Secondary battery and device

Abstract

The invention provides a secondary battery which comprises a positive pole piece, a negative pole piece and electrolyte, the electrolyte comprises lithium oxalate salt, and the positive pole piece comprises a positive active material and a solid electrolyte interface film located on the surface of the positive active material. The positive electrode active material comprises lithium transition metal phosphate and a carbon coating layer covering the surface of the lithium transition metal phosphate, based on the mass of the positive electrode active material, the mass content of the carbon coating layer is P%, P is more than 0 and less than 5, and an X-ray photoelectron spectrometer is adopted for testing under the sputtering etching time of 0 second. The mass content of oxalate in a solid electrolyte interface film on the surface of the positive electrode active material is NCx%, the mass content of carbonate is NCn%, and P * NCx / NCn is more than 2 and less than 5. The secondary battery can inhibit dissolution of transition metal in the active material, and has improved cycle performance, rate capability and high-temperature safety.

Description

Technical Field

[0001] This application relates to the field of lithium batteries, and particularly to a secondary battery and a device. Background Art

[0002] Lithium iron phosphate batteries have been widely used in the fields of energy storage and power batteries due to their advantages of low cost and long cycle life. However, with the increasing requirements for battery service life, it is still of great value to further improve the storage performance, cycle performance, safety performance, and kinetic performance of lithium-ion batteries at a relatively low cost.

[0003] However, due to the characteristics of the lithium iron phosphate cathode material itself, during the charge and discharge process, it undergoes a redox reaction of mutual transformation between Fe

[0006] / Fe 3+ . And Fe 3+ has a certain solubility in the electrolyte, and will dissolve out from the positive electrode and shuttle to the surface of the negative electrode to be reduced to form Fe单质 and Fe 2+ , while Fe 2+ can shuttle to the surface of the positive electrode and be oxidized to form Fe 3+ . The whole process repeats continuously, resulting in the occurrence of battery self-discharge, ultimately causing a decrease in battery capacity and attenuation of cycle life. Summary of the Invention

[0004] [[ID=​​​​​​​​​​​​​A second aspect of the present application provides an electrical device comprising the secondary battery described in the first aspect.

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

[0008] The secondary battery of the present application significantly reduces the dissolution of transition metals from the positive electrode active material, improving the secondary battery's room temperature cycling performance, high temperature cycling performance, and rate performance. The addition of lithium oxalate salt to the electrolyte improves the thermal stability of the electrolyte. Simultaneously, the controlled coordination of the oxalate and carbonate groups in the positive electrode CEI membrane and the carbon coating on the surface of the positive electrode active material reduces the dissolution of transition metals from the active material, improving the battery's cycling performance and high-temperature safety. 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] 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).

[0012] A list of items connected by the terms "at least one of", "at least one", "at least one kind of" or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, then 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, then 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 include a single component or multiple components. Item B may include a single component or multiple components. Item C may include a single component or multiple components.

[0013] The terms "above" and "below" include the corresponding numbers.

[0014] I. Primary and secondary batteries

[0015] One or more embodiments of the present application provide a secondary battery, which includes a positive electrode plate, a negative electrode plate, and an electrolyte. The electrolyte includes lithium oxalate salt. The positive electrode plate includes a positive electrode active material and a solid electrolyte interface film on the surface of the positive electrode active material. The positive electrode active material includes lithium transition metal phosphate and a carbon coating layer covering the surface of the lithium transition metal phosphate. Based on the mass of the positive electrode active material, the mass content of the carbon coating layer is P%, 0 < P < 5. Tested by an X-ray photoelectron spectrometer at a sputtering etching time of 0 seconds, the mass content of lithium oxalate derivative in the solid electrolyte interface film on the surface of the positive electrode active material is N Cx , and the mass content of lithium carbonate is N Cn , where 2 < P × N Cx / N Cn < 5.

[0016] If the value of P is too low, it means that the carbon coating layer is too thin to effectively improve the conductivity of the lithium transition metal phosphate itself, resulting in poor rate performance of the lithium-ion battery; if the value of P is too high, it means that the carbon coating layer is too thick, which will significantly reduce the energy density of the lithium-ion battery, and at the same time, the non-uniformity of the coating will be more obvious and the defects will increase, affecting the cycle performance of the lithium-ion battery. In the present application, the content of the carbon coating layer on the surface of the lithium transition metal phosphate can be measured by conventional testing methods in the art, such as obtained by thermogravimetric TG testing.

