Sodium battery

Abstract

To provide a sodium battery that suppresses growth of a dendrite extending from a negative electrode layer toward a positive electrode layer through a separator during battery charging, and thus hardly generates short-circuiting between the positive electrode layer and the negative electrode layer.SOLUTION: Sodium batteries 100, 200 have positive electrode layers 11, 21, separators 12, 22, and negative electrode layers 13, 23, respectively, in this order, and the sodium batteries are impregnated with an electrolyte. The sodium battery is the sodium metal battery 100 in which sodium metal is deposited during charging, or the sodium ion battery 200 having hard carbon serving as a negative active material. The separator is formed of a plurality of porous layers. Pore diameters of the plurality of the porous layers are made different from the positive electrode layer side to the negative electrode layer side with every layer so as to have two maximum values in a thickness direction of the separator. In the two maximum values, the maximum value on the positive electrode layer side is larger than the maximum value on the negative electrode layer side.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to sodium batteries.

Background Art

[0002] Lithium-ion batteries are used as power sources for mobile devices and vehicles by taking advantage of their characteristics of high capacity and light weight. On the other hand, in recent years, from the perspective of resource quantity, sodium batteries using sodium as a material to replace lithium have attracted attention.

[0003] For example, Patent Document 1 discloses a sodium-ion battery using hard carbon for the negative electrode.

[0004] By the way, in a secondary battery, dendrites may precipitate on the negative electrode layer during charging of the battery, and due to the growth of these dendrites, a short circuit may occur between the positive electrode layer and the negative electrode layer. Therefore, technologies for suppressing the growth of these dendrites have been developed.

[0005] For example, Patent Document 2 discloses a secondary battery capable of suppressing the growth of dendrites formed on the surface of the negative electrode beyond the separator.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

[0007] In a sodium battery, a technology is desired that suppresses the growth of dendrites that go from the negative electrode layer through the separator to the positive electrode layer during charging of the battery, thereby preventing a short circuit between the positive electrode layer and the negative electrode layer.

Problems to be Solved by the Invention

[0008] The present disclosure aims to provide a sodium battery in which the growth of dendrites from the negative electrode layer through the separator toward the positive electrode layer during charging of the battery is suppressed, thereby making it difficult for a short circuit to occur between the positive electrode layer and the negative electrode layer.

Means for Solving the Problems

[0009] The present inventors have found that the above problems can be solved by the following means. 〈Aspect 1〉 A sodium battery having a positive electrode layer, a separator, and a negative electrode layer in this order, and these being impregnated with an electrolytic solution, wherein the sodium battery is a sodium metal battery in which sodium metal is deposited during charging, or a sodium ion battery having hard carbon as a negative electrode active material, the separator is composed of a plurality of porous layers, in the thickness direction of the separator, the pore diameters of the respective layers of the plurality of porous layers are different so as to have two maximum values from the side of the positive electrode layer toward the side of the negative electrode layer, and the maximum value on the side of the positive electrode layer among the two maximum values is larger than the maximum value on the side of the negative electrode layer, sodium battery. 〈Aspect 2〉 The sodium battery according to Aspect 1, wherein the maximum value on the side of the positive electrode layer is 1.1 times or more and 1.7 times or less the maximum value on the side of the negative electrode layer. 〈Aspect 3〉 The sodium battery according to Aspect 1 or 2, wherein the maximum value on the side of the positive electrode layer is 100 nm or more and 150 nm or less.

Advantages of the Invention

[0010] According to the present disclosure, it is possible to provide a sodium battery in which the growth of sodium deposited on the negative electrode layer is suppressed and a short circuit is less likely to occur between the positive electrode layer and the negative electrode layer.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and can be variously modified and implemented within the scope of the gist of the disclosure. Also, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect the actual dimensional relationships.

[0013] 《SODIUM BATTERY》 As shown in FIGS. 1 and 2, the sodium batteries 100 and 200 of the present disclosure have a positive electrode layer 11, 21, a separator 12, 22, and a negative electrode layer 13, 23 in this order, and these are impregnated with an electrolytic solution. The sodium battery of the present disclosure is a sodium metal battery 100 in which sodium metal is deposited during charging, or a sodium ion battery 200 having hard carbon as a negative electrode active material. The separators 12 and 22 are composed of a plurality of porous layers, and in the thickness direction of the separator, the pore diameters of the respective layers of the plurality of porous layers are different so as to have two maximum values from the side of the positive electrode layers 11, 21 toward the side of the negative electrode layers 13, 23, and among the two maximum values, the maximum value on the side of the positive electrode layers 11, 21 is larger than the maximum value on the side of the negative electrode layers 13, 23.

