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

Abstract

To improve cycle characteristics.SOLUTION: In a negative electrode collector including resin and a fiber-like conductive filler, the aspect ratio of the fiber-like conductive filler is at least 20, the content of the resin in the negative electrode collector is at least 60 mass% and less than 90 mass%, and the content of the fiber-like conductive filler in the negative electrode collector is larger than 10 mass% and is not larger than 40 mass%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Japanese Patent Application Laid-Open No. 2019-21384 (Patent Document 1) discloses a battery including a collector layer containing a conductive filler and a resin, an electrode layer, and a displacement absorbing section located between the collector layer and the electrode layer and including an elastic body made of metal or metal fibers. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-21384 Summary of the Invention [Problem to be solved by the invention]

[0004] Metal foils have traditionally been used as electrode current collectors. For example, there is a demand for a reduction in the amount of metal, from the viewpoints of reducing material costs, mass energy density, etc. For example, resin current collectors containing conductive fillers and resins have been proposed as an alternative to metal foils.

[0005] Lithium metal secondary batteries are also being considered. Lithium metal secondary batteries have a higher energy density than conventional lithium-ion secondary batteries. The negative electrode reactions in lithium metal secondary batteries are the dissolution and precipitation reactions of lithium. During charging, lithium ions accept electrons on the surface of the negative electrode current collector, causing lithium to precipitate.

[0006] 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, which may result in a deterioration in cycle characteristics.

[0007] An object of the present disclosure is to improve cycle characteristics. [Means for solving the problem]

[0008] [1] A negative electrode current collector comprising a resin and a fibrous conductive filler, The aspect ratio of the fibrous conductive filler 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, A negative electrode current collector, wherein the content of the fibrous conductive filler in the negative electrode current collector is more than 10% by mass and 40% by mass or less.

[0009] The use of a fibrous conductive filler having a predetermined aspect ratio prevents the fibrous conductive filler from falling off even when lithium precipitates from the negative electrode current collector during charging, which is expected to improve cycle characteristics. Furthermore, by including a resin and a fibrous conductive filler in a predetermined ratio, excellent moldability and suppression of an increase in resistance are also expected.

[0010] [2] The negative electrode current collector according to [1], wherein the fibrous conductive filler is fibrous carbon.

[0011] [3] The negative electrode current collector according to [1] or [2], wherein the resin is a polyolefin resin.

[0012] [4] A negative electrode comprising the negative electrode current collector according to any one of [1] to [3].

[0013] [5] A lithium metal secondary battery comprising the negative electrode according to [4]. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram showing an example of the negative electrode current collector of this embodiment. [Figure 2] FIG. 2 is a schematic diagram showing an example of the negative electrode of this embodiment. [Figure 3]FIG. 3 is a schematic diagram showing an example of the lithium metal secondary battery of this embodiment. [Figure 4] FIG. 4 is a graph showing the capacity retention rate versus the number of cycles of the lithium metal secondary batteries in the examples and comparative examples. [Figure 5] FIG. 5 is a table showing the configurations of the negative electrode current collectors in the examples and comparative examples, and the evaluation results. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present disclosure (hereinafter may be abbreviated as "the present embodiment") and an example of the present disclosure (hereinafter may be abbreviated as "the present example") will be described. However, the present embodiment and the example do not limit the technical scope of the present disclosure.

[0016] In this specification, the term "lithium metal secondary battery" refers to 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 indicates a reversible capacity. For example, lithium metal may be precipitated on the negative electrode at an SOC of 1 to 100%, 1 to 75%, 1 to 50%, or 1 to 25%. At an SOC of 0% (fully discharged), all of the lithium metal may be dissolved in the electrolyte. At an SOC of 0%, some of the lithium metal may remain in the negative electrode.

[0017] In this specification, the lithium metal secondary battery may be, for example, a liquid battery or an all-solid-state battery. The lithium metal secondary battery may be, for example, a monopolar battery (unipolar battery) or a bipolar battery.

[0018] <Negative electrode current collector> 1 is a schematic diagram showing an example of a negative electrode current collector of this embodiment. The negative electrode current collector 21 contains a resin 1 and a fibrous conductive filler 2. The aspect ratio of the fibrous conductive filler 2 is 20 or more. The content of the resin 1 in the negative electrode current collector 21 is 60 mass% or more and less than 90 mass%, and the content of the fibrous conductive filler 2 in the negative electrode current collector 21 is more than 10 mass% and 40 mass% or less. The negative electrode current collector 21 is a resin current collector for a lithium metal secondary battery.

