Lithium secondary batteries and electrical devices

Abstract

This application provides a lithium secondary battery and an electrical device. The lithium secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative current collector or a negative current collector and a lithium metal layer disposed on the surface of the negative current collector. The negative current collector includes a buffer layer and metal layers disposed on both sides of the buffer layer. The elastic modulus of the buffer layer is less than that of the metal layer. This lithium secondary battery has improved cycle life and initial coulombic efficiency.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a lithium secondary battery and an electrical device thereof. Background Technology

[0002] In recent years, with the increasingly wide application of lithium secondary batteries, they have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in many fields such as power tools, electric bicycles, electric motorcycles, electric cars, and aerospace.

[0003] Due to the significant advancements in lithium-ion batteries, higher requirements have been placed on their cycle life and initial coulombic efficiency. Summary of the Invention

[0004] This application was made in view of the above-mentioned problems, and its object is to provide a lithium secondary battery and an electrical device. The lithium secondary battery of this application has improved cycle life and initial coulombic efficiency.

[0005] To achieve the above objectives, the first aspect of this application provides a lithium secondary battery, including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes a negative current collector or the negative electrode includes a negative current collector and a lithium metal layer disposed on the surface of the negative current collector, the negative current collector includes a buffer layer and metal layers disposed on both sides of the buffer layer, and the elastic modulus of the buffer layer is less than the elastic modulus of the metal layer.

[0006] In this application, a buffer layer is added between the two metal layers. Because the elastic modulus of the buffer layer is lower than that of the metal layers, it can easily deform during lithium-ion deposition, thereby absorbing or dispersing the volumetric strain and internal stress caused by lithium-ion deposition. This reduces the risk of cracking in the negative electrode current collector and extends the cycle life of the lithium-ion secondary battery. Furthermore, the presence of the buffer layer reduces the probability of cracking in the current collector, which helps to reduce the likelihood of irreversible lithium-ion reactions, thus reducing the loss of active material and improving the initial coulombic efficiency of the lithium-ion secondary battery.

[0007] In some implementations, the elastic modulus of the buffer layer is between 1000 MPa and 6000 MPa. This is beneficial for further improving the cycle life and initial coulombic efficiency of lithium secondary batteries.

[0008] In some implementations, the ratio of the thickness of the metal layer on one side of the buffer layer to the thickness of the buffer layer is 1:(1-10). This is beneficial for improving the cycle life and initial coulombic efficiency of the lithium secondary battery.

[0009] In some embodiments, the ratio of the thickness of the metal layer on one side of the buffer layer to the thickness of the buffer layer is 1:(1.7-10). A ratio within this range indicates a thinner metal layer and a thicker buffer layer. A thinner metal layer can reduce the overall density of the current collector, increasing the energy density of the lithium-ion battery. The presence of a thicker buffer layer helps improve the toughness of the negative electrode current collector, reducing cracking caused by stress resulting from volume expansion and contraction during charging and discharging, thereby extending the cycle life of the lithium-ion battery.

[0010] In some embodiments, the buffer layer is made of a polymer material. Polymer materials possess excellent mechanical strength, and using a buffer layer containing a polymer material helps to further reduce the risk of cracking in the negative electrode current collector, thereby further extending the cycle life of the lithium-ion battery. Optionally, the polymer material includes at least one of polyethylene terephthalate, polypropylene, and polyimide. These polymer materials have good chemical stability, and selecting them further improves the cycle life of the lithium-ion battery.

[0011] In some embodiments, the buffer layer thickness is 2 μm to 14 μm. Controlling the buffer layer thickness within this range facilitates achieving a high elastic modulus, reducing the risk of cracking in the negative electrode current collector. It also facilitates miniaturization of the negative electrode current collector, improving the volumetric energy density of the lithium-ion battery. Optionally, the buffer layer thickness is 5 μm to 10 μm.

[0012] In some embodiments, the metal layer is made of one or more of copper, nickel, iron, and titanium. These metal elements possess good electrochemical stability (i.e., they are not easily corroded at low potentials) and high ionic conductivity. Using metals containing these elements helps to further extend the cycle life of lithium-ion batteries. Optionally, the metal layer is made of one or more of copper and nickel.

[0013] In some embodiments, the thickness of the metal layer located on one side of the buffer layer is 0.5 μm to 4 μm. By controlling the thickness of the metal layer within this range, it helps the negative electrode current collector maintain sufficient mechanical strength, thereby improving the cycle life of the lithium secondary battery. Furthermore, it facilitates the miniaturization of the negative electrode current collector, increasing the volumetric energy density of the lithium secondary battery. Optionally, the thickness of the metal layer located on one side of the buffer layer is 1 μm to 3 μm.

[0014] In some embodiments, the negative electrode further includes a protective layer attached to the surface of the metal layer away from the buffer layer, and the protective layer comprises a carbon material with a BET specific surface area of ​​50 m². 2 / g to 500m 2 / g. A protective layer containing carbon material is added to the surface of the metal layer. Since the BET specific surface area of ​​carbon material is 50m², 2 / g to 500m 2 The carbon material, with its abundant porosity ( / g), facilitates the rapid passage of lithium ions through the protective layer, reducing lithium ion aggregation and further inhibiting lithium dendrite growth. This, in turn, extends the cycle life of the lithium-ion battery. Furthermore, the carbon material imparts high mechanical strength to the protective layer, effectively resisting lithium dendrite penetration and preventing excessive growth that could puncture the battery's internal structure, thus further enhancing the battery's cycle life.