[0014] In a sodium metal battery in which sodium metal is deposited during charging, since the negative electrode potential during charging is equal to or lower than the sodium deposition potential, dendrites tend to grow from the negative electrode layer through the separator toward the positive electrode layer. Similarly, in a sodium ion battery having hard carbon as a negative electrode active material, since the negative electrode potential during charging is close to the sodium deposition potential, dendrites tend to grow from the negative electrode layer through the separator toward the positive electrode layer.

[0015] Regarding this, the present inventors unexpectedly found that by adjusting the pore diameter of the separator constituting the sodium battery, the growth of dendrites from the negative electrode layer through the separator toward the positive electrode layer can be suppressed. Specifically, although not intended to be bound by any theory, the pore diameters of the respective layers in a separator composed of a plurality of porous layers are different so as to have two maximum values, and among the two maximum values, the maximum value on the positive electrode layer side is larger than the maximum value on the negative electrode layer side, whereby it is considered that sodium deposition in the separator is likely to occur on the positive electrode layer side of the separator. Thereby, the growth of dendrites from the negative electrode layer through the separator toward the positive electrode layer can be suppressed, and as a result, it is considered that a short circuit is less likely to occur between the positive electrode layer and the negative electrode layer.

[0016] The sodium battery of the present disclosure has a positive electrode layer, a separator, and a negative electrode layer in this order, and these are impregnated with an electrolytic solution.

[0017] Regarding the present disclosure, the "positive electrode layer" means a laminate of positive electrode current collectors 11a and 21a and positive electrode active material layers 11b and 21b. Further, the "negative electrode layer" means a laminate of a negative electrode current collector 13a or negative electrode current collectors 23a and a negative electrode active material layer 23b. In particular, the "negative electrode layer" may mean the negative electrode current collector 13a in a sodium metal battery and a laminate of the negative electrode current collector 23a and the negative electrode active material layer 23b in a sodium ion battery. Note that the positive electrode active material layer and the negative electrode active material layer among the positive electrode layer and the negative electrode layer are disposed on the separator side.

[0018] As shown in FIG. 3(b), the separators 12 and 22 in the sodium battery of the present disclosure are composed of a plurality of porous layers. In the thickness direction of the separators 12 and 22, the pore diameters of the respective layers of the plurality of porous layers are different such that they have two maximum values from the side of the positive electrode layers 11 and 21 toward the side of the negative electrode layers 13 and 23, and among the two maximum values, the maximum value on the side of the positive electrode layers 11 and 21 is larger than the maximum value on the side of the negative electrode layers 13 and 23. On the other hand, FIG. 3(a) is a diagram related to a comparative example showing the separators 12 and 22 having one maximum value. Regarding the present disclosure, the maximum value means the value of the pore diameter in a porous layer having a larger pore diameter than the porous layers on both adjacent sides in a separator composed of a plurality of porous layers. Note that in FIG. 3(b), a separator composed of six layers of porous layers is illustrated, but the number of layers of the separator is not limited thereto.

[0019] Regarding the pore diameter, the maximum value on the side of the positive electrode layer may be 1.1 times or more and 1.7 times or less the maximum value on the side of the negative electrode layer. The maximum value on the side of the positive electrode layer may be 1.2 times or more, or 1.3 times or more, and may be 1.6 times or less, 1.5 times or less, 1.4 times or less, or 1.3 times or less.

[0020] Regarding the pore diameter, the maximum value on the side of the positive electrode layer may be 100 nm or more and 150 nm or less. The maximum value on the side of the positive electrode layer may be 110 nm or more, 120 nm or more, or 130 nm or more, and may be 140 nm or less, 130 nm or less, or 120 nm or less.

[0021] Regarding the pore diameter, the maximum value on the side of the negative electrode layer may be 60 nm or more and 90 nm or less. The maximum value on the side of the negative electrode layer may be 70 nm or more, or 80 nm or more.

