Secondary battery and electric apparatus

Abstract

A secondary battery and an electric apparatus. 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 has a first surface away from the negative electrode current collector and a second surface opposite to the first surface, and the thickness of the negative electrode film layer is denoted as H. A region within the thickness range from the first surface of the negative electrode film layer to 0.3H is denoted as a first region of the negative electrode film layer, and a region within the thickness range from the second surface of the negative electrode film layer to 0.3H is denoted as a second region of the negative electrode film layer. The first region comprises a first negative electrode active material, and the second region comprises a second negative electrode active material. The first negative electrode active material comprises a base and a carbon coating layer formed on at least part of the surface of the base. Moreover, the lithium-intercalation platform voltage of the first negative electrode active material is 0.118 V to 0.140 V. The gram capacity of the second negative electrode active material is greater than the gram capacity of the first negative electrode active material.

Description

Secondary battery and power consuming device

[0001] Cross-reference to related applications

[0002] The present disclosure is based on and claims priority to Chinese Patent Application No. 202411301523.X, filed on September 18, 2024, entitled "Secondary battery and power consuming device", the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates to the technical field of batteries, and in particular to a secondary battery and a power consuming device. BACKGROUND

[0004] In recent years, with the application range of secondary batteries becoming more and more extensive, secondary batteries are widely used in energy storage power supply systems such as hydropower, thermal power, wind power and solar power stations, and in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, etc. Due to the great development of secondary batteries, higher requirements are put forward for their kinetic performance and energy density. SUMMARY

[0005] The present disclosure is made in view of the above-mentioned problems, and aims to provide a secondary battery and a power consuming device. The secondary battery has good kinetic performance and high energy density.

[0006] To achieve the above-mentioned purpose, the present disclosure provides a secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising 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 having a first surface away from the negative electrode current collector and a second surface opposite to the first surface, the thickness of the negative electrode film layer being denoted as H, a region within the thickness range of 0.3H from the first surface of the negative electrode film layer being denoted as a first region of the negative electrode film layer, and a region within the thickness range of 0.3H from the second surface of the negative electrode film layer being denoted as a second region of the negative electrode film layer, 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 comprising a matrix and a carbon coating layer formed on at least part of the surface of the matrix; and the lithium intercalation platform voltage of the first negative electrode active material being 0.118V-0.140V, wherein the lithium intercalation platform voltage is obtained by charging and discharging test on a button cell prepared by using the test material under the conditions that the lithium extraction rate is 0.1C and the lithium intercalation rate is 0.05C, and the charging and discharging curve within the range of 0.005V-2.0V is obtained, wherein the ratio of the total lithium intercalation energy within 0.005V-2.0V to the total lithium intercalation capacity is defined as the lithium intercalation platform voltage of the test material; and the gram capacity of the second negative electrode active material is greater than that of the first negative electrode active material. Thus, the secondary battery has good kinetic performance and high energy density.

[0007] In some embodiments, the lithium intercalation platform voltage of the first negative active material is 0.123V-0.135V. This is beneficial for further improving the kinetic performance of the secondary battery.

[0008] In some embodiments, the carbon coating layer comprises soft carbon. This is beneficial for providing more active ion channels, thereby improving the kinetic performance of the secondary battery.

[0009] In some embodiments, the first negative active material comprises primary particle graphite material and secondary particle graphite material. By mixing the primary particle graphite material and the secondary particle graphite material, the kinetic performance of the secondary battery can be further improved.

[0010] In some embodiments, the proportion of the secondary particle graphite material in the first negative active material is greater than or equal to 60%. By using secondary particles as the main component in the first negative active material, the isotropy is high, and the expansion of the secondary battery during the cycle process can be dispersed in all directions, reducing the expansion of the pole piece and improving the cycle performance.

[0011] In some embodiments, the first negative active material powder OI value is 2.0-6.0. In this way, lithium ions can be inserted from all directions, improving the fast charging performance of the secondary battery.

[0012] In some embodiments, the gram capacity of the second negative active material is greater than or equal to 359mAh / g. This is beneficial for improving the energy density of the secondary battery.

[0013] In some embodiments, the gram capacity of the second negative active material is 360mAh / g-366mAh / g. This is beneficial for improving the energy density of the secondary battery.

[0014] In some embodiments, the powder compaction density of the first negative active material under a pressure of 50000N is 1.74g / cc-1.79g / cc. This is beneficial for providing appropriate porosity in the first region, thereby facilitating the infiltration of the electrolyte. In some embodiments, the powder compaction density of the second negative active material under a pressure of 50000N is 1.95g / cc-2.04g / cc. This is beneficial for providing appropriate porosity in the second region while improving the energy density of the secondary battery. In some embodiments, the volume distribution particle size Dv50 of the first negative active material is 7.8μm-15.8μm. By having the volume distribution particle size Dv50 of the first negative active material in the above range, it is beneficial to form a better pore distribution state in the first region of the pole piece, thereby more beneficially improving the transmission performance of ions and electrons and improving the fast charging performance of the battery.

[0015] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.90 to 1.50. The particle size distribution in this range indicates good distribution of the negative electrode active material particles, which is conducive to the upper layer region of the negative electrode film layer having a suitable pore structure, thereby reducing the difficulty of ion liquid phase transmission and further improving the kinetic performance of the secondary battery.

[0016] In some embodiments, the BET specific surface area of the first negative electrode active material is 0.6 m 2 / g to 1.3 m 2 / g. Thereby, it is conducive to reducing the surface side reaction activity of the first negative electrode active material and improving the cycle performance of the secondary battery.

[0017] In some embodiments, the specific capacity of the first negative electrode active material is 350.5 mAh / g to 358.5 mAh / g. Thereby, it is conducive to improving the energy density of the secondary battery.

[0018] In some embodiments, the La(110) of the second negative electrode active material is 130 nm to 175 nm, and the Lc(002) is 30 nm to 42 nm, wherein La(110) represents the crystallite size along the a-axis in the (110) crystal plane of the material, and Lc(002) represents the crystallite size along the c-axis in the (002) crystal plane of the material. Thereby, it is conducive to the secondary battery having high energy density.

[0019] In some embodiments, La(110) / Lc(002) is 3.5 to 5.5. Thereby, it is conducive to the secondary battery having high energy density.

[0020] In some embodiments, the charge-discharge test is performed on the button cell prepared from the second negative electrode active material at a delithiation rate of 0.1C and a lithium intercalation rate of 0.05C, respectively, to obtain a charge-discharge curve in the range of 0.005V to 2.0V, and in the discharge curve of the second negative electrode active material, there is a lithium intercalation platform in the voltage range of 0.005V to 0.070V, and the proportion X1 of the discharge capacity corresponding to the lithium intercalation platform in the total discharge capacity of the button cell is 43% or more. Thereby, it is conducive to improving the energy density of the secondary battery.

[0021] In some embodiments, the proportion X1 of the discharge capacity corresponding to the lithium intercalation platform in the total discharge capacity of the button cell is 43% to 47%.

[0022] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.9 to 1.25. Thereby, it is conducive to improving the active ion and electron transport performance in the negative electrode film layer, and further improving the kinetic performance of the secondary battery.

[0023] In some embodiments, the BET specific surface area of the second negative active material is 0.8 m2 / g to 2.1 m2 / g. 2 / g to 2.1 m 2 / g. Thus, it is beneficial to reduce the surface side reaction activity of the negative active material, thereby improving the cycle performance and the initial coulombic efficiency of the secondary battery.

[0024] In some embodiments, the mass ratio of the first negative active material and the second negative active material is 4:6 to 6:4. Thus, it is beneficial to improve the kinetic performance of the secondary battery while taking into account the energy density.

[0025] In some embodiments, the compaction density of the negative electrode film layer is 1.50 g / cc to 1.90 g / cc. Thus, it is beneficial for the negative electrode film layer to have high capacity, good active ion and electron transport performance, thereby benefiting the secondary battery to have high energy density and good kinetic performance.

[0026] In some embodiments, the area density of the negative electrode film layer is 6.0 mg / cm 2 / 24.0 mg / cm 2 . Thus, it is beneficial for the negative electrode film layer to have high capacity, good active ion and electron transport performance, thereby benefiting the secondary battery to have high energy density and good kinetic performance.

[0027] In some embodiments, the thickness of the negative electrode film layer is 60 μm to 240 μm. Thus, it is beneficial for the secondary battery to have good kinetic performance while taking into account the energy density.

[0028] In some embodiments, the porosity of the first region is greater than the porosity of the second region. By providing the negative electrode film layer as a double layer, in which the porosity of the upper region (the first region) is greater than the porosity of the lower region (the second region), the contact opportunity of the first region with active ions in the electrolyte is increased, so that the active ions can migrate faster during the charging and discharging process, improving the lithium intercalation kinetics of the material, thereby improving the kinetic performance of the battery.

[0029] In some embodiments, the preparation method of the first negative active material comprises:

[0030] providing a base material;

[0031] mixing the base material and the organic carbon source, and then performing heat treatment to obtain the first negative active material;

[0032] wherein the mass ratio of the base material to the organic carbon source is 100:1.5 to 100:8.

[0033] Thus, by setting the base material and the organic carbon source in the above-mentioned mass ratio range, the obtained first negative electrode active material has a lithium intercalation plateau voltage of 0.118 V to 0.140 V.

[0034] A second aspect of the present disclosure provides a power consuming device including the secondary battery of the first aspect of the present disclosure. Since the power consuming device of the present disclosure includes the secondary battery provided by the present disclosure, it has at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF DRAWINGS

[0035] FIG. 1 is a schematic view of an embodiment of the negative electrode sheet of the present disclosure.

[0036] FIG. 2 is a schematic view of a battery cell of an embodiment of the present disclosure.

[0037] FIG. 3 is an exploded view of the battery cell of the embodiment of the present disclosure shown in FIG. 1.

[0038] FIG. 4 is a schematic view of a battery module of an embodiment of the present disclosure.

[0039] FIG. 5 is a schematic view of a battery pack of an embodiment of the present disclosure.

[0040] FIG. 6 is an exploded view of the battery pack of the embodiment of the present disclosure shown in FIG. 5.

[0041] FIG. 7 is a schematic view of a power consuming device using the secondary battery of an embodiment of the present disclosure as a power source.

[0042] FIG. 8 is a charge-discharge curve obtained by performing charge-discharge tests on button cells prepared from materials 1-1 and 1-6, respectively.

