Positive plate and lithium ion secondary battery

Abstract

The invention belongs to the technical field of batteries, and particularly relates to a positive plate and a lithium ion secondary battery. According to the positive plate provided by the invention, the positive active material comprises the ternary material and the lithium manganese iron phosphate material, the lithium manganese iron phosphate material is good in thermal stability, so that the safety performance of the battery is improved, and W and R meet the condition that W / R is more than or equal to 0.02 and less than or equal to 60 in cooperation with special regulation and control on the thickness W of a carbon coating layer in the first lithium manganese iron phosphate material and the pipe diameter R of a carbon nanotube; the carbon nanotubes can be inserted among primary particles of the positive electrode active material, agglomeration of the carbon nanotubes is avoided, a good conductive network is formed, and energy density and rate capability are considered while safety of a needling test and an overcharge test of the lithium ion secondary battery is improved finally.

Description

Technical Field

[0001] The present application belongs to the field of battery technology, and specifically relates to a positive electrode sheet and a lithium-ion secondary battery. Background Art

[0002] In some specific usage scenarios, lithium-ion secondary batteries need to pass safety tests such as needle penetration tests and overcharge tests. For example, electric tools such as logging machines may have relatively hard needle-shaped foreign objects (such as sawdust, etc.) in the usage scenario. When the logging machine containing lithium-ion secondary batteries is placed incorrectly, it may be pierced by sawdust, causing internal short circuits in the lithium-ion secondary batteries, local heating, thermal runaway, oxygen release and other problems, and may even catch fire and explode; overcharge tests will also cause local heating, thermal runaway, oxygen release and other problems. Therefore, lithium-ion secondary batteries that can pass safety tests such as needle penetration tests and overcharge tests are crucial to the safety of application scenarios.

[0003] At present, most of the lithium-ion secondary batteries used in power tools are mainly high-nickel (Ni molar content accounts for 80% or more of the transition metal molar content) ternary materials for positive electrode active materials to improve energy density. However, the thermal runaway temperature of pure high-nickel ternary materials decreases with the increase of nickel content. Generally, the thermal runaway temperature is above 200°C. When the heat generation temperature during puncture or overcharging is higher than this temperature, it is easy to cause decomposition and oxygenation of the positive electrode active material and subsequent thermal runaway, making it unable to pass the puncture test or overcharge test. In order to solve the above problems, the prior art generally uses positive electrode active materials with higher thermal runaway temperatures in combination with high-nickel ternary materials, but this will cause a loss of rate performance and energy density. Summary of the invention

[0004] Therefore, the present application provides a positive electrode sheet and a lithium-ion secondary battery, wherein the lithium-ion secondary battery can achieve both rate performance and energy density while improving the safety of the puncture test or overcharge test.

[0005] To this end, this application provides the following technical solutions:

[0006] According to one aspect of the present application, a positive electrode sheet is provided, comprising:

[0007] Positive electrode current collector,

[0008] A positive electrode active layer, disposed on at least one surface of the positive electrode current collector;

[0009] Wherein, the positive electrode active layer comprises a positive electrode active material, a conductive agent and a binder, the positive electrode active material comprises a ternary material and a first lithium manganese iron phosphate material, and the conductive agent comprises carbon nanotubes;

[0010] The first lithium iron manganese phosphate material includes single crystal particles, and the ternary material includes single crystal particles;

[0011] The chemical formula of the ternary material is: Li e Ni a Co b Mn c M d O 2 , 0.95 ≤ e ≤ 1.1, 0.8 ≤ a ≤ 0.95, 0.01 ≤ b ≤ 0.2, 0.01 ≤ c ≤ 0.1, 0 ≤ d ≤ 0.05; M includes at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb;

[0012] The first lithium iron manganese phosphate material includes a core and a carbon coating layer covering at least part of the surface of the core,

[0013] The chemical formula of the core is: Li v Mn x Fe y A z PO 4 , where A includes at least one of Al, Mg, Zn, Cu, Co, Ni, V, Zr, and Ti elements, where 0.9 ≤ v ≤ 1.1; 0.2 ≤ x ≤ 0.8; 0.2 ≤ y ≤ 0.8; 0.01 ≤ z ≤ 0.05;

[0014] The thickness of the carbon coating layer is W nm, the diameter of the carbon nanotube is R nm, 1 ≤ R ≤ 50, and the thickness of the carbon coating layer and the diameter of the carbon nanotube satisfy the following relationship: 0.02 ≤ W / R ≤ 60.

[0015] In some alternative embodiments, the thickness of the carbon coating layer and the diameter of the carbon nanotube satisfy the following relationship: 0.16 ≤ W / R ≤ 40;

[0016] And / or, the thickness of the carbon coating layer is 1 ≤ W ≤ 60, preferably 5 ≤ W ≤ 40;

[0017] And / or, the diameter of the carbon nanotube is 1 ≤ R ≤ 30.

[0018] In some alternative embodiments, based on the mass of the positive electrode active material, the mass ratio of the first lithium iron manganese phosphate material is 20% - 60%, preferably 30% - 50%;

[0019] And / or, in the chemical formula of the core, 0.4 ≤ x ≤ 0.6, and / or, 0.4 ≤ y ≤ 0.6;

[0020] And / or, the median voltage of the positive electrode active material is 3.55 - 3.87 V;

[0021] And / or, based on the total mass of the positive electrode active layer, the mass ratio of the carbon nanotubes is 0.1%-1.2%.

[0022] In some alternative embodiments, the positive electrode active layer is disposed on at least one surface in the thickness direction of the positive electrode current collector, and the thickness of the positive electrode sheet is H μm, where 70 ≤ H ≤ 120;

[0023] The thickness of the positive electrode sheet and the tube diameter of the carbon nanotubes satisfy the following relationship: 1.6 ≤ H / R ≤ 120, preferably 3.2 ≤ H / R ≤ 105.

[0024] In some alternative embodiments, the element A includes Ti element and Mg element. Based on the total mass of the first lithium iron manganese phosphate material, the content range of the Ti element is 100 ppm - 1000 ppm, preferably 300 ppm - 700 ppm;

[0025] The content range of the Mg element is 100 ppm - 5000 ppm, preferably 2000 ppm - 4000 ppm;

[0026] Preferably, based on the total mass of the first lithium iron manganese phosphate material, the sum of the contents of the Ti element and the Mg element ≤ 4500 ppm.

[0027] In some alternative embodiments, based on the total mass of the first lithium iron manganese phosphate material, the content range of carbon element is 0.5% - 4%, preferably 1.5% - 2.5%.

[0028] In some alternative embodiments, in the positive electrode active layer, at least part of the first lithium iron manganese phosphate material is located on at least part of the surface of the ternary material. The Dv50 of the first lithium iron manganese phosphate material is 3 - 8 μm, and the Dv50 of the ternary material is 1 - 5 μm;

[0029] And / or, the specific surface area of the ternary material is 0.3 m 2 / g - 1.5 m 2 / g, and the specific surface area of the first lithium iron manganese phosphate material is 10 m 2 / g - 30 m 2 / g.

[0030] In some alternative embodiments, the positive electrode sheet further includes a bottom coating, which is disposed between the positive electrode current collector and the positive electrode active layer, and the bottom coating includes a second lithium iron manganese phosphate material or a lithium iron phosphate material;

[0031] And / or, the total thickness of the bottom coating is 1 μm - 8 μm;

[0032] And / or, the thickness of the positive electrode current collector is 6 μm - 15 μm.