[0019] "resin" Resin 1 may include at least one selected from the group consisting of, for example, polyolefin resins, urethane resins, polyamide resins, cellulose resins, polyether resins, acrylic resins, and polyester resins. Resin 1 may include at least one selected from the group consisting of, for example, 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, polyacrylic acid ester, polymethacrylic acid ester, polystyrene, AS resin, ABS resin, polyphenylene ether (PPE), and silicone resin. Resin 1 is preferably PE or PP. Resin 1 may be used singly or in combination of two or more.

[0020] The content of 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 resin 1 in the negative electrode current collector 21 is within the above range, excellent moldability and suppression of an increase in resistance are expected. The content of resin 1 in the negative electrode current collector 21 is preferably 70% by mass or more and 80% by mass or less.

[0021] <<Fiber-like conductive filler>> The fibrous conductive filler 2 is made of a conductive material. Examples of the conductive material include carbon, metal, and metal plating. Examples of carbon include carbon black (CB), graphite, vapor grown carbon fiber (VGCF), carbon nanotubes (CNT), carbon nanofibers, and carbon nanospheres. Examples of metal include nickel (Ni), aluminum (Al), copper (Cu), silver (Ag), and stainless steel. Examples of metal plating include nickel plating, aluminum plating, copper plating, and silver plating. Carbon is preferred as the conductive material, and VGCF and CNT are more preferred. The fibrous conductive filler 2 may be used alone or in combination of two or more types.

[0022] The conductive filler has a fibrous shape. Conventionally, conductive fillers used in resin current collectors have generally been particulate. However, particulate conductive fillers tend to fall off from the resin due to the stress caused by lithium (Li) precipitation during charging. Therefore, the inventors have decided to use a fibrous conductive filler instead of a particulate conductive filler. By using a fibrous conductive filler, the fibrous conductive filler is prevented from falling off from the resin even when Li precipitates during charging, and as a result, improved cycle characteristics are expected.

[0023] 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, further improvement in cycle characteristics is expected. The aspect ratio of the fibrous conductive filler 2 may be 25 or more, or 30 or more. The aspect ratio of the fibrous conductive filler 2 may be 50 or less, or 40 or less.

[0024] The aspect ratio is the ratio of length to diameter. In this embodiment, the "aspect ratio" is determined 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 average diameter may each be the arithmetic mean value of measurements taken on 10 or more fibrous conductive fillers 2. The length and diameter of each individual fibrous conductive filler 2 may be measured using a scanning electron microscope (SEM) or a scanning probe microscope (SPM).

[0025] The fibrous conductive filler 2 may have an average diameter of, for example, 50 nm or more and 200 nm or less, and an average length of, for example, 1 μm or more and 10 μm or less.

[0026] The content of the fibrous conductive filler 2 in the negative electrode current collector 21 is more than 10% by mass and not more than 40% by mass. If the content of the fibrous conductive filler 2 in the negative electrode current collector 21 is not more than 10% by mass, the conductivity may be insufficient. If the content of the fibrous conductive filler 2 in the negative electrode current collector 21 exceeds 40% by mass, the negative electrode current collector may not be able to be molded, or even if it can be molded, problems such as the fibrous conductive filler 2 agglomerating to create voids may occur. The content of the fibrous conductive filler 2 in the negative electrode current collector 21 is preferably not less than 20% by mass and not more than 30% by mass.

[0027] The negative electrode current collector 21 may consist essentially of a resin 1 and a fibrous conductive filler 2, or may consist of a resin 1 and a fibrous conductive filler 2. Note that "consisting essentially of a resin 1 and a fibrous conductive filler 2" means that the content of the resin 1 and the fibrous conductive filler 2 in the negative electrode current collector 21 is 95 mass % or more.

[0028] Other ingredients The negative electrode current collector 21 may contain a conductive filler other than the fibrous conductive filler 2, a dispersant, and the like. Examples of the conductive filler other than the fibrous conductive filler 2 include carbon particles, metal particles, and metal-plated particles. Examples of the dispersant include a surfactant. The content 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.

[0029] <Method of manufacturing negative electrode current collector> The negative electrode current collector can be produced by the following method, however, the following production method is an example and is not intended to be limiting.

[0030] A resin composition is formed by mixing a resin, a fibrous conductive filler, and other components as needed. The resin composition may also be called, for example, a "compound." The resin composition may be, for example, in the form of pellets. The resin composition may be formed by any method. For example, melt-kneading may be performed. For example, melt-kneading may be performed using a twin-screw extrusion kneader.

[0031] The resin composition is molded into a negative electrode current collector by any method. The molding method is not particularly limited, and examples thereof include known methods such as a T-die method, an inflation method, and a calendar method.

[0032] <Negative electrode> 2 is a schematic diagram showing an example of the negative electrode of this embodiment. The negative electrode 20 includes a negative electrode current collector 21 and a lithium metal layer 22. The negative electrode current collector 21 is as described above.

