阅读说明：本技术 一种负极及其锂离子二次电池 (Negative electrode and lithium ion secondary battery thereof ) 是由 郭明奎 王天生 于 2019-03-06 设计创作，主要内容包括：本发明涉及锂离子二次电池用负极及包含该负极的锂离子二次电池,更具体地涉及含孔隙率和孔径不同的活性物质层的负极及包含该负极的锂离子二次电池,第一活性物质层为靠近集流体的层,第二活性物质层为远离集流体的层,该第一活性物质层采用高孔隙率高孔径类椭圆形多孔颗粒作为活性物质,极大增加了第一活性物质层的吸液储液能力,可保证负极尤其是第一活性物质层在循环过程中持续膨胀仍能保持较高的电解液含量,防止因电解液不足导致的离子传导断桥而降低循环性能,同时可保证电芯能量密度不降低的情况下,极大程度地提升电芯的循环性能和循环寿命。(The invention relates to a negative electrode for a lithium ion secondary battery and the lithium ion secondary battery comprising the same, in particular to a negative electrode comprising active material layers with different porosities and apertures and a lithium ion secondary battery comprising the same.)

1. A negative electrode, comprising:

a current collector;

a first active material layer adjacent to the current collector, the first active material layer comprising a first active material; and a second active material layer provided over the first active material layer, the second active material layer containing a second active material,

the active material is characterized in that the first active material and the second active material are both ellipse-like particles with through holes and/or semi-through holes, and the average pore diameter of the through holes and/or semi-through holes of the first active material is larger than that of the through holes and/or semi-through holes of the second active material.

2. The negative electrode according to claim 1, wherein the first active material particle diameter D is₅₀8-18 μm, preferably 10-16 μm; and/or

The second active material particle diameter D₅₀Is 5-15 μm, preferably 6-12 μm.

3. The negative electrode according to claim 2, wherein the first active material has an average pore diameter of 70 to 140nm, preferably 80 to 120 nm; and/or

The average pore diameter of the second active material is 10-30nm, preferably 15-25 nm.

4. The negative electrode according to claim 2, wherein the ratio of the short-diameter length to the long-diameter length of the first active material is 0.5 to 1, preferably 0.6 to 0.9;

the ratio of the short diameter length to the long diameter length of the second active material is 0.3-1, preferably 0.4-0.8.

5. The negative electrode according to claim 3, wherein the specific surface area of the first active material is 15 to 30g/m²Preferably 18 to 28g/m²；

The specific surface area of the second active material is 3-13g/m²Preferably 5 to 10g/m²。

6. The negative electrode according to claim 3, wherein the first active material has an apparent density of 0.5 to 1.2g/cm³Preferably 0.6 to 1.0g/cm³；

The apparent density of the second active substance is 1.5-2.0g/cm³Preferably 1.6 to 1.9g/cm³。

7. The anode according to claim 1, wherein each of the unit coating weights of the first active material layer and the second active material layer is 20 to 100g/m²Preferably 30 to 90g/m²；

And the ratio of the unit coating weight of the first active material layer to the unit coating weight of the second active material layer is 0.3 to 3, preferably 0.5 to 2.

8. The negative electrode according to any one of claims 1 to 7, wherein the second active material and the first active material are selected from materials capable of receiving and extracting lithium ions, and comprise one or any combination of soft carbon, hard carbon, artificial graphite, natural graphite, silicon-oxygen compound, silicon-carbon composite, lithium titanate and metal capable of forming an alloy with lithium.

9. The negative electrode according to claim 8, wherein the second active material and the first active material are both artificial graphite.

10. A lithium ion secondary battery characterized by comprising:

a positive electrode;

the negative electrode of any one of claims 1 to 7, 9;

an electrolyte; and

and (3) a barrier film.

Technical Field

The present invention relates to a negative electrode for a lithium ion secondary battery and a lithium ion secondary battery including the same, and more particularly, to a negative electrode including active material layers having different porosities and pore diameters and a lithium ion secondary battery including the same.

Background

As the market demand of power batteries is gradually expanded, the demand of consumers on the electrochemical performance and the dynamic performance of the power batteries is higher and higher. Lithium ion secondary batteries are one of the major members of the power battery family, and improvements in energy density and cycle life are crucial to battery performance improvement.

