Lithium-ion secondary batteries

Abstract

This invention relates to the field of battery technology, specifically to a lithium-ion secondary battery. The positive electrode of the lithium-ion secondary battery comprises electrodes containing Li... x Ni a1 Co b1 Mn c1 M 1 d1 The first particle of O2 has the chemical formula Li. y Ni a2 Co b2 Mn c2 M 2 d2 The second particle of O2; the negative electrode sheet includes a negative electrode active material, which includes a silicon-carbon material, and the silicon-carbon material includes a porous carbon matrix and silicon material located in the pores of the porous carbon matrix; the specific surface area A of the positive electrode active material, the oil absorption value B of the silicon-carbon material in the negative electrode sheet, and the mass content C of lithium difluorophosphate in the electrolyte satisfy: 0.13≤C / A≤5.45, 0.002≤C / B≤0.23. The lithium-ion secondary battery of the present invention has good high-temperature storage and high-temperature cycling performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and specifically to a lithium-ion secondary battery. Background Technology

[0002] With the development of the new energy industry, the demand and performance requirements for lithium-ion rechargeable batteries are gradually increasing, such as low cost, high energy density, long cycle life, and high power density. In lithium-ion rechargeable batteries, silicon-carbon materials are often used as the negative electrode and positive electrode, respectively, due to their high specific capacity and low cost and high energy density (based on the sum of the molar content of transition metals in the positive electrode active material, the molar content of Ni is >80%). They are favored by the power tool market. However, high-nickel materials also have problems such as low compaction density, poor rate performance, and serious gas generation, while silicon-carbon materials have the problem of cycle expansion, both of which are detrimental to the performance improvement of lithium-ion rechargeable batteries. Summary of the Invention

[0003] The purpose of this invention is to overcome the problem in lithium-ion secondary batteries where, when high-nickel materials are used as the positive electrode and silicon-carbon materials as the negative electrode, both materials reduce the battery's rate performance and cause gas generation, which is detrimental to battery safety. This problem is particularly pronounced at high voltages. This invention provides a lithium-ion secondary battery (hereinafter referred to as the battery) that uses a positive electrode sheet containing first and second particles of positive electrode material, while simultaneously controlling the specific surface area A (in m²) of the positive electrode active material. 2 The oil absorption value B (in mL / 100g) of silicon-carbon material and the mass content C (in %) of lithium difluorophosphate in the electrolyte satisfy the following conditions: 0.13≤C / A≤5.45, 0.002≤C / B≤0.23. The battery obtained by this invention has reduced gas production and good high-temperature cycling and high-temperature storage capabilities.

[0004] In existing technologies, batteries containing high-nickel materials (based on the sum of the molar contents of transition metals in the positive electrode active material, with Ni molar content > 80%) typically improve their energy density through two methods: reducing the average particle size of the high-nickel material particles in the positive electrode active material, or increasing the molar content of nickel in the high-nickel material to over 90% to increase the compaction density of the high-nickel material. However, both of these methods reduce the rate performance of the battery and exacerbate gas generation, thereby reducing the battery's safety performance. Furthermore, in existing technologies, positive electrodes containing high-nickel materials are often paired with negative electrodes containing silicon-carbon materials. Although this combination results in a battery with high energy density, the battery is not only affected by the volume expansion of the negative electrode, leading to a decrease in safety performance, but also causes a decrease in rate performance and increased gas generation, further deteriorating its high-temperature storage and high-temperature cycling performance.

[0005] To address the above problems, the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising a positive electrode, a negative electrode, and an electrolyte; the positive electrode comprises a positive electrode active material, the positive electrode active material comprising a first particle and a second particle;

[0006] The first particle comprises Li x Ni a1 Co b1 Mn c1 M 1 d1 For the positive electrode active material of O2, 0.9≤x≤1.1, 0.8≤a1≤0.98, 0.01≤b1≤0.2, 0.01≤c1≤0.14, 0 <d1≤0.08,M 1 The first particle comprises at least one of Al, Zr, B, Y, Sr, W, Ti, Mg, and Nb; the second particle comprises a single crystal particle; and the third particle comprises a particle with the chemical formula Li. y Ni a2 Co b2 Mn c2 M 2 d2 For the positive electrode active material of O2, 0.9≤y≤1.1, 0.8≤a²≤0.98, 0.01≤b²≤0.3, 0.01≤c²≤0.12, 0 <d2≤0.1,M 2 Including at least one of Al, Zr, B, Y, Sr, W, Ti, Mg and Nb, wherein the second particle comprises polycrystalline particles;

[0007] The negative electrode sheet includes a negative electrode active material, the negative electrode active material includes a silicon-carbon material, the silicon-carbon material includes a porous carbon matrix and silicon material located in the pores of the porous carbon matrix;

[0008] The electrolyte includes lithium difluorophosphate;

[0009] The specific surface area of ​​the positive electrode active material is A, with units of m². 2 / g, the oil absorption value of the silicon-carbon material is B, in mL / 100g, the mass content of the lithium difluorophosphate in the electrolyte is C, in %, 0.13≤C / A≤5.45, 0.002≤C / B≤0.23.

[0010] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0011] (1) In this invention, by adjusting the composition and crystal structure of the first and second particles in the positive electrode active material, the first particle includes single crystal particles and the second particle includes polycrystalline particles, thereby increasing the compaction density of the positive electrode sheet, reducing the porosity of the positive electrode sheet, suppressing gas production, and increasing the energy density of the battery.

[0012] (2) In this invention, by adjusting the relationship between the specific surface area of ​​the positive electrode active material, the oil absorption value of the silicon-carbon material and the mass content of lithium iron difluorophosphate in the electrolyte, the battery liquid retention and SEI film stability are improved, while the energy density and cycle stability of the battery are also improved.

[0013] (3) In this invention, the lithium-ion secondary battery has high energy density, improved high-temperature cycle retention rate, and reduced battery gas generation during the cycle process, thus improving high-temperature storage performance.

[0014] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0015] Figure 1 The image shown is an XRD diffraction pattern of a silicon-carbon material in one embodiment of the present invention.

[0016] Figure 2 The image shown is a Raman spectrum of a silicon-carbon material in one embodiment of the present invention.

[0017] Figure 3 The diagram shown is a schematic diagram of a multi-pole ear winding structure in one embodiment of the present invention.

[0018] Figure 4 The diagram shown is a schematic diagram of the battery core structure in one embodiment of the present invention. Detailed Implementation

[0019] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0020] A lithium-ion secondary battery includes a positive electrode, a negative electrode, and an electrolyte; the positive electrode includes a positive electrode active material, which includes a first particle and a second particle.

[0021] In this invention, the first particle comprises a chemical formula of Li x Nia1 Co b1 Mn c1 M 1 d1 a positive electrode active material of O2, 0.9 ≤ x ≤ 1.1 (such as 0.9, 0.92, 0.94, 0.96, 0.98, 1, 1.02, 1.04, 1.06, 1.08 or 1.1), 0.8 ≤ a1 ≤ 0.98 (such as 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.97 or 0.98), 0.01 ≤ b1 ≤ 0.2 (such as 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18 or 0.2), 0.01 ≤ c1 ≤ 0.14 (such as 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.1, 0.12, 0.13 or 0.14), 0 < d1 ≤ 0.08 (such as 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 or 0.08), M 1 including at least one of Al, Zr, B, Y, Sr, W, Ti, Mg and Nb, and the first particles include single crystal particles.

[0022] In the present invention, the second particles include a chemical formula of Li y Ni a2 Co b2 Mn c2 M 2 d2 a positive electrode active material of O2, 0.9 ≤ y ≤ 1.1 (such as 0.9, 0.92, 0.94, 0.96, 0.98, 1, 1.02, 1.04, 1.06, 1.08 or 1.1), 0.8 ≤ a2 ≤ 0.98 (such as 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.97 or 0.98), 0.01 ≤ b2 ≤ 0.3 (such as 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, 0.22, 0.24, 0.26, 0.28 or 0.3), 0.01 ≤ c2 ≤ 0.12 (such as 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08, 0.1, 0.11 or 0.12), 0 < d2 ≤ 0.1 (such as 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 or 0.1), M 2The second particle includes at least one of Al, Zr, B, Y, Sr, W, Ti, Mg, and Nb, and the second particle includes polycrystalline particles.

[0023] In one embodiment, 0.86≤a1≤0.97, 0.83≤a2≤0.95.

[0024] In this invention, by controlling the crystal structure and chemical composition of the first and second particles, the positive electrode active material is made to have a mixture of single-crystal and polycrystalline particles, and the amounts of Co and other metal elements M in the positive electrode active material are also controlled. 1 and M 2 The molar content allows the positive electrode active material to have high capacity while also having high rate performance, improving the battery's charge and discharge cycle capacity retention rate, especially significantly improving cycle stability at high temperatures and enhancing high-temperature storage performance.