[0043] FIG. 9 is a charge-discharge curve obtained by performing charge-discharge tests on a button cell prepared from material 2-1.

[0044] FIG. 10 is an image obtained by scanning material 1-1 using a transmission electron microscope (TEM).

[0045] FIG. 11 is an image obtained by scanning the negative electrode sheet prepared in Example 1 of the present disclosure using a scanning electron microscope (SEM).

[0046] REFERENCE NUMERALS: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 battery cell; 51 case; 52 electrode assembly; 53 top cap assembly; 10 negative electrode sheet; 101 negative electrode current collector; 102 negative electrode film layer; 102a first surface; 102b second surface; 1021 first region; 1022 second region; 1023 intermediate region. DETAILED DESCRIPTION

[0047] Hereinafter, embodiments of a secondary battery and an electric device of the present disclosure are specifically disclosed in detail with appropriate reference to the accompanying drawings. However, there are cases where unnecessary detailed description is omitted. For example, there are cases where detailed description of matters known well, repeated description of substantially the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter recited in the claims.

[0048] The "ranges" disclosed in the present disclosure are defined in the form of lower and upper limits, and a given range is defined by selecting one lower limit and one upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The ranges defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if the ranges 60-120 and 80-110 are listed for a particular parameter, it is understood that the ranges 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4 and 5 are listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present disclosure, unless otherwise stated, a numerical range "a-b" represents a shorthand manner of describing each and every numerical value that is contained within the range, wherein a and b are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely a shorthand manner of describing those numerical combinations. In addition, when it is stated that a parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0049] If not specifically stated, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions.

[0050] If not specifically stated, all technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions.

[0051] If not specifically stated, the terms used in the present disclosure have the commonly understood meanings understood by those skilled in the art.

[0052] If not specifically stated, the numerical values of the parameters mentioned in the present disclosure can be measured by various test methods commonly used in the art, for example, can be measured according to the test methods given in the present disclosure.

[0053] If not specifically stated, in the present disclosure, the term "active ion" refers to an ion that can be reversibly intercalated and deintercalated between the positive electrode and the negative electrode of a secondary battery, including but not limited to lithium ion.

[0054] Currently, in order to improve the energy density of the secondary battery, the negative electrode plate is usually thickly coated to prepare a high-load electrode. However, although the thick coating design improves the energy density of the battery cell by increasing the total amount of active material, the increase in thickness leads to an increase in the charge transfer path, and the transmission dynamics of electrons and active ions are limited, resulting in poor kinetic performance, such as fast charging performance, of the secondary battery.

[0055] Based on this, the present disclosure provides a secondary battery and a power utilization device. The secondary battery has improved kinetic performance while taking into account high energy density. The present disclosure and optional embodiments are described in more detail below.

[0056] Secondary battery

[0057] The present disclosure provides a secondary battery. The term "secondary battery" mentioned herein refers to a battery monomer, a battery module or a battery pack. The following are described respectively.

[0058] The secondary battery monomer includes a positive electrode plate, a negative electrode plate, an electrolyte and a separator. During the charging and discharging process of the battery, active ions are embedded and extracted between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The separator is arranged between the positive electrode plate and the negative electrode plate, mainly to prevent short circuiting of the positive and negative electrodes, while allowing ions to pass through.

[0059] Negative electrode plate

[0060] The secondary battery of the present disclosure includes a negative electrode plate, the negative electrode plate includes 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 has a first surface away from the negative electrode current collector and a second surface arranged opposite to the first surface, the thickness of the negative electrode film layer is denoted as H, the region within the thickness range of 0.3H from the first surface of the negative electrode film layer is denoted as the first region of the negative electrode film layer, and the region within the thickness range of 0.3H from the second surface of the negative electrode film layer is denoted as the second region of the negative electrode film layer, the first region includes a first negative electrode active material, the second region includes a second negative electrode active material, the first negative electrode active material includes a matrix and a carbon coating layer formed on at least part of the surface of the matrix; and the lithium intercalation platform voltage of the first negative electrode active material is 0.118V-0.140V, wherein the lithium intercalation platform voltage is obtained by charging and discharging test of the test material on the prepared button cell under the conditions of 0.1C of the delithiation rate and 0.05C of the lithium intercalation rate, respectively, to obtain the charging and discharging curve within the range of 0.005V-2.0V, wherein the ratio of the total lithium intercalation energy within 0.005V-2.0V to the total lithium intercalation capacity within the range of 0.005V-2.0V is defined as the lithium intercalation platform voltage of the test material; the gram capacity of the second negative electrode active material is greater than that of the first negative electrode active material.

[0061] In the present disclosure, by setting the first negative electrode active material in the upper layer (first region) to include a carbon coating layer, on the one hand, due to its good electrical conductivity and ion transmission capacity, it can provide more ion transmission channels on the surface of the matrix, thereby increasing the lithium intercalation channel, on the other hand, the carbon coating layer reduces the outer surface area of the matrix, effectively reducing the co-intercalation phenomenon of the electrolyte, thereby reducing the interface impedance, but the presence of the carbon coating layer will have a certain impact on the specific capacity of the material. The lithium intercalation channel and the interface impedance are important factors affecting the speed and efficiency of lithium ion transmission, and thus affect the lithium intercalation platform voltage. In the present disclosure, by making the first negative electrode active material have a lithium intercalation platform voltage of 0.118V-0.140V (greater than 0.118V), it means that the material can make the active ions quickly deintercalate on the surface of the first negative electrode active material and be less likely to occur lithium precipitation while maintaining a high specific capacity, thereby effectively improving the kinetic performance of the secondary battery. At the same time, further, by setting the specific capacity of the second negative electrode active material in the lower layer (second region) to be greater than that of the first negative electrode active material, the secondary battery has high energy density.

[0062] Referring to FIG. 1, a schematic diagram of an embodiment of the negative electrode sheet of the present disclosure is shown. As shown in FIG. 1, 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 opposite to the first surface 102a, and 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 on one side of the negative electrode current collector. The region within the range of 0.3H in thickness from the first surface 102a of the negative electrode film layer is denoted as the first region 1021 (upper layer) of the negative electrode film layer. The region within the range of 0.3H in thickness from the second surface 102b of the negative electrode film layer is denoted as the second region 1022 (lower layer) of the negative electrode film layer. The first region 1021 includes the first negative electrode active material, and the second region 1022 includes the 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 easy to understand that within the range of the intermediate region 1023, it can only contain the first negative electrode active material, only contain the second negative electrode active material, contain both the first negative electrode active material and the second negative electrode active material, or contain other negative electrode active materials known in the art in addition to the first negative electrode active material and the second negative electrode active material of the present disclosure.

[0063] It should be understood that the second surface 102b is in contact with the upper surface of the negative current collector 101 in the embodiment shown in FIG. 1, but the structure of the negative electrode sheet of the present disclosure is not limited thereto. For example, there can be an additional layer between the negative film layer 102 and the negative current collector 101. In this case, the second surface 102b is not in direct contact with the negative current collector 101.

[0064] It should also be understood that although clear boundaries between regions are shown in FIG. 1, such clear interfaces can not exist in the product.

[0065] In some embodiments, the lithium intercalation plateau voltage of the first negative active material is 0.123V-0.135V. The lithium intercalation rate of the battery is positively correlated with the electrode potential, i.e., the larger the electrode potential, the larger the lithium intercalation rate of the battery. The lithium intercalation plateau voltage in this range allows lithium intercalation to occur at a relatively high voltage, thereby facilitating the improvement of the kinetic performance of the secondary battery and reducing the likelihood of lithium precipitation.

[0066] In some embodiments, the carbon coating layer comprises soft carbon. The lithium storage process of soft carbon is mainly the adsorption of active lithium ions on its structural defects, edges and micropores, and has relatively fast reaction kinetics. Thus, it is beneficial to improve the kinetic performance of the secondary battery.

[0067] In the present disclosure, the matrix of the first negative active material and the carbon coating layer can be distinguished by transmission electron microscopy (TEM) of the material. In the transmission electron microscopy (TEM) image of the first negative active material, the matrix has regular lattice fringes, while the carbon coating layer has no obvious lattice fringes, which is soft carbon.

[0068] In the present disclosure, both primary particles and secondary particles are well-known meanings in the art. Among them, the primary particle refers to a particle in a non-agglomerated state, and the secondary particle refers to a particle in an agglomerated state formed by the aggregation of two or more primary particles. Primary particles and secondary particles can be distinguished by using scanning electron microscopy (SEM) images.

[0069] In some embodiments, the amount of the secondary particle graphite material in the first negative electrode active material is greater than or equal to 60%, for example, 60%, 65%, 70%, 75%, 80%, or a value between any two of these values. Optionally, the amount of the secondary particle graphite material is 75%, and the two types of graphite materials (i.e., the primary particle graphite material and the secondary particle graphite material) are used in combination by being arranged in the region of the negative electrode film layer away from the negative electrode current collector (i.e., the first region), wherein the granulation structure of the secondary particle graphite material is conducive to reducing the OI value of the material, thereby being conducive to fast charging performance, and the single particle graphite material (i.e., the primary particle graphite material) is conducive to increasing the specific surface area of the active material and reducing the charge transfer resistance (Rct), further improving the fast charging performance of the battery. By using the secondary particle as the main material in the first negative electrode active material, the isotropy is high, and the expansion of the secondary battery during the cycle process can be dispersed in all directions, reducing the expansion of the electrode sheet and improving the cycle performance.

[0070] In some embodiments, the powder OI value of the first negative electrode active material is 2.0-6.0. Illustratively, the powder OI value of the first negative electrode active material can be 2, 3, 4, 4.5, 5, 6, or a value between any two of these values. Optionally, the powder OI value of the first negative electrode active material is 4.5-6.0. In this way, lithium ions can be inserted from all directions, improving the charging capacity of the secondary battery.

[0071] In the present disclosure, the powder optimization index (Optimization Index, referred to as OI) is a quantitative index for characterizing the degree of crystal orientation of a powder sample, which evaluates the growth and arrangement of crystals by comparing the ratio of the integral area of the diffraction peak of the (004) crystal plane of crystalline carbon in the powder sample to the integral area of the diffraction peak of the (110) crystal plane of crystalline carbon. The OI value can be determined by instruments and methods known in the art. For example, the X-ray diffraction pattern of the powder sample can be obtained by referring to JIS K 0131-1996 and JB / T 4220-2011, and the powder OI value of the sample can be calculated according to OI value = I(004) / I(110). I(004) is the integral area of the diffraction peak of the (004) crystal plane of crystalline carbon in the powder sample, and I(110) is the integral area of the diffraction peak of the (110) crystal plane of crystalline carbon in the powder sample. In the X-ray diffraction analysis test of the embodiments of the present disclosure, a copper target can be used as an anode target, CuKα rays are used as the radiation source, the wavelength of the rays is 1.5406 A, the scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min. The scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min.

