Lithium secondary battery, negative electrode sheet and electric device

Abstract

A lithium secondary battery, comprising a negative electrode sheet, wherein the negative electrode sheet comprises 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 comprising a negative electrode active material and a fluorine-containing polymer containing -CF2CF(CF3)- repeating units. The fluorine-containing polymer contains a large number of F-containing groups, and can thus spontaneously react with by-products such as dead lithium generated by side reactions of the battery to convert the by-products into an inorganic component LiF that is conducive to the improvement of electrode performance; therefore, the loss of lithium can be effectively reduced, and the problem of reduced battery capacity can thus be ameliorated.

Description

Lithium secondary battery, negative electrode sheet, and electric device

[0001] The present application claims priority to the Chinese patent application No. 202411387971.6, filed on September 30, 2024, and entitled "A lithium secondary battery, negative electrode sheet, and electric device", the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of batteries, in particular to a lithium secondary battery, a negative electrode sheet, and an electric device. BACKGROUND

[0003] The cycle performance of a battery has always been the focus of product development, and one important manifestation of the deterioration of the cycle performance is the capacity reduction. During the cycle of the battery, active lithium ions participate in electrochemical reactions to provide capacity for the battery; however, various side reactions also occur in the battery to form by-products such as dead lithium on the negative electrode sheet, resulting in lithium loss and causing capacity reduction. Moreover, these by-products will accumulate on the surface of the negative electrode sheet, affecting the structure of the negative electrode sheet and also causing capacity attenuation. SUMMARY

[0004] The present application is made in view of the above technical problems, and aims to improve the problem of capacity attenuation of the battery.

[0005] To achieve the above-mentioned purpose, the present application provides a lithium secondary battery, a negative electrode sheet, and an electric device.

[0006] The first aspect of the present application provides a lithium secondary battery, comprising a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active layer arranged on at least one side of the negative electrode current collector, the negative electrode active layer comprising a negative electrode active material and a fluorine-containing polymer containing a -CH2CF(CF3)- repeating unit.

[0007] The embodiments of the present application add a fluorine-containing polymer containing a -CH2CF(CF3)- repeating unit to the negative electrode active layer, and the results show that the problem of battery capacity attenuation can be effectively improved. This may be due to the fact that this fluorine-containing polymer contains a large number of F-containing groups, which can spontaneously react with the by-products such as dead lithium generated by the side reactions of the battery, and convert these by-products into inorganic components LiF that are beneficial to the improvement of the electrode performance.

[0008] LiF has good mechanical strength, which can improve the stability of SEI film, thereby reducing the repeated extraction and embedding of lithium ions due to the rupture of SEI film, continuously consuming active lithium, thereby reducing lithium loss; at the same time, the by-product is converted into LiF by the fluorine-containing polymer, so the by-product accumulated on the negative electrode will be reduced, thereby the volume expansion of the negative electrode can be alleviated, the stability of the SEI film can be improved, which is also beneficial to reduce lithium loss, thereby improving the capacity fading problem.

[0009] Moreover, the by-product will continue to be generated during the battery cycle process, and this way of changing the by-product into LiF which is beneficial to improve the electrode performance can exist all the time during the battery cycle process, that is, the components in the negative electrode can be changed from waste to treasure during the dynamic cycle process, the secondary utilization of the by-product of the side reaction is completed, the lithium loss of the battery is continuously improved, and the capacity fading speed of the battery is slowed down.

[0010] In some embodiments, the F element mass content of the fluorine-containing polymer is 2% to 30%, and optionally 10% to 15%. Under a suitable F element mass content, the fluorine-containing polymer has sufficient F-containing groups to react with the by-products such as dead lithium to form LiF.

[0011] In some embodiments, the fluorine-containing polymer includes one or more of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyperfluoroethylene-propylene (F46, tetrafluoroethylene-hexafluoropropylene copolymer), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer (THV), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). These fluorine-containing polymers all contain -CH2CF(CF3)- repeating units, which can improve the capacity fading problem of the battery.

[0012] In some embodiments, in a lithium secondary battery, the mass content of the fluorine-containing polymer in the negative active layer is 0.3% to 3%, and optionally 0.8% to 1.2%. Under this mass content of the fluorine-containing polymer, the fluorine-containing polymer can be effectively utilized to improve the stability of the SEI film, reduce lithium loss, and slow down the capacity fading speed, and at the same time, the negative active material has sufficient content, so that the lithium secondary battery has sufficiently high energy density.

[0013] In some embodiments, the weight average molecular weight of the fluorine-containing polymer is 50×10 3 to 455×10 3 , and optionally 80×10 3 to 110×10 3The molecular weight is one of the factors affecting the performance of the fluorine-containing polymer, for example, affecting the conductivity, mechanical strength, etc. of the fluorine-containing polymer. In the embodiments of the present application, a fluorine-containing polymer with a certain molecular weight is selected to be added to the negative electrode sheet, and the fluorine-containing polymer has good conductivity and mechanical strength, which can improve the capacity decay of the battery and make the negative electrode sheet have good conductivity and structural stability.

[0014] In some embodiments, the negative active layer further comprises a binder, and the binder comprises one or more of polyacrylic acid (PAA) and polytetrafluoroethylene (PTFE). Generally, the preparation process of the negative active layer adopts an aqueous system (using water as the solvent), and the fluorine-containing polymer is insoluble in water, so that the negative electrode slurry has low viscosity and is difficult to adhere to the current collector. By adding the binder such as PAA and PTFE, the problem can be effectively improved.

[0015] In some embodiments, the binder comprises PAA, and the mass content of the PAA in the negative active layer is 0.1% to 1%, and optionally 0.2% to 0.25%. Under the mass content, the PAA can be used to improve the negative electrode process problem, and the proportion of the PAA in the negative active layer is not too high, so that the negative active material has sufficient content, and the lithium secondary battery has a high enough energy density.

