WO2024077522A1 - Negative electrode active material preparation method, negative electrode active material, secondary battery and electric apparatus - Google Patents

Abstract

The present application relates to a negative electrode active material preparation method, comprising the following steps: carrying out first negative-pressure roasting on natural graphite; after the first negative-pressure roasting of the natural graphite, coating same with a first liquid hard-carbon coating substance precursor, and pre-sintering same to form a primary coating layer; and coating the surface of the primary coating layer with a second liquid hard-carbon coating substance precursor, and carbonizing same to form a secondary coating layer. The present application also relates to a corresponding negative electrode active material, a secondary battery and an electric apparatus. The coating layers of the negative electrode active material have lower coating amounts and better coating uniformity, and can improve the storage performance, dynamic performance and cycle performance of batteries.

Description

Method for preparing negative electrode active material, negative electrode active material, secondary battery and electric device Technical Field

The present application relates to the technical field of secondary batteries, and in particular to a method for preparing a negative electrode active material, a negative electrode active material, a secondary battery and an electrical device.

Background technique

In recent years, as the application scope of secondary ion batteries has become more and more extensive, secondary batteries are widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields. As secondary batteries have made great progress, higher requirements have been put forward for their dynamic performance, storage performance and cycle performance.

The negative electrode is an important component of the secondary battery. The negative electrode active material has an important influence on the kinetic performance, storage performance and cycle performance of the secondary battery. With the rapid development of secondary batteries, higher requirements are also put forward for the performance of negative electrode active materials. Therefore, seeking negative electrode active materials with better performance is one of the research directions that technicians in this field focus on.

Summary of the invention

The present application is made in view of the above-mentioned problems, and one of its purposes is to provide a method for preparing a negative electrode active material. The coating layer of the negative electrode active material prepared by the preparation method has a low coating amount and good coating uniformity, which can improve the storage performance, kinetic performance and cycle performance of the battery.

In order to achieve the above object, the first aspect of the present application provides a method for preparing a negative electrode active material, comprising the following steps:

The natural graphite is subjected to a first negative pressure roasting;

Coating a first liquid hard carbon coating precursor on the natural graphite after the first negative pressure roasting, pre-burning, and forming a primary coating layer; and

A second liquid hard carbon coating precursor is coated on the surface of the primary coating layer and carbonized to form a secondary coating layer.

The present application performs a first negative pressure calcination treatment on the natural graphite before coating the first liquid hard carbon coating precursor on the natural graphite to form a primary coating layer; the first negative pressure calcination can discharge the moisture and gas adsorbed in the internal pores of the natural graphite, which is beneficial for the liquid hard carbon coating precursor to be evenly attached to the natural graphite in the subsequent coating step, reducing the coating amount, achieving the effect of uniform coating at a low coating amount, and enabling the battery using the negative electrode active material to have good storage performance and dynamic performance; moreover, the hard carbon coating layer can better fill the pores of the natural graphite to play a role in stabilizing the skeleton, effectively inhibiting the cyclic expansion of the natural graphite, and enabling the battery to have good cycle performance.

In any embodiment of the present application, the vacuum degree of the first negative pressure roasting is 0 to 0.5 atmospheres, the roasting temperature is 120 to 300° C., and the roasting time is 2 to 8 hours. Thus, it can be ensured that the moisture and gas adsorbed in the pores of natural graphite are fully removed without affecting the dynamic performance or increasing energy consumption.

In any embodiment of the present application, coating the first liquid hard carbon coating precursor on the natural graphite after the first negative pressure roasting comprises the following steps: transferring the natural graphite after the first negative pressure roasting to a vacuum coating device containing the first liquid hard carbon coating precursor under vacuum conditions, and vacuuming and stirring at 120 to 200° C. for 4 to 7 hours. In this way, it can be ensured that moisture and gas will not re-enter the pores during the transfer and coating process of the natural graphite, which is more conducive to uniform coating.

In any embodiment of the present application, the pre-firing temperature is 400-600°C and the pre-firing time is 0.5-2h. Thus, the first liquid hard carbon coating precursor can be better induced to form a uniform and thin primary coating layer on the natural graphite, and the primary coating layer can be well filled into the pores of the natural graphite.

In any embodiment of the present application, after forming the primary coating layer and before coating the surface of the primary coating layer with the second liquid hard carbon coating precursor, the preparation method further comprises the step of performing a second negative pressure roasting on the natural graphite. Thus, the moisture and gas adsorbed in the pores of the natural graphite can be discharged, which is conducive to the subsequent formation of a secondary coating layer with a lower coating amount and uniformity.

In any embodiment of the present application, the vacuum degree of the second negative pressure roasting is 0.1-0.3 atmospheres, the roasting temperature is 100-150°C, and the roasting time is 1-3 hours. In this way, it can be ensured that the moisture and gas adsorbed in the pores of natural graphite are fully removed, the uniformity of the secondary coating layer is improved, and the dynamic performance is not affected, and the energy consumption is not increased.

In any embodiment of the present application, coating the surface of the primary coating layer with a second liquid hard carbon coating precursor comprises the following steps: transferring the natural graphite after the second negative pressure roasting to a vacuum coating device containing the second liquid hard carbon coating precursor under vacuum conditions, and vacuuming and stirring at 120-200° C. for 2-4 hours. In this way, it can be ensured that moisture and gas will not re-enter the pores during the natural graphite transfer and secondary coating process, which is more conducive to uniform coating.

In any embodiment of the present application, the carbonization temperature is 800-1300°C, the carbonization time is 4-8 hours, and the carbonization is performed under an inert gas atmosphere. Thus, the secondary coating layer can be well filled into the pores of the natural graphite, better play the role of coating reinforcement, and improve the coating effect.

In any embodiment of the present application, the first liquid hard carbon coating precursor includes one or more of phenolic resin, epoxy resin, and petroleum resin, and the mass fraction of the resin in the first liquid hard carbon coating precursor is 30-70%.

In any embodiment of the present application, the second liquid hard carbon coating precursor includes one or more of phenolic resin, epoxy resin, and petroleum resin, and the mass fraction of the resin in the second liquid hard carbon coating precursor is 10-30%. Thus, the mass fraction of the resin in the second liquid hard carbon coating precursor is less than the mass fraction of the resin in the first liquid hard carbon coating precursor.

