A positive electrode sheet and a lithium-ion secondary battery

Abstract

The application belongs to the technical field of batteries, and particularly relates to a positive plate and a lithium ion secondary battery. The positive plate provided by the application is characterized in that the positive active material comprises a ternary material and a lithium manganese iron phosphate material. The lithium manganese iron phosphate material has good thermal stability, which improves the safety performance of the battery. In addition, the thickness W of the carbon coating layer and the tube diameter R of the carbon nanotube in the first lithium manganese iron phosphate material are specially controlled, so that W and R satisfy the condition of 0.02<=W / R<=60. The carbon nanotube can be inserted between the primary particles of the positive active material, the agglomeration of the carbon nanotube can be avoided, a good conductive network can be formed, and finally the safety of the lithium ion secondary battery in the needle test and the overcharge test can be improved while the energy density and the rate performance are taken into account.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of batteries, and particularly relates to a positive electrode sheet and a lithium ion secondary battery. BACKGROUND

[0002] In some specific use scenarios, the lithium ion secondary battery needs to pass safety tests such as needle test and overcharge test. For example, power tools such as loggers may have relatively hard needle-shaped foreign matter (such as wood chips) in the use scenario. When the logger containing the lithium ion secondary battery is placed incorrectly, it may be pierced by wood chips, causing internal short circuit of the lithium ion secondary battery, and problems such as local heating, thermal runaway, oxygen release, and even fire and explosion may occur. Overcharge test may also cause problems such as local heating, thermal runaway, oxygen release, and the like. Therefore, the lithium ion secondary battery capable of passing safety tests such as needle test and overcharge test is crucial to the safety of the use scenario.

[0003] At present, in order to improve the energy density, the positive electrode active material of the lithium ion secondary battery used by most power tools is mainly high-nickel (referring to the molar content of Ni accounting for the molar content of transition metal being 80% or more) ternary material. However, the thermal runaway temperature of pure high-nickel ternary material decreases with the increase of nickel content, and the thermal runaway temperature is generally above 200℃. When the heat production temperature is higher than this temperature during needle test or overcharge test, it is easy to cause decomposition and oxygen release of the positive electrode active material and subsequent thermal runaway, so that it cannot pass the needle test or overcharge test. In order to solve the above problems, the prior art generally uses a positive electrode active material with a higher thermal runaway temperature in combination with a high-nickel ternary material, but this will cause loss of rate performance and energy density. SUMMARY

[0004] Therefore, the present application provides a positive electrode sheet and a lithium ion secondary battery, which can improve the safety of needle test or overcharge test while achieving a balance between rate performance and energy density.

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

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

[0007] a 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 iron manganese phosphate material, and the conductive agent comprises carbon nanotubes;

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

[0011] 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 comprises at least one of Al, Zr, B, Y, Sr, W, Ti and Nb;

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

[0013] The chemical formula of the inner 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, Ti elements, wherein 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 tube diameter of the carbon nanotube is R nm, 1≤R≤50, and the thickness of the carbon coating layer and the tube diameter of the carbon nanotube satisfy the following relationship: 0.02≤W / R≤60.

[0015] In some optional embodiments, the thickness of the carbon coating layer and the tube 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 tube diameter of the carbon nanotube is 1≤R≤30.

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

[0019] And / or, in the chemical formula of the inner 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 3.55-3.87V;

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

[0022] In some alternative embodiments, the positive electrode active layer is arranged on at least one side surface of the positive electrode current collector in the thickness direction of the positive electrode current collector, the thickness of the positive electrode sheet is H μm, and 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 A element includes a Ti element and a Mg element, the content of the Ti element ranges from 100 ppm to 1000 ppm, preferably from 300 ppm to 700 ppm, based on the total mass of the first lithium iron manganese phosphate material;

[0025] The content of the Mg element ranges from 100 ppm to 5000 ppm, preferably from 2000 ppm to 4000 ppm;

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

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

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

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

[0031] and / or, the total thickness of the primer layer 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, comprising the above positive electrode sheet, wherein the positive electrode sheet, a separator and a negative electrode sheet are laminated and wound.

[0034] In some optional embodiments, the separator comprises a base film, one side surface of the base film is provided with a first adhesive layer, the other side surface of the base film is provided with a ceramic layer, the ceramic layer is provided with a second adhesive layer away from the one side surface of the base film, and the ceramic layer is opposite to the positive electrode sheet.

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

[0036] The ceramic layer comprises inorganic material, and the inorganic material comprises at least one of boehmite, magnesium oxide, magnesium hydroxide, BaSO4, CaSiO3, CaSiO4, Al2O3 and TiO2.

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

[0038] The present application has the following advantages:

