Lithium secondary battery and electric device

Abstract

The application provides a lithium secondary battery and an electric device. The lithium secondary battery comprises a positive electrode sheet, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one side of the positive electrode current collector, the lithium ion diffusion coefficient Ds of the positive electrode active material layer is 10 ‑16 cm 2 / s-10 ‑5 cm 2 / s; an electrolyte, the electrolyte comprises a solvent, the solvent comprises at least one of the following formula (I) compounds, the mass fraction W1 of the formula (I) compound in the solvent is 20%-80%; in formula (I), R1 and R2 independently comprise at least one of alkyl with 1-3 carbon atoms and halogenated alkyl with 1-3 carbon atoms. The lithium secondary battery can effectively improve the transmission rate of lithium ions in the electrolyte, and improve the energy density and fast charging performance of the lithium secondary battery by adjusting the lithium ion diffusion coefficient Ds of the positive electrode material and the amount of use of the formula (I) compound.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 202380011591.X, filed on February 24, 2023, entitled "Lithium Secondary Battery and Electric Device", the disclosure of which is incorporated herein by reference in its entirety. TECHNICAL FIELD

[0002] The present application relates to the technical field of batteries, in particular to a lithium secondary battery and an electric device. BACKGROUND

[0003] In recent years, secondary batteries have been widely used in various electronic products and electric vehicles due to their high power density and volume specific capacity. With the great development and wide application of secondary batteries, higher requirements are put forward for their energy density, fast charging performance, etc.

[0004] The fast charging of a lithium secondary battery relies on the rapid extraction of lithium ions from the positive active material, the movement of the lithium ions to the negative electrode through the electrolyte, and the rapid insertion of the lithium ions into the negative active material. Therefore, the fast charging performance of the lithium secondary battery is closely related to the transport rate of lithium ions in the electrode material and the electrolyte. The diffusion coefficient of lithium ions in the electrode material and the electrolyte is one of the parameters that reflect the transport rate of lithium ions in the electrode material and the electrolyte. However, the diffusion coefficient of lithium ions in the electrode material and the diffusion coefficient of lithium ions in the electrolyte are difficult to match, which seriously affects the charge and discharge performance of lithium ions. Therefore, how to improve the charge and discharge performance of the lithium secondary battery has become a technical problem to be solved. SUMMARY

[0005] The present application is made in view of the above technical problem, and aims to provide a lithium secondary battery and an electric device, which can improve the transport rate of lithium ions in the positive active material layer and the electrolyte, thereby helping to improve the fast charging performance of the lithium secondary battery.

[0006] The transport rate of lithium ions in the positive active material layer and the electrolyte.

[0007] In a first aspect, the present application provides a lithium secondary battery, comprising: a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive active material layer disposed on at least one side of the positive electrode current collector, the lithium ion diffusion coefficient of the positive active material layer being Ds; an electrolyte, the electrolyte comprising a solvent, the solvent comprising at least one of the following compounds of formula (I), the mass fraction of the compound of formula (I) in the solvent being W1, the mass fraction W1 of the compound of formula (I) and the lithium ion diffusion coefficient Ds of the positive active material layer satisfying: 2x10 -18 cm 2 / s≤W1xDs≤8x10 -6 cm 2 / s; optionally, 3x10 -14 cm2 / s≤ W1 x Ds≤ 7 x 10 -10 cm 2 / s;

[0008]

[0009] wherein R1 and R2 each independently include at least one of an alkyl group having 1-3 carbon atoms, a haloalkyl group having 1-3 carbon atoms.

[0010] In embodiments of the present application, the compound represented by formula (I) as a solvent or as part of a solvent can enable the electrolyte to have a higher electrical conductivity. When the lithium secondary battery is charged, lithium ions are released from the positive active material, move to the surface of the negative active material layer through the electrolyte, and then are embedded in the negative active material. According to the lithium ion diffusion coefficient Ds of the positive material in the lithium secondary battery, by controlling the amount of the compound represented by formula (I), the transmission rate of lithium ions in the electrolyte can be effectively improved, so that the electrolyte can timely transport a large amount of lithium ions released from the positive active material to the negative electrode, thereby improving the fast charging performance of the lithium secondary battery.

[0011] In some embodiments, R1 and R2 each independently include at least one of a methyl group, an ethyl group, a propyl group, a fluoromethyl group, a fluoroethyl group, a fluoropropyl group.

[0012] In some embodiments, 10 -16 cm 2 / s≤ Ds≤ 10 -5 cm 2 / s; optionally, 10 -14 cm 2 / s≤ Ds≤ 10 -9 cm 2 / s.

[0013] In some embodiments, 20%≤ W1≤ 80%; optionally, 30%≤ W1≤ 70%.

[0014] In some embodiments, the compound of formula (I) includes at least one of the following compounds:

[0015]

[0016] Optionally, the compound of formula (I) includes at least one of the compound of formula (I-I), the compound of formula (I-II), the compound of formula (I-V), and the compound of formula (I-VIII).

[0017] In some embodiments, the electrolyte includes a first additive, and the first additive includes at least one of the compounds of formula (II):

[0018] In some embodiments, the electrolyte includes a first additive, and the first additive includes at least one of the compounds of formula (II):

[0019] wherein R3 includes at least one of an alkylene group having 2 to 10 carbon atoms which is substituted with Ra, a heteroalkylene group having 2 to 10 carbon atoms which is substituted with Ra, an arylene group having 6 to 18 carbon atoms which is substituted with Ra, a heteroarylene group having 6 to 18 carbon atoms which is substituted with Ra, an alicyclyl group having 3 to 18 carbon atoms which is substituted with Ra, a heteroalicyclyl group having 3 to 18 carbon atoms which is substituted with Ra; and Ra includes at least one of a halogen atom, a cyano group, an isocyanate group, a hydroxyl group, a carboxyl group, a sulfonic acid group, an ester group, an alkyl group having 2 to 10 carbon atoms, an alkenyl group having 2 to 10 carbon atoms, an alkynyl group having 2 to 10 carbon atoms, an oxaalkyl group having 2 to 10 carbon atoms.

[0020] Embodiments of the present application help form a SEI (Solid Electrolyte Interface, SEI) film having a protective effect on the surface of the negative active material layer of the lithium secondary battery by introducing the first additive into the electrolyte, can effectively improve the stability of the solid-liquid interface between the negative electrode and the electrolyte, and improve the problem of gas production of the electrolyte at the interface between the negative active material layer and the electrolyte during the fast charging process of the lithium secondary battery, thereby helping to improve the cycle performance of the lithium secondary battery.

[0021] In some embodiments, the mass fraction of the first additive in the electrolyte is W2, the W2, the W1, and the Ds satisfy: 2x10 -22 cm 2 / s≤W2xW1xDs≤1.6x10 -6 cm 2 / s; optionally, 2x10 -21 cm 2 / s≤W2xW1xDs≤8x10 -7 cm 2 / s; optionally, 4x10 -21 cm 2 / s≤W2xW1xDs≤4x10 -7 cm 2 / s.

[0022] In some embodiments, the W2 satisfies: 0.01%≤W2≤20%; optionally, 0.1%≤W2≤10%; optionally, 0.2%≤W2≤5%.

[0023] In some embodiments, the first additive includes at least one of the following compounds:

[0024]

[0025]

[0026] In some embodiments, the electrolyte comprises a second additive, the second additive comprising M a+ [F w PO t ] b- , M a+ [BF4] - a , M a+ [FSO3] - a and at least one of the compounds of formula (III):

[0027]

[0028] wherein M a+ comprises at least one of lithium ion, sodium ion, potassium ion, rubidium ion, cesium ion, magnesium ion, calcium ion, barium ion, aluminum ion, iron ion, copper ion, nickel ion, organic cation, a, b, c each represents a natural number; t represents an integer of 0-3, w represents an integer of 1-6; X comprises at least one of halogen atoms, n represents an integer of 0-4; Y comprises at least one of boron atom, phosphorus atom; R4 comprises substituted or unsubstituted alkylene group with carbon atom number of 1-10, substituted or unsubstituted halogenated alkylene group with carbon atom number of 1-10, substituted or unsubstituted arylene group with carbon atom number of 6-20, substituted or unsubstituted halogenated arylene group with carbon atom number of 6-20, q represents an integer of 0-1, m represents an integer of 1-3.

[0029] In the embodiments of the present application, by introducing the second additive into the electrolyte, it is helpful to form SEI film on the surface of the negative active material layer, further improve the stability of the solid-liquid interface between the negative electrode and the electrolyte, and thus further improve the cycle performance of the lithium secondary battery.

[0030] In some embodiments, the mass fraction of the second additive in the electrolyte is W3, the W3, the W1, and the Ds satisfy: 2x10 -22 cm 2 / s≤W3xW1xDs≤1.6x10 -6 cm 2 / s; optionally, 2x10 -21 cm 2 / s≤W3xW1xDs≤8x10 -7 cm 2 / s; optionally, 4x10 -21 cm 2 / s≤W3xW1xDs≤4x10 -7 cm2 / s.

[0031] In some embodiments, 0.01%≤W3≤20%; optionally, 0.1%≤W3≤10%; optionally, 0.2%≤W3≤5%.

[0032] In some embodiments, in M a+ [FSO3] - a M a+ includes at least one of Li + , Na + , K + , Rb + , Cs + , Mg 2+ , Ca 2+ , Ba 2+ , Al 3 + , Fe 2+ , Cu 2+ , Fe 3+ , Ni 2+ , Ni 3+ .

[0033] In some embodiments, the positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material; the average particle size Dv501 of the first positive electrode active material satisfies: 8 μm≤Dv501≤50 μm; the average particle size Dv502 of the second positive electrode active material satisfies: 0.02 μm≤Dv502≤8 μm.

[0034] In embodiments of the present application, by matching the first active material with a larger average (volume) particle size and the second active material with a smaller average (volume) particle size, the powder compaction density of the positive electrode sheet is improved, thereby helping to improve the energy density of the lithium secondary battery.

[0035] In some embodiments, the mass fraction W4 of the second positive electrode active material in the positive electrode active material layer satisfies: 0%≤W4≤60%; optionally, 20%≤W4≤40%.

[0036] In some embodiments, the lithium secondary battery includes: a negative electrode sheet, the negative electrode sheet including a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer including a negative electrode active material; the average particle size Dv503 of the negative electrode active material satisfies: 6 μm≤Dv503; optionally, 15 μm≤Dv503≤20 μm.

