Secondary battery and device

Abstract

The invention provides a secondary battery, the secondary battery comprises a positive pole piece, a negative pole piece and an electrolyte, the electrolyte comprises a fluorine-containing sulfimide lithium salt, based on the total mass of the electrolyte, the mass percentage content of the fluorine-containing sulfimide lithium salt is L%, the negative electrode plate comprises a negative electrode active material layer and a solid electrolyte interface film located on the surface of the negative electrode active material layer, the negative electrode active material layer comprises a negative electrode active material, the compaction density of the negative electrode plate is Y g / cm < 3 >, the thickness of the solid electrolyte interface film is M nm, the OI value of the negative electrode active material is N, and the OI value of the negative electrode active material is N; the secondary battery meets the condition that # imgabs0 # meets the limited relation, and the secondary battery has excellent low-temperature performance and rate capability.

Description

Technical Field

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

[0002] Excellent low-temperature performance and high-rate performance have long been pain points in the power battery industry. In recent years, with the rapid development of power batteries, the industry's demand for lithium-ion batteries with excellent low-temperature and high-rate performance has become increasingly strong. However, most related research and reports focus on the electrolyte and electrodes themselves, respectively, with few studies synergizing the electrodes and electrolyte to create lithium-ion power batteries with excellent low-temperature and high-rate performance. Summary of the Invention

[0003] In view of the aforementioned problems in the prior art, the present application proposes a secondary battery and device. The secondary battery of the present application improves the low-temperature performance and rate performance of lithium-ion secondary batteries by controlling the mass percentage of a fluorinated sulfonyl imide lithium salt in the electrolyte, the thickness of the solid electrolyte interface film formed on the surface of the negative electrode active material layer, the compaction density of the negative electrode sheet, and the OI value of the negative electrode active material.

[0004] The first aspect of the present application provides a secondary battery, which includes a positive electrode plate, a negative electrode plate and an electrolyte, wherein the electrolyte includes a fluorinated sulfonyl imide lithium salt, and the mass percentage of the fluorinated sulfonyl imide lithium salt is L% based on the total mass of the electrolyte; the negative electrode plate includes a negative electrode active material layer and a solid electrolyte interface membrane located on the surface of the negative electrode active material layer; the negative electrode active material layer includes a negative electrode active material, and the compaction density of the negative electrode plate is Y g / cm 3 , the thickness of the solid electrolyte interface film is M nm, the OI value of the negative electrode active material is N, and the secondary battery satisfies:

[0005]

[0006] A second aspect of the present application provides a device including the aforementioned secondary battery.

[0007] The technical solution of this application can achieve the following beneficial effects:

[0008] The secondary battery of the present application controls the mass percentage of the fluorinated sulfonyl imide lithium salt in the electrolyte, the compaction density of the negative electrode sheet, the thickness of the solid electrolyte interface film formed on the surface of the negative electrode active material layer, and the OI value of the negative electrode active material to meet a specific relationship, thereby synergistically optimizing both the electrolyte formulation and the negative electrode sheet. At the same time, the lithium ions have excellent lithium ion transmission rates in the electrolyte, at the interface between the electrolyte and the negative electrode sheet, and in the negative electrode sheet, thereby improving the low-temperature performance and rate performance of the secondary battery. DETAILED DESCRIPTION

[0009] To make the purpose, technical solutions, and advantages of this application more clear, the technical solutions of this application will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of this application, rather than all the embodiments. The relevant embodiments described herein are illustrative and are used to provide a basic understanding of this application. The embodiments of this application should not be interpreted as limiting this application.

[0010] For the sake of clarity, only some numerical ranges are specifically disclosed herein. However, any lower limit may be combined with any upper limit to form an unspecified range; and any lower limit may be combined with other lower limits to form an unspecified range, and similarly, any upper limit may be combined with any other upper limit to form an unspecified range. In addition, each individually disclosed point or single value may itself serve as a lower limit or upper limit and be combined with any other point or single value, or with other lower limits or upper limits, to form an unspecified range.

[0011] In the description herein, unless otherwise specified, “above” and “below” include the number itself.

