JP2017010802A - Sodium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress expansion of a negative electrode and ensure a high cycle characteristic in a sodium ion secondary battery using negative electrode active material containing tin.SOLUTION: A sodium ion secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and nonaqueous electrolyte having sodium ion conductivity. The negative electrode includes a negative electrode current collector and a negative electrode mixture layer carried on the negative electrode current collector. The negative electrode mixture layer includes a negative electrode active material, a conductive auxiliary agent, and a binder. The negative electrode active material includes metal particles containing tin and hard carbon. The negative electrode mixture layer has voids, and under a discharge state, the rate Rof the volume of the metal particles to the volume of the voids satisfies 0.05≤R<0.24. The occupational rate of the metal particles in the total amount of the metal particles and the hard carbon ranges from 15 mass% to 60 mass%.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a sodium ion secondary battery using tin as a negative electrode active material.

  In recent years, technology that converts natural energy such as sunlight or wind power into electric energy has attracted attention. In addition, demand for lithium-ion secondary batteries, lithium-ion capacitors, and the like as power storage devices that can store a large amount of electrical energy is increasing. However, with the expansion of the market for these electricity storage devices, the price of lithium resources is also increasing.

  Compared to lithium resources, sodium resources are cheaper. Therefore, sodium ion batteries using sodium ions have been studied (for example, Patent Document 1). The sodium ion battery includes a positive electrode, a negative electrode, and a sodium ion conductive nonaqueous electrolyte. As the active material for the positive electrode and the negative electrode, for example, a material that occludes and releases (or inserts and desorbs) sodium ions is used. In Patent Document 1, hard carbon is used as the negative electrode active material. Further, negative electrode active materials (alloy-based negative electrode active materials) that are alloyed with sodium such as silicon and tin are also being studied.

JP 2013-48077 A

Since the alloy-based negative electrode active material can occlude many sodium ions, it is advantageous from the viewpoint of increasing the capacity. However, an alloy-based negative electrode active material, especially tin, has a very large expansion during occlusion of sodium ions, and therefore, when the charge and discharge are repeated, the deterioration of the negative electrode becomes significant. Therefore, a sufficient cycle life cannot be ensured.
An object of the present invention is to suppress expansion of a negative electrode and ensure high cycle characteristics in a sodium ion secondary battery using a negative electrode active material containing tin.

One aspect of the present invention includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte having sodium ion conductivity.
The negative electrode includes a negative electrode current collector and a negative electrode mixture layer carried on the negative electrode current collector,
The negative electrode mixture layer includes a negative electrode active material, a conductive additive, and a binder,
The negative electrode active material includes metal particles containing tin and hard carbon,
The negative electrode mixture layer has voids,
In the discharged state, the ratio R _{p / v} of the volume of the metal particles to the volume of the voids is 0.05 ≦ R _{p / v} <0.24, and the ratio of the metal particles and the hard carbon to the total amount is The ratio of the metal particles relates to a sodium ion secondary battery that is 15% by mass to 60% by mass.

  According to the above aspect of the present invention, in a sodium ion secondary battery using a negative electrode active material containing tin, expansion of the negative electrode during charging can be suppressed, and deterioration of the negative electrode during charge / discharge cycles can be suppressed. Therefore, high cycle characteristics can be ensured.

1 is a longitudinal sectional view schematically showing a sodium ion secondary battery according to an embodiment of the present invention.

[Description of Embodiment of the Invention]
First, the contents of the embodiment of the present invention will be listed and described.
One embodiment of the present invention relates to a sodium ion secondary battery including a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte having sodium ion conductivity. The negative electrode includes a negative electrode current collector and a negative electrode mixture layer carried on the negative electrode current collector, and the negative electrode mixture layer includes a negative electrode active material, a conductive additive, and a binder. The negative electrode active material includes metal particles containing tin and hard carbon, and the negative electrode mixture layer has voids. In the discharge state, the ratio R _{p / v} of the volume of the metal particles to the volume of the gap is 0.05 ≦ R _{p / v} <0.24, and the ratio of the metal particles to the total amount of the metal particles and the hard carbon is 15 mass% to 60 mass%.

  Since tin and tin alloys can reversibly store and release many sodium ions, high capacity is expected by using them as negative electrode active materials for sodium ion secondary batteries. However, when tin or a tin alloy is used as the negative electrode active material, the volume change accompanying expansion / contraction during charging / discharging is extremely large, so that the negative electrode deteriorates and a sufficient cycle life cannot be ensured.

In the present embodiment, although the negative electrode active material containing metal particles containing tin is used, by setting the ratio R _{p / v} within the above range, even if the metal particles expand greatly during charging, the negative electrode expands. Is suppressed. Therefore, even if charging / discharging is repeated, deterioration of the negative electrode is suppressed, so that excellent cycle characteristics can be obtained. Further, the use of a negative electrode active material containing a large amount of metal particles can increase the capacity of the battery.

  In addition, the negative electrode in the “discharge state” is a negative electrode in a state containing only irreversible capacity sodium ions (that is, a negative electrode in a fully discharged state), for example, a charge / discharge from the first to ten times in a sodium ion secondary battery. It is the negative electrode of the discharge state after performing.

  The porosity of the negative electrode mixture layer is preferably 25% by volume to 55% by volume in the discharged state. Moreover, it is preferable that the porosity of the negative mix layer in a full charge state is 0 volume%-25 volume%. When the void ratio in the discharged state and / or the fully charged state is in such a range, it is easy to suppress the expansion of the negative electrode during charging while securing a high capacity.

Note that the negative electrode in the “fully charged state” refers to a state where the sodium ion secondary battery is charged until the potential of the negative electrode does not substantially drop. For example, a negative electrode in which the potential of the negative electrode is 0.1 V or less relative to metal sodium may be used as a fully charged negative electrode. The fully charged negative electrode can be considered as a state in which substantially all Sn contained in the negative electrode mixture layer has been changed to the Na ₁₅ Sn ₄ alloy.

