Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. The positive plate of the lithium ion secondary battery comprises a first particle with the chemical formula of Li < x > Ni < 1 > Co < 1 > Mn < c > < 1 > M < 1 > d < 1 > O < 2 > and a second particle with the chemical formula of Li < y > Ni < 2 > Co < 2 > Mn < c > < 2 > M < 2 > d < 2 > O < 2 >, the negative plate comprises a negative active material, the negative active material comprises a silicon-carbon material, and the silicon-carbon material comprises a porous carbon matrix and a silicon material located in a pore channel of the porous carbon matrix; the specific surface area A of the positive active material, the oil absorption value B of the silicon carbon material in the negative plate and the mass content C of the lithium difluorophosphate in the electrolyte meet the following conditions: 0.13 < = C / A < = 5.45, 0.002 < = C / B < = 0.23. The lithium ion secondary battery provided by the invention has good high-temperature storage and high-temperature cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a lithium ion secondary battery. Background Art

[0002] With the development of the new energy industry, the demand and performance requirements for lithium-ion secondary batteries are gradually increasing, such as low cost, high energy density, long cycle life and high power density. In lithium-ion secondary batteries, silicon-carbon materials are often used for the negative and positive electrodes of lithium-ion secondary batteries due to their high specific capacity and high nickel (based on the sum of the molar content of transition metals in the positive electrode active material, the molar content of Ni>80%) materials. They are favored by the power tool market, but at the same time, high nickel materials also have problems such as low compaction density, poor rate performance, and severe gas production, while silicon-carbon materials have cyclic expansion problems, which are not conducive to the performance improvement of lithium-ion secondary batteries. Summary of the invention

[0003] The purpose of the present invention is to overcome the problem that when high nickel material is used for the positive electrode and silicon carbon material is used for the negative electrode in a lithium ion secondary battery, both the high nickel material and the silicon carbon material will reduce the rate performance of the battery and cause the battery to produce gas, which is not conducive to the safety performance of the battery. This problem is particularly significant under high voltage. The present invention provides a lithium ion secondary battery (hereinafter referred to as a battery), by using a positive electrode sheet containing a positive electrode material of a first particle and a second particle, and at the same time regulating the specific surface area A (unit: m 2 / g), the oil absorption value B of the silicon-carbon material (unit is mL / 100g) and the mass content C of lithium difluorophosphate in the electrolyte (unit is %) so that they meet the following conditions: 0.13≤C / A≤5.45, 0.002≤C / B≤0.23. The battery obtained by the present invention has reduced gas production and good high-temperature cycle and high-temperature storage capabilities.

[0004] In the prior art, in order to improve the energy density of batteries containing high-nickel materials (based on the sum of the molar contents of transition metals in the positive electrode active materials, the molar content of Ni is greater than 80%), the following two methods are usually used: reducing the average particle size of the high-nickel material particles in the positive electrode active materials, or increasing the molar content of nickel in the high-nickel material to more than 90% to increase the compaction density of the high-nickel material, but this will cause the battery's rate performance to decrease, and at the same time make the battery gas production more serious, thereby reducing the battery's safety performance; and in the prior art, the positive electrode containing high-nickel materials is often paired with the negative electrode containing silicon-carbon materials. Although the battery obtained by this combination has a high energy density, the battery will not only be affected by the volume expansion of the negative electrode, resulting in a decrease in safety performance, but will also cause a decrease in rate performance and increased gas production, which will further deteriorate the high-temperature storage and high-temperature cycle performance.

[0005] In order to solve the above problems, the present invention provides a lithium ion secondary battery, the lithium ion secondary battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte; the positive electrode sheet comprises a positive electrode active material, and the positive electrode active material comprises a first particle and a second particle;

[0006] The first particle includes a chemical formula of Li x Ni a1 Co b1 Mn c1 M 1 d1 O2 positive electrode active material, 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 particles include at least one of Al, Zr, B, Y, Sr, W, Ti, Mg and Nb, the first particles include single crystal particles, and the second particles include a chemical formula of Li y Ni a2 Co b2 Mn c2 M 2 d2 O2 positive electrode active material, 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 comprising at least one of Al, Zr, B, Y, Sr, W, Ti, Mg and Nb, wherein the second particles comprise polycrystalline particles;

[0007] The negative electrode sheet includes a negative electrode active material, the negative electrode active material includes a silicon-carbon material, and the silicon-carbon material includes a porous carbon matrix and a 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, in m 2 / g, the oil absorption value of the silicon-carbon material is B, the unit is mL / 100g, the mass content of the lithium difluorophosphate in the electrolyte is C, the unit is %, 0.13≤C / A≤5.45, 0.002≤C / B≤0.23.

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

[0011] (1) In the present invention, by adjusting the composition and crystal structure of the first particle and the second particle in the positive electrode active material so that the first particle includes single crystal particles and the second particle includes polycrystalline particles, the compaction density of the positive electrode sheet is increased, the porosity of the positive electrode sheet is reduced, gas generation is suppressed, and the energy density of the battery is increased.

[0012] (2) In the present 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 the lithium iron phosphate difluorophosphate in the electrolyte, the battery's liquid retention capacity and the stability of the SEI film are improved, while the battery's energy density and cycle stability are improved.

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

[0014] The endpoints and any values ​​of the ranges disclosed in this article 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Shown is the XRD diffraction pattern of the silicon-carbon material in one embodiment of the present invention.

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

[0017] Figure 3 Shown is a schematic diagram of a multi-electrode winding structure in one embodiment of the present invention.

[0018] Figure 4 FIG. 1 is a schematic diagram of a battery core structure according to an embodiment of the present invention. DETAILED DESCRIPTION

[0019] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

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

[0021] In the present 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 (for example, 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 (for example, 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 (for example, 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 (for example, 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 (for example, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06 or 0.08), M 1 Comprises at least one of Al, Zr, B, Y, Sr, W, Ti, Mg and Nb, and the first particles comprise single crystal particles.

