Secondary battery and electric device

Abstract

The application discloses a secondary battery and a power utilization device, and belongs to the technical field of batteries. The secondary battery provided by the application controls the outermost corner area of the winding core after full charging, and the maximum height and the maximum width of the first color area are respectively within specific ranges from the negative electrode tab side to the center direction of the negative electrode tab, so that the low-temperature capacity retention rate and the normal-temperature cycle performance of the secondary battery are improved.

Description

Technical Field

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

[0002] The production of secondary batteries mainly employs winding and stacking technologies for electrode assembly. Winded secondary batteries typically consist of a positive electrode, a separator, and a negative electrode wound together to form a bare cell. This cell then undergoes processes such as encapsulation, electrolyte injection, formation, and charging to ultimately become the finished product. During charging and discharging, the expansion of the electrodes can cause large gaps at the outermost corners of the wound core, affecting electrolyte wetting and hindering the intercalation of active cells. This can lead to phenomena such as purple spots at the corners and even lithium plating, which deteriorates the low-temperature capacity retention and room-temperature cycling performance of the secondary battery. Summary of the Invention

[0003] The purpose of this application is to solve the technical problems of poor low-temperature capacity retention and poor room-temperature cycling performance of secondary batteries in the prior art, and to propose a secondary battery and power supply device.

[0004] To achieve the above objectives, a first aspect of this application provides a secondary battery, including a winding core. The winding core includes a positive electrode sheet and a negative electrode sheet. The negative electrode sheet includes a negative electrode tab. A separator is provided between the positive electrode sheet and the negative electrode sheet. The winding core is formed by winding the positive electrode sheet, the separator, and the negative electrode sheet. The winding core includes a straight area and a corner area connected together. After the secondary battery is fully charged, the outermost corner area of ​​the winding core and the negative electrode sheet have a first color area and a second color area in the direction from the negative electrode tab side to the center of the negative electrode sheet. The first color area and the second color area are different. The maximum height of the first color area is less than or equal to 21 mm, and the maximum width of the first color area is less than or equal to 24 mm.

[0005] In some embodiments of this application, the secondary battery satisfies: 0.01≤Y≤0.105; Where Y = (H × L) / (D × W); H mm is the maximum height of the first color area; L mm is the maximum width of the first color area; D mm is the thickness of the core; W mm is the width of the core.

[0006] In some embodiments of this application, D is 15 to 32.

[0007] In some embodiments of this application, W is 90~180.

[0008] In some embodiments of this application, the width of the negative electrode tab is A mm, and the thickness of the negative electrode tab is T μm, satisfying: 20≤A≤60, 3≤T≤8.

[0009] In some embodiments of this application, the positive electrode includes a positive active material layer and a positive solid electrolyte interface film located on the surface of the positive active material layer, wherein the positive solid electrolyte interface film contains F and B elements.

[0010] In some embodiments of this application, the mass percentage of element F is CF and the mass percentage of element B is CB, based on the total mass of the positive electrode solid electrolyte interface membrane, satisfying: 0.1%≤CF≤10% and 0.01%≤CB≤5%.

[0011] In some embodiments of this application, the negative electrode sheet includes a negative electrode active material layer and a negative electrode solid electrolyte interface film located on the surface of the negative electrode active material layer, wherein the negative electrode solid electrolyte interface film contains F and B elements.

[0012] In some embodiments of this application, the mass percentage of element F is AF and the mass percentage of element B is AB, based on the total mass of the negative electrode solid electrolyte interface membrane, satisfying: 1%≤AF≤30% and 0.01%≤AB≤10%.

[0013] In some embodiments of this application, the secondary battery has a charging DC internal resistance (DCR) of less than or equal to 1 mΩ and a charge transfer impedance (Rct) of less than or equal to 0.8 mΩ at 50% SOC.

[0014] In some embodiments of this application, the lithium-ion diffusion coefficient of the secondary battery is D-Li, where D-Li ≥ 1 × 10⁻⁶. -18 cm 2 / s.

[0015] In some embodiments of this application, the secondary battery further includes an electrolyte containing additives, including tris(pentafluorophenyl)borane (TPFPB) and lithium fluoride.

[0016] In some embodiments of this application, the molar ratio of tris(pentafluorophenyl)borane to lithium fluoride is 0.5 to 2.

[0017] In some embodiments of this application, the sum of the masses of tris(pentafluorophenyl)borane and lithium fluoride is 1% to 8% based on the mass of the electrolyte.

[0018] In some embodiments of this application, the electrolyte further includes an organic solvent, which includes at least one of dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, and methyl ethyl carbonate.

[0019] In some embodiments of this application, the electrolyte further includes a lithium salt, wherein the molar concentration of the lithium salt is 0.8 mol / L to 2 mol / L.

[0020] In some embodiments of this application, the core further includes a winding end and an insulating tape, the insulating tape being bonded to at least one of the winding end, the corner area, and the straight area.

[0021] A second aspect of this application provides an electrical device comprising the secondary battery described in this application.

[0022] Compared with the prior art, the beneficial effects of this application are: The secondary battery provided in this application improves the low-temperature capacity retention rate and room-temperature cycle performance by controlling the outermost corner area of ​​the core after full charging, and ensuring that the maximum height and maximum width of the first color area of ​​the negative electrode sheet from the negative electrode tab side to the center of the negative electrode sheet are within a specific range. Attached Figure Description

[0023] Figure 1 This is a schematic diagram showing the maximum height (H) and maximum width (L) of the first color area of ​​the outermost corner region of the core of the secondary battery prepared in Example 1 after it is fully charged. The negative electrode sheet is located from the negative electrode tab side to the center of the negative electrode sheet.

[0024] Figure 2 This is a schematic diagram of the width of the negative electrode tab of the secondary battery prepared in Example 1.