[0017] In some embodiments, P is exemplarily 0.2, 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8 or a range composed of any two of the above values. In some embodiments, 1 ≤ P ≤ 3.5. In some embodiments, 1 ≤ P ≤ 1.8.

[0018] P×N Cx / N Cn If the value is too low, the electronic conductivity of the electrode will decrease, the lithium ion rate performance will decrease and the dissolution of transition metals will increase; if the value is too high, the surface defects of the electrode active material will increase, the side reactions between the electrolyte and the electrode will increase, affecting the cycle performance of the battery, and it will increase the impedance of the CEI membrane and reduce the kinetic performance of lithium ions.

[0019] In some embodiments, P×N Cx / N Cn Exemplarily, it is 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, 4.2, 4.4, 4.6, 4.8, or a range consisting of any two of the above values. In some embodiments, 2.2≤P×N Cx / N Cn ≤4.8.

[0020] In some embodiments, 1.5≤N Cx / N Cn ≤3.2.N Cx / N Cn If the value is too low, the dissolution of transition metals will increase, affecting the battery cycle performance; if the value is too high, the impedance of the positive electrode CEI will increase and the battery kinetics will decrease. Cx / N Cn Illustratively, it is 1.5, 1.7, 1.9, 2.1, 2.3, 2.5, 2.7, 2.9, 3.1, 3.2 or a range consisting of any two of the above values.

[0021] In some embodiments, 1≤N Cx ≤5. N Cx If the value is too high, the impedance of the electrode CEI increases, affecting the kinetic performance of the battery; if the value is too low, the inhibitory effect on the dissolution of transition metals is weakened. Cx Illustratively, it is 1, 1.5, 2., 2.5, 3, 3.5, 4, 4.5, 5, or a range consisting of any two of the above values.

[0022] In some embodiments, 0.5≤N Cn ≤5. N Cn If the value is too high, the dissolution of transition metals will increase; if the value is too low, the electrode CEI impedance will increase, affecting the kinetic performance of the battery. Cn Illustratively, it is 0.5, 1, 1.5, 2., 2.5, 3, 3.5, 4, 4.5, 5, or a range consisting of any two of the above values.

[0023] In the present application, the carbon coating layer at least covers a portion of the surface of the lithium transition metal phosphate.

[0024] In some embodiments, the lithium oxalate salt is selected from at least one of lithium oxalatoborate and lithium oxalatophosphate. In some embodiments, the lithium oxalate salt is selected from at least one of the following lithium bis(oxalatoborate) (LiBOB), lithium difluorooxalatoborate (LiODFB), lithium tetrafluorooxalatophosphate (LiTFOP), and lithium difluorobis(oxalatophosphate) (LiBODFP):

[0025]

[0026] In some embodiments, the lithium oxalate salt is lithium bis(oxalatoborate) (LiBOB).

[0027] In some embodiments, based on the mass of the electrolyte, the mass content of the lithium oxalatoborate salt is 0.2%-5%, exemplified by 0.2%, 0.4%, 0.6%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3%, 3.2%, 3.4%, 3.6%, 3.8%, 4%, 4.2%, 4.4%, 4.6%, 4.8%, 5% or a range consisting of any two of the above values.

[0028] In some embodiments, the electrolyte further comprises lithium hexafluorophosphate, and the mass content of the lithium hexafluorophosphate is 13% to 19% based on the total weight of the electrolyte, for example, 13%, 14%, 15%, 16%, 17%, 18%, 19% or a range consisting of any two of the above values. If the content of lithium hexafluorophosphate is too low, the conductivity of the electrolyte will be insufficient, which will affect the performance of the battery; if the content is too high, the viscosity of the electrolyte will increase, affecting the wetting of the electrolyte to the electrode, which will also affect the battery performance. In some embodiments, the mass content of the lithium hexafluorophosphate is 15% to 17% based on the total weight of the electrolyte.

