CN110690436B - Negative electrode material, preparation method thereof, prepared negative electrode plate and lithium ion battery - Google Patents

Negative electrode material, preparation method thereof, prepared negative electrode plate and lithium ion battery Download PDF

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CN110690436B
CN110690436B CN201910995725.1A CN201910995725A CN110690436B CN 110690436 B CN110690436 B CN 110690436B CN 201910995725 A CN201910995725 A CN 201910995725A CN 110690436 B CN110690436 B CN 110690436B
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hard carbon
negative electrode
graphite
particle size
ion battery
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CN110690436A (en
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姜倩荣
沈舒宜
崔亚青
吕正中
刘金成
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Hubei Eve Power Co Ltd
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Hubei Eve Power Co Ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/364—Composites as mixtures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0567—Liquid materials characterised by the additives
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
    • H01M10/4235—Safety or regulating additives or arrangements in electrodes, separators or electrolyte
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/04—Processes of manufacture in general
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/133—Electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
    • H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

The invention discloses a negative electrode material, a preparation method thereof, a prepared negative electrode plate and a lithium ion battery. The negative electrode material comprises graphite and hard carbon, the particle size of the graphite is smaller than that of the hard carbon, and the mass content of the hard carbon in the negative electrode material is less than or equal to that of the graphite. According to the invention, the hard carbon with larger particle size and the graphite with smaller particle size are compounded as the negative electrode material, and the content relationship of the hard carbon and the graphite in the negative electrode material is controlled, so that the problem that the charging capacity of the power lithium ion battery for the new energy vehicle at a low temperature of-40 ℃ is far lower than the rated capacity is solved, the lithium precipitation resistance of the battery can be increased, and the charging and discharging safety of the battery at the low temperature is improved. The module assembled by the battery can still ensure the cruising ability of the electric automobile without losing self heating energy consumption, has high system energy density, and can meet the normal use requirement of the electric automobile in winter in cold areas.

Description

Negative electrode material, preparation method thereof, prepared negative electrode plate and lithium ion battery
Technical Field
The invention belongs to the technical field of battery materials, and particularly relates to a negative electrode material, a preparation method of the negative electrode material, a prepared negative electrode plate and a lithium ion battery.
Background
Driven by policies and markets, new energy electric vehicles are gradually popularized in China, and in 2018, the yield and sales of the new energy electric vehicles reach 127 ten thousand and 125.6 ten thousand. Seven cities ranked in the top ten are sold and belong to the southern area, and various data show that the popularization rate of new energy vehicles in the southern area is far higher than that in the northern area. The main reason is that the temperature in winter in the north of China is basically below 0 ℃, and the lowest temperature of 12 to 2 months in the east-san province and the inner Mongolia region even reaches-40 ℃. The low temperature reduces the migration rate of lithium ions in electrolyte and the diffusion rate of lithium ions in a negative electrode, the polarization phenomenon is serious during charging, and the actually charged electric quantity is far lower than the rated capacity when the charge cut-off voltage is reached. In addition, lithium ions which are not timely inserted into the graphite layers of the negative electrode are gathered on the surface of the negative electrode and reduced into metal lithium, and lithium dendrites formed by precipitation exist and penetrate through the diaphragm to cause the safety risk of short circuit of the battery. Under such low temperature environment, the conventional power battery for the electric automobile is difficult to meet the use requirement, and the safety performance of the electric automobile cannot be ensured.
At present, one of the most common ways to solve the problem of poor low-temperature charging performance is to improve the low-temperature performance of the cell. The current methods for improving the low-temperature charging performance mainly focus on the aspects of electrolyte additives and negative electrode material selection. In addition, another solution is to provide a battery module heating device, which generally requires a heating system to rapidly and uniformly heat the module in a short time, and the battery core is in a temperature range capable of working normally through external heating. However, the module heating scheme not only sacrifices partial energy of the battery system, but also reduces the energy density of the system in the space occupied by the heating device, and is not beneficial to the mileage promotion of the new energy automobile.