[0037] In the embodiments of the present application, by setting the average (volume) particle size of the negative active material within a suitable range, the average volume particle size of the surface layer of the negative active material is increased and the specific surface area is reduced after the first additive and / or the second additive form a certain thickness of SEI film on the surface of the negative active material layer, thereby reducing the contact area between the surface layer of the negative active material and the electrolyte, reducing the probability of side reactions of the electrolyte on the surface of the negative active material layer, and improving the cycle performance of the lithium secondary battery.

[0038] In some embodiments, the specific surface area of the negative active material satisfies: 0.5 m 2 / g≤BET≤2.0 m 2 / g; optionally, 0.8 m 2 / g≤BET≤1.5 m 2 / g.

[0039] In the embodiments of the present application, by setting the specific surface area of the negative active material within a suitable range, the specific surface area of the surface layer of the negative active material is reduced after the first additive and / or the second additive form a certain thickness of SEI film on the surface of the negative active material layer, thereby reducing the contact area between the negative active material and the electrolyte, reducing the probability of side reactions of the electrolyte on the surface of the negative active material layer, and improving the cycle performance of the lithium secondary battery.

[0040] In a second aspect, a power device is provided, which comprises the lithium secondary battery according to any one of the embodiments of the first aspect.

[0041] Therefore, the lithium secondary battery provided by the present application has good fast charging performance and cycle performance, and the power device provided by the present application can be efficiently fast charged and has a long service life. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required to be used in the embodiments of the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by the drawings without creative labor for those skilled in the art. In the drawings, the drawings are not drawn according to the actual proportion.

[0043] Figure 1 is a schematic diagram of a secondary battery according to the present application.

[0044] Figure 2 is a schematic structural diagram of a secondary battery according to the present application.

[0045] Figure 3 is a schematic structural diagram of a battery module according to the present application.

[0046] Figure 4 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0047] Figure 5 is a schematic structural diagram of a battery pack according to an embodiment of the present application.

[0048] Figure 6 is a schematic diagram of an electric device according to an embodiment of the present application. DETAILED DESCRIPTION

[0049] Hereinafter, embodiments of a lithium secondary battery and an electric device according to the present application will be specifically described with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters that are well known and repeated description of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present application and are not intended to limit the subject matter recited in the claims.

[0050] The "ranges" disclosed herein are defined by the lower and upper limits of the range, and the ranges are defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. Ranges defined in this manner can be inclusive or exclusive of the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 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, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise stated, a numerical range "a-b" indicates a shorthand manner of describing all of the individual real combinations of a to b, where a and b are both real numbers. For example, the numerical range "0-5" indicates that all of the real numbers between "0-5" have been listed herein, and "0-5" is merely a shorthand manner of describing these numerical combinations. In addition, when it is stated that a parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0051] In the description of the present application, it is necessary to explain that, unless otherwise specified, the meaning of "a plurality of" is more than two; the orientation or position relationship indicated by the terms "upper", "lower", "left", "right", "inner", "outer" and the like only for the convenience of describing the present application and simplifying the description, and not to indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, therefore, it cannot be understood as a limitation on the present application. In addition, the terms "first", "second", "third" and the like are only for the purpose of description, and cannot be understood as indicating or implying relative importance. "Vertical" is not strictly vertical, but within the allowable range of error. "Parallel" is not strictly parallel, but within the allowable range of error.

[0052] If not specifically stated, "including" and "comprising" mentioned in the present application represent open type, and can also be closed type. For example, "including" and "comprising" can mean that other components not listed can also be included or contained, or only the listed components can be included or contained.

[0053] If not specifically stated, in the present application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": 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 A and B are both true (or exist).

[0054] If not specifically stated, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method can also comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0055] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0056] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0057] If not specifically stated, the following terms have the following meanings. Any undefined term has its technically recognized meaning.

[0058] The term "alkyl" has its ordinary meaning referring to a monovalent saturated hydrocarbon radical having one or more carbon atoms, optionally, from 1 to 3. For example, alkyl includes straight chain and branched chain hydrocarbon radicals such as methyl, ethyl, n-propyl, i-propyl, and the like.

[0059] The term "haloalkyl" has its ordinary meaning referring to an alkyl radical in which one or more hydrogen atoms on one or more carbon atoms in the straight chain, branched, or cyclic skeleton are replaced by a halogen atom. For example, chloromethyl, fluoromethyl, trifluoromethyl, fluoroethyl, fluoro-propyl, and the like.

[0060] The term "alkylene" has its ordinary meaning referring to a divalent hydrocarbon radical having one or more carbon atoms, optionally, from 1 to 10. For example, alkylene includes divalent straight chain and branched chain hydrocarbon radicals such as ethylene, methyl ethylene, n-propylene, and the like.

[0061] The term "alkylene substituted with Ra" refers to an alkylene in which one or more hydrogen atoms on one or more carbon atoms in the straight chain, branched, or cyclic skeleton are replaced by Ra.

[0062] The term "heteroalkylene" has its ordinary meaning referring to an alkylene in which one or more carbon atoms in the straight chain, branched, or cyclic skeleton are each independently replaced by the same or different heteroatom or group. Heteroatoms or groups include, but are not limited to, halogen atoms, oxygen atoms, sulfur atoms, peroxy groups, persulfido groups, oxysulfido groups, phenyl groups, and the like.

[0063] The term "heteroalkylene substituted with Ra" refers to a heteroalkylene in which one or more hydrogen atoms on one or more carbon atoms in the straight chain, branched, or cyclic skeleton are replaced by Ra.

[0064] The term "arylene" has its ordinary meaning referring to a divalent aromatic radical derived by removing one hydrogen atom from each of two different carbon atoms of an aromatic compound or by removing two hydrogen atoms from one carbon atom of an aromatic compound.

[0065] The term "arylene substituted with Ra" refers to an arylene in which one or more hydrogen atoms on one or more carbon atoms in the straight chain, branched, or cyclic skeleton are replaced by Ra.

[0066] The term "heteroarylene" has its ordinary meaning referring to an arylene in which one or more carbon atoms are each independently replaced by the same or different heteroatom or group.

[0067] The term "heteroarylene substituted with Ra" refers to a heteroarylene in which one or more hydrogen atoms on one or more carbon atoms in the straight chain, branched, or cyclic skeleton are replaced by Ra.

[0068] The term "cycloalkyl" has its ordinary meaning, referring to a saturated carbocyclic radical.

[0069] The term "substituted cycloalkyl" refers to a cycloalkyl group in which one or more hydrogen atoms on one or more carbon atoms in the linear, branched, or cyclic skeleton are replaced with the same or different heteroatom or group.

[0070] The term "heterocycloalkyl" has its ordinary meaning, referring to a heterocycloalkyl group in which one or more carbon atoms in the linear, branched, or cyclic skeleton are each independently replaced with the same or different heteroatom or group.

[0071] The term "substituted heterocycloalkyl" refers to a heterocycloalkyl group in which one or more hydrogen atoms on one or more carbon atoms in the linear, branched, or cyclic skeleton are replaced with the same or different heteroatom or group.

[0072] The term "halogen atom" has its ordinary meaning, referring to a fluorine atom, a chlorine atom, a bromine atom, an iodine atom, aastatine atom.

[0073] The term "alkenyl" has its ordinary meaning, referring to a hydrocarbon group having one or more carbon atoms and having at least one site of double bond unsaturation. Optionally, the number of carbon atoms is in the range of from 2 to 10. For example, alkenyl groups include straight chain alkenyl and branched chain alkenyl groups. For example, ethenyl, propenyl, n-buttenyl, and the like. In some other examples, alkenyl groups can also include cyclic alkenyl groups, bicyclic alkenyl groups, and the like. For example, cyclohexenyl.

[0074] The term "alkynyl" has its ordinary meaning, referring to a hydrocarbon group having one or more carbon-carbon triple bonds. Optionally, the number of carbon atoms is in the range of from 2 to 10. For example, ethynyl.

[0075] The term "oxaalkyl" has its ordinary meaning, referring to an alkyl group in which one or more carbon atoms in the linear, branched, or cyclic skeleton are replaced with an oxygen atom.

[0076] The term "substituted alkylene" has its ordinary meaning, referring to an alkylene group in which one or more hydrogen atoms on one or more carbon atoms in the linear, branched, or cyclic skeleton are replaced with the same or different heteroatom or group.

[0077] The term "haloalkylene" has its ordinary meaning, referring to an alkylene group in which one or more hydrogen atoms on one or more carbon atoms in the linear, branched, or cyclic skeleton are replaced with the same or different halogen atom.

[0078] The term "substituted haloalkylene" has its ordinary meaning, referring to a haloalkylene group in which one or more hydrogen atoms on one or more carbon atoms in the linear, branched, or cyclic skeleton are replaced with the same or different heteroatom or group.

[0079] The term "substituted aryl" refers to an aryl group in which one or more hydrogen atoms are replaced by the same or different heteroatoms or groups.

[0080] The term "haloaryl" refers to an aryl group in which one or more hydrogen atoms are replaced by the same or different halogen atoms or groups.

[0081] The term "substituted haloaryl" refers to a haloaryl group in which one or more hydrogen atoms are replaced by the same or different heteroatoms or groups.

[0082] The term "lithium ion diffusion coefficient" refers to the average number of lithium ions per unit area of medium per unit time, which can represent the penetration speed of lithium ions in the medium. The diffusion process of lithium ions in the solid phase includes diffusion based on the translocation mechanism in the ion crystal, diffusion caused by the concentration gradient, and diffusion caused by the chemical potential. In the embodiments of the present application, the lithium ion diffusion coefficient can also be represented by the chemical diffusion coefficient of lithium ions, which covers the above-mentioned multiple diffusion processes.

[0083] In the embodiments of the present application, the average (volume) particle size of the material is reflected by the particle size distribution.

[0084] The term "Dv50" refers to the particle size corresponding to the cumulative particle size distribution of 50% of the material. Its physical meaning is that the particles larger or smaller than this particle size account for 50% of the total number of particles in the material. The "1" in "Dv501" is a subscript, indicating the Dv50 of a certain material, used to distinguish the Dv50 of other materials, and does not have other limiting meanings.

[0085] Next, the embodiments of the present application are introduced.

[0086] In recent years, secondary batteries have been widely used in electric tools, electronic products, electric vehicles, aerospace and other fields due to their high energy density and long service life, and have made great progress. Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the charging and discharging process of the battery, active ions are embedded and extracted between the positive electrode sheet and the negative electrode sheet. Among them, the electrolyte plays a role in conducting active ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, which prevents the short circuit of the positive and negative electrodes while allowing the active ions to pass through, so that the secondary battery electrochemical reaction can proceed normally.

