WO2024128034A1 - Sodium ion secondary battery negative electrode and sodium ion secondary battery - Google Patents

Abstract

Provided is a sodium ion secondary battery negative electrode which makes it possible to improve battery characteristics such as a quick charging/discharging property of a secondary battery.　This sodium ion secondary battery negative electrode contains a negative electrode active substance including a carbon material, and has a peak located in the range of 1800-1850 cm^-1 in a Raman spectrum measured by Raman spectroscopy.

Description

Negative electrode for sodium ion secondary battery and sodium ion secondary battery

The present invention relates to a negative electrode for a sodium ion secondary battery and a sodium ion secondary battery.

Lithium-ion secondary batteries are essential for mobile devices and electric vehicles, and have established a position as a high-capacity, lightweight power source. However, current lithium-ion secondary batteries mainly use flammable organic electrolytes as the electrolyte, raising concerns about the risk of fire. As a method to solve this problem, development of all-solid-state batteries such as all-solid-state lithium-ion batteries that use solid electrolytes instead of organic electrolytes is underway. However, oxide-based solid electrolytes with lithium ion conductivity have the problem that they have low ionic conductivity and are difficult to operate at low temperatures or achieve high output. Therefore, as an alternative, development of all-solid-state sodium-ion secondary batteries that use oxide-based solid electrolytes with sodium ion conductivity is underway (for example, Patent Document 1).

JP 2010-15782 A

However, sodium ion secondary batteries such as those described in Patent Document 1 have the problem that their battery characteristics, such as rapid charge and discharge characteristics, are still insufficient.

The object of the present invention is to provide a negative electrode for a sodium ion secondary battery that can improve battery characteristics such as the rapid charge/discharge characteristics of the secondary battery, and a sodium ion secondary battery using the negative electrode for the sodium ion secondary battery.

We will explain various aspects of negative electrodes for sodium ion secondary batteries and sodium ion secondary batteries that solve the above problems.

The negative electrode for a sodium ion secondary battery according to aspect 1 of the present invention is a negative electrode for a sodium ion secondary battery containing a negative electrode active material including a carbon material, and is characterized in that it has a peak located in the range of 1800 cm ^-1 to 1850 cm ^-1 in a Raman spectrum measured by Raman spectroscopy.

In the negative electrode for a sodium ion secondary battery of aspect 2, in aspect 1, it is preferable that the carbon material is hard carbon.

The negative electrode for a sodium ion secondary battery of aspect 3 is preferably in aspect 1 or aspect 2, and further contains a solid electrolyte.

In the negative electrode for a sodium ion secondary battery of aspect 4, in aspect 3, it is preferable that the solid electrolyte contains at least one selected from the group consisting of β''-alumina, β-alumina, and NASICON crystal.

The negative electrode for a sodium ion secondary battery of aspect 5 is preferably used in an all-solid-state sodium ion secondary battery in any one of aspects 1 to 4.

The sodium ion secondary battery of aspect 6 preferably includes a negative electrode for a sodium ion secondary battery of any one of aspects 1 to 4.

A sodium ion secondary battery according to a seventh aspect of the present invention includes a sodium ion secondary battery negative electrode, and is characterized in that, in a Raman spectrum of the sodium ion secondary battery negative electrode measured by Raman spectroscopy, the peak intensity located in the range of 1800 cm ^-1 to 1850 cm ^-1 increases during charging, and the peak intensity located in the range of 1800 cm ^-1 to 1850 cm ^-1 decreases during discharging.

The present invention provides a negative electrode for a sodium ion secondary battery that can improve battery characteristics such as the rapid charge/discharge characteristics of the secondary battery, and a sodium ion secondary battery using the negative electrode for the sodium ion secondary battery.

FIG. 1 is a schematic cross-sectional view showing a windowed laminate cell used when measuring the Raman spectrum of a negative electrode for a sodium ion secondary battery according to one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing a sodium ion secondary battery according to one embodiment of the present invention. FIG. 3 is a diagram showing a Raman spectrum of the negative electrode constituting the test battery obtained in Example 1 when the charge depth is 0%. FIG. 4 is a diagram showing a Raman spectrum of the negative electrode constituting the test battery obtained in Example 1 when the charge depth is 100%. FIG. 5 is a graph showing the relationship between the charge depth and the voltage of the test battery obtained in Example 1. FIG. 6 is a graph showing the relationship between the depth of charge of the test battery obtained in Example 1 and the intensity at 1816 cm ⁻¹ in the Raman spectrum of the negative electrode. FIG. 7 is a graph showing the relationship between the depth of charge of the test battery obtained in Example 1 and the intensity at 1836 cm ⁻¹ in the Raman spectrum of the negative electrode. FIG. 8 is a graph showing the relationship between the depth of discharge and the voltage of the test battery obtained in Example 1. FIG. 9 is a graph showing the relationship between the depth of discharge of the test battery obtained in Example 1 and the intensity at 1816 cm ⁻¹ in the Raman spectrum of the negative electrode. FIG. 10 is a graph showing the relationship between the depth of discharge of the test battery obtained in Example 1 and the intensity at 1836 cm ⁻¹ in the Raman spectrum of the negative electrode. FIG. 11 is a diagram showing the discharge curve of the test battery obtained in Example 1. FIG. 12 is a diagram showing a Raman spectrum of the negative electrode constituting the test battery obtained in Comparative Example 1 when the depth of charge is 100%. FIG. 13 is a diagram showing the discharge curve of the test battery obtained in Comparative Example 1.

Below, preferred embodiments are described. However, the following embodiments are merely examples, and the present invention is not limited to the following embodiments. In addition, in each drawing, components having substantially the same functions may be referred to by the same reference numerals.

[Negative electrode for sodium ion secondary battery]
The negative electrode for a sodium ion secondary battery of the present invention contains a negative electrode active material containing a carbon material. Also, the negative electrode for a sodium ion secondary battery of the present invention has a peak located in the range of 1800 cm ⁻¹ to 1850 cm ⁻¹ in a Raman spectrum measured by Raman spectroscopy.

The negative electrode for a sodium ion secondary battery of the present invention has the above-mentioned configuration, and therefore can improve the battery characteristics, such as the rapid charge/discharge characteristics, of the secondary battery.

　Conventionally, sodium-ion secondary batteries have had the problem that their battery characteristics, such as rapid charge and discharge characteristics, are still insufficient.

In response to this, the present inventors have found that a negative electrode for a sodium ion secondary battery having a peak in the range of 1800 cm ^-1 to 1850 cm ^-1 in a Raman spectrum measured by Raman spectroscopy can improve battery characteristics such as rapid charge/discharge characteristics of the secondary battery.

The reason for this is unclear, but it is thought that this is because the diffusion coefficient of sodium ions in the negative electrode active material is high, promoting the movement of sodium ions in the negative electrode, or because the amount of expansion and contraction of the particles that accompanies charging and discharging is small, allowing a large contact area (conductive network) to be maintained during charging and discharging, resulting in low internal resistance.

