CN112072071B - Positive pole piece and lithium ion battery comprising same - Google Patents

Positive pole piece and lithium ion battery comprising same Download PDF

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CN112072071B
CN112072071B CN202010924317.XA CN202010924317A CN112072071B CN 112072071 B CN112072071 B CN 112072071B CN 202010924317 A CN202010924317 A CN 202010924317A CN 112072071 B CN112072071 B CN 112072071B
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layer
active material
positive electrode
lithium
safety
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CN112072071A (en
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曾家江
樊亚楠
伍鹏
李素丽
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Zhuhai Cosmx Battery 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
    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
    • 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
    • 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
    • H01M4/366—Composites as layered products
    • 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
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • 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
    • 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
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • 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
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
    • 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

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  • Battery Electrode And Active Subsutance (AREA)
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Abstract

The invention provides a lithium ion battery positive pole piece and a preparation method thereof, wherein the positive pole piece comprises a positive pole current collector, a safety layer, a conducting layer and an active substance layer, wherein the thermal decomposition temperature T1 of a first positive pole active substance in the safety layer under 100% SOC is more than or equal to the thermal decomposition temperature T2 of a second positive pole active substance in the active substance layer under 100% SOC; by using the novel multi-layer coating pole piece structure design, the pole piece has higher safety and better cycle performance than the pole piece of the conventional two-layer coating structure; by using a multilayer coating technology, the interface problem caused by poor electronic conductivity of a safety layer and an active material layer in the positive pole piece in the circulating process can be improved by utilizing the high conductivity of the conductive layer, so that the safety of the high-energy-density lithium ion battery is ensured, and the circulating performance of the lithium ion battery and the problem of large DCIR (direct current infrared) increase change rate in the circulating process can be further improved.

Description

Positive pole piece and lithium ion battery comprising same
Technical Field
The invention belongs to the technical field of lithium ion batteries, and particularly relates to a positive pole piece and a lithium ion battery comprising the same.
Background
Power batteries and high voltage digital batteries are currently rapidly developing and widely used in the 3C consumer digital field such as mobile phones, notebook computers, tablet computers, bluetooth small batteries and the like and in the electric vehicle field. The requirements for the safety performance of lithium ion batteries are increasing no matter in the digital or power fields. In recent years, the development of volume energy density and mass energy density of a digital lithium ion battery reaches the limit of materials quickly, the safety performance of the corresponding battery is poor due to poor safety performance of high-voltage lithium cobalt oxide (more than or equal to 4.4V) and high-nickel materials, and the lithium ion battery uses flammable organic solvent as electrolyte solvent and has potential safety hazard.
Moreover, the safety performance and the cycle performance of the lithium ion battery are two important performance indexes of the battery, when the lithium ion battery in the existing base coat mode obtains better safety performance, the cycle performance of the lithium ion battery is usually sacrificed, and because the base coat layer usually has the condition of large direct current internal resistance when solving the safety problem, the cycle performance of the lithium ion battery is influenced, so that how to realize the safety performance and the cycle performance simultaneously is a problem which needs to be solved urgently in the research process of the lithium ion battery.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a positive pole piece and a lithium ion battery comprising the same, wherein the positive pole piece comprises a positive current collector, a safety layer, a conducting layer and an active substance layer; the safety layer, the conductive layer and the active substance layer are sequentially arranged on a first surface of the positive current collector and further sequentially arranged on a second surface opposite to the first surface; the positive pole piece with the structure can solve the safety problem of the high-energy-density lithium ion battery, and avoid the problems of large Direct Current Internal Resistance (DCIR) increase change rate and poor cycle performance in the cycle process.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a positive pole piece comprises a positive current collector, a safety layer, a conductive layer and an active substance layer; the safety layer is arranged on the first surface of the positive current collector, the conductive layer is arranged on the surface of the safety layer, and the active substance layer is arranged on the surface of the conductive layer;
the safety layer includes a first positive electrode active material, and the active material layer includes a second positive electrode active material; wherein a thermal decomposition temperature (DSC) T1 of the first positive electrode active material in the safety layer at 100% SOC (full charge) is equal to or higher than a thermal decomposition temperature (DSC) T2 of the second positive electrode active material in the active material layer at 100% SOC (full charge), that is, the thermal stability of the first positive electrode active material in the safety layer is higher than that of the second positive electrode active material in the active material layer.
