Graphite negative electrode active material and preparation method thereof, secondary battery and electric device

Abstract

The invention provides a graphite negative electrode active material and a preparation method thereof, a secondary battery and an electric device, the graphitization degree of the graphite negative electrode active material is less than or equal to 93%, the graphite negative electrode active material meets the condition that La / Lc is greater than or equal to 4.5, La is the crystallite size in the a-axis direction obtained through XRD, and Lc is the crystallite size in the c-axis direction obtained through XRD.

Description

Technical Field

[0001] The present application relates to the technical field of battery materials, and in particular to a graphite negative electrode active material and a preparation method thereof, a secondary battery and an electrical device. Background Art

[0002] In recent years, secondary batteries have been widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace, and other fields. With the application and promotion of secondary batteries, people have increasingly higher requirements for their energy density, cycle performance, and high-rate charging performance. As a key component of secondary batteries, the performance of negative electrode active materials is influenced to a certain extent by their properties. Graphite, with its regular layered structure and excellent conductivity, has a theoretical specific capacity of 372 mAh / g and high efficiency, making it the current mainstream negative electrode active material. However, as a negative electrode active material in secondary batteries, graphite expands significantly during charge and discharge, resulting in severe capacity decay and a short cycle life. Summary of the Invention

[0003] The present application is made in view of the above-mentioned problems, and its purpose is to provide a graphite negative electrode active material and a preparation method thereof, a secondary battery containing the graphite negative electrode active material and an electrical device containing the secondary battery. The graphite negative electrode active material in the present application can improve the cycle performance of the secondary battery.

[0004] To achieve the above-mentioned objectives, the first aspect of the present application provides a graphite negative electrode active material, wherein the graphitization degree of the graphite negative electrode active material is less than or equal to 93%, and the graphite negative electrode active material satisfies the following conditions: La / Lc≥4.5, wherein La is the crystallite size in the a-axis direction obtained by XRD, and Lc is the crystallite size in the c-axis direction obtained by XRD. In the present application, the graphitization degree of the graphite negative electrode active material is controlled within the above-mentioned range, so that the secondary battery has a higher capacity. On this basis, controlling the aspect ratio of the graphite negative electrode active material within the above-mentioned range can reduce the number of carbon layers in the graphite negative electrode active material (i.e., reduce the size of the graphite at the end face) and increase the size of the graphite at the basal plane. In this way, the expansion at the end face can be reduced without reducing the storage capacity, thereby facilitating an increase in the cycle life of the secondary battery while also achieving a higher energy density.

[0005] In some embodiments, the graphite negative electrode active material has a degree of graphitization in the range of 88%-92%; and / or, the graphite negative electrode active material satisfies: 4.5≤La / Lc≤4.8. In the present application, the graphitization degree of the graphite negative electrode active material is controlled to be within the above range, so that the secondary battery has a more suitable high capacity. In addition, the aspect ratio of the graphite negative electrode active material is controlled to be within the above range, so that the crystallites of the graphite negative electrode active material have a more suitable aspect ratio, which is more conducive to reducing its expansion at the end face, thereby reducing the microscopic expansion degree of the graphite particles, which can further improve the cycle life of the secondary battery while also taking into account high energy density.

[0006] In some embodiments, 110 nm ≤ La ≤ 130 nm; and / or, 24 nm ≤ Lc ≤ 28 nm. In the present application, the crystallite sizes La and Lc of the graphite negative electrode active material are controlled within the above ranges, so that the graphite negative electrode active material has the characteristics of small crystallite size and large aspect ratio, so that the graphite crystallites have a more suitable crystallite size, which is more conducive to increasing the isotropy of the particles, reducing damage to the solid electrolyte membrane caused by isotropic expansion, and further improving the cycle life of the secondary battery.

[0007] In some embodiments, the graphite negative electrode active material has a first peak angle k on the (002) crystal plane obtained by XRD at 100% SOC, and a second peak angle h on the (002) crystal plane obtained by XRD at 0% SOC; the second peak angle h and the first peak angle k satisfy the following relationship: 2.3≤hk≤2.8. The difference between the first peak angle k of the (002) crystal plane of the graphite negative electrode active material at 100% SOC and the second peak angle h of the (002) crystal plane of the graphite negative electrode active material at 0% SOC reflects the lithium insertion capacity, that is, reflects the capacity characteristics of the material. When the above relationship is satisfied, it indicates that the graphite negative electrode active material of the present application has a high capacity.

[0008] In some embodiments, the graphite negative electrode active material is D / I G is 0.1-0.18; among them, I D Indicates that the Raman spectrum is at 1350±50cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±50cm -1 The G peak intensity at I D / I G The value is 0.1-0.18, which reflects that the carbon surface of the graphite negative electrode active material has a high degree of order and fewer defects, so there are fewer side reactions between it and the electrolyte, which is beneficial to the cycle stability of the secondary battery.

[0009] In some embodiments, the tap density of the graphite negative electrode active material is in the range of 0.95 g / cc to 1.25 g / cc. When the tap density of the graphite negative electrode active material is within the above range, it is beneficial to increase the compaction density of the negative electrode film layer and improve the energy density of the secondary battery.

[0010] In some embodiments, the specific surface area of ​​the graphite negative electrode active material is in the range of 0.96 m 2 / g-1.21m 2 The specific surface area of ​​the graphite negative electrode active material is within the above range, which can provide a larger reaction surface, facilitate the reaction of active ions, and improve the cycle performance of the secondary battery.

[0011] In some embodiments, the particle size Dv50 of the graphite negative electrode active material is 8 μm-15 μm. When the particle size Dv50 of the graphite negative electrode active material is within the above range, it is beneficial to reduce its specific surface area and reduce the occurrence of side reactions, thereby improving the cycle stability of the secondary battery.

