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 C10 of the graphitization degree is 0.92-1.10, 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] This application is a divisional application of the invention patent application filed on April 23, 2025, with application number 202510512090.0 and invention title "Lithium-ion secondary battery, battery device and power consumption device".

[0002] Cross-references This application incorporates, in its entirety, PCT international application filed on March 28, 2025, entitled "Lithium-ion secondary battery, battery device, electrical device, method for preparing positive electrode active material and method for preparing positive electrode sheet", application number: PCT / CN2025 / 085940, which is hereby incorporated by reference. Technical Field

[0003] This application relates to the field of lithium-ion battery technology, and in particular to a lithium-ion secondary battery, battery device, power device, a method for preparing positive electrode active material, and a method for preparing positive electrode sheet. Background Technology

[0004] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, military equipment, and aerospace.

[0005] Positive electrode active materials are a crucial component of lithium-ion secondary batteries. Lithium-containing transition metal phosphate materials possess characteristics such as structural stability, good safety, and long cycle life, demonstrating broad development prospects. However, with the increasing market demands for energy density and kinetics in lithium-containing transition metal phosphate system secondary batteries, existing technologies struggle to simultaneously improve these properties, making this a pressing technical challenge in the field. Summary of the Invention

[0006] This application is made in view of the above-mentioned issues, and its purpose is to provide a lithium-ion secondary battery that has both high energy density and good kinetic performance.

[0007] The first aspect of this application provides a lithium-ion secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode film disposed on at least one side of the positive current collector. The positive electrode film includes a positive active material, comprising lithium transition metal phosphate particles with at least a portion of their surface coated with carbon material. In the cumulative distribution curve of the graphitization degree C value obtained by laser confocal Raman spectroscopy in surface scanning mode, the median C of the graphitization degree is... 50 The concentration of C values ​​is greater than or equal to 0.95 and less than or equal to 1.20. 90 -C 10 ) / C50 The value is 0.01-0.04; where the degree of graphitization C is I. G / I D I G This indicates that the Raman spectrum is at 1580±100 cm⁻¹ -1 The intensity of peak G at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

[0008] The median C of the degree of graphitization in the embodiments of this application 50 The concentration of C value is greater than or equal to 0.95 and less than or equal to 1.20, while controlling the concentration of C value (C 90 -C 10 ) / C 50 A value of 0.01-0.04 indicates high graphitization degree and high graphitization uniformity of the particles in the positive electrode film. This signifies good coating uniformity and consistency of the positive electrode active material, reducing particle slippage obstruction and subsequent local stress concentration caused by low graphitization. Therefore, uniform slippage between positive electrode active material particles allows the electrode to achieve a high overall compaction density under relatively low rolling pressure. Simultaneously, uniform graphitization facilitates uniform lithium-ion insertion and extraction, improving both the electrode compaction density and battery energy density while maintaining good battery kinetic performance. In any embodiment, the median C of the graphitization degree C value cumulative distribution curve obtained in laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The range is 0.96-1.15, and can be further selected as 0.98-1.13.

[0009] The median C of the graphitization degree of the positive electrode film 50 Within the aforementioned range, it is beneficial to further improve the slippage between particles, thereby increasing the compaction density of the electrode while maintaining the high kinetic performance of the battery, thus achieving a balance between battery kinetic performance and energy density.

[0010] In any embodiment, the concentration of C values ​​(C0.05) in the cumulative distribution curve of graphitization degree C values ​​obtained by laser microscopy confocal Raman spectroscopy instrument scanning mode of the positive electrode film layer is... 90 -C 10 ) / C 50 The range is 0.02-0.038, and can be set to 0.02-0.036.

[0011] Concentration of C-values ​​(C 90 -C 10 ) / C 50Within the aforementioned range, it is beneficial to further improve the uniformity of graphitization degree of particles in the positive electrode film, improve the uniformity of slip between particles, reduce the inconsistent lithium intercalation rate caused by poor graphitization degree in the positive electrode film, and thus prevent local polarization. The battery can further improve its dynamic performance while maintaining good energy density.

[0012] In any embodiment, in the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode, the C value of the graphitization degree is... 90 The value is 1.0-1.30, and can be selected as 1.02-1.15.

[0013] C of graphitization degree 90 Within the above range, the median C of the degree of graphitization 50 The relatively close distribution indicates that the graphitization degree of the particles in the positive electrode film is within a narrow range, which is conducive to uniform sliding between particles and thus improves the compaction density of the positive electrode sheet.

[0014] In any embodiment, in the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopic confocal Raman spectroscopy instrument scanning mode, the C value of the graphitization degree is... 10 The range is 0.92-1.10, and can be set to 0.98-1.08.

[0015] C of graphitization degree 10 Within the above range, it is indicated that different sites in the positive electrode film have a high degree of graphitization, which is beneficial to the uniform slippage of particles, reduces the probability of local stress concentration, and further improves the compaction density of the electrode.

[0016] In any embodiment, the area of ​​the cross-section of the positive electrode film layer along the electrode thickness direction is 0.001 μm. 2 -0.06μm 2 The particle area ratio is 21.00%-27.00%, and the area is 1.0μm. 2 -4.0μm 2 The area ratio of the particles is 12.00%-20.00%.

[0017] The surface area of ​​the positive electrode active material is controlled to be 0.001 μm. 2 -0.06μm 2 The particle area ratio and area are 1.0 μm. 2 -4.0μm 2 With the particle area ratio within the above range, combined with a uniform high degree of graphitization, uniform particle slippage can be achieved during the electrode compaction process. This improves the electrode compaction density while reducing the negative impact on other properties such as battery dynamics, cycle life, and processing performance, thus comprehensively improving battery performance.

[0018] In any embodiment, in the cross-section of the positive electrode film along the electrode thickness direction, the D of the particles... A50 The range is 600nm-800nm, with options for 650nm-750nm, where D A50 It refers to the particle size at which the cumulative area distribution of particles reaches 50% in the cumulative area distribution curve of particles.

[0019] Particle size D A50 Within the above range, it indicates that the positive electrode film contains a certain number of large-sized particles. The median particle size D is controlled. A50 Within the aforementioned range, the large contact area between large-sized particles can improve the efficiency of roller pressure transmission between electrode particles, fully utilize the skeletal support of large-sized particles, enable the electrode to withstand higher roller pressure, and increase the compaction density of the electrode. At the same time, it can reduce the kinetic decline caused by excessively large particle size, thus maintaining the kinetic performance of the battery while increasing the compaction density of the electrode.

[0020] In any embodiment, the median roughness R in the cumulative distribution curve of particle roughness area obtained from a cross-section of the positive electrode film along the electrode thickness direction is... A50 It is 0.92-0.96.

[0021] Median roughness R A50 Within the aforementioned range, the particle surface is relatively smooth, and the friction between particles is relatively small. Under the action of external force, it is easy to slip. When combined with particles with high graphitization degree, the electrode compaction density can be improved under low rolling pressure, thereby further improving the energy density of the battery.

[0022] In any embodiment, in the cumulative distribution curve of the sphericity area of ​​the particles obtained by slicing the positive electrode film along the thickness direction of the electrode sheet, the sphericity L A90 The range is 0.80-0.95, and can be set to 0.85-0.93.

[0023] In any embodiment, the median L of the sphericity in the cumulative distribution curve of the particle sphericity area obtained from a cross-section of the positive electrode film along the electrode thickness direction is... A50 The value is 0.65-0.85, and can be selected as 0.70-0.80.

[0024] Median L of sphericity A90 L A50 Within the aforementioned range, the particles are approximately spherical, and under external force, they are prone to slippage between each other. When combined with particles of high graphitization, the electrode compaction density can be increased under low rolling pressure, thereby further improving the energy density of the battery.

[0025] In any embodiment, the positive electrode active material includes iron, and the iron dissolution rate of the positive electrode film is 500ppm-2000ppm, optionally 500ppm-1500ppm.

[0026] Positive electrode active materials with iron dissolution rates within the aforementioned range possess a relatively complete and dense carbon coating layer. This enhances the electrical contact between positive electrode active materials, improves their conductivity, reduces polarization, and further optimizes the kinetic performance of lithium-ion secondary batteries. Simultaneously, the densely coated carbon layer has a low space occupancy rate, making the interparticle gaps easier to compress under stress during rolling. Combined with a carbon coating layer of high graphitization, this further improves the compaction density of the electrode and the energy density of the battery.

[0027] In any embodiment, the carbon content is 0.8%-1.8% based on the total mass of the positive electrode active material, and can be selected as 0.90%-1.5%.

[0028] Compared with existing lithium transition metal phosphate cathode active materials, this cathode active material has a relatively low carbon coating content, which can further increase the loading of lithium transition metal phosphate in the cathode sheet and improve the energy density of lithium-ion secondary batteries.

[0029] In any embodiment, the positive electrode active material includes iron, and the lithium iron antisite defect concentration of the positive electrode active material is 0.1%-1.5%, optionally 0.3%-1.0%.

[0030] The positive electrode active material in this embodiment has low lithium iron antisite 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.

[0031] In any embodiment, the lithium-containing transition metal phosphate includes a component 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, and Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0032] Selecting an appropriate modifying element Q can improve the ion diffusion pathway 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.

[0033] In any embodiment, the positive electrode active material includes one or more of lithium iron phosphate and its doped and modified materials, and coated and modified materials.

[0034] In any embodiment, the positive electrode active material includes titanium, and the mass content of titanium is 2000ppm-6000ppm based on the total mass of the positive electrode active material.

[0035] The positive electrode active material in this application embodiment has a high titanium content. Surprisingly, the high amount of titanium added did not form a harmful impurity phase that negatively affects the battery's energy density and kinetic performance. Although the reason is not yet clear, it is speculated that titanium, lithium, phosphate, and other elements (e.g., lithium) together form a fast ion conductor, which actually improves the battery's kinetic performance.

[0036] In any embodiment, the tap density of the positive electrode active material powder is 0.70 g / cm³. 3 -1.50g / cm 3 0.70 g / cm³ is an option. 3 -1.20g / cm 3 .

[0037] The positive electrode active material in this embodiment has a relatively low powder tap density. Due to the high graphitization degree and good graphitization consistency of the positive electrode film, it is easy to slip under external force to improve the powder tap density.

[0038] In any embodiment, the compacted density of the positive electrode active material powder under 3T pressure is 2.50 g / cm³. 3 -2.70g / cm 3 The option is 2.52 g / cm³. 3 -2.68g / cm 3 .

[0039] Thanks to its high degree of graphitization and good consistency of graphitization, the positive electrode active material can still achieve high compaction density under external force, providing a material basis for improving electrode compaction density and preparing high-energy-density lithium-ion secondary batteries.

[0040] In any embodiment, 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 selected as 2 Ω·cm-20 Ω·cm.

[0041] This positive electrode active material has a high degree of graphitization; therefore, by utilizing the sp... 2The structure facilitates rapid electron conduction between particles, resulting in low powder resistivity in the positive electrode active material. This is beneficial for improving the solid-phase electron transport rate and further enhancing the battery's kinetic performance.

[0042] In any embodiment, the discharge specific capacity of the positive electrode active material at a 1C discharge rate at room temperature is 135mAh / g-150mAh / g.

[0043] The positive electrode active material exhibits a high discharge specific capacity at a 1C rate, indicating that it has good charge and discharge capabilities, which is beneficial for improving the battery's dynamic performance.

[0044] In any embodiment, the discharge capacity η of the positive electrode active material discharged to 3.2V accounts for ≥85%, where η is defined as follows: at room temperature, a coin cell containing the positive electrode active material is charged and discharged twice at a constant current of 0.1C within a voltage range of 2.0V to 3.75V, followed by a constant current charge and discharge once at a constant current of 1C. In the 1C charge and discharge test, the capacity value extracted at a discharge voltage of 3.2V is recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V is recorded as C2, where η = C1 / C2. The charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0045] The high discharge capacity ratio of the positive electrode active material used in the lithium-ion secondary battery of this application to 3.2V indicates that the positive electrode active material has good kinetic performance. At the same time, the high η value indicates that the lithium-ion secondary battery containing the positive electrode active material still has a high voltage when discharged to a low state of charge (SOC), which is beneficial for maintaining good power performance.

[0046] In any embodiment, the positive electrode film layer further includes a conductive agent, the mass content of which is 0.1%-1.5% based on the total mass of the positive electrode film layer.

[0047] The carbon layer of this positive electrode active material has a high degree of graphitization and good graphitization consistency, which gives the positive electrode active material good electronic conductivity. This can reduce or even eliminate the use of conductive agents in the positive electrode film, which is conducive to further increasing the loading of the positive electrode active material and improving the energy density of lithium-ion secondary batteries.

[0048] In any embodiment, the positive electrode film layer does not include a conductive agent.

[0049] This positive electrode active material has extremely high electronic conductivity, which means that conductive agents can be omitted from the positive electrode film, thus improving the loading of the positive electrode active material and enhancing the energy density of lithium-ion secondary batteries.

[0050] In any embodiment, the positive electrode film layer further includes a binder, and 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%.

[0051] In any embodiment, the one-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 .

[0052] Positive electrode films with areal densities within the above range can help improve the energy density of lithium-ion secondary batteries.

[0053] In any embodiment, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is 2.51 g / cm³. 3 -2.73g / cm 3 .

[0054] A compaction density of the positive electrode film within the above-mentioned range is beneficial to improving the energy density of lithium-ion secondary batteries.

[0055] In any embodiment, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is 2.55 g / cm³. 3 -2.70g / cm 3 .

[0056] A compaction density of the positive electrode film within the above-mentioned range is beneficial to improving the energy density of lithium-ion secondary batteries.

[0057] In any embodiment, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is 2.51 g / cm³. 3 -2.73g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%.

[0058] In any embodiment, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery is 2.55 g / cm³. 3 -2.70g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-20%.

[0059] The lower the porosity in the cross-section of the positive electrode film in this application embodiment, the better the gradation of large, medium and small particles in the positive electrode film and the higher the compaction density. On the other hand, under the same gradation and roller pressure, if the porosity is low, it means that the particles are more likely to slide against each other, thereby reducing the risk of overpressure and stress concentration in the film, further reducing the probability of the positive electrode film demolding during long cycle, which is beneficial to improving the long cycle performance of the battery.

[0060] In any embodiment, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode current collector; the base coating layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating layer is ≤10 pcs / 10 μm.

