Lithium-ion secondary battery

Abstract

The present invention relates to the technical field of lithium batteries. Provided is a lithium-ion secondary battery. In the lithium-ion secondary battery, a negative electrode sheet comprises a negative electrode active material, which comprises a carbon material and a silicon material, wherein the silicon material comprises a silicon-carbon composite material, the mass percentage content of the silicon-carbon composite material is denoted as b on the basis of the total mass of the negative electrode active material, and the ratio of the mass percentage content x of silicon in a first region of the silicon-carbon composite material to the mass percentage content y of silicon in a second region is denoted as B, where 0<B<1; and an electrolyte comprises lithium bis(fluorosulfonyl)imide and fluoroethylene carbonate, wherein on the basis of the total mass of the electrolyte, the mass percentage content of the lithium bis(fluorosulfonyl)imide is denoted as m1, and the mass percentage content of the fluoroethylene carbonate is denoted as m2. When B, b, m1 and m2 meet the relational expression [(m1+m2) / B]*b≥0.2, the battery is enabled to have a high energy density, a low thickness expansion rate and fast charging capability.

Description

A lithium ion secondary battery

[0001] The present application claims priority to the Chinese patent application No. 202411326144.6, filed on September 23, 2024, and entitled "A lithium ion secondary battery", the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application relates to the technical field of lithium batteries, in particular to a lithium ion secondary battery. BACKGROUND

[0003] The development background of high-voltage lithium ion battery electrolyte is mainly due to the pursuit of higher energy density and higher safety performance. With the development of technology and the improvement of market demand, the application field of lithium ion batteries is expanding, from consumer electronic products to electric vehicles, energy storage systems and other fields, and the requirements for battery performance are also getting higher and higher. The negative active material in the current commercial lithium ion battery is mainly graphite material, but the actual specific capacity of the existing graphite material is relatively low (within 360 mAh / g) and has approached its theoretical limit (372 mAh / g), and the development space is limited. Silicon-based materials have a very high theoretical specific capacity (4200 mAh / g), and are one of the ideal materials for further improving the energy density of lithium ion batteries. However, the existing silicon-based materials have a large particle volume expansion during charging and discharging, which makes the volume energy density of lithium ion batteries relatively low, and easily causes fast cycle decay and large volume change rate of the battery; at the same time, the lithium ion diffusion coefficient of silicon-based materials is low, which limits the rapid charging capacity of silicon-containing lithium ion batteries.

[0004] With the continuous increase of voltage (>4.55V), the oxidation of oxygen released by the positive active material to the electrolyte is continuously intensified, leading to solvent decomposition, gas generation, and decomposition of lithium salt, which will also increase, generating more HF, destroying the SEI film (solid electrolyte interface film) and corroding the electrode material; at the same time, the transition metal ions of the positive electrode are continuously dissolved and migrated to the negative electrode surface, causing certain damage to the negative electrode interface film.

[0005] Therefore, the development background of lithium ion battery electrolyte is closely related to the urgent demand of today's society for efficient, environmentally friendly and safe energy storage and conversion technology. SUMMARY

[0006] The present application aims to overcome the above problems existing in the prior art, and provides a lithium ion secondary battery, wherein a silicon-carbon composite material is added in a negative electrode sheet, the silicon content in the silicon-carbon composite material is gradiently distributed from the center to the surface layer, and an electrolyte containing lithium bisfluorosulfonylimide and fluoroethylene carbonate is used in combination, so that when the mass percentage content of lithium bisfluorosulfonylimide, the mass percentage content of fluoroethylene carbonate, the mass percentage content of the silicon-carbon composite material, and the mass percentage content ratio of silicon elements in the first region and the second region of the silicon-carbon composite material satisfy a specific relationship, the dissolution problem of the positive electrode material at high voltage can be solved, the cycle stability and high-temperature storage performance of the battery at high voltage are improved, and high energy density, low thickness expansion rate, and fast charging capability are considered.

[0007] To achieve the above-mentioned purpose, the present application provides a lithium ion secondary battery, which comprises a positive electrode sheet, a negative electrode sheet, and an electrolyte.

[0008] The negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises a carbon material and a silicon material; the silicon material comprises a silicon-carbon composite material; the mass percentage content of the silicon-carbon composite material is denoted as b based on the total mass of the negative electrode active material.

[0009] The mass percentage content of silicon elements in the first region of the silicon-carbon composite material is denoted as x, the mass percentage content of silicon elements in the second region is denoted as y, the ratio of y to x is denoted as B, and 0 < B < 1.

[0010] The electrolyte comprises an electrolyte lithium salt, and the electrolyte lithium salt comprises lithium bisfluorosulfonylimide; the mass percentage content of lithium bisfluorosulfonylimide is denoted as m1 based on the total mass of the electrolyte.

[0011] The electrolyte comprises a first additive, and the first additive comprises fluoroethylene carbonate; the mass percentage content of fluoroethylene carbonate is denoted as m2 based on the total mass of the electrolyte.

[0012] B, b, m1, and m2 satisfy the relationship: [(m1+m2) / B]*b≥0.2.

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

[0014] The lithium ion secondary battery provided in the application has a silicon-carbon composite material with a small volume expansion rate in the negative electrode sheet, and by adding lithium bisfluorosulfonylimide and fluoroethylene carbonate in the electrolyte, adjusting the mass percentage content (m1) of lithium bisfluorosulfonylimide, the mass percentage content (m2) of fluoroethylene carbonate in the electrolyte, and the mass percentage content (b) of the silicon-carbon composite material in the negative active material and the mass percentage content ratio (B) of silicon in the first region and the second region in the silicon-carbon composite material, when [(m1+m2) / B]*b≥0.2 is met, the problem of positive material dissolution at high voltage can be solved, the cycle stability and high-temperature storage performance of the battery at high voltage are improved, and high energy density, low thickness expansion rate and fast charging capability are considered.

