Secondary battery and electric device

Abstract

The embodiment of the application discloses a secondary battery, which comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, the electrolyte comprises an organic solvent, the organic solvent comprises a cyclic ester solvent and a chain ester solvent; and the secondary battery parameters satisfy a relationship formula (1). The secondary battery has good low-temperature kinetic performance, good capacity retention rate and low impedance under a low-temperature condition through the joint action of the electrolyte and the negative electrode sheet.

Description

[0001] This application is a divisional application of the original application filed on October 14, 2022, with application number 202211260215.8, entitled "A Secondary Battery", wherein the original application is incorporated herein by reference. Technical Field

[0002] This application relates to the field of battery technology, specifically to a secondary battery and an electrical device. Background Technology

[0003] Secondary batteries, such as lithium-ion batteries, have become the most popular energy storage system due to their high energy density, long lifespan, and environmental friendliness, and are now widely used in consumer electronics, electric vehicles, energy storage, and other fields. With the increasing penetration rate of new energy vehicles and the rapid development of the energy storage market, people's requirements for lithium-ion battery performance are constantly increasing; developing lithium-ion batteries with low cost, high safety, and wide applicability is currently the top priority for the lithium battery industry.

[0004] Secondary batteries suffer from drawbacks such as low capacity retention and high impedance at low temperatures, which limits their large-scale use. Therefore, it is necessary to improve the low-temperature kinetic performance of secondary batteries. Summary of the Invention

[0005] Therefore, the technical problem to be solved by this application is to overcome the defects of poor low-temperature kinetic performance of lithium-ion batteries in the prior art, thereby providing a lithium-ion battery.

[0006] Therefore, this application provides the following technical solution.

[0007] This application provides a secondary battery, including a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative current collector and a negative active material layer disposed on the negative current collector. The electrolyte includes an organic solvent, which includes cyclic ester solvents and chain ester solvents.

[0008] The secondary battery satisfies equation (1).

[0009]

[0010] Where Mel. represents the weight of the electrolyte in the secondary battery, in grams.

[0011] σ is the conductivity of the electrolyte, and the unit of conductivity is mS / cm;

[0012] Cap. is the theoretical capacity value of the secondary battery design, and the unit of the theoretical capacity is Ah;

[0013] PD is the compaction density of the negative electrode active material layer, with the unit of compaction density being g / cm³.3 ;

[0014] η is the viscosity of the electrolyte, with the unit of viscosity being mPa·s;

[0015] β is the mass ratio of cyclic ester solvent to chain ester solvent in the electrolyte;

[0016] The value of Mel. / Cap. is 3 to 6.

[0017] Optionally, the secondary battery satisfies at least one of (1)-(4):

[0018] (1) The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.25-0.85, preferably 0.33-0.82;

[0019] (2) The conductivity σ of the electrolyte is 9-13, preferably 9.5-12.5;

[0020] (3) The viscosity η of the electrolyte is 2-5, preferably 2.5-4.5;

[0021] (4) The weight of the electrolyte is 11-16, preferably 13-15.

[0022] Optionally, the value of PD is 1.1-1.65, preferably 1.2-1.6.

[0023] Optionally, the chain ester solvent includes at least one of chain carbonates and chain carboxylic acid esters;

[0024] The chain carbonate comprises at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate.

[0025] The chain carboxylic acid ester comprises at least one of ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, and ethyl butyrate.

[0026] Optionally, the cyclic ester solvent comprises at least one of ethylene carbonate, propylene carbonate, and butene carbonate.

[0027] Optionally, the electrolyte may further include additives, which may include at least one of silicon-containing additives, cyclic carbonate additives, or sulfur-containing additives.

[0028] Optionally, the additive satisfies at least one of the following characteristics:

[0029] (a) The silicon-containing additive comprises at least one of tris(trimethylsilane) phosphite, tris(trimethylsilane) phosphate, tris(trimethylsilyl) borate, trimethylfluorosilane or heptamethyldisilazane, and the mass fraction of the silicon-containing additive is 0.1%-4% based on the mass of the electrolyte;

[0030] (b) The cyclic carbonate additive comprises at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate or difluoroethylene carbonate, and the mass fraction of the cyclic carbonate additive is 0.05%-3% based on the mass of the electrolyte;

[0031] (c) The sulfur-containing additive comprises at least one of 1,3-propanesulfonyl lactone or vinyl sulfate, and the mass fraction of the sulfur-containing additive is 0.05%-3% based on the mass of the electrolyte.

[0032] Optionally, the electrolyte additive includes a silicon-containing additive and a sulfur-containing additive, wherein the content of the silicon-containing additive is W based on the mass of the electrolyte. si %, the content of the sulfur-containing additive is W s %, satisfying 0.3≤W si +W s ≤4, 0.1≤W si / W s ≤5.

[0033] Optionally, the electrolyte further includes lithium salt additives, which include at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalateborate, lithium difluorophosphate, and lithium difluorooxalateborate.