[0061] The undercoat layer helps improve the conductivity and adhesion between the positive electrode film and the current collector, reduces the likelihood of the positive electrode film detaching from the current collector during cycling, and improves the battery's dynamic performance. In the high compaction density electrode sheet of this application embodiment, controlling the distribution density of carbon-based particles with a particle size greater than 100 nm in the undercoat layer to ≤10 pcs / 10 μm helps reduce the probability of damage to the current collector in the high compaction density electrode sheet, further improving the ultimate compaction density of the positive electrode sheet.

[0062] In any embodiment, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode current collector; the compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.4 g / cm³. 3 The thickness of the base coating layer on one side is 1μm-4μm.

[0063] In any embodiment, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode current collector; the compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.5 g / cm³. 3 The thickness of the base coating layer on one side is 2μm-4μm.

[0064] As the compaction density of the electrode increases, the compressive effect of large lithium phosphate particles (e.g., particles larger than 1 μm) on the undercoat becomes more significant. Therefore, stress concentration easily occurs at large particle sites, and these particles can even penetrate the undercoat and damage the current collector. Increasing the thickness of the undercoat helps to mitigate stress concentration in the electrode, further improving the electrode's ultimate compaction density.

[0065] The second aspect of this application provides a battery device including the lithium-ion secondary battery provided in the first aspect of this application, wherein the battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0066] A third aspect of this application also provides an electrical device, which includes the lithium-ion secondary battery provided in the first aspect of this application or the battery device provided in the second aspect of this application.

[0067] The fourth aspect of this application also provides a method for preparing a positive electrode active material: obtaining a mixed raw material comprising a carbon source, a lithium source, an iron source, and a phosphorus source; wherein the carbon source comprises polyethylene glycol; the iron source comprises ferrous 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; grinding in a solvent to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material; wherein the sintering comprises at least two stages of isothermal sintering, wherein the sintering temperature of the high-temperature stage is 750℃-800℃.

[0068] The positive electrode film prepared by this method has a high degree of graphitization and good graphitization consistency. It is easy to improve the compaction density of the electrode sheet through uniform and consistent slip between particles, which is beneficial to improve the energy density of the battery while improving the battery dynamic performance.

[0069] The fifth aspect of this application provides a method for preparing a positive electrode sheet, the method comprising sequentially adding a binder, a conductive agent, and a positive active material prepared by the method of the fourth aspect, dry mixing them, adding a solvent, stirring to obtain a slurry; transferring the slurry to at least one side of a current collector, drying and hot pressing to obtain a positive electrode sheet.

[0070] In any embodiment, the revolution speed of the dry mix is ​​20 rpm-30 rpm, and the rotation speed of the dry mix is ​​750 rpm-850 rpm.

[0071] In any embodiment, hot pressing includes at least three hot roller pressings, with the hot roller pressure increasing sequentially to 20-50 tons, 50-70 tons, and 70-90 tons; the hot roller temperature is 40℃-80℃, and the positive electrode sheet is heated to 40℃-50℃ before the first hot roller compaction.

[0072] The positive electrode active material prepared by the above-mentioned hot pressing process in combination with the preparation method in the fourth aspect is beneficial to further reduce the cross-sectional porosity of the positive electrode film, increase the ultimate compaction density of the electrode sheet, and improve the energy density of the battery. Attached Figure Description

[0073] Figure 1 This is a scanning electron microscope image of a cross-section of the positive electrode film layer along the electrode thickness direction according to an embodiment of this application; Figure 2 This is a schematic diagram of a lithium-ion secondary battery according to one embodiment of this application; Figure 3This is an exploded view of a lithium-ion secondary battery according to one embodiment of this application; Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application; Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application; Figure 6 yes Figure 5 An exploded view of the battery pack shown. Figure 7 This is a schematic diagram of an electrical device using a lithium-ion secondary battery as a power source according to an embodiment of this application. Figure 8 This is a porosity test diagram of a cross-section along the thickness direction of the positive electrode film layer according to an embodiment of this application.

[0074] Explanation of reference numerals in the attached figures: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Lithium-ion secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0075] The following detailed description, with appropriate reference to the accompanying drawings, discloses embodiments of the lithium-ion secondary battery, battery device, power-consuming device, method for preparing positive electrode active material, and method for preparing positive electrode sheet of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0076] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; 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, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0077] Unless otherwise specified, all embodiments and optional embodiments of this application may 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] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions, and such technical solutions shall be deemed to be included in the disclosure of this application.

[0079] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), indicating that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0080] In this application, the terms "multiple" or "various" refer to two or more kinds of things.

[0081] Unless otherwise stated, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0082] Unless otherwise stated, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in the embodiments of this application. Unless otherwise stated, the test temperature for each parameter is 25°C.

[0083] The battery mentioned in the embodiments of this application can be a single physical module comprising one or more lithium-ion secondary batteries to provide higher voltage and capacity. For example, the battery mentioned in this application may include lithium-ion secondary batteries, battery modules, or battery packs.

[0084] A lithium-ion secondary battery is the smallest unit that makes up a battery, and it can independently perform the functions of charging and discharging. Lithium-ion secondary batteries can be cylindrical, cuboid, or other shapes, and the embodiments of this application are not limited in this respect. Figure 2 This is an example of a cuboid-structured lithium-ion secondary battery.

[0085] Lithium-ion secondary batteries consist of electrode components and electrolyte.

[0086] Lithium-ion secondary batteries may also include an outer packaging that encapsulates the electrode components and electrolyte. The outer packaging can be a rigid shell, such as a hard plastic shell, aluminum shell, or steel shell. It can also be a flexible package, such as a pouch. The material of the flexible package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0087] In some implementations, such as Figure 3 As shown, the outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates enclosing a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 is used to cover the opening to close the receiving cavity. Electrode assemblies 52 are encapsulated in the receiving cavity. The number of electrode assemblies 52 contained in the lithium-ion secondary battery 5 may be one or more, and can be adjusted according to requirements.

[0088] Electrode assemblies typically include positive and negative electrodes. The negative electrode is the electrode that absorbs or lithiates lithium ions during charging and releases or delithiates lithium during discharging. The positive electrode is the electrode that absorbs or delithiates lithium ions during charging and absorbs or lithiates lithium during discharging.

[0089] When multiple lithium-ion secondary batteries are present, they are connected in series, parallel, or mixed via a busbar. In some embodiments, the battery can be a battery module; when multiple lithium-ion secondary batteries are present, they are arranged and fixed to form a battery module. In some embodiments, the battery can be a battery pack, which includes a housing and lithium-ion secondary batteries, with the lithium-ion secondary batteries or battery modules housed within the housing. In some embodiments, the housing can be part of the vehicle's chassis structure. For example, a portion of the housing can be at least part of the vehicle's floor, or a portion of the housing can be at least part of the vehicle's crossbeams and longitudinal beams.

[0090] In some implementations, the battery can be an energy storage device. Energy storage devices include energy storage containers, energy storage cabinets, etc.

[0091] In some implementations, lithium-ion secondary batteries can be assembled into battery modules, and the number of lithium-ion secondary batteries contained in a battery module can be multiple, the specific number of which can be adjusted according to the application and capacity of the battery module. Figure 4 This is a schematic diagram of battery module 4 as an example. Figure 4 As shown, in battery module 4, multiple lithium-ion secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple lithium-ion secondary batteries 5 can be fixed in place using fasteners.

[0092] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of lithium-ion secondary batteries 5 are received.

[0093] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0094] Figure 5 and Figure 6 This is a schematic diagram of battery pack 1 as an example. Figure 5 and Figure 6 As shown, the battery pack 1 may include a housing and multiple battery modules 4 disposed within the housing. The housing includes an upper housing 2 and a lower housing 3. The upper housing 2 covers the lower housing 3, forming a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the housing.

[0095] 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 cycle performance. However, they suffer from low electronic conductivity and low stacking efficiency, which makes it difficult to further increase the loading of lithium-containing transition metal phosphates per unit volume of battery, thus failing to meet the needs of high energy density batteries.

[0096] To further improve battery energy density and increase electrode compaction density, the industry commonly uses particle size distribution improvement. Improving particle size distribution often requires increasing the particle size or proportion of large and small particles. However, increasing large particles reduces battery kinetic performance, while increasing small particles significantly increases processing costs and exacerbates side reactions, worsening cycle life. How to further improve electrode compaction to achieve high-energy-density battery fabrication without significantly sacrificing other battery performance characteristics and meeting design and processing requirements is a pressing technical problem in this field.

[0097] The first aspect of this application provides a lithium-ion secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive current collector and a positive electrode film disposed on at least one side of the current collector. The positive electrode film includes a positive active material, comprising lithium transition metal phosphate particles with at least a portion of their surface coated with carbon. In the cumulative distribution curve of the C-value obtained under laser confocal Raman spectroscopy scanning mode, the median C0 of the graphitization degree of the positive electrode film is... 50 The concentration of C values ​​is greater than or equal to 0.95 and less than or equal to 1.20. 90 -C 10 ) / C 50 The value is 0.01-0.04; where the degree of graphitization C is I. G / I D I G This indicates that the Raman spectrum is at 1580±100 cm⁻¹ -1 The intensity of peak G at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

[0098] In the cumulative distribution curve of graphitization degree C obtained by laser microscopy confocal Raman spectroscopy in instrument scanning mode, the median C of graphitization degree is... 50A value less than 0.95 indicates poor slipability between particles in the positive electrode film. Under pressure, it is difficult for particles to slip and form a dense packing, leading to stress concentration and making it difficult to achieve high compaction density in the electrode. Furthermore, research shows that increasing the graphitization degree of the positive electrode film often requires increasing the sintering temperature of the positive electrode active material. While upgrading process conditions is beneficial for improving the graphitization degree of the positive electrode film, limitations such as the uniformity of the temperature field can easily affect the uniformity of particle graphitization. Studies have shown that the concentration of C-values ​​(C...) 90 -C 10 ) / C 50 When the value is greater than 0.04, the lithium insertion rate will be inconsistent due to poor graphitization of the positive electrode film, which will lead to local polarization and is not conducive to improving the battery dynamic performance.

[0099] This application study shows that in the cumulative distribution curve of graphitization degree C value obtained by laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of graphitization degree is... 50 The concentration of C value is greater than or equal to 0.95 and less than or equal to 1.20, while controlling the concentration of C value (C 90 -C 10 ) / C 50 A value of 0.01-0.04 indicates that the particles in the positive electrode film have a high degree of graphitization and a high degree of graphitization uniformity. This means that the positive electrode active material has good coating uniformity and consistency, which can reduce the particle slippage obstruction caused by the low degree of graphitization in the positive electrode active material and the resulting local stress concentration. Thus, the uniform and consistent slippage between the positive electrode active material particles allows the electrode to achieve a high overall compaction density under relatively low rolling pressure. At the same time, the uniform and consistent degree of graphitization is conducive to the uniform and consistent insertion and extraction of lithium ions. While maintaining good dynamic performance of the battery, it improves the compaction density of the electrode and the energy density of the battery.

[0100] 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 can easily achieve uniform particle sliding by means of the carbon coating material of the particles during the roll forming process. While maintaining other performance levels of the battery, the compaction density of the electrode sheet is further improved, thereby improving the overall performance of the battery.

[0101] Those skilled in the art can adjust the graphitization concentration of the cathode film in any known manner, such as by selecting a carbon source, adjusting the carbon coating amount, changing the particle size distribution, controlling the sintering temperature, and adjusting the sintering heating rate.

[0102] In this application, the term "particle" refers to a particle in the positive electrode film layer that has a recognizable complete boundary in the field of view at a certain magnification, such as 10,000x. Defects and scratches may exist inside the particle, but a complete boundary sufficient to divide the particle cannot be identified inside the particle.

[0103] The particle identification method is as follows: The positive electrode film layer is cut along the thickness direction of the electrode sheet using an argon ion beam (for example, a Leica EM TIC 3X CP device can be used, operating voltage: 6kV, operating time: 6h). After exposing the cut surface, a scanning electron microscope (SEM) is used (for example, a Hitachi SU8230 device can be used, operating voltage: 3kV, beam current: high, probe model: U (LA100), working distance <5mm) to observe the cut surface along the thickness direction of the positive electrode film layer. Images are acquired in secondary electron mode at a non-edge position in the cut surface of the positive electrode film layer using a field emission scanning electron microscope (after observing the electrode edge under the SEM, the field of view is adjusted to the center of the sample). Electron micrographs are taken at 10kx magnification, and the particles in the electron micrographs are analyzed using ImageJ software (1.46r, win64 version). The specific usage of ImageJ software is as follows: Load the SEM image to be analyzed, such as... Figure 1As shown, the Cellpose plugin software was used to identify particles, and manual corrections were performed based on this. ImageJ was used to read and analyze the data. The specific method for identifying particles using the Cellpose plugin software is as follows: Set the segmentation diameter parameter (diameter in the Segmentation module) to 15 pixels, click "run cyto3" to identify particles, and manually mark the particles in the image that were not identified by the software, were not fully identified by the software, or had identification errors. The particles that were not identified by the software, were not fully identified by the software, or had identification errors mainly include the following types: 1. Particles that are too large or have scratches on their surface, making them unidentifiable or incompletely identifiable; 2. During argon ion beam cutting, scratches may be generated on the particle surface, and the software may misjudge the scratches as particle boundaries during the identification process, resulting in identification errors; 3. Particles that are too small and were not successfully identified; 4. Particles located at the edge of the electron microscope field of view, with the interior of the particle penetrated by the edge, and the morphology not fully displayed, resulting in identification errors due to partial identification replacing the whole. For the unidentified or misidentified particles mentioned above, manual calibration is performed. The specific process is as follows: 1. Delete large particles located around the edges of the scanning electron microscope that are not fully displayed. 2. Determine if any unidentified or misidentified particles have internal cracks or scratches. If no cracks or scratches are found, classify it as a single particle and manually mark it based on the observed particle boundary. 3. If cracks or scratches are found inside the particle, determine if they penetrate the particle. If not, classify it as a single particle and manually mark it. 4. If cracks or scratches penetrate the particle, determine if they are linear or irregular. 5. If the cracks or scratches are irregular, classify them as the boundary between particles and divide the particles along this boundary. 6. If the cracks or scratches are linear, perform contrast comparison. 7. If the contrast is not obvious and there is no crack-like appearance, classify it as a scratch and mark it as a single particle. 8. If the contrast is strong and there is a crack-like appearance, classify it as the boundary between particles and mark it as two particles. After manual marking, delete information irrelevant to the particles from the automatic image processing, thus completing the particle identification and marking in the image.

