Positive electrode sheet and lithium ion secondary battery

Abstract

The application relates to the battery technical field and discloses a positive plate and a lithium ion secondary battery. The positive plate comprises a first positive active material layer, the first positive active material layer comprises a first positive active material, a conductive agent and a binder, the first positive active material comprises a ternary material and a lithium manganate material, and the conductive agent comprises carbon nanotubes. According to the application, the particle size A of the first positive active material and the length L of the carbon nanotubes are regulated to meet the following conditions: 2.14 <= A <= 12.5, A = Dv99 / Dv50 PAM and 5 mu m >= L >= 30 mu m, so that the energy density, thermal stability, high-temperature cycle and rate performance of the battery can be simultaneously improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive plate and a lithium ion secondary battery. BACKGROUND

[0002] In new energy vehicles, the battery cost accounts for about 42% of the overall cost, and the positive material accounts for about 40% of the battery cost. It can be said that the positive material is an important variable affecting the cost of new energy vehicles.

[0003] The current commercialized positive active materials of lithium ion secondary batteries mainly include lithium manganate, lithium cobaltate, two or three element materials obtained by replacing part of cobalt in lithium cobaltate with nickel or manganese, etc. Compared with the high cost of lithium cobaltate, lithium manganate and three element materials have a price advantage. Moreover, lithium manganate batteries have almost no gas evolution during storage and charge-discharge process, and have good high temperature resistance, but the shortcoming is that the compaction density of lithium manganate is low, which is not conducive to the cycle performance of the battery. Although the three element material has high energy density, it is prone to thermal runaway at high temperature, resulting in poor safety performance of the battery. As can be seen, single positive active material has advantages and disadvantages, and cannot better meet the growing multifunctional requirements of different electric products. SUMMARY

[0004] The present application provides a positive plate, which aims to solve the problem that the existing lithium ion secondary battery containing single positive active material cannot balance high energy density, good safety performance, and high temperature cycle and rate performance to some extent.

[0005] The present application also provides a lithium ion secondary battery comprising the above positive plate.

[0006] According to the embodiments of the present application, in the first aspect, the present application provides a positive plate, comprising a positive current collector and a first positive active material layer arranged on at least one side surface of the positive current collector, the first positive active material layer comprising a first positive active material, a conductive agent and a binder;

[0007] The first positive active material comprises lithium manganate material and three element material, and the particle size A of the first positive active material satisfies: 2.14≤A≤12.5 and A=Dv99 / Dv50 PAM , wherein Dv50 PAM is the particle size corresponding to the cumulative volume distribution percentage of 50% of the first positive active material, 2μm≤Dv50 PAM ≤7μm; Dv99 is the particle size corresponding to the cumulative volume distribution percentage of 99% of the first positive active material, 15μm≤Dv99≤25μm;

[0008] The conductive agent includes carbon nanotubes, a length L of the carbon nanotubes satisfying 5 μm≤L≤30 μm.

[0009] In some optional embodiments, the content of the lithium manganate material is 10%-60%, preferably 30%-60%, based on the mass of the first positive electrode active material.

[0010] In some optional embodiments, the lithium manganate material has a chemical formula of LiNi a Mn 2-a O4, 0≤a≤0.2.

[0011] In some optional embodiments, the lithium manganate material includes a single-crystal lithium manganate material.

[0012] In some optional embodiments, the content of the ternary material is 40%-90%, preferably 40%-70%, based on the mass of the first positive electrode active material.

[0013] In some optional embodiments, the ternary material has a chemical formula of LiNi x Co y Mn 1-x-y O2, 0.8≤x<1, 0<y<0.2.

[0014] In some optional embodiments, the ternary material includes a single-crystal ternary material.

[0015] In some optional embodiments, the content of the carbon nanotubes is 0.1%-1.2%, preferably 0.1%-0.5%, based on the mass of the first positive electrode active material layer.

[0016] In some optional embodiments, the carbon nanotubes have a tube diameter of 7 nm-11 nm.

[0017] In some optional embodiments, the carbon nanotubes have a specific surface area of 200 m 2 / g-280 m 2 / g.

[0018] In some optional embodiments, the XRD pattern of the first positive electrode active material has characteristic peaks P1 and P3 at a position of 2θ of 35°-38° and characteristic peaks P2 and P4 at a position of 2θ of 42°-46°, peak intensities of P1, P2, P3 and P4 are I1, I2, I3 and I4 respectively, and satisfy: 0.05≤I1≤0.2, 0.05≤I2≤0.18, 0.1≤I3≤0.25, 0.2≤I4≤0.5.

[0019] In some alternative embodiments, the positive electrode sheet further comprises a plurality of positive electrode tabs and an insulation layer, the plurality of positive electrode tabs are arranged at intervals along the length direction of the positive electrode sheet and protrude from the positive electrode current collector; in the width direction of the positive electrode sheet, the insulation layer is arranged along the side edge of the positive electrode sheet, and the insulation layer and the plurality of positive electrode tabs are located on the same side of the positive electrode sheet, the insulation layer is adjacent to but does not overlap with the first positive electrode active material layer, and the insulation layer is also arranged on the part of the positive electrode tab close to the connection with the positive electrode current collector, and the head of the positive electrode tab is left with an empty foil area.

[0020] In some alternative embodiments, along the length direction of the positive electrode tab, the length of the insulation layer on the positive electrode tab is 4mm≥h1≥2mm.

[0021] In some alternative embodiments, the width of the insulation layer on the positive electrode current collector is 4mm≥h2≥0.5mm.

[0022] In some alternative embodiments, based on the mass of the insulation layer, the insulation layer comprises 70%-98% of borten, 0%-20% of aluminum oxide and 2%-10% of a binder.

[0023] In some alternative embodiments, the positive electrode sheet further comprises a second positive electrode active material layer, the first positive electrode active material layer is located between the positive electrode current collector and the second positive electrode active material layer; the second positive electrode active material layer comprises a ternary material.

[0024] In some alternative embodiments, the ratio of the volume median diameter D1 of the lithium manganate material in the first positive electrode active material layer to the volume median diameter D2 of the ternary material in the second positive electrode active material layer is 1.6-7.5, D1 is 8μm-15μm, and D2 is 2μm-5μm.

[0025] In some alternative embodiments, the second positive electrode active material layer comprises a ternary material and a high-temperature lithium manganate material, and the first positive electrode active material layer comprises a capacity-type lithium manganate material.

[0026] In some alternative embodiments, the ratio of the thickness of the second positive electrode active material layer to the thickness of the first positive electrode active material layer is 0.12-0.4.

[0027] In some alternative embodiments, the thickness of the first positive electrode active material layer is 40μm-100μm.

[0028] In some alternative embodiments, the thickness of the second positive electrode active material layer is 12μm-16μm.

[0029] According to the embodiments of the present application, in a second aspect, a lithium ion secondary battery is provided, comprising a negative electrode sheet, a separator, an electrolyte, and the positive electrode sheet of the first aspect of the present application.

[0030] In some optional embodiments, the negative electrode sheet comprises a negative electrode active material, the negative electrode active material comprises graphite with a carbon coating layer; the graphitization degree of the graphite is ≥ 93%, and / or the volume median diameter Dv50 of the graphite is 10 μm-25 μm, and / or the thickness of the carbon coating layer is 0.1 mm-1 mm. G In some optional embodiments, the volume median diameter Dv50 of the graphite is 10 μm-25 μm, and / or the thickness of the carbon coating layer is 0.1 mm-1 mm.