[0012] Unless otherwise specified, the terms used in this application have the commonly understood meanings commonly understood by those skilled in the art. Unless otherwise specified, the numerical values of the various parameters mentioned in this application can be measured using various measurement methods commonly used in the art (for example, they can be tested according to the methods given in the examples of this application).

[0013] A list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0014] Primary and secondary batteries

[0015] One or more embodiments of the present application provide a secondary battery, the secondary battery comprising a positive electrode plate, a negative electrode plate, and an electrolyte, wherein the electrolyte comprises a fluorinated sulfonyl imide lithium salt, and the mass percentage of the fluorinated sulfonyl imide lithium salt is L% based on the total mass of the electrolyte, the negative electrode plate comprises a negative electrode active material layer and a solid electrolyte interface membrane located on the surface of the negative electrode active material layer, the negative electrode active material layer comprises a negative electrode active material, and the compaction density of the negative electrode plate is Y g / cm 3 , the thickness of the solid electrolyte interface film is M nm, the OI value of the negative electrode active material is N, and the secondary battery satisfies:

[0016]

[0017] The present application regulates the matching relationship between the mass percentage of fluorinated sulfonyl imide lithium salt, the compaction density of the negative electrode plate, the thickness of the solid electrolyte interface film of the negative electrode and the OI value of the negative electrode active material to meet the above-mentioned numerical range. Under this limited condition, lithium ions have excellent lithium ion transmission rate in the electrolyte, at the interface between the electrolyte and the negative electrode plate, and in the negative electrode plate, thereby significantly improving the low-temperature performance and rate performance of the secondary battery.

[0018] If the value is too low, it means that the comprehensive transmission rate of lithium ions in the electrolyte, at the interface between the electrolyte and the negative electrode sheet, and in the negative electrode sheet is low, and the battery's low temperature and rate performance are poor; The value is too high and the cycle stability of the lithium battery is poor.

[0019] In some embodiments, 2.5, 3, 3.2, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 9, 10, 11, 13, 15, 17 or a range consisting of any two of the above values.

[0020] In some embodiments, Within this limited range, the lithium ion transfer rate can be further optimized, which is beneficial to improving the low-temperature performance and rate performance of the secondary battery.

[0021] In some embodiments, the fluorine-containing sulfonyl imide lithium salt includes at least one of lithium bis(fluorosulfonyl)imide (LiFSI), lithium (trifluoromethylsulfonyl)(perfluorobutylsulfonyl)imide (LiFNFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0022] In some embodiments, 0.2≤L≤25. Since the anion radius of the fluorinated sulfonyl imide lithium salt is large, it is easy to dissociate lithium ions, which helps the migration of lithium ions and weakens the concentration polarization phenomenon. On the one hand, it can increase the transmission rate of lithium ions in the electrolyte. On the other hand, it is beneficial for the release and embedding of lithium ions at the interface between the electrolyte and the negative electrode, thereby improving the low-temperature performance and fast charging performance of the battery. However, when the mass percentage of the fluorinated sulfonyl imide lithium salt is too high, it is easy to chemically react with the positive electrode current collector aluminum foil at high potential, causing it to corrode, thereby affecting the battery performance. In some embodiments, 1≤L≤14.

[0023] In some embodiments, 5≤M≤100. When the thickness of the solid electrolyte interface film on the surface of the negative electrode is too high, the impedance of the secondary battery is high, the dynamic performance is limited, and the low temperature performance and fast charging performance of the secondary battery are affected. When the thickness of the solid electrolyte interface film on the surface of the negative electrode is too low, the continuous side reaction between the surface of the negative electrode and the electrolyte cannot be suppressed, resulting in poor cycle stability. The solid electrolyte interface film within this limited range can ensure that the secondary battery has excellent low temperature performance and fast charging performance while having high cycle stability. In the present application, the thickness of the solid electrolyte interface film can be adjusted by conventional technical means in the field, such as controlling the formation temperature, formation time and adjusting the electrolyte formula. In some embodiments, 10≤M≤30.