The porosity of the negative electrode mixture layer can be defined by the following formula.
Porosity (%) = (1−d / d ₀ ) × 100
(In the formula, d is the apparent density of the negative electrode mixture layer, and d ₀ is the true density of the negative electrode mixture layer.)
The apparent density d can be calculated from the volume and mass of the negative electrode mixture layer in the negative electrode. The true density d ₀ can be calculated from the specific gravity and density of the constituent components of the negative electrode mixture layer and the mass ratio.

The ratio R _{p / v} is preferably 0.10 ≦ R _{p / v} <0.24. In this case, it is easy to ensure a high capacity.

The metal particles may include aggregated particles obtained by aggregating primary particles of metal including tin. The negative electrode mixture layer is in a discharged state, and the number of aggregated particles having a particle diameter of 10 μm or more is 2 or less in the region of an arbitrarily selected area of 2500 μm ² in the cross section in the thickness direction. It is preferable. That is, the dispersibility of the metal particles containing tin in the negative electrode mixture layer is high, and the proportion of aggregated particles having a large size is small. Thereby, expansion of the negative electrode can be further effectively suppressed.

In addition, the number of aggregated particles having a particle diameter of 10 μm or more can be obtained from a scanning electron microscope (SEM) image of a cross section in the thickness direction of the negative electrode mixture layer. The number of aggregated particles may be an average value obtained by selecting a plurality of arbitrary regions (for example, 10 locations) having an area of 2500 μm ² , obtaining the number of each region, and averaging them. In addition, the diameter of the equivalent circle having the same area as the area of the aggregated particles in the SEM image is assumed to be the particle diameter of the aggregated particles.

  It is preferable that the quantity of a conductive support agent is 5 mass parts-45 mass parts with respect to 100 mass parts of negative electrode active materials. When the amount of the conductive auxiliary is in such a range, even if the negative electrode expands, the conductive path can be prevented from being cut. Therefore, a decrease in cycle characteristics is further suppressed by suppressing an increase in resistance of the negative electrode.

  The average particle diameter of the metal particles is preferably 100 nm to 500 nm in the discharged state. When the average particle size is in such a range, expansion of the negative electrode during charging can be further suppressed.

  The average particle diameter of the metal particles can be obtained from an SEM image of a cross section in the thickness direction of the negative electrode mixture layer. Specifically, in the SEM image, an arbitrary plurality (for example, 50) of metal particles can be selected, and the particle diameter of each metal particle can be measured and averaged to obtain the average particle diameter. In addition, let the diameter of the equivalent circle which has the same area as the area of the metal particle in a SEM image be a particle diameter of a metal particle.

When the thickness of the negative electrode in the discharged state is T, the thickness of the negative electrode in the fully charged state is preferably 1.25 T or less. In this case, since the expansion and contraction of the negative electrode accompanying charging / discharging is small, deterioration of the negative electrode can be further suppressed even if charging / discharging is repeated.
The thickness of the negative electrode can be obtained from an SEM image in a cross section in the thickness direction of the negative electrode. The thickness of the negative electrode may be an average value obtained by measuring the thickness of the negative electrode at an arbitrary plurality of locations in the SEM image and averaging the measured values.

  The conductive additive is preferably at least one selected from the group consisting of carbon black and carbon nanofibers. When such a conductive additive is used, it is easy to increase the conductivity of the negative electrode, and it is easy to suppress an increase in resistance even if the negative electrode expands during charging.

[Details of the embodiment of the invention]
Specific examples of the sodium ion secondary battery according to the embodiment of the present invention will be described below with reference to the drawings as appropriate. In addition, this invention is not limited to these illustrations, is shown by the attached claim, and is intended that all the changes within the meaning and range equivalent to the claim are included. .

(Negative electrode)
The negative electrode includes a negative electrode current collector and a negative electrode mixture layer carried on the negative electrode current collector.
As the negative electrode current collector, for example, a metal foil can be used. The thickness of the metal foil is, for example, 10 μm to 50 μm.

  The material of the negative electrode current collector is not particularly limited, but is not alloyed with sodium and is stable at the negative electrode potential, so aluminum, aluminum alloy, copper, copper alloy, nickel, nickel alloy, and / or stainless steel Etc. are preferable.

The negative electrode active material contained in the negative electrode mixture layer contains metal particles containing tin and hard carbon.
(Metal particles)
As a metal which comprises a metal particle, a tin simple substance or a tin alloy is mentioned, for example. Examples of tin alloys include alloys of tin and elements other than tin (second element). Examples of the second element include metal elements such as titanium, zinc, indium, lead, and bismuth, and nonmetal elements such as silicon and phosphorus. The tin alloy may contain one kind of the second element, or may contain two or more kinds. One kind of metal particles may be used, or two or more kinds of metal particles having different kinds may be used.

The tin content in the metal particles is, for example, 80% by mass to 100% by mass, and may be 90% by mass to 100% by mass. When the tin content is in such a range, it is easy to ensure a high capacity, but expansion and contraction during charge / discharge tends to increase. In the present embodiment, even when such metal particles containing a large amount of tin are used, the expansion and contraction of the negative electrode can be reduced by setting the ratio R _{p / v} in the above range. The metal particles may contain impurities in addition to the elements that form tin and tin alloys (specifically, tin and the second element), but the content of impurities in the metal particles is 1 mass. % Or less is preferable. Note that the contents of tin and impurities in the metal particles referred to here are those at the time of preparation.

The metal particles may be included in the negative electrode mixture layer in the form of primary particles, or may be included in the state of aggregated particles (or secondary particles) in which the primary particles are aggregated. In the present embodiment, even when the negative electrode mixture layer includes aggregated particles, expansion and contraction of the negative electrode due to expansion and contraction of the aggregated particles can be suppressed, but it is preferable that the ratio of large size aggregated particles is small. For example, the negative electrode mixture layer is preferably in a discharged state, and the number of agglomerated particles having a particle diameter of 10 μm or more is 2 or less in an arbitrarily selected area of 2500 μm ² in the cross section in the thickness direction. It may be 1 or less or 0. When the number of aggregated particles having a particle size of 10 μm or more is in such a range, the effect of suppressing the expansion of the negative electrode during charging is further enhanced.