[0039] The positive electrode sheet provided by the present application comprises: a positive electrode current collector; and a positive electrode active layer provided on at least one surface of the positive electrode current collector; wherein the positive electrode active layer comprises positive electrode active material, conductive agent and binder, the positive electrode active material comprises ternary material and first lithium-iron-manganese phosphate material, the conductive agent comprises carbon nanotubes, the first lithium-iron-manganese phosphate material comprises single-crystal particles, and the ternary material comprises 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 comprises at least one of Al, Zr, B, Y, Sr, W, Ti and Nb; the first lithium-iron-manganese phosphate material comprises a core and a carbon coating layer coating at least part of the surface of the core, and the chemical formula of the core of the first lithium-iron-manganese phosphate material is: Li v Mn x Fe y A zPO4, wherein A comprises at least one of Mg, Zn, Cu, Co, Ni, V, Zr, 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 W nm, the tube diameter of the carbon nanotube is R nm, 1≤R≤50, and the thickness of the carbon coating layer and the tube diameter of the carbon nanotube satisfy the following relationship: 0.02≤W / R≤60. In the positive electrode sheet provided in the application, the positive electrode active material comprises a ternary material and a lithium manganese iron phosphate material. Since the lithium manganese iron phosphate material has good thermal stability, the safety performance of the battery is improved. In combination with the special regulation of the thickness W of the carbon coating layer and the tube diameter R of the carbon nanotube in the first lithium manganese iron phosphate material, W and R satisfy: 0.02≤W / R≤60, which can enable the carbon nanotube to be inserted between the primary particles of the positive electrode active material, avoid the agglomeration of the carbon nanotube, form a good conductive network, and ultimately achieve the improvement of the safety of the needle test and the overcharge test while taking into account the energy density and the rate performance. The use of the high-energy-density ternary material and the lithium manganese iron phosphate material which has good thermal stability itself can prevent the heat generated during the needle test or the overcharge test from spreading, improve 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 in the needle test and the overcharge test. Through the special regulation of the thickness W of the carbon coating layer and the tube diameter R of the carbon nanotube in the first lithium manganese iron phosphate material, W and R satisfy: 0.02≤W / R≤60, which can enable the carbon nanotube to be inserted between the primary particles of the positive electrode active material, avoid the agglomeration of the carbon nanotube, form a good conductive network, improve the overall conductivity of the positive electrode sheet, facilitate the transmission of lithium ions, thereby improving the rate performance of the lithium ion secondary battery, relieving the impact of the use of the lithium manganese iron phosphate material with lower ionic conductivity in the positive electrode active material on the rate performance, and achieving the consideration of safety performance, energy density and rate performance.

[0040] Additional aspects and advantages of the embodiments disclosed in the application will be described and shown in part in the following description, or will be explained or illustrated by the implementation of the embodiments disclosed in the application. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the specific embodiments of the application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the description of the specific embodiments or the prior art. Obviously, the drawings described below are some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor on the basis of these drawings.

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

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

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

[0045] Reference signs:

[0046] 1, positive electrode current collector; 2, positive electrode active layer; 3, tab. DETAILED DESCRIPTION

[0047] The following examples are provided to better enable those skilled in the art to further understand the application, and are not intended to limit the content and scope of the application. Any person skilled in the art, based on the disclosure of the present application or combining the present application with other prior art features, can obtain any product identical or similar to the present application, which falls within the scope of the present application.

[0048] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application; the term "comprising" and its derivatives, as used herein, are intended to be open-ended and to mean including, but not limited to.

[0049] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and 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 explicitly and specifically limited. The meaning of the term "at least one" is one or more than two, unless otherwise explicitly and specifically limited.

[0051] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, nor is it necessarily independent or alternative to other embodiments. It is explicitly and implicitly understood by those skilled in the art 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 the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone.

[0053] Unless otherwise indicated, conventional methods of chemistry, molecular biology, recombinant technology, and biochemistry, including the generation of recombinant DNA molecules, the insertion of DNA molecules into cells, the expression of DNA molecules in cells, and the purification of proteins, were used in accordance with conventional techniques. Unless otherwise indicated, the reagents and instruments used were conventional reagents and instruments available from commercial sources.

[0054] The application provides a positive electrode sheet and a lithium ion secondary battery, which can effectively solve the defect that the means for improving the safety of the needle test and the overcharge test in the prior art causes the loss of the rate performance and the energy density of the lithium ion secondary battery. The technical solution adopted by the application is as follows.

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

[0056] a positive electrode current collector,

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

[0058] 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 iron manganese phosphate material, and the conductive agent comprises carbon nanotubes.

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

[0060] 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 comprises at least one of Al, Zr, B, Y, Sr, W, Ti, and Nb.

[0061] The first lithium iron manganese phosphate material comprises 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 manganese phosphate material 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, Ti elements, wherein 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 tube diameter of the carbon nanotube is R nm, 1≤R≤50, and the thickness of the carbon coating layer and the tube 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 tube diameter of the carbon nanotube satisfy the following relationship: 0.16≤W / R≤40.

[0065] For example, the ratio of the thickness of the carbon coating layer W to the tube diameter R of the carbon nanotube W / R is 0.02, 0.1, 0.16, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, or within a range defined by any of the above values; and the tube 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 a range defined by any of the above values.

