Artificial SEI film suitable for silicon-based negative electrode material and preparation method and application thereof

Abstract

The invention provides an artificial SEI film suitable for a silicon-based negative electrode material and a preparation method and application thereof. The material of the artificial SEI membrane comprises a polymer, and the structure of the polymer comprises a combination of a triazole ring and a pyrazine ring. The artificial SEI membrane suitable for the silicon-based negative electrode material provided by the invention has high elastic deformation capability and good ion conduction capability and interface bonding capability, so that the cycle life of a silicon-based secondary battery is prolonged, and the quick charge performance of the silicon-based secondary battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and in particular relates to an artificial SEI film suitable for silicon-based negative electrode materials, a preparation method thereof, and an application thereof. Background Art

[0002] With the rapid development of mobile electronic devices, electric vehicles, and grid energy storage, the development of lithium-ion batteries with high energy density, high power density, long cycle life, and high safety has become a research hotspot in the field of energy storage today.

[0003] Traditional graphite negative electrodes have gradually failed to meet the needs of high-performance batteries, which has prompted researchers to look for more advantageous alternative materials. Among them, silicon-based negative electrode materials are considered to be ideal negative electrode materials for the next generation of high-energy-density lithium-ion batteries due to their extremely high theoretical specific capacity (about 4200mAh / g) and abundant reserves. However, silicon-based negative electrode materials will undergo significant volume expansion during the lithiation / delithiation process, with an expansion rate of up to 300% to 400%. This drastic volume change will lead to destruction of the electrode structure, particle pulverization and SEI film rupture, thereby reducing the cycle life and coulombic efficiency of lithium-ion batteries. In addition, the interface between silicon-based negative electrode materials and the electrolyte is unstable, resulting in continuous rupture and reconstruction of the SEI film during the cycle, continuous consumption of active lithium, and ultimately resulting in capacity loss.

[0004] At present, although the existing technology discloses that the use of coating technology can improve the cycle performance of silicon-based negative electrode materials to a certain extent, most of the SEI membrane materials used have disadvantages such as insufficient flexibility, poor ionic conductivity and weak binding ability, and therefore cannot effectively deal with problems such as volume expansion and insufficient interface stability of silicon-based negative electrode materials.

[0005] Therefore, in this field, it is necessary to develop an SEI membrane material with good flexibility, high ionic conductivity and strong bonding ability with silicon-based negative electrode materials. This has also become the key to solving the application bottleneck of silicon-based negative electrode materials. Summary of the Invention

[0006] To address the shortcomings of the prior art, the present invention aims to provide an artificial SEI membrane suitable for silicon-based anode materials, as well as its preparation method and application. The artificial SEI membrane suitable for silicon-based anode materials provided by the present invention exhibits high elastic deformation capability, good ion conductivity, and interfacial bonding ability, thereby improving the cycle life and fast-charging performance of silicon-based secondary batteries.

[0007] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides an artificial SEI film suitable for silicon-based negative electrode materials, wherein the material of the artificial SEI film comprises a polymer, and the structure of the polymer comprises a combination of a triazole ring and a pyrazine ring.

[0009] The polymer artificial SEI film provided by the present invention not only has good compatibility with silicon-based negative electrode materials and electrolytes, but also can adaptively adjust the volume change of the negative electrode, thereby comprehensively improving the interface stability and ion transfer rate. Among them, the polymer has a specific chemical structure, which can interact with the ions in the electrolyte, optimize the interface structure between the electrolyte and the silicon-based negative electrode material, thereby reducing the interface resistance between the two and increasing the transmission rate of lithium ions. In addition, the polymer artificial SEI film can also effectively reduce the volume expansion of the silicon-based negative electrode material during the charging and discharging process, avoid the pulverization and shedding of the negative electrode material, and thus significantly improve the cycle stability and fast charging performance of the silicon-based negative electrode material.

[0010] Specifically, the triazole ring and pyrazine ring groups contained in the polymer monomer structure can preferentially form a film on one side of the silicon-based negative electrode material during the secondary battery formation process, and the formed polymer artificial SEI film is stable, dense, flexible, highly elastic, and not easy to break. It can change accordingly with the volume change of the silicon-based negative electrode material, thereby improving the volume expansion of silicon during the repeated charge and discharge of the secondary battery. Among them, the reduction product of the nitrogen-rich triazole ring group can make the SEI film more uniform, dense and stable, and ultimately further improve the interface stability between the electrolyte and the silicon-based negative electrode material. The nitrogen atoms in the pyrazine ring can coordinate with the lithium ions in the electrolyte, optimize the transmission path of the lithium ions, and thus reduce the interface resistance.

