Secondary battery negative electrode sheet and preparation method thereof

Abstract

The present application relates to a secondary battery negative electrode sheet and a preparation method thereof. The present application also relates to a secondary battery negative electrode sheet, which comprises: a negative electrode active material, a binder, a conductive agent, and a pore former, wherein the pore former comprises hollow polymer microspheres, and the polymer comprises an elastic polymer.

Description

Technical Field

[0001] The present application relates to a secondary battery negative electrode sheet and a method for preparing the same. More specifically, the present application relates to the following aspects: a secondary battery negative electrode sheet, a method for preparing a slurry for a secondary battery negative electrode, a method for preparing a secondary battery negative electrode sheet, and a secondary battery. Background Art

[0002] With the development of new energy vehicles and energy storage business, the market has higher and higher requirements for batteries. In new energy vehicles, secondary batteries are currently widely used battery products. Secondary batteries have high energy ratio, low self-discharge, and high operating voltage. In addition, secondary batteries also have long cycle life and better safety than other batteries. Therefore, secondary batteries are widely used in new energy vehicles and energy storage business.

[0003] Secondary batteries mainly rely on the movement of lithium ions between the positive and negative electrodes to work. During the charge and discharge process, lithium ions move back and forth between the two electrodes. More specifically, when charging the battery, lithium ions leave the positive electrode and pass through the electrolyte to the negative electrode, leaving the negative electrode in a lithium-rich state. On the other hand, the opposite process occurs during discharge. For the negative electrode of a secondary battery, the diffusion and transmission rate of lithium ions therein is an important parameter for evaluating battery performance. The open porosity in the negative electrode determines the diffusion and transmission rate of lithium ions. The higher the porosity, the higher the diffusion and transmission rate of lithium ions (and electrolytes), which is conducive to achieving better battery performance.

[0004] However, if the porosity is increased blindly, it will also lead to a decrease in the volume density of the active material, and then lead to a decrease in the energy density of the battery. Therefore, in the prior art, the negative electrode sheet of the secondary battery usually needs to be pressed to increase the compaction density, thereby increasing the battery energy density. However, this results in a decrease in the porosity of the compacted negative electrode sheet, and in particular, it will form a closed porosity on the surface of the negative electrode sheet. This closed porosity makes it difficult for the electrolyte to penetrate into the interior, which will also lead to a decrease in the performance of the secondary battery.

[0005] In the prior art, there are mainly the following methods for making holes in negative electrode sheets:

[0006] 1) Physical hole making: Use a roller plate to pierce the pole piece. However, this method makes the pole piece very susceptible to physical damage, and can only produce straight through holes on the surface of the pole piece, and cannot achieve hole making inside the pole piece.

[0007] 2) Chemical pore forming agent: pore forming by chemical reaction.

[0008] For example, the prior art (CN 103515607B) discloses a secondary battery negative electrode slurry and a secondary battery negative electrode and a battery prepared using the negative electrode slurry. In the patent document, the slurry used to prepare the negative electrode of the secondary battery contains a negative electrode active material, a binder, a solvent and a pore former. In the technical solution of CN 103515607B, the pore former is selected from one or more of azo compounds and / or sulfonyl hydrazide compounds.

[0009] However, the performance of the above pore formers is very unstable and the gas generation rate is difficult to control. Therefore, the pore morphology cannot be regulated. In addition, some pore formers will decompose to produce harmful products, which needs to be avoided for environmental protection requirements.

[0010] 3) Freeze-drying and pore-forming: This method has a complex production process and high requirements on equipment. In addition, low-temperature freezing will damage the electrode.

[0011] Therefore, there is an urgent need in the art for an environmentally friendly negative electrode sheet with controllable internal pores and high porosity, and a secondary battery comprising such a negative electrode sheet. Summary of the invention

[0012] In order to solve the above problems existing in the prior art, the present application adopts hollow polymer microspheres, and mixes the hollow polymer microspheres as pore-forming agents in the slurry used to prepare the negative electrode plate, and performs pore formation during the drying stage of the plate. The production process is simple and reliable, and pore formation inside the plate can be achieved. The preparation process of the present application does not produce harmful products during the production process and is environmentally friendly.

