Artificial SEI film and silicon-based anode material for lithium-ion batteries

Abstract

The present application belongs to the lithium ion battery negative electrode material, and particularly relates to an artificial SEI film and a lithium ion battery negative electrode material. The composition of the film comprises a polymer, a conductive agent and a lithium salt; the polymer comprises a crosslinked copolymer modified polyethylene oxide. The silicon-based negative electrode material for the lithium ion battery comprises a silicon-based material and an artificial SEI film wrapped on the surface of the silicon-based material. As the lithium ion battery negative electrode material, the surface artificial SEI film 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%, and in the process of manufacturing the silicon-based negative electrode material, it is not necessary to add high-cost single-walled carbon nanotubes. The synthesis process is simple, easy to control, safe and reliable, low in production cost, high in yield, and suitable for industrialized mass production.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery negative electrode materials, and particularly relates to an artificial SEI film and a silicon-based negative electrode material for lithium ion batteries. BACKGROUND

[0002] Silicon is the second most abundant element in the earth's crust, and has the advantages of low cost, environmental friendliness, low lithium intercalation potential, and high theoretical specific capacity, and is considered to be one of the most promising negative electrode materials for lithium ion batteries. In theory, one silicon atom can form Li 4.4 Si with 4.4 lithium atoms, and the specific capacity can reach 4200 mAh 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 intercalation of lithium. With the progress of lithium deintercalation, the silicon particles continuously undergo volume expansion and contraction, generating a huge stress, and ultimately leading to the failure of the silicon-based negative electrode. The failure mechanism of silicon mainly includes: (1) destroying the structural integrity of the electrode. The volume change of the silicon particles in the cycle process causes the loss of electrical contact between the silicon particles, the conductive agent and the current collector, destroys the electronic conduction network of the electrode, and leads to rapid capacity decay. (2) breakage and pulverization of silicon particles. The volume change of the silicon particles generates a large stress, which destroys the structure of the silicon particles themselves. (3) formation of an unstable, thick solid-electrolyte interphase (SEI) film. Under the action of 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 transport distance of lithium ions and the impedance of the electrode will be affected, leading to poor rate performance and cycle performance of the battery.

[0003] These problems of silicon-based negative electrodes greatly limit their commercial application. In order to solve these problems, researchers mainly proceed with research from the following aspects: (1) structural design of nano-silicon materials. There is a critical size (150 nm) for silicon particles, and when the particle diameter is less than this value, the silicon particles are not easy to break during the deintercalation of lithium. (2) synthesis of silicon-carbon or silicon-oxide composite materials. The introduction of carbon or amorphous SiO x in silicon can act as a swelling buffer layer to reduce the volume change during intercalation and deintercalation of lithium, and improve the structural stability. (3) development of conductive additives, electrolytes and binders suitable for silicon negative electrodes. For example, modification of the electrolyte by electrolyte additives is beneficial to the formation of a stable SEI film, and improves the electrochemical performance of the silicon-based negative electrode. Among them, the strategy of combining the modification of the surface of silicon with high-performance binders has been proved to form additional chemical bonds between the silicon particles and the binders, further enhancing the adhesion strength of the binders, and showing great advantages in maintaining the structural integrity and / or conductive network of the electrode.

[0004] However, the silicon-based negative electrode material is often mixed with graphite, and the binder acts not only on the silicon-based negative electrode material but also on the graphite material, which will affect the performance of the graphite material to some extent. Moreover, the combination between the binder and the silicon-based particles is only physical combination, which is difficult to guarantee high combination strength, leading to easy falling off of the binder in the expansion process. SUMMARY

[0005] In view of the problems of the silicon-based negative electrode material at present, the application provides a way of modifying the surface of the silicon-based negative electrode material by coating a polymer and a conductive agent, and prepares an artificial SEI film to solve the problems of structure breaking and SEI repeated growth caused by the volume expansion of the silicon-based particles. The polymer used has a highly cross-linked structure, a smooth ion conduction path and good flexibility, and the three-dimensional cross-linked network structure and high elasticity formed can inhibit the volume expansion of the 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 electron conduction path, guarantee the electrical contact of the negative electrode particles and improve the cycle and rate performance. The silicon-carbon negative electrode material prepared has higher initial efficiency and more stable cycle performance, and the process steps are simple and efficient.

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

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

[0008] The polymer includes a cross-linked copolymerized modified polyethylene oxide.

[0009] The polymer is a cross-linked copolymerized modified polyethylene oxide, which forms a three-dimensional network structure after modification, greatly improves the flexibility and strength, and is conducive to buffering the volume expansion of the silicon-based particles, thereby stabilizing the structure. At the same time, the cross-linked copolymerization modification can also reduce the crystallinity of the polyethylene oxide, optimize the carrier concentration of the polyethylene oxide, and further improve the ion conductivity. The conductive agent forms a conductive network to guarantee the electrical contact of the negative electrode and improve the cycle. At the same time, the presence of the conductive agent can act as a skeleton to improve the mechanical strength of the polymer film and is conducive to maintaining the structural integrity of the silicon-based particles. The lithium salt is easy to dissociate in the polymer matrix to provide more Li + to obtain an electrolyte with higher ion conductivity and improve the initial coulomb efficiency of the negative electrode material; the specific structure diagram is shown in Figure 1 .

