Negative pole piece, secondary battery and electric device

Abstract

The invention discloses a negative pole piece, a secondary battery and an electric device, and belongs to the technical field of batteries. According to the negative electrode plate of the secondary battery, the first interface protection layer and the second interface protection layer are arranged on the surfaces of the first negative electrode active material and the second negative electrode active material respectively, the thickness of the first interface protection layer is smaller than that of the second interface protection layer, and the interface protection layers can inhibit side reaction between electrolyte and the negative electrode active layer; the polymer layer in the interface protection layer can inhibit decomposition of electrolyte and improve the mechanical property of the interface protection layer, the lithium salt layer in the interface protection layer promotes uniform deposition of lithium ions, and the lithium salt layer and the interface protection layer interact with each other, so that the mechanical property and the electrochemical property of the interface protection layer of the secondary battery are improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] Secondary batteries have advantages such as high energy density and long service life, and are widely used in electric vehicles and energy storage.

[0003] During the initial charge and discharge of a rechargeable battery, the active material in the electrode material reacts with the electrolyte to form a passivation layer, or solid electrolyte interphase (SEI), covering the surface of the electrode material. The formation of the SEI consumes some lithium ions, increasing the irreversible capacity during the initial charge and discharge and reducing the charge and discharge efficiency of the electrode material. Existing research indicates that the SEI layer on the surface of the negative electrode active material has a significant impact on the performance of the rechargeable battery.

[0004] Currently, the main methods to improve the performance of secondary batteries are to form an SEI coating layer (such as an inorganic layer or a polymer layer) on the surface of the negative electrode active material, or to use electrolyte additives. However, the above two improvement methods have the following problems: (1) The SEI coating layer on the graphite surface has a single composition and uniform thickness, which cannot adapt to the internal reaction gradient in the negative electrode active layer, resulting in the concentration of lithium ion flux in the voids of the negative electrode active layer and the initiation of dendrite growth; (2) The inorganic layer has high mechanical strength but is easy to crack, and the polymer layer has good flexibility but low ionic conductivity, which means that the SEI coating layer cannot take into account both mechanical and electrochemical performance; (3) The decomposition products of electrolyte additives are prone to interfacial side reactions, making it impossible to accurately control the spatial part of the SEI film; (4) The SEI film and the negative electrode active material are physically adsorbed and bonded. During the battery charge and discharge cycle, the volume of the negative electrode active material changes, which makes the SEI film unable to be repaired after cracking and falling off, resulting in continuous consumption of active lithium and pulverization of the negative electrode active layer, thus causing the cycle performance of the secondary battery to deteriorate. Summary of the Invention

[0005] The purpose of this application is to overcome the shortcomings of the prior art and provide a secondary battery and an electrical device.

[0006] To achieve the above objectives, the technical solution adopted in this application is as follows: In the first aspect, a negative electrode sheet is provided, the negative electrode sheet including a negative current collector and a first negative active layer and a second negative active layer disposed on at least one side of the surface of the negative current collector, the first negative active layer being located between the negative current collector and the second negative active layer. The first negative electrode active layer includes a first negative electrode active material, and the surface of the first negative electrode active material is covered with a first interface protective layer; along the direction from the first negative electrode active material to the first interface protective layer, the first interface protective layer includes a first polymer layer and a first lithium salt layer disposed sequentially. The second negative electrode active layer includes a second negative electrode active material, and the surface of the second negative electrode active material is covered with a second interface protective layer; along the direction from the second negative electrode active material to the second interface protective layer, the second interface protective layer includes a second polymer layer and a second lithium salt layer disposed sequentially. The thickness of the first interface protective layer is less than the thickness of the second interface protective layer.

[0007] In some embodiments, the first polymer layer and the second polymer layer comprise a polyethylene glycol diacrylate polymer; the first lithium salt layer and the second lithium salt layer each independently comprise at least one of lithium fluoride and lithium carbonate.

[0008] In some embodiments, the first negative electrode active layer further includes boron nitride nanosheets located in the pores of the first negative electrode active layer and / or the first lithium salt layer.

[0009] In some embodiments, the second negative electrode active layer further includes boron nitride nanosheets located in the pores of the second negative electrode active layer and / or the second lithium salt layer.

[0010] In some embodiments, the first polymer layer and the second polymer layer each independently include sulfonic acid groups and / or disulfide bonds.

[0011] In some embodiments, the porosity of the first negative electrode active layer is less than that of the second negative electrode active layer.

[0012] In some embodiments, the thickness of the first negative electrode active layer is greater than the thickness of the second negative electrode active layer.

[0013] In some embodiments, the pore size of the first negative electrode active layer is smaller than that of the second negative electrode active layer.

[0014] In some embodiments, the porosity of the first negative electrode active layer is 20-30%, and the porosity of the second negative electrode active layer is 40-50%.

[0015] In some embodiments, the thickness of the first negative electrode active layer is 50-100 μm, and the thickness of the second negative electrode active layer is 20-50 μm.

[0016] In some embodiments, the pore size of the first negative electrode active layer is 10-50 nm; the pore size of the second negative electrode active layer is 50-200 nm.

[0017] In some embodiments, the thickness of the first interface protective layer is 20-30 nm; the thickness of the second interface protective layer is 50-80 nm.

[0018] In some embodiments, the first polymer layer and the second polymer layer further include lithium salts, wherein the mass percentage of the lithium salt in the first polymer layer is less than the mass percentage of the lithium salt in the first lithium salt layer, and the mass percentage of the lithium salt in the second polymer layer is less than the mass percentage of the lithium salt in the second lithium salt layer. The first lithium salt layer and the second lithium salt layer further include polymers, wherein the mass percentage of the polymer in the first polymer layer is greater than the mass percentage of the polymer in the first lithium salt layer, and the mass percentage of the polymer in the second polymer layer is greater than the mass percentage of the polymer in the second lithium salt layer.

[0019] Secondly, a secondary battery is provided, the secondary battery including the aforementioned negative electrode sheet.

[0020] Thirdly, an electrical device is provided, including the aforementioned secondary battery.

