Sodium battery

Abstract

The invention relates to a sodium battery. The sodium battery has a positive electrode layer, a separator, and a negative electrode layer in this order, and the positive electrode layer, the separator, and the negative electrode layer are impregnated with an electrolyte solution. The sodium battery according to the present disclosure 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 configured from a plurality of porous layers, the pore diameters of the plurality of porous layers differ so as to have two maximum values in the thickness direction of the separator from the positive electrode layer side to the negative electrode layer side, and the maximum value on the positive electrode layer side of the two maximum values is greater than the maximum value on the negative electrode layer side.

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, taking advantage of their high capacity and light weight. On the other hand, sodium batteries using sodium have attracted attention in recent years as a material to replace lithium from the perspective of resource availability.

[0003] For example, Japanese Patent Application Laid-Open No. 2017-050148 discloses a sodium ion battery using hard carbon in a negative electrode.

[0004] In a secondary battery, dendrites may be deposited on the negative electrode layer during battery charging, and the growth of the dendrites may cause a short circuit between the positive electrode layer and the negative electrode layer. Therefore, a technology for suppressing the growth of the dendrites has been developed.

[0005] For example, Japanese Patent Application Laid-Open No. 2022-048597 discloses a secondary battery capable of suppressing the growth of dendrites formed on the surface of a negative electrode across a separator. Summary of the invention

[0006] In sodium batteries, a technology is expected to suppress the growth of dendrites from the negative electrode layer through the separator to the positive electrode layer during battery charging, thereby preventing a short circuit between the positive electrode layer and the negative electrode layer.

[0007] An object of the present disclosure is to provide a sodium battery that suppresses the growth of dendrites from the negative electrode layer through the separator to the positive electrode layer during battery charging, thereby making it difficult for a short circuit to occur between the positive electrode layer and the negative electrode layer.

[0008] The inventors of the present application have found that the following means can solve the above-mentioned problems.

[0009] Solution 1

[0010] A sodium battery having a positive electrode layer, a separator and a negative electrode layer in sequence, wherein the positive electrode layer, the separator and the negative electrode layer are impregnated with an electrolyte, wherein the sodium battery is a sodium metal battery in which sodium metal is precipitated during charging, or a sodium ion battery having hard carbon as a negative electrode active material, wherein the separator is composed of a plurality of porous layers, and in the thickness direction of the separator, from the positive electrode layer side to the negative electrode layer side, the pore diameters of the plurality of porous layers are different in a manner having two maximum values, and the maximum value on the positive electrode layer side of the two maximum values ​​is larger than the maximum value on the negative electrode layer side.

[0011] Solution 2

[0012] The sodium battery according to aspect 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.

[0013] Solution 3

[0014] The sodium battery according to aspect 1 or 2, wherein the maximum value on the positive electrode layer side is 100 nm or more and 150 nm or less.

[0015] According to the present disclosure, it is possible to provide a sodium battery in which the growth of sodium deposited in the negative electrode layer is suppressed and a short circuit is unlikely to occur between the positive electrode layer and the negative electrode layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention are described below with reference to the accompanying drawings, in which like reference numerals represent like elements, and in which:

[0017] Figure 1 is a schematic cross-sectional view of a sodium metal battery showing an example of the sodium battery disclosed herein;

[0018] Figure 2 is a schematic cross-sectional view of a sodium ion battery showing an example of the sodium battery disclosed herein;

[0019] Figure 3A is a schematic diagram showing the maximum value of the pore diameter in the separator of the comparative example;

[0020] Figure 3B A schematic diagram showing the maximum value of the pore diameter in the separator of the embodiment;

[0021] Figure 4A is a graph showing the XPS measurement results on the negative electrode layer of the comparative example;

[0022] Figure 4B A graph showing the XPS measurement results on the negative electrode layer of the embodiment;

[0023] Figure 5A is a graph showing the measurement results of XPS on a separator of a comparative example; and

[0024] Figure 5B It is a graph showing the measurement results of XPS on the separator of the example. DETAILED DESCRIPTION

[0025] The following is a detailed description of the embodiments of the present disclosure. It should be noted that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the disclosed subject matter. In addition, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect the actual dimensional relationships.

