Electrode, battery, battery pack and electric equipment

Abstract

The invention provides an electrode, a battery, a battery pack and electric equipment. The electrode comprises a current collector and an active layer arranged on at least one functional surface of the current collector, wherein the active layer comprises a first active layer close to the current collector and a second active layer arranged on one surface, far away from the current collector, of the first active layer; the first active layer comprises a first binder, the first binder comprises a polyvinylidene fluoride copolymer, and the crystallinity of the polyvinylidene fluoride copolymer is not higher than 25%; the second active layer comprises a second binder, and the crystallinity of the second binder is higher than that of the first binder. After the electrode is applied to the battery, the power performance and the cycle performance of a battery cell can be guaranteed while the surface density is improved.

Description

Technical Field

[0001] The present invention relates to an electrode, in particular to an electrode, a battery, a battery pack and an electrical device, belonging to the field of secondary batteries. Background Art

[0002] With the rapid development of the new energy industry, lithium-ion batteries are becoming increasingly popular in our daily lives. Consumers are also placing higher demands on the performance of lithium-ion batteries. Among them, improving the energy density of batteries is one of the key factors in promoting technological progress and meeting the needs of modern applications.

[0003] At present, developing thick electrodes to increase the surface density of the electrode sheet has become an important means to quickly and effectively improve the energy density of iron-lithium batteries. However, at this stage, directly increasing the thickness of the electrode often causes the power performance of the battery cell to deteriorate while increasing the energy density. Moreover, thick electrodes may experience greater volume changes during the charge and discharge cycle, resulting in increased mechanical stress, thereby affecting the cycle performance of the electrode.

[0004] How to improve energy density while ensuring battery cell power performance and cycle performance has become a topic of close attention. Summary of the Invention

[0005] The present invention provides an electrode, wherein the first active layer and the second active layer of the electrode include a specific binder. Under the synergistic effect of the first active layer and the second active layer, the electrode has a higher energy density, relatively excellent power performance and cycle performance.

[0006] The present invention also provides a battery, which includes the above-mentioned electrode and thus has higher energy density, relatively excellent power performance and cycle performance.

[0007] The present invention also provides a battery pack comprising the above-mentioned battery.

[0008] The present invention also provides an electrical device, which includes the battery or battery pack and has good performance.

[0009] In one aspect, the present invention provides an electrode comprising a current collector and an active layer disposed on at least one functional surface of the current collector;

[0010] Wherein, the active layer comprises a first active layer close to the current collector and a second active layer arranged on a side of the first active layer away from the current collector;

[0011] The first active layer includes a first binder, the first binder includes a polyvinylidene fluoride copolymer, and the crystallinity of the polyvinylidene fluoride copolymer is not higher than 25%;

[0012] The second active layer includes a second binder, and the second binder has a higher crystallinity than the first binder.

[0013] In the electrode as described above, the polyvinylidene fluoride copolymer is formed by copolymerizing a first monomer and a second monomer, wherein the first monomer includes 1,1-difluoroethylene, and the second monomer includes at least one of olefins and olefin derivatives.

[0014] In the electrode as described above, the molar proportion of the second monomer is not higher than 30%.

[0015] The electrode as described above has a weight average molecular weight of 50W-150W.

[0016] In the electrode as described above, the second binder includes at least one of polyvinylidene fluoride-hexafluoropropylene copolymer, a polyvinylidene fluoride-hexafluoropropylene copolymer derivative, polyvinylidene fluoride-tetrafluoroethylene copolymer, and a polyvinylidene fluoride-tetrafluoroethylene copolymer derivative.

[0017] The electrode as described above, wherein the first active layer comprises, by weight, 100 parts of positive electrode active material, 2.5-3.5 parts of first binder, and 1-2 parts of conductive agent;

[0018] And / or, based on parts by mass, the second active layer includes 100 parts of positive electrode active material, 2-3 parts of second binder, and 1-2 parts of conductive agent.

[0019] The electrode as described above, wherein the thickness of the electrode is 170 μm-260 μm;

[0020] And / or, the surface density of the electrode is 400g / m 2 ~600g / m 2 .

[0021] The surface density of the active layer of the electrode is 200 g / m 2 -300g / m 2 ;

[0022] And / or, the surface density of the first active layer is 20g / m 2 -200g / m 2 ;

[0023] And / or, the surface density of the second active layer is 0.1 g / m 2 -280g / m 2 .

[0024] The electrode as described above has a compaction density of 2.3 g / cm 3 -2.6g / cm 3 .