According to the present invention, the difference T1-T2 between the thermal decomposition temperature T1 at 100% SOC of the first positive electrode active material in the safety layer and the thermal decomposition temperature T2 at 100% SOC of the second positive electrode active material in the active material layer satisfies: T1-T2>10 ℃.
According to the invention, the security layer further comprises a first conductive agent and a first binder; the active material layer further includes a third conductive agent and a third binder.
According to the present invention, the conductive layer includes a second conductive agent and a second binder.
According to the invention, the positive current collector is selected from aluminium foil.
According to the invention, the thickness of the positive electrode current collector is 8-12 μm.
According to the invention, the security layer has a thickness of 1 to 10 μm, preferably 1 to 5 μm, for example 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm.
According to the invention, the mass of the first positive electrode active substance in the security layer amounts to 80 to 92 wt.%, preferably 86 to 90 wt.%, for example 80 wt.%, 81 wt.%, 82 wt.%, 83 wt.%, 84 wt.%, 85 wt.%, 86 wt.%, 87 wt.%, 88 wt.%, 89 wt.%, 90 wt.%, 91 wt.% or 92 wt.%, of the total mass of the security layer.
According to the invention, the mass ratio of the first binder to the first conductive agent in the security layer is 2-6:1, such as 2:1, 3:1, 4:1, 5:1 or 6: 1.
According to the present invention, the ratio of the charge capacity of the first positive electrode active material in the charge voltage range of 2.5V to 4.3V to the total capacity of the first positive electrode active material in the charge voltage range of 2.5V to 3.85V is not less than 75% at a charge rate of 0.2C and in the charge voltage range of 2.5V to 4.3V.
According to the present invention, the first positive electrode active material is selected from one or more of lithium iron phosphate, lithium cobaltate, lithium nickel cobalt manganese oxide (nickel molar content < 60%), lithium manganese oxide, preferably at least one of lithium iron phosphate, lithium manganese oxide, lithium nickel cobalt manganese oxide (nickel molar content < 60%).
According to the present invention, the particle diameter D of the first positive electrode active material in the safety layer507 μm or less, and illustratively, the particle diameter D of the lithium iron phosphate50<3 mu m, the particle diameter D of the lithium cobaltate, the lithium nickel cobalt manganese oxide or the lithium manganese oxide50≤7μm。
According to the invention, the thickness of the conductive layer is 1-5 μm, preferably 1-3 μm, for example 1 μm, 2 μm, 3 μm, 4 μm, 5 μm.
According to the invention, the mass ratio of the second conductive agent to the second binder in the conductive layer is 1-3:1, such as 1:1, 2:1, 3: 1.
According to the invention, the conductive layer further comprises an ion-conducting compound.
According to the invention, the ion conducting compound is selected from at least one of lithium iron phosphate, lithium cobaltate, lithium nickel cobalt manganese oxide material (the molar content of nickel is less than 60%), lithium manganese oxide, lithium titanate, lithium titanium aluminum phosphate, lanthanum lithium titanate, lanthanum lithium tantalate, lithium germanium aluminum phosphate, diboron trioxide doped lithium phosphate, lithium lanthanum zirconium oxygen, lanthanum zirconium aluminum lithium oxygen, niobium doped lithium lanthanum zirconium oxygen, tantalum doped lithium lanthanum zirconium oxygen, niobium doped lithium lanthanum zirconium oxygen.
