JP2013171798A - Negative electrode material for sodium secondary battery, method for producing the same, negative electrode for sodium secondary battery, sodium secondary battery, and electrical equipment including the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode material for a sodium secondary battery, capable of exhibiting excellent cycle characteristics while maintaining high discharge capacity.SOLUTION: A negative electrode material for a sodium secondary battery according to the present invention includes soft carbon fired at 500-1,500°C. Specifically, the negative electrode material comprises: a simple substance of the soft carbon; the soft carbon having a surface on which a carbon coating film is formed; the soft carbon coated on a surface of a substance capable of being alloyed with sodium; or the like.

Description

  The present invention relates to a negative electrode material for a sodium secondary battery and a method for producing the same, and a negative electrode for a sodium secondary battery, a sodium secondary battery, and an electric device using the same.

Lithium secondary batteries such as lithium ion batteries and lithium polymer batteries have a higher voltage and a higher capacity and are lighter than nickel cadmium batteries and nickel metal hydride batteries. Therefore, in recent years, the use as a main power source for mobile communication devices, portable electronic devices, electric bicycles, electric motorcycles, electric vehicles and the like has been expanded.
For example, current lithium ion batteries generally use lithium-containing transition metal composite oxides such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), and lithium iron phosphate (LiFePO ₄ ) as the positive electrode. For this, graphite, hard carbon or the like capable of inserting and extracting lithium is used. In addition, electrolytes used for lithium ion batteries are mainly cyclic carbonates such as propylene carbonate (PC) and ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (MEC). Lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium perchlorate (LiClO ₄₎ ), Lithium bistrifluoromethanesulfonylamide (LiN (CF ₃ SO ₂ ) ₂ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), and other electrolyte salts are used.

  A major problem with current lithium ion batteries is the uneven distribution of lithium resources. In recent years, research on non-aqueous electrolyte secondary batteries using sodium ions instead of lithium ions has begun. Sodium is abundant in seawater, the sixth most abundant element on earth, and is an inexpensive and readily available element. In other words, it can be said to be a very attractive element from the recent rare earth-free trend. The negative electrode current collector uses a copper foil in a lithium ion battery, but has an advantage that an inexpensive aluminum foil can be used in a sodium ion battery. Further, sodium is an alkali metal element similar to lithium and has similar properties, and the theory of sodium ion batteries has been studied for a long time.

However, sodium ion batteries have significant challenges.
For example, a lithium ion battery is an intercalation in which lithium ions move between lithium-containing transition metal oxides such as graphite as a negative electrode active material and LiCoO _{2 as a} positive electrode active material, and move between molecules of each material. Charging / discharging is performed by causing a phenomenon. Graphite has a layered molecular structure, and the structure of graphite is rarely destroyed even when lithium ions enter and exit between the layers. Theoretically, 372 mAh / g of lithium ions can be occluded.
However, since sodium ions have a large ionic radius and cannot enter between graphite layers, they do not exhibit capacity.

  Patent Document 1 discloses an invention of a secondary battery using an alkali metal as a negative electrode material. Specifically, it is described that lithium metal is used as the alkali metal. For example, if sodium metal (Na) is used as the negative electrode material as the alkali metal, it is theoretically expected that a high capacity can be obtained. Is done. However, when sodium metal (Na) is used as the negative electrode material, there is a great disadvantage that dendrites are deposited on the negative electrode during charging and reach the positive electrode side by repeating charging and discharging, causing an internal short circuit phenomenon. In addition, the deposited dendrites have high reaction activity due to their large specific surface area, and an interfacial film consisting of a decomposition product of a solvent having no electron conductivity is formed on the surface, thereby increasing the internal resistance of the battery. Reduces charge / discharge efficiency. For these reasons, sodium ion batteries using sodium metal have the disadvantages of low reliability and short cycle life.

From such a background, a negative electrode material that is made of a material other than sodium metal and does not cause an internal short circuit is desired in a sodium ion secondary battery.
Patent Document 2 describes an invention relating to a sodium ion secondary battery using a fibrous carbon material having a diameter of 0.1 μm to 1.0 μm as a negative electrode.
However, in the case of the negative electrode using the straight fibrous carbon material described in Patent Document 2, the cycle life is good, but there is a problem that the energy density is small.

Patent Document 3 and Non-Patent Document 1 describe using a hard carbon electrode in a sodium ion secondary battery.
However, since hard carbon has a filling plateau near 0 V (vs. Na ⁺ / Na), it is highly likely that sodium dentite will precipitate on the negative electrode surface when fast charging or filling in a low temperature environment is performed. There's a problem.

Patent Document 4 discloses an example in which sodium ions are contained in a nonaqueous electrolyte in a nonaqueous electrolyte secondary battery using a negative electrode containing Sn alone or Ge alone.
However, in the case of the negative electrode containing Sn alone or Ge alone described in Patent Document 4, large volume expansion / contraction occurs due to sodium occlusion / release occurring during charging / discharging. As a result, there is a problem that the electrode itself may be broken and the cycle life is poor.

JP 58-73968 A Japanese Patent Laid-Open No. 3-155062 WO2010 / 109889A1 JP 2006-244976 A

Murata Wataru et al., Hard Carbon for Sodium Ion Battery, 50th Battery Discussion Meeting, 1D05, pp. 233, issued on November 30, 2009

  The present invention has been made in view of the current state of the prior art, and its main object is a negative electrode material for sodium secondary batteries capable of exhibiting excellent cycle characteristics while maintaining a high discharge capacity, and a method for producing the same. And a negative electrode for a sodium secondary battery, a sodium secondary battery, and an electric device using the same.

The negative electrode material for a sodium secondary battery according to the present invention contains soft carbon baked at 500 to 1500 ° C. (hereinafter sometimes referred to as “the present soft carbon”).
The negative electrode material for a sodium secondary battery according to the present invention is composed of the present soft carbon alone, a carbon coating film formed on the surface of the present soft carbon, and the present soft carbon on the surface of a substance that can be alloyed with sodium. Examples include those coated with carbon.