The power battery completes the charging and discharging process by the insertion and extraction of ions between the positive and negative active materials, wherein the design of the negative electrode directly influences the performance of the lithium ion secondary battery. At present, for the improvement of the service life of the lithium ion secondary battery, the content of the electrolyte in the electrode in the circulating process is ensured mainly by reducing the compaction density of the cathode and improving the liquid absorption capacity of the cathode or by adding the electrolyte with higher content. However, reducing the compaction density leads to increased thickness of the negative electrode, reduced energy density, and increased distance between active material particles on the negative electrode may lead to poor contact and affect electron conduction; the electrolyte with higher content is added, so that the internal pressure of the battery core is increased, the expansion problem is caused, the safety performance of the battery core is influenced, and the production material cost of the battery core is increased.

How to obtain a lithium ion secondary battery with the cycle life, energy density and safety performance by reasonably designing a cathode is a problem generally faced in the industry at present.

Disclosure of Invention

In view of the problems of the background art, an object of the present invention is to provide a negative electrode for a lithium ion secondary battery having a long cycle life, a high energy density, and a high electron conductivity, and a lithium ion secondary battery.

To achieve the above object, in a first aspect of the present invention, there is provided a negative electrode comprising:

a current collector;

a first active material layer adjacent to the current collector, the first active material layer comprising a first active material; and a second active material layer provided over the first active material layer, the second active material layer containing a second active material,

the active material is characterized in that the first active material and the second active material are both ellipse-like particles with through holes and/or semi-through holes, and the average pore diameter of the through holes and/or semi-through holes of the first active material is larger than that of the through holes and/or semi-through holes of the second active material.

In a second aspect of the present invention, there is provided a lithium ion secondary battery comprising a positive electrode, a negative electrode, an electrolytic solution, and a separator, wherein the negative electrode is the negative electrode according to the first aspect of the present invention.

Compared with the prior art, the invention at least comprises the following beneficial effects:

the negative electrode of the present invention comprises two active material layers, wherein the first active material layer and the second active material layer are selected from the group consisting of elliptical particles having through-holes and/or semi-through-holes, and the pore diameter of the through-holes and/or semi-through-holes of the first active material layer is larger than the pore diameter of the through-holes and/or semi-through-holes of the second active material layer, thereby ensuring that the lithium ion secondary battery has long cycle life, high energy density and strong electron conductivity.

Drawings

Fig. 1 is a schematic structural view of a double active material layer pole piece in this embodiment, in which the average pore diameter of a through hole and/or a half through hole of a first active material layer is larger than the pore diameter of a through hole and/or a half through hole of a second active material layer;

FIG. 2 is a schematic diagram of a second active material layer single-layer pole piece structure without a through hole and/or a semi-through hole structure, which is described in the comparative example;

fig. 3 is a schematic structural diagram of a double active material layer pole piece in this embodiment, in which the average pore diameter of the through hole and/or the half through hole of the first active material layer is equal to the average pore diameter of the through hole and/or the half through hole of the second active material layer.

Detailed Description

The negative electrode and the lithium ion secondary battery of the present invention will be described in detail below.

First, a negative electrode provided by a first aspect of the present invention is explained, the negative electrode including: a negative current collector; a first active material layer adjacent to the negative electrode current collector, the first active material layer containing a first active material; and a second active material layer provided over the first active material layer, the second active material layer containing a second active material, the first active material and the second active material each being an ellipse-like particle having through-holes and/or semi-through-holes, and an average pore diameter of the through-holes and/or semi-through-holes of the first active material being larger than an average pore diameter of the through-holes and/or semi-through-holes of the second active material.

The negative electrode comprises two active material layers, wherein the first active material layer and the second active material layer are selected from ellipse-like particles with through holes and/or half through holes, wherein the half through holes are blind holes which do not penetrate through the ellipse-like particles, and the average pore diameter of the first active material through holes and/or the half through holes is larger than that of the second active material through holes and/or the half through holes. The coating of more than two active material layers reduces the internal stress of the active material layers in the drying process of the negative pole piece, and solves the cracking problem caused by the increase of unit coating weight, so that the cycle performance of the battery is improved, and the energy density, cycle life and dynamic performance of the battery are effectively improved.

The first active material and the second active material are both elliptical-like particles with through holes and/or semi-through holes, the average pore diameter of the through holes and/or semi-through holes of the first active material is larger than that of the through holes and/or semi-through holes of the second active material, and the first active material has larger pore diameter and higher porosity compared with the second active material particles, so that the first active material has stronger liquid absorption and retention force on electrolyte, the second active material particles are more compact, the pore diameter is smaller, the porosity is lower, and the negative pole piece can still keep higher liquid retention capacity in an expansion state due to the combined structural characteristics, so that the battery has higher energy density and excellent dynamic performance.

