Secondary battery and electric device

Abstract

The invention provides a secondary battery, which comprises a positive electrode plate, a negative electrode plate and an electrolyte, the positive electrode plate comprises a positive electrode film layer, the positive electrode film layer contains a positive electrode active material, the electrolyte comprises a solvent, a lithium salt and an additive, and the additive comprises a compound containing an isocyanate group. The mass fraction of the compound containing the isocyanate group in the electrolyte is a, n / 10000 a is more than or equal to 11 and less than or equal to 96600, and n is equal to b * c * d; n is the real surface area of the unit apparent surface area of the positive electrode active material; b is the BET specific surface area of the positive electrode active material, and the unit is cm < 2 > / g; c is the mass fraction of the positive electrode active material in the positive electrode film layer; d is the coating surface density of the positive electrode film layer, and the unit is g / cm < 2 >; the invention also relates to a corresponding electric device. The secondary battery can effectively remove water in the electrolyte, and not only has good cycle performance and storage performance, but also has relatively good charging performance.

Description

Secondary batteries and electrical devices Technical Field

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

[0002] In recent years, as the application scope of secondary batteries has become increasingly wider, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, aerospace and other fields.

[0003] With the continuous development of secondary battery technology, higher requirements are placed on the service life and charging performance of secondary batteries. Therefore, seeking a secondary battery with a longer service life and better charging performance is one of the focuses of those skilled in the art.

[0004] Summary of the Invention

[0005] The present application is made in view of the above-mentioned problems, and one of its purposes is to provide a secondary battery that not only has good cycle performance and storage performance, but also has low DC resistance and good charging performance.

[0006] To achieve the above-mentioned object, the first aspect of the present application provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet comprises a positive electrode film layer, the positive electrode film layer comprises a positive electrode active material, the electrolyte comprises a solvent, a lithium salt, and an additive, the additive comprises a compound containing an isocyanate group, the mass fraction of the compound containing an isocyanate group in the electrolyte is a, and 11≤n / 10000a≤96600, n=b×c×d;

[0007] Where n is the real surface area per unit apparent surface area of ​​the positive electrode active material; b is the BET specific surface area of ​​the positive electrode active material, in cm 2 / g; c is the mass fraction of the positive electrode active material in the positive electrode film layer; d is the coating surface density of the positive electrode film layer, unit is g / cm 2 .

[0008] This application adopts a compound containing an isocyanate group as an electrolyte additive, and according to the BET specific surface area b of the positive electrode active material in the secondary battery, the mass fraction c of the positive electrode active material in the positive electrode film layer, and the coating surface density d of the positive electrode film layer, the mass fraction a of the compound containing an isocyanate group in the electrolyte is determined according to the formula 11≤n / 10000a≤96600, n=b×c×d. In this way, the amount of the compound containing an isocyanate group in the electrolyte of the secondary battery can be determined according to the residual moisture of the positive electrode active material of the secondary battery (the residual moisture of the positive electrode active material is related to the above-mentioned bcd), so that the content of the compound containing an isocyanate group in the electrolyte of the secondary battery is moderate; it can effectively remove moisture from the electrolyte of the secondary battery and reduce the impact of moisture on the service life of the secondary battery; and it will not significantly increase the DC impedance of the secondary battery due to the excessive amount of the compound containing an isocyanate group, and will not have a significant impact on the charging performance of the secondary battery.

[0009] In any embodiment, 243≤n / 10000a≤96525. In this way, the initial DC resistance of the secondary battery can be further reduced.

[0010] In any embodiment, 243≤n / 10000a≤927. In this way, when the secondary battery has a smaller initial DC resistance, the high-temperature cycle performance and high-temperature storage performance of the secondary battery can be further improved, so that the secondary battery has more excellent comprehensive performance.

[0011] In any embodiment, the BET specific surface area b of the positive electrode active material is in the range of 50000 cm 2 / g~300000cm 2 / g.

[0012] In any embodiment, the mass fraction c of the positive electrode active material in the positive electrode film layer is in a range of 90% to 99%.

[0013] In any embodiment, the coating area density d of the positive electrode film layer of the secondary battery is in the range of 0.013 g / cm 2 ~0.0325g / cm 2 .

[0014] In any embodiment, the mass fraction a of the compound containing an isocyanate group in the electrolyte is 0.001% to 0.5%.

[0015] In any embodiment, the compound containing an isocyanate group comprises one or more compounds of the following structural formulas: Among them, isocyanate compounds containing benzene rings have greater steric hindrance and volume, forming thicker films and generating higher resistance SEI films. However, the lone pair of electrons in the isocyanate will conjugate with the benzene ring, reducing its ability to remove water and acid. Isocyanate compounds containing the element F can increase the wettability of the SEI film. Monoisocyanate compounds are not easy to condense into a film and are not easily involved in film formation, which can reduce the resistance caused by film formation. The above compounds containing isocyanate groups can be selected according to actual needs.

[0016] In any embodiment, the additive further includes a film-forming aid, so that a solid electrolyte interface film can be formed on the surface of the electrode during the charge and discharge process of the battery.

[0017] In any embodiment, the film-forming aid comprises an unsaturated carbonate.