[0087] For example, lithium secondary batteries are a typical kind of secondary batteries. Lithium secondary batteries are also known as rocking chair batteries because they rely on the chemical reaction of lithium ions deintercalating between the positive and negative electrodes to charge and discharge. During the charging process of lithium secondary batteries, lithium ions are deintercalated from the positive active material, move to the negative electrode through the electrolyte, and are intercalated into the negative active material. During the discharging process, lithium ions are deintercalated from the negative active material, move to the positive electrode through the electrolyte, and are intercalated into the positive active material.

[0088] It should be understood that the "intercalation of lithium" and "intercalation" processes described in the present application refer to the process of lithium ions being intercalated into the positive active material or the negative active material due to an electrochemical reaction. The "deintercalation" and "deintercalation of lithium" processes described in the present application refer to the process of lithium ions being deintercalated from the positive active material or the negative active material due to an electrochemical reaction.

[0089] With the increasingly wide application of lithium secondary batteries, the performance requirements for lithium secondary batteries in different fields are also becoming higher and higher. For example, in the field of electric vehicles, lithium secondary batteries are required to have high energy density and fast charging capability. As known from the above, during the charging process of lithium secondary batteries, lithium ions are deintercalated from the positive active material, move to the negative electrode through the electrolyte, and are intercalated into the negative active material. On the one hand, in order to achieve high energy density, more positive and negative active materials need to be loaded on the positive and negative electrode plates, which makes the thickness of the positive and negative active material layers of the electrode plates thicker than those of ordinary lithium secondary batteries. For example, in order to ensure that lithium ions in the positive active material far from the surface of the positive active material layer can be smoothly deintercalated, the lithium ion diffusion coefficient of the positive active material layer is usually large. On the other hand, the positive active material layer and the electrolyte have different abilities to conduct lithium ions. Generally speaking, the lithium ion diffusion coefficient of lithium ions in the positive active material layer (solid phase) is greater than the lithium ion diffusion coefficient of lithium ions in the electrolyte (liquid phase). The mismatch between the solid-phase diffusion coefficient and the liquid-phase diffusion coefficient of lithium ions results in that, during the fast charging process of lithium secondary batteries, a large amount of lithium ions deintercalated from the positive active material are limited by the lithium ion conduction ability of the electrolyte and cannot be timely transported to the negative electrode, thereby limiting the fast charging performance of lithium secondary batteries.

[0090] Therefore, embodiments of the present application provide a lithium secondary battery. In the lithium secondary battery, the solvent of the electrolyte includes a linear carboxylate compound as shown in formula (I). The compound as shown in formula (I) has a high dielectric constant and a low viscosity, thereby improving the ability of the electrolyte to conduct lithium ions. In the lithium secondary battery, the mass fraction of the compound as shown in formula (I) is adjusted to be within a suitable range according to the lithium ion diffusion coefficient of the positive active material layer, so that the lithium secondary battery has high energy density and good fast charging performance at the same time.

[0091] In one embodiment of the present application, a lithium secondary battery is provided. Generally, the lithium secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator. Next, the lithium secondary battery provided by the present application and each part in the lithium secondary battery are introduced.

[0092] In one embodiment of the present application, a lithium secondary battery is provided, which includes a positive electrode sheet and an electrolyte.

[0093] The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one side of the positive electrode current collector, and the lithium ion diffusion coefficient of the positive electrode active material layer is Ds.

[0094] The electrolyte includes a solvent, the solvent includes at least one of the compounds of formula (I) shown below, and the mass fraction of the compound of formula (I) in the electrolyte is W1. The mass fraction W1 of the compound of formula (I) and the lithium ion diffusion coefficient Ds of the positive electrode active material layer satisfy: 2x10 -18 cm 2 / s≤W1x Ds≤8x10 -6 cm 2 / s; optionally, 3x10 -14 cm 2 / s≤W1x Ds≤7x10 -10 cm 2 / s.

[0095]

[0096] R1 and R2 each independently include at least one of an alkyl group with 1-3 carbon atoms and a halogenated alkyl group with 1-3 carbon atoms.

[0097] Specifically, W1x Ds can be 2x10 -18 cm 2 / s, 2x10 -17 cm 2 / s, 2x10 -16 cm 2 / s, 2x10 - 15 cm 2 / s, 2x10 -14 cm 2 / s, 3x10 -14 cm 2 / s, 2x10 -12 cm 2 / s, 2x10 -11 cm 2 / s, 7x10 -10 cm 2 / s, 2x10 -9 cm 2 / s, 2 x 10 -8 cm 2 / s, 2 x 10 -7 cm 2 / s, 8 x 10 -6 cm 2 / s, or a value within a range obtained by any two of the above value combinations.

[0098] In the embodiment, the lithium secondary battery has a positive electrode active material layer with a lithium ion diffusion coefficient of Ds, and has an electrolyte matched with the positive electrode active material layer, the solvent of the electrolyte includes a compound shown in formula (I), so that the electrolyte has a higher electrical conductivity, the mass fraction W1 of the compound in the electrolyte is regulated according to the lithium ion diffusion coefficient Ds of the positive electrode active material layer, so that the product of the two is within a suitable range, which can improve the problem of mismatching of lithium ion diffusion capacity between the positive electrode active material layer and the electrolyte in the lithium secondary battery, and improve the fast charging capacity of the lithium secondary battery while realizing higher energy density of the lithium secondary battery.

[0099] The lithium ion diffusion coefficient Ds of the positive electrode active material is related to the particle size of the positive electrode active material, the morphology of the positive electrode active material, the ion conduction ability of the positive electrode active material itself and other factors. The lithium ion diffusion coefficient Ds of the positive electrode active material can be regulated by regulating the particle size of the positive electrode active material, changing the morphology of the positive electrode active material and other methods.

[0100] As an example, the lithium ion diffusion coefficient Dsof the positive electrode active material can be regulated by the following method: raw materials Li2CO3, H3PO4, FeCl2·4H2O are added into anhydrous ethanol in a molar ratio of 1:1:1, respectively, stirred uniformly, and after the raw material reaction is completed, the stirring is stopped, and the sol is immediately converted into a gray gel. After standing at room temperature, the gray gel is transferred into a 80°C blast drying oven for drying for 12h. Then the dried gel is placed in an alumina magnetic boat and moved into a tube furnace for heat treatment in an argon atmosphere. The heat treatment can adopt a two-step calcination method, i.e. pre-calcination and final calcination. The heat treatment conditions for pre-calcination are 350°C for 5h. The heat treatment conditions for final calcination are 500-800°C for 10-25h. In order to regulate the lithium ion diffusion coefficient Ds, an appropriate amount of citric acid (for example, 0mol, 0.2mol, 0.5mol, 1mol or 2mol) can be added to the sol after the raw material reaction is completed, and the stirring time can be extended by 3h. By adding citric acid to the sol after the raw material reaction, the amorphous carbon obtained by pyrolysis of citric acid can be coated on the surface of the particles. The presence of this carbon coating layer will limit the further diffusion of the reactants, play a steric hindrance effect, thereby reducing the growth rate of the crystal, and the introduction of carbon is conducive to the refinement of the grain, and thus by regulating the amount of citric acid added, positive electrode materials of different particle sizes can be obtained, and thus positive electrode materials with different lithium ion diffusion coefficients can be obtained. The lithium ion diffusion coefficient Dscan be directly tested by the constant current intermittent titration method (GITT method).

[0101] In an embodiment of the present application, the solvent can only include the compound of formula (I). When the solvent only includes the compound of formula (I), the high electrical conductivity and low viscosity of the compound of formula (I) benefit the electrode liquid to have good electrical conductivity and flowability, and also can improve the problem that the lithium ion diffusion capacity of the positive electrode active material layer does not match the electrolyte in the lithium secondary battery, and at the same time realizes the high energy density of the lithium secondary battery, and improves the fast charging capability of the lithium secondary battery.

[0102] Optionally, R1 and R2 each independently include at least one of methyl, ethyl, propyl, fluoromethyl, fluoroethyl, fluoropropyl.

[0103] Optionally, the compound of formula (I) includes at least one of the following compounds:

[0104]

[0105] Optionally, the compound of formula (I) includes at least one of the compounds of formula (I-I), formula (I-II), formula (I-V) and formula (I-VIII).

[0106] Next, the positive electrode sheet and the electrolyte in the lithium secondary battery are described in detail.

[0107] [Electrolyte]

[0108] The electrolyte plays a role of conducting lithium ions between the positive electrode sheet and the negative electrode sheet. The electrolyte includes a solute and a solvent, and the solute is dissolved in the solvent to form a bulk homogeneous electrolyte.

[0109] In the lithium secondary battery provided by the present application, the solvent of the electrolyte includes a compound shown in formula (I).

[0110] Optionally, the mass fraction W1 of the compound shown in formula (I) in the electrolyte is positively correlated with the lithium ion diffusion coefficient Dsof the positive active material layer.

[0111] Specifically, as the positive active material loading increases, the positive active material with a larger lithium ion diffusion coefficient Dsis selected, and a higher proportion of the compound shown in formula (I) is required to improve the lithium ion conduction ability of the electrolyte, so as to match the lithium ion conduction ability of the positive active material layer.

[0112] Optionally, the mass fraction W1 of the compound shown in formula (I) in the electrolyte satisfies: 20%≤W1≤80%; further optionally, 30%≤W1≤70%.

[0113] Specifically, W1 can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or a value within the range obtained by any two of the above combinations.

[0114] Optionally, in some other embodiments, the solvent can also include at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, butane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0115] Optionally, the solute of the electrolyte includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoroboric oxalate, lithium boric oxalate, lithium difluorophosphoric oxalate, and lithium tetrafluorophosphoric oxalate.

[0116] Optionally, the electrolyte includes a first additive, and the first additive includes at least one of a compound shown in formula (II):

[0117]

[0118] R3 includes at least one of the following: an alkylene group having 2-10 carbon atoms substituted or unsubstituted with Ra; a heteroalkylene group having 2-10 carbon atoms substituted or unsubstituted with Ra; an aryl group having 6-18 carbon atoms substituted or unsubstituted with Ra; a heteroaryl group having 6-18 carbon atoms substituted or unsubstituted with Ra; an alicyclic group having 3-18 carbon atoms substituted or unsubstituted with Ra; and a heteroalicyclic group having 3-18 carbon atoms substituted or unsubstituted with Ra.

