WO2024169402A1 - Negative electrode additive, negative electrode sheet, secondary battery, and electric device - Google Patents

Abstract

Provided are a negative electrode additive, a negative electrode sheet, a secondary battery, and an electric device. The negative electrode additive comprises: a core part and a shell part covering the outer side of the core. The core part comprises a first polymer and a dead-lithium activating agent embedded in the first polymer. The dead-lithium activating agent is distributed inside the first polymer. The shell part comprises a second polymer. The swelling ratio of the first polymer is denoted as C1, the swelling ratio of the second polymer is denoted as C2, and the swelling ratio of the first polymer and the swelling ratio of the second polymer satisfy: C1<C2. The addition of the negative electrode additive can improve the safety of the secondary battery.

Description

Negative electrode additive, negative electrode sheet, secondary battery and electric device

This application refers to Chinese Patent Application No. 202310116975.X filed on February 15, 2023, entitled “Negative Electrode Additive, Negative Electrode Plate, Secondary Battery and Electrical Device”, which is incorporated herein by reference in its entirety.

Technical Field

The present application relates to the technical field of secondary batteries, and in particular to a negative electrode additive, a negative electrode plate, a secondary battery and an electrical device.

Background Art

Lithium-ion batteries are widely used in portable electronic devices, electric vehicles and other fields. With the development of the current society, people have higher and higher requirements for the safety of lithium-ion batteries. During the initial cycle of lithium-ion batteries, lithium ions can form a good SEI film on the surface of the negative electrode; but after multiple cycles, metallic lithium will precipitate on the surface of the negative electrode to form metallic lithium dendrites. The metallic lithium dendrites will pierce the diaphragm, causing the positive and negative electrodes to contact, causing serious safety problems.

Summary of the invention

The present application is made in view of the above-mentioned problems, and its purpose includes providing a negative electrode additive, a negative electrode plate, a secondary battery and an electrical device to reduce or eliminate metal lithium dendrites and improve the safety of the secondary battery.

In order to achieve the above-mentioned object, the first aspect of the present application provides a negative electrode additive, comprising:

A core part, comprising a first polymer and a dead lithium activator embedded in the first polymer; and

An outer shell portion, covering the outer side of the inner core portion, wherein the outer shell portion comprises a second polymer;

The swelling degree of the first polymer is recorded as C1, and the swelling degree of the second polymer is recorded as C2. Then the swelling degree of the first polymer and the swelling degree of the second polymer satisfy: C1＜C2.

Since the swelling degree of the second polymer located in the outer shell is greater than that of the first polymer located in the inner core, when the negative electrode additive is used in the negative electrode active material layer of the secondary battery, the second polymer located in the outer shell is infiltrated by the electrolyte at the beginning of the cycle and swells. As the cycle progresses to the later stage, the first polymer located in the inner core is gradually infiltrated by the electrolyte, and the dead lithium activator in the inner core is released from the first polymer and reacts with the metallic lithium dendrites on the surface of the negative electrode, thereby restoring the activity of some metallic lithium dendrites, reducing or preventing the metallic lithium dendrites from piercing the diaphragm, and improving the safety of the secondary battery. At the same time, the reactivated metallic lithium dendrites can be further utilized, thereby improving the cycle performance of the secondary battery. In addition, the negative electrode additive The introduction of additives can make the film formed on the surface of the negative electrode contain more inorganic components and less organic components. Since the organic components need to consume a large amount of active lithium, the reduction of organic components reduces the consumption of active lithium, thereby improving the first coulombic efficiency in the battery.

In some embodiments, the swelling degree of the first polymer satisfies: 100%≤C1≤105%.

In some embodiments, the swelling degree of the second polymer satisfies: C2 ≥ 110%; preferably 110% ≤ C2 ≤ 200%.

In some embodiments, the shell portion further comprises a lithium salt embedded in the second polymer.

In some embodiments, the mass ratio of the dead lithium activator to the lithium salt is 1:(1-20).

In some embodiments, the mass ratio of the inner core portion to the outer shell portion is (0.1-10):1.

In some embodiments, the core portion satisfies at least one of the following features:

(1) The diameter of the core portion is 0.3 μm-1.5 μm;

(2) The mass ratio of the dead lithium activator to the first polymer is (0.05-10):1;

(3) The dead lithium activator is solid, and the volume average particle size Dv50 is 10nm-1000nm;

(4) The dead lithium activator includes one or more of an inorganic dead lithium activator and an organic dead lithium activator;

Optionally, the inorganic dead lithium activator includes one or more of lithium polysulfide, iron oxide, titanium disulfide, tin iodide and phosphorus pentoxide; further optionally, the inorganic dead lithium activator includes one or more of lithium polysulfide, iron oxide, tin iodide and phosphorus pentoxide;

Optionally, the organic dead lithium activator includes one or more of iodide, organic sulfide, ferrocene, 10-methylphenothiazine, 5,10-dimethyldihydrophenazine, tri[(diethylamino)phenyl]amine, tetraphenylcobalt porphyrin, thianthrene, tetrathiafulvalene, 2,2,6,6-tetramethylpiperidinoxide and bis(4-methoxyphenyl)phenylphosphine; further optionally, the organic dead lithium activator includes one or more of ferrocene, 10-methylphenothiazine, 2,2,6,6-tetramethylpiperidinoxide and bis(4-methoxyphenyl)phenylphosphine;

(5) The first polymer includes one or more of polyamide, polytetrafluoroethylene and polydopamine.

In some embodiments, the housing portion has at least one of the following features:

(1) The thickness of the outer shell portion is 0.5 μm-5 μm;

(2) The mass ratio of the lithium salt to the second polymer is (0.5-10):1;

(3) The volume average particle size Dv50 of the lithium salt is 10 nm to 1000 nm;

(4) The lithium salt includes one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(fluorosulfonyl imide) and lithium bis(trifluoromethylsulfonyl imide);

(5) The second polymer includes one or more of polyacrylic acid, poly(vinylidene fluoride-co-hexafluoropropylene), polyacrylonitrile, polyvinylidene fluoride and polyethylene oxide.

The second aspect of the present application provides a negative electrode plate, comprising:

Anode current collector;

The negative electrode active material layer is located on at least one side of the negative electrode current collector, and the negative electrode active material layer includes the negative electrode additive of the first aspect.

In some embodiments, the negative electrode additive accounts for 0.1%-5% by mass in the negative electrode active material layer.

A third aspect of the present application provides a secondary battery, the secondary battery comprising the negative electrode sheet of the second aspect; or

The secondary battery is prepared from the negative electrode sheet of the second aspect.

In some embodiments, the mass ratio of the dead lithium activator to the lithium salt is 1:(0.5-18).

A fourth aspect of the present application provides an electrical device, comprising the secondary battery of the third aspect.

The details of one or more embodiments of the present application are set forth in the following drawings and description. Other features, objects, and advantages of the present application will become apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments of the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on the drawings without creative work.