[0016] In some embodiments, the weight average molecular weight of the PAA is 50×10 3 to 500×10 3 , and optionally 80×10 3 to 100×10 3 . The molecular weight is one of the key factors affecting the viscosity of the PAA. By selecting PAA with a certain molecular weight as the binder, the PAA has high viscosity, which can effectively improve the low viscosity problem caused by the presence of the fluorine-containing polymer.

[0017] In some embodiments, the negative active material comprises one or more of graphite, soft carbon, hard carbon, silicon-based material, tin-based material, and titanium-based material. During the battery cycle, by-products such as dead lithium appear on the surface of the negative active material. The scheme of the embodiments of the present application has an improvement effect on the capacity decay of the battery using various negative active materials, and has wide applicability.

[0018] The second aspect of the present application provides a negative electrode sheet, which is the negative electrode sheet in the lithium secondary battery of the first aspect.

[0019] That is, the negative electrode sheet comprises a negative electrode current collector and a negative active layer arranged on at least one side of the negative electrode current collector, and the negative active layer comprises a negative active material and a fluorine-containing polymer containing a -CH2CF(CF3)- repeating unit.

[0020] The application embodiment can effectively improve the battery capacity decay problem by adding the fluorine-containing polymer containing -CH2CF(CF3)- repeating units in the negative active layer. This may be due to the fact that the fluorine-containing polymer contains a large amount of F-containing groups, which can spontaneously react with the by-products such as dead lithium generated by the battery side reaction, and convert these by-products into inorganic components LiF that are beneficial to the improvement of electrode performance.

[0021] 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 the rupture of the SEI film, and continuously consuming active lithium, thereby reducing lithium loss; at the same time, the fluorine-containing polymer converts by-products into LiF, which reduces the accumulation of by-products on the negative electrode, thereby alleviating the volume expansion of the negative electrode, improving the stability of the SEI film, and also reducing lithium loss, thereby improving the capacity decay problem.

[0022] Moreover, the side reaction to generate by-products will continue to occur during the battery cycle process, and this way of converting by-products into LiF that is beneficial to the improvement of electrode performance can exist throughout the battery cycle process, that is, the components inside the negative electrode can be converted from waste to treasure during dynamic cycling, completing the secondary use of by-products of side reactions, continuously improving the lithium loss of the battery, and slowing down the battery capacity decay rate.

[0023] The third aspect of the application provides a power utilization device comprising the above lithium secondary battery.

[0024] The above battery can be used as a power supply for the power utilization device, or as an energy storage unit for the power utilization device. The battery of the application embodiment has the advantages of slow capacity decay rate, good cycle performance and long service life, and therefore, when the battery is applied to the power utilization device, it is beneficial to improve the use experience of the power utilization device. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0026] Fig. 1 is a SOH curve of the battery of the application embodiment 1, the comparative example 1 and the comparative example 2 at different cycle numbers;

[0027] Fig. 2 is a PVDF-HFP improvement mechanism diagram for battery life in an embodiment of the application;

[0028] Fig. 3 is a schematic diagram of a battery cell in an embodiment of the application;

[0029] FIG. 4 is an exploded view of the battery cell of the embodiment of the present application shown in FIG. 3;

[0030] FIG. 5 is a schematic view of a battery module of the embodiment of the present application;

[0031] FIG. 6 is a schematic view of a battery pack of the embodiment of the present application;

[0032] FIG. 7 is an exploded view of the battery pack of the embodiment of the present application shown in FIG. 6;

[0033] FIG. 8 is a schematic view of an electric device using the battery of the embodiment of the present application as a power source.

[0034] Reference numerals: 01 - case, 02 - cover plate, 03 - electrode assembly, 04 - battery cell, 05 - battery module, 06 - upper case, 07 - lower case. DETAILED DESCRIPTION

[0035] Hereinafter, embodiments of the detection system and the detection method of the present application are specifically disclosed while appropriately referring to the accompanying drawings. However, there are cases where unnecessary detailed explanations are omitted. For example, there are cases where detailed explanations of matters that are already well known, repeated explanations of actually identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following explanations are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0036] The "ranges" disclosed in the present application are defined in the form of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The ranges defined in this manner can include or exclude the end values, and can be arbitrarily combined, i.e., 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 particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. 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 contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise stated, a numerical range "a-b" indicates a shorthand manner of describing each and every numeral between a and b, wherein a and b are both real numbers. For instance, the numerical range "0-5" indicates that all real numbers between "0 and 5", inclusive of "0 and 5", have been listed herein. "0-5" is merely a shorthand manner of describing each and every numeral that is contained in the range. In addition, when a parameter is stated to be an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and the like.

[0037] If not particularly specified, all the embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0038] If not particularly specified, all the technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0039] If not particularly specified, all the steps of the present application can be performed in sequence or randomly, and preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method also comprises step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0040] If not particularly specified, the "comprise" and "include" mentioned in the present application are open-ended, and can also be closed. For example, the "comprise" and "include" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0041] If not particularly specified, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any one of the following conditions satisfies the condition "A or B": 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 A and B are both true (or exist).