[0031] In some optional embodiments, the lithium ion secondary battery comprises a multi-tab winding structure.

[0032] In some optional embodiments, the negative electrode active material further comprises a silicon-based material, the content of the silicon-based material is 0-10% based on the mass of the negative electrode active material,

[0033] In some optional embodiments, the silicon-based material is SiC and / or SiO, the volume median diameter of the SiC is 5 μm-12 μm, and / or the volume median diameter of the SiO is 4 μm-10 μm.

[0034] In some optional embodiments, the electrolyte comprises propyl propionate, the content W of the propyl propionate in the electrolyte is 5%-60%, and satisfies: R=Dv50 G *W / Dv50 PAM , 0.07≤R≤7.5, 2 μm≤Dv50 PAM ≤7 μm; Dv50 G is 10 μm-25 μm.

[0035] In some optional embodiments, the separator comprises a base film, a ceramic layer and a glue coating layer are arranged on one side surface of the base film in sequence, and the ceramic layer is located between the base film and the glue coating layer.

[0036] In some optional embodiments, the glue coating layer comprises polyvinylidene fluoride and polymethyl methacrylate with a mass ratio of 1:9-9:1, and the thickness H of the glue coating layer and the area ratio S of the glue coating layer on the base film satisfy: Q=H / S, 0.7≤Q≤15, H is 0.5 μm-3 μm, and S is 20%-70%.

[0037] In some optional embodiments, the S is 30%-40%.

[0038] In some optional embodiments, the volume median diameter of the polyvinylidene fluoride is 0.3 μm-2.5 μm.

[0039] In some alternative embodiments, the volume median diameter of the polymethyl methacrylate is 0.1-0.2 μm.

[0040] In some alternative embodiments, the porosity of the separator is 40%-50%.

[0041] In some alternative embodiments, the air permeability of the separator is 70-170 sec / 100 cc.

[0042] In some alternative embodiments, the pore size of the base film is 32-58 nm.

[0043] In some alternative embodiments, the pore size of the base film is 45-58 nm.

[0044] In some alternative embodiments, the ceramic layer comprises boehmite and / or alumina.

[0045] In an alternative embodiment, the electrolyte comprises 10%-20% of lithium salt, 60%-80% of organic solvent and 1%-5% of additives, based on the mass of the electrolyte.

[0046] The technical solution of the present application has the following advantages:

[0047] The positive electrode sheet provided by the present application comprises a positive electrode current collector and a first positive electrode active material layer arranged on at least one side surface of the positive electrode current collector, wherein the first positive electrode active material layer comprises a first positive electrode active material, a conductive agent and a binder; the first positive electrode active material (referred to as a blended material) comprises a ternary material and a low-cost lithium manganate material, and the conductive agent comprises carbon nanotubes. The present application regulates the particle size of the positive electrode active material and the length L of the carbon nanotubes in the positive electrode sheet to satisfy the following conditions:

[0048] 2.14≤A≤12.5, A=Dv99 / Dv50 PAM ; 5 μm≥L≥30 μm.

[0049] In the first aspect, the large-particle lithium manganate material and the small-particle ternary material can be uniformly distributed in the positive electrode sheet after rolling, and the positive electrode sheet has a high compaction density (≥3.25 g / cm 3 ), thereby improving the energy density of the battery.

[0050] In the second aspect, the lithium manganate structure is more stable than the ternary material, the lithium manganate can effectively inhibit the problem of uncontrollable combustion of the ternary material, improve the thermal stability of the blended material, and optimize the safety performance of the battery.

[0051] In a third aspect, the ternary material is mixed with lithium manganate, reducing the contact between lithium manganate and electrolyte, which can inhibit manganese dissolution to a certain extent, and improve the high-temperature cycle and rate performance of the battery.

[0052] In a fourth aspect, carbon nanotubes are used as a conductive agent, the length of the carbon nanotubes is regulated, the ionic conductivity of the positive plate is increased, and the cycle performance of the battery is improved, which is crucial for the stability of the electrode material during the charging and discharging process and the long-term cycle life.

[0053] Additional aspects and advantages of the embodiments of the present application will be in part illustrated and described in the following description, or will be apparent from the description, or will be learned from the practice of the embodiments of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0054] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or the prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0055] Figure 1 is a SEM image of a cross section of a positive plate in an embodiment of the present application.

[0056] Figure 2 is a SEM image of a surface of a positive plate in an embodiment of the present application.

[0057] Figure 3 is a plan view of a positive plate in an embodiment of the present application.

[0058] Figure 4 is a SEM image of a surface of a negative plate in an embodiment of the present application.

[0059] Figure 5 is an XRD spectrum of a negative active material in an embodiment of the present application.

[0060] Figure 6 is a SEM image of a separator in an embodiment of the present application.

[0061] Figure 7 is a comparison diagram of XRD spectra of positive active materials in Example 1 and Comparative Example 1 of the present application.

[0062] Figure 8 is a certain local enlarged view of Figure 7 .

[0063] Figure 9 is another local enlarged view of Figure 7 .

[0064] Figure 10 is a cycle retention rate curve comparison chart of embodiment 1 and comparative example 1-2 of the present application.

[0065] In the drawings, the reference signs are explained as follows:

[0066] 1, positive electrode active material layer; 2, insulating layer; 3, positive electrode tab. DETAILED DESCRIPTION

[0067] The following examples are provided to better enable those skilled in the art to further understand the present application, and are not intended to limit the contents and protection scope of the present application, and do not limit the contents and protection scope of the present application, and any person who obtains any product same or similar to the present application under the enlightenment of the present application or combines the present application with other prior art features falls within the protection scope of the present application.

[0068] In the description of the present application, it should be noted that the terms "first" and "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance. In the technical solutions of the present application, the terms "blended material" and "first positive electrode active material" have the same meaning and can be replaced with each other.

[0069] The "range" disclosed in the present application is limited in the form of lower limit and upper limit, a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a particular range. The range defined in this way is inclusive of the end point values, and can be arbitrarily combined, i.e. any lower limit can be combined with any upper limit to form a range. For example, if a range of 1-100 is listed for a particular parameter, it is understood that a range of 1-100 is contemplated. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3, 4 and 5 are listed, the following ranges can all be contemplated: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.

[0070] The specific experimental steps or conditions not specified in the examples can be carried out according to the conventional experimental steps described in the literature in the art. The reagents or instruments not specified by the manufacturer are all conventional reagent products that can be obtained by purchase.

[0071] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as there is no conflict between them.

[0072] To solve the technical problems existing in the above-mentioned related technologies, according to the first aspect of the present application, a positive electrode sheet is provided, which comprises a positive electrode current collector and a first positive electrode active material layer arranged on at least one side surface of the positive electrode current collector, the first positive electrode active material layer comprising a first positive electrode active material, a conductive agent and a binder;

[0073] The first positive electrode active material includes lithium manganese oxide material and ternary material, and the particle size A of the first positive electrode active material satisfies: 2.14≤A≤12.5 and A=Dv99 / Dv50 PAM Among them, Dv50 PAM It is the particle size corresponding to the cumulative volume distribution percentage of the first positive electrode active material reaching 50%, 2μm≤Dv50 PAM ≤7μm; Dv99 is the particle size corresponding to the cumulative volume distribution percentage of the first positive electrode active material reaching 99%, 15μm≤Dv99≤25μm;

[0074] The conductive agent includes carbon nanotubes, and the length L of the carbon nanotubes satisfies: 5μm≤L≤30μm.