The method for producing a negative electrode material for a sodium secondary battery according to the present invention comprises a method of forming a composite powder composed of the present soft carbon and carbon by coating the surface of the present soft carbon with carbon.
Here, the composite powder is preferably formed by subjecting the soft carbon to a heat treatment in which the soft carbon is heated in a non-oxidizing gas atmosphere containing a carbon precursor.
Another aspect of the method for producing a negative electrode material for a sodium secondary battery according to the present invention is to form a composite powder comprising the substance and the soft carbon by coating the soft carbon on the surface of the substance that can be alloyed with sodium. It consists of the method of doing.
Here, the composite powder is preferably formed by mechanically milling a substance that can be alloyed with sodium and the present soft carbon.

The negative electrode for a sodium secondary battery according to the present invention is a negative electrode using the negative electrode material for a sodium secondary battery or a negative electrode using a negative electrode material for a sodium secondary battery obtained by the above production method.
It is preferable that the negative electrode for sodium secondary batteries which concerns on this invention contains 5-30 mass% of binders which consist of polyimides. Moreover, it is preferable to provide the electrical power collector which consists of stainless steel.

  The manufacturing method of the negative electrode for sodium secondary batteries which concerns on this invention is a manufacturing method of the negative electrode for sodium secondary batteries containing 5-30 mass% of the binder which consists of polyimides, The temperature is 200-400 degreeC by non-oxidizing atmosphere. It includes a step of thermally cross-linking polyimide.

The sodium secondary battery according to the present invention uses the above-described negative electrode for a sodium secondary battery.
The sodium secondary battery according to the present invention preferably uses a graphite positive electrode. Moreover, it is preferable that the lower limit value of the cut-off potential is 0.05 V or more with respect to Na ⁺ / Na.
The electric apparatus according to the present invention uses these sodium secondary batteries.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode material for sodium secondary batteries which can exhibit the outstanding cycling characteristics, maintaining its high discharge capacity, its manufacturing method, the negative electrode for sodium secondary batteries, a sodium secondary battery, and this were used. Electrical equipment can be provided.

It is the figure which showed the charging / discharging curve of the electrode using the negative electrode material (soft carbon 3) of Example 3. FIG. It is the figure which showed the charging / discharging curve of the electrode using the negative electrode material (hard carbon) of the comparative example 4. It is the figure which showed the charging / discharging curve of the electrode using the negative electrode material (spheroidized graphite) of the comparative example 5. It is the figure which showed the relationship between the discharge capacity regarding the negative electrode material of Examples 1-10 and Comparative Examples 1-3, and heat processing temperature. Relationship between discharge capacity and heat treatment (firing) temperature (lithium) for test results using negative electrode materials made of soft carbon (Examples 1 to 10 and Comparative Examples 1 to 3) as negative electrodes for lithium secondary batteries FIG. It is the figure which showed the high rate discharge test result using the negative electrode material of Example 3. It is the figure which showed the high rate discharge test result using the negative electrode material of Example 11. It is the figure which showed the charging / discharging curve of Example 12. It is the figure which showed the discharge curve of this whole battery produced using the negative electrode material which consists of Example 11 as an active material, and using the soft carbon baked at 1800 degreeC as a positive electrode active material.

Hereinafter, embodiments of a negative electrode material for a sodium secondary battery and a manufacturing method thereof, and a negative electrode for a sodium secondary battery, a sodium secondary battery, and an electric device using the same will be described.
The negative electrode material for a sodium secondary battery (sodium ion secondary battery) of the present invention contains soft carbon (this soft carbon) fired at 500 to 1500 ° C.

<Negative electrode material of first embodiment>
1st embodiment of the negative electrode material for sodium secondary batteries of this invention consists of a soft carbon single-piece | unit baked at 500-1500 degreeC.

In the present invention, soft carbon (highly crystalline carbon) means “carbon that becomes a graphite crystal by performing a treatment required for graphitization”. In other words, soft carbon is a carbon that easily develops a graphite structure (a structure in which hexagonal mesh planes composed of carbon atoms are stacked regularly) when heat treatment is performed in an inert atmosphere. Also called graphitizable carbon.
On the other hand, hard carbon (low crystallinity carbon) means “carbon that cannot be converted into graphite crystals even if a treatment required for graphitization (for example, high temperature treatment) is performed”. In other words, hard carbon is carbon having an irregular structure in which the development of the above-described graphite structure is suppressed, and is also called non-graphitizable carbon.

The reason for using soft carbon in the present invention is that, as described above, hard carbon has a filling plateau in the vicinity of 0 V (vs. Na ⁺ / Na). While there is a high possibility that dentrite is deposited on the negative electrode surface, hard carbon does not have a filling plateau in the vicinity of 0 V (vs. Na ⁺ / Na), causing a problem of precipitation of sodium dentite. This is because there is not.
Hard carbon tends to decrease discharge capacity when crystallinity is improved by increasing the heat treatment temperature, whereas soft carbon increases discharge capacity (sodium by increasing the heat treatment temperature (firing temperature) and improving crystallinity. This is because the (occlusion amount) is improved. However, when the heat treatment temperature exceeds a predetermined temperature (about 800 ° C.), the discharge capacity (sodium storage amount) tends to decrease. Therefore, it is preferable to set the heat treatment temperature within a specific range sandwiching the predetermined temperature.
In the present invention, it is possible to obtain a sodium secondary battery having a high discharge capacity by using soft carbon (the present soft carbon) baked at 500 to 1500 ° C.

<Negative electrode material of second embodiment>
In the second embodiment of the negative electrode material for sodium secondary batteries according to the present invention, the soft carbon is subjected to a conductive treatment, specifically, a carbon coating film is formed on the surface of the soft carbon by heat treatment. It is a thing.
The carbon coating film is a thin film having high conductivity and ionic conductivity, and has an effect of improving the conductivity of the negative electrode material.