[0042] The current limiting factor of battery cycle life includes capacity decay caused by structural performance deterioration of the negative electrode sheet. The structural performance deterioration is manifested as: in normal cycling, lithium loss occurs due to the formation of SEI film (solid electrolyte film) on the surface of the negative electrode sheet, causing capacity reduction; the by-products formed during the lithium loss process accumulate on the negative electrode sheet, causing rapid expansion of the negative electrode sheet. After the expansion is limited, the pore structure of the negative electrode sheet collapses rapidly, causing the transport path of the electrolyte and active lithium inside the negative electrode sheet structure to rapidly decrease, and the polarization to rapidly increase, thereby further triggering capacity decay and rapid life decay. At the same time, during the cycling process, the SEI film cannot exist stably due to the repeated extraction and insertion of lithium ions, and the SEI film is continuously consumed and decomposed and then reformed, which consumes a large amount of active lithium ions, and the continuous loss of active lithium material causes capacity reduction and rapid life decay.

[0043] In the face of the above-mentioned battery cycle life dilemma, the related technology tries to select low lithium consumption negative electrode active material (such as low lithium consumption graphite), use high-conductivity electrolyte, cathode lithium supplement, and other methods, hoping to improve the overall dynamics of the battery, so as to realize the long cycle life performance of the battery. However, even if these methods are used, side products will still appear on the surface of the negative electrode plate during the battery cycle process. The form of these side products does not change, so as the number of battery cycles increases, the side product accumulation on the surface of the negative electrode plate will also cause the volume expansion of the negative electrode plate, which will further cause the instability of the SEI film and cause capacity attenuation; and the lost lithium cannot be reused.

[0044] Based on this, the embodiments of the present application propose a battery, which can be a lithium ion battery. By adding a fluorine-containing polymer containing -CH2CF(CF3)- repeating units (HFP repeating units) in the active layer of the negative electrode plate, the side products such as dead lithium can be converted into inorganic components that are beneficial to the performance of the plate, the side products are reused, thereby reducing the loss of active lithium, and the stability of the SEI film can be improved, thereby reducing the lithium consumption caused by the rupture of the SEI film and improving the problem of battery capacity attenuation.

[0045] The embodiments of the present application provide a lithium secondary battery, which can be a lithium ion battery, hereinafter referred to as a battery. Generally, the battery includes a negative electrode plate, a positive electrode plate, an electrolyte, and a separator. During the charging and discharging process of the battery, active ions are embedded and extracted between the positive electrode plate and the negative electrode plate. The electrolyte plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The separator is arranged between the positive electrode plate and the negative electrode plate, mainly to prevent the short circuit of the positive and negative electrodes, and at the same time to allow ions to pass through.

[0046] [Negative electrode plate]

[0047] The battery of the embodiments of the present application includes a negative electrode plate, which includes a negative electrode current collector and a negative electrode active layer arranged on at least one side of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material and a fluorine-containing polymer containing -CH2CF(CF3)- repeating units (HFP repeating units).

[0048] The -CH2CF(CF3)- repeating unit can be a repeating unit formed after polymerization of HFP (hexafluoropropylene CF2=CF-CF3). Whether the negative electrode active layer contains the fluorine-containing polymer containing the HFP repeating unit can be determined by testing the negative electrode active layer by Fourier transform infrared spectroscopy or X-ray diffraction (XRD) to detect whether there is a characteristic peak of the HFP repeating unit. For example, in the infrared spectrum, 721 cm -1characteristic peaks near the characteristic peaks of CF2=CF-CF3, indicating the presence of HFP repeat units. Understandably, because the negative electrode sheet contains a variety of substances, and the fluorine-containing polymer in the negative electrode sheet also interacts with the electrolyte or other substances, in the detection, in addition to the above-listed characteristic peaks, characteristic peaks that can reflect the interaction between the fluorine-containing polymer and other substances can also be observed.

[0049] The embodiments of the present application add a fluorine-containing polymer containing HFP repeat units to the negative active layer, and the results show that the battery capacity decay problem can be effectively improved. This may be due to the fact that this fluorine-containing polymer contains a large number of F-containing groups, which can spontaneously react with the by-products such as dead lithium generated by the battery side reaction, and convert these by-products into inorganic components LiF that are beneficial to the improvement of electrode performance.

[0050] LiF has good mechanical strength and can improve the stability of the SEI film, thereby reducing the repeated extraction and insertion of lithium ions due to the rupture of the SEI film, and continuously consuming active lithium, thereby reducing lithium loss; at the same time, the fluorine-containing polymer converts by-products into LiF, which reduces the accumulation of by-products 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, thereby improving the capacity decay problem.

[0051] Moreover, the side reaction to generate by-products will continue to occur during the battery cycle process, and this way of converting by-products into LiF that is beneficial to the improvement of electrode performance can exist throughout the battery cycle process, i.e., the components inside the negative electrode can be converted from waste to treasure during dynamic cycling, completing the secondary use of by-products of side reactions, continuously improving the lithium loss of the battery, and slowing down the battery capacity decay rate.

[0052] In some embodiments, the mass content of F element of the fluorine-containing polymer is 2% to 30%, optionally 10% to 15%, for example, it can be any one of 2%, 3%, 5%, 10%, 15%, 20%, 25%, 30% or a range between any two of them. The mass content of F element can be tested by X-ray fluorescence spectroscopy (XRF), nuclear magnetic resonance (NMR) and the like. At a suitable F element mass content, the fluorine-containing polymer has enough F-containing groups to react with by-products such as dead lithium to form LiF.

[0053] In some embodiments, the fluorine-containing polymer includes one or more of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyperfluoroalkylalkyl acrylate (F46, tetrafluoroethylene-hexafluoropropylene copolymer), tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymer (THV), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), optionally including PVDF-HFP. The presence of these fluorine-containing polymers in the negative active layer can be determined by Fourier transform infrared spectroscopy or X-ray diffraction (XRD) tests on the negative active layer to detect the presence of characteristic peaks corresponding to these fluorine-containing polymers. For example, in XRD tests, the presence of characteristic peaks near 18°, 20°, and 40° indicates the presence of PVDF-HFP. For example, in infrared spectroscopy, the presence of characteristic peaks near 1402 cm -1 , 1171 cm -1 , 1072 cm -1 , 876 cm -1 , 835 cm -1 , 761 cm -1 indicates the presence of PVDF-HFP. These fluorine-containing polymers all contain HFP repeating units, which can improve the problem of battery capacity decay.

