A winding type lithium ion battery and a method for manufacturing the same

Abstract

The present application relates to the technical field of lithium ion batteries, and provides a winding type lithium ion battery and a preparation method thereof, the battery comprising a negative electrode sheet and an electrolyte; the negative electrode sheet comprises a silicon material, and the electrolyte comprises fluoroethylene carbonate; when the battery is in a 0% SOC state, the X-ray photoelectron spectroscopy test is performed on the negative electrode sheet A in the third fold of the head in the winding direction and the negative electrode sheet B in the first fold of 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 X1, 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 X2; X1 and X2 satisfy: X1>X2. In the present application, the FEC content in the winding core is relatively high, the FEC content outside the winding core is relatively low, the FEC is unevenly distributed in the lithium battery, the cycle performance of the lithium battery can be improved, and the furnace temperature failure caused by the FEC content being too high in the winding core can be avoided, so that the battery can have both cycle performance and high-temperature performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium batteries, in particular to a wound lithium ion battery and a preparation method thereof. BACKGROUND

[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 negative materials, silicon materials have been widely concerned due to their high theoretical capacity. However, silicon materials will experience significant volume expansion during charging and discharging, which will cause the deformation of the battery structure, thereby seriously affecting the cycle stability of the battery. In order to solve this problem, researchers introduced fluoroethylene carbonate (FEC) as an additive, which 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 a content of FEC will cause the oven temperature failure problem of the battery under high temperature conditions. Therefore, how to balance the cycle performance and high temperature stability of the battery is a key problem in current battery technology. SUMMARY

[0004] The present application aims to overcome the above-mentioned problems existing in the prior art, and provides a wound lithium ion battery, by adjusting the relationship between the C-F peak ratio X1 in the third fold of the negative electrode sheet at the head of the winding core and the C-F peak ratio X2 in the first fold of the negative electrode sheet at the tail, the FEC content in the winding core is higher, the FEC content outside the winding core is lower, the FEC is unevenly distributed in the lithium battery, which can improve the cycle performance of the lithium battery; at the same time, the oven temperature failure caused by too high FEC content in the winding core is avoided, and the cycle performance and high temperature performance of the battery are considered.

[0005] In order to achieve the above-mentioned purpose, the present application provides a wound lithium ion battery in the first aspect, the battery comprises a negative electrode sheet and an electrolyte;

[0006] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer arranged on at least one side surface of the negative electrode current collector;

[0007] The negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon material; the electrolyte comprises fluoroethylene carbonate;

[0008] The battery is in the 0% SOC state, the X-ray photoelectron spectroscopy test is carried out on the negative plate A of the third fold of the head in the winding direction and the negative plate B of the first fold of the tail 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.

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

[0010] The second aspect of the present application provides a method for preparing the winding type lithium ion battery, comprising the following steps:

[0011] (1) the first injection of electrolyte, in the electrolyte, the content of fluoroethylene carbonate is 10%-30%, the injection volume is 5 / 10-8 / 10 of the total amount of electrolyte;

[0012] (2) the second injection of electrolyte, in the electrolyte, the content of fluoroethylene carbonate is 0%-5%, the injection volume is 2 / 10-5 / 10 of the total amount of electrolyte.

[0013] The present application has the following beneficial effects by adopting the above technical scheme:

[0014] The winding type lithium ion battery provided by the present application can make the FEC content in the winding core higher and the FEC content outside the winding core lower by limiting the relationship X1>X2, the FEC is unevenly distributed in the lithium battery, the SEI film formed in the winding core is thinner and better, the resistance of lithium ion crossing the SEI film is smaller, and the cycle performance of the lithium battery is improved; at the same time, due to the uneven distribution of FEC, the FEC content in the winding core under high temperature conditions will diffuse to the outside of the winding core, avoiding the serious gas production caused by the high FEC content in the winding core, which leads to the failure of the furnace temperature. The present application can balance the cycle performance and high temperature performance of the battery by adjusting the uneven distribution of FEC in the lithium battery.

[0015] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited. The ranges and values should be construed as having a range of values including the values recited. For values that are presented as a range, the endpoints of the range are included in the range. For values that are presented as a range, the endpoints of the range are included in the range. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The winding core structure is shown in an example of the present application.

[0017] Figure 2 Fig. 1 shows a structure diagram of a negative electrode sheet in an example of the present application.

[0018] Figure 3 Fig. 2 shows a C spectrum in a first double-sided negative electrode arc segment in an example of the present application.

[0019] Figure 4 Fig. 3 shows a C spectrum in a second double-sided negative electrode arc segment in an example of the present application.

