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; the electrolyte comprises FEC and DFEC; the battery is pressurized to extract electrolyte, the content of FEC in the electrolyte extracted for the first time is M1%, and the content of DFEC is marked as N1%; the content of FEC in the electrolyte extracted for the second time is M2%, and the content of DFEC is recorded as N2%; n1 is larger than N2, and M1 is smaller than M2. The FEC content inside the roll core is adjusted to be higher than the FEC content outside the roll core, the DFEC content outside the roll core is adjusted to be higher than the DFEC content inside the roll core, the FEC and the DFEC are unevenly distributed in the lithium battery, and the cycle performance of the lithium battery can be improved; meanwhile, furnace temperature failure caused by too high FEC content in the roll core is avoided, and the cycle performance and the high-temperature performance of the battery are both considered.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and particularly 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 experience 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, 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 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 performing pressure extraction of the electrolyte on the battery twice successively, when the content N1 of difluoroethylene carbonate (DFEC) extracted for the first time is greater than the content N2 of DFEC extracted for the second time, and the content M1 of fluoroethylene carbonate (FEC) extracted for the first time is less than the content M2 of FEC extracted for the second time, the content of FEC inside the wound core of the wound lithium-ion battery is relatively high, the content of FEC outside the wound core is relatively low, and the content of DFEC inside the wound core is relatively low, and the content of DFEC outside the wound core is relatively high. This non-uniform distribution of FEC and DFEC in the lithium battery can improve the cycle performance of the lithium battery; at the same time, it avoids the furnace temperature failure caused by too high a content of FEC inside the wound core, and due to the concentration difference, DFEC diffuses into the wound core prior to FEC to form a SEI film on the surface of the silicon-containing anode, reducing the consumption of FEC, further improving the cycle performance of the battery, and taking into account the high-temperature performance.

[0005] To achieve the above object, a first aspect of the present invention provides a wound lithium-ion battery, the battery comprising a negative electrode sheet and an electrolyte; the negative electrode sheet comprises a negative electrode current collector, and a negative electrode active layer provided on at least one 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 and difluoroethylene carbonate; the battery is pressurized to extract the electrolyte. In the electrolyte extracted for the first time, the content of fluoroethylene carbonate is denoted as M1%, and the content of difluoroethylene carbonate is denoted as N1%; in the electrolyte extracted for the second time, the content of fluoroethylene carbonate is denoted as M2%, and the content of difluoroethylene carbonate is denoted as N2%; wherein, N1 and N2 satisfy: N1 > N2; M1 and M2 satisfy: M1 < M2.

[0006] A second aspect of the present invention provides a method for preparing the wound lithium-ion battery, comprising the following steps:

[0007] (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 - 8 / 10 of the total volume of the electrolyte;

[0008] (2) Inject the electrolyte for the second time. In the electrolyte, the content of difluoroethylene carbonate is 3% - 9%, and the injection volume is 2 / 10 - 5 / 10 of the total volume of the electrolyte.

[0009] The present invention adopts the above technical solutions and has the following beneficial effects:

[0010] In the wound-type lithium-ion battery provided by the present invention, electrolyte is pumped out under pressure twice at certain positions inside the membrane shell and at the left and right edges of the wound core. That is, when the content N1 of DFEC extracted for the first time is greater than the content N2 of DFEC extracted for the second time, and the content M1 of FEC extracted for the first time is less than the content M2 of FEC extracted for the second time, the content of FEC inside the wound core of the wound-type lithium-ion battery is relatively high, the content of FEC outside the wound core is relatively low, the content of DFEC inside the wound core is relatively low, and the content of DFEC outside the wound core is relatively high. The relatively high content of FEC inside the wound core can make the SEI film formed on the surface of the silicon-containing negative electrode sheet inside the wound core thinner (there is a C-F bond in FEC, and the reduction potential is relatively high. A high concentration of FEC will form an elastic polymer containing many C-F bonds on the surface of silicon material particles, and the formed SEI film is thinner), with better quality, and the resistance of lithium ions to penetrate the SEI film is smaller, improving the cycle performance of the lithium battery; at the same time, it avoids the serious gas generation caused by too high content of FEC inside the wound core, resulting in furnace temperature failure, and takes into account the high-temperature performance of the battery; DFEC in the electrolyte has higher reactivity. As a film-forming additive, it can preferentially form an SEI film on the surface of the silicon-containing negative electrode sheet (the film-forming mechanism is similar to that of FEC); further, due to the uneven distribution of DFEC, the content of DFEC outside the wound core is relatively high, which can ensure the cycle performance of the area with a relatively low content of FEC outside the wound core (such as the edge of the wound core). At the same time, DFEC outside the wound core will also diffuse into the wound core, thereby reducing the film-forming consumption of FEC and further improving the cycle performance of the battery.