[0025] Figure 3 This is a schematic diagram of the adhesive application at the outermost corner of the secondary battery core prepared in Example 1. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0027] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0028] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0029] In one embodiment of this application, a secondary battery is provided, including a core. The core includes a positive electrode and a negative electrode. The negative electrode includes a negative electrode tab. A separator is provided between the positive electrode and the negative electrode. The core is formed by winding the positive electrode, the separator, and the negative electrode. The core includes a straight area and a corner area connected together. After the secondary battery is fully charged, the outermost corner area of ​​the core and the negative electrode have a first color area and a second color area from the negative electrode tab side towards the center of the negative electrode. The first color area and the second color area are different. The maximum height of the first color area is less than or equal to 21 mm, and the maximum width of the first color area is less than or equal to 24 mm.

[0030] The secondary battery provided in this application, by controlling the maximum height and width of the first color area of ​​the negative electrode sheet from the negative electrode tab side towards the center of the negative electrode sheet within a specific range at the outermost corner region of the wound core after full charging, can ensure that the secondary battery has excellent low-temperature capacity retention and room-temperature cycling performance. Specifically, in wound-structured batteries, due to the low tension of the outermost negative electrode sheet, a large gap easily forms between the negative and positive electrodes in the outermost corner region after lithium insertion / extraction expansion, hindering lithium ion insertion and leading to corner purple spots. The negative electrode active material in the corner purple spot region cannot effectively exert its capacity. As the electrode sheet expands larger after cycling, the corner gap and corner purple spot region gradually increase, leading to accelerated capacity decay. This application's research found that the smaller the corner purple spot region, the lower the internal resistance of the secondary battery, the better the kinetics, and the better the low-temperature capacity retention and room-temperature cycling performance.

[0031] For example, the maximum height of the first color area can be any point value or any two-point range value below 21 mm, such as 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, 21 mm, etc.

[0032] For example, the maximum width of the first color area can be any point value or any two-point range value between 24 mm and below, such as 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 12 mm, 14 mm, 16 mm, 18 mm, 20 mm, 22 mm, 24 mm, 24 mm, etc.

[0033] It should be noted that in this application, the ordinal numbers "first" and "second" for the first color region and the second color region are used only to distinguish different color regions. As an example, the first color region is roughly purple, while the second color region is roughly black, but this application is not limited to these two colors and may also be any other different colors.

[0034] It should be noted that the test method for the maximum width and maximum height of the first color area in this application is as follows: (1) The secondary battery is charged at a constant current of 1C rate to the upper voltage limit Vmax, and then charged at a constant voltage of Vmax to a current of 0.05C (the upper voltage limit of lithium iron phosphate battery is 3.65V; the upper voltage limit of nickel cobalt manganese ternary battery is 4.3V); (2) The secondary battery is disassembled under dry conditions, and the maximum height (H) and maximum width (L) of the first color area of ​​the negative electrode plate at the outermost corner are measured from the negative electrode tab side to the center of the negative electrode plate using a transparent soft ruler. The maximum height (H) is defined as: the maximum distance from the edge of the negative electrode plate to the edge of the arc contour. If there are multiple arc contours, the maximum height is taken as the standard. The maximum width (L) is defined as: the maximum width of the arc contour. If there are multiple arc contours, the maximum width is taken as the standard. The maximum height (H) and maximum width (L) of the first color area are as follows: Figure 1 As shown.

[0035] In some embodiments, the secondary battery satisfies 0.01≤Y≤0.105; Where Y = (H × L) / (D × W); H mm is the maximum height of the first color area; L mm is the maximum width of the first color area; D mm is the thickness of the core; W mm is the width of the core.

[0036] For example, Y can be any point value between 0.01 and 0.105 or a range value between any two points, such as 0.01, 0.015, 0.020, 0.025, 0.030, 0.035, 0.040, 0.045, 0.050, 0.055, 0.060, 0.065, 0.070, 0.075, 0.080, 0.085, 0.090, 0.095, 0.100, 0.105, etc.

[0037] This study found that when the maximum height and width of the first color region at the outermost corner of the core after the secondary battery is fully charged, along with the thickness and width of the core, satisfy 0.01 ≤ (H×L) / (D×W) ≤ 0.105, the resulting secondary battery exhibits better low-temperature capacity retention and room-temperature cycling performance. Specifically, D and W can control the area and gap at the outermost corner of the secondary battery core to a certain extent, while H and L can reflect the size of the first color region, i.e., the area of ​​the negative electrode sheet that is not fully intercalated with lithium ions. Controlling the value of (H×L) / (D×W) within a certain range can effectively reduce the corner gap and the area of ​​the first color region, increase the number of lithium intercalation sites on the negative electrode sheet at the corner, thereby improving the low-temperature capacity retention and room-temperature cycling performance of the secondary battery.

[0038] In some embodiments, Y is 0.02 to 0.10. For example, it can be 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, etc.

[0039] This study found that when Y is further selected to be 0.02~0.10, the overall performance of the secondary battery is better.

[0040] In some embodiments, D is 15 to 32.

[0041] For example, D can be any point value between 15 and 32 or a range value between any two points, such as 15, 18, 20, 22, 25, 28, 30, 32, etc.

[0042] In some embodiments, D is 20.4 to 24.4. For example, it can be 20.4, 20.8, 21.2, 21.6, 22, 22.4, 22.8, 23.2, 23.6, 24, 24.4, etc.

[0043] It should be noted that the test method for the thickness (D) of the core is as follows: disassemble the secondary battery to obtain the core, use vernier calipers to measure the thickness at the middle position of the core, and take the average of the three measurements as the thickness of the core.