[0033] <Lithium metal layer> The lithium metal layer 22 contains lithium metal. The lithium metal layer 22 is a layer formed by the accumulation of lithium metal that is deposited on the negative electrode 20. The thickness of the lithium metal layer changes depending on whether the SOC increases or decreases.

[0034] <Lithium metal secondary battery> 3 is a schematic diagram showing an example of a lithium metal secondary battery (hereinafter simply referred to as "battery") according to this embodiment. Hereinafter, a liquid-type monopolar battery will be described as an example, but the present invention is not limited to this.

[0035] The battery 100 may include an exterior body (not shown). The exterior body may house the power generating element 50 and an electrolyte (not shown). The exterior body may have any shape. For example, the exterior body may be a metal case or a pouch made of a metal foil laminate film. The exterior body may contain, for example, Al or the like.

[0036] The battery 100 includes a power generating element 50. The power generating element 50 may also be referred to as an electrode assembly or an electrode group. The power generating element 50 includes a positive electrode 10, a separator 30, and a negative electrode 20. The power generating element 50 may have any structure. For example, the power generating element 50 may be wound. The positive electrode 10, the separator 30, and the negative electrode 20 may all be strip-shaped sheets. The power generating element 50 may be formed, for example, by stacking 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 generating element 50 may be formed into a flat shape.

[0037] 《Positive electrode》 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 include, for example, aluminum (Al) foil. The positive electrode active material layer 12 includes a positive electrode active material. The positive electrode active material layer 12 may further include, for example, a conductive material, a binder, etc.

[0038] The positive electrode active material may be, for example, particulate. The positive electrode active material may have a D50 of, for example, 1 to 30 μm. The positive electrode active material may contain, for example, at least one selected from the group consisting of LiCoO2, LiNiO2, LiMnO2, Li(NiCoMn)O2, and Li(NiCoAl)O2. For example, "(NiCoMn)" in "Li(NiCoMn)O2" indicates that the sum of the composition ratios in parentheses is 1. The amounts of each component are arbitrary as long as the sum is 1.

[0039] The conductive material may include, for example, acetylene black (AB), etc. The binder may include, for example, PVdF, etc. The conductive material and the binder may be present in an amount of, for example, 0.1% by mass or more and 10% by mass or less with respect to the positive electrode active material layer 12.

[0040] <Separator> The separator 30 is porous. The separator 30 is permeable to the electrolyte. The separator 30 separates the positive electrode 10 and the negative electrode 20. The separator 30 is electrically insulating. The separator 30 may contain, for example, a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator 30 may have, for example, a single-layer structure or a multi-layer structure. The separator 30 may, for example, be substantially composed of a PE layer, or may be formed by laminating a PP layer, a PE layer, and a PP layer in this order.

[0041] 《Electrolyte》 The electrolyte solution includes a solvent and a Li salt. The solvent is aprotic. The solvent may include any component. For example, the solvent may include at least one selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0042] The Li salt is a supporting electrolyte. The Li salt is dissolved in a solvent. The Li salt may include, for example, at least one selected from the group consisting of LiPF6 and LiBF4. The Li salt may have a molar concentration of, for example, 0.5 mol / L or more and 2.0 mol / L or less.

[0043] The electrolytic solution may further contain an optional additive. For example, the electrolytic solution may contain 0.01% by mass or more and 5% by mass or less of the additive. The additive may include, for example, at least one selected from the group consisting of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC). [Example]

[0044] No. 1 A Cu foil (thickness: 10 μm) was prepared as a material for the negative electrode current collector, and the Cu foil was used as the negative electrode.

[0045] As a positive electrode material, layered LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (average particle size: 10 μm) (90 mass%), AB (8 mass%) as a conductive material, and PVdF (2 mass%) as a binder were prepared. These materials were mixed with N-methyl-2-pyrrolidone (NMP) as a solvent so that the solid content was 56 mass%, and the mixture was mixed using a planetary mixer to obtain a positive electrode slurry.

[0046] The resulting positive electrode slurry was applied to the surface of an Al foil positive electrode current collector using a die coater. After application, the positive electrode slurry was dried at 120°C and compressed using a roll press to obtain a positive electrode.

[0047] A porous resin (PP / PE / PP) was prepared as the separator, with PP layers laminated on both sides of a PE layer. The positive electrode, separator, and negative electrode were laminated in this order to form a power generating element.

[0048] A pouch made of laminated film was prepared as the exterior body. The power generating element was housed in the exterior body. The electrolyte was prepared by dissolving a supporting salt (LiPF6) at a concentration of 1.0 mol / L in a mixed solvent containing EC and DMC in a volume ratio of 1:1. The electrolyte was poured into the exterior body. After the electrolyte was poured, the exterior body was sealed. This completed the assembly of the test battery.

