Battery jelly roll and cylindrical battery

Abstract

A first aspect of the present application provides a battery jelly roll, which is formed by winding a positive electrode sheet, a negative electrode sheet, and a separator between the positive electrode sheet and the negative electrode sheet. At least one surface of the positive electrode sheet is provided with a positive electrode coating, and at least one surface of the negative electrode sheet is provided with a negative electrode coating. The negative electrode coating comprises a negative electrode active material; the negative electrode active material at least comprises a silicon material and a carbon material, and the weight percentage of the silicon material in the negative electrode active material is Y%; and the tortuosity of the separator is X, X being greater than or equal to 0.15Y0.5+1.8. A second aspect of the present invention provides a cylindrical battery, comprising the battery jelly roll, a cap, and a steel shell. The interior of the steel shell accommodates the battery jelly roll, and the top of the steel shell is sealed with the cap. In the present application, the tortuosity of the separator is designed on the basis of the proportion of the silicon-based material in the negative electrode active material, which can greatly reduce the penetration of iron and nickel elements into the central area, thereby significantly improving the cycle performance and safety performance of the battery cell.

Description

Battery roll core and cylindrical battery TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a battery roll core and a cylindrical battery. BACKGROUND

[0002] The cylindrical lithium battery includes a cylindrical steel shell, and the inside of the steel shell is used to accommodate a battery roll core. The battery roll core is formed by winding a positive electrode sheet, a negative electrode sheet and a separator. The positive electrode sheet includes a positive electrode coating area and a positive electrode uncoated area, and the positive electrode uncoated area is a positive electrode tab. The negative electrode sheet includes a negative electrode coating area and a negative electrode uncoated area, and the negative electrode uncoated area is a negative electrode tab. The negative electrode coating contains a negative electrode active material, mainly graphite. In order to improve the energy density, silicon material can also be introduced into the negative electrode active material to obtain a silicon-based negative electrode.

[0003] The application of the silicon-based negative electrode greatly improves the energy density of the cylindrical battery, but at the same time, its huge volume expansion will cause each component of the battery core to bear strain. The common shell of the cylindrical battery is a steel shell with a nickel plating layer for protection, which can avoid iron dissolution during application. However, after the proportion of silicon increases, the plating layer will fall off under the action of strain, thereby accelerating the dissolution of iron. The dissolution of iron is accompanied by the deterioration of the electrolyte, increasing the side reactions. Further, the deteriorated electrolyte will continue to react with the positive electrode material, thereby causing the degradation of the positive electrode interface and the dissolution of nickel. These dissolved iron and nickel will diffuse through the separator to the negative electrode, thereby rapidly deteriorating the performance of the battery core, and even forming a local micro-short circuit, thereby causing safety problems. SUMMARY

[0004] To solve the above problems, the first aspect of the present application provides a battery roll core formed by winding a positive electrode sheet, a negative electrode sheet and a separator therebetween, at least one surface of the positive electrode sheet has a positive electrode coating, at least one surface of the negative electrode sheet has a negative electrode coating, the negative electrode coating includes a negative electrode active material; the negative electrode active material at least includes a silicon material and a carbon material, the weight percentage of the silicon material in the negative electrode active material is Y%, and the tortuosity of the separator is X, X satisfies X≥0.15Y 0.5 +1.8.

[0005] In some embodiments, the porosity of the separator is Z, and X satisfies X≤3-lnZ.

[0006] In some embodiments, after the battery roll core completes at least 100 cycles, at least 3 segments are taken on the separator in the direction from the beginning of winding to the end of winding, the length of each segment in the width direction of the separator is not less than 3 cm, and the interval between each two segments in the length direction of the separator is not less than 200 cm, each segment is divided into three regions, namely region A, region B and region C, by equally dividing the center line in the width direction of the separator to the end of the positive tab, the average value of the Fe element content of the three regions of all segments is denoted as Fe A , Fe B , Fe C , the average value of the Ni element content of the three regions of all segments is denoted as Ni A , Ni B , Ni C , and the following relationship is satisfied: Ni A / Fe A >Ni B / Fe B >Ni C / Fe C .

