Positive pole piece and application thereof

Abstract

The invention provides a positive pole piece and application thereof. The positive pole piece comprises a positive current collector, a negative current collector and a positive electrode, the positive electrode active layer is at least arranged on one side of the positive electrode current collector, and the positive electrode active layer comprises a first positive electrode active material and a second positive electrode active material; wherein the positive electrode active layer is analyzed through a scanning electron microscope, and the area of the bright area is 10%-70% of the test area in the area with the test area of 50 microns * 40 microns. According to the positive pole piece and the application thereof provided by the invention, the problem of sudden increase of a voltage platform and direct-current impedance of the lithium ion battery can be improved, and the performance of the lithium ion battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly relates to a positive electrode plate and its application. Background Art

[0002] With the rapid development of lithium-ion batteries in the fields of electric vehicles and large-scale energy storage, higher requirements are put forward for the capacity and performance stability of lithium-ion batteries. Among them, lithium iron phosphate batteries have become one of the mainstream directions of lithium-ion battery development due to their advantages of low cost and long life. However, the specific capacity of the current lithium iron phosphate materials has approached its theoretical value, making the energy density of lithium iron phosphate batteries tend to reach the limit.

[0003] By replacing part of the iron in lithium iron phosphate with manganese to obtain lithium manganese iron phosphate, while not increasing the cost of the positive electrode active material, the energy density of lithium manganese iron phosphate is improved, and it has a higher voltage platform. However, the improvement of the energy density is limited, and problems such as unstable power output are prone to occur, restricting the development of lithium-ion batteries. Summary of the Invention

[0004] The present invention provides a positive electrode plate and its application. Through the positive electrode plate and its application provided by the present invention, the voltage platform of the lithium-ion battery can be made gentle, the problem of sudden increase in the DC impedance of the manganese-iron conversion platform in the lithium manganese iron phosphate material can be improved, and a high-performance lithium-ion battery can be obtained.

[0005] To solve the above technical problems, the present invention provides a positive electrode plate, which at least includes:

[0006] A positive electrode current collector;

[0007] A positive electrode active layer, which is at least disposed on one side of the positive electrode current collector, and the positive electrode active layer includes a first positive electrode active material and a second positive electrode active material;

[0008] Wherein, by analyzing the positive electrode active layer with a scanning electron microscope, in a region with a test area of 50μm×40μm, the area of the bright region is 10%-70% of the test area.

[0009] In an embodiment of the present invention, the first positive electrode active material includes LiMn m Fe 1-m PO 4 , where 0.4≤m≤0.7.

[0010] In an embodiment of the present invention, the second positive electrode active material includes Li 1+a [Ni x Co y M z O 2-b A b, 0.7 ≤ x < 1, 0 ≤ y < 0.3, 0 ≤ z < 0.3, -0.2 < a < 0.2, 0 ≤ b < 0.2, and x + y + z = 1; wherein, the M element includes one or more of Mn, Al, Ti, Zr, Mg, Sr, W, Mo, B, V, Se, Nb, Ru, Rh, Pd, Sb, Te, Ce, Ca, Zn, Y, and W, and the A element includes one or more of F, N, Cl, S, and P.

[0011] In an embodiment of the present invention, the mass of the second positive electrode active material accounts for 5% - 95% of the total mass of the first positive electrode active material and the second positive electrode active material.

[0012] In an embodiment of the present invention, the Dv90 of the first positive electrode active material is 1 μm - 15 μm;

[0013] Wherein, Dv90 represents the particle size at which the volume cumulative reaches 90% starting from the small particle size in the particle size distribution based on volume.

[0014] In an embodiment of the present invention, the Dv90 of the second positive electrode active material is 2 μm - 15 μm;

[0015] Wherein, Dv90 represents the particle size at which the volume cumulative reaches 90% starting from the small particle size in the particle size distribution based on volume.

[0016] In an embodiment of the present invention, the area of the bright region is 10% - 50% of the test area.

[0017] In an embodiment of the present invention, the bright region represents the scanning electron microscope image corresponding to the second positive electrode active material.

[0018] In an embodiment of the present invention, the scanning electron microscope image is processed by image processing software. During the processing, the selected area of the positive electrode active layer is the test area; after being processed by the image processing software, the region with a gray threshold of 135 - 254 is the bright region.