  Although there is no restriction | limiting in particular about the thickness of a carbon coating film, For example, it is preferable to set it as the range of 0.01-5 micrometers. When the thickness of the carbon coating film is less than 0.01 μm, the improvement in conductivity is insufficient, and the action as a lithium ion adsorption / desorption reaction field is insufficient, and charging / discharging on the surface of the active material is performed. Current concentration tends to occur and it is difficult to improve the high rate charge / discharge characteristics. On the other hand, when the thickness of the carbon coating film exceeds 5 μm, the electrode capacity density may be reduced.

  The coverage of the carbon coating film needs to be 0.1 to 10% by mass with respect to 100% by mass of the present soft carbon. When the coverage is less than 0.1% by mass, the improvement in conductivity is insufficient, current concentration at the time of charging / discharging on the active material surface tends to occur, and it becomes difficult to improve the high rate charging / discharging characteristics. . In addition, there is a problem that it is difficult to act as a lithium ion adsorption / desorption reaction field. On the other hand, when the coverage exceeds 10% by mass, there is a problem in that the electrode capacity density is reduced. The minimum with a preferable coverage is 0.2 mass%, and a more preferable minimum is 0.5 mass%. Moreover, the upper limit with preferable coverage is 5 mass%, and a more preferable upper limit is 2 mass%.

<Negative electrode material of third embodiment>
The third embodiment of the negative electrode material for sodium secondary batteries of the present invention is a composite powder (hereinafter referred to as “soft”) in which the surface of a substance that can be alloyed with sodium (hereinafter referred to as an alloying substance) is coated with the present soft carbon. Carbon / alloying substance composite (or composite powder) ”.
Here, “composite” is a different concept from “mixing”. Whereas the mixed powder is simply an assembly of the alloying substance and the powder of the present soft carbon, the composite powder contains both the alloying substance and the present soft carbon in one particle constituting the powder.

Examples of alloying materials that can be used in the present invention include Sn, Sb, Ge, SnO, SnO ₂ , SnS, SnS ₂ , SnF ₂ , SnCl ₂ , SnBr ₂ , SnI ₂ , Sb ₂ S ₃ , GeO ₂ , CuO, NiO etc. are mentioned, These 1 type may be used independently and 2 or more types may be used together. Among these, it is particularly preferable to use Sn, Sb, SnS, SnS ₂ , and Sb ₂ S ₃ . The reason is that a high capacity negative electrode can be obtained.

  In the present invention (third embodiment), the reason why soft carbon is used is that it is softer than hard carbon or graphite, so that soft carbon and an alloying substance are easily combined by mechanical milling. On the other hand, since hard carbon and graphite are harder than soft carbon, they are difficult to be combined even if mechanical milling is performed.

  Another reason for using soft carbon is that the negative electrode material with soft carbon coated on the surface of the alloying substance and the negative electrode using polyimide binder are less deteriorated even when charged and discharged in a high temperature environment. is there. For example, in the case of an alloying material negative electrode using the same binder or a graphite negative electrode using the same binder, the active material in a charged state reacts with sodium and polyimide in a high temperature environment to deteriorate the binder. That is, since the ability as a binder falls, a favorable cycle life characteristic cannot be obtained.

According to the negative electrode material for a sodium secondary battery of the present invention (third embodiment), the soft carbon is covered with soft carbon around the alloyed substance as a nucleus, so that the soft carbon is occluded during charging and discharging. -The volume change (expansion / shrinkage) of the alloying substance caused by the release can be alleviated, and the crack caused by the volume change of the alloying substance can be suppressed.
As a result, the cycle characteristics can be improved even with an active material having a short cycle life with only the alloying material, and therefore the cycle life characteristics are good even with an active material having a high capacity (large volume expansion). Therefore, it can function as a negative electrode material having a high capacity and a good cycle life. In addition, the soft carbon prevents the alloying substance from coming into direct contact with the electrolytic solution, so that it can function as a negative electrode material having excellent high-temperature durability.

Next, the manufacturing method of the negative electrode material for sodium secondary batteries which concerns on this invention is demonstrated.
The negative electrode material for sodium secondary batteries according to the present invention can be broadly classified into the first to third embodiments as described above.

<The manufacturing method of the negative electrode material of 1st embodiment>
The negative electrode material of the first embodiment is manufactured by firing a soft carbon precursor.
Examples of the soft carbon precursor include coal-based heavy oil such as tar and pitch or petroleum-based heavy oil.
The firing temperature of the soft carbon precursor is set to 500 to 1500 ° C, preferably 600 to 1400 ° C.
When the firing temperature of the soft carbon precursor is less than 400 ° C., the soft carbon precursor is hardly carbonized (carbonized) and it is difficult to obtain soft carbon. Soft carbon is generated when the temperature is in the range of 400 ° C. or higher and lower than 500 ° C., but soft carbon having a large irreversible capacity is obtained. On the other hand, when the firing temperature rises, soft carbon gradually graphitizes from around 2800 ° C. Since graphite has a low reaction voltage range of sodium occlusion (discharge) of about 0.1 V, sodium dendride is likely to be generated under a low temperature environment or rapid charge.
If soft carbon is fired within the range of 500-1500 ° C., the above-mentioned problems do not occur, so the irreversible capacity is small, the discharge capacity is 90 mAh / g or more, and the output characteristics and cycle life characteristics are good. It becomes soft carbon. Moreover, when the firing temperature is set to 600 to 1400 ° C., a discharge capacity of 100 mAh / g or more can be obtained.