[0018] In any embodiment, the solvent includes one or more of a cyclic ester and a chain ester.

[0019] In any embodiment, the cyclic ester includes one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, and γ-butyrolactone.

[0020] In any embodiment, the chain ester includes one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl formate, methyl acetate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, methyl butyrate and ethyl butyrate.

[0021] In any embodiment, the lithium salt includes one or more of LiPF6, LiAsF6, LiBF4, LiSCN, LiTaF6, LiSnF6 and LiCF3SO3.

[0022] In any embodiment, the positive electrode active material includes one or more of lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium manganese oxide, lithium manganese phosphate, and lithium manganese iron phosphate.

[0023] A second aspect of the present application provides an electrical device comprising the secondary battery of the first aspect of the present application.

[0024] The secondary battery of the present application adopts a compound containing an isocyanate group in the electrolyte, and determines the mass fraction of the compound containing an isocyanate group in the electrolyte of the secondary battery according to a specific formula based on the BET specific surface area of ​​the positive electrode active material in the secondary battery, the mass fraction of the positive electrode active material in the positive electrode film layer, and the coating surface density of the positive electrode film layer in the positive electrode sheet; so that the content of the compound containing an isocyanate group in the electrolyte of the secondary battery is moderate, which can effectively remove moisture from the electrolyte of the secondary battery, reduce the impact of moisture on the cycle performance and storage performance of the secondary battery, and extend the service life of the secondary battery; and will not make the initial DC resistance of the secondary battery too large due to excessive isocyanate, and will not have a significant impact on the charging performance of the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to better describe and illustrate the embodiments and / or examples of the present application, reference may be made to one or more of the accompanying drawings. The additional details or examples used to describe the accompanying drawings should not be considered as limiting the scope of the disclosed application, the presently described embodiments and / or examples, and any of the best modes currently understood for these applications.

[0026] FIG1 is a schematic diagram of a secondary battery according to an embodiment of the present application;

[0027] FIG2 is an exploded view of the secondary battery according to one embodiment of the present application shown in FIG1 ;

[0028] FIG3 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0029] Explanation of reference numerals: 5, secondary battery; 51, housing; 52, electrode assembly; 53, cover plate; 6, electrical device. DETAILED DESCRIPTION

[0030] Below, the embodiments of the secondary battery and the electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there may be cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0031] The "ranges" disclosed herein are defined in terms of lower and upper limits, where a given range is defined by selecting a lower limit and an upper limit, and the selected lower and upper limits define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values ​​and can be combined arbitrarily, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also contemplated. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, the following ranges are all contemplated: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise indicated, the numerical range "a to b" is a shorthand representation of any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0032] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0033] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0034] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0035] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0036] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0037] The weights described in this specification may be weight units known in the chemical industry, such as μg, mg, g, and kg.

[0038] At present, due to the great development of secondary batteries, higher requirements have been put forward for the service life and electrical performance of secondary batteries. Due to the large specific surface area and high surface energy of the positive electrode active material (for example, lithium iron phosphate (LFP)), it is easy to absorb water and difficult to completely dry. The residual water will slowly release into the electrolyte, react with the lithium salt to generate HF, catalyze the reaction of the lithium salt with the solvent, dissolve the SEI film, metal dissolution, accelerate the aging of the battery cell and components, etc., leading to increased battery gas production, deterioration of the interface, increased reversible lithium loss, and rapid life decay. Compounds containing isocyanate groups have a large number of lone pairs of electrons and have a strong ability to remove water and bind acids. Adding them to the electrolyte can reduce the impact of moisture on the performance of the battery cell and can also participate in the formation of anode and cathode films, thereby improving the battery life. However, compounds containing isocyanate groups have a large impedance. If they are used in excessive amounts, it is easy to increase the DC impedance of the secondary battery and deteriorate the battery's charging capacity.

[0039] In this regard, the inventors have discovered a secondary battery, in which a compound containing an isocyanate group is used in its electrolyte. The mass fraction of the compound containing an isocyanate group in the electrolyte is determined according to a specific formula based on the BET specific surface area of ​​the positive electrode active material in the secondary battery, the mass fraction of the positive electrode active material in the positive electrode film layer, and the coating surface density of the positive electrode film layer in the positive electrode sheet. This can ensure that the content of the compound containing an isocyanate group in the electrolyte of the secondary battery is moderate, which can effectively remove moisture from the electrolyte and reduce the impact of moisture on the performance of the secondary battery; and will not cause a significant impact on the electrical performance of the secondary battery due to excessive isocyanate.

[0040] In some embodiments, the first aspect of the present application provides a secondary battery, comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, wherein the positive electrode sheet comprises a positive electrode film layer, and the positive electrode film layer contains a positive electrode active material; the electrolyte comprises a solvent, a lithium salt, and an additive, the additive comprises a compound containing an isocyanate group, the mass fraction of the compound containing an isocyanate group in the electrolyte is a, and 11≤n / 10000a≤96600, n=b×c×d;

[0041] Where n is the real surface area per unit apparent surface area of ​​the positive electrode active material in the positive electrode film layer; b is the BET specific surface area of ​​the positive electrode active material, in cm 2 / g; c is the mass fraction of the positive electrode active material in the positive electrode film layer; d is the coating surface density of the positive electrode film layer in the positive electrode sheet, in g / cm 2 .

