CN114300735B - lithium secondary battery - Google Patents

Abstract

In order to overcome the poor high-temperature cycle performance of existing high-voltage batteries, the present invention provides a lithium secondary battery, including a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the surface of the positive electrode active material is doped or coated with a first metal element and a third Compounds of dimetallic elements, non-aqueous electrolytes include solvents, electrolyte salts and compounds represented by structural formula 1: Wherein, R ₁ is selected from an unsaturated hydrocarbon group with 3-6 carbon atoms, R ₂ is selected from an alkylene group with 2-5 carbon atoms, n=1 or 2; the lithium secondary battery meets the following conditions: 0.05 ≤ a It can slow down the transmission problems of lithium ions caused by doped metal elements during the battery charge and discharge cycle, reduce battery impedance, battery polarization, and slow down lithium deposition in the battery, so that the battery can have better high-temperature cycle performance.

Description

Lithium secondary battery

Technical Field

The invention belongs to the technical field of energy storage battery devices, and in particular relates to a lithium secondary battery.

Background Art

Lithium-ion batteries have been widely used in the fields of 3C digital products such as mobile phones and laptops, as well as new energy vehicles, due to their advantages such as high operating voltage, wide operating temperature range, high energy density and power density, no memory effect and long cycle life. In recent years, with the continuous development of 3C digital products becoming thinner and lighter, the battery industry has increasingly higher requirements for high energy density of lithium-ion batteries. Therefore, it is urgent to improve the energy density of lithium-ion batteries.

There are currently two main methods to increase battery energy density: one is to increase the positive electrode charge cutoff voltage, and the other is to pressurize the active material layer of the electrode to achieve high density. However, after increasing the positive electrode charge cutoff voltage, the activity of the positive electrode will be further improved, and the side reactions between the positive electrode and the electrolyte will also intensify, which will lead to the dissolution of the positive electrode transition metal ions, thereby causing the high temperature performance of the battery to deteriorate. In addition, when using high-density electrodes, due to the low porosity of the high-density electrodes, the battery's liquid retention will also be reduced, making it difficult for the electrolyte to penetrate the low-porosity electrode interface, thereby increasing the contact internal resistance between the electrolyte and the electrode. During long-term circulation, the charge and discharge polarization becomes larger, causing a sudden drop due to lithium precipitation. Therefore, how to improve the long-term cycle performance of high-voltage, high-density lithium-ion batteries is an industry problem that requires improvements from various levels such as electrode materials and electrolytes.

In terms of positive electrode materials, surface coating modification is an important means to improve the performance of positive electrode materials for lithium-ion batteries. Surface coating materials can effectively reduce the corrosion of electrolytes on the positive electrode and reduce the dissolution of metal ions. At the same time, surface coating materials can also physically isolate the contact between the electrolyte and the positive electrode surface active materials. However, when using positive electrode surface coating materials such as aluminum oxide, the problem is that the coated metal oxides that are not electrochemically active inhibit the transmission of lithium ions at the positive electrode interface, resulting in increased impedance, which is not conducive to the long-term circulation of lithium-ion batteries.

There is no good solution for the electrolyte at present. Therefore, for high-voltage and high-density high-energy-density batteries, ensuring the long-term cycle performance of the battery from the perspective of the electrolyte and ensuring that lithium is not deposited in the later stage of the cycle is a major problem for high-voltage and high-density lithium-ion batteries.

Summary of the invention

In view of the problems of poor high temperature cycle performance and impedance growth of high voltage and high density lithium ion batteries in the prior art, the present invention provides a lithium secondary battery.

The technical solution adopted by the present invention to solve the above technical problems is as follows:

The present invention provides a lithium secondary battery, comprising a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises _LiCoO2 and a doped or coated compound containing a first metal element and a second metal element;

The first metal element includes at least one of Mg, Al and W;

The second metal element includes at least one of Ti, Cr, Zr, Mo, La, Ce and rare earth elements;

The non-aqueous electrolyte includes a lithium salt, an organic solvent and a compound shown in structural formula 1:

Wherein, _R1 is selected from an unsaturated hydrocarbon group having 3 to 6 carbon atoms, _R2 is selected from an alkylene group having 2 to 5 carbon atoms, and n=1 or 2;

The lithium secondary battery meets the following conditions:

0.05≤a×(b+c)≤3;

Wherein, a is the mass percentage of the compound represented by structural formula 1 in the non-aqueous electrolyte, in wt%;

b is the doping amount or coating amount of the first metal element relative to the mass of _LiCoO2 , in wt%,

c is the doping amount or coating amount of the second metal element relative to the mass of LiCoO ₂ , in wt %.

Furthermore, the lithium secondary battery meets the following conditions:

0.08≤a×(b+c)≤2.

