Pre-lithiated negative electrode, composite negative electrode, preparation method, and use

Abstract

The negative electrode active material layer of the negative electrode of a lithium-ion battery comprises negative electrode active substances (2) and lithium-carbon materials (3) of a silkworm cocoon structure, the lithium-carbon materials (3) of a silkworm cocoon structure being distributed between the negative electrode active substances (2). The lithium-carbon materials (3) of a silkworm cocoon structure each comprise a cocoon body formed of a structural carbon material and one or more lithium-containing particles contained in the cocoon body. The negative electrode of the lithium-ion battery has a stable structure and a controllable pre-lithiation degree, and is used in conjunction with a conventional positive electrode, thereby significantly improving the initial coulombic efficiency and cycle life of the lithium-ion battery. The preparation method for the negative electrode of the lithium-ion battery is simple, and can implement large-scale industrial production.

Description

Pre-lithiation negative electrode, composite negative electrode and preparation method and use thereof Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a pre-lithiated negative electrode, a composite negative electrode, and a preparation method and use thereof. Background Art

[0002] Lithium-ion batteries (LIBs) are currently the most widely used secondary batteries due to their long cycle life, high energy density, and lack of memory effect. Currently, graphite, silicon-carbon materials, and silicon-oxygen materials are commonly used as negative electrodes in LIBs. However, during the initial charge and discharge process, SEI formation on the negative electrode surface consumes a significant amount of lithium, resulting in a low initial coulombic efficiency (CE). This significantly reduces the actual specific energy of LIBs, severely restricting their large-scale commercial application. Therefore, improving CE has become a hot topic of research.

[0003] Some progress has been made in pre-lithiation using metallic lithium powder. Tesla's patent (CN 108780887 B) discloses an electrode membrane comprising carbon particles, a single metal, and a fibrillated binder; the electrode membrane is self-supporting and substantially free of residual solvent. The carbon particles include graphite particles, and the single metal includes single lithium metal particles. The preparation method comprises combining single lithium metal, a plurality of carbon particles, and a fibrillated binder to form an electrode membrane mixture; and forming an electrode membrane from the electrode membrane mixture, i.e., pre-lithiation using a mixture of metallic lithium powder and graphite. Lithium metal powder is highly reactive and requires stabilization before use. A common method involves forming a lithium carbonate or lithium fluoride layer on its surface, such as passivated lithium powder from FMC. However, these layers have low conductivity, and the metallic lithium powder, after functioning, forms voids in the negative electrode, reducing its conductivity and potentially causing lithium dendrites to form in thinner regions of the negative electrode, hindering negative electrode performance.

[0004] CN114765254A discloses a method for pre-lithiation of negative electrode sheets. This method uses a dry process to mix metallic lithium powder into the electrode sheet to compensate for lithium consumption caused by SEI formation during the initial charge of the battery, thereby effectively increasing the battery's energy density. However, after the metallic lithium powder undergoes inert treatment, the resulting passivation layer has poor conductivity, reducing the utilization rate of the metallic lithium powder. Furthermore, the metallic lithium powder can form holes or cavities in the negative electrode, resulting in uneven current density and the formation of lithium dendrites during cycling. Furthermore, pre-lithiation only addresses the problem of low initial coulombic efficiency and does not increase the negative electrode capacity.

[0005] CN102201565A discloses a high-capacity lithium metal powder composite negative electrode, a preparation method, and a multilayer composite electrode. The high-capacity negative electrode is prepared by mixing lithium metal powder, a binder such as PVDF, and a negative electrode material such as graphite. However, the lithium metal powder reacts with PVDF, degrading or even rendering the PVDF adhesive ineffective and causing some lithium metal loss. Furthermore, the problem of lithium metal volume expansion and dendrite growth during cycling cannot be resolved.

[0006] How to develop a pre-lithiation negative electrode that can precisely control the amount of lithium replenishment, maintain a stable structure after pre-lithiation, maintain a significant decrease in conductivity, and inhibit dendrite growth, as well as a high-capacity composite negative electrode that can fully utilize metallic lithium powder and solve the problems of metallic lithium volume expansion and dendrite growth during the cycle, while taking into account specific capacity, cycle performance, and rate performance, has become an urgent problem to be solved at this stage.

[0007] Summary of the Invention

[0008] In view of the problems existing in the prior art, the present invention provides a pre-lithiation negative electrode, a composite negative electrode, and a preparation method and use thereof. Among them, the pre-lithiation negative electrode selects a lithium-carbon material with a cocoon structure as a lithium supplement, which significantly improves the first coulombic efficiency of the lithium-ion battery. The lithium-carbon material with a cocoon structure contains a structural carbon material with good conductivity and a certain rigidity, which improves the utilization rate of the lithium-containing core, can achieve the improvement of the conductivity and structural stability of the pre-lithiation negative electrode, and can accurately supplement lithium. In addition, a lithium-carbon material with a cocoon structure is formed by constructing a structural carbon material on the surface of lithium-containing particles such as metallic lithium powder and / or lithium alloy powder. When the lithium-carbon material with a cocoon structure is used in a large amount in the negative electrode active material layer, the structural carbon material can cross-entangle with each other to form a stable porous skeleton, which can effectively solve the problems of metallic lithium volume expansion and lithium dendrite growth during the cycle, and at the same time improve the utilization rate of lithium-containing particles.

[0009] To achieve this object, the present invention adopts the following technical solutions:

[0010] A lithium-ion battery negative electrode, wherein the negative electrode active material layer of the negative electrode comprises a negative electrode active material and a cocoon-structured lithium-carbon material, wherein the cocoon-structured lithium-carbon material is distributed between the negative electrode active materials, wherein the cocoon-structured lithium-carbon material comprises a cocoon body formed of a structural carbon material and one or more lithium-containing particles contained in the cocoon body.

[0011] The lithium-ion battery negative electrode of the present invention can be divided into a pre-lithiation negative electrode (with a small content of lithium-carbon material, mainly used for pre-lithiation) and a composite negative electrode (with a large content of lithium-carbon material, in which the structural carbon material forms a porous skeleton in the negative electrode active material layer) according to the content of the cocoon-structured lithium-carbon material.

[0012] In a first aspect, the negative electrode of the lithium-ion battery of the present invention is a pre-lithiated negative electrode, which includes a negative electrode active material and a cocoon-structured lithium-carbon material, wherein the cocoon-structured lithium-carbon material is distributed between the negative electrode active material;

[0013] The mass ratio of the negative electrode active material to the cocoon-structured lithium-carbon material is 1 g: (1-300) mg;

[0014] The mass ratio of the structural carbon material to the lithium-containing particles is 1:(1-1000).

[0015] In the present invention, lithium replenishment is achieved using a cocoon-structured lithium-carbon material, with lithium-containing particles serving as a lithium source, to precisely replenish the negative electrode. The cocoon structure formed by the structural carbon material possesses a certain degree of rigidity, providing good contact with the negative electrode active material, providing support and constructing an electron pathway. Even after the lithium-containing particles are consumed, significant voids are not formed, which would increase the internal resistance of the negative electrode. Furthermore, the cocoon structure provides a good lithium ion pathway, improving the ionic conductivity of the pre-lithiated negative electrode and meeting the requirements of high-rate charge and discharge. The structural carbon material cocoon structure can also absorb and retain electrolyte, enabling the lithium-containing particles to fully utilize their capacity, thus overcoming the drawbacks of lithium replenishment using metallic lithium powder. The pre-lithiated negative electrode offers advantages such as high structural stability, long cycle life, and excellent rate performance.

[0016] Optionally, the negative electrode active material includes graphite, tin-based materials, hard carbon, soft carbon, activated carbon, mesophase carbon microbeads, silicon carbon (Si / C) materials or silicon oxide (SiO x (0<x≤2)) Any one or a combination of at least two of the materials.

[0017] Optionally, the structural carbon material includes any one or a combination of at least two of granular, linear or flaky nanocarbon materials, preferably acetylene black, Ketjen black, Cabot BP2000, Super P, single-walled carbon nanotubes, few-walled carbon nanotubes, multi-walled carbon nanotubes, doped carbon nanotubes, graphene, doped graphene, carbon nanofibers or doped carbon nanofibers, preferably any one or a combination of at least two of single-walled carbon nanotubes, few-walled carbon nanotubes, multi-walled carbon nanotubes, doped carbon nanotubes, graphene or doped graphene.

[0018] Optionally, the doping elements in the doped carbon nanotubes, doped graphene or doped carbon nanofibers independently include any one or a combination of at least two of fluorine, nitrogen, oxygen, phosphorus, silver, silicon, tin or zinc.

