Secondary battery and electric device

Abstract

The application provides a secondary battery and a power utilization device. The secondary battery comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, the negative electrode film layer 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, 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 comprises a first negative electrode active material, the second region comprises a second negative electrode active material, the first negative electrode active material comprises 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, and the gram capacity of the second negative electrode active material is greater than that of the first negative electrode active material.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Art

[0002] In recent years, the application of secondary batteries has become increasingly widespread. They are widely used in energy storage power systems such as hydropower, thermal power, wind power, and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace, and other fields. As secondary batteries have achieved great development, higher requirements have been placed on their dynamic performance and energy density. Summary of the Invention

[0003] The present application is made in view of the above-mentioned problems, and its purpose is to provide a secondary battery and an electric device. The secondary battery has high energy density while maintaining good dynamic performance.

[0004] In order to achieve the above-mentioned object, the present application provides a secondary battery, comprising a negative electrode plate, the negative electrode plate 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 arranged opposite to the first surface, the thickness of the negative electrode film layer is recorded as H, the area within the thickness range from the first surface of the negative electrode film layer to 0.3H is recorded as the first area of ​​the negative electrode film layer, and the area within the thickness range from the second surface of the negative electrode film layer to 0.3H is recorded as the second area of ​​the negative electrode film layer, the first area includes a first negative electrode active material, and the second area includes a second negative electrode active material. The first negative electrode active material includes a substrate and a carbon coating formed on at least a portion of the surface of the substrate; the first negative electrode active material has a lithium insertion platform voltage of 0.118V to 0.140V, wherein the lithium insertion platform voltage is obtained by charging and discharging button cells prepared with the test material at a delithiation rate of 0.1C and a lithium insertion rate of 0.05C, respectively, to obtain a charge and discharge curve within the range of 0.005V-2.0V, wherein the ratio of the total lithium insertion energy to the total lithium insertion capacity within the range of 0.005V-2.0V is defined as the lithium insertion platform voltage of the tested material; the gram capacity of the second negative electrode active material is greater than the gram capacity of the first negative electrode active material. As a result, the secondary battery has good kinetic performance while also taking into account high energy density.

[0005] In some embodiments, the lithium insertion platform voltage of the first negative electrode active material is 0.123 V to 0.135 V. This is beneficial for further improving the dynamic performance of the secondary battery.

[0006] In some embodiments, the carbon coating layer includes soft carbon, which is beneficial for providing more active ion channels, thereby improving the kinetic performance of the secondary battery.

[0007] In some embodiments, the first negative electrode active material includes primary graphite particles and secondary graphite particles. By mixing the primary graphite particles and the secondary graphite particles, the dynamic performance of the secondary battery can be further improved.

[0008] In some embodiments, the secondary graphite particles comprise 60% or more of the first negative electrode active material. By predominantly comprising secondary particles in the first negative electrode active material, the material exhibits high isotropy, allowing expansion of the secondary battery during cycling to be dispersed in all directions, reducing electrode expansion and improving cycling performance.

[0009] In some embodiments, the OI value of the first negative electrode active material powder is 2.0 to 6.0. Thus, lithium ions can be embedded from all directions, improving the fast charging performance of the secondary battery.

[0010] In some embodiments, the second negative electrode active material has a gram capacity of 359 mAh / g or greater, thereby facilitating an increase in the energy density of the secondary battery.

[0011] In some embodiments, the gram capacity of the second negative electrode active material is 360 mAh / g to 366 mAh / g, thereby facilitating improvement of the energy density of the secondary battery.

[0012] In some embodiments, the powder compaction density of the first negative electrode active material under a pressure of 50,000 N is 1.74 g / cc to 1.79 g / cc. This is beneficial to provide appropriate porosity in the first region, which is beneficial to the infiltration of the electrolyte. In some embodiments, the powder compaction density of the second negative electrode active material under a pressure of 50,000 N is 1.95 g / cc to 2.04 g / cc. This is beneficial to provide appropriate porosity in the second region, and is beneficial to improving the energy density of the secondary battery. In some embodiments, the volume distribution particle size Dv50 of the first negative electrode active material is 7.8 μm to 15.8 μm. By making the volume distribution particle size Dv50 of the first negative electrode active material within the above range, it is beneficial to form a better pore distribution state in the first region of the electrode, which is more beneficial to improving the transmission performance of ions and electrons and improving the fast charging performance of the battery.

[0013] In some embodiments, the particle size distribution of the first negative electrode active material (Dv90-Dv10) / Dv50 is 0.90 to 1.50. A particle size distribution within this range indicates a good distribution of negative electrode active material particles, which facilitates a suitable pore structure in the upper region of the negative electrode film layer, thereby reducing the difficulty of ion liquid phase transport and further improving the kinetic performance of the secondary battery.

[0014] In some embodiments, the BET specific surface area of ​​the first negative electrode active material is 0.6 m 2 / g to 1.3m 2 This is beneficial to reducing the side reaction activity on the surface of the first negative electrode active material and improving the cycle performance of the secondary battery.

[0015] In some embodiments, the gram capacity of the first negative electrode active material is 350.5 mAh / g to 358.5 mAh / g, thereby facilitating improvement of the energy density of the secondary battery.

[0016] In some embodiments, the second negative electrode active material has a La(110) of 130 nm to 175 nm and a Lc(002) of 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. This facilitates a high energy density of the secondary battery.

[0017] In some embodiments, La(110) / Lc(002) is 3.5 to 5.5, which is beneficial for the secondary battery to have a high energy density.

[0018] In some embodiments, charge and discharge tests were conducted on a button cell prepared using the second negative electrode active material at a delithiation rate of 0.1C and a lithium insertion rate of 0.05C, yielding charge and discharge curves within the range of 0.005V to 2.0V. In the discharge curve of the second negative electrode active material, a lithium insertion platform existed within the voltage range of 0.005V to 0.070V, and the proportion (X1) of the discharge capacity corresponding to the lithium insertion platform to the total discharge capacity of the button cell was greater than 43%. This facilitates improving the energy density of the secondary battery.

[0019] In some embodiments, the ratio X1 of the discharge capacity corresponding to the lithium embedded platform to the total discharge capacity of the button battery is 43% to 47%.

[0020] In some embodiments, the particle size distribution of the second negative electrode active material (Dv90-Dv10) / Dv50 is 0.9 to 1.25, thereby improving the active ion and electron transport performance in the negative electrode film layer, thereby enhancing the kinetic performance of the secondary battery.

[0021] In some embodiments, the BET specific surface area of ​​the second negative electrode active material is 0.8 m 2 / g~2.1m 2 This is beneficial to reducing the side reaction activity on the surface of the negative electrode active material, thereby improving the cycle performance and the first coulombic efficiency of the secondary battery.

[0022] In some embodiments, the mass ratio of the first negative electrode active material to the second negative electrode active material is 4:6 to 6:4, thereby improving the dynamic performance of the secondary battery while taking into account the energy density.

[0023] In some embodiments, the compacted density of the negative electrode film is 1.50 g / cc to 1.90 g / cc, which is beneficial for the negative electrode film to have both high capacity and good active ion and electron transport properties, thereby facilitating the secondary battery to have both high energy density and good kinetic performance.

[0024] In some embodiments, the surface density of the negative electrode film layer is 6.0 mg / cm 2 ~24.0mg / cm 2 This is beneficial for the negative electrode film layer to have both high capacity and good active ion and electron transport properties, and further beneficial for the secondary battery to have both high energy density and good kinetic performance.

[0025] In some embodiments, the thickness of the negative electrode film layer is 60 μm-240 μm, which is beneficial for the secondary battery to have good dynamic performance while taking into account energy density.

[0026] In some embodiments, the porosity of the first region is greater than the porosity of the second region. By configuring the negative electrode film layer as a double layer, wherein the porosity of the upper region (the first region) is greater than the porosity of the lower region (the second region), the first region is more likely to come into contact with active ions in the electrolyte, allowing the active ions to migrate faster during charge and discharge, thereby improving the lithium insertion kinetics of the material and thereby improving the kinetic performance of the battery.

[0027] In some embodiments, the method for preparing the first negative electrode active material includes:

[0028] Providing base material;

[0029] mixing the matrix material and the organic carbon source, and then performing heat treatment to obtain the first negative electrode active material;

[0030] Wherein, the mass ratio of the matrix material to the organic carbon source is 100:1.5 to 100:8.

[0031] Thus, by setting the base material and the organic carbon source in the above mass ratio range, the obtained first negative electrode active material has a lithium intercalation plateau voltage of 0.118 V to 0.140 V.