[0011] Preferably, the triazole ring in the polymer structure comprises an electron-donating group.

[0012] Preferably, the electron donating group comprises a thiol group.

[0013] In the present invention, the sulfur atom in the thiol group has electron-donating properties, which can form a coordination structure with the unoccupied 2s orbital of lithium ions, ultimately significantly improving the lithium ion conductivity of the artificial SEI film.

[0014] Preferably, the monomers forming the polymer include a compound containing a combination of at least one carbon-carbon double bond, at least one triazole ring, and at least one pyrazine ring.

[0015] Preferably, the structure of the monomer forming the polymer further includes at least one electron-donating group.

[0016] Preferably, the electron donating group comprises a thiol group.

[0017] Preferably, the monomers forming the polymer are selected from 4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol.

[0018] The present invention further improves the overall performance of the artificial SEI membrane by preferentially selecting a 4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol polymer monomer. In this monomer structure, the thiol group on the triazole ring is adjacent to the unsaturated bond, and the pyrazine ring and triazole ring are in a suitable conjugated position. This facilitates electron delocalization and transfer, thereby enhancing the interaction between the artificial SEI membrane and the electrolyte, thereby improving interfacial stability and ion transport efficiency.

[0019] Preferably, the number average molecular weight of the polymer is 5000Da to 20000Da, preferably 7000Da to 18000Da, for example, it can be 5000Da, 6000Da, 7000Da, 8000Da, 9000Da, 10000Da, 12000Da, 15000Da, 18000Da or 20000Da, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0020] By regulating the number-average molecular weight of the polymer, the present invention balances the structural stability of the artificial SEI membrane and the lithium ion transfer rate. Using a polymer with a lower number-average molecular weight results in poor film-forming properties, further weakening the stability of the artificial SEI membrane and shortening the battery's cycle life. Using a polymer with a higher number-average molecular weight increases the viscosity of the polymer solution, hindering uniform coating and potentially leading to a decrease in lithium ion transfer efficiency and a corresponding increase in interfacial resistance, ultimately affecting the electrochemical performance of the assembled secondary battery.

[0021] In a second aspect, the present invention provides a method for preparing the artificial SEI film suitable for silicon-based negative electrode materials according to the first aspect, the method comprising the following steps:

[0022] A monomer comprising at least one carbon-carbon double bond, at least one triazole ring and at least one pyrazine ring, an initiator and an organic solvent are mixed, and after a polymerization reaction, the artificial SEI film suitable for silicon-based negative electrode materials is obtained.

[0023] Preferably, the polymerization reaction is carried out under an inert atmosphere.

[0024] In the present invention, the inert atmosphere illustratively includes argon and / or nitrogen.

[0025] Preferably, the polymerization reaction temperature is 60°C to 90°C, for example, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C or 90°C, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0026] Preferably, the polymerization reaction time is 5 h to 12 h, for example, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h or 12 h, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0027] In the present invention, the initiator illustratively includes at least one of azobisisobutyronitrile, azobisisoheptanenitrile, or dibenzoyl peroxide (BPO). Furthermore, based on the total mass of the monomers as 100%, the mass percentage of the initiator is 0.5% to 1%, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%, etc., and is not limited to the values ​​listed above. Other values ​​not listed within this numerical range are also applicable.

[0028] In the present invention, the organic solvent illustratively includes at least one of benzene, tetrahydrofuran (THF), N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0029] In the present invention, after the polymerization reaction, the following steps are further included: adding the reaction solution after the polymerization reaction into a precipitation solvent to obtain a polymer precipitate, and then washing and drying the polymer precipitate to obtain the polymer.

[0030] In the present invention, the precipitation solvent illustratively includes at least one of propanol, isopropanol or acetone.

[0031] In a third aspect, the present invention provides a silicon-based negative electrode material, comprising a silicon substrate and an artificial SEI film layer coated on the surface of the silicon substrate, wherein the artificial SEI film layer comprises the artificial SEI film suitable for silicon-based negative electrode materials according to the first aspect.

[0032] Preferably, the material of the silicon substrate includes silicon-carbon material.