[0024] In some embodiments, in some implementations, 0.5≤Y≤5. If the compaction density of the negative electrode plate is too high, the electrolyte wettability will be poor, the desolvation process of lithium ions will be hindered, and the kinetic performance of the secondary battery will be affected. Too low a compaction density will lead to low conductivity and volume energy density of the negative electrode plate, and it is easy to cause continuous side reactions between the electrolyte and the negative electrode plate, and the cycle stability of the secondary battery is poor. The compaction density within this limited range can ensure that the secondary battery has high cycle stability while having excellent low temperature performance and fast charging performance. In this application, the compaction density of the negative electrode plate can be adjusted according to the characteristics of the selected active material by conventional technical means in the field, such as controlling the rolling pressure, rolling temperature and rolling speed of the plate. In some embodiments, 1≤Y≤3.

[0025] In some embodiments, 0.5≤N≤15. If the OI value of the negative electrode active material is too high, the negative electrode active material (such as graphite particles) tends to be arranged parallel to the negative electrode current collector, and there are fewer end faces available for active lithium ions to be deintercalated. The battery is prone to lithium plating during the cycle, and the battery's kinetic performance is poor. When the OI value of the negative electrode active material is too small, although the formation of unfavorable lithium dendrites is suppressed, the negative electrode pole piece is prone to wrinkling and powdering during the cycle, and the cycle stability is poor. The OI value of the negative electrode active material within this limited range can ensure that the negative electrode pole piece has a high cycle stability, while making the graphite particles tend to be arranged perpendicular to the negative electrode current collector, ensuring that lithium ions can be quickly deintercalated at the interface between the negative electrode pole piece and the electrolyte, improving the kinetic performance of the secondary battery, and thus improving the low temperature performance and fast charging performance of the system. In some embodiments, 1≤N≤5. The OI value of the negative electrode active material can be controlled by the sintering temperature and sintering time during the graphite production process.

[0026] In some embodiments, the positive electrode sheet includes a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material. In some embodiments, the positive electrode active material includes a lithium iron phosphate material, and the lithium iron phosphate material includes a lithium iron phosphate material having a formula of Li x Fe y R (1-y) At least one of the materials represented by PO4, wherein R includes at least one of the elements Mn, Co, Ti, Mg, Ca, Cr, Cu, Ni, V, Mo, Zn, Al, B and Nb, 0.05≤x≤1.2, 0<y≤1. In some embodiments, the lithium iron phosphate material is a lithium manganese iron phosphate material. The conductivity and lithium ion diffusion capacity of the lithium iron phosphate material with olivine structure are poor, especially at low temperatures, the lithium ion transmission rate is even worse. Therefore, it is even more necessary to improve the transmission rate of lithium ions in the electrolyte, at the interface between the electrolyte and the negative electrode sheet, and within the negative electrode sheet to improve the overall lithium ion transmission rate of the secondary battery and obtain better low temperature performance and rate performance.

[0027] In some embodiments, the surface of the lithium iron phosphate material has a carbon coating layer; based on the mass of the lithium iron phosphate material, the mass percentage of the carbon coating layer is 1% to 3%. It is understandable that if the carbon coating layer is too thin, it will not effectively improve the inherent conductivity of the positive electrode material, and the rate performance of the lithium-ion battery will be poor; if the carbon coating layer is too thick, the energy density of the lithium-ion battery will be significantly reduced, and the unevenness of the coating will be more obvious and the defects will increase, affecting the cycle performance of the lithium-ion battery. In some embodiments, the mass percentage of the carbon coating layer is 1%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 3%, or a range consisting of any two of the above values.

[0028] In some embodiments, in addition to the aforementioned positive electrode active materials, the positive electrode active material may further include lithium nickel oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, lithium nickel manganese cobalt magnesium oxide, and lithium nickel manganese oxide.

[0029] In some embodiments, the positive electrode active material layer further includes a binder, and optionally a conductive material. The binder improves the bonding between the positive electrode active material particles and also improves the bonding between the positive electrode active material and the current collector.

[0030] In some embodiments, the binder includes, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0031] In some embodiments, the conductive material includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0032] In some embodiments, the positive electrode further includes a positive electrode current collector, which can be a metal foil or a composite current collector. For example, aluminum foil can be used. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer substrate.