[0104] Lithium-containing transition metal phosphates refer to phosphate materials containing lithium and transition metal elements, and can be detected by any method known in the art. For example, they can be detected by combining X-ray diffraction (XRD) with energy dispersive spectroscopy (EDS) or inductively coupled plasma mass spectrometry (ICP-MS). As examples, lithium-containing transition metal phosphates include, but are not limited to, lithium iron phosphate, lithium manganese iron phosphate, and their doped materials.

[0105] Carbon coatings deposited on at least a portion of the surface of lithium-containing transition metal phosphates can be detected using any method known in the art. As an example, carbon coatings deposited on at least a portion of the surface of lithium-containing transition metal phosphates can be observed by characterizing the phosphates using transmission electron microscopy coupled with energy dispersive spectroscopy. It should be noted that the elements in the carbon coating are not limited to carbon, but may also include other non-carbon elements. Carbon coatings are not limited to film-like forms, but also include island-like, irregular, or discontinuous coatings.

[0106] Carbon has excellent electrical conductivity, which is beneficial for electron transport. The carbon coating layer can significantly improve the electronic conductivity of lithium transition metal phosphate materials.

[0107] In some embodiments, the median C of the graphitization degree C in the cumulative distribution curve of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The concentration of C values ​​is greater than or equal to 0.95 and less than or equal to 1.20. 90 -C 10 ) / C 50 The value is 0.01-0.04; where the degree of graphitization C is I. G / I D , where I G This indicates that the Raman spectrum is at 1580±100 cm⁻¹ -1 The intensity of peak G at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

[0108] In this application, the graphitization degree C value of the positive electrode film can be obtained by surface scanning mode of a laser confocal Raman spectrometer. As an example, specifically, a laser confocal Raman spectrometer (high-precision Renishaw laser confocal Raman spectrometer) is used, with an excitation wavelength of 532 nm. An appropriate amount of the positive electrode film is taken and surface scanned on its surface or along the thickness direction of the electrode. The scanning area is 45 μm × 45 μm, divided into 10 × 10 grids, with grid vertices as test points, a step size of 5 μm, and a total of 100 scan points. Thus, the C values ​​at different sites and the cumulative distribution curve of the C values ​​in the surface scan area are obtained.

[0109] The positive electrode film in this application can be either a freshly prepared positive electrode film or a positive electrode film obtained from disassembly of a battery. The surface of a positive electrode film obtained from disassembly of a battery inevitably contains residual electrolyte salt particles. To improve testing accuracy, it is preferable to perform a surface scan on a cross-section of the positive electrode film along the electrode thickness direction to characterize the degree of graphitization of the positive electrode film.

[0110] The graphitization degree C of the positive electrode film was obtained by the ratio of the peak intensities of the G-band and D-band peaks in the Raman spectrum. The position of the G-band peak was 1580±100 cm. -1 Its characterization of carbon sp 2 Hybrid structure; D peak position is 1350±100 cm⁻¹ -1 It characterizes the disordered structure of carbon, where disorder refers to the irregular arrangement of carbon atoms within the structure. In graphite crystals, carbon atoms in the same layer are arranged in an sp... 2 Hybridization forms covalent bonds, while interlayer bonding is facilitated by van der Waals forces, making the carbon in the graphite structure prone to slippage. Therefore, the C value can characterize the degree of graphitization of the cathode film. A higher C value indicates a higher degree of graphitization of the carbon material. It is understandable that the degree of graphitization in the cathode film mainly originates from the graphitized carbon material within the cathode film, i.e., the carbon coating layer of the cathode active material. Although rich in sp... 2 Hybridized carbon nanotube conductive agents also have relatively high I0 G / I D However, due to its low content and small tube diameter, its addition to the positive electrode film results in an extreme value in the Raman surface scan test of the positive electrode film, and does not affect the graphitization degree C in the positive electrode film. 50 It has an impact.

[0111] Therefore, the degree of graphitization of the cathode film can also be used to characterize the degree of graphitization of the cathode active material. The higher the degree of graphitization of carbon on the surface of the cathode active material, the higher the proportion of graphitic carbon in the cathode film, and the easier it is for particles to slip during the rolling process by means of the highly graphitized carbon structure in the coating layer, thus achieving an increase in electrode compaction density under low rolling pressure.

[0112] The cumulative distribution curve of graphitization degree C value refers to the curve obtained by arranging at least 100 C values ​​in ascending order, with graphitization degree as the horizontal axis and the cumulative percentage as the vertical axis. 50 This represents the C value corresponding to a cumulative percentage of 50% on the vertical axis of the cumulative distribution curve for graphitization degree C. The median C value for graphitization degree. 50 Compared to point values, it can reflect the overall graphitization degree of particles in the positive electrode film, i.e., the degree of slippage; compared to the mean value, it can reduce the influence of extreme values ​​during the test and improve the confidence of the test results.

[0113] Those skilled in the art can control the degree of graphitization of active material particles using 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 achieve the adjustment of the degree of graphitization of active material particles.

[0114] In some embodiments, the median C of the graphitization degree C in the cumulative distribution curve of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The value can be selected from 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 any value range between the two.

[0115] Referring to the above, and by analogy, C 90 C represents the C value corresponding to a cumulative percentage of 90% on the vertical axis of the cumulative distribution curve of graphitization degree C. 10 This represents the C-value on the vertical axis of the cumulative distribution curve of graphitization degree C, corresponding to a cumulative percentage of 10%. The concentration of C-values ​​is expressed as (C... 90 -C 10 ) / C 50 Indicates. (C) 90 -C 10 ) / C 50 It can reflect the magnitude of most C values, is unaffected by extreme values, and also reflects the width of the graphitization distribution of particles in the cathode film. A low concentration of C values ​​in the cathode film indicates a narrow width and good concentration of the graphitization distribution of particles in the cathode film.

[0116] In some embodiments, the concentration of C values ​​(C0) in the cumulative distribution curve of graphitization degree C values ​​obtained by laser microscopy confocal Raman spectroscopy in instrument scanning mode is... 90 -C 10 ) / C 50 The value can be selected as 0.01, 0.02, 0.03, 0.04 or any range between two of them.

[0117] In some embodiments, the median C of the graphitization degree C in the cumulative distribution curve of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode is... 50 The range is 0.96-1.15, and can be further selected as 0.98-1.13.

[0118] The median C of the graphitization degree of the positive electrode film 50 Within the aforementioned range, it is beneficial to further improve the slippage between particles, thereby increasing the compaction density of the electrode while maintaining the high kinetic performance of the battery, thus achieving a balance between battery kinetic performance and energy density.

[0119] In some embodiments, the concentration of C values ​​(C0.05) in the cumulative distribution curve of graphitization degree C values ​​obtained by laser microscopy confocal Raman spectroscopy in instrument scanning mode is... 90 -C 10 ) / C 50 The range is 0.02-0.038, and can be set to 0.02-0.036.

[0120] Concentration of C-values ​​(C 90 -C 10 ) / C 50 Within the aforementioned range, it is beneficial to further improve the uniformity of graphitization degree of particles in the positive electrode film, improve the uniformity of slip between particles, reduce the inconsistent lithium intercalation rate caused by poor graphitization degree in the positive electrode film, and thus prevent local polarization. The battery can further improve its dynamic performance while maintaining good energy density.

[0121] In some embodiments, the cumulative distribution curve of graphitization degree C value obtained by laser microscopy confocal Raman spectroscopy instrument scanning mode of the positive electrode film layer, the C value of graphitization degree 90 The value is 1.0-1.3, and can be selected as 1.02-1.15.

[0122] In some embodiments, the cumulative distribution curve of graphitization degree C value obtained by laser microscopy confocal Raman spectroscopy instrument scanning mode of the positive electrode film layer, the C value of graphitization degree 90 The value can be selected from 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 any value range between the two.

[0123] C of graphitization degree 90 Within the above range, the median C of the degree of graphitization 50 The relatively close distribution indicates that the graphitization degree of the particles in the positive electrode film is within a narrow range, which is conducive to uniform sliding between particles and thus improves the compaction density of the positive electrode sheet.

[0124] In some embodiments, the cumulative distribution curve of graphitization degree C value obtained by laser microscopy confocal Raman spectroscopy instrument scanning mode of the positive electrode film layer, the C value of graphitization degree 10 The value is 0.92-1.1, and can be selected as 0.96-1.08.

[0125] In some embodiments, the cumulative distribution curve of graphitization degree C value obtained by laser microscopy confocal Raman spectroscopy instrument scanning mode of the positive electrode film layer, the C value of graphitization degree 10 The value can be selected from 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 any value range between the two.

[0126] C of graphitization degree 10 Within the above range, it is indicated that different sites in the positive electrode film have a high degree of graphitization, which is beneficial to the uniform slippage of particles, reduces the probability of local stress concentration, and further improves the compaction density of the electrode.

[0127] In some embodiments, the area of ​​the positive electrode film layer in a cross-section along the electrode thickness direction is 0.001 μm. 2 -0.06μm 2 The particle area ratio is 21.00%-27.00%, and the area is 1.0μm. 2 -4.0μm 2 The area ratio of the particles is 12.00%-20.00%.

[0128] The specific method for statistically analyzing the particle area in the cross-section along the thickness direction of the positive electrode film is as follows. After particle identification and labeling, the image is imported into ImageJ software for analysis. The scale is set based on the scanning electron microscope image, and the particle diameter and area are analyzed using the "Feret diameter" and "Area" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Feret" parameter represents the maximum spacing between all parallel lines in the two-dimensional projection of the particle, thus characterizing the particle diameter; the "Area" parameter represents the pixel area of ​​the particle, thus characterizing its area. Since particles smaller than 50 nm have a large error in the statistical process and are difficult to accurately identify, and the particle size of conductive agents is generally smaller than 50 nm, which will introduce a large error into the statistical results, particles smaller than 50 nm are not counted in the particle size statistics of this application, and the particle statistics corresponding to Areas displaying "NaN" are deleted. Following the above method, to ensure a statistically significant sample size, at least 10 non-overlapping scanning electron microscope images were acquired for each electrode, and the area of ​​at least 5000 particles was calculated, with the area being 0.001 μm. 2 -0.06μm 2 The sum of the "Area" parameters of the particles and the sum of the "Area" parameters of all particles are respectively used as the area of ​​0.001μm.2 -0.06μm 2 The area of ​​the particles and the total area of ​​the particles were statistically analyzed. The area was set at 0.001 μm. 2 -0.06μm 2 The sum of the areas of the particles divided by the total statistical area of ​​the particles is used as the area of ​​the positive electrode film in the cross-section along the thickness direction of the electrode sheet, which is 0.001 μm. 2 -0.06μm 2 The area ratio of the particles is 1.0 μm. 2 -4.0μm 2 The area ratio of the particles can be obtained similarly.

[0129] The cross-sectional morphology of the positive electrode film along the thickness direction of the electrode sheet is shown in the figure below. Figure 1 As shown, the state of the positive electrode active material differs from that observed by Malvern laser scattering and also from that observed directly by scanning electron microscopy. Under roller pressure, the particles in the positive electrode film exhibit good dispersion, making observation of the positive electrode film beneficial for effectively characterizing the particle size, area, and quantity.

[0130] During the compaction process, the positive electrode film undergoes compaction along its thickness. Therefore, compared to the surface of the positive electrode film, a cross-section along the thickness direction better reflects the actual compaction status of the particles within the film on a spatial scale. The cross-section of the positive electrode film along the thickness direction has an area of ​​0.001 μm. 2 -0.06μm 2 The particle area ratio and area are 1.0 μm. 2 -4.0μm 2 The area ratio of particles can intuitively reflect the ratio of some particles to the total number of particles in that area, and reflect the amount of particles in that area.

[0131] It is understandable that the particles in the cross-section of the positive electrode film along the thickness direction of the electrode sheet, especially those larger than 50 nm, mainly originate from the positive electrode active material. Therefore, the embodiments of this application can accurately and objectively reflect the distribution of lithium-containing transition metal phosphate particles in the electrode sheet by observing and statistically analyzing the particle size and area in the cross-section of the positive electrode film.

[0132] Those skilled in the art can control the particle size using any known process. For example, the growth rate and time of the cathode material can be controlled by adjusting the temperature and time during the preparation process. The particle size can be adjusted by utilizing the mechanical force of crushing and grinding processes to process the raw materials to the target particle size distribution range; particle size separation can be achieved by using sieving and grading equipment to obtain the required particle size ratio; and precise control of the feed rate, adjusting the residence time and stress state of the particles within the equipment, also helps to control the particle size.

[0133] In some embodiments, the area of ​​the positive electrode film layer in a cross-section along the electrode thickness direction is 0.001 μm. 2 -0.06μm 2 The particle area percentage can be selected as 21%, 22%, 23%, 24%, 25%, 26%, 27% or any value between two of these, with an area of ​​1.0 μm. 2 -4.0μm 2 The area percentage of the particles can be selected as 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or any value between the two.

[0134] The surface area of ​​the positive electrode active material is controlled to be 0.001 μm. 2 -0.06μm 2 The particle area ratio and area are 1.0 μm. 2 -4.0μm 2 With the particle area ratio within the above range, combined with a uniform high degree of graphitization, uniform particle slippage can be achieved during the electrode compaction process. This improves the electrode compaction density while reducing the negative impact on other properties such as battery dynamics, cycle life, and processing performance, thus comprehensively improving battery performance.

[0135] In some embodiments, the particle size D of the particles in a cross-section of the positive electrode film along the electrode thickness direction is... A50 The range is 600nm-800nm, with options for 650nm-750nm, where D A50 It refers to the particle size at which the cumulative area distribution of particles reaches 50% in the cumulative area distribution curve of particles.

[0136] In a cross-section of the positive electrode film along the thickness direction of the electrode sheet, the particle size D is... A50 The specific testing method is as follows. Referring to the method described above, the particle size of no less than 5000 particles is counted. The particle sizes of the obtained at least 5000 particles are arranged in ascending order. Plotting the particle size on the horizontal axis and the cumulative area distribution of the particles on the vertical axis, the particle size corresponding to the cumulative area distribution curve of the particles is obtained when the cumulative area ratio of the vertical axis is 50%.

[0137] In some embodiments, the particle size D is obtained from the cumulative distribution curve of the positive electrode active material particle size area obtained by slicing the positive electrode film along the thickness direction of the electrode sheet. A50 The range can be 600nm, 610nm, 620nm, 630nm, 640nm, 650nm, 660nm, 670nm, 680nm, 690nm, 700nm, 710nm, 720nm, 730nm, 740nm, 750nm, 760nm, 770nm, 780nm, 790nm, 800nm, or any value range between the two.