[0033] According to another aspect of the present application, a lithium-ion secondary battery is provided, including the above-mentioned positive electrode sheet, wherein the positive electrode sheet, the separator, and the negative electrode sheet are stacked and wound.

[0034] In some alternative embodiments, the separator includes a base film, a first adhesive layer is provided on one surface of the base film, a ceramic layer is provided on the other surface of the base film, and a second adhesive layer is provided on the surface of the ceramic layer away from the base film, and the ceramic layer faces the positive electrode sheet;

[0035] The thickness of the ceramic layer is 0.5 μm - 3 μm, preferably 1 μm - 2 μm;

[0036] The ceramic layer includes an inorganic material, and the inorganic material includes boehmite, magnesium oxide, magnesium hydroxide, BaSO 4 , CaSiO 3 , CaSiO 4 , Al 2 O 3 and TiO 2 at least one of;

[0037] The Dv50 of the inorganic material is 100 nm - 2 μm.

[0038] The technical solution of the present application has the following advantages:

[0039] The positive electrode sheet provided by the present application includes: a positive electrode current collector; a positive electrode active layer provided on at least one surface of the positive electrode current collector; wherein, the positive electrode active layer includes a positive electrode active material, a conductive agent, and a binder, the positive electrode active material includes a ternary material and a first lithium iron phosphate manganese material, the conductive agent includes carbon nanotubes; the first lithium iron phosphate manganese material includes single crystal particles, and the ternary material includes single crystal particles; the chemical formula of the ternary material is: Li e Ni a Co b Mn c M d O 2 , 0.95 ≤ e ≤ 1.1, 0.8 ≤ a ≤ 0.95, 0.01 ≤ b ≤ 0.2, 0.01 ≤ c ≤ 0.1, 0 ≤ d ≤ 0.05; M includes at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb; the first lithium iron phosphate manganese material includes a core and a carbon coating layer covering at least part of the surface of the core, and the chemical formula of the core of the first lithium iron phosphate manganese material is: Li v Mn x Fe y A z PO4 , wherein A includes at least one of Mg, Zn, Cu, Co, Ni, V, Zr, and Ti elements, wherein 0.9≤v≤1.1; 0.2≤x≤0.8; 0.2≤y≤0.8; 0.01≤z≤0.05; the thickness of the carbon coating layer is Wnm, the diameter of the carbon nanotube is Rnm, 1≤R≤50, and the thickness of the carbon coating layer and the diameter of the carbon nanotube satisfy the following relationship: 0.02≤W / R≤60. In the positive electrode sheet provided in the present application, the positive electrode active material includes a ternary material and a lithium iron manganese phosphate material. Since the lithium iron manganese phosphate material has good thermal stability, the safety performance of the battery is improved. Combined with the special regulation of the thickness W of the carbon coating layer in the first lithium iron manganese phosphate material and the diameter R of the carbon nanotubes, W and R satisfy: 0.02≤W / R≤60, the carbon nanotubes can be inserted between the primary particles of the positive electrode active material to avoid agglomeration of the carbon nanotubes and form a good conductive network, ultimately achieving both energy density and rate performance while improving the safety of the puncture test and the overcharge test. Among them, the high-energy-density ternary material is used in combination with the lithium iron manganese phosphate material with good thermal stability, which can prevent the spread of heat generated during puncture or overcharging, increase the thermal runaway temperature of the positive electrode sheet as a whole, delay the occurrence of thermal runaway of the lithium-ion secondary battery, and improve the safety of the positive electrode sheet puncture test and overcharge test; by specially regulating the thickness W of the carbon coating layer in the first lithium iron manganese phosphate material and the diameter R of the carbon nanotubes, W and R satisfy: 0.02≤W / R≤60, the carbon nanotubes can be inserted between the primary particles of the positive electrode active material, avoiding the agglomeration of the carbon nanotubes, forming a good conductive network, improving the overall conductivity of the positive electrode sheet, and facilitating the transmission of lithium ions, thereby improving the rate performance of the lithium-ion secondary battery, alleviating the influence of the use of the lithium iron manganese phosphate material with lower ionic conductivity in the positive electrode active material on the rate performance, and achieving a balance between safety performance, energy density and rate performance.

[0040] Additional aspects and advantages of the embodiments of the present application will be described and shown in part in the subsequent description, or explained through the implementation of the embodiments of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0042] Figure 1 is a SEM image of the positive electrode sheet provided in Example 1 of the present application;

[0043] Figure 2 It is a schematic structural diagram of the positive electrode sheet provided in Embodiment 1 of the present application;

[0044] Figure 3 It is a schematic structural diagram of the wound battery cell in the embodiment of the present application;

[0045] Reference numerals:

[0046] 1. Positive current collector; 2. Positive active layer; 3. Tab. Detailed implementation manners

[0047] The following embodiments are provided to better further understand the present application. It is not limited to the described optimal implementation manner, and does not limit the content and protection scope of the present application. Any product obtained by anyone under the inspiration of the present application or by combining the features of the present application with those of other existing technologies that is the same as or similar to the present application falls within the protection scope of the present application.

[0048] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present application; the term "including" and any of its variations are intended to cover non-exclusive inclusion.

[0049] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary-secondary relationship of the indicated technical features.

[0050] In the description of the embodiments of the present application, the meaning of "several" is one or more than two, unless otherwise clearly and specifically defined. The meaning of the term "at least one" is one or more than two, unless otherwise clearly and specifically defined.

[0051] Referring to "embodiment" herein means that the specific features, structures or characteristics described in connection with the embodiment may be included in at least one embodiment of the present application. The phrase appears in various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0052] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist simultaneously, and B exists alone.

[0053] For those without specific experimental procedures or conditions noted in the examples, the operations or conditions of the conventional experimental procedures described in the literature in this field can be followed. For reagents or instruments without the manufacturer noted, they are all conventional reagent products that can be obtained through commercial purchase.

[0054] This application provides a positive electrode sheet and a lithium-ion secondary battery. The positive electrode sheet and the lithium-ion secondary battery containing the same can effectively solve the defect in the prior art that the means for improving the safety of the needle penetration test and overcharge test will cause loss of the rate performance and energy density of the lithium-ion secondary battery. The technical solution adopted in this application is as follows.

[0055] According to one aspect of this application, a positive electrode sheet is provided, including:

[0056] A positive electrode current collector,

[0057] A positive electrode active layer disposed on at least one surface of the positive electrode current collector;

[0058] Wherein, the positive electrode active layer includes a positive electrode active material, a conductive agent, and a binder. The positive electrode active material includes a ternary material and a first lithium iron phosphate manganese material. The conductive agent includes carbon nanotubes;

[0059] The first lithium iron phosphate manganese material includes single crystal particles, and the ternary material includes single crystal particles;

[0060] The chemical formula of the ternary material is: Li e Ni a Co b Mn c M d O 2 , 0.95 ≤ e ≤ 1.1, 0.8 ≤ a ≤ 0.95, 0.01 ≤ b ≤ 0.2, 0.01 ≤ c ≤ 0.1, 0 ≤ d ≤ 0.05; M includes at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb;

[0061] The first lithium iron phosphate manganese material includes a core and a carbon coating layer covering at least part of the surface of the core,

[0062] The chemical formula of the core of the first lithium iron phosphate manganese material is: Li v Mn x Fe y A z PO 4 , where A includes at least one of Al, Mg, Zn, Cu, Co, Ni, V, Zr, and Ti elements, where 0.9 ≤ v ≤ 1.1; 0.2 ≤ x ≤ 0.8; 0.2 ≤ y ≤ 0.8; 0.01 ≤ z ≤ 0.05;

[0063] The thickness of the carbon coating layer is W nm, the diameter of the carbon nanotube is R nm, 1 ≤ R ≤ 50, and the thickness of the carbon coating layer and the diameter of the carbon nanotube satisfy the following relationship: 0.02 ≤ W / R ≤ 60.

