CN115084477B - Preparation method of graphite negative electrode material, graphite negative electrode and application thereof - Google Patents

Abstract

The present invention discloses a method for preparing a graphite negative electrode material, a graphite negative electrode and its application. The preparation method comprises the following steps: (1) preparing coke aggregates by subjecting several kinds of cokes to coarse crushing, grinding and grading to obtain corresponding coke aggregates; (2) preparing modified cokes by adding acidic solutions to several kinds of coke aggregates respectively, stirring to obtain slurry, washing, filtering, dehydrating and drying the slurry to obtain corresponding first materials; (3) preparing a second material by mixing at least two first materials to obtain a second material, or by mixing at least one coke aggregate and at least one first material to obtain a second material; (4) coating and graphitizing by mixing the second material with a coating agent and then granulating and graphitizing. The present invention firstly subjects the coke aggregates treated in the early stage to an acidic solution modification treatment, thereby obtaining a graphite negative electrode material having both high dynamics and high granulation performance and improving the cycle performance of a secondary battery.

Description

Preparation method of graphite negative electrode material, graphite negative electrode and application thereof

Technical Field

The present invention relates to the technical field of battery material preparation, and in particular to a method for preparing a graphite negative electrode material, a graphite negative electrode and applications thereof.

Background technique

Secondary batteries are widely used in the fields of power and consumer new energy due to their high energy density, excellent kinetic performance, long cycle life, clean and environmentally friendly characteristics. Among them, graphite anode is still the mainstream of the current market demand for anode materials, but as consumers' requirements for secondary batteries with high specific capacity and high kinetic performance become more and more stringent, graphite anodes with only a single high specific capacity or high kinetic performance are increasingly difficult to meet consumer demand.

At present, the conventional solution for graphite negative electrodes with both high specific capacity and high kinetic performance is to simply mix a certain proportion of graphite materials with high specific capacity and graphite materials with high kinetic performance in order to achieve the best performance of both. However, in actual battery applications, this simple mixing of graphitized products is prone to uneven charging and discharging, resulting in the risk of obvious lithium dendrite precipitation, and even battery safety accidents.

On the other hand, the combination of high specific capacity raw materials and high kinetic characteristics raw materials, mixed granulation to form a composite secondary particle structure has become a promising research direction. However, the existing single or mixed granulation technology of high specific capacity raw materials and high kinetic characteristics raw materials often faces a series of problems such as low granulation degree, low bonding strength and uneven granulation in the actual production process, which will further affect the performance of graphite negative electrode such as cycle expansion and cycle life during battery cycling.

In order to improve the low bonding strength and poor stability of secondary particles after granulation of single or mixed raw materials, insufficient battery kinetic performance, high risk of cycle expansion and cycle life deterioration, it is urgent to develop a graphite negative electrode material with high kinetics, low expansion and long life performance.

Summary of the invention

Based on the above problems, the present invention can obtain a graphite negative electrode material with high dynamic performance and granulation performance by modifying the raw material coke, which can greatly improve the cycle performance of the secondary battery.

To achieve the above object, the present invention provides a method for preparing a graphite negative electrode material in a first aspect, comprising the steps of:

(1) Preparation of coke aggregate

The cokes are roughly crushed, ground and classified to obtain corresponding coke aggregates;

(2) Preparation of modified coke

Acidic solutions are added to the several types of coke aggregates respectively, and then stirred to obtain slurry, and the slurry is washed, filtered, dehydrated and dried to obtain the corresponding several types of first materials;

(3) Preparation of the second material

Mixing at least two of the first materials to obtain a second material, or mixing at least one of the coke aggregates and at least one of the first materials to obtain a second material;

(4) Coating and graphitization

The second material is mixed with the coating agent and then granulated and graphitized.