[0015] In some embodiments, the carbon material includes at least one of hard carbon materials, carbon nanotubes, soft carbon materials, and carbon black. These carbon materials possess excellent mechanical strength, and using them as a protective layer can further suppress the growth of lithium dendrites. Furthermore, these carbon materials also exhibit good ionic conductivity, which facilitates the rapid passage of lithium ions through the protective layer, thereby inhibiting the growth of lithium dendrites. Therefore, the cycle life of the lithium secondary battery is further extended.

[0016] In some embodiments, the ID / IG ratio of the hard carbon material is 1.68 to 1.72, where ID represents the Raman spectrum of the hard carbon material at 1350 ± 50 cm⁻¹. -1 The intensity of the D peak at 1580±50 cm⁻¹ represents the Raman spectrum of hard carbon materials. -1 The intensity of the G peak at the location; the ID / IG value represents the ratio of the D peak intensity to the G peak intensity, which helps to reduce the defects of hard carbon materials, reduce the consumption of active ions during the cycling process of lithium secondary batteries, and thus improve the cycle life and first coulombic efficiency of lithium secondary batteries.

[0017] In some embodiments, the BET specific surface area of ​​the hard carbon material is 50 m². 2 / g to 500m 2 / g, which helps maintain a good ion pathway, thereby improving the cycle life and first coulombic efficiency of lithium secondary batteries.

[0018] In some embodiments, the particle size distribution Dv50 of the hard carbon material is from 50 nm to 2000 nm. By keeping the particle size distribution Dv50 of the hard carbon material within the above range, it is beneficial to improve the cycle life of the lithium secondary battery.

[0019] In some embodiments, the compaction density of the hard carbon material powder at 50,000 N is 0.75 g / cm³. 3 Up to 0.9 g / cm 3 By compacting the hard carbon material powder to within the aforementioned range at 50,000 N, it is beneficial to improve the cycle life and energy density of lithium secondary batteries.

[0020] In some embodiments, the thickness of the negative electrode current collector is between 3 μm and 18 μm. By controlling the thickness of the negative electrode current collector within this range, it is beneficial to further improve the initial coulombic efficiency and cycle life of the lithium secondary battery.

[0021] In some implementations, the elastic modulus of the negative electrode current collector is between 1000 MPa and 10000 MPa. This is beneficial for further balancing cycle life and initial coulombic efficiency.

[0022] In some embodiments, the electrolyte includes a solid electrolyte. Solid electrolytes effectively inhibit the growth of lithium dendrites, which helps prevent internal short circuits and extends the cycle life of the lithium-ion battery. Furthermore, the current collector used in this embodiment can deform during battery charging and discharging, thereby alleviating internal stress and reducing the risk of solid electrolyte cracking.

[0023] In some implementations, the negative electrode is also a negative current collector. This design allows for a more compact internal battery structure, enabling the storage of more energy within the same volume or weight, thus increasing the energy density of lithium-ion batteries.

[0024] The second aspect of this application provides an electrical device including the lithium secondary battery provided in the first aspect. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the negative electrode current collector according to one embodiment of this application.

[0026] Figure 2 This is a schematic diagram of a battery cell according to one embodiment of this application.

[0027] Figure 3 yes Figure 2 An exploded view of a battery cell according to one embodiment of this application is shown.

[0028] Figure 4 This is a schematic diagram of a battery module according to one embodiment of this application.

[0029] Figure 5 This is a schematic diagram of a battery pack according to one embodiment of this application.

[0030] Figure 6 yes Figure 5 An exploded view of a battery pack according to one embodiment of this application is shown.

[0031] Figure 7 This is a schematic diagram of an electrical device that uses a lithium secondary battery as a power source according to an embodiment of this application.

[0032] Explanation of reference numerals in the attached figures:

[0033] 1 Battery pack; 2 Upper casing; 3 Lower casing; 4 Battery module; 5 Battery cell; 51 Housing; 52 Electrode assembly; 53 Top cover assembly; 10 Negative current collector; 101 Metal layer; 102 Buffer layer. Detailed Implementation

[0034] The following detailed description, with appropriate reference to the accompanying drawings, specifically discloses embodiments of the lithium secondary battery and power-consuming device of this application. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided for the purpose of enabling those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0035] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0036] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0037] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0038] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, preferably sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, if the method may also include step (c), it means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0039] Unless otherwise specified, the terms used in this application have the common meanings as commonly understood by those skilled in the art.

[0040] Unless otherwise specified, the values ​​of the parameters mentioned in this application can be determined using various testing methods commonly used in the art, for example, according to the testing methods given in this application.

[0041] The negative electrode, as a crucial component of lithium-ion batteries, significantly impacts the overall performance of the battery. Currently, to improve the energy density of lithium-ion batteries, related technologies are being developed to design them as such (including negative electrode-less batteries and lithium metal batteries). For example, negative electrode-less batteries consist only of a negative current collector, eliminating traditional negative electrode materials. This means that more positive electrode material can be accommodated in the same volume or weight of the lithium-ion battery, thus increasing its overall energy density. As another example, lithium metal batteries have a lithium metal layer on the surface of the negative current collector. Lithium metal has a very high theoretical specific capacity, allowing lithium metal batteries to store more charge and achieve high energy density.

[0042] For lithium-ion rechargeable batteries without a negative electrode, active ions are reduced and deposited on the surface of the negative electrode current collector or lithium metal layer. This results in significant volumetric strain and increased internal stress within the battery. Excessive internal stress can lead to current collector cracking, deterioration of battery cycle life, and reduced initial coulombic efficiency.