[0054] In some embodiments, the fluorine-containing polymer is present in the negative active layer in an amount of 0.5% to 3%, optionally 0.8% to 1.2%, for example, in an amount of any one 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%, 3%, or in a range between any two of these values. Understandably, the amount of fluorine-containing polymer in the negative active layer can change after battery cycling, as the fluorine-containing polymer can react with by-products such as dead lithium. However, as the amount of by-products is much less than the amount of fluorine-containing polymer, the change in the amount of fluorine-containing polymer should be relatively small. In a battery that has not been cycled (or a negative electrode sheet that has not been cycled), the amount of fluorine-containing polymer in the negative active layer is 0.5% to 3%, and in a cycled battery, the amount of fluorine-containing polymer in the negative active layer can be 0.3% to 3%, optionally 0.8% to 1.2%, for example, in an amount of any one 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%, 3%, or in a range between any two of these values. In a cycled battery, the amount of fluorine-containing polymer in the negative active layer can or can not have changed compared to a negative electrode sheet that has not been cycled.

[0055] The mass content of the fluorine-containing polymer in the negative active layer can be obtained by thermal gravimetric analysis (TGA). For example, by performing TGA tests on a fluorine-containing polymer standard, the temperature range and the mass loss of the fluorine-containing polymer during thermal weight loss are obtained; and by performing TGA tests on the negative active layer, the thermal weight loss stage and the mass loss of the fluorine-containing polymer in the TG-DTG curve of the negative active layer can be identified according to the TGA test results of the fluorine-containing polymer standard, and then the mass content of the fluorine-containing polymer in the negative active layer can be calculated by the mass loss and the mass of the negative active layer. Of course, the mass content of the fluorine-containing polymer in the negative active layer can also be calculated according to the amounts of the raw materials of the negative active layer during the preparation of the negative electrode sheet.

[0056] With the mass content of the fluorine-containing polymer, the stability of the SEI film can be effectively improved, the loss of lithium can be reduced, and the speed of capacity decay can be slowed down, and at the same time, the negative active material has a sufficient content, so that the negative electrode has a sufficiently high energy density.

[0057] In some embodiments, the weight average molecular weight of the fluorine-containing polymer is 50x10 3 ~ 455x10 3 , optionally 80x10 3 ~ 110x10 3 , for example, can be any one of 50x10 3 , 60x10 3 , 80x10 3 , 100x10 3 , 110x10 3 , 150x10 3 , 200x10 3 , 250x10 3 , 300x10 3 , 350x10 3 , 400x10 3 , 450x10 3 , 455x10 3 , or a range between any two of them. The weight average molecular weight of the fluorine-containing polymer can be obtained by viscosity method, light scattering method, etc. For example, the sample can be dissolved in a suitable solvent, the viscosity or refractive index of the solution is measured, and the weight average molecular weight is calculated by the viscosity or refractive index. The molecular weight is one of the factors affecting the performance of the fluorine-containing polymer, for example, affecting the electrical conductivity and mechanical strength of the fluorine-containing polymer. The fluorine-containing polymer with a certain molecular weight selected by the embodiments of the present application is added to the negative electrode sheet, which has good electrical conductivity and mechanical strength, can improve the capacity decay of the battery while making the negative electrode sheet have good conductivity and structural stability.

[0058] In some embodiments, the fluorine-containing polymer is dispersed on the surface of the negative active material. The distribution relationship between the fluorine-containing polymer and the negative active material can be observed by electron microscopy. Generally, the by-products such as dead lithium generated during the battery cycle are mainly deposited on the surface of the negative active material, and therefore, by dispersing the fluorine-containing polymer on the surface of the negative active material, the fluorine-containing polymer can be in good contact with the by-products such as dead lithium after the by-products appear, so as to react and convert the by-products into inorganic components beneficial to the performance of the electrode. At the same time, the fluorine-containing polymer has a certain absorption and swelling effect on the electrolyte, and by dispersing the fluorine-containing polymer on the surface of the negative active material, the contact area between the fluorine-containing polymer and the negative active material can be improved during the swelling of the fluorine-containing polymer, so as to achieve better contact.

[0059] It should be understood that the dispersion of the fluorine-containing polymer on the surface of the negative active material includes the dispersion of all the fluorine-containing polymers on the surface of the negative active material, and also includes the dispersion of part of the fluorine-containing polymers on the surface of the negative active material, which is mainly related to the uniformity of the dispersion of the fluorine-containing polymer during the preparation of the negative electrode. In the case where all the fluorine-containing polymers are dispersed on the surface of the negative active material, the contact between the fluorine-containing polymer and the by-products such as dead lithium is better.

[0060] At the same time, since the fluorine-containing polymer is water-insoluble, and the negative active layer is usually prepared in an aqueous system, the fluorine-containing polymer has one or more of granular, fibrous and other morphologies in the negative active layer. The morphology of the fluorine-containing polymer can be observed by electron microscopy. The fluorine-containing polymer with granular, fibrous and other morphologies is well dispersed on the surface of the negative active material and fully contacts with the by-products such as dead lithium on the surface of the negative electrode.

