JP2013229319A - Electrode for sodium secondary battery and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for a sodium secondary battery capable of achieving excellent initial discharge capacity and cycle durability, a method for manufacturing the same, and a sodium secondary battery.SOLUTION: An electrode for a sodium secondary battery includes an electrode mixture layer containing an electrode active material and a binder including polycarboxylic acid and/or alkali metal polycarboxylate. A content of the binder in the electrode mixture layer is less than 15 mass%. A method for manufacturing an electrode for a sodium secondary battery includes the steps of: (1) heating and drying an electrode active material; (2) manufacturing an electrode mixture slurry containing the heated and dried electrode active material and a binder including polycarboxylic acid and/or alkali metal polycarboxylate, the step being performed after the step (1); and (3) forming an electrode mixture layer using the manufactured electrode mixture slurry, the step being performed after the step (2).

Description

  The present invention relates to an electrode for a sodium secondary battery and a method for producing the same. More specifically, the present invention relates to a sodium secondary battery electrode capable of realizing excellent initial discharge capacity and cycle durability, a method for producing the same, and a sodium secondary battery. Examples of the sodium secondary battery in the present invention include a sodium ion secondary battery and a sodium metal secondary battery.

  In recent years, in order to cope with air pollution and global warming, reduction of carbon dioxide emissions has been strongly desired. In the automobile industry, there is a great expectation for reducing carbon dioxide emissions by introducing electric vehicles (EV) and hybrid electric vehicles (HEV), and the development of secondary batteries for motor drive that will be the key to the practical application of these technologies is thriving. Has been done. As a secondary battery for driving a motor, a lithium ion secondary battery having a high theoretical energy is attracting attention, and is currently being developed rapidly. However, lithium is expensive because it is not resource-rich compared to, for example, sodium. Therefore, in order to reduce the cost of the battery and provide a stable supply, a sodium ion secondary battery that replaces the lithium ion secondary battery is currently under development.

  Conventionally, even when a carbon material is used as a negative electrode active material, in order to suppress a decrease in overdischarge cycle life, the negative electrode includes a negative electrode and a sodium ion non-aqueous electrolyte, and the negative electrode includes a negative electrode active material and aluminum or aluminum. A sodium ion battery having a negative electrode current collector made of an alloy has been proposed (see Patent Document 1).

  Patent Document 1 describes that it is preferable to apply a polymer of a fluorine compound as a binder, and an example in which polyvinylidene fluoride is applied.

JP 2010-225522 A

  However, when the present inventors examined the discharge capacity and cycle durability in a sodium secondary battery, when polyvinylidene fluoride was applied as a binder, the battery characteristics of moisture compared to a lithium ion battery were improved. The technical knowledge that the adverse effect is great was obtained. Specifically, polyvinylidene fluoride reacts with moisture to produce hydrogen fluoride, which causes decomposition of the electrolyte and active material, resulting in excellent initial discharge capacity and cycle durability The technical knowledge that it is not possible to realize.

  The present invention has been made based on such technical knowledge. And the place made into the objective of this invention is providing the electrode for sodium secondary batteries which can implement | achieve the outstanding initial discharge capacity and cycling durability, its manufacturing method, and a sodium secondary battery.

  The inventors of the present invention have made extensive studies in order to achieve the above object. And as a result, it has an electrode mixture layer containing an electrode active material and a binder containing polycarboxylic acid and / or alkali metal salt of polycarboxylic acid, and the content of the binder in the electrode mixture layer The present invention has been completed by finding that the above-mentioned object can be achieved by making the composition less than 15% by mass.

  That is, the sodium secondary battery electrode of the present invention has an electrode mixture layer containing an electrode active material and a binder containing a polycarboxylic acid and / or a polycarboxylic acid alkali metal salt. And in the electrode for sodium secondary batteries of this invention, content of the binder in an electrode mixture layer is less than 15 mass%.

  Moreover, the manufacturing method of the electrode for sodium secondary batteries of this invention is the process (1) which heat-drys an electrode active material, the electrode active material which was implemented after the process (1), and was heat-dried, and polycarboxylic acid And / or step (2) of producing an electrode mixture slurry containing a binder containing an alkali metal salt of polycarboxylic acid, and using the produced electrode mixture slurry after step (2). It is a manufacturing method including the process (3) which forms an electrode mixture layer.

  Furthermore, the sodium secondary battery of this invention has the said electrode for sodium secondary batteries of this invention as at least one of a negative electrode and a positive electrode.

  According to this invention, it has an electrode mixture layer containing an electrode active material and a binder containing a polycarboxylic acid and / or a polycarboxylic acid alkali metal salt, and contains the binder in the electrode mixture layer An electrode whose amount is less than 15% by mass was applied. Therefore, it is possible to provide a sodium secondary battery electrode and a sodium secondary battery that can realize excellent initial discharge capacity and cycle durability. In addition, the electrode active material was heated and dried before preparing an electrode mixture slurry containing the electrode active material and a binder containing polycarboxylic acid and / or alkali metal salt of polycarboxylic acid. Therefore, the manufacturing method of the electrode for sodium secondary batteries which can implement | achieve the outstanding initial stage discharge capacity and cycling durability can be provided.

