CN115663116B - Negative pole piece and lithium ion battery containing same - Google Patents
Negative pole piece and lithium ion battery containing same Download PDFInfo
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- CN115663116B CN115663116B CN202211672410.1A CN202211672410A CN115663116B CN 115663116 B CN115663116 B CN 115663116B CN 202211672410 A CN202211672410 A CN 202211672410A CN 115663116 B CN115663116 B CN 115663116B
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Abstract
The invention discloses a battery negative pole piece and a lithium ion battery containing the same, which comprise a negative current collector and a negative active material layer coated on the surface of the current collector, wherein the negative active material comprises natural graphite, and the nano indentation hardness H of the negative pole piece is 0.005-0.035 GPa. The indentation hardness of the negative pole piece is the comprehensive reflection of the porosity of the pole piece, the crystallinity of the negative active particles and the densification degree of the interior of the particles, and is closely related to the actual production and use process of the battery. When the hardness H of the negative pole piece is within the range of 0.005-0.035 GPa, the comprehensive performance of the negative pole piece can be better balanced, and the corresponding lithium ion battery has better dynamic performance and cycle service life.
Description
Technical Field
The invention relates to the field of lithium ion batteries, in particular to a negative pole piece and a lithium ion battery containing the same.
Background
The negative electrode material used by commercial lithium ion batteries is mainly artificial graphite which is still the main negative electrode material in a period of time in the future, but the artificial graphite has high cost in the production process due to the high-temperature graphitization process of the artificial graphite, so that the cost of the battery is increased, natural graphite receives more and more attention in order to reduce the production cost of the battery, but certain failures often occur in the use or transportation process of a natural graphite electrode, the failures can influence the service life of the lithium ion battery, the cycle life of a chemical energy storage battery can be prolonged and the cost of the lithium ion battery can be reduced by prolonging the service life of a graphite negative electrode, and the method has great significance for popularizing a new energy technology.
The natural graphite negative electrode mainly comprises active material graphite, adhesive, conductive agent and current collector. The graphite cathode material has different sources and processing processes, and has different physical and chemical properties. Therefore, in the research and production processes, in order to obtain a graphite negative electrode material with excellent electrochemical properties, the physicochemical indexes of the graphite negative electrode material in all aspects need to be effectively controlled, so that the relationship between the physicochemical indexes and the electrochemical properties of the graphite negative electrode material is deeply researched. The technical indexes of the graphite negative electrode material are numerous and difficult to consider, and mainly comprise specific surface area, particle size distribution, tap density, compacted density, true density, first charge-discharge specific capacity, first efficiency and the like. In addition, there are electrochemical indices such as cycle performance, rate performance, swelling, and the like.
In addition, for how to prolong the service life of the graphite negative electrode, besides reducing the generation of side reactions in the aspects of active material modification and electrolyte design, the electrode design needs to be optimized to reduce the influence of factors such as mechanical stress, electrode polarization, thermal runaway and the like on the service life. The main parameters of current electrode design include: electrode thickness, porosity, active material particle size, etc.
Disclosure of Invention
In order to further improve the performance of the battery cathode, the invention provides a battery cathode pole piece, which comprises a cathode current collector and a cathode active material layer coated on the surface of the current collector, wherein the cathode active material is graphite, and the nano indentation hardness H of the cathode pole piece is 0.005-0.035 GPa.
In the research, the application discovers that the nano indentation hardness H of the negative pole piece has important influence on the performance of the natural graphite negative pole battery except that the common parameters such as the thickness, the porosity and the like are similar to the design of the artificial graphite electrode.
Firstly, the indentation hardness of the negative pole piece is closely related to the porosity of the pole piece. The greater the porosity, the lower the pole piece hardness. Certain pores (porosity) exist among the graphite particles, and along with the increase of the indentation depth, the pores among the particles are compressed, so that the hardness of the pole piece is reduced. The high porosity can improve the wettability of the electrolyte, so that a transmission channel of lithium ions in the pole piece is smoother, the charging and discharging performance under high multiplying power can be facilitated, but the contact interface of active substance particles and the electrolyte can be increased, and side reaction is easily caused with the electrolyte to cause the loss of active lithium, thereby reducing the available capacity. In addition, the large porosity is not beneficial to constructing a transmission channel of electrons, so that the electrochemical reaction is hindered, the small porosity can deteriorate the electrolyte wettability of the pole piece, the poor liquid retention capacity of the battery is reduced, lithium ions are difficult to shuttle in the pole piece, and the polarization in the circulation process is increased, so that the capacity attenuation of the battery is increased, and the increase of the internal resistance is particularly obvious.
Secondly, the indentation hardness of the negative electrode sheet is also related to the crystallinity of the graphite particles. The higher the crystallinity of graphite granule, the more regular is arranged to carbon atom's hexagon, and long-range degree of order grow, and the graphite flake interlamellar slides more easily, and granule hardness reduces, and pole piece hardness reduces. For a graphite negative electrode material, high crystallinity can enhance the toughness enhancement of particles, so that the particles can resist the deformation stress of lithium ions embedded into a graphite sheet layer, and the particle stability of graphite is improved, thereby prolonging the cycle life of a battery. The low crystallinity is advantageous for the quick charge performance, but the carbon layer on the surface exists in the form of a graphite crystallite, generating a large number of defects (broken bonds, micropores and oxygen-containing functional groups), which are active sites for reacting with the electrolyte during the cycle, consuming a large amount of active lithium at the same time as the electrolyte, deteriorating the cycle performance of the battery.