[0022] In the present invention, the second particles comprise 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 (for example, 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 (for example, 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 (for example, 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 (for example, 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 (for example, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.08 or 0.1), M 2The present invention comprises at least one of Al, Zr, B, Y, Sr, W, Ti, Mg and Nb, and the second particles comprise polycrystalline particles.

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

[0024] In the present invention, by regulating the crystal structure and chemical composition of the first particle and the second particle, the positive electrode active material has a mixture of single crystal particles and polycrystalline particles, and the content of Co, other metal elements M in the positive electrode active material is regulated. 1 and M 2 The molar content can achieve high rate performance while having high capacity of the positive electrode active material. The battery's charge and discharge cycle capacity retention rate is improved, especially the cycle stability at high temperature is significantly improved, and the high temperature storage performance is improved.

[0025] In the present invention, the chemical composition of the first particle and the second particle can be measured by conventional testing methods in the art, for example, by an ICP spectrometer test, specifically as follows: take about 0.1g of the sample, add 10mL of concentrated hydrochloric acid, and digest it on a 350°C hot plate for 10min, cool it, and make the volume up to 100ml with deionized water, take part of the solution and analyze it with an ICP spectrometer, and finally determine the final content of the measured element in each sample through the spectrum to obtain the test result.

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

[0027] In the present invention, the electrolyte includes lithium difluorophosphate; the specific surface area of ​​the positive electrode active material is A (in m 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 (for example, 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 (for example, 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 the present invention, by regulating C / A and / or C / B within a certain range, 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 can maintain a good synergistic relationship, so that the generated SEI film is more stable, the impedance is low, the battery cycle and rate performance are good, and gas production is suppressed. Specifically, when C / A>3.74 and / or C / B is greater than 5.45, the mass content of lithium difluorophosphate in the electrolyte is too high to reduce the ionic conductivity, 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 small, resulting in insufficient electrolyte infiltration, and the liquid retention of the negative electrode sheet is reduced, the stored electrolyte is reduced, which is not conducive to the formation of the SEI film, resulting in increased battery impedance, gas production, and deterioration of cycle and rate performance; when C / A<0.13 and / or C / B<0.00 2, the mass content of lithium difluorophosphate in the electrolyte is too low, which 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 the specific surface area of ​​the positive electrode active material being too large and excessively in contact with the electrolyte, which aggravates the occurrence of side reactions, the oil absorption value of the negative electrode sheet is too large, the porosity of the negative electrode sheet is increased, and the formed SEI film is too thick, resulting in increased consumption of lithium ions and reduced transmission efficiency, which is not conducive to the cycle performance and rate performance of the battery.

[0030] In the present 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, in m 2 / g.

[0032] In the present invention, regulating the specific surface area A of the positive electrode active material is beneficial to regulating the contact area between the positive electrode active material and the electrolyte and improving the wettability of the electrolyte; when A>1.2m 2 / g, the specific surface area of ​​the positive electrode active material is too large, and excessive contact with the electrolyte leads to aggravated side reactions, resulting in poor battery cycle performance and low energy density; when A<0.5m 2 / g, the specific surface area of ​​the positive electrode active material is too small, the electrolyte is difficult to infiltrate, the capacity of the positive electrode active material is difficult to fully exert, the energy density of the battery is low, and the capacity retention rate is poor.

[0033] In the present invention, the specific surface area A of the positive electrode active material can be obtained by conventional testing methods in the art, for example, the Brunauer-Emmett-Teller (BET) test method is used, specifically as follows: after the lithium-ion secondary battery is discharged to 0% SOC, the positive electrode plate is disassembled and taken out, and then soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then rinsed with DMC to remove the lithium salt attached to the plate, the aluminum foil is removed, and the active material is baked at 300°C for 30 minutes to obtain a positive electrode active material layer powder as a test sample, which is measured using a TriStarⅡ specific surface analyzer, and N2 is the adsorbed gas.

[0034] In the present 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, in units of mL / 100 g.

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

[0036] In the present invention, the oil absorption value of the silicon-carbon material is related to the liquid retention capacity of the battery and matches the positive electrode active material. A sufficient liquid retention capacity of the battery is a sufficient condition to support long cycles. With the consumption of electrolyte lithium salts and the occurrence of side reactions during the cycle, when the electrolyte is reduced, lithium ions lose the channel for migration between the positive and negative electrode active materials, and safety issues such as lithium precipitation, volume expansion, poor cycle stability, and even short circuit fire usually occur.

[0037] At the same time, temperature will also have a significant impact on battery performance. On the one hand, high temperature will accelerate the transmission speed of lithium ions inside the battery and improve the performance of lithium-ion batteries, but on the other hand, it will also lead to the intensification of battery interface side reactions, causing capacity loss and electrical performance degradation; in particular, the crystal structure of high-nickel materials is affected during storage, resulting in increased polarization. During full-charge storage, the electrolyte will undergo oxidative decomposition on the surface of the high-nickel material, thereby producing an interfacial phase change layer, resulting in increased impedance and consumption of active lithium; while silicon-carbon materials can increase the liquid retention capacity of the battery system and delay further deterioration of the battery due to accelerated consumption of the electrolyte at high temperatures.

[0038] Therefore, regulating 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 negative electrode active materials, and reduce the occurrence of side reactions. When B>100, the oil absorption value of silicon-carbon materials is too large, resulting in excessive specific surface area and porosity of the negative electrode sheet, forming a large number of SEI films, which is not conducive to the transmission of lithium ions and aggravates the consumption of lithium ions; when B<10, the oil absorption value of silicon-carbon materials is too small, the stored electrolyte is too little, and it is difficult to match the positive electrode sheet. The electrolyte is constantly decomposed or participates in the circulation, and the stored electrolyte is completely consumed, causing lithium precipitation.

