Lithium ion secondary battery, battery device, and power device

Abstract

The invention provides a lithium ion secondary battery, a battery device and an electric device. The lithium ion secondary battery comprises a positive pole piece, a negative pole piece and an electrolyte, the positive pole piece comprises a positive current collector and a positive film layer arranged on at least one side of the positive current collector, the positive film layer comprises a positive active material, and the positive active material comprises lithium-containing transition metal phosphate particles of which at least partial surfaces are provided with carbon coating materials; in a graphitization degree C value cumulative distribution curve obtained by the positive electrode film layer in a surface scanning mode of a laser microscopic confocal Raman spectrometer, the median C50 of the graphitization degree is greater than or equal to 0.95 and less than or equal to 1.20, and the concentration ratio (C90-C10) / C50 of the C value is 0.01-0.04; wherein the graphitization degree C value is IG / ID, IG represents the G peak intensity of the Raman spectrum at the position of 1580 + / -100 cm <-1 >, and ID represents the D peak intensity of the Raman spectrum at the position of 1350 + / -100 cm <-1 >.

Description

[0001] Cross-reference This application incorporates by reference in its entirety the PCT international application entitled "Lithium-ion secondary battery, battery device, power-consuming device, preparation method of positive electrode active material, and preparation method of positive electrode sheet" filed on March 28, 2025, with application number PCT / CN2025 / 085940. Technical field

[0002] This application relates to the technical field of lithium-ion batteries, and particularly to a lithium-ion secondary battery, a battery device, a power-consuming device, a preparation method of a positive electrode active material, and a preparation method of a positive electrode sheet. Background art

[0003] In recent years, secondary batteries have been widely used in energy storage power systems such as hydroelectric, thermal, wind, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace.

[0004] The positive electrode active material is an important component of a lithium-ion secondary battery. The lithium-containing transition metal phosphate material has the characteristics of stable structure, good safety, and long cycle life, and has broad development prospects. With the increasing requirements of the market for the energy density and kinetics of lithium-containing transition metal phosphate-based secondary batteries, it is difficult to simultaneously improve the above performances in the prior art, which has become a technical problem that urgently needs to be solved in this field. Summary of the invention

[0005] This application is made in view of the above problems, and its purpose is to provide a lithium-ion secondary battery with both high energy density and good kinetic performance.

[0006] The first aspect of this application provides a lithium-ion secondary battery, which includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. Among them, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one side of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material. The positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material provided on at least part of the surface. In the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser confocal Raman spectrometer for the positive electrode film layer, the median C of the graphitization degree 50 is greater than or equal to 0.95 and less than or equal to 1.20, and the concentration of the C value (C 90 - C 10 ) / C 50 is 0.01 - 0.04; among them, the graphitization degree C value is I G / I D , I G represents the Raman spectrum at 1580 ± 100 cm -1The intensity of the G peak at [location], I D represents the intensity of the D peak in the Raman spectrum at 1350 ± 100 cm -1 [location].

[0007] The median C of the graphitization degree of the examples of this application 50 is greater than or equal to 0.95 and less than or equal to 1.20 and simultaneously controls the concentration of the C value (C 90 - C 10 ) / C 50 is 0.01 - 0.04, indicating a high graphitization degree and a high degree of graphitization consistency of the particles in the positive electrode film layer, meaning that the positive electrode active material has good coating uniformity and consistency, which can reduce the hindrance of particle slip caused by the low graphitization degree of the particles in the positive electrode active material and the resulting local stress concentration. Therefore, through the uniform and consistent slip between the positive electrode active material particles, the electrode sheet can achieve a relatively high compaction density under a relatively low rolling pressure as a whole. At the same time, the uniform and consistent graphitization degree is conducive to the uniform and consistent insertion and extraction of lithium ions, maintaining the good kinetic performance of the battery and improving the compaction density of the electrode sheet and the energy density of the battery. In any implementation manner, in the cumulative distribution curve of the graphitization degree C value obtained by the positive electrode film layer in the surface scanning mode of the laser confocal Raman spectrometer, the median C of the graphitization degree 50 is 0.96 - 1.15, and further optionally 0.98 - 1.13.

[0008] The median C of the graphitization degree of the positive electrode film layer 50 within the above range is beneficial to further improve the easy slip degree between particles, further improve the compaction density of the electrode sheet while maintaining the high kinetic performance of the battery, and achieve the balance between the battery kinetic performance and the energy density.

[0009] In any implementation manner, in the cumulative distribution curve of the graphitization degree C value obtained by the positive electrode film layer in the surface scanning mode of the laser confocal Raman spectrometer, the concentration of the C value (C 90 - C 10 ) / C 50 is 0.02 - 0.038, and optionally 0.02 - 0.036.

[0010] The concentration of the C value (C 90 - C 10 ) / C 50 within the above range is beneficial to further improve the consistency of the graphitization degree of the particles in the positive electrode film layer, improve the consistency of particle slip, reduce the inconsistent lithium insertion rate in the positive electrode film layer due to the poor graphitization degree consistency, and further cause local polarization, and further improve the kinetic performance of the battery on the basis of maintaining the good energy density of the battery.

[0011] In any embodiment, in the cumulative distribution curve of the graphitization degree C value obtained by the laser confocal Raman spectrometer in the surface scanning mode of the positive electrode film layer, the C of the graphitization degree 90 is 1.0 - 1.30, and can be optionally 1.02 - 1.15.

[0012] The C of the graphitization degree 90 Within the above range, it is relatively close to the median C of the graphitization degree 50 indicating that the distribution range of the graphitization degree of the particles in the positive electrode film layer is narrow, which is conducive to the uniform slip between the particles to improve the compaction density of the positive electrode sheet.

[0013] In any embodiment, in the cumulative distribution curve of the graphitization degree C value obtained by the laser confocal Raman spectrometer in the surface scanning mode of the positive electrode film layer, the C of the graphitization degree 10 is 0.92 - 1.10, and can be optionally 0.98 - 1.08.

[0014] The C of the graphitization degree 10 Within the above range indicates that the graphitization degree is high at different sites in the positive electrode film layer, which is conducive to the uniform slip of the particles and reduces the occurrence probability of local stress concentration, further improving the compaction density of the electrode sheet.

[0015] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of the particles with an area of 0.001μm 2 - 0.06μm 2 is 21.00% - 27.00%, and the area ratio of the particles with an area of 1.0μm 2 - 4.0μm 2 is 12.00% - 20.00%.

[0016] Controlling the area ratio of the particles with an area of 0.001μm 2 - 0.06μm 2 and the area ratio of the particles with an area of 1.0μm 2 - 4.0μm 2 within the above range, combined with uniform high graphitization degree, can achieve uniform slip of the particles during the compaction of the electrode sheet, improve the compaction density of the electrode sheet, and reduce the negative impact on other performances such as the battery kinetics performance, cycle life, and processing performance, comprehensively improving the battery performance.

[0017] In any embodiment, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the D of the particles A50 is 600nm - 800nm, and can be optionally 650nm - 750nm, where D A50It refers to the particle size corresponding to 50% of the cumulative area distribution of particles in the area cumulative distribution curve of particles.

[0018] The particle size D of the particles A50 Within the above range, it indicates that there are a certain number of large-sized particles in the positive electrode film layer. Controlling the median particle size D A50 Within the above range, the transfer efficiency of the rolling pressure between the particles in the electrode can be improved through the large contact area between the large-sized particles, and the skeleton support effect of the large-sized particles can be fully exerted, enabling the electrode to withstand a higher rolling pressure, increasing the compaction density of the electrode, and at the same time reducing the kinetic decline caused by overly large particle sizes, maintaining the kinetic performance of the battery while increasing the compaction density of the electrode.

[0019] In any implementation manner, in the particle roughness area cumulative distribution curve obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode, the median roughness R A50 is 0.92 - 0.96.

[0020] The median roughness R A50 Within the above range, the surface of the particles is relatively smooth, the friction force between the particles is relatively small, and it is easy to slip under the action of external forces. When combined with particles with a high degree of graphitization, the compaction density of the electrode can be increased even under a low rolling pressure, further improving the energy density of the battery.

[0021] In any implementation manner, in the particle sphericity area cumulative distribution curve obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode, the sphericity L A90 is 0.80 - 0.95, and can be optionally 0.85 - 0.93.

[0022] In any implementation manner, in the particle sphericity area cumulative distribution curve obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode, the median sphericity L A50 is 0.65 - 0.85, and can be optionally 0.70 - 0.80.

[0023] The median sphericity L A90 、L A50 Within the above range, the particles are approximately spherical, and it is easy for the particles to slip between each other under the action of external forces. When combined with particles with a high degree of graphitization, the compaction density of the electrode can be increased even under a low rolling pressure, further improving the energy density of the battery.

[0024] In any implementation manner, the positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer is 500 ppm - 2000 ppm, and can be optionally 500 ppm - 1500 ppm.

[0025] The positive electrode active material with an iron dissolution rate within the above range has a relatively complete and dense carbon coating layer, which can improve the electrical contact between positive electrode active materials, improve the conductivity of the positive electrode active materials, reduce the polarization of the positive electrode active materials, and further optimize the kinetic performance of the lithium-ion secondary battery. At the same time, the densely coated carbon layer has a low space occupancy rate, and the particle gaps are easily compressed by stress during the rolling process. With a highly graphitized carbon coating layer, it is more conducive to improving the compaction density of the electrode sheet and the energy density of the battery.

[0026] In any embodiment, based on the total mass of the positive electrode active material, the mass content of carbon element is 0.8% - 1.8%, and can be optionally 0.90% - 1.5%.

[0027] Compared with the lithium-containing transition metal phosphate positive electrode active material in the prior art, this positive electrode active material has a relatively low content of carbon coating, which can further increase the loading amount of lithium-containing transition metal phosphate in the positive electrode sheet and improve the energy density of the lithium-ion secondary battery.

[0028] In any embodiment, the positive electrode active material contains iron element, and the lithium-iron antisite defect concentration of the positive electrode active material is 0.1% - 1.5%, and can be optionally 0.3% - 1.0%.

[0029] The positive electrode active material in the embodiment of this application has low lithium-iron antisite defects, which is beneficial to the uniform transmission of lithium ions in the solid phase and further improves the kinetic performance of the lithium-ion secondary battery.

[0030] In any embodiment, the lithium-containing transition metal phosphate includes components with the following general formula: Li m Fe x P y O j Q q , where Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8 ≤ m ≤ 1.15, 0.9 ≤ x ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 < q ≤ 0.1.

[0031] Selecting appropriate modified element Q can improve the ion diffusion path of the positive electrode active material, improve the lithium-ion diffusion rate of the positive electrode active material, and improve the kinetic performance of the battery.

[0032] In any embodiment, the positive electrode active material includes one or more of lithium iron phosphate and its doped and modified materials, coated and modified materials.

[0033] In any embodiment, the positive electrode active material includes titanium element, and based on the total mass of the positive electrode active material, the mass content of the titanium element is 2000 ppm - 6000 ppm.

[0034] The positive electrode active material in the embodiments of the present application has a high content of titanium element. Moreover, surprisingly, the high addition amount of titanium element does not form harmful impurity phases that have a negative impact on the battery energy density and kinetic performance. Although the reason is not yet clear, it is speculated that it may be that the titanium element, lithium element, phosphate group, and other elements (for example, lithium element) jointly form a fast ion conductor, which instead has a promoting effect on the kinetic performance of the battery.

[0035] In any embodiment, the tapped density of the powder of the positive electrode active material is 0.70 g / cm 3 -1.50 g / cm 3 , and may be optionally 0.70 g / cm 3 -1.20 g / cm 3 .

[0036] The positive electrode active material in the embodiments of the present application has a relatively low tapped density of the powder. With the high graphitization degree and good graphitization degree consistency of the positive electrode film layer, it is easy to slip under external force to achieve an increase in the compacted density of the powder.

[0037] In any embodiment, the compacted density of the powder of the positive electrode active material under 3T pressure is 2.50 g / cm 3 -2.70 g / cm 3 , and may be optionally 2.52 g / cm 3 -2.68 g / cm 3 .

[0038] With the high graphitization degree and good graphitization degree consistency of this positive electrode active material, the positive electrode active material can still achieve a high compacted density under external force, providing a material basis for improving the compacted density of the electrode sheet and preparing a high energy density lithium ion secondary battery.

[0039] In any embodiment, the powder resistivity of the positive electrode active material under 8 MPa pressure is 0.5 Ω·cm - 30 Ω·cm, and may be optionally 2 Ω·cm - 20 Ω·cm.

[0040] This positive electrode active material has a high graphitization degree. Therefore, with the sp 2 structure of the surface carbon, it is easy to achieve rapid conduction of electrons between particles, making the positive electrode active material have a low powder resistivity, which is beneficial to improving the solid-phase transmission rate of electrons and further improving the kinetic performance of the battery.

[0041] In any embodiment, the discharge specific capacity of the positive electrode active material at room temperature at a discharge rate of 1C is 135 mAh / g - 150 mAh / g.

[0042] The positive electrode active material has a high discharge specific capacity at a rate of 1C, indicating its good charge-discharge ability and being beneficial to improving the kinetic performance of the battery.

[0043] In any embodiment, the discharge capacity ratio η of the positive electrode active material discharged to 3.2V ≥ 85%, where η is defined as: at room temperature, a coin cell containing the positive electrode active material is charged and discharged at a constant current twice within a voltage range of 2.0V - 3.75V at a rate of 0.1C, and then charged and discharged at a constant current once at a rate of 1C. In the charge-discharge test at a rate of 1C, the capacity value at a discharge voltage of 3.2V is extracted and denoted as C 1 , and the capacity value of the discharge voltage to 2.0V is extracted as C 2 , η = C 1 / C 2 , where the charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cut-off current of 50 μA.

[0044] The high discharge capacity ratio of the positive electrode active material used in the lithium-ion secondary battery of the embodiments of the present application discharged to 3.2V means that the positive electrode active material has good kinetic performance. At the same time, a high η value indicates that when the lithium-ion secondary battery containing the positive electrode active material is discharged to a low state of charge (SOC), it still has a high voltage, which is beneficial to maintaining good power performance.

[0045] In any embodiment, the positive electrode film layer further includes a conductive agent, and based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.1% - 1.5%.

[0046] The carbon layer of the positive electrode active material has a high degree of graphitization and good graphitization consistency, enabling the positive electrode active material to have good electronic conductivity, capable of reducing or even eliminating the use of the conductive agent in the positive electrode film layer, being beneficial to further increasing the loading amount of the positive electrode active material and improving the energy density of the lithium-ion secondary battery.

[0047] In any embodiment, the positive electrode film layer does not include a conductive agent.

[0048] The positive electrode active material has extremely high electronic conductivity, such that the positive electrode film layer can even not add a conductive agent, which is beneficial to further increasing the loading amount of the positive electrode active material and improving the energy density of the lithium-ion secondary battery.

[0049] In any embodiment, the positive electrode film layer further includes a binder. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 95.5% - 99.5%, optionally 96.5% - 99.5%; the mass content of the binder is 0.5% - 3%.

[0050] In any embodiment, the areal density of one side of the positive electrode film layer is 300 mg / 1540 mm 2 - 450 mg / 1540 mm 2 。

[0051] The positive electrode film layer with an areal density within the above range can help improve the energy density of the lithium-ion secondary battery.

[0052] In any embodiment, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.51 g / cm 3 - 2.73 g / cm 3 。

[0053] When the tap density of the positive electrode film layer is within the above range, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0054] In any embodiment, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.55 g / cm 3 - 2.70 g / cm 3 。

[0055] When the tap density of the positive electrode film layer is within the above range, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0056] In any embodiment, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.51 g / cm 3 - 2.73 g / cm 3 , and in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the porosity of the positive electrode film layer is 10% - 22%.

[0057] In any embodiment, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.55 g / cm 3 - 2.70 g / cm 3 , and in the cross-section of the positive electrode film layer along the thickness direction of the electrode plate, the porosity of the positive electrode film layer is 10% - 20%.

