Winding type lithium ion battery and preparation method thereof

Abstract

The invention relates to the technical field of lithium ion batteries, and provides a winding type lithium ion battery and a preparation method thereof. The negative plate comprises a silicon material, and the electrolyte comprises fluoroethylene carbonate; when the battery is in a 0% SOC state, performing an X-ray photoelectron spectroscopy test on the negative plate A of the third fold at the head part in the winding direction and the negative plate B of the first fold at the tail part in the winding direction; the ratio of the peak area of the C-F peak in the negative plate A to the total peak area of the negative plate A is X1, and the ratio of the peak area of the C-F peak in the negative plate B to the total peak area of the negative plate B is X2; x1 and X2 satisfy: X1gt; x2. The FEC content in the roll core is high, the FEC content outside the roll core is low, and the FEC is unevenly distributed in the lithium battery, so that the cycle performance of the lithium battery can be improved, furnace temperature failure caused by too high FEC content in the roll core is avoided, and the battery has both cycle performance and high-temperature performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and particularly relates to a wound lithium ion battery and a preparation method thereof. Background Art

[0002] With the rapid development of electric vehicles and portable electronic devices, higher requirements are put forward for the performance of lithium batteries. In the selection of battery anode materials, silicon materials have been widely concerned due to their high theoretical capacity. However, silicon materials will undergo significant volume expansion during charge and discharge, which will cause the deformation of the battery structure and seriously affect the cycle stability of the battery. To solve this problem, researchers have introduced fluoroethylene carbonate (FEC) as an additive. FEC can form a stable solid electrolyte interface (SEI) film to inhibit the expansion and cracking of silicon, thereby improving the cycle performance of the battery.

[0003] However, the stability of FEC in the electrolyte is relatively poor, and too high content of it will cause the problem of furnace temperature failure of the battery under high temperature conditions. Therefore, how to find a balance between ensuring good cycle performance and high temperature stability of the battery is a key issue in current battery technology. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above problems existing in the prior art, and provide a wound lithium ion battery. By adjusting the relationship between the ratio X1 of the C-F peak in the anode sheet of the third fold at the head of the wound core and the ratio X2 of the C-F peak in the anode sheet of the first fold at the tail of the wound core, the content of FEC inside the wound core can be relatively high, and the content of FEC outside the wound core can be relatively low. The non-uniform distribution of FEC in the lithium battery can improve the cycle performance of the lithium battery; at the same time, it can avoid the furnace temperature failure caused by too high content of FEC inside the wound core, and take into account the cycle performance and high temperature performance of the battery.

[0005] To achieve the above purpose, in the first aspect of the present invention, a wound lithium ion battery is provided. The battery includes an anode sheet and an electrolyte;

[0006] The anode sheet includes an anode current collector and an anode active layer provided on at least one surface of the anode current collector;

[0007] The anode active layer includes an anode active material, and the anode active material includes silicon material; the electrolyte includes fluoroethylene carbonate;

[0008] When the battery is in the 0% SOC state, X-ray photoelectron spectroscopy tests are performed on the negative electrode sheet A at the third fold at the head in the winding direction and the negative electrode sheet B at the first fold at the tail in the winding direction; the ratio of the peak area of the C-F peak in the negative electrode sheet A to the total peak area of the negative electrode sheet A is denoted as X1, and the ratio of the peak area of the C-F peak in the negative electrode sheet B to the total peak area of the negative electrode sheet B is denoted as X2;

[0009] The relationship between X1 and X2 satisfies: X1 > X2.

[0010] The second aspect of the present invention provides a method for manufacturing the wound lithium-ion battery, including the following steps:

[0011] (1) Inject the electrolyte for the first time. In the electrolyte, the content of fluoroethylene carbonate is 10% - 30%, and the injection volume is 5 / 10 to 8 / 10 of the total volume of the electrolyte;

[0012] (2) Inject the electrolyte for the second time. In the electrolyte, the content of fluoroethylene carbonate is 0% - 5%, and the injection volume is 2 / 10 to 5 / 10 of the total volume of the electrolyte.

[0013] The present invention adopts the above technical solutions and has the following beneficial effects:

[0014] For the wound lithium-ion battery provided by the present invention, by defining the relationship of X1 > X2, the content of FEC inside the wound core of the wound lithium-ion battery can be made higher, and the content of FEC outside the wound core can be made lower. The non-uniform distribution of FEC in the lithium battery can make the SEI film formed inside the wound core thinner and of better quality, and the resistance for lithium ions to penetrate the SEI film is smaller, improving the cycle performance of the lithium battery; at the same time, due to the non-uniform distribution of FEC, when the lithium battery is at a high temperature, the relatively high content of FEC inside the wound core will diffuse to the outside of the wound core, avoiding the furnace temperature failure caused by excessive gas production due to too high a content of FEC inside the wound core. By adjusting the non-uniform distribution of FEC in the lithium battery, the present invention can take into account both the cycle performance and the high-temperature performance of the battery.

[0015] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein. In this article, unless otherwise specified, the data ranges include the endpoints. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Shown is a schematic structural diagram of a wound core in an example of the present invention.

[0017] Figure 2 The figure shows a schematic structural diagram of a negative electrode sheet in an example of the present invention.

[0018] Figure 3 The figure shows a C spectrum in the first double-sided negative electrode arc segment in an example of the present invention.

[0019] Figure 4 The figure shows a C spectrum in the second double-sided negative electrode arc segment in an example of the present invention.