And thirdly, the indentation hardness of the negative pole piece is also related to the densification degree of the interior of the graphite negative pole particles. In the case of the natural graphite negative electrode particles, a plurality of pores are formed inside the particles, and the pores inside the particles are compressed by an external force, thereby reducing the hardness of the material. The lower the compactness of the natural graphite is, the larger the internal porosity of the natural graphite is, the more interfaces in the particles are, the electrolyte can easily react with the exposed interfaces to generate an SEI film, and meanwhile, internal stress which is difficult to eliminate can be generated in the cyclic charge-discharge process, and the internal stress is easily concentrated on internal pore defects to induce the generation of cracks, so that the particles are broken to cause the loss of active substances, and the cycle life of the battery is shortened.
In addition, in the circulation process, the negative electrode can generate repeated expansion stress, if the hardness of the negative electrode pole piece is too low, the pole piece coating is easy to generate uneven plastic deformation, is difficult to be tightly attached to the diaphragm, is easy to generate polarization to cause lithium precipitation, and reduces the service life of the battery. It is worth noting that the actual working conditions of the battery are complex except for the expansion stress generated by circulation, and a certain external force action also exists, so that the hardness of the negative pole piece cannot be too small; the hardness of the negative pole piece is too high, the toughness of the pole piece is reduced, the pole piece is easy to fracture irreparably, and therefore the service life of the battery is shortened. Therefore, the hardness of the pole piece cannot be too high.
In conclusion, the indentation hardness of the negative electrode plate is a comprehensive expression of the porosity of the plate, the crystallinity of the negative active particles and the degree of densification inside the particles, and is closely related to the actual production and use process of the battery. When the hardness H of the negative pole piece is within the range of 0.005-0.035 GPa, the comprehensive performance of the negative pole piece can reach better balance, and the corresponding lithium ion battery has better dynamic performance and cycle service life.
On the other hand, the invention also provides a lithium ion battery containing the negative pole piece. The lithium ion battery has good cycle performance and dynamic performance, does not separate lithium, and has lower direct current impedance.
Drawings
FIG. 1 shows the hardness curve of the pole piece of example 4 of the present invention.
Detailed Description
For better understanding and implementation, the technical solutions of the present invention will be clearly and completely described below with reference to the examples.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.
Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as being modified by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that can vary depending upon the desired properties to be obtained.
As used herein, "and/or" means one or all of the referenced elements.
As used herein, "comprising" and "comprises" encompass the presence of only the recited elements as well as the presence of other, non-recited elements in addition to the recited elements.
All percentages in the present invention are by weight unless otherwise indicated.
As used in this specification, the terms "a", "an" and "the" are intended to include "at least one" or "one or more" unless otherwise indicated. For example, "a component" refers to one or more components, and thus more than one component may be considered and may be employed or used in the practice of the described embodiments.
The nanoindentation hardness referred to herein is understood to mean the ability of a material to resist elastic deformation, plastic deformation, or failure, and is also expressed as the ability of a material to resist residual deformation and failure. The nano indentation hardness is not a simple physical concept, but is a comprehensive index of mechanical properties such as elasticity, plasticity, strength and toughness of the material. The nano indentation hardness used by the invention has no conversion relation with other testing methods such as microscopic degree, vickers hardness, rockwell hardness and the like, and the adopted testing principle is different.
The physical explanation of the curve of the nano indentation hardness-indentation depth of the negative pole piece adopted by the invention is as follows: the hardness of the material in the test process usually shows stronger indentation depth dependence, higher indentation hardness can be obtained by smaller indentation depth, the phenomenon is called nanometer indentation hardness size effect (ISE), and the common ISE phenomenon is that the hardness value is reduced along with the increase of the indentation depth. Those skilled in the art have explained the ISE phenomenon using the concept of geometric essential dislocation (GDN) and proposed a correlation model. The indenter used in the nanoindentation instrument was a standard Berkovich indenter with a spherical tip having a radius of about 50 nm. With the increase of the indentation depth, the contact between the triangular cone indenter and the graphite particles is changed into surface-to-surface contact, and the projection of the indentation in the horizontal direction is a regular triangle with the size of micron, which shows that the triangular cone indenter simultaneously acts with a plurality of graphite particles in the pole piece, so that the nano indentation test can be used for representing the hardness of the pole piece. When external force load is applied, after the whole pole piece is subjected to stable plastic deformation, the hardness tends to a stable value, and the value can be used for expressing the hardness of the pole piece.
In the invention, the nano indentation hardness H of the negative pole piece is 0.005-0.035 GPa, preferably 0.010-0.028 GPa. Within the range, the comprehensive performance of the negative pole piece reaches better balance, and the corresponding lithium ion battery has better dynamic performance and cycle service life.
In a further preferred embodiment of the present invention, the negative electrode active material has a crystallinity of 0.020 to 0.400, and more preferably 0.040 to 0.340.
