Positive electrode sheet, lithium ion battery, and electronic device

Abstract

To provide a positive electrode sheet and an application thereof.SOLUTION: A positive electrode sheet includes a positive electrode current collector and a positive electrode active layer disposed on at least one side of the positive electrode current collector. The positive electrode active layer includes a first positive electrode active material and a second positive electrode active material. The analysis of the positive electrode active layer with a scanning type electron microscope indicates that, in a region with a test area of 50 μm×40 μm, the area of a bright region is 10% to 70% of the test area. By the positive electrode sheet and the application thereof according to the present invention, the sudden increase in DC resistance and voltage platform of a lithium ion battery can be enhanced and the performance of the lithium ion battery can be improved.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a positive electrode sheet and its uses. [Background technology]

[0002] With the rapid development of lithium-ion batteries in electric vehicles and large-scale energy storage, the demand for stable capacity and performance of lithium-ion batteries is increasing. Lithium iron phosphate batteries have the advantages of low cost and long life, and have become one of the mainstream lithium-ion battery developments. However, as the specific capacity of existing lithium iron phosphate materials approaches the theoretical value, the energy density of lithium iron phosphate batteries is also approaching its limit.

[0003] By substituting manganese for part of the iron in lithium iron phosphate to obtain lithium manganese iron phosphate, it is possible to improve the energy density of lithium manganese iron phosphate and obtain a higher voltage platform without increasing the cost of the positive electrode active material. However, there is a limit to how much energy density can be increased, and output tends to be unstable, which has limited the development of lithium-ion batteries. Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention provides a positive electrode sheet and its use, which can smooth the voltage platform of a lithium ion battery and mitigate the rapid increase in DC resistance in the manganese-iron conversion platform of lithium manganese iron phosphate material, thereby achieving high-performance lithium ion batteries. [Means for solving the problem]

[0005] To solve the above technical problems, the present invention provides a positive electrode sheet including at least a positive electrode current collector and a positive electrode active layer disposed on at least one side of the positive electrode current collector. The positive electrode active layer includes a first positive electrode active material and a second positive electrode active material. When the positive electrode active layer is analyzed with a scanning electron microscope, in a region with a test area of 50 μm × 40 μm, the area of the bright region is 10% to 70% of the test area.

[0006] In one embodiment of the present invention, the first positive electrode active material is LiMn m Fe 1-m PO4, where 0.4 ≦ m ≦ 0.7.

[0007] In one embodiment of the present invention, the second positive electrode active material is Li 1+a [Ni x Co y M z O 2-b A b and includes, 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. Here, the element M 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 element A includes one or more of F, N, Cl, S, and P.

[0008] In one embodiment of the present invention, the mass of the second positive electrode active material is 5% to 95% of the total mass of the first positive electrode active material and the second positive electrode active material.

[0009] In one embodiment of the present invention, the Dv90 of the first positive electrode active material is 1 μm to 15 μm. Dv90 indicates the particle size at the time of integrating 90% of the volume from the small particle size in the volume-based particle size distribution.

[0010] In one embodiment of the present invention, the Dv90 of the second positive electrode active material is 2 μm to 15 μm. Dv90 indicates the particle size at the time of integrating 90% of the volume from the small particle size in the volume-based particle size distribution.

[0011] In one embodiment of the present invention, the area of ​​the bright region is 10% to 50% of the test area.

[0012] In one embodiment of the present invention, the bright areas represent scanning electron microscope images corresponding to the second positive electrode active material.

[0013] In one embodiment of the present invention, the scanning electron microscope image is processed by image processing software. In the processing step, a selected area of ​​the positive electrode active layer is the test area, and after processing by the image processing software, the area with a gray scale threshold range of 135 to 254 is the bright area.

[0014] The present invention further provides a lithium ion battery including the above-described positive electrode sheet.