[0021] Compared with the prior art, the beneficial effects of this application are as follows: In the negative electrode sheet of the secondary battery of this application, a first interface protection layer and a second interface protection layer are respectively provided on the surface of the first negative electrode active material and the second negative electrode active material. The thickness of the first interface protection layer is less than the thickness of the second interface protection layer. The interface protection layer can suppress the side reaction between the electrolyte and the negative electrode active layer. The polymer layer in the interface protection layer inhibits the decomposition of the electrolyte and improves the mechanical properties of the interface protection layer. The lithium salt layer in the interface protection layer promotes the uniform deposition of lithium ions. The lithium salt layer and the interface protection layer interact to improve the mechanical and electrochemical properties of the interface protection layer of the secondary battery. Detailed Implementation

[0022] To facilitate understanding of this application, a more complete description will be provided below. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0023] As used in this article: "Prepared from" is synonymous with "comprising". The terms "comprising", "including", "having", "containing", or any other variations thereof as used herein are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0024] The conjunction "composed of..." excludes any unspecified elements, steps, or components. If used in a claim, this phrase makes the claim closed, excluding materials other than those described, except for associated conventional impurities. When the phrase "composed of..." appears in a clause of the body of a claim rather than immediately following it, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0025] When a quantity, concentration, or other value or parameter is expressed as a range, a preferred range, or a range defined by a series of upper and lower preferred values, this should be understood as specifically disclosing all ranges formed by any pair of any upper or preferred value with any lower or preferred value, regardless of whether the range is disclosed individually. For example, when the range “1-5” is disclosed, the described range should be interpreted as including ranges “1-4”, “1-3”, “1-2”, “1-2 and 4-5”, “1-3 and 5”, etc. When numerical ranges are described herein, unless otherwise stated, the range is intended to include its endpoints and all integers and fractions within that range.

[0026] In these embodiments, unless otherwise specified, the portions and percentages are all by weight.

[0027] "Parts by mass" refers to the basic unit of measurement that expresses the mass ratio of multiple components. One part can represent any unit mass, such as 1g or 2.689g. If we say that component A has 'a' parts by mass and component B has 'b' parts by mass, it means that the mass ratio of component A to component B is a:b. It is important to understand that, unlike mass percentage content, the sum of the mass parts of all components is not limited to 100 parts.

[0028] "And / or" is used to indicate that one or both of the described situations may occur, for example, A and / or B includes (A and B) and (A or B).

[0029] A first aspect of this application provides a secondary battery, including a negative electrode. The negative electrode sheet includes a negative current collector and a first negative active layer and a second negative active layer disposed on at least one side of the surface of the negative current collector, wherein the first negative active layer is located between the negative current collector and the second negative active layer. The first negative electrode active layer includes a first negative electrode active material, and the surface of the first negative electrode active material is covered with a first interface protective layer; along the direction from the first negative electrode active material to the first interface protective layer, the first interface protective layer includes a first polymer layer and a first lithium salt layer disposed sequentially. The second negative electrode active layer includes a second negative electrode active material, and the surface of the second negative electrode active material is covered with a second interface protective layer; along the direction from the second negative electrode active material to the second interface protective layer, the second interface protective layer includes a second polymer layer and a second lithium salt layer disposed sequentially. The thickness of the first interface protective layer is less than the thickness of the second interface protective layer.

[0030] In the negative electrode sheet of this application, a first interface protection layer and a second interface protection layer are respectively provided on the surfaces of the first negative electrode active material and the second negative electrode active material. The interface protection layer can suppress side reactions between the electrolyte and the negative electrode active layer. It is understood that the lithium salt layer can promote lithium ion migration, reduce interfacial impedance, and improve electrochemical performance. The polymer layer blocks the negative electrode active material (e.g., graphite) from the electrolyte, reducing the occurrence of side reactions. Therefore, the polymer layer in the interface protection layer inhibits electrolyte decomposition and improves the mechanical properties of the interface protection layer. The lithium salt layer in the interface protection layer promotes uniform lithium ion deposition. The interaction between the lithium salt layer and the interface protection layer improves the mechanical and electrochemical performance of the secondary battery interface protection layer.

[0031] Furthermore, the thickness of the first interface protective layer is less than that of the second interface protective layer. This implies that, along the thickness direction, the negative electrode current collector, the first negative electrode active layer, and the second negative electrode active layer are sequentially stacked. The second interface protective layer has a larger contact area with the electrolyte compared to the first interface protective layer, and its thickness is greater than that of the first interface protective layer, which helps suppress side reactions between the electrolyte and the second negative electrode material layer. Conversely, the first interface layer has a smaller contact area with the electrolyte, thus reducing its thickness and lowering the interfacial impedance. The interaction of these two elements ensures the battery's fast-charging capability and lifespan, improving overall performance.

[0032] In this application, the negative electrode active layer refers to the first negative electrode active layer and / or the second negative electrode active layer, the interface protection layer refers to the first interface protection layer and / or the second interface protection layer, the lithium salt layer refers to the first lithium salt layer and / or the second lithium salt layer, and the polymer layer refers to the first polymer layer and / or the second polymer layer.

[0033] In some embodiments, the first polymer layer and the second polymer layer comprise a polyethylene glycol diacrylate polymer; the first lithium salt layer and the second lithium salt layer each independently comprise at least one of lithium fluoride and lithium carbonate.

[0034] In this application, the polyethylene glycol diacrylate polymer forms a three-dimensional cross-linked network. The three-dimensional cross-linked network and the negative electrode active material are bonded by van der Waals forces, which improves the interfacial bonding strength between the interface protective layer and the negative electrode active material, and prevents the interface protective layer from separating from the negative electrode active material during the cycling process of the secondary battery.

[0035] In some embodiments, the first negative electrode active layer further includes boron nitride nanosheets located in the pores of the first negative electrode active layer and / or the first lithium salt layer.

[0036] In some embodiments, the second negative electrode active layer further includes boron nitride nanosheets located in the pores of the second negative electrode active layer and / or the second lithium salt layer.

[0037] The layered structure of boron nitride nanosheets directionally guides lithium ions to transport along the pores of the negative electrode active layer, suppressing local current density concentration in the negative electrode active layer and reducing the risk of dendrite formation in the negative electrode active layer. At the same time, boron nitride nanosheets form a thermally conductive pathway in the lithium salt layer, conducting and dispersing the heat generated during charging and discharging, and preventing local thermal decomposition of the interface protective layer.

[0038] In some embodiments, the first polymer layer and the second polymer layer each independently include sulfonic acid groups and / or disulfide bonds.

