Negative electrode for secondary battery and sodium ion secondary battery

Abstract

A negative electrode 100 for a secondary battery of the present disclosure is provided with a negative electrode current collector 110 and a negative electrode active material layer 120 that includes bismuth and nickel. The negative electrode active material layer 120 is provided with a first layer 121 and a second layer 122 that is disposed between the first layer 121 and the negative electrode current collector 110. A first ratio, which is obtained using an electron probe microanalyzer (EPMA), is the ratio of the characteristic X-ray intensity of nickel in the first layer 121 to the characteristic X-ray intensity of nickel in the negative electrode current collector 110 is less than 0.5, and a second ratio that is the ratio of the characteristic X-ray intensity of nickel in the second layer 122 to the characteristic X-ray intensity of nickel in the negative electrode current collector 110 is at least 0.5.

Description

Negative electrode for secondary batteries and sodium-ion secondary batteries

[0001] This disclosure relates to a negative electrode for a secondary battery and a sodium-ion secondary battery equipped with a negative electrode for a secondary battery.

[0002] Sodium-ion rechargeable batteries have several advantages, including a low risk of resource depletion and the potential for excellent input / output characteristics based on high ionic conductivity. Therefore, sodium-ion rechargeable batteries are attracting attention as a next-generation alternative to lithium-ion rechargeable batteries.

[0003] Patent Document 1 describes a sodium-ion secondary battery comprising a negative electrode capable of intercalating and releasing sodium ions and containing bismuth as a single component or as a main component, and a non-aqueous electrolyte containing sodium ions and a positive electrode. The negative electrode described in Patent Document 1 includes a negative electrode mixture containing bismuth that is coated on a current collector. Patent Document 2 describes a battery comprising an electrode having an active material layer containing BiNi.

[0004] Japanese Patent Publication No. 2007-123236, International Publication No. 2022 / 224571

[0005] This disclosure provides a negative electrode for a secondary battery that is suitable for suppressing the degradation of cycle characteristics.

[0006] This disclosure provides a negative electrode for a secondary battery, comprising: a negative electrode current collector; and a negative electrode active material layer containing Bi and Ni, wherein the negative electrode active material layer comprises a first layer and a second layer disposed between the first layer and the negative electrode current collector, and a first ratio, which is the ratio of the intensity of the Ni characteristic X-ray of the first layer to the intensity of the Ni characteristic X-ray of the negative electrode current collector obtained by an electron beam microanalyzer, is less than 0.5, and a second ratio, which is the ratio of the intensity of the Ni characteristic X-ray of the second layer to the intensity of the Ni characteristic X-ray of the negative electrode current collector, is 0.5 or more.

[0007] According to this disclosure, it is possible to provide a negative electrode for a secondary battery that is suitable for suppressing the deterioration of cycle characteristics.

[0008] Figure 1A is a cross-sectional view showing the schematic configuration of the negative electrode in the first embodiment. Figure 1B is a partially enlarged view of Figure 1A. Figure 2 is a cross-sectional view showing the schematic configuration of the sodium-ion secondary battery in the second embodiment. Figure 3 is a cross-sectional view showing the schematic configuration of the positive electrode of the sodium-ion secondary battery in Figure 2. Figure 4 is the powder X-ray diffraction pattern of the negative electrode active material layer of Examples 1 to 5. Figure 5 is a cross-sectional image of the negative electrode of Example 1 obtained by electron beam microanalyzer (EPMA) measurement. Figure 6 is a cross-sectional image of the negative electrode of Example 2 obtained by EPMA measurement. Figure 7 is a cross-sectional image of the negative electrode of Example 3 obtained by EPMA measurement. Figure 8 is a cross-sectional image of the negative electrode of Example 5 obtained by EPMA measurement.

[0009] (Knowledge forming the basis of this disclosure) Sodium-ion secondary batteries are known to have a lower energy density compared to, for example, lithium-ion secondary batteries. Methods to improve the capacity of the negative electrode have been investigated in order to improve the energy density of sodium-ion secondary batteries. For example, bismuth (Bi) has a higher energy density than carbon, which is a conventional negative electrode active material. On the other hand, Bi undergoes a large volume change when it reacts with sodium ions, which degrades the cycle characteristics of sodium-ion secondary batteries. Patent Document 1 has not adequately considered how to suppress the degradation of cycle characteristics of sodium-ion secondary batteries equipped with a negative electrode using Bi.

[0010] The inventors of this invention diligently studied the configuration of the negative electrode and electrolyte in order to realize a sodium secondary battery in which the deterioration of cycle characteristics is suppressed, and came up with the present invention.

[0011] Embodiments of this disclosure will be described below with reference to the drawings. This disclosure is not limited to the embodiments described below.

[0012] (First Embodiment) [Negative Electrode] Figure 1A is a cross-sectional view showing the schematic configuration of the negative electrode 100 in the first embodiment of the present disclosure. The negative electrode 100 comprises a negative electrode current collector 110 and a negative electrode active material layer 120. The negative electrode active material layer 120 contains Bi and Ni.

[0013] In this embodiment, the negative electrode active material layer 120 comprises a first layer 121 and a second layer 122. The second layer 122 is positioned between the first layer 121 and the negative electrode current collector 110, in contact with the negative electrode current collector 110. The first layer 121 and the second layer 122 are continuous. The first ratio, which is the ratio of the intensity of the Ni characteristic X-rays of the first layer 121 to the intensity of the Ni characteristic X-rays of the negative electrode current collector 110 obtained by electron beam microanalyzer (EPMA), is less than 0.5. The second ratio, which is the ratio of the intensity of the Ni characteristic X-rays of the second layer 122 to the intensity of the Ni characteristic X-rays of the negative electrode current collector 110 obtained by EPMA, is 0.5 or more.

[0014] As described above, the negative electrode active material layer 120 comprises a first layer 121 with a first ratio of less than 0.5 and a second layer 122 with a second ratio of 0.5 or more. This means that a second layer 122 with a higher Ni content than the first layer 121 exists near the interface between the negative electrode current collector 110 and the negative electrode active material layer 120. According to the inventors' studies, Bi has a higher energy density than the active materials used in conventional sodium-ion secondary batteries. On the other hand, Bi undergoes a large volume change when it reacts with sodium ions. Therefore, batteries using Bi as the active material tended to have reduced cycle characteristics. As a result of diligent research, the inventors have found that by using a negative electrode 100 in which a second layer 122 with a higher Ni content than the first layer 121 exists near the interface between the negative electrode current collector 110 and the negative electrode active material layer 120, excellent initial discharge capacity can be achieved in a sodium-ion secondary battery, and the reduction in cycle characteristics can be suppressed.

[0015] Here, EPMA is a method that involves irradiating a material with an accelerated electron beam, focusing on the characteristic X-ray spectrum to detect and identify constituent elements in a minute region irradiated by the electron beam, and analyzing the ratio (concentration) of the constituent elements. In this embodiment, for example, the first ratio and the second ratio can be determined by the following method. First, the negative electrode 100 is cut parallel to the thickness direction to prepare a negative electrode sample of approximately 1 cm × approximately 1 cm with the cross-section exposed. The negative electrode sample is set in the EPMA apparatus, and an EPMA measurement is performed using Ni-Kα rays as the characteristic X-rays.

[0016] The first and second ratios can be calculated by processing the cross-sectional image obtained by EPMA measurement, for example, as follows: For example, the count of Ni characteristic X-rays is measured for each of several points (e.g., 10 points) on the negative electrode current collector 110 within a 40 μm × 30 μm field of view. The average of the measured counts is taken as the intensity of Ni characteristic X-rays in the negative electrode current collector 110. Similarly, the count of Ni characteristic X-rays is measured for each of several points (e.g., 10 points) on the first layer 121. The average of the measured counts is taken as the intensity of Ni characteristic X-rays in the first layer 121. The count of Ni characteristic X-rays is measured for each of several points (e.g., 10 points) on the second layer 122. The average of the measured counts is taken as the intensity of Ni characteristic X-rays in the second layer 122. Based on the intensity of the Ni characteristic X-rays of the negative electrode current collector 110, the intensity of the Ni characteristic X-rays of the first layer 121, and the intensity of the Ni characteristic X-rays of the second layer 122, the first ratio and the second ratio can be determined.