[0015] The present invention further provides an electronic device including the lithium ion battery described above. [Effects of the Invention]

[0016] To summarize the above, the present invention provides a positive electrode sheet and its uses. By controlling the area ratio of bright areas in a scanning electron microscope image of the cross section of the positive electrode sheet, the voltage platform of the lithium-ion battery can be smoothed, the rapid increase in DC resistance of the manganese iron conversion platform in the lithium manganese iron phosphate material can be mitigated, and the output of the lithium-ion battery can be stabilized. The rate performance, cycle performance, and safety performance of the lithium-ion battery can be improved, and the cost of the positive electrode active material can be reduced. The output of the lithium-ion battery can be stabilized, making it compatible with existing battery management systems, while avoiding the impact of overcharge / discharge on the life of the lithium-ion battery, thereby extending the service life of the lithium-ion battery. The capacity, output, cycle life, and safety performance of the lithium-ion battery can be improved, resulting in a high-performance lithium-ion battery. [Brief explanation of the drawings]

[0017] In order to more clearly describe the technical solutions of the embodiments of the present invention, the drawings necessary for describing the embodiments are briefly described below. Obviously, the drawings described below only show some embodiments of the present invention. Those skilled in the art can obtain other drawings based on these drawings without any creative efforts.

[0018] [Figure 1] 1 is a scanning electron microscope image of a cross section of a positive electrode sheet according to one embodiment of the present invention. [Figure 2] 1 is a scanning electron microscope image of a cross section of a positive electrode sheet according to one embodiment of the present invention, processed using image processing software. [Figure 3] 1 shows the voltage platform of the lithium-ion battery provided in Example 1. [Figure 4] 1 shows the voltage platform of the lithium-ion battery provided in Example 3. [Figure 5] 1 shows the DC resistance of the lithium ion battery provided in Example 1 at different states of charge. [Figure 6] 1 shows the DC resistance of the lithium ion battery provided in Example 2 at different states of charge. [Figure 7] 1 shows the DC resistance at different states of charge of the lithium ion battery provided in Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0019] The following specific examples are illustrative of embodiments of the present invention, and those skilled in the art can easily understand other advantages and effects of the present invention based on the disclosure of this specification. The present invention may be implemented or applied according to other different specific embodiments, and various details of the specification may be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0020] It should be noted that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, this description of the embodiments is provided so that this invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0021] The technical solutions of the present invention will be further described in detail with reference to the embodiments and accompanying drawings. Obviously, the described embodiments only form a part, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0022] The present invention provides an electronic device that includes at least one lithium ion battery, which serves to supply electrical energy. The electronic device may be a vehicle, a mobile phone, a portable device, a laptop, a boat, a spacecraft, an electronic toy, a power tool, or the like. In one embodiment of the present invention, the vehicle may be, for example, a new energy vehicle, such as a pure electric vehicle, a hybrid vehicle, or a range-extended vehicle. The spacecraft may include an airplane, a rocket, a space shuttle, a spaceship, or the like. The electronic toys may include game consoles, electric toy cars, electric toy boats, electric toy airplanes, and other stationary or mobile electric toys. The power tools may include electric metal cutting tools such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, as well as electric grinding tools, electric assembly tools, and electric railroad tools. The embodiments of the present application are not limited to the above-mentioned electric devices.

[0023] The present invention further provides a lithium ion battery including a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. A separator is disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte is filled between the positive electrode sheet, the negative electrode sheet, and the separator. The present invention does not particularly limit the type or shape of the lithium ion battery. In one embodiment of the present invention, the lithium ion battery is a primary battery or a secondary battery. The secondary battery may be, for example, a pouch-type battery, a prismatic battery, or a cylindrical battery. In this embodiment, a pouch-type secondary battery is used as an example, and the cells of the pouch-type secondary battery are formed by stacking or rolling.

[0024] The present invention provides a positive electrode sheet including a positive electrode current collector and a positive electrode active layer disposed on at least one surface of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material, an adhesive, and a conductive agent. The positive electrode active layer has a thickness greater than 30 μm, for example, 60 μm to 90 μm, or for example, 65 μm, 68 μm, 70 μm, 75 μm, 78 μm, 80 μm, 84 μm, or 87 μm. The positive electrode active material includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material includes, for example, lithium manganese iron phosphate, which has a general formula of LiMn m Fe 1-m PO4, where 0.4≦m≦0.7. The second positive electrode active material may include, for example, a ternary material, the general formula of which is, for example, Li 1+a [Ni x Co y M z ]O 2-b A bwhere 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 element M 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 element A 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 sheet, and the first positive electrode active material can improve the rate characteristics, cycle characteristics, and safety performance of the lithium ion battery, and can reduce the cost of the positive electrode active material compared to the case where it consists only of the second positive electrode active material.