[0019] The present invention also provides a lithium-ion battery, including the positive electrode tab described above.

[0020] The present invention also provides an electronic device, including the lithium-ion battery described above.

[0021] In summary, the present invention provides a positive electrode sheet and its application. By controlling the area ratio of the bright regions in the scanning electron microscope image of the cross-section of the positive electrode sheet, the voltage plateau of the lithium-ion battery can be made gentle, the sudden increase in the DC impedance of the manganese-iron conversion plateau in the lithium iron phosphate material can be improved, and the power output of the lithium-ion battery can be stabilized. It can improve the rate performance, cycle performance and safety performance of the lithium-ion battery, and reduce the cost of the positive electrode active material. It can stabilize the output power of the lithium-ion battery, be compatible with the existing battery management system, and at the same time can avoid the impact of overcharging and over-discharging on the life of the lithium-ion battery, thereby extending the service life of the lithium ions in the battery. It can improve the capacity, power, cycle life and safety performance of the lithium-ion battery, and obtain a high-performance lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for describing the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0023] Figure 1 Scanning electron microscope image of the cross-section of the positive electrode sheet in an embodiment of the present invention.

[0024] Figure 2 Scanning electron microscope image of the cross-section of the positive electrode sheet processed by image processing software in an embodiment of the present invention.

[0025] Figure 3 Voltage plateau of the lithium-ion battery provided in Embodiment 1.

[0026] Figure 4 Voltage plateau of the lithium-ion battery provided in Embodiment 3.

[0027] Figure 5 DC impedance of the lithium-ion battery provided in Embodiment 1 under different charge states.

[0028] Figure 6 DC impedance of the lithium-ion battery provided in Embodiment 2 under different charge states.

[0029] Figure 7 DC impedance of the lithium-ion battery provided in Embodiment 4 under different charge states. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand the other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0031] It should be understood that the present invention can be implemented in different forms and should not be construed as limited to the embodiments presented herein. On the contrary, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the present invention to those skilled in the art.

[0032] The technical solutions of the present invention will be further described in detail below in conjunction with the embodiments and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the scope of protection of the present invention.

[0033] The present invention provides an electronic device, which includes at least one lithium-ion battery for providing electrical energy. Among them, the electronic device can be a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, an electric tool, etc. In an embodiment of the present invention, the vehicle is, for example, a new energy vehicle, which can be a pure electric vehicle, a hybrid electric vehicle, an extended-range electric vehicle, etc. The spacecraft includes an airplane, a rocket, a space shuttle, a spaceship, etc. The electric toy includes a fixed or mobile electric toy, such as a game console, an electric vehicle toy, an electric ship toy, an electric airplane toy, etc. The electric tool includes a metal cutting electric tool, a grinding electric tool, an assembly electric tool, and a railway electric tool, such as a drill, an electric grinder, an electric wrench, an electric screwdriver, a hammer drill, an impact drill, a concrete vibrator, and a planer. The embodiments of the present application do not impose special restrictions on the above-mentioned power-consuming devices.

[0034] The present invention also provides a lithium-ion battery, which includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. The separator is located between the positive electrode plate and the negative electrode plate, and the electrolyte is filled between the positive electrode plate, the negative electrode plate, and the separator. The present invention does not limit the type and shape of the lithium-ion battery. In an embodiment of the present invention, the lithium-ion battery is a primary battery or a secondary battery, and the secondary battery is, for example, a soft-pack battery, a hard-shell battery, a cylindrical battery, etc. In this embodiment, for example, a soft-pack secondary battery is taken as an example for illustration, and the battery cell of the soft-pack secondary battery is formed by stacking or winding, etc.