[0033] Preferably, the average particle size of the silicon-carbon material is 3μm to 9μm, for example, it can be 3μm, 3.2μm, 3.5μm, 3.8μm, 4μm, 4.2μm, 4.5μm, 4.8μm, 5μm, 5.2μm, 5.5μm, 5.8μm, 6μm, 6.2μm, 6.5μm, 6.8μm, 7μm, 7.2μm, 7.5μm, 7.8μm, 8μm, 8.2μm, 8.5μm, 8.8μm or 9μm, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0034] The present invention facilitates subsequent processing and improves the ion transport performance of silicon-based anode materials by regulating the average particle size of the silicon-carbon material. Using a silicon-carbon material with a smaller average particle size makes it difficult to disperse the particles during the subsequent homogenization process. Using a silicon-carbon material with a larger average particle size degrades the ion transport performance of the resulting silicon-based anode material, and the rate performance of the resulting secondary battery deteriorates accordingly.

[0035] Preferably, the specific surface area of ​​the silicon-carbon material is 1 m 2 / g~8m 2 / g, for example, 1m 2 / g, 1.2m 2 / g, 1.5m 2 / g, 1.8m 2 / g, 2m 2 / g, 2.2m 2 / g, 2.5m 2 / g, 2.8m 2 / g、3m 2 / g, 3.2m 2 / g, 3.5m 2 / g, 3.8m 2 / g、4m 2 / g, 4.2m 2 / g, 4.5m 2 / g, 4.8m 2 / g、5m 2 / g, 5.2m 2 / g, 5.5m 2 / g, 5.8m 2 / g、6m 2 / g, 6.2m 2 / g, 6.5m 2 / g, 6.8m 2 / g、7m 2 / g, 7.2m 2 / g, 7.5m 2 / g、7.8m 2 / g or 8m2 / g, etc., are not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0036] Preferably, the mass percentage of silicon material in the silicon-carbon material is 43% to 59%, for example, it can be 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58% or 59%, etc., and is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0037] By regulating the mass percentage of silicon in the silicon-carbon material, the present invention enables the silicon-carbon material to achieve better electrochemical performance at a high silicon content. Using a silicon-carbon material with a lower silicon content would result in a lower gram capacity, failing to meet the requirements of high-energy-density batteries. Using a silicon-carbon material with a higher silicon content would result in significant volume expansion.

[0038] Preferably, based on the total mass of the silicon-based negative electrode material as 100%, the mass percentage of the artificial SEI film layer is 1% to 6%, for example, it can be 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.2%, 4.5%, 4.8%, 5%, 5.2%, 5.5%, 5.8% or 6%, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0039] The present invention regulates the mass percentage of the material of the artificial SEI film layer to form a coating layer with good flexibility, thereby further optimizing the interface stability and ion conductivity, and reducing the occurrence of electrolyte decomposition and side reactions.

[0040] Preferably, the thickness of the artificial SEI film layer is 30 nm to 80 nm, for example, it can be 30 nm, 32 nm, 35 nm, 38 nm, 40 nm, 42 nm, 45 nm, 48 nm, 50 nm, 52 nm, 55 nm, 58 nm, 60 nm, 62 nm, 65 nm, 68 nm, 70 nm, 72 nm, 75 nm, 78 nm or 80 nm, etc., but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0041] The present invention achieves comprehensive improvements in the cycle and rate performance of the assembled secondary battery by regulating the thickness of the artificial SEI membrane to a moderate level, ensuring both good structural stability and high ionic conductivity. A thinner artificial SEI membrane is more susceptible to mechanical stress rupture, exposing new active surfaces and further accelerating the formation and consumption of the SEI membrane. A thicker artificial SEI membrane significantly increases the length of the lithium ion transmission path, increasing the lithium ion transfer impedance and subsequently reducing the cycle and rate performance of the assembled secondary battery.

[0042] The present invention also provides a method for preparing the silicon-based negative electrode material, which comprises the following steps:

[0043] The silicon-based negative electrode material is obtained by mixing a silicon substrate and a solution containing the artificial SEI film suitable for the silicon-based negative electrode material according to the first aspect, and spray drying the mixture.

[0044] Preferably, the mass concentration of the solution containing the artificial SEI film suitable for silicon-based negative electrode materials according to the first aspect is 5% to 25%, for example, it can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24% or 25%, etc., not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0045] In the present invention, the solution containing the artificial SEI film suitable for silicon-based negative electrode materials according to the first aspect includes an artificial SEI film suitable for silicon-based negative electrode materials and a solvent, and the solvent exemplarily includes at least one of benzene, tetrahydrofuran (THF), N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0046] Preferably, the mixing temperature is 60°C to 100°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C or 100°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0047] Preferably, the mixing time is 5 h to 10 h, for example, 5 h, 6 h, 7 h, 8 h, 9 h or 10 h, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0048] Preferably, the inlet temperature of the spray drying is 130°C to 200°C, for example, it can be 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C or 200°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0049] Preferably, the outlet temperature of the spray drying is 60°C to 100°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C or 100°C, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0050] In a fourth aspect, the present invention provides a secondary battery comprising a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises the silicon-based negative electrode material according to the third aspect.

