Secondary battery and electric device using the same

Abstract

The present application relates to the technical field of electrode sheet of secondary battery, and more particularly to a secondary battery and an electric device using the same. By arranging a conductive adhesive layer between the second active material layer and the separator of the last fold of the electrode sheet of the secondary battery, the adhesive layer can be effectively melted during the formation of the secondary battery, thereby enhancing the adhesion between the weak area of the last fold of the wound-type secondary battery and the separator, strengthening the adhesion of the interface, effectively improving the cycle performance of the battery cell and preventing the separator from folding when falling. At the same time, the increased adhesive layer has a lithium ion transmission capacity comparable to that of other areas of the electrode sheet during the cycle process. In addition, the introduction of the adhesive layer increases the impedance of the electrode sheet at this position, which can weaken the reaction activity when applied to the weak area, thereby prolonging the cycle life.

Description

Technical Field

[0001] This invention relates to the field of electrode technology for secondary batteries, and more specifically to a secondary battery and its power supply device. Background Technology

[0002] With societal development, smartphones and laptops are playing increasingly important roles in our lives, while the market for wearable devices and smart homes is also booming. Lithium-ion batteries, due to their high energy density and environmental friendliness, are widely used in these fields, leading to a rapid increase in market demand. To meet market demands, shortening charging time for terminal devices and improving user experience has been a key development direction for lithium-ion batteries in recent years. Simultaneously, to meet the demand for portability and lightweight design, prismatic pouch batteries are gradually evolving towards higher energy density technologies such as high-nickel cathodes, silicon anodes, high voltage, high density, and thicker electrodes.

[0003] Currently, most consumer lithium-ion batteries adopt a wound cell design. Existing winding technologies include center-mounted tab welding, conventional single-tab welding, and multi-tab winding. The methods employed typically involve winding the electrode with the separator covering the negative electrode and the positive electrode covering the negative electrode. The last fold of the electrode with the active material layer is usually a single-sided electrode, meaning the active material layer is applied to the side where the current collector faces the inner side of the electrode assembly. However, electrolyte tends to accumulate at this last fold, weakening the adhesion between the active material layer and the separator. This results in low binding force on the single-sided area of ​​the electrode at the tail, making the last fold with the active material layer a significant weak point. The resulting failure modes include: ① electrolyte bridging during cycling; ② excessive free electrolyte causing separator folding during drop tests due to cell movement; ③ lithium deposition at the electrode edges during cycling. These phenomena can be found in [reference needed]. Figure 1 The photo shown illustrates this point. Therefore, improving the interfacial adhesion in this weak area to enhance the cell's cycle performance and drop safety has become a pressing issue. Summary of the Invention

[0004] Therefore, there is a need to provide a secondary battery and its power supply device to solve the problems in the prior art, such as the formation of voids due to electrolyte accumulation at the last fold of the active material layer at the end of the electrode, which weakens the interfacial adhesion, poor interfacial contact caused by lithium plating at the end and edge of the electrode during cycling, shrinkage of the tail separator after falling, and reduced cell cycle performance.

[0005] To achieve the above objectives, in a first aspect, this application provides a secondary battery, comprising an electrode assembly formed by sequentially stacking and winding a first electrode, a separator, and a second electrode. The first electrode includes a first current collector and an active material layer. The first current collector comprises a first portion and a second portion, which are sequentially connected along the winding direction (i.e., the first portion is closer to the winding center and is wound before the second portion). The first active material layer is disposed on both sides of the first portion, and the second active material layer is disposed only on the surface of the second portion facing the winding center of the electrode assembly. An adhesive layer is disposed between the second active material layer and the separator, and the conductivity of the adhesive layer is 0.01 S / cm to 0.06 S / cm. In the above solution, the second portion of the first current collector has the second active material layer disposed only on the surface facing the winding center of the electrode assembly; this portion of the electrode is the outermost layer with the active material layer in the electrode assembly. It is worth noting that the first and second active material layers are only distinguished by their position; their compositions can be the same or different, but are preferably the same. The inventors discovered that by setting a conductive adhesive layer between the second active material layer and the separator, this adhesive layer can effectively melt during the formation of the secondary battery. This enhances the adhesion between the last fold of the active material layer and the separator in the wound secondary battery, strengthens the interfacial adhesion, improves the cycle performance of the cell, and prevents the separator from folding during drops. Furthermore, the inventors found that when the conductivity of this adhesive layer is between 0.01 S / cm and 0.06 S / cm, it can maintain the adhesion strength of the last fold of the active material layer while maximizing the normal lithium-ion transport capacity during the secondary battery cycle. This allows the active material layer covered by the adhesive layer to participate in the electrochemical reaction, preventing a reduction in the amount of active material participating in the electrochemical reaction and thus preventing a decrease in energy density due to the adhesive layer.