[0072] In some embodiments, the first negative active material has a powder compaction density under 50000N pressure that is less than the powder compaction density under 50000N pressure of the second negative active material. This results in the first region having a porosity that is greater than the porosity of the second region, which is beneficial for increasing the contact opportunity of the upper region with active ions in the electrolyte, allowing the active ions to migrate faster during the charge and discharge process, improving the lithium intercalation kinetics of the material, and thus improving the fast charging performance of the battery.

[0073] In some embodiments, the first negative active material has a powder compaction density under 50000N pressure of 1.74g / cc to 1.79g / cc. For example, the first negative active material has a powder compaction density under 50000N pressure of 1.74g / cc, 1.75g / cc, 1.76g / cc, 1.79g / cc, or a value between any two of these values. This is beneficial for providing an appropriate porosity in the first region, which is beneficial for the infiltration of the electrolyte, and thus for improving the kinetic performance of the secondary battery.

[0074] In some embodiments, the second negative active material has a powder compaction density under 50000N pressure of 1.95g / cc to 2.04g / cc. By having the powder compaction density of the second negative active material in the above range, it is beneficial to provide an appropriate porosity in the first region, while also beneficial to improve the energy density of the secondary battery. Illustratively, the second negative active material has a powder compaction density under 50000N pressure of 1.95g / cc, 1.96g / cc, 1.97g / cc, 1.98g / cc, 1.99g / cc, 2.00g / cc, 2.01g / cc, 2.02g / cc, 2.03g / cc, 2.04g / cc, or a value between any two of these values.

[0075] In the present disclosure, the powder compaction density of a material is the mass per unit volume of the powder under specified conditions. It can be measured using instruments and methods known in the art. For example, it can be measured according to GB / T 24533-2009, using an electronic pressure testing machine (e.g., a UTM7305 electronic pressure testing machine). An illustrative testing method is as follows: 1g of sample powder is weighed and added into a mold with a bottom area of 1.327cm 2 The powder compaction density of the material under 50000N pressure is then recorded and calculated.

[0076] In some embodiments, the first negative active material has a volume distribution particle size Dv50 of 7.8 μm to 15.8 μm. By having the volume distribution particle size Dv50 of the first negative active material in the above range, it is beneficial to form a better pore distribution state in the first region of the electrode tab, thereby more beneficial to improve the transmission performance of ions and electrons, and improve the fast charging performance of the battery. For example, the volume distribution particle size Dv50 of the first negative active material is 7.8 μm, 8.0 μm, 8.5 μm, 9.0 μm, 9.5 μm, 10.0 μm, 10.5 μm, 11.0 μm, 11.5 μm, 12.0 μm, 12.5 μm, 15.8 μm, or a value between any two of these values.

[0077] In some embodiments, the first negative active material has a particle size distribution (Dv90-Dv10) / Dv50 of 0.90 to 1.50. With the particle size distribution in this range, it indicates a good distribution of negative active material particles, which is beneficial to the upper region of the negative film layer to have a suitable pore structure, thereby reducing the difficulty of ion liquid phase transmission, and further improving the kinetic performance of the secondary battery. For example, the particle size distribution (Dv90-Dv10) / Dv50 of the first negative 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 between any two of these values.

[0078] In the present disclosure, the volume distribution particle sizes Dv10, Dv50, Dv90 of a material are the meanings commonly known in the art, which respectively represent the particle sizes corresponding to the cumulative volume distribution percentages of 10%, 50%, and 90% of the material, and can be measured by instruments and methods known in the art. For example, GB / T 19077-2016 can be referred to, and a laser particle size analyzer can be used for measurement. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0079] In some embodiments, the first negative active material has a BET specific surface area of 0.6 m 2 / g to 1.3 m 2 / g. By having the specific surface area of the first negative active material in the above range, on the one hand, the first negative active material can have a lower surface side reaction activity, thereby being able to reduce the consumption of active ions by SEI film formation, and improve the initial coulombic efficiency of the secondary battery; on the other hand, it can also have a higher active ion transmission performance, and improve the kinetic performance of the secondary battery. For example, the BET specific surface area of the first negative active material is 0.6 m 2 / g, 0.7 m 2 / g, 0.8 m 2 / g, 0.9 m 2 / g, 1.0 m 2 / g, 1.1 m 2 / g, 1.2 m 2 / g, 1.3 m 2 / g, or a value between any two of these values.

[0080] In the present disclosure, the specific surface area of a material is in the meaning known in the art and can be determined using instruments and methods known in the art. For example, it can be tested according to GB / T 19587-2017 using nitrogen adsorption specific surface area analysis test method and calculated using BET (Brunauer Emmett Teller) method. The testing instrument can be Tri-Star 3020 specific surface area pore size analyzer from Micromeritics, USA.

[0081] In some embodiments, the gram capacity of the first negative electrode active material is 350.5 mAh / g to 358.5 mAh / g, for example, the gram capacity of the first negative electrode active material is 350.5 mAh / g, 351.0 mAh / g, 352.0 mAh / g, 353.0 mAh / g, 354.0 mAh / g, 355.0 mAh / g, 356.0 mAh / g, 357.0 mAh / g, 358.0 mAh / g, 358.5 mAh / g, or a value between any two of these values. By having the gram capacity of the first negative electrode active material within the above range, it is beneficial for the secondary battery to have good dynamic performance while taking into account the energy density.

[0082] In the present disclosure, the gram capacity of a material is the ratio of the dischargeable capacity of the active material to the mass of the active material, which can be tested using methods known in the art. An exemplary testing method is as follows: a sample powder is mixed with a conductive agent, a binder, and optionally other additives in a certain mass ratio with a solvent to form a slurry; the prepared slurry is coated on the surface of a negative electrode current collector copper foil, dried in an oven, and then used; an electrolytic salt is dissolved in an organic solvent to prepare an electrolyte of a certain concentration; then a lithium metal sheet is used as a counter electrode, a polyethylene (PE) film is used as a separator film, and the electrolyte is assembled into a CR2430 type button cell in an argon glove box. After the obtained button cell is left for 12 h, it is discharged at 25°C at a constant current of 0.15 mA to 0.005 V, left for 10 min, discharged at a constant current of 50 μA to 0.005 V again, left for 10 min, and discharged at a constant current of 10 μA to 0.005 V; then it is charged at a constant current of 0.3 mA to 2.0 V, and the charge capacity is recorded. The ratio of the charge capacity to the mass of the sample is the gram capacity of the corresponding material. In some embodiments, the gram capacity of the second negative electrode active material is 359 mAh / g or more. Alternatively, the gram capacity of the second negative electrode active material is 360 mAh / g to 366 mAh / g. By having the gram capacity of the second negative electrode active material in the above range, the gram capacity of the negative electrode film layer is improved, and thus the energy density of the secondary battery is improved. Exemplarily, the gram capacity of the second negative electrode active material can be 359 mAh / g, 360 mAh / g, 361 mAh / g, 362 mAh / g, 363 mAh / g, 364 mAh / g, 365 mAh / g, 366 mAh / g, or a value between any two of them. In some embodiments, the La(110) of the second negative electrode active material is 130 nm to 175 nm, and the Lc(002) is 30 nm to 42 nm, where La(110) represents the crystallite size along the a axis in the (110) crystal plane of the material, and Lc(002) represents the crystallite size along the c axis in the (002) crystal plane of the material.

[0083] In the present disclosure, a relatively large La of the second negative electrode active material indicates that the crystallite size of the second negative electrode active material is relatively large, so that the surface lithium intercalation sites increase, and thus the gram capacity is high. In addition, a large crystallite size can also reduce the number of material crystalline grain boundaries, further increase the lithium intercalation sites on the surface of the material, thereby reducing the adverse effects of the gram capacity and the powder compaction density, and thus improving the gram capacity and the powder compaction density of the material. The Lc of the second negative electrode active material is in the above range, which is beneficial to reducing the stacking of graphite crystallites, thereby reducing the number of grain boundaries between the crystallites and increasing the lithium intercalation sites on the surface of the material, thereby improving the gram capacity of the material. Thus, the second negative electrode active material located in the second region has a high gram capacity and a high compaction density, which is beneficial to the secondary battery having a high energy density.

[0084] The La(110) of the second negative active material is 130 nm to 175 nm, and the Lc(002) is 30 nm to 42 nm. Illustratively, the La(110) is 130 nm, 135 nm, 140 nm, 145 nm, 150 nm, 155 nm, 160 nm, 165 nm, 170 nm, 172 nm, 173 nm, 175 nm, or a value between a range consisting of any two of them, and the Lc(002) is 30 nm, 31 nm, 32 nm, 33 nm, 34 nm, 35 nm, 36 nm, 37 nm, 38 nm, 39 nm, 40 nm, 41 nm, 42 nm, or a value between a range consisting of any two of them. Alternatively, the La(110) of the second negative active material is 132 nm to 172 nm, and the Lc(002) is 30 nm to 36 nm. By making the La(110) and the Lc(002) of the second negative active material within the above ranges, the specific capacity and the tap density of the second negative active material are more likely to be improved, thereby facilitating the secondary battery to have a high energy density.

[0085] In the present disclosure, the La(110) and the Lc(002) of the material can be determined using instruments and methods known in the art. For example, the negative electrode sheet detached from the secondary battery can be cleaned with an organic solvent such as DMC, and then dried. Then the cleaned negative electrode sheet is immersed in NMP and ultrasonically treated, and the copper foil is separated to obtain the negative electrode material. The negative electrode material is dried, and then powdering is performed at 350°C to 500°C, and then washed with water multiple times, and dried at 80°C to obtain the first negative active material. The X-ray diffractometer (such as Bruker D8 Discover) is used for testing, and the test can refer to JIS K 0131-1996, JB / T 4220-2011, to obtain the peak intensity and full width at half maximum (FWHM) of the diffraction peak corresponding to the (110) crystal face, and the peak intensity and full width at half maximum (FWHM) of the diffraction peak corresponding to the (002) crystal face of the first negative active material, and then calculated according to the Scherrer formula.

