Lithium secondary battery, negative pole piece and electric device

Abstract

The invention relates to the technical field of batteries, in particular to a lithium secondary battery, a negative pole piece and an electric device. The lithium secondary battery comprises a negative electrode plate, the negative electrode plate comprises a negative electrode current collector and a negative electrode active layer arranged on at least one side of the negative electrode current collector, and the negative electrode active layer comprises a negative electrode active material and a fluorine-containing polymer containing-CH2CF (CF3)-repeating units. The fluorine-containing polymer containing-CH2CF (CF3)-repetitive units is added into the negative electrode active layer, and the fluorine-containing polymer contains a large number of F-containing groups and can spontaneously react with dead lithium and other byproducts generated by side reaction of the battery, so that the byproducts are converted into an inorganic component LiF which is beneficial to improvement of the electrode performance; the lithium loss can be effectively reduced, so that the problem of capacity reduction is improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a lithium secondary battery, a negative electrode sheet,

[0002] Electrical appliances. Background Technology

[0003] Battery cycle performance has always been a key focus in product development, and a decline in cycle performance is a significant factor.

[0004] A key manifestation is capacity reduction. During battery cycling, active lithium ions participate in electrochemical processes.

[0005] The reaction provides capacity to the battery; however, various side reactions also occur in the battery, especially at the negative electrode.

[0006] The formation of dead lithium and other byproducts on the chip causes lithium loss and reduces capacity. Furthermore, these byproducts...

[0007] Material will accumulate on the surface of the negative electrode, affecting the structure of the negative electrode and causing capacity decay. Summary of the Invention

[0008] This application is made in view of the above-mentioned technical problems, and its purpose is to improve the capacity of batteries.

[0009] The problem of quantity decay.

[0010] To achieve the above objectives, this application provides a lithium secondary battery, a negative electrode sheet, and...

[0011] Electrical appliances.

[0012] The first aspect of this application provides a lithium secondary battery, including a negative electrode sheet and a negative electrode...

[0013] The sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side of the negative electrode current collector.

[0014] The active layer comprises a negative electrode active material and a fluoropolymer containing -CH2CF(CF3)- repeating units.

[0016] This application embodiment adds a repeating -CH2CF(CF3)- compound to the negative electrode active layer.

[0017] Fluoropolymers in the cells showed that they could effectively improve the problem of battery capacity decay.

[0018] This may be because this fluoropolymer contains a large number of F-containing groups, which can react with battery components.

[0019] The generated dead lithium and other byproducts should undergo spontaneous reactions, transforming these byproducts into beneficial electrical components.

[0020] LiF, an inorganic component with improved polar properties.

[0021] LiF possesses good mechanical strength, which can improve the stability of SEI films, thereby reducing...

[0022] Because the SEI film rupture causes repeated lithium ion extraction and insertion, continuously consuming active lithium, lithium loss can be reduced. At the same time, the fluoropolymer converts by-products into LiF, which reduces the by-products accumulated on the negative electrode, thereby alleviating the volume expansion of the negative electrode, improving the stability of the SEI film, and also helping to reduce lithium loss, thus improving the capacity decay problem.

[0023] Moreover, since side reactions and byproducts will continue to occur during battery cycling, this method of turning byproducts into LiF, which is beneficial to improving electrode performance, can continue to exist throughout the battery cycling process. In other words, the components inside the negative electrode can be turned into valuable resources during dynamic cycling, and the byproducts can be reused to continuously improve the lithium loss of the battery and slow down the battery capacity decay rate.

[0024] In some embodiments, the F element content of the fluoropolymer is 2% to 30% by mass, optionally 10% to 15%. At a suitable F element content, the fluoropolymer has enough F-containing groups to react with byproducts such as dead lithium to form LiF.

[0025] In some embodiments, the fluoropolymer includes one or more of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), perfluoroethylene propylene (F46, tetrafluoroethylene-hexafluoropropylene copolymer), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer (THV), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). These fluoropolymers all contain -CH2CF(CF3)- repeating units, which can improve the problem of battery capacity decay.

[0026] In some embodiments, in the lithium secondary battery, the mass content of the fluoropolymer in the negative electrode active layer is 0.3% to 3%, optionally 0.8% to 1.2%. At this mass content of the fluoropolymer, the fluoropolymer can be effectively utilized to improve the stability of the SEI film, reduce lithium loss, and slow down the rate of capacity decay, while also ensuring that the negative electrode active material has a sufficient content, thereby enabling the lithium secondary battery to have a sufficiently high energy density.

[0027] In some embodiments, the weight-average molecular weight of the fluoropolymer is 50 × 10⁻⁶. 3 ~455×10 3 Optionally 80×10 3 ~110×10 3Molecular weight is one of the factors affecting the properties of fluoropolymers, such as their electrical conductivity and mechanical strength. In this application, a fluoropolymer with a specific molecular weight is added to the negative electrode. This fluoropolymer has good electrical conductivity and mechanical strength, which can improve battery capacity decay while giving the negative electrode good conductivity and structural stability.

[0028] In some embodiments, the negative electrode active layer further includes a binder, which includes one or more of polyacrylic acid (PAA) and polytetrafluoroethylene (PTFE). Typically, the preparation process of the negative electrode active layer uses an aqueous system (using water as a solvent). However, fluoropolymers are insoluble in water, resulting in a low viscosity of the negative electrode slurry that is difficult to adhere to the current collector. Adding binders such as PAA and PTFE can effectively improve this situation.

[0029] In some embodiments, the binder includes PAA, and the mass content of PAA in the negative electrode active layer is 0.1% to 1%, optionally 0.2% to 0.25%. At this mass content, PAA can be used to improve the negative electrode process, while ensuring that PAA does not occupy too much of the proportion of the negative electrode active layer, so that the negative electrode active material has a sufficient content, thereby enabling the lithium secondary battery to have a sufficiently high energy density.