[0025] In this invention, the chemical composition of the first particle and the second particle can be determined by conventional testing methods in the art, such as by ICP spectrometer analysis, as follows: Take about 0.1g of sample and add 10mL of concentrated hydrochloric acid for digestion on a 350℃ hot plate for 10min. After cooling, dilute to 100ml with deionized water. Take a portion of the solution and analyze it with ICP spectrometer analysis. Finally, determine the final content of the elements measured in each sample by the spectrum to obtain the test results.

[0026] In this invention, the negative electrode sheet includes a negative electrode active material, which includes a silicon-carbon material, and the silicon-carbon material includes a porous carbon matrix and silicon material located in the pores of the porous carbon matrix.

[0027] In this invention, the electrolyte comprises lithium difluorophosphate; the specific surface area of ​​the positive electrode active material is A (unit: m²). 2 The oil absorption value of the silicon-carbon material is B (mL / 100g), and the mass content of lithium difluorophosphate in the electrolyte is C (%), where 0.13 ≤ C / A ≤ 5.45 (e.g., 0.13, 0.15, 0.17, 0.2, 0.3, 0.4, 0.6, 0.8, 1, 1.2, 1.4, 1.8, 2.2, 2.6, 3, 3.5, 4, 4.5, 5, 5.1). 5.2, 5.3, 5.4 or 5.45), 0.002≤C / B≤0.23 (e.g. 0.002, 0.004, 0.006, 0.008, 0.01, 0.012, 0.014, 0.016, 0.018, 0.02, 0.025, 0.03, 0.04, 0.05, 0.06, 0.08, 0.1, 0.14, 0.18, 0.22 or 0.23).

[0028] In one embodiment, 1.025 ≤ C / A ≤ 3.74, 0.035 ≤ C / B ≤ 0.05.

[0029] In this invention, by adjusting C / A and / or C / B within a certain range, a good synergistic relationship can be maintained between the specific surface area A of the positive electrode active material, the oil absorption value B of the silicon-carbon material, and the mass content C of lithium difluorophosphate in the electrolyte. This results in a more stable SEI film with low impedance, good battery cycle and rate performance, and suppression of gas generation. Specifically, when C / A > 3.74 and / or C / B > 5.45, the excessively high mass content of lithium difluorophosphate in the electrolyte reduces ionic conductivity, or the excessively low specific surface area A of the positive electrode active material and / or the insufficient oil absorption value B of the silicon-carbon material leads to insufficient electrolyte wetting and a decrease in the electrolyte retention of the negative electrode, resulting in less stored electrolyte. This is detrimental to SEI film formation, leading to increased battery impedance, gas generation, and deterioration of cycle and rate performance. When C / A < 0.13 and / or C / B < 0.00... If the mass content of lithium difluorophosphate in the electrolyte is too low, it is not conducive to the construction of the interface film. The impedance of the SEI film is high, or the specific surface area A of the positive electrode active material and / or the oil absorption value B of the silicon-carbon material are too large, resulting in an excessively large specific surface area of ​​the positive electrode active material, excessive contact with the electrolyte, and aggravation of the occurrence of side reactions. If the oil absorption value of the negative electrode is too large, the porosity of the negative electrode increases, and the SEI film formed is too thick, resulting in increased lithium ion consumption and reduced transport efficiency, which is not conducive to the cycle performance and rate performance of the battery.

[0030] In this invention, A is 0.5-1.2, for example, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, or 1.2, and the unit is m. 2 / g.

[0031] In one embodiment, A is 0.8-0.98, and the unit is m. 2 / g.

[0032] In this invention, controlling the specific surface area A of the positive electrode active material is beneficial for adjusting the contact area between the positive electrode active material and the electrolyte, thereby improving the wettability of the electrolyte; when A > 1.2m 2 / g, if the specific surface area of ​​the positive electrode active material is too large, excessive contact with the electrolyte will lead to intensified side reactions, resulting in poor battery cycle performance and low energy density; when A < 0.5m 2 When the specific surface area of ​​the positive electrode active material is too small, the electrolyte cannot wet it, the capacity of the positive electrode active material cannot be fully utilized, the energy density of the battery is low, and the capacity retention rate is poor.

[0033] In this invention, the specific surface area A of the positive electrode active material can be obtained by conventional testing methods in the art, such as the Brunauer-Emmett-Teller (BET) test method, as follows: After discharging the lithium-ion secondary battery to 0% SOC, the positive electrode sheet is disassembled and removed. After soaking in dimethyl carbonate (DMC) solvent for 12 hours, it is rinsed with DMC to remove the lithium salt attached to the electrode sheet. The aluminum foil is removed, and the active material is baked at 300°C for 30 minutes to obtain the positive electrode active material layer powder as a test sample. The specific surface area is measured using a TriStar II specific surface area analyzer, with N2 as the adsorbed gas.

[0034] In this invention, the oil absorption value B of the silicon-carbon material is 10-100, for example, 10, 12, 14, 16, 18, 20, 25, 30, 40, 50, 60, 70, 80, 90, 92, 94, 96, 98 or 100, with units of mL / 100g.

[0035] In one embodiment, the oil absorption value B of the silicon-carbon material is 20-70, in mL / 100g.

[0036] In this invention, the oil absorption value of silicon-carbon material is related to the liquid retention capacity of the battery and is matched with the positive electrode active material. Sufficient liquid retention capacity of the battery is a sufficient condition to support long-term cycling. As the electrolyte lithium salt and other electrolytes are consumed during cycling, and as side reactions occur, when the electrolyte decreases, lithium ions lose the channel to migrate between the positive and negative electrode active materials, which usually leads to safety problems such as lithium plating, increased volume expansion, poor cycle stability, and even short circuit fire.

[0037] Meanwhile, temperature also has a significant impact on battery performance. High temperatures can accelerate the transport of lithium ions inside the battery, improving the performance of lithium-ion batteries. However, they can also exacerbate interfacial side reactions, leading to capacity loss and electrical performance degradation. In particular, the crystal structure of high-nickel materials is affected during storage, resulting in increased polarization. During fully charged storage, the electrolyte will undergo oxidative decomposition on the surface of high-nickel materials, thereby generating an interfacial phase change layer, which leads to increased impedance and consumption of active lithium. Silicon-carbon materials, on the other hand, can improve the electrolyte retention of the battery system and delay further battery deterioration caused by the accelerated consumption of electrolyte at high temperatures.

[0038] Therefore, controlling the oil absorption value of silicon-carbon materials within a certain range can make the formed SEI film more complete and uniform, improve the structural stability of silicon-carbon materials in the negative electrode active material, and reduce the occurrence of side reactions. When B > 100, the oil absorption value of silicon-carbon materials is too large, resulting in an excessively large specific surface area and porosity of the negative electrode, forming a large amount of SEI film, which is not conducive to the transport of lithium ions and aggravates the consumption of lithium ions. On the other hand, when B < 10, the oil absorption value of silicon-carbon materials is too small, and the stored electrolyte is too small, making it difficult to match with the positive electrode. The electrolyte is continuously decomposed or participates in the cycle, and the stored electrolyte is completely consumed, resulting in lithium plating.

[0039] In this invention, the oil absorption value of the silicon-carbon material can be measured by conventional testing methods in the art, for example, by scraping off the negative electrode active material layer of the negative electrode sheet, washing away the adhesive, drying (vacuum drying oven 100℃, 12h) to obtain negative electrode active material powder, weighing the clean beaker and glass rod (m1), adding 5g of negative electrode active material powder and recording the total mass (m2), adding dioctyl phthalate (DOP) dropwise with a titration bottle and stirring thoroughly, stopping the addition of (DOP) when a clump is formed, and weighing the total weight of the beaker (m3) at this time. Then, the oil absorption value = (m3-m2) / (m2-m1)×100, and the oil absorption value of the silicon-carbon material is obtained by weighted average, wherein the oil absorption value of graphite is 10g / 100g-50g / 100g.

[0040] In this invention, C is 0.1-5, and the unit is %, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.2, 4.4, 4.6, 4.8 or 5.

[0041] In one embodiment, C is 0.1-3, in percentages.

[0042] In yet another embodiment, C is 1-3, in percentages.

