JP7188333B2 - sodium ion conductor - Google Patents

Description

The present invention relates to sodium ion conductors.

Lithium-ion secondary batteries are used as power sources for mobile devices and vehicles, taking advantage of their high-capacity and light weight characteristics. Since the organic electrolyte used as the electrolyte is flammable and has a risk of ignition, the use of a solid electrolyte instead of the electrolyte is being studied.

However, there is a concern that the price of raw materials for lithium will soar. Therefore, as a material to replace lithium, sodium-ion solid-state batteries using sodium, which is an abundant resource, are attracting attention. A sodium ion solid state battery requires a sodium ion conductor with sodium ion conductivity.

Non-Patent Document 1 discloses that NaCB ₉ H ₁₀ and NaCB ₁₁ H ₁₂ are mixed at a molar ratio of 50:50 in a sodium ion conductor.

Wan Si Tang et al., "Stabilizing Superionic-Conducting Structures Via Mixed-Anion Solid Solutions of Monocarba-closo-borate Salts", Acs Energy Letters, 2016, 1, 659-664

Sodium ion conductors have room for improvement in the conductivity of sodium ions. The present application has been made in view of the above circumstances, and a main object of the present application is to provide a sodium ion conductor with enhanced sodium ion conductivity.

The present inventors have focused on the fact that the molecular crystal has a large unit cell when the molar ratio of NaCB ₉ H ₁₀ and _{NaCB 11} H ₁₂ is 50:50 as in Non-Patent Document 1. It was found that by increasing the proportion of H ₁₀ and relatively decreasing the proportion of NaCB ₁₁ H ₁₂ , the unit cell can be made smaller, thereby increasing the conductivity of sodium ions.

Based on the above findings, the present application provides a sodium ion conductor composed of a solid solution of NaCB ₉ H ₁₀ and NaCB ₁₁ H ₁₂ as one means for solving the above problems, wherein NaCB ₉ H ₁₀ and NaCB ₁₁ H ₁₂ and 100 mol %, a sodium ion conductor in which the mol % of NaCB ₁₁ H ₁₂ is 30 or more and less than 50.

The sodium ion conductor disclosed by the present application can improve the conductivity of sodium ions.

1 is a schematic cross-sectional view of a sodium ion solid state battery having a solid electrolyte layer produced using the sodium ion conductor of the present application; FIG. It is a graph showing the result of an Example and a comparative example.

[Sodium ion conductor]
The sodium ion conductor of the present application is a mixture of NaCB ₉ H ₁₀ and NaCB ₁₁ H ₁₂ thereby forming, at least in part, a crystalline structure. Further, when the total of NaCB ₉ H ₁₀ and NaCB ₁₁ H ₁₂ is 100 in mol %, the mol % of NaCB ₁₁ H ₁₂ is 30 or more and less than 50. As a result, the conductivity of sodium ions is improved as compared with the conventional one.

It can be confirmed by X-ray diffraction (XRD) measurement or the like that the sodium ion conductor has a predetermined crystal structure. The crystalline state of the sodium ion conductor can be confirmed, for example, by subjecting the sodium ion conductor to powder X-ray diffraction measurement using CuKα rays.

A sodium ion conductor can be obtained by subjecting a raw material composition (eg, NaCB ₉ H ₁₀ , NaCB ₁₁ H ₁₂ ) to solid-phase reaction treatment. Examples of solid phase reaction treatment include ball mill treatment. The solid phase reaction treatment is preferably performed under an inert atmosphere.

The shape of the sodium ion conductor is preferably particulate from the viewpoint of good handleability. Also, the average particle diameter (D50) of the particles of the sodium ion conductor is not particularly limited, but can be 0.5 μm or more and 2 μm or less.
In the present application, unless otherwise specified, the average particle diameter of particles is the value of the volume-based median diameter (D50) measured by laser diffraction/scattering particle size distribution measurement. The median diameter (D50) is the diameter (volume average diameter) at which the cumulative volume of particles becomes half (50%) of the total when the particles are arranged in order from the smallest particle diameter.

