Negative electrode sheet and secondary battery

Abstract

The application provides a negative pole piece, which comprises a negative pole current collector and a negative pole active material layer arranged on at least one side surface of the negative pole current collector, and the negative pole active material layer contains a negative pole active material, and the following parameter k is defined: and k is in the range of 10.00-20.00; wherein Dv10, Dv50 and Dv90 respectively represent particle size values corresponding to 10%, 50% and 90% of the cumulative volume distribution percentage of the negative pole active material, and the unit is μm; and x represents the average length-diameter ratio of the particles of the negative pole active material. The tortuosity of the above negative pole piece is suitable, and the cycle performance, safety performance and energy density of the battery made of the negative pole piece are good. The application further provides a secondary battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a negative electrode sheet and a secondary battery. BACKGROUND

[0002] The fast charging performance and the service life of the battery are related to the migration path of the active ions in the negative electrode active material layer, and the tortuosity of the negative electrode active material layer can directly reflect the length of the migration path. If the tortuosity is too high, the active ions may not be able to be embedded in the negative electrode or be directly reduced and precipitated on the surface of the negative electrode, affecting the safety performance and the cycle life of the battery, and if the tortuosity is too low, the compaction density of the electrode sheet is small, the internal resistance of the battery is large, and the energy performance is affected. Therefore, it is necessary to control the tortuosity of the negative electrode sheet to improve the fast charging performance and the service life of the battery. SUMMARY

[0003] In view of this, the present application provides a negative electrode sheet and a secondary battery to solve the problem of poor fast charging performance and short cycle life caused by too high tortuosity of the negative electrode sheet.

[0004] In a first aspect, the present application provides a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode active material layer arranged on at least one side surface of the negative electrode current collector, and defines a parameter k as follows:

[0005] and the k is in the range of 10.00-20.00;

[0006] wherein Dv10, Dv50 and Dv90 represent the particle size values corresponding to the cumulative volume distribution percentages of 10%, 50% and 90% of the negative electrode active material contained in the negative electrode active material layer, respectively, and the unit is μm; x represents the average aspect ratio of the particles of the negative electrode active material.

[0007] In the negative electrode sheet provided by the first aspect of the present application, by controlling the negative electrode active material layer of the negative electrode sheet to satisfy the defined parameter k in the range of 10.00-20.00, the tortuosity of the negative electrode sheet can be appropriate, the active ions can be smoothly embedded / extracted in the negative electrode active material layer, the migration path is relatively short, and the negative electrode sheet surface is less likely to have lithium precipitation phenomenon, so that the fast charging performance, the cycle performance and the safety performance of the battery containing the negative electrode sheet are better, and at the same time, the tortuosity of the negative electrode sheet will not be too low to cause the compaction density of the electrode sheet to be too low, the internal resistance of the battery to be large, and the energy density to be low, etc.

[0008] In a second aspect, the present application provides a secondary battery, which comprises the negative electrode sheet as described above. The secondary battery can maintain high capacity and has good cycle stability when cycled at a high rate. BRIEF DESCRIPTION OF DRAWINGS

[0009] Figure 1A This is a schematic diagram of a negative electrode sheet provided in an embodiment of this application.

[0010] Figure 1B This is a schematic diagram of another structure of the negative electrode sheet provided in the embodiments of this application. Detailed Implementation

[0011] The embodiments of this application will now be described in conjunction with the accompanying drawings.

[0012] Please refer to the following: Figure 1A and Figure 1B This application provides a negative electrode sheet 10, which includes a current collector 11 and a negative electrode active material layer 12 sequentially disposed on at least one side surface of the negative electrode current collector 11. The negative electrode active material layer 12 contains a negative electrode active material, and the following parameter k is defined:

[0013] And the value of k is in the range of 10.00-20.00;

[0014] Wherein, Dv10, Dv50, and Dv90 represent the particle size values ​​corresponding to the cumulative volume distribution percentage of the negative electrode active material contained in the negative electrode active material layer 12 reaching 10%, 50%, and 90%, respectively, with the unit being μm; x represents the average aspect ratio of the negative electrode active material particles.

