CN111276690A - Low-porosity positive pole piece, preparation method thereof and application of positive pole piece in solid-state lithium metal battery - Google Patents

Low-porosity positive pole piece, preparation method thereof and application of positive pole piece in solid-state lithium metal battery Download PDF

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CN111276690A
CN111276690A CN202010101561.6A CN202010101561A CN111276690A CN 111276690 A CN111276690 A CN 111276690A CN 202010101561 A CN202010101561 A CN 202010101561A CN 111276690 A CN111276690 A CN 111276690A
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lithium
pole piece
positive pole
binder
porosity
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CN111276690B (en
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张兰
潘科成
张琪鹏
巫湘坤
张锁江
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Institute of Process Engineering of CAS
Langfang Institute of Process Engineering of CAS
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Institute of Process Engineering of CAS
Langfang Institute of Process Engineering of CAS
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    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
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    • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
    • H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
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    • H01M2004/021—Physical characteristics, e.g. porosity, surface area
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Abstract

The invention relates to a preparation method of a low-porosity positive pole piece suitable for a solid-state battery and a solid-state lithium metal battery thereof. The positive pole piece is composed of an active material, a conductive agent, a binder (with lithium ion conduction capacity) and a current collector, and is characterized in that the positive active material is primary particles or large single crystals, and the typical size of the positive active material is 50 nm-30 mu m; the adhesive, the conductive agent and the current collector act synergistically to ensure the integrity of ions and electronic channels in the pole piece; and the porosity of the pole piece is less than 20%. The pole piece provides a transmission channel for ions and electrons, so that the pole piece is in good interface contact with a solid electrolyte and is suitable for a solid lithium metal battery.

Description

Low-porosity positive pole piece, preparation method thereof and application of positive pole piece in solid-state lithium metal battery
Technical Field
The invention relates to the technical field of battery pole pieces and lithium metal battery manufacturing, in particular to a low-porosity positive pole piece, a preparation method thereof and application thereof in a solid lithium metal battery.
Background
With the development of economic society, lithium ion batteries have become one of necessities of daily life, and the energy density, safety and cost of the lithium ion batteries also gradually become the most concerned hot problems of the whole society.
Most of the existing lithium ion batteries adopt a transition metal oxide anode, a graphite cathode and liquid electrolyte. On one hand, the specific capacity of the graphite is lower, so that the energy density of the battery can hardly break through the upper limit of 300 Wh/kg; on the other hand, the electrolyte adopts an organic solvent with a lower flash point, so that the leakage and combustion risks exist at high temperature, and the safety of the battery is not guaranteed.
Solid-state lithium metal batteries are the most effective way to solve the above problems, and in recent years, solid-state electrolyte-related studies have yielded good results, and the positive electrode/electrolyte interface has gradually become a bottleneck that limits the practical development of lithium metal batteries. How to solve the problem of compatibility between the positive electrode and the solid electrolyte and reduce the interfacial resistance has become a research hotspot in the academic and industrial fields. For example, CN201710357012.3 discloses a method for in-situ preparing a plastic crystal modified solid-state battery positive electrode, which comprises mixing plastic crystal, lithium salt, polymer, inorganic filler and organic solvent, heating or irradiating for in-situ polymerization, mixing the obtained plastic crystal/polymer composite gel with a positive electrode material and a conductive agent, coating, and drying to obtain a positive electrode with lower interface impedance. CN201810231238.3 and CN201710462747.2 adopt a method of heating the battery to a temperature higher than the melting point of the electrolyte to promote the penetration of the electrolyte into the positive electrode. In the method disclosed in CN201880008286.4, they incorporate electrolyte (sulfide) particles with smaller particle size into the electrode sheet, and by applying pressure, promote the fusion of the electrolyte and the interface of the positive electrode material, and at the same time, they coat the positive electrode material in order to reduce the oxidation of the electrolyte by the positive electrode material.
However, without any exception, the above invention does not make an intensive study on the porosity of the pole piece, especially the positive pole piece, and in CN201880009551.0, the inventors made a relatively large study on the porosity of the pole piece with respect to the volume change of the Si negative pole during the cycle. For the positive electrode, the problem of interfacial resistance with the electrolyte is solved more, especially in practical applications, due to the high surface capacity (typically, 2 mAh/cm)2In the above), the transportation of lithium ions and electrons in the positive electrode becomes a determining factor of the performance of the solid-state battery, and aiming at the problem, the invention provides a composition and a preparation method of a low-porosity positive electrode piece and a solution of a lithium metal battery.