[0006] In a preferred embodiment of this application, the first electrode is a positive electrode. By providing the aforementioned conductive adhesive layer on the positive electrode, the positive electrode activity can be weakened while retaining its lithium ion insertion / extraction performance, thereby improving the lithium plating phenomenon on the corresponding negative electrode.

[0007] In a preferred embodiment of this application, along the winding direction, the first current collector further includes an empty foil area connected to the second portion. Neither surface of the empty foil area has an active material layer, and an adhesive layer covers the empty foil area and the second active material layer. Typically, when winding the battery cell, after the last fold at the end of the bare battery cell, empty aluminum foil is used for finishing. By covering the empty foil area and the second active material layer with the adhesive layer, the empty aluminum foil finishing area after the last fold of the active material layer also provides adhesion and reinforcement to the separator.

[0008] In a preferred embodiment of this application, the ratio of the width of the adhesive layer to the width of the first electrode is 0.03 to 1.00 in the width direction of the first electrode. The width direction of the first electrode is perpendicular to the winding direction of the battery cell. It is understood that in the above configuration, when the ratio of the adhesive layer width to the width of the first electrode is less than 1, the adhesive layer may not completely overlap the second active material layer in the width direction of the first electrode. However, to obtain a better bonding effect, it is preferable to place the adhesive layer at the end of the second active material layer and at the edge in the width direction of the first electrode. When the ratio of the adhesive layer width to the width of the first electrode is 1, and the adhesive layer extends continuously along the winding direction to the end of the second electrode or even to the end of the second active material layer and the connecting empty foil area, the overlap between the adhesive layer and the second active material layer or the second active material layer and the connecting empty foil area is the highest. Without considering cost, this achieves the best bonding effect at the interface, but it adversely affects the energy density and the specific capacity of the last fold of the active material layer. Meanwhile, in order to ensure the adhesion of the adhesive layer, the lower limit of the ratio of the width of the adhesive layer to the width of the first electrode is selected as 0.03.

[0009] Through extensive comparison of experimental results, the inventors discovered that the ratio of the width of the adhesive layer to the width of the first electrode is 0.05 to 0.30 in the width direction of the first electrode. This ensures that the adhesive layer strengthens the adhesion between the electrode and the separator, improves lithium deposition at the electrode edges, enhances the electrode's cycle life, and simultaneously maintains the battery's energy density.

[0010] In a preferred embodiment of this application, the thickness of the adhesive layer is 500 nm to 1600 nm. An adhesive layer that is too thin or too thick will not effectively achieve the purpose of this application. This design ensures two things: first, that the adhesive layer can effectively melt and bond under the high temperature and high pressure of cell formation; and second, that an excessively thick adhesive layer avoids a decrease in battery energy density and capacity loss due to the last fold of active material not fully participating in the electrochemical reaction during charge and discharge.

[0011] In a preferred embodiment of this application, the adhesive layer contains a conductive agent and an adhesive, with a weight ratio of (20-80):(80-20). By setting the above-mentioned reasonable ratio of conductive agent and adhesive, an optimal balance between adhesion and conductivity between the electrode and the separator is achieved.