[0017] Figure 1B is an enlarged view of section IB of Figure 1A. As shown in Figure 1B, in the negative electrode 100, the interface between the negative electrode current collector 110 and the second layer 122, and the interface between the first layer 121 and the second layer 122 may not be flat. Therefore, in this embodiment, the thickness of each layer is measured at multiple locations (for example, 10 locations) of the cross-sectional image obtained by EPMA measurement, and the average of the measured thicknesses is considered to be the thickness of each layer. For example, when the thickness of the first layer 121 is defined as T1, T1 is the average of the thicknesses T11, T12, T13, ... at multiple locations of the first layer 121. When the thickness of the second layer 122 is defined as T2, T2 is the average of the thicknesses T21, T22, T23, ... at multiple locations of the second layer 122. The thickness of the negative electrode active material layer 120 and the thickness of the negative electrode current collector 110 are determined in the same manner.

[0018] In this embodiment, the midpoints of the measured thicknesses are used as multiple points (for example, 10 points) for each layer when determining the intensity of the Ni characteristic X-rays of the negative electrode current collector 110, the first layer 121, and the second layer 122. For example, for the first layer 121, the count of Ni characteristic X-rays is measured at each of the midpoints of thickness T11 (P11), thickness T12 (P12), thickness T13 (P13), etc., and the average of the measured counts is taken as the intensity of the Ni characteristic X-rays of the first layer 121. For the second layer 122, the count of Ni characteristic X-rays is measured at each of the midpoints of thickness T21 (P21), thickness T22 (P22), thickness T23 (P23), etc., and the average of the measured counts is taken as the intensity of the Ni characteristic X-rays of the second layer 122.

[0019] Let the first ratio be defined as X1 and the second ratio as X2. In this case, the ratio of X1 to X2 (X1 / X2) may be in the range of 0.4 or more and 0.8 or less. With a negative electrode 100 that satisfies 0.4 ≤ X1 / X2 ≤ 0.8, the deterioration of the cycle characteristics in the sodium-ion secondary battery is easily suppressed.

[0020] The ratio (X1 / X2) may be in the range of greater than 0.45 and less than or equal to 0.8. With a negative electrode 100 that satisfies 0.45 < X1 / X2 ≤ 0.8, the deterioration of the cycle characteristics in the sodium-ion secondary battery is more easily suppressed.

[0021] The ratio (X1 / X2) may be in the range of 0.6 or more and 0.7 or less. With a negative electrode 100 that satisfies 0.6 ≤ X1 / X2 ≤ 0.7, the deterioration of the cycle characteristics in the sodium-ion secondary battery is more easily suppressed.

[0022] The ratio (X1 / X2) may be in the range of 0.5 or more and 0.7 or less. With a negative electrode 100 that satisfies 0.5 ≤ X1 / X2 ≤ 0.7, the deterioration of cycle characteristics in sodium-ion secondary batteries is more easily suppressed.

[0023] When the thickness of the negative electrode active material layer 120 is defined as T, T may be in the range of 0.5 µm or more and 10 µm or less. According to the negative electrode active material layer 120 satisfying 0.5 µm ≤ T ≤ 10 µm, a negative electrode 100 suitable for suppressing deterioration of cycle characteristics is easily realized.

[0024] T may be in the range of 0.5 µm or more and 9 µm or less, 0.5 µm or more and 8 µm or less, 0.5 µm or more and 7 µm or less, 0.5 µm or more and 6 µm or less, and further 0.5 µm or more and 5 µm or less.

[0025] T may be in the range of 0.5 µm or more and 4 µm or less. According to the negative electrode active material layer 120 satisfying 0.5 µm ≤ T ≤ 4 µm, a negative electrode 100 suitable for suppressing deterioration of cycle characteristics is more easily realized.

[0026] As described above, when the thickness of the first layer 121 is defined as T1 and the thickness of the second layer 122 is defined as T2, in the present embodiment, T1 is larger than T2. According to the negative electrode active material layer 120 satisfying T1 > T2, a negative electrode 100 suitable for suppressing deterioration of cycle characteristics is easily realized.

[0027] The ratio of T2 to T (T2 / T) is, for example, less than 0.5.

[0028] The ratio (T2 / T) may be in the range of 0.01 or more and 0.3 or less. According to the negative electrode active material layer 120 satisfying 0.01 ≤ T2 / T ≤ 0.3, a negative electrode 100 suitable for suppressing deterioration of cycle characteristics is easily realized.

[0029] The ratio (T2 / T) may be in the range of 0.02 or more and 0.3 or less, or may be in the range of 0.03 or more and 0.3 or less.

[0030] The ratio (T2 / T) may be in the range of 0.04 or more and 0.3 or less. According to the negative electrode active material layer 120 satisfying 0.04 ≤ T2 / T ≤ 0.3, a negative electrode 100 suitable for suppressing deterioration of cycle characteristics is more easily realized.

[0031] A ratio (T2 / T) may be in the range of more than 0.04 and 0.3 or less. According to the negative electrode active material layer 120 satisfying 0.04 < T2 / T ≤ 0.3, it is further easier to realize a negative electrode 100 suitable for suppressing deterioration of cycle characteristics.

[0032] T1 may be in the range of more than 0.5 µm and less than 10 µm. According to the negative electrode active material layer 120 satisfying 0.5 µm < T1 < 10 µm, it is easy to realize a negative electrode 100 suitable for suppressing deterioration of cycle characteristics.

[0033] T2 may be in the range of 0.05 µm or more and 0.5 µm or less. According to the negative electrode active material layer 120 satisfying 0.05 µm ≤ T2 ≤ 0.5 µm, it is easy to realize a negative electrode 100 suitable for suppressing deterioration of cycle characteristics.

[0034] As described above, the negative electrode active material layer 120 contains Bi and Ni. The negative electrode active material layer 120 may contain an alloy including Bi and Ni. Examples of the alloy including Bi and Ni include BiNi, Bi3Ni, and the like. BiNi and Bi3Ni may be intermetallic compounds formed between Bi and Ni.

[0035] The negative electrode active material layer 120 may contain at least one selected from the group consisting of BiNi and Bi3Ni. According to such a configuration, deterioration of cycle characteristics in a sodium ion secondary battery is easily suppressed.

[0036] That the negative electrode active material layer 120 contains BiNi and / or Bi3Ni can be confirmed, for example, by the presence of a peak in a powder X-ray diffraction pattern of the negative electrode active material layer 120. Specifically, when a peak attributed to BiNi exists in the powder X-ray diffraction pattern, it can be determined that the negative electrode active material layer 120 contains BiNi. When a peak attributed to Bi3Ni exists in the powder X-ray diffraction pattern, it can be determined that the negative electrode active material layer 120 contains Bi3Ni. When a peak attributed to BiNi and a peak attributed to Bi3Ni both exist in the powder X-ray diffraction pattern, it can be determined that the negative electrode active material layer 120 contains BiNi and Bi3Ni.

[0037] The negative electrode active material layer 120 may contain only BiNi, only Bi3Ni, or both BiNi and Bi3Ni.

[0038] The negative electrode active material layer 120 may contain at least one selected from the group consisting of BiNi and Bi3Ni as its main component. With such a configuration, the deterioration of the cycle characteristics in the sodium-ion secondary battery is more easily suppressed. In this embodiment, "the negative electrode active material layer 120 contains at least one selected from the group consisting of BiNi and Bi3Ni as its main component" is synonymous with "the content ratio of at least one selected from the group consisting of BiNi and Bi3Ni in the negative electrode active material layer 120 is 50% by mass or more." The content ratio of at least one selected from the group consisting of BiNi and Bi3Ni in the negative electrode active material layer 120 can be determined, for example, by calculating the ratio of the compounds contained by performing Rietveld analysis on the powder X-ray diffraction pattern of the negative electrode active material layer 120.

[0039] The negative electrode active material layer 120 contains a negative electrode active material having the property of intercalating and releasing sodium ions. The Bi and Ni contained in the negative electrode active material layer 120 function as negative electrode active material. In other words, the negative electrode active material layer 120 contains Bi and Ni as negative electrode active material. With this configuration, the deterioration of the cycle characteristics in sodium ion secondary batteries is easily suppressed.