[0043] In this invention, by controlling the mass content C of lithium difluorophosphate in the electrolyte, lithium ions in lithium difluorophosphate can participate in the construction of the interfacial film, improving lithium-ion conductivity, reducing interfacial impedance, and increasing the transport rate of lithium ions between the positive and negative electrodes, thereby improving the rate performance of the battery. Introducing lithium difluorophosphate into the electrolyte, due to the electron-withdrawing effect of fluorine atoms, replacing hydrogen atoms with fluorine atoms can lower the highest occupied orbital and lowest unoccupied orbital energy levels of the solvent in the electrolyte, thereby enhancing the antioxidant capacity of the solvent in the electrolyte and decomposing it on the surface of silicon-carbon materials to form a lithium fluoride (LiF)-rich SEI film. This SEI can uniformly and densely cover the silicon-carbon surface, suppressing side reactions at the interface and remaining stable during cycling, inhibiting gas generation, and exhibiting good high-temperature storage performance. When C < 0.1%, the mass content of lithium difluorophosphate in the electrolyte is too small, and its effect on improving the conductivity of the electrolyte is not obvious. When C > 5%, due to the low solubility of lithium difluorophosphate in the electrolyte, the conductivity will actually decrease when the mass content of lithium difluorophosphate in the electrolyte is too large.

[0044] In this invention, the mass content of lithium difluorophosphate in the electrolyte can be obtained by conventional testing methods in the art, such as by liquid phase ion chromatography, using the electrolyte in a fresh battery as a sample for testing.

[0045] In this invention, the mass content of the first particle in the positive electrode active material is 70%-90%, for example, 70%, 72%, 74%, 76%, 78%, 80%, 82%, 84%, 86%, 88% or 90%.

[0046] In this invention, the average particle size of the first particle is 0.5 μm-5 μm, for example, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, or 5 μm. The average particle size of the second particle is 5 μm-20 μm, for example, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, or 20 μm.

[0047] In this invention, the particle size Dv10 of the first particle is 0.5μm-2μm, for example, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, or 2μm; the median particle size Dv50 of the first particle is 0.5μm-5μm, for example, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.5μm, or 2μm. The particle size of the first particle is 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, or 5μm; the particle size Dv90 of the first particle is ≤10μm, for example, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2μm, 3μm, 5μm, 5μm, 6μm, 7μm, 8μm, 9μm, 9.5μm, 9.6μm, 9.8μm, or 10μm.

[0048] In this invention, the particle size Dv10 of the second particle is 3μm-12μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm or 12μm; the median particle size Dv50 of the second particle is 5μm-20μm, for example, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 1 The particle size of the second particle is 6μm, 17μm, 18μm, 19μm, or 20μm; the particle size Dv90 of the second particle is ≤25μm, for example, 0.5μm, 0.6μm, 0.8μm, 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2μm, 3μm, 5μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 14μm, 18μm, 22μm, 24μm, or 25μm.

[0049] In this invention, the first particle includes a monocrystalline particle, and the second particle includes a polycrystalline particle. Adjusting the mass content of the first particle in the positive electrode active material can control the ratio of monocrystalline to polycrystalline particles in the positive electrode active material. At the same time, the particle size distribution of the first and second particles can be controlled within a reasonable range, so that the amount and particle size of the first and second particles are matched, thereby ensuring that a good surface structure can be maintained during mixing. Furthermore, the first particle containing monocrystalline particles can fill and embed between the second particle containing polycrystalline particles, thereby increasing the overall compaction density of the positive electrode sheet, improving the stacking utilization rate of the positive electrode active material, reducing the porosity of the positive electrode sheet, reducing the contact between the electrolyte and the positive electrode active material, thereby mitigating the occurrence of side reactions, improving the volumetric energy density and cycle stability of the battery, and delaying and reducing battery gas production.

[0050] In this invention, the monocrystalline particles in the first particle and the polycrystalline particles in the second particle can be measured by conventional testing methods in the art, such as scanning electron microscopy imaging of the surface of the positive electrode and image analysis, and distinguishing monocrystalline particles and polycrystalline particles from the SEM image.

[0051] In this invention, the mass fraction of the first particle and the second particle in the positive electrode active material can be obtained by conventional testing methods in the art, such as the following method: first, disassemble the battery to obtain the positive electrode sheet, process it with an argon ion polisher ArFar100 to obtain a cross-sectional sample of the positive electrode sheet, acquire a high-resolution image by scanning electron microscopy, and then calculate the cross-sectional area (P1) of the first particle and the cross-sectional area (P2) of the second particle by electron microscopy image analysis software such as ImageJ, and calculate the volume fraction of the first particle in the positive electrode active material by the formula r=P1 / (P1+P2). Combined with density calculation, the mass fraction of the first particle in the positive electrode active material can be obtained.

[0052] In this invention, the particle size Dv10, median particle size Dv50, and particle size Dv90 of the first particle, as well as the particle size Dv10, median particle size Dv50, and particle size Dv90 of the second particle, can be obtained using conventional testing methods in the art, such as by using a laser particle size analyzer. Specifically, after discharging the lithium-ion secondary battery to 0% SOC, the positive electrode sheet is disassembled and removed. After soaking in dimethyl carbonate (DMC) solvent for 12 hours, it is rinsed with DMC to remove the lithium salts attached to the electrode sheet. The aluminum foil is removed, and the active material is baked at 300°C for 30 minutes to obtain the positive electrode active material layer powder. The positive electrode active material layer powder is measured using a Malvern particle size analyzer. The testing steps are as follows: the positive electrode active material layer powder is dispersed in deionized water containing a dispersant (such as nonylphenol polyoxyethylene ether, content 0.02-0.03wt%) to form a mixture. The mixture is sonicated for 2 minutes and then placed in a Malvern particle size analyzer for testing, and the corresponding particle size distribution data is obtained.

[0053] In this invention, the specific surface area of ​​the first particle is 0.6 m². 2 / g-1.2m 2 / g, for example, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g、1m 2 / g, 1.1m 2 / g or 1.2m 2 / g.

[0054] In this invention, the specific surface area of ​​the second particle is 0.35 m².2 / g-0.6m 2 / g, for example, 0.35m 2 / g, 0.36m 2 / g, 0.37m 2 / g, 0.38m 2 / g, 0.39m 2 / g, 0.4m 2 / g, 0.42m 2 / g, 0.44m 2 / g, 0.46m 2 / g, 0.48m 2 / g, 0.5m 2 / g, 0.52m 2 / g, 0.54m 2 / g, 0.56m 2 / g, 0.58m 2 / g or 0.6m 2 / g.

[0055] In this invention, the second particle has a lower specific surface area than the first particle. When the two are mixed in a suitable ratio, the porosity of the positive electrode sheet can be reduced, the direct contact between the electrolyte and the positive electrode active material can be reduced, and the occurrence of side reactions can be reduced.

[0056] In this invention, the specific surface area of ​​the first particle and the second particle can be obtained by conventional testing methods in the art, such as the Brunauer-Emmett-Teller (BET) test method, measured using a TriStar II specific surface area analyzer, with N2 being the adsorbed gas.

[0057] In this invention, M 1 Including Zr.

[0058] In one embodiment, the mass content of the element Zr in the first particle is 500ppm-2500ppm, for example, 500ppm, 520ppm, 540ppm, 560ppm, 580ppm, 600ppm, 650ppm, 700ppm, 750ppm, 800ppm, 900ppm, 1000ppm, 1200ppm, 140ppm, 1600ppm, 1800ppm, 2000ppm, 2200ppm, 2400ppm, or 2500ppm.

[0059] In this invention, M 2 Including Al.

[0060] In one embodiment, the mass content of element Al in the second particle is 1000ppm-3500ppm, for example, 1000ppm, 12000ppm, 1400ppm, 1600ppm, 1800ppm, 2000ppm, 2200ppm, 2400ppm, 2600ppm, 2800ppm, 2900ppm, 3000ppm, 3200ppm, 3400ppm or 3500ppm.

[0061] In this invention, the first particle contains Zr, and the second particle contains Al. Zr promotes the growth of single-crystal particles, enhances structural stability, and reduces the likelihood of breakage, thereby improving the battery's cycle performance and high-temperature storage performance. Al stabilizes the crystal lattice, slows down the phase transition from the ternary layered phase to the spinel phase in the positive electrode active material during cycling, reduces the volume change of the positive electrode active material during lithium-ion charging and discharging, and suppresses the appearance of microcracks in the second particle, significantly improving the battery's cycle stability. The introduction of Zr and Al creates a stable protective film on the surfaces of the first and second particles, increasing the capacity of the positive electrode active material and improving surface ion transport efficiency, thus enhancing the battery's high-temperature storage performance and high-temperature cycle performance.

[0062] In this invention, the mass content of element Al in the second particle and the mass content of element Zr in the first particle can be measured by conventional testing methods in the art, such as by ICP (inductively coupled plasma spectrometry).

[0063] In this invention, the electrolyte further includes ethyl 2,2-difluoroacetate.

[0064] In this invention, the ethyl 2,2-difluoroacetate in the electrolyte has a mass content of 2%-40%, for example, 2%, 2.5%, 3%, 3.5%, 4%, 4.2%, 4.4%, 4.6%, 4.8%, 5%, 7%, 10%, 15%, 20%, 25%, 30%, 35%, or 40%.