Furthermore, the compound represented by the structural formula 1 is selected from one or more of the following compounds:

Furthermore, the mass percentage a of the compound represented by structural formula 1 in the non-aqueous electrolyte is 0.1-5wt%; preferably, the mass percentage a of the compound represented by structural formula 1 in the non-aqueous electrolyte in the lithium secondary battery is 0.1-2wt%.

Further, the doping amount or coating amount b of the first metal element relative to the mass of LiCoO ₂ is 0.1-1.2 wt%; preferably, the doping amount or coating amount b of the first metal element relative to the mass of LiCoO ₂ is 0.5-0.8 wt%.

Furthermore, the doping amount or coating amount c of the second metal element relative to the mass of LiCoO ₂ is 0.001-0.1 wt%; preferably, the doping amount or coating amount c of the second metal element relative to the mass of LiCoO ₂ is 0.005-0.05 wt%.

Furthermore, the charging cut-off voltage of the lithium secondary battery is greater than or equal to 4.45V.

Furthermore, the compaction density of the positive electrode material layer is greater than or equal to 3.8 g/cm ³ ; preferably, the compaction density of the positive electrode material layer is 3.9 g/cm ³ -4.15 g/cm ³ ;

The compaction density of the negative electrode material layer is greater than or equal to 1.6 g/cm ³ ; preferably, the compaction density of the negative electrode material layer is 1.65 g/cm ³ -1.8 g/cm ³ ;

Further, the lithium salt is selected from one or more of LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ , LiBOB, LiClO ₄ , LiCF ₃ SO ₃ , LiDFOB, LiN(SO ₂ CF ₃ ) ₂ and LiN(SO ₂ F) ₂ ;

The concentration of the lithium salt in the non-aqueous electrolyte is 0.5 to 3.5 mol/L.

Furthermore, the non-aqueous electrolyte further comprises an auxiliary additive, wherein the auxiliary additive is at least one of a cyclic sulfate compound, a sultone compound, a cyclic carbonate compound, an unsaturated phosphate compound and a nitrile compound;

Furthermore, based on the total mass of the non-aqueous electrolyte being 100%, the amount of the auxiliary additive added is 0.01% to 30%.

Further, the cyclic sulfate ester compound is selected from at least one of vinyl sulfate, propylene sulfate or methyl vinyl sulfate;

The sultone compound is selected from at least one of 1,3-propane sultone, 1,4-butane sultone or 1,3-propylene sultone;

The cyclic carbonate compound is selected from at least one of vinylene carbonate, ethylene carbonate, fluoroethylene carbonate or the compound shown in structural formula 2.

In the structural formula 2, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , and R ₂₆ are each independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group;

The unsaturated phosphate compound is selected from at least one of the compounds shown in Structural Formula 3:

In the structural formula 3, R ₃₁ , R ₃₂ , and R ₃₃ are each independently selected from a C1-C5 saturated hydrocarbon group, an unsaturated hydrocarbon group, a halogenated hydrocarbon group, and -Si(C _m H _2m+1 ) ₃ , m is a natural number of 1 to 3, and at least one of R ₃₁ , R ₃₂ , and R ₃₃ is an unsaturated hydrocarbon group;

The nitrile compound includes one or more of succinonitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrinitrile, adiponitrile, pimelonitrile, suberonitrile, azelaic acid dinitrile and sebaconitrile.

Beneficial effects of the present invention:

The lithium secondary battery provided by the present invention has a compound shown in structural formula 1 added to a non-aqueous electrolyte while coating or doping a compound containing a first metal element and a second metal element on the surface of a _LiCoO2 positive electrode active material. Through a large number of studies, it is found that the doping amount or coating amount of the first metal and the second metal element doped or coated on the surface of the positive electrode active material is closely related to the content of the compound shown in structural formula 1 in the electrolyte. When 0.05≤a×(b+c)≤3 is satisfied, the dissolution of cobalt ions can be effectively inhibited and the damage of the cobalt ions dissolved from the positive electrode to the SEI film of the negative electrode can be repaired. At the same time, the doping amount or coating amount of the first metal element and the second metal element can stabilize the high-voltage characteristics of the _LiCoO2 positive electrode active material and reduce the transmission problem of lithium ions caused by it during the battery charge and discharge cycle, thereby reducing the battery impedance and battery polarization, so that the battery has a higher energy density and better high-temperature cycle performance.

DETAILED DESCRIPTION

In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

An embodiment of the present invention provides a lithium secondary battery, comprising a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises _LiCoO2 and a doped or coated compound containing a first metal element and a second metal element;

The first metal element includes at least one of Mg, Al, Zr, and W;

The second metal element includes at least one of Ti, Cr, Mo, and a rare earth element;

The negative electrode includes a negative electrode material layer; the non-aqueous electrolyte includes a lithium salt, an organic solvent and a compound shown in structural formula 1:

Wherein, _R1 is selected from an unsaturated hydrocarbon group having 3 to 6 carbon atoms, _R2 is selected from an alkylene group having 2 to 5 carbon atoms, and n is 1 or 2;

The lithium secondary battery meets the following conditions:

0.05≤a×(b+c)≤3;

Wherein, a is the mass percentage of the compound represented by structural formula 1 in the non-aqueous electrolyte, in wt%;

b is the doping amount or coating amount of the first metal element relative to the mass of _LiCoO2 , in wt%,

c is the doping amount or coating amount of the second metal element relative to the mass of LiCoO ₂ , in wt %.