[0007] In some embodiments, 2≤Y≤30.

[0008] In some embodiments, 33%≤Z≤48%.

[0009] In some embodiments, the separator comprises a base film, and the thickness of the base film is 5-20 μm.

[0010] In some embodiments, at least one surface of the base film is coated with a coating layer, and the thickness of the coating layer is 1-4 μm.

[0011] In some embodiments, the coating layer is selected from a mixture of one or more of boehmite, aluminum trioxide, barium oxide, polyvinylidene fluoride, and polyaryne.

[0012] In some embodiments, the silicon material is a silicon-carbon material and / or a silicon-oxygen material; and the carbon material is selected from a mixture of one or more of artificial graphite, natural graphite, hard carbon, and soft carbon.

[0013] In some embodiments, the diameter of the battery roll core is 20-60 mm, the height is 60-200 mm, and the ratio of the height to the diameter is greater than 1.7.

[0014] The second aspect of the present application provides a cylindrical battery comprising the battery roll core, the cap, and the steel shell described above; the inside of the steel shell contains the battery roll core, and the top is packaged with the cap.

[0015] The application designs the tortuosity of the separator based on the proportion of silicon-based material in the negative active material, which can greatly reduce the penetration of iron and nickel elements to the center area, thereby greatly improving the cycle and safety performance of the battery cell. When further using a separator with reasonable porosity and thickness, as well as reasonable coating material and thickness, the separator can obtain more reliable performance. In addition, the application can be effectively applied to cylindrical battery cells of different sizes. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings needed to be used in the description of the present application.

[0017] Fig. 1 is a schematic diagram of the structure of a cylindrical lithium battery in the present application;

[0018] Fig. 2 is a schematic diagram of the structure of a battery roll core in the present application;

[0019] Fig. 3 is a schematic diagram of the expansion of a battery roll core in the present application;

[0020] Fig. 4 is a schematic diagram of the structure of a positive electrode sheet in the present application;

[0021] Fig. 5 is a schematic diagram of the structure of a negative electrode sheet in the present application;

[0022] Fig. 6 is a schematic diagram of the determination of the proportion of the separator element in the present application.

[0023] Reference signs: 1-positive electrode end, 11-positive electrode column, 12-negative electrode end, 2-steel shell, 3-negative electrode sheet, 4-separator, 5-positive electrode sheet, 6-negative electrode coating area, 7-negative electrode uncoated area, 8-positive electrode uncoated area, 9-positive electrode coating area, 10-battery roll core. DETAILED DESCRIPTION

[0024] The embodiments of the present application will be described in detail below, which are only used to explain the present application and cannot be understood as a limitation of the present application.

[0025] In the description of the present application, several meanings are one or more, and the meaning of multiple is two or more. Greater than, less than, more than, etc. are understood as not including the number, and above, below, etc. are understood as including the number. If it is described as first, second, it is only used for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features or the sequence of indicated technical features.

[0026] Figure 1 shows a schematic diagram of a cylindrical lithium battery according to the present application, which comprises a cylindrical steel shell 2, inside which a battery core is accommodated, a positive electrode end 1 at the top and a negative electrode end 12 at the bottom, and a positive electrode post 11 arranged on the positive electrode end 1. Figure 2 shows a schematic diagram of a battery core according to the present application, and Figure 3 shows a schematic diagram of an unrolled battery core according to the present application. The positive electrode sheet 5, the negative electrode sheet 3 and the separator 4 are stacked as shown in Figure 3, and then wound to form the cylindrical battery core 10 shown in Figure 2. The initial winding is the end of the electrode sheet at the cylindrical axis, and the final winding is the end of the electrode sheet at the outer surface of the cylinder. Figure 4 and Figure 5 respectively show schematic diagrams of a positive electrode sheet and a negative electrode sheet according to the present application. The negative electrode sheet comprises a negative electrode coating region 6 and a negative electrode uncoating region 7, and the negative electrode uncoating region 7 is a negative electrode tab. The positive electrode sheet comprises a positive electrode coating region 9 and a positive electrode uncoating region 8, and the positive electrode uncoating region 8 is a positive electrode tab.