[0022] The separator in the present disclosure can be obtained, for example, by laminating a plurality of porous layers having different pore diameters such that the pore diameter has two maximum values. As will be described later, the separator in the present disclosure can be obtained, for example, by stacking two separators each composed of three layers of PP / PE / PP.

[0023] The pore diameter of the separator can be measured, for example, by the mercury intrusion method using a porosimeter. When each layer of the separator in the present disclosure is joined to each other, each layer of the separator can be peeled off, and the pore diameter can be measured for each peeled layer by the above method.

[0024] Hereinafter, each layer constituting the battery will be described.

[0025] 〈Positive electrode layer〉 (Positive electrode current collector) Examples of the material of the positive electrode current collector include SUS, aluminum, nickel, iron, titanium, carbon, etc. The positive electrode current collector may be, for example, in the form of a foil, a mesh, or a porous material.

[0026] (Positive electrode active material layer) The positive electrode active material layer contains a positive electrode active material and may optionally contain a conductive assistant and a binder.

[0027] Examples of the positive electrode active material include Na-containing oxides such as layered active materials, spinel-type active materials, and olivine-type active materials. Specifically, NaFeO 2 , NaNiO 2 , NaCoO 2 , NaMnO 2 , NaVO 2 , Na(Ni X Mn 1-X )O 2 (0 < X < 1), Na(Fe X Mn 1-X )O2(0 < X < 1), NaVPO 4 F, Na 2 FePO 4 F, Na 3 V 2 (PO 4 ) 3Examples include the above. The shape of the positive electrode active material is not particularly limited. The positive electrode active material may be particulate. In this case, the average particle diameter may be, for example, 1 nm or more, or 10 nm or more, and may be 100 μm or less, or 30 μm or less. The higher the content of the positive electrode active material in the positive electrode active material layer, the higher the capacity of the positive electrode. The positive electrode active material layer may contain the positive electrode active material, for example, at 50% by mass or more, or 70% by mass or more, and 99% by mass or less, or 95% by mass or less.

[0028] The conductive aid may be, for example, a carbon material, a metal material, or the like. Specific examples of the carbon material include carbon blacks such as acetylene black, ketjen black, furnace black, thermal black; carbon fibers such as VGCF; graphite; hard carbon n; coke, etc. can be mentioned. Examples of the metal material include Fe, Cu, Ni, Al, etc. The content of the conductive aid in the positive electrode active material layer is not particularly limited. For example, the positive electrode active material layer may contain the conductive aid at 1% by mass or more and 50% by mass or less.

[0029] As the binder, those that are chemically and electrically stable may be used. Specific examples of the binder include, for example, fluorine-based binders such as polyvinylidene fluoride (PVDF)-based binders, polytetrafluoroethylene (PTFE)-based binders, rubber-based binders such as styrene-butadiene rubber (SBR)-based binders, olefin-based binders such as polypropylene (PP)-based binders, polyethylene (PE)-based binders, cellulose-based binders such as carboxymethyl cellulose (CMC)-based binders, or polyacrylic acid (PAA)-based binders. The content of the binder in the positive electrode active material layer is not particularly limited and may be appropriately determined according to the desired binding property.

[0030] The positive electrode active material layer may have a certain thickness. The thickness of the positive electrode active material layer is not particularly limited, but may be, for example, 0.1 μm or more and 1 mm or less.

[0031] <Separator> The material of the separator is not particularly limited as long as it has the function of electrically separating the positive electrode layer and the negative electrode layer. For example, a porous sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide, etc., a non-woven fabric, a porous insulating material such as a glass fiber non-woven fabric, or a combination thereof can be mentioned. The thickness of the separator is not particularly limited, and for example, it may be 5 μm or more and 1 mm or less.

[0032] <Negative electrode layer> (Negative electrode current collector) When the sodium battery of the present disclosure is a sodium metal battery, the negative electrode current collector is sodium metal. In a sodium metal battery, sodium metal is deposited during charging.

[0033] When the sodium battery of the present disclosure is a sodium ion battery, examples of the material of the negative electrode current collector include SUS, aluminum, copper, nickel, carbon, etc. The negative electrode current collector may be, for example, in the form of a foil, a mesh, or a porous shape.

[0034] (Negative electrode active material layer) When the sodium battery of the present disclosure is a sodium ion battery, the negative electrode layer has a negative electrode active material layer. The negative electrode active material layer contains a negative electrode active material and may optionally contain a conductive assistant and a binder.