[0030] In some embodiments, the weight-average molecular weight of PAA is 50 × 10⁻⁶. 3 ~500×10 3 Optionally 80×10 3 ~100×10 3 Molecular weight is one of the key factors affecting the viscosity of PAA. By selecting PAA with a certain molecular weight as a binder, this PAA has high viscosity, which can effectively improve the low viscosity problem caused by the presence of fluoropolymers.

[0031] In some embodiments, the negative electrode active material includes one or more of graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and titanium-based materials. During battery cycling, byproducts such as dead lithium appear on the surface of these negative electrode active materials. The solution of this application embodiment improves the capacity decay of batteries using various negative electrode active materials and has wide applicability.

[0032] The second aspect of this application provides a negative electrode sheet, which is the negative electrode sheet in the lithium secondary battery of the first aspect mentioned above.

[0033] That is, the negative electrode includes a negative current collector and a negative active layer disposed on at least one side of the negative current collector. The negative active layer includes a negative active material and a fluoropolymer containing -CH2CF(CF3)- repeating units.

[0034] This application demonstrates that by adding a fluoropolymer containing -CH2CF(CF3)- repeating units to the negative electrode active layer, the battery capacity decay problem can be effectively improved. This is likely because the fluoropolymer contains a large number of F-containing groups, which can spontaneously react with byproducts such as dead lithium generated by battery side reactions, transforming these byproducts into inorganic components, LiF, which are beneficial for improving electrode performance.

[0035] LiF has good mechanical strength, which can improve the stability of the SEI film, thereby reducing the repeated extraction and insertion of lithium ions due to SEI film rupture and the continuous consumption of active lithium, thus reducing lithium loss. At the same time, the fluoropolymer converts by-products into LiF, which reduces the by-products accumulated on the negative electrode, thereby alleviating the volume expansion of the negative electrode, improving the stability of the SEI film, and also helping to reduce lithium loss, thus improving the capacity decay problem.

[0036] Moreover, since side reactions and byproducts will continue to occur during battery cycling, this method of turning byproducts into LiF, which is beneficial to improving electrode performance, can continue to exist throughout the battery cycling process. In other words, the components inside the negative electrode can be turned into valuable resources during dynamic cycling, and the byproducts can be reused to continuously improve the lithium loss of the battery and slow down the battery capacity decay rate.

[0037] A third aspect of this application provides an electrical device including the aforementioned lithium secondary battery.

[0038] The aforementioned battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The battery in this embodiment exhibits the advantages of slow capacity decay, good cycle performance, and long lifespan. Therefore, applying this battery to an electrical device improves the user experience of the device. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 The above are SOH curves of the batteries of Example 1, Comparative Example 1 and Comparative Example 2 of this application at different cycle numbers;

[0041] Figure 2 This is a diagram illustrating the mechanism by which PVDF-HFP improves battery life in one embodiment of this application.

[0042] Figure 3This is a schematic diagram of a battery cell according to one embodiment of this application;

[0043] Figure 4 for Figure 3 An exploded view of a battery cell according to one embodiment of this application is shown.

[0044] Figure 5 This is a schematic diagram of a battery module according to one embodiment of this application;

[0045] Figure 6 This is a schematic diagram of a battery pack according to one embodiment of this application;

[0046] Figure 7 for Figure 6 An exploded view of a battery pack according to one embodiment of this application is shown;

[0047] Figure 8 This is a schematic diagram of an electrical device in which a battery is used as a power source, according to one embodiment of this application.

[0048] Figure label:

[0049] 01-Housing, 02-Cover plate, 03-Electrode assembly, 04-Battery cell, 05-Battery module, 06-Upper housing, 07-Lower housing. Detailed Implementation

[0050] The following detailed description of the implementation of the detection system and detection method of this application is provided with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand this application and are not intended to limit the subject matter of the claims.

[0051] 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 understood that ranges of 60–110 and 80–120 are also expected. 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 "a–b" 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.

[0052] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0053] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0054] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates 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.

[0055] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0056] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0057] Currently, limiting factors for battery cycle life include capacity decay caused by the deterioration of the structural performance of the negative electrode. This structural performance deterioration manifests in the following ways: During normal cycling, lithium loss occurs due to the formation of an SEI film (solid electrolyte membrane) on the surface of the negative electrode, leading to a decrease in capacity; byproducts formed during lithium loss accumulate on the negative electrode, causing its volume to expand rapidly. Once this expansion is restricted, the pore structure of the negative electrode collapses rapidly, resulting in a rapid reduction in the transport paths between the electrolyte and active lithium within the negative electrode structure, and a rapid increase in polarization, further triggering capacity decay and a rapid decline in lifespan. Simultaneously, during cycling, the SEI film cannot remain stable due to the repeated extraction and insertion of lithium ions. The SEI film is continuously consumed, decomposed, and regenerated, a process that consumes a large number of active lithium ions. The continuous loss of active lithium material leads to a decrease in capacity and a rapid decline in lifespan.

[0058] To address the cycle life challenges of these batteries, related technologies have attempted to improve overall cell dynamics by using low-lithium-consumption negative electrode active materials (such as low-lithium-consumption graphite), employing highly conductive electrolytes, and adding lithium to the cathode. However, even with these methods, byproducts still appear on the surface of the negative electrode during battery cycling. These byproducts do not change form, and therefore, with increasing cycle counts, they accumulate on the negative electrode surface, causing volume expansion and destabilizing the SEI film, leading to capacity decay. Furthermore, the lost lithium cannot be reused.

[0059] Based on this, this application proposes a battery, which can be a lithium-ion battery. By adding a fluoropolymer containing -CH2CF(CF3)- repeating units (HFP repeating units) to the active layer of the negative electrode, byproducts such as dead lithium can be converted into inorganic components that are beneficial to the electrode performance, and byproducts can be reused, thereby reducing the loss of active lithium and improving the stability of the SEI film, thereby reducing lithium consumption caused by SEI film rupture and improving the problem of battery capacity decay.