[0064] In some alternative embodiments, the thickness of the carbon coating layer and the diameter of the carbon nanotube satisfy the following relationship: 0.16 ≤ W / R ≤ 40.

[0065] As an example, the ratio W / R of the thickness W of the carbon coating layer to the diameter R of the carbon nanotube is 0.02, 0.1, 0.16, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, or within the range composed of any of the above values; the diameter of the carbon nanotube can be 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, or within the range composed of any of the above values.

[0066] In the positive electrode sheet provided by the present application, the positive electrode active material includes a ternary material and a lithium iron manganese phosphate material. Due to the good thermal stability of the lithium iron manganese phosphate material, the safety performance of the battery is improved. In combination with the special regulation of the thickness W of the carbon coating layer in the first lithium iron manganese phosphate material and the diameter R of the carbon nanotube, W and R satisfy: 0.02≤W / R≤60, the carbon nanotubes can be inserted between the primary particles of the positive electrode active material to avoid the agglomeration of the carbon nanotubes and form a good conductive network, and finally achieve the energy density and rate performance while improving the safety of the lithium ion secondary battery needle puncture test and overcharge test. Among them, the high energy density ternary material is used in combination with the lithium iron manganese phosphate material with good thermal stability itself, which can prevent the heat spread generated during needle puncture or overcharge, increase the thermal runaway temperature of the positive electrode sheet as a whole, delay the occurrence of thermal runaway of the lithium ion secondary battery, and improve the safety of the positive electrode sheet needle puncture test and overcharge test. However, due to the low intrinsic ionic conductivity of lithium manganese iron phosphate (LMFP), by carbon coating LMFP, a fast conductive network can be constructed using the carbon coating layer and lithium manganese iron phosphate, so that electrons can quickly migrate between active materials during the charge and discharge process, reducing the internal resistance and charge and discharge polarization of the battery. At the same time, carbon nanotubes are used as conductive agents, and the thickness W of the carbon coating layer in the first lithium manganese iron phosphate material and the diameter R of the carbon nanotubes are specially regulated to make W and R meet: 0.02≤W / R≤60, so that the carbon nanotubes can be inserted between the primary particles of the positive electrode active material, avoiding the agglomeration of carbon nanotubes, forming a good conductive network, improving the overall conductivity of the positive electrode sheet, and facilitating the transmission of lithium ions, thereby improving the rate performance of the lithium ion secondary battery, alleviating the impact of the use of lithium manganese iron phosphate materials with lower ionic conductivity in the positive electrode active material on the rate performance, and achieving a balance between safety performance, energy density and rate performance. When the W / R ratio is too large, the diameter of the carbon nanotubes is small relative to the thickness of the coating layer, and it is unable to effectively compensate for the adverse effects of the excessive thickness of the coating layer on lithium ion transmission. In addition, the structure of the carbon nanotubes is unstable and they are easy to agglomerate together, making it difficult to form a good conductive network. When the W / R ratio is too small, the diameter of the carbon nanotubes is large relative to the thickness of the coating layer, making it difficult for the carbon nanotubes to be inserted between the primary particles of the positive electrode active material, thus weakening the conductivity. In addition, carbon coating can also reduce the contact between the positive electrode active material and the electrolyte, thereby avoiding side reactions with the electrolyte and improving its high temperature performance and cycle performance. And the surface carbon coating can also effectively inhibit the agglomeration and growth of the modified material particles, thereby maintaining the nanostructure of the particles and effectively reducing Li +The diffusion distance inside the active particles enables the material to have better rate performance. Additionally, it should be noted that lithium iron manganese phosphate has a stable olivine-type structure, which determines the high stability of lithium iron manganese phosphate and a one-dimensional lithium-ion transport channel, that is, a relatively low ionic conductivity. Through special regulation of the manganese element content, it is possible to improve the rate performance and cycle stability while ensuring the energy density. If the value of x in the material is higher than 0.8, the median voltage of the material is higher, but there will be more defects and pores in the material, which prolongs the lithium-ion insertion / extraction path, reduces the ion migration rate, and decreases the conductivity; and due to the Jahn-Teller effect of manganese ions, manganese dissolution may occur, which may deposit on the surface of the negative electrode and damage the SEI film; at the same time, manganese dissolution causes changes in the crystal structure of the lithium iron manganese phosphate material, thereby leading to a decrease in cycle stability and thermal stability. If x is lower than 0.2, the manganese content decreases, and the median voltage of the material also decreases accordingly, resulting in a significant drop in energy density.

[0067] It can be understood that the thickness of the carbon coating layer and the diameter of the carbon nanotubes can be measured by conventional methods and equipment in the field. As an example, TEM is used to measure the thickness of the carbon coating layer. At least 50 lithium iron manganese phosphate material particles are randomly selected in the field of view, and at least 10 sites are randomly selected on the surface of each particle to measure the thickness of the coating layer corresponding to each site, and the average value is taken. The diameter of the carbon nanotubes is measured by TEM. At least 50 carbon nanotubes are randomly selected within the field of view, and the outer diameter of each carbon nanotube is measured, and the average value is taken.

[0068] It should be noted that when calculating the values of the relationships defined in this application, the corresponding numerical values of the parameters are substituted into the relationships for calculation, and the units of the parameters are not included.

[0069] In some alternative embodiments, the conductive agent further includes at least one of conductive carbon black, acetylene black, and graphene, and the binder includes at least one of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polystyrene-acrylate, and polyacrylate; the carbon nanotubes include at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes. In this application, based on the total mass of the positive electrode active layer, the conductive agent in the positive electrode active layer accounts for 0.1%-3%, and the binder accounts for 0.1%-5%.

[0070] In some alternative embodiments, based on the mass of the positive electrode active material, the mass percentage M% of the first lithium iron manganese phosphate material is 20%-60%, preferably 30%-50%;

[0071] As an example, based on the mass of the positive electrode active material, the mass percentage of the first lithium iron manganese phosphate material can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or within the range composed of any of the above values; if the proportion of the lithium iron manganese phosphate material is too low, the improvement of safety performance is not obvious, and occasional failures in the needle penetration test or fast charging test may occur; if the proportion of the lithium iron manganese phosphate material is too high, the median voltage decreases, the energy density loss is obvious, the conductivity of the positive electrode active material decreases, and the surface resistance of the electrode increases, resulting in poor rate performance of the battery.

[0072] In some optional embodiments, in the chemical formula of the core, 0.4 ≤ x ≤ 0.6, and / or, 0.4 ≤ y ≤ 0.6;

[0073] In some optional embodiments, the thickness of the carbon coating layer is 1 ≤ W ≤ 60, preferably 5 ≤ W ≤ 40;

[0074] and / or, the diameter of the carbon nanotube is 1 ≤ R ≤ 30, and the thickness of the carbon coating layer and the diameter of the carbon nanotube satisfy the following relationship: 0.16 ≤ W / R ≤ 40;

[0075] As an example, the thickness of the carbon coating layer can be 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, or within the range composed of any of the above values.