CN108306018A discloses a lithium iron phosphate power battery with improved low-temperature charging performance, which adds additives such as Vinylene Carbonate (VC) and fluoroethylene carbonate (FEC) into an electrolyte, uses common artificial graphite as a negative electrode, and adopts a novel negative electrode binder polystyrene acrylic emulsion to improve the charging performance of the battery. But only the electrolyte and the cathode binder are changed, the charging temperature is as low as-10 ℃, and the initial charging and starting of the electric automobile in the high and cold regions cannot be met. The diffusion dynamic condition of lithium ions in the carbon negative electrode material is deteriorated, which is a main reason for limiting the low-temperature performance of the lithium ion battery, so that the electrochemical polarization of the negative electrode is obviously accelerated in the charging process, and metal lithium is easily precipitated on the surface of the negative electrode. And the diffusion dynamic condition of lithium ions in the common graphite negative electrode material is poor at low temperature, the lithium ions gathered outside the negative electrode are difficult to diffuse to the graphite layer, lithium dendrites are easy to form, and the safety performance cannot be guaranteed.
CN104578295B discloses a low-temperature charging and heating system and method for a vehicle power battery, which ensure that a battery module can be charged normally at low temperature and is safe and fast through a heating system mode. However, the heating system comprises a battery management unit, an off-board charger, a heater, a fan and the like, so that the energy density of the system is reduced, self-heating energy consumption is consumed, and the cruising ability of the battery is further reduced.
Therefore, there is a need in the art for a low temperature resistant battery that can be charged and discharged normally at a low temperature of-40 ℃ without an external heating device.
Disclosure of Invention
The invention aims to provide a negative electrode material, a preparation method thereof, a prepared negative electrode plate and a lithium ion battery. The lithium ion battery can be normally charged and discharged at the low temperature of-40 ℃ without an external heating device, and the preparation process is simple and can be industrially produced.
In order to achieve the purpose, the invention adopts the following technical scheme:
one of the purposes of the invention is to provide a negative electrode material, which comprises graphite and hard carbon, wherein the particle size of the graphite is smaller than that of the hard carbon, and the mass content of the hard carbon in the negative electrode material is less than or equal to that of the graphite.
According to the invention, the hard carbon with larger particle size and the graphite with smaller particle size are compounded to be used as the negative electrode material, and the content relation of the hard carbon and the graphite in the negative electrode material is controlled, so that the problem that the charging capacity of the power lithium ion battery for the new energy vehicle is far lower than the rated capacity at the low temperature of minus 40 ℃ is solved. The selected hard carbon has a long-range disordered and short-range ordered layered structure, the amorphous degree is large, the mechanical anisotropy is relatively weak, and the large-particle-size hard carbon and the small-particle-size graphite can play a good force conduction role after being uniformly mixed. The force conduction effect of the hard carbon ensures that the compacted density of each part of the rolled pole piece is consistent, and the problem that the compacted density of the interior close to a current collector is low due to the overvoltage of the surface of the pole piece is solved, so that the rebound rate of the negative pole piece is low, the contact internal resistance of an electrode is low, the polarization degree of the battery is low in the charging process even under the low-temperature condition of minus 40 ℃, and the charging voltage platform is low. The module assembled by the battery can still ensure the cruising ability of the electric automobile without losing self heating energy consumption, has high system energy density, and can meet the normal use requirement of the electric automobile in winter in cold areas.
According to the invention, the hard carbon material is added into the negative electrode, and the particle size and content of the hard carbon material and graphite are controlled, so that the lithium precipitation resistance of the battery can be increased, and the safety of charging and discharging of the battery under a low-temperature condition is improved. Because the spacing between the hard carbon layers is larger than that of graphite, the difficulty of lithium ions in being embedded into the negative electrode is smaller, and the lithium ions can be more quickly embedded into the negative electrode layers during charging, so that lithium dendrites formed on the surface of the negative electrode in an aggregation manner are avoided; in addition, the structural characteristics of short-range order and long-range disorder of the hard carbon enable the hard carbon to have a large number of end surface defects, more embedding channels are provided for lithium ions, concentration polarization on the surface of the negative electrode is reduced, and deposition of lithium metal on the surface of the negative electrode is reduced. Based on the structural characteristics of the materials, when the composite carbon material mixed with hard carbon and graphite is used as a negative electrode, the lithium precipitation resistance of the composite negative electrode material is strong, lithium dendrite which is easy to pierce a diaphragm to cause short circuit is difficult to form in the low-temperature charging process of the lithium iron phosphate full battery prepared by adopting the negative electrode material, and the use safety of an electric automobile at the extremely low temperature of minus 40 ℃ is ensured.
