to verify the impact of changes in the "mass content of element B in the first particle".

[0162] This set of embodiments is based on Embodiment 1, except that the mass content of element B in the first particle is changed, as follows:

[0163] In Example 6a, the mass content of element B in the first particle is 500 ppm, and the coating thickness in the first particle is 16 nm; in the positive electrode active material, the coating thickness T is approximately 18 nm, and y / T is approximately 1.

[0164] In Example 6b, the mass content of element B in the first particle is 1300 ppm, and the coating thickness in the first particle is 46 nm; in the positive electrode active material, the coating thickness T is approximately 45 nm, and y / T is approximately 0.4.

[0165] Example 7 group

[0166] This set of examples is used to verify the effect of changing the "average particle size D1 of the first particle".

[0167] This set of embodiments is based on Embodiment 1, except that the average particle size of the first particle is changed, as follows:

[0168] Example 7a, D1 is 1.5 μm, and the specific surface area of ​​the first particle is 1.22 m². 2 / g; D2 / D1 is 7.33; the specific surface area B of the positive electrode active material is 1.143m². 2 / g; the compaction density of the positive electrode is 3.32 g / cm³. 3 The B / L ratio is approximately 0.157.

[0169] Example 7b, D1 is 5 μm, and the specific surface area of ​​the first particle is 0.87 m². 2 / g; D2 / D1 is 2.2; the specific surface area B of the positive electrode active material is 0.828m². 2 / g; the compaction density of the positive electrode is 3.37 g / cm³. 3 The B / L ratio is approximately 0.113.

[0170] Example 8 group

[0171] This set of examples is used to verify the impact of changes in the "average particle size D2 of the second particle".

[0172] This set of embodiments is based on Embodiment 1, except that the average particle size of the second particle is changed, as follows:

[0173] Example 8a, D2 is 5 μm, and the specific surface area of ​​the second particle is 0.58 m². 2 / g; D2 / D1 is 1.67; the specific surface area B of the positive electrode active material is 0.976m². 2 / g; the compaction density of the positive electrode is 3.3 g / cm³. 3 The B / L ratio is approximately 0.134.

[0174] Example 8b, D2 is 20 μm, and the specific surface area of ​​the second particle is 0.36 m². 2 / g; D2 / D1 is 6.67; the specific surface area B of the positive electrode active material is 0.954m². 2 / g; the compaction density of the positive electrode is 3.4 g / cm³. 3 The B / L ratio is approximately 0.131.

[0175] Example 9 group

[0176] This set of examples is used to verify the impact of changes to "D2 / D1".

[0177] This set of embodiments refers to Embodiment 2 or Embodiment 3, except that D2 / D1 is changed by changing the second particle, as detailed below:

[0178] Example 9a was performed in accordance with Example 2, except that the second particle was replaced with the same mass of the second particle used in Example 3, wherein D2 / D1 was 6.5, the coating thickness T of the positive electrode active material was approximately 23.5 nm, and the specific surface area B was 1.014 m². 2 / g; the compaction density of the positive electrode is 3.43 g / cm³. 3 The y / T ratio is approximately 0.979, and the B / L ratio is approximately 0.225.

[0179] Example 9b was performed in accordance with Example 3, except that the second particle was replaced with the same mass of the second particle used in Example 2, wherein D2 / D1 was 2.25, the coating thickness T of the positive electrode active material was approximately 42 nm, and the specific surface area B was 0.844 m². 2 / g; the compaction density of the positive electrode is 3.45 g / cm³. 3 The porosity is 25%; y / T is approximately 0.119, and B / L is approximately 0.089.

[0180] Example 10 group

[0181] This set of examples is used to verify the impact of changes in the "mass content of element Al in the second particle".

[0182] This set of embodiments is based on Embodiment 1, except that the mass content of element Al in the second particle is changed, as follows:

[0183] In Example 10a, the mass content of elemental Al in the second particle is 1000 ppm, and the coating thickness in the second particle is 10 nm; the coating thickness T of the positive electrode active material is approximately 32.5 nm; y / T is approximately 0.554.

[0184] In Example 10b, the mass content of elemental Al in the second particle is 3500 ppm, and the coating thickness in the second particle is 48 nm; the coating thickness T of the positive electrode active material is approximately 36.5 nm; and y / T is approximately 0.493.