[0051] In the present invention, the lithium salt in the electrolyte can be selected from the types of lithium salts commonly used in the art, for example, it can be at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroborate (LiBF6), lithium bis(difluorosulfonyl imide) (LiFSI), lithium bis(oxalatoborate) (LiBOB) or lithium bis(trifluoromethanesulfonyl imide) (LiTFSI).

[0052] In the present invention, the electrolyte additive includes vinylene carbonate (VC). The polymer artificial SEI membrane and the vinylene carbonate additive in the present invention have a synergistic effect, which can promote the formation of a more stable SEI film with an appropriately thick layer, thereby effectively reducing the interfacial resistance and improving the transmission efficiency of lithium ions.

[0053] Furthermore, the mass percentage of vinylene carbonate in the electrolyte is 0.2% to 0.8%, for example, it can be 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75% or 0.8%, etc., and is not limited to the listed values. Other values ​​not listed within this numerical range are also applicable.

[0054] In the present invention, the solvent in the electrolyte includes a nitrile solvent (such as acetonitrile, etc.), which can interact with the pyrazine ring and triazole ring in the polymer artificial SEI membrane structure, thereby improving the density and stability of the SEI membrane, thereby effectively inhibiting the decomposition reaction of the electrolyte and improving the cycle stability of the secondary battery.

[0055] Furthermore, the volume percentage of the nitrile solvent in the solvent is 5% to 15%, for example, it can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%, etc., and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0056] Compared with the prior art, the present invention has the following beneficial effects:

[0057] The present invention provides an artificial SEI film suitable for silicon-based negative electrode materials, which not only has good compatibility with silicon-based negative electrode materials and electrolytes, but also can adaptively adjust the volume change of the negative electrode, thereby comprehensively improving the interface stability and ion transmission rate. Among them, the polymer has a specific chemical structure, which can interact with the ions in the electrolyte, optimize the interface structure between the electrolyte and the silicon-based negative electrode material, thereby reducing the interface resistance between the two and increasing the transmission rate of lithium ions. In addition, the polymer artificial SEI film can also effectively reduce the volume expansion of the silicon-based negative electrode material during the charging and discharging process, avoid the pulverization and shedding of the negative electrode material, and thus significantly improve the cycle stability and fast charging performance of the silicon-based negative electrode material.

[0058] Specifically, the triazole ring and pyrazine ring groups contained in the polymer monomer structure can preferentially form a film on one side of the silicon-based negative electrode material during the secondary battery formation process, and the formed polymer artificial SEI film is stable, dense, flexible, highly elastic, and not easy to break. It can change accordingly with the volume change of the silicon-based negative electrode material, thereby improving the volume expansion of silicon during the repeated charge and discharge of the secondary battery. Among them, the reduction product of the nitrogen-rich triazole ring group can make the SEI film more uniform, dense and stable, and ultimately further improve the interface stability between the electrolyte and the silicon-based negative electrode material. The nitrogen atoms in the pyrazine ring can coordinate with the lithium ions in the electrolyte, optimize the transmission path of the lithium ions, and thus reduce the interface resistance. DETAILED DESCRIPTION

[0059] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0060] Example 1

[0061] This embodiment provides an artificial SEI film suitable for a silicon-carbon negative electrode material. The artificial SEI film is coated on the surface of the silicon-carbon material, thereby obtaining a silicon-carbon negative electrode material.

[0062] The material of the artificial SEI film includes poly-4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol with a number average molecular weight of 12,000 Da. The thickness of the artificial SEI film is 55 nm. Based on the total mass of the silicon-carbon negative electrode material as 100%, the mass percentage of the artificial SEI film layer is 3.5%. The average particle size of the silicon-carbon material is 6 μm, and the specific surface area is 4.5 m 2 / g, and the mass percentage of silicon material in the silicon-carbon material is 51%.