[0015] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The ranges and values should be construed to be approximations that allow for significant variation. It is noted that various ranges and values are stated herein as being approximate, and it is meant that the range and values can vary from the stated approximate values. It is also noted that the data ranges include the endpoints. BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG. 1 shows a cross-sectional schematic diagram of a silicon-carbon composite material.

[0017] Reference signs: 1-first region; 2-second region. DETAILED DESCRIPTION

[0018] The specific embodiments of the application are described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and explanatory and are not intended to limit the application.

[0019] Unless otherwise defined, all scientific and technical terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0020] In the present application, the terms "battery", "lithium battery", "lithium ion battery", "lithium ion secondary battery" all have the same meaning, all refer to lithium ion secondary batteries, which generally include an electrode assembly (such as a positive electrode sheet, a negative electrode sheet and a separator), a container (a shell) containing the electrode assembly, and an electrolyte.

[0021] In the present application, the term "lithium dendrite" refers to an undesirable phenomenon that may occur during fast charging, which can pierce the separator and cause a short circuit in the battery.

[0022] In the present application, the term "Dv50" refers to the particle size corresponding to the cumulative volume particle size distribution percentage of 50% of a sample, and the Dv50 can be tested using a laser particle size analyzer.

[0023] In the present application, the term "closed pores" refers to the small, closed pore structure present inside the silicon-carbon composite material, which is not connected to the outside of the material.

[0024] In the present application, the term "HOMO value" refers to the energy value of the highest occupied molecular orbital, which is the highest energy level orbital occupied by electrons in a molecule.

[0025] The present application provides a lithium ion secondary battery, which comprises a positive electrode sheet, a negative electrode sheet and an electrolyte;

[0026] The negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises a carbon material and a silicon material; the silicon material comprises a silicon-carbon composite material; the mass percentage of the silicon-carbon composite material in the total mass of the negative electrode active material is denoted as b;

[0027] The mass percentage of silicon in the first region of the silicon-carbon composite material is denoted as x, the mass percentage of silicon in the second region is denoted as y, and the ratio of y to x is denoted as B, 0 < B < 1;

[0028] The electrolyte comprises an electrolyte lithium salt, and the electrolyte lithium salt comprises lithium bisfluorosulfonylimide; the mass percentage of lithium bisfluorosulfonylimide in the total mass of the electrolyte is denoted as m1;

[0029] The electrolyte comprises a first additive, and the first additive comprises fluoroethylene carbonate; the mass percentage of fluoroethylene carbonate in the total mass of the electrolyte is denoted as m2;

[0030] B, b, m1 and m2 satisfy the relationship: [(m1+m2) / B]*b≥0.2.

[0031] In the present application, the mass percentage of silicon refers to the average content of silicon in the first region or the second region of the silicon-carbon composite material. The test method of the average content of silicon comprises: using a spectrometer to test at least 5 points in any region of the first region or the second region to obtain the average value of the content of silicon.

[0032] In the present application, the method for measuring the mass percentage content b of the silicon-carbon composite material includes the following steps: using energy spectrum backscattering point scanning mode to test the silicon and carbon content in more than 10 silicon-carbon particles, taking the average value of the test results as the silicon-carbon ratio of the silicon-carbon particles, and recording the silicon content therein as a%; then using a thermogravimetric method to measure the silicon content in the entire negative electrode sheet, and recording the measured silicon content as β%; the mass percentage content b of the silicon-carbon composite material = β / ɑ.

[0033] In the present application, fluoroethylene carbonate (FEC) and lithium bisfluorosulfonylimide (LiFSI) are added to the electrolyte. The fluoroethylene carbonate can inhibit the expansion of the first region and the second region of the silicon-carbon composite material in the battery negative electrode sheet during the charge and discharge cycle process, so that the silicon-carbon composite material has a smaller volume expansion rate; at the same time, the FEC has a lower impedance and charge exchange impedance for the SEI film of the negative electrode. During the charging process of the battery, the volume deformation of the first region will cause a certain stress to the second region, and the interface film on the surface of the negative electrode is also extruded by stress, causing damage to the SEI film; the further addition of lithium bisfluorosulfonylimide in the electrolyte can help to repair the solid electrolyte interface SEI film layer formed on the outer layer of the second region, prevent direct contact between lithium ions in the electrolyte and the negative electrode material, and slow down the volume expansion of the negative electrode material.

[0034] In addition to solving the above problems, the fluoroethylene carbonate and lithium bisfluorosulfonylimide in the electrolyte are used in combination with the silicon-carbon composite material in the negative electrode, and the B, b, m1 and m2 satisfy the relationship [(m1+m2) / B]*b≥0.2, which can reduce the viscosity of the electrolyte and the impedance of the interface, promote the formation of the SEI film, improve the wettability of the electrolyte to the silicon-containing negative electrode, so that the electrolyte can more uniformly cover the surface of the negative electrode, improve the overall performance of the battery, and further improve the volume energy density, the cycle retention rate, the thickness expansion rate and the good rapid discharge capacity of the battery.

[0035] Further, lithium dendrites are prone to occur on the silicon-carbon negative electrode during fast charging of the battery. The addition of fluoroethylene carbonate in the electrolyte can reduce the risk of lithium dendrite growth by stabilizing the SEI film and reducing the internal impedance of the battery, reduce the risk of lithium dendrites piercing the separator and causing short circuit of the battery, and improve the safety performance of the battery.