According to the invention, the mass of the ion-conducting compound in the electrically conductive layer amounts to 30-70 wt.%, preferably 50-70 wt.%, for example 30 wt.%, 40 wt.%, 50 wt.%, 15 wt.%, 52 wt.%, 53 wt.%, 54 wt.%, 55 wt.%, 60 wt.%, 65 wt.%, 68 wt.%, 70 wt.%, of the total mass of the electrically conductive layer.
In the invention, a multilayer coating technology is used, and the interface problem caused by poor electronic conductivity and poor ionic conductivity of a safety layer and an active material layer in the positive pole piece in the circulating process can be improved by utilizing the high conductivity and the ionic conductivity of the conductive layer, so that the safety of the high-energy-density lithium ion battery is ensured, and the circulating performance of the lithium ion battery and the problem of large increase change rate of DCIR in the circulating process can be further improved. The ion conducting compound not only has the ion conducting property, but also can effectively solve the interface problem generated by poor ion migration rate and conductivity of the safety layer and the active material layer in the circulation process by combining with a conductive agent with high conductivity, thereby improving the circulation performance and the problem of large increase change rate of DCIR.
According to the invention, the thickness of the active substance layer is 50 to 130 μm, preferably 70 to 90 μm, for example 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm or 130 μm.
According to the invention, the second positive electrode active material in the active material layer is selected from one or more of lithium cobaltate, lithium nickel cobalt manganese oxide (the molar content of nickel is more than or equal to 60 percent) and lithium nickel cobalt aluminate (the molar content of nickel is more than or equal to 75 percent).
According to the present invention, the mass of the second positive electrode active material in the active material layer accounts for 90 to 99wt%, preferably 96 to 99wt%, for example, 90 wt%, 91 wt%, 92wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt%, 97 wt%, 98 wt%, 99wt% of the total mass of the active material layer.
According to the present invention, the mass ratio of the third conductive agent to the third binder in the active material layer is 0.5 to 2:1, for example, 0.5:1, 1:1, 2: 1.
According to the present invention, the particle diameter D of the second positive electrode active material in the active material layer50Is 3-18 μm, illustratively, the particle diameter D of the lithium cobaltate50D of the lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminate is 10-18 mu m50The grain diameter is 3-12 μm.
According to the invention, the first conductive agent, the second conductive agent and the third conductive agent are the same or different and are independently selected from one or more of conductive carbon black, acetylene black, carbon nanotubes (such as single-walled carbon nanotubes and multi-walled carbon nanotubes), carbon nanofibers and graphene, preferably graphene and conductive carbon black.
According to the invention, the first binder, the second binder and the third binder are the same or different and are selected independently from one or more of sodium carboxymethylcellulose, styrene-butadiene latex, polytetrafluoroethylene, polyvinylidene fluoride (PVDF) and polyethylene oxide.
The invention provides a preparation method of the positive pole piece, which comprises the following steps:
1) respectively preparing slurry for forming a safety layer, slurry for forming a conductive layer and slurry for forming an active material layer;
2) and coating the slurry for forming the safety layer, the slurry for forming the conductive layer and the slurry for forming the active substance layer on the first surface of the positive current collector to prepare the positive pole piece.
According to the invention, the method further comprises the steps of:
3) and coating the slurry for forming the safety layer, the slurry for forming the conductive layer and the slurry for forming the active substance layer on a second surface, opposite to the first surface, of the positive current collector to prepare the positive pole piece.
According to the invention, the method further comprises the steps of:
4) and (3) rolling the positive pole piece coated in the step 2) or the step 3) to obtain a rolled positive pole piece.
According to the present invention, in step 1), the solid contents of the security layer-forming paste, the conductive layer-forming paste, and the active material layer-forming paste are 30 wt% to 80 wt%.
According to the invention, in step 2), the coating may be, for example, extrusion coating, spray coating, or the like.
According to the invention, in the step 3), the surface density of the positive pole piece is 14-27mg/cm2The porosity of the positive pole piece is 14-30%, and the compaction density of the positive pole piece is 3.0-4.3g/cm3。
The invention provides a lithium ion battery which comprises the positive pole piece.