[0075] This application uses low-cost lithium manganese oxide as the first positive electrode active material by doping a ternary material with it. When the particle size A satisfies the above relationship, it can ensure that the large particles of lithium manganese oxide and the small particles of ternary material are evenly distributed in the rolled positive electrode sheet, which has the following advantages:

[0076] Firstly, it ensures that using blended materials as positive electrode materials results in a positive electrode sheet with a high compaction density (≥3.25 g / cm³). 3 This increases the energy density of the battery.

[0077] Secondly, lithium manganese oxide has a more stable structure than ternary materials. Lithium manganese oxide can effectively suppress the problem of flammability and runaway of ternary materials, improve the thermal stability of the mixed materials, improve overcharge and furnace temperature performance, and optimize the safety performance of the battery.

[0078] Thirdly, mixing ternary materials with lithium manganese oxide reduces the contact between lithium manganese oxide and the electrolyte, which can inhibit manganese dissolution to a certain extent and improve the high-temperature cycle and rate performance of the battery.

[0079] Meanwhile, this application uses carbon nanotubes as a conductive agent. Controlling the length L of the carbon nanotubes within the aforementioned range can significantly improve the battery's conductivity and cycle stability. This is because if the carbon nanotubes are too long (>30 μm), they are prone to entanglement, making it difficult to bond with the positive electrode active material and resulting in an unstable internal structure of the positive electrode sheet; if the carbon nanotubes are too short (<5 μm), it is difficult to form a three-dimensional conductive network, resulting in poor conductivity and affecting the stability and long-term cycle life of the electrode material.

[0080] by Figure 1 Taking an example, the positive electrode structure of this application will be illustrated. Figure 1As shown, the middle black line is the positive electrode current collector. On both surfaces of the positive electrode current collector, a first positive electrode active material layer is provided. In this positive electrode active material layer, the dark gray patches are lithium manganate materials, and the light gray patches represent ternary materials. It can be seen that the large-particle lithium manganate materials and the small-particle ternary materials are evenly distributed. As an example, the volume median diameter Dv50 of the first positive electrode active material PAM For example, it can be 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, etc., or within the range value composed of any two of the above values. The Dv99 of the first positive electrode active material can be 15μm, 17.5μm, 20μm, 22.5μm, 25μm, etc., or within the range value composed of any two of the above values. The Dv99 and Dv50 of the first positive electrode active material PAM The value of the ratio A can be 2.14, 3, 5, 7, 9, 11, 12.5, etc., or within the range value composed of any two of the above values.

[0081] See Figure 2 , the rod-like structure in the figure is a carbon nanotube, which forms a connected conductive network. As an example, the length L of the carbon nanotube can be 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, etc., or within the range value composed of any two of the above values.

[0082] In this application, the particle size of the sample can be measured by a conventional laser particle size analyzer, and the length of the carbon nanotube can be measured by a scanning electron microscope (SEM).

[0083] In some embodiments, based on the mass of the first positive electrode active material, the content of the lithium manganate material is 10%-60%, and the content of the ternary material is 40%-90%. As an example, the content of the lithium manganate material can be 10%, 20%, 30%, 40%, 50%, 60%, etc., or within the range value composed of any two of the above values. Correspondingly, the content of the ternary material can be 40%, 50%, 60%, 70%, 80%, 90%, etc., or within the range value composed of any two of the above values. It is found in this application that when the content of the lithium manganate material is within the above range, especially between 30%-60%, it can better balance the high energy density, rate, good safety and cycle performance of the battery.

[0084] It should be noted that the chemical formula of the ternary material in this application is LiNi x Co y Mn 1-x-y O2, 0.8≤x<1, 0<y<0.2. Selecting a high-nickel ternary material can ensure that the positive electrode active material has a high specific capacity. The chemical formula of the lithium manganate material in this application is LiNi aMn 2-a Doping nickel element in lithium manganate structure can improve capacity and cycle performance.

[0085] In some embodiments, the lithium manganate material comprises a single-crystal lithium manganate material, and the ternary material comprises a single-crystal ternary material. Single-crystal materials are selected because the crystal structure of single-crystal materials is highly ordered and has no grain boundaries. Compared with polycrystalline materials, single-crystal materials are more stable during the cycle charging and discharging process, and are less likely to break, corrode or peel due to the presence of grain boundaries, which helps to improve the cycle stability and safety of the battery.

[0086] In some embodiments, the content of the carbon nanotubes is 0.1%-1.2% based on the mass of the first positive electrode active material layer. For example, the content of the carbon nanotubes can be 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1.1%, 1.2%, or within a range formed by any two of the above values. The tube diameter of the carbon nanotubes is 7nm-11nm. For example, the tube diameter of the carbon nanotubes can be 7nm, 8nm, 9nm, 10nm, 11nm, or within a range formed by any two of the above values. The specific surface area of the carbon nanotubes is 200m 2 / g-280m 2 / g. For example, the specific surface area of the carbon nanotubes can be 200m 2 / g, 220m 2 / g, 240m 2 / g, 260m 2 / g, 280m 2 / g, or within a range formed by any two of the above values. The carbon nanotubes meeting the above conditions can further ensure the conductivity and cycle performance of the battery.

[0087] X-ray diffraction (XRD) technology has been widely used in the detection of positive active materials. Through XRD analysis, the crystal structure type of the material and the occupation and proportion of elements in the material can be determined. In the XRD spectrum of the first positive active material formed by the ternary material mixed with lithium manganate material, characteristic peaks P1 and P2 of lithium manganate appear at positions of 2θ of 35°-38° and 42°-46°, the crystal face index of P1 is (311), and the crystal face index of P2 is (400). At the same time, characteristic peaks P3 and P4 of the ternary material also appear at positions of 2θ of 35°-38° and 42°-46°, the crystal face index of P3 is (101), and the crystal face index of P2 is (104). Among them, the peak intensity of P1 is I1, the peak intensity of P2 is I2, the peak intensity of P3 is I3, and the peak intensity of P4 is I4, and satisfy: 0.05≤I1≤0.2, 0.05≤I2≤0.18, 0.1≤I3≤0.25, 0.2≤I4≤0.5. The first positive active material with the above XRD diffraction peak characteristics can better balance the high energy density and rate of the battery, good safety and cycle performance.

[0088] For example, I1 can be 0.05, 0.1, 0.15, 0.2, etc. or within the range of any two values, I2 can be 0.05, 0.08, 0.1, 0.13, 0.15, 0.18, etc. or within the range of any two values, I3 can be 0.1, 0.13, 0.15, 0.18, 0.2, 0.22, 0.25, etc. or within the range of any two values, and I4 can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, etc. or within the range of any two values.

[0089] It can be understood that in high-power applications, the battery needs to be able to quickly provide large current, and the multi-tab structure distributes more contact points on the positive / negative electrode sheet of the battery, which helps to reduce the risk of current concentration in a single point, thereby improving the current carrying capacity and stability of the battery; and the multi-tab design can also effectively reduce the contact resistance between the battery and the external circuit, which is particularly important for high-power batteries, because low resistance can reduce energy loss and heat generation, improve the efficiency and power output capacity of the battery; in addition, high-power batteries generate more heat when working, and the multi-tab structure can provide more heat dissipation surface to help more effective heat dissipation, thereby keeping the temperature of the battery within a controllable range, prolonging the service life of the battery and ensuring safety; in addition, the multi-tab design can also generally improve the mechanical strength and durability of the battery, especially when experiencing repeated charge and discharge cycles and vibration environment, it can reduce the risk of fatigue and damage of the connection part, thereby prolonging the service life of the battery. Therefore, in order to ensure the uniformity of current distribution during high-power operation, reduce contact resistance, optimize thermal management, and improve the reliability and durability of the battery, in some embodiments of the present application, the lithium ion secondary battery adopts a multi-tab winding structure.

