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] This disclosure relates to sodium batteries.

Background Art

[0002] Lithium-ion batteries are used as power sources for mobile devices and vehicles, taking advantage of their high capacity and lightweight characteristics. 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 secondary batteries, 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 sodium batteries, a technology is desired that suppresses the growth of dendrites from the negative electrode layer through the separator toward 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 to the positive electrode layer is suppressed during battery charging, thereby making it less likely for short circuits to occur between the positive and negative electrode layers. [Means for solving the problem]

[0009] The Disclosing Party has found that the above-mentioned 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 that order, and having these impregnated with an electrolyte, 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 the negative electrode active material. The separator is composed of multiple porous layers, In the thickness direction of the separator, the pore diameters of each of the plurality of porous layers differ from the positive electrode layer side toward the negative electrode layer side, such that they have two maximum values, Of the two aforementioned maximum values, the maximum value on the positive electrode layer side is greater than the maximum value on the negative electrode layer side. Sodium battery. <Aspect 2> The sodium battery according to embodiment 1, wherein the maximum value on the positive electrode layer side is 1.1 times or more and 1.7 times or less the maximum value on the negative electrode layer side. <Aspect 3> A sodium battery according to embodiment 1 or 2, wherein the maximum value on the positive electrode layer side is 100 nm or more and 150 nm or less. [Effects of the Invention]

[0010] According to this disclosure, it is possible to provide a sodium battery in which the growth of sodium deposited in the negative electrode layer is suppressed, and short circuits between the positive electrode layer and the negative electrode layer are less likely to occur. [Brief explanation of the drawing]

[0011] [Figure 1] FIG. 1 is a schematic cross-sectional view of a sodium metal battery showing an example of the sodium battery of the present disclosure. [Figure 2] FIG. 2 is a schematic cross-sectional view of a sodium ion battery showing an example of the sodium battery of the present disclosure. [Figure 3] FIG. 3 is a schematic view showing the maximum value of the pore diameter in the separator ((a): comparative example, (b): example). [Figure 4] FIG. 4 is a graph showing the measurement results of XPS on the negative electrode layer ((a): comparative example, (b): example). [Figure 5] FIG. 5 is a graph showing the measurement results of XPS on the separator ((a): comparative example, (b): example).

Mode for Carrying Out 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 various modifications can be made 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 electrolyte. 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 each layer of the plurality of porous layers are different so as to have two maximum values from the side of the positive electrode layer 11, 21 toward the side of the negative electrode layer 13, 23, and the maximum value on the side of the positive electrode layer 11, 21 of the two maximum values is larger than the maximum value on the side of the negative electrode layer 13, 23.

[0014] In a sodium metal battery in which sodium metal is deposited during charging, since the negative electrode potential during charging is below the sodium deposition potential, dendrites are likely 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 are likely 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 intending to be bound by any theory, the pore diameters of the respective layers in the 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 a positive electrode current collector 11a, 21a and a positive electrode active material layer 11b, 21b. Further, the “negative electrode layer” means a laminate of a negative electrode current collector 13a or a negative electrode current collector 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 Figure 3(b), the separators 12 and 22 in the sodium battery of this disclosure are composed of a plurality of porous layers. In the thickness direction of the separators 12 and 22, the pore diameter of each of the plurality of porous layers differs from the positive electrode layer 11 and 21 side toward the negative electrode layer 13 and 23 side, with each having two maximum values, and the maximum value on the positive electrode layer 11 and 21 side being larger than the maximum value on the negative electrode layer 13 and 23 side. In contrast, Figure 3(a) is a figure relating to a comparative example, showing separators 12 and 22 having one maximum value. In this disclosure, a maximum value means the pore diameter value of a porous layer in a separator composed of a plurality of porous layers, where the pore diameter is larger than that of the adjacent porous layers on both sides. Although Figure 3(b) shows a separator composed of six porous layers, the number of layers in the separator is not limited to this.

[0019] With respect to pore diameter, the maximum value on the positive electrode layer side may be 1.1 times or more and 1.7 times or less the maximum value on the negative electrode layer side. The maximum value on the positive electrode layer side may be 1.2 times or more, or 1.3 times or more the maximum value on the negative electrode layer side, 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] With respect to pore size, the maximum value on the positive electrode layer side may be 100 nm or more and 150 nm or less. The maximum value on the positive electrode layer side 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 pore size, the maximum value on the negative electrode layer side may be between 60 nm and 90 nm. The maximum value on the negative electrode layer side may be 70 nm or more, or 80 nm or more.