FIG. 1 is a schematic diagram of the structure of a negative electrode additive provided in one embodiment of the present application.

FIG. 2 is a schematic diagram of the surface morphology of the negative electrode when the discharge capacity of the secondary battery in Example 1 of the present application decays to 80% of the initial capacity.

FIG3 is a schematic diagram of the negative electrode surface morphology when the discharge capacity of the secondary battery in Comparative Example 1 of the present application decays to 80% of the initial capacity.

FIG. 4 is a schematic diagram of a secondary battery according to an embodiment of the present application.

FIG. 5 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 4 .

FIG. 6 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

Description of reference numerals:
111: first polymer; 112: second polymer; 113: dead lithium activator; 114: lithium salt.
1: secondary battery; 11: housing; 12: electrode assembly; 13: cover plate; 2: electrical device.

DETAILED DESCRIPTION

In order to facilitate the understanding of the present application, the present application will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present application are given in the drawings. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

In the present application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

In this application, when it comes to numerical ranges, unless otherwise specified, the above numerical ranges are deemed to be continuous and include the minimum and maximum values of the range, as well as each value between such minimum and maximum values. Further, when a range refers to an integer, each integer between the minimum and maximum values of the range is included. In addition, when multiple ranges are provided to describe features or characteristics, the ranges can be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges included therein.

In this application, when referring to the unit of a data range, if there is a unit only after the right endpoint, it means that the units of the left endpoint and the right endpoint are the same. For example, 10-1000nm means that the units of the left endpoint "10" and the right endpoint "1000" are both nm (nanometers).

In this application, "multiple", "multiple", "multiple times" and the like, unless otherwise specified, refer to a number greater than 2 or equal to 2. For example, "multiple" means greater than or equal to two. Only some numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form an unambiguous range; and any lower limit can be combined with other lower limits to form an unambiguous range, and similarly, any upper limit can be combined with any other upper limit to form an unambiguous range. In addition, each separately disclosed point or single value itself can be combined as a lower limit or upper limit with any other point or single value or with other lower limits or upper limits to form an unambiguous range.

The "range" disclosed in this application is defined in the form of a lower limit and an upper limit, and a given range is 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. The range defined in this way can be inclusive or exclusive of the end values.

The temperature parameters in this application, unless otherwise specified, allow for both constant temperature treatment and treatment within a certain temperature range. The constant temperature treatment allows the temperature to fluctuate within the accuracy range controlled by the instrument.

In recent years, lithium-ion batteries have been increasingly used in portable electronic devices, electric vehicles and other fields. Graphite negative electrodes are the most commonly used negative electrode materials for lithium-ion batteries. In the late cycle of lithium-ion batteries, metallic lithium dendrites will form on the surface of the graphite negative electrode. The metallic lithium dendrites will pierce the diaphragm, causing the positive and negative electrodes to contact each other, causing serious safety problems. In related technologies, negative electrode additives are often added to the negative electrode material, but the added negative electrode additives can often only play a role in the early stage of battery life, and are gradually consumed as the battery cycles.

In order to solve the above problems, the present application provides a negative electrode additive, including a core part and a shell part coated on the outside of the core, the core part includes a first polymer and a dead lithium activator embedded in the first polymer, and the shell The first polymer part includes a second polymer; the swelling degree of the first polymer is recorded as C1, and the swelling degree of the second polymer is recorded as C2, then the swelling degree of the first polymer and the swelling degree of the second polymer satisfy: C1＜C2.

It should be noted that the "first polymer", "second polymer" and the like in the present application are used for descriptive purposes only and cannot be understood as indicating or implying relative importance or quantity, nor can they be understood as implicitly indicating the importance or quantity of the indicated technical features.

Dead lithium is lithium that is formed during the cycle of a secondary battery and can no longer participate in electrochemical reactions. "Activation" means that the dead lithium can participate in electrochemical reactions again. The dead lithium activator mentioned in this application refers to a substance used to react with dead lithium to restore its activity; after the dead lithium regains its activity, it can participate in chemical reactions again. The dead lithium activator can be an inorganic substance or an organic substance.

The "dead lithium activator embedded in the first polymer" mentioned above means that the dead lithium activator is dispersed in the first polymer, and the dead lithium activator can be uniformly dispersed in the first polymer or randomly dispersed in the first polymer. The dead lithium activator is dispersed in the first polymer as solid particles.

The swelling degree of the first polymer and/or the swelling degree of the second polymer mentioned in the present application are measured after the first polymer and/or the second polymer are immersed in dimethyl carbonate for 12 hours at 25°C. As an example, the swelling degree of the first polymer and/or the swelling degree of the second polymer can be measured by the following method: obtain the first polymer of its core part and the second polymer of its shell part from the negative electrode additive, then place the polymer on a glass plate at 25°C and scrape the film, take a 1cm×1cm polymer film, measure its thickness with a vernier caliper or a micrometer, calculate its volume as V1, add the polymer film to dimethyl carbonate and soak it for 12 hours, take out and measure the length, width and height of the polymer film, and calculate its volume as V2; then calculate the swelling degree of the polymer according to the formula C=V2/V1×100%.

It can be understood that, since the swelling degree of the second polymer located in the outer shell is greater than the swelling degree of the first polymer located in the inner core, when both the first polymer and the second polymer are infiltrated by the electrolyte, the volume swelling degree of the second polymer is greater than the volume swelling degree of the first polymer, and the swelling of the second polymer can provide space for the swelling of the first polymer. At the same time, coating the second polymer on the outside of the first polymer can delay the infiltration of the electrolyte into the first polymer, thereby delaying the release time of the dead lithium activator, so that it can play a role in the late cycle when the amount of metal lithium dendrites formed is large.

When the negative electrode additive is used in the negative electrode active material layer of the secondary battery, the second polymer located in the outer shell is swollen after being infiltrated by the electrolyte during the cycle. As the cycle reaches the later stage, the first polymer located in the inner core is gradually infiltrated by the electrolyte, and the dead lithium activator in the inner core is released from the first polymer, reacting with the metal lithium dendrites on the surface of the negative electrode, thereby restoring the activity of some metal lithium dendrites, reducing or avoiding the metal lithium dendrites from piercing the diaphragm, and improving the safety of the secondary battery; and the restored activity of the metal lithium dendrites can be further utilized, improving the cycle performance of the secondary battery. In addition, the introduction of the negative electrode additive can make the film on the surface of the negative electrode have more inorganic components and fewer organic components. Since the organic components need to consume a large amount of active lithium, the reduction of organic components reduces the consumption of active lithium, thereby improving the first coulomb efficiency in the battery.

The structure of the negative electrode additive mentioned above can be determined by the following method: the sample of the negative electrode additive to be tested is pressed into a tablet, the tablet is imaged using a Zeiss X-ray microscope Xradia610, and the structure of the negative electrode additive is determined based on the imaging, as well as the diameter of the core part and the thickness of the shell part.