[0058] In the cross-section of the positive electrode film layer of the embodiments of the present application, the lower the porosity, on the one hand, it means that the particle size grading of large, medium and small particles in the positive electrode film layer is better, and the compaction density is high. On the other hand, after the same grading and roll pressure, if the porosity is low, it means that the particles are easy to slide relative to each other, thereby reducing the risk of overpressure and stress concentration in the film layer, further reducing the probability of the positive electrode film peeling off during long-term cycling, which is beneficial to improving the long-term cycling performance of the battery.

[0059] In any implementation manner, the positive electrode tab includes a bottom coating, and the bottom coating is disposed between the positive electrode film layer and the positive electrode current collector; the bottom coating includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating is ≤ 10 pcs / 10 μm.

[0060] The bottom coating is beneficial to improving the conductivity and adhesion between the positive electrode film layer and the current collector, reducing the peeling off of the positive electrode film layer from the current collector during cycling, and simultaneously improving the kinetic performance of the battery. In the high-compaction density positive electrode tabs of the embodiments of the present application, controlling the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating to be ≤ 10 pcs / 10 μm is beneficial to reducing the probability of damage to the current collector in the high-compaction density positive electrode tabs, and further improving the ultimate compaction density of the positive electrode tab.

[0061] In any implementation manner, the positive electrode tab includes a bottom coating, and the bottom coating is disposed between the positive electrode film layer and the positive electrode current collector; the compaction density of the positive electrode tab in the fully discharged state is greater than or equal to 2.4 g / cm 3 , and the single-sided thickness of the bottom coating is 1 μm - 4 μm.

[0062] In any implementation manner, the positive electrode tab includes a bottom coating, and the bottom coating is disposed between the positive electrode film layer and the positive electrode current collector; the compaction density of the positive electrode tab in the fully discharged state is greater than or equal to 2.5 g / cm 3 , and the single-sided thickness of the bottom coating is 2 μm - 4 μm.

[0063] With the increase in the compaction density of the positive electrode tab, the extrusion effect of the lithium-containing phosphate material of large particles (such as those with a particle size greater than 1 μm) in the positive electrode film layer on the bottom coating becomes more significant. Therefore, stress concentration is likely to occur at the large particle sites, and even penetrate through the bottom coating to damage the current collector. Increasing the thickness of the bottom coating is beneficial to improving the stress concentration phenomenon in the positive electrode tab and further increasing the ultimate compaction density of the positive electrode tab.

[0064] The second aspect of the present application provides a battery device, including the lithium-ion secondary battery provided by the first aspect of the present application, and the battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0065] The third aspect of the present application further provides an electrical device, which includes the lithium-ion secondary battery provided by the first aspect of the present application or the battery device provided by the second aspect of the present application.

[0066] The fourth aspect of the present application further provides a method for preparing a positive electrode active material: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; the carbon source includes polyethylene glycol; the iron source includes divalent iron; the molar ratio of iron to phosphorus in the mixed raw material is greater than or equal to 0.95 and less than or equal to 1; obtaining a mixed slurry after grinding in a solvent; obtaining a precursor powder after drying the mixed slurry; sintering the precursor powder to obtain a positive electrode active material; the sintering includes at least two-stage isothermal sintering, wherein the sintering temperature in the high-temperature stage is 750°C - 800°C.

[0067] The positive electrode film layer prepared from the positive electrode active material prepared by this preparation method has a high degree of graphitization and good consistency of graphitization degree, and it is easy to improve the compaction density of the electrode sheet through uniform and consistent slippage between particles, which is beneficial to improving the energy density of the battery while improving the battery kinetic performance.

[0068] The fifth aspect of the present application provides a method for preparing a positive electrode sheet, which includes adding a binder, a conductive agent, and the positive electrode active material prepared by the method of the fourth aspect in sequence, dry-mixing them, then adding a solvent, and stirring to obtain a shipping slurry; transferring and coating the shipping slurry on at least one side of a current collector, and drying and hot-pressing to obtain a positive electrode sheet.

[0069] In any embodiment, the revolution speed of the dry-mixing is 20 rpm - 30 rpm, and the rotation speed of the dry-mixing is 750 rpm - 850 rpm.

[0070] In any embodiment, the hot-pressing includes at least three hot roll pressings, the hot roll pressure increases in sequence, and the hot roll pressures are 20 tons - 50 tons, 50 tons - 70 tons, and 70 tons - 90 tons in sequence; the hot roll temperature is 40°C - 80°C, and before the first entry into the hot roll compaction, the positive electrode sheet is heated, and the heating temperature is 40°C - 50°C.

[0071] Using the above hot-pressing process in combination with the preparation method of the fourth aspect to prepare the positive electrode active material is beneficial to further reducing the porosity of the cut surface of the positive electrode film layer, increasing the ultimate compaction density of the electrode sheet, and improving the energy density of the battery. Description of the Drawings

[0072] Figure 1 It is a scanning electron microscope image of the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet in an embodiment of the present application; Figure 2 It is a schematic diagram of a lithium-ion secondary battery in an embodiment of the present application; Figure 3It is an exploded view of a lithium-ion secondary battery according to an embodiment of the present application; Figure 4 It is a schematic diagram of a battery module according to an embodiment of the present application; Figure 5 It is a schematic diagram of a battery pack according to an embodiment of the present application; Figure 6 is Figure 5 an exploded view of the battery pack shown; Figure 7 It is a schematic diagram of an electrical device using the lithium-ion secondary battery according to an embodiment of the present application as a power source; Figure 8 It is a porosity test chart of a cross-section of a positive electrode film layer in the thickness direction of a positive electrode sheet according to an embodiment of the present application.

[0073] Explanation of reference numerals: 1 Battery pack; 2 Upper box body; 3 Lower box body; 4 Battery module; 5 Lithium-ion secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed embodiments

[0074] Hereinafter, embodiments of the lithium-ion secondary battery, battery device, electrical device, method for preparing a positive electrode active material, and method for preparing a positive electrode sheet of the present application will be specifically disclosed in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually identical structures are omitted. This is to prevent the following description from becoming unnecessarily long and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter recited in the claims.

[0075] The "ranges" disclosed in this application are defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can include or exclude the end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 - 120 and 80 - 110 are listed for a specific parameter, ranges of 60 - 110 and 80 - 120 are understood to be contemplated. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all contemplated: 1 - 3, 1 - 4, 1 - 5, 2 - 3, 2 - 4, and 2 - 5. In this application, unless otherwise specified, the numerical range "a - b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0 - 5" means that all real numbers between "0 - 5" are fully listed herein, and "0 - 5" is only an abbreviated representation of these numerical combinations. Additionally, when stating that a certain parameter is an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0076] If there is no special indication, all embodiments and optional embodiments of this application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0077] If there is no special indication, all technical features and optional technical features of this application can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure of this application.

[0078] If there is no special indication, all steps of this application can be carried out in sequence or randomly, preferably in sequence. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out in sequence, or can also include steps (b) and (a) carried out in sequence. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b), and (c), or can also include steps (a), (c), and (b), or can also include steps (c), (a), and (b), etc.

[0079] In this application, the terms "a plurality of", "a variety of" mean two or more than two.

[0080] Unless otherwise specified, the terms used in this application have the well-known meanings commonly understood by those skilled in the art.

[0081] Unless otherwise specified, the values of the various parameters mentioned in this application can be measured by various commonly used testing methods in the art. For example, they can be measured according to the testing methods given in the embodiments of this application. Unless otherwise specified, the testing temperature of each parameter is 25°C.

[0082] The battery mentioned in the embodiments of this application can be a single physical module including one or more lithium-ion secondary batteries to provide higher voltage and capacity. For example, the battery mentioned in this application can include lithium-ion secondary batteries, battery modules, battery packs, etc.

[0083] The lithium-ion secondary battery is the smallest unit that makes up the battery and can independently perform the functions of charging and discharging. The lithium-ion secondary battery can be in the shape of a cylinder, a cuboid, or other shapes, and the embodiments of this application do not limit this. For example, Figure 2 is a lithium-ion secondary battery 5 in the shape of a cuboid as an example.

[0084] The lithium-ion secondary battery includes an electrode assembly and an electrolyte.

[0085] The lithium-ion secondary battery can also include an outer package, which can be used to encapsulate the electrode assembly and the electrolyte. The outer package can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0086] In some embodiments, as Figure 3 shown, the outer package can include a housing 51 and a cover plate 53. The housing 51 can include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The electrode assembly 52 is encapsulated in the receiving cavity. The number of electrode assemblies 52 included in the lithium-ion secondary battery 5 can be one or more, which can be adjusted according to requirements.

[0087] The electrode assembly generally includes a positive electrode sheet and a negative electrode sheet. The negative electrode sheet is the electrode where the reaction of absorbing or lithiating lithium ions occurs during charging and releasing or delithiating lithium during discharging. The positive electrode sheet is the electrode where the reaction of releasing or delithiating lithium ions occurs during charging and absorbing or lithiating lithium during discharging.

[0088] When there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are connected in series, parallel or in a combined series-parallel manner through a busbar component. In some embodiments, the battery can be a battery module; when there are multiple lithium-ion secondary batteries, the multiple lithium-ion secondary batteries are arranged and fixed to form a battery module. In some embodiments, the battery can be a battery pack, and the battery pack includes a box body and lithium-ion secondary batteries, and the lithium-ion secondary batteries or battery modules are accommodated in the box body. In some embodiments, the box body can be part of the chassis structure of a vehicle. For example, part of the box body can become at least part of the vehicle floor, or part of the box body can become at least part of the cross beams and longitudinal beams of the vehicle.

[0089] In some embodiments, the battery can be an energy storage device. The energy storage device includes an energy storage container, an energy storage cabinet, etc.

[0090] In some embodiments, the lithium-ion secondary batteries can be assembled into a battery module, and the number of lithium-ion secondary batteries included in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Figure 4 is a schematic diagram of a battery module 4 as an example. As Figure 4 shown, in the battery module 4, multiple lithium-ion secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other way. Further, the multiple lithium-ion secondary batteries 5 can be fixed by fasteners.

[0091] Optionally, the battery module 4 can further include a housing having an accommodation space, and the multiple lithium-ion secondary batteries 5 are accommodated in the accommodation space.

[0092] In some embodiments, the above battery module can be further assembled into a battery pack, and the number of battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0093] Figure 5 and Figure 6 is a schematic diagram of a battery pack 1 as an example. As Figure 5 and Figure 6 shown, the battery pack 1 can include a box body and a plurality of battery modules 4 disposed in the box body. The box body includes an upper box body 2 and a lower box body 3. The upper box body 2 is used to cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in the box body in any way.

[0094] Lithium-containing transition metal phosphate materials have been widely used in lithium-ion batteries due to their high capacity, stable structure, good safety performance, and excellent cycling performance. However, they have problems of low electronic conductivity and low packing efficiency, which further lead to difficulty in further increasing the loading amount of lithium-containing transition metal phosphate in a unit volume of the battery and cannot meet the requirements of high-energy density batteries.

[0095] In order to further improve the energy density of the battery and increase the compaction density of the electrode sheet, the commonly used method in the industry is to improve the particle size distribution. Improving the particle size distribution often requires increasing the particle size or proportion of large and small particles. However, the increase in large particles will reduce the kinetic performance of the battery, and the increase in small particles will significantly increase the processing cost and cause serious side reactions in the battery, deteriorating the cycle life of the battery. How to further increase the compaction of the electrode sheet on the basis of not significantly sacrificing other battery performances and meeting the design and processing requirements to achieve the preparation of high-energy density batteries is a technical problem urgently to be solved in this field.

[0096] In a first aspect of the present application, there is provided a lithium-ion secondary battery, which includes a positive electrode sheet, a negative electrode sheet, and an electrolyte. Among them, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one side of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material provided on at least part of the surface. In the C value cumulative distribution curve obtained by the positive electrode film layer in the surface scanning mode of a laser confocal Raman spectrometer, the median C of the graphitization degree 50 is greater than or equal to 0.95 and less than or equal to 1.20, and the concentration degree of the C value (C 90 -C 10 ) / C 50 is 0.01 - 0.04; wherein, the graphitization degree C value is I G / I D , I G represents the intensity of the G peak of the Raman spectrum at 1580 ± 100 cm -1 , and I D represents the intensity of the D peak of the Raman spectrum at 1350 ± 100 cm -1 .

[0097] In the C value cumulative distribution curve of the graphitization degree obtained by the positive electrode film layer in the surface scanning mode of a laser confocal Raman spectrometer, the median C of the graphitization degree 50Less than 0.95 indicates poor slidability between the particles in the positive electrode film layer. Under pressure, it is difficult for the particles to slide and form close packing, which is prone to stress concentration and difficult to achieve a high tap density of the electrode sheet. At the same time, research shows that improving the graphitization degree of the positive electrode film layer often requires increasing the sintering temperature of the positive electrode active material. Although the upgrade of process conditions is beneficial to the improvement of the graphitization degree of the positive electrode film layer, limited by conditions such as the uniformity of the temperature field, increasing the graphitization degree of the particles is likely to affect the uniformity of the graphitization degree of the particles. Research shows that when the concentration of the C value (C 90 -C 10 ) / C 50 is greater than 0.04, it will lead to inconsistent lithium insertion rates in the positive electrode film layer due to poor graphitization degree consistency, which will further cause local polarization and is not conducive to the improvement of battery kinetic performance.

[0098] Research in this application shows that in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser confocal Raman spectrometer for the positive electrode film layer, the median C of the graphitization degree 50 is greater than or equal to 0.95 and less than or equal to 1.20, and at the same time, controlling the concentration of the C value (C 90 -C 10 ) / C 50 to be 0.01 - 0.04 indicates that the graphitization degree of the particles in the positive electrode film layer is high and the graphitization consistency degree is high, which means that the positive electrode active material has good coating uniformity and consistency, can reduce the hindrance of particle sliding caused by low graphitization degree of the particles in the positive electrode active material and the resulting local stress concentration. Therefore, through the uniform sliding between the particles of the positive electrode active material, the electrode sheet can achieve a relatively high tap density under a relatively low rolling pressure as a whole. At the same time, the uniform graphitization degree is conducive to the uniform insertion and extraction of lithium ions, improving the tap density of the electrode sheet and the energy density of the battery while maintaining good battery kinetic performance.

[0099] In the embodiments of this application, by uniformly increasing the graphitization degree of the particles in the positive electrode film layer, the positive electrode active material is prone to achieve uniform particle sliding with the help of the carbon coating material of the particles during the rolling and film-forming process, further improving the tap density of the electrode sheet while maintaining other performance levels of the battery and improving the comprehensive performance of the battery.

[0100] Those skilled in the art can adjust the concentration of the graphitization degree of the positive electrode film layer by any well-known method, such as by selecting a carbon source, adjusting the carbon coating amount, changing the particle size distribution, controlling the sintering temperature, and the sintering heating rate-related processes to achieve the adjustment of the graphitization degree concentration.

[0101] In the present application, the term "particle" refers to a particle with an identifiable complete boundary in the field of view of the positive electrode film layer at a certain magnification, such as 10,000 times. There may be defects and scratches inside the particle, but no complete boundary sufficient to divide the particle can be identified inside the particle.