According to the invention, the lithium ion battery also comprises a negative pole piece, a diaphragm and electrolyte.
According to the invention, the negative electrode plate comprises a negative active material, and the negative active material comprises a graphite material or a mixture of graphite and a silicon material.
According to the present invention, the separator is a separator known in the art, for example, a separator for a commercial lithium ion battery known in the art.
According to the present invention, the graphite material is at least one of artificial graphite, natural graphite, and the like.
According to the invention, the silicon material is, for example, Si, SiC and SiOx(0<x<2) One or more of (a).
According to the invention, the silicon material accounts for 0-20 wt% of the total mass of the graphite material and the silicon material, and the pure graphite material is preferably used as a negative electrode.
According to the present invention, the electrolyte is a conventional electrolyte known in the art, and the solvent contains ethylene carbonate (abbreviated as EC), diethyl carbonate (abbreviated as DEC), propylene carbonate (abbreviated as PC), fluoroethylene carbonate (abbreviated as FEC), and the like.
The invention provides a preparation method of the lithium ion battery, which comprises the step of assembling the positive pole piece, the negative pole piece, the electrolyte and the diaphragm into the lithium ion battery.
The invention has the beneficial effects that:
the invention provides a lithium ion battery positive pole piece and a preparation method thereof, wherein the positive pole piece comprises a positive current collector, a safety layer, a conducting layer and an active substance layer; the safety layer, the conducting layer and the active substance layer are sequentially arranged on the first surface of the positive current collector; wherein the safety layer includes a first positive electrode active material, a first conductive agent, and a first binder; the conductive layer includes a second conductive agent and a second binder; the active material layer includes a second positive electrode active material, a third conductive agent, and a third binder. Compared with the pole piece with the conventional two-layer coating structure, the pole piece has higher safety and better cycle performance by using the novel multi-layer coating pole piece structure design; by using a multilayer coating technology, the interface problem caused by poor electronic conductivity and poor ionic conductivity of a safety layer and an active material layer in the positive pole piece in the circulating process can be improved by utilizing the high conductivity and the ionic conductivity of the conductive layer, so that the safety of the high-energy-density lithium ion battery is ensured, and the circulating performance of the lithium ion battery and the problem of large DCIR (direct current infrared) increase change rate in the circulating process can be further improved. The ion conducting compound not only has the ion conducting property, but also can effectively solve the interface problem generated by poor ion migration rate and conductivity of the safety layer and the active material layer in the circulation process by combining with a conductive agent with high conductivity, thereby improving the circulation performance and the problem of large increase change rate of DCIR.
Drawings
FIG. 1 is a schematic view of the structure of a positive electrode plate according to the present invention;
wherein, 1 is a positive electrode current collector, 2 is a safety layer, 3 is a conductive layer, and 4 is an active material layer.
Detailed Description
The present invention will be described in further detail with reference to specific examples. It is to be understood that the following examples are only illustrative and explanatory of the present invention and should not be construed as limiting the scope of the present invention. All the technologies realized based on the above-mentioned contents of the present invention are covered in the protection scope of the present invention.
The experimental methods used in the following examples are all conventional methods unless otherwise specified, and the lithium nickel cobalt manganese oxide NCM used in the following examples is exemplified by NCM 523; reagents, materials and the like used in the following examples are commercially available unless otherwise specified.
The composition of the 5130 gum used in the examples described below was polyvinylidene fluoride, which is normally commercially available.
Example 1
(1) Lithium iron phosphate (D) as a positive electrode active material50<3 μm), conductive agent SP and adhesive 5130 glue, mixed in a weight ratio of 86% to 4% to 10%, the mixture was dispersed in NMP, and after double planetary stirring, a security layer slurry was obtained. And coating the safety layer slurry on the front side and the back side of an aluminum foil current collector with the thickness of 12 mu m, and drying after coating, wherein the thickness of a single-side coating layer is 5 mu m, so as to obtain the positive pole piece containing the safety layer.