[Sodium ion solid state battery]
The sodium ion conductor of the present application can be suitably used in a sodium ion solid state battery by applying it to a solid electrolyte layer using this as an electrolyte. The sodium ion conductor of the present application has improved sodium ion conductivity, and is suitable as a solid electrolyte for a sodium ion solid battery.

FIG. 1 shows a schematic cross-sectional view of a sodium ion all-solid-state battery 100 having a solid electrolyte layer 10 produced using the sodium ion conductor of the present application as a solid electrolyte. The sodium ion all-solid-state battery 100 shown in FIG. It has a positive current collector 24 that collects current from the material layer 22 and a negative current collector 34 that collects current from the negative electrode active material layer 32 . The positive electrode active material layer 22 and the positive electrode current collector 24 constitute the positive electrode 20 , and the negative electrode active material layer 32 and the negative electrode current collector 34 constitute the negative electrode 30 .

<Solid electrolyte layer 10>
By using the above-described sodium ion conductor of the present application as an electrolyte, a solid electrolyte layer of a sodium ion all-solid-state battery is formed. The lower limit of the content of the sodium ion conductor of the present application in the solid electrolyte layer is preferably 80.0% by mass or more, more preferably 90.0% by mass or more, when the total mass of the solid electrolyte layer is 100% by mass. It is preferably 95.0% by mass or more, more preferably 99.0% by mass or more. The upper limit is not particularly limited, and the solid electrolyte layer may be formed only from the sodium ion conductor of the present application.

The solid electrolyte layer may contain a binder that binds the sodium ion conductor particles together from the viewpoint of exhibiting plasticity. However, from the viewpoint of preventing a decrease in the ion conductivity of the solid electrolyte layer, the binder content is preferably 20% by mass or less, more preferably 10% by mass or less, when the total mass of the solid electrolyte layer is 100% by mass. , is more preferably 5% by mass or less, and more preferably 1% by mass or less.

The thickness of the solid electrolyte layer is appropriately adjusted depending on the configuration of the battery and is not particularly limited, and is usually 0.1 μm or more and 1 mm or less.

As for the method for forming such a solid electrolyte layer, for example, the solid electrolyte layer may be formed by pressure-molding a powder of the material for the solid electrolyte layer containing a sodium ion conductor and, if necessary, other components. As another method, a solid electrolyte layer may be formed by applying a solid electrolyte layer slurry containing a binder onto a support, drying the solid electrolyte layer slurry, and peeling off the support.

<Positive electrode active material layer 22>
The positive electrode active material layer 22 contains a positive electrode active material. More specifically, it may optionally contain a conductive material and a binder in addition to the positive electrode active material.

(Positive electrode active material)
The positive electrode active material is a composite oxide containing Na, and any known positive electrode active material for sodium ion solid state batteries can be used. The “composite oxide containing Na” means an oxide containing, in addition to Na, metallic elements other than Na (transition metal elements, etc.) and/or non-metallic elements (P, S, etc.). Examples include layered compounds, spinel compounds, polyanion type compounds, and the like. Specifically, Na _x MO ₂ (0<x≦1, M is at least one of Fe, Ni, Co, Mn, V, and Cr) as a layered compound and a spinel compound, and a polyanion type compound , Na ₃ V ₂ (PO ₄ ) ₃ , Na ₂ Fe ₂ (SO ₄ ) ₃ , NaFePO ₄ , NaFeP ₂ O ₇ , Na ₂ MP ₂ O ₇ (M is at least one of Fe, Ni, Co and Mn above), Na ₄ M ₃ (PO ₄ ) ₂ P ₂ O ₇ (M is at least one of Fe, Ni, Co and Mn) and other known positive electrode active materials can be employed.

The shape of the positive electrode active material is preferably particulate. Also, the average particle diameter (D50) of the positive electrode active material is, for example, within the range of 1 nm to 100 μm, preferably within the range of 10 nm to 30 μm. The content of the positive electrode active material in the positive electrode is not particularly limited. % or more and 95 mass % or less is more preferable.