[0119] Ra includes at least one of the following: halogen atom, cyano group, isocyanate group, hydroxyl group, carboxyl group, sulfonic acid group, ester group, alkyl group with 2-10 carbon atoms, alkenyl group with 2-10 carbon atoms, alkynyl group with 2-10 carbon atoms, and oxaalkyl group with 2-10 carbon atoms.

[0120] Specifically, the electrolyte may also include additives, such as the first additive. The first additive is a negative electrode film-forming additive that can form a stable SEI (Solid Electrolyte Interface) film on the surface of the negative electrode active material layer, reducing direct contact between the negative electrode active material and the electrolyte. Introducing the compound shown in formula (I) into the electrolyte as a solvent or as part of a solvent helps improve the lithium-ion conductivity of the electrolyte. However, the compound shown in formula (I) may undergo side reactions on the surface of the negative electrode active material layer during the charging and discharging process of the lithium secondary battery, generating gas. This will consume the electrolyte in the lithium secondary battery and cause damage to the negative electrode active material and the structure of the negative electrode active material layer, affecting the cycle performance and safety performance of the lithium secondary battery.

[0121] Therefore, in this embodiment, by introducing a first additive into the electrolyte, the negative electrode active material layer can be effectively protected from direct contact with the electrolyte, the stability of the solid-liquid interface between the negative electrode active material layer and the electrolyte can be improved, and the possibility of gas generation on the surface of the negative electrode active material layer of the compound shown in formula (I) can be reduced. This helps to improve the cycle performance of the lithium secondary battery while achieving high energy density and fast charging performance.

[0122] Optionally, the first additive includes at least one of the following compounds:

[0123]

[0124]

[0125] Optionally, the mass fraction W2 of the first additive in the electrolyte satisfies: 0.01% ≤ W2 ≤ 20%; optionally, 0.1% ≤ W2 ≤ 10%; optionally, 0.2% ≤ W2 ≤ 5%.

[0126] Specifically, W2 can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or a value thereof within a range obtained by any two of the above numerical combinations.

[0127] Optionally, the mass fraction W2 of the first additive in the electrolyte, the mass fraction W1 of the compound of formula (I) in the electrolyte, and the lithium ion diffusion coefficient Dsof the positive active material layer satisfy: 2 x 10 -22 cm 2 / s≤ W2 x W1 x Ds≤ 1.6 x 10 -6 cm 2 / s; optionally, 2 x 10 -21 cm 2 / s≤ W2 x W1 x Ds≤ 8 x 10 -7 cm 2 / s; optionally, 4 x 10 -21 cm 2 / s≤ W2 x W1 x Ds≤ 4 x 10 -7 cm 2 / s.

[0128] Specifically, W2 x W1 x Dsmay be 2 x 10 -22 cm 2 / s, 2 x 10 -21 cm 2 / s, 4 x 10 -21 cm 2 / s, 2 x 10 -20 cm 2 / s, 2 x 10 -19 cm 2 / s, 2 x 10 -18 cm 2 / s, 2 x 10 -17 cm 2 / s, 2 x 10 -16 cm 2 / s, 2 x 10 -15 cm 2 / s, 2 x 10 -14 cm 2 / s, 2 x 10 -13 cm 2 / s, 2 x 10-12 cm 2 / s, 2 x 10 -11 cm 2 / s, 2 x 10 -10 cm 2 / s, 2 x 10 -9 cm 2 / s, 2 x 10 - 8 cm 2 / s, 4 x 10 -7 cm 2 / s, 8 x 10 -7 cm 2 / s, 1.6 x 10 -6 cm 2 / s, or a value within a range defined by any two of the above value combinations.

[0129] Optionally, the mass fraction W2 of the first additive in the electrolyte is positively correlated with the mass fraction W1 of the compound of formula (I) in the electrolyte.

[0130] Specifically, as the content of the compound of formula (I) in the electrolyte increases, the possibility of the compound of formula (I) generating gas on the surface of the negative active material layer increases, and therefore, more first additive is needed to form a film on the surface of the negative active material layer to reduce the possibility of the compound of formula (I) generating gas on the surface of the negative active material layer by reduction reaction.

[0131] Optionally, the electrolyte comprises a second additive, and the second additive comprises at least one of M a+ [F w PO t ] b- , M a+ [BF4] - a , M a+ [FSO3] - a and a compound of formula (III):

[0132]

[0133] wherein M a+at least one of lithium ion, sodium ion, potassium ion, rubidium ion, cesium ion, magnesium ion, calcium ion, barium ion, aluminum ion, iron ion, copper ion, nickel ion, organic cation, a, b, c each represent a natural number; t represents an integer of 0-3, w represents an integer of 1-6; X includes at least one of halogen atoms, n represents an integer of 0-4; Y includes at least one of boron atoms, phosphorus atoms; R4 includes substituted or unsubstituted alkylene group with carbon number of 1-10, substituted or unsubstituted haloalkylene group with carbon number of 1-10, substituted or unsubstituted arylene group with carbon number of 6-20, substituted or unsubstituted haloarylene group with carbon number of 6-20, q represents an integer of 0-1, m represents an integer of 1-3.

[0134] Specifically, the electrolyte can further include another negative electrode film-forming additive, i.e., the second additive. The second additive can also form a stable SEI film on the surface of the negative electrode active material layer, thereby isolating the electrolyte and the negative electrode active material layer, protecting the negative electrode active material, reducing the possibility of gas generation of the electrolyte on the surface of the negative electrode active material layer, and further improving the cycle performance of the lithium secondary battery. In addition, the second additive has little effect on the impedance of the lithium secondary battery after film formation, reduces the possibility of lithium deposition of the lithium secondary battery in the charging and discharging process due to excessive impedance, and helps to improve the charging and discharging performance of the battery.

[0135] It should be understood that the electrolyte can only include the first additive, only include the second additive, or include both the first additive and the second additive. In the case where the electrolyte includes both the first additive and the second additive, the first additive and the second additive exert a synergistic effect, and together form an SEI film on the surface of the negative electrode active material layer, which has a good film formation effect and little effect on the impedance of the lithium secondary battery.

[0136] Optionally, the mass fraction W3 of the second additive in the electrolyte satisfies: 0.01%≤W3≤20%; optionally, 0.1%≤W3≤10%; optionally, 0.2%≤W3≤5%.

[0137] Specifically, W3 can be 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or a value within the range obtained by any two of the above combinations.

[0138] Optionally, the mass fraction W3 of the second additive in the electrolyte, the mass fraction W1 of the compound represented by formula (I) in the electrolyte, and the lithium ion diffusion coefficient Ds of the positive electrode active material layer satisfy: 2 x 10 -22 cm 2 / s≤ W3 x W1 x Ds≤ 1.6 x 10 -6 cm 2 / s; optionally, 2 x 10 -21 cm 2 / s≤ W3 x W1 x Ds≤ 8 x 10 -7 cm 2 / s; optionally, 4 x 10 -21 cm 2 / s≤ W3 x W1 x Ds≤ 4 x 10 -7 cm 2 / s.

[0139] Specifically, W3 x W1 x Ds can be 2 x 10 -22 cm 2 / s, 2 x 10 -21 cm 2 / s, 4 x 10 -21 cm 2 / s, 2 x 10 -20 cm 2 / s, 2 x 10 -19 cm 2 / s, 2 x 10 -18 cm 2 / s, 2 x 10 -17 cm 2 / s, 2 x 10 -16 cm 2 / s, 2 x 10 -15 cm 2 / s, 2 x 10 -14 cm 2 / s, 2 x 10 -13 cm 2 / s, 2 x 10 -12 cm 2 / s, 2 x 10 -11 cm 2 / s, 2 x 10 -10 cm 2 / s, 2 x 10 -9 cm 2 / s, 2 x 10 - 8 cm 2 / s, 4 x 10 -7 cm 2 / s, 8 x 10 -7 cm 2 / s, 1.6 x 10 -6 cm 2 / s, or a value within a range obtained by any two of the above value combinations.

[0140] Optionally, the mass fraction W3 of the second additive in the electrolyte is positively correlated with the mass fraction W1 of the compound of formula (I) in the electrolyte.

[0141] Similar to the first additive, as the content of the compound of formula (I) in the electrolyte increases, the possibility of the compound of formula (I) generating gas on the surface of the negative active material layer increases, and therefore, more negative film-forming additives are needed to form a film on the surface of the negative active material layer to reduce the possibility of the compound of formula (I) generating gas on the surface of the negative active material layer by reduction reaction.

[0142] Optionally, in M a+ [FSO3] - a M a+ includes at least one of Li + , Na + , K + , Rb + , Cs + , Mg 2+ , Ca 2+ , Ba 2+ , Al 3+ , Fe 2+ , Cu 2 + , Fe 3+ , Ni 2+ , Ni 3+ .

[0143] Optionally, the electrolyte can further include additives capable of improving certain performance of the battery, such as additives for improving overcharge performance of the battery, additives for improving high-temperature or low-temperature performance of the battery, etc.

[0144] [Positive electrode sheet]

[0145] The positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector.

[0146] As an example, the positive current collector has two opposite surfaces in the thickness direction of itself, and the positive active material layer is disposed on any one or both of the two opposite surfaces of the positive current collector.

[0147] Optionally, the positive electrode current collector can employ a metal foil or a composite current collector. For example, as the metal foil, an aluminum foil can be employed. The composite current collector can 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 can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0148] Optionally, the positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material.

[0149] Optionally, the first positive electrode active material can employ a positive electrode active material for a battery known in the art. As an example, the first positive electrode active material can include at least one of a lithium-containing phosphate of an olivine structure, a lithium transition metal oxide, and a modified compound of each thereof. However, the present application is not limited to these materials, and other conventional materials that can be used as a positive electrode active material for a battery can also be used. These positive electrode active materials can be used alone only one or in combination of two or more. Among them, examples of the lithium transition metal oxide can 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 (which can also be referred to as NCM333), LiNi 0.5 Co 0.2 Mn 0.3 O2 (which can also be referred to as NCM523), LiNi 0.5 Co 0.25 Mn 0.25 O2 (which can also be referred to as NCM211), LiNi 0.6 Co 0.2 Mn 0.2 O2 (which can also be referred to as NCM622), LiNi 0.8 Co 0.1 Mn 0.1 O2 (which can also be referred to as NCM811), lithium nickel cobalt aluminum oxide (such as LiNi 0.85 Co 0.15 Al 0.05LiFePO4, also can be referred to as LFP, a composite of lithium iron phosphate and carbon, lithium manganese phosphate (e.g., LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon.

[0150] Optionally, the average (volume) particle diameter Dv501 of the first positive electrode active material satisfies: 8 μm ≤ Dv501 ≤ 50 μm.