[0102] The method for identifying particles is as follows: Cut the positive electrode film layer along the thickness direction of the electrode plate by an argon ion beam (as an example, the following can be selected: equipment model: Leica EM TIC 3X CP, working voltage: 6 kV, working duration: 6 h). After exposing the cross-section, use a scanning electron microscope (as an example, the following can be selected: equipment model: Hitachi SU8230, working voltage: 3 kV, beam current: high, probe model: U(LA100), working distance < 5 mm) to observe the cross-section of the positive electrode film layer along the thickness direction of the electrode plate. Collect images in the secondary electron mode at a non-edge position on the cross-section of the positive electrode film layer by a field emission scanning electron microscope (after observing the edge of the electrode plate under the scanning electron microscope, adjust the field of view to the central part of the sample). Take an electron microscope image at a magnification of 10,000 times, and analyze the particles in the electron microscope image using ImageJ software (version 1.46r, win64). The usage method of ImageJ software is as follows: Load the scanning electron microscope image to be analyzed, such as Figure 1As shown; use the Cellpose plug-in software to identify particles, and perform manual correction on this basis; use Image J to read and count data. The specific method of using the Cellpose plug-in software to identify particles is as follows: set the segmentation diameter parameter (diameter in the Segmantation module) to 15 pixels, click "run cyto3" to perform particle identification, and then manually mark the particles that are not recognized by the software, not fully recognized by the software, or have recognition errors in the image. The particles that are not recognized by the software, not fully recognized by the software, or have recognition errors in the image mainly include the following types: 1. Due to the particle being too large or having scratches on the particle surface, the particle cannot be recognized or cannot be fully recognized; 2. During the argon ion beam cutting process, scratches will be generated on the particle surface, and the software may misjudge the scratches as the particle boundary during the recognition process, resulting in recognition errors; 3. Due to the particle being too small, it is not successfully recognized; 4. The particle is located at the edge of the electron microscope field of view, and the particle interior is penetrated by the edge, and the morphology cannot be completely displayed, and the local part is recognized instead of the whole, resulting in recognition errors. For the above unrecognized or misrecognized particles, manual calibration is performed, and the specific process is as follows: delete the large particles that are located at the edges of the scanning electron microscope and cannot be completely displayed; judge whether there are gap scratches inside other unrecognized or misrecognized particles. If there are no gap scratches inside the particle, judge it as a single particle, and manually mark it according to the particle boundary observed manually; in response to the presence of gap scratches inside the particle, judge whether the gap scratches penetrate the particle. If they do not penetrate the particle, judge it as a single particle and perform manual marking; in response to the gap scratches penetrating the particle, judge whether the gap scratches are linear or irregular; in response to the gap scratches being irregular, judge it as the boundary between particles and divide the particles along this boundary; in response to the gap scratches being linear, perform contrast of contrast; in response to the contrast being not obvious and there being no sense of crack, judge it as a scratch and mark it as a single particle; in response to the contrast being strong and there being a sense of crack, judge it as the boundary between particles and mark it as two particles. After manual marking, delete the information irrelevant to the particles during the automatic image processing process, and the determination and marking of the particles in the picture are completed.

[0103] Lithium-containing transition metal phosphates refer to phosphate materials containing lithium elements and transition metal elements, and can be detected by any well-known method in the art. For example, it can be detected by combining an X-ray diffractometer (XRD), an energy spectrometer, and an inductively coupled plasma mass spectrometer. As an example, lithium-containing transition metal phosphates include, but are not limited to, lithium iron phosphate, lithium manganese iron phosphate, and their doped materials.

[0104] The carbon coating layer provided on at least a part of the surface of the lithium-containing transition metal phosphate can be detected by any commonly known method in the art. As an example, the carbon coating layer provided on at least a part of the surface of the lithium-containing transition metal phosphate can be observed by characterizing the lithium-containing transition metal phosphate by using a transmission electron microscope and an energy spectrometer in combination. It should be noted that the elements in the carbon coating layer are not limited to carbon elements, and there may also be other non-carbon elements. The carbon coating layer is not limited to a film shape, and also includes island-shaped, irregular-shaped or discontinuous coating layers.

[0105] Carbon has excellent electrical conductivity, which is beneficial to the transmission of electrons. The setting of the carbon coating layer can significantly improve the electronic conductivity of the lithium-containing transition metal phosphate material.

[0106] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained by the laser confocal Raman spectrometer in the surface scanning mode of the positive electrode film layer, the median C of the graphitization degree 50 is greater than or equal to 0.95 and less than or equal to 1.20, and the concentration degree of the C value (C 90 - C 10 ) / C 50 is 0.01 - 0.04; wherein the graphitization degree C value is I G / I D , where I G represents the intensity of the G peak of the Raman spectrum at 1580 ± 100 cm -1 , and I D represents the intensity of the D peak of the Raman spectrum at 1350 ± 100 cm -1 .

[0107] In the present application, the graphitization degree C value of the positive electrode film layer can be obtained by the surface scanning mode of the laser confocal Raman spectrometer. As an example, specifically, a laser confocal Raman spectrometer (high-precision Renishaw laser confocal Raman spectrometer) is used, the excitation wavelength of 532 nm is selected, an appropriate amount of the positive electrode film layer is taken, and its surface or a cross-section along the thickness direction of the electrode plate is subjected to surface scanning. The scanning area is 45 μm × 45 μm, which is divided into 10 × 10 grids. The grid vertices are used as test points, the step size is 5 μm, and the total number of scanning points is 100 points. Thus, the C values at different positions and the cumulative distribution curve of the C value of the surface scanning area are obtained.

[0108] The positive electrode film layer in the present application can be either a freshly prepared positive electrode film layer or a positive electrode film layer disassembled from a battery. It is inevitable that there are residual electrolyte salt particles on the surface of the positive electrode film layer disassembled from the battery. In order to improve the test accuracy, it is preferable to perform surface scanning on the cross-section of the positive electrode film layer along the thickness direction of the electrode plate to characterize the graphitization degree of the positive electrode film layer.

[0109] The C value of the graphitization degree of the positive electrode film layer is obtained from the peak intensity ratio of the G peak (G-band) and the D peak (D-band) of Raman spectroscopy. The position of the G peak is 1580 ± 100 cm -1 , which characterizes the carbon sp 2 hybrid structure; the position of the D peak is 1350 ± 100 cm -1 , which characterizes the disordered structure of carbon, where disorder means that there is no regular arrangement between carbon atoms in the structure. In a graphite crystal, carbon atoms in the same layer form covalent bonds through sp 2 hybridization, and the intermolecular force between layers is van der Waals force, making the carbon in the graphite structure easy to slip. Therefore, the C value can characterize the graphitization degree of the positive electrode film layer. The larger the value, the higher the graphitization degree of the carbon material. It can be understood that the graphitization degree in the positive electrode film layer mainly comes from the carbon material treated by graphitization in the positive electrode film layer, that is, the carbon coating layer of the positive electrode active material. Although carbon nanotube conductive agents rich in sp 2 hybrid structures also have a relatively high I G / I D , but due to their low addition content and small tube diameter, their addition in the positive electrode film layer shows an extreme value in the Raman surface scan test of the positive electrode film layer and will not affect the graphitization degree C 50 in the positive electrode film layer.

[0110] Therefore, the graphitization degree of the positive electrode film layer can also be used to characterize the graphitization degree of the positive electrode active material. The higher the graphitization degree of the carbon on the surface of the positive electrode active material, the higher the proportion of carbon with a graphite structure in the positive electrode film layer, and the easier it is for the particles to slip during the rolling process with the help of the carbon structure with a high graphitization degree in the coating layer, and the compaction density of the electrode sheet can be increased at a low rolling pressure.

[0111] The cumulative distribution curve of the graphitization degree C value refers to a curve obtained by arranging at least 100 obtained C values in ascending order, with the graphitization degree on the horizontal axis and the cumulative quantity proportion on the vertical axis. C 50 is the C value corresponding to the cumulative quantity proportion of 50% on the vertical axis in the cumulative distribution curve of the graphitization degree C value. The median C 50 of the graphitization degree can reflect the overall graphitization degree of the particles in the positive electrode film layer, that is, the ease of slipping, compared with the point value; compared with the mean value, it can reduce the influence of extreme values during the test and improve the confidence level of the test results.

[0112] Those skilled in the art can adjust the graphitization degree of the active material particles through any known process. As an example, adjusting the carbon source, optimizing the nucleation process, sintering temperature, sintering time, sintering pressure, and sintering atmosphere can all adjust the graphitization degree of the active material particles.

[0113] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser microscopic confocal Raman spectrometer for the positive electrode film layer, the median C of the graphitization degree 50 can be optionally 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20 or the numerical range between any two of them.

[0114] As described above, and so on, C 90 is the C value corresponding to when the cumulative quantity ratio on the vertical axis in the cumulative distribution curve of the graphitization degree C value is 90%, and C 10 is the C value corresponding to when the cumulative quantity ratio on the vertical axis in the cumulative distribution curve of the graphitization degree C value is 10%. The concentration degree of the C value is represented by (C 90 - C 10 ) / C 50 . (C 90 - C 10 ) / C 50 can not only reflect the magnitudes of most C values, but also be unaffected by extreme values, and can also reflect the width of the distribution of the graphitization degree of particles in the positive electrode film layer. A small concentration degree of the C value of the positive electrode film layer indicates a narrow width of the distribution of the graphitization degree of particles in the positive electrode film layer and good concentration.

[0115] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser microscopic confocal Raman spectrometer for the positive electrode film layer, the concentration degree (C 90 - C 10 ) / C 50 of the C value can be optionally 0.01, 0.02, 0.03, 0.04 or the numerical range between any two of them.

[0116] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser microscopic confocal Raman spectrometer for the positive electrode film layer, the median C of the graphitization degree 50 is 0.96 - 1.15, and further optionally 0.98 - 1.13.

[0117] The median C of the graphitization degree of the positive electrode film layer 50 within the above range is beneficial to further improving the easy slip degree between particles, further increasing the compaction density of the electrode sheet while maintaining high battery kinetic performance, and achieving the balance between battery kinetic performance and energy density.

[0118] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the area scanning mode of a laser microscopic confocal Raman spectrometer for the positive electrode film layer, the concentration degree (C 90 -C 10 ) / C 50 is 0.02 - 0.038, and can be optionally 0.02 - 0.036.

[0119] The concentration degree (C 90 -C 10 ) / C 50 Within the above range, it is beneficial to further improve the consistency of the graphitization degree of the particles in the positive electrode film layer, improve the consistency of the slip between the particles, reduce the inconsistent lithium intercalation rate in the positive electrode film layer due to the poor consistency of the graphitization degree, thereby causing local polarization, and further improve the kinetic performance of the battery on the basis of maintaining good energy density.

[0120] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the area scanning mode of a laser microscopic confocal Raman spectrometer for the positive electrode film layer, the C of the graphitization degree 90 is 1.0 - 1.3, and can be optionally 1.02 - 1.15.

[0121] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the area scanning mode of a laser microscopic confocal Raman spectrometer for the positive electrode film layer, the C of the graphitization degree 90 can be optionally 1.0, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.2, 1.21, 1.22, 1.23, 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.3 or the numerical range between any two of them.

[0122] The C of the graphitization degree 90 Within the above range, it is relatively close to the median C of the graphitization degree 50 , indicating that the distribution range of the graphitization degree of the particles in the positive electrode film layer is narrow, which is beneficial to the uniform slip between the particles to improve the compaction density of the positive electrode plate.

[0123] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the area scanning mode of a laser microscopic confocal Raman spectrometer for the positive electrode film layer, the C of the graphitization degree 10 is 0.92 - 1.1, and can be optionally 0.96 - 1.08.

[0124] In some embodiments, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of a laser confocal Raman spectrometer for the positive electrode film layer, the C of the graphitization degree 10 can be optionally 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.1 or the numerical range between any two of them.

[0125] The C of the graphitization degree 10 Within the above range, it indicates that different sites in the positive electrode film layer have a high graphitization degree, which is beneficial to the uniform slip of particles, reduces the occurrence probability of local stress concentration phenomenon, and further improves the compaction density of the electrode sheet.

[0126] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of particles with an area of 0.001 μm 2 -0.06 μm 2 is 21.00% - 27.00%, and the area ratio of particles with an area of 1.0 μm 2 -4.0 μm 2 is 12.00% - 20.00%.

[0127] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area statistical method of particles is as follows. After importing the picture after particle determination and identification into ImageJ software for analysis, set the scale according to the scanning electron microscope image, and analyze the particle size and the area of the particle in the picture through the "Feret diameter" and "Area" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter obtained by analysis represents the maximum distance between all parallel lines in the two-dimensional projection of the particle, so as to characterize the particle size of the particle; the "Area" parameter represents the pixel area of the particle, so as to characterize the area of the particle. Since particles with a particle size less than 50 nm have large errors in the statistical process and are difficult to accurately identify, and the particle size of the conductive agent is generally less than 50 nm, which will cause large errors to the statistical results. Therefore, in the particle size statistics process of this application, particles with a particle size less than 50 nm are not counted, and the particle statistical data corresponding to "NaN" displayed by Area is deleted. According to the above method, to meet the sample number with statistical significance, each electrode sheet collects no less than 10 non-overlapping scanning electron microscope images of the field of view, counts the areas of no less than 5000 particles, calculates the sum of the "Area" parameters of particles with an area of 0.001 μm 2 -0.06 μm 2 and the sum of the "Area" parameters of all particles, respectively, as the area of 0.001 μm2 -0.06 μm 2 and the total area of the counted particles. Taking the sum of the areas of the particles with an area of 0.001 μm 2 -0.06 μm 2 divided by the total area of the counted particles as the area ratio of the particles with an area of 0.001 μm 2 -0.06 μm 2 in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet. The area ratio of the particles with an area of 1.0 μm 2 -4.0 μm 2 can be obtained in the same way.

[0128] The cross-sectional morphology diagram of the positive electrode film layer along the thickness direction of the electrode sheet is as shown in Figure 1 . Different from the state of the positive electrode active material in the Malvern laser scattering method and also different from the state of the positive electrode active material when directly observing the positive electrode active material by scanning electron microscopy. The particles in the positive electrode film layer show a good dispersion state under the action of the roll pressure. Observing the positive electrode film layer is beneficial to effectively characterizing the objective conditions of the particle size, particle area and quantity in the positive electrode film layer.

[0129] During the compaction process of the positive electrode film layer, compaction occurs in the thickness direction. Therefore, the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet can reflect the real compaction condition of the particles inside the film layer on the spatial scale more than the surface of the positive electrode film layer. In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the area ratio of the particles with an area of 0.001 μm 2 -0.06 μm 2 and the area ratio of the particles with an area of 1.0 μm 2 -4.0 μm 2 can intuitively reflect the proportional relationship between the particles in this area segment and the total number of particles, and reflect the number of particles in this area segment.

[0130] It can be understood that the particles in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, especially the particles above 50 nm, mainly come from the positive electrode active material. Therefore, through the observation and statistics of the particle size and area of the particles in the cross-section of the positive electrode film layer in this embodiment of the present application, the distribution of the lithium-containing transition metal phosphate particles in the positive electrode film layer in the electrode sheet can be accurately and objectively reflected.

[0131] Those skilled in the art can adjust the particle size of the particles through any known process. As an example, by controlling the temperature and time during the preparation of the cathode material, the growth rate and time of the cathode material are controlled. Using the mechanical force of the crushing and grinding processes, the raw materials are processed to the target particle size distribution range to achieve the adjustment of the particle size; screening and classification equipment is used to separate the particle sizes of the particle system to obtain the particle size ratio that meets the requirements; by precisely controlling the feeding rate and adjusting the residence time and stress state of the particles in the equipment, it also helps to achieve the control of the particle size.

[0132] In some embodiments, in the cross-section of the cathode film layer along the thickness direction of the electrode sheet, the area of the particles is 0.001 μm 2 -0.06 μm 2 The area ratio of the particles can be selected as 21%, 22%, 23%, 24%, 25%, 26%, 27% or the numerical range between any two of them. The area of the particles is 1.0 μm 2 -4.0 μm 2 The area ratio of the particles can be selected as 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% or the numerical range between any two of them.

[0133] Controlling the area ratio of the cathode active material particles with an area of 0.001 μm 2 -0.06 μm 2 and the area ratio of the particles with an area of 1.0 μm 2 -4.0 μm 2 within the above range, and cooperating with a uniform high degree of graphitization, uniform slippage of the particles during the compaction of the electrode sheet can be achieved. While improving the compaction density of the electrode sheet, the negative impacts on other performances such as the battery kinetics performance, cycle life, and processing performance are reduced, and the battery performance is comprehensively improved.

[0134] In some embodiments, in the cross-section of the cathode film layer along the thickness direction of the electrode sheet, the particle size D of the particles A50 is 600 nm - 800 nm, and can be selected as 650 nm - 750 nm, where D A50 refers to the particle size corresponding to when the cumulative area distribution of the particles reaches 50% in the area cumulative distribution curve of the particles.

[0135] In the cross-section of the cathode film layer along the thickness direction of the electrode sheet, the test method for the particle size D of the particles A50 is as follows. Referring to the method described above, the particle sizes of no less than 5000 particles are counted. The at least 5000 particle sizes obtained are arranged in ascending order. Taking the particle size as the horizontal axis and the cumulative area distribution of the particles as the vertical axis, the particle size corresponding to when the cumulative area ratio on the vertical axis in the area cumulative distribution curve of the particles reaches 50% is obtained.