[0012] In a preferred embodiment of this application, the conductive agent includes at least one selected from conductive carbon black, acetylene black, carbon nanotubes, Ketjen black, conductive graphite, or graphene; the binder includes at least one selected from polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, or styrene-butadiene rubber. In a preferred embodiment of this application, the conductive agent is conductive carbon black, and the binder is styrene-butadiene rubber.

[0013] In a second aspect, the inventors provide an electrical device comprising a plurality of secondary batteries as provided in the first aspect of the present invention. The electrical device includes the secondary batteries of the first aspect of this application, as well as other electrical components and parts, with the secondary batteries providing electrical energy to the electrical components and parts. The electrical device described in this application is not particularly limited and may include, but is not limited to, the following types: laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors, etc.

[0014] Unlike existing technologies, the above-mentioned technical solution incorporates a conductive adhesive layer between the second active material layer and the separator at the final fold of the electrode in the secondary battery. This adhesive layer effectively melts during battery formation, enhancing the adhesion between the weakest area of ​​the final fold and the separator in the wound secondary battery. This strengthens interfacial bonding, effectively improving the cell's cycle performance and preventing separator folding during drops. Simultaneously, the added adhesive layer ensures that the lithium-ion transport capacity of the electrode during cycling is comparable to that of other areas of the electrode. Furthermore, the introduction of this adhesive layer increases the electrode's impedance at that location, which, when applied to weak areas, weakens reactivity, preventing lithium plating and extending cycle life. Attached Figure Description

[0015] Figure 1 The image shows lithium plating in a wound-structured battery cell under the failure / problem state described in the background art.

[0016] Figure 2 This is a schematic diagram illustrating the bonding layer configuration of a wound secondary battery electrode sheet according to a specific embodiment of this application;

[0017] Figure 3This is a schematic diagram illustrating the bonding layer configuration of another type of wound secondary battery electrode sheet according to a specific embodiment of this application;

[0018] Figure 4 This is a schematic diagram of the electrode structure corresponding to the first part of the first current collector in a wound secondary battery electrode according to a specific embodiment of this application;

[0019] Figure 5 This is a schematic diagram of the electrode structure corresponding to the second part of the second current collector in a wound secondary battery electrode according to a specific embodiment of this application;

[0020] Figure 6 This is a schematic diagram of the bare cell structure of a wound secondary battery after winding, as described in a specific embodiment of this application.

[0021] Explanation of reference numerals in the attached figures:

[0022] 1. First electrode; 10. First part; 101. First active material layer;

[0023] 20. Part Two; 201. Second Active Substance Layer;

[0024] 30. Empty foil area;

[0025] 40. Adhesive layer;

[0026] 2. Second electrode;

[0027] 3. Diaphragm;

[0028] 50. Anode tab;

[0029] 60. Cathode tab. Detailed Implementation

[0030] To explain in detail the technical content, structural features, objectives, and effects of the technical solution, the following description is provided in conjunction with specific embodiments and accompanying drawings.

[0031] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0032] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0033] In the description of this application, the term "and / or" is used to describe the logical relationship between objects, indicating that three relationships can exist. For example, A and / or B means: A exists, B exists, and A and B exist simultaneously. Additionally, the character " / " in this document generally indicates that the preceding and following objects have an "or" logical relationship.

[0034] In this application, terms such as “first” and “second” are used only to distinguish one entity or operation from another, and do not necessarily require or imply any actual quantity, hierarchy or order relationship between these entities or operations.

[0035] Unless otherwise specified, the use of terms such as “comprising,” “including,” “having,” or other similar expressions in this application is intended to cover non-exclusive inclusion, which does not exclude the presence of additional elements in a process, method, or product that includes the stated elements, such that a process, method, or product that includes a list of elements may include not only those defined elements but also other elements not expressly listed, or elements inherent to such a process, method, or product.