[0039] In the present invention, the oil absorption value of the silicon-carbon material can be measured by conventional testing methods in the art, for example, by the following method: scrape off the negative electrode active material layer of the negative electrode plate, wash off the binder, and dry (vacuum drying oven 100°C, 12h) to obtain the negative electrode active material powder, weigh the mass m1 of the clean beaker and the glass rod, add 5g of the negative electrode active material powder and record the total mass m2, add dioctyl phthalate (DOP) dropwise with a titration bottle, and stir well, stop adding (DOP) when a lump is formed, weigh the total weight m3 of the beaker 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 the present 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, and the unit is %.

[0042] In yet another embodiment, C is 1-3, and the unit is %.

[0043] In the present invention, the mass content C of lithium difluorophosphate in the electrolyte is regulated, and the lithium ions in the lithium difluorophosphate can participate in the construction of the interface film, improve the lithium ion conductivity, reduce the interface impedance, and increase the transfer rate of lithium ions between the positive and negative electrodes to improve the rate performance of the battery. Lithium difluorophosphate is introduced into the electrolyte. Due to the electron-withdrawing effect of fluorine atoms, the highest occupied orbital and the lowest unoccupied orbital energy levels of the solvent in the electrolyte can be reduced by replacing hydrogen atoms with fluorine atoms, thereby enhancing the antioxidant properties of the solvent in the electrolyte and decomposing on the surface of the silicon-carbon material to form a lithium fluoride (LiF)-rich SEI film. This SEI can be evenly and densely covered on the silicon-carbon surface, inhibiting side reactions at the interface and maintaining stability during the cycle, inhibiting gas production, and having good high-temperature storage performance. When C is less than 0.1%, the mass content of lithium difluorophosphate in the electrolyte is too small, and the effect of improving the conductivity of the electrolyte is not obvious. When C is greater than 5%, due to the low solubility of lithium difluorophosphate in the electrolyte, when the mass content of lithium difluorophosphate in the electrolyte is too large, the conductivity will decrease instead.

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

[0045] In the present invention, the mass content of the first particles 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 the present invention, the average particle size of the first particles 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 particles 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 the present invention, the particle size Dv10 of the first particles is 0.5 μm-2 μm, for example, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm or 2 μm; the median particle size Dv50 of the first particles 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 , 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm or 5μm; the particle size Dv90 of the first particles 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 the present invention, the particle size Dv10 of the second particles is 3 μm-12 μm, for example, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm or 12 μm; the median particle size Dv50 of the second particles 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, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm The particle size of the second particles is Dv90≤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 the present invention, the first particles include single crystal particles, and the second particles include polycrystalline particles. The ratio of single crystal particles to polycrystalline particles in the positive electrode active material can be regulated by adjusting the mass content of the first particles in the positive electrode active material. At the same time, the particle size distribution of the first particles and the second particles is regulated within a reasonable range, so that the amount and particle size of the first particles and the second particles are matched, thereby ensuring that a good surface structure can be maintained during mixing, and the first particles containing single crystal particles can be filled and embedded between the second particles containing polycrystalline particles, so that the overall compaction density of the positive electrode sheet is improved, the stacking utilization rate of the positive electrode active material is improved, the porosity of the positive electrode sheet is reduced, and the contact between the electrolyte and the positive electrode active material is reduced, thereby alleviating the occurrence of side reactions, improving the volume energy density and cycle stability of the battery, and delaying and reducing battery gas production.

[0050] In the present invention, the single crystal particles in the first particles and the polycrystalline particles in the second particles can be measured by conventional testing methods in the art, such as scanning electron microscopy imaging of the surface of the positive electrode sheet and image analysis to distinguish single crystal particles from polycrystalline particles from the SEM image.

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

[0052] In the present invention, the particle size Dv10, median particle size Dv50 and particle size Dv90 of the first particles and the particle size Dv10, median particle size Dv50 and particle size Dv90 of the second particles can be obtained by conventional testing methods in the art, for example, by a laser particle size tester, specifically as follows: after the lithium-ion secondary battery is discharged to 0% SOC, the positive electrode plate is disassembled and taken out, and after being soaked in dimethyl carbonate (DMC) solvent for 12 hours, it is then rinsed with DMC to remove the lithium salt attached to the plate, the aluminum foil is removed, and the active material is baked at 300°C for 30 minutes to obtain a positive electrode active material layer powder; the Malvern particle size tester is used for measurement, and the test steps are as follows: the positive electrode active material layer powder is dispersed in deionized water containing a dispersant (such as nonylphenol polyoxyethylene ether, with a content of 0.02-0.03wt%) to form a mixture, the mixture is ultrasonicated for 2 minutes, and then placed in a Malvern particle size tester for testing, and the corresponding particle size distribution data is obtained.

[0053] In the present invention, the specific surface area of ​​the first particles is 0.6 m 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 the present invention, the specific surface area of ​​the second particles is 0.35 m2 / 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 the present invention, the second particles have a lower specific surface area than the first particles. The two particles are mixed in a suitable ratio, which can reduce the porosity of the positive electrode sheet, reduce the direct contact between the electrolyte and the positive electrode active material, and reduce the occurrence of side reactions.

[0056] In the present invention, the specific surface areas of the first particles and the second particles can be obtained by conventional testing methods in the art, such as using the Brunauer-Emmett-Teller (BET) test method, using a TriStarⅡ specific surface analyzer for measurement, and using N2 as the adsorbed gas.

[0057] In the present 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 the present invention, M 2 Including Al.

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

[0061] In the present invention, the first particle includes the element Zr, and the second particle includes the element Al. The element Zr can promote the growth of single crystal particles, enhance structural stability, and is not prone to breakage, thereby improving the cycle performance and high-temperature storage performance of the battery; the element Al can stabilize the lattice, slow down the phase transition from the ternary layered phase to the spinel phase of the positive electrode active material during the cycle, reduce the volume change of the positive electrode active material during the lithium ion charging and discharging and deintercalation process, inhibit the appearance of microcracks in the second particle, and significantly improve the cycle stability of the battery; the introduction of the element Zr and the element Al generates a stable protective film on the surface of the first particle and the second particle, increases the capacity of the positive electrode active material, and improves the surface ion transmission efficiency, thereby improving the high-temperature storage performance and high-temperature cycle performance of the battery.

