Battery cell, method for preparing battery cell, battery device and power utilization device

Abstract

The invention provides a battery monomer, a method for preparing the battery monomer, a battery device and a power utilization device. The battery monomer comprises an electrode assembly, the electrode assembly comprises a positive pole piece, a negative pole piece and a diaphragm, and the diaphragm is located between the positive pole piece and the negative pole piece; the positive pole piece comprises a positive current collector and a positive active layer positioned on at least one side of the positive current collector, and the positive active layer comprises a positive active material; the compaction density of the positive electrode active layer is 2.3 g / cm < 3 >-3.35 g / cm < 3 >; the diaphragm comprises a base layer and a polymer layer located on the side, facing the positive pole piece, of the base layer, and the polymer layer comprises a fluorine-containing polymer with the crystallinity smaller than or equal to 50%. The fluorine-containing polymer layer of the battery monomer can melt and wrap burrs of the positive electrode current collector and the positive electrode active layer, so that the cycle performance and the safety performance of the battery monomer are improved.

Description

Technical Field

[0001] The present application relates to the technical field of batteries, and in particular, to a battery cell, a method for preparing the battery cell, a battery device, and an electrical device. Background Art

[0002] In recent years, with the increasingly wide range of applications of batteries, batteries are widely used in energy storage power systems such as hydroelectric, thermal, wind, and solar power stations, as well as in multiple fields such as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. Due to the great development of batteries, higher requirements are also put forward for their cycle performance and safety performance. Summary of the Invention

[0003] The present application is made in view of the above problems, and its purpose is to provide a battery cell, a method for preparing the battery cell, a battery device, and an electrical device. The fluoropolymer layer of the battery cell of the present application can melt and coat the burrs of the positive current collector and the positive active layer, improving the cycle performance and safety performance of the battery cell.

[0004] To achieve the above object, a first aspect of the present application provides a battery cell, including an electrode assembly, the electrode assembly including a positive electrode sheet, a negative electrode sheet, and a separator, the separator being located between the positive electrode sheet and the negative electrode sheet; The positive electrode sheet includes a positive current collector and a positive active layer located on at least one side of the positive current collector, the positive active layer including a positive active material; the tap density of the positive active layer is 2.3 g / cm 3 - 3.35 g / cm 3 ; The separator includes a base layer and a polymer layer located on the side of the base layer facing the positive electrode sheet, the polymer layer including a fluoropolymer with a crystallinity less than or equal to 50%.

[0005] Thus, the fluoropolymer with a crystallinity less than or equal to 50% in the polymer layer can melt and coat the burrs at the edges of the positive current collector and the positive active layer and the burrs in the non-edge regions of the positive active layer. Moreover, the tap density of the positive active layer within the above range is beneficial to reducing the number of burrs in the edge and non-edge regions of the positive active layer and improving the structural stability of the positive electrode sheet, thereby reducing the risk of short circuit caused by burrs piercing the separator, so as to improve the cycle performance and safety performance of the battery cell.

[0006] In any embodiment of the first aspect, the crystallinity of the fluoropolymer is 20% - 50%.

[0007] Thus, for the crystallinity range of the above-mentioned fluoropolymer, on the one hand, it can melt and coat the burrs at the edges of the positive current collector and the positive active layer as well as the burrs in non-edge areas of the positive active layer, so as to reduce the risk of short circuit caused by the burrs piercing the separator, and improve the cycling performance and safety performance of the battery cell. On the other hand, as the crystallinity of the fluoropolymer decreases, its structural stability decreases, and it is easily dissolved in the electrolyte, resulting in an increase in the viscosity of the electrolyte, a decrease in the lithium ion transmission rate, an increase in the impedance of the battery cell, and an impact on the cycling performance of the battery cell. The above range reduces this impact.

[0008] In any embodiment of the first aspect, the weight-average molecular weight of the fluoropolymer is 120,000 - 800,000.

[0009] In any embodiment of the first aspect, the weight-average molecular weight of the fluoropolymer is 150,000 - 500,000.

[0010] Thus, for the weight-average molecular weight range of the above-mentioned fluoropolymer, on the one hand, the viscosity of the polymer is beneficial to coating the burrs at the edges of the positive current collector and the positive active layer as well as the burrs in non-edge areas of the positive active layer, so as to reduce the risk of short circuit caused by the burrs piercing the separator and improve the cycling performance and safety performance of the battery cell. On the other hand, a decrease in the weight-average molecular weight may cause the fluoropolymer to be easily dissolved in the electrolyte, resulting in an increase in the viscosity of the electrolyte and an impact on the cycling performance of the battery cell. The above range reduces this impact.

[0011] In any embodiment of the first aspect, the thickness of the polymer layer is 0.2 μm - 5 μm.

[0012] In any embodiment of the first aspect, the thickness of the polymer layer is 0.5 μm - 3 μm.

[0013] Thus, on the one hand, the above thickness range of the polymer layer can coat the burrs at the edges of the positive current collector and the positive active layer as well as the burrs in non-edge areas of the positive active layer to a greater extent, so as to reduce the damage to the separator by the burrs and improve the cycling performance and safety performance of the battery cell. On the other hand, an increase in the thickness of the polymer layer may make the lithium ion transmission distance longer, thus affecting the fast charging performance of the battery cell, and an increase in the thickness of the polymer layer may also affect the energy density of the battery cell. The above thickness range reduces these impacts.

[0014] In any embodiment of the first aspect, the ratio of the area of the polymer layer to the area of the base layer is 65% - 100%.

[0015] Thus, it is beneficial for the burrs at the edges of the positive electrode current collector and the positive electrode active layer and the burrs in the non-edge regions of the positive electrode active layer to be covered and wrapped to a greater extent, thereby reducing the risk of the burrs piercing the separator, improving the cycle performance and safety performance of the battery cell, and at the same time being beneficial for the firm adhesion between the electrode sheets and the separator; moreover, when the polymer layer partially covers the base layer in the above proportion, it is beneficial to form a gap between the positive electrode sheet and the separator membrane, which helps the electrolyte to penetrate and improve the lithium ion transmission effect.

[0016] In any implementation manner of the first aspect, the fluorine-containing polymer includes one or more of polyvinylidene fluoride, vinylidene fluoride-chlorotrifluoroethylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-acrylonitrile copolymer, and vinylidene fluoride-acrylate copolymer.

[0017] Thus, the above-mentioned fluorine-containing polymer can melt and wrap the burrs at the edges of the positive electrode current collector and the positive electrode active layer and the burrs in the non-edge regions of the positive electrode active layer, reducing the risk of the burrs piercing the separator, thereby improving the cycle performance and safety performance of the battery cell.

[0018] In any implementation manner of the first aspect, the separator further includes an inorganic material layer, and the inorganic material layer is located between the base layer and the polymer layer.

[0019] Thus, the inorganic material layer has high mechanical strength and strong thermal shrinkage resistance, which can reduce the risk of the burrs at the edges of the positive electrode current collector and the positive electrode active layer and the burrs in the non-edge regions of the positive electrode active layer piercing the separator during high-temperature cycling, improving the cycle performance and safety performance of the battery cell. Moreover, the inorganic material layer has a high electrolyte infiltration rate, which can reduce the impedance and is beneficial for the transmission of lithium ions.

[0020] In any implementation manner of the first aspect, the inorganic material layer includes one or more inorganic materials such as alumina, boehmite, titanium oxide, and silica.

[0021] In any implementation manner of the first aspect, the average particle size of the positive electrode active material is 0.35 μm - 2.1 μm.

[0022] In any implementation manner of the first aspect, the average particle size of the positive electrode active material is 0.5 μm - 1.5 μm.

[0023] Thus, on the one hand, the average particle size of the positive electrode active material not exceeding the upper limit is beneficial to reducing the number of burrs in the edge and non-edge regions of the positive electrode active layer, reducing the risk of burrs piercing the separator, improving the cycle performance and safety performance of the battery cell, and at the same time being beneficial to improving the energy density of the battery cell; on the other hand, the decrease in the average particle size of the positive electrode active material may cause the positive electrode active material particles to pass through the pores of the separator, which has an impact on the self-discharge performance and safety performance of the battery cell. The above average particle size range reduces this impact.

[0024] In any implementation manner of the first aspect, the areal density of the positive electrode active layer is 0.15 mg / mm 2 - 0.4mg / mm 2 .

[0025] Thus, on the one hand, the areal density of the positive electrode active layer not being lower than the lower limit is beneficial to improving the energy density of the battery cell; on the other hand, the increase in the areal density of the positive electrode active layer leads to an increase in the thickness of the positive electrode active layer, which may easily generate edge burrs during the processing. The above areal density range of the positive electrode active layer reduces this impact.

[0026] In any implementation manner of the first aspect, the positive electrode active material includes lithium iron phosphate without doped elements or lithium iron phosphate doped with elements, wherein the doped elements include one or more of Ti, Zr, Mn, Co, and V.

[0027] Thus, among the commonly used positive electrode active materials, the particle size of the lithium iron phosphate material is relatively small, and the number of burrs at the edge and non-edge of its positive electrode active layer is less, reducing the risk of burrs piercing the separator and improving the cycle performance and safety performance of the battery cell.

[0028] In any implementation manner of the first aspect, the mass content of the doped elements in the doped lithium iron phosphate is 500 - 5000 ppm.

[0029] Thus, the above range of the content of the doped elements is beneficial to improving the lithium ion diffusion coefficient of the positive electrode active layer and enhancing the discharge power of the battery cell.

[0030] In any implementation manner of the first aspect, the thickness of the base layer is 5μm - 12μm.

[0031] Thus, on the one hand, the above range of the base layer thickness can improve the liquid retention capacity of the battery cell, reduce the impedance of the battery cell, and also improve the mechanical strength of the separator through the micropores and capillary action of the base layer, improving the rate performance and cycle performance of the battery cell; on the other hand, the increase in the base layer thickness may increase the resistance of lithium ion transmission, affecting the rate performance of the battery cell. The above range reduces this impact.

[0032] In any embodiment of the first aspect, the porosity of the separator is 19% - 50%. Thus, on the one hand, the porosity of the separator not being lower than the above lower limit is beneficial to improving the liquid-phase transport of the battery cell and enhancing the discharge performance of the battery cell. On the other hand, an increase in the porosity of the separator may lead to a weakening of the insulating effect of the separator, affecting the safety of the battery cell. The above range reduces this impact.

[0033] In any embodiment of the first aspect, the positive electrode active layer further includes a positive electrode conductive agent, and the positive electrode conductive agent includes single-walled carbon nanotubes.

[0034] Thus, through the entanglement effect of the single-walled carbon nanotubes, the positive electrode active material particles can be entangled and anchored on the positive electrode current collector, and the single-walled carbon nanotubes can buffer the volume change of the positive electrode active material particles during cycling to reduce particle shedding. Therefore, it is beneficial for the positive electrode active material particles to adhere to the positive electrode current collector and is beneficial to improving the electron conductivity to enhance the discharge power of the battery cell.

[0035] In any embodiment of the first aspect, the positive electrode active layer further includes a polymer-based positive electrode binder, and the weight-average molecular weight of the polymer-based positive electrode binder is greater than or equal to the weight-average molecular weight of the fluoropolymer.

