Lithium ion battery and preparation method thereof

Abstract

The invention provides a lithium ion battery and a preparation method thereof. The lithium ion battery comprises a positive pole piece, a negative pole piece, a diaphragm, electrolyte and a packaging film, wherein the diaphragm is arranged between the positive pole piece and the negative pole piece; the negative electrode piece comprises a negative electrode current collector and a negative electrode active substance layer arranged on the surface of the negative electrode current collector, the negative electrode active substance layer comprises a silicon-based negative electrode material, a negative electrode conductive agent and a negative electrode binder, the silicon-based negative electrode material is of a core-shell structure, the core is nano silicon particles, and the shell is a composite layer composed of carbon nanotubes and graphene. According to the invention, the surface of the nano silicon particle is coated with the composite layer which has high conductivity and flexibility and is composed of the carbon nano tube and the graphene, and the composite layer can effectively buffer the volume expansion of the nano silicon, maintain the integrity of the electrode structure, and facilitate the improvement of the electron conduction rate of the electrode; therefore, the lithium ion battery can stably work under high-rate charging and discharging conditions, and the energy density of the battery is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and in particular relates to a lithium ion battery and a preparation method thereof. Background Art

[0002] With the rapid development of modern technology, various electronic devices and electric vehicles are placing increasingly stringent demands on battery performance. Traditional lithium-ion batteries are increasingly unable to meet the energy density requirements of, for example, long-range electric vehicles. In terms of cycle life, capacity decay is evident after repeated charge and discharge cycles, impacting the long-term use of the device. Furthermore, safety risks include overheating and fire. For example, in some existing liquid lithium-ion batteries, the electrolyte easily evaporates or even burns in high-temperature environments or when overcharged, posing a threat to the user's life and property. Furthermore, energy density limitations force electronic devices to increase battery size or weight to ensure battery life, which becomes a significant obstacle in applications such as wearable devices and drones, where weight and space requirements are extremely high.

[0003] Therefore, it is urgent to develop a lithium-ion battery with better performance. Summary of the Invention

[0004] In response to the shortcomings of the prior art, the present invention aims to provide a lithium-ion battery and a method for preparing the same. This invention optimizes the structure of the silicon-based negative electrode material in the negative electrode plate, coating the surface of the nano-silicon particles with a highly conductive and flexible composite layer composed of carbon nanotubes and graphene. This composite layer effectively buffers the volume expansion of the nano-silicon, maintains the integrity of the electrode structure, and helps improve the electrode's electronic conduction rate, enabling the lithium-ion battery to operate stably under high-rate charge and discharge conditions. This significantly increases the battery's energy density and meets the demand for high-performance batteries in modern electronic devices and electric vehicles.

[0005] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte, and a packaging film, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet.

[0007] Among them, the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector. The negative electrode active material layer includes a silicon-based negative electrode material, a negative electrode conductor and a negative electrode binder. The silicon-based negative electrode material is a core-shell structure, the core is nano-silicon particles, and the shell is a composite layer composed of carbon nanotubes and graphene.

[0008] The present invention optimizes the structure of the silicon-based negative electrode material in the negative electrode plate, and coats the surface of the nano-silicon particles with a composite layer composed of carbon nanotubes and graphene with high conductivity and flexibility. The composite layer can effectively buffer the volume expansion of the nano-silicon, maintain the integrity of the electrode structure, and is conducive to improving the electronic conduction rate of the electrode, so that the lithium-ion battery can operate stably under high-rate charge and discharge conditions, greatly improving the energy density of the battery, and meeting the demand for high-performance batteries in modern electronic equipment and electric vehicles.

[0009] The present invention optimizes the structure of the silicon-based negative electrode material in the negative electrode plate, so that the battery can still maintain a high capacity retention rate after multiple charge and discharge cycles, significantly extending the service life of the battery and reducing the cost of use.

[0010] The present invention does not limit the types of negative electrode conductor and negative electrode binder. Exemplarily, the negative electrode conductor is carbon nanotube, conductive carbon black or conductive graphite, and the negative electrode binder is carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid or polyimide.

[0011] Preferably, the particle size D50 of the nano-silicon particles is 50-200 nm, for example, 50 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm or 200 nm.

[0012] Preferably, the thickness of the composite layer is 5-150 nm, for example, it can be 5 nm, 8 nm, 10 nm, 12 nm, 15 nm, 18 nm, 20 nm, 30 nm, 40 nm, 50 nm, 100 nm or 150 nm.

[0013] In the present invention, a suitable composite layer thickness helps to fully buffer the volume expansion of nano-silicon during the charge and discharge process, while not causing an excessively long electron transmission path due to excessive thickness, thereby affecting the overall performance of the battery.

[0014] Preferably, the composite layer is formed of graphene on a side close to the core, and gradually transitions to carbon nanotubes in a direction away from the core.

[0015] In the present invention, the side of the composite layer close to the core is designed to be graphene, and gradually transitions to carbon nanotubes along the direction away from the core. The graphene close to the core, with its high plane coverage, high mechanical strength and flexibility, can serve as a "rigid skeleton" to constrain the initial expansion of silicon, while improving the interface bonding force; when transitioning outward to carbon nanotubes, the elastic deformation ability of its one-dimensional tubular structure can buffer larger volume changes, forming a "rigid-flexible transition" stress release mechanism, reducing the interface peeling between the composite layer and the core, and improving structural stability. In addition, this gradient design can construct a continuous conductive channel, avoiding the conductive bottleneck caused by phase interface resistance in traditional composite materials, significantly reducing the electron transmission resistance of the entire composite layer, and having significant advantages in improving battery rate performance and cycle stability.