In any embodiment of the present application, the coating mass ratio of the primary coating layer to the natural graphite is 2-5%, which is a relatively low coating amount.

In any embodiment of the present application, the coating mass ratio of the secondary coating layer to the natural graphite is 1-3%. In this way, the preparation method of the present application can adopt a lower coating amount and achieve uniform coating of each coating layer.

In any embodiment of the present application, before the first negative pressure roasting is performed, the natural graphite satisfies at least one of the following conditions a to c:

a. The volume average particle size Dv ₅₀ is 6 to 15 μm;

Optionally, the volume average particle size Dv ₅₀ is 8 to 12 μm;

b. Volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.4;

Optionally, the volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.2;

c. Tap density TD is 0.7 to 1.1 g/cm ³ ;

Optionally, the tap density TD is 0.8 to 1.0 g/cm ³ .

In any embodiment of the present application, the negative electrode active material satisfies at least one of the following conditions d to g:

d. The volume average particle size _Dv50 is 7 to 17 μm;

Optionally, the volume average particle size Dv ₅₀ is 9 to 13 μm;

e. Volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.3;

Optionally, the volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.2;

f. Tap density TD is 0.7 to 1.1 g/cm ³ ;

Optionally, the tap density TD is 0.85 to 0.95 g/cm ³ ;

g, BET specific surface area is 1.0 to 5.0 m ² /g;

Optionally, the BET specific surface area is 1.5 to 3.5 m ² /g.

The second aspect of the present application further provides a negative electrode active material, which is prepared by the preparation method of the negative electrode active material described in the first aspect of the present application.

Therefore, the negative electrode active material has a lower coating amount and better coating uniformity, so that the battery using the negative electrode active material can have good storage performance, dynamic performance and cycle performance.

The third aspect of the present application also provides a secondary battery, comprising the negative electrode active material described in the second aspect of the present application.

The fourth aspect of the present application also provides an electrical device, comprising a secondary battery selected from the third aspect of the present application.

The preparation method of the negative electrode material of the present application can discharge the moisture and gas adsorbed in the pores of natural graphite by performing a first negative pressure calcination treatment on natural graphite before coating the first liquid hard carbon coating precursor, which is beneficial to enable the liquid hard carbon coating precursor to be evenly attached to the natural graphite in the subsequent coating step, reduce the coating amount, and achieve the effect of uniform coating at a low coating amount, so that the battery using the negative electrode active material has good storage performance and dynamic performance; moreover, the hard carbon coating layer can be better filled into the pores of natural graphite to play a role in stabilizing the skeleton, effectively inhibiting the cyclic expansion of natural graphite, so that the battery has good cycle performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope image of a negative electrode active material according to an embodiment of the present application.

FIG. 2 is a schematic diagram of a secondary battery according to an embodiment of the present application.

FIG. 3 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 2 .

FIG. 4 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

Description of reference numerals:

5. Secondary battery; 51. Casing; 52. Electrode assembly; 53. Cover plate; 6. Electrical device.

Detailed ways

Hereinafter, the method for preparing the negative electrode active material, the negative electrode active material, the secondary battery, the battery module, the battery pack and the implementation mode of the electric device of the present application will be specifically disclosed with appropriate reference to the drawings. However, there will be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following descriptions are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

The "range" disclosed in this application is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. The range defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60 to 120 and 80 to 110 is listed for a specific parameter, it is understood that the range of 60 to 110 and 80 to 120 is also expected. In addition, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4 and 5 are listed, the following ranges can all be expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4 and 2 to 5. In this application, unless otherwise specified, the numerical range "a-b" represents an abbreviation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0 to 5" means that all real numbers between "0 to 5" have been fully listed in this article, and "0 to 5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

If not otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

If there is no special explanation, all steps of the present application can be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

If there is no special explanation, the "include" and "comprising" mentioned in this application represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

If not specifically stated, in this application, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or A and B are both true (or exist).

At present, due to the great development of secondary batteries, higher requirements are also put forward for the kinetic performance, storage performance and cycle performance of secondary batteries. Secondary batteries with excellent performance have very high requirements for negative electrode active materials. Therefore, seeking negative electrode active materials with better performance is one of the research directions that technicians in this field focus on. In the traditional method of soft carbon coating on the surface of graphite, the coating layer is easily converted into graphite during the graphitization process, which may deteriorate the kinetic performance of the battery; the traditional method of using solid phase coating cannot guarantee the uniformity of the coating, and it is easy to bond to form extremely high active reaction sites, resulting in deterioration of storage performance. The inventor has studied and discovered a method for preparing a negative electrode active material, which uses natural graphite as a substrate, first performs a first negative pressure roasting on the natural graphite, and then forms a primary coating layer and a secondary coating layer on the natural graphite in sequence. The preparation method can obtain a coating layer with a low coating amount and uniform coating, which can improve the kinetic performance, storage performance and cycle performance of the battery.

In some embodiments, the first aspect of the present application provides a method for preparing a negative electrode active material, comprising the following steps:

The natural graphite is subjected to a first negative pressure roasting;

Coating a first liquid hard carbon coating precursor on the natural graphite after the first negative pressure roasting, pre-burning, and forming a primary coating layer; and

A second liquid hard carbon coating precursor is coated on the surface of the primary coating layer and carbonized to form a secondary coating layer.

The preparation method of the negative electrode active material of the present application performs a first negative pressure calcination treatment on the natural graphite before coating the first liquid hard carbon coating precursor on the natural graphite to form a primary coating layer, so as to discharge the moisture and gas adsorbed in the internal pores of the natural graphite, which is beneficial for the liquid hard carbon coating precursor to be evenly attached to the natural graphite in the subsequent coating step, thereby reducing the coating amount and achieving the effect of uniform coating at a low coating amount, so that the battery using the negative electrode active material can have good storage performance and dynamic performance.

Moreover, the first negative pressure roasting process can make the hard carbon coating layer better fill the pores of natural graphite, play a role in stabilizing the skeleton, effectively inhibit the cyclic expansion of natural graphite, and enable the battery using the negative electrode active material to have good cycle performance. Compared with artificial graphite, natural graphite has higher compaction density and capacity, which can improve the energy density of the battery.

