Carbon-based negative electrode active material, preparation method, secondary battery and electric device

Abstract

A carbon-based negative electrode active material, a preparation method, a secondary battery and an electric device. The gram volume of the carbon-based negative electrode active material is 345-355 mAh / g, and the powder compaction density under the pressure of 20,000 N is greater than 1.55 g / cm3 and less than 1.65 g / cm3. The secondary battery prepared from the carbon-based negative electrode active material has both good cycle performance and high energy density.

Description

Carbon-based negative electrode active material, preparation method, secondary battery and electrical device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202311675585.2, filed on December 7, 2023, entitled “Carbon-based negative electrode active materials, preparation methods, secondary batteries and electrical devices,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to the technical field of secondary batteries, and in particular to a carbon-based negative electrode active material, a preparation method, a secondary battery, and an electrical device. Background Art

[0004] In recent years, secondary batteries have been increasingly used in energy storage power systems such as hydropower, thermal, wind and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields. The market has also put forward higher requirements on the service life and energy storage performance of secondary batteries.

[0005] Negative electrode active materials are one of the important raw materials for secondary batteries and have a significant impact on the electrical performance of secondary batteries. In order to meet the ever-increasing market requirements, it is necessary to provide a negative electrode active material to improve the cycle performance of secondary batteries.

[0006] Summary of the Invention

[0007] The present disclosure provides a carbon-based negative electrode active material, the gram capacity of the carbon-based negative electrode active material is 345mAh / g to 355mAh / g, and the powder compaction density under a pressure of 20000N is 1.55g / cm 3 Above and less than 1.65g / cm 3 Secondary batteries made of carbon-based negative electrode active materials have good cycle performance and high energy density.

[0008] In any embodiment, the powder compaction density of the carbon-based negative electrode active material under a pressure of 20,000 N is 1.57 g / cm 3 ~1.62g / cm 3 .

[0009] In any embodiment, the powder compaction density of the carbon-based negative electrode active material under a pressure of 49000N is 1.73g / cm 3 Above and less than 1.84g / cm 3 The carbon-based negative electrode active materials within this range are beneficial to better improve the cycle performance of secondary batteries.

[0010] In any embodiment, the gram capacity of the carbon-based negative electrode active material is 346 mAh / g to 353 mAh / g. The gram capacity of the carbon-based negative electrode active material within this range helps to improve the stability of the negative electrode active material during the cycle process, thereby improving the cycle performance of the secondary battery.

[0011] In any embodiment, the carbon-based negative electrode active material includes primary particles and secondary particles. Optionally, the number of primary particles in the carbon-based negative electrode active material is greater than or equal to the number of secondary particles in the carbon-based negative electrode active material, which helps to reduce the structural fragmentation of the active material caused by expansion and contraction of the carbon-based negative electrode active material during the cycle, improve the structural stability of the carbon-based negative electrode active material, and thus improve the cycle performance of the secondary battery.

[0012] In any embodiment, the carbon-based negative electrode active material satisfies at least one of the following conditions:

[0013] a) Particle size distribution K is 1.0 to 1.6, and can be selected from 1.1 to 1.50, K = (Dv90-Dv10) / Dv50;

[0014] b) Volume distribution particle size Dv50 is 8 μm to 13 μm, optionally 9 μm to 12 μm;

[0015] c) The degree of graphitization is 88% to 95%, and can be optionally 90% to 95%.

[0016] Controlling the particle size and particle size distribution to meet this range helps reduce side reactions of the carbon-based negative electrode active material and irreversible consumption of active ions (such as lithium ions) during the cycle, thereby improving the cycle performance of the secondary battery. The graphitization degree of the carbon-based negative electrode active material within this range helps to increase the energy density of the negative electrode active material and improve the cycle performance of the secondary battery.

[0017] In any embodiment, the specific surface area of ​​the carbon-based negative electrode active material is 1.25 m 2 / g~1.95m 2 / g, optional 1.25m 2 / g~1.85m 2 / g; tap density is 1.07g / cm 3 ~1.27g / cm 3 The specific surface area and / or tap density of the carbon-based negative electrode active material meeting this range is helpful to reduce the side reactions of the negative electrode active material during the cycle and improve the cycle performance of the secondary battery.

[0018] In any embodiment, the carbon-based negative electrode active material is artificial graphite. Negative electrode active materials are widely available and low in cost, which helps to control the production cost of secondary batteries.

[0019] A second aspect of the present disclosure provides a method for preparing a carbon-based negative electrode active material, comprising the following steps:

[0020] Provide raw materials;

[0021] Processing raw materials to obtain intermediates;

[0022] performing graphitization treatment on the intermediate to obtain a graphitized product;

[0023] Screening the graphitized product to obtain a carbon-based negative electrode active material;

[0024] The gram capacity of the carbon-based negative electrode active material is 345mAh / g to 355mAh / g, and the powder compaction density under a pressure of 20,000N is 1.55g / cm 3 Above and less than 1.65g / cm 3 .

[0025] The carbon-based negative electrode active material prepared by this method has suitable energy density and good cycle stability, and the secondary battery prepared using this material has good cycle performance.

[0026] In any embodiment, the maximum power of the graphitization process is 70% to 90% of the rated power of the graphitization process equipment.

[0027] In any embodiment, the temperature of the graphitization treatment is 2600°C to 3000°C.

[0028] In any embodiment, the graphitization treatment time is 10 hours to 50 hours.

[0029] In any embodiment, the raw material includes one or more of petroleum coke, needle coke, and pitch coke, and needle coke can be selected.

[0030] In any embodiment, based on the total volume of the raw material, the volume proportion of the fibrous structure in the raw material is greater than or equal to 55%, and can be optionally 58%-70%.

[0031] Raw materials with a high volume fraction of fiber-type structures are beneficial for increasing the compaction density and specific capacity of graphite negative electrode active materials, allowing the graphite negative electrode active materials to retain a high degree of integrity during the compaction process, resulting in batteries with both a long cycle life and good energy density. However, an excessively high proportion of fiber-type structures will increase the cost and expansion rate of the graphite negative electrode active materials, and deteriorate the kinetic performance. Raw materials with a volume fraction of fiber-type structures within the above range have both lower costs and good specific capacity of the graphite, allowing the battery cell to have a full life cycle kinetic window, thereby comprehensively improving the long-term cycle life and electrochemical performance of the battery.

[0032] In any embodiment, processing the raw materials specifically includes the following steps: crushing, shaping and classifying the raw materials to obtain a first precursor; granulating the first precursor to obtain a second precursor; and low-temperature carbonizing the first precursor and the second precursor to obtain an intermediate product.

[0033] In any embodiment, the first precursor satisfies at least one of the following conditions:

[0034] (1) The Dv50 particle size of the first precursor is 6.5 μm to 10.5 μm;

[0035] (2) the particle size distribution of the first precursor (Dv90-Dv10) / Dv50 is 1.05 to 1.75;

[0036] (3) The tap density of the first precursor is 0.55 g / cm 3 ~0.75g / cm 3 .

[0037] In any embodiment, the volume distribution particle size Dv50 of the second precursor is 11 μm to 15 μm.

[0038] By controlling the particle size and particle size distribution of the raw materials or intermediate materials of the negative electrode active material, a negative electrode active material with good specific surface area and particle size distribution can be obtained, which helps to obtain a negative electrode active material with a suitable powder compaction density, improves the wettability of the negative electrode active material, and improves the cycle performance of secondary batteries prepared with carbon-based negative electrode active materials.

[0039] In any embodiment, the graphitized product of the first precursor and the second precursor are mixed in a mass ratio of (1:1) to (6:4), which can improve the structural stability of the negative electrode active material during the cycle process and help improve the cycle performance of the secondary battery.