[0020] Fig. 1 shows a structure diagram of a negative electrode sheet in an example of the present application. DETAILED DESCRIPTION

[0021] The specific embodiments of the present application are described in detail below. It should be understood that the specific embodiments described herein are merely intended to illustrate and explain the present application, and are not intended to limit the present application.

[0022] Unless otherwise defined, all scientific and technical terms used in the present application have the same meanings as commonly understood by one of ordinary skill in the art to which the present application pertains.

[0023] In the present application, the terms "winding type lithium ion battery", "battery", "lamination battery", "lithium battery", "lithium ion battery", "lithium ion secondary battery" all have the same meaning, and all refer to a winding type lithium ion battery, which generally includes a winding core (such as a positive electrode sheet, a negative electrode sheet, and a separator), a container (a shell) containing the winding core, and an electrolyte.

[0024] The present application provides a winding type lithium ion battery in a first aspect, the battery comprising a negative electrode sheet and an electrolyte;

[0025] The negative electrode sheet comprises a negative electrode current collector, and a negative electrode active layer arranged on at least one side surface of the negative electrode current collector;

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

[0027] The battery is subjected to X-ray photoelectron spectroscopy test at 0% SOC state, wherein the negative electrode sheet A at the third fold of the head of the winding direction and the negative electrode sheet B at the first fold of the tail of the winding direction are subjected to the X-ray photoelectron spectroscopy test; 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 of X1 and X2 satisfies: X1>X2.

[0029] In some embodiments, the winding type lithium ion battery comprises positive electrode sheets, separators and negative electrode sheets which are stacked and wound multiple turns along a winding direction to form a winding core. The third fold negative electrode sheet A at the head of the winding direction and the first fold negative electrode sheet B at the tail of the winding direction are as shown in the middle mark. Figure 1 The third fold negative electrode sheet A at the head of the winding direction is located inside the winding core, and the first fold negative electrode sheet B at the tail of the winding direction is located outside the winding core.

[0030] In the winding type lithium ion battery of the present application, the ratio X1 of the C-F peak in the third fold negative electrode sheet A at the head can represent the FEC content inside the winding core of the winding type lithium ion battery, and the ratio X2 of the C-F peak in the first fold negative electrode sheet B at the tail can represent the FEC content outside the winding core of the winding type lithium ion battery. By adjusting X1>X2, the FEC content inside the winding core of the winding type lithium ion battery can be higher, and the FEC content outside the winding core can be 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 in a high temperature condition, the FEC content inside the winding core that is too high will diffuse to the outside of the winding core, avoiding the failure of the oven temperature caused by the FEC content inside the winding core that is too high, 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 a semi-quantitative analysis. The X-ray photoelectron spectroscopy (XPS) is used to test the negative electrode sheet. The peak area of the C-F peak (carbon peak, binding energy between 290-291 eV) refers to the peak area of the C-F peak in the spectrum. The total peak area of the negative electrode sheet refers to the total area of all peaks appearing on the negative electrode sheet after XPS testing. All appearing peaks include but are not limited to 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 content of FEC in the test area. The larger the ratio of the peak area of the C-F peak, the higher the content of FEC.

[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 point value in the range formed by any two of the above point values, preferably 6%≤X1≤10%.

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

[0034] Under the condition that X1>X2, further adjusting the values ​​of X1 and X2 within the above range can make the FEC content inside and outside the core more suitable. This can make the SEI film formed by FEC inside the core thinner and denser, better suppressing silicon expansion and cracking, and reducing lithium-ion transport distance, thus further improving the cycle performance of the battery. It can also make FEC diffuse better from the inside to the outside of the core at high temperatures, further reducing the risk of battery furnace temperature failure.

[0035] In some embodiments, the wound lithium-ion battery includes a first arc and a second arc, and a flat region connecting the first arc and the second arc. In the width direction of the wound cell, the negative electrode includes a first double-sided negative electrode arc segment and a second double-sided negative electrode arc segment away from the flat region. The first double-sided negative electrode arc segment and the second double-sided negative electrode arc segment are subjected to X-ray photoelectron spectroscopy (XPS). 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 denoted as R1. 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 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 point value within the range of the above two points), preferably 3%-5%.

[0036] In some implementations, such as Figure 1 As shown, the core 1 includes a negative electrode sheet 2, a separator 3, and a positive electrode sheet 4, which are stacked and wound together. 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 located on both sides, and a flat region 13 located in the middle and connecting the first arc 11 and the second arc 12. In the width direction of the wound cell, 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 away from the flat region 13. The first double-sided negative electrode arc segment 21 is located on the first arc 11, and the second double-sided negative electrode arc segment 22 is located on 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 on both sides of the arc.