[0015] By controlling the negative electrode active material layer 12 of the negative electrode 10 to satisfy the defined parameter k within the range of 10.00-20.00, the tortuosity of the negative electrode 10 can be made appropriate, and active ions can be smoothly inserted / extracted in the negative electrode active material layer 12 with a shorter migration path. Lithium plating is less likely to occur on the surface of the negative electrode 10, resulting in better fast charging performance, cycle performance, and safety performance of the battery containing this negative electrode. At the same time, it avoids the phenomenon that the internal resistance of the battery is high and the energy density is low due to insufficient contact between the active material particles in the electrode due to the tortuosity of the negative electrode 10 being too low.

[0016] The aforementioned Dv10, Dv50, and Dv90 values ​​can be obtained by testing the particle size distribution of the negative electrode active material. Specific testing methods can be found in GB / T 19077-2016 / ISO 13320:2009, "Particle Size Distribution by Laser Diffraction," and the testing instrument can be a laser particle size analyzer (such as a Malvern 3000). The values ​​of Dv10, Dv50, and Dv90 can be obtained from the obtained laser particle size distribution diagram of the negative electrode active material.

[0017] The above x can be obtained by averaging the ratio of the longest side to the shortest side of multiple particles on a scanning electron microscope (SEM) photo of the negative electrode active material. x is dimensionless. Specifically, for the SEM photo of the negative electrode active material (containing a certain number (generally greater than 1000) of particles of the negative electrode active material), the longest side and the shortest side of each particle are measured using Image J software, the ratio of the longest side to the shortest side of each particle is calculated, and the ratio of multiple particles in the picture is averaged to obtain x. In some embodiments, the average aspect ratio x of the particles of the negative electrode active material is in the range of 1-6.

[0018] It should be noted that in this application, one side surface of the negative current collector can have a negative electrode active material layer 12 (as shown in Figure 1A The opposite side surface of the negative current collector can also have a negative electrode active material layer 12 (as shown in Figure 1B When the opposite side surface of the negative current collector has a negative electrode active material layer, as long as the negative electrode active material layer on any one surface satisfies the above parameter k in the range of 10.00-20.00, of course, both negative electrode active material layers can also be in the range of 10.00-20.00.

[0019] In addition, the negative electrode active material contained in the negative electrode active material layer 12 can be one material or multiple materials. Specifically, the negative electrode active material includes one or more of lithium titanate, carbon material, silicon-based material, and tin-based material, but is not limited thereto. Among them, when the negative electrode active material includes multiple materials (for example, both graphite and elemental silicon), the above Dv10, Dv50, and Dv90 refer to the related particle size values of the mixed negative electrode active material, and x is also calculated according to the SEM photo of the mixed negative electrode active material.

[0020] Among them, the carbon material can be one or more of graphite, carbon fiber, soft carbon, hard carbon, mesocarbon microbeads, and graphene, but is not limited thereto, and graphite is preferred. The silicon-based material can include elemental silicon, silicon alloy, silicon oxide, silicon-carbon composite material, etc. The tin-based material can include elemental tin, tin oxide, tin-based alloy, etc. In some embodiments of the application, the negative electrode active material includes multiple materials of different materials, and contains graphite, and the mass fraction of the graphite in the negative electrode active material is 40-95%. This helps to improve the rate performance of the overall negative electrode active material.

[0021] In embodiments of the present application, the Dv50 can be in the range of 7-18 pm. In some embodiments, the Dv50 can be in the range of 8-18 pm. Preferably, the Dv50 is in the range of 9-16 pm. The Dv50 of the particles can reflect the distribution of the overall particles of the negative active material, which can affect the speed of lithium ion extraction / insertion during the cycle, the cycle stability, etc., and a suitable Dv50 can make the battery made by the negative electrode sheet have more excellent performance.

[0022] In embodiments of the present application, the Dv10 can be in the range of 2-15 pm. In some embodiments, the Dv10 can be in the range of 2-10 pm. Preferably, the Dv10 is in the range of 3-9 pm.

[0023] In embodiments of the present application, the Dv90 is in the range of 12-30 pm. In some embodiments, the Dv90 can be in the range of 18-30 pm. Preferably, the Dv90 is in the range of 18-28 pm.

[0024] Controlling the Dv10 and Dv90 in the above range can help to ensure that the negative active material has an appropriate number of large and small particles, which is conducive to the formation of a tight packing between the particles of the negative active material and has a suitable porosity. The Dv50 / (Dv90-Dv10) described above can reflect the particle size distribution concentration of the particles of the negative active material. The larger the value, the higher the particle size distribution concentration of the particles of the negative active material.

