Secondary battery and electric device

Abstract

The invention provides a secondary battery and a power utilization device, the secondary battery comprises a positive pole piece and a negative pole piece, a negative active layer of the negative pole piece comprises a silicon-based material, and a positive active layer of the positive pole piece comprises a lithium transition metal oxide and lithium-containing phosphate of an olivine structure, the lithium-containing phosphate comprises secondary particles I formed by primary particles I, pores exist among the primary particles I in the secondary particles I, and the secondary particles I are of a porous structure; and in the cross section of the secondary particles I, the area of the area filled with the primary particles I accounts for 60%-85%. The secondary battery prepared by the invention has both low-temperature performance and energy density.

Description

Cross Reference to Related Applications

[0001] This application is based on the international patent application No. PCT / CN2025 / 095453, filed on May 16, 2025, entitled "Secondary battery and power consuming device", and claims priority to the international patent application, the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of batteries, in particular to a secondary battery and a power consuming device. BACKGROUND

[0003] In recent years, with the application range of secondary batteries becoming more and more extensive, secondary batteries are widely used in energy storage power supply systems such as water, fire, wind and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, etc.

[0004] In the field of secondary batteries, the battery system using silicon-based material as negative electrode and lithium transition metal oxide and olivine structure lithium-containing phosphate as positive electrode shows significant advantages. This system can have high energy density while considering good low temperature performance.

[0005] However, in this system: the expansion rate of silicon-based material is high, which can squeeze the positive electrode sheet and easily squeeze out the electrolyte from the positive electrode sheet, and the normal infiltration of electrolyte is crucial to the performance of the positive electrode sheet. The electrolyte being squeezed out will affect the electrolyte infiltration effect of the positive electrode sheet, and thus increase the internal resistance of the secondary battery and deteriorate the low temperature performance of the secondary battery. Therefore, how to balance the low temperature performance and energy density of the secondary battery becomes a technical problem to be solved. SUMMARY

[0006] The present application is made in view of the above-mentioned problems, and aims to provide a secondary battery and a power consuming device, which balance the low temperature performance and energy density.

[0007] To achieve the above-mentioned purpose, the first aspect of the present application provides a secondary battery, comprising a positive electrode sheet and a negative electrode sheet, the negative electrode active layer of the negative electrode sheet comprising a silicon-based material, the positive electrode active layer of the positive electrode sheet comprising a lithium transition metal oxide and an olivine structure lithium-containing phosphate, the lithium-containing phosphate comprising secondary particles I formed by primary particles I, the secondary particles I having a porous structure, and there being pores between the primary particles I in the secondary particles I, and in the cross section of the secondary particles I, the area ratio of the region filled with the primary particles I is 60%-85%.

[0008] In this application, the negative electrode comprises a silicon-based material, and the positive electrode comprises a lithium transition metal oxide, which is beneficial for achieving high energy density in the secondary battery. Adding lithium phosphate to the positive electrode can improve the low-temperature kinetic performance of the secondary battery. Furthermore, by controlling the area ratio to 60%-85%, it is beneficial for the formation of continuous capillary channels within the secondary particles I, thereby improving the electrolyte wetting performance of the positive electrode, increasing the lithium-ion transport efficiency within the positive electrode, reducing the internal resistance of the secondary battery, and further improving the low-temperature performance of the secondary battery, enabling the secondary battery to achieve both low-temperature performance and high energy density.

[0009] In some embodiments, the area filled by the primary particles I in the cross-section of the secondary particles I accounts for 60%-70% of the total area. This is more conducive to the formation of continuous capillary channels inside the secondary particles I, thereby improving the electrolyte wetting performance of the positive electrode and further enhancing the low-temperature performance of the secondary battery.

[0010] In some embodiments, the porosity of secondary particles I is 15%-40%. This indicates that secondary particles I have continuous capillary channels inside, which is beneficial for improving the electrolyte wetting performance of the positive electrode, thereby reducing the internal resistance of the secondary battery and improving the low-temperature performance of the secondary battery.

[0011] In some embodiments, the average pore size of secondary particles I is 20 nm-500 nm. This facilitates capillary action of secondary particles I, thereby increasing the flow rate of the electrolyte in the channels, improving the wetting effect of the positive electrode on the electrolyte, reducing the internal resistance of the secondary battery, and improving the low-temperature performance of the secondary battery.

[0012] In some embodiments, the average particle size of primary particles I is 20 nm-800 nm. Therefore, primary particles I can provide more ion insertion / extraction reaction sites, thereby further reducing the interfacial reaction impedance of the positive electrode, which is more conducive to reducing the internal resistance of the secondary battery, and thus improving the low-temperature performance of the secondary battery.

[0013] In some embodiments, the average particle size of secondary particles I is 4μm-13μm. This helps to maintain the particle size distribution of the positive electrode active material, thereby ensuring the high compaction density of the positive electrode sheet.

[0014] In some implementations, the mass percentage of lithium phosphate relative to the total mass of lithium transition metal oxide and lithium phosphate is 2%-10%. This allows for improved electrolyte wetting performance of the positive electrode without affecting battery energy density, thereby reducing the internal resistance of the secondary battery and improving its low-temperature performance.

[0015] In some embodiments, the chemical formula of the lithium phosphate includes: Li1+x Fe 1-y A y P 1-z E z O4, wherein -0.1≤x≤0.1, 0≤y≤0.5, 0≤z≤0.1; A is selected from one or more of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge; E is selected from one or more of B, Si, N, S, F, Cl, and Br. The above-mentioned lithium phosphates exhibit stable lattice structures and good kinetic performance at low temperatures, which can further improve the low-temperature performance of secondary batteries.

[0016] In some embodiments, the lithium transition metal oxide includes nickel, cobalt, and M, where M includes manganese and / or aluminum. Based on the total molar number of nickel, cobalt, and M, the molar percentage of nickel is 30%-95%. This is beneficial for improving the energy density of the secondary battery.

[0017] In some embodiments, the lithium transition metal oxide comprises primary particles II and / or secondary particles II. This is beneficial for improving the structural stability and ion insertion / extraction kinetics of the lithium transition metal oxide, thereby enhancing the cycle performance and kinetic performance of the secondary battery. It also allows for optimized particle size distribution, increasing the compaction density of the lithium transition metal oxide, thereby improving the energy density of the secondary battery.

[0018] In some embodiments, the average particle size of primary particles II is 1 μm-20 μm, and / or the average particle size of secondary particles II is 1 μm-20 μm. This helps to reduce the occurrence of crystal structure instability problems caused by excessively small sizes of primary and / or secondary particles II, and to reduce the compaction density of the electrode due to difficulties in particle gradation caused by excessively large sizes of primary and / or secondary particles II.

[0019] In some embodiments, the absolute value of the difference between the average particle size of secondary particles I and the average particle size of primary particles II is 0 μm-16 μm. This is beneficial for optimizing the particle size distribution of different positive electrode active materials in the positive electrode sheet, increasing the compaction density of the positive electrode sheet, and thus improving the energy density of the secondary battery.

[0020] In some embodiments, the absolute value of the difference between the average particle size of secondary particles II and the average particle size of secondary particles I is 0 μm-10 μm. This is beneficial for optimizing the particle size distribution of different active materials in the electrode, increasing the compaction density of the positive electrode, and thus improving the energy density of the secondary battery.

[0021] In some embodiments, the average particle size of secondary particles II is greater than that of secondary particles I, and / or, the average particle size of secondary particles I is greater than that of primary particles II. This particle size gradient can create a favorable particle size distribution, thereby optimizing the pore structure, promoting capillary penetration of the electrolyte, and further improving the wettability of the positive electrode sheet to the electrolyte, thus reducing the internal resistance of the secondary battery and enhancing its low-temperature performance.

[0022] In some embodiments, the mass ratio of primary particle II, secondary particle II, and secondary particle I is (20-50):(48-70):(2-10). This improves the low-temperature performance of the battery without affecting its energy density, and also enhances the wettability of the positive electrode to the electrolyte, thereby improving the low-temperature performance of the secondary battery.

[0023] In some implementations, the positive electrode sheet satisfies one or more of the following characteristics: (1) The compaction density of the positive electrode sheet is 3.55 g / cm³. 3 -3.7g / cm 3 (2) The coating weight on one side of the positive electrode sheet is 10 mg / cm². 2 -30mg / cm 2 (3) The specific surface area of ​​the positive electrode is 1.0 m². 2 / g-1.3m 2 / g.

[0024] Therefore, the positive electrode has a high porosity and good pore connectivity, which is beneficial to improving the wettability of the positive electrode to the electrolyte, thereby reducing the internal resistance of the secondary battery and improving the low-temperature performance of the secondary battery.

[0025] In some embodiments, the silicon-based material includes one or more of elemental silicon, silicon-carbon composites, silicon-oxygen materials, and silicon alloys.

[0026] In some embodiments, the silicon-based material includes a silicon-carbon composite that satisfies one or more of the following characteristics: (1) Silicon-carbon composites include porous carbon and silicon-containing materials dispersed in the pores of the porous carbon; (2) The silicon-carbon composite also includes a carbon-containing coating layer, which is located on the surface of porous carbon and / or silicon-containing materials; (3) The mass percentage of silicon in the silicon-carbon composite is 1%-70%; (4) The average particle size of the silicon-carbon composite is 2 μm-15 μm; (5) The resistivity of silicon-carbon composite powder at 8 MPa is 4 Ω·cm-17 Ω·cm; (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m².2 / g-6.7m 2 / g.

