A lithium-ion battery

Abstract

The application belongs to the technical field of batteries, and particularly relates to a lithium ion battery. Compared with the prior art, the application reduces the interface impedance of the negative plate and the electrolyte and improves the lithium ion transmission rate on the basis of avoiding the separation of the negative active material layer and the current collector by comprehensively controlling the content of isothiocyanate additive in the electrolyte, the porosity and adhesion of the negative active layer in the negative plate, and thus the overall kinetic performance is improved.

Description

Technical Field

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

[0002] Lithium-ion batteries are widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power plants, as well as in power tools, electric bicycles, electric motorcycles, electric cars, military equipment, aerospace and many other fields due to their advantages such as high specific energy, fast charging and discharging capabilities and low self-discharge.

[0003] In lithium-ion batteries, the oxidative decomposition of the electrolyte degrades battery performance, and the dissolution of metal ions during cycling and high-temperature storage often leads to a decline in battery performance. As market demand for lithium-ion batteries continues to grow, higher requirements are being placed on the overall performance of batteries. The use of additives is one of the effective ways to improve the overall performance of lithium-ion batteries. Studies have shown that the presence of isocyanate-based additives can eliminate trace amounts of HF and H2O in the battery, reduce electrolyte decomposition, and improve battery performance. Furthermore, isothiocyanate additives can be reduced at the negative electrode to form polythioamides, which are heat-resistant and have low impedance; however, their addition can cause significant gas generation inside the battery. Summary of the Invention

[0004] In view of this, the technical problem to be solved by the present invention is to provide a lithium-ion battery with long life and high power performance.

[0005] This invention provides a lithium-ion battery, comprising an electrolyte and a negative electrode sheet;

[0006] The electrolyte includes isothiocyanate additives as shown in formula (I);

[0007]

[0008] Where M is selected from C1-C10 alkyl or C1-C10 fluoroalkyl;

[0009] The mass content of the isothiocyanate additive shown in formula (I) in the electrolyte is a%;

[0010] The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer;

[0011] The porosity of the negative electrode active layer is c%; the adhesion force between the negative electrode active layer and the negative electrode current collector is bN / m.

[0012] The electrolyte and the negative electrode sheet satisfy the following relationship: 0.2≤100×a×b / c≤120.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] This invention improves the overall kinetic performance by comprehensively controlling the content of isothiocyanate additives in the electrolyte, the porosity and adhesion of the negative electrode active layer in the negative electrode sheet, and avoiding separation between the negative electrode active material layer and the current collector. Attached Figure Description

[0015] Figure 1 The mass spectrum is that of the isothiocyanate additives represented by formula (5) used in the embodiments of the present invention. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0017] This invention provides a lithium-ion battery, comprising an electrolyte and a negative electrode; the electrolyte comprises isothiocyanate additives as shown in formula (I);

[0018]

[0019] Where M is a C1-C10 alkyl group or a C1-C10 fluoroalkyl group;

[0020] The content of isothiocyanate additives represented by formula (I) in the electrolyte is a%; the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer; the porosity of the negative electrode active layer is c%; the adhesion force between the negative electrode active layer and the negative electrode current collector is b N / m; the electrolyte and the negative electrode sheet satisfy the relationship: 0.2≤100a×b / c≤100.

[0021] In this invention, the adhesion between the negative electrode active layer and the negative electrode current collector can be adjusted by the content and type of the negative electrode binder in the negative electrode active layer. Adhesion represents the bonding force between the active material layer and the current collector, affecting electron transport between them. If the adhesion is too low, electron transport between the current collector and the active material layer is poor, impedance increases, and it also makes it easy for the active material layer and the current collector to detach. If the adhesion is too high, it means a larger amount of binder needs to be added, affecting lithium-ion transport and resulting in a low lithium-ion transport rate. In this invention, the adhesion between the negative electrode active layer and the negative electrode current collector is b N / m; b is preferably 5–50, more preferably 10–45, even more preferably 10–40, and most preferably 10–30; in some embodiments provided by this invention, b is specifically 30, 12, 28.8, 35, 5.1, 29.4, 48.5, 10.5, 33, 10, or 30.