[0022] The separator in this disclosure can be obtained, for example, by stacking multiple porous layers with different pore sizes such that the pore size has two maximum values. As will be described later, the separator in this disclosure can be obtained, for example, by stacking two separators made of three layers of PP / PE / PP.

[0023] The pore diameter of the separator can be measured, for example, by using a porosimeter with the mercury intrusion method. 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 form.

[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, NaFeO2, NaNiO2, NaCoO2, NaMnO2, NaVO2, Na(Ni X Mn 1-X )O2(0 < X < 1), Na(Fe X Mn 1-X )O2(0 < X < 1), NaVPO4F, Na2FePO4F, Na3V2(PO4)3, etc. The shape of the positive electrode active material is not particularly limited. The positive electrode active material may be in the form of particles. In this case, its average particle diameter may be, for example, 1 nm or more or 10 nm or more, and may also 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] Conductive additives may be, for example, carbon materials or metallic materials. Specific examples of carbon materials include carbon black such as acetylene black, Ketjen black, furnace black, and thermal black; carbon fibers such as VGCF; graphite; and hard carbon. Examples of conductive materials include coke. Examples of metallic materials include Fe, Cu, Ni, Al, etc. The content of the conductive additive in the positive electrode active material layer is not particularly limited. For example, the positive electrode active material layer may contain 1% by mass or more and 50% by mass or less of the conductive additive.

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

[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 for example, it may be between 0.1 μm and 1 mm.

[0031] <Separator> The separator material is not particularly limited as long as it has the function of electrically separating the positive electrode layer and the negative electrode layer. Examples include porous sheets made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide, porous insulating materials such as nonwoven fabrics and glass fiber nonwoven fabrics, or combinations thereof. The thickness of the separator is not particularly limited and may be, for example, 5 μm or more and 1 mm or less.

[0032] <Negative electrode layer> (Negative electrode current collector) If the sodium battery in this 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] If the sodium battery of this disclosure is a sodium-ion battery, examples of materials for the negative electrode current collector include SUS, aluminum, copper, nickel, carbon, etc. The negative electrode current collector may be in the form of foil, mesh, or porous material, for example.

[0034] (Negative electrode active material layer) If the sodium battery of this 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 additive and a binder.

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

[0036] Hard carbon can be produced, for example, by carbonizing a raw material containing carbon. The carbonization temperature may be, for example, around 1000 to 2000°C. Furthermore, carbonization can be carried out under 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, organic compounds such as phenols and aldehydes such as formaldehyde can be used as raw materials. In addition, resins such as phenolic resins, polyacrylonitrile, and polyimide can also be used as raw materials. These raw materials may be used individually or in combination of multiple types.

[0037] For conductive additives and binders, refer to the above description of the positive electrode layer in this 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 for example, it may be between 0.1 μm and 1 mm.

[0039] Furthermore, if the sodium battery of this disclosure is a sodium metal battery, the negative electrode layer does not need to have a negative electrode active material layer.

[0040] <Electrolyte> The electrolyte may contain a sodium salt and a non-aqueous solvent. Examples of sodium salts include inorganic sodium salts such as NaPF6, NaBF4, NaClO4, and NaAsF6; and organic sodium salts such as NaCF3SO3, NaN(CF3SO2)2, NaN(C2F5SO2)2, NaN(FSO2)2, and NaC(CF3SO2)3.

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

[0042] The electrolyte may contain additives such as fluoroethylene carbonate (FEC).

[0043] <Other components> The sodium battery of this disclosure may include a battery case that houses each layer of the battery, and terminals connected to a current collector or the like. Furthermore, the sodium battery of this disclosure may include a restraining member that constrains each layer along the stacking direction in order to reduce contact resistance. These can be the same as those used in conventional batteries.