[0136] In some embodiments, in the cumulative particle size area distribution curve of the positive active material obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the particle size D A50 can be optionally 600 nm, 610 nm, 620 nm, 630 nm, 640 nm, 650 nm, 660 nm, 670 nm, 680 nm, 690 nm, 700 nm, 710 nm, 720 nm, 730 nm, 740 nm, 750 nm, 760 nm, 770 nm, 780 nm, 790 nm, 800 nm or the numerical range between any two of them.

[0137] The particle size D of the particles A50 Within the above range, it indicates that there are a certain number of large-sized particles in the positive electrode film layer. Controlling the median particle size D A50 Within the above range, it can not only improve the transfer efficiency of the rolling pressure between the electrode sheet particles through the large contact area between the large-sized particles, give full play to the skeleton support role of the large-sized particles, enable the electrode sheet to withstand a higher rolling pressure, and improve the compaction density of the electrode sheet, but also reduce the kinetic decline caused by too large particle size, and maintain the kinetic performance of the battery while improving the compaction density of the electrode sheet.

[0138] In some embodiments, in the cumulative particle roughness area distribution curve of the positive electrode film layer obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median roughness R A50 is 0.92 - 0.96.

[0139] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the method for testing the roughness of the particles is as follows: Identify the particles in the cross-section of the positive electrode film layer according to the method described above in this application. Import the images after the particles are determined and marked into the ImageJ software for analysis. Complete the scale setting according to the scanning electron microscope image. Analyze the particle size, the area of the particle, and the roughness of the particles in the image through the "Feret diameter", "Area", and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained by analysis represents the ratio of the pixel area of the particle to the convex area. Therefore, the "Solidity" parameter of the particles obtained by analysis is used to characterize the roughness of the particles. According to the definition, the closer the roughness is to 1, the smoother the particle. Since particles with a particle size less than 50 nm have large errors in the statistical process and are difficult to accurately identify, and the particle size of the conductive agent is generally less than 50 nm, which will cause large errors in the statistical results. Therefore, in the particle size statistics process of this application, particles with a particle size less than 50 nm are not statistically counted, and the particle statistical data corresponding to "NaN" displayed by Solidity is deleted. According to the above method, to meet the number of samples with statistical significance, each electrode sheet collects at least 10 non-overlapping scanning electron microscope images in the field of view. Arrange the roughness of at least 5000 particles obtained in ascending order. Use the roughness as the horizontal axis and the cumulative area ratio as the vertical axis to obtain the roughness cumulative distribution curve of the particles in the positive electrode film layer. R A50 is the roughness R value corresponding to the cumulative area ratio of 50% on the vertical axis in the roughness R value cumulative distribution curve.

[0140] In some embodiments, in the particle roughness area cumulative distribution curve obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median roughness R A50 can be selected as 0.92, 0.93, 0.94, 0.95, 0.96 or the numerical range between any two of them.

[0141] Those skilled in the art can adjust the roughness of the particles through any known process. As an example, the roughness of the particles can be adjusted through processes such as grinding, polishing, grinding, megajoule energy, electroplating, calendering, etc. and by adjusting the parameters of each process.

[0142] The median roughness R A50 For particles within the above range, the particle surface is relatively smooth, the friction between particles is relatively small, and it is easy to slip under the action of external force. When paired with particles with a high degree of graphitization, the compaction density of the electrode sheet can be increased under a low rolling pressure, further improving the energy density of the battery.

[0143] In some embodiments, in the cumulative area distribution curve of particle sphericity obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the L of sphericity A90 is 0.80 - 0.95, and optionally 0.85 - 0.93.

[0144] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the method for testing the sphericity of particles is as follows: Identify the particles in the cross-section of the positive electrode film layer according to the method described above in this application. Import the pictures after particle determination and identification into ImageJ software for analysis. Complete the scale setting according to the scanning electron microscope image. Analyze the particle size, the area of the particle, and the sphericity of the particles in the picture through the "Feret diameter", "Area", and "Round" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Round" parameter obtained by analysis represents the ratio of the pixel area of the particle to the area of a circle with the fitted major axis as the diameter, and can be used to characterize the sphericity of the particle. When the particle is closer to a spherical shape, the ratio of the pixel area to the area of a circle with the fitted major axis as the diameter is closer to 1. Therefore, the "Round" parameter of the analyzed particle is used to characterize the sphericity of the particle. Since particles with a particle size less than 50 nm have large errors in the statistical process and are difficult to accurately identify, and the particle size of the conductive agent is generally less than 50 nm, which will cause large errors in the statistical results. Therefore, particles with a particle size less than 50 nm are not counted in the particle size statistics process of this application, and the particle statistical data corresponding to "NaN" displayed by Round is deleted. According to the above method, to meet the sample number with statistical significance, each electrode sheet collects at least 10 non-overlapping scanning electron microscope images of the field of view. Arrange the sphericities of at least 5000 obtained particles in ascending order. Take the sphericity as the horizontal axis and the cumulative area ratio as the vertical axis to obtain the cumulative distribution curve of particle sphericity in the positive electrode film layer. L A90 is the sphericity L value corresponding to the cumulative area ratio of 90% on the vertical axis in the cumulative distribution curve of the sphericity L value.

[0145] In some embodiments, in the cumulative area distribution curve of particle sphericity obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the L of sphericity A90 is optionally 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92, 0.93, 0.94, 0.95 or the numerical range between any two of them.

[0146] Those skilled in the art can adjust the sphericity of the particles through any known process. As an example, the sphericity of the particles can be adjusted by processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, adding surfactants, etc., as well as by adjusting the parameters of each process.

[0147] In some embodiments, in the cumulative area distribution curve of particle sphericity obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the median sphericity L A50 is 0.65 - 0.85, and can be optionally 0.70 - 0.80.

[0148] L A50 is the sphericity L value corresponding to the case where the cumulative area ratio of the vertical axis in the cumulative distribution curve of the sphericity L value accounts for 50%.

[0149] In some embodiments, in the cumulative area distribution curve of particle sphericity obtained from the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the L of the sphericity A50 can be optionally 0.65, 0.66, 0.67, 0.68, 0.69, 0.70, 0.705, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85 or any numerical range between any two of them.

[0150] The median sphericity L A90 , L A50 The particles within the above range are approximately spherical, and it is easy for the particles to slip between each other under an external force. When combined with particles with a high degree of graphitization, the compaction density of the electrode sheet can be increased under a low rolling pressure, further improving the energy density of the battery.

[0151] In some embodiments, the positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer is 500 ppm - 2000 ppm, and can be optionally 500 ppm - 1500 ppm.

[0152] The iron dissolution rate of the positive electrode film layer can be tested in the following manner. Specifically, after disassembling and washing the electrode sheet from the battery, it is punched into small round pieces with a diameter of 14 mm. Multiple small round piece samples are taken so that the total mass of the samples is about 5 g. These are added to 100.3 g of ascorbic acid solution with a mass concentration of 0.3% (the solvent is ultrapure water). After stirring at a speed of 500 revolutions per minute for 5 minutes, 5 mL of the solution is quickly aspirated using a syringe, and the solution is filtered through a 0.45 μm pore size filter head into a test tube. 1 mL of the supernatant is aspirated using a pipette and added to a glass volumetric flask and diluted 50 times. It is tested using an inductively coupled plasma optical emission spectrometer (ICP-OES) to obtain the iron element concentration in the solution. Through the formula: [(ICP measured iron element concentration × solution volume / mass of the solution involved in volume fixation) × 100.3 g / (mass of the electrode sheet of the small round piece - mass of the current collector of the small round piece)], the solution volume is 50 mL, and the mass of the solution involved in volume fixation is 1 g, the iron dissolution rate of the positive electrode film layer is calculated. Preferably, the mass of the current collector of the small round piece is obtained by multiplying the thickness of the small round piece by the area and the density. The thickness of the small round piece can be equivalently measured by measuring the thickness of the current collector in the uncoated area with a thickness gauge. It can be understood that although the current collector will be extended during the compaction process in the coated area, resulting in a slightly lower thickness compared to the uncoated area, since the reduction amplitude is negligible, it will not have a significant impact on the test results. More preferably, when the current collector is aluminum foil, the density is 2.7 g / cm 3 .

[0153] In some embodiments, the positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer can be selected from 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm, 1900 ppm, 2000 ppm or the numerical range between any two of them.

[0154] Those skilled in the art can regulate the iron dissolution rate of the positive electrode film layer through any known process. As an example, the iron dissolution rate of the positive electrode film layer is regulated by regulating the surface coating mass of the positive electrode film layer, the temperature, time, and pressure during the preparation process.

[0155] The iron dissolution rate can reflect the integrity and density of the carbon coating on the surface of the cathode active material from the side. The lower the iron dissolution rate, the less likely the iron ions after acid dissolution are to precipitate from the carbon coating layer, that is, the more complete and dense the carbon coating layer on the surface of the cathode active material. The cathode active material with an iron dissolution rate within the above range has a relatively complete and dense carbon coating layer, which can improve the electrical contact between cathode active materials, improve the conductivity of the cathode active material, reduce the polarization of the cathode active material, and further optimize the kinetic performance of the lithium-ion secondary battery. At the same time, the densely coated carbon layer has a low space occupancy rate, and the particle gaps are easily compressed by stress during the rolling process. With a highly graphitized carbon coating layer, it is more conducive to improving the compaction density of the electrode sheet and the energy density of the battery.

[0156] In some embodiments, based on the total mass of the cathode active material, the mass content of carbon element is 0.8% - 1.8%, and can be optionally 0.90% - 1.5%.

[0157] Based on the total mass of the cathode active material, the mass content of carbon element can be measured by methods and equipment known in the art. For example, referring to GB / T 21023 - 2006 "Determination of total carbon and sulfur content in steel - Infrared absorption method after combustion in high - frequency induction furnace", it is measured using a Dekai HCS infrared carbon - sulfur analyzer.

[0158] In some embodiments, based on the total mass of the cathode active material, the mass content of carbon element can be optionally 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8% or any value range between any two of them.

[0159] Compared with the lithium - containing transition metal phosphate cathode active material in the prior art, this cathode active material has a relatively low content of carbon coating, which can further increase the loading of lithium - containing transition metal phosphate in the cathode electrode sheet and improve the energy density of the lithium - ion secondary battery.

[0160] In some embodiments, the cathode active material includes iron element, and the lithium - iron antisite defect concentration of the cathode active material is 0.1% - 1.5%, and can be optionally 0.3% - 1.0%.

[0161] XRD data of the sample was collected using an X-ray diffractometer, and the phase analysis of the sample was carried out. The CIF file of this phase obtained from an open-source website was used as the initial crystal structure model, including defining unit cell parameters, atomic positions, and occupancy probabilities, etc. In the initial crystal structure model, considering the possibility of Fe-Li anti-site, the possible Li content at the Fe position and the possible Fe content at the Li position were set, and the initial value was set to 0.1%. The collected XRD data was fitted and refined using the FullProf Suite software, and the parameters were refined in the order of background parameters, peak intensity, unit cell parameters, and peak shape. When the fitted peak shape and the experimental peak shape were best matched and Rwp was less than 10, the refined occupancy probabilities of Li and Fe were obtained, which were used as the concentration of Fe-Li anti-site defects.

[0162] In some embodiments, the concentration of Fe-Li anti-site defects in the positive electrode active material can be selected from 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or the numerical range between any two of them.

[0163] Those skilled in the art can regulate the Fe-Li anti-site defects of the positive electrode active material through any known process. As an example, the regulation of Fe-Li anti-site defects of the positive electrode active material can be achieved by regulating the sintering temperature, sintering time, preparation method, raw material metering ratio, etc.

[0164] During the preparation and cycling process, there will inevitably be a certain amount of lithium vacancies in the crystal structure of the positive electrode active material. Lithium vacancies will not only cause the oxidation of ferrous ions to ferric ions, but also induce the partial migration of ferric ions to the lithium position, forming Fe-Li anti-site defects and blocking the one-dimensional diffusion channels of lithium ions, which has an adverse effect on the solid-phase transport of lithium ions. The positive electrode active material in the embodiments of the present application has low Fe-Li anti-site defects, which is beneficial to the uniform transport of lithium ions in the solid phase and further improves the kinetic performance of lithium-ion secondary batteries.

[0165] In some embodiments, the lithium-containing transition metal phosphate includes components having the following general formula: Li m Fe x P y O j Q q , wherein, Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8 ≤ m ≤ 1.15, 0.9 ≤ x ≤ 1, 0.95 ≤ y ≤ 1, 3.5 ≤ j ≤ 4, 0 < q ≤ 0.1.

[0166] In some embodiments, m can be optionally 0.8, 0.85, 0.9, 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15 or a numerical range between any two of them; x can be optionally 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0 or a numerical range between any two of them; y can be optionally 0.95, 0.96, 0.97, 0.98, 0.99, 1.00 or a numerical range between any two of them; j can be optionally 3.5, 3.6, 3.7, 3.8, 3.9, 4 or a numerical range between any two of them; q can be optionally 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or a numerical range between any two of them.

[0167] Selecting an appropriate modification element Q can improve the ion diffusion path of the positive electrode active material, improve the lithium ion diffusion rate of the positive electrode active material, and improve the kinetic performance of the battery.

[0168] In some embodiments, the positive electrode active material includes one or more of lithium iron phosphate and its doped modification materials and coated modification materials.

[0169] In some embodiments, the positive electrode active material includes titanium element, and based on the total mass of the positive electrode active material, the mass content of titanium element is 2000 ppm - 6000 ppm.

[0170] The types and contents of elements in the positive electrode active material can be tested by any well-known method in the art. As an example, the inductively coupled plasma emission spectrometry is used to test the titanium element and its content with reference to Appendix C of GB / T 33822 - 2017.

[0171] In some embodiments, based on the total mass of the positive electrode active material, the mass content of titanium element can be optionally 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 5500 ppm, 6000 ppm or a numerical range between any two of them.

[0172] The doping of titanium element in the positive electrode active material is beneficial to cause lattice distortion, reduce the Li - O bond energy, improve the lithium ion transmission rate, and improve the kinetic performance of the lithium ion secondary battery. However, in the prior art, the doping content of titanium element in lithium-containing transition metal phosphates often cannot exceed 3000 ppm, because too much titanium element is difficult to completely enter the bulk phase of lithium-containing transition metal phosphates and is easy to become a harmful impurity phase remaining on the surface, which has a negative impact on the battery performance.

[0173] The positive electrode active material in the embodiments of the present application has a high titanium element content. Moreover, surprisingly, the high addition amount of titanium element does not form harmful impurity phases that have a negative impact on the battery energy density and kinetic performance. Although the reason is not yet clear, it is speculated that it may be possible that titanium element, lithium element, phosphate radical, and other elements (for example, lithium element) jointly form a fast ion conductor, which instead has a promoting effect on the kinetic performance of the battery.

[0174] In some embodiments, the tapped density of the powder of the positive electrode active material is 0.70 g / cm 3 -1.50 g / cm 3 , and may be optionally 0.7 g / cm 3 -1.20 g / cm 3 .

[0175] The tapped density of the powder can be obtained by testing in any well-known manner in the art.

[0176] As an example, turn on the electronic balance. First, use a conical flask as a base and place it on the electronic balance, then zero the electronic balance; place the tapped density measuring cylinder on the conical flask and weigh it to record the weight of the measuring cylinder; open the sample bag, use a clean sample spoon to stir the sample in the sample bag for 3 - 5 circles to mix it evenly, and then transfer the sample smoothly into the measuring cylinder; wipe the powder contaminated on the surface of the connecting part with dust-free paper, and then put it into the conical flask that has been zeroed and weigh it; seal the mouth of the measuring cylinder with sealing film, place the tapped density measuring cylinder in the matching instrument rubber ring, ensure that the tapped density measuring cylinder fits tightly with the rubber ring and is perpendicular to the instrument surface; set the vibration frequency to 250 times / min on the instrument, the number of vibrations to 5000 times, press the button, and vibrate for 20 min; then remove the TD tube, use a flashlight to irradiate the surface of the measuring cylinder, and by visual inspection, read the highest scale V 1 and the lowest scale V 2 , and take the average value V of the two; the mass m 1 of the measuring cylinder and the sample minus the mass m 0 of the measuring cylinder to obtain the powder mass m, and the tapped density of the sample can be obtained from the density formula ρ = m / v.