[0185] Example 11 group

[0186] This set of examples is used to verify the impact of changes in the "mass content of the first particle in the positive electrode active material".

[0187] This set of embodiments is based on Embodiment 1, except that the mass content of the first particle is changed, as follows:

[0188] In Example 11a, the positive electrode active material is a mixture of first particles and second particles in a mass ratio of 5:5; in the positive electrode active material, x is approximately 0.934, the thickness T of the coating layer is approximately 36.5 nm, and the specific surface area B is 0.735 m².2 / g; y / x is approximately 19.28, y / T is approximately 0.493, and B / L is approximately 0.101;

[0189] In Example 11b, the positive electrode active material is a mixture of first particles and second particles at a mass ratio of 99:1; in the positive electrode active material, x is approximately 0.939, the thickness T of the coating layer is approximately 35 nm, and the specific surface area B is 1.014 m². 2 / g; y / x is approximately 19.16, y / T is approximately 0.514, and B / L is approximately 0.139.

[0190] Example 12

[0191] This embodiment is used to verify the impact of changes in "y / T".

[0192] The procedure was carried out in accordance with Example 1, except that y / T was changed by altering the conductive agent, as detailed below:

[0193] The mass contents of single-walled carbon nanotubes (1 μm in length and 5 nm in diameter), multi-walled carbon nanotubes (10 μm in length and 5 nm in diameter), and conductive carbon black (average particle size 20 nm) in the negative electrode active material layer are 0.05%, 0.9%, and 0.45%, respectively. Among them, the diameter y of the carbon nanotubes is approximately 5 nm, and the length L is approximately 9.5 μm; therefore, y / x is approximately 5.33, y / T is approximately 0.142, B / L is approximately 0.101, and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black is 0.25:1.

[0194] Example 13 group

[0195] This set of examples is used to verify the impact of changes in the "mass content of single-walled carbon nanotubes".

[0196] This set of embodiments is based on Embodiment 1, except that the mass content of single-walled carbon nanotubes is changed, as follows:

[0197] In Example 13a, the mass content of single-walled carbon nanotubes was 0, meaning that the positive electrode active material, polyvinylidene fluoride, multi-walled carbon nanotubes, and conductive carbon black were mixed uniformly in a mass ratio of 97.4:1.2:0.7:0.7; wherein, the diameter y of the carbon nanotubes was approximately 20 nm, and the length L was approximately 8 μm; therefore, y / x was approximately 21.32, y / T was approximately 0.568, B / L was approximately 0.12, and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black was 0.133:1;

[0198] In Example 13b, the mass content of single-walled carbon nanotubes was 0.2%, i.e., the positive electrode active material, polyvinylidene fluoride, single-walled carbon nanotubes, multi-walled carbon nanotubes, and conductive carbon black were mixed uniformly in a mass ratio of 97.4:1.2:0.2:0.7:0.5; wherein, the diameter y of the carbon nanotubes was approximately 16 nm and the length L was approximately 6.5 μm; then y / x was approximately 17.05, y / T was approximately 0.455, B / L was approximately 0.148, and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black was 0.107:1.

[0199] Example 14 group

[0200] This set of examples is used to verify the impact of changing the diameter of the single-walled carbon nanotubes.

[0201] This set of embodiments is based on Embodiment 1, except that the diameter of the single-walled carbon nanotubes is changed, as follows:

[0202] Example 14a: The diameter of the single-walled carbon nanotubes is 0.5 nm;

[0203] In Example 14b, the diameter of the single-walled carbon nanotubes is 10 nm; the diameter y of the carbon nanotubes is approximately 19 nm; y / x is approximately 20.25, y / T is approximately 0.54, and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black is 0.127:1.

[0204] Example 15 group

[0205] This set of examples is used to verify the impact of changing the length of the single-walled carbon nanotubes.

[0206] This set of embodiments is based on Embodiment 1, except that the length of the single-walled carbon nanotubes is changed, as follows:

[0207] Example 15a: The length of the single-walled carbon nanotube is 0.5 μm; the length L of the carbon nanotube is approximately 7 μm; the B / L ratio is approximately 0.138.

[0208] Example 15b: The length of the single-walled carbon nanotube is 3.5 μm; the length L of the carbon nanotube is approximately 7.5 μm; and the B / L ratio is approximately 0.128.