[0036] If the mass percentage content of silicon element in the second region of the silicon-carbon composite material is greater than the mass percentage content of silicon element in the first region, i.e. y>x, the silicon content inside the silicon-carbon composite material is higher than the surface, and the expansion stress is concentrated inside the silicon-carbon composite material, so that the composite material is easy to crack during the process of lithium extraction, resulting in poor cycle retention rate of the battery. To overcome the above problems, the relationship between x and y should satisfy yx, i.e. the ratio B of y to x should satisfy 0B<1, for example, B can be 0.001, 0.003, 0.005, 0.008, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.66, 0.7, 0.8, 0.9, 0.99 or any point value in the range composed of any two of the above point values, preferably 0.2≤B≤0.8, and further preferably 0.2≤B≤0.66.

[0037] As shown in FIG. 1, the silicon-carbon composite material includes a first region (1) and a second region (2), the first region (1) refers to the region of 0.01-0.5 μm from the edge to the core of the cross section of the silicon-carbon composite material, and the second region (2) refers to the region of more than 0.5 μm from the edge on the cross section of the silicon-carbon composite material.

[0038] In some embodiments, the mass percentage content b of the silicon-carbon composite material satisfies: 3%≤b≤99%, for example, b can be 3%, 5%, 8%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 93%, 96%, 99% or any point value in the range composed of any two of the above point values, and preferably 5%≤b≤50%. When B, b, m1 and m2 satisfy [(m1+m2) / B]*b≥0.2, adjusting the mass percentage content b of the silicon-carbon composite material to satisfy the above range can avoid the problems of low specific capacity of the silicon-carbon composite material and low volume energy density of the battery when b<3%, and can also avoid the problems of large volume expansion of the silicon-carbon composite material, poor structural stability of the silicon-carbon composite material, poor cycle retention rate of the battery and high thickness expansion rate when b>99%. Thus, the energy density and expansion rate of the battery are further considered, so that the battery has high energy density and low thickness expansion rate.

[0039] In some embodiments, the mass percentage content m1 of lithium bisfluorosulfonylimide satisfies: 5%≤m1≤20%; for example, m1 can be, for example, 5%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, or any point value in the range formed by any two of the above point values. When m1 satisfies the above range, the SEI film layer formed on the outer layer of the silicon-containing negative electrode can be better repaired, the direct contact between lithium ions in the electrolyte and the negative electrode material can be further prevented, and the volume expansion of the negative electrode material can be slowed down.

[0040] In some embodiments, the mass percentage content m2 of fluoroethylene carbonate satisfies: 3%≤m2≤20%; for example, m2 can be, for example, 3%, 5%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, or any point value in the range formed by any two of the above point values. When m2 satisfies the above range, the expansion of the battery negative electrode during the charge and discharge cycle can be better inhibited, the SEI film impedance and charge exchange impedance of the negative electrode can be lower, the internal impedance of the battery can be further reduced, the risk of lithium dendrite piercing the separator and causing battery short circuit can be reduced, and the safety performance of the battery can be improved.

[0041] It should be noted that when calculating the value of the formula [(m1+m2) / B]*b, m1, m2, and b are expressed in percentages, and are converted to decimals when substituted into the formula for calculation. For example, when m1=20%, m2=20%, b=60%, and B=0.02, [(m1+m2) / B]*b= [(0.2+0.2) / 0.02]*0.6=12.

[0042] To improve the energy density of the battery, the prior art often increases the silicon doping ratio of the negative electrode material. An appropriate amount of silicon doping may, under certain conditions, help to promote the graphitization process and make the graphite crystal structure more perfect, thereby increasing the OI value. However, although increasing the silicon content of the negative electrode material can result in higher specific capacity, a high silicon content ratio can also cause the battery system to expand significantly, affecting the safety performance of the battery.

[0043] To further improve the safety performance of the battery, the present application further proposes that in the lithium ion secondary battery, the OI value of the negative electrode active material is denoted as A, and A, b, and B satisfy the relationship: B*b*A≥3. The value of B*b*A can be any value greater than or equal to 3, for example, 3, 4, 5, 6, 7, 8, etc.

[0044] As can be seen from FIG. 1, the second region of the silicon-carbon composite material has a larger area than the first region, and because y < x, the average silicon content of the first region is higher than that of the second region. When the mass percentage content b of the silicon-carbon composite material increases, the average silicon content of the first region has a relatively higher increase than that of the second region, resulting in a relatively smaller ratio B of y to x. Therefore, as the mass percentage content b of the silicon-carbon composite material increases, the ratio B of the mass percentage content of silicon in the second region to that in the first region decreases, i.e., b and B are negatively correlated.

[0045] In the present application, by changing the mass percentage content b of the silicon-carbon composite material in the negative electrode active material, the ratio B of the mass percentage content of silicon in the second region to that in the first region, and adjusting the OI value of the negative electrode active material to satisfy the relationship B*b*A≥3, as the content b of the silicon-carbon composite material increases, the ratio B of the average silicon content of the second region to that of the first region decreases, and the increase of b is beneficial to improve the specific capacity of the battery, but the larger the b is, the larger the expansion of the battery system will be, which affects the safety performance of the battery. Because the silicon content of the second region of the silicon-carbon composite material is low and there are many pores, it brings higher specific capacity, at the same time, the expansion of the particles inside the silicon-carbon composite material is small when it reacts with lithium, and the expansion effect caused by the increase of b can also be partially buffered by the pores inside the silicon-carbon composite material, and the silicon-carbon composite material has higher structural stability, so that the negative electrode active material has a relatively low OI value. Therefore, when B*b*A≥3, the battery can be reduced in thickness expansion and improved in safety performance under a high voltage (>4.55V) system.