[0076] By optimizing the proportion of the first lithium iron manganese phosphate material, the thickness W of the carbon coating layer, the diameter R of the carbon nanotube, and the ratio between the two, the present application can further improve the safety of the needle penetration test and overcharge test, the energy density, and the rate performance of the battery;

[0077] and / or, based on the total mass of the positive electrode active layer, the mass percentage of the carbon nanotube is 0.1% - 2%. As an example, the mass percentage of the carbon nanotube can be 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1%, 1.3%, 1.5%, 1.8%, 2%, or within the range composed of any of the above values.

[0078] Those skilled in the art can understand that in order to mitigate the impact on the rate performance and energy density of lithium-ion secondary batteries caused by the low intrinsic ionic conductivity of lithium iron phosphate manganese after the ternary material and lithium iron phosphate manganese are used in combination, in this application, through special regulation of the carbon nanotube content, the above-mentioned performance can be further optimized; if the carbon nanotube content is too low, the conductive network is not perfect, the surface resistance of the electrode is large, which is not conducive to the transmission of lithium ions; if the carbon nanotube content is too high, it will occupy the mass ratio of the positive active material in the positive active layer, resulting in a reduction of the active substance and a decrease in energy density.

[0079] In some alternative embodiments, the positive active layer is disposed on at least one surface in the thickness direction of the positive current collector, and the thickness of the positive electrode sheet is H μm, where 70 ≤ H ≤ 120, preferably 95 ≤ H ≤ 105; as an example, the thickness of the positive electrode sheet can be 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, 105 μm, 110 μm, 115 μm, 120 μm, or within the range composed of any of the above values;

[0080] The thickness H of the positive electrode sheet and the tube diameter R of the carbon nanotubes satisfy the following relationship: 1.6 ≤ H / R ≤ 120, preferably 3.2 ≤ H / R ≤ 105.

[0081] As an example, the value of H / R can be 1.6, 2, 3.2, 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110, 115, 120, or within the range composed of any of the above values; the thickness of the positive electrode sheet can be 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, 105 μm, 110 μm, 115 μm, 120 μm, or within the range composed of any of the above values.

[0082] It should be noted that after the ternary material is used in combination with the lithium iron manganese phosphate material, due to the relatively low intrinsic ionic conductivity of the lithium iron manganese phosphate, as the thickness of the positive electrode sheet increases, the transport rate of lithium ions in the positive electrode sheet may be further restricted. Therefore, it is necessary to simultaneously control the relationship between the thickness of the positive electrode sheet and the diameter of the carbon nanotubes to further improve the lithium ion transport speed and rate performance. When this ratio H / R is too large, the diameter of the carbon nanotubes is relatively small compared to the thickness of the positive electrode sheet. At this time, the contact effect with the positive electrode active material will decrease, and the carbon nanotubes cannot effectively compensate for the increase in electrode polarization caused by the increase in the lithium ion transport path due to the too large thickness of the electrode sheet, and the conductive network of the electrode sheet becomes worse; in addition, too small a diameter of the carbon nanotubes is prone to agglomeration and it is difficult to disperse evenly in the positive electrode sheet to form a conductive network; when this ratio H / R is too small, the diameter of the carbon nanotubes is relatively large compared to the thickness of the positive electrode sheet, resulting in difficulty for the carbon nanotubes to insert between the primary particles of the positive electrode active material, weakening the conductivity (loss of rate performance and reduction of specific capacity). In this application, through special regulation of the thickness of the positive electrode sheet, because the toughness of the electrode sheet decreases as the thickness of the electrode sheet decreases, process problems such as tape breakage are likely to occur during the winding process. At the same time, special regulation of the thickness of the electrode sheet can also reduce the tape breakage of the positive electrode sheet caused by the expansion of the negative electrode during the cycling process.

[0083] And / or, the median voltage of the positive electrode active material is 3.55 - 3.87V; as an example, the median voltage of the positive electrode active material can be 3.55V, 3.59V, 3.60V, 3.61V, 3.62V, 3.63V, 3.64V, 3.65V, 3.66V, 3.67V, 3.69V, 3.72V, 3.75V, 3.77V, 3.80V, 3.83V, 3.85V, 3.87V, or within the range composed of any of the above values. If the median voltage of the positive electrode active material is too high, there will be more defects and pores in the material, prolonging the insertion / extraction path of lithium ions, reducing the ion migration rate, and reducing the rate performance of the battery; if the median voltage of the positive electrode active material is relatively low, the energy density will decrease significantly. By limiting the median voltage of the positive electrode active material to the above range in this application, the rate performance and energy density can be further optimized.

[0084] In some optional embodiments, the A element includes Ti and Mg elements. Based on the total mass of the first lithium iron manganese phosphate material, the content range of the Ti element is 100 ppm - 1000 ppm, preferably 300 ppm - 700 ppm; as an example, the content of the Ti element is 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, or within the range composed of any of the above values;

[0085] The content range of Mg element is 100 ppm - 5000 ppm, preferably 2000 ppm - 4000 ppm; as an example, the content of Mg element is 100 ppm, 50000 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, or within the range composed of any of the above values;

[0086] Preferably, based on the total mass of the first lithium iron manganese phosphate material, the sum of the contents of Ti element and Mg element ≤ 4500 ppm.

[0087] It should be noted that after the ternary material and the lithium iron manganese phosphate material are used in combination in the positive electrode sheet, due to the low ionic conductivity of the lithium iron manganese phosphate material, the rate performance is affected. In this application, element doping is used to increase the lattice size to improve the lithium ion transport speed of the material, and combined with surface coating with an appropriate amount of carbon material, the overall conductivity of the material is improved, thereby improving the rate performance of the material. Among them, Mg doping is a homovalent doping. Since the valence states are the same, it will not cause vacancies in the lattice structure, and the material structure can maintain good integrity during the cycling process; Ti doping can improve the crystallinity and refine the grains, change the conductivity and ion diffusion performance of the material from the inside of the lattice, and can inhibit the Jahn-Teller effect, thereby improving the cycling performance of the material and reducing adverse reactions such as SEI film dissolution. If the doping ratios of Ti and Mg are too high, the specific capacity per gram will be lost due to the replacement of active elements, and the discharge specific capacity will decrease; if the doping ratios are too low, the effect of improving conductivity is not obvious.

[0088] And / or, based on the total mass of the first lithium iron manganese phosphate material, the content range of carbon element is 0.5% - 4%, preferably 1.5 - 2.5%; as an example, the content of carbon element is 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or within the range composed of any of the above values.

[0089] Those skilled in the art can understand that carbon is a good conductive material. In order to alleviate the influence brought by the low conductivity of the lithium iron manganese phosphate material itself, this application uses the carbon coating layer and lithium iron manganese phosphate to construct a fast conductive network, so that electrons can migrate rapidly between active substances during charge and discharge, reducing the internal resistance and charge-discharge polarization of the battery. At the same time, carbon coating can also reduce the contact surface between the active substance and the electrolyte, thereby avoiding side reactions with the electrolyte and improving its high-temperature performance and cycling performance. And surface carbon coating can also effectively inhibit the agglomeration and growth of active material particles, thereby maintaining the nano-structure of the particles and effectively reducing Li +The diffusion distance inside the active particles enables the material to have better rate performance. If the carbon coating content is too high, the mass energy density will be lost; if the carbon coating content is too low, the conductivity of the material is poor, resulting in capacity loss due to contact.

[0090] In this application, the content of each element can be tested by methods and equipment known in the art, such as EDS, ICP, etc.

[0091] In some alternative embodiments, in the positive electrode active layer, at least part of the first lithium iron phosphate manganese material is located on at least part of the surface of the ternary material. The Dv50 of the first lithium iron phosphate manganese material is 3 - 8 μm, and the Dv50 of the ternary material is 1 - 5 μm. As an example, the Dv50 of the first lithium iron phosphate manganese material can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or within the range composed of any of the above values; the Dv50 of the ternary material can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, or within the range composed of any of the above values.