[0042] The residual water content in a secondary battery's positive electrode active material is related not only to the specific surface area of ​​the positive electrode active material, but also to the coating density of the positive electrode film layer in the positive electrode sheet and the mass fraction of the positive electrode active material in the positive electrode film layer (i.e., the loading value). The product of these three factors (n) is the true surface area per unit apparent surface area of ​​the positive electrode active material. Furthermore, a larger n value indicates a stronger water absorption capacity and a greater amount of residual water in the positive electrode active material.

[0043] In the secondary battery of the present application, a compound containing an isocyanate group is used as an additive in the electrolyte, and the mass fraction a of the compound containing an isocyanate group in the electrolyte is determined according to the BET specific surface area b of the positive electrode active material in the secondary battery, the mass fraction c of the positive electrode active material in the positive electrode film layer, and the coating surface density d of the positive electrode film layer in the positive electrode sheet, according to the formula 11≤n / 10000a≤96600, n=b×c×d. The above technical solution can determine the content of the compound containing an isocyanate group in the electrolyte based on the residual moisture of the positive electrode active material of the secondary battery, so that the content of the compound containing an isocyanate group in the electrolyte of the secondary battery is moderate, which can effectively remove moisture from the electrolyte and reduce the impact of moisture on the service life of the secondary battery; at the same time, the DC impedance of the secondary battery will not be significantly increased due to excessive isocyanate compounds, and the charging performance of the secondary battery will not be significantly affected.

[0044] It should be noted that the apparent surface area is the geometric surface area, which can be measured with a ruler; the real surface area is the microscopic surface area, which can be tested by a BET surface area tester; the real surface area per unit apparent surface area of ​​the positive electrode active material is 1 cm 2 The actual surface area of ​​the positive electrode active material in the positive electrode film layer.

[0045] In some embodiments, 243 ≤ n / 10000a ≤ 96525. Research has found that controlling the value of n / a within the range of 243 to 96525 can further reduce the initial direct current resistance (DCR) of the secondary battery, achieving an initial DCR below 639 mohm.

[0046] In some embodiments, 243 ≤ n / 10000a ≤ 927. Further research has found that controlling the value of n / a within the range of 243 to 927 can further improve the high-temperature cycling and storage performance of the secondary battery. The capacity retention rate of the secondary battery after 1000 cycles at 60°C can reach over 89.9%, and the capacity retention rate of the secondary battery after 200 days of storage at 60°C can reach over 94%.

[0047] In some embodiments, the BET specific surface area b of the positive electrode active material is in the range of 50,000 cm 2 / g~300000cm 2 / g. It is understood that the BET specific surface area of ​​the positive electrode active material can be but is not limited to 50000 cm 2 / g、80000cm 2 / g、100000cm 2 / g、120000cm 2 / g、140000cm 2 / g、160000cm 2 / g、180000cm 2 / g、200000cm 2 / g、220000cm 2 / g、240000cm 2 / g, 260000cm 2 / g、280000cm 2 / g and 300000cm 2 / g.

[0048] In some embodiments, the mass fraction c of the positive electrode active material in the positive electrode film layer ranges from 90% to 99%. It is understood that the mass fraction of the positive electrode active material in the positive electrode film layer can be, but is not limited to, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, and 99%.

[0049] In some embodiments, the coating area density d of the positive electrode film layer in the positive electrode sheet of the secondary battery is in the range of 0.013 g / cm 2 ~0.0325g / cm 2 It is understood that the coating area density of the positive electrode film layer in the positive electrode sheet of the secondary battery can be but is not limited to 0.013 g / cm 2 , 0.015g / cm 2 、0.0175g / cm 2 , 0.02g / cm 2 、0.0225g / cm 2 , 0.025g / cm 2, 0.0275g / cm 2 , 0.03g / cm 2 and 0.0325g / cm 2 .

[0050] In some embodiments, the mass fraction a of the compound containing an isocyanate group in the electrolyte is 0.001% to 0.5%. It is understood that the mass fraction of the compound containing an isocyanate group in the electrolyte may be, but is not limited to, 0.001%, 0.005%, 0.01%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, and 0.5%.

[0051] In some embodiments, the compound containing an isocyanate group includes one or more compounds of the following structural formulas:

[0052] The use of the above-mentioned compounds containing isocyanate groups in the electrolyte can effectively remove water from the electrolyte. Among them, isocyanate compounds containing benzene rings have greater steric hindrance and volume, forming thicker films and generating a higher impedance SEI film. However, the lone pair of electrons in the isocyanate will conjugate with the benzene ring, reducing its ability to remove water and acid. Isocyanate compounds containing the element F can increase the wettability of the SEI film. Monoisocyanate compounds are not easy to condense into a film and are not easy to participate in film formation, which can reduce the impedance caused by film formation. The above-mentioned compounds containing isocyanate groups can be selected according to actual needs.

[0053] In some embodiments, the electrolyte additives further include a film-forming aid, which can form a solid electrolyte interface film (SEI film) on the surface of the electrode during the battery charge and discharge process.

[0054] In some embodiments, the film-forming aid includes, but is not limited to, unsaturated carbonates.