[0027] In some embodiments, the compaction density of the negative electrode sheet is 1.65 g / cm³. 3 -1.85g / cm 3 And / or, the coating weight of the negative electrode is 7 mg / cm³. 2 -15mg / cm 2 This is beneficial for obtaining higher energy density.

[0028] In some embodiments, the secondary battery further includes an electrolyte comprising a lithium salt and a solvent, and the electrolyte satisfies one or more of the following characteristics: (1) The conductivity of the electrolyte is 10 mS / cm-15 mS / cm. This is beneficial to improving the migration ability of lithium ions in the liquid phase, thereby reducing the lithium ion diffusion resistance of the lithium battery and obtaining better power performance.

[0029] (2) Lithium salts include one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium tri(trifluoromethanesulfonyl)methyl, lithium hexafluorosilicate, lithium bis(oxalateborate)borate, and lithium difluoroborate.

[0030] (3) The solvents include carboxylic acid ester solvents and / or cyclic carbonate solvents.

[0031] The second aspect of this application provides an electrical device, which includes the secondary battery of the first aspect. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of a battery cell according to one embodiment of this application; Figure 2 yes Figure 1 An exploded view of a battery cell according to one embodiment of this application is shown. Figure 3 This is a schematic diagram of a battery module according to one embodiment of this application; Figure 4 This is a schematic diagram of a battery pack according to one embodiment of this application; Figure 5 yes Figure 4 An exploded view of a battery pack according to one embodiment of this application is shown; Figure 6 This is a schematic diagram of an electrical device that uses a secondary battery as a power source according to one embodiment of this application; Figure 7 This is a scanning electron microscope image of the positive electrode film layer in Embodiment 1 of this application; Figure 8This is another scanning electron microscope image of the positive electrode film layer in Embodiment 1 of this application.

[0033] Explanation of reference numerals in the attached figures: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly. Detailed Implementation

[0034] Hereinafter, embodiments of the secondary battery and power-consuming device of this application will be described in detail with appropriate reference to the accompanying drawings. 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 making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0035] 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.

[0036] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0037] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0038] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means 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.

[0039] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0040] Unless otherwise specified, 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 this application.

[0041] In this application, the term "primary particle" refers to a particle that appears as a single, integral unit under an electron microscope. The particle has complete boundaries, and while defects may exist within the particle, there are no complete boundaries within the particle sufficient to divide it into two or more particles.

[0042] In this application, the term "secondary particle" refers to a particle formed by multiple primary particles. Secondary particles typically have a relatively regular shape, which may be spherical or quasi-spherical. Grain boundaries may exist between adjacent primary particles in a secondary particle.

[0043] To improve the energy density of secondary batteries, lithium transition metal oxides (i.e., ternary materials) are typically used as the positive electrode active material. The crystal structure of ternary materials shrinks to some extent at low temperatures, making the insertion / extraction process of lithium ions more difficult. Lithium ions need to overcome greater resistance to escape or insert into the electrode material's lattice, thus limiting the battery's charge / discharge rate and power output.

[0044] The related technology simultaneously adds ternary materials and lithium phosphate with olivine structure to the positive electrode. Under low temperature conditions, the lithium phosphate with olivine structure can still maintain a stable crystal structure, providing effective sites for lithium ion extraction or insertion, thereby improving the low temperature performance of the battery.

[0045] Based on this, in order to further improve the energy density of the battery, related technologies have been disclosed, which use silicon-carbon materials as the negative electrode active material in the negative electrode sheet.

[0046] The applicant's research revealed that silicon-based materials have a high expansion rate, which can compress the positive electrode sheet and easily squeeze the electrolyte out. Proper electrolyte wetting is crucial for the performance of the positive electrode sheet; electrolyte squeezing affects the wetting effect, leading to increased internal resistance in the secondary battery and limiting the improvement of low-temperature performance. Therefore, balancing the energy density and low-temperature performance of secondary batteries has become an urgent technical problem to be solved.

[0047] Based on this, this application provides a secondary battery and an electrical device that balance low-temperature performance and energy density.

[0048] Secondary batteries The first aspect of this application provides a secondary battery, which includes a positive electrode and a negative electrode. The negative electrode active layer of the negative electrode includes a silicon-based material, and the positive electrode active layer of the positive electrode includes a lithium transition metal oxide and a lithium phosphate with an olivine structure. The lithium phosphate includes secondary particles I formed from primary particles I. The secondary particles I have a porous structure, and there are pores between the primary particles I in the secondary particles I. In the cross-section of the secondary particles I, the area of ​​the region filled by the primary particles I accounts for 60%-85%.

[0049] In this application, the negative electrode comprises a silicon-based material, and the positive electrode comprises a lithium transition metal oxide, which is beneficial for achieving high energy density in the secondary battery. The positive electrode also includes a lithium phosphate with an olivine structure. The lithium phosphate with an olivine structure has a low discharge potential, and in the low SOC (State of Change) stage, the lithium phosphate with an olivine structure can preferentially discharge the lithium transition metal oxide. Furthermore, since the lithium phosphate has good kinetic performance at low temperatures, adding lithium phosphate to the positive electrode can improve the low-temperature kinetic performance of the secondary battery.

[0050] Furthermore, in this application, the "area percentage of the region filled by primary particles I" can be used to reflect the pore size and connectivity of the pores in secondary particles I. When the area percentage is greater than or equal to 60%, it means that the primary particles I are arranged relatively tightly. At this time, the pore size between primary particles I is usually small, and these small pores can easily form channels with capillary function. When the area percentage is less than or equal to 85%, it indicates that the space occupied by primary particles I in the cross-section of secondary particles I is relatively small, and correspondingly, the remaining pore area will be larger. A larger pore area helps the pores to connect with each other, forming continuous channels for ion conduction. Therefore, by controlling the area percentage to 60%-85%, it is beneficial to form continuous capillary channels inside secondary particles I, thereby improving the electrolyte wetting performance of the positive electrode, thereby improving the lithium ion transport efficiency of the positive electrode, reducing the internal resistance of the secondary battery, and further improving the low-temperature performance of the secondary battery, enabling the secondary battery to achieve both low-temperature performance and high energy density.

[0051] For example, in the cross-section of secondary particle I, the area percentage of the region filled by primary particle I is a value between 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, or any combination thereof.

[0052] In some embodiments, the area filled by the primary particles I in the cross-section of the secondary particles I accounts for 60%-70%. This is more conducive to the formation of continuous capillary channels inside the secondary particles I, improving the electrolyte wetting performance of the positive electrode, giving the positive electrode high wetting performance, thereby improving the ion migration resistance and interfacial reaction resistance of the positive electrode, further reducing the internal resistance of the secondary battery, and further improving the low-temperature performance of the secondary battery.

[0053] In this application, the area ratio of the region filled by primary particle I in the cross-section of secondary particle I can be determined by scanning electron microscopy (SEM). Specifically: the positive electrode film is peeled off from the lithium battery, the peeled positive electrode film is fixed on the sample stage, the sample stage is installed and locked in place on the sample holder, the power of the argon ion cross-section polisher (e.g., JEOL IB-09010 CP type argon ion cross-section polisher) is turned on and a vacuum is drawn (e.g., 10...). -7Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to swing mode to begin polishing. After polishing, observe the cross-section of the positive electrode film using a scanning electron microscope. Granular olivine-structured lithium phosphate and lithium transition metal oxides can be observed. The porous particles are secondary particles I, which are lithium phosphate particles with an olivine structure. In a cross-section of a secondary particle I, measure the area of ​​the cross-section and the area of ​​the region filled by primary particles I within that cross-section, and calculate the area percentage of the region filled by primary particles I in the cross-section of the secondary particle I. Reselect secondary particles I and test the area percentage of the region filled by primary particles I in the cross-section of the secondary particle I using the same method. Average the multiple area percentages obtained from the tests; this average represents the area percentage of the region filled by primary particles I in the cross-section of the secondary particle I.

[0054] In some embodiments, the porosity of secondary particles I is 15%-40%, optionally 30%-40%. A porosity within this range indicates that the secondary particles I have continuous capillary channels, which is beneficial for improving the electrolyte wetting performance of the positive electrode, resulting in high wetting performance. This improves the ion migration resistance and interfacial reaction resistance of the positive electrode, thereby reducing the internal resistance of the secondary battery and improving its low-temperature performance. Exemplarily, the porosity of secondary particles I is a value within a range of 15%, 20%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, or any combination thereof.