[0029] In some embodiments, the electrolyte includes lithium hexafluorophosphate and lithium bis(fluorosulfonyl imide) salt, and the mass ratio of lithium hexafluorophosphate to lithium bis(fluorosulfonyl imide) salt is 5-20. Controlling the mass ratio of lithium bis(fluorosulfonyl imide) salt to lithium hexafluorophosphate within the above range can further improve the high-temperature cycling performance and high-temperature safety performance of the battery.

[0030] In some embodiments, the electrolyte further includes other lithium salts, such as at least one of lithium tetrafluoroborate (LiBF4), lithium trifluorosulfonyl (LiOTf), lithium (trifluoromethylsulfonyl)(perfluorobutylsulfonyl)imide (LiFNFSI), lithium bis(pentafluoroethylsulfonate)imide (LiBETI), and lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI).

[0031] In some embodiments, the electrolyte further comprises vinylene carbonate, and the mass content of the vinylene carbonate is 0.5%-1.5% based on the mass of the electrolyte, illustratively 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or a range consisting of any two of the above values. If the content of vinylene carbonate is too high, a thicker CEI film will form on the surface of the lithium transition metal phosphate, reducing the kinetic performance of the battery and even leading to the occurrence of lithium plating at the negative electrode, thereby worsening the cycle life and safety performance of the battery.

[0032] In some embodiments, the electrolyte includes vinylene carbonate and at least one additive selected from fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), 1,3-propane sultone (PS), vinyl sulfate (DTD), 1,3-propene sultone (PST), vinyl sulfite (ES), tris(trimethylsilyl) phosphate (TMSP), tris(trimethylsilyl) borate (TMSB) and methylene methanedisulfonate (MMDS), wherein the mass ratio of the additive to the vinylene carbonate is 1-4, exemplarily 1, 1.5, 2, 2.5, 3, 3.5, 4 or a range consisting of any two of the above values.

[0033] In some embodiments, the electrolyte includes vinylene carbonate and at least one additive selected from fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, and 1,3-propene sultone, and the mass ratio of the additive to the vinylene carbonate is 1-4.

[0034] In some embodiments, the electrolyte comprises vinylene carbonate and fluoroethylene carbonate, and the mass ratio of the fluoroethylene carbonate to the vinylene carbonate is 1-4.

[0035] In some embodiments, the electrolyte further comprises a solvent, the solvent being selected from at least one of linear carbonate, cyclic carbonate and carboxylate. The linear carbonate includes but is not limited to one or more of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC). The cyclic carbonate includes but is not limited to one or more of ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC). The carboxylate 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, decanolactone, valerolactone or caprolactone.

[0036] In some embodiments, the electrolyte includes a solvent comprising at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, wherein the sum of the mass content of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate is 50% to 70% based on the mass of the solvent. Using 50% to 70% by weight of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate as the solvent can further improve the wetting of the high-density lithium transition metal phosphate cathode, thereby improving the cycle performance and rate performance of the battery.

[0037] In some embodiments, the electrolyte includes a solvent comprising at least one of ethyl acetate and methyl acetate, wherein the combined weight content of ethyl acetate and methyl acetate is 10% to 20% based on the weight of the solvent. Using 10% to 20% by weight of ethyl acetate and methyl acetate as the solvent can further improve the wetting of the high-density lithium transition metal phosphate cathode, thereby improving the cycling performance and rate performance of the battery.

[0038] In some embodiments, the electrolyte includes cyclic carbonate, chain carbonate, lithium hexafluorophosphate and lithium oxalate. Based on the mass of the electrolyte, the mass content of the cyclic carbonate is 15%-30%, the mass content of the chain carbonate is 50%-70%, the mass content of the lithium hexafluorophosphate is 13%-19%, and the mass content of the lithium oxalate is 0.2%-5%. The chain carbonate is selected from at least one of dimethyl carbonate, ethyl methyl carbonate and diethyl carbonate.