[0055] In some embodiments, the solvent includes one or more of a cyclic ester and a chain ester. Cyclic esters include, but are not limited to, one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate, and γ-butyrolactone; chain esters include, but are not limited to, one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl formate, methyl acetate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, methyl butyrate, and ethyl butyrate.

[0056] In some embodiments, the lithium salt includes one or more of LiPF6, LiAsF6, LiBF4, LiSCN, LiTaF6, LiSnF6, and LiCF3SO3.

[0057] In some embodiments, the positive electrode active material includes one or more of lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium manganese oxide, lithium manganese phosphate, and lithium manganese iron phosphate.

[0058] A second aspect of the present application further provides an electrical device, which includes the secondary battery according to the first aspect of the present application.

[0059] The secondary battery and the electric device of the present application will be described below with reference to the accompanying drawings as appropriate.

[0060] Unless otherwise specified, the components, material types, or contents of the batteries mentioned are applicable to both lithium-ion secondary batteries and sodium-ion secondary batteries.

[0061] In one embodiment of the present application, a secondary battery is provided.

[0062] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0063]

Positive electrode

[0064] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector.

[0065] As an example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0066] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material on a polymer material substrate. Among them, the metal material includes but is not limited to aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc. Polymer material substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.)

[0067] In some embodiments, the positive electrode active material may include a positive electrode active material for a battery known in the art.

[0068] As an example, the positive electrode active material of a lithium-ion secondary battery may include at least one of the following materials: a lithium-containing phosphate with an olivine structure, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide (such as LiCoO2), lithium nickel oxide (such as LiNiO2), lithium manganese oxide (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05 O2) and its modified compounds. Examples of olivine-structured lithium-containing phosphates may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO4), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

[0069] As an example, the positive electrode active material of a sodium ion secondary battery may include at least one of the following materials: a sodium transition metal oxide, a polyanionic compound, and a Prussian blue compound. However, the present application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for sodium ion batteries may also be used.

[0070] As an optional technical solution of the present application, in the sodium transition metal oxide, the transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Sodium transition metal oxide is, for example, Na x MO2, wherein M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, and 0<x≤1.

[0071] As an optional technical solution of the present application, the polyanionic compound can be a compound having sodium ions, transition metal ions and tetrahedral (YO4) n- A class of compounds with anionic units. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce; Y can be at least one of P, S and Si; n represents (YO4) n- valence.

[0072] Polyanionic compounds can also be sodium ions, transition metal ions, tetrahedral (YO4) n- A class of compounds containing anion units and halogen anions. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si, and n represents (YO4) n- valence state; the halogen may be at least one of F, Cl and Br.

[0073] Polyanionic compounds can also be sodium ions, tetrahedral (YO4) n- Anion unit, polyhedron unit (ZO y ) m+ and an optional halogen anion. Y can be at least one of P, S and Si, and n represents (YO4) n- valence state; Z represents a transition metal, which may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce, and m represents (ZO y ) m+ valence state; the halogen may be at least one of F, Cl and Br.

[0074] Polyanionic compounds include NaFePO4, Na3V2(PO4)3 (sodium vanadium phosphate, abbreviated as NVP), Na4Fe3(PO4)2(P2O7), NaM'PO4F (M' is one or more of V, Fe, Mn and Ni) and Na3(VO y )2(PO4)2F 3-2y At least one of (0≤y≤1).

[0075] Prussian blue compounds can be sodium ions, transition metal ions and cyanide ions (CN - ) compounds. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Prussian blue compounds are, for example, Na a Me b Me' c (CN)6, wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co and Zn, 0<a≤2, 0<b<1, 0<c<1.

[0076] The weight ratio of the positive electrode active material in the positive electrode film layer is 80 to 100 weight percent based on the total weight of the positive electrode film layer.

[0077] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin. The weight ratio of the binder in the positive electrode film layer is 0 to 20 weight%, based on the total weight of the positive electrode film layer.

[0078] In some embodiments, the positive electrode film layer may further optionally include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the positive electrode film layer is 0-20% by weight, based on the total weight of the positive electrode film layer.

[0079] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry, wherein the positive electrode slurry has a solid content of 40% by mass (wt%) to 80% by weight, and the viscosity at room temperature is adjusted to 5000 mPa·s to 25000 mPa·s, the positive electrode slurry is coated on the surface of the positive electrode collector, and after drying, the positive electrode sheet is formed by cold rolling; the positive electrode powder coating unit area density is 150 mg / m2 (mg / m 2 )~350mg / m 2 The compacted density of the positive electrode is 3.0 g / cm3 (g / cm 3 )~3.6g / cm 3 , optional 3.3g / cm 3 ~3.5g / cm 3 .

[0080] The calculation formula of the compacted density is:

[0081] Compaction density = coating surface density / (thickness of the electrode after extrusion - thickness of the current collector).

[0082] The mass M of the positive electrode active material per unit area of ​​the positive electrode membrane can be obtained by weighing using a standard balance.

[0083] The thickness T of the positive electrode film can be measured using a micrometer, for example, a Mitutoyo 293-100 with an accuracy of 0.1 μm. It should be noted that the thickness of the positive electrode film described in this application refers to the thickness of the positive electrode film in the positive electrode sheet after cold pressing and used in battery assembly.

