CN111276690B - 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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CN111276690B
CN111276690B CN202010101561.6A CN202010101561A CN111276690B CN 111276690 B CN111276690 B CN 111276690B CN 202010101561 A CN202010101561 A CN 202010101561A CN 111276690 B CN111276690 B CN 111276690B
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pole piece
binder
lithium
porosity
positive pole
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CN111276690A (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
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Abstract

本发明涉及一种适用于固态电池的低孔隙率正极极片制备方法及其固态锂金属电池。所述正极极片由活性材料、导电剂、粘结剂(具备锂离子导通能力)和集流体构成，其特点在于，其中的正极活性材料为一次颗粒或大单晶，其典型尺寸为50nm～30μm；粘结剂与导电剂、集流体协同作用，保障极片内部离子、电子通道的完整性；且极片孔隙率＜20％。该极片由于自身为离子、电子提供了传输通道，因此与固态电解质间界面接触良好，适用于固态锂金属电池。

Figure 202010101561

The invention relates to a preparation method of a low-porosity positive electrode plate suitable for solid-state batteries and a solid-state lithium metal battery. The positive electrode plate is composed of an active material, a conductive agent, a binder (with lithium ion conductivity) and a current collector, and is characterized in that the positive electrode active material is a primary particle or a large single crystal, and its typical size is 50nm ~30μm; the binder, the conductive agent and the current collector act synergistically to ensure the integrity of the ion and electron channels inside the pole piece; and the pole piece porosity is less than 20%. Since the pole piece itself provides a transmission channel for ions and electrons, it has good interface contact with the solid electrolyte, and is suitable for solid lithium metal batteries.

Figure 202010101561

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 (. degree. C.), dryThe drying time is not less than 12 h.
(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 has a mass fraction of 80% of D50Lithium cobaltate with the particle size of 5 mu m and the particle size of D90 of 8 mu m is taken as an active substance, 5 percent of Super P is added, and the adhesive consists of [8 percent PAALi +0.2 percent 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 (7)

1.一种低孔隙率正极极片，其特征在于，所述正极极片由活性物质颗粒、导电剂、具有锂离子导通能力的粘结剂所构成的涂层及集流体构成，其中粘结剂在涂层中的质量占比为5％～20％；且该粘结剂特点在于：1. a low porosity positive pole piece, it is characterized in that, described positive pole piece is made up of active material particle, conductive agent, the coating that the binder that has lithium ion conduction ability is formed and current collector, wherein sticky The mass proportion of the binder in the coating is 5% to 20%; and the binder is characterized by: 1)由聚合物、锂盐、离子液体和无机锂离子导体组成；1) It is composed of polymer, lithium salt, ionic liquid and inorganic lithium ion conductor; 2)所述聚合物是聚氧化乙烯(PEO)、聚乙二醇(PEG)、聚偏氟乙烯(PVDF)、聚偏氟乙烯-六氟丙烯共聚物(PVDF-HFP)、聚丙烯腈(PAN)和聚丙烯酸锂(PAALi)中的一种或两种以上的组合，其在粘结剂中所占质量比为50％～90％；2) The polymer is polyethylene oxide (PEO), polyethylene glycol (PEG), polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyacrylonitrile ( One or more combinations of PAN) and lithium polyacrylate (PAALi), which account for 50% to 90% by mass in the binder; 3)所述锂盐为LiPF6、LiBF4、LiBOB、LiDFOB、LiTFSI和LiFSI中的一种或两种以上的组合，其在粘结剂中的质量比为1％～20％；3) The lithium salt is one or a combination of two or more selected from LiPF 6 , LiBF 4 , LiBOB, LiDFOB, LiTFSI and LiFSI, and the mass ratio in the binder is 1% to 20%; 4)所述离子液体在粘结剂中质量比为1％～50％，其阴离子为BF4 -、BOB-、DFOB-、TFSI-和FSI-中一种或两种以上的组合；4) The mass ratio of the ionic liquid in the binder is 1% to 50%, and its anion is one or a combination of two or more of BF 4 - , BOB - , DFOB - , TFSI - and FSI - ; 5)所述无机锂离子导体选自氧化物纳米颗粒、无机快锂离子导体或它们的组合，且其在粘结剂中的质量比在0.5％～10％之间；5) The inorganic lithium ion conductor is selected from oxide nanoparticles, inorganic fast lithium ion conductors or combinations thereof, and the mass ratio thereof in the binder is between 0.5% and 10%; 该极片的特点还在于，且其孔隙率低于20％，电极内含有完整的锂离子和电子通道，因而适用于锂金属电池。The pole piece is also characterized in that, and its porosity is less than 20%, the electrode contains complete lithium ion and electron channels, so it is suitable for lithium metal batteries. 2.如权利要求1所述的正极极片，其特征在于，所述活性物质为颗粒尺寸在50nm～30μm的一次颗粒或大单晶，颗粒内部由于晶体缺陷造成的孔隙率体积分数＜2％。2 . The positive electrode piece according to claim 1 , wherein the active material is a primary particle or a large single crystal with a particle size of 50 nm to 30 μm, and the volume fraction of porosity in the particle due to crystal defects is less than 2%. 3 . . 3.如权利要求1所述的正极极片，其特征在于，所述活性物质为磷酸铁锂(LFP)、钴酸锂(LCO)、镍锰酸锂(LNMO)或镍钴锰三元材料大单晶(NCM)，其占涂层的质量占比在50％～95％之间。3. The positive electrode piece according to claim 1, wherein the active material is lithium iron phosphate (LFP), lithium cobalt oxide (LCO), lithium nickel manganese oxide (LNMO) or a nickel cobalt manganese ternary material Large single crystal (NCM), which accounts for between 50% and 95% of the mass of the coating. 4.如权利要求1所述的正极极片，其特征在于，所述具有锂离子导通能力的粘结剂的室温离子电导率不低于5*10-5S/cm。4 . The positive electrode piece according to claim 1 , wherein the ionic conductivity at room temperature of the binder with lithium ion conductivity is not lower than 5*10 −5 S/cm. 5 . 5.如权利要求1所述的正极极片，其特征在于，所述的氧化物纳米颗粒为Al2O3、SiO2、ZrO2或TiO2。5 . The positive pole piece according to claim 1 , wherein the oxide nanoparticles are Al 2 O 3 , SiO 2 , ZrO 2 or TiO 2 . 6 . 6.如权利要求1所述的正极极片，其特征在于，所述的无机快锂离子导体为含有锂离子的硫化物或氧化物。6 . The positive electrode piece according to claim 1 , wherein the inorganic fast lithium ion conductor is a sulfide or oxide containing lithium ions. 7 . 7.如权利要求1所述的正极极片的制备方法，其特征在于，所述制备方法是通过将一定比例的活性物质颗粒、导电剂、粘结剂混合于乙腈、N-甲基甲酰胺、碳酸乙烯酯、碳酸丙烯酯、碳酸二甲酯、碳酸二乙酯、碳酸甲乙酯、水之中的一种或两种以上的组合后，采用传统湿法将浆料涂布于集流体，干燥、辊压来完成。7. The preparation method of the positive pole piece as claimed in claim 1, wherein the preparation method is to mix a certain proportion of active material particles, conductive agent and binder in acetonitrile, N-methylformamide , ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, water, one or more of the combination of two or more, using traditional wet method to coat the slurry on the current collector , drying and rolling to complete.
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