In the preparation of the graphite negative electrode material of the present invention, the coke aggregate treated in the early stage is first modified by an acidic solution. The acidic solution modification of the coke aggregate can enhance the isotropy of the coke aggregate, especially the coke aggregate with a high degree of orientation. The acidic solution modification can reduce the degree of orientation of the surface layer and improve the isotropy. At the same time, the lithium ion embedding end face can be increased on the coke aggregate to improve the dynamic performance of the obtained graphite negative electrode material. In addition, after the coke aggregate is modified by an acidic solution, the surface -CHn (n = 1, 2, 3) and other group structures are reduced, and the oxygen-containing functional group carbonyl (-C = O) and the like are increased, so the surface active sites of the coke aggregate can be enhanced, and the coating agent can form a filling and pinning effect on the surface of the coke aggregate, thereby increasing the degree of granulation, improving the uniformity of granulation, etc., and enhancing the bonding strength between particles. In other words, the structural strength of the secondary particles formed by granulation can be improved, so when used as a negative electrode active material, it can show lower cycle expansion and excellent long cycle performance during the secondary battery cycle. In conclusion, the present invention can obtain a graphite negative electrode material having high kinetics, high granulation performance and improved secondary battery cycle performance by first subjecting the previously treated coke aggregate to an acid solution modification treatment.

The second aspect of the present invention provides a graphite negative electrode material, the particle size D50 of which is 15 to 21 μm. The graphite negative electrode material has better high dynamics and high granulation performance.

The third aspect of the present invention provides an application of a graphite negative electrode material in a negative electrode active material of a secondary battery, which can enable the secondary battery to exhibit lower cycle expansion and excellent long cycle performance, that is, the secondary battery has better cycle performance.

Detailed ways

The method for preparing the graphite negative electrode material of the present invention comprises the steps of:

(1) Preparation of coke aggregate

The cokes are roughly crushed, ground and classified to obtain corresponding coke aggregates;

(2) Preparation of modified coke

Acidic solutions are added to several kinds of coke aggregates respectively, and then slurries are prepared after stirring, and the slurries are washed, filtered, dehydrated and dried to obtain corresponding first materials;

(3) Preparation of the second material

Mixing at least two first materials to obtain a second material, or mixing at least one coke aggregate and at least one first material to obtain the second material;

(4) Coating and graphitization

The second material is mixed with the coating agent and then granulated and graphitized.

Wherein, the coke in step (1) is at least one of needle coke, ordinary coke and asphalt coke, that is, one or more cokes can be used to prepare the corresponding one or more coke aggregates. The volatile matter content of the coke is 0.2-15.0wt%, and the coke can be raw coke or cooked coke. Generally, raw coke has a high volatile matter (greater than 1.5%) and has higher dynamic properties, while cooked coke has a low volatile matter (less than 1%) and has a higher specific capacity. It is preferred to use at least one raw coke and at least one cooked coke to prepare the corresponding coke aggregate. The graphite negative electrode material prepared by mixing raw coke and cooked coke can take into account both high specific capacity and high dynamic properties. The particle size D50 of the coke aggregate obtained after the coke is coarsely crushed, ground and graded is 5-15μm, preferably 8μm.

The acidic solution in step (2) is at least one of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, carbonic acid, acetic acid and oxalic acid, preferably nitric acid, which has high production safety, easy material acquisition, convenient transportation and storage. The concentration of the acidic solution is 0.2-2.5 mol/L, preferably 1.5 mol/L. Each acidic solution accounts for 40-120% of the mass of the corresponding coke aggregate, preferably 60%. Selecting an acidic solution of a specific concentration and mass can increase the degree of lithium ion embedding end faces and surface active sites, thereby improving the surface modification effect of the coke aggregate. The increase in lithium ion embedding end faces can promote the improvement of the battery's kinetic performance, and the increase in surface active sites can improve the granulation effect, which is beneficial to the improvement of the cycle performance. The stirring speed is 50-200r/min, preferably 100r/min, and the stirring time is 1.5-6h, preferably 3h.

In step (3), the second material may be a mixture of two different first materials. The second material may also be obtained by mixing at least one coke aggregate with at least one first material. Preferably, the coke aggregate in the second material is different from the coke aggregate used in the first material. The mass fraction ratio of the coke aggregate to the first material may be 0-100:100-0, and both are not 0, specifically, but not limited to, 30:70, 50:50, 70:30.

In step (4), the coating agent includes asphaltenes having a softening point of 100 to 280°C, preferably 150°C. The particle size of the asphaltenes is 3.0 to 8.0 μm, preferably 7.0 μm. The coating agent accounts for 5 to 20 wt%, preferably 14%, of the sum of the mass of the second material and the coating agent. The equipment used for granulation is a carbon material granulation reactor, which can be, but is not limited to, a vertical reactor, a horizontal reactor, a drum reactor or a continuous granulation reactor, preferably a horizontal reactor. The equipment used for graphitization is an Acheson graphitization furnace, a box graphitization furnace, an inner string graphitization furnace or a continuous graphitization furnace, preferably an Acheson graphitization furnace, and the graphitization temperature is 2400 to 3200°C.