[0138] Particle size D A50 Within the above range, it indicates that the positive electrode film contains a certain number of large-sized particles. The median particle size D is controlled. A50 Within the aforementioned range, the large contact area between large-sized particles can improve the efficiency of roller pressure transmission between electrode particles, fully utilize the skeletal support of large-sized particles, enable the electrode to withstand higher roller pressure, and increase the compaction density of the electrode. At the same time, it can reduce the kinetic decline caused by excessively large particle size, thus maintaining the kinetic performance of the battery while increasing the compaction density of the electrode.

[0139] In some embodiments, the median roughness R in the cumulative distribution curve of particle roughness area obtained from a cross-section of the positive electrode film along the electrode thickness direction is... A50 It is 0.92-0.96.

[0140] The specific method for testing the roughness of particles in the cross-section of the positive electrode film along the thickness direction of the electrode sheet is as follows: Particles in the cross-section of the positive electrode film are identified using the method described above in this application. The images after particle identification and labeling are imported into ImageJ software for analysis. The scale is set based on the scanning electron microscope image. The particle diameter, area, and roughness in the image are analyzed using the "Feret Diameter," "Area," and "Solidity" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Solidity" parameter obtained from the analysis represents the ratio of the pixel area to the convex area of ​​the particle. Therefore, the "Solidity" parameter of the analyzed particles is used to characterize the roughness of the particles. By definition, the closer the roughness is to 1, the smoother the particles. Because particles smaller than 50 nm present significant errors during statistical analysis and are difficult to accurately identify, and because conductive agents typically have particle sizes smaller than 50 nm, which can also introduce substantial errors into the statistical results, particles smaller than 50 nm are not counted in the particle size statistics of this application, and the statistical data for particles with a Solidity value of "NaN" are deleted. Following the above method, to ensure a statistically significant sample size, at least 10 non-overlapping scanning electron microscope (SEM) images were acquired for each electrode. The roughness of at least 5000 particles was arranged in ascending order, and the cumulative roughness distribution curve of the particles in the positive electrode film was obtained with roughness as the horizontal axis and cumulative area percentage as the vertical axis. R A50 This is the roughness R value when the cumulative area ratio of the vertical axis in the cumulative distribution curve of roughness R value is 50%.

[0141] In some embodiments, the median roughness R in the cumulative distribution curve of particle roughness area obtained from a cross-section of the positive electrode film along the electrode thickness direction is... A50 The value can be selected as 0.92, 0.93, 0.94, 0.95, 0.96 or any range between two of them.

[0142] Those skilled in the art can control the roughness of particles using any known process. For example, particle roughness can be adjusted through processes such as grinding, polishing, abrasion, milligram energy processing, electroplating, and calendering, as well as by adjusting the parameters of each process.

[0143] Median roughness R A50 Within the aforementioned range, the particle surface is relatively smooth, and the friction between particles is relatively small. Under the action of external force, it is easy to slip. When combined with particles with high graphitization degree, the electrode compaction density can be improved under low rolling pressure, thereby further improving the energy density of the battery.

[0144] In some embodiments, the cumulative distribution curve of the spheroidal area of ​​the particles obtained from the cross-section of the positive electrode film along the electrode thickness direction, where L represents the spheroidal area... A90 The range is 0.80-0.95, and can be set to 0.85-0.93.

[0145] The specific method for testing the sphericity of particles in the cross-section of the positive electrode film along the thickness direction is as follows: Particles in the cross-section of the positive electrode film are identified using the method described above in this application. The images after particle identification and labeling are imported into ImageJ software for analysis. A scale is set based on the scanning electron microscope image. The particle diameter, area, and sphericity of the particles in the image are analyzed using the "Feret Diameter," "Area," and "Round" analysis functions. According to the software manual (ImageJ User Guide IJ 1.46r), the "Round" parameter obtained from the analysis represents the ratio of the pixel area of ​​the particle to the area of ​​a circle with the fitted major axis as its diameter, and can be used to characterize the sphericity of the particle. The closer the particle is to a sphere, the closer the ratio of the pixel area to the area of ​​the circle with the fitted major axis as its diameter is to 1. Therefore, the "Round" parameter of the particle obtained from the analysis is used to characterize the sphericity of the particle. Because particles smaller than 50 nm are prone to significant errors during statistical analysis and are difficult to identify accurately, and because the particle size of conductive agents is generally smaller than 50 nm, which can also introduce large errors into the statistical results, particles smaller than 50 nm are not counted in the particle size statistics of this application, and the statistical data of particles with a Round value of "NaN" are deleted. Following the above method, to ensure a statistically significant sample size, at least 10 non-overlapping scanning electron microscope images were acquired for each electrode. The sphericity of at least 5000 particles was arranged in ascending order, and the cumulative sphericity distribution curve of the particles in the positive electrode film was obtained with sphericity as the horizontal axis and the cumulative area percentage as the vertical axis. A90 The L-value is the L-value corresponding to the cumulative area under the vertical axis of the cumulative distribution curve of sphericity L-values ​​being 90%.

[0146] In some embodiments, the cumulative distribution curve of the spheroidal area of ​​the particles obtained from the cross-section of the positive electrode film along the electrode thickness direction, where L represents the spheroidal area... A90 The value can be selected as 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 any range between the two.

[0147] Those skilled in the art can control the sphericity of particles using any known process. For example, the sphericity of particles can be adjusted through processes such as grinding, polishing, chemical etching, mechanical stirring, extrusion, coating, granulation, and adding surfactants, as well as by adjusting the parameters of each process.

[0148] In some embodiments, the median L of the spheroidal area distribution curve of the particles obtained from the cross-section of the positive electrode film along the electrode thickness direction is... A50 The value is 0.65-0.85, and can be selected as 0.70-0.80.

[0149] L A50 This is the L-value of sphericity when the cumulative area under the vertical axis of the cumulative distribution curve of L-values ​​accounts for 50%.

[0150] In some embodiments, the cumulative distribution curve of the spheroidal area of ​​the particles obtained from the cross-section of the positive electrode film along the electrode thickness direction, where L represents the spheroidal area... A50 The value can be selected from 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 value range between the two.

[0151] Median L of sphericity A90 L A50 Within the aforementioned range, the particles are approximately spherical, and under external force, they are prone to slippage between each other. When combined with particles of high graphitization, the electrode compaction density can be increased under low rolling pressure, thereby further improving the energy density of the battery.

[0152] In some embodiments, the positive electrode active material includes iron, and the iron dissolution rate of the positive electrode film is 500ppm-2000ppm, optionally 500ppm-1500ppm.

[0153] The iron dissolution rate of the positive electrode film can be tested in the following ways. Specifically, after disassembling and cleaning the electrode from the battery, it is formed into small discs with a diameter of 14 mm. Multiple small disc samples are taken to make the total sample mass about 5 g. These are added to 100.3 g of a 0.3% ascorbic acid solution (using ultrapure water as the solvent). The solution is stirred at 500 rpm for 5 minutes, and then the solution is quickly aspirated using a 5 mL syringe. The solution is filtered into a test tube using a 0.45 μm pore size filter. 1 mL of the supernatant is pipetted into a glass volumetric flask and diluted 50 times. The iron concentration in the solution is measured using an inductively coupled plasma mass spectrometer (ICP-OES). The iron dissolution rate of the positive electrode film is calculated using the formula: [(ICP test iron concentration × solution volume / mass of the solution used for volume adjustment) × 100.3 g / (mass of small disc electrode - mass of small disc current collector)]. The solution volume is 50 mL, and the mass of the solution used for volume adjustment is 1 g. Preferably, the mass of the current collector in the small disc is obtained by multiplying the disc's thickness by its area and density. The disc's thickness can be equivalently measured by using a thickness gauge to measure the current collector thickness in the uncoated area. It is understood that although the current collector in the coated area will expand during compaction, resulting in a slight decrease in thickness compared to the uncoated area, this decrease is negligible and will not significantly affect the test results. More preferably, when the current collector is aluminum foil, the density is 2.7 g / cm³. 3 .

[0154] In some embodiments, the positive electrode active material includes iron, and the iron dissolution rate of the positive electrode film can be selected from 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1600ppm, 1700ppm, 1800ppm, 1900ppm, 2000ppm or any value range between the two.

[0155] Those skilled in the art can control the iron dissolution rate of the positive electrode film using any known process. For example, the iron dissolution rate of the positive electrode film can be controlled by adjusting the surface coating quality, temperature, time, and pressure during the preparation process.

[0156] Iron dissolution rate can indirectly reflect the integrity and density of the carbon coating on the surface of the positive electrode active material. A lower iron dissolution rate means that iron ions dissolved by acid are less likely to precipitate from the carbon coating layer, indicating a more complete and dense carbon coating layer on the surface of the positive electrode active material. Positive electrode active materials with iron dissolution rates within the above range have relatively complete and dense carbon coating layers, which can improve the electrical contact between positive electrode active materials, improve the conductivity of the positive electrode active material, reduce the polarization of the positive electrode active material, and further optimize the kinetic performance of lithium-ion secondary batteries. At the same time, the densely coated carbon layer has a low space occupancy rate, and the interparticle gaps are easily compressed by stress during the rolling process. Combined with a carbon coating layer with a high degree of graphitization, it is more conducive to improving the compaction density of the electrode sheet and the energy density of the battery.

[0157] In some embodiments, the carbon content is 0.8%-1.8% by mass, and optionally 0.90%-1.5%, based on the total mass of the positive electrode active material.

[0158] Based on the total mass of the positive electrode active material, the mass content of carbon can be measured using methods and equipment known in the art. For example, it can be determined using a Dekai HCS infrared carbon and sulfur analyzer, referring to GB / T 21023-2006 "Determination of Total Carbon and Sulfur Content in Iron and Steel - Infrared Absorption Method After Combustion in a High-Frequency Induction Furnace".

[0159] In some embodiments, based on the total mass of the positive electrode active material, the mass content of carbon can be selected as 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 the two.

[0160] Compared with existing lithium transition metal phosphate cathode active materials, this cathode active material has a relatively low carbon coating content, which can further increase the loading of lithium transition metal phosphate in the cathode sheet and improve the energy density of lithium-ion secondary batteries.

[0161] In some embodiments, the positive electrode active material includes iron, and the lithium iron antisite defect concentration of the positive electrode active material is 0.1%-1.5%, optionally 0.3%-1.0%.

[0162] XRD data of the samples were collected using an X-ray diffractometer, and phase analysis was performed. The CIF file of the phase obtained from an open-source website was used as the initial crystal structure model, including the definition of unit cell parameters, atomic positions, and occupancy probabilities. In the initial crystal structure model, considering the possibility of Fe-Li antisites, the possible Li content at Fe sites and the possible Fe content at Li sites were set to an initial value of 0.1%. The collected XRD data were fitted and refined using FullProf Suite software, refining the parameters 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 optimally matched, and Rwp was less than 10, the refined Li and Fe occupancy probabilities were obtained, which were used as the concentration of lithium-iron antisite defects.

[0163] In some embodiments, the lithium iron ion site defect concentration of the positive electrode active material can be selected as 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 any value range between the two.

[0164] Those skilled in the art can control the antisite defects of lithium iron phosphate in cathode active materials using any known process. For example, the antisite defects of lithium iron phosphate in cathode active materials can be controlled by adjusting the sintering temperature, sintering time, preparation method, and raw material stoichiometry.

[0165] During preparation and cycling, lithium vacancies inevitably exist within the crystal structure of the positive electrode active material. These vacancies not only lead to the oxidation of ferrous ions to ferric ions but also induce partial migration of ferric ions to lithium sites, forming lithium-iron antisite defects. This blocks the one-dimensional diffusion channels of lithium ions, adversely affecting the solid-phase transport of lithium ions. The positive electrode active material in this application has low lithium-iron antisite defects, which is beneficial for the uniform transport of lithium ions in the solid phase and further improves the kinetic performance of the lithium-ion secondary battery.

[0166] In some embodiments, the lithium-containing transition metal phosphate comprises a component 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, and Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 0<q≤0.1.

[0167] In some implementations, m can be selected as 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 any value range between two of these; x can be selected as 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1.0, or any value range between two of these. y can be 0.95, 0.96, 0.97, 0.98, 0.99, 1.00 or any value between two of these; j can be 3.5, 3.6, 3.7, 3.8, 3.9, 4 or any value between two of these; q can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or any value between two of these.

[0168] Selecting an appropriate modifying element Q can improve the ion diffusion pathway 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.

[0169] In some embodiments, the positive electrode active material includes one or more of lithium iron phosphate and its doped and modified materials, and coated and modified materials.

[0170] In some embodiments, the positive electrode active material includes titanium, and the mass content of titanium is 2000ppm-6000ppm based on the total mass of the positive electrode active material.

[0171] The types and contents of elements in positive electrode active materials can be tested using any method known in the art. As an example, inductively coupled plasma atomic emission spectrometry (ICP-AES) is used to test the titanium content, referring to Appendix C of GB / T 33822-2017.

[0172] In some embodiments, the mass content of titanium element, based on the total mass of the positive electrode active material, can be selected as 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm or any value range between the two.

[0173] Doping titanium in cathode active materials can induce lattice distortion, reduce Li-O bond energy, increase lithium-ion transport rate, and improve the kinetic performance of lithium-ion secondary batteries. However, in existing technologies, the doping content of titanium in lithium-containing transition metal phosphates often cannot exceed 3000 ppm, because excessive titanium is difficult to completely enter the bulk phase of lithium-containing transition metal phosphates and easily becomes a harmful impurity phase remaining on the surface, negatively impacting battery performance.

[0174] The positive electrode active material in this application embodiment has a high titanium content. Surprisingly, the high amount of titanium added did not form a harmful impurity phase that negatively affects the battery's energy density and kinetic performance. Although the reason is not yet clear, it is speculated that titanium, lithium, phosphate, and other elements (e.g., lithium) together form a fast ion conductor, which actually improves the battery's kinetic performance.

[0175] In some embodiments, the tap density of the positive electrode active material is 0.70 g / cm³. 3 -1.50g / cm 3 0.7g / cm³ is an option. 3 -1.20g / cm 3 .

[0176] The tap density of powder can be obtained by any method known in the art.