[0066] The positive electrode sheet provided in the application, the positive electrode active material includes ternary material and lithium manganese iron phosphate material, the lithium manganese iron phosphate material has good thermal stability, which improves the safety performance of the battery, and the thickness W of the carbon coating layer in the first lithium manganese iron phosphate material and the tube diameter R of the carbon nanotube are specially controlled, so that W and R satisfy: 0.02≤W / R≤60, which can make the carbon nanotube insert between the primary particles of the positive electrode active material, avoid the agglomeration of the carbon nanotube, form a good conductive network, and finally realize the improvement of the safety of the lithium ion secondary battery in the needle test and the overcharge test, while taking into account the energy density and the rate performance. Among them, the high-energy-density ternary material is used in combination with the lithium manganese iron phosphate material which has good thermal stability, which can prevent the spread of heat generated during the needle test or overcharge, improve the thermal runaway temperature of the whole positive electrode sheet, delay the occurrence of thermal runaway of the lithium ion secondary battery, and improve the safety of the positive electrode sheet in the needle test and the overcharge test. However, due to the low intrinsic ionic conductivity of lithium manganese iron phosphate (LMFP), carbon coating can be performed on LMFP to construct a fast conductive network with the lithium manganese iron phosphate through the carbon coating layer, so that electrons can rapidly migrate between active materials during charging and discharging, thereby reducing the internal resistance and charging and discharging polarization of the battery. At the same time, the carbon nanotube is used as a conductive agent, and the thickness W of the carbon coating layer in the first lithium manganese iron phosphate material and the tube diameter R of the carbon nanotube are specially controlled, so that W and R satisfy: 0.02≤W / R≤60, which can make the carbon nanotube insert between the primary particles of the positive electrode active material, avoid the agglomeration of the carbon nanotube, form a good conductive network, improve the overall conductivity of the positive electrode sheet, and facilitate the transmission of lithium ions, thereby improving the rate performance of the lithium ion secondary battery, relieving the influence of the use of lithium manganese iron phosphate material with low ionic conductivity on the rate performance, and achieving the balance of safety performance, energy density and rate performance. When the W / R ratio is too large, the tube diameter of the carbon nanotube is relatively small compared with the thickness of the coating layer, which cannot effectively compensate for the adverse effects of the too large thickness of the coating layer on the transmission of lithium ions. In addition, the structure of the carbon nanotube is unstable and tends to agglomerate together, making it difficult to form a good conductive network. When the W / R ratio is too small, the tube diameter of the carbon nanotube is relatively large compared with the thickness of the coating layer, which makes it difficult for the carbon nanotube to insert between the primary particles of the positive electrode active material, and weakens the conductivity. In addition, carbon coating can reduce the contact between the positive electrode active material and the electrolyte, thereby avoiding side reactions with the electrolyte and improving the high-temperature performance and cycle performance. Moreover, surface carbon coating can effectively inhibit the agglomeration and growth of the modified material particles, thereby maintaining the nanostructure of the particles and effectively reducing the Li +The diffusion distance inside the active particles enables the material to have more excellent rate performance. In addition, it should be noted that the manganese iron lithium phosphate has a stable olivine structure, which determines the high stability and one-dimensional lithium ion transmission channel of the manganese iron lithium phosphate, i.e. low ion conductivity. Through special regulation of the content of manganese element, the rate performance and cycle stability can be improved 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 the material has more defects and pores, which prolongs the embedding and extraction path of lithium ions, reduces the ion migration rate, and reduces the conductivity. Due to the Jahn-Teller effect of manganese ions, manganese is dissolved out and may be deposited on the surface of the negative electrode, damaging the SEI film. At the same time, the dissolution of manganese leads to changes in the crystal structure of the manganese iron lithium phosphate material, thereby reducing the cycle stability and thermal stability. If the value of x is lower than 0.2, the manganese content is reduced, and the median voltage of the material is also reduced, and the energy density is significantly reduced.

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

[0068] It should be noted that when calculating the value of the relationship defined in the present application, the corresponding numerical value of the parameter is substituted into the relationship for calculation, and the unit of the parameter is not included.

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

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

[0071] As an example, the mass percentage of the first lithium iron manganese phosphate material can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, or within a range consisting of any of the above values, based on the mass of the positive electrode active material; if the percentage of lithium iron manganese phosphate material is too low, the safety performance is not significantly improved, and occasional failures may occur in the needle test or the fast charging test; if the percentage of lithium iron manganese phosphate material is too high, the median voltage is reduced, the energy density is significantly lost, the positive electrode active material conductivity is reduced, the electrode sheet surface resistance is increased, and the battery rate performance is deteriorated.

[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 tube diameter of the carbon nanotube is 1≤R≤30, and the thickness of the carbon coating layer and the tube 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 a range consisting of any of the above values.

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

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

[0078] The skilled in the art can understand that, in order to alleviate the influence of the low intrinsic ionic conductivity of the lithium iron manganese phosphate on the rate capability and energy density of the lithium ion secondary battery after the ternary material is used in combination with the lithium iron manganese phosphate material, the content of the carbon nanotubes is particularly controlled in the present application to further optimize the above performance; if the content of the carbon nanotubes is too low, the conductive network is not perfect enough, the surface resistance of the pole piece is large, and the transmission of lithium ions is not conducive; if the content of the carbon nanotubes is too high, the mass ratio of the positive active material in the positive active layer will be occupied, resulting in a decrease in active material and a decrease in energy density.

[0079] In some optional embodiments, the positive active layer is arranged on at least one side surface of the positive current collector in the thickness direction of the positive current collector, the thickness of the positive pole piece is H μm, 70≤H≤120, preferably 95≤H≤105; for example, the thickness of the positive pole piece 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 a range consisting of any of the above values.

[0080] The thickness H of the positive pole piece 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] For 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 a range consisting of any of the above values; the thickness of the positive pole piece can be 80 μm, 85 μm, 90 μm, 95 μm, 100 μm, 105 μm, 110 μm, 115 μm, 120 μm, or within a range consisting of any of the above values.

[0082] It should be noted that after the ternary material is used in combination with the lithium manganese iron phosphate material, due to the low intrinsic ionic conductivity of the lithium manganese iron phosphate, as the thickness of the positive electrode sheet increases, the transmission rate of lithium ions in the positive electrode sheet can be further limited, and therefore the relationship between the thickness of the positive electrode sheet and the diameter of the carbon nanotube needs to be controlled at the same time to further improve the lithium ion transmission speed and the rate performance. When the ratio H / R is too large, the diameter of the carbon nanotube is relatively small with respect to the thickness of the positive electrode sheet, at this time the contact effect between the carbon nanotube and the positive electrode active material will be reduced, the carbon nanotube cannot effectively compensate for the increase in the lithium ion transmission path caused by the excessive thickness of the electrode sheet, and the conductive network of the electrode sheet is poor; in addition, the diameter of the carbon nanotube is too small to cause agglomeration and is difficult to disperse uniformly in the positive electrode sheet to form a conductive network; and when the ratio H / R is too small, the diameter of the carbon nanotube is relatively large with respect to the thickness of the positive electrode sheet, which makes it difficult for the carbon nanotube to insert between the primary particles of the positive electrode active material, and the conductivity is weakened (rate performance loss and specific capacity reduction). The present application controls the thickness of the positive electrode sheet, because the toughness of the electrode sheet decreases with the decrease of the thickness of the electrode sheet, and process problems such as belt breakage are prone to occur during winding, and special control of the thickness of the electrode sheet can also reduce the electrode sheet breakage caused by the expansion of the negative electrode during the cycle 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 a range formed by any of the above values. If the median voltage of the positive electrode active material is too high, the material will have more defects and pores, which will prolong the lithium ion insertion and extraction path, reduce the ion migration rate, and reduce the rate performance of the battery; if the median voltage of the positive electrode active material is too low, the energy density will decrease significantly. By limiting the median voltage of the positive electrode active material to the above range, the present application can further optimize the rate performance and energy density.