[0151] Specifically, Dv501 can be 8 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, or a value within a range obtained by combining any two of the above values.

[0152] Optionally, the second positive electrode active material can also be a positive electrode active material for a battery known in the art, as long as the average (volume) particle diameter of the second positive electrode active material is smaller than the average (volume) particle diameter of the selected first positive electrode active material. As an example, the second positive electrode active material can include at least one of: a lithium-containing phosphate with an olivine structure, a lithium transition metal oxide, and a modified compound of each thereof. Among them, examples of the lithium transition metal oxide can include, but are not limited to, at least one of: a lithium cobalt oxide, a lithium nickel oxide, a lithium manganese oxide, a lithium nickel cobalt oxide, a lithium manganese cobalt oxide, a lithium nickel manganese oxide, a lithium nickel cobalt manganese oxide, a lithium nickel cobalt aluminum oxide, and a modified compound thereof. Examples of the lithium-containing phosphate with an olivine structure can include, but are not limited to, at least one of: lithium iron phosphate, a composite of lithium iron phosphate and carbon, lithium manganese phosphate, a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon. These positive electrode active materials can be used alone or in combination of two or more.

[0153] Optionally, the average (volume) particle diameter Dv502 of the second positive electrode active material satisfies: 0.02 μm ≤ Dv502 ≤ 8 μm.

[0154] Specifically, Dv502 can be 0.02 μm, 0.04 μm, 0.06 μm, 0.08 μm, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 4 μm, 6 μm, 8 μm, or a value within a range obtained by combining any two of the above values.

[0155] Optionally, the mass fraction W4 of the second positive electrode active material in the positive electrode active material layer satisfies: 0% ≤ W4 ≤ 60%; further optionally, 20% ≤ W4 ≤ 40%.

[0156] Specifically, W4 can be 0%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 25%, 30%, 40%, 50%, 60%, or a numerical range within any two of the above-mentioned numerical combinations.

[0157] It should be understood that the first positive electrode active material and the second positive electrode active material can be the same kind of active material. For example, the first positive electrode active material is lithium iron phosphate with a micron-level average volume particle size, and the second positive electrode active material is lithium iron phosphate with a nanometer-level average volume particle size. The first positive electrode active material and the second positive electrode active material can also be different kinds of active materials. For example, the first positive electrode active material is lithium iron phosphate with a micron-level average particle size, and the second positive electrode active material is lithium iron manganese phosphate with a nanometer-level average volume particle size.

[0158] In this embodiment, by matching the first active material with a larger average (volume) particle size and the second active material with a smaller average (volume) particle size, the powder compaction density of the positive electrode active material layer is improved, thereby improving the energy density of the lithium secondary battery.

[0159] Optionally, the positive electrode active material layer further includes a binder. As an example, the binder can 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.

[0160] Optionally, the positive electrode active material layer further includes a conductive agent. As an example, the conductive agent can include at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0161] In some embodiments, the positive electrode sheet can be prepared by forming the above-mentioned components for preparing the positive electrode sheet into a positive electrode slurry, respectively. For example, the first positive electrode active material and / or the second positive electrode active material, the conductive agent, the binder, and any other components are dispersed in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry. Then, the positive electrode slurry is coated on at least one side of the positive electrode current collector, and after processes such as drying, cold pressing, etc., the positive electrode sheet is obtained.

[0162] [Negative electrode sheet]

[0163] The negative electrode sheet generally includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material.

[0164] As an example, the negative current collector has two surfaces opposite in the thickness direction thereof, and the positive active material layer is provided on either one or both of the two surfaces of the negative current collector.

[0165] Optionally, the negative current collector can adopt a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be adopted. The composite current collector can include a high polymer material base layer and a metal layer formed on at least one surface of the high polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a high polymer material base layer (such as a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0166] Optionally, the average (volume) particle size Dv503 of the negative active material satisfies: 6 μm ≤ Dv503; optionally, 15 μm ≤ Dv503 ≤ 20 μm.

[0167] Specifically, Dv503 can be 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, or a value within a range obtained by combining any two of the above values.

[0168] In the embodiment, by setting the average (volume) particle size of the negative active material within a suitable range, the powder compaction density of the negative active material layer is improved. After the first additive and / or the second additive form the SEI film on the surface of the negative active material layer, the average (volume) particle size of the negative active material on the surface of the negative active material layer increases, and the specific surface area decreases, which helps to further reduce the contact between the negative active material and the electrolyte, thereby reducing the probability of gas generation on the surface of the negative active material layer by the electrolyte, and helping to improve the cycle performance of the lithium secondary battery.

[0169] Optionally, the specific surface area of the negative active material satisfies: 0.5 m 2 / g ≤ BET ≤ 2.0 m 2 / g; optionally, 0.8 m 2 / g ≤ BET ≤ 1.5 m 2 / g.

[0170] Specifically, BET can be 0.5 m 2 / g, 0.6 m 2 / g, 0.7 m 2 / g, 0.8 m 2 / g, 0.9 m 2 / g, 1.0 m 2 / g, 1.1 m 2 / g, 1.2 m 2 / g, 1.3 m 2 / g, 1.5 m 2 / g, 1.6 m 2 / g, 1.7 m 2 / g, 1.8 m 2 / g, 1.9 m 2 / g, 2.0 m 2 / g, or a value within a range obtained by combining any two of the above-mentioned values.

[0171] In this embodiment, by setting the specific surface area of the negative electrode active material within a suitable range, after the first additive and / or the second additive form the SEI film on the surface of the negative electrode active material layer, the specific surface area of the negative electrode active material on the surface of the negative electrode active material layer is further reduced, which helps to reduce the probability of gas generation on the surface of the negative electrode active material layer, thereby further improving the cycle performance of the lithium secondary battery.

[0172] Optionally, the negative electrode active material layer further comprises a binder. The binder can 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).

[0173] Optionally, in an embodiment, the negative electrode film layer further comprises a conductive agent. The conductive agent can be selected from at least one of super-conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0174] Optionally, the negative electrode film layer further comprises other auxiliary agents, such as thickening agents (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0175] In some embodiments, the negative electrode sheet can be prepared by forming the above-mentioned components for preparing the negative electrode sheet into a negative electrode slurry, respectively. For example, the negative electrode active material, the conductive agent, the binder, and any other components are dispersed in a solvent (e.g., N-methyl pyrrolidone) to form a negative electrode slurry. Then, the negative electrode slurry is coated on a negative electrode current collector, and after processes such as drying and cold pressing, a negative electrode sheet is obtained.

[0176] [Separator]

[0177] Optionally, in an embodiment, the lithium secondary battery further comprises a separator. The type of the separator is not particularly limited in the present application, and for example, any known porous structure separator film having good chemical stability and mechanical stability can be used.

[0178] Optionally, in one embodiment, the material of the separator film can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator film can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator film is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0179] Optionally, in one embodiment, the positive electrode tab, the negative electrode tab, and the separator film can be used to make an electrode assembly through a winding process or a stacking process.

[0180] Optionally, in one embodiment, the lithium secondary battery includes an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0181] Optionally, in one embodiment, the outer package of the lithium secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the lithium secondary battery can also be a soft package, such as a pouch-type soft package. The material of the soft package can be plastic, and as the plastic, polypropylene, polybutylene terephthalate, and polybutylene succinate, etc. can be listed.

[0182] The shape of the lithium secondary battery is not particularly limited in the present application, and can be cylindrical, square, or any other shape. For example, Figure 1 is a square structure lithium secondary battery 10 as an example.

[0183] In some embodiments, referring to Figure 2 , the outer package can include a shell 22 and a cover plate 21. The shell 22 can 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 22 has an opening communicating with the receiving cavity, and the cover plate 21 can be arranged on the opening to close the receiving cavity. The positive electrode tab, the negative electrode tab, and the separator can be used to form an electrode assembly 23 through a winding process or a stacking process. The electrode assembly 23 is packaged in the receiving cavity. The electrode assembly 23 is soaked in the electrolyte. The number of electrode assemblies 23 contained in the lithium secondary battery 10 can be one or more, and the person skilled in the art can select according to the specific actual needs.

[0184] In some embodiments, the lithium secondary battery 10 can be assembled into a battery module, and the number of lithium secondary batteries 10 contained in the battery module can be one or more, and the specific number can be selected by the person skilled in the art according to the application and capacity of the battery module.

[0185] Figure 3 is a battery module 300 as an example. Referring to Figure 3In the battery module 300, the plurality of lithium secondary batteries 10 can be arranged in sequence along the length direction of the battery module 300. Of course, the plurality of lithium secondary batteries 10 can also be arranged in any other manner. The plurality of lithium secondary batteries 10 can be further fixed by fasteners.

[0186] Optionally, in an embodiment, the battery module 300 can further include a housing having an accommodation space, and the plurality of lithium secondary batteries 300 are accommodated in the accommodation space.

[0187] Optionally, in an embodiment, the battery module 300 described above can also be assembled into a battery pack, and the number of battery modules 300 contained in the battery pack can be one or more, and the specific number can be selected by a person skilled in the art according to the application and capacity of the battery pack.

[0188] Figure 4 And Figure 5 is a battery pack 400 as an example. Referring to Figure 4 And Figure 5 In the battery pack 400, a battery box and a plurality of battery modules 300 arranged in the battery box can be included. The battery box includes an upper box body 401 and a lower box body 402, and the upper box body 401 can cover the lower box body 402 and form a closed space for accommodating the battery module 300. The plurality of battery modules 300 can be arranged in the battery box in any manner.

[0189] The lithium secondary battery 10 described in the present application can also be referred to as a lithium battery cell. Optionally, in an embodiment, a plurality of lithium battery cells are first integrated into a battery module, and then the battery module is installed in a battery box to form a battery pack. In other production and processing technologies, a plurality of lithium battery cells can also be directly installed in a battery box to form a battery pack, eliminating the intermediate state of the battery module, thereby reducing the mass of the battery pack and improving the energy density of the battery pack.

[0190] Figure 6 is a schematic view of an electrical device 600 provided by the present application.

[0191] It should be understood that the electrical device 600 includes at least one of the lithium secondary battery 10, the battery module 300, or the battery pack 400 provided by the present application. The lithium secondary battery 10, the battery module 300, or the battery pack 400 can be used as a power source of the electrical device 600, or can be used as an energy storage unit of the electrical device 600. The electrical device 600 can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0192] As an electrical device 600, a lithium secondary battery 10, a battery module 300, or a battery pack 400 can be selected according to its usage requirements.