[0060] This application provides a lithium secondary battery, which can be a lithium-ion battery, hereinafter referred to as a battery. Typically, a battery includes a negative electrode, a positive electrode, an electrolyte, and a separator. During charging and discharging, active ions repeatedly insert and extract between the positive and negative electrodes. The electrolyte acts as a conductor between the positive and negative electrodes. The separator is disposed between the positive and negative electrodes, primarily to prevent short circuits between the positive and negative electrodes, while simultaneously allowing ions to pass through.

[0061] [Negative electrode plate]

[0062] The battery in this application embodiment includes a negative electrode sheet, which includes a negative current collector and a negative active layer disposed on at least one side of the negative current collector. The negative active layer includes a negative active material and a fluoropolymer containing -CH2CF(CF3)- repeating units (HFP repeating units).

[0063] The -CH2CF(CF3)- repeating unit can be a repeating unit formed after the polymerization of HFP (hexafluoropropylene CF2=CF-CF3). The presence of characteristic peaks of HFP repeating units can be detected by Fourier transform infrared spectroscopy or X-ray diffraction (XRD) of the negative electrode active layer, thus determining whether a fluoropolymer containing HFP repeating units exists in the negative electrode active layer. For example, a peak at 721 cm⁻¹ was observed in the infrared spectrum. -1 The characteristic peaks near the peaks originating from CF2=CF-CF3 indicate the presence of HFP repeating units. Understandably, since the negative electrode contains multiple substances, and the fluoropolymers in the negative electrode can interact with the electrolyte or other substances, in addition to the characteristic peaks listed above, it is possible to observe characteristic peaks reflecting the interaction between the fluoropolymers and other substances during detection.

[0064] This application demonstrates that by adding a fluoropolymer containing HFP repeating units to the negative electrode active layer, the battery capacity decay problem can be effectively improved. This is likely because the fluoropolymer contains a large number of F-containing groups, which can spontaneously react with byproducts such as dead lithium generated by battery side reactions, transforming these byproducts into inorganic components, LiF, which are beneficial for improving electrode performance.

[0065] LiF has good mechanical strength, which can improve the stability of the SEI film, thereby reducing the repeated extraction and insertion of lithium ions due to SEI film rupture and the continuous consumption of active lithium, thus reducing lithium loss. At the same time, the fluoropolymer converts by-products into LiF, which reduces the by-products accumulated on the negative electrode, thereby alleviating the volume expansion of the negative electrode, improving the stability of the SEI film, and also helping to reduce lithium loss, thus improving the capacity decay problem.

[0066] Moreover, since side reactions and byproducts will continue to occur during battery cycling, this method of turning byproducts into LiF, which is beneficial to improving electrode performance, can continue to exist throughout the battery cycling process. In other words, the components inside the negative electrode can be turned into valuable resources during dynamic cycling, and the byproducts can be reused to continuously improve the lithium loss of the battery and slow down the battery capacity decay rate.

[0067] In some embodiments, the fluorinated polymer has a sulfur (F) content of 2% to 30% by mass, optionally 10% to 15%, for example, any one or a range between 2%, 3%, 5%, 10%, 15%, 20%, 25%, and 30%. The F content can be determined by methods such as X-ray fluorescence spectrometry (XRF) and nuclear magnetic resonance (NMR). At a suitable F content, the fluorinated polymer has sufficient F-containing groups to react with byproducts such as dead lithium to form LiF.

[0068] In some embodiments, the fluoropolymers include one or more of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), perfluoroethylene propylene (F46, tetrafluoroethylene-hexafluoropropylene copolymer), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer (THV), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP), optionally including PVDF-HFP. The presence of these fluoropolymers in the negative electrode active layer can be determined by detecting the presence of characteristic peaks corresponding to these fluoropolymers through Fourier transform infrared spectroscopy or X-ray diffraction (XRD) testing. For example, in XRD testing, the detection of characteristic peaks near 18°, 20°, and 40° indicates the presence of PVDF-HFP; similarly, the observation of a peak at 1402 cm⁻¹ in the infrared spectrum... -1 1171cm -1 1072cm -1 876cm -1 835cm -1 761cm -1 The nearby characteristic peaks indicate the presence of PVDF-HFP. These fluoropolymers all contain repeating HFP units, which can improve the problem of battery capacity decay.

[0069] In some embodiments, the mass content of the fluoropolymer in the negative electrode active layer is 0.5% to 3%, optionally 0.8% to 1.2%, for example, it can be any one or a range between any two of 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, and 3%. Understandably, during battery cycling, the mass content of the fluoropolymer in the negative electrode active layer will change compared to the mass content of the fluoropolymer in a new negative electrode active layer that has not undergone battery cycling, because the fluoropolymer reacts with byproducts such as dead lithium. However, since the amount of byproducts is very small compared to the amount of fluoropolymer, this change in mass content should be relatively small. In an uncharged battery (or an uncharged negative electrode), the mass content of fluoropolymer in the negative electrode active layer is 0.5% to 3%. For a charged-discharged battery, the mass content of fluoropolymer in the negative electrode active layer can be 0.3% to 3%, optionally 0.8% to 1.2%, for example, any one or any combination of 0.3%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, and 3%. The mass content of fluoropolymer in the negative electrode active layer of a charged-discharged battery may or may not change compared to an uncharged negative electrode.

[0070] The mass content of fluoropolymers in the negative electrode active layer can be obtained through thermogravimetric analysis (TGA). For example, by performing TGA tests on fluoropolymer standards, the temperature range and mass loss of the fluoropolymer during thermal weight loss can be obtained. Then, TGA tests can be performed on the negative electrode active layer. Based on the TGA test results of the fluoropolymer standards, the thermal weight loss stage and mass loss related to the fluoropolymer in the TG-DTG curve of the negative electrode active layer can be identified. This mass loss can then be used to calculate the mass content of the fluoropolymer in the negative electrode active layer. Alternatively, the mass content of the fluoropolymer in the negative electrode active layer can also be calculated based on the amounts of various raw materials used in the negative electrode preparation process.