[0039] In this application, the sulfonic acid groups in the polymer can provide a fast lithium-ion transport channel, improving the electrochemical performance of the secondary battery. During the cycling process of the secondary battery, the disulfide bonds in the polymer layer can repair the microcracks in the interface protective layer through disulfide bond breaking / recombination, thereby improving the long-term stability of the interface protective layer.

[0040] In this application, the test method for sulfonic acid groups is as follows: A small amount of active layer sample from the negative electrode is taken, mixed with KBr, and pressed into a pellet. Fourier transform infrared spectroscopy (FTIR) is used for testing, with a scanning range of 4000-400 cm⁻¹. -1 Observe whether there is a height of 1170-1200 cm. -1 (S=O asymmetric stretching) and 1030-1060 cm -1 (S=O symmetric stretching) Characteristic peak, its presence indicates the presence of sulfonic acid groups.

[0041] The testing method for disulfide bonds is as follows: Fix the sample on a silicon wafer or glass slide, purge the sample surface with high-purity nitrogen to remove dust; turn on confocal mode (pinhole diameter 100 μm), select more than 5 micro-regions (≤2 μm²) on the sample surface, and use Fourier transform infrared spectroscopy (FTIR) to test the selected micro-regions. If the 510-550 cm⁻¹ region in the spectrum... -1 The presence of SS stretching vibration peaks (containing disulfide bonds) indicates that the sample contains disulfide bonds.

[0042] In some embodiments, the porosity of the first negative electrode active layer is less than that of the second negative electrode active layer.

[0043] In some embodiments, the thickness of the first negative electrode active layer is greater than the thickness of the second negative electrode active layer.

[0044] In some embodiments, the pore size of the first negative electrode active layer is smaller than that of the second negative electrode active layer.

[0045] In this application, the porosity of the first negative electrode active layer is less than that of the second negative electrode active layer, which can improve the wetting of the electrolyte, improve the conductivity of lithium ions, and thus improve the electrochemical performance of the secondary battery.

[0046] In this application, the thickness of the second negative electrode active layer is greater than that of the second negative electrode active layer, which is beneficial to improving the energy density and cycle life of the electrode.

[0047] In this application, the pore size of the first negative electrode active layer is smaller than that of the second negative electrode active layer, which helps to improve electrolyte penetration and increase lithium-ion transport rate.

[0048] In some embodiments, the porosity of the first negative electrode active layer is 20-30%, for example, it can be a range of one or any two of 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%.

[0049] In some embodiments, the porosity of the second negative electrode active layer is 40-50%, for example, it can be one or any two of the following values: 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%.

[0050] In some embodiments, the thickness of the first negative electrode active layer is 50-100 μm, for example, it can be one or any two of the following values: 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, 75 μm, 80 μm, 85 μm, 90 μm, 95 μm, 100 μm.

[0051] In some embodiments, the thickness of the second negative electrode active layer is 20-50 μm, for example, it can be one or any two of 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm.

[0052] In some embodiments, the pore size of the first negative electrode active layer is 10-50 nm, for example, it can be one or any two of the following values: 10 nm, 12 nm, 15 nm, 17 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 43 nm, 45 nm, 48 nm, 50 nm.

[0053] The pore size of the second negative electrode active layer is 50-200 nm, for example, it can be one of 50 nm, 70 nm, 90 nm, 110 nm, 130 nm, 150 nm, 180 nm, 200 nm or any value between two of them; In some embodiments, the thickness of the first interface protective layer is 20-30 nm, for example, it can be a range of one or any two of 20 nm, 21 nm, 22 nm, 23 nm, 24 nm, 25 nm, 26 nm, 27 nm, 28 nm, 29 nm, and 30 nm; the thickness of the second interface protective layer is 50-80 nm, for example, it can be a range of one or any two of 50 nm, 52 nm, 55 nm, 57 nm, 60 nm, 65 nm, 70 nm, 73 nm, 75 nm, 78 nm, and 80 nm.

[0054] The negative electrode sheet was taken, soaked in DMC or EMC to remove residual electrolyte, dried, and then subjected to cross-sectional argon ion polishing. After sample preparation, cross-sectional SEM testing was performed. Utilizing the potential contrast differences among the different components in the negative electrode sheet, a Thermo Fisher Scientific Apreo 2C scanning electron microscope was used. By mixing the signals received by T1 and T2 detectors (T1 detector receives backscattered electron signals, mainly distinguishing different components within the sample, separating the main material and conductive agent in the negative electrode sheet; T2 detector is a high-resolution detector, highly sensitive to the potential contrast of the sample, distinguishing the poorly conductive binder in the negative electrode sheet), different phase components in the electrode sheet could be distinguished. The images were then colored to more intuitively characterize the distribution of the main material, conductive agent, and binder in the negative electrode sheet. The binder content was then quantitatively analyzed using Avizo image analysis software, calculated as an area fraction. Based on porosity and morphological differences, the first and second negative electrode active layers were distinguished.

[0055] In this application, the porosity of the first negative electrode active layer and the second negative electrode active layer is tested by the following method: according to the SEM image of the cross-section of the negative electrode sheet, the pore area is measured, and the porosity is equal to the percentage of pore area.

[0056] In this application, the thickness of the first negative electrode active layer and the second negative electrode active layer are tested by the following method: based on the SEM image of the negative electrode cross-section, the thickness of the upper and lower layers is measured by a scale.

[0057] In this application, the pore size of the first negative electrode active layer and the second negative electrode active layer is tested by the following method: based on the above SEM image, a T1 / T2 detector is used to obtain a mixed image, which is then colored to identify the number of pores and their individual areas. The average area of ​​the pores is calculated, and the average pore size is calculated by approximating them as circles.

[0058] In this application, the thickness of the first interface protective layer and the second interface protective layer is tested by the following method: based on the cross-sectional sample of the negative electrode sheet, observe under a high-resolution SEM electron microscope, take more than 100 active material particles of each of the first negative electrode active layer and the second negative electrode active layer, observe and measure the thickness of the protective layer on the particle surface according to the different color contrast, and take the average thickness as the thickness of the first interface protective layer and the second interface protective layer.