[0016] Preferably, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer arranged on the surface of the positive electrode current collector, the positive electrode active material layer includes a composite positive electrode material, a positive electrode conductor and a positive electrode binder, the composite positive electrode material includes a core and a coating layer arranged on the surface of the core, the core includes a ternary positive electrode material, and the coating layer includes a MOF-derived material.

[0017] In the present invention, the coating layer constructed from MOF-derived materials can act as a mechanical buffer skeleton to inhibit structural distortion, and can also reduce the reaction between residual lithium on the surface of the ternary material and the electrolyte, thereby reducing interfacial impedance.

[0018] The present invention does not limit the types of the positive electrode conductor and the positive electrode binder. Exemplarily, the positive electrode conductor is graphene, carbon nanotubes, conductive carbon black or conductive graphite, and the positive electrode binder is polyvinylidene fluoride.

[0019] Preferably, the chemical formula of the ternary cathode material is LiNi x Mn y Co 1-x-y O2, 0<x<1, for example, it can be 0.2, 0.4, 0.6 or 0.8, etc., 0<y<1, for example, it can be 0.2, 0.4, 0.6 or 0.8, etc., x+y<1.

[0020] In the present invention, LiNi x Mn y Co 1-x-y O2's ternary positive electrode material has excellent lithium ion insertion and extraction efficiency, and the material has good stability, which is beneficial to improving the battery's charge and discharge performance and cycle life.

[0021] Preferably, the coating layer has a thickness of 5-10 nm, for example, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm or 10 nm.

[0022] Preferably, the coating layer is a porous structure.

[0023] In the present invention, the porous coating layer can effectively inhibit the side reaction between the positive electrode material and the electrolyte, while promoting the rapid diffusion of lithium ions, further improving the battery performance.

[0024] Preferably, the MOF-derived material comprises any one or a combination of at least two of a MOF-derived carbon material, a MOF-derived metal material, a MOF-derived metal oxide material, or an amino-functionalized MOF derivative. For example, the MOF-derived carbon material may be, for example, a ZIF-67-derived carbon material, the MOF-derived metal material may be, for example, a MOF-derived cobalt metal, the MOF-derived metal oxide material may be, for example, a MOF-derived manganese dioxide, and the amino-functionalized MOF derivative may be, for example, NH2-MIL-101(Fe).

[0025] Preferably, the diaphragm comprises a ceramic diaphragm.

[0026] In the present invention, the ceramic diaphragm has high mechanical strength and good thermal stability. It can not only effectively prevent short circuits between the positive and negative electrodes, but also further isolate the positive and negative electrodes through its own thermal expansion characteristics when the battery temperature rises, preventing the occurrence of thermal runaway and significantly improving the safety of the battery.

[0027] Preferably, the electrolyte comprises an ionic liquid, a lithium salt, and a functional additive. The ionic liquid comprises an imidazole ionic liquid and a pyridine ionic liquid. For example, the imidazole ionic liquid may be, for example, 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid, the pyridine ionic liquid may be, for example, N-butylpyridinium tetrafluoroborate ionic liquid, and the lithium salt may be, for example, lithium hexafluorophosphate.

[0028] In the present invention, pyridine ionic liquids, due to their excellent thermal stability and low viscosity, can optimize the electrolyte's ion migration number and reduce interfacial impedance when mixed with imidazole ionic liquids. Furthermore, the high safety characteristics of ionic liquid-based electrolytes enable stable battery operation under extreme conditions such as high temperature, overcharge, and over-discharge, significantly improving battery safety and reducing the risk of safety accidents. Furthermore, their high ionic conductivity ensures rapid lithium ion transfer between the positive and negative electrodes, improving the battery's charge and discharge efficiency.

[0029] In the present invention, the addition of functional additives further improves the interface compatibility between the electrolyte and the electrode material, reduces the interface resistance, and helps to extend the cycle life of the battery.

[0030] Preferably, the mass ratio of the imidazole ionic liquid to the pyridine ionic liquid is (2-5):1, for example, 2:1, 3:1, 4:1 or 5:1.

[0031] Preferably, the ionic liquid further comprises a quaternary phosphonium salt ionic liquid. For example, the quaternary phosphonium salt ionic liquid may be, for example, a tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide salt ionic liquid.

[0032] The present invention can also add quaternary phosphonium salt ionic liquid to the ionic liquid, which broadens the electrochemical window of the electrolyte by adjusting the anion and cation structure, enhances the adaptability to the electrode material, and thus significantly improves the overall performance of the battery.

[0033] Preferably, the mass ratio of the imidazole ionic liquid to the quaternary phosphonium salt ionic liquid is (3-6):1, for example, 3:1, 4:1, 5:1 or 6:1.

[0034] Preferably, the functional additive includes any one of fluoroethylene carbonate, tris(trimethylsilyl)phosphate, hexamethyldisiloxane, dimethyl carbonate, ethyl methyl carbonate or vinylene carbonate, or a combination of at least two thereof.

[0035] Preferably, based on the total mass of the electrolyte, the content of the functional additive is 4-6 wt%, for example, 4 wt%, 4.5 wt%, 5 wt%, 5.5 wt% or 6 wt%.

[0036] Preferably, the concentration of the lithium salt in the electrolyte is 0.5-1.5 mol / L, for example, 0.5 mol / L, 0.75 mol / L, 1 mol / L, 1.25 mol / L or 1.5 mol / L.

