Method for heat-pressing a core, secondary battery, and electric device

Abstract

The application discloses a hot-pressing method of a roll core, a secondary battery and an electric device. The hot-pressing method comprises the following steps: placing positive and negative electrode sheets on two sides of a diaphragm respectively, winding to obtain a roll core, and arranging a preheating support in the center of the roll core; and hot-pressing the roll core. Based on the hot-pressing method, at least the following beneficial effects are achieved: the hot-pressing method provided by the application prearranges a preheating support in the center of the roll core before hot-pressing, so that the inner and outer rings of the roll core are heated more uniformly during hot-pressing forming, the stress between the electrode sheet and the diaphragm is more balanced, the release of the stress of the electrode sheet and the improvement of the interface are facilitated, the thickness consistency of the roll core after hot-pressing can be obviously improved, the roll core is prevented from being abnormally fluffy, and the shell entry yield of the roll core is improved. In addition, the positive and negative electrode sheets and the diaphragm are well attached, so that the wrinkling and lithium precipitation of the electrode sheet interface after the battery is charged and discharged can be effectively alleviated, and the cycle performance of the battery is improved.

Description

Technical Field

[0001] This application relates to the field of secondary battery technology, and in particular to a hot pressing method for a winding core, a secondary battery, and an electrical device. Background Technology

[0002] With the development of the new energy industry, electric vehicles are being used more and more widely, and lithium batteries have become the preferred choice for electric vehicles due to their advantages such as high energy density, long lifespan, and mature technology. Lithium batteries mainly consist of several parts, including the positive electrode, negative electrode, separator, and electrolyte. The separator, while separating the positive and negative electrodes and preventing direct contact between them that could cause a short circuit, also provides a channel for the free conduction of lithium ions, making it a crucial component of lithium batteries.

[0003] Improving the separator is a feasible method to enhance the performance of lithium batteries. Existing coated separators involve introducing an adhesive layer onto the separator. After hot pressing, the adhesive layer bonds the electrode and separator together, reducing thermal deformation caused by separator shrinkage, improving interface smoothness, and enhancing the core's forming performance. However, the introduction of the adhesive layer significantly increases production costs. Therefore, the selection of separators tends towards lower-cost, uncoated separators. However, when using uncoated separators for cores, tight bonding between internal layers cannot be achieved during hot pressing. This makes the electrode prone to waviness, affecting flatness and resulting in poor core thickness consistency and a loose core after hot pressing. Simultaneously, during battery charging and discharging, the lack of effective bonding between the positive and negative electrodes easily leads to electrode wrinkling and lithium plating, severely impacting the battery's long-term cycle performance.

[0004] Therefore, it is necessary to improve the hot-pressing interface of the core and effectively enhance the electrochemical performance of the battery by modifying the hot-pressing method of the core without changing the battery design. Summary of the Invention

[0005] This application aims to address at least one of the technical problems existing in the prior art. To this end, this application proposes a hot-pressing method for a winding core, a secondary battery, and an electrical device, which can achieve effective bonding of the internal electrode sheets of the winding core without introducing an adhesive layer, thereby improving the hot-pressing interface and the electrochemical performance of the battery.

[0006] To clarify this application, the terms "battery," "cell," and "wound core" are explained herein. A wound core is formed by winding or stacking a positive electrode, a separator, and a negative electrode. A cell refers to a single electrochemical cell containing a positive and a negative electrode; it is generally not used directly and mainly consists of a positive electrode, a separator, a negative electrode, and an electrolyte. A cell is formed by winding or stacking the positive electrode, separator, and negative electrode, packaging them, filling them with electrolyte, and sealing them. A wound core is a component of a cell. A battery refers to a cup, tank, or other container or composite container containing an electrolyte solution and metal electrodes to generate current; it is a device that converts chemical energy into electrical energy. A cell is a component of a battery.