[0044] This study found that the thickness of the core affects its ability to release internal stress caused by the expansion and contraction of the electrode material. A suitable thickness reduces wrinkling during charging and discharging, alleviating poor local contact and thus reducing internal resistance and polarization in the secondary battery. This stress relief also reduces gaps at the core corners, facilitating lithium-ion diffusion and electrolyte wetting, thereby reducing the size of the first color region. Further selection of the core thickness within the aforementioned range, and especially within a more preferred range, enables the secondary battery to exhibit excellent low-temperature capacity retention and room-temperature cycling performance.

[0045] In some embodiments, W is 90~180.

[0046] For example, W can be any point value between 90 and 180 or a range value between any two points, such as 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, etc.

[0047] In some embodiments, W is 120~150. For example, it can be 120, 125, 130, 135, 140, 145, 150, etc.

[0048] It should be noted that the test method for the width (W) of the core is as follows: disassemble the secondary battery to obtain the core, use vernier calipers to measure the width at the middle position of the core, and take the average of the three measurements as the core width.

[0049] This study found that the width of the core affects its ability to release internal stress caused by the expansion and contraction of the electrode material. A suitable width reduces wrinkles on the electrode during charging and discharging, alleviating poor local contact and thus reducing internal resistance and polarization in the secondary battery. This stress relief also reduces gaps at the core corners, facilitating lithium-ion diffusion and electrolyte wetting, thereby reducing the size of the first color region. When the core width is further selected within the aforementioned range, especially when W is further within the range of 120~150, the resulting secondary battery exhibits better low-temperature capacity retention and room-temperature cycling performance.

[0050] In some embodiments, the width of the negative electrode tab is A mm, and the thickness of the negative electrode tab is T μm, satisfying: 20≤A≤60, 3≤T≤8.

[0051] For example, the width of the negative electrode tab can be any value between 20 and 60 mm, or a range between any two points, such as 20 mm, 30 mm, 40 mm, 50 mm, 60 mm, etc. The thickness of the negative electrode tab can be any value between 3 and 8 μm, or a range between any two points, such as 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, etc. It should be noted that the testing method for the width and thickness of the negative electrode tab is as follows: The secondary battery is disassembled under dry conditions, and the width of the negative electrode tab is measured using a transparent measuring tape. The width of the negative electrode tab is defined as: the maximum width of the area at the base of the tab, such as... Figure 2 As shown. Use a micrometer to measure the thickness of the negative electrode tab.

[0052] This study found that the width and thickness of the negative electrode tab affect the internal resistance of the secondary battery, and consequently the size of the first color region. Appropriate width and thickness of the negative electrode tab can increase electron transport speed, alleviate internal resistance and polarization phenomena in the secondary battery, and thus reduce the size of the first color region. Further selection of the width and thickness of the negative electrode tab within the aforementioned range results in a secondary battery with superior low-temperature capacity retention and room-temperature cycling performance.

[0053] In some embodiments, the positive electrode includes a positive active material layer and a positive solid electrolyte interface (CEI) film located on the surface of the positive active material layer, wherein the positive solid electrolyte interface film contains F and B elements.

[0054] This study found that including F and B elements in the CEI film on the surface of the positive electrode active material layer can improve the ion transport rate at the positive electrode interface, reduce the DCR and Rct of the secondary battery, increase the lithium ion diffusion coefficient, and reduce the maximum height and maximum width of the first color region, thereby improving the low-temperature capacity retention rate and room-temperature cycling performance.

[0055] In some embodiments, the mass percentage of element F is CF and the mass percentage of element B is CB, based on the total mass of the positive electrode solid electrolyte interface membrane, satisfying: 0.1%≤CF≤10% and 0.01%≤CB≤5%.

[0056] This study found that when the mass percentages of F and B elements in the CEI membrane are further controlled within the above-mentioned range, the low-temperature capacity retention and room-temperature cycling performance of the secondary battery are better.

[0057] For example, the mass percentage of the F element, based on the total mass of the positive electrode solid electrolyte interface membrane, can be any point value or a range between any two points between 0.1% and 10%, such as 0.1%, 0.5%, 1%, 2%, 4%, 6%, 8%, 10%, etc.

[0058] For example, based on the total mass of the positive electrode solid electrolyte interface membrane, the mass percentage of element B is any point value or range between any two points between 0.01% and 5%, such as 0.01%, 0.05%, 0.1%, 0.2%, 0.4%, 0.6%, 0.8%, 1%, 2%, 3%, 4%, 5%, etc.

[0059] In some embodiments, the negative electrode sheet includes a negative electrode active material layer and a negative electrode solid electrolyte interface (SEI) film located on the surface of the negative electrode active material layer, wherein the negative electrode solid electrolyte interface (SEI) film contains F and B elements.

[0060] This study found that the SEI film on the surface of the negative electrode active material layer contains F and B elements, which can improve the ion transport rate at the negative electrode interface, reduce the DCR and Rct of the secondary battery, increase the lithium ion diffusion coefficient, reduce the maximum height and maximum width of the first color region, and thus improve the low-temperature capacity retention rate and room-temperature cycling performance.

[0061] In some embodiments, the mass percentage of element F is AF and the mass percentage of element B is AB, based on the total mass of the negative electrode solid electrolyte interface membrane, satisfying: 1%≤AF≤30% and 0.01%≤AB≤10%.

[0062] This study found that when the mass percentages of F and B elements in the SEI film are further controlled within the above-mentioned range, the low-temperature capacity retention and room-temperature cycling performance of the secondary battery are better.

[0063] For example, the mass percentage of the F element, based on the total mass of the negative electrode solid electrolyte interface membrane, can be any point value or a range between any two points between 1% and 30%, such as 1%, 2%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, etc.

[0064] For example, the mass percentage of element B, based on the total mass of the negative electrode solid electrolyte interface membrane, is any point value or range between any two points between 0.01% and 10%, such as 0.01%, 0.05%, 0.1%, 0.4%, 0.6%, 0.8%, 1%, 2%, 4%, 6%, 8%, 10%, etc.