(2) PVDF and carbon nanotubes are mixed according to the weight ratio of 40% to 60%, the mixture is dispersed in NMP, and conductive layer slurry is obtained after double-planet stirring. And coating the conductive layer slurry on the surface of the positive pole piece containing the safety layer, and drying after coating, wherein the thickness of the single-side coating layer is 2 microns, so as to obtain the positive pole piece containing the conductive layer and the safety layer.
(3) Mixing 97% to 1.5% of lithium cobaltate, PVDF and carbon nanotubes by weight, dispersing the mixture in NMP, and stirring by double planets to obtain active material layer slurry. And coating the active substance layer slurry on the surface of a positive pole piece containing a conductive layer and a safety layer, and drying after coating, wherein the thickness of a single-side coating layer is 80 mu m, so as to obtain the positive pole piece.
(4) Rolling the obtained positive pole piece, wherein the compaction density is 3.9g/cm3The positive electrode sheet obtained by roll pressing has a structure as shown in fig. 1, and a safety layer 2, a conductive layer 3, and an active material layer 4 are respectively provided on both side surfaces of a positive electrode current collector 1.
(5) Mixing graphite serving as a negative electrode active material, styrene diene rubber (SBR), sodium carboxymethylcellulose and conductive carbon black in a weight ratio of 94% to 3% to 2% to 1%, dispersing the mixture in water, and mixing by double planets to obtain negative electrode slurry. And coating the negative electrode slurry on the surfaces of the two sides of the copper foil current collector, and then rolling and drying to obtain a negative electrode piece for later use.
The electrolyte used includes a solvent containing ethylene carbonate (abbreviated as EC), diethyl carbonate (abbreviated as DEC), propylene carbonate (abbreviated as PC), fluoroethylene carbonate (abbreviated as FEC), and a lithium salt containing lithium hexafluorophosphate (1M).
And then winding in a winding mode to obtain a winding core, packaging in an aluminum plastic bag, injecting electrolyte, and performing hot pressing to obtain a soft package battery core, wherein the capacity of the soft package battery core is 4000 mAh.
And measuring the capacity retention rate of the soft package battery cell in each week cycle, and testing the internal resistance change rate and the performances of acupuncture and heavy object impact in the cycle process.
(1) Test conditions and methods for capacity retention:
the lithium ion battery is placed in an environment with the temperature of 25 ℃, under the conditions of 0.7C charging and 0.7C discharging, the charging and discharging temperature is 25 ℃, and the voltage range is 3.0-4.45V.
(2) The test condition and method of the internal resistance change rate are as follows:
the lithium ion battery was placed in an environment at 25 ℃ and subjected to 0.7C charging and 0.7C discharging cycles with a cut-off current of 0.05C, and the DCR was tested at 50 weeks per cycle (charging to 3.6V followed by constant voltage charging with a cut-off current of 0.05C; 100mA discharging for 10s, 2000mA discharging for 1s), and the internal DC resistance was (V1-V2)/(I2-I1)).
The rate of change of internal resistance is current DCR/initial DCR 100%.
(3) Test conditions and methods for needling:
the lithium ion battery is placed in a high-temperature resistant steel needle (the conical angle of the needle point is 45-60 ℃, the surface of the needle is smooth and has no rust, oxide layer or oil stain) with the diameter phi of (3 +/-0.5) mm, the lithium ion battery penetrates through the lithium ion battery from the direction vertical to the pole plate of the battery cell at the speed of (100mm/s +/-5 mm/s), and the puncture position is close to the geometric center of the punctured surface (the steel needle stays in the battery cell). And observing for 1 hour or stopping the test when the highest temperature of the surface of the battery core is reduced to 10 ℃ or below the peak temperature. The lithium ion battery is not fired and is not exploded and is recorded as passing. 10 lithium ion batteries are tested in each case, and the nail penetration test passing rate is used as an index for evaluating the needling safety of the lithium ion batteries.