[0062] In the present invention, the mass content of the element Al in the second particle and the mass content of the 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 the present invention, the electrolyte further comprises ethyl 2,2-difluoroacetate.

[0064] In the present invention, the mass content of ethyl 2,2-difluoroacetate in the electrolyte is 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 mass content of ethyl 2,2-difluoroacetate in the electrolyte is 5%-30%.

[0066] In the present invention, ethyl 2,2-difluoroacetate (DFEA) is introduced into the electrolyte. On the one hand, it can cooperate with lithium difluorophosphate to decompose on the surface of the silicon-carbon material to form a LiF-containing SEI film and a CEI film on the surface of the positive electrode sheet, respectively, and the formed interface film is more stable; on the other hand, for a battery system including single crystal particles and polycrystalline particles of positive electrode active materials, DFEA has good chemical stability and electrochemical properties, can form a stable interface film at the positive terminal, reduce crystal defects and phase changes of single crystal particles, improve the agglomeration of polycrystalline particles, improve material stability, reduce thermal decomposition, and at the same time can reduce the polarization voltage of single and polycrystalline particles, improve the charge and discharge efficiency of the battery, reduce energy loss, and improve battery performance; in addition, due to the gas production problem of the battery under high voltage, the high temperature cycle and high temperature storage performance attenuation are aggravated, and by adding ethyl 2,2-difluoroacetate, in the presence of lithium difluorophosphate, the stability of the interface film is further improved, so that the battery can adapt to high voltage film formation, inhibit the oxidative decomposition of the electrolyte, and enable the battery to still have good high temperature storage performance and high temperature cycle performance under high voltage.

[0067] In the present 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-mass spectrometry.

[0068] In the present 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 the silicon-carbon material in the negative electrode active material is 20%-60%.

[0070] In the present 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 the present invention, the mass content of silicon in the silicon-carbon material is 20%-50%, for example, 20%, 22%, 24%, 26%, 28%, 30%, 32%, 36%, 40%, 45% or 50%.

[0072] In the present invention, the silicon material has a serious volume expansion (~300%) during the battery charging and discharging process, and the conductivity is low, while the carbon material has a small volume change during the charging and discharging process, has a good cycle stability performance, and the carbon material itself is a mixed conductor of ions and electrons, and has good conductivity; therefore, in the silicon-carbon material, the silicon material is used as an active substance to provide lithium storage capacity, and the carbon material can not only buffer the volume change of the silicon negative electrode during the charging and discharging process, but also improve the conductivity of the silicon material, and can also avoid the silicon material from agglomerating during the charging and discharging cycle; the silicon-carbon material in the negative electrode active material layer combines the advantages of silicon material and carbon material, and exhibits high specific capacity and long cycle life. When the mass content of the silicon-carbon material in the negative electrode active material is too small, the energy density of the battery is not significantly improved, and when the mass content of the silicon-carbon material in the negative electrode active material is too large, the overall volume expansion of the battery will be too large, resulting in poor cycle stability and conductivity of the battery.

[0073] In the present invention, the mass content of the silicon-carbon material in the negative electrode active material can be obtained by conventional testing methods in the art, for example, by thermogravimetric analysis, using a Shimadzu DTG-60 thermogravimetric analyzer as the equipment, and the specific method is as follows: after the lithium-ion secondary battery is discharged to 0% SOC, the negative electrode plate is disassembled and taken out, and then soaked in a dimethyl carbonate (DMC) solvent for 12 hours and then rinsed with DMC to remove the lithium salt attached to the plate, and then the plate is dried and then treated at a high temperature of 400°C in an inert atmosphere for 2 hours (such as a tubular furnace, nitrogen or argon atmosphere), and the negative electrode active material layer can be peeled off from the current collector, and the negative electrode active material layer powder is collected; in the mass of the silicon element In the test of the amount content, a thermogravimetric analyzer (such as TGA550 thermogravimetric analyzer) is used, 5-15 mg of the collected negative electrode active material layer powder is taken, and the temperature is increased from room temperature to 900°C at a heating rate of 10°C / min in an air or oxygen atmosphere, and kept at 900°C for 40 minutes, so that the non-silicon components in the negative electrode active material layer can be volatilized and the silicon can be fully oxidized to silicon dioxide. The weight percentage of the substance finally obtained 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 the present invention, the specific surface area of ​​the silicon-carbon material is 0.5 m 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 m 2 / g-3m 2 / g.

[0076] In the present invention, the specific surface area of ​​the silicon-carbon material can be measured by conventional testing methods in the art, such as using the Brunauer-Emmett-Teller (BET) test method, such as using a TriStarⅡ specific surface area analyzer for measurement, with N2 as the adsorbed gas.

[0077] In the present 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-peak width W1 of the first diffraction peak and the half-peak width 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 the present invention, the Raman spectrum of the silicon-carbon material is at 470 cm -1 -480cm -1 The first characteristic peak is 1330cm -1 -1350cm -1 With a second characteristic peak, 1590cm -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 the present 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 shown Figure 2 As described above, it can be seen that the silicon-carbon material consists of amorphous silicon and amorphous carbon.

[0080] In the present 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 degree of defect of the material. Then (I1+I2) / I3 can reflect the crystallinity of the silicon-carbon material and the defects on the surface of the silicon-carbon material. When (I1+I2) / I3<0.5, the crystallinity of silicon in the silicon-carbon material is high, which will lead to large cycle expansion of the battery and poor cycle capacity retention rate. When (I1+I2) / I3>3, the surface defects of the silicon-carbon material are too many, resulting in its first coulombic efficiency being too low, which is not conducive to the improvement of the battery energy density.