[0037] Preferably, the positive electrode sheet, separator and negative electrode sheet are stacked or wound to form a battery cell, the battery cell is covered by the packaging film, and the electrolyte is filled in the gap between the packaging film and the battery cell, as well as the interior of the battery cell.

[0038] The present invention forms a battery core by laminating or winding, which can ensure the precise alignment and tight fit of the electrode sheets and the diaphragm.

[0039] Preferably, the packaging film comprises an aluminized polyester film layer, a thermoplastic elastomer layer and a polyvinylidene fluoride layer which are stacked, and the aluminized polyester film layer is close to one side of the battery core.

[0040] In the present invention, the polyvinylidene fluoride layer has excellent chemical corrosion resistance and mechanical strength, and can resist erosion by external chemical substances and physical impact; the thermoplastic elastomer layer has good flexibility and sealing properties, and can adapt to the deformation of the battery during use, ensuring the sealing effect; the aluminum-plated polyester film layer (VMPET) has excellent oxygen and moisture barrier properties, which can effectively isolate external moisture and oxygen, preventing the volatilization of the electrolyte inside the battery and the oxidation of the electrode material. The three layers of material are tightly compounded to form an integrated packaging structure (i.e., packaging film) with excellent airtightness, waterproofness, and chemical corrosion resistance. This integrated packaging structure effectively reduces the contact between the interior of the battery and the external environment, avoids the volatilization of the electrolyte and the intrusion of moisture, and extends the service life of the battery. At the same time, it improves the overall mechanical strength of the battery, making it able to adapt to more complex usage environments.

[0041] Preferably, the material of the thermoplastic elastomer layer includes ethylene-vinyl acetate copolymer.

[0042] Preferably, the thickness ratio of the aluminum-plated polyester film layer, the thermoplastic elastomer layer and the polyvinylidene fluoride layer is (2-5):(2-4):(1-3), wherein the selection range of the aluminum-plated polyester film layer "2-5" can be, for example, 2, 3, 4 or 5, etc., the selection range of the thermoplastic elastomer layer "2-4" can be, for example, 2, 3 or 4, etc., and the selection range of the polyvinylidene fluoride layer "1-3" can be, for example, 1, 2 or 3, etc.

[0043] In the present invention, the appropriate thickness ratio of the three layers of materials can fully utilize the advantages of each layer of material and ensure the overall performance of the packaging film.

[0044] In a second aspect, the present invention provides a method for preparing a lithium-ion battery as described in the first aspect, the method comprising the following steps:

[0045] (1) Preparation of negative electrode sheet:

[0046] The nano-silicon organic dispersion is placed in a CVD reactor, and a mixture of carbon source gas and catalyst gas is introduced for chemical vapor deposition, so that a composite layer composed of carbon nanotubes and graphene is formed on the surface of the nano-silicon particles to obtain a silicon-based negative electrode material.

[0047] The silicon-based negative electrode material, the negative electrode conductive agent and the negative electrode binder are mixed in a solvent to obtain a negative electrode slurry, and then the negative electrode slurry is coated on a negative electrode current collector and dried to obtain a negative electrode sheet.

[0048] (2) Assembling the negative electrode plate, the separator and the positive electrode plate, then packaging them with a packaging film, and injecting an electrolyte to obtain the lithium-ion battery.

[0049] In the present invention, a composite layer composed of carbon nanotubes and graphene is grown on the surface of nano-silicon particles using chemical vapor deposition (CVD) technology, which can accurately control the structure and thickness of the composite layer, ensure its uniformity and stability, and thus effectively improve the performance of silicon-based negative electrode materials.

[0050] Preferably, the method for preparing the nano-silicon organic dispersion includes:

[0051] The nano-silicon particles are dispersed in an organic solvent to obtain the nano-silicon organic dispersion. For example, the organic solvent may be an ethanol solution.

[0052] Preferably, the dispersion process is accompanied by ultrasound, which is used to disperse the nano-silicon particles uniformly.

[0053] Preferably, the carbon source gas includes any one of methane, ethylene or acetylene, or a combination of at least two of them.

[0054] Preferably, the catalyst gas comprises hydrogen.

[0055] Preferably, the volume ratio of the carbon source gas and the catalyst gas is (1-6): (1-6), wherein the selection range of the carbon source gas "1-6" can be, for example, 1, 2, 3, 4, 5 or 6, etc., and the selection range of the catalyst gas "1-6" can be, for example, 1, 2, 3, 4, 5 or 6, etc.

[0056] In the present invention, an appropriate volume ratio of the carbon source gas to the catalyst gas ensures efficient chemical vapor deposition (CVD) reaction, facilitating the formation of a carbon nanotube and graphene composite layer with good structure and performance on the surface of the nano-silicon particles. An inappropriate volume ratio can result in an excessively fast or slow reaction rate, affecting the quality of the composite layer.

[0057] Preferably, during the chemical vapor deposition process, the volume ratio of the carbon source gas and the catalyst gas and / or the deposition temperature are dynamically controlled so that in the formed composite layer, the side close to the nano-silicon particles is graphene, and gradually transitions to carbon nanotubes in the direction away from the nano-silicon particles.

[0058] The present invention realizes a gradual structural transition by dynamically regulating the deposition temperature and / or gas volume ratio during the chemical vapor deposition process, so that in the formed composite layer, the side close to the nano-silicon particles is graphene, and gradually transitions to carbon nanotubes in the direction away from the nano-silicon particles.

[0059] It should be noted that the gas volume ratio and / or deposition temperature when forming graphene is lower than the gas volume ratio and / or deposition temperature when forming carbon nanotubes.