  In the negative electrode mixture layer, the average particle diameter of the metal particles in the discharged state is, for example, 100 nm to 800 nm, preferably 100 nm to 500 nm, or 100 nm to 300 nm, and more preferably 100 nm to 250 nm. When the average particle diameter is in such a range, there are few portions that are partially expanded and contracted during charging and discharging, which is advantageous in improving cycle characteristics.

In the discharged state, the ratio of the metal particles to the total amount of the metal particles and the hard carbon is 15% by mass to 60% by mass. In such a ratio, the high capacity of the metal particles can be effectively used, and the capacity of the battery can be increased. Although the volume change due to the expansion and contraction of the negative electrode active material becomes large, since the expansion and contraction of the negative electrode is suppressed by adjusting the ratio R _{p / v} , excellent cycle characteristics can be obtained. From the viewpoint of increasing the capacity, the proportion of metal particles in the discharged state is preferably 20% by mass to 60% by mass, and more preferably 30% by mass to 60% by mass or 35% by mass to 55% by mass. preferable.

In the discharged state, the ratio R _{p / v} of the volume of the metal particles to the volume of the voids of the negative electrode mixture layer is 0.05 or more, and is 0.10 or more or 0.12 or more from the viewpoint of increasing the capacity. It is preferable. The ratio R _{p / v} is less than 0.24, and is preferably 0.23 or less or 0.22 or less from the viewpoint of further improving the cycle characteristics. These lower limit values and upper limit values can be arbitrarily combined. The ratio _{R p / v,} for _{example, 0.05 ≦ R p / v ≦} 0.23,0.10 ≦ R p / v <0.24,0.12 ≦ R p / v ≦ 0.23 , or 0, .10 ≦ R _{p / v} ≦ 0.23.

When the ratio R _{p / v} is less than 0.05, the effect of increasing the capacity by using metal particles containing tin cannot be sufficiently obtained. When the ratio R _{p / v} is 0.24 or more, the volume change accompanying the expansion and contraction of the metal particles becomes large, and the negative electrode is deteriorated due to repeated charge and discharge, resulting in deterioration of cycle characteristics.
The ratio R _{p / v} can be adjusted by changing the degree of compression at the time of forming the negative electrode, the density of the negative electrode mixture layer, the ratio of the constituent components, and the like.

(Hard carbon)
Hard carbon is a carbonaceous material having a turbostratic structure in which carbon network surfaces are overlapped in a three-dimensionally shifted state. Hard carbon does not change from a turbulent structure to a graphite structure even by heat treatment at a high temperature (for example, 3000 ° C.), and the development of graphite crystallites is not observed. Therefore, hard carbon is also referred to as non-graphitizable carbon.

The average interplanar spacing d ₀₀₂ of the (002) plane measured by the hard carbon X-ray diffraction spectrum is, for example, 0.37 nm to 0.42 nm. The average specific gravity of the hard carbon is, for ^{example, 1.4g / cm 3 ~1.7g / cm 3} .

The average particle diameter _{D 50} of the hard carbon, for example, a 3Myuemu～20myuemu, preferably 5Myuemu～15myuemu. When the average particle size of the hard carbon is within such a range, it is easy to achieve high filling while leaving a certain amount of voids in the negative electrode mixture layer, and thus it is easy to balance high capacity and cycle characteristics.
The average particle diameter D ₅₀ is accumulated in the particle size distribution on a volume basis means a particle diameter at 50% (that is, median diameter).

  The negative electrode active material may contain an active material (third active material) other than these in addition to the metal particles containing tin (first active material) and hard carbon (second active material) as necessary. Examples of the third active material include metal compounds (such as lithium titanate and sodium titanate), silicon, silicon compounds (such as oxides), and soft carbon. From the viewpoint of sufficiently exerting the effects of the metal particles and the hard carbon, the amount of the third active material in the negative electrode active material is preferably 5% by mass or less.

(Conductive aid)
The conductive auxiliary agent is not particularly limited, and examples thereof include carbon black (acetylene black, ketjen black, etc.), graphite, carbon fiber (carbon nanofiber such as vapor grown carbon fiber, etc.), and / or carbon nanotube. Can be mentioned. From the viewpoint of increasing conductivity, the conductive auxiliary agent may be coated on the surface of the negative electrode active material particles. A conductive support agent may be used individually by 1 type, and may be used in combination of 2 or more type. From the viewpoint of easily suppressing an increase in resistance of the negative electrode, carbon black and / or carbon nanofibers are preferable as the conductive additive. From the viewpoint of further improving the cycle characteristics, it is advantageous to use carbon nanofibers such as vapor grown carbon fibers.

  It is preferable that the quantity of a conductive support agent is 5 mass parts-45 mass parts with respect to 100 mass parts of negative electrode active materials. In this case, it is easy to suppress an increase in resistance of the negative electrode. From the viewpoint of more effectively suppressing the conductive path in the negative electrode mixture layer from being cut off as the negative electrode expands during charging, the amount of the conductive auxiliary is 10 parts by weight with respect to 100 parts by weight of the negative electrode active material. It is preferable that it is mass part-45 mass parts, and it is preferable that it is 10 mass parts-40 mass parts from a viewpoint from which a high capacity | capacitance is easy to be obtained.

When the conductive auxiliary agent is in the form of particles, the average particle diameter D50 is, for example, 20 nm to 70 nm, and preferably 30 nm to ₅₀ nm. Moreover, when a conductive support agent is fibrous form, an average aspect-ratio (fiber length / fiber diameter) is 30-300, for example, and it is preferable that it is 50-200. When the average particle diameter and the average aspect ratio are in such ranges, even if the negative electrode mixture layer expands during charging, the conductive path is hardly cut.