[0033] In some embodiments, the negative electrode active material layer includes a negative electrode active material, wherein the negative electrode active material includes a carbon-based material including at least one of graphite, soft carbon, hard carbon, carbon nanotubes, and graphene. Exemplarily, the tin-based material includes at least one of tin, tin oxide, and a tin alloy. Exemplarily, the phosphorus-based material includes at least one of phosphorus and a phosphorus complex.

[0034] In some embodiments, the negative electrode active material layer further includes a binder and a conductive agent. The binder includes, but is not limited to: at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon. The conductive agent includes, but is not limited to: at least one of carbon-based materials, metal-based materials, conductive polymers and mixtures thereof. In some embodiments, the carbon-based material includes natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber or any combination thereof. In some embodiments, the metal-based material includes metal powder, metal fiber, copper, nickel, aluminum or silver. In some embodiments, the conductive polymer includes a polyphenylene derivative.

[0035] In some embodiments, the negative electrode further includes a negative electrode current collector, which includes at least one of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0036] In some embodiments, the porosity of the negative electrode sheet is ε1, which satisfies: 28% ≤ ε1 ≤ 45%. If the porosity of the negative electrode sheet is too low, it will affect the infiltration of the electrolyte and will not be conducive to the formation of a stable solid electrolyte interface film on the surface of the negative electrode sheet. Controlling the porosity of the negative electrode sheet within the above range is beneficial to ensure that a stable solid electrolyte interface film is formed on the surface of the negative electrode sheet while ensuring a higher energy density, thereby improving the cycle performance of the secondary battery. In some embodiments, ε1 is 28%, 30%, 32%, 34%, 36%, 38%, 40%, 42%, 44%, 45% or a range consisting of any two of the above values.

[0037] In some embodiments, the electrolyte further comprises other lithium salts, including at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium trifluorosulfonyl (LiOTf), lithium bis(oxalatoborate) (LiBOB), lithium bis(fluoromalonate)borate (LiBFMB), and lithium difluorooxalatoborate (LiDFOB). In some embodiments, the other lithium salt is LiPF6.

[0038] In some embodiments, the electrolyte further includes a solvent, and the solvent includes at least one of a chain carbonate compound, a cyclic carbonate compound, and a carboxylate compound.

[0039] In some embodiments, the linear carbonate compound includes, but is not limited to, one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), or ethylmethyl carbonate (MEC).

[0040] In some embodiments, the cyclic carbonate compound includes, but is not limited to, one or more of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC).

[0041] In some embodiments, the carboxylate compound includes, but is not limited to, one or more of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerolactone, or caprolactone.

[0042] In some embodiments, a separator is provided between the positive and negative electrode sheets to prevent short circuits. The material and shape of the separator used in the embodiments of the present application are not particularly limited and may be any known prior art material. In some embodiments, the separator comprises a polymer or inorganic material, for example, formed from a material that is stable to the electrolyte of the present application.

[0043] In some embodiments, the isolation film includes a base film and a coating disposed on the base film. The base film includes at least one of a polyethylene film, a polypropylene film, a PP / PE / PP composite film, a polyimide film, an aramid film, a polyethylene terephthalate film, or a non-woven fabric. In some embodiments, the coating includes at least one of a polymer layer, an inorganic ceramic layer, or a hybrid layer of a polymer and an inorganic ceramic layer.

[0044] In some embodiments, the inorganic ceramic layer includes inorganic particles and a binder, wherein the inorganic particles include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene.

[0045] The polymer layer includes a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0046] In some embodiments, a method for preparing a secondary battery includes providing an electrode assembly, injecting liquid, packaging, and forming. In some embodiments, the forming includes: charging at a constant current of 0.05C for 100 minutes, charging at a constant current of 0.1C for 50 minutes, and charging at a constant current of 0.33C for 50 minutes at a temperature between 30°C and 60°C, for example, 45°C, and a pressure between 150kgf and 400kgf, for example, 300kgf.