It is sectional drawing which shows the outline of an example of the sodium ion secondary battery which concerns on one Embodiment of this invention. 3 is an X-ray photoelectron spectrum using the radiation of the working electrode of Comparative Example 1. FIG. It is a graph which shows the charge / discharge capacity | capacitance and the irreversible capacity | capacitance in the sodium secondary battery of each example.

  Hereinafter, the electrode for sodium secondary batteries, the manufacturing method thereof, and the sodium secondary battery of the present invention will be described in detail. In addition, it is suitable to use the electrode for sodium secondary batteries of this invention as a negative electrode of the sodium ion secondary battery which is a sodium secondary battery, for example. Therefore, the sodium secondary battery electrode and the sodium secondary battery according to an embodiment of the present invention will be described by taking a sodium ion secondary battery negative electrode and a sodium ion secondary battery as examples.

  First, a negative electrode for a sodium ion secondary battery and a sodium ion secondary battery according to an embodiment of the present invention will be described in detail with reference to the drawings. In addition, the dimension ratio of drawing quoted by the following embodiment is exaggerated on account of description, and may differ from an actual ratio.

[Configuration of sodium ion secondary battery]
FIG. 1 is a cross-sectional view schematically showing an example of a sodium ion secondary battery according to an embodiment of the present invention. As shown in FIG. 1, the sodium ion secondary battery 1 of the present embodiment has a configuration in which a battery element 10 to which a positive electrode tab 21 and a negative electrode tab 22 are attached is enclosed in an exterior body 30. In the present embodiment, the positive electrode tab 21 and the negative electrode tab 22 are led out in the opposite direction from the inside of the exterior body 30 to the outside. Although not shown, the positive electrode tab and the negative electrode tab may be led out in the same direction from the inside of the exterior body to the outside. Moreover, such a positive electrode tab and a negative electrode tab can each be attached to the positive electrode collector and negative electrode collector which are mentioned later by ultrasonic welding, resistance welding, etc., for example.

[Positive electrode tab and negative electrode tab]
The positive electrode tab 21 and the negative electrode tab 22 are made of, for example, materials such as aluminum (Al), copper (Cu), titanium (Ti), nickel (Ni), alloys thereof, and stainless steel (SUS). However, the material is not limited to these, and a conventionally known material that can be used as a tab for a sodium ion secondary battery can be used. The positive electrode tab and the negative electrode tab may be made of the same material or different materials. In addition, as in the present embodiment, separately prepared tabs may be connected to a positive electrode current collector and a negative electrode current collector described later, respectively, and each positive electrode current collector and each negative electrode current collector described later are foil-shaped. If so, the tabs may be formed by extending each of them.

[Exterior body]
The exterior body 30 is preferably formed of a film-like exterior material from the viewpoint of miniaturization and weight reduction, but is not limited thereto, and is used for a sodium ion secondary battery. What was formed with the conventionally well-known material which can be used for an exterior body can be used. When applied to an automobile, for example, a polymer-metal composite laminate sheet having excellent thermal conductivity is used from the viewpoint of efficiently transferring heat from the automobile heat source and heating the inside of the battery to the battery operating temperature quickly. Is preferred.

[Battery element]
As shown in FIG. 1, the battery element 10 in the sodium ion secondary battery 1 of the present embodiment has a configuration in which a plurality of positive electrodes 11, electrolyte layers 13, and negative electrodes 12 are stacked. The positive electrode 11 has a configuration in which a positive electrode mixture layer 11B is formed on both surfaces of the positive electrode current collector 11A. The negative electrode 12 has a configuration in which a negative electrode mixture layer 12B is formed on both surfaces of the negative electrode current collector 12A.

  At this time, the positive electrode mixture layer 11B formed on one surface of the positive electrode current collector 11A of one positive electrode 11 and the negative electrode current collector 12A of the negative electrode 12 adjacent to the one positive electrode 11 are formed on one surface. A plurality of layers of a positive electrode, an electrolyte layer, and a negative electrode are laminated in order so that the negative electrode mixture layer 12 </ b> B faces the electrolyte layer 13.

  Thereby, the adjacent positive electrode mixture layer 11B, the electrolyte layer 13 and the negative electrode mixture layer 12B constitute one unit cell layer. Therefore, the sodium ion secondary battery 1 of the present embodiment has a configuration in which a plurality of the single battery layers 14 are stacked so that they are electrically connected in parallel. A negative electrode mixture layer 12B is formed on only one side of the negative electrode current collector 12a located in the outermost layer of the battery element 10.