The Raman spectrum of the carbon (graphite) material has two peaks, one peak is located at 1580cm -1 The vicinity (line G) is a characteristic scattering peak inherent to single crystal graphite; the other at 1360cm -1 The vicinity (line D) is caused by graphite lattice defects, edge disorder arrangement, low-symmetry carbon structure, and the like. Thus, the degree of crystallinity of the carbon material can be characterized by ID/IG. The smaller the ID/IG of each carbon material, the fewer the carbon defects and the higher the crystallinity.
The crystallinity of the graphite particles is generally controlled by controlling the heat treatment process and the coating layer. In the heat treatment process, the temperature is raised from room temperature to 1000 to 3200 ℃ at the speed of 2 to 10 ℃/min, the heat preservation time is 1 to 12 hours, and the heat treatment equipment is one of a roller kiln, a box type carbonization furnace, a low-temperature graphitization furnace, a graphitization furnace, an Acheson furnace and a discontinuous graphitization furnace. In the coating process, the coating layer is an indefinite-type material obtained by carbonizing asphalt (one or two of coal-series asphalt and petroleum asphalt), resin (phenolic resin, epoxy resin, furan resin, furfural resin or a mixture thereof) or polymer (one or two of PNS, IPN, CMC and SBR). The coating is preferably selected from asphalt, and the softening point is 150-250 ℃.
As described above, the higher the crystallinity of the graphite particles is, the better it is, and it is more advantageous to improve the comprehensive performance of the negative electrode sheet only if the crystallinity is kept within the range limited by the present invention.
In a further preferred embodiment of the present invention, the porosity of the negative electrode sheet is 21% to 59%, and more preferably 25% to 50%.
The porosity of the pole piece is realized by controlling the temperature (100-160 ℃) and the time (2-10 h) of the pore-forming agent and the vacuum baking. The pore-forming agent is any one or a combination of a plurality of paraffin wax microspheres, refined naphthalene, ammonium carbonate, ammonium bicarbonate, ammonium chloride, benzoic acid, oxalic acid, nano-pills, polyethylene oxide or Polymethacrylate (PMMA).
As mentioned above, the higher the porosity of the electrode sheet is, the better the porosity of the electrode sheet is, the more favorable the improvement of the comprehensive performance of the negative electrode sheet is when the porosity of the electrode sheet is kept within the range limited by the present invention.
As a further preferred aspect of the present invention, wherein the graphite is subjected to densification. More preferably, the graphite and the asphalt are mixed and then subjected to densification treatment.
The densification mode comprises impregnation filling, isostatic pressing filling and mechanical extrusion, wherein the mechanical extrusion equipment is cold isostatic pressing equipment, hot isostatic pressing equipment, fusion extrusion equipment or mixing kneading equipment, the extrusion time is 1 to 4 hours, and the isostatic pressing use pressure is 90 to 200MPa.
As a further preferable mode of the present invention, the graphite is one or more of artificial graphite, natural graphite and compound graphite. The artificial graphite cathode is mainly prepared by calcining coke raw materials at a certain temperature, crushing, grading and graphitizing at a high temperature; the natural graphite cathode is a high-purity graphite product obtained by purifying natural graphite raw ore in a series of processes. The artificial graphite has the main advantages of good cycle performance, good compatibility with electrolyte and relatively balanced indexes in all aspects; while the main advantages of natural graphite are high capacity, high compacted density and low price.
As a further optimization of the invention, the compacted density of the negative pole piece is 1.62-1.68g/cm 3 。
Example 1
1) Preparation of negative electrode material
Taking 10KG spherical natural graphite (D50 =11 μm) and 1KG asphalt (the softening point is 220 ℃) to be put into a VC mixer with nitrogen to be fully mixed and coated, setting the temperature at 220 ℃, the mixing speed at 70rpm and the time at 6 hours, and discharging after the mixing is finished. And (2) placing the obtained materials into a box-type carbonization furnace with nitrogen protection for heat treatment, heating to 500 ℃ at a heating rate of 10 ℃/min, keeping the temperature of 500 ℃ for 2 hours, naturally cooling to room temperature, transferring to an Acheson graphitization furnace for heat treatment, heating to 1250 ℃ at a heating rate of 10 ℃/min, then heating to 3000 ℃ at a heating rate of 5 ℃/min, keeping the temperature of 3000 ℃ for 4 hours, naturally cooling to room temperature, taking out, scattering and screening to obtain the natural spherical graphite particles with the core-shell structure, wherein the crystallinity (ID/IG strength ratio) of the natural spherical graphite particles is 0.021.
2) Preparation of negative pole piece
Mixing the slurry: mixing the prepared negative electrode active material, acetylene black as a conductive agent, CMC as a thickening agent and SBR as a binder according to a mass ratio of 96.4.
Coating: the negative electrode slurry was uniformly coated on both surfaces of a copper foil having a current collector of 6 μm of a negative electrode, and completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process (improving the plastic deformation of the polymer pore-forming agent to improve the distribution uniformity of pores and reducing the rebound rate of the pole piece caused by the volatilization of the subsequent pore-forming agent), setting the rolling pressure to be 45T, and setting the compaction density of the pole piece to be 1.65g/cm 3 And the hot pressing temperature is 95 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and (3) placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 150 ℃ for 6 hours at the vacuum degree of-80 Kpa.