(Binder)
The binder is not particularly limited, and examples thereof include fluorine resins such as polyvinylidene fluoride, polyolefin resins, rubbery polymers such as styrene butadiene rubber, polyamide resins, polyimide resins such as polyamideimide, polyvinylpyrrolidone, polyvinyl alcohol, and cellulose. Ether (carboxymethylcellulose and its salt etc.) etc. are mentioned. A binder may be used individually by 1 type and may be used in combination of 2 or more type.

  The amount of the binder is not particularly limited, but is, for example, 1 part by mass to 20 parts by mass and 5 parts by mass to 15 parts by mass with respect to 100 parts by mass of the negative electrode active material from the viewpoint of easily ensuring high binding properties and capacity. It is preferable that it is a mass part.

The negative electrode can be formed, for example, by coating or filling a negative electrode current collector with a negative electrode mixture and compressing (or rolling) it in the thickness direction as necessary. A drying treatment may be performed at an appropriate stage. The negative electrode active material may be pre-doped with sodium ions as necessary.
The negative electrode mixture is usually used in the form of a slurry (or paste) containing a dispersion medium. As the dispersion medium, for example, an organic solvent such as N-methyl-2-pyrrolidone (NMP) and / or water is used.

  The negative electrode mixture slurry is prepared by dispersing the components of the negative electrode mixture in a dispersion medium. When the component, particularly metal particles containing tin, are finely dispersed, the effect of reducing expansion and contraction of the negative electrode due to charge and discharge is enhanced. In preparing the negative electrode mixture, a known mixer, kneader, disper, mill or the like can be used.

  From the viewpoint of enhancing the dispersibility of the metal particles, it is preferable to use an apparatus that stirs the constituent components using centrifugal force. For example, it is preferable to form a thin film of a mixture of a component and a dispersion medium using centrifugal force using a thin film swirl type high-speed mixer and finely disperse the components in the thin film by swirling at high speed. It is advantageous to disperse the constituents using centrifugal force because it is easy to reduce the proportion of large agglomerated particles contained in the negative electrode mixture, such as a particle diameter of 10 μm or more. When the thin film of the mixture is swirled at a high speed, the rotation speed of the stirring device is, for example, 10 m / s to 50 m / s, and preferably 20 m / s to 40 m / s. The stirring time may be, for example, 10 seconds to 60 seconds, or 15 seconds to 45 seconds. When the rotational speed and / or the stirring time is within such a range, it is more advantageous to finely disperse the components.

  The solid content concentration of the negative electrode mixture slurry (or paste) is, for example, 30% by mass to 70% by mass, and is preferably 35% by mass to 65% by mass from the viewpoint of improving dispersibility, for example. It is more preferable to set it as mass%-55 mass%.

The degree of compression when producing the negative electrode is preferably adjusted so that the density of the negative electrode mixture layer, the porosity of the negative electrode mixture layer in the discharge state, and the like are within a desired range. The degree of compression, because it affects the ratio R _{p / v,} the ratio R _{p / v} may be adjusted and the number of times that the pressure and / or compression during the compression to be in the range described above.

  The porosity of the negative electrode mixture layer is, for example, 25% by volume to 55% by volume, preferably 27% by volume to 50% by volume, and 30% by volume to 46% by volume or 32% by volume in the discharged state. More preferably, it is 46 volume%. When the porosity in the discharged state is in such a range, in addition to further enhancing the effect of suppressing the expansion of the negative electrode during charging, it is easy to ensure a high capacity.

  The porosity of the negative electrode mixture layer in the fully charged state is, for example, 0 volume% to 25 volume%, preferably 1 volume% to 22 volume%, and more preferably 2 volume% to 21 volume%. preferable. When the fully charged porosity is in such a range, expansion of the negative electrode due to charging can be further effectively suppressed, and high capacity can be easily ensured.

  In this embodiment, since the expansion of the negative electrode during charging can be suppressed, deterioration of the negative electrode associated with charge / discharge can be suppressed. When the thickness of the negative electrode in the discharged state is T, the thickness of the negative electrode in the fully charged state is, for example, 1.26 T or less, and preferably 1.25 T or less (specifically, T to 1.25 T). , T to 1.20T or T to 1.15T. When the thickness of the fully charged negative electrode is in such a range, deterioration of the negative electrode due to charge / discharge is further suppressed, and thus excellent cycle characteristics are easily obtained.

(Positive electrode)
The positive electrode includes a positive electrode active material, and may include a positive electrode current collector and a positive electrode active material (or a positive electrode mixture) supported on the positive electrode current collector.
As the positive electrode current collector, for example, a metal foil, a metal porous body, or the like as described for the negative electrode current collector can be used. The material of the positive electrode current collector is not particularly limited, but aluminum and / or an aluminum alloy are preferable from the viewpoint of stability at the positive electrode potential. The thickness of the positive electrode current collector can be appropriately selected from the range described for the case of the negative electrode current collector.

  Examples of the positive electrode active material include materials that reversibly occlude and release sodium ions. Examples of such a material include a compound containing an alkali metal (sodium, potassium, etc.) and a transition metal (transition metal of the fourth period of the periodic table such as Cr, Mn, Fe, Co, Ni). In such a compound, a part of at least one of an alkali metal and a transition metal may be substituted with a typical metal element such as Al.

The positive electrode active material preferably contains a transition metal compound such as a sodium-containing transition metal compound. As the transition metal compound, known compounds that can be used as the positive electrode active material of the molten salt battery, for example, sulfides (transition metal sulfides such as TiS ₂ and FeS ₂ ; sodium-containing transition metal sulfides such as NaTiS ₂ ), Oxides [sodium-containing transition metals such as sodium chromite (NaCrO ₂ ), NaNi _0.5 Mn _0.5 O ₂ , sodium iron manganate (Na _2/3 Fe _1/3 Mn _2/3 O ₂ etc.) Oxides, etc.], sodium transition metal oxyacid salts, and / or sodium-containing transition metal halides (such as Na ₃ FeF ₆ ) and the like. Of these, sodium chromite and sodium ferromanganate are preferred. A part of Cr or Na in sodium chromite may be substituted with other elements, and a part of Fe, Mn or Na in sodium ferromanganate may be substituted with other elements.
A positive electrode active material may be used individually by 1 type, and may be used in combination of 2 or more type.