[0090] Specific to the positive electrode sheet of the present application, please refer to Figure 3 , which includes a plurality of positive tabs 3, a plurality of said positive tabs 3 are arranged at intervals along the length direction of the positive electrode sheet and protrude from the positive current collector; the surface of the positive current collector is coated with a positive active material layer 1, and an insulating layer 2 is arranged on the positive electrode sheet, the insulating layer 2 is arranged along the side edge of the positive electrode sheet in the width direction of the positive electrode sheet, and the insulating layer 2 and a plurality of the positive tabs 3 are located on the same side of the positive electrode sheet, and the insulating layer 2 is adjacent to but does not overlap with the positive active material layer 1. The surface of the positive tab 3 is also covered with the insulating layer 2, and the head of the positive tab 3 is left with an empty foil area, that is, the positive tab 3 is only provided with the insulating layer 2 at its root (close to the connection between the positive tab and the positive current collector), and the head of the positive tab 3 (away from the connection between the positive tab and the positive current collector) is not provided with the insulating layer 2.

[0091] In this way, the burrs generated after die cutting of the positive current collector can be avoided from contacting and short-circuiting with the negative electrode sheet, causing the battery to fail, and at the same time, the separator can also avoid not completely covering the positive electrode sheet after winding, causing the positive tab and the negative electrode sheet to contact and short-circuit, causing the battery to fail.

[0092] As shown in Figure 3 , h5 is the width of the positive active material layer, h3+h4+h5 is the projection of the corresponding negative electrode coating in the width direction on the positive electrode sheet, h3 is the overhang area, that is, the part of the negative electrode coating that exceeds the positive electrode coating in the width direction, h4 is the overhang area away from the end of the positive tab, h3≥2.5mm, h4≥2.5mm, and h3 and h4 can be the same or different.

[0093] In some embodiments, the length h1 of the insulating layer on the positive tab along the length direction of the positive tab satisfies 4mm≥h1≥2mm, and the width h2 of the insulating layer on the positive current collector along the width direction of the positive tab satisfies 4mm≥h2≥0.5mm. In this way, the battery can be better ensured from failure due to short circuit caused by contact between the positive and negative electrodes. For example, h1 can be 2mm, 2.5mm, 3mm, 3.5mm, 4mm, or within a range defined by any two of the above values, and h2 can be 0.5mm, 1mm, 2mm, 3mm, 4mm, or within a range defined by any two of the above values.

[0094] Regarding the composition of the insulating layer, in some embodiments, the insulating layer includes 70%-98% boron stone, 0%-20% aluminum oxide, and 2%-10% binder, based on the mass of the insulating layer. The cost of boron stone is lower, the hardness is lower, the mechanical wear is smaller, and the coating area is 1.25 times that of Al2O3 under the same weight. Therefore, the proportion of boron stone is the largest. Al2O3 can neutralize free HF in the electrolyte, improve the acid resistance and safety performance of the battery, and at the same time, in order to ensure that the insulating layer has good adhesion on the positive current collector, PVDF can be selected as the binder.

[0095] To further improve the cycle performance of the battery, the positive tab of the present application can adopt a double-layer coating technology, i.e., the positive tab further includes a second positive active material layer, the first positive active material layer is located between the positive current collector and the second positive active material layer, and the second positive active material layer includes a ternary material.

[0096] In some embodiments, the ratio of the volume median diameter D1 of lithium manganate in the first positive active material layer to the volume median diameter D2 of the ternary material in the second positive active material layer is 1.6-7.5, and D1 is 8-15μm and D2 is 2-5μm. This design can improve the cycle performance of the battery.

[0097] For example, the volume median diameter D1 of lithium manganate can be 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, or within a range defined by any two of the above values, the volume median diameter D2 of the ternary material in the second positive active material layer can be 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, or within a range defined by any two of the above values, and the ratio of D1 to D2 is, for example, 1.6, 2.5, 4, 5.5, 6, 7.5, or within a range defined by any two of the above values.

[0098] In other embodiments, the first positive electrode active material layer and the second positive electrode active material layer can also simultaneously use a ternary material mixed with lithium manganate. When the first positive electrode active material layer uses a ternary material mixed with capacity-type lithium manganate, the capacity-type lithium manganate has a higher manganese content, which can provide greater capacity, ensuring that the battery energy density is not lost, and because of the presence of the ternary material, the contact between the lithium manganate and the electrolyte is reduced, effectively inhibiting the dissolution of manganese elements in the lithium manganate; when the second positive electrode active material layer uses a ternary material mixed with high-temperature lithium manganate, the specific surface area of the high-temperature lithium manganate is smaller, which can improve the high-temperature cycle performance. The high-temperature lithium manganate can also be doped and / or coated with metal elements to improve the structural stability.

[0099] It should be noted that in the present application, "capacity-type lithium manganate" can be understood as doping lithium manganate with metal elements, and the doping amount of metal elements is in the range of 100-500 ppm; "high-temperature lithium manganate" can be understood as doping lithium manganate with metal elements, and the doping amount of metal elements is in the range of 2000-3000 ppm. Common metal elements include aluminum (Al), chromium (Cr), copper (Cu), zirconium (Zr), magnesium (Mg), cobalt (Co), titanium (Ti). The capacity of the capacity-type lithium manganate is about 5-10% higher than that of the high-temperature lithium manganate.

[0100] In summary, if the first positive electrode active material layer uses a ternary material mixed with capacity-type lithium manganate, the energy density of the battery is improved, and if the second positive electrode active material layer uses a ternary material mixed with high-temperature lithium manganate, the stability and cycle performance of the battery in a high-temperature environment are improved. This design ensures that the battery can maintain a high energy output and a longer service life during long-term use.

[0101] To reduce the cost of the battery, the thickness of the first positive electrode active material layer is much greater than the thickness of the second positive electrode active material layer. For example, the thickness of the first positive electrode active material layer is 40 μm-100 μm, and the thickness of the second positive electrode active material layer is 12 μm-16 μm. As an example, the thickness of the first positive electrode active material layer can be 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, etc., or within a range formed by any two of the above values, and the thickness of the second positive electrode active material layer can be 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, etc., or within a range formed by any two of the above values. It can be understood that in some embodiments, the ternary material in the first positive electrode active material layer and the second positive electrode active material layer uses the same material.

[0102] According to a second aspect of the present application, a lithium ion secondary battery is provided, which includes a negative electrode sheet, a separator, an electrolyte, and a positive electrode sheet according to the first aspect of the present application.

[0103] It is understood that the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material. In some embodiments of this application, the negative electrode active material includes graphite with a carbon coating layer, wherein the volume median diameter Dv50 of the graphite is... G The thickness is 10μm-25μm, for example, 10μm, 13μm, 15μm, 18μm, 20μm, 23μm, 25μm, etc., or within the range of any two of the above values, thereby ensuring that the graphite anode has a high compaction density.