[0025] In one embodiment of the present invention, as the positive electrode current collector, for example, a foil material such as nickel, titanium, aluminum, silver, stainless steel, or carbon that has been surface-treated can be used. In addition to the foil material, the positive electrode current collector may be used alone or in combination in various shapes such as a film material, a mesh material, a porous material, a foam material, or a non-woven fabric material. The thickness of the positive electrode current collector is, for example, 8 μm to 15 μm. In one embodiment of the present invention, the positive electrode current collector is, for example, an aluminum foil.

[0026] In one embodiment of the present invention, the adhesive is selected from one or more of polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinylether, polymethyl methacrylate (PMMA), ethylene propylene diene monomer (EPDM), polyhexafluoropropylene, and polymerized styrene butadiene rubber (SBR). The conductive agent is selected from one or more of conductive carbon black (Super P, SP), acetylene black, carbon nanotubes, and graphene.

[0027] In one embodiment of the present invention, the positive electrode active material is, for example, LiMn m Fe 1-m PO4 and LiNi x Co y Mn z The positive electrode slurry is a mixture of 0.4≦m≦0.7, 0.7≦x<1, 0≦y<0.3, and 0≦z<0.3. The adhesive is, for example, polyvinylidene fluoride, and the conductive agent is, for example, acetylene black. The positive electrode active material, acetylene black, and polyvinylidene fluoride are mixed in a mass ratio of, for example, 95:3:2, and then an organic solvent is added and stirred until the system becomes homogeneous to obtain a positive electrode slurry. The organic solvent is, for example, selected from N-methylpyrrolidone (NMP). The positive electrode slurry is uniformly applied to aluminum foil and dried. The dried aluminum foil is then processed, for example, by cold pressing, to obtain a positive electrode sheet.

[0028] 1 and 2, in one embodiment of the present invention, for example, a positive electrode sheet is cut with scissors into pieces of, for example, 2×5 mm 2A sample having dimensions of 100 μm is obtained. The cut surface of the sample can be polished to reduce the effects of cutting. For example, when processing the cut surface of the sample using ion milling, the cut surface is treated with a direct beam for 30 to 60 minutes. The processed sample is placed in a scanning electron microscope to obtain a scanning electron microscope image of the positive electrode active layer in the cross section of the positive electrode sheet. The scanning electron microscope image of the positive electrode active layer in the cross section of the positive electrode sheet has two levels of gradation (i.e., bright and dark areas with clear contrast). The white bright areas are "bright areas," and the rest are "dark areas." In this embodiment, the scanning electron microscope image can be processed using image processing software such as ImageJ software to measure the area ratio of bright areas in the scanning electron microscope image of the cross section of the positive electrode sheet, or it can be measured using equipment or processes known in the art. In one embodiment of the present invention, a selected area of ​​the positive electrode active layer in the processing step using ImageJ software is the test area. In this embodiment, the test area is, for example, an area of ​​50 μm × 40 μm. After processing with image processing software, the area where the gradation threshold range is 135 to 254 is defined as a bright area, and the rest is defined as a dark area. In one embodiment of the present invention, the area of ​​the bright area in the scanning electron microscope image is 10% to 70% of the test area, for example, 10% to 50% of the test area, or for example, 12% to 32% of the test area.

[0029] In one embodiment of the present invention, in a scanning electron microscope image of a cross section of a positive electrode sheet, the components primarily composed of the second positive electrode active material correspond to bright areas in the scanning electron microscope image, and the components primarily composed of the first positive electrode active material correspond to dark areas in the scanning electron microscope image. The dark areas may further include areas corresponding to conductive agents, adhesives, or voids. Controlling the area ratio of the bright areas can smooth the voltage platform of the lithium-ion battery, mitigate a sudden increase in the direct current resistance (DCR) of the LMFP's manganese-iron conversion platform, and stabilize the output of the lithium-ion battery.