[0007] A first aspect of this application provides a hot-pressing method for a winding core, the hot-pressing method comprising the following steps:

[0008] The positive and negative electrode sheets are placed on both sides of the diaphragm and wound to obtain a core. A preheating support gap is provided in the center of the core.

[0009] The core is hot-pressed.

[0010] The hot pressing method according to the embodiments of this application has at least the following beneficial effects:

[0011] The hot-pressing method provided in this application preheats the center of the core before hot pressing. This ensures more uniform heating of the inner and outer rings of the core during hot pressing, and more balanced stress between the electrode and the separator. This facilitates stress release and interface improvement, significantly improving the thickness consistency of the core after hot pressing, preventing abnormal fluffing, and increasing the core's casing yield. Furthermore, the better adhesion between the positive and negative electrodes and the separator effectively mitigates wrinkles and lithium plating at the electrode interface after battery charging and discharging, thereby increasing battery cycle performance.

[0012] In addition, when using the hot pressing method described above, hot pressing can be performed directly in one step, without the need for the two steps of pre-pressing and hot pressing as in traditional hot pressing processes. The overall process is simpler, does not change the cell design, and does not increase battery production costs.

[0013] In some embodiments of this application, the temperature of the preheating support is 60–120°C.

[0014] In some embodiments of this application, the temperature of the preheating support is 80–100°C.

[0015] In some embodiments of this application, the preheating support is in the shape of a sheet.

[0016] In some embodiments of this application, the thickness of the preheating support is 1 to 2 mm.

[0017] In some embodiments of this application, the preheating support meets the following requirements: Wherein, W1, H1, and T1 represent the width, thickness, and temperature of the preheating support, respectively; and W2, H2, and T2 represent the width, thickness, and hot-pressing temperature of the coil core, respectively. Furthermore, the preheating support meets the following requirements:

[0018] In some embodiments of this application, the material of the preheating support includes at least one of metal, alloy, and ceramic.

[0019] In some embodiments of this application, the material of the preheating support comprises at least one of the following I to III:

[0020] I. Any one of the following metals: silver, copper, gold, beryllium, aluminum, tungsten, zinc, nickel, iron, and indium;

[0021] Alloys of any one of the metals in I or II;

[0022] III. Any one of the following ceramics: alumina, aluminum nitride, silicon nitride, silicon carbide, and beryllium oxide.

[0023] In some embodiments of this application, the hot pressing pressure is 3000-15000 kgf and the hot pressing time is 10-200 s.

[0024] In some embodiments of this application, the separator forming the core includes at least one of polyolefin separator, polyamide separator, polyimide separator, polyester separator, and nonwoven separator.

[0025] In some embodiments of this application, the polyolefin separator is at least one of a polypropylene separator and a polyethylene separator.

[0026] In some embodiments of this application, the diaphragm does not contain an adhesive coating.

[0027] In some embodiments of this application, after hot pressing is completed, the preheated support is separated from the core.

[0028] A second aspect of this application provides a secondary battery, including a housing and a battery cell disposed within the housing, the battery cell comprising a wound core obtained after being processed by the above-described hot pressing method.

[0029] A third aspect of this application provides an electrical device comprising a secondary battery that serves as a power supply for the device. Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the application. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the hot pressing method for the core in an embodiment of this application.

[0031] Figure 2 This is a schematic diagram of the core after the preheating support is inserted in an embodiment of this application.

[0032] Figure 3 A and B are photographs showing the effect of hot pressing the core of Comparative Example 1 and Example 1 in this application, respectively.

[0033] Figure 4 A and B are comparative photographs of the negative electrode interface after disassembly of the core in Comparative Example 1 and Example 1 of this application, respectively.

[0034] Figure 5 This is a comparison of the charge-discharge cycle results of the batteries in Example 1 and Comparative Example 1 of this application.

[0035] Reference numerals: First pressure plate 110, Second pressure plate 120, Core 200, Positive electrode sheet 210, Negative electrode sheet 220, Diaphragm 230, Gap 240, Preheating support 241. Detailed Implementation

[0036] The following will clearly and completely describe the concept and technical effects of this application in conjunction with embodiments, so as to fully understand the purpose, features and effects of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are all within the scope of protection of this application.