Furthermore, the average particle size of the first active material and the average particle size of the second active material and the relationship between the average particle sizes have certain influence on the performance of the battery, and in order to more clearly characterize the overall morphology of the active material particles, the particle size D corresponding to the cumulative percentage of the particle size distribution of the active material particles reaching 50% is selected₅₀And (6) performing characterization. Cumulatively over 50% of D₅₀The particle diameter of the elliptic-like particles with larger numerical value is larger than D₅₀Particle diameter of the less elliptic particles, D₅₀Large particles are more loosely arranged to improve the liquid absorption capacity and the electron conduction capacity, but the particle size difference of the active material particles of the two active material layers is not too large, so that the difference is too large, and D is easy to be caused₅₀Small active substance particles inserted D₅₀Between large active material particles, the active material particles are too dense, lithium ion intercalation and deintercalation are affected, and the cycle performance and the dynamic performance of the negative electrode are reduced.

Further, the average particle diameter D of the second active material₅₀The performance of the negative electrode is also affected to some extent. The smaller the average particle size of the second active material is, the more sufficient the contact with the electrolyte is, the more favorable the charge exchange of the active ions with the electrons, and thus the rapid charging of the battery is, but the smaller the average particle size of the second active material is, the more difficult the preparation of the negative electrode slurry is, and thus the uniformity of the negative electrode may be affected.

Preferably, the first active material particle diameter D₅₀8-18 μm, preferably 10-16 μm; and/or the second active material particle diameter D₅₀Is 5-15 μm, preferably 6-12 μm.

Further, the pore diameters of the through-holes and/or semi-through-holes of the active materials of the two active material layers have certain influences on the energy density and the kinetic performance of the battery. The larger the pore diameter of the active material is, the larger the porosity of the active material layer is, the easier the electrolyte is to fully contact with the active material, and the more the lithium ion is easy to be desorbed, but too large porosity can cause the amount of the active material coated in a unit volume to be reduced, and can cause the energy density of the negative electrode plate to be reduced, conversely, the smaller the porosity of the active material layer is, the more easily the energy density of the negative electrode plate is improved, but too small the porosity of the active material layer can also cause the dynamic performance of the negative electrode plate to be reduced. The active material that the average pore size is bigger is selected to the first active material layer that is close to the negative pole mass collector, and the less active material of average pore size is selected to the second active material layer of setting on first active material layer, is favorable to electrolyte to soak the negative pole piece fast, is favorable to improving the imbibition rate of negative pole piece, simultaneously, the less porosity in second active material layer helps improving the stock solution ability of negative pole piece.

Preferably, the first active substance has an average pore size of 70 to 140nm, preferably 80 to 120 nm; and/or the second active substance has an average pore size of 10 to 30nm, preferably 15 to 25 nm.

Further, the more circular or more oblate elliptical shape of the active material particles also has some effect on the energy density and dynamic performance of the battery. Theoretically, the ratio of the short-diameter length to the long-diameter length of the ellipse-like active substance particles is less than or equal to 1, the shape of the particles tends to be more circular as the value is closer to 1, the shearing force between the particles is larger, and the ratio of the short-diameter length to the long-diameter length is smaller, which indicates that the shape of the particles tends to be more flat and elliptical, and the particles are easier to be mutually embedded. And selecting the active substance with the larger ratio of the short diameter length to the long diameter length to be arranged on the negative current collector, and selecting the active substance with the larger ratio of the short diameter length to the long diameter length to be arranged on the first active substance layer, thereby being beneficial to properly improving the liquid storage capacity of the negative pole piece.

Preferably, the ratio of the short diameter length to the long diameter length of the first active material is 0.5-1, preferably 0.6-0.9; the ratio of the short diameter length to the long diameter length of the second active material is 0.3-1, preferably 0.4-0.8.

Further, the specific surface area of the active material is an important index for its adsorption capacity to the electrolyte. The larger the specific surface area of the active material particles, the stronger the adsorption capacity to the electrolyte. The specific surface area of the first active material is larger than that of the second active material, so that the liquid absorption capacity and the liquid storage capacity of the battery can be improved.