  In addition, an insulating layer (not shown) for insulating between adjacent positive electrode current collectors and negative electrode current collectors may be provided on the outer periphery of the unit cell layer. Such an insulating layer is preferably formed of a material that retains the electrolyte contained in the electrolyte layer and the like and prevents electrolyte leakage from the outer periphery of the single cell layer. Specifically, general-purpose plastics such as polypropylene (PP), polyethylene (PE), polyurethane (PUR), polyamide resin (PA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polystyrene (PS), etc. Can be used. Moreover, thermoplastic olefin rubber, silicone rubber, etc. can also be used.

[Positive electrode current collector and negative electrode current collector]
The positive electrode current collector 11A and the negative electrode current collector 12A are made of, for example, a conductive material such as foil-shaped or mesh-shaped aluminum, copper, and stainless steel (SUS). However, it is not limited to these, The conventionally well-known material which can be used as a collector for sodium ion secondary batteries can be used.

[Negative electrode mixture layer]
The negative electrode mixture layer 12B contains a negative electrode active material and a polycarboxylic acid and / or alkali metal salt of a polycarboxylic acid as a binder, and may contain a conductive auxiliary agent as necessary. . Moreover, content of the binder in the negative mix layer 12B is less than 15 mass%.

  By setting it as such a negative electrode, it becomes a sodium ion secondary battery which can implement | achieve the outstanding initial stage discharge capacity and cycling durability. Further, the content of the binder is preferably 0.5% by mass or less and less than 15% by mass in the negative electrode mixture layer from the viewpoint of increasing the initial discharge capacity and improving the cycle durability. More preferably, it is contained in a proportion of 1% by mass or more and 10% by mass or less, and more preferably 1% by mass or more and 5% by mass or less.

  The negative electrode active material is not particularly limited as long as it can occlude and release sodium. For example, a negative electrode active material having a moisture content of preferably 500 ppm or less, more preferably 300 ppm or less, more preferably 100 ppm or less, more preferably 50 ppm or less, and even more preferably 30 ppm or less is applied by heating drying described below in detail. It is preferable. By forming a negative electrode mixture layer using a negative electrode active material with a reduced water content and a binder containing a polycarboxylic acid and / or polycarboxylic acid alkali metal salt that is a water-absorbing polymer, Since decomposition of the electrolytic solution is suppressed and the amount of decomposition products deposited on the negative electrode surface can be reduced, a decrease in capacity can be suppressed. Here, the moisture content of the negative electrode active material can be measured by, for example, a Karl Fischer moisture meter using the negative electrode active material powder before electrode formation as a measurement target. Moreover, as a negative electrode active material, a carbon material can be mentioned, for example. As the carbon material, hard carbon can be suitably used. When hard carbon is applied, a particularly excellent initial discharge capacity can be realized. In addition, hard carbon does not change in quality even when heated and dried, and has an advantage that it has good compatibility with polycarboxylic acids and polycarboxylic acid alkali metal salts.

  “Hard carbon” refers to non-graphitizable carbon that does not migrate to graphite and maintains a random structure even when fired at 3000 ° C. In contrast, “soft carbon” refers to graphitizable carbon that transitions to graphite when fired at 3000 ° C. These are sometimes classified as low crystalline carbon. Further, in the present invention, “containing hard carbon” should be interpreted to include the case of “consisting only of hard carbon”.

In addition, if the negative electrode active material is formed as a secondary battery, another negative electrode active material may be applied instead of or in addition to the hard carbon. Examples of other negative electrode active materials include soft carbon, fullerene, carbon nanomaterials in general, and polyacene, which are examples of low crystalline carbon. Other negative electrode active materials include carbon black (Ketjen black, acetylene black, channel black, lamp black, oil furnace black, thermal black, etc.), and other negative electrode active materials include, for example, Ge, Sn , Pb, In, Zn, Ca, Sr, Ba, Ru, Rh, Ir, Pd, Pt, Ag, Au, Cd, Hg, Ga, Tl, C, N, Sb, Bi, O, S, Se, Te Elemental elements and intermetallic compounds alloyed with sodium such as Cl, oxides containing these elements (silicon monoxide (SiO), SiO _x (0 <x <2), tin dioxide (SnO ₂ ), SnO) _x (0 <x <2), SnSiO ₃ etc.) and carbides (silicon carbide (SiC) etc.) etc. can also be mentioned. Furthermore, as other negative electrode active materials, for example, various kinds of titanium dioxide and sodium-titanium composite oxide (sodium titanate: Na ₂ Ti ₃ O ₇ , Na ₄ Ti ₅ O ₁₂ ) and other sodium-transition metal composite oxides are used. You can also list things. However, the material is not limited to these, and a conventionally known material that can be used as a negative electrode active material for a sodium ion secondary battery can be used. These negative electrode active materials may be used alone or in combination of two or more.

