Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. The lithium ion secondary battery comprises a battery cell and an electrolyte, wherein the battery cell comprises a pole piece; the pole piece comprises a current collector and a tab extending from one side of the current collector, and the ratio of the size of the tab in the width direction of the battery cell to the width of the battery cell is x1; the electrolyte comprises a fluorinated nitrile compound; the chemical formula of the fluorinated nitrile compound is shown in the specification, R is alkyl with the carbon atom number of 1-3, and n is an integer of 1-3; the mass content of the fluorinated nitrile compound in the electrolyte is c1; and x1 and c1 satisfy 0.03 < = c1 / x1 < = 5. The lithium ion secondary battery disclosed by the invention has relatively good cycling stability and relatively low thermal runaway risk under a quick charge condition; and the material has excellent low-temperature performance and high-temperature cycle 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 increasing market demand for faster charging speeds, there is a need to continuously improve the charging speed of lithium-ion rechargeable batteries. Electrolytes, as the lifeblood of lithium-ion batteries, play a crucial role in ion transport under fast-charging conditions. The application of chain-like carboxylic acid esters has further advanced the fast-charging capability of electrolytes. However, as battery energy density and voltage levels continue to rise, the application of chain-like carboxylic acid esters at high voltages is limited. This leads to higher temperatures in batteries under fast-charging conditions, which is detrimental to cycle stability and can even trigger thermal runaway.

[0003] Therefore, it is necessary to reduce the temperature rise of the battery under high voltage and fast charging conditions and improve cycle stability. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems of poor cycle stability and susceptibility to thermal runaway in lithium-ion secondary batteries under high voltage (e.g., 4.48V and above) and fast charging conditions, and to provide a lithium-ion secondary battery. The lithium-ion secondary battery of this invention (hereinafter referred to as the battery) improves the cycle stability of the battery under high voltage and fast charging conditions and effectively reduces the risk of thermal runaway by adjusting the electrodes and electrolyte; furthermore, by adjusting the electrodes and electrolyte, the low-temperature performance and high-temperature cycle performance of the battery can also be improved.

[0005] The inventors of this invention, through extensive research, discovered that fluoronitrile compounds can effectively replace chain-like carboxylic acid esters. Compared to chain-like carboxylic acid esters, fluoronitrile compounds exhibit higher ionic conductivity and lower melting points, significantly improving the conduction performance of lithium ions in the electrolyte and reducing the transport resistance of lithium ions. Furthermore, compared to traditional nitrile compounds, fluoronitrile compounds demonstrate higher stability and greater compatibility with the negative electrode. While traditional nitrile compounds (such as acetonitrile) can improve the ionic conductivity of the electrolyte, they are prone to side reactions with the negative electrode active material, thereby reducing battery stability (especially high-temperature stability). Fluorinated nitrile compounds, containing the element F, effectively prevent side reactions with the negative electrode active material, improving compatibility with the negative electrode sheet and thus enhancing the battery's cycle stability, especially high-temperature stability.

[0006] However, simply adding fluoronitriles to the electrolyte has limited effect on improving the battery's cycle stability under fast charging conditions. This is likely because the battery experiences a significant temperature rise during fast charging, especially near the tabs. This higher temperature rise reduces the battery's cycle stability, and adding fluoronitriles alone does not significantly improve the temperature rise near the tabs. Based on this, the inventors of this invention conducted extensive targeted research and discovered that by synergistically controlling the ratio of the tab's dimension in the cell width direction to the cell width, the battery's cycle stability under high voltage and fast charging conditions can be improved. The reason for this may be that the ratio of the tab's dimension in the cell width direction to the cell width leads to a difference in the battery's current density distribution. When this ratio is large, the current density on the edge side is large, and vice versa. A large ratio results in a higher temperature rise during charging and discharging. Higher temperatures can increase the ionic conductivity of the electrolyte, but they reduce the stability between the electrolyte and the active materials (including the positive and negative electrode active materials). Fluoronitriles, due to their low melting points, have relatively poor thermal stability. Therefore, when this ratio is large, the electrolyte has good ionic conductivity, and the content of fluoronitrile compounds in the electrolyte needs to be reduced to maintain the battery's cycle stability. Conversely, when this ratio is small, the battery temperature rise during charging and discharging is low. Lower temperatures reduce the electrolyte's ionic conductivity, so the content of fluoronitrile compounds in the electrolyte needs to be increased to achieve higher ionic conductivity to meet fast charging requirements. By adjusting the ratio of the tab's dimension in the cell width direction to the cell width and the mass content of fluoronitrile compounds in the electrolyte, the battery can achieve optimal fast charging performance while maintaining optimal stability. Based on this, the inventors of this invention propose the following solution:

[0007] This invention provides a lithium-ion secondary battery, comprising a cell and an electrolyte. The cell includes an electrode; the electrode includes a current collector and a tab extending from one side of the current collector, the ratio of the dimension of the tab in the width direction of the cell to the width of the cell is x1; the electrolyte includes a fluoronitrile compound; the chemical formula of the fluoronitrile compound is […]. Wherein, R is an alkyl group with 1-3 carbon atoms, and n is an integer from 1 to 3; the mass content of the fluoronitrile compound in the electrolyte is c1; x1 and c1 satisfy: 0.03≤c1 / x1≤5.

[0008] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0009] (1) The battery of the present invention can improve the cycle stability under high voltage and fast charging conditions and effectively reduce the risk of thermal runaway;

[0010] (2) The battery of the present invention has excellent low temperature performance, high temperature cycle performance and overcharge and over-discharge performance.

[0011] 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

[0012] Figure 1 The diagram shown is a schematic diagram of a battery cell in an embodiment of the present invention, wherein, Figure 1 (a) The battery cell is a wound core. Figure 1 (b) The battery cell is a stacked core.