[0063] This embodiment provides a method for preparing the artificial SEI film suitable for silicon-carbon negative electrode materials and the silicon-carbon negative electrode materials, which comprises the following steps:

[0064] 4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol was added to tetrahydrofuran solvent, and then azobisisobutyronitrile initiator was added, and the mixture was heated to 75° C. under the protection of argon atmosphere to carry out polymerization reaction for 8 hours to obtain a reaction solution;

[0065] The reaction solution is added to a propanol solvent to precipitate a polymer, and the polymer precipitate is washed and dried to obtain a polymer;

[0066] The polymer was dissolved in tetrahydrofuran solvent to obtain a polymer solution with a mass concentration of 15%. The silicon-carbon material was added to the polymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 165°C and an outlet temperature of 80°C to obtain a silicon-carbon negative electrode material.

[0067] Example 2

[0068] This embodiment provides an artificial SEI film suitable for a silicon-carbon negative electrode material. The artificial SEI film is coated on the surface of the silicon-carbon material, thereby obtaining a silicon-carbon negative electrode material.

[0069] The material of the artificial SEI film includes poly-4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol with a number average molecular weight of 15,000 Da. The thickness of the artificial SEI film is 65 nm. Based on the total mass of the silicon-carbon negative electrode material as 100%, the mass percentage of the artificial SEI film layer is 5%. The average particle size of the silicon-carbon material is 6 μm, and the specific surface area is 4.5 m 2 / g, and the mass percentage of silicon material in the silicon-carbon material is 56%.

[0070] This embodiment provides a method for preparing the artificial SEI film suitable for silicon-carbon negative electrode materials and the silicon-carbon negative electrode materials, which comprises the following steps:

[0071] 4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol was added to tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 85° C. under an argon atmosphere to carry out a polymerization reaction for 7 hours to obtain a reaction solution;

[0072] The reaction solution is added to a propanol solvent to precipitate a polymer, and the polymer precipitate is washed and dried to obtain a polymer;

[0073] The polymer was dissolved in tetrahydrofuran solvent to obtain a polymer solution with a mass concentration of 17%. The silicon-carbon material was added to the polymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 165°C and an outlet temperature of 80°C to obtain a silicon-carbon negative electrode material.

[0074] Example 3

[0075] This embodiment provides an artificial SEI film suitable for a silicon-carbon negative electrode material. The artificial SEI film is coated on the surface of the silicon-carbon material, thereby obtaining a silicon-carbon negative electrode material.

[0076] The material of the artificial SEI film includes poly-4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol with a number average molecular weight of 10,000 Da. The thickness of the artificial SEI film is 40 nm. Based on the total mass of the silicon-carbon negative electrode material as 100%, the mass percentage of the artificial SEI film layer is 2.5%. The average particle size of the silicon-carbon material is 6 μm, and the specific surface area is 4.5 m 2 / g, and the mass percentage of silicon material in the silicon-carbon material is 44%.

[0077] This embodiment provides a method for preparing the artificial SEI film suitable for silicon-carbon negative electrode materials and the silicon-carbon negative electrode materials, which comprises the following steps:

[0078] 4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol was added to tetrahydrofuran solvent, and then azobisisobutyronitrile was added as an initiator. The mixture was heated to 70° C. under an argon atmosphere to carry out a polymerization reaction for 10 hours to obtain a reaction solution.

[0079] The reaction solution is added to a propanol solvent to precipitate a polymer, and the polymer precipitate is washed and dried to obtain a polymer;

[0080] The polymer was dissolved in tetrahydrofuran solvent to obtain a polymer solution with a mass concentration of 10%. The silicon-carbon material was added to the polymer solution and stirred at 80°C for 7 hours to obtain a mixed solution. The mixed solution was spray-dried at an inlet temperature of 165°C and an outlet temperature of 80°C to obtain a silicon-carbon negative electrode material.

[0081] Example 4

[0082] The difference between this embodiment and embodiment 1 is that, based on the total mass of the silicon-carbon negative electrode material being 100%, the mass percentage of the artificial SEI film layer is adjusted to 10%, and the rest is the same as embodiment 1.

[0083] Example 5

[0084] The difference between this embodiment and embodiment 1 is that the number average molecular weight of the polymer is 2000 Da, wherein the change of the number average molecular weight of the polymer is achieved by adjusting the temperature and time of the polymerization reaction. The rest is the same as embodiment 1.

[0085] Example 6

[0086] The difference between this embodiment and embodiment 1 is that the number average molecular weight of the polymer is 50,000 Da, wherein the change of the number average molecular weight of the polymer is achieved by adjusting the temperature and time of the polymerization reaction. The rest is the same as embodiment 1.