[0055] In this application, the porosity of secondary particles I can be determined by scanning electron microscopy with an ion-polished cross section. Specifically: the positive electrode film is peeled off from the lithium battery, the peeled positive electrode film is fixed on the sample stage, the sample stage is installed in the sample holder and locked in place, the power of the argon ion cross section polisher (e.g., the IB-09010 CP type argon ion cross section polisher from JEOL Corporation of Japan) is turned on and a vacuum is drawn (e.g., 10...). -7Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to rocking mode to begin polishing. After polishing, observe the cross-section of the positive electrode film using a scanning electron microscope. Granular olivine-structured lithium phosphate and lithium transition metal oxides can be observed. The porous particles are secondary particles I, which are lithium phosphate particles with an olivine structure. Measure the area of ​​the cross-section of a secondary particle I and the area of ​​the pore region (the area not filled by the primary particle I) within that cross-section to calculate the porosity. Select another secondary particle I and test its porosity using the same method. Average the multiple porosities obtained to obtain the porosity of the secondary particle I.

[0056] In some embodiments, the average pore size of secondary particles I is 20 nm to 500 nm, optionally 50 nm to 200 nm. An average pore size within this range facilitates capillary action, thereby increasing the electrolyte flow rate within the pores and improving the wetting effect of the positive electrode on the electrolyte. This results in high wetting performance of the positive electrode, improving the ion migration resistance and interfacial reaction resistance of the positive electrode, and consequently reducing the internal resistance of the secondary battery, thus enhancing its low-temperature performance. For example, the average pore size of secondary particles I is a value within a range of 20 nm, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, or any combination thereof.

[0057] In this application, the average pore size of secondary particles I can be determined by scanning electron microscopy with an ion-polished cross section. Specifically: the positive electrode film is peeled off from the lithium battery, the peeled positive electrode film is fixed on the sample stage, the sample stage is installed and locked in place on the sample holder, the power of the argon ion cross section polisher (e.g., the IB-09010 CP type argon ion cross section polisher from JEOL Corporation of Japan) is turned on and a vacuum is applied (e.g., 10...). -7Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to rocking mode to begin polishing. After polishing, observe the cross-section of the positive electrode film using a scanning electron microscope. Granular olivine-structured lithium phosphate and lithium transition metal oxides can be observed. The porous particles are secondary particles I, consisting of lithium phosphate with an olivine structure. In the cross-section of a secondary particle I, measure the cross-sectional width of each pore (i.e., the maximum value between any two points on the edge of the pore's cross-section). Reselect secondary particle I and test the cross-sectional width of each pore using the same method. Average the measured cross-sectional widths of multiple pores to obtain the average pore diameter of secondary particle I.

[0058] In some embodiments, the average particle size of primary particle I is 20 nm-800 nm, optionally 50 nm-200 nm. Therefore, primary particle I has a smaller average particle size, resulting in a larger specific surface area, providing more ion insertion / extraction reaction sites, thereby further reducing the interfacial reaction impedance of the positive electrode, which is more conducive to reducing the internal resistance of the secondary battery, and thus improving the low-temperature performance of the secondary battery. Exemplarily, the average particle size of primary particle I is a value within a range of 20 nm, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, or any combination thereof.

[0059] In this application, the average particle size of primary particle I can be determined by scanning electron microscopy with an ion polished cross section. Specifically: the positive electrode film is peeled off from the lithium battery, the peeled positive electrode film is fixed on the sample stage, the sample stage is installed in the sample holder and locked in place, the power of the argon ion cross section polisher (e.g., the IB-09010 CP type argon ion cross section polisher from JEOL Corporation of Japan) is turned on and a vacuum is drawn (e.g., 10...). -7Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to swing mode to begin polishing. After polishing, observe the cross-section of the positive electrode film using a scanning electron microscope. Granular olivine-structured lithium phosphate and lithium transition metal oxides can be observed. The porous particles are secondary particles I of the olivine-structured lithium phosphate. In the cross-section of a secondary particle I, measure the cross-sectional width of each primary particle I in that cross-section (i.e., the maximum distance between any two points on the edge of a primary particle I is the cross-sectional width). Reselect secondary particles I and test the cross-sectional width of each primary particle I in the cross-section using the same method. Average the cross-sectional widths of the multiple primary particles I obtained to obtain the average particle size of the primary particles I.

[0060] In some embodiments, the average particle size of secondary particles I is 4 μm-13 μm. An average particle size of secondary particles I within this range is beneficial for maintaining the particle size distribution of the positive electrode active material, thereby ensuring the high compaction density of the positive electrode sheet. Exemplarily, the average particle size of secondary particles I is a value within a range of 4 μm, 6 μm, 8 μm, 10 μm, 13 μm, or any combination thereof.

[0061] In this application, the average particle size of secondary particles I can be tested using equipment and methods known in the art. For example, a scanning electron microscope (e.g., ZEISS Sigma 300) can be used. As an example, the test can be performed as follows: A small amount of positive electrode active material test sample is applied to the conductive adhesive on the SEM sample stage. Multiple test areas (e.g., 5) are randomly selected in the test sample, and the particle size of each secondary particle I in each test area is read at a certain magnification (e.g., 1000x). The distance between the two farthest points on the particle is taken as the particle size. The number and particle size of secondary particles I in each test area are counted, and the arithmetic mean of the secondary particles I in each test area is taken as the average particle size of secondary particles I in the test sample. To ensure the accuracy of the test results, the above test can be repeated with multiple test samples (e.g., 10), and the average value of each test sample is taken as the final test result.

[0062] In some embodiments, the mass percentage of lithium phosphate relative to the total mass of lithium transition metal oxide and lithium phosphate is 2%-10%, optionally 3%-5%. A mass percentage of lithium phosphate within this range can improve the electrolyte wetting performance of the positive electrode without affecting the battery energy density, resulting in high wetting performance of the positive electrode. This improves the ion migration resistance and interfacial reaction resistance of the positive electrode, thereby reducing the internal resistance of the secondary battery and improving its low-temperature performance. For example, the mass percentage of lithium phosphate relative to the total mass of lithium transition metal oxide and lithium phosphate is a value within the range of 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination thereof.

[0063] In some embodiments, the chemical formula of the lithium phosphate includes: Li 1+x Fe 1-y A y P 1-z E z O4, wherein -0.1≤x≤0.1, 0≤y≤0.5, 0≤z≤0.1; A is selected from one or more of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge; E is selected from one or more of B, Si, N, S, F, Cl, and Br. The above-mentioned lithium phosphates have stable crystal structures and good low-temperature kinetic performance, which can further improve the low-temperature performance of secondary batteries.

[0064] In some embodiments, the lithium transition metal oxide includes nickel, cobalt, and element M, where element M includes manganese and / or aluminum. Based on the total molar number of nickel, cobalt, and element M, the molar percentage of nickel is 30%-95%, optionally 60%-90%. Therefore, the higher nickel content in the lithium transition metal oxide is beneficial for improving the energy density of the secondary battery. For example, based on the total molar number of nickel, cobalt, and element M, the molar percentage of nickel is a value within a range of 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, or any combination thereof.

[0065] In some embodiments, the lithium transition metal oxide includes primary particles II and / or secondary particles II. Optionally, the lithium transition metal oxide includes both primary and secondary particles II. Primary particles II have high mechanical strength and exhibit minimal volume expansion during battery charging and discharging, while secondary particles II have finer grains, which can shorten the ion diffusion path. By mixing primary and secondary particles II, the structural stability and ion insertion / extraction kinetics of the lithium transition metal oxide can be improved, thereby enhancing the cycle performance and kinetic performance of the secondary battery. Furthermore, mixing primary and secondary particles II can optimize particle size distribution and increase the compaction density of the lithium transition metal oxide, thereby increasing the energy density of the secondary battery.

[0066] In some embodiments, the average particle size of primary particles II is 1 μm-20 μm, and / or the average particle size of secondary particles II is 1 μm-20 μm. Having the average particle sizes of primary and secondary particles II within these ranges helps to reduce the occurrence of crystal structure instability caused by excessively small sizes of primary and / or secondary particles II, and to reduce particle gradation difficulties caused by excessively large sizes of primary and / or secondary particles II, thereby reducing the compaction density of the electrode. For example, the average particle size of primary particles II is a value within a range of 1 μm, 3 μm, 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm, 20 μm, or any combination thereof, and the average particle size of secondary particles II is a value within a range of 1 μm, 3 μm, 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm, 20 μm, or any combination thereof.

[0067] In some embodiments, the absolute value of the difference between the average particle size of secondary particles I and the average particle size of primary particles II is 0 μm-16 μm. This facilitates optimization of the particle size distribution of different positive electrode active materials (secondary particles I and primary particles II) in the positive electrode sheet, increases the compaction density of the positive electrode sheet, and thus improves the energy density of the secondary battery. For example, the absolute value of the difference between the average particle size of secondary particles I and the average particle size of primary particles II is a value within a range of 0 μm, 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, or any combination thereof.

[0068] In some embodiments, the absolute value of the difference between the average particle size of secondary particles II and the average particle size of secondary particles I is 0 μm-10 μm. This facilitates optimization of the particle size distribution of different active materials in the electrode, increases the compaction density of the positive electrode, and thus improves the energy density of the secondary battery. For example, the absolute value of the difference between the average particle size of secondary particles II and the average particle size of secondary particles I is a value within a range of 0 μm, 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, or any combination thereof.

