A negative electrode sheet and a battery including the negative electrode sheet

Abstract

The present invention belongs to the technical field of lithium-ion battery negative electrodes and relates to a negative electrode sheet and a battery including the negative electrode sheet. The pore-forming method using the pore-forming agent is simple and only needs to be added during the negative electrode batching process, without the need for additional steps and equipment, thus avoiding increased pore-forming costs. A large-volume molecule-cage polysilsesquioxane is used as a pore-forming agent and added during the negative electrode batching process. After injection, the pore-forming agent can be rapidly dissolved from the negative electrode sheet into the electrolyte, effectively increasing the porosity of the negative electrode sheet, reducing the pore tortuosity, and increasing the lithium ion transmission rate, thereby improving the kinetics of the negative electrode sheet and reducing the risk of lithium plating.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery negative electrodes and relates to a negative electrode sheet and a battery comprising the negative electrode sheet. Background Art

[0002] After nearly two decades of development, lithium-ion batteries are becoming increasingly mature. With the increasing popularity of electric vehicles, mobile phones, laptops, and wearable electronic devices, the consumption of lithium ions has also seen exponential growth. At the same time, people are increasingly demanding the energy density of lithium-ion batteries. Especially in mobile phones, laptops, and wearable electronic devices, as these electronic devices are becoming increasingly miniaturized and ultra-thin, it is particularly urgent to increase the volume energy density of lithium-ion batteries. Currently, the more commonly used method to increase the volume energy density of lithium-ion batteries is to increase the thickness and compaction density of the positive and negative electrodes, especially to increase the compaction density of the negative electrode, which can effectively increase the volume energy density of the battery. Currently, the compaction density of the negative electrode of consumer batteries generally does not exceed 1.85g / cm 3 The maximum surface density does not exceed 12 mg / cm 2 When the negative electrode compaction density and surface density are increased, the electrode porosity will drop significantly, the pore tortuosity in the thickness direction of the electrode will increase, the electrolyte transmission path will become longer, the electrolyte wettability to the electrode will deteriorate, and the lithium ion transmission efficiency will decrease, which will easily cause the negative electrode kinetics to deteriorate and the risk of lithium plating to increase sharply. Therefore, it is necessary to increase the porosity of the negative electrode sheet, reduce the pore tortuosity, improve the lithium ion transmission efficiency, and improve the negative electrode kinetics.

[0003] There have been some studies on improving the porosity of negative electrodes, which can be mainly divided into laser pore formation, magnetic material pore formation, and pore formation with pore formers. Laser pore formation is costly, inefficient, and difficult to use on a large scale. In addition, a small amount of magnetic material or pore former is added to the electrode slurry, and then an external magnetic field is applied during the electrode coating process to control the electrode microstructure, or the pore former evaporates in the electrode piece during the coating process to produce pores. The above scheme reduces the pore tortuosity in the electrode thickness direction by 4 times, improving the electrochemical performance of the prepared battery. However, this scheme requires the introduction of magnetic material, which on the one hand reduces the proportion of electrode active material and sacrifices energy density; on the other hand, the introduced magnetic impurities can easily pierce the diaphragm during the charge and discharge process, causing safety hazards to the battery. Therefore, the development of a pore formation method with simple process, low cost, and no introduction of magnetic material and impurities is still of great practical value. Summary of the Invention

[0004] To address the deficiencies of the prior art, the present invention provides a negative electrode sheet and a battery comprising the negative electrode sheet. The negative electrode sheet employs a large-volume molecule, cage-type polysilsesquioxane, as a pore-forming agent. This pore-forming agent is added during the negative electrode batching process. After injection, the pore-forming agent rapidly dissolves from the negative electrode sheet into the electrolyte, effectively increasing the porosity of the negative electrode sheet, reducing pore tortuosity, and improving the lithium ion transmission rate, thereby enhancing the kinetics of the negative electrode sheet and reducing the risk of lithium precipitation. Furthermore, the pore-forming agent has stable electrochemical properties and contains groups that can coordinate with cobalt and nickel ions, preventing the dissolution of metal ions in the positive electrode material and providing a certain degree of protection for the positive electrode material.