[0087] Comparative Example 1

[0088] The difference between this comparative example and Example 1 is that the silicon-carbon material is not subjected to artificial SEI film coating treatment, and only one silicon-carbon material is provided, and the average particle size, specific surface area and mass percentage of the provided silicon-carbon material are the same as those in Example 1.

[0089] Comparative Example 2

[0090] The difference between this comparative example and Example 1 is that 4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol is replaced by an equal amount of 4-allyl-5-methyl-4H-1,2,4-triazole-3-thiol monomer (CAS No.: 6232-84-4), and the rest are the same as Example 1.

[0091] Comparative Example 3

[0092] The difference between this comparative example and Example 1 is that 4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol is replaced by an equal amount of 2-mercaptopyrazine monomer (CAS No.: 38521-06-1), and the rest are the same as Example 1.

[0093] Application Examples 1-6 and Comparative Application Examples 1-3

[0094] The silicon-carbon negative electrode materials provided in Examples 1 to 6 and Comparative Examples 1 to 3 were used to prepare negative electrode sheets, and then assembled to obtain lithium-ion batteries. The specific preparation method is as follows:

[0095] Preparation of negative electrode sheet:

[0096] The silicon-carbon negative electrode material, conductive carbon black Super-P, single-walled carbon nanotubes (SWCNTs) and polyacrylic acid binder (PAA) provided in the above embodiments and comparative examples were mixed and stirred uniformly with water in a mass ratio of 80:9:1:10 to obtain a negative electrode slurry, and the solid content was controlled to be 30%. The negative electrode slurry was then coated on a copper foil current collector through a coating process, and then vacuum dried and cold pressed to obtain a negative electrode sheet.

[0097] Preparation of positive electrode:

[0098] The ternary cathode material NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 The active material O2, polyvinylidene fluoride binder (PVDF) and conductive carbon black Super-P are mixed and stirred evenly with N-methylpyrrolidone solvent in a mass ratio of 96:2:2 to obtain a positive electrode slurry, and then the positive electrode slurry is coated on an aluminum foil through a coating process, and then dried and cold pressed to obtain a positive electrode sheet.

[0099] Electrolyte:

[0100] Ethylene carbonate, dimethyl carbonate, diethyl carbonate, fluoroethylene carbonate, and acetonitrile are mixed in a volume ratio of 20:40:20:10:10 to obtain an organic solvent. Thoroughly dried lithium salt LiPF6 is then dissolved in the organic solvent to prepare an electrolyte having a lithium salt concentration of 1 mol / L. Furthermore, the electrolyte also includes a vinylene carbonate additive, with the weight percentage of the vinylene carbonate additive being 0.5% based on the total weight of the electrolyte being 100%.

[0101] Preparation of lithium-ion batteries:

[0102] The positive electrode sheet, separator (including a polyethylene film and a ceramic coating on one side of the polyethylene film), and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrodes to provide isolation. The sheets are then wound to form a bare cell. The bare cell is then placed in an outer packaging shell, dried, and then injected with electrolyte. The lithium-ion battery is then produced through vacuum packaging, resting, forming, and shaping processes.

[0103] Comparative Application Example 4

[0104] The difference between this comparative application example and application example 1 is that no vinylene carbonate additive is added, and the content of the organic solvent is adaptively adjusted so that the total content of the electrolyte system is 100%. The rest is the same as application example 1.

[0105] Comparative Application Example 5

[0106] The difference between this comparative application example and application example 1 is that the acetonitrile solvent is replaced with an equal volume of fluoroethylene carbonate solvent, and the rest is the same as application example 1.

[0107] Test conditions

[0108] The lithium-ion batteries provided in Application Examples 1 to Application Examples 6 and Comparative Application Examples 1 to Comparative Application Examples 5 were subjected to performance tests. The rate performance of the batteries was tested under lithium-ion battery test conditions. The tests were conducted on the LAND battery test system of Wuhan Jinnuo Electronics Co., Ltd. at room temperature (25°C). The charge and discharge voltages were limited to 2.5V to 4.2V. The test conditions are as follows:

[0109] (1) First Coulombic efficiency

[0110] At 25°C, the lithium-ion battery was charged to 4.2V at a constant current and constant voltage rate of 0.33C and allowed to stand for 10 minutes. Then, the lithium-ion battery was discharged to 2.5V at a constant current rate of 0.33C and allowed to stand for 10 minutes. The first coulombic efficiency of the lithium-ion battery was calculated.