[0012] In some embodiments, the graphite negative electrode active material has a particle size distribution (Dv90-Dv10) / Dv50 of 1.25-1.6. When the particle size distribution of the graphite negative electrode active material is within this range, its particles have good packing properties, which is beneficial for increasing the compaction density of the negative electrode film layer and improving the energy density of the secondary battery. Furthermore, it is beneficial for forming a reasonable pore structure between the particles in the negative electrode film layer, improving the transport capacity of active ions and enhancing the cycle performance of the secondary battery.

[0013] The second aspect of the present application also provides a method for preparing a graphite negative electrode active material, the method comprising: selecting a coke material; pre-treating the coke material to obtain an intermediate material; and carbonizing the intermediate material to obtain the graphite negative electrode active material, the carbonization treatment comprising: heating the intermediate material from a first temperature to a second temperature at a first heating rate, and then heating the intermediate material from the second temperature to a third temperature at a second heating rate; wherein the first temperature is 1100°C-1200°C; the second temperature is 2000°C-2300°C; the third temperature is 2800°C-3100°C; the first heating rate is 11°C / min-25°C / min; and the second heating rate is 5°C / min-10°C / min. Therefore, by regulating the heating rate in different temperature ranges during the graphitization process, the growth trend of the microcrystals can be changed, so that the size La of the graphite microcrystals in the a-axis direction increases and the size Lc in the c-axis direction decreases, thereby increasing the aspect ratio of the formed microcrystals, reducing the number of carbon layers in the graphite microcrystals, and further reducing the end surface expansion rate of the graphite negative electrode active material, which is beneficial to improving the cycle life of the graphite negative electrode active material.

[0014] In some embodiments, the pretreatment includes heating the raw material from room temperature to 1100°C-1200°C and maintaining the temperature for 1 hour or less under an inert atmosphere. The high-temperature pretreatment can remove impurities and moisture from the raw material, improve the purity of the raw material, and improve the efficiency of the graphitization reaction.

[0015] In some embodiments, prior to the high-temperature pretreatment, the pretreatment further comprises: heating from room temperature to 1100°C-1200°C and maintaining the temperature for less than or equal to 0.5 hours under an inert atmosphere, followed by cooling to room temperature at a cooling rate of 25°C / min-30°C / min; and, cycling the heating and cooling processes. During the pretreatment, the degree of supercooling is increased by rapidly cooling at high temperatures, thereby increasing the number of crystal nuclei inside the particles.

[0016] As the number of crystal nuclei increases, the number of crystallites inside graphite particles of the same size increases, and the size of the corresponding crystallites decreases.

[0017] As a result, the expansion of individual crystallite end faces decreases, reducing their damaging effects on the solid electrolyte membrane, which helps improve the cycle life of the graphite anode. Furthermore, the increased number of crystallites makes the graphite particles more isotropic, and the particle-level expansion is more uniform in all directions. As a result, surface stress accumulation is reduced during cycling, further minimizing damage to the solid electrolyte membrane on the particle surface.

[0018] In some embodiments, the temperature increase and temperature decrease processes are cycled 1-4 times, thereby further increasing the number of crystal nuclei within the particles, thereby reducing the size of the crystallites and improving the cycle life of the secondary battery.

[0019] The third aspect of the present application provides a secondary battery, comprising a negative electrode plate, wherein the negative electrode plate comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises the graphite negative electrode active material described in the first aspect or the graphite negative electrode active material prepared according to the preparation method of the second aspect of the present application.

[0020] The negative electrode film layer in the secondary battery of the present application includes the graphite negative electrode active material provided by the present application, which can effectively improve the cycle performance of the secondary battery.

[0021] A fourth aspect of the present application provides an electrical device comprising the secondary battery according to the third aspect of the present application.

[0022] The electric device of the present application includes the secondary battery provided by the present application, and thus has at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1Schematic diagram of a battery cell according to one embodiment of the present application.

[0024] Figure 2 yes Figure 1 FIG. 1 is an exploded view of a battery cell according to an embodiment of the present application.

[0025] Figure 3 Schematic diagram of a battery module according to one embodiment of the present application.

[0026] Figure 4 Schematic diagram of a battery pack according to one embodiment of the present application.

[0027] Figure 5 yes Figure 4 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0028] Figure 6 FIG. 1 is a schematic diagram of an electrical device using a secondary battery according to an embodiment of the present application as a power source.

[0029] Description of reference numerals:

[0030] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 battery cell; 51 shell; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0031] Below, the embodiments of the graphite negative electrode active material and its preparation method, secondary battery and electric device of the present application are described in detail with appropriate reference to the drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures 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.

[0032] " 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.

[0033] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0034] 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.

[0035] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0036] Unless otherwise specified, the terms used in this application have the common meanings generally understood by those skilled in the art.

[0037] 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.

[0038] In this application, the term "active ions" has the conventional meaning in the art, and refers to ions that can be intercalated and deintercalated between the positive electrode and the negative electrode of a secondary battery, including but not limited to lithium ions.

[0039] In this application, the term "coke material" has the conventional meaning in the art, and refers to a carbonaceous fuel produced by heating and distilling coal under high temperature and low oxygen conditions.

[0040] For graphite negative electrode active materials, increasing the energy density of secondary batteries requires increasing the graphitization degree of graphite, thereby increasing the reversible capacity of graphite. However, highly graphitized graphite generally has a large anisotropy. High anisotropy means that during the lithium insertion process, the graphite lattice tends to expand in the same direction (the c-axis direction of the graphite crystal), resulting in a large volume expansion of the secondary battery.