[0026] Sodium battery

[0027] like Figure 1 and 2As shown, the sodium battery 100, 200 of the present disclosure has positive electrode layers 11, 21, separators 12, 22, and negative electrode layers 13, 23 in sequence, and they are impregnated (impregnated) with an electrolyte. The sodium battery of the present disclosure is a sodium metal battery 100 in which sodium metal is precipitated during charging, or a sodium ion battery 200 having hard carbon as a negative electrode active material. The separators 12, 22 are composed of a plurality of porous layers. In the thickness direction of the separator, from the positive electrode layer 11, 21 side to the negative electrode layer 13, 23 side, the pore diameters of each layer of the plurality of porous layers are different in a manner having two maximum values. Of the two maximum values, the maximum value on the positive electrode layer 11, 21 side is larger than the maximum value on the negative electrode layer 13, 23 side.

[0028] In a sodium metal battery in which sodium metal is deposited during charging, dendrites easily grow from the negative electrode layer through the separator to the positive electrode layer because the negative electrode potential during charging is below the sodium deposition potential. Similarly, in a sodium ion battery having hard carbon as a negative electrode active material, dendrites easily grow from the negative electrode layer through the separator to the positive electrode layer because the negative electrode potential during charging is close to the sodium deposition potential.

[0029] In this regard, the inventors of the present application unexpectedly discovered 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 to the positive electrode layer can be suppressed. Specifically, without intending to be bound by certain theories, it is believed to be as follows. (i) The pore diameters of each layer in the separator composed of a plurality of porous layers are different in a manner having two maximum values. (ii) The maximum value on the positive electrode layer side of the two maximum values ​​is larger than the maximum value on the negative electrode layer side. It is believed that through these (i) and (ii), the precipitation of sodium in the separator is likely to occur on the positive electrode layer side of the separator. Therefore, it is believed that the growth of dendrites from the negative electrode layer through the separator to the positive electrode layer can be suppressed, and as a result, it is difficult for a short circuit to occur between the positive electrode layer and the negative electrode layer.

[0030] The sodium battery of the present disclosure has a positive electrode layer, a separator, and a negative electrode layer in this order, and the positive electrode layer, the separator, and the negative electrode layer are impregnated (impregnated) with an electrolytic solution.

[0031] In the present disclosure, the "positive electrode layer" means a stack of positive electrode collectors 11a, 21a and positive electrode active material layers 11b, 21b. In addition, the "negative electrode layer" means a negative electrode collector 13a, or a stack of a negative electrode collector 23a and a negative electrode active material layer 23b. In particular, in a sodium metal battery, the "negative electrode layer" may mean a negative electrode collector 13a, and in a sodium ion battery, the "negative electrode layer" may mean a stack of a negative electrode collector 23a and a negative electrode active material layer 23b. Furthermore, the positive electrode active material layer and the negative electrode active material layer in the positive electrode layer and the negative electrode layer are arranged on the separator side.

[0032] like Figure 3BAs shown, 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 plurality of porous layers are different from each other in a manner having two maximum values ​​from the positive electrode layer 11 and 21 side to the negative electrode layer 13 and 23 side. Of the two maximum values, the maximum value on the positive electrode layer 11 and 21 side is larger than the maximum value on the negative electrode layer 13 and 23 side. In contrast, Figure 3A 1 and 2 are diagrams showing separators 12 and 22 having a maximum value, and are related to comparative examples. In the present disclosure, the so-called maximum value means the value of the pore diameter of a porous layer having a larger pore diameter than the porous layers on both sides of the separator composed of a plurality of porous layers. Figure 3B , a separator composed of six porous layers is shown, but the number of layers of the separator is not limited to this.