[0177] In some embodiments, the tapped density of the powder of the positive electrode active material may be optionally 0.70 g / cm 3 , 0.80 g / cm 3 , 0.90 g / cm 3 , 1.00 g / cm 3 , 1.10 g / cm 3 , 1.20 g / cm 3 , 1.30 g / cm 3 , 1.40 g / cm 3 , 1.50 g / cm 3or a numerical range between any two of them.

[0178] The positive electrode active material in the embodiments of the present application has a relatively low tapped density of powder. With the high degree of graphitization and good consistency of graphitization degree of the positive electrode film layer, it is easy to slide under the action of external force to improve the tap density of the powder.

[0179] In some embodiments, the tap density of the positive electrode active material under a pressure of 3T is 2.50 g / cm 3 - 2.70 g / cm 3 , and can be optionally 2.52 g / cm 3 - 2.68 g / cm 3 .

[0180] In the present application, the term "tap density of powder" refers to the density of a green compact with a certain density and strength formed during the external force compression process. As the powder moves and deforms, larger voids are filled, the contact area between particles increases, the attraction between atoms is generated, and the mechanical fitting effect between particles is enhanced. The unit is g / cm 3 .

[0181] The tap density of the positive electrode active material can be measured by methods and equipment known in the art. For example, reference can be made to GB / T24533-2009 and measured using a tap density instrument. Specifically, a certain amount of positive electrode active material is placed on a special compaction mold (the diameter of the mold is known), and there is a hollow in the middle of the mold with a metal disc at each end. The positive electrode active material is placed between the metal discs, and a metal cylinder is placed on the top. The mold is placed on the tap density instrument, and the bottom area of the mold is 1.327 cm 2 . The pressure is set to 3T, and the thickness of the positive electrode active material under a pressure of 3T can be read on the equipment. The tap density of the positive electrode active material is ρ = m / v, where v = (S×H), m is the mass of the positive electrode active material, S is the bottom area of the mold, and H is the thickness of the positive electrode active material after compaction.

[0182] In some embodiments, the tap density of the positive electrode active material under a pressure of 3T can be optionally 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3, 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 or the numerical range between any two of them.

[0183] Due to the high degree of graphitization and good consistency of graphitization of the positive electrode active material, the positive electrode active material can still achieve a high tap density under external force, providing a material basis for improving the tap density of the electrode sheet and preparing a lithium-ion secondary battery with a high energy density.

[0184] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 8 MPa is 0.5 Ω·cm - 30 Ω·cm, and can be optionally 2 Ω·cm - 20 Ω·cm.

[0185] The powder resistivity of the positive electrode active material can be measured by methods and equipment known in the art. For example, referring to GB / T33822-2017, it can be measured using a powder resistivity meter (Suzhou Crystal, model ST2722). Specifically, a certain amount of positive electrode active material (such as 1 g) is weighed and added to the feeding chamber of the powder resistivity meter, and a pressure of 8 MPa is applied. The forward resistivity and reverse resistivity of the positive electrode active material are respectively tested, and the average value of the two is taken as the powder resistivity of the positive electrode active material.

[0186] In some embodiments, the powder resistivity of the positive electrode active material under a pressure of 29400 N can be optionally 0.5 Ω·cm, 1 Ω·cm, 2 Ω·cm, 3 Ω·cm, 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 15 Ω·cm, 20 Ω·cm, 25 Ω·cm, 30 Ω·cm or the numerical range between any two of them.

[0187] The positive electrode active material has a high degree of graphitization. Therefore, due to the sp 2 structure of the surface carbon, it is easy to achieve rapid conduction of electrons between particles, making the positive electrode active material have a low powder resistivity, which is beneficial to improving the solid-phase transmission rate of electrons and further improving the kinetic performance of the battery.

[0188] In some embodiments, the discharge specific capacity of the positive electrode active material at room temperature at a discharge rate of 1 C is 135 mAh / g - 150 mAh / g.

[0189] In this application, the positive electrode active material is assembled into a button cell and the electrical performance is tested on a blue electric tester. At 25±5℃, within the voltage range of 2.0V~3.75V, it is charged to 3.75V at 1C constant current, then paused for 5 minutes, charged at constant voltage to a cut-off current of 50μA, and then discharged to 2.0V at 1C constant current. The discharge capacity of the button cell divided by the mass of the positive electrode active material is taken as the discharge gram capacity of the positive electrode active material at room temperature at a discharge rate of 1C.

[0190] The preparation and testing process of button cells is as follows: 2.0g of positive electrode active material, conductive carbon black, and PVDF are mixed at a mass ratio of 0.9:0.05:0.05, and then organic solvent NMP (N-methylpyrrolidone) is added. After fully mixing, a 150μm scraper is used for coating, and the mixture is dried at 100℃ for 2h. The compacted density is 2.0g / cm 3 -2.2g / cm 3 The positive electrode sheet was compacted and punched into a disc with a diameter of 14 mm using a hole puncher, then weighed and recorded, and the weighed positive electrode sheet was placed in a vacuum drying oven (105°C, 1-12 hrs, -90 kpa). After baking, the positive electrode sheet was placed in a glove box, and the battery was assembled in the order of negative electrode shell-nickel mesh-lithium sheet-diaphragm-positive electrode sheet-positive electrode shell, and 65-87 μL (pipette gun) of electrolyte (the electrolyte was a mixed solvent of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) in a volume ratio of 1:1, and the electrolyte LiPF 6 ), the negative electrode is on top and placed in the groove of the sealing machine, the sealing pressure is 650kg / cm 2 , use insulated tweezers to remove the button battery and put it into a dust-free bag, remove the glove box, and place it in a constant temperature room for 3 hours to obtain the button battery for testing.

[0191] It can be understood that the discharge capacity in grams of the positive electrode active material can also be obtained by disassembling the battery, obtaining the positive electrode sheets, assembling them into button cells according to the method described above, and then testing them.

[0192] In some embodiments, the discharge capacity in grams of the positive electrode active material at room temperature at a discharge rate of 1C may be selected to be 135 mAh / g, 136 mAh / g, 137 mAh / g, 138 mAh / g, 139 mAh / g, 140 mAh / g, 141 mAh / g, 142 mAh / g, 143 mAh / g, 144 mAh / g, 145 mAh / g, 146 mAh / g, 147 mAh / g, 148 mAh / g, 149 mAh / g, 150 mAh / g or any range of values ​​therebetween.

[0193] The positive electrode active material has a high discharge gram capacity at a 1C rate, indicating that it has good charge and discharge capabilities and is beneficial to improving the kinetic performance of the battery.

[0194] In some embodiments, the discharge capacity ratio η of the positive electrode active material discharged to 3.2V is η≥85%. η is defined as follows: at room temperature, a button cell containing the positive electrode active material is subjected to constant current charge and discharge twice at a rate of 0.1C within a voltage range of 2.0V to 3.75V, and then subjected to constant current charge and discharge once at a rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value when the discharge voltage is 3.2V is denoted as C1, and the capacity value when the discharge voltage reaches 2.0V is C2, and η = C1 / C2. During the charging process, constant voltage charging is included, with a constant voltage of 3.75V and a constant voltage cut-off current of 50μA.

[0195] The η value of the positive electrode active material can be measured by methods and equipment known in the art. As an example, first prepare a button cell according to the method described above, and test the electrical performance of the prepared button cell on a Blue Electric Tester. Specifically, at room temperature, the button cell is subjected to constant current charge and discharge twice at a rate of 0.1C within a voltage range of 2.0V to 3.75V, and after constant current charging to the cut-off voltage, it is charged at a constant voltage until the current is 50μA, and then subjected to constant current charge and discharge once at a rate of 1C. In the charge and discharge test at a rate of 1C, the capacity value discharged from 3.75V to a voltage of 3.2V is denoted as C1, and the capacity value discharged from 3.75V to 2.0V is C2, and η = C1 / C2.

[0196] In some embodiments, η can be selected as 85%, 86%, 87%, 88%, 88.1%, 89%, 90%, 90.1%, 91%, 92%, 92.2%, 93%, 94%, 94.1%, 94.5%, 95%, 95.1% or any numerical range between any two of them.

[0197] In some embodiments, the discharge capacity ratio η of the positive electrode active material in a freshly prepared lithium-ion secondary battery discharged to 3.2V is η≥88%. After the freshly prepared lithium-ion secondary battery is subjected to constant current charge and discharge at a rate of 0.1C within a voltage range of 2.0V to 3.75V for a period of time, the discharge capacity ratio η of the positive electrode active material discharged to 3.2V can remain ≥85%.

[0198] The high discharge capacity ratio of the positive electrode active material used in the lithium-ion secondary battery of the embodiments of the present application discharged to 3.2V means that the positive electrode active material has good kinetic performance. At the same time, a high η value indicates that when the lithium-ion secondary battery containing the positive electrode active material is discharged to a low state of charge (SOC), it still has a high voltage, which is beneficial to maintaining good power performance.

[0199] In some embodiments, the positive electrode film layer further includes a conductive agent. Based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.1% - 1.5%.

[0200] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the conductive agent may be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or a numerical range between any two of them.

[0201] In some embodiments, the conductive agent includes at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0202] The carbon layer of the positive electrode active material has a high degree of graphitization and good consistency in graphitization degree, enabling the positive electrode active material to have good electron conductivity, which can reduce or even eliminate the use of the conductive agent in the positive electrode film layer, facilitating further increase of the loading amount of the positive electrode active material and improving the energy density of the lithium-ion secondary battery.

[0203] In some embodiments, the positive electrode film layer does not include a conductive agent.

[0204] The positive electrode active material has extremely high electron conductivity, such that the conductive agent may not even be added to the positive electrode film layer, which is beneficial to further increasing the loading amount of the positive electrode active material and improving the energy density of the lithium-ion secondary battery.

[0205] In some embodiments, the positive electrode film layer further includes a binder. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 95.5% - 99.5%, and may be 96.5% - 99.5%; the mass content of the binder is 0.5% - 3%.

[0206] In some embodiments, the binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride - tetrafluoroethylene - propylene terpolymer, vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene terpolymer, tetrafluoroethylene - hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0207] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material may be 95.5%, 96%, 96.5%, 97%, 98%, 99%, 99.5%, or a numerical range between any two of them.

[0208] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the binder may be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or a numerical range between any two of them.

[0209] In some embodiments, the areal density of the single side of the positive electrode film layer is 300 mg / 1540 mm 2 -450 mg / 1540 mm 2 .

[0210] In this application, the areal density of the single side of the positive electrode film layer has the meaning well-known in the art and can be tested by methods known in the art. For example, take the positive electrode sheet after single-sided coating and compaction (if it is a double-sided coated positive electrode sheet, the positive electrode film layer on one side can be wiped off first), punch it into small round pieces with an area of S 1 , weigh it, and record it as M 1 . Then wipe off the positive electrode film layer of the above-mentioned weighed positive electrode sheet, weigh the weight of the current collector, and record it as M 0 . The areal density of the single side of the positive electrode film layer = (M 1 -M 0 ) / S 1 . To ensure the accuracy of the test results, multiple groups (for example, 10 groups) of samples to be tested can be tested, and the average value can be calculated as the test result.

[0211] In some embodiments, the areal density of the single side of the positive electrode film layer can be selected as 300 mg / 1540 mm 2 , 310 mg / 1540 mm 2 , 320 mg / 1540 mm 2 , 330 mg / 1540 mm 2 , 340 mg / 1540 mm 2 , 350 mg / 1540 mm 2 , 360 mg / 1540 mm 2 , 370 mg / 1540 mm 2 , 380 mg / 1540 mm 2 , 390 mg / 1540 mm 2 , 400 mg / 1540 mm 2 , 410 mg / 1540 mm 2 , 420 mg / 1540 mm 2 , 430 mg / 1540 mm 2 , 440 mg / 1540 mm 2 , 450 mg / 1540 mm 2 or any numerical range between any two of them.

[0212] The positive electrode film layer with the areal density within the above range can help improve the energy density of the lithium-ion secondary battery.

[0213] In some embodiments, when the lithium-ion secondary battery is in a fully discharged state, the tap density of the positive electrode film layer is 2.51 g / cm3 -2.73 g / cm 3 。

[0214] In some embodiments, in the fully discharged state of the lithium-ion secondary battery, the tap density of the positive electrode film layer is 2.55 g / cm 3 -2.70 g / cm 3 。

[0215] In this application, the fully discharged state means that the battery is placed in an oven environment at 25 °C, left standing for 2 h, and after the battery temperature is maintained at 25 °C, the battery is discharged at a constant current of 1 / 3C to 2.5 V and then discharged at a constant current of 0.1C to 2.0 V.

[0216] The tap density of the positive electrode film layer can be tested by methods known in the art. As an example, the battery is placed in an oven environment at 25 °C, left standing for 2 h, and after the battery temperature is maintained at 25 °C, the battery is discharged at a constant current of 1 / 3C to 2.5 V and then discharged at a constant current of 0.1C to 2.0 V. The battery is disassembled to obtain the positive electrode plate. The residual electrolyte is treated with dimethyl carbonate solvent, the electrode plate is dried, cut into small round pieces with an area of S, and its mass is obtained as W 1 , and the thickness T of the positive electrode plate is measured using a micrometer 1 , then the positive electrode film layer of the above-mentioned weighed electrode plate is wiped off, and the mass of the current collector is weighed and recorded as W 2 , and the thickness T of the current collector is measured using a micrometer 2 , then the tap density PD of the positive electrode film layer = (W 1 -W 2 ) / [(T 1 -T 2 ) × S].

[0217] In some embodiments, in the fully discharged state of the lithium-ion secondary battery, the tap density of the positive electrode film layer can be selected as 2.51 g / cm 3 , 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm 3 , 2.72 g / cm 3 , 2.73 g / cm 3 or the numerical range between any two of them.

[0218] When the compaction density of the positive electrode film layer is within the above range, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0219] In some embodiments, after being treated by the compaction process, the compaction density of the positive electrode film layer is 2.63 g / cm 3 -2.85 g / cm 3 .

[0220] In some embodiments, after being treated by the compaction process, the compaction density of the positive electrode film layer can be selected as 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm 3 , 2.72 g / cm 3 , 2.73 g / cm 3 , 2.74 g / cm 3 , 2.75 g / cm 3 , 2.76 g / cm 3 , 2.77 g / cm 3 , 2.78 g / cm 3 g / cm 3 , 2.79 g / cm 3 , 2.80 g / cm 3 , 2.81 g / cm 3 , 2.82 g / cm 3 , 2.83 g / cm 3 , 2.84 g / cm 3 , 2.85 g / cm 3 or the numerical range between any two of them.

[0221] In this application, compaction refers to the process of compacting the positive electrode film layer by mechanical pressure during battery assembly to improve its density and conductivity.

[0222] In some embodiments, after the formation process, the compaction density of the positive electrode film layer is 2.52 g / cm 3 -2.73 g / cm 3 .

[0223] In some embodiments, after the formation process, the compaction density of the positive electrode film layer can be selected from 2.52 g / cm 3 , 2.53 g / cm 3 , 2.54 g / cm 3 , 2.55 g / cm 3 , 2.56 g / cm 3 , 2.57 g / cm 3 , 2.58 g / cm 3 , 2.59 g / cm 3 , 2.60 g / cm 3 , 2.61 g / cm 3 , 2.62 g / cm 3 , 2.63 g / cm 3 , 2.64 g / cm 3 , 2.65 g / cm 3 , 2.66 g / cm 3 , 2.67 g / cm 3 , 2.68 g / cm 3 , 2.69 g / cm 3 , 2.70 g / cm 3 , 2.71 g / cm 3 , 2.72 g / cm 3 , 2.73 g / cm 3 or any value range between any two of them.

[0224] In this application, formation refers to the process of forming a stable solid electrolyte interface (SEI film) and electrode structure through electrochemical reactions during the first charge and discharge of the battery.

[0225] It can be understood that with the rebound of the electrode sheet during the cycling process, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is slightly lower than that of the positive electrode film layer after compaction and formation.