[0025] The electrode as described above, wherein the thickness of the first active layer is 10 μm-85 μm;

[0026] And / or, the thickness of the second active layer is 0.1 μm-120 μm.

[0027] In another aspect, the present invention provides a battery comprising the electrode as described above.

[0028] In another aspect, the present invention provides a battery pack comprising the battery described above.

[0029] In another aspect, the present invention provides an electrical device comprising the battery or the battery pack as described above.

[0030] The electrode provided by the present invention includes a first active layer and a second active layer arranged in a stacked manner, wherein the first binder in the first active layer includes a polyvinylidene fluoride copolymer with relatively low crystallinity. The polyvinylidene fluoride copolymer with low crystallinity has a large swelling degree, is easy to absorb electrolyte, has a strong liquid retention capacity, and can effectively improve the battery power performance; while the binder with relatively high crystallinity in the second active layer can control the swelling of the electrode to a reasonable level, prevent the electrode structure from being damaged due to excessive swelling, and further ensure the cycle stability of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the electrode structure in a specific embodiment of the present invention;

[0032] Figure 2 A bar graph of discharge ratios at different rates after batteries were prepared using the electrodes provided in Example 2, Example 4, Comparative Example 1, and Comparative Example 2 of the present invention;

[0033] Figure 3 These are the normal temperature cycle curves of the batteries provided in Examples 2 and 5 and Comparative Examples 1 and 2.

[0034] Description of reference numerals:

[0035] 1-active layer; 11-first active layer; 12-second active layer; 2-current collector. DETAILED DESCRIPTION

[0036] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0037] In the existing thick electrode design process, due to the increase in electrode thickness and compaction density, the infiltration of electrolyte inside the electrode becomes increasingly difficult. The closer the electrode is to the current collector, the greater the lithium ion liquid phase diffusion impedance, and the internal resistance of the battery cell will also increase. In addition, the stress accumulation of the thick electrode during the charge and discharge process may also cause the electrode structure to be unstable, resulting in a decrease in cycle performance.

[0038] Therefore, it is necessary to develop new electrodes that can effectively absorb the electrolyte near the current collector to ensure power performance while maintaining relatively excellent structural stability.

[0039] In one aspect, the present invention provides an electrode comprising a current collector and an active layer disposed on at least one functional surface of the current collector;

[0040] The active layer includes a first active layer close to the current collector and a second active layer arranged on a side of the first active layer away from the current collector;

[0041] The first active layer includes a first binder, the first binder includes a polyvinylidene fluoride copolymer, and the crystallinity of the polyvinylidene fluoride copolymer is not higher than 25%;

[0042] The second active layer includes a second binder having a higher crystallinity than the first binder.

[0043] In detail, the electrode includes a current collector and an active layer arranged on one functional surface or two functional surfaces of the current collector, and the active layer includes a first active layer and a second active layer stacked in sequence in a direction away from the current collector.

[0044] The functional surface of the current collector refers to the outermost surface in the length direction and the width direction of the current collector, that is, the two largest and opposite surfaces among the six surfaces of the current collector.

[0045] In one embodiment, Figure 1 As shown, the electrode includes a current collector 2 and an active layer 1 arranged on two functional surfaces of the current collector 2. The active layer 1 includes a first active layer 11 and a second active layer 12 stacked in sequence in a direction away from the current collector 2.

[0046] Common current collectors in the art, such as aluminum foil, nickel foil, etc., can be selected according to actual conditions.

[0047] The first active layer includes a first binder, and the first binder includes a polyvinylidene fluoride (PVDF) copolymer. The polyvinylidene fluoride copolymer is a polymer material formed by copolymerizing vinylidene fluoride with other monomers.

[0048] The crystallinity of the polyvinylidene fluoride copolymer can be tested using common testing methods in the art, such as differential scanning calorimetry (DSC). The crystallinity of the polyvinylidene fluoride copolymer in the present invention includes, but is not limited to, 0%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, or any other range not exceeding 25%.

[0049] The second active layer includes a second binder, and the crystallinity of the second binder is higher than that of the first binder. When the crystallinity of the second binder is higher than that of the first binder, it can provide higher mechanical strength and chemical stability, which helps to improve the structural integrity and durability of the electrode.