[0035] The negative electrode active material layer has hard carbon as the negative electrode active material. The negative electrode active material layer may contain hard carbon as the negative electrode active material, for example, 50% by mass or more, 70% by mass or more, 99% by mass or less, or 95% by mass or less. The content of hard carbon relative to the total amount of the negative electrode active material may be 50% by mass or more, 70% by mass or more, 90% by mass or more, 95% by mass or more, or 99% by mass, and may be 100% by mass. That is, the negative electrode active material may be hard carbon. The average particle size of the hard carbon is not particularly limited, but for example, it can be in the range of 50 nm to 100 μm.

[0036] Hard carbon can be produced, for example, by carbonizing a raw material containing a carbon element. The carbonization temperature may be, for example, about 1000 to 2000°C. Further, the carbonization can be carried out in an inert atmosphere. The raw material for hard carbon is not particularly limited as long as it is a raw material capable of producing hard carbon. For example, alcohols such as ethanol, phenols, aldehydes such as formaldehyde, and other organic compounds can be used as raw materials. In addition, phenolic resins, resins such as polyacrylonitrile and polyimide can be used as raw materials. These raw materials may be used alone or in combination of multiple types.

[0037] Regarding the conductive assistant and the binder, reference can be made to the above description of the positive electrode layer of the present disclosure.

[0038] The negative electrode active material layer may have a certain thickness. The thickness of the negative electrode active material layer is not particularly limited, but may be, for example, 0.1 μm or more and 1 mm or less.

[0039] In addition, when the sodium battery of the present disclosure is a sodium metal battery, the negative electrode layer does not necessarily have to have a negative electrode active material layer.

[0040] 〈Electrolyte〉 The electrolyte may contain a sodium salt and a non-aqueous solvent. Examples of the sodium salt include inorganic sodium salts such as NaPF 6 , NaBF 4 , NaClO 4 and NaAsF 6 ; and organic sodium salts such as NaCF 3 SO 3 , NaN(CF 3 SO 2 ) 2 , NaN(C 2 F 5 SO 2 ) 2 , NaN(FSO 2 ) 2 , NaC(CF 3 SO 2 ) 3Examples thereof include organic sodium salts and the like.

[0041] The non-aqueous solvent is not particularly limited as long as it can dissolve the sodium salt. For example, as the high dielectric constant solvent, cyclic esters (cyclic carbonates) such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), γ-butyrolactone, sulfolane, N-methylpyrrolidone (NMP), 1,3-dimethyl-2-imidazolidinone (DMI), and the like can be mentioned. On the other hand, as the low viscosity solvent, chain esters (chain carbonates) such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), acetates such as methyl acetate and ethyl acetate, ethers such as 2-methyltetrahydrofuran, and the like can be mentioned. A mixed solvent obtained by mixing a high dielectric constant solvent and a low viscosity solvent may also be used.

[0042] The electrolytic solution may contain an additive such as fluoroethylene carbonate (FEC).

[0043] <Other configurations> The sodium battery of the present disclosure may include a battery case that houses each layer of the battery, and terminals connected to a current collector or the like. Further, the sodium battery of the present disclosure may include a restraining member that restrains each layer along the stacking direction in order to reduce the contact resistance. For these, those similar to the conventional ones may be used.

[0044] Examples of the shape of the sodium battery of the present disclosure include coin type, laminate type, cylindrical type, and square type. The negative electrode active material layer and the positive electrode active material layer of the sodium battery can be produced, for example, by dry forming such as pressure powder molding or wet forming using a slurry. After laminating the layers constituting the sodium battery on each other, the sodium battery may be obtained through an optional pressing process.

Examples

[0045] 《Comparative Example》 〈Manufacture of cells〉 (Fabrication of the positive electrode layer) A positive electrode active material layer using a sodium-containing layered oxide as the positive electrode active material was formed on an aluminum (Al) foil serving as a positive electrode current collector to fabricate a positive electrode layer.

[0046] (Preparation of the separator) As the separator, a separator composed of the following three porous layers laminated from the positive electrode side to the negative electrode side was prepared (PP = polypropylene, PE = polyethylene): PP (pore diameter: 80 nm) / PE (pore diameter: 90 nm) / PP (pore diameter: 80 nm)

[0047] Note that the pore diameters of each layer of the separator were measured using a porosimeter by the mercury intrusion method. Specifically, each layer of the separator was peeled off, and the pore diameter of each peeled layer was measured by the above method.