[0059] In some embodiments, the first polymer layer and the second polymer layer further include lithium salts, wherein the mass percentage of lithium salts in the first polymer layer is less than the mass percentage of lithium salts in the first lithium salt layer, and the mass percentage of lithium salts in the second polymer layer is less than the mass percentage of lithium salts in the second lithium salt layer; it is understood that the lithium salts in the polymer layer can improve the transport efficiency of active ions in the polymer layer.

[0060] The first lithium salt layer and the second lithium salt layer also include polymers. The mass percentage of polymers in the first polymer layer is greater than that in the first lithium salt layer, and the mass percentage of polymers in the second polymer layer is greater than that in the second lithium salt layer. It can be understood that the polymer layers in the lithium salt layer can improve the structural stability of the lithium salt layer.

[0061] Test methods for polymer layer and lithium salt layer: ① Sample Preparation: Quickly rinse the negative electrode sheet with DMC solvent (3 times, 10 s each time) to remove residual electrolyte; dry with inert gas (high-purity Ar) to avoid dissolution or oxidation of the SEI film. Cut the negative electrode sheet into 1 cm × 1 cm pieces and fix it to the XPS sample stage with conductive tape. The entire process is carried out inside the glove box. Use a vacuum transfer chamber to transfer the sample from the glove box to the XPS analysis chamber, ensuring that the SEI film (interface protective layer) does not come into contact with air during the transfer (to prevent the formation of secondary products such as Li2CO3 due to H2O and CO2).

[0062] ② Acquire high-resolution spectra: Perform narrow scans on C 1s, O 1s, F 1s, and Li 1s: pass energy 20 eV, step size 0.05 eV, scan each spectrum 10 times and accumulate to improve the signal-to-noise ratio.

[0063] ③Etching conditions: Ar⁺ ion gun (energy 3 keV, beam spot 2 mm × 2 mm), etching rate needs to be calibrated with SiO2 standard sample (set to 1 nm / min).

[0064] ④ Cyclic mode: Etch for 30 s (corresponding to a depth of ~0.5 nm) → stop etching → acquire high-resolution spectrum, repeat this cycle until the intensity of the graphite substrate signal (CC, 284.8 eV) increases significantly, indicating that the graphite substrate has been etched.

[0065] ⑤ Plot the element chemical state depth distribution curve with etching time as the horizontal axis and the peak area of ​​each chemical state as the vertical axis. The Li1s peak area ratio represents the lithium content.

[0066] Using the above method, the elemental distribution at different depths of the interface protective layer can be obtained. Elemental analysis can then be used to distinguish between the polymer layer and the lithium salt layer at different depths of the interface protective layer.

[0067] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0068] In some embodiments, the first and second negative electrode active layers may each independently include other negative electrode active materials known in the art for lithium-ion batteries. As examples, other negative electrode active materials may include at least one of the following: soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for lithium-ion batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0069] In some embodiments, the first negative electrode active layer and the second negative electrode active layer may also optionally include an adhesive independently. As an example, the adhesive 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).

[0070] In some embodiments, the first negative electrode active layer and the second negative electrode active layer may also optionally include a 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.

[0071] In some embodiments, the first negative electrode active layer and the second negative electrode active layer may also optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose CMC-Na, lithium carboxymethyl cellulose CMC-Li).

[0072] In one embodiment, the secondary battery of this application further includes a positive electrode, an electrolyte, and a separator.

[0073] In one embodiment, the positive electrode sheet includes a positive current collector and a layer of positive active material disposed on at least one surface of the positive current collector.

[0074] In one embodiment, the positive electrode active material layer includes a positive electrode active material.

[0075] In one embodiment, the positive electrode active material includes at least one of lithium cobalt oxide, lithium iron phosphate, and ternary composite materials.

[0076] In one embodiment, the positive electrode active material includes lithium iron phosphate.

[0077] In one embodiment, the positive electrode active material includes a ternary composite material.

[0078] In one embodiment, the ternary composite material includes Li a Ni x Co y Mn z N b At least one of the compounds shown in O2; wherein N includes at least one of Al, Ti, Mg, Fe, V, Y, Nb, Sr, W, and B, 0.9 ≤ a ≤ 1.1, 0 < x < 1, 0 < y < 1, 0 < z < 1, and 0 ≤ b ≤ 0.12.

[0079] It should be noted that nitrogen (N) can be incorporated into the positive electrode active material through doping and / or surface coating. The testing method for nitrogen is based on EPA6010D-2018 Inductively Coupled Plasma Atomic Emission Spectrometry: the positive electrode sheet is cut into circular pieces with a diameter of 12±20 mm, and 20 circular pieces are taken for ICP testing.

[0080] In one embodiment, the positive electrode active material layer comprises the following components in weight percentage: 30wt%-98wt% positive electrode active material, 0.5wt%-30wt% conductive agent, and 0.5wt%-20wt% binder.

[0081] In one embodiment, the binder includes at least one of polytetrafluoroethylene (PTFE), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), nitrile rubber (NBR), styrene-ethylene-butene-styrene copolymer (SEBS), styrene-butadiene-styrene copolymer (SBS), lithium polyacrylate (LiPAA), sodium polyacrylate (NaPAA), sodium alginate, and lithium alginate.

[0082] In some embodiments, the conductive agent may include carbon-based materials, powdered nickel or other metal particles, or conductive polymers. Carbon-based materials may include particles such as carbon black, graphite, acetylene black (e.g., KETCHENTM black or DENKATM black), carbon fibers and carbon nanotubes, graphene, etc. Examples of conductive polymers include polyaniline, polythiophene, polyacetylene, polypyrrole, etc.

[0083] A positive current collector facilitates the flow of electrons between the positive electrode and an external circuit. The positive current collector may comprise a metal, such as a metal foil, metal grid, or screen, or a metal mesh. For example, the positive current collector may be formed from aluminum, stainless steel, and / or nickel, or any other suitable conductive agent known to those skilled in the art.

[0084] The electrolyte is a known electrolyte in the art that can be used in secondary batteries.

[0085] The separator is a type of membrane known in the art that can be used in secondary batteries and is stable to the electrolyte used. Specifically, it may include one or more of polyolefins, aromatic polyamides, polytetrafluoroethylene, and polyethersulfone. Further, the separator may include one or both of polyethylene and polypropylene. Moreover, the separator may be obtained by sequentially stacking multiple layers of materials; for example, the separator may include sequentially stacked polypropylene layers, polyethylene layers, and polypropylene layers.