  The positive electrode can be formed, for example, by applying or filling a positive electrode mixture containing a positive electrode active material to a positive electrode current collector and compressing (or rolling) in the thickness direction as necessary, according to the case of the negative electrode. . A drying treatment may be performed at an appropriate stage.

  In addition to the positive electrode active material, the positive electrode mixture can further contain a conductive additive and / or a binder. As a binder and a conductive support agent, it can respectively select suitably from what was illustrated about the negative electrode. The positive electrode mixture is usually used in the form of a slurry (or paste) containing a dispersion medium. As a dispersion medium, it can select suitably from what was illustrated about the negative electrode.

The amount of the conductive additive can be appropriately selected from the range of 1 to 25 parts by mass with respect to 100 parts by mass of the positive electrode active material, and may be 5 to 20 parts by mass.
The amount of the binder is not particularly limited, but can be selected from a range of, for example, about 0.5 parts by mass to 15 parts by mass with respect to 100 parts by mass of the positive electrode active material from the viewpoint of easily ensuring high binding properties and capacity. The amount may be preferably 1 to 12 parts by mass.

(Separator)
The separator is interposed between the positive electrode and the negative electrode. As the separator, for example, a resin microporous film and / or a nonwoven fabric can be used. The material of the separator can be selected in consideration of the operating temperature of the battery. Examples of the resin contained in the fibers forming the microporous film or the nonwoven fabric include polyolefin resin, polyphenylene sulfide resin, polyamide resin, and / or polyimide resin. The fibers forming the nonwoven fabric may be inorganic fibers such as glass fibers. The separator may include an inorganic filler such as ceramic particles. The inorganic filler may be coated on the separator.
Although the thickness of a separator is not specifically limited, For example, it can select from the range of about 10 micrometers-300 micrometers.

(Electrolytes)
As the electrolyte, a nonaqueous electrolyte containing sodium ions is used. Nonaqueous electrolytes include, for example, an electrolyte (organic electrolyte) in which a salt (sodium salt) of sodium ions and anions is dissolved in a nonaqueous solvent (or organic solvent), and an ionic liquid (molten) containing sodium ions and anions Salt electrolyte).

From the viewpoint of low temperature characteristics and the like, it is preferable to use an electrolyte containing a non-aqueous solvent (organic solvent). From the viewpoint of suppressing the decomposition of the electrolyte as much as possible, an electrolyte containing an ionic liquid is preferably used, and an electrolyte containing an ionic liquid and a nonaqueous solvent may be used.
The concentration of sodium salt or sodium ion in the electrolyte can be appropriately selected from the range of 0.3 to 10 mol / L, for example.

(Organic electrolyte)
The organic electrolyte can contain an ionic liquid and / or an additive in addition to a non-aqueous solvent (organic solvent) and a sodium salt. From the viewpoint of easily ensuring high low-temperature characteristics, the non-aqueous solvent in the electrolyte and 60 mass%-100 mass% may be sufficient as the sum total of content of a sodium salt, for example.

The kind of the anion (first anion) constituting the sodium salt is not particularly limited. For example, hexafluorophosphate ion, tetrafluoroborate ion, perchlorate ion, bis (oxalato) borate ion (B (C ₂ O _{4) 2} ^-), tris (oxalato) phosphate ions _{(P (C 2} O _{4) 3} ^-), trifluoromethanesulfonate ion _(CF 3 SO ₃ ^-), and the like bissulfonylamide anions. A sodium salt may be used individually by 1 type, and may use it in combination of 2 or more types of sodium salts from which the kind of 1st anion differs.

Examples of the bissulfonylamide anion include bis (fluorosulfonyl) amide anion (FSA: bis (fluorosulfonyl) amide anion), bis (trifluoromethylsulfonyl) amide anion (TFSA: bis (trifluoromethylsulfonyl) amide anion), (Fluorosulfonyl) (perfluoroalkylsulfonyl) amide anion [(FSO ₂ ) (CF ₃ SO ₂ ) N ⁻ etc.], bis (perfluoroalkylsulfonyl) amide anion [N (SO ₂ CF ₃ ) ₂ ⁻ , N ( SO ₂ C ₂ F ₅ ) ₂ ^{— and the} like]. Of these, FSA and / or TFSA are particularly preferable.

  The non-aqueous solvent is not particularly limited, and a known non-aqueous solvent used for a sodium ion secondary battery can be used. Non-aqueous solvents include, for example, cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; and γ-butyrolactone. The cyclic carbonate of the above can be preferably used. A non-aqueous solvent may be used individually by 1 type, and may be used in combination of 2 or more type.

(Molten salt electrolyte)
When an ionic liquid is used as the electrolyte, the electrolyte can include a nonaqueous solvent and / or an additive in addition to the ionic liquid containing a cation and an anion. However, from the viewpoint of easily suppressing the decomposition of the electrolyte, the content of the ionic liquid in the electrolyte is preferably 80% by mass or more (for example, 80% by mass to 100% by mass), and 90% by mass to 100% by mass. %.

  The ionic liquid can contain cations (third cation) other than sodium ions in addition to sodium ions (second cation). Examples of the third cation include organic cations and inorganic cations other than sodium ions. The ionic liquid may contain one kind of third cation or may contain two or more kinds in combination.

  Examples of the inorganic cation include alkali metal ions (potassium ions, etc.) other than sodium ions, and / or alkaline earth metal ions (magnesium ions, calcium ions, etc.), ammonium ions, and the like.

Organic cations include cations derived from aliphatic amines, alicyclic amines or aromatic amines (for example, quaternary ammonium cations), as well as cations having nitrogen-containing heterocycles (that is, derived from cyclic amines). Examples thereof include nitrogen-containing onium cations such as cations), sulfur-containing onium cations, and phosphorus-containing onium cations.
Among organic cations, in particular, a cation having a pyrrolidine, pyridine, or imidazole skeleton as a nitrogen-containing heterocyclic skeleton in addition to a quaternary ammonium cation is preferable.