[0060] Preferably, during the chemical vapor deposition process, the deposition temperature is in the range of 650-850°C, for example, 650°C, 700°C, 720°C, 740°C, 760°C, 780°C, 800°C or 850°C.

[0061] In the present invention, a suitable deposition temperature helps to promote the decomposition of the carbon source gas under the action of the catalyst gas and deposit it on the surface of the nano-silicon particles to form carbon nanotubes and graphene. If the temperature is too low, the reaction is difficult to proceed, and if it is too high, it may cause the nano-silicon particles to sinter or the composite layer structure to be destroyed.

[0062] Preferably, during the chemical vapor deposition process, the deposition pressure is in the range of 100-150 Pa, for example, it may be 100 Pa, 110 Pa, 120 Pa, 130 Pa, 140 Pa or 150 Pa.

[0063] In the present invention, an appropriate deposition pressure helps control the diffusion and reaction rate of the reaction gases within the reactor, ensuring uniform growth of the composite layer. Too low a pressure reduces the probability of gas molecule collisions and slows the reaction; too high a pressure results in an overly violent reaction, hindering the orderly formation of the composite layer.

[0064] Preferably, the deposition time of the chemical vapor deposition is 1-3 hours, for example, 1 hour, 1.5 hours, 2 hours, 2.5 hours or 3 hours.

[0065] Preferably, the method for preparing the positive electrode sheet in step (c) comprises:

[0066] (1) The ternary cathode material and the MOF-derived material are mixed to obtain a mixture, and then the mixture is calcined to obtain a composite cathode material.

[0067] (2) The composite positive electrode material, the positive electrode conductive agent and the positive electrode binder are mixed in a solvent to obtain a positive electrode slurry, and then the positive electrode slurry is coated on a positive electrode current collector, and dried to obtain a positive electrode sheet.

[0068] Preferably, the temperature of the calcination treatment in step (1) is 500-800°C, for example, 500°C, 600°C, 700°C or 800°C.

[0069] Preferably, the calcination treatment time in step (1) is 2-5 hours, for example, it can be 2 hours, 3 hours, 4 hours or 5 hours.

[0070] Preferably, after the electrolyte is injected, it is further dried and heat activated.

[0071] The present invention allows the electrolyte to fully infiltrate the electrode material through drying and heat activation treatment, optimizes the interface performance inside the battery, and improves the initial performance and consistency of the battery.

[0072] Preferably, the drying method includes vacuum drying.

[0073] Preferably, the temperature of the heat activation treatment is 50-70°C, for example, 50°C, 55°C, 60°C, 65°C or 70°C, and the time is 1.5-2.5h, for example, 1.5h, 2h or 2.5h.

[0074] The numerical range described in the present invention includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0075] Compared with the prior art, the present invention has the following beneficial effects:

[0076] (1) The present invention optimizes the structure of the silicon-based negative electrode material in the negative electrode plate, and coats the surface of the nano-silicon particles with a composite layer composed of carbon nanotubes and graphene with high conductivity and flexibility. The composite layer can effectively buffer the volume expansion of the nano-silicon, maintain the integrity of the electrode structure, and is conducive to improving the electronic conduction rate of the electrode, so that the lithium-ion battery can operate stably under high-rate charge and discharge conditions, greatly improving the energy density of the battery, and meeting the demand for high-performance batteries in modern electronic equipment and electric vehicles.

[0077] (2) The present invention optimizes the structure of the silicon-based negative electrode material in the negative electrode plate, so that the battery can still maintain a high capacity retention rate after multiple charge and discharge cycles, significantly extending the service life of the battery and reducing the cost of use. DETAILED DESCRIPTION

[0078] 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.

[0079] Example 1

[0080] This embodiment provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte, and a packaging film. The separator is arranged between the positive electrode sheet and the negative electrode sheet.

[0081] Among them, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector. The negative electrode active material layer includes a silicon-based negative electrode material, a negative electrode conductive agent and a negative electrode binder. The silicon-based negative electrode material is a core-shell structure, the core is nano-silicon particles, and the shell is a composite layer composed of carbon nanotubes and graphene; the negative electrode collector is copper foil, the negative electrode conductive agent is conductive carbon black, and the negative electrode binder is carboxymethyl cellulose; the particle size D50 of the nano-silicon particles is 80nm, the thickness of the composite layer is 150nm, the side of the composite layer close to the core is graphene, and gradually transitions to carbon nanotubes as it moves away from the core.

[0082] Among them, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer arranged on the surface of the positive electrode current collector. The positive electrode active material layer includes a composite positive electrode material, a positive electrode conductive agent and a positive electrode binder. The composite positive electrode material includes a core and a coating layer arranged on the surface of the core. The core includes a ternary positive electrode material, and the coating layer includes a MOF derivative material. The coating layer is a porous structure. The positive electrode current collector is aluminum foil, the positive electrode conductive agent is conductive carbon black, and the positive electrode binder is polyvinylidene fluoride. The chemical formula of the ternary positive electrode material is LiNi 0.3 Mn 0.4 Co 0.3 O2; the thickness of the coating layer is 8 nm, and the MOF-derived material is a ZIF-8-derived carbon material.

[0083] Among them, the diaphragm is a ceramic diaphragm; the electrolyte includes ionic liquid, lithium salt and functional additives, the ionic liquid includes imidazole ionic liquid, quaternary phosphonium salt ionic liquid and pyridine ionic liquid, the mass ratio of imidazole ionic liquid to pyridine ionic liquid is 3:1, and the mass ratio of imidazole ionic liquid to quaternary phosphonium salt ionic liquid is 5:1; the imidazole ionic liquid is 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid, the pyridine ionic liquid is N-butylpyridinium tetrafluoroborate ionic liquid, and the quaternary phosphonium salt ionic liquid is tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide salt ionic liquid; the functional additive is fluoroethylene carbonate, and the content of the functional additive is 5wt% based on the total mass of the electrolyte; the lithium salt is lithium hexafluorophosphate, and the concentration of the lithium salt in the electrolyte is 1 mol / L.

