Secondary battery, electrolyte, and device including the same

Abstract

Provided herein are a secondary battery, an electrolyte, and a device comprising the secondary battery. The secondary battery of the present application comprises an electrolyte, characterized in that the electrolyte comprises an organic solvent, the organic solvent comprises a cyclic carbonate and a chain carbonate; the mass ratio of the cyclic carbonate to the chain carbonate is 25:75-32:68; the chain carbonate comprises dimethyl carbonate; the mass percentage of the dimethyl carbonate in the chain carbonate is greater than or equal to 9wt% and less than 50wt%; and the mass percentage of carboxylic acid ester in the organic solvent is less than 5wt% based on the total mass of the organic solvent. The secondary battery of the present application can simultaneously achieve excellent low-temperature power, long service life and cycle performance.

Description

Technical Field

[0001] This application relates to the field of energy storage, and more specifically to a secondary battery, an electrolyte, and a device including the secondary battery. Background Technology

[0002] Secondary batteries have been widely used in various digital products, portable devices, electric vehicles, energy storage power supplies, etc. due to their advantages such as high energy density, long cycle life, safety and reliability, and no memory effect.

[0003] With the widespread use of rechargeable batteries in recent years, higher requirements have been placed on them. As the temperature of the operating environment decreases, the charge and discharge performance of rechargeable batteries drops sharply. Moreover, with each charge and discharge cycle, the battery expands, which in turn affects its cycle performance and lifespan. Summary of the Invention

[0004] The first aspect of this application relates to a secondary battery comprising an electrolyte comprising an organic solvent comprising cyclic carbonates and chain carbonates; the mass ratio of cyclic carbonates to chain carbonates being 25:75 to 32:68; the chain carbonates comprising dimethyl carbonate (also abbreviated herein as "DMC"); the mass percentage of dimethyl carbonate in the chain carbonates being greater than or equal to 9 wt% and less than 50 wt%; wherein, based on the total mass of the organic solvents, the mass percentage of carboxylic acid esters in the organic solvents is less than 5 wt%.

[0005] A second aspect of this application relates to an electrolyte comprising an organic solvent, said organic solvent comprising cyclic carbonates and chain carbonates; the mass ratio of cyclic carbonates to chain carbonates being 25:75 to 32:68; the chain carbonate comprising dimethyl carbonate; the mass percentage of said dimethyl carbonate in the chain carbonate being greater than or equal to 9 wt% and less than 50 wt%; wherein, based on the total mass of said organic solvent, the mass percentage of said organic solvent containing carboxylic acid esters is less than 5 wt%.

[0006] A third aspect of this application relates to an apparatus comprising a secondary battery as described in the first aspect of this application.

[0007] The inventors of this application have discovered that by specifically controlling the ratio of cyclic carbonates to chain carbonates in the electrolyte and simultaneously specifically controlling the dimethyl carbonate content in the chain carbonates, a secondary battery can achieve both excellent low-temperature power and 45°C cycle performance. Furthermore, the inventors have found that while the electrolyte meets the above conditions, the amount of carboxylic acid esters needs to be strictly controlled. When the carboxylic acid ester content is outside the range given in this application, the battery's cycle performance is severely affected. Therefore, in this application, by simultaneously controlling the ratio of cyclic carbonates to chain carbonates, the dimethyl carbonate content in the chain carbonates, and the amount of carboxylic acid esters, the secondary battery exhibits excellent low-temperature power and cycle performance. The device of this application includes the secondary battery provided in this application and thus has at least the same advantages as the aforementioned secondary battery. Attached Figure Description

[0008] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on the drawings without creative effort.

[0009] Figure 1 This is a schematic diagram of one embodiment of a secondary battery according to this application.

[0010] Figure 2 This is a schematic diagram of one embodiment of a battery module according to this application.

[0011] Figure 3 This is a schematic diagram of one embodiment of a battery pack according to this application.

[0012] Figure 4 yes Figure 3 An exploded view of the battery pack.

[0013] Figure 5 This is a schematic diagram of one embodiment of the device of this application.

[0014] The reference numerals in the attached figures are explained as follows:

[0015] 1. Battery pack;

[0016] 2. Upper box;

[0017] 3. Lower box;

[0018] 4. Battery module;

[0019] 5. Secondary battery. Detailed Implementation

[0020] To make the inventive objectives, technical solutions, and beneficial technical effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the embodiments described in this specification are merely illustrative and not intended to limit the scope of this application.

[0021] When a composition is described as including or comprising specific components, it is anticipated that optional components not covered by the present invention are not excluded from the composition, and that the composition may be constituted or composed of the components involved. Similarly, when a method is described as including or comprising specific process steps, it is anticipated that optional process steps not covered by the present invention are not excluded from the method, and that the method may be constituted or composed of the process steps involved.

[0022] For simplicity, this paper only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, although not explicitly stated, every point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can serve as its own lower or upper limit and be combined with any other point or individual value, or with other lower or upper limits, to form an unspecified range.

[0023] In the description of this article, it should be noted that, unless otherwise stated, "above" and "below" include the number itself, and "several" in "one or several" means two or more.

[0024] The terms "preferred" and "ideally" refer to embodiments of the invention that provide certain benefits in certain circumstances. However, other embodiments may also be preferred in the same or other circumstances. Furthermore, the description of one or more preferred embodiments does not imply that other embodiments are unavailable, nor is it intended to exclude other embodiments from the scope of the invention.

[0025] The foregoing description of this invention is not intended to describe every disclosed embodiment or implementation. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments. These embodiments can be used in various combinations, and the combined embodiments should be considered as specific embodiments or instances disclosed herein. In each example, the examples are listed only as representative groups and should not be construed as exhaustive.