As the binder, at least one of polycarboxylic acid and alkali metal salt of polycarboxylic acid is applied. Polycarboxylic acids and polycarboxylic acid alkali metal salts have many hydrogen bonding sites and can keep moisture inside the binder, so that decomposition of the electrolyte due to moisture is suppressed, and the negative electrode surface Since the accumulation amount of decomposition products in can be reduced, a decrease in capacity can be suppressed.
Here, the “polycarboxylic acid” refers to a polymer in which a carboxyl group is directly or indirectly bonded to an average of 10% or more of polymer structural units (for example, monomers). In addition, “polycarboxylic acid alkali metal salt” means that a carboxyl group is directly or indirectly bonded to an average of 10% or more of polymer constituent units (for example, monomers), and at least a part of the carboxyl group is an alkali carboxylate. A polymer substituted with a metal salt. In addition, as an alkali metal, it is not limited to sodium (Na), Other alkali metals, such as lithium (Li) and potassium (K), may be sufficient. In addition, the main chain (in some cases, a side chain) of the polymer in polycarboxylic acid or the like is substituted or unsubstituted aliphatic hydrocarbon group (for example, methylene group) or substituted or unsubstituted alicyclic hydrocarbon group (for example, β- Glucose), a substituted or unsubstituted aromatic hydrocarbon group (for example, a phenylene group), and the like can be used. The polycarboxylic acid and the polycarboxylic acid alkali metal salt may be a homopolymer or a copolymer. In the case of a copolymer, any of a random copolymer, an alternating copolymer, a block copolymer, and a graft copolymer may be used.
These may be used alone or in combination of two or more.

  For example, as a suitable example of polycarboxylic acid, polyacrylic acid and carboxymethylcellulose can be mentioned, and as a suitable example of polycarboxylic acid alkali metal salt, polyacrylic acid sodium salt and carboxymethylcellulose sodium salt can be mentioned. . When these are applied, excellent cycle durability can be realized. Among these, it is preferable to apply polyacrylic acid.

  Examples of the conductive aid include carbon materials such as carbon black such as acetylene black, graphite, and carbon fiber. However, the material is not limited to these, and a conventionally known material that can be used as a conductive additive for a sodium ion secondary battery can be used. These conductive assistants may be used alone or in combination of two or more.

  The amount of water contained in the negative electrode mixture layer is preferably 1100 ppm or less, more preferably 1000 ppm or less, and still more preferably 900 ppm or less. When the amount of water exceeds 1100 ppm, the initial discharge capacity and cycle durability may be worse than when polyvinylidene fluoride is applied and when polyvinylidene fluoride is applied. The amount of water in the negative electrode mixture layer can be measured by, for example, a Karl Fischer moisture meter using the negative electrode mixture layer before electrode formation as a measurement target.

[Positive electrode mixture layer]
The positive electrode mixture layer 11B contains one or more positive electrode materials capable of occluding and releasing sodium as a positive electrode active material, and a binder or a conductive aid as necessary. May be included. In addition, as the binder and the conductive auxiliary agent, those described above can be appropriately selected and used.

As a positive electrode material capable of occluding and releasing sodium, for example, a sodium-containing compound is preferable from the viewpoint of capacity and output characteristics. Examples of sodium-containing compounds include layered oxide-based materials such as sodium iron composite oxide (NaFeO ₂ ), sodium cobalt composite oxide (NaCoO ₂ ), sodium chromium composite oxide (NaCrO ₂ ), and sodium manganese composite oxide. (NaMnO ₂ ), sodium nickel composite oxide (NaNiO ₂ ), sodium nickel titanium composite oxide (NaNi _1/2 Ti _1/2 O ₂ ), sodium nickel manganese composite oxide (NaNi _1/2 Mn _1/2 O) ₂ ), sodium iron manganese composite oxide (Na _2/3 Fe _1/3 Mn _2/3 O ₂ ), sodium nickel cobalt manganese composite oxide (NaNi _1/3 Co _1/3 Mn _1/3 O ₂ ), And their solid solutions and non-stoichiometric compounds. Examples of the sodium-containing compound include sodium manganese composite oxide (Na _2/3 MnO ₂ , NaMn ₂ O ₄ ), sodium nickel manganese composite oxide (Na _2/3 Ni _1/3 Mn _2/3 O _2). , NaNi _1/2 Mn _3/2 O ₂ ) and the like. Furthermore, examples of the sodium-containing compound include olivine-based materials such as a sodium iron phosphate compound (NaFePO ₄ ), a sodium manganese phosphate compound (NaMnPO ₄ ), and a sodium cobalt phosphate compound (NaCoPO ₄ ). As the sodium-containing compound, may be mentioned for example, fluoride olivine material _{Na 2 FePO 4 F, Na 2} MnPO 4 F, and Na 2 CoPO ₄ F. Furthermore, organic active materials such as polymer radical compounds and π-conjugated polymers known from organic radical batteries can also be mentioned. Furthermore, the element which forms a compound with sodium, such as solid sulfur and sulfur-carbon composite material, can also be mentioned. However, the present invention is not limited to these, and other sodium-containing transition metal oxides, sodium-containing transition metal sulfides, sodium-containing transition metal fluorides, etc., as long as they can occlude and release sodium. Conventionally known materials can also be used.