[0041] In addition, during the cycling of secondary batteries, the graphite negative electrode repeatedly intercalates and deintercalates lithium, which is manifested microscopically as the graphite particles continuously expanding and contracting. The conventional solid electrolyte interphase membrane (SEI) formed on the graphite surface has general toughness and cannot withstand the large expansion of graphite, which eventually leads to the continuous rupture of the SEI. The fresh graphite negative electrode interface exposed after the SEI membrane rupture consumes active lithium, resulting in capacity loss during the cycle, that is, reducing the cycle performance. At the same time, the accumulation of SEI film during the graphite cycle affects the performance of the conductive network, which leads to a decrease in the electronic conductivity of the negative electrode plate and an increase in polarization, which to a certain extent also reduces the cycle performance of the secondary battery.

[0042] With the continuous deepening of graphite anode research and the continuous improvement of market requirements, the requirements for the cycle performance of graphite anode are becoming increasingly higher. Based on this, this application proposes a new graphite anode active material that has low expansion during the cycle process and can achieve a long cycle life.

[0043] negative electrode active material

[0044] In a first aspect of an embodiment of the present application, a graphite negative electrode active material is provided, wherein the graphitization degree of the graphite negative electrode active material is less than or equal to 93%, and the graphite negative electrode active material satisfies: La / Lc≥4.5, wherein La is the crystallite size in the a-axis direction obtained by XRD, and Lc is the crystallite size in the c-axis direction obtained by XRD.

[0045] In the present application, the graphitization degree range of the graphite negative electrode active material is within the above-mentioned range, so that the secondary battery has a higher capacity. On this basis, the aspect ratio of the graphite negative electrode active material is controlled to be within the above-mentioned range, which can reduce the number of carbon layers in the graphite negative electrode active material (i.e., reduce the size of the graphite at the end face) and increase the size of the graphite at the base face. In this way, the expansion at the end face can be reduced without reducing the storage capacity, which is beneficial to improving the cycle life of the secondary battery, while at the same time taking into account a higher energy density.

[0046] In the present application, the reduction in the size of the graphite end face can further reduce the consumption of active ions due to SEI aging and thickening during high-temperature storage, thereby improving the high-temperature storage performance of the secondary battery.

[0047] The XRD spectrum of the graphite negative electrode active material of this application can be measured using an X-ray diffractometer in accordance with JIS K 0131-1996. The test conditions are as follows: graphite powder is placed in a sample cell 0.5 mm deep and 25 mm in diameter, and the sample is prepared using the flat plate method. CuKα radiation is used as the radiation source, a copper target is used as the anode target, a voltage of 40 kV, a current of 40 mA, an anti-scatter slit of 1 mm, a scanning range of 20°-80° at a 2θ angle of 0.01671°, a step length of 0.24 s per step, and a scanning rate of 4° / min. The test instrument can be a Bruker D8 Discover X-ray diffractometer.

[0048] Calculation of graphite crystallite size: Substitute the half-width and peak angle of the (100) and (002) crystal planes into the Scherrer formula Dc = kλ / (Bcosθ), where k is the Scherrer constant, Dc is the average thickness of the grain perpendicular to the crystal plane, B is the half-width or integrated width of the diffraction peak in the graphite powder sample, θ is the Bragg angle, and λ is the X-ray wavelength, which is 1.54056 nm. The half-width and peak angle of the (100) and (002) crystal planes can be used to obtain the graphite crystallite sizes La and Lc.

[0049] In this application, the degree of graphitization of the graphite negative electrode active material has a well-known meaning in the art and can be tested 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 (002) crystal plane in the crystal structure of the graphite negative electrode active material. 002 Then according to the formula g=(0.344-d 002 ) / (0.344-0.3354)×100% to calculate the degree of graphitization. In the above formula, d 002It is the average interlayer spacing of the (002) crystal planes in the material's crystal structure expressed in nanometers (nm).

[0050] In some embodiments, the graphite negative electrode active material has a degree of graphitization ranging from 88% to 92%. In some embodiments, the graphite negative electrode active material satisfies the following conditions: 4.5 ≤ La / Lc ≤ 4.8. In some embodiments, the graphite negative electrode active material has a degree of graphitization ranging from 88% to 92%, and the graphite negative electrode active material satisfies the following conditions: 4.5 ≤ La / Lc ≤ 4.8.

[0051] In the present application, the graphitization degree of the graphite negative electrode active material is controlled within the above-mentioned range, so that the secondary battery has a more suitable high capacity. In addition, the aspect ratio of the graphite negative electrode active material is controlled within the above-mentioned range, so that the crystallites of the graphite negative electrode active material have a more suitable aspect ratio, which is more conducive to reducing its expansion at the end surface, thereby reducing the microscopic expansion of the graphite particles, which can further improve the cycle life of the secondary battery while also achieving a high energy density.

[0052] In some embodiments, 110 nm ≤ La ≤ 130 nm. In some embodiments, 24 nm ≤ Lc ≤ 28 nm. In some embodiments, 110 nm ≤ La ≤ 130 nm, and 24 nm ≤ Lc ≤ 28 nm.

[0053] In the present application, the crystallite sizes La and Lc of the graphite negative electrode active material are controlled within the above-mentioned range, so that the negative electrode material has the characteristics of small crystallite size and large aspect ratio, and the graphite crystallites have a more suitable crystallite size, which is more conducive to increasing the isotropy of the particles, reducing the damage to the solid electrolyte membrane caused by isotropic expansion, and can further improve the cycle life of the secondary battery.

[0054] In addition, the value of the crystallite size Lc of the graphite negative electrode active material satisfies the above range, so that the graphite has a smaller end face size, which can further reduce the consumption of active ions due to SEI aging and thickening during high-temperature storage and improve the high-temperature storage performance of the secondary battery.

[0055] This application addresses the root cause of expansion. On the one hand, it reduces the number of carbon layers in the graphite anode active material, thereby reducing the microscopic expansion of the graphite particles. On the other hand, it reduces the size of the crystallites and increases the number of grains to improve the isotropy of the particles and reduce the damage to the SEI film caused by expansion. These two aspects can improve the cycling performance of the graphite anode and achieve a longer cycle life.