[0036] Thus, it is beneficial to firmly adhere the positive electrode active material particles to the positive electrode current collector through the polymer-based positive electrode binder to reduce the loss of the positive electrode active material, thereby enhancing the cycle life of the battery cell.

[0037] In any embodiment of the first aspect, the negative electrode plate includes a negative electrode current collector and a negative electrode active layer located on at least one side of the negative electrode current collector, and the negative electrode active layer includes a negative electrode active material; wherein, The negative electrode active material includes a graphite material.

[0038] Thus, the use of a graphite material in the negative electrode active material is beneficial to enhancing the cycle life of the battery cell.

[0039] In any embodiment of the first aspect, the graphitization degree of the negative electrode active material is 90% - 94%.

[0040] Thus, on the one hand, the graphitization degree of the negative electrode active material not being lower than the above lower limit is beneficial to enhancing the rate performance, energy density, and cycle life of the battery cell; on the other hand, an increase in the graphitization degree of the negative electrode active material may lead to a decrease in the capacity of the battery cell and a deterioration of the fast charging performance. The above range reduces this impact.

[0041] In any embodiment of the first aspect, the average particle size of the negative electrode active material is 8μm - 20 μm.

[0042] Thus, on the one hand, the average particle size of the negative electrode active material being not less than the above lower limit is beneficial to reducing the specific surface area of the negative electrode active material, decreasing the contact area between the negative electrode active material and the electrolyte, reducing side reactions, and thereby improving the cycle life of the battery cell. On the other hand, an increase in the average particle size of the negative electrode active material may lead to gelation of the negative electrode slurry, making processing difficult, and may also result in poor fast charging performance of the battery cell. The above range reduces this influence.

[0043] In any implementation of the first aspect, the battery cell further includes an electrolyte, the electrolyte includes an electrolyte additive, and the electrolyte additive includes an unsaturated carbonate additive and / or a sulfonate additive. Thus, using the above electrolyte additive is beneficial to improving the cycle life of the battery cell.

[0044] In any implementation of the first aspect, the unsaturated carbonate additive includes one or more of vinylene carbonate, fluoroethylene carbonate, and vinyl vinylene carbonate; and / or, The sulfonate additive includes one or more of 1,3-propane sultone, 1,4-butane sultone, methylene methanedisulfonate, ethylene sulfate, and ethyl sulfite.

[0045] In any implementation of the first aspect, the electrolyte further includes a solvent, the solvent includes a cyclic carbonate solvent, and the mass of the cyclic carbonate solvent is 20% - 40% of the mass of the solvent.

[0046] Thus, on the one hand, the content of the cyclic carbonate solvent being not less than the above lower limit is beneficial to improving the cycle life of the battery cell. On the other hand, the content of the cyclic carbonate solvent not exceeding the above upper limit is beneficial to reducing the viscosity of the electrolyte in a low-temperature environment, thereby improving the low-temperature charge and discharge power of the battery cell.

[0047] In any implementation of the first aspect, the electrode assembly is rectangular parallelepiped-shaped, and the ratio of the length to the width of the electrode assembly is 2 – 8.

[0048] Since the crystallinity of the fluoropolymer in the polymer layer is less than or equal to 50%, the mechanical strength of the electrode assembly formed by bonding the separator to the positive electrode plate through the polymer layer is reduced. Using the above ratio of the length to the width of the electrode assembly is more convenient for loading and unloading the electrode assembly for subsequent processing.

[0049] In any implementation of the first aspect, the electrode assembly is a laminated structure.

[0050] The second aspect of the present application provides a method for preparing a battery cell, including the following steps: Arrange the positive electrode sheet, separator, and negative electrode sheet in sequence, and perform hot pressing at 80°C - 150°C with a hot pressing pressure of 2 kN - 8 kN to obtain an electrode assembly, and prepare a battery cell using the electrode assembly; wherein, The positive electrode sheet includes a positive current collector and a positive active layer located on at least one side of the positive current collector, and the positive active layer includes a positive active material; the tap density of the positive active layer is 2.3 g / cm 3 - 3.35 g / cm 3 ; The separator includes a base layer and a polymer layer located on the side of the base layer facing the positive electrode sheet, and the polymer layer includes a fluoropolymer with a crystallinity less than or equal to 50%.

[0051] Thus, hot pressing under the above conditions can cause the fluoropolymer with a crystallinity less than or equal to 50% to melt and coat the burrs at the edges of the positive current collector and the positive active layer and the non-edge burrs of the positive active layer. At the same time, limiting the tap density of the positive active layer within the above range is beneficial to reducing the number of burrs in the edge and non-edge regions of the positive active layer and improving the structural stability of the positive electrode sheet, thereby reducing the risk of short circuit caused by burrs piercing the separator and improving the cycle performance and safety performance of the battery cell.

[0052] In any implementation manner of the second aspect, the hot pressing time is 40 s - 180 s.

[0053] In any implementation manner of the second aspect, the battery cell is as described in the first aspect.

[0054] In any implementation manner of the second aspect, the separator further includes an inorganic material layer, and the inorganic material layer is located between the base layer and the polymer layer.

[0055] In any implementation manner of the second aspect, the polymer layer is prepared by the following steps: Mix the fluoropolymer with an oily solvent to obtain a polymer slurry; Coat the polymer slurry on the side of the base layer or the inorganic material layer facing the positive electrode sheet, and dry it to obtain the polymer layer.

[0056] In any implementation manner of the second aspect, the oily solvent includes one or more of N-methylpyrrolidone, dimethyl sulfoxide, dimethylformamide, and dimethylacetamide.

[0057] The third aspect of the present application provides a battery device, including the battery cell of the first aspect of the present application or the battery cell prepared by the method of the second aspect of the present application.

[0058] The fourth aspect of the present application provides an electrical device, including the battery cell of the first aspect of the present application or the battery device of the third aspect of the present application. Brief Description of the Drawings

[0059] Figure 1 is a schematic diagram of a battery cell according to an embodiment of the present application.

[0060] Figure 2 is Figure 1 an exploded view of the battery cell according to an embodiment of the present application shown in the figure.

[0061] Figure 3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0062] Figure 4 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0063] Figure 5 is Figure 4 an exploded view of the battery pack according to an embodiment of the present application shown in the figure.

[0064] Figure 6 is a schematic diagram of an electrical device using the battery cell according to an embodiment of the present application as a power source.

[0065] Description of the Reference Numerals: 1 Battery pack; 2 Upper box body; 3 Lower box body; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Cover plate. Detailed Embodiments

[0066] Hereinafter, embodiments of the battery cell, battery module, battery pack, and electrical device of the present application will be specifically disclosed in detail with reference to the accompanying drawings as appropriate. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the accompanying drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0067] If there is no special description, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0068] If there is no special description, all technical features and optional technical features of the present application can be combined with each other to form a new technical solution.

[0069] Unless otherwise specified, all steps of this application can be carried out sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out sequentially, or can also include steps (b) and (a) carried out sequentially. For example, when it is mentioned that the method may further include step (c), it means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b) and (c), or can also include steps (a), (c) and (b), or can also include steps (c), (a) and (b), etc.

[0070]

Battery cell

[0071] The battery cell can be a lithium-ion battery, a sodium-ion battery, a sodium-lithium-ion battery, a lithium metal battery, a sodium metal battery, a lithium-sulfur battery, a magnesium-ion battery, a nickel-metal hydride battery, a nickel-cadmium battery, a lead-acid battery, etc., and the embodiments of this application do not limit this.

[0072] The battery cell generally includes an electrode assembly. The electrode assembly includes a positive electrode, a negative electrode and a separator, and the separator is disposed between the negative electrode and the positive electrode. During the charge and discharge process of the battery cell, active ions (such as lithium ions) are embedded and extracted back and forth between the positive electrode and the negative electrode. The separator is disposed between the positive electrode and the negative electrode, which can prevent the short circuit between the positive and negative electrodes and at the same time allow active ions to pass through.

[0073] One embodiment of this application provides a lithium-ion battery cell, including an electrode assembly, the electrode assembly includes a positive electrode plate, a negative electrode plate and a separator, and the separator is located between the positive electrode plate and the negative electrode plate; The positive electrode plate includes a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector, and the positive electrode active layer includes a positive electrode active material; the compaction density of the positive electrode active layer is 2.3 g / cm 3 - 3.35 g / cm 3 (such as 2.3 g / cm 3 、2.4 g / cm 3 、2.48 g / cm 3 、2.5 g / cm 3 、2.55 g / cm 3 、2.6 g / cm 3 、2.63 g / cm 3 、2.7 g / cm 3 、2.8g / cm 3 、2.9 g / cm 3 、3.0 g / cm3 、 3.1 g / cm 3 、 3.2 g / cm 3 、 3.3 g / cm 3 、 3.35 g / cm 3 or a range composed of any of the above values); The separator includes a base layer and a polymer layer located on one side of the base layer facing the positive electrode sheet. The polymer layer includes a fluoropolymer with a crystallinity less than or equal to 50% (such as 10%, 18%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or a range composed of any of the above values).

[0074] In the battery cell, burrs are likely to occur at the edges of the positive current collector and the positive active layer and in the non-edge regions of the positive active layer in the positive electrode sheet. The burrs piercing the separator are likely to cause a short circuit, leading to safety problems of the battery.

[0075] Although the mechanism is not yet clear, the applicant unexpectedly found that: the fluoropolymer with a crystallinity less than or equal to 50% in the polymer layer can melt and coat the burrs at the edges of the positive current collector and the positive active layer and the burrs in the non-edge regions of the positive active layer, reducing the sharpness and conductivity of the burrs. At the same time, limiting the compaction density of the positive active layer within the above range is beneficial to reducing the number of burrs in the edge and non-edge regions of the positive active layer and improving the structural stability of the positive electrode sheet, so as to reduce the risk of short circuit caused by burrs piercing the separator, thereby improving the cycle performance and safety performance of the battery cell.

[0076] The electrode assembly of the battery cell includes various structures, and burr problems exist in the positive electrode sheets in the traditional electrode assembly structures. For example: burrs are likely to appear on the cut edges of the positive current collector in the wound electrode assembly structure. However, in the later stage of cycling of the wound electrode assembly structure, the pole piece spacing is likely to increase due to uneven stress distribution, and local polarization is likely to occur in the corner area, resulting in problems such as lithium deposition at the corner and increased internal resistance. The laminated electrode assembly structure can avoid the above corner problems. However, in the positive electrode sheet of the laminated electrode assembly structure, burrs are likely to occur at the edges of the positive current collector and the positive active layer and in the non-edge regions of the positive active layer. The fluoropolymer in the polymer layer of the present application can coat the burrs of the positive electrode sheet, and the number of burrs in the edge and non-edge regions of the positive active layer of the present application is reduced, so as to improve the cycle performance and safety performance of the battery cell.

[0077] In some embodiments, the crystallinity of the fluoropolymer is 20% - 50%.