[0104] like Figure 4 As shown, there are no obvious gaps between the graphite particles. In other embodiments, the graphite has a graphitization degree of ≥93%. Graphite with a high graphitization degree has better electrical conductivity, thermal stability, and mechanical strength, which can improve battery performance and cycle life.

[0105] Please refer to Figure 5 The JADE software analysis shows that d002 ≥ 3.354 nm, indicating that amorphous carbon is coated on the graphite surface, increasing the overall interlayer spacing of the anode material and thus benefiting Li. + Diffusion within it. The thickness of the carbon coating is 0.1mm-1mm, for example, 0.1mm, 0.3mm, 0.5mm, 0.7mm, 1mm, etc., or within any two of the above values.

[0106] To improve the energy density of the battery, in some embodiments of this application, the negative electrode active material may further include a silicon-based material, wherein the content of the silicon-based material is 0-10% based on the mass of the negative electrode active material. Specifically, the silicon-based material is SiC and / or SiO, wherein the volume median diameter of the SiC is 5μm-12μm, and the volume median diameter of the SiO may be 4μm-10μm.

[0107] As an example, the doping amount of silicon-based materials can be 0%, 2%, 4%, 6%, 8%, 10%, etc., or within any range of two of the above values. The median volume diameter of SiC can be, for example, 5μm, 7μm, 9μm, 10μm, 12μm, etc., or within any range of two of the above values. The median volume diameter of SiO can be, for example, 4μm, 6μm, 8μm, 10μm, etc., or within any range of two of the above values.

[0108] To better improve the low-temperature kinetic performance of the battery, in some embodiments of the present application, propyl propionate (PP) is added to the electrolyte, and the content W of propyl propionate in the electrolyte is 5%-60% based on the mass of the electrolyte. For example, the content W of propyl propionate can be 5%, 10%, 15%, 20%, 30%, 40%, 50%, 60%, or within a range formed by any two of the above values. Adding propyl propionate to the electrolyte can ensure that the electrolyte maintains good viscosity at low temperatures, ensuring that lithium ions can migrate smoothly and freely.

[0109] To further improve the low-temperature charge-discharge capacity and excellent cycle life of the battery, the volume median diameter Dv50 of the graphite G , the content W of propyl propionate in the electrolyte, and the volume median diameter Dv50 of the positive active material PAM satisfy: R=Dv50 G *W / Dv50 PAM , 0.07≤R≤7.5, Dv50 G is 10 μm-25 μm, 2 μm≤Dv50 PAM ≤7 μm.

[0110] It should be noted that when Dv50 PAM is small, the lithium ion diffusion path is short, the polarization is small, and the electrolyte is also easily attached to the surface layer of the original particles, reducing concentration polarization. However, when Dv50 PAM is too large, the lithium ion diffusion path is long, and during battery operation and discharge, lithium ions from the negative electrode to the positive electrode cannot compensate for the electrons flowing from the negative electrode to the positive electrode, resulting in an excess of electrons in the positive electrode, causing the electrode potential to shift negatively, causing the discharge voltage platform to be low or even unable to discharge.

[0111] It should be noted that when the positive electrode sheet includes two layers of positive active material layers, Dv50 PAM in the present application refers to the volume median diameter of the positive active material in the first positive active material layer, because the second positive active material layer has a small thickness and has a small impact.

[0112] For example, R can be 0.07, 0.5, 1, 2, 3, 4, 5, 6, 7.5, or within a range formed by any two of the above values; the volume median diameter Dv50 of the graphite G may be 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, or within a range formed by any two of the above values.

[0113] In order to ensure the high-power discharge characteristics while ensuring the cycle performance of the battery, in some embodiments of the present application, a large-pore separator is used, the air permeability of the separator is 70 sec / 100cc-170 sec / 100cc, for example, it can be 70 sec / 100cc, 100 sec / 100cc, 130 sec / 100cc, 150 sec / 100cc, 170 sec / 100cc, etc. or within the range value composed of any two of the above values, the porosity of the separator is 40%-50%, for example, it can be 40%, 42%, 45%, 48%, 50%, etc. or within the range value composed of any two of the above values.

[0114] Specifically, the separator includes a base film, the pore size of the base film is 32nm-58nm, for example, it can be 32nm, 35nm, 38nm, 42nm, 45nm, 48nm, 52nm, 55nm, 58nm, etc. or within the range value composed of any two of the above values, a glue coating layer and a ceramic layer are laminated on one side surface of the base film, the ceramic layer includes boehmite and / or aluminum oxide, and the ceramic layer is located between the base film and the glue coating layer.

[0115] Further, the glue layer comprises polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA) with a mass ratio of 1:9-9:1, and the mass ratio of PVDF to PMMA can be 1:9, 3:7, 5:5, 7:3, 9:1, or within a range formed by any two of the above values; wherein the volume median diameter of PVDF is 0.3-2.5 μm, such as 0.3 μm, 0.7 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or within a range formed by any two of the above values, and the volume median diameter of PMMA is 0.1-0.2 μm, such as 0.1 μm, 0.12 μm, 0.15 μm, 0.18 μm, 0.2 μm, or within a range formed by any two of the above values. The thickness H of the glue layer and the area ratio S of the glue layer on the base film satisfy: Q=H / S, 0.7≤Q≤15, H is 0.5-3 μm, and S is 20%-70%. The glue layer thus designed not only has excellent porosity, which can meet the requirements of high-power discharge, but also exhibits good adhesion in the battery, thereby ensuring the cycle performance of the battery. As an example, the thickness H of the glue layer can be 0.5 μm, 1.0 μm, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, or within a range formed by any two of the above values, the area ratio S of the glue layer on the base film can be 20%, 30%, 40%, 50%, 60%, 70%, or within a range formed by any two of the above values, and the value of Q can be 0.7, 2, 4, 6, 8, 10, 12, 15, or within a range formed by any two of the above values.

[0116] It should be noted that the above is the front structure of the separator, and only a glue layer is provided on the back of the separator. Please refer to Figure 6 wherein Figure 6 a is the front structure of the separator, Figure 6 b is the back structure of the separator.

[0117] In some embodiments of the present application, the electrolyte comprises 10-20% lithium salt, 60-80% organic solvent and 1-5% additive, based on the mass of the electrolyte. As an example, the lithium salt comprises at least one of lithium hexafluorophosphate (LiFP6), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorooxalato)borate (LiODFB), lithium bis(fluorosulfonyl)imide (LiFSI); the organic solvent comprises at least one of propyl propionate (PP), ethyl propionate (EP), ethyl difluoroacetate (EDFA), ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC); and the additive comprises at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC) and 1,3-propane sultone (1,3-PS) to ensure the film stability of the battery material. The electrolyte formed by the low-cost solvent and the lithium salt has good ionic conductivity, stable electrochemical window and long-term cycle life while reducing the cost of the electrolyte.

[0118] In summary, the lithium ion secondary battery composed of the above positive electrode sheet, negative electrode sheet, separator and electrolyte has high power discharge characteristics, excellent electrochemical performance and good low-temperature charge and discharge capability, while the battery cost is significantly reduced.

[0119] The present application will be further described in detail below in conjunction with specific examples, which should not be construed as limiting the scope of the present application. In all examples and comparative examples of the present application, the unit % represents the mass percentage.