[0043] Based on this, this application provides a lithium secondary battery and an electrical device, which has improved initial coulombic efficiency and cycle life. The invention and its optional embodiments are described in more detail below.

[0044] Lithium secondary batteries

[0045] The first aspect of this application provides a lithium secondary battery, including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes a negative current collector or the negative electrode includes a negative current collector and a lithium metal layer disposed on the surface of the negative current collector, and the negative current collector includes a buffer layer and metal disposed on both sides of the buffer layer.

[0046] In this application, when the negative electrode sheet is the negative current collector, the negative electrode sheet omits the negative electrode active material. This means that more positive electrode active material can be accommodated in the same volume or weight of the lithium secondary battery, thereby improving the overall energy density of the lithium secondary battery.

[0047] When the negative electrode sheet includes a negative current collector and a lithium metal layer disposed on the surface of the negative current collector, the high theoretical specific capacity of lithium metal allows the lithium secondary battery to store more charge and achieve high energy density.

[0048] Building upon this foundation, this application further improves the negative electrode current collector. The negative electrode current collector comprises two metal layers, with a buffer layer added between them. Because the elastic modulus of the buffer layer is lower than that of the metal layers, the buffer layer can easily deform during lithium-ion deposition, thereby absorbing or dispersing the volumetric strain and internal stress caused by lithium-ion deposition. This reduces the risk of cracking in the negative electrode current collector and extends the cycle life of the lithium-ion secondary battery. Furthermore, the presence of the buffer layer reduces the probability of cracking in the current collector, which helps to reduce the likelihood of irreversible lithium-ion reactions, thus reducing the loss of active material and improving the initial coulombic efficiency of the lithium-ion secondary battery.

[0049] In this application, "elastic modulus" refers to the ratio of stress to strain during the elastic deformation stage of the buffer layer. When the elastic modulus is low, it is prone to deformation under stress. In this application, the elastic modulus can be determined in the following way. The sample to be tested (e.g., the buffer layer) can be a pre-prepared sample or a sample obtained by disassembling a battery. The latter will be used as an example to illustrate the testing process. Specifically, the negative electrode current collector is obtained by disassembling the battery. The metal layer and buffer layer are peeled off from the negative electrode current collector, and then the buffer layer is cut into a sheet with a length of 50mm to 200mm and a width of 15mm. The two ends of the obtained sheet are fixedly connected to the upper and lower clamps of a tensile testing instrument, respectively. The initial distance between the upper and lower clamps is L0. The upper clamp is set to move at a constant speed away from the lower clamp until the sheet between the upper and lower clamps breaks. Plot the stress-strain curve. From the initial linear portion of the curve, such as when the elongation ranges from 0 to 1.0%, calculate the elastic modulus E (MPa) of the buffer layer using the following formula: E = σ / ε. Where: σ - stress; ε - strain.

[0050] In this application, the elastic modulus of the buffer layer is between 1000 MPa and 6000 MPa. This is beneficial for further improving the cycle life and initial coulombic efficiency of the lithium secondary battery. For example, the elastic modulus of the buffer layer is a value between 1000 MPa, 2000 MPa, 3000 MPa, 4000 MPa, 5000 MPa, 6000 MPa, or any two of these values.

[0051] See Figure 1 In this application, the negative electrode current collector 10 includes a buffer layer 102 and metal layers 101 disposed on both sides of the buffer layer 102, that is, the buffer layer 102 is stacked between two metal layers 101. In this application, the stacking direction of the buffer layer 102 and the metal layers 101 is consistent with the stacking direction of the positive electrode and the negative electrode. The negative electrode current collector 10 will be further described below.

[0052] In some implementations, the ratio of the thickness of the metal layer on one side of the buffer layer to the thickness of the buffer layer is 1:(1-10). This is beneficial for improving the cycle life and initial coulombic efficiency of the lithium secondary battery.

[0053] For example, the ratio of the thickness of the metal layer on one side of the buffer layer to the thickness of the buffer layer is 1:1, 1:1.7, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, or any value within a range of two values.

[0054] In some embodiments, the ratio of the thickness of the metal layer on one side of the buffer layer to the thickness of the buffer layer is 1:(1.7-10). A ratio within this range indicates a thinner metal layer and a thicker buffer layer. A thinner metal layer can reduce the overall density of the current collector, increasing the energy density of the lithium-ion battery. The presence of a thicker buffer layer helps improve the toughness of the negative electrode current collector, reducing cracking caused by stress resulting from volume expansion and contraction during charging and discharging, thereby extending the cycle life of the lithium-ion battery.

[0055] In this application, the ratio of the thickness of the metal layer to the thickness of the buffer layer on the side of the negative electrode current collector can be tested in the following way. The negative electrode current collector to be tested can be a prepared negative electrode current collector or a negative electrode current collector obtained by disassembling the battery. The latter is used as an example to illustrate the testing process below. Specifically, to obtain the negative electrode current collector by disassembling the battery, a cross-sectional polishing instrument can be used to test the cross-sectional morphology (CP) of the negative electrode current collector to obtain a cross-sectional SEM image of the negative electrode current collector in the thickness direction. In the cross-sectional SEM image, the regions at both ends are the metal layer, and the central region is the buffer layer. The thickness of the polymer layer and the thickness of the metal layer can be measured by the scale bar in the cross-sectional SEM image.