[0065] It should be noted that the test methods for the mass percentage content of element F and element B, based on the total mass of the positive electrode solid electrolyte interface film, and the test methods for the mass content of element F and element B, based on the total mass of the negative electrode solid electrolyte interface film, are as follows: The secondary battery was discharged at 1C to the lower voltage limit of 2.5V, and then disassembled in a drying room (humidity <1%) to obtain positive and negative electrode sheets. The positive and negative electrode sheets were cut into 2cm × 3cm pieces in the drying room (humidity <1%), immersed in ethylene carbonate (EC) solvent for 6 hours, and then dried in a 60℃ oven for 12 hours. XPS analysis was used to determine the F and B element content on the surface of the positive and negative active material layers.

[0066] In some embodiments, the secondary battery has a charging DC internal resistance of less than or equal to 1 mΩ and a charge transfer impedance of less than or equal to 0.8 mΩ at 50% SOC.

[0067] This application research found that the secondary battery has a DCR of less than 1mΩ and an Rct of less than 0.8mΩ when charged at 50% SOC, which is beneficial to improving the lithium-ion diffusion coefficient, reducing the maximum height and maximum width of the first color region, and thus improving the low-temperature capacity retention rate and room-temperature cycling performance.

[0068] For example, the charging DCR of the secondary battery at 50% SOC can be any value below 1 mΩ, such as 0.1 mΩ, 0.2 mΩ, 0.3 mΩ, 0.4 mΩ, 0.5 mΩ, 0.6 mΩ, 0.7 mΩ, 0.8 mΩ, 0.9 mΩ, 1 mΩ, etc.

[0069] For example, the Rct of the secondary battery at 50% SOC can be any value below 0.8 mΩ, such as 0.1 mΩ, 0.2 mΩ, 0.3 mΩ, 0.4 mΩ, 0.5 mΩ, 0.6 mΩ, 0.7 mΩ, 0.8 mΩ, etc.

[0070] It should be noted that the test method for the charging DCR and Rct of the secondary battery at 50% SOC is as follows: after discharging the secondary battery to 50% SOC with a 1 / 3C current, it is charged with a 2C constant current for 10s, and then left to stand for 40s for testing and calculation of charging DCR; the EIS spectrum is tested using an electrochemical workstation, and Rct is calculated by fitting.

[0071] In some embodiments, the lithium-ion diffusion coefficient of the secondary battery is D-Li, where D-Li ≥ 1 × 10⁻⁶. -18 cm 2 / s.

[0072] This application research found that the lithium-ion diffusion coefficient of the secondary battery is greater than or equal to 1×10⁻⁶. -18 cm 2 At a speed of / s, it is beneficial to improve internal resistance and interface ion transport, reduce the maximum height and maximum width of the first color region, and thus improve low-temperature capacity retention and room-temperature cycling performance.

[0073] For example, the lithium-ion diffusion coefficient of the secondary battery can be 1×10⁻⁶. -18 cm 2 Any point value above / s, for example, 1×10 -18 cm 2 / s, 1×10 -17 cm 2 / s, 1×10 -16 cm 2 / s, 1×10 -15 cm 2 / s, 1×10 -14 cm 2 / s, 1×10 -13 cm 2 / s etc.

[0074] It should be noted that the test method for the lithium-ion diffusion coefficient of the secondary battery is as follows: S1. Discharge the secondary battery to 2.5V at 1C; S2. Charge to 5% SOC using 0.33C, then let stand for 1 hour; S3. Charge to 10% SOC using 0.33C (i.e., add another 5% SOC), and let stand for 1 hour; S4. Repeat step S3 above, that is, increase SOC by 5% each time until it is charged to 100% SOC. S5. Calculate the lithium-ion diffusion coefficient using the GITT formula.

[0075] In some embodiments, the secondary battery further includes an electrolyte comprising additives, the additives including tris(pentafluorophenyl)borane and lithium fluoride.

[0076] This study found that by introducing tris(pentafluorophenyl)borane and lithium fluoride as additives into the electrolyte of secondary batteries, the combination of the two can effectively reduce the lithium fluoride deposition barrier, promote the formation of a dense, stable positive electrode interface CEI film and a negative electrode interface SEI film with high ionic conductivity, reduce the occurrence of side reactions, and improve the low-temperature capacity retention rate and room-temperature cycling performance of secondary batteries.

[0077] In some embodiments, the molar ratio of the tris(pentafluorophenyl)borane to the lithium fluoride is 0.5 to 2.

[0078] For example, the molar ratio of tris(pentafluorophenyl)borane to lithium fluoride can be any value between 0.5 and 2 or a range between any two values, such as 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, etc.

[0079] This study found that lithium fluoride (LiF) and tris(pentafluorophenyl)borane (TPFPB) electrolyte additives have the following effects: (1) Tris(pentafluorophenyl)borane can act as an anion acceptor to dissociate lithium fluoride, improve the utilization rate of lithium fluoride, and increase the content of lithium fluoride in CEI and SEI films while reducing the lithium fluoride deposition barrier, thereby improving the ionic conductivity of the positive and negative electrode interfaces; (2) Tris(pentafluorophenyl)borane participates in the formation of CEI and SEI films of positive and negative electrodes, forming a dense electrolyte film rich in B and F elements at the interface, composed of inorganic lithium salts such as LiF, LiBO2, and Li2B4O7, which improves the internal resistance of the secondary battery and reduces the maximum height and maximum width of the first color region, thereby helping to improve the low-temperature capacity retention rate and room-temperature cycling performance of the secondary battery.