The graphite negative electrode material prepared by the preparation method of the graphite negative electrode material of the present invention has a particle size D50 of 15 to 21 μm. The graphite negative electrode material of the present invention can be used as a negative electrode active material for a secondary battery. The secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator and an electrolyte.

Among them, the positive electrode is obtained by coating a positive electrode slurry containing a positive electrode active material, a binder and a conductive agent on a positive electrode collector, drying, and cold pressing. The positive electrode active material can be selected from lithium cobalt oxide positive electrode materials, lithium iron phosphate positive electrode materials, lithium manganate positive electrode materials, nickel cobalt manganate lithium positive electrode materials, and nickel cobalt aluminum manganate positive electrode materials. The binder is used to improve the bonding between the positive electrode active material particles and between the positive electrode active material particles and the aluminum foil current collector. It is selected from polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber and epoxy resin, and the binder is preferably polyvinylidene fluoride. The conductive agent is used to improve the conductivity of the positive electrode, and can be selected from carbon-containing materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or metal powders or metal fiber materials such as copper, nickel, aluminum, silver, or conductive polymers such as polyphenylene derivatives, or mixtures thereof. The solvent of the positive electrode slurry can be N-methylpyrrolidone. The positive electrode current collector can be aluminum foil.

The negative electrode is obtained by coating the negative electrode slurry containing the negative electrode active material and the binder on the negative electrode current collector, drying and cold pressing. The negative electrode active material can be the aforementioned graphite negative electrode material of the present invention, or a mixed material of the aforementioned graphite negative electrode material and other materials (silicon/carbon composite negative electrode material or silicon oxygen negative electrode material). The binder is used to improve the bonding between the negative electrode active material particles and the positive electrode active material particles and the aluminum foil current collector. It is selected from at least one of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, epoxy-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic styrene-butadiene rubber and epoxy resin, and the preferred binder is carboxymethyl cellulose and styrene-butadiene rubber. The graphite negative electrode material of the present invention itself has conductivity, so it can be selected to add or not add a conductive agent. The solvent of the negative electrode slurry can be N-methylpyrrolidone. The negative electrode current collector may be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and the like.

The electrolyte includes a non-aqueous organic solvent and a lithium salt. The non-aqueous organic solvent acts as a medium for transmitting ions involved in the electrochemical reaction of the battery. The non-aqueous organic solvent may include a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent or an alcohol solvent. The carbonate solvent may be a linear carbonate and/or a branched carbonate, and may specifically include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. The ester solvent may include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, butyl decanolactone, γ-valerolactone, γ-caprolactone, etc. The ether solvent may include dibutyl ether, tetraethylene glycol dimethyl ether, 1,2-dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc., and the ketone solvent may include cyclohexanone, etc. The alcohol solvent may include ethanol, isopropanol, etc. The lithium salt may be at least one of LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₂ , LiAlCl ₄ , LiN(C _x F _2x+1 SO ₂ )(C _y F _{2y+ 1} SO ₂ )(wherein x and y are natural numbers) and LiB(C ₂ O ₄ ) _2. The lithium salt concentration may be 0.1 to 2.0M. Additives may be added to the electrolyte to improve battery performance, which may be, but are not limited to, at least one of vinyl sulfite, fluoroethylene carbonate, vinylene carbonate, vinyl carbonate, 1,3-propane sultone and vinyl sulfate.

The separator can be a single layer or a combination of multiple layers of polyethylene, polypropylene, and polyvinylidene fluoride, such as a polyethylene/polypropylene double-layer separator, a polyethylene/polypropylene/polyethylene three-layer separator, or a polypropylene/polyethylene/polypropylene three-layer separator. A ceramic layer can also be provided on the separator to prevent the secondary battery from short-circuiting when thermal shrinkage occurs.

In order to better illustrate the purpose, technical scheme and beneficial effects of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be noted that the following implementation method is a further explanation of the present invention and should not be used as a limitation of the present invention.