Example 2
1) Preparing a negative electrode material:
taking 10KG spherical natural graphite (D50 =11 μm) and 1KG asphalt (the softening point is 150 ℃) to be put into a VC mixer with nitrogen to be fully mixed and coated, setting the temperature at 150 ℃, the mixing speed at 70rpm and the time at 6 hours, and discharging after the mixing is finished. And (2) placing the obtained material into a box-type carbonization furnace filled with nitrogen for heat treatment, heating to 500 ℃ at a heating rate of 10 ℃/min, keeping the temperature of 500 ℃ for 2 hours, naturally cooling to room temperature, transferring to a low-temperature graphitization furnace protected by argon for heat treatment, heating to 1250 ℃ at a heating rate of 10 ℃/min, then heating to 2000 ℃ at a heating rate of 5 ℃/min, keeping the temperature of 2000 ℃ for 4 hours, naturally cooling to room temperature, taking out, scattering and screening to obtain natural spherical graphite particles with a core-shell structure, wherein the crystallinity (ID/IG strength ratio) of the natural spherical graphite particles is 0.175.
2) Preparation of negative pole piece
Mixing the slurry: mixing the prepared negative electrode active material, acetylene black as a conductive agent, CMC as a thickening agent and SBR as a binder according to a mass ratio of 96.4.
Coating: the negative electrode slurry is evenly coated on two surfaces of a copper foil with a current collector of 6 mu m and is completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the pole piece compaction density to be 1.65g/cm 3 And the hot pressing temperature is 80 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 120 ℃ for 2 hours, wherein the vacuum degree is-80 Kpa.
Example 3
1) Preparation of negative electrode material
Placing 100KG spherical natural graphite (D50 =11 μm) in a cold isostatic pressing device for densification to obtain blocky graphite, wherein the pressure is 180MPa, and the extrusion time is 2 hours; depolymerizing, scattering and dispersing the massive graphite to obtain densified spherical graphite; and (3) putting the 10KG densified graphite and 1KG asphalt (the softening point is 150 ℃) into a VC mixer filled with nitrogen for fully mixing and coating, setting the temperature to be 150 ℃, the mixing rotating speed to be 70rpm, the time to be 6 hours, and discharging after the mixing is finished. And (3) placing the obtained material into a box-type carbonization furnace with nitrogen protection for heat treatment, heating to 1250 ℃ at the heating rate of 5 ℃/min, keeping the temperature of 1250 ℃ for 4 hours, naturally cooling to room temperature, taking out, scattering and screening to obtain the natural spherical graphite particles with the core-shell structure, wherein the crystallinity (ID/IG strength ratio) of the natural spherical graphite particles is 0.402.
2) Preparation of negative pole piece
Mixing the slurry: mixing the prepared negative electrode active material, acetylene black as a conductive agent, CMC as a thickening agent and SBR as a binder according to a mass ratio of 96.4.
Coating: the negative electrode slurry was uniformly coated on both surfaces of a copper foil having a current collector of 6 μm of a negative electrode, and completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the compaction density of the pole piece to be 1.65g/cm 3 And the hot pressing temperature is 90 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 150 ℃ for 4 hours, wherein the vacuum degree is-80 Kpa.
Example 4
1) Preparation of cathode material
Placing 10KG spherical natural graphite (D50 =11 μm) and 0.5KG asphalt (softening point is 180 ℃) in hot isostatic pressing equipment for filling and compacting to obtain blocky graphite, wherein the temperature is set to be 200 ℃, the pressure is 180MPa, and the extrusion time is 2 hours; depolymerizing, breaking up and dispersing the blocky graphite to obtain densified spherical graphite, and recording as the densified spherical graphite S; and (3) putting the 10.5KG densified graphite and 0.5KG asphalt (the softening point is 220 ℃) into a VC mixer filled with nitrogen for fully mixing and coating, setting the temperature at 220 ℃, the mixing rotating speed at 80rpm and the time at 6 hours, and discharging after the mixing is finished. And (2) placing the obtained material into a box-type carbonization furnace filled with nitrogen for heat treatment, heating to 500 ℃ at a heating rate of 10 ℃/min, keeping the temperature of 500 ℃ for 2 hours, naturally cooling to room temperature, transferring to a low-temperature graphitization furnace protected by argon for heat treatment, heating to 1250 ℃ at a heating rate of 10 ℃/min, then heating to 2400 ℃ at a heating rate of 5 ℃/min, keeping the temperature of 2400 ℃ for 4 hours, naturally cooling to room temperature, taking out, scattering and screening to obtain natural spherical graphite particles with a core-shell structure, wherein the crystallinity (ID/IG strength ratio) of the natural spherical graphite particles is 0.042.
2) Preparation of negative pole piece
Mixing the slurry: mixing the prepared negative electrode active material, acetylene black as a conductive agent, CMC as a thickening agent and SBR as a binder according to a mass ratio of 96.4.