[0036] Similar to the understanding in the Examination Guidelines, in this application, expressions such as "greater than," "less than," and "exceeding" are understood to exclude the stated number; expressions such as "above," "below," and "within" are understood to include the stated number. Furthermore, in the description of the embodiments in this application, "multiple" means two or more (including two), and similar expressions related to "multiple" are also understood in this way, such as "multiple groups" and "multiple times," unless otherwise explicitly specified.

[0037] In the description of the embodiments of this application, the space-related expressions used, such as "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "vertical," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential," indicate the orientation or positional relationship based on the orientation or positional relationship shown in the specific embodiments or drawings. They are only for the purpose of describing the specific embodiments of this application or for the reader's understanding, and do not indicate or imply that the device or component referred to must have a specific position, a specific orientation, or be constructed or operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of this application.

[0038] Unless otherwise expressly specified or limited, the terms "installation," "connection," "linking," "fixing," and "setting," as used in the description of the embodiments of this application, should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral setting; it can be a mechanical connection, an electrical connection, or a communication connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be the internal connection of two components or the interaction between two components. For those skilled in the art to which this application pertains, the specific meaning of the above terms in the embodiments of this application can be understood according to the specific circumstances.

[0039] A secondary battery can be used independently as a power source to output electrical energy, or multiple secondary batteries can be connected in series, parallel, or a hybrid connection to form a battery pack, which then outputs electrical energy as a power source. A hybrid connection refers to multiple secondary batteries being connected in both series and parallel. The secondary battery can be a lithium-ion battery. In this application, the secondary battery is an electrode assembly in the shape of a flat cylinder, produced by a winding method. The following explanation uses a lithium-ion battery as an example of a secondary battery.

[0040] The manufacturing method for each part of the secondary battery provided in the specific embodiments of this application includes the following steps:

[0041] Preparation of adhesive layer slurry

[0042] The adhesive slurry contains a conductive agent and a binder. In this application, the weight ratio of the conductive agent to the binder is (20-80):(80-20). The conductive agent is selected from at least one of conductive carbon black, acetylene black, carbon nanotubes, Ketjen black, conductive graphite, or graphene. The binder includes at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, or styrene-butadiene rubber.

[0043] Preparation of positive electrode

[0044] The positive electrode sheet of this application also includes a positive current collector. This application does not impose any particular limitation on the positive current collector, as long as it achieves the purpose of this application. For example, the positive current collector may comprise aluminum foil, aluminum alloy foil, or a composite current collector. In this application, the positive current collector includes a first portion 10 and a second portion 20, which are sequentially connected along the winding direction. The positive active material layer 101 is disposed on two surfaces in the thickness direction of the positive current collector in the first portion, and the positive active material layer 201 is disposed only on the surface in the thickness direction of the positive current collector in the second portion, facing the winding center of the electrode assembly.

[0045] This application does not impose any particular restrictions on the material of the positive electrode active material layer, as long as it can electrochemically absorb / release lithium ions. Examples include lithium-containing transition metal composite oxides, lithium-containing transition metal phosphate compounds, sulfides, and conductive polymers. Among these, lithium-containing transition metal composite oxides and lithium-containing transition metal phosphate compounds are preferred as the positive electrode active material, and lithium-containing transition metal composite oxides that produce high voltage are particularly preferred. Furthermore, the technical solution of this application is also applicable to sodium-ion batteries, meaning that the positive electrode active material layer can also be a sodium-ion-containing active material, such as Prussian compounds.

[0046] After selecting suitable positive electrode active material, binder, and conductive agent, they are dissolved in a solvent in a certain proportion and mixed evenly to prepare a positive electrode active material slurry. Specifically, 90%–98% of the positive electrode active material, 0.2%–4% of the conductive agent, and 0.2%–5% of the binder are dissolved in a solvent and mixed evenly to obtain the active material slurry. The viscosity is 2000 mPa·s–20000 mPa·s, and the solid content is 30%–80%.