[0056] In some embodiments, the (002) crystal plane of the graphite negative electrode active material obtained by XRD at 100% SOC has a first peak angle k, and the (002) crystal plane of the graphite negative electrode active material obtained by XRD at 0% SOC has a second peak angle h; the second peak angle h and the first peak angle k satisfy the following relationship: 2.3≤hk≤2.8.

[0057] The difference between the first peak angle k of the (002) crystal plane of the graphite negative electrode active material at 100% SOC and the second peak angle h of the (002) crystal plane of the graphite negative electrode active material at 0% SOC reflects the lithium insertion capacity, that is, the capacity characteristics of the material. When the above relationship is met, it indicates that the graphite negative electrode active material of the present application has a high capacity.

[0058] In this application, the XRD spectrum at 100% SOC can be measured using an X-ray diffractometer with reference to JIS K 0131-1996. Specifically, a graphite negative electrode active material is prepared into a negative electrode sheet and assembled into a secondary battery. The negative electrode sheet is charged to 3.65V at a constant current of 0.5C and to 0.05C at a constant voltage. Then, the secondary battery is disassembled and the negative electrode sheet is removed. The negative electrode sheet is soaked in dimethyl carbonate for a certain time (e.g., 2h-10h); next, the negative electrode sheet is removed and dried at a certain temperature and time (e.g., 60°C for more than 4h); the dried negative electrode sheet is baked at a certain temperature and time (e.g., 400°C for more than 2h); a sample of the negative electrode active material is sampled from an area of ​​the baked negative electrode sheet (a blade scraping sample can be used); the collected negative electrode active material is sieved (e.g., sieved with a 200-mesh sieve) to obtain a sample suitable for testing. The test conditions are consistent with the test conditions of the graphite negative electrode active material in the above embodiment.

[0059] In the present application, the XRD spectrum at 0% SOC can be measured using an X-ray diffractometer with reference to JIS K 0131-1996. The test method is as follows: a graphite negative electrode active material is prepared into a negative electrode sheet and assembled into a secondary battery. The negative electrode sheet is charged to 3.65V at a constant current of 0.5C, charged to 0.05C at a constant voltage, then discharged to 2.5V at 0.5C, and then discharged to 0.05C at a constant voltage. Next, the secondary battery is disassembled and the negative electrode sheet is removed. The negative electrode sheet is soaked in dimethyl carbonate for a certain time (e.g., 2h-10h). The negative electrode sheet is then removed and dried at a certain temperature and time (e.g., 60°C for more than 4h). The dried negative electrode sheet is baked at a certain temperature and time (e.g., 400°C for more than 2h). The negative electrode active material is sampled from any area of ​​the baked negative electrode sheet (scraping with a blade can be used for sampling). The collected negative electrode active material is sieved (e.g., sieved with a 200-mesh screen) to obtain a sample suitable for testing. The test conditions are consistent with the test conditions for the graphite negative electrode active material in the above embodiment.

[0060] In some embodiments, the diffraction peak of the (002) crystal plane of the graphite negative electrode active material obtained by XRD at 100% SOC can be divided into a first diffraction peak having a third peak angle e and a second diffraction peak having a fourth peak angle f; the third peak angle e and the fourth peak angle f satisfy the following relationship: 0.75≤fe≤1.15.

[0061] The difference between the third peak angle e of the first diffraction peak and the fourth peak angle f of the second diffraction peak in the diffraction peak of the (002) crystal plane at 100% SOC reflects the lithium insertion capacity, that is, the capacity characteristics of the material. When the above relationship is met, it indicates that the graphite negative electrode active material of the present application has a high capacity.

[0062] In some embodiments, the diffraction peak of the (002) crystal plane includes a first diffraction peak and a second diffraction peak, which are obtained by performing peak separation processing on the peak angle of the negative electrode (002) crystal plane using TOPAS at 100% SOC.

[0063] In this application, the XRD spectrum at 100% SOC can be measured using an X-ray diffractometer with reference to JIS K 0131-1996. Specifically, a graphite negative electrode active material is prepared into a negative electrode sheet and assembled into a secondary battery. The negative electrode sheet is charged to 3.65V at a constant current of 0.5C and to 0.05C at a constant voltage. Then, the secondary battery is disassembled and the negative electrode sheet is removed. The negative electrode sheet is soaked in dimethyl carbonate for a certain time (e.g., 2h-10h); next, the negative electrode sheet is removed and dried at a certain temperature and time (e.g., 60°C for more than 4h); the dried negative electrode sheet is baked at a certain temperature and time (e.g., 400°C for more than 2h); a sample of the negative electrode active material is sampled from an area of ​​the baked negative electrode sheet (a blade scraping sample can be used); the collected negative electrode active material is sieved (e.g., sieved with a 200-mesh sieve) to obtain a sample suitable for testing. The test conditions are consistent with the test conditions of the graphite negative electrode active material in the above embodiment.

[0064] In some embodiments, the graphite negative electrode active material is D / I G is 0.1-0.18; among them, I D Indicates that the Raman spectrum is at 1350±50cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±50cm -1 The G peak intensity at .

[0065] I of graphite negative electrode active material D / I G The value is 0.1-0.18, which reflects that the carbon surface of the graphite negative electrode active material has a high degree of order and fewer defects, so there are fewer side reactions between it and the electrolyte, which is beneficial to the cycle stability of the secondary battery.

[0066] In this application, the graphite negative electrode active material I D / I G The value can be tested using a Raman spectrometer. The test conditions are: excitation wavelength of 532nm, grating of 600 lines, objective lens of 50 times, integration time of 10s, accumulation times of 3 times, surface scanning, obtain the D peak and G peak intensity of 100 points, calculate the I of 100 points D / I G , remove the largest and smallest 30 I D / I G The average value of the remaining 40 points is the I D / I G The testing instrument may be a Horiba LabRAM HR800 Raman spectrometer.