[0034] The mass fraction of lithium salt additive in the electrolyte is 0.1%-5%.

[0035] Optionally, the positive electrode includes a positive current collector and a positive active material layer disposed in the positive current collector, wherein the positive active material layer contains a positive active material;

[0036] The positive electrode active material includes Li a Fe 1-y Mn y PO4;

[0037] Where 0.9≤a≤1.1, 0≤y≤1.

[0038] The active material layer in the negative electrode sheet contains at least one of graphite and silicon-based materials.

[0039] This application also provides an electrical device comprising the aforementioned secondary battery.

[0040] The technical solution of this application has the following advantages:

[0041] The secondary battery provided in this application includes a positive electrode, a negative electrode, and an electrolyte. The electrolyte includes an organic solvent, which includes cyclic ester solvents and chain ester solvents. The secondary battery satisfies the relationship (1). By optimizing the composition and content of the electrolyte and adjusting the active material layer of the negative electrode, the secondary battery exhibits good low-temperature kinetic performance, good capacity retention and low impedance under low-temperature conditions, and generally does not experience lithium plating. The electrolyte is one of the main materials of lithium-ion batteries and is one of the important factors affecting the performance of secondary batteries. The solvent is an important component of the electrolyte and has a significant impact on the cycling, impedance, and kinetic performance of the secondary battery. This application optimizes the electrolyte, the negative electrode, and the battery capacity, and comprehensively designs parameters such as the electrolyte solvent composition, electrolyte viscosity, conductivity, and battery capacity. This can greatly improve the low-temperature kinetic performance of the secondary battery, reduce the impedance of the secondary battery, and improve the low-temperature capacity retention.

[0042] Among them, cyclic ester solvents have high dielectric constants and high viscosity, while chain ester solvents have low dielectric constants and low viscosity. Controlling the ratio of cyclic ester solvents to chain ester solvents within a suitable range helps to improve the kinetic performance of secondary batteries.

[0043] The secondary battery provided in this application has an electrolyte viscosity that affects the migration rate of lithium ions in the electrolyte. Within the range of this application, the electrolyte viscosity further improves the transport of lithium ions, thereby reducing the impedance of the secondary battery and improving the battery cycle performance. Based on satisfying the relationship (1), a lower electrolyte viscosity will accelerate the transport of lithium ions in the electrolyte, further improving the kinetic performance.

[0044] By limiting the conductivity of the electrolyte, it can be further ensured that secondary batteries will not experience phenomena such as lithium plating under high current conditions.

[0045] The amount of electrolyte injected into a secondary battery is affected by the battery's design capacity. If the amount injected is too small, it is difficult to completely wet the positive and negative electrode plates, affecting the degree of lithium ion insertion and extraction during charging and discharging, and affecting the secondary battery's impedance, capacity, cycle life, lithium plating, and other issues. If the amount injected is too large, it increases side reactions and causes gas production and expansion.

[0046] If the compaction density (PD) of the negative electrode active material layer in the negative electrode sheet is too low, it will affect the energy density of the battery and make it difficult to control the negative electrode slurry coating process. If the compaction density (PD) is too high, the electrolyte will not easily wet the negative electrode active material layer, affecting the battery's kinetic performance. By setting an appropriate compaction density (PD), the battery's kinetic performance and energy density can be further improved. Detailed Implementation

[0047] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0048] This application provides a secondary battery, including a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative current collector and a negative active material layer disposed on the negative current collector. The electrolyte includes an organic solvent, which includes cyclic ester solvents and chain ester solvents.

[0049] The secondary battery satisfies equation (1).

[0050]

[0051] Where Mel. represents the weight of the electrolyte in the secondary battery, in grams.

[0052] σ is the conductivity of the electrolyte, and the unit of conductivity is mS / cm;

[0053] Cap. is the theoretical capacity value of the secondary battery design, and the unit of the theoretical capacity is Ah;

[0054] PD is the compaction density of the negative electrode active material layer, with the unit of compaction density being g / cm³. 3 ;

[0055] η is the viscosity of the electrolyte, with the unit of viscosity being mPa·s;

[0056] β is the mass ratio of cyclic ester solvent to chain ester solvent in the electrolyte. For example, the value of formula (1) can be 18.1, 18.3, 18.5, 18.7, 19.0, 19.3, 19.5, 19.7, 20.0, 20.2, 20.4, 20.6, 20.8, 21.0, 21.3, 21.5, 21.7, 22.0, 22.3, 22.5, 22.7, 23.0, 23.5, 24.0, 24.3, 24.7, 25.0, 25.5, 25.7, 26.0, 26.3, 26.5, 26.9 or any two of these values. When equation (1) is within the above range, the electrolyte has good kinetic behavior. Due to its high conductivity and low viscosity, lithium ions have a good diffusion rate. Furthermore, when the compaction density is within a suitable range, lithium ions can migrate rapidly in the negative electrode. In addition, an appropriate amount of electrolyte can replenish the SEI in time to repair the consumed lithium ions, so that the electrolyte also has good cycle performance.