[0193] As an example, electrical device 600, such as Figure 6 As shown, the electrical device 600 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. To meet the high power and high energy density requirements of the lithium secondary battery 10 for the electrical device 600, a battery pack 400 or a battery module 300 can be used.

[0194] As another example, the power-consuming device 600 could be a mobile phone, tablet computer, laptop computer, etc. This device typically requires a slim and lightweight design and can use a lithium-ion battery 10 as its power source.

[0195] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0196] Example 1

[0197] (1) Preparation of positive electrode sheet

[0198] Lithium iron phosphate with a Dv50 of 10 μm was selected as the positive electrode active material. The positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride (PVDF) were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 90:5:5. After thorough mixing, a slurry was obtained. This slurry was uniformly coated onto the positive electrode current collector aluminum foil, and then dried, cold-pressed, and slit to obtain the positive electrode sheet. The lithium-ion diffusion coefficient Ds of the positive electrode active material layer in the positive electrode sheet was 3.4 × 10⁻⁶. -12 cm 2 / s, the compacted density of the positive electrode powder is 2.5 g / cm³. 3 .

[0199] (2) Preparation of negative electrode sheet

[0200] Natural graphite was selected as the negative electrode active material. The negative electrode active material, conductive agent acetylene black, binder styrene-butadiene rubber (SBR), and thickener sodium carboxymethyl cellulose (CMC) were dissolved in deionized water at a mass ratio of 90:4:4:2. After thorough mixing, a negative electrode slurry was obtained. This slurry was uniformly coated onto the negative electrode current collector copper foil, and then dried, cold-pressed, and slit to obtain negative electrode sheet 1. The specific surface area of ​​the negative electrode active material in negative electrode sheet 1 is 0.8 m². 2 / g.

[0201] (3) Preparation of electrolyte

[0202] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), ethylene carbonate (EC) and methyl ethyl carbonate (EMC) were mixed uniformly in a volume ratio of 3:7, and then the compound represented by formula (I-II) was added to prepare a solvent. The amount of the compound represented by formula (I-II) was adjusted so that the mass fraction W1 of the compound represented by formula (I-II) in the solvent was 60%, and then mixed uniformly. Then, the solute lithium hexafluorophosphate (LiPF6) was added to be dissolved in the solvent so that the mass fraction of the solute in the electrolyte was 12.5%, and then stirred uniformly to obtain the electrolyte.

[0203] (4) Assembly of lithium secondary battery

[0204] The positive electrode sheet, the separator, and the negative electrode sheet were stacked in order so that the separator was between the positive electrode sheet and the negative electrode sheet and could separate the positive electrode sheet and the negative electrode sheet. Then, the stacked components were wound to obtain an electrode assembly. The electrode assembly was arranged in a housing, and after drying, the electrolyte was injected. After processes such as formation and standing, the lithium secondary battery of Example 1 was obtained.

[0205] Example 2

[0206] Compared with the lithium secondary battery of Example 1, Example 2 selected lithium iron phosphate with a Dv50 of 10 μm as the positive electrode active material, and the lithium ion diffusion coefficient Ds of the positive electrode active material layer in the positive electrode sheet was 2.7 × 10 -9 cm 2 / s, and the mass fraction W1 of the compound represented by formula (I-II) in the solvent was 70%.

[0207] Example 3

[0208] Compared with the lithium secondary battery of Example 1, Example 3 selected lithium iron phosphate with a Dv50 of 10 μm as the positive electrode active material, and the lithium ion diffusion coefficient Ds of the positive electrode active material layer in the positive electrode sheet was 2.7 × 10 -9 cm 2 / s.

[0209] Example 4

[0210] Compared with Example 1, Example 4 included the compound represented by formula (I-V) in the solvent, and the mass fraction W1 of the compound represented by formula (I-V) in the solvent was 60%.

[0211] Example 5

[0212] Compared with Example 1, the solvent of Example 5 comprises a compound represented by formula (I-VIII), and the mass fraction W1 of the compound represented by formula (I-VIII) in the solvent is 60%.

[0213] Example 6

[0214] Compared with Example 1, the electrolyte of Example 6 comprises a first additive, which is a compound represented by formula (II-XXXII), and the mass fraction W2 of the compound represented by formula (II-XXXII) in the electrolyte is 1.0%.

[0215] Example 7

[0216] Compared with Example 1, the first additive in Example 7 is a compound represented by formula (II-XVII), and the mass fraction W2 of the compound represented by formula (II-XVII) in the electrolyte is 1.0%.

[0217] Example 8

[0218] Compared with Example 6, the first additive in Example 8 is a compound represented by formula (II-XXXII), and the mass fraction W2 of the compound represented by formula (II-XXXII) in the electrolyte is 1.5%.

[0219] Example 9

[0220] Compared with Example 1, the electrolyte of Example 9 comprises a second additive, which is lithium difluoro(oxalato)borate LiDFOB, and the mass fraction W3 of LiDFOB in the electrolyte is 0.05%.

[0221] Example 10

[0222] Compared with Example 1, the second additive in Example 10 is lithium difluorodioxalate phosphate LiBODFP, and the mass fraction W3 of LiBODFP in the electrolyte is 1.0%.

[0223] Example 11

[0224] Compared with Example 2, the second additive in Example 11 is LiDFOB, the mass fraction W1 of the compound represented by formula (I-II) in the solvent is 70%, and the mass fraction W3 of LiDFOB in the electrolyte is 1.0%.

[0225] Example 12

[0226] Compared with Example 1, the first additive in Example 12 is a compound represented by Formula (I-II), the second additive is LiDFOB, the mass fraction W2 of the compound represented by Formula (I-II) in the electrolyte is 0.5%, and the mass fraction W3 of LiDFOB in the electrolyte is 0.5%.

[0227] Example 13

[0228] Compared with Example 1, Example 13 selects lithium iron phosphate with a Dv50 of 10 μm as the first positive electrode active material, lithium iron phosphate with a Dv50 of 2 μm as the second positive electrode active material, dissolves the first positive electrode active material, the second positive electrode active material, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 63:27:5:5 in the solvent N-methyl pyrrolidone (NMP), uniformly mixes after sufficient stirring, uniformly coats the slurry on the positive electrode current collector aluminum foil, and then performs drying, cold pressing, and slitting to obtain a positive electrode sheet. The mass fraction W4 of the second positive electrode active material in the positive electrode active material layer is 30%, and the powder compacted density of the positive electrode sheet is 2.6 g / cm 3 .

[0229] Example 14

[0230] Compared with Example 13, the mass fraction W4 of the second positive electrode active material in the positive electrode active material layer in Example 14 is 10%, and the powder compacted density of the positive electrode sheet is 2.55 g / cm 3 .

[0231] Example 15

[0232] Compared with Example 1, Example 15 selects artificial graphite with a specific surface area of 1.5 m 2 / g as the negative electrode active material.

[0233] Example 16

[0234] Compared with Example 1, the solvent in Example 16 simultaneously includes a compound represented by Formula (I-II) and a compound represented by Formula (I-I). The mass ratio of the compound represented by Formula (I-II) to the compound represented by Formula (I-I) is 30:30.

[0235] Comparative Example 1

[0236] Compared with Example 1, Comparative Example 1 selects dimethyl carbonate (DMC) as the solvent.

[0237] Comparative Example 2

[0238] Compared with Example 1, lithium iron phosphate is selected as the positive electrode active material, and the lithium ion diffusion coefficient Ds of the positive electrode active material layer is 7.3 x 10-20 cm 2 / s, the mass fraction W1 of the compound represented by formula (I-II) in the solvent is 60%.

[0239] The product parameters of the examples and comparative examples are shown in Table 1.

[0240] Table 1: Product parameters of different examples and comparative examples

[0241]

[0242]

[0243] In Table 1, Ds is the lithium ion diffusion coefficient of the positive active material layer of the positive electrode tab, the positive electrode powder compaction density is the powder compaction density of the positive electrode tab, the negative electrode BET is the BET of the negative active material, the compound is the compound included in the solvent, W1 is the mass fraction of the compound in the solvent, W2 is the mass fraction of the first additive in the electrolyte, and W3 is the mass fraction of the second additive in the electrolyte.

[0244] It should be understood that the difference in lithium ion diffusion coefficient Ds is usually directly reflected in the order of magnitude, so the lithium ion diffusion coefficient Ds of the positive active material layer in the above table shows the order of magnitude of the lithium ion diffusion coefficient. For example, Ds in Example 1 is specifically 3.4 x 10 -12 , and only 10 -12 is shown in Table 1.

[0245] The battery performance test results of Examples 1-16 and Comparative Examples 1-2 are shown in Table 2.

[0246] Table 2: Battery performance test results of different examples and comparative examples

[0247]

[0248]

[0249] As can be seen from the comparison of Example 1, Example 4, Example 5, and Example 16 with Comparative Example 1, by introducing the compounds represented by formula (I-II), formula (I-V), formula (I-VIII), and formula (I-I) in the solvent, the problem of mismatch between the lithium ion diffusion capacity of the positive active material layer and the electrolyte in the lithium secondary battery is improved, so that the discharge capacity, fast charging capacity, and cycle life of the lithium secondary battery in Example 1, Example 4, Example 5, and Example 16 are all better than those of Comparative Example 1 which does not contain the compound represented by formula (I) in the solvent, and the lithium secondary battery in Example 1, Example 4, Example 5, and Example 16 also has a lower volume expansion rate and a significantly improved safety performance.

[0250] A comparison between Example 1 and Comparative Example 2 shows that, when the compound of formula (I) is introduced into the solvent, the product of the mass fraction W1 of the compound in the solvent and the lithium-ion diffusion coefficient Ds of the positive electrode active material layer in Example 1, W1×Ds, is 6×10. -11 cm 2 / s, in 2×10 -18 cm 2 / s-8×10 -6 cm 2 Within the interval of / s, further, in 3×10 -14 cm 2 / s-7×10 -10 cm 2 Within the range of / s. In Comparative Example 2, W1×Ds is 9×10. -19 cm 2 / s, exceeding the aforementioned range, resulted in Comparative Example 2 exhibiting inferior discharge capacity, fast charging capability, and cycle life compared to Example 1. This demonstrates that controlling Ds×W1 within a suitable range can mitigate the mismatch between the positive electrode active material layer and the electrolyte's lithium-ion diffusion capacity in lithium-ion secondary batteries, thereby improving the discharge capacity, fast charging capability, cycle life, and safety performance of lithium-ion secondary batteries.

