WO2024120225A1

WO2024120225A1 - Lithium-rich positive electrode, preparation method therefor and secondary battery

Info

Publication number: WO2024120225A1
Application number: PCT/CN2023/134027
Authority: WO
Inventors: 林律欢; 万远鑫; 孔令涌
Original assignee: 深圳市德方创域新能源科技有限公司; 佛山市德方创界新能源科技有限公司; 曲靖德方创界新能源科技有限公司
Priority date: 2022-12-05
Filing date: 2023-11-24
Publication date: 2024-06-13
Also published as: CN115939317A

Abstract

Disclosed are a lithium-rich positive electrode, a preparation method therefor and a secondary battery. The lithium-rich positive electrode comprises a positive electrode current collector and a positive electrode active layer bonded onto the positive electrode current collector. The positive electrode active layer comprises a positive electrode active material, a lithium replenishment additive, a polymer binder and a free radical inhibitor. In the present disclosure, the addition of the lithium replenishment additive and the free radical inhibitor into the positive electrode active layer of the lithium-rich positive electrode can compensate for active lithium loss and increase the reversible capacity, and, during the processes of preparation and charging and discharging of lithium ion batteries, continuously reduce or inhibit the reaction between active oxygen and polymer binders, so as to inhibit or quench peroxy radicals and reduce degradation of the polymer binders, thereby inhibiting gas generation, and improving the cycle performance of the lithium-rich positive electrode.

Description

A lithium-rich positive electrode and preparation method thereof and secondary battery

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on the Chinese patent application with application number 202211549640.9 and application date December 5, 2022, and claims the priority of the Chinese patent application. The entire content of the Chinese patent application is hereby introduced into this application as a reference.

Technical Field

The present disclosure relates to the technical field of lithium-ion battery positive electrodes, and in particular, to a lithium-rich positive electrode and a preparation method thereof, and in particular, also to a secondary battery.

Background technique

As a green and environmentally friendly energy storage technology, lithium-ion batteries have the characteristics of high operating voltage and energy density, relatively small self-discharge level, and ultra-long cycle life, and are increasingly attracting widespread attention. However, during the first charging process of lithium-ion batteries, the negative electrode surface is usually accompanied by the formation of a solid electrolyte membrane SEI film. This process consumes a large amount of Li ⁺ , resulting in irreversible consumption of part of the Li ⁺ released from the positive electrode material, and the corresponding reversible specific capacity of the battery cell is reduced. For example, in a lithium-ion battery system using a graphite negative electrode, the first charge will consume about 10% of the lithium source. When using high-specific capacity negative electrode materials, such as alloys (silicon, tin, etc.), oxides (silicon oxide, tin oxide) and amorphous carbon negative electrodes, the consumption of the positive electrode lithium source will be further aggravated, resulting in a decrease in the first coulomb efficiency of the lithium-ion battery and the capacity of the lithium-ion battery.

At present, by adding lithium supplement additives to the positive electrode material or negative electrode material to offset the irreversible lithium loss caused by the formation of SEI film during the first charge, the battery capacity and reversible specific capacity can be improved. However, in the lithium-rich positive electrode, since the lithium supplement additives are easy to produce oxygen or active oxygen, the active oxygen is easy to react with the polymer binder to form peroxy radicals, causing the binder to decompose and affect the electrochemical performance of the positive electrode. Therefore, it is necessary to provide a lithium-rich positive electrode to solve the problem of poor electrochemical performance such as gas production and reduced cycle performance caused by the easy production of oxygen by the lithium supplement additive.

Summary of the invention

The present disclosure is based on the inventors' discovery and understanding of the following facts and problems: lithium supplement additives are decomposed or lattice oxygen escapes during charging and discharging, which easily generates oxygen or active oxygen.

The lithium-rich positive electrode of lithium-ion batteries contains polymer binders. The oxygen or active oxygen generated by the lithium supplement additive will attack the double bonds, hydrocarbon groups, hydrogen groups or atoms on the tertiary carbon atoms on the main chain of the polymer binder to form high molecular weight peroxyl radicals or peroxides, which will then cause the main chain to break at this location, leading to the decomposition of the polymer binder. The decomposition of the polymer binder will reduce the mechanical properties of the electrode sheet, easily produce microcracks or even pulverize the active layer, resulting in poor cycle performance of the positive electrode.

The present disclosure aims to solve at least one of the technical problems in the related art to a certain extent. To this end, the embodiments of the present disclosure provide a lithium-rich positive electrode and a preparation method thereof and a secondary battery, which can make up for the loss of active lithium and improve the reversible capacity by adding a lithium supplement additive and a free radical inhibitor to the positive electrode active layer of the lithium-rich positive electrode. At the same time, during the preparation and charging and discharging process of the lithium-ion battery, the reaction of active oxygen with the polymer binder is continuously reduced or inhibited, free radicals are inhibited or quenched, the degradation of the polymer binder is reduced, gas production is inhibited, and the cycle performance of the lithium-rich positive electrode is improved.