[0035] The present invention provides a positive electrode plate, which includes a positive electrode current collector and a positive electrode active layer disposed at least on one surface of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material, a binder, a conductive agent, etc., and the thickness of the positive electrode active layer is greater than 30 μm, for example, 60 μm - 90 μm, for example, 65 μm, 68 μm, 70 μm, 75 μm, 78 μm, 80 μm, 84 μm or 87 μm, etc. The positive electrode active material includes a first positive electrode active material and a second positive electrode active material, etc., and the first positive electrode active material, for example, includes lithium manganese iron phosphate, etc. The general formula of lithium manganese iron phosphate is LiMn m Fe 1-m PO 4 , where 0.4 ≤ m ≤ 0.7. The second positive electrode active material, for example, includes a ternary material, etc., and the general formula of the ternary material is, for example, Li 1+a [Ni x Co y M z O 2-b A b , where 0.7 ≤ x < 1, 0 ≤ y < 0.3, 0 ≤ z < 0.3, -0.2 < a < 0.2, 0 ≤ b < 0.2, and x + y + z = 1; where the M element includes one or more of Mn, Al, Ti, Zr, Mg, Sr, W, Mo, B, V, Se, Nb, Ru, Rh, Pd, Sb, Te, Ce, Ca, Zn, Y, and W, and the A element includes one or more of F, N, Cl, S, and P. By providing the first positive electrode active material and the second positive electrode active material, the second positive electrode active material can improve the energy density of the positive electrode plate, and the first positive electrode active material can improve the rate performance, cycle performance, and safety performance of the lithium-ion battery. At the same time, compared with the pure second positive electrode active material, the cost of the positive electrode active material can be reduced.

[0036] In an embodiment of the present invention, the positive electrode current collector can be, for example, a foil formed after surface treatment of nickel, titanium, aluminum, nickel, silver, stainless steel, or carbon, etc. In addition to the foil, the positive electrode current collector can also be used in any one or more combinations of various forms such as film-like, net-like, porous, foam, or non-woven fabric. Among them, the thickness of the positive electrode current collector is, for example, 8 μm - 15 μm. In an embodiment of the present invention, the positive electrode current collector is, for example, an aluminum foil.

[0037] In an embodiment of the present invention, the binder is selected from, for example, any one or more of polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene, or polymerized styrene butadiene rubber (SBR). The conductive agent is selected from, for example, any one or more of conductive carbon black (Super P, SP), acetylene black, carbon nanotubes, and graphene.

[0038] In an embodiment of the present invention, the positive electrode active material is, for example, LiMn m Fe 1-m PO 4 and LiNi x Co y Mn Z O 2 The mixture is such that 0.4 ≤ m ≤ 0.7, 0.7 ≤ x < 1, 0 ≤ y < 0.3, and 0 ≤ z < 0.3. The binder is, for example, polyvinylidene fluoride, and the conductive agent is, for example, acetylene black. After mixing the positive electrode active material, acetylene black, and polyvinylidene fluoride in a mass ratio of, for example, 95:3:2, an organic solvent is added and stirred until the system becomes homogeneous to obtain a positive electrode slurry. Among them, the organic solvent is selected from, for example, N-methylpyrrolidone (NMP). After uniformly coating the positive electrode slurry on the aluminum foil, it is dried, and then the dried aluminum foil is subjected to processes such as cold pressing to obtain a positive electrode plate.

[0039] Please refer to Figures 1 to 2 As shown, in an embodiment of the present invention, the positive electrode plate is cut, for example, with scissors to obtain a size of, for example, 2 × 5 mm 2For the sample, the cut surface of the sample is polished to reduce the adverse effects caused by cutting. For example, the cut surface of the sample is treated by ion milling. For example, the cut surface is facing the light beam for 30 min - 60 min. The treated sample is placed in a scanning electron microscope for testing to obtain a scanning electron microscope image of the positive electrode active layer of the cross-section of the positive electrode sheet. Among them, in the scanning electron microscope image of the positive electrode active layer of the cross-section of the positive electrode sheet, it is divided into two gray-scale intervals, a bright area and a dark area, and the contrast is obvious. The obvious white bright area is the bright area, and the rest of the area is the dark area. In this embodiment, through an image processing software, such as Image J software, the scanning electron microscope image is processed to measure the area ratio of the bright area in the scanning electron image of the cross-section of the positive electrode sheet, or any device or process known in the relevant field can be used for measurement. In an embodiment of the present invention, during the processing by Image J software, the selected area of the positive electrode active layer is used as the test area. In this embodiment, the test area is, for example, a region of 50 μm × 40 μm. After being processed by the image processing software, the region with a gray-scale threshold (Threhold) of 135 - 254 is defined as the bright area, and the rest of the region is defined as the dark area. In an embodiment of the present invention, in the scanning electron microscope image, the area of the bright area is 10% - 70% of the test area, and for another example, it is 10% - 70%, and for another example, it is 12% - 32%.