[0037] The embodiments of this application are described in detail below. The described embodiments are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0038] In the description of this application, "several" means one or more, "multiple" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0039] In the description of this application, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0040] refer to Figure 1 The present application illustrates a hot-pressing method for a core. The hot-pressing method relates to a core 200, which is typically formed by winding together a positive electrode 210, a negative electrode 220, and a diaphragm 230 separating the positive electrode 210 and the negative electrode 220.

[0041] In some embodiments, the positive electrode 210 typically includes a positive current collector and a positive active material layer located on the positive current collector. In some specific embodiments, the positive current collector includes, but is not limited to, metal foil (such as at least one of aluminum foil, silver foil, tin foil, iron foil, titanium foil, nickel foil, copper foil, or alloy foil of the above metals), metal mesh (such as at least one of aluminum mesh, silver mesh, tin mesh, iron mesh, titanium mesh, nickel mesh, copper mesh, or alloy mesh of the above metals), and other composite materials based on metal foil or metal mesh. In some embodiments, the positive active material layer is made of a positive electrode slurry, which typically contains a positive active material and may also contain at least one of a conductive agent and a binder; specifically, the positive active material includes, but is not limited to, LiMn2O4, LiFePO4, LiCoO2, LiMnO2, and Li... a Ni x Co y Mn z O2, Li a Ni x Co y Al z O2 (a = 0.8–1.2, x + y + z = 1) and materials obtained by doping, modification, or coating of the above materials, etc.; the conductive agent includes, but is not limited to, at least one of graphite, acetylene black, carbon black, carbon nanotubes, carbon fibers, etc.; the binder includes, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyurethane (PU), polyvinyl alcohol (PVA), polyvinyl butyral (PVB), etc. In some embodiments, the positive electrode active material layer includes 70–99 wt% positive electrode active material, 0.1–6 wt% conductive agent, and 0.1–20 wt% binder.

[0042] In some embodiments, the negative electrode 220 typically comprises a negative electrode current collector and a layer of negative electrode active material located on the negative electrode current collector. The negative electrode active material layer is made of a negative electrode slurry, which typically contains a negative electrode active material and may also contain at least one of a conductive agent and a binder. Specifically, the negative electrode active material includes, but is not limited to, at least one of natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composites, Li-Sn alloys, Li-Sn-O alloys, Sn, SnO, SnO2, and spinel-structured lithium titanate. Similarly, the conductive agent, binder, and negative electrode current collector can be selected from at least one of the same selection range as the conductive agent and binder in the aforementioned positive electrode. In some embodiments, the negative electrode active material layer comprises 70–99 wt% negative electrode active material, 0.1–6 wt% conductive agent, and 0.1–20 wt% binder.

[0043] In some embodiments, the diaphragm 230 typically comprises at least one of a polyolefin diaphragm, a polyamide diaphragm, a polyimide diaphragm, a polyester diaphragm, or a nonwoven fabric diaphragm. Preferably, a polyolefin material with good mechanical strength, chemical stability, and low thickness is selected as the diaphragm, such as a polymer diaphragm containing ethylene and / or propylene as monomers, specifically including but not limited to at least one of a polyethylene diaphragm, a polypropylene diaphragm, or an ethylene-propylene copolymer diaphragm, which can be a single-layer diaphragm or a composite diaphragm.

[0044] In some embodiments, the core 200 is obtained by a square winding process. The winding process of square batteries is relatively simple, with high energy density, high packing efficiency, and strong protection for the battery cell.

[0045] refer to Figure 1 When the diaphragm 230 does not contain an adhesive layer, the diaphragm 230 cannot be tightly bonded to the positive electrode 210 and the negative electrode 220. As a result, during hot pressing, the positive electrode 210 and the negative electrode 220 are prone to becoming wavy, affecting their flatness. (Refer to...) Figure 3 Grade A has a loose, 200mm core with poor thickness consistency. Furthermore, during operation, it is prone to electrode wrinkling and lithium plating, severely impacting the battery's cycle performance.