[0011] 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, data ranges include endpoints. Description of the Drawings

[0012] Figure 1 The figure shows a schematic structural diagram of a wound core in an example of the present invention. Detailed Embodiments

[0013] 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 for explaining and illustrating the present invention, and are not used to limit the present invention.

[0014] 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.

[0015] 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, referring to a wound lithium-ion battery, which generally includes a wound core (such as a positive electrode sheet, a negative electrode sheet, and a separator), a container (a case) for accommodating the wound core, and an electrolyte.

[0016] In a first aspect of the present invention, there is provided a wound lithium-ion battery, the battery including a negative electrode sheet and an electrolyte; 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; 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 and difluoroethylene carbonate; the battery is pressurized to extract the electrolyte. In the electrolyte extracted for the first time, the content of fluoroethylene carbonate is denoted as M1%, and the content of difluoroethylene carbonate is denoted as N1%; in the electrolyte extracted for the second time, the content of fluoroethylene carbonate is denoted as M2%, and the content of difluoroethylene carbonate is denoted as N2%; wherein, N1 and N2 satisfy: N1 > N2; M1 and M2 satisfy: M1 < M2.

[0017] In the wound lithium-ion battery provided by the present invention, the content of FEC inside the wound core is relatively high, the content of FEC outside the wound core is relatively low, and the content of DFEC inside the wound core is relatively low, and the content of DFEC outside the wound core is relatively high. The non-uniform distribution of FEC and DFEC 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 content of FEC inside the wound core will diffuse to the outside of the wound core, avoiding the furnace temperature failure caused by too high content of FEC inside the wound core. The relatively high content of DFEC outside the wound core will diffuse to the inside of the wound core, preferentially forming a SEI film on the surface of the silicon-containing negative electrode prior to FEC, reducing the consumption of FEC, further improving the cycle performance of the battery, and taking into account the high-temperature performance.

[0018] In some embodiments, the content of DFEC N1 extracted for the first time and the content of DFEC N2 extracted for the second time satisfy: 1 < N1 / N2 ≤ 10, and the value of N1 / N2 can be, for example, 1.001, 1.01, 1.1, 2, 4, 6, 8, 10, or any point value within the range composed of the above two values. When N1 / N2 satisfies the above range, the content of DFEC inside the wound core of the wound lithium-ion battery can be relatively low, and the content of DFEC outside the wound core of the wound lithium-ion battery can be relatively high. The non-uniform distribution of DFEC in the lithium battery can take into account the cycle performance and high-temperature performance of the battery; at the same time, when N1 / N2 ≤ 1, the content of DFEC outside the wound core is relatively lower than that inside the wound core, and the improvement of the battery cycle performance is not obvious; when N1 / N2 > 10, the content of DFEC outside the wound core is relatively too high compared to that inside the wound core, and the furnace temperature safety of the battery cannot be improved.

[0019] In some embodiments, the FEC content M1 extracted for the first time and the FEC content M2 extracted for the second time satisfy: 1% ≤ M1 / M2 ≤ 99%. The value of M1 / M2 can be, for example, the ratio of M1 to M2, such as 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 99%, or any value within the range formed by any two of the above values. Preferably, 30% ≤ M1 / M2 ≤ 90%. When the value of M1 / M2 satisfies the above range, the FEC content inside the core of the wound lithium-ion battery can be relatively high, while the FEC content outside the core of the wound lithium-ion battery can be relatively low, and the FEC is unevenly distributed in the lithium battery, which can take into account the cycle performance and high-temperature performance of the battery; at the same time, when M1 / M2 < 1%, the FEC content inside the core is too high relative to the FEC content outside the core, resulting in gas generation at high temperatures of the battery, and the improvement of the furnace temperature performance and high-temperature cycle performance of the battery is not obvious; at the same time, when M1 / M2 > 99%, the FEC content outside the core is too high relative to the FEC content inside the core, and the normal-temperature cycle performance of the battery cannot be effectively improved.