[0251] A comparison between Example 3 and Example 2 shows that when Ds increases, Example 2 also increases W1, while Example 3 does not. The performance of the lithium secondary battery in Example 3 is slightly inferior to that in Example 2. Therefore, controlling the positive correlation between Ds and W1 helps to improve the discharge capacity, fast charging capability, cycle life, and safety performance of the lithium secondary battery.

[0252] A comparison of Examples 6 and 7 with Example 1 shows that Examples 6 and 7 introduced compounds of formulas (II-XXXII) and (II-XVII) as first additives into the electrolyte, respectively. The lithium secondary batteries in Examples 6 and 7 exhibited significantly better cycle performance and lower volume expansion rates than the lithium secondary battery in Example 1. This demonstrates that introducing a first additive into the electrolyte can improve the lithium-ion conductivity of the electrolyte while effectively reducing side reactions on the surface of the negative electrode active material layer, thereby improving the cycle performance and safety performance of the lithium secondary battery.

[0253] A comparison of Example 8 and Example 6 shows that when W1 increases, W2 increases in Example 8. The volume expansion rate of the lithium secondary battery in Example 8 is lower than that in Example 6. Therefore, by controlling the positive correlation between W2 and W1, it is helpful to further improve the safety performance of the lithium secondary battery.

[0254] As can be seen from the comparison of Example 9, Example 10 and Example 1, the lithium secondary batteries in Example 9 and Example 10 have the second additive LiDFOB and LiBODFP introduced into the electrolyte respectively, and the cycle performance of the lithium secondary batteries in Example 9 and Example 10 is obviously superior to that of the lithium secondary battery in Example 1, and the volume expansion rate of the lithium secondary batteries in Example 9 and Example 10 is less than that of the lithium secondary battery in Example 1. Therefore, it is illustrated that the introduction of the second additive into the electrolyte can effectively reduce the side reaction of the electrolyte on the surface of the negative active material layer while improving the lithium ion conduction ability of the electrolyte, thereby improving the cycle performance and safety performance of the lithium secondary battery.

[0255] As can be seen from the comparison of Example 11 and Example 9, in the case of increasing W1, Example 11 increases W3. The volume expansion rate of the lithium secondary battery in Example 11 is lower than that of the lithium secondary battery in Example 9. Therefore, by controlling W3 to be positively correlated with W1, it is helpful to further improve the safety performance of the lithium secondary battery.

[0256] As can be seen from the comparison of Example 12 and Example 6, Example 9, Example 12 simultaneously introduces the first additive and the second additive into the electrolyte, and the cycle performance of the lithium secondary battery in Example 12 is significantly improved, and the volume expansion rate is significantly reduced. Therefore, it can be seen that the first additive and the second additive have a synergistic effect in the electrolyte, which can further improve the cycle performance and safety performance of the lithium secondary battery.

[0257] As can be seen from the comparison of Example 13 and Example 1, the second positive active material with a smaller Dv502 is introduced into the positive active material layer of the lithium secondary battery in Example 13, and the powder compaction density of the positive electrode sheet is increased. Compared with Example 1, the discharge capacity of the lithium secondary battery in Example 13 is improved. Therefore, it is illustrated that by matching positive active materials with different particle sizes, it is helpful to improve the powder compaction density of the positive active material layer, thereby helping to improve the discharge capacity of the lithium secondary battery.

[0258] As can be seen from the comparison of Example 14 and Example 13, the mass fraction W4 of the second positive active material with a smaller Dv502 in the positive active material layer in Example 14 is reduced, the powder compaction density of the positive electrode sheet is reduced, and the discharge capacity of the lithium secondary battery is also reduced. Therefore, it is illustrated that by controlling the mass fraction W4 of the second positive active material with a smaller average volume particle size in the positive active material layer, the powder compaction density of the positive electrode sheet can be affected, thereby affecting the energy density of the lithium secondary battery and further affecting the discharge capacity of the lithium secondary battery.

[0259] It should be understood that the average volume particle size of a material and its specific surface area are interrelated physical quantities. The larger the average volume particle size of a material, the smaller its BET is generally; and the smaller the average volume particle size of a material, the larger its BET is generally.

[0260] As can be seen by comparing Example 15 with Example 1, Example 15 uses a negative active material with a smaller Dv503 and a larger BET, and the discharge capacity, fast-charging performance and cycle performance of the lithium secondary battery in Example 1 are all superior to those of the lithium secondary battery in Example 15, and the volume expansion rate of the lithium secondary battery in Example 1 is smaller. Thus, it can be seen that the average volume particle size or specific surface area of the negative active material has an impact on the performance of the lithium secondary battery, and the negative active material with a larger Dv503 and a smaller BET has a smaller contact area with the electrolyte, which can reduce the probability of side reactions of the electrolyte on the surface of the negative active material layer, thereby helping to improve the performance of the lithium secondary battery.

[0261] Next, the test methods of the physical parameters and performance parameters mentioned in the examples of the present application are briefly introduced.

[0262] 1. Test method of lithium ion diffusion coefficient

[0263] The diffusion coefficient of lithium ions can be tested by using conventional methods in the art, such as cyclic voltammetry (CV method), electrochemical impedance method (EIS method), constant potential intermittent titration method (PITT method), constant current intermittent titration method (GITT method), etc., and is calculated according to Fick's first law and Fick's second law.

[0264] As an example, the steps of testing the lithium ion diffusion coefficient of the positive electrode active material layer by electrochemical impedance method are as follows: the positive electrode active material, acetylene black conductive agent and polyvinylidene fluoride binder prepared in the examples and comparative examples of the present application are mixed uniformly in a mass ratio of 85:10:5, N-methyl pyrrolidone is added to prepare a slurry, which is coated on an aluminum sheet, and after vacuum drying at 60°C for 5h, a positive electrode sheet sample is prepared. The prepared sample is used as the positive electrode, a lithium metal sheet is used as the negative electrode, a Celgard2400 polypropylene microporous membrane is used as the separator, a 1 mol / L LiPF6, ethylene carbonate (EC) and diethyl carbonate (DEC) solution is mixed in a volume ratio of 1:1 and used as the electrolyte, and a CR2032 button-type half-cell sample is assembled in a high argon environment. The electrochemical impedance (EIS) test of the battery is carried out by using a Shanghai Chenhua CHI660b electrochemical workstation, the amplitude is 5mV, and the frequency range is 10mHz-100KHz. The electrochemical impedance spectrum (Nyquist spectrum) of the battery sample can be obtained. By curve fitting the electrochemical impedance spectrum, the Warburg constant of the battery sample can be calculated according to the second Fick's law and the Nernst equation, and the lithium ion diffusion coefficient of the positive electrode active material layer can be calculated by the following formula combined with the Bulter-Volmer equation.

[0265]

[0266] wherein σ is the Warburg constant, F is the Faraday constant, S is the surface area of the positive electrode (unit: cm 2 ), V m is the molar volume of the positive electrode active material (unit: cm 2 / mol), and dE / dx is the positive electrode coulometric titration straight line slope. The coulometric titration straight line can be tested by a method known in the art. For example, it can be tested by a constant current electrolysis device to obtain the coulometric titration straight line.

[0267] 2. Test method of average (volume) particle size

[0268] In the present application, the average volume particle size Dv50 of the active material has the meaning known in the art and can be determined by using an instrument and method known in the art. For example, it can be determined by referring to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method and using a laser particle size analyzer (e.g. Master Size 3000).

[0269] 3. Test method of BET

[0270] The test method refers to the standard GB / T19587-2004 Gas Adsorption BET Method for Determining Specific Surface Area of Solid Substances.

[0271] Take 8g-15g of the sample to be tested and load it into the sample tube, and record the initial mass of the sample to be tested. Weigh the sample to be tested and load it into the device NOVA2000e. Then start the degassing, and after heating the sample to be tested to 200℃, keep it for 2h. Then record the mass of the sample to be tested after degassing. Then re-load the degassed sample into the device, pour into liquid nitrogen for BET test. Set the nitrogen pressure to 0.08MPa-0.12MPa, and the heating temperature to 40℃-350℃. After the test, read the specific surface area from the test results.

[0272] 4. Test method of powder compacted density

[0273] Take a circular positive electrode tab with an area of 1540.25mm 2 as the basic unit, wherein the total weight of the positive electrode tab coated with positive electrode active material on both sides is A(g), the weight of the positive electrode current collector is B(g), the thickness of the positive electrode current collector is T(mm), and the thickness of the aluminum foil is U(mm). The powder compacted density (PD) of the positive electrode tab can be calculated by the following formula.

[0274]

[0275] 5. Battery capacity test

[0276] At 25℃, charge the lithium secondary battery at 0.5C constant current to 3.65V, then charge at 3.65V constant voltage until the current is less than 0.05C, then discharge the lithium secondary battery at 0.5C constant current to 2.5V, and get the discharge capacity at 0.5C.

[0277] 6. Fast charging time test

[0278] At 25℃, charge to 10% state of charge (SOC) at 0.33C, charge for s minutes according to the given fast charging procedure, charge to 80% SOC, stand for 30 minutes, then discharge at 1C, then stand for 30 minutes, after 20 cycles, disassemble the lithium secondary battery after full charge, and observe whether the negative electrode is lithiumized. If not, it indicates that the lithium secondary battery has s minutes of fast charging capability.

[0279] 7. Cycle performance test

[0280] At 60℃, charge the lithium secondary battery at 0.5C constant current to 3.65V, then charge at 3.65V constant voltage until the current is less than 0.05C, then discharge the lithium secondary battery at 0.5C constant current to 2.5V, which is one charge and discharge process. Repeat the charging and discharging, and calculate the number of cycles when the lithium secondary battery decays to 80%.

[0281] 8. Volume expansion rate test

[0282] The prepared lithium secondary battery was charged to 3.65 V at a constant current of 0.33 C at 25 °C, further charged to a current of 0.05 C at a constant voltage of 3.65 V, then discharged to 2.5 V at a constant current of 0.33 C, and the discharge capacity was the discharge capacity before high-temperature storage of the lithium secondary battery; then the lithium secondary battery was charged to 3.65 V at a constant current of 0.33 C, and charged to a current of 0.05 C at a constant voltage of 3.65 V, and the lithium secondary battery was fully charged. The volume of the battery was tested by the drainage method. Then the lithium secondary battery was stored at 60 °C for 60 days, and after the storage was completed, the lithium secondary battery was placed in an environment at 25 °C, and the volume of the battery was tested by the drainage method. The battery volume expansion rate = (volume after storage / volume before storage-1) %.