[0067] In some embodiments, the tap density of the graphite negative electrode active material is in the range of 0.95 g / cc to 1.25 g / cc. When the tap density of the graphite negative electrode active material is within the above range, it is beneficial to increase the compaction density of the negative electrode film layer and improve the energy density of the secondary battery.

[0068] In this application, the tap density of a graphite negative electrode active material has a meaning known in the art and can be measured using instruments and methods known in the art. For example, it can be measured using a powder tap density tester in accordance with GB / T 5162-2006. The test instrument can be a Dandong Better BT-301, with the following test parameters: vibration frequency 250±15 times / minute, amplitude 3±0.2mm, vibration number 5000 times, and a 25mL graduated cylinder.

[0069] In some embodiments, the specific surface area of ​​the graphite negative electrode active material is in the range of 0.96 m 2 / g-1.21m 2 / g.

[0070] The specific surface area of ​​the graphite negative electrode active material is within the above range, which can provide a larger reaction surface, is conducive to the reaction of active ions, and improves the cycle performance of the secondary battery.

[0071] In this application, the BET specific surface area of ​​a graphite negative electrode active material is well 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 test method in accordance with GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be a Micromeritics Tri-Star 3020 specific surface area pore size analyzer.

[0072] In some embodiments, the particle size Dv50 of the graphite negative electrode active material is 8 μm-15 μm.

[0073] When the particle size Dv50 of the graphite negative electrode active material is within the above range, it is beneficial to reduce its specific surface area and reduce the occurrence of side reactions, thereby improving the cycle stability of the secondary battery.

[0074] In some embodiments, the particle size distribution of the graphite negative electrode active material (Dv90-Dv10) / Dv50 is 1.25-1.6, optionally 1.3-1.6.

[0075] When the particle size distribution (Dv90-Dv10) / Dv50 of the graphite negative electrode active material is within the above range, its particle stacking performance is better, which is beneficial to improving the compaction density of the negative electrode film layer and improving the energy density of the secondary battery; in addition, it is also beneficial to form a reasonable pore structure between the particles of the negative electrode film layer, improving the transmission capacity of active ions and improving the cycle performance of the secondary battery.

[0076] In this application, the volume distribution particle sizes Dv10, Dv50, and Dv90 of the graphite negative electrode active 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.

[0077] In some embodiments, the expansion rate of the negative electrode sheet prepared from the graphite negative electrode active material is 6%-9% at 100% SOC.

[0078] The graphite negative electrode active material provided in the present application has small expansion, which is beneficial to improving the cycle stability of the secondary battery.

[0079] In the present application, the expansion rate of the negative electrode sheet at 100% SOC is determined by the following method: first, the graphite negative electrode active material is prepared into a negative electrode sheet, and the thickness of the negative electrode sheet after cold pressing is measured, which is recorded as d0. Then, the negative electrode sheet is assembled into a secondary battery, and is charged to 3.65V at a constant current of 0.1C and to 0.05C at a constant voltage. Next, the secondary battery is disassembled and the negative electrode sheet is taken out, and the thickness of the negative electrode sheet at 100% SOC is measured, which is recorded as d1. The expansion rate at 100% SOC is (d1-d0) / d1.

[0080] In the present application, the thickness of the negative electrode film layer has a well-known meaning in the art and can be measured using methods known in the art, such as using a micrometer (such as Mitutoyo 293-100, with an accuracy of 0.1 μm).

[0081] Preparation method of negative electrode active material

[0082] According to a second aspect of the embodiment of the present application, a method for preparing a negative electrode active material is provided, the method comprising: step 1, selecting a coke material; step 2, pre-treating the coke material to obtain an intermediate material; step 3, carbonizing the intermediate material to obtain the graphite negative electrode active material, the carbonization treatment comprising: heating the intermediate material from a first temperature to a second temperature at a first heating rate, and then heating the intermediate material from the second temperature to a third temperature at a second heating rate; wherein the first temperature is 1100°C-1200°C; the second temperature is 2000°C-2300°C; the third temperature is 2800°C-3100°C; the first heating rate is 11°C / min-25°C / min; and the second heating rate is 5°C / min-10°C / min. The graphitization degree of the graphite negative electrode active material is less than or equal to 93%, and the graphite negative electrode active material satisfies: La / Lc≥4.5, wherein La is the crystallite size in the a-axis direction obtained by XRD, and Lc is the crystallite size in the c-axis direction obtained by XRD.

[0083] Thus, by controlling the gradient temperature rise during the graphitization process and regulating the heating rate in different temperature ranges, the growth trend of the microcrystals can be changed, so that the size La of the graphite microcrystals in the a-axis direction increases and the size Lc in the c-axis direction decreases, thereby reducing the number of carbon layers in the graphite, increasing the aspect ratio of the graphite microcrystals, and reducing the end-face expansion rate of the graphite negative electrode active material, which is beneficial to improving the cycle life of the graphite negative electrode active material. In addition, the reduction in the size Lc of the graphite microcrystals in the c-axis direction can further reduce the consumption of active ions due to aging and thickening of the SEI film during high-temperature storage, thereby improving the high-temperature storage performance of the secondary battery.

[0084] In the present application, the first heating rate affects the size of Lc, and the second heating rate affects the size of La. By controlling the first heating rate to be greater than the second heating rate, the aspect ratio of the graphite crystallites can be effectively regulated, thereby improving the expansion of the formed graphite negative electrode active material and improving the cycle performance.

[0085] This application addresses the crystal structure of graphite and modulates the graphitization process to reduce the number of carbon layers in graphite crystallites, which in turn reduces crystallite expansion during cycling. From crystallites to particles, due to the phenomenon of mixed packing, the expansion of graphite particles becomes more isotropic, which reduces damage to the SEI film caused by expansion during charge and discharge, thereby improving battery cycling performance.