[0086] In some embodiments, the La(110) / Lc(002) of the second negative electrode active material is 3.5-5.5. In the present disclosure, by making the La(110) / Lc(002) of the second negative electrode active material in the above range, the number of crystallites in the material is small, and thus the number of grain boundaries between the crystallites is small, which is conducive to increasing the lithium-embeddable sites on the surface of the material and improving the specific capacity of the material, thereby improving the energy density of the secondary battery. Illustratively, the La(110) / Lc(002) can be 3.5, 3.8, 4.0, 4.2, 4.4, 4.6, 4.8, 5.0, 5.2, 5.5, or a value between any two of them. In some alternative embodiments, the La(110) / Lc(002) of the second negative electrode active material is 4.5-5.5.

[0087] In some embodiments, the charge-discharge test is performed on a button cell prepared from the second negative electrode active material at a delithiation rate of 0.1C and a lithium-embedding rate of 0.05C, respectively, to obtain a charge-discharge curve in the range of 0.005V-2.0V. In the discharge curve, there is a lithium-embedding platform in the voltage range of 0.005V-0.07V, and the proportion X1 of the lithium-embedding platform in the total discharge capacity of the button cell is 43% or more.

[0088] In the present disclosure, the charge-discharge test is performed as follows: the sample powder is mixed uniformly with the conductive agent carbon black (Super P), the binder polyvinylidene fluoride (PVDF) in a mass ratio of 91.6:1.8:6.6, and the solvent N-methyl pyrrolidone (NMP) to prepare a slurry; the prepared slurry is coated on the surface of a negative electrode current collector copper foil, dried in an oven, and reserved; ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 is dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L; then, a lithium metal sheet is used as the counter electrode, a polyethylene (PE) film is used as the separator, and the above electrolyte is assembled into a CR2430 type button cell in an argon glove box; the obtained button cell is left to stand for 12h, then discharged at a rate of 0.05C at 25°C, and discharged to 0.005V; left to stand for 10 minutes, then discharged again at a current of 50μA, discharged to 0.005V, left to stand for 10 minutes, and finally discharged at a current of 10μA to 0.005V; the charging process is constant-current charging at 0.1C to 2.0V, to obtain the relationship between the charge-discharge capacity and the voltage of the material, i.e. the charge-discharge curve.

[0089] In the present disclosure, the second negative active material is prepared into a button cell, and the above charge-discharge test is performed to obtain a charge-discharge curve. In the discharge curve, the proportion X1 of the discharge capacity corresponding to the lithium intercalation platform in the voltage range of 0.005 V to 0.07 V to the total discharge capacity of the button cell is 43% or more, the gram capacity of the second negative active material is high, and the energy density of the secondary battery is improved. Although the mechanism is not clear, the inventors believe that during the lithium intercalation process of the graphite material, as the amount of lithium ions intercalated increases, different orders of graphite interlayer compounds will gradually form, such as LiC 24 , LiC 12 , LiC6, etc. The formation of different orders of graphite interlayer compounds corresponds to the lithium intercalation platform in different voltage ranges in the charge-discharge curve of the graphite material. Since the theoretical capacity of different orders of graphite interlayer compounds is different, for example, the theoretical capacity of LiC6 is 372 mAh / g, and the theoretical specific capacity of LiC 12 is 186 mAh / g, therefore, during the lithium intercalation process, the more LiC6 (i.e., corresponding to the lithium intercalation platform in the voltage range of 0.005 V to 0.07 V) formed, the more capacity contributed, and the higher the gram capacity of the graphite material. Exemplarily, X1 can be 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or a value between any two of them.

[0090] In some embodiments, the proportion X1 of the discharge capacity corresponding to the lithium intercalation platform to the total discharge capacity of the button cell is 43% to 50%, and optionally 43% to 47%.

[0091] In some embodiments, the graphitization degree of the second negative active material is 94.0% to 96.0%. By making the graphitization degree of the second negative active material in the above range, the first negative active material has a high tap density and gram capacity, thereby improving the energy density of the secondary battery. Exemplarily, the graphitization degree of the second negative active material can be 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, or a value between any two of them. In some optional embodiments, the graphitization degree of the second negative active material is 94.2% to 95.8%.

[0092] In the present disclosure, the graphitization degree of a material is the proportion of carbon elements in the material existing in the form of a graphite structure, which can be tested by instruments and methods known in the art. For example, an X-ray diffractometer (such as Bruker D8 Discover) can be used for testing, and the test can refer to JIS K 0131-1996, JB / T 4220-2011 to obtain the average interlayer spacing d 002 of the C (002) crystal plane in the crystal structure of the material, and then the graphitization degree g is calculated according to the formula g = (0.344-d002 ) / (0.344-0.3354) x 100% to calculate the graphitization degree. In the above formula, d 002 is the average interlayer spacing of the C (002) crystal plane in the crystal structure of the material, expressed in nanometers (nm).

[0093] In some embodiments, the second negative electrode active material has a volume distribution particle size Dv50 of 14.5 μm to 18.0 μm, which can be measured by the method described above. Illustratively, the volume distribution particle size Dv50 of the second negative electrode active material can be 14.5 μm, 15.0 μm, 15.5 μm, 16.0 μm, 16.5 μm, 17.0 μm, 17.5 μm, 18.0 μm, or a value between any two of these values. In some alternative embodiments, the volume distribution particle size Dv50 of the second negative electrode active material is 15.0 μm to 17.5 μm.

[0094] In some embodiments, the second negative electrode active material has a particle size distribution (Dv90-Dv10) / Dv50 of 0.90 to 1.25. Illustratively, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material can be 0.90, 0.95, 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, or a value between any two of these values. In some alternative embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.90 to 1.20.

[0095] In some embodiments, the second negative electrode active material has a specific surface area of 0.8 m 2 / g to 2.1 m 2 / g, which can be measured by the method described above. By having the specific surface area of the second negative electrode active material within the above range, on the one hand, the second negative electrode active material can have a lower surface side reaction activity, thereby reducing the consumption of active ions by SEI film formation and improving the initial coulombic efficiency of the secondary battery; on the other hand, it can also have a higher active ion transport performance, improving the kinetic performance of the secondary battery. Illustratively, the specific surface area of the second negative electrode active material can be 0.8 m 2 / g, 1.0 m 2 / g, 1.2 m 2 / g, 1.4 m 2 / g, 1.6 m 2 / g, 1.8 m 2 / g, 2.0 m 2 / g, 2.1 m 2 / g, or a value between any two of these values.

[0096] In some embodiments, the mass ratio of the first negative active material and the second negative active material in the negative electrode film layer is 4:6 to 6:4. By having the mass ratio of the first negative active material and the second negative active material in the above range, it is beneficial for the secondary battery to have excellent kinetic performance while taking into account the energy density. Illustratively, the mass ratio of the first negative active material and the second negative active material can be 4:6, 5:5, 6:4, or a ratio between a range consisting of any two of the values.

[0097] In some embodiments, the compaction density of the negative electrode film layer is 1.50 g / cc to 1.90 g / cc. By having the powder compaction density of the negative electrode film layer in the above range, it is beneficial for the negative electrode film layer to have high capacity, good active ion and electron transport performance, and further beneficial for the secondary battery to have high energy density and good kinetic performance. Illustratively, the compaction density of the negative electrode film layer can be 1.50 g / cc, 1.55 g / cc, 1.60 g / cc, 1.65 g / cc, 1.70 g / cc, 1.75 g / cc, 1.80 g / cc, 1.85 g / cc, 1.90 g / cc, or a value between a range consisting of any two of the values. In some alternative embodiments, the compaction density of the negative electrode film layer is 1.55 g / cc to 1.85 g / cc.

[0098] In some embodiments, the areal density of the negative electrode film layer is 6.0 mg / cm 2 to 24.0 mg / cm 2 . By having the areal density of the negative electrode film layer in the above range, it is beneficial for the negative electrode film layer to have high capacity, good active ion and electron transport performance, and further beneficial for the secondary battery to have high energy density and good kinetic performance. Illustratively, the areal density of the negative electrode film layer can be 6.0 mg / cm 2 , 6.5 mg / cm 2 , 8.0 mg / cm 2 , 10.0 mg / cm 2 , 12.0 mg / cm 2 , 14.0 mg / cm 2 , 16.0 mg / cm 2 , 18.0 mg / cm 2 , 20.0 mg / cm 2 , 22.0 mg / cm 2 , 24.0 mg / cm 2 , or a value between a range consisting of any two of the values. In some alternative embodiments, the areal density of the negative electrode film layer is 6.5 mg / cm 2 to 22.5 mg / cm 2 .

[0099] In some embodiments, the thickness of the negative electrode film layer is 60-240 μm. By having the thickness of the negative electrode film layer in the above range, the negative electrode film layer can have high capacity, high active ion and electron transport performance, and thus the secondary battery can have high energy density and good storage performance and kinetic performance. For example, the thickness of the negative electrode film layer can be 60 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 140 μm, 160 μm, 180 μm, 200 μm, 220 μm, 240 μm or a value between any two of the above values. In some alternative embodiments, the thickness of the negative electrode film layer is 90-220 μm.

[0100] In the present disclosure, the thickness of the negative electrode film layer has the meaning known in the art and can be tested by methods known in the art, for example, by using a micrometer (e.g. Mitutoyo 293-100, accuracy 0.1 μm). The thickness range given in the present disclosure is the thickness range of the negative electrode film layer on one side of the negative electrode current collector.

[0101] The film layer is provided in a double layer, and the porosity of the upper layer region (first region) is greater than the porosity of the lower layer region (second region), thereby increasing the contact opportunity of the upper layer region with active ions in the electrolyte, so that the active ions can migrate faster during the charging and discharging process, improving the lithium intercalation kinetics of the material, and thus improving the fast charging performance of the battery.