Disclosure of Invention
In view of the problems in the prior art, the invention provides a composition and a preparation method of a low-porosity positive pole piece and a solution of a lithium metal battery thereof, and is expected to increase the electrode surface density and reduce the electrolyte ratio, thereby improving the energy density of the battery and ensuring the battery performance.
In a first aspect, the invention provides a positive pole piece suitable for a solid-state lithium metal battery and a preparation method thereof, wherein the positive pole piece is formed by coating a coating consisting of an active substance, a conductive agent and a binder with a lithium ion transmission function on a current collector, and is characterized in that the porosity of the positive pole piece is lower than 20%.
Specifically, the active substance is primary particles or large single crystals of lithium iron phosphate (LFP), Lithium Cobaltate (LCO), Lithium Nickel Manganese Oxide (LNMO) or ternary material (NCM) with the particle size of 50 nm-30 μm, the porosity caused by crystal defects in the particles is less than 2% (vol, volume fraction), and the mass percentage of the active substance in the coating is 50-95%.
Preferably, the mass ratio of the components is between 60 and 90 percent.
The conductive agent is a combination of at least 2 of conductive graphite, conductive carbon black, carbon nano tubes, carbon nano fibers, reduced graphene oxide and graphene, and the mass of the conductive agent accounts for 2-20% of the coating.
Preferably, the conductive agent comprises at least one of carbon nanotubes or carbon nanofibers in combination with at least 1 of conductive graphite, conductive carbon black, reduced graphene oxide, and graphene.
In a second aspect, the invention provides a binder with a lithium ion transmission function and a preparation method thereof, wherein the binder accounts for 5-20% of a coating, and consists of a polymer, a lithium salt, an ionic liquid and an inorganic lithium ion conductor; the polymer is selected from one or the combination of more than two of polyethylene oxide (PEO), polyethylene glycol (PEG), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), Polyacrylonitrile (PAN) and lithium Polyacrylate (PAALi), and the proportion of the polymer in the binder is 50-90% (wt); the lithium salt is LiPF6、LiBF4One or the combination of more than two of LiBOB, LiDFOB, LiTFSI and LiFSI, wherein the proportion of the LiBOB, the LiDFOB, the LiTFSI and the LiFSI in the binder is 1-20% (wt); the ionic liquid has the functions of improving the stability of an electrode material/binder interface and promoting the dispersion of a positive coating, and is characterized in that the ionic liquid accounts for 1-50% of the binder, and the anion of the ionic liquid is BF4 -、BOB-、DFOB-、 TFSI-And FSI-One or a combination of two or more of them; the inorganic lithium ion conductor is selected from oxide nanoparticles, such as Al2O3、SiO2、ZrO2、TiO2Or other inorganic fast lithium ion conductors, such as Lithium Phosphorus Sulfide (LPS), Lithium Germanium Phosphorus Sulfide (LGPS), lithium tin phosphorus sulfide (LSnPS), Lithium Lanthanum Zirconium Oxygen (LLZO), Lithium Lanthanum Zirconium Tantalum Oxygen (LLZTO) and lithium aluminum titanium phosphorus oxygen (LATP), wherein the inorganic lithium ion conductors account for 0.5-10% of the binder by mass.
Preferably, the lithium salt is LiPF6、LiBF4Any one or a combination of two or more of LiTFSI, LiFSI, LiBOB and lidpob, wherein typical but non-limiting combinations are: a combination of LiTFSI and LiFSI, a combination of LiTFSI and LiBOB, a combination of LiTFSI and LiDFOB, a combination of LiTFSI and LiBF4A combination of LiTFSI, LiFSI and LiBF4Combinations of (a), (b), and the like.
Preferably, the ionic liquid cation is of a saturated structure, i.e., free of double bonds, triple bonds, benzene rings, and the like. Typical ionic liquids include, but are not limited to: PP13TFSI, PYR14TFSI, PYR12BF4PP13DFOB, Li (G3) FSI, Li (G4) TFSI, and the like.