[0177] As an example, turn on the electronic balance, first place the conical flask as a base on the electronic balance, then zero the electronic balance; place the graduated cylinder on the conical flask, weigh it, and record the weight of the graduated cylinder; open the sample bag, use a clean sample spoon to stir the sample in the sample bag 3-5 times to mix it well, then smoothly transfer the sample into the graduated cylinder; wipe the powder adhering to the surface of the graduated cylinder with lint-free paper, then place it into the zeroed conical flask and weigh it; seal the mouth of the graduated cylinder with sealing film, and place the graduated cylinder into the matching instrument rubber ring to maintain its position. Ensure the graduated cylinder fits snugly against the rubber ring and remains perpendicular to the instrument surface. Set the vibration frequency to 250 times / min and the number of vibrations to 5000. Press the button and vibrate for 20 minutes. Then remove the TD tube, illuminate the surface of the graduated cylinder with a flashlight, and visually read the highest scale V1 and the lowest scale V2. Take the average of the two, V. Subtract the mass of the graduated cylinder m0 from the mass of the sample m1 to obtain the powder mass m. Use the density formula ρ=m / v to obtain the tapped density of the sample.

[0178] In some embodiments, the tap density of the positive electrode active material powder can be selected as 0.70 g / cm³. 3 0.80g / cm 3 0.90g / cm 3 1.00g / cm 3 1.10 g / cm 3 1.20g / cm 3 1.30g / cm 3 1.40 g / cm 3 1.50g / cm 3 Or the range of values ​​between any two.

[0179] The positive electrode active material in this embodiment has a relatively low powder tap density. Due to the high graphitization degree and good graphitization consistency of the positive electrode film, it is easy to slip under external force to improve the powder tap density.

[0180] In some embodiments, the compacted density of the positive electrode active material at a pressure of 3T is 2.50 g / cm³. 3 -2.70g / cm 3 The option is 2.52 g / cm³. 3 -2.68g / cm 3 .

[0181] In this application, the term "powder compaction density" refers to the density of a compacted compact with a certain density and strength, formed during the external force compression process. This density is measured in g / cm³, as the powder moves and deforms, larger voids are filled, the contact area between particles increases, resulting in attractive forces between atoms and enhanced mechanical cohesion between particles. 3 .

[0182] The compacted density of the positive electrode active material powder can be measured using methods and equipment known in the art. For example, it can be measured using a compaction density instrument, referring to GB / T24533-2009. Specifically, a certain amount of positive electrode active material is placed on a compaction mold (the mold diameter is known). The mold is hollow in the middle and has a metal disc at the top and bottom. The positive electrode active material is placed between the metal discs, and a metal cylinder is placed on top. The mold is then placed on a compaction density instrument. The bottom area of ​​the mold is 1.327 cm². 2 The pressure is set to 3T. The thickness of the positive electrode active material under 3T pressure can be read on the equipment. The compaction density of the positive electrode active material powder 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.

[0183] In some embodiments, the compaction density of the positive electrode active material under 3T pressure can be selected as 2.52 g / cm³. 3 2.53g / cm 3 2.54 g / cm 3 2.55g / cm 3 2.56 g / cm 3 2.57g / cm 3 2.58g / cm 3 2.59 g / cm 3 2.60g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm3 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 range of values ​​between any two.

[0184] Thanks to its high degree of graphitization and good consistency of graphitization, the positive electrode active material can still achieve high compaction density under external force, providing a material basis for improving electrode compaction density and preparing high-energy-density lithium-ion secondary batteries.

[0185] 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 selected as 2 Ω·cm-20 Ω·cm.

[0186] The powder resistivity of the positive electrode active material can be measured using methods and equipment known in the art. For example, it can be measured using a powder resistivity meter (Suzhou Jingge, ST2722 type) according to GB / T33822-2017. Specifically, a certain amount of positive electrode active material (e.g., 1g) is weighed and added to the feeding chamber of the powder resistivity meter. A pressure of 8MPa is applied, and the forward and reverse resistivity of the positive electrode active material are measured separately. The average value of the two is taken as the powder resistivity of the positive electrode active material.

[0187] In some embodiments, the powder resistivity of the positive electrode active material at a pressure of 29400N can be selected as 0.5Ω·cm, 1Ω·cm, 2Ω·cm, 3Ω·cm, 4Ω·cm, 5Ω·cm, 6Ω·cm, 7Ω·cm, 8Ω·cm, 9Ω·cm, 10Ω·cm, 15Ω·cm, 20Ω·cm, 25Ω·cm, 30Ω·cm, or any value range between the two.

[0188] This positive electrode active material has a high degree of graphitization; therefore, by utilizing the sp... 2 The structure facilitates rapid electron conduction between particles, resulting in low powder resistivity in the positive electrode active material. This is beneficial for improving the solid-phase electron transport rate and further enhancing the battery's kinetic performance.

[0189] In some embodiments, 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.

[0190] In this application, the positive electrode active material is assembled into a coin cell, and its electrical performance is tested using a blue electric current tester. At 25±5℃ and within a voltage range of 2.0V to 3.75V, it is charged at a constant current of 1C to 3.75V, paused for 5 minutes, charged at a constant voltage until the cutoff current reaches 50μA, and then discharged at a constant current of 1C to 2.0V. The discharge capacity of the coin cell is divided by the mass of the positive electrode active material to obtain the specific discharge capacity of the positive electrode active material at a 1C discharge rate at room temperature.

[0191] The preparation and testing process of the coin cell is as follows: 2.0g of positive electrode active material, conductive carbon black, and PVDF are mixed in a mass ratio of 0.9:0.05:0.05. Then, the organic solvent NMP (N-methylpyrrolidone) is added and thoroughly mixed. The mixture is then coated using a 150μm doctor blade, dried at 100℃ for 2 hours, and compacted to a density of 2.0g / cm³. 3 -2.2g / cm 3 Compact the positive electrode sheet, punch it into a 14mm diameter circle, weigh it and record the weight. Place the weighed positive electrode sheet in a vacuum drying oven (105℃, 1-12hrs, -90kPa). After drying, place the positive electrode sheet in a glove box. Assemble the battery in the following order: negative electrode shell - nickel mesh - lithium sheet - separator - positive electrode sheet - positive electrode shell. Add 65-87μL (pipette) of electrolyte (a 1:1 volume ratio of EC (ethylene carbonate) and DMC (1,2-dimethyl carbonate) mixed solvent, with LiPF6 as the electrolyte). Place the negative electrode on top and put it in the groove of the sealing machine. The sealing pressure is 650kg / cm. 2 The button was removed with insulated tweezers, placed in a cleanroom bag, removed from the glove box, and left to stand in a constant temperature room for 3 hours to obtain the button for testing.

[0192] It is understandable that the discharge capacity of the positive electrode active material can also be obtained by disassembling the battery, obtaining the positive electrode sheet, assembling it into a coin cell according to the method described above, and then testing it.

[0193] In some embodiments, the discharge specific capacity of the positive electrode active material at a 1C discharge rate at room temperature can be selected as 135mAh / g, 136mAh / g, 137mAh / g, 138mAh / g, 139mAh / g, 140mAh / g, 141mAh / g, 142mAh / g, 143mAh / g, 144mAh / g, 145mAh / g, 146mAh / g, 147mAh / g, 148mAh / g, 149mAh / g, 150mAh / g, or any value range between the two.

[0194] The positive electrode active material exhibits a high discharge specific capacity at a 1C rate, indicating that it has good charge and discharge capabilities, which is beneficial for improving the battery's dynamic performance.

[0195] In some embodiments, the discharge capacity η of the positive electrode active material discharged to 3.2V accounts for ≥85%, where η is defined as follows: at room temperature, a coin cell containing the positive electrode active material is charged and discharged twice at a constant current of 0.1C within a voltage range of 2.0V to 3.75V, followed by a constant current charge and discharge once at a constant current of 1C. In the 1C charge and discharge test, the capacity value extracted at a discharge voltage of 3.2V is recorded as C1, and the capacity value extracted at a discharge voltage of 2.0V is recorded as C2, where η = C1 / C2. The charging process includes constant voltage charging, with a constant voltage of 3.75V and a constant voltage cutoff current of 50μA.

[0196] The η value of the positive electrode active material can be measured using methods and equipment known in the art. As an example, a coin cell is first prepared according to the method described above. The electrical performance of the prepared coin cell is then tested using a blue-light tester. Specifically, at room temperature, the coin cell is charged and discharged twice at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V. After constant current charging to the cutoff voltage, it is charged at a constant voltage until the current reaches 50μA, followed by a single constant current charge and discharge at a rate of 1C. In the 1C charge and discharge test, the capacity value discharged from 3.75V to 3.2V is recorded as C1, and the capacity value discharged from 3.75V to 2.0V is recorded as C2, where η = C1 / C2.

[0197] In some implementations, 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 value range between the two.

[0198] In some embodiments, the discharge capacity percentage η of the positive electrode active material in the freshly prepared lithium-ion secondary battery when discharged to 3.2V is ≥88%. After the freshly prepared lithium-ion secondary battery is charged and discharged at a constant current rate of 0.1C within a voltage range of 2.0V to 3.75V for a period of time, the discharge capacity percentage η of the positive electrode active material when discharged to 3.2V can be maintained at ≥85%.

[0199] The high discharge capacity ratio of the positive electrode active material used in the lithium-ion secondary battery of this application to 3.2V indicates that the positive electrode active material has good kinetic performance. At the same time, the high η value indicates that the lithium-ion secondary battery containing the positive electrode active material still has a high voltage when discharged to a low state of charge (SOC), which is beneficial for maintaining good power performance.

[0200] In some embodiments, the positive electrode film layer further includes a conductive agent, the mass content of which is 0.1%-1.5% based on the total mass of the positive electrode film layer.

[0201] In some embodiments, based on the total mass of the positive electrode film, the mass content of the conductive agent can be selected as 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 any value range between the two.

[0202] 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.

[0203] The carbon layer of this positive electrode active material has a high degree of graphitization and good graphitization consistency, which gives the positive electrode active material good electronic conductivity. This can reduce or even eliminate the use of conductive agents in the positive electrode film, which is conducive to further increasing the loading of the positive electrode active material and improving the energy density of lithium-ion secondary batteries.

[0204] In some implementations, the positive electrode film does not include a conductive agent.

[0205] This positive electrode active material has extremely high electronic conductivity, which means that conductive agents can be omitted from the positive electrode film, thus improving the loading of the positive electrode active material and enhancing the energy density of lithium-ion secondary batteries.

[0206] In some embodiments, the positive electrode film layer further includes a binder, and the mass content of the positive electrode active material is 95.5%-99.5%, optionally 96.5%-99.5%, based on the total mass of the positive electrode film layer; the mass content of the binder is 0.5%-3%.

[0207] In some embodiments, the adhesive includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0208] In some embodiments, based on the total mass of the positive electrode film, the mass content of the positive electrode active material can be selected as 95.5%, 96%, 96.5%, 97%, 98%, 99%, 99.5%, or any value range between the two.

[0209] In some embodiments, based on the total mass of the positive electrode film, the mass content of the binder can be selected as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or any value range between the two.

[0210] In some embodiments, the one-sided density of the positive electrode film is 300 mg / 1540 mm².2 -450mg / 1540mm 2 .

[0211] In this application, the unilateral density of the positive electrode film layer has a meaning known in the art and can be tested using methods known in the art. For example, take a positive electrode sheet that has been coated on one side and compacted (if it is a double-sided coated positive electrode sheet, the positive electrode film layer on one side can be wiped off first), cut it into a small circular piece with an area of ​​S1, weigh it, and record its weight as M1. Then wipe off the positive electrode film layer of the above-weighed positive electrode sheet, weigh the current collector, and record it as M0. The unilateral density of the positive electrode film layer = (M1-M0) / S1. To ensure the accuracy of the test results, multiple sets (e.g., 10 sets) of samples can be tested, and the average value can be calculated as the test result.

[0212] In some embodiments, the areal density of the positive electrode film layer on one side may be selected as 300 mg / 1540 mm². 2 310mg / 1540mm 2 320mg / 1540mm 2 330mg / 1540mm 2 340mg / 1540mm 2 350mg / 1540mm 2 360mg / 1540mm 2 370mg / 1540mm 2 380mg / 1540mm 2 390mg / 1540mm 2 400mg / 1540mm 2 410mg / 1540mm 2 420mg / 1540mm 2 430mg / 1540mm 2 440mg / 1540mm 2 450mg / 1540mm 2 Or the range of values ​​between any two.

[0213] Positive electrode films with areal densities within the above range can help improve the energy density of lithium-ion secondary batteries.

[0214] In some embodiments, the positive electrode film layer of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.51 g / cm³. 3 -2.73g / cm 3 .

[0215] In some embodiments, the positive electrode film layer of the lithium-ion secondary battery, in its fully discharged state, has a compaction density of 2.55 g / cm³.3 -2.70g / cm 3 .

[0216] In this application, the fully discharged state refers to the state after the battery is placed in a 25°C oven environment, left to stand for 2 hours, and the battery temperature is maintained at 25°C, and then discharged at a constant current of 1 / 3C to 2.5V and then discharged at a constant current of 0.1C to 2.0V.

[0217] The compaction density of the positive electrode film can be tested using methods known in the art. As an example, the battery is placed in a 25°C oven and left to stand for 2 hours. Once the battery temperature is maintained at 25°C, it is discharged at a constant current of 1 / 3C to 2.5V, then discharged at a constant current of 0.1C to 2.0V. The battery is then disassembled to obtain the positive electrode sheet. The residual electrolyte is treated with dimethyl carbonate solvent, the electrode sheet is dried, and it is cut into small circular pieces with an area of ​​S, yielding a mass of W1. The thickness T1 of the positive electrode sheet is measured using a micrometer. The positive electrode film layer is then wiped off, and the mass of the current collector is weighed and recorded as W2. The thickness T2 of the current collector is measured using a micrometer. The compaction density PD of the positive electrode film layer is then calculated as follows: PD = (W1 / (W2)) / (W1 / (W2)) W2) / [(T1-T2)×S。

[0218] In some embodiments, the compaction density of the positive electrode film layer in the fully discharged state of the lithium-ion secondary battery can be selected as 2.51 g / cm³. 3 2.52g / cm 3 2.53g / cm 3 2.54 g / cm 3 2.55g / cm 3 2.56 g / cm 3 2.57g / cm 3 2.58g / cm 3 2.59g / cm 3 2.60g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / 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 3Or the range of values ​​between any two.

[0219] A compaction density of the positive electrode film within the above-mentioned range is beneficial to improving the energy density of lithium-ion secondary batteries.

[0220] In some embodiments, the compaction density of the positive electrode film layer after the compaction process is 2.63 g / cm³. 3 -2.85g / cm 3 .