[0013] In addition, the hollow polymer microspheres can also buffer the expansion of the pole piece as an elastic support material. As a further improvement, the hollow polymer microspheres can also be loaded with flame retardant materials to release flame retardants to prevent combustion when the battery cell thermally runs away.

[0014] More specifically:

[0015] The first aspect of the present application relates to a secondary battery negative electrode plate, which includes a current collector and an electrode layer, wherein the electrode layer contains: a negative electrode active material, a binder, a conductive agent, and a pore former, wherein the pore former includes hollow polymer microspheres, and the polymer includes an elastic polymer.

[0016] The second aspect of the present application relates to a method for preparing a slurry for a secondary battery negative electrode, which comprises the following steps:

[0017] 1) Providing negative electrode active materials, binders, conductive agents and pore-forming agents;

[0018] 2) dispersing the negative electrode active material, the binder, the conductive agent and the pore former in a solvent to obtain a slurry;

[0019] The pore-forming agent is hollow polymer microspheres, the pore-forming agent accounts for 1 to 20 mass fractions of the total weight of the slurry, and the pore-forming agent accounts for 1 to 10 volume fractions of the total volume of the slurry before expansion.

[0020] The third aspect of the present application relates to a method for preparing a negative electrode sheet of a secondary battery, which comprises the following steps:

[0021] 1) providing a slurry comprising a negative electrode active material, a binder, a conductive agent, and a pore former, wherein the pore former comprises hollow polymer microspheres, and the polymer comprises an elastic polymer;

[0022] 2) coating the slurry on a current collector to form an electrode layer;

[0023] 3) performing heating, drying and rolling to form a negative electrode sheet for a secondary battery.

[0024] A fourth aspect of the present application relates to a secondary battery, which includes the secondary battery negative electrode, positive electrode, electrolyte and separator as described in the first aspect.

[0025] Compared with the prior art, the preparation process of the present application provides a simple and environmentally friendly method for making anode sheets of secondary batteries. This method can form pores of adjustable size in the anode diaphragm, and the process is simple to implement, is friendly to the active materials in the anode diaphragm, has adjustable pore size, is easy to reverse in structure, and is simple in process, without changing the existing production process.

[0026] These and other features and advantages will become apparent by reading the following detailed description and by reference to the associated drawings.It is to be understood that the foregoing general description and the following detailed description are illustrative only and are not restrictive of the aspects of what is claimed. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic cross-sectional view of the hollow polymer microsphere of the present application.

[0028] Figure 2 It is a flow chart of the manufacturing process of the negative electrode plate of the secondary battery of the present application.

[0029] Figure 3 It is a schematic structural diagram of the negative electrode plate of the secondary battery of the present application.

[0030] Figure 4 Schematic diagram of an embodiment of a device using the secondary battery of the present application as a power system. DETAILED DESCRIPTION

[0031] "Scope" disclosed herein is defined in the form of lower limit and upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a particular range. The scope defined in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a particular parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0032] In this application, if there is no special description, all embodiments and preferred embodiments mentioned herein can be combined with each other to form a new technical solution. In this application, if there is no special description, all technical features and preferred features mentioned herein can be combined with each other to form a new technical solution.

[0033] In this application, unless otherwise specified, the terms "include" and "comprising" mentioned herein may be open-ended or closed-ended. For example, the terms "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0034] In the description herein, unless otherwise specified, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0035] The first aspect

[0036] The first aspect of the present application relates to a secondary battery negative electrode plate, which includes a current collector and an electrode layer, wherein the electrode layer contains: a negative electrode active material, a binder, a conductive agent, and a pore former, wherein the pore former includes hollow polymer microspheres, and the polymer includes an elastic polymer.