[0013] Figure 2 The diagram shown is a schematic diagram of the positive electrode sheet in an example of the present invention. Detailed Implementation

[0014] 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.

[0015] This invention provides a lithium-ion secondary battery, which may include a battery cell and an electrolyte. The battery cell may include an electrode. The electrode may include a current collector and a tab extending from one side of the current collector. The ratio of the dimension of the tab in the width direction of the battery cell to the width of the battery cell is x1. The electrolyte may include a fluoronitrile compound. The chemical formula of the fluoronitrile compound can be... Wherein, R can be an alkyl group with 1-3 carbon atoms (e.g., 1, 2, or 3 carbon atoms), and n can be an integer from 1 to 3 (e.g., 1, 2, or 3). The mass content of the fluoronitrile compound in the electrolyte is c1. x1 and c1 satisfy: 0.03 ≤ c1 / x1 ≤ 5, for example, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, or 5.

[0016] In one instance, 0.12 ≤ c1 / x1 ≤ 1.

[0017] In one instance, 0.13 ≤ c1 / x1 ≤ 0.9.

[0018] In this invention, the ratio of the dimension of the electrode in the width direction of the battery cell to the width of the battery cell has a conventional meaning in the art. For example... Figure 1 The diagram shown is a schematic diagram of a battery cell in an embodiment of the present invention, wherein, Figure 1 (a) The battery cell is a wound core. Figure 1 (b) The battery cells are stacked. Figure 1 (a) It can be seen that the battery cell includes electrode sheets. When the electrode sheets include a positive electrode sheet and a negative electrode sheet, the negative electrode sheet includes a negative electrode tab 11, and the positive electrode sheet includes a positive electrode tab 21. The width of the battery cell is a, and the dimension of the negative electrode tab 11 in the width direction of the battery cell is a1. Then, the ratio x1 of the dimension of the negative electrode tab 11 in the width direction of the battery cell to the width of the battery cell is a1 / a. The dimension of the positive electrode tab 21 in the width direction of the battery cell is a2. Then, the ratio x1 of the dimension of the positive electrode tab 21 in the width direction of the battery cell to the width of the battery cell is a2 / a. From Figure 1 (b) It can be seen that the battery cell includes electrode sheets. When the electrode sheets include a positive electrode sheet and a negative electrode sheet, the negative electrode sheet 1 includes a negative electrode tab 11, the positive electrode sheet 2 includes a positive electrode tab 21, the width of the battery cell is a, the dimension of the negative electrode tab 11 in the width direction of the battery cell is a1, then the ratio x1 of the dimension of the negative electrode tab 11 in the width direction of the battery cell to the width of the battery cell is a1 / a; the dimension of the positive electrode tab 21 in the width direction of the battery cell is a2, then the ratio x1 of the dimension of the positive electrode tab 21 in the width direction of the battery cell to the width of the battery cell is a2 / a.

[0019] In this invention, x1 can be 0.05-0.3, for example, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2 or 0.3.

[0020] In one instance, x1 is 0.08-0.24.

[0021] In one instance, x1 is 0.15-0.17.

[0022] In this invention, the size of the electrode tab in the width direction of the battery cell can be 4mm-12mm, for example, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, 11mm or 12mm.

[0023] In this invention, the width of the battery cell can be 20mm-150mm, for example, 20mm, 30mm, 40mm, 50mm, 60mm, 70mm, 80mm, 90mm, 100mm, 110mm, 120mm, 130mm, 140mm or 150mm.

[0024] In one example, the width of the battery cell is 35mm-80mm.

[0025] In this invention, the dimensions of the tab in the width direction of the battery cell and the width of the battery cell can be obtained by conventional methods in the art, such as by using measuring tools.

[0026] In this invention, c1 can be 0.5%-50%, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, or 50%.

[0027] In one instance, c1 is 2%–15%.

[0028] In this invention, the mass content of the fluoronitrile compound in the electrolyte can be obtained by methods conventional in the art, such as gas chromatography (GC).

[0029] The inventors of this invention have discovered that the 'n' element in the structural formula of fluoronitrile compounds has a significant impact on improving the cycle stability of batteries under high voltage and fast charging conditions. When n is 1, the content of element F is suitable, which can improve the structural stability of fluoronitrile compounds without significantly affecting their conductivity. When n is larger (e.g., 2 or 3), it not only fails to further improve the structural stability of fluoronitrile compounds but also significantly reduces their conductivity.

[0030] In one instance, n is 1.

[0031] The inventors of this invention have discovered that the 'R' in the structural formula of fluoronitrile compounds also has a significant impact on improving the cycle stability of batteries under high voltage and fast charging conditions. When R is an alkyl group with one carbon atom, the 'F' atom attached to R can more effectively prevent the side reaction between the cyano group and the negative electrode active material, thereby improving the cycle stability of the battery. When R has a larger number of carbon atoms (e.g., 2 or 3), the increased carbon chain length weakens the barrier effect of the 'F' atom on the cyano group and the negative electrode active material, thus affecting the cycle stability of the battery.

[0032] In one example, R is an alkyl group with 1 carbon atom.

[0033] In this invention, the fluoronitrile compound may include At least one of them.

[0034] In one example, the fluoronitrile compound includes

[0035] In this invention, the electrode can be a negative electrode and / or a positive electrode.