An embodiment of the present disclosure provides a lithium-rich positive electrode, including a positive electrode current collector and a positive electrode active layer bonded to the positive electrode current collector, wherein the positive electrode active layer contains a positive electrode active material, a lithium supplement additive, a polymer binder and a free radical inhibitor.

The lithium-rich positive electrode includes a lithium supplement additive. Since the lithium supplement additive is mostly decomposed in the form of _Li2O to form active lithium, it can make up for the irreversible capacity loss caused by the formation of SEI film during the first charging process of the lithium-ion battery, so that the positive electrode has a higher specific capacity; however, the decomposition process of the lithium supplement additive is accompanied by the generation of a large amount of oxygen or active oxygen, and the active oxygen easily reacts with the polymer binder to form peroxy radicals, which triggers the decomposition of the polymer binder and deteriorates the electrochemical performance of the positive electrode. In the embodiment of the present disclosure, by adding a free radical inhibitor to the positive active layer of the lithium-rich positive electrode while adding a lithium supplement additive, during the preparation and charging and discharging process of the lithium-ion battery, the active oxygen generated by the lithium supplement additive is reduced or inhibited from reacting with the polymer binder, the peroxy radical is continuously inhibited or quenched, the degradation of the polymer binder is reduced, the gas production is inhibited, and the cycle performance of the lithium-rich positive electrode is improved.

In some embodiments, the free radical inhibitor is an organic small molecule, and the free radical inhibitor contains or is capable of generating at least one of N· free radical, N-O· free radical or C-O· free radical.

In some embodiments, the free radical inhibitor comprises at least one of the following structural formulae I ₁ to I ₅ :

Wherein, _R1 and _R2 are selected from hydrogen atom or C1-C10 alkyl group; _X1 , _X2 , _X3 and _X4 are selected from C1-C15 alkyl group or C1-C15 substituted alkyl group; _Y1 and _Y2 are selected from C, N, O, S, B, Si atom or its derivative group; _X5 , _X6 , _X7 and _X8 are selected from C1-C15 alkyl group or conjugated cyclic group; _R3 , _R4 and _R5 are selected from C1-C15 alkyl group or C1-C15 substituted alkyl group; _R6 , _R7 and _R8 are selected from C1-C15 alkyl group or C1-C15 substituted alkyl group.

In some embodiments, the general structural formula _I2 comprises at least one of the following molecular structural formulas _I6 - _I9 :

Among them, X ₁ , X ₂ , X ₃ and X ₄ are selected from C1-C15 alkyl groups or C1-C15 substituted alkyl groups, and R ₃ is selected from a hydrogen atom, a C1-C15 alkyl group or a C1-C15 substituted alkyl group.

In some embodiments, the free radical inhibitor comprises at least one of the following molecular structural formulas I ₁₀ -I ₁₇ :

In some embodiments, the polymer binder and the free radical inhibitor are physically and/or chemically bonded to each other.

In some embodiments, the polymer binder is connected to the free radical inhibitor via at least one of π-π stacking, van der Waals force, or hydrogen bonding.

In some embodiments, the polymer binder includes a carboxyl-containing polymer binder, and the carboxyl-containing polymer binder is connected to the free radical inhibitor via hydrogen bonding.

In some embodiments, the free radical inhibitor is attached to the polymer binder by grafting.

In some embodiments, the mass ratio of the positive electrode active material, the lithium supplement additive, the polymer binder and the free radical inhibitor is 100:(1-5):(1-5):(0.01-0.5).

In some embodiments, the molar ratio of the lithium supplement additive, the polymer binder and the free radical inhibitor per unit area of the lithium-rich positive electrode is 1:(0.00001-0.002):(0.002-0.1).

In some embodiments, the lithium supplement additive includes a material with a molecular formula of Li _x My _N _z O _q and a material with a molecular formula of Li _w O At least one of the materials, wherein M includes at least one of Fe, Co, Ni, Mn, V, Cu, Mo, Al, Ti and Mg, N includes at least one of Fe, Co, Mn, Ni, Si and Al, and 0＜x≤6, 0＜y≤1, 0≤z≤2, 0＜q≤5, 1≤w≤3.

In some embodiments, the lithium supplement additive is an inverse fluorite structure.

In some embodiments, the polymer binder includes at least one of polyvinylidene fluoride or a derivative of polyvinylidene fluoride, polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and a chitosan derivative.

In some embodiments, the positive electrode active material includes at least one of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, NCA ternary material, NCM ternary material, and lithium manganese iron phosphate.

In some embodiments, the positive electrode active layer further includes a conductive agent.

An embodiment of the present disclosure provides a method for preparing a lithium-rich positive electrode, comprising:

Mixing a positive electrode active material, a lithium supplement additive, a polymer binder, a free radical inhibitor, and a solvent to obtain a lithium-rich positive electrode slurry;

The lithium-rich positive electrode slurry is coated on the positive electrode current collector and dried to obtain a lithium-rich positive electrode.