Example 1

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the common coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Nitric acid with a concentration of 0.2 mol/L is added to the coke aggregate A, and the mass of the nitric acid is 60% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a moisture content of less than 0.2% to obtain a first material A.

Add nitric acid with a concentration of 0.2 mol/L to the coke aggregate B, and the mass of the nitric acid is 60% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Example 2

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the ordinary coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Nitric acid with a concentration of 1.0 mol/L is added to the coke aggregate A, and the mass of the nitric acid is 60% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a moisture content of less than 0.2% to obtain a first material A,

Add nitric acid with a concentration of 1.0 mol/L to the coke aggregate B, and the mass of the nitric acid is 60% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Example 3

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the common coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Nitric acid with a concentration of 1.5 mol/L is added to the coke aggregate A, and the mass of the nitric acid is 60% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a moisture content of less than 0.2% to obtain a first material A,

Add nitric acid with a concentration of 1.5 mol/L to the coke aggregate B, and the mass of the nitric acid is 60% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Example 4

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the common coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Nitric acid with a concentration of 2.0 mol/L is added to the coke aggregate A, and the mass of the nitric acid is 60% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a moisture content of less than 0.2% to obtain a first material A,

Add nitric acid with a concentration of 2.0 mol/L to the coke aggregate B, and the mass of the nitric acid is 60% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Example 5

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the common coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Nitric acid with a concentration of 2.5 mol/L is added to the coke aggregate A, and the mass of the nitric acid is 60% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a moisture content of less than 0.2% to obtain a first material A,

Add nitric acid with a concentration of 2.5 mol/L to the coke aggregate B, and the mass of the nitric acid is 60% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Example 6

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the common coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Nitric acid with a concentration of 1.5 mol/L is added to the coke aggregate A, and the mass of the nitric acid is 40% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a moisture content of less than 0.2% to obtain a first material A,

Add nitric acid with a concentration of 1.5 mol/L to the coke aggregate B, and the mass of the nitric acid is 40% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Example 7

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the common coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Nitric acid with a concentration of 1.5 mol/L is added to the coke aggregate A, and the mass of the nitric acid is 80% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a moisture content of less than 0.2% to obtain a first material A,

Add nitric acid with a concentration of 1.5 mol/L to the coke aggregate B, and the mass of the nitric acid is 80% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Example 8

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the common coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Nitric acid with a concentration of 1.5 mol/L is added to the coke aggregate A, and the mass of the nitric acid is 100% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a moisture content of less than 0.2% to obtain a first material A,

Add nitric acid with a concentration of 1.5 mol/L to the coke aggregate B, and the mass of the nitric acid is 100% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Example 9

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the common coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Nitric acid with a concentration of 1.5 mol/L is added to the coke aggregate A, and the mass of the nitric acid is 120% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a moisture content of less than 0.2% to obtain a first material A,

Add nitric acid with a concentration of 1.5 mol/L to the coke aggregate B, and the mass of the nitric acid is 120% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Example 10

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The asphalt coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 10.0 μm, and the common coke with a volatile content of 2.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Add 0.2 mol/L oxalic acid to the coke aggregate A, and the mass of nitric acid is 60% of the mass of the coke aggregate A, stir at a speed of 150 r/min for 2 hours to obtain slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain the first material A,

Add nitric acid with a concentration of 0.2 mol/L to the coke aggregate B, and the mass of the nitric acid is 60% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The first material A and the first material B are uniformly mixed in a mass fraction ratio of 40:60 to obtain the second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

Embodiment 11

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 6.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate with a particle size D50 of 8.0 μm;

(2) Preparation of modified coke

Add nitric acid with a concentration of 0.2 mol/L and a mass of nitric acid of 60% of the mass of the coke aggregate, stir and bubble at a speed of 180 r/min for 2 hours to obtain slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain the first material;

(3) Preparation of the second material

The coke aggregate and the first material are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 200°C and a particle size of 5.5 μm is added to the second material, with the asphaltene accounting for 20% of the sum of the weight of the asphaltene and the second material. After being evenly mixed, granulation is performed in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in a chamber graphitization furnace at 3200°C to obtain a graphite negative electrode material.