[0086] In one embodiment, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and electrolyte.

[0087] In one embodiment, the outer casing of the secondary battery can be a rigid shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0088] This application does not impose any particular restrictions on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape.

[0089] In one embodiment, the charging cutoff voltage of the secondary battery in this application is not less than 4.2V, meaning the battery can be used at a high voltage of not less than 4.2V. For example, the battery can operate in the range of 4.2V to 4.9V, or it can operate in the range of 4.3V to 4.8V.

[0090] In one embodiment, the secondary battery of this application can operate at a temperature greater than 45°C, meaning the battery can be used at a high temperature of not less than 45°C. For example, the battery can operate in the range of 45 to 70°C.

[0091] Secondly, a method for preparing a secondary battery is provided, comprising the following steps: A negative electrode material is added to a solution of polyethylene glycol diacrylate and a photoinitiator, and then irradiated with ultraviolet light to form a polyethylene glycol diacrylate polymer on the surface of the negative electrode material, resulting in a negative electrode material coated with a first polyethylene glycol diacrylate polymer and a negative electrode material coated with a second polyethylene glycol diacrylate polymer. A first negative electrode slurry is prepared, comprising a negative electrode material coated with a first polyethylene glycol diacrylate polymer and lithium difluorooxalate borate (LiDFOB). A second negative electrode slurry is prepared, the second negative electrode slurry comprising a negative electrode material coated with a second polyethylene glycol diacrylate polymer and lithium difluorooxalate borate; A first negative electrode slurry and a second negative electrode slurry are coated on the surface of the negative electrode current collector, with the first negative electrode slurry located between the negative electrode current collector and the second negative electrode slurry, to obtain a negative electrode sheet; The negative electrode, positive electrode, and separator are assembled into a battery cell, and the battery cell is placed in a package shell. Electrolyte is injected into the package shell to form a secondary battery.

[0092] In this application, taking the first negative electrode active layer as an example, during the UV curing process, polyethylene glycol diacrylate (PEGDA) undergoes cross-linking, forming a first polymer layer on the surface of the first negative electrode active material. This reduces the loss of active lithium during the formation stage (i.e., the first polymer layer reduces the contact between the electrolyte and the first negative electrode active material, reducing side reactions and thus reducing the loss of active lithium). During charging, lithium difluorooxalate borate decomposes to generate lithium salt at voltages above 4.0V, forming a first lithium salt layer on the surface of the first polymer layer. The first polymer layer and the first lithium salt layer constitute a first interface protective layer. Similarly, the second negative electrode active layer also undergoes the same reaction. Furthermore, lithium difluorooxalate borate also generates ·DFOB during decomposition. - Free radicals can further initiate PEGDA crosslinking, improving the density of the first polymer layer.

[0093] It is understood that in some embodiments, some of the PEGDA may migrate to the first lithium salt layer, resulting in the presence of a polymer in the first lithium salt layer. In other embodiments, lithium salts in the electrolyte may also migrate to the first polymer layer, resulting in the presence of lithium salts in the first polymer layer.

[0094] In some embodiments, with the mass percentage of the dispersed phase in the first negative electrode slurry being 100%, the mass percentage of the polyethylene glycol diacrylate polymer is 4-6%, for example, it can be one or any two of 4%, 4.2%, 4.4%, 4.6%, 4.8%, 5%, 5.2%, 5.4%, 5.6%, 5.8%, 6%; the mass percentage of the lithium difluorooxalate borate is 3-5%, for example, it can be one or any two of 3%, 3.2%, 3.5%, 3.7%, 4%, 4.3%, 4.5%, 4.8%, 5%. With the mass percentage of the dispersed phase in the second negative electrode slurry being 100%, the mass percentage of the polyethylene glycol diacrylate polymer is 2-3%, for example, it can be one or any two of 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%; the mass percentage of the lithium difluorooxalate borate is 1-2%, for example, it can be one or any two of 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%.

[0095] In this application, the thickness of the first interface protective layer and the second interface protective layer is controlled by controlling the mass percentage content of polyethylene glycol diacrylate and lithium difluorooxalate borate in the dispersed phase of the first negative electrode active slurry and the second negative electrode active slurry.

[0096] In some embodiments, the first negative electrode slurry and the second negative electrode slurry each independently include boron nitride nanosheets; based on the mass percentage of the dispersed phase in the first negative electrode slurry being 100%, the mass percentage of the boron nitride nanosheets is 0.5-1%; for example, it can be one of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% or any value between two of them. With the mass percentage of the dispersed phase in the second negative electrode slurry being 100%, the mass percentage of the boron nitride nanosheets is 0.5-1%, for example, it can be one of 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, or any value between two of them.

[0097] In some embodiments, the first negative electrode slurry and the second negative electrode slurry each independently include dithiothreitol; the mass percentage of dithiothreitol is 0.2-0.8% based on the mass percentage of the dispersed phase in the first negative electrode slurry being 100%; for example, it can be one of 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8% or any value between two of them; With the mass percentage of the dispersed phase in the second negative electrode slurry being 100%, the mass percentage of the dithiothreitol is 0.2-0.8%, for example, it can be one of 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, or any value between two of them.

[0098] The reduction potential of the polymer layer in the interface protective layer is lower than that of the lithium salt layer. During the initialization stage, it promotes the formation of disulfide bonds by the thiol groups of dithiothreitol through oxidation. During the cycling process of the secondary battery, the microcracks in the interface protective layer can be repaired by the breaking / recombining of disulfide bonds, thereby improving the long-term stability of the interface protective layer.

[0099] In this application, the dispersed phase in the first and second negative electrode slurries refers to substances other than the solvent. The solvent is water or an organic solvent; as a specific example of an organic solvent, it could be N-methylpyrrolidone.

[0100] In some embodiments, the specific steps for coating the surface of the negative electrode current collector with the first negative electrode slurry and the second negative electrode slurry, wherein the first negative electrode slurry is located between the negative electrode current collector and the second negative electrode slurry, and then performing ultraviolet curing to obtain the negative electrode sheet are as follows: The first negative electrode slurry is coated on the surface of the negative electrode current collector and then subjected to first ultraviolet curing to form the first negative electrode active layer. Then, the second negative electrode slurry is coated on the surface of the first negative electrode active layer, and a second ultraviolet curing is performed to form the second negative electrode active layer, thus obtaining the negative electrode sheet.