[0102] In the present disclosure, porosity represents the number of pores per unit area in the film layer into the base channel, wherein the porosity of the first region and the second region can be tested by any method known in the art. Illustratively, after sampling the cold-pressed pole piece, the pole piece is subjected to cross-section polishing treatment by a cross-section ion polisher (for example, a model IB-09010CP argon ion cross-section polisher of JEOL Co., Ltd. of Japan) device, and then a longitudinal section of the negative pole piece is scanned by a scanning electron microscope (for example, a Sigma 300 scanning electron microscope of ZEISS Co., Ltd. of Germany); a plurality of test regions are randomly taken in the test sample, and images of the plurality of test regions are obtained by using a scanning electron microscope, and finally the scanning images in the first region and the second region are subjected to threshold segmentation by using ImageJ software, the gray-white contrast region is a particle, and the black contrast region is a pore, and the porosity of the pole piece surface in different depth regions is obtained by computer image recognition and threshold division method. Alternatively, the battery can be disassembled to obtain the negative pole piece, the pole piece is cut into a size of 6mm*6mm, fixed on a sample holder, and a cross-section of the negative pole piece is prepared by a cross-section ion polisher (for example, a model IB-09010CP argon ion cross-section polisher of JEOL Co., Ltd. of Japan). Then, scanning is performed by using a scanning electron microscope (for example, a Sigma 300 scanning electron microscope of ZEISS Co., Ltd. of Germany), and the porosity of the pole piece surface in different depth regions is obtained by computer image recognition and threshold division method.

[0103] In some embodiments, in the first negative electrode active material, the mass percentage content of the carbon coating layer relative to the mass of the substrate is 0.52% to 2.35%,

[0104] In some embodiments, the preparation method of the first negative electrode active material comprises:

[0105] providing a substrate material;

[0106] mixing the substrate material and the organic carbon source, and then performing heat treatment to obtain the first negative electrode active material;

[0107] wherein the mass ratio of the substrate material to the organic carbon source is 100:1.5 to 100:8.

[0108] Therefore, by setting the substrate material and the organic carbon source in the above mass ratio range, the lithium intercalation platform voltage of the obtained first negative electrode active material is 0.118V-0.140V.

[0109] positive electrode pole piece

[0110] The positive electrode pole piece comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, and the positive electrode film layer comprises a positive electrode active material.

[0111] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode film layer is disposed on either one or both of the two opposite surfaces of the positive electrode current collector.

[0112] In some embodiments, the positive electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0113] In some embodiments, when the battery cell is a lithium secondary battery, the positive electrode active material can employ a positive electrode active material for a lithium secondary battery known in the art. As an example, the positive electrode active material can include at least one of a lithium-containing phosphate having an olivine structure, a lithium transition metal oxide, and a modified compound of each thereof. However, the present disclosure is not limited to these materials, and other conventional materials that can be used as a battery positive electrode active material can also be used. These positive electrode active materials can be used alone only in one kind, or two or more kinds can be used in combination. Among them, examples of the lithium transition metal oxide can include, but are not limited to, lithium cobalt oxide (e.g., LiCoO2), lithium nickel oxide (e.g., LiNiO2), lithium manganese oxide (e.g., LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (which can also be referred to simply as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be referred to simply as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (which can also be referred to simply as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (which can also be referred to simply as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (which can also be referred to simply as NCM 811 ), lithium nickel cobalt aluminum oxide (e.g., LiNi 0.85 Co0.1 Al 0.05 O2) and modified compounds thereof, etc. Examples of the olivine-structured lithium-containing phosphate can include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (which can also be referred to as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite of lithium manganese iron phosphate and carbon.

[0114] In some embodiments, when the battery cell is a sodium secondary battery, the positive active material can employ a positive active material known in the art for use in sodium secondary batteries. As an example, the positive active material can include a sodium transition metal oxide, a polyanion compound, a prussian blue compound, and the like.

[0115] The battery will be accompanied by Li deintercalation and consumption during charging and discharging, and the molar content of Li is different when the battery is discharged to different states. In the enumeration of the positive active material in the present disclosure, the molar content of Li is the initial state of the material, i.e., the state before feeding, and the positive active material is applied to the battery system. After charging and discharging cycles, the molar content of Li will change.

[0116] In the enumeration of the positive active material in the present disclosure, the molar content of O is only the theoretical state value, and the release of oxygen from the lattice will cause the molar content of oxygen to change, and the actual molar content of O will float.

[0117] In some embodiments, the positive electrode film layer can further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0118] In some embodiments, the positive electrode film layer can further optionally include a conductive agent. As an example, the conductive agent can include at least one of super P carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0119] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as the positive active material, the conductive agent, the binder, and any other components, in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on a positive electrode current collector, and after processes such as drying, cold pressing, etc., the positive electrode sheet can be obtained.

[0120] Electrolyte

[0121] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of electrolyte is not particularly limited in the present disclosure and can be selected as needed. For example, the electrolyte can be liquid, gel, or all-solid.

[0122] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0123] In some embodiments, the electrolyte salt can include at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro oxalate borate, lithium di-oxalate borate, lithium difluoro di-oxalate phosphate, and lithium tetrafluoro oxalate phosphate.

[0124] In some embodiments, the solvent can include at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0125] In some embodiments, the electrolyte solution can optionally further include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain properties of the battery, such as an additive for improving overcharge performance of the battery, an additive for improving high-temperature or low-temperature performance of the battery, etc.

[0126] Separator film

[0127] In some embodiments, the battery cell further includes a separator film. The type of separator film is not particularly limited in the present disclosure and any known porous structure separator film having good chemical stability and mechanical stability can be used.

[0128] In some embodiments, the material of the separator film can include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator film can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator film is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0129] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator film can be made into an electrode assembly through a winding process or a stacking process.

[0130] In some embodiments, the battery cell can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0131] In some embodiments, the outer package of the battery cell can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, or the like. The outer package of the battery cell can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, or the like can be listed.

[0132] The present disclosure does not have a particular limitation on the shape of the battery cell, which can be cylindrical, square, or any other arbitrary shape. For example, FIG. 2 is a battery cell 5 of a square structure as an example.

[0133] In some embodiments, referring to FIG. 3, the outer package can include a shell 51 and a top cover assembly 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be provided on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, which can be selected by a person skilled in the art according to the specific actual needs.

[0134] In some embodiments, the battery cell can be assembled into a battery module, and the number of battery cells contained in the battery module can be one or more, and the specific number can be selected by a person skilled in the art according to the application and capacity of the battery module.

[0135] FIG. 4 is a battery module 4 as an example. Referring to FIG. 4, in the battery module 4, a plurality of battery cells 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangements can also be made. Further, the plurality of battery cells 5 can be fixed by fasteners.

[0136] Optionally, the battery module 4 can also include a housing having a receiving space, and the plurality of battery cells 5 are received in the receiving space.

[0137] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by a person skilled in the art according to the application and capacity of the battery pack.

[0138] FIGS. 5 and 6 are a battery pack 1 as an example. Referring to FIGS. 5 and 6, the battery pack 1 can include a battery box and a plurality of battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be provided on the lower box body 3 to form a closed space for receiving the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0139] Method for manufacturing secondary battery

[0140] The present disclosure also provides a method for manufacturing a secondary battery, which comprises manufacturing a negative electrode sheet.

[0141] The negative electrode sheet can be manufactured by providing a first slurry comprising a first negative electrode active material and a second slurry comprising a second negative electrode active material; simultaneously extruding the first slurry and the second slurry, the first slurry being coated on a negative electrode current collector copper foil and the second slurry being coated on the first slurry; and obtaining the negative electrode sheet after drying and cold pressing.

[0142] The method for manufacturing the first negative electrode active material comprises:

[0143] providing a base material;

[0144] mixing the base material and an organic carbon source, and performing heat treatment to obtain the first negative electrode active material;

[0145] The mass ratio of the base material to the organic carbon source is 100:1.5 to 100:8. For example, the mass ratio is 100:1.5, 100:2, 100:3, 100:4, 100:5, 100:6, 100:8, or a value between any two of these values.

[0146] Therefore, by setting the mass ratio of the base material and the organic carbon source in the above range, the mass percentage content of the carbon coating layer in the obtained first negative electrode active material is 0.52% to 2.35% relative to the mass of the base material, and the lithium intercalation platform voltage is in the range of 0.118V to 0.140V.

[0147] In the present disclosure, the "surface residual carbon content" after heat treatment is taken as the "amount of carbon coating layer", and the calculation process of the mass percentage content of the carbon coating layer is exemplarily shown by taking pitch as the raw material of the carbon coating layer in the manufacturing process.

[0148] The surface residual carbon content = the added amount of pitch x the coking value of pitch.

[0149] The mass percentage content of the carbon coating layer = the surface residual carbon content / the mass of the base material x 100%.

[0150] Exemplarily, for the method for manufacturing the first negative electrode active material, the following method can be used, which comprises steps (11) to (12):

[0151] Step (11) provides a base material.

[0152] In some embodiments, step (11) comprises steps (111) to (113).

[0153] Step (111): crushing, shaping, and classifying the artificial graphite precursor to obtain primary particles.

[0154] In some embodiments, the artificial graphite precursor in step (111) comprises at least one of calcined coke and needle-shaped green coke.

[0155] In some embodiments, the volume average particle size Dv50 of the primary particles in step (111) is 6.0 μm to 9.0 μm, for example, 6.0 μm, 6.5 μm, 7.0 μm, 7.5 μm, 8.0 μm, 8.5 μm, 9.0 μm, or a value between any two of these values.

[0156] Step (112): mixing the first binder and the primary particles, granulating, and then performing first heat treatment on the granulated product to obtain secondary particle graphite material.

[0157] The present disclosure does not have specific limitations on the type of the first binder, and a binder known in the art can be used. For example, the second binder can include solid pitch, liquid pitch, etc.

[0158] In some embodiments, the mass ratio of the primary particles to the first binder is 100:5 to 100:9. For example, 100:5, 100:6, 100:7, 100:8, 100:9, or a value between any two of these values.

[0159] In some embodiments, the first heat treatment comprises graphitization at 2800°C to 3800°C.

[0160] The skilled in the art can adjust the graphitization time as needed. Exemplarily, the graphitization treatment can be performed using an Acheson graphitization furnace, and the graphitization time can be 2 to 8 days when the temperature is above 2800°C.

[0161] Step (113): mixing the primary particles and the secondary particles to obtain a matrix material.

[0162] The purpose of mixing the primary particles and the secondary particles is to fill the primary particles between the secondary particles. Since the primary particles have a small particle size and the secondary particles have a large particle size, the two have complementary advantages in packing, so as to improve the migration rate of active ions on the surface / bulk phase of the obtained first negative electrode active material.

[0163] In some embodiments, the proportion of the number of the secondary particles in the matrix material is greater than 60%.

[0164] Step (12) mixes the matrix material and the organic carbon source, and then performs second heat treatment to obtain the first negative electrode active material.