[0036] In this invention, the electrode is a positive electrode. The positive electrode may further include an insulating layer. The insulating layer may be located at at least one surface edge of the current collector (positive current collector) on the same side as the tab (positive electrode tab). Figure 2 The figure shows a schematic diagram of the structure of the positive electrode sheet in an embodiment of the present invention. As can be seen from the figure, the positive electrode sheet includes a positive current collector 11 and a positive electrode tab 12 extending from one side of the positive current collector 11. The positive electrode sheet also includes an insulating layer 13, which is located at at least one side surface edge of the positive current collector 11 on the same side as the positive electrode tab 12.

[0037] In this invention, the ratio of the dimension of the insulating layer in the width direction of the current collector to the width of the current collector is x2. x2 and c1 satisfy: 0.003≤x2+0.1×c1≤0.09, for example, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08 or 0.09.

[0038] In one instance, 0.005 ≤ x2 + 0.1 × c1 ≤ 0.05.

[0039] In one instance, 0.015 ≤ x2 + 0.1 × c1 ≤ 0.04.

[0040] Placing an insulating layer at the edge of the positive electrode tab effectively prevents burrs on the positive electrode tab (especially when the number of positive electrode tabs is greater than one). However, the size of the insulating layer in the width direction of the positive current collector affects battery performance. A larger size means the insulating layer covers a larger area of ​​the thinned region at the edge of the positive electrode. In this case, the cell balance (CB) value at the battery edge is more consistent with that in the middle region, resulting in smaller differences in kinetic performance across the battery. Simultaneously, since the current density at the edge of the positive electrode is higher than that in the middle region, covering a larger area of ​​the high current density region at the edge significantly reduces the risk of lithium plating at the edge. Conversely, a smaller size means the insulating layer covers a smaller area of ​​the thinned region at the edge of the positive electrode. In this case, the CB value at the edge fluctuates more compared to that in the middle region, requiring higher system kinetics to compensate for the resulting fast-charging risk. Fluorinated nitrile compounds can significantly improve system kinetics and reduce the risk caused by CB value fluctuations. However, the content of fluorinated nitrile compounds should not be too high; x2 and c1 need to be controlled to satisfy a specific relationship. Otherwise, it will lead to decreased interface stability, thereby degrading the battery's cycle stability.

[0041] In this invention, the insulating layer may include inorganic particles and a binder. The inorganic particles may include at least one selected from BaSO4, CaSiO3, γ-AlOOH, CaSiO4, and alumina. The binder may include at least one selected from polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polymethyl methacrylate (PMMA), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), and polytetrafluoroethylene (PTFE).

[0042] In this invention, x2 can be 0.001-0.09, for example, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08 or 0.09.

[0043] In one instance, x2 is 0.003-0.05.

[0044] In this invention, the dimension of the insulating layer in the width direction of the current collector can be 0.1mm-5mm, for example, 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm or 5mm.

[0045] In one example, the insulating layer has a dimension of 0.2 mm to 2 mm in the width direction of the current collector.

[0046] In this invention, the dimension of the insulating layer in the width direction of the current collector can be obtained by conventional methods in the art. For example, take a positive electrode sheet, randomly select at least 10 sites on the positive electrode sheet, measure the dimension of the insulating layer in the width direction of the current collector at each site, and take the average value.

[0047] In this invention, the positive electrode sheet may further include a positive electrode active material. The positive electrode active material may include element La. The mass content of element La in the positive electrode active material is x3. x3 and c1 satisfy: 0.0005≤x3 / c1≤0.09, for example, 0.0005, 0.0006, 0.0007, 0.0008, 0.0009, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, or 0.09.

[0048] In one instance, 0.0015 ≤ x3 / c1 ≤ 0.028.

[0049] In one instance, 0.003 ≤ x3 / c1 ≤ 0.015.

[0050] Adding an appropriate amount of element La to the positive electrode active material can significantly improve its structural and thermal stability, while also enhancing its surface stability and reducing the risk of side reactions with the electrolyte. However, excessively high La content in the positive electrode active material can lead to high interfacial impedance, reducing lithium-ion transport speed and degrading the battery's fast-charging performance. It can also increase the overpotential of the positive electrode active material during charging and discharging, resulting in decreased stability and increasing the risk of side reactions between the positive electrode active material and the electrolyte. Fluorinated nitrile compounds, on the other hand, have high conductivity, which can reduce interfacial impedance. Furthermore, fluorinated nitrile compounds can suppress side reactions between the electrolyte and element La, improving the stability between the positive electrode active material and the electrolyte, inhibiting interfacial impedance growth, and enabling the battery to maintain superior fast-charging performance while improving long-cycle performance during long-term cycling.

[0051] In this invention, x3 can be 150ppm-1500ppm, for example, 150ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm or 1500ppm.

[0052] In one instance, x3 is 200ppm-550ppm.

[0053] In this invention, the mass content x3 of element La in the positive electrode active material can be obtained by conventional methods in the art, such as inductively coupled plasma (ICP) metal element content testing.

[0054] In this invention, the positive electrode sheet may further include a positive electrode active material layer. The positive electrode active material layer includes the positive electrode active material. The positive electrode active material layer is located on at least one surface of the positive electrode current collector. The positive electrode active material layer is disposed in contact with the insulating layer. The positive electrode active material may include materials with the chemical formula Li. 1+x Ni y Co z M mThe substance is O2, wherein -0.1 ≤ x ≤ 1 (e.g., -0.1, 0, 0.1, 0.5, or 1), 0 ≤ y ≤ 1 (e.g., 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1), 0 ≤ z ≤ 1 (e.g., 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1), and 0 ≤ m ≤ 1 (e.g., 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1); M can be at least one of Mg, Zn, Ga, Ba, Al, Fe, Cr, Sn, V, Mn, Sc, Ti, Nb, Mo, La, and Zr. The positive electrode active material layer may further include a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent may include at least one of conductive carbon black, acetylene black, Ketjen black, carbon nanotubes, and conductive carbon fibers. The positive electrode binder may include at least one of PVDF, styrene-butadiene rubber, polyacrylic acid, polymethyl methacrylate, carboxymethyl cellulose, and sodium carboxymethyl cellulose.