[0039] In some embodiments, the electrolyte includes a cyclic carbonate, a chain carbonate, lithium hexafluorophosphate, a lithium salt of bis(fluorosulfonyl)imide, a lithium oxalate, vinylene carbonate, and at least one additive selected from fluoroethylene carbonate, 1,3-propane sultone, vinyl sulfate, and 1,3-propene sultone. Based on the mass of the electrolyte, the mass content of the cyclic carbonate is 15%-30%, the mass content of the chain carbonate is 50%-70%, the mass content of the lithium hexafluorophosphate is 13%-19%, the mass content of the lithium salt of bis(fluorosulfonyl)imide is 1%-3%, the mass content of the lithium oxalate is 0.2%-5%, the mass content of the vinylene carbonate is 0.2%-1%, and the mass content of the additive is 0%-2.5%; the chain carbonate is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0040] In some embodiments, the compacted density of the positive electrode sheet is 2.45 g / cm 3 to 2.7g / cm 3 The inventors of this application found that the compaction density of the positive electrode sheet is 2.45g / cm 3When it is above, it will reduce the capacity performance of the battery, increase charge-discharge polarization, increase the side reactions between the electrolyte and the active material, and also increase the dissolution of transition metals, thereby shortening the battery life. By controlling the content of oxalate and carbonate in the cathode CEI film in cooperation with the carbon coating layer on the surface of the cathode active material, the performance deterioration caused by the above-mentioned high-compaction cathode electrode can be improved.

[0041] In this application, the solid electrolyte interface film is a non-artificial film. In some embodiments, the solid electrolyte interface film is formed by the reaction of the cathode active material and the electrolyte. In some embodiments, the solid electrolyte interface film is obtained through formation.

[0042] In this application, the oxalate in the solid electrolyte interface film comes from lithium oxalate derivatives, and the carbonate comes from lithium carbonate.

[0043] In some embodiments, the compaction density of the cathode electrode is exemplarily 2.45 g / cm 3 、2.5 g / cm 3 、2.55 g / cm 3 、2.6 g / cm 3 、2.65 g / cm 3 、2.7 g / cm 3 or the range composed of any two of the above values. In some embodiments, the compaction density of the cathode electrode is 2.5 g / cm 3 to 2.7 g / cm 3 . In some embodiments, the compaction density of the cathode electrode is 2.55 g / cm 3 to 2.7 g / cm 3 .

[0044] In some embodiments, the lithium transition metal phosphate is selected from at least one of the compounds represented by the formula Li x Fe y M (1-y) PO4, where -0.5 ≤ x ≤ 0.5, 0 < y ≤ 1, and M is selected from at least one of Mn, Ni, Co, Al, Mg, Ti, Ca, Zn, Ga, Cu, V, Nb, Zr, In, and Y. In some embodiments, x is -0.5, -0.4, -0.3, -0.2, -0.1, 0, 0.1, 0.2, 0.3, 0.4, 0.5 or the range composed of any two of these values. In some embodiments, y is 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95 or the range composed of any two of these values.

[0045] In some embodiments, the lithium transition metal phosphate includes lithium iron phosphate and lithium manganese iron phosphate (such as LiMn 0.6 Fe 0.4 PO4 or LiMn 0.8 Fe 0.2 PO4).

[0046] In some embodiments, the positive electrode plate further includes a binder and optionally a conductive material. The binder includes but is not limited to polyvinylidene fluoride and polytetrafluoroethylene, and the conductive agent includes but is not limited to carbon nanotubes, carbon black, and acetylene black. In some embodiments, the positive electrode plate further includes a positive current collector. The positive current collector includes but is not limited to metal foil (such as aluminum foil) and a composite current collector (such as a current collector formed by a metal material composited on a polymer substrate).

[0047] In some embodiments, the negative electrode plate includes at least one of artificial graphite and natural graphite.

[0048] In some embodiments, the negative electrode plate further includes a binder and, optionally, a conductive agent. The binder includes, but is not limited to, styrene-butadiene rubber (SBR) or acrylated SBR, and the conductive agent includes, but is not limited to, carbon nanotubes, carbon black, or acetylene black. In some embodiments, the negative electrode plate further includes a negative electrode current collector. The negative electrode current collector includes, but is not limited to, copper foil, nickel foil, nickel foam, or copper foam.