In this specification, the Raman spectrum of the negative electrode for sodium ion secondary batteries measured by Raman spectroscopy (Raman spectroscopy measurement) can be obtained by measuring the measurement range of 100 cm ⁻¹ to 2500 cm ⁻¹ using a laser Raman microscope. As the laser Raman microscope, for example, a model number "RamanTouch" (excitation wavelength: 532 nm) manufactured by Nanophoton Corporation can be used.

In order to further suppress damage to the sample caused by the laser irradiation, it is preferable that the energy density of the laser irradiated to the sample is low, and the laser energy density can be, for example, 100 kW/cm ² or less. Similarly, in order to suppress damage to the sample caused by the laser irradiation, it is preferable that the laser irradiation time to the sample is short, and the laser irradiation time can be, for example, 10 seconds or less.

Furthermore, the Raman spectroscopy measurement of the negative electrode for a sodium ion secondary battery may be performed by operando measurement using a windowed laminate cell 1 as shown in FIG. 1. The windowed laminate cell 1 can be formed by disposing the cell body 2 in a glove box 3, providing a window material 4, and sealing with a sealant 5. The cell body 2 is preferably charged and discharged in an inert gas atmosphere in the glove box 3. As the inert gas, argon gas, nitrogen gas, helium gas, etc. can be used. As the cell body 2, a cell can be used in which the negative electrode 7 (the negative electrode for a sodium ion secondary battery of the present invention) is laminated on one main surface of the solid electrolyte layer 6, and metallic sodium 8 is laminated on the other main surface. At this time, the negative electrode 7 is placed on the window material 4 side. A current collector can be formed on the surface of the negative electrode 7 on the window material 4 side, but it is preferable to provide a measurement area with a diameter of φ1 mm or more where no current collector is formed so that laser light can pass through. As the window material 4, for example, a quartz plate or calcium fluoride can be used. Additionally, the sealing material 5 can be a polyethylene film, a polypropylene film, a lead sealant film, etc.

When the sodium ion secondary battery negative electrode of the present invention is subjected to Raman spectroscopy while charging and discharging using the windowed laminate cell 1 as described above, the peak intensity located in the range of 1800 cm ^-1 to 1850 cm ^-1 increases during charging, and the peak intensity located in the range of 1800 cm ^-1 to 1850 cm ^-1 decreases during discharging.

In addition, when the sodium ion secondary battery negative electrode of the present invention is subjected to Raman spectroscopy while charging and discharging using the windowed laminate cell 1 as described above, the peaks of the D band (near 1360 cm ^-1 ) and the G band (near 1580 cm ^-1 ) of the carbon material shift to the lower wavenumber side during charging, and the peaks of the D band and the G band shift back to their positions before charging during discharging. The reason for this is believed to be that, as described above, the negative electrode has a peak located in the range of 1800 cm ^-1 to 1850 cm ^-1 , which facilitates the diffusion of sodium ions into the carbon material during charging and discharging.

In the negative electrode for a sodium ion secondary battery of the present invention, at a charge depth of 100%, the intensity of the highest peak located in the range of 1800 cm ^-1 to 1850 cm ^-1 normalized with the peak intensity of the D band (near 1360 cm ^-1 ) of the carbon material as 1 is preferably 0.1 or more, more preferably 0.2 or more, and even more preferably 0.4 or more, and is preferably 3.0 or less, more preferably 2.0 or less, and even more preferably 1.0 or less, and may be 0.7 or less, or may be 0.5 or less. In this case, the battery characteristics such as the rapid charge and discharge characteristics of the secondary battery can be further improved.

In addition, in the Raman spectrum measured by Raman spectroscopy, the peak located in the range of 1800 cm ⁻¹ to 1850 cm ⁻¹ is considered to be due to the reductive decomposition product of the solid electrolyte, for example, due to a substance containing sodium and phosphorus.

In addition, in the Raman spectrum measured by Raman spectroscopy, the peak located in the range of 1800 cm ^-1 to 1850 cm ^-1 can be adjusted, for example, by electrochemically reacting metallic sodium in an inert atmosphere with an electrode (negative electrode) in which Na _3.4 Zr ₂ Si _2.4 P _0.6 O ₁₂ powder and a conductive assistant are mixed.

The negative electrode for a sodium ion secondary battery of the present invention is preferably used in an all-solid-state sodium ion secondary battery that uses a solid electrolyte as the electrolyte. However, the negative electrode for a sodium ion secondary battery of the present invention may also be used in a liquid-based sodium ion secondary battery that uses an organic electrolyte as the electrolyte, and is not particularly limited.

The details of the negative electrode for sodium ion secondary batteries are explained below.

The negative electrode for a sodium ion secondary battery (hereinafter also simply referred to as the negative electrode) contains a negative electrode active material. The negative electrode active material includes a carbon material. As the carbon material, hard carbon, soft carbon, etc. can be used. Of these, the carbon material is preferably hard carbon. However, the negative electrode active material may contain metals capable of absorbing sodium, such as tin, bismuth, lead, phosphorus, or alloys of these, or metallic sodium. It is preferable that the negative electrode is not a negative electrode consisting of a single phase of metallic sodium.

The negative electrode may further contain a solid electrolyte and a conductive additive. The ratio of each material in the negative electrode, for example, in mass %, can be: negative electrode active material 60% to 99%, solid electrolyte 1% to 35%, and conductive additive 0% to 20%.

The content of the negative electrode active material in the negative electrode is, in mass %, preferably 60% or more, more preferably 65% or more, even more preferably 70% or more, and is preferably 99% or less, more preferably 93% or less, even more preferably 90% or less. When the content of the negative electrode active material in the negative electrode is within the above range, the battery characteristics such as the charge/discharge capacity of the secondary battery can be improved even more effectively.

The content of the solid electrolyte in the negative electrode is, in mass %, preferably 1% or more, more preferably 5% or more, even more preferably 10% or more, and is preferably 35% or less, more preferably 30% or less, even more preferably 25% or less. When the content of the solid electrolyte in the negative electrode is within the above range, the ionic conductivity can be further improved, and the battery characteristics of the secondary battery can be further effectively improved.

The content of the conductive assistant in the negative electrode is, in mass %, preferably 0% or more, more preferably 0.1% or more, even more preferably 0.5% or more, and is preferably 20% or less, more preferably 10% or less, even more preferably 5% or less. When the content of the conductive assistant in the negative electrode is within the above range, the electronic conductivity can be further improved, and the battery characteristics of the secondary battery can be further effectively improved.

As the solid electrolyte, for example, a sodium ion conductive oxide can be used. Examples of sodium ion conductive oxides include compounds containing at least one selected from Al, Y, Zr, Si, and P, Na, and O. Specific examples of sodium ion conductive oxides include beta-alumina or NASICON crystal, which have excellent sodium ion conductivity. In particular, the sodium ion conductive oxide is preferably at least one sodium ion conductive oxide selected from the group consisting of β''-alumina, β-alumina, and NASICON crystal. It is more preferable that the sodium ion conductive oxide is NASICON crystal. These are even more excellent in that the production cost is low due to the low firing temperature.