[0047] Specifically, this application takes lithium cobalt oxide as an example. (1) The conductive agent and lithium cobalt oxide are placed in a planetary high-energy ball mill and dry-milled for 10 min to 100 min; (2) The material obtained in (1) is transferred to a self-rotating and revolution-rotating mixer, and all the binder and 1 / 3 to 2 / 3 of the dispersion medium according to the formula weight are added to the mixer. The mixture is stirred at high speed for 5 min to 30 min, and after stirring, the bubbles are removed for 2 min to 10 min; (3) The remaining 1 / 3 to 2 / 3 of the dispersion medium according to the formula weight is added to the material obtained in (2), and the mixture is stirred at high speed for 5 min to 30 min. After stirring, the bubbles are removed for 1 min to 5 min to obtain the positive electrode slurry. The dispersion medium is N-methylpyrrolidone (NMP), the conductive agent is conductive carbon black and carbon nanotubes; the binder is polyvinylidene fluoride; the solid content of the positive electrode slurry is 75%. The above slurry is coated on an aluminum foil with a thickness of 10 μm (i.e., the positive electrode current collector), dried at 100°C, and rolled to obtain the positive electrode.

[0048] Specifically, an adhesive layer 40 is provided between the positive electrode active material layer 201 and the separator 3 on the surface of the second portion 20 of the positive electrode current collector facing the winding center of the electrode assembly. The conductivity of this adhesive layer is 0.01 S / cm-0.06 S / cm. In some preferred embodiments, along the winding direction, the positive electrode current collector also includes an empty foil region 30 connected to the second portion, on both surfaces of which no active material layer is provided. The adhesive layer 40 covers the empty foil region 30 and the positive electrode active material layer 201 of the second portion 20 of the positive electrode current collector.

[0049] In some embodiments of the present application, the thickness of the adhesive layer is 100 nm to 3 μm, and in preferred embodiments, the thickness of the adhesive layer is 500 nm to 1600 nm.

[0050] Specifically, in some embodiments of the present application, as Figure 2 shown, the width of the first electrode tab 1 is set to 72 mm. The position where the adhesive layer 40 is provided on the second part of the active material layer 201 is at the edge of the width of the first electrode tab, but does not completely cover the second part of the active material layer 201, and the width of the adhesive layer is 0.5 mm to 72 mm to ensure an effective bonding area.

[0051] As Figure 3 shown, in other embodiments of the present application, the upper limit of the width of the adhesive layer ≤ the width of the first electrode tab, and the ratio of the width of the adhesive layer to the width of the first electrode tab is 0.03 to 1, all of which meet the invention object of the present application. As Figure 4 shown in the cross-sectional structure, no adhesive layer is provided in the first part, and active material layers are only provided on both sides of the current collector. As Figure 5 shown in the cross-sectional structure, an adhesive layer is provided in the second part. An active layer is provided on one side of the current collector facing the winding center, and then an adhesive layer is provided on the side opposite to the current collector on this active material layer. More preferably, the ratio of the width of the adhesive layer to the width of the first electrode tab is 0.05 to 0.30.

[0052] Preparation of the negative electrode

[0053] The present application does not particularly limit the negative electrode current collector, as long as the object of the present application can be achieved. For example, the negative electrode current collector may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or composite current collector, etc.

[0054] The present application does not particularly limit the type of the negative electrode active material layer material, as long as the object of the present application can be achieved. For example, the negative electrode active material layer material may include at least one of natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiOx (0 < x), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithium titanate Li4Ti5O 12 , Li-Al alloy or metallic lithium.

[0055] Artificial graphite (anode active material), acetylene black (conductive agent), sodium carboxymethyl cellulose (CMC) (thickener), and styrene-butadiene rubber (SBR) (binder) were mixed in a weight ratio of 96:1:1.5:1.5. Deionized water was added, and the mixture was stirred in a vacuum mixer to obtain a cathode slurry with a solid content of 54 wt%. The cathode slurry was uniformly coated onto a copper foil with a thickness of 8 μm. The coated copper foil was dried at 85 °C, then cold-pressed, cut, and slit, and subsequently dried under vacuum at 120 °C for 12 h to obtain the cathode.