[0055] In this invention, the electrolyte may further include fluoroethylene carbonate (FEC). The mass content of the fluoroethylene carbonate in the electrolyte is c2. c1 and c2 satisfy: 0.1≤c2 / c1≤30, for example, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20 or 30.

[0056] In one instance, 1 ≤ c2 / c1 ≤ 5.

[0057] Compared to other nitrile compounds (such as acetonitrile), fluoronitrile compounds, due to their inclusion of the sulfur (F) element, exhibit significantly improved compatibility with negative electrode sheets. However, when the mass content of fluoronitrile compounds in the electrolyte is too high, trace structural decomposition can occur under over-discharge and overcharge conditions. The decomposed nitrile structures can undergo side reactions with the negative electrode active material, leading to a deterioration in the battery's electrochemical performance. Furthermore, due to the high current density at the negative electrode tab, the side reactions between the negative electrode active material and the nitrile structures near the tab are significantly aggravated, thus affecting the protective effect of the negative electrode active material layer on the negative electrode current collector. During battery manufacturing, the rolling process compresses the negative electrode current collector, making it thinner and more susceptible to corrosion and breakage. At this point, the negative electrode current collector near the tab, lacking sufficient protection from the negative electrode active material layer, is corroded by the electrolyte (such as HF in the electrolyte), causing the negative electrode sheet to break and severely impacting the battery's cycle performance and safety. In contrast, fluoropolymers (FECs) in the electrolyte possess excellent negative electrode film-forming properties and continuous SEI repair capabilities. Therefore, when nitrile structures undergo side reactions with the negative electrode active material, FEC can promptly repair SEI damage, reducing the risk of side reactions between nitrile structures and the negative electrode active material. Simultaneously, FEC can capture free nitrile structures, further inhibiting side reactions between them and the negative electrode active material. Thus, by controlling the content relationship between FEC and fluorinated nitrile compounds, it is possible to ensure that the electrolyte exhibits high conductivity and fast-charging performance while maintaining long-cycle stability.

[0058] In this invention, the mass content c2 of FEC in the electrolyte can be 4%-30%, for example, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25% or 30%.

[0059] In one instance, c2 is 10%-20%.

[0060] In this invention, the mass content c2 of FEC in the electrolyte can be obtained by methods conventional in the art, such as GC.

[0061] In this invention, the electrolyte may further include an organic solvent. The organic solvent may include carbonates and / or carboxylic acid esters. The carbonates may include at least one selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. The carboxylic acid esters may include at least one selected from propyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, and n-ethyl butyrate.

[0062] In the present invention, the electrolyte may further include additives. The additives may include at least one of vinylene carbonate, vinyl ethylene carbonate, ethylene sulfate, succinonitrile, glutaronitrile, adiponitrile, pimelonitrile, suberonitrile, sebaconitrile, decanedinitrile, 1,3,6-hexanetricarbonitrile, glycerol trinitrile, 1,2-bis(2-cyanoethoxy)ethane, 1,3-propane sultone, and allyl-1,3-sulfonic acid lactone.

[0063] In the present invention, the electrolyte may further include a lithium salt. The lithium salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium difluorophosphate (LiPO2F2), lithium difluorooxalate borate (LiDFOB), lithium bis(trifluoromethylsulfonyl)imide, lithium difluorobis(oxalate)phosphate, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, and lithium tris(trifluoromethylsulfonyl)methyl.

[0064] In the present invention, the electrode sheet may also be a negative electrode sheet. The negative electrode sheet may further include a negative electrode active material. The negative electrode active material may include at least one of a carbon-based material and a silicon-based material. The carbon-based material may include at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon, and soft carbon. The silicon-based material may include at least one of nanosilicon, silicon oxide (SiO x (0 < x < 2)), silicon carbide, and silicon alloy. The silicon carbide refers to a material including elemental silicon and elemental carbon.

[0065] In one example, the negative electrode active material includes the carbon-based material and the silicon-based material.

[0066] In one example, the silicon-based material includes silicon carbide. The silicon carbide includes a material formed by silicon and / or partially oxidized silicon filled (including partially filled or completely filled) in the pores of porous amorphous carbon or porous crystalline carbon.

[0067] In the present invention, based on the total mass of the negative electrode active material, the content of the silicon-based material may be 2% - 80%, for example, 2%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, or 80%.

[0068] In one example, based on the total mass of the negative electrode active material, the content of the silicon-based material is 5% - 30%.

[0069] The inventors of this invention have discovered that the battery system of this invention is particularly suitable for silicon-doped batteries. This is because the silicon-based material in silicon-doped batteries tends to shrink and expand in volume during battery charge-discharge cycles, resulting in poor conductivity of the negative electrode, affecting the overall internal resistance of the battery, and causing a high temperature rise. However, when the battery system of this invention is used in batteries containing silicon-based materials, it can effectively reduce the internal resistance of the battery, reduce the temperature rise of the battery during cycling, and improve cycle stability.

[0070] In this invention, the battery may further include a separator. The separator may include separators conventionally used in the art.

[0071] 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.

[0072] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0073] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0074] Example 1

[0075] The battery is prepared according to the following method:

[0076] (1) Preparation of positive electrode sheet

[0077] Lithium cobalt oxide (La content 413 ppm), PVDF, conductive carbon black and carbon nanotubes were added to a vacuum mixer in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone (NMP) was added and the mixture was thoroughly mixed under vacuum until a uniform and fluid positive electrode slurry (solid content 55 wt%) was formed. The positive electrode slurry was then uniformly coated onto aluminum foil, dried and rolled.