Preferably, the firstThe specific surface area of the active substance is 15-30m²A/g, preferably from 18 to 28m²(ii)/g; the specific surface area of the second active material is 3-13m²A/g, preferably of 5 to 10m²/g。

The apparent density value is the ratio of the mass of the material to the volume of water drained by the material, and the smaller the value is, the larger the pore volume contained in the material is, the stronger the liquid storage capacity is, so that the apparent density value has a certain influence on the energy density of the battery.

Preferably, the first active substance has an apparent density of 0.5 to 1.2g/cm³Preferably 0.6 to 1.0g/cm³(ii) a The apparent density of the second active substance is 1.5-2.0g/cm³Preferably 1.6 to 1.9g/cm³。

Further, the unit coating weight of the two active material layers has a great influence on the consistency, energy density and cycle life of the negative electrode plate. An excessively small coating weight tends to result in low energy density and poor cycle life of the battery, and an excessively large coating weight may result in reduced fluidity of the applied slurry, thereby reducing the uniformity of the negative electrode sheet.

Preferably, the first active material layer and the second active material layer each have a coating weight of 20g/m²-100g/m²Preferably 30g/m²-90g/m²(ii) a The inventors have further studied and found that when the ratio of the coating weight per unit of the first active material layer to the coating weight per unit of the second active material layer is 0.3 to 3, the energy density and cycle performance of the battery can be further improved, and preferably, the ratio of the coating weight per unit of the first active material layer to the coating weight per unit of the second active material layer is 0.5 to 2.

In the negative electrode of the present invention, the second active material and the first active material are selected from materials capable of receiving and extracting lithium ions, including one or any combination of soft carbon, hard carbon, artificial graphite, natural graphite, silicon oxy compound, silicon carbon composite, lithium titanate, and metal capable of forming an alloy with lithium.

Preferably, the second active material and the first active material are both artificial graphite.

Next, a lithium ion secondary battery according to a second aspect of the present invention is described, which includes a positive electrode, a negative electrode, an electrolyte solution, and a separator, wherein the negative electrode is the negative electrode according to the first aspect of the present invention.

In the lithium ion secondary battery of the second aspect of the invention, the positive electrode includes a positive electrode current collector, a positive electrode membrane that is provided on at least one surface of the positive electrode current collector and includes a positive electrode active material, a conductive agent, and a binder. The specific type and composition of the positive pole piece are not particularly limited and can be selected according to actual requirements.

The positive active material is selected from but not limited to Li_aM_1-xM’_xO₂The layered lithium transition metal oxide and the chemical formula of which is LiFe_yMn_1-y-zM”_zPO₄/C_bWherein a is more than or equal to 0.9 and less than or equal to 1.1, x is more than or equal to 0 and less than or equal to 0.1, y is more than or equal to 0.1 and less than or equal to 0.9, z is more than or equal to 0 and less than or equal to 0.9, B is more than or equal to 0, M is at least one of Co, Mn and Ni, M 'is one or more of Al, Mg, B, Zr, Si, Ti, Cr, Fe, V, Cu, Ca, Zn, Nb, Mo, Sr, Sb, W and Bi, and M' is one or more of Cr, Mg, Ti, Al, Zn, W, Nb and Zr.

In the lithium ion secondary battery according to the second aspect of the present invention, the electrolyte solution includes a solvent and a solute, and the specific kind and composition of the solvent and the solute are not particularly limited and can be selected according to actual needs, for example, the solvent can be selected from one or more organic carbonates such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate that are electronically insulating but ionically conducting, and the solute can be selected from LiPF₆、LiBF₄、LiBOB、LiAsF6、Li(CF₃SO₂)₂N、LiCF₃SO₃、LiClO₄One or more lithium salts of (a).

In the lithium ion secondary battery of the second aspect of the invention, the kind of the separator is not particularly limited, and may be any separator material used in existing batteries, such as polyethylene, polypropylene, polyvinylidene fluoride, nonwoven fabric, a film of a fibrous material, and a multilayer composite film thereof.

In order to explain technical contents, structural features, and objects and effects of the technical means in detail, the following detailed description is given with reference to specific embodiments. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present application.

Referring to fig. 1 to 3, the batteries of examples 1 to 19 and comparative examples 1 and 2 were fabricated as follows.

