A silicon composite anode material, its preparation method, and a secondary battery

Abstract

This invention provides a silicon composite anode material, its preparation method, and a secondary battery. The silicon composite anode material includes a silicon core and an artificial SEI film coating the silicon core; the artificial SEI film is a copolymer, and the copolymer structure contains a combination of cyclic ether groups and nitro groups. This invention designs a copolymer artificial SEI film containing specific types of active groups, which not only improves the flexibility, structural stability, and ionic conductivity of the SEI film but also reduces its interfacial impedance.

Description

Technical Field

[0001] This invention belongs to the field of anode material technology, specifically relating to a silicon composite anode material, its preparation method, and a secondary battery. Background Technology

[0002] Silicon is a promising alternative to graphite as an anode material in lithium-ion batteries. However, during the lithium insertion / extraction process, the significant volume change of silicon anode materials leads to persistent side reactions with the electrolyte, severely hindering their further applications. While fabricating nanostructured silicon materials can effectively mitigate this volume change, it drastically increases the specific surface area of ​​silicon, exacerbating the side reactions between silicon and the electrolyte. Furthermore, silicon's low electronic conductivity is a major contributing factor to its rapid capacity decay. Therefore, constructing a complete interface layer with good electronic / ionic conductivity on the surface of silicon particles can effectively alleviate these problems.

[0003] CN113540395A discloses a method for preparing an artificial SEI film on the surface of a rechargeable magnesium battery negative electrode. The method involves immersing metallic magnesium in a film-forming solution to form the artificial SEI film on the magnesium electrode surface. Although the method is simple, the uniformity and integrity of the film formation are difficult to guarantee, resulting in poor electrical performance. CN110289448A discloses a lithium metal negative electrode with an artificially constructed SEI film. However, the method for preparing the artificial SEI film in this solution is more complex, and the SEI film has poor flexibility, making it prone to cracking during charge and discharge due to expansion and contraction.

[0004] Therefore, there is an urgent need in this field to develop an SEI film with good structural stability, uniformity and flexibility to improve the electrical performance of silicon anode materials. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a silicon composite anode material, its preparation method, and a secondary battery. The present invention designs a copolymer artificial SEI film containing specific types of active groups, which not only improves the flexibility, structural stability, and ionic conductivity of the SEI film but also reduces its interfacial impedance.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a silicon composite anode material, the silicon composite anode material comprising a silicon material core and an artificial SEI film coating the silicon material core;

[0008] The artificial SEI membrane is made of a copolymer, and the copolymer contains a combination of cyclic ether groups and nitro groups in its structure.

[0009] This invention involves coating a silicon material surface with a copolymer artificial SEI film containing a combination of cyclic ether groups and nitro groups. Firstly, it exhibits excellent flexibility. The nitro groups can form hydrogen bonds with the hydroxyl groups on the silicon anode material surface, improving the adhesion between the artificial SEI film and the silicon anode material, thus preventing the artificial SEI film from cracking during the volume expansion of the silicon anode material. Secondly, the copolymer contains nonpolar ions (-O-), with Lewis oxygen (-O-) acting as ligands to coordinate with lithium ions, providing the necessary solvation energy for the formation of the polymer-lithium ion complex. This allows for high ionic conductivity through repeated coordination dissociation processes. The interaction between polar groups and lithium ions enables lithium ion conduction, significantly improving the lithium ion transport rate. Finally, the high volume fraction of cyclic ether groups in the polymer chain results in good affinity at the interface with the silicon anode material. Therefore, the silicon composite material with the artificial SEI film combines low impedance, high coulombic efficiency, and long cycle performance.

[0010] Preferably, the copolymer's polymeric monomers include a first monomer and a second monomer.

[0011] Preferably, the structure of the first monomer contains a cyclic ether group.

[0012] Preferably, the structure of the second monomer contains a nitro group.

[0013] In this invention, the selection of the aforementioned specific types of first and second monomers results in higher stability of the SEI film, thereby improving the cycle performance of the battery and reducing the expansion rate of the silicon anode material.

[0014] Preferably, the first monomer comprises any one or a combination of at least two of 2-vinyl-1,3-dioxolane, 2-cyclopenten-1-ketoacetaldehyde, and 2-ethyl-2-vinyl-1,3-dioxolane.

[0015] Preferably, the second monomer comprises any one or a combination of at least two of 2,3-dimethoxy-b-nitrostyrene and 5-methoxy-3-nitrovinylindole.