[0071] At this fluoropolymer content, the fluoropolymer can be effectively used to improve the stability of the SEI film, reduce lithium loss, and slow down the capacity decay rate. At the same time, it can ensure that the negative electrode active material has a sufficient content, thereby enabling the negative electrode to have a sufficiently high energy density.

[0072] In some embodiments, the weight-average molecular weight of the fluoropolymer is 50 × 10⁻⁶. 3 ~455×10 3 Optionally 80×10 3~110×10 3 For example, it could be 50×10 3 60×10 3 80×10 3 100×10 3 110×10 3 150×10 3 200×10 3 250×10 3 300×10 3 350×10 3 400×10 3 450×10 3 455×10 3 The weight-average molecular weight of fluoropolymers can be determined by methods such as viscosity analysis and light scattering. For example, the sample can be dissolved in a suitable solvent, and the viscosity or refractive index of the solution can be measured. The weight-average molecular weight can then be calculated using the viscosity or refractive index. Molecular weight is one of the factors affecting the properties of fluoropolymers, such as their electrical conductivity and mechanical strength. In this application, a fluoropolymer with a certain molecular weight is selected and added to the negative electrode. This fluoropolymer has good electrical conductivity and mechanical strength, which can improve battery capacity decay while giving the negative electrode good conductivity and structural stability.

[0073] In some embodiments, the fluoropolymer is dispersed on the surface of the negative electrode active material. The positional distribution between the fluoropolymer and the negative electrode active material can be observed using an electron microscope. Typically, byproducts such as dead lithium generated during battery cycling mainly deposit on the surface of the negative electrode active material. Therefore, by dispersing the fluoropolymer on the surface of the negative electrode active material, the fluoropolymer can effectively contact these byproducts after they appear, thereby reacting and converting the byproducts into inorganic components beneficial to electrode performance. Simultaneously, the fluoropolymer has a certain absorption and swelling effect on the electrolyte. Dispersing it on the surface of the negative electrode active material increases the contact area between the fluoropolymer and the negative electrode active material during the swelling process, achieving better contact.

[0074] It is important to understand that the dispersion of fluoropolymers on the surface of the negative electrode active material includes both the dispersion of all fluoropolymers on the surface and the dispersion of only a portion of the fluoropolymers on the surface. This is mainly related to the uniformity of the dispersion of the fluoropolymers during the negative electrode preparation process. If all fluoropolymers are dispersed on the surface of the negative electrode active material, the contact between the fluoropolymers and byproducts such as dead lithium will be better.

[0075] Meanwhile, because fluoropolymers are non-water-soluble, while the negative electrode active layer is usually made of an aqueous system, the fluoropolymers in the negative electrode active layer have one or more morphologies, such as particulates and fibers. The morphology of the fluoropolymers can be observed using an electron microscope. The fluoropolymers, distributed in particulate or fibrous morphologies in the negative electrode active layer, can be well dispersed on the surface of the negative electrode active material and can fully contact the dead lithium and other byproducts that appear on the negative electrode surface.

[0076] In some embodiments, the negative electrode active layer further includes a binder, which includes one or more of PAA (polyacrylic acid) and PTFE (polytetrafluoroethylene). Optionally, the binder includes PAA. The presence or absence of these binders can be determined by detecting the characteristic peaks corresponding to these binders in the negative electrode active layer through Fourier transform infrared spectroscopy or XRD. Typically, the preparation process of the negative electrode active layer uses an aqueous system (using water as a solvent), but fluoropolymers are insoluble in water, resulting in a low viscosity of the negative electrode slurry that is difficult to adhere to the current collector. Adding binders such as PAA and PTFE can effectively improve this situation.

[0077] In some embodiments, the mass content of PAA in the negative electrode active layer is 0.1% to 1%, optionally 0.2% to 0.25%, for example, it can be any one or a range between any two of 0.1%, 0.2%, 0.25%, 0.4%, 0.6%, 0.8%, and 1%. Similarly, the mass content of PAA in the negative electrode active layer can also be obtained by thermogravimetric analysis (TGA). At this mass content, PAA can be used to improve the negative electrode process while ensuring that PAA does not occupy an excessive proportion of the negative electrode active layer, thus ensuring that the negative electrode active material has a sufficient content, thereby giving the negative electrode a sufficiently high energy density.

[0078] Understandably, the binder in the negative electrode active layer may also include one or more of SBR (styrene-butadiene rubber), PAAS (sodium polyacrylate), PAM (polyacrylamide), PVA (polyvinyl alcohol), SA (sodium alginate), PMAA (polymethacrylic acid), and CMCS (carboxymethyl chitosan). The mass content of all binders in the negative electrode active layer may be 0.1% to 10%, for example, any one or a range between 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, and 10%.

[0079] In some embodiments, the weight-average molecular weight of PAA is 50 × 10⁻⁶. 3 ~500×10 3 Optionally 80×10 3 ~100×10 3For example, it could be 50×10 3 60×10 3 80×10 3 100×10 3 110×10 3 150×10 3 200×10 3 250×10 3 300×10 3 350×10 3 400×10 3 450×10 3 500×10 3 The weight-average molecular weight of PAA can be any one of the following or any two of these values. Similarly, the weight-average molecular weight of PAA can be obtained through tests such as viscosity analysis and light scattering. Molecular weight is one of the key factors affecting the viscosity of PAA. By selecting PAA with a certain molecular weight as a binder, this PAA has high viscosity, which can effectively improve the low viscosity problem caused by the presence of PVDF-HFP.

[0080] In some embodiments, the negative electrode active layer also includes LiF. The presence of LiF can be determined by XRD analysis of the negative electrode active layer; the presence of characteristic peaks for LiF (around 16.36°, 18.87°, 24.09°, and 31.99°) confirms its presence. The LiF in the negative electrode active layer may be formed by the reaction of substances in the electrolyte with active lithium, or it may originate from the reaction of fluoropolymers with byproducts such as dead lithium. LiF possesses good mechanical strength, which can improve the stability of the SEI film, thereby reducing the repeated extraction and insertion of lithium ions due to SEI film rupture, thus continuously consuming active lithium and reducing lithium loss. Simultaneously, the fluoropolymer converts byproducts such as dead lithium into LiF, reducing the amount of byproducts accumulating on the negative electrode, thereby alleviating the volume expansion of the negative electrode, improving the stability of the SEI film, reducing lithium loss, and thus mitigating capacity decay.

