Positive plate and lithium ion battery

Abstract

The invention provides a positive plate and a lithium ion battery. The positive plate comprises a positive current collector and an active layer arranged on at least one surface of the positive current collector; according to the direction away from the positive electrode current collector, the active layer comprises a positive electrode active layer and a positive electrode lithium supplementing layer which are stacked in sequence, the positive electrode active layer comprises a first positive electrode active material, and the positive electrode lithium supplementing layer comprises a positive electrode lithium supplementing agent and a second positive electrode active material; the Dv50 of the positive electrode lithium supplementing agent is not higher than 10 microns; the second positive electrode active material comprises lithium cobalt oxide Li Co < x > M O < 2 >, and the Dv < 50 > of the lithium cobalt oxide is 8-25 [mu] m; the lithium cobalt oxide has a bimodal distribution characteristic, the peak position of a first peak is 2-8 [mu] m, the peak position of a second peak is 12-25 [mu] m, a is greater than or equal to 0.9 and less than or equal to 1.1, x is greater than or equal to 0.8 and less than or equal to 1.05, and b is greater than or equal to 0 and less than or equal to 0.05. The positive plate is favorable for improving the rate capability, the cycle performance and the storage performance of the lithium ion battery.

Description

Technical Field

[0001] The present application relates to a positive electrode sheet, and in particular to a positive electrode sheet and a lithium-ion battery, belonging to the technical field of secondary batteries. Background Art

[0002] Lithium-ion batteries have greatly changed people's lives since their invention. The rapid development of electric vehicles, grid energy storage, and portable electronic devices has made the research and development of higher energy density lithium batteries more urgent. In order to develop high-energy-density lithium batteries, the research and application of a new generation of high-capacity negative electrode active materials such as silicon and tin-based materials have been widely carried out. However, since high-capacity negative electrode active materials will consume a considerable amount of active lithium due to interfacial side reactions during the initial charge and discharge process, the high specific capacity characteristics of these negative electrode active materials cannot be fully utilized in actual battery applications. Pre-lithiation (lithium compensation) technology can provide additional active lithium to the battery and is considered to be one of the effective strategies to solve the loss of active lithium in high-capacity batteries and improve battery energy density.

[0003] Pre-lithiation generally involves two methods: negative electrode recharge and positive electrode recharge. Negative electrode recharge primarily involves mechanically pressing lithium metal foil onto the negative electrode sheet, spraying lithium powder onto the negative electrode sheet surface, or coating the negative electrode sheet with a slurry of lithium powder. However, negative electrode recharge is complex and carries serious safety risks.

[0004] Positive electrode lithium replenishment primarily involves adding lithium-rich compounds as positive electrode replenishers to the positive electrode sheet. While this approach is simpler than negative electrode lithium replenishment and ensures battery energy density and safety, it often degrades the battery's rate capability, cycling performance, and storage performance, limiting the application of secondary batteries. Summary of the Invention

[0005] The present invention provides a positive electrode sheet and a lithium ion battery. The special structure of the positive electrode sheet helps to improve the rate performance, cycle performance and storage performance of the lithium ion battery.

[0006] The present invention provides a positive electrode sheet, comprising a positive electrode current collector and an active layer arranged on at least one surface of the positive electrode current collector; in a direction away from the positive electrode current collector, the active layer comprises a positive electrode active layer and a positive electrode lithium replenishing layer stacked in sequence, the positive electrode active layer comprises a first positive electrode active material, the positive electrode lithium replenishing layer comprises a positive electrode lithium replenishing agent and a second positive electrode active material; the D v50 Not higher than 10μm;

[0007] The second positive electrode active material includes lithium cobaltate Li a Co x M b O2, the lithium cobalt oxide D v50The lithium cobalt oxide has a bimodal distribution characteristic, wherein the peak position of the first peak is 2-8 μm, the peak position of the second peak is 12-25 μm, 0.9≤a≤1.1, 0.8≤x≤1.05, 0≤b≤0.05, and M includes at least one of Mg, Ti, Zr, La, Y, Te, W, Al, B, P, S, Se, K, Rb, and Cs.

[0008] The positive electrode sheet as described above, wherein the ratio of the surface density of the positive electrode lithium replenishing layer to the surface density of the positive electrode active layer is (0.1-1):1.

[0009] The positive electrode sheet as described above, wherein the compaction density of the positive electrode sheet is 3.5-4.5 g / cm 3 and / or,

[0010] The porosity of the active layer of the positive electrode sheet is 10%-40%; and / or,

[0011] The thickness of the active layer is 80-120 μm; and / or,

[0012] In the active layer, the mass ratio of the positive electrode lithium supplement to the positive electrode active material is (0.005-0.2):1.

[0013] The positive electrode sheet as described above, wherein the conductivity of the positive electrode lithium supplement is greater than 1×10 -4 S / cm; and / or, the specific surface area of ​​the positive electrode lithium supplement agent is not less than 5m 2 / g.

[0014] The positive electrode sheet as described above, wherein the positive electrode lithium supplement comprises a positive electrode lithium supplement core and a functional material layer covering at least a portion of the surface of the positive electrode lithium supplement core, wherein the functional material layer comprises at least one of a carbon material, a metal oxide, and a metal carbide;

[0015] Preferably, in the functional material layer, the D v50 100-500nm;

[0016] Preferably, the positive electrode lithium replenisher includes lithium squarate; preferably, the positive electrode lithium replenisher includes lithium squarate and other lithium replenishers, and the mass ratio of the lithium squarate and other lithium replenishers is 5:5-9:1.

[0017] The positive electrode sheet as described above, wherein the thickness of the functional material layer is 5-200 nm; and / or,

[0018] The mass ratio of the functional material layer to the positive electrode lithium supplement agent is (0.001-0.2):1; and / or,

[0019] The coverage area of ​​the functional material layer is 50-90% of the surface area of ​​the positive electrode lithium supplement core; and / or,

[0020] The lithium quarate has a (110) diffraction peak at 2θ of 25.0-25.6°, a (112) diffraction peak at 2θ of 29.1-29.7°, and a (202) diffraction peak at 2θ of 31.3-31.9°, and I 112 >I 202 >I 110 , I 112 / I 202 >1.1, where I 112 , I 202 , I 110 They are the peak intensities of the (110) diffraction peak, (112) diffraction peak, and (202) diffraction peak, respectively.

[0021] The present invention provides a lithium-ion battery, comprising the positive electrode sheet described in any one of the above items.

[0022] The present invention provides a lithium-ion battery, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode current collector and an active layer, wherein the surface of the active layer away from the positive electrode current collector comprises a plurality of holes, and the average diameter of the holes does not exceed 10 μm; the positive electrode active material in the active layer comprises lithium cobalt oxide Li a Co x M b O2, the lithium cobalt oxide D v50 The lithium cobalt oxide has a bimodal distribution characteristic, wherein the peak position of the first peak is 2-8 μm, and the peak position of the second peak is 12-25 μm, wherein 0.9≤a≤1.1, 0.8≤x≤1.05, 0≤b≤0.05, and M includes at least one of Mg, Ti, Zr, La, Y, Te, W, Al, B, P, S, Se, K, Rb, and Cs.

[0023] The lithium-ion battery as described above, wherein the lithium-ion battery comprises an electrolyte, the electrolyte comprises ethyl propionate, and the mass percentage of the ethyl propionate in the electrolyte is 10-40%.

[0024] In the lithium-ion battery as described above, the porosity of the active layer of the positive electrode sheet is 15-40%.