In some embodiments, the positive electrode active material, the lithium supplement additive, the polymer binder, the free radical inhibitor, and the solvent are mixed to obtain a lithium-rich positive electrode slurry, including: mixing the polymer binder, the free radical inhibitor, and the second solvent, refluxing, and obtaining a binder slurry; mixing the positive electrode active material, the lithium supplement additive, the binder slurry, and the first solvent to obtain a lithium-rich positive electrode slurry.

In some embodiments, the reflux temperature is 100-200° C., and the reflux time is 5-20 h.

In the preparation method of the lithium-rich positive electrode provided in the embodiment of the present disclosure, a lithium-rich positive electrode containing both a lithium-supplementing additive and a free radical inhibitor is prepared, which can compensate for the loss of active lithium and improve the reversible capacity during the preparation and charging and discharging process of the lithium-ion battery. At the same time, the active oxygen generated by the lithium-supplementing additive can be continuously reduced or inhibited from reacting with the polymer binder, thereby inhibiting or quenching peroxyl radicals, reducing the degradation of the polymer binder, inhibiting gas production, and improving the cycle performance of the lithium-rich positive electrode.

The embodiment of the present disclosure provides a secondary battery, including a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet includes the lithium-rich positive electrode of the embodiment of the present disclosure, or the lithium-rich positive electrode prepared by the preparation method of the embodiment of the present disclosure. The secondary battery of the embodiment of the present disclosure, by simultaneously containing a lithium supplement additive and a free radical inhibitor in the positive electrode active layer of the lithium-rich positive electrode, improves the electrochemical performance of the lithium-rich positive electrode while supplementing lithium, thereby improving the first efficiency and cycle performance of the secondary battery, reducing gas production, and reducing impedance.

Detailed ways

Embodiments of the present disclosure are described in detail below. The embodiments are exemplary and intended to be used to explain the present disclosure but should not be construed as limiting the present disclosure.

In the embodiment of the present disclosure, the lithium-rich positive electrode includes a lithium supplement additive. Since the lithium supplement additive is mostly decomposed in the form of _Li2O to form active lithium, it can make up for the irreversible capacity caused by the formation of SEI film during the first charging process of the lithium-ion battery. loss, so that the positive electrode has a higher specific capacity; however, the decomposition of the lithium supplement additive is accompanied by the generation of a large amount of oxygen or active oxygen, which easily reacts with the polymer binder to form peroxy radicals, causing the polymer binder to decompose and deteriorate the electrochemical performance of the positive electrode. In the embodiments of the present disclosure, by adding a free radical inhibitor to the positive active layer of the lithium-rich positive electrode while adding a lithium supplement additive, during the preparation and charging and discharging of the lithium-ion battery, the reaction of the active oxygen generated by the lithium supplement additive with the polymer binder is reduced or inhibited, the peroxy radicals are continuously inhibited or quenched, the degradation of the polymer binder is reduced, the gas production is inhibited, and the cycle performance of the lithium-rich positive electrode is improved.

In some embodiments, the free radical inhibitor is an organic small molecule; the free radical inhibitor contains or can produce at least one of N· free radicals, N-O· free radicals or C-O· free radicals. In the embodiments of the present disclosure, the free radical inhibitor can effectively inhibit the generation of free radicals in the charging and discharging process of the lithium-ion battery, and can effectively capture the free radicals generated in the charging and discharging process of the lithium-ion battery to form stable compounds, especially for the peroxy radicals formed by the reaction of active oxygen and polymer binders. The N· free radicals or N-O· free radicals and C-O· free radicals contained or generated in the free radical inhibitor can be coupled with peroxy radicals to form relatively stable chemical bonds and play a role in consuming peroxy radicals. Therefore, in the embodiments of the present disclosure, by adding a lithium supplement additive to the positive active layer of the lithium-rich positive electrode and adding a free radical inhibitor, the reaction of active oxygen generated by the addition of the lithium supplement additive with the polymer binder is reduced or inhibited, the peroxy radicals generated by the addition of the lithium supplement additive are inhibited or quenched, and the degradation of the polymer binder is reduced to stabilize or improve the electrochemical performance of the lithium-rich positive electrode. In the embodiments of the present disclosure, the free radical inhibitor is preferably an organic small molecule. Compared with high molecular weight polymers, the organic small molecule free radical inhibitor has better solubility and dispersibility in the lithium-rich positive electrode, is easier to establish a connection with the polymer binder, and is more efficient in inhibiting or quenching peroxyl radicals, reducing or inhibiting the reaction of active oxygen with the polymer binder, and further improving the electrochemical properties of the lithium-rich positive electrode, such as the cycle performance.