[0081] In some embodiments, the negative electrode active material includes one or more of graphite (artificial graphite, natural graphite), soft carbon, hard carbon, silicon-based materials, tin-based materials, and titanium-based materials. Silicon-based materials may include one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxide compounds, and tin alloys. Titanium-based materials may include lithium titanate. It is understood that this application is not limited to these materials, and other materials that can be used as negative electrode active materials in batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0082] During battery cycling, byproducts such as dead lithium can appear on the surface of these negative electrode active materials. The solution in this application improves the capacity decay of batteries using various negative electrode active materials and has wide applicability.

[0083] The mass content of the negative electrode active material in the negative electrode active layer can range from 70% to 98%, or even 90% to 98%, for example, any one of the values ​​of 70%, 75%, 80%, 85%, 90%, 92%, 94%, 96%, or 98%, or any range between two of these values. A high content of negative electrode active material can provide high energy density for the battery.

[0084] In some embodiments, the negative electrode active layer further includes a conductive agent. The conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The mass content of the conductive agent in the negative electrode active layer includes 0.5% to 10%, for example, any one of 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%, or a range between any two.

[0085] In some embodiments, the negative electrode active layer may optionally include other additives, such as thickeners, like CMC (carboxymethyl cellulose), CMC-Na (sodium carboxymethyl cellulose), etc.

[0086] The negative electrode sheet in the battery of this application also includes a negative electrode current collector. As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode active layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0087] In some embodiments, the negative current collector includes one or more of a metal foil and a composite current collector. For example, copper foil can be used as the metal foil. The composite current collector may include a polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymeric material substrate [such as a substrate of PP (polypropylene), PET (polyethylene terephthalate), PBT (polybutylene terephthalate), PS (polystyrene), PE (polyethylene), etc.].

[0088] In some implementations, the negative electrode sheet can be prepared in the following manner:

[0089] The components used to prepare the negative electrode sheet, such as negative electrode active material, fluoropolymer, binder, and conductive agent (and may also include any other components), are dispersed in a solvent (e.g., water) to form a negative electrode slurry. The negative electrode slurry is coated on at least one side of the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0090] In some embodiments, during the preparation of the negative electrode sheet, the PAA in the binder is selected as a PAA aqueous solution with a solid content of 20% to 50%, optionally 30% to 35%. For example, the solid content of this PAA aqueous solution is any one or a range between any two of 20%, 25%, 30%, 35%, 40%, 45%, and 50%. During the fabrication of the negative electrode sheet, adding PAA in the form of an aqueous solution to the negative electrode slurry can effectively improve its dispersibility in the negative electrode slurry, thereby increasing the viscosity of the negative electrode slurry and improving the process of negative electrode fabrication.

[0091] [Positive electrode plate]

[0092] The battery in this application embodiment includes a positive electrode sheet, which includes a positive current collector and a positive active layer disposed on at least one side of the positive current collector. The positive active layer includes a positive active material, a conductive agent, and a binder.

[0093] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0094] In some embodiments, the positive current collector includes one or more of a metal foil and a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymeric material substrate and a metal layer formed on at least one surface of the polymeric material substrate. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymeric material substrate [such as a substrate of PP (polypropylene), PET (polyethylene terephthalate), PBT (polybutylene terephthalate), PS (polystyrene), PE (polyethylene), etc.].

[0095] In some embodiments, the positive electrode active material includes one or more of lithium phosphates having an olivine structure and their modified compounds, lithium transition metal oxides and their modified compounds. Examples of lithium phosphates having an olivine structure include lithium iron phosphate (such as LiFePO4, i.e., LFP), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium iron manganese phosphate, and lithium iron manganese phosphate and carbon composites. Examples of lithium transition metal oxides include 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 / 3 Mn 1 / 3 O2, or NCM 333 LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM 523 LiNi 0.5 Co 0.25 Mn 0.25 O2(NCM 211 LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM 622 LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 One or more of O2 and its modified compounds. These positive electrode active materials can be used alone or in combination of two or more.

[0096] The mass content of the positive electrode active material in the positive electrode active layer can be, but is not limited to, 70% to 98%, or 80% to 98%, for example, any one of 70%, 75%, 80%, 85%, 90%, 92%, 94%, 96%, 98%, or any range between two.

[0097] In some embodiments, the binder in the positive electrode active layer may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0098] In some embodiments, the conductive agent in the positive electrode active layer may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0099] The mass content of binder and conductive agent in the positive electrode active layer can be independently, including but not limited to 0.5% to 10%, or 1% to 10%, for example, any one of 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or any range between the two.

[0100] In some embodiments, the positive electrode active layer may optionally include additives. These additives may include those capable of improving certain properties of the positive electrode, such as additives with lithium replenishment effects or additives that can improve and regulate the CEI composition. For example, the additive may include a fluoropolymer containing HFP repeating units. Using a fluoropolymer containing HFP repeating units as a positive electrode additive can improve and regulate the CEI composition, resulting in a more superior lithium transport capability for the positive electrode, thereby contributing to improved overall battery performance.

[0101] In some implementations, the positive electrode sheet can be prepared in the following manner:

[0102] The components used to prepare the positive electrode sheet, such as positive active material, conductive agent, binder and any other components, are dispersed in a solvent (e.g. N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0103] [Electrolytes]

[0104] The electrolyte acts as a conductor of ions between the positive and negative electrodes. In the battery of this embodiment, the type of electrolyte can be selected according to requirements. For example, the electrolyte can be a solid electrolyte or a liquid electrolyte.