[0056] diaphragm

[0057] The secondary battery of this application also includes a separator 3 to separate the positive electrode 1 and the negative electrode 2, preventing internal short circuits in the secondary battery. The separator allows electrolyte ions to pass freely, completing the electrochemical charging and discharging process. There are no particular limitations on the separator used in this application, as long as it achieves the purpose of this application. For example, it can be at least one of the following: polyethylene (PE), polypropylene (PP) based polyolefin (PO) separators, polyester membranes (e.g., polyethylene terephthalate (PET) membranes), cellulose membranes, polyimide (PI) membranes, polyamide (PA) membranes, spandex, aramid membranes, woven membranes, nonwoven membranes (non-woven fabrics), microporous membranes, composite membranes, separator paper, rolled membranes, or spun membranes. For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer can be a non-woven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane can be used. In the following embodiments of this application, a PE porous polymer film is used as the separator.

[0058] electrolyte

[0059] The electrolyte of this application includes lithium salts and non-aqueous solvents. This application does not particularly limit the type of lithium salt, as long as it achieves the purpose of this application. For example, the lithium salt may include at least one of lithium hexafluorophosphate (LiPF6), LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, or LiSiF6. This application does not particularly limit the non-aqueous solvent, as long as it achieves the purpose of this application. For example, the non-aqueous solvent may include at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may be at least one of chain carbonate compounds or cyclic carbonate compounds. The aforementioned chain carbonate compounds may include at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). Cyclic carbonate compounds may include at least one of ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC). The aforementioned carboxylic acid ester compounds may include at least one of ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, or propyl propionate. The aforementioned ether compounds may include at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, or phosphate esters. In the following embodiments of this application, ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3:7. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent at a ratio of 1 mol / L. Based on the above basic electrolyte, 2 wt.% of fluoroethylene carbonate (FEC) is added to prepare the electrolyte.

[0060] The positive electrode sheet, separator, and negative electrode sheet prepared by the above method are stacked sequentially, with the separator acting as an insulator between the positive and negative electrodes, and then wound to obtain a bare battery cell. After the winding process, the positions and structural relationships of the positive electrode sheet 1, negative electrode sheet 2, separator 3, empty foil area 30, anode tab 50, and cathode tab 60 are as follows: Figure 6 The diagram shows the bare cell structure. The bare cell is placed in an outer packaging foil, and the prepared electrolyte is injected into the dried battery. After vacuum sealing, settling, formation, and shaping, the lithium-ion battery is complete. Formation activates the cell at a temperature ≥50℃ and a pressure ≥0.8MPa. The positive electrode is bonded to the separator and negative electrode through the melting of the adhesive layer.

[0061] The finished battery was then subjected to the following tests:

[0062] 1. Thickness test

[0063] The testing equipment used is a laser thickness gauge with a spot size of 25μm*1400μm. The thickness of the object can be obtained by measuring the distance between the two laser displacement sensors, the distance from the upper sensor to the object, and the distance from the lower sensor to the object.

[0064] 2. Compacted density test

[0065] The equipment used was the Sansi Zongheng UTM7305, and the mold used was CARVER#3619. The mold was used to take samples, and the mass of the test samples was weighed. The displacement sensor recorded the height of the pressed tablet and the bottom area of ​​the fixed pressed tablet. A total of 32 parallel data were collected to obtain the compaction density result.

[0066] 3. Lithium plating test

[0067] After the test is completed, the battery is fully charged at room temperature using the standard charging method (0.5CCC to the cutoff voltage, CV to 0.02C). The battery is then disassembled to check the black spots and lithium plating distribution on the surface of the negative electrode.