[0032] A second aspect of the present application provides a power consuming device comprising the secondary battery of the first aspect of the present application. Since the power consuming device of the present application comprises the secondary battery provided by the present application, it has at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a schematic view of an embodiment of the negative electrode sheet of the present application.

[0034] Figure 2 is a schematic view of a battery cell of an embodiment of the present application.

[0035] Figure 3 Figure 1 is an exploded view of a battery cell of an embodiment of the present application.

[0036] Figure 4 is a schematic view of a battery module of an embodiment of the present application.

[0037] Figure 5 is a schematic view of a battery pack of an embodiment of the present application.

[0038] Figure 6 is Figure 5 is an exploded view of a battery pack of an embodiment of the present application.

[0039] Figure 7 is a schematic view of a power consuming device using the secondary battery of an embodiment of the present application as a power source.

[0040] Figure 8 is a charge-discharge curve obtained by performing charge-discharge test on a button cell prepared from material 1-1 and material 1-6.

[0041] Figure 9 is a charge-discharge curve obtained by performing charge-discharge test on a button cell prepared from material 2-1.

[0042] Figure 10 is an image obtained by scanning material 1-1 by transmission electron microscopy (TEM).

[0043] Figure 11 is an image obtained by scanning the negative electrode sheet prepared in Example 1 of the present application by scanning electron microscopy (SEM).

[0044] BRIEF DESCRIPTION OF DRAWINGS

[0045] 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 battery cell; 51 shell; 52 electrode assembly; 53 top cover assembly; 10 negative electrode plate; 101 negative electrode current collector; 102 negative electrode film layer; 102a first surface; 102b second surface; 1021 first region; 1022 second region; 1023 middle region. DETAILED DESCRIPTION

[0046] Below, the embodiments of the secondary battery and the electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0047] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0048] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0049] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0050] Unless otherwise specified, the terms used in this application have the common meanings generally understood by those skilled in the art.

[0051] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods given in this application.

[0052] Unless otherwise specified, in this application, the term "active ions" refers to ions that can be intercalated and extracted back and forth between the positive electrode and the negative electrode of a secondary battery, including but not limited to lithium ions.

[0053] At present, in order to improve the energy density of secondary batteries, the negative electrode sheets are usually thickly coated to prepare high-load electrodes. However, although the thick coating design improves the energy density of the battery cell by increasing the total amount of active materials, the increase in thickness will lead to an increase in the charge migration path and limited electron and active ion transmission kinetics, resulting in poor kinetic performance of the secondary battery, such as fast charging performance.

[0054] Based on this, the present application proposes a secondary battery and an electrical device. The secondary battery has improved dynamic performance while also having high energy density. The present invention and optional embodiments are described in more detail below.

[0055] secondary batteries

[0056] This application provides a secondary battery. The term "secondary battery" herein refers to a battery cell, battery module, or battery pack. Each of these is described below.

[0057] A secondary battery cell consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0058] Negative electrode

[0059] The secondary battery of the present application 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 recorded as H, the area within the thickness range from the first surface of the negative electrode film layer to 0.3H is recorded as the first area of ​​the negative electrode film layer, the area within the thickness range from the second surface of the negative electrode film layer to 0.3H is recorded as the second area of ​​the negative electrode film layer, the first area includes a first negative electrode active material, the second area includes a second negative electrode active material, the first negative electrode active material includes a matrix and a substrate formed on the matrix. a carbon coating layer on at least a portion of the surface of the body; and the lithium insertion platform voltage of the first negative electrode active material is 0.118V to 0.140V, wherein the lithium insertion platform voltage is a charge and discharge test of a button cell prepared for the test material at a delithiation rate of 0.1C and a lithium insertion rate of 0.05C, respectively, to obtain a charge and discharge curve in the range of 0.005V-2.0V, wherein the ratio of the total lithium insertion energy in the range of 0.005V-2.0V to the total lithium insertion capacity in the range of 0.005V-2.0V is defined as the lithium insertion platform voltage of the tested material; and the gram capacity of the second negative electrode active material is greater than the gram capacity of the first negative electrode active material.

[0060] In the present application, 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 conductivity and ion transport ability, it can provide more ion transport channels on the surface of the substrate, thereby increasing the lithium insertion channel, and on the other hand, the carbon coating layer reduces the surface area of ​​the substrate, effectively reducing the co-intercalation phenomenon of the electrolyte, thereby reducing the interface impedance, but the presence of the carbon coating layer will also have a certain effect on the gram capacity of the material. Lithium insertion channels and interface impedance are important factors affecting the speed and efficiency of lithium ion transmission, and thus affect the lithium insertion platform voltage. In the present application, by making the first negative electrode active material have a lithium insertion platform voltage of 0.118V to 0.140V (greater than 0.118V), it means that the material enables the active ions to be quickly deintercalated and less prone to lithium plating on the surface of the first negative electrode active material while maintaining a high gram capacity, thereby effectively improving the kinetic performance of the secondary battery. At the same time, further, by setting the gram capacity of the second negative electrode active material in the lower layer (second region) to be greater than the gram capacity of the first negative electrode active material, the secondary battery has a high energy density.

[0061] See also Figure 1 , which shows a schematic diagram of an embodiment of the negative electrode sheet of the present application. Figure 1As shown, the negative electrode sheet 10 includes a negative electrode current collector 101 and a negative electrode film 102 formed on at least one surface of the negative electrode current collector 101. The negative electrode film 102 has a first surface 102a distal from the negative electrode current collector 101 and a second surface 102b opposite the first surface 102a. The thickness of the negative electrode film 102 is denoted as H. The thickness H of the negative electrode film refers to the thickness of the negative electrode film located on a single side of the negative electrode current collector. The region from the first surface 102a of the negative electrode film to a thickness of 0.3H is denoted as the first region 1021 (upper layer) of the negative electrode film. The region from the second surface 102b of the negative electrode film to a thickness of 0.3H is denoted as the second region 1022 (lower layer) of the negative electrode film. The first region 1021 includes a first negative electrode active material, and the second region 1022 includes a second negative electrode active material. The region between the first region 1021 and the second region 1022 is denoted as the intermediate region 1023. It is easy to understand that within the scope of the middle region 1023, it may contain only the first negative electrode active material, only the second negative electrode active material, both the first negative electrode active material and the second negative electrode active material, or 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 application.

[0062] It should be understood that Figure 1 In the embodiment shown in FIG, the second surface 102b is in contact with the upper surface of the negative electrode current collector 101, but the structure of the negative electrode sheet of the present application is not limited thereto. For example, there may be an additional layer between the negative electrode film layer 102 and the negative electrode current collector 101. In this case, the second surface 102b is not in direct contact with the negative electrode current collector 101.

[0063] It should also be understood that Figure 1 Although the diagram shows clear boundaries between areas, such clear interfaces may not exist in the product.

[0064] In some embodiments, the first negative electrode active material has a lithium insertion plateau voltage of 0.123V to 0.135V. The battery's lithium insertion rate is positively correlated with the electrode potential; that is, the higher the electrode potential, the higher the lithium insertion rate. A lithium insertion plateau voltage within this range allows lithium insertion at a relatively high voltage, thereby improving the secondary battery's kinetic performance and reducing the risk of lithium plating.

[0065] In some embodiments, the carbon coating layer comprises soft carbon. The lithium storage process of soft carbon is primarily due to the adsorption of active lithium ions within its structural defects, edges, and micropores, resulting in relatively fast reaction kinetics. This can improve the kinetic performance of the secondary battery.

[0066] In the present application, the matrix and the carbon coating layer of the first negative electrode active material can be distinguished by transmission electron microscopy (TEM) of the material. In the transmission electron microscopy (TEM) image of the first negative electrode active material, the matrix has regular lattice fringes, while the carbon coating layer has no obvious lattice fringes and is soft carbon.

[0067] In this application, primary particles and secondary particles have meanings well known in the art. Primary particles refer to non-agglomerated particles, and secondary particles refer to agglomerated particles formed by the aggregation of two or more primary particles. Primary and secondary particles can be distinguished using scanning electron microscopy (SEM) images.

[0068] In some embodiments, in the first negative electrode active material, the amount of secondary particle graphite material accounts for greater than or equal to 60%, such as 60%, 65%, 70%, 75%, 80% or a value within the range formed by any two of these values. Optionally, the amount of secondary particle graphite material accounts for 75%, and is used in combination by arranging two graphite materials (i.e., primary particle graphite material and secondary particle graphite material) in the negative electrode film layer region away from the negative electrode current collector (i.e., the first region), wherein the granulation structure of the secondary particle graphite material is beneficial to reducing the OI value of the material, thereby facilitating fast charging performance, while the single particle graphite material (i.e., primary particle graphite material) is beneficial to increasing the active specific surface area, reducing the charge transfer resistance (Rct), and further improving the fast charging performance of the battery. By using secondary particles as the main component in the first negative electrode active material, the isotropy is high, so that the expansion of the secondary battery during the cycle can be dispersed in all directions, reducing the expansion of the pole piece and improving the cycle performance.