[0025] In the present application, the negative active material layer can include a binder in addition to the negative active material. In some cases, it can also include a conductive agent. Optionally, the mass of the negative active material accounts for 80%-98% of the mass of the negative active material layer, preferably 90-98%. This can make the negative electrode sheet 10 have a higher loading of negative active material to improve the energy density of the battery. The binder and the conductive agent are conventional choices in the battery field. Exemplarily, the binder can include one or more of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate (such as polymethyl methacrylate, polymethyl acrylate, polyethyl acrylate, etc.), polyolefin (such as polypropylene, polyethylene, etc.), carboxymethyl cellulose (CMC), sodium alginate, etc. Exemplarily, the conductive agent includes one or more of carbon nanotubes, graphene, carbon fibers, carbon black (such as acetylene black, ketjen black), etc. In addition, the negative current collector carrying the negative active material layer can include but is not limited to copper foil, stainless steel foil, copper alloy foil, carbon-coated copper foil, or copper-plated film, etc.

[0026] The embodiment of the present application also provides a secondary battery, which comprises the negative electrode tab.

[0027] The secondary battery further comprises a positive electrode tab, and a separator and an electrolyte arranged between the positive electrode tab and the negative electrode tab.

[0028] The secondary battery contains the negative electrode tab with the appropriate tortuosity, so that the secondary battery has good cycle performance, high safety performance and high energy density. Specifically, the secondary battery can be a lithium secondary battery, a sodium secondary battery, a potassium secondary battery, a magnesium secondary battery, an aluminum secondary battery, a zinc secondary battery, etc.

[0029] The positive electrode tab comprises a positive electrode current collector and a positive electrode active material layer arranged on the positive electrode current collector, and the positive electrode active material layer comprises a positive electrode active material, a binder and an optional conductive agent. The positive electrode active material can be selected according to the active ions on which the energy storage of the specific secondary battery depends. The active ions can include lithium ions, sodium ions, potassium ions, magnesium ions, aluminum ions, zinc ions, etc. For a lithium ion battery, the positive electrode active material can include but is not limited to one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium-containing phosphates with an olivine structure. For a sodium ion battery, the positive electrode active material includes but is not limited to one or more of transition metal oxides, polyanion compounds, organic polymers, and Prussian blue materials.

[0030] The separator can be various separators suitable for secondary batteries in the art, and specifically can be a polymer separator, a non-woven fabric, etc., wherein the polymer separator includes but is not limited to a single-layer PP (polypropylene) separator, a single-layer PE (polyethylene) separator, a double-layer PP / PE separator, a double-layer PP / PP separator, and a three-layer PP / PE / PP separator, etc. The electrolyte includes an electrolyte salt and an organic solvent, wherein the specific types and compositions of the electrolyte salt and the organic solvent are conventional choices in the battery field, and can be selected according to actual needs.

[0031] The technical solutions of the present application will be further described below in combination with multiple specific embodiments.

[0032] Embodiment 1

[0033] A preparation method of a negative electrode tab comprises:

[0034] The negative active material (specifically graphite, particle size and size parameters are shown in Table 1) is mixed with a conductive agent (specifically Ketjen black, a binder (CMC and PVDF, mass ratio is 1:2) according to a mass ratio of 96:1:3. The mixed powder is placed in a vacuum stirrer, deionized water is added and stirred to obtain a negative electrode slurry; and uniformly coated on the coated foil. The negative electrode slurry is coated on the opposite two sides of the negative electrode current collector-copper foil, and after drying, rolling, cutting, a negative electrode sheet is obtained, which includes a copper foil and a negative electrode active material layer arranged on the two side surfaces of the copper foil. The calculation results of the aforementioned definition parameter k are also summarized in Table 1.

[0035] A method for preparing a lithium secondary battery, comprising:

[0036] 1) Preparation of a positive electrode sheet:

[0037] The positive active material-lithium iron phosphate (LiFePO4), a conductive agent (specifically Ketjen black), and a binder (specifically PVDF) are mixed according to a mass ratio of 96:1.5:2.5. The mixed powder is placed in a vacuum stirrer, a solvent-N-methyl pyrrolidone (NMP) is added, and stirring is performed to obtain a positive electrode slurry. The positive electrode slurry is coated on a positive electrode current collector aluminum foil, and after drying, rolling, and cutting, a positive electrode sheet is obtained.