[0120] Example 1

[0121] The present embodiment provides a preparation method of a lithium ion secondary battery, comprising the following steps:

[0122] (1) Preparation of the positive electrode sheet

[0123] The single-crystal ternary material (chemical formula LiNi 0.8 Co 0.1 Mn 0.1 O2) and single-crystal lithium manganate (chemical formula LiMn2O4) are mixed uniformly at a mass ratio of 7:3 to form a positive electrode active material, which has a Dv50 PAM of 5.1 μm and a Dv99 of 24.5 μm. The XRD pattern of the positive electrode active material is shown in Figures 7-9(Atlas is normalized with the highest peak intensity as the denominator), it can be seen from the figure that characteristic peaks P1 and P3 appear at 2θ of 35°-38° position and characteristic peaks P2 and P4 appear at 42°-46° position, P1 peak intensity I1 = 0.127, P2 peak intensity I2 = 0.139, P3 peak intensity I3 = 0.2, P4 peak intensity I4 = 0.4.

[0124] The above positive electrode active material, conductive carbon black and binder PVDF are uniformly dispersed in N-methyl pyrrolidone at a mass ratio of 98.5:0.5:1 to obtain a uniform slurry. Carbon nanotubes (length 5.3 μm, tube diameter 7 nm, specific surface area 240 m 2 / g, volume median diameter 1 μm) are added to the slurry, and the amount of carbon nanotubes added accounts for 0.5% of the total mass of the positive electrode active material layer, and the solid content of the final positive electrode slurry is controlled at 70%. Subsequently, the positive electrode slurry is uniformly coated on both surfaces of the aluminum foil, and then dried and rolled to finally obtain the positive electrode sheet. A ceramic adhesive (composed of 80% boehmite, 10% alumina and 10% binder PVDF by mass ratio) is coated on the tab end of the positive electrode sheet, so that the formed insulating layer is in contact with but not overlapped with the positive electrode active material layer on one side in the width direction of the insulating layer, and the other end extends to the root of the positive electrode tab, the length h1 of the insulating layer on the positive electrode tab is 2.5 mm, and the width h2 of the insulating layer on the positive electrode current collector is 0.8 mm.

[0125] (2) Preparation of negative electrode sheet

[0126] The negative electrode active material, binder SBR, thickening agent CMC and conductive carbon black are dispersed in deionized water at a mass ratio of 96:1.5:1:1.5 to obtain a uniform negative electrode slurry, and the solid content of the negative electrode slurry is 50%. The negative electrode slurry is uniformly coated on both surfaces of the copper foil, and then dried and rolled to obtain the negative electrode sheet. The negative electrode active material is composed of 95% graphite with a carbon coating layer and 5% silicon carbide, the volume median diameter of the silicon carbide is 10 μm, the graphitization degree of the graphite is ≥93%, the volume median diameter Dv50 G of the graphite is 10 μm, and the thickness of the carbon coating layer is 0.5 mm.

[0127] (3) Selection of separator

[0128] A large-pore separator is used, the base film has a pore size of 45-58 nm, the base film is coated with a boehmite ceramic layer and a water-based adhesive layer, the adhesive layer is coated with a mixture of water-based PVDF (volume median diameter 0.3 μm) and PMMA (volume median diameter 0.2 μm) at a mass ratio of 5:5, the adhesive layer coverage is 30%, and the adhesive layer thickness is 1.5 μm. The air permeability of the separator is 70 sec / 100cc, and the porosity of the separator is 45%.

[0129] (4) Preparation of electrolyte

[0130] LiFP6, PP, DMC and FEC were mixed uniformly at a mass ratio of 15:60:22:3 to form an electrolyte.

[0131] (5) Preparation of battery

[0132] After the above prepared positive electrode sheet, negative electrode sheet were cut, sheeted, wound, packaged, baked, injected, formed and tested, a multi-tab wound lithium ion secondary battery was obtained.

[0133] The preparation method of lithium ion secondary battery in Examples 2-9 was the same as that in Example 1, and the differences were shown in Table 1.

[0134] Table 1

[0135]

[0136]

[0137] Example 10

[0138] The positive electrode sheet used a double-layer coating technology, and the first positive electrode active material layer selected the same positive electrode active material as that in Example 1, wherein the volume median diameter D1 of lithium manganate was 15 μm, and the thickness was 100 μm.

[0139] The second positive electrode active material layer was a ternary material (chemical formula LiNi 0.8 Co 0.1 Mn 0.1 O2) with a volume median diameter D2 of 2 μm, and the thickness was 12 μm.

[0140] The remaining steps and process parameters were the same as those in Example 1.

[0141] Example 11

[0142] The positive electrode sheet used a double-layer coating technology, and the first positive electrode active material layer selected the same positive electrode active material as that in Example 1, wherein the volume median diameter D1 of lithium manganate was 12 μm.

[0143] The second positive electrode active material layer was a ternary material (chemical formula LiNi 0.8 Co 0.1 Mn 0.1 O2) with a volume median diameter D2 of 2.5 μm, and the thickness was 15 μm.

[0144] The remaining steps and process parameters were the same as those in Example 1.

[0145] Example 12

[0146] The positive electrode sheet is coated with a double layer, the first positive electrode active material layer is made of the same positive electrode active material as in Example 1, wherein the volume median diameter D1 of the lithium manganate is 8 μm and the thickness is 40 μm;

[0147] The second positive electrode active material layer is a ternary material (chemical formula LiNi 0.8 Co 0.1 Mn 0.1 O2) with a volume median diameter D2 of 5 μm and a thickness of 16 μm.

[0148] The remaining steps and process parameters are the same as in Example 1.

[0149] Example 13

[0150] The positive electrode sheet is coated with a double layer, the first positive electrode active material layer is made of a positive electrode active material comprising 70% of a ternary material (chemical formula LiNi 0.8 Co 0.1 Mn 0.1 O2) and 30% of a capacity-type lithium manganate (chemical formula LiMg 0.001 Mn 1.999 O4), and has a thickness of 70 μm.

[0151] The second positive electrode active material layer is made of a positive electrode active material comprising 60% of a ternary material (chemical formula LiNi 0.8 Co 0.1 Mn 0.1 O2) and 40% of a high-temperature-type lithium manganate (chemical formula LiMg 0.05 Mn 1.95 O4), and has a thickness of 14 μm.

[0152] The remaining steps and process parameters are the same as in Example 1.

[0153] The lithium ion secondary batteries in Examples 14-21 are prepared in the same way as in Example 1, except for the differences shown in Table 2.

[0154] Table 2

[0155] Dv50 G (μm) W R H (pm) S Q Example 14 15 6.8％ 0.2 1.5 30％ 5.0 Example 15 25 60％ 2.94 1.5 30％ 5.0 Example 16 58.7 33％ 3.8 1.5 30％ 5.0 Example 17 76.5 50％ 7.5 1.5 30％ 5.0 Example 18 10 60％ 1.18 3 0.3 10 Example 19 10 60％ 1.18 3 0.2 15 Example 20 10 60％ 1.18 0.5 0.7 0.7 Example 21 10 60％ 1.18 1 0.4 2.5

[0156] Comparative Example 1

[0157] The difference from Example 1 is that only a ternary material is used as the positive electrode active material in this comparative example, and the XRD pattern thereof is shown in Figures 7-9 (normalized with the highest peak intensity as the denominator).

[0158] As can be seen from the figure, characteristic peaks P3 and P4 appear at positions of 2θ = 35°-38° and 42°-46°, respectively, the P3 peak intensity I3 = 0.32, and the P4 peak intensity I4 = 0.60.