[0016] Preferably, the molar ratio of the first monomer and the second monomer in the copolymer is 1:(1-5), more preferably 1:(1.5-3), for example, it can be 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2, 1:2.2, 1:2.5, 1:2.8, 1:3, 1:3.2, 1:3.5, 1:3.8, 1:4, 1:4.2, 1:4.5, 1:4.8, 1:5, etc.

[0017] In this invention, by controlling the molar ratio of the first monomer to the second monomer in the polymerized monomer, the SEI film has good stability. If the molar ratio is too low, the bonding strength between the artificial SEI film and the silicon anode will be poor. Conversely, if the molar ratio is too high, the artificial SEI film will be too rigid, and its flexibility and stability will decrease accordingly.

[0018] Preferably, based on the total mass of the silicon composite anode material as 100%, the mass percentage of the artificial SEI film in the silicon composite anode material is 1-4%, 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%, etc.

[0019] In this invention, by adjusting the mass percentage content of the artificial SEI film, the battery cell can have good rate performance and cycle performance. If the content is too low, the SEI film will rupture, which will affect the cycle performance of the battery. Conversely, if the content is too high, the energy density of the battery will be reduced.

[0020] Preferably, the thickness of the artificial SEI film is 50-150 nm, for example, it can be 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, 100 nm, 105 nm, 110 nm, 115 nm, 120 nm, 125 nm, 130 nm, 135 nm, 140 nm, 145 nm, 150 nm, etc.

[0021] In this invention, by adjusting the thickness of the artificial SEI film, the SEI film can have good stability and excellent rate performance. If the thickness is too small, the SEI film will break, while if the thickness is too large, the lithium ion transport distance will increase and the rate performance of the battery will decrease accordingly.

[0022] Preferably, the silicon material core comprises any one or a combination of at least two of silicon suboxide, silicon-carbon, or elemental silicon.

[0023] In a second aspect, the present invention provides a method for preparing the silicon composite anode material according to the first aspect, the method comprising the following steps:

[0024] The first monomer, the second monomer, the first solvent and the initiator are mixed once, and the copolymer material is obtained by reaction.

[0025] The copolymer material and the second solvent are mixed twice to obtain a copolymer solution. The copolymer solution and the silicon material are mixed three times, and the mixture is spray-dried to obtain the silicon composite anode material.

[0026] Preferably, the first solvent comprises benzene and / or N-methylpyrrolidone.

[0027] Preferably, the initiator comprises azobisisobutyronitrile and / or azobisisoheptanenitrile.

[0028] Preferably, the initiator has a mass percentage content of 0.02% to 1%, based on the total mass of the first monomer, the second monomer, and the first solvent as 100%, for example, 0.02%, 0.05%, 0.08%, 0.1%, 0.2%, 0.4%, 0.5%, 0.8%, 1%, etc.

[0029] Preferably, the reaction temperature is 60-120°C, for example, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, 120°C, etc.; the time is 10-24h, for example, 10h, 12h, 15h, 18h, 20h, 22h, 24h, etc.

[0030] Preferably, the reaction is carried out under an inert atmosphere.

[0031] In this invention, the inert atmosphere includes, but is not limited to, nitrogen or argon.

[0032] In this invention, after the reaction, the reaction solution is added to the precipitation solvent to obtain a copolymer precipitate, which is then washed and dried to obtain the copolymer material.

[0033] In this invention, the precipitation solvent includes, but is not limited to, any one of n-propanol, isopropanol, or acetone.

[0034] Preferably, the second solvent comprises benzene and / or N-methylpyrrolidone.

[0035] Preferably, the mass concentration of the copolymer solution is 5% to 15%, for example, it can be 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, etc.

[0036] In this invention, by adjusting the mass concentration of the copolymer solution, the copolymer can completely and densely coat the negative electrode material.

[0037] Preferably, the temperature of the three mixing processes is 40-80℃, for example, 40℃, 50℃, 60℃, 70℃, 80℃, etc.; the time is 5-10h, for example, 5h, 6h, 7h, 8h, 9h, 10h, etc.

[0038] Preferably, the inlet temperature of the spray dryer is 100-200℃, for example, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, etc.; and the outlet temperature is 60-90℃, for example, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, etc.

[0039] Thirdly, the present invention provides a secondary battery, the secondary battery comprising a positive electrode, a negative electrode, an electrolyte and a separator, wherein the negative electrode comprises a current collector and a negative electrode active material layer disposed on at least one side of the current collector.