As an example, Figure 1 is a schematic diagram of the structure of a negative electrode additive provided in one embodiment of the present application. As shown in Figure 1, the negative electrode additive includes a core portion composed of a first polymer 111 and a dead lithium activator 113 located inside thereof, and an outer shell portion composed of a second polymer 112 and a lithium salt 114 located inside thereof, and the inner shell portion is coated on the outside of the outer shell portion.

The inventors of the present application have found through in-depth research that when the negative electrode additive of the present application satisfies the above-mentioned design conditions and optionally satisfies one or more of the following conditions, the coulombic efficiency and cycle performance of the secondary battery can be further improved.

In some embodiments, the swelling degree of the first polymer satisfies: 100% ≤ C1 ≤ 105%; for example, it may be 101% ≤ C1 ≤ 105%, 101% ≤ C1 ≤ 104%, 102% ≤ C1 ≤ 103% or 100% ≤ C1 ≤ 104%, etc., without specific limitation. When the swelling degree of the first polymer is greater than the above range, the dead lithium activator may be released from the first polymer in advance, resulting in the dead lithium activator being consumed in the early stage of the cycle, which may result in the inability to persist until the metal lithium dendrites are generated to take effect. As an example, the swelling degree of the first polymer may be 100%, 101%, 102%, 103%, 104% or 105%, etc., without specific limitation.

In some embodiments, the first polymer includes one or more of polyamide, polytetrafluoroethylene, and polydopamine.

In some embodiments, the swelling degree of the second polymer satisfies: C2 ≥ 110%.

In some embodiments, the swelling degree of the second polymer satisfies: 110% ≤ C2 ≤ 200%; for example, it may be 120% ≤ C2 ≤ 200%, 120% ≤ C2 ≤ 190%, 130% ≤ C2 ≤ 180%, 140% ≤ C2 ≤ 170%, 150% ≤ C2 ≤ 160% or 110% ≤ C2 ≤ 190%, etc. When the swelling degree of the second polymer is within the above range, sufficient space can be provided for the swelling of the first polymer. As an example, the swelling degree of the second polymer may be 110%, 120%, 130%, 140%, 150%, 160%, 170%, 180%, 190% or 200%, etc., without specific limitation.

In some embodiments, the second polymer includes one or more of polyacrylic acid, poly(vinylidene fluoride-co-hexafluoropropylene), polyacrylonitrile, polyvinylidene fluoride, and polyethylene oxide.

In some embodiments, the outer shell also includes a lithium salt, which is distributed inside the second polymer. When the negative electrode additive is used in the negative electrode active material layer of a secondary battery, the second polymer located in the outer shell is swollen after being infiltrated by the electrolyte during the cycle, and the lithium salt is gradually released from the second polymer to supplement the lithium salt consumed in the process of forming the SEI film. Under the dual effects of lithium salt supplementation and the elimination of metal lithium dendrites by the dead lithium activator, the cycle performance of the secondary battery can be further improved.

It should be noted that when the swelling degree of the second polymer is within the above range, the lithium salt can be dissolved more in the electrolyte in the initial stage of the cycle, promoting the formation of the SEI film. If the swelling degree of the second polymer exceeds the above range, the lithium salt in the second polymer may be released all at once, which may make it impossible to ensure the stability of the subsequent film formation.

In some embodiments, the mass ratio of the dead lithium activator to the lithium salt is 1:(1-20); for example, it can be 1:(1-18), 1:(1-15), 1:(1-12), 1:(1-10), 1:(1-8), 1:(1-5) or 1:(1-2), etc. When the mass ratio of the dead lithium activator to the lithium salt is within the above range, the inorganic component in the SEI film formed on the surface of the negative electrode can be dominant, and at the same time, the metallic lithium dendrites can be effectively utilized in the later stage of the cycle. As an example, the mass ratio of the dead lithium activator to the lithium salt can be 1:1, 1:2, 1:(1-20), 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19 or 1:20, etc., without specific limitation.

In some embodiments, the mass ratio of the core part to the shell part is (0.1-10):1; for example, it can be (0.1-9):1, (0.1-7):1, (0.1-5):1, (0.1-3):1, (0.1-2.5):1, (0.1-2):1, (0.1-1.5):1, (0.1-1):1 or (0.5-2.5):1, etc. When the mass ratio of the core part to the shell part is within the above range, the inorganic component in the SEI film formed on the surface of the negative electrode can be further dominated, and at the same time, the metal lithium dendrites can be effectively utilized in the later stage of the cycle. As an example, the mass ratio of the core part to the shell part can be 0.1:1, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1 or 10:1, etc., without specific limitation.

In some embodiments, the mass ratio of the dead lithium activator in the core part to the first polymer is (0.05-10):1; for example, it can be (0.05-8):1, (0.05-5):1, (0.05-3):1, (0.05-1):1, (0.05-0.5):1 or (0.05-0.1):1, etc. When the mass ratio of the dead lithium activator in the core part to the first polymer is within the above range, the inorganic component in the SEI film formed on the surface of the negative electrode can be further dominated, and at the same time, the metal lithium dendrites can be effectively utilized in the later stage of the cycle. As an example, the mass ratio of the dead lithium activator in the core part to the first polymer can be 0.05:1, 0.1:1, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1 or 10:1, etc., without specific limitation.

In some embodiments, the diameter of the core portion is 0.3 micrometers to 1.5 micrometers (μm); for example, it can be 0.4μm-1.5μm, 0.4μm-1.4μm, 0.5μm-1.3μm, 0.6μm-1.2μm, 0.7μm-1.1μm, 0.8μm-1.0μm, 0.8μm-0.9μm or 0.3μm-0.5μm, etc. As an example, the diameter of the core portion can be 0.3μm, 0.4μm, 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, 1.0μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm or 1.5μm, etc., without specific limitation.

In some embodiments, the volume average particle size Dv50 of the dead lithium activator is 10 nanometers to 1000 nanometers (nm); for example, it can be 10nm-900nm, 10nm-800nm, 10nm-700nm, 10nm-600nm, 10nm-500nm, 10nm-400nm, 10nm-300nm, 10nm-250nm, 50nm-250nm, 100nm-250nm or 50nm-100nm, etc.; preferably 50nm-250nm. As an example, the volume average particle size Dv50 of the dead lithium activator can be 50nm, 70nm, 100nm, 120nm, 150nm, 170nm, 200nm, 230nm or 250nm, etc., without specific limitation.