In some embodiments, the _LiCoO2 positive electrode of the lithium secondary battery is doped or coated with metal elements such as Mg, Al, W, Ti, Cr, Zr, Mo, La, Ce and rare earth elements. Compared with the undoped or coated _LiCoO2 positive electrode, Mg, Al, W and other elements improve the stability of the _LiCoO2 positive electrode at high temperature and high pressure. Ti, Cr, Zr, Mo, La, Ce and rare earth elements form a unique layered distribution due to slow diffusion dynamics during sintering, which is mainly enriched on the top surface and can form strong ionic bonds with oxygen ions, thereby inhibiting the loss of surface oxygen, and can further improve the high-pressure structural stability of the cobalt-containing and lithium compound positive electrode active material particles. However, the problem is that the coated metal elements that do not have electrochemical activity inhibit the transmission of lithium ions at the positive electrode interface, resulting in an increase in positive electrode impedance, which is not conducive to the long-term cycle of lithium secondary batteries.

Based on the above-mentioned deficiencies, the non-aqueous electrolyte of the lithium secondary battery of the present invention contains a substance shown in structural formula 1, and its mechanism of action is not very clear, but the inventors speculate that it is because it can effectively complex on the surface of the positive electrode to inhibit the dissolution of cobalt ions. At the same time, due to its special structure, it can effectively repair the damage of the cobalt ions dissolved from the positive electrode to the negative electrode SEI film, thereby effectively inhibiting the impedance growth of the battery during high temperature and high pressure cycling.

Through extensive research, the inventors found that there is a certain relationship between the content of the compound shown in Structural Formula 1 in the electrolyte and the doping coating amount. The compound shown in Structural Formula 1 can effectively inhibit the dissolution of cobalt ions in the positive electrode and indirectly improve the stability of the positive electrode. Therefore, the doping coating amount of the positive electrode material can be appropriately reduced, thereby indirectly reducing the amount of non-electrochemically active metal elements, alleviating the problem of inhibiting lithium ion conduction caused by them, thereby improving the high temperature and high pressure cycle performance of the battery. However, excessive use of the compound shown in Structural Formula 1 will cause the battery performance to deteriorate due to too high viscosity. Through a large number of studies, it is found that when the doping amount or coating amount of the first metal and the second metal element doped or coated on the surface of the positive electrode active material and the content of the compound shown in structural formula 1 in the electrolyte satisfy 0.05≤a×(b+c)≤3, the compound shown in structural formula 1 in the electrolyte can effectively inhibit the dissolution of cobalt ions and repair the damage of the dissolved cobalt ions to the negative electrode SEI film. At the same time, the doping amount or coating amount of the first and second metal elements can stabilize the high-voltage characteristics of the positive electrode active material while slowing down the transmission problem of lithium ions during the battery charge and discharge cycle, reducing battery impedance and battery polarization, so that the battery can have better high-temperature cycle performance.

In a preferred embodiment, the lithium secondary battery meets the following conditions:

0.08≤a×(b+c)≤2;

In some embodiments, the lithium secondary battery provided by the present application associates the content of the compound shown in the non-aqueous electrolyte structural formula 1 with the doping amount or coating amount of the compound containing the first metal element and the second metal element on the surface of the positive electrode active material in the lithium secondary battery, which can to a certain extent comprehensively consider the effects of the first metal element, the second metal element and the compound shown in the structural formula 1 on the battery performance, that is, weaken the battery polarization, reduce the battery impedance, and slow down the battery lithium plating, so that the battery can have better high-temperature cycle performance.

In some embodiments, the compound represented by structural formula 1 is selected from one or more of the following compounds:

In some embodiments, the doping amount or coating amount b of the first metal element relative to the mass of LiCoO ₂ is 0.1-1.2 wt %.

In a preferred embodiment, the doping amount or coating amount b of the first metal element relative to the mass of LiCoO ₂ is 0.5-0.8 wt %.

In some embodiments, the doping amount or coating amount c of the second metal element relative to the mass of LiCoO ₂ is 0.001-0.1 wt %.

In a preferred embodiment, the doping amount or coating amount c of the second metal element relative to the mass of LiCoO ₂ is 0.005-0.05 wt %.