[0040] The negative electrode active material may contain at least one selected from the group consisting of BiNi and Bi3Ni as its main component. With such a configuration, the deterioration of the cycle characteristics in the sodium-ion secondary battery is more easily suppressed. In this embodiment, "the negative electrode active material contains at least one selected from the group consisting of BiNi and Bi3Ni as its main component" is synonymous with "the content ratio of at least one selected from the group consisting of BiNi and Bi3Ni in the negative electrode active material is 50% by mass or more."

[0041] The negative electrode active material layer 120 may contain only at least one selected substantially from the group consisting of BiNi and Bi3Ni as the negative electrode active material. With such a configuration, the deterioration of cycle characteristics in sodium-ion secondary batteries is more easily suppressed. In this embodiment, "the negative electrode active material layer 120 contains only at least one selected substantially from the group consisting of BiNi and Bi3Ni as the negative electrode active material" means that in the negative electrode active material layer 120, the content of other active materials other than the at least one selected from the group consisting of BiNi and Bi3Ni is 1% by mass or less.

[0042] Bi is a metallic element that alloys with sodium. On the other hand, Ni does not alloy with sodium, so it is presumed that alloys containing Ni reduce the load on the crystal structure of the negative electrode active material during the desorption and insertion of sodium atoms during charging and discharging, thereby suppressing the decrease in the capacity retention rate of sodium-ion secondary batteries. When at least one selected from the group consisting of BiNi and Bi3Ni functions as the negative electrode active material, sodium is adsorbed during charging by Bi forming an alloy with sodium. That is, a sodium-bismuth alloy is generated in the negative electrode active material layer 120 during charging of the sodium-ion secondary battery. The generated sodium-bismuth alloy includes at least one selected from the group consisting of Na3Bi and NaBi. In other words, during charging of the sodium-ion secondary battery, the negative electrode active material layer 120 includes at least one selected from the group consisting of Na3Bi and NaBi. During discharge of the sodium-ion secondary battery, sodium is released from the sodium-bismuth alloy, and the sodium-bismuth alloy returns to BiNi and / or Na3Bi.

[0043] It is presumed that BiNi, as the negative electrode active material, reacts during charging and discharging of sodium-ion secondary batteries as follows. Note that the following example of reaction is for the case where the sodium-bismuth alloy produced during charging is Na3Bi. Charging: BiNi + 3Na + +3e - → Na3Bi+Ni Discharge: Na3Bi+Ni → BiNi+3Na + +3e -

[0044] Bi₃Ni as a negative electrode active material is presumed to react as follows, for example, during charging and discharging of a sodium ion secondary battery. Note that the following reaction example is a case where the sodium bismuth alloy generated during charging is Na₃Bi. (First cycle) Charging: Bi₃Ni + 9Na + + 9e - → 3Na₃Bi + Ni Discharging: 3Na₃Bi + Ni → 2Bi + BiNi + 9Na + + 9e - (Second and subsequent cycles) Charging: 2Bi + BiNi + 9Na + + 9e - → 3Na₃Bi + Ni Discharging: 3Na₃Bi + Ni → 2Bi + BiNi + 9Na + + 9e -

[0045] In the present embodiment, the negative electrode active material layer 120 does not contain a solid electrolyte.

[0046] In the present embodiment, the negative electrode active material layer 120 is arranged such that the second layer 122 is in direct contact with the surface of the negative electrode current collector 110. The negative electrode active material layer 120 may be in the form of a thin film. The negative electrode active material layer 120 may be composed of, for example, at least one selected from the group consisting of BiNi and Bi₃Ni formed into a thin film. Hereinafter, such a thin film is referred to as a "Bi-Ni thin film".

[0047] The negative electrode active material layer 120 composed of a Bi-Ni thin film can be produced, for example, by electroplating. The method for producing the negative electrode 100 in the present embodiment may include producing the negative electrode active material layer 120 by electroplating. An example of the production method will be described below.

[0048] The method for producing the negative electrode 100 includes, for example, forming a Bi plating layer on a Ni-containing negative electrode current collector 110 by an electroplating method, and performing a heat treatment on the negative electrode current collector 110 and the Bi plating layer to diffuse Ni contained in the negative electrode current collector 110 into the Bi plating layer, thereby forming a Bi-Ni thin film on the negative electrode current collector 110.

[0049] The heating temperature of the negative electrode current collector 110 and the Bi plating layer during the heat treatment is, for example, 200°C or higher. The heating temperature may be 240°C or higher, 300°C or higher, or even 350°C or higher. The upper limit of the heating temperature is, for example, 500°C.

[0050] The heating time for the negative electrode current collector 110 and the Bi plating layer during the heat treatment is, for example, 10 hours or more. The heating time may be 12 hours or more, 18 hours or more, or even 24 hours or more. The upper limit for the heating time is, for example, 60 hours.

[0051] The manufacturing method for the negative electrode 100 will be explained in more detail below.

[0052] First, the substrate for electroplating is prepared. In the case of the negative electrode 100, for example, the negative electrode current collector 110 serves as the substrate. As an example, a current collector containing Ni is prepared as the negative electrode current collector 110. As the current collector containing Ni, for example, nickel foil can be used. After pre-degreasing the nickel foil with an organic solvent, one side is masked and the foil is degreased by immersion in an acidic solvent to activate the surface of the nickel foil. The activated nickel foil is connected to a power supply so that current can be applied. The nickel foil connected to the power supply is immersed in a bismuth plating bath. As the bismuth plating bath, for example, Bi3 + An organic acid bath containing ions and organic acids is used. Then, by applying a current to the nickel foil while controlling the current density and application time, Bi is electroplated onto the surface of the unmasked nickel foil. After electroplating, the nickel foil is recovered from the plating bath, the masking is removed, and then it is washed with pure water and dried. By these methods, a Bi plating layer is created on the surface of the nickel foil. The bismuth plating bath used to create the Bi plating layer is not particularly limited and can be appropriately selected from known bismuth plating baths capable of depositing a thin film of elemental Bi. In the bismuth plating bath, an organic sulfonic acid bath, a gluconic acid and ethylenediaminetetraacetic acid (EDTA) bath, or a citric acid and EDTA bath can be used as the organic acid bath. In addition, a sulfuric acid bath may be used as the bismuth plating bath, for example. Additives may also be added to the bismuth plating bath.

[0053] The negative electrode active material layer 120 may be a heat-treated plated layer. The negative electrode active material layer 120 may be a heat-treated plated layer provided in direct contact with the surface of the negative electrode current collector 110. In other words, the negative electrode active material layer 120 may be a layer formed by heat-treating a Bi plated layer formed on the Ni-containing negative electrode current collector 110. With such a configuration, the deterioration of the cycle characteristics in the sodium-ion secondary battery is more easily suppressed.

[0054] If the negative electrode active material layer 120 is a heat-treated plating layer provided in direct contact with the surface of the negative electrode current collector 110, the negative electrode active material layer 120 adheres firmly to the negative electrode current collector 110. This suppresses the deterioration of the current collection characteristics of the negative electrode 100 that occurs when the negative electrode active material layer 120 repeatedly expands and contracts. As a result, the cycle characteristics of the sodium-ion secondary battery are improved. Furthermore, if the negative electrode active material layer 120 is the heat-treated plating layer described above, the negative electrode active material layer 120 contains a high density of Bi that alloys with Na, thus enabling higher capacity.

[0055] The negative electrode active material layer 120 may contain materials other than the negative electrode active material.

[0056] The negative electrode active material layer 120 may contain a conductive material.

[0057] Examples of conductive materials include carbon materials, metals, inorganic compounds, and conductive polymers. Examples of carbon materials include graphite; carbon black such as acetylene black and Ketjen black; carbon whiskers; needle coke; and carbon fibers. Examples of graphite include natural graphite and artificial graphite. Examples of natural graphite include lump graphite and flake graphite. Examples of metals include copper, nickel, aluminum, silver, and gold. Examples of inorganic compounds include tungsten carbide, titanium carbide, tantalum carbide, molybdenum carbide, titanium boride, and titanium nitride. These materials may be used individually or in combination.