Beta alumina has two crystal forms, β-alumina (theoretical formula: Na ₂ O·11Al ₂ O ₃ ) and β″-alumina (theoretical formula: Na ₂ O·5.3Al ₂ O ₃ ). β″-alumina is a metastable substance, and is usually used with Li ₂ O or MgO added as a stabilizer. Since β″-alumina has a higher sodium ion conductivity than β-alumina, it is preferable to use β″-alumina alone or a mixture of β″-alumina and β-alumina, and it is more preferable to use Li ₂ O-stabilized β″-alumina (Na _1.7 Li _0.3 Al _10.7 O ₁₇ ) or MgO-stabilized β″-alumina ((Al _10.32 Mg _0.68 O ₁₆ )(Na _1.68 O)).

_{The NASICON crystals include Na3Zr2Si2PO12 , Na3.2Zr1.3Si2.2P0.7O10.5 , Na3Zr1.6Ti0.4Si2PO12 , Na3Hf2Si2PO12 , Na3.4Zr2Si2.4P0.6O12 , Na3.4Zr1.9Zn0.1Si2.4P0.6O12 , Na3.4Zr1.9Mg0.1Si2.2P0.8O12 , Na2.8Zr2Si2.4P0.6O12 , Na3.4 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Examples of the NASICON crystal include Zr0.9Hf1.4Al0.6Si1.2P1.8O12 , Na3Zr1.7Nb0.24Si2PO12 , Na3.6Ti0.2Y0.7Si2.8O9 , Na3Zr1.88Y0.12Si2PO12 , Na3.12Zr1.88Y0.12Si2PO12 , and Na3.6Zr0.13Yb1.67Si0.11P2.9O12 . As the NASICON crystal , Na3.4Zr2Si2.4P0.6O12 is particularly preferred because of its excellent ionic conductivity . ​ ​}

The average particle size of the solid electrolyte powder is preferably 0.05 μm or more and 3 μm or less, more preferably 0.05 μm or more and less than 1.8 μm, even more preferably 0.05 μm or more and 1.5 μm or less, particularly preferably 0.1 μm or more and 1.2 μm or less, and most preferably 0.1 μm or more and 0.7 μm or less.

As the conductive additive, for example, conductive carbon can be used. Examples of conductive carbon include acetylene black, carbon black, ketjen black, vapor grown carbon fiber (VGCF), etc. The conductive additive is preferably a carbon-based conductive additive made of the above-mentioned materials.

Below, an example of a method for producing a negative electrode for a sodium ion secondary battery according to the present invention will be described.

(Method of Manufacturing Negative Electrode)
First, a paste containing a carbon material precursor and a solid electrolyte is prepared. In preparing the paste, a solid electrolyte precursor is first prepared. At this stage, it is preferable to prepare a solid electrolyte precursor solution. Specific examples of the solid electrolyte precursor and its solution will be described later. Furthermore, a carbon material precursor (a precursor of a carbon material made of hard carbon) is prepared. The carbon material precursor can be an appropriate sugar, biomass, polymer, or the like.

Next, the solid electrolyte precursor solution and the carbon material precursor are mixed and then dried. This results in a powder mixture of the solid electrolyte precursor and the carbon material precursor. Next, the powder mixture is pulverized and further mixed with a conductive assistant and a binder in an organic solvent as required. For example, N-methyl-2-pyrrolidone can be used as the organic solvent. This results in a paste.

In addition, when the powder mixture of the solid electrolyte precursor and the carbon material precursor is used as the first negative electrode component, the paste may contain a second negative electrode component. Examples of the second negative electrode component include other carbon material powders, metals capable of absorbing sodium, such as tin, bismuth, lead, phosphorus, or alloys of these, or metallic sodium. Examples of other carbon material powders include hard carbon after sintering. In this case, the discharge capacity of the secondary battery can be increased, and a negative electrode with superior energy density can be formed.

In this case, the content ratio of the first negative electrode component to the second negative electrode component (first negative electrode component:second negative electrode component) is preferably 9:1 to 2:8, more preferably 8:2 to 3:7, and even more preferably 7:3 to 4:6, by mass. In this case, the first negative electrode component can provide excellent electrode forming ability, thereby further improving the rapid charge/discharge characteristics and charge/discharge cycle performance of the battery.

Next, paste is applied to one main surface of the solid electrolyte layer, which will be described later. In this manner, a lamination process is performed in which the pastes serving as the solid electrolyte layer and the negative electrode material layer are laminated. Next, the laminate of the pastes serving as the solid electrolyte layer and the negative electrode material layer is fired. This allows the negative electrode (negative electrode layer) to be formed on the solid electrolyte layer.

The firing temperature (maximum temperature) during the firing is preferably 600°C or higher, more preferably 650°C or higher, and preferably 1200°C or lower, more preferably 1000°C or lower. The holding time at the firing temperature can be, for example, 10 minutes to 12 hours.

The firing may be performed, for example, under an inert atmosphere. For example, the firing may be performed under an N ₂ , Ar, Ne or He atmosphere, or in a vacuum. The firing may be performed under a reducing atmosphere containing H _2. When firing is performed under an inert atmosphere or a reducing atmosphere, the initial charge/discharge efficiency of the negative electrode active material can be further improved, which is preferable. Note that the atmosphere may contain a small amount of oxygen as long as the negative electrode material layer is not oxidized or oxidatively decomposed during firing. The oxygen concentration may be, for example, 1 ppm or more and 1000 ppm or less, but is not limited thereto.

Carbon material precursor;
When sugar is used as the carbon material precursor, examples of the carbon material precursor include sucrose, cellulose, D-glucose, sucrose, etc. When biomass is used as the carbon material precursor, examples of the carbon material precursor include corn stalk, sorghum stalk, pine cone, mangosteen, argan shell, rice husk, dandelion, cereal straw core, ramie fiber, cotton, kelp, coconut endocarp, etc. When polymer is used as the carbon material precursor, examples of the carbon material precursor include polyacrylonitrile (PAN), pitch, polyvinyl chloride (PVC) nanofiber, polyaniline, sodium polyacrylate, tires (polymers for tires), phosphorus-doped PAN, etc.

Solid electrolyte precursors and solutions thereof;
When the solid electrolyte is beta-alumina, the solid electrolyte precursor can be obtained by mixing, for example, aluminum nitrate, sodium nitrate, lithium nitrate, etc. At this time, the ratio of each of the above materials is adjusted to obtain the composition ratio of the target solid electrolyte.

When the solid electrolyte is a NASICON type crystal or a Na ₅ XSi ₄ O ₁₂ type crystal (X is at least one selected from group 3 transition metal elements, preferably rare earth elements), the solid electrolyte precursor solution may be a solution containing sodium element and transition metal element constituting the solid electrolyte, and carbonate ions. In this solution, the sodium element is contained in the form of sodium ions, and the transition metal element is contained in the form of transition metal ions. The solid electrolyte precursor is, for example, a gelled or dried product of the solid electrolyte precursor solution. And the solid electrolyte is a fired product of the solid electrolyte precursor.