Doping or coating the surface of the positive electrode active material _LiCoO2 with a compound containing a first metal element and a second metal element can effectively increase the battery voltage, but doping or coating with excessive metal elements hinders the insertion or extraction of lithium in the positive electrode active material, thereby reducing the battery capacity and increasing the impedance of the battery. After a lot of research, the inventor found that the doping amount or coating amount b of the first metal element relative to the mass of _LiCoO2 in the positive electrode material layer reaches 0.1-1.2wt%, and the doping amount or coating amount c of the second metal element relative to the mass of _LiCoO2 reaches 0.001-0.1wt%. The first metal element, the second metal element and the compound shown in structural formula 1 in the electrolyte work together, which can not only effectively increase the battery voltage, but also alleviate the inhibition of lithium ion transmission of the positive electrode active material.

In some embodiments, the mass percentage a of the compound represented by structural formula 1 in the non-aqueous electrolyte is 0.1-5 wt %.

In a preferred embodiment, the mass percentage a of the compound represented by structural formula 1 in the non-aqueous electrolyte is 0.1-2 wt %.

The positive electrode active material includes a lithium cobalt oxide compound, which makes the lithium secondary battery have good rate performance. However, when the mass percentage of the Co element in the positive electrode material layer is too large, it is easy to cause the positive electrode active material to react with the non-aqueous electrolyte under high voltage conditions to cause the dissolution of the Co element. By adding the compound shown in Structural Formula 1, the reaction between the non-aqueous electrolyte and the positive electrode active material can be effectively inhibited, thereby inhibiting the dissolution of the Co element in the positive electrode active material. However, the amount of the compound shown in Structural Formula 1 added should not be too large, because the initial impedance of the SEI film formed at the negative electrode of the battery is large, which will cause the battery impedance to increase, and excessive addition of the compound shown in Structural Formula 1 will increase the viscosity of the electrolyte and reduce the conductivity, which is not conducive to the cycle performance of the battery.

The above analysis is only based on the influence of each parameter or multiple parameters on the battery when they exist alone, but in the actual battery application process, the above three parameters are interrelated and inseparable. The relationship given by the present invention relates the three, and the three jointly affect the electrochemical performance of the battery. Therefore, the mass percentage a of the compound shown in structural formula 1, the doping amount or coating amount b of the first metal element in the positive electrode material layer relative to the mass of _LiCoO2 , and the doping amount or coating amount c of the second metal element in the positive electrode material layer relative to the mass of _LiCoO2 are adjusted to make 0.05≤a×(b+c)≤3, which can ensure that the lithium secondary battery has a higher stable voltage characteristic while improving the high temperature cycle performance of the battery; if it exceeds the above range, the battery will experience kinetic deterioration, thereby shortening the cycle life of the battery under high temperature conditions, and even causing failure.

Furthermore, the charging cut-off voltage of the lithium secondary battery is greater than or equal to 4.45 V. The positive electrode of the battery of the present invention is doped or coated with a metal element that isolates the electrolyte from contacting the positive electrode surface active material, and the electrolyte is added with a compound shown in structural formula 1. The two cooperate to make the charging cut-off voltage of the battery greater than or equal to 4.45 V, meeting the demand for high energy density of lithium-ion batteries.

Furthermore, the compaction density of the positive electrode material layer is greater than or equal to 3.8 g/cm ³ ; preferably, the compaction density of the positive electrode material layer is 3.9 g/cm ³ -4.15 g/cm ³ ;

The compaction density of the negative electrode material layer is greater than or equal to 1.6 g/cm ³ ; preferably, the compaction density of the negative electrode material layer is 1.65 g/cm ³ -1.8 g/cm ³ ;

The porosity of the positive electrode material layer and the negative electrode material layer is less than or equal to 50%; preferably, the porosity of the positive electrode material layer and the negative electrode material layer is 10%-35%.

The compaction density of the positive electrode material layer and the compaction density of the negative electrode material layer have a certain influence on the electrical performance of the secondary battery. For the compaction density of the positive electrode material layer, in general, if the compaction density of the positive electrode material layer is too low, the positive electrode material is easy to fall off during the preparation of the pole piece, affecting the consistency of the overall battery capacity; if the compaction density of the positive electrode material layer is too high, the pole piece preparation process is easy to break, process problems occur, and production efficiency is reduced. At the same time, during the battery charging and discharging process, lithium ions are difficult to escape, increasing battery impedance, battery capacity, and battery cycle performance. For the compaction density of the negative electrode material layer, in general, if the compaction density of the negative electrode material layer is too low, it is more likely to have process abnormalities such as powder loss during the preparation of the pole piece, reducing production efficiency; if the compaction density of the negative electrode material layer is too large, it not only affects the immersion of the electrolyte, but also affects the embedding of the electrolyte, reducing the battery capacity, increasing the lithium precipitation of the battery, and reducing the safety performance of the battery. The greater the porosity of the positive and negative electrode material layers, the more conducive it is to the escape or embedding of lithium ions, but if the porosity is too large, the volume of the battery increases, reducing the energy density of the battery.