[0058] The negative electrode active material layer 120 may contain a binder.

[0059] Examples of binders include fluororesins, thermoplastics, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, polyacrylic acid (PAA) and its derivatives, sodium polyacrylate, styrene butadiene rubber (SBR), and natural butyl rubber (NBR). Examples of fluororesins include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and fluororubber. Examples of thermoplastics include polypropylene and polyethylene. These materials may be used individually or in combination.

[0060] The material of the negative electrode current collector 110 is, for example, a single metal or alloy. More specifically, it may be a single metal or alloy containing at least one selected from the group consisting of copper, chromium, nickel, titanium, platinum, gold, aluminum, tungsten, iron, and molybdenum. The negative electrode current collector 110 may also be stainless steel.

[0061] The negative electrode current collector 110 may contain Ni. With this configuration, the deterioration of the cycle characteristics in the sodium-ion secondary battery is easily suppressed.

[0062] The negative electrode current collector 110 may be in the form of a plate or foil. From the viewpoint of easily ensuring high conductivity, the negative electrode current collector 110 may be a metal foil, or a metal foil containing Ni. Examples of metal foils containing Ni include nickel foil and nickel alloy foil. The Ni content in the metal foil may be 50% by mass or more, or 80% by mass or more. In particular, the metal foil may be nickel foil containing substantially only Ni as the metal. The negative electrode current collector 110 may be a metal foil formed of a metal other than Ni, with a Ni layer, such as a Ni plating layer, formed on its surface.

[0063] The negative electrode current collector 110 may be a laminated film in which multiple layers are stacked.

[0064] The thickness of the negative electrode 100 is, for example, in the range of 10 μm to 500 μm. If the thickness of the negative electrode 100 is 10 μm or more, a sufficient energy density can be ensured in the sodium-ion secondary battery. If the thickness of the negative electrode 100 is 500 μm or less, the sodium-ion secondary battery can operate at high power.

[0065] (Second Embodiment) [Sodium-ion secondary battery] Figure 2 is a cross-sectional view showing the schematic configuration of a sodium-ion secondary battery 500 according to the second embodiment of the present disclosure. The sodium-ion secondary battery 500 (hereinafter sometimes simply referred to as battery 500) comprises a negative electrode 100, a positive electrode 200, and an electrolyte 300 as described in the first embodiment. The positive electrode 200 comprises a positive electrode current collector 210 and a positive electrode active material layer 220.

[0066] As shown in Figure 2, the battery 500 comprises a container 10 and an electrode group 40. The electrode group 40 has a wound structure. The electrode group 40 is housed in the container 10. The electrode group 40 includes a negative electrode 100, a positive electrode 200, and a pair of separators 70.

[0067] The electrode group 40 is impregnated with electrolyte 300. The opening of the container 10 is sealed with a sealing plate 20. One end of the negative electrode lead 130 is connected to the negative electrode 100. The other end of the negative electrode lead 130 is connected to the bottom surface of the container 10. One end of the positive electrode lead 230 is connected to the positive electrode 200. The other end of the positive electrode lead 230 is connected to the back surface of the sealing plate 20. An insulating packing 30 is placed around the sealing plate 20. Insulating rings 80 are placed on the top and bottom surfaces of the electrode group 40, respectively.

[0068] During charging and discharging of the battery 500, the metal ions absorbed and released in the negative electrode active material layer 120 of the negative electrode 100 and the positive electrode active material layer 220 of the positive electrode 200 are sodium ions.

[0069] In this embodiment, the container 10 has negative polarity and the sealing plate 20 has positive polarity. However, the container 10 may have positive polarity and the sealing plate 20 may have negative polarity.

[0070] In the battery 500, the negative electrode active material layer 120 of the negative electrode 100 contains Bi and Ni. The electrolyte 300 contains two or more non-aqueous solvents, including vinylene carbonate. As a result of diligent research, the inventors have found that the battery 500 having the above configuration achieves excellent initial discharge capacity and suppresses deterioration of cycle characteristics. It is known that the electrolyte undergoes reductive decomposition at the electrodes during battery operation, forming a polymer film on the electrode surface. The polymer film formed by the electrolyte 300 in this embodiment suppresses the peeling of the negative electrode active material from the negative electrode current collector 110 even when a volume change occurs in the negative electrode active material layer 120, compared to polymer films formed by conventional electrolytes. As a result, deterioration of the negative electrode 100 is suppressed. In addition, the polymer film formed by the electrolyte 300 in this embodiment is less likely to cause resistance in the negative electrode active material layer 120, and excellent electrical characteristics are maintained in the negative electrode active material layer 120. According to the inventors' studies, the electrolyte 300 exhibits the above-mentioned effects particularly when the negative electrode active material layer 120 contains Bi and Ni as negative electrode active materials.

[0071] [Electrolyte] The electrolyte 300 contains two or more non-aqueous solvents and sodium salts dissolved in two or more non-aqueous solvents. The concentration of sodium salts in the electrolyte 300 is, for example, 0.5 mol / liter or more and 2.5 mol / liter or less. As described above, the two or more non-aqueous solvents include vinylene carbonate.

[0072] The two or more non-aqueous solvents may further contain chain-like carbonates (chain-like carbonate esters). With such a configuration, the deterioration of the cycle characteristics in battery 500 is more easily suppressed. The chain-like carbonates are not particularly limited. Examples of chain-like carbonates include dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate.

[0073] The electrolyte 300 may contain vinylene carbonate and at least one chain-like carbonate selected from the above as a non-aqueous solvent. The non-aqueous solvent contained in the electrolyte 300 may consist only of vinylene carbonate and chain-like carbonate.

[0074] The electrolyte 300 may contain vinylene carbonate and methyl ethyl carbonate as non-aqueous solvents. The non-aqueous solvent contained in the electrolyte 300 may consist only of vinylene carbonate and methyl ethyl carbonate.

[0075] The ratio of the volume of vinylene carbonate to the volume of two or more non-aqueous solvents contained in the electrolyte 300 is, for example, 5% to 80%.

[0076] The ratio of the volume of vinylene carbonate to the volume of two or more non-aqueous solvents contained in the electrolyte 300 may be 5% to 70%, 5% to 60%, or 5% to 50%. With such a configuration, the cycle characteristics of the battery 500 are improved.

[0077] The ratio of the volume of vinylene carbonate to the volume of two or more non-aqueous solvents contained in the electrolyte 300 may be 5% or more and less than 50%. With such a configuration, the cycle characteristics of the battery 500 are further improved.

[0078] The lower limit of the ratio of the volume of vinylene carbonate to the volume of two or more non-aqueous solvents contained in the electrolyte 300 may be 6%, 7%, 8%, 9%, or even 10%. The upper limit of the ratio of the volume of vinylene carbonate to the volume of two or more non-aqueous solvents contained in the electrolyte 300 may be 40%.

[0079] The electrolyte 300 may further contain other aprotic solvents as additives, excluding vinylene carbonate and linear carbonates. Examples of other aprotic solvents include cyclic carbonate solvents, cyclic ether solvents, linear ether solvents, cyclic ester solvents, linear ester solvents, and fluorine solvents. Examples of cyclic carbonate solvents include fluoroethylene carbonate, ethylene carbonate, propylene carbonate, and butylene carbonate. Examples of cyclic ether solvents include tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, and 1,3-dioxolane. Examples of linear ether solvents include 1,2-dimethoxyethane and 1,2-diethoxyethane. An example of a cyclic ester solvent is γ-butyrolactone. An example of a linear ester solvent is methyl acetate. Examples of fluorine solvents include methyl fluoropropionate, fluorobenzene, fluoroethylmethyl carbonate, and fluorodimethylene carbonate.

[0080] The ratio of the mass of other aprotic solvents to the mass of electrolyte 300 is, for example, 0.5% to 10%.

[0081] The electrolyte 300 may contain fluoroethylene carbonate as an additive. With such a configuration, the deterioration of the cycle characteristics in the battery 500 is easily suppressed.

[0082] The ratio of the mass of fluoroethylene carbonate to the mass of the electrolyte 300 may be 1% or more. With such a configuration, the deterioration of cycle characteristics is more easily suppressed.