[0020] In the present invention, the extracted FEC content is a mass content, that is, in the extracted electrolyte, the mass percentage of FEC. Similarly, the extracted DFEC content is a mass content, that is, in the extracted electrolyte, the mass percentage of DFEC.

[0021] In some embodiments, the values of M1 and M2 are independently 0.5% - 30%, for example, they can be independently 0.5%, 1%, 5%, 8%, 10%, 12%, 15%, 16%, 18%, 20%, 22%, 25%, 28%, 30%, or any value within the range formed by 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 in a more appropriate range, which can not only facilitate the lithium-ion transmission inside the core, improve the cycle performance of the battery, but also improve the high-temperature performance of the battery, and better take into account the cycle performance and high-temperature performance of the battery.

[0022] In some embodiments, the values of N1 and N2 independently range from 0.1% to 5%, preferably from 0.1% to 3%. The values of N1 and N2 can be, for example, 0.1%, 0.2%, 0.5%, 0.75%, 0.95%, 1%, 1.2%, 1.5%, 1.6%, 1.7%, 2%, 2.2%, 2.5%, 3%, 4%, 5% or any value within the range formed by any two of the above values. Further preferably, when the ratio of the content of DFEC extracted for the first time (N1) to the content of DFEC extracted for the second time (N2) is within the above range, the content of DFEC inside the core can be within a more appropriate range, which can better reduce the consumption of FEC film formation inside the core while maintaining the battery cycling performance in the area with less FEC content outside the core, thereby further improving the cycling performance of the battery.

[0023] In some embodiments, the pressure for pressurizing during electrolyte extraction is 4 Kpa - 20 Kpa. For example, it can be 4 Kpa, 8 Kpa, 10 Kpa, 12 Kpa, 14 Kpa, 16 Kpa, 18 Kpa, 20 Kpa or any value within the range formed by any two of the above values.

[0024] 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. For example, it can be 0.2 mL, 0.3 mL, 0.4 mL, 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 ratios of N1 / N2 and M1 / M2 can be calculated more precisely, and the distribution of the contents of FEC and DFEC in the battery can be better reflected.

[0025] In some embodiments, the method for testing the electrolyte content: Electrolyte extraction is carried out at 25°C and with a humidity below 1%. The finished battery is fixed in a hydraulic press, and the pressure is gradually increased to 2 Mpa. Fresh electrolyte (for example, 0.2 mL is extracted) 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 / DFEC content is measured by gas chromatography GC. Subsequently, the pressure is further increased to extract more electrolyte (for example, 0.5 mL), and the FEC / DFEC content is measured by GC.

[0026] In some embodiments, 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; the relationship between X1 and X2 satisfies: X1 > X2.

[0027] In some embodiments, the wound-type 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 in the winding direction and the negative electrode sheet B at the first fold at the tail in the winding direction are as shown in the identification in Figure 1 . The negative electrode sheet A at the third fold at the head in the winding direction is located inside the core, and the negative electrode sheet B at the first fold at the tail in the winding direction is located outside the core.

[0028] In the wound-type lithium-ion battery of the present invention, the ratio X1 of the C-F peak in the negative electrode sheet A at the third fold at the head can represent the content of FEC and DFEC inside the core of the wound-type lithium-ion battery, and the ratio X2 of the C-F peak in the negative electrode sheet B at the first fold at the tail can represent the content of FEC and DFEC outside the core of the wound-type lithium-ion battery. On the basis of controlling that the ratio of the DFEC content N1 to the FEC content M1 in the first extraction is greater than the ratio of the DFEC content N2 to the FEC content M2 in the second extraction, further adjusting X1>X2 can make the sum of the FEC content and the DFEC content inside the core of the wound-type lithium-ion battery higher than the sum of the FEC content and the DFEC content outside the core. In this way, the SEI film formed inside the core can be made thinner and of better quality, and the resistance of lithium ions to penetrate the SEI film is smaller, improving the cycle performance of the lithium battery. At the same time, due to the uneven distribution of FEC, when the lithium battery is at a high temperature, it can avoid the furnace temperature failure caused by excessive gas generation due to too high FEC content inside the core; the uneven distribution of DFEC enables the DFEC outside the core to form a film on the silicon-containing negative electrode prior to FEC, reducing the consumption of FEC. Further, due to the simultaneous presence of DFEC and FEC, the relatively high DFEC concentration outside the core can also ensure the cycle performance of the area with a relatively low FEC content outside the core (such as the edge of the core), thereby further improving the cycle performance of the battery. By adjusting the uneven distribution of FEC and DFEC in the lithium battery, the present invention can take into account the cycle performance and high-temperature performance of the battery.