If the particle size of the graphite is larger than or equal to that of the hard carbon, the hard carbon cannot better conduct force, and the polarization phenomenon of the negative plate during low-temperature charging cannot be reduced.
Preferably, the particle size of the graphite is 30 to 60% of the particle size of the hard carbon, such as 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, and the like.
The particle size of the graphite is 30-60% of the particle size of the hard carbon, and if the particle size of the graphite is too large different from the particle size of the hard carbon, the hard carbon cannot be uniformly dispersed in the slurry; if the difference between the particle size of the graphite and the particle size of the hard carbon is too small, the effect of stress conduction of the hard carbon is not good, and the low-temperature charging efficiency is reduced.
Preferably, the graphite is any one of or a combination of at least two of scale graphite, natural aphanitic graphite and artificial graphite.
Preferably, the particle size D50 of the graphite is 5-15 μm, such as 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm or 14 μm.
Preferably, the hard carbon is any one of or a combination of at least two of furfuryl ketone resin hard carbon, unsaturated polyester resin hard carbon, acrylic resin hard carbon, phenolic resin hard carbon, polyformaldehyde resin hard carbon, epoxy resin hard carbon, furfural resin hard carbon and asphalt hard carbon.
Preferably, the particle size D50 of the hard carbon is 10-20 μm, such as 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm or 19 μm.
Preferably, the content of the hard carbon in the anode material is 10 wt% to 50 wt%, such as 12 wt%, 15 wt%, 18 wt%, 20 wt%, 22 wt%, 25 wt%, 28 wt%, 30 wt%, 32 wt%, 35 wt%, 38 wt%, 40 wt%, 42 wt%, 45 wt%, or 48 wt%, etc.
The negative electrode material is a mixed material of graphite and hard carbon, wherein the content of the hard carbon is 10-50 wt%.
Another object of the present invention is to provide a method for preparing the anode material according to the first object, the method comprising: and mixing the graphite and the hard carbon for the first time to obtain the cathode material.
Preferably, the primary mixing mode is stirring mixing, and preferably stirring mixing with revolution speed of 10-50 r/min and rotation speed of 200-3000 r/min. The revolution speed is 15r/min, 20r/min, 25r/min, 30r/min, 35r/min, 40r/min or 45r/min, etc.; the rotation speed is, for example, 500r/min, 800r/min, 1000r/min, 1200r/min, 1500r/min, 1800r/min, 2000r/min, 2200r/min, 2500r/min or 2800 r/min.
Preferably, the time for the first mixing is 1 to 5 hours, such as 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours or 4.5 hours.
Preferably, the apparatus used for the primary mixing is a double planetary mixer.
The invention also aims to provide a preparation method of the anode slurry, which comprises the following steps: and mixing the negative electrode material, the binder, the thickening agent, the conductive agent and the solvent for the second time to obtain negative electrode slurry.
Preferably, the secondary mixing is stirring mixing, and preferably stirring mixing with revolution speed of 10-50 r/min and rotation speed of 200-3000 r/min. The revolution speed is 15r/min, 20r/min, 25r/min, 30r/min, 35r/min, 40r/min or 45r/min, etc.; the rotation speed is, for example, 500r/min, 800r/min, 1000r/min, 1200r/min, 1500r/min, 1800r/min, 2000r/min, 2200r/min, 2500r/min or 2800 r/min.
Preferably, the time for the secondary mixing is 5 to 10 hours, such as 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours, 8 hours, 8.5 hours, 9 hours or 9.5 hours.
Preferably, the apparatus used for the secondary mixing is a double planetary mixer.
Preferably, the binder is any one of styrene-butadiene rubber, acrylic acid-polyacrylic acid copolymer, polyethylene oxide and acrylonitrile multipolymer or the combination of at least two of the styrene-butadiene rubber, the acrylic acid-polyacrylic acid copolymer and the acrylonitrile multipolymer.
Preferably, the thickener is sodium carboxymethylcellulose and/or sodium polyacrylate.
Preferably, the conductive agent is any one of graphene, carbon nanotubes, SP, and KS-6, or a combination of at least two thereof.
Preferably, the solvent is water.
The fourth purpose of the invention is to provide a preparation method of a negative pole piece, which comprises the following steps: and coating the negative electrode slurry prepared by the third method on a current collector, drying, and then carrying out primary cold pressing, die cutting and stripping to prepare a negative electrode plate.