Preferably, the inorganic lithium ion conductor is a combination of a nanostructured oxide, which is closer to the positive electrode material due to surface tension, and a fast lithium ion conductor, which is dispersed in a polymer binder, typically but not limited to the combination: al (Al)2O3With LGPS, SiO2With LLZO, Al2O3With LLZO, SiO2LGPS and the like.
Preferably, the binder has an ionic conductivity of not less than 5 x 10 at room temperature-5S/cm, where the conductivity can be measured by AC impedance method after coating the binder, preparing a stainless steel symmetrical cell.
In a third aspect, the invention provides a method for preparing the positive electrode plate of the first aspect. When the adhesive contains sulfide, the active substance, the conductive agent and the adhesive can be uniformly dispersed, pressed on a current collector by a hot pressing method and then rolled to obtain the conductive paste; when the binder does not contain sulfide, the above substances can be dispersed in a good solvent of the binder, such as acetonitrile, N-methylformamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, water and the like, and then the slurry is coated on a current collector by adopting a traditional wet method, and the preparation is finished by drying and rolling.
Preferably, the single-side capacity of the pole piece is 1mAh/cm2Above, and the porosity is less than or equal to 20 percent.
In a fourth aspect, the invention also provides a preparation method of a lithium metal battery using the pole piece, which comprises the following steps:
(1) further vacuum drying the positive pole piece, and when the melting point of the main polymer of the adhesive is T1At (. degree.C.) drying temperature T2In the range of (. degree. C.) should have T2＜T1-10 deg.C, and drying for no less than 12 hr.
(2) The counter electrode adopts metal lithium or lithium alloy, and the current collector adopts surface roughening or porous copper foil.
(3) The capacity ratio (N/P ratio) of the negative electrode to the positive electrode of the battery is not less than 1.2.
(4) And assembling and packaging the battery by adopting a lamination or winding process in an environment with a dew point of less than-60 ℃.
(5) After the encapsulation is completed, the cell is left at temperature T3(T3≤T2) The electrolyte and the positive electrode are promoted to be fully fused in the environment of not less than 12 hours, and the preparation of the lithium metal battery is completed.
Compared with the prior art, the invention has at least the following beneficial effects:
the invention provides a low-porosity positive pole piece suitable for a lithium metal battery and a preparation method thereof, and on one hand, the electrode material is primary particles or large single crystals, so that the transmission resistance of lithium ions and electrons in active substance particles is small; on the other hand, the binder can provide a channel for providing lithium ions, and the lithium ions and the conductive agent form a complete ion and electron transport channel in the electrode; the lower porosity of the electrode can further guarantee the transport of ionic electrons, so that the internal impedance of the electrode is lower. Meanwhile, the adhesive exposed on the surface of the electrode can reduce the interface impedance between the electrode and the solid electrolyte, so that the overall internal resistance of the lithium metal battery is ensured to be lower, the utilization rate of active substances is ensured to be higher, and the energy density of the battery is improved.
Drawings
Fig. 1 is a schematic structural diagram of a low-porosity pole piece provided by the present invention.
Detailed Description
The technical scheme of the invention is further explained by the specific implementation mode in combination with the attached drawings. The following examples are merely illustrative of the present invention and do not represent or limit the scope of the claims, which are defined by the claims.
Example 1
The electrode takes 80% of lithium cobaltate with D50 of 5 mu m and D90 of 8 mu m as active substances, 5% of Super P is added, and the binder is composed of [ 8% PAALi + 0.2% of nano Al2O3+ 1% LGPS + 1% LiFSI and 4.8% ionic liquid]The ionic liquid has the following structure:
Figure BDA0002387026100000051
the preparation method comprises the following steps:
1) the adhesives in the proportion are uniformly mixed in the environment with the dew point lower than minus 60 ℃ to obtain the viscous composite adhesive with the room-temperature conductivity more than 10-4S/cm；
2) Uniformly mixing lithium cobaltate and Super P by using a dry powder mixer;
3) dry-mixing the binder and the mixture obtained in the step 2), heating and uniformly mixing the double screws, and extruding the mixture onto the surface of the aluminum foil to finish double-side coating;
4) rolling the obtained pole piece to obtain the pole piece with the porosity of about 15 percent and the single-side design surface capacity of 1.5mAh/cm2。
5) A button cell is formed by a LLZO solid electrolyte sheet with the thickness of 0.5mm, the anode and the lithium sheet, and the charge and discharge performance of the button cell is tested at the temperature of 60 ℃ and 0.1 ℃.