[0046] Therefore, refer to Figure 1 and Figure 2This application proposes to pre-install a preheating support 241 at the center of the core 200 before hot pressing. After the core 200 is wound, a gap 240 is formed at the center of the core 200, and at least one preheating support 241 is placed in the gap 240 before hot pressing. The gap 240 is generated by the positive electrode 210, the negative electrode 220, and the diaphragm 230 during the winding process, for example, by the retraction of the winding needle. Specifically, the gap 240 is formed when the winding needle between the two inner layers of diaphragm 230 is retracted after winding, resulting in incomplete adhesion between the upper and lower layers of diaphragm. The preheating support 241 refers to a component that, after preheating, has a temperature higher than room temperature and provides a certain supporting function.

[0047] In some embodiments, when the first pressure plate 110 and the second pressure plate 120 of the hot pressing device apply pressure to the core 200 at a certain temperature, the force between the positive electrode 210, the negative electrode 220 and the diaphragm 230 is more uniform due to the action of the preheating support 241. Moreover, since the preheating support 241 has a certain temperature, it can also heat the positive electrode 210, the negative electrode 220 and the diaphragm 230 of the inner ring to a certain extent. Therefore, the heating of the inner and outer rings of the core 200 is also more uniform. In this way, it is beneficial to release the electrode stress and improve the interface. After hot pressing, the thickness uniformity of the core becomes higher, the fluffing of the core 200 is greatly reduced, and the yield of the core 200 in the shell is also significantly improved. Moreover, due to more uniform heating and stress, the adhesion between the positive and negative electrodes and the separator is also higher. When the lithium battery is working, the possibility of wrinkles or lithium plating at the electrode interface is also lower, and the cycle performance of the battery is significantly improved.

[0048] Conventional hot pressing methods include pre-pressing and shaping, and secondary hot pressing. However, in the embodiments of this application, by setting up a preheating support, the two hot pressing processes of pre-pressing and shaping and secondary hot pressing are integrated into one-step hot pressing. Only one hot pressing is needed to achieve a better shaping effect than the previous two hot pressing processes. The battery performance is better, the process is simpler, and there is no need to change the cell design or increase the battery production cost.

[0049] In some embodiments, the preheating support needs to meet certain temperature conditions so that, after being placed into the core, it can increase the heating of the inner electrode and the separator. Therefore, the temperature of the preheating support is 60–120°C. It should be noted that, to ensure the separator can effectively separate the positive and negative electrodes and provide a channel for free lithium-ion conduction, the temperature of the preheating support should generally not be too high to avoid deformation, denaturation, softening, or melting of the separator, which would affect its physical and chemical properties. Simultaneously, it should ensure as uniform a heating process as possible with the outer ring during hot pressing. Therefore, its temperature setting can be adjusted according to the actual separator type. In some embodiments, since the separator is made of polyolefin or further polypropylene material, the temperature of the preheating support is generally no higher than 120°C, 110°C, or 100°C, and the further preheating support temperature is 70–110°C, 80–100°C, or 85–95°C.

[0050] In some embodiments, the preheating support is sheet-shaped. Specifically, the size and thickness of the preheating support are matched to the size and height of the gap inside the core, for example, slightly smaller than the size and height of the gap so that the preheating support can be easily inserted into the gap. In some embodiments, the thickness of the preheating support is 1 to 2 mm.