[0078] Boehmite and PVDF were added to a vacuum mixer at a mass ratio of 8.5:1.5, along with NMP. The mixture was thoroughly stirred under vacuum until a uniform, fluid insulating slurry was formed. This insulating slurry was then coated onto one side of an aluminum foil along its length (on the same side as the positive electrode tab). The foil was then dried, rolled, slit, and punched (to obtain the positive electrode tab) to obtain the positive electrode sheet. The width of the insulating layer was 1.2 mm, the width of the positive electrode tab was 8 mm, and x2 was 0.019.

[0079] (2) Preparation of negative electrode sheet

[0080] The negative electrode active material (a combination of artificial graphite and silicon carbon, wherein the mass content of silicon carbon in the negative electrode active material is 15% and the silicon content of silicon carbon is 30%), styrene-butadiene rubber, sodium carboxymethyl cellulose, and conductive carbon black are added to a vacuum mixer in a mass ratio of 96.5:1:1:1.5. Deionized water is added, and the mixture is thoroughly mixed under the action of the vacuum mixer to form a uniform and fluid negative electrode slurry (solid content of 45wt%). The above negative electrode slurry is uniformly coated on copper foil, dried, rolled, and die-cut (to obtain negative electrode tabs) to obtain a negative electrode sheet, wherein the width of the negative electrode tab is 8mm.

[0081] (3) Preparation of electrolyte

[0082] In an argon glove box with a water content of <0.1ppm and an oxygen content of <0.1ppm, ethylene carbonate, propylene carbonate, and propyl propionate were mixed evenly in a mass ratio of 2:3:8. Thoroughly dried LiPF6 was added and stirred to dissolve, with the amount of LiPF6 added being 15% of the total electrolyte mass. Then, 8% of a fluoronitrile compound I-1, 15% of FEC, 2% of 1,3-propanesulfonate lactone, 2% of 1,3,6-hexanetrionitrile, 1% of adiponitrile, and 1% of succinate were added based on the total electrolyte mass. The mixture was stirred evenly, and after passing physical property testing, the electrolyte was obtained. The sum of the mass contents of the above substances in the electrolyte was 100%.

[0083] (4) Preparation of lithium-ion secondary batteries

[0084] The positive electrode obtained in step (1), the negative electrode obtained in step (2), and the separator (8μm thick polyethylene film) are wound together to obtain a battery cell (ensuring that the separator is between the positive and negative electrodes to provide isolation). The tabs are welded, the cell is placed in an aluminum-plastic film, and the electrolyte prepared in step (3) is injected. After vacuum sealing, settling, formation, shaping, and sorting, a lithium-ion secondary battery is obtained. The width of the battery cell is 50mm, x1 is 0.16, and the values ​​of the relevant relationships are shown in Table 1.

[0085] Example 2

[0086] The battery is prepared according to the following method:

[0087] (1) Preparation of positive electrode sheet

[0088] Lithium cobalt oxide (La content 233 ppm), PVDF, conductive carbon black and carbon nanotubes were added to a vacuum mixer in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone (NMP) was added and the mixture was thoroughly mixed under vacuum until a uniform and fluid positive electrode slurry (solid content 55 wt%) was formed. The positive electrode slurry was then uniformly coated onto aluminum foil, dried and rolled.

[0089] Boehmite and PVDF were added to a vacuum mixer at a mass ratio of 8.5:1.5, along with NMP. The mixture was thoroughly mixed under vacuum until a uniform, fluid insulating slurry was formed. This insulating slurry was then coated onto one side of an aluminum foil along its length (on the same side as the positive electrode tab). The foil was then dried, rolled, slit, and punched (to obtain the positive electrode tab) to obtain the positive electrode sheet. The width of the insulating layer was 2 mm, the width of the positive electrode tab was 7.5 mm, and x2 was 0.032.

[0090] (2) Preparation of negative electrode sheet

[0091] The negative electrode active material (a combination of artificial graphite and silicon carbon, wherein the silicon carbon content in the negative electrode active material is 15% by mass), styrene-butadiene rubber, sodium carboxymethyl cellulose, and conductive carbon black are added to a vacuum mixer in a mass ratio of 96.5:1:1:1.5. Deionized water is added, and the mixture is thoroughly mixed under the action of the vacuum mixer to form a uniform and fluid negative electrode slurry (solid content of 45wt%). The above negative electrode slurry is uniformly coated on copper foil, dried, rolled, and die-cut (to obtain negative electrode tabs) to obtain a negative electrode sheet, wherein the width of the negative electrode tab is 7.5mm.

[0092] (3) Preparation of electrolyte

[0093] In an argon glove box with a water content of <0.1ppm and an oxygen content of <0.1ppm, ethylene carbonate, propylene carbonate, and propyl propionate were mixed evenly in a mass ratio of 2:3:8. Thoroughly dried LiPF6 was added and stirred to dissolve, with the amount of LiPF6 added being 15% of the total electrolyte mass. Then, 2% of a fluoronitrile compound I-1, 10% of FEC, 2% of 1,3-propanesulfonic acid lactone, 2% of 1,3,6-hexanetrionitrile, 1% of adiponitrile, and 1% of succinate were added based on the total electrolyte mass. The mixture was stirred evenly, and after passing physical property testing, the electrolyte was obtained. The sum of the mass contents of the above substances in the electrolyte was 100%.