Coating: the negative electrode slurry is evenly coated on two surfaces of a copper foil with a current collector of 6 mu m and is completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the compaction density of the pole piece to be 1.65g/cm 3 And the hot pressing temperature is 85 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 120 ℃ for 4 hours, wherein the vacuum degree is-80 Kpa.
Example 5
1) Preparation of negative electrode material
Taking 10.5KG of the densified graphite S and 0.5KG of the asphalt (the softening point is 250 ℃) in the embodiment 4, putting the densified graphite S and the asphalt into a VC mixer filled with nitrogen for full mixing and coating, setting the temperature to be 250 ℃, the mixing rotating speed to be 90rpm and the time to be 8 hours, and discharging after the mixing is finished. The obtained material was placed in a box-type carbonization furnace with nitrogen gas protection for heat treatment, and the setting parameters of the heat treatment were the same as those in example 3. And after heat treatment, scattering and screening are carried out to obtain the natural spherical graphite particles with the core-shell structure, wherein the crystallinity (ID/IG strength ratio) of the natural spherical graphite particles is 0.339.
2) Preparation of negative pole piece
Mixing the slurry: mixing the prepared negative electrode active material, acetylene black as a conductive agent, CMC as a thickening agent and SBR as a binder according to a mass ratio of 96.4.
Coating: the negative electrode slurry is evenly coated on two surfaces of a copper foil with a current collector of 6 mu m and is completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the pole piece compaction density to be 1.65g/cm 3 And the hot pressing temperature is 90 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 150 ℃ for 5 hours, wherein the vacuum degree is-80 Kpa.
Example 6
1) Preparation of negative electrode material
Taking 10.5KG of the densified graphite S and 0.5KG of the asphalt (the softening point is 180 ℃) in the embodiment 4, putting the densified graphite S and the asphalt into a VC mixer filled with nitrogen for full mixing and coating, setting the temperature to be 180 ℃, the mixing speed to be 75rpm and the time to be 6 hours, and discharging after the mixing is finished. And placing the obtained materials into a box-type carbonization furnace with nitrogen protection for heat treatment, heating to 500 ℃ at the heating rate of 10 ℃/min, keeping the temperature of 500 ℃ for 2 hours, naturally cooling to room temperature, transferring to a low-temperature graphitization furnace with argon protection for heat treatment, heating to 1250 ℃ at the heating rate of 10 ℃/min, then heating to 2000 ℃ at the heating rate of 5 ℃/min, keeping the temperature of 2000 ℃ for 4 hours, naturally cooling to room temperature, taking out, scattering and screening to obtain the natural spherical graphite particles with the core-shell structure. And (2) uniformly mixing the obtained natural graphite and artificial graphite according to the mass ratio of 1.
2) Preparation of negative pole piece
Mixing the slurry: mixing the prepared negative electrode active material, acetylene black as a conductive agent, CMC as a thickening agent and SBR as a binder according to a mass ratio of 96.4.
Coating: the negative electrode slurry was uniformly coated on both surfaces of a copper foil having a current collector of 6 μm of a negative electrode, and completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the compaction density of the pole piece to be 1.65g/cm 3 And the hot pressing temperature is 95 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and (3) placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 150 ℃ for 6 hours at the vacuum degree of-80 Kpa.
Example 7
1) Preparation of negative electrode material
Taking 10.5KG of the densified graphite S and 0.5KG of the asphalt (the softening point is 250 ℃) in the embodiment 4, putting the graphite S and the asphalt into a VC mixer filled with nitrogen for full mixing and coating, setting the temperature to be 250 ℃, the mixing speed to be 100rpm and the time to be 8 hours, and discharging after the mixing is finished. The parameters of the remaining 500 ℃ pretreatment and the 3000 ℃ high temperature graphitization heat treatment were set as in example 1. After the heat treatment, the graphite particles were dispersed and sieved to obtain natural spherical graphite particles having a core-shell structure, the crystallinity (ID/IG strength ratio) of which was 0.018.
2) Preparation of negative pole piece
Mixing the slurry: mixing the prepared negative electrode active material, acetylene black as a conductive agent, CMC as a thickening agent and SBR as a binder according to a mass ratio of 96.4.
Coating: the negative electrode slurry is evenly coated on two surfaces of a copper foil with a current collector of 6 mu m and is completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the pole piece compaction density to be 1.65g/cm 3 And the hot pressing temperature is 90 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 150 ℃ for 3 hours, wherein the vacuum degree is-80 Kpa.
Example 8
1) Preparation of negative electrode material
Taking 10.5KG of the densified graphite S and 0.5KG of the asphalt (the softening point is 180 ℃) in the embodiment 4, putting the graphite S and the asphalt into a VC mixer filled with nitrogen for full mixing and coating, setting the temperature to be 180 ℃, the mixing speed to be 75rpm and the time to be 6 hours, and discharging after the mixing is finished. And placing the obtained materials into a box-type carbonization furnace with nitrogen protection for heat treatment, heating to 500 ℃ at the heating rate of 10 ℃/min, keeping the temperature of 500 ℃ for 2 hours, naturally cooling to room temperature, transferring to a low-temperature graphitization furnace with argon protection for heat treatment, heating to 1250 ℃ at the heating rate of 10 ℃/min, then heating to 2500 ℃ at the heating rate of 5 ℃/min, keeping the temperature of 2500 ℃ for 4 hours, naturally cooling to room temperature, taking out, scattering and screening to obtain the natural spherical graphite particles with the core-shell structure. And (3) uniformly mixing the obtained natural graphite and artificial graphite according to the mass ratio of 2.
