Artificial SEI film and silicon-based negative electrode material for lithium ion battery

Abstract

The invention belongs to a lithium ion battery negative electrode material, and particularly relates to an artificial SEI film and a lithium ion battery negative electrode material. The thin film comprises a polymer, a conductive agent and a lithium salt, the polymer comprises polyethylene oxide subjected to crosslinking copolymerization modification. The silicon-based negative electrode material for the lithium ion battery comprises a silicon-based material and an artificial SEI film wrapping the surface of the silicon-based material. As the negative electrode material of the lithium ion battery, the artificial SEI film on the surface can reduce the expansion rate of the pole piece of the silicon-based negative electrode material by about 5%, improve the cycle performance by more than 10% and reduce the internal resistance by more than 15%, and in the manufacturing process of the silicon-based negative electrode pole piece, high-cost single-walled carbon nanotubes do not need to be added. The synthesis process is simple, easy to control, safe, reliable, low in production cost, high in yield and suitable for industrial batch production.

Description

Technical Field

[0001] The invention belongs to the field of negative electrode materials for lithium ion batteries, and in particular relates to an artificial SEI film and a silicon-based negative electrode material for lithium ion batteries. Background Art

[0002] Silicon is the second most abundant element in the earth's crust. It has the advantages of low cost, environmental friendliness, low lithium insertion potential, and high theoretical specific capacity. It is considered to be one of the most promising negative electrode materials for lithium-ion batteries. Theoretically, one silicon atom can form Li with 4.4 lithium atoms. 4.4 Si, the specific capacity can reach 4200mAh g -1 , which is more than 10 times that of traditional graphite negative electrodes. However, silicon will undergo a large volume expansion (~350%) during the lithium insertion process. As the lithium insertion and deinsertion process proceeds, the silicon particles continue to expand and shrink in volume, generating huge stress, which eventually leads to the failure of the silicon-based negative electrode. The failure mechanism of silicon is mainly divided into: (1) Destruction of the integrity of the electrode structure. The silicon particles continue to change in volume during the cycle, resulting in loss of electrical contact between the silicon particles, the conductive agent and the current collector, destroying the electronic conductive network of the electrode, and causing rapid capacity decay. (2) Fracture and pulverization of silicon particles. The volume change of silicon particles will generate large stress and destroy the structure of the silicon particles themselves. (3) Formation of an unstable and thick solid-electrolyte interface (SEI) film. Under the stress caused by volume expansion, the structure of the SEI film is destroyed, causing the exposed silicon particles to continue to react with the electrolyte, continuously consuming lithium ions and electrolyte, and forming a new SEI film. As the thickness of the SEI film gradually increases, the transmission distance of lithium ions and the electrode impedance will be affected, resulting in poor rate performance and cycle performance of the battery.

[0003] These problems of silicon-based negative electrodes have greatly limited their commercial application. In order to solve these problems, researchers have mainly conducted research from the following aspects: (1) Structural design of nano-silicon materials. Silicon particles have a critical size (150nm). When the particle diameter is smaller than this value, the silicon particles are not easy to break during the lithium insertion and extraction process. (2) Synthesis of silicon-carbon or silicon oxide composite materials. Introducing carbon or amorphous SiO into silicon x It can act as an expansion buffer layer to reduce the volume change during the lithium insertion and extraction process and improve the structural stability. (3) Develop conductive additives, electrolytes and binders suitable for silicon negative electrodes. For example, modifying the electrolyte by electrolyte additives is conducive to the formation of a stable SEI film and improving the electrochemical performance of silicon-based negative electrodes. Among them, the strategy of combining silicon surface modification with high-performance binders has been shown to be able to form additional chemical bonds between silicon particles and binders, further enhancing the adhesion strength of the binder, and showing great advantages in maintaining the structural integrity and / or conductive network of the electrode.