[0026] Secondary batteries

[0027] The first aspect of this application provides a secondary battery comprising an electrolyte, characterized in that the electrolyte comprises an organic solvent, the organic solvent comprising cyclic carbonates and chain carbonates; the mass ratio of cyclic carbonates to chain carbonates is 25:75 to 32:68; the chain carbonates comprise dimethyl carbonate; the mass percentage of dimethyl carbonate in the chain carbonates is greater than or equal to 9 wt% and less than 50 wt%; wherein, based on the total mass of the organic solvents, the mass percentage of carboxylic acid esters in the organic solvents is less than 5 wt%.

[0028] As used herein, the term "organic solvent" may have the meaning commonly understood in the battery field. Generally, the term "organic solvent" can be understood as a non-aqueous, aprotic solvent that can be used as a carrier of active ions in a battery. For example, organic solvents include cyclic carbonates, linear carbonates, carboxylic acid esters, etc.

[0029] In some embodiments, the cyclic carbonate includes one or more of ethylene carbonate (also abbreviated as “EC” herein), propylene carbonate (also abbreviated as “PC” herein), and butyl carbonate (also abbreviated as “BC” herein); preferably, the cyclic carbonate includes ethylene carbonate.

[0030] In some embodiments, the chain carbonate further includes one or more of diethyl carbonate (also abbreviated as "DEC" herein) and ethyl methyl carbonate (also abbreviated as "EMC" herein); preferably, the chain carbonate further includes ethyl methyl carbonate.

[0031] The inventors of this application discovered that the relative content of cyclic carbonates and chain carbonates needs to be limited to a specific range. They do not wish to be limited by theory. On the one hand, an excessively high proportion of cyclic carbonates not only increases the viscosity of the electrolyte at low temperatures, affecting its low-temperature conductivity and reducing the battery's low-temperature charge / discharge power, but also allows for oxidation at the positive electrode, leading to increased gas production, which in turn affects the charging interface, deteriorates charging capacity, and consequently impacts 45°C cycle performance. On the other hand, an excessively low proportion of cyclic carbonates reduces the electrolyte's ability to dissociate electrolyte salts, affecting its high-temperature conductivity and resulting in greater polarization during 45°C cycling. Under the influence of expansion forces, this leads to deterioration in cycle performance. Through extensive experiments and research, the inventors discovered that the mass ratio of cyclic carbonates to chain carbonates should be controlled between 25:75 and 32:68, which can advantageously balance the high and low-temperature conductivity of the electrolyte, thereby enabling the battery to achieve superior 45°C cycle performance and low-temperature power performance.

[0032] In some preferred embodiments, the mass percentage of ethylene carbonate in the cyclic carbonate is greater than 90%, preferably 92% to 100%. The inventors have discovered that ethylene carbonate has a film-forming protective effect on the negative electrode active material, and controlling its content within the given range can further improve the cycle performance of the battery. Particularly preferably, the mass percentage of ethylene carbonate in the cyclic carbonate is greater than or equal to 94%, and even more preferably greater than or equal to 98%. For example, in some preferred embodiments, the mass percentage of ethylene carbonate in the cyclic carbonate even reaches 100%.

[0033] In some preferred embodiments, the mass percentage of dimethyl carbonate in the chain carbonate is 10 wt% to 48 wt%, more preferably 15 wt% to 45 wt%. In exemplary embodiments, the mass percentage of dimethyl carbonate in the chain carbonate can be about 14 wt%, 18 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, or 40 wt%. The inventors have found that using the amount of dimethyl carbonate defined herein can achieve good low-temperature conductivity and 45°C cycle performance. Adding a specific amount of dimethyl carbonate to the electrolyte effectively mitigates the deterioration of charging capacity due to increased swelling force during battery cycling, which helps to suppress the increase of swelling force within the battery and thus improves battery cycle performance. However, excessive dimethyl carbonate leads to a significant decrease in the low-temperature conductivity and low-temperature power of the electrolyte. Moreover, excessive dimethyl carbonate increases the decomposition and gas production at the positive electrode interface, resulting in severe battery swelling, which particularly severely deteriorates the battery's 45°C cycle performance.

[0034] In some embodiments, the carboxylic acid ester may include one or more of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. Specifically, the carboxylic acid ester may include one or more of ethyl acetate and ethyl propionate. Generally, carboxylic acid esters have advantages such as low viscosity and high dielectric constant compared to chain carbonates, and their room temperature and low temperature conductivity are usually higher than those of carbonate solvents. However, the inventors have found that adding carboxylic acid esters to the electrolyte of a specific component in this application can disrupt the structural stability of the SEI film on the graphite anode surface, leading to deterioration in cycle performance. The inventors have experimentally discovered that, based on the total mass of the organic solvent, when the mass percentage of carboxylic acid esters in the organic solvent is less than 5 wt%, the deterioration of battery performance by carboxylic acid esters is suppressed. In some preferred embodiments, the electrolyte includes 3 wt% or less of carboxylic acid esters based on the total mass of the organic solvent. Even more preferably, based on the total mass of the organic solvent, the organic solvent does not contain carboxylic acid esters (i.e., the mass percentage of carboxylic acid esters in the organic solvent is 0 wt%).

[0035] In some preferred embodiments, the electrolyte has a conductivity greater than or equal to 2.5 mS / cm at -20°C. More preferably, the electrolyte has a conductivity of 2.6 mS / cm to 3.5 mS / cm at -20°C. For example, the electrolyte conductivity at -20°C can be about 2.6 mS / cm, 2.8 mS / cm, 3.0 mS / cm, 3.2 mS / cm, or 3.4 mS / cm.