(4) Test conditions and methods for weight impact:
placing the battery core on the surface of the platform, transversely placing a metal rod with the diameter of 15.8mm +/-0.2 mm on the upper surface of the geometric center of the battery core, impacting the surface of the battery core with the metal rod in a free falling state from a high position of 610mm +/-25 mm by adopting a weight of 9.1kg +/-0.1 kg, and observing for 6 hours. The broad face was subjected to impact testing. Only one impact test was performed on 1 sample. The lithium ion battery is not ignited and not exploded, and is recorded as passing. 10 lithium ion batteries are tested in each case, and the weight test passing rate is used as an index for evaluating the weight impact safety of the lithium ion batteries.
Example 2
The preparation method of example 2 is the same as that of example 1 except that step (2):
(2) PVDF and SP were mixed at a weight ratio of 35% to 65%, and the mixture was dispersed in NMP and stirred by a double planetary mixer to obtain a conductive layer slurry. And coating the conductive layer slurry on the surface of the positive pole piece containing the safety layer, drying after coating, and coating the conductive layer slurry on one side to obtain the positive pole piece containing the conductive layer and the safety layer, wherein the thickness of the coating on one side is 2 microns.
Comparative example 1
The preparation method of comparative example 1 is the same as that of example 1 except that step (2) is omitted and the positive electrode sheet is coated with only two layers, i.e., the safety layer and the active material layer.
Comparative example 2
The preparation method of comparative example 2 is the same as that of example 1 except that step (1) is omitted and the positive electrode sheet is coated with only two layers, i.e., only the conductive layer and the active material layer.
Comparative example 3
The preparation method of comparative example 3 is the same as that of example 1 except that steps (1) and (2) are omitted and the positive electrode sheet has only an active material layer.
TABLE 1.1 compositions of positive electrode sheets of examples 1-2 and comparative examples 1-3
Figure BDA0002667789190000091
TABLE 1.2 test results of Performance of lithium ion batteries of examples 1-2 and comparative examples 1-3
Figure BDA0002667789190000101
As can be seen from table 1.2, the internal resistance change rate of the lithium ion battery assembled by the positive electrode plate with the three-layer structure of the invention in the cycle process is obviously reduced compared with the two-layer positive electrode plate without the conductive layer structure (comparative example 1), the cycle performance is improved, and the safety performance is greatly improved compared with the conventional structure (comparative example 3). The lithium ion battery of the invention has obvious safety improvement and improvement of cycle performance and internal resistance change rate in the cycle process.
Specifically, it can be seen from the comparison between example 1 and comparative example 1 that the capacity retention rate and the internal resistance change rate of the lithium ion battery during the cycle process are significantly improved after the conductive layer is added. From a comparison of example 1 and comparative example 2, it can be seen that the provision of the security layer provides a very significant improvement in the security properties. The comparison between example 1 and comparative example 3 shows that the three-layer structure design can further improve the safety performance of the lithium ion battery on the basis of ensuring the cycle performance.
It can be seen from examples 1-2 that, after the addition of the ion-conducting compound, the effect level of the ion-conducting compound is equivalent to that of a single conductive agent in terms of the cycle capacity retention rate and the internal resistance increase rate of change, but the ion-conducting compound is slightly better in terms of the weight impact and the pin prick test, because the introduction of the ion-conducting compound reduces the overall content of the conductive agent in the conductive layer, and the safety performance is improved to a certain extent.
Examples 3 to 8
Examples 3 to 8 were prepared in the same manner as in example 1, except that the selection of the first conductive agent, the first binder and the first positive electrode active material and the additives in step (1) were different, as specifically shown in table 2.1.