[0029] 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 in 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 content of FEC and DFEC in the test area. The larger the ratio of the peak area of the C-F peak, the higher the total content of FEC and DFEC can be represented.

[0030] In some embodiments, the ratio X1 of the peak area of the C-F peak in the negative electrode sheet A of the third fold of the head to the total peak area of the negative electrode sheet B 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%.

[0031] In some embodiments, the ratio X2 of the peak area of the C-F peak in the negative electrode sheet B of the first fold of the tail to the total peak area of the negative electrode sheet B satisfies: 0.8% ≤ X2 ≤ 14%. For example, X2 can be 0.8%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or any value within the range composed of any two of the above values. Preferably, 4% ≤ X2 ≤ 9%.

[0032] To a certain extent, the ratio of the peak area of the C-F peak in the XPS of the negative electrode sheet to the total peak area represents the uniformity of the SEI film. Under the condition of X1 > X2, when further adjusting the values of X1 and X2 within the above range, the contents of FEC and DFEC 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. The DFEC outside the core preferentially forms a film on the silicon-containing negative electrode inside the core rather than FEC, reducing the consumption of FEC, better inhibiting the swelling and cracking of silicon, and reducing the lithium-ion transmission distance, thereby improving the cycle performance of the battery. Also, it can prevent the content of FEC inside the core from being too high at high temperatures, further reducing the risk of battery furnace temperature failure. The simultaneous presence of DFEC and FEC, and the relatively high concentration of DFEC outside the core can also ensure the cycle performance of the area with a relatively low FEC content outside the core (such as the edge of the core), thus further improving the cycle performance of the battery.

[0033] 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; along the width direction of the wound lithium-ion battery, the negative electrode sheet includes a first double-sided negative electrode arc section and a second double-sided negative electrode arc section away from the straight section; X-ray photoelectron spectroscopy tests are performed on the first double-sided negative electrode arc section and the second double-sided negative electrode arc section. The ratio of the peak area of the C-F peak in the first double-sided negative electrode arc section to the total peak area of the first double-sided negative electrode arc section is denoted as R1, and the ratio of the peak area of the C-F peak in the second double-sided negative electrode arc section to the total peak area of the second double-sided negative electrode arc section is denoted as R2. The difference between R1 and R2 is 1% - 10% (the difference can be, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% or any value within the range composed of any two of the above values), preferably 1% - 5%.

[0034] 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 located on both sides, and a flat area 13 located in the middle and connecting the first arc 11 and the second arc 12. In the width direction of the wound battery 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 flat area 13. The first double-sided negative electrode arc 21 segment 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 on both sides of the arc.

[0035] 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 both surfaces of the negative electrode current collector in the negative electrode sheet of this arc segment.

[0036] 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 contents of FEC and DFEC 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 contents of FEC and DFEC on the outermost side of the second arc. The difference between R1 and R2 represents the difference in the total contents of FEC and DFEC on the outer sides of the arcs on both sides of the wound battery. On the basis of controlling that the ratio of the DFEC content N1 to the FEC content M1 extracted for the first time is greater than the ratio of the DFEC content N2 to the FEC content M2 extracted for the second time, when the difference between R1 and R2 is 1%-10%, the FEC and DFEC in the wound lithium-ion battery can be unevenly distributed. When the difference in the total contents of FEC and DFEC on the outer sides of the arcs on both sides is within the above range, the cycle performance and furnace temperature performance of the battery can be improved. If the difference in the total contents of FEC and DFEC on the outer sides of the arcs on both sides is too small (<1%), the FEC or DFEC in the wound lithium-ion battery is relatively evenly mixed. When the FEC content inside the core is high at high temperatures, some gas will be generated, and the low DFEC content inside the core cannot reduce the film-forming consumption of FEC, and the effect of improving the battery cycle performance is not obvious; if the difference in the total contents of FEC and DFEC on the outer sides of the arcs on both sides is too large (>10%), the main reason may be that the wettability during the first liquid injection is poor and the FEC content inside the core is low. At this time, the effect of improving the battery cycle performance is also not obvious.

[0037] 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 ≤ 18%. For example, R1 can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, or any value within the range formed by any two of the above values. Preferably, 7% ≤ R1 ≤ 10%.

[0038] 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 formed by any two of the above values. Preferably, 4% ≤ R2 ≤ 8%.