[0027] In order to avoid the adverse effects of silicon expansion on the battery, the first aspect of the present application provides a battery core, which is wound by a positive electrode sheet, a negative electrode sheet and a separator therebetween, at least one surface of the positive electrode sheet is provided with a positive electrode coating, and at least one surface of the negative electrode sheet is provided with a negative electrode coating. It can be understood that the present application is based on the problem of silicon expansion of the silicon-containing negative electrode material, and the improvement is mainly in the negative electrode and the separator. Therefore, the design of the positive electrode sheet can be any design known to those skilled in the art, for example, the positive electrode coating comprises a positive electrode active material, a conductive agent, a binder and the like, and the selection of each raw material can also be any selection known to those skilled in the art.

[0028] The negative electrode coating generally comprises a negative electrode active material, a conductive agent, a thickening agent and a binder, and the present application improves the negative electrode active material therein. In some embodiments, the negative electrode active material at least comprises a silicon material and a carbon material, and the silicon material accounts for 2-30% of the negative electrode active material in terms of weight percentage; preferably, the silicon material accounts for 5-20% of the negative electrode active material. The silicon material and the carbon material are uniformly mixed to obtain the negative electrode active material. The addition amount of the silicon particles in the negative electrode active material is a key factor to determine the expansion and contraction degree of the negative electrode material, and controlling the amount can limit the grain expansion caused by the lithium ion insertion into the silicon material as a whole, and can as much as possible improve the battery capacity. When the proportion of silicon is too high, the silicon expansion phenomenon is difficult to be effectively controlled, and the cycle performance and safety performance of the battery are reduced, and vice versa, when the proportion of silicon is too low, the energy storage capacity of the battery is reduced, and the use demand cannot be met.

[0029] In some embodiments, the silicon material is selected from silicon-carbon material and / or silicon-oxygen material; the carbon material is selected from a mixture of one or more of artificial graphite, natural graphite, hard carbon, and soft carbon. Silicon has a higher theoretical specific capacity and specific energy density, and can achieve a higher capacity of lithium ion insertion and extraction, but the insertion can cause volume expansion and the extraction can cause volume contraction. In the present application, the silicon-carbon material can be a carbon-coated silicon material, and the coated silicon material can be crystalline silicon or amorphous silicon. The silicon oxide can be represented by the chemical formula SiOx, where 0 < x ≤ 2, and can be obtained commercially. The silicon oxide can also be further coated with carbon. Generally, the carbon coating process can improve the electrical conductivity and stability of the silicon material. The method of carbon coating can be selected from any method known to those skilled in the art, such as chemical vapor deposition, physical vapor deposition, solution method, etc. In the selection of carbon material, the principle is to meet the requirements of electrical conductivity, stability, etc. From the perspective of balanced performance, artificial graphite is preferred. x

[0030] In some embodiments, the separator includes a base film, and the thickness of the base film is 5-20 μm; preferably, the thickness of the base film is 7-15 μm. Within the appropriate thickness range of the base film, the battery has higher battery power and energy density, lower internal resistance, good cycle life, and safety. When the thickness of the base film is too thin, the separator can provide a greater rate of electron and ion transmission, but it can also accelerate metal deposition and increase the risk of perforation; when the thickness of the base film is too thick, although it can ensure the isolation of the positive and negative electrodes inside, reduce the risk of short circuit and thermal runaway, but the transmission rate is significantly reduced, increasing the internal resistance of the battery. The material of the base film in the present application can be any material known to those skilled in the art, such as polyethylene (PE) or polypropylene (PP).

[0031] In some embodiments, at least one surface of the base film is coated with a coating layer, and the thickness of the coating layer is 1-4 μm. The design of the coating layer can further improve the mechanical strength of the separator and prolong the cycle life of the battery, but too thick a thickness can also affect the power and energy density of the battery.