[0040] Preferably, the negative electrode active material layer comprises the silicon composite negative electrode material according to the first aspect.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] This invention provides a silicon composite anode material, which has an artificial SEI film layer containing a combination of cyclic ether groups and nitro groups coated on the surface of silicon material. Firstly, it exhibits good flexibility. The nitro groups can form hydrogen bonds with the hydroxyl groups on the surface of the silicon anode material, improving the adhesion between the artificial SEI film layer and the silicon anode material, thus preventing the artificial SEI film from cracking during the volume expansion of the silicon anode material. Secondly, the copolymer contains nonpolar ions (-O-), with Lewis oxygen (-O-) acting as ligands to coordinate with lithium ions, providing the necessary solvation energy for the formation of the polymer-lithium ion complex. This allows for high ionic conductivity through repeated coordination dissociation processes. The interaction between polar groups and lithium ions enables lithium ion conduction, significantly improving the lithium ion transport rate. Finally, the high volume fraction of cyclic ether groups in the polymer chain results in good affinity at the interface with the silicon anode material. Therefore, the silicon composite material with the artificial SEI film layer combines low impedance, high coulombic efficiency, and long cycle performance. Detailed Implementation

[0043] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0044] Example 1

[0045] This embodiment provides a silicon composite anode material, which includes a silicon-carbon material core and an artificial SEI film (100 nm thick) coating the silicon-carbon material core; the material of the artificial SEI film is a copolymer.

[0046] The copolymer is obtained by copolymerizing 2-vinyl-1,3-dioxane and 2,3-dimethoxy-b-nitrostyrene in a molar ratio of 1:2.5. The mass percentage of the artificial SEI film in the silicon composite anode material is 3% based on the total mass of the silicon composite anode material as 100%.

[0047] This embodiment also provides a method for preparing the above-mentioned silicon composite anode material, which includes the following steps:

[0048] 2-Vinyl-1,3-dioxane, 2,3-dimethoxy-b-nitrostyrene, N-methylpyrrolidone solvent, and azobisisobutyronitrile initiator (based on the total mass of monomer, solvent, and initiator being 100%, with the initiator comprising 0.2% by mass) were mixed once, heated to 90°C under argon protection, and reacted for 12 hours. The reaction solution was then added to n-propanol to obtain a polymer precipitate, which was washed and dried to obtain the copolymer material.

[0049] The copolymer material and N-methylpyrrolidone solvent were mixed twice to obtain a copolymer solution with a mass concentration of 10%. The copolymer solution and silicon carbon material were stirred at 60°C for 7 hours. The mixed liquid was spray-dried at an inlet temperature of 150°C and an outlet temperature of 75°C to obtain the silicon composite anode material.

[0050] Example 2

[0051] This embodiment provides a silicon composite anode material, which includes a silicon-carbon material core and an artificial SEI film (75 nm thick) coating the silicon-carbon material core; the artificial SEI film is made of a copolymer.

[0052] The copolymer is obtained by copolymerizing 2-vinyl-1,3-dioxane and 2,3-dimethoxy-b-nitrostyrene in a molar ratio of 1:1.5. The mass percentage of the artificial SEI film in the silicon composite anode material is 2% based on the total mass of the silicon composite anode material as 100%.

[0053] This embodiment also provides a method for preparing the above-mentioned silicon composite anode material, which includes the following steps:

[0054] 2-Vinyl-1,3-dioxane, 2,3-dimethoxy-b-nitrostyrene, N-methylpyrrolidone solvent, and azobisisobutyronitrile initiator (based on a total mass of 100% of monomer, solvent, and initiator, with the initiator comprising 0.2% by mass) were mixed once, heated to 90°C under argon protection, and reacted for 12 hours. The reaction solution was then added to propanol to obtain a polymer precipitate, which was washed and dried to obtain the polymer material.

[0055] The copolymer material and N-methylpyrrolidone solvent were mixed twice to obtain a copolymer solution with a mass concentration of 8%. The copolymer solution and silicon carbon material were stirred at 60°C for 7 hours. The mixed liquid was spray-dried at an inlet temperature of 150°C and an outlet temperature of 75°C to obtain the silicon composite anode material.

[0056] Example 3

[0057] This embodiment provides a silicon composite anode material, which includes a silicon-carbon material core and an artificial SEI film (120 nm thick) covering the silicon-carbon material core; the material of the artificial SEI film is a copolymer.

[0058] The copolymer is obtained by copolymerizing 2-vinyl-1,3-dioxane and 2,3-dimethoxy-b-nitrostyrene in a molar ratio of 1:3. Based on the total mass of the silicon composite anode material as 100%, the mass percentage of the artificial SEI film in the silicon composite anode material is 4%.