[0092] and / or, the specific surface area of the ternary material is 0.3 m 2 / g - 1.5 m 2 / g, and the specific surface area of the first lithium iron phosphate manganese material is 10 m 2 / g - 30 m 2 / g. As an example, the specific surface area of the ternary material can be 0.3 m 2 / g, 0.5 m 2 / g, 0.7 m 2 / g, 0.9 m 2 / g, 1 m 2 / g, 1.2 m 2 / g, 1.5 m 2 / g, or within the range composed of any of the above values; the specific surface area of the first lithium iron phosphate manganese material can be 10 m 2 / g, 12 m 2 / g, 15 m 2 / g, 17 m 2 / g, 20 m 2 / g, 23 m 2 / g, 25 m 2 / g, 28 m 2 / g, 30 m 2 / g, or within the range composed of any of the above values.

[0093] In this application, through the special regulation of the first lithium iron manganese phosphate material and the ternary material Dv50, the lithium iron manganese phosphate can be coated on the surface of the ternary material, the overall specific surface area of the composite material is reduced, and the above special regulation of the specific surface areas of the ternary material and the lithium iron manganese phosphate material is beneficial to controlling the solid content and viscosity of the slurry during homogenization, facilitating the uniform dispersion of carbon nanotubes, further optimizing the conductive network, improving the overall conductivity of the electrode sheet, facilitating the rapid transmission of lithium ions, and enhancing the rate performance.

[0094] In this application, the lithium iron manganese phosphate can be coated on the surface of the ternary material by dry coating, and the process of dry coating can be realized by means of a high-speed mixer.

[0095] In some alternative embodiments, the positive electrode sheet further includes a bottom coating disposed between the positive electrode current collector and the positive electrode active layer, and the bottom coating includes a second lithium iron manganese phosphate material or a lithium iron phosphate material;

[0096] and / or, the total thickness of the bottom coating is 1 μm - 8 μm. As an example, the total thickness of the bottom coating (if it is coated on both sides of the positive electrode current collector, it refers to the total thickness of the two bottom coatings) can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or within the range composed of any of the above values;

[0097] It should be noted that in order to improve the puncture safety of the lithium-ion secondary battery, the related art will introduce a bottom coating in the positive electrode sheet, that is, first coat the bottom coating on the positive electrode current collector, and then coat the positive electrode active material on the bottom coating; due to the low thermal runaway temperature and high heat generation of the ternary material, in order to further balance the instability of the ternary material, a low-conductivity material such as lithium iron phosphate or lithium iron manganese phosphate is used as the bottom coating material to be coated on the surface of the positive electrode current collector. During the puncture test, by increasing the internal short-circuit resistance, reducing the short-circuit current, reducing heat generation, avoiding thermal runaway, and increasing the puncture test or overcharge test safety of the battery cell.

[0098] and / or, the thickness of the positive electrode current collector is 6 - 15 μm; as an example, the thickness of the positive electrode current collector can be 6 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or within the range composed of any of the above values.

[0099] In this application, by adjusting the thickness of the positive current collector, it is possible to avoid the fracture of the electrode sheet while taking into account the utilization efficiency of the cell space. If the thickness of the positive current collector is too small, when the positive electrode sheet is rolled, the hard positive active material is subjected to pressure in the vertical direction and will be embedded into the positive current collector, which may cause local perforation. And due to the uneven surface of the material or unreasonable rolling parameters, the material will be deformed locally, thus forming a fracture of the electrode sheet. If the thickness of the positive current collector is too large, the utilization efficiency of the cell space will be significantly reduced.

[0100] In some optional embodiments, it further includes a plurality of tabs extending from the width direction of the positive electrode sheet. The tabs are distributed along the length direction of the positive electrode sheet, and the number of the tabs is 1 - 46, preferably 20 - 40. As an example, the number of the tabs can be 1, 2, 5, 10, 20, 25, 30, 35, 40, 46, or within the range composed of any of the above values.

[0101] Preferably, the tabs extend from one side of the width direction of the current collector;

[0102] It can be understood that due to the low ionic conductivity of the lithium iron manganese phosphate material, when used in combination with ternary materials, it will reduce the rate performance of the battery. In this application, by increasing the number of tabs provided on the positive electrode sheet, it is possible to increase the transmission path of lithium ions between the electrode sheets, improve the conductivity of the electrode sheet, make up for the problem of insufficient conductivity of the composite positive electrode material, enhance the electrical contact of the electrode sheet, improve the rate discharge performance, and prevent obvious loss of the cell capacity.

[0103] Those skilled in the art can understand that the "width direction" refers to the direction along the short side of the positive electrode sheet.

[0104] According to another aspect of the present application, a lithium-ion secondary battery is provided, including the above-mentioned positive electrode sheet, separator, and negative electrode sheet. The lithium-ion secondary battery has the same effects as the above-mentioned positive electrode sheet, which will not be elaborated here.

[0105] In some optional embodiments, the lithium-ion secondary battery can be a wound battery. After the positive electrode sheet, separator, and negative electrode sheet are stacked in sequence and wound, a wound-type electrode core can be formed.

[0106] In some optional embodiments, the separator includes a base film. One side surface of the base film is provided with an adhesive layer, and the other side surface of the base film is provided with a ceramic layer. The ceramic layer faces the positive electrode sheet;

[0107] The thickness of the ceramic layer is 0.5 μm - 3 μm, preferably 1 - 2 μm. As an example, the thickness of the ceramic layer is 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, or within the range composed of any of the above values.

[0108] It should be noted that the ceramic layer includes inorganic materials, and the inorganic materials include boehmite (hydrated aluminum hydroxide), magnesium oxide, magnesium hydroxide, BaSO 4 , CaSiO 3 , CaSiO 4 , Al 2 O 3 and TiO 2 at least one of them; the Dv50 of the inorganic material is 100 nm - 2 μm; as an example, the Dv50 of the material of the ceramic layer can be 100 nm, 300 nm, 500 nm, 0.1 μm, 0.2 μm, 0.5 μm, 1 μm, 1.2 μm, 1.5 μm, 1.8 μm, 2 μm, or within the range composed of any of the above values. The adhesive layer includes PVDF or PMMA, etc.

[0109] In this application, in order to improve the pinprick safety of lithium - ion secondary batteries, a separator containing a ceramic layer is used, and the ceramic layer is arranged facing the positive electrode sheet. When the battery cell fails, lithium ions on the surface of the negative electrode sheet cannot be embedded, and are likely to be deposited on the surface in the form of lithium dendrites. The continuous growth of lithium dendrites may pierce the separator, resulting in short - circuit between the positive and negative electrodes, and serious safety problems such as fire and explosion. The ceramic layer of the separator can effectively prevent lithium dendrites from piercing the separator, improving the battery safety. At the same time, the presence of the ceramic layer can increase the internal short - circuit resistance, reduce the short - circuit current, reduce heat generation, avoid thermal runaway, and increase the battery safety. If the ceramic layer is too thin, the increase in safety is not obvious, and there may be a situation where probabilistic lithium dendrites lead to thermal runaway or failure in the pinprick test; if the ceramic layer is too thick, it will lead to a decrease in the energy density of the battery cell.

[0110] Those skilled in the art can understand that during the charge and discharge process of the battery, lithium ions shuttle back and forth between the positive electrode sheet and the negative electrode sheet, and the electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly playing a role in preventing short - circuit between the positive and negative electrodes, and at the same time allowing lithium ions to pass through.