[0051] In some implementations, combined Figure 2 The width W1, thickness H1, and temperature T1 of the preheating support 241, and the width W2, thickness H2, and hot-pressing temperature of the core 200, shall meet the following conditions: When the preheated support meets this condition, hot pressing has a better shaping and performance improvement effect. Here, W1, H1, and T1 are the width, thickness, and temperature of the preheated support, respectively, and W2, H2, and T2 are the width, thickness, and hot pressing temperature of the core, respectively. Furthermore, the preheated support meets…

[0052] In some embodiments, the preheating support needs to meet certain thermal conductivity requirements to allow heat to be conducted from the preheating support to the inner diaphragm, and a certain hardness to ensure that the diaphragms and electrodes on both sides of the preheating support are subjected to uniform stress during hot pressing for better fit. For example, the thermal conductivity of the preheating support is greater than 20 W / (m·K), and further greater than 50 W / (m·K), 100 W / (m·K), 120 W / (m·K), 150 W / (m·K), and 200 W / (m·K); the hardness of the preheating support is greater than HBS5, HBS10, HBS15, HBS20, HBS25, HBS30, HBS35, HBS40, HBS45, and HBS50.

[0053] In some embodiments, for simplicity, the method for obtaining a temperature above room temperature for the preheating support can be, for example, resistance heating, where the preheating support is used as a heating element and a power source is connected to both ends of the preheating support for heating. Therefore, to improve heat generation and transfer efficiency, the preheating support preferably meets certain resistivity requirements, such as higher than 10 nΩ·m, higher than 15 nΩ·m, 20 nΩ·m, 25 nΩ·m, 50 nΩ·m, 100 nΩ·m, 1000 nΩ·m, etc.

[0054] In some embodiments, in order to enable the preheating support to simultaneously achieve the function of more uniform heating and stress distribution on the core, the material of the preheating support can be at least one of metal, alloy, ceramic, etc. In some embodiments, the material of the preheating support includes at least one of the following I to III: I, any one of the metals selected from silver, copper, gold, beryllium, aluminum, tungsten, zinc, nickel, iron, and indium; II, an alloy of any one of the metals selected from I; III, any one of the ceramics selected from alumina, aluminum nitride, silicon nitride, silicon carbide, and beryllium oxide.

[0055] In some embodiments, the preheating support is made of aluminum sheet. It offers good heat generation and transfer performance, while being low in cost, widely available, and able to meet production requirements.

[0056] In some embodiments, the hot-pressing pressure is 3000–15000 kgf. Further, the hot-pressing pressure is 5000–15000 kgf, 8000–12000 kgf, or 10000–1200 kgf.

[0057] In some embodiments, the hot-pressing time is 10–200 s. Further, the hot-pressing time is 50–200 s, 100–200 s, or 100–150 s.

[0058] In some embodiments, the hot pressing pressure is 3000–15000 kgf, and the time is 10–200 s. Further, the hot pressing pressure is 5000–15000 kgf, and the time is 50–200 s; the hot pressing pressure is 8000–12000 kgf, and the time is 100–200 s; the hot pressing pressure is 10000–1200 kgf, and the time is 100–150 s.

[0059] In some embodiments, after hot pressing, the preheated support is separated from the core.

[0060] In some embodiments, inserting the preheating support into the gap means that the core is relatively fixed, while the preheating support is moved to the gap of the core to complete the insertion; in this case, separating the preheating support from the core can be accomplished by removing the preheating support from the core. It is understood that in some embodiments, inserting the preheating support into the gap can also be considered as fixing the preheating support relatively, while moving the core to align the preheating support with the gap to complete the relative insertion; in this case, separating the preheating support from the core can be accomplished by pulling the core out of the preheating support.

[0061] An embodiment of this application also provides a secondary battery, which includes a casing and a battery cell disposed within the casing. The battery cell includes a wound core obtained after being processed by the above-described hot pressing method.

[0062] In some embodiments, the core is processed after a hot-pressing process. The processed core is then encapsulated, baked, injected with electrolyte, formed, and capacity-tested to obtain the secondary battery. In some embodiments, the casing can be an aluminum casing or a steel casing. Since aluminum casings are lighter and offer better safety, they are preferred.

[0063] In this application embodiment, an electrical device is also provided, which includes the above-mentioned secondary battery, and the secondary battery serves as the power supply for the electrical device.