[0221] In some embodiments, the compaction density of the positive electrode film layer after the compaction process can be selected as 2.63 g / cm³. 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / 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 2.75g / cm 3 2.76 g / cm 3 2.77 g / cm 3 2.78g / cm 3 g / cm 3 2.79 g / cm 3 2.80g / cm 3 2.81 g / cm 3 2.82 g / cm 3 2.83 g / cm 3 2.84 g / cm 3 2.85g / cm 3 Or the range of values ​​between any two.

[0222] In this application, compaction refers to the process of compacting the positive electrode film layer with mechanical pressure during battery assembly to improve its density and conductivity.

[0223] In some embodiments, the compaction density of the positive electrode film layer after formation processing is 2.52 g / cm³. 3 -2.73g / cm 3 .

[0224] In some embodiments, the compaction density of the positive electrode film layer after formation processing can be selected as 2.52 g / cm³.3 2.53g / cm 3 2.54 g / cm 3 2.55g / cm 3 2.56 g / cm 3 2.57g / cm 3 2.58g / cm 3 2.59g / cm 3 2.60g / cm 3 2.61 g / cm 3 2.62 g / cm 3 2.63 g / cm 3 2.64 g / cm 3 2.65g / cm 3 2.66 g / cm 3 2.67 g / cm 3 2.68g / 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 range of values ​​between any two.

[0225] In this application, formation refers to the formation of a stable solid electrolyte interface (SEI film) and electrode structure through electrochemical reactions during the first charge and discharge of the battery.

[0226] It is understandable that, with the rebound of the electrode during the cycle, the compaction density of the positive electrode film in the fully discharged state of the lithium-ion secondary battery is slightly lower than that of the positive electrode film after compaction and formation.

[0227] A compaction density of the positive electrode film within the above-mentioned range is beneficial to improving the energy density of lithium-ion secondary batteries.

[0228] In some embodiments, the compaction density of the positive electrode film is 2.51 g / cm³. 3 -2.73g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%.

[0229] In some embodiments, the compaction density of the positive electrode film is 2.55 g / cm³. 3 -2.70g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-20%.

[0230] In some embodiments, the porosity of the positive electrode film layer in a cross-section along the thickness direction of the electrode sheet can be selected as 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, or any value range between the two.

[0231] The porosity of the positive electrode film layer in a cross-section along the electrode thickness direction can be tested as follows: Import the scanning electron microscope (SEM) image of the positive electrode film layer in a cross-section along the electrode thickness direction obtained as described above into ImageJ software. Select the line tool and use a line to mark the length of the scale bar in the image. Click "Analyze Set Scale" and set the scale parameters in the software according to the scale bar length in the image. Select the rectangle tool and select the part of the image outside the scale bar area. Use "Image Duplicate" to copy the selected area and use "Image Type 8 bit" to adjust the image format. Select "Analyze Set Measurements" and choose the following 5 options: "Area", "Mean gray value", "Area Fraction", "Limit to threshold", and "Feret's diameter". Select 3 for "Decimal places". Then select "Image" - "Adjust" - "Threshold" in sequence and set 0 and 100 in the "Threshold" box. You can then use the Analyze-Measure function to export the porosity data in the SEM image of this cross-section. Export using "Image" - "Overlay" - "Flatten" to obtain a pore image; click "Apply" in "Threshold", then click "Analyze" - "Analyze Particles", check the four columns on the left to obtain pore statistics.

[0232] like Figure 8 As shown, in this embodiment of the application, "pores" in the cross-section of the positive electrode film are identified by image color difference and threshold. These "pores" are not the pore data obtained from the exhaust test, but are mainly used to characterize the cross-sectional area between particles in the cross-section of the positive electrode film. This method is superior to the exhaust method because the porosity obtained by the exhaust method is related to the pores between particles and the pores in the carbon layer covering the surface of the lithium iron phosphate particles, thus failing to objectively reflect the pores between particles.

[0233] The lower the porosity in the cross-section of the positive electrode film tested by this method, the better the gradation of large, medium and small particles in the positive electrode film and the higher the compaction density. On the other hand, under the same gradation and roller pressure, if the porosity is low, it means that the particles are more likely to slide against each other, thereby reducing the risk of overpressure and stress concentration in the film, further reducing the probability of the positive electrode film demolding during long cycles, which is beneficial to improving the long cycle performance of the battery.

[0234] In some embodiments, the positive electrode sheet includes a base coating layer disposed between the positive electrode film layer and the positive electrode current collector; the base coating layer includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating layer is ≤10 pcs / 10 μm.

[0235] Carbon-based particles refer to particles whose main component is carbon, including but not limited to conductive carbon and carbon black.

[0236] The undercoat layer helps improve the conductivity and adhesion between the positive electrode film and the current collector, reduces the likelihood of the positive electrode film detaching from the current collector during cycling, and improves the battery's dynamic performance. In the high compaction density electrode sheets of this application embodiment, for example, the compaction density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.4 g / cm³. 3 At this time, the current collector is easily damaged during the high-pressure compaction process of the electrode sheet, and large-sized particles are prone to creating pits on the current collector. Controlling the distribution density of carbon-based particles with a particle size greater than 100nm in the bottom coating layer to ≤10pcs / 10μm is beneficial to reducing the probability of damage to the current collector in the high-pressure compaction electrode sheet and further improving the ultimate compaction density of the positive electrode sheet.

[0237] The distribution density of carbon-based particles with a diameter greater than 100 nm in the undercoat can be determined by the method described above. The positive electrode film is cut along the thickness direction of the electrode by an argon ion beam, and scanning electron microscope or microscopic image is taken. The size of carbon particles in the undercoat is detected by statistical methods, and the number of carbon-based particles with a diameter greater than 100 nm per 10 μm in the undercoat is counted. The count is repeated at least 5 times and the average value is calculated.

[0238] The base coating in this embodiment can be achieved through any known preparation process, such as pre-sieving or centrifuging during the preparation of carbon-based particles to remove large carbon-based material particles, thereby reducing the D of the carbon-based particles added during the preparation of the base coating. V50 In the 20nm-60nm range, D V90 For materials with a wavelength of 70 nm or less, a base coating is obtained by mixing, stirring, and applying the carbon-based material with a binder onto the current collector.

[0239] In some embodiments, the compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.4 g / cm³.3 The thickness of the base coating layer on one side is 1μm-4μm.

[0240] In some embodiments, the compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.5 g / cm³. 3 The thickness of the base coating layer on one side is 2μm-4μm.

[0241] As the compaction density of the electrode increases, the compressive effect of large lithium phosphate particles (e.g., particles larger than 1 μm) on the undercoat becomes more significant. Therefore, stress concentration easily occurs at large particle sites, and these particles can even penetrate the undercoat and damage the current collector. Increasing the thickness of the undercoat helps to mitigate stress concentration in the electrode, further improving the electrode's ultimate compaction density.

[0242] The thickness of the base coating on one side can be tested as follows: As described above, the positive electrode film is cut along the thickness direction of the electrode using an argon ion beam, and a scanning electron microscope image is taken. The thickness of the base coating on one side is measured at 1m intervals along the length of the electrode. After measuring the thickness of the base coating at 10 points, the average value is calculated. It is important to note that outliers should be avoided during the measurement process, namely, areas with a thickness less than 50nm and areas with a thickness greater than 4m. These outliers are mainly due to extreme thickness fluctuations in individual areas caused by abnormal stress concentration and compression during electrode compaction, and are not statistically significant.

[0243] In some embodiments, the thickness of the positive current collector is less than or equal to 17 μm, and can be selected as 13 μm-15 μm.

[0244] In some embodiments, the thickness of the positive current collector is 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, or any value between the two.

[0245] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0246] In some embodiments of the lithium-ion secondary battery, the negative electrode includes a negative current collector and a negative electrode film layer disposed on at least one side of the negative current collector, wherein the areal density of the negative electrode film layer on one side is 140 mg / 1540 mm². 2-221mg / 1540mm 2 ; and / or the compaction density of the negative electrode film is 1.40 g / cm³. 3 -1.75g / cm 3 .

[0247] The unilateral density and compaction density of the negative electrode film can be tested using a method similar to that used for the positive electrode film described above.

[0248] Having the areal density and compaction density of the negative electrode film within the above-mentioned range is beneficial for improving the energy density of lithium-ion secondary batteries.

[0249] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0250] In some embodiments, the negative electrode film layer includes a negative electrode active material. The negative electrode active material may be any negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may 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 may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0251] In some embodiments, the negative electrode film layer may optionally include an adhesive. The adhesive may 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).

[0252] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0253] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0254] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and after drying, compaction and other processes, the negative electrode sheet can be obtained.

[0255] In some embodiments, the lithium-ion secondary battery includes an electrolyte. The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific limitations on the type of electrolyte, which can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, or entirely solid-state.

[0256] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0257] 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 difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0258] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl 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.

[0259] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0260] In some embodiments, the lithium-ion secondary battery also includes a separator. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0261] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can 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 can be the same or different, without particular limitation.

[0262] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0263] In some embodiments, the lithium-ion secondary battery may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0264] In some implementations, the outer packaging of the lithium-ion secondary battery can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0265] The second aspect of this application provides a battery device including the lithium-ion secondary battery provided in the first aspect of this application. The battery device includes at least one of a battery module, a battery pack, and an energy storage battery.

[0266] A third aspect of this application provides an electrical device, including the lithium-ion secondary battery provided in the first aspect of this application.

[0267] The fourth aspect of this application provides a method for preparing a positive electrode active material, comprising: obtaining a mixed raw material including a carbon source, a lithium source, an iron source, and a phosphorus source; wherein the carbon source includes polyethylene glycol; the iron source includes ferrous 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; grinding in a solvent to obtain a mixed slurry; drying the mixed slurry to obtain a precursor powder; sintering the precursor powder to obtain a positive electrode active material; wherein the sintering includes at least two stages of isothermal sintering, wherein the sintering temperature of the high-temperature stage is 750℃-800℃.

[0268] The preparation method provided in this application improves the graphitization degree of the positive electrode active material by adjusting the molar ratio of iron and phosphorus in the mixed raw materials. Furthermore, it uses polyethylene glycol as a carbon source, and combines this with sintering temperature control and ferrous iron catalytic reduction to uniformly increase the graphitization degree of the particles in the positive electrode film. The median C for the prepared graphitization degree is... 50 Concentration of C values ​​greater than or equal to 0.95 and less than or equal to 1.20 (C 90 -C 10 ) / C 50 It provides a material basis for positive electrode film layers with a thickness of 0.01-0.04.

[0269] The positive electrode film prepared by this method has a high degree of graphitization and good graphitization consistency. It is easy to improve the compaction density of the electrode sheet through uniform and consistent slip between particles, which is beneficial to improve the energy density of the battery while improving the battery dynamic performance.

[0270] In some embodiments, the iron source includes ferrous iron, which may be one or more of ferrous oxalate, ferrous carbonate, and ferrous nitrate.

[0271] During sintering, the ferrous iron source preferentially decomposes to generate a large amount of ferrous oxide, which serves as a nucleation site for the formation of lithium transition metal phosphate nanocrystals. Simultaneously, the polymer carbon source has a relatively low decomposition temperature, and the iron elements on the surface of the nanocrystals further catalyze the decomposition of the carbon source. This allows the carbon coating layer on the surface of the cathode active material to achieve a relatively high degree of graphitization even at a lower sintering temperature, reducing the resistivity of the cathode active material and improving the density and uniformity of the carbon coating layer on the lithium transition metal phosphate surface.

[0272] In some embodiments, the phosphorus source includes one or more of lithium dihydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate.

[0273] In some implementations, the lithium source and the phosphorus source can be the same substance.

[0274] In some embodiments, the iron source includes ferrous oxalate, the lithium and phosphorus sources include lithium dihydrogen phosphate, and the carbon source includes polyethylene glycol.

[0275] In some embodiments, the atomic molar ratio of iron to phosphorus in the iron source and the phosphorus source is 0.95:1.0-1.0:1.0.

[0276] In some embodiments, the atomic molar ratio of iron to phosphorus 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 any range between the two.

[0277] Both excessively high and low iron-phosphorus ratios can lead to structural instability. A low iron-phosphorus ratio may result in incomplete lithium occupancy, reducing the material's ionic conductivity and structural stability; while a high ratio may lead to excessive iron, affecting the electrochemical stability and capacity output of LiFePO4. An appropriate iron-phosphorus ratio promotes uniform crystal growth, preventing particle aggregation or excessive size variations during synthesis. If the iron-phosphorus ratio is too high, excessive iron ions may form large particles during the reaction, affecting particle size uniformity; conversely, if the ratio is too low, phosphate may not participate fully in the reaction, leading to incomplete particle growth and affecting particle uniformity.

[0278] In some embodiments, the particle size D of ferrous oxalate 10 ≥3μm, particle size D 50 Particle size D: 50μm-80μm 90 Less than or equal to 150 μm.

[0279] In this application, the term "D" 10 “D” 50 "and "D 90 "These correspond to the particle sizes at which the cumulative particle size distribution percentage of the sample obtained by the Malvern laser scattering method reaches 10%, 50%, and 90%, respectively."

[0280] Controlling the particle size D of ferrous oxalate 10 A particle size of 3 μm or larger can reduce the proportion of small-diameter ferrous oxalate particles and control their reactivity during the grinding process. Controlling the particle size D of ferrous oxalate... 50 D 90 It helps to uniformly mix the raw materials during the grinding process, obtain a mixed slurry with consistent composition and uniform particle size, and improve the particle size consistency of the prepared lithium transition metal phosphate.

[0281] In some implementations, the mass content of ferric iron is less than or equal to 0.08%.

[0282] In some implementations, the mass content of ferric iron can be selected as 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, or any value range between the two.

[0283] Controlling the mass content of ferric iron helps improve the uniformity and consistency of the carbon coating. Excessive ferric iron will preferentially consume the carbon source, resulting in poor consistency in the mass and thickness of the carbon coating between particles. On the one hand, the uneven thickness of the carbon coating will affect the compaction between particles; on the other hand, local carbon depletion will affect the overlap of the conductive network between particles, which is not conducive to effectively improving the compaction density of the electrode and the improvement of kinetics.

[0284] In some embodiments, the lithium source includes one or more of lithium dihydrogen phosphate, lithium phosphate, lithium carbonate, and lithium acetate.

[0285] In some embodiments, the carbon source includes a polymeric carbon source, which may be one or more of polyethylene glycol and polyvinyl alcohol.

[0286] In some implementations, the carbon source content is 1-4% based on the total mass of the positive electrode film.