[0038] 2) Assembly of the battery:

[0039] The above negative electrode sheet is used as the negative electrode, the above positive electrode sheet is used as the positive electrode, the separator is a polypropylene separator, and the electrolyte is a 1:1:1 mixed solution containing 1 mol / L LiPF6, ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC). The assembly is performed in an argon-filled glove box to obtain a laminated lithium battery with a rated capacity of 1.5 Ah.

[0040] The following electrochemical properties of the lithium secondary battery in Example 1 are tested, and the results are summarized in Table 2 below.

[0041] Among them, the test method of charge efficiency and negative electrode lithium precipitation is: 1) at 25°C, the battery is charged to full charge state at 1 / 3C rate, then discharged at 1 / 3C rate, and the above charge and discharge process is repeated 3 times, and the third charge capacity is recorded as C0; 2) the battery is charged to full charge state at 4C rate, then discharged at 1C rate, and the above charge and discharge process is repeated 15 times, and the 15th charge capacity is recorded as C 15 , and the charge efficiency is calculated as C 15 / C0. 3) Then the battery is charged to full charge state at 4C rate, and the battery is disassembled to check the negative electrode lithium precipitation.

[0042] The test method of cycle performance is as follows: at 25℃, the battery is charged to full state at 1C rate, then discharged at 1C rate, and the above charging and discharging process is repeated, and the capacity retention rate after 1000 cycles is recorded.

[0043] The test method of impedance is as follows: 1) the battery is charged to full state at 1 / 3C rate, then discharged at 1 / 3C rate, and the charging and discharging process is repeated for 3 times, and the third charging capacity is recorded as C0; and the battery is charged at 1 / 3C rate to adjust the state of charge (SOC) to 50% SOC; 2) at-10℃, the battery is stored for 12h, and the direct current resistance (DCIR) is tested at 1.5C rate for 30s.

[0044] Other embodiments

[0045] The negative electrode sheets and batteries of the remaining embodiments are prepared according to the parameters listed in Table 1, and the related performances are tested, and the test results are also summarized in Table 2 below.

[0046] In addition, in order to highlight the beneficial effects of the technical solutions of the present application, the following Comparative Examples 1-6 shown in Table 1 are also provided, and the related results are summarized in Table 2. It should be noted that when testing the cycle performance of the battery, if the cycle life of the battery cannot reach 1000 cycles, the test is stopped when the discharge capacity of the battery decays to 80% of the initial discharge capacity (i.e. the cycle life is reached), and the capacity retention rate at the cycle life is recorded.

[0047] Table 1: Composition parameters of negative electrode sheets of each example

[0048]

[0049] Table 2: Electrochemical performance of batteries prepared from negative electrode sheets of each example

[0050]

[0051] In combination with Table 1, it can be known from Table 2 that when the composition of the negative electrode sheet is such that the self-defined parameter k is in the range of 10.00-20.00, the safety performance of the battery containing the negative electrode sheet is better, the negative electrode is less likely to lithiumize, the fast charging performance of the battery is good, the charging efficiency is higher, and the cycle performance is better, and in addition, the impedance of the battery is also lower.

[0052] The above-described embodiments only express several exemplary embodiments of the present application, and the description is more specific and detailed, but it should not be understood as limiting the scope of the patent of the present application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are all within the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material, and the following parameter k is defined: And the value of k is in the range of 10.00-20.00; Wherein, Dv10, Dv50, and Dv90 represent the particle size values ​​corresponding to the cumulative volume distribution percentage of the negative electrode active material reaching 10%, 50%, and 90%, respectively, in μm; x represents the average aspect ratio of the particles of the negative electrode active material.

2. The negative electrode sheet as described in claim 1, characterized in that, The value of x is in the range of 1-6.

3. The negative electrode sheet as described in claim 1, characterized in that, The Dv50 is in the range of 7-18 μm.

4. The negative electrode sheet as described in claim 1, characterized in that, The Dv10 is in the range of 2-15 μm.

5. The negative electrode sheet as described in claim 1, characterized in that, The Dv90 is in the range of 12-30 μm.

6. The negative electrode sheet as described in any one of claims 1-5, characterized in that, The negative electrode active material includes one or more of lithium titanate, carbon materials, silicon-based materials, and tin-based materials.

7. The negative electrode sheet as described in claim 6, characterized in that, The negative electrode active material includes carbon materials, and the carbon materials include graphite, wherein the mass percentage of graphite in the negative electrode active material is 40-95%.

8. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in any one of claims 1-7.

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