[0025] The positive electrode sheet provided by the present invention has an active layer comprising a stacked positive electrode active layer and a positive electrode lithium replenishing layer, and the active layer comprises lithium cobalt oxide having a special particle size composition and special distribution characteristics. Since the positive electrode lithium replenishing agent with a special particle size of the present invention is only distributed in the positive electrode lithium replenishing layer away from the positive electrode current collector, and is paired with the lithium cobalt oxide with a special distribution characteristic, it not only effectively reduces the degree of disordered distribution of holes in the positive electrode sheet after the positive electrode lithium replenishing agent is chemically decomposed, but also improves the integrity of the conductive network in the positive electrode sheet and its ability to absorb electrolyte, while also helping to reduce the decomposition voltage of the positive electrode lithium replenishing agent, thereby facilitating the optimization of the rate performance, cycle performance, and storage performance of the lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a schematic structural diagram of an embodiment of a positive electrode sheet of the present invention;

[0027] Figure 2 is the XRD diffraction pattern of Li2C4O4 powder in Example 1 of the present invention;

[0028] Figure 3 This is a SEM image of the positive electrode lithium supplement agent in Example 1 of the present invention;

[0029] Figure 4 This is a SEM image of the positive electrode lithium supplement agent in Example 1 of the present invention;

[0030] Figure 5 This is a particle size distribution diagram of lithium cobalt oxide in Example 1 of the present application;

[0031] Figure 6 is the diffraction pattern of Li2C4O4 powder in Example 15 of the present invention;

[0032] Figure 7 This is a first-cycle specific capacity-voltage curve of the positive electrode lithium supplement in Example 1 of the present invention;

[0033] Figure 8 This is a SEM image of the cross section of the positive electrode sheet of the lithium-ion battery after formation in Example 1;

[0034] Figure 9 This is an SEM image of the cross section of the positive electrode sheet after the lithium-ion battery in Comparative Example 1 is formed. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the solution of the present invention, this application is further described in detail below. The specific embodiments listed below are only descriptions of the principles and features of the present invention. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements.

[0036] During the positive electrode lithium replenishment process, although the lithium replenisher in the positive electrode sheet can provide more active lithium for the lithium-ion battery after chemical decomposition, the rate performance and cycle performance of the lithium-ion battery have been significantly degraded. The inventors have conducted extensive research on this issue and believe that the pores generated by the positive electrode lithium replenisher after chemical decomposition may affect the ion conductivity of the positive electrode sheet to a certain extent, thereby resulting in poor rate performance and cycle performance of the lithium-ion battery. Therefore, the inventors believe that if the research and development entry point is to optimize the distribution of pores in the positive electrode sheet, the negative impact of the positive electrode lithium replenisher on the electrical performance of the lithium-ion battery, including rate performance and cycle performance, can be effectively reduced.

[0037] Based on this, Figure 1 As shown, the first aspect of the present invention provides a positive electrode sheet, including a positive electrode current collector 1 and an active layer 2 arranged on at least one surface of the positive electrode current collector 1; in the direction away from the positive electrode current collector 1, the active layer 2 includes a positive electrode active layer 21 and a positive electrode lithium replenishing layer 22 stacked in sequence, the positive electrode active layer 21 includes a first positive electrode active material, and the positive electrode lithium replenishing layer 22 includes a positive electrode lithium replenishing agent and a second positive electrode active material; the D of the positive electrode lithium replenishing agent v50 Not higher than 10μm; the second positive electrode active material includes lithium cobaltate Li a Co x M b O2, D of lithium cobalt oxide v50 The lithium cobalt oxide has a bimodal distribution characteristic, wherein the peak position of the first peak is 2-8 μm, the peak position of the second peak is 12-25 μm, 0.9≤a≤1.1, 0.8≤x≤1.05, 0≤b≤0.05, and M includes at least one of Mg, Ti, Zr, La, Y, Te, W, Al, B, P, S, Se, K, Rb, and Cs.

[0038] Specifically, at least one surface of the positive electrode current collector 1 is provided with an active layer 2 ( Figure 1The active layer 2 is provided only on one surface of the positive electrode current collector 1, and the active layer 2 includes two parts in the thickness direction. The part close to the positive electrode current collector 1 is the positive electrode active layer 21 including the first positive electrode active material, and the part away from the positive electrode current collector 1 is the positive electrode lithium replenishing layer 22 including the second positive electrode active material and the positive electrode lithium replenishing agent. v50 Not more than 10 μm. In the present invention, the D v50 It is the particle size when the cumulative volume reaches 50% of the total volume of the material.

[0039] The lithium cobalt oxide Li of the present invention a Co x M b In O2, M is at least one of Mg, Ti, Zr, La, Y, Te, W, Al, B, P, S, Se, K, Rb, Cs, etc. The volume particle size distribution curve of the lithium cobalt oxide of the present invention has a bimodal distribution characteristic, that is, a bimodal distribution curve. In detail, after the material is tested by a laser particle size analyzer, its volume particle size distribution curve has obvious bimodal peaks. Even if the lithium cobalt oxide exists in the positive electrode lithium replenishing layer, the battery can be disassembled, and the positive electrode lithium replenishing layer can be soaked in dimethyl carbonate (DMC) for 3 hours. After naturally drying in a drying room, it is placed in a muffle furnace and calcined at 300°C for 3 hours. After that, the material can be separated from the current collector, the material powder can be ultrasonicated for 2 minutes, and then tested using a laser particle size analyzer.

[0040] During the initial formation process of a lithium-ion battery including the positive electrode sheet of the present invention, the positive electrode lithium replenisher decomposes under the drive of voltage, releasing lithium ions, carbon dioxide and other gases, and some carbon remains in the positive electrode lithium replenisher layer 22. The lithium ions can replenish the active lithium lost due to SEI film formation, thereby increasing the energy density of the battery. Gases such as carbon dioxide are collected by the gas bag provided during the formation stage and discharged from the battery system.

[0041] In addition, the positive electrode sheet of the present invention is also beneficial to improving the rate performance and cycle performance of the battery. In detail, the positive electrode lithium replenisher of the present invention is only present in the positive electrode lithium replenisher layer 22 away from the positive electrode current collector 1. Therefore, when the positive electrode lithium replenisher decomposes, accompanied by the generation of lithium ions and gas, holes will be generated in the positive electrode lithium replenisher layer 22. Compared with the positive electrode lithium replenisher dispersed in the entire active layer 2, the holes generated by the decomposition of the positive electrode lithium replenisher of the present invention are only dispersed on the surface of the active layer 2, which reduces the degree of disorderly distribution of the holes in the active layer 2. On the one hand, it ensures the integrity of the conductive network of the active layer 2 to a certain extent, reduces the transmission resistance of ions and electrons of the electrode sheet, and on the other hand, it can also avoid the problem of too deep holes causing too fast consumption of electrolyte in the cycle, thereby improving the rate performance and cycle performance of the lithium-ion battery.

[0042] At the same time, due to the D v50 Not exceeding 10 μm, it not only ensures the smoothness of lithium removal, but more importantly, the size of the pores generated by the decomposition is also small, which not only does not destroy the conductive network of the active layer 2, but also can further increase the degree of electrolyte infiltration into the positive electrode lithium replenishing layer 22, so that more lithium ions can be deintercalated in the positive electrode sheet, thereby improving the cycle performance of the battery.

[0043] It should be noted that the present invention also defines lithium cobalt oxide particles with special bimodal distribution characteristics, and the lithium cobalt oxide and D v50 Matching the positive electrode lithium replenisher with a diameter not exceeding 10μm can not only ensure that the positive electrode lithium replenisher is more evenly distributed in the positive electrode lithium replenisher layer, thereby improving the distribution uniformity of the pores formed after decomposition and further reducing the impedance between the electrolyte, but also lithium cobalt oxide can help reduce the decomposition energy barrier of the positive electrode lithium replenisher, ensuring that the decomposition is completed during the formation process, avoiding the problem of battery expansion and deformation caused by decomposition during the later cycle and storage process, and optimizing the battery's electrical performance including cycle and storage.