[0086] In some embodiments, in step 1, the coke material may be petroleum coke and / or coal-based coke; optionally, the petroleum coke is needle coke.

[0087] In some embodiments, the particle size of the char material powder is 9 μm-13 μm.

[0088] In some embodiments, the pretreatment in step 2 includes: in an inert atmosphere, heating from room temperature to 1100°C-1200°C and maintaining for less than or equal to 1 hour to perform high temperature pretreatment; optionally, heating at a heating rate of 5°C / min-15°C / min.

[0089] In the present application, the high-temperature pretreatment process can remove impurities and moisture in the raw materials, improve the purity of the raw materials, and be beneficial to the efficiency of the graphitization reaction.

[0090] In some embodiments, the carbonization treatment in step 3 is maintained at the third temperature for 1 hour to 4 hours after the temperature is raised to the third temperature.

[0091] In some embodiments, before the high-temperature pretreatment, the pretreatment in step 2 further includes: heating from room temperature to 1100°C-1200°C under an inert atmosphere and maintaining the temperature for less than or equal to 0.5h, and then cooling to room temperature at a cooling rate of 25°C / min-30°C / min, optionally, heating at a heating rate of 5°C / min-15°C / min; and, cycling the above heating process and cooling process.

[0092] In this application, during the pretreatment process, the temperature is rapidly lowered at high temperatures to increase the degree of supercooling, thereby increasing the number of crystal nuclei within the particles. As the number of crystal nuclei increases, the number of crystallites within the same-sized graphite particles increases, and the corresponding crystallites become smaller in size. As a result, the end face expansion of individual crystallites decreases, reducing their destructive effect on the solid electrolyte membrane, which is beneficial for improving the cycle life of the graphite negative electrode. In addition, the increase in the number of crystallites makes the graphite particles more isotropic, and the expansion of the particle level is more uniform in all directions. Therefore, during the cycle, the surface stress accumulation is reduced, and the damage to the solid electrolyte membrane on the particle surface is further reduced.

[0093] In some embodiments, the temperature increasing process and the temperature decreasing process are cycled 1-4 times.

[0094] This is beneficial to further increase the number of crystal nuclei inside the particles, thereby reducing the size of the crystallites and improving the cycle life of the secondary battery.

[0095] secondary batteries

[0096] According to a third aspect of the embodiments of the present application, a secondary battery is provided. The secondary battery of the present application will be described below with reference to the accompanying drawings as appropriate.

[0097] The term "secondary battery" mentioned herein refers to a battery cell, a battery module, or a battery pack. Each of these is described below.

[0098] Typically, 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.

[0099] [Negative electrode]

[0100] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes the negative electrode active material provided in the above embodiment, or the negative electrode active material prepared according to the preparation method provided in the above embodiment.

[0101] As an example, the negative electrode current collector has two surfaces opposite to each other 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.

[0102] 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.).

[0103] 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).

[0104] 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.

[0105] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0106] 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, 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 current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0107] [Positive electrode]

[0108] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer provided on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0109] 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.

[0110] 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.).

[0111] In some embodiments, when the battery cell is a lithium-ion battery, the positive electrode active material may adopt the positive electrode active material for lithium-ion batteries known in the art. As an example, the positive electrode active material may include at least one of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides 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 ), LiNi0.5 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 LiNi 0.85 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.

[0112] In some embodiments, when the battery cell is a sodium ion battery, the positive electrode active material may be a positive electrode active material for sodium ion batteries known in the art. As an example, the positive electrode active material may include sodium transition metal oxides, polyanionic compounds, Prussian blue compounds, etc., but the present application is not limited to these materials, and other traditionally known materials that can be used as positive electrode active materials for sodium ion batteries may also be used. For example, as an optional technical solution of the present application, in the sodium transition metal oxide, the transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Sodium transition metal oxide is, for example, Na x MO2, wherein M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, 0 <x≤1。

[0113] As an optional technical approach of the present application, the polyanionic compound can be a compound having sodium ions, transition metal ions and tetrahedral (YO4) n- A class of compounds with anionic units. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce; Y can be at least one of P, S and Si; n represents (YO4) n- The polyanionic compound can also be a compound with sodium ions, transition metal ions, tetrahedral (YO4) n-A class of compounds containing anion units and halogen anions. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si, and n represents (YO4) n- The halogen can be at least one of F, Cl and Br. The polyanionic compound can also be a compound with sodium ions, tetrahedral (YO4) n- Anion unit, polyhedron unit (ZO y ) m+ and an optional halogen anion. Y can be at least one of P, S and Si, and n represents (YO4) n- Valence state: Z represents a transition metal, which can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce, and m represents (ZO y ) m+ The halogen can be at least one of F, Cl and Br. Examples of polyanionic compounds are NaFePO4, Na3V2(PO4)3, NaM'PO4F (M' is one or more of V, Fe, Mn and Ni) and Na3(VO y )2(PO4)2F 3-2y (0≤y≤1). The Prussian blue compound can be a compound having sodium ions, transition metal ions and cyanide ions (CN-). The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. For example, the Prussian blue compound is Na a Me b Me' c (CN)6, wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co and Zn, 0 <a≤2,0<b<1,0<c<1。

[0114] The battery's charge and discharge processes are accompanied by the deintercalation and consumption of active ions (Li or Na). The molar content of Li or Na varies when the battery is discharged to different states. The molar content of Li or Na in the list of positive electrode active materials in this application refers to the material's initial state, i.e., the state before the materials are added. When the positive electrode active material is used in a battery system, the molar content of Li or Na will change after charge and discharge cycles.

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

[0122] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0123] In some embodiments, the solvent can be selected from 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 further 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 can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[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 1 The battery cell 5 is a square structure as an example.