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

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

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

[0013] As a preferred technical solution of the present application, the lithium salt comprises lithium bis-trifluoromethanesulfonimide, lithium trifluoromethanesulfonate, lithium bis-oxalate borate, lithium difluoro-oxalate borate, lithium nitrate, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate.

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

[0015] In a second aspect, the present application provides the application of the above artificial SEI film to a lithium ion battery.

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

[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 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 the lithium salt in the artificial SEI film to the polymer is 0.5-5%.

[0022] In a fourth aspect, the present application provides a preparation method of the above silicon-based negative electrode material for a lithium ion battery, 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) to obtain a slurry by dispersion;

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

[0026] (4) after cross-linking and curing the powder material of step (3), a silicon-based negative electrode material for lithium ion batteries.

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

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

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

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

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

[0032] Advantages of the present application

[0033] (1) The polymer is a cross-linked and copolymerized modified polyethylene oxide, which forms a three-dimensional network structure after modification, greatly improves the flexibility and strength, and is conducive to buffering the volume expansion of silicon-based particles, thereby stabilizing the structure. At the same time, cross-linking and copolymerization 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 the electrical contact of the negative electrode and improve the cycle. At the same time, the presence of the conductive agent can play a role as a skeleton to improve the mechanical strength of the polymer film, which is conducive to maintaining the structural integrity of the silicon-based particles. Lithium salt is easily dissociated in the polymer matrix, providing more Li + to obtain an electrolyte with higher ionic conductivity, and to improve the first coulombic efficiency of the negative electrode material.

[0034] (2) Excellent electrochemical performance: as a lithium ion battery negative electrode material, the surface artificial SEI film 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%, and in the process of making silicon-based negative electrode sheets, there is no need to add high-cost single-walled carbon nanotubes.

[0035] (3) The synthesis process of the present application is simple, easy to control, safe and reliable, low in production cost, high in yield, and suitable for industrialized batch production. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the specific embodiments or the prior art of the present application, the drawings needed to be used in the description of the specific embodiments or the prior art will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0037] Figure 1 Structure diagram of the artificial SEI film on the surface of the silicon-based material in the present application patent;

[0038] Figure 2 Scanning electron microscope image of the sample of Example 1;

[0039] Figure 3 First charge-discharge diagram of Example 1 and Comparative Example 1 as the negative electrode of a lithium ion battery;

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

[0041] Example 1

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

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

[0044] (2) 100g of commercial silicon-carbon composite material, 0.05g of carbon nanotube conductive agent and 0.25g of lithium bis-trifluoromethanesulfonimide are added to the above solution, and ultrasonic dispersion is performed to form a uniform slurry;

[0045] (3) The above slurry is subjected to spray drying treatment, and the spray drying temperature is 250℃, to obtain a powder material in the form of particles;

[0046] (4) After the powder material is treated at 150℃ for 3h for crosslinking and curing, the surface modified silicon-based negative electrode material is obtained after magnetic screening and screening.

[0047] Figure 2 The scanning electron microscope results show that after surface modification, the silicon-carbon particles are attached to a dense polymer and carbon nanotube network on the surface, forming a tight coating layer. The electrochemical results show that Figure 3 、 Figure 4The first charge-discharge efficiency of the surface-modified silicon-carbon material is 93%, which is much higher than the first charge-discharge efficiency of 88% of the unmodified silicon-carbon material, the first charge specific capacity is 2150 mAh / g, and the capacity retention rate is 88% after 1000 cycles, 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) 2.5g of polyethylene oxide (PEO) and 0.5g of polyethylene glycol (PEG) are heated and dissolved in 100mL of N-methyl pyrrolidone solvent to form a uniform solution;

[0051] (2) 100g of commercial silicon-oxygen / carbon composite material, 0.1g of graphene conductive agent and 0.06g of lithium difluoroborate are added to the above solution, and ultrasonic dispersion is performed to form a uniform slurry;

[0052] (3) The slurry is subjected to spray drying treatment, and the spray drying temperature is 220°C, to obtain a powder material in the form of particles;

[0053] (4) After crosslinking and curing treatment of the powder material at 200°C for 1h, the surface-modified silicon-based negative electrode material is obtained by magnetic screening and screening.

[0054] Example 3

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

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

[0057] (2) 500g of commercial silicon particle material, 5g of conductive carbon black conductive agent and 0.075g of lithium nitrate are added to the above solution, and ultrasonic dispersion is performed to form a uniform slurry;

[0058] (3) The slurry is subjected to spray drying treatment, and the spray drying temperature is 280°C, to obtain a powder material in the form of particles;

[0059] (4) After crosslinking and curing treatment of the powder material at 100°C for 5h, the surface-modified silicon-based negative electrode material is obtained by magnetic screening and screening.