[0084] Among them, the positive electrode plate, the separator and the negative electrode plate are stacked or wound to form a battery cell, the battery cell is covered with a packaging film, and the electrolyte is filled in the gap between the packaging film and the battery cell, as well as the inside of the battery cell; the packaging film includes a stacked aluminum-plated polyester film layer, an ethylene-vinyl acetate copolymer layer and a polyvinylidene fluoride layer, the aluminum-plated polyester film layer is close to the side of the battery cell, and the thickness ratio of the aluminum-plated polyester film layer, the ethylene-vinyl acetate copolymer layer and the polyvinylidene fluoride layer is 2:3:1.

[0085] This embodiment also provides a method for preparing the above-mentioned lithium-ion battery, the method comprising the following steps:

[0086] (1) Preparation of negative electrode sheet:

[0087] (i) Ultrasonic dispersion of nano-silicon particles in an ethanol solution for 30 min to obtain a nano-silicon organic dispersion.

[0088] (ii) placing a nano-silicon organic dispersion in a CVD reactor, and introducing a mixture of a carbon source gas and a catalyst gas in a volume ratio of 1:5, and performing chemical vapor deposition at 700°C for 0.5 h to form graphene; then gradually increasing the volume ratio of the carbon source gas to the catalyst gas to 1:3, and raising the deposition temperature to 850°C to curl the graphene edge and induce the nucleation of carbon nanotubes; maintaining the deposition temperature and the volume ratio of the carbon source gas to the catalyst gas constant, and reacting for 1 h to vertically grow carbon nanotubes on the graphene surface to form a composite layer consisting of carbon nanotubes and graphene, thereby obtaining a silicon-based negative electrode material.

[0089] The carbon source gas is methane and the catalyst gas is hydrogen. During the chemical vapor deposition process, the deposition pressure is constant at 100 Pa and the total deposition time is 2 h.

[0090] (iii) mixing a silicon-based negative electrode material, a negative electrode conductive agent, and a negative electrode binder in a solvent to obtain a negative electrode slurry, then coating the negative electrode slurry on a copper foil, and drying to obtain a negative electrode sheet.

[0091] (2) Preparation of positive electrode sheet:

[0092] (a) Weigh lithium acetate, cobalt acetate, nickel acetate and manganese acetate, and 0.3 Mn 0.4 Co 0.3 The stoichiometric ratio of O2 is dissolved in ethylene glycol methyl ether to form a uniformly mixed precursor solution; citric acid is added to the precursor solution as a chelating agent, and after stirring evenly, the solvent is evaporated at 80°C to form a gel; the gel is dried at 120°C for 12 hours to obtain a precursor material; the precursor material is placed in a high-temperature furnace, and in an oxygen atmosphere, the temperature is first increased to 400°C at a heating rate of 5°C / min, and kept warm for 2 hours, and then the temperature is increased to 800°C at a heating rate of 3°C / min, and kept warm for 5 hours to obtain a ternary positive electrode material.

[0093] (b) mixing the ternary cathode material and the MOF-derived material to obtain a mixture, and then calcining the mixture to obtain a composite cathode material; wherein the calcination temperature is 600° C. and the calcination time is 3 hours.

[0094] (c) The composite positive electrode material, the positive electrode conductive agent and the positive electrode binder are mixed in a solvent to obtain a positive electrode slurry, and then the positive electrode slurry is coated on an aluminum foil and dried to obtain a positive electrode sheet.

[0095] (3) Preparation of electrolyte:

[0096] 1-Ethyl-3-methylimidazolium tetrafluoroborate ionic liquid, N-butylpyridinium tetrafluoroborate ionic liquid, tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide salt ionic liquid and lithium hexafluorophosphate were mixed to prepare a solution with a concentration of 1 mol / L lithium hexafluorophosphate, and then 5% by mass of vinylene carbonate was added as a functional additive, and the mixture was stirred evenly to obtain an electrolyte.

[0097] (4) Using automated lamination equipment, the negative electrode sheets, ceramic diaphragms, and positive electrode sheets are laminated and assembled, ensuring that the alignment accuracy of the electrode sheets and diaphragms is within ±0.1 mm to obtain a battery cell.

[0098] The battery core is packaged and integrated using a hot pressing molding technology to form a packaging film on the outside of the battery core; wherein the temperature of the hot pressing molding technology is 150° C. and the pressure is 5 MPa.

[0099] Inject electrolyte and then place in a vacuum of 1×10 -3 The lithium ion battery was obtained by vacuum drying at 400 nm and 800 nm under the conditions of 1000 nm and 1000 nm, respectively, for 12 h and then performing heat activation treatment at 60° C. for 2 h.

[0100] Example 2

[0101] The difference between this embodiment and embodiment 1 is that in step (2), LiNi 0.3 Mn 0.4 Co 0.3 O2 replaced by LiNi 0.2 Mn 0.5 Co 0.3 O2; in step (3), 5% by mass of vinylene carbonate is replaced by 3% by mass of fluoroethylene carbonate; in step (4), the lamination assembly process is replaced by a winding process at a winding speed of 50 mm / s.

[0102] The rest of the preparation methods and parameters remained the same as in Example 1.