[0019] Optionally, the lithium-containing particles include metallic lithium particles and / or metallic lithium alloy particles, preferably metallic lithium alloy particles. A small amount of metallic lithium alloy particles still remain after lithium replenishment to form a skeleton, which cooperates with the structural carbon material and does not produce large voids in the negative electrode, thereby increasing the internal resistance of the negative electrode. At the same time, the deposition of metallic lithium can be regulated, thereby improving the cycle performance of the negative electrode while achieving lithium replenishment.

[0020] Optionally, the mass content of metallic lithium in the metallic lithium alloy particles is not less than 70%, for example, it can be 70%, 72%, 75%, 80%, 83%, 85%, 88%, 90%, 93%, 95%, 97% or 99%, etc., including but not limited to the listed point values. If the mass content is too low, more inactive lithium substances will be introduced, affecting the energy density of the negative electrode, and preferably not less than 85%.

[0021] Optionally, the average particle size of the lithium-containing particles is 1-100 microns, for example, 1 micron, 3 microns, 5 microns, 10 microns, 15 microns, 20 microns, 25 microns, 30 microns, 40 microns, 50 microns, 60 microns, 70 microns, 80 microns, 90 microns or 100 microns, including but not limited to the listed point values, preferably 5-30 microns.

[0022] Optionally, the metallic lithium alloy particles include binary lithium alloys and / or ternary lithium alloys, the binary lithium alloy powder includes any one of lithium magnesium alloys, lithium boron alloys, lithium silver alloys, lithium indium alloys, lithium silicon alloys, lithium aluminum alloys or lithium tin alloys, or a combination of at least two thereof; the ternary lithium alloy powder includes any one of lithium tin silver alloys, lithium silver silicon alloys or lithium boron zinc alloys, or a combination of at least two thereof.

[0023] Optionally, the mass ratio of the structural carbon material to the lithium-containing particles is 1:(1-1000), for example, it can be 1:1, 1:5, 1:10, 1:20, 1:50, 1:55, 1:60, 1:80, 1:90, 1:100, 1:120, 1:150, 1:200, 1:300, 1:400, 1:450, 1:480, 1:500, 1:550, 1:600, 1:700, 1:800, 1:900, 1:950 or 1:1000, etc., including but not limited to the point values, preferably 1:(10-500).

[0024] Optionally, an organic conductive layer is provided between the lithium-containing particles and the structural carbon material, the organic conductive layer comprising an organic lithium ion conductive agent and a conductive agent. The organic conductive layer facilitates the diffusion of lithium ions and further improves the ionic conductivity of the pre-lithiated negative electrode.

[0025] The conductive agent includes at least one of carbon nanotubes, carbon black, graphene, and acetylene black. The organic lithium ion conductive agent includes polyethylene oxide, polyethylene glycol, polysiloxane, polytrimethylene carbonate, polycarbonate, polyethylene carbonate, polypropylene carbonate, polyvinylene carbonate, polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, poly(vinylidene fluoride-hexafluoropropylene), polyphenylene sulfide, p-benzoquinone, and copolymers or mixtures of at least two of the above polymers.

[0026] The mass ratio of the lithium-containing particles, the organic lithium ion transport agent and the conductive agent is (80-95): (1-10): (0.1-10), preferably (90-95): (5-10) (0.5-5).

[0027] Optionally, the mass ratio of the negative electrode active material to the cocoon-structured lithium-carbon material includes but is not limited to 1g:(1-300)mg, for example, it can be 1g:1mg, 1g:3mg, 1g:5mg, 1g:10mg, 1g:15mg, 1g:20mg, 1g:30mg, 1g:50mg, 1g:100mg, 1g:120mg, 1g:150mg, 1g:180mg, 1g:200mg, 1g:240mg or 1g:300mg, etc., including but not limited to the point values. The mass ratio can be selected according to the type of negative electrode active material, the type of cocoon-structured lithium-carbon material, the first coulombic efficiency and actual needs. Generally, the first coulombic efficiency of graphite negative electrode material is higher than that of silicon-containing negative electrode material. For the same type of cocoon-structured lithium-carbon material, the actual demand for graphite negative electrode material is smaller, preferably 1g:(10-200)mg. When the mass ratio is too small, that is, when the mass of the lithium-carbon material in the cocoon structure is relatively large, the pre-lithiation negative electrode not only has the effect of pre-lithiation, but also has the function of storing lithium sources, showing the characteristics of a composite negative electrode. During the battery cycle, the lithium source can be continuously replenished, further improving the cycle performance and energy density of the pre-lithiation negative electrode.

[0028] In a second aspect, the present invention provides a method for preparing a pre-lithiated negative electrode as described in the first aspect above, the method comprising the following steps:

[0029] (1) mixing a structural carbon material, lithium-containing particles, and an organic solvent, and then spray drying or stirring at a rotation speed of more than 5000 rpm to obtain a cocoon-structured lithium-carbon material;

[0030] (2) mixing the negative electrode active material, the conductive agent and the first binder, and then fiberizing the binder to obtain a first mixture, then mixing the first mixture with a lithium-carbon material having a cocoon structure to obtain a second mixture, hot pressing the second mixture into a negative electrode active material layer, and then compounding it with a current collector to obtain the pre-lithiation negative electrode;

[0031] Alternatively, the negative electrode active material, the conductive agent, the second binder, the cocoon-structured lithium-carbon material and the organic solvent are mixed to prepare a negative electrode slurry, which is then coated on the surface of the current collector to obtain the pre-lithiation negative electrode.

[0032] In the present invention, regardless of whether a dry method (without using an organic solvent) or a wet method (using an organic solvent) is used, a cocoon-structured lithium-carbon material can be used as a lithium source, which is mixed into the electrode system during the mixing process to directly prepare a pre-lithiated negative electrode. Preferably, the dry method is used to prepare a pre-lithiated negative electrode. The dry method can prepare a high-energy-density negative electrode and the morphology of the cocoon-structured lithium-carbon material remains more intact.

[0033] Optionally, step (1) further comprises the following operations: (a) mixing lithium-containing particles, an organic lithium ion transport agent, and a conductive agent to form particles consisting of lithium-containing particles and an organic conductive layer coated on the surface thereof;

[0034] (b) uniformly mixing the resulting particles with a structured carbon material in an organic solvent, followed by spray drying or high-speed stirring to obtain a cocoon-structured lithium-carbon material. This process forms an organic ion-conducting layer on the surface of the lithium-containing particles, which facilitates the diffusion of lithium ions and further improves the ionic conductivity of the pre-lithiated negative electrode.

[0035] Alternatively, the lithium-containing particles, the organic lithium ion transport agent, and the conductive agent may be mixed in the presence of an organic solvent, and then the solvent may be removed. The organic solvent may include N-methylpyrrolidone (NMP), ethanol, n-hexane, liquid paraffin, tetrahydrofuran, p-xylene, and the like.

[0036] Optionally, the organic solvent in step (1) is any one of N-methylpyrrolidone, liquid alkanes with 5 to 10 carbon atoms, benzene, p-xylene or petroleum ether, or a combination of at least two thereof.

[0037] Optionally, the spray drying conditions in step (1) include: an air inlet temperature of 180-240°C, for example, 180°C, 185°C, 190°C, 195°C, 200°C, 205°C, 215°C or 220°C, etc., preferably 200-220°C; an air outlet temperature of 90-110°C, for example, 90°C, 95°C, 100°C, 105°C or 110°C, etc., preferably 100-110°C; and an atomization pressure of 0.1-0.5 MPa, for example, 0.2 MPa, 0.25 MPa, 0.3 MPa, 0.35 MPa or 0.4 MPa, etc., preferably 0.2-0.4 MPa.

[0038] Optionally, the speed of the high-speed stirring in step (1) is 6000-15000 rpm, and the time is 0.5-10 min. For example, the speed can be 6000 rpm, 6500 rpm, 7000 rpm, 75000 rpm, 8000 rpm, 8500 rpm, 9000 rpm, 10000 rpm, 11000 rpm, 11500 rpm, 12000 rpm, 12500 rpm, 13000 rpm, 14000 rpm, 14500 rpm or 15000 rpm, and the time can be 0.5 min, 1 min, 1.5 min, 2 min, 2.5 min, 3 min, 3.5 min, 4 min, 4.5 min, 5 min, 6 min, 7 min, 8 min, 9 min, 9.5 min or 10 min, etc., preferably the speed is 8000-12000 rpm, and the time is 1-5 min.