[0005] The object of the present invention is achieved through the following technical solutions:

[0006] A negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer coated on one side or both sides of the negative electrode current collector. The negative electrode active material layer comprises a negative electrode active material, a conductive agent, a binder and a cage-type polysilsesquioxane.

[0007] According to an embodiment of the present invention, the molecular formula of the cage-type polysilsesquioxane is (RSiO 1.5 )n, wherein R is a reactive group directly connected to the silicon atom, and n is any one of 6, 8, 10 or 12.

[0008] According to an embodiment of the present invention, the R is *-(CH2) x -R1; wherein the * end is connected to the silicon atom, x is an integer between 1 and 12 (such as an integer between 1 and 6, such as 1, 2, 3, 4, 5 or 6), and R1 is selected from carboxyl (-COOH), nitrile (-CN), pyridine Imidazole or thiazole The wavy lines represent connecting lines.

[0009] According to an embodiment of the present invention, the R1 group can form coordination with cobalt and nickel ions, which is beneficial to preventing the dissolution of metal ions in the positive electrode material and has a certain protective effect on the positive electrode material.

[0010] According to an embodiment of the present invention, the cage-type polysilsesquioxane has a structure as shown in Formula 1:

[0011]

[0012] In Formula 1, R is as defined above.

[0013] According to an embodiment of the present invention, the median particle size of the cage-type polysilsesquioxane is 1 nm to 5 nm, for example, 1 nm, 2 nm, 3 nm, 4 nm or 5 nm.

[0014] According to an embodiment of the present invention, the pore former is added during the negative electrode preparation process. After injection, the pore former can be quickly dissolved from the negative electrode sheet into the electrolyte (the dissolution rate is 5 to 10 g / 30 min, and the dissolution rate means that 5 to 10 g of the pore former is completely dissolved in 100 g of electrolyte (conventional commercial electrolyte or the electrolyte described in the specific embodiment) at 25°C in only 30 minutes. Such a dissolution rate can ensure that the dissolution time of the pore former is much lower than the aging standing time after the battery injection (generally 24h to 48h)), effectively improving the porosity of the negative electrode sheet (the porosity of the negative electrode sheet can be increased by 0.5% to 10%), reducing the pore tortuosity, and increasing the lithium ion transmission rate, thereby improving the kinetics of the negative electrode sheet and reducing the risk of lithium plating.

[0015] According to an embodiment of the present invention, the negative electrode active material layer includes the following components in percentage by weight: 85-98.99 wt % of negative electrode active material, 0.5-5 wt % of conductive agent, 0.5-5 wt % of binder, and 0.01-5 wt % of cage-type polysilsesquioxane.

[0016] Illustratively, the mass percentage of the negative electrode active material content to the total mass of the negative electrode active material layer is 85wt%, 86wt%, 87wt%, 88wt%, 89wt%, 90wt%, 91wt%, 92wt%, 93wt%, 94wt%, 95wt%, 96wt%, 97wt%, 98wt% or 98.99wt%.

[0017] Illustratively, the content of the conductive agent in the total mass of the negative electrode active material layer is 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% or 5 wt%.

[0018] For example, the content of the cage-type polysilsesquioxane in the total mass of the negative electrode active material layer is 0.01wt%, 0.05wt%, 0.1wt%, 0.15wt%, 0.25wt%, 0.55wt%, 0.65wt%, 0.70wt%, 0.75wt%, 0.85wt%, 0.90wt%, 1.0wt%, 1.2wt%, 1.5wt%, 2wt%, 3wt%, 4wt% or 5wt%. When the content of the cage-type polysilsesquioxane is greater than 5wt%, that is, the content of the cage-type polysilsesquioxane is too high, the negative electrode active material will be reduced, thereby reducing the energy density of the battery; when the content of the cage-type polysilsesquioxane is less than 0.01wt%, that is, the content of the cage-type polysilsesquioxane is too low, the number of pores formed by the cage-type polysilsesquioxane is small, and the migration rate of lithium ions is not significantly improved.