In some embodiments, the dead lithium activator includes an inorganic dead lithium activator and an organic dead lithium activator. Optionally, the inorganic dead lithium activator includes one or more of lithium polysulfide, iron oxide, titanium disulfide, tin iodide and phosphorus pentoxide; preferably includes one or more of lithium polysulfide, iron oxide, tin iodide and phosphorus pentoxide. Optionally, the organic dead lithium activator includes iodide, organic sulfide, ferrocene, 10-methylphenothiazine, 5,10-dimethyldihydrophenazine, tris[(diethyl)-]-pyridinium]-iodide, 1,10-dimethyl- ... The invention relates to one or more of 2,2,6,6-tetramethylpiperidinyl oxide and bis(4-methoxyphenyl)phenylphosphine; preferably one or more of ferrocene, 10-methylphenothiazine, 2,2,6,6-tetramethylpiperidinyl oxide and bis(4-methoxyphenyl)phenylphosphine.

In some embodiments, the thickness of the outer shell portion is 0.5 μm-5 μm; for example, it can be 0.5 μm-4 μm, 1 μm-4 μm, 1.5 μm-3.5 μm or 2 μm-3 μm, etc., without specific limitation. As an example, the thickness of the outer shell portion can be 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm or 5 μm, etc., without specific limitation.

In some embodiments, the mass ratio of the lithium salt to the second polymer is (0.5-10):1; for example, it can be (0.5-8):1, (0.5-5):1, (0.5-3):1, (0.5-2):1 or (0.5-1):1, etc. When the mass ratio of the lithium salt to the second polymer is within the above range, the inorganic component in the SEI film formed on the surface of the negative electrode can be further dominated, and at the same time, the metal lithium dendrites can be effectively utilized in the later stage of the cycle. As an example, the mass ratio of the lithium salt to the second polymer can be 0.5:1, 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1 or 10:1, etc., without specific limitation.

In some embodiments, the volume average particle size Dv50 of the lithium salt is 10nm-1000nm; for example, it can be 10nm-900nm, 10nm-800nm, 10nm-700nm, 10nm-600nm, 10nm-500nm, 10nm-400nm, 10nm-300nm, 10nm-250nm, 50nm-250nm, 100nm-250nm or 50nm-100nm, etc.; preferably 50nm-250nm. As an example, the volume average particle size Dv50 of the lithium salt can be 10nm, 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 8000nm, 9000nm or 1000nm, etc., without specific limitation.

In some embodiments, the lithium salt includes one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bisoxalatoborate, lithium difluorooxalatoborate, lithium bisfluorosulfonyl imide, and lithium bistrifluoromethylsulfonyl imide.

It should be noted that the various components of the above-mentioned negative electrode additives, the mass ratio of the dead lithium activator and the lithium salt, the mass ratio of the core part and the shell part, the mass ratio of the dead lithium activator and the first polymer, and the mass ratio of the lithium salt and the second polymer can all be determined by the following method: using dilute nitric acid as a solvent, the sample to be tested is prepared into a solution, and then the above items are determined using inductively coupled plasma technology (ICP).

The volume average particle size Dv50 of the lithium salt and/or dead lithium additive mentioned above refers to the particle size corresponding to 50% in the volume distribution. As an example, Dv50 can be conveniently measured by a laser particle size analyzer with reference to GB/T 19077-2016 particle size distribution laser diffraction method, such as the Mastersizer 2000E laser particle size analyzer of Malvern Instruments Ltd., UK.

The diameter of the core part and the thickness of the shell part mentioned above can be determined by the following method: the sample of the negative electrode additive to be tested is pressed into a tablet, and the tablet is imaged using a Zeiss X-ray microscope Xradia610, and the structure of the negative electrode additive is determined based on the image, as well as the diameter of the core part and the thickness of the shell part.

The present application also provides a method for preparing a negative electrode additive, comprising the following steps: mixing a dead lithium activator, a first polymer and a first solvent to obtain a first mixed solution; removing the first solvent from the first mixed solution to obtain a core portion; mixing the core portion, a lithium salt, a second polymer and a second solvent to obtain a second mixed solution; removing the second solvent from the second mixed solution; In some embodiments, the first solvent and the second solvent each independently include one or more of acetone, ethanol, N-methylpyrrolidone and acetonitrile.

It should be noted that when the dead lithium activator is an organic dead lithium activator, the first mixed solution may be a solution; when the dead lithium activator is an inorganic dead lithium activator, the first mixed solution may be a suspension. In addition, the second mixed solution is a suspension.

In some embodiments, the mass ratio of the first polymer to the first solvent is (0.005-1):10; for example, it can be (0.005-0.7):10, (0.005-0.5):10, (0.005-0.3):10, (0.005-0.1):10, (0.005-0.05):10, or (0.05-0.1):10, etc. The mass ratio of the second polymer to the second solvent is (0.005-1):10; for example, it can be (0.005-0.7):10, (0.005-0.5):10, (0.005-0.3):10, (0.005-0.1):10, (0.005-0.05):10, or (0.05-0.1):10, etc.

The present application also provides a negative electrode sheet, comprising: a negative electrode current collector and a negative electrode active material layer located on at least one side of the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode additive provided as described above or the negative electrode additive prepared by the method described above.

It should be noted that when determining the above-mentioned items of the negative electrode additive from the negative electrode pole piece, the Zeiss X-ray microscope Xradia610 can be used to determine the structure of the negative electrode additive in the negative electrode pole piece as well as the diameter of the core part and the thickness of the shell part; the inductively coupled plasma technology (ICP) is used to determine the various components of the negative electrode additive, the mass ratio of the dead lithium activator and the lithium salt, the mass ratio of the core part and the shell part, the mass ratio of the dead lithium activator and the first polymer, and the mass ratio of the lithium salt and the second polymer.

In some embodiments, the mass proportion of the negative electrode additive in the negative electrode active material layer is 0.1%-5%; for example, it can be 0.1%-4%, 0.5%-3.5%, 1%-3%, 1.5%-2.5%, 2%-2.5% or 1.2%-2%, etc.

The negative electrode active material layer further includes a negative electrode active material, a binder and a conductive agent.

In some embodiments, the negative electrode active material may adopt the negative electrode active material for the battery known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based material, tin-based material and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more. The weight ratio of the negative electrode active material in the negative electrode film layer is 70% by weight to 100% by weight, based on the total weight of the negative electrode film layer. Further, the weight ratio of the negative electrode active material in the negative electrode film layer is 90% by weight to 98% by weight, based on the total weight of the negative electrode film layer.

In some embodiments, 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). The weight ratio of the binder in the negative electrode film layer is 0-30 weight%, based on the total weight of the negative electrode film layer. Further, the weight ratio of the binder in the negative electrode film layer is 0.5 weight%-5 weight%. The amount is %, based on the total weight of the negative electrode film layer.

In some embodiments, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers. The weight ratio of the conductive agent in the negative electrode film layer is 0-20 weight%, based on the total weight of the negative electrode film layer. Further, the weight ratio of the conductive agent in the negative electrode film layer is 0.3 weight%-5 weight%, based on the total weight of the negative electrode film layer.

The negative electrode sheet further includes a negative electrode current collector, and the negative electrode active material layer is on at least one surface of the negative electrode current collector.

