Negative electrode collector, negative electrode, and lithium metal secondary battery

Abstract

The invention discloses a negative electrode current collector, a negative electrode, and a lithium metal secondary battery. The negative electrode current collector contains a resin and a fibrous conductive filler, the aspect ratio of the fibrous conductive filler is 20 or more, and the content of the resin in the negative electrode current collector is 60 mass% or more but less than 90 mass%. The content of the fibrous conductive filler in the negative electrode current collector is more than 10% by mass but not more than 40% by mass.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode current collector, a negative electrode, and a lithium metal secondary battery. Background Art

[0002] Japanese Unexamined Patent Application Publication No. 2019-21384 discloses a battery including a current collector layer containing a conductive filler and a resin, an electrode layer, and a displacement absorption portion located between the current collector layer and the electrode layer and containing an elastomer made of a metal or metal fiber. Summary of the Invention

[0003] Conventionally, a metal foil has been used as an electrode current collector. For example, from the viewpoints of reducing material costs, mass energy density, etc., a reduction in the amount of metal has been required. For example, as an alternative to the metal foil, a resin current collector containing a conductive filler and a resin has been proposed.

[0004] In addition, lithium metal secondary batteries have been studied. A lithium metal secondary battery has a higher energy density than a conventional lithium ion secondary battery. The negative electrode reaction in a lithium metal secondary battery is a dissolution reaction and a precipitation reaction of lithium. During charging, lithium ions receive electrons on the surface of the negative electrode current collector, and thus lithium precipitates.

[0005] On the other hand, when a resin current collector is used as the negative electrode current collector of a lithium metal secondary battery, the conductive filler may fall off. As a result, the cycle characteristics may deteriorate.

[0006] An object of the present disclosure is to improve the cycle characteristics.

[0007] [1] A negative electrode current collector containing a resin and a fibrous conductive filler,

[0008] The aspect ratio of the fibrous conductive filler is 20 or more.

[0009] The content rate of the resin in the negative electrode current collector is 60% by mass or more and less than 90% by mass.

[0010] The content rate of the fibrous conductive filler in the negative electrode current collector exceeds 10% by mass and is 40% by mass or less.

[0011] By using a fibrous conductive filler having a specified aspect ratio, even when lithium precipitates from the negative electrode current collector during charging, the falling off of the fibrous conductive filler can be suppressed. As a result, an improvement in the cycle characteristics can be expected. In addition, by containing a resin and a fibrous conductive filler in a specified ratio, the moldability is excellent, and an increase in resistance is also expected to be suppressed.

[0012] [2][1] The negative electrode current collector according to [1], wherein the fibrous conductive filler is fibrous carbon.

[0013] [3] The negative electrode current collector according to [1] or [2], wherein the resin is a polyolefin resin.

[0014] [4] A negative electrode comprising the negative electrode current collector according to any one of [1] to [3].

[0015] [5] A lithium metal secondary battery comprising the negative electrode according to [4]. Description of the Drawings

[0016] Hereinafter, the features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described with reference to the drawings, in which the same reference numerals denote the same elements, and in which:

[0017] Figure 1 is a schematic diagram showing an example of the negative electrode current collector of the present embodiment;

[0018] Figure 2 is a schematic diagram showing an example of the negative electrode of the present embodiment;

[0019] Figure 3 is a schematic diagram showing an example of the lithium metal secondary battery of the present embodiment;

[0020] Figure 4 is a graph showing the capacity retention rate of the lithium metal secondary battery in the examples and comparative examples with respect to the number of cycles;

[0021] Figure 5 is a table showing the configurations and evaluation results of the negative electrode current collectors in the examples and comparative examples. Detailed Embodiments

[0022] Hereinafter, embodiments of the present disclosure (hereinafter may be simply referred to as "the present embodiment") and examples of the present disclosure (hereinafter may be simply referred to as "the present example") will be described. However, the present embodiment and the present example do not limit the technical scope of the present disclosure.