[0209] Example 16 group

[0210] This set of examples is used to verify the impact of changes in the "mass content of multi-walled carbon nanotubes".

[0211] This set of embodiments is based on Embodiment 1, except that the mass content of multi-walled carbon nanotubes is changed, as follows:

[0212] In Example 16a, the mass content of multi-walled carbon nanotubes was 0, meaning that the positive electrode active material, polyvinylidene fluoride, single-walled carbon nanotubes, and conductive carbon black were mixed uniformly in a mass ratio of 97.4:1.2:0.1:1.3; wherein, the diameter y of the carbon nanotubes was approximately 3 nm, and the length L was approximately 2 μm; therefore, y / x was approximately 3.2, y / T was approximately 0.085, B / L was approximately 0.482, and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black was 0.02:1;

[0213] In Example 16b, the mass content of single-walled carbon nanotubes was 1%, i.e., the positive electrode active material, polyvinylidene fluoride, single-walled carbon nanotubes, multi-walled carbon nanotubes, and conductive carbon black were mixed uniformly in a mass ratio of 97.4:1.2:0.1:1:0.3; wherein, the diameter y of the carbon nanotubes was approximately 18 nm and the length L was approximately 7.5 μm; then y / x was approximately 19.18, y / T was approximately 0.511, B / L was approximately 0.128, and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black was 0.12:1.

[0214] Example 17 group

[0215] This set of examples is used to verify the impact of changing the diameter of the multi-walled carbon nanotubes.

[0216] This set of embodiments is based on Embodiment 1, except that the diameter of the multi-walled carbon nanotubes is changed, as follows:

[0217] Example 17a: The diameter of the multi-walled carbon nanotubes is 2 nm; the diameter y of the carbon nanotubes is approximately 2 nm; y / x is approximately 2.13; y / T is approximately 0.057; and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black is 0.013:1.

[0218] In Example 17b, the diameter of the multi-walled carbon nanotubes is 80 nm; the diameter y of the carbon nanotubes is approximately 70 nm; y / x is approximately 74.61, y / T is approximately 1.989, and the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black is 0.467:1.

[0219] Example 18 group

[0220] This set of examples is used to verify the impact of changing the length of the multi-walled carbon nanotubes.

[0221] This set of embodiments is based on Embodiment 1, except that the length of the multi-walled carbon nanotubes is changed, as follows:

[0222] Example 18a: The length of the multi-walled carbon nanotube is 3.5 μm; the length L of the carbon nanotube is approximately 3 μm; the B / L ratio is approximately 0.321.

[0223] In Example 18b, the length of the multi-walled carbon nanotube was 12.5 μm; the length L of the carbon nanotube was approximately 11 μm; and the B / L ratio was approximately 0.088.

[0224] Example 19 group

[0225] This set of examples is used to verify the impact of changes in the "average particle size of conductive carbon black".

[0226] This set of embodiments is based on Embodiment 1, except that the average particle size of the conductive carbon black is changed, as follows:

[0227] In Example 19a, the average particle size of the conductive carbon black was 10 nm; the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black was 1.8:1.

[0228] In Example 19b, the average particle size of the conductive carbon black was 800 nm; the ratio of the diameter of the carbon nanotubes to the average particle size of the conductive carbon black was 0.023:1.

[0229] The first particle in all the above embodiments satisfies Dv 1 10 is 0.5μm-4μm, Dv 1 50 is 1.2μm-6μm, Dv 1 99 ≤ 10 μm. The second particle in all the above embodiments satisfies Dv. 2 10 is 3μm-16μm, Dv 2 50 is 4μm-21μm, Dv 2 99≤25μm, and the second particle has pores inside, with pore size of 5nm-25nm.

[0230] Comparative Example 1

[0231] The process was carried out in accordance with Example 1, except that the conductive agent was entirely conductive carbon black, that is, the positive electrode active material, polyvinylidene fluoride and conductive carbon black were mixed evenly in a mass ratio of 97.4:1.2:1.4.

[0232] Comparative Example 2

[0233] The procedure was carried out in accordance with Example 5b, except that the conductive agent was changed, as follows:

[0234] The length of a single-walled carbon nanotube is 2 μm and the diameter is 0.5 nm; the length of a multi-walled carbon nanotube is 8 μm and the diameter is 2 nm; the diameter y of the carbon nanotube is approximately 1.5 nm and the y / x ratio is approximately 1.55.