[0050] The electrode provided by the present invention has a high energy density and excellent power performance and cycle stability. Among them, the first active layer in the electrode of the present invention includes a polyvinylidene fluoride copolymer with low crystallinity. The proportion of amorphous regions in the polyvinylidene fluoride copolymer is high, with more free volume and intermolecular pores, providing more liquid absorption space, which can better absorb electrolyte, has a strong liquid retention capacity, and effectively improves the battery power performance; and the binder with higher crystallinity in the second active layer can control the electrode swelling to a reasonable level, prevent the electrode from damaging the structure due to excessive swelling, and further ensure the cycle stability of the battery.

[0051] The type of comonomer of the polyvinylidene fluoride copolymer will have a certain impact on its liquid absorption capacity. In a specific embodiment, the polyvinylidene fluoride copolymer is formed by copolymerizing a first monomer and a second monomer, the first monomer includes 1,1-difluoroethylene, and the second monomer includes at least one of olefins and olefin derivatives.

[0052] In one specific embodiment, the second monomer is at least one of hexafluoropropylene, 1-chloropentafluoropropylene, and 1.1-dichlorotetrafluoropropylene.

[0053] The present invention does not limit the source of the polyvinylidene fluoride copolymer. In one embodiment, the polyvinylidene fluoride copolymer is commercially available.

[0054] When the first and second monomers in the polyvinylidene fluoride copolymer are selected from the aforementioned types, the structure of hexafluoropropylene and its derivatives makes it difficult for them to form crystals as a whole. This increases the amorphous region of the material, effectively adjusting the crystallinity of the polyvinylidene fluoride copolymer and ensuring the liquid absorption capacity of the first binder. Furthermore, the copolymer formed by the copolymerization of the first and second monomers exhibits excellent chemical resistance and flexibility, maintaining the mechanical properties of the copolymer.

[0055] By adjusting the ratio of the first monomer to the second monomer, the properties of the copolymer can be further optimized. In one embodiment, the molar ratio of the second monomer is no more than 30%.

[0056] Specifically, the molar percentage of the second monomer includes but is not limited to 1%, 5%, 10%, 15%, 20%, 25%, 30% or any range therebetween.

[0057] When the molar ratio of the first monomer to the second monomer is within the above range, not only can the crystallinity of the polyvinylidene fluoride copolymer be ensured at a lower level, but the relationship between the swelling degree and the crystallinity of the polyvinylidene fluoride copolymer can also be better balanced, thereby improving the power performance while ensuring the cycle performance.

[0058] Furthermore, in one specific embodiment, the weight average molecular weight of the polyvinylidene fluoride copolymer is 500,000 to 1.5 million.

[0059] The molecular weight of the polyvinylidene fluoride copolymer can be measured by a common method in the art, such as gel permeation chromatography.

[0060] Specifically, the weight average molecular weight of the polyvinylidene fluoride copolymer includes but is not limited to 500,000, 600,000, 700,000, 800,000, 900,000, 1,000,000, 1,100,000, 1,200,000, 1,300,000, 1,400,000, 1,500,000 or a range between any two thereof.

[0061] When the weight-average molecular weight of the polyvinylidene fluoride copolymer is too low, the overall peeling force of the electrode is low, which may easily cause failures such as electrode falling off. When the weight-average molecular weight of the polyvinylidene fluoride copolymer is too high, the slurry viscosity is too high during the early slurrying process, and normal coating cannot be performed, affecting normal production. Only when the weight-average molecular weight of the polyvinylidene fluoride copolymer is moderate can it meet the electrode peeling force requirements and the slurry viscosity requirements, ensuring normal production.

[0062] The second active layer includes a second binder with lower crystallinity than the first binder, and thus has a good protective effect on the electrode. Therefore, the type of the second binder directly affects the cycle performance of the electrode.

[0063] In a specific embodiment, the second binder includes at least one of polyvinylidene fluoride-hexafluoropropylene copolymer, a polyvinylidene fluoride-hexafluoropropylene copolymer derivative, a polyvinylidene fluoride-tetrafluoroethylene copolymer, and a polyvinylidene fluoride-tetrafluoroethylene copolymer derivative.

[0064] The present invention does not limit the source of the second adhesive, which can be obtained through commercial channels or prepared by conventional preparation methods in the art.

[0065] The above-mentioned type of second binder has a high degree of crystallinity, which gives the electrode higher mechanical strength and structural integrity, better helps the electrode maintain a stable structure during the charge and discharge cycle, avoids the first active layer absorbing liquid and swelling, which causes the active material to expand, shrink and fall off, and improves the cycle stability of the electrode.

[0066] It can be understood that the first active layer and the second active layer include not only the binder but also the positive electrode active material and the conductive agent.