[0039] Optionally, in step (2), the first binder includes any one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, polyisobutylene, polytetrafluoroethylene or sodium carboxymethyl cellulose, or a combination of at least two thereof.

[0040] Optionally, the organic solvent in step (2) includes any one or a combination of at least two of N-methylpyrrolidone, liquid alkanes with 5-10 carbon atoms, benzene, p-xylene or petroleum ether, preferably any one or a combination of at least two of p-xylene, n-hexane or heptane.

[0041] In the present invention, the type of conductive agent is not specifically limited, and the conductive agent includes but is not limited to any one or a combination of at least two of acetylene black, Ketjen black, conductive graphite, Cabot black, or Super P. Any type commonly used by those skilled in the art can be used in the present invention.

[0042] In the present invention, the type of binder is not specifically limited. The second binder includes, but is not limited to, any one or a combination of at least two of polyvinylidene fluoride, styrene-butadiene rubber, polystyrene, polyisobutylene, styrene-butadiene-styrene block copolymer, or styrene-ethylene-butylene-styrene block copolymer. Any type commonly used by those skilled in the art can be used in the present invention.

[0043] Optionally, the mass ratio of the conductive agent, the first binder or the second binder and the negative electrode active material is (1-10): (1-10): (80-98), preferably (2-5): (2-5): (90-96).

[0044] As a preferred technical solution of the present invention, the steps for preparing the negative electrode slurry in step (2) are as follows:

[0045] (a) mixing the negative electrode active material, the conductive agent, the second binder and the organic solvent for a primary mixing, and then adding the cocoon-structured lithium-carbon material for a secondary mixing to prepare a negative electrode slurry;

[0046] The secondary mixing time is 30-60 min, and the secondary mixing speed is 3000-5000 rpm; the mixing time can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min, etc., and the speed can be 3000 rpm, 3200 rpm, 3500 rpm, 4000 rpm, 4500 rpm, 4800 rpm or 5000 rpm, etc. The mixing time and speed include but are not limited to the listed values ​​and are adjusted according to the actual process.

[0047] (b) coating the negative electrode slurry on the surface of the current collector to obtain the pre-lithiation negative electrode.

[0048] Compared with the prior art, the pre-lithiation negative electrode and the preparation method thereof of the present invention have at least one of the following beneficial effects:

[0049] (1) The pre-lithiation negative electrode provided by the present invention uses a lithium-carbon material with a cocoon structure as a lithium supplement, which is evenly mixed with the negative electrode active material and has good contact, and does not form obvious voids in the negative electrode to increase the internal resistance. The pre-lithiation negative electrode has a high first coulombic efficiency and structural stability;

[0050] (2) The present invention further improves the initial coulombic efficiency and cycle performance of the pre-lithiation negative electrode by selecting the type of lithium-containing particles and regulating parameters such as the mass ratio of lithium-carbon material to lithium-containing particles;

[0051] (3) The preparation method provided by the present invention is simple to operate and has high production efficiency. In particular, the coating process is different from the existing process. The solvent and binder used are friendly to metallic lithium, providing a new idea for accurate lithium replenishment of the negative electrode.

[0052] In a third aspect, the negative electrode of the lithium-ion battery of the present invention is a composite negative electrode, which includes a current collector and a negative electrode active material layer arranged on at least one side of the current collector, and the weight composition of the negative electrode active material layer is as follows: 40-78 parts of negative electrode material; 20-50 parts of cocoon structure lithium carbon material; 1-5 parts of conductive agent; 1-5 parts of binder, and the sum of the weight parts of all components is 100 parts by weight; wherein the cocoon structure lithium carbon material includes a cocoon body formed by a structural carbon material and one or more lithium-containing particles contained in the cocoon body.

[0053] In the present invention, the structured carbon material forms a cocoon on the lithium-containing particles, creating a continuous, stable, three-dimensional electron network channel. This provides excellent conductivity and improves the utilization rate of the lithium-containing particles in the composite negative electrode. The cocoon acts on the lithium-containing particles in a limited manner, effectively alleviating the volume expansion of the lithium-containing particles in the composite negative electrode during cycling. The surface of the cocoon-structured lithium-carbon material in the composite negative electrode has micro-nanoscale textures formed by the structured carbon material, resulting in a relatively rough surface with a certain degree of flexibility and ductility. This provides good contact with the negative electrode material, conductive agent, and binder. Even if the lithium-containing particles form holes or cavities after cycling, the cocoon maintains an unobstructed electron path, regulates the current density uniformly, and effectively solves the problem of lithium dendrite growth caused by uneven current density, resulting in excellent cycling and rate performance.

[0054] The design of the weight ratios of the cocoon-structured lithium-carbon material and the negative electrode material can not only act as a pre-lithium agent to improve the initial coulombic efficiency of the negative electrode material, but can also act as a lithium source to continuously provide lithium ions during the cycle, thereby improving the capacity and cycle performance of the composite negative electrode. If the weight ratio of the cocoon-structured lithium-carbon material is too small, it can only act as a pre-lithium agent and cannot replenish the loss of lithium ions during the cycle; if the weight ratio of the cocoon-structured lithium-carbon material is too large, the composite negative electrode will exhibit performance similar to that of a metallic lithium negative electrode, and the cycle life will be significantly reduced. The use of the cocoon-structured lithium-carbon material in combination with the negative electrode material can not only overcome the problems existing in the use of a single negative electrode material or a cocoon-structured lithium-carbon material, but also reduce the amount of conductive agent and binder used in the composite negative electrode.

[0055] In the present invention, there is no specific limitation on the type of current collector. It can be ordinary copper foil, carbon-coated copper foil, composite copper foil, copper mesh, nickel foil, nickel mesh. As long as it is a type commonly used by technicians in this field, it is applicable to the present invention.

[0056] Optionally, the negative electrode active material layer comprises, by weight, 44-68 parts negative electrode material; 30-50 parts cocoon-structured lithium-carbon material; 1-3.5 parts conductive agent; and 1-2.5 parts binder. This composition fully demonstrates the advantages of the cocoon-structured lithium-carbon material, which can serve as a pre-lithiation agent to improve the initial coulombic efficiency of the negative electrode material, as well as enhance the capacity and cycle performance of the negative electrode. The lithium-containing particles are sticky, reducing the amount of binder used in the negative electrode. The cocoon's excellent conductivity reduces the amount of conductive agent used in the negative electrode.

[0057] Optionally, the lithium-containing particles include metallic lithium powder and / or lithium alloy powder, preferably lithium alloy powder. The lithium alloy powder will have some residue after its use, which can guide the deposition of metallic lithium during the cycle and maintain the structural stability of the composite negative electrode.

[0058] Optionally, the lithium alloy powder includes any one or a combination of at least two of lithium boron alloy, lithium silver alloy, lithium tin alloy, lithium silicon alloy, lithium zinc alloy, lithium indium alloy, lithium aluminum alloy or lithium magnesium alloy, preferably any one or a combination of at least two of lithium boron alloy, lithium indium alloy, lithium silver alloy or lithium magnesium alloy.

[0059] Optionally, the average particle size of the lithium-containing particles is 1-100 μm, for example, 1 μm, 3 μm, 5 μm, 7 μm, 10 μm, 15 μm, 20 μm, 30 μm, 40 μm, 45 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, or 100 μm, and preferably 10-50 μm. The average particle size of the metallic lithium powder and / or lithium alloy powder can be measured using a laser particle size analyzer under a protective atmosphere.

[0060] Optionally, the structural carbon material includes a micro-nano carbon material, as long as one dimension is micrometer or nanometer. The micro-nano carbon material includes any one or a combination of at least two of graphene nanosheets, graphene nanoribbons, multi-walled carbon nanotubes, single-walled carbon nanotubes, carbon fibers or few-walled carbon nanotubes, including but not limited to graphene nanosheets and few-walled carbon nanotubes, multi-walled carbon nanotubes and single-walled carbon nanotubes, graphene nanoribbons and carbon fibers, single-walled carbon nanotubes and nickel nanowires, etc., preferably any one or a combination of at least two of graphene nanosheets, single-walled carbon nanotubes or few-walled carbon nanotubes.

[0061] Optionally, the mass ratio of the lithium-containing particles to the structural carbon material is (1-1000):1, for example, it can be 1:1, 5:1, 10:1, 30:1, 50:1, 100:1, 120:1, 150:1, 200:1, 300:1, 400:1, 500:1, 600:1, 700:1, 750:1, 800:1, 850:1, 900:1, 950:1 or 1000:1, etc. If the mass ratio is too small, the micro-nano carbon material is easy to entangle itself, unevenly distributed on the surface of the lithium-containing particles, and the energy density of the composite material will be reduced; if the mass ratio is too large, a stable porous skeleton cannot be formed, preferably (10-500):1.