[0061] In some embodiments, the negative active layer further comprises a binder, and the binder comprises one or more of PAA (polyacrylic acid) and PTFE (polytetrafluoroethylene). Optionally, the binder comprises PAA. Whether the binder exists or not can be determined by detecting whether the characteristic peaks corresponding to the binder exist in the negative active layer by Fourier transform infrared spectroscopy or XRD test. Generally, the preparation process of the negative active layer adopts an aqueous system (using water as a solvent), and the fluorine-containing polymer is insoluble in water, so that the negative electrode slurry has low viscosity and is difficult to adhere to the current collector. By adding the binder such as PAA and PTFE, this situation can be effectively improved.

[0062] In some embodiments, the PAA has a mass content in the negative active layer of 0.1% to 1%, optionally 0.2% to 0.25%, for example, it can be any one of 0.1%, 0.2%, 0.25%, 0.4%, 0.6%, 0.8%, 1% or a range between any two of them. Similarly, the mass content of PAA in the negative active layer can also be obtained by thermal gravimetric analysis (TGA). At this mass content, both the negative process problem can be improved by using PAA, and the proportion of PAA in the negative active layer is not too high, so that the negative active material has sufficient content, so that the negative electrode has a high enough energy density.

[0063] It can be understood that the binder in the negative active layer can also include one or more of SBR (styrene-butadiene rubber), PAAS (sodium polyacrylate), PAM (polyacrylamide), PVA (polyvinyl alcohol), SA (sodium alginate), PMAA (polymethyl methacrylate), CMCS (carboxymethyl chitosan). The mass content of all binders in the negative active layer can be 0.1% to 10%, for example, any one of 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or a range between any two of them.

[0064] In some embodiments, the PAA has a weight average molecular weight of 50 x 10 3 to 500 x 10 3 , optionally 80 x 10 3 to 100 x 10 3 , for example, it can be any one of 50 x 10 3 , 60 x 10 3 , 80 x 10 3 , 100 x 10 3 , 110 x 10 3 , 150 x 10 3 , 200 x 10 3 , 250 x 10 3 , 300 x 10 3 , 350 x 10 3 , 400 x 10 3 , 450 x 10 3 , 500 x 10 3 or a range between any two of them. Similarly, the weight average molecular weight of PAA can be obtained by viscosity method, light scattering method, etc. Molecular weight is one of the key factors affecting the viscosity of PAA. By selecting PAA with a certain molecular weight as a binder, the PAA has high viscosity, which can effectively improve the low viscosity problem caused by the presence of PVDF-HFP.

[0065] In some embodiments, the negative active layer further comprises LiF. The presence of LiF can be determined by XRD test of the negative active layer. If there are characteristic peaks of LiF (around 16.36°, 18.87°, 24.09°, 31.99°), it can be determined that there is LiF. LiF in the negative active layer can be formed by the reaction of substances in the electrolyte with active lithium, or can be derived from the reaction of fluorine-containing polymers with by-products such as dead lithium. LiF has good mechanical strength and can improve the stability of the SEI film, thereby reducing the repeated extraction and insertion of lithium ions due to the rupture of the SEI film, continuously consuming active lithium, i.e. reducing lithium loss. At the same time, the fluorine-containing polymer converts by-products such as dead lithium into LiF, so the accumulation of by-products on the negative electrode will be reduced, thereby alleviating the volume expansion of the negative electrode, improving the stability of the SEI film, reducing lithium loss, and thus improving the capacity decay problem.

[0066] In some embodiments, the negative active material comprises one or more of graphite (artificial graphite, natural graphite), soft carbon, hard carbon, silicon-based material, tin-based material, titanium-based material. The silicon-based material can comprise one or more of elemental silicon, silicon oxide compound, silicon-carbon composite, silicon-nitrogen composite, silicon alloy. The tin-based material can comprise one or more of elemental tin, tin oxide compound, tin alloy. The titanium-based material can comprise lithium titanate. It can be understood that the present application is not limited to these materials, and other materials that can be used as battery negative active materials can also be used. These negative active materials can be used alone or in combination with two or more.

[0067] During the cycle of the battery, by-products such as dead lithium will appear on the surface of the negative active material. The scheme of the present application has an improvement effect on the capacity decay of batteries using various negative active materials, and has wide applicability.

[0068] The mass content of the negative active material in the negative active layer can include 70% to 98%, for example 90% to 98%, for example 70%, 75%, 80%, 85%, 90%, 92%, 94%, 96%, 98% of any one point value or a range value between any two. The negative active material at a high content can provide high energy density for the battery.

[0069] In some embodiments, the negative active layer further comprises a conductive agent. The conductive agent can comprise one or more of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, carbon nanofibers. The mass content of the conductive agent in the negative active layer includes 0.5% to 10%, for example 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% of any one point value or a range value between any two.

[0070] In some embodiments, the negative active layer can also optionally include other auxiliary agents, such as thickening agents, for example, CMC (carboxymethyl cellulose), CMC-Na (sodium carboxymethyl cellulose), and the like.

[0071] The negative electrode sheet in the battery of the present application also includes a negative current collector, for example, the negative current collector has two surfaces opposite in the thickness direction of the negative current collector, and the negative active layer is disposed on either one or both of the two surfaces of the negative current collector.

[0072] In some embodiments, the negative current collector includes one or more of a metal foil, a composite current collector. For example, as the metal foil, a copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, and the like) on a polymer material base layer, such as a base layer of PP (polypropylene), PET (polyethylene terephthalate), PBT (polybutylene terephthalate), PS (polystyrene), PE (polyethylene), and the like.

[0073] In some embodiments, the negative electrode sheet can be prepared by the following method:

[0074] The components used to prepare the negative electrode sheet, such as the negative active material, the fluorine-containing polymer, the binder, the conductive agent (and any other components can also be included), are dispersed in a solvent (such as water) to form a negative electrode slurry; the negative electrode slurry is coated on at least one side of the negative current collector, and after processes such as drying, cold pressing, and the like, the negative electrode sheet is obtained.