[0078] Thus, for the crystallinity range of the above-mentioned fluoropolymer, on the one hand, it can melt and coat the burrs at the edges of the positive current collector and the positive active layer as well as the burrs at non-edge positions of the positive active layer, so as to reduce the risk of short circuit caused by burrs piercing the separator, and improve the cycle performance and safety performance of the battery cell. On the other hand, as the crystallinity of the fluoropolymer decreases, its structural stability decreases, and it is easily dissolved in the electrolyte, resulting in an increase in the viscosity of the electrolyte, a decrease in the lithium ion transmission rate, an increase in the impedance of the battery cell, and an impact on the cycle performance of the battery cell. The above range reduces this impact.

[0079] In this application, the crystallinity of the fluoropolymer is tested by conventional methods in the art. For example, disassemble the battery cell, take out the separator, and scrape the polymer on the surface of the separator facing the positive electrode plate, which is the fluoropolymer. Use a differential scanning calorimeter (DSC) model Discovery 250 from TA Instruments in the United States to test the fluoropolymer in a nitrogen atmosphere at a heating rate of 10 °C / min in the temperature range from -100 °C to 400 °C. The purge gas flow rate is 50 ml / min, and the protective gas flow rate is 70 ml / min to obtain a DSC curve. The crystallinity of the fluoropolymer is calculated according to the following formula. Among them, the peak area of the DSC curve is the enthalpy of fusion ΔH (unit: J / g), and ΔHm100% is the standard enthalpy of fusion of the fluoropolymer (heat of fusion of the crystalline state, unit: J / g). For example, the ΔHm100% of polyvinylidene fluoride is 104.7 J / g; Crystallinity of fluoropolymer = 100% × ΔH / (ΔHm100%).

[0080] In some embodiments, the weight-average molecular weight of the fluoropolymer is 120,000 - 800,000, such as 120,000, 150,000, 200,000, 250,000, 300,000, 350,000, 400,000, 450,000, 500,000, 550,000, 600,000, 650,000, 700,000, 750,000, 800,000 or the range composed of any of the above values.

[0081] In some embodiments, the weight-average molecular weight of the fluoropolymer is 150,000 - 500,000.

[0082] Thus, for the weight-average molecular weight range of the above-mentioned fluoropolymer, on the one hand, the viscosity of the polymer is beneficial to coating the burrs at the edges of the positive current collector and the positive active layer as well as the burrs at non-edge positions of the positive active layer, so as to reduce the risk of short circuit caused by burrs piercing the separator, and improve the cycle performance and safety performance of the battery cell. On the other hand, a decrease in the weight-average molecular weight may cause the fluoropolymer to be easily dissolved in the electrolyte, resulting in an increase in the viscosity of the electrolyte and an impact on the cycle performance of the battery cell. The above range reduces this impact.

[0083] In this application, the weight-average molecular weight of the fluoropolymer is measured by conventional methods in the art. For example, disassemble the battery monomer, take out the separator, scrape the polymer on the surface of the separator facing the positive electrode sheet, which is the fluoropolymer, and prepare a 3.0% by mass fluoropolymer colloidal solution with a purified solvent (such as N-methylpyrrolidone), let it stand for one day and reserve it; use a gel permeation chromatograph (such as Waters 2695 Isocratic HPLC type, differential refractive index detector 2141) for detection, the chromatographic column is oily: Styragel HT5 DMF7.8*300mm + Styragel HT4, the injection volume is 5 ml, and a 3.0% by mass polystyrene colloidal solution sample is used as a reference. The gel permeation chromatograph can calculate the weight-average molecular weight of the polymer to be measured based on the weight-average molecular weight of the reference sample.

[0084] In some embodiments, the thickness of the polymer layer is 0.2 μm - 5 μm, such as 0.2 μm, 0.5μm, 0.8μm, 1μm, 2μm, 3μm, 4μm, 5μm or the range composed of any of the above values.

[0085] In some embodiments, the thickness of the polymer layer is 0.5 μm - 3 μm.

[0086] Thus, on the one hand, the above thickness range of the polymer layer can cover the burrs at the edges of the positive current collector and the positive active layer and the burrs in the non-edge area of the positive active layer to a greater extent, so as to reduce the damage of the burrs to the separator and improve the cycle performance and safety performance of the battery monomer. On the other hand, an increase in the thickness of the polymer layer may make the lithium ion transmission distance farther, thus affecting the fast charging performance of the battery monomer, and an increase in the thickness of the polymer layer may also affect the energy density of the battery monomer. The above thickness range reduces these effects.

[0087] In this application, the thickness of the polymer layer is measured by conventional methods in the art; for example, disassemble the battery monomer, take out the separator, cut it along the thickness direction of the separator, and use a scanning electron microscope to observe the cross-section along the thickness direction, so as to distinguish the base layer, inorganic material layer and polymer layer of the separator, and thus directly measure the thickness of the polymer layer; it is also possible to measure the thickness of the polymer layer at different sites multiple times, and then take the average value as the thickness of the polymer layer to improve the measurement accuracy.

[0088] In some embodiments, the polymer layer is located on the side of the base layer facing the positive electrode sheet, which means that the polymer layer can cover the edge and non-edge areas of the side of the base layer facing the positive electrode sheet.

[0089] In some embodiments, the ratio of the area of the polymer layer to the area of the base layer is 65% - 100% or 70% - 100% or 70% - 99%, such as 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99%, 100% or a range composed of any of the above values.

[0090] Thus, it is beneficial for the burrs at the edges of the positive current collector and the positive active layer and the burrs in the non-edge regions of the positive active layer to be covered and wrapped to a greater extent, thereby reducing the risk of the burrs piercing the separator, improving the cycling performance and safety performance of the battery cell, and at the same time being beneficial for the firm adhesion between the electrode sheets and the separator; and when the polymer layer partially covers the base layer in the above ratio, it is beneficial to form a gap between the positive electrode sheet and the separator membrane, which helps the electrolyte to penetrate and improve the lithium ion transport effect.

[0091] In some embodiments, the fluoropolymer includes one or more of polyvinylidene fluoride, vinylidene fluoride-trifluorochloroethylene copolymer, vinylidene fluoride-trifluorochloroethylene-tetrafluoroethylene copolymer, vinylidene fluoride-trifluorochloroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-trifluorochloroethylene-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-acrylonitrile copolymer, and vinylidene fluoride-acrylate copolymer.

[0092] Thus, the above-mentioned fluoropolymer can melt and wrap the burrs at the edges of the positive current collector and the positive active layer and the burrs in the non-edge regions of the positive active layer, reducing the risk of the burrs piercing the separator, thereby improving the cycling performance and safety performance of the battery cell.

[0093] In this application, the structure of the fluoropolymer is confirmed by conventional methods in the art. For example, disassemble the battery cell, take out the separator, cut a 0.5 cm × 0.5 cm separator sample, and test it with a scanning electron microscope & energy spectrometer (equipment model Sigma300). According to JY / T010-1996, it can be confirmed whether fluorine elements are contained; scrape the material on the side of the separator facing the positive electrode sheet (excluding the material of the base film and the inorganic material layer), and test the material with an infrared spectrometer (model: IS10) according to the national standard GB / T6040-2002. The main characteristic peaks in the polymer can be confirmed through the characteristic peaks of fluorocarbon bonds (the symmetric stretching vibration peak of CF2, the asymmetric stretching vibration peak of CF2, the bending vibration peak of CF2, and the torsional vibration peak of CF2 are located at about 1140 cm -1 or so, 1210 cm -1 or so, 650 cm -1 or so, and 540 cm -1 or so, and the C-F stretching vibration peak is located at about 1200 cm-1) and the characteristic peaks of carbon-hydrogen bonds, and combined with nuclear magnetic resonance (such as 1 H NMR,13 13C NMR and 19 19F NMR) to identify the repeating unit structure of the polymer, thereby confirming the structure of the fluoropolymer.

[0094] In some embodiments, the separator further includes an inorganic material layer located between the base layer and the polymer layer.

[0095] Thus, the inorganic material layer has high mechanical strength and strong thermal shrinkage resistance, which can reduce the risk of burrs at the edges of the positive electrode current collector and the positive electrode active layer and burrs in the non-edge region of the positive electrode active layer piercing the separator during high-temperature cycling, improving the cycling performance and safety performance of the battery cell. Moreover, the inorganic material layer has a high electrolyte infiltration rate, which can reduce impedance and is beneficial to the transport of lithium ions.

[0096] In some embodiments, the inorganic material layer includes one or more inorganic materials such as alumina, boehmite, titanium oxide, and silicon dioxide.

[0097] In this application, the specific substance of the inorganic material is confirmed by conventional methods in the art. For example, disassemble the battery cell, take out the separator, scrape off the polymer on the side of the separator facing the positive electrode sheet, and then take the powder of the inorganic material layer (additives such as binders do not produce interfering diffraction peaks). Through XRD testing, the obtained XRD pattern is compared with the standard PDF card to confirm the specific substance of the inorganic material.

[0098] In some embodiments, the average particle size of the positive electrode active material is 0.35 μm - 2.1 μm, such as 0.35 μm, 0.5 μm, 0.6 μm, 0.8 μm, 1 μm, 1.3 μm, 1.5 μm, 1.7 μm, 2 μm, 2.1 μm, or a range composed of any of the above values.

[0099] In some embodiments, the average particle size of the positive electrode active material is 0.5 μm - 1.5 μm.

[0100] Thus, on the one hand, the average particle size of the positive electrode active material not exceeding the upper limit is beneficial to reducing the number of burrs at the edges and non-edge regions of the positive electrode active layer, reducing the risk of burrs piercing the separator, improving the cycling performance and safety performance of the battery cell, and at the same time being beneficial to improving the energy density of the battery cell; on the other hand, reducing the average particle size of the positive electrode active material may cause the positive electrode active material particles to pass through the pores of the separator, affecting the self-discharge performance and safety performance of the battery cell. The above average particle size range reduces this effect.

[0101] In this application, the average particle size of the positive electrode active material is measured by conventional methods in the art. For example, disassemble the battery cell, take out the positive electrode plate, cut it into pieces of 6 mm × 6 mm, and cut the positive electrode plate along the thickness direction of the electrode plate by an argon ion beam (as an example, the equipment model can be selected: Leica EM TIC 3X CP, working voltage: 6 kV, working duration: 6 h). After exposing the cut surface, use a scanning electron microscope (as an example, the equipment model can be selected: Hitachi SU8230, working voltage: 3 kV, beam current: high, probe model: U(LA100), working distance < 5 mm) to observe the cut surface of the positive electrode active layer along the thickness direction of the electrode plate. Collect images of the non-edge positions on the cut surface of the positive electrode active layer through the secondary electron mode using a field emission scanning electron microscope, and take a number of electron microscope images at a certain magnification (for example, 10,000 times magnification). Analyze the particles in the electron microscope images using Avizo software to obtain the particle size information of "EqDiameter (nm)". Since additives such as conductive agents are usually less than 50 nm, particles with "EqDiameter (nm)" less than 50 nm should be removed to avoid interference when calculating the average particle size. At the same time, to ensure the accuracy of the data, the number of particles counted should be ≥ 5000. Summing up and averaging the "EqDiameter (nm)" parameters of the positive electrode active material particles counted will obtain the average particle size.

[0102] In some embodiments, the areal density of the positive electrode active layer is 0.15 mg / mm 2 - 0.4 mg / mm 2 , for example 0.15 mg / mm 2 , 0.2 mg / mm 2 , 0.276 mg / mm 2 , 0.3 mg / mm 2 , 0.364 mg / mm 2 , 0.4 mg / mm 2 or a range composed of any of the above values.