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

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

In some embodiments, the negative electrode film layer may further include other additives, such as thickeners, etc., wherein the thickeners include but are not limited to sodium carboxymethyl cellulose (CMC-Na), etc. The weight ratio of the other additives in the negative electrode film layer is 0-15% by weight, based on the total weight of the negative electrode film layer.

In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent to form a negative electrode slurry, wherein the solvent includes but is not limited to deionized water, etc., the solid content of the negative electrode slurry is 30wt%-70wt%, and the viscosity at room temperature is adjusted to 2000mPa·s-10000mPa·s, and the room temperature refers to 25℃-30℃; the obtained negative electrode slurry is coated on the negative electrode collector, and after a drying process, cold pressing such as rolling, a negative electrode sheet is obtained. The unit area density of the negative electrode powder coated on one side is 75mg/ ^m2-220mg / ^m2 , and the compaction density of the negative electrode sheet is 1.2g/ ^m3-2.65g / ^m3 .

The above raw materials not specifically mentioned can be obtained from the market.

Secondary battery

The present application also provides a secondary battery, comprising the negative electrode sheet provided above in the present application; or prepared from the negative electrode sheet provided above in the present application.

Thus, the obtained secondary battery has high safety, high first coulombic efficiency and good cycle performance.

In some embodiments, the mass ratio of the dead lithium activator to the lithium salt is 1:(0.5-18); for example, it can be 1:(0.5-17), 1:(0.5-15), 1:(0.5-12), 1:(0.5-10), 1:(0.5-8), 1:(0.5-5), 1:(0.5-3), 1:(0.5-2), or 1:(0.5-1), etc. No specific limitation.

It should be noted that, due to the process of electrolyte infiltrating the outer shell of the negative electrode additive during the formation process after the secondary battery is assembled, the second polymer swells under the infiltration of the electrolyte, causing part of the lithium salt to be released from the second polymer; resulting in a change in the mass ratio of the dead lithium activator and the lithium salt in the negative electrode sheet of the secondary battery after formation compared with the negative electrode sheet assembled into the secondary battery.

The secondary battery, battery module, battery pack, and electric device of the present application are described below with reference to the accompanying drawings as appropriate.

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

Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is set between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

Positive electrode

The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

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

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

In some embodiments, the positive electrode active material may include a positive electrode active material for a battery known in the art. As an example, the positive electrode active material may include at least one of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to _, lithium cobalt oxide (such as _LiCoO2 ), lithium nickel oxide (such as _LiNiO2 ), lithium manganese oxide (such as _LiMnO2 , _LiMn2O4 ), lithium nickel cobalt oxide, lithium manganese cobalt _{oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (such as LiNi1/3Co1/3Mn1/3O2 ( also referred to as NCM333 ) , LiNi0.5Co0.2Mn0.3O2 ( also referred to} as _{NCM523 )} , _{LiNi0.5Co0.25Mn0.25O2} (also referred to as _NCM211 ), _{LiNi0.6Co0.2Mn0.2O2} (also referred _{to as NCM622} ), _{LiNi0.8Co0.1Mn0.1O2} (also referred to _{as NCM811} ), At least one of lithium nickel cobalt aluminum oxide (such as LiNi _0.80 Co _0.15 Al _0.05 O ₂ ) and modified compounds thereof. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO ₄ (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO ₄ ), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon. The weight ratio of the positive electrode active material in the positive electrode film layer is 80-100% by weight, based on the total weight of the positive electrode film layer.

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

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

In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry, wherein the positive electrode slurry has a solid content of 40wt%-80wt%, and the viscosity at room temperature is adjusted to 5000mPa·s-25000mPa·s, the positive electrode slurry is coated on the surface of the positive electrode collector, and after drying, it is cold-pressed by a cold rolling mill to form a positive electrode sheet; the unit surface density of the positive electrode powder coated on one side is 150mg/ ^m2-350mg / ^m2 , and the compaction density of the positive electrode sheet is 1.5g/ ^cm3-3.6g / ^cm3 , and can be 2.5g/ ^cm3-3.5g / ^cm3 . The calculation formula of the compaction density is:

Compacted density = coating surface density/(thickness of the electrode after extrusion - thickness of the current collector).

Electrolytes

The electrolyte plays the role of conducting ions between the positive electrode and the negative electrode. The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

In some embodiments, the electrolyte salt may be selected from one or more of lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO ₂ F ₂ ), lithium difluorobis(oxalatophosphate) (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP). The concentration of the electrolyte salt is generally 0.5 mol/L-5 mol/L.

In some embodiments, the solvent may be selected from fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), methyl formate One or more of: (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

In some embodiments, the electrolyte may further include additives, such as negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high or low temperature performance, etc.

Isolation film

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

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

In some embodiments, the isolation film has a thickness of 6 μm-40 μm, and optionally 12 μm-20 μm.

In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and examples of the plastic include polypropylene, polybutylene terephthalate, and polybutylene succinate.

In some embodiments, the secondary battery of the present application is a lithium ion secondary battery.

The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. For example, FIG4 is a secondary battery 5 of a square structure as an example.

In some embodiments, referring to FIG5 , the outer package may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 11 has an opening connected to the receiving cavity, and the cover plate 13 can be covered on the opening to close the receiving cavity.

The positive electrode sheet, the negative electrode sheet and the separator can be wound or laminated to form an electrode assembly 12. The electrode assembly 12 is encapsulated in the housing cavity. The electrolyte is infiltrated in the electrode assembly 12. The number of electrode assemblies 12 contained in the secondary battery 1 can be one or more, which can be adjusted according to demand.

In some embodiments, secondary batteries may be assembled into a battery module. The battery module may contain multiple secondary batteries, and the specific number may be adjusted according to the application and capacity of the battery module.

In the battery module, the plurality of secondary batteries may be arranged in sequence along the length direction of the battery module. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries may be fixed by fasteners.

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

In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

The battery pack may include a battery box and a plurality of battery modules disposed in the battery box. The battery box includes an upper box body and a lower box body, and the upper box body can be covered on the lower box body to form a closed space for accommodating the battery modules. The plurality of battery modules can be arranged in the battery box in any manner.

Electrical devices

The present application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack. The secondary battery, battery module, or battery pack can be used as a power source for the device, or as an energy storage unit for the device. The device can be, but is not limited to, a mobile device, an electric vehicle, an electric train, a ship and a satellite, an energy storage system, etc.; wherein the mobile device may include, but is not limited to, at least one of a mobile phone, a laptop computer, etc.; the electric vehicle may include, but is not limited to, at least one of 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.

The device can select a secondary battery, a battery module or a battery pack according to its usage requirements.

Fig. 6 is an example of an electric device 2. The electric device 2 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the device's requirements for high power and high energy density of secondary batteries, a battery pack or a battery module may be used.

Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

The beneficial effects of the present application are further illustrated below with reference to the embodiments.