[0235] Comparative Example 3

[0236] The procedure was carried out in accordance with Example 5c, except that the conductive agent was changed, as follows:

[0237] The length of a single-walled carbon nanotube is 2 μm and the diameter is 10 nm; the length of a multi-walled carbon nanotube is 8 μm and the diameter is 80 nm; the diameter y of the carbon nanotube is approximately 71 nm and the y / x ratio is approximately 80.87.

[0238] Test case

[0239] (1) Gram volume test

[0240] The batteries prepared in the examples and comparative examples were placed in a constant temperature environment of 25°C, charged at 0.5C with a cutoff current of 0.02C and a cutoff voltage range of 2.5V-4.3V, and then discharged at a rate of 0.2C to 3.0V. The discharge capacity was recorded, and the specific capacity was obtained by dividing the discharge capacity by the mass of the positive electrode active material. The results are recorded in Table 1.

[0241] (2) Ratio Test

[0242] The batteries prepared in the examples and comparative examples were placed in a constant temperature environment of 25°C and charged at 0.5C with a cutoff voltage range of 3.0V-4.3V. They were then discharged at 0.5C and 1C rates to 3.0V respectively. The capacity data during the process were recorded. The rate retention rate was obtained by dividing the 1C rate discharge capacity by the 0.5C rate discharge capacity. The results are recorded in Table 1.

[0243] (3) Surface resistance test

[0244] The batteries prepared in the examples and comparative examples were disassembled to obtain positive electrode sheets. The sheet resistance of the positive electrode sheets was tested, and the specific test method is as follows:

[0245] After punching the positive electrode sheet into a small disc, place the disc on the testing device and make its surface flat. Adjust the pressure gauge to fix the test sample and ensure that the probe makes good contact with the sample. Use the ST2258C multi-functional digital four-probe tester to perform the test, read the values ​​and record the results in Table 1.

[0246] (4) Loop Test

[0247] The batteries prepared in the examples and comparative examples were placed in a constant temperature environment of 45°C and charged and discharged at a rate of 1.5C / 4C. The cutoff voltage range was 3.0V-4.3V, and the batteries were cycled 500 times. The cycle discharge capacity was recorded and divided by the discharge capacity of the first cycle to obtain the cycle capacity retention rate. The thickness data per 100T was also recorded. The thickness expansion rate was obtained by dividing the thickness data after 500 cycles by the initial thickness. The results of the 500T cycle capacity retention rate and thickness expansion rate are recorded in Table 1.

[0248] Table 1

[0249]

[0250]

[0251] As can be seen from Table 1, the positive electrode sheet of the present invention has a lower sheet resistance compared to the comparative example, indicating that the positive electrode sheet of the present invention has a good conductive network and can improve charge transport efficiency. Compared with the comparative example, the battery prepared by the positive electrode sheet of the present invention can achieve both higher cycle stability and rate performance while ensuring energy density.

[0252] 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 positive electrode plate, characterized in that, The positive electrode sheet includes a positive electrode active material and a conductive agent; The positive electrode active material includes a first particle and a second particle; The first particle includes a single crystal particle, and the second particle includes a polycrystalline particle; The first particle comprises Li x1 Ni a1 Co b1 Mn c1 M 1 d1 For O2, 0.9 ≤ x1 ≤ 1.1, 0.8 ≤ a1 ≤ 0.99, 0.01 ≤ b1 ≤ 0.1, 0.01 ≤ c1 ≤ 0.1, 0 ≤ d1 ≤ 0.05, M 1 Including at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb; The second particle comprises Li x2 Ni a2 Co b2 Mn c2 M 2 d2 For O2, the following conditions apply: 0.9 ≤ x² ≤ 1.1, 0.8 ≤ a² ≤ 0.95, 0.01 ≤ b² ≤ 0.2, 0.01 ≤ c² ≤ 0.1, 0 ≤ d² ≤ 0.05, M 2 Including at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb; In the positive electrode active material, the ratio of the number of moles of element Ni to the total number of moles of elements Ni, Co, and Mn is x; The conductive agent includes carbon nanotubes, the diameter of which is y, in nm. x and y satisfy the condition that y / x is 1-75.