Comparative Example 1

A method for preparing a graphite negative electrode material comprises the following steps:

(1) Preparation of coke aggregate

The needle coke with a volatile content of 5.0 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate A with a particle size D50 of 8.0 μm, and the common coke with a volatile content of 0.5 wt% is subjected to coarse crushing, grinding and classification to obtain a coke aggregate B with a particle size D50 of 8.0 μm;

(2) Preparation of coke slurry

Distilled water is added to the coke aggregate A, and the mass of the distilled water is 60% of the mass of the coke aggregate A, and the mixture is stirred at a speed of 100 r/min for 3 hours to obtain a slurry, and the slurry is washed to neutrality, filtered, dehydrated, and dried to a water content of less than 0.2% to obtain a first material A.

Add distilled water to the coke aggregate B, and the mass of the distilled water is 60% of the mass of the coke aggregate B, stir at a speed of 100 r/min for 3 hours to obtain a slurry, wash the slurry to neutrality, filter, dehydrate, and dry until the water content is less than 0.2% to obtain a first material B;

(3) Preparation of the second material

The coke aggregate A and the first material B are uniformly mixed in a mass fraction ratio of 50:50 to obtain a second material;

(4) Coating and graphitization

Asphaltene with a softening point of 150°C and a particle size of 7.0 μm is added to the second material, the asphaltene accounting for 14% of the sum of the weight of the asphaltene and the second material. After mixing evenly, granulation is carried out in a horizontal reactor. After granulation, the material is loaded into a graphite crucible and heat-treated in an Acheson graphitization furnace at 3000°C to obtain a graphite negative electrode material.

The average particle size D50 of the graphite negative electrode materials in Examples 1 to 11 and Comparative Example 1 was tested, and each graphite negative electrode material was respectively composed into a standard button cell to detect its first discharge specific capacity and first coulombic efficiency. The results are shown in Table 1.

The graphite negative electrode materials in Examples 1 to 11 and Comparative Example 1 were used as negative electrode active materials, respectively, and the conductive agent SuperP, carboxymethyl cellulose, and styrene-butadiene rubber were mixed in a mass ratio of 97:1:1:1 to prepare negative electrode slurry, and the mixed slurry was applied on both sides of the copper foil, and then dried and rolled to obtain the negative electrode sheet. Lithium cobalt oxide, adhesive PVDF, and conductive agent SuperP were mixed in a mass ratio of 98:1:1 to prepare a secondary battery positive electrode slurry of a certain viscosity, and the mixed slurry was applied on both sides of the aluminum foil, and then dried and rolled to obtain the positive electrode sheet. A polyethylene/polypropylene double-layer isolation film was set between the positive electrode sheet and the negative electrode sheet, and the bare battery cell was stacked and automatically wound. After being packaged with aluminum-plastic film, the electrolyte was injected, and the processes of standing, forming, shaping, and capacity division were carried out, and the 2C charging constant current ratio and cycle performance were carried out. The test conditions are as follows, and the test results are shown in Table 1.

(1) 2C charging constant current ratio

At room temperature (25℃), the battery is charged and discharged as follows (1C capacity is C0): ① Charge to 4.4V at 0.2C constant current and constant voltage, with a cut-off current of 0.05C; ② Discharge to 3.0V at 0.5C constant current; ③ Charge to 4.4V at 0.2C constant current and constant voltage, with a cut-off current of 0.05C; ④ Discharge to 3.0V at 0.5C constant current; ⑤ Charge to 4.4V at 2.0C constant current and constant voltage, with a cut-off current of 0.05C; Record the charging capacity C1 of the constant current section in step ⑤ and the total charging capacity C2 in step ⑤. C1/C2 is the 2C charging constant current ratio.

(2) Cycle performance

At room temperature (25°C), the battery is charged and discharged at 1.2C/1.0C (1C capacity is C0), the upper limit voltage is 4.4V, and the thickness of the middle of the battery tab at 50% SOC in the discharge stage is d0. Then, at room temperature, 1.2C/1.0C charging and discharging is performed for 100, 300, 500, and 800 weeks. The corresponding battery discharge capacity is C1, and the corresponding thickness of the middle of the battery tab at 50% SOC in the discharge stage is d1. Among them,