[0040] In an embodiment of the present invention, in the scanning electron microscope image of the cross-section of the positive electrode sheet, the scanning electron microscope image corresponding to the component mainly composed of the second positive electrode active material is the bright area, and the scanning electron microscope image corresponding to the component mainly composed of the first positive electrode active material is the dark area. And the dark area, for example, also includes the regions corresponding to the conductive agent, the binder or the voids. By controlling the area ratio of the bright area, the voltage platform of the lithium-ion battery can be made gentle, the sudden increase in the direct current resistance (DCR) of the manganese-iron conversion platform of LMFP can be improved, and the power output of the lithium-ion battery can be stabilized.

[0041] In an embodiment of the present invention, in the positive electrode active material, the mass of the second positive electrode active material, for example, accounts for 5% - 95% of the total mass of the first positive electrode active material and the second positive electrode active material, and the balance is the first positive electrode active material, that is, the mass of the second positive electrode active material, for example, accounts for 5% - 95% of the total mass of the positive electrode active material. Among them, the mass of the second positive electrode active material, for another example, accounts for 25% - 75% of the total mass of the positive electrode active material, and for another example, it is 30%, 50% or 70%, etc. By adjusting the ratio of the mass of the second positive electrode active material in the total mass of the positive electrode active material, the area ratio of the bright area formed in the positive electrode sheet can be controlled, thereby improving the performance of the lithium-ion battery.

[0042] In an embodiment of the present invention, the Dv90 of the first positive electrode active material is, for example, 1 μm - 15 μm, or for example, 2 μm - 10 μm, and the Dv90 of the second positive electrode active material is, for example, 2 μm - 15 μm, or for example, 3 μm - 8 μm. Herein, Dv90 represents the particle size at which the volume cumulative reaches 90% starting from the smaller particle size in the particle size distribution based on volume. By controlling the Dv90 of the first positive electrode active material and the second positive electrode active material, the agglomeration properties of different active materials are made different, thereby enabling the control of the area ratio of the bright regions in the formed positive electrode sheet and improving the performance such as the voltage platform and DC impedance of the lithium-ion battery.

[0043] In an embodiment of the present invention, the negative electrode sheet includes, for example, a negative electrode current collector and at least a negative electrode active layer coated on one side of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, a binder, a conductive agent, a thickening agent, etc. Among them, the negative electrode current collector is, for example, selected from one of a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foam copper current collector, or a stainless steel current collector, etc., and the thickness of the negative electrode current collector is, for example, 8 μm - 15 μm. In an embodiment of the present invention, the negative electrode current collector is, for example, a copper foil.

[0044] In an embodiment of the present invention, the negative electrode active material is, for example, selected from any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide, silicon carbide, or lithium titanate, etc. The binder is, for example, selected from any one or more of polyvinylidene fluoride, polyamide, polypropylene, polyacrylate, polyethylene ether, polymethyl methacrylate, polyhexafluoropropylene, or styrene-butadiene rubber, etc. The conductive agent is, for example, selected from any one or more of conductive carbon black, acetylene black, carbon nanotubes, and graphene, etc. In an embodiment of the present invention, the negative electrode current collector is, for example, selected from a copper foil, the negative electrode active material is, for example, selected from graphite, the conductive agent is, for example, selected from acetylene black, the binder is, for example, selected from styrene-butadiene rubber, and the thickening agent is, for example, selected from sodium carboxymethyl cellulose. In an embodiment of the present invention, graphite, acetylene black, styrene-butadiene rubber, and sodium carboxymethyl cellulose are, for example, mixed in a mass ratio of 96:2:1:1, deionized water is added, and the mixture is fully stirred to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on the copper foil, and after processes such as drying and cold pressing, a negative electrode sheet is obtained.

[0045] In an embodiment of the present invention, the separator is, for example, a polyethylene film (Polyethylene, PE), a polypropylene film (Polypropylene, PP), a glass fiber film, a polyethylene film, or a composite film, etc. And the thickness of the separator is, for example, 9 μm - 15 μm. In an embodiment of this aspect, the separator selects, for example, a polyethylene with a thickness of 8 μm - 10 μm as the base film, and a nano-aluminum oxide coating with a thickness of 2 μm - 4 μm is coated on the base film to obtain the separator.