[0080] It should be noted that the method for testing the molar ratio of tris(pentafluorophenyl)borane to lithium fluoride is as follows: the secondary battery is disassembled to obtain the electrolyte. Then, the concentration of lithium fluoride in the electrolyte is detected by ion chromatography. The amount of lithium fluoride is obtained by dividing the molar mass of lithium fluoride by (lithium fluoride concentration × electrolyte volume). The concentration of tris(pentafluorophenyl)borane in the electrolyte is detected by gas chromatography-mass spectrometry. The amount of tris(pentafluorophenyl)borane is obtained by dividing the molar mass of tris(pentafluorophenyl)borane by (pentafluorophenyl)borane concentration × electrolyte volume) / tris(pentafluorophenyl)borane. The molar ratio of tris(pentafluorophenyl)borane to lithium fluoride is obtained by dividing the amount of tris(pentafluorophenyl)borane by the amount of lithium fluoride.

[0081] In some embodiments, the sum of the masses of tris(pentafluorophenyl)borane and lithium fluoride is 1% to 8% based on the mass of the electrolyte.

[0082] For example, based on the mass of the electrolyte, the sum of the masses of tris(pentafluorophenyl)borane and lithium fluoride can be any point value or a range between any two points between 1% and 8%, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc.

[0083] This application research found that the combined mass ratio of tris(pentafluorophenyl)borane and lithium fluoride affects the thickness and ionic conductivity of the subsequently formed CEI and SEI films, thereby affecting the lithium-ion intercalation capability; when the combined mass ratio of tris(pentafluorophenyl)borane and lithium fluoride is further selected within the above range, the resulting secondary battery exhibits better low-temperature discharge capacity retention and room-temperature cycling performance.

[0084] It should be noted that the test method for the mass ratio of tris(pentafluorophenyl)borane and lithium fluoride is as follows: the secondary battery is disassembled to obtain the electrolyte, the mass ratio of lithium fluoride in the electrolyte is detected by ion chromatography, and the mass ratio of tris(pentafluorophenyl)borane is detected by gas chromatography-mass spectrometry. The sum of the two is the mass ratio of tris(pentafluorophenyl)borane and lithium fluoride.

[0085] In some embodiments, the electrolyte further includes an organic solvent, which includes at least one of dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, and methyl ethyl carbonate.

[0086] For example, the organic solvent may be ethylene carbonate: methyl ethyl carbonate: dimethyl carbonate = 1:1:1, ethylene carbonate: dimethyl carbonate = 1:1, ethylene carbonate: methyl ethyl carbonate = 1:1, propylene carbonate: dimethyl carbonate = 1:1, propylene carbonate: dimethyl carbonate = 1:1, etc.

[0087] This study found that the type of organic solvent affects the stability and ionic conductivity of the CEI and SEI films, influences the battery's internal resistance and the size of the first color region, and consequently affects the low-temperature and cycle performance of the secondary battery. When the solvent type is further selected within the above range, the overall performance of the secondary battery is better.

[0088] In some embodiments, the electrolyte further includes a lithium salt, wherein the molar concentration of the lithium salt is 0.8 mol / L to 2 mol / L.

[0089] For example, the molar concentration of the lithium salt can be any point value or a range between any two points between 0.8 mol / L and 2 mol / L, such as 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.4 mol / L, 1.6 mol / L, 1.8 mol / L, 2 mol / L, etc.

[0090] This study found that the molar concentration of lithium salt affects the stability and ionic conductivity of the SEI and CEI films, influences the battery's internal resistance and the size of the first color region, and consequently affects the low-temperature and cycle performance of the secondary battery. When the molar concentration of lithium salt is further selected within the above range, the overall performance of the secondary battery is better.

[0091] It should be noted that the method for testing the molar concentration of lithium salt is as follows: disassemble the secondary battery to obtain the electrolyte, use ion chromatography to detect the concentration of lithium salt in the electrolyte, and obtain the molar concentration of lithium salt based on the concentration / molar mass of lithium salt.

[0092] In some embodiments, see Figure 3The core further includes a winding end and an insulating tape, the insulating tape being bonded to at least one of the winding end, the corner area, and the straight area.

[0093] This application research found that, based on insulation considerations, the winding end of the core is usually a diaphragm. However, using insulating tape to bond the winding end and bonding at least one of the corner area and the straight area along the winding direction helps to reduce the corner gap caused by the expansion of the electrode during the charging and discharging of the secondary battery, improves the electrolyte wetting effect, and thus reduces the size of the first color area.

[0094] In some embodiments, the core further includes a winding end and an insulating tape, the insulating tape being bonded to at least two of the winding end, the corner area, and the straight area.

[0095] This application research found that, based on insulation considerations, the winding end of the core is usually a diaphragm. However, using insulating tape to bond the winding end and bonding at least two of the corner area and the straight area along the winding direction helps to further reduce the corner gap caused by the expansion of the electrode during the charging and discharging of the secondary battery, improve the electrolyte wetting effect, and thus reduce the size of the first color area.

[0096] In some embodiments, see Figure 3 The core also includes a winding end and an insulating tape, the insulating tape being bonded to the winding end, the corner area and the straight area.

[0097] This application research found that, based on insulation considerations, the winding end of the core is usually a diaphragm. However, using insulating tape to bond the winding end and bonding the corner area and the straight area along the winding direction helps to reduce the outermost corner gap caused by the expansion of the electrode during the charging and discharging of the secondary battery, improves the electrolyte wetting effect, and thus reduces the size of the first color area.

[0098] In some embodiments, the negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder.

[0099] This application does not impose any particular restrictions on the selection of the negative electrode active material, and conventional negative electrode active materials in the art can be used. For example, the negative electrode active material may be artificial graphite, natural graphite, amorphous carbon, carbon nanotubes, mesophase carbon microspheres, etc.

[0100] This application does not impose any particular restrictions on the selection of the negative electrode conductive agent; conventional negative electrode conductive agents in the art can be used. For example, the negative electrode conductive agent may be acetylene black, graphene, carbon nanotubes (CNTs), etc.

