Secondary battery and electric device

Abstract

A secondary battery and an electric device. The secondary battery comprises a negative electrode sheet; the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector; the negative electrode film layer comprises a first area and a second area; the second area is located between the first area and the negative electrode current collector; the first area comprises a first negative electrode active material; the second area comprises a second negative electrode active material; the first negative electrode active material comprises a substrate and a carbon coating layer formed on at least partial surface of the substrate; thermogravimetric analysis testing is performed on the first negative electrode active material in an air atmosphere; an initial weight loss temperature T0 of the first negative electrode active material is 730°C-780°C; and the gram capacity of the second negative electrode active material is 357.0 mAh / g or above.

Description

Secondary batteries and electrical appliances

[0001] Cross-reference to related applications

[0002] This disclosure is based on and claims priority to Chinese Patent Application No. 202411214880.2, filed on August 30, 2024, entitled “Secondary Battery and Electrical Device”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure relates to the field of secondary battery technology, and more particularly to a secondary battery and an electrical device. Background Technology

[0004] In recent years, with the increasingly wide application of secondary batteries, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, and many other fields. Due to the significant development of secondary batteries, higher requirements have been placed on their energy density, cycle performance, and kinetic performance. Summary of the Invention

[0005] This disclosure is made in view of the above-mentioned problems, and its object is to provide a secondary battery and an electrical device. The secondary battery of this disclosure has high energy density, good cycle performance and excellent kinetic performance.

[0006] To achieve the above objectives, this disclosure provides a secondary battery, including a negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer includes a first region and a second region, with the second region located between the first region and the negative current collector. The first region includes a first negative electrode active material, and the second region includes a second negative electrode active material. The first negative electrode active material includes a substrate and a carbon coating layer formed on at least a portion of the surface of the substrate. The first negative electrode active material is subjected to thermogravimetric analysis in air, and the initial weight loss temperature T0 of the first negative electrode active material is 730℃-780℃. The specific capacity of the second negative electrode active material is 357.0 mAh / g or higher. Therefore, the secondary battery achieves both high energy density and good kinetic and cycle performance.

[0007] In some embodiments, the first negative electrode active material is subjected to thermogravimetric analysis in air, and the temperature Tmax corresponding to the maximum weight loss rate of the first negative electrode active material is 785℃-825℃. This results in the secondary battery exhibiting good kinetic and cycle performance.

[0008] In some embodiments, the carbon coating layer comprises amorphous carbon. This facilitates the provision of more active ion channels, thereby improving the kinetic performance of the secondary battery.

[0009] In some embodiments, the matrix comprises graphite. In some embodiments, the matrix comprises synthetic graphite. Synthetic graphite has a stable structure, which is beneficial for improving the cycle performance of the secondary battery.

[0010] In some embodiments, the carbon coating layer comprises 0.4%-1% of the substrate by mass. This is beneficial for improving the kinetic and cycle performance of the secondary battery. In some embodiments, the carbon coating layer comprises 0.5%-0.95% of the substrate by mass.

[0011] In some embodiments, the specific capacity of the second negative electrode active material is 357.0–365 mAh / g. This is beneficial for further improving the energy density of the secondary battery.

[0012] In some embodiments, the specific surface area of ​​the first negative electrode active material is 0.5 m². 2 / g-1.8m 2 / g; This helps reduce surface side reactions, thereby reducing the consumption of active ions during cycling and improving the cycle life of the secondary battery. In some embodiments, the specific surface area of ​​the first negative electrode active material is 0.5m². 2 / g-1.6m 2 / g.

[0013] In some embodiments, the specific surface area of ​​the second negative electrode active material is 0.5 m². 2 / g-2.2m 2 / g. This helps reduce surface side reactions and improve the energy density of the secondary battery. In some embodiments, the specific surface area of ​​the second negative electrode active material is 0.5m². 2 / g-2.0m 2 / g.

[0014] In some embodiments, the first negative electrode active material I D / I G The value is greater than the I of the second negative electrode active material. D / I G Value, where I D This indicates that the Raman spectrum of the material is at 1350±50 cm⁻¹ -1 The intensity of peak D at I G This indicates that the Raman spectrum of the material is at 1580±50 cm⁻¹ -1 The intensity of peak G at point I; D / I GThe value represents the ratio of D-band strength to G-band strength. This helps to improve the problem of localized lithium plating on the negative electrode and enhance the cycle performance of the secondary battery.

[0015] In some embodiments, the first negative electrode active material I D / I G The value is 0.10-0.25; and / or, the I of the second negative electrode active material D / I G The value is 0.02-0.17. In some embodiments, the I of the first negative electrode active material... D / I G The value is 0.11-0.23; and / or, the I of the second negative electrode active material D / I G The value is 0.02-0.14. This is beneficial for improving the kinetic and cycle performance of secondary batteries.

[0016] In some embodiments, the OI value of the first negative electrode active material is lower than that of the second negative electrode active material. This is beneficial for improving the kinetic performance of the secondary battery, increasing its energy density, and simultaneously ensuring excellent cycle performance.

[0017] In some embodiments, the powder OI value of the first negative electrode active material is 2.5-6.5. This is beneficial for improving the kinetic and cycle performance of the secondary battery. In some embodiments, the powder OI value of the first negative electrode active material is 2.8-6.3.

[0018] In some embodiments, the OI value of the second negative electrode active material powder is 5.5-19.5. This is beneficial for improving the energy density of the secondary battery. In some embodiments, the OI value of the second negative electrode active material powder is 5.0-20.5.

[0019] In some embodiments, the graphitization degree of the first negative electrode active material is lower than that of the second negative electrode active material. This is beneficial for improving the structural stability of the SEI film, thereby improving the cycle life of the secondary battery, while also increasing the energy density of the secondary battery.

[0020] In some embodiments, the degree of graphitization of the first negative electrode active material is 92.0%-94.0%. This is beneficial for improving the cycle performance of the secondary battery. In some embodiments, the degree of graphitization of the first negative electrode active material is 92.2%-93.9%.

[0021] In some embodiments, the degree of graphitization of the second negative electrode active material is 94.0%-96.5%. This is beneficial for improving the energy density of the secondary battery. In some embodiments, the degree of graphitization of the second negative electrode active material is 94.2%-96.0%.

[0022] In some embodiments, the first negative electrode active material comprises secondary particles; the second negative electrode active material comprises secondary particles. This is beneficial for improving the cycle performance of the secondary battery.

[0023] In some embodiments, the X-ray diffraction patterns of the first and / or second negative electrode active materials do not exhibit diffraction peaks of the 3R phase 101 crystal plane at 43°-44°. This helps reduce the continuous consumption of active ions, thereby improving the cycle performance of the secondary battery.

[0024] In some embodiments, the volumetric particle size distribution Dv50 of the first negative electrode active material is 9.0 μm-15.0 μm. This is beneficial for improving the kinetic performance of the secondary ion battery. In some embodiments, the volumetric particle size distribution Dv50 of the first negative electrode active material is 9.5-14.5 μm.

[0025] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.8-1.3. This is beneficial for improving the kinetic and cycle performance of the secondary battery. In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.85-1.25.

[0026] In some embodiments, the powder compaction density of the first negative electrode active material at a pressure of 50,000 N is 1.72-1.87 g / cc. This is beneficial for improving the energy density of the secondary battery. In some embodiments, the powder compaction density of the first negative electrode active material at a pressure of 50,000 N is 1.75-1.85 g / cc.

[0027] In some embodiments, the specific capacity of the first negative electrode active material is 352.2-359.6 mAh / g. This improves the energy density of the secondary battery. In some embodiments, the specific capacity of the first negative electrode active material is 353.0-359.0 mAh / g.

[0028] In some embodiments, the tap density of the first negative electrode active material is 1.00-1.25 g / cc. This is beneficial for improving the processing performance of the negative electrode active material and also for increasing the energy density of the secondary battery. In some embodiments, the tap density of the first negative electrode active material is 1.05-1.20 g / cc.

[0029] In some embodiments, the volumetric particle size distribution Dv50 of the second negative electrode active material is 13.5 μm-19.5 μm. This is beneficial for improving the energy density and kinetic performance of the secondary battery. In some embodiments, the volumetric particle size distribution Dv50 of the second negative electrode active material is 14.0 μm-19.0 μm.

[0030] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.9-1.5. This is beneficial for improving the kinetic performance of the secondary battery. In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.95-1.45.

[0031] In some embodiments, the powder compaction density of the second negative electrode active material at 50,000 N pressure is 1.93 g / cc to 2.06 g / cc. This is beneficial for increasing the specific capacity of the second negative electrode active material, thereby improving the energy density of the secondary battery. In some embodiments, the powder compaction density of the second negative electrode active material at 50,000 N is 1.95 g / cc to 2.04 g / cc.

[0032] In some embodiments, the first negative electrode material and / or the second negative electrode active material further include one or more of silicon materials, silicon-carbon materials, and silicon composite materials. This is beneficial for further improving the energy density of the secondary battery.

[0033] In some embodiments, the mass ratio of the negative electrode active material in the first region to the negative electrode active material in the second region of the negative electrode film is 30:70 to 70:30. This is beneficial for the secondary battery to achieve both high energy density and excellent cycle performance.

[0034] In some implementations, the specific capacity of the negative electrode is 355 mAh / g-365 mAh / g. This is beneficial for improving the energy density of the secondary battery.

[0035] In some embodiments, the secondary battery includes a positive electrode sheet, which includes a positive electrode active material, comprising at least one of a lithium phosphate with an olivine structure, a lithium transition metal oxide, and a modified compound thereof.