[0020] Reference numerals: 1 - winding core; 11 - first arc; 12 - second arc; 13 - flat area; 2 - negative electrode sheet; 21 - first double-sided negative electrode arc segment; 22 - second double-sided negative electrode arc segment; 23 - first side; 231 - first edge area; 24 - second side; 241 - second edge area; 25 - middle area; 26 - negative electrode tab; 3 - separator; 4 - positive electrode sheet. Detailed Embodiments

[0021] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0022] Unless otherwise defined, all scientific and technical terms used in the present invention have the same meaning as commonly understood by those skilled in the technical field to which the present invention pertains.

[0023] In the present invention, the terms "wound lithium-ion battery", "battery", "laminated battery", "lithium battery", "lithium-ion battery", and "lithium-ion secondary battery" all have the same meaning, and all refer to a wound lithium-ion battery, which generally includes a winding core (such as a positive electrode sheet, a negative electrode sheet, and a separator), a container (housing) for accommodating the winding core, and an electrolyte.

[0024] The first aspect of the present invention provides a wound lithium-ion battery, and the battery includes a negative electrode sheet and an electrolyte;

[0025] The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer provided on at least one surface of the negative electrode current collector;

[0026] The negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a silicon material; the electrolyte includes fluoroethylene carbonate (FEC);

[0027] When the battery is in the 0% SOC state, X-ray photoelectron spectroscopy tests are performed on the negative electrode sheet A at the third fold at the head in the winding direction and the negative electrode sheet B at the first fold at the tail in the winding direction; the ratio of the peak area of the C-F peak in the negative electrode sheet A to the total peak area of the negative electrode sheet A is denoted as X1, and the ratio of the peak area of the C-F peak in the negative electrode sheet B to the total peak area of the negative electrode sheet B is denoted as X2;

[0028] The relationship between X1 and X2 satisfies: X1 > X2.

[0029] In some embodiments, the wound lithium-ion battery includes a positive electrode sheet, a separator, and a negative electrode sheet that are stacked and wound multiple times in a winding direction to form a core. The negative electrode sheet A at the third fold at the head of the winding direction and the negative electrode sheet B at the first fold at the tail of the winding direction are as Figure 1 indicated by the markings. The negative electrode sheet A at the third fold at the head of the winding direction is located inside the core, and the negative electrode sheet B at the first fold at the tail of the winding direction is located outside the core.

[0030] In the wound lithium-ion battery of the present invention, the ratio X1 of the peak area of the C-F peak in the negative electrode sheet A at the third fold at the head can represent the FEC content inside the core of the wound lithium-ion battery, and the ratio X2 of the peak area of the C-F peak in the negative electrode sheet B at the first fold at the tail can represent the FEC content outside the core of the wound lithium-ion battery. Adjusting X1 > X2 can make the FEC content inside the core of the wound lithium-ion battery higher and the FEC content outside the core lower. The uneven distribution of FEC in the lithium battery can improve the cycle performance of the lithium battery; at the same time, when the lithium battery is under high-temperature conditions, the relatively high FEC content inside the core will diffuse to the outside of the core, avoiding the furnace temperature failure caused by the too high FEC content inside the core, and taking into account the cycle performance and high-temperature performance of the battery.

[0031] The "ratio of the peak area of the C-F peak in the negative electrode sheet to the total peak area of the negative electrode sheet" can be understood as semi-quantitative analysis. The negative electrode sheet is tested by X-ray photoelectron spectroscopy (XPS). The peak area of the C-F peak (carbon peak, binding energy between 290-291 eV) refers to the area where the C-F peak appears on the spectrum, and the total peak area of the negative electrode sheet refers to the total area of all the peaks that appear on the negative electrode sheet after XPS testing. All the peaks that appear include but are not limited to the C-F peak, C-O peak, C=O double bond peak, fluorine-lithium peak, etc. The ratio of the peak area of the C-F peak can reflect the FEC content in the test area. The larger the ratio of the peak area of the C-F peak, the higher the FEC content.

[0032] In some embodiments, the ratio X1 of the peak area of the C-F peak in the negative electrode sheet A to the total peak area of the negative electrode sheet A satisfies: 1% ≤ X1 ≤ 15%. For example, X1 can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or any value within the range composed of any two of the above values. Preferably, 6% ≤ X1 ≤ 10%.

[0033] In some embodiments, the ratio X2 of the peak area of the C-F peak in the negative electrode sheet B to the total peak area of the negative electrode sheet B satisfies: 0.5% ≤ X1 ≤ 12%. For example, X2 can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12% or any value within the range formed by any two of the above values. Preferably, 4% ≤ X1 ≤ 9%.

[0034] Under the condition of X1 > X2, when further adjusting the values of X1 and X2 within the above range, the FEC content inside and outside the core can be within a more appropriate range. This can make the SEI film formed by FEC inside the core thin and dense, which can better inhibit the expansion and cracking of silicon, and reduce the lithium ion transmission distance, further improving the cycle performance of the battery. Also, it can make FEC diffuse better from the inside of the core to the outside of the core at high temperatures, further reducing the risk of the battery failing due to overheating.

[0035] In some embodiments, the wound lithium ion battery includes a first arc and a second arc, and a straight section connecting the first arc and the second arc. In the width direction of the wound core, the negative electrode sheet includes a first double-sided negative electrode arc segment and a second double-sided negative electrode arc segment that are away from the straight section. X-ray photoelectron spectroscopy tests are performed on the first double-sided negative electrode arc segment and the second double-sided negative electrode arc segment. The ratio of the peak area of the C-F peak in the first double-sided negative electrode arc segment to the total peak area of the first double-sided negative electrode arc segment is denoted as R1, and the ratio of the peak area of the C-F peak in the second double-sided negative electrode arc segment to the total peak area of the second double-sided negative electrode arc segment is denoted as R2. The difference between R1 and R2 is 1% - 8% (the difference can be, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8% or any value within the range formed by any two of the above values). Preferably, it is 3% - 5%.