[0046] In an embodiment of the present invention, the electrolyte at least includes a non-aqueous solvent and a lithium salt, etc. The non-aqueous solvent includes, for example, any one or a combination of at least two of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), or diethyl carbonate (DEC), etc. In an embodiment of the present invention, the mass content of the non-aqueous solvent in the electrolyte is, for example, 60%-85%. In an embodiment of the present invention, the lithium salt is, for example, selected from lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(oxalato)borate (LiBOB), lithium difluorobis(oxalato)phosphate (LiODFP), lithium difluoro(oxalato)borate (LiODFB), lithium difluorophosphate (LiPO 2 F 2 ), or lithium trifluoromethanesulfonate (LiCF 3 SO 3 ), etc., or any one or a combination of at least two of them. In an embodiment of the present invention, the concentration of the lithium salt in the electrolyte is 0.1 mol / L - 2 mol / L. In an embodiment of the present invention, the electrolyte also includes, for example, additives. The additives include, for example, at least one of film-forming additives such as 1,3-propanesultone (PS), fluoroethylene carbonate (FEC), or vinylene carbonate (VC). The mass content of the additives in the electrolyte is 0.1 wt - 15 wt%. In an embodiment of the present invention, the electrolyte selects any applicable commercial electrolyte.

[0047] In an embodiment of the present invention, the above-mentioned positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed in the middle between the positive electrode sheet and the negative electrode sheet to play an isolating role, and a bare battery cell is obtained by stacking or winding. The bare battery cell is placed into an aluminum-plastic film, and after baking to remove water, the electrolyte is injected and sealed. Then, after processes such as standing, hot and cold pressing, formation, clamping, and grading, a finished lithium-ion battery is obtained.

[0048] Hereinafter, the present invention will be more specifically explained by citing embodiments, and these embodiments should not be construed as restrictive. Within the scope consistent with the gist of the present invention, appropriate modifications can be made, and they all fall within the technical scope of the present invention.

[0049] Example 1

[0050] Preparation of positive electrode sheet: positive electrode active material is LiMn 0.6 Fe 0.4 PO 4 , and LiMn 0.6 Fe 0.4 PO 4 's Dv90 is 2.10μm. The positive electrode active material, the conductive agent acetylene black and the binder polyvinylidene fluoride are mixed in a mass ratio of 95:3:2. After the positive electrode active material, the binder and the conductive agent are evenly mixed, the solvent N-methylpyrrolidone is added and stirred until it is uniform and transparent to obtain the positive electrode slurry. The positive electrode slurry is evenly coated on the aluminum foil, dried, and then cold pressed to obtain the positive electrode sheet. In the scanning electron microscope image of the cross section of the positive electrode sheet, the area of ​​the bright area is 1% of the test area.

[0051] Preparation of negative electrode sheet: Mix the negative electrode active material graphite, the conductive agent acetylene black, the adhesive styrene butadiene rubber and the thickener sodium carboxymethyl cellulose in a mass ratio of 96:2:1:1, add deionized water, and stir thoroughly to obtain the negative electrode slurry. The negative electrode slurry is evenly coated on the copper foil, and the negative electrode sheet is obtained after drying and cold pressing.

[0052] Preparation of electrolyte: Under the conditions that the nitrogen content in the glove box is 99.999%, the actual oxygen content in the glove box is 0.1ppm, and the moisture content is 0.1ppm, the non-aqueous solvent is composed of ethylene carbonate, ethyl methyl carbonate and carbonic acid in a mass ratio of 2:4:4. Lithium hexafluorophosphate is added, and the concentration of the lithium salt is 1mol / L. Taking the total mass of the electrolyte as 100%, 3% VC is added to the electrolyte to obtain the electrolyte.

[0053] Selection of diaphragm: Select 9μm polyethylene as the base film, and coat the base film with 3μm nano-aluminum oxide coating to obtain the diaphragm.

[0054] Battery preparation: stack the positive electrode sheet, separator, and negative electrode sheet in sequence, with the separator placed between the positive and negative electrodes to isolate them, and stack them to obtain a bare battery cell. Then wrap it with aluminum plastic film, dry and remove water, inject electrolyte and seal it, and after standing, hot and cold pressing, formation, clamping and capacity separation, a lithium-ion battery is prepared.