[0056] In some embodiments, the buffer layer is made of a polymer material. Polymer materials possess excellent mechanical strength, and selecting a polymer material helps to further reduce the risk of cracking in the negative electrode current collector, thereby further extending the cycle life of the lithium-ion secondary battery. Optionally, the polymer material includes at least one of polyethylene terephthalate, polypropylene, and polyimide. These polymer materials have good chemical stability, and selecting them further helps to improve the cycle life of the lithium-ion secondary battery.

[0057] In this application, the polymer material used in the buffer layer can be tested in the following ways. The buffer layer to be tested can be a pre-prepared buffer layer or a buffer layer obtained by disassembling the battery. The latter will be used as an example to illustrate the testing process below. Specifically, the negative electrode current collector is obtained by disassembling the battery, the buffer layer is separated from the metal layer, and the composition of the buffer layer is tested according to the infrared spectroscopy analysis method in GB / T6040-2019.

[0058] In some embodiments, the thickness of the buffer layer is between 2 μm and 14 μm. Controlling the thickness of the buffer layer within this range facilitates achieving a high elastic modulus, thereby reducing the risk of cracking in the negative electrode current collector. It also facilitates the miniaturization of the negative electrode current collector, thereby increasing the volumetric energy density of the lithium-ion battery. Exemplarily, the buffer layer thickness is a value between 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, or any two of these values. Optionally, the thickness of the buffer layer is between 5 μm and 10 μm.

[0059] In some embodiments, the metal layer is made of one or more of copper, nickel, iron, and titanium. These metal elements possess good electrochemical stability and high ionic conductivity. By selecting a metal layer containing these metal elements, it is beneficial to further extend the cycle life of the lithium-ion secondary battery. Optionally, the metal layer is made of one or more of copper and nickel.

[0060] In this application, the material used in the metal layer of the negative electrode current collector can be tested in the following way. The metal layer to be tested can be a pre-prepared metal layer or a metal layer obtained by disassembling the battery. The latter is used as an example to illustrate the testing process below. Specifically, the negative electrode current collector is obtained by disassembling the battery, and the metal layer is separated from the buffer layer. The metal layer is dissolved using a digestion reagent, and the metal elements contained in the metal layer are tested using inductively coupled plasma atomic emission spectrometry (ICP-AES) according to EPA 6010D-2018.

[0061] In some embodiments, the thickness of the metal layer is from 0.5 μm to 4 μm. Controlling the thickness of the metal layer within this range helps maintain sufficient mechanical strength in the negative electrode current collector, thereby improving the cycle life of the lithium-ion battery. It also facilitates the miniaturization of the negative electrode current collector. Exemplarily, the thickness of the metal layer is 0.5 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm, 3 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4 μm, or any value within a range of two such values. Optionally, the thickness of the metal layer is from 1 μm to 3 μm.

[0062] In some embodiments, the negative electrode further includes a protective layer attached to the surface of the metal layer away from the buffer layer, and the protective layer is coated with carbon material. The BET specific surface area of ​​the carbon material is 50 m². 2 / g to 500m 2 / g. A protective layer containing carbon material is added to the surface of the metal layer. The BET specific surface area of ​​the carbon material is 50m². 2 / g to 500m 2 The carbon material, with a surface area of ​​50 m² / g, possesses a rich porous structure. This abundant porosity facilitates the rapid passage of lithium ions through the protective layer, reducing lithium ion aggregation and further inhibiting lithium dendrite growth, thereby extending the cycle life of the lithium-ion battery. Furthermore, the carbon material imparts high mechanical strength to the protective layer, effectively resisting lithium dendrite penetration and preventing excessive growth that could puncture the battery's internal structure, thus further improving the battery's cycle life. For example, the carbon material has a BET specific surface area of ​​50 m² / g. 2 / g、60m 2 / g、70m 2 / g、80m 2 / g、90m 2 / g, 100m 2 / g, 150m 2 / g、200m 2 / g、300m 2 / g、400m 2 / g、500m 2 / g, or the value between any two values ​​within a range.

[0063] In this application, the BET specific surface area of ​​the material is the total area per unit mass of material, which can be determined using instruments and methods known in the art. For example, it can be tested using the nitrogen adsorption BET specific surface area analysis method according to GB / T 19587-2017, and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be the Tri-Star 3020 BET specific surface area pore size analyzer from Micromeritics, USA.

[0064] In some embodiments, the carbon material includes at least one of hard carbon materials, carbon nanotubes, soft carbon materials, and carbon black. These carbon materials possess excellent mechanical strength, and their use can further suppress the growth of lithium dendrites. Furthermore, these carbon materials also exhibit good ionic conductivity, which facilitates the rapid passage of lithium ions through the protective layer, thereby inhibiting lithium dendrite growth. Therefore, the cycle life of the lithium secondary battery is further extended.

[0065] In some embodiments, the hard carbon material I D / I G The values ​​range from 1.68 to 1.72, where I D This indicates that the Raman spectrum of hard carbon materials is at 1350±50 cm⁻¹ -1 D peak intensity at I G This indicates that the Raman spectrum of hard carbon materials is at 1580±50 cm⁻¹ -1 The intensity of the G peak at I; D / I G The value represents the ratio of the D peak intensity to the G peak intensity. This helps reduce defects in hard carbon materials, decrease active ion consumption during lithium-ion battery cycling, and thus improve the cycle life and initial coulombic efficiency of lithium-ion batteries. For example, the I value of hard carbon materials... D / I G The value is between 1.68, 1.69, 1.70, 1.71, 1.72, or any two values.