[0062] Optionally, the lithium-containing particles include metallic lithium powder and lithium alloy powder, and the mass ratio of the metallic lithium powder to the lithium alloy powder is (0.5-50):1, for example, it can be 0.5:1, 0.8:1, 1:1, 1.5:1, 3:1, 5:1, 8:1, 10:1, 15:1, 20:1, 25:1, 30:1, 35:1, 40:1, 45:1 or 50:1, etc. If the mass ratio is too small, the specific capacity of the composite negative electrode is low, which is not conducive to the improvement of energy density; if the mass ratio is too large, the cycle performance of the composite negative electrode is limited, preferably (1-30):1.

[0063] Optionally, the negative electrode material includes any one or a combination of at least two of graphite, tin-based materials, hard carbon, soft carbon, mesocarbon microbeads, silicon-carbon materials or silicon-oxygen materials.

[0064] Optionally, the conductive agent includes any one of acetylene black, Ketjen black, Cabot black, conductive graphite, Super P, carbon nanotubes, carbon fibers or graphene, or a combination of at least two thereof.

[0065] Optionally, the binder includes any one of polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene copolymer, sodium carboxymethyl cellulose, styrene-butadiene rubber, polystyrene, polyisobutylene, styrene-butadiene-styrene block copolymer or styrene-ethylene-butylene-styrene block copolymer, or a combination of at least two thereof.

[0066] In a fourth aspect, the present invention provides a method for preparing the above-mentioned composite negative electrode, the method comprising the following steps:

[0067] (1) mixing a negative electrode material, a conductive agent, and a binder, and then fibrillating the binder to obtain a mixture;

[0068] (2) The mixture in step (1) is mixed with the cocoon-structured lithium-carbon material, and then hot-pressed into a negative electrode active material layer, and the negative electrode active material layer is arranged on at least one side of the current collector to obtain the composite negative electrode.

[0069] In the present invention, a dry process is adopted to mix the cocoon structure lithium carbon material and the mixture after the binder is fiberized, which can better maintain the morphology of the cocoon structure. The cocoon structure lithium carbon material is not consumed during the preparation process, and the obtained composite negative electrode has a higher energy density.

[0070] In the present invention, the preparation method of the cocoon-structured lithium-carbon material is not specifically limited. As long as the cocoon-structured lithium-carbon material is composed of a cocoon body formed by a structural carbon material and one or more lithium-containing particles contained in the cocoon body, it falls within the protection scope of the present invention.

[0071] For example, a cocoon-structured lithium-carbon material can be obtained by dispersing the structured carbon material in an organic solvent such as n-hexane or p-xylene, then adding lithium-containing particles and stirring at a speed of 5000 rpm or more and / or spray granulating. The lithium-containing particles include metallic lithium powder and / or lithium alloy powder, which are highly active in air. The method is carried out in a protective atmosphere, such as argon and / or helium.

[0072] Optionally, the equipment for fiberizing the binder in step (1) is not specifically limited, including but not limited to any one of a planetary high-speed dispersion device, an ultrasonic gasification device or an ultra-strong shearing device or a combination of at least two thereof, preferably an ultra-strong shearing device.

[0073] Optionally, the temperature of the binder fiberization in step (1) is 50-150°C, for example, it can be 50°C, 55°C, 60°C, 70°C, 80°C, 90°C, 95°C, 100°C, 105°C, 110°C, 120°C, 130°C, 140°C or 150°C, etc., including but not limited to the listed point values. As long as it is within the above numerical range, it belongs to the protection scope of the present invention, preferably 90-110°C.

[0074] Optionally, the mixing equipment in step (2) is not specifically limited, including but not limited to any one or a combination of at least two of a V-type mixing equipment, a drum mixing equipment or a planetary high-speed dispersing equipment, preferably a V-type mixing equipment and / or a drum mixing equipment.

[0075] Optionally, the mixing time in step (2) is 10-60 min, for example, it can be 10 min, 15 min, 20 min, 25 min, 30 min, 40 min, 45 min, 50 min or 60 min, including but not limited to the listed point values. As long as it is within the above numerical range, it belongs to the protection scope of the present invention, preferably 20-45 min.

[0076] Optionally, the mixing speed in step (2) is 2000-5000 rpm, for example, it can be 2000 rpm, 2500 rpm, 3000 rpm, 3500 rpm, 4000 rpm, 4500 rpm or 5000 rpm, including but not limited to the listed point values. As long as it is within the above numerical range, it belongs to the protection scope of the present invention, preferably 3000-4500 rpm.

[0077] Optionally, the temperature of the hot pressing in step (2) is 80-120°C, for example, it can be 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C or 120°C, including but not limited to the listed point values. As long as it is within the above numerical range, it belongs to the protection scope of the present invention, preferably 90-110°C.

[0078] Optionally, the hot pressing pressure in step (2) is 10-50t, for example, it can be 10t, 15t, 20t, 25t, 30t, 35t, 40t, 45t or 50t, etc., preferably 20-40t.

[0079] As a further preferred technical solution of the present invention, the method comprises the following steps:

[0080] (1) mixing a negative electrode material, a conductive agent, and a binder according to parts by weight, and then fibrillating the binder at 90-110° C. to obtain a mixture;

[0081] The negative electrode material includes any one or a combination of at least two of graphite, tin-based materials, hard carbon, soft carbon, mesophase carbon microspheres, silicon-carbon materials or silicon-oxygen materials, with a weight ratio of 44-68 parts;

[0082] The conductive agent includes any one or a combination of at least two of acetylene black, Ketjen black, Cabot black, conductive graphite, Super P, carbon nanotubes, carbon fibers or graphene, with a weight ratio of 1 to 3.5 parts;

[0083] The binder comprises any one or a combination of at least two of polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene copolymer, sodium carboxymethyl cellulose, styrene-butadiene rubber, polystyrene, polyisobutylene, styrene-butadiene-styrene block copolymer or styrene-ethylene-butylene-styrene block copolymer, with a weight portion of 1-2.5 parts;

[0084] (2) The mixture in step (1) and the cocoon structure lithium carbon material are mixed at 3000-4500 rpm for 20-45 min, wherein the weight portion of the cocoon structure lithium carbon material is 30-50 parts, and then hot pressed into a negative electrode active material layer under conditions of 90-110° C. and 20-40t, and the negative electrode active material layer is arranged on at least one side of the current collector to obtain the composite negative electrode.

[0085] The composite negative electrode and preparation method thereof of the present invention have at least one of the following beneficial technical effects:

[0086] 1. In the composite negative electrode provided by the present invention, a structural carbon material with good conductivity is constructed on the surface of the cocoon-structured lithium-carbon material, which improves the utilization rate of the lithium-containing particles while alleviating the problem of their volume expansion; the surface of the cocoon-structured lithium-carbon material is relatively rough and has certain flexibility and ductility, and has good contact with the negative electrode material, conductive agent and binder, which can adjust the current density and effectively solve the problem of lithium dendrite growth due to uneven current density, so that the composite negative electrode has good cycle performance and rate performance;

[0087] 2. In the composite negative electrode provided by the present invention, the cocoon-structured lithium-carbon material is uniformly dispersed, which can serve as a lithium replenisher to improve the initial coulombic efficiency of the negative electrode, and can also serve as a lithium source to increase the specific capacity of the negative electrode while replenishing the lithium ions continuously consumed during the cycle. The reasonable design of the weight ratio of the cocoon-structured lithium-carbon material to the negative electrode material reduces the amount of conductive agent and binder in the composite negative electrode, which can take into account the specific capacity, cycle performance and rate performance.

[0088] 3. The present invention further improves the cycle performance and rate performance of the composite negative electrode by optimizing parameters such as the mass ratio of lithium-containing particles to structural carbon materials in the cocoon-structured lithium-carbon material and the weight ratio of each component in the negative electrode active material layer;

[0089] 4. The composite negative electrode preparation method provided by the present invention adopts a dry process, and the cocoon-structured lithium-carbon material will not be consumed during the preparation process. The structural design of the cocoon-structured lithium-carbon material enables good contact between it and the negative electrode material, conductive agent, and binder, thereby improving the specific capacity and structural stability of the composite negative electrode. The preparation method is simple and easy to implement and easy to industrialize.