[0040] The third aspect of the present disclosure provides a secondary battery including a negative electrode plate, the negative electrode plate including a negative electrode collector and a negative electrode film layer arranged on at least one surface of the negative electrode collector, the negative electrode film layer including the carbon-based negative electrode active material of the first aspect of the present disclosure or the carbon-based negative electrode active material prepared by the method of the second aspect of the present disclosure.

[0041] In any embodiment, the negative electrode plate satisfies at least one of the following conditions:

[0042] (1) The compaction density of the negative electrode film is 1.5 g / cm 3 ~1.65g / cm 3 , optional 1.55g / cm 3 ~1.65g / cm 3 ;

[0043] (2) The specific surface area of ​​the negative electrode film layer is 0.7m 2 / g~2m 2 / g, optional 1.0m 2 / g~1.6m 2 / g;

[0044] (3) The porosity of the negative electrode film layer is 0.22 to 0.42, and can be optionally 0.27 to 0.37.

[0045] The negative electrode sheet meeting the above conditions helps the secondary battery obtain good energy density, reduces the side reactions of the negative electrode active material during the cycle, and / or the migration resistance of active ions (such as lithium ions), thereby improving the cycle performance of the secondary battery.

[0046] A fourth aspect of the present disclosure provides an electric device including the secondary battery according to the third aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] FIG. 1 is a schematic diagram of a secondary battery according to an embodiment of the present disclosure.

[0048] FIG. 2 is an exploded view of the secondary battery according to the embodiment of the present disclosure shown in FIG. 1 .

[0049] FIG3 is a schematic diagram of a battery module according to an embodiment of the present disclosure.

[0050] FIG4 is a schematic diagram of a battery pack according to an embodiment of the present disclosure.

[0051] FIG. 5 is an exploded view of the battery pack shown in FIG. 4 according to an embodiment of the present disclosure.

[0052] FIG6 is a schematic diagram of an electric device using a secondary battery according to an embodiment of the present disclosure as a power source.

[0053] Description of reference numerals:

[0054] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 secondary battery; 51 shell; 52 electrode assembly; 53 cover plate. DETAILED DESCRIPTION

[0055] Below, the embodiments of the positive electrode active material and its manufacturing method, positive electrode sheet, secondary battery, battery module, battery pack and electrical device disclosed in the present invention 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 disclosure and are not intended to limit the subject matter described in the claims.

[0056] The "ranges" disclosed in this disclosure are defined in the form of lower limits and upper limits, where a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. In addition, if the minimum range values ​​listed are 1 and 2, and if the maximum range values ​​listed are 3, 4, and 5, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this disclosure, unless otherwise specified, the numerical range "a to b" represents an abbreviation for any combination of real numbers between a and b, where a and b are both 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.

[0057] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions.

[0058] Unless otherwise specified, all technical features and optional technical features disclosed herein can be combined with each other to form a new technical solution.

[0059] Unless otherwise specified, all steps of the present disclosure may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means 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), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0060] Unless otherwise specified, the terms "include" and "comprising" mentioned in this disclosure may be open-ended or closed-ended. For example, the terms "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0061] Unless otherwise specified, the term "or" is inclusive in this disclosure. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0062] Carbon-based negative electrode active materials are one of the important raw materials for secondary batteries and have a significant impact on the electrical performance of secondary batteries. During the cycling process of secondary batteries, active ions (such as lithium ions and sodium ions) are inserted and deintercalated into the carbon-based negative electrode active materials, causing the carbon-based negative electrode active materials to continuously expand and contract. After the negative electrode active materials are graphitized, the order of the material's crystal structure is improved, which helps to improve the energy density of the negative electrode active materials. However, after the degree of graphitization of the negative electrode active materials increases, the interlayer spacing of the material's crystal structure becomes narrower. During the cycling process of the secondary battery, the interlayer spacing of the negative electrode active materials expands, which reduces the stability of the negative electrode active material's crystal structure, causing the negative electrode active material to crack and break, thereby reducing the cycling performance of the secondary battery. In addition, as the interlayer spacing of the negative electrode active material's crystal structure narrows, the powder compaction density of the negative electrode active material also increases. The increased powder compaction density can improve the energy density of the negative electrode active material, but it can also reduce the wettability of the negative electrode active material or the negative electrode sheet with the electrolyte, increase the electron transfer resistance and polarization loss, and affect the cycling performance of the secondary battery.

[0063] Based on this, the present disclosure provides a carbon-based negative electrode active material that can improve the cycle performance of secondary batteries.

[0064] [Negative electrode active material]

[0065] The present disclosure provides a carbon-based negative electrode active material, wherein the gram capacity of the carbon-based negative electrode active material is 345 mAh / g to 355 mAh / g, and the powder compaction density under a pressure of 20000 N is 1.55 g / cm 3 Above and less than 1.65g / cm 3 .

[0066] In this article, the term "gram capacity" refers to the ratio of the amount of electricity released by the carbon-based negative electrode active material to the mass of the carbon material. Generally speaking, a higher gram capacity is more conducive to improving the energy density of the secondary battery.

[0067] The gram capacity of the carbon-based negative electrode active material is in the range of 345mAh / g to 355mAh / g. The lattice expansion during the material cycle is relatively small, the crystal structure is stable, and the irreversible consumption of active ions is reduced. At the same time, the carbon-based negative electrode active material has excellent capacitance, which can increase the energy density of the secondary battery, thereby improving the cycle performance of the secondary battery.

[0068] In some embodiments, the gram capacity of the carbon-based negative electrode active material is 346 mAh / g to 353 mAh / g. In some embodiments, the gram capacity of the carbon-based negative electrode active material is 347 mAh / g to 353 mAh / g. In some embodiments, the gram capacity of the carbon-based negative electrode active material is 345 mAh / g, 346 mAh / g, 347 mAh / g, 348 mAh / g, 349 mAh / g, 350 mAh / g, 351 mAh / g, 352 mAh / g, 353 mAh / g, 354 mAh / g or 355 mAh / g, or a range between any two of the above values.

[0069] When the gram capacity is within the above range, the carbon-based negative electrode active material has excellent crystal structure stability and capacitance, and significantly improves the cycle performance of the secondary battery.

[0070] The gram capacity can be measured using any method known in the art. For example, a negative electrode active material sample can be thoroughly stirred and mixed with the conductive agent carbon black and polyvinylidene fluoride (PVDF) in a suitable amount of NMP solvent at a mass ratio of 91.6:1.8:6.6 to form a uniform negative electrode slurry. The negative electrode slurry is evenly coated on the surface of the negative electrode current collector copper foil and dried and cold pressed. Then, a metal lithium sheet is used as the counter electrode and a polypropylene (PP) film is used as the separator. An electrolyte solution is injected. The electrolyte formula used is as follows: dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) are mixed at a weight ratio of 1:1:1 to obtain an organic solvent, and LiPF6 is dissolved in the organic solvent to prepare an electrolyte solution with a concentration of 1.0 mol / L. CR2430 button cells are assembled in an argon-protected glove box. At 25°C, the prepared button cell was first discharged at a constant current of 0.05C to 0.005V, then discharged at a constant current of 10μA to 0.005V. The cell was allowed to rest for 5 minutes, and the first-cycle discharge capacity was recorded. The cell was then charged at a constant current of 0.1C to 2.0V, and the charge capacity was recorded. The ratio of the charge capacity of the button cell to the mass of the negative electrode active material sample is the gram capacity of the negative electrode active material.