[0032] In some embodiments, the coating layer is selected from a mixture of one or more of boehmite, aluminum trioxide, barium oxide, PVDF (polyvinylidene fluoride), and PAA (polyaryne). Preferably, the coating layer is a mixture of one of barium oxide, PVDF (polyvinylidene fluoride), and PAA (polyaryne) and boehmite, with a weight ratio of (1-3):(7-9). In the selection of coating layer material, the principle is to improve the mechanical strength of the separator, followed by the ease of operation of the coating process and the adhesion of the coating layer.

[0033] In some embodiments, the weight percentage of the silicon material in the negative active material is Y%, and the tortuosity of the separator is X, both of which satisfy X ≥ 0.15Y. 0.5 ​+1.8. For example, the weight percentage of the silicon material in the negative active material is 10%, 0.15Y 0.5 +1.8 is 2.27, then X satisfies X≥2.27. In cylindrical lithium batteries, the tortuosity of the separator refers to the degree of bending or folding exhibited by the separator. A certain tortuosity can reduce the diffusion of iron, nickel elements and transition metal elements through the separator to the negative electrode, thereby reducing the adverse effects of silicon expansion. Under the above inequality relationship, the above effect can be achieved. If the tortuosity of the separator is too low, the diffusion of metal elements will accelerate, thereby more quickly deteriorating the performance of the battery cell, and even forming a local micro-short circuit, causing safety problems.

[0034] In some embodiments, the porosity of the separator is 33-48%; preferably, the porosity of the separator is 36-45%. A separator with a reasonable porosity is beneficial to improve the cycle and safety performance of the battery. When the porosity is too low, the ion diffusion resistance will increase, causing cycle deterioration; when the porosity is too high, the diffusion rate of metal ions will accelerate, and the mechanical reliability of the separator will be reduced, thereby causing cycle deterioration and safety failure.

[0035] In some embodiments, the porosity of the separator is Z, and the tortuosity of the separator is X, which satisfies X≤3-lnZ. For example, the porosity of the separator is 42%, and the value of 3-lnZ is 3.87 to the second decimal place, then X satisfies X≤3.87. In order to reduce the penetration of Fe and Ni elements to the center area, it is necessary to appropriately increase the tortuosity of the separator. However, the greater the tortuosity of the separator, the fewer the straight-through holes, which is not conducive to the diffusion and transmission of lithium ions in the separator, causing poor current flow, thereby reducing the capacity of the battery, and possibly affecting the safety of the battery, such as increasing the risk of battery short circuit, liquid leakage, etc., and even possibly causing the battery to explode or catch fire. Therefore, the maximum value of the tortuosity of the separator is controlled, which can solve the problem of element dissolution caused by silicon expansion while ensuring the normal use of the battery.

[0036] In some embodiments, after the battery roll core completes at least 100 cycles, at least 3 sections are taken on the separator in the direction from the beginning of winding to the end of winding, each section has a length in the width direction of the separator of not less than 3 cm, and the interval between each two sections in the length direction of the separator is not less than 200 cm. Each section is divided into three regions by the center line in the width direction of the separator to the end of the positive electrode tab, which are region A, region B and region C, respectively. The Fe element content and the Ni element content of multiple region A, multiple region B and multiple region C are obtained by sampling and testing in each region of each section of the separator. The average value of the Fe element content of region A, region B and region C is denoted as Fe A , Fe B , Fe C , and the average value of the Ni element content of region A, region B and region C is denoted as Ni ANi B Ni C , satisfying the following relationship: Ni A / Fe A >Ni B / Fe B >Ni C / Fe C .

[0037] In some embodiments, the battery roll core has a diameter of 20-60 mm, a height of 60-200 mm, and a ratio of height to diameter greater than 1.7.