<The manufacturing method of the negative electrode material of 2nd embodiment>
The negative electrode material of the second embodiment is manufactured by forming a carbon coating film on the surface of the present soft carbon by subjecting the present soft carbon to a heat treatment.
As the heat treatment, for example, the soft carbon is housed in a heat treatment furnace such as a rotary kiln, and a carbon precursor gas such as butane gas is introduced into the heat treatment furnace to maintain 400 to 1000 ° C. in a non-oxidizing gas atmosphere. However, the process which hold | maintains for 0.1 to 5 hours is employable. When the heat treatment temperature is less than 400 ° C., the carbon precursor is difficult to be carbonized and the conductivity improvement effect of soft carbon may be small. On the other hand, if the temperature exceeds 1000 ° C., the apparatus becomes large and not only the cost increases, but also the active material may be damaged. In addition, when the heat treatment time is less than 0.1 hour, it may be difficult to uniformly coat the carbon. On the other hand, if it exceeds 5 hours, the heat source must be driven for a long time, which may increase the cost.
The heat treatment atmosphere is not limited to butane gas, but methane gas, ethane gas, propane gas, or a mixed gas thereof can be adopted, and carbon diluted with nitrogen, helium, neon, argon, hydrogen, carbon dioxide, or a mixed gas thereof. A precursor gas may be used. Moreover, it is not limited to gas, and it may be in any physical state such as liquid or solid.

<The manufacturing method of the negative electrode material of 3rd embodiment>
The negative electrode material of the third embodiment is manufactured by coating the surface of a substance that can be alloyed with sodium (alloying substance) with the present soft carbon to form a composite powder composed of the alloying substance and the present soft carbon. Is done.
Specifically, the alloying substance and the present soft carbon are mechanically milled to form a soft carbon / alloying substance composite in which the particle surface of the alloying substance is coated with the present soft carbon.
The gravitational acceleration of the mechanical milling process is preferably 20 to 200G, and more preferably 40 to 60G. When the gravitational acceleration is less than 20 G, it is difficult to coat the surface of the alloying substance particles with soft carbon, resulting in poor life characteristics. On the other hand, if it exceeds 200 G, the alloying substance may be exposed on the surface of the composite particles, and the high-temperature durability deteriorates.

Here, the mechanical milling process is a method capable of imparting external forces such as impact, tension, friction, compression, and shear to the raw material powder (at least the alloying substance and soft carbon). , Planetary mill, rocking mill, horizontal mill, attritor mill, jet mill, crusher, homogenizer, fluidizer, paint shaker, mixer and the like.
For example, in the method using a planetary mill, the raw material powder can be pulverized / mixed or subjected to solid phase reaction by mechanical energy generated by putting both the raw material powder and balls in a container and rotating and revolving. According to this method, it is known that the raw material powder is pulverized to the nano-order.
In the present invention, the raw material powder of the negative electrode material contains at least an alloying substance and soft carbon. Since soft carbon has lower mechanical strength than alloyed materials, soft carbon is more easily pulverized than alloyed materials. Therefore, the soft carbon powder that has become fine particles is pressed against the surface of the alloying substance powder by a ball or the like, and the alloying substance can be covered with soft carbon.

  As the soft carbon coated on the surface of the alloying substance powder, one produced by the same method as that used in the first embodiment is used.

Any of the negative electrode materials of the first to third embodiments described above can be used effectively as a negative electrode active material (negative electrode material) for a sodium secondary battery.
By forming these negative electrode materials of the present invention on a current collector, the negative electrode for sodium secondary batteries according to the present invention can be obtained.

  The formation of deposition means fixing the current collector and the negative electrode material of the present invention in contact with each other. Specifically, filling of the negative electrode material, fixing of the negative electrode material by a metal net as a current collector, and the like are applicable. The deposition method is not particularly limited, and examples thereof include a pressure bonding method, a slurry method, a paste method, an electrophoresis method, a dipping method, a spin coating method, and an aerosol deposition method. In particular, when a metal foam such as foamed nickel is used as a current collector, a slurry method or a paste method is preferable from the viewpoints of packing density, electrode production rate, and the like.

In addition to the negative electrode material of the present invention, the negative electrode may contain a conductive auxiliary agent for imparting conductivity and a binder for imparting binding properties, if necessary.
For example, an appropriate solvent (N-methyl-2-pyrrolidone (NMP), water, alcohol, xylene, toluene, etc.) is added to a mixture (negative electrode mixture) containing a conductive additive and a binder in addition to the negative electrode material. The negative electrode mixture paste composition, the negative electrode mixture slurry, and the like obtained by sufficiently kneading and applying to the surface of the current collector are dried, and further pressed to form a negative electrode material-containing layer on the current collector surface. It can be formed into a negative electrode.

The sodium secondary battery according to the present invention is a sodium secondary battery equipped with this negative electrode.
This sodium secondary battery is assembled into a sodium secondary battery such as a square, cylindrical or coin type according to a conventional method using battery elements (positive electrode, separator, electrolyte, etc.) of a known sodium secondary battery. Can be manufactured.

  As the conductive auxiliary agent, those usually used, for example, those described above can be used. When a carbon material is included, the type (structure, etc.) of the carbon material is not particularly limited. For example, carbon materials such as acetylene black (AB), ketjen black (KB), graphite, carbon fiber, carbon tube, graphene, and amorphous carbon may be used alone or in combination of two or more. May be. More preferably, those capable of forming a conductive three-dimensional network structure in the composite powder (for example, flaky conductive material (flaked copper powder, flake nickel powder, etc.), carbon fiber, carbon tube, amorphous carbon, etc.) Is preferred. If a conductive three-dimensional network structure is formed, a sufficient current collecting effect can be obtained as a negative electrode material for sodium secondary batteries, and the volume expansion of electrodes (particularly alloy components) during Na occlusion can be effectively suppressed. it can.

  Binders are also commonly used, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyimide (PI), polyamide, polyamideimide, polyacryl, styrene butadiene rubber (SBR), styrene-ethylene- Materials such as butylene-styrene copolymer (SEBS), carboxymethylcellulose (CMC), polyacryl, PVA, PVB and EVA may be used singly or in combination of two or more. However, since the volume expansion accompanying charging / discharging is large when the active material capacity is large, PI is preferably used as the binder.