The solid electrolyte precursor solution may be, for example, a solution containing nitrate ions instead of carbonate ions, and is not particularly limited.

In addition, it is preferable that the carbonate ions are bidentate to the transition metal element in the solid electrolyte precursor solution. In this case, the transition metal element tends to exist stably in the solution.

In addition, it is preferable that the solution contains NR ⁴⁺ (wherein each R is independently at least one substituent selected from the group consisting of H, CH ₃ , C ₂ H ₅ and CH ₂ CH ₂ OH) as a counter ion of the sodium ion, which makes it easier for the transition metal element to exist stably in the solution.

The solid electrolyte precursor solution can be obtained, for example, by mixing water glass (sodium silicate), sodium tripolyphosphate, and an aqueous solution of ammoniated zirconium carbonate.

Binders are materials used to bind raw materials (raw material powders) together. Examples of binders include cellulose derivatives such as carboxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxypropyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose, and water-soluble polymers such as polyvinyl alcohol; thermosetting resins such as thermosetting polyimide, phenolic resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; polycarbonate resins such as polypropylene carbonate; polyvinylidene fluoride; and acrylic resins such as polyacrylic acid.

[Sodium ion secondary battery]
2 is a schematic cross-sectional view showing a sodium ion secondary battery according to one embodiment of the present invention. As shown in Fig. 2, the sodium ion secondary battery 10 includes a solid electrolyte layer 20, a positive electrode layer 30, a negative electrode layer 40, a first current collector layer 50, and a second current collector layer 60.

The solid electrolyte layer 20 has a first main surface 20a and a second main surface 20b that face each other. A positive electrode layer 30 is provided on the first main surface 20a of the solid electrolyte layer 20. A first current collector layer 50 is provided on the main surface of the positive electrode layer 30 opposite the solid electrolyte layer 20.

The negative electrode layer 40 is provided on the second main surface 20b of the solid electrolyte layer 20. The negative electrode layer 40 is the above-mentioned negative electrode for a sodium ion secondary battery. In addition, a second current collector layer 60 is provided on the main surface of the negative electrode layer 40 opposite the solid electrolyte layer 20. Note that the first current collector layer 50 and the second current collector layer 60 do not necessarily have to be provided.

The sodium ion secondary battery 10 has a negative electrode layer 40 made of the above-mentioned sodium ion secondary battery negative electrode.

In particular, in the sodium ion secondary battery 10, in the Raman spectrum of the sodium ion secondary battery negative electrode measured by Raman spectroscopy, the peak intensity located in the range of 1800 cm ^-1 to 1850 cm ^-1 increases during charging, and the peak intensity located in the range of 1800 cm ^-1 to 1850 cm ^-1 decreases during discharging.

The inventors have discovered that by using such a negative electrode for a sodium ion secondary battery, it is possible to improve the battery characteristics, such as the rapid charge and discharge characteristics, of the sodium ion secondary battery 10.

The details of each layer in the sodium ion secondary battery of the present invention are described below.

(Solid electrolyte layer)
The solid electrolyte constituting the solid electrolyte layer 20 is preferably formed from a sodium ion conductive oxide. As the sodium ion conductive oxide, those described in the section on the negative electrode for sodium ion secondary batteries can be used.

The solid electrolyte layer 20 can be manufactured by mixing raw material powders, molding the mixed raw material powders, and then firing them. For example, it can be manufactured by forming a green sheet by turning the raw material powders into a slurry, and then firing the green sheet. It may also be manufactured by the sol-gel method.

The thickness of the solid electrolyte layer 20 is preferably in the range of 5 μm to 1000 μm, and more preferably in the range of 10 μm to 200 μm. If the thickness of the solid electrolyte layer 20 is too thin, the mechanical strength decreases and it becomes more susceptible to breakage, which makes it more likely for an internal short circuit to occur. If the thickness of the solid electrolyte layer 20 is too thick, the sodium ion conduction distance associated with charging and discharging becomes longer, which increases the internal resistance and makes it more likely for the discharge capacity and operating voltage to decrease. In addition, the energy density per unit volume of the sodium ion secondary battery also tends to decrease.

(Positive electrode layer)
The positive electrode active material contained in the positive electrode layer 30 is not particularly limited, but is preferably a positive electrode active material made of crystallized glass containing crystals represented by the general formula Na _x M _y P ₂ O _z (1≦x≦2.8, 0.95≦y≦1.6, 6.5≦z≦8, M is at least one selected from the group consisting of Fe, Ni, Co, Mn, and Cr). In particular, it is more preferable that the positive electrode active material is made of crystallized glass containing crystals represented by the general formula Na _x MP ₂ O ₇ (1≦x≦2, M is at least one selected from the group consisting of Fe, Ni, Co, Mn, and Cr). As such a positive electrode active material crystal, for example, Na ₂ FeP ₂ O ₇ , Na ₂ CoP ₂ O ₇ , Na ₂ NiP ₂ O ₇ , etc. can be used.

Crystalline glass refers to glass that has been obtained by heating (firing) a precursor glass containing an amorphous phase to cause crystals to precipitate (crystallize). The entire amorphous phase may be transformed into a crystalline phase, or the amorphous phase may remain. One type of crystal may be precipitated, or two or more types of crystals may be precipitated. For example, it is possible to determine whether or not crystalline glass is crystallized by the peak angle shown by powder X-ray diffraction (XRD).

The positive electrode layer 30 may also contain a solid electrolyte and a conductive additive. The ratio of each material in the positive electrode layer 30, for example, in mass %, can be 60% to 99.9% positive electrode active material, 0% to 30% solid electrolyte, and 0.1% to 10% conductive additive.

As the solid electrolyte and conductive additive, for example, those described above in the section on negative electrodes for sodium ion secondary batteries can be used.

The positive electrode layer 30 can be formed, for example, by forming an electrode material layer containing a positive electrode active material precursor and, if necessary, a solid electrolyte powder and a conductive assistant on one main surface of the solid electrolyte layer 20, and then firing the electrode material layer. The electrode material layer can be obtained, for example, by applying a paste containing a positive electrode active material precursor and, if necessary, a solid electrolyte powder and a conductive assistant, and then drying the paste. Note that the paste may contain a binder, a plasticizer, a solvent, or the like, if necessary. Note that the electrode material layer may be a compressed powder.

The drying temperature of the paste is not particularly limited, but can be, for example, 30°C or higher and 150°C or lower. The drying time of the paste is not particularly limited, but can be, for example, 5 minutes or higher and 600 minutes or lower.

Furthermore, it is preferable that the atmosphere during firing is a reducing atmosphere. The firing temperature (maximum temperature) can be, for example, 400°C to 600°C, and the holding time at that temperature can be, for example, 5 minutes to less than 3 hours.