Example 2
The active material adopts 150nm lithium iron phosphate D50, 3% Keqin black is added as a conductive agent, PEO with the number average molecular weight of 20 ten thousand is adopted as a main polymer body of the binder, Li (G3) TFSI with the mass fraction of 6.8% is adopted as ionic liquid, and the other proportions are the same as those of the active material in example 1: conductive agent: binder 80:3: 17.
The preparation steps of the pole piece are the same as those of the example 1, but the current collector adopts the nickel foam, and the pole piece is prepared by one-time coating. The capacity of the rolled back surface of the pole piece is 3.2mAh/cm2(two-sided), porosity 19%.
A button cell is formed by a LLZO solid electrolyte sheet with the thickness of 0.5mm, the anode and the lithium sheet, and the charge and discharge performance of the button cell is tested at 50 ℃ and 0.1 ℃.
Example 3
The anode adopts NCM622 single crystal with D50 of 3 μm, 2% Ketjen black and 3% carbon nano tube as conductive agents, and the binder is 8% PVDF-HFP and 0.5% LiBF4+0.5％LiBOB+0.2％LiPF6+4％PP13TFSI+0.3％SiO2+ 1.5% LLZO composition, i.e. active substance: conductive agent: binder 80:5: 15.
In the preparation process, the components of the binder are added into NMP according to the proportion, stirred at room temperature for 5h, added with active substances and conductive agents which are uniformly mixed in dry powder, the solid content of the system is controlled to be 50-70%, continuously stirred for 6h, coated on an aluminum foil with roughened surface in the environment that the dew point is lower than minus 40 ℃, coated on two sides, dried and hot-pressed at 80 ℃ to obtain the aluminum foil with 18.7 percent of porosity and 2.2mAh/cm of single-side capacity2The pole piece of (2).
A button cell is formed by a LLZO solid electrolyte sheet with the thickness of 0.5mm, the anode and the lithium sheet, and the charge and discharge performance of the button cell is tested at the temperature of 60 ℃ and 0.1 ℃.
Example 4
The same as example 3, but using LATP as inorganic fast lithium ion conductor and PC as solvent for coating. The obtained pole piece has the porosity of 13.2 percent and the single-side capacity of 2.3mAh/cm2。
A button cell is formed by a LLZO solid electrolyte sheet with the thickness of 0.5mm, the anode and the lithium sheet, and the charge and discharge performance of the button cell is tested at the temperature of 60 ℃ and 0.1 ℃.
Comparative example 1
Lithium cobaltate: super P: PVDF 92:3:5(wt), after being coated on a common aluminum foil and rolled by a traditional method, the porosity of a pole piece is 30 percent, and the single-side design surface capacity is 1.5mAh/cm2。
Comparative example 2
Lithium iron phosphate: super P: PVDF 92:3:5(wt), after being coated on a common aluminum foil and rolled by a traditional method, the porosity of a pole piece is 34 percent, and the single-side design surface capacity is 2.3mAh/cm2。
Comparative example 3
NCM622 single crystal particle: super P: PVDF (polyvinylidene fluoride) 92:3:5(wt), the porosity of a pole piece is 32 percent after being coated on a common aluminum foil and rolled by a traditional method, and the capacity of a single-side design surface is 2.5mAh/cm2。
The charge and discharge performance of the button cell is tested by adopting a LLZO solid electrolyte sheet with the thickness of 0.5mm, and forming the button cell with the positive electrode and the lithium sheet at the temperature of 60 ℃ and the temperature of 0.1 ℃.
Effects of the implementation
The first-effect, first-release and 10-week discharge capacities of the examples and comparative examples are shown in table 1.
TABLE 1
Figure BDA0002387026100000081
As can be seen from table 1, the low porosity pole piece according to the present invention effectively improves the cycling stability of the lithium metal battery.
The applicant states that the present invention is illustrated by the above examples to show the detailed process equipment and process flow of the present invention, but the present invention is not limited to the above detailed process equipment and process flow, i.e. it does not mean that the present invention must rely on the above detailed process equipment and process flow to be implemented. It should be understood by those skilled in the art that any modification of the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific modes, etc., are within the scope and disclosure of the present invention.