[0036] In some implementations, the positive electrode active material includes lithium iron phosphate.

[0037] A second aspect of this disclosure provides an electrical device including the secondary battery described in the first aspect. Therefore, the electrical device of this disclosure possesses at least the advantages of the secondary battery of this disclosure. Attached Figure Description

[0038] Figure 1 is a schematic diagram of one embodiment of the negative electrode sheet of this disclosure.

[0039] Figure 2 is a schematic diagram of a secondary battery according to an embodiment of the present disclosure.

[0040] Figure 3 is an exploded view of a secondary battery according to an embodiment of the present disclosure shown in Figure 2.

[0041] Figure 4 is a schematic diagram of a battery module according to one embodiment of the present disclosure.

[0042] Figure 5 is a schematic diagram of a battery pack according to one embodiment of the present disclosure.

[0043] Figure 6 is an exploded view of a battery pack according to an embodiment of the present disclosure, as shown in Figure 5.

[0044] Figure 7 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present disclosure.

[0045] Figure 8 is a thermogravimetric (TG) curve and thermogravimetric differential (DTG) curve of the first negative electrode active material 1-2 according to an embodiment of the present disclosure.

[0046] Figure 9 is a transmission electron microscope (TEM) image of the first negative electrode active material 1-1 in Embodiment 1 of this disclosure.

[0047] Explanation of reference numerals in the attached drawings: 1 Battery pack; 2 Upper housing; 3 Lower housing; 4 Battery module; 5 Secondary battery; 51 Housing; 52 Electrode assembly; 53 Top cover assembly; 10 Negative electrode sheet; 101 Negative current collector; 102 Negative electrode film; 102a First surface; 102b Second surface; 1021 First region; 1022 Second region; 1023 Intermediate region. Detailed Implementation

[0048] Hereinafter, embodiments of the secondary battery and electrical device of this disclosure are specifically disclosed 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 unnecessarily lengthy descriptions 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 disclosure and are not intended to limit the subject matter of the claims.

[0049] The "range" disclosed in this disclosure is defined by a lower limit and an upper limit, whereby a given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the 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 expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​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 disclosure, 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.

[0050] Unless otherwise specified, all embodiments and optional embodiments of this disclosure can be combined to form new technical solutions.

[0051] Unless otherwise specified, all technical features and optional technical features of this disclosure can be combined to form new technical solutions.

[0052] Unless otherwise specified, the terminology used in this disclosure has the common meaning as commonly understood by those skilled in the art.

[0053] Unless otherwise specified, the values ​​of the parameters mentioned in this disclosure can be determined using various test methods commonly used in the art, for example, according to the test methods given in this disclosure.

[0054] Unless otherwise specified, in this disclosure, the term "active ion" refers to ions that can be inserted and extracted back and forth between the positive and negative electrodes of a secondary battery, including but not limited to lithium ions.

[0055] This disclosure does not impose any particular limitation on the type of secondary battery; for example, a secondary battery can be a lithium-ion battery. Typically, a single secondary battery cell includes a positive electrode, a negative electrode, an electrolyte, and a separator. During the charging and discharging process of the secondary battery, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor of ions between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0056] Currently, the electric vehicle industry is developing rapidly, and in order to achieve longer driving ranges, people need batteries with higher energy density. At present, rechargeable batteries struggle to achieve both high energy density and good kinetic and cycle performance.

[0057] Based on this, the present disclosure proposes a secondary battery and an electrical device, which has high energy density, excellent kinetic performance, and cycle performance.

[0058] Negative electrode sheet

[0059] In the secondary battery disclosed herein, the negative electrode sheet includes a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector. The negative electrode film layer includes a first region and a second region, with the second region located between the first region and the negative current collector. The first region includes a first negative electrode active material, and the second region includes a second negative electrode active material. The first negative electrode active material includes a substrate and a carbon coating layer formed on at least a portion of the surface of the substrate. The first negative electrode active material is subjected to thermogravimetric analysis in an air atmosphere, and the initial weight loss temperature T0 of the first negative electrode active material is 730℃-780℃. The specific capacity of the second negative electrode active material is 357.0 mAh / g or higher.

[0060] In this disclosure, by including a carbon coating layer in the first negative electrode active material in the upper layer (first region), the surface carbon coating can increase lithium intercalation channels and reduce interfacial impedance. Furthermore, the carbon coating layer can improve the wettability of the electrode, thereby enhancing the kinetic and cycle performance of the secondary battery. The initial weight loss temperature T0 of the first negative electrode active material is 730℃-780℃, indicating that the first negative electrode active material has good surface activity and stability. The good surface activity of the first negative electrode active material facilitates the rapid intercalation and deintercalation of active ions on the surface of the first negative electrode active material, thereby improving the kinetic performance of the secondary battery. In addition, the good surface stability of the first negative electrode active material helps reduce the volume expansion of the active material during cycling and improves the stability of the SEI film formed on the surface of the negative electrode active material, thus enabling the secondary battery to have good cycle performance. For example, the initial weight loss temperature T0 of the first negative electrode active material is a value between 730°C, 735°C, 740°C, 745°C, 750°C, 755°C, 760°C, 765°C, 770°C, 775°C, 780°C, or any two of these values. In some alternative embodiments, the initial weight loss temperature T0 of the first negative electrode active material is 740°C-770°C. By making the specific capacity of the second negative electrode active material in the lower layer (second region) 357.0 mAh / g or higher, the secondary battery has a high energy density.

[0061] In addition, by setting the negative electrode film layer as a double-layer structure, the negative electrode sheet can still maintain a good pore structure, low tortuosity, short ion transport path and high ion conduction efficiency even under high compaction density. This is beneficial for the secondary battery to achieve both high energy density and excellent kinetic performance.

[0062] In this disclosure, the initial weight loss temperature T0 of the first negative electrode active material is the temperature corresponding to the intersection of the tangent at the horizontal level before the step in the thermogravimetric (TG) curve obtained by thermogravimetric analysis of the first negative electrode active material and the tangent at the maximum weight loss rate. The thermogravimetric analysis is performed as follows: Referring to JY / T 014-1996, a certain amount of carbon material is weighed and placed in a flat-bottomed crucible, shaken evenly, left open, and purged with air at a flow rate of 60 mL / min. The heating rate is 5 °C / min, and thermogravimetric analysis is performed within the range of 35 °C–950 °C to obtain the thermogravimetric (TG) curve and thermogravimetric differential (DTG) curve of the carbon material. The testing instrument can be a NETZSCH STA 449F3 simultaneous thermal analyzer from NETZSCH Instruments GmbH, Germany.

[0063] One embodiment of the negative electrode sheet disclosed herein is shown in FIG1. ​​The negative electrode sheet 10 includes a negative electrode current collector 101 and a negative electrode film layer 102 formed on at least one surface of the negative electrode current collector 101. The negative electrode film layer 102 has a first surface 102a away from the negative electrode current collector 101 and a second surface 102b disposed opposite to the first surface 102a. The thickness of the negative electrode film layer 102 is denoted as H. The thickness H of the negative electrode film layer refers to the thickness of the negative electrode film layer located on one side of the negative electrode current collector. The region from the first surface 102a of the negative electrode film layer to a thickness of 0.3H is denoted as the first region 1021 of the negative electrode film layer. The region from the second surface 102b of the negative electrode film layer to a thickness of 0.3H is denoted as the second region 1022 of the negative electrode film layer. The first region 1021 includes a first negative electrode active material, and the second region 1022 includes a second negative electrode active material. The region between the first region 1021 and the second region 1022 is denoted as the intermediate region 1023. It is readily understood that within the intermediate region 1023, there may be only a first negative electrode active material, only a second negative electrode active material, both a first negative electrode active material and a second negative electrode active material, or other negative electrode active materials known in the art besides the first and second negative electrode active materials disclosed herein.

[0064] It should be understood that in the embodiment shown in FIG1, the second surface 102b is in contact with the upper surface of the negative electrode current collector 101, but the structure of the negative electrode sheet of this disclosure is not limited thereto. For example, there may be an additional layer between the negative electrode film layer 102 and the negative electrode current collector 101. In this case, the second surface 102b is not in direct contact with the negative electrode current collector 101.

[0065] It should also be understood that although Figure 1 shows clear boundaries between the areas, such clear interfaces may not exist in the product.

[0066] In some embodiments, the first negative electrode active material is subjected to thermogravimetric analysis in an air atmosphere, and the temperature Tmax corresponding to the maximum weight loss rate of the first negative electrode active material is 785℃-825℃. By keeping the temperature Tmax corresponding to the maximum weight loss rate of the first negative electrode active material within this range, it is demonstrated that the first negative electrode active material has good surface activity and stability, and good kinetic and cycling performance. Exemplarily, the temperature Tmax corresponding to the maximum weight loss rate of the first negative electrode active material can be 785℃, 790℃, 795℃, 800℃, 805℃, 810℃, 815℃, 820℃, 825℃, or a value within a range consisting of any two of these values. In some optional embodiments, the temperature Tmax corresponding to the maximum weight loss rate of the first negative electrode active material is 790℃-820℃.

[0067] In this disclosure, the temperature Tmax corresponding to the maximum weight loss rate of the first negative electrode active material is the peak temperature of the thermogravimetric differential curve obtained by thermogravimetric analysis of the first negative electrode active material. The thermogravimetric analysis is performed using the same method described above.