[0101] This application does not impose any particular restrictions on the selection of the negative electrode binder; conventional negative electrode binders in the art can be used. For example, the negative electrode binder may be polyvinylidene fluoride, styrene-butadiene rubber, etc.

[0102] In some embodiments, the positive electrode sheet further includes a positive current collector, and the positive active material layer is disposed on at least one surface of the positive current collector; the positive active material layer includes a positive active material, a positive binder, and a positive conductive agent.

[0103] In some embodiments, a carbon layer is disposed on the surface of the positive electrode current collector. This application research has found that further disposing of a carbon layer on the surface of the positive electrode current collector can effectively improve the conductivity of the positive electrode, thereby further enhancing the overall performance of the secondary battery.

[0104] This application does not impose any particular restrictions on the selection of the positive electrode active material; conventional positive electrode active materials in the art can be used. For example, the positive electrode active material may be lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium nickel cobalt manganese oxide, etc.

[0105] This application does not impose any particular restrictions on the selection of the positive electrode binder; conventional positive electrode binders in the art can be used. For example, the positive electrode binder may be polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), sodium polyacrylate (PAANa), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), etc.

[0106] This application does not impose any particular restrictions on the selection of the positive electrode conductive agent; conventional positive electrode conductive agents in the art can be used. For example, the positive electrode conductive agent may be acetylene black, Super P, carbon nanotubes, graphene, etc.

[0107] This application does not impose any particular restrictions on the selection of lithium salts, and conventional lithium salts in the art can be used. For example, the lithium salt may be lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorosulfonate imine, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, etc.

[0108] In some embodiments, the method for preparing the secondary battery includes the following steps: (1) Preparation of positive electrode sheet: The positive active material, positive conductive agent and positive binder are mixed and then added to N-methylpyrrolidone to obtain positive electrode slurry. The positive electrode slurry is coated on at least one surface of the positive current collector, and then dried, rolled, slit and cut to obtain positive electrode sheet. (2) Preparation of negative electrode sheet: The negative electrode active material, negative electrode conductive agent and negative electrode binder are mixed and added to water to obtain negative electrode slurry. The negative electrode slurry is coated on at least one surface of the negative electrode current collector, and then dried, rolled, slit and cut to obtain negative electrode sheet. (3) Preparation of electrolyte: Lithium fluoride was added to tris(pentafluorophenyl)borane under an inert atmosphere, and then heated and stirred to dissolve to obtain a composite additive; then lithium salt and composite additive were added to an organic solvent to obtain an electrolyte; (4) Preparation of secondary battery: The positive electrode, negative electrode, separator and other components are assembled and then subjected to processes such as winding, hot pressing, super welding, core assembly, casing, baking, liquid injection, high temperature immersion, formation, aging, sealing and capacity testing to obtain secondary battery.

[0109] It should be noted that by adjusting the thickness (D) of the core, the width (W) of the core, the width (A) of the negative electrode tab, the thickness (T) of the negative electrode tab, the molar ratio (P) of tris(pentafluorophenyl)borane and lithium fluoride, the amount of tris(pentafluorophenyl)borane and lithium fluoride added (Q), the type of organic solvent, and the concentration of electrolyte lithium salt, the DCR and Rct of the secondary battery can be controlled, thereby controlling the lithium-ion diffusion coefficient, and further controlling the maximum height and maximum width of the first color area of ​​the negative electrode sheet from the tab side to the center of the negative electrode sheet after the secondary battery is fully charged.

[0110] For example, increasing the thickness (D) and decreasing the width (W) of the core can increase the maximum height and maximum width of the first color area.

[0111] For example, increasing the width (A) and thickness (T) of the negative electrode tab to a certain extent can reduce the maximum height and maximum width of the first color region.

[0112] For example, increasing the molar ratio (P) of tris(pentafluorophenyl)borane and lithium fluoride and the amount (Q) of tris(pentafluorophenyl)borane and lithium fluoride added to a certain extent can reduce the maximum height and maximum width of the first color region.

[0113] For example, increasing the concentration of the electrolyte lithium salt to a certain extent can reduce the maximum height and maximum width of the first color region.

[0114] In some embodiments, the width of the positive electrode is smaller than the width of the negative electrode.

[0115] This application research found that having a width smaller than that of the negative electrode sheet can further improve the utilization rate of the positive electrode sheet and reduce the generation of purple spots.

[0116] In some embodiments, the width of the positive electrode is smaller than the width of the negative electrode, and the difference between the width of the positive electrode and the width of the negative electrode is less than or equal to 10 mm.

[0117] In some embodiments, the width of the positive electrode is smaller than the width of the negative electrode, and the difference between the width of the positive electrode and the width of the negative electrode is less than or equal to 8 mm.

[0118] In some embodiments, the width of the positive electrode is smaller than the width of the negative electrode, and the difference between the width of the positive electrode and the width of the negative electrode is less than or equal to 6 mm.

[0119] This application research found that controlling the difference between the width of the positive electrode and the width of the negative electrode within the above-mentioned range can further improve the utilization rate of the positive electrode and reduce the generation of purple spots.

[0120] In one embodiment of this application, an electrical device is provided, which includes the secondary battery described in this application.

[0121] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0122] Example 1 This application provides a secondary battery, the preparation method of which includes the following steps: (1) Preparation of positive electrode sheet The positive electrode active material (lithium iron phosphate), positive electrode conductive agent (Super P), and positive electrode binder (PVDF) were mixed in a weight ratio of 97:0.7:2.3. Then, N-methylpyrrolidone (NMP) was added and thoroughly mixed to obtain a positive electrode slurry with a solid content of 69%. The positive electrode slurry was then coated on both sides of a 15μm carbon-coated aluminum foil, with the width of the positive electrode sheet controlled to be 5mm less than the width of the negative electrode sheet, and the coating weight on one side of the positive electrode controlled to be 205g / m². 2 The electrode sheets are then dried, rolled, slit, and cut to obtain the positive electrode sheet; during the rolling process, the compaction density of the positive electrode sheet is controlled to be 2.65 g / cm³. 3 .