[0019] Exemplarily, the mass percentage of the binder in the total mass of the negative electrode active material layer is 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% or 5 wt%.

[0020] According to an embodiment of the present invention, the negative electrode active material is selected from silicon-based materials and / or carbon-based materials.

[0021] Among them, the carbon-based material is selected from at least one of artificial graphite, natural graphite, hard carbon, soft carbon, and mesophase microspheres.

[0022] Among them, the silicon-based material is selected from at least one of nano-silicon, silicon monoxide, and SiOx (0 < x < 2).

[0023] According to an embodiment of the present invention, the conductive agent is selected from one or several of conductive carbon black, Ketjen black, conductive fiber, conductive polymer, acetylene black, carbon nanotube, graphene, flake graphite, conductive oxide, and metal particles.

[0024] According to an embodiment of the present invention, the binder is selected from at least one of polyvinylidene fluoride and its copolymer derivatives, polytetrafluoroethylene and its copolymer derivatives, polyacrylic acid and its copolymer derivatives, polyvinyl alcohol and its copolymer derivatives, polybutadiene rubber and its copolymer derivatives, polyimide and its copolymer derivatives, polyethyleneimine and its copolymer derivatives, polyacrylate and its copolymer derivatives, sodium carboxymethyl cellulose and its copolymer derivatives.

[0025] According to an embodiment of the present invention, the single-sided areal density of the negative electrode sheet is 1 to 25 mg / cm 2 , for example, 1 mg / cm[[ID=2{]] 2 , 2 mg / cm 2 , 3 mg / cm 2 , 5 mg / cm 2 , 6 mg / cm 2 , 8 mg / cm 2 , 9 mg / cm 2 , 10 mg / cm 2 , 12 mg / cm 2 , 15 mg / cm 2 , 18 mg / cm 2 , 20 mg / cm 2 , 22 mg / cm 2 , 25 mg / cm 2 or 25 mg / cm 2 .

[0026] According to an embodiment of the present invention, the tap density of the negative electrode sheet is 1.60 to 1.85 g / cm 3 , for example, 1.60 g / cm 3, 1.61g / cm 3 、1.62g / cm 3 , 1.63g / cm 3 , 1.64g / cm 3 , 1.65g / cm 3 , 1.66g / cm 3 , 1.67g / cm 3 , 1.68g / cm 3 , 1.69g / cm 3 , 1.70g / cm 3 、1.71g / cm 3 , 1.72g / cm 3 , 1.73g / cm 3 , 1.74g / cm 3 , 1.75g / cm 3 , 1.76g / cm 3 , 1.77g / cm 3 , 1.78g / cm 3 , 1.79g / cm 3 , 1.80g / cm 3 、1.81g / cm 3 , 1.82g / cm 3 , 1.83g / cm 3 , 1.84g / cm 3 , 1.85g / cm 3 .

[0027] According to an embodiment of the present invention, the thickness of the negative electrode active material layer (the thickness of the single-sided negative electrode active material layer after rolling) is 10μm to 150μm, preferably 30μm to 100μm, such as 10μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, 70μm, 80μm, 90μm, 100μm, 110μm, 120μm, 130μm, 140μm or 150μm.

[0028] The present invention also provides a method for preparing the above-mentioned negative electrode sheet, the method comprising the following steps:

[0029] The solvent, negative electrode active material, conductive agent, binder and cage-type polysilsesquioxane are uniformly mixed to prepare negative electrode slurry; the negative electrode slurry is coated on the surface of the negative electrode current collector and dried to prepare the negative electrode sheet.

[0030] According to an embodiment of the present invention, the negative electrode slurry contains 100-300 parts by mass of a solvent, 85-98.99 parts by mass of a negative electrode active material, 0.5-5 parts by mass of a conductive agent, 0.01-5 parts by mass of cage polysilsesquioxane, and 0.5-5 parts by mass of a binder.

[0031] According to an embodiment of the present invention, the solvent is selected from at least one of water, acetonitrile, benzene, toluene, xylene, acetone, tetrahydrofuran, hydrofluoroether, and N-methylpyrrolidone.