[0055] Example 2

[0056] The positive electrode active material is LiMn 0.6 Fe 0.4 PO 4 With LiNi 0.8 Co 0.1 Mn 0.1 O 2 Mixture of LiMn0.6 Fe 0.4 PO 4 has a Dv90 of 2.10 μm, and LiNi 0.8 Co 0.1 Mn 0.1 O 2 has a Dv90 of 5.20 μm, and LiMn 0.6 Fe 0.4 PO 4 and LiNi 0.8 Co 0.1 Mn 0.1 O 2 have a mass ratio of 8:2. In the scanning electron microscope image of the cross-section of the obtained positive electrode sheet, the area of the bright region is 12% of the test area. Other steps are the same as those in Example 1.

[0057] Example 3

[0058] The positive electrode active material is a mixture of LiMn 0.6 Fe 0.4 PO 4 and LiNi 0.8 Co 0.1 Mn 0.1 O 2 The Dv90 of LiMn 0.6 Fe 0.4 PO 4 is 2.10 μm, and the Dv90 of LiNi 0.8 Co 0.1 Mn 0.1 O 2 is 5.0 μm. The mass ratio of LiMn 0.6 Fe 0.4 PO 4 and LiNi 0.8 Co 0.1 Mn 0.1 O 2 is 8:2. In the scanning electron microscope image of the cross-section of the obtained positive electrode sheet, the area of the bright region is 13% of the test area. Other steps are the same as those in Example 1.

[0059] Example 4

[0060] The positive electrode active material is a mixture of LiMn 0.6 Fe 0.4 PO 4 and LiNi 0.8 Co 0.1 Mn 0.1 O 2 The Dv90 of LiMn 0.6 Fe 0.4 PO 4The Dv90 of LiNi 0.8 Co 0.1 Mn 0.1 O 2 is 2.10 μm, and the Dv90 of LiMn 0.6 Fe 0.4 PO 4 is 5.20 μm. The mass ratio of LiMn 0.8 Co 0.1 Mn 0.1 O 2 to LiNi

[0061] is 7:3. In the scanning electron microscope image of the cross-section of the obtained positive electrode sheet, the area of the bright region is 18% of the test area. Other steps are the same as those in Example 1.

[0062] Please refer to Figure 3 and Figure 4 as shown. They are the voltage platforms of the lithium-ion batteries of Example 1 and Example 3 respectively. When the positive electrode active material is LiMn 0.6 Fe 0.4 PO 4 , there are two voltage platforms near 3.8 V and 3.3 V for the lithium-ion battery, and there is a problem of sudden voltage drop during discharge, resulting in unstable power output. As can be seen from Figure 3 and Figure 4 , as the area of the bright region in the scanning electron microscope image of the cross-section of the positive electrode sheet increases, the obtained lithium-ion battery has a high voltage platform, and the discharge platform becomes slower, which can stabilize the output power of the lithium-ion battery, be compatible with the existing battery management system, and at the same time can avoid the influence of overcharge and overdischarge on the life of the lithium-ion battery, thereby extending the service life of the lithium-ion battery.

[0063] Please refer to Figures 5 to 7 as shown. They are the DC impedances of the lithium-ion batteries of Example 1, Example 2 and Example 4 under different charge states. As can be seen fromFigures 5 to 7 It can be seen that as the area of the bright region increases in the scanning electron microscope image of the cross-section of the positive electrode plate, the DC impedance of the lithium-ion battery decreases at different states of charge. Moreover, as the area of the bright region increases in the scanning electron microscope image of the cross-section of the positive electrode plate, the change amplitude of the DC impedance of the lithium-ion battery becomes slower at different states of charge, that is, the sudden increase in DCR of the manganese-iron conversion platform in the first positive active material is improved, which is beneficial to improving the capacity, power, cycle life and safety performance of the lithium-ion battery, and obtaining a high-performance lithium-ion battery.