[0084]

Negative electrode

[0085] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0086] As an example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0087] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material on a polymer material substrate. Among them, the metal material includes but is not limited to copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, silver and silver alloys, etc., and the polymer material substrate includes but is not limited to polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE) and other substrates.

[0088] In some embodiments, the negative electrode active material may be a negative electrode active material for batteries known in the art.

[0089] As an example, the negative electrode active material of a lithium-ion secondary battery may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxides, and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0090] As an example, the negative electrode active material of a sodium ion secondary battery is generally a hard carbon material, a two-dimensional metal carbide or a nitride. Preferably, the negative electrode active material of a sodium ion secondary battery is generally a hard carbon material.

[0091] The weight ratio of the negative electrode active material in the negative electrode film layer is 70 to 100 weight percent based on the total weight of the negative electrode film layer.

[0092] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The weight ratio of the binder in the negative electrode film layer is 0 to 30% by weight, based on the total weight of the negative electrode film layer.

[0093] In some embodiments, the negative electrode film layer may further include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the negative electrode film layer is 0 to 20% by weight, based on the total weight of the negative electrode film layer.

[0094] In some embodiments, the negative electrode film layer may further include other additives, such as a thickener (e.g., sodium carboxymethyl cellulose (CMC-Na)). The weight ratio of the other additives in the negative electrode film layer is 0 to 15% by weight, based on the total weight of the negative electrode film layer.

[0095] In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry, wherein the solid content of the negative electrode slurry is 30wt% to 70wt%, and the viscosity at room temperature is adjusted to 2000mPa·s to 10000mPa·s; the obtained negative electrode slurry is coated on the negative electrode current collector, and after a drying process, cold pressing, such as rolling, is performed to obtain the negative electrode sheet. The negative electrode powder coating unit area density is 75g / cm 2 ~220g / cm 2 The compacted density of the negative electrode is 1.2 g / m3 (g / m 3 )~2.0g / m 3 .

[0096] The mass M of the negative electrode active material per unit area of ​​the negative electrode membrane can be obtained by weighing using a standard balance.

[0097] The thickness T of the negative electrode film can be measured using a micrometer, such as a Mitutoyo 293-100 with an accuracy of 0.1 μm. It should be noted that the thickness of the negative electrode film described in this application refers to the thickness of the negative electrode film after cold pressing and compaction and used in the negative electrode sheet assembled into the battery.

[0098] Electrolytes

[0099] The electrolyte plays a role in conducting ions between the positive electrode and the negative electrode. The electrolyte in the secondary battery of the present application adopts the electrolyte specified in the present application.

[0100]

Isolation film

[0101] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0102] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0103] In some embodiments, the thickness of the isolation film is 6 μm to 40 μm, and optionally 12 μm to 20 μm.

[0104] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0105] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0106] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0107] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG1 shows a secondary battery 5 with a square structure as an example.

[0108] In some embodiments, referring to Figure 2, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0109] In some embodiments, the secondary batteries 5 can be assembled into a battery module. The number of secondary batteries 5 contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0110] In the battery module, the plurality of secondary batteries 5 can be arranged in sequence along the length of the battery module. Of course, they can also be arranged in any other manner. Furthermore, the plurality of secondary batteries 5 can be fixed by fasteners.

[0111] Optionally, the battery module may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0112] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0113] A battery pack may include a battery box and multiple battery modules disposed within the box. The battery box comprises an upper case and a lower case. The upper case can be placed over the lower case to form an enclosed space for accommodating the battery modules. The multiple battery modules can be arranged in any manner within the battery box.

[0114] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0115] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0116] Figure 3 shows an example of an electric device. This device can be a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0117] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0118] The following are some examples and comparative examples.

[0119] In order to make the technical problems, technical solutions and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of this application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present application and its applications. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0120] If no specific techniques or conditions are specified in the examples, the experiments were carried out according to the techniques or conditions described in the literature in the field or according to the product instructions. All reagents or instruments used without specifying the manufacturer are commercially available conventional products.

[0121] 1. Implementation

[0122] Example 1:

[0123] 1) Preparation of positive electrode sheet

[0124] Lithium iron phosphate (LFP) with an olivine structure is used as the positive electrode active material. Lithium iron phosphate, polyvinylidene fluoride (PVDF), and carbon black (SP) are mixed and stirred to form a uniformly dispersed positive electrode slurry. The mass ratio of PVDF to carbon black in the positive electrode slurry is 5:1. The positive electrode slurry is evenly coated on both surfaces of aluminum foil, dried, cold pressed, and cut to obtain a positive electrode sheet. The positive electrode sheet consists of aluminum foil and a positive electrode film formed on both surfaces of the aluminum foil.

[0125] Among them, the BET specific surface area of ​​the positive electrode active material is 5m 2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 90.0%, and the coating area density of the positive electrode film layer in the positive electrode sheet is 0.013g / cm 2 .