[0123] (2) Preparation of negative electrode sheet The negative electrode active material (graphite), negative electrode conductive agent (Super P), negative electrode binder (CMC), and negative electrode binder (SBR) were mixed in a mass ratio of 96.3:0.7:1.1:1.9, and then thoroughly mixed in water to obtain a negative electrode slurry with a solid content of 50%. The negative electrode slurry was coated on both surfaces of a 4.5μm copper foil, with the single-sided coating weight controlled at 100g / m². 2The electrode sheets are then dried, rolled, slit, and cut to obtain negative electrode sheets; during the rolling process, the compaction density of the negative electrode sheets is controlled to be 1.6 g / cm³. 3 .

[0124] (3) Preparation of electrolyte LiF was added to tris(pentafluorophenyl)borane in an inert gas atmosphere (nitrogen) with a molar ratio of 1:1. The mixture was then heated and stirred at 40°C for 5 hours until the LiF was completely dissolved, yielding a composite additive. Subsequently, ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed in a mass ratio of 1:1:1, followed by the addition of lithium salt (lithium hexafluorophosphate). After thorough mixing, the composite additive was added and stirred. In the electrolyte, the molar concentration of lithium salt was 1 mol / L, and the mass percentage of the composite additive was 4%. (4) Preparation of secondary batteries The positive electrode sheet, negative electrode sheet, and separator (polyethylene film) prepared above are wound to obtain a core, wherein the core thickness is 24.4mm and the core width is 150mm. The core undergoes hot pressing, super welding, core assembly, casing, baking, liquid injection, high temperature wetting, formation, aging, sealing, and capacity testing to obtain a secondary battery. The diagram shows the maximum height (H) and maximum width (L) of the first color area at the outermost corner of the secondary battery after full charging, from the tab side towards the center of the negative electrode. Figure 1 As shown.

[0125] Examples 2-4 Embodiments 2-4 of this application provide a secondary battery. The difference between the preparation method of the secondary battery and that of Embodiment 1 is that the thickness of the core is changed by adjusting the number of winding electrode layers and the circumference of the winding needle, so as to achieve the parameters in Table 1.

[0126] Examples 5-8 Embodiments 5-8 of this application provide a secondary battery. The difference between the preparation method of the secondary battery and that of Embodiment 1 is that the circumference of the winding needle is adjusted to change the width of the winding core, so as to achieve the parameters in Table 1.

[0127] Examples 9-12 Examples 9-12 of this application provide a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the molar ratio of tris(pentafluorophenyl)borane to lithium fluoride in the electrolyte additive is adjusted to change the mass percentage content of F and B elements in the CEI and SEI films, so as to achieve the parameters in Table 1.

[0128] Examples 13-16 Examples 13-16 of this application provide a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of tris(pentafluorophenyl)borane and lithium fluoride added is adjusted to change the ratio of their total mass in the electrolyte, so as to achieve the parameters in Table 1.

[0129] Examples 17-20 Examples 17-20 of this application provide a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of lithium salt added is adjusted to change the molar concentration of lithium salt, so as to achieve the parameters in Table 1.

[0130] Examples 21-24 Examples 21-24 of this application provide a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the type of organic solvent is adjusted to change the mass percentage content of F and B elements in the CEI and SEI films, so as to achieve the parameters in Table 1.

[0131] Examples 25-26 Embodiments 25-26 of this application provide a secondary battery. The difference between the preparation method of the secondary battery and that of Embodiment 1 is that the width (A) and thickness (T) of the negative electrode tab are adjusted to change the DCR and lithium-ion diffusion coefficient of the secondary battery, so as to achieve the parameters in Table 1.

[0132] Comparative Example 1 Comparative Example 1 of this application provides a secondary battery. The difference between the preparation method of the secondary battery and Example 1 is that the thickness (D) and width (W) of the core are adjusted to achieve the parameters in Table 1.

[0133] Comparative Example 2 Comparative Example 2 of this application provides a secondary battery. The difference between the preparation method of the secondary battery and Example 1 is that the molar ratio of tris(pentafluorophenyl)borane to lithium fluoride in the electrolyte additive is adjusted to achieve the parameters in Table 1.

[0134] Comparative Example 3 Comparative Example 3 of this application provides a secondary battery. The difference between the preparation method of the secondary battery and Example 1 is that the amount of tris(pentafluorophenyl)borane and lithium fluoride added is adjusted to change the ratio of their total mass in the electrolyte, so as to achieve the parameters in Table 1.

[0135] Comparative Example 4 Comparative Example 4 of this application provides a secondary battery. The difference between the preparation method of the secondary battery and Example 1 is that tris(pentafluorophenyl)borane and lithium fluoride electrolyte additives are not added to achieve the parameters in Table 1.

[0136] Comparative Example 5 Comparative Example 5 of this application provides a secondary battery. The difference between the preparation method of the secondary battery and Example 1 is that the amount of lithium salt added is adjusted to change the molar concentration of lithium salt, so as to achieve the parameters in Table 1.

[0137] Comparative Example 6 Comparative Example 6 of this application provides a secondary battery. The difference between the preparation method of the secondary battery and Example 1 is that the type of organic solvent is adjusted to achieve the parameters in Table 1.