[0071] In this article, the "compacted density" of a powder refers to the density of the powder under a preset pressure. The higher the powder compaction density, the higher the mass of the powder per unit volume. For negative electrode active materials, a higher powder compaction density increases the capacitance of the negative electrode active material, which is more beneficial for increasing the energy density of the secondary battery. However, increasing the powder compaction density also reduces the interlayer spacing of the negative electrode active material, increasing lattice expansion variations, which is detrimental to the structural stability of the negative electrode active material.

[0072] In addition, after the compaction density of the negative electrode active material powder is increased, the porosity of the negative electrode active material is correspondingly reduced, and the wettability of the negative electrode active material in the battery electrode with the secondary battery electrolyte is affected, which is not conducive to the insertion and deinsertion of active ions in the negative electrode active material and the free migration of electrons, and affects the cycle performance of the secondary battery.

[0073] The method for measuring the compacted density of the powder can be any method known in the art. For example, referring to GB / T 24533-2009, 1g of negative electrode active material powder is weighed and added to a container with a bottom area of ​​1.327cm 2 The mold is pressurized to a specific pressure, such as 20000N or 49000N, and the pressure is maintained for 30s, then the pressure is released and maintained for 10s. The powder compaction density of the negative electrode active material under the selected pressure is measured by an electronic pressure testing machine (such as UTM7305 electronic pressure testing machine).

[0074] In the prior art, when the gram capacity of the negative electrode active material is in the range of 345 mAh / g to 355 mAh / g, it has a viscosity of not less than 1.65 g / cm2 under a pressure of 20,000 N. 3 The compaction density of the carbon-based negative electrode active material provided by the present disclosure has a reduced powder compaction density within the aforementioned gram capacity range, which not only gives the carbon-based negative electrode active material an ideal energy density, but also enables the negative electrode active material to have a lower volume expansion rate during the cycle, thereby improving the structural stability of the negative electrode active material and improving the cycle life of the secondary battery.

[0075] In some embodiments, the powder compaction density of the carbon-based negative electrode active material under a pressure of 20,000 N is 1.55 g / cm 3 , 1.56g / cm 3 , 1.57g / cm 3 、1.58g / cm 3 , 1.59g / cm 3 , 1.60g / cm 3 , 1.61g / cm 3 , 1.62g / cm 3 , 1.63g / cm 3 , 1.64g / cm 3 , 1.65g / cm 3 Or the range between any two of the above values. In some embodiments, the powder compaction density of the carbon-based negative electrode active material under a pressure of 20000N is 1.57g / cm 3 ~1.62g / cm 3 , which can further improve the capacity and structural stability of the negative electrode active material and enhance the cycle performance of the secondary battery.

[0076] In some embodiments, the powder compaction density of the carbon-based negative electrode active material under a pressure of 49000N is 1.73g / cm 3 Above and less than or equal to 1.84g / cm 3 .

[0077] In some embodiments, the powder compaction density of the carbon-based negative electrode active material under a pressure of 49000N is 1.73g / cm 3 , 1.74g / cm 3 , 1.75g / cm 3 , 1.76g / cm 3 , 1.77g / cm 3 , 1.78g / cm 3 , 1.79g / cm 3 , 1.80g / cm 3 、1.81g / cm 3 , 1.82g / cm 3 , 1.83g / cm 3 、1.84g / cm 3 Or the range between any two of the above values ​​is helpful to better improve the capacity and structural stability of the negative electrode active material and improve the cycle performance of the secondary battery.

[0078] In some embodiments, the carbon-based negative electrode active material includes primary particles and secondary particles. In some embodiments, the proportion of the primary particles in the carbon-based negative electrode active material is greater than or equal to the proportion of the secondary particles in the carbon-based negative electrode active material, which helps reduce the breakage of the negative electrode active material particles due to expansion and contraction during the cycle of the carbon-based negative electrode active material, improves the chemical and mechanical stability of the carbon-based negative electrode active material, reduces the irreversible consumption of active ions, and achieves improved cycle performance of the secondary battery.

[0079] As used herein, primary particles and secondary particles have meanings well known in the art. As used herein, "primary particles" refer to particles that are not aggregated. As used herein, "secondary particles" refer to particles that are aggregated from two or more primary particles.

[0080] Primary particles and secondary particles can be distinguished by observing the particle cross-section of the graphite negative electrode active material using a scanning electron microscope (SEM). In the present disclosure, the proportion of the number of secondary particles in the graphite negative electrode active material can be tested by methods known in the art. As an example, a cross-section polisher (such as the IB-09010CP argon ion cross-section polisher of Japan's JEOL company) can be used to prepare the cross-section of the negative electrode sheet; then, referring to JY / T010-1996, a scanning electron microscope (such as the Sigma 300 scanning electron microscope of Germany's ZEISS company) is used to scan the cross-section of the negative electrode sheet, and multiple test areas are randomly selected in the test sample. Images of the multiple test areas are obtained using a scanning electron microscope, and the number of graphite negative electrode active materials with secondary particle morphology in each image is counted as the ratio of the total number of graphite negative electrode active material particles. The average value of the multiple statistical results is the number proportion of secondary particles in the graphite negative electrode active material.

[0081] In some embodiments, based on the total number of primary particles and secondary particles in the graphite negative electrode active material, the proportion of primary particles can be selected as 50%, 52%, 54%, 56%, 58%, 60% or any numerical range therebetween.

[0082] During the cycle, the negative electrode active material undergoes repeated expansion and contraction deformation processes, resulting in the breakage of the negative electrode active material particles, which severely reduces the mechanical integrity of the primary and secondary particles. At the same time, the broken particles increase the electrolyte penetration and side reaction levels, increasing the irreversible consumption of active ions. Graphite negative electrode active materials with a low proportion of secondary particles are beneficial for maintaining the particle integrity of the graphite negative electrode active material during battery preparation, reducing the generation of new interfaces, reducing the consumption of active lithium during the cycle, and further improving the cycle stability of the secondary battery. At the same time, a certain number of secondary particles can reduce the expansion of the negative electrode while taking into account the kinetic performance of the secondary battery, thereby giving the battery cell a kinetic window throughout its life cycle. This prevents the battery capacity from rapidly decreasing or the battery life from rapidly deteriorating due to lithium plating caused by uneven current distribution, thereby comprehensively improving the battery's cycle stability.

[0083] In some embodiments, the particle size distribution K of the carbon-based negative electrode active material is 1.0 to 1.6, K = (Dv90-Dv10) / Dv50. In some embodiments, the particle size distribution K of the carbon-based negative electrode active material is 1.1 to 1.5. In some embodiments, the particle size distribution K of the carbon-based negative electrode active material is 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.6 or a range between any two of the above values, which helps to improve the concentration of the particle size of the negative electrode active material, reduce the uneven distribution of active ions in the negative electrode active material due to the large difference in particle size, reduce the side reaction of the negative electrode active material with smaller particle size and the electrolyte, and improve the cycle performance of the secondary battery.

[0084] In some embodiments, the volume distribution particle size Dv50 of the carbon-based negative electrode active material is 8 μm to 13 μm. In some embodiments, the volume distribution particle size Dv50 of the carbon-based negative electrode active material is 9 to 12 μm. In some embodiments, the volume distribution particle size Dv50 of the carbon-based negative electrode active material is 8.0 μm, 8.5 μm, 9.0 μm, 9.5 μm, 10.0 μm, 10.5 μm, 11.0 μm, 11.5 μm, 12.0 μm, 12.5 μm, 13.0 μm or a range between any two of the above values, thereby shortening the path for active ion embedding and de-embedding, which is beneficial to improving the transport performance of active ions and electrons, and is also beneficial to forming a reasonable pore structure between the particles of the negative electrode film layer, thereby further improving the cycle performance and / or rate performance of the secondary battery.