[0057] In some embodiments of this application, When the above range is met, its low-temperature performance is further improved, mainly because lithium ions migrate rapidly in the battery, effectively preventing the negative electrode potential from dropping too quickly and causing the formation of lithium dendrites.

[0058] In some embodiments of this application, the mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.25-0.85. For example, it can be a range of 0.25, 0.30, 0.33, 0.35, 0.38, 0.40, 0.43, 0.45, 0.47, 0.50, 0.53, 0.55, 0.57, 0.60, 0.63, 0.65, 0.67, 0.70, 0.72, 0.74, 0.78, 0.80, 0.82, 0.85, or any two of these values. When the mass ratio β of the cyclic ester solvent and the chain ester solvent is within the above range, the viscosity and conductivity of the electrolyte remain relatively stable, which is beneficial for lithium ion diffusion at low temperatures, maintains the kinetic behavior of lithium ions, and reduces the risk of lithium plating.

[0059] In some embodiments of this application, the mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.33-0.82.

[0060] In some embodiments of this application, the conductivity σ of the electrolyte is 9-13. For example, it can be 9, 9.3, 9.5, 9.7, 10.0, 10.4, 10.7, 11.0, 11.3, 11.5, 11.7, 12.0, 12.2, 12.4, 12.5, 12.7, 13.0, or any two of these values. When the conductivity of the electrolyte is within the above range, lithium ions in the electrolyte diffuse rapidly, and lithium ions extracted from the positive electrode reach the negative electrode more quickly, increasing the potential of the negative electrode. This effectively improves lithium ion kinetics and further improves low-temperature lithium plating.

[0061] In some embodiments of this application, the conductivity σ of the electrolyte is 9.5-12.5.

[0062] The conductivity of the electrolyte can be measured using a DDSJ-308A conductivity meter.

[0063] In some embodiments of this application, the viscosity η of the electrolyte is 2-5. For example, it can be 2, 2.2, 2.4, 2.6, 2.8, 3.0, 3.3, 3.5, 3.7, 3.9, 4.0, 4.2, 4.4, 4.6, 4.8, 5.0, or any two of these values. When the viscosity η of the electrolyte is within the above range, a higher η indicates a higher viscosity of the electrolyte, which is unfavorable for the diffusion of lithium ions at low temperatures, reduces the kinetic behavior of lithium ions, and thus increases the risk of lithium plating.

[0064] In some embodiments of this application, the viscosity η of the electrolyte is 2.5-4.5.

[0065] In this application, the viscosity of the electrolyte can be measured using an 1835 Ubbelohde viscometer.

[0066] In some embodiments of this application, the weight of the electrolyte (Mel.) is 11-16. For example, it can be 11, 11.2, 11.5, 11.7, 12, 12.2, 12.4, 12.8, 13, 13.3, 13.5, 13.7, 14, 14.3, 14.5, 14.7, 15, 15.5, 16, or a range of any two of these values. When the electrolyte weight is within the above range, the battery's gas production during cycle is within a suitable range, preventing excessive increase in internal battery pressure that could cause the explosion-proof valve to burst and the battery to fail. Simultaneously, it reduces the appearance of bubbles and black spots at the negative electrode interface and allows the electrolyte to better facilitate lithium-ion migration, thereby improving the battery's cycle performance and kinetic performance.

[0067] In some embodiments of this application, the weight of the electrolyte (Mel.) is 13-15. When Mel. is within this range, the electrolyte can provide better lithium-ion migration while producing less gas and resulting in better overall battery performance.

[0068] In some embodiments of this application, the value of Mel. / Cap. is 3-6.

[0069] In some embodiments of this application, the PD value is 1.1-1.65. When the PD is within the above range, the compaction density affects the migration of lithium ions at the negative electrode. When the compaction density is too high, the lithium ion diffusion rate decreases, especially at low temperatures, which will have a certain impact on low-temperature performance and lead to lithium plating. Moreover, too high a compaction density will have a certain impact on the wettability of the electrolyte.

[0070] In some embodiments of this application, the value of PD is 1.2-1.6.

[0071] In some embodiments of this application, the chain ester solvent includes at least one of chain carbonates and chain carboxylic esters.

[0072] In some embodiments of this application, the chain carbonate comprises at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate.

[0073] In some embodiments of this application, the chain carboxylic acid ester comprises at least one of ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, and ethyl butyrate.

[0074] In some embodiments of this application, the cyclic ester solvent comprises at least one of ethylene carbonate, propylene carbonate, and butene carbonate.

[0075] In some embodiments of this application, the electrolyte further includes additives, which include at least one of silicon-containing additives, cyclic carbonate additives, or sulfur-containing additives.