[0046] In some embodiments, the OI value A of the negative electrode active material is controlled to be 5-30, and A can be, for example, 5, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30 or any point value in the range formed by any two of the above point values. When A, b and B satisfy the relationship B*b*A≥3, and the OI value of the negative electrode active material is adjusted to be in the above range, the battery energy density can be improved while avoiding the battery system from expanding too much due to the too high silicon content of the negative electrode sheet, thereby improving the safety performance of the battery.

[0047] In the present application, the test method of the OI value A includes the following specific steps: after the lithium ion secondary battery is discharged to 0% SOC, the negative electrode sheet is taken out, soaked in dimethyl carbonate (DMC) solvent for 12 hours, then washed with DMC to remove the lithium salt attached to the sheet, dried, and then tested by an X-ray powder diffraction instrument (such as a Rigaku XRD-6100 X-ray diffractometer). The diffraction peak appearing at 2θ = 54°-55° in the obtained diffraction spectrum is the (004) peak of graphite, and the intensity is denoted as I 004The diffraction peak appearing at 2θ = 77°-78° is the (110) peak of graphite, and the intensity is denoted as I 110 The OI value A of the negative electrode sheet is I 004 / I 110 .

[0048] A higher true density in the silicon-carbon composite material means that more active material is contained in a unit volume, which helps to improve the energy density of the battery. However, when the true density is high, cracks are more likely to occur during the preparation of the silicon-carbon composite material, mainly because the high-density silicon-carbon composite material is subjected to greater stress during processing, thereby increasing the risk of crack formation. The presence of cracks can seriously affect the cycle life and safety of the battery. Moreover, a larger true density also limits the transmission of electric charges within the silicon-carbon composite material, causing blockage or lengthening of the electric charge transmission path, thereby reducing the charge and discharge efficiency and power output of the battery.

[0049] To further improve the charge and discharge efficiency of the battery and reduce the risk of lithium precipitation, the present application further proposes that in a lithium ion secondary battery, the true density of the silicon-carbon composite material is denoted as p, and p and B satisfy the relationship: 0.3≤p*B≤0.7. As can be seen from the above, the mass percentage content b of the silicon-carbon composite material is inversely proportional to the mass percentage content ratio B of silicon in the first region and the second region. Since the density of silicon is greater than that of carbon, as the value of b increases, the true density p of the silicon-carbon composite material increases, and B decreases, i.e., p and B are negatively correlated. When p and B satisfy 0.3≤p*B≤0.7, the structure of the silicon-carbon composite material can be made more compact, which helps to reduce the permeability of lithium ions to the negative electrode material during the charge and discharge process, improve the ion transmission efficiency, and effectively control the distribution of lithium ions inside the battery, thereby improving the charge and discharge efficiency of the battery and reducing the risk of lithium precipitation.

[0050] In some embodiments, p satisfies: 1.4g / cm 3 ≤p≤2.2g / cm 3 For example, p can be 1.4g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.7g / cm 3 , 1.8g / cm 3 , 1.9g / cm 3 , 2.0g / cm 3 , 2.1g / cm 3 , 2.2g / cm 3or any point value in the range consisting of any two of the above-mentioned point values. When p and B satisfy 0.3≤p*B≤0.7, and p is adjusted to satisfy the above-mentioned condition, the silicon-carbon composite material contains a suitable amount of closed pores with a size of 5 nm-80 nm (for example, 5 nm, 6 nm, 8 nm, 10 nm, 12 nm, 16 nm, 20 nm, 25 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 75 nm, 77 nm, 78 nm, 79 nm, 80 nm, or any point value in the range consisting of any two of the above-mentioned point values); when p<1.4, the composite material contains a large amount of closed pores, which can easily cause the volume energy density of the battery to be low; when p>2.2, the composite material contains a small amount of closed pores, which can cause the volume effect of the buffer active silicon to be low when lithium is deintercalated, and the volume expansion rate of the silicon-carbon composite material to be relatively high, and the expansion rate of the negative electrode sheet and the thickness expansion rate of the battery to be relatively high. Thus, when p and B satisfy 0.3≤p*B≤0.7, and p and B are adjusted to satisfy the above-mentioned condition, the charge-discharge efficiency of the battery can be further improved, and the risk of lithium precipitation can be further reduced.

[0051] In the present application, the size of the closed pores can be characterized and measured by various methods, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), etc. Other methods or forms that can directly observe the morphology and size of the closed pores can also be used for characterization and measurement of the size of the closed pores.

[0052] In some embodiments, the mass percentage content x of silicon in the first region of the silicon-carbon composite material satisfies: 60%≤x≤83%, for example, x can be 60%, 65%, 70%, 75%, 80%, 83%, or any point value in the range consisting of any two of the above-mentioned point values; and / or, the mass percentage content y of silicon in the second region satisfies: 16%≤y≤40%, for example, y can be 16%, 20%, 25%, 30%, 35%, 40%, or any point value in the range consisting of any two of the above-mentioned point values.

[0053] In some embodiments, the Dv50 of the silicon-carbon composite material is 3 μm-20 μm, and the value of Dv50 may, for example, be 3 μm, 4 μm, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 19 μm, 20 μm, or any point value in the range consisting of any two of the above-mentioned point values; more preferably, 6 μm-12 μm. When the particle size of the silicon-carbon composite material is in the above-mentioned range, the electrode structure of the silicon-carbon composite material can be optimized, the volume expansion under high pressure can be reduced, which helps to reduce the silicon-carbon composite stress of the material, and the technical effects of optimizing the battery performance, ion transmission rate, and structural stability can be achieved.