[0159] Comparative Example 2

[0160] The difference from Example 1 is that only lithium manganate is used as the positive electrode active material in this comparative example.

[0161] Comparative Example 3

[0162] The difference from Example 1 is that the particle size Dv50 of the positive electrode active material in this comparative example is 1.9 μm, so A is 12.9. PAM

[0163] Comparative Example 4

[0164] The difference from Example 1 is that the particle size Dv99 of the positive electrode active material in this comparative example is 10.2 μm, so A is 2.

[0165] Comparative Example 5

[0166] The difference from Example 1 is that the length L of the carbon nanotube in this comparative example is 32 μm.

[0167] Comparative Example 6

[0168] The difference from Example 1 is that the length L of the carbon nanotube in this comparative example is 4 μm.

[0169] Test Example

[0170] 1. Particle size test

[0171] Select ultrapure water as the dispersion medium, add 1 g of the sample to be tested into 1 L of the dispersion medium, and ultrasonicate for 60 min to make the sample fully dispersed to form a uniform suspension or dispersion. Pour the prepared sample suspension or dispersion into a sample cell, turn on the laser particle size meter to test, and after the test is completed, the computer generates a particle size distribution graph and a related data report.

[0172] 2. Coating thickness test

[0173] Measure the electrode sheet after the coating roll is pressed using a micrometer, randomly select 10 positions on the electrode sheet to test, and after the test, take the average of the 10 data to be the thickness H1 of the electrode sheet. Measure the thickness H2 of the current collector before coating using the micrometer according to the above method, and then the thickness value of the coating is H1-H2.

[0174] 3. Compaction density test

[0175] Compaction density = (total weight of the electrode sheet - weight of the current collector) / [area of the electrode sheet * (thickness of the electrode sheet - thickness of the current collector)].

[0176] 4. XRD test

[0177] ​XRD diffractometer model is Bruker D8 Advance, and the Highscore software is used to refine the XRD pattern after testing (Rietvald method).

[0178] 5. Volume energy density test

[0179] The lithium ion secondary battery is charged to full at 25°C under constant current and constant voltage, and then discharged to 3V at 0.2C. The discharged capacity is recorded as the discharge capacity (also known as battery capacity), and the discharged energy is recorded as the discharge energy. The average discharge voltage is calculated as discharge energy / discharge capacity;

[0180] The lithium ion secondary battery is charged to 30% SOC at 25°C, and the battery thickness is tested using 600g PPG;

[0181] The volume energy density is calculated as battery capacity*platform voltage / battery length / width / thickness.

[0182] 6. Low temperature discharge performance test

[0183] The lithium ion secondary battery is charged to full at 25°C under constant current and constant voltage, and then discharged to 3V at 0.2C. The discharged capacity is recorded as the initial discharge capacity. The lithium ion secondary battery is charged to full at 25°C under constant current and constant voltage, and then placed in a low temperature box at -20°C. After 2h, it is discharged to 3V at 10C. The discharged capacity is recorded as the final discharge capacity. The low temperature discharge capacity retention rate is calculated as final discharge capacity / initial discharge capacity*100%.

[0184] 7. Cycle performance test

[0185] The lithium ion battery is tested at 35°C under 1C / 5C cycle. Specifically, it is charged to 4.2V at 1C, cut off at 0.05C, discharged to 3V at 5C, and repeated 500 times. Every 100 cycles, it is charged to 4.2V at 0.2C, cut off at 0.05C, and discharged to 3V at 0.2C. The capacity retention rate and thickness expansion rate are tested for 500 cycles.

[0186] 500th cycle capacity retention rate = residual capacity / initial capacity*100%.

[0187] 8. Overcharge performance test

[0188] The lithium ion secondary battery is tested at 25°C under 2C-5V overcharge. After 0.5C emptying, it is charged to 5V at 2C, and then stopped at constant voltage for 7h or the temperature decreases to room temperature.

[0189] 9. Furnace temperature test

[0190] The lithium ion secondary battery was charged at 25°C to the upper limit voltage at 0.5C, and the battery was put into a temperature oven, which was heated to 140°C±2°C at a rate of 5°C / min, and kept at this temperature for 60 min.

[0191] The test results are shown in Table 3, and the cycle retention rate curve is shown in Figure 10 .

[0192] Table 3

[0193]

[0194]

[0195] As can be seen from Table 3, after the positive electrode active material formed by blending an appropriate amount of lithium manganate in the ternary material is rolled, the compaction density of the positive electrode sheet obtained is larger, and for example, the compaction density of Example 1 can reach 3.25 g / cm 3 At the same time, the battery thickness increases less, and the average discharge voltage is improved, so as to ensure that the energy density of the battery in Examples 1-21 is not lost. Compared with Example 1, since 100% lithium manganate is used as the positive electrode active material in Comparative Example 2, the battery thickness is greatly increased, and the volume energy density of the battery is obviously lost. In Comparative Examples 3-4, the particle size A value of the positive electrode active material exceeds the range of 2.14-12.5, resulting in poor packing effect between the positive electrode active material particles, thereby affecting the compaction density of the positive electrode sheet, and reducing the volume energy density of the battery.

[0196] At -20°C, all the discharges of Example 10C can release more than 90.5% capacity, indicating that the low-temperature discharge performance of the battery after blending lithium manganate is obviously improved, which benefits from the specific chemical structure of lithium manganate, so that the positive electrode material can still maintain good ion conduction and electron transmission performance in a low-temperature environment. In contrast, although the ternary lithium ion battery of Comparative Example 1 has an advantage in energy density, its ion conduction is more affected under low-temperature conditions, which is not conducive to the discharge performance of the battery. In Comparative Examples 3-4, the particle size A value of the positive electrode active material exceeds the range of 2.14-12.5, resulting in poor contact between the positive electrode active material particles, thereby affecting the lithium ion transmission rate, and reducing the discharge capacity of the battery.

[0197] In combination with Figure 10It can be seen that due to the existence of Jahn-Teller effect, the manganese is continuously dissolved out, so that the comparative example 2 only circulates 350T at 35℃, and the capacity of the battery jumps due to swelling, while the capacity retention of all examples still reaches more than 90% after 500T of cycling. This shows that by blending lithium manganate and ternary material, the defects of lithium manganate material are compensated for, the structural stability of the material is improved, and the high-temperature cycle stability of the battery is improved. The particle size A value of the positive active material in comparative examples 3-4 exceeds the range of 2.14-12.5, which leads to poor contact between positive active material particles, thereby affecting the lithium ion transmission rate, and the cycle performance of the battery is reduced. The length of the carbon nanotube in comparative examples 5-6 exceeds the range of 5-30 μm, which increases the positive plate surface resistance and affects the lithium ion transmission rate, thereby reducing the cycle performance of the battery.

[0198] The blended material of ternary material and lithium manganate has a significant improvement in safety performance compared to pure ternary material. All examples and comparative example 2 have similar safety performance and can pass 2C-5V overcharge and 140℃ oven temperature. The blended material has relatively less oxygen released by lithium manganate during charging and discharging, reducing the possibility of gas accumulation in the battery, thereby reducing the risk of gas explosion in the battery. In addition, the blended material has better overcharge resistance under high-voltage charging compared to traditional nickel-cobalt-manganese (NCM) or nickel-cobalt-aluminum (NCA) materials, which means that even if the battery is charged to a higher voltage, the blended material can maintain the stability of the material structure and reduce the safety risk during charging. The particle size A value of the positive active material in comparative examples 3-4 exceeds the range of 2.14-12.5, which causes the ternary material particles to aggregate in the positive plate, resulting in a decrease in the thermal stability of the battery.