[0103] Thus, on the one hand, the areal density of the positive electrode active layer not being lower than the lower limit is beneficial to improving the energy density of the battery cell. On the other hand, an increase in the areal density of the positive electrode active layer leads to an increase in the thickness of the positive electrode active layer, which may easily cause edge burrs during the processing. The above areal density range of the positive electrode active layer reduces this influence.

[0104] In this application, the tap density and surface density of the positive electrode active layer can be measured by conventional methods in the art. For example, disassemble the battery cell, take out the positive electrode plate, clean and dry it with an organic solvent, cut a positive electrode plate with a fixed area and weigh it, and weigh the positive electrode current collector with the same area in advance; subtract the weight of the positive electrode current collector from the weight of the positive electrode plate, and then divide by the fixed area to obtain the surface density of the positive electrode active layer; then measure the average thickness of the positive electrode active layer on the positive electrode plate through a scanning electron microscope, and divide the surface density of the positive electrode active layer by the average thickness of the positive electrode active layer to obtain the tap density of the positive electrode active layer.

[0105] In some embodiments, the surface of the positive electrode active material has a carbon-containing coating layer. When the average primary particle size of the positive electrode active material becomes smaller and the coating layer thickness remains unchanged, the carbon coating layer content in the positive electrode active material increases, resulting in the positive electrode active material particles being less likely to be compacted, causing the tap density of the positive electrode active layer to decrease; on the contrary, when the average primary particle size of the positive electrode active material becomes larger and the coating layer thickness remains unchanged, the positive electrode active material particles are more likely to be compacted, causing the tap density of the positive electrode active layer to increase.

[0106] In some embodiments, the positive electrode active material includes lithium iron phosphate without doped elements or lithium iron phosphate doped with elements, wherein the doped elements include one or more of Ti, Zr, Mn, Co, and V.

[0107] Therefore, among the commonly used positive electrode active materials, the particle size of the lithium iron phosphate material is relatively small, and the number of burrs at the edge and non-edge of the positive electrode active layer is less, reducing the risk of burrs piercing the separator and improving the cycle performance and safety performance of the battery cell.

[0108] In some embodiments, the mass content of the doped elements in the doped lithium iron phosphate is 500 - 5000 ppm, such as 500 ppm, 800 ppm, 900 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm or a range composed of any of the above values.

[0109] Therefore, the above range of doped element content is beneficial to improving the lithium ion diffusion coefficient of the positive electrode active layer and enhancing the discharge power of the battery cell.

[0110] In this application, conventional methods in the field are used to confirm the content of the positive electrode active material and the doping elements therein. For example, a battery cell is placed at 25°C for 30 minutes, then discharged at 0.33C to 2.0V to obtain a fully discharged battery cell. The battery cell is disassembled, the positive electrode sheet is removed, and the material on the surface of the positive electrode current collector is scraped. The material is subjected to XRD testing. The obtained XRD pattern is compared with a standard PDF card to confirm the specific substance of the positive electrode active material; the scraped material is placed in an appropriate amount of concentrated nitric acid solvent and digested using a plate digestion method, and the digested solution is then diluted with a 7% by volume hydrochloric acid solution. The resulting solution is then fixed to volume and tested using an inductively coupled plasma optical emission spectrometer (ICP=OES). The mass content of the doping element is calculated based on the test results.

[0111] Another embodiment of the present application provides a method for preparing a battery cell, comprising the following steps: Arrange the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and perform hot pressing at 80°C to 150°C (e.g., 80°C, 90°C, 100°C, 110°C, 120°C, 130°C, 140°C, 150°C, or any range thereof), with a hot pressing pressure of 2 kN to 8 kN (e.g., 2 kN, 3 kN, 4 kN, 5 kN, 6 kN, 7 kN, 8 kN, or any range thereof), to obtain an electrode assembly, and use the electrode assembly to prepare a battery cell; wherein, The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector, wherein the positive electrode active layer includes a positive electrode active material; the compaction density of the positive electrode active layer is 2.3 g / cm 3 - 3.35 g / cm 3 ; The diaphragm includes a base layer and a polymer layer located on the side of the base layer facing the positive electrode plate, and the polymer layer includes a fluorine-containing polymer with a crystallinity less than or equal to 50%.

[0112] Therefore, hot pressing under the above conditions can enable the fluorine-containing polymer with a crystallinity of less than or equal to 50% to melt and cover the burrs on the edges of the positive electrode current collector and the positive electrode active layer and the non-edge burrs of the positive electrode active layer. At the same time, limiting the compaction density of the positive electrode active layer to within the above range is beneficial to reducing the number of burrs on the edges and non-edge areas of the positive electrode active layer and improving the structural stability of the positive electrode sheet, so as to reduce the risk of burrs piercing the diaphragm and causing a short circuit, thereby improving the cycle performance and safety performance of the battery cell.

[0113] In some embodiments, the hot pressing time is 40 s - 180 s or 50 s - 150 s, such as 40 s, 50 s, 60 s, 70 s, 80 s, 90 s, 100 s, 120 s, 130 s, 140 s, 150 s, 160 s, 170 s, 180 s, or a range composed of any of the above values.

[0114] In some embodiments, the battery cell is as described above.

[0115] In some embodiments, the separator further includes an inorganic material layer, and the inorganic material layer is located between the base layer and the polymer layer.

[0116] In some embodiments, the polymer layer is prepared by the following steps: Mix the fluoropolymer with an oily solvent to obtain a polymer slurry; Coat the polymer slurry on the side of the base layer or the inorganic material layer facing the positive electrode sheet, and dry it to obtain the polymer layer.

[0117] Thus, using an oily solvent when formulating the polymer slurry can make the coverage of the slurry on the base layer or the inorganic material layer higher, so as to obtain a polymer layer with a higher coverage, which is beneficial to reducing the risk of the burrs on the positive electrode sheet piercing the separator, and improving the cycle performance and safety performance of the battery cell.

[0118] In some embodiments, the oily solvent includes one or more of N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), dimethylformamide (DMF), and dimethylacetamide (DMAc).

[0119]

Separator

[0120] In some embodiments, the thickness of the base layer is 5μm - 12μm, such as 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, or a range composed of any of the above values.

[0121] Thus, on the one hand, the above base layer thickness range can improve the liquid retention capacity of the battery cell through the micropores of the base layer and their capillary action, reduce the impedance of the battery cell, and also improve the mechanical strength of the separator and the rate performance and cycle performance of the battery cell. On the other hand, an increase in the base layer thickness may increase the resistance to lithium ion transmission and affect the rate performance of the battery cell, and the above range reduces this influence.

[0122] In this application, the thickness test method of the base layer is similar to the aforementioned polymer layer thickness test method.

[0123] In some embodiments, the porosity of the separator is 19% - 50%, such as 19%, 19.9%, 20%, 24%, 26%, 26.6%, 28%, 29.8%, 30%, 31.4%, 33%, 35%, 35.8%, 37%, 37.7%, 39%, 40%, 40.2%, 41%, 44%, 46%, 48%, 50% or a range composed of any of the above values.

[0124] Thus, on the one hand, the porosity of the separator not being lower than the above lower limit is beneficial to improving the liquid-phase transport of the battery cell and enhancing the discharge performance of the battery cell. On the other hand, an increase in the porosity of the separator may lead to a weakening of the insulating effect of the separator, affecting the safety of the battery cell. The above range reduces this impact.

[0125] In this application, the porosity of the separator is measured by conventional methods in the art. For example, disassemble the battery cell, take out the separator, soak it in a solvent (such as dimethyl carbonate) for 20 minutes, then rinse it twice, and after drying, use a fully automatic mercury intrusion porosimeter (instrument model: Quantachrome PoreMaster-33) to measure the porosity of the separator.

[0126] As an example, the main material of the base layer can be selected from at least one of glass fiber, non-woven fabric, polyethylene, and polypropylene. The base layer can be a single-layer film or a multi-layer composite film, without particular limitation. The separator can be a single component located between the positive and negative electrodes or attached to the surfaces of the positive and negative electrodes.

[0127] In some embodiments, the positive electrode sheet, negative electrode sheet, and separator film can be made into an electrode assembly by a winding process or a stacking process.

[0128]

Positive Electrode Sheet

[0129] As an example, the positive current collector can be made of a metal foil, a conductive polymer material, a carbon material, or a composite current collector. For example, as a metal foil, pure metals, alloys, and metals with surface treatment can be used, including but not limited to stainless steel, copper, aluminum, nickel, nickel, titanium, or silver, etc. The composite current collector can include a polymer material base layer and a metal layer. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polyethylene, etc.).

[0130] As an example, the positive electrode active material may further include at least one of the following materials: lithium-containing phosphates, lithium transition metal oxides, and their respective modified compounds. Among them, examples of the lithium-containing phosphates may include, but are not limited to, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and at least one of a composite material of lithium iron manganese phosphate and carbon. Examples of the lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (which can also be abbreviated as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be abbreviated as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (which can also be abbreviated as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (which can also be abbreviated as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (which can also be abbreviated as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.8 Co 0.15 Al 0.05 O2) and at least one of their modified compounds, etc. The modified compound refers to a substance obtained by modification means such as doping or coating on the basis of the above substances. However, the present application is not limited to these materials, and other conventional materials that can be used as the positive electrode material of the battery can also be used. These positive electrode materials can be used alone or in combination of two or more.

[0131] During the charge and discharge process of the battery, the deintercalation and consumption of Li will occur, and the molar content of Li is different when the battery is discharged to different states. In the listing of the positive electrode material in the present application, the molar content of Li is the initial state of the material, that is, the state before feeding. When the positive electrode material is applied to the battery system and undergoes charge and discharge cycles, the molar content of Li will change.

[0132] In the listing of the positive electrode material in the present application, the molar content of O is only the theoretical state value, and the release of oxygen from the lattice will cause the molar content of oxygen to change, and the actual molar content of O will fluctuate.

[0133] In some embodiments, the polymeric cathode binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride - tetrafluoroethylene - propylene terpolymer, vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene terpolymer, tetrafluoroethylene - hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0134] In some embodiments, the cathode conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0135] In some embodiments, the cathode active layer further includes a cathode conductive agent, and the cathode conductive agent includes single - walled carbon nanotubes.

[0136] Thus, through the entanglement effect of single - walled carbon nanotubes, the cathode active material particles can be entangled and anchored on the cathode current collector, and the single - walled carbon nanotubes can buffer the volume change of the cathode active material particles during cycling to reduce particle shedding. Thereby, it is beneficial for the cathode active material particles to adhere to the cathode current collector and beneficial for improving the electron conductivity to enhance the discharge power of the battery cell.

[0137] In some embodiments, the cathode active layer further includes a polymeric cathode binder, and the weight - average molecular weight of the polymeric cathode binder is greater than or equal to the weight - average molecular weight of the fluorinated polymer.

[0138] Thus, it is beneficial to firmly adhere the cathode active material particles to the cathode current collector through the polymeric cathode binder to reduce the loss of the cathode active material, thereby enhancing the cycle life of the battery cell.