[0068] In some embodiments, the carbon coating layer comprises amorphous carbon. The ungraphitized carbon coating layer exhibits high surface activity, providing more active ion channels, thereby improving the kinetic performance of the secondary battery.

[0069] In some embodiments, the matrix comprises graphite, for example, synthetic graphite. Synthetic graphite has a stable structure, which is beneficial for improving the cycle performance of the secondary battery.

[0070] In this disclosure, the matrix and carbon coating of the first negative electrode active material can be distinguished by transmission electron microscopy (TEM). In the TEM image of the first negative electrode active material, the matrix has regular lattice stripes, while the carbon coating has no lattice stripes and is amorphous.

[0071] In some embodiments, the carbon coating layer comprises 0.4%-1% by mass relative to the substrate. Maintaining the carbon coating layer content within this range facilitates the increase of surface lithium intercalation channels in the first negative electrode active material and reduces the reaction between the active material of the carbon coating layer and the active lithium and electrolyte, thereby improving the kinetic and cycle performance of the secondary battery. Exemplarily, the carbon coating layer may comprise 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 0.95%, 1.0%, or any two of these values. In some alternative embodiments, the carbon coating layer comprises 0.5%-0.95% by mass relative to the substrate.

[0072] In this disclosure, the specific capacity of the second negative electrode active material can be 357.0 mAh / g, 358 mAh / g, 359 mAh / g, 360 mAh / g, 361 mAh / g, 362 mAh / g, 363 mAh / g, 364 mAh / g, 365 mAh / g, 366 mAh / g, 367 mAh / g, or a value within a range of any two of these values. In some optional embodiments, the specific capacity of the second negative electrode active material is 357.0–365 mAh / g. This is beneficial for further improving the energy density of the secondary battery.

[0073] In some embodiments, the specific surface area of ​​the first negative electrode active material is 0.5 m². 2 / g-1.8m 2 / g. The first negative electrode active material has a low specific surface area, which reduces the active ions consumed during SEI film formation in the secondary battery, thereby improving the capacity of the secondary battery. Furthermore, the low specific surface area of ​​the first negative electrode active material results in fewer surface side reactions during secondary battery cycling, which helps reduce the consumption of active ions during cycling, thus improving the cycle life of the secondary battery. For example, the specific surface area of ​​the first negative electrode active material can be 0.5m². 2 / g, 0.8m 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m 2 / g or a value within a range of any two of these values. In some alternative embodiments, the specific surface area of ​​the first negative electrode active material is 0.5m². 2 / g-1.6m 2 / g.

[0074] In some embodiments, the specific surface area of ​​the second negative electrode active material is 0.5 m². 2 / g-2.2m 2 / g. By ensuring the specific surface area of ​​the second negative electrode active material is within the aforementioned range, it is beneficial to reduce the occurrence of surface side reactions, thereby improving the energy density of the secondary battery. For example, the specific surface area of ​​the second negative electrode active material can be 0.5m². 2 / g, 0.8m 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.2m 2 / g or a value within a range of any two of these values. In some alternative embodiments, the specific surface area of ​​the second negative electrode active material is 0.5m². 2 / g-2.0m 2 / g.

[0075] In some embodiments, the first negative electrode active material I D / I G The value is greater than the I of the second negative electrode active material. D / I G Value, where I D This indicates that the Raman spectrum of the material is at 1350±50 cm⁻¹ -1 The intensity of peak D at I G This indicates that the Raman spectrum of the material is at 1580±50 cm⁻¹ -1 The intensity of peak G at point I; D / I G The value represents the ratio of the D-band intensity to the G-band intensity. This is achieved by increasing the I-band strength of the first negative electrode active material located in the first region. D / I G The value is greater than the I of the second negative electrode active material in the second region. D / I G Value, in the initial stage of charging, the lithium-ion concentration in the first region is higher than that in the second region, when the I of the first negative electrode active material... D / I G The value is greater than the I of the second negative electrode active material. D / I G This value is beneficial for uniform lithium intercalation at various positions in the negative electrode, reducing the polarization of the negative electrode, improving the problem of local lithium plating, and enhancing the kinetic and cycle performance of the secondary ion battery.

[0076] In some embodiments, the first negative electrode active material I D / IG The value is 0.10-0.25. This is achieved by increasing the Ig of the first negative electrode active material. D / I G Values ​​within the aforementioned range are beneficial for balancing good kinetic and cycle performance. For example, the Ia of the first negative electrode active material... D / I G The value can be 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.20, 0.22, 0.23, 0.25, or a range consisting of any two of these values. In some alternative embodiments, the I of the first negative electrode active material... D / I G The value is 0.11-0.23. In some embodiments, the I of the second negative electrode active material... D / I G The value is 0.02-0.17. This is achieved by increasing the Ig of the second negative electrode active material. D / I G Values ​​within the aforementioned range are beneficial for improving the cycle performance of secondary batteries. For example, the I value of the second negative electrode active material... D / I G The value can be 0.02, 0.04, 0.06, 0.08, 0.10, 0.11, 0.12, 0.13, 0.14, 0.16, 0.17, or a value within a range of any two values ​​therein. In some alternative embodiments, the I of the second negative electrode active material... D / I G The value is 0.02-0.14.

[0077] In some embodiments, the powder OI value of the first negative electrode active material is lower than that of the second negative electrode active material. By making the first negative electrode active material in the first region have a smaller powder OI value, the first negative electrode active material has a higher isotropy. During the operation of the secondary battery, active ions are inserted into the first negative electrode active material from all directions. The resulting volume expansion can be dispersed in all directions, thereby reducing the generation of new interfaces on the negative electrode surface, reducing the consumption of active ions, and thus improving the cycle performance of the secondary battery. In addition, the high isotropy of the first negative electrode active material can provide more lithium intercalation channels, thereby improving the kinetic performance of the secondary battery. By making the second negative electrode active material in the second region have a higher powder OI value, it is beneficial to improve the energy density of the secondary battery. Thus, the secondary battery can achieve both high energy density and excellent cycle performance.

[0078] In some embodiments, the powder OI value of the first negative electrode active material is 2.5-6.5. By keeping the powder OI value of the first negative electrode active material within this range, it is beneficial for the first negative electrode active material to have higher isotropy, which helps reduce volume expansion and provides more lithium intercalation channels, thereby improving the kinetic and cycle performance of the secondary battery. Exemplarily, the powder OI value of the first negative electrode active material can be 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, or a value between any two of these values. In some optional embodiments, the powder OI value of the first negative electrode active material is 2.8-6.3. In some embodiments, the powder OI value of the second negative electrode active material is 5.0-20.5. By keeping the powder OI value of the second negative electrode active material within the above range, it is beneficial to improve the energy density of the secondary battery. For example, the OI value of the second negative electrode active material can be 5.0, 5.5, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0, 20.5, or a value between any two of these values. In some alternative embodiments, the OI value of the second negative electrode active material is 5.5-19.5.

[0079] In some embodiments, the graphitization degree of the first negative electrode active material is lower than that of the second negative electrode active material. By having a lower graphitization degree in the first negative electrode active material located in the first region, it is beneficial to reduce the volume expansion of the first negative electrode active material during cycling, which helps to improve the structural stability of the SEI film, thereby improving the cycle life of the secondary battery. Furthermore, the higher graphitization degree of the second negative electrode active material located in the second region helps to increase the energy density of the negative electrode film layer, thereby increasing the energy density of the secondary battery. Thus, the secondary battery can achieve both good cycle performance and high energy density.

[0080] In some embodiments, the graphitization degree of the first negative electrode active material is 92.0%-94.0%. Maintaining the graphitization degree of the first negative electrode active material within this range helps reduce volume expansion during cycling, improves the structural stability of the SEI film, and thus enhances the cycle performance of the secondary battery. Exemplarily, the graphitization degree of the first negative electrode active material can be 92.0%, 92.2%, 92.5%, 93.0%, 93.5%, 93.7%, 93.9%, 94.0%, or a value within a range of any two of these values. In some optional embodiments, the graphitization degree of the first negative electrode active material is 92.2%-93.9%.

[0081] In some embodiments, the graphitization degree of the second negative electrode active material is 94.0%-96.5%. Maintaining the graphitization degree of the second negative electrode active material within this range is beneficial for improving the energy density of the secondary battery. Exemplarily, the graphitization degree of the second negative electrode active material can be 94.0%, 94.2%, 94.5%, 94.7%, 95.0%, 95.5%, 95.7%, 96.0%, 96.5%, or a value within a range of any two of these values. In some optional embodiments, the graphitization degree of the second negative electrode active material is 94.2%-96.0%.

[0082] In some embodiments, the first negative electrode active material includes secondary particles; and / or the second negative electrode active material includes secondary particles. Secondary particles help reduce the orientation degree of the material, increase its isotropy, reduce its volume expansion, and thus help reduce the damage and growth of the SEI film due to volume expansion, thereby improving cycle performance.

[0083] In some embodiments, the X-ray diffraction patterns of the first and / or second negative electrode active materials do not exhibit diffraction peaks of the 3R phase 101 crystal plane at 43°-44°. These diffraction peaks at 43-44° correspond to the diffraction peaks of the graphite 3R phase (101) crystal plane, a characteristic crystal plane of natural graphite. In this disclosure, neither the first nor the second negative electrode active material exhibits diffraction peaks of the 3R phase 101 crystal plane, and both are artificial graphite materials. This helps reduce volume expansion during cycling, decreases the continuous consumption of active ions, and thus improves the cycle performance of the secondary battery.