[0069] In some embodiments, the average particle size of secondary particles II is greater than that of secondary particles I, and / or, the average particle size of secondary particles I is greater than that of primary particles II. This particle size gradient creates a good particle size distribution. The large-diameter secondary particles II serve as the framework supporting the electrode structure, the medium-diameter secondary particles I fill some of the pores, and the small-diameter primary particles II further fill the pores, resulting in a more compact overall structure and a more uniform pore distribution on the positive electrode. This optimizes the pore structure, promotes capillary penetration of the electrolyte, and further improves the wettability of the positive electrode to the electrolyte, giving it high wettability. This improves the ion migration resistance and interfacial reaction resistance of the positive electrode, thereby reducing the internal resistance of the secondary battery and enhancing its low-temperature performance.

[0070] In this application, the average particle size of primary particle II and secondary particle II can be determined, for example, by using the test method for the average particle size of primary particle I and secondary particle I described above.

[0071] In some embodiments, the porosity of primary particles II is 0%-10%. Exemplarily, the porosity of primary particles II is a value between 0%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination thereof.

[0072] In some embodiments, the porosity of the secondary particles II is 0%-10%. Exemplarily, the porosity of the secondary particles II is a value between 0%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any combination thereof.

[0073] In this application, the above-described method can be used to polish the positive electrode film. After polishing, the cross-section of the positive electrode film is observed using SEM, revealing granular olivine-containing lithium phosphate and lithium transition metal oxide structures. Using the porosity testing method for secondary particles I described above, the porosity of each particle in the cross-section of the positive electrode film is measured, and primary particles II, secondary particles II, and secondary particles I are distinguished based on their porosity.

[0074] In some embodiments, the mass ratio of primary particle II, secondary particle II, and secondary particle I is (20-50):(48-70):(2-10). This improves the low-temperature performance of the battery without affecting its energy density, enhances the wettability of the positive electrode to the electrolyte, improves the ion migration resistance and interfacial reaction resistance of the positive electrode, and thus reduces the internal resistance of the secondary battery, thereby improving its low-temperature performance.

[0075] In this application, the mass ratio of primary particle II, secondary particle II, and secondary particle I can be determined by inductively coupled plasma mass spectrometry (ICP) or SEM-EDS and SEM. Specifically, the mass ratio of primary particle II, secondary particle II, and Li can be estimated by measuring the Ni / Fe content using ICP or SEM-EDS. 1+x Fe 1-y A y P 1-z E z The mass ratio of O4 (secondary particles I) can be easily distinguished from primary particles II by observing the cross-sectional morphology of the electrode using SEM. The mass ratio of the two particles can be approximated by their distribution ratio in the electrode.

[0076] In some embodiments, the chemical formula of the lithium transition metal oxide includes: Li a Ni b Co c M d O f Where M includes Mn and / or Al; 0.8≤a≤1.1, 0.3≤b<1, 0.04≤c<1, 0.05≤d<1, 1.8≤f≤2.2, b+c+d =1, optionally, 0.3≤b≤0.96, 0.02≤c≤0.3, 0.02≤d≤0.3, more optionally, 0.6≤b≤0.9, 0.05≤c≤0.2, 0.05≤d≤0.2.

[0077] In some implementations, the positive electrode sheet satisfies one or more of the following characteristics: (1) The compaction density of the positive electrode sheet is 3.55 g / cm³. 3 -3.7g / cm 3 For example, the compaction density of the positive electrode sheet is 3.55 g / cm³. 3 3.56g / cm 3 3.58g / cm 3 3.6g / cm 3 3.62g / cm 3 3.64 g / cm 3 3.65g / cm 3 3.7g / cm3 Or the value between any two of them within a range; (2) The coating weight on one side of the positive electrode sheet is 10 mg / cm². 2 -30mg / cm 2 The coating weight on one side of the positive electrode is 10 mg / cm². 2 15mg / cm 2 20mg / cm 2 25mg / cm 2 30mg / cm 2 Or the value between any two of them within a range; (3) The specific surface area of ​​the positive electrode is 1.0 m². 2 / g-1.3m 2 / g, for example, the specific surface area of ​​the positive electrode is 1.0m². 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g or a range between the values ​​of either / g or any two of them.

[0078] The compaction density, single-sided coating weight, and specific surface area of ​​the positive electrode sheet are within the above range, which gives the positive electrode sheet a high porosity and good pore connectivity. This can shorten the transport path of the electrolyte in the positive electrode sheet, which is beneficial to improving the wettability of the positive electrode sheet to the electrolyte. This results in a high wettability of the positive electrode sheet, thereby improving the ion migration resistance and interfacial reaction resistance of the positive electrode sheet, reducing the internal resistance of the secondary battery, and thus improving the low-temperature performance of the secondary battery.

[0079] In this application, the compaction density of the positive electrode sheet has a meaning known in the art and can be measured using instruments and methods known in the art. The compaction density of the positive electrode sheet = the areal density of the positive electrode film layer / the thickness of the positive electrode film layer. The thickness of the positive electrode film layer can be measured using instruments and methods known in the art. For example, a micrometer (e.g., Mitutoyo 293-100, with an accuracy of 0.1 μm) can be used to measure the thickness of the positive electrode sheet at at least 12 different locations along the thickness direction. The average value is then taken as the thickness of the positive electrode sheet, and the thickness of the positive current collector is subtracted to obtain the thickness of the positive electrode film layer. The areal density of the positive electrode film layer can be measured using instruments and methods known in the art. For example, a cold-pressed positive electrode sheet or a positive electrode sheet obtained after disassembling a battery (if it is a double-sided coated positive electrode sheet, first wipe off the positive electrode film layer on one side), can be punched into small circular pieces with an area of ​​S1, weighed, and recorded as M1. Then, wipe off the positive electrode film layer of the weighed positive electrode sheet, weigh the positive current collector, and record it as M0. The surface density of the positive electrode film layer = (M1-M0) / S1.

[0080] In this application, the specific surface area of ​​the positive electrode sheet has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be measured using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017, and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0081] In some embodiments, the silicon-based material includes one or more of elemental silicon, silicon-carbon composites, silicon-oxygen materials, and silicon alloys.

[0082] In some embodiments, the silicon-based material includes a silicon-carbon composite that satisfies one or more of the following characteristics: (1) The silicon-carbon composite includes porous carbon and silicon-containing materials dispersed in the pores of the porous carbon; optionally, the porous carbon is hard carbon; (2) The silicon-carbon composite also includes a carbon-containing coating layer, which is located on the surface of porous carbon and / or silicon-containing materials; (3) The mass percentage of silicon in the silicon-carbon composite is 1%-70%; (4) The average particle size of the silicon-carbon composite is 2 μm-15 μm, and optionally, 7 μm-11 μm; (5) The resistivity of silicon-carbon composite powder at 8 MPa is 4 Ω·cm-17 Ω·cm; (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g.

[0083] In some embodiments, the silicon-carbon composite includes a core comprising porous carbon and silicon-containing material dispersed in the pores of the porous carbon. The porous carbon, acting as a carrier for the silicon-containing material, provides support for the nanoscale silicon-containing material and, simultaneously, provides expansion space for the expansion of the silicon nanoparticles, effectively mitigating the stress compression caused by expansion during charging. Especially when the silicon-containing particles are in the nanometer range, the specific capacity is higher and dispersion in the pores of the porous carbon is facilitated. Furthermore, the buffering effect of the porous carbon's pores on expansion can be more fully utilized. When this silicon-carbon composite is applied in a wound electrode assembly, it can significantly alleviate the stretching of the outer negative electrode sheet caused by silicon expansion.

[0084] In some implementations, the porous carbon is hard carbon. When the porous carbon is hard carbon, it has stronger support, a more stable pore structure, and is harder, thus providing better porosity for the negative electrode active layer, providing a smoother path for active ion transport, and improving the charging capability of the battery cell.

[0085] In some embodiments, the silicon-containing material includes at least one of elemental silicon, silicon oxides, silicon nitrides, and silicon alloys. In some embodiments, the silicon-containing material includes crystalline silicon, thereby further improving the structural stability of the silicon-containing material and the energy density of the battery cell.

[0086] In some embodiments, the silicon-carbon composite further includes a carbon-containing coating layer that coats the surface of the core. This can improve the conductivity of the silicon-carbon composite and reduce the internal impedance of the battery cell, while also effectively reducing the probability of direct contact between the silicon-containing material in the porous carbon channels and the external environment, thereby improving the chemical stability of the silicon-carbon composite.

[0087] In some embodiments, the mass percentage of silicon in the silicon-carbon composite is 1%-70%. This approach, while maximizing the specific capacity of the negative electrode active material using silicon, also facilitates the full dispersion of silicon in the carbon-containing porous material and helps control the expansion of silicon during charging. For example, the mass percentage of silicon in the silicon-carbon composite is a value within a range of 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, or any combination thereof.

[0088] In this application, the method for testing the mass percentage of silicon in the silicon-carbon composite can be a method known in the art. As an example, the following method can be used: take a certain amount of silicon-carbon composite, obtain the mass of silicon in the silicon-carbon composite by inductively coupled plasma optical emission spectrometry (ICP-OES), and calculate the mass percentage of silicon in the silicon-carbon composite.