[0044] Furthermore, the D of the positive lithium supplement of the present invention is v50 For example, it is 0.005μm, 0.008μm, 0.1μm, 0.2μm, 0.3μm, 0.5μm, 0.8μm, 1.0μm, 1.5m, 2.0μm, 2.5μm, 3.0μm, 3.5μm, 4.0μm, 4.5μm, 5.0μm, 5.5μm, 5.8μm, 6.0μm, 6.8μm, 7.0μm, 7.5m, 8.0μm, 8.5μm, 9.0μm, 9.5μm, 10.0μm or any value in the range formed by any two of them.

[0045] The first positive electrode active material of the present invention can be a common positive electrode active material in the art, for example, it can be at least one composite oxide of lithium and a metal selected from the group consisting of cobalt, manganese, nickel, and combinations thereof. Specifically, it can be at least one of lithium cobaltate, lithium nickelate, lithium manganate, lithium manganate, a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material, lithium iron phosphate (LFP), lithium nickel manganate, and a lithium-rich manganese-based material.

[0046] In a specific embodiment, the first positive electrode active material is the same as the second positive electrode active material.

[0047] The present invention does not limit the choice of positive electrode current collector 1, which can be at least one of aluminum foil and nickel foil. In addition, the positive electrode active layer 21 and the positive electrode lithium replenishing layer 22 also include a conductive agent and a binder. The conductive agent can include at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; the binder can include at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, and polyurethane.

[0048] The present invention does not specifically limit the preparation method of the positive electrode sheet. For example, the positive electrode active layer 21 slurry can be coated on at least one surface of the positive electrode current collector 1. After drying to form the positive electrode active layer 21, the positive electrode lithium replenisher layer slurry is coated on the surface of the positive electrode active layer 21, dried, and rolled to obtain the positive electrode sheet of the present invention.

[0049] The present invention does not limit the preparation method of lithium cobalt oxide having the above-mentioned bimodal distribution characteristics and Dv50. For example, D v50 Small particles of lithium cobalt oxide (2-8 μm) and D v50 Large-particle lithium cobalt oxide of 12-25 μm is mixed with small-particle lithium cobalt oxide at a mass ratio of (1.2-9):1.

[0050] To further reduce the degree of disordered distribution of pores in the active layer 2 and improve the wettability of the electrolyte on the surface of the active layer 2, the surface density ratio of the positive electrode lithium replenishing layer 22 to the positive electrode active layer 21 can be further controlled. Specifically, the surface density ratio of the positive electrode lithium replenishing layer 22 to the positive electrode active layer 21 can be set to (0.1-1):1. In this case, the pores formed by the decomposition of the positive electrode lithium replenishing agent tend to form only on the surface of the active layer 2 away from the positive electrode current collector 1. This not only further ensures the integrity of the conductive network and reduces electrolyte consumption, but also further optimizes the diffusion rate of lithium ions by improving the electrolyte wettability.

[0051] In one embodiment, the compaction density of the positive electrode sheet of the present invention is 3.5-4.5 g / cm 3 This compaction density helps to ensure the battery energy density while also taking into account the electrolyte absorption capacity, ensuring the smoothness of lithium ion insertion and extraction.

[0052] In order to ensure the electrical performance of the battery, the porosity of the active layer of the positive electrode can also be controlled to be between 10-40%. It should be noted that the porosity here is the coating porosity of the positive electrode before participating in the formation. When the positive electrode sheet of the present invention having the porosity of the above-mentioned active layer is formed into a battery cell and participates in the formation, as the positive electrode lithium replenisher decomposes, the porosity of the positive electrode sheet will continue to increase. The increased porosity not only further ensures the liquid absorption capacity of the electrode sheet, but also reduces the phenomenon of excessive consumption of the electrolyte and the instability of the stored charge, thereby further improving the cycle performance of the lithium-ion battery. In detail, based on the D of the positive electrode lithium replenisher not exceeding 10μm v50 Therefore, after the battery cell is formed, the porosity of the positive electrode sheet will increase to 15-40%.

[0053] Preferably, the porosity of the active layer of the positive electrode sheet is between 10-35%.

[0054] In one embodiment, to further balance the battery's energy density and rate performance, the thickness of the active layer 2 in the positive electrode sheet can be controlled to 80-120 μm. Furthermore, the thickness ratio of the positive electrode lithium replenishment layer 22 to the positive electrode active layer 21 is (0.1-1):1, which further helps to improve the integrity of the conductive network of the active layer 2 and enhance the battery's rate performance.

[0055] In addition, the mass ratio of the positive electrode lithium replenisher to the positive electrode active material in the active layer 2 can also be controlled to achieve further optimization of the battery's electrical performance. In detail, when the mass ratio of the positive electrode lithium replenisher in the active layer 2 to the positive electrode active material in the active layer 2 is (0.005-0.2): 1, accurate lithium replenishment of the battery can be achieved, so that the battery can further reduce the possibility of lithium precipitation in the system while ensuring the energy density, so that the battery can exhibit better cycle performance and safety performance. It should be noted that in this ratio, the mass of the positive electrode active material refers to the total mass of the first positive electrode active material and the second positive electrode active material.

[0056] Furthermore, in the specific implementation process, the conductivity can be selected to be greater than 1×10 -4 S / cm positive electrode lithium replenisher. The inventors found that the conductivity of the positive electrode lithium replenisher is closely related to the delithiation voltage of the positive electrode lithium replenisher. Within a certain range, the higher the conductivity of the positive electrode lithium replenisher, the more obvious the trend of reducing the delithiation voltage of the positive electrode lithium replenisher. As the delithiation voltage of the positive electrode lithium replenisher decreases, on the one hand, the impedance of the battery will also show a downward trend, thereby ensuring the battery energy density and cycle performance. On the other hand, the positive electrode lithium replenisher can also achieve large-scale decomposition during the battery formation process, reducing the residual amount of undecomposed positive electrode lithium replenisher, avoiding the phenomenon of battery bloating caused by the decomposition of a large amount of residual positive electrode lithium replenisher during the normal battery cycle, leading to the deterioration of the battery cycle performance and safety performance.

[0057] In order to further improve the wettability of the electrolyte and the transportability of lithium ions during the battery cycle, the specific surface area of ​​the positive electrode lithium supplement can be controlled to be no less than 5m 2 / g. For example, the specific surface area of ​​the positive electrode lithium supplement is 5m 2 / g-100m 2 / g, further 5m 2 / g-20m 2 / g, for example, it can be 5m 2 / g, 5.5m 2 / g、6m 2 / g、7m 2 / g, 7.5m 2 / g、8m 2 / g、9m 2 / g、10m 2 / g、12m 2 / g, 12.5m 2 / g、15m 2 / g、18m 2 / g, 20m 2 / g or any value in the range formed between any two.

[0058] In one embodiment, the positive electrode lithium supplement agent of the present invention includes a positive electrode lithium supplement agent core and a functional material layer covering at least a portion of the surface of the positive electrode lithium supplement agent core. The functional material layer includes at least one of a carbon material, a metal oxide, and a metal carbide. The functional material coating at least a portion of the surface of the positive electrode lithium supplement agent core is used to reduce the reaction activation energy of the positive electrode lithium supplement agent, increase its reactivity, and thereby reduce its delithiation voltage.

[0059] Illustratively, the carbon material may include but is not limited to at least one of acetylene black, carbon nanotubes, Supper P, Ketjen black, nanoporous carbon, ordered mesoporous carbon and the like; the metal oxide may include but is not limited to at least one of MnO2, NiO, Co3O4; and the metal carbide may include but is not limited to at least one of Mo2C, TaC, ZrC, and WC.