[0090] A fifth aspect of the present invention provides an energy storage device comprising the pre-lithiated negative electrode described in the first aspect or the composite negative electrode described in the third aspect. The energy storage device includes but is not limited to a lithium-ion battery and a lithium-ion supercapacitor. BRIEF DESCRIPTION OF THE DRAWINGS

[0091] FIG1 is a schematic diagram of a pre-lithiation negative electrode of the present invention;

[0092] 1-current collector, 2-negative electrode active material, 3-cocoon structure lithium carbon material

[0093] FIG2 is a SEM image of the cocoon-structured lithium-carbon material of Example 4;

[0094] FIG3 is a cross-sectional SEM image of the pre-lithiated negative electrode in Example 4;

[0095] FIG4 is a SEM image of the cocoon-structured lithium-carbon material in Example 9;

[0096] FIG5 is a cross-sectional SEM image of the composite negative electrode in Example 9;

[0097] FIG6 is a cross-sectional SEM image of the composite negative electrode in Comparative Example 4. DETAILED DESCRIPTION

[0098] For the convenience of understanding the present invention, the present invention is given below with examples. It should be understood by those skilled in the art that the examples are only for the purpose of helping to understand the present invention and should not be regarded as specific limitations of the present invention.

[0099] For example, the present invention provides a pre-lithiation negative electrode, a schematic diagram of which is shown in FIG1 . The pre-lithiation negative electrode comprises a current collector 1 , on the surface of which a negative electrode active material 2 and a cocoon-structured lithium-carbon material 3 are provided. The cocoon-structured lithium-carbon material 3 is distributed between the negative electrode active materials 2 and is evenly mixed. The cocoon-structured lithium-carbon material comprises a cocoon body formed of a structural carbon material and one or more lithium-containing particles contained in the cocoon body (refer to the SEM image shown in FIG2 ). The cocoon body is in good contact with the negative electrode active material 2, constructing a good electron path. After lithium replenishment, no obvious voids are formed in the negative electrode, thereby increasing the internal resistance. The pre-lithiation negative electrode has a high first coulombic efficiency and structural stability.

[0100] Example 1

[0101] This embodiment provides a pre-lithiation negative electrode, which includes graphite and a lithium-carbon material with a cocoon structure, wherein the lithium-carbon material with a cocoon structure is distributed between the graphite.

[0102] The method for preparing the pre-lithiation negative electrode comprises the following steps:

[0103] (1) Graphene (XF001W 7440-44-0, Jiangsu Xianfeng Nanomaterial Technology Co., Ltd.), multi-walled carbon nanotubes (XFM01 1333-86-4, Jiangsu Xianfeng Nanomaterial Technology Co., Ltd.), Cabot BP2000, metallic lithium powder with an average particle size of 5 μm, and n-hexane were mixed, with the mass ratio of the total mass of graphene, multi-walled carbon nanotubes, and Cabot BP2000 to metallic lithium powder being 1:1000, and then high-speed dispersion was carried out at a rotation speed of 5000 rpm for 10 min to obtain a lithium-carbon material with a cocoon structure;

[0104] (2) Graphite, acetylene black, and styrene-butadiene rubber (SBR, 1502, Sinopec Jilin Chemical) were mixed in quantities of 800 g, 100 g, and 100 g, respectively, with n-hexane as the solvent to prepare a first slurry. 8 g of a cocoon-structured lithium-carbon material was then added, and the mixture was stirred at 3000 rpm for 60 min to obtain a second slurry. The second slurry was then coated on a 6 μm carbon-coated copper foil surface and dried to obtain the pre-lithiation negative electrode.

[0105] Example 2

[0106] This embodiment provides a pre-lithiation negative electrode, which includes silicon-carbon and a lithium-carbon material with a cocoon structure, wherein the lithium-carbon material with a cocoon structure is distributed between the silicon-carbon.

[0107] The method for preparing the pre-lithiation negative electrode comprises the following steps:

[0108] (1) Acetylene black, polyethylene oxide (Aladdin, P432440) and metallic lithium powder were mixed in p-xylene at a mass ratio of 5:5:90, and the p-xylene was removed by drying to form metallic lithium powder containing an organic conductive layer;

[0109] (2) mixing single-walled carbon nanotubes, metal lithium powder with an average particle size of 45 μm containing an organic conductive layer, and n-hexane, wherein the mass ratio of the single-walled carbon nanotubes to the metal lithium powder is 1:500, and then high-speed dispersion is performed at a speed of 8000 rpm for 8 minutes to obtain a lithium-carbon material with a cocoon structure;

[0110] (3) Silicon carbon (Bai Te Rui, DXB8), carbon fiber, and polyvinylidene fluoride (Su Wei, 5130) were mixed in masses of 800g, 100g, and 100g, with N-methylpyrrolidone as the solvent to prepare a first slurry, and then 160g of cocoon-structured lithium carbon material was added. The mixture was stirred at a speed of 4500 rpm for 45 minutes to obtain a second slurry. The second slurry was then coated on the surface of 6μm carbon nanotube paper and dried to obtain the pre-lithiation negative electrode.

[0111] Example 3

[0112] This embodiment provides a pre-lithiation negative electrode, which includes silicon oxide and a lithium-carbon material with a cocoon structure, wherein the lithium-carbon material with a cocoon structure is distributed between the silicon oxide.

[0113] The method for preparing the pre-lithiation negative electrode comprises the following steps:

[0114] (1) Carbon nanofibers (Aladdin, C478418), conductive graphite KS-6, lithium-magnesium alloy particles with an average particle size of 100 μm (magnesium mass fraction of 10%), and p-xylene were mixed, with the mass ratio of the total mass of single-walled carbon nanotubes and conductive carbon black to the metal lithium powder being 1:100, and then high-speed dispersed at a speed of 12000 rpm for 5 minutes to obtain a lithium-carbon material with a cocoon structure;

[0115] (2) Silicon oxide (Betray, DXA5), Super P, and polyisobutylene (Aladdin, P418837) were mixed in masses of 800 g, 100 g, and 100 g, with heptane as the solvent, to prepare a first slurry, and then 240 g of cocoon-structured lithium-carbon material was added and stirred at a speed of 4500 rpm for 45 min to obtain a second slurry. The second slurry was then coated on the surface of a 6 μm perforated copper foil and dried to obtain the pre-lithiation negative electrode.

[0116] Example 4

[0117] This embodiment provides a pre-lithiation negative electrode, which includes graphite and a lithium-carbon material with a cocoon structure, wherein the lithium-carbon material with a cocoon structure is distributed between the graphite.

[0118] The method for preparing the pre-lithiation negative electrode comprises the following steps:

[0119] (1) Single-walled carbon nanotubes (XFS02 1333-86-4, Jiangsu Xianfeng Nanomaterial Technology Co., Ltd.), metallic lithium powder with an average particle size of 50 μm, and n-hexane were mixed, with the mass ratio of single-walled carbon nanotubes to metallic lithium powder being 1:500, and then spray-dried with an inlet air temperature of 220°C, an outlet air temperature of 100°C, and an atomization pressure of 0.5 MPa to obtain a lithium-carbon material with a cocoon structure; the SEM characterization of the lithium-carbon material with a cocoon structure is shown in FIG2 . It can be clearly seen from the figure that the single-walled carbon nanotubes are wrapped around the surface of the metallic lithium powder to form a cocoon body, and the cocoon body contains metallic lithium powder to form a lithium-carbon material with a cocoon structure;

[0120] (2) Graphite, acetylene black, and polytetrafluoroethylene were mixed in quantities of 900 g, 50 g, and 50 g, and then processed through an ultra-strong shearing device to fiberize the polytetrafluoroethylene binder. 180 g of a cocoon-structured lithium-carbon material was added, and a 120 μm thick negative electrode film was obtained by extrusion and calendaring at 80°C. The negative electrode film was composited with a carbon-coated copper foil by hot pressing to obtain the pre-lithiation negative electrode. The cross-sectional SEM characterization of the pre-lithiation negative electrode is shown in FIG3 . It can be clearly seen from the figure that the cocoon-structured lithium-carbon material (the portion marked in the black circle in the figure) is evenly distributed between materials such as graphite (the portion marked outside the black circle in the figure), and a good cocoon structure can be maintained.

[0121] Example 5

[0122] Compared with Example 4, the only difference is that the mass of the cocoon-structured lithium-carbon material in step (2) is replaced with 0.9 g, and the other conditions are the same.

[0123] Example 6

[0124] Compared with Example 4, the only difference is that the mass of the cocoon-structured lithium-carbon material in step (2) is replaced with 300 g, and the other conditions are the same.