[0165] In some embodiments, the mass ratio of the base material to the organic carbon source is 100:1.5 to 100:8.

[0166] The present disclosure does not have specific limitations on the type of organic carbon source, and a carbon-containing substance known in the art can be used. For example, the organic carbon source includes at least one of coal tar pitch, petroleum pitch, and phenol formaldehyde resin.

[0167] In some embodiments, the second heat treatment includes carbonization treatment at 900°C to 1300°C for 1 h to 5 h.

[0168] The method for preparing the second negative electrode active material in the present disclosure can employ, for example, a method including the following steps (21) to (23):

[0169] Step (21): crushing and classifying a synthetic graphite precursor to obtain a first intermediate.

[0170] Step (22): mixing and granulating the first intermediate with a second binder, and then performing a third heat treatment to obtain a second intermediate.

[0171] Step (23): performing a fourth heat treatment on the second intermediate to obtain the second negative electrode active material.

[0172] In the present disclosure, the synthetic graphite precursor refers to a raw material used for preparing the second negative electrode active material. The present disclosure does not have specific limitations on the type of synthetic graphite precursor, and a synthetic graphite precursor known in the art can be used. For example, the synthetic graphite precursor includes one or more of calcined coke, petroleum-based non-acicular coke, petroleum-based acicular coke, coal-based non-acicular coke, and coal-based acicular coke. The calcined coke includes coke formed by calcining the above-mentioned petroleum-based non-acicular coke, petroleum-based acicular coke, coal-based non-acicular coke, and coal-based acicular coke. In some alternative embodiments, calcined acicular coke can be used.

[0173] The present disclosure does not have specific limitations on the manner of crushing, shaping, and classifying, and a manner of crushing, shaping, and classifying known in the art can be used.

[0174] In some embodiments, calcined coke can be used as the synthetic graphite precursor in step (21). The calcined coke has strong acicularity and few mosaic structures, and using it as the synthetic graphite precursor is helpful for the development of crystal grain structure during graphitization, so that the second negative electrode active material with larger La and / or higher graphitization degree can be obtained, thereby facilitating the intercalation of lithium ions between the layers of the second negative electrode active material, and also being conducive to the formation of a large amount of LiC6 during the intercalation of lithium by the second negative electrode active material.

[0175] In some embodiments, the first intermediate obtained in step (21) has a volume average particle size Dv50 of 9.0 μm to 13.0 μm. For example, 9.0 μm, 10.0 μm, 11.0 μm, 12.0 μm, 13.0 μm, or a value between any two of these values.

[0176] In some embodiments, the mixing mass ratio of the second intermediate and the first binder is 100:8 to 100:12, for example, 100:8, 100:9, 100:10, 100:11, 100:12, or a value between any two of these values.

[0177] In some embodiments, the second intermediate has a volume average particle size Dv50 of 15 μm to 18 μm. For example, 15 μm, 16 μm, 17 μm, 18 μm, or a value between any two of these values.

[0178] In some embodiments, the second binder comprises a pitch that is strong in adhesion, low in residual carbon, and easy to graphitize. By selecting the above pitch as the second binder, the residual amount of pitch in the second negative active material can be reduced, and based on the characteristics of the pitch being easy to graphitize, the residual carbon of the pitch after graphitization has a higher graphitization degree, reducing the impact of residual carbon on the specific capacity and the tap density of the second negative active material.

[0179] In some embodiments, the third heat treatment comprises a pre-carbonization treatment at 1000°C to 1500°C for 1 h to 4 h.

[0180] In some embodiments, the fourth heat treatment comprises a graphitization treatment at 3000°C to 3200°C.

[0181] The skilled person can adjust the graphitization time as needed. Exemplarily, an Acheson graphitization furnace can be used for the graphitization treatment, and the graphitization time can be 2 to 8 days when the temperature is above 2800°C.

[0182] In some embodiments, the negative electrode current collector has two opposite surfaces in the thickness direction of the negative electrode current collector, and the negative electrode film layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector.

[0183] In some embodiments, the negative current collector can employ a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0184] In some embodiments, the negative electrode film layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0185] In some embodiments, the negative electrode film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of superconducting carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0186] In some embodiments, the negative electrode film layer can further optionally include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0187] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative active material (artificial graphite), the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative current collector, and then performing processes such as drying, cold pressing, and the like to obtain the negative electrode sheet.

[0188] Electric device

[0189] In addition, the present disclosure also provides an electric device including the secondary battery provided by the present disclosure. The secondary battery can be used as a power source of the electric device, or can be used as an energy storage unit of the electric device. The electric device can include a mobile device (e.g., a mobile phone, a notebook computer, etc.), an electric vehicle (e.g., a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0190] As the electric device, the battery cell, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0191] FIG. 7 is an example of a power consuming device. The power consuming device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the demand of the power consuming device for high power and high energy density of the secondary battery, a battery pack or a battery module can be used.

[0192] The device as another example can be a mobile phone, a tablet, a notebook computer, etc. The device generally requires thinning, and a battery monomer can be used as a power source.

[0193] Embodiments

[0194] Hereinafter, embodiments of the present disclosure will be described. The embodiments described below are exemplary and are for the purpose of explaining the present disclosure only and are not to be understood as limiting the present disclosure. In the embodiments, specific techniques or conditions not mentioned are performed in accordance with the techniques or conditions described in the literature in the field or in accordance with the product manual. The reagents or instruments used, for which the manufacturer is not mentioned, are all conventional products that can be obtained commercially.

[0195] Preparation of the first negative electrode active material

[0196] Preparation Example 1-1

[0197] Step (11): providing the matrix material includes: step (111) shaping the needle-shaped green coke by breaking with an air jet mill to obtain primary particles with a volume average particle size Dv50 of 8.5 μm. Step (112) mixing the primary particles with pitch at a mass ratio of 100:8, then granulating, and then heat treating the granulated product at 3000°C for 7 days to obtain secondary particles. Step (113) blending the secondary particles and the primary particles at a mass ratio of 3:1 to obtain the matrix material.

[0198] Step (22) mixing the blended matrix material and pitch (coking value 33%) at a mass ratio of 100:2, then carbonizing at 1150°C in a nitrogen atmosphere for 4h, and finally screening to obtain the first negative electrode active material with a soft carbon coating layer structure on the surface.

[0199] Preparation Examples 1-2 to 1-6

[0200] The preparation method of Preparation Examples 1-2 to 1-6 is similar to that of Preparation Example 1-1, except that the mass ratio of the blended material to the organic carbon source is adjusted according to the values in Table 1 below.

[0201] Table 1

[0202] Test of the parameters of the first negative electrode active material

[0203] (1) Test of the lithium intercalation plateau voltage X2:

[0204] The first negative electrode active material (material 1-1) prepared above, a conductive agent carbon black (Super P), and a binder polyvinylidene fluoride (PVDF) were uniformly mixed in a mass ratio of 91.6:1.8:6.6 to obtain a slurry; the prepared slurry was coated on the surface of a negative electrode current collector copper foil, which was dried in an oven and then used; ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L; then, a CR2430 button cell was assembled in an argon glove box using a lithium sheet as a counter electrode, a polyethylene (PE) film as a separator, and the electrolyte; and the obtained button cell was left to stand for 12 h.

[0205] At 25°C, the 0.05C rate constant current was discharged to 0.005V, left to stand for 5 minutes, then discharged again to 0.005V at a current of 50μA, left to stand for 5 minutes, then discharged again to 0.005V at a current of 10μA; then, the 0.1C rate constant current was charged to 2.0V to obtain the relationship between the charge-discharge capacity and the voltage of the material, i.e., the charge-discharge curve. The total lithium intercalation energy in the range of 0.005V-2V in the discharge curve was recorded as E1, and the total lithium intercalation capacity in the range of 0.005V-2.0V was recorded as C3, and the lithium intercalation platform voltage X2=E1 / C3.

[0206] The charge-discharge curve of material 1-1 measured by the above method is shown as a black curve in FIG. 8, and the total lithium intercalation energy E1 in the range of 0.005V-2V and the total lithium intercalation capacity C3 in the range of 0.005V-2.0V were calculated using a blue cell test system (CT2001A type), and then the lithium intercalation platform voltage was calculated as 0.123V. The test results are recorded in Table 2.

[0207] The charge-discharge curve of material 1-6 measured by the above method is shown as a red curve in FIG. 8, and the total lithium intercalation energy E1 in the range of 0.005V-2V and the total lithium intercalation capacity C3 in the range of 0.005V-2.0V were calculated using a blue cell test system (CT2001A type), and then the lithium intercalation platform voltage was calculated as 0.095V. The test results are recorded in Table 2.

[0208] As can be seen from FIG. 8, compared with material 1-6, material 1-1 has a carbon coating layer on the surface and an improved lithium intercalation platform voltage.

[0209] (2) Powder OI value test:

[0210] The X-ray diffraction pattern of the powder sample can be obtained by referring to JIS K 0131-1996 and JB / T 4220-2011, and the powder OI value of the sample can be calculated according to the OI value = I(004) / I(110). I(004) is the integral area of the diffraction peak of the (004) crystal plane of the crystalline carbon in the powder sample, and I(110) is the integral area of the diffraction peak of the (110) crystal plane of the crystalline carbon in the powder sample. In the X-ray diffraction analysis test, a copper target is used as an anode target, CuKα ray is used as a radiation source, and the wavelength of the ray is 0.15406 nm. The scanning 2θ angle range is 20°-80°, and the scanning rate is 4° / min. The test results are recorded in Table 2.

[0211] (3) Test of BET specific surface area:

[0212] The nitrogen adsorption specific surface area analysis test can be performed by using the nitrogen adsorption specific surface area analysis test method in GB / T 19587-2017, and the BET (Brunauer Emmett Teller) method is used for calculation. The nitrogen adsorption specific surface area analysis test can be performed by using the Tri-Star 3020 specific surface area and pore size analyzer of the American Micromeritics company.

[0213] (4) Test of compacted density:

[0214] 1 g of sample powder is weighed and added to a mold with a bottom area of 1.327 cm 2 , and is pressed to 50,000 N, and is kept for 30 s, and then is unloaded, and is kept for 10 s, and then the powder compacted density of the material under the pressure of 50,000 N is recorded and calculated. The test results are recorded in Table 4.