[0060] In the specific implementation process, you can choose D v50 The core of the positive electrode lithium supplement agent is coated with a functional material with a particle size of 100-500nm. This particle size helps to increase the contact area between the core of the positive electrode lithium supplement agent and the functional material, thereby further reducing the delithiation voltage of the positive electrode lithium supplement agent. In addition, in order to achieve efficient coating, the D v50 The ratio is (20-100):1.

[0061] During the coating process, a certain amount of functional materials and the core of the positive electrode lithium supplement agent are weighed, dispersed in deionized water, stirred at room temperature, and the resulting mixed slurry is transferred to a spray dryer and dried at 150-230°C at a rate of 10-50mL / min to finally obtain a positive electrode lithium supplement agent with a coating structure.

[0062] In order to further reduce the delithiation voltage of the positive electrode lithium supplement, the positive electrode lithium supplement in the present invention can be selected from an organic lithium supplement. In a specific embodiment, the positive electrode lithium supplement of the present invention includes lithium squarate. In addition to lithium squarate, the core of the positive electrode lithium supplement of the present invention can also include at least one of other lithium supplements such as lithium oxalate, lithium carbonate, and tetrahydroxybenzoquinone lithium salt. When other lithium supplements are also included, the mass ratio of lithium squarate to other lithium supplements is 5:5-9:1, so as to enhance the lithium supplement effect.

[0063] The inventors discovered that by controlling the thickness of the functional material layer coating the core of the positive electrode lithium supplement, the battery's energy density can be further controlled. Specifically, when the thickness of the functional material layer is 5-200 nm, while ensuring that the quality of the positive electrode active material in the battery is not affected, the delithiation voltage of the positive electrode lithium supplement can be further reduced. For example, it can be reduced to below 4.3V, or even not higher than 4.1V, thereby improving the battery's energy density.

[0064] Furthermore, the mass ratio of the functional material layer to the positive electrode lithium replenisher can be controlled to (0.001-0.2):1. This can further reduce the delithiation voltage of the positive electrode lithium replenisher without increasing the mass of the positive electrode active material, allowing more active lithium to be generated during the formation process and participate in the deintercalation and intercalation of lithium ions, thereby improving the battery's energy density. Furthermore, the mass ratio of the functional material layer to the positive electrode lithium replenisher is (0.05-0.15):1.

[0065] As mentioned above, the functional material layer is coated on the outer surface of the positive electrode lithium supplement core. In order to further reduce the delithiation voltage of the positive electrode lithium supplement, the coverage area of ​​the functional material layer can be controlled to be 50-90% of the surface area of ​​the positive electrode lithium supplement core.

[0066] In a specific embodiment, in the diffraction pattern of lithium quarate, there is a (110) diffraction peak at 2θ of 25.0-25.6°, a (112) diffraction peak at 2θ of 29.1-29.7°, and a (202) diffraction peak at 2θ of 31.3-31.9°, and I 112 >I 202 >I 110 , I 112 / I 202 >1.1, where I 112 , I 202 , I 110The peak intensities of the (110) diffraction peak, the (112) diffraction peak, and the (202) diffraction peak are respectively. The lithium squarate is beneficial to maximize the capacity of the lithium supplement agent.

[0067] The present invention is not limited to the preparation method of the above-mentioned lithium squarate. For example, a certain amount of squaric acid (purity 99.9%) and lithium salt (purity 99.9%, for example, Li2CO3) are weighed in a molar ratio of (1-1.2): 1 and dispersed in deionized water. The mixture is stirred at 50-80°C for 1-4h, filtered, washed with ethanol, and dried at 80-120°C to obtain Li2C4O4 powder, which is further crushed by air flow milling to obtain crushed Li2C4O4.

[0068] A second aspect of the present invention provides a lithium-ion battery, which includes the positive electrode sheet according to the first aspect.

[0069] Since in the positive electrode sheet of the present invention, the positive electrode lithium replenishing agent is only distributed in the positive electrode lithium replenishing layer 22 away from the current collector, and the D v50 The diameter of the positive electrode lithium replenishing agent does not exceed 10 μm. Therefore, as the positive electrode lithium replenishing agent decomposes during the formation process, the holes left in the positive electrode lithium replenishing layer 22 will not only not damage the conductive network of the positive electrode sheet, but will also improve the positive electrode sheet's ability to absorb electrolyte to a certain extent. Therefore, the lithium-ion battery of the present invention not only has a higher energy density, but also has outstanding performance in rate performance, cycle performance and storage performance.

[0070] The third aspect of the present invention provides a lithium ion battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and an active layer, the surface of the active layer away from the positive electrode current collector comprising a plurality of pores, the average diameter of the pores not exceeding 10 μm; the positive electrode active material in the active layer comprises lithium cobalt oxide Li a Co x M b O2, the lithium cobalt oxide D v50 The lithium cobalt oxide has a bimodal distribution characteristic, wherein the peak position of the first peak is 2-8 μm, and the peak position of the second peak is 12-25 μm, wherein 0.9≤a≤1.1, 0.8≤x≤1.05, 0≤b≤0.05, and M includes at least one of Mg, Ti, Zr, La, Y, Te, W, Al, B, P, S, Se, K, Rb, and Cs.

[0071] Specifically, in the direction away from the positive electrode current collector, the active layer of the positive electrode sheet includes a positive electrode active layer and a positive electrode lithium replenishing layer stacked in sequence, wherein both the positive electrode active layer and the positive electrode lithium replenishing layer include the above-mentioned lithium cobalt oxide material, and the positive electrode lithium replenishing agent in the positive electrode lithium replenishing layer decomposes during the formation stage, thereby leaving holes on the surface of the active layer (i.e., the surface of the positive electrode lithium replenishing layer).v50 The average diameter of the holes does not exceed 10 μm, so the average diameter of the holes does not exceed 10 μm. The average diameter of the holes is the maximum size of the holes opening on the surface of the active layer.

[0072] The pores of the positive electrode sheet are only distributed on the surface of the active layer, so not only will they not damage the conductive network of the positive electrode sheet, but they will also improve the positive electrode sheet's ability to absorb electrolyte to a certain extent. Therefore, the lithium-ion battery of the present invention not only has a high energy density, but also has outstanding performance in rate performance, cycle performance and storage performance.

[0073] The present invention does not limit the specific structure of the lithium-ion battery mentioned in the second and third aspects above. For example, it can be a square shell battery, a cylindrical battery, etc.

[0074] In addition to the positive electrode sheet, the lithium-ion battery of the present invention also includes an electrolyte, a negative electrode sheet and a separator. Among them, the electrolyte includes one or more solvents commonly used in lithium-ion battery electrolytes, as well as electrolyte lithium salts commonly used in current lithium-ion electrolytes. For example, the solvent can be ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, ethyl methyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc.; the lithium salt can be, for example, one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0075] In one embodiment, the electrolyte of the present invention includes ethyl propionate, and the weight percentage of ethyl propionate in the electrolyte is 10-40%. Specifically, the electrolyte composition helps to further reduce the viscosity of the electrolyte, thereby improving the electrolyte's wettability to a certain extent, facilitating the improvement of the rate performance and cycle performance of the lithium-ion battery. In addition, when the positive electrode lithium supplement includes lithium squarate, the above electrolyte can effectively suppress the dissolution loss of lithium squarate in the electrolyte, thereby reducing its delithiation voltage.