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

[0085] The content of Mg element is in the range of 100 ppm-5000 ppm, preferably 2000 ppm-4000 ppm; for 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 in the range consisting of any of the above values;

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

[0087] It should be noted that after the ternary material and the lithium iron manganese phosphate material are used in combination in the positive sheet, the lithium ion transmission speed of the material is improved by increasing the lattice size through element doping, and the overall conductivity of the material is improved by combining with the surface coating of an appropriate amount of carbon material, so as to improve the rate performance of the material. The Mg doping is homovalent doping, and since the valence states are consistent, it will not cause vacancies in the crystal structure, and the material structure can maintain good integrity during the cycle process; the Ti doping can improve the crystallinity and refine the crystal 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 cycle performance of the material and reducing adverse reactions such as SEI film dissolution. If the proportion of Ti and Mg doping is too high, the specific capacity will be lost due to the replacement of active elements, and the discharge specific capacity will decrease; if the proportion of doping is too low, the effect of improving the conductivity is not obvious.

[0088] Preferably, the content of carbon element is in the range of 0.5%-4%, preferably 1.5-2.5%, based on the total mass of the first lithium iron manganese phosphate material; for example, the content of carbon element is 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or in the range consisting of any of the above values.

[0089] As can be understood by those skilled in the art, carbon is a good conductive material, and in order to alleviate the influence caused by the low electrical conductivity of the lithium iron manganese phosphate material itself, the present application utilizes the carbon coating layer and the lithium iron manganese phosphate to construct a fast conductive network, so that electrons can rapidly migrate between active substances during the charging and discharging process, thereby reducing the internal resistance and charging and discharging polarization of the battery. At the same time, the carbon coating can also reduce the contact surface between the active substance and the electrolyte, thereby avoiding side reactions with the electrolyte and improving the high temperature performance and cycle performance. In addition, the surface carbon coating can effectively inhibit the agglomeration and growth of the active material particles, thereby maintaining the nano structure of the particles, effectively reducing Li +The diffusion distance inside the active particles makes the material have more excellent rate performance. If the carbon coating content is too high, the mass energy density is lost; if the carbon coating content is too low, the material has poor conductivity, resulting in contact loss of capacity.

[0090] In the present application, the content of each element can be tested by methods and devices known in the art, for example, EDS, ICP, etc.

[0091] 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; as an example, the Dv50 of the first lithium iron manganese phosphate material can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or within a range consisting 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 a range consisting 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 manganese phosphate 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 a range consisting of any of the above values; the specific surface area of the first lithium iron manganese phosphate 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 a range consisting of any of the above values.

[0093] In the present application, by special regulation of the Dv50 of the first lithium manganese iron phosphate material and the ternary material, the lithium manganese iron phosphate can be coated on the surface of the ternary material, the overall specific surface area of the composite material is reduced, and the special regulation of the specific surface area of the ternary material and the lithium manganese iron phosphate material is beneficial to control the slurry solid content and viscosity during homogenization, which is beneficial to the uniform dispersion of carbon nanotubes, further optimizes the conductive network, improves the overall conductivity of the pole piece, and is beneficial to the rapid transmission of lithium ions and improves the rate performance.

[0094] In the present application, the lithium manganese iron 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 optional embodiments, the positive pole piece further comprises a primer layer, the primer layer is arranged between the positive current collector and the positive active layer, and the primer layer comprises a second lithium manganese iron phosphate material or a lithium iron phosphate material.

[0096] And / or, the total thickness of the primer layer is 1-8 μm, for example, the total thickness of the primer layer (if coated on both sides of the positive current collector, it refers to the total thickness of the two primer layers) can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, or within a range consisting of any of the above values.

[0097] It should be noted that in order to improve the needle safety of lithium ion secondary batteries, the related technology introduces a primer layer in the positive pole piece, that is, a primer layer is coated on the positive current collector, and then a positive active material is coated on the primer layer. Since the thermal runaway temperature of the ternary material is low and the heat production is high, in order to further balance the instability of the ternary material, a low-conductivity material such as lithium iron phosphate or lithium manganese iron phosphate is used as a primer material to coat on the surface of the positive current collector. During the needle test, by increasing the internal short-circuit resistance, reducing the short-circuit current, and reducing the heat generation, the thermal runaway is avoided, and the safety of the needle test or overcharge test of the battery cell is increased.

[0098] And / or, the thickness of the positive current collector is 6-15 μm; for example, the thickness of the positive current collector can be 6 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, or within a range consisting of any of the above values.

[0099] In the present application, by adjusting the thickness of the positive current collector, the utilization efficiency of the space of the battery cell can be considered while avoiding the striping of the electrode sheet. If the thickness of the positive current collector is too small, when the positive electrode sheet is rolled, the harder positive active material will be embedded into the positive current collector under the vertical pressure, which may cause local puncture, and due to the uneven surface of the material or unreasonable rolling parameters, the material may be deformed locally, thereby forming a striping of the electrode sheet. If the thickness of the positive current collector is too large, the utilization efficiency of the space of the battery cell will be significantly reduced.