TABLE 2.1 compositions of positive electrode sheets of examples 1, 3-8 and comparative examples 2-3
Figure BDA0002667789190000111
TABLE 2.2 test results of Performance of lithium ion batteries of examples 1, 3 to 8 and comparative examples 2 to 3
Figure BDA0002667789190000112
As can be seen from table 2.2, the types of the positive active materials in the safety layer in the positive electrode sheet having the three-layer structure of the present invention are different, and there is a large difference in the safety performance of the lithium ion battery, and it is found through comparative studies that when the charging capacity of the positive active material in the safety layer in the charging voltage range of 2.5V to 4.3V occupies more than 75% of the total capacity in the charging voltage range of 2.5V to 4.3V in the charging voltage range of 2.5V to 4.85V under the charging voltage range of 2.5V to 4.3V, the lithium ion battery can have a high pass rate due to the arrangement of the safety layer.
Specifically, it can be seen from comparison between examples 3 to 8 and comparative examples 2 to 3 that the safety performance of the lithium ion battery is improved to a certain extent by the design of the safety layer, but the type of the positive active material in the safety layer has a large influence on the safety performance, and it can be seen from comparison between examples 3 to 5 and examples 6 to 8 that when the charging capacity of the positive active material in the safety layer is more than 75% of the total capacity in the charging voltage range of 2.5 to 4.3V at the charging voltage range of 2.5 to 3.85V at the charging voltage range of 2.5 to 4.3V at the charging rate of 0.2V, the safety performance is improved well, and the requirement for improving the safety performance can be satisfied.
Examples 9 to 12 and comparative examples 4 to 6
Examples 9 to 12 and comparative examples 4 to 6 were prepared in the same manner as in example 1, except that the selection of the first positive electrode active material in step (1) was different, and/or the selection of the second positive electrode active material in step (3) was different as shown in table 3.1.
TABLE 3.1 compositions of positive electrode sheets of examples 1, 9-12 and comparative examples 2, 4-6
Figure BDA0002667789190000121
TABLE 3.2 Performance test results of the lithium ion batteries of examples 1, 9 to 12 and comparative examples 2, 4 to 6
Figure BDA0002667789190000122
As can be seen from table 3.2, the positive active materials are used in the safety layer and the active material layer, but the combination of different types of positive active materials has a greater influence on the cycle performance and safety performance of the positive electrode sheet, for example, the LCO material has a higher energy density and a certain cycle performance, but the safety performance is poorer, if the LCO material is further coated on the safety layer, the safety performance is not significantly improved, because the thermal decomposition temperature (DSC) of the LCO material determines the safety performance of the LCO material; thus, examples 1, 9 to 12 and comparative examples 2, 4 to 6 examined the influence of the thermal decomposition temperature of different types of positive electrode active materials in the safety layer and the active material layer on the cycle performance and safety performance of the battery;
specifically, it can be seen from comparison of examples 9 to 12, comparative examples 4 to 6, and comparative example 2 that the safety performance of the electrode sheet is improved to some extent by the design of the safety layer, but the difference between the thermal decomposition temperature of the positive electrode active material in the safety layer and the thermal decomposition temperature of the positive electrode active material in the active material layer has a large influence on the safety performance, and it can be seen from examples 9 to 12 and comparative examples 4 to 6 that when the thermal decomposition temperature of the positive electrode active material in the safety layer is higher than the thermal decomposition temperature of the positive electrode active material in the active material layer, the safety performance is improved well, and the safety performance improvement requirement can be satisfied, that is, when the thermal decomposition temperature of the positive electrode active material in the safety layer is higher than the thermal decomposition temperature of the positive electrode active material in the active material layer, the safety performance improvement effect is optimal.