[0138] The following parameters are given in Table 1: maximum height H mm, maximum width L mm, core thickness D mm, core width W mm, Y, negative electrode tab width A, negative electrode tab thickness T, positive electrode CEI film F element mass percentage CF%, positive electrode CEI film B element mass percentage CB%, negative electrode SEI film F element mass percentage AF%, negative electrode SEI film B element mass percentage AB%, molar ratio P of tris(pentafluorophenyl)borane and lithium fluoride, mass percentage of electrolyte of tris(pentafluorophenyl)borane and lithium fluoride Q%, molar concentration of lithium salt M mol / L, and types and ratios of organic solvents after the secondary battery is fully charged in the examples and comparative examples. Table 1. Parameters of Secondary Batteries The performance of the secondary batteries prepared in the examples and comparative examples was tested, including the following aspects: 1. Low-Temperature Capacity Retention Rate: At 25℃, the battery is charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage of 3.65V until the current is less than 0.05C. After standing for 5 minutes, it is discharged at a current of 1 / 3C to 2.5V, yielding the battery capacity C0. At 25℃, after charging at a constant current of 1 / 3C to 3.65V, the temperature is adjusted to -20℃, and the battery is left to stand for 24 hours. Then, it is discharged at a current of 1 / 3C to 2.5V, yielding the battery capacity C1. Battery capacity retention rate at -20℃ = C1 / C0 × 100%; where the voltage range of lithium iron phosphate batteries is 2.5V~3.65V.

[0139] 2. Room temperature cycling performance: At 25℃, it is charged at a constant current of 2C to 3.65V, then charged at a constant voltage of 3.65V until the current is less than or equal to 0.05C. After resting for 5 minutes, it is discharged at a current of 1C to 2.5V. This charge-discharge cycle is repeated 1500 times. The capacity retention rate after the 1500th cycle = (capacity after the 1500th cycle / capacity after the first cycle) × 100%; the voltage range of the lithium iron phosphate battery is 2.5V~3.65V.

[0140] The results are shown in Table 2; Table 2 Performance Data of Secondary Batteries As can be seen from the above embodiments and comparative examples, when the technical solution provided in this application is adopted, the resulting secondary battery has excellent low-temperature performance and room-temperature cycling performance; specifically, the low-temperature capacity retention rate is above 75%, and the capacity retention rate after 1500 cycles at room temperature is above 85%.

[0141] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a winding core, the winding core including a positive electrode and a negative electrode, the negative electrode including a negative electrode tab, a separator between the positive electrode and the negative electrode, the winding core being formed by winding the positive electrode, the separator, and the negative electrode, the winding core including a straight section and a corner section connected together, characterized in that, After the secondary battery is fully charged, in the outermost corner area of ​​the core, there is a first color area and a second color area in the negative electrode sheet from the negative electrode tab side to the center of the negative electrode sheet. The first color area and the second color area are different. The maximum height of the first color area is less than or equal to 21mm and the maximum width of the first color area is less than or equal to 24mm.

2. The secondary battery according to claim 1, characterized by The secondary battery satisfies 0.01≤Y≤0.105; Where Y = (H × L) / (D × W); H mm is the maximum height of the first color area; L mm is the maximum width of the first color area; D mm is the thickness of the core; W mm is the width of the core.

3. The secondary battery according to claim 2, characterized by The value of D is 15~32; And / or, the W is 90~180.

4. The secondary battery according to claim 1, characterized by The width of the negative electrode tab is A mm, and the thickness of the negative electrode tab is T μm, satisfying: 3≤T≤8, 20≤A≤60.

5. The secondary battery according to claim 1, characterized by The positive electrode includes a positive active material layer and a positive solid electrolyte interface film located on the surface of the positive active material layer. The positive solid electrolyte interface film contains F and B elements.

6. The secondary battery according to claim 5, characterized in that, Based on the total mass of the positive electrode solid electrolyte interface membrane, the mass percentage of element F is CF, and the mass percentage of element B is CB, satisfying: 0.1%≤CF≤10%, 0.01%≤CB≤5%.

7. The secondary battery according to claim 1, characterized in that, The negative electrode sheet includes a negative electrode active material layer and a negative electrode solid electrolyte interface film located on the surface of the negative electrode active material layer. The negative electrode solid electrolyte interface film contains F and B elements.

8. The secondary battery according to claim 7, characterized in that, Based on the total mass of the negative electrode solid electrolyte interface membrane, the mass percentage of element F is AF, and the mass percentage of element B is AB, satisfying: 1%≤AF≤30%, 0.01%≤AB≤10%.

9. The secondary battery according to claim 1, characterized in that, The secondary battery has a charging DC internal resistance of less than or equal to 1mΩ and a charge transfer impedance of less than or equal to 0.8mΩ at 50% SOC.

10. The secondary battery according to claim 1, characterized in that, The lithium ion diffusion coefficient of the secondary battery is D-Li, D-Li≥1×10 -18 cm 2 / s.

11. The secondary battery according to claim 1, characterized in that, The secondary battery also includes an electrolyte, which includes additives, including tris(pentafluorophenyl)borane and lithium fluoride.

12. The secondary battery according to claim 11, characterized in that, The molar ratio of the tris(pentafluorophenyl)borane to the lithium fluoride is 0.5~2; And / or, based on the mass of the electrolyte, the sum of the masses of the tris(pentafluorophenyl)borane and the lithium fluoride is 1% to 8%.

13. The secondary battery according to claim 11 or 12, characterized in that, The electrolyte also includes an organic solvent, which includes at least one of dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, and methyl ethyl carbonate. And / or, the electrolyte further includes a lithium salt, wherein the molar concentration of the lithium salt is 0.8 mol / L to 2 mol / L.

14. The secondary battery according to claim 1, characterized in that, The core also includes a winding end and an insulating tape, the insulating tape being bonded to at least one of the winding end, the corner area, and the straight area.

15. An electrical appliance, characterized in that, The electrical device includes a secondary battery as described in any one of claims 1 to 14.

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