2) Preparation of negative electrode plate
Mixing the slurry: mixing the prepared negative electrode active material, acetylene black as a conductive agent, CMC as a thickening agent and SBR as a binder according to a mass ratio of 96.4.
Coating: the negative electrode slurry is evenly coated on two surfaces of a copper foil with a current collector of 6 mu m and is completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the pole piece compaction density to be 1.68g/cm 3 And the hot pressing temperature is 95 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 150 ℃ for 6 hours, wherein the vacuum degree is-80 Kpa.
Comparative example 1
1) Preparation of negative electrode material
Taking 10.5KG of the densified graphite S and 0.5KG of the asphalt (the softening point is 280 ℃) in the embodiment 4, putting the graphite S and the asphalt into a VC mixer filled with nitrogen for full mixing and coating, setting the temperature to be 280 ℃, the mixing speed to be 120rpm and the time to be 8 hours, and discharging after the mixing is finished. The parameters of the remaining 500 ℃ pretreatment and the 3000 ℃ high temperature graphitization heat treatment were set as in example 1. After the heat treatment, the graphite particles were dispersed and sieved to obtain natural spherical graphite particles having a core-shell structure, and the crystallinity (ID/IG strength ratio) of the particles was 0.014.
2) Preparation of negative pole piece
Mixing the slurry: mixing the prepared negative electrode active material, the conductive agent acetylene black, the thickening agent CMC and the binder SBR according to a mass ratio of 9.4.
Coating: the negative electrode slurry is evenly coated on two surfaces of a copper foil with a current collector of 6 mu m and is completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the compaction density of the pole piece to be 1.65g/cm 3 And the hot pressing temperature is 95 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and (3) placing the obtained pole piece into a vacuum drying oven for baking, wherein the temperature is set to be 150 ℃, the time is 8 hours, and the vacuum degree is-80 Kpa.
Comparative example 2
1) Preparation of cathode material
Taking 10KG spherical natural graphite (D50 =11 μm) and 1KG asphalt (the softening point is 280 ℃) to be put into a VC mixer with nitrogen to be fully mixed and coated, setting the temperature at 280 ℃, the mixing speed at 120rpm and the time at 10 hours, and discharging after the mixing is finished. The parameters of the remaining 500 ℃ pretreatment and 3000 ℃ high temperature graphitization heat treatment were set as in example 1. And after heat treatment, scattering and screening to obtain the natural spherical graphite particles with the core-shell structure, wherein the crystallinity (ID/IG strength ratio) of the natural spherical graphite particles is 0.017.
2) Preparation of negative electrode plate
Mixing the slurry: mixing the prepared negative electrode active material, acetylene black as a conductive agent, CMC as a thickening agent and SBR as a binder according to a mass ratio of 96.4.
Coating: uniformly coating the negative electrode slurry on two surfaces of a copper foil with a current collector of 6 mu m of a negative electrode, and completely drying in an oven at 80 ℃;
rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the compaction density of the pole piece to be 1.65g/cm 3 And the hot pressing temperature is 80 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 120 ℃ for 2 hours, wherein the vacuum degree is-80 Kpa.
Comparative example 3
1) Preparation of cathode material
Taking 10KG spherical natural graphite (D50 =11 μm) and 1KG asphalt (the softening point is 150 ℃) to be put into a VC mixer with nitrogen to be fully mixed and coated, setting the temperature at 150 ℃, the mixing speed at 70rpm and the time at 6 hours, and discharging after the mixing is finished. The obtained material was placed in a box-type carbonization furnace with nitrogen gas protection for heat treatment, and the parameters for the heat treatment were set as in example 3. After the heat treatment, the graphite particles were dispersed and sieved to obtain natural spherical graphite particles having a core-shell structure, the crystallinity (ID/IG strength ratio) of which was 0.495.
2) Preparation of negative pole piece
Mixing the slurry: mixing the prepared negative electrode active material, the conductive agent acetylene black, the thickening agent CMC and the binder SBR according to a mass ratio of 96.4.
Coating: the negative electrode slurry was uniformly coated on both surfaces of a copper foil having a current collector of 6 μm of a negative electrode, and completely dried in an oven at 80 ℃.
Rolling: adopting a hot pressing process, setting the rolling pressure to be 45T, and setting the compaction density of the pole piece to be 1.65g/cm 3 And the hot pressing temperature is 90 ℃, the rolling speed is 20m/min, and then the negative pole piece is obtained after slitting.
Baking: and placing the obtained pole piece in a vacuum drying oven for baking at the temperature of 150 ℃ for 7 hours, wherein the vacuum degree is-80 Kpa.