In some embodiments, the vacuum degree of the first negative pressure roasting is 0 to 0.5 atmospheres, the roasting temperature is 120 to 300°C, and the roasting time is 2 to 8 hours. Thus, it can be ensured that the moisture and gas adsorbed in the pores of natural graphite are fully removed. When the vacuum degree is insufficient, the roasting temperature is too low, or the roasting time is too short, the moisture and gas in the pores of natural graphite will not be removed cleanly; when the roasting temperature is too high or the roasting time is too long, the functional groups on the surface of natural graphite will react, affecting the kinetic properties and increasing energy consumption.

It can be understood that in the above-mentioned "the vacuum degree is 0 to 0.5 atmospheres", the values include the minimum and maximum values of the range, and each value between the minimum and maximum values, and specific examples include but are not limited to the point values in the embodiments and: 0 atmosphere, 0.1 atmosphere, 0.15 atmosphere, 0.2 atmosphere, 0.25 atmosphere, 0.3 atmosphere, 0.35 atmosphere, 0.4 atmosphere, 0.45 atmosphere, 0.5 atmosphere. Similarly, in the above-mentioned "the calcination temperature is 120 to 300°C", the values include the minimum and maximum values of the range, and each value between the minimum and maximum values, and specific examples include but are not limited to the point values in the embodiments and: 120°C, 150°C, 180°C, 200°C, 220°C, 240°C, 260°C, 280°C, 300°C. In the above “calcination time is 2h to 8h”, the values include the minimum and maximum values of the range, and every value between the minimum and maximum values. Specific examples include but are not limited to the point values in the embodiments and: 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, and 8h.

In any embodiment of the present application, coating the first liquid hard carbon coating precursor on the natural graphite after the first negative pressure roasting includes the following steps: transferring the natural graphite after the first negative pressure roasting to a vacuum coating device containing the first liquid hard carbon coating precursor under vacuum conditions through a sealed channel, and vacuuming and stirring at 120 to 200° C. for 4 to 7 hours. Thus, after the first negative pressure roasting, the natural graphite is transferred to the vacuum coating device containing the first liquid hard carbon coating precursor through a sealed channel for coating, which can ensure that moisture and gas will not re-enter the pores during the transfer and coating of the natural graphite, which is more conducive to uniform coating.

It can be understood that one end of the sealed channel is connected to the roasting furnace for the first negative pressure roasting, and the other end is connected to the vacuum coating container. In the process of transferring the roasted natural graphite to the vacuum coating container, the vacuum state is maintained, and external moisture and gas are prevented from entering the sealed channel, thereby preventing moisture and gas from being re-adsorbed in the pores of the natural graphite. It can be understood that in the above "vacuuming and stirring at 120-200°C for 4-7h", the values of temperature and stirring time include the minimum and maximum values of the range, and each value between the minimum and maximum values. Specific examples include but are not limited to the temperature point values in the embodiment and: 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C; but not limited to the stirring time point values in the embodiment and: 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h.

In any embodiment of the present application, the pre-firing temperature is 400-600°C and the pre-firing time is 0.5-2h. Thus, the first liquid hard carbon coating precursor can be better induced to form a uniform and thin primary coating layer on the natural graphite, and the primary coating layer can be well filled into the pores of the natural graphite.

It can be understood that the above-mentioned "pre-burning temperature is 400-600°C", the value includes the minimum and maximum values of the range, and each value between the minimum and maximum values, and specific examples include but are not limited to the point values in the embodiment and: 400°C, 420°C, 440°C, 460°C, 480°C, 500°C, 520°C, 540°C, 560°C, 580°C, 600°C. The above-mentioned "pre-burning time is 0.5-2h", the value includes the minimum and maximum values of the range, and each value between the minimum and maximum values, and specific examples include but are not limited to the point values in the embodiment and: 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1.0h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h, 2h.

In any embodiment of the present application, after forming the primary coating layer and before coating the surface of the primary coating layer with the second liquid hard carbon coating precursor, the preparation method further comprises the step of performing a second negative pressure roasting on the natural graphite. Thus, the moisture and gas adsorbed in the pores of the natural graphite can be discharged before coating with the second liquid hard carbon coating precursor, which is conducive to the subsequent formation of a secondary coating layer with a lower coating amount and uniformity.

In any embodiment of the present application, the vacuum degree of the second negative pressure roasting is 0.1 to 0.3 atmospheres, the roasting temperature is 100 to 150°C, and the roasting time is 1 to 3 hours. In this way, it can be ensured that the moisture and gas adsorbed in the pores of natural graphite are fully removed and the uniformity of the secondary coating layer is improved. When the vacuum degree is insufficient, the roasting temperature is too low, or the roasting time is too short, the moisture and gas in the pores of natural graphite will not be removed cleanly; when the roasting temperature is too high or the roasting time is too long, the functional groups on the surface of natural graphite will react, affecting the kinetic properties and increasing energy consumption.

It can be understood that the above "vacuum degree is 0.1-0.3 atmospheres", the value includes the minimum and maximum values of the range, and each value between the minimum and maximum values, and specific examples include but are not limited to the point values in the embodiment and: 0.1 atmosphere, 0.15 atmosphere, 0.2 atmosphere, 0.25 atmosphere, 0.3 atmosphere. Similarly, the above "calcination temperature is 100-150°C", the value includes the minimum and maximum values of the range, and each value between the minimum and maximum values, and specific examples include but are not limited to the point values in the embodiment and: 100°C, 110°C, 120°C, 130°C, 140°C, 150°C. In the above “calcination time is 1 to 3 hours”, the values include the minimum and maximum values of the range, and every value between the minimum and maximum values. Specific examples include but are not limited to the point values in the embodiments and: 1h, 1.2h, 1.5h, 1.8h, 2.0h, 2.2h, 2.5h, 2.8h, 3h.

In any embodiment of the present application, coating the surface of the primary coating layer with a second liquid hard carbon coating precursor comprises the following steps: transferring the natural graphite after the second negative pressure roasting to a vacuum coating device containing the second liquid hard carbon coating precursor through a sealed channel under vacuum conditions, and vacuuming and stirring at 120 to 200° C. for 2 to 4 hours. Thus, after the second negative pressure roasting, the natural graphite is transferred to the vacuum coating device containing the second liquid hard carbon coating precursor through a sealed channel for coating, which can ensure that moisture and gas will not re-enter the pores during the natural graphite transfer and secondary coating process, which is more conducive to uniform coating.