[0039] When the difference between R1 and R2 is 1% - 10%, further adjusting the values of R1 and R2 within the above ranges can further improve the cycling performance and furnace temperature performance of the battery.

[0040] In some embodiments, in the electrolyte, the content of fluoroethylene carbonate is 0.5% - 30%. For example, it can be 1%, 5%, 10%, 15%, 20%, 25%, 30%, or any value within the range formed by any two of the above values. Preferably, 8% - 25%. The test method for the FEC content includes: pressurizing the battery to extract all the added electrolyte for quantitative analysis of the FEC content.

[0041] When the FEC content is unevenly distributed inside and outside the core, further adjusting the FEC content in the electrolyte within the above ranges can maintain the FEC content in the battery within a suitable range, ensuring sufficient FEC content inside the core to improve the cycling performance of the battery, while avoiding poor furnace temperature performance of the battery due to too high FEC content, and taking into account both the cycling performance and high-temperature performance of the battery.

[0042] In some embodiments, based on the total mass of the electrolyte, the content of difluoroethylene carbonate is 0.1%-5%, for example, it can be 0.1%, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or any value within the range composed of any two of the above values. Further controlling the content of DFEC not to exceed 5% can ensure that DFEC can preferentially form a SEI film on the surface of the silicon-containing negative electrode, reduce the consumption of FEC, and at the same time avoid excessive gas generation in the battery caused by high-temperature DFEC, resulting in poor furnace temperature performance of the battery, thus taking into account the cycle performance and high-temperature performance of the battery. In addition, controlling the content of DFEC > 0.1% can avoid the complete consumption of FEC in the later stage of battery cycling. The test method for the content of DFEC includes: pressurizing the battery to extract all the added electrolyte for quantitative analysis of the content of DFEC.

[0043] 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 of the negative electrode, and the higher the risk of negative electrode swelling and cracking. More FEC is required to form a stable SEI film to inhibit the swelling and cracking of silicon. However, too high a content of FEC will cause greater swelling of the battery and poor high-temperature performance of the battery. Although DFEC can form a SEI film on the surface of the silicon-containing negative electrode and reduce the consumption of FEC, it can only be substituted in a small amount. Too high a content of DFEC will cause more serious gas generation in the battery, affecting the furnace temperature performance. Therefore, when adjusting the mass ratio of silicon element within the above range, the energy density of the battery can be effectively improved, and the high-temperature performance of the battery can be taken into account.

[0044] 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 tested by thermogravimetric analysis. For example, using a Shimadzu DTG-60 thermogravimetric analyzer for testing, the test conditions are: sample amount 5mg, with air as the atmosphere, heating rate 10°C / min, heating from room temperature to 900°C and holding for 40min. The relationship between the mass ratio of silicon element (x) and the final weight residue percentage (y) of the whole test is: x = 7y / 15.

[0045] In some embodiments, the silicon material includes at least one of silicon oxide, silicon carbide, nanosilicon, and silicon alloy.

[0046] 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.

[0047] In some embodiments, the tap density of the negative electrode sheet is 1.5 g / cm 3 -1.8 g / cm 3 , and the tap 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 3 or any value within the range composed of any two of the above values. When the tap density of the negative electrode sheet is within the above range, it is possible to avoid too large a tap density, poor infiltration of the electrolyte, and difficulty in meeting the requirement of a high FEC content inside the wound core, resulting in poor battery cycle performance; if the tap 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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 membranes, polypropylene microporous membranes, woven membranes, non-woven membranes (non-woven fabrics), composite membranes, separator papers, rolled membranes, etc.

[0052] In some embodiments, the electrolyte includes an organic solvent, and the organic solvent includes at least one of carbonate esters and carboxylic acid esters. The carbonate ester 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 carboxylic acid ester 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 ethyl n-butyrate.

[0053] In some embodiments, the electrolyte includes additives, and the additives include 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-sultone. Preferably, the mass percentage of the additives in the total mass of the electrolyte is 0.1%-15%.

[0054] 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 difluoro(oxalato)phosphate (LiPF2(C2O4)2), lithium tetrafluoro(oxalato)phosphate (LiPF4C2O4), lithium oxalato(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%.