[0067] In a specific embodiment, the first active layer includes, by weight, 100 parts of positive electrode active material, 2.5-3.5 parts of first binder, and 1-2 parts of conductive agent.

[0068] In another specific embodiment, the second active layer includes, by mass, 100 parts of positive electrode active material, 2-3 parts of second binder, and 1-2 parts of conductive agent.

[0069] The difference in the intensity of PVDF characteristic peaks can be estimated through infrared testing, or by taking the upper and lower surface layer materials, measuring thermal weight loss, and calculating the component difference based on the decomposition temperature and weight loss (PVDF decomposition).

[0070] The present invention does not limit the type of positive electrode active material, and can be any positive electrode active material commonly used in lithium-ion batteries, such as at least one composite oxide of lithium and a metal selected from the group consisting of cobalt, manganese, nickel, and combinations thereof. Specifically, the positive electrode active material can be at least one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium manganese oxide, nickel-cobalt-manganese ternary materials, nickel-cobalt-aluminum ternary materials, lithium iron phosphate (LFP), lithium nickel manganese oxide, and lithium-rich manganese-based materials.

[0071] The present invention does not limit the type of the conductive agent. For example, the conductive agent can be selected from at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and graphene.

[0072] This ratio design can optimize battery performance, including higher capacity, longer cycle life and better safety, by precisely controlling the proportion and distribution of each component.

[0073] The thickness and surface density of the electrode have a certain impact on the energy density and other properties of the electrode. The present invention does not limit the test method of the electrode thickness and surface density, and the thickness and surface density can be tested using common methods in the field.

[0074] In a specific embodiment, the thickness of the electrode is 170 μm-260 μm; in detail, the thickness of the electrode includes but is not limited to 170 μm, 180 μm, 190 μm, 200 μm, 210 μm, 220 μm, 230 μm, 240 μm, 250 μm, 260 μm or a range between any two thereof.

[0075] When the thickness of the electrode is within the above range, more positive electrode active material can be accommodated, thereby increasing the energy density of the battery.

[0076] In another embodiment, the surface density of the electrode is 400 g / m 2 ~600g / m 2 ; In detail, the surface density of the electrode is 400g / m 2 , 420g / m 2 , 440g / m 2 , 460g / m 2 , 480g / m 2 , 500g / m 2 , 520g / m 2 , 540g / m 2 , 560g / m 2 , 580g / m 2 , 600g / m 2 Or a range between any two of them.

[0077] A higher areal density means that the electrode contains more active material per unit area, thereby increasing the energy density of the battery.

[0078] In one embodiment, the surface density of the active layer is 200 g / m 2 -300g / m 2 In detail, the surface density of the active layer includes but is not limited to 200g / m 2 , 220g / m 2 , 240g / m 2 , 260g / m 2 , 280g / m 2 , 300g / m 2 Or a range between any two of them.

[0079] In another embodiment, the surface density of the first active layer is 20 g / m 2 -200g / m 2 In detail, the surface density of the first active layer includes but is not limited to 20g / m 2 , 40g / m 2 , 60g / m 2 , 80g / m 2, 100g / m 2 , 120g / m 2 , 140g / m 2 , 160g / m 2 , 180g / m 2 , 200g / m 2 Or a range between any two of them.

[0080] In another embodiment, the surface density of the second active layer is 0.1 g / m 2 -280g / m 2 In detail, the surface density of the second active layer includes but is not limited to 0.1g / m 2 , 1g / m 2 , 10g / m 2 , 50g / m 2 , 100g / m 2 , 150g / m 2 , 200g / m 2 , 250g / m 2 , 280g / m 2 Or a range between any two of them.

[0081] The surface density of the first active layer and the second active layer can be tested using common testing methods in this field. For example, when the added amounts of the first binder and the second binder are inconsistent, the EDS of the electrode cross section can be tested to check the F element distribution. Based on the difference in F element distribution, the thickness difference between the first active layer and the second active layer can be roughly estimated, and the surface density difference can be deduced.

[0082] Furthermore, in one embodiment, the compaction density of the electrode is 2.3 g / cm 3 -2.6g / cm 3 In detail, the compaction density of the electrode includes but is not limited to 2.3g / cm 3 , 2.35g / cm 3 , 2.4g / cm 3 , 2.45g / cm 3 , 2.5g / cm 3 , 2.55g / cm 3 , 2.6g / cm 3 Or a range between any two of them.