In addition, the compaction density of the positive electrode material layer, the compaction density of the negative electrode material layer, the porosity of the positive electrode material layer and the negative electrode material layer are interrelated with the compound of the electrolyte structural formula 1. After the compound shown in structural formula 1 is added to the non-aqueous electrolyte, its ion conductivity and viscosity change, which affects the permeability of the non-aqueous electrolyte to the positive and negative electrode material layers, and further affects the insertion and extraction efficiency of lithium ions; and the compaction density of the positive electrode material layer and the negative electrode material layer also affects the density of the SEI film formed by the decomposition of the compound shown in structural formula 1 on the surface of the negative electrode material layer, affecting the protective effect of the SEI film on free Co.

In some embodiments, the lithium salt in the non-aqueous electrolyte is one or more of LiPF ₆ , LiPO ₂ F ₂ , LiBF ₄ , LiBOB, LiClO ₄ , LiCF ₃ SO ₃ , LiDFOB, LiN(SO ₂ CF ₃ ) ₂ and LiN(SO ₂ F) _2. In a preferred embodiment, the lithium salt is LiPF ₆ .

In the non-aqueous electrolyte, lithium salt contains lithium ions, which are beneficial to the electrochemical reaction of the battery during the charge and discharge process of the battery, replenish the consumption of a small amount of lithium ions, and promote the cycle performance of the battery.

In some embodiments, the conductivity of the non-aqueous electrolyte at 25°C is greater than or equal to 6mS/cm. The conductivity of the non-aqueous electrolyte has a certain influence on the cycle performance and storage performance of the lithium secondary battery. After a lot of research, the inventors found that when the conductivity of the non-aqueous electrolyte is low at room temperature, the conductivity in the electrolyte is low, the ion transfer rate is reduced, the concentration polarization increases during the charge and discharge process of the lithium secondary battery, and the battery impedance also increases, reducing the cycle performance and storage performance of the battery. The study found that when the conductivity of the non-aqueous electrolyte is greater than 6mS/cm at room temperature of 25°C, it does not affect the transmission of lithium ions and can minimize the concentration polarization. It works together with the compound shown in structural formula 1 to effectively inhibit the dissolution of cobalt ions and repair the damage of the dissolved cobalt ions to the negative electrode SEI film, thereby improving the high-temperature cycle and high-temperature storage performance of the battery.

In some embodiments, the concentration of the lithium salt in the non-aqueous electrolyte is 0.5-3.5 mol/L; in a preferred embodiment, the concentration of the lithium salt in the non-aqueous electrolyte is 0.8-2.0 mol/L.

In the non-aqueous electrolyte, if the concentration of lithium salt is too low, the concentration polarization in the battery electrochemical reaction will increase, which is not conducive to the battery electrochemical reaction; if the concentration of lithium salt is too high, the concentration of non-aqueous electrolyte will increase, affecting the ion transmission efficiency of the compound shown in Structural Formula 1, affecting the compound shown in Structural Formula 1 to repair the damage of the dissolved cobalt ions to the negative electrode SEI film, and inhibiting the reduction of the dissolution efficiency of cobalt ions.

In some embodiments, the organic solvent in the non-aqueous electrolyte is one or more of cyclic carbonates, linear carbonates, carboxylates and ethers; preferably, the cyclic carbonates include one or more of vinylene carbonate, propylene carbonate and ethylene carbonate; preferably, the linear carbonates include one or more of dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate; preferably, the carboxylates include one or more of methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylacetate and ethyl trimethylacetate; preferably, the ethers include one or more of ethylene glycol dimethyl ether, 1,3-dioxolane and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether.

In some embodiments, the non-aqueous electrolyte further comprises an auxiliary additive, and the auxiliary additive is at least one of a cyclic sulfate ester compound, a sultone compound, a cyclic carbonate compound, an unsaturated phosphate compound and a nitrile compound.

In a preferred embodiment, the cyclic sulfate ester compound is selected from at least one of vinyl sulfate, propylene sulfate or methyl vinyl sulfate;

The cyclic carbonate compounds include but are not limited to cyclic carbonate compounds with fluorine atoms, such as fluoroethylene carbonate and/or difluoroethylene carbonate; preferably, the cyclic carbonate compounds include but are not limited to cyclic carbonate compounds with carbon-carbon unsaturated bonds, such as vinylene carbonate, vinyl ethylene carbonate, vinyl ethylene carbonate, methyl vinylene carbonate or one or more of the compounds shown in Structural Formula 2;

In the structural formula 2, R ₂₁ , R ₂₂ , R ₂₃ , R ₂₄ , R ₂₅ , and R ₂₆ are each independently selected from a hydrogen atom, a halogen atom, and a C1-C5 group.