[0059] This embodiment also provides a method for preparing the above-mentioned silicon composite anode material, which includes the following steps:

[0060] 2-Vinyl-1,3-dioxane, 2,3-dimethoxy-b-nitrostyrene, N-methylpyrrolidone solvent, and azobisisobutyronitrile initiator (based on a total mass of 100% of monomer, solvent, and initiator, with the initiator comprising 0.2% by mass) were mixed once, heated to 90°C under argon protection, and reacted for 12 hours. The reaction solution was then added to propanol to obtain a polymer precipitate, which was washed and dried to obtain the polymer material.

[0061] The copolymer material and N-methylpyrrolidone solvent were mixed twice to obtain a copolymer solution with a mass concentration of 12%. The copolymer solution and silicon carbon material were stirred at 60°C for 7 hours. The mixed liquid was spray-dried at an inlet temperature of 150°C and an outlet temperature of 75°C to obtain the silicon composite anode material.

[0062] Example 4

[0063] The difference between this embodiment and Example 1 is that the molar ratio of 2-vinyl-1,3-dioxane to 2,3-dimethoxy-b-nitrostyrene in the polymer monomers is 1:0.5, while all other aspects are the same as in Example 1.

[0064] Example 5

[0065] The difference between this embodiment and Example 1 is that the molar ratio of 2-vinyl-1,3-dioxane to 2,3-dimethoxy-b-nitrostyrene in the polymer monomers is 1:10, while all other aspects are the same as in Example 1.

[0066] Example 6

[0067] The difference between this embodiment and Embodiment 1 is that the mass percentage of the artificial SEI film in the silicon composite anode material is 0.2%, while all other aspects are the same as in Embodiment 1.

[0068] Example 7

[0069] The difference between this embodiment and Embodiment 1 is that the mass percentage of the artificial SEI film in the silicon composite anode material is 6%, while all other aspects are the same as in Embodiment 1.

[0070] Example 8

[0071] The difference between this embodiment and Embodiment 1 is that the thickness of the artificial SEI film is 20 nm, while all other aspects are the same as in Embodiment 1.

[0072] Example 9

[0073] The difference between this embodiment and Embodiment 1 is that the thickness of the artificial SEI film is 200 nm, while all other aspects are the same as in Embodiment 1.

[0074] Example 10

[0075] The difference between this embodiment and Example 1 is that the mass concentration of the copolymer solution is 2%, while everything else is the same as in Example 1.

[0076] Example 11

[0077] The difference between this embodiment and Example 1 is that the mass concentration of the copolymer solution is 20%, while everything else is the same as in Example 1.

[0078] Comparative Example 1

[0079] The difference between this comparative example and Example 1 is that the material of the artificial SEI film is poly(2-vinyl-1,3-dioxane), while all other aspects are the same as in Example 1.

[0080] Comparative Example 2

[0081] The difference between this comparative example and Example 1 is that the material of the artificial SEI film is 2,3-dimethoxy-b-nitrostyrene, while all other aspects are the same as in Example 1.

[0082] Application Examples 1 to 11 and Comparative Application Examples 1 to 2

[0083] The silicon composite anode materials provided in Examples 1 to 11 and Comparative Examples 1 to 2 were assembled into lithium-ion batteries, and the preparation method is as follows:

[0084] A slurry was prepared by compounding silicon composite anode material, conductive agent Super P, and binder polyacrylic acid in a mass ratio of 8:1:1. The slurry was then coated onto a copper foil current collector and vacuum dried to obtain an electrode. A 12 μm thick polyethylene film was used as a separator, and a solution of ethylene carbonate and dimethyl carbonate of LiPF6 was used as an electrolyte. Fluorinated ethylene carbonate was added as an electrolyte additive (the molar ratio of fluorinated ethylene carbonate to ethylene carbonate and dimethyl carbonate was 1:10). The electrode, lithium metal sheet, and separator were assembled into a CR2032 half-cell in a glove box filled with hydrogen.

[0085] Test conditions

[0086] The lithium-ion half-cells provided in Application Examples 1 to 11 and Comparative Application Examples 1 to 2 were tested using the following methods:

[0087] (1) Initial coulombic efficiency: 0.1C constant current and constant voltage charging to 5mV, then 0.1C discharge to 1.5V;

[0088] (2) Cyclic performance: At 25℃, it is charged to 5mV by 1C constant current and constant voltage, and then discharged to 1.5V by 1C.

[0089] Perform charge-discharge cycles, 300 cycles in total.