[0226] When the compaction density of the positive electrode film layer is within the above range, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0227] In some embodiments, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73 g / cm3 In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10% - 22%.

[0228] In some embodiments, the tap density of the positive electrode film layer is 2.55 g / cm 3 - 2.70 g / cm 3 In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10% - 20%.

[0229] In some embodiments, in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer can be selected from 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22% or any numerical range between any two of them.

[0230] In the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer can be tested in the following manner. Import the scanning electron microscope image of the cross-section of the positive electrode film layer obtained in the manner described above along the thickness direction of the electrode sheet into the ImageJ software. Select the straight line tool, use the straight line to mark the scale length in the picture, click "Analyze SetScale", and set the scale parameters in the software according to the scale length in the picture. Select the rectangular tool, select the part of the picture outside the scale area, use "Image Duplicate" to copy the selected area, and use "Image Type 8 bit" to adjust the picture format; select "Analyze Set Measurements", and select the following 5 options: "Area", "Mean gray value", "Area Fraction", "Limit to threshold", "Feret’s diameter", where "Decimal places" is selected as 3, and then select "Image" - "Adjust" - "Threshold" in sequence. Set 0 and 100 in the "Threshold" box selection position in sequence, and then the pore data in the scanning electron microscope image of this cross-section can be exported using the Analyze-Measure function. Use "Image" - "Overlay" - "Flatten" to export to obtain the pore picture; click "Apply" in "Threshold", and then click "Analyze" - "Analyze Particles", and check the left four columns to obtain the pore statistical data.

[0231] Such as Figure 8As shown, it can be understood that in the embodiments of the present application, the "pores" in the cross-section of the positive electrode film layer are identified through the color difference of the picture and the threshold. This "pore" is not the pore data obtained from the exhaust test, but is mainly used to characterize the cross-sectional area between the particles in the cross-section of the positive electrode film layer. This method is superior to the exhaust method because the porosity obtained by the exhaust method is related to the pores between the particles and the mesopores in the carbon layer coated on the surface of the lithium iron phosphate particles, so it cannot objectively reflect the pores between the particles.

[0232] The lower the porosity in the cross-section of the positive electrode film layer measured by this method means, on the one hand, that the grading of large, medium and small particles in the positive electrode film layer is better and the compaction density is high. On the other hand, after the same grading and roll pressure, if the porosity is low, it means that the particles are easy to slide relative to each other, thus reducing the risk of overpressure and stress concentration in the film layer, further reducing the probability of the positive electrode film peeling off during the long cycle process, which is beneficial to improving the long cycle performance of the battery.

[0233] In some embodiments, the positive electrode tab includes a bottom coating, and the bottom coating is disposed between the positive electrode film layer and the positive electrode current collector; the bottom coating includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating is ≤ 10 pcs / 10 μm.

[0234] Among them, the carbon-based particles refer to particles mainly composed of carbon elements, including but not limited to conductive carbon, carbon black, etc.

[0235] The bottom coating is beneficial to improving the conductivity and adhesion between the positive electrode film layer and the current collector, reducing the peeling off of the positive electrode film layer from the current collector during the cycle, and at the same time improving the kinetic performance of the battery. In the high compaction density electrode tab of the embodiments of the present application, for example, when the compaction density of the positive electrode tab in the fully discharged state is greater than or equal to 2.4 g / cm 3 At this time, the current collector is prone to damage during the compaction process of the high-pressure electrode tab, and large-sized particles are easy to produce pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating to be ≤ 10 pcs / 10 μm is beneficial to reducing the probability of damage to the current collector in the high compaction density electrode tab and further improving the ultimate compaction density of the positive electrode tab.

[0236] The distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating can adopt the method described above. The positive electrode film layer is cut along the thickness direction of the electrode tab by an argon ion beam, and a scanning electron microscope image or a microscope image is taken. The size of the carbon particles in the bottom coating is detected by a statistical method, and the number of carbon-based particles with a particle size greater than 100 nm contained in every 10 μm of the bottom coating is counted. The statistics are carried out no less than 5 times, and the average value is obtained.

[0237] The bottom coating in the embodiments of the present application can be realized through any well-known preparation process. For example, during the preparation process of carbon-based particles, operations such as sieving or centrifugation are pre-performed to remove large particle carbon-based materials, so that the D of the carbon-based particles added during the bottom coating preparation process V50 is in the range of 20 nm - 60 nm, and D V90 is less than or equal to 70 nm. The carbon-based material and the binder are mixed, stirred, and coated on the current collector to obtain the bottom coating.

[0238] In some embodiments, the compaction density of the positive electrode sheet in the fully discharged state is greater than or equal to 2.4 g / cm 3 , and the single-sided thickness of the bottom coating is 1 μm - 4 μm.

[0239] In some embodiments, the compaction density of the positive electrode sheet in the fully discharged state is greater than or equal to 2.5 g / cm 3 , and the single-sided thickness of the bottom coating is 2 μm - 4 μm.

[0240] With the increase in the compaction density of the electrode sheet, the extrusion effect of large particle lithium-containing phosphate materials (such as particle size greater than 1 μm) in the positive electrode film layer on the bottom coating becomes more significant. Therefore, stress concentration is prone to occur at large particle sites, and even penetrate through the bottom coating to damage the current collector. Increasing the thickness of the bottom coating is beneficial to improving the stress concentration phenomenon in the electrode sheet and further increasing the ultimate compaction density of the electrode sheet.

[0241] The single-sided thickness of the bottom coating can be tested in the following way. As described above, the positive electrode film layer is cut along the thickness direction of the electrode sheet by an argon ion beam, and a scanning electron microscope image is taken. In the length direction of the electrode sheet, points are taken every 1 m to measure the single-sided bottom coating thickness. After measuring the bottom coating thickness at 10 points, the average value is calculated. It should be noted that during the process of taking measurement points, abnormal points need to be avoided, that is, the bottom coating areas with a thickness less than 50 nm and a thickness greater than 4 m; these abnormal points are mainly caused by extreme fluctuations in the thickness of individual areas due to abnormal stress concentration during the compaction process of the electrode sheet and do not have statistical significance.

[0242] In some embodiments, the thickness of the positive electrode current collector is less than or equal to 17 μm, and can be selected as 13 μm - 15 μm.

[0243] In some embodiments, the thickness of the positive electrode current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm or the numerical range between any two of them.

[0244] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0245] In some embodiments of the lithium-ion secondary battery, the negative electrode sheet includes a negative current collector and a negative electrode film layer provided on at least one side of the negative current collector. The areal density of the single-sided negative electrode film layer is 140 mg / 1540 mm 2 -221 mg / 1540 mm 2 ; and / or the tap density of the negative electrode film layer is 1.40 g / cm 3 -1.75 g / cm 3 .

[0246] The areal density and tap density of the negative electrode film layer can be tested by a method similar to that of the positive electrode film layer described above.

[0247] When the areal density and tap density of the negative electrode film layer are within the above ranges, it is beneficial to improve the energy density of the lithium-ion secondary battery.

[0248] In some embodiments, the negative current collector may be a metal foil or a composite current collector. For example, as the metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0249] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material can be a negative electrode active material for batteries well-known in the art. As an example, the negative electrode active material can include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material can be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as the negative electrode active material of the battery can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0250] In some embodiments, the negative electrode film layer may further optionally include a binder. The binder can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0251] In some embodiments, the negative electrode film layer may further optionally include a conductive agent. The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0252] In some embodiments, the negative electrode film layer may further optionally include other additives, such as thickeners (such as sodium carboxymethyl cellulose (CMC-Na)), etc.

[0253] In some embodiments, the negative electrode plate can be prepared in the following manner: dispersing the above components for preparing the negative electrode plate, such as the negative electrode active material, conductive agent, binder, and any other components, in a solvent (such as deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative electrode current collector, and after processes such as drying and compaction, the negative electrode plate can be obtained.

[0254] In some embodiments, the lithium-ion secondary battery includes an electrolyte. The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The present application has no specific limitation on the type of the electrolyte, and it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or all-solid-state.

[0255] In some embodiments, the electrolyte uses an electrolytic solution. The electrolytic solution includes an electrolyte salt and a solvent.

[0256] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro(oxalato)borate, lithium bis(oxalato)borate, lithium difluoro bis(oxalato)phosphate, and lithium tetrafluoro(oxalato)phosphate.

[0257] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0258] In some embodiments, the electrolyte may further optionally include additives. For example, the additives may include anode film-forming additives, cathode film-forming additives, and may also include additives that can improve certain battery performance, such as additives for improving the overcharge performance of the battery, additives for improving the high-temperature or low-temperature performance of the battery, and the like.

[0259] In some embodiments, the lithium-ion secondary battery further includes a separator. The present application does not particularly limit the type of the separator, and any well-known porous structure separator with good chemical stability and mechanical stability can be selected.

[0260] In some embodiments, the material of the separator may be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0261] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be made into an electrode assembly by a winding process or a stacking process.

[0262] In some embodiments, the lithium-ion secondary battery may include an outer package. The outer package can be used to encapsulate the above-mentioned electrode assembly and electrolyte.

[0263] In some embodiments, the outer package of the lithium-ion secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery may also be a soft package, such as a pouch-type soft package. The material of the soft package may be plastic, and examples of the plastic may include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0264] The second aspect of the present application provides a battery device, including the lithium-ion secondary battery provided by the first aspect of the present application. The battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0265] The third aspect of the present application provides an electrical device, including the lithium-ion secondary battery provided by the first aspect of the present application.

[0266] The fourth aspect of the present application provides a method for preparing a positive electrode active material, including: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; the carbon source includes polyethylene glycol; the iron source includes divalent iron; the molar ratio of iron to phosphorus in the mixed raw material is greater than or equal to 0.95 and less than or equal to 1; obtaining a mixed slurry after grinding in a solvent; obtaining a precursor powder after drying the mixed slurry; sintering the precursor powder to obtain the positive electrode active material; the sintering includes at least two-stage isothermal sintering, wherein the sintering temperature in the high-temperature stage is 750°C - 800°C.

[0267] The preparation method provided by the embodiments of the present application improves the graphitization degree of the positive electrode active material by regulating the molar ratio of iron to phosphorus in the mixed raw material. Further, polyethylene glycol is used as the carbon source, and at the same time, sintering temperature control and divalent iron-catalyzed reduction are combined to uniformly increase the graphitization degree of the particles in the positive electrode film layer. To prepare a positive electrode film layer with a median C 50 greater than or equal to 0.95 and less than or equal to 1.20 and the concentration of the C value (C 90 -C 10 ) / C 50 being 0.01 - 0.04 provides a material basis.

[0268] The positive electrode film layer prepared from the positive electrode active material prepared by this preparation method has a high graphitization degree and good graphitization degree consistency, and it is easy to improve the compaction density of the electrode sheet through the uniform and consistent slippage between the particles, which is beneficial to improving the energy density of the battery while improving the battery kinetic performance.

[0269] In some embodiments, the iron source includes divalent iron, and can be selected from one or more of ferrous oxalate, ferrous carbonate, and ferrous nitrate.

[0270] During the sintering process, the divalent iron source will preferentially decompose to generate a large amount of ferrous oxide, which serves as a nucleation site to generate nanocrystalline nuclei of lithium-containing transition metal phosphate. At the same time, the polymer carbon source has a relatively low decomposition temperature, and the iron element on the surface of the nanocrystalline nuclei will further catalyze the decomposition of the carbon source, so that the carbon coating layer on the surface of the positive electrode active material can have a relatively high graphitization degree at a relatively low sintering temperature, reducing the resistivity of the positive electrode active material and at the same time improving the compactness and uniformity of the carbon coating layer coated on the surface of the lithium-containing transition metal phosphate.

[0271] In some embodiments, the phosphorus source includes one or more of lithium dihydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate.

[0272] In some embodiments, the lithium source and the phosphorus source can be the same substance.

[0273] In some embodiments, the iron source includes iron oxalate, the lithium source and the phosphorus source include lithium dihydrogen phosphate, and the carbon source includes polyethylene glycol.

[0274] In some embodiments, the atomic molar ratio of iron element to phosphorus element in the iron source and the phosphorus source is 0.95:1.0 - 1.0:1.0.

[0275] In some embodiments, the atomic molar ratio of iron element to phosphorus element in the iron source and the phosphorus source can be selected as 0.95:1.0, 0.96:1.0, 0.97:1.0, 0.98:1.0, 0.99:1.0, 1.0:1.0, or the numerical range between any two of them.

[0276] Both too high or too low iron - phosphorus ratio will lead to the instability of the structure. Too low iron - phosphorus ratio may cause incomplete occupation of lithium, reducing the ionic conductivity and structural stability of the material; while too high iron - phosphorus ratio may lead to excessive iron, affecting the 4 electrochemical stability and capacity output of LiFePO. An appropriate iron - phosphorus ratio can promote the uniform growth of crystals, avoiding the aggregation of particles or too large differences in particle size during the synthesis process. If the iron - phosphorus ratio is too high, it may cause excessive iron ions to form large particles in the reaction, affecting the particle size consistency; while if the iron - phosphorus ratio is too low, phosphate may not fully participate in the reaction, resulting in incomplete particle growth and also affecting particle uniformity.

[0277] In some embodiments, the particle size D of iron oxalate 10 is greater than or equal to 3μm, the particle size D 50 is 50μm - 80μm, and the particle size D 90 is less than or equal to 150μm.

[0278] In the present application, the terms "D 10 ", "D 50 ", and "D 90 " respectively correspond to the particle sizes when the cumulative particle size distribution percentage of the sample measured by the Malvern laser scattering method reaches 10%, 50%, and 90%.

[0279] Controlling the particle size D of iron oxalate 10 to be greater than or equal to 3μm can reduce the proportion of small - particle - size iron oxalate particles and control its reaction activity during the grinding process. Controlling the particle size D of iron oxalate 50 、D 90It helps to uniformly mix raw materials during the grinding process, obtain a mixed slurry with consistent components and uniform particle sizes, and improve the particle size consistency of the prepared lithium-containing transition metal phosphate.

[0280] In some embodiments, the mass content of trivalent iron element is less than or equal to 0.08%.

[0281] In some embodiments, the mass content of trivalent iron element can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08% or the numerical range between any two of them.

[0282] Controlling the mass content of trivalent iron element helps to improve the uniformity and consistency of the carbon coating layer. An excessive content of trivalent iron element will preferentially consume the carbon source, resulting in poor consistency in the mass and thickness of the carbon layer coated between particles. On the one hand, the uneven carbon coating layer with different thicknesses will affect the compaction between particles, and on the other hand, local carbon deficiency will affect the connection of the conductive network between particles, which is not conducive to effectively improving the compaction density of the electrode sheet and improving the kinetics.

[0283] In some embodiments, the lithium source includes one or more of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, and lithium acetate.

[0284] In some embodiments, the carbon source includes a polymer carbon source, which can be one or more of polyethylene glycol and polyvinyl alcohol.

[0285] In some embodiments, based on the total mass of the positive electrode film layer, the mass content of the carbon source is 1-4%.

[0286] The polymer carbon source has a relatively low decomposition temperature and graphitization temperature, enabling the carbon coating layer on the surface of the positive electrode active material to decompose and form a carbon layer at a relatively low sintering temperature, hindering the growth and sintering of lithium-containing transition metal phosphate grains, and being conducive to reducing the particle size of the positive electrode active material particles.

[0287] At the same time, the polymer carbon source usually has a relatively high molecular weight or a long molecular chain, and it is easy to form a stable skeleton structure through cross-linking or orientation during the heat treatment process. This orderliness is retained during the high-temperature carbonization process, which is conducive to the directional growth of graphite crystals; at the same time, the entanglement and cross-linking between long chains are conducive to reducing structural defects and reducing lattice disorder caused by chain breakage during the carbonization process, thereby improving the graphitization degree.

[0288] The organic molecules in the carbon source will decompose at high temperatures, releasing carbon atoms. These carbon atoms can cover and fill the tiny voids or defects on the surface of the active material, reducing the surface roughness. The carbon coating layer formed by the polymer carbon source has a relatively high graphitization degree and a relatively tight carbon structure, which is conducive to optimizing the surface roughness of the positive electrode active material.