[0048] (Fabrication of the negative electrode layer) A negative electrode active material layer using hard carbon as the negative electrode active material was formed on an Al foil serving as a negative electrode current collector to fabricate a negative electrode layer.

[0049] (Fabrication of the cell) The positive electrode layer, the separator, and the negative electrode layer were laminated in this order to form a laminate. A sodium hexafluorophosphate (NaPF 6 ) electrolyte solution was prepared by dissolving it in a solution of ethylene carbonate (EC): diethyl carbonate (DEC) = 1:1 (volume ratio) and adding 0.1 mass% of fluoroethylene carbonate (FEC) as an additive. The laminate was impregnated with the electrolyte solution to fabricate a 2032-type coin cell of the comparative example.

[0050] 《Example》 A 2032-type coin cell of the example was fabricated in the same manner as the comparative example, except that in the preparation step of the separator, a separator composed of the following six porous layers laminated from the positive electrode side to the negative electrode side was prepared (PP = polypropylene, PE = polyethylene): PP (pore diameter: 80 nm) / PE (pore diameter: 120 nm) / PP (pore diameter: 80 nm) / PP (pore diameter: 80 nm) / PE (pore diameter: 90 nm) / PP (pore diameter: 80 nm)

[0051] Note that this separator was obtained by laminating the following two three-layer separators (PP = polypropylene, PE = polyethylene): PP (pore diameter: 80 nm) / PE (pore diameter: 120 nm) / PP (pore diameter: 80 nm) PP (pore diameter: 80 nm) / PE (pore diameter: 90 nm) / PP (pore diameter: 80 nm)

[0052] 《Evaluation》 〈Confirmation of the degree of sodium deposition〉 For each cell of each example, charge and discharge between 4.2 V and 3.0 V at 25°C and 0.1C were repeated 20 times. Thereafter, the cell was disassembled, and the degree of sodium deposition on the negative electrode layer and on the separator was confirmed by X-ray photoelectron spectroscopy (XPS) and visually.

[0053] 《Results》 The XPS measurement results on the negative electrode layer are shown in Fig. 4, and the XPS measurement results on the separator are shown in Fig. 5. Note that Fig. 4(a) and Fig. 5(a) are the results of the comparative example, and Fig. 4(b) and Fig. 5(b) are the results of the example. As shown in Fig. 4, the intensity of the binding energy of sodium on the negative electrode layer was smaller in the cells of the example than in the cells of the comparative example. In contrast, as shown in Fig. 5, the intensity of the binding energy of sodium on the separator was greater in the cells of the example than in the cells of the comparative example. These results suggest that in the cells of the example, sodium deposition on the negative electrode layer was suppressed more than in the cells of the comparative example, and more sodium was deposited on the separator. Also, it was confirmed visually that more sodium was deposited on the separator in the cells of the example than in the cells of the comparative example.

Explanation of symbols

[0054] 100, 200 Sodium batteries 11, 21 Positive electrode layer 11a, 21a Positive electrode current collector 11b, 21b Positive electrode active material layer 12, 22 Separator 13, 23 Negative electrode layer 13a, 23a Negative electrode current collector 23b Negative electrode active material layer

Claims

1. A sodium battery having a positive electrode layer, a separator, and a negative electrode layer, in that order, and impregnated with an electrolyte, the sodium battery is a sodium metal battery in which sodium metal precipitates during charging, or a sodium ion battery having hard carbon as a negative electrode active material, The separator is composed of a plurality of porous layers, In a thickness direction of the separator, the pore diameters of the layers of the plurality of porous layers are different from each other so as to have two maximum values ​​from the positive electrode layer side to the negative electrode layer side, and Of the two maximum values, the maximum value on the side of the positive electrode layer is greater than the maximum value on the side of the negative electrode layer. Sodium battery.

2. 2. The sodium battery according to claim 1, wherein the maximum value on the positive electrode layer side is 1.1 to 1.7 times the maximum value on the negative electrode layer side.

3. 3. The sodium battery according to claim 1, wherein the maximum value on the positive electrode layer side is 100 nm or more and 150 nm or less.

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