[0287] The polymer carbon source has a relatively low decomposition temperature and graphitization temperature, which allows the carbon coating layer on the surface of the positive electrode active material to decompose and form a carbon layer at a lower sintering temperature. This hinders the growth and sintering of lithium-containing transition metal phosphate grains, which is beneficial for reducing the particle size of the positive electrode active material.

[0288] Meanwhile, polymer carbon sources typically have high molecular weights or long molecular chains, which are easy to form stable framework structures through cross-linking or orientation during heat treatment. This orderliness is preserved during high-temperature carbonization, which is beneficial for the directional growth of graphite crystals. At the same time, the entanglement and cross-linking between long chains help reduce structural defects and reduce lattice disorder caused by chain breakage during carbonization, thereby improving the degree of graphitization.

[0289] Organic molecules in the carbon source decompose at high temperatures, releasing carbon atoms. These carbon atoms can cover and fill tiny gaps or defects on the surface of the active material, reducing surface roughness. The coating layer formed by the polymer carbon source has a high degree of graphitization and a denser carbon structure, which is beneficial for optimizing the surface roughness of the positive electrode active material.

[0290] In some embodiments, the weight-average molecular weight of polyethylene glycol is less than 10,000.

[0291] In some embodiments, the weight-average molecular weight of polyethylene glycol can be selected as 1500, 2000, 3000, 4000, 6000, 8000 or any range between the two.

[0292] Using polyethylene glycol with a weight-average molecular weight of less than 10,000 allows for control of the decomposition rate during sintering, resulting in a carbon coating layer of suitable and uniform thickness.

[0293] In some implementations, the water content of polyethylene glycol is less than or equal to 0.5%.

[0294] If the moisture content in polyethylene glycol is high, it may affect the decomposition process, leading to incomplete decomposition or uneven decomposition rate during sintering. Excessive moisture may also cause uneven distribution of molten polyethylene glycol during sintering, affecting the uniformity of the carbon layer and resulting in unstable carbon coating or peeling.

[0295] In some embodiments, the water content of polyethylene glycol can be selected as 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any value between two of these.

[0296] In some implementations, the pH of polyethylene glycol is 5-7.

[0297] Polyethylene glycol (PEG) with a pH of 5-7 exhibits high stability and will not degrade during mixing due to excessive acidity, especially under high-temperature conditions, which would lead to rapid decomposition and affect the quality of the coating layer. If PEG is alkaline, it may affect the stability of other components, causing metal ions to dissolve or oxidize, thus impacting the performance of the final positive electrode active material.

[0298] In some embodiments, the slurry further includes a titanium source, optionally including one or more of titanium dioxide, tetrabutyl titanate, titanium nitrate, and titanic acid.

[0299] Titanium sources often have low surface activity. Including titanium sources in the slurry can reduce the activity of lithium transition metal phosphate precursors and inhibit the particle growth of lithium transition metal phosphates during high-temperature sintering, resulting in smaller particles of lithium transition metal phosphates during sintering.

[0300] Titanium is used as a lattice stabilizer; the element titanium is usually represented by Ti. 4+ In the form of lithium transition metal phosphate, some titanium ions can replace iron ions in the crystal lattice, making the crystal structure more stable and reducing the possibility of lithium and iron ions being reversed, especially at high temperatures or during high-current charging and discharging.

[0301] In some embodiments, lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide are mixed evenly in an organic solvent and then ground to obtain a mixed raw material.

[0302] Organic solvents can effectively reduce the occurrence of side reactions and improve the purity and consistency of materials. Furthermore, organic solvents have good volatility, making them easier to remove during subsequent drying processes and preventing residues inside the material that could create pores and affect its density and structural stability.

[0303] In some embodiments, the carbon source in the mixed raw materials accounts for 5%-7% of the total mass of the mixed raw materials.

[0304] In some embodiments, based on the total mass of the mixed raw materials, the mass percentage of the carbon source in the mixed raw materials can be selected as 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 value range between the two.

[0305] By controlling the quality of the carbon source and keeping the lithium content within the aforementioned range, the conductivity of the material can be enhanced, and the negative impact on the specific capacity of the positive electrode and the energy density of the battery can be reduced. An excessively thick carbon layer not only occupies valuable space for the active material but may also lead to structural instability.

[0306] In some embodiments, the solvent includes water and mixtures thereof.

[0307] In some embodiments, obtaining the mixed raw material including carbon source, lithium source, iron source and phosphorus source includes: adding carbon source, lithium source, phosphorus source, iron source and carbon source to a solvent and mixing and stirring, wherein the stirring speed is 1400rpm-2200rpm.

[0308] In some embodiments, obtaining precursor powder after drying the mixed slurry includes obtaining precursor powder after spray drying the mixed slurry.

[0309] In some embodiments, the product is subjected to air jet milling after sintering the precursor to obtain the positive electrode active material.

[0310] In some embodiments, the grading frequency of the air jet mill is 18Hz-24Hz, and the milling pressure is 0.45MPa-0.65MPa.

[0311] In air jet milling, the classification frequency refers to the operating frequency of the classification device, which is typically related to the classification efficiency and particle size distribution. A higher classification frequency results in more frequent sieving of particles in the airflow, removing larger particles and leaving smaller ones. Furthermore, a higher classification frequency may increase the number of particle collisions, subjecting irregular particles to further impact, resulting in smoother particle surfaces and a more spherical shape.

[0312] High air pressure causes particles to be subjected to greater impact force, and the collisions between particles are more intense. This results in stronger impact and wear on the particle surface, which can crush large particles into smaller particles. The more intense collisions between particles make the surface easier to be trimmed, improving the sphericity and surface smoothness of the particles.

[0313] However, excessively high classification frequency and pulverizing pressure can cause agglomerated particles to disperse into primary particles, leading to further cracking and breakage. This affects the intended particle size distribution and results in an incomplete carbon coating, manifested as increased iron dissolution. This negatively impacts particle slippage during rolling and increases the contact and reaction between lithium-containing transition metal phosphates and external factors such as the electrolyte, which is detrimental to battery cycle performance and lifespan. Therefore, it is necessary to control the classification frequency and pulverizing pressure of the air jet pulverizer within a suitable range.

[0314] The fifth aspect of this application provides a method for preparing a positive electrode sheet, the method comprising sequentially adding a binder, a conductive agent, and a positive active material prepared by the method of the fourth aspect, dry mixing them, adding a solvent, stirring, adjusting the viscosity, and obtaining a slurry; transferring the slurry to at least one side of a current collector, drying, and hot pressing to obtain a positive electrode sheet.

[0315] In some embodiments, the revolution speed of the dry mix is ​​20 rpm-30 rpm, and the rotation speed of the dry mix is ​​750 rpm-850 rpm.

[0316] In some embodiments, hot pressing includes at least three hot roller pressings, with the hot roller pressure increasing sequentially to 20-50 tons, 50-70 tons, and 70-90 tons; the hot roller temperature is 40°C-80°C, and the positive electrode sheet is heated to 40°C-50°C before the first hot roller compaction.

[0317] The positive electrode active material prepared by the above-mentioned hot pressing process in combination with the preparation method of the fourth aspect in this application embodiment is beneficial to further reduce the cross-sectional porosity of the positive electrode film, increase the ultimate compaction density of the electrode sheet, and improve the energy density of the battery.

[0318] In addition, this application also provides an electrical device, which includes at least one of the lithium-ion secondary battery, battery module, or battery pack provided in this application. The lithium-ion secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0319] As the electrical device, a lithium-ion secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0320] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of lithium-ion secondary batteries for this device, a battery pack or battery module can be used.

[0321] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a lithium-ion rechargeable battery as their power source.

[0322] Example The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0323] Example 1 (1) Preparation of positive electrode active material Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide were mixed evenly in a solvent and then ground. The ratio of ferrous oxalate to lithium dihydrogen phosphate was such that the molar ratio of iron to phosphorus was 0.965:1.0.

[0324] The mixed raw materials were ball-milled multiple times and demagnetized in a ball mill to obtain a mixed slurry. The number of grinding cycles and time were controlled, and the particle size Dv50 of the ground mixed slurry was 3.0 μm.

[0325] The spray-dried slurry yields a dried precursor powder, which is light yellow in appearance and uniform in color.

[0326] The precursor powder was placed in a sintering furnace and heated from 25°C to 350°C at a rate of 2°C / min under a nitrogen atmosphere and held at that temperature for 3 hours. Then, the temperature was increased to a second temperature of 770°C at a rate of 5°C / min and held at that temperature for 10 hours. After the process, the temperature was lowered and cooled.

[0327] The obtained lithium iron phosphate cathode material was crushed by air jet milling to obtain carbon-coated lithium iron phosphate cathode active material.

[0328] The carbon content of the positive electrode active material is 1.103% by mass, and the median sphericity L A50 It is 0.720, L A90 The median roughness R is 0.895. A50 The concentration of lithium iron phosphate (LFP) antisite defects was 0.941, the concentration of LFP antisite defects was 0.55%, and the tap density of the powder was 1.04 g / cm³. 3 The compacted density of the powder under 3T pressure is 2.572 g / cm³. 3 The powder resistivity at 8MPa pressure is 5.90Ω·cm; the discharge capacity at 1C discharge rate is 143.8mAh / g; and the discharge capacity at 3.2V discharge platform accounts for 91.0%.

[0329] (2) Preparation of the positive electrode sheet: 2.2 wt% PVDF, 0.8 wt% conductive carbon black, and 97.0 wt% positive electrode active material were sequentially added and dry-mixed, followed by the addition of N-methylpyrrolidone. The mixture was stirred and the viscosity adjusted to obtain a slurry. This slurry was then transferred and coated onto a base coating of current collector aluminum foil. The base coating consisted of carbon black and PVDF in a 1:1 mass ratio. The density of carbon-based particles larger than 100 nm in the base coating was ≤10 pcs / 10 μm, and the base coating thickness was 2 μm. After drying and hot pressing, a single-sided surface density of 350 mg / 1540 cm³ was obtained.2 The positive electrode film layer.

[0330] The dry mix has a revolution speed of 25 rpm and a rotation speed of 800 rpm.

[0331] The hot pressing process includes three hot roller pressing processes, with the hot roller pressing pressure increasing sequentially to 35 tons, 55 tons, and 75 tons. The hot roller temperature is 65°C. Before the first entry into the hot roller for compaction, the electrode sheet is heated to 50°C.

[0332] The compacted density of the electrode sheet is the ultimate compacted density of the electrode sheet. The test method for the ultimate compacted density of the electrode sheet is described below; in this example, the ultimate compacted density of the electrode sheet is 2.68 g / cm³. 3 .

[0333] 17,857 particles were counted in a cross-section of the positive electrode film along the thickness direction of the electrode sheet, indicating an area of ​​0.001 μm. 2 -0.06μm 2 The particle area accounted for 23.84%, with an area of ​​1.0 μm. 2 -4.0μm 2 The area ratio of the particles is 15.01%. The median C of the graphitization degree of the positive electrode film obtained by laser microscopy confocal Raman spectroscopy in surface scanning mode is... 50 C is 1.018. 90 C is 1.041. 10 The concentration of C is 1.007. 90 -C 10 ) / C 50 It is 0.033.

[0334] The iron dissolution rate of the positive electrode film is 1041 ppm.

[0335] (3) Preparation of negative electrode sheet: A mixture of 95.5 wt% negative electrode active material (artificial graphite), 1.0 wt% conductive agent (conductive carbon black), 2.0 wt% binder (styrene-butadiene rubber (SBR)), and 1.5 wt% thickener (sodium carboxymethyl cellulose (CMC)) was prepared by mixing with deionized water and stirring to disperse the mixture into a negative electrode slurry. The negative electrode slurry was then coated onto both sides of a Cu foil. After coating both sides, the foil was dried, compacted, slit, and sheeted to obtain the negative electrode sheet. The coating density on one side was 165 mg / 1540 mm². 2 The compacted density is 1.60 g / cm³. 3 .

[0336] (4) Preparation of the separating membrane Polypropylene film is used as the separator.

[0337] (5) Preparation of electrolyte In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) and dimethyl carbonate (DMC) organic solvents were mixed evenly at a volume ratio of 1 / 1. Lithium salt LiPF6 was added and dissolved in the organic solvent. The concentration of LiPF6 in the solution was 1 mol / L. The mixture was stirred evenly to obtain the electrolyte.

[0338] (6) Battery fabrication: The positive electrode, separator, and negative electrode are stacked in sequence. The separator must be able to isolate the positive and negative electrodes. The bare cell is obtained by winding. The bare cell is placed in the outer packaging, electrolyte is injected, and it goes through processes such as encapsulation, formation, and degassing to finally obtain a lithium-ion battery.

[0339] The preparation methods of Examples 2 and 3 are basically the same as those of Example 1, except that the carbon source in the raw materials is adjusted.

[0340] Example 2 Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and glucose (mixed in a mass ratio of 3:1), along with titanium dioxide, were mixed evenly in a solvent and then ground. The ratio of lithium dihydrogen phosphate to ferrous oxalate resulted in a molar ratio of iron to phosphorus of 0.965:1.0.

[0341] Example 3 Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and glucose (mixed in a mass ratio of 1:3), along with titanium dioxide, were mixed evenly in a solvent and then ground. The ratio of lithium dihydrogen phosphate to ferrous oxalate resulted in a molar ratio of iron to phosphorus of 0.965:1.0.

[0342] 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.

[0343] Example 4 Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and glucose (mixed in a mass ratio of 1:3), along with titanium dioxide, were mixed evenly in a solvent and then ground. The ratio of lithium dihydrogen phosphate to ferrous oxalate resulted in a molar ratio of iron to phosphorus of 0.965:1.0.

[0344] The precursor powder was placed in a sintering furnace and sintered in two stages under a nitrogen atmosphere to obtain lithium iron phosphate cathode material: the temperature was increased from 25°C to 350°C at a heating rate of 2°C / min and held for 3 hours; the temperature was increased from 350°C to 780°C at a heating rate of 5°C / min and held for 10 hours; and then the temperature was cooled.

[0345] The preparation methods of Examples 5 and 6 are basically the same as those of Example 1, except that the sintering temperature of the precursor powder is adjusted.

[0346] Example 5 The precursor powder was placed in a sintering furnace and sintered in two stages under a nitrogen atmosphere to obtain lithium iron phosphate cathode material: the temperature was increased from 25°C to 350°C at a heating rate of 2°C / min and held for 3 hours; the temperature was increased from 350°C to 755°C at a heating rate of 5°C / min and held for 10 hours; and then the temperature was cooled.