[0037] It needs to be explained that the "double-sided negative electrode circular arc segment" refers to a certain circular arc segment in the negative electrode sheet located in the circular arc region, and the surfaces on the opposite sides of the negative electrode current collector in the negative electrode sheet of the circular arc segment are both provided with a negative electrode active material layer.

[0038] Further, in the present application, the peak area ratio R1 of the C-F peak in the first double-sided negative electrode circular arc segment can represent the FEC content of the outermost side of the first circular arc, the peak area ratio R2 of the C-F peak in the second double-sided negative electrode circular arc segment can represent the FEC content of the outermost side of the second circular arc, and the difference between R1 and R2 represents the difference in FEC content of the outer sides of the circular arcs on both sides of the winding type battery. When the difference between R1 and R2 is within the range of 1% to 8%, the FEC in the winding type lithium ion battery can be unevenly distributed, and when the difference in FEC content of the outer sides of the circular 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 FEC content of the outer sides of the circular arcs on both sides is too small (<1%), the FEC in the winding type lithium ion battery is mixed more uniformly, and the FEC in the inner part of the winding core cannot be diffused at high temperature, which can cause serious gas production and lead to oven temperature failure. If the difference in FEC content of the outer sides of the circular arcs on both sides is too large (>8%), the first liquid injection wets poorly, the FEC content in the inner part of the winding core is low, and the cycle performance of the battery can be deteriorated.

[0039] In some embodiments, the ratio R1 of the peak area of the C-F peak in the first double-sided negative electrode circular arc segment to the total peak area of the first double-sided negative electrode circular 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 point value within the range formed by any two of the above point 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 circular arc segment to the total peak area of the second double-sided negative electrode circular 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 point value within the range formed by any two of the above point values, and preferably 4%≤R2≤8%.

[0041] Under the condition that the difference between R1 and R2 is within the range of 1% to 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 comprises a first edge region and a second edge region, and a middle region between the first edge region and the second edge region:

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

[0044] For example, the ratio of h1 to h2 can be 1.05, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6 or any point value within the range of the above pairs of point values.

[0045] It should be explained that "SEI film thickness" refers to the thickness of the SEI film on the surface of silicon material particles in the corresponding test area. The test method includes: selecting the corresponding position in the negative electrode, cutting a cross section for electron microscopy test, counting the thickness of the SEI film on the surface of at least 30-40 silicon material particles in each corresponding area, taking the average value, and recording it as the thickness of the SEI film.

[0046] In some implementations, such as Figure 2 As shown (the red dashed line in the middle is the center line of the negative electrode), along the width direction of the negative electrode, the negative electrode 2 includes a first side 23 close to the negative electrode tab 26 and a second side 24 away from the negative electrode tab 26. The first edge region 231 covers the first side 23 and extends toward the second side 24, the second edge region 241 covers the second side 24 and extends toward 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 application, the thickness h1 of the SEI film in the first edge region or the second edge region represents the SEI film thickness of the edge region of the negative electrode sheet, and the thickness h2 of the SEI film in the middle region represents the SEI film thickness of the middle region of the negative electrode sheet. The SEI film thickness is related to the FEC content, and the higher the FEC content, the thinner the SEI film formed (FEC has a C-F bond and a relatively high reduction potential, and a high FEC concentration can form an elastic polymer containing many C-F bonds on the surface of the silicon material particles, and the SEI film formed is relatively thin), and the lower the FEC content, the thicker the SEI film formed (the FEC content is relatively low, and other solvents can be decomposed to form the SEI film, and the SEI film is relatively thick). When h1>h2 and the ratio of h1 and h2 is within the above range, the FEC content in the middle region of the negative electrode sheet is relatively high, the FEC content at the edge of the negative electrode sheet is relatively low, and the FEC content in the inside of the core is relatively high, and the FEC content in the outside of the core is relatively low, which can make the FEC in the lithium battery unevenly distributed, the SEI film formed in the middle region of the negative electrode sheet and the inside of the core is thinner and better in quality, and the resistance of lithium ions passing through the SEI film is smaller, thereby improving the cycle performance of the lithium battery; and when the lithium battery is in a high-temperature condition, the FEC in the middle region of the negative electrode sheet and the inside of the core is more easily diffused to the edge of the negative electrode sheet and the outside of the core, thereby avoiding the failure of the furnace temperature caused by serious gas production due to the FEC content being too high in the middle region of the negative electrode sheet and the inside of the core. By adjusting the FEC content distribution in the middle region and the edge region of the negative electrode sheet, and in the inside and the outside of the core, the FEC in the lithium battery is unevenly distributed, and the cycle performance and high-temperature performance of the battery can be considered.