[0030] In one embodiment of the present invention, the mass of the second positive electrode active material in the positive electrode active material is, for example, 5% to 95% of the total mass of the first positive electrode active material and the second positive electrode active material, with the remainder being the first positive electrode active material. That is, the mass of the second positive electrode active material is, for example, 5% to 95% of the total mass of the positive electrode active material. Furthermore, the mass of the second positive electrode active material is, for example, 25% to 75% of the total mass of the positive electrode active material, e.g., 30%, 50%, or 70% of the total mass of the positive electrode active material. By adjusting the mass ratio of the second positive electrode active material to the total mass of the positive electrode active material, the area ratio of the bright region in the formed positive electrode sheet can be controlled, thereby improving the performance of the lithium-ion battery.

[0031] In one embodiment of the present invention, the Dv90 of the first positive electrode active material is controlled to be, for example, 1 μm to 15 μm, or, for example, 2 μm to 10 μm. The Dv90 of the second positive electrode active material is controlled to be, for example, 2 μm to 15 μm, or, for example, 3 μm to 8 μm. Here, Dv90 refers to the particle size when the volume is accumulated from the smallest particle size to 90% in the volume-based particle size distribution. By controlling the Dv90 of the first and second positive electrode active materials, different cohesion characteristics can be created between the different active materials, thereby controlling the area ratio of the bright region of the formed positive electrode sheet and improving the performance of the lithium-ion battery, such as the voltage platform and DC resistance.

[0032] In one embodiment of the present invention, the negative electrode sheet includes, for example, a negative electrode current collector and a negative electrode active layer coated on at least one surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, an adhesive, a conductive agent, and a thickener. 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 foamed copper current collector, and a stainless steel current collector. The thickness of the negative electrode current collector is, for example, 8 μm to 15 μm. In one embodiment of the present invention, the negative electrode current collector is, for example, copper foil.

[0033] In one embodiment of the present invention, the negative electrode active material is selected from, for example, one or a combination of two or more of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide compounds, silicon carbide compounds, and lithium titanate. The adhesive is selected from, for example, one or more of polyvinylidene fluoride, polyamide, polypropylene, polyacrylate, polyvinyl ether, polymethyl methacrylate, polyhexafluoropropylene, and polymerized styrene butadiene rubber. The conductive agent is selected from, for example, one or more of conductive carbon black, acetylene black, carbon nanotubes, and graphene. In one embodiment of the present invention, the negative electrode current collector is selected from, for example, copper foil, the negative electrode active material is selected from, for example, graphite, the conductive agent is selected from, for example, acetylene black, the adhesive is selected from, for example, polymerized styrene butadiene rubber, and the thickener is selected from, for example, sodium carboxymethyl cellulose. In one embodiment of the present invention, graphite, acetylene black, polymerized styrene butadiene rubber, and sodium carboxymethyl cellulose are mixed in a mass ratio of 96:2:1:1, deionized water is added, and the mixture is thoroughly stirred to obtain a negative electrode slurry. The negative electrode slurry is then uniformly coated on copper foil, followed by drying, cold pressing, and other processes to obtain a negative electrode sheet.

[0034] In one embodiment of the present invention, the separator is, for example, a polyethylene (PE) film, a polypropylene (PP) film, a glass fiber film, or a composite film. The thickness of the separator is, for example, 9 μm to 15 μm. In one embodiment of the present invention, for example, a polyethylene film with a thickness of 8 μm to 10 μm is selected as the base film of the separator, and a nano-alumina coating with a thickness of 2 μm to 4 μm is applied to the base film to obtain the separator.