[0023] In the present specification, a "lithium metal secondary battery" means a battery in which the negative electrode reaction includes a dissolution and precipitation reaction of lithium metal. For example, the dissolution and precipitation reaction of lithium metal may account for 1 to 100%, 25 to 100%, 50 to 100%, or 75 to 100% of the negative electrode capacity. The negative electrode capacity represents the reversible capacity. For example, when the SOC is 1 to 100%, 1 to 75%, 1 to 50%, or 1 to 25%, lithium metal can precipitate on the negative electrode. At 0% SOC (fully discharged), lithium metal can also completely dissolve in the electrolyte. At 0% SOC, a part of lithium metal may remain on the negative electrode.

[0024] In this specification, the lithium metal secondary battery may be, for example, a liquid-based battery or a all-solid-state battery. The lithium metal secondary battery may be, for example, a monopolar battery (single-polarity battery) or a bipolar battery.

[0025] Negative electrode current collector

[0026] Figure 1 It is a schematic diagram showing an example of the negative electrode current collector of the present embodiment. The negative electrode current collector 21 includes a resin 1 and fibrous conductive fillers 2. The aspect ratio of the fibrous conductive fillers 2 is 20 or more. The content of the resin 1 in the negative electrode current collector 21 is 60% by mass or more and less than 90% by mass, and the content of the fibrous conductive fillers 2 in the negative electrode current collector 21 exceeds 10% by mass and is 40% by mass or less. The negative electrode current collector 21 is a resin current collector for a lithium metal secondary battery.

[0027] Resin

[0028] The resin 1 may include, for example, at least one selected from the group consisting of polyolefin resins, polyurethane resins, polyamide resins, cellulose resins, polyether resins, acrylic resins, and polyester resins. The resin 1 may include, for example, at least one selected from the group consisting of polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyamide (PA), polyamideimide (PAI), polyimide (PI), aromatic polyamide (aramid), nylon, liquid crystal polyester, polyacrylate, polymethacrylate, polystyrene, AS resin, ABS resin, polyphenylene ether (PPE), and silicone resin. The resin 1 is preferably PE and PP. The resin 1 may be used alone or in combination of two or more.

[0029] The content of the resin 1 in the negative electrode current collector 21 is 60% by mass or more and less than 90% by mass. When the content of the resin 1 in the negative electrode current collector 21 is within the above range, the moldability is excellent, and an increase in resistance can also be expected to be suppressed. The content of the resin 1 in the negative electrode current collector 21 is preferably 70% by mass or more and 80% by mass or less.

[0030] Fibrous conductive filler

[0031] The fibrous conductive filler 2 is made of a conductive material. Examples of the conductive material include carbon, metal, metal plating, etc. Examples of carbon include carbon black (CB), graphite, vapor-grown carbon fiber (VGCF), carbon nanotube (CNT), carbon nanofiber, carbon nanosphere, etc. Examples of metal include nickel (Ni), aluminum (Al), copper (Cu), silver (Ag), stainless steel, etc. Examples of metal plating include nickel plating, aluminum plating, copper plating, silver plating, etc. As the conductive material, carbon is preferred, and VGCF and CNT are more preferred. The fibrous conductive filler 2 can be used alone or in combination of two or more kinds.

[0032] The shape of the conductive filler is fibrous. Conventionally, the shape of the conductive filler used in the resin current collector is generally particulate. On the other hand, when lithium (Li) is precipitated during charging in the particulate conductive filler, there is a tendency to fall off from the resin due to the stress. Therefore, the inventors used a fibrous conductive filler instead of the particulate conductive filler. By using the fibrous conductive filler, even when Li is precipitated during charging, it is possible to suppress the fibrous conductive filler from falling off from the resin, and as a result, an improvement in the cycle characteristics can be expected.

[0033] The aspect ratio of the fibrous conductive filler 2 is 20 or more. When the aspect ratio of the fibrous conductive filler 2 is 20 or more, an improvement in the cycle characteristics is more expected. The aspect ratio of the fibrous conductive filler 2 can be 25 or more, or can be 30 or more. The aspect ratio of the fibrous conductive filler 2 can be 50 or less, or can be 40 or less.

[0034] The aspect ratio is the ratio of the length to the diameter. The "aspect ratio" in the present embodiment is obtained by dividing the average length of the fibrous conductive filler 2 by the average diameter of the fibrous conductive filler 2. The average length and the average diameter can be the arithmetic means of the measured values of 10 or more fibrous conductive fillers 2, respectively. The length and the diameter of each fibrous conductive filler 2 can be measured by a scanning electron microscope (SEM) or a scanning probe microscope (SPM).