[0065] In one embodiment, the ethyl 2,2-difluoroacetate in the electrolyte has a mass content of 5%-30%.

[0066] In this invention, ethyl 2,2-difluorophosphate (DFEA) is introduced into the electrolyte. On the one hand, it can synergistically work with lithium difluorophosphate to decompose and form a LiF-containing SEI film on the surface of silicon-carbon materials and a CEI film on the surface of the positive electrode, resulting in a more stable interfacial film. On the other hand, for battery systems with positive electrode active materials including monocrystalline and polycrystalline particles, DFEA has good chemical stability and electrochemical performance. It can form a stable interfacial film at the positive electrode, reduce crystal defects and phase transitions in monocrystalline particles, improve the aggregation phenomenon of polycrystalline particles, enhance material stability, and reduce thermal decomposition. At the same time, it can reduce the polarization voltage of monocrystalline and polycrystalline particles, improve the charge and discharge efficiency of the battery, reduce energy loss, and improve battery performance. In addition, due to the accelerated gas generation, high-temperature cycling, and high-temperature storage performance degradation of batteries under high voltage, the addition of ethyl 2,2-difluorophosphate, in the presence of lithium difluorophosphate, further improves the stability of the interfacial film, enabling the battery to adapt to high-voltage film formation, suppressing the oxidative decomposition of the electrolyte, and ensuring that the battery still has good high-temperature storage and high-temperature cycling performance under high voltage.

[0067] In this invention, the mass content of ethyl 2,2-difluoroacetate in the electrolyte can be obtained by conventional testing methods in the art, such as by gas chromatography or gas chromatography coupled with mass spectrometry.

[0068] In this invention, the mass content of silicon-carbon material in the negative electrode active material is 10%-80%, for example, 10%, 12%, 14%, 16%, 18%, 20%, 24%, 28%, 32%, 36%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80%.

[0069] In one embodiment, the mass content of silicon-carbon material in the negative electrode active material is 20%-60%.

[0070] In this invention, the negative electrode sheet includes a negative electrode active material layer, and the negative electrode active material layer includes the silicon-carbon material.

[0071] In this invention, the silicon content of the silicon-carbon material is 20%-50% by mass, for example, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 36%, 40%, 45%, or 50%.

[0072] In this invention, silicon materials exhibit significant volume expansion (~300%) and low conductivity during battery charging and discharging, while carbon materials show smaller volume changes and better cycle stability. Furthermore, carbon is a mixed conductor of ions and electrons, exhibiting excellent conductivity. Therefore, in silicon-carbon materials, silicon acts as the active material, providing lithium storage capacity, while carbon buffers the volume changes of the silicon anode during charging and discharging, improves the conductivity of silicon, and prevents silicon agglomeration during charge-discharge cycles. The silicon-carbon material in the anode active material layer combines the advantages of both silicon and carbon, exhibiting high specific capacity and long cycle life. When the mass content of silicon-carbon material in the anode active material is too low, the energy density improvement of the battery is not significant; conversely, when the mass content is too high, it causes excessive volume expansion of the battery, leading to poor cycle stability and conductivity.

[0073] In this invention, the mass content of silicon-carbon material in the negative electrode active material can be obtained by conventional testing methods in the art, such as thermogravimetric analysis (TGA). The equipment used is a Shimadzu DTG-60 thermogravimetric analyzer. The specific method is as follows: After discharging the lithium-ion secondary battery to 0% SOC, the negative electrode sheet is disassembled and removed. It is then soaked in dimethyl carbonate (DMC) solvent for 12 hours, followed by rinsing with DMC to remove the lithium salt adhering to the electrode sheet. After drying, the electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active material layer can then be peeled off from the current collector, and the negative electrode active material layer powder is collected. The mass content of silicon element... In the mass content test, a thermogravimetric analyzer (such as a TGA550 thermogravimetric analyzer) is used. 5-15 mg of the collected negative electrode active material layer powder is taken and heated from room temperature to 900℃ at a heating rate of 10℃ / min in an air or oxygen atmosphere. The temperature is then maintained at 900℃ for 40 min, so that the non-silicon components in the negative electrode active material layer volatilize while the silicon is fully oxidized to silicon dioxide. The final weight percentage of the substance is the ash content v1 of the negative electrode active material layer. The mass content of silicon-carbon material in the negative electrode active material is recorded as v2, and the mass content of silicon element in the silicon-carbon material is recorded as v3. Then, v2 = (7 × v1) / (15 × v3).

[0074] In this invention, the specific surface area of ​​the silicon-carbon material is 0.5 m². 2 / g-10m 2 / g, for example, 0.5m 2 / g, 0.52m 2 / g, 0.54m 2 / g, 0.56m 2 / g, 0.58m 2 / g, 0.6m 2 / g, 0.65m 2 / g, 0.7m2 / g, 0.75m 2 / g, 0.8m 2 / g, 0.85m 2 / g, 0.9m 2 / g, 0.95m 2 / g or 10m 2 / g.

[0075] In one embodiment, the specific surface area of ​​the silicon-carbon material is 0.7 m². 2 / g-3m 2 / g.

[0076] In this invention, the specific surface area of ​​the silicon-carbon material can be determined by conventional testing methods in the art, such as the Brunauer-Emmett-Teller (BET) test method, or by using a TriStar II specific surface area analyzer, with N2 being the adsorbed gas.

[0077] In this invention, the XRD diffraction pattern of the silicon-carbon material has a first diffraction peak at 27.4°-29.4° and a second diffraction peak at 42.4°-44.4°; the half-width at half maximum (W1) of the first diffraction peak and the half-width at half maximum (W2) of the second diffraction peak satisfy: 3°≤W1+W2≤35°, for example, 3°, 4°, 5°, 6°, 8°, 10°, 12°, 14°, 16°, 18°, 20°, 25°, 30° or 35°.

[0078] In this invention, the Raman spectrum of the silicon-carbon material is at 470 cm⁻¹. -1 -480cm -1 It has the first characteristic peak at 1330 cm⁻¹ -1 -1350cm -1 It has a second characteristic peak at 1590 cm⁻¹ -1 -1610cm -1 It has a third characteristic peak; the intensity I1 of the first characteristic peak, the intensity I2 of the second characteristic peak and the intensity I3 of the third characteristic peak satisfy: 0.5≤(I1+I2) / I3≤3, for example, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8 or 3.

[0079] In this invention, the XRD diffraction pattern of the silicon-carbon material is as follows: Figure 1 As shown, the Raman spectrum of the silicon-carbon material is as follows: Figure 2 As can be seen from the above, the silicon-carbon material is composed of amorphous silicon and amorphous carbon.

[0080] In this invention, in the Raman spectrum of the negative electrode active material, the first characteristic peak represents amorphous carbon, the second characteristic peak represents crystalline silicon, and the third characteristic peak represents the defect degree of the material. Then, (I1+I2) / I3 can reflect the crystallinity of silicon-carbon material and the defects on the surface of silicon-carbon material. When (I1+I2) / I3<0.5, the silicon in silicon-carbon material has high crystallinity, which will lead to large cycle expansion of the battery and poor cycle capacity retention. When (I1+I2) / I3>3, there are too many surface defects of silicon-carbon material, resulting in low initial coulombic efficiency, which is not conducive to improving the energy density of the battery.

[0081] During lithiation / delithiation, the volume expansion / contraction of silicon-carbon materials causes significant stress, leading not only to cracking and fragmentation of the silicon-carbon material, disrupting the originally regular spacing between them, but also causing the pulverized and broken silicon-carbon material to detach from the conductive network and lose electrical contact with the current collector, resulting in decreased battery capacity and eventual failure. Simultaneously, it causes the originally intact SEI film to rupture, requiring the silicon-carbon material to re-form the SEI film upon re-contact with the electrolyte. This repeated SEI film formation-rupture-formation cycle, with each cycle, thickens the film, leading to decreased active Li-carbon content. + The loss of energy and the increase in interface resistance shorten the battery cycle life.