[0101] Through first UV curing and second UV curing, the polyethylene glycol diacrylate in the first negative electrode active layer and the second negative electrode active layer have different degrees of crosslinking.

[0102] Specifically, the power of the first UV curing is 6-10 W / cm², and the time is 15-25s. The parameters of the first UV curing are within the above range, so that the degree of crosslinking of polyethylene glycol diacrylate in the first negative electrode active layer is 10-20%, thereby allowing the first negative electrode active layer to retain ion transport channels. Specifically, the power of the second UV curing is 10-14 W / cm², and the time is 35-45s. The parameters of the second UV curing are within the above range, so that the degree of crosslinking of polyethylene glycol diacrylate in the second negative electrode active layer is 45-55%, and the polyethylene glycol diacrylate forms a dense prepolymer network.

[0103] Thirdly, an electrical device is provided, comprising the aforementioned secondary battery or a secondary battery prepared by the method for preparing a secondary battery.

[0104] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, 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 and satellites, energy storage systems, etc., but are not limited thereto.

[0105] Example 1 This embodiment provides a method for preparing a secondary battery, including the following steps: (1) Preparation method of PEGDA UV-cured coated negative electrode (graphite) particles: PEGDA and a photoinitiator (water-soluble Irgacure 2959, an α-hydroxy ketone free radical UV photoinitiator) were mixed at a mass ratio of 95:5, and water was added to prepare a 30% solution. Graphite:PEGDA was added at a mass ratio of 93.5%:3%, and the mixture was ultrasonically dispersed for 20 min after adding graphite particles. Under irradiation with a 400 W high-pressure mercury lamp (UVA 320-400 nm, light intensity 800 μW / cm²), the mixture was magnetically stirred (300 rpm) in a quartz reactor, and cured in a nitrogen atmosphere for 30 min after oxygen removal. The product was then vacuum filtered through a 0.45 μm filter membrane, washed three times with anhydrous ethanol, and vacuum dried at 60℃ for 12 h to obtain the coated particles, referred to as the first PEGDA polymer-coated graphite (the first polyethylene glycol diacrylate polymer-coated negative electrode material).

[0106] By adjusting the mass ratio of graphite to PEGDA in the above preparation method to 89% to 5%, a second PEGDA polymer-coated graphite (a negative electrode material coated with a second polyethylene glycol diacrylate polymer) is obtained.

[0107] (2) Preparation of negative electrode sheet 96.5% by weight of the first PEGDA polymer-coated graphite, 1.5% by weight of LiDFOB, 1% by weight of Super P conductive agent, and 1% by weight of SBR binder were added to deionized water and mixed evenly to obtain the first negative electrode slurry. 94% by weight of second PEGDA polymer-coated graphite, 4% by weight of LiDFOB, 1% by weight of Super P conductive agent and 1% by weight of CMC binder were added to deionized water and mixed evenly to obtain the second negative electrode slurry. The first negative electrode slurry is coated on a copper foil with a thickness of 8 μm to obtain the first negative electrode active layer; The second negative electrode slurry is coated on the surface of the first negative electrode active layer to obtain the second negative electrode active layer, and then rolled to obtain the negative electrode sheet.

[0108] (3) Preparation of positive electrode sheet LiFePO4, conductive carbon black, and polyvinylidene fluoride (PVDF) binder were mixed uniformly in a mass ratio of lithium iron phosphate: conductive carbon black: PVDF = 96:2:2, and then uniformly dispersed in 1-methyl-2-pyrrolidone (NMP) to form a uniform positive electrode slurry. The obtained positive electrode slurry was coated on the positive electrode current collector aluminum foil, and after baking, rolling, and cutting, the positive electrode sheet was obtained, wherein the thickness of the positive electrode active material layer was 130 μm. (4) Preparation of the diaphragm A 12μm thick polyethylene microporous film was used as the porous membrane substrate. Inorganic alumina powder, polyvinylpyrrolidone, and acetone solvent were mixed evenly in a weight ratio of 3:1.5:5.5 to prepare an inorganic slurry. The inorganic slurry was then coated on both sides of the substrate and dried to form an inorganic layer with a thickness of 5μm, thus obtaining the membrane. (5) Preparation of electrolyte At room temperature, in a glove box filled with argon (H2O < 1 ppm, O2 < 1 ppm), ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed uniformly at a volume ratio of 1:1, and water was removed using a 4 Å molecular sieve to obtain a mixed solvent. Lithium salt LiPF6 was added to a mixed solvent and mixed thoroughly. Then, fluoroethylene carbonate (FEC) was added to obtain the electrolyte. The mass concentration of LiPF6 was 1 mol / L, and the mass percentage of fluoroethylene carbonate was 5%. (6) Preparation of secondary batteries The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes. After winding, hot pressing and shaping, and electrode tab welding, a bare cell is obtained. The bare cell is placed in an outer packaging aluminum-plastic film and baked in an oven at 85±10℃ for 24 hours. The electrolyte prepared above is injected into the dried battery, and the battery is allowed to stand, form, and be capacity tested to complete the preparation of the secondary battery. The initial charging voltage during formation is 0.3-0.8V, and the constant current is 0.1C. The secondary charging voltage is 3.8-4.2V, and the constant current is 0.05C.

[0109] Example 2 This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: the second PEGDA polymer coating on graphite is reduced to 93.5%, and 0.5% boron nitride nanosheets are added.

[0110] Example 3 This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: the second PEGDA polymer coating on graphite is reduced to 93%, and then 1% boron nitride nanosheets are added.

[0111] Example 4 This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: the second PEGDA polymer coating on graphite is reduced to 92.5%, and 1.5% boron nitride nanosheets are added.

[0112] Example 5 This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: the first PEGDA polymer coating of graphite is reduced to 95.5%, and then 1% boron nitride nanosheets are added.

[0113] Example 6 This embodiment provides a method for preparing a secondary battery, which differs from Embodiment 1 in that: the first PEGDA polymer coating on graphite is reduced to 95.5%, and then 1% boron nitride nanosheets are added; Reduce the second PEGDA polymer coating of graphite to 93%, and then add 1% boron nitride nanosheets.