[0289] In some embodiments, the weight-average molecular weight of polyethylene glycol is less than 10,000.

[0290] In some embodiments, the weight-average molecular weight of polyethylene glycol can be optionally 1500, 2000, 3000, 4000, 6000, 8000 or the numerical range between any two of them.

[0291] Using polyethylene glycol with a weight-average molecular weight below 10,000 can control the decomposition rate during sintering to form a carbon coating layer with appropriate and uniform thickness.

[0292] In some embodiments, the water content of polyethylene glycol is less than or equal to 0.5%.

[0293] If the water content in polyethylene glycol is high, the water may affect the decomposition process, making it decompose incompletely or at a non-uniform rate during sintering. Excessive water may also cause the molten polyethylene glycol to be unevenly distributed during sintering, affecting the uniformity of the carbon layer and resulting in an unstable or peeling carbon coating layer.

[0294] In some embodiments, the water content of polyethylene glycol can be optionally 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or the numerical range between any two of them.

[0295] In some embodiments, the pH of polyethylene glycol is 5 - 7.

[0296] Polyethylene glycol with a pH of 5 - 7 has relatively high stability and will not degrade during the mixing process due to excessive acidity. Especially under high-temperature conditions, it will cause too fast decomposition, affecting the quality of the coating layer. If polyethylene glycol is alkaline, it may affect the stability of other components, leading to the dissolution or oxidation reaction of metal ions and affecting the performance of the final cathode active material.

[0297] In some embodiments, the slurry further includes a titanium source. Optionally, the titanium source includes one or more of titanium dioxide, tetrabutyl titanate, titanium nitrate, and titanic acid.

[0298] Titanium sources often have relatively low surface activity. Including a titanium source in the slurry can reduce the activity of the lithium-containing transition metal phosphate precursor, inhibit the grain growth of the lithium-containing transition metal phosphate during high-temperature sintering, and enable the lithium-containing transition metal phosphate to form smaller particles during sintering.

[0299] As a lattice stabilizer, titanium usually enters the lattice of the lithium-containing transition metal phosphate in the form of Ti 4+ Some titanium ions can replace the position of iron ions, making the crystal structure more stable and reducing the possibility of lithium and iron ions being in the wrong position, especially during high-temperature or high-current charge and discharge.

[0300] In some embodiments, lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide are uniformly mixed and ground in an organic solvent to obtain a mixed raw material.

[0301] The organic solvent can effectively reduce the occurrence of side reactions, improve the purity and consistency of the material. Moreover, the organic solvent has good volatility and is easier to remove during the subsequent drying process, without remaining inside the material, causing pores to form inside the material and affecting the denseness and structural stability of the material.

[0302] In some embodiments, based on the total mass of the mixed raw material, the mass proportion of the carbon source in the mixed raw material is 5% - 7%.

[0303] In some embodiments, based on the total mass of the mixed raw material, the mass proportion of the carbon source in the mixed raw material can be selected from 5%, 5.1%, 5.2%, 5.3%, 5.4%, 5.5%, 5.6%, 5.7%, 5.8%, 5.9%, 6.0%, 6.1%, 6.2%, 6.3%, 6.4%, 6.5%, 6.6%, 6.7%, 6.8%, 6.9%, 7% or any numerical range between any two of them.

[0304] By controlling the mass of the carbon source containing lithium within the above range, the conductivity of the material can be enhanced, and the negative impact on the specific capacity of the positive electrode sheet and the energy density of the battery can be reduced. An overly thick carbon layer not only occupies the space of the effective active material but may also cause the structure of the material to be unstable.

[0305] In some embodiments, the solvent includes water and its mixtures.

[0306] In some embodiments, obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source includes: adding the carbon source, the lithium source, the phosphorus source, the iron source, and the carbon source into a solvent and mixing and stirring, and the rotation speed of the stirring is 1400 rpm - 2200 rpm.

[0307] In some embodiments, obtaining the precursor powder after drying the mixed slurry includes spray - drying the mixed slurry to obtain the precursor powder.

[0308] In some embodiments, after sintering the precursor, the product is subjected to air - flow crushing to obtain the positive electrode active material.

[0309] In some embodiments, the classification frequency of the air - flow crushing is 18 Hz - 24 Hz, and the crushing air pressure is 0.45 MPa - 0.65 MPa.

[0310] The classification frequency in airflow milling refers to the working frequency of the classification device in airflow milling, which is usually related to the classification efficiency and particle size distribution of the particles. A higher classification frequency will screen the particles in the airflow more times, so that larger particles are screened out and smaller particles are left. And a higher classification frequency may increase the number of particle collisions, causing irregular particles to be further impacted, making the particle surface smoother and the shape tending to be spherical.

[0311] High air pressure will cause the particles to be subjected to greater impact force, and the collisions between particles will be more intense, which will cause the surface of the particles to be subjected to stronger impact and wear, and can crush large particles into small particles. The collisions between particles will be more intense, and the surface will be easier to be trimmed, thereby improving the sphericity and surface flatness of the particles.

[0312] However, too high classification frequency and crushing pressure will cause the agglomerated particles to further crack and break after being dispersed into primary particles, affecting the predetermined particle gradation distribution and making the carbon coating incomplete, which is manifested as increased iron dissolution, negatively affecting the slip of particles during roller pressing, and increasing the contact and reaction between lithium-containing transition metal phosphates and external factors such as electrolytes, which is not conducive to the cycle performance and life of the battery. Therefore, it is necessary to control the classification frequency and crushing pressure of air flow crushing within a suitable range.

[0313] The fifth aspect of the present application provides a method for preparing a positive electrode plate, which comprises sequentially adding a binder, a conductive agent, and a positive electrode active material prepared by the preparation method of the fourth aspect, dry-mixing the mixture, adding a solvent, stirring, and adjusting the viscosity to obtain a shipping slurry; transfer-coating the shipping slurry to at least one side of a current collector, and obtaining a positive electrode plate after drying and hot pressing.

[0314] In some embodiments, the orbital speed of the dry mix is ​​20 rpm-30 rpm, and the rotational speed of the dry mix is ​​750 rpm-850 rpm.

[0315] In some embodiments, hot pressing includes at least three hot roller pressings, and the hot roller pressure increases successively, and the hot roller pressure is 20 tons-50 tons, 50 tons-70 tons, and 70 tons-90 tons respectively; the hot roller temperature is 40℃-80℃, and the positive electrode sheet is heated before entering the hot roller compaction for the first time, and the heating temperature is 40℃-50℃.

[0316] The positive electrode active material prepared by the above-mentioned hot pressing process in combination with the preparation method of the fourth aspect in the embodiment of the present application is beneficial to further reduce the cross-sectional porosity of the positive electrode film layer, increase the ultimate compaction density of the electrode piece, and improve the energy density of the battery.

[0317] In addition, the present application also provides an electrical device, which includes at least one of the lithium-ion secondary battery, battery module, or battery pack provided by the present application. The lithium-ion secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device can include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc., but is not limited thereto.

[0318] As the electrical device, the lithium-ion secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0319] Figure 7 This is an example of an electrical device. The electrical device is a pure electric vehicle, hybrid electric vehicle, or plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the electrical device for the lithium-ion secondary battery, a battery pack or battery module can be used.

[0320] Another example of the device can be a mobile phone, tablet computer, laptop, etc. This device usually requires being thin and light, and a lithium-ion secondary battery can be used as the power source.

[0321] Embodiment Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation of the present application. For those not specified in the embodiments regarding specific technologies or conditions, they shall be carried out according to the technologies or conditions described in the literature in the art or according to the product specifications. For reagents or instruments not specified as to the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0322] Embodiment 1 (1) Preparation of the positive electrode active material Lithium dihydrogen phosphate, iron oxalate, polyethylene glycol, and titanium dioxide are mixed evenly and ground in a solvent. Among them, the ratio of iron oxalate to lithium dihydrogen phosphate is such that the molar ratio of iron to phosphorus is 0.965:1.0.

[0323] The mixed raw materials are ball-milled multiple times in a ball mill and demagnetized to obtain a mixed slurry. The grinding times and time are controlled, and the particle size Dv50 of the mixed slurry after grinding is 3.0 μm.

[0324] The mixed slurry is spray-dried to obtain a dried precursor powder material, and the appearance of the dried precursor powder material is light yellow and the color is uniform.

[0325] Place the precursor powder in a sintering furnace. Under a nitrogen atmosphere, heat it from 25°C to 350°C at a rate of 2°C / min and hold at this temperature for 3 h. Then heat it to the second temperature of 770°C at a rate of 5°C / min and hold at this temperature for 10 h. After that, cool it down.

[0326] Use the method of airflow pulverization to crush the obtained lithium iron phosphate cathode material to obtain the carbon-coated lithium iron phosphate cathode active material.

[0327] The mass content of carbon element in the cathode active material is 1.103%. The median value of the sphericity L A50 is 0.720, and L A90 is 0.895. The median value of the roughness R A50 is 0.941. The concentration of lithium-iron antisite defects is 0.55%. The tapped density of the powder is 1.04 g / cm 3 , and the compacted density of the powder under a pressure of 3 T is 2.572 g / cm 3 . The powder resistivity under a pressure of 8 MPa is 5.90 Ω·cm; the discharge specific capacity at a discharge rate of 1 C is 143.8 mAh / g; the proportion of the discharge capacity of the 3.2 V discharge platform is 91.0%.

[0328] (2) Preparation of the cathode electrode: Add 2.2 wt% of PVDF, 0.8 wt% of conductive carbon black, and 97.0 wt% of the cathode active material in sequence, dry-mix them, and then add N-methylpyrrolidone, stir, and adjust the viscosity to obtain the outgoing slurry. Transfer the outgoing slurry and coat it on the bottom coating of the current collector aluminum foil. The bottom coating includes carbon black and PVDF, and the mass ratio of the two is 1:1. The distribution density of carbon-based particles with a particle size greater than 100 nm in the bottom coating ≤ 10 pcs / 10 μm, and the thickness of the bottom coating is 2 μm. After drying and hot pressing, a cathode film layer with a single-sided density of 350 mg / 1540 cm 2 is obtained.

[0329] Among them, the revolution speed of the dry mixing is 25 rpm, and the rotation speed of the dry mixing is 800 rpm.

[0330] The hot pressing process includes three hot roll pressing processes, and the hot roll pressing pressure increases in sequence. The hot roll pressures are 35 tons, 55 tons, and 75 tons in sequence; the hot roll temperature is 65°C. Before the first entry into the hot roll compaction, heat the electrode, and the heating temperature is 50°C.

[0331] The compacted density of the electrode is the ultimate compacted density of the electrode. The test method for the ultimate compacted density of the electrode is as follows; in this example, the ultimate compacted density of the electrode is 2.68 g / cm 3 .

[0332] 17,857 particles were counted in the cross-section of the positive electrode film layer along the thickness direction of the electrode sheet. The results showed that the area ratio of particles with an area of 0.001 μm 2 -0.06 μm 2 was 23.84%, and the area ratio of particles with an area of 1.0 μm 2 -4.0 μm 2 was 15.01%. The median degree of graphitization C 50 obtained by the positive electrode film layer in the surface scanning mode of the laser confocal Raman spectrometer was 1.018, C 90 was 1.041, C 10 was 1.007, and the concentration of the C value ((C 90 - C 10 ) / C 50 was 0.033.

[0333] The iron dissolution rate of the positive electrode film layer was 1041 ppm.

[0334] (3) Preparation of the negative electrode sheet: 95.5 wt% of negative electrode active material (artificial graphite), 1.0 wt% of conductive agent (conductive carbon black), 2.0 wt% of binder (styrene-butadiene rubber (SBR)), and 1.5 wt% of thickener (sodium carboxymethyl cellulose (CMC)) were mixed, and deionized water was added and stirred to disperse and prepare a negative electrode slurry. Then, the negative electrode slurry was coated on both sides of the Cu foil. After both sides were completed, it was dried, compacted, slit, and sliced to prepare a negative electrode sheet. The single-sided coating areal density was 165 mg / 1540 mm 2 , and the tap density was 1.60 g / cm 3 .

[0335] (4) Preparation of the separator A polypropylene film was used as the separator.

[0336] (5) Preparation of the electrolyte In an argon atmosphere glove box (H 2 O < 0.1 ppm, O 2 < 0.1 ppm), organic solvents ethylene carbonate (EC) / dimethyl carbonate (DMC) were mixed evenly according to a volume ratio of 1 / 1, and lithium salt LiPF 6 was dissolved in the organic solvent. The content of LiPF 6 in the solution was 1 mol / L, and it was stirred evenly to obtain the electrolyte.

[0337] (6) Preparation of the battery: Stack the positive electrode sheet, separator, and negative electrode sheet in sequence. The separator should be able to isolate the positive and negative electrodes. Wind them to obtain a bare battery cell. Place the bare battery cell in an outer package, inject the electrolyte, and go through processes such as encapsulation, formation, and degassing to finally obtain a lithium-ion battery.

[0338] The preparation methods of Examples 2 and 3 are basically the same as that of Example 1, except that the carbon source in the raw materials is adjusted.

[0339] Example 2 Mix lithium dihydrogen phosphate, iron oxalate, polyethylene glycol and glucose (mixed in a mass ratio of 3:1), and titanium dioxide evenly in a solvent and grind them. Among them, the ratio of lithium dihydrogen phosphate to iron oxalate makes the molar ratio of iron to phosphorus 0.965:1.0.

[0340] Example 3 Mix lithium dihydrogen phosphate, iron oxalate, polyethylene glycol and glucose (mixed in a mass ratio of 1:3), and titanium dioxide evenly in a solvent and grind them. Among them, the ratio of lithium dihydrogen phosphate to iron oxalate makes the molar ratio of iron to phosphorus 0.965:1.0.

[0341] The preparation method of Example 4 is basically the same as that of Example 3, except that the sintering temperature of the precursor powder is adjusted.

[0342] Example 4 Mix lithium dihydrogen phosphate, iron oxalate, polyethylene glycol and glucose (mixed in a mass ratio of 1:3), and titanium dioxide evenly in a solvent and grind them. Among them, the ratio of lithium dihydrogen phosphate to iron oxalate makes the molar ratio of iron to phosphorus 0.965:1.0.

[0343] Place the precursor powder in a sintering furnace and perform two-stage sintering in a nitrogen atmosphere to obtain a lithium iron phosphate cathode material: heat from 25°C to 350°C at a heating rate of 2°C / min and hold for 3 h; heat from 350°C to 780°C at a heating rate of 5°C / min and hold for 10 h; after completion, cool down.

[0344] The preparation methods of Examples 5 and 6 are basically the same as that of Example 1, except that the sintering temperature of the precursor powder is adjusted.

[0345] Example 5 Place the precursor powder in a sintering furnace and perform two-stage sintering in a nitrogen atmosphere to obtain a lithium iron phosphate cathode material: heat from 25°C to 350°C at a heating rate of 2°C / min and hold for 3 h; heat from 350°C to 755°C at a heating rate of 5°C / min and hold for 10 h; after completion, cool down.

[0346] Example 6 Place the precursor powder in a sintering furnace and perform two-stage sintering under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material: heat from 25°C to 350°C at a heating rate of 2°C / min and hold for 3 h; heat from 350°C to 790°C at a heating rate of 5°C / min and hold for 10 h; after completion, cool down.

[0347] The preparation methods of Examples 7 and 8 are basically the same as that of Example 1, except that the molar ratio of iron to phosphorus is adjusted.

[0348] Example 7 Mix lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide evenly in a solvent and grind them. Among them, the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the molar ratio of iron to phosphorus is 0.955:1.0.

[0349] Example 8 Mix lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide evenly in a solvent and grind them. Among them, the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the molar ratio of iron to phosphorus is 0.975:1.0.

[0350] The preparation method of Example 9 is basically the same as that of Example 1, except that conductive carbon black is not added when preparing the cathode electrode sheet: Mix 97.8 wt% of the cathode active material and 2.2 wt% of PVDF, then add N-methylpyrrolidone and stir and disperse to make the cathode slurry.

[0351] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that the carbon source in the raw materials and the sintering temperature of the precursor powder are adjusted.