[0064] In summary, the present invention proposes a positive electrode plate and its application. By controlling the proportion of the area of the bright region in the scanning electron microscope image of the cross-section of the positive electrode plate, the voltage platform of the lithium-ion battery can be made gentle, the problem of sudden increase in DC impedance of the manganese-iron conversion platform in the lithium iron phosphate material can be improved, and the power output of the lithium-ion battery can be stabilized. The second positive active material can improve the energy density of the positive electrode plate, and the first positive active material can improve the rate performance, cycle performance and safety performance of the lithium-ion battery and reduce the cost of the positive active material. By adjusting the ratio of the mass of the second positive active material in the total mass of the positive active material, and the Dv90 of the first positive active material and the second positive active material, the proportion of the area of the bright region formed in the positive electrode plate can be controlled, thereby improving the performance of the lithium-ion battery. It can stabilize the output power of the lithium-ion battery, be compatible with the existing battery management system, and at the same time can avoid the influence of overcharge and overdischarge on the life of the lithium-ion battery, thereby extending the service life of the lithium ions in the battery. It can improve the capacity, power, cycle life and safety performance of the lithium-ion battery, and obtain a high-performance lithium-ion battery.

[0065] The above description is only the preferred embodiments of the present application and the explanation of the applied technical principles. Those skilled in the art should understand that the scope of the invention involved in the present application is not limited to the technical solutions formed by the specific combination of the above technical features, and should also cover other technical solutions formed by any combination of the above technical features or their equivalent features without departing from the inventive concept, such as the technical solutions formed by mutually replacing the above features with the (but not limited to) technical features with similar functions disclosed in the present application.

[0066] Except for the technical features described in the specification, the rest of the technical features are known to those skilled in the art. To highlight the innovative features of the present invention, the rest of the technical features are not described herein again.

Claims

1. A positive electrode sheet, characterized in that: At least: Positive electrode current collector; A positive electrode active layer, disposed at least on one side of the positive electrode current collector, the positive electrode active layer comprising a first positive electrode active material and a second positive electrode active material; The positive electrode active layer was analyzed by scanning electron microscopy. In a test area of ​​50 μm×40 μm, the area of ​​the bright area was 10%-70% of the test area.

2. The positive electrode sheet according to claim 1, characterized in that: The first positive electrode active material includes LiMn m Fe 1- m PO4, where 0.4≤m≤0.

7.

3. The positive electrode sheet according to claim 1, characterized in that: The second positive electrode active material includes Li 1+a [Ni x Co y M z O 2-b A b , where 0.7 ≤ x < 1, 0 ≤ y < 0.3, 0 ≤ z < 0.3, -0.2 < a < 0.2, 0 ≤ b < 0.2, and x + y + z = 1; wherein, the M element includes one or more of Mn, Al, Ti, Zr, Mg, Sr, W, Mo, B, V, Se, Nb, Ru, Rh, Pd, Sb, Te, Ce, Ca, Zn, Y, and W, and the A element includes one or more of F, N, Cl, S, and P.

4. The positive electrode sheet according to claim 1, characterized in that: The mass of the second positive electrode active material accounts for 5% to 95% of the total mass of the first positive electrode active material and the second positive electrode active material.

5. The positive electrode sheet according to claim 1, characterized in that: The Dv90 of the first positive electrode active material is 1 μm-15 μm; Here, Dv90 represents a particle size at which the volume accumulation from the smallest particle size reaches 90% in a volume-based particle size distribution.

6. The positive electrode sheet according to claim 1, characterized in that: The Dv90 of the second positive electrode active material is 2 μm-15 μm; Here, Dv90 represents a particle size at which the volume accumulation from the smallest particle size reaches 90% in a volume-based particle size distribution.

7. The positive electrode sheet according to claim 1, characterized in that: The area of ​​the bright area is 10%-50% of the test area.

8. The positive electrode sheet according to claim 1, characterized in that: The bright area represents a scanning electron microscope image corresponding to the second positive electrode active material.

9. The positive electrode sheet according to claim 1, characterized in that: The scanning electron microscope image is processed by image processing software. During the processing, the area of ​​the positive electrode active layer is selected as the test area. After the image processing software is processed, the area with a grayscale threshold of 135-254 is the bright area.

10. A lithium ion battery, characterized in that: It comprises the positive electrode sheet as described in any one of claims 1 to 9.

11. An electronic device, characterized in that: Includes the lithium ion battery as claimed in claim 10.

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