[0090] The test results are shown in Table 1:

[0091] Table 1

[0092]

[0093] As can be seen from Table 1, by comparing Application Example 1 with Application Examples 4-5, it can be seen that when the molar ratio of 2-vinyl-1,3-dioxane to 2,3-dimethoxy-b-nitrostyrene is too high, the content of nitro groups is too low, resulting in a weak hydrogen bonding force between the artificial SEI film and the silicon anode, and the SEI film cannot suppress the volume expansion of the silicon anode; when the molar ratio of the two monomers is too low, the content of nitro groups is too high, resulting in excessive rigidity and poor flexibility of the artificial SEI film, making the SEI film prone to breakage.

[0094] By comparing Application Example 1 with Application Examples 6-7, it can be seen that when the mass percentage of the SEI film is too low, the stability of the SEI film is not effectively improved, thus affecting the cycle performance of the battery; when the mass percentage of the SEI film is too high, the content of inactive materials is too high, which reduces the energy density of the battery.

[0095] By comparing Application Example 1 with Application Examples 8-9, it can be seen that when the thickness of the SEI film is too low, the SEI film is easily broken by the expansion of the silicon anode, resulting in the deterioration of the battery's electrical performance; when the thickness of the SEI film is too high, the lithium ion transport distance increases, polarization increases, and the battery's rate performance and cycle performance also deteriorate.

[0096] By comparing Application Example 1 with Application Examples 10-11, it can be seen that when the mass concentration of the polymer solution is too high or too low, the uniformity of the SEI film coating deteriorates, leading to the degradation of the battery's electrical performance.

[0097] By comparing Application Example 1 with Comparative Application Examples 1-2, it can be seen that when the monomer is only 2-vinyl-1,3-dioxane, the artificial SEI film has good flexibility but poor rigidity. Its interaction with the silicon anode is mainly intermolecular force, which leads to the deterioration of electrical performance. When the monomer is only 2,3-dimethoxy-b-nitrostyrene, the artificial SEI film has strong rigidity but poor flexibility, which also leads to the deterioration of electrical performance.

[0098] The applicant declares that the present invention is illustrated by the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A silicon composite negative electrode material, characterized by, The silicon composite anode material includes a silicon core and an artificial SEI film coating the silicon core. The artificial SEI membrane is made of a copolymer, and the copolymer contains a combination of cyclic ether groups and nitro groups in its structure. The copolymer's polymerizable monomers include a first monomer and a second monomer; The first monomer comprises any one or a combination of at least two of 2-vinyl-1,3-dioxolane, 2-cyclopenten-1-ketoacetal, and 2-ethyl-2-vinyl-1,3-dioxolane. The second monomer includes any one or a combination of at least two of 2,3-dimethoxy-b-nitrostyrene and 5-methoxy-3-nitrovinylindole; The molar ratio of the first monomer and the second monomer in the copolymer is 1:(1~5). 2.The silicon composite negative electrode material of claim 1, characterized in that, The molar ratio of the first monomer and the second monomer in the copolymer is 1:(1.5~3). 3.The silicon composite negative electrode material of claim 1, wherein, Based on the total mass of the silicon composite anode material being 100%, the mass percentage of the artificial SEI film in the silicon composite anode material is 1-4%. 4.The silicon composite negative electrode material of claim 1, wherein, The thickness of the artificial SEI film is 50~150nm; The silicon material core includes any one or a combination of at least two of silicon suboxide, silicon-carbon, or elemental silicon.

5. A method of producing the silicon composite negative electrode material according to any one of claims 1 to 4, characterized by, The method includes the following steps: The first monomer, the second monomer, the first solvent and the initiator are mixed once, and the copolymer material is obtained by reaction. The copolymer material and the second solvent are mixed twice to obtain a copolymer solution. The copolymer solution and the silicon material are mixed three times, and the mixture is spray-dried to obtain the silicon composite anode material.

6. The method of claim 5, wherein, The first solvent includes benzene and / or N-methylpyrrolidone; The initiator includes azobisisobutyronitrile and / or azobisisoheptanenitrile; The initiator has a mass percentage content of 0.02% to 1%, based on the total mass of the first monomer, the second monomer, and the first solvent as 100%. The reaction is carried out at a temperature of 60~120℃ for a time of 10~24h. The reaction was carried out under an inert atmosphere.

7. The method according to claim 5, characterized in that, The second solvent includes benzene and / or N-methylpyrrolidone; The copolymer solution has a mass concentration of 5-15%; The three mixing processes are carried out at a temperature of 40~80℃ for 5~10 hours. The inlet temperature of the spray dryer is 100-200℃, and the outlet temperature is 60-90℃.

8. A secondary battery, characterized in that, The secondary battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a current collector and a layer of negative electrode active material disposed on at least one side of the current collector. The negative electrode active material layer comprises the silicon composite negative electrode material according to any one of claims 1-4.

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