[0064] Based on the aforementioned hot-pressing method for the core, the heating of the inner and outer rings of the core is more uniform during hot pressing, and the stress between the electrode and the separator is more balanced. This is beneficial for releasing electrode stress and improving the interface, significantly improving the thickness consistency of the core after hot pressing, preventing abnormal fluffing of the core, and increasing the core's casing yield. Furthermore, the adhesion between the positive and negative electrodes and the separator is better, effectively mitigating wrinkles and lithium plating at the electrode interface after charging and discharging, thereby increasing the cycle performance of the secondary battery. Since the electrical equipment uses this secondary battery as its power source, its battery life and service life are longer.

[0065] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0066] Example 1

[0067] This embodiment provides a hot-pressing method for a winding core, wherein the method for preparing the winding core is as follows:

[0068] Lithium iron phosphate (LiFePO4), conductive carbon black (SP), and PVDF binder were dispersed in NMP solvent at a mass ratio of 97:0.5:2.5 and mixed thoroughly to form a positive electrode slurry. The positive electrode slurry was then uniformly coated onto carbon-coated aluminum foil for the positive electrode current collector, and the positive electrode sheet was prepared through processes such as baking, rolling, die-cutting, and slitting.

[0069] The negative electrode active material graphite, conductive carbon black (SP), thickener CMC, and binder SBR are dispersed in deionized water at a mass ratio of 97:0.2:1:1.8 and mixed evenly to form a negative electrode slurry. The negative electrode slurry is then uniformly coated onto the negative electrode current collector copper foil, and the negative electrode sheet is prepared through processes such as baking, rolling, die cutting, and slitting.

[0070] Select the prepared positive electrode sheet, negative electrode sheet, and uncoated polypropylene (PP) separator and wind them. The positive and negative electrode sheets are wound with a tension of 400±50g and a decrease of 3.5g per turn, and the separator is wound with a tension of 140±30g and a decrease of 1.2g per turn. After being wound into a complete core, glue is applied and the core is removed from the machine.

[0071] The core is 150mm wide, 108mm long, and 20mm thick.

[0072] The method for hot-pressing the wound core is as follows:

[0073] (1) Take an aluminum sheet (120mm wide, 105mm long, and 1mm thick) that matches the size of the gap between the inner rings at the center of the core. Use it as an electric heating element and connect it to the power supply at both ends for heating. Heat it to 95℃ and maintain it within that temperature.

[0074] (2)Reference Figure 1 A preheated aluminum sheet (thermal conductivity 247 W / (m·K), resistivity 26.55 nΩ·m, hardness HBS25) is inserted into the gap 240 at the center of the core 200. Then the core is placed between the first pressure plate 110 and the second pressure plate 120 of the hot pressing equipment. Without the pre-pressing and shaping process, it is directly hot-pressed in one step. The hot pressing temperature of the first pressure plate and the second pressure plate is set to 85℃, the hot pressing pressure is 11000 kgf, and the hot pressing time is 110 s.

[0075] (3) After hot pressing, remove the aluminum sheet, measure the thickness of the core after hot pressing, and assemble it into the shell.

[0076] in,

[0077] Example 2

[0078] This embodiment provides a hot pressing method for a winding core, which differs from Embodiment 1 in that the hot pressing temperature is 90°C.

[0079] Example 3

[0080] This embodiment provides a hot pressing method for a core, which differs from Embodiment 1 in that the hot pressing temperature is 90°C and the hot pressing pressure is 11300 kgf.

[0081] Comparative Example 1

[0082] This comparative example provides a conventional hot-pressing method. After the core is prepared according to the preparation method in Example 1, it is hot-pressed using the following method:

[0083] (1) Place the wound core between the first pressure plate 110 and the second pressure plate 120 of the hot pressing equipment. First, set the pre-pressing temperature to 90±5℃, the pre-pressing pressure to 150±50kgf, and the hot pressing time to 360s for pre-pressing and shaping.