All of the above examples and comparative examples were used to prepare and test cells according to the following process steps:
(1) Preparation of positive pole piece
The positive active material NCM333 (including other positive LiCoO materials) 2 、LiNiO 2 、LiMnO 2 、LiMn 2 O 4 、LiNi 1/ 3 Co 1/3 Mn 1/3 O 2 (NCM333)、LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523)、LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622)、LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811)、LiNi 0.85 Co 0.15 Al 0.05 O 2 、LiFePO 4 、LiMnPO 4 Etc.), mixing acetylene black serving as a conductive agent and PVDF serving as a binder according to a mass ratio of 96; the anode slurry is evenly coated on two surfaces of the anode current collector aluminum foilAnd transferring the dried positive electrode plate to an oven for continuous drying after warm air drying, and then obtaining the positive electrode plate through cold pressing and slitting.
(2) Preparation of negative pole piece
The negative pole piece is obtained in the above examples and comparative examples.
(3) Preparation of the electrolyte
Ethylene Carbonate (EC), ethyl Methyl Carbonate (EMC), diethyl carbonate (DEC) were mixed in a volume ratio of 1 6 Dissolving in the mixed organic solvent to prepare the electrolyte with the concentration of 1 mol/L.
(4) Preparation of the separator
Selected from polyethylene films as barrier films.
(5) Preparation of lithium ion battery
The positive pole piece, the isolating membrane and the negative pole piece are sequentially stacked, the isolating membrane is positioned between the positive pole piece and the negative pole piece to play an isolating role, then the bare cell is obtained by winding, the bare cell is placed in an outer packaging shell, electrolyte is injected after drying, and the lithium ion battery is obtained through the procedures of vacuum packaging, standing, formation, shaping and the like.
Physical and chemical index test
(1) Raman (Raman) testing
Preparing a sample: preparing sample powder into a very flat sample in the same horizontal plane;
and (3) testing: connecting an instrument with the equipment model of i-Raman Prime with a computer and BWSpec software for Raman testing, collecting a Raman spectrum after adjusting the distance between the laser and the tested sample, testing 8 points of each sample for focusing the optimal testing position when the Raman peak intensity is maximum, storing data, and subtracting a background baseline by the BWSpec software for analysis to obtain a Raman curve spectrum;
data processing: i (D)/I (G) is the ratio of the intensities of the D peak and the G peak, and represents the degree of defects on the surface of the graphite particles.
(2) Pole piece porosity test
The porosity epsilon of the negative pole piece is obtained by a mass difference method, and the pole piece is cut into halves by a pole piece punching machineA wafer with the diameter r of 0.95cm, and a thickness gauge is used for respectively measuring the thicknesses L and L of the pole piece and the current collector 0 Calculating the volume V = pi x r of the negative active material on the cut pole piece 2 *(L- L 0 ) (ii) a Weighing the cut pole piece by a balance with the accuracy of 0.00001g, and recording the mass m of the cut pole piece 1 Soaking the pole piece with hexadecane for 1h (completely soaking the pole piece therein), taking out the pole piece with tweezers, drying with filter paper until the mass is constant, and weighing the mass m 2 And substituting experimental data into a formula to calculate: ε = (m) 2 -m 1 )/(V* ρ 0 ) 100% of where ρ 0 Is hexadecane with the density of 0.7734g/cm 3 And obtaining the porosity epsilon of the pole piece.
(3) Pole piece hardness test
The hardness of the negative pole piece was tested by a nanoindentation instrument using a standard berkovich indenter. The method comprises the steps of firmly fixing a negative pole piece on a glass slide, then placing the glass slide on an object stage of a nano indentation instrument, fixing the glass slide in a vacuum adsorption mode, observing a sample test surface by using a device with a microscope, selecting a position to be tested (a cross cursor at the center of a visual field), selecting a test method of a continuous rigidity mode, setting experiment parameters, starting a click test, moving away an optical lens after the test is started, moving a probe to a target position, touching the surface of the sample with very small contact force, then staying on the surface of the sample for waiting, starting the test according to a set method after a system is stabilized, finishing the test, and withdrawing the probe from the surface of the sample to obtain a curve of nano indentation hardness H-displacement depth d, wherein d is more than or equal to 4 mu m.
Performance testing
(1) And (3) testing cycle performance:
the lithium ion batteries prepared in the examples and comparative examples were subjected to cycle testing at 25 ℃ according to the following procedure: and fully charging at the multiplying power of 1C until the capacity of the lithium ion battery is less than 80% of the initial capacity, recording the number of cycles, taking down the lithium ion battery, and testing the direct current impedance after taking down the lithium ion battery.
(2) DC impedance testing
The cells of examples and comparative examples were charged to 50% SOC at 25 ℃, discharged at a current of 1C rate for 18S, recorded as cell voltage U2 before the termination of discharge, current I and cell voltage U1 after the cell voltage was stabilized, and calculated according to the formula R = (U2-U1)/I to obtain direct current internal resistance R; recording direct current internal resistances of the battery before and after circulation as R0 and R1 respectively, and recording the direct current resistance change rate = (R1-R0)/R0;
(3) Dynamic performance test
And (3) fully charging the lithium ion batteries prepared in the examples and the comparative examples at-10 ℃ by 0.2C and fully discharging the lithium ion batteries at 0.33C, repeating the steps for 10 times, fully charging the lithium ion batteries at 0.2C, disassembling the negative pole piece, and observing the lithium precipitation condition on the surface of the negative pole piece. Wherein, the lithium precipitation area of the surface of the negative electrode of less than 10 percent is considered to be slightly lithium precipitation, the lithium precipitation area of the surface of the negative electrode of 10 percent to 50 percent is considered to be moderately lithium precipitation, and the lithium precipitation area of the surface of the negative electrode of more than 50 percent is considered to be severely lithium precipitation.