[0038] A second aspect of the present application provides a method for preparing the battery roll core described above, comprising the following steps:

[0039] S1 - making a positive electrode sheet: applying a positive electrode coating to at least one surface of an aluminum foil to obtain a positive electrode sheet;

[0040] S2 - making a negative electrode sheet: applying a negative electrode coating to at least one surface of a copper foil to obtain a negative electrode sheet;

[0041] S3 - making a battery roll core: rolling the positive electrode sheet and the negative electrode sheet after being respectively rolled, slitting, and die cutting, together with a separator to obtain a cylindrical battery roll core.

[0042] A third aspect of the present application provides a cylindrical battery comprising the battery roll core described above, a cap, and a steel shell, wherein the inside of the steel shell contains the battery roll core, and the top is packaged with the cap.

[0043] A method for preparing the cylindrical battery by assembling the battery roll core described above is as follows:

[0044] (1) making an electrolyte: dissolving a fully dried lithium salt in an organic solvent to prepare an electrolyte;

[0045] (2) assembling: assembling the battery roll core into the steel shell, welding the tab of the battery roll core with the electrical connection piece, injecting the electrolyte, sealing, and forming to obtain the cylindrical battery.

[0046] A fourth aspect of the present application provides an electrical device comprising the cylindrical battery described above for providing power.

[0047] The present application is further explained and described in the following examples.

[0048] Example 1

[0049] Example 1 provides a battery roll core formed by rolling a positive electrode sheet, a negative electrode sheet, and a separator therebetween, the positive electrode sheet having a positive electrode coating on at least one surface thereof, and the negative electrode sheet having a negative electrode coating on at least one surface thereof.

[0050] The positive electrode coating includes 2% conductive agent, 1wt% binder, and the rest is positive active material; wherein the conductive agent is carbon nanotube and carbon black in a weight ratio of 1:4, the binder is polyvinylidene fluoride, and the positive active material is lithium nickel cobalt manganese oxide.

[0051] The negative electrode coating includes 1wt% conductive agent, 1wt% thickening agent, 2wt% binder, and the rest is negative active material; wherein the conductive agent is acetylene black, the thickening agent is carboxymethyl cellulose, and the binder is polyacrylate. The negative active material includes 10% silicon-oxygen material (SiO) and the rest is artificial graphite in terms of weight percentage. The silicon-oxygen material and the artificial graphite are uniformly mixed to obtain the negative active material.

[0052] The separator includes a base film and a coating layer coated on at least one surface of the base film, wherein the base film is made of polypropylene and has a thickness of 9μm; the coating layer is boehmite and has a thickness of 3μm. The separator has a tortuosity of 2.69 and a porosity of 42%.

[0053] The diameter of the battery roll core is 21mm, and the height is 70mm.

[0054] Example 1 also provides a preparation method of the above battery roll core, including the following steps:

[0055] (1) Making a positive electrode sheet: taking 97% lithium nickel cobalt manganese oxide, 0.4% carbon nanotube conductive agent, 1.6% conductive carbon black, and 1% polyvinylidene fluoride (PVDF) binder as solid substances of the positive electrode slurry in terms of weight percentage, dispersing the above solid substances in N-methyl-2-pyrrolidone (NMP solvent) with a solid content of 60%, uniformly mixing in a homogenizer, coating the slurry on both sides of an aluminum foil, and drying to obtain the positive electrode sheet;

[0056] (2) Making a negative electrode sheet: taking 96% negative active material, 1% acetylene black conductive agent, 1% thickening agent (hydroxymethyl cellulose), and 2% polyacrylate binder as solid substances of the negative electrode slurry in terms of weight percentage, dispersing the above solid substances in deionized water with a solid content of 40%, uniformly mixing in a homogenizer, coating the slurry on both sides of a copper foil, and drying to obtain the negative electrode sheet;

[0057] (3) Making a battery roll core: rolling the positive electrode sheet and the negative electrode sheet respectively, cutting and die-cutting according to the size requirement, and winding together with a separator to obtain a cylindrical battery roll core.

[0058] Example 2

[0059] Example 2 provides a battery roll core, which is different from that of Example 1 in that the silicon-oxygen material accounts for 5% of the negative active material in terms of weight percentage.