[0022] Porosity refers to the percentage of void volume in the negative electrode of a battery. Appropriate porosity helps improve the battery's energy density because pores can hold more electrolyte, allowing ions to transport more smoothly during charging and discharging, thus improving the battery's fast-charging performance. Porosity also affects the battery's cycle life. However, excessively high porosity can lead to a loose electrode structure, reducing the battery's mechanical strength and causing poor electron transport between particles, increasing contact resistance and consequently increasing electron transport impedance. Conversely, excessively low porosity affects lithium-ion transport, thus impacting the battery's fast-charging performance. According to this invention, the porosity of the negative electrode active layer is c%; preferably, c% is 10% to 40%, more preferably 12% to 35%; in some embodiments provided by this invention, c% is specifically 35%, 12%, 24%, 40%, 35.5%, 26%, 60%, 33%, 27.5%, or 30%. In addition, the present invention does not limit the method of adjusting the porosity. Specifically, it can be adjusted by the compaction density of the negative electrode sheet.

[0023] According to the present invention, the electrolyte includes isothiocyanate additives as shown in formula (I); the isothiocyanate additives, as film-forming additives, form a solid electrolyte interphase (SEI) film at the negative electrode parameters. The SEI film formed has good high-temperature resistance and low impedance. In formula (I), M is preferably a C1-C8 alkyl or a C1-C8 fluoroalkyl, more preferably a C1-C6 alkyl or a C1-C6 fluoroalkyl, and even more preferably a C1-C4 alkyl or a C1-C4 fluoroalkyl. The number of fluorine atoms in the fluoroalkyl group can be one or more, without any special limitation. Fluorination can increase the wettability of the electrolyte, while other halogen substitutions will deteriorate the electrical performance of the battery. In the present invention, most preferably, M is fluoromethyl, fluoroethyl, fluoropropyl, fluorobutyl, methyl, ethyl, propyl, or butyl.

[0024] In one specific embodiment of the present invention, the isothiocyanate additive is selected from one or more of formulas (1) to (16):

[0025]

[0026]

[0027] According to the present invention, the content of the isothiocyanate additive shown in formula (I) in the electrolyte is a%. The isothiocyanate additive can generate polythioamide on the negative electrode surface, which is heat resistant and has good stability. However, its addition amount cannot be too small. If the addition amount is too small, it cannot play a role in reducing interfacial impedance and forming a stable SEI film. The addition amount cannot be too large either, because the isothiocyanate additive has serious gas generation problems, because a large amount of gas (hydrogen sulfide) will be generated when isothiocyanate is electrochemically reduced to form SEI at the negative electrode. In the present invention, the a% is preferably 0.01% to 1.5%, more preferably 0.05% to 1.2%, and even more preferably 0.1% to 1.0%. In some embodiments provided by the present invention, the a% is specifically 1%, 0.1%, 0.5%, 1.2%, 0.015%, 1.48%, 0.18%, 1.3%, or 0.2%.

[0028] This invention improves overall kinetic performance by comprehensively controlling the content of isothiocyanate additives in the electrolyte, the porosity and adhesion of the negative electrode active layer in the negative electrode sheet, and avoiding separation between the negative electrode active material layer and the current collector. This reduces the interfacial impedance between the negative electrode sheet and the electrolyte, while simultaneously increasing the lithium-ion transport rate. In this invention, the electrolyte and the negative electrode sheet preferably satisfy the relationship 5 ≤ 100 × a × b / c ≤ 100, more preferably 10 ≤ 100 × a × b / c ≤ 100, even more preferably 10 ≤ 100 × a × b / c ≤ 90, even more preferably 25 ≤ 100 × a × b / c ≤ 76, and most preferably 25 ≤ 100 × a × b / c ≤ 60. In some embodiments provided by the present invention, 85.71, 10, 60.00, 105, 0.22, 113.08, 119.63, 5.73, 120, 3.33, 60 or 85.71.