[0083] Examples of sodium salts include NaPF6, NaBF4, NaN(SO2F)2, NaN(SO2CF3)2, NaPO2F2, and NaSO3F. One or more combinations of these can be used as the sodium salt. These sodium salts can dissolve in two or more of the non-aqueous solvents mentioned above. From the viewpoint of solubility, cost, and battery characteristics, one or more combinations of NaPF6, NaN(SO2F)2, NaPO2F2, and NaSO3F are desirable. For example, combinations of NaPF6 and NaN(SO2F)2, NaN(SO2F)2 and NaPO2F2, and combinations of NaN(SO2F)2 and NaSO3F can be used.

[0084] [Positive Electrode] Figure 3 is a cross-sectional view showing the schematic configuration of the positive electrode 200 of the battery 500. As described above, the positive electrode 200 comprises a positive electrode current collector 210 and a positive electrode active material layer 220. The positive electrode active material layer 220 may be disposed in direct contact with the surface of the positive electrode current collector 210.

[0085] The positive electrode active material layer 220 contains a material capable of intercepting and releasing sodium ions. This material is, for example, a positive electrode active material. The positive electrode active material layer 220 contains a positive electrode active material.

[0086] Examples of positive electrode active materials include sodium-containing transition metal oxides, sodium-containing transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, and transition metal sulfides. The positive electrode active material may contain sodium-containing transition metal oxides, or may consist solely of sodium-containing transition metal oxides. Sodium-containing transition metal oxides can achieve a high average discharge voltage.

[0087] The sodium-containing transition metal oxide may have a composition represented by the following formula (1).

[0088] Na x M y O (2-z) ...Formula (1)

[0089] In equation (1), x, y, and z satisfy 0 < x ≤ 1.2, 0 < y ≤ 1, and 0 ≤ z ≤ 0.1, and M includes at least one selected from the group consisting of Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, and B.

[0090] Sodium-containing transition metal oxides having the composition represented by formula (1) are desirable from the viewpoint of achieving a high average discharge voltage.

[0091] In formula (1), M may include at least one selected from the group consisting of Ti, Fe, and Ni, and Mn. Sodium-containing transition metal oxides having such a composition can achieve a high average discharge voltage.

[0092] In formula (1), M may contain Ti, Ni, and Mn, or Fe, Ni, and Mn. Sodium-containing transition metal oxides having such a composition can achieve a high average discharge voltage. Examples of sodium-containing transition metal oxides having such a composition include NaNi. 1 / 2 Mn 1 / 6 Ti 1 / 3 O2, NaNi 1 / 3 Mn 1 / 3 Fe 1 / 3 O2, NaNi 2 / 3 Mn 1 / 6 Fe 1 / 6 O2 is one example.

[0093] In formula (1), M includes Fe and Mn, but may not include Ni. Sodium-containing transition metal oxides having such a composition can achieve a high average discharge voltage. Examples of sodium-containing transition metal oxides having such a composition include NaMn. 1 / 2 Fe 1 / 2 O2 is one example.

[0094] In equation (1), x, y, and z may satisfy x=1, y=1, and z=0. Sodium-containing transition metal oxides having such a composition can achieve a high average discharge voltage.

[0095] The average particle size of the positive electrode active material may be in the range of 1 μm to 20 μm. The average particle size of the positive electrode active material may be 2 μm or more, 3 μm or more, 4 μm or more, and even 5 μm or more. The average particle size of the positive electrode active material may be 19 μm or less, 18 μm or less, 17 μm or less, 16 μm or less, 15 μm or less, 14 μm or less, 13 μm or less, 12 μm or less, 11 μm or less, and even 10 μm or less. In this disclosure, unless otherwise specified, the average particle size is the median diameter (D50) at which the cumulative volume in the volume-based particle size distribution is 50%.

[0096] The positive electrode active material layer 220 may contain a solid electrolyte. Known solid electrolytes used in sodium-ion secondary batteries can be used as the solid electrolyte. For example, sulfide solid electrolytes and oxide solid electrolytes may be used. For example, Na3PS4 can be used as a sulfide solid electrolyte. For example, β-alumina can be used as an oxide solid electrolyte.

[0097] The thickness of the positive electrode active material layer 220 is not particularly limited. For example, the thickness of the positive electrode active material layer 220 is in the range of 0.1 μm to 200 μm.

[0098] The positive electrode active material layer 220 may contain a conductive material for the purpose of enhancing electronic conductivity.

[0099] The positive electrode active material layer 220 may contain a binder.

[0100] The same materials that can be used for the negative electrode active material layer 120 may be used as the conductive material and binder.

[0101] Examples of materials for the positive electrode current collector 210 include metal materials. Examples of metal materials include copper, stainless steel, iron, and aluminum.

[0102] The thickness of the positive electrode 200 is, for example, in the range of 10 μm to 500 μm. If the thickness of the positive electrode 200 is 10 μm or more, a sufficient energy density can be ensured in the sodium-ion secondary battery. If the thickness of the positive electrode 200 is 500 μm or less, the sodium-ion secondary battery can operate at high power.

[0103] [Separator] The separator 70 has sodium ion conductivity. The material of the separator 70 is not particularly limited as long as the passage of sodium ions is permitted. The material of the separator 70 may be at least one selected from the group consisting of solid electrolytes, gel electrolytes, ion exchange resin membranes such as sodium cation exchange resins, semipermeable membranes and porous membranes. If the separator 70 is made of these materials, the safety of the battery 500 can be sufficiently ensured. As for the solid electrolyte, sulfide solid electrolytes such as Li2S-P2S5 and Li7La3Zr2O 12 Examples of oxide solid electrolytes include (LLZ). Examples of gel electrolytes include gel electrolytes containing fluororesins such as PVdF. Examples of ion exchange resin membranes include cation exchange membranes and anion exchange membranes. Examples of porous membranes include porous membranes made of polyolefin resin and porous membranes made of glass paper obtained by weaving glass fibers into a nonwoven fabric.

[0104] [Container] Container 10 is a metal container such as aluminum, aluminum alloy, or stainless steel. Container 10 may have a cylindrical shape or a rectangular tube shape.

[0105] The electrode group 40, consisting of a negative electrode 100, an electrolyte 300, and a positive electrode 200 as its basic components, is sealed inside a sealed container 10 to prevent contamination from air and moisture. The electrode group 40 may be wound in a cylindrical shape or in an elliptical shape.

[0106] The shape of the sodium-ion secondary battery 500 is not particularly limited. Various shapes such as cylindrical and rectangular can be used for the sodium-ion secondary battery 500.

[0107] (Other Embodiments) (Note) The above description of embodiments discloses the following technologies.

[0108] (Technical 1) A negative electrode for a secondary battery comprising: a negative electrode current collector; and a negative electrode active material layer containing Bi and Ni, wherein the negative electrode active material layer comprises a first layer and a second layer disposed between the first layer and the negative electrode current collector; and a first ratio, which is the ratio of the intensity of the Ni characteristic X-ray of the first layer to the intensity of the Ni characteristic X-ray of the negative electrode current collector obtained by an electron beam microanalyzer, is less than 0.5, and a second ratio, which is the ratio of the intensity of the Ni characteristic X-ray of the second layer to the intensity of the Ni characteristic X-ray of the negative electrode current collector, is 0.5 or more.

[0109] According to the negative electrode for secondary batteries of Technology 1, excellent initial discharge capacity is achieved in sodium-ion secondary batteries, and the deterioration of cycle characteristics is suppressed.

[0110] (Technology 2) The negative electrode for secondary batteries according to Technology 1, wherein the negative electrode active material layer includes at least one selected from the group consisting of BiNi and Bi3Ni. With such a configuration, the deterioration of cycle characteristics in sodium-ion secondary batteries is easily suppressed.

[0111] (Technology 3) The negative electrode for a secondary battery according to Technology 1 or 2, wherein the negative electrode active material layer includes a negative electrode active material, and the negative electrode active material includes at least one selected from the group consisting of BiNi and Bi3Ni as a main component. With such a configuration, the deterioration of cycle characteristics in a sodium-ion secondary battery is more easily suppressed.