[0069] In some embodiments, the OI value of the first negative electrode active material powder is 2.0 to 6.0. For example, the OI value of the first negative electrode active material powder can be 2, 3, 4, 4.5, 5, 6, or a range consisting of any two of these values. Alternatively, the OI value of the first negative electrode active material powder is 4.5 to 6.0. This allows lithium ions to be intercalated from all directions, improving the rechargeability of the secondary battery.

[0070] In the present application, the powder optimization index (OI) is a quantitative index used to characterize the degree of crystal orientation of a powder sample. It evaluates the quality of crystal growth and arrangement 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 measured using instruments and methods known in the art. For example, referring to JIS K 0131-1996 and JB / T 4220-2011, the X-ray diffraction pattern of the powder sample can be obtained, 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 embodiment of the present application, a copper target can be used as an anode target, and CuKα rays can be used as a radiation source. The ray wavelength is The scanning 2θ angle range was from 20° to 80°, and the scanning rate was 4° / min.

[0071] In some embodiments, the powder compaction density of the first negative electrode active material under a pressure of 50,000 N is less than the powder compaction density of the second negative electrode active material under a pressure of 50,000 N. This results in a greater porosity in the first region than in the second region, which helps increase the contact opportunity between the upper region and the active ions in the electrolyte, allowing the active ions to migrate faster during the charge and discharge process, improving the lithium insertion kinetics of the material, and thus improving the fast charging performance of the battery.

[0072] In some embodiments, the first negative electrode active material has a powder compaction density of 1.74 g / cc to 1.79 g / cc at a pressure of 50,000 N. For example, the first negative electrode active material has a powder compaction density of 1.74 g / cc, 1.75 g / cc, 1.76 g / cc, 1.79 g / cc, or a range between any two of these values ​​at a pressure of 50,000 N. This helps provide an appropriate porosity in the first region, thereby facilitating electrolyte infiltration and, in turn, improving the kinetic performance of the secondary battery.

[0073] In some embodiments, the powder compaction density of the second negative electrode active material under a pressure of 50,000 N is 1.95 g / cc to 2.04 g / cc. By making the powder compaction density of the second negative electrode active material within the above range, it is beneficial to provide an appropriate porosity in the first region, while also helping to improve the energy density of the secondary battery. Exemplarily, the powder compaction density of the second negative electrode active material under a pressure of 50,000 N can be 1.95 g / cc, 1.96 g / cc, 1.97 g / cc, 1.98 g / cc, 1.99 g / cc, 2.00 g / cc, 2.01 g / cc, 2.02 g / cc, 2.03 g / cc, 2.04 g / cc, or a value between any two of these values.

[0074] In this application, 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 using an electronic pressure testing machine (for example, a UTM7305 electronic pressure testing machine) with reference to GB / T 24533-2009. An exemplary test method is as follows: Weigh 1g of sample powder and add a 1.327cm 2 In the mold, pressurize to 50000N, hold the pressure for 30s, then release the pressure, hold for 10s, and then record and calculate the powder compaction density of the material under 50000N pressure.

[0075] In some embodiments, the volume distribution particle size Dv50 of the first negative electrode active material is 7.8μm to 15.8μm. By making the volume distribution particle size Dv50 of the first negative electrode active material within the above range, it is beneficial to form a better pore distribution state in the first region of the electrode, thereby being more conducive to improving the transmission performance of ions and electrons and improving the fast charging performance of the battery. For example, the volume distribution particle size Dv50 of the first negative electrode active material is 7.8μm, 8.0μm, 8.5μm, 9.0μm, 9.5μm, 10.0μm, 10.5μm, 11.0μm, 11.5μm, 12.0μm, 12.5μm, 15.8μm or a value between the ranges consisting of any two of these values.

[0076] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.90 to 1.50. Thus, the particle size distribution within this range indicates that the negative electrode active material particles are well distributed, which is conducive to the upper region of the negative electrode film layer having a suitable pore structure, thereby reducing the difficulty of ion liquid phase transmission, thereby improving the kinetic performance of the secondary battery. For example, the particle size distribution (Dv90-Dv10) / Dv50 of the first negative electrode active material is 0.90, 0.95, 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, 1.40, 1.45, 1.50 or a value between the ranges consisting of any two of these values.

[0077] In this application, the volume distribution particle sizes Dv10, Dv50, and Dv90 of a material are generally known in the art and represent the particle sizes corresponding to the 10%, 50%, and 90% cumulative volume distribution percentages, respectively. These can be measured using instruments and methods known in the art. For example, measurements can be made using a laser particle size analyzer, as per GB / T 19077-2016. The testing instrument may be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0078] In some embodiments, the BET specific surface area of ​​the first negative electrode active material is 0.6 m 2 / g to 1.3m 2 / g. By making the specific surface area of ​​the first negative electrode active material within the above range, on the one hand, the first negative electrode active material can have lower surface side reaction activity, thereby reducing the consumption of active ions in SEI film formation and improving the initial coulombic efficiency of the secondary battery; on the other hand, it can also have higher active ion transport performance, thereby improving the kinetic performance of the secondary battery. For example, the BET specific surface area of ​​the first negative electrode active material is 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g, 1.0m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g or a value within a range consisting of any two of these values.

[0079] In this application, the specific surface area of ​​a material is generally known in the art and can be measured using instruments and methods known in the art. For example, it can be measured using the nitrogen adsorption specific surface area analysis method in accordance with GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be a Tri-Star 3020 Specific Surface Area Pore Size Analyzer from Micromeritics, Inc., USA.

[0080] 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 range consisting of any two of these values. By making the gram capacity of the first negative electrode active material within the above range, it is beneficial for the secondary battery to have good power performance while taking into account energy density.

[0081] In this application, the gram capacity of a material is the ratio of the capacitance that can be released by the active material to the mass of the active material, and can be tested using methods known in the art. An exemplary test method is as follows: the sample powder is mixed with a conductive agent, a binder, and other optional additives in a certain mass ratio with a solvent to form a slurry; the prepared slurry is applied to the surface of the negative electrode current collector copper foil, dried in an oven, and then used; the electrolytic salt is dissolved in an organic solvent to prepare an electrolyte of a certain concentration; then, a metal lithium sheet is used as the counter electrode, a polyethylene (PE) film is used as the separator, and the electrolyte is assembled into a CR2430 button cell in an argon-protected glove box. After the obtained button battery was left to stand for 12 hours, it was discharged at a constant current of 0.15mA to 0.005V at 25°C, left to stand for 10 minutes, and then discharged at a constant current of 50μA to 0.005V. After standing for 10 minutes, it was discharged at a constant current of 10μA to 0.005V; then it was charged at a constant current of 0.3mA to 2.0V, and the charge capacity was recorded. The ratio of the charge capacity to the sample mass is the gram capacity of the corresponding material. In some embodiments, the gram capacity of the second negative electrode active material is greater than 359mAh / g. Optionally, the gram capacity of the second negative electrode active material is 360mAh / g to 366mAh / g. By ensuring that the gram capacity of the second negative electrode active material is within the above range, it is beneficial to increase the gram capacity of the negative electrode film layer, thereby increasing the energy density of the secondary battery. For example, 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 within a range formed by any two of these values. 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.

[0082] In the present application, the 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, thereby increasing the number of lithium-insertion sites on the surface, and thus the gram capacity is higher. In addition, the larger crystallite size can also reduce the number of crystallite grain boundaries of the material, further increasing the number of lithium-insertion sites, thereby reducing the adverse effects of the gram capacity and powder compaction density, and thus improving the gram capacity and powder compaction density of the material. The Lc of the second negative electrode active material is within the above range, which is conducive to reducing the stacking of graphite crystallites, thereby reducing the number of grain boundaries between crystallites, increasing the lithium-insertion sites on the surface of the material, and thus improving the gram capacity of the material. As a result, the second negative electrode active material located in the second region has a high gram capacity and a high compaction density, which is beneficial for the secondary battery to have a high energy density.

[0083] 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. Exemplarily, 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 range consisting of any two of these values, and 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 range consisting of any two of these values. Optionally, 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. By making La(110) and Lc(002) of the second negative electrode active material within the above ranges, it is more conducive to improving the gram capacity and compaction density of the second negative electrode active material, thereby facilitating the secondary battery to have a high energy density.