[0094] (4) Preparation of lithium-ion secondary batteries

[0095] The positive electrode obtained in step (1), the negative electrode obtained in step (2), and the separator (8μm thick polyethylene film) are wound together to obtain a battery cell (ensuring that the separator is between the positive and negative electrodes to provide isolation). The tabs are welded, the cell is placed in an aluminum-plastic film, and the electrolyte prepared in step (3) is injected. After vacuum sealing, settling, formation, shaping, and sorting, a lithium-ion secondary battery is obtained. The width of the battery cell is 50mm, x1 is 0.15, and the values ​​of the relevant relationships are shown in Table 1.

[0096] Example 3

[0097] The battery is prepared according to the following method:

[0098] (1) Preparation of positive electrode sheet

[0099] Lithium cobalt oxide (La content of 529 ppm), PVDF, conductive carbon black, and carbon nanotubes were added to a vacuum mixer in a mass ratio of 96:2:1.5:0.5. N-methylpyrrolidone (NMP) was added, and the mixture was thoroughly mixed under vacuum until a uniform and fluid positive electrode slurry (solid content of 55 wt%) was formed. The positive electrode slurry was then uniformly coated onto aluminum foil, dried, and rolled.

[0100] Boehmite and PVDF were added to a vacuum mixer at a mass ratio of 8.5:1.5, along with NMP. The mixture was thoroughly stirred under vacuum until a uniform, fluid insulating slurry was formed. This insulating slurry was then coated onto one side of an aluminum foil along its length (on the same side as the positive electrode tab). The foil was then dried, rolled, slit, and punched (to obtain the positive electrode tab) to obtain the positive electrode sheet. The width of the insulating layer was 0.2 mm, the width of the positive electrode tab was 8.5 mm, and x2 was 0.003.

[0101] (2) Preparation of negative electrode sheet

[0102] The negative electrode active material (a combination of artificial graphite and silicon carbon, wherein the silicon carbon content in the negative electrode active material is 15% by mass), styrene-butadiene rubber, sodium carboxymethyl cellulose, and conductive carbon black are added into a vacuum mixer in a mass ratio of 96.5:1:1:1.5. Deionized water is added, and the mixture is thoroughly mixed under the action of the vacuum mixer to form a uniform and fluid negative electrode slurry (solid content of 45wt%). The above negative electrode slurry is uniformly coated on copper foil, dried, rolled, and die-cut (to obtain negative electrode tabs) to obtain a negative electrode sheet, wherein the width of the negative electrode tab is 8.5mm.

[0103] (3) Preparation of electrolyte

[0104] In an argon glove box with a water content of <0.1ppm and an oxygen content of <0.1ppm, ethylene carbonate, propylene carbonate, and propyl propionate were mixed evenly in a mass ratio of 2:3:8. Thoroughly dried LiPF6 was added and stirred to dissolve, with the amount of LiPF6 added being 15% of the total mass of the electrolyte. Then, 15% of a fluoronitrile compound I-1, 20% of FEC, 2% of 1,3-propanesulfonic acid lactone, 2% of 1,3,6-hexanetrionitrile, 1% of adiponitrile, and 1% of succinate were added, and the mixture was stirred evenly. After passing physical property testing, the electrolyte was obtained, and the sum of the mass contents of the above substances in the electrolyte was 100%.

[0105] (4) Preparation of lithium-ion secondary batteries

[0106] The positive electrode obtained in step (1), the negative electrode obtained in step (2), and the separator (8μm thick polyethylene film) are wound together to obtain a battery cell (ensuring that the separator is between the positive and negative electrodes to provide isolation). The tabs are welded, the cell is placed in an aluminum-plastic film, and the electrolyte prepared in step (3) is injected. After vacuum sealing, settling, formation, shaping, and sorting, a lithium-ion secondary battery is obtained. The width of the battery cell is 50mm, x1 is 0.17, and the values ​​of the relevant relationships are shown in Table 1.

[0107] Example 4 group

[0108] This set of examples is used to verify the impact of changes to "c1 / x1".

[0109] This set of embodiments refers to Embodiments 2 and 3, the difference being that x1 is changed by adjusting the dimensions of the positive and negative electrode tabs in the width direction of the cell, thereby changing c1 / x1, as detailed below:

[0110] Example 4a is carried out with reference to Example 3, except that the width of the positive electrode tab is 7.5 mm, the width of the negative electrode tab is 7.5 mm, and x1 is 0.15;

[0111] Example 4b is carried out with reference to Example 2, except that the width of the positive electrode tab is 8.5 mm, the width of the negative electrode tab is 8.5 mm, and x1 is 0.17.

[0112] Example 5 group

[0113] This set of examples is used to verify the impact of changes to "x1".

[0114] This set of embodiments refers to Embodiment 1, except that x1 is changed by altering the dimensions of the positive and negative electrode tabs in the width direction of the cell, as detailed below:

[0115] Example 5a: The width of the positive electrode tab is 4mm, the width of the negative electrode tab is 4mm, and x1 is 0.08;

[0116] In Example 5b, the width of the positive electrode tab is 12mm, the width of the negative electrode tab is 12mm, and x1 is 0.24.

[0117] Example 6 group

[0118] This set of examples is used to verify the effects of changes to "fluoronitriles".

[0119] This set of examples is based on Example 1, except that the fluoronitrile compounds are changed, as follows:

[0120] Example 6a, in which the fluoronitrile compound I-1 was replaced with the same mass of I-2;

[0121] Example 6b, in which the fluoronitrile compound I-1 was replaced with the same mass of I-3;

[0122] Example 6c: The fluoronitrile compound I-1 was replaced with the same mass of I-4.

[0123] Example 7 group

[0124] This set of examples is used to verify the effect of changing the "mass content c1 of fluoronitrile compounds in the electrolyte".