[0089] In addition to providing structural support and buffering for the expansion of silicon materials, the pores in silicon-carbon composites also form between the particles, further improving the ion flow effect of intraparticle and interparticle pores. In some embodiments, the average particle size of the silicon-carbon composite is 2μm-15μm, optionally 7μm-11μm, and more preferably 5μm-10μm. This facilitates increasing the compactness of the negative electrode active layer by utilizing the interparticle gaps, thereby further improving the energy density of the battery cell.

[0090] The average particle size of the aforementioned silicon-carbon composite can be tested using equipment and methods known in the art. For example, a scanning electron microscope (e.g., ZEISS Sigma 300) can be used, referring to JY / T010-1996, to obtain a scanning electron microscope (SEM) image of the negative electrode sheet. As an example, the test can be performed as follows: Randomly select a test sample of length × width = 50 mm × 100 mm on the negative electrode sheet. Randomly select multiple test areas (e.g., 5 areas) within the test sample, and at a certain magnification (e.g., 1000x when measuring silicon-carbon composites), read the particle size of each silicon-carbon composite particle in each test area (i.e., take the distance between the two farthest points on the silicon-carbon composite particle as the particle size). Count the number and particle size values ​​of silicon-carbon composite particles in each test area, and take the arithmetic mean of the silicon-carbon composite particles in each test area, which is the average particle size of the silicon-carbon composite particles in the test sample. To ensure the accuracy of the test results, multiple test samples (e.g., 10) can be taken and the above test can be repeated. The average value of each test sample can be taken as the final test result.

[0091] In some embodiments, the silicon-carbon composite powder resistivity at 8 MPa is 4 Ω·cm to 17 Ω·cm. Controlling the powder resistivity as described above improves the conductivity of the silicon-carbon composite, thereby increasing the charging rate of the battery cell. Exemplarily, the silicon-carbon composite powder resistivity at 8 MPa is a value within a range of 4 Ω·cm, 5 Ω·cm, 6 Ω·cm, 7 Ω·cm, 8 Ω·cm, 9 Ω·cm, 10 Ω·cm, 11 Ω·cm, 12 Ω·cm, 13 Ω·cm, 14 Ω·cm, 15 Ω·cm, 16 Ω·cm, 17 Ω·cm, or any combination thereof.

[0092] In this application, the powder resistivity of silicon-carbon composites can be determined using methods known in the art. As an example, a four-probe method can be used, where two probes apply voltage and the other two probes measure current. The powder resistivity can be calculated by measuring the resistance value. Models of four-probe semiconductor powder resistivity testers include the ST-2722.

[0093] In some embodiments, the BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g. For example, the specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g, 2.0m 2 / g, 3.0m 2 / g, 4.0m 2 / g, 5.0m 2 / g, 6.0m 2 / g, 6.7m 2 / g or a range between the values ​​of either / g or any two of them.

[0094] In this application, the method for testing the BET specific surface area of ​​the silicon-carbon composite can be a method known in the art. As an example, referring to GB / T 19587-2017, the nitrogen adsorption specific surface area analysis method can be used. The sample tube containing the first graphite material sample is immersed in liquid nitrogen at -196℃, and the amount of nitrogen adsorbed on the surface of the solid sample at different pressures of 0.05-0.30 is measured. Based on the BET multilayer adsorption theory and calculation formula, the amount of monolayer adsorption of the sample is obtained, and thus the BET specific surface area is obtained. This test can be performed using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA.

[0095] In some embodiments, the compaction density of the negative electrode sheet is 1.65 g / cm³. 3 -1.85g / cm 3 And / or, the coating weight of the negative electrode is 7 mg / cm³. 2 -15mg / cm 2 The compaction density and coating weight of the negative electrode sheet are within the above range, which is beneficial for obtaining a higher energy density. For example, the compaction density of the negative electrode sheet is 1.65 g / cm³. 3 1.7g / cm 3 1.75g / cm 3 1.8g / cm 3 1.85g / cm 3 Or a value between any two of these ranges, and / or, the coating weight of the negative electrode is 7 mg / cm³. 2 9mg / cm 2 10mg / cm 2 13mg / cm 2 15mg / cm 2 Or the value between any two of them within a range.

[0096] In this application, the compaction density of the negative electrode sheet can be determined, for example, by using the test method for the compaction density of the positive electrode sheet described above.

[0097] In some embodiments, the secondary battery also includes an electrolyte comprising a lithium salt and a solvent.

[0098] In some embodiments, the conductivity of the electrolyte is between 10 mS / cm and 15 mS / cm. An electrolyte conductivity within this range is beneficial for improving the migration ability of lithium ions in the liquid phase, thereby reducing the lithium-ion diffusion resistance of the lithium battery and achieving better power performance. Exemplarily, the electrolyte conductivity is a value within a range of 10 mS / cm, 11 mS / cm, 12 mS / cm, 13 mS / cm, 14 mS / cm, 15 mS / cm, or any combination thereof.

[0099] In this application, the conductivity of the electrolyte has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be tested using a conductivity meter according to HG / T 4067-2015. Specifically, the electrolyte sample is poured into a dry and clean plastic centrifuge tube and placed in a constant temperature bath at 25°C for 30 minutes. The platinum black electrode (DJS-1C type) is washed with pure water and dried. The dried electrode is vertically placed into the uniform sample to be tested, and the conductivity is measured using a conductivity meter (Leici, DDSJ-318). After the test value stabilizes, the reading is taken. The test is repeated three times, and the average value is taken.

[0100] In some embodiments, the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium tris(trifluoromethanesulfonyl)methyl lithium, lithium hexafluorosilicate, lithium bis(oxalato)borate, and lithium difluoroborate.

[0101] In some embodiments, the lithium salt constitutes 8%-20% of the electrolyte by mass. Exemplarily, the lithium salt constitutes 8%, 10%, 12%, 14%, 16%, 18%, 20% of the electrolyte by mass, or a range of any two of these.

[0102] In some embodiments, the solvent includes carboxylic acid ester solvents and / or cyclic carbonate solvents. Optionally, the solvent includes carboxylic acid ester solvents.

[0103] In some embodiments, the carboxylic acid ester solvent includes one or more compounds having the following structure: R1-(C=O)-O-R2, wherein R1 and R2 each independently comprise a substituted or unsubstituted alkyl group.

[0104] In some embodiments, the carboxylic acid ester solvent includes one or more of ethyl acetate, methyl acetate, methyl formate, butyl acetate, methyl propionate, ethyl propionate, methyl butyrate, propyl butyrate, butyl butyrate, isopropyl acetate, and isoamyl acetate.

[0105] In some embodiments, the carboxylic acid ester solvent accounts for 5%-80% of the electrolyte by mass. Exemplarily, the carboxylic acid ester solvent accounts for 5%, 10%, 20%, 40%, 60%, 80% of the electrolyte by mass, or a range of any two of these.

[0106] In some embodiments, the cyclic carbonate solvent includes one or more of ethylene carbonate, propylene carbonate, and butylene carbonate.

[0107] In some embodiments, the cyclic carbonate solvent constitutes 10%-95% of the electrolyte by mass. Exemplarily, the cyclic carbonate solvent constitutes 10%, 20%, 40%, 60%, 80%, 90%, 95% of the electrolyte by mass, or a range consisting of any two of these.

[0108] The olivine-structured lithium phosphate in this application can be prepared, for example, by the following method: S1, after uniformly mixing lithium source, iron source, phosphorus source and first carbon source, the first sintering treatment is carried out in an inert gas atmosphere to obtain the first sintered product.

[0109] In some implementations, the lithium source includes one or more of lithium phosphate, lithium carbonate, and lithium nitrate.

[0110] In some embodiments, the phosphorus source and iron source include one or more of ferric phosphate, ferric phosphate dihydrate, and ammonium dihydrogen phosphate.

[0111] In some embodiments, the first carbon source includes one or more of ascorbic acid, cellulose, polypropylene, epoxy resin, sucrose, glucose, fructose, citric acid, polyethylene glycol, starch, and phenolic resin.

[0112] In some embodiments, the inert gas includes one or more of nitrogen, argon, and helium.

[0113] In some embodiments, the first sintering treatment is performed at 200°C-500°C for 2-8 hours. Exemplarily, the temperature of the first sintering treatment is a value within a range of 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, or any combination thereof. The duration of the first sintering treatment is a value within a range of 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, or any combination thereof.

[0114] In some embodiments, the molar ratio of lithium source to iron source is (1-1.05):(0.95-1.05), and the amount of phosphorus source added is not specifically limited. A first carbon source is added to the mixed powder of lithium source, iron source, and phosphorus source, and the first carbon source accounts for 1%-5% of the mass of the powder, optionally 1.5%-3%.

[0115] S2, the first sintered product prepared above is mixed with a pore-forming agent and a solvent, and then subjected to ultrafine grinding to obtain primary particles I containing lithium phosphate with an olivine structure.

[0116] This application does not specify the type of solvent; solvents known in the art may be used.

[0117] In some embodiments, the pore-forming agent includes one or more of methylcellulose, natural fibers, carboxymethylcellulose, polystyrene, and polyethylene glycol.

[0118] In some embodiments, the solvent includes deionized water and / or methanol.

[0119] In some embodiments, the mass ratio of the first sintered product to the pore-forming agent is 1:(0.02-0.05).