Similarly, one end of the sealed channel is connected to the roasting furnace for the second negative pressure roasting, and the other end is connected to the vacuum coating container for the secondary coating. In the process of transferring the roasted natural graphite to the vacuum coating container, the vacuum state is maintained, and external moisture and gas are prevented from entering the sealed channel, thereby preventing moisture and gas from being re-adsorbed in the pores of the natural graphite. It can be understood that in the above "vacuuming and stirring at 120-200°C for 2-4h", the values of temperature and stirring time include the minimum and maximum values of the range, as well as every value between the minimum and maximum values. Specific examples include but are not limited to the temperature point values in the embodiment and: 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C; but are not limited to the stirring time point values in the embodiment and: 2h, 2.5h, 3h, 3.5h, 4h.

In any embodiment of the present application, the carbonization temperature is 800-1300°C, the carbonization time is 4-8h, and the carbonization is carried out under an inert gas atmosphere. Thus, the second liquid hard carbon coating precursor can be better induced to form a uniform and thin secondary coating layer on the natural graphite, so that the secondary coating layer can be well filled into the pores of the natural graphite, better play the role of coating reinforcement, and improve the coating effect.

It is understood that the carbonization temperature includes but is not limited to the following specific values: 800° C., 850° C., 900° C., 950° C., 1000° C., 1050° C., 1100° C., 1150° C., 1200° C., 1250° C., 1300° C. The carbonization time includes but is not limited to the following specific values: 4h, 4.4h, 4.8h, 5h, 5.4h, 5.8h, 6h, 6.4h, 6.8h, 7h, 7.4h, 7.8h, 8h.

In any embodiment of the present application, the first liquid hard carbon coating precursor includes one or more of phenolic resin, epoxy resin, and petroleum resin, and the mass fraction of the resin in the first liquid hard carbon coating precursor is 30-70%. That is, the first liquid hard carbon coating precursor may contain only any one of phenolic resin, epoxy resin, and petroleum resin, or may contain any two or a combination of three of the above three resins. The mass fraction of the resin in the first liquid hard carbon coating precursor may be, but is not limited to, the following specific values: 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%.

In any embodiment of the present application, the second liquid hard carbon coating precursor includes one or more of phenolic resin, epoxy resin, and petroleum resin, and the mass fraction of the resin in the second liquid hard carbon coating precursor is 10 to 30%. Generally, the mass fraction of the resin in the second liquid hard carbon coating precursor is less than the mass fraction of the resin in the first liquid hard carbon coating precursor.

In any embodiment of the present application, the coating mass ratio of the primary coating layer to the natural graphite is 2-5%. Since the preparation method of the present application adopts negative pressure roasting, the moisture and gas in the pores of the natural graphite are eliminated, so that the liquid hard carbon coating precursor can easily enter the pores; therefore, the primary coating layer of the present application can achieve uniform coating at a low coating amount of 2-5%.

In any embodiment of the present application, the coating mass ratio of the secondary coating layer to the natural graphite is 1-3%. The secondary coating layer mainly plays a role in reinforcing the coating of the primary coating layer and improving the coating effect. Similarly, the preparation method of the present application can use a lower secondary coating amount to achieve a good coating effect.

In any embodiment of the present application, before the first negative pressure roasting is performed, the natural graphite satisfies at least one of the following conditions a to c:

a. The volume average particle size Dv ₅₀ is 6 to 15 μm;

Optionally, the volume average particle size Dv ₅₀ is 8 to 12 μm;

b. Volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.4;

Optionally, the volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.2;

c. Tap density TD is 0.7 to 1.1 g/cm ³ ;

Optionally, the tap density TD is 0.8 to 1.0 g/cm ³ .

It can be understood that the volume average particle size Dv ₅₀ of natural graphite can be, but is not limited to, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm. The volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ can be, but is not limited to, 1.0, 1.1, 1.2, 1.3, 1.4. The tap density TD can be, but is not limited to, 0.7 g/cm ³ , 0.8 g/cm ³ , 0.9 g/cm ³ , 1.0 g/cm ³ , 1.1 g/cm ³ .

In any embodiment of the present application, the negative electrode active material satisfies at least one of the following conditions d to g:

d. The volume average particle size _Dv50 is 7 to 17 μm;

Optionally, the volume average particle size Dv ₅₀ is 9 to 13 μm;

e. Volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.3;

Optionally, the volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.2;

f. Tap density TD is 0.7 to 1.1 g/cm ³ ;

Optionally, the tap density TD is 0.85 to 0.95 g/cm ³ ;

g, BET specific surface area is 1.0 to 5.0 m ² /g;

Optionally, the BET specific surface area is 1.5 to 3.5 m ² /g.

It can be understood that the volume average particle size Dv ₅₀ of the negative electrode active material can be, but is not limited to, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm. The volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ can be, but is not limited to, 1.0, 1.1, 1.2, 1.3. The tap density TD can be, but is not limited to, 0.7 g/cm ³ , 0.8 g/cm ³ , 0.9 g/cm ³ , 1.0 g/cm ³ , 1.1 g/cm ³ . The BET specific surface area may be, but is not limited to, 1.0 ^m2 /g, ^1.4 m2/g, 1.8 m2/g, ^{2.0 m2/g, 2.4} m2/g, 2.8 m2/g ^{, 3.0} m2/g, 3.4 ^m2 /g, ^{3.8 m2} /g ^{, 4.0 m2} /g, ^4.4 m2/g ^, 4.8 m2/g, and ^{5.0 m2} /g.

The second aspect of the present application further provides a negative electrode active material, which is prepared by the preparation method of the negative electrode active material described in the first aspect of the present application.

Therefore, the negative electrode active material has a lower coating amount and better coating uniformity, so that the battery using the negative electrode active material can have good storage performance, dynamic performance and cycle performance.

The third aspect of the present application also provides a secondary battery, comprising the negative electrode active material described in the second aspect of the present application.

The fourth aspect of the present application also provides an electrical device, comprising a secondary battery selected from the third aspect of the present application.

The secondary battery and the electric device of the present application will be described below with reference to the drawings as appropriate.

Unless otherwise specified, the components, material types or contents of the batteries mentioned are applicable to both lithium-ion secondary batteries and sodium-ion secondary batteries.

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

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

Positive electrode

The positive electrode plate includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, wherein the positive electrode film layer includes the positive electrode active material of the first aspect of the present application.