[0004] However, silicon-based negative electrode materials are often mixed with graphite. The binder not only acts on the silicon-based negative electrode materials, but also on the graphite materials, which will affect the performance of the graphite materials to a certain extent. Moreover, the bond between the binder and the silicon-based particles is only a physical bond, which is difficult to ensure high bonding strength, resulting in the binder easily falling off during the expansion process. Summary of the invention

[0005] In view of the problems existing in silicon-based negative electrode materials at this stage, the present invention provides a method of modifying the surface of silicon-based negative electrode materials by coating polymers and conductive agents to prepare an artificial SEI film to solve the problems of structural fragmentation and repeated growth of SEI caused by the volume expansion of silicon-based particles. The polymer used has a highly cross-linked structure, a smooth ion conduction path and good flexibility. The formed three-dimensional cross-linked network structure and high elasticity can inhibit the volume expansion of silicon-based particles and improve the structural stability and electrochemical performance of the electrode. The conductive agent used is directly attached to the surface of the silicon-based particles to form an efficient electronic conduction path, ensure the electrical contact of the negative electrode particles, and improve the cycle and rate performance. The prepared silicon-carbon negative electrode material has a higher first effect and a more stable cycle performance, and its process steps are simple and efficient.

[0006] The technical solution of this application is as follows:

[0007] In a first aspect, the present application provides an artificial SEI film, wherein the raw materials of the film include a polymer, a conductive agent, and a lithium salt;

[0008] The polymer comprises cross-linked copolymer-modified polyethylene oxide.

[0009] The polymer is cross-linked copolymer modified polyethylene oxide, which forms a three-dimensional network structure after modification, achieving a significant increase in flexibility and strength, which is beneficial to buffer the volume expansion of silicon-based particles, thereby stabilizing their structure. At the same time, cross-linked copolymer modification can also reduce the crystallinity of polyethylene oxide, optimize the carrier concentration of polyethylene oxide, and thus improve its ionic conductivity. The conductive agent forms a conductive network to ensure electrical contact with the negative electrode and improve the cycle. At the same time, the presence of the conductive agent can act as a skeleton, improve the mechanical strength of the polymer film, and help maintain the structural integrity of the silicon-based particles. Lithium salts are easily dissociated in the polymer matrix, providing more Li + Thereby obtaining an electrolyte with higher ionic conductivity and improving the first coulombic efficiency of the negative electrode material; see the schematic diagram of the specific structure Figure 1 .

[0010] As a preferred technical solution of the present application, the cross-linked copolymer modified polyethylene oxide includes a cross-linked copolymer and polyethylene oxide.

[0011] As a preferred technical solution of the present application, the cross-linked copolymer includes at least one of polyacrylic acid, polyacrylate, polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylonitrile, polymethyl methacrylate, and polyvinylidene fluoride.

[0012] As a preferred technical solution of the present application, the conductive agent includes at least one of carbon nanotubes, graphene, conductive carbon black, carbon fiber, and graphite.

[0013] As a preferred technical solution of the present application, the lithium salt includes lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium borate oxalate, lithium difluorooxalate borate, lithium nitrate, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, and lithium hexafluorophosphate.

[0014] As a preferred technical solution of the present application, the ratio of the cross-linked copolymer to the polyethylene oxide is 0.2 to 5:1.

[0015] In a second aspect, the present application provides the application of the above-mentioned artificial SEI film in lithium-ion batteries.

[0016] As a preferred technical solution of the present application, it is used as a silicon-based negative electrode material for lithium-ion batteries.

[0017] In a third aspect, the present application provides a silicon-based negative electrode material for a lithium-ion battery, wherein the negative electrode material comprises a silicon-based material and the above-mentioned artificial SEI film wrapped on the surface of the silicon-based material.

[0018] As a preferred technical solution of the present application, the silicon-based material is at least one of silicon carbon, silicon oxide / carbon, silicon / polymer, and silicon particles.

[0019] As a preferred technical solution of the present application, the mass percentage of the polymer in the artificial SEI film to the silicon-based material is 0.1-3%.