[0081] During the lithiation / delithiation process, the volume expansion / contraction of the silicon-carbon material causes great stress, which not only causes the cracking and crushing of the silicon-carbon material, but also destroys the originally regular distance between the silicon-carbon materials. The powdered and broken silicon-carbon materials are separated from the conductive network and lose electrical contact with the current collector, resulting in a decrease in battery capacity and eventual failure. At the same time, the originally intact SEI film is ruptured, and the silicon-carbon material re-contacts with the electrolyte to form the SEI film again. As the cycle progresses, the SEI film is repeatedly formed-ruptured-formed, making it thicker and thicker, resulting in the active Li + The loss of charge and the increase of interface resistance shorten the battery cycle life.

[0082] In the present 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, and thus has a higher specific surface area and oil absorption value, and has a stronger adsorption capacity for electrolyte; amorphous silicon has no clear crystal structure, exhibits isotropic physical properties, has a loose structure and a large number of pores. The silicon-carbon material composed of amorphous silicon and amorphous carbon has a large specific surface area, many pores with reasonable distribution, a high oil absorption value, a stronger adsorption capacity for electrolyte, and improved liquid retention; compared with the silicon-carbon material with a crystalline structure, amorphous silicon carbon has the function of dispersing stress when its volume expands during the charging and discharging process, and the silicon-carbon material is not easy to crack and break, avoiding the collapse of the material after expansion and contraction, which is beneficial to improving the cycle performance of the battery, delaying the further deterioration of the battery caused by the accelerated consumption of the electrolyte at high temperature, and having good high-temperature storage performance; on the other hand, amorphous silicon carbon has a high oil absorption value, and lithium difluorophosphate in the electrolyte is beneficial to improving the formation efficiency of the SEI film. The combination of the two can optimize the formation of the SEI film and increase the deintercalation rate of lithium ions on the surface of the silicon-carbon material, which is beneficial to the realization of fast charging.

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

[0084] In the present invention, the Raman spectrum test method of the silicon-carbon material is as follows: Raman spectroscopy (Raman) is used for testing, for example, a ThermoFisher Raman spectrometer is used, and the test wave number range is 400-4000cm -1 .

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

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

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

[0088] In the present 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 comprises a multi-electrode winding structure.

[0091] In the present invention, a multi-electrode tab structure is adopted. The multi-electrode tab winding structure refers to cutting a fixed pole tab shape in a carrier, and then welding the pole tabs to the carrier after winding is completed to form a multi-electrode tab battery; Figure 3 The figure shows a schematic diagram of a multi-electrode winding structure. The multi-electrode winding pole piece has multiple pole ears, and the pole ear positions are more evenly distributed, which can not only further reduce the battery impedance, improve the battery's high-rate charge and discharge performance, support 5C-10C discharge, but also effectively increase the lithium ion deintercalation speed in the battery, reduce the temperature rise of the battery under high-rate discharge, reduce battery heat, and significantly increase the battery cycle life.

[0092] In the present invention, the electrolyte may further include a lithium salt, and the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiDFOB), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(trifluoromethylsulfonyl)imide, lithium difluorobis(oxalatophosphate), lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(pentafluoroethylsulfonyl)imide, tris(trifluoromethylsulfonyl)methyl lithium and lithium bis(trifluoromethylsulfonyl)imide.

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

[0094] In the present invention, the electrolyte may further include other additives, including at least one of fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), 1,3-acrylonitrile lactone (PST), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), succinonitrile (SN), adiponitrile (ADN) and 1.3.6-hexanetrinitrile (HTCN).

[0095] In the present 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 the present invention, the other additives can form films on the surfaces of the positive electrode sheet and the negative electrode sheet, and have a protective effect on the positive electrode sheet and the negative electrode sheet.

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

[0098] In the present 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 ethyl methyl carbonate (EMC).

[0099] In the present invention, EC in the organic solvent can participate in the formation of SEI film during the first charge and discharge cycle of the battery, can improve the efficiency of subsequent lithium ion deintercalation at the negative electrode, and reduce the occurrence of side reactions; PC has a higher dielectric constant, can promote the dissociation of lithium salts, and greatly improve the ion conductivity in the solution; lithium salt determines the basic physical and chemical properties of the electrolyte, is the most important component in the electrolyte that affects the characteristics of lithium-ion batteries, can conduct lithium ions, and increase the conductivity of the electrolyte.

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

[0101] In the present invention, the negative electrode sheet also includes a negative electrode current collector, a negative electrode conductor and a negative electrode binder, and all of them are conventional choices of 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 conductor 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 expressions such as "first" and "second" in the present invention are only used to distinguish different substances or usages, and do not represent the difference in order.

[0103] The present invention will be described in detail below by way of examples. The examples described in the present invention are only a part of the examples of the present invention, rather than all of the examples. Based on the examples in the present invention, all other examples obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

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

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

[0106] Embodiment 1:

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

[0108] 1.2% PVDF, 0.4% single-walled carbon nanotubes and 35% NMP were stirred respectively (solid content was 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 particle (chemical formula LiNi 0.93 Co 0.04 Mn 0.02 Zr 0.01 O2, Zr content in the first particle is 2000ppm) and 17% in the second particle (chemical formula is LiNi 0.89 Co 0.05 Mn 0.04 Al 0.02 O2, Al mass content in the second particle is 2700ppm); the positive electrode active slurry is evenly coated on a 10μm aluminum foil using a coating machine, baked at a temperature of 120°C for 12h, and then rolled and die-cut to obtain a positive electrode sheet.

[0109] The average particle size of the first particle is 3.2 μm and the specific surface area is 1 m 2 / g, the average particle size of the second particles is 13.2μm, and the specific surface area is 0.48m 2 / g; At this time, the specific surface area A of the positive electrode active material is 0.91m 2 / g.