[0084] In the present application, the La(110) and Lc(002) of the material can be measured using instruments and methods known in the art. For example, the negative electrode plate removed from the secondary battery can be cleaned with an organic solvent such as DMC and then dried. The cleaned negative electrode plate is then immersed in NMP for ultrasonic treatment to separate the copper foil to obtain the negative electrode material. After the negative electrode material is dried, it is calcined at 350°C to 500°C, then washed with water several times, and dried at 80°C to obtain the first negative electrode active material. An X-ray diffractometer (such as Bruker D8 Discover) is used for testing. The test can refer to JIS K 0131-1996 and JB / T4220-2011 to obtain the peak intensity and full width at half maximum (FWHM) of the diffraction peak corresponding to the (110) crystal plane of the first negative electrode active material and the peak intensity and full width at half maximum (FWHM) of the diffraction peak corresponding to the (002) crystal plane, and then calculated according to the Scherrer formula.

[0085] In some embodiments, the La(110) / Lc(002) of the second negative electrode active material is 3.5 to 5.5. In the present application, by making the La(110) / Lc(002) of the second negative electrode active material within the above range, the number of crystallites in the material is small, and therefore the number of grain boundaries between the grains is small, which is beneficial to increase the lithium embedding sites on the surface of the material, improve the gram capacity of the material, and thus improve the energy density of the secondary battery. Exemplarily, 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 within the range consisting of any two of these values. In some optional embodiments, in the second negative electrode active material, La(110) / Lc(002) is 4.5 to 5.5.

[0086] In some embodiments, charge and discharge tests are performed on a button cell prepared from the second negative electrode active material at a delithiation rate of 0.1C and a lithium insertion rate of 0.05C, respectively, to obtain a charge and discharge curve in the range of 0.005V to 2.0V. In the discharge curve, a lithium insertion platform exists in the voltage range of 0.005V to 0.07V, and the discharge capacity corresponding to the lithium insertion platform accounts for a proportion X1 of more than 43% of the total discharge capacity of the button cell.

[0087] In this application, the charge and discharge test was carried out as follows: the sample powder was mixed with the conductive agent carbon black (SuperP), the binder polyvinylidene fluoride (PVDF) in a mass ratio of 91.6:1.8:6.6 and the solvent N-methylpyrrolidone (NMP) to form a slurry; the prepared slurry was coated on the surface of the negative electrode current collector copper foil, dried in an oven and set aside; ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 was dissolved in the above organic solvent to prepare a concentration of 1 mol / L. L electrolyte; then, a metal lithium sheet is used as a counter electrode, a polyethylene (PE) film is used as an isolation membrane, and the above-mentioned electrolyte is assembled into a CR2430 button battery in an argon-protected glove box; after the obtained button battery is allowed to stand for 12 hours, it is discharged at a constant current rate of 0.05C at 25°C to 0.005V, allowed to stand for 10 minutes, and then discharged at a constant current of 50μA again to 0.005V, allowed to stand for 10 minutes, and finally discharged at a current of 10μA to 0.005V. The charging process is charged at a constant current of 0.1C to 2.0V, and the relationship between the charge and discharge capacity of the material and voltage, that is, the charge and discharge curve, is obtained.

[0088] In this application, the button cell prepared with the second negative electrode active material was subjected to the above-mentioned charge and discharge test to obtain the charge and discharge curve. In the discharge curve, the discharge capacity corresponding to the lithium insertion platform in the voltage range of 0.005V to 0.07V accounts for more than 43% of the total discharge capacity of the button cell. The high gram capacity of the second negative electrode active material is conducive to improving the energy density of the secondary battery. Although the mechanism is not clear, the inventors believe that during the lithium insertion process of the graphite material, as the amount of lithium ion insertion increases, graphite intercalation compounds of different orders, such as LiC 24 、LiC 12 , LiC6, etc. The formation of graphite intercalation compounds of different orders corresponds to lithium insertion platforms in different voltage ranges in the charge and discharge curves of graphite materials. Due to the different theoretical capacities of graphite intercalation compounds of different orders, for example, the theoretical capacity of LiC6 is 372mAh / g, while LiC 12 The theoretical specific capacity is 186 mAh / g. Therefore, during the lithium insertion process, the more LiC6 is formed (i.e., corresponding to the lithium insertion platform in the voltage range of 0.005V to 0.07V), the greater the corresponding capacity contribution, and the higher the specific capacity of the graphite material. For example, X1 can be 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, or a range consisting of any two of these values.

[0089] In some embodiments, the ratio X1 of the discharge capacity corresponding to the lithium embedded platform to the total discharge capacity of the button cell is 43% to 50%, optionally 43% to 47%.

[0090] In some embodiments, the degree of graphitization of the second negative electrode active material is 94.0% to 96.0%. By setting the degree of graphitization of the second negative electrode active material within the above range, it is beneficial for the first negative electrode active material to have a high compaction density and gram capacity, thereby improving the energy density of the secondary battery. Exemplarily, the degree of graphitization of the second negative electrode active material can be 94.0%, 94.5%, 95.0%, 95.5%, 96.0%, or a range between any two of these values. In some optional embodiments, the degree of graphitization of the second negative electrode active material is 94.2% to 95.8%.

[0091] In this application, the graphitization degree of a material is the proportion of carbon in the material in the form of a graphite structure, which can be measured using instruments and methods known in the art. For example, an X-ray diffractometer (such as Bruker D8 Discover) can be used for testing. The test can refer to JIS K 0131-1996 and JB / T 4220-2011 to obtain the average interlayer spacing d of the C(002) crystal plane in the material crystal structure. 002 Then according to the formula g=(0.344-d002 ) / (0.344-0.3354)×100% to calculate the degree of graphitization. In the above formula, d 002 It is the average interlayer spacing of the C(002) planes in the material's crystal structure expressed in nanometers (nm).

[0092] In some embodiments, the volume distribution particle size Dv50 of the second negative electrode active material is 14.5 μm to 18.0 μm, which can be measured by the above method. For example, the volume distribution particle size Dv50 of the first 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 range between any two of these values. In some optional embodiments, the volume distribution particle size Dv50 of the second negative electrode active material is 15.0 μm to 17.5 μm.

[0093] In some embodiments, the particle size distribution of the second negative electrode active material (Dv90-Dv10) / Dv50 is 0.90-1.25. Exemplarily, the particle size distribution of the second negative electrode active material (Dv90-Dv10) / Dv50 is 0.90, 0.95, 1.00, 1.05, 1.10, 1.15, 1.20, 1.25, or a range consisting of any two of these values. In some optional embodiments, the particle size distribution of the second negative electrode active material (Dv90-Dv10) / Dv50 is 0.90-1.20.

[0094] In some embodiments, the specific surface area of ​​the second negative electrode active material is 0.8 m 2 / g~2.1m 2 / g, which can be measured by the above method. By making 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 in SEI film formation and improving the first 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. For example, the specific surface area of ​​the second negative electrode active material can be 0.8m 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.1m 2 / g or a value within the range consisting of any two of them.

[0095] In some embodiments, 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. By ensuring that the mass ratio of the first negative electrode active material to the second negative electrode active material is within this range, the secondary battery can have excellent kinetic performance while also taking into account energy density. For example, the mass ratio of the first negative electrode active material to the second negative electrode active material can be 4:6, 5:5, 6:4, or a range consisting of any two of these values.

[0096] In some embodiments, the compaction density of the negative electrode film layer is 1.50g / cc to 1.90g / cc. By making the compaction density of the powder of the negative electrode film layer within the above range, it is beneficial for the negative electrode film layer to take into account high capacity, good active ion and electron transport performance, and thus it is beneficial for the secondary battery to take into account high energy density and good kinetic performance. Exemplarily, the compaction density of the negative electrode film layer is 1.50g / cc, 1.55g / cc, 1.60g / cc, 1.65g / cc, 1.70g / cc, 1.75g / cc, 1.80g / cc, 1.85g / cc, 1.90g / cc or a value between the ranges consisting of any two of these values. In some optional embodiments, the compaction density of the negative electrode film layer is 1.55g / cc to 1.85g / cc.

[0097] In some embodiments, the surface density of the negative electrode film layer is 6.0 mg / cm 2 ~24.0mg / cm 2 By making the surface density of the negative electrode film layer within the above range, it is beneficial for the negative electrode film layer to have both high capacity and good active ion and electron transport performance, thereby helping the secondary battery to have both high energy density and good kinetic performance. For example, the surface density of the negative electrode film layer can be 6.0 mg / cm 2 、6.5mg / cm 2 、8.0mg / cm 2 、10.0mg / cm 2 、12.0mg / cm 2 、14.0mg / cm 2 、16.0mg / cm 2 、18.0mg / cm 2 , 20.0mg / cm 2 , 22.0mg / cm 2 , 24.0mg / cm 2 Or a value between any two of the values. In some optional embodiments, the surface density of the negative electrode film layer is 6.5 mg / cm 2 ~22.5mg / cm 2 .