Preferably, the current collector is a copper foil, preferably a copper foil with a thickness of 6-9 μm, such as 6.2 μm, 6.5 μm, 6.8 μm, 7 μm, 7.2 μm, 7.5 μm, 7.8 μm, 8 μm, 8.2 μm, 8.5 μm, or 8.8 μm.
The fifth purpose of the invention is to provide a negative pole piece, and the negative pole piece is prepared by the method of the fourth purpose.
Preferably, the compaction density of the negative pole piece is 1-1.4 g/cm3E.g. 1.1g/cm3、1.2g/cm3Or 1.3g/cm3And the like.
The sixth purpose of the invention is to provide a low-temperature-resistant lithium ion battery, which comprises the negative pole piece of the fifth purpose.
The low-temperature-resistant lithium ion battery can meet the use requirement at the temperature of more than or equal to-40 ℃.
Preferably, the lithium ion battery further comprises an electrolyte containing an additive, a positive electrode and a separator.
Preferably, the additive in the electrolyte comprises any one or a combination of at least two of ethylene sulfite, vinylene carbonate, propylene sulfite, fluoroethylene carbonate, lithium dioxalate borate, lithium difluorooxalate borate and lithium difluorophosphate.
According to the invention, the impedance of the negative electrode is reduced through the electrolyte of the low-temperature type carbonate additive, and the low-temperature charging performance of the power battery is improved. The additive can reduce the viscosity of the electrolyte, improve the migration rate of lithium ions in the electrolyte and reduce the internal polarization of the battery cell; in addition, the additive has a relatively obvious negative electrode film forming effect on a negative electrode, and a thin and compact SEI film is formed, so that the impedance of the negative electrode can be effectively reduced, and Li at low temperature is ensured+Can still have a larger diffusion coefficient in the negative electrode. Based on the two points, the electrolyte containing the low-temperature type carbonate additive can improve the low-temperature charging performance of the power lithium ion battery.
Preferably, the concentration of the additive in the electrolyte is 0.04-0.08 mol/L, such as 0.05mol/L, 0.06mol/L or 0.07 mol/L.
Preferably, the positive electrode material in the positive electrode is lithium iron phosphate.
The seventh purpose of the present invention is to provide a method for increasing the low-temperature charging capacity of a lithium ion battery, wherein a negative electrode plate in the lithium ion battery is the fifth purpose of the negative electrode plate.
Compared with the prior art, the invention has the following beneficial effects:
(1) the invention can solve the problem of low-temperature charge capacity: according to the invention, the hard carbon with larger particle size and the graphite with smaller particle size are compounded as the negative electrode material, so that the problem that the charged quantity of the power lithium ion battery for the new energy vehicle is far lower than the rated capacity at the low temperature of-40 ℃ is solved, and even under the low temperature condition of-40 ℃, the polarization degree of the battery in the charging process is small, the charging voltage platform is low, and the charging efficiency is high. Meanwhile, the impedance of the negative electrode is reduced through the electrolyte of the low-temperature type carbonate additive, and the low-temperature charging performance of the power battery is further improved. In addition, the additive has a relatively obvious negative electrode film forming effect on a negative electrode, and a thin and compact SEI film is formed, so that the impedance of the negative electrode can be effectively reduced, and Li at low temperature is ensured+Can still have a larger diffusion coefficient in the negative electrode. Based on the two points, the electrolyte containing the low-temperature type carbonate additive can improve the low-temperature charging performance of the power lithium ion battery.
(2) The invention can ensure the safety of the new energy electric automobile in the charging process: according to the invention, the hard carbon material is added into the negative electrode and the content of the hard carbon material is controlled to increase the lithium precipitation resistance of the battery, so that the safety of charging and discharging of the battery under a low-temperature condition is improved. Since the hard carbon layer spacing is larger than graphite, lithium ions are less difficult to intercalate into the negative electrode. During charging, lithium ions can be more rapidly embedded into the negative electrode layer, and lithium dendrites are prevented from being formed on the surface of the negative electrode by aggregation.
(3) The module assembled by the battery can still ensure the cruising ability of the electric automobile without losing self heating energy consumption, has high system energy density, and can meet the normal use requirement of the electric automobile in winter in cold areas.