[0110] (2) Preparation of negative electrode sheet:

[0111] 96.1% of negative electrode active material, 0.25% of single-walled carbon tube, 0.15% of multi-walled carbon tube and 2.9% of SBR were mixed, and EC accounting for 1% of the mass fraction of the negative electrode active material was added, and the negative electrode active slurry was obtained after stirring evenly; wherein the negative electrode active material was composed of 54.5% of artificial graphite and 45.5% of silicon-carbon material. The negative electrode active slurry was evenly coated on a 4μm high-strength carbon-coated copper foil, and after drying, it was rolled, die-cut and sheeted to obtain a negative electrode sheet.

[0112] At this time, the specific surface area of ​​the silicon carbon material is 2.5m 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 < 0.1ppm, O2 < 0.1ppm), EC, PC, DEC, and propyl propionate are mixed 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, 1.25mol / L of fully dried lithium hexafluorophosphate (LiPF6), 15% DFEA and 2.2% lithium difluorophosphate (i.e., C = 2.2%) are quickly added and dissolved in the non-aqueous organic solvent, stirred evenly, and finally 0.5% succinonitrile, 2% HTCN, 8% FEC, and 2.5% PS additives are added. After the moisture and free acid tests are qualified, the desired electrolyte is obtained.

[0115] (4) Preparation of diaphragm:

[0116] A conventional commercial diaphragm was selected, in which the thickness of the substrate, ceramic and PVDF adhesive were 5μm, 1μm and 1.5μm respectively, and the surface density was 5.8g / m 2 .

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

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

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

[0120] like Figure 3 The diagram shows a multi-electrode winding structure. Figure 4 Shown is a schematic diagram of a battery roll core.

[0121] Embodiment 2:

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

[0123] 1.2% PVDF, 0.4% single-walled carbon nanotubes and 35% NMP were stirred respectively (solid content was 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 particle (chemical formula LiNi 0.86 Co 0.07 Mn 0.04 Zr 0.03 O2, Zr content in the first particle is 1800ppm) and 15% of the second particle (chemical formula is LiNi 0.83 Co 0.06 Mn 0.05 Al 0.06 O2, Al mass content in the second particle is 2500ppm); the positive electrode active slurry is evenly coated on a 10μm aluminum foil using a coating machine, baked at 120°C for 12h, and then rolled and die-cut to obtain a positive electrode sheet.

[0124] The average particle size of the first particle is 4.3 μm and the specific surface area is 0.85 m 2 / g, the average particle size of the second particles is 16.5μm, and the specific surface area is 0.53m 2 / g; at this time, the specific surface area A of the positive electrode active material is 0.8m 2 / g.

[0125] (2) Preparation of negative electrode sheet:

[0126] 96.1% of negative electrode active material, 0.25% of single-walled carbon tube, 0.15% of multi-walled carbon tube and 2.9% of SBR were mixed, and EC accounting for 1% of the mass fraction of the negative electrode active material was added, and stirred evenly to obtain negative electrode active slurry; wherein the negative electrode active material is composed of 60% of artificial graphite and 40% of silicon carbon material. The negative electrode active slurry was evenly coated on a 4μm high-strength carbon-coated copper foil, and after drying, it was rolled, die-cut and sheeted to obtain a negative electrode sheet.

[0127] At this time, the specific surface area of ​​the silicon-carbon material is 3m 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 < 0.1ppm, O2 < 0.1ppm), EC, PC, DEC, and propyl propionate are mixed 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, 1.25mol / L of fully dried lithium hexafluorophosphate (LiPF6), 5% DFEA and 3% lithium difluorophosphate (i.e., C = 3%) are quickly added and dissolved in the non-aqueous organic solvent, stirred evenly, and finally 0.5% succinonitrile, 2% HTCN, 8% FEC, and 2.5% PS are added. After passing the moisture and free acid tests, the desired electrolyte is obtained.

[0130] (4) Preparation of diaphragm:

[0131] A conventional commercial diaphragm was selected, in which the thickness of the substrate, ceramic and PVDF adhesive were 5μm, 1μm and 1.5μm respectively, and the surface density was 5.8g / m 2 .

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

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

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

[0135] Embodiment 3:

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

[0137] 1.2% PVDF, 0.4% single-walled carbon nanotubes and 35% NMP were stirred (solid content was 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 particle (chemical formula LiNi 0.97 Co 0.015 Mn 0.01 Zr 0.005 O2, Zr content in the first particle is 2200ppm) and 20% of the second particle (chemical formula is LiNi 0.95 Co 0.03 Mn 0.01 Al 0.01O2, Al mass content in the second particle is 2900ppm); the positive electrode active slurry is evenly coated on a 10μm aluminum foil using a coating machine, baked at 120°C for 12h, and then rolled and die-cut to obtain a positive electrode sheet.

[0138] The average particle size of the first particle is 1.6 μm and the specific surface area is 1.12 m 2 / g, the average particle size of the second particles is 10.7 μm, and the specific surface area is 0.4 m 2 / g; at this time, the specific surface area A of the positive electrode active material is 0.98m 2 / g.

[0139] (2) Preparation of negative electrode sheet:

[0140] 96.1% of negative electrode active material, 0.25% of single-walled carbon tube, 0.15% of multi-walled carbon tube and 2.9% of SBR were mixed, and EC accounting for 1% of the mass fraction of the negative electrode active material was added, and the negative electrode active slurry was obtained after stirring evenly; wherein the negative electrode active material was composed of 70% of artificial graphite and 30% of silicon-carbon material. The negative electrode active slurry was evenly coated on a 4μm high-strength carbon-coated copper foil, and after drying, it was rolled, die-cut and sheeted to obtain a negative electrode sheet.

[0141] At this time, the specific surface area of ​​the silicon-carbon material is 0.7 m 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 < 0.1ppm, O2 < 0.1ppm), EC, PC, DEC, and propyl propionate are mixed 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, 1.25mol / L of fully dried lithium hexafluorophosphate (LiPF6), 30% DFEA and 1% lithium difluorophosphate (i.e., C = 1%) are quickly added and dissolved in the non-aqueous organic solvent, stirred evenly, and finally 0.5% succinonitrile, 2% HTCN, 8% FEC, and 2.5% PS are added. After the moisture and free acid tests are qualified, the desired electrolyte is obtained.