(1) Preparation of positive pole piece

The positive active material-nickel cobalt manganese ternary active material LiNi_1/3Co_1/3Mn_1/3O₂Fully stirring the conductive carbon black Super-P and the PVDF binder in an N-methyl pyrrolidone solvent system at a weight ratio of 94:3:3 in a vacuum stirrer, wherein the stirring linear speed is controlled to be 4-10m/min, and the stirring and mixing time is 60-150min until the system is uniform, so as to obtain anode slurry; and coating the positive electrode slurry on two surfaces of the Al foil substrate, and drying, cold pressing, slitting and cutting to obtain the positive electrode piece.

(2) Preparation of negative pole piece

i. Preparation of Single-layer active Material layer negative electrode (comparative examples 1 and 2 prepared according to the method)

Mixing the negative electrode active materials shown in comparative examples 1 and 2 in the table 1, conductive carbon black Super-P, Styrene Butadiene Rubber (SBR) serving as a binder and sodium carboxymethyl cellulose (CMC) serving as a thickening agent according to a weight ratio of 96:1:2:1, and fully stirring and uniformly mixing in a deionized water solvent system to obtain negative electrode slurry; and uniformly coating the negative electrode slurry on two surfaces of a negative electrode current collector Cu foil, coating according to unit coating weight values shown in comparative examples 1 and 2 in the table 1, airing at room temperature, transferring to an oven for continuous drying, and then performing cold pressing and slitting to obtain a negative electrode piece.

ii. Preparation of double-layer active Material layer negative electrodes (negative electrodes of examples 1 to 19 prepared according to the method)

Mixing a first active substance shown in examples 1-19 in Table 1, conductive carbon black Super-P, Styrene Butadiene Rubber (SBR) serving as a binder and sodium carboxymethyl cellulose (CMC) serving as a thickening agent according to a weight ratio of 96:1:2:1, and fully stirring and uniformly mixing in a deionized water solvent system to obtain slurry A;

mixing the second active substance shown in examples 1-19 in Table 1, conductive carbon black Super-P, Styrene Butadiene Rubber (SBR) as a binder and sodium carboxymethyl cellulose (CMC) as a thickening agent according to a weight ratio of 96:1:2:1, and fully stirring and uniformly mixing in a deionized water solvent system to obtain slurry B;

uniformly coating the slurry A on two surfaces of a negative current collector Cu foil, coating by referring to a unit coating weight value of the slurry A shown in examples 1-19 in table 1, airing at room temperature, transferring to an oven, and continuously drying to obtain a pole piece A coated with a first active material layer; and then coating the slurry B on the two surfaces of the pole piece A, coating according to the unit coating weight value of the slurry B shown in examples 1-19 in the table 1, drying, and then carrying out cold pressing and slitting to obtain the negative electrode with two active material layers.

(3) Preparation of the electrolyte

Mixing Ethylene Carbonate (EC) and Ethyl Methyl Carbonate (EMC) according to a volume ratio of 3:7 to obtain an organic solvent, and then fully drying LiPF₆Dissolving the mixture in the mixed organic solvent to prepare electrolyte with the concentration of 1 mol/L.

(4) Preparation of the separator

A PE/PP/PE three-layer porous polymer film is selected as an isolating film.

(5) Preparation of lithium ion secondary battery

Stacking the positive pole piece, the isolating film and the negative pole piece in sequence to enable the isolating film to be positioned between the positive pole piece and the negative pole piece to play an isolating role, and then winding to obtain a bare cell; and welding the qualified bare cell on the top cover through a tab, placing the qualified bare cell in an outer packaging shell, drying the qualified bare cell, injecting electrolyte, and performing vacuum packaging, standing, formation, shaping and other processes to obtain the lithium ion secondary battery.

Next, the methods for testing the apparent density, average pore diameter, particle diameter, and specific surface area of the active material and the methods for testing the characterization indices porosity, liquid-absorbing capacity, liquid-retaining capacity, cycle life, and cycle performance of the lithium ion secondary battery will be explained.

(1) The apparent density was measured using the following method:

taking active substance particles with certain mass, placing the active substance particles in a specific gravity bottle, soaking the active substance particles in a proper amount of kerosene, pouring out the excessive kerosene after a certain period of time, measuring the volume of the kerosene absorbed by the particles, wherein the volume is the pore volume, the apparent density of the particles is the weight of the particles/(the true volume and the pore volume), the true volume is calculated through the true density, and the true volume is the weight of the particles/the true density.

(2) The pore diameter was measured by the following method:

the particles are taken and placed in an SEM, and the pore size of the active material particles is read after the particles are amplified.

(3) Average particle diameter (D)₅₀) The following methods were used for the test of (1):

the average particle size of the active material particles was measured using a particle size tester.