[0068] 4. Test method for conductivity of adhesive layer

[0069] The test was conducted using a film resistance meter provided by Yuaneng Technology. The equipment power supply was maintained at 220V, and the air pressure was greater than 0.7MPa. The prepared adhesive layer was placed in the sample stage. Then, the sample stage was placed in the test chamber of the equipment, and the test began. Throughout the entire test, the test air pressure was set to "0".

[0070] 5. Adhesion test

[0071] Take the dried electrode and cut a sample of a certain width using a blade. Apply special double-sided tape to the steel plate and attach the cut sample to the tape, test side down. Insert a paper strip with the same width as the electrode and a length greater than the sample length under the electrode and secure it with wrinkle adhesive.

[0072] Lithium-ion secondary batteries of Examples 1 to 11 and Comparative Example 1 were prepared according to the above preparation method.

[0073] In the examples and comparative examples of lithium-ion secondary battery formulations and binder layer settings shown in Table 1, the positive electrode active material layer uses a positive electrode slurry prepared by the same method, which mainly includes a formulation of lithium cobalt oxide, conductive carbon black, polyvinylidene fluoride, and carbon nanotubes. The binder layer is made by coating a mixture of conductive carbon black and styrene-butadiene rubber in different proportions.

[0074] Table 1. Lithium-ion secondary battery formulations and binder layer configurations for Examples 1 to 11 and Comparative Example 1

[0075]

[0076] The compaction density, bonding strength and conductivity of the lithium-ion secondary batteries obtained in Examples 1 to 11 and Comparative Example 1 were measured, and the results are shown in Table 2.

[0077] Table 2 shows the compaction density, adhesion strength, and electrical conductivity of Examples 1 to 11 and Comparative Example 1.

[0078]

[0079]

[0080] The lithium-ion secondary batteries obtained in Examples 1 to 11 and Comparative Example 1 were tested for lithium deposition under different cell cycle counts. The test results are shown in Table 3.

[0081] Table 3 shows the cycle performance test results of the battery cells prepared in the examples and comparative examples.

[0082]

[0083] As shown in Table 3, compared with Comparative Example 1, Examples 1 to 11 show that Comparative Example 1 experienced severe lithium plating after 300 cycles, while Examples 1 to 11 showed varying degrees of improvement. This indicates that introducing a bonding layer can significantly improve the phenomenon of cyclic lithium plating.

[0084] Comparing Examples 1-3, 6, and 10-13, it can be seen that, due to the narrow coating width of the adhesive layer, although the adhesion strength of Examples 1-2 is improved to some extent compared with Comparative Example 1, it is still lower than that of the other examples. This results in lower binding force on the last fold of the positive electrode active material layer of the electrode assembly. The battery of Example 1 exhibited slight lithium plating after 300 cycles, and the battery of Example 2 also showed slight lithium plating after 500 cycles. Poor interface contact easily leads to cycle failure, and the risk of failure upon drop is also higher. The adhesive layer adhesion strength of the secondary batteries in Examples 3, 6, and 10-13 is significantly better than that of Examples 1-2, and the cycle performance of the batteries is also significantly better than that of Examples 1-2. However, the adhesion strength of Example 3 is still lower than that of Examples 6 and 10-13, and it still shows slight lithium plating after 700 cycles. While Examples 12-13 exhibit strong adhesive bonding and good battery cycle performance, their relatively large width hinders the lithium-ion intercalation / deintercalation of the covered positive electrode active material during charge and discharge. This negatively impacts the full utilization of the positive electrode active material's specific capacity and thus affects the overall energy density of the battery. Overall, Examples 6 and 10-11 demonstrate superior comprehensive performance compared to Examples 1-3 and 12-13.

[0085] Comparing Examples 4, 5, and 6, when the content of conductive agent in the binder layer increases (Example 4), the adhesion between the electrode and the separator decreases, and the conductivity of the binder layer increases. Increased conductivity of the binder layer is beneficial for fully utilizing the specific capacity of the positive electrode active material, but decreased adhesion is detrimental to cycle performance and lithium plating improvement. Considering all factors, Examples 4-6 show that the preferred binder content in the binder layer is 50wt%-80wt%.