[0126] 2) Preparation of negative electrode sheet

[0127] The negative electrode active material (artificial graphite): carbon black (SP): styrene-butadiene rubber (SBR): carboxymethyl fiber (CMC) is mixed with a solvent in a weight ratio of 93:1.5:3.5:2, and stirred to obtain a uniformly dispersed negative electrode slurry. The negative electrode slurry is evenly coated on both surfaces of the copper foil, and then dried, cold pressed, and cut to obtain a negative electrode sheet.

[0128] 3) Isolation film

[0129] Polyethylene film is used as the isolation film.

[0130] 4) Preparation of electrolyte

[0131] In an argon atmosphere glove box, 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.5% hexamethylene diisocyanate (HDI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder was supplemented with diethyl carbonate (DEC) to 100%, and mixed to obtain the corresponding electrolyte.

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

[0133] The positive electrode sheet, the negative electrode sheet and the separator are made into an electrode assembly through a winding process or a lamination process, and placed in a shell made of an aluminum shell, an aluminum-plastic film, etc. The above-mentioned electrolyte is injected, allowed to stand at high temperature, formed, and divided into volumes to obtain the secondary battery of Example 1.

[0134] Example 2:

[0135] This embodiment is basically the same as the embodiment 1, and differs from the embodiment 1 in that:

[0136] The BET specific surface area of ​​the positive electrode active material in the positive electrode sheet is 10m 2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 92.0%, and the coating area density of the positive electrode film layer in the positive electrode sheet is 0.0195g / cm 2 ;

[0137] In the preparation step of the electrolyte, the electrolyte mass % includes 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.3% hexamethylene diisocyanate (HDI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder is supplemented to 100% with diethyl carbonate (DEC).

[0138] Example 3:

[0139] This embodiment is basically the same as the embodiment 1, and differs from the embodiment 1 in that:

[0140] The BET specific surface area of ​​the positive electrode active material in the positive electrode sheet is 15m 2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 93.0%, and the coating area density of the positive electrode film layer in the positive electrode sheet is 0.0195g / cm 2 ;

[0141] In the preparation step of the electrolyte, the electrolyte mass % includes 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.1% hexamethylene diisocyanate (HDI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder is supplemented to 100% with diethyl carbonate (DEC).

[0142] Example 4:

[0143] This embodiment is basically the same as the embodiment 1, and differs from the embodiment 1 in that:

[0144] The BET specific surface area of ​​the positive electrode active material in the positive electrode sheet is 25m 2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 95.0%, and the coating area density of the positive electrode film layer in the positive electrode sheet is 0.0195g / cm 2 ;

[0145] In the preparation step of the electrolyte, the electrolyte mass % includes 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.05% hexamethylene diisocyanate (HDI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder is supplemented to 100% with diethyl carbonate (DEC).

[0146] Example 5:

[0147] This embodiment is basically the same as the embodiment 1, and differs from the embodiment 1 in that:

[0148] The BET specific surface area of ​​the positive electrode active material in the positive electrode sheet is 25m 2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 96.0%, and the coating surface density of the positive electrode film layer in the positive electrode sheet is 0.026g / cm 2 ;

[0149] In the preparation step of the electrolyte, the electrolyte mass % includes 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.005% hexamethylene diisocyanate (HDI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder is supplemented to 100% with diethyl carbonate (DEC).

[0150] Example 6:

[0151] This embodiment is basically the same as the embodiment 1, and differs from the embodiment 1 in that:

[0152] The BET specific surface area of ​​the positive electrode active material in the positive electrode sheet is 25m 2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 98.0%, and the coating area density of the positive electrode film layer in the positive electrode sheet is 0.026g / cm 2 ;

[0153] In the preparation step of the electrolyte, the electrolyte mass % includes 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.001% hexamethylene diisocyanate (HDI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder is supplemented to 100% with diethyl carbonate (DEC).

[0154] Example 7:

[0155] This embodiment is basically the same as the embodiment 1, and differs from the embodiment 1 in that:

[0156] The BET specific surface area of ​​the positive electrode active material in the positive electrode sheet is 30m 2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 99.0%, and the coating area density of the positive electrode film layer in the positive electrode sheet is 0.0325g / cm 2 ;

[0157] In the preparation step of the electrolyte, the electrolyte mass % includes 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.001% hexamethylene diisocyanate (HDI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder is supplemented to 100% with diethyl carbonate (DEC).

[0158] Example 8:

[0159] This embodiment is basically the same as the embodiment 1, and differs from the embodiment 1 in that:

[0160] The BET specific surface area of ​​the positive electrode active material in the positive electrode sheet is 13m 2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 96.0%, and the coating area density of the positive electrode film layer in the positive electrode sheet is 0.0195g / cm 2 ;

[0161] In the preparation step of the electrolyte, the electrolyte mass % includes 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.1% toluene diisocyanate (TDI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder is supplemented to 100% with diethyl carbonate (DEC).

[0162] Example 9:

[0163] This embodiment is basically the same as the embodiment 1, and differs from the embodiment 1 in that:

[0164] The BET specific surface area of ​​the positive electrode active material in the positive electrode sheet is 13m 2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 96.0%, and the coating area density of the positive electrode film layer in the positive electrode sheet is 0.0195g / cm 2 ;

[0165] In the preparation step of the electrolyte, the electrolyte mass % includes 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.1% 4-fluorophenyl isocyanate (FPI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder is supplemented to 100% with diethyl carbonate (DEC).