[0066] In this application, the I of hard carbon material D / I G Raman spectroscopy can be used for testing. The testing conditions are: excitation wavelength 532 nm, grating 600 lines, objective lens 50x, integration time 10 s, cumulative scan 3 times, area scan, obtaining the D and G peak intensities at 100 points, and calculating the I at 100 points. D / I G Remove the largest and smallest 30 I's. D / I G The average of the remaining 40 points is the material's I. D / IG The testing instrument can be a Horiba LabRAM HR800 Raman spectrometer.

[0067] In some embodiments, the BET specific surface area of ​​the hard carbon material is 50 m². 2 / g to 500m 2 / g; This helps reduce the side reaction activity on the surface of hard carbon materials, reduces the consumption of active ions during SEI film formation, and thus improves the cycle life and first coulombic efficiency of lithium secondary batteries. For example, the BET specific surface area of ​​the hard carbon material is 50m². 2 / g、60m 2 / g、70m 2 / g、80m 2 / g、90m 2 / g, 100m 2 / g, 150m 2 / g、200m 2 / g or any value within a range formed by two numerical values.

[0068] In some embodiments, the particle size distribution Dv50 of the hard carbon material is from 50 nm to 2000 nm. By keeping the particle size distribution Dv50 of the hard carbon material within this range, it is beneficial to improve the cycle life of the lithium-ion secondary battery. For example, the particle size distribution Dv50 of the hard carbon material is a value between 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 200 nm, 300 nm, 500 nm, 1000 nm, 2000 nm, 50000 nm, or any two of these values.

[0069] In this application, the particle size distribution Dv50 of the hard carbon material represents the particle size corresponding to a cumulative volume distribution percentage of 50%, which can be determined using instruments and methods known in the art. For example, it can be determined using a laser particle size analyzer in accordance with GB / T19077-2016. The testing instrument can be a Mastersizer 3000 laser particle size analyzer from Malvern Instruments Ltd., UK.

[0070] In some embodiments, the compaction density of the hard carbon material powder at 50,000 N is 0.75 g / cm³. 3 Up to 0.9 g / cm 3 By maintaining the powder compaction density of the hard carbon material within the aforementioned range, it is beneficial to reduce volume expansion during battery cycling, thereby improving the cycle life of the lithium secondary battery. For example, the powder compaction density of the hard carbon material is 0.75 g / cm³. 3 0.80g / cm 3 0.85g / cm 3 0.90g / cm3 Or the value between any two values ​​within a range.

[0071] In this application, the compacted density of hard carbon material powder is the mass per unit volume of the powder under specified conditions, which can be determined using instruments and methods known in the art. For example, it can be determined using an electronic pressure testing machine (e.g., a UTM7305 type electronic pressure testing machine) according to GB / T 24533-2009. An exemplary test method is as follows: Weigh 1g of sample powder and add it to a container with a bottom area of ​​1.327cm². 2 In the mold, the pressure is increased to 50,000 N, held for 30 seconds, then the pressure is released and held for 10 seconds. The compaction density of the powder under 50,000 N pressure is then recorded and calculated.

[0072] In some embodiments, the thickness of the negative electrode current collector is from 3 μm to 18 μm. By controlling the thickness of the negative electrode current collector within this range, it is beneficial to further improve the initial coulombic efficiency and cycle life of the lithium secondary battery. Exemplarily, the thickness of the negative electrode current collector is 3 μm, 2 μm, 4 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, or a value between any two of these values. Optionally, the thickness of the negative electrode current collector is from 5 μm to 15 μm.

[0073] In this application, the thickness of the negative electrode current collector has a meaning known in the art and can be tested using methods known in the art. The negative electrode current collector to be tested can be a prepared negative electrode current collector or a negative electrode current collector obtained by disassembling the battery. The latter will be used as an example to illustrate the testing process below. Specifically, the negative electrode current collector is obtained by disassembling the battery, and its thickness is measured using a micrometer (e.g., a Mitutoyo 293-100 model with an accuracy of 0.1 μm).

[0074] In some implementations, the elastic modulus of the negative electrode current collector is between 1000 MPa and 10000 MPa. For example, the elastic modulus of the negative electrode current collector is a value between 1000 MPa, 2000 MPa, 3000 MPa, 4000 MPa, 5000 MPa, 6000 MPa, 7000 MPa, 8000 MPa, 9000 MPa, 10000 MPa, or any two of these values.

[0075] The term "lithium secondary battery" used in this article refers to a single battery cell, a battery module, or a battery pack. These will be explained separately below.

[0076] Typically, a lithium-ion battery cell includes a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and extracting. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0077] Negative electrode sheet

[0078] In one embodiment, the negative electrode sheet does not include a negative electrode active material. Instead, it includes the negative electrode current collector disclosed in this application. During battery charging, lithium ions are reduced on the negative electrode side and deposited on the negative electrode current collector to form a lithium metal layer. The design of the negative electrode sheet as a negative electrode current collector eliminates the need to reserve space for the negative electrode active material, allowing for a more compact internal battery structure. This enables the storage of more energy within the same volume or weight, thus significantly improving the energy density of lithium-ion rechargeable batteries.

[0079] In one embodiment, the negative electrode sheet does not include a negative electrode active material, and the negative electrode sheet includes the negative electrode current collector disclosed in this application and a lithium metal layer disposed on the surface of the negative electrode current collector; during battery charging, an external power source forces the current to flow in the reverse direction, and Li + It migrates from the positive electrode to the negative electrode and is deposited on the surface of the lithium metal layer.