[0084] (2) After completing the pre-pressing and shaping, increase the pressure for hot pressing. Set the hot pressing plate temperature to 85±5℃, the pressure to 11000±300kgf, and the hot pressing time to 110s.

[0085] (3) After hot pressing, measure the thickness of the core and assemble it into the shell.

[0086] The cores after hot pressing according to Example 1 and Comparative Example 1 were tested in the following manner:

[0087] 1. Core internal resistance and thickness testing

[0088] Cells prepared, wound, and hot-pressed according to the methods of Examples 1-3 and Comparative Example 1 were taken respectively, and their internal resistance and thickness were measured. The test was repeated 32 times, and their distribution range, mean, and range were calculated.

[0089] 2. Test of the rate at which the core absorbs the electrolyte

[0090] The cores of Examples 1-3 and Comparative Example 1 with different hot pressing schemes were injected with the same 350g electrolyte. After standing for different times, the amount of residual free electrolyte in the shell was observed after full disassembly. Thus, the time required for the core to absorb the electrolyte was determined.

[0091] 3. Cell assembly

[0092] After being encased, the cores in Examples 1-3 and Comparative Example 1 underwent welding, baking, and other processes. They were then injected with the same mass of electrolyte under the same pressure conditions and left at 45°C for 18-26 hours. After that, they were charged, formed, and tested using the same current steps.

[0093] 4. Cell disassembly

[0094] The battery cells assembled in Examples 1-3 and Comparative Example 1 were charged to 3.65V at 0.5C and then disassembled to observe the wrinkling of the negative electrode interface and the uniformity of the interface.

[0095] 5. Cell Cycle Test

[0096] The cells from Examples 1-3 and Comparative Example 1 were subjected to charge-discharge cycle tests. The specific steps are as follows: the cells were left to stand at room temperature for 30 minutes, charged to 3.65V at 1C constant current and constant voltage, cut off at 0.05C, and then discharged to 2.5V at 1C constant current after resting for 30 minutes. The above steps were repeated for 2000 cycles to test the capacity retention rate under different cycle numbers.

[0097] Table 1. Core Performance Test Results

[0098]

[0099]

[0100] As can be seen from the results in Table 1, compared with Comparative Example 1, Examples 1-3 have lower mean and range of core thickness, indicating that the Examples are better than the Comparative Example in terms of thickness consistency. Simultaneously, the mean and range of core internal resistance are also lower, indicating that the internal resistance distribution of the cell is more concentrated and the range is smaller. Furthermore, the core electrolyte absorption time can be shortened by 4-6 hours, which is beneficial to improving the overall yield of battery production. Combined with... Figure 3 In the embodiment, the electrode diaphragms inside the hot-pressed core are more regularly arranged, while in the comparative example, the hot-pressed core is fluffy and has poor flatness. Therefore, the method provided in the embodiment can significantly improve the shaping effect of the core by omitting pre-pressing and shaping.

[0101] The negative terminal interface of the battery cell after full charging is shown below. Figure 4 As shown, in Comparative Example 1, after the battery cell using the traditional hot-pressing method was fully charged and disassembled, the negative electrode sheet showed significant wrinkling, and some areas exhibited purple spots. The main reason is likely that after hot pressing, the adhesion between the electrode sheet and the separator was poor, preventing lithium ions in the electrolyte from effectively migrating between the electrode sheet and the separator, leading to interface deterioration and poor consistency of the negative electrode sheet. In contrast, Examples 1-3, using the optimized one-step hot-pressing molding method, showed only slight wrinkling of the negative electrode sheet after full charging and disassembly, with higher interface color consistency. The direct adhesion between the positive and negative electrodes and the separator was better, facilitating sufficient wetting and absorption of the electrolyte. During charging and discharging, lithium ions could freely insert into and extract from the electrode sheet, improving the overall electrical performance of the battery.