[0082] In this invention, the silicon-carbon material is amorphous silicon-carbon composed of amorphous silicon and amorphous carbon. Amorphous carbon is a disordered carbon atom structure without a clear crystal structure, thus having a high specific surface area and oil absorption value, and a stronger adsorption capacity for electrolyte. Amorphous silicon does not have a clear crystal structure, exhibits isotropic physical properties, and has a loose structure and a large number of pores. Amorphous silicon and carbon composite silicon-carbon materials have a large specific surface area, numerous and rationally distributed pores, and a high oil absorption value, resulting in stronger electrolyte adsorption capacity and improved electrolyte retention. Compared to crystalline silicon-carbon materials, amorphous silicon-carbon disperses stress during volume expansion during charging and discharging, making the silicon-carbon material less prone to cracking and breakage. This prevents the material from collapsing after expansion and contraction, which is beneficial to improving battery cycle performance and delaying further battery deterioration caused by accelerated electrolyte consumption at high temperatures, resulting in good high-temperature storage performance. On the other hand, the high oil absorption value of amorphous silicon-carbon, combined with lithium difluorophosphate in the electrolyte, helps improve the formation efficiency of the SEI film. The combination of these two can optimize SEI film formation and increase the lithium-ion insertion / extraction rate on the silicon-carbon material surface, which is conducive to the realization of fast charging.

[0083] In this invention, the XRD diffraction pattern of the silicon-carbon material is obtained by the following method: X-ray diffraction (XRD) is used, for example, with a Shimadzu XRD-6100 X-ray diffractometer, and the sample amount used for testing is 0.5 g / cm³. 2The Kα line of Cu was used as the incident X-ray. The working voltage of the X-ray source was 40kV, the test power was 2kW, 2θ was used as the abscissa and the signal intensity was used as the ordinate. The test range was 10°-80°, the scanning rate was 4° / min, and the data point interval was 0.02°.

[0084] In this invention, the Raman spectrum of the silicon-carbon material is tested using the following method: Raman spectroscopy is employed, for example, using a Thermo Fisher Raman spectrometer, with a wavenumber range of 400-4000 cm⁻¹. -1 .

[0085] In this invention, the lithium-ion secondary battery also includes a battery cell.

[0086] In this invention, the positive electrode sheet includes a positive current collector and a positive electrode tab extending from the positive current collector.

[0087] In this invention, the negative electrode sheet includes a negative current collector and a negative electrode tab extending from the negative current collector.

[0088] In this invention, the total number of the positive electrode tabs and the negative electrode tabs is greater than or equal to 2.

[0089] In one embodiment, the total number of the positive electrode tabs and the negative electrode tabs is greater than 2.

[0090] In yet another embodiment, the battery cell includes a multi-electrode winding structure.

[0091] In this invention, a multi-tab structure is employed. The multi-tab winding structure refers to cutting fixed tab shapes into the carrier, winding them, and then welding the tabs onto the carrier to form a multi-tab battery; for example... Figure 3 The diagram shows a multi-tab wound electrode structure. The multi-tab wound electrode has multiple tabs, and the tabs are more evenly distributed. This not only further reduces battery impedance and improves the battery's high-rate charge / discharge performance, supporting 5C-10C discharge, but also effectively increases the lithium-ion insertion / extraction rate in the battery, reduces the temperature rise under high-rate discharge, reduces battery heat generation, and significantly increases the battery's cycle life.

[0092] In this invention, the electrolyte may further comprise a lithium salt, which includes at least one of lithium hexafluorophosphate (LiPF6), lithium difluorooxalate borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiTFSI), lithium bis(trifluoromethyl)sulfonyl)imide, lithium difluorobis(oxalate) phosphate, lithium tetrafluoroborate, lithium bis(oxalate) borate, lithium hexafluoroantimonyate, lithium hexafluoroarsenate, lithium di(pentafluoroethyl)imide, lithium tri(trifluoromethyl)sulfonyl)methyl, and lithium di(trifluoromethyl)imide.

[0093] In this invention, the lithium salt content is ≤21% based on the total weight of the electrolyte, for example, 21%, 20%, 18%, 16%, 14%, 12%, 10%, 8%, 6%, 4%, 2%, 1% or 0.5%.

[0094] In this invention, the electrolyte may further include other additives, including at least one of fluoroethylene carbonate (FEC), 1,3-propanesulfonyl lactone (PS), 1,3-acrylonitrile lactone (PST), ethylene carbonate (VC), ethylene ethylene carbonate (VEC), succinic acid (SN), adiponitrile (ADN), and 1,3,6-hexanetrionitrile (HTCN).

[0095] In this invention, based on the total weight of the electrolyte, the content of the other additives is ≤25%, for example, 25%, 24%, 22%, 20%, 18%, 16%, 14%, 12%, 10%, 8%, 6%, 4%, 2%, 1%, or 0.5%.

[0096] In this invention, the other additives can form films on the surfaces of the positive and negative electrodes, providing protection for the positive and negative electrodes.

[0097] In this invention, the electrolyte may further include an organic solvent, including ethylene carbonate (EC) and propylene carbonate (PC).

[0098] In this invention, the organic solvent may further include at least one of propyl propionate (PP), ethyl propionate (EP), ethyl butyrate (EB), ethyl acetate (EA), diethyl carbonate (DEC), propyl propionate (PP), and methyl ethyl carbonate (EMC).

[0099] In this invention, the EC in the organic solvent can participate in the formation of the SEI film during the first charge-discharge cycle of the battery, which can improve the efficiency of subsequent lithium ion insertion / extraction at the negative electrode and reduce the occurrence of side reactions; PC has a high dielectric constant, which can promote the dissociation of lithium salt and significantly improve the ionic conductivity in the solution; lithium salt determines the basic physicochemical properties of the electrolyte and is the most important component in the electrolyte composition that affects the characteristics of lithium-ion batteries. It can conduct lithium ions and increase the conductivity of the electrolyte.

[0100] In this invention, the positive electrode sheet further includes a positive current collector, a positive conductive agent, and a positive binder, all of which are conventional choices for those skilled in the art. For example, the positive current collector includes aluminum foil, the positive binder includes at least one of polyvinylidene fluoride (PVDF), polyacrylate, polyimide, and styrene-butadiene rubber, and the positive conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0101] In this invention, the negative electrode sheet further includes a negative electrode current collector, a negative electrode conductive agent, and a negative electrode binder, all of which are conventional choices for those skilled in the art. For example, the negative electrode current collector includes copper foil, the negative electrode binder includes at least one of polyacrylic acid (PAA), sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose, polyimide, styrene-butadiene rubber (SBR), and polyvinylidene fluoride, and the negative electrode conductive agent includes at least one of acetylene black, conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and graphene.

[0102] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0103] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0104] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0105] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0106] Example 1:

[0107] (1) Preparation of the positive electrode:

[0108] 1.2% PVDF, 0.4% single-walled carbon nanotubes, and 35% NMP were stirred separately (solid content 65%), and then 1% multi-walled carbon nanotubes and 97.4% positive electrode active material were added and stirred to obtain a positive electrode active slurry; wherein, the positive electrode active material includes 83% of the first particles (chemical formula LiNi). 0.93 Co 0.04 Mn 0.02 Zr 0.01 O2 and Zr content in the first particle is 2000 ppm, and 17% of the second particle (chemical formula LiNi) is added. 0.89 Co 0.05 Mn 0.04 Al 0.02 O2 and Al content in the second particle are 2700 ppm. The positive electrode active slurry is evenly coated onto a 10 μm aluminum foil using a coating machine. After baking at 120°C for 12 h, it is rolled, die-cut, and sheeted to obtain the positive electrode sheet.

[0109] The first particle has an average particle size of 3.2 μm and a specific surface area of ​​1 m². 2 / g, the average particle size of the second particle is 13.2μm, and the specific surface area is 0.48m². 2 / g; at this time, the specific surface area A of the positive electrode active material is 0.91m². 2 / g.

[0110] (2) Preparation of negative electrode:

[0111] A mixture of 96.1% negative electrode active material, 0.25% single-walled carbon nanotubes, 0.15% multi-walled carbon nanotubes, and 2.9% SBR was prepared, along with 1% EC (electrolyte by mass) of the negative electrode active material. The mixture was stirred until homogeneous to obtain a negative electrode active slurry. The negative electrode active material consisted of 54.5% artificial graphite and 45.5% silicon-carbon material. The negative electrode active slurry was uniformly coated onto a 4μm high-strength carbon-coated copper foil, dried, and then rolled, die-cut, and sheeted to obtain the negative electrode sheet.

[0112] At this point, the specific surface area of ​​the silicon-carbon material is 2.5 m². 2 / g, the oil absorption value B of silicon-carbon material is 63mL / 100g.

[0113] (3) Preparation of electrolyte:

[0114] In a glove box filled with inert gas (argon) (H2O < 0.1 ppm, O2 < 0.1 ppm), EC, PC, DEC, and propyl propionate were mixed evenly in a mass ratio of 3:2:2:13 to obtain a non-aqueous organic solvent. Then, based on the total weight of the electrolyte, 1.25 mol / L of fully dried lithium hexafluorophosphate (LiPF6), 15% DFEA, and 2.2% lithium difluorophosphate (i.e., C = 2.2%) were quickly added and dissolved in the non-aqueous organic solvent and stirred evenly. Finally, 0.5% succinate, 2% HTCN, 8% FEC, and 2.5% PS additives were added. After passing the moisture and free acid tests, the desired electrolyte was obtained.