[0048] In some embodiments, the width of the first edge region is 5mm-10mm, for example, it can be 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, or any point value in the range composed of any two of the above point values. As shown in FIG. 1, the width of the first edge region can be interpreted as the size of the first edge region along the width direction of the negative electrode sheet. Figure 2

[0049] In some embodiments, the width of the second edge region is 5mm-10mm, for example, it can be 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, or any point value in the range composed of any two of the above point values. As shown in FIG. 1, the width of the second edge region can be interpreted as the size of the second edge region along the width direction of the negative electrode sheet. Figure 2

[0050] ​​In some embodiments, the width of the middle region is 4mm-6mm, for example, it can be 4mm, 4.2mm, 4.5mm, 4.8mm, 5mm, 5.2mm, 5.5mm, 5.8mm, 6mm or any point value in the range consisting of any two of the above point values. The middle region can be understood as a region with a vertical distance of 2mm-3mm above and below the center line of the length direction of the negative electrode sheet, and the width of the middle region can be explained as the vertical distance of 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 FEC content in the first extracted electrolyte is denoted as M1, the FEC content in the second extracted electrolyte is denoted as M2, and the ratio of M1 and M2 is 1%-99%. The ratio of M1 and M2 may, for example, be 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 in the range consisting of any two of the above point values, preferably 60%-80%.

[0052] Further, in the present application, electrolyte is extracted twice at a certain position inside the film shell and on both sides of the winding core. During the pressurization of the battery, FEC will diffuse into the battery film shell, reducing the FEC concentration inside the winding core and increasing the FEC concentration outside the winding core. When the ratio of the FEC content M1 of the first extraction and the FEC content M2 of the second extraction is in the range of 1%-99%, the FEC content inside the winding core of the wound lithium ion battery is higher, the FEC content outside the winding core of the wound lithium ion battery is lower, and the FEC is unevenly distributed in the lithium battery, which can balance the cycle performance and high temperature performance of the battery; at the same time, it avoids the case where the ratio of M1 / M2 is <1%, the FEC content inside the winding core is too high, causing the battery to produce gas seriously at high temperature, thereby causing the oven temperature performance and high temperature cycle performance of the battery to be too poor; at the same time, it avoids the case where the ratio of M1 / M2 is >99%, the FEC content inside the winding core diffuses more to the outside of the winding core, resulting in a lower FEC content inside the winding core, which cannot effectively improve the normal temperature cycle performance of the battery.

[0053] In the present application, the FEC content extracted is the mass content, i.e., the mass percentage content of FEC in the extracted electrolyte.

[0054] In some embodiments, M1 and M2 are independently 0.5% to 30%, for example, can be independently 0.5%, 1%, 5%, 8%, 10%, 12%, 15%, 16%, 18%, 20%, 22%, 25%, 28%, 30%, or any point value in the range consisting of any two of the above-mentioned point values, preferably 5% to 20%. It is further preferred that the ratio of the FEC content M1 of the first extraction and the FEC content M2 of the second extraction is in the above-mentioned range, which can make the FEC content in the core in a more suitable range, which can not only be beneficial to the transmission of lithium ions in 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 during extraction of the electrolyte is 4 KPa to 20 KPa. When the pressure is in the above-mentioned range, it can avoid too small pressure causing FEC to be unable to diffuse, affecting the accuracy of the ratio of M1 / M2, and also can avoid too large pressure causing safety problems of the battery.

[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 are independently 0.2 mL to 0.5 mL. The volume of the electrolyte extracted for the first time and the second time can be the same or different, preferably the same. The same volume of electrolyte extracted for the first and second time can make the ratio of M1 / M2 more accurate, and can better reflect the distribution of FEC content in the battery.

[0057] In some embodiments, the method for testing the content of the electrolyte: the extraction of the electrolyte is carried out at 25°C and humidity below 1%, the finished battery is fixed in a hydraulic machine, gradually pressurized to 2 Mpa, 0.5 mL of fresh electrolyte is extracted from the periphery of the battery (for example, a certain position is selected on the left and right side edges of the core), the FEC content is measured by gas chromatography GC, and the electrolyte is continuously extracted by further pressurization, and 2 mL is extracted, and the FEC content is measured by GC.

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

[0059] When the FEC content is unevenly distributed inside and outside the winding core, further adjusting the FEC content in the electrolyte to be within the above range can maintain the FEC content in the battery within a suitable range, which not only ensures that the FEC content inside the winding core is sufficient to improve the cycle performance of the battery, but also avoids too high FEC content causing poor furnace temperature performance of the battery, and balances the cycle performance and high temperature performance of the battery.