[0036] In some preferred embodiments, the electrolyte may further include additives. These additives include, but are not limited to, one or more of fluoroethylene carbonate (FEC), vinylene carbonate (VC), ethylene sulfate (DTD), tris(trimethylsilane)phosphate (TMSP), lithium difluorooxalate borate (LiDFOB), and lithium bis(fluorosulfonyl)imide (LiFSI). These additives can further improve the chemical stability of the electrolyte, improve the film-forming stability at the positive and negative electrode interfaces, modify the lithium-ion transport pathways of the interface film to achieve lower interfacial impedance, and repair the positive and negative electrode interfaces throughout the battery's lifespan. Furthermore, the inventors have found that, in terms of battery performance, they can comprehensively improve the battery's power output at room temperature / low temperature, cycle performance at room temperature / high temperature, storage life at room temperature / high temperature, gas generation during high-temperature storage, and fast charging capability at room temperature / low temperature.

[0037] In some preferred embodiments, the total amount of the additive does not exceed 10 wt% of the total mass of the electrolyte. Based on the total mass of the electrolyte, the amount of each additive component can be 0.05-5 wt%, preferably 0.1-3 wt%. For example, based on the total mass of the electrolyte, the amount of each additive component can be 0.1 wt%, 0.3 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, or 2.5 wt%. Insufficient additive content will lead to incomplete film formation at the electrode interface and unstable structure; excessive additive content will lead to increased film resistance or residual redox decomposition causing battery swelling.

[0038] The electrolyte described in this application further includes an electrolyte salt as a solute. As an example, the electrolyte salt may be selected from one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), Li(CF3SO2)2N, LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium dioxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate). In some preferred embodiments, one or more of LiPF6, LiFSI, LiPO2F2, LiDFOB, and LiDFOP may be used as the electrolyte salt. In some specific embodiments, LiPF6 may be used as the electrolyte salt. Preferably, the total mass of the electrolyte salt constitutes ≤20% of the total mass of the electrolyte. More preferably, the total mass of the electrolyte salt accounts for 10% to 15% of the total mass of the electrolyte.

[0039] The secondary battery of this application further includes a negative electrode sheet. The negative electrode sheet comprises a negative electrode active material, which can be selected from substances known in the art that can be used as negative electrode active materials. Preferably, the negative electrode active material comprises artificial graphite; when the negative electrode active material comprises artificial graphite, the improvement effect of the electrolyte is more significant.

[0040] When the negative electrode active material includes artificial graphite, the performance of the battery can be further improved if the artificial graphite also satisfies one or more of the following parameters.

[0041] In some preferred embodiments, the surface of the artificial graphite does not have an amorphous carbon coating layer; when the surface of the artificial graphite does not have an amorphous carbon coating layer, the side reactions between it and the electrolyte during battery cycling can be further reduced, the increase in SEI film thickness can be reduced, thereby reducing the cycle expansion of the battery.

[0042] In some preferred embodiments, the artificial graphite I D / I G Preferably less than or equal to 0.25. For example, the I of artificial graphite. D / I G The stoichiometry can be 0.23, 0.2, 0.18, 0.16, 0.15, 0.12, 0.1, or 0.08. Preferably, the stoichiometry of the artificial graphite is... D / I GThe value is 0.1 to 0.2. The inventors have discovered that using artificial graphite within the above-mentioned preferred range can reduce the volume expansion of the battery during cycling. It is not expected that, limited by theory, the I of artificial graphite... D / I G It is suitable for improving its surface stability and reducing side reactions of the electrolyte on its surface, thereby further reducing the volume expansion of the battery during cycling.

[0043] The I D / I G The peak intensity (I) of the D peak in artificial graphite D ) and the peak intensity of G peak (I G The ratio of the peaks (D and G) is significant. The D and G peaks are characteristic Raman peaks of graphite materials. The D and G peaks of artificial graphite can be measured using laser Raman spectroscopy, such as the Advantage 785TM Raman spectrometer. In the Raman spectrum of the artificial graphite of this application measured by a Raman spectrometer, the D peak is at 1300 cm⁻¹. -1 Up to 1400cm -1 Within the range, the G peak is at 1580 cm⁻¹. -1 Up to 1620cm -1 Within the range.

[0044] In some preferred embodiments, the artificial graphite also satisfies the requirement of a number average particle size D. n The particle size of 10 is 1μm to 3μm, preferably 1μm to 2μm. The number average particle size D within the above range is used. n When the artificial graphite is 10%, the specific capacity of the artificial graphite can be further improved; and in the negative electrode sheet prepared by the artificial graphite, the artificial graphite and additives such as binders can be evenly dispersed, the overall adhesion of the electrode sheet is high, and the cycle expansion of the battery can be further reduced.

[0045] In some preferred embodiments, the artificial graphite also satisfies the requirement of a volume average particle size D. v 50 is 15μm to 20μm, preferably 15μm to 18μm.

[0046] In some preferred embodiments, the volume average particle size D of the artificial graphite v 10 ≥ 6 μm, preferably 6.5 μm to 10.5 μm. For example, the D of artificial graphite. v 10 can be 6μm or larger, 6.5μm or larger, 7μm or larger, or 7.5μm or larger.

[0047] Using the above-mentioned range of D v 50 and / or D v The 10% synthetic graphite has high active ion and electron transport performance, reducing the side reactions of the electrolyte at the negative electrode; at the same time, it is also beneficial to improve its own powder compaction density.

[0048] In some preferred embodiments, the artificial graphite also satisfies the particle size distribution (D). v 90-D v 10) / D v The particle size distribution is 1.1–1.8, preferably 1.2–1.5. When the artificial graphite also satisfies the given particle size distribution, it can improve the cohesive force between particles, thereby reducing the electrode expansion rate during battery cycling. In addition, a suitable particle size distribution is also beneficial for the artificial graphite to have an appropriate specific surface area (SSA), giving it both high electrochemical reactivity and high surface stability, thereby further improving cycle performance.

[0049] In this application, the D of the artificial graphite n 10. D v 10. D v 50. D v 90 can be determined using a laser particle size analyzer (such as Malvern Master Size 3000) in accordance with standard GB / T 19077.1-2016.