[0032] According to an embodiment of the present invention, the temperature of the drying process is 70 to 110° C., and the time of the drying process is 12 to 36 hours.

[0033] The present invention also provides a battery, comprising the above-mentioned negative electrode sheet.

[0034] According to an embodiment of the present invention, the battery further includes a positive electrode sheet and an electrolyte.

[0035] According to an embodiment of the present invention, the electrolyte also includes the above-mentioned cage-type polysilsesquioxane.

[0036] According to an embodiment of the present invention, the cage-type polysilsesquioxane in the electrolyte is obtained by dissolving the cage-type polysilsesquioxane in the negative electrode sheet into the electrolyte.

[0037] According to an embodiment of the present invention, the charge cut-off voltage of the battery is greater than 4.5V.

[0038] The present invention also provides a use of the cage-type polysilsesquioxane as a pore-forming agent in a negative electrode sheet.

[0039] According to an embodiment of the present invention, the single surface density of the negative electrode sheet is 1 to 25 mg / cm 2 .

[0040] According to an embodiment of the present invention, the compaction density of the negative electrode sheet is 1.60 to 1.85 g / cm 3 .

[0041] Beneficial effects of the present invention:

[0042] The pore-forming method using this pore-forming agent is simple. It only needs to be added during the negative electrode batching process. There is no need to increase the process and equipment, which avoids increasing the cost of pore-forming. A large-volume molecule - cage-type polysilsesquioxane is used as a pore-forming agent and added during the negative electrode batching process. After injection, the pore-forming agent can quickly dissolve from the negative electrode sheet into the electrolyte, effectively increasing the porosity of the negative electrode sheet, reducing the pore tortuosity, and increasing the lithium ion transmission rate, thereby improving the kinetics of the negative electrode sheet and reducing the risk of lithium precipitation. In addition, the electrochemical properties of the pore-forming agent are stable, and it contains groups that can coordinate with cobalt and nickel ions, which can prevent the dissolution of metal ions in the positive electrode material and has a certain protective effect on the positive electrode material. The application of this pore-forming agent to high-area-density and high-density negative electrode sheets has a particularly significant improvement in the kinetics of the negative electrode sheet. DETAILED DESCRIPTION

[0043] The present invention will be described in further detail below with reference to specific examples. It should be understood that the following examples are merely illustrative and explanations of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are encompassed within the scope of protection that the present invention is intended to protect.

[0044] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; the reagents, materials, etc. used in the following examples are all commercially available unless otherwise specified.

[0045] The lithium-ion batteries used in the following examples were prepared by the following method:

[0046] (1) Preparation of positive electrode sheet

[0047] The positive electrode active material lithium cobalt oxide (LCO), the binder polyvinylidene fluoride (PVDF), and the conductive agent acetylene black were mixed in a weight ratio of 97:1.5:1.5, and N-methylpyrrolidone (NMP) was added. The mixture was stirred under the action of a vacuum mixer until the mixed system became a positive electrode slurry with uniform fluidity; the positive electrode slurry was evenly coated on the current collector aluminum foil (aluminum foil thickness 10 μm) with an area density of 20.35 mg / cm 2 The coated aluminum foil was baked in an oven and then dried in an oven at 120°C for 8 hours, and then roller pressed (compacted density 4.15g / cm 3 ), cut to obtain the required positive electrode sheet.

[0048] (2) Preparation of negative electrode sheet

[0049] Graphite, dispersant CMC (solid content 1.3%), binder SBR (solid content 40%), conductive agent SP, and pore-forming agent (cage-type polysilsesquioxane shown in Formula 1) were dispersed in deionized water to form a slurry. The solid content of the slurry was 43%. The mass ratio of graphite, dispersant CMC (calculated on a dry basis), binder SBR (calculated on a dry basis), conductive agent SP, and pore-forming agent was 96.5%-Z%:1.5%:1.5%:0.5%:Z%. The slurry was coated on the surface of copper foil, dried, and roller-pressed. The surface density was 11.1 mg / cm 2 , compacted density is 1.82g / cm 3 , the battery cell CB value is about 1.06.