[0076] In some embodiments of this application, the silicon-containing additive comprises at least one of tris(trimethylsilane) phosphite, tris(trimethylsilane) phosphate, tris(trimethylsilyl)borate, trimethylfluorosilane, or heptamethyldisilazane.

[0077] In some embodiments of this application, the mass fraction of the silicon-containing additive is 0.1%-4% based on the mass of the electrolyte.

[0078] In some embodiments of this application, the cyclic carbonate additive comprises at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, or difluoroethylene carbonate.

[0079] In some embodiments of this application, the mass fraction of the cyclic carbonate additive is 0.05%-3% based on the mass of the electrolyte;

[0080] In some embodiments of this application, the sulfur-containing additive comprises at least one of 1,3-propanesulfonyl lactone or vinyl sulfate.

[0081] In some embodiments of this application, the mass fraction of the sulfur-containing additive is 0.05%-3% based on the mass of the electrolyte.

[0082] In some embodiments of this application, the electrolyte additive comprises a silicon-containing additive and a sulfur-containing additive, and the content of the silicon-containing additive is W based on the mass of the electrolyte. si %, the content of the sulfur-containing additive is W s %, satisfying 0.3≤W si +W s ≤4. When the electrolyte contains both silicon-containing additives and sulfur-containing additives, they can form a SEI film with low impedance, allowing lithium ions to pass through the SEI film quickly. When the sum of their contents is within the above range, the film formation is better, and the quality and thickness of the protective film formed are both optimal, improving lithium ion diffusion.

[0083] In some embodiments of this application, the electrolyte additive comprises a silicon-containing additive and a sulfur-containing additive, and the content of the silicon-containing additive is W based on the mass of the electrolyte. si %, the content of the sulfur-containing additive is W s%, satisfying 0.1≤W si / W s ≤5. When W si / W s Within the aforementioned range, the composition and thickness of the formed protective film can be further optimized, resulting in better overall battery performance.

[0084] In some embodiments of this application, the electrolyte further includes lithium salt additives, which include at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalateborate, lithium difluorophosphate, and lithium difluorooxalateborate. When the electrolyte contains the above-mentioned lithium salt additives, it can further reduce membrane impedance, improve film formation stability, thereby enhancing lithium ion diffusion and maintaining good cycle performance.

[0085] In some embodiments of this application, the mass fraction of lithium salt additive in the electrolyte is 0.1%-5%.

[0086] In some embodiments of this application, the positive electrode includes a positive current collector and a positive active material layer disposed in the positive current collector;

[0087] The positive electrode active material includes Li a Fe 1-y Mn y PO4;

[0088] Where 0.9≤a≤1.1, 0≤y≤1.

[0089] In some embodiments of this application, the positive electrode active material may also contain doping elements. The type of doping element is not limited, as long as it can improve the performance of the positive electrode active material.

[0090] In some embodiments of this application, the surface of the positive electrode active material has a coating layer.

[0091] In some embodiments of this application, the coating layer includes coating elements, which are not limited to any particular element, as long as they can improve the stability of the positive electrode active material.

[0092] In some embodiments of this application, the surface of the positive electrode active material comprises a carbon material layer. The carbon material layer comprises amorphous carbon.

[0093] Phosphate-based positive electrode active materials have advantages such as low cost and high safety, and are therefore widely used in lithium batteries. However, phosphate-based positive electrode active materials have poor low-temperature mechanical properties. When phosphate-based positive electrode active materials are used in combination with the electrolyte and negative electrode sheet of this application, the low-temperature performance of the battery can be significantly improved. The electrolyte of this application has good compatibility with phosphate-based positive electrode active materials and negative electrode sheets, which can result in a film with low impedance and good stability, while also taking into account cycle performance and effectively improving low-temperature lithium plating.

[0094] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0095] Example 1

[0096] This embodiment provides a lithium-ion secondary battery with a theoretical capacity of 3.5Ah, comprising:

[0097] Positive electrode sheet: The positive electrode active material LiFePO4, conductive agent SP, and binder PVDF (Arkema HSV900, hereinafter the same) are mixed in a mass ratio of 96:2:2. After adding NMP, the mixture is stirred under vacuum until the system becomes homogeneous, resulting in a positive electrode slurry. The positive electrode slurry is coated onto the positive electrode current collector aluminum foil, dried, and cold-pressed to obtain a positive electrode sheet with a positive electrode active material layer. The compaction density of the positive electrode active material layer is 2.45 g / cm³. 3 .

[0098] Negative electrode sheet: Graphite, conductive agent SP, thickener CMC, and binder SBR (Zeon 451B, hereinafter the same) are mixed in a mass ratio of 96.2:1.2:1.2:1.4. Deionized water is added, and the mixture is stirred under vacuum until the system becomes homogeneous to obtain a negative electrode slurry. The negative electrode slurry is coated onto a copper foil for the negative electrode current collector, dried, and cold-pressed to obtain a negative electrode sheet with a negative electrode active material layer. The compaction density of the negative electrode active material layer is 1.55 g / cm³. 3 .