[0111] As an example, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer. The negative electrode current collector has two opposite surfaces in its own thickness direction, and the negative electrode active material layer is provided on any one or both of the two opposite surfaces of the negative electrode current collector. The materials, compositions, and manufacturing methods of the negative electrode sheets used in the lithium - ion secondary batteries of this application can include any technologies disclosed in the prior art.

[0112] There are no particular limitations on the material and shape of the separator used in the lithium-ion secondary battery of the present application, and it can include any technology disclosed in the prior art.

[0113] The electrolyte used in the lithium-ion secondary battery of the present application can include any technology disclosed in the prior art.

[0114] The present application will be further described in detail below in conjunction with specific embodiments, and these embodiments should not be construed as limiting the scope claimed in the present application. In all embodiments and comparative examples of the present application, the unit % represents mass percentage.

[0115] Example 1

[0116] This example provides a positive electrode sheet and a lithium-ion secondary battery containing the positive electrode sheet. The structural schematic diagram of the positive electrode sheet is as Figure 2 shown, including: a positive electrode current collector 1, and a positive electrode active layer 2 provided on at least one surface of the positive electrode current collector 1; it also includes a plurality of tabs 3 extending from the width direction of the positive electrode sheet of the positive electrode current collector; the specific composition and preparation method of the lithium-ion secondary battery are as follows:

[0117] (1) Preparation of the positive electrode sheet

[0118] A bottom coating with a thickness of 1.5 μm was prepared. The specific preparation method was: mixing lithium iron phosphate, conductive carbon black, and binder PVDF in a mass ratio of 97.4:1.5:1.1, using NMP as a dispersion medium, preparing a slurry with a certain viscosity through high-speed stirring, and coating the slurry on both sides of the positive electrode current collector (10-μm-thick aluminum foil) through a coating device and drying to form the bottom coating, controlling the thickness of the bottom coating on one side of the positive electrode current collector to be 1.5 μm.

[0119] The positive electrode active material (the first particles and the second particles were premixed in a mass ratio of 1:1, and the proportion of lithium manganese iron phosphate in the positive electrode active material was M%, and the proportion in this example was 50%), polyvinylidene fluoride, single-walled carbon nanotubes with a diameter of 3 nm, multi-walled carbon nanotubes with a diameter of 20 nm (the average diameter of the single-walled carbon nanotubes and the multi-walled carbon nanotubes (i.e., the diameter of the carbon nanotubes) was 10 nm), and conductive carbon black were mixed evenly in a mass ratio of 97.4:1.2:0.1:0.7:0.6, adding N-methylpyrrolidone (NMP) to obtain a positive electrode slurry with a solid content of 65%; the positive electrode slurry was evenly coated on the aluminum foil with the bottom coating using a coater, dried, rolled, die-cut, and sliced to obtain a positive electrode sheet with a thickness of 100 μm; among them, the tap density of the positive electrode sheet was 2.9 g / cm 3 , and the areal density was 13 g / cm 2 ; the first particles were a ternary material, and the chemical formula of the first particles was LiNi0.93 Co 0.03 Mn 0.02 Al 0.02 O 2 , the specific surface area of the first particles is 0.65 m 2 / g, and the particle size Dv50 is 4.7 μm; the second particles are lithium iron manganese phosphate materials, and the second particles include a core and a carbon coating layer. The chemical formula of the core is LiMn 0.4 Fe 0.58 A 0.02 PO 4 , A is Mg and Ti. The mass content of element Mg in the second particles is 3000 ppm, the mass content of element Ti in the second particles is 500 ppm, and the mass content of element C in the second particles is 1.75%. The specific surface area of the second particles is 18.5 m 2 / g, the thickness of the coating layer in the second particles is 38 nm, and the particle size Dv50 of the second particles is 2.5 μm. Using a die cutter, 16 corresponding tabs are punched out in the reserved empty foil (aluminum foil) area to make a multi-tab electrode. Figure 1 Figure 23 is the SEM image of the positive electrode. As can be seen from the figure, the positive active material is evenly distributed in the electrode. The darker black part in the figure is the lithium iron manganese phosphate material, and the lighter white part is the ternary material.

[0120] (2) Preparation of the negative electrode

[0121] Mix artificial graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, styrene-butadiene rubber, and sodium carboxymethyl cellulose evenly according to the mass ratio of 96.1:0.25:0.15:2.9:0.6 to obtain a material. Add ethylene carbonate accounting for 1% of the total mass of the material, and add deionized water to obtain a negative electrode slurry (solid content: 45%); evenly coat the negative electrode slurry on a 6-μm high-strength carbon-coated copper foil, dry it, then roll, die-cut, and cut into pieces to obtain a negative electrode with a thickness of 83 μm. Using a die cutter, 16 corresponding tabs are punched out in the reserved empty foil (aluminum foil) area to make a multi-tab electrode. The tap density of the negative electrode is 1.55 g / cm 3 , and the areal density is 6 g / cm 2 .

[0122] (3) Preparation of the electrolyte

[0123] In a glove box filled with inert gas (argon) (H 2 O < 0.1 ppm, O 2(with a content of <0.1 ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, and propyl propionate are mixed evenly in a mass ratio of 15:10:10:65. Then, 1.25 mol / L of fully dried lithium hexafluorophosphate is quickly added thereto and stirred evenly. Finally, succinonitrile accounting for 0.5% of the total mass of the electrolyte is added. After passing the moisture and free acid tests, the required electrolyte is obtained.)

[0124] (4) Preparation of the battery

[0125] The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (including a 5-μm-thick polyethylene substrate, with a 2-μm-thick ceramic layer (made of alumina, Dv50 is 1 μm) on one side of the substrate and a 2-μm-thick polyvinylidene fluoride adhesive layer on the other side of the substrate) are wound by a winding machine to obtain a multi-tab wound battery cell with the positive and negative electrode sheets separated by the separator (the structural schematic diagram is as shown in Figure 3 (shown), and then through processes such as welding, encapsulation, liquid injection, formation, airbag cutting, and sorting, a lithium-ion secondary battery is obtained.)

[0126] Examples 2 - 25

[0127] Compared with Example 1, Examples 2 - 25 have the same preparation method, and the differences are shown in Table 1.)

[0128] Example 26

[0129] The positive electrode sheet provided in this example and the lithium-ion secondary battery containing this positive electrode sheet are different from Example 1 only in that, in the positive electrode sheet, an equal mass of LiNi 0.8 Co 0.1 Mn 0.08 Al 0.02 O 2 is used to replace LiNi 0.93 Co 0.03 Mn 0.02 Al 0.02 O 2 .

[0130] Example 27

[0131] The positive electrode sheet provided in this example and the lithium-ion secondary battery containing this positive electrode sheet are different from Example 1 only in that, in the positive electrode sheet, an equal mass of LiNi 0.95 Co 0.02 Mn 0.02 Al 0.01 O 2 is used to replace LiNi 0.93 Co 0.03 Mn 0.02 Al 0.02 O 2 .

[0132] Example 28

[0133] The positive electrode sheet provided in this example and the lithium-ion secondary battery containing the positive electrode sheet are only different from Example 1 in that LiMn 0.48 Fe 0.5 A 0.02 PO 4 is used instead of LiMn 0.4 Fe 0.58 A 0.02 PO 4 .