[0144] (4) Preparation of diaphragm:

[0145] A conventional commercial diaphragm was selected, in which the thickness of the substrate, ceramic and PVDF adhesive were 5μm, 1μm and 1.5μm respectively, and the surface density was 5.8g / m 2 .

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

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

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

[0149] Embodiment 4 group:

[0150] This group of examples is used to verify the impact of the change of "a1 and a2", which is achieved by changing the chemical formula and element composition of the first particle and the second particle, as follows:

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

[0152] Example 4b is based on Example 1, except that a1=0.975, 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 chemical formula of the second particle is LiNi 0.978 Co 0.01 Mn 0.01 Zr 0.002 O2.

[0153] Embodiment 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 0.8 m 2 / g, B is 20mL / 100g, and C is 3%. At this time, C / A is 3.75 and C / B is 0.15.

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

[0157] Embodiment 6 group:

[0158] This group of examples is used to verify the effect of the change of "A", which is achieved by changing the specific surface area of ​​the first particle and the second particle, as follows:

[0159] Example 6a is based on Example 1, except that the specific surface area of ​​the first particles is 0.6 m 2 / g, the specific surface area of ​​the second particle is 0.35m 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 particles is 1.2 m 2 / g, the specific surface area of ​​the second particle is 0.6m 2 / g, at this time, A is 1.1m 2 / g, C / A is 2.004.

[0161] Embodiment 7 group:

[0162] This group of examples is used to verify the impact of the change of "B", as follows:

[0163] Example 7a is based on Example 1, except that B is 10.5 mL / 100 g. At this time, the specific surface area of ​​the silicon-carbon material is 1.75 m 2 / g, C / B is 0.21.

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

[0165] Embodiment 8 group:

[0166] This group of examples is used to verify the impact of the change of "C", as follows:

[0167] Example 8a is based on Example 1, except that C is 0.12%, then 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] Embodiment 9 group:

[0170] This group of examples is used to verify the impact of the change in "the mass fraction of the first particles in the positive electrode active material", as follows:

[0171] Example 9a is based on Example 1, except that the mass fraction of the first particles in the positive electrode active material is 71.2%. In this case, A is 0.85 m 2 / g, C / A is 2.588.

[0172] Example 9b is based on Example 1, except that the mass fraction of the first particles in the positive electrode active material is 89.6%. In this case, A is 0.95 m 2 / g, C / A is 2.326.

[0173] Example 10 Group:

[0174] This group of examples is used to verify the influence of the change of "the average particle size of the first particles and the second particles", as follows:

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

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

[0177] Embodiment 11 group:

[0178] This group of examples is used to verify the influence of the change of "the mass content of Zr in the first particle and the mass content of Al in the second particle", which is as follows:

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

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

[0181] Embodiment 12 group:

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

[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] Embodiment 13 group:

[0186] This group 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 follows:

[0187] Example 13a is based on Example 1, except that the negative electrode active material is composed of 80% artificial graphite and 20% silicon-carbon material. In this case, the specific surface area of ​​the silicon-carbon material is 0.78 m 2 / g, the oil absorption value B of silicon carbon material is 23mL / 100g, and C / B 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 this case, the specific surface area of ​​the silicon-carbon material is 3m 2 / g, the oil absorption value B of silicon-carbon material is 70mL / 100g, and C / B is 0.031.

[0189] Example 13c is based on Example 1, except that the negative electrode active material is composed of 89.4% artificial graphite and 10.6% silicon-carbon material. At this time, the specific surface area of ​​the silicon-carbon material is 0.65 m 2 / g, the oil absorption value B of silicon carbon material is 18mL / 100g, and C / B is 0.122.

[0190] Example 13d is based on Example 1, except that the negative electrode active material is composed of 20.2% artificial graphite and 79.8% silicon-carbon material. At this time, the specific surface area of ​​the silicon-carbon material is 9.5 m 2 / g, the oil absorption value B of silicon-carbon material is 80mL / 100g, and C / B is 0.028.

[0191] Embodiment 14 group:

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

[0193] Example 14a is based on Example 1, except that the specific surface area of ​​the silicon-carbon material is 0.53 m 2 / g, at this time, B is 10.5mL / 100g, and 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 m 2 / g, at this time, B is 100mL / 100g, and C / B is 0.022.

[0195] Embodiment 15:

[0196] Based on Example 1, the difference is that the battery adopts 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≤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≤25μm.

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

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

[0200] Comparative Example 1:

[0201] This group of examples is used to verify the influence of the composition of the positive electrode active material, as follows:

[0202] Comparative Example 1a is based on Example 1, except that the mass fraction of the first particles in the positive electrode active material is 0%, and the positive electrode active material is all the second particles, and A is 0.48 m 2 / g, then C / A is 4.583.

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

[0204] Comparative Example 2 Group:

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

[0206] Comparative Example 1a is based on Example 1, except that A is 1.1 m 2 / g, B is 99.6mL / 100g, C is 0.12%, then C / A is 0.109 and C / B is 0.001.

[0207] Comparative Example 1b is based on Example 1, except that A is 0.52 m 2 / g, B is 10.5mL / 100g, C is 4.95%, then C / A is 9.519 and 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. At this time, C / A is 0 and C / B is 0.

[0210] Test example:

[0211] (1) Cycle life test:

[0212] The batteries obtained from the embodiments of the present invention and the comparative examples were placed in a constant temperature environment of 45°C and charged and discharged at a rate of 1.8C / 4.0C. The cut-off voltage range was 2.5V-4.3V. The charge and discharge cycles were 500 times. 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 of every 100T was recorded. The thickness data divided by the initial thickness of the battery was the thickness expansion rate.