[0098] In some embodiments, the thickness of the negative electrode film layer is 60μm to 240μm. By making the thickness of the negative electrode film layer within the above range, it is beneficial for the negative electrode film layer to take into account high capacity, high active ion and electron transport performance, and thus it is beneficial for the secondary battery to take into account high energy density and good storage performance and kinetic performance. Exemplarily, the thickness of the negative electrode film layer can be 60μm, 80μm, 90μm, 100μm, 110μm, 120μm, 140μm, 160μm, 180μm, 200μm, 220μm, 240μm or a value between the ranges consisting of any two of them. In some optional embodiments, the thickness of the negative electrode film layer is 90μm to 220μm.

[0099] In this application, the thickness of the negative electrode film layer has a meaning well known in the art and can be measured using methods known in the art, such as using a micrometer (e.g., Mitutoyo 293-100, with an accuracy of 0.1 μm). The thickness range given in this application is the thickness range of the negative electrode film layer on one side of the negative electrode current collector.

[0100] The membrane layer is set as a double layer, and the porosity of the upper region (first region) is greater than the porosity of the lower region (second region), which is beneficial to increase the contact opportunity between the upper region and the active ions in the electrolyte, so that the active ions can migrate faster during the charging and discharging process, improve the lithium insertion kinetics of the material, and thus improve the fast charging performance of the battery.

[0101] In the present application, porosity refers to the number of pores per unit area in the channel from the film layer to the substrate, wherein the porosity of the first region and the second region can be tested by any method known in the art. For example, the cold-pressed electrode is sampled and the electrode is subjected to cross-sectional polishing by a cross-sectional ion polisher (e.g., the IB-09010CP argon ion cross-sectional polisher of JEOL, Japan), and then the longitudinal cross-sectional view of the negative electrode is scanned with a scanning electron microscope (e.g., the Sigma 300 scanning electron microscope of ZEISS, Germany); a plurality of test areas are randomly selected from the test sample, and images of the plurality of test areas are obtained by scanning electron microscope. Finally, the scanned images representing the first region and the second region are threshold segmented by using ImageJ software, the gray-white contrast area is the particle, and the black contrast area is the pore. The pore part is analyzed by computer image recognition and the threshold segmentation method to obtain the porosity of different depth areas on the electrode surface. Alternatively, the battery can be disassembled to obtain the negative electrode sheet. The sheet is cut into 6 mm x 6 mm pieces, fixed to a sample holder, and a cross-section of the negative electrode sheet is prepared using a cross-section ion polisher (e.g., the IB-09010CP argon ion cross-section polisher from JEOL, Japan). The cross-section is then scanned using a scanning electron microscope (e.g., the Sigma 300 scanning electron microscope from ZEISS, Germany). Computer image recognition and thresholding are used to analyze the pores and determine the porosity of regions at different depths on the surface of the electrode sheet.

[0102] In some embodiments, in the first negative electrode active material, the mass percentage of the carbon coating layer relative to the mass of the substrate is 0.52% to 2.35%.

[0103] In some embodiments, the method for preparing the first negative electrode active material includes:

[0104] Providing base material;

[0105] mixing the matrix material and the organic carbon source, and then performing heat treatment to obtain the first negative electrode active material;

[0106] Wherein, the mass ratio of the matrix material to the organic carbon source is 100:1.5 to 100:8.

[0107] Therefore, by setting the matrix material and the organic carbon source within the above mass ratio range, the lithium insertion platform voltage of the obtained first negative electrode active material is made to be 0.118V to 0.140V.

[0108] Positive electrode

[0109] The positive electrode sheet includes 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 includes a positive electrode active material.

[0110] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0111] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0112] In some embodiments, when the battery cell is a lithium secondary battery, the positive electrode active material may adopt the positive electrode active material for lithium secondary batteries known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphate containing olivine structure, lithium transition metal oxide and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include but are not limited to lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi0.85 Co 0.1 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0113] In some embodiments, when the battery cell is a sodium secondary battery, the positive electrode active material may be a positive electrode active material known in the art for sodium secondary batteries. For example, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc.

[0114] The battery's charge and discharge processes involve the intercalation and deintercalation of lithium, leading to different molar Li contents at different discharge states. The molar Li contents listed in this application for the positive electrode active materials refer to the initial state of the material, i.e., the state before addition of the materials. When the positive electrode active materials are used in a battery system, the molar Li content will change after charge and discharge cycles.

[0115] In the list of positive electrode active materials in this application, the molar content of O is only a theoretical value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual molar content of O will fluctuate.

[0116] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may 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 acrylate resin.

[0117] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0118] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0119] electrolytes

[0120] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0121] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0122] In some embodiments, the electrolyte salt may include at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonyl imide, lithium bistrifluoromethanesulfonyl imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium bisoxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0123] In some embodiments, the solvent may include at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl 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, cyclopentane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0124] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0125] Isolation film

[0126] In some embodiments, the battery cell further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0127] In some embodiments, the material of the separator may include at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator may be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0128] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0129] In some embodiments, the battery cell may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0130] In some embodiments, the outer packaging of the battery cell may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the battery cell may be a soft shell, such as a pouch-type soft shell. The soft shell may be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0131] The present application has no particular restrictions on the shape of the battery cell, which can be cylindrical, square or any other shape. For example, Figure 2 The battery cell 5 is a square structure as an example.

[0132] In some embodiments, reference Figure 3 , the outer packaging may include a shell 51 and a top cover assembly 53. The shell 51 may 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 connected to the receiving cavity, and the top cover assembly 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0133] In some embodiments, battery cells may be assembled into a battery module. The battery module may contain one or more battery cells. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0134] Figure 4 4 is an example of a battery module. Figure 4 In the battery module 4, the plurality of battery cells 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. The plurality of battery cells 5 may further be fixed by fasteners.

[0135] Optionally, the battery module 4 may further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0136] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0137] Figure 5 and Figure 6 The battery pack 1 is used as an example. Figure 5 and Figure 6The battery pack 1 may include a battery box and multiple battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner in the battery box.

[0138] Method for preparing secondary battery

[0139] The present application also provides a method for preparing a secondary battery, which includes preparing a negative electrode sheet.

[0140] The negative electrode sheet can be prepared in the following manner: providing a first slurry containing a first negative electrode active material and a second slurry containing a second negative electrode active material; extruding the first slurry and the second slurry simultaneously, coating the first slurry on the negative electrode current collector copper foil, and coating the second slurry on the first slurry; after drying and cold pressing, a negative electrode sheet is obtained.

[0141] Wherein, preparing the first negative electrode active material comprises:

[0142] Providing base material;

[0143] mixing the matrix material and the organic carbon source, and then performing heat treatment to obtain the first negative electrode active material;

[0144] The mass ratio of the matrix material to the organic carbon source is 100:1.5 to 100:8, for example, 100:1.5, 100:2, 100:3, 100:4, 100:5, 100:6, 100:8, or a range between any two of these values.

[0145] Thus, by setting the matrix material and the organic carbon source within the above-mentioned mass ratio range, the mass percentage of the carbon coating layer in the obtained first negative electrode active material relative to the mass of the matrix is ​​0.52% to 2.35%, thereby making its lithium insertion platform voltage within the range of 0.118V to 0.140V.

[0146] In this application, the "surface carbon residue amount" after heat treatment is taken as the "amount of carbon coating layer", and the calculation process of the mass percentage of the carbon coating layer is illustrated by taking asphalt as the raw material of the carbon coating layer in the preparation process as an example.

[0147] Surface carbon residue = amount of asphalt added × coking value of asphalt.

[0148] The mass percentage of the carbon coating layer = the amount of residual carbon on the surface / the mass of the substrate × 100%.

[0149] For example, the preparation method of the first negative electrode active material may be, for example, the following method, including steps (11) to (12):

[0150] Step (11) provides a base material.

[0151] In some embodiments, step (11) includes steps (111) to (113).

[0152] Step (111): crushing, shaping, and classifying the artificial graphite precursor to obtain primary particles.

[0153] In some embodiments, the artificial graphite precursor in step (111) includes at least one of calcined coke and needle coke.

[0154] 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 within a range consisting of any two of these values.

[0155] Step (112): mixing the first binder and the primary particles, granulating, and then performing a first heat treatment on the granulated product to obtain a secondary particle graphite material.

[0156] The present application does not specifically limit the type of the first binder, and any binder known in the art may be used. For example, the second binder may include solid asphalt, liquid asphalt, etc.