[0050] Among them, D n 10. D v 10. D v 50. D v The physical definition of 90 is as follows:

[0051] D n 10: The particle size corresponding to a cumulative quantity distribution percentage of 10% for the material;

[0052] D v 10: The particle size corresponding to a cumulative volumetric distribution percentage of 10%;

[0053] D v 50: The particle size corresponding to a cumulative volumetric distribution percentage of 50%;

[0054] D v 90: The particle size corresponding to a cumulative volume distribution percentage of 90% for the material.

[0055] In some preferred embodiments of this application, the specific surface area (SSA) of the artificial graphite is 1.0 m². 2 / g~1.5m 2 / g. Without being limited by theory, artificial graphite possesses an appropriate specific surface area, enabling it to exhibit high electrochemical reactivity in secondary batteries. This meets the kinetic requirements of secondary batteries while reducing side reactions of the electrolyte on the material surface and decreasing gas production, thereby reducing the volume expansion of the battery during cycling. Artificial graphite with an appropriate specific surface area also exhibits strong bonding forces with binders, thereby improving the cohesion and adhesion of the electrode sheets, further reducing the battery's cyclic expansion.

[0056] The specific surface area of ​​artificial graphite can be tested using methods known in the art. For example, it can be tested using the nitrogen adsorption specific surface area analysis method according to GB / T19587-2017, and calculated using the BET (Brunauer Emmett Teller) method. The nitrogen adsorption specific surface area analysis can be performed using the Tri-Star 3020 specific surface area and pore size analyzer from Micromeritics, Inc., USA.

[0057] In some preferred embodiments, the degree of graphitization G of the artificial graphite can be 90% to 95%, preferably 92% to 94%. An appropriate degree of graphitization G can enable the artificial graphite to have a high specific capacity while also making the bulk structure of the artificial graphite more stable.

[0058] The degree of graphitization of artificial graphite can be tested using methods known in the art. For example, the degree of graphitization can be tested using an X-ray diffractometer (Bruker D8 Discover), and the testing can be referenced in JIS K 0131-1996 and JB / T 4220-2011 to measure d. 002 The size is then determined according to the formula G = (0.344 - d). 002 The degree of graphitization is calculated by d / (0.344-0.3354)×100%, where d 002 It is the interlayer spacing in the artificial graphite crystal structure, expressed in nanometers (nm).

[0059] In some preferred embodiments, the compacted density of the artificial graphite powder under 2000 kg pressure is 1.65 g / cm³. 3 ~1.85g / cm 3 The preferred value is 1.68 g / cm³. 3 ~1.83g / cm 3 Artificial graphite has a high powder compaction density under 2000 kg pressure. The negative electrode sheet using this artificial graphite can have a high compaction density, thus giving the battery a high energy density.

[0060] The compacted density of the artificial graphite powder can be tested using methods known in the art. For example, it can be tested using an electronic pressure testing machine (such as UTM7305) in accordance with GB / T24533-2009.

[0061] In some preferred embodiments, the compaction density of the negative electrode film is 1.55 g / cm³. 3 ~1.75g / cm 3 More preferably, the compaction density of the negative electrode film is 1.6 g / cm³. 3 ~1.7g / cm 3 This allows the negative electrode film to have both high compaction density and porosity suitable for full electrolyte wetting. Therefore, the battery's capacity can be utilized more effectively, and the battery achieves better kinetic performance.

[0062] In some preferred embodiments, when the negative electrode film is within the compaction density range given above, the orientation OI value of the negative electrode sheet is 8 to 15, more preferably 8 to 12. When the OI value of the electrode sheet is within the given range, it can have a high degree of isotropy, thereby dispersing the expansion of the artificial graphite in the electrode sheet during lithium intercalation in various directions, thereby further reducing the cyclic expansion of the electrode sheet and the battery.

[0063] The orientation OI value of the negative electrode sheet is the peak area C of the 004 diffraction peak of the negative electrode active material in the X-ray diffraction pattern of the negative electrode sheet. 004 Peak area C of the 110 diffraction peak 110 The ratio. Therefore, the orientation OI value of the negative electrode = C 004 / C 110 X-ray diffraction analysis is performed according to the JIS K 0131-1996 standard, using an X-ray diffractometer (e.g., a Bruker D8 Discover X-ray diffractometer) with CuK... α X-rays are the radiation source, and the wavelength of the X-rays is... The scanning 2θ angle range is 20° to 80°, and the scanning rate is 4° / min.

[0064] In some preferred embodiments, the areal density of the negative electrode film of this application is 7.5 mg / cm³. 2 ~14.0 mg / cm 2 The preferred concentration is 9.5 mg / cm³. 2 ~12.0mg / cm 2The areal density of the negative electrode film characterizes the weight of the coating per unit area on the electrode after cold pressing. It can be measured according to the methods described in the embodiments, or according to other methods known in the art. In some exemplary embodiments, the mass of a specific area of ​​the negative electrode film is weighed using a standard balance, and then the mass per unit area of ​​the negative electrode film, i.e., the areal density, is calculated.

[0065] It should be noted that the negative electrode film parameters (e.g., negative electrode film compaction density and areal density) given in this application refer to the parameter range of a single-sided film. When the negative electrode film is simultaneously disposed on both surfaces of the current collector, if the film parameters on either surface meet the requirements of this application, it is considered to fall within the protection scope of this application. Furthermore, the compaction density, areal density, and other ranges mentioned in this invention refer to the parameter range after cold pressing and compaction for use in battery assembly.

[0066] In some exemplary embodiments, the artificial graphite described above in this application can be prepared by the following methods:

[0067] (1) The raw coke is crushed and graded;

[0068] (2) Shape the product obtained in step (1);

[0069] (3) Granulate the product obtained in step (2), wherein the amount of binder added in the granulation process does not exceed 5% of the total weight of the raw coke.