[0036] In some embodiments, as Figure 1 shown, the core 1 includes a negative electrode sheet 2, a separator 3, and a positive electrode sheet 4 that are stacked and wound. Along the width direction (Y-axis direction) of the core 1, the wound lithium ion battery includes a first arc 11 and a second arc 12 on both sides, and a straight section 13 in the middle that connects the first arc 11 and the second arc 12. In the width direction of the wound core, the negative electrode sheet 2 includes a first double-sided negative electrode arc segment 21 and a second double-sided negative electrode arc segment 22 that are away from the straight section 13. The first double-sided negative electrode arc segment 21 is located in the first arc 11, and the second double-sided negative electrode arc segment 22 is located in the second arc 12. The first double-sided negative electrode arc segment 21 and the second double-sided negative electrode arc segment 22 correspond to the outermost regions of the negative electrode sheet 2 at both sides of the arc.

[0037] It should be noted that the "double-sided negative electrode arc segment" refers to a certain arc in the negative electrode sheet located in the arc area, and negative electrode active material layers are provided on the surfaces of the negative electrode current collector on both opposite sides of the negative electrode sheet of this arc segment.

[0038] Furthermore, in the present invention, the peak area ratio R1 of the C-F peak in the first double-sided negative electrode arc segment can represent the FEC content on the outermost side of the first arc, and the peak area ratio R2 of the C-F peak in the second double-sided negative electrode arc segment can represent the FEC content on the outermost side of the second arc. The difference between R1 and R2 represents the difference in the FEC content on the outer sides of the arcs on both sides of the wound battery. When the difference between R1 and R2 is between 1% and 8%, the FEC in the wound lithium-ion battery can be unevenly distributed. When the difference in the FEC content on the outer sides of the arcs on both sides is within the above range, the cycle performance and oven temperature performance of the battery can be improved. If the difference in the FEC content on the outer sides of the arcs on both sides is too small (<1%), the FEC in the wound lithium-ion battery is relatively evenly mixed, and the FEC inside the wound core cannot diffuse at high temperature, resulting in serious gas generation and oven temperature failure; if the difference in the FEC content on the outer sides of the arcs on both sides is too large (>8%), the wettability during the first liquid injection is poor, and the FEC content inside the wound core is low, resulting in deterioration of the battery cycle performance.

[0039] In some embodiments, the ratio R1 of the peak area of the C-F peak in the first double-sided negative electrode arc segment to the total peak area of the first double-sided negative electrode arc segment satisfies: 1% ≤ R1 ≤ 15%. For example, R1 can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or any value within the range composed of any two of the above values, and preferably 7% ≤ R1 ≤ 10%.

[0040] In some embodiments, the ratio R2 of the peak area of the C-F peak in the second double-sided negative electrode arc segment to the total peak area of the second double-sided negative electrode arc segment satisfies: 0.5% ≤ R2 ≤ 12%. For example, R2 can be 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12% or any value within the range composed of any two of the above values, and preferably 4% ≤ R2 ≤ 8%.

[0041] Under the condition that the difference between R1 and R2 is between 1% and 8%, further adjusting the values of R1 and R2 within the above range can further improve the cycle performance and oven temperature performance of the battery.

[0042] In some embodiments, along the width direction of the negative electrode sheet, the negative electrode sheet includes a first edge area, a second edge area, and an intermediate area located between the first edge area and the second edge area:

[0043] Wherein, the thickness of the SEI film in the first edge region or the second edge region is denoted as h1, the thickness of the SEI film in the middle region is denoted as h2, and the ratio of h1 to h2 is 1.05 - 1.6.

[0044] Exemplarily, the ratio of h1 to h2 can be, for example, 1.05, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, or any value within the range formed by any two of the above values.

[0045] It should be explained that the "thickness of the SEI film" refers to the thickness of the SEI film on the surface of silicon material particles in the corresponding region during testing. The testing method includes: selecting the corresponding position in the negative electrode sheet, cutting a cross-section for electron microscopy testing, counting the thicknesses of the SEI films on the surfaces of at least 30 - 40 silicon material particles in each corresponding region, and taking the average value, which is denoted as the thickness of the SEI film.

[0046] In some embodiments, as Figure 2 shown (the middle red dashed line is the center line of the negative electrode sheet), along the width direction of the negative electrode sheet, the negative electrode sheet 2 includes a first side 23 close to the negative electrode tab 26 and a second side 24 far from the negative electrode tab 26. The first edge region 231 covers the first side 23 and extends towards the second side 24, the second edge region 241 covers the second side 24 and extends towards the first side 23, and the middle region 25 is located between the first edge region 231 and the second edge region 241.

[0047] Further, in the present invention, the thickness h1 of the SEI film in the first edge region or the second edge region represents the thickness of the SEI film in the edge region of the negative electrode sheet, and the thickness h2 of the SEI film in the middle region represents the thickness of the SEI film in the middle region of the negative electrode sheet. The thickness of the SEI film is related to the content of FEC. When the content of FEC is high, the formed SEI film is thinner (there is a C-F bond in FEC, and the reduction potential is relatively high. When the FEC concentration is high, an elastic polymer containing many C-F bonds will be formed on the surface of the silicon material particles, and the formed SEI film is thinner). When the content of FEC is low, the formed SEI film is thicker (when the content of FEC is relatively small, other solvents will decompose to form the SEI film, and the SEI film is thicker). When h1>h2 and the ratio of h1 to h2 is within the above range, the content of FEC in the middle region of the negative electrode sheet can be made relatively high, and the content of FEC at the edge of the negative electrode sheet can be made relatively low. Combined with the relatively high content of FEC inside the winding core and the relatively low content of FEC outside the winding core, the FEC is unevenly distributed in the lithium battery, which can make the SEI film formed in the middle region of the negative electrode sheet and inside the winding core thinner, of better quality, and the resistance of lithium ions to penetrate the SEI film smaller, thereby improving the cycle performance of the lithium battery; and when the lithium battery is under high-temperature conditions, the FEC in the middle region of the negative electrode sheet and inside the winding core is more likely to diffuse to the edge of the negative electrode sheet and outside the winding core, avoiding the furnace temperature failure caused by excessive gas generation due to too high content of FEC in the middle region of the negative electrode sheet and inside the winding core. By adjusting the content distribution of FEC in the middle region and the edge region of the negative electrode sheet, as well as inside the winding core and outside the winding core, the FEC is unevenly distributed in the lithium battery, which can take into account the cycle performance and high-temperature performance of the battery.