[0139] In this application, the test method for the weight - average molecular weight of the polymeric cathode binder is similar to the test method for the weight - average molecular weight of the fluorinated polymer.

[0140] In some embodiments, the cathode electrode sheet can be prepared in the following manner: Dispersing the above - mentioned components for preparing the cathode electrode sheet, such as the cathode active material, the cathode conductive agent, the cathode binder, and any other components, in a solvent (such as N - methylpyrrolidone) to form a cathode slurry; Coating the cathode slurry on the cathode current collector, and after processes such as drying and cold pressing, the cathode electrode sheet can be obtained.

[0141]

Negative electrode sheet

[0142] As an example, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0143] As an example, the negative electrode active material may further include at least one of the following materials: natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based materials may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based materials may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other conventional materials that can be used as the negative electrode active material of the battery cell can also be used. These negative electrode active materials can be used alone or in combination of two or more.

[0144] In some embodiments, the negative electrode active material includes a graphite material; and / or, the negative electrode active material includes artificial graphite material.

[0145] Therefore, using a graphite material in the negative electrode active material is beneficial to improving the cycle life of the battery cell.

[0146] In some embodiments, the graphitization degree of the negative electrode active material is 90% - 94%, such as 90%, 92%, 92.6%, 93%, 94%, or a range composed of any of the above values.

[0147] Thus, on the one hand, the graphitization degree of the negative electrode active material not being lower than the above lower limit is beneficial to improving the rate performance, energy density, and cycle life of the battery cell; on the other hand, an increase in the graphitization degree of the negative electrode active material may lead to a decrease in the capacity of the battery cell and a deterioration in the fast charging performance, and the above range reduces this influence.

[0148] In this application, the graphitization degree of the negative electrode active material is tested by conventional methods in the art. For example, the battery cell is disassembled, the negative electrode plate is taken out, the material on the negative electrode current collector is scraped off, and sintered at a certain temperature for a period of time (for example, sintered at 250 °C for 3 hours) to remove additives such as binders and SEI film substances, obtaining the negative electrode active material. Refer to Appendix E in the national standard GB / T 24533-2019 "Graphite-based Negative Electrode Materials for Lithium-ion Batteries" to test the negative electrode active material. Artificial graphite is also applicable to the Chinese mechanical industry standard JB / T 4220-2011 "Determination Method for Lattice Parameters of Artificial Graphite"; specific test steps: The graphitization degree G of the negative electrode active material uses an XRD diffractometer to automatically record the diffraction line patterns of the 002, 004, 110, and 112 of carbon, and at the same time reads the diffraction angle (2θobs)c, and obtains the corrected diffraction angle (2θcor)c by the internal standard method, and substitutes it into the crystal plane spacing formula to calculate the data of the graphite layer spacing d002; substitute the d002 data value into the Mering-Maire formula: G = [(3.440 - d002) / (3.440 - 3.354)] × 100%, and obtain the graphitization degree value G.

[0149] In some embodiments, the average particle size of the negative electrode active material is 8 μm - 20 μm, such as 8 μm, 8.7 μm, 9 μm, 10 μm, 11 μm, 13 μm, 15 μm, 17 μm, 18 μm, 19 μm, 20 μm or the range composed of any of the above values.

[0150] Thus, on the one hand, the average particle size of the negative electrode active material not being lower than the above lower limit is beneficial to reducing the specific surface area of the negative electrode active material, reducing the contact area between the negative electrode active material and the electrolyte, reducing side reactions, and thus improving the cycle life of the battery cell. On the other hand, an increase in the average particle size of the negative electrode active material may lead to gelation of the negative electrode slurry, making processing difficult, and may also lead to poor fast charging performance of the battery cell. The above range reduces this influence.

[0151] In this application, the test method for the average particle size of the negative electrode active material is similar to the test method for the average particle size of the positive electrode active material.

[0152] As an example, the negative electrode material can be filled or / and deposited in the negative electrode current collector.

[0153] In some embodiments, the material of the positive electrode current collector can be aluminum, and the material of the negative electrode current collector can be copper.

[0154] In some embodiments, the anode material may further optionally include an anode binder. As an example, the binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0155] In some embodiments, the anode material may further optionally include an anode conductive agent. As an example, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0156] In some embodiments, the anode material may further optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)), etc.

[0157] In some embodiments, the anode electrode sheet can be prepared in the following manner: Dispersing the above components for preparing the anode electrode sheet, such as the anode active material, anode conductive agent, anode binder, and any other components, in a solvent (e.g., deionized water) to form an anode slurry; Coating the anode slurry on the anode current collector, and after processes such as drying and cold pressing, the anode electrode sheet can be obtained.

[0158]

Electrolyte

[0159] Among them, the liquid electrolyte includes an electrolyte salt and a solvent.

[0160] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium bis(oxalate) borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0161] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone. The solvent may also be an ether solvent. The ether solvent may include one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, tetrahydrofuran, methyltetrahydrofuran, diphenyl ether and crown ether.

[0162] In some embodiments, the electrolyte further includes a solvent, and the solvent includes a cyclic carbonate solvent. The mass of the cyclic carbonate solvent is 20% - 40% of the mass of the solvent, such as 20%, 25%, 30%, 34%, 36%, 40% or the range composed of any of the above values.

[0163] Thus, on the one hand, the content of the cyclic carbonate solvent not being lower than the above lower limit is beneficial to improving the cycle life of the battery cell. On the other hand, the content of the cyclic carbonate solvent not exceeding the above upper limit is beneficial to reducing the viscosity of the electrolyte in a low-temperature environment, thereby improving the low-temperature charge and discharge power of the battery cell.

[0164] In some embodiments, the cyclic carbonate solvent includes one or more of ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (γ-GBL).

[0165] In some embodiments, the electrolyte may further optionally include an additive. For example, the additive may include a negative electrode film-forming additive, or may include a positive electrode film-forming additive, or may also include an additive capable of improving certain properties of the battery cell, such as an additive for improving the overcharge / quick charge performance of the battery cell, an additive for improving the high-temperature performance of the battery cell, an additive for improving the low-temperature performance of the battery cell, etc.

[0166] In some embodiments, the battery cell further includes an electrolyte, and the electrolyte includes an electrolyte additive. The electrolyte additive includes an unsaturated carbonate additive and / or a sulfonate additive. Thus, adopting the above electrolyte additive is beneficial to improving the cycle life of the battery cell.

[0167] In some embodiments, the unsaturated carbonate additive includes one or more of vinylene carbonate, fluoroethylene carbonate, vinyl vinylene carbonate.

[0168] In some embodiments, the sulfonate additives include one or more of 1,3 - propane sultone, 1,4 - butane sultone, methylene methanedisulfonate, ethylene sulfate, and ethyl sulfite.

[0169] Among them, the gel - state electrolyte includes a polymer as the backbone network and can be used in combination with an ionic liquid - lithium salt.

[0170]

Structure of the electrode assembly

[0171] In some embodiments, the electrode assembly is a wound structure. The positive electrode sheet and the negative electrode sheet are wound into a wound structure.

[0172] In some embodiments, the electrode assembly is cuboid - shaped, and the ratio of the length to the width of the electrode assembly is 2 - 8, such as 2, 3, 4, 5, 6, 7, 8, or the range composed of any of the above values.

[0173] Since the crystallinity of the fluoropolymer in the polymer layer is less than or equal to 50%, the mechanical strength of the electrode assembly formed by bonding the separator with the positive electrode sheet through the polymer layer is reduced. Using the above - mentioned ratio of the length to the width of the electrode assembly is more convenient for loading and unloading the electrode assembly for subsequent processing.

[0174] In some embodiments, the cuboid - shaped electrode assembly has three directions: length, width, and thickness. And, the dimension in the length direction > the dimension in the width direction > the dimension in the thickness direction. Therefore, the length dimension and the width dimension of the electrode assembly can be determined according to the size differences in the three directions of the electrode assembly.

[0175] In some embodiments, the electrode assembly is a stacked structure.

[0176] As an example, multiple positive electrode sheets and multiple negative electrode sheets can be respectively provided, and the multiple positive electrode sheets and the multiple negative electrode sheets are alternately stacked.

[0177] As an example, multiple positive electrode sheets can be provided, and the negative electrode sheet is folded to form multiple stacked folding segments, and a positive electrode sheet is clamped between adjacent folding segments.

[0178] As an example, both the positive electrode sheet and the negative electrode sheet are folded to form multiple stacked folding segments.

[0179] As an example, multiple separators can be provided and are respectively arranged between any adjacent positive electrode sheet or negative electrode sheet.

[0180] As an example, the separators can be continuously arranged and are arranged between any adjacent positive electrode sheet or negative electrode sheet by folding or winding.

[0181] In some embodiments, the shape of the electrode assembly can be cylindrical, flat, prismatic, or the like.

[0182] In some embodiments, the electrode assembly is provided with tabs, and the tabs can conduct the current out of the electrode assembly. The tabs include a positive tab and a negative tab.

[0183]

Housing

[0184] As an example, the battery cell can be a cylindrical battery cell, a prismatic battery cell, a soft-pack battery cell, or a battery cell of other shapes. The prismatic battery cell includes a square-shell battery cell, a blade-shaped battery cell, a multi-prismatic battery, and the multi-prismatic battery is, for example, a hexagonal-prismatic battery, etc., and there is no particular limitation in this application.

[0185] In some embodiments, the housing includes an end cap and a housing body. The housing body is provided with an opening, and the end cap covers the opening. The housing body can be provided with one or more openings. One or more end caps can also be provided.

[0186]

Electrode Terminal

[0187]

Pressure Relief Mechanism

[0188] As an example, it is actuated to release the internal pressure or temperature when the internal pressure or temperature of the battery cell reaches a predetermined threshold. When the internal pressure or temperature of the battery cell reaches a predetermined threshold, the pressure relief mechanism performs an action or a weak structure provided in the pressure relief mechanism is damaged, thereby forming an opening or a channel for the internal pressure or temperature to be released. This threshold design varies according to different design requirements. The threshold may depend on one or several of the materials of the positive electrode sheet, negative electrode sheet, electrolyte, and separator in the battery cell.

[0189] As an example, the pressure relief mechanism can be integrally formed with the housing.

[0190] As an example, the pressure relief mechanism can also be separately provided and connected to the housing.

[0191] The "actuation" mentioned in this application refers to the pressure relief mechanism generating an action or being activated to a certain state, so that the internal pressure and temperature of the battery cell can be relieved. The actions generated by the pressure relief mechanism can include but are not limited to: components in the pressure relief mechanism moving to form an exhaust passage, at least a part of the pressure relief mechanism breaking, shattering, being torn or opened, etc. When the pressure relief mechanism is actuated, the high-temperature and high-pressure substances inside the battery cell will be discharged outward from the actuated part as emissions. In this way, the battery cell can be depressurized and cooled under controlled pressure or temperature, thus avoiding potential more serious accidents.

[0192] In some embodiments, when the housing is a non-sealed structure, the pressure relief mechanism can be set as a through hole for discharging the gas inside the battery cell.

[0193] The emissions from the battery cell mentioned in this application include but are not limited to: electrolytes, dissolved or split positive and negative electrode plates, fragments of separators, high-temperature and high-pressure gases generated by reactions, flames, etc.