The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiment. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (12)

1. A positive pole piece comprises a positive pole current collector, a safety layer, a conductive layer and an active substance layer; the safety layer is arranged on the first surface of the positive current collector, the conductive layer is arranged on the surface of the safety layer, and the active substance layer is arranged on the surface of the conductive layer;
the safety layer includes a first positive electrode active material, and the active material layer includes a second positive electrode active material;
the first positive active material is selected from one or more of nickel cobalt lithium manganate and lithium manganate with the nickel molar content of less than 60%;
the second positive active material is selected from one or more of lithium cobaltate, nickel cobalt lithium manganate with the nickel molar content being more than or equal to 60 percent and nickel cobalt lithium aluminate with the nickel molar content being more than or equal to 75 percent;
wherein a difference T1-T2 between a thermal decomposition temperature T1 of the first positive electrode active material in the safety layer at 100% SOC and a thermal decomposition temperature T2 of the second positive electrode active material in the active material layer at 100% SOC satisfies: T1-T2>10 ℃.
2. The positive electrode sheet according to claim 1, further comprising a safety layer disposed on a second surface of the positive electrode current collector opposite to the first surface, a conductive layer disposed on a surface of the safety layer, and an active material layer disposed on a surface of the conductive layer.
3. The positive electrode sheet according to claim 1, wherein the safety layer further comprises a first conductive agent and a first binder; the active material layer further includes a third conductive agent and a third binder;
the conductive layer includes a second conductive agent and a second binder,
the first conductive agent, the second conductive agent and the third conductive agent are the same or different and are independently selected from one or more of conductive carbon black, acetylene black, carbon nano tubes, nano carbon fibers and graphene;
the first binder, the second binder and the third binder are the same or different and are independently selected from one or more of sodium carboxymethylcellulose, styrene-butadiene latex, polytetrafluoroethylene, polyvinylidene fluoride and polyethylene oxide.
4. The positive electrode sheet according to claim 3, wherein the safety layer has a thickness of 1 to 10 μm; and/or the presence of a gas in the gas,
the mass of the first positive electrode active material in the safety layer accounts for 80-92wt% of the total mass of the safety layer; and/or the presence of a gas in the gas,
the mass ratio of the first binder to the first conductive agent in the security layer is 2-6: 1.
5. The positive electrode tab of claim 3, wherein the conductive layer further comprises an ion-conducting compound.
6. The positive electrode plate according to claim 5, wherein the ion conducting compound is selected from at least one of lithium iron phosphate, lithium cobaltate, lithium nickel cobalt manganese oxide material with a molar content of nickel less than 60%, lithium manganate, lithium titanate, lithium titanium aluminum phosphate, lanthanum lithium titanate, lanthanum lithium tantalate, lithium germanium aluminum phosphate, diboron trioxide doped lithium phosphate, lithium lanthanum zirconium oxygen, lanthanum zirconium aluminum lithium oxygen, niobium doped lithium lanthanum zirconium oxygen, tantalum doped lithium lanthanum zirconium oxygen, niobium doped lithium lanthanum zirconium oxygen; and/or the presence of a gas in the gas,
the mass of the ion conducting compound in the conducting layer accounts for 30-70wt% of the total mass of the conducting layer; and/or the presence of a gas in the gas,
the thickness of the conductive layer is 1-5 μm.
7. The positive electrode sheet according to claim 3, wherein the mass ratio of the second conductive agent to the second binder in the conductive layer is 1-3: 1.
8. The positive electrode sheet according to claim 3, wherein the active material layer has a thickness of 50 to 130 μm; and/or the presence of a gas in the gas,
the mass of the second positive electrode active material in the active material layer accounts for 90-99wt% of the total mass of the active material layer.
9. The positive electrode sheet according to claim 3, wherein the mass ratio of the third conductive agent to the third binder in the active material layer is 0.5 to 2: 1.
10. The positive electrode sheet according to claim 3, wherein the particle diameter D of the second positive electrode active material in the active material layer50Is 3-18 μm.
11. The positive electrode sheet according to claim 3, wherein the ratio of the charge capacity of the first positive electrode active material in the charge voltage range of 2.5V to 4.3V to the total capacity of the first positive electrode active material in the charge voltage range of 2.5V to 4.3V is not less than 75% at a charge rate of 0.2C and in the charge voltage range of 2.5V to 3.85V.
12. A lithium ion battery comprising the positive electrode sheet of any one of claims 1-11.
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