[0048] In some embodiments, the width of the first edge region is 5 mm - 10 mm, for example, it can be 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm or any value within the range composed of any two of the above point values. As Figure 2 shown, the width of the first edge region can be interpreted as: along the width direction of the negative electrode sheet, the size of the first edge region.

[0049] In some embodiments, the width of the second edge region is 5 mm - 10 mm, for example, it can be 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm or any value within the range composed of any two of the above point values. As Figure 2 shown, the width of the second edge region can be interpreted as: along the width direction of the negative electrode sheet, the size of the second edge region.

[0050] In some embodiments, the width of the middle region is 4 mm - 6 mm, and for example, it can be 4 mm, 4.2 mm, 4.5 mm, 4.8 mm, 5 mm, 5.2 mm, 5.5 mm, 5.8 mm, 6 mm, or any point value within the range formed by any two of the above values. The middle region can be understood as the region where the vertical distance from the center line in the length direction of the negative electrode sheet is within 2 mm - 3 mm, and the width of the middle region can be interpreted as: the vertical distance between the upper and lower sides of the middle region in the width direction of the negative electrode sheet.

[0051] In some embodiments, the battery is pressurized to extract electrolyte. The content of FEC in the electrolyte extracted for the first time is denoted as M1, and the content of FEC in the electrolyte extracted for the second time is denoted as M2. The ratio of M1 to M2 is 1% - 99%. The ratio of M1 to M2 can be, for example, 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99%, or any point value within the range formed by any two of the above values, and preferably 60% - 80%.

[0052] Furthermore, in the present invention, electrolyte is extracted twice at certain positions inside the membrane shell and at the left and right edges of the wound core. During the pressurization of the battery, FEC will diffuse into the battery membrane shell, reducing the FEC concentration inside the wound core and increasing the FEC concentration outside the wound core. When the ratio of the FEC content M1 extracted for the first time to the FEC content M2 extracted for the second time is in the range of 1% - 99%, the FEC content inside the wound core of the wound-type lithium-ion battery can be relatively high, and the FEC content outside the wound core of the wound-type lithium-ion battery can be relatively low. The non-uniform distribution of FEC in the lithium battery can take into account the cycle performance and high-temperature performance of the battery; at the same time, when the ratio of M1 / M2 < 1%, it can be avoided that the FEC content inside the wound core is too high, resulting in serious gas generation at high temperatures of the battery, thus leading to poor furnace temperature performance and high-temperature cycle performance of the battery; at the same time, when the ratio of M1 / M2 > 99%, it can be avoided that too much of the FEC content inside the wound core diffuses to the outside of the wound core, making the FEC content inside the wound core relatively low and unable to effectively improve the normal-temperature cycle performance of the battery.

[0053] In the present invention, the extracted FEC content is the mass content, that is, in the extracted electrolyte, the mass percentage of FEC.

[0054] In some embodiments, the values of M1 and M2 independently range from 0.5% to 30%, for example, they can independently be 0.5%, 1%, 5%, 8%, 10%, 12%, 15%, 16%, 18%, 20%, 22%, 25%, 28%, 30% or any value within the range composed of any two of the above values. Preferably, it is 5% - 20%. Further preferably, when the ratio of the FEC content M1 extracted for the first time to the FEC content M2 extracted for the second time is within the above range, the FEC content inside the core can be within a more appropriate range, which can not only facilitate the lithium-ion transport inside the core and improve the cycle performance of the battery, but also improve the high-temperature performance of the battery, and better balance the cycle performance and high-temperature performance of the battery.

[0055] In some embodiments, the pressure for pressurizing during the electrolyte extraction is 4 Kpa - 20 Kpa. When the pressurizing pressure is within the above range, it can avoid the situation that the pressure is too small to cause the FEC to not diffuse, affecting the accuracy of the M1 / M2 ratio, and can also avoid the safety problems of the battery caused by too high pressure.

[0056] In some embodiments, the volume of the electrolyte extracted for the first time and the volume of the electrolyte extracted for the second time independently range from 0.2 mL to 0.5 mL. The volumes of the electrolyte extracted for the first time and the second time can be the same or different, preferably the same. When the volumes of the electrolyte extracted for the first time and the second time are the same, the calculation of the M1 / M2 ratio can be more accurate, and it can better reflect the distribution of the FEC content in the battery.

[0057] In some embodiments, the method for testing the electrolyte content: The electrolyte extraction is carried out at 25°C and a humidity lower than 1%. The finished battery is fixed in a hydraulic press, and the pressure is gradually increased to 2 Mpa. Fresh electrolyte of 0.5 mL is extracted from the periphery of the battery (for example, a certain position is selected on the left and right edges of the core), and the FEC content is measured by gas chromatography GC. Subsequently, the pressure is continued to be increased to extract the electrolyte, 2 mL is extracted, and the FEC content is measured by GC.