[0132] In some embodiments, reference Figure 2 , 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 3 4 is an example of a battery module. Figure 3 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 4 and Figure 5 The battery pack 1 is used as an example. Figure 4 and Figure 5 The 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] Electrical devices

[0139] A fourth aspect of the embodiment of the present application further provides an electrical device, and the electrical device of the present application will be described below with reference to the accompanying drawings as appropriate.

[0140] The electrical device mentioned in the embodiments of the present application includes the secondary battery provided in the present application. The secondary battery can be used as a power source for the electrical device, and can also be used 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.

[0141] As the electrical device, a battery cell, a battery module or a battery pack can be selected according to its usage requirements.

[0142] Figure 6 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.

[0143] 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.

[0144] Example

[0145] 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.

[0146] Example 1

[0147] Preparation of graphite negative electrode active material:

[0148] Petroleum coke was selected as the coke material; under an argon atmosphere, the temperature was raised to 1100°C (first temperature) at a heating rate of 10°C / min and kept warm for 0.5h for high-temperature pretreatment; the product after high-temperature pretreatment was heated to 2200°C (second temperature) at a first heating rate of 13°C / min, and then heated to 2900°C (third temperature) at a second heating rate of 5°C / min, kept warm for 2h, and then cooled naturally to obtain a graphite negative electrode active material.

[0149] Characterization of graphite anode activity

[0150] The XRD spectrum of the graphite negative electrode active material prepared above was measured using a Bruker D8 Discover X-ray diffractometer in accordance with JIS K 0131-1996. The test conditions were as follows: the graphite negative electrode active material powder was placed in a sample cell 0.5 mm deep and 25 mm in diameter. Sample preparation was performed using the flat plate method, with CuKα radiation as the radiation source and a copper target as the anode target. The voltage was 40 kV, the current was 40 mA, an anti-scatter slit was 1 mm, and the 2θ scanning range was 20°-80°, with a step size of 0.01671°, a step duration of 0.24 s per step, and a scan rate of 4° / min.

[0151] Calculation of graphite crystallite size: Substitute the half-width and peak angle of the (100) and (002) crystal planes into the Scherrer formula Dc = kλ / (Bcosθ), where k is the Scherrer constant, Dc is the average thickness of the crystals perpendicular to the crystal plane, B is the half-width or integrated width of the diffraction peak in the graphite powder sample, θ is the Bragg angle, and λ is the X-ray wavelength, which is 1.54056 nm. The half-width and peak angle of the (100) and (002) crystal planes indicate that the size of the graphite negative electrode active material along the a-axis, La, is 122 nm, and along the c-axis, Lc, is 27 nm, with a La / Lc ratio of 4.52.

[0152] The graphitization degree of the graphite negative electrode active material prepared above can be tested using a Bruker D8 Discover X-ray diffractometer. The test can refer to JIS K 0131-1996 and JB / T 4220-2011 to obtain the average interlayer spacing d of the (002) crystal plane in the graphite negative electrode active material. 002 Then according to the formula g=(0.344-d 002 ) / (0.344-0.3354)×100%, and the graphitization degree of the graphite negative electrode active material prepared above is calculated to be 90.9%.

[0153] Preparation of secondary batteries:

[0154] Positive electrode sheet preparation: Mix the positive electrode active material, lithium iron phosphate (LiFePO4), the binder polyvinylidene fluoride, and the conductive agent Super P in a mass ratio of 97:2:1. Add an appropriate amount of solvent N-methylpyrrolidone (NMP) and stir evenly to obtain a positive electrode slurry. The positive electrode slurry is coated on both surfaces of the positive electrode current collector aluminum foil. After drying and cold pressing, the positive electrode sheet is obtained.

[0155] Preparation of the negative electrode sheet: The negative electrode active material (artificial graphite), the conductive agent (Super P), the binder (styrene-butadiene rubber), and the thickener (sodium carboxymethyl cellulose) were thoroughly mixed in a suitable amount of deionized water at a mass ratio of 96.2:0.6:1.3:1.9 to obtain a negative electrode slurry. The negative electrode slurry was applied to both surfaces of the negative electrode current collector copper foil. After drying and cold pressing, the negative electrode sheet was obtained.

[0156] Isolation film: Polypropylene film

[0157] Electrolyte: Dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) are mixed in a mass 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 1.0 mol / L, and then vinylene carbonate (VC) is added, and the content of vinylene carbonate is 1 wt% of the total mass of the electrolyte.

[0158] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation to obtain an electrode assembly; the electrode assembly is placed in an outer package, and the prepared electrolyte is injected into the dried electrode assembly. After vacuum packaging, standing, formation, shaping and other processes, a secondary battery is obtained.

[0159] Secondary battery performance test

[0160] 1. 45℃ cycle test

[0161] 1) At 45°C, the secondary battery prepared above was charged at a constant current of 1.0C to 3.65V, then at a constant voltage of 0.05C, and allowed to stand for 5 minutes. Then, the battery was discharged at a constant current of 1.0C to 2.5V, and then at a constant voltage of 0.1C. This process was considered one cycle, and the discharge capacity D0 (i.e., the initial discharge capacity) was recorded.

[0162] 2) Perform the above process 1000 times and record the discharge capacity D1 at the 1000th cycle.

[0163] The capacity retention rate of a secondary battery after 1000 cycles of 1C charge and 1C discharge is % = D1 / D0*100%.

[0164] 2. 45℃ full charge storage performance test

[0165] 1) At 25°C, the prepared secondary battery was charged at a constant current of 1C to 3.65V, then charged at a constant voltage to a current of 0.05C. After standing for 5 minutes, the secondary battery was discharged at a constant current of 1C to 2.5V. The discharge capacity at this time was recorded, which was the discharge capacity before storage C0.