[0055] The second aspect of the present invention provides a method for preparing a wound lithium-ion battery, including the following steps:

[0056] (1) Inject the electrolyte for the first time. In the electrolyte, the content of fluorinated ethylene carbonate is 10%-30%, and the injection volume is 5 / 10-8 / 10 of the total volume of the electrolyte;

[0057] (2) Inject the electrolyte for the second time. In the electrolyte, the content of difluorinated ethylene carbonate is 3%-9%, and the injection volume is 2 / 10-5 / 10 of the total volume of the electrolyte.

[0058] 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.

[0059] In some embodiments, the content of DFEC in the second-injected electrolyte can be 3%, 4%, 5%, 6%, 7%, 8%, 9% or any value within the range composed of any two of the above values. 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.

[0060] In some embodiments, in step (1), the first-injected electrolyte does not contain DFEC; in step (2), the second-injected electrolyte does not contain FEC.

[0061] The present invention provides a secondary liquid injection method. First, a high-concentration FEC electrolyte is injected at the initial stage of battery manufacturing, and after aging for a certain time, the electrolyte is fully absorbed by the electrode sheets. Then, after the battery formation is completed, an electrolyte containing a lower concentration of DFEC and no FEC is injected. This secondary liquid injection method results in a lower total amount of FEC in the battery, 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, when measuring the furnace temperature in the fully charged state, the FEC inside the wound core will diffuse to the outside of the wound core, avoiding the furnace temperature failure caused by excessive gas generation due to too high FEC content inside the wound core; in addition, due to the presence of DFEC in the second-injected electrolyte, as a film-forming additive, DFEC can also form a stable SEI film on the surface of the silicon-containing negative electrode, ensuring the cycle performance of the region with a lower FEC content outside the wound core (such as the edge region of the wound core); further, since the DFEC concentration outside the wound core is higher than that inside the wound core, DFEC will diffuse into the wound core and form an SEI film on the surface of the silicon-containing negative electrode prior to FEC, reducing the consumption of FEC, thereby comprehensively improving the cycle performance of the battery. The secondary liquid 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.

[0062] In some embodiments, electrolyte is injected for the first time. In the electrolyte, the content of fluoroethylene carbonate can be, for example, 10%, 15%, 20%, 25%, 30%, or any value within the range composed of any two of the above values. The injection volume is 5 / 10 to 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 is formed, FEC will be consumed to some extent, and the content of FEC after battery formation will be lower than that of FEC when the electrolyte is injected for the first time.

[0063] In some embodiments, electrolyte is injected for the second time. In the electrolyte, the content of difluoroethylene carbonate is 3% - 9%, for example, it can be 3%, 5%, 7%, 9%, or any value within the range composed of any two of the above values. The injection volume is 2 / 10 to 5 / 10 of the total electrolyte volume. In the electrolyte injected for the second time, the content of DFEC should not be too high. If it is too high (>9%), it will cause gas generation at high temperature. Nor should it be too low. If it is too low (<3%), it may cause problems such as rapid capacity decay and rapid increase in internal resistance of the battery when FEC is completely consumed in the later stage of cycling, resulting in a "dive" in the cycle performance of the battery (i.e., a significant decline in cycle performance).

[0064] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts belong to the scope of protection of the present invention.

[0065] The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels without special instructions.

[0066] The present invention will be described in detail below in conjunction with specific embodiments. These embodiments are for understanding rather than limiting the present invention.

[0067] Example 1-1:

[0068] 1. Preparation of the positive electrode sheet

[0069] 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.

[0070] 2. Preparation of the negative electrode sheet

[0071] Mix silicon-carbon anode, graphite, conductive carbon black, and SBR in a mass ratio of 7:91:1:1, dissolve them in deionized water, coat the solution 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 to obtain a compaction density of 1.71 g / cm 3 Subsequently, cut the negative electrode into a size of 53 mm * 674 mm to obtain a negative electrode sheet.

[0072] 3. Separator preparation

[0073] 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.

[0074] 4. Winding battery preparation

[0075] 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 at about 90 °C and a pressure of 250 kgf for 100 s using hot pressing. Subsequently, the electrode assembly is accommodated in an aluminum-plastic film.

[0076] 5. Electrolyte preparation:

[0077] Electrolyte 1: Ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl propionate (EP) are mixed in a mass ratio of 1:1:1, and FEC (mass fraction 15%) is added; LiPF6 (concentration 1 mol / L) is added, and 1,3-propane sultone PS (mass fraction 2%) and 1,3,6-hexanetricarbonitrile (mass fraction 3%) are added.