[0352] Comparative Example 1 Mix lithium dihydrogen phosphate, ferrous oxalate, glucose, and titanium dioxide evenly in a solvent and grind them. Among them, the ratio of lithium dihydrogen phosphate to ferrous oxalate is such that the molar ratio of iron to phosphorus is 0.965:1.0.

[0353] Place the precursor powder in a sintering furnace and perform two-stage sintering under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material: heat from 25°C to 350°C at a heating rate of 2°C / min and hold for 3 h; heat from 350°C to 740°C at a heating rate of 5°C / min and hold for 10 h; after completion, cool down.

[0354] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that the molar ratio of iron to phosphorus and the sintering temperature of the precursor powder are adjusted.

[0355] Comparative Example 2 Mix lithium dihydrogen phosphate, iron oxalate, polyethylene glycol, and titanium dioxide evenly in a solvent and grind them. Among them, the ratio of lithium dihydrogen phosphate to iron oxalate is such that the molar ratio of iron to phosphorus is 0.955:1.0.

[0356] Place the precursor powder in a sintering furnace and perform two-stage sintering in a nitrogen atmosphere to obtain the lithium iron phosphate cathode material: heat from 25°C to 350°C at a heating rate of 2°C / min and hold for 3 h; heat from 350°C to 810°C at a heating rate of 5°C / min and hold for 10 h; after completion, cool down.

[0357] Performance testing 1. Energy density test Let the lithium-ion secondary battery stand at 25°C for 2 h to ensure the temperature of the lithium-ion secondary battery is 25°C. After charging the lithium-ion secondary battery to the charging cut-off voltage of 3.65 V at 0.33C at 25°C, continue to perform constant-voltage charging at this charging cut-off voltage until the current is 0.05C, and then stop charging (where C represents the rated capacity of the lithium-ion secondary battery). After the lithium-ion secondary battery stands at 25°C for 1 h, discharge the lithium-ion secondary battery to the discharge cut-off voltage of 3.65 V at 0.33C at 25°C, and record the total discharge energy of the lithium-ion secondary battery as E 0 。

[0358] Measure the length, width, and height of the battery cell and calculate the volume value V of the battery cell 0 = length * width * height. The volume energy density of the lithium-ion secondary battery = the discharge energy E of the lithium-ion secondary battery 0 / the volume V of the lithium-ion secondary battery 0 。

[0359] 2. DCR test method At 25°C, after constant-current charging to 3.65 V at 0.33C, perform constant-voltage charging until the current is 0.05C, then discharge at 0.33C to 20% SOC, stand for 5 min, then perform 3C pulsed discharge for 30 s, stand for 40 s, then charge at 3C for 40 s, stand for 5 min, then charge at 0.33C to 3.65 V, perform constant-voltage charging to 0.05C, then discharge at 0.33C to 10% SOC, stand for 5 min, perform 3C pulsed discharge for 30 s, stand for 40 s, charge at 3C for 40 s, stand for 5 min, then charge to full capacity at 0.33C, then discharge at 0.33C to 50% SOC, then stand at -25°C for 2 h and perform 1C pulsed discharge for 30 s, stand for 10 min, then stand at 25°C for 2 h, charge at 0.33C to 3.65 V and perform constant-voltage charging to 0.05C, then discharge at 0.33C to 20% SOC, then stand at -25°C for 2 h and perform 1C pulsed discharge for 30 s, stand for 10 min.

[0360] Record the voltage before and after each pulse discharge, and calculate the DCR under different conditions. The calculation formula is DCR = (voltage before pulse discharge after standing - voltage before standing after pulse discharge) / pulse current.

[0361] 3. Ultimate compaction density of the electrode Compress the double-sided coated electrode through a roller press, test the elongation rate of the compressed electrode, and evaluate the flexibility of the electrode after compaction. By increasing the pressure of the roller press, electrodes with different compaction densities can be obtained. As the pressure increases, the compaction density of the electrode increases, the elongation rate of the electrode increases, and the flexibility of the electrode decreases. An overly high elongation rate of the electrode is likely to cause warping of the electrode, and an overly low flexibility of the electrode is likely to lead to brittle fracture of the electrode. Therefore, define the smaller value of the compaction density corresponding to an elongation rate of 8% of the electrode or a folding number of 3 times for the flexibility of the electrode as the ultimate compaction density of the electrode.

[0362] The compaction density is calculated by the mass of the positive electrode film layer / the volume of the positive electrode film layer.

[0363] The test method for the elongation rate is as follows: Lay the electrode flat on a horizontal tabletop, cut the electrode into segments, and each electrode is about 100 cm long; remove the base copper foil at the edge of the electrode, and pay attention to keeping the cut edge of the electrode parallel to the MD direction of the electrode (perpendicular to the direction of the pressure roller), ensure that the electrode part is completely covered by the coating, use a steel ruler to measure the length between the marked points at the head and tail of the electrode and at the same position in the width direction of the length direction, estimate to 0.1 mm, and record the length before compaction; record the length after compaction between the corresponding marked points after compaction, and use (length after compaction - length before compaction) / length before compaction as the elongation rate of the electrode.

[0364] The test method for the flexible folding number is as follows.

[0365] Cut the positive electrode into test specimens with dimensions of 20×100 mm 2 ; After folding it forward, flatten it with a 2 kg pressure roller, and unfold it to check whether there is light transmission through the gap against the light. If there is no light transmission, then fold it backward, flatten it with a 2 kg pressure roller, and check again against the light. Repeat this process until there is light transmission through the gap, and record the number of folds; repeat the test three times and take the average value as the reference data for the flexibility of the electrode.

[0366] Test results Table 1

[0367] Table 2

[0368] From the comparison between the examples and the comparative examples, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer for the positive electrode film layer, the median C of the graphitization degree 50 is greater than or equal to 0.95 and less than or equal to 1.20, and the concentration of the C value (C 90 -C 10 ) / C 50 is 0.01 - 0.04; while the battery maintains a low internal resistance (especially having a low impedance at low SOC), the compaction density of the positive electrode sheet is increased, so that on the basis of the battery maintaining good kinetic performance, the compaction density of the electrode sheet and the energy density of the battery are improved.

[0369] From the comparison between Example 5 and Examples 1 - 4, Examples 6 - 9, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer for the positive electrode active material, when the median C of the graphitization degree 50 is 0.98 - 1.13, it is beneficial to increase the compaction density of the electrode sheet on the basis of maintaining the low impedance of the battery, and to improve the energy density of the battery on the basis of maintaining the good kinetic performance of the battery.

[0370] From the comparison between Examples 4, 6 and Example 2, in the cumulative distribution curve of the graphitization degree C value obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer for the positive electrode active material, when the concentration of the C value (C 90 -C 10 ) / C 50 is 0.02 - 0.038, it indicates that the coating layer of the positive electrode active material has a high uniformity, which is beneficial to further improve the kinetic performance of the battery on the basis of maintaining the good compaction density of the electrode sheet and the energy density of the battery.

[0371] From the comparison between Example 9 and Example 1, it can be seen that the lithium-ion secondary battery of the embodiment of the present application still has good kinetic performance without adding a conductive agent, and at the same time, the energy density of the lithium-ion secondary battery is further improved.

[0372] It should be noted that the present application is not limited to the above embodiments. The above embodiments are only examples, and the embodiments having the same constitution and the same effect as the technical idea within the technical solution scope of the present application are all included in the technical scope of the present application. In addition, within the scope not departing from the gist of the present application, various deformations that can be thought of by those skilled in the art are applied to the embodiments, and other ways constructed by combining some constituent elements of the embodiments are also included in the scope of the present application.

Claims

1. A lithium ion secondary battery, characterized in that: Including positive electrode sheet, negative electrode sheet and electrolyte, The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one side of the positive electrode current collector. The positive electrode film layer includes a positive electrode active material, and the positive electrode active material includes lithium-containing transition metal phosphate particles with a carbon coating material disposed on at least a portion of the surface. In the cumulative distribution curve of the graphitization degree C of the positive electrode film layer obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the median value of the graphitization degree C 50 Greater than or equal to 0.95 and less than or equal to 1.20, and the concentration of C values ​​(C 90 -C 10 ) / C 50 0.01-0.04; Among them, the graphitization degree C value is I G / I D , I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at .

2. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C of the positive electrode film layer obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the median value of the graphitization degree C 50 It is 0.96-1.

15.

3. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C of the positive electrode film layer obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the median value of the graphitization degree C 50 It is 0.98-1.

13.

4. The lithium-ion secondary battery according to claim 1, characterized in that: In the graphitization degree C value cumulative distribution curve of the positive electrode film layer obtained in the surface scanning mode of the laser micro-confocal Raman spectrometer, the concentration of C value (C 90 -C 10 ) / C 50 It is 0.02-0.

038.

5. The lithium ion secondary battery according to claim 1, characterized in that: In the graphitization degree C value cumulative distribution curve of the positive electrode film layer obtained in the surface scanning mode of the laser micro-confocal Raman spectrometer, the concentration of C value (C 90 -C 10 ) / C 50 It is 0.02-0.

036.

6. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the graphitization degree C 90 It is 1.0-1.

30.

7. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the graphitization degree C 90 It is 1.02-1.

15.

8. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the graphitization degree C 10 It is 0.92-1.

10.

9. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the surface scanning mode of the laser microscopic confocal Raman spectrometer, the graphitization degree C 10 It is 0.98-1.

08.

10. The lithium ion secondary battery according to claim 1, characterized in that: The area of ​​the cross section of the positive electrode film along the thickness direction of the electrode piece is 0.001 μm 2 -0.06μm 2 The area of ​​the particles is 21.00%-27.00%, and the area is 1.0μm 2 -4.0μm 2 The area proportion of the particles is 12.00%-20.00%.

11. The lithium ion secondary battery according to claim 1, characterized in that: In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the D A50 600nm-800nm, where D A50 It refers to the particle size corresponding to when the cumulative area distribution of the particles reaches 50% in the particle area cumulative distribution curve.

12. The lithium ion secondary battery according to claim 11, characterized in that: In the cross section of the positive electrode film along the thickness direction of the electrode sheet, the D A50 650nm-750nm, where D A50 It refers to the particle size corresponding to when the cumulative area distribution of the particles reaches 50% in the particle area cumulative distribution curve.

13. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of the particle roughness area obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the roughness R A50 It is 0.92-0.

96.

14. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of particle sphericity obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the sphericity L A90 It is 0.80-0.

95.

15. The lithium ion secondary battery according to claim 14, characterized in that: In the cumulative distribution curve of particle sphericity obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the sphericity L A90 It is 0.85-0.

93.

16. The lithium ion secondary battery according to claim 1, characterized in that: In the cumulative distribution curve of particle sphericity area obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 It is 0.65-0.

85.

17. The lithium ion secondary battery according to claim 16, characterized in that: In the cumulative distribution curve of particle sphericity area obtained from the cross section of the positive electrode film along the thickness direction of the electrode sheet, the median of the sphericity L A50 It is 0.70-0.

80.

18. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material includes iron element, and the iron dissolution rate of the positive electrode film layer is 500ppm-2000ppm.

19. The lithium ion secondary battery according to claim 18, characterized in that: The iron dissolution rate of the positive electrode film layer is 500ppm-1500ppm.

20. The lithium ion secondary battery according to claim 1, characterized in that: Based on the total mass of the positive electrode active material, the mass content of carbon element is 0.8%-1.8%.

21. The lithium ion secondary battery according to claim 20, characterized in that: Based on the total mass of the positive electrode active material, the mass content of carbon element is 0.9%-1.5%.

22. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material includes iron element, and the lithium iron antisite defect concentration of the positive electrode active material is 0.1%-1.5%.

23. The lithium ion secondary battery according to claim 22, characterized in that: The lithium iron antisite defect concentration of the positive electrode active material is 0.3%-1.0%.

24. The lithium ion secondary battery according to claim 1, characterized in that: The lithium-containing transition metal phosphate includes a component having the following general formula: The m Fe x P y O j Q q , Among them, Q includes one or more of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, 0<q≤0.

1.

25. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material includes one or more of lithium iron phosphate and its doped modified materials and coated modified materials.

26. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material includes titanium element, and the mass content of the titanium element is 2000ppm-6000ppm based on the total mass of the positive electrode active material.

27. The lithium ion secondary battery according to claim 1, characterized in that: The tap density of the positive electrode active material powder is 0.70 g / cm 3 -1.50g / cm 3 .

28. The lithium ion secondary battery according to claim 27, characterized in that: The tap density of the positive electrode active material powder is 0.70 g / cm 3 -1.20g / cm 3 .

29. The lithium ion secondary battery according to claim 1, characterized in that: The powder compaction density of the positive electrode active material under a pressure of 3T is 2.50 g / cm 3 -2.70g / cm 3 .

30. The lithium ion secondary battery according to claim 29, characterized in that: The powder compaction density of the positive electrode active material under a pressure of 3T is 2.52 g / cm 3 -2.68g / cm 3 .

31. The lithium ion secondary battery according to claim 1, characterized in that: The powder resistivity of the positive electrode active material under a pressure of 8 MPa is 0.5 Ω·cm-30 Ω·cm.

32. The lithium ion secondary battery according to claim 31, characterized in that: The powder resistivity of the positive electrode active material under a pressure of 8 MPa is 2Ω·cm-20Ω·cm.

33. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode active material has a discharge capacity of 135 mAh / g to 150 mAh / g at a 1C discharge rate at room temperature.

34. The lithium ion secondary battery according to claim 1, characterized in that: The discharge capacity of the positive electrode active material discharged to 3.2V accounts for η≥85%, and η is defined as: at room temperature, the button battery containing the positive electrode active material is charged and discharged twice at a constant current rate of 0.1C in the voltage range of 2.0V~3.75V, and then charged and discharged once at a constant current rate of 1C. In the charge and discharge test at the rate of 1C, the capacity value extracted at the discharge voltage of 3.2V is recorded as C1, and the capacity value extracted to the discharge voltage of 2.0V is C2, η=C1 / C2, wherein the charging process includes constant voltage charging, a constant voltage of 3.75V, and a constant voltage cut-off current of 50μA.

35. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer further includes a conductive agent, and based on the total mass of the positive electrode film layer, the mass content of the conductive agent is 0.1%-1.5%.

36. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer does not include a conductive agent.

37. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer also includes a binder. Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 95.5%-99.5%; the mass content of the binder is 0.5%-3%.

38. The lithium ion secondary battery according to claim 37, characterized in that: Based on the total mass of the positive electrode film layer, the mass content of the positive electrode active material is 96.5%-99.5%.

39. The lithium ion secondary battery according to claim 1, characterized in that: The single-side density of the positive electrode film layer is 300 mg / 1540 mm 2 -450mg / 1540mm 2 .

40. The lithium ion secondary battery according to claim 1, characterized in that: When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 .

41. The lithium ion secondary battery according to claim 1, characterized in that: When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.70g / cm 3 .

42. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode film layer satisfies at least one of the following conditions: (1) When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.51 g / cm 3 -2.73g / cm 3 , in a cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-22%; (2) When the lithium-ion secondary battery is fully discharged, the compaction density of the positive electrode film layer is 2.55 g / cm 3 -2.70g / cm 3 In the cross section of the positive electrode film layer along the thickness direction of the electrode sheet, the porosity of the positive electrode film layer is 10%-20%.

43. The lithium ion secondary battery according to claim 1, characterized in that: The positive electrode plate includes a primer layer, which is disposed between the positive electrode film layer and the positive electrode current collector; the primer layer satisfies at least one of the following conditions: (1) The primer layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the primer layer is ≤10 pcs / 10 μm; (2) The compaction density of the positive electrode sheet in the fully charged state is greater than or equal to 2.4 g / cm 3 , the thickness of the primer layer on one side is 1 μm-4 μm; (3) The compaction density of the positive electrode sheet in the fully charged state is greater than or equal to 2.5 g / cm 3 The single-side thickness of the primer layer is 2 μm-4 μm.

44. A battery device, characterized in that: A lithium-ion secondary battery comprising any one of claims 1 to 43, wherein the battery device comprises at least one of a battery module, a battery pack, and an energy storage battery.

45. An electrical device, characterized in that: A lithium ion secondary battery according to any one of claims 1 to 43 or a battery device according to claim 44.

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