Specific examples of the organic cation include tetraethylammonium cation (TEA ⁺ : tetraethylammonium cation), methyltriethylammonium cation (TEMA ⁺ : methyltriethylammonium cation) and the like; 1-methyl-1-propylpyrrolidinium cation (MPPY) ⁺ : 1-methyl-1-propylpyrrolidinium cation, 1-butyl-1-methylpyrrolidinium cation (MBPY ⁺ : 1-butyl-1-methylpyrrolidinium cation); 1-ethyl-3-methylimidazolium cation (EMI ⁺ : 1-ethyl-3-methylimidazolium c 1-butyl-3-methylimidazolium cation (BMI ⁺ : 1-butyl-3-methylimidazolium cation) and the like.

  As the anion, a bissulfonylamide anion is preferably used. The bissulfonylamide anion can be appropriately selected from those exemplified for the organic electrolyte. Of the bissulfonylamide anions, FSA and / or TFSA are particularly preferable.

  A sodium ion secondary battery includes, for example, (a) a step of forming an electrode group with a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode; and (b) an electrode group and an electrolyte are accommodated in a battery case. It can manufacture by passing through the process to do.

  FIG. 1 is a longitudinal sectional view schematically showing a sodium ion secondary battery according to an embodiment of the present invention. The sodium ion secondary battery includes a stacked electrode group, an electrolyte (not shown), and a rectangular aluminum battery case 10 that houses them. The battery case 10 includes a bottomed container body 12 having an upper opening and a lid 13 that closes the upper opening.

  In the center of the lid 13, a safety valve 16 is provided for releasing gas generated inside when the internal pressure of the battery case 10 rises. An external positive electrode terminal that penetrates the lid body 13 is provided near the one side of the lid body 13 with the safety valve 16 at the center, and an external negative electrode that penetrates the lid body 13 at a position near the other side of the lid body 13. A terminal 14 is provided.

  The stacked electrode group is composed of a plurality of positive electrodes 2, a plurality of negative electrodes 3, and a plurality of separators 1 interposed between them, each having a rectangular sheet shape. In FIG. 1, the separator 1 is formed in a bag shape so as to surround the positive electrode 2, but the form of the separator is not particularly limited. The plurality of positive electrodes 2 and the plurality of negative electrodes 3 are alternately arranged in the stacking direction within the electrode group.

  A positive electrode lead piece 2 a may be formed at one end of each positive electrode 2. The plurality of positive electrodes 2 are connected in parallel by bundling the positive electrode lead pieces 2 a of the plurality of positive electrodes 2 and connecting them to an external positive terminal provided on the lid 13 of the battery case 10. Similarly, a negative electrode lead piece 3 a may be formed at one end of each negative electrode 3. The plurality of negative electrodes 3 are connected in parallel by bundling the negative electrode lead pieces 3 a of the plurality of negative electrodes 3 and connecting them to the external negative terminal 14 provided on the lid 13 of the battery case 10. The bundle of the positive electrode lead pieces 2a and the bundle of the negative electrode lead pieces 3a are desirably arranged on the left and right sides of one end face of the electrode group with an interval so as to avoid mutual contact.

  Both the external positive terminal and the external negative terminal 14 are columnar, and at least a portion exposed to the outside has a screw groove. A nut 7 is fitted in the screw groove of each terminal, and the nut 7 is fixed to the lid 13 by rotating the nut 7. A flange 8 is provided in a portion of each terminal accommodated in the battery case 10, and by rotation of the nut 7, the flange 8 attaches an O-ring-shaped gasket 9 to the inner surface of the lid 13. Fixed through.

  The electrode group is not limited to the laminated type, and may be formed by winding a positive electrode and a negative electrode through a separator. From the viewpoint of preventing metallic sodium from being deposited on the negative electrode, the size of the negative electrode may be made larger than that of the positive electrode.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
(1) Production of negative electrode Using a thin-film swirling mixer, tin powder (average particle diameter D ₅₀ : 120 nm) and hard carbon (average particle diameter D ₅₀ : 9 μm) as a negative electrode active material, and acetylene as a conductive additive A negative electrode mixture paste was prepared by rotating black (average particle diameter D ₅₀ : 48 nm), polyamideimide as a binder, and NMP at a rotational speed of 30 m / s for 30 seconds. At this time, the mass ratio of tin powder, hard carbon, acetylene black, and polyamideimide was 35: 47: 10: 8. The solid content concentration of the negative electrode mixture paste was 45% by mass. The ratio of the tin powder to the total amount of the tin powder and the hard carbon was 42.7% by mass, and the amount of acetylene black relative to 100 parts by mass of the negative electrode active material was 12.2 parts by mass.
The obtained negative electrode mixture paste is applied to an aluminum foil as a negative electrode current collector, dried, compressed, vacuum dried, and then punched to produce a disc-shaped negative electrode (diameter 12 mm, thickness 65 μm). did.

(2) Production of Half Cell The negative electrode obtained in (1) was placed on the inner bottom of a coin-type battery container, and a separator was placed on the negative electrode. Next, a metal sodium electrode (counter electrode) was disposed with a separator interposed so as to face the negative electrode. A coin-type half cell (battery a1) was produced by injecting a nonaqueous electrolyte into the battery container and fitting a lid provided with an insulating gasket on the periphery into the opening of the battery container. The half cell was charged at a temperature value of 40 ° C. at a current value of 0.1C hour rate to 0.005V and discharged to 0.7V at a current value of 0.05C rate. Precharging / discharging was completed by repeating this charging and discharging cycle five times.
As the separator, a polyolefin microporous film (thickness 50 μm) was used. As the non-aqueous electrolyte, a mixture of NaFSA and MPPYFSA at a molar ratio of 4: 6 was used.