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

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

In some embodiments, the positive electrode active material may include a positive electrode active material for a battery known in the art. As an example, the positive electrode active material may include at least one of the following materials: an olivine-structured lithium-containing phosphate, a lithium transition metal oxide, and their respective modified compounds. However, the present application is not limited to these materials, and other conventional materials that can be used as positive electrode active materials for batteries may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to _, lithium cobalt oxide (such as _LiCoO2 ), lithium nickel oxide (such as _LiNiO2 ), lithium manganese oxide (such as _LiMnO2 , _LiMn2O4 ), lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel _cobalt manganese oxide (such as LiNi1 _/ 3Co1 _{/ 3Mn1 / 3O2} (also referred to as _NCM333 ), _{LiNi0.5Co0.2Mn0.3O2} (also referred _{to as NCM523 ) , LiNi0.5Co0.25Mn0.25O2} (also referred _to as _{NCM211 )} , _{LiNi0.6Co0.2Mn0.2O2} (also referred to as _NCM622 ), _{LiNi0.8Co0.1Mn0.1O2} (also referred to _{as NCM811} ), lithium _{nickel cobalt} aluminum oxide ( _such as LiNi _0.85 Co _0.15 Al _0.05 O ₂ ) and at least one of modified compounds thereof. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, at least one of lithium iron phosphate (such as LiFePO ₄ (also referred to as LFP)), a composite material of lithium iron phosphate and carbon, lithium manganese phosphate (such as LiMnPO ₄ ), a composite material of lithium manganese phosphate and carbon, lithium iron manganese phosphate, and a composite material of lithium iron manganese phosphate and carbon.

As an example, the positive electrode active material of the sodium ion secondary battery may include at least one of the following materials: at least one of a sodium transition metal oxide, a polyanionic compound, and a Prussian blue compound. However, the present application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for sodium ion batteries may also be used.

As an optional technical solution of the present application, in the sodium transition metal oxide, the transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. The sodium transition metal oxide is, for example, Na _x MO ₂ , wherein M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, and 0＜x≤1.

As an optional technical solution of the present application, the polyanionic compound can be a class of compounds having sodium ions, transition metal ions and tetrahedral (YO ₄ ) ^n- anion units. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce; Y can be at least one of P, S and Si; n represents the valence state of (YO ₄ ) ^n- .

The polyanionic compound may also be a compound having sodium ions, transition metal ions, tetrahedral (YO ₄ ) ^n- anion units and halogen anions. The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce; Y may be at least one of P, S and Si, and n represents the valence state of (YO ₄ ) ^n- ; the halogen may be at least one of F, Cl and Br.

The polyanionic compound may also be a compound having sodium ions, tetrahedral (YO ₄ ) ^n- anion units, polyhedral units (ZO _y ) ^m+ and optional halogen anions. Y may be at least one of P, S and Si, and n represents the valence state of (YO ₄ ) ^n- ; Z represents a transition metal, and may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce, and m represents the valence state of (ZO _y ) ^m+ ; the halogen may be at least one of F, Cl and Br.

The polyanionic compound is, for example, at _{least one of NaFePO4, Na3V2(PO4)3 (sodium vanadium phosphate, abbreviated as NVP), Na4Fe3(PO4)2 ( P2O7 ) , NaM'PO4F ( M '} is one or more of V, Fe, _Mn and Ni) and _Na3 ( _VOy ) ₂ ( _PO4 ) _2F3-2y ( _0≤y≤1 ).

The Prussian blue compound may be a compound having sodium ions, transition metal ions and cyanide ions (CN ^- ). The transition metal may be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. The Prussian blue compound is, for example, Na _a Me _b Me' _c (CN) ₆ , wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co and Zn, 0＜a≤2, 0＜b＜1, 0＜c＜1.

The weight ratio of the positive electrode active material in the positive electrode film layer is 80 to 100 weight %, based on the total weight of the positive electrode film layer.

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

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

In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry, wherein the positive electrode slurry has a solid content of 40-80wt%, and the viscosity at room temperature is adjusted to 5000-25000mPa·s, the positive electrode slurry is coated on the surface of the positive electrode collector, and after drying, it is cold-pressed by a cold rolling mill to form a positive electrode sheet; the positive electrode powder coating unit area density is 150-350mg/ ^m2 , and the positive electrode sheet compaction density is 3.0-3.6g/ ^cm3 , and can be optionally 3.3-3.5g/ ^cm3 .

The calculation formula of the compacted density is:

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

Negative electrode

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

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

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

In some embodiments, the negative electrode active material is the negative electrode active material described above in this application, or the negative electrode active material prepared by the preparation method described above in this application.

In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS). The weight ratio of the binder in the negative electrode film layer is 0 to 30% by weight, based on the total weight of the negative electrode film layer.

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

In some embodiments, the negative electrode film layer may further include other additives, such as a thickener (such as sodium carboxymethyl cellulose (CMC-Na)), etc. The weight ratio of the other additives in the negative electrode film layer is 0 to 15 weight %, based on the total weight of the negative electrode film layer.

In some embodiments, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent (such as deionized water) to form a negative electrode slurry, wherein the solid content of the negative electrode slurry is 30-70wt%, and the viscosity at room temperature is adjusted to 2000-10000mPa·s; the obtained negative electrode slurry is coated on the negative electrode collector, and after a drying process, cold pressing such as rolling, a negative electrode sheet is obtained. The negative electrode powder coating unit area density is 75-220mg/ ^m2 , and the negative electrode sheet compaction density is 1.2-2.0g/ ^m3 .

Electrolytes

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

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

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

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

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

Isolation film

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

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

In some embodiments, the isolation film has a thickness of 6 to 40 μm, and may be 12 to 20 μm.

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

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

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

The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square or any other shape. For example, FIG2 is a secondary battery 5 of a square structure as an example.

In some embodiments, referring to FIG. 3 , the outer package may include a shell 51 and a cover plate 53. Among them, the shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can form an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

In some embodiments, the secondary batteries 5 can be assembled into a battery module. The number of secondary batteries 5 contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

In the battery module, the plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

Optionally, the battery module may further include a housing having a housing space, and the plurality of secondary batteries 5 are housed in the housing space.

In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art according to the application and capacity of the battery pack.