[0083] When the compaction density of the electrode is within the above range, the battery can maintain high energy density, cycle performance and electrochemical performance.

[0084] Furthermore, in a specific embodiment, the thickness of the first active layer is 10 μm-85 μm; in detail, the thickness of the first active layer includes but is not limited to 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 85 μm or a range between any two thereof.

[0085] In another specific embodiment, the thickness of the second active layer is 0.1 μm-120 μm. Specifically, the thickness of the second active layer includes but is not limited to 0.1 μm, 1 μm, 10 μm, 20 μm, 40 μm, 60 μm, 80 μm, 100 μm, 120 μm or a range between any two thereof.

[0086] When the thickness of the first active layer and the thickness of the second active layer are within the above range, not only can the relationship between the liquid absorption capacity and swelling of the first active layer be better balanced, but also unnecessary impedance can be avoided while effectively protecting the electrode, thereby ensuring the coordinated development of cycle performance and power performance.

[0087] The present invention does not limit the preparation process of the electrode, and the electrode can be prepared using a common preparation process in the art.

[0088] In one embodiment, the battery preparation process includes the following steps:

[0089] 1) Preparation of the first active layer slurry: Add 2.5-3.5 kg of low-crystalline PVDF binder to a blender, along with 45 kg to 155 kg of NMP solvent, and stir at high speed for 2 hours. Then, add 1-2 kg of conductive agent to the adhesive and continue stirring for 1 hour. Then, add 100 kg of lithium iron phosphate powder in three portions, stirring for 1 hour after each addition, to obtain the first active layer slurry.

[0090] 2) Preparation of the second active layer slurry: 2-3 kg of conventional homopolymer PVDF binder was added to a blender, along with 45-155 kg of NMP solvent. The mixture was stirred at high speed for 2 hours. 1-2 kg of a conductive agent was then added to the adhesive solution, stirring continued for 1 hour. Then, 100 kg of lithium iron phosphate powder was added in three portions, stirring for 1 hour after each addition, to obtain the second active layer slurry.

[0091] 3) The slurry is then passed through a double-layer coating die to simultaneously coat the first active layer and the second active layer on at least one side of the current collector, followed by compaction and drying to obtain the final electrode.

[0092] The above-mentioned electrode preparation process is basically consistent with the existing battery cell production process, is fully compatible with existing production equipment, and has a simple production process, making it easier to achieve industrial promotion.

[0093] In another aspect, the present invention provides a battery comprising the electrode as described above.

[0094] Since the battery provided by the present invention includes the above-mentioned electrodes, it has higher energy density, excellent power performance and cycle performance.

[0095] It is conceivable that when a positive electrode active material is included in the electrode as described above, the polarity of the electrode is positive.

[0096] The battery of the present invention includes, in addition to the above-mentioned electrodes, a negative electrode sheet, an electrolyte and a separator.

[0097] The present invention is not strictly limited to the negative electrode active material in the negative electrode sheet, and can be at least one of the negative electrode active materials commonly used in batteries, such as graphite, hard carbon, soft carbon, mesophase carbon microbeads, silicon-based negative electrode materials (mainly including silicon oxide, silicon-carbon negative electrode), tin-based negative electrode materials (mainly including tin, tin alloy), etc.

[0098] The present invention is not strictly limited to the choice of electrolyte, and can include one or more of the solvents commonly used in current battery electrolytes, and the electrolyte salts commonly used in current electrolytes. Taking lithium-ion batteries as an example: the solvent can be ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, ethyl methyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc.; the lithium salt can be, for example, one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0099] The present invention does not strictly limit the material selection of the diaphragm, which can be a diaphragm material commonly used in current batteries, such as polypropylene diaphragm (PP), polyethylene diaphragm (PE), polypropylene / polyethylene double-layer composite film (PP / PE), polyimide electrospun diaphragm (PI), polypropylene / polyethylene / polypropylene three-layer composite film (PP / PE / PP), cellulose non-woven fabric diaphragm, and one of the diaphragms with ceramic coating.

[0100] To prepare the battery, the electrodes, separator, and negative electrode sheets are wound or stacked to form a bare cell. These cells are then packaged in pre-stamped aluminum-plastic film bags. The packaged cells are dried at 85°C, and then the electrolyte is injected into the dried cells. After the cells are allowed to stand, undergo formation, and undergo secondary sealing, the lithium-ion battery is complete.

[0101] In another aspect, the present invention provides a battery pack comprising the battery described above.