In some embodiments, the polymer binder and the free radical inhibitor are connected to each other through physical and/or chemical action. Specifically, the polymer binder and the free radical inhibitor are connected through at least one of π-π stacking, van der Waals force or hydrogen bonding; or, the free radical inhibitor is connected by grafting onto the polymer binder. In the embodiments of the present disclosure, the polymer binder and the free radical inhibitor are connected to each other so that the two are close to each other and not easy to move away from each other, thereby enabling the free radical inhibitor to more effectively inhibit the decomposition of the polymer binder.

In some embodiments, the mass ratio of the positive electrode active material, the lithium supplement additive, the polymer binder and the free radical inhibitor is 100:(1-5):(1-5):(0.01-0.5), optionally, 100:1:1:0.01, 100:2:2:0.1, 94:2:2:0.1, 100:3:3:0.3, 100:4:4:0.4, 100:5:5:0.5. In the embodiments of the present disclosure, when the dosage of the free radical inhibitor is too low, it is not possible to fully reduce or inhibit the reaction of active oxygen with the polymer binder, and inhibit or quench the free radicals; when the dosage is too high, the proportion of active materials in the positive electrode sheet is reduced, resulting in a decrease in energy density; when the dosage of the lithium supplement additive is too low, the lithium supplement effect cannot be fully exerted, and the first efficiency of the battery is reduced; when the dosage is too high, the reversible capacity is reduced; by optimizing the mass ratio of the positive electrode active material, the lithium supplement additive, the polymer binder and the free radical inhibitor, the first efficiency and cycle performance of the lithium-ion battery can be further improved, and gas production can be suppressed.

In some embodiments, the molar ratio of the lithium supplement additive, the polymer binder and the free radical inhibitor per unit area of the lithium-rich positive electrode is 1:(0.00001-0.002):(0.002-0.1), optionally, 1:0.00001:0.002, 1:0.0001:0.005, 1:0.001:0.01, 1:0.002:0.001, 1:0.002:0.02, 1:0.002:0.1. In the embodiments of the present disclosure, the molar ratio of the lithium supplement additive, the polymer binder and the free radical inhibitor per unit area of the lithium-rich positive electrode is further preferred, and the free radical inhibitor content corresponding to the lithium supplement additive and the polymer binder per unit area is within the range, which can effectively reduce or inhibit the reaction of the active oxygen generated by the lithium supplement additive with the polymer binder, and inhibit or quench the peroxyl radicals generated by the addition of the lithium supplement additive, reduce the degradation of the polymer binder, so as to stabilize or improve the electrochemical performance of the lithium-rich positive electrode. When the content of free radical inhibitors corresponding to the lithium supplement additive and the polymer binder is too low, the lithium supplement additive, the polymer binder and the free radical inhibitor in the lithium-rich positive electrode are far apart, and the cycle performance and gas production improvement effects are poor.

In some embodiments, the polymer binder includes at least one of polyvinylidene fluoride (PVDF) or a derivative of PVDF, polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and chitosan derivatives. Preferably, a carboxyl-containing polymer binder (modified PVDF). In the embodiments of the present disclosure, the polymer binder is used in the electrode to bond the electrode active material and the conductive agent so that the pole piece components have good contact with the current collector; at the same time, it stabilizes the internal structure of the pole piece and alleviates the volume shrinkage and expansion of the electrode material during the lithium insertion and extraction process. However, as a polymer material, the polymer binder is more susceptible to oxygen attack and aging. In the embodiments of the present disclosure, since the lithium-rich positive electrode contains a lithium-supplementing additive that can produce a large amount of oxygen or active oxygen, it will further aggravate the aging of the polymer binder. Specifically, oxygen or active oxygen will attack the double bonds, hydrocarbon groups, hydrogen groups or atoms on tertiary carbon atoms on the main chain of the polymer binder to form high molecular weight peroxyl radicals or peroxides, which will then cause the main chain to break at this location, leading to the decomposition of the polymer binder. The decomposition of the polymer binder will reduce the mechanical properties of the electrode sheet, easily generate microcracks or even pulverize the active layer, resulting in poor cycle performance and increased impedance of the positive electrode.

In some embodiments, the lithium supplementing additive includes at least one of a material with a molecular formula of Li _x M _y N _z O _q and a material with a molecular formula of Li _w O, wherein M includes at least one of Fe, Co, Ni, Mn, V, Cu, Mo, Al, Ti and Mg, N includes at least one of Fe, Co, Mn, Ni, Si and Al, and 0＜x≤6, 0＜y≤1, 0≤z≤2, 0＜q≤5, 1≤w≤3. Preferably, Li _x Fe _y Al _z O _q , further preferably, Li ₅ Fe _0.98 Al _0.02 O _4. Preferably, Li ₂ NiO ₂ , Li ₅ FeO ₄ , LiCoO ₂ , Li ₂ MnO ₂ , LiMn ₂ O ₄ , LiFePO _4. Preferably, the lithium supplementing additive is an anti-fluorite structure. In the embodiments of the present disclosure, according to the element type represented by M in the molecular formula Li _x _My N _z O _q , the lithium supplement material can be at least one of an iron-based lithium supplement material, a manganese-based lithium supplement material, a nickel-based lithium supplement material, etc. The lithium supplement additive is rich in lithium and can be used in the first cycle of charging. The release of lithium ions during the process plays an effective role in replenishing lithium. In the embodiments of the present disclosure, when the lithium replenishing additive is an inverse fluorite structure, it can also improve the unidirectional capacity characteristics of the lithium replenishing material, thereby ensuring the lithium replenishing effect of the positive electrode lithium replenishing additive. When the lithium replenishing additive contains aluminum doping, Al atoms exist in the form of replacing iron atom lattices. Al atoms in this form can broaden the transmission channel of lithium ions and increase the rate of lithium ion escape.