[0035] The fibrous conductive filler 2 can also have an average diameter of 50 nm or more and 200 nm or less, for example. The fibrous conductive filler 2 can have an average length of 1 μm or more and 10 μm or less, for example.

[0036] The content rate of the fibrous conductive filler 2 in the negative electrode current collector 21 is more than 10% by mass and 40% by mass or less. When the content rate of the fibrous conductive filler 2 in the negative electrode current collector 21 is 10% by mass or less, the conductivity may become insufficient. When the content rate of the fibrous conductive filler 2 in the negative electrode current collector 21 exceeds 40% by mass, problems such as being unable to be formed into the negative electrode current collector or, even if it can be formed, the fibrous conductive filler 2 agglomerating to generate voids may occur. The content rate of the fibrous conductive filler 2 in the negative electrode current collector 21 is preferably 20% by mass or more and 30% by mass or less.

[0037] The negative electrode current collector 21 may be substantially composed of the resin 1 and the fibrous conductive filler 2, or may be composed of the resin 1 and the fibrous conductive filler 2. In addition, "substantially composed of the resin 1 and the fibrous conductive filler 2" means that the content rates of the resin 1 and the fibrous conductive filler 2 in the negative electrode current collector 21 are 95% by mass or more.

[0038] Other components

[0039] The negative electrode current collector 21 may also contain conductive fillers other than the fibrous conductive filler 2, dispersants, etc. As the conductive fillers other than the fibrous conductive filler 2, for example, carbon particles, metal particles, metal-coated particles, etc. can be cited. As the dispersant, for example, surfactants, etc. can be cited. The content rate of other components in the negative electrode current collector 21 is, for example, 0.1% by mass or more and 5% by mass or less.

[0040] Manufacturing method of the negative electrode current collector

[0041] The negative electrode current collector can be manufactured by the following method. However, the following manufacturing method is an example and is not limited thereto.

[0042] By mixing a resin, a fibrous conductive filler, and other components as needed, a resin composition is formed. The resin composition may also be referred to as, for example, a "composite", etc. The resin composition can be, for example, granular. The resin composition can be formed by any method. For example, melt kneading can be performed. For example, melt kneading can be carried out by a twin-screw extrusion kneader.

[0043] The resin composition is formed into a negative electrode current collector by any method. As the forming method, there is no particular limitation, and for example, known methods such as the T-die method, the blow molding method, and the calendering method can be cited.

[0044] Negative electrode

[0045] Figure 2This is a schematic diagram showing an example of the negative electrode of the present embodiment. The negative electrode 20 includes a negative electrode current collector 21 and a lithium metal layer 22. Regarding the negative electrode current collector 21, as described above.

[0046] Lithium metal layer

[0047] The lithium metal layer 22 contains lithium metal. The lithium metal layer 22 is a layer formed by depositing the lithium metal precipitated on the negative electrode 20. The thickness of the lithium metal layer changes with the increase or decrease of the SOC.

[0048] Lithium metal secondary battery

[0049] Figure 3 This is a schematic diagram showing an example of the lithium metal secondary battery (hereinafter, also simply referred to as "battery") of the present embodiment. Hereinafter, as an example, a liquid-based single-pole battery will be described, but it is not limited thereto.

[0050] The battery 100 may include an exterior body (not shown). The exterior body may also house a power generation element 50 and an electrolytic solution (not shown). The exterior body may have any form. For example, the exterior body may be a metal case, or may be a bag made of a metal foil laminated film, etc. The exterior body may contain, for example, Al or the like.

[0051] The battery 100 includes a power generation element 50. The power generation element 50 may also be referred to as an electrode body or an electrode group. The power generation element 50 includes a positive electrode 10, a separator 30, and a negative electrode 20. The power generation element 50 has an arbitrary structure. For example, the power generation element 50 may be a wound type. The positive electrode 10, the separator 30, and the negative electrode 20 may all be strip-shaped sheets. For example, the power generation element 50 may also be formed by laminating the positive electrode 10, the separator 30 (first sheet), the negative electrode 20, and the separator 30 (second sheet) in this order. After winding, the power generation element 50 may be formed into a flat shape.