[0075] In some embodiments, in the preparation of the negative electrode sheet, the PAA in the binder is selected from a PAA aqueous solution with a solid content of 20% to 50%, and optionally 30% to 35%. For example, the solid content of the PAA aqueous solution is any one of 20%, 25%, 30%, 35%, 40%, 45%, 50%, or a range between any two of them. In the process of making the negative electrode sheet, the PAA is added to the negative electrode slurry in the form of an aqueous solution, which can effectively improve its dispersibility in the negative electrode slurry, thereby improving the viscosity of the negative electrode slurry and improving the process problems of the negative electrode.

[0076] [Positive electrode sheet]

[0077] The battery of the embodiments of the present application includes a positive electrode sheet, the positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side of the positive current collector, and the positive active layer includes a positive active material, a conductive agent, and a binder.

[0078] As an example, the positive electrode current collector has two surfaces opposite in the thickness direction thereof, and the positive electrode active layer is provided on either one or both of the two surfaces of the positive electrode current collector.

[0079] In some embodiments, the positive electrode current collector includes one or more of a metal foil, a composite current collector. As the metal foil, for example, an aluminum foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material such as a base material of PP (polypropylene), PET (polyethylene terephthalate), PBT (polybutylene terephthalate), PS (polystyrene), PE (polyethylene), etc.

[0080] In some embodiments, the positive electrode active material includes one or more of a lithium-containing phosphate having an olivine structure and a modified compound thereof, a lithium transition metal oxide and a modified compound thereof. Among them, examples of the lithium-containing phosphate having an olivine structure can include one or more of lithium iron phosphate (such as LiFePO4, LFP), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon. Examples of the lithium transition metal oxide can include lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, 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 0.05One or more of O2 and its modified compounds. These positive electrode active materials can be used alone or in combination of two or more.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] In some implementations, the positive electrode sheet can be prepared in the following manner:

[0087] 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.

[0088] [Electrolytes]

[0089] The electrolyte serves to conduct ions between the positive electrode and the negative electrode. In the battery of the present embodiment, the type of electrolyte can be selected as desired. For example, the electrolyte can be a solid electrolyte or an electrolyte solution.

[0090] The solid electrolyte includes one or more of a polymer electrolyte, an inorganic solid-state electrolyte. Illustratively, the polymer electrolyte can include a fluorine-containing polymer containing HFP repeating units. Such a fluorine-containing polymer can be used to form a polymer gel electrolyte, which can be used in a polymer solid-state battery, and the fluorine-containing polymer containing HFP repeating units can also convert by-products such as dead lithium formed on the surface of the negative electrode in contact with the electrolyte, thereby enabling reuse of the by-products such as dead lithium.

[0091] The electrolyte solution includes a solvent and a lithium salt dissolved in the solvent.

[0092] The solvent can be a non-aqueous organic solvent, and for example, can include one or more of, preferably two or more of, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl 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).

[0093] The lithium salt can include one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bisfluorosulfonylimide), LiTFSI (lithium bis-trifluoromethanesulfonylimide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium bis(oxalato)borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluoro(oxalato)phosphate), and LiTFOP (lithium tetrafluoro(oxalato)phosphate).

[0094] The concentration of the lithium salt in the electrolyte solution can include 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 a range between any two of them.

[0095] The electrolyte solution can also optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can also include an additive that can improve certain properties of the battery, for example, an additive that improves overcharge performance of the battery, an additive that improves high-temperature or low-temperature performance of the battery, etc.

[0096] [Separator]

[0097] The separator in the battery is usually stacked between the positive electrode sheet and the negative electrode sheet, used to separate the positive electrode sheet and the negative electrode sheet, so that the electrons in the battery cannot pass freely, preventing the two poles from short circuiting, while allowing the ions in the electrolyte to pass freely between the positive and negative electrodes.

[0098] In the battery of the present application, the type of separator can be selected from any known porous structure separator with good chemical stability and mechanical stability.

[0099] The material of the separator can include one or more of glass fiber, non-woven fabric, polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride (PVDF). The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of the layers can be the same or different, without particular limitation.

[0100] The positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly through a winding process or a stacking process.

[0101] [Outer package]

[0102] The battery can include an outer package, which can be used to package the electrode assembly containing the positive electrode sheet, the negative electrode sheet, and the separator, as well as the electrolyte.

[0103] The outer package of the battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc.; or a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate, etc.

[0104] The shape of the outer package can be cylindrical, square, or any other shape. For example, FIG. 3 is a battery cell with a square structure as an example of the shape of the outer package.

[0105] Referring to FIG. 4, the outer package can include a shell 01 and a cover plate 02. The shell 01 can include a bottom plate and a side plate connected to the bottom plate, which enclose a receiving cavity. The shell 01 has an opening communicating with the receiving cavity, and the cover plate 02 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly 03 through a winding process or a stacking process. One or more electrode assemblies 03 are packaged in the receiving cavity. The electrolyte is impregnated in the electrode assembly 03.

[0106] [Battery cell, battery module, battery pack]

[0107] The battery of the embodiments of the present application can be at least one of a battery monomer, a battery module, and a battery pack. According to different packaging forms, the battery is divided into a battery monomer, a battery module, and a battery pack. The battery monomer is the most basic unit of a secondary battery, which includes an electrode assembly and an electrolyte. The electrode assembly is usually composed of a positive electrode sheet, a negative electrode sheet, and a separator. The positive electrode sheet and the negative electrode sheet are alternately stacked, and a separator film is arranged between the positive electrode sheet and the negative electrode sheet to play a separating role, thereby obtaining an electrode assembly (also referred to as a battery cell). The battery cell can also be obtained after winding. The battery cell is placed in a shell, electrolyte is injected, and the shell is sealed to obtain a battery monomer. The battery monomer mainly works by moving active metal ions in the electrolyte between the positive electrode sheet and the negative electrode sheet.