[0125] This set of embodiments is based on Embodiment 1, except that c1 is changed, as follows:

[0126] Example 7a, c1 is 0.5%;

[0127] Example 7b, c1 is 25%;

[0128] Example 7c, c1 is 50%.

[0129] Example 8 group

[0130] This set of examples is used to verify the impact of changes in the "mass content of FEC in the electrolyte c2".

[0131] This set of embodiments is based on Embodiment 1, except that c2 is changed, as follows:

[0132] Example 8a, c2 is 4%;

[0133] Example 8b, c2 is 30%.

[0134] Example 9 group

[0135] This set of examples is used to verify the impact of the change in "x2+0.1×c1".

[0136] This set of embodiments refers to Embodiments 2 and 3, the difference being that x2+0.1×c1 is changed by altering the width of the insulating layer, as detailed below:

[0137] Example 9a is carried out with reference to Example 2, except that the width of the insulating layer is 0.2 mm and x2 is 0.003;

[0138] Example 9b is performed with reference to Example 3, except that the width of the insulating layer is 2 mm and x2 is 0.032.

[0139] Example 10 group

[0140] This set of examples is used to verify the effect of changing the ratio of the dimension of the insulating layer in the current collector width direction to the current collector width x2.

[0141] This set of embodiments is based on Embodiment 1, except that x2 is changed by altering the width of the insulating layer, as detailed below:

[0142] Example 10a: No insulating layer is coated, i.e., x2 is 0;

[0143] Example 10b: The width of the insulating layer is 0.1 mm, and x2 is 0.002.

[0144] Example 10c: The width of the insulating layer is 5 mm, and x2 is 0.081.

[0145] Example 11 group

[0146] This set of examples is used to verify the impact of the change in "x3 / c1".

[0147] This set of embodiments refers to Embodiments 2 and 3, the difference being that the x3 / c1 ratio is changed by altering the mass content x3 of element La in lithium cobalt oxide, as detailed below:

[0148] Example 11a, x3 is 529 ppm;

[0149] Example 11b, x3 is 233 ppm.

[0150] Example 12 group

[0151] This set of examples is used to verify the impact of changing "the mass content of element La in the positive electrode active material x3".

[0152] This set of embodiments is based on Embodiment 1, except that x3 is changed, as follows:

[0153] Example 12a: Lithium cobalt oxide contains no element La, i.e., x3 is 0;

[0154] In Example 12b, the mass content of element La in lithium cobalt oxide is 152 ppm, and x3 is 152 ppm;

[0155] In Example 12c, the mass content of element La in lithium cobalt oxide was 1467 ppm, and x3 was 1467 ppm.

[0156] Example 13 group

[0157] This set of examples is used to verify the impact of changes in the "mass content of silicon-based materials in the negative electrode active material".

[0158] This set of embodiments is based on Embodiment 1, except that the mass content of silicon-based material in the negative electrode active material is changed, as follows:

[0159] In Example 13a, the mass content of silicon-carbon in the negative electrode active material is 2%;

[0160] In Example 13b, the mass content of silicon-carbon in the negative electrode active material is 5%;

[0161] Example 13c: The mass content of silicon-carbon in the negative electrode active material is 30%;

[0162] In Example 13d, the mass content of silicon-carbon in the negative electrode active material was 80%.

[0163] Table 1

[0164]

[0165]

[0166] Comparative Example 1

[0167] The procedure was carried out in accordance with Example 1, except that the fluoronitrile compound was replaced with the same mass of acetonitrile.

[0168] Comparative Example 2

[0169] The procedure was carried out in accordance with Example 1, except that the width of the positive electrode tab was 4 mm, the width of the negative electrode tab was 4 mm, x1 was 0.08, c1 was 50%, and c1 / x1 was 6.25.

[0170] Comparative Example 3

[0171] The procedure was carried out in accordance with Example 1, except that the width of the positive electrode tab was 12 mm, the width of the negative electrode tab was 12 mm, x1 was 0.24, c1 was 0.5%, and c1 / x1 was 0.02.

[0172] Comparative Example 4

[0173] The procedure was carried out in accordance with Example 1, except that no fluoronitrile compounds were added to the electrolyte.

[0174] Test case

[0175] (1) Room temperature cycling test

[0176] The batteries prepared in the examples and comparative examples were subjected to room temperature cycling tests. The specific test methods are as follows:

[0177] At 25℃, the battery, after capacity testing, is charged to 4.5V at a constant current and constant voltage of 0.7C, with a cutoff current of 0.05C. Then, it is discharged to 3.0V at a constant current of 0.5C. This cycle is repeated for 800 charge-discharge cycles. The capacity retention rate at the 800th cycle is calculated using the following formula:

[0178] The 800th cycle capacity retention rate (%) = (800th cycle discharge capacity / first cycle discharge capacity) × 100%, and the results are recorded in Table 2.

[0179] (2) Temperature rise test

[0180] The batteries prepared in the examples and comparative examples were subjected to temperature rise tests. The specific test methods are as follows:

[0181] 1. Let stand at (25±2)℃ for 10 min;

[0182] 2. Discharge at 0.2C to the lower limit voltage (3.0V) and let stand for 1 hour;

[0183] 3. Charge to 4.5V (cutoff at 0.02C) using 3C, then let stand for 1 hour;

[0184] 4. Discharge at 0.7C to the lower limit voltage (3.0V) and let stand for 1 hour;

[0185] 5. Repeat steps 3-4 three times. Monitor the temperature at the center of the battery body, testing the temperature every 5 seconds. Record the highest temperature of the last cycle in Table 2.