[0103] Example 3

[0104] The difference between this embodiment and embodiment 1 is that the thickness of the composite layer is 3 nm.

[0105] The rest of the preparation methods and parameters remained the same as in Example 1.

[0106] Example 4

[0107] The difference between this embodiment and embodiment 1 is that the thickness of the composite layer is 200 nm.

[0108] The rest of the preparation methods and parameters remained the same as in Example 1.

[0109] Example 5

[0110] The difference between this embodiment and embodiment 1 is that the side of the composite layer close to the core is carbon nanotubes, and gradually transitions to graphene in the direction away from the core. That is, a mixture of carbon source gas and catalyst gas with a volume ratio of 1:3 is first introduced, and chemical vapor deposition is performed at 850°C to form carbon nanotubes, and then the volume ratio of carbon source gas and catalyst gas is gradually reduced to 1:5, and the deposition temperature is gradually reduced to 700°C, thereby gradually transitioning to graphene in the direction away from the nano-silicon particles.

[0111] The rest of the preparation methods and parameters remained the same as in Example 1.

[0112] Example 6

[0113] The difference between this embodiment and embodiment 1 is that the electrolyte does not contain a pyridine ionic liquid.

[0114] The rest of the preparation methods and parameters remained the same as in Example 1.

[0115] Example 7

[0116] The difference between this embodiment and embodiment 1 is that the electrolyte does not contain quaternary phosphonium salt ionic liquid.

[0117] The rest of the preparation methods and parameters remained the same as in Example 1.

[0118] Example 8

[0119] The difference between this embodiment and embodiment 1 is that the packaging film is composed of an ethylene-vinyl acetate copolymer layer and a polyvinylidene fluoride layer stacked together.

[0120] The rest of the preparation methods and parameters remained the same as in Example 1.

[0121] Example 9

[0122] The difference between this embodiment and embodiment 1 is that the packaging film is composed of a laminated aluminum-coated polyester film layer and a polyvinylidene fluoride layer.

[0123] The rest of the preparation methods and parameters remained the same as in Example 1.

[0124] Example 10

[0125] The difference between this embodiment and embodiment 1 is that the packaging film is a polyvinylidene fluoride layer.

[0126] The rest of the preparation methods and parameters remained the same as in Example 1.

[0127] Example 11

[0128] The difference between this embodiment and embodiment 1 is that the volume ratio of the carbon source gas and the catalyst gas in step (1) is constant and is 1:5.

[0129] The rest of the preparation methods and parameters remained the same as in Example 1.

[0130] Example 12

[0131] The difference between this embodiment and embodiment 1 is that during the chemical vapor deposition process in step (1), the deposition temperature is constant, which is 700°C.

[0132] The rest of the preparation methods and parameters remained the same as in Example 1.

[0133] Example 13

[0134] The difference between this embodiment and embodiment 1 is that the electrolyte in step (3) is a 1 mol / L lithium hexafluorophosphate solution, and the solvent is composed of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1.

[0135] The rest of the preparation methods and parameters remained the same as in Example 1.

[0136] Comparative Example 1

[0137] The difference between this comparative example and Example 1 is that steps (i) and (ii) are not performed, and the silicon-based negative electrode material is replaced by a graphite negative electrode material; step (a) is not performed, and the ternary positive electrode material is replaced by a lithium cobalt oxide positive electrode material; in step (3), the electrolyte is a 1 mol / L lithium hexafluorophosphate solution, and the solvent is composed of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1.

[0138] The rest of the preparation methods and parameters remained the same as in Example 1.

[0139] Comparative Example 2

[0140] The difference between this comparative example and Example 1 is that the silicon-based negative electrode material is only nano-silicon material, that is, steps (i) and (ii) are not performed in step (1).

[0141] The rest of the preparation methods and parameters remained the same as in Example 1.

[0142] Comparative Example 3

[0143] The difference between this comparative example and Example 1 is that in the silicon-based negative electrode material, the shell is only carbon nanotubes, that is, in step (1), during chemical vapor deposition, only methane is introduced as the carbon source gas, and no catalyst gas hydrogen is introduced, so that only a carbon nanotube layer is formed on the surface of the nano-silicon particles, without graphene, and a silicon-based negative electrode material with only carbon nanotube shell is obtained.

[0144] The rest of the preparation methods and parameters remained the same as in Example 1.

[0145] Comparative Example 4

[0146] The difference between this comparative example and Example 1 is that in the silicon-based negative electrode material, the shell is only graphene, that is, low-pressure chemical vapor deposition is adopted in step (1), the deposition temperature is 1000°C, the deposition pressure is 10Pa, the carbon source gas is ethylene, and no catalyst gas is introduced, so that only a graphene layer is formed on the surface of the nano-silicon particles, and a silicon-based negative electrode material with a shell of graphene is obtained.

[0147] The rest of the preparation methods and parameters remained the same as in Example 1.

[0148] Performance Testing

[0149] The electrochemical performance of the lithium-ion battery prepared above was tested.

[0150] Energy density test: Using the Xinwei battery test system, charge the lithium-ion battery at a constant current of 0.2C to 4.2V, then charge at a constant voltage until the current drops to 0.05C. Discharge the battery at a constant current of 0.2C to 2.5V, and record the discharge capacity C (in Ah). Calculate the energy density based on the battery mass m and the rated voltage U using the formula: energy density.

[0151] Cycling performance test: Using the Blue Power battery test system, the lithium-ion battery was charged at a constant current of 1C to 4.2V, then charged at a constant voltage until the current dropped to 0.05C, and then discharged at a constant current of 1C to 2.5V. The cycle was repeated and the number of cycles at which the capacity retention rate reached 80% was recorded.