[0105] Solid electrolytes include one or more of polymer electrolytes and inorganic solid electrolytes. For example, a polymer electrolyte may include a fluoropolymer containing HFP repeating units. Such fluoropolymers can be used to form polymer gel electrolytes, which can be used in polymer solid-state batteries. The fluoropolymer containing HFP repeating units can also convert byproducts such as dead lithium formed on the surface of the negative electrode in contact with the electrolyte, enabling the reuse of these byproducts.

[0106] The electrolyte consists of a solvent and a lithium salt dissolved in the solvent.

[0107] The solvent may be a non-aqueous organic solvent, such as one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB), preferably two or more.

[0108] Lithium salts may include one or more of the following: LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiBOB (lithium bis(oxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorooxalate phosphate), and LiTFOP (lithium tetrafluorooxalate phosphate).

[0109] The concentration of lithium salt in the electrolyte can be 0.5 to 1.5 mol / L, for example, any one of 0.5 mol / L, 0.6 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.4 mol / L, 1.5 mol / L or any range between two of them.

[0110] The electrolyte may also optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0111] [Isolation membrane]

[0112] In a battery, a separator is typically stacked between the positive and negative electrodes to separate them, preventing electrons from passing freely and thus preventing short circuits. At the same time, it allows ions in the electrolyte to pass freely between the positive and negative electrodes.

[0113] In the battery of this application, the type of separator can be any known porous structure separator with good chemical and mechanical stability.

[0114] The material of the separator membrane may include one or more of the following: glass fiber, non-woven fabric, polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF). The separator membrane can be a single-layer film or a multi-layer composite film; there are no particular restrictions. When the separator membrane is a multi-layer composite film, the materials of each layer can be the same or different; there are no particular restrictions.

[0115] Positive electrode, negative electrode and separator can be made into electrode assembly by winding process or stacking process.

[0116] [Outer Packaging]

[0117] The battery may include an outer packaging that can be used to encapsulate an electrode assembly containing a positive electrode, a negative electrode, a separator, and an electrolyte.

[0118] The outer packaging of batteries can be a hard shell, such as a hard plastic shell, aluminum shell, or steel shell; or it can be a soft pack, such as a pouch. The material of the soft pack can be plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0119] The outer packaging can be cylindrical, square, or any other shape. For example, Figure 3 The battery cell, as an example, has a square outer packaging shape.

[0120] Reference Figure 4 The outer packaging may include a housing 01 and a cover plate 02. The housing 01 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 01 has an opening communicating with the receiving cavity, and the cover plate 02 can be placed over the opening to close the receiving cavity. A positive electrode, a negative electrode, and a separator can be formed into an electrode assembly 03 via a winding or stacking process. One or more electrode assemblies 03 are encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 03.

[0121] [Battery cells, battery modules, battery packs]

[0122] The battery in this application embodiment can be at least one of a battery cell, a battery module, or a battery pack. Depending on the packaging form, batteries are classified as battery cells, battery modules, or battery packs. A battery cell is the most basic unit of a secondary battery, including an electrode assembly and an electrolyte. The electrode assembly typically consists of a positive electrode, a negative electrode, and a separator. The positive and negative electrodes are alternately stacked, with a separator placed between them for isolation, to obtain the electrode assembly (also called a cell). Alternatively, the cell can be obtained by winding. The cell is placed in a casing, injected with electrolyte, and sealed to obtain a battery cell. A battery cell primarily functions by the movement of active metal ions in the electrolyte between the positive and negative electrodes.

[0123] In some battery packaging technologies, one or more individual battery cells can be integrated into a battery module, and then one or more battery modules can be assembled into a battery pack. In other battery packaging technologies, one or more individual battery cells can be directly installed in a housing to form a battery pack, eliminating the intermediate state of battery modules, thereby reducing the weight of the battery pack and increasing the energy density of the battery.

[0124] refer to Figure 5 This is an example battery module. In the battery module, multiple battery cells 04 can be arranged sequentially along the length of the battery module. Of course, they can also be arranged in any other arbitrary way. Furthermore, these multiple battery cells 04 can be secured using fasteners.

[0125] Optionally, the battery module may also include a housing with a receiving space in which multiple battery cells 04 are received.

[0126] refer to Figure 6 and Figure 7 This is an example of a battery pack. The battery pack may include a battery compartment and multiple battery modules 05 disposed within the battery compartment. The battery compartment includes an upper body 06 and a lower body 07, with the upper body 06 covering the lower body 07 to form a closed space for accommodating the battery modules 05. The multiple battery modules 05 can be arranged in any manner within the battery compartment.

[0127] [Electrical appliances]

[0128] This application also provides an electrical device, which includes the battery described above.

[0129] The aforementioned battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The battery in this embodiment exhibits advantages such as slow capacity loss, good cycle performance, and long lifespan. Therefore, applying this battery to an electrical device improves the user experience.

[0130] Electrical devices may include, but are not limited to, 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.

[0131] As an electrical device, the battery can be selected as a single battery cell, battery module, or battery pack according to its usage requirements.

[0132] Figure 8This 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 device's requirements for high power and high energy density, a battery pack or battery module can be used.

[0133] The embodiments of this application are described in detail below. 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 in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0134] Example 1

[0135] 1. Preparation of positive electrode sheet

[0136] Lithium iron phosphate, conductive carbon black, and PVDF were mixed in a mass ratio of 8:1:1, and then N-methylpyrrolidone solvent was added to obtain a positive electrode slurry. The positive electrode slurry was coated on both sides of an aluminum foil, and after cold pressing and cutting, a positive electrode sheet was obtained.

[0137] 2. Preparation of negative electrode sheet

[0138] Artificial graphite, conductive carbon black, CMC, PAA, PVDF-HFP, and solvent water were uniformly mixed at a mass ratio of 93.75:2:3:0.25:1:100 to obtain a negative electrode slurry. The negative electrode slurry was coated on both sides of a copper foil, and after cold pressing and cutting, a negative electrode sheet was obtained. The mass content of PVDF-HFP in the negative electrode active layer was 1%.