[0166] Example 10:

[0167] This embodiment is basically the same as the embodiment 1, and differs from the embodiment 1 in that:

[0168] The BET specific surface area of ​​the positive electrode active material in the positive electrode sheet is 13m2 / g, the mass fraction of the positive electrode active material in the positive electrode film layer is 96.0%, and the coating area density of the positive electrode film layer in the positive electrode sheet is 0.0195g / cm 2 ;

[0169] In the preparation step of the electrolyte, the electrolyte mass % includes 8% lithium hexafluorophosphate (LiPF6), 2% vinylene carbonate (VC), 0.1% 4-cyanophenyl isocyanate (CPI), 25% ethylene carbonate (EC), 10% dimethyl carbonate (DMC), 20% ethyl methyl carbonate (EMC), and the remainder is supplemented to 100% with diethyl carbonate (DEC).

[0170] 2. Comparative Example

[0171] Comparative Example 1:

[0172] This comparative example is basically the same as Example 1, except that:

[0173] The electrolyte did not contain hexamethylene diisocyanate (HDI), and the amount of diethyl carbonate (DEC) was adjusted accordingly.

[0174] Comparative Example 2:

[0175] This comparative example is basically the same as Example 2, except that:

[0176] The electrolyte did not contain hexamethylene diisocyanate (HDI), and the amount of diethyl carbonate (DEC) was adjusted accordingly.

[0177] Comparative Example 3:

[0178] This comparative example is basically the same as Example 3, except that:

[0179] The electrolyte did not contain hexamethylene diisocyanate (HDI), and the amount of diethyl carbonate (DEC) was adjusted accordingly.

[0180] Comparative Example 4:

[0181] This comparative example is substantially the same as Example 4, except that:

[0182] The electrolyte did not contain hexamethylene diisocyanate (HDI), and the amount of diethyl carbonate (DEC) was adjusted accordingly.

[0183] Comparative Example 5:

[0184] This comparative example is substantially the same as Example 5, except that:

[0185] The electrolyte did not contain hexamethylene diisocyanate (HDI), and the amount of diethyl carbonate (DEC) was adjusted accordingly.

[0186] Comparative Example 6:

[0187] This comparative example is substantially the same as Example 6, except that:

[0188] The electrolyte did not contain hexamethylene diisocyanate (HDI), and the amount of diethyl carbonate (DEC) was adjusted accordingly.

[0189] Comparative Example 7:

[0190] This comparative example is substantially the same as Example 7, except that:

[0191] The electrolyte did not contain hexamethylene diisocyanate (HDI), and the amount of diethyl carbonate (DEC) was adjusted accordingly.

[0192] 3. Performance Testing

[0193] 1) Initial Direct Current Resistance (DCR) measurement

[0194] At room temperature, the secondary batteries of each embodiment and comparative example were charged at a constant current of 0.5C to 3.65V, then charged at a constant voltage to a current of 0.05C. The secondary batteries were discharged at a constant current of 0.5C for 30 minutes to adjust the battery to 50% SOC. The battery voltage at this time was recorded as U1. The battery was discharged at a constant current of 4C for 30 seconds, with a sampling point at 0.1 seconds. The voltage at the end of discharge was recorded as U2. The discharge DCR of the battery at 50% SOC represents the initial DCR of the battery: initial DCR = (U1 - U2) / 4C.

[0195] 2) 60℃ high temperature cycle performance test

[0196] At 60°C, the secondary batteries of each embodiment and comparative example were charged at a constant current of 1 / 3C to 3.65V, then charged at a constant voltage to a current of 0.05C. The batteries were allowed to rest for 5 minutes and then discharged at a constant current of 1 / 3C to 2.5V. This was the first charge-discharge cycle of the battery, and the discharge capacity at this time was recorded as the discharge capacity of the battery in the first cycle. The battery was charged and discharged 1000 times according to the above method, and the discharge capacity after 1000 cycles was recorded. The capacity retention rate (%) of the battery after 1000 cycles at 60°C = (discharge capacity after 1000 cycles / discharge capacity of the battery in the first cycle) × 100%.

[0197] 3) 60℃ high temperature storage performance test

[0198] At 25°C, the secondary batteries of each embodiment and comparative example were charged to 3.65V at a constant current of 1 / 3C, and then charged at a constant voltage to a current of 0.05C; the 100% SOC battery cells were placed in a 60°C high and low temperature box, and the true capacity of the battery cells was tested every 30 days (1 / 3C discharge to 2.5V, 0.04C discharge to 2.0V, recording the true capacity of the battery cells and charging to 3.65V at 1 / 3C) until they were stored at 60°C for 200 days.

[0199] The electrolyte parameters of the secondary batteries of Examples 1 to 10 and Comparative Examples 1 to 7 are detailed in Table 1. The positive electrode plate parameters and performance test data of the secondary batteries of Examples 1 to 10 and Comparative Examples 1 to 7 are detailed in Table 2.

[0200] Table 1

[0201] Table 2

[0202] In Table 2, b is the BET specific surface area of ​​the positive electrode active material; c is the mass fraction of the positive electrode active material in the positive electrode film layer; d is the coating surface density of the positive electrode film layer in the positive electrode sheet of the secondary battery; and the mass fraction of the compound containing an isocyanate group in the secondary battery electrolyte is a.