[0047] In some embodiments, the secondary battery is a lithium secondary battery or a sodium secondary battery. In some embodiments, the lithium secondary battery includes, but is not limited to, a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0048] In some embodiments, the secondary battery may include an outer packaging, which may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0049] In some embodiments, the shape of the secondary battery is not particularly limited and can be cylindrical, square, or any other shape.

[0050] In some embodiments, the present application also provides a battery module. The battery module includes the aforementioned secondary battery. The battery module of the present application utilizes the aforementioned secondary battery and therefore has at least the same advantages as the secondary battery. The battery module of the present application may include multiple secondary batteries, the specific number of which can be adjusted based on the application and capacity of the battery module.

[0051] In some embodiments, the present application further provides a battery pack comprising the aforementioned battery module. The number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0052] 2. Device

[0053] The present application also provides a device comprising at least one of the above-mentioned secondary battery, battery module or battery pack.

[0054] In some embodiments, the device includes, but is not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, power storage systems, etc. To meet the device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module may be used.

[0055] In other embodiments, the device may be a mobile phone, a tablet computer, a laptop computer, etc. The device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0056] Hereinafter, the secondary battery of the present application will be further described in combination with specific embodiments and comparative examples.

[0057] Examples and Comparative Examples

[0058] Hereinafter, the present application will be described in more detail with reference to Examples and Comparative Examples. However, the present application is not limited to these Examples unless departing from the gist of the present application.

[0059] The materials and solvents used in the following examples and comparative examples were all commercially available.

[0060] Example 1

[0061] Preparation of positive electrode sheet: The positive electrode active material LiFePO, the conductive agent carbon nanotube (CNT) and acetylene black (ACET), and the binder polyvinylidene fluoride (PVDF) are fully homogenized in an N-methylpyrrolidone (NMP) solvent system in a weight ratio of LiFePO: CNT: ACET: PVDF = 97: 0.5: 1: 1.5, and then evenly coated on one side of a positive electrode current collector aluminum foil with a thickness of 12 μm. After drying, the above steps are repeated on the other side of the aluminum foil to obtain a double-sided coated positive electrode sheet. After cold pressing, cutting, and welding the pole ears, the positive electrode sheet is obtained.

[0062] Preparation of the negative electrode sheet: The negative electrode active material graphite, conductive agent acetylene black (ACET), binder styrene butadiene rubber (SBR), thickener sodium carboxymethyl cellulose CMCNa, and polyacrylic acid PAA are fully homogenized in deionized water in a weight ratio of graphite:ACET:SBR:CMCNa:PAA=95:2:1.5:1:0.5, and then coated on one side of the negative electrode current collector copper foil with a thickness of 8μm. After drying, the above steps are repeated on the other side of the copper foil to obtain a double-sided coated negative electrode sheet. After cold pressing, cutting, and welding the pole ears, the negative electrode sheet is obtained.

[0063] Preparation of electrolyte: In an argon-protected glove box (water <0.1ppm, oxygen <0.1ppm), solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:EMC:DEC = 25:55:20. Based on the mass of the solvent, lithium salts (LiFSI and LiPF6) were added to a molar concentration of 1.2 mol / L, of which the mass percentage of LiFSI was 8%.

[0064] Isolation film: PP / PE / PP three-layer composite film is used as the isolation film.

[0065] Preparation of a lithium-ion secondary battery: The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrode sheets, and then wound to form a bare cell. The bare cell is placed in an aluminum-plastic film outer packaging, thoroughly dried, and then injected with the prepared lithium-ion battery electrolyte. The battery is then aged at 45°C for 48 hours, followed by fixture formation at 45°C, secondary sealing, and conventional capacity separation. After formation, the solid electrolyte interface film on the surface of the negative electrode active material layer is 20 nm thick.

[0066] Examples 2-14

[0067] Examples 2-14 were achieved based on Example 1 by adjusting the content of the fluorinated sulfonyl imide lithium salt in the electrolyte, the compaction density of the negative electrode plate, the thickness of the solid electrolyte interface film on the surface of the negative electrode plate, and the OI value of the negative electrode active material. Specific adjustment measures and detailed data are shown in Table 1. The rest of the preparation process is the same as that of Example 1.