[0035] In one embodiment of the present invention, the electrolyte solution contains at least a non-aqueous solvent and a lithium salt. The non-aqueous solvent is selected from, for example, one or a combination of two or more of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC). In one embodiment of the present invention, the mass content of the non-aqueous solvent in the electrolyte solution is, for example, 60% to 85%. In one embodiment of the present invention, the lithium salt is selected from one or a combination of two or more of lithium hexafluorophosphate (LiPF), lithium tetrafluoroborate (LiBF), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium bis(oxalato)borate (LiBOB), lithium difluorobis(oxalato)phosphate (LiODFP), lithium difluoro(oxalato)borate (LiODFB), lithium difluorophosphate (LiPOF), and lithium trifluoromethanesulfonate (LiCFSO). In one embodiment of the present invention, the concentration of the lithium salt in the electrolyte is 0.1 mol / L to 2 mol / L. In one embodiment of the present invention, the electrolyte further contains, for example, an additive. The additive includes at least one film-forming additive, such as 1,3-propanesultone (PS), fluoroethylene carbonate (FEC), or vinylene carbonate (VC). The mass content of the additive in the electrolyte is 0.1% to 15% by weight. In one embodiment of the present invention, the electrolyte is selected from, for example, any suitable commercially available electrolyte.

[0036] In one embodiment of the present invention, the positive electrode sheet, separator, and negative electrode sheet are stacked in this order, with a separator placed between the positive and negative electrode sheets to separate them. The stack is then stacked or rolled to obtain a bare cell. The bare cell is then placed in an aluminum plastic film and baked to remove moisture, after which an electrolyte solution is injected and sealed. The finished lithium-ion battery is obtained through processes such as standing, hot pressing, cold pressing, battery molding, clamping, and capacity grading.

[0037] The present invention will be described in more detail with reference to the following examples. These examples should not be understood as limiting. Appropriate modifications are possible within the scope of the present invention, and all such modifications are within the technical scope of the present invention.

[0038] Example 1

[0039] Preparation of positive electrode sheet: The positive electrode active material is LiMn 0.6 Fe 0.4 PO4 and LiMn 0.6 Fe 0.4 The Dv90 of PO4 was 2.10 μm. The positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride as an adhesive were mixed in a mass ratio of 95:3:2. After uniformly mixing the positive electrode active material, adhesive, and conductive agent, N-methylpyrrolidone was added as a solvent and stirred until uniformly transparent to obtain a positive electrode slurry. The positive electrode slurry was uniformly applied to aluminum foil, dried, and then cold-pressed to obtain a positive electrode sheet. In a scanning electron microscope image of the cross section of the positive electrode sheet, the area of ​​the bright region was 1% of the test area.

[0040] Preparation of negative electrode sheet: Graphite as the negative electrode active material, acetylene black as the conductive agent, polymerized styrene butadiene rubber as the adhesive, and sodium carboxymethyl cellulose as the thickener were mixed in a mass ratio of 96:2:1:1. Deionized water was added and the mixture was stirred thoroughly to obtain negative electrode slurry. The negative electrode slurry was uniformly applied to copper foil, dried, and cold-pressed to obtain a negative electrode sheet.

[0041] Preparation of electrolyte: The nitrogen content in the glove box was 99.999%, the actual oxygen content in the glove box was 0.1 ppm, and the water content was 0.1 ppm. The non-aqueous solvent was composed of ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a mass ratio of 2:4:4. Lithium hexafluorophosphate was added as the lithium salt, and the concentration of the lithium salt was 1 mol / L. The electrolyte was obtained by adding 3% VC, with the total mass of the electrolyte being 100%.

[0042] Selection of separator: A 9 μm thick polyethylene film was selected as the base film, and a 3 μm nano-alumina film was coated on the base film to obtain a separator.

[0043] Battery fabrication: The positive electrode sheet, separator, and negative electrode sheet were stacked in this order, and a separator was placed between the positive electrode sheet and the negative electrode sheet to separate them. A bare cell was obtained by stacking them. The bare cell was wrapped in aluminum plastic film, dried to remove moisture, and then injected with electrolyte and sealed. A lithium-ion battery was obtained after going through processes such as leaving it to stand, hot pressing, cold pressing, battery molding, mold clamping, and capacity grading.

[0044] Example 2

[0045] The positive electrode active material is LiMn 0.6 Fe 0.4 PO4 and LiNi 0.8 Co 0.1 Mn 0.1 O2 mixture, LiMn 0.6 Fe 0.4 The Dv90 of PO4 is 2.10 μm, and LiNi 0.8 Co 0.1 Mn 0.1 The Dv90 of O2 is 5.20 μm, and that of LiMn 0.6 Fe 0.4 PO4 and LiNi 0.8 Co 0.1 Mn 0.1 The mass ratio of O2 was 8:2. In a scanning electron microscope image of the cross section of the obtained positive electrode sheet, the area of ​​the bright region was 12% of the test area. The other steps were the same as in Example 1.