[0120] In some implementations, the ultrafine grinding process satisfies one or more of the following characteristics: (1) The grinding speed is 5 r / min to 20 r / min. For example, the grinding speed is a value between 5 r / min, 10 r / min, 15 r / min, 20 r / min or any combination thereof; (2) The grinding time is 20 min to 200 min. For example, the grinding time is a value between 20 min, 50 min, 100 min, 150 min, 200 min or any combination thereof.

[0121] In some embodiments, step S2 further includes: mixing the first sintered product prepared above with a dispersant, a pore-forming agent, a second carbon source, and a solvent, and then performing ultrafine grinding to obtain primary particles I of lithium phosphate with an olivine structure. By adding a second carbon source, the conductivity of the lithium phosphate can be further improved.

[0122] In some embodiments, the second carbon source includes one or more of ascorbic acid, cellulose, polypropylene, epoxy resin, sucrose, glucose, fructose, citric acid, polyethylene glycol, starch, and phenolic resin.

[0123] In some embodiments, the mass ratio of the first sintered product, the pore-forming agent, and the second carbon source is 1:(0.02-0.05):(0.01-0.05).

[0124] S3, the primary particles I containing lithium phosphate prepared above are spray-dried to form monodisperse microspheres, and then the microspheres are subjected to a second sintering treatment under an inert gas atmosphere to obtain lithium phosphate, which includes secondary particles I with a porous structure.

[0125] In some implementations, the process parameters for spray drying are within a certain range.

[0126] In some embodiments, the second sintering process is carried out at 500°C-800°C for 2-10 hours. Exemplarily, the temperature of the second sintering process is a value within a range of 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, or any combination thereof. The duration of the second sintering process is a value within a range of 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or any combination thereof.

[0127] The term "secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0128] Typically, a single secondary battery cell includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0129] Negative electrode sheet The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0130] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0131] 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.).

[0132] 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).

[0133] 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.

[0134] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0135] 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 a negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0136] Positive electrode sheet The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including the positive electrode material of this application.

[0137] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the opposite surfaces of the positive current collector. 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 material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0138] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0139] In the examples of positive electrode active materials in this application, the molar content of O is only a theoretical value. Oxygen release from the crystal lattice will cause changes in the molar content of oxygen, and the actual molar content of O will fluctuate.

[0140] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0141] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0142] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0143] electrolytes The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0144] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0145] 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.

[0146] Separating membrane In some embodiments, the battery cell 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.

[0147] 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.

[0148] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0149] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0150] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0151] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 1 The example shown is a square-structured battery cell 5.

[0152] In some implementations, refer to Figure 2 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can cover the opening to close the receiving cavity. A positive electrode sheet, a negative electrode sheet, and a separator can be formed into an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in a single battery cell 5 can be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0153] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0154] Figure 3 This is battery module 4, used as an example. (See reference...) Figure 3 In battery module 4, multiple battery cells 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 battery cells 5 can be fixed in place using fasteners.

[0155] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0156] 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 one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0157] Figure 4and Figure 5 This is battery pack 1 as an example. (See reference...) Figure 4 and Figure 5 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0158] Electrical appliances The second aspect of this application provides an electrical device, which includes the secondary battery provided in the first aspect of this application.

[0159] Secondary batteries can be used as the power source for an electrical device or as the energy storage unit of an electrical device. Electrical devices can include, but are not limited to, mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0160] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0161] Figure 6 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 the secondary battery for this device, a battery pack or battery module can be used.

[0162] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0163] 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.

[0164] Example 1 Preparation of lithium phosphates: S1, lithium source (lithium carbonate), iron source (ferrous oxalate), and phosphorus source (ammonium dihydrogen phosphate) are mixed in a molar ratio of Li, Fe, and P of 1.05:1:1 to obtain powder. Then, 4% of the first carbon source (glucose) relative to the mass of the powder is added. After uniform mixing, the mixture is reacted at 400°C for 4 hours under a nitrogen atmosphere to carry out the first sintering treatment and obtain the first sintered product. S2, the first sintering product and pore-forming agent (methylcellulose) are mixed at a mass ratio of 1:0.05, and then solvent (deionized water) is added. The mixture is then ground at a speed of 5 r / min for 2 h to obtain primary particles of lithium iron phosphate I. S3, the above-mentioned primary particle I slurry of lithium iron phosphate is loaded into a spray dryer for spray drying to form monodisperse microspheres. Then, the microspheres are reacted at 700°C for 5 hours under a nitrogen atmosphere to carry out a second sintering treatment to obtain lithium iron phosphate LiFePO4. The lithium iron phosphate includes secondary particle I with a porous structure.

[0165] Preparation of positive electrode active materials: LiNi 0.8 Co 0.1 Mn 0.1 O2 and the lithium iron phosphate prepared above were mixed at a mass ratio of 95:5 to obtain the positive electrode active material, wherein LiNi 0.8 Co 0.1 Mn 0.1 O2 consists of primary particles II and secondary particles II. The mass ratio of primary particles II, secondary particles II, and secondary particles I of lithium iron phosphate is 30:65:5.

[0166] Preparation of secondary batteries: 1. Preparation of the positive electrode sheet: The positive electrode active material, polyvinylidene fluoride, and conductive carbon black were mixed in a mass ratio of 95:1.5:3.5, and then added to the solvent N-methylpyrrolidone. The mixture was stirred until homogeneous to form a positive electrode slurry. The positive electrode slurry was coated onto the positive electrode current collector aluminum foil to form a positive electrode film. After drying, cold rolling, slitting, and cutting, the positive electrode sheet was obtained. The single-sided coating weight of the positive electrode sheet was 15 mg / cm². 2 .

[0167] Parameter testing of the positive electrode: (1) Creep test Cut the positive electrode sheet into strips 3cm wide and 15cm long. Secure the cut positive electrode sample and a steel ruler together with tape, then place them vertically into a beaker, ensuring the ruler and electrode sample just touch the bottom of the beaker, without bending or sticking to the sides. Pour the electrolyte into the beaker to a height of 1cm, then seal the beaker with plastic wrap. Record the time and the height of the electrolyte rise on the electrode sample.

[0168] In the positive electrode of Example 1, the electrolyte rise height was 14 mm after 10 minutes. This reflects that the positive electrode of Example 1 has good wetting properties for the electrolyte.

[0169] (2) Microscopic morphology test Peel the positive electrode film from the positive electrode current collector, fix the peeled positive electrode film onto the sample stage, install the sample stage into the sample holder and lock it in place, turn on the power of the argon ion cross-section polisher (JEOL IB-09010 CP type argon ion cross-section polisher from Japan) and perform vacuuming (10 -7 The argon flow rate (0.12 MPa) and polishing time (90 min) were set, and the sample stage was adjusted to swing mode to begin polishing. After polishing, the cross-section of the positive electrode film was observed using SEM to obtain the microstructure of the positive electrode active material.

[0170] Figure 7 The image shown is a scanning electron microscope (SEM) image of the positive electrode film in Example 1. Figure 7 As shown, the positive electrode film layer includes lithium transition metal oxide and lithium-containing phosphate with olivine structure. The lithium transition metal oxide includes primary particles II and secondary particles II, and the lithium-containing phosphate includes secondary particles I with porous structure.

[0171] (3) Area proportion test The positive electrode film was polished using the method described above. After polishing, the cross-section of the positive electrode film was observed using SEM. Figure 8 This is another scanning electron microscope image of the positive electrode in Example 1. (See image below.) Figure 8 As shown, a cross-section of a secondary particle I includes primary particles I and the pores between them. The cross-sectional area of ​​the secondary particle I and the area of ​​the region filled by primary particles I in the cross-section are measured, and the area ratio of the region filled by primary particles I in the cross-section of the secondary particle I is calculated. A second secondary particle I is selected, and the area ratio of the region filled by primary particles I in the cross-section of the secondary particle I is measured again using the same method. The average of the multiple area ratios obtained is taken as the area ratio of the region filled by primary particles I in the cross-section of the secondary particle I.

[0172] In Example 1, in the cross-section of secondary particle I, the area A of the region filled by primary particle I is 76%.

[0173] (4) Porosity test Porosity can be measured using an ion-polished cross-section scanning electron microscope (SEM). Specifically: the positive electrode film is peeled off from the lithium battery, the peeled-off positive electrode film is fixed on the sample stage, the sample stage is installed in the sample holder and locked in place, the power of the argon ion cross-section polisher (e.g., the JEOL IB-09010 CP type argon ion cross-section polisher) is turned on and a vacuum is applied (e.g., 10...). -7 Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to rocking mode to begin polishing. After polishing, observe the cross-section of the positive electrode film using a scanning electron microscope. Granular olivine-structured lithium phosphate and lithium transition metal oxides can be observed. The porous particles are secondary particles I, consisting of lithium phosphate with an olivine structure. Measure the cross-sectional area and the area of ​​the pore region within a cross-section of a secondary particle I, and calculate the porosity of that cross-section. Repeat the process with a different secondary particle I, testing its porosity using the same method. Averaging the multiple porosities obtained yields the final porosity of the secondary particle I.

[0174] In Example 1, the porosity of secondary particles I is 24%.