[0037] In one embodiment, the secondary battery negative electrode sheet of the first aspect is related, wherein the hollow polymer microsphere comprises a shell-core structure, the shell comprises the elastic polymer, and the elastic polymer comprises polyethylene, polystyrene, polycarbonate, polyacrylate. The hollow polymer microsphere can produce uniform pores throughout the slurry and the layer formed thereby in an efficient, simple, and low-cost manner.

[0038] In one embodiment, the negative electrode sheet of the secondary battery of the first aspect is related to the core body, wherein the core body contains an inert gas filled and a filler with a flame retardant effect. The gas and filler can release the flame retardant when the battery cell thermally runs away, thereby playing a flame retardant role.

[0039] In one embodiment, the negative electrode plate of the secondary battery of the first aspect is provided, wherein the gas is selected from one or more of the following groups: N2, CO2, helium, and argon. The gas is heated and expanded during the drying process of the plate, so that the polymer hollow microspheres expand and occupy a certain space inside the plate. When the plate is dried and cooled to room temperature, the volume of the gas shrinks and the volume of the polymer hollow microspheres decreases as a whole, forming pores inside the plate.

[0040] In one embodiment, the negative electrode plate of the secondary battery related to the first aspect, wherein the filler comprises one or more flame retardants selected from the group consisting of tributyl phosphate, tri(2-ethylhexyl) phosphate, tri(2-chloroethyl) phosphate, tri(2,3-dichloropropyl) phosphate, tri(2,3-dibromopropyl) phosphate, Pyrol99, toluene-diphenyl phosphate, tricresyl phosphate, triphenyl phosphate, (2-ethylhexyl)-diphenyl phosphate, dicyclopentadiene, chlordanic anhydride, aliphatic halogenated hydrocarbons, dibromomethane, trichlorobromomethane, dichlorobromomethane, octabromodiphenyl oxide, pentabromoethylbenzene, tetrabromobisphenol A, tellurium compounds, hydroxyaluminum, magnesium hydroxide, and borates. The flame retardant plays a flame retardant role when the battery cell is thermally runaway.

[0041] In one embodiment, the negative electrode sheet of the secondary battery related to the first aspect, wherein the electrode layer contains the pore former in an amount of 1%-20%, 1%-5%, 1%-2% by mass, and any range therebetween. The greater the mass fraction of the pore former, the greater the porosity of the prepared electrode sheet, which can meet the requirements of different products for charging speed.

[0042] In one embodiment, the negative electrode sheet of the secondary battery of the first aspect is provided, wherein the electrode layer comprises pores with a pore size of 1-100 μm, 20-60 μm, and any range therebetween. The different pore sizes of the prepared pores of the electrode sheet can meet the requirements of different products for charging speed.

[0043] In one embodiment, the negative electrode sheet of the secondary battery of the first aspect, wherein the porosity of the electrode layer is 20%-40%, 25-35%, 27-35% and any range therebetween. The larger the porosity of the prepared electrode sheet, the faster the charging speed can be.

[0044] The second aspect

[0045] The second aspect of the present application relates to a method for preparing a slurry for a secondary battery negative electrode, which comprises the following steps:

[0046] 1) Providing negative electrode active materials, binders, conductive agents and pore-forming agents;

[0047] 2) dispersing the negative electrode active material, the binder, the conductive agent and the pore former in a solvent to obtain a slurry;

[0048] The pore-forming agent is hollow polymer microspheres, the pore-forming agent accounts for 1 to 20 mass fractions of the total weight of the slurry, and the pore-forming agent accounts for 1 to 10 volume fractions of the total volume of the slurry before expansion.

[0049] In one embodiment, the method for preparing a slurry for a negative electrode of a secondary battery related to the second aspect, wherein the hollow polymer microspheres include a shell-core structure, the shell comprises an elastic polymer, and the elastic polymer includes polyethylene, polystyrene, polycarbonate, polyacrylate. The hollow polymer microspheres can produce uniform pores throughout the slurry and the layer formed thereby in an efficient, simple, and low-cost manner.