[0050] (3) Preparation of electrolyte

[0051] In a glove box filled with inert gas (argon) (H2O <0.1ppm, O2 <0.1ppm), EC (ethylene carbonate), EMC (ethyl methyl carbonate), DEC (diethyl carbonate), and FEC (fluorinated ethylene carbonate) were prepared in a mass ratio of 20:50:20:10. Then, fully dried lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) were quickly added thereto with mass fractions of 11.4% and 3.1% in the system, respectively. They were dissolved in a non-aqueous organic solvent and stirred evenly. After passing the moisture and free acid tests, the basic electrolyte was obtained.

[0052] (4) Preparation of isolation membrane

[0053] 8μm thick (5+3) Zhuogao mixed coating diaphragm is selected.

[0054] (5) Preparation of lithium-ion batteries

[0055] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in order, ensuring that the separator is between the positive and negative electrode sheets to play an isolating role, and then a bare cell without liquid injection is obtained by winding; the bare cell is placed in an outer packaging foil, and the corresponding electrolyte prepared above is injected into the dried bare cell. After vacuum packaging, standing, formation, shaping, sorting and other processes, the corresponding lithium-ion battery is obtained.

[0056] Comparative Example 1 and Examples 1 to 16

[0057] Comparative Example 1: No pore former was used in the negative electrode preparation process (Z%=0%), and the battery cell was prepared as described above.

[0058] Examples 1-8: In the negative electrode preparation process, a cage-type polysilsesquioxane shown in Formula I is used as a pore-forming agent, wherein R is -(CH2)6-COOH, and is named as pore-forming agent 1; the mass proportion Z% of pore-forming agent 1 in the negative electrode is: 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 5%, and 10%, respectively, and the battery cell is prepared as described above.

[0059] Example 9-16: In the negative electrode preparation process, a cage-type polysilsesquioxane of formula I is used as a pore-forming agent, wherein R is -(CH2)6-R2, and R2 is selected from It is named as pore-forming agent 2; the mass proportion Z% of pore-forming agent 2 in the negative electrode is: 0.01%, 0.05%, 0.1%, 0.5%, 1%, 2%, 5%, and 10%, respectively. The battery cell is prepared as described above.

[0060] Porosity of electrode and cycle life of lithium-ion battery after injection:

[0061] (1) After the battery cell is filled with liquid and sealed, it is left to stand for 48 hours to allow the pore-forming agent to fully dissolve in the electrolyte. The battery cell is disassembled and the negative electrode sheet is immersed in an electrolyte solvent without lithium salt (EC (ethylene carbonate), EMC (ethyl methyl carbonate), DEC (diethyl carbonate), FEC (fluorinated ethylene carbonate) with a mass ratio of 20:50:20:10) for 30 minutes and washed three times to fully remove the pore-forming agent and lithium salt in the negative electrode sheet. The cell is vacuum dried at room temperature for 48 hours. After fully removing the electrolyte solvent, the porosity of the negative electrode sheet is measured (BET method).

[0062] (2) 25℃ cycle test: After the OCV is measured, the battery with 50% SOC is tested for voltage, internal resistance and thickness T1 when it is received. Then the battery is placed in a constant temperature environment at 25℃ and charged to 4.2V at 2C constant current, then charged to 4.5V at 0.7C constant current and constant voltage, with a cut-off current of 0.05C, and then discharged to 3V at 0.5C. The charge and discharge cycle is repeated for 1000 cycles. The cycle discharge capacity is recorded and divided by the discharge capacity of the first cycle to obtain the normal temperature cycle capacity retention rate. After the cycle is completed for 100 / 400 / 800 cycles, the fully charged battery is taken out of the 25℃ constant temperature box and immediately tested for its fully charged thickness T2 after 100 / 400 / 800 cycles. The cycle capacity retention rate at the 100th / 400 / 800th cycle and the cycle thickness expansion rate at the 100th / 400 / 800th cycle of the battery are recorded respectively, as shown in Table 2.