[0134] Example 29

[0135] The positive electrode sheet provided in this example and the lithium-ion secondary battery containing the positive electrode sheet are only different from Example 1 in that LiMn 0.4 Fe 0.57 A 0.03 PO 4 is used instead of LiMn 0.4 Fe 0.58 A 0.02 PO 4 .

[0136] Comparative Example 1

[0137] The positive electrode sheet provided in this comparative example and the lithium-ion secondary battery containing the positive electrode sheet are only different from Example 1 in that the positive electrode sheet does not include carbon nanotubes, and an equal amount of conductive carbon black is used instead of single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0138] Comparative Examples 2 - 3

[0139] The positive electrode sheet provided in this comparative example and the lithium-ion secondary battery containing the positive electrode sheet are only different from Example 20 in that by adjusting the diameter of the carbon nanotubes and the thickness of the carbon coating layer, W / R is not within the scope defined in this application. See Table 1 for details.

[0140] Table 1

[0141]

[0142]

[0143] Test Example

[0144] 1. Penetration test (the smaller the diameter of the iron nail, the smaller the resistance and the more difficult it is to pass the test; the slower the penetration speed, not only the temperature of the steel needle part rises, but also the temperature of the tabs of the short-circuited electrode sheets rises, and the heat spreads from both the steel needle and the tabs as the starting points, making it more difficult to pass the test):

[0145] (1) Under the environment of 25℃±5℃, charge the lithium-ion secondary batteries provided in each example and comparative example at a rate of 0.7C to the upper limit voltage of 4.2V, perform constant voltage charging until the cut-off current is 0.02C, and let it stand for 10 min; use an iron nail with a diameter of 10 mm and a tip length of 15 mm; invert the iron nail, and use the flat head surface of the iron nail to pass through the center position of the battery respectively at a needle speed of 30 mm / s, and leave the nail in the battery. Keep the time for 5 min. Test 10 batteries respectively. The batteries that do not smoke, catch fire or explode are considered to pass. Record the number of batteries N that pass the test. The larger this value is, the higher the safety of the battery is proved.

[0146] (2) Under the environment of 25℃±5℃, charge the lithium-ion secondary batteries provided in each example and comparative example at a rate of 0.7C to the upper limit voltage of 4.2V, perform constant voltage charging until the cut-off current is 0.02C, and let it stand for 10 min; use an iron nail with a diameter of 8 mm and a tip length of 10 mm; invert the iron nail, and use the flat head surface of the iron nail to pass through the center position of the battery respectively at a needle speed of 30 mm / s, and leave the nail in the battery. Keep the time for 5 min. Test 10 batteries respectively. The batteries that do not smoke, catch fire or explode are considered to pass. Record the number of batteries N that pass the test. The larger this value is, the higher the safety of the battery is proved.

[0147] (3) Under the environment of 25℃±5℃, charge the lithium-ion secondary batteries provided in each example and comparative example at a rate of 0.7C to the upper limit voltage of 4.2V, perform constant voltage charging until the cut-off current is 0.02C, and let it stand for 10 min; use an iron nail with a diameter of 8 mm and a tip length of 10 mm; invert the iron nail, and use the flat head surface of the iron nail to pass through the center position of the battery respectively at a needle speed of 5 mm / s, and leave the nail in the battery. Keep the time for 5 min. Test 10 batteries respectively. The batteries that do not smoke, catch fire or explode are considered to pass. Record the number of batteries N that pass the test. The larger this value is, the higher the safety of the battery is proved.

[0148] (4) Under the environment of 25℃±5℃, charge the lithium-ion secondary batteries provided in each example and comparative example at a rate of 0.7C to the upper limit voltage of 4.2V, perform constant voltage charging until the cut-off current is 0.02C, and let it stand for 10 min; use an iron nail with a diameter of 10 mm and a tip length of 15 mm; invert the iron nail, and use the flat head surface of the iron nail to pass through the center position of the battery respectively at a needle speed of 5 mm / s, and leave the nail in the battery. Keep the time for 5 min. Test 10 batteries respectively. The batteries that do not smoke, catch fire or explode are considered to pass. Record the number of batteries N that pass the test. The larger this value is, the higher the safety of the battery is proved.

[0149] 2. Overcharge test (the higher the charging rate and the upper limit cut-off voltage of charging, the more difficult it is to pass the test):

[0150] (1) The lithium-ion secondary batteries provided in each example and comparative example were discharged at 0.5C to the lower cut-off voltage of 2V at room temperature; the batteries were charged at 2C to the cut-off voltage of 10V, and the charging continued for more than 24h or the maximum surface temperature of the battery dropped to 20% or less of the peak temperature, then the experiment was stopped. Among them, the batteries that did not smoke, catch fire, or explode were considered to pass, and the number of batteries passing the test, M, was recorded. The larger this value, the higher the safety of the battery.

[0151] (2) The lithium-ion secondary batteries provided in each example and comparative example were discharged at 0.5C to the lower cut-off voltage of 2V at room temperature; the batteries were charged at 3C to the cut-off voltage of 10V, and the charging continued for more than 24h or the maximum surface temperature of the battery dropped to 20% or less of the peak temperature, then the experiment was stopped. Among them, the batteries that did not smoke, catch fire, or explode were considered to pass, and the number of batteries passing the test, M, was recorded. The larger this value, the higher the safety of the battery.

[0152] (3) The lithium-ion secondary batteries provided in each example and comparative example were discharged at 0.5C to the lower cut-off voltage of 2V at room temperature; the batteries were charged at 8C to the cut-off voltage of 6V, and the charging continued for more than 24h or the maximum surface temperature of the battery dropped to 20% or less of the peak temperature, then the experiment was stopped. Among them, the batteries that did not smoke, catch fire, or explode were considered to pass, and the number of batteries passing the test, M, was recorded. The larger this value, the higher the safety of the battery.

[0153] (4) The lithium-ion secondary batteries provided in each example and comparative example were discharged at 0.5C to the lower cut-off voltage of 2V at room temperature; the batteries were charged at 10C to the cut-off voltage of 6V, and the charging continued for more than 24h or the maximum surface temperature of the battery dropped to 20% or less of the peak temperature, then the experiment was stopped. Among them, the batteries that did not smoke, catch fire, or explode were considered to pass, and the number of batteries passing the test, M, was recorded. The larger this value, the higher the safety of the battery.

[0154] The electrical performance parameters were tested using a Neware electronic cabinet, and the specific test parameters are as follows:

[0155] 3. Energy density test

[0156] The mass of the lithium-ion secondary batteries prepared in each example and comparative example was measured with a balance. In a constant temperature environment of 25°C, they were charged at 0.5C with a cut-off current of 0.02C and a cut-off voltage range of 2.5V - 4.2V, and then discharged at a rate of 0.2C to 2.5V. The discharge capacity and working voltage were recorded. Through the formula: energy density = discharge capacity × working voltage / mass, the energy density was calculated and the results were recorded in the following table.

[0157] 4. Rate performance test

[0158] The lithium-ion secondary batteries prepared in each example and comparative example were placed in a constant temperature environment of 25°C and charged at 0.5C with a cut-off voltage range of 3.0V - 4.3V. Then, they were discharged at 0.5C and 1C rates respectively until 3.0V, and the capacity data during the process was recorded. The discharge capacity at 1C rate divided by the discharge capacity at 0.5C rate is the rate retention, and the results are recorded in the following table.

[0159] 5. Sheet Resistance Test

[0160] The lithium-ion secondary batteries prepared in each example and comparative example were disassembled to obtain the positive electrode sheets, and the sheet resistance of the positive electrode sheets was tested. The specific test method is as follows:

[0161] After punching the positive electrode sheet into small round pieces, place the small round piece on the test device to make its surface flat, adjust the pressure gauge to fix the test sample, ensure good contact between the probe and the sample, and use a ST2258C type multi-functional digital four-probe tester for testing. Read the data and record the results in the following table.