(3) Evaluation The following evaluation was performed using the coin-type half cell obtained in the above (2) or the negative electrode taken out from the half cell (that is, the negative electrode in a discharged state).
(A) Density, porosity, and ratio R _{p / v} of the negative electrode mixture layer, and the theoretical capacity of the negative electrode The diameter of the negative electrode in the discharged state and the thickness of the negative electrode mixture layer are measured, and these are used to form the negative electrode mixture layer The apparent volume (cm ³ ) of was determined. And when the density (apparent density) of the negative electrode mixture layer in the discharge state was determined by dividing the mass (g) of the negative electrode mixture layer by the apparent volume, it was 1.50 g / cm ³ .

From the specific gravity, density and mass ratio of the constituent components of the negative electrode mixture layer, the true density of the negative electrode mixture layer in the discharged state was determined. The porosity of the negative electrode mixture layer in the discharged state was determined from the true density and the apparent density using the above-described formula, and found to be 32% by volume.
The porosity of the negative electrode mixture layer in the fully charged state was determined as described above according to the case of the porosity in the discharged state. However, the apparent density was assumed to be the same as the discharge state, the true density is as if all of the Sn in the anode is converted into _Na 15 Sn _{4 alloy,} the density of 2.4 g / cm ³ of _Na 15 Sn ₄ alloy Used to calculate.

From the specific gravity, density, and mass ratio of the constituent components of the negative electrode mixture layer, the total volume of tin powder contained in the discharged negative electrode mixture layer was determined. From the true density and the apparent density of the negative electrode mixture layer, the volume of voids in the negative electrode mixture layer was determined. The ratio R _{p / v} was determined by dividing the total volume of the tin powder by the volume of the voids in the negative electrode mixture layer, and was 0.22.
The theoretical capacity of the negative electrode was determined from the mass ratio of tin powder and hard carbon in the negative electrode, with the theoretical capacity of tin being 500 mAh / g and the theoretical capacity of hard carbon being 250 mAh / g. As a result, the theoretical capacity of the negative electrode was 357 mAh / g.

(B) Number of aggregated particles and average particle diameter of metal particles In the SEM image of the cross section in the thickness direction of the negative electrode mixture layer portion of the negative electrode in the discharged state, a region having an area of 2500 μm ² is arbitrarily selected, and the particles in this region The number of aggregated particles having a diameter of 10 μm or more was counted. The same operation was performed for a total of 10 regions, and an average value was calculated.
In the SEM image of the above-mentioned cross section, the average particle size was obtained by measuring and averaging the particle size of 50 arbitrarily selected metal particles.

(C) Thickness ratio of negative electrode By charging the half cell after pre-charging / discharging at a temperature of 40 ° C. until the current value at a time rate of 0.05 C is 0.005 V, sodium ions are contained in the negative electrode active material. Occluded. An SEM photograph of a cross section in the thickness direction of the half cell in this state (during initial charging) was taken, and the thickness of the negative electrode (that is, the negative electrode in a fully charged state) was measured. The thickness ratio of the negative electrode (that is, the ratio of the thickness of the negative electrode in the fully charged state to the thickness of the negative electrode in the discharged state) was determined by dividing this thickness by the thickness of the negative electrode after pre-charging and discharging (that is, in the discharged state). .

(D) Initial capacity The half-cell after the pre-charging / discharging is charged at a temperature value of 40 ° C. at a current value of a rate of 0.1 C at a current rate of 0.005 V, and a current value of 0.05 C rate is set to 0.00. Discharge up to 7V was performed, and the discharge capacity at this time (at the first discharge) was measured. The discharge capacity was determined by the half cell capacity (mAh / g) per unit mass of the negative electrode active material and the half cell capacity (Ah / L) per apparent volume of the negative electrode.

(E) Cycle characteristics The discharge capacity when the charge / discharge of (d) above was repeated 50 times was determined in the same manner as in the case of the discharge capacity at the first discharge. Then, the ratio (capacity maintenance ratio) (%) when the charge capacity at the first charge was 100% was calculated and used as an index of cycle characteristics.

(4) Preparation of sodium ion secondary battery 100 parts by mass of NaCrO ₂ (positive electrode active material), 11.8 parts by mass of acetylene black (conducting aid) and 5.9 parts by mass of polyvinylidene fluoride (binder) are mixed together with NMP. Thus, a positive electrode mixture paste was prepared. The obtained positive electrode mixture paste was applied to one surface of an aluminum foil as a positive electrode current collector, compressed after drying, and punched out after vacuum drying at 150 ° C., thereby forming a disk-shaped positive electrode (diameter 12 mm, thickness 85 μm). Produced.

  A coin-type sodium ion secondary battery was produced in the same manner as in the above (2) except that the obtained positive electrode was used instead of the metal sodium electrode. The sodium ion secondary battery was charged at a constant current at a temperature of 40 ° C. at a current value of a rate of 0.1 C until the voltage reached 3.4 V, charged at a constant voltage of 3.4 V, and a time rate of 0. Discharging was carried out at a current value of 05C rate until the voltage reached 1.8V. This charging / discharging cycle was repeated 5 times to perform preliminary charging / discharging, and the charging / discharging cycle was further performed several times to confirm that the sodium ion secondary battery could be repeatedly charged / discharged.

Example 2
A negative electrode and a half cell were prepared and evaluated in the same manner as in Example 1 except that the degree of compression was adjusted so that the density of the negative electrode mixture layer was 1.20 g / cm ³ .

Example 3
While changing the mass ratio of tin powder, hard carbon, acetylene black, and polyamideimide to 15: 67: 10: 8, the degree of compression so that the density of the negative electrode mixture layer is 1.25 g / cm ³ A negative electrode and a half cell were produced and evaluated in the same manner as in Example 1 except that was adjusted.

Example 4
The degree of compression so that the mass ratio of tin powder, hard carbon, acetylene black, and polyamideimide is changed to 35: 32: 25: 8, and the density of the negative electrode mixture layer is 1.30 g / cm ^3. A negative electrode and a half cell were produced and evaluated in the same manner as in Example 1 except that was adjusted.

Example 5
A negative electrode and a half cell were produced and evaluated in the same manner as in Example 1 except that the solid content concentration of the negative electrode mixture paste was changed to 70% by mass.