The unsaturated phosphate compound is selected from at least one of the compounds shown in Structural Formula 3:

In the structural formula 3, R ₃₁ , R ₃₂ and R ₃₃ are independently selected from C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halogenated hydrocarbon groups, and -Si(C _m H _2m+1 ) ₃ , m is a natural number of 1 to 3, and at least one of R ₃₁ , R ₃₂ and R ₃₃ is an unsaturated hydrocarbon group.

In a preferred embodiment, the unsaturated phosphate compound may be at least one of tripropargyl phosphate, dipropargyl methyl phosphate, dipropargyl ethyl phosphate, dipropargyl propyl phosphate, dipropargyl trifluoromethyl phosphate, dipropargyl-2,2,2-trifluoroethyl phosphate, dipropargyl-3,3,3-trifluoropropyl phosphate, dipropargyl hexafluoroisopropyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, diallyl trifluoromethyl phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, and diallyl hexafluoroisopropyl phosphate.

Preferably, the sultone compound includes but is not limited to cyclic sulfonate compounds, for example, it may be at least one of 1,3-propane sultone, propylene sulfite, 1,4-butane sultone or 1,3-propene sultone;

Preferably, the nitrile compound may be, but is not limited to, one or more of succinonitrile, adiponitrile, ethylene glycol bis(propionitrile) ether, hexanetrinitrile, adiponitrile, pimelonitrile, suberonitrile, azelaic acid dinitrile and sebaconitrile.

It should be noted that, unless otherwise specified, in general, the amount of any one of the optional substances in the auxiliary additives added to the non-aqueous electrolyte is 0.05-10%, preferably, the amount of addition is 0.1-5%, and more preferably, the amount of addition is 0.1%-3%. Specifically, the amount of any one of the optional substances in the auxiliary additives can be 0.05%, 0.08%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 7.8%, 8%, 8.5%, 9%, 9.5%, 10%.

In some embodiments, when the auxiliary additive is selected from fluoroethylene carbonate, the amount of the fluoroethylene carbonate added is 0.05% to 30% based on the total mass of the non-aqueous electrolyte as 100%.

Setting the content of the auxiliary additive of the non-aqueous electrolyte within this range can avoid the reduction in conductivity due to the reduction in the dielectric constant of the non-aqueous electrolyte, and can easily make the large current discharge characteristics, stability relative to the negative electrode, and cycle characteristics of the non-aqueous electrolyte battery reach a good range; it can improve the oxidation/reduction resistance of the non-aqueous electrolyte, thereby helping to improve the stability of the battery during high-temperature cycling and high-temperature storage.

In the non-aqueous electrolyte, compared with a single addition or a combination of other existing additives, when the compound shown in Structural Formula 1 is added together with the auxiliary additive, a significant synergistic improvement effect is shown in improving the high-temperature storage performance of the battery, indicating that the compound shown in Structural Formula 1 and the auxiliary additive form a film together on the electrode surface to make up for the film-forming defects of a single addition, and obtain a passivation film that is more stable under high temperature conditions.

The present invention is further described below by way of examples.

The compounds shown in the structural formula 1 used in the following examples and comparative examples are as follows:

Examples 1 to 30 and Comparative Examples 1 to 12

This embodiment is used to illustrate the battery disclosed in the present invention and the preparation method thereof. The positive electrode active material is lithium cobalt oxide, and the negative electrode active material is graphite. The embodiment includes the following steps:

(1) Preparation of positive electrode

The positive electrode active material is LiCoO ₂ , the doping amount or coating amount of the first metal element doped or coated on the positive electrode active material is b, and the doping amount or coating amount of the second metal element doped or coated on the positive electrode active material is c, wherein the values of b and c refer to the tables, the surface doped or coated positive electrode active material LiCoO ₂ , the conductive agent and the binder PVDF are dispersed in the solvent NMP and mixed uniformly to obtain a positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector aluminum foil, and after drying, rolling and cutting, a positive electrode sheet is obtained, wherein the compaction density of the obtained positive electrode sheet is 4.1 g/cm ³ , and the mass ratio of the positive electrode active material, the conductive carbon black and the binder PVDF is 96:2:2.

(2) Preparation of negative electrode

The negative electrode active material graphite, conductive agent, binder CMC and SBR are dispersed in deionized water at a mass ratio of 96:1:1:2 and stirred to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode collector copper foil, and after drying, rolling and cutting, a negative electrode sheet with a compaction density of 1.7g/ ^cm3 is obtained.

(3) Preparation of electrolyte

Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed uniformly at a mass ratio of 3:7, and then lithium hexafluorophosphate (LiPF ₆ ) was added to a molar concentration of 1 mol/L, based on the total weight of the non-aqueous electrolyte being 100%, and then additives were added according to the tables.