[0215] (5) Morphology test

[0216] The material 1-1 prepared in Example 1 is subjected to focused ion beam (FIB) thinning pretreatment, Pt nanoparticles are used as a substrate, and high-resolution test is performed by using the Thermo Scientific-Talos F200S G2 field emission transmission electron microscope (TEM). The test results can be seen from FIG. 10.

[0217] As can be seen from FIG. 10, the material 1-1 has a graphite crystal nucleus and an amorphous carbon layer (no obvious lattice fringes) on the surface, i.e., a soft carbon layer.

[0218] Table 2

[0219] Preparation Example 2-1

[0220] Preparation of the second negative electrode active material

[0221] Step (21) The oil-based calcined needle coke raw material (carbon content of 98.5%, volatile content of 1.0%, sulfur element content of 0.5%) was broken and shaped by using an air jet mill to obtain a shaped material (first intermediate) with a Dv50 of 11 μm.

[0222] Step (22) The above shaped material was granulated together with granulating pitch (softening point of 200°C; coking value of 60%) as a granulating agent by using a granulating kettle, to obtain a granulated material, wherein the mass ratio of the shaped material and the granulating agent was 100:8, and a second intermediate (volume distribution particle size Dv50 of 16.5 μm) was obtained when pre-carbonizing at 1150°C in a nitrogen atmosphere for 2h.

[0223] Step (23) The second intermediate was subjected to graphitization treatment at a high temperature of 3020°C, and the graphitized particles were screened and demagnetized to obtain a second negative electrode active material (material 2-1).

[0224] Preparation Examples 2-2 and 2-3

[0225] The preparation methods of Preparation Examples 2-2 and 2-3 were similar to those of Preparation Example 2-1, except that the raw materials used in the preparation steps of the second negative electrode active material and the graphitization temperature were adjusted according to the values in Table 3 below.

[0226] Table 3

[0227] Test of parameters of the second negative electrode active material

[0228] (1) Test of La and Lc:

[0229] The test was performed using an X-ray diffractometer of the Bruker D8 Discover model according to the test method of JIS K 0131-1996, wherein a copper target was used as an anode target, CuKα ray was used as a radiation source, and the wavelength of the ray was The X-ray diffraction pattern of the powder sample was obtained by scanning the 2θ angle range of 20°-80° at a scanning rate of 4° / min. The La(110) calculation formula was:

[0230] Wherein: K is a shape factor, which is 1.84; λ is the wavelength (nm); β110 is the half-peak width of the 110 crystal plane corresponding to ~43° in the X-ray diffraction pattern; and θ110 is the diffraction angle corresponding to the 110 crystal plane in the X-ray diffraction pattern.

[0231] The Lc(002) calculation formula was:

[0232] wherein: K is a shape factor, taking 0.89; λ is wavelength (nm); β002 is the half-peak width of the 002 crystal plane corresponding to ~26° in the X-ray diffraction pattern; θ002 is the diffraction angle corresponding to the 002 crystal plane in the X-ray diffraction pattern. The test results are shown in Table 4.

[0233] (2) Test of the gram capacity of the material:

[0234] The second negative electrode active material prepared above, the conductive agent carbon black (Super P), and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 91.6:1.8:6.6 with the solvent N-methyl pyrrolidone (NMP) to obtain a slurry; the prepared slurry was coated on the surface of a negative electrode current collector copper foil, which was dried in an oven and then reserved; ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L; then a CR2430 button cell was assembled in an argon-filled glove box with lithium metal as the counter electrode, a polyethylene (PE) film as the separator, and the electrolyte above; the obtained button cell was left to stand for 12 h.

[0235] At 25°C, the button cell prepared above was first discharged at a constant current of 0.15 mA to 0.005 V, left to stand for 5 minutes, discharged at a constant current of 50 μA to 0.005 V, left to stand for 5 minutes, and then discharged at a constant current of 10 μA to 0.005 V, and the first-cycle discharge capacity of the button cell was recorded; then the button cell was charged at a constant current of 0.3 mA to 2.0 V, and the first-cycle charge capacity of the button cell was recorded, and the ratio of the first-cycle charge capacity to the mass of the material was the gram capacity of the material. The test results are recorded in Table 4.

[0236] (3) Test of the proportion X1 of the lithium intercalation capacity of the platform in the whole:

[0237] A CR2430 button cell was prepared by the same method as in the gram capacity test above. After the CR2430 button cell prepared above was left to stand for 12 h, it was first discharged at a constant current of 0.15 mA to 0.005 V at 25°C, left to stand for 5 minutes, and then discharged at a constant current of 50 μA to 0.005 V again, left to stand for 5 minutes, and then discharged at a constant current of 10 μA to 0.005 V again; then the button cell was charged at a constant current of 0.3 mA to 2.0 V, and the charge-discharge capacity of the material versus voltage, i.e., the charge-discharge curve, was obtained. The lithium intercalation capacity of the lithium intercalation platform appearing in the discharge curve within 0.005 V-0.070 V was recorded as C1, and the total lithium intercalation capacity in the range of 0.005 V-2.0 V was recorded as C2, and then the proportion X1 of the lithium intercalation capacity of the platform in the whole was C1 / C2*100%.

[0238] The charge-discharge curve of material 2-1 measured by the above method is shown in FIG. 9. The lithium intercalation capacity of the lithium intercalation platform appearing in the range of 0.005 V-0.070 V in the discharge curve is recorded as C1, the total lithium intercalation capacity in the range of 0.005 V-2.0 V is recorded as C2, and then the proportion of the lithium intercalation capacity of the platform is calculated as X1=44.1%. The test results are recorded in Table 4.

[0239] (4) Powder compaction density:

[0240] 1 g of sample powder was weighed into a mold with a bottom area of 1.327 cm 2 , and was pressed to 50,000 N, kept for 30 s, then released, kept for 10 s, and then the powder compaction density of the material under a pressure of 50,000 N was recorded and calculated. The test results are recorded in Table 4.

[0241] Table 4

[0242] Example 1

[0243] Preparation of secondary battery

[0244] (1) Preparation of negative electrode sheet:

[0245] The negative electrode active material (material 1-1) used in the first region, the conductive agent carbon black (Super P), the thickening agent carboxymethyl cellulose sodium, and the binder styrene-butadiene rubber were mixed in a weight ratio of 97.0:0.80:1.0:1.2 in an appropriate amount of solvent deionized water to form a first negative electrode slurry.

[0246] The negative electrode active material (material 2-1) used in the second region, the conductive agent carbon black (Super P), the thickening agent carboxymethyl cellulose sodium, and the binder styrene-butadiene rubber were mixed in a weight ratio of 96.4:1.0:1.2:1.4 in an appropriate amount of solvent deionized water to form a second negative electrode slurry.

[0247] The same mass of the first negative electrode slurry and the second negative electrode slurry were simultaneously extruded by an extrusion coating device, the first negative electrode slurry was coated on the negative electrode current collector copper foil, and the second negative electrode slurry was coated on the first negative electrode slurry; after drying and cold pressing, a negative electrode sheet was obtained. The coating weight of the first negative electrode slurry and the second negative electrode slurry was the same, the compaction density of the negative electrode film layer was 1.75 g / cc, the area density of the negative electrode film layer was 11.5 mg / cm 2 , and the thickness of the negative electrode film layer was 65 μm.

[0248] (2) Preparation of positive electrode sheet:

[0249] Lithium iron phosphate, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96:2:2, and then added to a solvent N-methyl pyrrolidone to obtain a positive electrode slurry after stirring. The positive electrode slurry was coated on both surfaces of a positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained.

[0250] (3) Isolation film:

[0251] A 12 μm polyethylene film was used as the isolation film.

[0252] (4) Preparation of electrolyte:

[0253] Ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0254] (5) Assembly of secondary battery:

[0255] The positive electrode sheet and the negative electrode sheet prepared above were placed in order, with the isolation film between the positive electrode sheet and the negative electrode sheet to serve as a separation function, and then wound to obtain an electrode assembly; the electrode assembly was placed in an outer package, dried, and then injected with electrolyte, and then subjected to processes such as vacuum packaging, standing, formation, and shaping to obtain a secondary battery.

[0256] Test of negative electrode sheet

[0257] The cold-pressed negative electrode sheet was sampled and subjected to cross-section polishing treatment by a cross-section ion polishing instrument (e.g., an argon ion cross-section polishing instrument of IB-09010CP type from JEOL Co., Ltd., Japan), and then a longitudinal section of the negative electrode sheet was scanned by a scanning electron microscope (e.g., a Sigma 300 scanning electron microscope from ZEISS Co., Ltd., Germany) to obtain an SEM image as shown in FIG. 11.

[0258] As can be seen from FIG. 11, the porosity of the first region of the negative electrode sheet prepared in Example 1 is greater than that of the second region.

[0259] Test of secondary battery performance

[0260] (1) Fast charging capability (fast charging) test:

[0261] ① At 25°C, the secondary battery was charged at 0.33C to 3.8V, and then charged at a constant voltage to a current of 0.05C, and after standing for 5 min, the secondary battery was discharged at 0.33C to 2.0V, and the actual capacity was recorded as C0.

[0262] ②Then the secondary battery is sequentially charged at 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, 3.0C0 constant current to 3.8V or 0V negative electrode cut-off potential (whichever is reached first), and after each charging is completed, it needs to be discharged to 2.0V at 1C0, and the negative electrode potential corresponding to the charging to 10% SOC, 20% SOC, 30% SOC, 40% SOC, 50% SOC, 60% SOC, 70% SOC, 80% SOC (State of Charge, state of charge) at different charging rates is recorded.

[0263] ③Draw the charging rate-negatve electrode potential curve at different SOC states, and linearly fit to obtain the charging rate corresponding to the negative electrode potential of 0V at different SOC states. This charging rate is the charging window at this SOC state, and is denoted as C 10％SOC , C 20％SOC , C 30％SOC , C 40％SOC , C 50％SOC , C 60％SOC , C 70％SOC , C 80％SOC .

[0264] ④According to the following formula, the charging time T of the secondary battery from 10% SOC to 80% SOC is calculated (under the premise that the secondary battery does not lithiumize), and the unit is min. (60 / C 20％SOC +60 / C 30％SOC +60 / C 40％SOC +60 / C 50％SOC +60 / C 60％SOC +60 / C 70％SOC +60 / C 80％SOC )×10%.