[0033] Regarding the pore diameter, the maximum value on the positive electrode layer side may be 1.1 times or more and 1.7 times or less of 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, and may be 1.6 times or less, 1.5 times or less, 1.4 times or less, or 1.3 times or less of the maximum value on the negative electrode layer side.

[0034] The pore diameter may have a maximum value of 100 nm to 150 nm on the positive electrode layer side, 110 nm to 120 nm, or 130 nm to 140 nm, 130 nm to 120 nm, or 120 nm to 150 nm.

[0035] Regarding the pore diameter, the maximum value on the negative electrode layer side may be 60 nm or more and 90 nm or less. The maximum value on the negative electrode layer side may be 70 nm or more or 80 nm or more.

[0036] The separator of the present disclosure can be obtained, for example, by stacking a plurality of porous layers having different pore diameters so that the pore diameter has two local maxima. As described later, the separator of the present disclosure can be obtained, for example, by stacking two separators consisting of three layers of PP / PE / PP.

[0037] The pore diameter of the separator can be measured, for example, by mercury intrusion porosimeter. When the layers of the separator in the present disclosure are bonded to each other, the layers of the separator may be peeled off and the pore diameter of each peeled layer may be measured by the above method.

[0038] Each layer constituting the battery will be described below.

[0039] Positive electrode layer

[0040] Positive electrode collector

[0041] 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 in a foil, mesh, or porous form, for example.

[0042] Positive electrode active material layer

[0043] The positive electrode active material layer contains a positive electrode active material, and may optionally contain a conductive auxiliary agent and a binder.

[0044] Examples of positive electrode active materials include oxides containing Na, 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. There is no particular limitation on the shape of the positive electrode active material. The positive electrode active material may be in a particulate form. In this case, its average particle size may be, for example, greater than 1 nm or greater than 10 nm, and may be less than 100 μm or less than 30 μm. The more the positive electrode active material content in the positive electrode active material layer, the higher the capacity of the positive electrode. The positive electrode active material layer may contain, for example, greater than 50 mass % or greater than 70 mass %, less than 99 mass % or less than 95 mass % of the positive electrode active material.

[0045] The conductive aid may be, for example, a carbon material, a metal material, or the like. Specific examples of carbon materials include carbon black such as acetylene black, ketjen black, furnace black, and pyrolytic black; carbon fibers such as VGCF; graphite; hard carbon; coke, and the like. Metal materials include Fe, Cu, Ni, and Al, and the like. 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 a conductive aid in an amount of 1% by mass or more and 50% by mass or less.

[0046] As an adhesive, a chemically and electrically stable adhesive can be used. As a specific example of an adhesive, for example, a fluorine-based adhesive such as a polyvinylidene fluoride (PVDF)-based adhesive, a polytetrafluoroethylene (PTFE)-based adhesive, a rubber-based adhesive such as a styrene butadiene rubber (SBR)-based adhesive, an olefin-based adhesive such as a polypropylene (PP)-based adhesive, a polyethylene (PE)-based adhesive, a cellulose-based adhesive such as a carboxymethyl cellulose (CMC)-based adhesive, or a polyacrylic acid (PAA)-based adhesive can be cited. The content of the adhesive in the positive electrode active material layer is not particularly limited, and can be appropriately determined according to the adhesiveness of the target.

[0047] The positive electrode active material layer may have a certain thickness. The thickness of the positive electrode active material layer is not particularly limited, and may be, for example, 0.1 μm or more and 1 mm or less.

[0048] Separator

[0049] 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. The material of the separator can be, for example, a porous sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide, etc.; a porous insulating material such as a non-woven fabric, a non-woven fabric such as a glass fiber non-woven fabric, etc.; or a combination thereof. The thickness of the separator is not particularly limited, and can be, for example, 5 μm or more and 1 mm or less.