  In addition, you may use active materials other than the above, for example, a sodium metal and a sodium alloy can also be used. When sodium metal is used as the active material, since sodium metal has a lower potential than, for example, hard carbon, the battery has a negative electrode on the sodium metal side and a positive electrode on the hard carbon side.

  In addition, when the optimum particle diameter is different in expressing the unique effect of each active material, the optimum particle diameters may be mixed and used for expressing each unique effect. There is no need to make the particle size of the material uniform. For example, when using hard carbon in the form of particles as the negative electrode active material, the average particle diameter of the hard carbon may be the same as the average particle diameter of the negative electrode active material contained in the existing negative electrode mixture layer, and is not particularly limited. . From the viewpoint of increasing the output, it is preferably in the range of 1 to 20 μm. However, it is not limited to such a range at all, and it goes without saying that it may be outside this range as long as the effects of the present embodiment can be expressed effectively.

[Electrolyte layer]
As the electrolyte layer 13, for example, a layer structure is formed using a nonaqueous electrolytic solution or a polymer gel electrolyte held in a separator described later, and further, a laminated structure is formed using a polymer gel electrolyte. The thing etc. can be mentioned.
As the non-aqueous electrolyte, for example, it is preferable that it is usually used in a sodium ion secondary battery. Specifically, a form in which a sodium salt is dissolved in a non-aqueous solvent that is an organic solvent, and further if necessary The additive to be added has a dissolved form.
Examples of the sodium salt include inorganic acid anion salts such as NaPF ₆ , NaBF ₄ , NaClO ₄ , NaAsF ₆ , NaTaF ₆ , NaAlCl ₄ , Na ₂ B ₁₀ Cl ₁₀ , NaCF ₃ SO ₃ , Na (CF ₃ SO ₂ ) ₂ N, at least one sodium salt selected from organic acid anion salts such as Na (C ₂ F ₅ SO ₂ ) ₂ N, and the like.
As the non-aqueous solvent, for example, a non-aqueous solvent composed of a saturated cyclic carbonate (excluding a non-aqueous solvent composed only of ethylene carbonate) or a non-aqueous solvent composed of a saturated cyclic carbonate and a chain carbonate is applied. Can do.
Examples of the saturated cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like.
Examples of the chain carbonate include dimethyl carbonate (DMC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC).
In addition, other nonaqueous solvents may be included, for example, ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-dibutoxyethane; Use lactones such as butyrolactone; nitriles such as acetonitrile; esters such as methyl propionate; amides such as dimethylformamide; one or a mixture of two or more selected from methyl acetate and methyl formate You can also.
In addition, as a separator, the microporous film which consists of polyolefins, such as polyethylene and a polypropylene, can be mentioned, for example.
Examples of the polymer gel electrolyte include those containing a polymer constituting the polymer gel electrolyte and a non-aqueous electrolyte in a conventionally known ratio.
The polymer gel electrolyte is a solid polymer electrolyte having ion conductivity containing the above-mentioned electrolytic solution usually used in a sodium ion secondary battery, but is not limited to this. A structure in which a similar electrolyte solution is held in a polymer skeleton having no conductivity is also included.
Examples of polymers having no sodium ion conductivity used in polymer gel electrolytes include polyvinylidene fluoride (PVdF), polyvinyl chloride (PVC), polyacrylonitrile (PAN), and polymethyl methacrylate (PMMA). it can.
However, it is not necessarily limited to these. Note that PAN, PMMA, etc. are in a class that has almost no ionic conductivity, and therefore can be a polymer having the above ionic conductivity, but here, they are used for a polymer gel electrolyte. This is exemplified as a polymer having no sodium ion conductivity.
Examples of the solid polymer electrolyte include those obtained by dissolving the sodium salt in polyethylene oxide (PEO), polypropylene oxide (PPO), and the like.
The thickness of the electrolyte layer is preferably thinner from the viewpoint of reducing internal resistance. The thickness of the electrolyte layer is usually 1 to 100 μm, preferably 5 to 50 μm.

Next, an example of the manufacturing method of the sodium ion secondary battery of this embodiment mentioned above is demonstrated.
First, a positive electrode is produced. For example, when a granular positive electrode active material is used, the positive electrode active material and, if necessary, a conductive additive, a binder, and a viscosity adjusting solvent are mixed to prepare a positive electrode mixture slurry.
Next, this positive electrode mixture slurry is applied to a positive electrode current collector, dried, and compression molded to form a positive electrode mixture layer.

  Moreover, a negative electrode is produced. For example, when using a granular negative electrode active material, a negative electrode active material, a predetermined binder, and a conductive additive and a viscosity adjusting solvent as necessary are mixed to prepare a negative electrode mixture slurry. Thereafter, the negative electrode mixture slurry is applied to a negative electrode current collector, dried, and compression molded to form a negative electrode mixture layer.