[0114] Example 7 This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: the second PEGDA polymer coating on graphite is reduced to 93.8%, and 0.2% dithiothreitol is added.

[0115] Example 8 This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: the second PEGDA polymer coating on graphite is reduced to 93.5%, and 0.5% dithiothreitol is added.

[0116] Example 9 This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: the second PEGDA-coated graphite is reduced to 93.2%, and 0.8% dithiothreitol is added.

[0117] Example 10 This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: the first PEGDA polymer coating of graphite is reduced to 96%, and then 0.5% dithiothreitol is added.

[0118] Example 11 This embodiment provides a method for preparing a secondary battery, which differs from Embodiment 1 in that: the first PEGDA polymer coating on graphite is reduced to 96%, and then 0.5% dithiothreitol is added; Reduce the amount of graphite coated with the second PEGDA polymer to 93.5%, and then add 0.5% dithiothreitol.

[0119] Example 12 This embodiment provides a method for preparing a secondary battery, which differs from Example 1 in that: the second PEGDA polymer coating on graphite is reduced to 92.5%, and 1% boron nitride nanosheets and 0.5% dithiothreitol are added.

[0120] Example 13 This embodiment provides a method for preparing a secondary battery, which differs from Embodiment 1 in that the mass ratio of graphite:PEGDA in the above preparation method is adjusted to 89%:6% to obtain a second PEGDA polymer-coated graphite.

[0121] 95% by weight of second PEGDA polymer-coated graphite, 3% by weight of LiDFOB, 1% by weight of Super P conductive agent, and 1% by weight of CMC binder were added to deionized water and mixed evenly to obtain the second negative electrode slurry.

[0122] Example 14 This embodiment provides a method for preparing a secondary battery, which differs from Embodiment 1 in that the mass ratio of graphite:PEGDA in the above preparation method is adjusted to 89%:4% to obtain a second PEGDA polymer-coated graphite.

[0123] 93% by weight of second PEGDA polymer-coated graphite, 5% by weight of LiDFOB, 1% by weight of Super P conductive agent, and 1% by weight of CMC binder were added to deionized water and mixed evenly to obtain the second negative electrode slurry.

[0124] Example 15 This embodiment provides a method for preparing a secondary battery, which differs from Embodiment 1 in that: the mass ratio of graphite:PEGDA in the above preparation method is adjusted to 94%:2% to obtain a first PEGDA polymer-coated graphite.

[0125] 96% by weight of the first PEGDA polymer-coated graphite, 2% by weight of LiDFOB, 1% by weight of Super P conductive agent, and 1% by weight of SBR binder were added to deionized water and mixed evenly to obtain the first negative electrode slurry.

[0126] Example 16 This embodiment provides a method for preparing a secondary battery, which differs from Embodiment 1 in that: the mass ratio of graphite:PEGDA in the above preparation method is adjusted to 94.5%:2.5% to obtain a first PEGDA polymer-coated graphite.

[0127] 97% by weight of the first PEGDA polymer-coated graphite, 1% by weight of LiDFOB, 1% by weight of Super P conductive agent, and 1% by weight of SBR binder were added to deionized water and mixed evenly to obtain the first negative electrode slurry.

[0128] Example 17 This embodiment provides a method for preparing a secondary battery, which differs from Embodiment 16 in that the porosity, thickness, and pore size of the negative electrode active layer are changed. The thickness of the first and second active layers is adjusted by coating weight, and the porosity and pore size of the negative electrode active layer are adjusted by adjusting the electrode baking rate and electrode compaction density.

[0129] Comparative Example 1 This comparative example provides a method for preparing a secondary battery, which differs from Example 1 in that the preparation method of the negative electrode sheet is different. The preparation method of the negative electrode sheet in this comparative example is as follows: 96% by mass of graphite, 2% by mass of polyvinylidene fluoride, 1% by mass of Super P conductive agent, and 1% by mass of CMC binder are added to N-methylpyrrolidone and mixed evenly to obtain a negative electrode slurry; the obtained negative electrode slurry is coated on a copper foil with a thickness of 8 μm, dried at room temperature, transferred to an oven for further drying, and then cold-pressed and slit to obtain the negative electrode sheet; wherein, the thickness of the negative electrode active material layer is 150 μm.

[0130] Comparative Example 2 This comparative example provides a method for preparing a secondary battery, which differs from Example 1 in that: based on the mass percentage of the dispersed phase in the first negative electrode slurry being 100%, it includes the following components by mass percentage: 96% graphite, 2% polyvinylidene fluoride, 1% Super P conductive agent, and 1% SBR binder.

[0131] Comparative Example 3 This comparative example provides a method for preparing a secondary battery, which differs from Example 1 in that: based on the mass percentage of the dispersed phase in the second negative electrode slurry being 100%, it includes the following components by mass percentage: 96% graphite, 2% polyvinylidene fluoride, 1% Super P conductive agent, and 1% CMC binder.

[0132] Comparative Example 4 This comparative example provides a method for preparing a secondary battery, which differs from Example 17 in that the thickness of the first interface protective layer is greater than the thickness of the second interface protective layer.

[0133] Performance testing (1) Charge transfer resistance test of symmetrical battery with negative electrode: Take a negative electrode containing active material and cut it into multiple rectangular small electrodes with a size of 2.5*4cm. Assemble the small electrodes into a symmetrical battery by combining the negative electrode, separator (impregnated with electrolyte), adhesive tape with a 16mm hole in the middle, and the negative electrode. Test the AC impedance of the symmetrical battery by applying a small sinusoidal AC voltage (5 mV) with a frequency scanning range of 0.01 Hz–100 kHz, measuring the current response and calculating the impedance, and outputting the Nyquist plot. Analyze the interface resistance (such as ohmic resistance, charge transfer resistance, and diffusion impedance) by fitting the equivalent circuit.

[0134] (2) SEI film thickness (protective layer thickness) test method: Based on the cross-sectional sample of the negative electrode sheet, observe under a high-resolution SEM electron microscope, take more than 100 active material particles of the first negative electrode active layer and the second negative electrode active layer respectively, observe and measure the thickness of the protective layer on the particle surface according to the different color contrast, and take the average thickness as the thickness of the first interface protective layer and the second interface protective layer.