[0283] It should be noted that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration and playing the same role and effect as the technical idea within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, within the scope of the gist of the present application, various modifications that can be thought of by those skilled in the art, other modes constructed by combining part of the configuration elements of the embodiments are also included in the scope of the present application.

Claims

1. A lithium secondary battery, wherein, The lithium secondary battery comprises: A positive electrode sheet including a positive electrode current collector and a positive electrode active material layer provided on at least one side of the positive electrode current collector, the positive electrode active material layer having a lithium ion diffusion coefficient Ds in the range of 10 -16 cm 2 / s-10 -5 cm 2 / s. an electrolyte, the electrolyte comprising a solvent, the solvent comprising at least one of compounds of formula (I) in a mass fraction W1 in the range of 60%-70% in the solvent; (I) wherein R1 and R2 each independently comprise at least one of an alkyl group with a carbon atom number of 1-3, a haloalkyl group with a carbon atom number of 1-3; the electrolyte further comprising a first additive and / or a second additive; the first additive comprising at least one of compounds of formula (II): (I) wherein R3 comprises at least one of an alkylene group with a carbon atom number of 2-10 substituted or unsubstituted by Ra, a heteroalkylene group with a carbon atom number of 2-10 substituted or unsubstituted by Ra, an arylene group with a carbon atom number of 6-18 substituted or unsubstituted by Ra, a heteroarylene group with a carbon atom number of 6-18 substituted or unsubstituted by Ra, an alicyclylene group with a carbon atom number of 3-18 substituted or unsubstituted by Ra, a heteroalicyclylene group with a carbon atom number of 3-18 substituted or unsubstituted by Ra; Ra comprises at least one of a halogen atom, a cyano group, an isocyanate group, a hydroxyl group, a carboxyl group, a sulfonic acid group, an ester group, an alkyl group with a carbon atom number of 2-10, an alkenyl group with a carbon atom number of 2-10, an alkynyl group with a carbon atom number of 2-10, an oxaalkyl group with a carbon atom number of 2-10; said second additive comprises M a+ [BF4] w PO t ] b- , M a+ [BF4] - a , M a+ [FSO3] - a and at least one of the compounds of formula (III) (III) wherein M a+ comprises at least one of lithium ion, sodium ion, potassium ion, rubidium ion, cesium ion, magnesium ion, calcium ion, barium ion, aluminum ion, iron ion, copper ion, nickel ion, organic cation, a, b, c each represent a natural number; t represents an integer of 0-3, and w represents an integer of 1-6; X comprises at least one of a halogen atom, and n represents an integer of 0-4; Y comprises at least one of a boron atom and a phosphorus atom; R4 comprises a substituted or unsubstituted alkylene group with a carbon atom number of 1-10, a substituted or unsubstituted haloalkylene group with a carbon atom number of 1-10, a substituted or unsubstituted arylene group with a carbon atom number of 6-20, a substituted or unsubstituted haloarylene group with a carbon atom number of 6-20, q represents an integer of 0-1, and m represents an integer of 1-3.

2. The lithium secondary battery according to claim 1, wherein R1 and R2 each independently comprise at least one of a methyl group, an ethyl group, a propyl group, a fluoromethyl group, a fluoroethyl group, a fluoropropyl group.

3. The lithium secondary battery according to claim 1, wherein, 10 -14 cm 2 / s≤Ds≤10 -9 cm 2 / s.

4. The lithium secondary battery according to claim 1, wherein 30%≤W1≤70%。 5. The lithium secondary battery according to claim 1, wherein, The compound of formula (I) comprises at least one of the following compounds: (I-I) (I-II) (I-III) (I-IV) (I-V) (I-VI) (I-VII) (I-VIII) (I-IX) (I-X) (I-XI).

6. The lithium secondary battery according to claim 5, wherein The compound of formula (I) comprises at least one of the compound of formula (I-I), the compound of formula (I-II), the compound of formula (I-V), and the compound of formula (I-VIII).

7. The lithium secondary battery according to claim 5, wherein The compound of formula (I) comprises the compound of formula (I-I) and the compound of formula (I-II).

8. The lithium secondary battery according to any one of claims 1 to 7, wherein A mass fraction W2 of the first additive in the electrolyte satisfies: 0.01%≤W2≤20%.

9. The lithium secondary battery according to claim 8, wherein, A mass fraction W2 of the first additive in the electrolyte satisfies: 0.1%≤W2≤10%.

10. The lithium secondary battery according to claim 8, wherein, A mass fraction W2 of the first additive in the electrolyte satisfies: 0.2%≤W2≤5%.

11. The lithium secondary battery according to claim 8, wherein, A mass fraction W2 of the first additive in the electrolyte satisfies: 0.50%≤W2≤1.50%.

12. The lithium secondary battery according to any one of claims 1 to 7, wherein, The first additive includes at least one of the following compounds: (II-I) (II-II) (II-III) (II-IV) (II-V) (II-VI) (II-VII) (II-VIII) (II-IX) (II-X) (II-XI) (II-XII) (II-XIII) (II-XIV) (II-XV) (II-XVI) (II-XVII) (II-XVIII) (II-XIX) (II-XX) (II-XXI) (II-XXII) (II-XXIII) (II-XXIV) (II-XXV) (II-XXVI) (II-XXVII) (II-XXVIII) (II-XXIX) (II-XXX) (II-XXXI) (II-XXXII).

13. The lithium secondary battery according to any one of claim 12, wherein, The first additive includes at least one of formula II-XXXII and II-XVII.

14. The lithium secondary battery according to any one of claims 1-7, wherein, The compound of formula (III) includes at least one of lithium difluoro(oxalato)borate LiDFOB, lithium difluorodioxalate phosphate LiBODFP.

15. The lithium secondary battery according to any one of claims 1-7, wherein, The mass fraction W3 of the second additive in the electrolyte satisfies: 0.01%≤W3≤20%.

16. The lithium secondary battery according to claim 15, wherein, The mass fraction W3 of the second additive in the electrolyte satisfies: 0.1%≤W3≤10%.

17. The lithium secondary battery according to any one of claims 1-7, wherein, The mass fraction W3 of the second additive in the electrolyte satisfies: 0.2%≤W3≤5%.

18. The lithium secondary battery according to any one of claims 1-7, wherein, The mass fraction W3 of the second additive in the electrolyte satisfies: 0.05%≤W3≤1.0%.

19. The lithium secondary battery according to any one of claims 1-7, wherein, In M a+ [FSO3] - a , M a+ includes at least one of Li + , Na + , K + , Rb + , Cs + , Mg 2+ , Ca 2+ , Ba 2+ , Al 3+ , Fe 2+ , Cu 2+ , Fe 3+ , Ni 2+ , Ni 3+ .

20. The lithium secondary battery according to any one of claims 1-7, wherein, The solvent of the electrolyte further includes at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, butane sulfone, methyl ethyl sulfone, and diethyl sulfone.

21. The lithium secondary battery according to claim 20, wherein, The solvent further includes ethylene carbonate and methyl ethyl carbonate.

22. The lithium secondary battery according to any one of claims 1-7, wherein, The electrolyte further includes a solute, and the solute includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethylsulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro(oxalato)borate, lithium difluorodioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorodioxalate phosphate.

23. The lithium secondary battery according to any one of claims 1-7, wherein, The positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material, the first positive electrode active material and the second positive electrode active material are the same kind of material or different kinds of material, The average particle size Dv501 of the first positive electrode active material satisfies: 8 μm≤Dv501≤50 μm, The average particle size Dv502 of the second positive electrode active material satisfies: 0.02 μm≤Dv502≤8 μm.

24. The lithium secondary battery according to claim 23, wherein, The first positive electrode active material includes one or more of a lithium transition metal oxide and an olivine-structured lithium-containing phosphate, and the second positive electrode active material includes one or more of a lithium transition metal oxide and an olivine-structured lithium-containing phosphate.

25. The lithium secondary battery according to claim 24, wherein, The first positive electrode active material includes a lithium nickel cobalt manganese oxide and / or a lithium iron phosphate, The second positive electrode active material includes one or more of a lithium nickel cobalt manganese oxide, a lithium iron phosphate, and a lithium manganese iron phosphate.

26. The lithium secondary battery according to claim 25, wherein, The first positive electrode active material is a lithium iron phosphate with a micron-level average volume particle size, and the second positive electrode active material is a lithium iron phosphate with a nanometer-level average volume particle size.

27. The lithium secondary battery according to claim 25, wherein, The first positive electrode active material is a lithium iron phosphate with a micron-level average volume particle size, and the second positive electrode active material is a lithium manganese iron phosphate with a nanometer-level average volume particle size.

28. The lithium secondary battery according to claim 23, wherein, The mass fraction W4 of the second positive electrode active material in the positive electrode active material layer satisfies: 0%≤W4≤60%.

29. The lithium secondary battery according to claim 28, wherein, The mass fraction W4 of the second positive electrode active material in the positive electrode active material layer satisfies: 20%≤W4≤40%.

30. The lithium secondary battery according to any one of claims 1-7, wherein, The lithium secondary battery includes: The negative electrode tab includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, and the negative electrode active material layer includes a negative electrode active material; The average particle size Dv503 of the negative electrode active material satisfies: 6 μm ≤ Dv503.

31. The lithium secondary battery according to claim 30, wherein, The average particle size Dv503 of the negative electrode active material satisfies: 8 μm ≤ Dv503≤ 20 μm.

32. The lithium secondary battery according to claim 30, wherein, The average particle size Dv503 of the negative electrode active material satisfies: 8 μm ≤ Dv503≤ 18 μm.

33. The lithium secondary battery according to claim 30, wherein, The average particle size Dv503 of the negative electrode active material satisfies: 15 μm ≤ Dv503≤ 20 μm.

34. The lithium secondary battery according to claim 30, wherein, The specific surface area of the negative electrode active material satisfies: 0.5 m 2 / g ≤ BET ≤ 2.0 m 2 / g.

35. The lithium secondary battery according to claim 34, wherein, The specific surface area of the negative electrode active material satisfies: 0.8 m 2 / g ≤ BET ≤ 1.5 m 2 / g.

36. The lithium secondary battery according to claim 30, wherein, The lithium secondary battery includes an electrode assembly and the electrolyte accommodated in an outer package, the electrode assembly includes a separator, the positive electrode tab, and the negative electrode tab, and the separator, the positive electrode tab, and the negative electrode tab are made into the electrode assembly through a roll-pressing process or a stacking process.

37. An electrical device, comprising: The electric device includes the lithium secondary battery according to any one of claims 1-36.

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