[0058] In some embodiments, in the electrolyte, the content of fluoroethylene carbonate ranges from 0.5% to 30%, for example, it can be 1%, 5%, 10%, 15%, 20%, 25%, 30% or any value within the range composed of any two of the above values. Preferably, it is 2% - 20%. The test method for the FEC content includes: Pressurizing the battery to extract all the electrolyte for quantitative analysis of the FEC content.

[0059] When the content of FEC is unevenly distributed inside and outside the core, further adjusting the content of FEC in the electrolyte within the above range can maintain the content of FEC in the battery within a suitable range, ensuring sufficient FEC content inside the core to improve the cycle performance of the battery, while avoiding poor furnace temperature performance of the battery caused by too high FEC content, and taking into account the cycle performance and high temperature performance of the battery.

[0060] In some embodiments, in the negative electrode active material, the mass ratio of silicon element is 3%-50%, for example, it can be 3%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or any value within the range composed of any two of the above values. The higher the mass ratio of silicon element, the higher the silicon doping amount in the negative electrode, and the higher the risk of negative electrode expansion and cracking. More FEC is required to form a stable SEI film to inhibit the expansion and cracking of silicon. However, too high FEC content will cause relatively large expansion of the battery and poor high temperature performance of the battery. Adjusting the mass ratio of silicon element within the above range can effectively improve the energy density of the battery and take into account the high temperature performance of the battery.

[0061] Exemplarily, in the negative electrode active material, the mass ratio of silicon element can be obtained by acid-treating the negative electrode active material and then performing ICP analysis and calculation. Or the test method for the mass ratio of silicon element can be carried out by thermogravimetric analysis. For example, using a Shimadzu DTG-60 thermogravimetric analyzer for testing, the test conditions are: sample amount 5mg, air atmosphere, heating rate 10°C / min, heating from room temperature to 900°C and holding for 40min. The relationship between the mass ratio (x) of the silicon-carbon material and the final weight residue percentage (y) of the whole test is: x = 7y / 15.

[0062] In some embodiments, the silicon material includes at least one of silicon oxide, silicon carbon, nano-silicon, and silicon alloy.

[0063] In some embodiments, the negative electrode active material includes a carbon material, and the carbon material includes at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon, and soft carbon.

[0064] In some embodiments, the compaction density of the negative electrode sheet is 1.5 g / cm 3 -1.8 g / cm 3 , and the compaction density can be, for example, 1.5 g / cm 3 、1.55 g / cm 3 、1.6 g / cm 3 、1.65 g / cm 3 、1.7 g / cm 3 、1.75 g / cm 3 、1.8 g / cm 3Or any point value within the range formed by the above two point values. When the compaction density of the negative electrode sheet is within the above range, it can avoid the situation where the compaction density is too large, resulting in poor electrolyte infiltration and difficulty in meeting the requirement of a high FEC content inside the wound core, and the battery has poor cycling performance; when the compaction density is too low, the porosity of the negative electrode active layer is too large, which will reduce the actual contact area of the negative electrode material, thereby reducing the overall capacity of the battery and making it easy to deposit lithium during the cycling process.

[0065] In some embodiments, the negative electrode active layer includes a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The types of the negative electrode conductive agent and the negative electrode binder are not specifically limited, and conventional conductive agents and binder types in the art can be selected. The types of the negative electrode conductive agent include, but are not limited to, at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, and metal powder. The negative electrode binder includes, for example, but is not limited to, at least one of styrene-butadiene rubber latex, polytetrafluoroethylene latex, sodium carboxymethyl cellulose, sodium alginate, polyvinyl alcohol, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, and carboxylated chitosan.

[0066] In some embodiments, the battery includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector, and a positive electrode active layer provided on at least one surface of the positive electrode current collector, and the positive electrode active layer includes lithium cobaltate or a positive electrode active material commonly used in the art.

[0067] In some embodiments, the positive electrode active layer includes a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The positive electrode conductive agent is not specifically limited. For example, conventional conductive agents in the art can be selected, including but not limited to one or more of acetylene black, conductive carbon black, Ketjen black, conductive graphite, carbon nanotube, conductive carbon fiber, and graphene. The positive electrode binder is not specifically limited. For example, conventional binders in the art can be selected, including but not limited to one or more of polyvinylidene fluoride, polytetrafluoroethylene, sodium carboxymethyl cellulose, styrene-butadiene rubber, and polyethylene oxide.

[0068] In some embodiments, the type of the separator is not specifically limited. For example, conventional lithium-ion battery separators in the art can be selected, including but not limited to polyethylene microporous membrane, polypropylene microporous membrane, woven membrane, non-woven membrane (non-woven fabric), composite membrane, separator paper, and calendared membrane.

[0069] In some embodiments, the electrolyte includes an organic solvent, and the organic solvent includes at least one of carbonate and carboxylate. The carbonate solvents include one or more of fluorinated or unsubstituted ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate, diethyl carbonate (DEC), and ethyl methyl carbonate; the carboxylate solvents include one or more of fluorinated or unsubstituted propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isopentyl acetate, propyl propionate (PP), ethyl propionate (EP), methyl butyrate, and n-ethyl butyrate.

[0070] In some embodiments, the electrolyte includes an additive, and the additive includes one or more of vinylene carbonate, vinyl ethylene carbonate, ethylene sulfate, 1,4-dicyano-2-butene, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, sebaconitrile, 1,3,6-hexanetricarbonitrile, glycerol trinitrile, 1,2-bis(2-cyanoethoxy)ethane, 1,3-propane sultone, and allyl-1,3-sulfonic acid lactone. Preferably, the mass percentage of the additive in the total mass of the electrolyte is 0.1%-15%.