[0052] Positive electrode

[0053] The positive electrode 10 may include a positive electrode current collector 11 and a positive electrode active material layer 12. The positive electrode current collector 11 may also contain, for example, aluminum (Al) foil or the like. The positive electrode active material layer 12 contains a positive electrode active material. The positive electrode active material layer 12 may also contain, for example, a conductive material, an adhesive, etc.

[0054] The positive electrode active material may be, for example, particulate. The positive electrode active material may have a D50 of 1 to 30 μm, for example. The positive electrode active material may contain at least one selected from the group consisting of LiCoO2, LiNiO2, LiMnO2, Li(NiCoMn)O2, and Li(NiCoAl)O2. For example, in "Li(NiCoMn)O2", "(NiCoMn)" indicates that the total of the composition ratios in the parentheses is 1. As long as the total is 1, the respective component amounts are arbitrary.

[0055] The conductive material may contain, for example, acetylene black (AB), etc. The binder may include, for example, PVdF, etc. The conductive material and the binder may be 0.1% by mass or more and 10% by mass or less with respect to the positive electrode active material layer 12, for example.

[0056] Separator

[0057] The separator 30 is porous. The separator 30 can permeate the electrolyte. The separator 30 separates the positive electrode 10 and the negative electrode 20. The separator 30 has electrical insulation. The separator 30 may also contain, for example, polyolefin resins such as polyethylene (PE), polypropylene (PP), etc. The separator 30 may have a single-layer structure, or may have a multi-layer structure, for example. The separator 30 may be substantially composed of a PE layer, or may be formed by laminating a PP layer, a PE layer, and a PP layer in sequence.

[0058] Electrolyte

[0059] The electrolyte contains a solvent and a Li salt. The solvent is aprotic. The solvent may include optional components. The solvent may contain, for example, at least one selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0060] The Li salt is a supporting electrolyte. The Li salt is dissolved in the solvent. The Li salt may contain, for example, at least one selected from the group consisting of LiPF6 and LiBFz. The Li salt may have a molar concentration of 0.5 mol / L or more and 2.0 mol / L or less, for example.

[0061] The electrolyte may further contain optional additives. The electrolyte may contain, for example, 0.01% by mass or more and 5% by mass or less of additives. The additives may contain, for example, at least one selected from the group consisting of vinylene carbonate (VC) and vinylethylene carbonate (VEC), etc.

[0062] No.1

[0063] As the material for the negative electrode current collector, a Cu foil (thickness: 10 μm) was prepared. This Cu foil was used as the negative electrode.

[0064] As the material for the positive electrode, layered-structured LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (average particle size: 10 μm) (90% by mass), AB (8% by mass) as the conductive material, and PVdF (2% by mass) as the binder were prepared. These materials were mixed in the solvent N-methyl-2-pyrrolidone (NMP) so that the solid content became 56% by mass, and mixed using a planetary mixer to obtain a positive electrode slurry.

[0065] The obtained positive electrode paste was coated on the surface of an Al foil serving as a positive electrode current collector using a metallic mold coater. After coating, the positive electrode paste was dried at 120 °C and compressed by a roll press to obtain a positive electrode.

[0066] As the separator, a porous resin (PP / PE / PP) having PP layers laminated on both sides of a PE layer was prepared. A power generation element was formed by laminating the positive electrode, the separator, and the negative electrode in this order.

[0067] As the exterior body, a bag made of a laminated film was prepared. The power generation element was housed in the exterior body. As the electrolytic solution, an electrolytic solution prepared by dissolving a supporting salt (LiPF6) at a concentration of 1.0 mol / L in a mixed solvent containing EC and DMC at a volume ratio of 1:1 was prepared. The electrolytic solution was injected into the exterior body. After injecting the electrolytic solution, the exterior body was sealed. Through the above, a test battery was assembled.

[0068] In a temperature environment of 25 °C, the test battery was charged at a constant current of 0.4 mA / cm 2 until the positive electrode potential reached 4.3 V. Subsequently, the test battery was discharged at a constant current of 0.4 mA / cm 2 until the positive electrode potential reached 3.0 V. As a result, a lithium metal layer was formed on the surface of the negative electrode current collector. Through the above, the test battery No. 1 was manufactured. In addition, the test battery No. 1 is a reference battery using the same metal foil (Cu foil) as in the past as the negative electrode current collector.