[0049] In some embodiments, a separator is provided between the positive and negative electrode plates to prevent short circuits. The material and shape of the separator that can be used in the embodiments of the present application are not particularly limited and can be any material disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic material formed from a material that is stable to the electrolyte of the present application. For example, the separator may include a substrate layer and a surface treatment layer.

[0050] In some embodiments, the method for preparing the secondary battery includes providing an electrode assembly, injecting liquid, packaging, and forming. In some embodiments, the temperature of the formation is 30°C to 60°C, for example, 30°C, 32°C, 34°C, 36°C, 38°C, 40°C, 42°C, 44°C, 46°C, 48°C, 50°C, 52°C, 54°C, 56°C, 58°C, or 60°C. In some embodiments, the temperature of the formation is 30°C to 50°C.

[0051] In some embodiments, the secondary battery is formed. In some embodiments, the solid electrolyte interface membrane is obtained by the formation of the secondary battery. During the formation process of the secondary battery, the lithium oxalate salt is oxidized to form a film on the surface of the positive electrode active material to form a CEI membrane, which inhibits the continuous reaction between the electrolyte and the positive electrode active material. After the secondary battery has been cycled multiple times, the absolute content of oxalate and carbonate in the CEI membrane will be slightly different from the absolute content of oxalate and carbonate in the CEI membrane after formation, but the relative content, that is, the ratio of the mass percentage of oxalate and carbonate in the CEI membrane, is still between 1.5 and 3.2.

[0052] In some embodiments, the secondary battery has a filling coefficient of 3 g / Ah to 3.8 g / Ah, exemplified by 3 g / Ah, 3.1 g / Ah, 3.2 g / Ah, 3.3 g / Ah, 3.4 g / Ah, 3.5 g / Ah, 3.6 g / Ah, 3.7 g / Ah, 3.8 g / Ah, or a range consisting of any two of the foregoing values. If the filling coefficient of the secondary battery is too high, the energy density of the battery will be reduced, and if the filling coefficient is too low, the cycle performance of the battery will be degraded.

[0053] 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 pouch-type soft package. In some embodiments, the shape of the secondary battery is not particularly limited, and may be cylindrical, square, or any other shape.

[0054] 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 aforementioned 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.

[0055] In some embodiments, the present application further provides a battery pack comprising the above-mentioned 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.

[0056] 2. Electrical devices

[0057] The present application also provides an electrical device, which includes at least one of the above-mentioned secondary battery, battery module or battery pack.

[0058] In some embodiments, the electrical 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, battery packs or battery modules may be used.

[0059] In other embodiments, the power-consuming device may be a mobile phone, a tablet computer, a laptop computer, etc. Such a device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0060] Examples and Comparative Examples

[0061] 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.

[0062] The materials and reagents used in the following examples and comparative examples were all commercially available.

[0063] Example 1

[0064] Preparation of positive electrode sheet: The positive electrode active material LiFePO4, conductive agent carbon nanotube (CNT), conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 95:2:1:2, and then dispersed in N-methylpyrrolidone (NMP) solvent and fully homogenized. The resulting slurry was evenly coated on one side of a carbon-coated aluminum foil with a thickness of 12 μm. After drying, the above steps were repeated on the other side of the carbon-coated aluminum foil. After drying, rolling twice (rolling load 1000 N / m), and stripping, a compaction density of 2.55 g / cm was obtained. 3 The positive electrode.

[0065] The steps for preparing the negative electrode sheet are as follows: the negative electrode active material artificial graphite, the conductive agent acetylene black, the binder styrene-butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMCNa) are mixed in a weight ratio of 96:2:2:1, and then dispersed in a deionized water solvent and fully homogenized. The resulting slurry is evenly coated on one side of an 8μm thick copper foil. After drying, the above steps are repeated on the other side of the copper foil. After drying, rolling, and slitting, the negative electrode sheet is obtained.

[0066] Diaphragm: PP / PE / PP three-layer composite diaphragm.