[0086] A comparison of the results from Examples 6, 7, 8, and 9 reveals that the coating thickness affects the cycling results. When the coating thickness is 1600 nm (Example 9), the cathode density exceeds the upper limit of the formulation design by 4.25, leading to failure during cycling. However, when the coating thickness is 1200 nm (Example 8), there are no cycling abnormalities. Therefore, the thickness design should match the formulation density, preferably between 500 nm and 1200 nm.

[0087] The comparison of the results of the examples and the comparative examples (without using a conductive adhesive layer) shows that placing a conductive adhesive layer between the positive electrode active material layer and the separator in the last fold of the wound cell electrode sheet can effectively melt during the formation activation process, thereby strengthening the interfacial adhesion. In addition, the introduction of this adhesive layer increases the impedance of the electrode sheet at that location, and when applied to weak areas, it can weaken the reactivity, extend the cycle life, and significantly improve the cell's cycle performance and safety performance after drop.

[0088] This application also proposes an electrical device that includes several of the aforementioned secondary batteries. The electrical device includes the secondary batteries provided in this application, as well as other electrical components and parts, with the secondary batteries providing power to the electrical components and parts. There are no particular limitations on the electrical devices using the aforementioned secondary batteries in this application; they may include, but are not limited to, the following types: laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0089] It should be noted that although the above embodiments have been described herein, this does not limit the scope of patent protection of the present invention. Therefore, any changes and modifications made to the embodiments described herein based on the innovative concept of the present invention, or equivalent structural or procedural transformations made using the content of the present invention's specification and drawings, directly or indirectly applying the above technical solutions to other related technical fields, are all included within the scope of patent protection of the present invention.

Claims

1. A secondary battery comprising an electrode assembly formed by sequentially stacking and winding a first electrode sheet, a separator, and a second electrode sheet; the first electrode sheet comprising a first current collector and an active material layer, the first current collector comprising a first portion and a second portion connected in sequence along a winding direction, both surfaces of the first portion being provided with a first active material layer, and only a surface of the second portion facing a center of the winding of the electrode assembly being provided with a second active material layer; characterized in that: a bonding layer is provided between the second active material layer and the separator, the bonding layer containing an electrically conductive agent and a bonding agent, the weight ratio of the electrically conductive agent to the bonding agent being (20-80):(80-20), and the electrical conductivity of the bonding layer being 0.01 S / cm-0.06 S / cm.

2. The secondary battery according to claim 1, characterized by the first electrode sheet is a positive electrode sheet.

3. The secondary battery according to claim 1, characterized by along the winding direction, the first current collector further comprises an empty foil region connected to the second portion, both surfaces of the empty foil region being not provided with an active material layer, and the bonding layer being provided on the empty foil region and the second active material layer.

4. The secondary battery according to claim 1, characterized by in the width direction of the first electrode sheet, the ratio of the width of the bonding layer to the width of the first electrode sheet is 0.03-1.

00.

5. The secondary battery according to claim 4, characterized by in the width direction of the first electrode sheet, the ratio of the width of the bonding layer to the width of the first electrode sheet is 0.05-0.

30.

6. The secondary battery according to claim 1, characterized by the thickness of the bonding layer is 500 nm-1600 nm.

7. The secondary battery according to claim 1, characterized by the electrically conductive agent comprises at least one of electrically conductive carbon black, carbon nanotubes, electrically conductive graphite, or graphene; and the bonding agent comprises at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylic acid salt, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, polyhexafluoropropylene, or butadiene-styrene rubber.

8. The secondary battery according to claim 1, characterized by the electrically conductive agent is electrically conductive carbon black, and the bonding agent is butadiene-styrene rubber.

9. An electrical device, characterized by: a secondary battery as claimed in any one of claims 1 to 8.

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