[0111] First coulombic efficiency (%)=(first discharge total capacity of the lithium-ion battery at a rate of 0.33C / first charge total capacity of the lithium-ion battery at a rate of 0.33C)×100%.

[0112] (2) Capacity retention after 1000 cycles at 1C / 2C at room temperature

[0113] At 25°C, the lithium-ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C, and allowed to stand for 10 minutes. Then, the lithium-ion battery is discharged to 2.5V at a constant current rate of 2C and allowed to stand for 10 minutes. This is considered one charge and discharge cycle. The lithium-ion battery is charged and discharged 1200 times according to the above method. The capacity retention rate of the lithium-ion battery after 1000 charge and discharge cycles at 1C / 2C is calculated.

[0114] Capacity retention rate (%) of a lithium-ion battery after N cycles = (discharge capacity at the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery.

[0115] (3) Room temperature 6C rate performance - constant current charging ratio

[0116] At 25°C, the lithium-ion battery was discharged at a constant current rate of 1C to 2.5V and allowed to stand for 10 minutes. The lithium-ion battery was then charged at a constant current and constant voltage rate of 6C to 4.2V with a cut-off current of 0.05C and allowed to stand for 10 minutes. The constant current charging capacity Q1 and the total constant current and constant voltage charging capacity Q2 of the lithium-ion battery were recorded. The constant current charging ratio of the 6C rate charging was calculated according to the following formula: 6C rate charging constant current charging ratio = (constant current charging capacity Q1 / total constant current and constant voltage charging capacity Q2) × 100%.

[0117] (4) 1C / 8C discharge capacity retention rate at room temperature

[0118] At 25°C, the divided lithium-ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C; it is left to stand for 10 minutes; then the lithium-ion battery is discharged to 2.5V at a constant current rate of 1C, and its discharge capacity Q is recorded. 1C As the initial discharge capacity; then at 25 ° C, the lithium ion battery is charged to 4.2V at a constant current and constant voltage rate of 1C, with a cut-off current of 0.05C; let it stand for 10 minutes; then the fully charged lithium ion battery is discharged to 2.5V at a constant current rate of 8C, and its discharge capacity Q is recorded. 8C ; Calculate the discharge capacity retention rate (%) of lithium-ion batteries at 1C / 8C rate = (discharge capacity Q at 8C rate) 8C / Discharge capacity Q at 1C rate 1C )×100%.

[0119] (5) Initial expansion rate of negative electrode

[0120] ① Before assembling the lithium-ion battery, use a micrometer to measure the initial thickness of the negative electrode sheet and record it as h1. The thickness of the negative electrode current collector is recorded as h0.

[0121] ② Fully charged: At 25°C, after assembling the battery cells, charge the lithium-ion battery at a constant current and constant voltage rate of 0.33C to 4.2V, with a cut-off current of 0.05C, and let it stand for 120 minutes;

[0122] ③ Disassemble the fully charged lithium-ion battery to obtain the negative electrode sheet, clean the negative electrode sheet with dimethyl carbonate, and measure the thickness of the cleaned negative electrode sheet, which is recorded as h2;

[0123] ④The initial expansion rate of the negative electrode is: (h2-h1) / (h1-h0)×100%.

[0124] The test results are shown in Table 1:

[0125] Table 1

[0126]

[0127]

[0128] It can be seen from Table 1 that compared with Comparative Application Example 1, the polymer artificial SEI film provided by Application Examples 1 to 3 of the present invention can not only optimize the interface structure between the electrolyte and the silicon-carbon negative electrode material, thereby reducing the interface resistance between the two, but also effectively reduce the volume expansion of the silicon-carbon negative electrode material during the charging and discharging process, avoid the pulverization and shedding of the negative electrode material, and thus significantly improve the cycle stability and fast charging performance of the silicon-carbon negative electrode material.

[0129] By comparing Application Examples 1 to 4, it can be seen that the present invention achieves a better coating effect by regulating the mass percentage of the artificial SEI film layer, thereby optimizing the interface stability and ion conductivity, and reducing the occurrence of electrolyte decomposition and side reactions.

[0130] Comparing Application Examples 1, 5, and 6, we can see that the number-average molecular weight of the polymer has a significant impact on the film-forming properties, interfacial stability, ion transport properties, and volume expansion suppression of the artificial SEI film. If the number-average molecular weight of the polymer is low, film formation is not conducive, and the artificial SEI film's ability to suppress silicon particle volume expansion is weakened, resulting in poor cycle performance of the assembled lithium-ion battery. If the number-average molecular weight of the polymer is high, lithium-ion transport efficiency may decrease and interfacial resistance may increase, ultimately affecting the fast-charging performance of the assembled lithium-ion battery.