  Next, the positive electrode tab is attached to the positive electrode and the negative electrode tab is attached to the negative electrode, and then the positive electrode, the separator, and the negative electrode are stacked. Further, the laminated product is sandwiched between polymer-metal composite laminate sheets, and the outer peripheral edge except one side is heat-sealed to form a bag-like exterior body.

  After that, prepare a non-aqueous electrolyte containing a sodium salt such as sodium hexafluorophosphate and a non-aqueous solvent such as propylene carbonate, and inject it into the interior of the exterior body through the opening of the exterior body. Heat seal and seal. Thereby, a laminate type sodium ion secondary battery is completed.

  In addition, for example, in manufacturing an electrode for a sodium secondary battery, the step (1) of heating and drying the electrode active material, the electrode active material that is performed after the step (1) and dried by heating, a polycarboxylic acid, and Step (2) for producing an electrode mixture slurry containing a binder containing at least one of polycarboxylic acids, and an electrode mixture using the produced electrode mixture slurry after step (2) It is preferable to apply a manufacturing method including the step (3) of forming a layer. Before producing an electrode mixture slurry containing an electrode active material and a binder containing a polycarboxylic acid and / or a polycarboxylic acid alkali metal salt, the electrode active material is heated and dried to obtain an electrolytic solution based on moisture. In addition, since the amount of decomposition products deposited on the electrode surface can be reduced, a decrease in capacity can be suppressed.

  Furthermore, for example, it is preferable to apply hard carbon as the electrode active material. Hard carbon does not change even when dried by heating, and has an advantage of good compatibility with polycarboxylic acid and polycarboxylic acid alkali metal salt.

  In addition, for example, in the production of an electrode for a sodium secondary battery, in the step (1), it is preferably heat-dried at 100 ° C. or higher and 600 ° C. or lower, more preferably 150 ° C. or higher and 550 ° C. or lower. It is further preferable to heat dry at 200 ° C. or higher and 500 ° C. or lower. By heating and drying the electrode active material within the above temperature range before forming the electrode, it is possible to reduce the amount of moisture that cannot be removed even if heated and dried after the electrode is manufactured, and suppress the decrease in capacity. Can do.

  Furthermore, for example, in manufacturing an electrode for a sodium secondary battery, it is preferable to heat and dry under vacuum conditions in the step (1). By heating and drying the electrode active material under vacuum conditions before forming the electrode, it is possible to reduce the amount of moisture that cannot be removed even if heated and dried after the electrode is manufactured, and suppress the decrease in capacity. Can do.

  Further, for example, in manufacturing an electrode for a sodium secondary battery, an electrode mixture and water are contained, and the electrode mixture includes an electrode active material and a polycarboxylic acid and / or a polycarboxylic acid alkali metal salt. When the electrode mixture layer is formed using the electrode mixture slurry containing the binder and the binder content in the electrode mixture is less than 15% by mass, adversely affecting the battery characteristics, contrary to conventional concerns Thus, it is possible to obtain a sodium secondary battery electrode or a sodium secondary battery that can realize excellent initial discharge capacity and cycle durability. Moreover, since water is used as the solvent, there is an advantage that the environmental load in the production process can be reduced.

  In the present invention, a desired electrode or battery can be produced by appropriately selecting the conditions in the production steps (1) to (3).

  The reason why it is possible to obtain a sodium secondary battery electrode or a sodium secondary battery that can achieve excellent initial discharge capacity and cycle durability is not clear at this time, but the binder absorbs hard carbon water. It is thought that it was suppressed. In addition, conventionally, hard carbon having a fine pore structure once absorbed water cannot be removed at the temperature exposed in the normal electrode manufacturing process, there is a concern about adverse effects on electrode characteristics, Water management during storage and manufacturing processes was difficult.

  In the sodium ion secondary battery described above, when charged, sodium ions are released from the positive electrode mixture layer and inserted in the negative electrode mixture layer through the electrolyte layer. When discharging is performed, sodium ions are released from the negative electrode mixture layer and inserted in the positive electrode mixture layer through the electrolyte layer.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.

(Example 1-1)
5 parts by mass of polyacrylic acid (manufactured by Sigma-Aldrich, PAA (viscosity average molecular weight (Mv) = 750,000)) as a binder is added to N-methyl-2-pyrrolidone (NMP) which is a viscosity adjusting solvent. Furthermore, 95 parts by mass of hard carbon (Carbotron P, manufactured by Kureha Co., Ltd.) as an electrode active material was added and mixed and stirred with a defoaming kneader to obtain a working electrode mixture slurry.
The obtained working electrode mixture slurry was applied onto a copper foil as a working electrode current collector, dried in a vacuum dryer at 120 ° C., and punched into a circle (φ15 mm) to obtain a working electrode of this example. .
On the other hand, sodium metal foil (circular: φ16 mm) was used as the counter electrode.
Further, a glass filter (thickness: 0.38 mm) was used as the separator.
The working electrode and the counter electrode were laminated in the order of the working electrode, the separator, and the counter electrode through one glass filter (thickness: 0.38 mm) to obtain a single cell layer having a three-layer structure.
The obtained single cell layer was placed in one case of a coin-type battery, a gasket was attached to maintain insulation between the electrodes, and the following non-aqueous electrolyte was injected using a syringe, Spacers were stacked, the other case was overlapped, and caulking was performed to obtain a sodium secondary battery of this example. As the non-aqueous electrolyte, a solution obtained by dissolving sodium perchlorate (NaClO ₄ ) as an electrolyte salt in propylene carbonate (PC) as a non-aqueous solvent so as to have a concentration of 1 mol / L was used. .