[0139] PAA has a weight-average molecular weight of approximately 100 × 10⁻⁶. 3 The weight-average molecular weight of PVDF-HFP is approximately 80 × 10⁻⁶. 3 The F element content of PVDF-HFP is 10%.

[0140] 3. Separating membrane

[0141] A polyethylene film with a thickness of 7 μm was used as the separator.

[0142] 4. Electrolyte

[0143] Ethylene carbonate, diethyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 1:1:1 to obtain a mixed solvent. LiPF6 was then dissolved in this mixed solvent to obtain an electrolyte. The concentration of LiPF6 in the electrolyte was 1 mol / L.

[0144] 5. Battery assembly

[0145] The electrodes are arranged in the order of "separator-negative electrode-separator-positive electrode". One end of the positive electrode, negative electrode, and two separators is fixed to the discharge roller, and the other end is stacked together and fixed to the winding shaft. The winding shaft is rotated by a motor to wind the positive electrode, negative electrode, and two separators to obtain a wound battery. After the battery is coated with electrolyte, it is charged with a 0.1C current for 5 hours to complete the formation process.

[0146] Example 2

[0147] The difference between this embodiment and Embodiment 1 is that the F element mass content of PVDF-HFP is 3%.

[0148] Example 3

[0149] The difference between this embodiment and Embodiment 1 is that the F element mass content of PVDF-HFP is 5%.

[0150] Example 4

[0151] The difference between this embodiment and Embodiment 1 is that the F element mass content of PVDF-HFP is 15%.

[0152] Example 5

[0153] The difference between this embodiment and Embodiment 1 is that the F element mass content of PVDF-HFP is 20%.

[0154] Example 6

[0155] The difference between this embodiment and Embodiment 1 is that the F element mass content of PVDF-HFP is 30%.

[0156] Example 7

[0157] The difference between this embodiment and Embodiment 1 is that the mass ratio of artificial graphite, conductive carbon black, CMC, PAA, and PVDF-HFP in the negative electrode sheet is 94.25:2:3:0.25:0.5. That is, the mass content of PVDF-HFP in the negative electrode active layer is 0.5%.

[0158] Example 8

[0159] The difference between this embodiment and Embodiment 1 is that the mass ratio of artificial graphite, conductive carbon black, CMC, PAA, and PVDF-HFP in the negative electrode sheet is 93.25:2:3:0.25:1.5. That is, the mass content of PVDF-HFP in the negative electrode active layer is 1.5%.

[0160] Example 9

[0161] The difference between this embodiment and Embodiment 1 is that the mass ratio of artificial graphite, conductive carbon black, CMC, PAA, and PVDF-HFP in the negative electrode sheet is 92.75:2:3:0.25:2. That is, the mass content of PVDF-HFP in the negative electrode active layer is 2%.

[0162] Example 10

[0163] The difference between this embodiment and Embodiment 1 is that the mass ratio of artificial graphite, conductive carbon black, CMC, PAA, and PVDF-HFP in the negative electrode sheet is 91.75:2:3:0.25:3. That is, the mass content of PVDF-HFP in the negative electrode active layer is 3%.

[0164] Comparative Example 1

[0165] The difference between this comparative example and Example 1 is that the mass ratio of artificial graphite, conductive carbon black, CMC, PAA, and PVDF-HFP in the negative electrode is 98:2:3:0:0. That is, the negative electrode does not contain PAA and PVDF-HFP.

[0166] Comparative Example 2

[0167] The difference between this comparative example and Example 1 is that the mass ratio of artificial graphite, conductive carbon black, CMC, PAA, and PVDF-HFP in the negative electrode is 94.75:2:3:0.25:0. That is, the negative electrode does not contain PVDF-HFP.

[0168] Comparative Example 3

[0169] The difference between this comparative example and Example 1 is that the PVDF-HFP in the negative electrode is replaced with an equal mass of PVDF, and the F element mass content of the PVDF is 14%.

[0170] Comparative Example 4

[0171] The difference between this comparative example and Example 1 is that the PVDF-HFP in the negative electrode is replaced with an equal mass of PTFE, and the F element mass content of PTFE is 17%.

[0172] The performance of the batteries assembled in each embodiment and comparative example was tested as follows:

[0173] (1) Loop test

[0174] At room temperature (25℃), charge at a constant current of 1C to 3.65V, then switch to constant voltage testing until the current is less than 0.05C, then discharge at 1C to 2.5V, repeating this cycle 1000 times. Record the discharge capacity of the first cycle as C0, and the discharge capacity of each subsequent cycle as C1. Calculate SOH = C1 / C0 for different number of cycles.

[0175] SOH (State of Health) represents the battery's health status, measuring the degree of wear and tear and aging accumulated during long-term use. It reflects the proportion of the battery's remaining capacity relative to its initial design capacity. SOH is typically represented by a value between 0 and 1, where 0 indicates extremely poor battery performance and 1 indicates good battery performance. As battery use and the number of charge cycles increase, SOH gradually decreases, leading to a decline in battery performance.

[0176] (2) DC impedance DCR

[0177] Test steps:

[0178] 1) Charge at a constant current of 0.33C to 3.65V, then charge at a constant voltage until the current is less than 0.05C;

[0179] 2) Discharge at a constant current of 0.33C for 18 minutes, then adjust the SOC to 90%;

[0180] 3) Record the initial voltage as OCV1, discharge with a current of I = 2C for 30s, and record the final voltage as OCV2;

[0181] Calculation formula:

[0182] DCR=(OCV1-OCV2) / I.

[0183] The test results are shown in the table below.

[0184] [Table 1]

[0185]

[0186] Note: In Table 1, the mass content of PVDF-HFP and the mass content of PAA represent the mass content of PVDF and PAA in the negative electrode active layer, respectively.