Example

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

If no specific techniques or conditions are specified in the examples, the techniques or conditions described in the literature in the field or the product instructions are used. If no manufacturer is specified for the reagents or instruments used, they are all conventional products that can be obtained commercially.

It should be noted that the NMP mentioned in the following examples and comparative examples is N-methylpyrrolidone, and _LiPF6 is hexafluoro Lithium phosphate, PVDF-HFP is poly(vinylidene fluoride-co-hexafluoropropylene), LiFSI represents lithium bisfluorosulfonyl imide, LiBOB represents lithium bisoxalatoborate, LiTFSI represents lithium bistrifluoromethylsulfonyl imide, LiDFOB represents lithium difluorooxalatoborate, _LiBF4 represents lithium tetrafluoroborate, _LiAsF6 represents lithium hexafluoroarsenate, and _LiClO4 represents lithium perchlorate.

1. Preparation of negative electrode additives

It should be noted that when the following examples and comparative examples contain both a dead lithium activator and a lithium salt, the total mass of the dead lithium activator and the lithium salt contained in each example and comparative example is the same.

The swelling degree of each first polymer and second polymer mentioned below is measured by the following method: place the polymer on a glass plate at 25°C and scrape the film, take a 1cm×1cm polymer film, measure its thickness with a vernier caliper or a micrometer, and calculate its volume as V1, add the polymer film into dimethyl carbonate and soak it for 12 hours, take it out and measure the length, width and height of the polymer film, and calculate its volume as V2; then calculate the swelling degree of the polymer according to the formula C=V2/V1×100%.

Example 1

2,2,6,6-tetramethylpiperidinyl oxide (as a dead lithium activator) and polytetrafluoroethylene (as a first polymer) are weighed separately, the mass ratio of 2,2,6,6-tetramethylpiperidinyl oxide to polytetrafluoroethylene is 0.05:1, wherein the volume average particle size Dv50 of 2,2,6,6-tetramethylpiperidinyl oxide is 100 nm; polytetrafluoroethylene and N-methylpyrrolidone (as a first solvent) are stirred and mixed at a mass ratio of 0.05:1, and then 2,2,6,6-tetramethylpiperidinyl oxide is added, and the solvent is dried to obtain a core portion with a diameter of 0.5 μm.

Weigh _LiPF6 (as lithium salt) and PVDF-HFP (as the second polymer), wherein the mass ratio of _LiPF6 to 2,2,6,6-tetramethylpiperidinyl oxide is 1:1, and the mass ratio of _LiPF6 to PVDF-HFP is 0.5:1; mix _LiPF6 and the core part to prepare a mixture, mix PVDF-HFP and acetone (as the second solvent) in a mass ratio of 1:10, then add the mixture to dissolve, stir and dry for 12 hours, form an outer shell part on the outside of the core part, and prepare a negative electrode additive.

Example 2-14

The preparation method of the negative electrode additive in Examples 2-14 is basically the same as the preparation method of the negative electrode additive in Example 1, and the main difference is that at least one of the type of the first polymer, the type of the second polymer, the type of the dead lithium activator and/or the volume average particle size Dv50, the type and/or the volume average particle size Dv50 of the lithium salt, the mass ratio of the dead lithium activator to the first polymer, the mass ratio of the lithium salt to the second polymer, the diameter of the core part, and the thickness of the shell part is different, as described in Table 1.

The preparation method of the negative electrode additive in Examples 2-4 and the preparation method of the negative electrode additive in Example 1 also include the following differences:

In Example 2, the mass ratio of the first polymer to the first solvent is 0.1:1; in Example 3, the mass ratio of the second polymer to the second solvent is 1:100; and in Example 4, the mass ratio of the first polymer to the first solvent is 0.1:1.

Comparative Examples 1-6

The difference between the negative electrode additive in Comparative Example 1 and the negative electrode additive in Example 1 is that the negative electrode additive in Comparative Example 1 is only Contains the core part but not the shell part.

The difference between the negative electrode additive in Comparative Example 2 and the negative electrode additive in Example 2 is that the negative electrode additive in Comparative Example 2 contains only the outer shell portion but does not contain the inner core portion.

The difference between the negative electrode additive in Comparative Example 3 and the negative electrode additive in Example 3 is that the negative electrode additive in Comparative Example 3 contains only a core portion and does not contain a shell portion.

The difference between the negative electrode additive in Comparative Example 4 and the negative electrode additive in Example 4 is that the negative electrode additive in Comparative Example 4 only contains the outer shell part and does not contain the inner core part.

The difference between the negative electrode additive in Comparative Example 5 and the negative electrode additive in Example 1 is that the second polymer replaces the first polymer to form the core part together with the dead lithium activator, and the first polymer replaces the second polymer to form the shell part together with the lithium salt.

The difference between the negative electrode additive in Comparative Example 6 and the negative electrode additive in Example 1 is that the core part uses the second polymer of the same mass to replace the first polymer, and the rest are the same.

The preparation parameters in the above embodiments and comparative examples are shown in Table 1.

Among them, n1 represents the mass ratio of the dead lithium activator in the core part and the first polymer; D1 represents the volume average particle size Dv50 of the dead lithium activator; n2 represents the mass ratio of the lithium salt in the outer shell part and the second polymer; D2 represents the volume average particle size Dv50 of the lithium salt; n3 represents the mass ratio of the dead lithium activator and the lithium salt.

It should be noted that the structure of the negative electrode additives mentioned in Table 1 can be tested by the following method: the sample of the negative electrode additive to be tested is pressed into a tablet, the tablet is imaged using a Zeiss X-ray microscope Xradia610, and the structure of the negative electrode additive is determined based on the imaging, as well as the diameter of the core part and the thickness of the shell part.

2. Preparation of secondary batteries

1. Preparation of negative electrode sheet

In Examples 1-2, 5-14, Comparative Examples 1-2 and 5-6, graphite is used as the negative electrode active material, and in Examples 3-4 and Comparative Examples 3-4, elemental silicon is used as the negative electrode active material; the specific preparation method is as follows:

The negative electrode active material, negative electrode additive, sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR) and conductive carbon black are mixed in a ratio of 8:0.1:0.5:0.5:0.9, water is added and stirred into a uniform slurry, the slurry is coated on one surface of the copper foil, transferred to a vacuum drying oven for complete drying, and then punched to obtain the negative electrode sheet.

2. Preparation of positive electrode

Lithium iron phosphate (as positive electrode active material), polyvinylidene fluoride (PVDF) and conductive carbon black are mixed in a ratio of 8:1:1, and N-methylpyrrolidone is added and stirred to form a uniformly dispersed slurry; the slurry is evenly coated on one surface of an aluminum foil, and then transferred to a vacuum drying oven for complete drying; the obtained electrode sheet is rolled and then punched to obtain a positive electrode sheet.