(4) Preparation of lithium secondary batteries

The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence using the lamination process, and then a soft-pack battery is made after processes such as top and side sealing and injection of a certain amount of electrolyte.

Performance Testing

The lithium secondary battery prepared above was subjected to the following performance tests:

1. High temperature cycle performance test: At 45°C, charge the formed battery with 1C constant current and constant voltage to the cut-off voltage, then charge it with constant voltage until the current drops to 0.05C, and then discharge it with 1C constant current to 4.45V. Repeat this cycle for 500 times, and record the first discharge capacity and the last discharge capacity.

The capacity retention rate of high temperature cycle is calculated as follows:

Capacity retention rate = last discharge capacity/first discharge capacity × 100%.

2. Internal resistance growth rate after 500 cycles of high-temperature cycling: At 45°C, charge the formed battery at a constant current of 1C at room temperature to the cut-off voltage, then charge it at constant current and constant voltage until the current drops to 0.05C, test the internal resistance of the battery, and then discharge it at a constant current of 1C to 4.45V. After 500 cycles, record the internal resistance of the battery at the first full charge and the internal resistance of the battery after the last full charge.

The internal resistance growth rate of the battery after 500 cycles of high-temperature cycling = (the internal resistance of the battery after the last full charge - the internal resistance of the battery after the first full charge) / the internal resistance of the battery after the first full charge × 100%.

1.1. The test results obtained in Examples 1 to 23 and Comparative Examples 1 to 8 are entered in Table 2.

Table 2

It can be seen from the test results of Examples 1-23 and Comparative Examples 1-8 that when the mass percentage a of compound 1 in the electrolyte, the doping amount b of the first metal element in the positive electrode active material, and the doping amount c of the second metal element in the positive electrode active material satisfy the preset relationship 0.05≤a×(b+c)≤3, the lithium secondary battery has a higher high temperature cycle performance.

It can be seen from the test results of Examples 1-23 that as the value of a×(b+c) increases, the high-temperature cycle performance of the lithium secondary battery first increases and then decreases, indicating that the content of the compound shown in structural formula 1 in the electrolyte, the doping amount of the first metal element and the doping amount of the second metal element in the lithium cobalt oxide positive electrode are related to the high-temperature performance of the lithium secondary battery, especially when 0.08≤a×(b+c)≤2, the lithium-ion battery has the best high-temperature cycle performance.

In the lithium secondary battery of comparative example 4, since the content of the first metal element in the positive electrode is very low, although the internal resistance growth is not particularly obvious, it may be because the content of the compound shown in structural formula 1 in the electrolyte is not high, which is not enough to inhibit the dissolution of cobalt ions in the positive electrode, and the stability of the positive electrode is poor, resulting in a large impedance growth rate and low high-temperature cycle performance of the battery. In the lithium secondary batteries of comparative examples 5-8, the improvement of high-temperature cycle performance and the inhibition of internal resistance growth rate are much lower than the effect when the battery satisfies the preset relationship 0.05≤a×(b+c)≤3. This may be because the content of the compound shown in structural formula 1 in the electrolyte is high, which is easy to increase the viscosity of the electrolyte and reduce the conductivity, which is not conducive to the cycle performance of the battery. At the same time, due to the high content of the first metal element and the second metal element in the positive electrode, the insertion and extraction of lithium in the positive electrode active material are hindered, the battery impedance increases significantly, and the high-temperature cycle performance of the battery is poor.

1.2. The test results obtained in Examples 12, 24 to 26 are entered in Table 3.

Table 3

It can be seen from the test results in Table 3 that for different compounds represented by structural formula 1, when the content of the first metal element, the content of the second metal element and the content of the compound represented by structural formula 1 in the positive electrode material layer satisfy the preset relationship of 0.05≤a×(b+c)≤3, the effects they play are similar, and both have a certain improvement effect on the internal resistance growth and high-temperature cycle performance of the battery, indicating that the relationship provided by the present invention is applicable to different compounds represented by structural formula 1.

1.3. The test results obtained in Examples 27 to 30 and Comparative Examples 9 to 12 are entered in Table 4.

Table 4

It can be seen from the test results in Table 4 that in the battery provided by the present invention, adding the above-mentioned additives PS (1,3-propane sultone), VC (ethylene carbonate), DTD (ethylene sulfate) or FEC (fluoroethylene carbonate) to the non-aqueous electrolyte can further improve the high temperature cycle performance of the battery and reduce the thermal internal resistance growth. It is speculated that the compound shown in structural formula 1 and the above-mentioned additives jointly participate in the formation of the passivation film on the electrode surface, thereby obtaining a passivation film with excellent thermal stability, inhibiting the dissolution of cobalt ions, and improving the stability of the positive electrode, thereby effectively reducing the reaction of the electrolyte on the electrode surface and improving the high temperature cycle performance of the battery.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims (10)