[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) 200 g of a commercial silicon / carbon composite material, 2.0 g of carbon fiber, and 0.05 g of lithium hexafluorophosphate were added to the above solution and ultrasonically dispersed to form a uniform slurry;

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

[0065] (4) After the powder material was treated at 150°C for 2 h for crosslinking and curing, the surface-modified silicon-based anode material was obtained after magnetic screening and sieving.

[0066] Comparative Example 1 - without surface modification

[0067] The commercial silicon / carbon composite material was not subjected to surface modification and was directly used as an anode material.

[0068] Comparative Example 2 - without adding a conductive agent

[0069] Compared with Example 1, 0.1 g of graphene conductive agent was not added, and the other conditions were the same as in Example 1.

[0070] Comparative Example 3 - without adding a lithium salt

[0071] Compared with Example 1, 0.06 g of lithium difluorophosphate was not added, and the other conditions were the same as in Example 1.

[0072] Comparative Example 4 - single polymer

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

[0074] Effect Example

[0075] The modified silicon-based materials prepared in Examples 1-4 and Comparative Examples 1-3 were used as anode materials for lithium ion batteries, and the electrochemical performance was compared. The specific experimental process was as follows: the modified silicon-based materials prepared in Examples 1-4 and Comparative Examples 1-3, carbon nanotubes, and CMC binder were mixed in a ratio of 93:2:5; lithium sheet was used as the counter electrode and reference electrode; 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 performance of anode materials prepared in Examples 1-4 and Comparative Examples 1-3

[0077]

[0078] The above merely describes the preferred embodiments of the present application, and is not intended to limit the patent scope of the present application. Any equivalent structural variations or direct / indirect applications in other related technical fields based on the content of the present application description are included in the patent protection scope of the present application.

Claims

1. An application of an artificial SEI thin film in lithium-ion batteries, characterized in that, The film comprises a polymer, a conductive agent, and a lithium salt; the polymer includes cross-linked copolymerized polyethylene oxide. The crosslinked copolymerized modified polyethylene oxide includes crosslinked copolymers and polyethylene oxide; The crosslinked copolymer includes at least one of polyacrylic acid, polyacrylate, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, polyacrylonitrile, polymethyl methacrylate, and polyvinylidene fluoride; The ratio of crosslinked copolymer to polyethylene oxide is 0.2–5:1; The lithium salts include lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium dioxalate borate, lithium difluorooxalate borate, lithium tetrafluoroborate, lithium hexafluoroarsenate, and lithium hexafluorophosphate.

2. The application according to claim 1, characterized in that, The conductive agent includes at least one of carbon nanotubes, graphene, conductive carbon black, carbon fiber, and graphite.

3. The application according to claim 2, characterized in that, Applications as silicon-based anode materials for lithium-ion batteries.

4. A silicon-based anode material for lithium-ion batteries, characterized in that, The negative electrode material includes a silicon-based material and an artificial SEI film as described in claim 1 or 2, which is wrapped around the surface of the silicon-based material.

5. The silicon-based anode material for lithium-ion batteries according to claim 4, characterized in that, The silicon-based material is at least one of silicon-carbon, silicon oxide / carbon, silicon / polymer, and silicon particles.

6. The silicon-based anode material for lithium-ion batteries according to claim 4 or 5, characterized in that, The polymer in the artificial SEI film accounts for 0.1% to 3% of the mass of the silicon-based material; And / or, the conductive agent in the artificial SEI film accounts for 0.05 to 1% of the mass of the silicon-based material; And / or, the lithium salt in the artificial SEI film accounts for 0.5% to 5% of the polymer by mass.

7. A method for preparing a silicon-based anode material for lithium-ion batteries according to any one of claims 4-6, comprising the following steps: (1) Dissolve the polymer to obtain a solution; (2) Add conductive agent, lithium salt and silicon-based material to the solution in step (1) and disperse to obtain slurry; (3) The slurry from step (2) is dried to obtain powder material; (4) After the powder material in step (3) is subjected to cross-linking and curing treatment, silicon-based anode material for lithium-ion batteries is obtained.

8. The preparation method according to claim 7, characterized in that, The conditions for dissolving in step (1) include heating in an organic solvent; And / or, the drying in step (3) includes spray drying; And / or, the heating conditions in step (4) are: heating temperature 100-200℃, heating time 1-5h.

9. The preparation method according to claim 8, characterized in that, The heating conditions are: temperature 50–150°C, polymer solid content 0.5–5%.

10. The preparation method according to claim 8 or 9, characterized in that, The organic solvent is one or more of dimethyl sulfoxide, N-methylpyrrolidone, and tetrahydrofuran.

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