[0108] In some battery packaging technologies, one or more battery monomers 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 battery monomers can be directly installed in a box to form a battery pack, eliminating the intermediate state of the battery module, thereby reducing the mass of the battery pack and improving the energy density of the battery.

[0109] Referring to FIG. 5, it is a battery module as an example. In the battery module, a plurality of battery monomers 04 can be arranged in sequence along the length direction of the battery module. Of course, they can also be arranged in other arbitrary ways. Further, the plurality of battery monomers 04 can be fixed by fasteners.

[0110] Optionally, the battery module can further include a shell having an accommodation space, and the plurality of battery monomers 04 are accommodated in the accommodation space.

[0111] Referring to FIGS. 6 and 7, it is a battery pack as an example. The battery pack can include a battery box and a plurality of battery modules 05 arranged in the battery box. The battery box includes an upper box 06 and a lower box 07, and the upper box 06 can be arranged on the lower box 07 to form a closed space for accommodating the battery modules 05. The plurality of battery modules 05 can be arranged in the battery box in any manner.

[0112] [Electric device]

[0113] The embodiments of the present application also provide an electric device, which includes the above-mentioned battery.

[0114] The above-mentioned battery can be used as a power supply of the electric device, or can be used as an energy storage unit of the electric device. The battery of the embodiments of the present application has the advantages of slow capacity loss, good cycle performance, and long service life. Therefore, when the battery is applied to the electric device, it is beneficial to improve the use experience of the electric device.

[0115] The power consuming device can include a mobile device (e.g., a cell phone, a laptop, etc.), an electric vehicle (e.g., a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0116] As the power consuming device, a battery cell, a battery module, or a battery pack among the batteries can be selected according to the use requirements thereof.

[0117] FIG. 8 is a power consuming device as an example. The power consuming device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the requirements of high power and high energy density of the power consuming device for the battery, a battery pack or a battery module can be used.

[0118] The embodiments of the present application are described in detail below. The embodiments described below are exemplary only, and are for the purpose of explanation of the present application, and are not to be understood as limiting the present application. In the embodiments, the specific techniques or conditions not mentioned are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be obtained by purchase on the market.

[0119] Example 1

[0120] 1. Preparation of positive electrode sheet

[0121] Lithium iron phosphate, conductive carbon black, and PVDF were mixed in a mass ratio of 8:1:1, and then added to a solvent N-methyl pyrrolidone 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.

[0122] 2. Preparation of negative electrode sheet

[0123] Artificial graphite, conductive carbon black, CMC, PAA, PVDF-HFP, and water as a solvent were uniformly mixed in 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%.

[0124] The weight average molecular weight of PAA is about 100x10 3 The weight average molecular weight of PVDF-HFP is about 80x10 3 The mass content of F element of PVDF-HFP is 10%.

[0125] 3. Separation film

[0126] A polyethylene film with a thickness of 7 μm was used as a separation film.

[0127] 4. Electrolyte

[0128] Ethylene carbonate, diethyl carbonate, dimethyl carbonate were mixed in a volume ratio of 1:1:1 to obtain a mixed solvent. LiPF6 was dissolved in the mixed solvent to obtain an electrolyte. In the electrolyte, the concentration of LiPF6 was 1 mol / L.

[0129] 5. Battery assembly

[0130] The positive electrode sheet, the negative electrode sheet and one end of the two separators were fixed on the discharge roller in the order of "separator-negative electrode sheet-separator-positive electrode sheet", and the other end was fixed on the winding shaft after being stacked together. The winding shaft was rotated by a motor to wind the positive electrode sheet, the negative electrode sheet and the two separators to obtain a wound battery. After the battery was coated with a film, injected with liquid and charged with a current of 0.1C for 5 hours, the formation process was completed.

[0131] Example 2

[0132] The difference between this example and Example 1 is that the mass content of F element in PVDF-HFP is 3%.

[0133] Example 3

[0134] The difference between this example and Example 1 is that the mass content of F element in PVDF-HFP is 5%.

[0135] Example 4

[0136] The difference between this example and Example 1 is that the mass content of F element in PVDF-HFP is 15%.

[0137] Example 5

[0138] The difference between this example and Example 1 is that the mass content of F element in PVDF-HFP is 20%.

[0139] Example 6

[0140] The difference between this example and Example 1 is that the mass content of F element in PVDF-HFP is 30%.

[0141] Example 7

[0142] The difference between this example and Example 1 is that in the negative electrode sheet, the mass ratio of artificial graphite, conductive carbon black, CMC, PAA and PVDF-HFP 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%.

[0143] Example 8

[0144] The difference between this example and Example 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%.

[0145] Example 9

[0146] The difference between this example and Example 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%.

[0147] Example 10

[0148] The difference between this example and Example 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%.

[0149] Comparative Example 1

[0150] 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 sheet is 98:2:3:0:0. That is, the negative electrode sheet does not contain PAA and PVDF-HFP.

[0151] Comparative Example 2

[0152] 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 sheet is 94.75:2:3:0.25:0. That is, the negative electrode sheet does not contain PVDF-HFP.

[0153] Comparative Example 3

[0154] The difference between this comparative example and Example 1 is that the PVDF-HFP in the negative electrode sheet is replaced with an equal mass of PVDF, and the mass content of the F element in the PVDF is 14%.

[0155] Comparative Example 4

[0156] The difference between this comparative example and Example 1 is that the PVDF-HFP in the negative electrode sheet is replaced with an equal mass of PTFE, and the mass content of the F element in the PTFE is 17%.