[0085] In this disclosure, the volume distribution particle size Dv50 of the carbon-based negative electrode material is well known in the art and represents the particle size corresponding to the cumulative volume distribution percentage of the material reaching 50%. It can be measured using instruments and methods known in the art. For example, it can be measured using a laser particle size analyzer with reference to GB / T 19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd. in the UK.

[0086] In some embodiments, the degree of graphitization of the carbon-based negative electrode active material is 88% to 95%. In some embodiments, the degree of graphitization of the carbon-based negative electrode active material is 88%, 89%, 90%, 90.4%, 91.1%, 92.3%, 93.5%, 94.1%, 94.7%, 95%, or a range between any two of the above values, which is conducive to the negative electrode active material having excellent specific capacity and good active ion transport performance, thereby facilitating the secondary battery to have both high energy density and good kinetic performance.

[0087] In the present disclosure, the degree of graphitization of carbon-based negative electrode active materials 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 JISK 0131-1996 and JB / T 4220-2011 to obtain the average interlayer spacing d002 of the C(002) crystal plane in the material crystal structure, and then the degree of graphitization is calculated according to the formula g=(0.344-d002) / (0.344-0.3354)×100%. In the above formula, d002 is the average interlayer spacing of the C(002) crystal plane in the material crystal structure expressed in nanometers (nm).

[0088] The degree of graphitization reflects the completeness of the graphite crystal structure in the material, that is, the regularity of the arrangement of carbon atoms in the graphite structure of the material. A material with a high degree of graphitization indicates that the distance between graphite layers is small, the smaller the lattice rotation, the less scattered stacking of layers, the more orderly the arrangement, and the higher the specific capacity of the material. A material with a low degree of graphitization indicates that the distance between graphite layers is large, which is conducive to the rapid intercalation and deintercalation of active ions and is conducive to improving the rate performance of the battery. Moreover, the large distance between graphite layers means that the material expands less when lithium is inserted, which has the effect of shallow charge and shallow discharge. At the same time, the material has a low degree of graphitization, and the SP in the graphite material is low. 3 The large number of bonds makes the layers of graphite material restrain each other, making the structure of graphite material more stable. The graphite material has excellent stability during the cycle and is conducive to long-term circulation.

[0089] In some embodiments, the specific surface area of ​​the carbon-based negative electrode active material is 1.25 m 2 / g~1.95m 2 In some embodiments, the specific surface area of ​​the carbon-based negative electrode active material is 1.25 m 2 / g~1.85m 2 In some embodiments, the specific surface area of ​​the carbon-based negative electrode active material is 1.25 m 2 / g, 1.3m 2 / g, 1.35m 2 / g, 1.45m 2 / g, 1.5m 2 / g, 1.55m 2 / g, 1.6m 2 / g, 1.65m 2 / g, 1.7m 2 / g, 1.75m 2 / g, 1.8m 2 / g, 1.9m 2 / g, 1.95m 2 / g or a range between any two of the above values, which helps to reduce the side reaction between the negative electrode active material and the electrolyte, reduce the irreversible active consumption of metal ions, and improve the cycle performance of the secondary battery.

[0090] In some embodiments, the tap density of the carbon-based negative electrode active material is 1.07 g / cm 3 ~1.27g / cm 3 In some embodiments, the tap density of the carbon-based negative electrode active material is 1.07 g / cm 3 , 1.1g / cm 3 , 1.15g / cm 3 , 1.18g / cm 3 , 1.2g / cm 3 , 1.23g / cm 3 , 1.27g / cm 3 Or the range between any two of the above values ​​is helpful to improve the transmission performance of active ions and electrons and improve the kinetic performance of the secondary battery.

[0091] In some embodiments, the carbon-based negative electrode active material is artificial graphite.

[0092] [Method for preparing negative electrode active material]

[0093] The present disclosure also provides a method for preparing a carbon-based negative electrode active material, the preparation method comprising the following steps: providing a raw material; processing the raw material to obtain an intermediate product; graphitizing the intermediate product to obtain a carbon-based negative electrode active material; the carbon-based negative electrode active material has a gram capacity of 345 mAh / g to 355 mAh / g, and a powder compaction density of 1.55 g / cm under a pressure of 20,000 N. 3 Above and less than 1.65g / cm 3 The secondary battery prepared by the carbon-based negative electrode active material has a high energy density while having good cycle performance.

[0094] As used herein, the term "graphitization" refers to the high-temperature heat treatment of carbon materials, whereby the carbon material undergoes a transformation from a two-dimensional carbon network structure to a three-dimensional ordered structure through crystallite growth at high temperatures.

[0095] In some embodiments, the maximum power of the graphitization process is 70% to 90% of the rated power of the graphitization process equipment.

[0096] In some embodiments, the power of the graphitization process may be selected to be 70%, 75%, 80%, 85%, 90% of the rated power of the equipment, or any range of values ​​therebetween.

[0097] It is understood that graphitization equipment refers to any device capable of graphitization, including but not limited to Acheson furnaces, box furnaces, internal furnaces, continuous graphitization furnaces, electric calcining furnaces, medium frequency furnaces, and tubular furnaces. The rated power of graphitization equipment produced by different manufacturers may vary, so you can select the right one based on your actual needs.

[0098] The graphitization treatment power used in the present disclosure needs to be lower than the rated power of the graphitization treatment equipment to achieve uniformity of the temperature field during the graphitization process, ensure the consistency of the material's gram capacity, and help improve the cycle life of the battery.

[0099] In some embodiments, the graphitization treatment equipment is an inner string furnace, and the rated power of the inner string furnace is 25000W-32000W.

[0100] In some embodiments, the graphitization treatment equipment is an Acheson furnace, and the rated power of the Acheson furnace is 28,000W-30,000W.

[0101] In some embodiments, the temperature of the graphitization treatment is 2600°C to 3000°C.

[0102] In some embodiments, the temperature of the graphitization treatment is 2600° C., 2700° C., 2800° C., 2900° C., 3000° C., or any range therebetween.

[0103] In some embodiments, the graphitization treatment time is 10 hours to 50 hours.

[0104] In some embodiments, the graphitization treatment time is 10 h, 15 h, 20 h, 25 h, 30 h, 35 h, 40 h, 45 h, 50 h, or any range therebetween.

[0105] In some embodiments, the graphitization treatment equipment is an internal series furnace, and the graphitization treatment time at maximum power is 10 hours to 30 hours.

[0106] In some embodiments, the graphitization treatment equipment is an Acheson furnace, and the graphitization treatment is performed at maximum power for 30 hours to 50 hours.

[0107] In some embodiments, the raw material includes at least one of petroleum coke, needle coke, and pitch coke. In some embodiments, the raw material is needle coke.

[0108] In this article, the term "petroleum coke" refers to the coke formed by high-temperature carbonization of petroleum residue or petroleum asphalt.

[0109] In this article, the term "needle coke" refers to coal tar pitch or petroleum pitch, which, after undergoing liquid phase carbonization to generate an anisotropic mesophase, can produce coke with a needle-like texture through processes such as high-temperature carbonization.

[0110] As used herein, the term "pitch coke" refers to the solid material produced by carbonizing coal tar pitch at high temperatures.

[0111] In some embodiments, the volume proportion of the fibrous structure in the raw material is greater than or equal to 55%. In some embodiments, the volume proportion of the fibrous structure in the raw material is 58%-70%.

[0112] Needle coke has a range of advantages, including low thermal expansion coefficient, low porosity, low sulfur, low ash, low metal content, high electrical conductivity, and easy graphitization. The graphite material after graphitization can achieve a high ultimate compaction density and a low cyclic expansion rate.