[0084] In some embodiments, the volumetric particle size distribution Dv50 of the first negative electrode active material is 9.0 μm-15.0 μm. By keeping the volumetric particle size distribution Dv50 of the first negative electrode active material within this range, it is beneficial to reduce the solid-phase diffusion distance of active ions in the graphite particles, thereby improving the transport efficiency of active ions and thus improving the kinetic performance of the secondary ion battery. Exemplarily, the volumetric particle size distribution Dv50 of the first negative electrode active material is 9.0 μm, 9.5 μm, 10.0 μm, 10.5 μm, 11.0 μm, 11.5 μm, 12.0 μm, 12.5 μm, 13.0 μm, 13.5 μm, 14.0 μm, 14.5 μm, 15.0 μm, or a value between any two of these values. In some optional embodiments, the volumetric particle size distribution Dv50 of the first negative electrode active material is 9.5-14.5 μm.

[0085] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.8-1.3. This facilitates the formation of a reasonable pore structure between the particles in the negative electrode film, improving the transport performance of active ions and electrons in the negative electrode film, thereby enhancing the kinetic and cycle performance of the secondary battery. Exemplarily, the particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is a value between 0.8, 0.85, 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, or any two of these values. In some optional embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.85-1.25.

[0086] In some embodiments, the powder compaction density of the first negative electrode active material at a pressure of 50,000 N is 1.72-1.87 g / cc. Maintaining the powder compaction density of the first negative electrode active material within this range helps maintain a better porosity distribution and lower electrode tortuosity in the negative electrode sheet; it also helps increase the compaction density of the negative electrode sheet, thereby improving the energy density of the secondary battery. For example, the powder compaction density of the first negative electrode active material at a pressure of 50,000 N can be 1.72 g / cc, 1.75 g / cc, 1.77 g / cc, 1.79 g / cc, 1.80 g / cc, 1.82 g / cc, 1.84 g / cc, 1.85 g / cc, 1.87 g / cc, or a value within a range of any two of these values. In some optional embodiments, the powder compaction density of the first negative electrode active material at a pressure of 50,000 N is 1.75-1.85 g / cc.

[0087] In some embodiments, the specific capacity of the first negative electrode active material is 352.2-359.6 mAh / g. By keeping the specific capacity of the first negative electrode active material within the above range, it is beneficial to increase the specific capacity of the negative electrode sheet, thereby improving the energy density of the secondary battery. For example, the specific capacity of the first negative electrode active material can be 352.2 mAh / g, 352.5 mAh / g, 352.8 mAh / g, 353.0 mAh / g, 353.2 mAh / g, 353.5 mAh / g, 353.8 mAh / g, 354.0 mAh / g, 354.5 mAh / g, 355.0 mAh / g, 355.5 mAh / g, 356.0 mAh / g, 356.5 mAh / g, 357.0 mAh / g, 357.5 mAh / g, 358.0 mAh / g, 358.5 mAh / g, 359.0 mAh / g, 359.2 mAh / g, 359.4 mAh / g, 359.6 mAh / g, or a value within a range of any two of these values. In some optional embodiments, the specific capacity of the first negative electrode active material is 353.0-359.0 mAh / g.

[0088] In some embodiments, the tap density of the first negative electrode active material is 1.00-1.25 g / cc. Maintaining the tap density of the first negative electrode active material within this range facilitates better particle packing, improves the processing performance of the negative electrode active material, and also helps to increase the compaction density of the negative electrode sheet, thereby increasing the energy density of the secondary battery. For example, the tap density of the first negative electrode active material can be 1.00 g / cc, 1.02 g / cc, 1.05 g / cc, 1.07 g / cc, 1.09 g / cc, 1.10 g / cc, 1.12 g / cc, 1.14 g / cc, 1.15 g / cc, 1.17 g / cc, 1.18 g / cc, 1.20 g / cc, 1.22 g / cc, 1.24 g / cc, 1.25 g / cc, or any two of these values ​​within a range. In some alternative embodiments, the tap density of the first negative electrode active material is 1.05-1.20 g / cc.

[0089] In some embodiments, the volumetric particle size distribution Dv50 of the second negative electrode active material is 13.5 μm-19.5 μm. By ensuring the volumetric particle size distribution Dv50 of the second negative electrode active material is within this range, it is beneficial for the negative electrode sheet to have a higher compaction density, thereby improving the energy density of the secondary battery. Furthermore, it is also beneficial for improving the kinetic performance of the secondary battery. Exemplarily, the volumetric particle size distribution Dv50 of the second negative electrode active material can be 13.5 μm, 14.0 μm, 14.5 μm, 15.0 μm, 15.5 μm, 16.0 μm, 16.5 μm, 17.0 μm, 17.5 μm, 18.0 μm, 18.5 μm, 19.0 μm, 19.5 μm, or a value within a range consisting of any two of these values. In some optional embodiments, the volumetric particle size distribution Dv50 of the second negative electrode active material is 14.0 μm-19.0 μm.

[0090] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.9-1.5. This is beneficial for increasing the compaction density of the negative electrode film and improving the energy density of the secondary battery. Furthermore, it facilitates the formation of a reasonable pore structure between the particles in the negative electrode film, improving the transport performance of active ions and electrons in the negative electrode film, thereby enhancing the kinetic performance of the secondary battery. For example, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.90, 0.95, 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50, or a value within a range of any two of these values. In some optional embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.95-1.45.

[0091] In some embodiments, the powder compaction density of the second negative electrode active material at a pressure of 50,000 N is 1.93 g / cc to 2.06 g / cc. Maintaining the powder compaction density of the second negative electrode active material within this range is beneficial for increasing the specific capacity of the second negative electrode active material, thereby improving the energy density of the secondary battery. For example, the powder compaction density of the second negative electrode active material at a pressure of 50,000 N can be 1.93 g / cc, 1.94 g / cc, 1.95 g / cc, 1.96 g / cc, 1.97 g / cc, 1.98 g / cc, 1.99 g / cc, 2.00 g / cc, 2.01 g / cc, 2.02 g / cc, 2.03 g / cc, 2.04 g / cc, 2.05 g / cc, 2.06 g / cc, or any value between two of these values. In some alternative embodiments, the powder compaction density of the second negative electrode active material at a pressure of 50,000 N is 1.95 g / cc to 2.04 g / cc.

[0092] In some embodiments, the first negative electrode material and / or the second negative electrode active material further include one or more of silicon materials, silicon-carbon materials, and silicon composite materials. Adding one or more of silicon materials, silicon-carbon materials, and silicon composite materials to the first negative electrode active material and / or the second negative electrode active material is beneficial for further improving the energy density of the secondary battery.

[0093] In some embodiments, the mass ratio of the negative electrode active material in the first region to the negative electrode active material in the second region of the negative electrode film is 30:70 to 70:30. Maintaining this mass ratio within the aforementioned range is beneficial for the secondary battery to achieve both high energy density and excellent cycle performance. Exemplarily, the mass ratio of the negative electrode active material in the first region to the negative electrode active material in the second region can be 30:70, 40:60, 50:50, 60:40, 70:30, or a range consisting of any two of these values.

[0094] In some embodiments, the specific capacity of the negative electrode is 355 mAh / g to 365 mAh / g. Maintaining the specific capacity of the negative electrode within this range is beneficial for improving the energy density of the secondary battery. Exemplarily, the specific capacity of the negative electrode is a value between 355 mAh / g, 357 mAh / g, 360 mAh / g, 362 mAh / g, 365 mAh / g, or any two of these values. In some alternative embodiments, the specific capacity of the negative electrode is 355 mAh / g to 363 mAh / g.

[0095] 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 material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0096] In some embodiments, the negative electrode film layer may optionally include a binder. The binder 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).

[0097] 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.

[0098] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0099] In this disclosure, the specific surface area of ​​a material has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be tested 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.

[0100] In this disclosure, material I D / I G The test can be performed using a Raman spectrometer. D This indicates that the Raman spectrum of the material is at 1350±50 cm⁻¹. -1 The intensity of peak D at I G This indicates that the Raman spectrum of the material is at 1580±50 cm⁻¹. -1 The intensity of the G peak at the location was measured. The test conditions were: excitation wavelength 532 nm, grating 600 lines, objective lens 50x, integration time 10 s, cumulative scan 3 times, area scan, obtaining the D and G peak intensities at 100 points, and calculating the I at 100 points. D / I GRemove the largest and smallest 30 I's. D / I G The average of the remaining 40 points is the material's I. D / I G The testing instrument can be a Horiba LabRAM HR800 Raman spectrometer.

[0101] In this disclosure, the powder OI value of a material refers to the graphite orientation degree of the material, describing the uniformity of crystal orientation within the graphite particles. The powder OI value is the ratio of the peak area of ​​plane (004) to the peak area of ​​plane (110) obtained by X-ray diffraction (XRD). In this disclosure, the powder OI value of the material can be determined using instruments and methods known in the art. For example, an X-ray diffractometer (such as a Bruker D8 Discover) can be used for testing, and the testing can be performed with reference to JIS K 0131-1996 and JB / T 4220-2011 to obtain the X-ray diffraction pattern of the powder sample. The OI value is calculated as follows: OI = I 004 / I 110 The OI value of the powder in the sample was calculated. 004 I is the integrated area of ​​the diffraction peak of the crystalline carbon 004 crystal plane in the powder sample. 110 This represents the integrated area of ​​the diffraction peak on the 110 crystal plane of crystalline carbon in the powder sample. In the X-ray diffraction analysis of this disclosure, a copper target can be used as the anode target, with CuKα rays as the radiation source, and the ray wavelength... The scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min.