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

In addition, the present application also provides an electrical device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, and can also be used as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

As the electrical device, a secondary battery, a battery module or a battery pack may be selected according to its usage requirements.

FIG4 is an example of an electric device. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the electric device's requirements for high power and high energy density of secondary batteries, a battery pack or a battery module may be used.

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

Example

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

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

1. Preparation Example

1. Preparation of negative electrode active materials

Preparation Example 1

(1) natural graphite green balls with a volume average particle size _Dv50 of 10 μm were calcined at 200°C for 3 h under a vacuum of 0.1 atmosphere;

(2) transferring the calcined natural graphite from the calcining device to a vacuum coating reactor containing a first liquid hard carbon coating precursor through a sealed pipe, evacuating the reactor at 180° C. and continuously stirring the reactor for 4 hours; wherein the mass fraction of the resin in the first liquid hard carbon coating precursor is 50%;

(3) collecting the natural graphite coated with the first liquid hard carbon coating precursor, and pre-calcining it under a nitrogen atmosphere at 500° C. for 1 h to induce the formation of a primary coating layer filling the pores of the natural graphite; the coating mass ratio of the primary coating layer is 3%;

(4) calcining the natural graphite with the primary coating layer at a vacuum of 0.1 atmosphere and a temperature of 200° C. for 1 hour;

(5) transferring the calcined natural graphite from the calcining device to a vacuum coating reactor containing a second liquid hard carbon coating precursor (20%) through a sealed pipe, and continuously stirring for 2 hours under vacuum at 120° C.; wherein the mass fraction of the resin in the second liquid hard carbon coating precursor is 20%;

(6) Collect natural graphite coated with the second liquid hard carbon coating precursor, and carbonize it under a nitrogen atmosphere at 1100°C for 6 hours to form a thin and uniform secondary coating layer; the coating mass ratio of the secondary coating layer is 2%; and obtain a negative electrode active material having a primary coating layer and a secondary coating layer. The scanning electron microscope image of the negative electrode active material is shown in Figure 1.

Preparation Example 2:

This preparation example is substantially the same as preparation example 1, except that the carbonization temperature in step (6) is 1200°C.

Preparation Example 3:

This preparation example is substantially the same as preparation example 1, except that the carbonization temperature in step (6) is 1300°C.

Preparation Example 4:

This preparation example is basically the same as preparation example 1, except that the carbonization treatment time in step (6) is 7 hours.

Preparation Example 5:

This preparation example is basically the same as preparation example 1, except that the carbonization treatment time in step (6) is 5 hours.

Preparation Example 6:

This preparation example is substantially the same as preparation example 1, except that in step (1), the volume average particle size Dv ₅₀ of the natural graphite green balls is 12 μm.

Preparation Example 7:

This preparation example is substantially the same as preparation example 1, except that in step (1), the vacuum degree is adjusted to 0.15 atmospheres.

Preparation Example 8:

This preparation example is basically the same as preparation example 1, except that the calcination temperature in step (1) is adjusted to 250°C.

Preparation Example 9:

This preparation example is basically the same as preparation example 1, except that the calcination treatment time in step (1) is adjusted to 5 h.

Preparation Example 10:

This preparation example is basically the same as preparation example 1, except that in step (2), the vacuum is drawn and the stirring temperature is adjusted to 200°C.

Preparation Example 11:

This preparation example is basically the same as preparation example 1, except that the vacuum stirring time in step (2) is adjusted to 6 hours.

Preparation Example 12:

This preparation example is basically the same as the preparation example 1, except that the mass fraction of the resin in the first liquid hard carbon coating precursor selected in step (2) is adjusted to 70%.

Preparation Example 13:

This preparation example is basically the same as preparation example 1, except that the pre-sintering temperature in step (3) is adjusted to 600°C.

Preparation Example 14:

This preparation example is basically the same as preparation example 1, except that the pre-calcination treatment time in step (3) is adjusted to 2 h.

Preparation Example 15:

This preparation example is basically the same as preparation example 1, except that in step (3), the coating mass ratio of the primary coating layer is 4%.

Preparation Example 16:

This preparation example is substantially the same as preparation example 1, except that in step (4), the vacuum degree of the natural graphite with the primary coating layer during calcination is adjusted to 0.15 atmospheres.

Preparation Example 17:

This preparation example is basically the same as preparation example 1, except that in step (4), the calcination time of the natural graphite with the primary coating layer is adjusted to 1.5 h.

Preparation Example 18:

This preparation example is basically the same as preparation example 1, except that in step (4), the calcination time of the natural graphite with the primary coating layer is adjusted to 2 h.

Preparation Example 19:

This preparation example is substantially the same as preparation example 1, except that the temperature during vacuuming and continuous stirring in step (5) is adjusted to 150°C.

Preparation Example 20:

This preparation example is basically the same as preparation example 1, except that the time for continuous stirring during vacuuming in step (5) is adjusted to 2.5 h.

Preparation Example 21:

This preparation example is basically the same as the preparation example 1, except that the mass fraction of the resin in the second liquid hard carbon coating precursor selected in step (5) is adjusted to 10%.

Preparation Example 22:

This preparation example is basically the same as preparation example 1, except that the negative pressure calcination conditions in step (1) are calcination temperature of 300° C. and calcination time of 8 h.

Preparation Comparative Example 1:

This comparative preparation example is substantially the same as the preparation example 1, except that in step (1), the volume average particle size Dv ₅₀ of the natural graphite is 18 μm.

Preparation Comparative Example 2:

The preparation comparative example is basically the same as the preparation example 1, except that the natural graphite green balls are not subjected to the negative pressure calcination treatment in step (1), but are directly coated with the first liquid hard carbon coating precursor.

2. Negative electrode active material performance test

2.1 Material Gram Capacity Test

The prepared negative electrode active material, conductive agent Super P, binder (PVDF) and solvent NMP (N-methylpyrrolidone) are mixed uniformly in a mass ratio of 91.6:1.8:6.6 to prepare slurry; the prepared slurry is coated on a copper foil current collector, dried in an oven and cold pressed for standby use, with a density range of 1.4-1.6 g/cm ³ ; a metal lithium sheet is used as a counter electrode; a polyethylene (PE) film is used as an isolation membrane; ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1, and then LiPF ₆ is uniformly dissolved in the above solution to obtain an electrolyte.