[0029] In one specific embodiment of the present invention, the electrolyte further includes unsaturated ester additives; the unsaturated ester additives are preferably one or more selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene ethylene carbonate (VEC), allyl ethyl carbonate (AEC), vinyl acetate (VA), and catechol carbonate (CC); the content of unsaturated ester additives in the electrolyte is d%, preferably 0.5 ≤ d ≤ 3, more preferably 0.5 ≤ d ≤ 2.5, and even more preferably 0.8 ≤ d ≤ 2.5; further specifically, 0.2 ≤ a / d ≤ 2; further specifically, 0.5 ≤ a + d ≤ 4; in some embodiments provided by the present invention, The a / d ratio is specifically 1, 0.1, 0.5, 1.2, 0.3, 1.48, 0.18, 2, 0.17, or 2.5. Combining unsaturated carbonate with isothiocyanate additives can reduce the amount of isothiocyanate additives used and improve gas production. This is because unsaturated carbonate can reduce side reactions between the electrolyte and the negative electrode during the initial formation of the CEI film, thus reducing gas production and allowing for less adhesion to the negative electrode. However, if the unsaturated carbonate is not completely consumed during the initial film formation, it will still generate a large amount of gas during later cycles. Furthermore, the addition of unsaturated carbonate increases impedance. Therefore, the unsaturated carbonate content cannot be too high, the isothiocyanate content cannot be too low, and the negative electrode adhesion cannot be too low. In this invention, more specifically, when the electrolyte includes unsaturated ester additives, the electrolyte and the negative electrode preferably satisfy the relationship: 0.5 ≤ 100a × b / c ≤ 70. When unsaturated carbonates and isothiocyanates are used in combination, the formula is within the above range to further improve gas production and maintain low impedance. In a specific embodiment of the present invention, the unsaturated ester additive is vinylene carbonate (VC). The reason for using VC and isothiocyanate additives in combination is that VC can also form an SEI film on the negative electrode surface, and VC can achieve better coating of the negative electrode, reduce side reactions between the negative electrode and the electrolyte, reduce gas production, and improve battery cycle performance. However, if the VC content is too high, it will cause excessive impedance and affect the battery's fast charging performance. The combined use of unsaturated ester additives and isothiocyanate additives reduces impedance and also avoids excessive addition of isothiocyanates, which would cause severe gas production. Further controlling the ratio within the range of 0.2 ≤ a / d ≤ 2 balances low impedance and low gas production.

[0030] According to the present invention, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer; the negative electrode current collector can be any negative electrode current collector well known to those skilled in the art, and there are no special limitations, but copper foil is preferred in the present invention; the negative electrode active layer comprises a negative electrode active material; the mass concentration of the negative electrode active material in the negative electrode active layer is preferably 90% to 98%; optionally, the mass concentration of the negative electrode active material in the negative electrode active layer is specifically 90%, 92%, 94%, 96%, 97%, 98% or any two of the above values; the negative electrode active material can be any negative electrode active material well known to those skilled in the art, and there are no special limitations, but natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, silicon, SiO are preferred in the present invention. x Silicon-carbon or Li4Ti5O 12 One or more of the following are used: In the embodiments provided by this invention, artificial graphite with a particle size of 9-12 μm is used as an example; the negative electrode active layer further includes a negative electrode conductive agent; the mass concentration of the negative electrode conductive agent in the negative electrode active layer is preferably 0.5%-5%; optionally, the mass concentration of the negative electrode conductive agent in the negative electrode active layer is 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or any two of the above values; the negative electrode conductive agent can be any negative electrode conductive agent well known to those skilled in the art, and there are no special limitations. In this invention, conductive agent SP is preferred; the negative electrode active layer further includes a negative electrode binder; the mass concentration of the negative electrode binder in the negative electrode active layer is preferably 1%-5%; optionally, the mass concentration of the negative electrode binder in the negative electrode active layer is 1%, 1.5%, 2%, 2.5%. The negative electrode binder is 3%, 3.5%, 4%, 4.5%, 5%, or any two of the above values; the negative electrode binder can be any negative electrode binder known to those skilled in the art, and there are no special limitations. In this invention, it is preferably one or more of polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR); the molecular weight of the PAA is preferably 30-100W; optionally, the molecular weight of the PAA is 30W, 40W, 50W, 60W, 70W, 80W, 90W, 100W, or any two of the above values; the particle size of the SBR is preferably 120-180nm; optionally, the particle size of the SBR is 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, or any two of the above values.

[0031] According to the present invention, the electrolyte further includes a solvent; the solvent can be any organic solvent well known to those skilled in the art, and there are no special limitations. Preferably, the solvent is one or more of carboxylic acid ester solvents, cyclic carbonates, and chain carbonates. The carboxylic acid ester solvent is preferably one or more of methyl acetate, ethyl formate, and ethyl acetate. The cyclic carbonate is any cyclic carbonate well known to those skilled in the art, and there are no special limitations. Preferably, the solvent is ethylene carbonate (EC) and / or propylene carbonate (PC). The chain carbonate is any chain carbonate well known to those skilled in the art, and there are no special limitations. Preferably, the solvent is one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). In the embodiments provided by the present invention, EC and EMC are specifically used. Taking DEC as an example of an electrolyte solvent, the following description is provided: The mass of EC is preferably 20% to 40% of the electrolyte solvent mass; optionally, the mass of EC is 20%, 25%, 30%, 35%, 40% of the electrolyte solvent mass, or any two of the above values; the mass of EMC is preferably 20% to 50% of the electrolyte solvent mass; optionally, the mass of EMC is 20%, 25%, 30%, 35%, 40%, 45%, 50% of the electrolyte solvent mass, or any two of the above values; the mass of DEC is preferably 10% to 40% of the electrolyte solvent mass; optionally, the mass of DEC is 10%, 15%, 20%, 25%, 30%, 35%, 40% of the electrolyte solvent mass, or any two of the above values.