[0080] In some embodiments, a coating may also be applied to the surface of the negative electrode current collector. The coating may optionally include a binder. 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), polymethyl methacrylate (PMAA), and carboxymethyl chitosan (CMCS). The coating may also optionally include a conductive agent. 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. The coating may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)). In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned conductive agent, binder, and any other components in a solvent (e.g., deionized water) to form a slurry; coating the slurry onto the negative electrode current collector to form a coating; and then obtaining the negative electrode sheet through processes such as drying and cold pressing.

[0081] Positive electrode sheet

[0082] The positive electrode includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer including a positive electrode active material.

[0083] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0084] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0085] In some embodiments, when the battery cell is a lithium-ion battery, the positive electrode active material may be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, this application is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co0.1 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0086] During the charging and discharging process of a battery, Li undergoes insertion / extraction and consumption, resulting in varying molar Li content at different discharge states. In the examples of positive electrode active materials in this application, the molar Li content refers to the initial state of the material, i.e., before feeding. When the positive electrode active material is applied to the battery system, the molar Li content changes after charge-discharge cycles.

[0087] In some embodiments, the positive electrode film layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0088] In some embodiments, the positive electrode film may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0089] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0090] electrolytes

[0091] During battery charging and discharging, active ions repeatedly insert and extract between the positive and negative electrode plates. The electrolyte acts as a conductor of ions between the positive and negative electrode plates.

[0092] In some embodiments, the electrolyte includes a solid electrolyte. Solid electrolytes effectively inhibit the growth of lithium dendrites, which helps prevent internal short circuits and extends the cycle life of the lithium-ion battery. Furthermore, the current collector used in this embodiment can deform during battery charging and discharging, thereby alleviating internal stress and reducing the risk of solid electrolyte cracking.

[0093] In some embodiments, the solid electrolyte includes, but is not limited to, Li2S-P2S5 type electrolytes (such as Li3PS) 4、 Li7P3S 11 Li₂S-P₂S₅-LiCl solid electrolytes (such as Li₆PS₅Cl), Li 10 GeP2S 12 The solid electrolyte comprises at least one of the following: lithium lanthanum zirconium oxide (LLZO), lithium lanthanum zirconium titanium oxide solid electrolyte (LLZTO), lithium aluminum silicate solid electrolyte (LAGP), and lithium phosphosulfosilicate solid electrolyte (LPSC). The aforementioned solid electrolytes have the advantage of high ion mobility coefficients, and their use helps to reduce charge transport resistance and improve the initial coulombic efficiency of lithium secondary batteries.

[0094] In some embodiments, the battery cell may include an outer packaging. This outer packaging can be used to encapsulate the electrode assembly and electrolyte described above.

[0095] In some embodiments, the outer packaging of the battery cell can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the battery cell can also be a flexible package, such as a pouch. The material of the flexible package can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0096] This application does not impose any particular limitation on the shape of the battery cell; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 The example shown is a square-structured battery cell 5.

[0097] In some implementations, refer to Figure 3 The outer packaging may include a housing 51 and a top cover assembly 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the top cover assembly 53 can be placed over the opening to close the receiving cavity. The positive electrode sheet, negative electrode sheet, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 may be one or more, which can be selected by those skilled in the art according to specific practical needs.

[0098] In some implementations, individual battery cells can be assembled into a battery module. The number of individual battery cells contained in a battery module can be one or more, and the specific number can be selected by those skilled in the art based on the application and capacity of the battery module.

[0099] Figure 4 This is battery module 4, used as an example. (See reference...) Figure 4In battery module 4, multiple battery cells 5 can be arranged sequentially along the length of battery module 4. Of course, they can also be arranged in any other manner. Furthermore, these multiple battery cells 5 can be fixed in place using fasteners.

[0100] Optionally, the battery module 4 may also include a housing with a receiving space in which multiple battery cells 5 are received.

[0101] In some embodiments, the battery modules described above can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, the specific number of which can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0102] Figure 5 and Figure 6 This is battery pack 1 as an example. (See reference...) Figure 5 and Figure 6 The battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box includes an upper body 2 and a lower body 3, with the upper body 2 covering the lower body 3 to form a closed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0103] Electrical appliances

[0104] The second aspect of this application provides an electrical device, which includes the lithium secondary battery provided in the first aspect of this application. The lithium secondary battery can be used as a power source for the electrical device or as an energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0105] As an electrical device, you can choose individual battery cells, battery modules, or battery packs according to your usage requirements.

[0106] Figure 7 This is an example of an electrical device. The device could be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the lithium-ion battery for this device, a battery pack or battery module can be used.

[0107] Another example device could be a mobile phone, tablet, or laptop. These devices typically require a slim and lightweight design and can use a single battery cell as their power source.

[0108] Example

[0109] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0110] Example 1

[0111] (1) Preparation of negative electrode sheet

[0112] 1) Preparation of negative electrode current collector

[0113] Copper (Cu) nanoparticles were sputtered onto the surface of a polyethylene terephthalate (PET) film (buffer layer) using a magnetron sputtering target to obtain a negative electrode current collector. The magnetron sputtering target had a power of 200 W and a pressure of 0.5 Pa. The PET film was 5 μm thick, and the thickness of the sputtered copper layer (metal layer) on one side was 1 μm.

[0114] 2) Preparation of the protective layer

[0115] PVDF and N-methylpyrrolidone (NMP) were mixed at a mass ratio of 2:98 to obtain a PVDF-NMP solution. Hard carbon material was then added to the aforementioned PVDF-NMP solution to prepare a slurry with a solid content of 25%.