[0050] In one embodiment, the method for preparing a slurry for a negative electrode of a secondary battery according to the second aspect, wherein the core comprises an inert gas filled and a filler having a flame retardant function. By filling the gas and the filler, the flame retardant can be released when the battery cell thermally runs away, thereby playing a flame retardant role.

[0051] In one embodiment, the method for preparing a slurry for a negative electrode of a secondary battery related to the second aspect, wherein the gas is selected from one or more of the following groups: N2, CO2, helium, and argon. The gas is heated and expanded during the drying process of the pole piece, causing the polymer hollow microspheres to expand and occupy a certain space inside the pole piece. When the pole piece is dried and cooled to room temperature, the volume of the gas shrinks, and the volume of the polymer hollow microspheres decreases as a whole, forming pores inside the pole piece.

[0052] In one embodiment, the method for preparing a slurry for a secondary battery negative electrode of the second aspect, wherein the filler comprises one or more flame retardants selected from the group consisting of tributyl phosphate, tri(2-ethylhexyl) phosphate, tri(2-chloroethyl) phosphate, tri(2,3-dichloropropyl) phosphate, tri(2,3-dibromopropyl) phosphate, Pyrol99, toluene-diphenyl phosphate, tricresyl phosphate, triphenyl phosphate, (2-ethylhexyl)-diphenyl phosphate, dicyclopentadiene, chlordanic anhydride, aliphatic halogenated hydrocarbons, dibromomethane, trichlorobromomethane, dichlorobromomethane, octabromodiphenyl oxide, pentabromoethylbenzene, tetrabromobisphenol A, tellurium compounds, hydroxyaluminum, magnesium hydroxide, and borates. The flame retardant plays a flame retardant role when the battery cell is thermally runaway.

[0053] The third aspect

[0054] The third aspect of the present application relates to a method for preparing a negative electrode sheet of a secondary battery, which comprises the following steps:

[0055] 1) providing a slurry comprising a negative electrode active material, a binder, a conductive agent, and a pore former, wherein the pore former comprises hollow polymer microspheres, and the polymer comprises an elastic polymer;

[0056] 2) coating the slurry on a current collector to form an electrode layer;

[0057] 3) performing heating, drying and rolling to form a negative electrode sheet for a secondary battery.

[0058] In one embodiment, the method for preparing a negative electrode sheet of a secondary battery related to the third aspect, wherein the heating and drying temperature is 60-200 degrees Celsius, and the drying time is 5-120 minutes. For a specific heating temperature and drying time, if the time and temperature are insufficient, the slurry may not be formed, and if the time and temperature are too long or too high, it may lead to excessive curing.

[0059] In one embodiment, the method for preparing a negative electrode sheet of a secondary battery related to the third aspect, wherein the porosity of the electrode layer is 20%-40%, 25-35%, 27-35% and any range therebetween. The larger the porosity of the prepared electrode sheet, the faster the charging speed can be.

[0060] In one embodiment, the method for preparing a negative electrode sheet of a secondary battery according to the third aspect, wherein the electrode layer comprises pores with a pore size of 1-100 μm, optionally 20-60 μm. The different pore sizes of the prepared electrode sheet can meet the requirements of different products for charging speed.

[0061] The fourth aspect

[0062] A fourth aspect of the present application relates to a secondary battery, which includes the secondary battery negative electrode, positive electrode, electrolyte and separator as described in the first aspect.

[0063] The present invention will be further described below in conjunction with the accompanying drawings, but it should be understood that they are only used to illustrate the present invention rather than to limit the scope of the present invention.

[0064] Attached to this application Figure 1 is a schematic cross-sectional view of a pore former (more specifically, a hollow polymer microsphere). Figure 1 , the hollow polymer microspheres include a shell-core structure, which corresponds to Figure 1 The polymer shell and the inner space in the polymer shell. The material of the polymer shell may include a polymer, such as an elastic polymer. The elastic polymer is selected from one or more of the following groups: polyethylene, polystyrene, polycarbonate, polyacrylate, etc. The inner space may be filled with gas and may optionally also be filled with filler.