[0100] In some alternative embodiments, a plurality of tabs extending from the width direction of the positive electrode sheet are further included, 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. For example, the number of the tabs can be 1, 2, 5, 10, 20, 25, 30, 35, 40, 46, or within a range 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, since the lithium iron manganese phosphate material has a relatively low ionic conductivity, the use of the ternary material will reduce the rate performance of the battery. By increasing the number of tabs on the positive electrode sheet, the present application can increase the transmission path of lithium ions between the electrode sheets, improve the conductivity of the electrode sheet, compensate for the insufficient conductivity of the composite positive electrode material, enhance the electrical contact of the electrode sheet, and improve the rate discharge performance, so that the capacity of the battery cell is not significantly lost.

[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 still another aspect of the present application, a lithium ion secondary battery is provided, which includes the above-mentioned positive electrode sheet, a separator, and a negative electrode sheet. The lithium ion secondary battery has the same effects as the above-mentioned positive electrode sheet, and will not be described here.

[0105] In some alternative embodiments, the lithium ion secondary battery can be a wound battery, and the positive electrode sheet, the separator, and the negative electrode sheet can be sequentially stacked and wound to form a wound battery cell.

[0106] In some alternative embodiments, the separator includes a base film, a glue layer is provided on one side surface of the base film, and a ceramic layer is provided on the other side surface of the base film, and the ceramic layer faces the positive electrode sheet;

[0107] The ceramic layer has a thickness of 0.5 pm to 3 pm, preferably 1 to 2 pm. As an example, the ceramic layer has a thickness of 0.5 pm, 1 pm, 1.5 pm, 2 pm, 2.5 pm, 3 pm, or within a range consisting of any of the above values.

[0108] It should be noted that the ceramic layer includes an inorganic material including at least one of boehmite (hydrated aluminum hydroxide), magnesium oxide, magnesium hydroxide, BaS04, CaSi03, CaSi04, AI2O3, and Ti02; the inorganic material has a Dv50 of 100 nm to 2 pm; as an example, the ceramic layer can have a Dv50 of 100 nm, 300 nm, 500 nm, 0.1 pm, 0.2 pm, 0.5 pm, 1 pm, 1.2 pm, 1.5 pm, 1.8 pm, 2 pm, or within a range consisting of any of the above values. The adhesive layer includes PVDF or PMMA, etc.

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

[0110] As understood by those skilled in the art, during the charging and discharging of the battery, lithium ions are embedded and extracted between the positive and negative electrode sheets, and the electrolyte plays a role in conducting ions between the positive and negative electrode sheets. The separator is arranged between the positive and negative electrode sheets and mainly plays a role in preventing a short circuit between the positive and negative electrodes, while 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, and the negative electrode current collector has two opposite surfaces in the thickness direction thereof, and the negative electrode active material layer is arranged on any one or both of the two opposite surfaces of the negative electrode current collector. The material, composition, and manufacturing method of the negative electrode sheet used in the lithium ion secondary battery of the present application can include any technology disclosed in the prior art.

[0112] The material and shape of the separator used in the lithium ion secondary battery of the present application are not particularly limited, and 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 of the techniques disclosed in the prior art.

[0114] The present application will be further described in detail below in connection with specific examples, which are not to be understood as limiting the scope of the application as claimed. In all examples and comparative examples of the present application, the unit % represents the mass percentage.

[0115] Example 1

[0116] The present example provides a positive electrode sheet and a lithium ion secondary battery containing the same, wherein the structural diagram of the positive electrode sheet is shown as Figure 2 including: a positive electrode current collector 1, a positive electrode active layer 2, provided on at least one surface of the positive electrode current collector 1; further including a plurality of tabs 3 extending out of the positive electrode sheet in the width direction of the positive electrode sheet; the specific composition and preparation method of the lithium ion secondary battery are as follows:

[0117] (1) Preparation of positive electrode sheet

[0118] A bottom coating layer with a thickness of 1.5 μm is prepared, and the specific preparation method is as follows: lithium iron phosphate, conductive carbon black and binder PVDF are mixed together in a mass ratio of 97.4:1.5:1.1, NMP is used as the dispersion medium, and a slurry with a certain viscosity is prepared by high-speed stirring, then the slurry is coated on both sides of the positive electrode current collector (10 μm thick aluminum foil) by a coating device and dried to form a bottom coating layer, and the thickness of the bottom coating layer on one side of the positive electrode current collector is controlled to be 1.5 μm.