[0070] (4) The product obtained in step (3) is subjected to graphitization treatment at a temperature of 2800℃~3200℃ to obtain the artificial graphite.

[0071] In the above preparation method, preferably, the raw coke raw material can be selected from one or more of raw petroleum coke and raw pitch coke; more preferably, it includes raw petroleum coke.

[0072] In the above preparation method, preferably, the raw coke is non-needle coke.

[0073] In the above preparation method, preferably, the volatile matter content of the raw petroleum coke is 7% to 10%; an appropriate volatile matter content in the raw coke raw material can give the artificial graphite high structural strength.

[0074] In the above preparation method, preferably, the sulfur content of the raw petroleum coke is ≤2%. The low sulfur content of the raw coke raw material can improve the surface stability of artificial graphite.

[0075] In the above preparation method, preferably, after shaping in step (2), a fine powder removal step is also included, which can remove D n 10. Adjusted to the range given in this application.

[0076] In the above preparation method, preferably, the product obtained in step (2) is granulated without the addition of a binder, which can further improve the specific capacity and structural strength of artificial graphite.

[0077] In the above preparation method, preferably, the product obtained in step (3) is graphitized at a temperature of 2900℃~3100℃.

[0078] The secondary battery of this application further includes a positive electrode sheet, which comprises a positive electrode active material. The positive electrode active material can be selected from substances known in the art that can be used as positive electrode active materials. Preferably, the positive electrode active material comprises one or more of lithium transition metal oxides and their modified compounds, and the modified compound can be a doping modification and / or coating modification of the lithium transition metal oxide. Preferably, the lithium transition metal oxide comprises one or more of lithium nickel manganese oxide and lithium nickel cobalt aluminum oxide. When the positive electrode active material comprises the above-mentioned lithium transition metal oxides and their modified compounds, the improvement effect on the electrolyte is more significant.

[0079] In the secondary battery of this application, the positive and negative electrode sheets may optionally include a binder. This application does not impose specific limitations on the type of binder, and those skilled in the art can select one according to actual needs. As an example, the binder used for the positive electrode sheet may include one or more of polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE).

[0080] In the secondary battery of this application, the positive and negative electrode sheets may optionally include a conductive agent. This application does not impose specific limitations on the type of conductive agent, and those skilled in the art can select one according to actual needs. As an example, the conductive agent used for the positive electrode sheet may include one or more of the following: artificial graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, artificial graphene, and carbon nanofibers.

[0081] The secondary battery of this application also includes a separator. The separator is disposed between the positive electrode and the negative electrode to provide isolation. This application does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected. In some embodiments, the material of the separator can be selected from one or more 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. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0082] The secondary battery of this application can be prepared according to conventional methods in the art. For example, the negative electrode active material and optional conductive agent and binder are dispersed in a solvent (e.g., water) to form a uniform negative electrode slurry. The negative electrode slurry is coated on a negative electrode current collector, and after drying, cold pressing and other processes, a negative electrode sheet is obtained. The positive electrode active material and optional conductive agent and binder are dispersed in a solvent (e.g., N-methylpyrrolidone, abbreviated as NMP) to form a uniform positive electrode slurry. The positive electrode slurry is coated on a positive electrode current collector, and after drying, cold pressing and other processes, a positive electrode sheet is obtained. The positive electrode sheet, separator, and negative electrode sheet are wound or stacked in sequence, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play a role in isolation, to obtain a battery cell. The battery cell is placed in an outer packaging and injected with the electrolyte of this application to obtain the secondary battery of this application.

[0083] In some embodiments, a secondary battery may include an outer packaging and battery cells and electrolyte encapsulated within the outer packaging. The number of battery cells in a secondary battery may be one or more, and can be adjusted as needed.

[0084] In some embodiments, the outer packaging of the secondary battery can be a soft pack (e.g., a bag, the material of which can be plastic, such as polypropylene PP, polybutylene terephthalate PBT, polybutylene succinate PBS, etc.) or a hard shell (e.g., an aluminum shell, etc.).

[0085] This application does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. Figure 1 This is an example of a square-structured secondary battery 5.

[0086] In some embodiments, secondary batteries can be assembled into battery modules, and the number of secondary batteries contained in a battery module can be multiple, with the specific number adjustable according to the application and capacity of the battery module.

[0087] Figure 2 This is battery module 4, used as an example. (See reference...) Figure 2 In battery module 4, multiple secondary batteries 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple secondary batteries 5 can be fixed in place using fasteners.

[0088] Optionally, the battery module 4 may also include a housing with a receiving space in which a plurality of secondary batteries 5 are received.

[0089] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0090] Figure 3 and Figure 4 This is battery pack 1 as an example. (See reference...) Figure 3 and Figure 4 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0091] electrolyte

[0092] This application also provides an electrolyte comprising an organic solvent, the organic solvent comprising cyclic carbonates and chain carbonates; the mass ratio of the cyclic carbonates to the chain carbonates is 25:75 to 32:68; the chain carbonates comprise dimethyl carbonate; the mass percentage of dimethyl carbonate in the chain carbonates is greater than or equal to 9 wt% and less than 50 wt%;

[0093] Wherein, based on the total mass of the organic solvent, the mass percentage of carboxylic acid esters in the organic solvent is less than 5 wt%.

[0094] The preferred embodiments regarding the components and amounts of the electrolyte discussed above are also applicable to the electrolyte described here and constitute the various specific implementations of the electrolyte. For the sake of brevity, they will not be elaborated upon here.

[0095] The electrolyte can be prepared according to conventional methods in the art. It can be obtained by uniformly mixing an organic solvent, an electrolyte salt, and optional additives. There are no particular restrictions on the order in which the materials are added. For example, the electrolyte salt and optional additives can be added to an organic solvent and mixed uniformly to obtain the electrolyte. Alternatively, the electrolyte salt can be added to the organic solvent first, followed by the optional additives, either individually or simultaneously, added to the organic solvent.