[0175] (5) Average aperture test The average pore size can be determined using an ion-polished cross-section scanning electron microscope (SEM). Specifically: the positive electrode film is peeled off from the lithium battery, the peeled positive electrode film is fixed on the sample stage, the sample stage is installed and locked in place on the sample holder, the power of the argon ion cross-section polisher (e.g., the JEOL IB-09010 CP argon ion cross-section polisher) is turned on, and a vacuum is applied (e.g., 10...). -7 Set the argon flow rate (e.g., 0.12 MPa) and polishing time (e.g., 90 min), and adjust the sample stage to rocking mode to begin polishing. After polishing, observe the cross-section of the positive electrode film using a scanning electron microscope. Granular olivine-structured lithium phosphate and lithium transition metal oxides can be observed. The porous particles are secondary particles I, which are lithium phosphate particles with an olivine structure. Measure the cross-sectional width of each pore in the cross-section of a secondary particle I. Select another secondary particle I and measure the cross-sectional width of each pore using the same method. Average the cross-sectional widths of the measured pores to obtain the average pore diameter of the secondary particle I.

[0176] In Example 1, the average pore size of secondary particle I is 180 nm.

[0177] (6) Average particle size test A test sample with dimensions of 50mm x 100mm was randomly selected from the positive electrode. Five test areas were randomly selected from the test sample, and the particle sizes of primary and secondary lithium iron phosphate particles I and lithium transition metal oxide (LiNi) were read at 1000x magnification in each test area. 0.8 Co 0.1 Mn 0.1 The particle sizes of primary particles I and secondary particles II of O2 are calculated (i.e., the distance between the two farthest points on the particle is taken as the particle size). The number and particle size values ​​of primary particles I, secondary particles I, primary particles II and secondary particles II in each test area are counted. The arithmetic mean of primary particles I, secondary particles I, primary particles II and secondary particles II in each test area is taken as the average particle size of primary particles I and secondary particles I of lithium iron phosphate, and primary particles II and secondary particles II of lithium transition metal oxide in the test sample.

[0178] In Example 1, the average particle size of primary lithium iron phosphate particles I was 220 nm, the average particle size of secondary lithium iron phosphate particles I was 13 μm, and the lithium transition metal oxide LiNi 0.8 Co 0.1 Mn 0.1 The average particle size of primary particles II of O2 is 3.5 μm, the average particle size of secondary particles II is 14.5 μm, the absolute value of the difference between the average particle size of secondary particles I and the average particle size of primary particles II is 9.5 μm, and the absolute value of the difference between the average particle size of secondary particles II and the average particle size of secondary particles I is 1.5 μm.

[0179] (7) Compacted density test The compaction density of the positive electrode sheet = the areal density of the positive electrode film / the thickness of the positive electrode film.

[0180] The thickness of the positive electrode film was measured using a micrometer (Mitutoyo 293-100, with an accuracy of 0.1 μm). Specifically, the positive electrode film on one side of the positive electrode was wiped off first, and the thickness of the positive electrode at 12 different positions along the thickness direction of the positive electrode was measured. The average value was then taken as the thickness of the positive electrode, and the thickness of the positive current collector was subtracted to obtain the thickness of the positive electrode film.

[0181] The method for testing the areal density of the positive electrode film is as follows: First, wipe off the positive electrode film on one side of the positive electrode sheet, then cut the positive electrode sheet 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 on the other side of the weighed positive electrode sheet, weigh the positive current collector, and record its weight as M0. The areal density of the positive electrode film = (M1 - M0) / S1.

[0182] In Example 1, the compaction density of the positive electrode sheet was 3.6 g / cm³. 3 .

[0183] (8) Specific surface area test The specific surface area and pore size analyzer of the positive electrode was tested using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA, following the standard procedure GB / T 19587-2017. The specific surface area was calculated using the BET method.

[0184] In Example 1, the specific surface area of ​​the positive electrode sheet is 1.2 m². 2 / g.

[0185] 2. Preparation of negative electrode sheet: The negative electrode active material is a carbon-silicon composite (containing 5 wt% silicon, with an average particle size of 8.5 μm, a powder resistivity of 10 Ω·cm at 8 MPa, and a specific surface area of ​​1.51 m²). 2 A mixture of conductive carbon black, styrene-butadiene rubber, and sodium carboxymethyl cellulose in a mass ratio of 97.3:0.7:1.2:0.8 is thoroughly mixed in an appropriate amount of deionized water to form a negative electrode slurry. The negative electrode slurry is then coated onto a copper foil current collector to form a negative electrode film. After drying, cold rolling, slitting, and cutting, a negative electrode sheet is obtained. The coating weight of the negative electrode sheet is 9.5 mg / cm³. 2 The compacted density is 1.7 g / cm³. 3 .

[0186] 3. Preparation of electrolytes: Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1 to form an organic solvent. Lithium hexafluorophosphate (LiPF6) was dissolved in the organic solvent to obtain an electrolyte with a LiPF6 concentration of 1 mol / L and a conductivity of 10 mS / cm.

[0187] 4. Preparation of the separating membrane: A polyethylene film with a thickness of 13 μm was used as the separator.

[0188] 5. Preparation of secondary batteries: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The electrodes are then bent and wound to form an electrode assembly. The electrode assembly is placed in outer packaging, dried, and then injected with the prepared electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery is obtained.

[0189] Performance testing of secondary batteries: (1) Direct Current Resistance (DCR) Test At -25℃, the secondary battery was charged at a constant current of 1 / 3C to the cutoff voltage of 4.35V, and then charged at a constant voltage of 4.35V until the current was 0.05C. At this point, the secondary battery was fully charged. After the fully charged secondary battery was left to stand for 5 minutes, it was discharged at a current of 1 / 3C to a state of charge of 10% and the voltage V1 was recorded. Then it was discharged at a current of 0.33C for 10 seconds and the voltage V2 was recorded. The DC internal resistance of the battery DCR was calculated as (V2-V1) / current. The test results are recorded in Table 2 below.

[0190] The discharge rate (DCR) at -25℃ reflects the battery's low-temperature performance. A lower DCR indicates lower internal resistance at low temperatures, allowing for more efficient discharge and thus better low-temperature performance. Conversely, a higher DCR indicates higher internal resistance and reduced discharge efficiency at low temperatures, resulting in poorer low-temperature performance.

[0191] (2) Energy density test At 25℃, the secondary battery was charged to 4.35V with a constant current and constant voltage at 1 / 3C. After standing for 30 minutes, it was discharged to 2.8V with a current of 1 / 3C. The discharge capacity D1 of the secondary battery was recorded. The energy density of the battery was calculated as D1 / mass of the secondary battery. The test results are recorded in Table 2 below.

[0192] Examples 2 to 4 The secondary battery was prepared using the same method as in Example 1, except that the amount of pore-forming agent added in step S2 of the lithium iron phosphate preparation was adjusted so that the area ratio of the region filled by the primary particle I in the cross-section of the secondary particle I was different. Please refer to Table 1 below for details.

[0193] Example 5 The secondary battery was prepared using the same method as in Example 1, except that LiNi was used in the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2 contains only primary particles II.

[0194] Example 6 The secondary battery was prepared using the same method as in Example 1, except that LiNi was used in the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2 contains only secondary particles II.

[0195] Example 7 The secondary battery was prepared using the same method as in Example 1, except that the preparation steps for the lithium phosphate were as follows: S1, lithium source (lithium carbonate), iron source (ferrous oxalate), manganese source (manganese carbonate), and phosphorus source (ammonium dihydrogen phosphate) are mixed in a molar ratio of Li, Fe, Mn, and P of 1.05:0.6:0.4:1 to obtain powder. Then, 5% of the first carbon source (glucose) relative to the mass of the powder is added. After uniform mixing, the mixture is reacted at 400°C for 6 hours under a nitrogen atmosphere to carry out the first sintering treatment and obtain the first sintered product. S2, the first sintering product and pore-forming agent (methylcellulose) are mixed at a mass ratio of 1:0.05, and then solvent (deionized water) is added. The mixture is then ground at a speed of 5 r / min for 2 h to obtain primary particles of lithium manganese iron phosphate I'. S3, the above-mentioned primary granular slurry of lithium manganese iron phosphate (LiFePO4) is loaded into a spray dryer for spray drying to form monodisperse microspheres. Then, the microspheres are subjected to a second sintering treatment at 700°C for 4 hours under a nitrogen atmosphere to obtain lithium manganese iron phosphate (LiFePO4). 0.6 Mn 0.4 PO4, lithium manganese iron phosphate, includes secondary particles I' with a porous structure.

[0196] Comparative Example 1 The secondary battery was prepared using the same method as in Example 1, except that the positive electrode active material consisted only of lithium transition metal oxide LiNi. 0.8 Co 0.1 Mn 0.1 O2, excluding lithium iron phosphate.

[0197] Comparative Examples 2 and 3 The secondary battery was prepared using the same method as in Example 1, except that the amount of pore-forming agent in step S2 of the lithium iron phosphate preparation was adjusted so that the area ratio of the region filled by primary particle I in the cross-section of secondary particle I was different. Please refer to Table 1 below for details.

[0198] Comparative Example 4 The secondary battery was prepared using the same method as in Example 1, except that the carbon-silicon composite material used as the negative electrode was replaced with graphite material during the preparation of the negative electrode sheet.