[0187] The SOH of batteries in Example 1, Comparative Example 1, and Comparative Example 2 at different cycle numbers are as follows: Figure 1 As shown in Table 1 and Figure 1 The test results show that after 1000 cycles, the SOH of the battery in Example 1 was higher than that in Comparative Examples 1 and 2, and the battery capacity decayed more slowly. This indicates that adding PVDF-HFP to the negative electrode can effectively improve the battery capacity decay problem, which will help improve the battery's cycle life. See also Figure 2This is likely because during battery charging and discharging, active lithium ions in the battery gradually undergo side reactions, forming byproducts such as dead lithium on the surface of graphite particles in the negative electrode, resulting in lithium loss. However, adding PVDF-HFP to the active layer of the negative electrode allows the F-containing groups in PVDF-HFP to spontaneously react with these byproducts, converting them into LiF. This forms a LiF-rich SEI film on the graphite particle surface. LiF possesses good mechanical strength, improving the stability of the SEI film and reducing the repeated extraction and insertion of lithium ions due to SEI film rupture, thus continuously consuming active lithium and reducing lithium loss, thereby improving capacity decay. Simultaneously, the conversion of dead lithium and other byproducts by PVDF-HFP into LiF reduces the amount of byproducts accumulating on the graphite particles, mitigating graphite volume expansion and improving SEI film stability, which also helps reduce lithium loss and improve capacity decay.

[0188] At the same time, from Figure 1 It can be seen that, under different cycle numbers, the SOH of the battery in Example 1 is higher than that of Comparative Example 1 and Comparative Example 2. It can be inferred that the process of PVDF-HFP converting byproducts into LiF, which is beneficial to the improvement of electrode performance, can continue to exist during the battery cycling process.

[0189] [Table 2]

[0190]

[0191] The data in Table 2 show that adding PVDF-HFP with different F element mass contents to the negative electrode can improve the battery capacity decay. Furthermore, within the range of 3% to 30% F element mass content, the improvement effect on battery capacity decay is better with increasing F element mass content.

[0192] [Table 3]

[0193]

[0194] Examples 1 and 7-10, by adding different mass contents of PVDF-HFP to the negative electrode, all improved the capacity decay of the battery. Furthermore, within the PVDF-HFP mass content range of 0.5% to 3%, as the PVDF-HFP mass content increased, the state of equilibrium (SOH) of the battery increased after the same number of cycles, indicating less capacity decay. Specifically, when the PVDF-HFP mass content was below 10%, the SOH improved significantly with increasing PVDF-HFP mass content; as the PVDF-HFP mass content increased to above 10%, the SOH leveled off.

[0195] [Table 4]

[0196]

[0197] Note: In Table 4, the DCR growth rate refers to the DCR growth rate compared to Comparative Example 1.

[0198] Considering that PVDF-HFP mainly reacts with byproducts such as dead lithium through its fluorine-containing groups, the experiment was conducted by replacing PVDF-HFP with other fluorine-containing substances, such as PVDF and PTFE. The results showed that, with similar fluorine content, the state of oxygen (SOH) after the same number of cycles was similar to that without any fluorine-containing substances (Comparative Example 1), showing no improvement, and even causing a significant increase in capacity decay (DCR). This indicates that adding PVDF and PTFE to the negative electrode does not effectively improve the battery's capacity decay problem, but rather significantly degrades other battery performance characteristics. In contrast, adding PVDF-HFP to the negative electrode effectively improves the battery's capacity decay problem, with minimal impact on DCR and no significant degradation of other battery performance characteristics. This is preliminarily speculated to be related to the presence of repeating HFP cells.

[0199] 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 by comprising: The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one side of the negative electrode current collector, the negative electrode active layer including a negative electrode active material and a fluorine-containing polymer including -CH2CF(CF3)- repeating units.

2. The lithium secondary battery according to claim 1, wherein The fluorine-containing polymer has a F element mass content of 2% to 30%.

3. The lithium secondary battery according to claim 1 or 2, wherein The fluorine-containing polymer has a F element mass content of 10% to 15%.

4. The lithium secondary battery according to any one of claims 1 to 3, characterized by, The fluorine-containing polymer includes one or more of a polyvinylidene fluoride-hexafluoropropylene copolymer, a polyperfluoroethypropylene, a tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer, and a tetrafluoroethylene-hexafluoropropylene copolymer.

5. The lithium secondary battery according to any one of claims 1 to 4, characterized by, The fluorine-containing polymer has a mass content of 0.3% to 3% in the negative electrode active layer.

6. The lithium secondary battery according to any one of claims 1 to 5, wherein The fluorine-containing polymer has a mass content of 0.8% to 1.2% in the negative electrode active layer.

7. The lithium secondary battery according to any one of claims 1 to 6, wherein The weight average molecular weight of the fluoropolymer is 50 x 10 3 ~ 455 x 10 3 .

8. The lithium secondary battery according to any one of claims 1 to 7, characterized by, The weight average molecular weight of the fluoropolymer is 80 x 10 3 ~ 110 x 10 3 .

9. The lithium secondary battery according to any one of claims 1 to 8, characterized by, The negative electrode active layer further includes a binder including one or more of a polyacrylic acid and a polytetrafluoroethylene.

10. The lithium secondary battery according to claim 9, wherein The binder includes a polyacrylic acid having a mass content of 0.1% to 1% in the negative electrode active layer.

11. The lithium secondary battery according to claim 10, wherein The polyacrylic acid has a mass content of 0.2% to 0.25% in the negative electrode active layer.

12. The lithium secondary battery according to claim 10 or 11, wherein The weight average molecular weight of the polyacrylic acid is 50 x 10 3 ~ 500 x 10 3 .

13. The lithium secondary battery according to any one of claims 10 to 12, wherein The weight average molecular weight of the polyacrylic acid is 80 x 10 3 ~ 100 x 10 3 .

14. The lithium secondary battery according to any one of claims 1 to 13, wherein The negative electrode active material includes one or more of graphite, soft carbon, hard carbon, a silicon-based material, a tin-based material, and a titanium-based material.

15. A negative electrode sheet characterized by comprising: The negative electrode sheet is for use in a lithium secondary battery as described in any one of claims 1 to 14.

16. An electrical device, comprising: The lithium secondary battery includes the negative electrode sheet as described in any one of claims 1 to 14.

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