Drawings
FIG. 1 is a schematic view of the structure of a negative electrode sheet obtained in comparative example 1 of the present invention;
fig. 2 is a schematic structural diagram of a negative electrode sheet obtained in example 1 of the present invention.
Detailed Description
The technical solution of the present invention is further explained by the following embodiments. It should be understood by those skilled in the art that the examples are only for the understanding of the present invention and should not be construed as the specific limitations of the present invention.
Example 1
(1) Adding scaly graphite with D50 being 5 mu m and furfuryl ketone resin hard carbon with D50 being 10 mu m into a stirring cylinder of a double planetary stirrer in a mass ratio of 9:1, and dispersing for 1h at a revolution speed of 50r/min and a rotation speed of 1500r/min to obtain a hard carbon-graphite composite negative electrode material;
(2) sequentially adding the composite negative electrode material, the binder SBR, the thickening agent CMC and the conductive agent SP in the step (1) into a stirring cylinder according to the mass ratio of 94:2:2:2, stirring and dispersing for 1h, adding solvent deionized water with the same mass as the powder, and stirring for 6h to obtain hard carbon-graphite composite negative electrode slurry;
(3) uniformly coating the composite negative electrode slurry on copper foil, drying in an oven, and performing primary cold pressing, die cutting and stripping to prepare a negative electrode plate, wherein the surface density of the plate is 15.8mg/cm2Thickness of 125 μm and compacted density of 1.4g/cm3；
(4) A winding core prepared by winding a positive plate (lithium iron phosphate: conductive agent SP: binder PVDF in a mass ratio of 95:3:2), a diaphragm and a negative plate is placed in a square aluminum shell, and a low-temperature electrolyte (a solvent is 1mol/L LiPF with EC: EMC of 1: 2) is injected6The electrolyte contains 0.05mol/L of propylene sulfite additive), and the 100Ah square aluminum-shell battery is prepared after encapsulation and formation.
Fig. 2 is a schematic structural view of the negative electrode plate obtained in this embodiment, in which the circular particles are hard carbon, the hard carbon hardly deforms due to small anisotropy, and the compacted density of the graphite in the direction perpendicular to the current collector of the composite negative electrode plate doped with the hard carbon is uniform due to the stress conduction effect of the hard carbon.
Example 2
(1) Adding artificial graphite with D50 being 10 μm and acrylic resin hard carbon with D50 being 15 μm into a stirring cylinder of a planetary stirrer in a mass ratio of 7.5:2.5, and dispersing for 3 hours at a revolution speed of 30r/min and a rotation speed of 3000r/min to obtain a hard carbon-graphite composite negative electrode material;
(2) sequentially adding the composite negative electrode material, the binder SBR, the thickening agent CMC and the conductive agent SP in the step (1) into a stirring cylinder according to the mass ratio of 94:2:2:2, stirring and dispersing for 1h, adding solvent deionized water with the same mass as the powder, and stirring for 6h to obtain hard carbon-graphite composite negative electrode slurry;
(3) uniformly coating the composite negative electrode slurry on copper foil, drying in an oven, and performing primary cold pressing, die cutting and stripping to prepare a negative electrode plate, wherein the surface density of the plate is 12mg/cm2Thickness of 108 μm and compacted density of 1.2g/cm3；
(4) A winding core prepared by winding a positive plate (lithium iron phosphate: conductive agent SP: binder PVDF mass ratio is 96:2:2), a diaphragm and a negative plate is placed in a square aluminum shell, and low-temperature electrolyte (the solvent is 1mol/L LiPF with EC: EMC being 1: 1) is injected6The electrolyte contains 0.05mol/L VC additive), and the 100Ah square aluminum-shell battery is prepared after encapsulation and formation.