[0065] For example, the gas filled is selected from one or more of the following groups: N2, CO2, helium, argon, etc., but not limited thereto. For example, the filler filled is selected from one or more flame retardants in the following group: tributyl phosphate, tri(2-ethylhexyl) phosphate, tri(2-chloroethyl) phosphate, tri(2,3-dichloropropyl) phosphate, tri(2,3-dibromopropyl) phosphate, Pyrol99, toluene-diphenyl phosphate, tricresyl phosphate, triphenyl phosphate, (2-ethylhexyl)-diphenyl phosphate, dicyclopentadiene, chlordanic anhydride, aliphatic halogenated hydrocarbons, dibromomethane, trichlorobromomethane, dichlorobromomethane, octabromodiphenyl oxide, pentabromoethylbenzene, tetrabromobisphenol A, tellurium compounds, hydroxyaluminum, magnesium hydroxide, borates, etc., but not limited thereto.

[0066] Attached to this application Figure 2 This is a flow chart of the manufacturing process of the negative electrode of a secondary battery. Figure 2 In the manufacturing process, in the first step, a slurry containing a negative electrode active material, a binder, a conductive agent and a pore former is provided. The pore former includes hollow polymer microspheres, and the hollow polymer microspheres include a shell-core structure, that is, corresponding to Figure 1 The polymer shell and the inner space in the polymer shell. The material of the polymer shell may include a polymer, such as an elastic polymer. The elastic polymer is selected from one or more of the following groups: polyethylene, polystyrene, polycarbonate, polyacrylate, etc. The inner space may be filled with gas and may optionally also be filled with filler.

[0067] In the second step, the negative electrode active material, the binder, the conductive agent, and the pore former are dispersed in a solvent (eg, deionized water) and stirred to obtain a slurry.

[0068] In the third step, the slurry obtained in the second step is applied to a current collector (also referred to herein as a current collector, a current collector, etc., which terms are used interchangeably).

[0069] In the fourth step, the current collector coated with the slurry is dried and roll-pressed.

[0070] In the fifth step, after the drying process at a suitable temperature and time and rolling in the fourth step, a negative electrode sheet of a secondary battery is obtained.

[0071] Attached to this application Figure 3 Schematic diagram of the structure of the negative electrode sheet of the secondary battery of the present application. Figure 3 It can be seen that the prepared current collector includes an active material layer, in which negative electrode active materials and pores are contained, wherein the pores contain hollow polymer microspheres.

[0072] Attached to this application Figure 4 Schematic diagram of an embodiment of a device using a secondary battery as a power system. Specifically, the secondary battery includes: a secondary battery positive electrode, a secondary battery negative electrode, an electrolyte, and a separator separating the secondary battery positive electrode from the secondary battery negative electrode.

[0073] For the negative electrode of the secondary battery, it includes a negative electrode current collector and a negative electrode active material disposed on at least one surface of the negative electrode current collector. For the negative electrode of the secondary battery, it can be:

[0074] 1) A secondary battery negative electrode comprising the negative electrode slurry as described herein;

[0075] 2) A secondary battery negative electrode comprising a negative electrode slurry prepared by the negative electrode slurry preparation method as described herein;

[0076] 3) comprising or consisting of a secondary battery negative electrode as described herein;

[0077] The secondary battery positive electrode includes a positive electrode collector and a positive electrode active material disposed on at least one surface of the positive electrode collector.

[0078] An example of a device using a secondary battery as a power system is a car, including new energy vehicles, hybrid vehicles, etc., but not limited thereto. For example, various vehicles (including but not limited to power-assisted vehicles, cars, trucks, aircraft, drones, etc.) can use the secondary battery described herein (which includes the secondary battery negative electrode, the secondary battery positive electrode, the separator, etc. as described herein).

[0079] The secondary battery negative electrode slurry, the secondary battery negative electrode and the negative electrode active material of the secondary battery used in the present application include but are not limited to: natural graphite, artificial graphite, graphene, carbon nanotubes, soft carbon, hard carbon and the like.