[0032] According to the present invention, the electrolyte preferably further comprises a lithium salt; the lithium salt is preferably one or more selected from lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium fluorosulfonyl (perfluorobutylsulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(oxalateborate), lithium difluorooxalateborate, lithium trifluoromethanesulfonate, and lithium tetrafluorooxalate phosphate; the concentration of the lithium salt in the electrolyte is preferably 0.5 to 2 mol / L; optionally, the concentration of the lithium salt in the electrolyte is 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 2 mol / L, or any two of the above values.

[0033] According to the present invention, the lithium-ion battery further includes a positive electrode sheet; the positive electrode sheet includes a positive current collector and a positive active layer; the positive active layer includes a positive active material; the mass concentration of the positive active material in the positive active layer is preferably 90% to 98%; optionally, the mass concentration of the positive active material in the positive active layer is 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or any two of the above values.

[0034] In a specific embodiment of the present invention, the positive electrode active material preferably includes lithium iron phosphate; the particle size of the positive electrode active material is preferably 0.05-10 μm, more preferably 0.1-5 μm, even more preferably 0.1-3 μm, and most preferably 0.1-1.5 μm; the negative electrode also has a significant impact on the fast charging performance of lithium iron phosphate batteries. After the positive electrode lithium ions reach the negative electrode, they need to be able to quickly reach the interior of the negative electrode active material layer from the negative electrode surface. Therefore, it is necessary to control the porosity and adhesion of the negative electrode to improve fast charging. Therefore, by further controlling the formula range of 0.2≤100×a×b / c≤105, lithium iron phosphate batteries can have higher fast charging performance. More specifically, the electrolyte and the negative electrode preferably satisfy the relationship 0.2≤100×a×b / c≤80, more preferably the relationship 5≤100×a×b / c≤80, even more preferably the relationship 10≤100×a×b / c≤80, even more preferably the relationship 25≤100×a×b / c≤80, and most preferably the relationship 25≤100×a×b / c≤60.

[0035] In another specific embodiment of the present invention, the positive electrode active material includes a ternary positive electrode material. Because the ternary positive electrode material has poor structural stability, severe dissolution of transition metal ions, and damage to the SEI film stability, an excessively thick SEI film increases interfacial impedance. To further prevent excessive impedance in the ternary material battery, the formula range is further controlled within the range of 5 ≤ 100 × a × b / c ≤ 120. More specifically, the electrolyte and the negative electrode preferably satisfy the relationship 5 ≤ 100 a × b / c ≤ 100, more preferably satisfying the relationship 10 ≤ 100. The preferred order is a×b / c≤100, further preferably satisfying the relationship 10≤100a×b / c≤90, further preferably satisfying the relationship 25≤100a×b / c≤76, and most preferably satisfying the relationship 25≤100a×b / c≤60; the ternary cathode material can be any ternary cathode material well known to those skilled in the art, and there are no special restrictions. In this invention, it is preferred to be mainly composed of salts of nickel, cobalt, and manganese, usually represented as NCM (lithium nickel cobalt manganese oxide) or NCA (lithium nickel cobalt aluminum oxide); specifically, the structural formula of the ternary cathode material is LiNi x Co y Mn z O2; where x is greater than or equal to 0.5 and less than 1, y is greater than 0 and less than 0.5, z is greater than 0 and less than 0.5, and x+y+z=1; in the embodiments provided by the present invention, x is specifically 0.6, y is 0.2, and z is 0.2.