Example 3
(1) Adding natural aphanitic graphite with D50 being 15 micrometers and epoxy resin hard carbon with D50 being 20 micrometers into a stirring cylinder of a planetary stirrer in a mass ratio of 5:5, and dispersing for 5 hours at a revolution speed of 10r/min and a rotation speed of 200r/min to obtain a hard carbon-graphite composite negative electrode material;
(2) sequentially adding the composite negative electrode material, the binder SBR, the thickening agent CMC and the conductive agent SP in the step (1) into a stirring cylinder according to the mass ratio of 94:2:2:2, stirring and dispersing for 1h, adding solvent deionized water with the same mass as the powder, and stirring for 6h to obtain hard carbon-graphite composite negative electrode slurry;
(3) uniformly coating the composite negative electrode slurry on copper foil, drying in an oven, and performing primary cold pressing, die cutting and stripping to prepare a negative electrode plate, wherein the surface density of the plate is 9.5mg/cm2A thickness of 98 μm and a compacted density of 1g/cm3；
(4) Mixing a positive plate (lithium iron phosphate: conductive agent SP: binder PVDF mass ratio is 95:2:3), a diaphragm and a negative plateThe wound core is placed in a square aluminum shell, and a low-temperature electrolyte (1 mol/L LiPF with EC: EMC 1:1 as a solvent) is injected6The electrolyte contains 0.05mol/L lithium difluorophosphate additive), and the 100Ah square aluminum-shell battery is prepared after packaging and formation.
Example 4
The difference from example 1 is that the flake graphite of step (1) has a D50 of 6 μm and the furfuryl ketone resin hard carbon has a D50 of 20 μm, i.e., the particle size of the graphite is 30% of the particle size of the hard carbon.
Example 5
The difference from example 1 is that the flake graphite of step (1) has a D50 of 12 μm and the furfuryl ketone resin hard carbon has a D50 of 20 μm, i.e., the particle size of the graphite is 60% of the particle size of the hard carbon.
Example 6
The difference from example 1 is that the flake graphite of step (1) has a D50 of 4 μm and the furfuryl ketone resin hard carbon has a D50 of 20 μm, i.e., the particle size of the graphite is 20% of the particle size of the hard carbon.
Example 7
The difference from example 1 is that the flake graphite of step (1) has a D50 of 14 μm and the furfuryl ketone resin hard carbon has a D50 of 20 μm, i.e., the particle size of the graphite is 70% of the particle size of the hard carbon.
Example 8
The difference from example 1 is that the mass ratio of the scaly graphite and the furfuryl ketone resin hard carbon in step (1) is 95: 5.
Example 9
The difference from the embodiment 1 is that the electrolyte in the step (4) does not contain additives.
Comparative example 1
The difference from example 1 is that the furfuryl ketone resin hard carbon in step (1) is replaced by the same amount of scaly graphite with the particle size of D50 ═ 5 μm.
Fig. 1 is a schematic structural diagram of the negative electrode plate obtained in the present comparative example, and it can be seen from the diagram that the internal compaction density near the current collector is low due to the surface overpressure of the negative electrode plate obtained by only using graphite as the negative electrode material.
Comparative example 2
The difference from example 1 is that the particle size of the scaly graphite in step (1) is 15 μm, and the particle size of the furfuryl ketone resin hard carbon is 5 μm, i.e., D50.
Comparative example 3
The difference from example 1 is that the scaly graphite in step (1) is replaced by the same amount of furfuryl ketone resin hard carbon with the particle size of D50 ═ 10 μm.
Comparative example 4
The difference from example 1 is that the scaly graphite in step (1) is replaced by the same amount of furfuryl ketone resin hard carbon with the particle size of D50 ═ 5 μm.
Comparative example 5
The difference from the example 1 is that the mass ratio of the scaly graphite of the step (1) to the furfuryl ketone resin hard carbon is 4: 6.
And (3) performance testing:
the obtained battery was subjected to the following performance tests:
(1) negative electrode rebound rate: measuring the thickness of the rolled negative pole piece and recording the thickness as T1Standing at 25 deg.C for 48 hr, measuring the thickness again, and recording as T2The negative electrode rebound rate is (T)2-T1)/T1；
(2)1C charging efficiency: in each of the examples and comparative examples, 10 cells were taken, and the obtained cells were first subjected to 1C capacity calibration at 25 ℃ using a Xinwei capacity cabinet, and the discharge capacity C was recordedDCThen, the mixture was allowed to stand at-20 ℃ (5) and-40 ℃ (5) for 24h, and was charged at constant current and constant voltage at 1C, and the charging capacity C 'of the battery at-20 ℃ and-40 ℃ was recorded'CCAnd 1C charging efficiency is C'CC/CDCX 100%, calculating the average value of the results;
(3)1C discharge efficiency: in each of the examples and comparative examples, 10 cells were taken, and the obtained cells were first subjected to 1C capacity calibration at 25 ℃ using a Xinwei capacity cabinet, and the charge capacity C was recordedCCThen, the mixture was left standing at-20 ℃ (5) and-40 ℃ (5) for 24h, and constant-current and constant-voltage discharge was carried out at 1C, and the discharge capacity C 'was recorded'DCAnd 1C discharge efficiency is C'DC/CCCX 100%, calculating the average value of the results;
(4) the first efficiency is as follows: at 1C current density, the first efficiency is the first discharge specific capacity/first charge specific capacity x 100%.