[0080] The secondary battery negative electrode slurry, secondary battery negative electrode and secondary battery binder used in the present application include but are not limited to: polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin, etc.

[0081] The secondary battery negative electrode slurry, the secondary battery negative electrode and the conductive agent of the secondary battery used in the present application include but are not limited to: superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers, etc.

[0082] The secondary battery negative electrode slurry, the secondary battery negative electrode and the current collector of the secondary battery used in the present application include but are not limited to: copper foil, silver foil, iron foil, or foils composed of alloys of the above metals. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer, which can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material base layer (such as a base layer made of polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and copolymers thereof).

[0083] The secondary battery positive electrode and the positive electrode active materials of the secondary battery used in the present application include but are not limited to: lithium-containing phosphates with olivine structure, lithium transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for secondary batteries can also be used. These positive electrode active materials can be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include but are not limited to lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2(NCM211), LiNi0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (NCM811)), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and its modified compounds, etc. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate (such as LiFePO4 (LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0084] The binder used for the secondary battery positive electrode and the secondary battery of the present application includes but is not limited to: polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer and fluorine-containing acrylate resin, etc.

[0085] The conductive materials of the secondary battery positive electrode and the secondary battery used in the present application include but are not limited to: superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0086] The secondary battery positive electrode and the current collector of the secondary battery used in the present application include but are not limited to: copper foil, silver foil, iron foil, or foils composed of alloys of the above metals. The composite current collector may include a polymer material base and a metal layer formed on at least one surface of the polymer material base, which can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material base (such as a base made of polypropylene PP, polyethylene terephthalate PET, polybutylene terephthalate PBT, polystyrene PS, polyethylene PE and its copolymers).

[0087] The electrolyte used for the secondary battery of the present application includes but is not limited to: one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalatoborate), LiBOB (lithium dioxalatoborate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorooxalatophosphate) and LiTFOP (lithium tetrafluorooxalatophosphate). In one embodiment of the present application, the solvent may be selected from one or more of the following: ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0088] The separator used for the secondary battery of the present application includes, but is not limited to: one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different. In an embodiment of the present application, the separator is selected from a polyolefin separator, a polyester separator, a polyimide separator, a polyamide separator and a cellulose separator.

[0089] The secondary battery of the present application may be manufactured by a winding process or a stacking process using the secondary battery positive electrode, the secondary battery negative electrode, the electrolyte, and the separator described above.

[0090] Example

[0091] The present invention can be better understood according to the following examples. However, it is easy for those skilled in the art to understand that the specific material ratios, process conditions and results described in the examples are only used to illustrate the present invention, and should not and will not limit the present invention described in detail in the claims.

[0092] Example 1

[0093] Preparation of negative electrode

[0094] Select the negative electrode active material (graphite, 96.5% mass fraction), binder (styrene-butadiene rubber, 2% mass fraction), conductive agent (conductive carbon black, 1%) and pore-forming agent (0.5% mass fraction, polyethylene shell, shell thickness 300nm, nitrogen filled inside, diameter 5 microns), dispersed in deionized water to form a negative electrode slurry, wherein the mass fraction of deionized water in the negative electrode slurry is 30%. The negative electrode slurry is prepared with the pore-forming agent content shown in Table 1 below. With 130mg / 1540.25mm 2 The negative electrode slurry is applied to the surface of the current collector with a coating weight per unit area of ​​10000 t / cm. After that, it is dried at 130 degrees Celsius for 20 minutes. Then, it is rolled to manufacture the negative electrode sheet of the secondary battery.

[0095] Preparation of lithium-ion secondary batteries

[0096] 7-micron polyethylene (PE) is used as a separator. The positive electrode active material NCM523, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are dissolved in the solvent N-methylpyrrolidone (NMP) at a weight ratio of 96.5:1.5:2, and the positive electrode slurry is obtained after being fully stirred and mixed; then the positive electrode slurry is evenly coated on the positive electrode collector, and then dried, cold pressed, and cut to obtain the positive electrode sheet. The positive electrode sheet, the separator, and the negative electrode sheet prepared above are wound into a battery cell, and a lithium-ion secondary battery is obtained by polar ear welding, aluminum shell packaging, liquid injection, packaging formation, and vacuum molding. The battery cell has a width of 148mm, a thickness of 28mm, a height of 98mm, and a capacity of 40Ah. Among them, the electrolyte is a solution of 1M LiPF6 ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1:2).