[0049] 0.4mA / cm at 25℃ 2 The test battery was charged in a constant current mode with a current of 0.4 mA / cm until the positive electrode potential reached 4.3 V. 2 The test battery was discharged at a constant current until the positive electrode potential reached 3.0 V. This resulted in the formation of a lithium metal layer on the surface of the negative electrode current collector. This resulted in the production of test battery No. 1. Note that test battery No. 1 is a reference battery that uses the same metal foil (Cu foil) as the negative electrode current collector.

[0050] No.2 As materials for the negative electrode current collector, PE (Novatec HD HF560, manufactured by Japan Polyethylene Co., Ltd.) was prepared as a resin, and CB, which is an aggregate of particulate carbon, was prepared as a conductive filler.

[0051] PE and CB were weighed out to a mass ratio of 80:20 and melt-kneaded in a twin-screw extruder to form a compound, which was then extruded by a T-die method to obtain a negative electrode current collector No. 2 (thickness: 50 μm).

[0052] Test battery No. 2 was fabricated using the same materials and method as No. 1, except that the negative electrode current collector No. 2 was used as the negative electrode.

[0053] No.3 The materials for the negative electrode current collector were prepared using the same PE resin as in No. 2 and particulate Ni as a conductive filler. PE and Ni were weighed out to a mass ratio of 95:5 and melt-kneaded using a twin-screw extruder to form a compound. The compound was extrusion-molded using a T-die method to obtain a negative electrode current collector No. 3 (thickness: 50 μm).

[0054] Test battery No. 3 was fabricated using the same materials and method as No. 1, except that the negative electrode current collector No. 3 was used as the negative electrode.

[0055] No.4-6 The materials for the negative electrode current collector were prepared as follows: the same PE as in No. 2 as the resin; and VGCF (VGCF-H, manufactured by Resonac Corporation) (aspect ratio: 26.7) as a fibrous conductive filler. PE and VGCF were weighed out to a predetermined mass ratio and melt-kneaded using a twin-screw extruder to form a compound. The compound was extrusion-molded using a T-die method to obtain negative electrode current collectors Nos. 4 to 6 (thickness: 50 μm). For No. 4, the PE and VGCF were weighed out to a mass ratio of 90:10; for No. 5, the PE and VGCF were weighed out to a mass ratio of 80:20; and for No. 6, the PE and VGCF were weighed out to a mass ratio of 70:30.

[0056] Test batteries Nos. 4 to 6 were fabricated using the same materials and method as No. 1, except that the negative electrode current collectors Nos. 4 to 6 were used as the negative electrodes.

[0057] <Evaluation> 1.0mA / cm at 25℃ 2 Each test battery was charged at a constant current of 1.0 mA / cm until the positive electrode potential reached 4.3 V. 2The test battery was discharged at a constant current until the positive electrode potential reached 3.0 V. This cycle was considered as one cycle, and the cycle test was performed for 20 cycles. The discharge capacity retention rate was calculated by dividing the discharge capacity at each cycle by the discharge capacity at the first cycle. The results are shown in Figure 4. The discharge capacity retention rate at the 20th cycle is also shown in Figure 5. Note that for No. 4, it was not possible to charge or discharge it, and it did not function as a battery, so the discharge capacity retention rate could not be calculated.

[0058] <Result> As shown in Figures 4 and 5, it can be seen that Nos. 5 and 6 have higher capacity retention rates than Nos. 2 and 3. It can also be seen that Nos. 5 and 6 have equivalent capacity retention rates to the reference battery No. 1.

[0059] The present embodiment and examples are illustrative in all respects. The present embodiment and examples are not limiting. The technical scope of the present disclosure encompasses all modifications within the meaning and scope equivalent to the claims. For example, it is also intended from the beginning that any configuration may be extracted from the present embodiment and examples and that they may be combined in any desired manner. [Explanation of symbols]

[0060] 1 resin, 2 fibrous conductive filler, 10 positive electrode, 11 positive electrode current collector, 12 positive electrode active material layer, 20 negative electrode, 21 negative electrode current collector, 22 lithium metal layer, 30 separator, 50 power generating element, 100 lithium metal secondary battery.

Claims

1. A negative electrode current collector including a resin and a fibrous conductive filler, The aspect ratio of the fibrous conductive filler 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, a content of the fibrous conductive filler in the negative electrode current collector being more than 10% by mass and not more than 40% by mass;

2. The negative electrode current collector according to claim 1 , wherein the fibrous conductive filler is 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 comprising the negative electrode according to claim 4.

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