[0186] (3) Low-temperature discharge test

[0187] The batteries prepared in the examples and comparative examples were subjected to low-temperature discharge tests. The specific test methods are as follows:

[0188] Under 25℃ conditions, the capacity-graded battery was discharged at 0.5C to 3.0V and left to stand for 5 minutes; then charged at 0.2C to 4.48V. When the battery voltage reached 4.48V, it was switched to constant voltage charging at 4.48V until the charging current was less than or equal to the given cutoff current of 0.05C, and left to stand for 5 minutes. The fully charged battery was then transferred to a high-low temperature chamber, set to -10℃, and left to stand for 120 minutes after the chamber temperature was reached. Then, it was discharged at 0.2C to the cutoff voltage of 3.0V and left to stand for 5 minutes. The high-low temperature chamber temperature was then adjusted to (25±3)℃ and left to stand for 60 minutes. It was then charged at 0.2C to 4.48V. When the battery voltage reached 4.48V, it was switched to constant voltage charging at 4.48V until the charging current was less than or equal to the given cutoff current of 0.05C, and left to stand for 5 minutes. The capacity retention rate at -10℃ low-temperature discharge of 3.0V was calculated. The calculation formula is as follows:

[0189] -10℃ discharge to 3.0V capacity retention rate (%) = (-10℃ discharge capacity to 3.0V / 25℃ discharge capacity to 3.0V) × 100%, and the results are recorded in Table 2.

[0190] (4) High temperature cycling test

[0191] The batteries prepared in the examples and comparative examples were subjected to high-temperature cycling tests. The specific test methods are as follows:

[0192] At 45℃, the battery, after capacity testing, is charged to 4.48V at a constant current and constant voltage of 0.7C, with a cutoff current of 0.05C. Then, it is discharged to 3.0V at a constant current of 0.5C. This cycle is repeated 500 times. The capacity retention rate at the 500th cycle is calculated using the following formula:

[0193] The 500th cycle capacity retention rate (%) = (500th cycle discharge capacity / first cycle discharge capacity) × 100%, and the results are recorded in Table 2.

[0194] Table 2

[0195]

[0196]

[0197] As shown in Table 2, compared with the comparative example, the battery of the present invention can improve cycle stability and reduce the risk of thermal runaway under fast charging conditions, and also has both low-temperature performance and high-temperature cycle performance. In Example 13d, due to the high silicon-carbon content (80%) in the negative electrode active material, the battery of Example 13d has poor cycle stability, high temperature rise, and poor low-temperature and high-temperature cycle performance under fast charging conditions.

[0198] 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 by comprising: The lithium ion secondary battery comprises a cell and an electrolyte, the cell comprises a pole piece; The pole piece comprises a current collector and a tab extending from one side of the current collector, the ratio of the size of the tab in the width direction of the current collector to the width of the current collector is x1; The electrolyte comprises a fluorinated nitrile compound; the fluorinated nitrile compound has a chemical formula of wherein R is an alkyl group with a carbon atom number of 1-3, and n is an integer of 1-3; the mass content of the fluorinated nitrile compound in the electrolyte is c1; x1 and c1 satisfy: 0.03≤c1 / x1≤5.

2. The lithium-ion secondary battery according to claim 1, wherein 0.12≤c1 / x1≤1; Preferably, 0.13≤c1 / x1≤0.

9.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein x1 is 0.05-0.3; preferably 0.08-0.24; more preferably 0.15-0.17; And / or, c1 is 0.5%-50%; preferably 2%-15%; And / or, n is 1; And / or, R is an alkyl group with 1 carbon atom.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein The fluoro-nitrile compound includes at least one of Preferably, the fluoro-nitrile compound comprises 5. The lithium-ion secondary battery according to claim 1 or 2, wherein The pole piece is a positive pole piece; the positive pole piece further comprises an insulating layer, the insulating layer is located at least one side surface edge of the current collector on the same side as the tab; The ratio of the size of the insulating layer in the width direction of the current collector to the width of the current collector is x2; x2 and c1 satisfy: 0.003≤x2+0.1×c1≤0.09; Preferably, 0.005≤x2+0.1×c1≤0.

05. More preferably, 0.015≤x2+0.1×c1≤0.

04.

6. The lithium-ion secondary battery according to claim 5, wherein x2 is 0.001-0.09; preferably 0.003-0.05; And / or, the size of the insulating layer in the width direction of the current collector is 0.1mm-5mm; preferably 0.2mm-2mm.

7. The lithium-ion secondary battery according to claim 1 or 2, wherein The pole piece is a positive pole piece; the positive pole piece further comprises a positive active material, the positive active material comprises an element La; the mass content of the element La in the positive active material is x3; x3 and c1 satisfy: 0.0005≤x3 / c1≤0.09; Preferably, 0.0015≤x3 / c1≤0.

028. More preferably, 0.003≤x3 / c1≤0.

015.

8. The lithium-ion secondary battery according to claim 7, wherein x3 is 150ppm-1500ppm; Preferably, x3 is 200ppm-550ppm.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein The electrolyte further comprises fluoroethylene carbonate, the mass content of the fluoroethylene carbonate in the electrolyte is c2; c1 and c2 satisfy: 0.1≤c2 / c1≤30; preferably, 1≤c2 / c1≤5; And / or, c2 is 4%-30%; preferably 10%-20%.

10. The lithium-ion secondary battery according to claim 1 or 2, wherein The pole piece is a negative pole piece; the negative pole piece further comprises a negative active material, the negative active material comprises at least one of a carbon-based material and a silicon-based material; Preferably, the negative active material comprises the carbon-based material and the silicon-based material, the content of the silicon-based material is 2%-80% based on the total mass of the negative active material; Preferably, the silicon-based material comprises at least one of silicon element, silicon oxide, silicon carbon and silicon alloy; more preferably, the silicon-based material comprises the silicon carbon.

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