In some embodiments, the positive electrode active layer further includes a conductive agent, which includes at least one of conductive carbon black, conductive graphite, vapor-grown carbon fiber, carbon nanotubes, graphene, etc., preferably Super P; the mass proportion of the conductive agent in the positive electrode active layer is 0.5-5%, optionally 0.5%, 1%, 2%, 3%, 4%, 5%. In the embodiments of the present disclosure, the conductive agent can construct an electron transmission channel between active materials, improve the conductivity of the pole piece and the current collection effect.

A method for preparing a lithium-rich positive electrode in an embodiment of the present disclosure includes the following steps:

S101, mixing the positive electrode active material, the lithium supplement additive, the polymer binder, the free radical inhibitor and the first solvent to obtain a lithium-rich positive electrode slurry;

S102, coating the lithium-rich positive electrode slurry on the positive electrode current collector, and drying to obtain a lithium-rich positive electrode.

In the preparation method of the lithium-rich positive electrode provided in the embodiment of the present disclosure, a lithium-rich positive electrode containing both a lithium-supplementing additive and a free radical inhibitor is prepared, which can compensate for the loss of active lithium and improve the reversible capacity during the preparation and charging and discharging process of the lithium-ion battery. At the same time, the reaction of the active oxygen generated by the lithium-supplementing additive with the polymer binder is continuously reduced or inhibited, the peroxyl free radicals are inhibited or quenched, the reaction of the active oxygen with the polymer binder is reduced or inhibited, the degradation of the polymer binder is reduced, the gas production is inhibited, and the cycle performance of the lithium-rich positive electrode is improved.

In some embodiments, in step S101, the first solvent includes at least one of N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF).

In some embodiments, before step S101, it also includes step S100: mixing a polymer binder, a free radical inhibitor and a second solvent to obtain a binder slurry; step S101: mixing the positive electrode active material, the lithium supplement additive, the binder slurry and the first solvent to obtain a lithium-rich positive electrode slurry.

In some embodiments, before step S101, it also includes step S100: mixing the polymer binder, the free radical inhibitor and the second solvent, refluxing to obtain a binder slurry; step S101: mixing the positive electrode active material, the lithium supplement additive, the binder slurry and the first solvent to obtain a lithium-rich positive electrode slurry.

In some embodiments, the reflux temperature is 100-200°C, optionally, 120°C, 150°C, 180°C; the reflux time is 5-20h, optionally, 8h, 10h, 12h, 15h.

In some embodiments, in step S100, the second solvent includes at least one of N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF).

In some embodiments, in step S100, the polymer binder and the free radical inhibitor are connected to each other through physical or chemical action. Preferably, the polymer binder and the free radical inhibitor are connected through at least one of π-π stacking, van der Waals force or hydrogen bonding; or, the free radical inhibitor is connected by grafting on the polymer binder. Further preferably, the grafting is formed by the formation of hydrogen bonds between N or O in the free radical inhibitor and OH in the polymer binder; the benzene ring structure in the free radical inhibitor and the benzene ring structure in the polymer binder form π-π stacking; the polar groups in the free radical inhibitor and the polar groups in the polymer binder form van der Waals force; the hydroxyl and/or amine groups in the free radical inhibitor and the carboxyl groups in the polymer binder form ester bonds or amide bonds.

In some embodiments, a conductive agent is added to the lithium-rich positive electrode slurry.

In some embodiments, in step S101, the conductive agent is mixed with the positive electrode active material, the lithium supplement additive, the polymer binder, the free radical inhibitor, and the solvent to obtain a lithium-rich positive electrode slurry.

In some embodiments, the drying temperature is 70 to 150°C, optionally 80°C, 90°C, 100°C, 110°C, 120°C, 130°C. In some embodiments, the drying method is baking. In the embodiments of the present disclosure, the lithium-rich positive electrode obtained by drying contains both lithium supplement additives and free radical inhibitors, which can make up for the loss of active lithium, increase the reversible capacity, inhibit gas production, and improve the cycle performance of the lithium-rich positive electrode.