Comparative Example 1
A negative electrode and a half cell were produced and evaluated in the same manner as in Example 1 except that the mass ratio of tin powder, hard carbon, acetylene black, and polyamideimide was changed to 55: 27: 10: 8.

Comparative Example 2
A negative electrode and a half cell were produced and evaluated in the same manner as in Comparative Example 1 except that the degree of compression was adjusted so that the density of the negative electrode mixture layer was 1.65 g / cm ³ .

Comparative Example 3
A negative electrode and a half cell were produced and evaluated in the same manner as in Example 1 except that the mass ratio of tin powder, hard carbon, acetylene black, and polyamideimide was changed to 35: 54: 3: 8.

Comparative Example 4
A negative electrode and a half cell were produced and evaluated in the same manner as in Comparative Example 3 except that the solid content concentration of the negative electrode mixture paste was changed to 70% by mass.

  The results of Examples and Comparative Examples are shown in Table 1. Examples 1 to 5 are A1 to A5, and Comparative Examples 1 to 4 are B1 to B4. In Table 1, tin powder is represented by Sn and hard carbon is represented by HC. The amount of Sn, HC, and conductive additive is the amount of raw material charged. The density of the negative electrode mixture layer is the apparent density of the negative electrode mixture layer in the discharged negative electrode. The porosity is the porosity of the negative electrode mixture layer. The number of agglomerated particles is the average number of agglomerated particles having a particle diameter of 10 μm or more contained in a predetermined area in the discharged negative electrode mixture, and the Sn average particle diameter is in the discharged negative electrode mixture layer. The average particle size of the metal particles.

  As shown in Table 1, in the examples, a high capacity retention rate of 80% to 99% was obtained even after repeated charge and discharge. From this, and the thickness ratio of the negative electrode is small, it can be seen that in the examples, deterioration of the negative electrode is suppressed. On the other hand, in Comparative Examples 1 to 4, the capacity retention rate is low and the cycle characteristics are inferior compared to the examples. In Comparative Examples 1, 2, and 4, since the thickness ratio of the negative electrode is large, it can be seen that the deterioration of the negative electrode was significant due to the expansion and contraction of the negative electrode during charging and discharging. In Comparative Example 3, it is considered that the cycle characteristics deteriorated because the conductive path in the negative electrode decreased due to expansion and contraction during charge and discharge, and the resistance increased. From the results of Example 5 and Comparative Example 4, when the number of aggregated particles having a particle diameter of 10 μm or more increases, the cycle characteristics tend to be lower than those of the corresponding Example 1 and Comparative Example 3. Therefore, from the viewpoint of obtaining higher cycle characteristics, it is preferable that the number of aggregated particles having a particle diameter of 10 μm or more is smaller.

  The porosity of the fully charged negative electrode mixture layer is calculated based on the apparent density calculated on the assumption that the volume of the negative electrode mixture layer is the same as the volume of the negative electrode mixture layer in the discharged state. . Therefore, it may be a negative numerical value as in B1 and B2. B1 and B2 indicate that the thickness of the negative electrode mixture layer increases even if the gap in the negative electrode mixture layer can be ideally filled by charging. Actually, since the voids are not ideally filled, the thickness of the negative electrode mixture layer increases during charging even when the fully charged void ratio is a positive value. In the example, the increase in thickness at this time is greatly suppressed as compared with the comparative example.

  The sodium ion secondary battery according to an embodiment of the present invention is excellent in cycle characteristics and can have a high capacity. Therefore, the sodium ion secondary battery is useful, for example, as a power source for home or industrial large power storage devices, electric vehicles, and hybrid vehicles.

1: Separator 2: Positive electrode 2a: Positive electrode lead piece 3: Negative electrode 3a: Negative electrode lead piece 7: Nut 8: Hook 9: Gasket 10: Battery case 12: Container body 13: Lid 14: External negative electrode terminal 16: Safety valve

Claims (9)

A positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte having sodium ion conductivity,
The negative electrode includes a negative electrode current collector and a negative electrode mixture layer carried on the negative electrode current collector,
The negative electrode mixture layer includes a negative electrode active material, a conductive additive, and a binder,
The negative electrode active material includes metal particles containing tin and hard carbon,
The negative electrode mixture layer has voids,
In the discharged state, the ratio R _{p / v} of the volume of the metal particles to the volume of the voids is 0.05 ≦ R _{p / v} <0.24, and occupies the total amount of the metal particles and the hard carbon. The ratio of metal particles is a sodium ion secondary battery which is 15 mass%-60 mass%.   The sodium ion secondary battery according to claim 1, wherein a porosity of the negative electrode mixture layer is 25% by volume to 55% by volume in a discharged state.   The sodium ion secondary battery according to claim 1 or 2, wherein a porosity of the negative electrode mixture layer is 0% by volume to 25% by volume in a fully charged state. 4. The sodium ion secondary battery according to claim 1, wherein the ratio R _{p / v} is 0.10 ≦ R _{p / v} <0.24. 5. The metal particles include aggregated particles in which primary particles of metal including tin are aggregated,
The negative electrode mixture layer is in a discharged state, and the number of aggregated particles having a particle diameter of 10 μm or more among the aggregated particles is 2 or less in an area of an arbitrarily selected area of 2500 μm ² in the cross section in the thickness direction. The sodium ion secondary battery according to any one of claims 1 to 4.   The amount of the said conductive support agent is a sodium ion secondary battery of any one of Claims 1-5 which are 5 mass parts-45 mass parts with respect to 100 mass parts of said negative electrode active materials.   The average particle diameter of the said metal particle is a sodium ion secondary battery of any one of Claims 1-6 which are 100 nm-500 nm in a discharge state.   The sodium ion secondary battery according to any one of claims 1 to 7, wherein when the thickness of the negative electrode in a discharged state is T, the thickness of the negative electrode in a fully charged state is 1.25 T or less. The sodium ion secondary battery according to any one of claims 1 to 8, wherein the conductive auxiliary agent is at least one selected from the group consisting of carbon black and carbon nanofibers.

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