[0036] According to the present invention, the positive electrode active layer preferably further includes a positive electrode conductive agent and a positive electrode binder; the mass of the positive electrode conductive agent is preferably 0.01% to 3% of the mass of the positive electrode active layer; optionally, the mass of the positive electrode conductive agent is 0.01%, 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3% of the mass of the positive electrode active layer, or a range between any two of the above values; the positive electrode conductive agent can be any positive electrode conductive agent well known to those skilled in the art, and there are no special limitations. In the present invention, carbon black and / or carbon nanotubes are preferred; the mass of the positive electrode binder... Preferably, the positive electrode active layer comprises 0.01% to 5% of its mass; optionally, the mass of the positive electrode binder is 0.01%, 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5% or any two of the above values; the positive electrode binder is preferably PVDF; the molecular weight of the PVDF is preferably 60 to 100W; optionally, the molecular weight of the PVDF is 60W, 70W, 80W, 90W, 100W or any two of the above values.

[0037] According to the present invention, the lithium-ion battery further includes a separator for separating the positive electrode and the negative electrode; the separator is preferably one or more of polypropylene (PP), polyethylene (PE) and polypropylene-phenolic resin composite materials.

[0038] The present invention also provides a method for preparing the above-mentioned lithium-ion battery, comprising the following steps:

[0039] S1) Preparation of positive electrode sheet: The positive active material, positive binder and positive conductive agent are mixed evenly and dispersed in NMP to obtain a positive slurry; the positive slurry is coated on aluminum foil to obtain a double-sided coated positive electrode sheet; then it is rolled and cut to obtain a positive electrode sheet.

[0040] S2) Preparation of negative electrode sheet: The negative electrode active material, negative electrode conductive agent and negative electrode binder are mixed evenly and dispersed in deionized water to obtain a negative electrode slurry; the negative electrode slurry is coated on copper foil to obtain a double-sided coated electrode sheet; then it is rolled and cut to obtain a negative electrode sheet.

[0041] S3) Preparation of electrolyte: Mix organic solvent, lithium salt and isothiocyanate additives as shown in formula (I) to obtain electrolyte.

[0042] S4) Assembly and formation: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrode. Then, they are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, standing, formation, and shaping, a lithium-ion battery is obtained.

[0043] To further illustrate the present invention, the following describes in detail a lithium-ion battery provided by the present invention with reference to embodiments; the molecular weight of PVDF is about 70W; the molecular weight of PAA is about 60W; the particle size of SBR is 120-180nm; and the particle size of artificial graphite is 9-12μm.

[0044] All reagents used in the following examples are commercially available.

[0045] Example 1

[0046] 1) Preparation of positive electrode sheet: Lithium iron phosphate (particle size approximately 1 μm, purchased from Guizhou Anda Technology Energy Co., Ltd.) was used as the main material. The main material, binder PVDF, and conductive agent SP were mixed uniformly at a ratio of 97%:1.2%:1.8%; dispersed in NMP to obtain a positive electrode slurry; the positive electrode slurry was coated on aluminum foil to obtain a double-sided coated positive electrode sheet; then rolled and cut to obtain the positive electrode sheet with a compaction density of 2.5 g / cm³. 3 .

[0047] 2) Negative electrode preparation: Artificial graphite was mixed evenly with 1.5% conductive agent SP, 1.25% binder PAA, and 0.75% binder SBR at a mass ratio of 96.5%, and dispersed in deionized water to obtain a negative electrode slurry; the negative electrode slurry was coated onto copper foil to obtain a double-sided coated electrode; then it was rolled and compacted to a density of 1.5 g / cm³. 3 Then it is cut to obtain the negative electrode sheet.

[0048] 3) Preparation of electrolyte

[0049] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of 3:4:3 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L. Additives A and VC were then added at a mass ratio of 1.0%, as shown in Table 1.

[0050] 4) Preparation of the separating membrane

[0051] The coated PP is used as the separator, with a base film thickness of 7μm; both surfaces are coated with an aluminum oxide coating with a coating thickness of 1μm.

[0052] 5) Assembly, formation, and volume determination

[0053] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. The cells are then wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, electrolyte injection, settling, formation, and volume adjustment, a lithium-ion battery is obtained.

[0054] Examples 2 to 15 and Comparative Examples 1 to 2

[0055] The preparation of lithium-ion batteries is the same as in Example 1, except that the types and amounts of positive electrode materials, negative electrode materials, binders and additives are different, as shown in Tables 1 and 2.

[0056] Figure 1 The mass spectrum of the isothiocyanate additives shown in formula (5) is shown.

[0057] The performance of the lithium-ion batteries obtained in Examples 1-9 and Comparative Examples 1-2 was tested according to the following methods, and the test results are shown in Table 1.