[0099] Electrolyte: Ethylene carbonate (EC, cyclic ester solvent), ethyl methyl carbonate (EMC, chain carbonate solvent), and dimethyl carbonate (DMC, chain carbonate solvent) are mixed in a ratio of 30:40:30 to obtain an organic solvent. LiPF6 is added to the organic solvent and mixed evenly. Then, vinylene carbonate, vinyl sulfate, lithium difluorophosphate, and tris(trimethylsilane) phosphate are added to obtain the electrolyte. The mass percentages of LiPF6, vinylene carbonate, vinyl sulfate, lithium difluorophosphate, and tris(trimethylsilane) phosphate are 12%, 1%, 1.5%, 0.5%, and 1%, respectively. The mass ratio β of the cyclic ester solvent to the chain ester solvent is 0.43. The conductivity of the electrolyte is 11.28 mS / cm, and the viscosity is 3.46 mPa·s.

[0100] The positive electrode, separator (14PP material from Xingyuan) and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrode. After winding, a battery cell is obtained. The battery cell is then encapsulated in an aluminum-plastic film, dried, and injected with 14g of electrolyte. After encapsulation, settling, formation, and hot pressing, a secondary battery is obtained.

[0101] Example 2

[0102] The method is basically the same as in Example 1, except that the density of the negative electrode active material layer is 1.6 g / cm³. 3 .

[0103] Example 3

[0104] The method is basically the same as in Example 1, except that the density of the negative electrode active material layer is 1.45 g / cm³. 3 .

[0105] Example 4

[0106] The preparation method is basically the same as in Example 1, except that the electrolyte is mixed in a ratio of 30:30:30:10. The mass ratio β of the cyclic ester solvent to the chain ester solvent in the electrolyte is 0.43, the conductivity of the electrolyte is 11.44 mS / cm, and the viscosity of the electrolyte is 3.32 mPa·s.

[0107] Example 5

[0108] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, the electrolyte is prepared by mixing ethylene carbonate (EC, cyclic ester solvent), ethyl methyl carbonate (EMC, chain carbonate solvent), and dimethyl carbonate (DMC, chain carbonate solvent) in a ratio of 27:40:33. The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.37, the conductivity of the electrolyte is 11.09 mS / cm, and the viscosity of the electrolyte is 3.37 mPa·s.

[0109] Example 6

[0110] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, the electrolyte is prepared by mixing ethylene carbonate (EC, cyclic ester solvent), ethyl methyl carbonate (EMC, chain carbonate solvent), and dimethyl carbonate (DMC, chain carbonate solvent) in a ratio of 25:45:30. The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.33, the conductivity of the electrolyte is 10.75 mS / cm, and the viscosity of the electrolyte is 3.3 mPa·s.

[0111] Example 7

[0112] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, the electrolyte is prepared by mixing ethylene carbonate (EC, cyclic ester solvent), ethyl methyl carbonate (EMC, chain carbonate solvent), and dimethyl carbonate (DMC, chain carbonate solvent) in a ratio of 32:40:28. The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.47, the conductivity of the electrolyte is 11.52 mS / cm, and the viscosity of the electrolyte is 3.48 mPa·s.

[0113] Example 8

[0114] The preparation method is basically the same as in Example 1, except that the electrolyte does not include vinyl sulfate. The mass ratio β of the cyclic ester solvent and the chain ester solvent in the obtained electrolyte is 0.43, the conductivity of the electrolyte is 11.18 mS / cm, and the viscosity of the electrolyte is 3.34 mPa·s.

[0115] Example 9

[0116] The preparation method is basically the same as in Example 1, except that the electrolyte does not include vinyl sulfate and tris(trimethylsilane) phosphate. The mass ratio β of the cyclic ester solvent and the chain ester solvent in the obtained electrolyte is 0.43, the conductivity of the electrolyte is 11.02 mS / cm, and the viscosity of the electrolyte is 3.4 mPa·s.

[0117] Example 10

[0118] The preparation method is basically the same as in Example 1, except that the electrolyte does not include lithium difluorophosphate and tris(trimethylsilane)phosphate. The mass ratio β of the cyclic ester solvent and the chain ester solvent in the obtained electrolyte is 0.43, the conductivity of the electrolyte is 10.85 mS / cm, and the viscosity of the electrolyte is 3.42 mPa·s.

[0119] Example 11

[0120] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, 0.5% lithium difluorophosphate, 1% tris(trimethylsilane) phosphate, and 1.5% vinyl sulfate are added. The mass ratio β of the cyclic ester solvent and the chain ester solvent in the resulting electrolyte is 0.43, the conductivity of the electrolyte is 11.12 mS / cm, and the viscosity of the electrolyte is 3.39 mPa·s.