[0116] The above slurry was uniformly coated onto the above negative electrode current collector (PET-based copper foil current collector). It was then placed in an oven and baked for 12 hours to completely remove the solvent, resulting in the negative electrode sheet.

[0117] Among them, hard carbon materials I D / I G The value is 1.70, and the specific surface area of ​​BET is 300m². 2 The compacted density of the powder at 50000N is 0.8 g / cm³. 3 The Dv50 is 1.1μm.

[0118] (2) Preparation of positive electrode sheet

[0119] The positive electrode active material (LiNi) 0.8 Co 0.1 Mn 0.1 O2, NCM811), lithium phosphorus sulfur chlorine solid electrolyte (Li6PS5Cl), carbon fiber (VGCF), and polytetrafluoroethylene (PTFE) are mixed evenly in a mass ratio of 70:25.5:2.5:2 to obtain composite cathode powder.

[0120] 3g of the above-mentioned composite cathode powder was heated on a heating table at 80°C and rolled back and forth to obtain a cathode film. Then, the cathode film was rolled onto aluminum foil and positive electrode tabs were welded on, resulting in an areal capacity of 3mAh / cm². 2 The positive electrode sheet.

[0121] (3) Preparation of solid electrolytes

[0122] Solid electrolyte salt (Li6PS5Cl) and polytetrafluoroethylene (PTFE) were mixed evenly at a mass ratio of 98:2.

[0123] Take 3g of a mixture of Li6PS5Cl and PTFE and heat it on a heating table at 80℃. Roll it back and forth until the thickness of the electrolyte sheet is 200μm to obtain the electrolyte sheet (solid electrolyte).

[0124] (4) Preparation of lithium secondary batteries

[0125] The positive electrode, electrolyte sheet, and negative electrode are assembled in an alternating stacked manner and hot-pressed. Then, they are encapsulated with aluminum-plastic film under negative pressure to obtain a lithium secondary battery (solid-state battery).

[0126] Elastic modulus test

[0127] The sample to be tested (negative electrode current collector, buffer layer) is cut into a sheet 150 mm long and 15 mm wide. Both ends of the sheet are fixedly connected to the upper and lower clamps of the tensile testing instrument, respectively, with an initial distance L0 between them. The upper clamp is set to move at a constant speed away from the lower clamp until the sheet between the upper and lower clamps breaks. A stress-strain curve is plotted. From the initial straight portion of the curve, if the elongation ranges from 0 to 1.0%, the elastic modulus E (MPa) is calculated using the following formula: E = σ / ε. Where: σ - stress; ε - strain.

[0128] Electrical performance testing of lithium secondary batteries

[0129] 1) First test of Coulomb efficiency

[0130] The following test procedure was conducted at a test temperature of 45℃ and a test pressure of 5MPa. The test steps are as follows:

[0131] ① Charge the capacitor to 4.5V with a constant current of 0.1C, then charge it to 0.05C with a constant voltage of 4.5V. Record the initial charge capacity C1.

[0132] ② Let stand for 5 minutes.

[0133] ③ Discharge at a constant current of 0.1C to 2.5V and record the first discharge capacity D1.

[0134] First charge specific capacity = first charge capacity C1 / mass of positive electrode active material.

[0135] Initial discharge specific capacity = initial discharge capacity D1 / mass of positive electrode active material.

[0136] Initial coulombic efficiency = initial discharge specific capacity / initial charge specific capacity. The test results are recorded in Table 1.

[0137] 2) Cycle life testing

[0138] ④ Charge the battery from step ③ above to 4.5V at a constant current of 0.3C, and then charge it to 0.05C at a constant voltage of 4.5V.

[0139] ⑤ Let stand for 5 minutes.

[0140] ⑥ Discharge to 2.5V with a constant current of 0.3C.

[0141] ⑦ Repeat steps ④ to ⑥ above for 500 cycles, and record the discharge capacity D. 500 .

[0142] Discharge specific capacity after 500 cycles = D 500 The quality of the positive electrode active material and the test results are recorded in Table 1.

[0143] Example 2

[0144] Example 2 and Example 1 use the same method to prepare lithium secondary batteries, the only difference being that step 2 is not performed during the preparation of the negative electrode sheet.

[0145] Comparative Example 1

[0146] Comparative Example 1 and Example 1 were prepared using the same method, the only difference being that...

[0147] A copper foil with a thickness of 7μm was used as the negative electrode current collector.

[0148] The elastic modulus of the negative electrode current collectors prepared in Example 2 and Comparative Example 1 were tested in the same manner as in Example 1, and the test results are recorded in Table 1.

[0149] The battery performance of the lithium secondary batteries prepared in Example 2 and Comparative Example 1 was tested in the same manner as in Example 1, and the test results are recorded in Table 1.

[0150] Table 1:

[0151]

[0152] In the data shown in Table 1, " / " indicates that the battery had already degraded and reached the failure criteria before completing 500 cycles.

[0153] The data in Table 1 shows that, compared to Comparative Example 1 (which uses copper sheets as the negative electrode current collector), the lithium secondary batteries in Examples 1 and 2 use a buffer layer between the two metal layers as the negative electrode current collector, and the elastic modulus of the buffer layer is smaller than that of the metal layers. As a result, the lithium secondary batteries have improved cycle life and initial coulombic efficiency.

[0154] Examples 3 to 8

[0155] Examples 3 to 8 were prepared in the same manner as Example 1, with the only difference being that during the preparation of the negative electrode current collector, the thickness of the metal layer and / or the thickness of the buffer layer were adjusted according to the description in Table 2-1.