3. Preparation of Electrolyte

The preparation method of the electrolyte in Example 1-2, Example 5-14 and Comparative Examples 1-2 and Comparative Examples 5-6 is as follows: LiTFSI (as a lithium salt) and LiNO ₃ (as a film-forming additive) are dissolved in a mixture of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (DME) in a volume ratio of 1:1 (as a solvent), and stirred evenly to obtain an electrolyte with a LiTFSI concentration of 1 mol/L and a LiNO ₃ mass percentage concentration of 2%.

The preparation method of the electrolyte in Example 3-4 and Comparative Example 3-4 is as follows: LiTFSI (as a lithium salt) and fluoroethylene carbonate (FEC, as a film-forming additive) are dissolved in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7 (as a solvent), and stirred evenly to obtain an electrolyte with a LiTFSI concentration of 1 mol/L and a FEC mass percentage concentration of 2%.

4. Preparation of Isolation Membrane

A polyethylene film is used as the isolation film.

The positive electrode sheet, the separator, and the negative electrode sheet are stacked in order to form a battery cell, and the separator is placed between the positive and negative electrode sheets to play an isolating role. The bare battery cell is placed in an outer package, and the electrolyte is added and packaged to obtain a secondary battery.

2. Performance Test

1. First Coulomb efficiency

At 25°C, the lithium ion batteries in the above embodiments and comparative examples were charged to 4.0V at a rate of 0.04C, and the first charging capacity was recorded; then they were discharged to 2.0V at a rate of 0.04C, and the first discharge capacity was recorded; the ratio of the first discharge capacity to the first charging capacity was calculated to obtain the first coulombic efficiency.

2. Cycle life

At 25°C, the lithium ion batteries in the above embodiments and comparative examples were charged to 4.0V at a rate of 0.33C, and then discharged to 2.0V at a rate of 0.33C. The cycle test was performed in this full charge and discharge form until the discharge capacity of the lithium ion battery decayed to 80% of the initial capacity, and the number of cycles at this time was recorded.

After the cycle life test of the secondary battery in Example 1 and Comparative Example 1 was completed, the secondary battery was disassembled to obtain the negative electrode sheet, and the surface morphology of the negative electrode sheet was scanned by an electron scanning microscope. The result of Example 1 is shown in FIG2, and the structure of Comparative Example 1 is shown in FIG3. The scale bars in FIG2 and FIG3 are both 200 nm.

As shown in FIG2 , no dendritic metal lithium is precipitated on the surface of the negative electrode plate after the cycle life test in Example 1; while dendritic metal lithium is precipitated on the surface of the negative electrode plate after the cycle life test in Comparative Example 1.

3. Proportion of inorganic components in SEI film

The electrode obtained after the cycle was made into a 5mm*5mm electrode, and then the electrode was measured using X-ray photoelectron spectroscopy (XPS) to obtain the Li 1s spectrum, and the proportion of inorganic components in the SEI film was obtained based on the spectrum.

4. The mass ratio of dead lithium activator and lithium salt in the negative electrode additive after secondary battery formation

The lithium-ion batteries in the above-mentioned embodiments and comparative examples were charged at a rate of 0.04C to 30% residual power to complete the formation. After the formation, the lithium-ion batteries were disassembled to obtain the negative electrode plates, and the negative electrode active material layer samples were scraped from the negative electrode plates. The samples to be tested were prepared into solutions using dilute nitric acid as a solvent, and then measured using inductively coupled plasma technology (ICP).

The performance test results of the above embodiments and comparative examples are shown in Table 2 below.

Table 2

Among them, m in Table 2 represents the mass ratio of the dead lithium activator and the lithium salt in the negative electrode additive after the secondary battery is formed; the ratio can be measured by the following method: the secondary battery after formation is disassembled to obtain the negative electrode sheet, dilute nitric acid is used as a solvent to prepare the test solution, and then the mass ratio of the dead lithium additive and the lithium salt is determined by inductively coupled plasma technology (ICP).

From the results of Examples 1-14 in Table 2, it can be seen that by adding the negative electrode additive of the present application to the negative electrode plate, the cycle performance and the first coulomb efficiency of the secondary battery can be effectively improved, and the proportion of inorganic components in the SEI film can be increased. And compared with Examples 1-2 and Examples 5-14, the cycle life of the secondary battery and the proportion of inorganic components in the SEI film in Examples 3-4 are significantly reduced. The technicians analyzed that the reason may be that in Examples 3-4, silicon powder is used as the negative electrode active material, and there is a significant volume expansion in the silicon film formation, which causes the original SEI film to rupture. Therefore, the first efficiency of the battery is still low compared to the graphite negative electrode after improvement, and the number of cycles is poor.

From the comparison of the results of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2, Example 3 and Comparative Example 3, and Example 4 and Comparative Example 4, it can be seen that compared with the secondary battery using the negative electrode additive containing only the core part or the shell part, the secondary battery using the negative electrode additive containing both the core part and the shell part has an improved first coulombic efficiency, and at the same time, the proportion of inorganic components in the SEI film and the cycle life are significantly improved; it is speculated that this is because: in the negative electrode additive of the present application, the dead lithium activator can react with the metallic lithium dendrites on the surface of the negative electrode to restore the activity of some metallic lithium dendrites so that they can be further utilized; the lithium salt can replenish the lithium salt consumed in the process of forming the SEI film; under the dual effects of the dead lithium activator eliminating metallic lithium dendrites and the lithium salt replenishment, the cycle performance and first coulombic efficiency of the secondary battery can be significantly improved, and at the same time, the inorganic components in the SEI film formed on the surface of the negative electrode can be dominant.

The main difference between Example 1 and Examples 9-12 is that the mass ratio of the dead lithium activator and the lithium salt in the negative electrode additive is different; the mass ratio of the dead lithium activator and the lithium salt in Example 11 is the smallest, and the mass ratio of the dead lithium activator and the lithium salt in Example 12 is the largest. Compared with Example 1 and Examples 9-10, the cycle life (number of cycles) of the secondary battery in Examples 11 and 12 is significantly reduced; technicians analyzed that the reason may be that when the mass ratio is larger, the more dead lithium activators are more, the longer the battery cycle can be extended, but too much dead lithium activators will cause instability in the early stage of the cycle; and the more lithium salts are added, the higher the first efficiency of the battery, the higher the proportion of inorganic components in SEI, but the dead lithium generated cannot be solved in the later stage; then the addition of too much dead lithium activator or lithium salt will cause the cycle life of the secondary battery to deteriorate. It can be seen from the results of Examples 1 and 9-12 that when the mass ratio of the dead lithium activator to the lithium salt is 1:(1-20), the inorganic components in the SEI film formed on the negative electrode surface can dominate, and at the same time, the metallic lithium dendrites can be effectively utilized in the later stage of the cycle.