[0199] As can be seen, the lithium ion secondary battery with high power discharge characteristics, excellent electrochemical performance and good low-temperature charge and discharge capability can be prepared, and the cost of battery materials is reduced, providing a new and more competitive battery solution for electric vehicles, energy storage systems and other fields.

[0200] Obviously, the above examples are only examples for clarity and do not limit the embodiments. Based on the above description, other different forms of changes or variations can be made by those skilled in the art. Here, all embodiments need not and cannot be exhaustively enumerated. The obvious changes or variations derived therefrom are still within the scope of protection of the present application.

Claims

1. A positive electrode sheet, comprising a positive electrode current collector and a first positive electrode active material layer disposed on at least one side surface of the positive electrode current collector, the first positive electrode active material layer comprising a first positive electrode active material, a conductive agent, and a binder; characterized in that: the conductive agent comprises carbon nanotubes, a length L of the carbon nanotubes satisfying 5 μm≤L≤30 μm. The first positive electrode active material includes a lithium manganate material and a ternary material having a chemical formula of LiNi x Co y Mn 1-x-y O2, 0.8≤x<1, 0<y<0.2; The particle size A of the first positive electrode active material satisfies 2.14 ≤ A ≤ 12.5 and A = Dv99 / Dv50 PAM wherein Dv50 PAM is a particle size at which the cumulative volume distribution percentage of the first positive electrode active material reaches 50%, 2 μm ≤ Dv50 PAM ≤ 7 μm; and Dv99 is a particle size at which the cumulative volume distribution percentage of the first positive electrode active material reaches 99%, 15 μm ≤ Dv99 ≤ 25 μm. the content of the lithium manganate material is 10%-60% based on the mass of the first positive electrode active material; 2. The positive electrode sheet according to claim 1, characterized by and / or, the lithium manganate material comprises a single-crystal lithium manganate material; and / or the lithium manganese oxide material has a chemical formula of LiNi a Mn 2-a O4, 0≤a≤0.2; and / or, the content of the ternary material is 40%-90% based on the mass of the first positive electrode active material; and / or, the ternary material comprises a single-crystal ternary material. the content of the lithium manganate material is 30%-60% based on the mass of the first positive electrode active material; 3. The positive electrode sheet according to claim 2, characterized by and / or, the content of the ternary material is 40%-70% based on the mass of the first positive electrode active material. the content of the carbon nanotubes is 0.1%-1.2% based on the mass of the first positive electrode active material layer; 4. The positive electrode sheet according to claim 1, characterized by and / or, a tube diameter of the carbon nanotubes is 7 nm-11 nm; and / or, an XRD pattern of the first positive electrode active material has characteristic peaks P1 and P3 at a position of 2θ of 35°-38° and characteristic peaks P2 and P4 at a position of 2θ of 42°-46°, peak intensities of P1, P2, P3, and P4 are I1, I2, I3, and I4 respectively, and satisfy 0.05≤I1≤0.2, 0.05≤I2≤0.18, 0.1≤I3≤0.25, and 0.2≤I4≤0.

5. and / or the specific surface area of the carbon nanotubes is 200 m 2 / g - 280 m 2 / g; the content of the carbon nanotubes is 0.1%-0.5% based on the mass of the first positive electrode active material layer.

5. The positive electrode sheet according to claim 4, characterized by The positive electrode sheet further comprises a second positive electrode active material layer, the first positive electrode active material layer is located between the positive electrode current collector and the second positive electrode active material layer, and the second positive electrode active material layer comprises a ternary material.

6. The positive electrode sheet according to any one of claims 1 to 5, characterized by, a ratio of a volume median diameter D1 of the lithium manganate material in the first positive electrode active material layer to a volume median diameter D2 of the ternary material in the second positive electrode active material layer is 1.6-7.5, D1 is 8 μm-15 μm, and D2 is 2 μm-5 μm; 7. The positive electrode sheet according to claim 6, characterized by and / or, a ratio of a thickness of the second positive electrode active material layer to a thickness of the first positive electrode active material layer is 0.12-0.

4. a thickness of the first positive electrode active material layer is 40 μm-100 μm; 8. The positive electrode sheet according to claim 7, characterized by and / or, a thickness of the second positive electrode active material layer is 12 μm-16 μm. The lithium ion secondary battery comprises a negative electrode sheet, a separator, an electrolyte, and the positive electrode sheet according to any one of claims 1-8.

9. A lithium-ion secondary battery characterized by comprising: The negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises graphite with a carbon coating layer; 10. The lithium-ion secondary battery according to claim 9, characterized by The lithium ion secondary battery comprises a multi-tab winding structure. The graphitization degree of the graphite is ≥ 93%, and / or the volume median diameter Dv50 of the graphite is 10 μm-25 μm, and / or the thickness of the carbon coating layer is 0.1 mm-1 mm. G The graphitization degree of the graphite is ≥ 93%, and / or the volume median diameter Dv50 of the graphite is 10 μm-25 μm, and / or the thickness of the carbon coating layer is 0.1 mm-1 mm.

11. The lithium-ion secondary battery according to claim 9, characterized by 12.The lithium ion secondary battery according to claim 10, wherein the negative electrode active material further comprises a silicon-based material, and a content of the silicon-based material is 0-10% based on the mass of the negative electrode active material. ​ 13. The lithium-ion secondary battery according to claim 12, characterized by The silicon-based material is SiC and / or SiO, the volume median diameter of the SiC is 5-12 mu m, and / or the volume median diameter of the SiO is 4-10 mu m.

14. The lithium-ion secondary battery according to claim 9, characterized by The electrolyte comprises propyl propionate, the content W of the propyl propionate is 5%-60% based on the mass of the electrolyte, and the following conditions are satisfied: R=Dv50 G *W / Dv50 PAM , 0.07≤R≤7.5, 2μm≤Dv50 PAM ≤7μm; Dv50 G is 10μm-25μm; And / or, the diaphragm comprises a base film, a ceramic layer and a glue coating layer are arranged on one side surface of the base film in sequence, and the ceramic layer is located between the base film and the glue coating layer.

15. The lithium-ion secondary battery according to claim 14, characterized by The glue coating layer comprises polyvinylidene fluoride and polymethyl methacrylate with a mass ratio of 1:9-9:1, and the thickness H of the glue coating layer and the area ratio S of the glue coating layer on the base film satisfy: Q=H / S, 0.7≤Q≤15, H is 0.5-3 mu m, and S is 20%-70%.

16. The lithium-ion secondary battery according to claim 15, characterized by S is 30%-40%; And / or, the volume median diameter of the polyvinylidene fluoride is 0.3-2.5 mu m; And / or, the volume median diameter of the polymethyl methacrylate is 0.1-0.2 mu m.

17. The lithium-ion secondary battery according to any one of claims 14 to 16, characterized by The porosity of the diaphragm is 40%-50%; And / or, the air permeability of the diaphragm is 70-170 sec / 100cc; And / or, the pore size of the base film is 32-58 nm; And / or, the ceramic layer comprises boehmite and / or alumina.

18. The lithium-ion secondary battery according to claim 17, characterized by The pore size of the base film is 45-58 nm.

Images