[0113] Coke raw materials typically have at least one of the following structures: mosaic, regional, and fibrous. Typically, based on the morphological characteristics and isochromatic zone size of coke raw materials under a polarizing microscope, isochromatic zones smaller than 30 μm are classified as mosaic; isochromatic zones larger than 30 μm are classified as regional; and anisotropic, banded isochromatic zones are classified as fibrous.

[0114] In this article, "fibrous structure" is also called streamlined structure, which refers to the structure of the raw material with obvious fibrous texture observed under a microscope.

[0115] In the present disclosure, the volume percentage of the fibrous structure in the raw material can be tested using methods known in the art. For example, according to the provisions of GB 1997-89, the raw material is crushed to 1mm and mixed, and 40g to 50g is separated. A square hole sieve is used to take 4g to 5g of a 0.07mm to 1.0mm grade sample for film making; according to the provisions of MT 116.1-86, powder coke and block coke optical films are prepared. The diameter of the powder coke optical film shall not be less than 22mm, and the volume occupied by the cement shall be less than 1 / 3; the sample is placed on a slide with clay, flattened, and then placed on the stage for focusing. After calibrating the microscope, the polarizer and analyzer are adjusted to make them orthogonal. Insert the azurite inspection plate (1λ) so that the field of view shows the interference color of the first order red; determine the step length of the moving ruler to ensure that more than 400 effective measuring points are evenly distributed, with a point spacing of 0.3 to 0.5mm and a line spacing of 0.5 to 0.8mm. Starting from one end of the sample, determine the microstructure category at the intersection of the crosshairs, and divide the number of effective measurement points of the fiber-type optical structure by the total number of test points as the volume proportion of the fiber-type structure in the raw material.

[0116] In some embodiments, based on the total volume of the raw material structure, the proportion of the fiber-type structure in the coke raw material can be selected to be 55%, 58%, 60%, 63%, 65%, 68%, 70% or any range of values ​​therebetween. Raw materials with more fiber-type structures are beneficial to improving the compaction density and gram capacity of the graphite negative electrode active material, so that the graphite negative electrode active material retains a high degree of integrity during the compaction process, and the battery has a good energy density while having a long cycle life; but an excessively high proportion of the fiber-type structure will increase the cost and expansion rate of the graphite negative electrode active material and deteriorate the kinetic performance. Raw materials with a volume proportion of the fiber-type structure within the above range have both low cost and good gram capacity of the graphite, and the battery cell has a kinetic window for the entire life cycle, thereby comprehensively improving the long-term cycle life and electrochemical performance of the battery.

[0117] By selecting coke raw materials with a fiber-type structure volume ratio greater than or equal to 55% and less than or equal to 70%, under the same graphitization treatment conditions, the crystal structure interlayer spacing of the negative electrode active material is larger, and it has excellent structural stability while having good capacitance; at the same time, the powder compaction density of the carbon-based negative electrode active material is lower (less than 1.65g / cm 3 ), which is beneficial to taking into account both the structural stability and capacitance of carbon-based negative electrode active materials.

[0118] In some embodiments, the maximum gram capacity that can be achieved by the raw material is greater than the gram capacity of the graphite negative electrode active material.

[0119] High-grade raw materials are used and the degree of graphitization is controlled so that the maximum gram capacity that the raw materials can achieve is not fully utilized, thereby reducing the cycle expansion of the graphite negative electrode active material and improving the cycle stability of the graphite negative electrode active material.

[0120] In some embodiments, the processing of raw materials specifically includes: crushing, shaping and grading the raw materials to obtain a first precursor; granulating the first precursor to obtain a second precursor; and low-temperature carbonizing a mixture of the first precursor and the second precursor to obtain the intermediate product.

[0121] Crushing is the process of reducing the particle size of raw materials. The raw materials can be crushed by any mechanical device such as crusher, mechanical mill, etc.

[0122] Shaping and grading is the process of adjusting the particle size distribution of the raw materials to obtain a first precursor that meets the particle size requirements. The particle size and particle size distribution of the first precursor can be controlled by adjusting the grading frequency and air intake volume. In some embodiments, the grading frequency is 40 Hz to 50 Hz, and the air damper opening is 20% to 70%.

[0123] Granulation involves adding a binding material to a predetermined amount of the first precursor to facilitate granulation. The binding material can be any substance known in the art for preparing negative electrode active materials, such as one or more of coal tar pitch, petroleum pitch, polymer compounds, and resins. After being heated and melted, the binding material exhibits low viscosity and maintains good fluidity, thereby reducing agglomeration of the raw material particles during subsequent preparation. This also reduces issues such as increased surface defects and surface active sites in the carbon-based negative electrode active material particles due to the need for a depolymerization step.

[0124] Low-temperature carbonization of the first precursor and the second precursor to obtain the intermediate product includes low-temperature carbonization of a mixture of the first precursor and the second precursor to obtain the intermediate product; and also includes low-temperature carbonization of the first precursor and the second precursor separately to obtain the first intermediate product and the second intermediate product respectively.

[0125] In some embodiments, the volume distribution particle size Dv50 of the first precursor is 6.5 μm to 10.5 μm. In some embodiments, the volume distribution particle size Dv50 of the first precursor is 6.5 μm, 7.0 μm, 8.0 μm, 9.0 μm, 10.0 μm, 10.5 μm, or any range therebetween.

[0126] In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 (or K) of the first precursor is 1.05-1.75. In some embodiments, the particle size distribution (Dv90-Dv10) / Dv50 (or K) of the first precursor is 1.05, 1.15, 1.25, 1.35, 1.45, 1.55, 1.65, 1.75 or any numerical range therebetween.

[0127] In some embodiments, the tap density of the first precursor is 0.55 g / cm 3 ~0.75g / cm 3 .

[0128] In some embodiments, the tap density of the first precursor is 0.55 g / cm 3 , 0.6g / cm 3 , 0.65g / cm 3 , 0.7g / cm 3 , 0.75g / cm 3 or any range of values ​​between them.

[0129] In some embodiments, the Dv50 particle size of the second precursor is 11.0 μm to 15.0 μm.

[0130] In some embodiments, the Dv50 particle size of the second precursor is 11.0 μm, 12.0 μm, 13.0 μm, 14.0 μm, 15.0 μm, or any range therebetween.

[0131] The second precursor is obtained by granulating the first precursor, and therefore, the second precursor mainly forms secondary particles in the graphite negative electrode active material.

[0132] Controlling the particle size of the first precursor and the second precursor within the above range helps to achieve the regulation of the particle size and particle size distribution of the graphite negative electrode active material and improve the cycle stability of the battery.

[0133] In some embodiments, the temperature of low-temperature carbonization is 900° C.-1300° C., and the time of low-temperature carbonization is 24 hours-240 hours.

[0134] In some embodiments, the temperature of the low-temperature carbonization may be selected to be 900° C., 1000° C., 1100° C., 1200° C., 1300° C., or any range therebetween.

[0135] In some embodiments, the low-temperature carbonization time may be 24 h, 50 h, 75 h, 100 h, 150 h, 200 h, 240 h, or any range therebetween.

[0136] In some embodiments, the first precursor and the graphitized product of the second precursor are mixed in a mass ratio of (1:1) to (6:4) to obtain a carbon-based negative electrode active material. This improves the chemical and mechanical properties of the negative electrode active material during cycling, and can also provide the negative electrode membrane with a suitable pore structure, reduce the irreversible consumption of active ions, and improve the transport performance of active ions, thereby improving the cycling performance of the secondary battery.

[0137] In some embodiments, after the precursor is graphitized, it is further screened and demagnetized.