Positive electrode active material precursor;
The positive electrode active material precursor (positive electrode active material precursor powder) is preferably made of an amorphous oxide material that generates active material crystals by firing. When the positive electrode active material precursor powder is made of an amorphous oxide material, active material crystals are generated during firing, and the material softens and flows to form a dense positive electrode layer 30. When the positive electrode layer 30 contains a solid electrolyte, the positive electrode active material and the solid electrolyte can be integrated. Alternatively, when the positive electrode layer 30 contacts the solid electrolyte layer 20, the two can be integrated. As a result, the ion conduction path is more favorably formed, which is preferable. In the present invention, the "amorphous oxide material" is not limited to a completely amorphous oxide material, but also includes a material that contains some crystals (for example, a crystallinity of 10% or less).

The positive electrode active material precursor powder preferably contains, in mole percent calculated on the oxide basis below, 25% to 55% Na ₂ O , 10% to 30% Fe ₂ O ₃ + Cr ₂ O ₃ + MnO + CoO + NiO, and 25% to 55% P ₂ O ₅ . The reason for limiting the composition in this way is explained below. In the following explanation of the content of each component, "%" means "mol percent" unless otherwise specified.

Na ₂ O is the main component of active material crystals represented by the general formula Na _x M _y P ₂ O _z (M is at least one transition metal element selected from Cr, Fe, Mn, Co and Ni, 1≦x≦2.8, 0.95≦y≦1.6, 6.5≦z≦8). The content of Na ₂ O is preferably 25% to 55%, and more preferably 30% to 50%. When the content of Na ₂ O is within the above range, the charge/discharge capacity of the sodium ion secondary battery 10 can be further increased.

_Fe2O3 , _Cr2O3 , MnO, CoO _and NiO are also _the main components of _the active material crystal represented by _the general formula _NaxMyP2Oz . The content of _Fe2O3 + _Cr2O3 +MnO+CoO+NiO is preferably 10% to 30%, and more preferably 15% to 25%. _When the content of _{Fe2O3 + Cr2O3} + _MnO +CoO+NiO is equal to or greater than _the lower limit, the charge _/ discharge capacity of the sodium ion secondary battery 10 can be further increased. On the other hand, when the content of Fe ₂ O ₃ + Cr ₂ O ₃ + MnO + CoO + NiO is equal to or less than the upper limit, it is possible to make it difficult to precipitate undesired crystals such as Fe ₂ O ₃ , Cr ₂ O ₃ , MnO, CoO or NiO. In order to further improve the cycle characteristics of the sodium ion secondary battery 10, it is preferable to actively contain Fe _{2 O 3. The content of Fe 2 O 3 is preferably} 1% to 30%, more preferably 5% to 30%, even more preferably 10% to 30%, and particularly preferably 15% to 25%. The contents of each of the components Cr ₂ O ₃ , MnO, CoO and NiO are preferably 0% to 30%, more preferably 10% to 30%, and even more preferably 15% to 25%. Furthermore, when at least two or more components selected from Fe ₂ O ₃ , Cr ₂ O ₃ , MnO, CoO and NiO are contained, the total amount thereof is preferably 10% to 30%, and more preferably 15% to 25%.

P ₂ O ₅ is also a main component of the active material crystal represented by the general formula Na _x M _y P ₂ O _z . The content of P ₂ O ₅ is preferably 25% to 55%, and more preferably 30% to 50%. When the content of P ₂ O ₅ is within the above range, the charge/discharge capacity of the sodium ion secondary battery 10 can be further increased.

In addition to the above components, the positive electrode active material precursor powder may contain V ₂ O ₅ , Nb ₂ O ₅ , MgO, Al ₂ O ₃ , TiO ₂ , ZrO ₂ , or Sc ₂ O ₃ . These components have the effect of increasing electrical conductivity (electronic conductivity), and the rapid charge/discharge characteristics of the sodium ion secondary battery 10 are likely to be improved. The total content of the above components is preferably 0% to 25%, and more preferably 0.2% to 10%. When the content of the above components is equal to or less than the upper limit, heterogeneous crystals that do not contribute to the battery characteristics are unlikely to be generated, and the charge/discharge capacity of the sodium ion secondary battery 10 can be further increased.

In addition to the above components, the positive electrode active material precursor powder may contain SiO ₂ , B ₂ O ₃ , GeO ₂ , Ga ₂ O ₃ , Sb ₂ O ₃ , or Bi ₂ O ₃ . By including these components, the glass forming ability is further improved, and it becomes easier to obtain a more homogeneous positive electrode active material precursor powder. The total content of the above components is preferably 0% to 25%, and more preferably 0.2% to 10%. Since these components do not contribute to the battery characteristics, if the content is too high, the charge/discharge capacity of the sodium ion secondary battery 10 tends to decrease.

The positive electrode active material precursor powder is preferably produced by melting and molding a raw material batch. This production method is preferable because it makes it easier to obtain amorphous positive electrode active material precursor powder with excellent homogeneity. Specifically, the positive electrode active material precursor powder can be produced as follows.

First, the raw materials are prepared to obtain a raw material batch having the desired composition. The obtained raw material batch is then melted. The melting temperature may be adjusted as appropriate so that the raw material batch is homogeneously melted. For example, the melting temperature is preferably 800°C or higher, and more preferably 900°C or higher. There is no particular upper limit to the melting temperature, but a melting temperature that is too high can lead to energy loss and evaporation of sodium components, etc., so it is preferably 1500°C or lower, and more preferably 1400°C or lower.

Then, the resulting molten material is molded. There are no particular limitations on the molding method, and for example, the molten material may be poured between a pair of cooling rolls and molded into a film while being rapidly cooled, or the molten material may be poured into a mold and molded into an ingot.

Then, the obtained molded body is pulverized to obtain a positive electrode active material precursor powder. The average particle size of the positive electrode active material precursor powder is preferably 0.01 μm or more and less than 0.7 μm, more preferably 0.03 μm or more and 0.6 μm or less, even more preferably 0.05 μm or more and 0.6 μm or less, and particularly preferably 0.1 μm or more and 0.5 μm or less.

Binder;
The binder is a material for integrating raw materials (raw material powders) together. Examples of the binder include water-soluble polymers such as cellulose derivatives such as carboxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxypropyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and hydroxymethyl cellulose, and polyvinyl alcohol; thermosetting resins such as thermosetting polyimide, phenol resin, epoxy resin, urea resin, melamine resin, unsaturated polyester resin, and polyurethane; polycarbonate resins such as polypropylene carbonate; and polyvinylidene fluoride.

The thickness of the positive electrode layer 30 is preferably 10 μm or more, more preferably 20 μm or more, even more preferably 40 μm or more, particularly preferably 70 μm or more, and most preferably 100 μm or more. In this case, the charge/discharge capacity of the sodium ion secondary battery 10 can be further increased. On the other hand, if the thickness of the positive electrode layer 30 is too thick, the resistance to electronic conduction increases, which may reduce the discharge capacity and operating voltage of the sodium ion secondary battery 10, and the stress due to shrinkage during firing increases, which may lead to peeling. Therefore, the thickness of the positive electrode layer 30 is preferably 1000 μm or less.