[0097] Examples 2-3 and Comparative Example 4

[0098] The preparation methods of Examples 2-3 and Comparative Example 4 are similar to those of Example 1, and the detailed preparation parameters are shown in Table 1 below.

[0099] Test Method

[0100] For the secondary battery negative electrode sheet prepared in the above preparation example, the gas adsorption method (BET) is used to characterize the pore-forming effect. The test process is as follows:

[0101] 1) Equipment used: TriStar II 3020;

[0102] 2) Take a clean sample tube, weigh it, and record the empty weight;

[0103] 3) Weigh the pole piece sample and place it in the sample tube.

[0104] 4) Load the sample tube containing the electrode sample into the degassing station of the equipment and vacuum degas for 120 minutes at 130 degrees Celsius. Weigh the degassed sample tube to obtain the degassed weight;

[0105] 5) Open the test software attached to the equipment and test the porosity of the electrode sample.

[0106] The test results are shown in Table 2. More specifically, Table 2 lists the electrode porosity and charge capacity retention rate (2C) of each embodiment.

[0107] Table 1

[0108]

[0109] Table 2

[0110] Pole Porosity Charge capacity retention rate (2C) Example 1 33.90％ 88.2％ Example 2 34.10％ 89.6％ Example 3 35.40％ 90.3％ Comparative Example 4 28.70％ 85.3％

[0111] As can be seen from Tables 1 and 2, the use of the pore-forming agent (hollow polymer microspheres) of the present application can achieve a pole piece porosity greater than 30%, more specifically greater than or equal to 33.90%, or greater than or equal to 34.10%, or even greater than or equal to 35.40%. For the above-mentioned pole piece porosity, a charge capacity retention rate greater than or equal to 88.2%, or greater than or equal to 89.6%, or even greater than or equal to 90.3% can be achieved. This is a significant improvement over Comparative Example 4 without a pore-forming agent.

[0112] Although the present invention has been described in detail for the purpose of clarity and understanding, after reading this application specification, those skilled in the art will understand that various modifications and changes can be made to the present invention without departing from the spirit and essence of the present invention, and these modifications and changes all fall within the scope of the attached claims and their equivalents.

Claims

1. A secondary battery negative electrode sheet, comprising a current collector and an electrode layer, wherein the electrode layer comprises: a negative electrode active material, a binder, a conductive agent, and a pore-forming agent, wherein: The pore former comprises hollow polymer microspheres, wherein the polymer comprises an elastic polymer, The hollow polymer microsphere comprises a shell-core structure, the shell comprises the elastic polymer, and the core comprises a filler having a flame retardant effect.

2. The negative electrode sheet of a secondary battery according to claim 1, wherein: The elastic polymer includes one or more selected from the following group: polyethylene, polystyrene, polycarbonate, polyacrylate.

3. The negative electrode sheet of a secondary battery according to claim 2, wherein: The core contains an inert gas filling.

4. The negative electrode sheet of a secondary battery according to claim 3, wherein: The gas is selected from one or more of the following groups: N2, CO2, helium, and argon.

5. The negative electrode sheet of a secondary battery according to claim 1, wherein: The filler contains one or more flame retardants selected from the following group: tributyl phosphate, tri(2-ethylhexyl) phosphate, tri(2-chloroethyl) phosphate, tri(2,3-dichloropropyl) phosphate, tri(2,3-dibromopropyl) phosphate, Pyrol99, toluene-diphenyl phosphate, tricresyl phosphate, triphenyl phosphate, (2-ethylhexyl)-diphenyl phosphate, dicyclopentadiene, chlordanic anhydride, aliphatic halogenated hydrocarbons, dibromomethane, trichlorobromomethane, dichlorobromomethane, octabromodiphenyl oxide, pentabromoethylbenzene, tetrabromobisphenol A, tellurium compounds, hydroxyaluminum, magnesium hydroxide, and borates.