[0102] Since the battery pack provided by the present invention includes the above-mentioned battery, it has higher energy density, excellent cycle performance and power performance.

[0103] In another aspect, the present invention provides an electrical device comprising the battery or the battery pack as described above.

[0104] The present invention is not limited to the specific types of electrical equipment, and can include large energy storage systems, electric vehicles, mobile phones, smart homes, robots, drones, electronic cigarettes, speakers, and any other equipment that requires batteries to power it.

[0105] The electrical equipment provided by the present invention includes the above-mentioned battery or the above-mentioned battery pack, and thus has good performance.

[0106] Hereinafter, the electrode provided by the present invention will be described in detail through specific embodiments.

[0107] Unless otherwise specified, the reagents, materials, and instruments used in the following examples are conventional reagents, conventional materials, and conventional instruments in the art and can be obtained commercially. The reagents involved can also be synthesized by conventional methods in the art.

[0108] Example 1

[0109] The parameters of the electrodes provided in this embodiment are as follows:

[0110] Formula for the first active layer: 100 parts lithium iron phosphate, 3.0 parts hexafluoropropylene vinylidene fluoride copolymer (the crystallinity of the hexafluoropropylene vinylidene fluoride copolymer is 20%; the molar proportion of hexafluoropropylene is 10%; the weight average molecular weight of the hexafluoropropylene vinylidene fluoride copolymer is 1 million), 1.5 parts carbon black, and 69 parts NMP;

[0111] The second active layer formula: lithium iron phosphate 100 parts, conventional homopolymer polyvinylidene fluoride 2.5 parts (crystallinity 40%, weight average molecular weight 1 million), carbon black 1.5 parts, NMP 69 parts;

[0112] Control electrode surface density is 500g / m 2 , the active layer density is 250g / m 2 , where the density of the first active surface is 35g / m 2 The second active layer density is 215g / m 2 The electrode compaction density is controlled at 2.3g / cm 3 ; The total thickness of the electrode is 229μm: the current collector thickness is 12μm, the electrode thickness is 217μm, of which the first active layer thickness is 15μm and the second active layer thickness is 93μm.

[0113] The preparation process of the electrode provided in this embodiment includes the following steps:

[0114] 1) Preparation of the first active layer slurry: 3.0 kg of copolymerized polyvinylidene fluoride was added to a blender, along with 69 kg of NMP solvent, and stirred at high speed for 2 hours. 1.5 kg of a conductive agent was then added to the glue solution, and stirring continued for 1 hour. Then, 100 kg of lithium iron phosphate powder was added in three portions, stirring for 1 hour after each addition, to obtain the first active layer slurry.

[0115] 2) Preparation of the second active layer slurry: 2.5 kg of conventional homopolymerized polyvinylidene fluoride binder was added to a blender, along with 69 kg of NMP solvent. The mixture was stirred at high speed for 2 hours. 1.5 kg of conductive agent was then added to the binder, followed by stirring for 1 hour. 100 kg of lithium iron phosphate powder was then added in three portions, stirring for 1 hour after each addition, to obtain the second active layer slurry.

[0116] 3) The slurry is then passed through a double-layer coating die, coated on the surface of the current collector, and dried to obtain an electrode.

[0117] Example 2

[0118] The preparation method and parameters of the electrode provided in this embodiment are basically the same as those in Example 1, except that:

[0119] The first active layer density of the electrode provided in this embodiment is 70 g / m 2 The density of the second active surface is 180g / m 2 .

[0120] Example 3

[0121] The preparation method and parameters of the electrode provided in this embodiment are basically the same as those in Example 1, except that:

[0122] The first active layer density of the electrode provided in this embodiment is 105 g / m 2 The density of the second active layer is 145g / m 2 .

[0123] Example 4

[0124] The preparation method and parameters of the electrode provided in this embodiment are basically the same as those in Example 1, except that:

[0125] The first active layer density of the electrode provided in this embodiment is 140 g / m 2 The density of the second active surface is 110g / m 2 .

[0126] Comparative Example 1

[0127] The formula of the electrode active layer slurry provided in this comparative example is as follows:

[0128] 100 parts of lithium iron phosphate, 2.5 parts of conventional homopolymer polyvinylidene fluoride (crystallinity of 40%, weight average molecular weight of 1 million), 1.5 parts of carbon black, and 69 parts of NMP.