[0213] (2) High temperature storage test:

[0214] 60℃ high temperature storage: At an ambient temperature of 25℃±3℃, the batteries obtained from the embodiments of the present invention and the comparative examples were discharged at 0.2C to a cut-off voltage of 3.0V and left to stand for 10min; 0.5C constant current constant voltage charge to an upper limit voltage of 4.3V, with a cut-off current of 0.05C, and the thickness of the fully charged state was tested at 25℃±3℃. The fully charged battery was placed in a test box, and the test box was heated at a temperature rise rate of (5±2)℃ / min. When the temperature in the box reached 60℃±2℃, it was kept at a constant temperature for 35 days; after the test was completed, 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 battery stored at high temperature was cooled to room temperature, discharged at 1C, and then charged and discharged at 1C again, and the capacity retention rate and capacity recovery rate of the battery were calculated respectively, and the formulas are as follows:

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

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

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

[0218] 1) Place the above battery at an ambient temperature of 25°C for 5 minutes;

[0219] 2) 0.5C discharge to 2.5V;

[0220] 3) Set aside for 1 hour;

[0221] 4) 0.5C charging, when the battery terminal voltage reaches the charging limit voltage of 4.3V, change to constant voltage charging, and stop charging when the charging current is ≤ the cut-off current;

[0222] 5) Leave for 30 minutes;

[0223] 6) Discharge at a rate to 2.5V, and record the capacity, 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 is completed. Step 6 discharges according to the specified rate and sequence.

[0224] (4) Energy density test:

[0225] Use a blue power test cabinet to charge the battery at a constant current of 0.5C with an upper voltage limit of 4.3V, then perform constant voltage charging with a cutoff current of 0.05C, let it stand for 10 minutes, and then discharge it at 0.5C to 2.0V. The discharge energy is E, unit: Wh. Use a balance to measure the mass of the battery as W, unit: Kg. Therefore, the mass energy density of the battery is D=E / W, unit: Wh / Kg.

[0226] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

[0227] The performance tests of the batteries obtained in the examples and comparative examples of the present invention are recorded in Table 1.

[0228] Table 1:

[0229]

[0230] It can be seen from Table 1 that the lithium ion battery prepared in the present invention has good high temperature storage performance and cycle life compared with the comparative example.

[0231] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A lithium ion secondary battery, characterized in that: Including positive electrode sheet, negative electrode sheet and electrolyte; The positive electrode sheet includes a positive electrode active material, the positive electrode active material includes a first particle and a second particle; the first particle includes a chemical formula of Li x Ni a1 Co b1 Mn c1 M 1 d1 O2 positive electrode active material, 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 comprising at least one of Al, Zr, B, Y, Sr, W, Ti, Mg and Nb, wherein the first particles comprise single crystal particles; The second particle includes a chemical formula of Li y Ni a2 Co b2 Mn c2 M 2 d2 O2 positive electrode active material, 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 comprising at least one of Al, Zr, B, Y, Sr, W, Ti, Mg and Nb, wherein the second particles comprise polycrystalline particles; The negative electrode sheet includes a negative electrode active material, the negative electrode active material includes a silicon-carbon material, and the silicon-carbon material includes a porous carbon matrix and a silicon material located in the pores of the porous carbon matrix; The electrolyte includes lithium difluorophosphate; The specific surface area of ​​the positive electrode active material is A, in m 2 / g, the oil absorption value of the silicon-carbon material is B, the unit is mL / 100g, the mass content of the lithium difluorophosphate in the electrolyte is C, 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. The lithium ion secondary battery according to claim 1 or 2, wherein: A is 0.5-1.2, unit is m 2 / g; and / or, B is 10-100, in mL / 100 g; preferably 20-70, in mL / 100 g; And / or, C is 0.1-5; preferably 0.1-3, the unit is %.

4. The lithium ion secondary battery according to claim 1 or 2, wherein: The mass content of the first particles in the positive electrode active material is 70%-90%; and / or, 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 average particle size of the first particles is 0.5 μm-5 μm, and the average particle size of the second particles is 5 μm-20 μm; and / or, the particle size Dv10 of the first particles is 0.5 μm-2 μm, the median particle size Dv50 is 0.5 μm-5 μm, and the particle size Dv90 is ≤10 μm; and / or, the particle size Dv10 of the second particles is 3 μm-12 μm, the median particle size Dv50 is 5 μm-20 μm, and the particle size Dv90 is ≤25 μm; And / or, the specific surface area of ​​the first particles is 0.6 m 2 / g-1.2m 2 / g; And / or, the specific surface area of ​​the second particles is 0.35m 2 / g-0.6m 2 / g.

6. The lithium ion secondary battery according to claim 1 or 2, wherein: M 1 Including Zr; preferably, the mass content of element Zr in the first particle is 500ppm-2500ppm; and / or, M 2 Comprises Al; preferably, the mass content of element Al in the second particles is 1000ppm-3500ppm.

7. The lithium ion secondary battery according to claim 1 or 2, wherein: The electrolyte also includes ethyl 2,2-difluoroacetate; Preferably, the mass content of ethyl 2,2-difluoroacetate in the electrolyte is 2%-40%; More preferably, the mass content of ethyl 2,2-difluoroacetate in the electrolyte is 5%-30%.

8. The lithium ion secondary battery according to claim 1 or 2, wherein: The mass content of the silicon-carbon material in the negative electrode active material is 10%-80%, preferably 20%-60%; And / or, the mass content of silicon in the silicon-carbon material is 20%-50%; And / or, the specific surface area of ​​the silicon-carbon material is 0.5 m 2 / g-10m 2 / g; preferably 0.7m 2 / g-3m 2 / g.

9. The lithium ion secondary battery according to claim 1 or 2, wherein: The XRD diffraction spectrum 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-peak width W1 of the first diffraction peak and the half-peak width 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 -480cm -1 The first characteristic peak is 1330cm -1 -1350cm -1 With a second characteristic peak, 1590cm -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.

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

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