[0076] In a specific embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one surface of the negative electrode current collector, the negative electrode active layer including a negative electrode active material, a conductive agent, and a binder. In a specific embodiment, the negative electrode active layer includes, by weight percentage, 70-99wt% of the negative electrode active material, 0.5-15wt% of the conductive agent, and 0.5-15wt% of the binder, and further includes 80-98wt% of the negative electrode active material, 1-10wt% of the conductive agent, and 1-10wt% of the binder. The negative electrode active material is selected from one or more of artificial graphite, natural graphite, hard carbon, mesophase carbon microbeads, lithium titanate, silicon carbon (the mass percentage of silicon is 3-30%), and silicon monoxide.

[0077] In a specific embodiment, the diaphragm is a material with polypropylene as the base material, or a rubber-coated diaphragm with ceramic coating on one side or both sides thereof.

[0078] Furthermore, with the generation of pores on the surface of the active layer during the formation stage, the porosity of the active layer of the positive electrode is 15-40%, which is beneficial for increasing the liquid absorption capacity of the electrode and extending the service life of the electrolyte, further avoiding the occurrence of liquid shortage.

[0079] The lithium-ion battery of the present invention is suitable for medium- and high-voltage systems, with a specific charge and discharge voltage range of 3-4.60 V. As the delithiation voltage of the positive electrode lithium supplement in the positive electrode sheet decreases, the lithium-ion battery exhibits superior cycle stability, rate capability, and high energy density at medium- and high voltages, meeting the demand for lightweight and thin high-end digital products.

[0080] Hereinafter, the positive electrode sheet and the lithium-ion battery of the present invention will be introduced through specific embodiments.

[0081] Example 1

[0082] The method for preparing the positive electrode sheet of this embodiment includes the following steps:

[0083] 1) A certain amount of squaric acid (purity 99.9%) and Li2CO3 (purity 99.9%) were weighed in a molar ratio of 1:1 and dispersed in deionized water. The mixture was stirred at 60°C for 2 h, filtered, washed with ethanol, and dried at 100°C to obtain Li2C4O4 powder. The crushed powder was further crushed by air flow to obtain D v50 The Li2C4O4 powder was tested by XRD. Figure 2 is the XRD diffraction pattern of Li2C4O4 powder in Example 1 of the present invention, as shown in Figure 2 As shown, there is a 110 diffraction peak at 25.3°, a 112 diffraction peak at 29.4°, a 202 diffraction peak at 31.5°, and I112 >I 202 >I 110 , I 112 / I 202 >1.1.

[0084] Ordered mesoporous carbon and Li2C4O4 powder were weighed and dispersed in deionized water at a mass ratio of 1:9. The mixture was stirred at room temperature for 1 h. The obtained mixed slurry was transferred to a spray dryer and dried at 190 °C at a rate of 15 mL / min to obtain D v50 It is a 0.3μm positive electrode lithium supplement Li2C4O4@C.

[0085] Figure 3 This is the SEM image of the positive electrode lithium supplement in Example 1 of the present invention. Figure 4 This is an SEM image of the positive electrode lithium supplement in Example 1 of the present invention. Figure 3 The scale bar is 100 μm. Figure 4 The scale bar is 5μm. Figure 4 It can be seen that the Li2C4O4 core A presents an uneven block morphology, and the surface coating B with a thickness of 100nm is aggregated in the form of particles on the surface of the Li2C4O4 core A. According to the test, the coverage area of ​​the ordered mesoporous carbon is 65% of the surface area of ​​the Li2C4O4 powder.

[0086] After testing, the conductivity of the positive electrode lithium supplement in this embodiment is 2.62×10 -1 S / cm, specific surface area is 16.06m 2 / g.

[0087] 2) Lithium cobalt oxide (D v50 4.47μm lithium cobalt oxide and D v50 The lithium cobalt oxide with a particle size of 18.18 μm was mixed in a mass ratio of 1:4), conductive carbon black, polyvinylidene fluoride were mixed in a mass ratio of 98.2:10:0.8 with N-methylpyrrolidone, and stirred for 3 hours at a revolution speed of 30 rpm and a rotation speed of 3000 rpm to obtain a positive electrode active layer slurry with a solid content of 75.5%; the D v50 The lithium cobalt oxide has a distribution peak at 4.47 μm and a distribution peak at 18.18 μm.

[0088] The positive electrode active layer slurry was coated on both surfaces of a 9 μm thick aluminum foil and dried to obtain the positive electrode active layer; the D v50 The lithium cobalt oxide has a first distribution peak at 4.47 μm and a second distribution peak at 18.18 μm; Figure 5 This is a particle size distribution diagram of lithium cobalt oxide in Example 1 of the present application;

[0089] 3) lithium cobalt oxide (same composition as in step 2), conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone were mixed in a mass ratio of 98.2:1:0.8, stirred at an orbital speed of 30 rpm and an autogenous speed of 3000 rpm for 3 hours, and then a positive electrode lithium replenisher was added, and stirred at an orbital speed of 30 rpm and an autogenous speed of 1500 rpm for 30 minutes to obtain a positive electrode lithium replenishing layer slurry with a solid content of 75.5%;

[0090] The positive electrode lithium supplement layer slurry was coated on the surface of the positive electrode active layer, dried and rolled to obtain an active layer with a porosity of 16.85% and a compaction density of 4.15 g / cm 3 The positive electrode sheet; in the positive electrode sheet, the thickness of the active layer is 85.4μm, wherein the surface density of the positive electrode active layer is 11.10mg / cm 2 , with a thickness of 59.8 μm; the surface density of the positive electrode lithium replenishment layer is 4.76 mg / cm 2 , with a thickness of 25.6μm; the total mass ratio of the positive electrode lithium supplement and lithium cobalt oxide is 0.014:1.

[0091] Example 2

[0092] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the ordered mesoporous carbon is replaced by NiO. The conductivity of the positive electrode lithium supplement of this embodiment is 5.4×10 -3 S / cm, specific surface area is 14.34m 2 / g.

[0093] Example 3

[0094] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the ordered mesoporous carbon is replaced with TaC in this embodiment. -3 S / cm, specific surface area is 14.57m 2 / g.

[0095] Example 4

[0096] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the surface density of the positive electrode active layer in the positive electrode sheet of this embodiment is 14.66 mg / cm 2 The surface density of the positive electrode lithium replenishment layer is 1.2 mg / cm 2 .

[0097] Example 5

[0098] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the surface density of the positive electrode active layer in the positive electrode sheet of this embodiment is 7.76 mg / cm 2The surface density of the positive electrode lithium replenishment layer is 9.1 mg / cm 2 .

[0099] Example 6

[0100] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the active layer porosity of the positive electrode sheet of this embodiment is 25.21%, and the compaction density is 4.10 g / cm 3 .

[0101] Example 7

[0102] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the active layer porosity of the positive electrode sheet of this embodiment is 38.04% and the compaction density is 4.07 g / cm 3 .

[0103] Example 8

[0104] The positive electrode sheet of this embodiment is basically the same as that of Example 1, except that the mass ratio of the positive electrode lithium replenisher to the positive electrode active material lithium cobalt oxide in the positive electrode sheet is 0.018:1.

[0105] Example 9

[0106] The positive electrode sheet of this embodiment is basically the same as that of Example 1, except that the mass ratio of the positive electrode lithium replenisher to the positive electrode active material lithium cobalt oxide in the positive electrode sheet is 0.003:1.

[0107] Example 10

[0108] The positive electrode sheet of this embodiment is basically the same as that of Example 1, except that the mass ratio of the positive electrode lithium replenisher to the positive electrode active material lithium cobalt oxide in the positive electrode sheet is 0.3:1.

[0109] Example 11

[0110] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the ordered mesoporous carbon coating of the lithium supplement agent in the positive electrode sheet is replaced with acetylene black. After testing, the conductivity of the positive electrode lithium supplement agent of this embodiment is 2.5*10 -2 S / cm, with a specific surface area of ​​4.5m 2 / g.