[0129] The preparation method of the electrode provided in this comparative example includes the following steps:

[0130] 2.5 kg of binder was added to the stirrer, and 69 kg of NMP solvent was added at the same time. The mixture was stirred at high speed for 2 hours. Then 1.5 kg of conductive agent was added to the glue solution and the stirring was continued for 1 hour. Then 100 kg of lithium iron phosphate powder was added in three times. After each addition, the mixture was stirred for 1 hour to finally obtain the active layer slurry. The slurry was then coated through a coating die and dried to obtain the electrode. The electrode surface density was 250 g / m 2 The electrode compaction density is controlled at 2.3g / cm 3 ;The thickness of the electrode is 109μm.

[0131] Comparative Example 2

[0132] The formula of the electrode active layer slurry provided in this comparative example is as follows:

[0133] 100 parts of lithium iron phosphate, 3.0 parts of hexafluoropropylene vinylidene fluoride copolymer (the crystallinity of the hexafluoropropylene vinylidene fluoride copolymer is 20%; the molar proportion of hexafluoropropylene is 10%; the weight average molecular weight of the hexafluoropropylene vinylidene fluoride copolymer is 1 million), 1.5 parts of carbon black, and 69 parts of NMP;

[0134] The preparation method of the electrode provided in this comparative example includes the following steps:

[0135] 3.0 kg of copolymerized polyvinylidene fluoride was added to the stirrer, and 69 kg of NMP solvent was added at the same time. The mixture was stirred at high speed for 2 hours. Then 1.5 kg of conductive agent was added to the glue solution and the stirring was continued for 1 hour. Then 100 kg of lithium iron phosphate powder was added in three times, and the mixture was stirred for 1 hour after each addition to finally obtain the active layer slurry. The slurry was then coated through a coating die and dried to obtain an electrode. The electrode active layer density was 250 g / m 2 .

[0136] Test Case

[0137] 1. Electrode liquid absorption rate test

[0138] The electrodes provided in all the embodiments and comparative examples were selected in turn, and discs with a diameter of 20 mm were cut and weighed. They were immersed in the electrolyte for 48 hours, and the ambient temperature was controlled to 60°C. After soaking, they were taken out, and then the excess electrolyte on the surface was wiped off. The electrodes were re-weighed to obtain the weight after soaking in the electrolyte. The weights before and after soaking in the electrolyte were subtracted and divided by the original weight, which was the final absorption rate of the electrode piece for absorbing the electrolyte. The specific data are shown in Table 1. According to Table 1, it can be seen that the electrode provided in the embodiment of the present invention has a significant improvement in the wettability of the electrolyte, and has a positive effect on the power improvement of the thick electrode.

[0139] Table 1

[0140]

[0141] 2. Electrical performance test

[0142] The electrodes provided in all the embodiments and comparative examples are sequentially selected and prepared into lithium-ion button batteries, comprising the following steps:

[0143] First, use a punching machine to cut the electrode into small discs with a diameter of 14mm. Then bake them in an oven for 1 day. Then, prepare the baked electrode, nickel foam, positive and negative battery cases, separator, lithium sheet, and electrolyte in advance and place them in a glove box. Once prepared, use tweezers to assemble the battery in the order of negative electrode case, nickel foam, lithium sheet, two drops of electrolyte, separator, two drops of electrolyte, positive electrode sheet, two drops of electrolyte, and positive electrode case. Align the positive and negative electrodes and separator. Once assembled, place the battery in a sealing machine and wipe any residual electrolyte on the surface with dust-free paper to obtain the final test button battery.

[0144] The rate performance was tested at room temperature: the prepared buckle battery was first charged to 3.8V at 25°C using a 0.1C-CCCV cycle, then cutoff at 0.02C. After 30 minutes, the battery was discharged to 2.0V using a 0.1C-CC cycle three times. The final discharge capacity was used as the actual usage of the buckle battery. The buckle battery was then charged to 3.8V at 0.5C-CCCV, then cutoff at 0.02C. After 30 minutes, the battery was discharged to 2.0V using a 0.5C-CCCV cycle, then cutoff at 0.02C. After 30 minutes, the battery was discharged to 2.0V using 0.5C, 1C, 2C, 3C, and 5C constant current cycles, respectively. The discharge capacity was recorded to evaluate the rate performance.

[0145] Record the discharge capacity of button cells with different pole pieces at different rates, where the discharge capacity at 0.5C rate is the benchmark discharge capacity, and the discharge capacity at 1C, 2C, 3C, and 5C is divided by the benchmark capacity to obtain the discharge ratio at different rates. The specific results are shown in Table 2. Figure 2 The discharge ratio bar graphs at different rates after the electrodes provided in Examples 1-4 and Comparative Examples 1 and 2 are prepared into batteries show that the rate performance of Comparative Example 1 is the worst. As the density of the first active layer surface gradually increases, the rate performance gradually improves.