[0046] Example 3

[0047] The positive electrode active material is LiMn 0.6 Fe 0.4 PO4 and LiNi 0.8 Co 0.1 Mn 0.1 O2 mixture, LiMn 0.6 Fe 0.4 The Dv90 of PO4 is 2.10 μm, and LiNi 0.8 Co 0.1 Mn 0.1 The Dv90 of O2 is 5.0 μm, and that of LiMn 0.6 Fe 0.4 PO4 and LiNi 0.8 Co 0.1 Mn 0.1 The mass ratio of O2 was 8:2. In a scanning electron microscope image of the cross section of the obtained positive electrode sheet, the area of ​​the bright region was 13% of the test area. The other steps were the same as in Example 1.

[0048] Example 4

[0049] The positive electrode active material is LiMn 0.6 Fe 0.4 PO4 and LiNi 0.8 Co 0.1 Mn 0.1 O2 mixture, LiMn 0.6 Fe 0.4 The Dv90 of PO4 is 2.10 μm, and LiNi 0.8 Co 0.1 Mn 0.1 The Dv90 of O2 is 5.20 μm, and that of LiMn 0.6 Fe 0.4 PO4 and LiNi 0.8 Co 0.1 Mn 0.1 The mass ratio of O2 was 7:3. In a scanning electron microscope image of the cross section of the obtained positive electrode sheet, the area of ​​the bright region was 18% of the test area. The other steps were the same as in Example 1.

[0050] In the present invention, performance tests were conducted on the lithium-ion batteries obtained in Examples 1 to 4. For example, the voltage platform and DCR of the lithium-ion batteries at different states of charge (SOC) were tested. In one embodiment of the present invention, the voltage platform test was conducted by charging the battery to 4.2 V at a constant current of 0.33 C, charging it at a constant voltage to a cutoff current of 0.05 C, leaving it for 30 minutes, and then discharging it to 2.5 V to 2.8 V at a constant current of 0.33 C to obtain a curve. In one embodiment of the present invention, the DCR test of the lithium-ion batteries at different SOCs was conducted by charging it to 4.2 V at a constant current of 0.33 C, charging it at a constant voltage to a cutoff current of 0.05 C, discharging it for 30 minutes, discharging it at a constant current of 0.33 C to a predetermined SOC, leaving it for a sufficient time, and then discharging it at a current of 1 C for 10 seconds to record the DCR. Here, 1 C represents the rated capacity of the lithium-ion battery.

[0051] 3 and 4, which respectively show the voltage platforms of the lithium ion batteries of Examples 1 and 3. The positive electrode active material is LiMn 0.6 Fe 0.4 In the case of PO4, the lithium-ion battery exhibited two voltage platforms near 3.8V and 3.3V, with a sudden voltage drop during discharge, resulting in unstable output. As can be seen from Figures 3 and 4, as the area of ​​the bright area in the cross-sectional scanning electron microscope image of the positive electrode sheet increased, the resulting lithium-ion battery's voltage platform became higher and the discharge platform became gentler, stabilizing the output of the lithium-ion battery and making it compatible with existing battery management systems. At the same time, this method avoided the impact of overcharge / discharge on the lifespan of the lithium-ion battery, thereby extending the service life of the lithium-ion battery.

[0052] 5 to 7, which show the DC resistances of the lithium-ion batteries of Examples 1, 2, and 4 at different states of charge, respectively. As can be seen from FIGS. 5 to 7, as the area of ​​the bright regions in the cross-sectional scanning electron microscope images of the positive electrode sheet increases, the DC resistance of the lithium-ion battery at each state of charge decreases. Furthermore, as the area of ​​the bright regions in the cross-sectional scanning electron microscope images of the positive electrode sheet increases, the change in the DC resistance of the lithium-ion battery at different states of charge becomes more gradual. This means that the rapid increase in the DCR of the manganese-iron conversion platform in the first positive electrode active material is mitigated, contributing to improvements in the capacity, output, cycle life, and safety performance of the lithium-ion battery, resulting in a high-performance lithium-ion battery.