The concentration of LiPF ₆ is 1 mol/L; the above parts are assembled into a CR2430 button cell in an argon-protected glove box. After the button cell is left to stand for 12 hours, it is discharged at a constant current of 0.05C to 0.005V, left to stand for 10 minutes, and then discharged at a constant current of 50μA to 0.005V, left to stand for 10 minutes, and then discharged at a constant current of 10μA to 0.005V. The sum of the three discharge capacities is the discharge capacity; then it is charged at a constant current of 0.1C to 2.000V, and the charge capacity is recorded. The ratio of the charge capacity to the mass of the negative electrode active material is the gram capacity of the prepared negative electrode active material, and the ratio of the charge capacity to the discharge capacity is the first coulomb efficiency.

2.2 Volume average particle size Dv ₅₀ test

Take 0.1g of the prepared negative electrode active material and add it to a clean beaker filled with 20mL of water for 5 minutes of ultrasound. After the parameters of the Malvern 3000 laser particle size analyzer are adjusted, test the background first, and then add the prepared sample to the equipment. After the shading range reaches 8% to 12%, the test begins. The particle size distribution is tested based on the diffraction or scattering phenomenon caused by laser irradiation to the particles. Large particles cause a small angle of scattered light, and the larger the angle, the smaller the particles. Through the Richter lens, a halo of different radii is formed in the focal plane. A halo with a large radius corresponds to a smaller particle size, and a small halo corresponds to a large particle size. The number information of the particle size can be analyzed by the signature of the halo light, and the particle size distribution is obtained by converting it into an electrical signal through a photoelectric receiver and processing it by a computer.

The meaning of Dv ₅₀ data: 50% of the total volume of particles have a diameter greater than this value, and another 50% of the total volume of particles have a diameter less than this value. Dv ₅₀ represents the median particle size of the powder.

2.3 Particle size distribution width test

Particle size distribution width = (Dv ₉₀ - Dv ₁₀ )/Dv ₅₀ ; volume distribution data obtained by laser particle size test can be calculated using the formula.

The different product parameters of the negative electrode active materials of the preparation examples 1 to 22 and the negative electrode active materials of the preparation comparative examples 1 to 2 are shown in Table 1.

Table 1 Different product parameters of the negative electrode active materials of Examples 1 to 22 and the negative electrode active materials of Comparative Examples 1 to 2

2. Application Examples

Example 1

(1) Preparation of positive electrode sheet

The positive electrode active material, conductive carbon black SP and binder PVDF were dispersed in solvent NMP at a weight ratio of 98:1:1 and mixed evenly to obtain positive electrode slurry; the positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and after drying and cold pressing, a positive electrode sheet was obtained, and the coating amount per unit area was 0.27g/ ^1540.25mm2 .

(2) Preparation of negative electrode sheet

The negative electrode active material prepared in Preparation Example 1 of the present application, the thickener sodium carboxymethyl cellulose, the binder styrene butadiene rubber, and the conductive agent acetylene black were mixed in a mass ratio of 97:1:1:1, and deionized water was added to obtain a negative electrode slurry under the action of a vacuum mixer; the negative electrode slurry was evenly coated on a copper foil; the copper foil was dried at room temperature and then transferred to a 120°C oven for drying for 1 hour, and then supercooled pressed and cut to obtain a negative electrode sheet, and the coating amount per unit area was 0.17g/ ^1540.25mm2 .

(3) Isolation film

A 12μm thick polypropylene isolation film was selected.

(4) Preparation of electrolyte

The organic solvent is a mixed solution containing ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC), wherein the volume ratio of EC, EMC and DEC is 20:20:60. In an argon atmosphere glove box with a water content of <10ppm, fully dried lithium salt LiPF ₆ is dissolved in the organic solvent and mixed evenly to obtain an electrolyte. The concentration of the lithium salt is 1 mol/L.

(5) Preparation of batteries

The positive electrode sheet, isolation film, and negative electrode sheet are stacked in order, so that the isolation film is placed between the positive and negative electrode sheets to play an isolating role. Then, they are wound into a square bare battery cell, loaded with aluminum-plastic film, and then baked at 80°C to remove water, and 10g of the corresponding non-aqueous electrolyte is injected and sealed. After standing, hot and cold pressing, formation, clamping, capacity division and other processes, a finished battery with a capacity of 4000mAh is obtained.

The secondary batteries of Examples 2 to 22 and the secondary batteries of Comparative Examples 1 to 2 were prepared in a similar manner to the secondary battery of Example 1, but the negative electrode active materials of the corresponding preparation examples were used.

3. Battery performance test

1. Storage days test

At 25°C, the soft-pack batteries prepared in the examples and comparative examples were left standing for 12 hours, then discharged at a constant current of 1C to 2.8V, left standing for 5 minutes, and then charged at a constant current of 0.33C to 4.2V, and charged at a constant voltage to 0.05C. After standing for 5 minutes, the batteries were discharged at a constant current of 1C to 2.8V, and the measured initial capacity C0 of the battery was obtained. After standing for 10 minutes, the battery was charged to 100% SOC (State of Charge) again according to the above charging process, and then stored in a 60°C thermostat until the capacity retention rate (Cn/C0×100%) was ≤80%, and the storage days were recorded. The more storage days, the better the storage life of the battery.

2. Fast charging performance test

Under 25°C, the secondary batteries prepared in the embodiments and comparative examples were charged to 4.25V at a constant current of 1C (i.e., the current value at which the theoretical capacity is completely discharged within 1h), then charged to 0.05C at a constant voltage, left to stand for 5min, and then charged to 4.25V or 0V negative electrode cutoff potential (whichever is reached first) at a constant current of 0.5C, 1C0, 1.5C0, 2C0, 2.5C0, 3C0, 3.5C0, 4C0, 4.5C0 (whichever is reached first). After each charging, it was discharged to 2.8V at 1C0. The corresponding negative electrode potentials when charged to 10%, 20%, 30% ... 80% SOC (state of charge) at different charging rates were recorded, and different charge states were plotted. The rate-negative electrode potential curve under SOC state is used to draw the charging window under different SOC states, and the corresponding critical rates C20% SOC, C30% SOC, C40% SOC, C50% SOC, C60% SOC, C70% SOC, and C80% SOC are recorded. The charging time T (min) of the battery from 10% SOC to 80% SOC can be calculated by the following formula (60/C20% SOC+60/C30% SOC+60/C40% SOC+60/C50% SOC+60/C60% SOC+60/C70% SOC+60/C80% SOC)×10%. The shorter the time, the better the fast charging performance of the battery.