[0199] The positive electrode sheets prepared in Examples 2 to 7 and Comparative Examples 1 to 4 were subjected to parameter tests using the same test method as in Example 1, and the secondary batteries prepared in Examples 2 to 7 and Comparative Examples 1 to 4 were subjected to performance tests using the same test method as in Example 1.

[0200] Table 1 below shows the relevant parameters of the positive and negative electrode active materials prepared in Examples 1 to 7 and Comparative Examples 1 to 4. Table 2 below shows the performance test results of the secondary batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 4.

[0201] Table 1

[0202] In Table 1, " / " indicates that it does not exist; A represents the area percentage of the region filled by primary particle I in the cross-section of secondary particle I.

[0203] Table 2

[0204] As can be seen from the data in Tables 1 and 2, compared with Comparative Example 1 (without lithium phosphate) and Comparative Example 2 (when A is higher than 85%), the secondary batteries prepared in Examples 1 to 7 have lower impedance at 10% SOC, which means that the low-temperature performance of the secondary batteries has been improved, while also maintaining high energy density.

[0205] Compared to Comparative Example 3 (when A is less than 60%) and Comparative Example 4 (when the negative electrode active material is graphite), the secondary batteries prepared in Examples 1 to 7 maintain high low-temperature performance while also achieving high energy density.

[0206] Examples 8 to 12 The secondary battery was prepared using the same method as in Example 1, except that the amount of pore-forming agent and grinding time in step S2 of the lithium iron phosphate preparation process, as well as the sintering time in step S3, were adjusted to make the average particle size, average pore size and porosity of the secondary lithium iron phosphate particles I different from that of the primary particles I. Please refer to Table 3 below for details.

[0207] And adjust the lithium transition metal oxide LiNi 0.8 Co 0.1 Mn 0.1 The average particle sizes of primary particles II and secondary particles II in O2 are shown in Table 4 below.

[0208] The parameters of the positive electrode sheets prepared in Examples 8 to 12 were tested according to the same test method as in Example 1, and the performance of the secondary batteries prepared in Examples 8 to 12 were tested according to the same test method as in Example 1.

[0209] Tables 3 and 4 below show the relevant parameters of the positive electrode active materials prepared in Examples 8 to 12. Table 5 below shows the performance test results of the secondary batteries prepared in Examples 8 to 12. For ease of comparison, the results of Example 1 are also shown in Tables 3 to 5 below.

[0210] Table 3

[0211] In Table 3, A represents the area percentage of the region filled by primary particle I in the cross-section of secondary particle I.

[0212] Table 4

[0213] In Table 4, X1 represents the absolute value of the difference between the average particle size of secondary particle I and the average particle size of primary particle II; X2 represents the absolute value of the difference between the average particle size of secondary particle II and the average particle size of secondary particle I.

[0214] Table 5

[0215] As can be seen from the data in Tables 3 to 5, when the average particle size of primary particle I is 20nm-800nm, the average pore size of secondary particle I is 20nm-500nm, and the average particle size of secondary particle I is 4μm-13μm, the prepared secondary battery can balance low-temperature performance and energy density.

[0216] Examples 13 and 14 The secondary battery was prepared using the same method as in Example 1, except that the mass ratio of primary particle II, secondary particle II and secondary particle I was adjusted according to Table 6 below.

[0217] The parameters of the positive electrode sheets prepared in Examples 13 and 14 were tested according to the same test method as in Example 1, and the performance of the secondary batteries prepared in Examples 13 and 14 was tested according to the same test method as in Example 1.

[0218] Table 6 below shows the performance test results of the secondary batteries prepared in Examples 13 and 14. For easy comparison, the results of Example 1 are also shown in Table 6 below.

[0219] Table 6

[0220] As can be seen from Table 6, when the mass ratio of primary particle II, secondary particle II and secondary particle I is (20-50):(48-70):(2-10), the prepared secondary battery can balance low-temperature performance and energy density.

[0221] 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 secondary battery, characterized in that, The secondary battery includes a positive electrode and a negative electrode, and the negative electrode active layer of the negative electrode includes a silicon-based material. The positive electrode active layer of the positive electrode sheet includes lithium transition metal oxide and lithium phosphate with olivine structure. The lithium phosphate includes secondary particles I formed from primary particles I. The secondary particles I have a porous structure. There are pores between the primary particles I in the secondary particles I. In the cross-section of the secondary particles I, the area of ​​the region filled by the primary particles I accounts for 60%-85%.

2. The secondary battery according to claim 1, characterized in that, In the cross-section of the secondary particle I, the area of ​​the region filled by the primary particle I accounts for 60%-70%.

3. The secondary battery according to claim 1, characterized in that, The porosity of the secondary particles I is 15%-40%.

4. The secondary battery according to claim 1, characterized in that, The average pore size of the secondary particles I is 20nm-500nm.

5. The secondary battery according to claim 1, characterized in that, The average particle size of the primary particle I is 20nm-800nm.

6. The secondary battery according to claim 1, characterized in that, The average particle size of the secondary particles I is 4μm-13μm.

7. The secondary battery according to claim 1, characterized in that, The lithium-containing phosphate accounts for 2%-10% of the total mass of the lithium transition metal oxide and the lithium-containing phosphate.

8. The secondary battery according to claim 1, characterized in that, The chemical formula of the lithium phosphate includes: Li 1+ x Fe 1-y A y P 1-z E z O4; Wherein, -0.1≤x≤0.1, 0≤y≤0.5, 0≤z≤0.1; A is selected from one or more of Mn, Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb, and Ge; E is selected from one or more of B, Si, N, S, F, Cl, and Br.

9. The secondary battery according to claim 1, characterized in that, The lithium transition metal oxide includes nickel, cobalt, and M, wherein the M element includes manganese and / or aluminum, and the molar percentage of nickel is 30%-95% based on the total molar number of the nickel, cobalt, and M elements.

10. The secondary battery according to claim 1, characterized in that, The lithium transition metal oxide comprises primary particles II and / or secondary particles II.

11. The secondary battery according to claim 10, characterized in that, The average particle size of the primary particles II is 1μm-20μm, and / or the average particle size of the secondary particles II is 1μm-20μm.

12. The secondary battery according to claim 10, characterized in that, The absolute value of the difference between the average particle size of the secondary particle I and the average particle size of the primary particle II is 0 μm-16 μm.

13. The secondary battery according to claim 10, characterized in that, The absolute value of the difference between the average particle size of secondary particle II and the average particle size of secondary particle I is 0 μm-10 μm.

14. The secondary battery according to claim 10, characterized in that, The average particle size of the secondary particle II is greater than the average particle size of the secondary particle I, and / or the average particle size of the secondary particle I is greater than the average particle size of the primary particle II.

15. The secondary battery according to claim 10, characterized in that, The mass ratio of the primary particle II, the secondary particle II and the secondary particle I is (20-50):(48-70):(2-10).

16. The secondary battery according to claim 1, characterized in that, The positive electrode sheet satisfies one or more of the following characteristics: (1) The compaction density of the positive electrode sheet is 3.55 g / cm³. 3 -3.7g / cm 3 ; (2) The coating weight on one side of the positive electrode sheet is 10 mg / cm². 2 -30mg / cm 2 ; (3) The specific surface area of ​​the positive electrode is 1.0 m². 2 / g-1.3m 2 / g.

17. The secondary battery according to claim 1, characterized in that, The silicon-based material includes one or more of elemental silicon, silicon-carbon composites, silicon-oxygen materials, and silicon alloys.

18. The secondary battery according to claim 1, characterized in that, The silicon-based material includes a silicon-carbon composite, which satisfies one or more of the following characteristics: (1) The silicon-carbon composite comprises porous carbon and silicon-containing material dispersed in the pores of the porous carbon; (2) The silicon-carbon composite further includes a carbon-containing coating layer, which is located on the surface of the porous carbon and / or the silicon-containing material; (3) The mass percentage of silicon in the silicon-carbon composite is 1%-70%; (4) The average particle size of the silicon-carbon composite is 2 μm-15 μm; (5) The powder resistivity of the silicon-carbon composite at 8 MPa is 4 Ω·cm-17 Ω·cm; (6) The BET specific surface area of ​​the silicon-carbon composite is 1.0 m². 2 / g-6.7m 2 / g.

19. The secondary battery according to claim 1, characterized in that, The compaction density of the negative electrode sheet is 1.65 g / cm³. 3 -1.85g / cm 3 And / or, the coating weight of the negative electrode sheet is 7 mg / cm³. 2 -15mg / cm 2 .

20. The secondary battery according to claim 1, characterized in that, The secondary battery further includes an electrolyte comprising a lithium salt and a solvent, and the electrolyte satisfies one or more of the following characteristics: (1) The conductivity of the electrolyte is 10 mS / cm-15 mS / cm; (2) The lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium perchlorate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium tri(trifluoromethanesulfonyl)methyl lithium, lithium hexafluorosilicate, lithium bis(oxalato)borate, and lithium difluoroborate. (3) The solvents include carboxylic acid ester solvents and / or cyclic carbonate solvents.

21. An electrical appliance, characterized in that, The secondary battery includes any one of claims 1 to 20.

Images