[0053] To summarize the above, the present invention provides a cathode sheet and its use. Controlling the area ratio of bright areas in a cross-sectional scanning electron microscope image of the cathode sheet can smooth the voltage platform of a lithium-ion battery and mitigate the sudden increase in DC resistance of the manganese iron conversion platform in the lithium manganese iron phosphate material, thereby stabilizing the output of the lithium-ion battery. The second cathode active material can improve the energy density of the cathode sheet, while the first cathode active material can improve the rate characteristics, cycle characteristics, and safety performance of the lithium-ion battery, reducing the cost of the cathode active material. By adjusting the mass ratio of the second cathode active material to the total mass of the cathode active material and by adjusting the Dv90 of the first and second cathode active materials, the area ratio of bright areas in the formed cathode sheet can be controlled, thereby improving the performance of the lithium-ion battery. This stabilizes the output of the lithium-ion battery and makes it compatible with existing battery management systems, while avoiding the impact of overcharge / discharge on the life of the lithium-ion battery, thereby extending the service life of the lithium-ion battery. The capacity, output, cycle life, and safety performance of lithium-ion batteries can be improved, resulting in high-performance lithium-ion batteries.

[0054] The above description merely illustrates exemplary embodiments and examples of the present application and explains relevant technical principles. Those skilled in the art should understand that the scope of the disclosure contained in the present invention is not limited to the technical solution formed by a specific combination of the above technical features, but also encompasses other technical solutions formed by any combination of the above technical features or their equivalent features without departing from the concept of the present invention, for example, but not limited to, technical solutions formed by replacing the above features with technical features having similar functions disclosed in the present invention.

[0055] Except for the technical features described herein, the remaining technical features are known to those skilled in the art, and in order to highlight the innovative features of the present invention, the remaining features will not be described again in this specification. [Industrial Applicability]

[0056] The positive electrode sheet of the present invention can be applied to lithium ion batteries and further to electronic devices, and can smooth the voltage platform of the lithium ion battery and mitigate the rapid increase in DC resistance in the manganese-iron conversion platform of the lithium manganese iron phosphate material, thereby achieving high-performance lithium ion batteries.

Claims

1. a positive electrode current collector; a positive electrode active layer disposed on at least one side of the positive electrode current collector and including a first positive electrode active material and a second positive electrode active material; At least the first positive electrode active material comprises LiMn m Fe 1-m PO 4 , where 0.4≦m≦0.7; the second positive electrode active material comprises Li 1+a [NixCoyMz]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 element M comprises 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 element A comprises one or more of F, N, Cl, S, and P; The first positive electrode active material has a Dv90 of 1 μm to 15 μm, The second positive electrode active material has a Dv90 of 2 μm to 15 μm, The Dv90 indicates a particle size when the volume is accumulated from the smallest particle size to 90% in a volume-based particle size distribution, When the positive electrode active layer is analyzed by a scanning electron microscope, in a test area of ​​50 μm × 40 μm, the area of ​​a scanning electron microscope image corresponding to the second positive electrode active material is 10% to 70% of the test area. Positive electrode sheet.

2. The mass of the second positive electrode active material is 5% to 95% of the total mass of the first positive electrode active material and the second positive electrode active material. The positive electrode sheet according to claim 1 .

3. the area of ​​the scanning electron microscope image corresponding to the second positive electrode active material is 10% to 50% of the test area; The positive electrode sheet according to claim 1 .

4. the scanning electron microscope image is processed by image processing software, and in the processing step, a selected area of ​​the positive electrode active layer is the test area, and after processing by the image processing software, a region in the gradation threshold range of 135 to 254 is the scanning electron microscope image corresponding to the second positive electrode active material; The positive electrode sheet according to claim 1 .

5. A lithium ion battery comprising the positive electrode sheet according to any one of claims 1 to 4.

6. An electronic device comprising the lithium ion battery according to claim 5.

Images