1. A lithium secondary battery comprising a positive electrode, a negative electrode, and a nonaqueous electrolyte, wherein the positive electrode comprises a positive electrode material layer comprising a positive electrode active material comprising LiCoO ₂ And a doped or coated compound containing a first metal element and a second metal element;

the first metal element includes at least one of Mg, al, and W;

the second metal element comprises at least one of Ti, cr, zr, mo, la, ce and rare earth elements;

the nonaqueous electrolyte includes a lithium salt, an organic solvent, and a compound represented by structural formula 1:

structure 1

Wherein R is ₁ Selected from unsaturated hydrocarbon groups having 3 to 6 carbon atoms, R ₂ Selected from hydrocarbylene groups having 2-5 carbon atoms, n=1 or 2;

the lithium secondary battery satisfies the following conditions:

0.05≤a×(b+c)≤3；

wherein a is the mass percentage content of a compound shown in a structural formula 1 in the nonaqueous electrolyte, and the unit is weight percent;

b is the relative LiCoO of the first metal element ₂ The doping amount or coating amount of the mass is expressed as weight percent,

c is the second metal element relative LiCoO ₂ The doping amount or cladding amount of the mass is expressed as weight percent;

a is 0.1 to 5 weight percent, b is 0.1 to 1.2 weight percent, c is 0.001 to 0.1 weight percent;

the lithium secondary battery has a charge cutoff voltage of 4.45V or more.

2. The lithium secondary battery according to claim 1, wherein the lithium secondary battery satisfies the following condition:

0.08≤a×(b+c)≤2。

3. the lithium secondary battery according to claim 1, wherein the compound represented by structural formula 1 is selected from one or more of the following compounds:

4. the lithium secondary battery according to claim 1, wherein,

the mass percentage content a of the compound shown in the structural formula 1 in the nonaqueous electrolyte is 0.1-2 wt%.

5. The lithium secondary battery according to claim 1, wherein,

the first metal element is relative to LiCoO ₂ The doping amount or coating amount b of the mass is 0.5 to 0.8 weight percent, and the second metal element phaseFor LiCoO ₂ The doping amount or coating amount c is 0.005-0.05 wt%.

6. The lithium secondary battery according to claim 1, wherein the positive electrode material layer has a compacted density of 3.8g/cm or more ³ The negative electrode material layer has a compacted density of 1.6g/cm or more ³ 。

7. The lithium secondary battery according to claim 6, wherein the positive electrode material layer has a compacted density of 3.9g/cm ³ -4.15g/cm ³ ；

The negative electrode material layer had a compacted density of 1.65g/cm ³ -1.8g/cm ³ 。

8. The lithium secondary battery according to claim 1, wherein the lithium salt is selected from LiPF ₆ 、LiPO ₂ F ₂ 、LiBF ₄ 、LiBOB、LiClO ₄ 、LiCF ₃ SO ₃ 、LiDFOB、LiN(SO ₂ CF ₃ ) ₂ And LiN (SO) ₂ F) ₂ One or more of the following;

the concentration of the lithium salt in the nonaqueous electrolyte is 0.5-3.5mol/L.

9. The lithium secondary battery according to claim 1, wherein the nonaqueous electrolyte further comprises an auxiliary additive including at least one of a cyclic sulfate compound, a sultone compound, a cyclic carbonate compound, an unsaturated phosphate compound, and a nitrile compound;

the addition amount of the auxiliary additive is 0.01% -30% based on 100% of the total mass of the nonaqueous electrolyte.

10. The lithium secondary battery according to claim 9, wherein the cyclic sulfate compound is selected from at least one of vinyl sulfate, propylene sulfate, or vinyl methyl sulfate;

the sultone compound is at least one selected from 1, 3-propane sultone, 1, 4-butane sultone or 1, 3-propylene sultone;

the cyclic carbonate compound is at least one selected from ethylene carbonate, fluoroethylene carbonate or a compound shown in a structural formula 2,

in the structural formula 2, R ₂₁ 、R ₂₂ 、R ₂₃ 、R ₂₄ 、R ₂₅ 、R ₂₆ Each independently selected from one of a hydrogen atom, a halogen atom, a C1-C5 group;

the unsaturated phosphate compound is at least one compound shown in a structural formula 3:

in the structural formula 3, R ₃₁ 、R ₃₂ 、R ₃₃ Each independently selected from the group consisting of C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halogenated hydrocarbon groups, -Si (C) _m H _2m+1 ) ₃ M is a natural number of 1 to 3, and R ₃₁ 、R ₃₂ 、R ₃₃ At least one of them is an unsaturated hydrocarbon group;

the nitrile compound comprises one or more of succinonitrile, glutaronitrile, ethylene glycol bis (propionitrile) ether, hexanedinitrile, adiponitrile, pimelic nitrile, suberonitrile, nonyldinitrile and decyldinitrile.