[0058] Test method:

[0059] Test methods for determining the structure and content of additive A:

[0060] 1) Discharge the battery completely:

[0061] Use a battery charging and discharging device to discharge the battery completely. Discharge conditions: current 0.3C, cutoff voltage 2.5V, and record the battery number / barcode.

[0062] 2) Collect the electrolyte:

[0063] The battery was disassembled and the electrolyte collected in a glove box (H2O≤0.1ppm, O2≤0.1ppm). There are three methods for collecting the electrolyte: After removing the battery cover, ① if there is free electrolyte, collect it into a 5mL sample tube using a pipette and seal it with adhesive tape to prevent leakage. ② if there is no free electrolyte, use a hydraulic press (Beijing Heng'ao Technology Co., Ltd.'s FY-30 hydraulic press) to continuously pressurize until free electrolyte appears, collect it into a sample tube, and seal it. ③ Add an appropriate amount of dichloromethane extractant to the battery and record the dichloromethane content. After adding dichloromethane, place the battery in an aluminum-plastic bag and seal it with a heat sealer. Transfer it to an ultrasonic vibrator and vibrate for 12 hours to allow the electrolyte in the electrode to mix thoroughly with the dichloromethane. Then, use a pipette to draw the mixture of dichloromethane and electrolyte into a 5mL sample tube and seal the sample tube with adhesive tape.

[0064] The collected electrolyte samples were injected into an Agilent Intuvo 9000 gas chromatograph-mass spectrometer using a microsyringe to obtain GC-MS spectra. Additives VC and A were prepared into EMC solutions of different concentrations and injected into the Agilent Intuvo 9000 gas chromatograph-mass spectrometer to obtain standard GC-MS spectra of the pure substances. The GC spectrum shows the peak area curves corresponding to different concentrations, and the MS curve shows the specific spectrum of the substance. The GC-MS spectrum of the electrolyte to be tested was compared with the standard GC-MS spectrum to determine whether the electrolyte to be tested contained additives VC and A (for example, if a peak appears in the standard spectrum at the position where additive VC is located in the spectrum of the electrolyte to be tested, then the electrolyte to be tested contains VC; the same applies to other components). Then, the content of each component was determined based on the peak area of ​​each component in the electrolyte to be tested.

[0065] Negative electrode adhesion test method:

[0066] Pretreatment process: Disassemble the empty battery, remove the negative electrode, soak the negative electrode in dimethyl carbonate (DMC) solution for 2 hours, and dry it at 80°C.

[0067] Take a standard steel plate (50mm×125mm) as the rigidity test base and wipe the surface of the steel plate clean with lint-free paper soaked in alcohol. Adhere one side of the 50mm×125mm 3M double-sided tape to the steel plate, ensuring a smooth, wrinkle-free adhesion. Cut the negative electrode sheet with an active coating into 50mm×125mm test samples. Adhere the electrode sheet to be tested to the other adhesive side of the double-sided tape, and then apply another layer of double-sided tape to the electrode sheet surface, ensuring smooth, wrinkle-free contact during the adhesion process. After pressing with a pressure roller, clamp the steel plate with one end of the universal testing machine's tensile clamps and the 3M tape with the other end. Set the tensile testing machine's stroke to 100mm and perform a tensile test at a speed of 300mm / min. Set the tensile testing machine's stroke to 100m and record the curve in the tensile testing machine's software graph until it flattens out and the displacement is greater than 80mm, then stop the machine. Read the average tensile force value of the flattened portion of the curve, which is the adhesion force.

[0068] Test method for porosity of negative electrode active layer:

[0069] The porosity ε of the negative electrode active material layer was obtained by the mass difference method. The electrode was cut into circular pieces with a radius r of 0.95 cm using an electrode punching machine. The thicknesses L and L0 of the electrode and current collector were measured using a thickness gauge, respectively. The volume of the negative electrode active material on the cut electrode was calculated as V = π × r. 2×(L-L0); Weigh the cut electrode sheet using a balance with an accuracy of 0.00001g and record the mass as m1. Soak the electrode sheet in hexadecane for 1 hour (completely immersed). Remove the electrode sheet with tweezers and blot it dry with filter paper until a constant weight is achieved. Weigh the electrode sheet again and record the mass as m2. Substitute the experimental data into the formula to calculate: ε=(m2-m1) / (V×ρ0)×100%, where ρ0 is the density of hexadecane, 0.7734g / cm³. 3 The porosity ε of the electrode is obtained.