The amount of the positive electrode active material carried in the positive electrode layer 30 is preferably 1 mg/ ^cm2 or more, more preferably 2.5 mg/ ^cm2 or more, even more preferably 5 mg/ ^cm2 or more, particularly preferably 9 mg/ ^cm2 or more, and most preferably 12 mg/ ^cm2 or more. In this case, the capacity of the sodium ion secondary battery 10 can be further increased. The upper limit of the amount of the positive electrode active material carried is not particularly limited, but can be, for example, 150 mg/ ^cm2 .

(Negative electrode layer)
As the negative electrode layer 40, the above-mentioned negative electrode for a sodium ion secondary battery can be used.

The thickness of the anode layer 40 is preferably 3 μm or more, more preferably 6 μm or more, even more preferably 10 μm or more, particularly preferably 20 μm or more, and most preferably 30 μm or more. In this case, the charge/discharge capacity of the sodium ion secondary battery 10 can be further increased. On the other hand, if the thickness of the anode layer 40 is too thick, the resistance to electronic conduction increases, and the discharge capacity and operating voltage of the sodium ion secondary battery 10 may decrease, so the thickness of the anode layer 40 is preferably 500 μm or less.

The amount of the negative electrode active material carried in the negative electrode layer 40 is preferably 0.5 mg/ ^cm2 or more, more preferably 1 mg/ ^cm2 or more, even more preferably 2 mg/ ^cm2 or more, particularly preferably 4 mg/ ^cm2 or more, and most preferably 6 mg/ ^cm2 or more. In this case, the capacity of the sodium ion secondary battery 10 can be further increased. The upper limit of the amount of the negative electrode active material carried is not particularly limited, but can be, for example, 100 mg/ ^cm2 .

(First Current Collector Layer and Second Current Collector Layer)
The materials for the first current collector layer 50 and the second current collector layer 60 are not particularly limited, but may be metal materials such as aluminum, titanium, silver, copper, stainless steel, or alloys thereof. The above metal materials may be used alone or in combination. These alloys are alloys containing at least one of the above metals. The thicknesses of the first current collector layer 50 and the second current collector layer 60 are not particularly limited, but may be 0.01 μm or more and 1000 μm or less, respectively.

The method for forming the first current collector layer 50 and the second current collector layer 60 is not particularly limited, and examples of the method include physical vapor phase methods such as vapor deposition or sputtering, and chemical vapor phase methods such as thermal CVD, MOCVD, and plasma CVD. Other methods for forming the first current collector layer 50 and the second current collector layer 60 include plating, the sol-gel method, and liquid phase film formation methods using spin coating. However, it is preferable to form the first current collector layer 50 and the second current collector layer 60 on the positive electrode layer 30 and the negative electrode layer 40, respectively, by the sputtering method, as this provides excellent adhesion.

The present invention will be described in more detail below based on specific examples, but the present invention is not limited to the following examples and can be modified as appropriate within the scope of the gist of the invention.

Example 1
<Preparation of Solid Electrolyte Precursor>
A total of ₂₅ g of water glass (sodium silicate: Na ₂ O.nSiO ₂ ), ammonium zirconium carbonate aqueous solution ((NH ₄ ) ₂ Zr( _OH ) ₂ (CO ₃ ) ₂ ), and sodium tripolyphosphate (Na _{5 P 3} O ₁₀ ) were weighed out so as to obtain a NASICON type crystal with _a composition ratio of Na 3.4 Zr 2 Si 2.4 P _0.6 O 12. These were added to 100 g of pure water and mixed with a planetary mixer. This resulted in a solid electrolyte precursor solution. Next, this solution was gelled by leaving it to stand overnight in a thermostatic bath at 5°C. Thus, a solid electrolyte precursor was prepared.

<Preparation of Mixture of Solid Electrolyte Precursor and Carbon Material Precursor>
A mixture was obtained by mixing sucrose (cane sugar), a hard carbon source, which is a carbon material precursor, and the solid electrolyte precursor obtained above in a weight ratio of 4:1 in a rotary mixer for 10 minutes. Next, this mixture was dried in a thermostatic bath at 60°C for 12 hours, and then vacuum-dried at 60°C for 6 hours. The mixture was then pulverized in an agate mortar and further pulverized in a planetary ball mill to produce a powder with an average particle size of 1 μm.

<Preparation of electrode paste>
The powder mixture of the solid electrolyte precursor and the carbon material precursor obtained above as the first negative electrode component and the hard carbon powder as the second negative electrode component were weighed out so that the mass ratio after firing was 3: 7. A PPC (polypropylene carbonate) binder was added to these in an external ratio of 15 mass%, and the mixture was mixed in an N-methylpyrrolidone solvent using a planetary mixer to prepare an electrode paste.

<Preparation of Battery Components>
A solid electrolyte layer made of β''-alumina having a thickness of 500 μm was separately prepared. The electrode paste obtained above was applied to one main surface of the solid electrolyte layer to a thickness of 70 μm, and dried in a dryer at 50 ° C. to form an electrode-forming material layer. After drying the electrode paste, it was fired at 800 ° C. for 2 hours in a nitrogen atmosphere (N ₂ : 99.99%) to precipitate a mixed phase of a solid electrolyte having a composition of Na _3.4 Zr ₂ Si _2.4 P _0.6 O ₁₂ and a carbon material made of hard carbon, thereby obtaining a sintered electrode that serves as a negative electrode. Next, a current collector made of an aluminum thin film was formed on the sintered electrode by a sputtering method. In addition, in order to enable Raman spectroscopy measurement, a φ2 mm mask was placed on the negative electrode to form a film, so that only this range of the negative electrode was exposed. As a result, a battery member was obtained.

Example 2
An electrode paste was prepared by adding a PPC (polypropylene carbonate) binder to a powder mixture of a solid electrolyte precursor and a carbon material precursor prepared in the same manner as in Example 1 in an external ratio of 15 mass % without adding hard carbon powder, and mixing the mixture in an N-methylpyrrolidone solvent using a planetary mixer.

Example 3
A solid electrolyte precursor prepared in the same manner as in Example 1 was dried in a thermostatic chamber at 60° C. for 12 hours, and then vacuum-dried at 60° C. for 6 hours. The solid electrolyte precursor was then pulverized in an agate mortar and further pulverized in a planetary ball mill to produce a powder having an average particle size of 2 μm.

The solid electrolyte precursor powder and hard carbon powder obtained above were weighed out so that the mass ratio after firing would be 1:1. PPC (polypropylene carbonate) binder was added to these in an external ratio of 15 mass%, and mixed in an N-methylpyrrolidone solvent using a planetary mixer to produce an electrode paste. In other respects, battery components were obtained in the same manner as in Example 1.