[0125] Example 7

[0126] Compared with Example 4, the only difference is that the metallic lithium powder in step (1) is replaced by lithium-indium alloy powder, the mass fraction of indium is 20%, and the other conditions are the same.

[0127] Comparative Example 1

[0128] Compared with Example 4, the only difference is that the cocoon-structured lithium-carbon material in step (2) is replaced by the metallic lithium powder in step (1), and the other conditions remain unchanged.

[0129] Comparative Example 2

[0130] Compared with Example 4, the only difference is that the lithium-carbon material with a cocoon structure is not added in step (2), and the other conditions remain unchanged.

[0131] Comparative Example 3

[0132] Compared with Example 4, the only difference is that the rotation speed in step (1) is replaced with 2000 rpm, and the other conditions remain unchanged. At this rotation speed, the single-walled carbon nanotubes can be fully mixed with the metallic lithium powder, but no cocoon structure is formed.

[0133] Negative electrode performance test:

[0134] The negative electrodes of Examples 1-7 and Comparative Examples 1-3 were assembled into soft-pack batteries with the positive electrodes of lithium nickel cobalt manganese oxide for testing. The first coulombic efficiency and cycle test data at 0.2C are shown in Table 1 below.

[0135] Table 1

[0136] From Table 1 we can see that:

[0137] (1) Based on Examples 4-6, it can be seen that the degree of pre-lithiation is closely related to the amount of cocoon-structured lithium-carbon material used. When the amount is small (Example 5), the improvement of the first coulombic efficiency and cycle performance is not obvious; when the amount is too much (Example 6), the first coulombic efficiency and cycle performance are significantly improved, but it is not conducive to the preparation of high energy density batteries.

[0138] (2) Combining Examples 4 and 7, it can be seen that the first coulombic efficiency of the battery corresponding to Example 4 is close to that of Example 7, but the cycle performance is worse than that of Example 7. This is because lithium-indium alloy is used to supplement lithium in Example 7. Indium can regulate the deposition of metallic lithium and conduct electrons at the same time, and cooperates with the structural carbon material single-walled carbon nanotubes, so that the negative electrode exhibits excellent cycle performance.

[0139] (3) From Example 4 and Comparative Examples 1-3, it can be seen that when the same mass of lithium replenisher is added, the degree of improvement of the first coulombic efficiency and cycle performance of the negative electrode by pre-lithiation agents of different structures is different. The core-shell structured lithium-carbon material of the present invention is used as the lithium replenisher. The cocoon-structured lithium-carbon material contains a structural carbon material with good conductivity and a certain rigidity, which can improve the conductivity and structural stability of the pre-lithiation negative electrode, so that the pre-lithiation negative electrode has a higher first coulombic efficiency and excellent cycle performance.

[0140] Example 8

[0141] This embodiment provides a composite negative electrode, which includes a copper foil and a negative electrode active material layer arranged on one side of the copper foil. The weight ratio of the negative electrode active material layer is as follows: 40 parts of graphite, 50 parts of cocoon-structured lithium-carbon material, 5 parts of multi-walled carbon nanotubes, and 5 parts of polytetrafluoroethylene; the cocoon-structured lithium-carbon material includes a cocoon body formed by graphene sheets and metallic lithium powder contained in the cocoon body.

[0142] The preparation method of the composite negative electrode comprises the following steps:

[0143] (1) 60 g of metallic lithium powder with an average particle size of 10 μm, 60 g of structural carbon material graphene sheet and 300 g of n-hexane were dispersed at a high speed of 8000 rpm for 30 min, filtered and dried to obtain a cocoon-structured lithium-carbon material;

[0144] (2) dry-mixing graphite (B&T, AGP-7), multi-walled carbon nanotubes, and polytetrafluoroethylene in parts by weight, and then performing binder fiberization in an ultra-strong shearing device at a temperature of 50° C. to obtain a mixture;

[0145] (3) The mixture in step (2) and the cocoon-structured lithium-carbon material are mixed in a V-type mixing device according to weight for 10 minutes at a mixing speed of 5000 rpm, and then hot-pressed at 80°C with a pressure of 50t to obtain a negative electrode active material layer, and then the negative electrode active material layer is laminated to the copper foil by pressure composite to obtain the composite negative electrode.

[0146] Example 9

[0147] This embodiment provides a composite negative electrode, comprising a carbon-coated copper foil and negative electrode active material layers disposed on both sides of the carbon-coated copper foil. The negative electrode active material layers are composed by weight of: 78 parts silicon carbon, 20 parts cocoon-structured lithium carbon material, 1 part Ketjen black, and 1 part styrene-butadiene-styrene block copolymer. The cocoon-structured lithium carbon material comprises a cocoon body formed of single-walled carbon nanotubes and a lithium-boron alloy powder contained in the cocoon body. The boron content of the lithium-boron alloy is 20 wt%.

[0148] The preparation method of the composite negative electrode comprises the following steps:

[0149] (1) 60 g of lithium-boron alloy powder with an average particle size of 50 μm, 0.15 g of single-walled carbon nanotubes (purchased from Okoshiel) and 300 g of p-xylene (purchased from Aladdin) were dispersed at a high speed of 12,000 rpm for 15 min, filtered, and dried at 80° C. to obtain a cocoon-structured lithium-carbon material; the cocoon-structured lithium-carbon material was characterized by SEM, and the characterization results are shown in FIG1 . It can be clearly seen from the figure that the single-walled carbon nanotubes are wrapped around the surface of the lithium-boron alloy powder to form a cocoon structure, which plays a binding role on the lithium-boron alloy powder;

[0150] (2) dry-mixing silicon carbon (Betray, DXB8), Ketjen black, and styrene-butadiene-styrene block copolymer in parts by weight, and then performing binder fiberization in an ultra-strong shearing device at a temperature of 100° C. to obtain a mixture;

[0151] (3) The mixture in step (2) and the cocoon-structured lithium-carbon material were mixed by weight in a V-type mixing device for 30 minutes at a mixing speed of 4000 rpm, and then hot-pressed at 100° C. using a pressure of 30 tons to obtain a negative electrode active material layer. The negative electrode active material layer was laminated to a carbon-coated copper foil by pressure lamination to obtain the composite negative electrode. The SEM of the test cross-section of the composite negative electrode is shown in FIG2 . It can be clearly seen from the figure that the components in the composite negative electrode are evenly distributed, the cocoon-structured lithium-carbon material maintains a cocoon structure, and is tightly combined with other components.

[0152] Example 10

[0153] This embodiment provides a metallic lithium composite negative electrode, comprising a copper mesh and negative electrode active material layers disposed on both sides of the copper mesh, wherein the negative electrode active material layers are composed by weight of: 55 parts of hard carbon, 40 parts of a cocoon-structured lithium-carbon material, 2.5 parts of Ketjen black, and 2.5 parts of a styrene-ethylene-butylene-styrene block copolymer; the cocoon-structured lithium-carbon material comprises a cocoon body formed of carbon nanofibers and a lithium-indium alloy powder contained in the cocoon body, wherein the indium content of the lithium-indium alloy is 20 wt%.

[0154] The composite negative electrode comprises the following steps:

[0155] (1) 60 g of lithium indium alloy powder with an average particle size of 100 μm and carbon nanofiber (purchased from Aladdin) were mixed.

[0156] 0.06g and 450g of tetrahydrofuran (purchased from Aladdin) were dispersed at high speed at a speed of 8000rpm for 20min, and then spray granulated at an inlet air temperature of 220°C, an outlet air temperature of 100°C, and a pressure of 0.5Mpa to obtain a cocoon structure lithium-carbon material;

[0157] (2) dry-mixing hard carbon (BHT, BHC-400), Ketjen black, and styrene-butadiene-styrene block copolymer in parts by weight, and then subjecting the binder to fiberization in an ultra-strong shearing apparatus at a temperature of 120° C. to obtain a mixture;

[0158] (3) The mixture in step (2) and the cocoon-structured lithium-carbon material are mixed in a drum mixing device according to weight for 30 minutes at a mixing speed of 4000 rpm, and then hot-pressed at 100° C. with a pressure of 30t to obtain a negative electrode active material layer, and the negative electrode active material layer is laminated to a copper mesh by pressure bonding to obtain the composite negative electrode.

[0159] Example 11

[0160] Compared with Example 9, the mass of the single-walled carbon nanotubes in step (1) was replaced with 65 g, and the other conditions remained unchanged.

[0161] Example 12

[0162] Compared with Example 9, the mass of the single-walled carbon nanotubes in step (1) was replaced with 0.05 g, and the other conditions remained unchanged.