[0071] In some embodiments, the electrolyte includes an electrolyte lithium salt, and the electrolyte lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiPF2O2), lithium difluorobis(oxalato)phosphate (LiPF2(C2O4)2), lithium tetrafluoroxalate phosphate (LiPF4C2O4), lithium oxalate phosphate (LiPO2C2O4), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(fluorosulfonyl)imide (LiFSI). Preferably, the mass percentage of the electrolyte salt in the total mass of the electrolyte is 10%-15%.

[0072] The second aspect of the present invention provides a method for preparing a wound lithium-ion battery, including the following steps:

[0073] (1) Injecting the electrolyte for the first time, in the electrolyte, the content of fluorinated ethylene carbonate is 10%-30%, and the injection volume is 5 / 10 to 8 / 10 of the total volume of the electrolyte;

[0074] (2) Injecting the electrolyte for the second time, in the electrolyte, the content of fluorinated ethylene carbonate is 0%-5%, and the injection volume is 2 / 10 to 5 / 10 of the total volume of the electrolyte.

[0075] In some embodiments, the content of FEC in the first-injected electrolyte can be 10%, 12%, 14%, 15%, 16%, 18%, 20%, 22%, 25%, 26%, 28%, 30% or any value within the range composed of any two of the above values, preferably 12%-30%. The volume of the first-injected electrolyte can be 5 / 10, 6 / 10, 7 / 10, 8 / 10 or any value within the range composed of any two of the above values.

[0076] In some embodiments, the content of FEC in the second-injected electrolyte can be 0%, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5% or any value within the range composed of any two of the above values, preferably 0%-1%, more preferably the FEC content is 0. The volume of the second-injected electrolyte can be 2 / 10, 3 / 10, 4 / 10, 5 / 10 or any value within the range composed of any two of the above values.

[0077] In some embodiments, after the first injection of the electrolyte, the battery is sealed and aged, and then the second injection of the electrolyte is carried out, and then sealed and aged again.

[0078] The present invention proposes a method for secondary electrolyte injection. First, a high-concentration FEC electrolyte is injected at the initial stage of battery manufacturing, and aged for a certain period of time so that the electrolyte is fully absorbed by the electrode sheets. Then, after the battery formation is completed, a low-concentration or FEC-free electrolyte is injected. This method of secondary electrolyte injection makes the total amount of FEC in the battery relatively low, but the FEC concentration inside the battery (especially in the inner region of the wound core) is still relatively high, thus effectively improving the cycle performance of the battery. At the same time, due to the diffusibility of FEC at high temperatures, when measuring the furnace temperature in the fully charged state, the FEC inside the wound core will diffuse outward, improving the furnace temperature performance of the battery. The secondary electrolyte injection technology provided by the present invention extends the service life of the battery and also improves the stability of the battery under high-temperature conditions, providing a more reliable solution for the application of lithium batteries.

[0079] In some embodiments, the content of FEC in the first-injected electrolyte is 10%-30%, and the injection volume is 5 / 10-8 / 10 of the total electrolyte volume. The higher the mass ratio of silicon element in the negative electrode, the higher the concentration of FEC injected for the first time should be to ensure the cycle stability of the battery. It should be noted that after the battery formation is completed, FEC will be consumed to some extent, and the content of FEC after battery formation will be lower than that of the first-injected electrolyte.

[0080] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0081] For the materials, reagents, etc. used in the following embodiments, unless otherwise specified, they can all be obtained from commercial channels.

[0082] The present invention will be described in detail below with reference to specific embodiments, and these embodiments are for understanding rather than limiting the present invention.

[0083] Example 1-1:

[0084] 1. Preparation of the positive electrode sheet

[0085] Mix LiCoO2: conductive carbon black: PVDF in a mass ratio of 97:2:1, dissolve it in NMP, and then stir it on a magnetic stirrer for 12 h; uniformly coat the positive electrode paste on an aluminum foil with a thickness of 10 μm, and place it in an oven at 60 °C for drying for 24 h. Subsequently, cut the positive electrode into a size of 50 mm * 670 mm to obtain the positive electrode sheet.

[0086] 2. Preparation of the negative electrode sheet

[0087] Mix silicon-carbon material: graphite: conductive carbon black: SBR in a mass ratio of 5:93:1:1, dissolve them in deionized water, coat them on a copper foil with a thickness of 8 μm, and place it in an oven at 60 °C for drying for 24 h. Roll-press the negative electrode sheet, and the obtained compaction density is 1.71 g / cm 3 Subsequently, cut the negative electrode into a size of 53 mm * 674 mm to obtain the negative electrode sheet.

[0088] 3. Preparation of the separator

[0089] A 5-μm polyethylene substrate, with a 2-μm-thick ceramic coated on one side and a 1-μm-thick PVDF adhesive coated on the other side, and the ceramic corresponds to the positive electrode sheet.

[0090] 4. Preparation of the wound battery

[0091] After the above-mentioned separator is wound with the positive electrode sheet and the negative electrode sheet to form an electrode assembly, the electrode assembly is stacked and pressed for 100 seconds under a pressure of about 90 °C and 250 kgf using hot pressing. Subsequently, the electrode assembly is accommodated in an aluminum-plastic film.

[0092] 5. Preparation of the electrolyte:

[0093] Electrolyte 1: Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl propionate (EP) are mixed in a mass ratio of 1:1:1, and fluoroethylene carbonate (FEC) is added (mass fraction 15%); lithium hexafluorophosphate (LiPF6) with a concentration of 1 mol / L is added, and 1,3 - propane sultone (PS) with a mass fraction of 2% and 1,3,6 - hexane trinitrile with a mass fraction of 3% are added.

[0094] Electrolyte 2: Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl propionate (EP) are mixed in a mass ratio of 1:1:1; FEC is not contained, lithium hexafluorophosphate (LiPF6) with a concentration of 1 mol / L is added, and 1,3 - propane sultone (PS) with a mass fraction of 2% and 1,3,6 - hexane trinitrile with a mass fraction of 3% are added.