[0102] In this disclosure, the volumetric particle sizes Dv10, Dv50, and Dv90 are known in the art, representing the particle sizes corresponding to a cumulative volumetric distribution percentage of 10%, 50%, and 90%, respectively, and can be determined using instruments and methods known in the art. For example, they can be determined using a laser particle size analyzer, referring to GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0103] In this disclosure, the powder compaction density of the material has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be determined using an electronic pressure testing machine (e.g., a UTM7305 type electronic pressure testing machine) in accordance with GB / T 24533-2009. An exemplary test method is as follows: Weigh 1g of sample powder and add it to a container with a bottom area of ​​1.327cm². 2In the mold, the pressure is increased to 50,000 N, held for 30 seconds, then the pressure is released and held for 10 seconds. The compaction density of the powder under 50,000 N pressure is then recorded and calculated.

[0104] In this disclosure, the specific capacity of the material has a meaning known in the art and can be tested using methods known in the art. An exemplary test method is as follows: Sample powder, conductive agent, binder, and solvent are mixed uniformly at a certain mass ratio to form a slurry; the prepared slurry is coated onto the surface of the copper foil of the negative electrode current collector and dried in an oven for later use; an electrolyte is prepared; then, using a lithium metal sheet as the counter electrode, the above electrolyte and separator are assembled into a CR2430 coin cell in an argon-protected glove box; after the obtained coin cell is left to stand for 12 hours, it is discharged at 25°C with a constant current of 0.05C to the cutoff voltage, left to stand for 5 minutes, then discharged again with a constant current of 50 μA to the cutoff voltage, left to stand for 5 minutes, and then discharged again with a constant current of 10 μA to the cutoff voltage; then it is charged with a constant current of 0.1C to the cutoff voltage, and the charging capacity is recorded. The ratio of the charging capacity to the sample mass is the specific capacity of the corresponding material (first negative electrode active material, second negative electrode active material, etc.).

[0105] In this disclosure, the specific capacity of the negative electrode sheet has a meaning known in the art and can be tested using methods known in the art. An exemplary test method is as follows: The active material on one side of the negative electrode sheet is wiped off and dried in an oven for later use; an electrolyte is prepared; then, using a lithium metal sheet as the counter electrode, the above electrolyte and separator are assembled into a CR2430 coin cell in an argon-protected glove box; after the resulting coin cell is left to stand for 12 hours, it is discharged at 25°C with a constant current of 0.05C to the cutoff voltage, left to stand for 5 minutes, then discharged again with a constant current of 50 μA to the cutoff voltage, left to stand for 5 minutes, and then discharged again with a constant current of 10 μA to the cutoff voltage; then it is charged with a constant current of 0.1C to the cutoff voltage, and the charging capacity is recorded. The ratio of the charging capacity to the mass of the active material is the specific capacity of the corresponding negative electrode sheet.

[0106] In this disclosure, the degree of graphitization of the material has a meaning known in the art and can be tested using instruments and methods known in the art. For example, it can be tested using an X-ray diffractometer (such as a Bruker D8 Discover), and the testing can be performed with reference to JIS K 0131-1996 and JB / T 4220-2011 to obtain the average interlayer spacing d of the C(002) crystal plane in the material's crystal structure. 002 Then, according to the formula g = (0.344 - d) 002 The degree of graphitization is calculated as d / (0.344-0.3354)×100%. In the above formula, d... 002 It is the average interlayer spacing of the C(002) crystal plane in the material crystal structure, expressed in nanometers (nm).

[0107] In this disclosure, the tap density of the material has a meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be determined using a powder tap density tester according to GB / T 5162-2006. The testing instrument can be Dandong Baite BT-301, with the following test parameters: vibration frequency 250±15 times / minute, amplitude 3±0.2mm, number of vibrations 5000 times, and a measuring cylinder of 25mL.

[0108] In this disclosure, the thickness of the negative electrode film layer has a meaning known in the art and can be tested using methods known in the art, such as a micrometer (e.g., a Mitutoyo 293-100 with an accuracy of 0.1 μm). The thickness range given in this disclosure refers to the thickness range of the negative electrode film layer on one side of the negative electrode current collector.

[0109] In this disclosure, primary particles and secondary particles have meanings known in the art. Primary particles refer to non-agglomerated particles. Secondary particles refer to aggregated particles formed by the aggregation of two or more primary particles. Primary particles and secondary particles can be distinguished using scanning electron microscopy (SEM) images.

[0110] Preparation method of negative electrode sheet

[0111] The negative electrode sheet in this disclosure can be prepared in the following manner: a first slurry containing a first negative electrode active material and a second slurry containing a second negative electrode active material are provided; the first slurry is coated on a negative electrode current collector, the second slurry is coated on the first slurry, and after drying and cold pressing, a negative electrode sheet is obtained.

[0112] The preparation method of the first negative electrode active material in this disclosure includes the following steps:

[0113] Preparation of the matrix for the first negative electrode active material:

[0114] 1) Preparation of the first artificial graphite component: The raw material with a sulfur content of less than or equal to 1.5% by mass is crushed, shaped, graded and screened, and then graphitized at 3000℃-3200℃ to produce primary particles with a volume distribution particle size Dv50 of 7μm-9μm, thus obtaining the first artificial graphite component.

[0115] 2) Preparation of the second artificial graphite component: The raw material with a sulfur content of less than or equal to 1.5% by mass is crushed and shaped to obtain primary particles with a volume distribution particle size Dv50 of 7μm-9μm. Then, a binder is added for granulation, and graphitization is carried out at 3000℃-3200℃ to produce secondary particles with (Dv90-Dv10) / Dv50 of 1.0-1.25 and Dv50 of 13μm-18μm, thus obtaining the second artificial graphite component;

[0116] 3) The first artificial graphite component and the second artificial graphite component are mixed in a mass ratio of (60-20:40-80) to obtain the matrix of the first negative electrode active material.

[0117] Preparation of the carbon coating layer of the first negative electrode active material: The matrix prepared above is mixed evenly with the coating agent; then heat-treated at 1000℃-1500℃ to form a coating layer on the matrix. After demagnetization and sieving, the first negative electrode active material is obtained.

[0118] In some embodiments, the coating agent includes one or more of oil-based bitumen, coal-based bitumen, and liquid residue oil. This facilitates the formation of an amorphous carbon coating layer, improving the kinetics and cycle performance of the first negative electrode active material.

[0119] In some embodiments, the mass ratio of the matrix to the coating agent is 100:X, and the coking value (Z) of the coating agent can be determined according to GB / T8727-2008, for example, Z can be 25%-70%. In the prepared negative electrode active material, the mass percentage (Y) of the carbon coating layer relative to the mass of the matrix is ​​X*Z / 100. For example, Y can be 0.4%-1%, optionally 0.5%-0.95%.

[0120] The preparation method of the second negative electrode active material in this disclosure includes the following steps:

[0121] Pretreatment steps: The calcined needle coke is crushed and shaped to obtain shaped material; the carbon content of the calcined needle coke is above 97%, the volatile component content is below 1.5%, and the sulfur content is below 1.5%;

[0122] Granulation step: Mix the shaping material with the binder to obtain granulated material;

[0123] Graphitization step: The granulated material is graphitized at 3050℃-3250℃ to obtain the second negative electrode active material.

[0124] In this disclosure, calcined needle coke is used as the raw material for preparing the second negative electrode active material. Calcined needle coke has a stronger needle-like structure and fewer embedded structures. Using calcined needle coke as a raw material helps reduce the impurity content in artificial graphite, thereby improving the graphitization degree of artificial graphite.

[0125] In this disclosure, the calcined needle coke has a carbon content of 97% or more, a volatile component content of 1.5% or less, and a sulfur content of 1.5% or less. Here, volatile components refer to hydrocarbons with low molecular weights, such as light alkanes. When the sulfur content and volatile components of the calcined needle coke are within the above-mentioned ranges, the raw material has better purity, which is beneficial to grain development during graphitization, reduces the number of defects during graphitization, increases the degree of graphitization of artificial graphite, and thus improves the specific capacity and compaction density of the negative electrode film, thereby increasing the energy density of the secondary battery.

[0126] In this disclosure, the graphitization temperature is 3050℃-3250℃. A graphitization temperature within this range is beneficial for the growth and development of graphite grains, increasing the degree of graphitization of the artificial graphite, thereby increasing the specific capacity of the second negative electrode active material and improving the energy density of the secondary battery. Exemplarily, the graphitization temperature can be 3050℃, 3100℃, 3150℃, 3200℃, 3250℃, or a value within a range of any two of these values.

[0127] In this disclosure, the graphitization treatment time is 2h-60h. For example, when the graphitization temperature is 3050°C, the graphitization time is 2h, 4h, 8h, 20h, 48h, 60h, or a value within a range of any two of these values.

[0128] In some embodiments, the softening point of the binder is 180°C-270°C. When the softening point of the binder is within this range, the molecular weight and content of the polycyclic aromatic hydrocarbons in the binder are large, which is beneficial to improving the adhesion of the binder, reducing the amount of binder used in the granulation step, and thus increasing the specific capacity and compaction density of the second negative electrode active material. Exemplarily, the softening point of the first binder can be 180°C, 200°C, 220°C, 240°C, 260°C, 270°C, or a value within a range consisting of any two of these values.