[0163] Example 13

[0164] Compared with Example 9, the weight composition of the negative electrode active material layer is replaced by: 68 parts of silicon carbon, 30 parts of cocoon-structured lithium carbon material, 1 part of conductive agent, and 1 part of binder.

[0165] Example 14

[0166] Compared with Example 9, the weight composition of the negative electrode active material layer is replaced by: 44 parts of silicon carbon, 50 parts of cocoon-structured lithium carbon material, 3.5 parts of conductive agent, and 2.5 parts of binder.

[0167] Example 15

[0168] Compared with Example 9, the lithium-boron alloy powder was replaced by a mixture of metallic lithium powder and lithium-boron alloy powder, the mass ratio of metallic lithium powder to lithium-boron alloy powder was 1:1, and other conditions remained unchanged.

[0169] Example 16

[0170] Compared with Example 9, the lithium-boron alloy powder was replaced by a mixture of metallic lithium powder and lithium-boron alloy powder, the mass ratio of metallic lithium powder to lithium-boron alloy powder was 30:1, and the other conditions remained unchanged.

[0171] Example 17

[0172] Compared with Example 9, the lithium-boron alloy powder was replaced by a mixture of metallic lithium powder and lithium-boron alloy powder, the mass ratio of metallic lithium powder to lithium-boron alloy powder was 60:1, and the other conditions remained unchanged.

[0173] Comparative Example 4

[0174] Compared to Example 9, the only difference is that the cocoon-structured lithium-carbon material in the negative electrode active material layer is replaced with metallic lithium powder; all other conditions remain unchanged. A SEM image of a cross-section of the composite negative electrode test is shown in Figure 3. The image clearly shows large pores between the metallic lithium powder, the negative electrode material, and the conductive agent, resulting in poor contact.

[0175] Comparative Example 5

[0176] Compared with Example 9, the only difference is that the cocoon-structured lithium-carbon material in the negative electrode active material layer is replaced by a lithium-boron alloy, the boron content in the lithium-boron alloy is 20 wt %, and the other conditions remain unchanged.

[0177] Comparative Example 6

[0178] Compared with Example 9, the only difference is that the cocoon-structured lithium-carbon material in the negative electrode active material layer is replaced by a mixture of lithium-boron alloy and single-walled carbon nanotubes. The single-walled carbon nanotubes and lithium-boron alloy in the mixture are randomly dispersed and do not form a cocoon structure. The other conditions remain unchanged.

[0179] Comparative Example 7

[0180] Compared with Example 8, the only difference is that the cocoon-structured lithium-carbon material in the negative electrode active material layer is replaced with a metallic lithium-lithiophilic skeleton composite material, and the other conditions remain unchanged;

[0181] The preparation method of the metal lithium-lithiophilic skeleton composite material comprises the following steps:

[0182] (1) Polyvinyl alcohol (Aladdin Reagent (Shanghai) Co., Ltd.), polystyrene microspheres (Suzhou Weimai New Materials Co., Ltd.), carbon nanotubes (Shandong Dazhan, carbon nanotube model: GTC-304), isothiazolinone (Aladdin Reagent (Shanghai) Co., Ltd.) and deionized water in a mass ratio of 5:7:10:10:500 were uniformly mixed to obtain a slurry with a solid content of 6%;

[0183] (2) The slurry was atomized and granulated by a two-fluid atomizer with a carrier gas pressure of 0.3 MPa and an atomization chamber temperature set at 220°C;

[0184] (3) placing the carbon particles obtained by the spray granulation into a crucible and subjecting them to a high-temperature treatment at 800° C. for 3 hours under an inert atmosphere;

[0185] (4) The carbon particles after high temperature treatment are mixed with molten metal lithium and stirred evenly to obtain a metal lithium-lithiophilic skeleton composite material.

[0186] The preparation method of the mixture comprises the following steps:

[0187] 60 g of lithium-boron alloy powder with an average particle size of 50 μm, 0.15 g of single-walled carbon nanotubes (purchased from Okoshiel) and 300 g of p-xylene (purchased from Aladdin) were mechanically stirred at a stirring speed of 1000 rpm for 15 minutes, filtered, and dried at 80°C to obtain a mixture of lithium-boron alloy and single-walled carbon nanotubes.

[0188] Material performance testing:

[0189] The negative electrodes provided in Examples 8-17 and Comparative Examples 4-7 were assembled with lithium sheets to test specific capacity, and assembled with NCM811 to test cycle performance and rate performance. The test results are shown in Table 1.

[0190] Table 1

[0191] From Table 1, we can see the following points:

[0192] (1) Comparing Example 9 and Examples 11-12, it can be seen that the structural carbon material used in Example 11 is more than that in Example 9, and the negative electrode specific capacity is lower; the structural carbon material used in Example 12 is less than that in Example 9. Although the negative electrode specific capacity is higher, the cycle performance and rate performance are worse than those in Example 9;

[0193] (2) By comparing Example 9 with Examples 13-14, it can be seen that by adjusting the proportion of the cocoon-structured lithium-carbon material in the negative electrode, the specific capacity, cycle performance, and rate performance of the composite negative electrode in Example 13 and Example 14 are better than those in Example 9. That is, the weight composition of the negative electrode material layer in the composite negative electrode in Example 13 and Example 14 is better than that in Example 9, and the specific capacity, cycle performance, and rate performance of the composite negative electrode can be taken into account;

[0194] (3) Comparing Example 9 with Examples 15-17, it can be seen that the lithium-containing particles used in Examples 15-17 are a mixture of lithium-boron alloy powder and metallic lithium powder. The specific capacity, cycle performance and rate performance of the composite negative electrode of Examples 15 and 16 are better than those of Example 9. This is because the ratio of metallic lithium powder to lithium-boron alloy powder is controlled within an appropriate range. Although the specific capacity of the composite negative electrode of Example 17 is higher than that of Example 9, its cycle performance and rate performance are worse than those of Example 9. This is because the ratio of metallic lithium powder in the lithium-containing particles in Example 10 is higher.

[0195] (4) By comparing Example 9 with Comparative Examples 4-6, it can be seen that the specific capacity of the composite negative electrode in Example 9 is lower than that in Comparative Example 4, but the cycle performance and rate performance are better than those in Comparative Example 4. This is because the cocoon-structured lithium-carbon material in Example 9 contains a lithium-boron alloy, which can serve as a skeleton to stabilize the composite negative electrode structure and guide the deposition of metallic lithium, making it difficult to form lithium dendrites. The structural carbon material also alleviates the volume expansion of the lithium-boron alloy and regulates the conductivity of the composite negative electrode. The metallic lithium powder in Comparative Example 4 easily forms holes after taking effect, causing the electronic path to be disconnected. The performance of the composite negative electrode in Comparative Examples 5 and 6 is worse than that in Example 9. This is because the lithium-boron alloy surface in the composite negative electrode of Comparative Examples 5 and 6 does not construct a cocoon structure.

[0196] (5) By comparing Example 8 with Comparative Example 7, it can be seen that the specific capacity, cycle performance and rate performance of the composite negative electrode in Example 8 are all better than those in Comparative Example 7. This is because the content of metallic lithium in the metallic lithium-lithiophilic skeleton material in Comparative Example 7 is relatively small (limited by the preparation method, molten metallic lithium is easily coated on the surface of the porous particles), and the specific capacity of the composite negative electrode is lower than that in Example 8; the lithiophilic skeleton is inside the metallic lithium, and its role in supporting and regulating the current density of the metallic lithium is more than its role in binding it, and it has good contact with the negative electrode material and the conductive agent, while the structural carbon material of the cocoon-structured lithium-carbon material in the present invention plays a role in binding the lithium-containing particles, effectively alleviating the problem of volume expansion of the lithium-containing particles; the cocoon-structured lithium-carbon material has good contact with the negative electrode material and the conductive agent, and the composite negative electrode structure has good stability.

[0197] The applicant declares that the present invention is intended to illustrate the detailed structural features of the present invention through the above-described embodiments, but the present invention is not limited to the above-described detailed structural features. This does not mean that the present invention must rely on the above-described detailed structural features in order to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent replacements for selected components, additions of auxiliary components, and selection of specific embodiments, etc., fall within the scope of protection and disclosure of the present invention.

Claims

1. A negative electrode for a lithium ion battery, characterized in that: The negative electrode active material layer of the negative electrode comprises a negative electrode active material and a lithium-carbon material with a cocoon structure, wherein the lithium-carbon material with a cocoon structure is distributed between the negative electrode active materials. The cocoon-structured lithium-carbon material comprises a cocoon body formed of a structural carbon material and one or more lithium-containing particles contained in the cocoon body.