[0265] (2) Test of energy density:

[0266] At 25℃, the secondary battery is charged at 1 / 3C constant current to 3.65V, then charged at 3.65V constant voltage to 0.05C current, and then discharged at 1 / 3C constant current to 2.5V. The battery discharge energy at this time is recorded. The battery discharge energy divided by the weight of the battery is the weight energy density of the battery, and the unit is Wh / kg. The measurement data is shown in Table 5.

[0267] (3) Test of cycle performance:

[0268] ①At 45℃, the above prepared secondary battery is placed for 5min, and then charged at 1C rate constant current to the upper cut-off voltage (corresponding to 100% SOC);

[0269] ②After the secondary battery is charged at a constant voltage to a current of 0.05C, and is left for 5 min, the secondary battery is discharged at a constant current of 1C to the lower limit cut-off voltage (corresponding to 0% SOC), and the discharge capacity at this time is recorded, which is the first cycle discharge capacity (D1).

[0270] The above steps ① and ② are cycled 1000 times, and the discharge capacity D of the 1000th cycle is recorded 1000 ;

[0271] The capacity retention rate (%) of the secondary battery after 1000 cycles = D1 / D 1000 . The test data are recorded in Table 5.

[0272] Examples 2-8

[0273] The secondary battery is prepared in the same manner as in Example 1, except that the types of active materials used in the second region and the first region are adjusted according to the records in Table 5.

[0274] The test results for the secondary battery of Example 2-8 are recorded in Table 5.

[0275] Comparative Examples 1-3

[0276] The secondary battery is prepared in the same manner as in Example 1, except that the types of active materials used in the second region and the first region are adjusted according to the records in Table 5.

[0277] Comparative Example 4

[0278] The secondary battery is prepared in the same manner as in Example 1, except that the negative electrode sheet is prepared by the following method:

[0279] The negative electrode active material (Material 1-1), the conductive agent carbon black (Super P), the thickening agent carboxymethyl cellulose sodium, and the binder styrene-butadiene rubber are mixed in a weight ratio of 97.0:0.80:1.0:1.2 in an appropriate amount of solvent deionized water by fully stirring, to form a negative electrode slurry.

[0280] The negative electrode slurry is coated on the negative electrode current collector copper foil by an extrusion coating device; after drying and cold pressing, a negative electrode sheet is obtained. The compaction density of the negative electrode film layer is 1.75 g / cc, the area density of the negative electrode film layer is 11.5 mg / cm 2 , and the thickness of the negative electrode film layer is 65 μm.

[0281] Comparative Example 5

[0282] The secondary battery is prepared in the same manner as in Example 1, except that the negative electrode sheet is prepared by the following method:

[0283] The negative active material (material 1-1 + material 2-2, mass ratio 1:1), conductive agent carbon black (Super P), thickening agent sodium carboxymethyl cellulose, binder styrene-butadiene rubber were mixed in a weight ratio of 97.0:0.80:1.0:1.2 in an appropriate amount of solvent deionized water, and the negative electrode slurry was formed by fully stirring.

[0284] The negative electrode slurry was coated on the negative electrode current collector copper foil by an extrusion coating device; after drying and cold pressing, the negative electrode sheet was obtained. The compaction density of the negative electrode film layer was 1.75 g / cc, the area density of the negative electrode film layer was 11.5 mg / cm 2 , and the thickness of the negative electrode film layer was 65 μm.

[0285] The test results of the secondary battery of Comparative Example 1-5 are recorded in Table 5.

[0286] Table 5

[0287] From the data in Table 5, it can be seen that compared with Comparative Example 1-5, the secondary batteries of Examples 1-8 according to the present disclosure can effectively improve the kinetic performance without affecting the cycle performance, while also taking into account the high energy density.

[0288] Examples 9-10

[0289] The secondary batteries were prepared and tested by a method similar to Example 1, except that the mass ratio of the first negative active material and the second negative active material was adjusted according to Table 6 when preparing the negative electrode sheet.

[0290] The secondary batteries were tested, and the test results are recorded in Table 6.

[0291] Table 6

[0292] From the data in Table 6, it can be seen that when the mass ratio of the first negative active material and the second negative active material is 4:6-6:4, the prepared secondary batteries have improved kinetic performance, while also taking into account the energy density.

[0293] It should be noted that the present disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role within the scope of the technical solutions of the present disclosure are also included in the technical scope of the present disclosure. In addition, within the scope of the main idea of the present disclosure, various modifications that can be thought of by those skilled in the art, and other ways constructed by combining part of the components of the embodiments are also included in the scope of the present disclosure.

Claims

1. A secondary battery, comprising a negative electrode tab, the negative electrode tab comprising 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 having a first surface away from the negative electrode current collector and a second surface located opposite to the first surface, a thickness of the negative electrode film layer is denoted as H, a region within a thickness range of 0.3H from the first surface of the negative electrode film layer is denoted as a first region of the negative electrode film layer, and a region within a thickness range of 0.3H from the second surface of the negative electrode film layer is denoted as a second region of the negative electrode film layer, the first region comprising a first negative electrode active material, and the second region comprising a second negative electrode active material, wherein the first negative electrode active material comprises a matrix and a carbon coating layer formed on at least part of a surface of the matrix, and a lithium intercalation plateau voltage of the first negative electrode active material is 0.118V-0.140V, wherein the lithium intercalation plateau voltage is obtained by charging and discharging a coin cell prepared by using the tested material at a lithium extraction rate of 0.1C and a lithium intercalation rate of 0.05C, respectively, to obtain a charging and discharging curve within a range of 0.005V-2.0V, wherein a ratio of a total lithium intercalation energy to a total lithium intercalation capacity within 0.005V-2.0V is defined as the lithium intercalation plateau voltage of the tested material; and a gram capacity of the second negative electrode active material is greater than a gram capacity of the first negative electrode active material. The lithium intercalation plateau voltage of the first negative electrode active material is 0.123V-0.135V. The carbon coating layer comprises soft carbon.

2. The secondary battery according to claim 1, wherein The first negative electrode active material comprises primary particle graphite material and secondary particle graphite material.

3. The secondary battery according to claim 1 or 2, wherein In the first negative electrode active material, a proportion of the number of the secondary particle graphite material is greater than or equal to 60%.

4. The secondary battery according to any one of claims 1 to 3, wherein The first negative electrode active material powder OI value is 2.0-6.

0.

5. The secondary battery according to claim 4, wherein The gram capacity of the second negative electrode active material is 359mAh / g or more.

6. The secondary battery according to any one of claims 1 to 5, wherein The gram capacity of the second negative electrode active material is 360mAh / g-366mAh / g.

7. The secondary battery according to any one of claims 1 to 6, wherein 9.The secondary battery according to any one of claims 1 to 8, wherein, the powder compaction density of the first negative electrode active material under a pressure of 50000N is 1.74g / cc-1.79g / cc; and / or the powder compaction density of the second negative electrode active material under a pressure of 50000N is 1.95g / cc-2.04g / cc.

8. The secondary battery according to any one of claims 1 to 7, wherein The first negative electrode active material satisfies one or more of the following: (1) a volume distribution particle size Dv50 of the first negative electrode active material is 7.8μm-15.8μm; (2) a particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.90-1.50; (4) a gram capacity of the first negative electrode active material is 350.5mAh / g-358.5mAh / g.

10. The secondary battery according to any one of claims 1 to 9, wherein The second negative electrode active material satisfies one or more of the following: ​ ​ (3) the BET specific surface area of the first negative electrode active material is 0.6 m 2 / g to 1.3 m 2 / g; ​ 11. The secondary battery according to any one of claims 1 to 10, wherein ​ (1) the La(110) of the second negative electrode active material is 130 nm to 175 nm, and the Lc(002) is 30 nm to 42 nm, wherein La(110) represents the crystallite size along the a axis in the (110) crystal plane of the material, and Lc(002) represents the crystallite size along the c axis in the (002) crystal plane of the material; (2) La(110) / Lc(002) is 3.5 to 5.5; (3) a charge-discharge test is performed on a button cell prepared from the second negative electrode active material at a delithiation rate of 0.1 C and a lithium intercalation rate of 0.05 C, respectively, to obtain a charge-discharge curve in the range of 0.005 V to 2.0 V, and in the discharge curve of the second negative electrode active material, there is a lithium intercalation platform in the voltage range of 0.005 V to 0.070 V, and the proportion X1 of the discharge capacity corresponding to the lithium intercalation platform in the total discharge capacity of the button cell is 43% or more; (4) the graphitization degree of the second negative electrode active material is 94.0% to 96.0%.

12. The secondary battery according to claim 11, wherein The second negative electrode active material meets one or more of the following: (1) the La(110) of the second negative electrode active material is 132 nm to 172 nm, and the Lc(002) is 30 nm to 36 nm; (2) La(110) / Lc(002) is 4.5 to 5.5; (3) the proportion X1 of the discharge capacity corresponding to the lithium intercalation platform in the total discharge capacity of the button cell is 43% to 47%; (4) the graphitization degree of the second negative electrode active material is 94.2% to 95.8%.

13. The secondary battery according to any one of claims 1 to 12, wherein The second negative electrode active material meets one or more of the following: (1) the volume distribution particle size Dv50 of the second negative electrode active material is 14.5 μm to 18.0 μm; (2) the particle size distribution (Dv90-Dv10) / Dv50 of the second negative electrode active material is 0.9 to 1.25; (3) the BET specific surface area of the second negative electrode active material is 0.8 m 2 / g ~ 2.1 m 2 / g.

14. The secondary battery according to any one of claims 1 to 13, wherein The mass ratio of the first negative electrode active material to the second negative electrode active material in the negative electrode film layer is 4:6 to 6:

4.

15. The secondary battery according to any one of claims 1 to 14, wherein The secondary battery meets one or more of the following conditions: The compaction density of the negative electrode film layer is 1.50 g / cc to 1.90 g / cc; The face density of the negative electrode film layer is 6.0 mg / cm 2 ~ 24.0 mg / cm 2 ; The thickness of the negative electrode film layer is 60 μm to 240 μm.

16. The secondary battery according to any one of claims 1 to 15, wherein The porosity of the first region is greater than the porosity of the second region.

17. The secondary battery according to any one of claims 1 to 16, wherein The preparation method of the first negative electrode active material comprises: providing a base material; mixing the base material and an organic carbon source, and then performing heat treatment to obtain the first negative electrode active material; wherein the mass ratio of the base material to the organic carbon source is 100:1.5 to 100:

8.

18. An electrical device comprising the secondary battery of any one of claims 1 to 17.

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