[0096] Device

[0097] A second aspect of this application provides an apparatus comprising a secondary battery as described in the first aspect of this application, the secondary battery providing power to the apparatus. The apparatus may be, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0098] The device can be configured to use a secondary battery, battery module, or battery pack, depending on its usage requirements.

[0099] Figure 5This is an example device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the device's requirements for high power and high energy density in its secondary batteries, a battery pack or battery module can be used.

[0100] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a rechargeable battery as their power source.

[0101] Example

[0102] The following embodiments describe the disclosure of this application in more detail. These embodiments are merely illustrative, as various modifications and variations within the scope of the disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on mass, and all reagents used in the embodiments are commercially available or synthesized using conventional methods and can be used directly without further processing. Furthermore, unless otherwise stated, the instruments used in the embodiments are commercially available.

[0103] The negative electrode active material used in the embodiments of this application is:

[0104] (1) Artificial graphite A: It can be obtained commercially or prepared by the following method:

[0105] Needle-shaped calcined petroleum coke was used as raw material and crushed; granulation was performed using asphalt as a binder (the amount of asphalt added was 8% of the total weight of the needle-shaped calcined petroleum coke raw material); then graphitization was carried out at 2800℃~3000℃; finally, it was mixed with asphalt and heat-treated at 950℃~1200℃ to obtain artificial graphite A with amorphous carbon coating. Artificial graphite A has the following characteristics: specific capacity of approximately 354mAh / g, and volume average particle size D... v 50 is approximately 12.8 μm, with a volume average particle size D v 10 is approximately 6.9 μm, and the particle size distribution (D) v 90-D v 10) / D v 50 is approximately 1.26, and the number average particle size D n 10 is approximately 4.3 μm, I D / I G The value is approximately 0.32, and the SSA value is approximately 0.95m. 2 / g.

[0106] (2) Artificial graphite B: can be prepared using the method described above in this application:

[0107] Non-needle-shaped raw petroleum coke (volatile matter approximately 9.5%, sulfur content approximately 0.6%) was used as raw material and subjected to crushing; granulation was carried out under binder-free conditions; and then graphitization was performed at 3000℃~3100℃ to obtain artificial graphite B. Artificial graphite B has the following characteristics: specific capacity approximately 354mAh / g, and volume average particle size D... v 50 is approximately 16.2 μm, with a volume average particle size D v 10 is approximately 7.2 μm, and the particle size distribution (D) v 90-D v 10) / D v 50 is approximately 1.37, and the number average particle size D n 10 is approximately 1.5 μm, I D / I G Approximately 0.18, SSA approximately 1.25m 2 / g.

[0108] Example 1

[0109] (1) Preparation of positive electrode sheet

[0110] LiNi, the positive electrode active material 0.5 Co 0.2 Mn 0.3 O2, conductive agent (Super P), and binder polyvinylidene fluoride (PVDF) are thoroughly mixed in N-methylpyrrolidone (NMP) at a mass ratio of 94:3:3 to prepare a positive electrode slurry. Then, the positive electrode slurry is uniformly coated onto a current collector Al foil, dried, and cold-pressed to obtain the positive electrode sheet.

[0111] (2) Preparation of negative electrode sheet

[0112] Artificial graphite A (negative electrode active material), conductive agent (Super P), styrene-butadiene rubber (SBR) (binder), and sodium carboxymethyl cellulose (CMC-Na) (thickener) were thoroughly mixed in a deionized water solvent system at a mass ratio of 95:2:2:1 to prepare a negative electrode slurry. The negative electrode slurry was then coated onto a Cu foil current collector, dried, and cold-pressed to obtain a negative electrode sheet. The compacted density of the negative electrode sheet was 1.65 g / cm³. 3 Its surface density is 10.7 mg / cm³. 2 The orientation OI value of the negative electrode is 22.

[0113] (3) Separating membrane

[0114] PE porous polymer film is used as the separator.

[0115] (4) Preparation of electrolyte

[0116] In an argon-filled glove box, the components were mixed according to the proportions shown in Tables 1 and 2 to obtain an organic solvent mixture. Then, LiPF6 electrolyte salt, comprising 13% of the total mass of the electrolyte, was added to the organic solvent mixture. Subsequently, functional additives 0.3 wt% VC, 1 wt% DTD, 1 wt% TMSP, 0.5 wt% LiDFOB, and 1 wt% LiFSI were added and mixed thoroughly.

[0117] (5) Assembly of secondary batteries

[0118] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between them for isolation. After winding, a bare cell is obtained, and tabs are welded on. The bare cell is placed in an outer packaging and the prepared electrolyte is injected. Battery assembly, settling, and formation are then completed. The battery is then charged at a constant current of 0.02C to 3.3V, followed by a constant current of 0.1C to 3.6V, shaped, and capacity tested to complete battery fabrication.

[0119] Examples 2-8 and Comparative Examples 1-8

[0120] The preparation steps of Examples 2-8 and Comparative Examples 1-8 were similar to those of Example 1, except for the composition and ratio of the organic solvent. Specific experimental parameters and results are shown in Table 1.

[0121] Examples 9-16

[0122] In Examples 9-16, the same preparation steps as in Examples 1-8 were used, with the difference that artificial graphite A was replaced with artificial graphite B in the preparation of the negative electrode sheet. Specific experimental parameters and results are shown in Table 2. It is noteworthy that when artificial graphite B was used in the preparation of the negative electrode sheet, the orientation OI value of the obtained negative electrode sheet was 10.5.

[0123] Test methods

[0124] (1) Electrical conductivity (mS / cm)

[0125] The conductivity of the electrolyte at -20℃ was tested using a conductivity meter (DDS-307A type Leici conductivity meter).