[0203] It can be seen from the data of the above embodiments and comparative examples that the electrolyte of the secondary battery of the present application uses a compound containing an isocyanate group as an additive, and the value of bcd / a is controlled within a specific range, so that the content of the compound containing an isocyanate group in the secondary battery electrolyte can be moderate, which can effectively remove moisture from the electrolyte and reduce the impact of moisture on the cycle performance and storage performance of the secondary battery, thereby extending the service life of the secondary battery; and the DC impedance of the secondary battery will not be significantly increased due to excessive isocyanate compounds, and will not have a significant impact on the charging performance of the secondary battery.

[0204] Comparing Example 1 with Comparative Example 1, Example 2 with Comparative Example 2, Example 3 with Comparative Example 3, Example 4 with Comparative Example 4, Example 5 with Comparative Example 5, Example 6 with Comparative Example 6, and Example 7 with Comparative Example 7, it can be seen that, under the same other conditions, adding a compound containing an isocyanate group to the electrolyte and controlling its content within the scope of this application can effectively improve the cycle performance and storage performance of the secondary battery, thereby extending the service life of the secondary battery. Moreover, although the initial direct current resistance (DCR) of the secondary battery is increased due to the addition of the compound containing an isocyanate group, the increase is not large and has little effect on the charging performance of the secondary battery.

[0205] By comparing the various examples, it can be seen that in Examples 3 to 10, controlling the bcd / 10000a value within the range of 243 to 96525 can further reduce the initial DC resistance of the secondary battery; the initial DC resistance can reach below 639 mohm. In Examples 1 and 2, the bcd / 10000a values ​​are 11.7 and 59.8, respectively, and the initial DC resistance is above 670 mohm.

[0206] By comparing the various embodiments, it can be seen that in Example 4 and Examples 8 to 10, controlling the value of bcd / 10000a within the range of 243 to 927 can not only reduce the initial DC resistance of the secondary battery, but also improve its cycle performance and storage performance. The 60°C cycle capacity retention rate is above 89.9%, and the 60°C storage capacity retention rate is above 94.4%. In Examples 5 to 7, the value of bcd / 10000a is not within the above range. Although its initial DC resistance is also small, its cycle performance and storage performance are significantly lower than those of Examples 4 and Examples 8 to 10. This shows that controlling the value of bcd / 10000a within the range of 243 to 927 can enable the secondary battery to obtain better comprehensive performance.

[0207] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode film layer, the positive electrode film layer contains a positive electrode active material, the electrolyte comprises a solvent, a lithium salt and an additive, the additive comprises a compound containing an isocyanate group, the mass fraction of the compound containing an isocyanate group in the electrolyte is a, and 11≤n / 10000a≤96600, n=b×c×d; in, n is the real surface area per unit apparent surface area of ​​the positive electrode active material; b is the BET specific surface area of ​​the positive electrode active material, in cm 2 / g; c is the mass fraction of the positive electrode active material in the positive electrode film layer; d is the coating surface density of the positive electrode film layer, in g / cm 2 .

2. The secondary battery according to claim 1, It is characterized in that 243≤n / 10000a≤96525.

3. The secondary battery according to claim 1, It is characterized in that 243≤n / 10000a≤927.

4. The secondary battery according to any one of claims 1 to 3, It is characterized in that The range of b is 50000cm 2 / g~300000cm 2 / g.

5. The secondary battery according to any one of claims 1 to 4, It is characterized in that The range of c is 90% to 99%.

6. The secondary battery according to any one of claims 1 to 5, It is characterized in that The range of d is 0.013 g / cm 2 ~0.0325g / cm 2 .

7. The secondary battery according to any one of claims 1 to 6, It is characterized in that The range of a is 0.001% to 0.5%.

8. The secondary battery according to any one of claims 1 to 7, It is characterized in that The compound containing an isocyanate group includes one or more of the following structural formula compounds:

9. The secondary battery according to any one of claims 1 to 8, It is characterized in that The additives further include a film-forming aid, and the film-forming aid includes an unsaturated carbonate.

10. The secondary battery according to any one of claims 1 to 9, It is characterized in that The solvent includes one or more of cyclic esters and chain esters.

11. The secondary battery according to claim 10, It is characterized in that The cyclic ester includes one or more of ethylene carbonate, propylene carbonate, fluoroethylene carbonate and γ-butyrolactone.

12. The secondary battery according to claim 10 or 11, It is characterized in that The chain ester includes one or more of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl formate, methyl acetate, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, methyl butyrate and ethyl butyrate.

13. The secondary battery according to any one of claims 1 to 12, It is characterized in that The lithium salt includes LiPF 6 、LiAsF 6 , LiBF 4 、LiSCN、LiTaF 6 、LiSnF 6 and LiCF 3 SO 3 One or more of .

14. The secondary battery according to any one of claims 1 to 13, It is characterized in that The positive electrode active material includes one or more of lithium iron phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium manganese oxide, lithium manganese phosphate and lithium manganese iron phosphate.

15. An electrical device comprising the secondary battery according to any one of claims 1 to 14.

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