(Example 1-2)
In producing the working electrode, the working electrode mixture slurry obtained by using distilled water instead of N-methyl-2-pyrrolidone (NMP) as the viscosity adjusting solvent was used, and the same as in Example 1-1. This operation was repeated to obtain a sodium secondary battery of this example.

(Example 1-3)
In the production of the working electrode, 10 ml of carboxymethylcellulose sodium salt (Daicel Chemical Industries, CMC (viscosity average molecular weight (Mv) = 1,000,000)) as a binder is added to distilled water which is a viscosity adjusting solvent. The working electrode mixture slurry obtained by adding 90 parts by mass of hard carbon (manufactured by Kureha Co., Ltd., Carbotron P (J)) as an electrode active material and mixing with a mortar and pestle is used. A sodium secondary battery of this example was obtained by repeating the same operation as in Example 1-1 except that the above was performed.

(Example 1-4)
In the production of the working electrode, polyacrylic acid sodium salt as a binder (manufactured by Kishida Chemical Co., Ltd., PAANA (viscosity average molecular weight (Mv) = 22,000-66,000)) is added to distilled water which is a viscosity adjusting solvent. The working electrode mixture slurry obtained by adding 5 parts by mass and further adding 95 parts by mass of hard carbon (manufactured by Kureha Co., Ltd., Carbotron P (J)) as an electrode active material and mixing with a mortar and pestle. Except having used, the same operation as Example 1-1 was repeated, and the sodium secondary battery of this example was obtained.

(Comparative Example 1-1)
10 parts by mass of polyvinylidene fluoride (PVdF) as a binder is added to N-methyl-2-pyrrolidone (NMP) which is a viscosity adjusting solvent, and hard carbon (manufactured by Kureha Corporation, Carbotron P (J)) was added in an amount of 90 parts by mass, and the same procedure as in Example 1-1 was repeated except that a working electrode mixture slurry obtained by mixing with a mortar and pestle was used. Sodium secondary battery was obtained.

(Comparative Example 1-2)
15 parts by mass of polyacrylic acid (manufactured by Sigma-Aldrich, PAA (viscosity average molecular weight (Mv) = 750,000)) as a binder as a binder is added to distilled water which is a viscosity adjusting solvent. Example 1 except that 85 parts by mass of hard carbon (Carbotron P, manufactured by Kureha Co., Ltd.) as an electrode active material was added and a working electrode mixture slurry obtained by mixing with a mortar and pestle was used. The same operation as 1 was repeated to obtain the sodium secondary battery of this example.
Table 1 shows a part of the specifications of the sodium secondary battery of each example.

[Performance evaluation]
(Charge / discharge cycle test)
For the sodium secondary battery of each example, a current density of 25 mA / g, a potential range of 0.0 to 2.0 Vvs. Constant current charging / discharging was performed with Na / Na ⁺ . This charging / discharging was made into 1 cycle, charging / discharging was performed to 20 cycles on the same charging / discharging conditions, and the discharge capacity in each cycle was measured. The discharge capacity (mAh / g) in the first cycle was used as an index of the initial discharge capacity in the evaluation. The discharge capacity is the result converted per 1 g of hard carbon. Further, as an index of cycle durability in the evaluation, a discharge capacity maintenance ratio (%) which is a ratio of the discharge capacity at the 20th cycle to the discharge capacity at the first cycle was used. The obtained results are also shown in Table 1.

  As can be seen from the results in Table 1, in the case of Comparative Example 1-1 using polyvinylidene fluoride (PVdF), which is a binder that is typically used in lithium ion secondary batteries, the discharge capacity retention rate is It turns out that it is low. Moreover, in the case of the comparative example 1-2 which used polyacrylic acid over the predetermined amount as a binder, although discharge capacity maintenance factor is excellent, it turns out that initial stage discharge capacity is low. Compared with these, in Examples 1-1 to 1-4 using polyacrylic acid, carboxymethylcellulose sodium salt, and polyacrylic acid sodium salt as the binder, the initial discharge capacity is 226 to 241 mAh / g. Furthermore, it can be seen that the discharge capacity retention rate is 90% or more, which is excellent. In addition, it is thought that the initial discharge capacity is excellent because hard carbon is used. In addition, polyacrylic acid can use both NMP and water as a solvent for the electrode mixture slurry, and there is no particular problem even with an electrode made from a slurry of an aqueous solution with a smaller environmental load, and it is as excellent as when NMP is used. It can be seen that the results are shown. Therefore, it turns out that the environmental load in a manufacturing process can be made smaller by using water as a solvent.