[0162] 6. Median Voltage Test

[0163] The lithium-ion secondary batteries prepared in each example and comparative example were placed in a constant temperature environment of 25°C and charged at 0.1C with a cut-off current of 0.01C and a cut-off voltage range of 2.5V - 4.2V. Then, they were discharged at 0.1C rate until 2.5V. Read the median voltage of this curve from the constant current charge and discharge data obtained from the Neware multi-channel battery test system. Record the results in the following table.

[0164] Table 2 Safety Performance Test Results

[0165]

[0166]

[0167] Table 3 Electrical Performance Test Results

[0168]

[0169]

[0170] As can be seen from Table 1, Table 2 and Table 3, after the electrical performance tests of the positive electrode sheets and lithium-ion secondary batteries provided in the examples and Comparative Examples 1-3, the surface resistance of the electrode sheets in the comparative examples is significantly higher than that in the examples, which in turn affects the rate performance of the batteries. This shows that the positive electrode sheet provided in this application can insert carbon nanotubes between the primary particles of the positive active material, avoid the aggregation of carbon nanotubes, form a good conductive network, improve the overall conductivity of the positive electrode sheet, facilitate the transmission of lithium ions, and alleviate the influence on the rate performance caused by the use of lithium iron phosphate manganese material with low ionic conductivity in the positive active material by compounding the ternary material with the lithium iron phosphate manganese material, using carbon nanotubes as the conductive agent, and particularly regulating the ratio between the thickness W of the carbon coating layer of the lithium iron phosphate manganese and the diameter R of the carbon nanotubes, so as to be able to take into account the energy density and rate performance while improving the safety of the needle penetration test and overcharge test.

[0171] Obviously, the above-mentioned examples are only for clear illustration and not for limitation of the implementation manners. For those of ordinary skill in the art, other different forms of changes or variations can be made on the basis of the above description. It is not necessary and impossible to list all the implementation manners here. The obvious changes or variations derived therefrom are still within the protection scope of the present invention.

Claims

1. A positive electrode sheet, characterized in that: include: Positive electrode current collector, A positive electrode active layer, disposed on at least one surface of the positive electrode current collector; Wherein, the positive electrode active layer comprises a positive electrode active material, a conductive agent and a binder, the positive electrode active material comprises a ternary material and a first lithium manganese iron phosphate material, and the conductive agent comprises carbon nanotubes; The first lithium manganese iron phosphate material includes single crystal particles, and the ternary material includes single crystal particles; The chemical formula of the ternary material is: Li e Ni a Co b Mn c M d O2, 0.95≤e≤1.1, 0.8≤a≤0.95, 0.01≤b≤0.2, 0.01≤c≤0.1, 0≤d≤0.05; M includes at least one of Al, Zr, B, Y, Sr, W, Ti and Nb; The first lithium manganese iron phosphate material includes a core and a carbon coating layer covering at least a portion of the surface of the core, The chemical formula of the core is: Li v Mn x Fe y A z PO4, wherein A comprises at least one of Al, Mg, Zn, Cu, Co, Ni, V, Zr, and Ti, wherein 0.9≤v≤1.1; 0.2≤x≤0.8; 0.2≤y≤0.8; 0.01≤z≤0.05; The thickness of the carbon coating layer is Wnm, the diameter of the carbon nanotube is Rnm, 1≤R≤50, and the thickness of the carbon coating layer and the diameter of the carbon nanotube satisfy the following relationship: 0.02≤W / R≤60.

2. The positive electrode sheet according to claim 1, characterized in that: The thickness of the carbon coating layer and the diameter of the carbon nanotube satisfy the following relationship: 0.16≤W / R≤40; and / or, the thickness of the carbon coating layer is 1≤W≤60, preferably 5≤W≤40; And / or, the diameter of the carbon nanotube is 1≤R≤30.

3. The positive electrode sheet according to claim 1, characterized in that: Based on the mass of the positive electrode active material, the mass proportion of the first lithium manganese iron phosphate material is 20%-60%, preferably 30%-50%; and / or, in the chemical formula of the core, 0.4≤x≤0.6, and / or, 0.4≤y≤0.6; and / or, the median voltage of the positive electrode active material is 3.55-3.87V; And / or, based on the total mass of the positive electrode active layer, the mass proportion of the carbon nanotubes is 0.1%-1.2%.

4. The positive electrode sheet according to claim 1, characterized in that: The positive electrode active layer is disposed on at least one side of the positive electrode current collector in the thickness direction, and the thickness of the positive electrode sheet is H μm, 70≤H≤120; The thickness of the positive electrode sheet and the diameter of the carbon nanotubes satisfy the following relationship: 1.6≤H / R≤120, preferably 3.2≤H / R≤105.

5. The positive electrode sheet according to claim 1, characterized in that: The A element includes Ti element and Mg element. Based on the total mass of the first lithium manganese iron phosphate material, the content of the Ti element ranges from 100ppm to 1000ppm, preferably from 300ppm to 700ppm. The content of the Mg element ranges from 100ppm to 5000ppm, preferably from 2000ppm to 4000ppm; Preferably, based on the total mass of the first lithium manganese iron phosphate material, the sum of the contents of the Ti element and the Mg element is ≤4500 ppm.

6. The positive electrode sheet according to claim 1, characterized in that: Based on the total mass of the first lithium manganese iron phosphate material, the content of carbon element is in the range of 0.5%-4%, preferably 1.5%-2.5%.

7. The positive electrode sheet according to claim 1, characterized in that: In the positive electrode active layer, at least part of the first lithium iron manganese phosphate material is located on at least part of the surface of the ternary material, the Dv50 of the first lithium iron manganese phosphate material is 3-8 μm, and the Dv50 of the ternary material is 1-5 μm; And / or, the specific surface area of ​​the ternary material is 0.3m 2 / g-1.5m 2 / g, the specific surface area of ​​the first lithium manganese iron phosphate material is 10m 2 / g-30m 2 / g.

8. The positive electrode sheet according to any one of claims 1, 2, 3, 5 or 6, characterized in that: The positive electrode sheet further includes a primer layer, which is disposed between the positive electrode current collector and the positive electrode active layer, and the primer layer includes a second lithium manganese iron phosphate material or a lithium iron phosphate material; and / or, the total thickness of the primer layer is 1 μm-8 μm; And / or, the thickness of the positive electrode current collector is 6 μm-15 μm.

9. A lithium ion secondary battery, characterized in that: The positive electrode sheet comprises the positive electrode sheet according to any one of claims 1 to 8, wherein the positive electrode sheet, the separator and the negative electrode sheet are stacked and wound.

10. The lithium ion secondary battery according to claim 9, characterized in that: The diaphragm comprises a base film, a first adhesive layer is arranged on one side surface of the base film, a ceramic layer is arranged on the other side surface of the base film, a second adhesive layer is arranged on the side surface of the ceramic layer away from the base film, and the ceramic layer faces the positive electrode sheet; The thickness of the ceramic layer is 0.5 μm-3 μm, preferably 1 μm-2 μm; The ceramic layer includes an inorganic material, and the inorganic material includes at least one of boehmite, magnesium oxide, magnesium hydroxide, BaSO4, CaSiO3, CaSiO4, Al2O3 and TiO2; The Dv50 of the inorganic material is 100nm-2μm.

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