[0060] Example 3

[0061] Example 3 provides a battery jelly-roll, which is different from Example 1 in that the silicon-oxygen material accounts for 20% of the negative active material in terms of weight percentage.

[0062] Example 4

[0063] Example 4 provides a battery jelly-roll, which is different from Example 1 in that the silicon-oxygen material accounts for 2% of the negative active material in terms of weight percentage.

[0064] Example 5

[0065] Example 5 provides a battery jelly-roll, which is different from Example 1 in that the silicon-oxygen material accounts for 30% of the negative active material in terms of weight percentage.

[0066] Example 6

[0067] Example 6 provides a battery jelly-roll, which is different from Example 1 in that the porosity of the separator is 33%.

[0068] Example 7

[0069] Example 7 provides a battery jelly-roll, which is different from Example 1 in that the porosity of the separator is 48%.

[0070] Example 8

[0071] Example 8 provides a battery jelly-roll, which is different from Example 1 in that the porosity of the separator is 36%.

[0072] Example 9

[0073] Example 9 provides a battery jelly-roll, which is different from Example 1 in that the porosity of the separator is 45%.

[0074] Example 10

[0075] Example 10 provides a battery jelly-roll, which is different from Example 1 in that the coating layer is aluminum trioxide.

[0076] Example 11

[0077] Example 11 provides a battery jelly-roll, which is different from Example 1 in that the coating layer is barium oxide and boehmite with a weight ratio of 1:9.

[0078] Example 12

[0079] Example 12 provides a battery jelly-roll, which is different from Example 1 in that the coating layer is polyvinylidene fluoride and boehmite with a weight ratio of 3:7.

[0080] Example 13

[0081] Example 13 provides a battery jelly-roll, which is different from Example 1 in that the thickness of the coating layer is 1 μm.

[0082] Example 14

[0083] Example 14 provides a battery jelly-roll, which is different from Example 1 in that the thickness of the coating layer is 4 μm.

[0084] Example 15

[0085] Example 15 provides a battery jelly-roll, which is different from Example 1 in that the diameter of the battery jelly-roll is 46 mm and the height is 95 mm.

[0086] Example 16

[0087] Example 16 provides a battery jelly-roll, which is different from Example 1 in that the diameter of the battery jelly-roll is 46 mm and the height is 120 mm.

[0088] Comparative Example 1

[0089] Comparative Example 1 provides a battery jelly-roll, which is different from Example 1 in that the silicon-oxygen material accounts for 37% of the negative active material by weight percentage.

[0090] Comparative Example 2

[0091] Comparative Example 2 provides a battery jelly-roll, which is different from Example 1 in that the tortuosity of the separator is 2.04.

[0092] Comparative Example 3

[0093] Comparative Example 3 provides a battery jelly-roll, which is different from Example 1 in that the porosity of the separator is 30%.

[0094] Comparative Example 4

[0095] Comparative Example 4 provides a battery jelly-roll, which is different from Example 1 in that the porosity of the separator is 55%.

[0096] The variable parameters of the above examples are compared in Table 1.

[0097] Table 1

[0098] Performance Evaluation

[0099] The battery jelly-rolls provided by the above examples are assembled into batteries for testing, and the preparation method is as follows:

[0100] (1) Preparation of electrolyte: mix ethylene carbonate, methyl ethyl carbonate and diethyl carbonate in a volume ratio of 1:1:1 to obtain an organic solvent, then dissolve the fully dried lithium salt LiPF6 in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L;

[0101] (2) Assembly: assemble the battery roll core into the shell, weld the battery roll core tab to the electrical connection sheet, inject electrolyte, seal, and form to obtain the experimental battery.