[0135] (3) Cycle retention rate: Initial capacity C0 determination: Under standard conditions (e.g., 25°C), the battery is charged and discharged 3 times at a rate of 0.33C, and the initial capacity C0 is recorded.

[0136] Cyclic test: Constant current charge and discharge at a 1C (=3C0) rate (charge to the upper limit voltage, then maintain constant voltage to the cutoff current of 0.05C, and discharge to the termination voltage), for a specified number of cycles of 1000. The ratio of the 1C capacity C2 after 1000 cycles to the initial 1C capacity C1. 1C capacity retention rate: Capacity retention rate = C2 / C1 × 100%.

[0137] (4) Expansion rate of the negative electrode: Take two cells, charge one to the upper voltage limit at 0.33C, and discharge the other to the lower voltage limit. Disassemble the secondary battery, remove the negative electrode, and use a multimeter to measure the average thickness of 50 points in different areas of the middle of the load negative electrode active material layer and the thickness of the copper foil, S foil. The thickness of the negative electrode of the fully charged secondary battery is denoted as S charge, and the thickness of the negative electrode of the discharged secondary battery is denoted as S discharge. The expansion rate f = (S charge - S discharge) / (S discharge - S foil) × 100%.

[0138] The test results are shown in Tables 1-3.

[0139] Table 1 Table 2 Table 3 As can be seen from the experimental data in Table 1-3, the expansion rate of the negative electrode sheet of this application is ≤16.5%, the negative electrode charge resistance after cycling is ≤42 mΩ·cm², and the cycle retention rate of the secondary battery containing the negative electrode sheet of this application is ≥84%.

[0140] Experimental data from Examples 1-6 show that the first negative electrode active layer comprising boron nitride nanosheets and / or the second negative electrode active layer comprising boron nitride nanosheets can significantly reduce the expansion rate of the negative electrode and the negative electrode charge resistance after cycling, as well as improve the cycle retention rate of the secondary battery containing the negative electrode of this application.

[0141] Experimental data from Examples 1 and 7-11 show that the first polymer layer and the second polymer layer each independently include disulfide bonds, which can further reduce the expansion rate of the negative electrode and the negative electrode charge resistance after cycling, as well as improve the cycle retention rate of the secondary battery containing the negative electrode of this application.

[0142] Experimental data from Examples 1 and Comparative Examples 1-3 show that having a first interface protection layer in the first negative electrode active layer and a second interface protection layer in the second negative electrode active layer helps to obtain a negative electrode sheet with low expansion rate and low negative electrode charge resistance after cycling, as well as a secondary battery with high cycle retention rate.

[0143] The experimental data from Example 17 and Comparative Example 4 show that the thickness of the first interface protective layer is smaller than that of the second interface protective layer, which helps to obtain a negative electrode sheet with low expansion rate and low negative electrode charge resistance after cycling, as well as a secondary battery with high cycle retention rate.

[0144] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative current collector and a first negative active layer and a second negative active layer disposed on at least one side of the surface of the negative current collector, wherein the first negative active layer is located between the negative current collector and the second negative active layer. The first negative electrode active layer includes a first negative electrode active material, and the surface of the first negative electrode active material is covered with a first interface protective layer; along the direction from the first negative electrode active material to the first interface protective layer, the first interface protective layer includes a first polymer layer and a first lithium salt layer disposed sequentially. The second negative electrode active layer includes a second negative electrode active material, and the surface of the second negative electrode active material is covered with a second interface protective layer; along the direction from the second negative electrode active material to the second interface protective layer, the second interface protective layer includes a second polymer layer and a second lithium salt layer disposed sequentially. The thickness of the first interface protective layer is less than the thickness of the second interface protective layer.

2. The negative electrode sheet as described in claim 1, characterized in that, The first polymer layer and the second polymer layer comprise polyethylene glycol diacrylate polymer; the first lithium salt layer and the second lithium salt layer each independently comprise at least one of lithium fluoride and lithium carbonate.

3. The negative electrode sheet as described in claim 1, characterized in that, The first negative electrode active layer further includes boron nitride nanosheets, which are located in the pores of the first negative electrode active layer and / or the first lithium salt layer; And / or, the second negative electrode active layer further includes boron nitride nanosheets, which are located in the pores of the second negative electrode active layer and / or the second lithium salt layer.

4. The negative electrode sheet as described in claim 1, characterized in that, The first polymer layer and the second polymer layer each independently comprise sulfonic acid groups and / or disulfide bonds.

5. The negative electrode sheet as described in claim 1, characterized in that, Meet at least one of the following: (a) The porosity of the first negative electrode active layer is less than that of the second negative electrode active layer; (b) The thickness of the first negative electrode active layer is greater than the thickness of the second negative electrode active layer; (c) The pore size of the first negative electrode active layer is smaller than that of the second negative electrode active layer.

6. The negative electrode sheet as described in claim 1, characterized in that, The porosity of the first negative electrode active layer is 20-30%, and the porosity of the second negative electrode active layer is 40-50%.

7. The negative electrode sheet as described in claim 1, characterized in that, Meet at least one of the following: (a) The thickness of the first negative electrode active layer is 50-100 μm, and the thickness of the second negative electrode active layer is 20-50 μm; (b) The pore size of the first negative electrode active layer is 10-50 nm; the pore size of the second negative electrode active layer is 50-200 nm; (c) The thickness of the first interface protective layer is 20-30 nm; the thickness of the second interface protective layer is 50-80 nm.

8. The negative electrode sheet as described in any one of claims 1 to 7, characterized in that, The first polymer layer and the second polymer layer further include lithium salt, wherein the mass percentage of lithium salt in the first polymer layer is less than the mass percentage of lithium salt in the first lithium salt layer, and the mass percentage of lithium salt in the second polymer layer is less than the mass percentage of lithium salt in the second lithium salt layer. The first lithium salt layer and the second lithium salt layer further include polymers, wherein the mass percentage of the polymer in the first polymer layer is greater than the mass percentage of the polymer in the first lithium salt layer, and the mass percentage of the polymer in the second polymer layer is greater than the mass percentage of the polymer in the second lithium salt layer.

9. A secondary battery, characterized in that, Includes the negative electrode sheet as described in any one of claims 1-8.

10. An electrical device, characterized in that, It includes the negative electrode sheet as described in any one of claims 1-8 or the secondary battery as described in claim 9.

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