[0095] 6. Secondary injection of the battery

[0096] Inject 4.23 g of Electrolyte 1 into the battery with an aluminum - plastic film, then vacuum - seal it and age for 24 h. Apply a pressure of 1.0 Mpa at 80 °C and charge at 0.2C to 70% SOC for the first charge. Cut open the air - bag belt of the formed battery, then inject 2.11 g of Electrolyte 2, seal it, and continue to age for 16 h. After secondary sealing and sorting, the finished battery is obtained.

[0097] Battery performance test:

[0098] (1) Cycle performance test method: Charge at 1C to 4.53V, constant voltage to 0.125C, and discharge at 0.7C to 3V.

[0099] Capacity retention rate = discharge capacity per cycle / max (discharge capacity of three - cycle cycling).

[0100] Swelling rate = thickness of the fully - charged battery later (such as 50T, 100T, 150T, 200T, 300T...) / thickness of the sampled battery (50% SOC) - 1.

[0101] (2) Oven temperature test: Place the battery in the tester, raise the temperature to the specified temperature at a heating rate of 5 °C / min, then keep it at a constant temperature for 60 min, and observe whether the battery catches fire.

[0102] (3) SEI film thickness test: Charge at 1C to 4.53V, constant voltage to 0.125C, and discharge at 0.7C to 3V. After cycling 300 times, disassemble the battery, take out the negative electrode sheet, select the corresponding position on the negative electrode sheet, cut the cross - section for electron microscopy test, count the thickness of the SEI film on the surface of at least 30 - 40 silicon material particles in each corresponding area, and take the average value, which is recorded as the thickness of the SEI film.

[0103] Examples 1 and 2, and Comparative Examples 1 - 5 were carried out with reference to Example 1 - 1. The main differences are shown in Table 1. Among them, in Example 1, the mass ratio of FEC in electrolyte 1 was adjusted; in Example 2, in Examples 2 - 1 and 2 - 2, the mass ratio of the silicon-carbon material was adjusted; Example 2 - 3 was carried out with reference to Example 1 - 3 to adjust the mass ratio of the silicon-carbon material. In Comparative Example 1, only one injection was performed, that is, 6.34 g of electrolyte (the mass ratio of FEC was 10%) was injected into the battery; in Comparative Example 2, only one injection was performed, that is, 4.23 g of electrolyte (the mass ratio of FEC was 15%, and the injection mass was 2 / 3 of that in Example 1 - 1) was injected into the battery; in Comparative Examples 3 and 4, the mass ratio of FEC in electrolyte 1 was adjusted; in Comparative Example 5, 4.23 g of electrolyte 2 was first injected, and then 2.11 g of electrolyte 1 was injected (reverse injection).

[0104] Table 1

[0105]

[0106] Note: " / " indicates not tested.

[0107] As can be seen from Table 1, by adjusting the content of FEC in electrolyte 1 during secondary injection in the present invention to make the relationship between X1 and X2 satisfy X1 > X2, the content of FEC inside the winding core of the wound lithium-ion battery can be higher, and the content of FEC outside the winding core can be lower. The uneven distribution of FEC in the lithium battery can better inhibit the expansion and cracking of silicon, improve the cycle performance of the lithium battery, and balance the cycle performance and high-temperature performance of the battery.

[0108] Examples 3 and 4 were carried out with reference to Example 1 - 1. The main differences are shown in Table 2. Among them, in Example 3, the mass ratio of FEC in electrolyte 1 was adjusted; in Example 4, in Example 4 - 1, the volume of the first injection was increased, the FEC content was decreased, and the volume of the second injection was decreased; in Example 4 - 2, the volume of the first injection was decreased, the FEC content was increased, and the volume of the second injection was increased.

[0109] Table 2

[0110]

[0111] As can be seen from Table 2, when the difference between R1 and R2 is within the protection scope of the present invention, the FEC in the wound lithium-ion battery can be unevenly distributed. When the difference in the FEC content on the outer sides of the two arcs is within the above range, the cycle performance and furnace temperature performance of the battery can be improved.

[0112] Figure 3 The spectrum of the C-F peak of the first double-sided negative electrode arc segment in Example 1 - 1 is shown as follows Figure 4The figure shows the spectrogram of the C-F peak of the second double-sided negative electrode arc segment in Example 1-1. The ratio R1 of the C-F peak of the first double-sided negative electrode arc segment is greater than the ratio R2 of the C-F peak of the second double-sided negative electrode arc segment.

[0113] Groups 5 and 6 of the examples were carried out with reference to Example 1-1, and the main differences are shown in Table 3. The width of the first edge region and the width of the second edge region are 6 mm; the width of the middle region is 5 mm. In Group 5 of the examples, the mass proportion of FEC in electrolyte 1 was adjusted to change h1 and h2; in Group 6 of the examples, the compaction density of the negative electrode sheet was adjusted, and h1 and h2 would also change.

[0114] Table 3

[0115]

[0116] As can be seen from Table 3, by adjusting h1>h2 in the present invention, the content of FEC in the middle region of the negative electrode sheet can be made higher, and the content of FEC at the edge of the negative electrode sheet can be made lower. The uneven distribution of FEC in the lithium battery improves the cycle performance of the lithium battery; and improves the furnace temperature performance of the battery, enabling the battery to take into account both the cycle performance and the high-temperature performance.