[0054] The Dv50 value can be measured by a laser particle size test method. For example, the measurement is performed using a Malvern particle size tester, and the specific test procedure includes: dispersing the silicon-carbon composite material in deionized water containing a dispersant (for example, nonylphenol polyoxyethylene ether, content 0.02-0.03wt%), forming a mixture, ultrasonicating the mixture for 2-5min, and then placing the mixture into the Malvern particle size tester for testing.

[0055] In some embodiments, the electrolyte further comprises a fluorinated solvent other than ethylene fluorinated carbonate. The fluorinated solvent has a low surface tension, preferably, the surface tension of the fluorinated solvent is 3mN / m-30mN / m, for example, it can be 3mN / m, 6mN / m, 9mN / m, 12mN / m, 15mN / m, 18mN / m, 21mN / m, 24mN / m, 27mN / m, 30mN / m, or any value within the range consisting of any two of the above-mentioned values, preferably 3mN / m-10mN / m, for example, it can be 3mN / m, 4mN / m, 5mN / m, 6mN / m, 7mN / m, 8mN / m, 9mN / m, 10mN / m, or any value within the range consisting of any two of the above-mentioned values. The fluorinated solvent with low surface tension helps to better wet the surface outer layer (first region) of the negative silicon-carbon composite material. The low surface tension makes the solvent molecules more easily spread on the surface of the silicon negative electrode, forming a uniform covering layer, reducing the interfacial resistance between the silicon negative electrode and the electrolyte, further improving the transmission efficiency of lithium ions, and improving the ionic conductivity of the silicon negative electrode.

[0056] Further, with the increase of the battery voltage system, the conventional solvent in the electrolyte is insufficient in oxidation resistance, while the fluorinated solvent has a lower HOMO value, preferably, the HOMO value of the fluorinated solvent is less than-11eV. The lower HOMO value of the fluorinated solvent makes it have better oxidation resistance and wetting performance. The good wetting effect helps to form a stable SEI film layer on the surface of the silicon-containing negative electrode, which can to some extent alleviate the volume expansion change of the silicon-containing negative electrode, and improve the cycle stability of the battery.

[0057] Preferably, the mass percentage content m3 of the fluorinated solvent in the total mass of the electrolyte satisfies: 5%≤m3≤20%, for example, m3 can be 5%, 6%, 7%, 8%, 10%, 12%, 14%, 16%, 17%, 18%, 19%, 20%, or any value within the range consisting of any two of the above-mentioned values. The addition of the fluorinated solvent can improve the solubility of lithium salt in the electrolyte, improve the chemical stability of the electrolyte, and reduce the decomposition reaction of the electrolyte.

[0058] In some embodiments, the fluorinated solvent comprises at least one of a fluorinated carbonate, a fluorinated carboxylate, and a fluorinated ether compound.

[0059] Exemplarily, the fluorinated carbonate includes but is not limited to at least one of fluorinated propylene carbonate, fluorinated methyl ethyl carbonate, fluorinated diethyl carbonate, fluorinated dimethyl carbonate.

[0060] Exemplarily, the fluorinated carboxylic acid ester includes but is not limited to at least one of ethyl difluoroacetate (DFEA), ethyl trifluoroacetate (TFEA), ethyl trifluoropropionate.

[0061] Exemplarily, the fluorinated ether compound includes but is not limited to at least one of bis(2,2,2-trifluoroethyl) ether, 2,2,2-trifluoroethyl-1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, 1H,1H,5H-octafluoropentyl-1,1,2,2-tetrafluoroethyl ether, methyl nonafluorobutyl ether, tris(trifluoroethoxy)methane, ethyl nonafluorobutyl ether.

[0062] Further, when B, b, m1 and m2 satisfy [(m1+m2) / B]*b≥0.2, adding fluorinated solvent in the electrolyte can reduce the interfacial resistance between the silicon negative electrode and the electrolyte, further improve the transmission efficiency of lithium ions, improve the ionic conductivity of the silicon-containing negative electrode, alleviate the volume change of the silicon-containing negative electrode, and improve the cycle stability of the battery.

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

[0064] Further, the silicon material optionally includes at least one of nanosilicon, silicon oxide (SiO x , 0

[0065] Further, the positive electrode sheet includes a positive electrode active material, and the positive electrode active material includes a lithium composite oxide; the chemical formula of the lithium composite oxide is Li x Co 1-y Me y O2, wherein 0.68

[0066] In some embodiments, the electrolyte further includes a second additive; the second additive includes an ester additive, a nitrile additive, a sulfonic acid additive; preferably, the mass percentage content of the second additive is 2%-20% based on the total mass of the electrolyte, for example, can be 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%.

[0067] Exemplarily, the ester additive in the second additive includes, but is not limited to, at least one of vinylene carbonate, vinyl ethylene carbonate, and vinyl sulfate. The addition of the ester additive in the electrolyte can reduce the hydrolysis reaction in the electrolyte, improve the cycle performance, capacity, and power performance of the battery, improve the overall performance of the battery, reduce the gelation tendency of the electrolyte, and prolong the service life of the battery.

[0068] Exemplarily, the nitrile additive in the second additive includes, but is not limited to, at least one of butanedinitrile, pentanedinitrile, hexanedinitrile, heptanedinitrile, octanedinitrile, nonanedinitrile, 1,3,6-hexanetricarbonitrile, glycerol trinitrile, 1,2-bis(2-cyanoethoxy)ethane, and derivatives thereof. The addition of the nitrile additive in the electrolyte can effectively protect the positive electrode material, prevent the electrolyte from being oxidized and decomposed, inhibit the generation of oxides, and reduce the possible thermal runaway situation.