[0060] In some embodiments, the mass percentage of silicon in the negative electrode active material is 3% to 50%, for example, 3%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, or any point value in the range formed by any two of the above point values. The higher the mass percentage of silicon, the higher the amount of silicon doped in the negative electrode, and the higher the risk of expansion and cracking of the negative electrode. More FEC is needed to form a stable SEI film to inhibit the expansion and cracking of silicon, but too high FEC content will cause the battery to expand too much and have poor high temperature performance. Adjusting the mass percentage of silicon to be within the above range can effectively improve the energy density of the battery and balance the high temperature performance of the battery.

[0061] For example, the mass percentage of silicon in the negative electrode active material can be calculated by acid treatment of the negative electrode active material and then ICP analysis. Alternatively, the test method for the mass percentage of silicon can use a thermogravimetric analysis method, for example, using a Shimadzu DTG-60 thermogravimetric analyzer for testing, with the following test conditions: sample amount 5 mg, air atmosphere, heating rate 10℃ / min from room temperature to 900℃ and constant temperature for 40 min. The relationship between the mass percentage of silicon-carbon material (x) and the final weight residue percentage of the entire test (y) 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 1.5g / cm 3 -1.8g / cm 3 For example, the compaction density can be 1.5g / cm 3 , 1.55g / cm 3 , 1.6g / cm 3 , 1.65g / cm 3 , 1.7g / cm 3 , 1.75g / cm 3 , 1.8g / cm 3or any point value in the range formed by any two of the above point values. When the compaction density of the negative electrode sheet is within the above range, it can avoid the compaction density being too large, the electrolyte being not well infiltrated, and the requirement of high FEC content in the core being difficult to achieve, and the battery cycle performance being poor; and the compaction density being too low, the porosity of the negative electrode active layer being too large, the actual contact area of the negative electrode material being reduced, thereby reducing the overall capacity of the battery, and lithium being easily precipitated during the cycle process.

[0065] In some embodiments, the negative electrode active layer comprises a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder. The type of negative electrode conductive agent and negative electrode binder is not specifically limited, and conventional conductive agents and binders in the art can be selected. The type of negative electrode conductive agent includes, but is 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 emulsion, polytetrafluoroethylene emulsion, sodium carboxymethyl cellulose, sodium alginate, polyvinyl alcohol, polyacrylic acid, lithium polyacrylate, sodium polyacrylate, and carboxylated chitosan.

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

[0067] In some embodiments, the positive electrode active layer comprises a positive electrode active material, a positive electrode conductive agent, and a positive electrode binder. The type of positive electrode conductive agent is not specifically limited, and 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 type of positive electrode binder is not specifically limited, and 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 separator is not specifically limited, and 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, calendered membrane, and the like.

[0069] In some embodiments, the electrolyte comprises an organic solvent, the organic solvent comprising at least one of carbonates, carboxylic acid esters. Carbonate solvents include one or more of fluorinated or unsubstituted ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate, diethyl carbonate (DEC), methyl ethyl carbonate; carboxylic acid ester solvents include one or more of fluorinated or unsubstituted propyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, propyl propionate (PP), ethyl propionate (EP), methyl butyrate, ethyl n-butyrate.

[0070] In some embodiments, the electrolyte comprises an additive, the additive comprising one or more of vinylene carbonate, vinyl ethylene carbonate, vinyl sulfate, 1,4-dicyano-2-butene, butanedinitrile, pentanedinitrile, hexanedinitrile, heptanedinitrile, octanedinitrile, nonanedinitrile, 1,3,6-hexanetrinitrile, glycerol trinitrate, 1,2-bis(2-cyanoethoxy)ethane, 1,3-propane sultone, and propenyl-1,3-sultone. Preferably, the additive has a mass percentage of 0.1%-15% of the total mass of the electrolyte.

[0071] In some embodiments, the electrolyte comprises an electrolyte lithium salt, the electrolyte lithium salt comprising at least one of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiPF2O2), lithium difluorobisoxalate phosphate (LiPF2(C2O4)2), lithium tetrafluoroxalate phosphate (LiPF4C2O4), lithium oxalate phosphate (LiPO2C2O4), lithium bisoxalate borate (LiBOB), lithium difluorooxalate borate (LiODFB), lithium tetrafluoroborate (LiBF4), lithium bisfluorosulfonylimide (LiTFSI), and lithium bisfluorosulfonylimide (LiFSI). Preferably, the electrolyte salt has a mass percentage of 10%-15% of the total mass of the electrolyte.