[0102] Test one: cycle performance test

[0103] Take a fresh battery and place it in a 25°C constant temperature box for more than 4 hours, and test according to the following steps:

[0104] (1) Discharge the battery at 0.1C to 2.5V cutoff, and stand for 5 minutes;

[0105] (2) Charge the battery at 0.2C to 4.2V cutoff, and charge at constant voltage to 0.05C cutoff, and stand for 5 minutes;

[0106] (3) Discharge the battery at 0.2C to 2.5V cutoff, and stand for 5 minutes, read the capacity value C0 at this time;

[0107] (4) Charge the battery at 1.0C to 4.2V, and charge at constant voltage to 0.05C cutoff, and stand for 5 minutes;

[0108] (5) Discharge the battery at 2.0C to 2.5V cutoff, and stand for 5 minutes;

[0109] (6) Repeat steps (4) and (5) 600 times;

[0110] (7) The cycle performance of a single battery, i.e. the capacity retention rate, is obtained by the ratio of the 600th discharge capacity to the 1st discharge capacity of steps (4) and (5).

[0111] Test two: determination of the proportion of separator elements

[0112] (1) Take a fresh battery roll core and cycle it according to the cycle performance test for 100 cycles, then discharge the battery core to 2.5V at a current of 0.2C, and stand at room temperature for 30 minutes;

[0113] (2) Disassemble the battery cell and obtain the separator. Refer to FIG. 6. The left end of the figure is the initial winding position, which is marked as 0, and the right end is the end winding position. Divide the separator into 4 segments of 5 cm in length at positions of 10-15 cm, 310-315 cm, 610-615 cm, and 910-915 cm using a ceramic knife, i.e., d1 = d2 = d3 = d4 = 5 cm and D1 = D2 = D3 = 295 cm in FIG. 6;

[0114] (3) Divide each segment into three equal-interval segments A, B, and C, with the center of each segment as the starting point and the direction of the positive electrode tab (upper part of FIG. 6) as the side;

[0115] (4) Take a 5*5 mm area sample (shaded part in FIG. 6) from the center area of each segment. Use SEM to measure and EDS to analyze the mass content of Ni and Fe elements (unit: ppm) in each area of each segment. Then calculate the average value of the Fe element content and the average value of the Ni element content in each area, and further calculate the nickel-iron ratio according to the average values. Take the calculation of the average value of the Fe element content in area A as an example, which is calculated from the Fe element content in area A of the 4 segments. A

[0116] Test three: determination of the tortuosity of the separator

[0117] (1) Disassemble a cylindrical battery and obtain the separator;

[0118] (2) Soak the separator in DMC (dimethyl carbonate) for 2 hours, then rinse it 3 times with DMC, and bake the separator in a vacuum oven at 60°C for 4 hours to dry;

[0119] (3) Take some newly prepared NCM811 positive electrode tabs after cold pressing, and assemble them with the separator to form symmetrical batteries (the active materials of the anode and the cathode are the same, and the properties and structures of the two electrodes are similar);

[0120] (4) The number of layers of the separator is 3, and the number of symmetrical batteries is not less than 3;

[0121] (5) Test the EIS of the above symmetrical batteries and obtain the ion impedance R ion of the separator;

[0122] (6) The tortuosity τ of the separator = (R ion *A*y*m) / 3d, where A is the tab area, y is the porosity of the separator, m is the conductivity of the electrolyte, and d is the thickness of a single layer of the separator (here, the average value of the ion impedance of the three-layer separator is used).

[0123] Test four: hot box test

[0124] ​Take a fresh battery, place it in a 25℃ constant temperature box for more than 4h, and test according to the following steps:

[0125] (1) Discharge the battery at 0.1C to 2.5V cut-off, and stand for 5min;

[0126] (2) Charge the battery at 0.2C to 4.2V cut-off, and constant voltage charge to 0.05C cut-off, stand for 5min;

[0127] (3) Put the battery into a constant temperature box, set the heating rate to 5K / min, heat to 130℃, and keep for 1h, then stop heating, and cool to below 30℃ by self-heating;

[0128] (4) The battery is considered to pass if it does not catch fire or smoke, otherwise it is considered to fail, at least 5 batteries are tested, and the pass rate is recorded.

[0129] The test two results (unit: ppm) are shown in Table 2, and other test results are shown in Table 3.