[0069] Exemplarily, the sulfonic acid additive in the second additive includes, but is not limited to, at least one of 1,3-propane sultone, 1-propene-1,3-sultone, 5-methylthiophene 2,2-dioxide, 1,3-propylene sultone, 2,4-butane sultone, 1,4-butane sultone, and derivatives thereof. The addition of the sulfonic acid additive in the electrolyte can enhance the oxidation resistance of the electrolyte, delay the aging process of the electrolyte, improve the stability of the battery, and improve the fast-charging performance of the battery.

[0070] In some embodiments, the electrolyte includes one or more of a carbonate solvent and / or a carboxylic acid ester solvent; preferably, the mass percentage of the carbonate solvent and / or the carboxylic acid ester solvent in the total mass of the electrolyte is 40%-60%, for example, can be 40%, 44%, 48%, 50%, 52%, 56%, 58%, 60%, or any point value in the range consisting of any two of the above point values. The addition of the carbonate solvent can form an SEI film at the negative electrode, inhibit the side reaction on the electrode surface, and improve the high-temperature performance of the battery.

[0071] Exemplarily, the carbonate solvent includes, but is not limited to, at least one of vinylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate, and fluorinated products thereof.

[0072] Exemplarily, the carboxylic acid ester solvent includes, but is not limited to, at least one of propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isopentyl acetate, ethyl propionate, n-propyl propionate, methyl butyrate, ethyl butyrate, and n-butyl butyrate, and derivatives thereof.

[0073] In some embodiments, the electrolyte lithium salt can further include at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium difluoro oxalate borate, lithium bis-trifluoromethylsulfonylimide, lithium difluorobisoxalate phosphate, lithium tetrafluoroborate, lithium bisoxalate borate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium tris(trifluoromethylsulfonyl)methide, or lithium bis(trifluoromethylsulfonyl)imide. The lithium salt additive is added to the electrolyte, which can improve the ionic conductivity of the electrolyte, make the lithium ion migrate faster in the electrolyte, and thus improve the fast-charging performance of the battery.

[0074] The technical solutions in the embodiments of the present application will be clearly and completely described below in combination with the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0075] The materials, reagents, etc. used in the following examples can be obtained from commercial channels unless otherwise specified.

[0076] The present application will be described in detail below in combination with specific examples, which are used for understanding rather than limiting the present application.

[0077] The batteries of the examples and comparative examples in the present application are prepared according to the following preparation method. The differences from Example 1-1 are described below.

[0078] Example 1-1

[0079] (1) Preparation of positive electrode sheet

[0080] The positive electrode active material Li x Co 1-y Me y O2(x = 0.7, y = 0.1, doped with Mn element), polyvinylidene fluoride (PVDF), SP (super P), and carbon nanotube (CNT) are mixed in a mass ratio of 96:2:1.5:0.5, and then put into a vacuum stirrer. N-methyl pyrrolidone (NMP) is added, and they are mixed under the action of the vacuum stirrer until a uniform and good-flowing positive electrode slurry is formed, wherein the solid content is 55 wt%. The above positive electrode slurry is uniformly coated on an aluminum foil with a thickness of 12 μm, dried, rolled, slitted, and punched to obtain a positive electrode sheet.

[0081] (2) Preparation of negative electrode sheet

[0082] The silicon-carbon composite material (the mass percentage of silicon element in the first region x and the mass percentage of silicon element in the second region y are respectively 77% and 23%), artificial graphite, sodium carboxymethyl cellulose, styrene-butadiene rubber, conductive carbon black and carbon nanotubes are put into a vacuum stirrer according to a mass ratio of 40:64.5:2.5:1.5:1:0.5, deionized water is added, and they are fully mixed under the action of the vacuum stirrer to finally form a negative electrode slurry with good flowability and uniformity, wherein the solid content is 45wt%; the above negative electrode slurry is uniformly coated on a copper foil with a thickness of 6μm, dried, rolled, die-cut, and a negative electrode sheet is obtained.

[0083] (3) Preparation of electrolyte

[0084] In an argon glove box with water content <0.1 ppm and oxygen content <0.1 ppm, ethylene carbonate (EC), propylene carbonate (PC), fluorinated methyl ethyl carbonate (FEMC), propyl propionate (PP) and fluorinated ethylene carbonate (FEC) are uniformly mixed, and fully dried lithium hexafluorophosphate (LiPF6) and lithium bisfluorosulfonylimide salt (LiFSI) are added and stirred to dissolve, wherein the amount of LiPF6 added is 12.5% and the amount of LiFSI added is 15% based on the total mass of the electrolyte, and then 1,3-propane sultone 4%, 1,3,6-hexanetricarbonitrile 2.5%, adiponitrile (ADN) 2%, and butanedinitrile SN 1% are added based on the total mass of the electrolyte, and stirred uniformly, the mass ratio of FEC to the electrolyte is 10%, the mass ratio of FEMC to the electrolyte is 15%, and the mass ratio of EC, PC and PP (1:2:4) to the total mass of the electrolyte can be calculated, after passing the physical property test, the electrolyte is obtained.

[0085] (4) Preparation of battery

[0086] The positive electrode sheet obtained in step (1), the negative electrode sheet obtained in step (2) and the separator (8μm thick polyethylene separator provided by Asahi Kasei Corporation) are obtained by a winding method to obtain a bare cell; the bare cell is welded with tabs, placed in a battery shell, and the electrolyte prepared in step (3) is injected into the dry qualified cell, and after standing, aging, formation, exhaust, aging and sorting processes, the battery is obtained.