[0044] Examples of sodium battery shapes in this disclosure include coin-type, laminate-type, cylindrical, and prismatic types. The negative electrode active material layer and positive electrode active material layer of the sodium battery can be manufactured, for example, by dry molding such as powder compaction or wet molding using slurry. The layers constituting the sodium battery may be stacked on top of each other and then optionally subjected to a pressing process to obtain the sodium battery. [Examples]

[0045] Comparative Example <Cell manufacturing> (Fabrication of the positive electrode layer) A positive electrode layer was fabricated by forming a positive electrode active material layer, using a sodium-containing layered oxide as the positive electrode active material, on an aluminum (Al) foil used as the positive electrode current collector.

[0046] (Preparing the separator) As a separator, we prepared one consisting of the following three porous layers stacked from the positive electrode side to the negative electrode side (PP = polypropylene, PE = polyethylene): PP (pore diameter: 80nm) / PE (pore diameter: 90nm) / PP (pore diameter: 80nm)

[0047] The pore size of each layer of the separator was measured using a porosimeter by the mercury intrusion method. Specifically, each layer of the separator was peeled off, and the pore size of each peeled layer was measured using the method described above.

[0048] (Fabrication of the negative electrode layer) A negative electrode layer was fabricated by forming a negative electrode active material layer using hard carbon as the negative electrode active material on an Al foil, which serves as the negative electrode current collector.

[0049] (Cell creation) A laminate was formed by stacking a positive electrode layer, a separator, and a negative electrode layer in that order. An electrolyte was prepared by dissolving 1 M sodium hexafluorophosphate (NaPF6) in ethylene carbonate (EC):diethyl carbonate (DEC) = 1:1 (volume ratio), and adding 0.1% by mass of fluoroethylene carbonate (FEC) as an additive. The laminate was impregnated with the electrolyte to produce a comparative example of type 2032 coin cell.

[0050] Examples In the separator preparation process, a separator consisting of the following six porous layers stacked from the positive electrode side to the negative electrode side (PP = polypropylene, PE = polyethylene) was prepared; otherwise, the 2032 type coin cell of the example was manufactured in the same manner as the comparative example: PP (pore diameter: 80nm) / PE (pore diameter: 120nm) / PP (pore diameter: 80nm) / PP (pore diameter: 80nm) / PE (pore diameter: 90nm) / PP (pore diameter: 80nm)

[0051] This separator was obtained by laminating the following two separators, each consisting of three layers (PP = polypropylene, PE = polyethylene): PP (pore diameter: 80nm) / PE (pore diameter: 120nm) / PP (pore diameter: 80nm) PP (pore diameter: 80nm) / PE (pore diameter: 90nm) / PP (pore diameter: 80nm)

[0052] "evaluation" <Confirmation of the degree of sodium precipitation> Each cell was subjected to 20 charge-discharge cycles at 25°C, 0.1C, and between 4.2V and 3.0V. Afterward, the cells were disassembled, and the degree of sodium deposition on the negative electrode layer and separator was confirmed by X-ray photoelectron spectroscopy (XPS) and visual inspection.

[0053] "result" Figure 4 shows the XPS measurement results on the negative electrode layer, and Figure 5 shows the XPS measurement results on the separator. Figures 4(a) and 5(a) show the results for the comparative example, and Figures 4(b) and 5(b) show the results for the example. As shown in Figure 4, the strength of the sodium binding energy on the negative electrode layer was lower in the example cell than in the comparative example cell. In contrast, as shown in Figure 5, the strength of the sodium binding energy on the separator was higher in the example cell than in the comparative example cell. These results suggest that sodium precipitation on the negative electrode layer was suppressed in the example cell compared to the comparative example cell, and more sodium precipitated on the separator. Furthermore, visual inspection on the separator confirmed that more sodium was precipitated in the example cell than in the comparative example cell. [Explanation of symbols]

[0054] 100, 200 sodium batteries 11, 21 Positive electrode layer 11a, 21a Positive electrode current collector 11b, 21b Cathode active material layer 12, 22 Separators 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 having these impregnated with an electrolyte, 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 the negative electrode active material. The separator is composed of multiple porous layers, In the thickness direction of the separator, the pore diameters of each of the plurality of porous layers differ from the positive electrode layer side toward the negative electrode layer side, such that each layer has two maximum values, Of the two aforementioned maximum values, the maximum value on the positive electrode layer side is greater than the maximum value on the negative electrode layer side. Sodium battery.

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

3. The sodium battery according to claim 1 or 2, wherein the maximum value on the positive electrode layer side is 100 nm or more and 150 nm or less.

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