[0129] In some embodiments, the coking value of the binder is 50%-70%. In this disclosure, the coking value can be tested using methods known in the art. Exemplarily, it can be determined with reference to GB / T 8727-2008. A carbon residue content in the binder within the above range is beneficial for improving the specific capacity and compaction density of the second negative electrode active material, thereby increasing the energy density of the secondary battery.

[0130] In some embodiments, the mass ratio of the shaping material to the binder is 100:(6-12). A ratio of shaping material to binder within this range helps reduce the amount of residual carbon from the binder, increases the specific capacity and compaction density of the second negative electrode active material, and thus improves the energy density of the secondary battery.

[0131] In some embodiments, the granulated material is pre-carbonized before the graphitization step. This pre-carbonization step helps reduce volatile components in the binder, thereby reducing the impurity content in the second negative electrode active material, increasing its specific capacity and compaction density, and ultimately improving the energy density of the secondary battery.

[0132] In some embodiments, pre-carbonization is performed under a protective atmosphere, which may, for example, be nitrogen or argon. In an alternative embodiment, pre-carbonization is performed under a nitrogen atmosphere.

[0133] In some embodiments, the pre-carbonization temperature is 1000-1500°C. For example, the pre-carbonization temperature can be 1000°C, 1100°C, 1200°C, 1300°C, 1400°C, 1500°C, or any two of these values. By keeping the pre-carbonization temperature within this range, volatile components in the raw material can be sufficiently removed, which helps reduce the number of impurities and defects in the artificial graphite, thereby increasing the specific capacity of the artificial graphite. Additionally, pre-carbonization can increase the density of the raw material, allowing for a larger furnace charge during graphitization. In an optional embodiment, the pre-carbonization temperature is 1000°C-1100°C.

[0134] In some implementations, the pre-carbonization time is 1-4 hours. Exemplary pre-carbonization times can be 1 hour, 2 hours, 3 hours, 4 hours, or any two of these values.

[0135] In some embodiments, the volumetric particle size distribution (Dv50) of the intermediate is 15 μm-18 μm. An intermediate particle size within this range is beneficial for increasing the particle size of the second negative electrode active material, thereby improving the specific capacity of the second negative electrode active material and increasing the energy density of the secondary battery.

[0136] Positive electrode sheet

[0137] 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 active material of the first aspect of the present disclosure.

[0138] 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 two opposite surfaces of the positive current collector.

[0139] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0140] In some embodiments, when the secondary battery is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this disclosure is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0141] In some implementations, the positive electrode active material includes lithium iron phosphate. Using lithium iron phosphate as the positive electrode active material is beneficial for secondary batteries to achieve both high energy density and excellent cycle performance.

[0142] electrolytes

[0143] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This disclosure does not impose specific limitations on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel-like, 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 salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0146] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0147] 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.

[0148] Separating membrane

[0149] In some embodiments, the secondary battery also includes a separator. This disclosure does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0150] 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.

[0151] In some implementations, the positive electrode, negative electrode, and separator can be fabricated into an electrode assembly using a winding or stacking process.

[0152] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0153] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0154] This disclosure does not impose any particular limitation on the shape of the secondary battery, which can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 shows a square-structured secondary battery 5 as an example.

[0155] In some embodiments, referring to FIG3, the outer packaging may include a housing 51 and a cover plate 53. The housing 51 may include a bottom plate and side plates connected to the bottom plate, the bottom plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by 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 secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0156] In some implementations, the secondary batteries can be assembled into a battery module, and the number of secondary batteries contained in the battery module 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 module.

[0157] Figure 4 shows a battery module 4 as an example. Referring to Figure 4, in the battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of the battery module 4. Of course, they can also be arranged in any other manner. Furthermore, the multiple secondary batteries 5 can be fixed in place using fasteners.

[0158] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0159] 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.

[0160] Figures 5 and 6 show a battery pack 1 as an example. Referring to Figures 5 and 6, 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 box 2 and a lower box 3, with the upper box 2 covering the lower box 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.

[0161] In addition, this disclosure also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in this disclosure. The secondary battery, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0162] As an electrical device, a secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0163] Figure 7 shows an example of an electrical device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. 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.

[0164] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0165] Example

[0166] The following describes embodiments of this disclosure. The embodiments described below are exemplary and are only used to explain this disclosure, and should not be construed as limiting this disclosure. 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 the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0167] Preparation of the first negative electrode active material

[0168] Material 1-1

[0169] Step 1: Preparation of the matrix for the first negative electrode active material

[0170] 1) The raw material with a sulfur content of 1.0% by mass is crushed, shaped, graded and screened, and then graphitized at 3150℃ to produce primary particles with a volume distribution particle size Dv50 of 8.7μm, thus obtaining the first artificial graphite component.

[0171] 2) The raw material with a sulfur content of 1.0% by mass is crushed and shaped to obtain primary particles with a volume distribution particle size Dv50 of 8.5 μm. Then, a binder is added for granulation, and graphitization is carried out at 3150℃ to produce secondary particles with a (Dv90-Dv10) / Dv50 of 1.15 and a Dv50 of 14.3 μm, thus obtaining the second artificial graphite component.

[0172] 3) The first artificial graphite component and the second artificial graphite component are mixed at a mass ratio of 50:50 to obtain the matrix of the first negative electrode active material.

[0173] Step 2: The matrix prepared in Step 1 is mixed with an oily asphalt coating agent with a softening point of 250℃ at a mass ratio of 99:1; then heat-treated at 1200℃ for 2 hours to form an amorphous carbon coating layer on the matrix. After demagnetization and sieving, the first negative electrode active material 1-1 is obtained.

[0174] Materials 1-2 to 1-5 and Materials 1-1' to 1-2'

[0175] The preparation methods of materials 1-2 to 1-5 and materials 1-1' to 1-2' are similar to those of material 1-1. The difference is that the type of coating agent, the mass ratio of the coating agent, and the heat treatment temperature in the preparation method are adjusted according to the preparation process parameters shown in Table 1.

[0176] The above materials were tested according to the following methods, and the results are shown in Table 1.

[0177] Calculation of the mass percentage (Y) of the carbon coating layer relative to the total mass of the matrix:

[0178] Y = X * Z / 100, where Z represents the coking value of the coating agent, which is determined according to GB / T 8727-2008; X represents the amount of coating agent added relative to 100 parts by mass of matrix.

[0179] The specific surface area of ​​the material was tested using a Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, USA, following the nitrogen adsorption specific surface area analysis method specified in GB / T 19587-2017.

[0180] The specific surface area of ​​the material was calculated using the BET (Brunauer Emmett Teller) method.

[0181] Thermogravimetric analysis of materials:

[0182] Referring to JY / T 014-1996, 10 mg of the first negative electrode active material was weighed and placed in a flat-bottomed crucible, shaken evenly, and left open. Air was used as the purging gas at a flow rate of 60 mL / min, and the heating rate was 5℃ / min. Thermogravimetric analysis was performed within the range of 35℃-950℃ to obtain the thermogravimetric (TG) curve and thermogravimetric differential (DTG) curve of the first negative electrode active material. The testing instrument was a NETZSCH STA 449F3 simultaneous thermal analyzer from NETZSCH Instruments, Germany. The initial weight loss temperature T0 of the first negative electrode active material is the temperature corresponding to the intersection of the tangent at the horizontal level before the step in the TG curve and the tangent at the maximum weight loss rate. The temperature Tmax corresponding to the maximum weight loss rate of the first negative electrode active material is the peak temperature of the thermogravimetric differential curve of the carbon material.

[0183] The thermogravimetric (TG) curve and thermogravimetric differential (DTG) curve of the first negative electrode active material 1-2 are shown in Figure 8. As can be seen from Figure 8, the initial weight loss temperature T0 of the first negative electrode active material 1-2 is 745.4℃, and the temperature Tmax corresponding to the maximum weight loss rate is 802.5℃.

[0184] Transmission electron microscopy (TEM) testing

[0185] The first anode active material 1-1 was subjected to TEM testing, and the obtained TEM image is shown in Figure 9. As can be seen from the figure, the part with regular lattice stripes is the graphite matrix, and the amorphous carbon without regular lattice stripes is the carbon coating layer.

[0186] Material I D / I G Value testing:

[0187] A Horiba LabRAM HR800 Raman spectrometer was used. The test conditions were as follows: excitation wavelength of 532 nm, grating of 600 lines, objective lens of 50x, integration time of 10 s, and 3 integration scans. A surface scan was performed to obtain the D and G peak intensities at 100 points. The I values ​​at these 100 points were then calculated. D / I G Remove the largest and smallest 30 I's. D / I G The average of the remaining 40 points is the material's I. D / I G .

[0188] Testing of the powder OI value of the material:

[0189] Following the testing method in JIS K 0131-1996, a Bruker D8 Discover X-ray diffractometer was used for testing, with a copper target as the anode and CuKα rays as the radiation source. The wavelength of the rays was... The X-ray diffraction pattern of the powder sample was obtained by scanning with a 2θ angle range of 20°-80° and a scanning rate of 4° / min. Based on the X-ray diffraction pattern, I0 was calculated. 004 and I 110 , where I 004 I is the integrated area of ​​the diffraction peak of the crystalline carbon 004 crystal plane in the powder sample. 110 This represents the integrated area of ​​the diffraction peak on the 110 crystal plane of crystalline carbon in the powder sample. Based on the OI value = I... 004 / I 110 The OI value of the powder in the sample was calculated.