2. The negative electrode according to claim 1, characterized in that The negative electrode is a pre-lithiation negative electrode, wherein the mass ratio of the negative electrode active material to the lithium-carbon material with a cocoon structure is 1 g: (1-300) mg; The mass ratio of the structural carbon material to the lithium-containing particles is 1:(1-1000).

3. The negative electrode according to claim 2, characterized in that The negative electrode active material includes any one or a combination of at least two of graphite, tin-based materials, hard carbon, soft carbon, activated carbon, mesophase carbon microspheres, silicon-carbon materials or silicon-oxygen materials; The structural carbon material is any one or a combination of at least two of granular, linear or flaky nanocarbon materials, and the structural carbon material includes any one or a combination of at least two of acetylene black, Ketjen black, Cabot BP2000, Super P, single-walled carbon nanotubes, few-walled carbon nanotubes, multi-walled carbon nanotubes, doped carbon nanotubes, graphene, doped graphene, carbon nanofibers or doped carbon nanofibers; The doping elements in the doped carbon nanotubes, doped graphene or doped carbon nanofibers independently include any one of fluorine, nitrogen, oxygen, phosphorus, silver, silicon, tin or zinc, or a combination of at least two of them.

4. The negative electrode according to claim 2, characterized in that The lithium-containing particles include metal lithium particles and / or metal lithium alloy particles, and the mass content of metal lithium in the metal lithium alloy particles is not less than 70%; Wherein, the average particle size of the lithium-containing particles is 1-100 microns; Among them, the metallic lithium alloy particles include binary lithium alloy and / or ternary lithium alloy, the binary lithium alloy powder includes any one of lithium magnesium alloy, lithium boron alloy, lithium silver alloy, lithium indium alloy, lithium silicon alloy, lithium aluminum alloy or lithium tin alloy, or a combination of at least two of them; the ternary lithium alloy powder includes any one of lithium tin silver alloy, lithium silver silicon alloy or lithium boron zinc alloy, or a combination of at least two of them.

5. The negative electrode according to claim 2, characterized in that The cocoon structure further comprises an organic conductive layer disposed between the lithium-containing particles and the structural carbon material, wherein the organic conductive layer comprises an organic ion conductive agent and a conductive agent; The conductive agent comprises at least one of carbon nanotubes, carbon black, graphene and acetylene black; The organic lithium ion transport agent includes polyethylene oxide, polyethylene glycol, polysiloxane, polytrimethylene carbonate, polycarbonate, polyethylene carbonate, polypropylene carbonate, polyvinylene carbonate, polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, poly(vinylidene fluoride-hexafluoropropylene), polyphenylene sulfide, para-benzoquinone, and a copolymer or mixture of at least two of the above polymers.

6. A method for preparing a negative electrode as claimed in any one of claims 1 to 5, characterized in that: The method comprises the following steps: (1) mixing a structural carbon material, lithium-containing particles and an organic solvent, and then spray drying or stirring at a rotation speed of more than 5000 rpm to obtain a lithium-carbon material with a cocoon structure; (2) mixing the negative electrode active material, the conductive agent and the first binder, and then fiberizing the binder to obtain a first mixture, and then mixing the first mixture with a lithium-carbon material with a cocoon structure to obtain a second mixture, hot pressing the second mixture into a negative electrode active material layer, and then compounding it with a current collector to obtain the pre-lithiation negative electrode; Alternatively, the negative electrode active material, the conductive agent, the second binder, the cocoon-structured lithium-carbon material and the organic solvent are mixed to prepare a negative electrode slurry, and then the negative electrode slurry is coated on the surface of the current collector to obtain the pre-lithiation negative electrode.

7. The method according to claim 6, characterized in that The organic solvent in step (1) is any one of N-methylpyrrolidone, liquid alkane with 5 to 10 carbon atoms, benzene, p-xylene or petroleum ether, or a combination of at least two thereof.

8. The method according to claim 6, characterized in that In step (2), the first binder includes any one of polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, polytetrafluoroethylene or sodium carboxymethyl cellulose, or a combination of at least two thereof; The organic solvent in step (2) includes any one of N-methylpyrrolidone, liquid alkanes with 5 to 10 carbon atoms, benzene, p-xylene or petroleum ether, or a combination of at least two thereof; The conductive agent in step (2) includes any one of acetylene black, Ketjen black, conductive graphite, Cabot black or Super P, or a combination of at least two thereof; In step (2), the second binder comprises any one of polyvinylidene fluoride, styrene-butadiene rubber, polystyrene, polyisobutylene, styrene-butadiene-styrene block copolymer or styrene-ethylene-butylene-styrene block copolymer, or a combination of at least two thereof; The mass ratio of the conductive agent, the first binder or the second binder and the negative electrode active material is (1-10): (1-10): (80-98).

9. The method according to claim 6, characterized in that The steps for preparing the negative electrode slurry in step (2) are as follows: (a) mixing a negative electrode active material, a conductive agent, a second binder and an organic solvent for a primary mixing, and then adding a lithium carbon material with a cocoon structure for secondary mixing to prepare a negative electrode slurry; The secondary mixing time is 30-60 min, and the secondary mixing speed is 3000-5000 rpm; (b) coating the negative electrode slurry on the surface of the current collector to obtain the pre-lithiation negative electrode.

10. The negative electrode according to claim 1, characterized in that The negative electrode is a composite negative electrode, comprising a current collector and a negative electrode active material layer disposed on at least one side of the current collector, wherein the negative electrode active material layer is composed of: The total weight of all components in the negative electrode active material layer is 100 parts by weight.

11. The negative electrode according to claim 10, characterized in that The negative electrode active material layer is composed of:

12. The negative electrode according to claim 10, characterized in that The lithium-containing particles include metallic lithium powder and / or lithium alloy powder; The lithium alloy powder includes any one or a combination of at least two of lithium boron alloy, lithium silver alloy, lithium tin alloy, lithium silicon alloy, lithium zinc alloy, lithium indium alloy, lithium aluminum alloy or lithium magnesium alloy; Wherein, the average particle size of the lithium-containing particles is 1-100 μm.

13. The negative electrode according to claim 10, characterized in that The structural carbon material includes micro-nano carbon material, and the micro-nano carbon material includes any one of graphene nanosheets, graphene nanoribbons, multi-walled carbon nanotubes, single-walled carbon nanotubes, carbon fibers or few-walled carbon nanotubes, or a combination of at least two thereof.

14. The negative electrode according to claim 10, characterized in that The mass ratio of the lithium-containing particles to the structural carbon material is (1-1000):1; When the lithium-containing particles include metallic lithium powder and lithium alloy powder, the mass ratio of the metallic lithium powder to the lithium alloy powder is (0.5-50):

1.

15. The negative electrode according to claim 10, characterized in that The negative electrode material includes any one or a combination of at least two of graphite, tin-based materials, hard carbon, soft carbon, mesophase carbon microspheres, silicon-carbon materials or silicon-oxygen materials; The conductive agent includes any one or a combination of at least two of acetylene black, Ketjen black, Cabot black, conductive graphite, Super P, carbon nanotubes, carbon fibers or graphene; The binder includes any one of polyvinylidene fluoride, polytetrafluoroethylene, polyvinylidene fluoride-hexafluoropropylene copolymer, sodium carboxymethyl cellulose, styrene-butadiene rubber, polystyrene, polyisobutylene, styrene-butadiene-styrene block copolymer or styrene-ethylene-butylene-styrene block copolymer, or a combination of at least two thereof.

16. A method for preparing the negative electrode according to any one of claims 10 to 15, characterized in that: The method comprises the following steps: (1) mixing a negative electrode material, a conductive agent and a binder, and then fibrillating the binder to obtain a mixture; (2) The mixture in step (1) is mixed with a cocoon-structured lithium-carbon material, and then hot-pressed into a negative electrode active material layer, and the negative electrode active material layer is disposed on at least one side of a current collector to obtain the composite negative electrode.

17. The method according to claim 16, characterized in that The temperature of the binder fiberization in step (1) is 50-150°C.

18. The method according to claim 16, characterized in that The mixing time in step (2) is 10-60 min, and the mixing speed is 2000-5000 rpm; The temperature of the hot pressing in step (2) is 80-120° C. The hot pressing pressure in step (2) is 10-50t.

19. An energy storage device, characterized in that: The energy storage device comprises the negative electrode according to any one of claims 1 to 5 or the negative electrode according to any one of claims 10 to 15.

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