A secondary battery of an embodiment of the present disclosure includes a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet includes the lithium-rich positive electrode of the embodiment of the present disclosure, or the lithium-rich positive electrode prepared by the preparation method of the embodiment of the present disclosure. The secondary battery of the embodiment of the present disclosure contains both a lithium supplement additive and a free radical inhibitor in the positive electrode active layer of the lithium-rich positive electrode, thereby improving the electrochemical performance of the lithium-rich positive electrode while supplementing lithium, thereby improving the first efficiency and cycle performance of the secondary battery, reducing gas production, and reducing impedance.

The present disclosure is described below with reference to specific embodiments. It should be noted that these embodiments are merely illustrative and do not limit the present disclosure in any way.

Example 1

1. Lithium-rich cathode

A lithium-rich positive electrode comprises a positive electrode current collector and a positive electrode active layer combined with the positive electrode current collector, wherein the positive electrode active layer comprises positive electrode active material LiFePO ₄ , lithium supplement additive Li ₅ FeO ₄ , polymer binder PVDF, free radical inhibitor I ₁₀ and conductive agent Super P.

2. Preparation of lithium-rich cathode

S1. The positive electrode active material LiFePO ₄ , the lithium supplement additive Li ₅ FeO ₄ , the polymer binder PVDF, the free radical inhibitor I ₁₀ , the solvent NMP, and the conductive agent Super P are mixed in a mass ratio of 94:2:2:0.1:100:2, and the mixing method is ball milling for 60 minutes; the rotation speed is set to 30 Hz to obtain a lithium-rich positive electrode slurry;

S2. The lithium-rich positive electrode slurry is coated on the positive electrode current collector through a coating-baking-cutting operation to prepare a positive electrode sheet, and the positive electrode sheet is baked in a vacuum oven at 100° C. to remove trace water to obtain a lithium-rich positive electrode.

3. Assemble lithium-ion batteries

1) Positive electrode sheet: The prepared lithium-rich positive electrode is used as the positive electrode sheet.

2) Negative electrode sheet: The negative electrode active material graphite, conductive agent Super P, thickener carboxymethyl cellulose (CMC), and binder styrene butadiene rubber (SBR) are mixed evenly in deionized water to form a negative electrode slurry, wherein the mass ratio of graphite: Super P: CMC: SBR is 95:2:0.5:2.5. The negative electrode slurry is coated on the current collector copper foil, and after drying-rolling-secondary drying process, the negative electrode sheet is made.

3) Diaphragm: Use polyethylene (PE) diaphragm.

4) Electrolyte: The electrolyte is a 1 mol/L LiPF ₆ solution, and the solvent is composed of EC (ethylene carbonate) and DEC (diethyl carbonate) in a volume ratio of 1:1.

5) Assembly of secondary batteries:

The positive electrode sheet, negative electrode sheet, electrolyte and separator are assembled into a lithium-ion soft-pack battery according to the lithium-ion battery assembly requirements.

Embodiment 2-8

The methods of Examples 2-8 are the same as those of Example 1, except that the types and amounts of the lithium-supplementing additive, polymer binder, and free radical inhibitor in the lithium-rich positive electrode are different, as shown in Table 1.

Example 9

1. Lithium-rich cathode

A lithium-rich positive electrode comprises a positive electrode current collector and a positive electrode active layer bonded to the positive electrode current collector, wherein the positive electrode active layer comprises positive electrode active material LiFePO ₄ , lithium supplement additive Li ₅ FeO ₄ , a carboxyl-containing polymer binder (modified PVDF), a free radical inhibitor I ₁₄ and a conductive agent Super P. The carboxyl-containing polymer binder (modified PVDF) and the free radical inhibitor I ₁₄ are connected by hydrogen bonding.

2. Preparation of lithium-rich cathode

S1. A carboxyl-containing polymer binder (modified PVDF), a free radical inhibitor I ₁₄ and a solvent NMP are mixed to obtain a binder slurry.

S2. The positive electrode active material LiFePO ₄ , the lithium supplement additive Li ₅ FeO ₄ , the above-mentioned binder slurry, the solvent NMP, and the conductive agent Super P are mixed, wherein the mass ratio of the positive electrode active material LiFePO ₄ , the lithium supplement additive Li ₅ FeO ₄ , the carboxyl-containing polymer binder (modified PVDF), the free radical inhibitor I ₁₄ , the solvent NMP and the conductive agent Super P is 94:2:2:0.1:100:2, the mixing method is ball milling, and the ball milling time is 60 min; the rotation speed is set to 30 Hz to obtain a lithium-rich positive electrode slurry;

S3. The lithium-rich positive electrode slurry is coated on the positive electrode current collector through a coating-baking-cutting operation to prepare a positive electrode sheet, and the positive electrode sheet is baked in a vacuum oven at 100° C. to remove trace water to obtain a lithium-rich positive electrode.

The method of assembling the lithium ion battery is the same as in Example 1.