Example 4
<Preparation of Solid Electrolyte Precursor>
A total of _{25 g} of water glass (sodium silicate: Na ₂ O.nSiO ₂ ), ammonium zirconium carbonate aqueous solution ((NH _{4 ) 2} Zr ₍ OH) ₂ (CO ₃ ) ₂ ), sodium tripolyphosphate (Na _{5 P 3 O 10} ), and zinc oxide (ZnO) were weighed out so as to obtain a NASICON type crystal with a composition ratio of Na 3.4 Zr 1.9 Zn 0.1 Si 2.4 P 0.6 O _12. These were added to 100 g of pure water and mixed with a planetary mixer. This resulted in a solid electrolyte precursor solution. Next, this solution was gelled by leaving it to stand overnight in a thermostatic bath at 5°C. Thus, a solid electrolyte precursor was prepared.

<Preparation of Mixture of Solid Electrolyte Precursor and Carbon Material Precursor>
A mixture was obtained by mixing sucrose (glucose) as a hard carbon source, which is a carbon material precursor, and the solid electrolyte precursor obtained above in a weight ratio of 4:1 in a rotary mixer for 10 minutes. Next, this mixture was dried in a thermostatic chamber at 60°C for 12 hours, and then vacuum-dried at 60°C for 6 hours. The mixture was then pulverized in an agate mortar and further pulverized in a planetary ball mill to produce a powder with an average particle size of 1 μm.

<Preparation of electrode paste>
The powder of the mixture of the solid electrolyte precursor and the carbon material precursor obtained above as the first negative electrode component and the hard carbon powder as the second negative electrode component were weighed out so as to have a mass ratio of 3 _{:7 after firing. In addition, as the solid electrolyte layer, a solid electrolyte layer having a composition of Na3.4Zr2Si2.4P0.6O12 and} a thickness of 500 _μm was used instead of _β ″ _-alumina . In other respects, a battery member was obtained in the same manner as in Example 1.

Example 5
<Preparation of Solid Electrolyte Precursor>
A total of _{25 g} of water glass (sodium silicate: Na _{2 O.nSiO 2} ), ammonium zirconium carbonate aqueous solution ((NH ₄ ) ₂ Zr ( _OH ) ₂ (CO ₃ ) ₂ ), sodium tripolyphosphate (Na _{5 P 3 O 10} ), and magnesium oxide (MgO) were weighed out so as to obtain a NASICON type crystal with a composition ratio of Na 3.4 Zr 1.9 Mg 0.1 Si 2.2 P 0.8 O _12. These were added to 100 g of pure water and mixed with a planetary mixer. This resulted in a solid electrolyte precursor solution. Next, this solution was gelled by leaving it to stand overnight in a thermostatic bath at 5°C. Thus, a solid electrolyte precursor was prepared.

Furthermore, the powder mixture of the solid electrolyte precursor as the first negative electrode component obtained above and the carbon material precursor obtained in the same manner as in Example 4, and the hard carbon powder as the second negative electrode component were weighed out so that the mass ratio after firing was 7:3. To these, PPC (polypropylene carbonate) binder was added in an external ratio of 15 mass%, and the mixture was mixed in N-methylpyrrolidone solvent using a planetary mixer to prepare an electrode paste.

Further, as the solid electrolyte layer, a solid electrolyte layer having a thickness of 500 μm and a composition of Na _3.4 Zr ₂ Si _2.4 P _0.6 O ₁₂ was used instead of β″-alumina. In other respects, a battery member was obtained in the same manner as in Example 1.

Example 6
<Preparation of Solid Electrolyte Precursor>
In order to obtain a NASICON type crystal with a composition ratio of Na _2.8 Zr ₂ Si _2.4 P _0.6 O ₁₂ , a total of 25 g of water glass (sodium silicate: Na ₂ O.nSiO ₂ ), an aqueous solution of ammonium zirconium carbonate ((NH ₄ ) ₂ Zr (OH) ₂ (CO ₃ ) ₂ ), polyphosphoric acid (condensed phosphoric acid P ₂ O ₅ ), and sodium carbonate (Na ₂ CO ₃ ) were weighed. These were added to 100 g of pure water and mixed with a rotary mixer. This resulted in a solid electrolyte precursor solution. Next, this solution was left to stand overnight in a thermostatic bath at 5°C to gel it. As described above, a solid electrolyte precursor was prepared. Other than that, a battery member was obtained in the same manner as in Example 4.

(Example 7)
A powder mixture of a solid electrolyte precursor and a carbon material precursor as a first negative electrode component obtained in the same manner as in Example 1, and tin powder (average particle size 500 nm) as a second negative electrode component were weighed out so that the mass ratio after firing would be 7:3. A polyacrylic acid (molecular weight 300,000) binder was added to these in an external ratio of 15 mass%, and the mixture was mixed in an N-methylpyrrolidone solvent using a planetary mixer to prepare an electrode paste.

Further, as the solid electrolyte layer, a solid electrolyte layer having a thickness of 500 μm and a composition of Na _3.4 Zr ₂ Si _2.4 P _0.6 O ₁₂ was used instead of β″-alumina. In other respects, a battery member was obtained in the same manner as in Example 1.

(Comparative Example 1)
Polyacrylic acid (molecular weight 250,000) as a carbon material precursor as the first negative electrode component and hard carbon powder as the second negative electrode component were weighed out so that the mass ratio after firing would be 1:9. These were mixed in an N-methylpyrrolidone solvent using a planetary mixer to prepare an electrode paste.

In addition, the above electrode paste was formed to a thickness of 70 μm on β″-alumina of 500 μm as a solid electrolyte layer. After drying at 60° C. for 2 hours, it was fired at 400° C. in a nitrogen atmosphere (N ₂ : 99.99%) to obtain a battery member.

(Comparative Example 2)
A carbon material precursor polyacrylonitrile (molecular weight 150,000) as the first negative electrode component and a hard carbon powder as the second negative electrode component were weighed out so that the mass ratio after firing was 1: 9. A battery member was obtained in the same manner as in Comparative Example 1 in other respects.

(Comparative Example 3)
A carbon material precursor polyacrylic acid (molecular weight 250,000) as the first negative electrode component and a tin powder (average particle size 500 nm) as the second negative electrode component were weighed out so that the mass ratio after firing was 1: 9. A battery member was obtained in the same manner as in Comparative Example 1 in other respects.

[evaluation]
<Preparation of test battery>
A test battery was fabricated in a glove box by attaching metallic sodium as a counter electrode to the other main surface of the solid electrolyte layer of the battery member, and the test battery was sealed in a laminate to fabricate a laminate cell.

<Charge/discharge test>
A charge and discharge test of the test battery was carried out in a thermostatic chamber at 30°C. Specifically, charging (insertion of sodium ions) was carried out at a cut-off voltage of 0.001V, and discharging (desorption of sodium ions) was carried out at a cut-off voltage of 2.5V, thereby measuring the charge capacity and discharge capacity. At this time, the actual capacity was set to the weight of the active material of the produced negative electrode x 300mAhg ^-1 , and the current value was set to be 0.02C rate, and charging and discharging were carried out. In addition, the charge capacity and discharge capacity were measured at a 1C rate in the same manner as above, and the output performance was evaluated.