In the negative electrode material-containing layer of the negative electrode, for example, the negative electrode material of the present invention is 50 to 99% by mass, the conductive auxiliary agent amount is 0.5 to 40% by mass, and the binder amount is 0.5 to 30% by mass. preferable.
The thickness of the negative electrode material-containing layer of the negative electrode is preferably 0.5 to 200 μm, for example, although it depends on the electrode capacity density. By setting the thickness of the negative electrode material-containing layer within this range, a practical electric capacity can be obtained while the current collector supports the negative electrode material.

The manufacturing process of the negative electrode includes a drying process in which the binder is heated and dried.
Although the heating temperature in a drying process changes with kinds of binder, for example, in the case of PVdF, 120-200 degreeC in the case of PTFE, 120-250 degreeC in the case of PI, 200-400 degreeC in the case of PI, 120-in the case of SBR In the case of 250 degreeC and CMC, 120-250 degreeC is preferable.
In particular, when PI is used as the binder, if the uncrosslinked portion of PI remains in the negative electrode, it reacts with the alloyed material in a charged state, and therefore it is preferable to thermally crosslink PI by drying at a high temperature. In order to thermally crosslink PI, the heating temperature in the drying step is preferably 200 to 400 ° C, more preferably 250 to 400 ° C. Note that the atmosphere in the drying step is preferably a non-oxidizing atmosphere in order to prevent oxidation of the current collector.

The material of the current collector used for the negative electrode is not particularly limited as long as it is a material that has electronic conductivity and can conduct electricity to the held negative electrode material. For example, conductive materials such as C, Ti, Cr, Ni, Mo, Ru, Rh, Ta, W, Os, Ir, Pt, Au, Al, and alloys containing two or more of these conductive materials (for example, Stainless steel) can be used. From the viewpoint of high electrical conductivity and good stability in the electrolyte, C, Ti, Cr, Ni, Cu, Au, Al, stainless steel and the like are preferable as the current collector, and C from the viewpoint of material cost. Al, stainless steel and the like are preferable. In particular, when the active material capacity exceeds 300 mAh / g, since the expansion / contraction associated with charging / discharging is severe, stainless steel having high mechanical strength is preferable.
The shape of the current collector includes linear, rod-like, plate-like, foil-like, net-like, woven fabric, non-woven fabric, expanded, porous body or foam, among which the packing density can be increased and the output characteristics are good Therefore, an expand, a porous body or a foam is preferable. From the viewpoint of cost, a versatile plate shape is preferable, and the thickness is preferably 5 to 40 μm.

As a positive electrode, sodium cobaltate (NaCoO ₂ ), sodium nickelate (NaNiO ₂ ), cobalt manganese sodium nickelate (NaCo _0.33 Ni _0.33 Mn _0.33 O ₂ ), sodium manganate (NaMn ₂ O ₄₎ ), Sodium ferrate (NaFeO ₂ ), sodium iron phosphate (NaFePO ₄ ), vanadium oxide-based materials, sulfur-based materials, graphite, and the like, and soft carbon may be used as the positive electrode material. it can.
In the present invention, it is preferable to use graphite or soft carbon. By using graphite or soft carbon, a 4 to 5 V class sodium secondary battery can be obtained. More preferably, it is soft carbon baked in a temperature range of 1300 to 2200 ° C. If the soft carbon is fired within the above temperature range, a positive electrode having a high potential and a high capacity can be obtained.

  The material of the current collector used for the positive electrode is not particularly limited as long as it is a material that has electronic conductivity and can conduct electricity to the held negative electrode material. For example, conductive materials such as C, Ti, Cr, Mo, Ru, Rh, Ta, W, Os, Ir, Pt, Au, and Al, and alloys containing two or more of these conductive materials (for example, stainless steel ) Can be used. The current collector is preferably C, Al, stainless steel or the like from the viewpoint of high electrical conductivity and good stability and oxidation resistance in the electrolytic solution, and Al or the like is more preferable from the viewpoint of material cost.

Although there is no restriction | limiting in particular in the shape of an electrical power collector, A foil-like base material, a three-dimensional base material, etc. can be used. When a three-dimensional base material (foamed metal, mesh, woven fabric, non-woven fabric, expanded, etc.) is used, an electrode with a high capacity density can be obtained even with a binder that lacks adhesion to the current collector. In addition, high rate charge / discharge characteristics are also improved.
Even in the case of a foil-like current collector, the capacity can be increased by forming a primer layer on the current collector surface in advance. The primer layer only needs to have good adhesion between the active material layer and the current collector and have conductivity. For example, a primer layer can be formed by applying a binder mixed with a carbon-based conductive additive on a current collector in a thickness of 0.1 μm to 50 μm.

  The conductive auxiliary for the primer layer is preferably carbon powder. If it is a metal-based conductive additive, it is possible to increase the capacity density, but the input / output characteristics deteriorate. If the conductive additive is carbon-based, the capacity density can be increased using CMC, and the input / output characteristics are improved. Examples of the carbon-based conductive assistant include KB, AB, VGCF, graphite, graphene, carbon tube, and the like. One of these may be used, or two or more may be used in combination. Among these, KB or AB is preferable from the viewpoint of conductivity and cost.

The binder for the primer layer is not limited as long as it can bind the carbon-based conductive additive. However, when the primer layer is formed using an aqueous binder such as PVA, CMC, or sodium alginate, the primer layer melts when the active material layer is formed, and the effect is often not exhibited remarkably. Therefore, when using such an aqueous binder, the primer layer may be crosslinked in advance. Examples of the cross-linking material include a zirconia compound, a boron compound, a titanium compound, and the like.
The primer layer thus prepared is a foil-shaped current collector, and not only can the capacity density be increased using CMC, but also the polarization becomes small and high even when charging / discharging with a high current. The rate charge / discharge characteristics are improved.
The primer layer is not only effective for the foil-shaped current collector but also has the same effect for a three-dimensional substrate.

As a separator, what is used for a well-known sodium secondary battery can be used.
For example, a porous sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide or the like, a glass filter, a nonwoven fabric, a woven fabric, or the like can be used, but is not limited thereto.