3. Cycle performance test

At 25°C, the soft-pack batteries prepared in the examples and comparative examples were left to stand for 12 hours, then discharged at a constant current of 1C to 2.8V, left to stand for 5 minutes, and then charged at a constant current of 0.33C to 4.2V, charged at a constant voltage to 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 1C to 2.8V to obtain the measured initial capacity C0 of the battery. Subsequently, the battery was charged to the cut-off voltage at 2C and discharged to the cut-off voltage at 1C, and the discharge capacity Cn after each cycle was recorded until the cycle capacity retention rate (Cn/C0×100%) was ≤80%, and the number of cycles was recorded. The more cycles, the better the cycle life of the battery.

4. Test results of various embodiments and comparative examples

The batteries of the embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The results are shown in Table 2 below.

Table 2 Performance parameters of the batteries of various embodiments and comparative examples

It can be learned from the above embodiments and comparative examples that: compared with Example 1, adjusting the average particle size of natural graphite raw balls to Dv ₅₀ of 12 μm in step (1) (Example 6) can increase the storage and cycle days of the battery, but will deteriorate its kinetic performance; adjusting the average particle size of natural graphite raw balls to Dv ₅₀ of 18 μm (Comparative Example 1) will deteriorate the storage, cycle days and kinetic performance of the battery; after increasing the solid content of the first liquid hard carbon coating in step (2) (Example 12), the storage, cycle and kinetic performance of the battery are all deteriorated; after changing the negative pressure roasting conditions in step (1) to a roasting temperature of 300°C and a roasting time of 8 hours (Example 22), the storage, cycle days and kinetic performance of the material are all improved; if the first negative pressure roasting is not performed in step (1) (Comparative Example 2), the storage, cycle and kinetic performance of the battery are all deteriorated.

It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims (17)

A method for preparing a negative electrode active material comprises the following steps: The natural graphite is subjected to a first negative pressure roasting; Coating a first liquid hard carbon coating precursor on the natural graphite after the first negative pressure roasting, pre-burning, and forming a primary coating layer; and A second liquid hard carbon coating precursor is coated on the surface of the primary coating layer and carbonized to form a secondary coating layer. The preparation method according to claim 1 is characterized in that the vacuum degree of the first negative pressure roasting is 0 to 0.5 atmospheres, the roasting temperature is 120 to 300° C., and the roasting time is 2 to 8 hours. The preparation method according to claim 1 is characterized in that coating the first liquid hard carbon coating precursor on the natural graphite after the first negative pressure roasting comprises the following steps: The natural graphite after the first negative pressure calcination is transferred to a vacuum coating device containing the first liquid hard carbon coating precursor under vacuum conditions, and vacuumed and stirred at 120-200° C. for 4-7 hours. The preparation method according to claim 1 is characterized in that the pre-firing temperature is 400 to 600° C. and the pre-firing time is 0.5 to 2 hours. The preparation method according to any one of claims 1 to 4 is characterized in that after forming a primary coating layer and before coating a second liquid hard carbon coating precursor on the surface of the primary coating layer, the preparation method further comprises a step of performing a second negative pressure calcination on the natural graphite. The preparation method according to claim 5 is characterized in that the vacuum degree of the second negative pressure roasting is 0.1 to 0.3 atmospheres, the roasting temperature is 100 to 150° C., and the roasting time is 1 to 3 hours. The preparation method according to claim 5 is characterized in that coating the surface of the primary coating layer with a second liquid hard carbon coating precursor comprises the following steps: The natural graphite after the second negative pressure calcination is transferred to a vacuum coating device containing the second liquid hard carbon coating precursor under vacuum conditions, and is vacuumed and stirred at 120-200° C. for 2-4 hours. The preparation method according to any one of claims 1 to 7, characterized in that the carbonization temperature is 800 to 1300° C., the carbonization time is 4 to 8 hours, and the carbonization is carried out under an inert gas atmosphere. The preparation method according to any one of claims 1 to 7 is characterized in that the first liquid hard carbon coating precursor comprises one or more of phenolic resin, epoxy resin, and petroleum resin, and the mass fraction of the resin in the first liquid hard carbon coating precursor is 30-70%. The preparation method according to any one of claims 1 to 7 is characterized in that the second liquid hard carbon coating precursor comprises one or more of phenolic resin, epoxy resin, and petroleum resin, and the mass fraction of the resin in the second liquid hard carbon coating precursor is 10-30%. The preparation method according to any one of claims 1 to 10, characterized in that the coating mass ratio of the primary coating layer to the natural graphite is 2-5%. The preparation method according to any one of claims 1 to 10, characterized in that the coating mass ratio of the secondary coating layer to the natural graphite is 1 to 3%. The preparation method according to any one of claims 1 to 12, characterized in that before the first negative pressure roasting, the natural graphite satisfies at least one of the following conditions a to c: a. The volume average particle size Dv ₅₀ is 6 to 15 μm; Optionally, the volume average particle size Dv ₅₀ is 8 to 12 μm; b. Volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.4; Optionally, the volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.2; c. Tap density TD is 0.7 to 1.1 g/cm ³ ; Optionally, the tap density TD is 0.8 to 1.0 g/cm ³ . The preparation method according to any one of claims 1 to 12, characterized in that the negative electrode active material satisfies at least one of the following conditions d to g: d. The volume average particle size _Dv50 is 7 to 17 μm; Optionally, the volume average particle size Dv ₅₀ is 9 to 13 μm; e. Volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.3; Optionally, the volume average particle size (Dv ₉₀ -Dv ₁₀ )/Dv ₅₀ is 1.0 to 1.2; f. Tap density TD is 0.7 to 1.1 g/cm ³ ; Optionally, the tap density TD is 0.85 to 0.95 g/cm ³ ; g, BET specific surface area is 1.0 to 5.0 m ² /g; Optionally, the BET specific surface area is 1.5 to 3.5 m ² /g. A negative electrode active material, characterized in that the negative electrode active material is prepared by the method for preparing a negative electrode active material according to any one of claims 1 to 14. A secondary battery, characterized by comprising the negative electrode active material according to claim 15. An electrical device, characterized by comprising the secondary battery according to claim 16.

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