[0068] Comparative Examples 1-6

[0069] Comparative Examples 1-6 were prepared based on Examples 1-14 by adjusting the content of the fluorinated sulfonyl imide lithium salt in the electrolyte, the compaction density of the negative electrode sheet, the thickness of the solid electrolyte interface film on the surface of the negative electrode sheet, and the OI value of the negative electrode active material (porosity omitted). Specific adjustments and detailed data are shown in Table 1. The molar concentration of the lithium salt in Comparative Example 1 was 2 mol / L. The rest of the preparation process was the same as in Example 1.

[0070] Test Method

[0071] 1. Pole piece porosity test

[0072] Porosity is measured using a mercury intrusion porosimeter. Specifically, the dried electrode sample is cut into thin strips of a certain size. A micrometer is used to measure the apparent volume of the electrode coating: apparent volume = sample coating thickness × sample length × sample width. The electrode is then vacuum-degassed, rolled, and placed in a sample cell. The sample volume must occupy 40%-70% of the effective volume of the sample tube to ensure measurement accuracy. A mercury intrusion porosimeter is then used to measure the sample's pore volume, i.e., the volume of mercury intruded into the sample. Porosity = pore volume / apparent volume.

[0073] 2. Test of the compaction density of the negative electrode

[0074] The compaction density of the negative electrode sheet = the surface density of the negative electrode sheet (g / cm 2) / thickness of the negative electrode active material layer (cm). Cut a piece of negative electrode sheet and weigh its mass, record it as M1. Then scrape off the negative electrode active material layer on the negative electrode sheet and weigh its mass, record it as M2. Measure the area of the negative electrode sheet and record it as V. The surface density of the negative electrode sheet = (M1-M2) / V. The thickness of the negative electrode active material layer is the total thickness of the negative electrode sheet minus the thickness of the negative electrode current collector (e.g., foil). The mass can be measured using a standard balance, and the thickness can be measured using a micrometer.

[0075] 3. Test of solid electrolyte interface film thickness

[0076] The electrode after cycling was placed in a dimethyl carbonate solution to wash away the solvent and residual organic small molecules, and then the electrode was sent to the sample chamber. The electrode was then characterized by cryo-electron microscopy. After obtaining specific cryo-electron microscopy digital photos, the imageJ software was used to measure the part with obvious SEI film characteristics on the negative electrode surface to obtain the SEI film thickness.

[0077] 4. Test of OI value of negative electrode active material

[0078] The OI value N of the negative electrode active material is used to characterize the orientation index of the negative electrode active material, and can be calculated by using the X-ray diffraction spectrum measured by an X-ray powder diffractometer, that is, N = C004 / C110, wherein C004 is the peak area of the characteristic diffraction peak of the

[004] crystal plane of the negative electrode active material, and C110 is the peak area of the characteristic diffraction peak of the

[110] crystal plane of the negative electrode active material. Specific test method: control the ambient temperature to 15-25°C and the ambient humidity to 20%-80%, stick the negative electrode active material to one end of a glass slide with double-sided tape, place it in a Bruker D8 Discover X-ray powder diffractometer, set the scanning angle range to 50-80°, the step length to 0.01°, and the time for each step to 0.9s to obtain the X-ray diffraction spectrum of the negative electrode active material, and use software to calculate the peak area of the diffraction peak of the

[004] crystal plane of the negative electrode active material and the peak area of the diffraction peak of the

[110] crystal plane, thereby using the formula N=C004 / C1110 to obtain the OI value N of the negative electrode active material.

[0079] 5. Capacity retention test after 2000 cycles at a normal rate at 25°C

[0080] At 25°C, the prepared lithium-ion secondary battery was charged at a constant current rate of 1C to 3.8V, followed by constant voltage charging until the current was less than 0.05C. After standing for 5 minutes, the battery was discharged at a rate of 1C to 2.7V, and the initial discharge capacity was recorded. The lithium-ion secondary battery was cycled 2000 times using the above method, and the discharge capacity was recorded each time. The capacity retention rate of the lithium-ion secondary battery after 2000 cycles at a conventional rate at 25°C = discharge capacity at the 2000th cycle / initial discharge capacity × 100%.