Claims (9)

1. The positive pole piece with low porosity is characterized in that the positive pole piece is composed of a coating layer and a current collector, wherein the coating layer is composed of active material particles, a conductive agent and a bonding agent with lithium ion conduction capability, the porosity of the positive pole piece is lower than 20%, and the electrode contains complete lithium ions and electron channels, so that the positive pole piece is suitable for a lithium metal battery.
2. The positive electrode sheet according to claim 1, wherein the active material is a primary particle or a large single crystal having a particle size of 50nm to 30 μm, and the volume fraction of porosity due to crystal defects inside the particle is < 2%.
3. The positive electrode sheet according to claim 1, wherein the active material is lithium iron phosphate (LFP), Lithium Cobaltate (LCO), Lithium Nickel Manganese Oxide (LNMO) or ternary material large single crystal (NCM), and the mass ratio of the active material to the coating is 50-95%.
Preferably, the mass ratio of the components is between 60 and 90 percent.
4. The positive electrode sheet according to claim 1, wherein the conductive agent is a combination of at least 2 of conductive graphite, conductive carbon black, carbon nanotubes, carbon nanofibers, reduced graphene oxide and graphene, and the mass ratio of the conductive agent in the coating is 2-20%.
Preferably, the conductive agent comprises at least one of carbon nanotubes or carbon nanofibers in combination with at least 1 of conductive graphite, conductive carbon black, reduced graphene oxide, and graphene.
5. The positive electrode sheet according to claim 1, wherein the mass ratio of the binder having lithium ion conductivity in the coating is 5-20%; the binder consists of a polymer, lithium salt, ionic liquid and an inorganic lithium ion conductor; the polymer is polyethylene oxide (PEO), polyethylene glycol (PEG), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), Polyacrylonitrile (PAN) andone or more than two of lithium Polyacrylate (PAALi) are combined, and the mass ratio of the lithium polyacrylate to the binder is 50-90%; the lithium salt is LiPF6、LiBF4One or the combination of more than two of LiBOB, LiDFOB, LiTFSI and LiFSI, wherein the mass ratio of the LiBOB, the LiDFOB, the LiTFSI and the LiFSI in the binder is 1-20%; the mass ratio of the ionic liquid in the adhesive is 1-50%, and the anion of the ionic liquid is BF4 -、BOB-、DFOB-、TFSI-And FSI-One or a combination of two or more of them; the inorganic lithium ion conductor is selected from oxide nanoparticles, preferably Al2O3、SiO2、ZrO2、TiO2Or other inorganic fast lithium ion conductors, preferably one or a combination of more than two of sulfide Lithium Phosphorus Sulfur (LPS), Lithium Germanium Phosphorus Sulfur (LGPS), lithium tin phosphorus sulfur (LSnPS), oxide Lithium Lanthanum Zirconium Oxygen (LLZO), Lithium Lanthanum Zirconium Tantalum Oxygen (LLZTO) and lithium aluminum titanium phosphorus oxygen (LATP), wherein the inorganic lithium ion conductors account for 0.5-10% of the mass ratio of the binder.
Preferably, the binder has an ionic conductivity of not less than 5 x 10 at room temperature-5S/cm。
6. The method for preparing the positive electrode plate according to claim 1, wherein the preparation process is performed by mixing active material particles, a conductive agent and a binder, dry pressing or spraying the mixture on a current collector, and performing roll pressing.
7. The method for preparing the positive electrode plate according to claim 1, wherein the preparation process is completed by mixing a certain proportion of active material particles, a conductive agent and a binder in one or more of solvents such as acetonitrile, N-methylformamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and water, coating the slurry on a current collector by a conventional wet method, drying and rolling.
8. The use of the positive electrode sheet of claim 1 in a solid state lithium metal battery.
9. A method of making a solid state lithium metal battery comprising the positive electrode tab of claim 1, comprising the steps of:
(1) preparing the positive pole piece by adopting a dry method or a wet method, wherein the porosity of the positive pole piece is lower than 20% after rolling;
(2) preparing the positive pole piece obtained in the step (1), a solid electrolyte and a metal lithium (alloy) negative pole into a battery cell in a winding or laminating manner;
(3) welding a tab, putting the battery cell into a shell, sealing the shell, standing the battery cell at room temperature for 24 hours, and then forming the battery cell;
(4) and after the formation is finished, the lithium metal battery adopting the low-porosity positive pole piece is obtained.
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