[0119] The positive electrode active material (the first particles and the second particles are premixed in a mass ratio of 1:1, and the proportion of lithium manganese iron phosphate in the positive electrode active material is M%, and the proportion in this example is 50%), polyvinylidene fluoride, single-walled carbon nanotubes with a tube diameter of 3 nm, multi-walled carbon nanotubes with a tube diameter of 20 nm (the average tube diameter of the single-walled carbon nanotubes and the multi-walled carbon nanotubes (i.e. the tube diameter of the carbon nanotubes) is 10 nm), and conductive carbon black are mixed uniformly in a mass ratio of 97.4:1.2:0.1:0.7:0.6, N-methyl pyrrolidone (NMP) is added, and a positive electrode slurry with a solid content of 65% is obtained; the positive electrode slurry is uniformly coated on the aluminum foil with the bottom coating layer using a coating machine, dried, rolled, die-cut, and sheeted to obtain a positive electrode sheet with a thickness of 100 μm; wherein the compaction density of the positive electrode sheet is 2.9 g / cm 3 , the area density is 13 g / cm 2 ; the first particles are ternary materials, the chemical formula of the first particles is LiNi 0.93 Co 0.03 Mn 0.02 Al 0.02 O2, the specific surface area of the first particles is 0.65 m2 / g, and the particle size Dv50 of the second particles was 4.7 μm; the second particles were lithium iron manganese phosphate material, the second particles included a core and a carbon coating layer, the chemical formula of the core was LiMn 0.4 Fe 0.58 A 0.02 PO4, A was Mg and Ti, the mass content of the element Mg in the second particles was 3000 ppm, the mass content of the element Ti in the second particles was 500 ppm, the mass content of the element C in the second particles was 1.75%, the specific surface area of the second particles was 18.5 m 2 / g, the thickness of the coating layer in the second particles was 38 nm, and the particle size Dv50 of the second particles was 2.5 μm. Sixteen corresponding tabs were punched out in the reserved foil (aluminum foil) area by a knife die, and a multi-tab tab was prepared. Figure 1 The SEM image of the positive tab was shown in FIG. 6, and it could be seen from the image that the positive active material was uniformly distributed in the tab. The darker black part in the image was the lithium iron manganese phosphate material, and the lighter white part was the ternary material.

[0120] (2) Preparation of the negative tab

[0121] The artificial graphite, single-walled carbon nanotubes, multi-walled carbon nanotubes, styrene-butadiene rubber and sodium carboxymethyl cellulose were mixed uniformly according to a mass ratio of 96.1:0.25:0.15:2.9:0.6 to obtain a material. 1% of the total material by mass of ethylene carbonate was added, and deionized water was added to obtain a negative electrode slurry (solid content of 45%). The negative electrode slurry was uniformly coated on a 6 μm high-strength coated copper foil, and after drying, rolling, die cutting and tab preparation, a negative tab with a thickness of 83 μm was obtained. Sixteen corresponding tabs were punched out in the reserved foil (aluminum foil) area by a knife die, and a multi-tab tab was prepared. The compaction density of the negative tab was 1.55 g / cm 3 , and the area density was 6 g / cm 2 .

[0122] (3) Preparation of the electrolyte

[0123] In an inert gas (argon) filled glove box (H2O <0.1 ppm, O2 <0.1 ppm), ethylene carbonate, propylene carbonate, diethyl carbonate and propyl propionate were mixed uniformly according to a mass ratio of 15:10:10:65, and then 1.25 mol / L of fully dried lithium hexafluorophosphate was quickly added thereto, stirred uniformly, and finally 0.5% of butanedinitrile based on the total mass of the electrolyte was added. After passing the water and free acid detection, the required electrolyte was 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 a separator (including a 5-μm-thick polyethylene base material, a 2-μm-thick ceramic layer (material: alumina, Dv50: 1 μm) on one side of the base material, and a 2-μm-thick polyvinylidene fluoride adhesive layer on the other side of the base material) were wound by a winding machine to obtain a multi-tab wound battery cell in which the positive and negative electrode sheets were separated by the separator (a structure thereof is schematically shown in Figure 3 The lithium ion secondary battery was obtained by further performing welding, packaging, liquid injection, formation, gas pocket cutting, and sorting processes.

[0126] Examples 2 to 25

[0127] Examples 2 to 25 were prepared by the same method as in Example 1, except for the differences shown in Table 1.

[0128] Example 26

[0129] The positive electrode sheet and the lithium ion secondary battery containing the same provided in this example differ from Example 1 only in that LiNi 0.8 Co 0.1 Mn 0.08 Al 0.02 O2was used instead of LiNi 0.93 Co 0.03 Mn 0.02 Al 0.02 O2in the same mass in the positive electrode sheet.

[0130] Example 27

[0131] The positive electrode sheet and the lithium ion secondary battery containing the same provided in this example differ from Example 1 only in that LiNi 0.95 Co 0.02 Mn 0.02 Al 0.01 O2was used instead of LiNi 0.93 Co 0.03 Mn 0.02 Al 0.02 O2in the same mass in the positive electrode sheet.

[0132] Example 28

[0133] The positive electrode sheet and the lithium ion secondary battery containing the same provided in this example differ from Example 1 only in that LiMn 0.48 Fe 0.5 A 0.02 PO4was used instead of LiMn 0.4 Fe 0.58 A 0.02 PO4in the positive electrode sheet.

[0134] Example 29

[0135] The positive electrode sheet and the lithium ion secondary battery containing the same provided by the present example differ from Example 1 only in that LiMn 0.4 Fe 0.57 A 0.03 PO4is replaced by LiMn 0.4 Fe 0.58 A 0.02 PO4.

[0136] Comparative Example 1

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

[0138] Comparative Examples 2-3

[0139] The positive electrode sheet and the lithium ion secondary battery containing the same provided by the present comparative example differ from Example 20 only in that the W / R is not within the range defined in the present application by adjusting the tube diameter of the carbon nanotubes and the thickness of the carbon coating layer, as shown in Table 1.

[0140] Table 1

[0141]

[0142]

[0143] Test Example

[0144] 1. Needle-punch 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 needle speed, not only the steel needle part has temperature rise, but also the tab of the short-circuited electrode sheet has temperature rise, and the heat will spread from the steel needle and the tab as starting points, and it is more difficult to pass the test):

[0145] (1) At 25℃±5℃, the lithium ion secondary batteries provided by each of the examples and comparative examples are charged at 0.7C rate to the upper limit voltage of 4.2V, and the constant voltage is charged to the cut-off current of 0.02C, and then it is left for 10 min; an iron nail with a diameter of 10 mm and a needle tip length of 15 mm is used; the iron nail is inverted, and the flat head of the iron nail is used to pass through the center position of the battery, the needle speed is 30 mm / s, and the nail is left in the battery. The holding time is 5 min. Ten batteries are tested respectively, and the batteries that do not smoke, do not catch fire, and do not explode are passed, and the number N of batteries that pass the test is recorded, and the larger the value, the higher the safety of the battery.