[0152] Safety performance test: Place at 120℃ for 2h.

[0153] The test results are shown in Table 1.

[0154] Table 1

[0155]

[0156]

[0157] analyze:

[0158] As can be seen from Table 1, the lithium-ion battery provided by the present invention has excellent energy density, cycle stability and safety performance.

[0159] By comparing Example 1 with Examples 3-4, it can be seen that if the thickness of the composite layer is too small, it cannot effectively buffer the volume expansion of the nano-silicon particles during the charge and discharge process, resulting in easy damage to the electrode structure, thereby reducing the energy density of the battery and the number of cycles; if the thickness of the composite layer is too large, although it can better buffer the volume expansion, it will increase the internal resistance of the battery and reduce the electron transmission efficiency, which will also lead to a decrease in energy density and cycle performance.

[0160] By comparing Example 1 with Example 5, it can be seen that if the side of the composite layer close to the core is carbon nanotubes and gradually transitions to graphene in the direction away from the core, the structural stability and conductivity of the silicon-based negative electrode material will deteriorate, affecting the high-rate charge and discharge performance of the battery, resulting in a decrease in energy density and number of cycles.

[0161] By comparing Example 1 with Examples 6-7, it can be seen that if the electrolyte does not contain a pyridine ionic liquid, the ionic conductivity of the electrolyte will be affected, resulting in poor low-temperature performance of the battery, and a decrease in energy density and cycle number; if the electrolyte does not contain a quaternary phosphonium salt ionic liquid, the high-temperature stability of the electrolyte is reduced, the performance of the battery decreases during high-temperature storage, and the energy density and cycle number are also affected.

[0162] By comparing Example 1 with Examples 8-10, it can be seen that if the packaging film does not contain an aluminum-coated polyester film layer, the barrier performance of the battery will decrease and it will be easily affected by the external environment, resulting in reduced battery flexibility, reduced energy density and cycle number; if the packaging film does not contain an ethylene-vinyl acetate copolymer layer, the bonding performance and flexibility of the packaging film will deteriorate, and the packaging integrity will be slightly reduced, affecting the battery performance; if the packaging film is a polyvinylidene fluoride layer, the overall performance of the battery will be significantly reduced, the energy density and cycle number will be greatly reduced, and it will be easily affected by the external environment.

[0163] By comparing Example 1 with Example 11, it can be seen that if the volume ratio of the carbon source gas and the catalyst gas is constant, the composite layer structure to be protected by the present application cannot be formed (that is, the side of the composite layer close to the core is graphene, and gradually transitions to carbon nanotubes in the direction away from the core), resulting in unstable performance of the silicon-based negative electrode material, affecting the energy density and cycle performance of the battery.

[0164] By comparing Example 1 with Example 12, it can be seen that if the deposition temperature is constant during the chemical vapor deposition process, it is not conducive to the formation of the composite layer structure to be protected by this application (that is, the side of the composite layer close to the core is graphene, and gradually transitions to carbon nanotubes in the direction away from the core), resulting in poor chemical vapor deposition effect and poor energy density and cycle performance of the battery.

[0165] By comparing Example 1 with Example 13, it can be seen that if the electrolyte is a 1 mol / L lithium hexafluorophosphate solution and the solvent is composed of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1, the overall performance of the battery is not as good as the electrolyte using multiple ionic liquids and functional additives in Example 1, which is manifested in reduced energy density and number of cycles, and the battery becomes slightly heated at high temperature, and a small amount of electrolyte volatilization occurs.

[0166] From the comparison between Example 1 and Comparative Example 1, it can be seen that the lithium ion battery prepared in Comparative Example 1 is inferior to the lithium ion battery prepared in the present application in terms of energy density, cycle life and safety.

[0167] From the comparison between Example 1 and Comparative Example 2, it can be seen that if the silicon-based negative electrode material is only nano-silicon material and no shell is provided, the volume expansion of nano-silicon during the charge and discharge process cannot be effectively buffered, resulting in rapid battery capacity decay and poor cycle performance.

[0168] By comparing Example 1 with Comparative Examples 3-4, it can be seen that if the outer shell of the silicon-based negative electrode material is only carbon nanotubes, the conductivity and structural stability of the battery are insufficient, the high-rate performance is poor, and it is not conducive to fast charging and discharging; if the outer shell of the silicon-based negative electrode material is only graphene, the mechanical properties of the battery are insufficient, and it cannot effectively buffer the volume expansion of nano-silicon, and it is easily affected by external forces, resulting in reduced battery performance.

[0169] It should be noted that while the present invention illustrates the process method through the above-described embodiments, the present invention is not limited to the above-described process steps, and does not necessarily rely on the above-described process steps for implementation. Those skilled in the art will appreciate that any improvements to the present invention, equivalent replacements 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. A lithium-ion battery, characterized in that: The lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte and a packaging film, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet; Among them, the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer arranged on the surface of the negative electrode current collector. The negative electrode active material layer includes a silicon-based negative electrode material, a negative electrode conductor and a negative electrode binder. The silicon-based negative electrode material is a core-shell structure, the core is nano-silicon particles, and the shell is a composite layer composed of carbon nanotubes and graphene.

2. The lithium-ion battery according to claim 1, wherein The particle size D50 of the nano-silicon particles is 50-200 nm; Preferably, the thickness of the composite layer is 5-150 nm.

3. The lithium-ion battery according to claim 1 or 2, characterized in that The side of the composite layer close to the core is graphene, and gradually transitions to carbon nanotubes in a direction away from the core.