[0087] (5) Test battery

[0088] Electrochemical performance test: the electrochemical performance test is carried out by the following test method using a blue electric charge and discharge test cabinet:

[0089] (i) High temperature storage test

[0090] The batteries prepared from the examples and comparative examples were subjected to high-temperature storage test, and the specific test method was as follows: at 25°C±5°C, the voltage, internal resistance and thickness d0 of the sample were tested,

[0091] Step 1: standing at 25°C±5°C for 10 min;

[0092] Step 2: discharging at 0.2C to the lower limit voltage (3.0V); standing for 10 min;

[0093] Step 3: charging at 0.7C to the upper limit voltage (4.55V), with a cutoff of 0.05C, and standing for 10 min;

[0094] Step 4: discharging at 0.2C to the lower limit voltage (for initial capacity test);

[0095] Step 5: standing for 10 min;

[0096] Step 6: charging at 0.7C to the upper limit voltage, with a cutoff of 0.05C;

[0097] Step 7: standing for 10 min;

[0098] Step 8: storing at 85°C±2°C for 6h;

[0099] After storage, the sample was immediately tested for thermal thickness d1, and the thickness change rate was calculated: (d1-d0) / d0.

[0100] (ii) Wettability test

[0101] The test method used the immersion time method, which evaluated the wettability by measuring the time required for the electrolyte to completely penetrate the negative electrode sheet. The specific operation steps were as follows:

[0102] (1) A certain amount of electrolyte was added to the negative electrode sheet.

[0103] (2) The time required for the electrolyte to completely penetrate the solid was recorded.

[0104] (3) The electrolyte wettability under different conditions was evaluated by the time difference.

[0105] (iii) Fast-charging capacity retention rate

[0106] Cycle test: the batteries prepared from the examples and comparative examples were subjected to cycle test, and the specific test method was as follows: at 45°C, the batteries were subjected to constant current charge-discharge cycle at 1C rate in the charge-discharge cutoff voltage range (3.0V-4.55V) for 600 cycles; the discharge capacity Q1 of the 1st week and the discharge capacity Q 600 of the 600th week were recorded, and the fast-charging capacity retention rate was calculated: (Q 600The capacity retention rate at 45℃ after 600 cycles and the thickness expansion rate were calculated based on Q1x100%.

[0107] (iv) Lithium precipitation test

[0108] The battery interface after 600T cycles was disassembled to observe whether the negative electrode sheet had lithium precipitation.

[0109] (v) Safety performance

[0110] Furnace temperature test: the batteries prepared from the examples and the comparative examples were subjected to furnace temperature test, and the specific test method was as follows:

[0111] 1) At 25℃, first discharge at 0.2C and charge at 0.5C to the specified voltage 4.55V;

[0112] 2) At 25℃, test the full voltage, internal resistance and thickness;

[0113] 3) At 25℃, hang the full battery in a gravity convection or circulating air oven (heat transfer is not allowed for non-integral battery components), and properly insulate the voltage and temperature leads (to avoid short circuit), start from room temperature (25℃) and heat at (5±2)℃ / min to 132℃, and keep at 132℃ for 60min, and observe whether the battery catches fire or explodes. If the battery does not catch fire or explode, it is judged as passing the furnace temperature test. If the battery catches fire or explodes, it is judged as failing the furnace temperature test.

[0114] Examples 1-4 groups and Comparative Examples 1-3 were carried out according to Example 1-1, and the main differences were shown in Table 1. Among them, in Example 1 group, the mass percentage content of LiFSI in the electrolyte was changed; in Example 2 group, the mass percentage content of FEC was changed; in Example 3 group, the ratio of the mass percentage content of silicon element in the second region to that in the first region of the silicon-carbon composite material was changed; in Example 4 group, the mass percentage content of the silicon-carbon composite material was changed; in Comparative Example 1, the mass percentage content of the silicon-carbon composite material b was too small; in Comparative Example 2, the ratio of the mass percentage content of silicon element in the second region to that in the first region was B>1; in Comparative Example 3, the mass percentage content of LiFSI was too small.

[0115] Table 1 Note: m1, m2 and b are converted into decimals and substituted into the formula.

[0116] As can be seen from Table 1, by adjusting the mass percentage content (m1) of lithium bisfluorosulfonylimide in the electrolyte, the mass percentage content (m2) of fluoroethylene carbonate, and the mass percentage content (b) of the silicon-carbon composite material in the negative electrode active material and the mass percentage content ratio (B) of silicon in the first region and the second region in the silicon-carbon composite material satisfying [(m1+m2) / B]*b≥0.2, the application can solve the dissolution problem of the positive electrode material under high voltage, improve the cycle stability and high-temperature storage performance of the battery under high voltage, and balance high energy density, low thickness expansion rate and fast charging capability.

[0117] Example 5 group is carried out by referring to Example 1-1, and the main difference is shown in Table 2. In Example 5 group, the OI value of the negative electrode sheet is changed.

[0118] Table 2

[0119] As can be seen from Table 2, by adjusting the OI value (A) of the negative electrode sheet, the mass percentage content (b) of the silicon-carbon composite material and the mass percentage content ratio (B) of silicon in the first region and the second region in the composite material satisfying B*b*A≥3, the application can avoid the large expansion of the battery system caused by the too high silicon content of the negative electrode sheet, thereby improving the safety performance of the battery.

[0120] Example 6 group is carried out by referring to Example 1-1, and the main difference is shown in Table 3. In Example 6 group, the true density of the silicon-carbon composite material is changed.

[0121] Table 3

[0122] As can be seen from Table 3, by adjusting the true density (ρ) of the silicon-carbon composite material and the mass percentage content ratio (B) of silicon in the first region and the second region in the composite material satisfying 0.3≤ρ*B≤0.7, the application can improve the ion transmission efficiency, improve the charge and discharge efficiency of the battery and reduce the risk of lithium precipitation.

[0123] Example 7 is carried out by referring to Example 1-1, and the main difference is shown in Table 4. In Example 7 group, the Dv50 of the silicon-carbon composite material is changed.