[0117] Group 7 of the examples was carried out with reference to Example 1-1, and the main differences are shown in Table 4. In Group 7 of the examples, the mass proportion of FEC in electrolyte 1 was adjusted. The method for testing the electrolyte content twice: At 25 °C and a humidity lower than 1%, the electrolyte was extracted. The finished battery was fixed in a hydraulic press and gradually pressurized to 4 Mpa. For the first time, 0.2 mL of fresh electrolyte was extracted at a certain position on the left edge of the core (periphery of the battery), and the FEC content was measured by gas chromatography GC. Later, the pressure was continued to be increased to extract the electrolyte for the second time, and 0.2 mL was extracted, and the FEC content was measured by GC.

[0118] Table 4

[0119]

[0120] As can be seen from Table 4, when the ratio of the FEC content M1 extracted for the first time to the FEC content M2 extracted for the second time is within the limited range of the present invention, the FEC content inside the core can be in a more appropriate range, improving the cycle performance of the battery and also improving the high-temperature performance of the battery, enabling the battery to better take into account both the cycle performance and the high-temperature performance.

[0121] It should be noted that in this text, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not explicitly listed, or elements inherent to such a process, method, article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the presence of additional identical elements in the process, method, article or device comprising that element. In addition, it should be pointed out that the scope of the methods and devices in the embodiments of the present application is not limited to performing functions in the order shown or discussed, but may also include performing functions in a substantially simultaneous manner or in the reverse order according to the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. Additionally, features described with reference to certain examples may be combined in other examples.

[0122] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A wound lithium-ion battery, characterized in that: The battery comprises a negative electrode sheet and an electrolyte; The negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector; The negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a silicon material; the electrolyte includes fluoroethylene carbonate; When the battery is in a 0% SOC state, the negative electrode sheet A at the third fold at the head of the winding direction and the negative electrode sheet B at the first fold at the tail of the winding direction are subjected to an X-ray photoelectron spectroscopy test; the ratio of the peak area of ​​the CF peak in the negative electrode sheet A to the total peak area of ​​the negative electrode sheet A is recorded as X1, and the ratio of the peak area of ​​the CF peak in the negative electrode sheet B to the total peak area of ​​the negative electrode sheet B is recorded as X2; The relationship between X1 and X2 satisfies: X1>X2.

2. The wound lithium-ion battery according to claim 1, characterized in that: X1 satisfies: 1%≤X1≤15%; and / or, X2 satisfies: 0.5%≤X1≤12%.

3. The wound lithium ion battery according to claim 1, characterized in that: The wound lithium-ion battery comprises a first arc and a second arc, and a straight area connecting the first arc and the second arc; along the width direction of the wound lithium-ion battery, the negative electrode sheet comprises a first double-sided negative electrode arc segment and a second double-sided negative electrode arc segment away from the straight area; the first double-sided negative electrode arc segment and the second double-sided negative electrode arc segment are subjected to an X-ray photoelectron spectroscopy test, and the ratio of the peak area of ​​the CF peak in the first double-sided negative electrode arc segment to the total peak area of ​​the first double-sided negative electrode arc segment is recorded as R1, and the ratio of the peak area of ​​the CF peak in the second double-sided negative electrode arc segment to the total peak area of ​​the second double-sided negative electrode arc segment is recorded as R2, and the difference between R1 and R2 is 1%-8%, preferably 3%-5%; R1 satisfies: 1%≤R1≤15%, and R2 satisfies: 0.5%≤R2≤12%.

4. The wound lithium-ion battery according to claim 1, characterized in that: Along the width direction of the negative electrode sheet, the negative electrode sheet includes a first edge region, a second edge region, and a middle region between the first edge region and the second edge region; The thickness of the SEI film in the first edge region or the second edge region is recorded as h1, the thickness of the SEI film in the middle region is recorded as h2, and the ratio of h1 to h2 is 1.05-1.

6.

5. The wound lithium ion battery according to claim 4, characterized in that: The width of the first edge zone is 5 mm-10 mm, and / or the width of the second edge zone is 5 mm-10 mm; and / or the width of the middle zone is 4 mm-6 mm.

6. The wound lithium ion battery according to claim 1, characterized in that: The battery is pressurized to extract electrolyte, the FEC content in the electrolyte extracted for the first time is recorded as M1, and the FEC content in the electrolyte extracted for the second time is recorded as M2, and the ratio of M1 to M2 is 1%-99%.

7. The wound lithium ion battery according to claim 6, characterized in that: The pressurized pressure is 4Kpa-20Kpa; And / or, the volume of the electrolyte extracted for the first time and the volume of the electrolyte extracted for the second time are independently 0.2 mL-0.5 mL.

8. The wound lithium-ion battery according to any one of claims 1 to 7, characterized in that: In the electrolyte, the content of fluoroethylene carbonate is 0.5%-30%, preferably 2%-20%; And / or, in the negative electrode active material, the mass percentage of silicon element is 3%-50%; And / or, the silicon material includes at least one of silicon oxygen, silicon carbon, nano silicon, and silicon alloy.

9. The wound lithium-ion battery according to any one of claims 1 to 7, characterized in that: The negative electrode active material includes a carbon material, and the carbon material includes at least one of artificial graphite, natural graphite, mesophase carbon microbeads, hard carbon and soft carbon; And / or, the compaction density of the negative electrode sheet is 1.5 g / cm 3 -1.8g / cm 3 .

10. A method for preparing a wound lithium-ion battery according to any one of claims 1 to 9, characterized in that: The following steps are involved: (1) injecting an electrolyte for the first time, wherein the content of fluoroethylene carbonate in the electrolyte is 10% to 30%, and the injection volume is 5 / 10 to 8 / 10 of the total amount of the electrolyte; (2) injecting electrolyte for the second time, wherein the content of fluoroethylene carbonate in the electrolyte is 0%-5%, and the injection volume is 2 / 10 to 5 / 10 of the total amount of the electrolyte.

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