[0020] As a preferred technical solution of the present application, the mass percentage of the conductive agent in the artificial SEI film to the silicon-based material is 0.05-1%.

[0021] As a preferred technical solution of the present application, the mass percentage of lithium salt in the artificial SEI film to the polymer is 0.5-5%.

[0022] In a fourth aspect, the present application provides a method for preparing the above-mentioned silicon-based negative electrode material for lithium-ion batteries, comprising the following steps:

[0023] (1) dissolving the polymer to obtain a solution;

[0024] (2) adding a conductive agent, a lithium salt, and a silicon-based material to the solution of step (1) and dispersing to obtain a slurry;

[0025] (3) drying the slurry of step (2) to obtain a powder material;

[0026] (4) The powder material of step (3) is cross-linked and cured to obtain a silicon-based negative electrode material for lithium-ion batteries.

[0027] As a preferred technical solution of the present application, the dissolving conditions in step (1) include heating in an organic solvent.

[0028] As a preferred technical solution of the present application, the heating conditions are: temperature of 50-150° C. and polymer solid content of 0.5-5%.

[0029] As a preferred technical solution of the present application, the organic solvent is one or more of dimethyl sulfoxide, N-methylpyrrolidone, and tetrahydrofuran.

[0030] As a preferred technical solution of the present application, the drying in step (3) includes spray drying.

[0031] As a preferred technical solution of the present application, the heating conditions in step (4) are: heating temperature 100-200° C., heating time 1-5 h.

[0032] Beneficial Effects of the Invention

[0033] (1) The polymer is cross-linked copolymer modified polyethylene oxide, which forms a three-dimensional network structure after modification, achieving a significant improvement in flexibility and strength, which is beneficial to buffer the volume expansion of silicon-based particles, thereby stabilizing their structure. At the same time, cross-linked copolymer modification can also reduce the crystallinity of polyethylene oxide, optimize the carrier concentration of polyethylene oxide, and thus improve its ionic conductivity. The conductive agent forms a conductive network to ensure electrical contact with the negative electrode and improve the cycle. At the same time, the presence of the conductive agent can act as a skeleton, improve the mechanical strength of the polymer film, and help maintain the structural integrity of the silicon-based particles. Lithium salts are easily dissociated in the polymer matrix, providing more Li + This results in an electrolyte with higher ionic conductivity and improves the first coulombic efficiency of the negative electrode material.

[0034] (2) Excellent electrochemical performance: As a negative electrode material for lithium-ion batteries, the artificial SEI film on the surface can reduce the expansion rate of the silicon-based negative electrode material by about 5%, improve the cycle performance by more than 10%, and reduce the internal resistance by more than 15%. In addition, there is no need to add high-cost single-walled carbon nanotubes during the production of silicon-based negative electrode sheets.

[0035] (3) The synthesis process of the present invention is simple, easy to control, safe and reliable, has low production cost and high yield, and is suitable for industrial mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0037] Figure 1 This is a schematic diagram of the structure of the artificial SEI film on the surface of the silicon-based material in the patent of this invention;

[0038] Figure 2 This is a scanning electron microscope image of the sample of Example 1;

[0039] Figure 3 The first charge and discharge diagram of Example 1 and Comparative Example 1 as negative electrodes of lithium-ion batteries;

[0040] Figure 4 This is a cycle performance diagram of Example 1 as a negative electrode of a lithium ion battery. DETAILED DESCRIPTION

[0041] Example 1

[0042] A surface modification method and application of a silicon-based negative electrode material, comprising the following steps:

[0043] (1) 2.5 g of polyethylene oxide (PEO) and 2.5 g of polyacrylic acid (PAA) were heated and dissolved in 200 mL of dimethyl sulfoxide solvent to form a uniform solution;

[0044] (2) adding 100 g of commercial silicon-carbon composite material, 0.05 g of carbon nanotube conductive agent and 0.25 g of lithium bis(trifluoromethanesulfonyl imide) to the above solution, and performing ultrasonic dispersion to form a uniform slurry;

[0045] (3) spray drying the slurry at a temperature of 250° C. to obtain a powder material in the form of particles;

[0046] (4) The powder material is treated at 150° C. for 3 h for cross-linking and curing, and then demagnetized and sieved to obtain a surface-modified silicon-based negative electrode material.