[0166] 2) At 25°C, the prepared secondary battery was charged at a constant current of 1C to 3.65V, and then charged at a constant voltage to a current of 0.05C. The secondary battery was then stored in a 45°C constant temperature box for 30 days, and then taken out and allowed to stand for 24 hours until the overall temperature of the secondary battery dropped to room temperature;

[0167] 3) At 25° C., the stored secondary battery was discharged at a constant current of 1 C to 2.5 V, and the discharge capacity at this time was recorded, which was the post-storage discharge capacity C1.

[0168] The capacity retention rate (%) of the secondary battery after storage at 45° C. for 30 days = discharge capacity C1 after storage / discharge capacity C0 before storage × 100%.

[0169] Example 2-3

[0170] A graphite negative electrode active material was prepared and characterized in a manner similar to Example 1, and assembled into a secondary battery for performance testing, with the only difference being that the first heating rate during the graphitization treatment was set as shown in Table 1 when preparing the graphite negative electrode active material.

[0171] Comparative Example 1

[0172] A graphite negative electrode active material was prepared and characterized in a manner similar to that of Example 1, and assembled into a secondary battery for performance testing, with the only difference being that the first heating rate and the second heating rate during the graphitization treatment were set as shown in Table 1 when preparing the graphite negative electrode active material.

[0173] Comparative Example 2

[0174] A graphite negative electrode active material was prepared and characterized in a manner similar to that of Example 1, and assembled into a secondary battery for performance testing, with the only difference being that the first heating rate, the second heating rate, and the third temperature during the graphitization treatment were set as shown in Table 1 when preparing the graphite negative electrode active material.

[0175] The parameters of the graphite negative electrode active materials prepared in Examples 1-3 and Comparative Examples 1 and 2, as well as the test results of the secondary batteries are shown in Table 1 below.

[0176] Table 1:

[0177]

[0178] It can be seen from Table 1 above that when the graphite negative electrode active material has a specific graphitization degree range and if La / Lc is also within the range given in this application, the battery can have both good cycle performance and storage performance.

[0179] 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 graphite negative electrode active material, characterized in that The graphitization degree of the graphite negative electrode active material is less than or equal to 93%, and the graphite negative electrode active material satisfies: La / Lc≥4.5, wherein La is the crystallite size in the a-axis direction obtained by XRD, and Lc is the crystallite size in the c-axis direction obtained by XRD.

2. The graphite negative electrode active material according to claim 1, characterized in that The graphitization degree of the graphite negative electrode active material is in the range of 88% to 92%; and / or, The graphite negative electrode active material satisfies: 4.5≤La / Lc≤4.

8.

3. The graphite negative electrode active material according to claim 1 or 2, characterized in that 110nm≤La≤130nm; and / or, 24nm≤Lc≤28nm.

4. The graphite negative electrode active material according to any one of claims 1 to 3, characterized in that The (002) crystal plane of the graphite negative electrode active material obtained by XRD at 100% SOC has a first peak angle k, and the (002) crystal plane of the graphite negative electrode active material obtained by XRD at 0% SOC has a second peak angle h; The second peak position angle h and the first peak position angle k satisfy the following relationship: 2.3≤hk≤2.

8.

5. The graphite negative electrode active material according to any one of claims 1 to 4, characterized in that The graphite negative electrode active material I D / I G is 0.1-0.18; among them, I D Indicates that the Raman spectrum is at 1350±50cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±50cm -1 The G peak intensity at .

6. The graphite negative electrode active material according to any one of claims 1 to 5, characterized in that The tap density of the graphite negative electrode active material is in the range of 0.95 g / cc to 1.25 g / cc.

7. The graphite negative electrode active material according to any one of claims 1 to 6, characterized in that The specific surface area of ​​the graphite negative electrode active material is in the range of 0.96 m 2 / g-1.21m 2 / g.

8. The graphite negative electrode active material according to any one of claims 1 to 7, characterized in that The volume distribution particle size Dv50 of the graphite negative electrode active material is 8 μm-15 μm.

9. The graphite negative electrode active material according to any one of claims 1 to 8, characterized in that The particle size distribution of the graphite negative electrode active material (Dv90-Dv10) / Dv50 is 1.25-1.

6.

10. A method for preparing a graphite negative electrode active material, characterized in that: The method comprises: Select coke material; Pre-processing the coke material to obtain an intermediate material; and The intermediate material is subjected to a carbonization treatment to obtain the graphite negative electrode active material, wherein the carbonization treatment comprises: heating the intermediate material from a first temperature to a second temperature at a first heating rate, and then heating the intermediate material from the second temperature to a third temperature at a second heating rate; wherein, The first temperature is 1100° C.-1200° C.; The second temperature is 2000° C.-2300° C.; The third temperature is 2800° C.-3100° C.; The first heating rate is 11°C / min-25°C / min; The second heating rate is 5°C / min-10°C / min.

11. The preparation method according to claim 10, characterized in that: The pretreatment includes: in an inert atmosphere, heating from room temperature to 1100° C.-1200° C. and maintaining the temperature for less than or equal to 1 hour to perform high-temperature pretreatment.

12. The preparation method according to claim 11, characterized in that Before the high temperature pretreatment, the pretreatment further comprises: Under an inert atmosphere, heating from room temperature to 1100°C-1200°C and maintaining for less than or equal to 0.5h, and then cooling to room temperature at a cooling rate of 25°C / min-30°C / min; and, The above-mentioned heating process and cooling process are repeated.

13. The preparation method according to claim 12, characterized in that The above-mentioned heating process and cooling process are cycled 1-4 times.

14. A secondary battery comprising a negative electrode plate, wherein the negative electrode plate comprises a negative electrode current collector and a negative electrode film layer located on at least one surface of the negative electrode current collector, wherein the negative electrode film layer comprises the graphite negative electrode active material according to any one of claims 1 to 9, or comprises the graphite negative electrode active material prepared by the preparation method according to any one of claims 10 to 13.

15. An electrical device, characterized in that: The secondary battery according to claim 14 is included.

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