[0138] In some embodiments, the sieved and demagnetized primary particle graphitization product and the secondary particle graphitization product are mixed in a mass ratio of (1:1) to (6:4), for example, 50:50, 55:45, or 60:40.

[0139] In addition, the secondary battery, battery module, battery pack, and electric device of the present disclosure will be described below with reference to the drawings as appropriate.

[0140] In one embodiment of the present disclosure, a secondary battery is provided.

[0141] Typically, a secondary battery 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.

[0142] [Positive electrode]

[0143] 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.

[0144] 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.

[0145] 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 layer and a metal layer formed on at least one surface of the polymer material base layer. 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 made of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0146] In some embodiments, the positive electrode active material may adopt the positive electrode active material for batteries that is well 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 disclosure 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 / 3 Mn 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 0.15 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.

[0147] 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.

[0148] 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.

[0149] 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.

[0150] [Negative electrode]

[0151] The negative electrode plate includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes the above-mentioned carbon-based negative electrode active material.

[0152] 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.

[0153] In some embodiments, the negative electrode film layer may further include other negative electrode active materials in addition to the above-mentioned carbon-based negative electrode active materials. In some embodiments, the other negative electrode active materials include but are not limited to one or more of conventional natural graphite, other artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials and lithium titanate. The silicon-based material may include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite and silicon alloy material. The tin-based material may include one or more of elemental tin, tin oxide and tin alloy material.

[0154] 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.).

[0155] 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).

[0156] 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.

[0157] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0158] 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, a negative electrode sheet including a negative electrode film layer can be obtained.

[0159] In some embodiments, the compaction density of the negative electrode film layer is 1.5 g / cm 3 ~1.65g / cm 3 In some embodiments, the compaction density of the negative electrode film layer is 1.55 g / cm 3 ~1.65g / cm 3 The compaction density of the negative electrode film layer within the above range helps to control the energy density of the secondary battery, thereby improving the cycle performance of the secondary battery.

[0160] In some embodiments, the specific surface area of ​​the negative electrode film layer is 0.7 m 2 / g~2m 2 In some embodiments, the specific surface area of ​​the negative electrode film layer is 1.0 m 2 / g~1.6m 2 The specific surface area of ​​the negative electrode film layer within the above range helps to control its contact area with the electrolyte, reduce the side reactions of the carbon-based negative electrode active material in the negative electrode sheet, reduce the irreversible consumption of active ions, and thus improve the cycle performance of the secondary battery.

[0161] In some embodiments, the porosity of the negative electrode film layer is 0.22 to 0.42. In some embodiments, the porosity of the negative electrode film layer is 0.27 to 0.37. A porosity of the negative electrode film layer within this range helps reduce the migration resistance of active ions and electrons, thereby improving the kinetic performance of the secondary battery.

[0162] The compaction density of the negative electrode film layer can be tested using methods known in the art. As an example, a negative electrode sheet test sample with an area of ​​S is weighed using an electronic balance, with the weight recorded as W1. The thickness of the negative electrode sheet is measured using a caliper to obtain the thickness T1 of the negative electrode sheet. The weighed electrode sheet film layer is then wiped off, the weight of the negative electrode current collector is weighed as W2, and the thickness of the negative electrode current collector is measured using a caliper to obtain the thickness T2 of the negative electrode current collector. The compaction density of the negative electrode film layer is then calculated as PD = (W1-W2) / [(T1-T2)×S].

[0163] The specific surface area of ​​the negative electrode film can be measured using methods known in the art. For example, the specific surface area of ​​the negative electrode film can be measured using a surface area analyzer (e.g., Micromeritics TriStar 3020) using the nitrogen adsorption / desorption method, referring to the specific surface area determination method in GB / T 19587-2017. The negative electrode film is dried in a vacuum drying oven, placed in a sample tube, and measured using the analyzer.

[0164] The porosity P of the negative electrode film layer can be tested using methods known in the art. As an example, referring to GB / T21650.2-2008, the porosity P of the negative electrode sheet is measured using the nitrogen adsorption method. This is the percentage of the pore volume in the negative electrode sheet to the total volume of the negative electrode sheet: the calculation formula is P = (V1-V2) / V1×100%, where V1 is the apparent volume of the electrode sheet sample and V2 is the actual volume of the electrode sheet sample. The apparent volume V1 of the electrode sheet sample is calculated using the formula V1 = S×H×A, where S is the area of ​​the negative electrode sheet (unit: cm2); H is the thickness of the negative electrode sheet (unit: cm); and A is the number of samples (unit: ea). The specific test steps are as follows: in a conventional laboratory environment, use tweezers to select no less than 20 electrode discs with good appearance and no powder falling on the edges and put them into the sample cup, record the number of pieces, and calculate the apparent volume V1; after the sample electrode is placed in a 3.5cm3 sample cup, place it in a true density tester, close the test system, introduce ammonia according to the procedure, detect the pressure of the gas in the sample chamber and the expansion chamber, and then calculate the true volume V2 according to Bohr's law (PV=nRT), so as to obtain the porosity of the negative electrode film layer to be tested.

[0165] [Electrolytes]

[0166] The electrolyte conducts ions between the positive and negative electrodes. This disclosure does not specifically limit the type of electrolyte, and the electrolyte can be selected based on needs. For example, the electrolyte can be liquid, gel, or solid.

[0167] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0168] 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.

[0169] 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.

[0170] 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.

[0171] [Isolation film]

[0172] In some embodiments, the secondary battery further includes a separator. The present disclosure has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0173] 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.

[0174] 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.

[0175] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0176] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0177] The present disclosure has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG1 shows a secondary battery 5 having a square structure as an example.

[0178] In some embodiments, referring to Figure 2, the outer packaging may include a shell 51 and a cover plate 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 cover plate 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 be formed into 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 secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0179] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0180] Figure 3 shows an example battery module 4. Referring to Figure 3 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0181] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0182] 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.

[0183] Figures 4 and 5 illustrate an example battery pack 1. Referring to Figures 4 and 5 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0184] In addition, the present disclosure further provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present disclosure. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric 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.

[0185] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0186] Figure 6 shows 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.

[0187] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0188] Example

[0189] The following examples are provided. The examples described below are illustrative and are intended only to explain the present disclosure and are not to be construed as limiting the present disclosure. Where specific techniques or conditions are not specified in the examples, the methods were performed according to those described in the literature in the art or according to the product specifications. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0190] 1. Negative electrode active materials and secondary battery performance testing

[0191] 1. Powder compaction density test

[0192] According to GB / T 24533-2009, 1g of negative electrode active material powder was weighed and added to a plate with a bottom area of ​​1.327cm 2 The mold is pressurized to 20000N, maintained for 30s, then released and maintained for 10s, and the powder compaction density of the negative electrode active material under a pressure of 20000N is measured by an electronic pressure testing machine (such as UTM7305 electronic pressure testing machine).

[0193] According to GB / T 24533-2009, weigh 1g of negative electrode active material powder and add it to a bottom area of ​​1.327cm 2 The mold is pressurized to 49000N, maintained for 30s, then released and maintained for 10s. The powder compaction density of the carbon material under a pressure of 49000N is measured by an electronic pressure testing machine (such as UTM7305 electronic pressure testing machine).

[0194] 2. Particle size test and particle size distribution test of powder

[0195] Determination method: Referring to GB / T 19077-2016 particle size distribution laser diffraction method, a Mastersizer 2000E laser particle size analyzer from Malvern Instruments Ltd., UK, was used for determination to obtain volume distribution particle sizes Dv10, Dv50, Dv90, and particle size distribution.