[0078] Electrolyte 2: Ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl propionate (EP) are mixed in a mass ratio of 1:1:1, and DFEC (mass fraction 6%) is added; LiPF6 (concentration 1 mol / L) is added, and 1,3-propane sultone PS (mass fraction 2%) and 1,3,6-hexanetricarbonitrile (mass fraction 3%) are added. Among them, electrolyte 2 does not contain FEC.

[0079] 6. Secondary electrolyte injection for the battery

[0080] Inject 4.23 g of electrolyte 1 into the battery with an aluminum-plastic film, then vacuum seal it and age it for 24 h. Apply a pressure of 1.0 Mpa at 80 °C and charge it at 0.2C to 70% SOC for the first charge. Cut open the airbag belt of the battery after formation, then inject 2.11 g of electrolyte 2, seal it, and continue to age it for 16 h. After secondary sealing and sorting, the finished battery is obtained.

[0081] Battery performance test:

[0082] (1) Cycle performance test method: Charge at 1C to 4.53V, hold at constant voltage until 0.125C, and discharge at 0.7C to 3V.

[0083] Capacity retention rate = discharge capacity per cycle / max(discharge capacity of three - cycle charge - discharge).

[0084] Swelling rate = thickness of the fully - charged battery in subsequent cycles (such as 50T, 100T, 150T, 200T, 300T...) / thickness of the sample battery (50% SOC) - 1.

[0085] (2) Furnace temperature test: Place the battery in the testing machine, raise the temperature to the specified temperature at a rate of 5°C / min, then hold the temperature constant for 60 min, and observe whether the battery catches fire.

[0086] Examples 1 - 2 groups and Comparative Examples 1 - 2 are carried out with reference to Example 1 - 1. The main differences are shown in Table 1. Among them, in the Example 1 group, the mass ratio of FEC injected for the first time is changed (the second liquid injection remains unchanged). Specifically, in Example 1 - 2, the mass ratio of FEC injected for the first time is about 12%; in Example 1 - 3, the mass ratio of FEC injected for the first time is about 23%; in Example 1 - 4, the mass ratio of FEC injected for the first time is about 11%. In the Example 2 group, the mass ratio of DFEC injected for the second time is changed (the first liquid injection remains unchanged). Specifically, in Example 2 - 1, the mass ratio of DFEC injected for the second time is about 10%; in Example 2 - 2, the mass ratio of DFEC injected for the second time is about 2.5%; in Example 2 - 3, the mass ratio of DFEC injected for the second time is about 3%; in Example 2 - 4, the mass ratio of DFEC injected for the second time is about 9%. In Comparative Example 1, only one liquid injection is carried out, and the mass ratio of FEC injected is about 10%. In Comparative Example 2, there are also two liquid injections. However, for the first injection, the electrolyte contains DFEC with a mass ratio of about 6%; for the second injection, the electrolyte contains FEC with a mass ratio of about 15%.

[0087] Table 1

[0088]

[0089]

[0090] Note: " / " indicates that the corresponding parameter is not tested or cannot be tested; the time interval between two extractions of the electrolyte is about 8 s, and about 0.2 mL of the electrolyte is extracted each time; the data are the 25°C cycle retention rate, 25°C thickness swelling rate, 45°C cycle retention rate, and 45°C thickness swelling rate at 400 cycles; the furnace temperature is the highest furnace temperature that the battery can pass.

[0091] As can be seen from Table 1, in the wound-type lithium-ion battery of the present invention, the content of FEC inside the wound core is relatively high, the content of FEC outside the wound core is relatively low, the content of DFEC inside the wound core is relatively low, and the content of DFEC outside the wound core is relatively high, which can improve the furnace temperature performance and cycling performance of the battery. In addition, in Comparative Example 2 in Table 1, since FEC was not contained in the first liquid injection, other solvent additives underwent a film-forming reaction during battery formation, and the formed SEI film had no elasticity and was easily broken, and lithium was easily deposited inside the battery. Subsequently, the battery experienced "diving" during the cycling process, so the battery data could not be measured.

[0092] Groups 3-4 of the examples were carried out with reference to Examples 1-1, and the main differences are shown in Table 2. Among them, the C-F peak ratio X1 of the negative electrode sheet at the third fold of the head and the C-F peak ratio X2 of the negative electrode sheet at the first fold of the tail in Group 3 of the examples were changed. In Group 4 of the examples, the mass proportion of silicon element in the negative electrode active material was changed.