<Raman spectroscopy measurement>
The test battery was taken out every 5% state of charge (SOC) during the charge/discharge test. The test battery was disassembled and taken out in an argon glove box. The test battery was sealed in an electrochemical cell (manufactured by EC Frontier, model number "VB1400") with a quartz window of 1 mm thickness so that the negative electrode could be seen, and measurement was performed by Raman spectroscopy (Raman spectroscopy measurement). The Raman spectroscopy measurement was performed using a laser Raman microscope (manufactured by nanophoton, model number "Ramantouch") with an excitation wavelength of 532 nm, measuring the measurement range from 100 cm ^-1 to 2500 cm ^-1 .

Figure 3 shows the Raman spectrum when the negative electrode constituting the test battery obtained in Example 1 has a charge depth of 0%. Figure 4 shows the Raman spectrum when the negative electrode constituting the test battery obtained in Example 1 has a charge depth of 100%. Also, Figure 12 shows the Raman spectrum when the negative electrode constituting the test battery obtained in Comparative Example 1 has a charge depth of 100%.

As shown in Figures 3 and 4, in the negative electrode obtained in Example 1, in addition to the peaks of the D band (1360 cm ^-1 ) and G band (1580 cm ^-1 ) of the carbon material, two peaks located in the range of 1800 cm ^-1 to 1850 cm ^-1 are observed in the Raman spectrum. It is also found that the peaks of the D band and G band of the carbon material in Example 1 are shifted to the lower wave number side when the charge depth is 100% compared to when the charge depth is 0%. On the other hand, as shown in Figure 12, in the negative electrode obtained in Comparative Example 1, no peaks located in the range of 1800 cm ^-1 to 1850 cm ^-1 were observed.

FIG. 5 is a diagram showing the relationship between the charge depth and voltage of the test battery obtained in Example 1. FIG. 6 is a diagram showing the relationship between the charge depth of the test battery obtained in Example 1 and the intensity at 1816 cm ⁻¹ in the Raman spectrum of the negative electrode. FIG. 7 is a diagram showing the relationship between the charge depth of the test battery obtained in Example 1 and the intensity at 1836 cm ⁻¹ in the Raman spectrum of the negative electrode. The intensity at 1816 cm ⁻¹ in FIG. 6 and the intensity at 1836 cm ⁻¹ in FIG. 7 are the intensities of the respective peak tops of the two peaks located in the range of 1800 cm ⁻¹ to 1850 cm ⁻¹ in FIG. 3 and FIG. 4. The peak intensities in FIG. 6 and FIG. 7 are intensities normalized with the peak intensity of the D band set to 1, and the maximum value on the vertical axis is 0.5.

As shown in FIGS. 5 to 7, in the negative electrode obtained in Example 1, it is seen that as the depth of charge increases, the peak intensity of the peak located in the range of 1800 cm ^-1 to 1850 cm ^-1 increases.

FIG. 8 is a diagram showing the relationship between the depth of discharge of the test battery obtained in Example 1 and the voltage. FIG. 9 is a diagram showing the relationship between the depth of discharge of the test battery obtained in Example 1 and the intensity at 1816 cm ⁻¹ in the Raman spectrum of the negative electrode. FIG. 10 is a diagram showing the relationship between the depth of discharge of the test battery obtained in Example 1 and the intensity at 1836 cm ⁻¹ in the Raman spectrum of the negative electrode. The intensity at 1816 cm ⁻¹ in FIG. 9 and the intensity at 1836 cm ⁻¹ in FIG. 10 are the intensities of the respective peak tops of the two peaks located in the range of 1800 cm ⁻¹ to 1850 cm ⁻¹ in FIG. 3 and FIG. 4. The peak intensities in FIG. 9 and FIG. 10 are intensities normalized with the peak intensity of the D band set to 1.

As shown in FIGS. 8 to 10, in the negative electrode obtained in Example 1, as the depth of discharge increases, the peak intensity of the peak located in the range of 1800 cm ^-1 to 1850 cm ^-1 decreases.

FIG. 11 shows the discharge curve of the test battery obtained in Example 1. Also, FIG. 13 shows the discharge curve of the test battery obtained in Comparative Example 1.

As shown in Figure 11, the test battery obtained in Example 1 has an increased capacity retention rate (1C discharge capacity/0.02C discharge capacity), and is therefore excellent in battery characteristics such as rapid charge and discharge characteristics. On the other hand, as shown in Figure 13, the test battery obtained in Comparative Example 1 has an insufficient capacity retention rate (1C discharge capacity/0.02C discharge capacity), and is therefore insufficient in battery characteristics such as rapid charge and discharge characteristics.

The evaluation results of the test batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 3 are shown in Tables 1 and 2 below. In Tables 1 and 2, the presence or absence of a peak is determined as follows: when peaks having peak tops in the ranges of 1800 cm ^-1 to 1820 cm ^-1 and 1820 cm ^-1 to 1850 cm ^-1 at a charge depth of 100%, and the intensity normalized to 1, taking the peak intensity of the D band in the Raman spectrum as 1, is 0.1 or more, the presence of a peak is determined; and when it is less than 0.1 or no peak is determined, the absence of a peak is determined.

Reference Signs List 1... Windowed laminate cell 2... Cell body 3... Glove box 4... Window material 5... Sealing material 6, 20... Solid electrolyte layer 7... Negative electrode 8... Metallic sodium 10... Sodium ion secondary battery 20a, 20b... First and second main surfaces 30... Positive electrode layer 40... Negative electrode layer 50, 60... First and second current collector layers

Claims (7)

A negative electrode for a sodium ion secondary battery, comprising a negative electrode active material including a carbon material,
A negative electrode for a sodium ion secondary battery, which has a peak located in the range of 1800 cm ^-1 to 1850 cm ^-1 in a Raman spectrum measured by Raman spectroscopy. The negative electrode for a sodium ion secondary battery according to claim 1, wherein the carbon material is hard carbon. The negative electrode for a sodium ion secondary battery according to claim 1 or 2, further comprising a solid electrolyte. The negative electrode for a sodium ion secondary battery according to claim 3, wherein the solid electrolyte contains at least one selected from the group consisting of β''-alumina, β-alumina, and NASICON crystals. The negative electrode for a sodium ion secondary battery according to claim 1 or 2, which is used in an all-solid-state sodium ion secondary battery. A sodium ion secondary battery comprising the negative electrode for a sodium ion secondary battery according to claim 1 or 2. A negative electrode for a sodium ion secondary battery is provided,
In the Raman spectrum of the sodium ion secondary battery negative electrode measured by Raman spectroscopy, the peak intensity located in the range of 1800 cm ^-1 to 1850 cm ^-1 increases during charging, and the peak intensity located in the range of 1800 cm ^-1 to 1850 cm ^-1 decreases during discharging.

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