[0111] Example 12

[0112] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the coating thickness of the functional material in the positive electrode lithium supplement is 75 nm and the Dv50 is 0.3 μm.

[0113] Example 13

[0114] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the coating thickness of the functional material in the positive electrode lithium supplement is 4 nm and the Dv50 is 0.3 μm.

[0115] Example 14

[0116] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that the coating thickness of the functional material in the positive electrode lithium supplement is 120 nm and the Dv50 is 0.3 μm.

[0117] Example 15

[0118] The positive electrode sheet of this embodiment is basically the same as that of Example 1, except that the preparation method of the lithium squarate of this embodiment includes the following steps:

[0119] A certain amount of squaric acid (purity 99.9%) and LiHCO3 (purity 99.9%) were weighed in a molar ratio of 1:2 and dispersed in deionized water respectively. The squaric acid aqueous solution was slowly added to the lithium bicarbonate aqueous solution in a 60°C water bath in several portions. After complete addition, stirring was continued for 2 hours to obtain a lithium squarate solution. The solution was filtered and washed with ethanol. The solution was dried at 80°C to obtain Li2C4O4 powder, which was further crushed by air flow milling to obtain Li2C4O4 with a Dv50 of 0.3μm.

[0120] The lithium quarate of this embodiment was subjected to XRD detection. Figure 6 is the diffraction pattern of the Li2C4O4 powder in Example 15 of the present invention, as shown in FIG. Figure 6 As shown, there is a 110 diffraction peak at 2θ of 25.5°, a 112 diffraction peak at 2θ of 29.4°, a 202 diffraction peak at 2θ of 31.5°, and I202>I112>I110, I112 / I202<1.0.

[0121] Example 16

[0122] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that step 1) is:

[0123] A certain amount of squaric acid (purity 99.9%) and Li2CO3 (purity 99.9%) were weighed in a molar ratio of 1:1 and dispersed in deionized water. The mixture was stirred at 60°C for 2h, filtered, washed with ethanol, and dried at 100°C to obtain Li2C4O4 powder. The crushed D v50 The diffraction data of Li2C4O4 is basically the same as that of Example 1;

[0124] Ordered mesoporous carbon and Li2C4O4 powder prepared in Example 1 were weighed and dispersed in deionized water at a mass ratio of 1:9. The mixture was stirred at room temperature for 1 h. The obtained mixed slurry was transferred to a spray dryer and dried at 190 ° C at a rate of 15 mL / min to obtain D v50 It is a 2.5μm positive electrode lithium supplement Li2C4O4@C.

[0125] After testing, the conductivity of the positive electrode lithium supplement in this embodiment is 2.52×10 -1 S / cm, specific surface area is 13.43m 2 / g.

[0126] Example 17

[0127] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that step 1) is:

[0128] A certain amount of squaric acid (purity 99.9%) and Li2CO3 (purity 99.9%) were weighed in a molar ratio of 1:1 and dispersed in deionized water. The mixture was stirred at 60°C for 2h, filtered, washed with ethanol, and dried at 100°C to obtain Li2C4O4 powder. The crushed D v50 The diffraction data of Li2C4O4 is basically the same as that of Example 1;

[0129] Ordered mesoporous carbon and Li2C4O4 powder prepared in Example 1 were weighed and dispersed in deionized water at a mass ratio of 1:9. The mixture was stirred at room temperature for 1 h. The obtained mixed slurry was transferred to a spray dryer and dried at 190 ° C at a rate of 15 mL / min to obtain D v50 It is a 5.3μm positive electrode lithium supplement Li2C4O4@C.

[0130] After testing, the conductivity of the positive electrode lithium supplement in this embodiment is 2.57×10 -1 S / cm, specific surface area is 10.07m 2 / g.

[0131] Example 18

[0132] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1, except that step 1) is:

[0133] A certain amount of squaric acid (purity 99.9%) and Li2CO3 (purity 99.9%) were weighed in a molar ratio of 1:1 and dispersed in deionized water. The mixture was stirred at 60°C for 2h, filtered, washed with ethanol, and dried at 100°C to obtain Li2C4O4 powder. The crushed D v50 The diffraction data of Li2C4O4 is basically the same as that of Example 1;

[0134] Ordered mesoporous carbon and Li2C4O4 powder prepared in Example 1 were weighed and dispersed in deionized water at a mass ratio of 1:9. The mixture was stirred at room temperature for 1 h. The obtained mixed slurry was transferred to a spray dryer and dried at 190 ° C at a rate of 15 mL / min to obtain D v50 It is a 9.7μm positive electrode lithium supplement Li2C4O4@C.

[0135] After testing, the conductivity of the positive electrode lithium supplement in this embodiment is 2.5×10 -1 S / cm, specific surface area is 7.63m 2 / g.

[0136] Example 19

[0137] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1. The difference is that the mass ratio of large-particle lithium cobalt oxide to small-particle lithium cobalt oxide is adjusted to 7:3, so that the D v50 The lithium cobalt oxide has a first distribution peak at 4.3 μm and a second distribution peak at 18.8 μm.

[0138] Example 20

[0139] The positive electrode sheet of this embodiment is basically the same as that of embodiment 1. The difference is that the mass ratio of large-particle lithium cobalt oxide to small-particle lithium cobalt oxide is adjusted to 6:1, so that the D v50 The lithium cobalt oxide has a first distribution peak at 5.8 μm and a second distribution peak at 18.2 μm.

[0140] Example 21

[0141] The positive electrode sheet of this comparative example is basically the same as that of Example 1, except that the positive electrode lithium supplement in this example is D-type lithium ion battery without the ordered mesoporous carbon coating. v50 It is 0.3μmLi2C4O4.

[0142] Comparative Example 1

[0143] The preparation method of the positive electrode sheet of this comparative example comprises the following steps:

[0144] 1) Lithium cobalt oxide (same as in Example 1), conductive carbon black, polyvinylidene fluoride and N-methylpyrrolidone were mixed in a ratio of 98.2:1:0.8, stirred at an orbital speed of 30 rpm and an autorotational speed of 3000 rpm for 3 h, and then the D prepared in Example 1 was added. v50 0.1 μm Li2C4O4 was used as the positive electrode lithium replenisher (i.e., the positive electrode lithium replenisher of this comparative example was not coated), and the mixture was stirred for 30 min at an orbital speed of 30 rpm and an autorotational speed of 1500 rpm to obtain an active layer slurry with a solid content of 75.5%;

[0145] The active layer slurry was coated on the surface of 9 μm thick aluminum foil, dried and rolled to obtain an active layer with a porosity of 17.03% and a compaction density of 4.15 g / cm 3 The positive electrode sheet has an active layer thickness of 85.4 μm and a surface density of 15.86 mg / cm 2 ; The total mass ratio of the positive electrode lithium supplement and lithium cobalt oxide is 0.014:1.

[0146] Comparative Example 2

[0147] This comparative example is basically the same as comparative example 1, except that the positive electrode lithium replenisher in Example 1 is used instead of the positive electrode lithium replenisher in comparative example 1.

[0148] Comparative Example 3

[0149] The positive electrode sheet of this comparative example is basically the same as that of Example 1, except that step 1) is:

[0150] A certain amount of squaric acid (purity 99.9%) and Li2CO3 (purity 99.9%) were weighed in a molar ratio of 1:1 and dispersed in deionized water. The mixture was stirred at 60°C for 2h, filtered, washed with ethanol, and dried at 100°C to obtain Li2C4O4 powder. The crushed D v50 The diffraction data of Li2C4O4 is basically the same as that of Example 1;

[0151] Ordered mesoporous carbon and Li2C4O4 powder prepared in Example 1 were weighed and dispersed in deionized water at a mass ratio of 1:9. The mixture was stirred at room temperature for 1 h. The obtained mixed slurry was transferred to a spray dryer and dried at 190 ° C at a rate of 15 mL / min to obtain D v50 It is a 12.2μm positive electrode lithium supplement Li2C4O4@C.