[0112] (Technology 4) The negative electrode for a secondary battery according to any one of the technologies 1 to 3, wherein the thickness of the negative electrode active material layer is in the range of 0.5 μm to 10 μm. With such a configuration, a negative electrode suitable for suppressing the deterioration of cycle characteristics is easily realized.

[0113] (Technical 5) The negative electrode for a secondary battery according to any one of Technical 1 to 4, wherein the thickness of the negative electrode active material layer is in the range of 0.5 μm or more and 4 μm or less. With such a configuration, a negative electrode suitable for suppressing the deterioration of cycle characteristics is more easily realized.

[0114] (Technical 6) A negative electrode for a secondary battery according to any one of Technical 1 to 5, wherein the thickness of the first layer is greater than the thickness of the second layer. With such a configuration, a negative electrode suitable for suppressing a decrease in cycle characteristics is easily realized.

[0115] (Technical 7) A negative electrode for a secondary battery according to any one of Technical 1 to 6, wherein the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer is in the range of 0.01 or more and 0.3 or less. With such a configuration, a negative electrode suitable for suppressing the deterioration of cycle characteristics is easily realized.

[0116] (Technical 8) A negative electrode for a secondary battery according to any one of Technical 1 to 7, wherein the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer is in the range of 0.04 or more and 0.3 or less. With such a configuration, a negative electrode suitable for suppressing the deterioration of cycle characteristics is more easily realized.

[0117] (Technical 9) A negative electrode for a secondary battery according to any one of Technical 1 to 8, wherein the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer is in the range of more than 0.04 and less than or equal to 0.3. With such a configuration, a negative electrode suitable for suppressing the deterioration of cycle characteristics can be realized more easily.

[0118] (Technical 10) A negative electrode for a secondary battery according to any one of Technical 1 to 9, wherein when the first ratio is defined as X1 and the second ratio is defined as X2, the ratio of X1 to X2 is in the range of 0.4 or more and 0.8 or less. With such a configuration, the deterioration of cycle characteristics in a sodium-ion secondary battery is easily suppressed.

[0119] (Technical 11) The negative electrode for a secondary battery according to Technical 10, wherein the ratio of X1 to X2 is in the range of more than 0.45 and less than or equal to 0.8. With such a configuration, the deterioration of the cycle characteristics in a sodium-ion secondary battery is more easily suppressed.

[0120] (Technical 12) A negative electrode for a secondary battery according to Technical 10 or 11, wherein the ratio of X1 to X2 is in the range of 0.6 or more and 0.7 or less. With such a configuration, the deterioration of cycle characteristics in a sodium-ion secondary battery is more easily suppressed.

[0121] (Technical 13) A negative electrode for a secondary battery according to any one of Technical 10 to 12, wherein the ratio of X1 to X2 is in the range of 0.5 or more and 0.7 or less. With such a configuration, the deterioration of cycle characteristics in a sodium-ion secondary battery is more easily suppressed.

[0122] (Technical 14) The negative electrode current collector is a negative electrode for a secondary battery according to any one of Technical 1 to 13, comprising Ni. With such a configuration, the deterioration of cycle characteristics in a sodium-ion secondary battery is easily suppressed.

[0123] (Technical 15) The negative electrode for a secondary battery according to any one of Technical 1 to 14, wherein the negative electrode active material layer is a heat-treated plating layer. With such a configuration, the deterioration of the cycle characteristics in a sodium-ion secondary battery is more easily suppressed.

[0124] (Technical 16) A sodium ion secondary battery comprising a negative electrode, a positive electrode, and an electrolyte, wherein the negative electrode is a negative electrode for a secondary battery as described in any one of Technical 1 to 15, and the electrolyte contains two or more non-aqueous solvents, the two or more non-aqueous solvents containing vinylene carbonate.

[0125] According to the sodium-ion secondary battery of Technology 16, excellent initial discharge capacity is achieved, and the deterioration of cycle characteristics is suppressed.

[0126] The details of this disclosure are disclosed below with reference to examples and reference examples. The following examples are illustrative and the disclosure is not limited to these examples.

[0127] [Regarding the effect of the negative electrode] The effect of a negative electrode comprising a negative electrode current collector and a negative electrode active material layer including a first layer with a first ratio (X1) of less than 0.5 and a second layer with a second ratio (X2) of 0.5 or more was investigated.

[0128] (Example 1) <Preparation of negative electrode> As a pretreatment, nickel foil (10 cm x 10 cm, thickness: 10 μm) was pre-degreased with an organic solvent, then one side was masked and degreased by immersion in an acidic solvent to activate the surface of the nickel foil. In 1.0 mol / L of methanesulfonic acid, bismuth methanesulfonate was added as a soluble bismuth salt (Bi3). + A plating bath was prepared by adding ions to a concentration of 0.18 mol / L. The activated nickel foil was connected to a power supply so that current could be applied, and then immersed in the plating bath. Subsequently, the current density was set to 2 A / dm 2 By controlling the process, Bi was electroplated to a thickness of 5 μm onto the surface of an unmasked nickel foil. After electroplating, the nickel foil was recovered from the acidic bath, the masking was removed, and it was washed and dried with pure water. Then, the nickel foil electroplated with Bi was heat-treated in an electric furnace under an argon atmosphere at 240°C for 12 hours. This resulted in a laminate consisting of a negative electrode current collector made of nickel foil and a negative electrode active material layer placed in direct contact with the surface of the negative electrode current collector. The obtained laminate was punched out to 2 cm × 2 cm to produce the negative electrode of Example 1. In Example 1, by performing heat treatment on the nickel foil electroplated with Bi, Ni contained in the negative electrode current collector was diffused into the Bi plating layer, and a negative electrode active material layer containing Bi and Ni was formed.

[0129] <Preparation of Test Cell> The prepared negative electrode was used as the working electrode. A 0.34 mm thick Na metal was used as the counter electrode. The working electrode corresponds to the negative electrode of a secondary battery. The Na metal was double-coated with a microporous separator (Asahi Kasei Corporation, Cellguard 3401). An electrolyte was obtained by dissolving NaPF6 at a concentration of 1.0 mol / L in a non-aqueous solvent. The non-aqueous solvent consisted only of vinylene carbonate (VC) and methyl ethyl carbonate (EMC). In the non-aqueous solvent, the volume ratio of VC to EMC was 3:7. Fluoroethylene carbonate (FEC) was added as an additive to the obtained electrolyte. The mass ratio of FEC to the mass of the electrolyte was 1%. The test cell of Example 1 was assembled using the working electrode, counter electrode, and electrolyte.

[0130] The test cell fabricated is a unipolar test cell using a working electrode and a counter electrode, and is used to test the performance of one electrode in a secondary battery. Specifically, the electrode under test is used as the working electrode, and a suitable amount of active material sufficient to support the reaction of the working electrode is used as the counter electrode. Since this test cell mainly tests the performance of the negative electrode, a large excess of Na metal was used as the counter electrode, as is commonly done. The negative electrode whose performance has been tested using such a test cell can be used as a sodium-ion secondary battery by combining it with a positive electrode active material, such as a sodium-containing transition metal oxide, as described in the above embodiment.

[0131] (Example 2) A nickel foil electroplated with Bi was heat-treated at 330°C for 12 hours in an electric furnace under an argon atmosphere. The negative electrode and test cell for Example 2 were prepared using the same method as in Example 1.

[0132] (Example 3) A nickel foil electroplated with Bi was heat-treated at 400°C for 12 hours in an electric furnace under an argon atmosphere. The negative electrode and test cell for Example 3 were prepared using the same method as in Example 1.

[0133] (Example 4) A nickel foil electroplated with Bi was heat-treated at 400°C for 60 hours in an electric furnace under an argon atmosphere. The negative electrode and test cell for Example 4 were prepared using the same method as in Example 1.

[0134] (Example 5) A nickel foil electroplated with Bi was heat-treated at 500°C for 12 hours in an electric furnace under an argon atmosphere. The negative electrode and test cell of Example 5 were prepared by the same method as in Example 1.