6. The negative electrode sheet of a secondary battery according to claim 1, wherein: The electrode layer contains the pore former in an amount of 1% to 20% by mass.

7. The negative electrode sheet of a secondary battery according to claim 6, wherein: The electrode layer contains the pore former in an amount of 1% to 5% by mass.

8. The negative electrode sheet of a secondary battery according to claim 7, wherein: The electrode layer contains the pore former in an amount of 1% to 2% by mass.

9. The negative electrode sheet of a secondary battery according to claim 1, wherein: The electrode layer contains pores with a pore size of 1-100 μm.

10. The negative electrode sheet of a secondary battery according to claim 9, wherein: The electrode layer contains pores with a pore size of 20-60 μm.

11. The negative electrode sheet of a secondary battery according to claim 1, wherein: The porosity of the electrode layer is 20%-40%.

12. A method for preparing a slurry for a secondary battery negative electrode, comprising the following steps: 1) Providing negative electrode active materials, binders, conductive agents and pore-forming agents; 2) dispersing the negative electrode active material, the binder, the conductive agent and the pore former in a solvent to obtain a slurry; in, The pore-forming agent is a hollow polymer microsphere, the pore-forming agent accounts for 1 to 20 mass fractions of the total weight of the slurry, and the pore-forming agent accounts for 1 to 10 volume fractions of the total volume of the slurry before expansion, The hollow polymer microsphere comprises a shell-core structure, wherein the shell comprises an elastic polymer and the core comprises a filler having a flame retardant effect.

13. The method of claim 12, wherein: The elastic polymer includes one or more selected from the following group: polyethylene, polystyrene, polycarbonate, polyacrylate.

14. The method of claim 13, wherein: The core contains an inert gas filling.

15. The method of claim 14, wherein: The gas is selected from one or more of the following groups: N2, CO2, helium, and argon.

16. The method of claim 12, wherein: The filler contains one or more flame retardants selected from the following group: tributyl phosphate, tri(2-ethylhexyl) phosphate, tri(2-chloroethyl) phosphate, tri(2,3-dichloropropyl) phosphate, tri(2,3-dibromopropyl) phosphate, Pyrol99, toluene-diphenyl phosphate, tricresyl phosphate, triphenyl phosphate, (2-ethylhexyl)-diphenyl phosphate, dicyclopentadiene, chlordanic anhydride, aliphatic halogenated hydrocarbons, dibromomethane, trichlorobromomethane, dichlorobromomethane, octabromodiphenyl oxide, pentabromoethylbenzene, tetrabromobisphenol A, tellurium compounds, hydroxyaluminum, magnesium hydroxide, and borates.

17. A method for preparing a negative electrode sheet of a secondary battery, comprising the following steps: 1) Providing a slurry comprising a negative electrode active material, a binder, a conductive agent, and a pore-forming agent, wherein: The pore-forming agent comprises hollow polymer microspheres, the polymer comprises an elastic polymer, the hollow polymer microspheres comprise a shell-core structure, the shell comprises the elastic polymer, and the core comprises a filler having a flame retardant effect; 2) coating the slurry on a current collector to form an electrode layer; 3) Heat drying and roll pressing to form a negative electrode sheet for a secondary battery.

18. The method of claim 17, wherein: The heating drying temperature is 60-200°C and the drying time is 5-120min.

19. The method of claim 17, wherein: The porosity of the electrode layer is 20%-40%.

20. The method of claim 17, wherein: The electrode layer contains pores with a pore size of 1-100 μm.

21. The method of claim 20, wherein: The electrode layer contains pores with a pore size of 20-60 μm.

22. A secondary battery comprising the secondary battery negative electrode sheet according to any one of claims 1 to 11, as well as a positive electrode, an electrolyte and a separator.

Images