4. The lithium-ion battery according to any one of claims 1 to 3, wherein: The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the surface of the positive electrode current collector, the positive electrode active material layer includes a composite positive electrode material, a positive electrode conductive agent and a positive electrode binder, the composite positive electrode material includes a core and a coating layer disposed on the surface of the core, the core includes a ternary positive electrode material, and the coating layer includes a MOF-derived material; Preferably, the chemical formula of the ternary cathode material is LiNi x Mn y Co 1-x-y O2, 0<x<1, 0<y<1, x+y<1; Preferably, the coating layer has a thickness of 5-10 nm; Preferably, the coating layer is a porous structure; Preferably, the MOF-derived material comprises any one or a combination of at least two of a MOF-derived carbon material, a MOF-derived metal material, a MOF-derived metal oxide material or an amino-functionalized MOF derivative.

5. The lithium-ion battery according to any one of claims 1 to 4, characterized in that: The diaphragm includes a ceramic diaphragm; Preferably, the electrolyte comprises an ionic liquid, a lithium salt and a functional additive, and the ionic liquid comprises an imidazole ionic liquid and a pyridine ionic liquid; Preferably, the mass ratio of the imidazole ionic liquid to the pyridine ionic liquid is (2-5):1; Preferably, the ionic liquid further comprises a quaternary phosphonium salt ionic liquid; Preferably, the mass ratio of the imidazole ionic liquid to the quaternary phosphonium salt ionic liquid is (3-6):1; Preferably, the functional additive comprises any one or a combination of at least two of fluoroethylene carbonate, tris(trimethylsilyl)phosphate, hexamethyldisiloxane, dimethyl carbonate, ethyl methyl carbonate or vinylene carbonate; Preferably, based on the total mass of the electrolyte, the content of the functional additive is 4-6 wt%; Preferably, the concentration of lithium salt in the electrolyte is 0.5-1.5 mol / L.

6. The lithium-ion battery according to any one of claims 1 to 5, characterized in that: The positive electrode sheet, the separator and the negative electrode sheet are stacked or wound to form a battery core, the battery core is covered by the packaging film, and the electrolyte is filled in the gap between the packaging film and the battery core, as well as the interior of the battery core; Preferably, the packaging film comprises an aluminized polyester film layer, a thermoplastic elastomer layer and a polyvinylidene fluoride layer which are stacked, and the aluminized polyester film layer is close to one side of the battery core; Preferably, the material of the thermoplastic elastomer layer comprises ethylene-vinyl acetate copolymer; Preferably, the thickness ratio of the aluminum-plated polyester film layer, the thermoplastic elastomer layer and the polyvinylidene fluoride layer is (2-5):(2-4):(1-3).

7. A method for preparing a lithium-ion battery according to any one of claims 1 to 6, characterized in that: The preparation method comprises the following steps: (1) Preparation of negative electrode sheet: The nano-silicon organic dispersion is placed in a CVD reactor, and a mixture of a carbon source gas and a catalyst gas is introduced to perform chemical vapor deposition, so that a composite layer composed of carbon nanotubes and graphene is formed on the surface of the nano-silicon particles to obtain a silicon-based negative electrode material; The silicon-based negative electrode material, the negative electrode conductive agent and the negative electrode binder are mixed in a solvent to obtain a negative electrode slurry, and then the negative electrode slurry is coated on a negative electrode current collector, and dried to obtain a negative electrode sheet; (2) Assembling the negative electrode plate, the separator and the positive electrode plate, then packaging them with a packaging film, and injecting an electrolyte to obtain the lithium-ion battery.

8. The preparation method according to claim 7, characterized in that The method for preparing the nano-silicon organic dispersion comprises: dispersing nano-silicon particles in an organic solvent to obtain the nano-silicon organic dispersion; Preferably, the dispersing process is accompanied by ultrasound; Preferably, the carbon source gas includes any one of methane, ethylene or acetylene, or a combination of at least two thereof; Preferably, the catalyst gas comprises hydrogen; Preferably, the volume ratio of the carbon source gas to the catalyst gas is (1-6):(1-6); Preferably, during the chemical vapor deposition process, the volume ratio of the carbon source gas and the catalyst gas and / or the deposition temperature are dynamically controlled so that in the formed composite layer, the side close to the nano-silicon particles is graphene and gradually transitions to carbon nanotubes in a direction away from the nano-silicon particles; Preferably, during the chemical vapor deposition process, the deposition temperature is in the range of 650-850°C; Preferably, during the chemical vapor deposition process, the deposition pressure is in the range of 100-150 Pa; Preferably, the deposition time of the chemical vapor deposition is 1-3 hours.

9. The preparation method according to claim 7 or 8, characterized in that The method for preparing the positive electrode sheet in step (c) comprises: (1) mixing a ternary cathode material and a MOF-derived material to obtain a mixture, and then calcining the mixture to obtain a composite cathode material; (2) mixing the composite positive electrode material, the positive electrode conductive agent and the positive electrode binder in a solvent to obtain a positive electrode slurry, then coating the positive electrode slurry on a positive electrode current collector, and drying to obtain a positive electrode sheet; Preferably, the temperature of the calcination treatment in step (1) is 500-800°C; Preferably, the calcination treatment time in step (1) is 2-5 hours.

10. The preparation method according to any one of claims 7 to 9, characterized in that: After the electrolyte is injected, it is further dried and heat activated; Preferably, the drying method includes vacuum drying; Preferably, the temperature of the thermal activation treatment is 50-70° C., and the time is 1.5-2.5 hours.

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