[0130] Table 2

[0131] Table 3

[0132] It can be found from Table 3 that when the silicon addition ratio and the separator tortuosity are in the preferred range, the cycle and safety performance of the battery core are both relatively excellent; when the tortuosity is low, the permeability of the separator is relatively strong, at this time, the ion penetration cannot be effectively blocked, thereby deteriorating the cycle and hot box performance. When the separator with reasonable porosity is used, excellent cycle and safety performance can be obtained, when the porosity is too low, the ion diffusion impedance is increased, causing cycle deterioration; when the porosity is too high, the metal ions diffuse faster, and the mechanical reliability of the separator is reduced, thereby causing cycle deterioration and safety failure. Secondly, reasonable coating material and thickness can make the separator obtain reliable performance, the cycle retention rate reaches 90% or more, and the hot box pass rate reaches 100%. In addition, the present application can be used in different sizes of cylindrical batteries and is effective.

[0133] In the description of the present specification, the reference term "some embodiments", "an embodiment" or similar description means that the specific features, structures, materials or characteristics described in connection with the embodiment are included in at least one embodiment or example. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0134] While the embodiments of the present embodiment have been shown and described, it is understood that the embodiments can be changed, modified, substituted, and varied, without departing from the principles and spirit of the present embodiment, and the scope of the present embodiment is defined by the claims and their equivalents.

Claims

1. A battery jelly-roll, formed by winding a positive electrode sheet, a negative electrode sheet, and a separator between the two, wherein at least one surface of the positive electrode sheet has a positive electrode coating layer, and at least one surface of the negative electrode sheet has a negative electrode coating layer, characterized in that: the negative electrode coating layer comprises a negative electrode active material; the negative electrode active material comprises at least a silicon material and a carbon material, and the weight percentage of the silicon material in the negative electrode active material is Y%; X satisfies X≤3-lnZ, wherein Z is the porosity of the separator. The separator comprises a base film, and the thickness of the base film is 5-20 μm. The tortuosity of the membrane is X, the X satisfies X≥0.15Y 0.5 +1.

8.

2. The battery jelly-roll of claim 1, wherein, At least one surface of the base film is coated with a coating layer, and the thickness of the coating layer is 1-4 μm.

3. The jelly-roll battery core according to claim 1 or 2, characterized by After the battery jelly-roll core is completed at least 100 cycles, at least 3 segments of a length not less than 3 cm in the width direction of the separator are taken on the separator in the direction from the winding start to the winding end, the interval between each two segments is not less than 200 cm in the length direction of the separator, each segment is divided into three regions, region A, region B and region C, by tri-sectioning the center line in the width direction of the separator to the end of the positive electrode tab, the average value of the Fe element content of the three regions of all the segments is recorded as Fe A , Fe B , Fe C , the average value of the Ni element content of the three regions of all the segments is recorded as Ni A , Ni B , Ni C , the following relationship is met: Ni A / Fe A >Ni B / Fe B >Ni C / Fe C .

4. The jelly-roll battery core according to claim 1 or 2, characterized by 2≤Y≤30。 5. The battery roll core of claim 2, wherein, 33％≤Z≤48％。 6. The jelly-roll battery core according to claim 1 or 2, characterized by The coating layer is selected from one or more of a mixture of boehmite, aluminum trioxide, barium oxide, polyvinylidene fluoride, and polyarylene.

7. The jelly-roll battery core according to claim 6, characterized in that The silicon material is a silicon-carbon material and / or a silicon-oxygen material; and the carbon material is selected from one or more of a mixture of artificial graphite, natural graphite, hard carbon, and soft carbon.

8. The jelly-roll battery core according to claim 7, characterized by The diameter of the battery jelly-roll is 20-60 mm, the height is 60-200 mm, and the ratio of the height to the diameter is greater than 1.

7.

9. The jelly-roll battery core according to claim 1 or 2, characterized by, The battery jelly-roll, a cap, and a steel shell are provided, the steel shell contains the battery jelly-roll inside, and the top is sealed with the cap.

10. The jelly-roll battery core according to claim 1 or 2, characterized by, ​ 11. A cylindrical battery, characterized by comprising: ​

Images