Since the electrolyte needs to contain sodium ions, the electrolyte is not particularly limited as long as it is used in a sodium secondary battery, but a sodium salt is preferable as the electrolyte salt. Examples of the sodium salt include NaPF ₆ , NaBF ₄ , NaClO ₄ , NaTiF ₄ , NaVF ₅ , NaAsF, NaSbF ₆ , NaCF ₃ SO ₃ , Na (C ₂ F ₅ SO ₂ ) ₂ N, NaB (C ₂ O ₄ ) ₂ , NaB ₁₀ Cl ₁₀ , NaB ₁₂ Cl ₁₂ , NaCF ₃ COO, Na ₂ S ₂ O ₄ , NaNO ₃ , Na ₂ SO ₄ , NaPF ₃ (C ₂ F ₅ ) ₃ , NaB (C ₆ F ₅ ) ₄ and salts such as Na (CF ₃ SO ₂ ) ₃ C can be used. In addition, 1 type may be used independently among the said salts, and 2 or more types may be combined. Since the sodium salt has high electronegative properties and is easily ionized, it has excellent charge / discharge cycle characteristics and can improve the charge / discharge capacity of the secondary battery. Among them, the electrolyte salt NaPF ₆ is preferred. By using NaPF ₆ as a salt, the effect of improving the discharge capacity and cycle life of the positive electrode and improving the cycle life of the negative electrode is enhanced. The concentration of the electrolytic solution (the concentration of the salt in the solvent) is not particularly limited, but is preferably 0.1 to 3 mol / L, and more preferably 0.5 to 2 mol / L.

Examples of the solvent for the electrolyte include propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), γ-butyrolactone, 2-methyltetrahydrofuran, 1,3 -Dioxolane, 4-methyl-1,3-dioxolane, 1,2-dimethoxyethane, 1,2-diethoxyethane, diethyl ether, sulfolane, methylsulfolane, nitromethane, N, N-dimethylformamide, dimethyl sulfoxide In particular, a mixture of ethylene carbonate and diethyl carbonate or γ-butyrolactone alone is preferable. The mixing ratio of the mixture of ethylene carbonate and diethyl carbonate can be arbitrarily adjusted in the range of 10 to 90 vol% for both ethylene carbonate and diethyl carbonate. Among them, the electrolyte solution solvent is preferably a solvent containing EC. By using a solvent containing EC, the effect of improving the cycle life of the negative electrode is enhanced.
Usually, since EC is solid at room temperature, EC alone does not function as an electrolyte. However, by using a mixed solvent with PC, DMC, DEC, EMC, etc., it functions as an electrolyte that can be used even at room temperature.
Alternatively, a solid electrolyte may be used without using a solvent.
Although it does not specifically limit as a structure of a sodium secondary battery, It can apply to the existing battery forms and structures, such as a laminated type battery and a wound type battery.

According to the sodium secondary battery having the above structure, it functions as a secondary battery.
Since the sodium secondary battery of the present invention has a high energy density and is configured without using a special material, it can be used as a power source for various electric devices (including vehicles using electricity).

  Examples of electrical equipment include air conditioners, washing machines, televisions, refrigerators, freezers, air conditioners, notebook computers, computer keyboards, computer displays, desktop computers, notebook computers, CRT monitors, computer racks, printers, integrated computers , Mouse, hard disk, computer peripherals, iron, clothes dryer, window fan, walkie-talkie, blower, ventilation fan, TV, music recorder, music player, oven, range, toilet seat with washing function, hot air heater, car component, car navigation system, pocket Electric light, humidifier, portable karaoke machine, ventilation fan, dryer, dry cell, air purifier, mobile phone, emergency light, game machine, blood pressure monitor, coffee mill, coffee maker, kotatsu, copy machine, disk changer, radio, shaver , Juicer, shredder, water purifier, lighting equipment, dehumidifier, dish dryer, rice cooker, stereo, stove, speaker, trouser press, vacuum cleaner, body fat scale, weight scale, health meter, movie player, electric carpet, electric kettle , Rice cooker, electric razor, table lamp, electric pot, electronic game machine, portable game machine, electronic dictionary, electronic notebook, microwave oven, electromagnetic cooker, calculator, electric cart, electric wheelchair, electric tool, electric toothbrush, red bean , Haircut equipment, telephones, watches, intercoms, air circulators, lightning insecticide, copying machines, hot plates, toasters, dryers, electric drills, water heaters, panel heaters, crushers, soldering irons, video cameras, video decks, facsimile , Fan heater, food processor, futon dryer, headphones Electric kettle, hot carpet, microphone, massage machine, miniature light bulb, mixer, sewing machine, mochi machine, floor heating panel, lantern, remote control, cold storage, water cooler, refrigeration stocker, cold air blower, word processor, whisk, electronic musical instrument , Motorcycles, toys, lawn mowers, walkers, bicycles, automobiles, hybrid cars, plug-in hybrid cars, electric cars, railways, ships, airplanes, emergency batteries, etc.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

<Soft carbon production>
Petroleum pitch (soft carbon precursor) was carbonized (carbonized) under the firing conditions shown in Table 1 below, and then pulverized to obtain soft carbons 1 to 13. The firing condition was that the pitch was heated to a predetermined temperature at a heating rate of 20 ° C./hour in a nitrogen gas atmosphere and then held at that temperature for 10 hours.

<Examples 1 to 10, Comparative Examples 1 to 5>
The carbon materials shown in Table 2 below were used as samples of Examples 1 to 10 and Comparative Examples 1 to 5.

<Evaluation of battery characteristics>
Using the negative electrode materials composed of Examples 1 to 10 and Comparative Examples 1 to 5 as active materials, 85% by mass of the negative electrode active material, 5% by mass of KB, and 10% by mass of PVdF binder were mixed to prepare a slurry mixture. After preparing and applying and drying on an aluminum foil (thickness 25 μm, 4.88 mg / cm ² ), the aluminum foil and the coating film are closely bonded with a roll press machine, and then heat treatment (under reduced pressure, 300 And a test electrode (negative electrode) was obtained.
As a counter electrode, metal sodium foil is used, and a glass filter (trade name “Advantech GA-100”, thickness 0.44 mm, porosity 90.1% is compressed to a thickness 0.35 mm, porosity 87.6% is used as a separator. And a coin cell (CR2032) having 1 mol / L NaPF ₆ / EC-DEC (1: 1 vol%) as an electrolytic solution.