[0196] 3. Negative electrode active material gram capacity test

[0197] The negative electrode active material sample, conductive agent carbon black, and polyvinylidene fluoride (PVDF) were thoroughly stirred in an appropriate amount of NMP solvent at a mass ratio of 91.6:1.8:6.6 to form a uniform negative electrode slurry. The slurry was evenly coated on the surface of the negative electrode current collector copper foil and dried and cold-pressed. Next, a lithium metal sheet was used as the counter electrode, and a polypropylene (PP) film was used as the separator. An electrolyte solution was injected. The electrolyte formulation used was as follows: dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) were mixed in a weight ratio of 1:1:1 to form an organic solvent. LiPF6 was dissolved in the organic solvent to prepare an electrolyte solution with a concentration of 1.0 mol / L. CR2430 button cells were assembled in an argon-protected glove box. At 25°C, the prepared button cell was first discharged at a constant current of 0.05C to 0.005V, then discharged at a constant current of 10μA to 0.005V. The cell was allowed to rest for 5 minutes, and the first-cycle discharge capacity was recorded. The cell was then charged at a constant current of 0.1C to 2.0V, and the charge capacity was recorded. The ratio of the charge capacity of the button cell to the mass of the negative electrode active material sample is the gram capacity of the negative electrode active material.

[0198] 4. Cycle performance test of secondary batteries

[0199] At 60°C, the prepared secondary battery was charged at a constant current of 1C to a voltage of 3.65V. It was then charged at a constant voltage of 3.65V to a current of ≤0.05C. The battery was then discharged at a constant current of 1C to a voltage of 2.5V. This constituted one charge and discharge cycle, and the discharge capacity (C1) of the first cycle was recorded. This charge and discharge cycle was repeated until the battery capacity decayed to 80% of the initial capacity (C1). The test was then stopped, and the number of test cycles was recorded.

[0200] 5. Test on the volume ratio of fiber structure in coke raw materials

[0201] Prepare raw materials according to GB 1997-89. Mix the raw materials crushed to 1mm and separate them into 40g-50g pieces. Use a square-hole sieve to select a 4g-5g sample of 0.07mm-1.0mm grade for slide preparation. Prepare powdered coke and block coke slides according to MT 116.1-86. The diameter of the powdered coke slide must not be less than 22mm, and the cementing material must occupy less than 1 / 3 of the volume. Place the sample on a slide with cement, flatten it, and then place it on the stage to align the focus. After calibrating the microscope, adjust the polarizer and analyzer so that they are orthogonal. Insert an azurite inspection plate (1λ) so that the field of view displays the interference color of primary red. Determine the step size of the moving ruler to ensure that at least 400 valid measurement points are evenly distributed. The point spacing should be 0.3-0.5mm, and the line spacing should generally be 0.5-0.8mm. Starting from one end of the sample, determine the microstructure category at the intersection of the crosshairs, and divide the number of effective measurement points of the fiber-type optical structure by the total number of test points as the volume proportion of the fiber-type structure in the raw material.

[0202] 2. Preparation of Secondary Batteries

[0203] Example 1

[0204] 1) Preparation of carbon-based negative electrode active materials

[0205] The needle coke with a volume ratio of 61.0% of the fiber structure was crushed; the crushed material was shaped and finely powdered to obtain the first precursor. The Dv50 particle size of the first precursor was 9.4 μm, the particle size distribution (Dv90-Dv10) / Dv50 was 1.19, and the tap density of the first precursor was 0.68 g / cm 3 .

[0206] The first precursor was granulated and reformed in a reactor to obtain a second precursor with a particle size Dv50 of 14.1 μm;

[0207] The first precursor and the second precursor were placed in a kiln for carbonization at a temperature of 1100°C and a high temperature zone time of 24 hours to obtain the first intermediate product and the second intermediate product, respectively. The tap density of the first intermediate product was 0.96 g / cm 3 The tap density of the second intermediate product is 0.92g / cm 3 ;

[0208] The first intermediate product and the second intermediate product are graphitized at a temperature of 2800°C, respectively. The graphitization treatment device is an internal string furnace with a rated power of 28000W. The maximum power of the graphitization treatment is 22400W. The constant power time of the maximum power is maintained for 24 hours, and primary particles and secondary particles are obtained respectively.

[0209] The primary particles and secondary particles were mixed evenly in a mass ratio of 50:50, and the mixture was sieved to remove magnetism to obtain the final product, graphite negative electrode active material.

[0210] The compaction density of the graphite negative electrode active material at 20000N is 1.61g / cm 3 , the graphitization degree is 92.48%, and the specific surface area is 1.47m 2 / g, the volume distribution particle size Dv50 is 10.3μm, and the powder compaction density under a pressure of 49000N is 1.82g / cm 3 , the gram capacity is 350mAh / g, ID / IG is 0.075, the interlayer spacing d1 of the surface area is 0.3373, the interlayer spacing d2 of the internal area is 0.3360nm, and the particle size distribution (Dv90-Dv10) / Dv50 is 1.36.

[0211] 2) Preparation of positive electrode sheet

[0212] The positive electrode active material lithium iron phosphate (LFP), the conductive agent Super P, and the binder PVDF were mixed in a mass ratio of 97:1:2, and the solvent N-methylpyrrolidone was added and stirred evenly to obtain a positive electrode slurry; the positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheet was obtained after drying, cold pressing, and slitting. The compacted density of the positive electrode sheet is 2.5g / cm 3 .

[0213] 3) Preparation of negative electrode sheet

[0214] The prepared carbon-based negative electrode active material, conductive agent Super P, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were mixed in a mass ratio of 96:1:1.2:1.8, and deionized water was added as a solvent and stirred to obtain a negative electrode slurry. The negative electrode slurry was evenly coated on the negative electrode current collector copper foil, dried, cold pressed, and cut to obtain a negative electrode sheet. The compacted density of the negative electrode film layer was 1.6 g / cm 3 , the surface density is 9.48mg / cm 2 .

[0215] 4) Preparation of electrolyte

[0216] In an argon atmosphere glove box with a water content of less than 10 ppm, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) were mixed in a weight ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1.0 mol / L. Vinylene carbonate (VC) was then added, and the VC content was 2% of the total mass of the electrolyte.

[0217] 5) Isolation film

[0218] A polypropylene film is selected as the isolation film.

[0219] 6) Preparation of secondary batteries

[0220] 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, and then wound to obtain an electrode assembly; the electrode assembly is placed in an outer package, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0221] Examples 2 to 7 and Comparative Examples 1 to 4

[0222] The preparation steps of Examples 2 to 7 and Comparative Examples 1 to 4 are similar to those of Example 1, but the preparation parameters of the negative electrode active materials are adjusted. For details, see Table 1-1 and Table 1-2.

[0223] Table 1-1: Preparation parameters of negative electrode active materials of Examples 1 to 7 and Comparative Examples 1 to 4

[0224] Table 1-2: Preparation parameters of negative electrode active materials of Examples 1 to 7 and Comparative Examples 1 to 4

[0225] The negative electrode active materials and secondary batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 4 were tested using the above test method. The results are shown in Table 2 below:

[0226] Table 2: Parameter test and performance test results of Examples 1 to 7 and Comparative Examples 1 to 4

[0227] It can be seen from Examples 1 to 7 and Comparative Examples 1 to 4 that when the gram capacity of the negative electrode active material is in the range of 345 mAh / g to 355 mAh / g, the powder compaction density of the negative electrode active material under a pressure of 20,000 N is 1.55 g / cm 3 ~1.65g / cm 3 The secondary battery prepared using the negative electrode active material has a high cycle number at 60°C, indicating that the secondary battery has good cycle performance.