[0194]

Battery Apparatus

[0195] In some embodiments, the battery cell assembly is usually formed by arranging a plurality of battery cells.

[0196] As an example, the battery cell assembly can be a battery module, and the battery module is formed by arranging and fixing a plurality of battery cells to form an independent module. As an example, the battery module can be formed by bundling a plurality of battery cells with cable ties.

[0197] In some embodiments, the battery apparatus can be a battery pack, and the battery pack includes a box body and one or more battery cell assemblies, and the battery cell assemblies are accommodated in the box body.

[0198] As an example, the battery cell assembly can be a battery module, and the battery cell assembly can be accommodated in the box body by fixing the battery module in the box body.

[0199] As an example, the battery cell assembly can also be accommodated in the box body by directly fixing a plurality of battery cells to the box body.

[0200] As an example, the box body can include a first box body and a second box body. The first box body and the second box body are buckled together so that a closed space is formed inside the box body to accommodate the battery cell assembly. Here, "closed" means covering or closing, which can be sealed or non-sealed. The first box body can be a top cover or a bottom plate.

[0201] As an example, the box body can include a top cover, a frame, and a bottom plate. The top cover and the bottom plate are respectively connected to the frame so that a closed space is formed inside the box body to accommodate the battery cell assembly.

[0202] In some embodiments, the box body can be part of the chassis structure of a vehicle. For example, part of the box body can become at least part of the floor of the vehicle, or part of the box body can become at least part of the cross beam and longitudinal beam of the vehicle.

[0203] The technical solutions described in the embodiments of the present application are applicable to various electrical devices using battery cells, such as mobile phones, portable devices, laptop computers, battery cars, electric toys, electric tools, vehicles, ships, and spacecrafts, etc. For example, spacecrafts include airplanes, rockets, space shuttles, and spaceships, etc.

[0204] For example, Figure 1 is a battery cell 5 with a square structure as an example.

[0205] In some embodiments, with reference to Figure 2 , the outer package can include a housing 51 and a cover plate 53. Among them, the housing 51 can include a bottom plate and side plates connected to the bottom plate. The bottom plate and the side plates enclose to form a containing cavity. The housing 51 has an opening communicating with the containing cavity, and the cover plate 53 can be covered on the opening to close the containing cavity. The positive electrode plate, the negative electrode plate, and the separator can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the containing cavity. The electrolyte is infiltrated in the electrode assembly 52. The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0206] In some embodiments, the battery cells can be assembled into a battery module. The number of battery cells contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0207] Figure 3 is a battery module 4 as an example. With reference to Figure 3, in the battery module 4, multiple battery cells 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other arbitrary manner. Further, the multiple battery cells 5 can be fixed by fasteners.

[0208] Optionally, the battery module 4 can further include a housing having an accommodation space, and the multiple battery cells 5 are accommodated in the accommodation space.

[0209] In some embodiments, the above battery module can also be assembled into a battery pack. The number of battery modules included in the battery pack can be one or more. Those skilled in the art can select the specific number according to the application and capacity of the battery pack.

[0210] Figure 4 and Figure 5 is the battery pack 1 as an example. Refer to Figure 4 and Figure 5 , in the battery pack 1, it can include a battery box and multiple battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in the battery box in any manner.

[0211] In addition, the present application also provides an electrical device. The electrical device includes at least one of the battery cell, battery module, or battery pack provided by the present application. The battery cell, battery module, or battery pack can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device can include mobile devices (such as mobile phones, laptop computers, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc., but not limited thereto.

[0212] As the electrical device, the battery cell, battery module, or battery pack can be selected according to its usage requirements.

[0213] Figure 6 is the electrical device as an example. The electrical device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the electrical device for the battery cell, a battery pack or a battery module can be adopted.

Embodiment

[0214] Hereinafter, embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. For those without specific technical or conditions noted in the embodiments, the techniques or conditions described in the literature in the art or according to the product specifications are followed. For the reagents or instruments without the manufacturer noted, they are all conventional products that can be obtained through commercial purchase.

[0215] Example 1 (1) Preparation of the positive electrode sheet: The lithium iron phosphate positive electrode active material with an average particle size of 0.8 μm and doped with Ti (the mass content of Ti element in lithium iron phosphate is 800 ppm and it has a carbon coating layer on its surface), polyvinylidene fluoride (PVDF) binder (weight average molecular weight of 7 million), single-walled carbon nanotube conductive agent and polyvinylpyrrolidone surfactant are mixed at a mass ratio of 97.2:1.8:0.4:0.6, and then a certain amount of solvent N-methylpyrrolidone (NMP) is added and stirred into a uniform slurry. The positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil, and after drying, cold pressing and slitting, the positive electrode sheet is prepared. The positive electrode sheet includes a positive electrode current collector and positive electrode active layers on both sides of the positive electrode current collector. The areal density of the positive electrode active layer is 0.364 mg / mm 2 , and the tap density is 2.55 g / cm 3 .

[0216] (2) Preparation of the separator: Prepare a polyethylene base layer with a thickness of 7 μm; the inorganic material boehmite, binder polyvinylidene fluoride (weight average molecular weight of 400,000), acrylic acid emulsion (concentration 35% by mass) and deionized water are mixed evenly according to a mass ratio of 40%, 1.8%, 4.7% and 54.5% to form an inorganic slurry. The inorganic slurry is coated on one side of the base layer facing the positive electrode sheet by slot coating, and after drying, an inorganic material layer loaded on one side of the base layer facing the positive electrode sheet is obtained, and the thickness of the inorganic material layer is 1 μm; Polyvinylidene fluoride (PVDF) with a crystallinity of 25% and a weight average molecular weight of 400,000 is mixed with the oily solvent N-methylpyrrolidone (NMP) to prepare a polymer slurry. The polymer slurry is coated on one side of the inorganic material layer facing the positive electrode sheet by slot coating, and after drying and slitting, the separator is obtained. The thickness of the polymer layer loaded on one side of the inorganic material layer facing the positive electrode sheet in the separator is 1 μm, the porosity of the separator is 37.7%, and the ratio of the area of the polymer layer to the area of the base layer is 87%.

[0217] (3) Preparation of the negative electrode sheet: The artificial graphite anode active material, the anode binder styrene-butadiene rubber (SBR), the anode thickener sodium carboxymethyl cellulose (CMC-Na), and the anode conductive agent carbon black (Super P) are fully stirred and mixed in deionized water as a solvent in a mass ratio of 96:1.5:0.5:2 to form a uniform anode slurry; the anode slurry is coated on the surface of the anode current collector copper foil, and the anode electrode sheet is prepared through drying, cold pressing, and slitting. The anode electrode sheet includes an anode current collector and anode active layers on both sides of the anode current collector. The anode active layers include anode active material, and the average particle size of the anode active material is 8.7 μm, and the graphitization degree is 92.6%.

[0218] (4) Preparation of the electrolyte: Ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:1:1 to obtain a non-aqueous solvent, and then the electrolyte salt LiPF6 and the electrolyte additive vinylene carbonate are dissolved in the above non-aqueous solvent to obtain the electrolyte. The concentration of the electrolyte salt LiPF6 in the electrolyte is 1 mol / L, the mass content of the additive vinylene carbonate in the electrolyte is 5%, and the mass content of ethylene carbonate (EC) in the solvent is 34%.

[0219] (5) Fabrication of the battery cell: The above-mentioned positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, and the three are bonded by hot pressing (hot pressing temperature 95°C, hot pressing pressure 4000 N, hot pressing time 120 s), and the polymer layer of the separator wraps the tip burrs of the edge and non-edge regions of the positive electrode sheet. Finally, the laminated battery cell is obtained through processes such as encapsulation, liquid injection, high-temperature standing, formation aging, and film coating.

[0220] Battery testing

[0221] (1) Energy density test of the battery cell: In a constant temperature environment of 25°C, the battery cell is charged at a constant current of 0.33C to the upper limit voltage of 3.8V, charged at a constant voltage until the current is 0.05C, and after standing for 10 min, it is discharged at a constant current of 0.33C to the lower limit voltage of 2.0V. Record the discharge capacity A0 and the discharge platform voltage V at this time; use a caliper to measure the length, thickness, and height of the battery cell (generally calculated based on the outer shell size of the battery cell, excluding the height of the electrode terminals and excluding the insulating film outside the outer shell), and calculate the volume V0 of the battery cell; the volume energy density VED of the battery cell = (A0 × V) / V0, with the unit Wh / L.

[0222] (2) DC impedance test of the battery cell at 20% SOC (-20°C): Under a constant temperature environment of 25°C, the battery cell is charged at a constant current of 0.33C to the upper limit voltage of 3.8V, then charged at a constant voltage of 3.8V until the current is 0.05C, left standing for 10 minutes, then discharged at a constant current of 0.33C to the lower limit voltage of 2.0V, and the discharge capacity C0 at this time is recorded. Subsequently, it is left standing for 10 minutes, then charged at a constant current of 0.33C to 0.20 C0 (i.e., 20% SOC), left standing for 60 minutes in an environment of -20°C and then discharged at 0.36C0 for 30 seconds. Calculate the DC impedance according to the following formula. The smaller the impedance value, the greater the discharge power and the higher the rate performance; DC impedance = (voltage before 30s of discharge - voltage after 30s of discharge) / discharge current.

[0223] (3)Self-discharge rate test of the battery cell: Under a constant temperature environment of 25°C, the battery cell is charged at a constant current of 0.33C to 3.8V, then charged at a constant voltage of 3.8V until the current is lower than 0.05C, left standing for 10 minutes, then discharged at a constant current of 0.33C to 2.0V to obtain the discharge capacity C0. Subsequently, it is left standing for 10 minutes, then charged at a constant current of 0.33C to 0.3C0 (i.e., 30% SOC), and then left standing for 10 minutes to test the open-circuit voltage of the battery cell as V1, with the unit of V. Then, the battery cell is placed in an oven at 45°C for 48 hours, and the open-circuit voltage of the battery is tested again as V2, with the unit of V. Then, the self-discharge rate of the battery cell = (V1 - V2) × 1000 / 48.

[0224] (4)High-temperature cycle performance test of the battery cell: Under a constant temperature environment of 45°C, the battery cell is charged at a constant current of 0.5C to the upper limit voltage of 3.8V, then charged at a constant voltage until the current ≤ 0.05C, left standing for 5 minutes, and then discharged at a constant current of 1C to the lower limit voltage of 2.0V. This is a cycle of charge and discharge process, and the discharge capacity recorded at this time is the discharge capacity of the first cycle. The battery cell is cycled for charge and discharge according to the above method, and the discharge capacity after each cycle is recorded until the discharge capacity of the battery cell decays to 75% of the discharge capacity of the first cycle, and the number of cycles at this time is recorded.