Examples 1 to 7 and comparative examples 1 to 3 were tested in the same manner, and the results of the tests of the respective examples and comparative examples are shown in Table 1. The pole piece hardness curve of example 4 is shown in figure 1.
Table 1: test results of examples 1 to 7 and comparative examples 1 to 3
It can be seen from the above test data that in examples 1 and 2, by reasonably setting the porosity of the pole piece (to cover or sacrifice the dynamic performance of part of the pole piece), and in example 3, by adjusting and controlling the densification, crystallinity and porosity of the pole piece (to give consideration to the dynamic performance of the material and the pole piece end), the hardness of the pole piece is in the range of 0.005 to 0.035, thereby improving the cycle performance of the battery.
Examples 4 to 6 further optimize the pole piece hardness to 0.010-0.028 by further optimizing the densification process (to make the interior of the natural graphite more dense), the crystallinity of the negative electrode material (to improve the structural stability of the material and the appropriate dynamic performance), and by regulating and controlling the appropriate pole piece porosity (the appropriate lithium ion transmission channel and the contact area with the electrolyte), so that the cycle life and the dynamic performance of the battery are further improved.
In example 7, although the crystallinity of natural graphite is out of the range required by the present patent, the lack of the dynamic performance can be compensated by setting a proper porosity of the pole piece, and simultaneously, the densification treatment is performed to improve the structural stability of the material phase, so that the overall pole piece hardness is in the optimized hardness range (0.010-0.028), and the battery still has excellent cycle life and dynamic performance.
In comparative example 1, the crystallinity of the obtained natural graphite was too small, and the lithium intercalation ability of the graphite surface was insufficient; the pole piece porosity is too big, and the active material leads to constantly consuming active lithium with the area of contact grow of electrolyte, and active material's area of contact reduces simultaneously, and the electron conduction of whole pole piece is obstructed, the increase polarization. Thus, the crystallinity and the hardness of the electrode sheet in comparative example 1 were not properly set, and the hardness of the electrode sheet as a whole was 0.005 or less, which deteriorated the service life of the battery.
In comparative example 2, the natural graphite is not subjected to densification treatment, and the internal interface of the bulk phase of the natural graphite continuously generates side reaction with the electrolyte, which is not beneficial to the cycle performance of the battery; the crystallinity of the natural graphite is too small, and the lithium ion is hindered in the process of embedding into the surface of the natural graphite; the porosity of the pole piece is too small, the liquid retention capacity of the battery is poor, lithium ions are difficult to transmit among graphite particles of the pole piece, the pole piece is enlarged, and the lithium ions are easy to separate out on the surface of the pole piece. Therefore, the crystallinity, the densification and the pole piece hardness in the comparative example 2 are unreasonable, so that the overall pole piece hardness is over 0.035, and the cycle life of the battery is shortened.
In comparative example 3, the natural graphite has excessively high crystallinity, the toughness of the surface thereof is deteriorated, which is not favorable for the constraint with the expansion stress, and the surface thereof has a large number of defects, and the electrolyte easily causes a side reaction with the electrolyte to aggravate the continuous growth of the SEI film, and the polarization phenomenon is deteriorated. In addition, natural graphite is not subjected to densification treatment, and an internal interface can continuously generate side reaction with electrolyte to consume active lithium. Although the porosity of the pole piece is in the range required by the patent, the intrinsic properties (densification and crystallinity) of the material are unreasonable, so that the hardness of the whole pole piece is over 0.035, and the cycle life of the battery is reduced.
As described above, the present invention is only a preferred embodiment, and is not limited in any way, and therefore, any simple modification, equivalent change and modification of the above embodiment according to the technical essence of the present invention will still fall within the scope of the technical solution of the present invention.
Claims (7)
1. The negative pole piece of the battery comprises a negative current collector and a negative pole coated on the surface of the current collector
The negative electrode active material layer comprises natural graphite, and the nano indentation hardness H of the negative electrode plate is 0.005-0.035 Gpa;
wherein the crystallinity of the negative electrode active material is 0.020 to 0.400;
wherein the porosity of the negative pole piece is 21-59%;
wherein the compaction density of the negative pole piece is 1.62-1.68g/cm < 3 >.
2. The battery negative pole piece of claim 1, wherein the nano-indentation hardness H of the negative pole piece is 0.010-0.028 GPa.
3. The battery negative electrode tab according to claim 1, wherein the negative electrode active material has a crystallinity of 0.040 to 0.340.
4. The battery negative electrode sheet of claim 1, wherein the porosity of the negative electrode sheet
25% -50%.
5. The battery negative electrode sheet of claim 1, wherein the natural graphite is densified.
6. The battery negative electrode sheet according to claim 1, wherein the natural graphite accounts for 20-50% of the mass fraction of the negative electrode active material layer.
7. A lithium ion battery comprising the battery negative electrode sheet of any one of claims 1 to 6.
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