[0156] The elastic modulus of the negative electrode current collectors prepared in Examples 3 to 8 were tested in the same manner as in Example 1, and the test results are recorded in Table 2-2.

[0157] The battery performance of the lithium secondary batteries prepared in Examples 3 to 8 was tested in the same manner as in Example 1, and the test results are recorded in Table 2-2.

[0158] Table 2-1

[0159]

[0160] Table 2-2

[0161]

[0162] The data in Tables 2-1 and 2-2 show that when the thickness of the buffer layer is between 2 μm and 14 μm, and the thickness of the metal layer on one side of the buffer layer is between 0.5 μm and 4 μm, the resulting lithium secondary battery has improved cycle life and initial coulombic efficiency.

[0163] Examples 9 to 11

[0164] Examples 9 to 11 were prepared in the same manner as Example 1, with the only difference being that during the preparation of the negative electrode current collector, the type of metal layer and / or the type of buffer layer were adjusted according to the description in Table 3-1.

[0165] The elastic modulus of the negative electrode current collectors prepared in Examples 9 to 11 was tested in the same manner as in Example 1, and the test results are recorded in Table 3-2.

[0166] The battery performance of the lithium secondary batteries prepared in Examples 9 to 11 was tested in the same manner as in Example 1, and the test results are recorded in Table 3-2.

[0167] Table 3-1

[0168]

[0169]

[0170] Table 3-2

[0171]

[0172] The data in Tables 3-1 and 3-2 show that the lithium secondary batteries prepared in Examples 9 to 11 have improved cycle life and initial coulombic efficiency.

[0173] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium secondary battery, characterized in that, The device includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative current collector, or the negative electrode includes a negative current collector and a lithium metal layer disposed on the surface of the negative current collector. The negative current collector includes a buffer layer and metal layers disposed on both sides of the buffer layer. The elastic modulus of the buffer layer is less than the elastic modulus of the metal layer.

2. The lithium secondary battery according to claim 1, characterized in that, The elastic modulus of the buffer layer is between 1000 MPa and 6000 MPa.

3. The lithium secondary battery according to claim 1 or 2, characterized in that, The ratio of the thickness of the metal layer located on one side of the buffer layer to the thickness of the buffer layer is 1:(1-10).

4. The lithium secondary battery according to claims 1 to 3, characterized in that, The ratio of the thickness of the metal layer located on one side of the buffer layer to the thickness of the buffer layer is 1:(1.7-10).

5. The lithium secondary battery according to any one of claims 1 to 4, characterized in that, The buffer layer is made of polymer materials.

6. The lithium secondary battery according to any one of claims 1 to 5, characterized in that, The polymeric material includes at least one of polyethylene terephthalate, polypropylene, and polyimide.

7. The lithium secondary battery according to any one of claims 1 to 6, characterized in that, The thickness of the buffer layer is 2 μm to 14 μm.

8. The lithium secondary battery according to any one of claims 1 to 7, characterized in that, The thickness of the buffer layer is 5 μm to 10 μm.

9. The lithium secondary battery according to any one of claims 1 to 8, characterized in that, The metal layer material includes one or more of copper, nickel, iron, and titanium.

10. The lithium secondary battery according to any one of claims 1 to 9, characterized in that, The thickness of the metal layer located on one side of the buffer layer is 0.5 μm to 4 μm.

11. The lithium secondary battery according to any one of claims 1 to 10, characterized in that, The thickness of the metal layer located on one side of the buffer layer is 1 μm to 3 μm.

12. The lithium secondary battery according to any one of claims 1 to 11, characterized in that, The negative electrode further includes a protective layer, which is attached to the metal layer away from the surface of the buffer layer, and the protective layer comprises a carbon material with a BET specific surface area of ​​50 m². 2 / g to 500m 2 / g.

13. The lithium secondary battery according to claim 12, characterized in that, The carbon material includes at least one of hard carbon material, carbon nanotubes, soft carbon material, and carbon black.

14. The lithium secondary battery according to claim 13, characterized in that, The hard carbon material satisfies one or more of the following: (1) The hard carbon material I D / I G The values ​​range from 1.68 to 1.72, where I D This indicates that the Raman spectrum of the hard carbon material is at 1350±50 cm⁻¹. -1 The intensity of peak D at I G This indicates that the Raman spectrum of the hard carbon material is at 1580±50 cm⁻¹. -1 The intensity of peak G at point I; D / I G The value represents the ratio of the D peak intensity to the G peak intensity; (2) The BET specific surface area of ​​the hard carbon material is 50 m². 2 / g to 500m 2 / g; (3) The particle size distribution Dv50 of the hard carbon material is from 50 nm to 2000 nm; (4) The compacted density of the hard carbon material at 50000N is 0.75 g / cm³. 3 Up to 0.9 g / cm 3 .

15. The lithium secondary battery according to any one of claims 1 to 14, characterized in that, The thickness of the negative electrode current collector is 3 μm to 18 μm.

16. The lithium secondary battery according to any one of claims 1 to 15, characterized in that, The elastic modulus of the negative electrode current collector is between 1000 MPa and 10000 MPa.

17. The lithium secondary battery according to any one of claims 1 to 16, characterized in that, The electrolyte includes a solid electrolyte.

18. The lithium secondary battery according to any one of claims 1 to 17, characterized in that, The negative electrode sheet is the negative current collector.

19. An electrical appliance, characterized in that, The lithium secondary battery includes any one of claims 1 to 18.

Images