[0121] Example 12

[0122] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, 1.0% vinylene carbonate, 0.5% lithium difluorophosphate, 0.2% vinyl sulfate, and 1% tris(trimethylsilane) phosphate are added. The resulting electrolyte has a mass ratio β of cyclic ester solvent to chain ester solvent of 0.43, an electrolyte conductivity of 11.14 mS / cm, and an electrolyte viscosity of 3.41 mPa·s.

[0123] Example 13

[0124] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, 1.0% vinylene carbonate, 0.5% lithium difluorophosphate, 0.7% vinyl sulfate, and 1.6% tris(trimethylsilane) phosphate are added. The resulting electrolyte has a mass ratio β of cyclic ester solvent to chain ester solvent of 0.43, an electrolyte conductivity of 11.25 mS / cm, and an electrolyte viscosity of 3.38 mPa·s.

[0125] Example 14

[0126] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, 1.0% vinylene carbonate, 0.5% lithium difluorophosphate, 2% vinyl sulfate, and 0.15% tris(trimethylsilane) phosphate are added. The resulting electrolyte has a mass ratio β of cyclic ester solvent to chain ester solvent of 0.43, an electrolyte conductivity of 11.21 mS / cm, and an electrolyte viscosity of 3.36 mPa·s.

[0127] Example 15

[0128] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, 1.0% vinylene carbonate, 0.5% lithium difluorophosphate, 1.5% vinyl sulfate, and 3% tris(trimethylsilane) phosphate are added. The resulting electrolyte has a mass ratio β of cyclic ester solvent and chain ester solvent of 0.43, an electrolyte conductivity of 11.33 mS / cm, and an electrolyte viscosity of 3.33 mPa·s.

[0129] Example 16

[0130] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, 1.0% vinylene carbonate, 0.5% lithium difluorophosphate, 2.5% vinyl sulfate, and 3% tris(trimethylsilane) phosphate are added. The resulting electrolyte has a mass ratio β of cyclic ester solvent to chain ester solvent of 0.43, an electrolyte conductivity of 11.32 mS / cm, and an electrolyte viscosity of 3.35 mPa·s.

[0131] Example 17

[0132] The preparation method is basically the same as in Example 1, except for the mass of the electrolyte, specifically the amount of electrolyte injected is 13g.

[0133] Example 18

[0134] The preparation method is basically the same as in Example 1, except for the electrolyte, specifically the amount of electrolyte injected is 16g.

[0135] Example 19

[0136] The preparation method is basically the same as in Example 1, except for the compaction density of the negative electrode, specifically, the density of the negative electrode active material layer is 1.68 g / cm³. 3 .

[0137] Comparative Example 1

[0138] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, the electrolyte is prepared by mixing ethylene carbonate (EC, cyclic ester solvent), ethyl methyl carbonate (EMC, chain carbonate solvent), and dimethyl carbonate (DMC, chain carbonate solvent) in a ratio of 40:30:30. The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.67, the conductivity of the electrolyte is 12.01 mS / cm, and the viscosity of the electrolyte is 3.74 mPa·s.

[0139] Comparative Example 2

[0140] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, the electrolyte is prepared by mixing ethylene carbonate (EC, cyclic ester solvent), ethyl methyl carbonate (EMC, chain carbonate solvent), and dimethyl carbonate (DMC, chain carbonate solvent) in a ratio of 40:40:20. The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.67, the conductivity of the electrolyte is 11.84 mS / cm, and the viscosity of the electrolyte is 4.05 mPa·s.

[0141] Comparative Example 3

[0142] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, the electrolyte is prepared by mixing ethylene carbonate (EC, cyclic ester solvent), ethyl methyl carbonate (EMC, chain carbonate solvent), and dimethyl carbonate (DMC, chain carbonate solvent) in a ratio of 40:15:45. The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.67, the conductivity of the electrolyte is 12.19 mS / cm, and the viscosity of the electrolyte is 3.66 mPa·s.

[0143] Comparative Example 4

[0144] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, the electrolyte is prepared by mixing ethylene carbonate (EC, cyclic ester solvent), ethyl methyl carbonate (EMC, chain carbonate solvent), and dimethyl carbonate (DMC, chain carbonate solvent) in a ratio of 48:26:26. The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.92, the conductivity of the electrolyte is 12.13 mS / cm, and the viscosity of the electrolyte is 4.43 mPa·s.

[0145] Comparative Example 5

[0146] The preparation method is basically the same as in Example 12, except for the electrolyte. Specifically, 3.5% vinylene carbonate, 0.5% lithium difluorophosphate, 1% tris(trimethylsilane) phosphate, and 1.5% vinyl sulfate are added. The resulting electrolyte has a mass ratio β of cyclic ester solvent to chain ester solvent of 0.43, an electrolyte conductivity of 10.97 mS / cm, and an electrolyte viscosity of 3.68 mPa·s.