[0050] Negative electrode layer

[0051] Negative electrode collector

[0052] When the sodium battery of the present disclosure is a sodium metal battery, the negative electrode current collector is sodium metal. In the sodium metal battery, sodium metal is deposited during charging.

[0053] 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 in a foil, mesh, or porous form, for example.

[0054] Negative electrode active material layer

[0055] 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 auxiliary agent and a binder.

[0056] The negative electrode active material layer has hard carbon as the negative electrode active material. The negative electrode active material layer may contain, for example, 50% by mass or more, 70% by mass or more, 99% by mass or less, or 95% by mass or less of hard carbon as the negative electrode active material. The content of hard carbon 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 or less, and may be 100% by mass relative to the total amount of the negative electrode active material. That is, the negative electrode active material may be hard carbon. The average particle size of the hard carbon is not particularly limited, and may be, for example, in the range of 50 nm to 100 μm.

[0057] Hard carbon can be produced, for example, by carbonizing a raw material containing carbon elements. The carbonization temperature can be, for example, about 1000 to 2000°C. In addition, carbonization can be carried out in an inert atmosphere. The raw material of hard carbon is not particularly limited as long as it is a raw material that can produce hard carbon. For example, organic compounds such as alcohols such as ethanol, phenols, and aldehydes such as formaldehyde can be used as raw materials. In addition, resins such as phenolic resins, polyacrylonitrile, and polyimide can be used as raw materials. These raw materials can be used alone or in combination.

[0058] Regarding the conductive auxiliary agent and the binder, reference can be made to the above description related to the positive electrode layer of the present disclosure.

[0059] The negative electrode active material layer may have a certain thickness. The thickness of the negative electrode active material layer is not particularly limited, and may be, for example, 0.1 μm or more and 1 mm or less.

[0060] Furthermore, when the sodium battery of the present disclosure is a sodium metal battery, the negative electrode layer may not have a negative electrode active material layer.

[0061] Electrolyte

[0062] The electrolyte may contain a sodium salt and a non-aqueous solvent. Examples of the sodium salt 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.

[0063] As non-aqueous solvent, as long as sodium salt is dissolved, then there is no particular limitation.For example, as high dielectric constant solvent, cyclic esters (cyclic carbonates), gamma-butyrolactone, cyclopentane, N-methyl pyrrolidone (NMP), 1,3-dimethyl-2-imidazolidinone (DMI) etc. such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) etc. can be listed.On the other hand, as low viscosity solvent, ethers such as acetates such as chain esters (chain carbonates), methyl acetate, ethyl acetate, 2-methyltetrahydrofuran etc. such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC) etc. can be listed.Also can use the mixed solvent that high dielectric constant solvent and low viscosity solvent are mixed.

[0064] The electrolyte may contain additives such as fluoroethylene carbonate (FEC).

[0065] Other components

[0066] The sodium battery of the present disclosure may include a battery case that accommodates each layer of the battery, and a terminal connected to a current collector, etc. In addition, in order to reduce contact resistance, the sodium battery of the present disclosure may include a restraining member that restrains each layer along the stacking direction. For these, the same members as in the past can be used.

[0067] The shape of the sodium battery disclosed in the present invention includes, for example, a coin shape, a laminated shape, a cylindrical shape, and a square shape. The negative electrode active material layer and the positive electrode active material layer of the sodium battery can be produced by dry molding such as powder pressing molding, or wet molding using slurry. After the layers constituting the sodium battery are stacked on each other, they can be optionally subjected to a pressing process to obtain a sodium battery.

[0068] Comparative Example

[0069] Manufacturing of battery cells

[0070] Fabrication of positive electrode layer

[0071] A positive electrode active material layer was formed on an aluminum (Al) foil as a positive electrode current collector. The positive electrode active material layer used a layered oxide containing sodium as a positive electrode active material, thereby producing a positive electrode layer.