[0157] The performance of the batteries assembled for each example and comparative example was tested as follows:

[0158] (1) Cycle test

[0159] At room temperature 25℃, charge to 3.65V at 1C constant current, then test at constant voltage until the current is less than 0.05C, then discharge to 2.5V at 1C, and so on for 1000 cycles. Record the discharge capacity of the first cycle as C0, and the discharge capacity of each subsequent cycle as C1. Calculate the SOH at different cycle numbers as C1 / C0.

[0160] Where SOH represents the state of health of the battery, used to measure the cumulative loss and aging degree of the battery during long-term use, reflecting the proportion of the remaining capacity of the battery relative to its initial design capacity. SOH is usually represented by a value between 0 and 1, where 0 represents poor battery performance, and 1 represents good battery performance. As the number of battery use and charge cycles increases, SOH gradually decreases, resulting in a decrease in battery performance.

[0161] (2) Direct current resistance DCR

[0162] Test steps:

[0163] 1) 0.33C constant current to 3.65V, constant voltage charge to current less than 0.05C;

[0164] 2) 0.33C constant current discharge for 18min, adjust SOC to 90%;

[0165] 3) Record the initial voltage as OCV1, discharge at I=2C current for 30s, record the end voltage as OCV2;

[0166] Calculation formula:

[0167] DCR=(OCV1-OCV2) / I.

[0168] Test results are shown in the following table.

[0169] [Table 1]

[0170] Note: In Table 1, the PVDF-HFP mass content and the PAA mass content represent the mass content of PVDF and PAA in the negative active layer, respectively.

[0171] The SOH of the batteries of Example 1, Comparative Example 1 and Comparative Example 2 at different cycle numbers is shown in Figure 1. The test results in Table 1 and Figure 1 show that, after 1000 cycles, the SOH of the battery in Example 1 is higher than that of Comparative Example 1 and Comparative Example 2, and the capacity decay of the battery is slower, indicating that the capacity decay problem of the battery can be effectively improved by adding PVDF-HFP in the negative electrode sheet, which will be beneficial to improve the cycle life of the battery. As shown in Figure 2, this may be due to the fact that, during the charging and discharging of the battery, the active lithium ions in the battery gradually undergo side reactions to form by-products such as dead lithium on the surface of the graphite particles in the negative electrode sheet, resulting in lithium loss. After adding PVDF-HFP in the active layer of the negative electrode sheet, the F-containing groups in PVDF-HFP can spontaneously react with these by-products to convert them into LiF, so that a SEI film rich in LiF can be formed on the surface of the graphite particles. LiF has good mechanical strength and can improve the stability of the SEI film, thereby reducing the repeated extraction and insertion of lithium ions due to the rupture of the SEI film and the continuous consumption of active lithium, i.e. reducing lithium loss, thereby improving the capacity decay. At the same time, the conversion of by-products such as dead lithium into LiF by PVDF-HFP will reduce the accumulation of by-products on the graphite particles, which in turn can alleviate the volume expansion of the graphite and improve the stability of the SEI film, which is also beneficial to reduce lithium loss and improve capacity decay.

[0172] At the same time, as can be seen from Figure 1, the SOH of the battery in Example 1 is higher than that of Comparative Example 1 and Comparative Example 2 at different cycle numbers, which can be inferred that the process of converting by-products into LiF that is beneficial to the improvement of electrode performance by PVDF-HFP can exist throughout the cycle process of the battery.

[0173] [Table 2]

[0174] The data in Table 2 shows that adding PVDF-HFP with different F element mass contents in the negative electrode sheet can improve the capacity decay of the battery. And within the range of F element mass content of 3% to 30%, the higher the F element mass content, the better the improvement effect on the capacity decay of the battery.

[0175] [Table 3]

[0176] Embodiment 1 and Embodiments 7-10 can improve the capacity decay of the battery by adding different mass contents of PVDF-HFP in the negative electrode sheet. When the mass content of PVDF-HFP is in the range of 0.5% to 3%, the SOH of the battery is higher, i.e., the capacity decay is less, after the same number of cycles, as the mass content of PVDF-HFP increases. When the mass content of PVDF-HFP is less than 10%, the SOH is improved more obviously as the mass content of PVDF-HFP increases. When the mass content of PVDF-HFP is more than 10%, the SOH tends to be flat.

[0177] [Table 4]

[0178] Note: In Table 4, the DCR growth rate refers to the DCR growth rate compared to Comparative Example 1.

[0179] It is considered that PVDF-HFP mainly reacts with by-products such as dead lithium through the F-containing groups therein. Therefore, PVDF-HFP is replaced by other substances containing F groups, such as PVDF and PTFE. The results show that, under similar F element mass contents, after adding PVDF and PTFE, the SOH of the battery after the same number of cycles is similar to that without adding any fluorine-containing substance (Comparative Example 1), and no improvement is observed. In addition, the DCR increases significantly, which reflects that adding PVDF and PTFE in the negative electrode sheet cannot well improve the capacity decay of the battery, but can cause a large degree of degradation of other performance of the battery. However, adding PVDF-HFP in the negative electrode sheet can well improve the capacity decay of the battery, and has less effect on the DCR, and does not cause obvious degradation of other performance of the battery. It is preliminarily speculated that this may be related to the presence of HFP repeating units.

[0180] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and embodiments having the same technical idea and playing the same role and effect within the scope of the technical solutions of the present application are all included in the technical scope of the present application. In addition, within the scope of the main idea of the present application, various modifications of the embodiments that can be thought of by those skilled in the art, and other ways constructed by combining part of the components of the embodiments are also included in the scope of the present 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, characterized by, 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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