[0152] After testing, the conductivity of the positive electrode lithium supplement in this embodiment is 2.43×10 -2 S / cm, specific surface area is 4.41m2 / g.

[0153] Comparative Example 4

[0154] The positive electrode sheet of this comparative example is basically the same as that of Example 1. The difference is that the mass ratio of large-particle lithium cobalt oxide to small-particle lithium cobalt oxide is adjusted to 9.2:0.8, so that the D v50 The lithium cobalt oxide has a first distribution peak at 1.5 μm and a second distribution peak at 22.4 μm.

[0155] Comparative Example 5

[0156] The positive electrode sheet of this comparative example is basically the same as that of the embodiment, except that the positive electrode lithium replenishing layer in this comparative example does not contain lithium cobaltate, and the lithium cobaltate is added to the positive electrode active layer.

[0157] Test Example 1

[0158] The positive electrode lithium supplement used in the examples and comparative examples was taken and assembled into a button battery according to the following method, and the button battery was subjected to charge and discharge treatment.

[0159] 1) The positive electrode lithium supplement agent in the embodiment and the comparative example was weighed with conductive carbon black and PVDF in a mass ratio of 8:1:1, and then mixed with N-methylpyrrolidone to obtain a slurry with a solid content of 45%. The obtained slurry was coated, dried, and cut to obtain the positive electrode sheet in Experimental Example 1.

[0160] 2) In a dry room, the aforementioned positive electrode sheets were assembled into button-type cells with a lithium metal sheet, a polypropylene (PP) separator, and an electrolyte (1 mol / L lithium hexafluorophosphate (LiPF6) dissolved in a 1:1 volume ratio mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). The test voltage range was 2.0–4.50 V, using a 0.1C constant current charge to 4.5 V and a 0.1C constant current discharge to 2 V. The initial charge gram capacity and the decomposition voltage (delithiation voltage) of the positive electrode lithium supplement were measured. The results are shown in Table 1.

[0161] Figure 7 The first cycle specific capacity-voltage curve of the positive electrode lithium supplement in Example 1 of the present invention is shown in FIG. Figure 7 It can be seen that the positive electrode lithium replenisher of Example 1 decomposes at around 4.1V, and most of its capacity has been released in the first cycle of charging. Therefore, the positive electrode lithium replenisher in Example 1 can basically complete the decomposition during the formation process, ensuring the safety performance, cycle performance and energy density of the battery.

[0162] Table 1

[0163]

[0164] Test Example 2

[0165] The positive electrode sheets in the embodiment and the comparative example were assembled into lithium ion batteries according to the following method, and the relevant electrical properties of the lithium ion batteries were tested. The results are shown in Table 2 and Table 3.

[0166] Preparation of the lithium-ion battery: A slurry of (artificial graphite + 10% silicon carbon): conductive carbon black: polyacrylic acid at a mass ratio of 97.2:1.45:1.35 was mixed with deionized water to obtain a 65% solids content. This slurry was then applied to the negative electrode current collector after uniform mixing, dried, and roller-pressed to produce the negative electrode sheet. The positive and negative electrode sheets, along with a separator (PP / PE / PP composite film, 8μm thick, 42% porosity, 2μm alumina ceramic coating + 2μm PVDF coating on both sides), and an electrolyte (1 mol / L lithium hexafluorophosphate, 2wt% 1,3,6-hexanetrionitrile HTCN, 1.5wt% succinonitrile SN, 14wt% fluoroethylene carbonate FEC, and 3.5wt% 1,3-propane sultone PS dissolved in a 1:1:1 volume ratio mixture of ethylene carbonate EC, diethyl carbonate DEC, and propyl propionate PP) were assembled into lithium-ion batteries. The lithium-ion battery was subjected to hot pressing treatment, and the hot pressing formation system was as follows: standing at 83±2℃, 2198±20kg.f for 2min, then charging at 0.2C constant current to 3.7V, standing for 2min, then charging at 0.5C constant current to 4.0V, standing for 2min, and charging at 0.7C constant current to 4.25V, standing for 5min.

[0167] Figure 8 This is a SEM image of the cross section of the positive electrode sheet after the lithium-ion battery in Example 1 is formed. Figure 9 This is a SEM image of the cross section of the positive electrode sheet after the lithium ion battery in Comparative Example 1 is formed. Figure 8 and 9 It can be seen that the lithium ion battery in Example 1 has no obvious holes in the active layer after formation, while the lithium ion battery in Comparative Example 1 has obvious holes after formation, which basically destroys the structure of the active layer and produces obvious gaps between the active layer and the current collector.

[0168] 1) Capacity and energy density

[0169] The above battery was left standing for 5 minutes, charged to 4.53V at 0.5C constant current and constant voltage, with a cut-off current of 0.025C. After standing for 5 minutes, it was discharged to 3V at 0.2C constant current, and the capacity Q1 at this time was recorded.

[0170] Energy density = Q1 × platform voltage / battery volume

[0171] 2) Cycling performance at 25°C

[0172] Place the battery in a (25±2)°C environment and let it rest for 10 minutes. Charge the battery to 4.53V at a constant current and constant voltage of 1.2C, with a cutoff current of 0.05C. After the battery is fully charged, let it rest for 5 minutes, then discharge it at a constant current of 0.5C to a cutoff voltage of 3.0V. Record the highest discharge capacity of the first three cycles as the initial capacity Q2. When the number of cycles reaches 600, record the last discharge capacity of the battery Q3. At the same time, record the initial thickness of the battery, recorded as H1. After 600 cycles, test the thickness of the battery cell and record it as H2.

[0173] Cycle retention rate = Q3 / Q2*100%

[0174] Cycle expansion ratio = (H2-H1) / H1*100%

[0175] 3) 45℃ cycle performance

[0176] Place the battery in a (45±2)°C environment and let it rest for 10 minutes. Charge the battery to 4.53V at a constant current and constant voltage of 1.2C, with a cutoff current of 0.05C. After the battery is fully charged, let it rest for 5 minutes, then discharge it at a constant current of 0.5C to a cutoff voltage of 3.0V. Record the highest discharge capacity of the first three cycles as the initial capacity Q4. When the number of cycles reaches 400, record the last discharge capacity of the battery Q5. At the same time, record the initial thickness of the battery, which is recorded as H3. After 400 cycles, test the thickness of the battery cell and record it as H4.

[0177] Cycle retention rate = Q5 / Q4*100%

[0178] Cycle expansion ratio = (H4-H3) / H3*100%

[0179] 4) 85℃ storage performance

[0180] Record the initial thickness of the battery, H5. Then, place the battery in an environment of (25±2)°C for 10 minutes, discharge the battery at 0.2C to 3V, let it stand for 10 minutes, and then charge it at 0.7C to 4.53V. The battery is further placed in an oven at 85°C±2°C for 8 hours. After storage, remove the sample and immediately measure the hot thickness, H6.

[0181] Battery expansion rate = (H6-H5) / H5*100%

[0182] 5) Rate performance

[0183] The battery was placed in an environment of (25±5)℃ for 10 min, discharged at 0.2C to the lower limit voltage, and allowed to stand for 10 min. It was then fully charged at 0.7C in a constant temperature room with a cutoff current of 0.025C. The battery was discharged at a certain rate (0.2C / 0.5C / 0.7C / 1C / 1.5C / 2C) to the cutoff voltage and allowed to stand for 10 min. The discharge capacity at different discharge rates was recorded. Based on the 0.2C charge and discharge rate, the capacity retention rate at 0.5C / 0.7C / 1C / 1.5C / 2C was calculated.