[0347] Example 6 The precursor powder was placed in a sintering furnace and sintered in two stages under a nitrogen atmosphere to obtain lithium iron phosphate cathode material: the temperature was increased from 25°C to 350°C at a heating rate of 2°C / min and held for 3 hours; the temperature was increased from 350°C to 790°C at a heating rate of 5°C / min and held for 10 hours; and then the temperature was cooled.

[0348] The preparation methods of Examples 7 and 8 are basically the same as those of Example 1, except that the molar ratio of iron and phosphorus is adjusted.

[0349] Example 7 Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide were mixed evenly in a solvent and then ground. The ratio of lithium dihydrogen phosphate to ferrous oxalate was such that the molar ratio of iron to phosphorus was 0.955:1.0.

[0350] Example 8 Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide were mixed evenly in a solvent and then ground. The ratio of lithium dihydrogen phosphate to ferrous oxalate was such that the molar ratio of iron to phosphorus was 0.975:1.0.

[0351] 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 positive electrode sheet: 97.8 wt% of positive electrode active material and 2.2 wt% of PVDF were mixed, and then N-methylpyrrolidone was added and stirred to disperse the mixture, thus preparing a positive electrode slurry.

[0352] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that the sintering temperature of the carbon source and precursor powder in the raw materials is adjusted.

[0353] Comparative Example 1 Lithium dihydrogen phosphate, ferrous oxalate, glucose, and titanium dioxide were mixed evenly in a solvent and then ground. The ratio of lithium dihydrogen phosphate to ferrous oxalate was such that the molar ratio of iron to phosphorus was 0.965:1.0.

[0354] The precursor powder was placed in a sintering furnace and sintered in two stages under a nitrogen atmosphere to obtain lithium iron phosphate cathode material: the temperature was increased from 25°C to 350°C at a heating rate of 2°C / min and held for 3 hours; the temperature was increased from 350°C to 740°C at a heating rate of 5°C / min and held for 10 hours; and then the temperature was cooled.

[0355] The preparation method of Comparative Example 2 is basically the same as that of Example 1, except that the molar ratio of iron and phosphorus and the sintering temperature of the precursor powder are adjusted.

[0356] Comparative Example 2 Lithium dihydrogen phosphate, ferrous oxalate, polyethylene glycol, and titanium dioxide were mixed evenly in a solvent and then ground. The ratio of lithium dihydrogen phosphate to ferrous oxalate was such that the molar ratio of iron to phosphorus was 0.955:1.0.

[0357] The precursor powder was placed in a sintering furnace and sintered in two stages under a nitrogen atmosphere to obtain lithium iron phosphate cathode material: the temperature was increased from 25°C to 350°C at a heating rate of 2°C / min and held for 3 hours; the temperature was increased from 350°C to 810°C at a heating rate of 5°C / min and held for 10 hours; and then the temperature was cooled.

[0358] Performance testing 1. Energy density test The lithium-ion secondary battery was left to stand at 25°C for 2 hours to ensure the battery temperature remained at 25°C. At 25°C, the battery was charged at 0.33C to the charging cutoff voltage of 3.65V, and then continued to be charged at this cutoff voltage under constant voltage until the current reached 0.05C, at which point charging was stopped (where C represents the rated capacity of the lithium-ion secondary battery). After leaving the battery to stand at 25°C for 1 hour, it was discharged at 0.33C at 25°C to the discharge cutoff voltage of 3.65V. The total discharge energy of the lithium-ion secondary battery was recorded as E0.

[0359] Measure the length, width, and height of each battery cell, and calculate the volume of the battery cell, V0 = length * width * height. The volumetric energy density of a lithium-ion secondary battery = discharge energy E0 / volume V0.

[0360] 2. DCR Test Method At 25℃, the capacitor was charged at a constant current of 0.33C to 3.65V, then charged at a constant voltage to a current of 0.05C, then discharged at 0.33C to 20% SOC. After resting for 5 minutes, it was pulsed discharged at 3C for 30 seconds, rested for 40 seconds, charged at 3C for 40 seconds, rested for 5 minutes, charged at a constant current of 0.33C to 3.65V, then charged at a constant voltage to 0.05C, then discharged at 0.33C to 10% SOC. After resting for 5 minutes, it was pulsed discharged at 3C for 30 seconds, and then rested for 40 seconds. After charging at 3C for 40 seconds, let it stand for 5 minutes, then fully charge it at 0.33C, then discharge it at 0.33C to 50% SOC, then let it stand at -25℃ for 2 hours, then pulse discharge it at 1C for 30 seconds, let it stand for 10 minutes, then let it stand at 25℃ for 2 hours, then charge it at 0.33C constant current to 3.65V, then charge it at constant voltage to 0.05C, then discharge it at 0.33C to 20% SOC, then let it stand at -25℃ for 2 hours, then pulse discharge it at 1C for 30 seconds, and let it stand for 10 minutes.

[0361] 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 resting - voltage before resting after pulse discharge) / pulse current.

[0362] 3. Ultimate compaction density of electrode sheets The double-coated electrode sheets were compacted using a roller press, and the elongation and flexibility of the compacted electrode sheets were tested. By increasing the pressure of the roller press, electrode sheets with different compaction densities were obtained. As the pressure increased, the compaction density of the electrode sheet increased, the elongation of the electrode sheet increased, and the flexibility of the electrode sheet decreased. Excessive elongation of the electrode sheet can easily lead to warping, while insufficient flexibility can easily lead to brittle fracture. Therefore, the lower of the compaction density corresponding to an elongation of 8% or the compaction density corresponding to a flexible folding number of times is defined as the ultimate compaction density of the electrode sheet.

[0363] The compaction density is calculated by dividing the mass of the positive electrode film by the volume of the positive electrode film.

[0364] The method for testing elongation is as follows: Lay the electrode flat on a horizontal table and cut it into sections, each about 100cm long. Remove the copper foil from the edges of the electrode, ensuring the cut edges are parallel to the MD direction (perpendicular to the pressure roller) to guarantee the electrode is completely covered by the coating. Use a steel ruler to measure the length between marked points at the beginning and end of the electrode, estimating to 0.1mm, and record the length before compaction. After compaction, record the length between the corresponding marked points. Use (compacted length - uncompacted length) / uncompacted length as the electrode's elongation.

[0365] The test method for the number of flexible folds is as follows.

[0366] Cut the positive electrode sheet to 20×100mm. 2 Test the size of the sample; fold it in half with the front side facing up, flatten it with a 2kg roller, unfold it and check the gap against the light to see if light passes through. If no light passes through, fold it in half again with the back side facing up, flatten it with a 2kg roller, and check it against the light again. Repeat this process until light passes through the gap. 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 sheet.

[0367] Test Results Table 1

[0368] Table 2

[0369] As can be seen from the comparison between the examples and the comparative examples, in the cumulative distribution curve of graphitization degree C obtained by laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of graphitization degree is... 50 The concentration of C values ​​is greater than or equal to 0.95 and less than or equal to 1.20. 90 -C 10 ) / C 50 The value is 0.01-0.04. While maintaining low internal resistance (especially low impedance at low SOC), the compaction density of the positive electrode is increased, which improves the compaction density of the electrode and the energy density of the battery while maintaining good dynamic performance.

[0370] As can be seen from the comparison between Example 5 and Examples 1-4 and Examples 6-9, in the cumulative distribution curve of graphitization degree C obtained by laser microscopy confocal Raman spectroscopy in instrument scanning mode, the median C of graphitization degree is... 50 When the value is 0.98-1.13, it is beneficial to increase the electrode compaction density while maintaining the low impedance of the battery, and to improve the energy density of the battery while maintaining good kinetic performance.

[0371] A comparison of Examples 4 and 6 with Example 2 shows that, in the cumulative distribution curve of graphitization degree C value obtained by laser microscopy confocal Raman spectroscopy in instrument scanning mode, the concentration of C value (C0) is relatively high. 90 -C 10 ) / C 50 When the value is 0.02-0.038, it indicates that the coating layer of the positive electrode active material has high uniformity, which is beneficial to further improve the dynamic performance of the battery while maintaining good electrode compaction density and battery energy density.

[0372] As can be seen from the comparison between Example 9 and Example 1, the lithium-ion secondary battery of this application still has good kinetic performance without the addition of conductive agent, and the energy density of the lithium-ion secondary battery is further improved.

[0373] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium-ion secondary battery, characterized in that, Includes positive electrode, negative electrode, and electrolyte. The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one side of the positive current collector. The positive electrode film layer includes a positive electrode active material, which comprises lithium transition metal phosphate particles with at least a portion of their surface coated with carbon material. In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of the graphitization degree is... 10 The concentration of C values ​​is 0.92-1.10, and the concentration of C values ​​(C 90 -C 10 ) / C 50 It ranges from 0.01 to 0.04; Among them, the degree of graphitization C is I G / I D I G This indicates that the Raman spectrum is at 1580±100 cm⁻¹ -1 The intensity of peak G at I D This indicates that the Raman spectrum is at 1350±100 cm⁻¹ -1 The intensity of peak D at that location.

2. 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 laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of the graphitization degree is... 50 Greater than or equal to 0.95 and less than or equal to 1.

20.

3. The lithium-ion secondary battery according to claim 2, characterized in that, In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of the graphitization degree is... 50 The value ranges from 0.96 to 1.

15.

4. The lithium-ion secondary battery according to claim 2, characterized in that, In the cumulative distribution curve of the graphitization degree C value of the positive electrode film obtained in the laser microscopy confocal Raman spectroscopy instrument scanning mode, the median C of the graphitization degree is... 50 The range is 0.98-1.

13.

5. The lithium-ion secondary battery according to claim 1, characterized in that, The concentration of C values ​​(C0.05) in the cumulative distribution curve of graphitization degree C values ​​obtained by laser microscopy confocal Raman spectroscopy in instrument scanning mode of the positive electrode film is shown to be high. 90 -C 10 ) / C 50 The value ranges from 0.02 to 0.

038.

6. The lithium-ion secondary battery according to claim 1, characterized in that, The concentration of C values ​​(C0.05) in the cumulative distribution curve of graphitization degree C values ​​obtained by laser microscopy confocal Raman spectroscopy in instrument scanning mode of the positive electrode film is shown to be high. 90 -C 10 ) / C 50 The value ranges from 0.02 to 0.

036.

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 laser microscopy confocal Raman spectroscopy instrument scanning mode, the graphitization degree C 90 It ranges from 1.0 to 1.

30.

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 laser microscopy confocal Raman spectroscopy instrument scanning mode, the graphitization degree C 90 It is 1.02-1.

15.

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 laser microscopy confocal Raman spectroscopy instrument scanning mode, the graphitization degree C 10 The value ranges from 0.98 to 1.

08.

10. The lithium-ion secondary battery according to claim 1, characterized in that, The area of ​​the positive electrode film layer in the cross-section along the thickness direction of the electrode sheet is 0.001 μm. 2 -0.06μm 2 The particle area ratio is 21.00%-27.00%, and the area is 1.0μm. 2 -4.0μm 2 The area ratio 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 of the particles A50 The range is 600nm-800nm, where D A50 It refers to the particle size at which the cumulative area distribution of particles reaches 50% in the cumulative area distribution curve of particles.

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 of the particles A50 The range is 650nm-750nm, where D A50 It refers to the particle size at which the cumulative area distribution of particles reaches 50% in the cumulative area distribution curve of particles.

13. The lithium-ion secondary battery according to claim 1, characterized in that, In the cumulative distribution curve of particle roughness area obtained from the cross-section of the positive electrode film along the electrode thickness direction, the median 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 area obtained from the cross-section of the positive electrode film along the electrode thickness direction, 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 area obtained from the cross-section of the positive electrode film along the electrode thickness direction, the sphericity L A90 It ranges from 0.85 to 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 electrode thickness direction, the median L of sphericity is... A50 It ranges from 0.65 to 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 electrode thickness direction, the median L of sphericity is... 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, and the iron dissolution rate of the positive electrode film 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 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 carbon content 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 carbon content 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, 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 concentration of lithium iron ion reverse defects in 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 comprises components having the following general formula: The 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, and Br, 0.8≤m≤1.15, 0.9≤x≤1, 0.95≤y≤1, 3.5≤j≤4, and 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 and modified materials, and coated and modified materials.

26. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode active material includes titanium, and the mass content of titanium 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 is 0.70 g / cm³. 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 is 0.70 g / cm³. 3 -1.20g / cm 3 .

29. The lithium-ion secondary battery according to claim 1, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.50 g / cm³. 3 -2.70g / cm 3 .

30. The lithium-ion secondary battery according to claim 29, characterized in that, The compacted density of the positive electrode active material under 3T pressure is 2.52 g / cm³. 3 -2.68g / cm 3 .

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

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

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

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

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

36. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode film 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%; and 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, 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-sided density of the positive electrode film is 300 mg / 1540 mm². 2 -450mg / 1540mm 2 .

40. The lithium-ion secondary battery according to claim 1, characterized in that, In the fully discharged state, the compaction density of the positive electrode film in the lithium-ion secondary battery is 2.51 g / cm³. 3 -2.73g / cm 3 .

41. The lithium-ion secondary battery according to claim 1, characterized in that, In the fully discharged state, the positive electrode film of the lithium-ion secondary battery has a compaction density of 2.55 g / cm³. 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) In the fully discharged state, the compaction density of the positive electrode film of the lithium-ion secondary battery is 2.51 g / cm³. 3 -2.73g / cm 3 In a cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-22%. (2) In the fully discharged state, the positive electrode film of the lithium-ion secondary battery has a compaction density of 2.55 g / cm³. 3 -2.70g / cm 3 In the cross-section along the thickness direction of the electrode sheet, the porosity of the positive electrode film is 10%-20%.

43. The lithium-ion secondary battery according to claim 1, characterized in that, The positive electrode sheet includes a base coating layer, which is disposed between the positive electrode film layer and the positive electrode current collector; the base coating layer satisfies at least one of the following conditions: (1) The base coating includes carbon-based particles, and the distribution density of carbon-based particles with a particle size greater than 100 nm in the base coating is ≤10 pcs / 10 μm; (2) The compacted density of the positive electrode sheet in its fully loaded state is greater than or equal to 2.4 g / cm³. 3 The thickness of the base coating on one side is 1μm-4μm; (3) The compacted density of the positive electrode sheet in the fully loaded state is greater than or equal to 2.5 g / cm³. 3 The thickness of the base coating layer on one side is 2μm-4μm.

44. A battery device, characterized in that, The battery device includes 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 appliance, characterized in that, Includes the lithium-ion secondary battery according to any one of claims 1 to 43 or the battery device according to claim 44.

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