(Conductive material)
The type of conductive material is not particularly limited, and any known conductive material for sodium ion solid state batteries can be used. For example, a carbon material is preferable, and a highly crystalline carbon material is particularly preferable. This is because if the carbon material has high crystallinity, it becomes difficult for sodium ions to be inserted into the carbon material, and the irreversible capacity due to sodium ion insertion can be reduced. As a result, it is possible to obtain a sodium ion solid state battery with even better cycle characteristics. The crystallinity of the carbon material can be defined, for example, by the interlayer distance d002 and the D/G ratio. The interlayer distance d002 refers to the interplanar distance between (002) planes in the carbon material, and specifically corresponds to the distance between graphene layers. The interlayer distance d002 can be determined from peaks obtained by, for example, an X-ray diffraction (XRD) method using CuKα rays. The D / G ratio is observed in Raman spectrometry (wavelength 532 nm), relative to the peak intensity of the G-band derived from the graphite structure near 1590 cm ^-1 , the D-band derived from the defect structure near 1350 cm ^-1 is the peak intensity of In the present invention, for example, the upper limit of d002 is preferably 3.54 Å or less, more preferably 3.50 Å or less. The lower limit is usually 3.36 Å or more. Also, the upper limit of the D/G ratio is preferably 0.90 or less, more preferably 0.80 or less, still more preferably 0.50 or less, and particularly preferably 0.20 or less. The content of the conductive material in the positive electrode active material layer 22 is not particularly limited.

(binder)
The binder is not particularly limited as long as it is chemically and electrically stable. For example, fluorine-based binders such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE); rubber binders such as (SBR), olefin binders such as polypropylene (PP) and polyethylene (PE), and cellulose binders such as carboxymethyl cellulose (CMC). The binder content in the positive electrode is not particularly limited.

The method for producing the positive electrode active material layer 22 is not particularly limited, and it can be easily produced by a dry method or a wet method. That is, the above components are added to an appropriate solvent to prepare a slurry, and the slurry is applied to the surface of a substrate (which may be a positive electrode current collector or a solid electrolyte layer described later) and then dried. A positive electrode active material layer 22 having a predetermined thickness (for example, 0.1 μm or more and 1000 μm or less) can be easily produced by a wet method. Alternatively, the positive electrode active material layer 22 may be obtained by dry-mixing the above components and press-molding the mixture.

(Positive collector 24)
The positive electrode is generally equipped with a positive current collector 24 . Examples of materials for the positive electrode current collector 24 include SUS, aluminum, nickel, iron, titanium, and carbon. Examples of the shape of the positive electrode current collector 24 include a foil shape, a mesh shape, a porous shape, and the like. The positive electrode 20 can be easily produced by laminating the positive electrode current collector 24 on the positive electrode active material layer 22 described above. However, depending on the material contained in the positive electrode active material layer 22, the positive electrode current collector 24 may be omitted in some cases. In this case, the positive electrode active material layer 22 itself becomes the positive electrode 20 .

<Negative electrode active material layer 32>
The negative electrode active material layer 32 contains a negative electrode active material. More specifically, it may optionally contain a conductive material and a binder in addition to the negative electrode active material.

(Negative electrode active material)
The negative electrode active material is not particularly limited, and any known negative electrode active material for sodium ion secondary batteries can be employed. For example, metal materials containing sodium such as sodium metal and sodium alloys; carbon materials such as graphite, hard carbon and carbon black; sodium-transition metal composite oxides such as sodium titanate; oxides composed of elements other than sodium such as SiOx things; and the like. The negative electrode active material is preferably particulate like the positive electrode active material.

(Conductive material and binder)
In the negative electrode active material layer 32, conductive materials and binders that can be used in the positive electrode active material layer 22 can be used. The conductive material and binder are optional components, and the content thereof is not particularly limited.