Example 10

1. Lithium-rich cathode

A lithium-rich positive electrode comprises a positive electrode current collector and a positive electrode active layer bonded to the positive electrode current collector, wherein the positive electrode active layer comprises positive electrode active material LiFePO ₄ , lithium supplement additive Li ₅ FeO ₄ , a carboxyl-containing polymer binder (modified PVDF), a free radical inhibitor I ₁₄ and a conductive agent Super P. The free radical inhibitor I ₁₄ is grafted to the carboxyl-containing polymer binder (modified PVDF).

2. Preparation of lithium-rich cathode

S1. A carboxyl-containing polymer binder (modified PVDF), a free radical inhibitor I ₁₄ and a solvent NMP are mixed and refluxed at 150° C. for 12 h to obtain a binder slurry.

The method of assembling the lithium ion battery is the same as in Example 1.

Comparative Example 1

The method is the same as that of Example 1, except that the lithium-rich positive electrode does not contain a free radical inhibitor.

Comparative Example 2

The method is the same as that of Example 1, except that the lithium-rich positive electrode does not contain a lithium supplement additive.

Comparative Example 3

The method is the same as that of Example 1, except that the lithium-rich positive electrode does not contain free radical inhibitors and lithium supplement additives.

The substances and amounts used in the lithium-rich positive electrodes prepared in Examples 1-10 and Comparative Examples 1-3 are shown in Table 1.

Table 1 Lithium-rich positive electrode compositions of various embodiments and comparative examples

The lithium ion batteries prepared in Examples 1-10 and Comparative Examples 1-3 were subjected to the following performance tests:

Normal temperature cycle test: The battery is placed at 25°C and charged and discharged at a voltage range of 3.0 to 4.3V using a 1C current. The initial thickness is recorded as T ₀ and the initial capacity is recorded as Q ₀ . The thickness after 300 cycles is recorded as T ₁ and the capacity is recorded as Q ₁ . The thickness change rate and capacity retention rate of the battery after 300 cycles at normal temperature are calculated using the following formula:

Thickness change rate after 300 cycles at room temperature (%) = (T ₁ -T ₀ )/T ₀ × 100%;

Capacity retention rate after 300 cycles at room temperature (%) = Q ₁ /Q ₀ × 100%.

High temperature cycle test: The battery is placed at 45°C and charged and discharged at a voltage range of 3.0 to 4.3V using a 1C current. The initial thickness is recorded as T ₂ and the initial capacity is recorded as Q _2. The thickness after 300 cycles is recorded as T ₃ and the capacity is recorded as Q _3. The thickness change rate and capacity retention rate after 300 cycles at high temperature (45°C) are calculated using the following formula:

Thickness change rate after 300 cycles at high temperature (45°C) = (T ₃ -T ₂ )/T ₂ × 100%;

Capacity retention rate after 300 cycles at high temperature (45°C) = Q ₃ /Q ₂ × 100%.

The performance test results of the lithium ion batteries prepared in Examples 1-10 and Comparative Examples 1-3 are shown in Table 2 below:

Table 2 Performance test results

It can be seen from the test results in Table 2 that in the lithium ion batteries prepared in Examples 1 to 10, when containing lithium supplement additives and free radical inhibitors, the thickness change rate of the lithium ion batteries after 300 cycles at room temperature is all below 4%, which is significantly less than that of Comparative Example 1 containing lithium supplement additives but not containing free radical inhibitors. The difference is more obvious at high temperatures, which fully demonstrates that the free radical inhibitor can play a role in inhibiting gas production; on the other hand, the capacity retention rate of Examples 1 to 10 is also significantly better than that of Comparative Examples 1 to 3, because the free radical inhibitor can continuously reduce or inhibit the reaction of active oxygen generated by the lithium supplement additive with the polymer binder, inhibit or quench peroxy radicals, reduce the degradation of the polymer binder, and improve the cycle performance of the lithium ion battery.

It is worth noting that the thickness change rate of Comparative Examples 2 and 3 is smaller than that of Examples 1 to 10. This is because Comparative Examples 2 and 3 do not contain lithium supplement additives, and the lithium supplement additives themselves are one of the important factors leading to gas production. Although Comparative Examples 2 and 3 produce slightly less gas than the embodiments, the lithium supplement additives have an important role in compensating for the loss of active lithium and improving the reversible capacity, and the capacity retention rate of Examples 1-10 is significantly higher than that of Comparative Examples 2 and 3.

In the present disclosure, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may refer to the descriptions in this specification in any suitable manner without contradiction. Different embodiments or examples and features of different embodiments or examples may be combined and combined.

Although the above embodiments have been shown and described, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present disclosure. Changes, modifications, substitutions and variations of the above embodiments by those of ordinary skill in the art are all within the scope of protection of the present disclosure.