[0228] In Comparative Example 1, the gram capacity of the negative electrode active material is 358.1 mAh / g, and the powder compaction density of the negative electrode active material under a pressure of 20,000 N is increased to 1.72 g / cm 3 However, the number of cycles of the secondary battery is only 1573, indicating that the powder compaction density of the negative electrode active material under a pressure of 20000N exceeds 1.65g / cm 3 When the gram capacity exceeds 355mAh / g, it is not conducive to improving the cycle performance of the secondary battery.

[0229] In Comparative Example 2, the gram capacity of the negative electrode active material is 348.5 mAh / g. When the negative electrode active material is entirely primary particles, the powder compaction density of the negative electrode active material under a pressure of 20,000 N is increased to 1.67 g / cm 3 However, the number of cycles of the secondary battery is only 1650, indicating that the powder compaction density of the negative electrode active material under a pressure of 20000N exceeds 1.65g / cm 3 After that, it can affect the cycle performance of the secondary battery.

[0230] In Comparative Example 3, when the gram capacity of the negative electrode active material is 353.7 mAh / g, the powder compaction density of the negative electrode active material under a pressure of 20000 N is increased to 1.69 g / cm 3 However, the number of cycles of the secondary battery is only 1621, indicating that the powder compaction density of the negative electrode active material under a pressure of 20000N exceeds 1.65g / cm 3 After that, it can affect the cycle performance of the secondary battery.

[0231] In Comparative Example 4, when the gram capacity of the negative electrode active material is 350.3 mAh / g, the powder compaction density of the negative electrode active material under a pressure of 20000 N is reduced to 1.54 g / cm 3 The number of cycles of the secondary battery is only 1605, indicating that the powder compaction density of the negative electrode active material under a pressure of 20000N is lower than 1.55g / cm 3 After that, the cycle performance of the secondary battery is poor.

[0232] It should be noted that the present disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and within the scope of the technical solution of the present disclosure, embodiments having substantially the same structure as the technical concept and exerting the same effects are all included in the technical scope of the present disclosure. In addition, within the scope of the present disclosure, various modifications that can be imagined by those skilled in the art to the embodiments, and other methods constructed by combining some of the constituent elements of the embodiments are also included in the scope of the present disclosure.

Claims

1. A carbon-based negative electrode active material, characterized in that: The gram capacity of the carbon-based negative electrode active material is 345 mAh / g to 355 mAh / g, and the powder compaction density under a pressure of 20000 N is 1.55 g / cm 3 Above and less than 1.65g / cm 3 .

2. The carbon-based negative electrode active material according to claim 1, characterized in that: The powder compaction density of the carbon-based negative electrode active material under a pressure of 20000N is 1.57g / cm 3 ~1.62g / cm 3 Alternatively, the powder compaction density of the carbon-based negative electrode active material under a pressure of 49000N is 1.73g / cm 3 Above and less than 1.84g / cm 3 .

3. The carbon-based negative electrode active material according to claim 1 or 2, characterized in that: The gram capacity of the carbon-based negative electrode active material is 346 mAh / g to 353 mAh / g.

4. The carbon-based negative electrode active material according to any one of claims 1 to 3, characterized in that: The carbon-based negative electrode active material includes primary particles and secondary particles; optionally, the amount of the primary particles in the carbon-based negative electrode active material is greater than or equal to the amount of the secondary particles in the carbon-based negative electrode active material.

5. The carbon-based negative electrode active material according to any one of claims 1 to 4, characterized in that: The carbon-based negative electrode active material satisfies at least one of the following conditions: a) Particle size distribution K is 1.0-1.6, and can be selected as 1.1-1.5, K = (Dv90-Dv10) / Dv50; b) Volume distribution particle size Dv50 is 8 μm to 13 μm, and can be 9 μm to 12 μm; c) The degree of graphitization is 88% to 95%, and can be optionally 90% to 95%.

6. The carbon-based negative electrode active material according to any one of claims 1 to 5, characterized in that: The specific surface area of ​​the carbon-based negative electrode active material is 1.25 m 2 / g~1.95m 2 / g, optional 1.25m 2 / g~1.85m 2 / g.

7. The carbon-based negative electrode active material according to any one of claims 1 to 6, characterized in that: The tap density of the carbon-based negative electrode active material is 1.07 g / cm 3 ~1.27g / cm 3 .

8. The carbon-based negative electrode active material according to any one of claims 1 to 7, characterized in that: The carbon-based negative electrode active material is artificial graphite.

9. A method for preparing a carbon-based negative electrode active material, characterized in that: The following steps are involved: Provide raw materials; Processing the raw materials to obtain intermediates; Performing graphitization on the intermediate to obtain a graphitized product; Screening the graphitized product to obtain a carbon-based negative electrode active material; The gram capacity of the carbon-based negative electrode active material is 345 mAh / g to 355 mAh / g, and the powder compaction density under a pressure of 20000 N is 1.55 g / cm 3 Above and less than 1.65g / cm 3 .

10. The preparation method according to claim 9, characterized in that: The graphitization treatment satisfies at least one of the following conditions: (1) The maximum power of the graphitization treatment is 70%-90% of the rated power of the graphitization treatment equipment; (2) The temperature of the graphitization treatment is 2600° C. to 3000° C.; (3) The graphitization treatment time is 10 h to 50 h.

11. The preparation method according to claim 9 or 10, characterized in that: The raw materials include one or more of petroleum coke, needle coke, and asphalt coke, and can optionally include needle coke.

12. The preparation method according to claims 9 to 11, characterized in that: Based on the total volume of the raw material, the volume proportion of the fiber-type structure in the raw material is greater than or equal to 55%, and can be optionally 58%-70%.

13. The preparation method according to any one of claims 9 to 12, characterized in that: The processing of raw materials specifically comprises the following steps: crushing, shaping and classifying the raw materials to obtain a first precursor; Granulating the precursor to obtain a second precursor; The first precursor and the second precursor are carbonized at low temperature to obtain the intermediate product.

14. The preparation method according to claim 13, characterized in that: The first precursor satisfies at least one of the following conditions: (1) The Dv50 particle size of the first precursor is 6.5 μm to 10.5 μm; (2) the particle size distribution of the first precursor (Dv90-Dv10) / Dv50 is 1.05 to 1.75; (3) The tap density of the first precursor is 0.55 g / cm 3 ~0.75g / cm 3 .

15. The preparation method according to claim 13, characterized in that: The volume distribution particle size Dv50 of the second precursor is 11 μm to 15 μm.

16. The preparation method according to any one of claims 13 to 15, characterized in that: The first precursor and the graphitized product of the second precursor are mixed in a mass ratio of (1:1) to (6:4).

17. A secondary battery, comprising a negative electrode plate, characterized in that: The negative electrode plate includes a negative electrode current collector and a negative electrode film layer arranged on at least one surface of the negative electrode current collector, and the negative electrode film layer includes the carbon-based negative electrode active material described in any one of claims 1 to 8 or the carbon-based negative electrode active material prepared by the method described in any one of claims 9 to 16.

18. The secondary battery according to claim 17, characterized in that: The negative electrode plate satisfies at least one of the following conditions: (1) The compaction density of the negative electrode film layer is 1.5 g / cm 3 ~1.65g / cm 3 , optional 1.55g / cm 3 ~1.65g / cm 3 ; (2) The specific surface area of ​​the negative electrode film layer is 0.7 m 2 / g~2m 2 / g, optional 1.0m 2 / g~1.6m 2 / g; (3) The porosity of the negative electrode film layer is 0.22 to 0.42, and can be optionally 0.27 to 0.

37.

19. An electrical device, characterized in that: A secondary battery comprising the secondary battery according to claim 17 or 18.

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