[0146] Table 2

[0147]

[0148] 3. Cycle performance test

[0149] The button cells prepared in the above examples and comparative examples were used in turn to perform a cycle performance test, comprising the following steps:

[0150] At 25°C, charge to 3.8V at a constant current and constant voltage of 0.5C, cut off at 0.02C, then discharge to 2.0V at a constant current of 0.5C, cycle 50 times, set the discharge capacity Q1 at the first cycle: the capacity retention rate of a specific number of cycles is equal to Qn / Q1*100%. The specific cycle results are as follows: Figure 3 shown.

[0151] in, Figure 3 The room-temperature cycling curves for the batteries provided in Examples 1-4 and Comparative Examples 1 and 2 are shown. Comparative Example 2 exhibits the worst cycling performance. Due to the excessive swelling of the low-crystalline binder, the interaction between active material particles is weakened compared to conventional binders. This, coupled with a certain volume change during cycling, accelerates degradation. Therefore, different low-crystalline binder addition ratios should be selected to meet the specific cell requirements.

[0152] In summary, the batteries provided in Examples 1-4 of the present invention can better balance the cycle performance and rate performance, so that the batteries have excellent performance. Although some performances of Comparative Examples 1 and 2 are relatively excellent, they cannot achieve a balance between cycle performance and rate performance, and cannot achieve the technical effect of practical application.

[0153] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electrode, characterized in that: comprising a current collector and an active layer disposed on at least one functional surface of the current collector; Wherein, the active layer comprises a first active layer close to the current collector and a second active layer arranged on a side of the first active layer away from the current collector; The first active layer includes a first binder, the first binder includes a polyvinylidene fluoride copolymer, and the crystallinity of the polyvinylidene fluoride copolymer is not higher than 25%; The second active layer includes a second binder, and the second binder has a higher crystallinity than the first binder.

2. The electrode according to claim 1, characterized in that The polyvinylidene fluoride copolymer is formed by copolymerizing a first monomer and a second monomer, wherein the first monomer includes 1,1-difluoroethylene, and the second monomer includes at least one of olefins and olefin derivatives.

3. The electrode according to claim 2, characterized in that The molar proportion of the second monomer is not higher than 30%.

4. The electrode according to any one of claims 1 to 3, characterized in that The weight average molecular weight of the polyvinylidene fluoride copolymer is 50W-150W.

5. The electrode according to any one of claims 1 to 4, characterized in that The second binder includes at least one of a polyvinylidene fluoride-hexafluoropropylene copolymer, a polyvinylidene fluoride-hexafluoropropylene copolymer derivative, a polyvinylidene fluoride-tetrafluoroethylene copolymer, and a polyvinylidene fluoride-tetrafluoroethylene copolymer derivative.

6. The electrode according to any one of claims 1 to 5, characterized in that In parts by mass, the first active layer includes 100 parts of positive electrode active material, 2.5-3.5 parts of first binder, and 1-2 parts of conductive agent; And / or, based on parts by mass, the second active layer includes 100 parts of positive electrode active material, 2-3 parts of second binder, and 1-2 parts of conductive agent.

7. The electrode according to any one of claims 1 to 6, characterized in that The thickness of the electrode is 170 μm-260 μm; And / or, the surface density of the electrode is 400g / m 2 ~600g / m 2 .

8. The electrode according to any one of claims 1 to 7, characterized in that The surface density of the active layer is 200 g / m 2 -300g / m 2 ; And / or, the surface density of the first active layer is 20g / m 2 -200g / m 2 ; And / or, the surface density of the second active layer is 0.1 g / m 2 -280g / m 2 .

9. The electrode according to any one of claims 1 to 8, characterized in that The compacted density of the electrode is 2.3 g / cm 3 -2.6g / cm 3 .

10. The electrode according to any one of claims 1 to 9, characterized in that The thickness of the first active layer is 10 μm-85 μm; And / or, the thickness of the second active layer is 0.1 μm-120 μm.

11. A battery, characterized in that: The electrode comprises the electrode according to any one of claims 1 to 10.

12. A battery pack, characterized in that: Including the battery according to claim 11.

13. An electrical device, characterized in that: The battery according to claim 11 or the battery pack according to claim 12 is included.

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