[0190] Material density testing:

[0191] The sample powder was mixed with conductive carbon black (Super P), binder polyvinylidene fluoride (PVDF) in a mass ratio of 91.6:1.8:6.6 and solvent N-methylpyrrolidone (NMP) to form a slurry. The prepared slurry was coated on the surface of the negative electrode current collector copper foil and dried in an oven for later use. Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, using a lithium metal sheet as the counter electrode, the CR2430 coin cell was assembled with the above electrolyte and separator in an argon-protected glove box.

[0192] After the obtained coin cells were allowed to stand for 12 hours, they were discharged at 25°C with a constant current of 0.05C to the cutoff voltage. After standing for 5 minutes, they were discharged again with a constant current of 50μA to the cutoff voltage. After standing for 5 minutes, they were discharged again with a constant current of 10μA to the cutoff voltage. Then, they were charged with a constant current of 0.1C to the cutoff voltage, and the charging capacity was recorded. The ratio of the charging capacity to the sample mass is the specific capacity of the material.

[0193] Table 1:

[0194] Preparation of the second negative electrode active material

[0195] Material 2-1

[0196] Step 1: The calcined needle coke raw material (volatile component content of 0.5% and sulfur content of 0.5%) is mechanically crushed and shaped to obtain a shaped material with a Dv50 of 11μm.

[0197] Step 2: Using granulated asphalt with a softening point of 200℃ as a binder, the above-mentioned shaping material is granulated together in a granulation kettle to obtain granulated material, wherein the mass ratio of shaping material to binder is 100:8.

[0198] Step 3: The above granulated material is pre-carbonized at 1150°C for 2 hours under a nitrogen atmosphere to obtain an intermediate.

[0199] Step 4: Graphitize the above intermediate at a high temperature of 3200℃, and then sieve and demagnetize the graphitized particles to obtain the second negative electrode active material 1-1.

[0200] Materials 2-2 to 2-4 and Materials 2-1' to 2-2'

[0201] The preparation methods of materials 2-2 to 1-4 and materials 2-1' to 2-2' are similar to those of material 2-1, the difference being that the types of raw materials and the graphitization temperature are adjusted.

[0202] For the above materials, the specific capacity, specific surface area, powder OI value, and I... D / I G The values ​​and other parameters were tested according to the method described in the preparation of the first negative electrode active material, and the results are shown in Table 2.

[0203] Table 2:

[0204] Example 1

[0205] (1) Preparation of the first negative electrode active material slurry

[0206] The first negative electrode active material, conductive agent Super P, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were thoroughly mixed in an appropriate amount of deionized water at a weight ratio of 97.0:0.80:1.0:1.2 to form a slurry of the first negative electrode active material.

[0207] (2) Preparation of the second negative electrode active material slurry

[0208] The above-mentioned second negative electrode active material, conductive agent Super P, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are mixed thoroughly in an appropriate amount of deionized water at a weight ratio of 96.6:0.80:1.0:1.4 to form a slurry of the second negative electrode active material.

[0209] (3) Preparation of negative electrode sheet

[0210] Using a dual-cavity coating device, a first negative electrode active material slurry and a second negative electrode active material slurry are simultaneously extruded, with a mass ratio of 5:5. The second negative electrode active material slurry is coated onto the negative electrode current collector copper foil, specifically in the lower coating area, while the first negative electrode active material slurry is coated onto the upper coating area. After drying and cold pressing, the negative electrode sheet is obtained.

[0211] (4) Preparation of positive electrode sheet

[0212] Lithium iron phosphate, conductive carbon black, and PVDF were mixed in a mass ratio of 96:2:2, and then N-methylpyrrolidone solvent was added and stirred until homogeneous to obtain a positive electrode slurry. The positive electrode slurry was coated onto both surfaces of the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0213] (5) Preparation of lithium-ion batteries

[0214] A polyethylene membrane is used as the separator. PVDF slurry is sprayed on both sides of the separator, and a ceramic coating of 1μm is sprayed on one side, with the ceramic coating side corresponding to the cathode electrode.

[0215] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0216] The positive and negative electrode sheets prepared above are placed in sequence, with the separator placed between the positive and negative electrode sheets to provide isolation. Then, the electrode assembly is wound up to obtain the electrode assembly. The electrode assembly is placed in the outer packaging, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, and shaping, a secondary battery is obtained.

[0217] Examples 2-4

[0218] The battery preparation methods in Examples 2-4 are similar to those in Example 1, except that the types of the first and second negative electrode active materials are adjusted in the negative electrode preparation steps, as shown in Table 3.

[0219] Comparative Example 1

[0220] The battery preparation method of Comparative Example 1 is similar to that of Example 1, except that in the preparation step of the negative electrode sheet, the first negative electrode active material slurry is coated on the lower region and the second negative electrode active material slurry is coated on the upper region.

[0221] Comparative Example 2

[0222] The battery preparation method of Comparative Example 2 is similar to that of Example 1, except that in the preparation step of the negative electrode sheet, the first negative electrode active material 1-1 and the negative electrode active material 2-1 are mixed evenly to prepare a negative electrode slurry, and a single-layer negative electrode film is formed on the negative electrode current collector.

[0223] Comparative Example 3

[0224] The battery preparation method of Comparative Example 3 is similar to that of Example 1, except that in the preparation step of the negative electrode sheet, only the first negative electrode active material 1-1 is used to form a single layer of negative electrode film on the negative electrode current collector.

[0225] Comparative Example 4-5

[0226] The battery preparation methods of Comparative Examples 4-5 are similar to those of Example 1, except that the types of the first and second negative electrode active materials are adjusted in the negative electrode preparation steps, as shown in Table 3.

[0227] Performance testing

[0228] (1) Specific capacity of the negative electrode sheet

[0229] A coin cell was assembled using a negative electrode and a lithium sheet. After the resulting coin cell was left to stand for 12 hours, it was discharged at 25°C with a constant current of 0.05C to 0.005V. After standing for 10 minutes, it was discharged again with a constant current of 50μA to 0.005V. After standing for 10 minutes, it was discharged again with a constant current of 10μA to 0.005V. The resulting capacity is the lithium insertion capacity of the negative electrode. Then, it was charged at a constant current of 0.1C to 2V. The resulting capacity is the lithium removal capacity of the negative electrode.

[0230] (2) Energy density

[0231] At 25℃, the secondary battery was charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage of 3.65V to a current of 0.05C, allowed to stand for 5 minutes, and then discharged at a constant current of 1 / 3C to 2.5V. The battery discharge energy was recorded at this point. The battery discharge energy divided by the battery weight is the battery's gravimetric energy density, expressed in Wh / kg.

[0232] (3) Cyclic performance test of secondary batteries

[0233] At 45℃, the secondary battery was charged at a constant current of 1C to the upper limit of 3.65V (corresponding to 100% SOC), then charged at a constant voltage to a current of 0.05C. After resting for 5 minutes, the secondary battery was discharged at a constant current of 1C to 2.5V (corresponding to 0% SOC), and the discharge capacity at this point was recorded, which is the discharge capacity of the first cycle. The secondary battery was subjected to a cyclic charge-discharge test using the above method, and the discharge capacity after each cycle was recorded.

[0234] The capacity retention rate (%) of a secondary battery after 1000 cycles at 45℃ = discharge capacity after 1000 cycles / discharge capacity of the first cycle × 100%.

[0235] (4) Fast charging performance test of secondary batteries

[0236] At 25°C, the secondary battery was charged to 3.65V at a constant current of 0.33C, then charged to 0.05C at a constant voltage. After standing for 5 minutes, the secondary battery was discharged to 2.0V at a constant current of 0.33C, and its actual capacity was recorded as C0.

[0237] Then, the secondary battery was sequentially charged at constant current rates of 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, and 3.0C0 until it reached 3.65V or 0V negative terminal cutoff potential (whichever comes first). After each charge, it was discharged to 2.0V at 1C0. The state of charge (SOC) was recorded at different charging rates until it reached 10%, 20%, 30%, ..., 80%. The negative electrode potential corresponding to the state of charge (SOC) is plotted, and the charging rate-negative electrode potential curves under different SOC states are obtained. After linear fitting, the charging rate corresponding to the negative electrode potential of 0V under different SOC states is obtained. This charging rate is the charging window under that SOC state, and is denoted as C10%SOC, C20%SOC, C30%SOC, C40%SOC, C50%SOC, C60%SOC, C70%SOC, and C80%SOC, respectively. The charging time T from 10%SOC to 80%SOC of the secondary battery (under the premise that the secondary battery does not plaque lithium) is calculated according to the following formula, in minutes. The shorter the charging time, the better the kinetic performance of the secondary battery: T = (60 / C10%SOC + 60 / C20%SOC + 60 / C30%SOC + 60 / C40%SOC + 60 / C50%SOC + 60 / C60%SOC + 60 / C70%SOC + 60 / C80%SOC) × 10%

[0238] The performance of the secondary batteries prepared in the above embodiments and comparative examples was tested, and the results are shown in Table 3.

[0239] Table 3:

[0240] Based on the above results, it can be seen that the secondary batteries prepared in Examples 1-4 have a double-layer design for the negative electrode sheet. The upper first negative electrode active material has a carbon coating layer and a matrix. The first negative electrode active material is subjected to thermogravimetric analysis in an air atmosphere, and the initial weight loss temperature T0 of the first negative electrode active material is measured to be 730℃-780℃. The lower layer uses a second negative electrode active material with high energy density. The energy density of the secondary battery is significantly improved, and the cycle performance and kinetic performance are also significantly improved.