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

[0073] (1) first injecting an electrolyte, the electrolyte having a content of fluorinated ethylene carbonate of 10%-30%, and the injection volume being 5 / 10-8 / 10 of the total volume of the electrolyte;

[0074] (2) second injecting an electrolyte, the electrolyte having a content of fluorinated ethylene carbonate of 0%-5%, and the injection volume being 2 / 10-5 / 10 of the total volume of the electrolyte.

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

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

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

[0078] The present application proposes a secondary injection method, which first injects a high-concentration FEC electrolyte at the initial stage of battery manufacturing, and ages for a certain period of time to allow the electrolyte to be fully absorbed by the electrode sheet. Then, after the completion of battery formation, a low-concentration or FEC-free electrolyte is injected. This secondary injection method allows the total amount of FEC in the battery to be low, but the FEC concentration in the internal region of the battery (especially the internal region of the winding core) is still high, thereby effectively improving the cycle performance of the battery. At the same time, due to the diffusion of FEC at high temperatures, when the oven temperature is measured at the full charge state, the FEC in the internal region of the winding core will diffuse outward, thereby improving the oven temperature performance of the battery. The secondary injection technology provided by the present application prolongs the service life of the battery and also improves the stability of the battery under high-temperature conditions, thereby providing a more reliable solution for the application of lithium batteries.

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

[0080] With reference to the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the scope of the present application.

[0081] The materials, reagents and the like used in the following examples can be obtained from commercial channels unless otherwise specified.

[0082] The present application will be described in detail below with reference to specific examples, which are used to understand rather than limit the present application.

[0083] Example 1-1:

[0084] 1. Preparation of positive electrode sheet

[0085] LiCoO2: conductive carbon black: PVDF were mixed in a mass ratio of 97:2:1, dissolved in NMP, and then stirred on a magnetic stirrer for 12 h; the positive electrode paste was uniformly coated on an aluminum foil with a thickness of 10 μm, and dried in an oven at 60°C for 24 h. Then the positive electrode was cut into a size of 50 mm*670 mm to obtain the positive electrode sheet.

[0086] 2. Preparation of negative electrode sheet

[0087] Silicon-carbon material: graphite: conductive carbon black: SBR were mixed in a mass ratio of 5:93:1:1, and they were dissolved in deionized water and coated on a copper foil with a thickness of 8 μm, and dried in an oven at 60°C for 24 h. The negative electrode sheet was rolled to obtain a compacted density of 1.71 g / cm 3 Then the negative electrode was cut into a size of 53 mm*674 mm to obtain the negative electrode sheet.

[0088] 3. Preparation of separator

[0089] 5 μm polyethylene base material, one side of the base material coated with 2 μm thick ceramic, the other side coated with 1 μm thick PVDF glue, the ceramic corresponding to the positive electrode sheet.

[0090] 4. Preparation of wound battery

[0091] The above separator was wound with the positive electrode sheet and the negative electrode sheet to form an electrode assembly, and then the electrode assembly was stacked and pressed using hot pressing at a pressure of about 90°C and 250 kgf for 100 seconds. Then, the electrode assembly was accommodated in an aluminum plastic film.

[0092] 5. Preparation of electrolyte:

[0093] Electrolyte 1: ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl propionate (EP) are mixed in a mass ratio of 1:1:1, FEC (15% by mass) is added; LiPF6 (concentration of 1 mol / L) is added, 1,3-propane sultone PS (2% by mass) and 1,3,6-hexane trinitrile (3% by mass) 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; no FEC is added, LiPF6 (concentration of 1 mol / L) is added, 1,3-propane sultone PS (2% by mass) and 1,3,6-hexane trinitrile (3% by mass) are added.

[0095] 6, battery secondary injection

[0096] 4.23g of electrolyte 1 is injected into a battery with an aluminum plastic film, and then vacuum sealed and aged for 24h. Apply a pressure of 1.0Mpa at 80℃, charge to 70% SOC at 0.2C for the first charge. The air bag of the formed battery is cut open, then 2.11g of electrolyte 2 is injected, sealed and continued to age for 16h. Two seals, after sorting, the finished battery is obtained.

[0097] Battery performance test:

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

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

[0100] Swelling rate = thickness of subsequent full battery (such as 50T, 100T, 150T, 200T, 300T…) / thickness of the sample battery (50% SOC) -1.

[0101] (2) Furnace temperature test: place the battery in the test machine, increase the temperature to the specified temperature at a rate of 5℃ / min, then keep the temperature constant for 60min, and observe whether the battery catches fire.

[0102] (3) SEI film thickness test, 1C charge to 4.53V, constant voltage to 0.125C, discharge 0.7C discharge to 3V, after 300 cycles, disassemble the battery, take out the negative electrode sheet, select the corresponding position in the negative electrode sheet, cut the cross section for electron microscope test, count the thickness of SEI film on the surface of at least 30-40 silicon material particles in each corresponding area, take the average value, and record it as the thickness of SEI film.