[0157] In some embodiments, the mixing 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 range between any two of these values.

[0158] In some embodiments, the first heat treatment includes graphitizing at 2800°C to 3800°C.

[0159] Those skilled in the art can adjust the graphitization time as needed. For example, an Acheson graphitization furnace can be used for graphitization treatment. When the temperature is above 2800° C., the graphitization time can be 2 to 8 days.

[0160] Step (113): mixing the primary particles and the secondary particles to obtain a matrix material.

[0161] The purpose of mixing primary particles with 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 stacking and filling advantages of the two complement each other, thereby improving the migration rate of active ions in the surface / bulk phase of the obtained first negative electrode active material.

[0162] In some embodiments, the secondary particles account for more than 60% of the matrix material.

[0163] Step (12): After mixing the matrix material and the organic carbon source, a second heat treatment is performed to obtain a first negative electrode active material.

[0164] In some embodiments, the mass ratio of the matrix material to the organic carbon source is 100:1.5 to 100:8.

[0165] The present application does not specifically limit the type of organic carbon source, and any carbon-containing material known in the art can be used. For example, the organic carbon source includes at least one of coal tar, petroleum tar, and phenolic resin.

[0166] In some embodiments, the second heat treatment includes a carbonization treatment at 900° C. to 1300° C. for 1 to 5 hours.

[0167] The preparation method of the second negative electrode active material in the present application may, for example, be the following method, including the following steps (21) to (23):

[0168] Step (21): crushing and classifying the artificial graphite precursor to obtain a first intermediate.

[0169] Step (22): The first intermediate is mixed with the second binder and granulated, and then subjected to a third heat treatment to obtain a second intermediate.

[0170] Step (23): subjecting the second intermediate to a fourth heat treatment to obtain a second negative electrode active material.

[0171] In this application, an artificial graphite precursor refers to a raw material used to prepare the second negative electrode active material. This application does not specifically limit the type of artificial graphite precursor, and artificial graphite precursors known in the art can be used. For example, the artificial graphite precursor includes one or more of calcined coke, petroleum non-needle coke, petroleum needle coke, coal-based non-needle coke, and coal-based needle coke. Among them, calcined coke includes coke formed by calcining the above-mentioned petroleum non-needle coke, petroleum needle coke, coal-based non-needle coke, and coal-based needle coke. In some optional embodiments, calcined needle coke can be used.

[0172] The present application does not specifically limit the methods of crushing, shaping and grading, and the methods of crushing, shaping and grading known in the art can be used.

[0173] In some embodiments, calcined coke can be used as an artificial graphite precursor in step (21). Calcined coke has strong needle-like properties and less mosaic structure. Using it as an artificial graphite precursor helps the development of the grain structure during the graphitization process, thereby obtaining a second negative electrode active material with a larger La and / or higher degree of graphitization, thereby promoting the embedding of lithium ions between the layers of the second negative electrode active material, and also facilitating the lithium embedding process of the second negative electrode active material to form a large amount of LiC6.

[0174] In some embodiments, the volume average particle size Dv50 of the first intermediate obtained in step (21) is 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 range consisting of any two of these values.

[0175] In some embodiments, the mixing mass ratio of the second intermediate to the first binder is 100:8 to 100:12, for example, 100:8, 100:9, 100:10, 100:11, 100:12, or a range consisting of any two of these values.

[0176] In some embodiments, the volume average particle size Dv50 of the second intermediate is 15 μm to 18 μm, for example, 15 μm, 16 μm, 17 μm, 18 μm, or a range consisting of any two of these values.

[0177] In some embodiments, the second adhesive includes asphalt with strong adhesion, low residual carbon and easy graphitization. By selecting the above asphalt as the second adhesive, the residual amount of asphalt in the second negative electrode active material can be reduced. Based on the characteristic of easy graphitization of asphalt, the residual carbon of the asphalt after graphitization has a higher degree of graphitization, reducing the influence of the residual carbon on the gram capacity and compaction density of the second negative electrode active material.

[0178] In some embodiments, the third thermal treatment includes performing a pre-carbonization treatment at 1000° C. to 1500° C. for 1 hour to 4 hours.

[0179] In some embodiments, the fourth heat treatment includes graphitizing at 3000° C. to 3200° C.

[0180] Those skilled in the art can adjust the graphitization time as needed. For example, an Acheson graphitization furnace can be used for graphitization treatment. When the temperature is above 2800° C., the graphitization time can be 2 to 8 days.

[0181] In some embodiments, the negative electrode current collector has two opposite surfaces in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0182] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0183] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0184] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0185] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0186] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material (artificial graphite), the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0187] Electrical devices

[0188] In addition, the present application also provides an electrical device, which includes the secondary battery provided in the present application. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0189] As the electrical device, a battery cell, a battery module or a battery pack can be selected according to its usage requirements.

[0190] Figure 7 This is an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0191] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be lightweight and thin, and may use a battery cell as a power source.

[0192] Example

[0193] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0194] Preparation of the first negative electrode active material

[0195] Preparation Example 1-1

[0196] Step (11): Providing a matrix material includes: Step (111): crushing and shaping needle-shaped green coke using a jet mill to obtain primary particles with a volume average particle size Dv50 of 8.5 μm. Step (112): mixing the primary particles with asphalt in a mass ratio of 100:8 and granulating the mixture, and then heat-treating the granulated product at 3000° C. for 7 days to obtain secondary particles. Step (113): blending the secondary particles with the primary particles in a quantitative ratio of 3:1 to obtain a matrix material.

[0197] Step (22) The blended base material is mixed with asphalt (coking value 33%) in a mass ratio of 100:2, and then carbonized at 1150°C in a nitrogen atmosphere for 4 hours. Finally, after screening, a first negative electrode active material having a soft carbon coating structure on the surface is obtained.

[0198] Preparation Examples 1-2 to 1-6

[0199] The preparation methods of Preparation Examples 1-2 to 1-6 are similar to Preparation Example 1-1, except that the mass ratio of the blended materials to the organic carbon source is adjusted according to the values ​​in Table 1 below.

[0200] Table 1

[0201] project raw material Mass ratio of matrix to organic carbon source serial number Preparation Example 1-1 Needle-shaped coke 100:2 Materials 1-1 Preparation Example 1-2 Needle-shaped coke 100:6 Materials 1-2 Preparation Examples 1-3 Needle-shaped coke 100:1.5 Ingredients 1-3 Preparation Examples 1-4 Needle-shaped coke 100:7 Ingredients 1-4 Preparation Examples 1-5 Needle-shaped coke 100:8 Ingredients 1-5 Preparation Examples 1-6 Needle-shaped coke 100:0 Materials 1-6

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

[0203] (1) Test of lithium embedded platform voltage X2:

[0204] The prepared first negative electrode active material (material 1-1), conductive agent carbon black (Super P), binder polyvinylidene fluoride (PVDF) are mixed uniformly with solvent N-methylpyrrolidone (NMP) in a mass ratio of 91.6:1.8:6.6 to form a slurry; the prepared slurry is coated on the surface of the negative electrode current collector copper foil, dried in an oven and set aside; ethylene carbonate (EC), ethyl methyl 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 metal lithium sheet is used as a counter electrode, a polyethylene (PE) film is used as an isolation membrane, and the above electrolyte is assembled into a CR2430 button battery in an argon-protected glove box; the obtained button battery is left to stand for 12 hours.

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

[0206] The charge-discharge curve of material 1-1 measured by the above method is as follows Figure 8 As shown by the black curve in Figure 2, the total lithium insertion energy E1 within the range of 0.005V-2V and the total lithium insertion capacity C3 within the range of 0.005V-2.0V were calculated using a Blue Power Battery Test System (CT2001A). Furthermore, the lithium insertion platform voltage was calculated to be 0.123V. The test results are recorded in Table 2.

[0207] The charge-discharge curves of materials 1-6 measured using the above method are as follows: Figure 8 As shown by the red curve in Figure 2, the total lithium insertion energy E1 within the range of 0.005V-2V and the total lithium insertion capacity C3 within the range of 0.005V-2.0V were calculated using a Blue Power Battery Test System (CT2001A). Furthermore, the lithium insertion platform voltage was calculated to be 0.095V. The test results are recorded in Table 2.

[0208] from Figure 8 It can be seen that compared with material 1-6, material 1-1 has a carbon coating layer on the surface, and material 1-1 has an improved lithium insertion 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 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 the anode target, CuKα rays are used as the radiation source, and the ray wavelength is The scanning range of 2θ angle was 20° to 80°, and the scanning rate was 4° / min. The test results are recorded in Table 2.