TABLE 1
Figure BDA0002239661510000121
Figure BDA0002239661510000131
As can be seen from the table 1, the proportion and content ratio of D50 between the hard carbon and the graphite are favorable for the conduction of hard carbon stress, and the compaction uniformity of the pole piece is ensured; through the release of the hard carbon stress, the rebound of graphite in the negative plate is limited, so that the polarization phenomenon of charging can be reduced, and the charging efficiency is improved. Under the condition that the first efficiency is more than 80 percent (the highest efficiency can reach 92.01 percent), the 1C charging efficiency can respectively reach 97.17 percent and 78.88 percent at the highest temperature of-20 ℃ and-40 ℃, the 1C discharging efficiency can respectively reach 94.31 percent and 86.96 percent at the highest temperature, the electrochemical performance is excellent, and the advantages are more prominent particularly under the extremely cold condition of-40 ℃.
As can be seen from table 1, in example 6 of the present invention, compared to example 1, the low temperature charging performance is poor, because the particle size of the graphite in example 6 is 20% of the particle size of the hard carbon, and the particle size of the graphite is greatly different from the particle size of the hard carbon, the hard carbon is easily dispersed unevenly, and the effect of reducing the charging polarization cannot be exerted, so the low temperature charging performance is poor; compared with the embodiment 1, the embodiment 7 of the invention has poorer low-temperature charging performance, because the particle size of the graphite in the embodiment 7 is 70% of the particle size of the hard carbon, the difference between the particle size of the graphite and the particle size of the hard carbon is smaller, the stress conduction effect of the hard carbon is not facilitated, the rebound rate of a pole piece is higher, the contact internal resistance is increased, and thus the low-temperature charging performance is poorer. As can be seen from table 1, example 8 of the present invention is inferior to example 1 in low-temperature charging performance because the mass ratio of the scale-like graphite to the furfuryl ketone resin hard carbon in example 8 is 95:5, the scale-like graphite content is too large, the furfuryl ketone resin hard carbon content is too small, and further the effect of reducing polarization cannot be exerted, thus resulting in poor low-temperature charging performance.
As can be seen from table 1, in example 9 of the present invention, compared to example 1, the low-temperature charge and discharge performance is poor, because the electrolyte of example 9 does not contain an additive, and further, the migration rate of lithium ions in the electrolyte is small, and the concentration polarization is large, thereby causing poor low-temperature electrical performance.
As can be seen from Table 1, comparative example 1 of the present invention is inferior in low-temperature charging performance to example 1 and hardly chargeable at-40 ℃. Since the hard carbon is not present in comparative example 1, the technical effect of example 1 is not achieved because the cut-off voltage is reached in a short time due to the large polarization during charging at low temperature; the particle size of the scaly graphite in the comparative example 2 of the present invention is larger than that of the furfuryl ketone resin hard carbon, and the hard carbon cannot perform a good force conduction function, so that the performance is poor.
As can be seen from table 1, comparative examples 3 to 4 according to the present invention have inferior first-effect and low-temperature discharge performance to example 1 because graphite is not present in comparative examples 3 to 4, the hard carbon has a large degree of amorphousness, and the intercalated lithium ions cannot completely leave the electrode surface in a short time during discharge. Even if hard carbons having different particle diameters were used, the technical effect of example 1 was not achieved, and thus the performance was poor.
As can be seen from Table 1, comparative example 5 of the present invention has inferior first-effect and low-temperature discharge performance compared to example 1, because the mass ratio of the scaly graphite to the furfuryl ketone resin hard carbon in comparative example 5 is 4:6, the graphite content is too low, and the hard carbon content is too high, lithium ions inside the electrode cannot rapidly and orderly leave the electrode surface, and thus the performance is poor.
The present invention is illustrated in detail by the examples described above, but the present invention is not limited to the details described above, i.e., it is not intended that the present invention be implemented by relying on the details described above. It should be understood by those skilled in the art that any modification of the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific modes, etc., are within the scope and disclosure of the present invention.