[0241] Compared to Comparative Example 1, the secondary battery of Example 1 uses a first negative electrode active material with a carbon coating in the first region (upper layer) of the negative electrode sheet, and a second negative electrode active material with high energy density in the second region (lower layer), which improves the cycle performance and kinetic performance of the secondary battery. Compared to Comparative Examples 2 and 3, the double-layer design of Examples 1-4 significantly improves the cycle performance and kinetic performance, while also increasing the energy density of the secondary battery. Compared to Comparative Example 4, the initial weight loss temperature T0 of the first negative electrode active material used in the first region of the negative electrode sheet of Examples 1-4 is less than or equal to 780°C, resulting in a significant increase in the energy density of the secondary battery, as well as significantly improved cycle performance and kinetic performance. Compared to Comparative Example 5, the initial weight loss temperature T0 of the first negative electrode active material used in the first region of the negative electrode sheet of Examples 1-4 is greater than or equal to 730°C, which significantly improves the energy density, cycle performance, and kinetic performance of the secondary battery.

[0242] Examples 5-10

[0243] The battery preparation methods in Examples 5-10 are similar to those in Example 1, except that the types of the first and second negative electrode active materials are adjusted in the negative electrode preparation steps, as shown in Table 4.

[0244] The performance of the secondary batteries prepared in the above embodiments was tested, and the results are shown in Table 4.

[0245] Table 4

[0246] As can be seen from the above results, the secondary battery of Embodiments 5-10 of this disclosure improves the energy density, cycle performance and kinetic performance of the secondary battery by making the negative electrode sheet a double-layer design, the first region including a first negative electrode active material including a carbon coating layer, the second region including a second negative electrode active material, and the specific capacity of the second negative electrode active material being 357.0 mAh / g or more.

[0247] Examples 11-14

[0248] The battery preparation methods in Examples 11-14 are similar to those in Example 1, except that the mass ratio of the first negative electrode active material and the second negative electrode active material is adjusted as shown in Table 5 in the negative electrode preparation step.

[0249] The performance of the secondary batteries prepared in the above embodiments was tested, and the results are shown in Table 5.

[0250] Table 5

[0251] As can be seen from the above results, by making the mass ratio of the negative electrode active material in the first region and the negative electrode active material in the second region of the secondary battery in Examples 11-14 30:70-70:30, the energy density, cycle performance and kinetic performance of the secondary battery can be improved.

[0252] It should be noted that this disclosure is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same essential structure and achieving the same effect as the technical concept within the scope of this disclosure are included in the technical scope of this disclosure. 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, are also included in the scope of this disclosure without departing from the spirit of this disclosure.

Claims

A secondary battery includes a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative electrode film layer located on at least one surface of the negative current collector, the negative electrode film layer comprising a first region and a second region, the second region being located between the first region and the negative current collector, the first region comprising a first negative electrode active material, and the second region comprising a second negative electrode active material. The first negative electrode active material includes a matrix and a carbon coating layer formed on at least a portion of the surface of the matrix; The first negative electrode active material was subjected to thermogravimetric analysis in air atmosphere, and the initial weight loss temperature T0 of the first negative electrode active material was 730℃-780℃. The specific capacity of the second negative electrode active material is above 357.0 mAh / g. According to claim 1, the secondary battery, wherein, The first negative electrode active material was subjected to thermogravimetric analysis in air atmosphere, and the temperature Tmax corresponding to the maximum weight loss rate of the first negative electrode active material was 785℃-825℃. The secondary battery according to claim 1 or 2, wherein, The carbon coating layer comprises amorphous carbon. The secondary battery according to any one of claims 1-3, wherein, The matrix includes graphite. The secondary battery according to any one of claims 1-4, wherein, The matrix includes artificial graphite. The secondary battery according to any one of claims 1-5, wherein, The carbon coating layer has a mass percentage content of 0.4%-1% relative to the mass of the substrate. The secondary battery according to any one of claims 1-6, wherein, The carbon coating layer has a mass percentage content of 0.5%-0.95% relative to the mass of the substrate. The secondary battery according to any one of claims 1-7, wherein, The specific capacity of the second negative electrode active material is 357.0–365 mAh / g. The secondary battery according to any one of claims 1-8, wherein, The specific surface area of ​​the first negative electrode active material is 0.5 m². 2 / g-1.8m 2 / g; and / or The specific surface area of ​​the second negative electrode active material is 0.5 m². 2 / g-2.2m 2 / g. The secondary battery according to claim 9, wherein, The specific surface area of ​​the first negative electrode active material is 0.5 m². 2 / g-1.6m 2 / g; The specific surface area of ​​the second negative electrode active material is 0.5 m². 2 / g-2.0m 2 / g. The secondary battery according to any one of claims 1-10, wherein, The first negative electrode active material I D / I G The value is greater than the I of the second negative electrode active material. D / I G Value, of which, I D This indicates that the Raman spectrum of the material is at 1350±50 cm⁻¹ -1 The intensity of peak D at I G This indicates that the Raman spectrum of the material is at 1580±50 cm⁻¹ -1 The intensity of peak G at point I; D / I G The value represents the ratio of the D-band strength to the G-band strength. The secondary battery according to any one of claims 1-11, wherein, The first negative electrode active material I D / I G The value is 0.10-0.25; and / or, The second negative electrode active material I D / I G The value is 0.02-0.

17. The secondary battery according to claim 12, wherein, The first negative electrode active material I D / I G The value is 0.11-0.23; and / or, The second negative electrode active material I D / I G The value is 0.02-0.

14. The secondary battery according to any one of claims 1-13, wherein, The OI value of the powder of the first negative electrode active material is less than that of the powder of the second negative electrode active material. The secondary battery according to any one of claims 1-14, wherein, The powder OI value of the first negative electrode active material is 2.5-6.5; and / or The OI value of the second negative electrode active material powder is 5.0-20.

5. The secondary battery according to any one of claims 1-15, wherein, The powder OI value of the first negative electrode active material is 2.8-6.3; and / or The OI value of the second negative electrode active material powder is 5.5-19.

5. The secondary battery according to any one of claims 1-16, wherein, The degree of graphitization of the first negative electrode active material is less than that of the second negative electrode active material. The secondary battery according to any one of claims 1-17, wherein, The degree of graphitization of the first negative electrode active material is 92.0%-94.0%; and / or, The degree of graphitization of the second negative electrode active material is 94.0%-96.5%. The secondary battery according to any one of claims 1-18, wherein, The graphitization degree of the first negative electrode active material is 92.2%-93.9%; and / or, The degree of graphitization of the second negative electrode active material is 94.2%-96.0%. The secondary battery according to any one of claims 1-19, wherein, The first negative electrode active material includes secondary particles; the second negative electrode active material includes secondary particles. The secondary battery according to any one of claims 1-20, wherein, In the X-ray diffraction patterns of the first negative electrode active material and / or the second negative electrode active material, there is no diffraction peak of the 3R phase 101 crystal plane at 43°-44°. The secondary battery according to any one of claims 1-21, wherein, The first negative electrode active material satisfies one or more of the following conditions: The volume distribution particle size Dv50 of the first negative electrode active material is 9.0 μm-15.0 μm; The particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.8-1.3; The first negative electrode active material has a powder compaction density of 1.72-1.87 g / cc under a pressure of 50,000 N; The specific capacity of the first negative electrode active material is 352.2-359.6 mAh / g; and / or The tap density of the first negative electrode active material is 1.00-1.25 g / cc. The secondary battery according to any one of claims 1-22, wherein, The first negative electrode active material satisfies one or more of the following conditions: The volume distribution particle size Dv50 of the first negative electrode active material is 9.5-14.5 μm; The particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.85-1.25; The first negative electrode active material has a powder compaction density of 1.75-1.85 g / cc under a pressure of 50,000 N. The specific capacity of the first negative electrode active material is 353.0-359.0 mAh / g; and / or The tap density of the first negative electrode active material is 1.05-1.20 g / cc. The secondary battery according to any one of claims 1-23, wherein, The second negative electrode active material satisfies one or more of the following conditions: The volume distribution particle size Dv50 of the second negative electrode active material is 13.5 μm-19.5 μm; The particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.9-1.5; The second negative electrode active material has a powder compaction density of 1.93 g / cc to 2.06 g / cc under a pressure of 50,000 N. The secondary battery according to any one of claims 1-24, wherein, The second negative electrode active material satisfies one or more of the following conditions: The volume distribution particle size Dv50 of the second negative electrode active material is 14.0 μm-19.0 μm; The particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.95-1.45; The second negative electrode active material has a powder compaction density of 1.95 g / cc to 2.04 g / cc at 50,000 N. The secondary battery according to any one of claims 1-25, wherein, The first negative electrode material and / or the second negative electrode active material further include one or more of silicon materials, silicon-carbon materials, and silicon composite materials. The secondary battery according to any one of claims 1-26, wherein, In the negative electrode film layer, the mass ratio of the negative electrode active material in the first region to the negative electrode active material in the second region is 30:70-70:

30. The secondary battery according to any one of claims 1-27, wherein, The specific capacity of the negative electrode sheet is 355mAh / g-365mAh / g. The secondary battery according to any one of claims 1-28, wherein, The secondary battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes at least one of a lithium phosphate with an olivine structure, a lithium transition metal oxide, and their respective modified compounds. The secondary battery according to claim 29, wherein, The positive electrode active material includes lithium iron phosphate. An electrical device comprising a secondary battery as described in any one of claims 1-30.

Images