[0211] (3) BET specific surface area test:

[0212] The nitrogen adsorption specific surface area analysis test method can be used with reference to GB / T 19587-2017, and the result can be calculated using the BET (Brunauer Emmett Teller) method. The nitrogen adsorption specific surface area analysis test can be performed using the Tri-Star 3020 specific surface area pore size analyzer from Micromeritics, USA.

[0213] (4) Compaction density test:

[0214] Weigh 1g of sample powder and add a bottom area of ​​1.327cm 2 In the mold, the pressure is increased to 50000N, the pressure is maintained for 30s, and then the pressure is released and maintained for 10s. Then the powder compaction density of the material under the pressure of 50000N is recorded and calculated. The test results are recorded in Table 4.

[0215] (5) Morphology test

[0216] After the material 1-1 prepared in Example 1 was pre-treated with a focused ion beam (FIB) thinning method, a high-resolution test was performed using a Thermo Scientific-Talos F200S G2 field emission transmission electron microscope (TEM) with Pt nanoparticles as the substrate. The test results can be seen in Figure 10 .

[0217] from Figure 10 It can be seen that material 1-1 has a graphite core and an amorphous carbon layer (without obvious lattice fringes) located on its surface, that is, a soft carbon layer.

[0218] Table 2

[0219]

[0220]

[0221] Preparation Example 2-1

[0222] Preparation of the second negative electrode active material

[0223] 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 using an air jet mill to obtain a shaped material (first intermediate) with a Dv50 of 11 μm.

[0224] 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 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.

[0225] Step (23) The second intermediate was graphitized at a high temperature of 3020°C, and the graphitized granules were sieved and removed of magnetic, to obtain a second negative electrode active material (material 2-1).

[0226] Preparation Examples 2-2 and 2-3

[0227] The preparation method of Preparation Examples 2-2 and 2-3 was similar to that of Preparation Example 2-1, except that the raw materials used in the preparation step of the second negative electrode active material and the graphitization temperature were adjusted according to the values in Table 3 below.

[0228] Table 3

[0229]

[0230] Test of parameters of the second negative electrode active material

[0231] (1) Test of La, Lc:

[0232] 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 1.5418 A. 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:

[0233]

[0234] Where: K is the 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; θ110 is the diffraction angle corresponding to the 110 crystal plane in the X-ray diffraction pattern.

[0235] Lc(002) calculation formula:

[0236] Where: K is the shape factor, which is 0.89; λ is the wavelength (nm); β002 is the half-height 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.

[0237] (2) Test of gram capacity of materials:

[0238] The prepared second negative electrode active material, conductive agent carbon black (Super P), binder polyvinylidene fluoride (PVDF) are mixed uniformly with solvent N-methylpyrrolidone (NMP) in a mass ratio of 91.6:1.8:6.6 to form a slurry; the prepared slurry is coated on the surface of the negative electrode current collector copper foil, dried in an oven and set aside; ethylene carbonate (EC), ethyl methyl 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 metal lithium sheet is used as a counter electrode, a polyethylene (PE) film is used as an isolation membrane, and the above electrolyte is assembled into a CR2430 button battery in an argon-protected glove box; the obtained button battery is left to stand for 12 hours.

[0239] At 25°C, the prepared button cell was first discharged at a constant current of 0.15 mA to 0.005 V, allowed to stand for 5 minutes, then discharged at a constant current of 50 μA to 0.005 V, allowed to stand for 5 minutes, and then discharged at a constant current of 10 μA to 0.005 V. The first-cycle discharge capacity of the button cell was recorded. The button cell was then 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. The ratio of the first-cycle charge capacity to the mass of the material is the gram capacity of the material. The test results are recorded in Table 4.

[0240] (3) Test of the proportion of the platform's lithium-ion capacity to the overall capacity X1:

[0241] The CR2430 button cell was prepared by the same method as in the above capacity test method. After the above CR2430 button cell was left standing for 12 h, at 25°C, it was first discharged at a current of 0.15 mA to 0.005 V, left standing for 5 min, then discharged again at a current of 50 μA to 0.005 V, left standing for 5 min, and then discharged again at a current of 10 μA to 0.005 V; then it was charged at a current of 0.3 mA to 2.0 V to obtain the relationship between the charge-discharge capacity and the voltage of the material, i.e. the charge-discharge curve. The lithium intercalation capacity of the lithium intercalation platform appearing in the discharge curve in the range of 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 of the lithium intercalation capacity of the platform was calculated as X1=C1 / C2*100%.

[0242] The charge-discharge curve of material 2-1 measured by the above method is shown in Figure 9 From Figure 9 it can be read that the lithium intercalation capacity of the lithium intercalation platform appearing in the discharge curve in the range of 0.005 V-0.070 V is recorded as C1, and 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.

[0243] (4) Powder compaction density:

[0244] 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, and the test results are recorded in Table 4.

[0245] Table 4

[0246]

[0247] Example 1

[0248] Preparation of secondary batteries

[0249] (1) Preparation of the negative electrode sheet:

[0250] 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 a suitable amount of solvent deionized water, and were fully stirred to form a first negative electrode slurry.

[0251] The negative electrode active material (material 2-1) used in the second region, conductive agent carbon black (Super P), thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were fully stirred and mixed in an appropriate amount of solvent deionized water at a weight ratio of 96.4:1.0:1.2:1.4 to form a second negative electrode slurry.

[0252] Using an extrusion coating machine, the first negative electrode slurry and the second negative electrode slurry of equal mass are extruded simultaneously. The first negative electrode slurry is coated on the negative electrode current collector copper foil, and the second negative electrode slurry is coated on the first negative electrode slurry. After drying and cold pressing, the negative electrode sheet is obtained. The coating weight of the first and second negative electrode slurries is the same. The compacted density of the negative electrode film layer is 1.75g / cc, and the surface density of the negative electrode film layer is 11.5mg / cm 2 , the thickness of the negative electrode film is 65μm.

[0253] (2) Preparation of positive electrode sheet:

[0254] Lithium iron phosphate, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 96:2:2, and then N-methylpyrrolidone was added as a solvent and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry was coated on both surfaces of the positive electrode current collector aluminum foil, dried, and cold pressed to obtain a positive electrode sheet.

[0255] (3) Isolation film:

[0256] A 12 μm polyethylene film was used as the separator.

[0257] (4) Preparation of electrolyte:

[0258] Ethylene carbonate (EC), ethyl methyl 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.

[0259] (5) Assembly of secondary batteries:

[0260] The positive electrode sheet and the negative electrode sheet prepared above are placed in order, so that the isolation film is placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, and then the electrode assembly is obtained by winding; the electrode assembly is placed in an outer package, and after drying, the electrolyte is injected, and a secondary battery is obtained after vacuum packaging, standing, forming, shaping and other processes.

[0261] Negative electrode testing

[0262] After cold pressing, the negative electrode is sampled and polished by a cross-section ion polisher (e.g., IB-09010CP argon ion cross-section polisher from JEOL, Japan). Then, a longitudinal cross-section of the negative electrode is scanned by a scanning electron microscope (e.g., Sigma 300 scanning electron microscope from ZEISS, Germany), and the following is obtained: Figure 11 SEM images shown.

[0263] from Figure 11 It can be seen from the figure that the porosity of the first region of the negative electrode sheet prepared in Example 1 is greater than the porosity of the second region.

[0264] Secondary battery performance testing

[0265] (1) Fast charging capability (fast charging) test:

[0266] ① At 25°C, charge the secondary battery at a constant current of 0.33C to 3.8V, then charge it at a constant voltage to a current of 0.05C. After standing for 5 minutes, discharge the secondary battery at a constant current of 0.33C to 2.0V, and record its actual capacity as C0.

[0267] ②Then the secondary battery is charged with a constant current of 1.0C0, 1.3C0, 1.5C0, 1.8C0, 2.0C0, 2.3C0, 2.5C0, and 3.0C0 in sequence to a negative electrode cutoff potential of 3.8V or 0V (whichever is reached first). After each charging is completed, it is discharged with 1C0 to 2.0V. The negative electrode potential corresponding to charging to 10% SOC, 20% SOC, 30% SOC, 40% SOC, 50% SOC, 60% SOC, 70% SOC, and 80% SOC (State of Charge) at different charge rates is recorded.

[0268] ③ Draw the charge rate-negative electrode potential curve under different SOC states, and obtain the charge rate corresponding to the negative electrode potential of 0V under different SOC states after linear fitting. The charge rate is the charging window under the SOC state, which is recorded 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 .

[0269] ④ According to the following formula, calculate the charging time T of the secondary battery from 10% SOC to 80% SOC (assuming that the secondary battery does not deposit lithium), in minutes.

[0270] (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%.