[0135] (Example 6) Bi was electroplated to a thickness of 10 μm on the surface of nickel foil that was not masked during electroplating. The nickel foil electroplated with Bi was heat-treated in an electric furnace under an argon atmosphere at 400°C for 12 hours. The negative electrode and test cell of Example 6 were prepared by the same method as in Example 1, except for these steps.

[0136] (Example 7) Bi was electroplated to a thickness of 1 μm on the surface of nickel foil that was not masked during electroplating. The nickel foil electroplated with Bi was heat-treated in an electric furnace under an argon atmosphere at 400°C for 12 hours. The negative electrode and test cell of Example 7 were prepared by the same method as in Example 1, except for these steps.

[0137] (Comparative Example 1) A solution containing Bi and Ni powders and a solvent was applied to the surface of an unmasked nickel foil to a thickness of 5 μm to form a coating film. The coating film was dried at 80°C for 12 hours. This resulted in a laminate composed of a negative electrode current collector made of nickel foil and a negative electrode active material layer placed in direct contact with the surface of the negative electrode current collector. In other words, Bi electroplating and subsequent heat treatment were not performed in Comparative Example 1. Except for this, the negative electrode and test cell of Comparative Example 1 were prepared by the same method as in Example 1.

[0138] (Comparative Example 2) No heat treatment was performed on the nickel foil electroplated with Bi. Otherwise, the negative electrode and test cell of Comparative Example 2 were prepared by the same method as in Example 1. In Comparative Example 2, since no heat treatment was performed on the nickel foil electroplated with Bi, the Ni contained in the negative electrode current collector did not diffuse into the Bi plating layer, and a negative electrode active material layer containing Bi and Ni was not formed.

[0139] Table 1 shows the configuration of the negative electrodes for Examples 1 to 7 and Comparative Examples 1 to 2.

[0140] <Thickness Measurement> For the negative electrodes of Examples 1 to 7 and Comparative Examples 1 to 2, the thickness of the negative electrode active material layer (T), the thickness of the first layer (T1), and the thickness of the second layer (T2) were determined based on the method described above. To measure the thickness of each layer, the thickness was measured at five locations on the cross-sectional image obtained by EPMA measurement described later, and the average of the measured thicknesses was taken as the thickness of each layer. The results are shown in Table 1.

[0141]

[0142] <Powder X-ray Diffraction Measurement> Powder X-ray diffraction measurements were performed on the pulverized negative electrode active material layers of Examples 1 to 5. The measurement conditions for the powder X-ray diffraction measurements were as follows.

[0143] Equipment name: MiniFlex 600 (manufactured by Rigaku Corporation) X-ray source: Cu-Kα Tube voltage: 40kV Tube current: 15mA Scan speed: 10deg / min Scan step: 0.01deg

[0144] Figure 4 shows the powder X-ray diffraction patterns of the negative electrode active material layers of Examples 1 to 5, along with the X-ray diffraction patterns of BiNi powder and Bi3Ni powder. As can be seen from Figure 4, it is understood that the negative electrode active material layer of Example 1 contained Bi3Ni. It is understood that the negative electrode active material layer of Example 2 contained both Bi3Ni and BiNi. It is understood that the negative electrode active material layers of Examples 3 to 5 contained BiNi.

[0145] <EPMA Measurement> For the negative electrodes of Examples 1 to 7 and Comparative Examples 1 to 2, the first ratio (X1), the second ratio (X2), and the ratio of X1 to X2 (X1 / X2) were determined based on the method described above. The measurement conditions for EPMA measurement were as follows. To measure the intensity of Ni characteristic X-rays in each layer, the count of Ni characteristic X-rays was measured at 10 points within a 40 μm × 30 μm field of view of the cross-sectional image obtained by EPMA measurement, and the average of the measured counts was taken as the intensity of Ni characteristic X-rays in each layer. The results are shown in Table 2.

[0146] Equipment name: JXA-8530F Plus (manufactured by JEOL) Acceleration voltage: 15kV Electron dose (irradiation current): 0.05μA Measurement time per point: 30ms Measurement range per point (1 pixel): 0.12μm Characteristic X-ray: Ni-Kα rays (1.66Å)

[0147] Figures 5 to 8 show cross-sectional images (10,000x magnification) of the negative electrodes of Examples 1 to 3 and 5 obtained by EPMA measurement. Figures 5 to 8 are cross-sectional images near the interface between the negative electrode current collector and the negative electrode active material layer.

[0148]

[0149] <Charge-Discharge Test> The following charge-discharge tests were performed on the test cells of Examples 1 to 7 and Comparative Examples 1 to 2 under conditions of 25°C. 0.8 mA (0.2 mA / cm²) 2 Charging was performed to 0V and discharging to 1.5V using a constant current value of ). This was considered one cycle, and the initial charge capacity and initial discharge capacity were evaluated. The initial efficiency was calculated using the formula (initial discharge capacity / initial charge capacity) × 100. The results are shown in Table 3.

[0150] <Charge-Discharge Cycle Test> For the test cells of Examples 1 to 7 and Comparative Examples 1 to 2, charging and discharging were repeatedly performed under the same conditions as in the charge-discharge test, with each cycle counting as one charge-discharge cycle. The discharge capacity after 10 cycles was evaluated. The capacity retention rate after 10 cycles was calculated using the formula (discharge capacity after 10 cycles / initial discharge capacity) × 100. The results are shown in Table 3.

[0151]

[0152] ≪Discussion≫ As can be seen from the results of the charge-discharge test and charge-discharge cycle test, the test cells of Examples 1 to 7 showed relatively high initial discharge capacity, and both the initial efficiency and the capacity retention rate after 10 cycles were excellent. In other words, in the test cells of Examples 1 to 7, excellent initial discharge capacity was achieved, and the deterioration of cycle characteristics was suppressed. This is thought to be because in the test cells of Examples 1 to 7, a second layer with a higher Ni content than the first layer existed near the interface between the negative electrode current collector and the negative electrode active material layer in the negative electrode. The test cells of Examples 2 to 5 and 7, in which the negative electrode active material layer contained BiNi, showed particularly suppressed deterioration of cycle characteristics compared to the test cell of Example 1, in which the negative electrode active material layer contained Bi3Ni. Note that the test cell of Comparative Example 1 did not operate after 2 cycles, so it was not possible to evaluate the discharge capacity after 10 cycles.

[0153] [Regarding the effectiveness of the battery] The effectiveness of a battery in which the negative electrode active material layer contains Bi and Ni, and the electrolyte contains two or more non-aqueous solvents including vinylene carbonate, was investigated.

[0154] (Sample 1) <Fabrication of the negative electrode> A nickel foil electroplated with Bi was heat-treated at 400°C for 24 hours in an electric furnace under an argon atmosphere. The negative electrode of Sample 1 was fabricated in the same manner as in Example 1. In Sample 1, the heat treatment of the nickel foil electroplated with Bi caused the Ni contained in the negative electrode current collector to diffuse into the Bi plating layer, forming a negative electrode active material layer containing Bi and Ni.

[0155] <Preparation of Test Cells> An electrolyte was obtained by dissolving NaPF6 at a concentration of 1.0 mol / L in a non-aqueous solvent. The non-aqueous solvent consisted only of vinylene carbonate (VC) and methyl ethyl carbonate (EMC). In the non-aqueous solvent, the volume ratio of VC to EMC was 0.5:9.5. Except for this, the test cell for Sample 1 was prepared by the same method as in Example 1.

[0156] (Sample 2) The negative electrode from Sample 1 was used as the negative electrode. In the non-aqueous solvent contained in the electrolyte, the volume ratio of VC to EMC was 1:9. Except for this, the test cell for Sample 2 was prepared using the same method as for Sample 1.

[0157] (Sample 3) The negative electrode from Sample 1 was used as the negative electrode. In the non-aqueous solvent contained in the electrolyte, the volume ratio of VC to EMC was 2:8. Except for this, the test cell for Sample 3 was prepared using the same method as for Sample 1.

[0158] (Sample 4) The negative electrode from Sample 1 was used as the negative electrode. In the non-aqueous solvent contained in the electrolyte, the volume ratio of VC to EMC was 3:7. Except for this, the test cell for Sample 4 was prepared using the same method as for Sample 1.