[0081] 6. -20℃ capacity retention test

[0082] At 25°C, the prepared lithium-ion secondary battery was charged to 3.8V at a constant current rate of 0.33C, and then charged at a constant voltage until the current was less than 0.05C. After standing for 5 minutes, it was discharged to 2.7V at a rate of 0.1C, and the room temperature discharge capacity was recorded. The battery was then cooled to -20°C, and the lithium-ion secondary battery was charged to 3.8V at a constant current rate of 0.33C, and then charged at a constant voltage until the current was less than 0.05C. After standing for 5 minutes, it was discharged to 2.7V at a rate of 0.1C, and the -20°C discharge capacity was recorded. Capacity retention rate of lithium-ion secondary battery at -20°C = discharge capacity at -20°C / initial discharge capacity × 100%.

[0083] 7. Capacity retention test after 750 cycles of fast charging at 25°C

[0084] At 25°C, the prepared lithium-ion secondary battery was charged at a constant current rate of 2C to 3.8V, followed by constant voltage charging until the current was less than 0.05C. After standing for 5 minutes, the battery was discharged at a rate of 0.5C to 2.7V, and the initial discharge capacity was recorded. The lithium-ion secondary battery was cycled 750 times using the above method, and the discharge capacity was recorded each time. The capacity retention rate of the lithium-ion secondary battery after 750 fast charge cycles at 25°C = discharge capacity at the 750th cycle / initial discharge capacity × 100%.

[0085] Please see Table 1 below for test data.

[0086] Table 1

[0087]

[0088]

[0089] It can be seen from Examples 1-14 that when the mass percentage of fluorinated sulfonyl imide lithium salt, the thickness of the solid electrolyte interface film on the surface of the negative electrode active material layer, the compaction density of the negative electrode plate, and the OI value of the negative electrode active material meet the preset relationship, the 25°C conventional rate cycle performance, the 25°C fast charge cycle performance and the -20°C capacity retention rate are all high.

[0090] Comparative Example 1 shows that when the mass percentage of fluorinated sulfonyl imide lithium salt is too high, the thickness of the solid electrolyte interface film formed between it and the surface of the negative electrode active material layer, the compaction density of the negative electrode plate, and the OI value of the negative electrode active material do not conform to the preset relationship. The high concentration of fluorinated sulfonyl imide lithium salt will cause corrosion to the positive electrode current collector, and have a significant negative impact on the 25°C conventional rate and 25°C fast charge cycle performance.

[0091] Comparative Example 2 shows that when the mass percentage of fluorinated sulfonyl imide lithium salt is too low, the thickness of the solid electrolyte interface film formed between it and the surface of the negative electrode active material layer, the compaction density of the negative electrode plate, and the OI value of the negative electrode active material do not conform to the preset relationship. At this time, the battery system has an extremely low lithium ion transmission rate due to the absence of highly dissociated lithium salt, and the fast charging cycle performance at 25°C and the capacity retention rate at -20°C are both poor.

[0092] Comparison with Comparative Example 3 shows that when the formation temperature is too high, the solid electrolyte interface film formed on the surface of the negative electrode active material layer is too thick, and its relationship with the mass percentage of the fluorinated sulfonylimide lithium salt, the compaction density of the negative electrode sheet, and the OI value of the negative electrode active material does not conform to the preset relationship. In this case, the system impedance is high, and the performance of the 25°C normal rate cycle, the 25°C fast charge cycle, and the -20°C capacity retention are all poor.

[0093] Comparison with Comparative Example 4 shows that when the OI value of the negative electrode active material is too high, the compaction density between it and the negative electrode sheet, the thickness of the solid electrolyte interface film formed on the surface of the negative electrode active material layer, and the mass percentage of the fluorinated sulfonyl imide lithium salt do not conform to the preset relationship. At this time, there are fewer end faces available for active lithium ions to be deintercalated and deintercalated, and the battery is prone to lithium deposition during the cycle. The battery's kinetic performance is extremely poor, and the 25°C conventional rate cycle, 25°C fast charge cycle performance, and -20°C capacity retention rate are all poor.