[0147] Comparative Example 6

[0148] The preparation method is basically the same as in Example 1, except for the mass of the electrolyte, specifically the amount of electrolyte injected is 10g.

[0149] Comparative Example 7

[0150] The preparation method is basically the same as in Example 1, except for the electrolyte. Specifically, the electrolyte is prepared by mixing ethylene carbonate (EC, cyclic ester solvent), ethyl methyl carbonate (EMC, chain carbonate solvent), and dimethyl carbonate (DMC, chain carbonate solvent) in a ratio of 20:40:40. The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.25, the conductivity of the electrolyte is 10.48 mS / cm, and the viscosity of the electrolyte is 3.11 mPa·s.

[0151] In the embodiments and comparative examples, the conductivity of the electrolyte was measured by a conductivity meter at 25°C, and the viscosity of the electrolyte was measured by a viscometer at 25°C.

[0152] Test case

[0153] This experimental example provides the performance test results of the lithium-ion secondary batteries provided in each embodiment and comparative example, as detailed below.

[0154] The test method for battery capacity retention is as follows: The battery is fully charged to the upper limit voltage, then discharged at 25°C with a 1C discharge current to the lower limit voltage, and the discharge capacity is recorded as C0. The battery is then fully charged again to the upper limit voltage at 25°C, and the temperature is adjusted to -20°C, discharging at a 1C discharge current to the lower limit voltage, and the discharge capacity is recorded as C1. The capacity retention rate of the lithium-ion secondary battery at -20°C is C1 / C0.

[0155] Battery DCR (Impedance) test method: At 25℃, adjust the charge to 50% SOC with a 1C discharge current, adjust the temperature to -25℃, discharge at 0.36C for 10s, and calculate the discharge DCR at -25℃. The calculation formula is △U / I, where △U is the change in voltage before and after discharge, and I is the discharge current.

[0156] Battery lithium plating test method: Adjust the temperature to -10℃, charge to the upper voltage limit with a 0.13C charging current, discharge to the lower voltage limit with a 1C discharging current, perform 10 charge-discharge cycles, adjust the temperature to 25℃, charge to the upper voltage limit with a 1C charging current, disassemble the battery, and observe whether lithium plating occurs at the interface. If the area of ​​lithium plating on the negative electrode surface is less than 2%, it is considered no lithium plating; if the area of ​​lithium plating on the negative electrode surface is greater than or equal to 2% and less than 5%, it is considered slight lithium plating; if the area of ​​lithium plating on the negative electrode surface is 5-50%, it is considered lithium plating; and if the area of ​​lithium plating on the negative electrode surface is greater than 50%, it is considered severe lithium plating.

[0157] Table 1. Performance test results of the lithium-ion secondary batteries provided in each embodiment and comparative example.

[0158]

[0159]

[0160]

[0161] Based on the experimental results recorded in Table 1, the parameters between the organic solvent, the negative electrode, and the cell in the secondary battery satisfy a specific relationship. The secondary battery has good capacity retention and low impedance under low temperature conditions, and basically no lithium plating occurs, except for slight lithium plating in some cases (such as in Example 15).

[0162] Specifically, a comparison of Comparative Example 1 with Examples 4-6, and Comparative Example 4, reveals that... Lithium plating occurred at values ​​below 18, primarily due to an excessively high β value. This resulted in a high content of cyclic esters in the electrolyte, leading to excessively high viscosity at low temperatures. This negatively impacted kinetics, causing a high DCR (displacement coefficient) and ultimately lithium plating. A comparison of Examples 1-3 shows that even with the same β value, the specific composition of the cyclic ester solvent directly affects the overall viscosity and conductivity of the electrolyte, thus influencing the performance of the secondary battery.

[0163] Compared with Example 1, in Comparative Example 7, at this time When the value exceeds 27, it exhibits good low-temperature performance and does not precipitate lithium. However, the capacity retention rate decreases significantly at this point, mainly due to the reduced amount of cyclic carbonate, which is detrimental to the capacity retention rate during cycling.

[0164] A comparison of Examples 1-3 and Example 19 reveals that, under conditions of excessively high compaction, although Example 19... However, slight lithium plating may still occur, mainly because the compaction density is too high at this time, which increases the difficulty of lithium ion insertion and extraction, affects the low-temperature performance of the secondary battery, and leads to a decrease in the low-temperature capacity retention rate of the secondary battery, resulting in slight lithium plating at the interface.

[0165] Examples 17-18, and Comparative Example 6 differ from Example 1 in the change of electrolyte injection volume. When too low an injection volume is selected, Below 18, the reduced electrolyte injection amount leads to difficulty in wetting the positive and negative electrode plates, increased impedance, and lithium plating at the interface; it also reduces the capacity retention rate of the secondary battery at low temperatures. Furthermore, there is an optimal range for electrolyte injection amount; excessive injection amount will also negatively impact the performance of the secondary battery.