[0126] (2) Cycling performance at 45℃

[0127] Five lithium-ion batteries from each embodiment and comparative example were taken, and the lithium-ion batteries were repeatedly charged and discharged through the following steps, and the discharge capacity retention rate and cycle number of the batteries were calculated.

[0128] The battery is assembled into a steel plate fixture and subjected to cyclic charging and discharging, with the battery's expansion force monitored during the process.

[0129] The first charge and discharge cycle was performed at 45°C. Constant current and constant voltage charging was conducted at a charging current of 1C (the current value required to completely discharge the theoretical capacity within 1 hour) until the upper limit voltage reached 4.3V. Then, constant current discharging was performed at a discharging current of 1C until the final voltage reached 2.8V, and the initial discharge capacity value was recorded. Subsequently, continuous charge and discharge cycles were performed, recording the discharge capacity value during each cycle and calculating the cycle capacity retention rate.

[0130] The capacity retention rate of the Nth cycle = (discharge capacity of the Nth cycle / discharge capacity of the first cycle) × 100%. When the cycle capacity retention rate drops to 80%, the number of cycles of the battery is recorded.

[0131] (3) Discharge power at -20℃

[0132] Five lithium-ion batteries from each embodiment and comparative example were taken, and the lithium-ion batteries were charged and discharged through the following steps, and the discharge power was calculated.

[0133] First, the battery was kept at 25°C for 120 minutes to ensure a constant internal and external temperature of 25°C. Then, it was charged at a constant current rate of 1C to 4.3V, followed by constant voltage charging at 4.3V until the current was ≤0.05C. Finally, the battery was discharged at 25°C at a 1C rate for 48 minutes to maintain it at 20% SOC capacity.

[0134] Next, the battery was kept at -20℃ for 120 minutes to ensure that the internal and external temperatures of the battery were constant at -20℃. Then, it was discharged at 400W for 10 seconds, and the discharge terminal voltage V was measured. When the terminal voltage V was within 2.1±0.05V, the current power was the power value of the battery, expressed in watts (W).

[0135] If the terminal voltage V is greater than 2.1 ± 0.05V, the battery will be readjusted to 20% SOC capacity at room temperature and discharged at higher power at low temperatures. If the terminal voltage V is less than 2.1 ± 0.05V, the battery will be readjusted to 20% SOC capacity at room temperature and discharged at lower power at low temperatures.

[0136] (4) Battery expansion force test

[0137] The battery is assembled into the steel plate clamp, with the bottom and four sides of the battery tightly against the clamp. The preload of the steel plate is adjusted to 2000N. A pressure sensor is placed between the steel plate clamp and the battery and connected to a computer to test the expansion force of the battery after 1000 cycles. The unit of expansion force is N.

[0138] The specific experimental parameters and performance results of Examples 1-16 and Comparative Examples 1-8 are shown in Tables 1 and 2 below.

[0139]

[0140] Comparing Examples 1-2 and 4-6 with Comparative Example 1, it can be seen that adding DMC to the electrolyte helps alleviate the deterioration of charging capacity caused by increased expansion force, thereby inhibiting further increase in expansion force to a certain extent and thus improving the 45°C cycle performance. However, as can be seen from Comparative Examples 2-3, with further increase in DMC content, the low-temperature viscosity of the electrolyte increases, leading to a significant decrease in conductivity and low-temperature power at low temperatures; in addition, the decomposition and gas production of DMC at the positive electrode interface increases, resulting in severe battery gas expansion, affecting the charging capacity at the battery interface, and thus severely deteriorating the 45°C cycle performance.

[0141] It can be seen that when the mass percentage of DMC in the chain carbonate is controlled at at least 9 wt% and less than 50 wt%, good low-temperature conductivity and 45°C cycling performance are obtained. Moreover, further controlling the amount of DMC in the chain carbonate to 15 wt% to 45 wt%, more preferably 25 wt% to 43 wt%, results in even better low-temperature power and 45°C cycling performance.

[0142] Comparing Examples 3-4 and Comparative Examples 4-5, the 45°C cycle performance of the battery was significantly improved with increasing cyclic carbonate (EC) content (e.g., to 30%). However, with further increases in the amount of cyclic carbonate in the organic solvent (e.g., exceeding 32%), the low-temperature viscosity of the electrolyte increased, leading to a decrease in the low-temperature conductivity of the electrolyte and the low-temperature power of the battery. Simultaneously, EC increased oxidation gas generation at the positive electrode interface, resulting in increased gas generation during 45°C cycling, which further deteriorated the 45°C cycle performance. It can be seen that when the amount of EC in the organic solvent was between 25 wt% and 32 wt%, relatively good low-temperature power and 45°C cycle performance were achieved.

[0143] Comparing Examples 4 and 7 with Comparative Example 8, it can be seen that adding a small amount of PC to cyclic carbonate can improve low-temperature conductivity, thereby improving low-temperature power. However, further increasing the PC content will cause graphite exfoliation, which will lead to increased battery expansion force and deterioration of cycle performance.

[0144] As can be seen from Examples 6 and 8 and Comparative Examples 6-7, the addition of a small amount of carboxylic acid ester to the organic solvent increases the battery's conductivity at both room temperature and low temperature; however, the battery expands more, and its cycle performance at 45°C deteriorates significantly. The addition of carboxylic acid ester disrupts the structural stability of the SEI film, leading to a deterioration in battery cycle performance. Furthermore, it can be seen that EA has a more significant impact on the structural stability of the SEI film than EP, resulting in a substantial reduction in battery cycle performance.