[0047] Figure 2 Scanning electron microscopy results show that after surface modification, a dense polymer and carbon nanotube network are attached to the surface of silicon carbon particles to form a tight coating layer. Electrochemical results show that Figure 3 , Figure 4The first charge and discharge efficiency of the surface-modified silicon-carbon material is 93%, which is much higher than the first charge and discharge efficiency of 88% of the unmodified silicon-carbon material. The first charge specific capacity is 2150mAh / g. After 1000 cycles, the capacity retention rate is 88%, and the cycle performance is stable.

[0048] Example 2

[0049] A surface modification method and application of a silicon-based negative electrode material, comprising the following steps:

[0050] (1) 2.5 g of polyethylene oxide (PEO) and 0.5 g of polyethylene glycol (PEG) were heated and dissolved in 100 mL of N-methylpyrrolidone solvent to form a uniform solution;

[0051] (2) adding 100 g of commercial silicon-oxygen / carbon composite material, 0.1 g of graphene conductive agent and 0.06 g of lithium difluorooxalate borate to the above solution, and ultrasonically dispersing to form a uniform slurry;

[0052] (3) spray drying the slurry at a temperature of 220° C. to obtain a powder material in the form of particles;

[0053] (4) The powder material is treated at 200° C. for 1 h for cross-linking and curing, and then demagnetized and sieved to obtain a surface-modified silicon-based negative electrode material.

[0054] Example 3

[0055] A surface modification method and application of a silicon-based negative electrode material, comprising the following steps:

[0056] (1) 2.5 g of polyethylene oxide (PEO) and 12.5 g of polymethyl methacrylate (PMMA) were heated and dissolved in 300 mL of tetrahydrofuran solvent to form a uniform solution;

[0057] (2) adding 500 g of commercial silicon granules, 5 g of conductive carbon black and 0.075 g of lithium nitrate to the above solution, and performing ultrasonic dispersion to form a uniform slurry;

[0058] (3) spray drying the slurry at a temperature of 280° C. to obtain a powder material in the form of particles;

[0059] (4) The powder material is treated at 100° C. for 5 h for cross-linking and curing, and then demagnetized and sieved to obtain a surface-modified silicon-based negative electrode material.

[0060] Example 4

[0061] A surface modification method and application of a silicon-based negative electrode material, comprising the following steps:

[0062] (1) 2.5 g of polyethylene oxide (PEO) and 1.25 g of polyvinylidene fluoride (PVDF) were heated and dissolved in 400 mL of dimethyl sulfoxide solvent to form a uniform solution;

[0063] (2) adding 200 g of commercial silicon / carbon composite material, 2.0 g of carbon fiber and 0.05 g of lithium hexafluorophosphate to the above solution, and performing ultrasonic dispersion to form a uniform slurry;

[0064] (3) spray drying the slurry at a temperature of 230° C. to obtain a powder material in the form of particles;

[0065] (4) The powder material is treated at 150° C. for 2 h for cross-linking and curing, and then demagnetized and sieved to obtain a surface-modified silicon-based negative electrode material.

[0066] Comparative Example 1 - No surface modification

[0067] Commercial silicon-carbon composite materials are used directly as negative electrode materials without surface modification.

[0068] Comparative Example 2-No conductive agent added

[0069] Compared with Example 1, 0.1 g of graphene conductive agent is not added, and the rest is the same as Example 1.

[0070] Comparative Example 3 - No lithium salt added

[0071] Compared with Example 1, 0.06 g of lithium difluorooxalate borate was not added, and the rest was the same as Example 1.

[0072] Comparative Example 4 - Polymer Single

[0073] Compared with Example 1, 2.5 g of polyacrylic acid (PAA) was not added, and the rest was the same as Example 1.