[0146] (2) 25℃±5℃ environment, the lithium ion secondary battery provided by each example and comparative example is charged to the upper limit voltage 4.2V at 0.7C rate, constant voltage charging to the cut-off current 0.02C, static 10min;Using the diameter 8mm, the needle tip length 10mm iron nail;Iron nail upside down, using the flat head of iron nail respectively through the center position of the battery, needle speed 30mm / s, nail left in the battery.Keep time 5min.10 batteries are tested respectively, and the battery that does not smoke, does not fire, does not explode is passed, and the number N of passed batteries is recorded, the larger the value is, the higher the safety of the battery is.

[0147] (3) 25℃±5℃ environment, the lithium ion secondary battery provided by each example and comparative example is charged to the upper limit voltage 4.2V at 0.7C rate, constant voltage charging to the cut-off current 0.02C, static 10min;Using the diameter 8mm, the needle tip length 10mm iron nail;Iron nail upside down, using the flat head of iron nail respectively through the center position of the battery, needle speed 5mm / s, nail left in the battery.Keep time 5min.10 batteries are tested respectively, and the battery that does not smoke, does not fire, does not explode is passed, and the number N of passed batteries is recorded, the larger the value is, the higher the safety of the battery is.

[0148] (4) 25℃±5℃ environment, the lithium ion secondary battery provided by each example and comparative example is charged to the upper limit voltage 4.2V at 0.7C rate, constant voltage charging to the cut-off current 0.02C, static 10min;Using the diameter 10mm, the needle tip length 15mm iron nail;Iron nail upside down, using the flat head of iron nail respectively through the center position of the battery, needle speed 5mm / s, nail left in the battery.Keep time 5min.10 batteries are tested respectively, and the battery that does not smoke, does not fire, does not explode is passed, and the number N of passed batteries is recorded, the larger the value is, the higher the safety of the battery is.

[0149] 2, overcharge test (the larger the charge rate is, the higher the upper limit cut-off voltage is, the more difficult the test is passed):

[0150] (1) the lithium ion secondary battery provided by each example and comparative example is discharged to the lower limit cut-off voltage 2V at 0.5C at room temperature;The battery is charged to 10V cut-off at 2C, and the experiment is stopped when the battery continues to charge for more than 24h or the maximum temperature of the battery surface drops to 20% or less of the peak temperature. Among them, the battery that does not smoke, does not fire, does not explode is passed, and the number M of passed batteries is recorded, the larger the value is, the higher the safety of the battery is.

[0151] (2) The lithium ion secondary batteries provided by each example and comparative example were discharged at 0.5C to the lower limit cutoff voltage 2V at room temperature; the batteries were charged at 3C to the 10V cutoff, and the experiment was stopped when the batteries were continuously charged for more than 24h or the maximum temperature of the battery surface dropped to 20% or less of the peak temperature. The batteries that did not smoke, catch fire or explode were passed, and the number M of batteries that passed the test was recorded. The larger this value, the higher the safety of the battery.

[0152] (3) The lithium ion secondary batteries provided by each example and comparative example were discharged at 0.5C to the lower limit cutoff voltage 2V at room temperature; the batteries were charged at 8C to the 6V cutoff, and the experiment was stopped when the batteries were continuously charged for more than 24h or the maximum temperature of the battery surface dropped to 20% or less of the peak temperature. The batteries that did not smoke, catch fire or explode were passed, and the number M of batteries that passed the test was recorded. The larger this value, the higher the safety of the battery.

[0153] (4) The lithium ion secondary batteries provided by each example and comparative example were discharged at 0.5C to the lower limit cutoff voltage 2V at room temperature; the batteries were charged at 10C to the 6V cutoff, and the experiment was stopped when the batteries were continuously charged for more than 24h or the maximum temperature of the battery surface dropped to 20% or less of the peak temperature. The batteries that did not smoke, catch fire or explode were passed, and the number M of batteries that passed the test was recorded. The larger this value, the higher the safety of the battery.

[0154] The electrical performance parameters were tested using a new Wei electric cabinet, and the specific test parameters were as follows:

[0155] 3. Energy density test

[0156] The lithium ion secondary batteries prepared from each example and comparative example were measured for mass using a balance, and were charged at 0.5C in a 25°C constant temperature environment, with a cutoff current of 0.02C and a cutoff voltage range of 2.5V-4.2V. Then the batteries were discharged at 0.2C to 2.5V, and the discharge capacity and working voltage were recorded. The energy density was calculated by the formula: energy density = discharge capacity x working voltage / mass, and the results were recorded in the table below.

[0157] 4. Rate test

[0158] The lithium ion secondary batteries prepared from each example and comparative example were placed in a 25°C constant temperature environment and charged at 0.5C, with a cutoff voltage range of 3.0V-4.3V. Then the batteries were discharged at 0.5C and 1C, respectively, to 3.0V, and the capacity data during the process was recorded. The rate retention was calculated by dividing the 1C discharge capacity by the 0.5C discharge capacity, and the results were recorded in the table below.

[0159] 5. Surface resistance test

[0160] The lithium ion secondary batteries prepared from each of the examples and the comparative examples were disassembled to obtain positive electrode sheets, and the positive electrode sheets were subjected to surface resistance testing. The specific testing method was as follows:

[0161] After the positive electrode sheets were punched into small round sheets, the small round sheets were placed on a testing device to make the surface flat, a pressure gauge was adjusted to fix the testing sample, and the probe was ensured to be in good contact with the sample. A ST2258C multifunctional digital four-probe tester was used for testing, and the readings were recorded in the following table.