2. The positive electrode according to claim 1, wherein, y / x is 5-25.5; preferably 5.3-24; And / or, x is 0.87-0.97; And / or, y is 0.5-80, in nm; preferably 2-70, in nm; more preferably 5-25, in nm.

3. The positive electrode according to claim 1 or 2, wherein, The carbon nanotubes are distributed in clusters; And / or, the carbon nanotubes include at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes; Preferably, the carbon nanotubes include single-walled carbon nanotubes and multi-walled carbon nanotubes.

4. The positive electrode according to claim 1 or 2, wherein, The positive electrode active material includes a coating layer located on the surface of the positive electrode active material, and the thickness of the coating layer is T; y and T satisfy: y / T is 0.05-2; And / or, T is 5-50, in nm; preferably 10-45, in nm; more preferably 25-40, in nm.

5. The positive electrode according to claim 1 or 2, wherein, The specific surface area of ​​the positive electrode active material is B, with units of m². 2 / g; the length of the carbon nanotube is L, in μm; B and L satisfy: B / L is 0.05-0.5; preferably 0.07-0.25; And / or, B is 0.5-1.5, in meters. 2 / g; And / or, L is 0.5-12.5 in μm; preferably 2-11 in μm; more preferably 4-10 in μm.

6. The positive electrode according to claim 1 or 2, wherein, 0.9≤a1≤0.99； And / or, based on the total mass of the positive electrode active material, the content of the first particle is C1, 50% ≤ C1 < 100%; preferably 70% ≤ C1 ≤ 90%; And / or, the particle size Dv of the first particle 1 10 has a particle size of 0.5μm-4μm and a median particle size Dv. 1 50 has a particle size of 1.2μm-6μm and a particle size Dv. 1 90≤10μm; And / or, the particle size Dv of the second particle 2 10 has a particle size of 3μm-16μm and a median particle size Dv. 2 50 has a particle size of 4μm-21μm, and a particle size Dv 2 90≤25μm; And / or, the second particle includes a particle composed of a plurality of primary particles, the primary particles having a particle size of 100nm-600nm.

7. The positive electrode according to claim 1 or 2, wherein, M 2 Including Al, the mass content of element Al in the second particle is 1000ppm-3500ppm; And / or, M 1 Including B, the mass content of element B in the first particle is 500ppm-1300ppm.

8. The positive electrode according to claim 1 or 2, wherein, The ratio of the average particle size D2 of the second particle to the average particle size D1 of the first particle, D2 / D1, satisfies 1≤D2 / D1≤10; preferably, 3≤D2 / D1≤4.5; And / or, 1.5μm≤D1≤5μm; preferably, 2μm≤D1≤4μm; And / or, 5μm≤D2≤20μm; preferably, 9μm≤D2≤13μm.

9. The positive electrode according to claim 1 or 2, wherein, The compaction density of the positive electrode is 3.3 g / cm³. 3 -3.5g / cm 3 ; And / or, the areal density of the positive electrode is 10 g / cm³. 2 -14g / cm 2 ; And / or, the porosity of the positive electrode is 20%-35%.

10. The positive electrode according to claim 1 or 2, wherein, The conductive agent also includes conductive carbon black; And / or, the average particle size of the conductive carbon black is 10nm-800nm; preferably 20nm-300nm; Preferably, the ratio of the diameter of the carbon nanotube to the average particle size of the conductive carbon black is (0.01-2):1; more preferably (0.05-0.25):

1.

11. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes the positive electrode sheet according to any one of claims 1-10.

12. The lithium-ion secondary battery according to claim 11, wherein, The lithium-ion secondary battery also includes a negative electrode sheet, which includes a negative electrode active material, and the negative electrode active material includes silicon-based particles. Preferably, the silicon-based particles comprise silicon-carbon; More preferably, the silicon-carbon comprises a material formed by filling the pores of porous amorphous carbon or porous crystalline carbon with silicon or partially oxidized silicon.

13. The lithium-ion secondary battery according to claim 12, wherein, Based on the total mass of the negative electrode active material, the content of the silicon-based particles is 1%-50%; preferably 4%-30%. Preferably, the negative electrode further includes a negative electrode conductive agent, which includes single-walled carbon nanotubes.

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