[0115] (4) Preparation of the diaphragm:

[0116] A commercially available diaphragm was selected, with the substrate, ceramic, and PVDF adhesive having thicknesses of 5μm, 1μm, and 1.5μm, respectively, and an areal density of 5.8g / m³. 2 .

[0117] (5) Preparation of lithium-ion secondary batteries:

[0118] The positive electrode, negative electrode, and separator are wound together by a winding machine to obtain a battery core in which the positive and negative electrode sheets are separated by the separator. After welding, encapsulation, liquid injection, formation, gas bag cutting, and sorting, a lithium-ion secondary battery is obtained, which has a multi-tab winding structure.

[0119] In this embodiment, C / A is 2.413 and C / B is 0.035.

[0120] like Figure 3 The diagram shown is a schematic of a multi-pole ear winding structure. Figure 4 The diagram shown is of a battery core.

[0121] Example 2:

[0122] (1) Preparation of the positive electrode:

[0123] 1.2% PVDF, 0.4% single-walled carbon nanotubes, and 35% NMP were stirred separately (solid content 65%), and then 1% multi-walled carbon nanotubes and 97.4% positive electrode active material were added and stirred to obtain a positive electrode active slurry; wherein, the positive electrode active material includes 85% of the first particles (chemical formula LiNi). 0.86 Co 0.07 Mn 0.04 Zr 0.03 O2 and Zr content in the first particle is 1800 ppm, and 15% of the second particle (chemical formula LiNi) is added. 0.83 Co 0.06 Mn 0.05 Al 0.06 O2 and Al content in the second particle are 2500 ppm. The positive electrode active slurry is evenly coated onto a 10 μm aluminum foil using a coating machine. After baking at 120°C for 12 h, it is rolled, die-cut, and sheeted to obtain the positive electrode sheet.

[0124] The first particle has an average particle size of 4.3 μm and a specific surface area of ​​0.85 m². 2 / g, the average particle size of the second particle is 16.5μm, and the specific surface area is 0.53m². 2 / g; at this point, the specific surface area A of the positive electrode active material is 0.8m². 2 / g.

[0125] (2) Preparation of negative electrode:

[0126] A mixture of 96.1% negative electrode active material, 0.25% single-walled carbon nanotubes, 0.15% multi-walled carbon nanotubes, and 2.9% SBR was prepared, along with 1% EC (electrolyte by mass) of the negative electrode active material. The mixture was stirred until homogeneous to obtain a negative electrode active slurry. The negative electrode active material consisted of 60% artificial graphite and 40% silicon-carbon material. The negative electrode active slurry was uniformly coated onto a 4μm high-strength carbon-coated copper foil, dried, and then rolled, die-cut, and sheeted to obtain the negative electrode sheet.

[0127] At this point, the specific surface area of ​​the silicon-carbon material is 3m². 2 / g, the oil absorption value B of silicon-carbon material is 70mL / 100g.

[0128] (3) Preparation of electrolyte:

[0129] In a glove box filled with inert gas (argon) (H2O < 0.1 ppm, O2 < 0.1 ppm), EC, PC, DEC, and propyl propionate were mixed evenly in a mass ratio of 3:2:2:13 to obtain a non-aqueous organic solvent. Then, based on the total weight of the electrolyte, 1.25 mol / L of fully dried lithium hexafluorophosphate (LiPF6), 5% DFEA, and 3% lithium difluorophosphate (i.e., C = 3%) were quickly added and dissolved in the non-aqueous organic solvent. The mixture was stirred evenly, and finally 0.5% succinate, 2% HTCN, 8% FEC, and 2.5% PS were added. After passing the moisture and free acid tests, the desired electrolyte was obtained.

[0130] (4) Preparation of the diaphragm:

[0131] A commercially available diaphragm was selected, with the substrate, ceramic, and PVDF adhesive having thicknesses of 5μm, 1μm, and 1.5μm, respectively, and an areal density of 5.8g / m³. 2 .

[0132] (5) Preparation of lithium-ion secondary batteries:

[0133] The positive electrode, negative electrode, and separator are wound together by a winding machine to obtain a battery core in which the positive and negative electrode sheets are separated by the separator. After welding, encapsulation, liquid injection, formation, gas bag cutting, and sorting, a lithium-ion secondary battery is obtained, which has a multi-tab winding structure.

[0134] In this embodiment, C / A is 3.741 and C / B is 0.043.

[0135] Example 3:

[0136] (1) Preparation of the positive electrode:

[0137] 1.2% PVDF, 0.4% single-walled carbon nanotubes, and 35% NMP were stirred separately (solid content 65%), and then 1% multi-walled carbon nanotubes and 97.4% positive electrode active material were added and stirred to obtain a positive electrode active slurry; wherein, the positive electrode active material includes 80% of the first particles (chemical formula LiNi). 0.97 Co 0.015 Mn 0.01 Zr 0.005 O2 and Zr content in the first particle is 2200 ppm, and 20% of the second particle (chemical formula LiNi) is added. 0.95 Co 0.03 Mn 0.01 Al 0.01O2 and Al content in the second particle are 2900 ppm. The positive electrode active slurry is evenly coated onto a 10 μm aluminum foil using a coating machine. After baking at 120°C for 12 h, it is rolled, die-cut, and sheeted to obtain the positive electrode sheet.

[0138] The first particle has an average particle size of 1.6 μm and a specific surface area of ​​1.12 m². 2 / g, the average particle size of the second particle is 10.7μm, and the specific surface area is 0.4m². 2 / g; at this point, the specific surface area A of the positive electrode active material is 0.98m². 2 / g.

[0139] (2) Preparation of negative electrode:

[0140] A mixture of 96.1% negative electrode active material, 0.25% single-walled carbon nanotubes, 0.15% multi-walled carbon nanotubes, and 2.9% SBR was prepared, along with 1% EC (electrolyte by mass) of the negative electrode active material. The mixture was stirred until homogeneous to obtain a negative electrode active slurry. The negative electrode active material consisted of 70% artificial graphite and 30% silicon-carbon material. The negative electrode active slurry was uniformly coated onto a 4μm high-strength carbon-coated copper foil, dried, and then rolled, die-cut, and sheeted to obtain the negative electrode sheet.

[0141] At this point, the specific surface area of ​​the silicon-carbon material is 0.7 m². 2 / g, the oil absorption value B of silicon-carbon material is 20mL / 100g.

[0142] (3) Preparation of electrolyte:

[0143] In a glove box filled with inert gas (argon) (H2O < 0.1 ppm, O2 < 0.1 ppm), EC, PC, DEC, and propyl propionate were mixed evenly in a mass ratio of 3:2:2:13 to obtain a non-aqueous organic solvent. Then, based on the total weight of the electrolyte, 1.25 mol / L of fully dried lithium hexafluorophosphate (LiPF6), 30% DFEA, and 1% lithium difluorophosphate (i.e., C = 1%) were quickly added and dissolved in the non-aqueous organic solvent. The mixture was stirred evenly, and finally 0.5% succinate, 2% HTCN, 8% FEC, and 2.5% PS were added. After passing the moisture and free acid tests, the desired electrolyte was obtained.

[0144] (4) Preparation of the diaphragm:

[0145] A commercially available diaphragm was selected, with the substrate, ceramic, and PVDF adhesive having thicknesses of 5μm, 1μm, and 1.5μm, respectively, and an areal density of 5.8g / m³. 2 .

[0146] (5) Preparation of lithium-ion secondary batteries:

[0147] The positive electrode, negative electrode, and separator are wound together by a winding machine to obtain a battery core in which the positive and negative electrode sheets are separated by the separator. After welding, encapsulation, liquid injection, formation, gas bag cutting, and sorting, a lithium-ion secondary battery is obtained, which has a multi-tab winding structure.

[0148] In this embodiment, C / A is 1.025 and C / B is 0.05.

[0149] Example 4 group:

[0150] This set of examples is used to verify the impact of changes in "a1 and a2", which is achieved by changing the chemical formulas and elemental composition of the first and second particles, as follows:

[0151] Example 4a, based on Example 1, differs in that a1 = 0.8, a2 = 0.8, and the chemical formula of the first particle is LiNi. 0.8 Co 0.12 Mn 0.06 Zr 0.02 O2, the second particle has the chemical formula LiNi 0.8 Co 0.08 Mn 0.07 Zr 0.05 O2.

[0152] Example 4b, based on Example 1, differs in that a1 = 0.975 and a2 = 0.96. In this case, the chemical formula of the first particle is LiNi. 0.975 Co 0.01 Mn 0.01 Zr 0.005 O2, the second particle has the chemical formula LiNi 0.978 Co 0.01 Mn 0.01 Zr 0.002 O2.