[0072] Preparation of separators

[0073] As a separator, a separator composed of the following three porous layers stacked from the positive electrode side to the negative electrode side (PP=polypropylene, PE=polyethylene) was prepared: PP (pore diameter: 80 nm) / PE (pore diameter: 90 nm) / PP (pore diameter: 80 nm).

[0074] Furthermore, the pore diameter of each layer of the separator was measured by mercury intrusion porosimetry using a porosimeter. Specifically, each layer of the separator was peeled off, and the pore diameter of each peeled layer was measured by the above-mentioned method.

[0075] Fabrication of negative electrode layer

[0076] A negative electrode active material layer using hard carbon as a negative electrode active material was formed on Al foil as a negative electrode current collector, thereby producing a negative electrode layer.

[0077] Battery cell manufacturing

[0078] The positive electrode layer, separator and negative electrode layer are stacked in sequence to form a stack. The following electrolyte was prepared: an electrolyte containing 0.1% by mass of fluoroethylene carbonate (FEC) as an additive was added to a solution of 1M sodium hexafluorophosphate (NaPF6) dissolved in ethylene carbonate (EC): diethyl carbonate (DEC) = 1:1 (volume ratio). The stack was impregnated (immersed) in the electrolyte to produce a comparative example 2032-type coin battery cell.

[0079] A 2032-type coin cell of the example was produced in the same manner as in the comparative example, except that a separator composed of the following six porous layers stacked from the positive electrode side to the negative electrode side was prepared in the separator preparation step (PP=polypropylene, PE=polyethylene).

[0080] PP (pore diameter: 80nm) / PE (pore diameter: 120nm) / PP (pore diameter: 80nm) / PP (pore diameter: 80nm) / PE (pore diameter: 90nm) / PP (pore diameter: 80nm)

[0081] The separator is obtained by laminating the following two separators consisting of three layers (PP=polypropylene, PE=polyethylene).

[0082] PP (pore diameter: 80nm) / PE (pore diameter: 120nm) / PP (pore diameter: 80nm)

[0083] PP (pore diameter: 80nm) / PE (pore diameter: 90nm) / PP (pore diameter: 80nm)

[0084] evaluate

[0085] Confirmation of the degree of sodium precipitation

[0086] For each battery cell, charge and discharge were repeated 20 times at 25°C, 0.1C, and between 4.2V and 3.0V. The battery cell was then disassembled, and the extent of sodium precipitation on the negative electrode layer and the separator was confirmed by X-ray photoelectron spectroscopy (XPS) and visual observation.

[0087] result

[0088] The XPS measurement results on the negative electrode layer are shown in Figure 4A , 4B The XPS measurement results on the separator are shown in Figure 5A , 5B . To explain, Figure 4A and Figure 5A The results of the comparative example are Figure 4B and Figure 5B The results of the embodiment. Figure 4A , 4B As shown in FIG. 1 , the strength of the binding energy of sodium on the negative electrode layer is smaller in the battery cell of the embodiment than in the battery cell of the comparative example. Figure 5A , 5BAs shown, the strength of the binding energy of sodium on the separator is greater in the battery cell of the embodiment than in the battery cell of the comparative example. These results suggest that, in the battery cell of the embodiment, the precipitation of sodium on the negative electrode layer is suppressed, and more sodium is precipitated on the separator than in the battery cell of the comparative example. In addition, it was also confirmed by visual observation that a larger amount of sodium was precipitated on the separator in the battery cell of the embodiment than in the battery cell of the comparative example.

Claims

1. A sodium battery comprising a positive electrode layer, a separator and a negative electrode layer in this order, wherein the positive electrode layer, the separator and the negative electrode layer are impregnated with an electrolyte, The sodium battery is a sodium metal battery in which sodium metal is precipitated 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 local values ​​from the positive electrode layer side to the negative electrode layer side, And, of the two local values, the local value on the positive electrode layer side is larger than the local value on the negative electrode layer side.

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

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