<Charge / discharge test>
The prepared test cell (sodium secondary battery) was subjected to a charge / discharge test at a rate of 0.5C. The test conditions were set such that the cut-off potential was 0 to ¹ V (vs. Na ⁺ / Na) and the temperature atmosphere was 30 ° C.
Table 3 shows the test results using the negative electrode materials of Examples 1 to 10 and Comparative Examples 1 to 5. As is apparent from Table 3, the batteries using the soft carbon of each example as the negative electrode material have an initial discharge capacity of 90 to 165 mAh / g. Further, the discharge capacity at the 100th charge / discharge cycle showed a value of 81 to 150 mAh / g, and the cycle life characteristics were good.

  Among these, the charge / discharge curves of the electrodes using the negative electrode materials of Example 3 and Comparative Examples 4 and 5 having particularly good characteristics are shown in FIGS. 1 is a charge / discharge curve of an electrode using the negative electrode material (soft carbon 3) of Example 3, FIG. 2 is a charge / discharge curve of an electrode using the negative electrode material (hard carbon) of Comparative Example 4, and FIG. 3 is a comparative example. 5 shows charge / discharge curves of electrodes using the negative electrode material 5 (spheroidized graphite).

In the case of the negative electrode made of hard carbon (Comparative Example 4), as shown in Table 3 and FIG. 2, a capacity of about 380 mAh / g was confirmed in the initial charge, and the reversible capacity was about 250 mAh / g. However, there is a large plateau in the vicinity of 0 V, and there is a concern about the precipitation of dendriides.
In the case of the negative electrode made of spheroidized graphite (Comparative Example 5), as shown in Table 3 and FIG. 3, the capacity of about 80 mAh / g can be confirmed in the initial charge, but the reversible capacity is about 10 mAh / g. It turns out that it cannot be used as a negative electrode.
On the other hand, in the case of the negative electrode made of soft carbon (Example 3), as shown in Table 3 and FIG. 1, a capacity of about 320 mAh / g can be confirmed in the initial charge, and the reversible capacity is about 160 mAh / g. It was. Although the reversible capacity is lower than that in the case of hard carbon (Comparative Example 4), it can be seen that there is no large plateau in the vicinity of 0 V, so that the possibility of dendride precipitation is low.

Moreover, in order to prevent dendride precipitation from a negative electrode, a normal battery is designed so that it may become positive electrode regulation. Therefore, the entire negative electrode capacity cannot be used. Further, as described above, since there is a concern about dendrid precipitation from the negative electrode, it is recommended not to use a potential near 0V.
Therefore, Table 4 shows the capacities of the batteries using the negative electrode materials of Example 3 and Comparative Example 4 by changing the lower limit of the cutoff potential of the test conditions to a predetermined value. As is clear from Table 4, when the lower limit value of the cutoff potential is 0.05 V (vs. Na ⁺ / Na) or more, the capacity retention rate of Example 3 may exceed the capacity retention rate of Comparative Example 4. Recognize. Therefore, in the present invention, the lower limit value of the cutoff potential is preferably 0.05 V or more, more preferably 0.08 V or more, based on Na ⁺ / Na.

  FIG. 4 shows the relationship (sodium) between the discharge capacity and the heat treatment (firing) temperature for the test results using the negative electrode material made of soft carbon (Examples 1 to 10 and Comparative Examples 1 to 3) as the negative electrode for a sodium secondary battery. Shown in

From FIG. 4, the negative electrode material made of soft carbon improves the discharge capacity (sodium storage amount) when the heat treatment temperature (calcination temperature) is increased, but the discharge capacity decreases when the heat treatment temperature exceeds a predetermined temperature (800 ° C.). It turns out that there is a tendency to. In particular, it can be seen that a sodium secondary battery having a high discharge capacity can be obtained by using soft carbon (Examples 1 to 10) baked at 500 to 1500 ° C.
Moreover, about the test result which used the negative electrode material (Examples 1-10 and Comparative Examples 1-3) consisting of soft carbon as a negative electrode for lithium secondary batteries, the relationship between discharge capacity and heat treatment (firing) temperature ( Lithium) is shown in FIG.
As is clear by comparing FIG. 4 and FIG. 5, the evaluation result as a sodium secondary battery is different from the evaluation result as a lithium secondary battery. Accordingly, it has been proved that even a negative electrode material having a high capacity as a lithium ion battery does not necessarily have a high capacity as a sodium secondary battery.

<High-rate discharge test (comparison with or without carbon coating)>
The prepared test cell (sodium secondary battery) was charged at a rate of 0.5 C, and then a high rate discharge test was performed at a predetermined discharge rate. In addition, the capacity | capacitance at the time of charging by 0.5C rate is set to 100% of battery capacity. The test conditions were set such that the cut-off potential was 0 to ¹ V (vs. Na ⁺ / Na) and the temperature atmosphere was 30 ° C.
The results of the high rate discharge test using the negative electrode material of Example 3 are shown in FIG.
In Example 11, a powder obtained by charging the powder of Example 3 in a rotary kiln (700 ° C., 1 hour, butane gas atmosphere) in advance to form 1% by mass of a conductive thin film (carbon coating film) was used. The production of electrodes and the test conditions are the same as in Example 3.
The results of the high rate discharge test using the negative electrode material of Example 11 are shown in FIG.
As is clear from comparison between FIG. 6 and FIG. 7, Example 11 in which the conductive thin film was formed had better high rate discharge characteristics (output characteristics) than Example 3 in which the conductive thin film was not formed. .