[0153] Example 5 group:

[0154] This set of examples is used to verify the impact of changes in "C / A and C / B", which is achieved by changing the values ​​of A, B, and C, as follows:

[0155] Example 5a is based on Example 1, except that A is 0.8m. 2 / g, B is 20mL / 100g, C is 3%, at this time C / A is 3.75, C / B is 0.15.

[0156] Example 5b is based on Example 1, except that A is 0.98m. 2 / g, B is 70mL / 100g, C is 1%, at this time C / A is 1.02, C / B is 0.014.

[0157] Example 6 group:

[0158] This set of embodiments is used to verify the impact of changes in "A", which is achieved by changing the specific surface area of ​​the first particle and the second particle, as follows:

[0159] Example 6a, based on Example 1, differs in that the specific surface area of ​​the first particle is 0.6 m². 2 / g, the specific surface area of ​​the second particle is 0.35m². 2 / g, at this time, A is 0.52m 2 / g, C / A is 4.231.

[0160] Example 6b is based on Example 1, except that the specific surface area of ​​the first particle is 1.2 m². 2 / g, the specific surface area of ​​the second particle is 0.6m². 2 / g, at this time, A is 1.1m. 2 / g, C / A is 2.004.

[0161] Example 7 group:

[0162] This set of examples is used to verify the impact of changes in "B", as detailed below:

[0163] Example 7a, based on Example 1, differs in that B is 10.5 mL / 100 g, and the specific surface area of ​​the silicon-carbon material is 1.75 m². 2 / g, C / B is 0.21.

[0164] Example 7b is based on Example 1, except that B is 99.6 mL / 100 g, and the specific surface area of ​​the silicon-carbon material is 2.3 m². 2 / g, C / B is 0.022.

[0165] Example 8 group:

[0166] This set of examples is used to verify the impact of changes in "C", as detailed below:

[0167] Example 8a is based on Example 1, except that C is 0.12%, C / A is 0.132, and C / B is 0.002.

[0168] Example 8b is based on Example 1, except that C is 4.95%, C / A is 5.43, and C / B is 0.079.

[0169] Example 9 group:

[0170] This set of examples is used to verify the impact of changes in the "mass fraction of the first particle in the positive electrode active material", as detailed below:

[0171] Example 9a, based on Example 1, differs in that the first particle accounts for 71.2% of the mass fraction of the positive electrode active material, and A is 0.85m. 2 / g, C / A is 2.588.

[0172] Example 9b is based on Example 1, except that the first particle accounts for 89.6% of the mass fraction of the positive electrode active material, and A is 0.95m. 2 / g, C / A is 2.326.

[0173] Example 10 group:

[0174] This set of examples is used to verify the effect of changes in the "average particle size of the first particle and the second particle", as detailed below:

[0175] Example 10a, based on Example 1, differs in that the average particle size of the first particle is 0.55 μm, and the average particle size of the second particle is 5.2 μm. In this case, the specific surface area of ​​the first particle is 1.13 m². 2 / g, the specific surface area of ​​the second particle is 0.58m². 2 / g, A is 1.04m 2 If / g, then C / A is 2.123.

[0176] Example 10b is based on Example 1, except that the average particle size of the first particle is 4.8 μm and the average particle size of the second particle is 19.6 μm. In this case, the specific surface area of ​​the first particle is 0.62 m². 2 / g, the specific surface area of ​​the second particle is 0.36m². 2 / g, A is 0.58m 2 If / g, then C / A is 3.821.

[0177] Example 11 group:

[0178] This set of examples is used to verify the impact of changes in the "mass content of Zr in the first particle and the mass content of Al in the second particle", as detailed below:

[0179] Example 11a is based on Example 1, except that the Zr content in the first particle is 506 ppm and the Al content in the second particle is 1012 ppm.

[0180] Example 11b is based on Example 1, except that the Zr content in the first particle is 2495 ppm and the Al content in the second particle is 3492 ppm.

[0181] Example 12 group:

[0182] This set of examples is used to verify the impact of changes in the "mass content of DFEA in the electrolyte", as detailed below:

[0183] Example 12a is based on Example 1, except that the mass content of DFEA in the electrolyte is 2.2%.

[0184] Example 12b is based on Example 1, except that the mass content of DFEA in the electrolyte is 39.6%.

[0185] Example 13 group:

[0186] This set of examples is used to verify the impact of changes in the "mass content of silicon-carbon material in the negative electrode active material", as detailed below:

[0187] Example 13a, based on Example 1, differs in that the negative electrode active material is composed of 80% artificial graphite and 20% silicon-carbon material, in which case the specific surface area of ​​the silicon-carbon material is 0.78 m². 2 / g, the oil absorption value B of the silicon-carbon material is 23mL / 100g, and the C / B ratio is 0.096.

[0188] Example 13b is based on Example 1, except that the negative electrode active material is composed of 40% artificial graphite and 60% silicon-carbon material, in which case the specific surface area of ​​the silicon-carbon material is 3m². 2 / g, the oil absorption value B of the silicon-carbon material is 70mL / 100g, and C / B is 0.031.

[0189] Example 13c, based on Example 1, differs in that the negative electrode active material consists of 89.4% artificial graphite and 10.6% silicon-carbon material, in which case the specific surface area of ​​the silicon-carbon material is 0.65 m². 2 / g, the oil absorption value B of the silicon-carbon material is 18mL / 100g, and the C / B ratio is 0.122.

[0190] Example 13d, based on Example 1, differs in that the negative electrode active material consists of 20.2% artificial graphite and 79.8% silicon-carbon material, with a specific surface area of ​​9.5 m² for the silicon-carbon material. 2 / g, the oil absorption value B of the silicon-carbon material is 80mL / 100g, and C / B is 0.028.

[0191] Example 14 group:

[0192] This set of examples is used to verify the impact of changes in the specific surface area of ​​silicon-carbon materials, as detailed below:

[0193] Example 14a is based on Example 1, except that the specific surface area of ​​the silicon-carbon material is 0.53 m². 2 / g, at this time, B is 10.5mL / 100g, then C / B is 0.21.

[0194] Example 14b is based on Example 1, except that the specific surface area of ​​the silicon-carbon material is 9.9 m². 2 / g, at this time, B is 100mL / 100g, then C / B is 0.022.

[0195] Example 15:

[0196] Based on Example 1, the difference is that the battery uses a conventional tab structure.

[0197] In the above embodiments, the particle size Dv10 of the first particles is in the range of 0.5μm-2μm, the median particle size Dv50 is in the range of 0.5μm-5μm, and the particle size Dv90 is ≤10μm; the particle size Dv10 of the second particles is in the range of 3μm-12μm, the median particle size Dv50 is in the range of 5μm-20μm, and the particle size Dv90 is ≤25μm.

[0198] In the above embodiments, in the XRD pattern of the silicon-carbon material, the sum of the full width at half maximum (W1) of the first diffraction peak and the full width at half maximum (W2) of the second diffraction peak is in the range of 3°-35°; in the Raman spectrum of the silicon-carbon material, (I1+I2) / I3 is in the range of 0.5-3.

[0199] In the above embodiments, the compaction density of the positive electrode sheet is 3.1 g / cm³. 3 -3.55g / cm 3 Within the specified range; the compaction density of the negative electrode is all within 1.3 g / cm³. 3 -1.6g / cm 3 Within the range.

[0200] Comparative Example 1:

[0201] This set of examples is used to verify the impact of the "composition of the positive electrode active material", as detailed below:

[0202] Comparative Example 1a, based on Example 1, differs in that the mass fraction of the first particle in the positive electrode active material is 0%, and the positive electrode active material consists entirely of the second particle, with A being 0.48m. 2 If / g, then C / A is 4.583.

[0203] Comparative Example 1b is based on Example 1, except that the mass fraction of the second particle in the positive electrode active material is 0%, and the positive electrode active material is entirely composed of the first particle, with A being 1m. 2 If / g, then C / A is 2.2.

[0204] Comparative Example 2:

[0205] This set of scales is used to verify the impact of changes in "C / A and C / B", achieved by changing the values ​​of A, B, and C, as follows:

[0206] Comparative Example 1a, based on Example 1, differs in that A is 1.1m. 2 / g, B is 99.6mL / 100g, C is 0.12%, at this time C / A is 0.109, C / B is 0.001.

[0207] Comparative Example 1b is based on Example 1, except that A is 0.52m. 2 / g, B is 10.5mL / 100g, C is 4.95%, at this time C / A is 9.519, C / B is 0.471.

[0208] Comparative Example 3:

[0209] Based on Example 1, the difference is that C = 0%, that is, no lithium difluorophosphate is added to the electrolyte. In this case, C / A is 0 and C / B is 0.