[0271] (2) Energy density test:

[0272] At 25°C, the secondary battery was charged at a constant current of 1 / 3C to 3.65V. Then, it was charged at a constant voltage at 3.65V to a current of 0.05C. After resting for 5 minutes, it was discharged at a constant current of 1 / 3C to 2.5V. The discharge energy of the battery was recorded. The battery discharge energy divided by the battery weight is the battery's gravimetric energy density, expressed in Wh / kg. The measurement data is shown in Table 5.

[0273] (3) Cyclic performance test:

[0274] ① After the secondary battery prepared above was allowed to stand for 5 minutes at 45°C, it was charged at a constant current rate of 1C to the upper cut-off voltage (corresponding to 100% SOC);

[0275] ② Charge the secondary battery at a constant voltage to a current of 0.05C. After standing for 5 minutes, discharge the secondary battery at a constant current of 1C to the lower cut-off voltage (corresponding to 0% SOC). Record the discharge capacity at this time, which is the first cycle discharge capacity (D1).

[0276] Repeat steps ① and ② above 1000 times and record the discharge capacity D at the 1000th cycle. 1000 ;

[0277] Capacity retention rate of secondary battery after 1000 cycles (%) = D1 / D 1000 The test data are recorded in Table 5.

[0278] Examples 2-8

[0279] A secondary battery was 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 were adjusted according to the description in Table 5.

[0280] The test results of the secondary batteries of Examples 2-8 are recorded in Table 5.

[0281] Comparative Examples 1-3

[0282] A secondary battery was 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 were adjusted according to the description in Table 5.

[0283] Comparative Example 4

[0284] A secondary battery was prepared in the same manner as in Example 1, except that the negative electrode sheet was prepared by the following method:

[0285] The negative electrode active material (Material 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 mixture was stirred to form a negative electrode slurry.

[0286] The negative electrode slurry was coated on the negative electrode current collector copper foil by an extrusion coating device; after drying and cold pressing, a negative electrode sheet was obtained. The compaction density of the negative electrode film layer was 1.75 g / cc, the surface 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.

[0287] Comparative Example 5

[0288] A secondary battery was prepared in the same manner as in Example 1, except that the negative electrode sheet was prepared by the following method:

[0289] The negative electrode 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 mixture was stirred to form a negative electrode slurry.

[0290] The negative electrode slurry was coated on the negative electrode current collector copper foil by an extrusion coating device; after drying and cold pressing, a negative electrode sheet was obtained. The compaction density of the negative electrode film layer was 1.75 g / cc, the surface 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.

[0291] The test results of the secondary batteries of Comparative Examples 1-5 are recorded in Table 5.

[0292] Table 5

[0293]

[0294]

[0295] From the data in Table 5, it can be seen that compared with Comparative Examples 1-5, Examples 1-8 according to the present application can effectively improve the kinetic performance without affecting the cycle performance, while taking into account the high energy density.

[0296] Examples 9-10

[0297] A secondary battery was prepared and tested using a method similar to that of Example 1, with the only difference being that when preparing the negative electrode sheet, the mass ratio of the first negative electrode active material to the second negative electrode active material was adjusted according to Table 6.

[0298] The secondary batteries were tested and the test results are recorded in Table 6.

[0299] Table 6

[0300]

[0301] It can be seen from the data in Table 6 that when the mass ratio of the first negative electrode active material to the second negative electrode active material is 4:6-6:4, the prepared secondary battery has improved kinetic performance while taking into account energy density.

[0302] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery comprising a negative electrode plate, characterized in that: The negative electrode sheet 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 having 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 recorded as H, the area within a thickness range from the first surface of the negative electrode film layer to 0.3H is recorded as the first area of ​​the negative electrode film layer, and the area within a thickness range from the second surface of the negative electrode film layer to 0.3H is recorded as the second area of ​​the negative electrode film layer, the first area includes a first negative electrode active material, and the second area includes a second negative electrode active material. The first negative electrode active material includes a substrate and a carbon coating layer formed on at least a portion of the surface of the substrate, wherein the carbon coating layer includes soft carbon, the lithium insertion platform voltage of the first negative electrode active material is 0.118V~0.140V, and the OI value of the first negative electrode active material powder is 4.48~6.0, wherein, The lithium insertion platform voltage is obtained by performing charge and discharge tests on button cells prepared from the test material at a delithiation rate of 0.1C and a lithium insertion rate of 0.05C, respectively, to obtain a charge and discharge curve in the range of 0.005V-2.0V, wherein the ratio of the total lithium insertion energy to the total lithium insertion capacity in the range of 0.005V-2.0V is defined as the lithium insertion platform voltage of the tested material; The gram capacity of the second negative electrode active material is greater than the gram capacity of the first negative electrode active material.

2. The secondary battery according to claim 1, wherein The lithium insertion platform voltage of the first negative electrode active material is 0.123V~0.135V.

3. The secondary battery according to claim 1, wherein The first negative electrode active material includes a primary particle graphite material and a secondary particle graphite material.

4. The secondary battery according to claim 3, wherein In the first negative electrode active material, the secondary particulate graphite material accounts for greater than or equal to 60%.

5. The secondary battery according to claim 1, wherein The OI value of the first negative electrode active material powder is 4.5-6.

0.

6. The secondary battery according to claim 1, wherein The gram capacity of the second negative electrode active material is greater than 359 mAh / g.

7. The secondary battery according to any one of claims 1 to 4, characterized in that The gram capacity of the second negative electrode active material is 360 mAh / g to 366 mAh / g.

8. The secondary battery according to any one of claims 1 to 4, characterized in that The first negative electrode active material has a powder compaction density of 1.74 g / cc to 1.79 g / cc under a pressure of 50,000 N; and / or The powder compaction density of the second negative electrode active material under a pressure of 50,000 N is 1.95 g / cc to 2.04 g / cc.

9. The secondary battery according to any one of claims 1 to 4, characterized in that The first negative electrode active material satisfies one or more of the following: (1) The volume distribution particle size Dv50 of the first negative electrode active material is 7.8 μm to 15.8 μm; (2) the particle size distribution of the first negative electrode active material (Dv90-Dv10) / Dv50 is 0.90 to 1.50; (3) The BET specific surface area of ​​the first negative electrode active material is 0.6 m 2 / g to 1.3m 2 / g; (4) The gram capacity of the first negative electrode active material is 350.5 mAh / g to 358.5 mAh / g.

10. The secondary battery according to any one of claims 1 to 4, characterized in that The second negative electrode active material satisfies one or more of the following: (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 3.5~5.5; (3) a button cell prepared from the second negative electrode active material is subjected to charge and discharge tests at a delithiation rate of 0.1C and a lithium insertion rate of 0.05C, respectively, to obtain a charge and discharge curve in the range of 0.005V to 2.0V. In the discharge curve of the second negative electrode active material, a lithium insertion platform exists in the voltage range of 0.005V to 0.070V, and the discharge capacity corresponding to the lithium insertion platform accounts for a ratio X1 of more than 43% of the total discharge capacity of the button cell; (4) The degree of graphitization of the second negative electrode active material is 94.0% to 96.0%.

11. The secondary battery according to claim 10, wherein: The second negative electrode active material satisfies one or more of the following: (1) The La(110) of the second negative electrode active material is 132nm~172nm, and the Lc(002) is 30nm~36nm; (2) La(110) / Lc(002) is 4.5~5.5; (3) The ratio X1 of the discharge capacity corresponding to the lithium embedded platform to the total discharge capacity of the button battery is 43% to 47%; (4) The degree of graphitization of the second negative electrode active material is 94.2% to 95.8%.

12. The secondary battery according to any one of claims 1 to 4, characterized in that The second negative electrode active material satisfies 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 of the second negative electrode active material (Dv90-Dv10) / Dv50 is 0.9-1.25; (3) The BET specific surface area of ​​the second negative electrode active material is 0.8 m 2 / g~2.1m 2 / g.

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

4.

14. The secondary battery according to any one of claims 1 to 4, characterized in that The secondary battery satisfies 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 surface density of the negative electrode film layer is 6.0 mg / cm 2 ~24.0mg / cm 2 ; The thickness of the negative electrode film layer is 60 μm to 240 μm.

15. The secondary battery according to any one of claims 1 to 4, characterized in that The porosity of the first region is greater than the porosity of the second region.

16. The secondary battery according to any one of claims 1 to 4, characterized in that The preparation method of the first negative electrode active material comprises: Providing base material; mixing the matrix material and the organic carbon source, and then performing heat treatment to obtain the first negative electrode active material; Wherein, the mass ratio of the matrix material to the organic carbon source is 100:1.5 to 100:

8.

17. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to any one of claims 1 to 16.

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