[0184] 6) Average diameter and porosity of the pores on the surface of the positive electrode sheet after formation. The results are shown in Table 4.

[0185] Average diameter of the holes: The positive electrode is tested under an electron microscope after being formed, and the hole diameter is measured using the electron microscope ruler.

[0186] Porosity measurement:

[0187] ①Measure the thickness, length and weight of the cut electrode

[0188] ② Calculate the apparent volume of the sample = measured sample thickness * sample length * sample width

[0189] ③ Seal the cut electrodes in a sample chamber of known volume

[0190] ④ The volume of the sample can be calculated by the pressure before and after gas diffusion, that is, the true volume of the sample

[0191] ⑤ Calculate the porosity of the sample = (sample apparent volume - sample true volume) / sample apparent volume * 100% Table 2

[0192]

[0193]

[0194] Table 3

[0195]

[0196]

[0197] According to Tables 1-3, the positive electrode sheet of the present invention helps to improve the gram capacity, cycle performance, storage performance and rate performance of lithium-ion batteries.

[0198] Table 4

[0199]

[0200]

[0201] Test Example 3

[0202] The positive electrode sheets in the embodiment and the comparative example were assembled into lithium ion batteries according to the following method, and the relevant electrical properties of the lithium ion batteries were tested. The results are shown in Table 5.

[0203] Preparation of the lithium-ion battery: A slurry of (graphite + 10% silicon carbon): conductive carbon black: polyacrylic acid at a mass ratio of 97.2:1.45:1.35 was mixed with deionized water to obtain a 65% solids content. This slurry was then applied to the negative electrode current collector after uniform mixing, dried, and roller-pressed to produce the negative electrode sheet. The positive and negative electrode sheets, separator (PP / PE / PP composite film, 8μm thick, 42% porosity, 2μm alumina ceramic coating + 2μm PVDF coating on both sides), and electrolyte (comprising, by mass percentage: lithium hexafluorophosphate (LiPF6) 15%, fluoroethylene carbonate (FEC) 12%, succinonitrile (SN) 2%, 1,3,6-hexanetricarbonitrile (HTCN) 1.5%, ethyl propionate (EP) 15%, propyl propionate (PP) 4.5%, ethylene carbonate (EC) 25%, and diethyl carbonate (DEC) 25%) were assembled into lithium-ion batteries.

[0204] The 45°C cycle performance was tested according to the method in Test Example 2. The results are shown in Table 5.

[0205] Table 5

[0206]

[0207]

[0208] According to Table 5, when the positive electrode sheet of the present invention is matched with a specific combination of electrolytes, the improvement on the electrical performance of the lithium-ion battery is more significant.

[0209] Finally, it should be noted that those skilled in the art will readily identify other embodiments of the present invention after considering the specification and practicing the invention disclosed herein. The present invention is intended to cover any variations, uses, or adaptations of the present invention that follow the general principles of the present invention and include common knowledge or customary techniques in the art not disclosed herein. The present invention is not limited to the precise structure described above and illustrated in the accompanying drawings, and various modifications and variations may be made without departing from the scope thereof. The scope of the present invention is limited solely by the appended claims.

Claims

1. A positive electrode sheet, characterized in that: The invention comprises a positive electrode current collector and an active layer arranged on at least one surface of the positive electrode current collector; in a direction away from the positive electrode current collector, the active layer comprises a positive electrode active layer and a positive electrode lithium replenishing layer stacked in sequence, the positive electrode active layer comprises a first positive electrode active material, the positive electrode lithium replenishing layer comprises a positive electrode lithium replenishing agent and a second positive electrode active material; the D v50 Not higher than 10μm; The second positive electrode active material includes lithium cobaltate Li a Co x M b O2, the lithium cobalt oxide D v50 The lithium cobalt oxide has a bimodal distribution characteristic, wherein the peak position of the first peak is 2-8 μm, the peak position of the second peak is 12-25 μm, 0.9≤a≤1.1, 0.8≤x≤1.05, 0≤b≤0.05, and M includes at least one of Mg, Ti, Zr, La, Y, Te, W, Al, B, P, S, Se, K, Rb, and Cs.

2. The positive electrode sheet according to claim 1, characterized in that The ratio of the surface density of the positive electrode lithium replenishing layer to the surface density of the positive electrode active layer is (0.1-1):

1.

3. The positive electrode sheet according to claim 1 or 2, characterized in that: The compaction density of the positive electrode sheet is 3.5-4.5 g / cm 3 and / or, The porosity of the active layer of the positive electrode sheet is 10%-40%; and / or, The thickness of the active layer is 80-120 μm; and / or, In the active layer, the mass ratio of the positive electrode lithium replenisher to the positive electrode active material is (0.005-0.2):

1.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The conductivity of the positive electrode lithium supplement is greater than 1×10 -4 S / cm; and / or, the specific surface area of ​​the positive electrode lithium supplement is not less than 5m 2 / g.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: The positive electrode lithium supplement agent includes a positive electrode lithium supplement agent core and a functional material layer covering at least a portion of the surface of the positive electrode lithium supplement agent core, wherein the functional material layer includes at least one of a carbon material, a metal oxide, and a metal carbide; Preferably, in the functional material layer, the D v50 100-500nm; Preferably, the positive electrode lithium replenisher includes lithium squarate; preferably, the positive electrode lithium replenisher includes lithium squarate and other lithium replenishers, and the mass ratio of the lithium squarate and other lithium replenishers is 5:5-9:

1.

6. The positive electrode sheet according to claim 5, characterized in that: The thickness of the functional material layer is 5-200 nm; and / or, The mass ratio of the functional material layer to the positive electrode lithium supplement agent is (0.001-0.2):1; and / or, The coverage area of ​​the functional material layer is 50-90% of the surface area of ​​the positive electrode lithium supplement core; and / or, The lithium quarate has a (110) diffraction peak at 2θ of 25.0-25.6°, a (112) diffraction peak at 2θ of 29.1-29.7°, and a (202) diffraction peak at 2θ of 31.3-31.9°, and I 112 >I 202 >I 110 , I 112 / I 202 >1.1, where I 112 , I 202 , I 110 They are the peak intensities of the (110) diffraction peak, (112) diffraction peak, and (202) diffraction peak, respectively.

7. A lithium-ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet according to any one of claims 1 to 6.

8. A lithium ion battery, characterized in that: The positive electrode sheet includes a positive electrode current collector and an active layer. The surface of the active layer away from the positive electrode current collector includes a plurality of holes, and the average diameter of the holes does not exceed 10 μm. The positive electrode active material in the active layer includes lithium cobalt oxide Li a Co x M b O2, the lithium cobalt oxide D v50 The lithium cobalt oxide has a bimodal distribution characteristic, wherein the peak position of the first peak is 2-8 μm, and the peak position of the second peak is 12-25 μm, wherein 0.9≤a≤1.1, 0.8≤x≤1.05, 0≤b≤0.05, and M includes at least one of Mg, Ti, Zr, La, Y, Te, W, Al, B, P, S, Se, K, Rb, and Cs.

9. The lithium-ion battery according to claim 7 or 8, characterized in that The lithium-ion battery comprises an electrolyte, the electrolyte comprises ethyl propionate, and the mass percentage of the ethyl propionate in the electrolyte is 10-40%.

10. The lithium-ion battery according to any one of claims 7 to 9, characterized in that: The porosity of the active layer of the positive electrode sheet is 15-40%.

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