[0070] Fast charging performance testing method:

[0071] The battery charging and discharging equipment performs a full discharge process on the battery. The discharge conditions are as follows: For batteries using lithium iron phosphate as the positive electrode active material: charge at a constant current of 0.33C to the upper limit voltage of 3.65V, charge at a constant voltage until the current is less than or equal to 0.05C, and then discharge at 0.33C to the lower limit voltage of 2.5V; this constitutes one cycle, and the charge-discharge cycle is repeated 3 times. The discharge capacity of the third cycle is taken as the battery capacity. For ternary lithium batteries: charge at a constant current of 0.33C to the upper limit voltage of 4.25V, charge at a constant voltage until the current is less than or equal to 0.05C, and then discharge at 0.33C to the lower limit voltage of 2.75V; this constitutes one cycle, and the charge-discharge cycle is repeated 3 times. The discharge capacity of the third cycle is taken as the battery capacity. Then, after charging at 0.33C to 5% SOC (which is 5% of the third discharge capacity), a fast charging test was conducted. Specifically, the charging time was recorded as t1 when charging at 1.5C to 10% SOC, t2 when charging at 1.5C to 20% SOC, and then left to rest for 5 minutes. The charging time was recorded as t3 when charging at 1.5C to 30% SOC, and then left to rest for 5 minutes. The charging time was recorded as t4 when charging at 1.5C to 40% SOC, and then left to rest for 5 minutes. The charging time was recorded as t5 when charging at 1.5C to 50% SOC, and then left to rest for 5 minutes. The charging time was recorded as t6 when charging at 1.2C to 60% SOC, and then left to rest for 5 minutes. The charging time was recorded as t7 when charging at 1.2C to 70% SOC, and then left to rest for 5 minutes. The charging time was recorded as t8 when charging at 1.2C to 80% SOC, and then left to rest for 5 minutes. Fast charging time t = t1 + t2 + t3 + t4 + t5 + t6 + t7 + t8.

[0072] Gas production test method:

[0073] Discharge - Record battery number. Use a battery charging and discharging device to discharge the battery completely. Discharge conditions: current 0.33C, cutoff voltage 2.5V. Then charge to full capacity according to the following conditions.

[0074] For batteries using lithium iron phosphate as the positive electrode active material, charge at a constant current of 0.33C to the upper limit voltage of 3.65V, then charge at a constant voltage until the current is less than or equal to 0.05C, and then discharge at 0.33C to the lower limit voltage of 2.5V; this constitutes one cycle. Perform three charge-discharge cycles, and use the discharge capacity of the third cycle as the battery capacity. For batteries using ternary lithium lithium as the positive electrode active material, charge at a constant current of 0.33C to the upper limit voltage of 4.25V, then charge at a constant voltage until the current is less than or equal to 0.05C, and then discharge at 0.33C to the lower limit voltage of 2.75V; this constitutes one cycle. Perform three charge-discharge cycles, and use the discharge capacity of the third cycle as the battery capacity.

[0075] Full charge: For cathode active materials using lithium iron phosphate: charge at a constant current of 0.33C to the upper limit voltage of 3.65V, and then charge at a constant voltage until the current is less than or equal to 0.05C;

[0076] For cathode materials using ternary cathodes, charge at a constant current of 0.33C to the upper limit voltage of 4.25V, and then charge at a constant voltage until the current is less than or equal to 0.05C.

[0077] After full charging, a gas production test was conducted.

[0078] Gas production test

[0079] Then, the volume of the battery is tested using the water displacement method and recorded as V0. After the battery is stored in a 60°C oven for a period of time, it is taken out and the battery temperature is reduced to room temperature. The volume of the battery is then tested again using the water displacement method and recorded as V1. The amount of gas produced by the battery during this storage period is V1-V0. This value divided by the battery's rated capacity is the amount of gas produced per ampere-hour.

[0080] Battery volume testing using the water displacement method:

[0081] 1) Add an appropriate amount of pure water to the container and test its density with a hydrometer and record it;

[0082] 2) Adjust the balance to level and tare the cells (tareing is performed before testing each cell);

[0083] 3) Submerge the battery cell body along with the tabs into the solution, ensuring that the battery cell does not contact the container wall. After stabilization, take a reading and record the data as T.

[0084] 4) Turn off the balance and seal the container to prevent the reagent from evaporating.