[0131] Comparison of Application Examples 1, 2, and 3 demonstrates that a single group cannot achieve the full benefits of the present invention, demonstrating a synergistic effect between the triazole and pyrazine rings within the polymer structure. By optimizing the structure of the polymerized monomers, the present invention further enhances the interfacial stability and ion transport properties of the artificial SEI membrane, thereby improving the overall performance of lithium-ion batteries.

[0132] Comparison of Application Examples 1, 4, and 5 demonstrates that the polymer artificial SEI membrane has a synergistic effect with specific additives and solvents in the electrolyte. By optimizing the electrolyte composition, the present invention promotes the formation of a more stable, dense, and appropriately thick SEI membrane, thereby effectively inhibiting electrolyte decomposition reactions, reducing interfacial resistance, and improving the cycling and fast-charging performance of lithium-ion batteries.

[0133] The applicant states that while the above-described embodiments illustrate the process of the present invention, the present invention is not limited to the above-described process steps, nor does it imply that the present invention must rely on the above-described process steps for implementation. Those skilled in the art will appreciate that any improvements to the present invention, equivalent substitutions for the raw materials used, additions of auxiliary components, and selection of specific methods, etc., fall within the scope of protection and disclosure of the present invention.

Claims

1. An artificial SEI film suitable for silicon-based negative electrode materials, characterized in that: The material of the artificial SEI film includes a polymer, and the polymer structure includes a combination of a triazole ring and a pyrazine ring.

2. The artificial SEI film suitable for silicon-based negative electrode materials according to claim 1, characterized in that: The triazole ring in the polymer structure contains an electron-donating group; Preferably, the electron donating group comprises a thiol group.

3. The artificial SEI film suitable for silicon-based negative electrode materials according to claim 1 or 2, characterized in that: The monomers forming the polymer include a compound containing a combination of at least one carbon-carbon double bond, at least one triazole ring, and at least one pyrazine ring; Preferably, the structure of the monomer forming the polymer further includes at least one electron-donating group; Preferably, the electron-donating group comprises a thiol group; Preferably, the monomers forming the polymer are selected from 4-(prop-2-en-1-yl)-5-(pyrazin-2-yl)-4H-1,2,4-triazole-3-thiol.

4. The artificial SEI film suitable for silicon-based negative electrode materials according to any one of claims 1 to 3, characterized in that: The number average molecular weight of the polymer is 5000Da to 20000Da, preferably 7000Da to 18000Da.

5. A method for preparing an artificial SEI film suitable for silicon-based negative electrode materials according to any one of claims 1 to 4, characterized in that: The method comprises the following steps: A monomer comprising at least one carbon-carbon double bond, at least one triazole ring and at least one pyrazine ring, an initiator and an organic solvent are mixed, and after a polymerization reaction, the artificial SEI film suitable for silicon-based negative electrode materials is obtained.

6. The method according to claim 5, characterized in that The polymerization reaction is carried out under an inert atmosphere; Preferably, the polymerization reaction temperature is 60°C to 90°C; Preferably, the polymerization reaction time is 5 h to 12 h.

7. A silicon-based negative electrode material, characterized in that: The silicon-based negative electrode material includes a silicon substrate and an artificial SEI film layer coated on the surface of the silicon substrate, and the artificial SEI film layer includes the artificial SEI film suitable for silicon-based negative electrode materials according to any one of claims 1-4.

8. The silicon-based negative electrode material according to claim 7, characterized in that The material of the silicon substrate includes silicon-carbon material; Preferably, the average particle size of the silicon-carbon material is 3 μm to 9 μm; Preferably, the specific surface area of ​​the silicon-carbon material is 1m 2 / g~8m 2 / g; Preferably, the mass percentage of silicon material in the silicon-carbon material is 43% to 59%.

9. The silicon-based negative electrode material according to claim 7 or 8, characterized in that: Based on the total mass of the silicon-based negative electrode material being 100%, the mass percentage of the artificial SEI film layer is 1% to 6%; Preferably, the thickness of the artificial SEI film layer is 30 nm to 80 nm.

10. A secondary battery, characterized in that: The secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. The negative electrode sheet includes a negative electrode active material. The negative electrode active material includes the silicon-based negative electrode material according to any one of claims 7 to 9.

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