Claims (26)

1. The negative electrode material is characterized by comprising graphite and hard carbon, wherein the particle size of the graphite is smaller than that of the hard carbon, and the mass content of the hard carbon in the negative electrode material is less than or equal to that of the graphite;
the particle size D50 of the graphite is 5-15 mu m; the particle size of the graphite is 50-60% of the particle size of the hard carbon;
the hard carbon is any one or a combination of at least two of furfuryl ketone resin hard carbon, unsaturated polyester resin hard carbon, acrylic resin hard carbon, phenolic resin hard carbon, polyformaldehyde resin hard carbon, epoxy resin hard carbon, furfural resin hard carbon and asphalt hard carbon; the graphite is any one or the combination of at least two of scale graphite, natural aphanitic graphite and artificial graphite;
the particle size D50 of the hard carbon is 10-20 μm;
the content of hard carbon in the negative electrode material is 10 wt% -50 wt%.
2. A method for preparing the anode material according to claim 1, wherein the method comprises: and mixing the graphite and the hard carbon for the first time to obtain the cathode material.
3. The method according to claim 2, wherein the primary mixing is performed by stirring.
4. The method according to claim 3, wherein the stirring and mixing is performed at a revolution speed of 10 to 50r/min and a rotation speed of 200 to 3000 r/min.
5. The method according to claim 2, wherein the time for the first mixing is 1 to 5 hours.
6. The method of claim 5, wherein the apparatus used for the primary mixing is a double planetary mixer.
7. A method for preparing anode slurry, the method comprising: mixing the negative electrode material of claim 1, a binder, a thickener, a conductive agent, and a solvent for the second time to obtain a negative electrode slurry.
8. The method of claim 7, wherein the secondary mixing is stirred mixing.
9. The method according to claim 8, wherein the stirring and mixing is performed at a revolution speed of 10 to 50r/min and a rotation speed of 200 to 3000 r/min.
10. The method of claim 7, wherein the time for the second mixing is 5 to 10 hours.
11. The method of claim 7, wherein the secondary mixing is performed using a double planetary mixer.
12. The method of claim 7, wherein the binder is any one of styrene-butadiene rubber, acrylic acid-polyacrylic acid copolymer, polyethylene oxide and acrylonitrile multipolymer or a combination of at least two of them.
13. The method of claim 7, wherein the thickener is sodium carboxymethylcellulose and/or sodium polyacrylate.
14. The production method according to claim 7, wherein the conductive agent is any one of graphene, carbon nanotubes, SP, and KS-6, or a combination of at least two thereof.
15. The method of claim 7, wherein the solvent is water.
16. A preparation method of a negative pole piece is characterized by comprising the following steps: coating the negative electrode slurry prepared by the method of claim 7 on a current collector, drying, and then carrying out primary cold pressing, die cutting and splitting to prepare a negative electrode plate.
17. The method of claim 16, wherein the current collector is a copper foil.
18. The method of claim 17, wherein the copper foil is a copper foil having a thickness of 6 to 9 μm.
19. A negative electrode plate, characterized in that it is prepared by the method of claim 16.
20. The negative pole piece of claim 19, wherein the negative pole piece has a compacted density of 1 to 1.4g/cm3。
21. A low temperature resistant lithium ion battery, characterized in that the lithium ion battery comprises the negative electrode tab of claim 19.
22. The low temperature resistant lithium ion battery of claim 21 further comprising an electrolyte containing additives, a positive electrode, and a separator.
23. The low temperature resistant lithium ion battery of claim 22 wherein the electrolyte additive comprises any one or a combination of at least two of ethylene sulfite, vinylene carbonate, propylene sulfite, fluoroethylene carbonate, lithium dioxalate borate, lithium difluorooxalate borate, and lithium difluorophosphate.
24. The low temperature resistant lithium ion battery of claim 22, wherein the concentration of the additive in the electrolyte is 0.04 to 0.08 mol/L.
25. The low temperature resistant lithium ion battery of claim 22, wherein the positive electrode material in the positive electrode is lithium iron phosphate.
26. A method for improving the charge capacity of a lithium ion battery at low temperature, which is characterized in that the negative pole piece in the lithium ion battery adopts the negative pole piece of claim 19.
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