[0162] 6. Median voltage testing

[0163] The lithium ion secondary batteries prepared from each of the examples and the comparative examples were placed in a constant temperature environment of 25°C, and were charged at 0.1C, with a cutoff current of 0.01C and a cutoff voltage range of 2.5V-4.2V, and then were discharged at 0.1C to 2.5V. The constant current charge-discharge data obtained from the new Wei multi-channel battery testing system was read out from the system to obtain the median voltage of the curve. The results are recorded in the following table.

[0164] Table 2. Safety performance test results

[0165]

[0166]

[0167] Table 3. Electrical performance test results

[0168]

[0169]

[0170] It can be seen from Tables 1, 2 and 3 that the surface resistance of the positive electrode sheets and the lithium ion secondary batteries provided by the examples and Comparative Example 1-Comparative Example 3 is significantly higher than that of the examples after the electrical performance test, which further affects the rate performance of the batteries. This shows that the positive electrode sheet provided by the present application can make the carbon nanotubes insert between the primary particles of the positive electrode active material, avoid the agglomeration of the carbon nanotubes, form a good conductive network, improve the overall conductivity of the positive electrode sheet, facilitate the transmission of lithium ions, alleviate the influence of the use of the manganese iron lithium phosphate material with lower ionic conductivity on the rate performance in the positive electrode active material, and achieve the ability to improve the needle test and overcharge test safety while taking into account the energy density and rate performance.

[0171] Obviously, the above embodiments are merely example for clearly illustrating but not limitation to 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 the embodiments need not and can not be enumerated. The obvious changes or variations derived from this still fall within the protection scope of the present application.

Claims

1. A positive electrode sheet characterized by comprising: The positive electrode active layer is arranged on at least one side surface of the positive electrode current collector in the thickness direction of the positive electrode current collector, the thickness of the positive electrode sheet is H pm, and 70≤H≤120. The thickness of the positive electrode sheet and the tube diameter of the carbon nanotube satisfy the following relationship: 1.6≤H / R≤120. The A element includes a Ti element and a Mg element, the content of the Ti element ranges from 100 ppm to 1000 ppm based on the total mass of the first lithium iron manganese phosphate material; The content of the Mg element ranges from 100 ppm to 5000 ppm. The content of the carbon element ranges from 0.5% to 4% based on the total mass of the first lithium iron manganese phosphate material. 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 mass ratio of the first lithium iron manganese phosphate material is 30%-50%; The chemical formula of the inner core is: Li v Mn x Fe y A z PO4, wherein A includes at least one of Al, Mg, Zn, Cu, Co, Ni, V, Zr, Ti elements, wherein 0.9≤v≤1.1; 0.2≤x≤0.8; 0.2≤y≤0.8; 0.01≤z≤0.05; And / or, the thickness of the positive electrode sheet and the tube diameter of the carbon nanotube satisfy the following relationship: 3.2≤H / R≤105; 2. The positive electrode sheet according to claim 1, characterized by And / or, the content of the Ti element ranges from 300 ppm to 700 ppm based on the total mass of the first lithium iron manganese phosphate material; And / or, the content of the Mg element ranges from 2000 ppm to 4000 ppm; And / or, the sum of the contents of the Ti element and the Mg element is ≤4500 ppm based on the total mass of the first lithium iron manganese phosphate material; 3. The positive electrode sheet according to claim 1, characterized by And / or, the content of the carbon element ranges from 1.5% to 2.5% based on the total mass of the first lithium iron manganese phosphate material. 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 pm, and the Dv50 of the ternary material is 1-5 pm. ​ ​ 4. The positive electrode sheet according to claim 1, characterized by ​ ​ 5. The positive electrode sheet according to claim 1, characterized by ​ ​ 6. The positive electrode sheet according to claim 1, characterized by ​ 7. The positive electrode sheet according to any one of claims 3 to 6, characterized by, ​ ​ ​ ​ ​ ​ 8. The positive electrode sheet according to claim 1, characterized by ​ and / or the specific surface area of the ternary material is 0.3 m 2 / g-1.5 m 2 / g, the specific surface area of the first lithium iron manganese phosphate material is 10 m 2 / g-30 m 2 / g.

9. The positive electrode sheet according to any one of claims 1, 2, 3, 5, 6, or 8, characterized by, The positive electrode sheet further comprises a base coating layer, the base coating layer is arranged between the positive electrode current collector and the positive electrode active layer, the base coating layer comprises a second lithium iron manganese phosphate material or a lithium iron phosphate material; And / or, the total thickness of the base coating layer is 1-8 μm; And / or, the thickness of the positive electrode current collector is 6-15 μm.

10. A lithium-ion secondary battery, characterized by comprising: The positive electrode sheet comprises the positive electrode sheet according to any one of claims 1-9, wherein the positive electrode sheet, the separator and the negative electrode sheet are laminated and wound.

11. The lithium-ion secondary battery according to claim 10, characterized by The separator comprises a base film, one side surface of the base film is provided with a first adhesive layer, the other side surface of the base film is provided with a ceramic layer, the ceramic layer is provided with a second adhesive layer away from the one side surface of the base film, and the ceramic layer faces the positive electrode sheet; The thickness of the ceramic layer is 0.5-3 μm; The ceramic layer comprises an inorganic material, the inorganic material comprises at least one of boehmite, magnesium oxide, magnesium hydroxide, BaSO4, CaSiO3, CaSiO4, Al2O3 and TiO2; The Dv50 of the inorganic material is 100 nm-2 μm.

12. The lithium-ion secondary battery according to claim 11, characterized by The thickness of the ceramic layer is 1-2 μm.

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