[0166] Data from Examples 1, 8-16, and Comparative Example 5 show that different additives have little effect on the viscosity and conductivity of the electrolyte. However, excessive amounts of a particular additive can still lead to lithium plating, as seen in Comparative Example 5. While secondary batteries made with different additive systems show virtually no lithium plating at low temperatures, they exhibit different capacity retention rates and impedance characteristics. Furthermore, the combined effects of cyclic carbonate additives, silicon-containing additives, sulfonate additives, and lithium salt additives, along with the rational design and quantification of parameters such as the electrolyte, the active material layer of the negative electrode, and the weight of the electrolyte in the secondary battery, allow for… A value greater than 18 can ensure that the secondary battery has a high capacity retention rate at -20℃ and will not experience lithium plating problems at -10℃, thus exhibiting good low-temperature performance.

[0167] Data from Examples 1 and 12-16 show that when 0.3 ≤ W si +W s ≤4, 0.1≤W si / W s When the value is ≤5, the secondary battery has better overall performance.

[0168] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this application.

Claims

1. A secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that, The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the negative current collector; the electrolyte includes an organic solvent, which includes cyclic ester solvents and chain ester solvents. The secondary battery satisfies equation (1). 27> >18 Equation (1); Where Mel. represents the weight of the electrolyte in the secondary battery, in grams. σ is the conductivity of the electrolyte at 25℃, and the unit of conductivity is mS / cm; Cap. is the theoretical capacity value of the secondary battery design, and the unit of the theoretical capacity is Ah; PD is the compaction density of the negative electrode active material layer, with the unit of compaction density being g / cm³. 3 ; η is the viscosity of the electrolyte at 25℃, and the unit of viscosity is mPa·s; β is the mass ratio of cyclic ester solvent to chain ester solvent in the electrolyte; The value of Mel. / Cap. is 3~6; The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.25-0.85; The conductivity σ of the electrolyte is 9-13; The viscosity η of the electrolyte is 2-5; The weight of the electrolyte is 11-16 (Mel). The value of PD is 1.1-1.

65.

2. The secondary battery according to claim 1, characterized in that, Satisfying at least one of (1)-(4): (1) The mass ratio β of the cyclic ester solvent and the chain ester solvent is 0.33-0.82; (2) The conductivity σ of the electrolyte is 9.5-12.5; (3) The viscosity η of the electrolyte is 2.5-4.5; (4) The weight of the electrolyte is 13-15.

3. The secondary battery according to claim 1 or 2, characterized in that, The value of PD is 1.2-1.

6.

4. The secondary battery according to claim 1, characterized in that, The chain ester solvent includes at least one of chain carbonates and chain carboxylic esters; The chain carbonate comprises at least one of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, and methyl propyl carbonate. The chain carboxylic acid ester comprises at least one of ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, and ethyl butyrate.

5. The secondary battery according to claim 1, characterized in that, The cyclic ester solvent comprises at least one of ethylene carbonate, propylene carbonate, and butene carbonate.

6. The secondary battery according to claim 1, characterized in that, The electrolyte also includes additives, which include at least one of silicon-containing additives, cyclic carbonate additives, or sulfur-containing additives.

7. The secondary battery according to claim 6, characterized in that, The additive satisfies at least one of the following characteristics: (a) The silicon-containing additive comprises at least one of tris(trimethylsilane) phosphite, tris(trimethylsilane) phosphate, tris(trimethylsilyl) borate, trimethylfluorosilane or heptamethyldisilazane, and the mass fraction of the silicon-containing additive is 0.1%-4% based on the mass of the electrolyte. (b) The cyclic carbonate additive comprises at least one of vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate or difluoroethylene carbonate, and the mass fraction of the cyclic carbonate additive is 0.05%-3% based on the mass of the electrolyte; (c) The sulfur-containing additive comprises at least one of 1,3-propanesulfonyl lactone or vinyl sulfate, and the mass fraction of the sulfur-containing additive is 0.05%-3% based on the mass of the electrolyte.

8. The secondary battery according to claim 6, characterized in that, The electrolyte additive includes a silicon-containing additive and a sulfur-containing additive, and the content of the silicon-containing additive is W based on the mass of the electrolyte. si %, the content of the sulfur-containing additive is W s %, satisfying 0.3≤W si +W s ≤4, 0.1≤W si / W s ≤5.

9. The secondary battery according to claim 1, characterized in that, The electrolyte also includes lithium salt additives, which include at least one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalateborate, lithium difluorophosphate, and lithium difluorooxalateborate. The mass fraction of lithium salt additive in the electrolyte is 0.1%-5%.

10. The secondary battery according to claim 1, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer disposed in the positive current collector, wherein the positive active material layer contains a positive active material; The positive electrode active material includes Li a Fe 1-y Mn y PO4; Where 0.9≤a≤1.1, 0≤y≤1.

11. An electrical device comprising a secondary battery as described in any one of claims 1-10.

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