[0225] Examples 2 - 7 (2)Preparation of the separator: Polyvinylidene fluoride (PVDF) with a crystallinity of 20% and a weight-average molecular weight of 350,000, polyvinylidene fluoride (PVDF) with a crystallinity of 50% and a weight-average molecular weight of 400,000, polyvinylidene fluoride (PVDF) with a crystallinity of 25% and a weight-average molecular weight of 150,000, polyvinylidene fluoride (PVDF) with a crystallinity of 25% and a weight-average molecular weight of 500,000, polyvinylidene fluoride (PVDF) with a crystallinity of 18% and a weight-average molecular weight of 120,000, and polyvinylidene fluoride (PVDF) with a crystallinity of 50% and a weight-average molecular weight of 800,000 were respectively mixed with the oily solvent N-methylpyrrolidone (NMP) to prepare polymer slurries, and the rest was the same as in step (2) of Example 1; (5) Fabrication of battery monomers: The hot pressing conditions used were: hot pressing temperature 95 °C, hot pressing pressure 4000 N, hot pressing time 90 s; hot pressing temperature 95 °C, hot pressing pressure 4000 N, hot pressing time 150 s; hot pressing temperature 95 °C, hot pressing pressure 4000 N, hot pressing time 90 s; hot pressing temperature 95 °C, hot pressing pressure 4000 N, hot pressing time 150 s; hot pressing temperature 95 °C, hot pressing pressure 4000 N, hot pressing time 90 s; hot pressing temperature 105 °C, hot pressing pressure 4000 N, hot pressing time 150 s; and the rest was the same as in step (5) of Example 1; Steps (1), (3)-(4) were the same as steps (1), (3)-(4) of Example 1.

[0226] Comparative Example 1 (2) Preparation of separator: Polyvinylidene fluoride (PVDF) with a crystallinity of 55% and a weight-average molecular weight of 400,000 was mixed with the oily solvent N-methylpyrrolidone (NMP) to prepare a polymer slurry, and the rest was the same as in step (2) of Example 1; (5) Fabrication of battery monomers: The hot pressing conditions used were hot pressing temperature 95 °C, hot pressing pressure 4000 N, hot pressing time 180 s; and the rest was the same as in step (5) of Example 1; Steps (1), (3)-(4) were the same as steps (1), (3)-(4) of Example 1.

[0227] Comparative Example 2 (2) Preparation of separator: Poly(methyl methacrylate) (PMMA) with a crystallinity of 25% and a weight-average molecular weight of 400,000 was mixed with the oily solvent N-methylpyrrolidone (NMP) to prepare a polymer slurry, and the rest was the same as in step (2) of Example 1; Steps (1), (3)-(5) were the same as steps (1), (3)-(5) of Example 1.

[0228] Table 1: Partial parameters and test results of Examples 1-7 and Comparative Examples 1-2 Number Polymer (also known as P) P crystallinity Mw of P Porosity of the separator Energy density (Wh / L) DC impedance at 20% SOC (mΩ) Self-discharge rate (mV / h) Number of cycles at 45°C, 75% Example 1 PVDF 25% 400,000 37.70% 427.6 10.1 0.013 5877 Example 2 Same as Example 1 20% 350,000 35.8% 427.6 9.6 0.014 5548 Example 3 Same as Example 1 50% 400,000 39% 427.6 10.4 0.021 5481 Example 4 Same as Example 1 25% 150,000 35% 427.6 10.2 0.01 5470 Example 5 Same as Example 1 25% 500,000 41% 427.6 10 0.018 5600 Example 6 Same as Example 1 18% 120,000 29.80% 427.6 12.6 0.011 5201 Example 7 Same as Example 1 50% 800,000 37.70% 427.6 9.9 0.022 5344 Comparative Example 1 Same as Example 1 55% Same as Example 1 39.10% 427.6 10.5 0.029 4587 Comparative Example 2 PMMA 25% Same as Example 1 37.70% 427.6 10.6 0.03 4379

[0229] As can be seen from the above table: Compared with Comparative Example 1 where the crystallinity of the fluoropolymer is greater than 50%, the safety performance and cycling performance of the battery monomers of Examples 1-7 of the present application are significantly improved.

[0230] Compared with Comparative Example 2 where the polymer layer uses a non-fluoropolymer, the safety performance and cycling performance of the battery monomers of Examples 1-7 of the present application are significantly improved.

[0231] Compared with Example 6 where the fluoropolymer has a smaller crystallinity and a smaller weight-average molecular weight, the DC impedance of the battery monomers of Examples 1-5 of the present application is significantly lower and the cycling performance is significantly improved.

[0232] Compared with Example 7 where the fluoropolymer has a larger weight-average molecular weight, the safety performance and cycling performance of the battery monomers of Examples 1-5 of the present application are significantly improved.

[0233] Examples 8-11 (2) Preparation of the separator: Adjust the coating amount of the polymer slurry coated on the side of the inorganic material layer facing the positive electrode plate by slit coating so that the thicknesses of the polymer layers are 0.5 μm, 3 μm, 0.2 μm, and 5 μm respectively, and the rest is the same as in step (2) of Example 1; (5) Fabrication of the battery monomer: The hot pressing conditions used are: hot pressing temperature 95°C, hot pressing pressure 4000 N, hot pressing time 90 s; hot pressing temperature 95°C, hot pressing pressure 4000 N, hot pressing time 120 s; hot pressing temperature 95°C, hot pressing pressure 4000 N, hot pressing time 90 s; hot pressing temperature 95°C, hot pressing pressure 4000 N, hot pressing time 150 s; the rest is the same as in step (5) of Example 1; Steps (1), (3)-(4) are the same as steps (1), (3)-(4) of Example 1.

[0234] Examples 12-13 (2) Preparation of the separator: Adjust the coating area of the polymer slurry coated on the side of the inorganic material layer facing the positive electrode plate by slit coating so that the ratio of the polymer layer area to the base layer area is 70% and 100% respectively, and the rest is the same as in step (2) of Example 1; (5) Fabrication of the battery monomer: The hot pressing conditions adopted are as follows: hot pressing temperature 95°C, hot pressing pressure 4000 N, hot pressing time 90 s; hot pressing temperature 95°C, hot pressing pressure 4000 N, hot pressing time 100 s; the rest is the same as that in step (5) of Example 1; Steps (1), (3)-(4) are the same as those in steps (1), (3)-(4) of Example 1.

[0235] Example 14 (2) Preparation of the separator: Polyvinylidene fluoride (PVDF) with a crystallinity of 25% and a weight average molecular weight of 400,000 is mixed with deionized water to prepare a polymer slurry, and the polymer slurry is coated on one side of the inorganic material layer facing the positive electrode plate by slit coating. The rest is the same as that in step (2) of Example 1; Steps (1), (3)-(5) are the same as those in steps (1), (3)-(5) of Example 1.

[0236] Comparative Example 3 (2) Preparation of the separator: The polymer slurry is coated on the four edges of the surface of the inorganic material layer facing the positive electrode plate by slit coating (the middle part of the positive electrode material layer is not coated). The ratio of the total area of the formed polymer layer to the area of the base layer is 45%. The rest is the same as that in step (2) of Example 1; (5) Fabrication of the battery cell: The hot pressing conditions adopted are as follows: hot pressing temperature 95°C, hot pressing pressure 4000 N, hot pressing time 90 s. The rest is the same as that in step (5) of Example 1; Steps (1), (3)-(4) are the same as those in steps (1), (3)-(4) of Example 1.

[0237] Table 2: Partial parameters and test results of Examples 1, 8-14 and Comparative Example 3 Number Thickness of the polymer layer (μm) Area ratio of the polymer layer to the base layer Porosity of the separator Energy density (Wh / L) DC impedance at 20% SOC (mΩ) Self-discharge rate (mV / h) Number of cycles at 45°C, 75% Example 1 1 87% 37.70% 427.6 10.1 0.013 5877 Example 8 0.5 Same as Example 1 40.20% 429.8 10.1 0.023 5289 Example 9 3 Same as Example 1 26.60% 422.6 10.6 0.01 5800 Example 10 0.2 Same as Example 1 41.00% 430.3 10 0.025 4899 Example 11 5 Same as Example 1 26% 419.9 13 0.01 5789 Example 12 Same as Example 1 70% 39% 427.6 9.9 0.016 5679 Example 13 Same as Example 1 100% 31.40% 427.6 10.7 0.011 5919 Example 14 Same as Example 1 65% 37.70% 427.6 9.6 0.022 5702 Comparative Example 3 Same as Example 1 45% 45% 427.6 9.6 0.035 4180

[0238] As can be seen from the above table: Compared with Comparative Example 3 where the polymer layer is only prepared at the edge of the inorganic material layer, the safety performance and cycling performance of the battery cells of Examples 1, 8-14 of the present application are significantly improved.

[0239] Compared with Example 10 where the polymer layer has a relatively thin thickness, the safety performance and cycling performance of the battery cells of Examples 1, 8-9 of the present application are significantly improved.

[0240] Compared with Example 11 where the polymer layer has a relatively thick thickness, the energy density of the battery cells of Examples 1, 8-9 of the present application is significantly increased and the DC impedance is significantly reduced.

[0241] Compared with Example 14 where deionized water was used to prepare the polymer slurry, in Examples 1, 12 - 13 of the present application, the area ratio of the polymer layer to the base layer is larger, and the safety performance and cycling performance of the battery monomer are significantly improved.

[0242] Example 15 (2)Preparation of the separator: Omit the preparation of the inorganic material layer, and the rest is the same as in step (2) of Example 1; the separator porosity is 37%; Steps (1), (3) - (5) are the same as steps (1), (3) - (5) of Example 1.

[0243] Examples 16 - 19 (1)Preparation of the positive electrode sheet: Use lithium iron phosphate positive electrode materials doped with Ti with average particle sizes of 0.5 μm, 1.5 μm, 0.35 μm, and 2.1 μm respectively (the mass content of Ti element in lithium iron phosphate is 800 ppm, and its surface has a carbon coating layer with a constant thickness), and the rest is the same as in step (1) of Example 1; the tap densities of the positive electrode active layers are 2.48 g / cm 3 、2.63 g / cm 3 、2.3 g / cm 3 、2.8 g / cm 3 ; Steps (2) - (5) are the same as steps (2) - (5) of Example 1.

[0244] Example 20 (5)Fabrication of the battery monomer: Adjust the hot pressing conditions to a hot pressing temperature of 95 °C, a hot pressing pressure of 4000 N, and a hot pressing time of 40 s, and the rest is the same as in step (5) of Example 1; the separator porosity is 37.7%; Steps (1) - (4) are the same as steps (1) - (4) of Example 1.

[0245] Example 21 (1)Preparation of the positive electrode sheet: Use the positive electrode active material LiNi 0.5 Co 0.2 Mn 0.3O2 (NCM523), polyvinylidene fluoride (PVDF) binder (weight-average molecular weight of 7 million), single-walled carbon nanotube conductive agent, and polyvinylpyrrolidone surfactant were mixed at a mass ratio of 97.2:1.8:0.4:0.6. Then, a certain amount of solvent N-methylpyrrolidone (NMP) was added and stirred into a uniform slurry. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and after drying, cold pressing, and slitting, the positive electrode sheet was prepared. The positive electrode sheet includes a positive electrode current collector and positive electrode active layers on both sides of the positive electrode current collector. The areal density of the positive electrode active layer is 0.276 mg / mm 2 , and the tap density is 3.35 g / cm 3 .

[0246] Steps (2)-(5) are the same as steps (2)-(5) of Example 1.