[0063] Here, thickness expansion ratio (%) = (T2-T1) / T1×100%.

[0064] Table 1 Lithium ion battery information and cycle performance test results of Comparative Example 1 and Examples 1-16

[0065]

[0066]

[0067] Among them, the negative electrode sheet of Comparative Example 1 does not contain a pore former, while Examples 1-8 and Examples 9-16 use pore former 1 and pore former 2 in the negative electrode ingredients, respectively. It can be seen from Comparative Example 1 that without using a pore former, the negative electrode sheet has a porosity of 35% when disassembled 48 hours after injection, and the capacity retention rates of 100T, 400T and 800T cycles at a high voltage of 4.5V are 97.1%, 91.0% and 80.6% respectively. After adding pore former 1 and pore former 2, as the pore former content increases, the porosity of the negative electrode sheet will continue to increase when disassembled 48 hours after injection. When the pore former dosage is 2%, the improvement effect on the battery capacity retention rate is better. As the content continues to increase, it may cause problems such as clogging of the diaphragm pores and increase the viscosity of the electrolyte, which is not conducive to lithium ion transmission. Therefore, the most suitable amount of pore former addition is around 2%.

[0068] In summary, it can be seen that the lithium-ion battery using the solution of the present invention has good cycle stability by adding a pore-forming agent, and shows extremely high application value.

[0069] The above describes the embodiments of the present invention. However, the present invention is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the scope of protection of the present invention.

Claims

1. A negative electrode sheet, characterized in that: The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer coated on one side or both sides of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material, a conductive agent, a binder and a cage-type polysilsesquioxane; The molecular formula of the cage-type polysilsesquioxane is (RSiO 1.5 )n, wherein n is any one of 6, 8, 10 or 12; R is *-(CH2) x -R1; wherein the * end is connected to the silicon atom, x is an integer between 1 and 12, and R1 is selected from carboxyl (-COOH), nitrile (-CN), pyridine, imidazole or thiazole.

2. The negative electrode sheet according to claim 1, characterized in that: The connection mode of pyridine is The connection mode of imidazole is The connection mode of thiazole is The wavy lines represent connecting lines.

3. The negative electrode sheet according to claim 2, characterized in that: The cage-type polysilsesquioxane has a structure as shown in Formula 1: In Formula 1, R is as defined above.

4. The negative electrode sheet according to claim 1, characterized in that: The median particle size of the cage-type polysilsesquioxane is 1 nm to 5 nm.

5. The negative electrode sheet according to any one of claims 1 to 4, characterized in that: The negative electrode active material layer comprises the following components in percentage by weight: 85-98.99 wt% of negative electrode active material, 0.5-5 wt% of conductive agent, 0.5-5 wt% of binder, and 0.01-5 wt% of cage-type polysilsesquioxane.

6. A battery, characterized in that: The battery comprises the negative electrode sheet according to any one of claims 1 to 5.

7. The battery according to claim 6, characterized in that The battery further comprises a positive electrode and an electrolyte; the electrolyte comprises the cage-type polysilsesquioxane according to any one of claims 1 to 5.

8. The battery according to claim 6, characterized in that The charging cut-off voltage of the battery is above 4.5V.

9. A use of a cage-type polysilsesquioxane as a pore-forming agent in a negative electrode sheet, wherein the cage-type polysilsesquioxane has the general molecular formula (RSiO 1.5 )n, where n is any one of 6, 8, 10 or 12; R is *-(CH2) x -R1; Wherein, the * end is connected to the silicon atom, x is an integer between 1 and 12, and R1 is selected from carboxyl (-COOH), nitrile (-CN), pyridine, imidazole or thiazole.

10. The use according to claim 9, characterized in that The single surface density of the negative electrode sheet is 1 to 25 mg / cm 2 ; And / or, the compaction density of the negative electrode sheet is 1.60 to 1.85 g / cm 3 .

11. The use according to claim 9, characterized in that The connection mode of pyridine is The connection mode of imidazole is The connection mode of thiazole is The wavy lines represent connecting lines.

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