Claims

A lithium-rich positive electrode comprises a positive electrode current collector and a positive electrode active layer bonded to the positive electrode current collector, wherein the positive electrode active layer comprises a positive electrode active material, a lithium supplement additive, a polymer binder and a free radical inhibitor.
The lithium-rich positive electrode according to claim 1, wherein the free radical inhibitor is an organic small molecule, and the free radical inhibitor contains or can produce at least one of N· free radicals, N-O· free radicals or C-O· free radicals.
The lithium-rich positive electrode according to claim 2, wherein the free radical inhibitor comprises at least one of the following structural formulas I 1 to I 5 :

Wherein, R1 and R2 are selected from hydrogen atom or C1-C10 alkyl group; X1 , X2 , X3 and X4 are selected from C1-C15 alkyl group or C1-C15 substituted alkyl group; Y1 and Y2 are selected from C, N, O, S, B, Si atom or its derivative group; X5 , X6 , X7 and X8 are selected from C1-C15 alkyl group or conjugated cyclic group; R3 , R4 and R5 are selected from C1-C15 alkyl group or C1-C15 substituted alkyl group; R6 , R7 and R8 are selected from C1-C15 alkyl group or C1-C15 substituted alkyl group.
The lithium-rich positive electrode according to claim 3, wherein the general structural formula I2 comprises at least one of the following molecular structural formulas I6 - I9 :

Among them, X 1 , X 2 , X 3 and X 4 are selected from C1-C15 alkyl groups or C1-C15 substituted alkyl groups, and R 3 is selected from a hydrogen atom, a C1-C15 alkyl group or a C1-C15 substituted alkyl group.
The lithium-rich positive electrode according to any one of claims 2 to 4, wherein the free radical inhibitor comprises at least one of the following molecular structural formulas I 10 -I 17 :
The lithium-rich positive electrode according to any one of claims 1 to 4, wherein the polymer binder and the free radical inhibitor are interconnected by physical and/or chemical action.
The lithium-rich positive electrode according to claim 6, wherein the polymer binder is connected to the free radical inhibitor through at least one of π-π stacking, van der Waals force or hydrogen bonding.
The lithium-rich positive electrode according to claim 7, wherein the polymer binder comprises a carboxyl-containing polymer binder, and the carboxyl-containing polymer binder is connected to the free radical inhibitor through hydrogen bonding.
The lithium-rich positive electrode according to claim 6, wherein the free radical inhibitor is connected to the polymer binder by grafting.
The lithium-rich positive electrode according to claim 1, wherein the mass ratio of the positive electrode active material, the lithium supplement additive, the polymer binder and the free radical inhibitor is 100:(1-5):(1-5):(0.01-0.5).
The lithium-rich positive electrode according to claim 1, wherein the lithium-supplementing additive The molar ratio of the additive, the polymer binder and the free radical inhibitor is 1:(0.00001-0.002):(0.002-0.1).
The lithium-rich positive electrode according to claim 1, wherein the lithium supplement additive includes at least one of a material with a molecular formula of Li x M y N z O q and a material with a molecular formula of Li w O, wherein M includes at least one of Fe, Co, Ni, Mn, V, Cu, Mo, Al, Ti and Mg, N includes at least one of Fe, Co, Mn, Ni, Si and Al, and 0＜x≤6, 0＜y≤1, 0≤z≤2, 0＜q≤5, 1≤w≤3.
The lithium-rich positive electrode according to claim 12, wherein the lithium supplement additive is an inverse fluorite structure.
The lithium-rich positive electrode according to claim 1, wherein the polymer binder includes at least one of polyvinylidene fluoride or a derivative of polyvinylidene fluoride, polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and a chitosan derivative.
The lithium-rich positive electrode according to claim 1, wherein the positive electrode active material includes at least one of lithium iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel manganese oxide, NCA ternary material, NCM ternary material, and lithium iron manganese phosphate.
The lithium-rich positive electrode according to claim 1, wherein the positive electrode active layer further comprises a conductive agent.
A method for preparing a lithium-rich positive electrode according to any one of claims 1 to 16, comprising:

Mixing a positive electrode active material, a lithium supplement additive, a polymer binder, a free radical inhibitor, and a solvent to obtain a lithium-rich positive electrode slurry;

The lithium-rich positive electrode slurry is coated on the positive electrode current collector and dried to obtain a lithium-rich positive electrode.
According to the preparation method of claim 17, wherein the positive electrode active material, the lithium supplement additive, the polymer binder, the free radical inhibitor, and the solvent are mixed to obtain a lithium-rich positive electrode slurry, comprising: mixing the polymer binder, the free radical inhibitor, and the second solvent, and refluxing to obtain a binder slurry; and mixing the positive electrode active material, the lithium supplement additive, the binder slurry, and the first solvent to obtain a lithium-rich positive electrode slurry.
The preparation method according to claim 18, wherein the reflux temperature is 100-200°C and the reflux time is 5-20h.
A secondary battery comprises a positive electrode sheet and a negative electrode sheet, wherein the positive electrode sheet comprises the lithium-rich positive electrode described in any one of claims 1 to 16, or the lithium-rich positive electrode prepared by the preparation method described in any one of claims 17 to 19.