The method for producing the negative electrode active material layer 32 is not particularly limited, and it can be easily produced by a dry method or a wet method like the positive electrode active material layer 22 .

(Negative electrode current collector 34)
The negative active material electrode 32 is generally provided with a negative electrode current collector 34 . Examples of materials for the negative electrode current collector 34 include SUS, aluminum, nickel, copper, and carbon. Examples of the shape of the negative electrode current collector 34 include a foil shape, a mesh shape, a porous shape, and the like. By laminating the negative electrode current collector 34 on the negative electrode active material layer 32 described above, the negative electrode 30 can be easily manufactured. However, depending on the material contained in the negative electrode active material layer 32, the negative electrode current collector 34 may be omitted in some cases. In this case, the negative electrode active material layer 32 itself becomes the negative electrode 30 .

<Other configurations>
A general battery case can be used as the battery case, and the battery case is not particularly limited. For example, a battery case made of SUS can be mentioned. In addition, the sodium ion solid state battery may be a primary battery or a secondary battery, but from the viewpoint of more effectively exhibiting the durability improvement effect of the present application, it is preferably a secondary battery. . This is because they can be repeatedly charged and discharged, and are useful, for example, as batteries for vehicles. In addition, the primary battery includes the use of a primary battery (use for the purpose of discharging only once after charging). In addition, examples of the shape of the sodium ion solid state battery in the present disclosure include coin type, laminate type, cylindrical type, rectangular type, and the like. Moreover, the manufacturing method of the sodium ion solid state battery is not particularly limited, and is the same as the manufacturing method of a general sodium ion solid state battery.

Hereinafter, the sodium ion conductor of the present disclosure will be described using examples.

[Production of sodium ion conductor]
NaCB ₉ H ₁₀ (Katchem, Na[1-CB ₉ H ₁₀ ], 10 g, CAS No. 94322-26-6) and NaCB ₁₁ H ₁₂ (Katchem, Na[CB ₁₁ H ₁₂ ], 10 g, CAS No. 92468-38-7) was vacuum-dried at 160° C. overnight, weighed at the mol % ratio shown in Table 1 for each example, and placed in a 45 mL ball mill pot (made of ZrO ₂ ).

20 ZrO ₂ spheres (5 mm in diameter) were added to the ball mill pot, mixed at 500 rpm for 20 hours, and ball milled. The resulting mixed powder, excluding the ZrO ₂ spheres, is the sodium ion conductor.

[Evaluation of Sodium Ion Conductivity]
Sodium ion conductivity was evaluated by measuring sodium ion conductivity. Specifically, it is as follows.
50 mg of each of the obtained sodium ion conductors of each example was weighed, placed in an evaluation cell, press-molded under conditions of 6000 kg (6 tons) for 3 minutes, and formed into a circle having an area of 1 cm ² on each of the upper and lower surfaces. After forming a columnar pellet, it was enclosed in a glass desiccator after restraint. Then, the impedance was measured under conditions of a temperature of 25° C., a measurement frequency of 0.01 MHz to 1 MHz, and an amplitude of 10 mV. Then, the ionic conductivity (mS/cm) was calculated from the impedance.

[result]
The results are shown in FIG. In this figure, the horizontal axis represents mol % of NaCB ₁₁ H ₁₂ and the vertical axis represents sodium ion conductivity (mS/cm). As can be seen from FIG. 2, when the mol % of NaCB ₁₁ H ₁₂ is 30 or more and less than 50, sodium conductivity is better than others.

100 Sodium ion solid state battery 10 Solid electrolyte layer 22 Positive electrode active material layer 24 Positive electrode current collector 32 Negative electrode active material layer 34 Negative electrode current collector

Claims (1)

In a sodium ion conductor consisting of a solid solution of NaCB ₉ H ₁₀ and NaCB ₁₁ H ₁₂ ,
A sodium ion conductor wherein the mol% of NaCB ₁₁ H ₁₂ is 30 or more and less than 50 when the total of NaCB ₉ H ₁₀ and NaCB ₁₁ H ₁₂ is 100.

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