[0074] Implementation effect example

[0075] The modified silicon-based materials prepared in Examples 1-4 and Comparative Examples 1-3 were used as negative electrode materials for lithium-ion batteries to compare their electrochemical performance. The specific experimental process was as follows: modified silicon-based materials prepared in Examples 1-4 and Comparative Examples 1-3: carbon nanotubes: CMC binder = 93:2:5; lithium sheets were used as counter electrodes and reference electrodes; the electrolyte was 1M LiPF6-EC / DMC (volume ratio 1:1), and the charge and discharge rate was 0.1C. The experimental results are shown in Table 1.

[0076] Table 1 Comparison of electrochemical properties of negative electrode materials prepared in Examples 1-4 and Comparative Examples 1-3

[0077]

[0078] The above description is only a preferred embodiment of the present invention, and does not limit the patent scope of the present invention. All equivalent structural changes made by using the contents of the present invention specification under the inventive concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. An artificial SEI film, characterized in that: The film comprises a polymer, a conductive agent and a lithium salt; The polymer comprises cross-linked copolymer-modified polyethylene oxide.

2. The artificial SEI film according to claim 1, characterized in that: The cross-linked copolymer modified polyethylene oxide includes a cross-linked copolymer and polyethylene oxide; Preferably, the cross-linked copolymer includes at least one of polyacrylic acid, polyacrylate, polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone, polyacrylonitrile, polymethyl methacrylate, and polyvinylidene fluoride; And / or, the conductive agent includes at least one of carbon nanotubes, graphene, conductive carbon black, carbon fiber, and graphite; And / or, the lithium salt includes lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium borate oxalate, lithium difluorooxalate borate, lithium nitrate, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, and lithium hexafluorophosphate.

3. The artificial SEI film according to claim 2, characterized in that: The ratio of the cross-linked copolymer to the polyethylene oxide is 0.2 to 5:

1.

4. Use of the artificial SEI film according to any one of claims 1 to 3 in a lithium ion battery.

5. The use according to claim 4, characterized in that: Application as silicon-based negative electrode material for lithium-ion batteries.

6. A silicon-based negative electrode material for lithium-ion batteries, characterized in that: The negative electrode material comprises a silicon-based material and an artificial SEI film according to any one of claims 1 to 3 wrapped on the surface of the silicon-based material.

7. The silicon-based negative electrode material for lithium-ion batteries according to claim 6, characterized in that: The silicon-based material is at least one of silicon carbon, silicon oxide / carbon, silicon / polymer, and silicon particles.

8. The silicon-based negative electrode material for lithium-ion batteries according to claim 6 or 7, characterized in that: The mass percentage of the polymer in the artificial SEI film to the silicon-based material is 0.1-3%; And / or, the mass percentage of the conductive agent in the artificial SEI film to the silicon-based material is 0.05-1%; And / or, the mass percentage of lithium salt in the artificial SEI film to the polymer is 0.5-5%.

9. A method for preparing a silicon-based negative electrode material for a lithium ion battery according to any one of claims 6 to 8, comprising the following steps: (1) dissolving the polymer to obtain a solution; (2) adding a conductive agent, a lithium salt, and a silicon-based material to the solution of step (1) and dispersing to obtain a slurry; (3) drying the slurry of step (2) to obtain a powder material; (4) The powder material of step (3) is cross-linked and cured to obtain a silicon-based negative electrode material for a lithium-ion battery.

10. The preparation method according to claim 9, characterized in that: The dissolving conditions in step (1) include heating in an organic solvent; Preferably, the heating conditions are: temperature of 50-150° C. and polymer solid content of 0.5-5%. Preferably, the organic solvent is one or more of dimethyl sulfoxide, N-methylpyrrolidone, and tetrahydrofuran; And / or, the drying in step (3) comprises spray drying; And / or, the heating conditions in step (4) are: heating temperature 100-200° C., heating time 1-5 h.

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