[0247] Example 22 Preparation of the positive electrode sheet: Lithium iron phosphate positive electrode active material with an average particle size of 0.8 μm (with a carbon coating layer on its surface), polyvinylidene fluoride (PVDF) binder (weight-average molecular weight of 3 million), Super P conductive agent, and polyvinylpyrrolidone surfactant were mixed at a mass ratio of 97.2:1.8:0.4:0.6. Then, a certain amount of solvent N-methylpyrrolidone (NMP) was added and stirred into a uniform slurry. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and after drying, cold pressing, and slitting, the positive electrode sheet was prepared. The positive electrode sheet includes a positive electrode current collector and positive electrode active layers on both sides of the positive electrode current collector. The areal density of the positive electrode active layer is 0.364 mg / mm 2 , and the tap density is 2.55 g / cm 3 .

[0248] Steps (2)-(5) are the same as steps (2)-(5) of Example 1.

[0249] Example 23 Preparation of the separator: The thickness of the polyethylene base layer was adjusted to 5 μm, and the rest was the same as in step (2) of Example 1; the porosity of the separator was 19.9%; Steps (1), (3)-(5) are the same as those in steps (1), (3)-(5) of Example 1.

[0250] Table 3: Partial parameters and test results of Examples 1, 15-23 Number Inorganic material layer Average particle size of the positive electrode active material (μm) Positive electrode active material Thickness of the base layer (μm) Positive electrode conductive agent Mw of the polymer-based positive electrode binder Hot pressing conditions Energy density (Wh / L) DC impedance at 20% SOC (mΩ) Self-discharge rate (mV / h) Number of cycles at 45°C, 75% Example 1 Yes 0.8 Doped LiFePO₄ with Ti 7 Single-walled carbon nanotubes 700,000 95°C, 4000N, 120s 427.6 10.1 0.013 5877 Example 15 No Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 95°C, 4000N, 90s 428.1 11.3 0.02 5407 Example 16 Yes 0.5 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 419 8.8 0.012 5856 Example 17 Yes 1.5 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 434.8 13 0.011 5805 Example 18 Yes 0.35 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 410.2 8.5 0.018 5338 Example 19 Yes 2.1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 443.2 14.5 0.021 5348 Example 20 Yes Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 95°C, 4000N, 40s 427.6 10.5 0.018 5670 Example 21 Yes 4.5 NCM523 Same as Example 1 Same as Example 1 Same as Example 1 Same as Example 1 543.1 13 0.02 4577 Example 22 Yes Same as Example 1 LiFePO₄ Same as Example 1 Super P 300,000 Same as Example 1 427.6 11.1 0.014 5566 Example 23 Yes Same as Example 1 Same as Example 1 5 Same as Example 1 Same as Example 1 Same as Example 1 429.7 11 0.012 5481

[0251] As can be seen from the above table: Compared with Example 15 without an inorganic material layer, the battery cell of Example 1 of the present application has lower impedance, higher safety performance and cycling performance.

[0252] Compared with Example 18 with a smaller average particle size of the positive electrode active material, the safety performance and cycling performance of the battery cells of Examples 1, 16 - 17 of the present application are significantly higher.

[0253] Compared with Example 19 with a larger average particle size of the positive electrode active material, the battery cells of Examples 1, 16 - 17 of the present application have lower DC impedance, higher safety performance and cycling performance.

[0254] Compared with Example 20 with a shorter hot pressing time, the safety performance and cycling performance of the battery cell of Example 1 of the present application are significantly higher.

[0255] Compared with Example 21 using lithium nickel cobalt manganese oxide as the positive electrode active material, the average particle size of the lithium iron phosphate positive electrode active material doped with Ti in Example 1 of the present application is smaller, and the safety performance and cycling performance of the battery cell are higher.

[0256] Compared with Example 22 using a lithium iron phosphate positive electrode active material without doping Ti element, a particulate positive electrode conductive agent and the weight average molecular weight of the positive electrode binder being less than that of the fluoropolymer, the battery cell of Example 1 of the present application has lower DC impedance and higher cycling performance.

[0257] Compared with Example 23 with a thinner base layer thickness, the battery cell of Example 1 of the present application has lower DC impedance and improved cycling performance.

[0258] It should be noted that the present application is not limited to the above - described embodiments. The above - described embodiments are only examples, and embodiments having the same structure as the technical idea in essence and achieving the same effect within the technical solution scope of the present application are all included in the technical scope of the present application. In addition, within the scope not departing from the gist of the present application, various modifications that can be thought of by those skilled in the art to the embodiments, and other ways constructed by combining some constituent elements of the embodiments are also included in the scope of the present application.

Claims

1. A battery cell, characterized in that, The battery cell includes an electrode assembly, the electrode assembly includes a positive electrode plate, a negative electrode plate and a separator, and the separator is located between the positive electrode plate and the negative electrode plate; The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector, and the positive electrode active layer includes a positive electrode active material; the tap density of the positive electrode active layer is 2.3 g / cm 3 - 3.35 g / cm 3 ; The separator includes a base layer and a polymer layer on one side of the base layer facing the positive electrode plate, and the polymer layer includes a fluoropolymer with a crystallinity less than or equal to 50%.

2. The battery cell according to claim 1, wherein The crystallinity of the fluoropolymer is 20% - 50%; and / or, The weight average molecular weight of the fluoropolymer is 120,000 - 800,000.

3. The battery cell according to claim 1, wherein The weight average molecular weight of the fluoropolymer is 150,000 - 500,000.

4. The battery cell according to claim 1, wherein, The thickness of the polymer layer is 0.2 μm - 5μm.

5. The battery cell according to claim 1, characterized in that, The thickness of the polymer layer is 0.5 μm - 3μm.

6. The battery cell according to claim 1, characterized in that, The ratio of the area of the polymer layer to the area of the base layer is 65% - 100%.

7. The battery cell according to claim 1, characterized in that, The fluoropolymer includes one or more of polyvinylidene fluoride, vinylidene fluoride-trifluorochloroethylene copolymer, vinylidene fluoride-trifluorochloroethylene-tetrafluoroethylene copolymer, vinylidene fluoride-trifluorochloroethylene-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-trifluorochloroethylene-tetrafluoroethylene-hexafluoropropylene copolymer, vinylidene fluoride-acrylonitrile copolymer, vinylidene fluoride-acrylate copolymer.

8. The battery cell according to claim 1, characterized in that, The separator further includes an inorganic material layer, and the inorganic material layer is located between the base layer and the polymer layer.

9. The battery cell according to claim 8, wherein, The inorganic material layer includes one or more inorganic materials such as alumina, boehmite, titanium oxide, and silicon dioxide.

10. The battery cell according to claim 1, characterized in that, The average particle size of the positive electrode active material is 0.35 μm - 2.1 μm.

11. The battery cell according to claim 1, characterized in that, The average particle size of the positive electrode active material is 0.5 μm - 1.5 μm.

12. The battery cell according to claim 1, characterized in that, The areal density of the positive electrode active layer is 0.15 mg / mm 2 - 0.4 mg / mm 2 .

13. The battery cell according to claim 1, wherein The positive electrode active material includes undoped lithium iron phosphate or doped lithium iron phosphate, and the doping element includes one or more of Ti, Zr, Mn, Co, and V.

14. The battery cell according to claim 13, characterized in that, The mass content of the doping element in the doped lithium iron phosphate is 500 - 5000 ppm.

15. The battery cell according to claim 1, characterized in that, The thickness of the base layer is 5μm - 12μm.

16. The battery cell according to claim 1, wherein The porosity of the separator is 19% - 50%.

17. The battery cell according to claim 1, characterized in that, The positive electrode active layer further includes a positive electrode conductive agent, and the positive electrode conductive agent includes single-walled carbon nanotubes.

18. The battery cell according to claim 1, characterized in that, The positive electrode active layer further includes a polymer-based positive electrode binder, and the weight average molecular weight of the polymer-based positive electrode binder is greater than or equal to the weight average molecular weight of the fluoropolymer.

19. The battery cell according to claim 1, characterized in that, The negative electrode plate includes a negative electrode current collector and a negative electrode active layer on at least one side of the negative electrode current collector, and the negative electrode active layer includes a negative electrode active material; wherein, The negative electrode active material includes a graphite material.

20. The battery cell according to claim 19, wherein, The graphitization degree of the negative electrode active material is 90% - 94%.

21. The battery cell according to claim 19, wherein, The average particle size of the negative electrode active material is 8μm - 20 μm.

22. The battery cell according to claim 1, characterized in that, The battery cell further includes an electrolyte, the electrolyte includes an electrolyte additive, and the electrolyte additive includes an unsaturated carbonate additive and / or a sulfonate additive.

23. The battery cell according to claim 22, wherein, The unsaturated carbonate additive includes one or more of vinylene carbonate, fluoroethylene carbonate, and vinyl vinylene carbonate; and / or, The sulfonate additives include one or more of 1,3 - propane sultone, 1,4 - butane sultone, methylene methanedisulfonate, ethylene sulfate, and ethyl sulfite.

24. The battery cell according to claim 22, wherein The electrolyte further includes a solvent, and the solvent includes cyclic carbonate solvents, and the mass of the cyclic carbonate solvents is 20% - 40% of the mass of the solvent.

25. The battery cell according to any one of claims 1 to 24, characterized in that, The electrode assembly is cuboid, and the ratio of the length to the width of the electrode assembly is 2 - 8; and / or, The electrode assembly is a laminated structure.

26. A method for preparing a battery cell, characterized in that, The method includes the following steps: Arrange the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and perform hot pressing at 80°C - 150°C, and the pressure of the hot pressing is 2 kN - 8 kN to obtain an electrode assembly, and use the electrode assembly to prepare a battery cell; wherein, The positive electrode plate includes a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector, and the positive electrode active layer includes a positive electrode active material; the tap density of the positive electrode active layer is 2.3 g / cm 3 - 3.35 g / cm 3 ; The separator includes a base layer and a polymer layer on one side of the base layer facing the positive electrode sheet, and the polymer layer includes a fluoropolymer with a crystallinity less than or equal to 50%.

27. The method according to claim 26, wherein The time of the hot pressing is 40 s - 180 s; and / or, The battery cell is the battery cell according to any one of claims 1 to 25.

28. The method according to claim 26, wherein The separator further includes an inorganic material layer, and the inorganic material layer is located between the base layer and the polymer layer.

29. The method according to claim 26, wherein The polymer layer is prepared by the following steps: Mix the fluoropolymer with an oily solvent to obtain a polymer slurry; Coat the polymer slurry on one side of the base layer facing the positive electrode sheet, and dry it to obtain the polymer layer.

30. The method according to claim 28, wherein The polymer layer is prepared by the following steps: Mix the fluoropolymer with an oily solvent to obtain a polymer slurry; Coat the polymer slurry on one side of the inorganic material layer facing the positive electrode sheet, and dry it to obtain the polymer layer.

31. The method according to claim 29 or 30, characterized in that, The oily solvent includes one or more of N - methylpyrrolidone, dimethyl sulfoxide, dimethylformamide, and dimethylacetamide.

32. A battery device, characterized in that, The battery device includes the battery cell according to any one of claims 1 to 25 or the battery cell prepared by the method according to any one of claims 26 to 31.

33. An electrical device, characterized in that, The electrical device includes the battery cell according to any one of claims 1 to 25 or the battery device according to claim 32.

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