[0094] Comparison with Comparative Example 5 shows that when the OI value of the negative electrode active material is too low, the compaction density of the negative electrode plate, the thickness of the solid electrolyte interface film formed on the surface of the negative electrode active material layer, and the mass percentage of the fluorinated sulfonyl imide lithium salt do not conform to the preset relationship. At this time, the negative electrode plate is prone to wrinkling and falling off during the cycle, so the performance of the 25°C conventional rate cycle and the 25°C fast charge cycle is poor.

[0095] By comparing with Comparative Example 6, it can be seen that when the compaction density of the negative electrode sheet is too high, the thickness of the solid electrolyte interface film formed between it and the surface of the negative electrode active material layer, the mass percentage of the fluorinated sulfonyl imide lithium salt, and the OI value of the negative electrode active material do not conform to the preset relationship. At this time, the electrolyte wettability is poor, the lithium ion desolvation process is hindered, and the 25°C conventional rate cycle, 25°C fast charge cycle performance and -20°C capacity retention rate are all poor.

[0096] Although some exemplary embodiments of the present application have been illustrated and described, the present application is not limited to the disclosed embodiments. On the contrary, those skilled in the art will recognize that some modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present application as described in the appended claims.

Claims

1. A secondary battery, characterized in that: The secondary battery includes a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the electrolyte includes a fluorinated sulfonyl imide lithium salt, and the mass percentage of the fluorinated sulfonyl imide lithium salt is 1% based on the total mass of the electrolyte; the negative electrode sheet includes a negative electrode active material and a solid electrolyte interface film located on the surface of the negative electrode active material; the negative electrode active material layer includes a negative electrode active material, and the compaction density of the negative electrode sheet is Yg / cm 3 , the thickness of the solid electrolyte interface film is M nm, the OI value of the negative electrode active material is N, and the secondary battery satisfies:

2. The secondary battery according to claim 1, wherein The secondary battery also meets the following requirements:

3. The secondary battery according to any one of claims 1 or 2, characterized in that The secondary battery further satisfies at least one of the following conditions: (1) 0.2≤L≤25； (2) 5≤M≤100； (3) 0.5≤Y≤5； (4) 0.5≤N≤15。 4. The secondary battery according to claim 3, wherein The secondary battery further satisfies at least one of the following conditions: (1) 1≤L≤14； (2) 10≤M≤30； (3) 1≤Y≤3； (4) 1≤N≤5。 5. The secondary battery according to claim 1, wherein The fluorine-containing sulfonyl imide lithium salt includes at least one of lithium bis(fluorosulfonyl)imide, lithium (trifluoromethylsulfonyl)(perfluorobutylsulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide.

6. The secondary battery according to any one of claims 1 or 2, characterized in that: The positive electrode plate includes a lithium iron phosphate material, and the lithium iron phosphate material includes a x Fe y R (1 -y)at least one of the materials represented by PO4, wherein R includes at least one of the elements Mn, Co, Ti, Mg, Ca, Cr, Cu, Ni, V, Mo, Zn, Al, B and Nb, 0.05≤x≤1.2, 0<y≤1.

7. The secondary battery according to any one of claims 6, characterized in that A carbon coating layer is provided on the surface of the lithium iron phosphate material. Based on the mass of the lithium iron phosphate material, the mass percentage of the carbon coating layer is 1% to 3%.

8. The secondary battery according to any one of claims 1 or 2, characterized in that: The secondary battery further satisfies at least one of the following conditions: (1) The negative electrode active material includes at least one of graphite, soft carbon, hard carbon, carbon nanotubes and graphene; (2) The porosity of the negative electrode plate is ε1, which satisfies the following: 28%≤ε1≤45%.

9. The secondary battery according to any one of claims 1 or 2, characterized in that: The electrolyte also includes a solvent and other lithium salts, the solvent includes at least one of a chain carbonate compound, a cyclic carbonate compound and a carboxylate compound; the other lithium salts include at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalatoborate), lithium bis(fluoromalonate)borate and lithium difluorooxalatoborate.

10. A device, characterized in that: The device includes the secondary battery according to any one of claims 1 to 9.

Images