[0103] Examples 1 group and 2 group, and Comparative Examples 1-Comparative Examples 5 refer to Example 1-1, the main difference is shown in Table 1. Among them, the mass percentage of FEC in electrolyte 1 is adjusted in Example 1 group; the mass percentage of silicon-carbon material is adjusted in Example 2-1 and Example 2-2 in Example 2 group; the mass percentage of silicon-carbon material is adjusted in Example 2-3 referring to Example 1-3. In Comparative Example 1, only one injection is performed, i.e. 6.34g of electrolyte (10% of the mass percentage of FEC) is injected into the battery; in Comparative Example 2, only one injection is performed, i.e. 4.23g of electrolyte (15% of the mass percentage of FEC, the injection mass is 2 / 3 of Example 1-1) is injected into the battery; the mass percentage of FEC in electrolyte 1 is adjusted in Comparative Examples 3 and 4; in Comparative Example 5, 4.23g of electrolyte 2 is first injected, and then 2.11g of electrolyte 1 is injected (reverse injection).

[0104] Table 1

[0105]

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

[0107] As can be seen from Table 1, the FEC content in electrolyte 1 is adjusted in the secondary injection, so that the relationship between X1 and X2 satisfies X1>X2, which can make the FEC content in the winding core of the winding type lithium ion battery higher and the FEC content outside the winding core lower, and the FEC is unevenly distributed in the lithium battery, which can better inhibit the expansion and cracking of silicon and improve the cycle performance of the lithium battery, and the cycle performance and high temperature performance of the battery are considered.

[0108] Examples 3 group and 4 group refer to Example 1-1, the main difference is shown in Table 2. Among them, the mass percentage of FEC in electrolyte 1 is adjusted in Example 3 group; in Example 4-1 in Example 4 group, the first injection volume is increased and the FEC content is reduced, and the second injection volume is reduced.

[0109] Table 2

[0110]

[0111] As can be seen from Table 2, when the difference between R1 and R2 is within the protection range of the application, the FEC in the winding type lithium ion battery can be unevenly distributed, and when the difference in the FEC content outside the two side 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-face negative electrode arc segment in Example 1-1 is shown in FIG. 1, Figure 4The spectrum of the C-F peak of the second double-sided negative electrode arc segment in Example 1-1 is shown. 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] Examples 5 and 6 are carried out with reference to Example 1-1, and the main difference is shown in Table 3. The first edge zone width and the second edge zone width are 6 mm; the middle zone width is 5 mm. In Example 5, the mass fraction of FEC in electrolyte 1 is adjusted to change h1 and h2; in Example 6, the compaction density of the negative electrode sheet is adjusted, and h1 and h2 will also change.

[0114] Table 3

[0115]

[0116] As can be seen from Table 3, adjusting h1>h2 can make the FEC content in the middle region of the negative electrode sheet higher, the FEC content in the edge of the negative electrode sheet lower, the FEC in the lithium battery unevenly distributed, improve the cycle performance of the lithium battery; and improve the furnace temperature performance of the battery, so that the battery takes into account the cycle performance and high temperature performance.

[0117] Example 7 is carried out with reference to Example 1-1, and the main difference is shown in Table 4. In Example 7, the mass fraction of FEC in electrolyte 1 is adjusted. The method of twice electrolyte content testing: at 25℃, and under the condition of humidity less than 1%, electrolyte extraction is carried out, the finished battery is fixed to the hydraulic machine, and gradually pressurized to 4Mpa, the fresh electrolyte 0.2mL is extracted for the first time at the periphery of the battery (select a position at the left edge of the winding core), and the FEC content is measured by gas chromatography GC, and the electrolyte is extracted again under continued pressure, and 0.2mL is extracted, and the FEC content is 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 and the FEC content M2 extracted for the second time is within the range defined in the application, the FEC content in the winding core can be within a more suitable range, the cycle performance of the battery can be improved, and the high temperature performance of the battery can also be improved, so that the battery better takes into account the cycle performance and high temperature performance.

[0121] It is to be understood that the terminology "including", "containing" or any other variation thereof does not exclude the presence of other elements or steps than those listed in the process, method, article, or apparatus. It is further understood that the steps and elements recited in any of the examples herein can be combined, removed or arranged in various ways without departing from the scope of the application. Further, the features described in relation to one example can be combined with features described in relation to other examples.

[0122] The above description is merely illustrative of the application, and is not to be taken in a limiting sense. Any modification, equivalent replacement, or the like which does not depart from the spirit and principle of the application is intended to be included in the scope of the application.