The difference between Comparative Example 5 and Example 1 is that the swelling degree of the polymer used in the core part of the negative electrode additive is greater than that of the outer part. The swelling degree of the polymer used in the shell part; compared with the secondary battery in Example 1, the first coulomb efficiency of the secondary battery in Example 5 is reduced, and its cycle life (number of cycles) and the proportion of inorganic components in the SEI film are significantly reduced; it is speculated that this is because: when the swelling degree of the first polymer in the core part is greater than the swelling degree of the second polymer in the shell part, the outer lithium salt cannot be effectively dissolved due to the solubility of the second polymer; and when the swelling degree of the first polymer in the core part is less than the swelling degree of the second polymer in the shell part, the lithium salt can be more dissolved in the electrolyte in the initial stage of the cycle, replenishing the lithium salt consumed in the process of forming the SEI film and promoting better formation of the SEI film; at the same time, the dead lithium activator can be released in the later stage of the cycle, so that some metal lithium dendrites can be restored to activity and further utilized; under the joint action of the two, the cycle performance of the secondary battery is significantly improved.

The difference between Example 14 and Example 1 is that the outer shell of the negative electrode additive in Example 14 does not contain lithium salt. Compared with the secondary battery in Example 1, the first coulombic efficiency of the secondary battery in Example 14 is reduced, and its cycle life (number of cycles) and the proportion of inorganic components in the SEI film are significantly reduced; it is speculated that this is because: lithium salt is added to the outer shell, and the lithium salt can replenish the lithium salt consumed in the formation of the SEI film during the cycle, and the inorganic components in the SEI film formed on the negative electrode surface are dominant, while improving the cycle performance of the secondary battery.

The difference between Example 14 and Comparative Example 1 is that a second polymer is also used in Example 14 to form an outer shell covering the core part. Compared with the secondary battery in Comparative Example 1, the cycle life (number of cycles) of the secondary battery in Example 14 is significantly improved; it is speculated that this is because: by coating the second polymer on the outside of the core part, the release of the dead lithium activator can be delayed, allowing it to play a role in the later stage of the cycle. In addition, compared with the secondary battery in Comparative Example 1, the inorganic components in the SEI in Example 14 are the same as those in Comparative Example 1, which also shows that adding lithium salts to the outer shell can make the inorganic components dominate in the SEI film formed on the negative electrode surface.

The difference between Comparative Example 6 and Example 1 is that the core part uses a second polymer of the same mass to replace the first polymer; compared with the secondary battery in Example 1, the first coulombic efficiency of the secondary battery in Comparative Example 6 is reduced, and its cycle life (number of cycles) and the proportion of inorganic components in the SEI film are significantly reduced; it is speculated that this is because: the swelling degree of the polymer in the core part is increased, which may cause the dead lithium activator to be consumed in the early stage of the cycle, and may cause it to be unable to persist until the metal lithium dendrites are generated to take effect.

The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent application shall be subject to the attached claims.

Claims (13)

1. A negative electrode additive, comprising:

   A core part, comprising a first polymer and a dead lithium activator embedded in the first polymer; and

   An outer shell portion, covering the outer side of the inner core portion, wherein the outer shell portion comprises a second polymer;

   The swelling degree of the first polymer is recorded as C1, and the swelling degree of the second polymer is recorded as C2. Then the swelling degree of the first polymer and the swelling degree of the second polymer satisfy: C1＜C2.
2. The negative electrode additive as claimed in claim 1, wherein the swelling degree of the first polymer satisfies: 100%≤C1≤105%.
3. The negative electrode additive according to any one of claims 1 to 2, wherein the swelling degree of the second polymer satisfies: C2 ≥ 110%; optionally 110% ≤ C2 ≤ 200%.
4. The negative electrode additive according to any one of claims 1 to 3, wherein the outer shell portion further comprises a lithium salt embedded in the second polymer.
5. The negative electrode additive as claimed in claim 4, wherein the mass ratio of the dead lithium activator to the lithium salt is 1: (1-20).
6. The negative electrode additive according to any one of claims 1 to 5, wherein the mass ratio of the core part to the shell part is (0.1-10):1.
7. The negative electrode additive according to any one of claims 1 to 6, wherein the core portion satisfies at least one of the following characteristics:

   (1) The diameter of the core portion is 0.3 μm-1.5 μm;

   (2) The mass ratio of the dead lithium activator to the first polymer is (0.05-10):1;

   (3) The dead lithium activator is solid, and the volume average particle size Dv50 is 10nm-1000nm;

   (4) The dead lithium activator includes one or more of an inorganic dead lithium activator and an organic dead lithium activator;

   Optionally, the inorganic dead lithium activator includes one or more of lithium polysulfide, iron oxide, titanium disulfide, tin iodide and phosphorus pentoxide; further optionally, the inorganic dead lithium activator includes one or more of lithium polysulfide, iron oxide, tin iodide and phosphorus pentoxide;

   Optionally, the organic dead lithium activator includes one or more of iodide, organic sulfide, ferrocene, 10-methylphenothiazine, 5,10-dimethyldihydrophenazine, tri[(diethylamino)phenyl]amine, tetraphenylcobalt porphyrin, thianthrene, tetrathiafulvalene, 2,2,6,6-tetramethylpiperidinoxide and bis(4-methoxyphenyl)phenylphosphine; further optionally, the organic dead lithium activator includes one or more of ferrocene, 10-methylphenothiazine, 2,2,6,6-tetramethylpiperidinoxide and bis(4-methoxyphenyl)phenylphosphine;

   (5) The first polymer includes one or more of polyamide, polytetrafluoroethylene and polydopamine.
8. The negative electrode additive according to any one of claims 1 to 7, wherein the outer shell portion has at least one of the following characteristics:

   (1) The thickness of the outer shell portion is 0.5 μm-5 μm;

   (2) The mass ratio of the lithium salt to the second polymer is (0.5-10):1;

   (3) The volume average particle size Dv50 of the lithium salt is 10 nm to 1000 nm;

   (4) The lithium salt includes one or more of lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(fluorosulfonyl imide) and lithium bis(trifluoromethylsulfonyl imide);

   (5) The second polymer includes one or more of polyacrylic acid, poly(vinylidene fluoride-co-hexafluoropropylene), polyacrylonitrile, polyvinylidene fluoride and polyethylene oxide.
9. A negative electrode sheet, comprising:

   Anode current collector;

   A negative electrode active material layer is located on at least one side of the negative electrode current collector, and the negative electrode active material layer comprises the negative electrode additive according to any one of claims 1 to 8.
10. The negative electrode plate according to claim 9, wherein the mass proportion of the negative electrode additive in the negative electrode active material layer is 0.1-5%.
11. A secondary battery, comprising the negative electrode sheet according to any one of claims 9 to 10; or

    The secondary battery is prepared using the negative electrode sheet as described in any one of claims 9 to 10.
12. The secondary battery according to claim 11, wherein the mass ratio of the dead lithium activator to the lithium salt is 1:(0.5-18).
13. An electrical device comprising the secondary battery as claimed in any one of claims 11 to 12.