[0210] Test example:

[0211] (1) Cyclic life test:

[0212] The batteries obtained in the embodiments and comparative examples of this invention were placed in a constant temperature environment of 45°C and charged and discharged at a rate of 1.8C / 4.0C. The cutoff voltage range was 2.5V-4.3V, and 500 charge and discharge cycles were performed. The cycle discharge capacity was recorded and divided by the discharge capacity of the first cycle to obtain the cycle capacity retention rate. The thickness data per 100T was also recorded. The thickness data was divided by the initial thickness of the battery to obtain the thickness expansion rate.

[0213] (2) High-temperature storage test:

[0214] High-temperature storage at 60℃: Under an ambient temperature of 25℃±3℃, the batteries obtained in the embodiments and comparative examples of this invention were discharged at 0.2C to a cutoff voltage of 3.0V and left to stand for 10 minutes; then charged at 0.5C constant current and constant voltage to the upper limit voltage of 4.3V, with a cutoff current of 0.05C. The thickness of the fully charged battery was tested at 25℃±3℃. The fully charged battery was placed in a test chamber, and the test chamber was heated at a temperature rise rate of (5±2)℃ / min. When the temperature inside the chamber reached 60℃±2℃, it was kept at a constant temperature for 35 days. After the test, the thickness of the battery was recorded, and the thickness data was divided by the initial thickness of the battery to obtain the thickness expansion rate; the batteries stored at high temperature were cooled to room temperature, discharged at 1C, and then charged and discharged at 1C. The capacity retention rate and capacity recovery rate of the batteries were calculated respectively, using the following formulas:

[0215] Capacity retention rate (%) = Retained capacity / Initial capacity × 100%;

[0216] Capacity recovery rate (%) = Recovered capacity / Initial capacity × 100%.

[0217] (3) Discharge tests at different rates:

[0218] 1) Let the above batteries rest at an ambient temperature of 25°C for 5 minutes;

[0219] 2) Discharge at 0.5C to 2.5V;

[0220] 3) Let it sit for 1 hour;

[0221] 4) 0.5C charging: When the battery terminal voltage reaches the charging limit voltage of 4.3V, switch to constant voltage charging until the charging current is ≤ the cutoff current, then stop charging.

[0222] 5) Let it sit for 30 minutes;

[0223] 6) Discharge to 2.5V at a rate of 1:1. Record the capacitance, internal resistance, voltage, and other data during the process. Rate = {0.5C / 1C / 2C / 3C / 4C / 5C / 6C / 7C / 8C / 9C}. Repeat steps 3-6 until the test rate ends. In step 6, discharge according to the specified rate and sequence.

[0224] (4) Energy density test:

[0225] The battery was charged at a constant current of 0.5C using a Blue Electric test cabinet, with a voltage upper limit of 4.3V. Then, it was charged at a constant voltage with a cutoff current of 0.05C. After resting for 10 minutes, it was discharged at 0.5C to 2.0V, with the discharge energy being E (Wh). The mass of the battery was measured using a balance and was measured as W (kg). Therefore, the mass energy density of the battery is D = E / W (Wh / kg).

[0226] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

[0227] The performance test records of the batteries obtained in the embodiments and comparative examples of this invention are shown in Table 1.

[0228] Table 1:

[0229]

[0230] As can be seen from Table 1, the lithium-ion battery prepared in this invention has better high-temperature storage performance and cycle life compared with the comparative example.

[0231] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that, Includes positive electrode, negative electrode, and electrolyte; The positive electrode sheet includes a positive electrode active material, which comprises a first particle and a second particle; the first particle comprises a material with the chemical formula Li. x Ni a1 Co b1 Mn c1 M 1 d1 For the positive electrode active material of O2, 0.9≤x≤1.1, 0.8≤a1≤0.98, 0.01≤b1≤0.2, 0.01≤c1≤0.14, 0 <d1≤0.08,M 1 The material includes at least one of Al, Zr, B, Y, Sr, W, Ti, Mg, and Nb, wherein the first particle comprises a single crystal particle, and the mass content of the first particle in the positive electrode active material is 70%-90%; The second particle comprises Li y Ni a2 Co b2 Mn c2 M 2 d2 For the positive electrode active material of O2, 0.9≤y≤1.1, 0.8≤a²≤0.98, 0.01≤b²≤0.3, 0.01≤c²≤0.12, 0 <d2≤0.1,M 2 Including at least one of Al, Zr, B, Y, Sr, W, Ti, Mg and Nb, wherein the second particle comprises polycrystalline particles; The average particle size of the first particle is 0.5 μm-5 μm, and the average particle size of the second particle is 5 μm-20 μm; The negative electrode sheet includes a negative electrode active material, which includes a silicon-carbon material. The silicon-carbon material comprises a porous carbon matrix and silicon material located in the pores of the porous carbon matrix; the mass content of the silicon-carbon material in the negative electrode active material is 10%-80%. The electrolyte includes lithium difluorophosphate; The specific surface area of ​​the positive electrode active material is A, where A is 0.5-1.2, and the unit is m². 2 / g, the oil absorption value of the silicon carbon material is B, B is 10-100, the unit is mL / 100g, the mass content of the lithium difluorophosphate in the electrolyte is C, C is 0.1-5, the unit is %, 0.13≤C / A≤5.45, 0.002≤C / B≤0.

23.

2. The lithium-ion secondary battery according to claim 1, wherein, 1.025≤C / A≤3.74; And / or, 0.035≤C / B≤0.

05.

3. In the lithium-ion secondary battery according to claim 1 or 2, B is 20-70, with units of mL / 100g; And / or, C is 0.1-3, in units of %.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein, 0.86≤a1≤0.97； And / or, 0.83≤a2≤0.

95.

5. The lithium-ion secondary battery according to claim 1 or 2, wherein, The first particle has a particle size Dv10 of 0.5μm-2μm, a median particle size Dv50 of 0.5μm-5μm, and a particle size Dv90 ≤10μm; And / or, the particle size Dv10 of the second particle is 3μm-12μm, the median particle size Dv50 is 5μm-20μm, and the particle size Dv90 is ≤25μm; And / or, the specific surface area of ​​the first particle is 0.6 m². 2 / g-1.2m 2 / g; And / or, the specific surface area of ​​the second particle is 0.35 m². 2 / g-0.6m 2 / g.

6. The lithium-ion secondary battery according to claim 1 or 2, wherein, M 1 Including Zr; And / or, M 2 Including Al.

7. The lithium-ion secondary battery according to claim 6, wherein, The mass content of element Zr in the first particle is 500ppm-2500ppm; And / or, the mass content of element Al in the second particle is 1000ppm-3500ppm.

8. The lithium-ion secondary battery according to claim 1 or 2, wherein, The electrolyte also includes ethyl 2,2-difluoroacetate.

9. The lithium-ion secondary battery according to claim 8, wherein, The ethyl 2,2-difluoroacetate in the electrolyte has a mass content of 2%-40%.

10. The lithium-ion secondary battery according to claim 8, wherein, The ethyl 2,2-difluoroacetate in the electrolyte contains 5%-30% by mass.

11. The lithium-ion secondary battery according to claim 1 or 2, wherein, The mass content of silicon-carbon material in the negative electrode active material is 20%-60%; And / or, the silicon content in the silicon-carbon material is 20%-50% by mass; And / or, the specific surface area of ​​the silicon-carbon material is 0.5 m². 2 / g-10m 2 / g.

12. The lithium-ion secondary battery according to claim 11, wherein, The specific surface area of ​​the silicon-carbon material is 0.7 m². 2 / g-3m 2 / g.

13. The lithium-ion secondary battery according to claim 1 or 2, wherein, The XRD diffraction pattern of the silicon-carbon material has a first diffraction peak at 27.4°-29.4° and a second diffraction peak at 42.4°-44.4°; the half-width at half maximum (W1) of the first diffraction peak and the half-width at half maximum (W2) of the second diffraction peak satisfy: 3°≤W1+W2≤35°; And / or, the Raman spectrum of the silicon-carbon material is at 470 cm⁻¹ -1 -480cm -1 It has the first characteristic peak at 1330 cm⁻¹ -1 -1350cm -1 It has a second characteristic peak at 1590 cm⁻¹ -1 -1610cm -1 It has a third characteristic peak; the intensity I1 of the first characteristic peak, the intensity I2 of the second characteristic peak and the intensity I3 of the third characteristic peak satisfy: 0.5≤(I1+I2) / I3≤3.

14. The lithium-ion secondary battery according to claim 1, wherein, The positive electrode plate includes a positive current collector and a positive electrode tab extending from the positive current collector; And / or, the negative electrode includes a negative current collector and a negative electrode tab extending from the negative current collector.

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