[0067] Preparation of the electrolyte: In an argon-protected glove box (H2O<0.1ppm, O2<0.1ppm), solvents ethylene carbonate (EC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) were mixed in a weight ratio of 25:20:55, and lithium hexafluorophosphate (LiPF6) was added to prepare a solution with a lithium hexafluorophosphate mass content of 17wt%. Then, 0.5% of lithium bis(oxalatoborate) (LiBOB) based on the total mass of the electrolyte was added according to the amount in Table 1, and the mixture was stirred evenly to obtain the lithium ion battery electrolyte of Example 1.

[0068] Preparation of a lithium-ion 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. After thorough drying, the prepared lithium-ion battery electrolyte is injected at an injection rate of 3.5Ah / g. After the battery is stored at 45°C, formed in a high-temperature fixture, and sealed again, conventional capacity separation is performed.

[0069] Examples 2-14 and Comparative Examples 1-7

[0070] Examples 2-14 and Comparative Examples 1-7 are achieved on the basis of Example 1 by adjusting the ratio of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide salt in the electrolyte, the type and content of lithium oxalate salt, the type and amount of other additives, the carbon coating amount on the surface of lithium iron phosphate, the injection coefficient and the formation temperature. Specific adjustment measures and detailed data are shown in Table 1.

[0071] Table 1

[0072]

[0073]

[0074] Test Method

[0075] 1. Determination of carbon coating amount on lithium iron phosphate surface

[0076] Take m0 = 0.1g of lithium iron phosphate powder and add it to a crucible. Use thermogravimetric (TG) testing. After heating to 800°C in air, record the mass of the sample at this time as m1, and the carbon coating amount value P% = (m0-m1) / m0%.

[0077] 2. Determination of pole piece compaction density

[0078] The compaction density of the pole piece = the surface density of the pole piece (g / cm 2 ) / thickness of the active material layer (cm). Cut a piece of positive electrode and weigh its mass as M1 and thickness as H1. Then scrape off the active material layer on the electrode and weigh its mass as M2 and thickness as H2. Measure the area of ​​the electrode and record it as V. The surface density of the electrode is then calculated as (M1-M2) / V / (H1-H2). The mass can be measured using a standard balance, and the thickness can be measured using a micrometer.

[0079] 3. XPS test

[0080] The lithium-ion battery was discharged at a current of 0.1C to 2.5V. The battery was then disassembled in an argon-filled glove box to obtain the electrode sheet. The obtained electrode sheet was cut into 8mm×8mm test samples and soaked and cleaned in a low-boiling-point dimethyl carbonate (DMC) solvent for half an hour. After complete drying, it was attached to the XPS sample stage with the surface of the positive electrode active material layer facing away from the current collector facing upward. The measurement was performed without exposure to the atmosphere. The specific test conditions and steps are as follows:

[0081] Single crystal spectroscopy AlKα radiation was used, and as for the X-ray spot, an elliptical form of 1000×1750 μm with an output of 10 KV and 22 mA was used, data when the sputter etching time was 0 seconds was selected, 284.8 eV was used for neutral carbon C1s, and as for data processing such as peak differentiation, 3-point smoothing, peak area measurement, background subtraction, and peak synthesis were used to calculate the percentage of carbonate and oxalate in the positive electrode CEI N. Cn % and N Cx %.

[0082] 4. Capacity retention test of 1000 cycles at 1C / 1C at 25°C

[0083] 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 1000 times using the above method, and the discharge capacity was recorded each time. The capacity retention rate of the lithium-ion secondary battery after 1000 cycles at 1C / 1C at 25°C = discharge capacity at 1000 times / initial discharge capacity × 100%.

[0084] 5. Capacity retention test after 200 cycles at 2C / 0.5C at 25°C

[0085] 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 200 times using the above method, and the discharge capacity was recorded each time. The capacity retention rate of the lithium-ion secondary battery after 200 cycles of 2C / 0.5C at 25°C = discharge capacity at the 200th cycle / initial discharge capacity × 100%.

[0086] 6. Capacity retention test of 500 cycles at 1C / 1C at 45°C

[0087] At 45°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 500 times using the above method, and the discharge capacity was recorded each time. The capacity retention rate of the lithium-ion secondary battery after 500 cycles at 1C / 1C at 45°C = discharge capacity at the 500th cycle / initial discharge capacity × 100%.