[0102] In addition, refer to Figure 5The cycle curves show that the capacity retention rate of the battery using the hot-pressing process of Example 1 is significantly higher than that of the battery cell using the traditional hot-pressing process represented by Comparative Example 1. This is because inserting a preheated support in the middle of the core before hot-pressing results in a smoother hot-pressing interface and more balanced heating and stress between the electrode and separator, which improves the electrode interface and increases the battery cycle life.

[0103] Example 4

[0104] This embodiment provides a hot pressing method, which differs from Embodiment 1 in that a copper sheet (thermal conductivity 398 W / (m·K), resistivity 16.73 nΩ·m, hardness HBS37) is used instead of an aluminum sheet.

[0105] Example 5

[0106] This embodiment provides a hot pressing method, which differs from Embodiment 1 in that an aluminum alloy sheet is used instead of an aluminum sheet.

[0107] Example 6

[0108] This embodiment provides a hot pressing method, which differs from Embodiment 1 in that a SiC ceramic sheet (thermal conductivity 270 W / (m·K), resistivity 0.1 Ω·m) is used instead of an aluminum sheet.

[0109] Example 7

[0110] This embodiment provides a hot pressing method, which differs from Embodiment 1 in that the diaphragm in the core is a polyimide diaphragm.

[0111] Example 8

[0112] This embodiment provides a hot pressing method, which differs from Embodiment 1 in that the hot pressing pressure is 5000 kgf and the hot pressing time is 200 s.

[0113] The core thickness consistency and cycle performance after hot pressing in Examples 4-8 are significantly improved compared to Comparative Example 1, which will not be elaborated further here.

[0114] The present application has been described in detail above with reference to the embodiments. However, the present application is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present application. Furthermore, unless otherwise specified, the embodiments and features in the embodiments of the present application can be combined with each other.

Claims

1. A hot-pressing method for a winding core, characterized in that, Includes the following steps: The positive and negative electrode sheets are placed on both sides of the separator and wound to form a core. A preheating support is provided at the center of the core. The separator is selected from polyolefin separators and does not contain an adhesive layer. The temperature of the preheating support is 70~110℃, which is lower than the softening temperature of the separator. The core is obtained using a square winding process. The preheating support is sheet-shaped and satisfies the condition 0.03 ≤ ≤0.3, where W1, H1, and T1 are the width, thickness, and temperature of the preheating support, respectively, and W2, H2, and T2 are the width, thickness, and hot-pressing temperature of the core, respectively; and the thermal conductivity of the preheating support is greater than 20 W / (m·K); The core is hot-pressed.

2. The hot pressing method according to claim 1, characterized in that, The temperature of the preheated support is 80~100℃.

3. The hot pressing method according to claim 1, characterized in that, The thickness of the preheating support is 1~2mm.

4. The hot pressing method according to claim 1, characterized in that, The material of the preheating support includes metal, alloy, etc. At least one of gold and ceramics.

5. The hot pressing method according to claim 4, characterized in that, The material of the preheating support includes at least one of the following I to III: I. Any one of the following metals: silver, copper, gold, beryllium, aluminum, tungsten, zinc, nickel, iron, and indium; Alloys of any one of the metals in I or II; III. Any one of the following ceramics: alumina, aluminum nitride, silicon nitride, silicon carbide, and beryllium oxide.

6. The hot pressing method according to claim 1, characterized in that, The hot pressing pressure is 3000~15000 kgf, and the hot pressing time is 10~200 s.

7. The hot pressing method according to claim 1, characterized in that, The polyolefin membrane is at least one of polypropylene membrane and polyethylene membrane.

8. The hot pressing method according to any one of claims 1 to 7, characterized in that, After hot pressing is completed, the preheated support is separated from the core.

9. A secondary battery, characterized in that, It includes a housing and a battery cell disposed within the housing, the battery cell comprising a wound core obtained by the hot pressing method according to any one of claims 1 to 8.

10. An electrical appliance, characterized in that, The device includes the secondary battery as described in claim 9, wherein the secondary battery serves as the power supply for the electrical equipment.

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