Claims

1. A wound-type lithium ion battery, characterized by comprising: 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 arranged on at least one side surface of the negative electrode current collector; The negative electrode active layer comprises a negative electrode active material, and the negative electrode active material comprises a silicon material; the electrolyte comprises fluoroethylene carbonate; When the battery is in a 0% SOC state, the negative electrode sheet A in the third fold of the head in the winding direction and the negative electrode sheet B in the first fold of the tail in the winding direction are subjected to X-ray photoelectron spectroscopy test; 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; The relationship between X1 and X2 satisfies: X1>X2; X1 satisfies: 1%≤X1≤15%, and X2 satisfies: 0.5%≤X1≤12%. The electrolyte comprises an organic solvent, and the organic solvent is selected from at least one of carbonates and carboxylic acid esters; the carbonate solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; and the carboxylic acid ester solvent is selected from one or more of propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isoamyl acetate, propyl propionate, ethyl propionate, methyl butyrate, and ethyl n-butyrate. The electrolyte comprises an additive, and the additive is selected from one or more of vinylene carbonate, vinyl ethylene carbonate, vinyl sulfate, 1,4-dicyano-2-butene, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, 1,3,6-hexanetricarbonitrile, glycerol tricarbonitrile, 1,2-bis(2-cyanoethoxy)ethane, 1,3-propane sultone, and propylene-1,3-sulfone.

2. The wound-type lithium-ion battery according to claim 1, characterized by The winding type lithium ion battery comprises a first circular arc and a second circular arc, and a flat area connecting the first circular arc and the second circular arc; along the width direction of the winding type lithium ion battery, the negative electrode sheet comprises a first double-sided negative electrode circular arc segment and a second double-sided negative electrode circular arc segment away from the flat area; the first double-sided negative electrode circular arc segment and the second double-sided negative electrode circular arc segment are subjected to X-ray photoelectron spectroscopy test, the ratio of the peak area of the C-F peak in the first double-sided negative electrode circular arc segment to the total peak area of the first double-sided negative electrode circular arc segment is denoted as R1, the ratio of the peak area of the C-F peak in the second double-sided negative electrode circular arc segment to the total peak area of the second double-sided negative electrode circular arc segment is denoted as R2, and the difference between R1 and R2 is 1%-8%, R1 satisfies: 1%≤R1≤15%, and R2 satisfies: 0.5%≤R2≤12%.

3. The wound-type lithium-ion battery according to claim 2, wherein The difference between R1 and R2 is 3%-5%.

4. The wound-type lithium-ion battery according to claim 1, wherein Along the width direction of the negative electrode sheet, the negative electrode sheet comprises a first edge area and a second edge area, and a middle area between the first edge area and the second edge area; The thickness of the SEI film in the first edge area or the second edge area is denoted as h1, the thickness of the SEI film in the middle area is denoted as h2, and the ratio of h1 to h2 is 1.05-1.

6.

5. The wound-type lithium-ion battery according to claim 4, wherein The width of the first edge area is 5mm-10mm, and / or the width of the second edge area is 5mm-10mm, and / or the width of the middle area is 4mm-6mm.

6. The wound-type lithium-ion battery according to claim 1, wherein The battery is extracted under pressure, the content of FEC in the first extracted electrolyte is recorded as M1, the content of FEC in the second extracted electrolyte is recorded as M2, and the ratio of M1 and M2 is 1%-99%.

7. The wound-type lithium-ion battery according to claim 6, wherein The pressure of the pressure is 4Kpa-20Kpa; And / or, the volume of the first extracted electrolyte and the volume of the second extracted electrolyte are independently 0.2mL-0.5mL.

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

9. The wound-type lithium-ion battery according to any one of claims 1 to 7, wherein The content of fluoroethylene carbonate in the electrolyte is 2%-20%.

10. The wound-type lithium-ion battery according to any one of claims 1 to 7, wherein 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. and / or the compacted density of the negative electrode sheet is 1.5 g / cm 3 -1.8 g / cm 3 .

11. A method of manufacturing the wound-type lithium-ion battery according to any one of claims 1 to 10, characterized by, The method comprises the following steps: (1) First injection of electrolyte, the content of fluoroethylene carbonate in the electrolyte is 10%-30%, the injection volume is 5 / 10~8 / 10 of the total amount of electrolyte; (2) After the first injection of electrolyte is completed, the battery is aged and formed; (3) Second injection of electrolyte, the content of fluoroethylene carbonate in the electrolyte is 0%-5%, the injection volume is 2 / 10~5 / 10 of the total amount of electrolyte.

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