[0088] 7. Storage of diaphragm Fe at 60℃ for 21 days 3+ Content determination

[0089] After disassembling the battery cell after storage at 60°C, accurately weigh 0.1g of the separator and place it in a 50mL quartz beaker. Add 4.0mL of aqua regia, cover with a watch glass, and heat on a hotplate until slightly boiling and dissolve for 15 minutes. Remove and cool. Transfer the solution to a 50mL plastic volumetric flask and shake thoroughly. Shake the solution thoroughly before measurement. After allowing it to stand for a while, use ICP to measure the Fe content in the sample in ppm.

[0090] Please see Table 2 below for test data.

[0091] Table 2

[0092]

[0093] a: NCx in each embodiment is between 1 and 5, and NCn is between 0.5 and 5.

[0094] 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 comprises a positive electrode sheet, a negative electrode sheet and an electrolyte. The electrolyte includes lithium oxalate salt, The positive electrode plate includes a positive electrode active material and a solid electrolyte interface membrane located on the surface of the positive electrode active material. The positive electrode active material includes a lithium transition metal phosphate and a carbon coating layer covering the surface of the lithium transition metal phosphate. Based on the mass of the positive electrode active material, the mass content of the carbon coating layer is P%, 0 <P<5, The X-ray photoelectron spectrometer was used to test the sputtering etching time at 0 seconds. The mass content of oxalate in the solid electrolyte interface film on the surface of the positive electrode active material was N Cx %, the mass content of carbonate ion is N Cn %, Among them, 2 <P×N Cx / N Cn <5.

2. The secondary battery according to claim 1, wherein The secondary battery satisfies at least one of the following conditions: (i) 1≤P≤3.5, (ii) 1.5≤N Cx / N Cn ≤3.2, (iii)2.2≤P×N Cx / N Cn ≤4.8。 3. The secondary battery according to claim 1 or 2, characterized in that The secondary battery satisfies at least one of the following conditions: (iv)1≤N Cx ≤5, (v)0.5≤N Cn ≤5。 4. The secondary battery according to claim 1, wherein The compaction density of the positive electrode sheet is 2.45 g / cm 3 to 2.7g / cm 3 and / or, The lithium transition metal phosphate includes at least one of lithium iron phosphate and lithium manganese iron phosphate.

5. The secondary battery according to claim 1 or 2, characterized in that The lithium oxalate salt is selected from at least one of lithium oxalate borate and lithium oxalate phosphate; and / or, Based on the mass of the electrolyte, the mass content of the lithium oxalate salt is 0.2%-5%.

6. The secondary battery according to claim 5, characterized in that The lithium oxalate salt is selected from at least one of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium tetrafluorooxalatophosphate and lithium difluorobis(oxalatophosphate).

7. The secondary battery according to claim 1 or 2, characterized in that: The electrolyte further comprises lithium hexafluorophosphate, and the mass content of the lithium hexafluorophosphate is 13%-19% based on the mass of the electrolyte; and / or, The electrolyte further comprises vinylene carbonate, and the mass content of the vinylene carbonate is 0.5%-1.5% based on the mass of the electrolyte.

8. The secondary battery according to claim 7, wherein: The electrolyte further comprises bis(fluorosulfonyl)imide lithium salt, and the mass ratio of the lithium hexafluorophosphate to the bis(fluorosulfonyl)imide lithium salt is 5-20; and / or, The electrolyte further includes an additive selected from fluoroethylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone, vinyl sulfate, 1,3-propene sultone, vinyl sulfite, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)borate and methylene methanedisulfonate, and the mass ratio of the additive to the vinylene carbonate is 1-4.

9. 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: (vi) the negative electrode sheet comprises at least one of artificial graphite and natural graphite; (vii) the electrolyte further comprises a solvent, wherein the solvent comprises 50 wt% to 70 wt% of a chain carbonate, wherein the chain carbonate is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; (viii) the electrolyte further comprises a solvent, wherein the solvent comprises 10 wt% to 20 wt% of a carboxylate ester, wherein the carboxylate ester is selected from at least one of ethyl acetate and methyl acetate; (ix) The secondary battery has a filling rate of 3 g / Ah to 3.8 g / Ah.

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

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