Capacity retention rate = (C1/C0)*100%

Battery expansion rate = (d1-d0)/d0*100%

Table 1 Material properties and battery performance tests of various embodiments

Note: A represents capacity retention rate, B represents battery expansion rate

From the results in Table 1, it can be seen that compared with Comparative Example 1, the particle size D50 of the graphite negative electrode material obtained by the acid solution modification treatment in Examples 1 to 11 is larger, and the 2C charge constant current ratio is higher. This is mainly because the acid solution modification treatment can enhance the isotropy of the coke aggregate, increase the number of lithium ion embedding end faces on the coke aggregate, and improve the kinetic properties of the obtained graphite negative electrode material, so that the charge constant current ratio can be increased. In addition, the active sites on the surface of the coke aggregate can be enhanced, and the coating agent can form a filling and pinning effect on the surface of the coke aggregate, thereby improving the degree of granulation, so it is easier to form secondary particles, and the obtained secondary particles have a larger particle size.

At the same time, it can be seen from the results in Table 1 that the cycle performance of the batteries prepared in Examples 1 to 11 is better, especially the greater the number of cycles, the more obvious the difference in cycle performance between Examples 1 to 11 and Comparative Example 1. This is because the modification treatment with the acidic solution can enhance the surface active sites of the coke aggregate and improve the structural strength of the secondary particles formed by granulation. Therefore, when used as a negative electrode active material, it can show lower cycle expansion and excellent long cycle performance during the secondary battery cycle.

In addition, it can be seen from the results in Table 1 that even if the graphite negative electrode material of the present invention is modified with an acidic solution during the preparation process, it does not affect the material's first discharge specific capacity and first coulombic efficiency. Therefore, the present invention, by subjecting the coke aggregate to an acidic solution modification treatment, can not only take into account high kinetics and high granulation performance, improve the cyclability of the secondary battery, but also have better first discharge specific capacity and first coulombic efficiency.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention is described in detail with reference to the preferred embodiments, it is not limited to those listed in the embodiments. Those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims (7)

1. A method for preparing a graphite negative electrode material, characterized in that it comprises the steps of: (1) Preparation of coke aggregate Several kinds of coke are subjected to coarse crushing, grinding and classification to obtain corresponding coke aggregates, wherein the coke is at least one of needle coke, ordinary coke and asphalt coke, and the volatile matter content of the coke is 0.2-15.0wt%; (2) Preparation of modified coke Acidic solutions are added to the several coke aggregates respectively, and then stirred to obtain slurry, and the slurry is washed, filtered, dehydrated and dried to obtain the corresponding several first materials, the acidic solution is at least one of hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, carbonic acid, acetic acid and oxalic acid, the concentration of the acidic solution is 0.2-2.5 mol/L, each of the acidic solutions accounts for 40-120% of the mass of the corresponding coke aggregate, the stirring speed is 50-200 r/min, and the time is 1.5-6h; (3) Preparation of the second material Mixing at least two of the first materials to obtain a second material, or mixing at least one of the coke aggregates and at least one of the first materials to obtain a second material; (4) Coating and graphitization The second material is mixed with the coating agent and then granulated and graphitized. 2. The method for preparing a graphite negative electrode material according to claim 1, wherein the particle size D50 of the coke aggregate is 5 to 15 μm. 3. The method for preparing a graphite negative electrode material according to claim 1 is characterized in that the coating agent comprises asphaltene with a softening point of 100-280°C, the particle size of the asphaltene is 3.0-8.0 μm, and the coating agent accounts for 5-20 wt% of the sum of the mass of the second material and the coating agent. 4. The method for preparing a graphite negative electrode material according to claim 1, characterized in that the equipment used for granulation is a carbon material granulation reactor. 5. The method for preparing a graphite negative electrode material according to claim 1, characterized in that the equipment used for graphitization is an Acheson graphitization furnace, a box graphitization furnace, an inner series graphitization furnace or a continuous graphitization furnace, and the graphitization temperature is 2400~3200℃. 6. The graphite negative electrode material prepared by the method for preparing a graphite negative electrode material according to any one of claims 1 to 5, characterized in that the particle size D50 of the graphite negative electrode material is 15 to 21 μm. 7. Use of the graphite negative electrode material prepared by the method for preparing the graphite negative electrode material according to any one of claims 1 to 5 or the graphite negative electrode material according to claim 6 in a negative electrode active material for a secondary battery.