[0124] Table 4

[0125] As can be seen from Table 4, by adjusting the Dv50 of the silicon-carbon composite material, the application can reduce the volume expansion under high pressure system, the silicon-carbon composite stress of less material, and optimize the battery performance, ion transmission rate and structural stability.

[0126] The main difference between the Example 8 group and the Example 9 reference Example 1-1 is shown in Table 5. In the Example 8 group, the mass percentage of the fluorinated solvent was changed, and in Example 8-4, no fluorinated solvent was added to the electrolyte. In Example 9, the type of fluorinated solvent was changed.

[0127] Table 5 Note: " / " means that the corresponding parameter was not tested.

[0128] As can be seen from Table 5, by adding a fluorinated solvent to the electrolyte and adjusting the mass percentage of the fluorinated solvent, the interface resistance between the silicon negative electrode and the electrolyte can be reduced, the transmission efficiency of lithium ions can be further improved, the ionic conductivity of the silicon-containing negative electrode can be improved, the volume change of the silicon-containing negative electrode can be alleviated, and the cycle stability of the battery can be improved.

[0129] It should be noted that the terms "comprising", "including", or any other variant thereof, are intended to cover a non-exclusive inclusion, such that processes, methods, articles, or apparatuses that comprise a list of elements are not limited to those elements, but can also include other elements not expressly listed, or inherent to such processes, methods, articles, or apparatuses. Without more limitations, an element defined by the phrase "comprising a" does not exclude the existence of additional identical elements in the process, method, article, or apparatus that includes the element. In addition, it should be pointed out that the scope of the methods and apparatuses in the embodiments of the present application is not limited to performing functions in the order shown or discussed, but can also include performing functions in a substantially simultaneous manner or in the opposite order, for example, the described methods can be performed in an order different from that described, and various steps can also be added, omitted, or combined. In addition, features described with reference to certain examples can be combined in other examples.

[0130] The above description is only the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A lithium-ion secondary battery, wherein, The battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte; The negative electrode sheet comprises a negative electrode active material, the negative electrode active material comprises a carbon material and a silicon material; the silicon material comprises a silicon-carbon composite material; the mass percentage of the silicon-carbon composite material in the total mass of the negative electrode active material is denoted as b; The mass percentage of silicon element in the first region of the silicon-carbon composite material is denoted as x, the mass percentage of silicon element in the second region is denoted as y, the ratio of y to x is denoted as B, and 0 < B < 1; The electrolyte comprises an electrolyte lithium salt, the electrolyte lithium salt comprises lithium bisfluorosulfonylimide; the mass percentage of lithium bisfluorosulfonylimide in the total mass of the electrolyte is denoted as m1; The electrolyte comprises a first additive, the first additive comprises fluoroethylene carbonate, and the mass percentage of fluoroethylene carbonate in the total mass of the electrolyte is denoted as m2; B, b, m1 and m2 satisfy the relationship: [(m1+m2) / B]*b≥0.

2.

2. The lithium-ion secondary battery according to claim 1, wherein b satisfies: 3%≤b≤99%, preferably 5%≤b≤50%; Preferably, m1 satisfies: 5%≤m1≤20%; and / or, 3%≤m2≤20%.

3. The lithium-ion secondary battery according to claim 1, wherein The OI value of the negative electrode active material is denoted as A, A, b and B satisfy the relationship: B*b*A≥3; Preferably, the OI value A of the negative electrode active material is 5-30.

4. The lithium-ion secondary battery according to claim 1, wherein The true density of the silicon-carbon composite material is denoted as p, p and B satisfy the relationship: 0.3≤p*B≤0.7; Preferably, p satisfies: 1.4 g / cm 3 ≤ p ≤ 2.2 g / cm 3 .

5. The lithium-ion secondary battery according to claim 1, wherein In the silicon-carbon composite material, x satisfies: 60%≤x≤83%; and / or, y satisfies: 16%≤y≤40%; Preferably, the Dv50 of the silicon-carbon composite material is 3-20 microns; more preferably 6-12 microns.

6. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein The electrolyte comprises a fluorinated solvent; Preferably, the mass percentage m3 of the fluorinated solvent in the total mass of the electrolyte satisfies: 5%≤m3≤20%.

7. The lithium-ion secondary battery according to claim 6, wherein The surface tension of the fluorinated solvent is 3-30 mN / m; and / or, the HOMO value of the fluorinated solvent is less than -11 eV; Preferably, the fluorinated solvent comprises at least one of a fluorinated carbonate, a fluorinated carboxylate, a fluoroether compound.

8. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein The carbon material comprises at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon and soft carbon; Preferably, the silicon material optionally comprises at least one of nanosilicon, silicon oxide and silicon alloy; Preferably, the positive electrode sheet includes a positive electrode active material including a lithium complex oxide; the lithium complex oxide has a chemical formula of Li x Co 1-y Me y O2, wherein 0.68 < x < 0.74, 0≤y≤0.15, the doping element Me comprises one or more of Al, Mg, Ti, Zr, Ni, Mn, Y, La, Sr, W, Sc, Ce, P, Nb, V, Ta and Te.

9. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein The electrolyte further comprises a second additive; the second additive comprises an ester additive, a nitrile additive, a sulfonic acid additive; Preferably, the mass percentage of the second additive in the total mass of the electrolyte is 2%-20%.

10. The lithium-ion secondary battery according to any one of claims 1 to 5, wherein The electrolyte comprises one or more of a carbonate solvent and / or a carboxylate solvent; Preferably, the mass percentage of the carbonate solvent and / or the carboxylate solvent in the total mass of the electrolyte is 40%-60%.

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