[0145] Table 2. Effects of improvements in artificial graphite and electrolyte composition on battery performance

[0146]

[0147] In addition to the effects of electrolyte composition and dosage ratio on battery performance discussed above, the inventors also surprisingly discovered that by using artificial graphite B, there are fewer defects on the graphite surface, and the rupture and repair of the SEI film on the graphite surface by the electrolyte is reduced. At the same time, when combined with the electrolyte composition and dosage discussed above, the battery's 45°C cycle performance and low-temperature power performance are further improved.

[0148] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and these modifications or substitutions should all be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A secondary battery, comprising an electrolyte, characterized in that, The electrolyte includes an organic solvent, which includes cyclic carbonates and chain carbonates; The mass ratio of the cyclic carbonate to the chain carbonate is 25:75 to 32:68; The chain carbonate includes dimethyl carbonate; the mass percentage of dimethyl carbonate in the chain carbonate is greater than or equal to 9 wt% and less than or equal to 45 wt%. Wherein, based on the total mass of the organic solvent, the mass percentage of carboxylic acid esters in the organic solvent is less than 5 wt%. Furthermore, the cyclic carbonate includes one or more of ethylene carbonate and propylene carbonate, wherein the mass percentage of ethylene carbonate in the cyclic carbonate is greater than 90%. The secondary battery further includes a negative electrode sheet, the negative electrode sheet comprising a negative electrode active material, the negative electrode active material comprising artificial graphite, and the artificial graphite satisfying the following: The D peak intensity I of the artificial graphite D With G peak intensity I G The following conditions must be met between them: I D / I G ≤0.25, the average particle size D of the artificial graphite anode material n The particle size distribution (D) of the artificial graphite anode material is 1μm to 3μm. v 90-D v 10) / D v 50 is 1.1 to 1.8, and the volume average particle size D of the artificial graphite anode material is... v 10≥6μm, the volume average particle size D of the artificial graphite anode material v The thickness of the graphite is 15μm to 20μm, and the specific surface area of ​​the artificial graphite is 1.0m². 2 / g~1.5m 2 / g, wherein the degree of graphitization G of the artificial graphite is 90% to 95%.

2. The secondary battery according to claim 1, characterized in that, The mass percentage of dimethyl carbonate in the chain carbonate is 15wt% to 45wt%.

3. The secondary battery according to claim 2, characterized in that, The dimethyl carbonate in the chain carbonate has a mass percentage of 25 wt% to 43 wt%.

4. The secondary battery according to any one of claims 1-3, characterized in that, The mass percentage of carboxylic acid esters in the organic solvent is less than 3 wt%.

5. The secondary battery according to claim 4, characterized in that, The organic solvent contains 0 wt% carboxylic acid esters.

6. The secondary battery according to claim 1 or 2, characterized in that, The ethylene carbonate comprises 92% to 100% by mass in the cyclic carbonate.

7. The secondary battery according to any one of claims 1-3, characterized in that, The chain carbonates also include one or more of diethyl carbonate and methyl ethyl carbonate.

8. The secondary battery according to any one of claims 1-3, characterized in that, The carboxylic acid esters include one or more of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate.

9. The secondary battery according to claim 8, characterized in that, The carboxylic acid ester includes one or more of ethyl acetate and ethyl propionate.

10. The secondary battery according to any one of claims 1-3, characterized in that, The electrolyte also includes additives, which include one or more of fluoroethylene carbonate, vinylene carbonate, ethylene sulfate, tris(trimethylsilane) phosphate, lithium difluorooxalate borate, and lithium difluorosulfonylimide.

11. The secondary battery according to claim 10, characterized in that, The total amount of the additives shall not exceed 10 wt% of the total mass of the electrolyte.

12. The secondary battery according to any one of claims 1-3, characterized in that, The electrolyte has a conductivity of ≥2.5 mS / cm at -20℃.

13. The secondary battery according to claim 12, characterized in that, The electrolyte has a conductivity of 2.6 mS / cm to 3.5 mS / cm at -20°C.

14. The secondary battery according to any one of claims 1-3, characterized in that, The artificial graphite satisfies one or more of the following (1) – (6): (1) The specific capacity of the artificial graphite is 350mAh / g ~358mAh / g; (2) The D peak intensity I of the artificial graphite D With G peak intensity I G The following condition must be met: 0.1 ≤ I D / I G ≤0.2 (3) The average particle size D of the artificial graphite anode material n 10 represents 1 μm to 2 μm; (4) The volume average particle size D of the artificial graphite anode material v 10 represents 6.5μm ~ 10.5μm; (5) The volume average particle size D of the artificial graphite anode material v 50 represents 15 μm to 18 μm; (6) Particle size distribution (D) of the artificial graphite anode material v 90-D v 10) / D v 50 represents 1.2 to 1.

5.

15. The secondary battery according to claim 14, characterized in that, The compaction density of the negative electrode sheet is 1.55 g / cm³. 3 ~ 1.75g / cm 3 .

16. The secondary battery according to claim 15, characterized in that, The compaction density of the negative electrode sheet is 1.6 g / cm³. 3 ~ 1.7g / cm 3 .

17. The secondary battery according to claim 15 or 16, characterized in that, The orientation OI value of the negative electrode sheet is 8~15; The orientation OI value of the negative electrode sheet is the ratio of the peak area of ​​the 004 diffraction peak to the peak area of ​​the 110 diffraction peak of the negative electrode active material in the X-ray diffraction pattern of the negative electrode sheet.

18. The secondary battery according to claim 7, characterized in that, The orientation OI value of the negative electrode sheet is 8~12.

19. The secondary battery according to any one of claims 1-3, characterized in that, The secondary battery further includes a positive electrode sheet, which includes a positive electrode active material, and the positive electrode active material includes one or more of lithium transition metal oxides and their modified compounds.

20. The secondary battery according to any one of claims 19, characterized in that, The positive electrode active material includes one or more of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide.

21. An apparatus, characterized in that, The device includes a secondary battery according to any one of claims 1-20.

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