[0159] (Sample 5) The negative electrode from Sample 1 was used as the negative electrode. In the non-aqueous solvent contained in the electrolyte, the volume ratio of VC to EMC was 4:6. Except for this, the test cell for Sample 5 was prepared using the same method as for Sample 1.

[0160] (Sample 6) The negative electrode from Sample 1 was used as the negative electrode. In the non-aqueous solvent contained in the electrolyte, the volume ratio of VC to EMC was 5:5. Except for this, the test cell for Sample 6 was prepared using the same method as for Sample 1.

[0161] (Sample 7) The negative electrode from Sample 1 was used as the negative electrode. In the non-aqueous solvent contained in the electrolyte, the volume ratio of VC to EMC was 8:2. Except for this, the test cell for Sample 7 was prepared using the same method as for Sample 1.

[0162] (Sample 8) <Negative electrode preparation> No heat treatment was performed on the nickel foil electroplated with Bi. Except for this, the negative electrode of Sample 8 was prepared using the same method as Sample 1. In Sample 8, since no heat treatment was performed on the nickel foil electroplated with Bi, the Ni contained in the negative electrode current collector did not diffuse into the Bi plating layer, and a negative electrode active material layer containing Bi and Ni was not formed.

[0163] <Preparation of Test Cell> The negative electrode from Sample 8 was used as the negative electrode. In the non-aqueous solvent contained in the electrolyte, the volume ratio of VC to EMC was 2:8. Except for this, the test cell for Sample 8 was prepared using the same method as for Sample 1.

[0164] (Sample 9) <Fabrication of the negative electrode> Instead of nickel foil, copper foil (10 cm x 10 cm, thickness: 10 μm) was used. Except for this, the negative electrode of Sample 9 was fabricated using the same method as Sample 1.

[0165] <Preparation of Test Cell> The negative electrode from Sample 9 was used as the negative electrode. In the non-aqueous solvent contained in the electrolyte, the volume ratio of VC to EMC was 2:8. Except for this, the test cell for Sample 9 was prepared using the same method as for Sample 1.

[0166] (Sample 10) <Fabrication of the negative electrode> Instead of nickel foil, copper foil (10 cm x 10 cm, thickness: 10 μm) was used. As a pretreatment, the copper foil was pre-degreased with an organic solvent, then degreased by masking one side and immersing it in an acidic solvent to activate the surface of the copper foil. A plating bath was prepared by mixing 100 g / L sulfuric acid, 100 g / L cresol sulfonic acid, and 2 g / L β-naphthol, and adding 50 g / L of stannous sulfate as a soluble tin salt. The activated copper foil was connected to a power supply so that current could be applied, and then immersed in the plating bath. After that, the current density was set to 2 A / dm 2 By controlling the process, Sn was electroplated onto the surface of an unmasked copper foil to a thickness of approximately 5 μm. After electroplating, the copper foil was recovered from the acidic bath, the masking was removed, and it was washed with pure water and dried. Subsequently, the Sn-electroplated copper foil was heat-treated at 400°C for 24 hours in an electric furnace under an argon atmosphere. This resulted in a laminate consisting of a negative electrode current collector made of copper foil and a negative electrode active material layer placed in direct contact with the surface of the negative electrode current collector. The obtained laminate was punched out to 2 cm × 2 cm to produce the negative electrode for sample 10.

[0167] <Preparation of Test Cell> The negative electrode from Sample 10 was used as the negative electrode. In the non-aqueous solvent contained in the electrolyte, the volume ratio of VC to EMC was 2:8. Except for this, the test cell for Sample 10 was prepared using the same method as for Sample 1.

[0168] Table 4 shows the configurations of the test cells for samples 1 through 10.

[0169]

[0170] <Charge-Discharge Test and Charge-Discharge Cycle Test> Charge-discharge tests and charge-discharge cycle tests were performed on test cells 1 to 10 under the same conditions as those performed on test cells 1 to 7 and comparative examples 1 to 2. The results are shown in Table 5.

[0171]

[0172] ≪Discussion≫ As can be seen from the results of the charge-discharge test and charge-discharge cycle test, the test cells of Samples 1 to 7 showed relatively high initial discharge capacity, and both the initial efficiency and capacity retention rate after 10 cycles were excellent. In other words, in the test cells of Samples 1 to 7, excellent initial discharge capacity and initial efficiency were achieved, and the deterioration of cycle characteristics was suppressed. This is thought to be because the negative electrode of the test cells of Samples 1 to 7 was equipped with an electrolyte containing a negative electrode active material layer containing Bi and Ni, and two or more non-aqueous solvents containing vinylene carbonate. In the test cells of Samples 1 to 5, where the ratio of the volume of vinylene carbonate to the volume of non-aqueous solvent was 5% or more and less than 50%, the capacity retention rate after 10 cycles was 70% or more, and the deterioration of cycle characteristics was particularly suppressed. The test cell of Sample 10 did not operate in the charge-discharge test. This is thought to be because, in the test cell of Sample 10, the Sn6Cu5 generated in the negative electrode active material layer did not function as a negative electrode active material.

[0173] The technology disclosed herein is useful for sodium-ion secondary batteries.

Claims

1. A negative electrode for a secondary battery, comprising: a negative electrode current collector; and a negative electrode active material layer containing Bi and Ni, wherein the negative electrode active material layer comprises a first layer and a second layer disposed between the first layer and the negative electrode current collector; and a first ratio, which is the ratio of the intensity of the Ni characteristic X-rays of the first layer to the intensity of the Ni characteristic X-rays of the negative electrode current collector obtained by an electron beam microanalyzer, is less than 0.5, and a second ratio, which is the ratio of the intensity of the Ni characteristic X-rays of the second layer to the intensity of the Ni characteristic X-rays of the negative electrode current collector, is 0.5 or more.

2. The negative electrode for a secondary battery according to claim 1, wherein the negative electrode active material layer comprises at least one selected from the group consisting of BiNi and Bi3Ni.

3. The negative electrode for a secondary battery according to claim 1, wherein the negative electrode active material layer comprises a negative electrode active material, and the negative electrode active material comprises at least one selected from the group consisting of BiNi and Bi3Ni as a main component.

4. The negative electrode for a secondary battery according to claim 1, wherein the thickness of the negative electrode active material layer is in the range of 0.5 μm or more and 10 μm or less.

5. The negative electrode for a secondary battery according to claim 4, wherein the thickness of the negative electrode active material layer is in the range of 0.5 μm or more and 4 μm or less.

6. The negative electrode for a secondary battery according to claim 1, wherein the thickness of the first layer is greater than the thickness of the second layer.

7. The negative electrode for a secondary battery according to claim 6, wherein the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer is in the range of 0.01 or more and 0.3 or less.

8. The negative electrode for a secondary battery according to claim 6, wherein the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer is in the range of 0.04 or more and 0.3 or less.

9. The negative electrode for a secondary battery according to claim 6, wherein the ratio of the thickness of the second layer to the thickness of the negative electrode active material layer is in the range of more than 0.04 and less than or equal to 0.

3.

10. The negative electrode for a secondary battery according to claim 1, wherein when the first ratio is defined as X1 and the second ratio is defined as X2, the ratio of X1 to X2 is in the range of 0.4 or more and 0.8 or less.

11. The negative electrode for a secondary battery according to claim 10, wherein the ratio of X1 to X2 is in the range of more than 0.45 and less than or equal to 0.

8.

12. The negative electrode for a secondary battery according to claim 10, wherein the ratio of X1 to X2 is in the range of 0.6 or more and 0.7 or less.

13. The negative electrode for a secondary battery according to claim 10, wherein the ratio of X1 to X2 is in the range of 0.5 or more and 0.7 or less.

14. The negative electrode for a secondary battery according to claim 1, wherein the negative electrode current collector contains Ni.

15. The negative electrode for a secondary battery according to claim 1, wherein the negative electrode active material layer is a heat-treated plating layer.

16. A sodium-ion secondary battery comprising a negative electrode, a positive electrode, and an electrolyte, wherein the negative electrode is a negative electrode for a secondary battery according to any one of claims 1 to 15, and the electrolyte comprises two or more non-aqueous solvents, the two or more non-aqueous solvents comprising vinylene carbonate.

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