[0093] Table 2

[0094]

[0095]

[0096] Note: The data are the 25°C cycling retention rate, 25°C thickness expansion rate, 45°C cycling retention rate, and 45°C thickness expansion rate at 400 cycles; the furnace temperature is the highest furnace temperature that the battery can pass through.

[0097] As can be seen from Table 2, by further controlling the magnitudes of the C-F peak ratio of the negative electrode sheet at the third fold of the head and the C-F peak ratio of the negative electrode sheet at the first fold of the tail, and controlling the mass proportion of silicon element in the negative electrode active material, the present invention can further balance the cycling performance and high-temperature performance of the battery.

[0098] Group 5 of the examples was carried out with reference to Examples 1-1, and the main differences are shown in Table 3. Among them, the ratio R1 of the C-F peak of the first arc and the ratio R2 of the C-F peak of the second arc in Group 5 of the examples were changed.

[0099] Table 3

[0100]

[0101] Note: The data are the 25°C cycling retention rate, 25°C thickness expansion rate, 45°C cycling retention rate, and 45°C thickness expansion rate at 400 cycles; the furnace temperature is the highest furnace temperature that the battery can pass through.

[0102] As can be seen from Table 3, by further adjusting the magnitudes of the ratio of the C-F peak of the first arc and the ratio of the C-F peak of the second arc, the present invention can further improve the cycling performance and furnace temperature performance of the battery.

[0103] The 6th group of examples was carried out with reference to Examples 1-1, and the main differences are shown in Table 4. Among them, the compaction density of the negative electrode was changed in the 6th group of examples.

[0104] Table 4

[0105]

[0106] Note: The data are the 25°C cycle retention rate, 25°C thickness expansion rate, 45°C cycle retention rate, and 45°C thickness expansion rate at 400 cycles; the furnace temperature is the highest furnace temperature that the battery can pass through.

[0107] As can be seen from Table 4, by further controlling the compaction density of the negative electrode, the present invention can avoid the situation where the compaction density of the negative electrode is too large, resulting in poor electrolyte infiltration and poor battery cycle performance; and avoid the situation where the compaction density of the negative electrode is too low, resulting in too large porosity of the negative active layer and reducing the overall capacity of the battery.

[0108] It should be noted that in this article, the terms "including", "comprising" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including one..." does not exclude the existence of another identical element in the process, method, article or device including 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, and may also include performing functions in a substantially simultaneous manner or in a reverse order according to the functions involved. For example, the described methods may be performed in an order different from that described, and various steps may be added, omitted, or combined. Additionally, the features described with reference to certain examples may be combined in other examples.

[0109] 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 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 and difluoroethylene carbonate; The battery is pressurized to extract electrolyte, and in the electrolyte extracted for the first time, the content of fluoroethylene carbonate is recorded as M1%, and the content of bisfluoroethylene carbonate is recorded as N1%; in the electrolyte extracted for the second time, the content of fluoroethylene carbonate is recorded as M2%, and the content of bisfluoroethylene carbonate is recorded as N2%; Among them, N1 and N2 satisfy: N1>N2; M1 and M2 satisfy: M1<M2.

2. The wound lithium-ion battery according to claim 1, characterized in that: N1 and N2 satisfy: 1<N1 / N2≤10; And / or, M1 and M2 satisfy: 1%≤M1 / M2≤99%, preferably 30%≤M1 / M2≤90%.

3. The wound lithium ion battery according to claim 1, 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.

4. The wound lithium-ion battery according to claim 1, characterized in that: 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.

5. The wound lithium ion battery according to claim 4, characterized in that: X1 satisfies: 1%≤X1≤15%; and / or, X2 satisfies: 0.8%≤X2≤14%.

6. 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%-10%, preferably 1%-5%; R1 satisfies: 1%≤R1≤18%, and R2 satisfies: 0.5%≤R2≤12%.

7. The wound lithium-ion battery according to any one of claims 1 to 6, characterized in that: Based on the total mass of the electrolyte, the content of fluoroethylene carbonate is 0.5%-30%, preferably 8%-25%; And / or, based on the total mass of the electrolyte, the content of the bisfluoroethylene carbonate is 0.1%-5%.

8. The wound lithium-ion battery according to any one of claims 1 to 6, characterized in that: 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 6, 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 bis(fluoroethylene carbonate) in the electrolyte is 3% to 9%, and the injection volume is 2 / 10 to 5 / 10 of the total amount of the electrolyte.

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