<Charge / discharge test (combining Sn)>
A soft carbon / Sn composite (mass ratio of 3: 7) in which Sn and soft carbon (firing temperature: 800 ° C.) are treated with a planetary ball mill in an atmospheric argon atmosphere to coat the Sn particle surface with soft carbon. ) Was formed (Example 12).
The conditions of the planetary ball mill process are a gravitational acceleration of 100 G and a processing time of 1 hour.
The conditions are the same as in Example 3 except that the binder is changed to PI.
The produced test cell (sodium secondary battery) was charged and discharged at a rate of 0.2C. The test conditions were set such that the cut-off potential was 0 to ¹ V (vs. Na ⁺ / Na) and the temperature atmosphere was 30 ° C.
In FIG. 8, the charging / discharging curve of Example 12 is shown.
As is apparent from FIG. 8, a high capacity of 300 to 400 mAh / g was reversibly obtained by combining soft carbon and Sn.

<All battery tests>
Using the negative electrode material consisting of Example 11 as an active material, 94% by mass of the negative electrode active material, 2% by mass of KB, 3.5% by mass of CMC binder, and 0.5% by mass of SBR binder were mixed to prepare a slurry mixture. Then, after coating and drying on an aluminum foil (thickness 25 μm, 4.88 mg / cm ² ), the aluminum foil and the coating film are closely bonded with a roll press machine, and then heat treatment (under reduced pressure, 200 ° C. 5 hours or more) to obtain a test electrode (negative electrode).
The positive electrode uses soft carbon baked at 1800 ° C. as an active material, and mixes 94% by mass of the positive electrode active material, 2% by mass of KB, and 4% by mass of the CMC binder to prepare a slurry-like mixture having a thickness of 1 μm. After applying and drying on the aluminum foil (thickness 26 μm, 4.88 mg / cm ² ) on which the carbon layer has been formed, the aluminum foil on which the carbon layer has been formed and the coating film are closely bonded by a roll press machine, Next, heat treatment was performed (under reduced pressure at 200 ° C. for 5 hours or longer) to obtain a positive electrode.
As a separator, a glass filter (trade name “Advaneck GA-100”, thickness 0.44 mm, porosity 90.1% compressed to a thickness 0.35 mm, porosity 87.6%), electrolyte A coin cell (CR2032) equipped with 1 mol / L NaPF ₆ / EC-DEC (1: 1 vol%) was produced.
The nominal capacity of all the batteries is 1 mAh, and the N / P ratio is 1.7. The prepared test cell (all sodium battery) was subjected to a charge / discharge test at a rate of 0.2 C, and the discharge curve is shown in FIG. The test conditions for all the batteries were set to a cut-off potential: 1.5-4.90 V (battery potential) and a temperature atmosphere: 30 ° C.
As is clear from FIG. 9, a large discharge plateau is confirmed at a high potential of 4 V or more by combining with a positive electrode using soft carbon fired at 1800 ° C. as an active material.

  The sodium secondary battery obtained by the present invention can be used for applications such as main power sources for mobile communication devices, portable electronic devices, electric bicycles, electric motorcycles, electric vehicles, and the like.

  The negative electrode for a sodium secondary battery obtained by the present invention has a high capacity and good cycle life characteristics, and a sodium secondary battery using the negative electrode can withstand practical use. Sodium is an element that is inexpensive and readily available, has no local uneven distribution like lithium resources, and because sodium ion batteries can use inexpensive aluminum foil as a negative electrode current collector, If it can be replaced with a lithium ion secondary battery that is the mainstream of secondary batteries, it will be possible to manufacture secondary batteries at a lower cost than before. Thus, the present invention greatly contributes to further development of the secondary battery market.

Claims (16)

  A negative electrode material for a sodium secondary battery containing soft carbon baked at 500 to 1500 ° C.   The negative electrode material for sodium secondary batteries according to claim 1, wherein a carbon coating film is formed on the surface of the soft carbon.   The negative electrode material for sodium secondary batteries according to claim 1, wherein the soft carbon is coated on the surface of a substance that can be alloyed with sodium.   The manufacturing method of the negative electrode material for sodium secondary batteries which forms the composite powder which consists of the said soft carbon and carbon by coat | covering carbon on the surface of the soft carbon baked at 500-1500 degreeC.   5. The sodium secondary battery according to claim 4, wherein the soft carbon is heat-treated in a non-oxidizing gas atmosphere containing a carbon precursor to form a composite powder in which the surface of the soft carbon is coated with carbon. Manufacturing method of negative electrode material.   A method for producing a negative electrode material for a sodium secondary battery, wherein a composite powder composed of the substance and the soft carbon is formed by coating the surface of a substance that can be alloyed with sodium with a soft carbon fired at 500 to 1500 ° C. .   The manufacturing method of the negative electrode material for sodium secondary batteries of Claim 6 which forms the composite powder which coat | covered the said substance surface with the said soft carbon by carrying out the mechanical milling process of the said substance and the said soft carbon.   The negative electrode for sodium secondary batteries using the negative electrode material for sodium secondary batteries in any one of Claims 1 thru | or 3.   The negative electrode for sodium secondary batteries using the negative electrode material for sodium secondary batteries obtained by the manufacturing method in any one of Claims 4 thru | or 6.   The sodium secondary battery negative electrode according to claim 8 or 9, comprising 5 to 30% by mass of a binder made of polyimide.   The negative electrode for sodium secondary batteries according to any one of claims 8 to 10, comprising a current collector made of stainless steel.   The method for producing a negative electrode for a sodium secondary battery according to claim 10, comprising a step of thermally cross-linking the polyimide of the binder at a temperature of 200 to 400 ° C. in a non-oxidizing atmosphere. .   The sodium secondary battery using the negative electrode for sodium secondary batteries in any one of Claims 8 thru | or 11.   The sodium secondary battery according to claim 13, wherein a graphite positive electrode is used. The sodium secondary battery according to claim 13, wherein the lower limit value of the cutoff potential is 0.05 V or more with respect to Na ⁺ / Na.   An electric device using the sodium secondary battery according to claim 13.

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