(4) The specific surface area (BET) was measured by the following method:

and testing the specific surface area of the active material particles by using a specific surface area tester.

(5) The porosity was measured using the following method:

and testing the pore volume of the pole piece by using a specific surface area tester, wherein the porosity of the pole piece is equal to the pore volume of the pole piece/the total volume of the pole piece.

(6) The liquid absorption capacity was measured by the following method:

the liquid absorption capacity of the pole piece is tested by adopting an electrolyte soaking method, after the negative pole piece is subjected to cold pressing, 3 negative poles obtained in examples 1-19 and comparative examples 1 and 2 are respectively taken, the negative pole piece is placed into electrolyte with the same weight under the same condition, the negative pole piece is taken out after being soaked for 12 hours at normal temperature, the weight of the pole piece before and after soaking is weighed, and the liquid absorption capacity of the pole piece is (the weight of the pole piece after soaking-the weight of the pole piece before soaking)/the weight of the pole piece before soaking.

(7) The following method is adopted for testing the liquid retention capacity:

and testing the residual quantity of the electrolyte in the negative pole piece under the pressing force of 100N to represent the liquid retention capacity of the pole piece. After the pole pieces are cold-pressed, 9 of the negative electrodes obtained in examples 1 to 19 and comparative examples 1 and 2 are respectively taken, the negative pole pieces are placed into the electrolyte with the same weight under the same condition, the negative pole pieces are soaked for 12 hours at normal temperature and then taken out, the weight of the pole pieces before and after soaking is weighed, and the initial liquid retention capacity of the pole pieces is (the weight of the pole pieces after soaking-the weight of the pole pieces before soaking)/the weight of the pole pieces before soaking. Then 6 negative pole pieces which are absorbed with electrolyte are respectively taken from the negative poles obtained in the examples 1-19 and the comparative examples 1 and 2 and are placed between two steel plates, the steel plate-negative pole piece-steel plate structure is clamped by a clamp, then the clamp is applied with about 100N force by a stamping device, 3 negative poles obtained in the examples 1-19 and the comparative examples 1 and 2 are respectively kept for 30min, and 3 negative poles are respectively kept for 60min, the force application is stopped, the clamp is taken out, the negative pole piece is taken out after being disassembled, the weight of the negative pole piece is measured, the liquid retention capacity of the pole piece under 100N pressing force is obtained, and the liquid retention capacity of the pole piece is pressing force (the weight of the pole piece after the force application, the weight of the pole piece before being soaked)/the weight of.

(8) The cycle life was tested using the following method:

under the same conditions, the cells containing the negative electrodes obtained in examples 1 to 19 and comparative examples 1 and 2 were subjected to a charge-discharge test, and the cell bol (before of life), i.e., the first-cycle discharge capacity Cb, was recorded. And then carrying out cycle life detection, wherein the test condition is a normal temperature condition, 1C/1C cycle is carried out, the discharge capacity Ce during the cell is recorded at any time, the ratio Ce/Cb of Ce to Cb is the capacity retention rate in the cycle process, the test is stopped when the capacity retention rate is lower than or equal to 80%, EOL (end of life) is considered to be reached, the number of cycle turns of the cell when the EOL is reached is recorded, and the cycle life is judged by using the value.

(9) The following method is adopted for testing the cycle performance:

at normal temperature, the lithium ion secondary batteries manufactured by the negative electrodes obtained by the methods of examples 1 to 19 and comparative examples 1 and 2 are charged and discharged for the first time by using a current of 0.5C (namely, a current value of which the theoretical capacity is completely discharged within 2H), the charging is constant-current constant-voltage charging, the end voltage is 4.2V, the cut-off current is 0.05C, the discharge end voltage is 3.0V, then after the batteries are placed for 24H, a charge-discharge test is firstly carried out under the same condition, the constant-current constant-voltage charging is carried out by using a current of 1C, the end voltage is 4.2V, the cut-off current is 0.05C, the discharging is carried out by using a current of 1C, the discharge end voltage is 3.0V, a battery cell BOL (before off), namely, the discharge capacity at the first cycle, Cb is then a cycle life test is carried out, the test condition is that at normal temperature, 1C/1C cycle is carried out, the voltage range is 3.0 to 4.2V, and recording the capacity Ce in the discharge process in the circulating process, wherein the ratio Ce/Cb of Ce to Cb is the capacity retention rate in the circulating process, and judging the attenuation degree of the circulating life by using the value.