[0085] The battery volume is T / ρ 液

[0086] Among them, tests were conducted before and after storage to obtain T0 and T1 respectively;

[0087] V1-V0=T0 / ρ 液 -T1 / ρ液

[0088] Gas production at 60℃ = (V1 - V0) / battery capacity

[0089] Table 1. Parameters and performance test results of lithium-ion batteries

[0090]

[0091]

[0092] Table 2. Parameters and performance test results of lithium-ion batteries

[0093]

[0094]

[0095] Comparing Examples 1-15 and Comparative Examples 1 and 2 in Tables 1 and 2, it can be seen that when 100×a×b / c meets the formula range of 0.2-100, the interfacial impedance between the negative electrode and the electrolyte is reduced, while the lithium-ion transport rate is improved. The batteries all exhibit excellent fast-charging performance, with a fast-charging time of less than 26 minutes and a gas production of less than 1.5 Ml / Ah. In Comparative Example 1, when 100×a×b / c is less than 0.2, the battery's fast-charging performance is poor, with a fast-charging time exceeding 30 minutes, and the gas production is greater than 1.5 Ml / Ah. In Comparative Example 2, when 100×a×b / c is greater than 120, the internal gas production of the battery is large.

[0096] Furthermore, the formula range in Examples 1 to 3 meets the preferred range of 0.2 to 105; and when each parameter meets the preferred range, the battery takes into account both better fast charging and low gas production, with fast charging time less than 23 minutes and gas production less than Ml / Ah.

[0097] Combining Examples 1-3 and Example 6, it can be seen that when the formula for lithium iron phosphate does not satisfy 0.2-105, the gas production of the battery in Example 6 is significantly higher than that in Examples 1-3, and its fast charging performance is also significantly worse than that in Examples 1-3.

[0098] In combination with Examples 8, 9 and 10, when the positive electrode active material is a ternary positive electrode material, Example 10 does not meet the further preferred range of 5 to 120, the fast charging is significantly worse than that of Examples 8 and 9, and the gas production is greater than that of Examples 8 and 9;

[0099] Based on Examples 3, 11, and 2, it can be seen that when the ratio of additive A content to VC additive content, i.e., a / d, does not meet the preferred range of 0.2 to 2, the fast charging performance of the battery is slightly worse, and the gas production increases.

[0100] In Example 13, without the addition of VC, the fast charging performance deteriorated compared to Example 3, and the gas production increased.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A lithium-ion battery, characterized by, Including electrolyte and negative electrode plate; The electrolyte includes isothiocyanate additives as shown in formula (I); Formula (I); Where M is selected from C1-C4 alkyl or C1-C4 fluoroalkyl; The mass content of the isothiocyanate additive shown in formula (I) in the electrolyte is a%; The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer; The porosity of the negative electrode active layer is c%; the adhesion force between the negative electrode active layer and the negative electrode current collector is b N / m. The electrolyte and the negative electrode sheet satisfy the following relationship: 60≤100×a×b / c≤85.71; The percentage a% is 0.1% to 1%; The value of b is 28.8~40; The c% is 12%~35%; The electrolyte further includes unsaturated ester additives; the unsaturated ester additives are selected from one or more of vinylene carbonate, fluoroethylene carbonate, ethylene ethylene carbonate, allyl ethylene carbonate, vinyl acetate and catechol carbonate; the content of unsaturated ester additives in the electrolyte is d%; 0.5 ≤ d ≤ 3. 0.2≤a / d≤1.

2. The lithium-ion battery of claim 1, wherein, M is selected from fluoromethyl, fluoroethyl, fluoropropyl, fluorobutyl, methyl, ethyl, propyl or butyl.

3. The lithium-ion battery of claim 2, wherein, The isothiocyanate additives are selected from one or more of formulas (1) to (16): Formula (1); Formula (2); Formula (3); Formula (4); Formula (5); Formula (6); Formula (7); Formula (8); Formula (9); Formula (10); Formula (11); Formula (12); Formula (13); Formula (14); Formula (15); Formula (16).

4. The lithium-ion battery of claim 1, wherein, It also includes a positive electrode sheet; the positive electrode sheet includes a positive electrode active material; the positive electrode active material includes lithium iron phosphate.

5. The lithium-ion battery of claim 1, wherein, It also includes a positive electrode sheet; the positive electrode sheet includes a positive electrode active material; the positive electrode active material includes a ternary positive electrode material.

6. The lithium-ion battery of claim 1, wherein, The electrolyte further includes a lithium salt; the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium fluorosulfonyl (perfluorobutylsulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(oxalateborate), lithium difluorooxalateborate, lithium trifluoromethanesulfonate, and lithium tetrafluorooxalate phosphate.

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