[0069] No. 2

[0070] As the material of the negative electrode current collector, PE (manufactured by Japan Polyethylene Corporation, Novactec HDHF560) as a resin and CB composed of aggregates of particulate carbon as a conductive filler were prepared.

[0071] PE and CB were weighed so that the mass ratio became 80:20, and a composite was formed by melt-kneading using a biaxial extrusion kneader. The composite was extruded and molded by the T-die method to obtain the negative electrode current collector No. 2 (thickness: 50 μm).

[0072] Except for using the negative electrode current collector No. 2 as the negative electrode, the test battery No. 2 was manufactured using the same materials and methods as those of No. 1.

[0073] No. 3

[0074] As the material for the negative electrode current collector, prepare the same PE as No. 2 as the resin and particulate Ni as the conductive filler. Weigh PE and Ni in a mass ratio of 95:5 and melt-knead them using a twin-screw extruder to form a composite. Extrusion-mold this composite by the T-die method to obtain the negative electrode current collector of No. 3 (thickness: 50 μm).

[0075] Manufacture the test cell of No. 3 using the same materials and method as No. 1, except that the negative electrode current collector of No. 3 is used as the negative electrode.

[0076] No. 4 to No. 6

[0077] As the material for the negative electrode current collector, prepare the same PE as No. 2 as the resin and VGCF (manufactured by Resonac Corporation, VGCF-H) (aspect ratio: 26.7) as the fibrous conductive filler. Weigh PE and VGCF in a specified mass ratio and melt-knead them using a twin-screw extruder to form a composite. Extrusion-mold this composite by the T-die method to obtain the negative electrode current collectors of No. 4 to No. 6 (thickness: 50 μm). Additionally, in No. 4, weigh PE and VGCF in a mass ratio of 90:10. In No. 5, weigh PE and VGCF in a mass ratio of 80:20. In No. 6, weigh PE and VGCF in a mass ratio of 70:30.

[0078] Manufacture the test cells of No. 4 to No. 6 using the same materials and method as No. 1, except that the negative electrode current collectors of No. 4 to No. 6 are used as the negative electrodes.

[0079] Evaluation

[0080] At a temperature of 25 °C, charge each No. test cell at a constant current of 1.0 mA / cm 2 until the positive electrode potential reaches 4.3 V. Subsequently, discharge the test cell at a constant current of 1.0 mA / cm 2 until the positive electrode potential reaches 3.0 V. Take this cycle as 1 cycle and perform a cycle test of 20 cycles. Calculate the discharge capacity retention rate by dividing the discharge capacity of each cycle by the discharge capacity of the first cycle. The results are shown in Figure 4 . Figure 5 The numerical values of the discharge capacity retention rate at the 20th cycle are shown. Additionally, for No. 4, since it cannot be charged and discharged and cannot function as a battery, the discharge capacity retention rate cannot be calculated.

[0081] Results

[0082] As Figure 4 and 5 shown, it can be seen that the capacity retention rates of No. 5 and No. 6 are higher than those of No. 2 and No. 3. In addition, it can be seen that the capacity retention rates of No. 5 and No. 6 are the same as that of No. 1 which is used as a reference battery.

[0083] This embodiment and this example are illustrative in all respects. This embodiment and this example are not limited. The technical scope of the present disclosure includes the meanings equivalent to the descriptions in the claims and all changes within the scope. For example, extracting any structures from this embodiment and this example and combining them arbitrarily are also what was originally intended.

Claims

1. A negative electrode current collector comprising a resin and fibrous conductive fillers, wherein the aspect ratio of the fibrous conductive fillers is 20 or more, the content of the resin in the negative electrode current collector is 60% by mass or more and less than 90% by mass, and the content of the fibrous conductive fillers in the negative electrode current collector exceeds 10% by mass and is 40% by mass or less.

2. The negative electrode current collector according to claim 1, wherein the fibrous conductive fillers are fibrous carbon.

3. The negative electrode current collector according to claim 1, wherein the resin is a polyolefin resin.

4. A negative electrode, comprising the negative electrode current collector according to any one of claims 1 to 3.

5. A lithium metal secondary battery, including the negative electrode according to claim 4.

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