In addition, in order to investigate the reason why the discharge capacity retention rate is low in Comparative Example 1-1, sodium perchlorate (NaClO ₄ ) as an electrolyte salt is added to propylene carbonate (PC) as a non-aqueous solvent as a non-aqueous electrolyte. X-ray photoelectron spectroscopy (HAX-PES, measurement depth) using synchrotron radiation from the working electrode before and after the charge / discharge cycle, using a solution that does not contain fluorine (F) dissolved so as to have a concentration of 1 mol / L. Is about 10 nm, and the chemical bond information of a substance deeper than about 1 nm of normal XPS is known.) The fluorine (F) spectrum was measured. The obtained results are shown in FIG. As shown in FIG. 2, a spectrum attributed to NaF that was not observed before use was observed at the sample electrode after one cycle. Since the non-aqueous electrolyte does not contain fluorine (F), it is assumed that this NaF is derived from polyvinylidene fluoride (PVdF) as a binder. This is supported by the fact that the spectrum attributed to CF of polyvinylidene fluoride (PVdF) observed at the sample electrode before use at the same time as the appearance of the spectrum attributed to NaF is very weak.

(Example 2-1)
First, hard carbon (Carbotron P (J) manufactured by Kureha Co., Ltd.) was heated and dried at 350 ° C. for 12 hours under vacuum conditions to obtain an electrode active material used in this example. Subsequently, 1 mass of polyacrylic acid (manufactured by Sigma-Aldrich, PAA (weight average molecular weight (Mw) = about 1.25 million)) as a binder is added to N-methyl-2-pyrrolidone (NMP) which is a viscosity adjusting solvent. In addition, 99 parts by mass of hard carbon as an electrode active material that had been dried by heating was added, and the mixture was stirred with a defoaming kneader to prepare an electrode mixture slurry. Further, the prepared electrode mixture slurry was applied on an aluminum foil as a current collector so that the electrode basis weight was 3.5 ± 5% mg / cm ² and the electrode density was 1.0 g / cc. It was dried at 0 ° C. for 8 hours and punched into a circle (φ = 15 mm) to obtain an electrode of this example. A sodium metal foil was used as the other electrode. A glass filter was used as the separator. A single cell was obtained by sandwiching a glass filter between two electrodes. The obtained unit cell is placed in one case of a coin-type battery (CR2032), a gasket is attached to maintain insulation between the electrodes, and the following non-aqueous electrolyte is injected using a syringe, A spring and a spacer were laminated, the other case was overlapped, and caulking was performed to obtain a negative electrode half cell (sodium secondary battery) of this example. As the non-aqueous electrolyte, a solution obtained by dissolving sodium hexafluorophosphate (NaPF ₆ ) as an electrolyte salt in propylene carbonate (PC), which is a non-aqueous solvent, to a concentration of 1 mol / L. Using.

(Example 2-2)
Hard carbon (manufactured by Kureha Co., Ltd., Carbotron P (J)) is used as an electrode active material used in this example without being heated and dried, and further, is bonded to N-methyl-2-pyrrolidone (NMP) which is a viscosity adjusting solvent. 1 part by mass of polyacrylic acid (made by Sigma-Aldrich, PAA (weight average molecular weight (Mw) = about 1.25 million)) as an adhesive is added, 99 parts by mass of hard carbon as an electrode active material is added, and defoamed The electrode of this example was obtained by repeating the same operation as in Example 2-1, except that the electrode mixture slurry was prepared by mixing and stirring with a kneader. Furthermore, except having used the electrode of this example, operation similar to Example 2-1 was repeated and the negative electrode half cell (sodium secondary battery) of this example was obtained.

(Comparative Example 2-1)
Hard carbon (manufactured by Kureha Co., Ltd., Carbotron P (J)) is heated and dried at 350 ° C. for 12 hours under vacuum conditions to form an electrode active material used in this example. 10 parts by mass of polyvinylidene fluoride (manufactured by Kureha Co., Ltd., PVDF (# 9200)) as a binder is added to 2-pyrrolidone (NMP), and 90 parts by mass of hard carbon as an electrode active material dried by heating. The electrode of this example was obtained by repeating the same operation as in Example 2-1, except that the electrode mixture slurry was prepared by mixing and stirring with a defoaming kneader. Furthermore, except having used the electrode of this example, operation similar to Example 2-1 was repeated and the negative electrode half cell (sodium secondary battery) of this example was obtained.

(Comparative Example 2-2)
Hard carbon (manufactured by Kureha Co., Ltd., Carbotron P (J)) is used as an electrode active material used in this example without being heated and dried, and further, is bonded to N-methyl-2-pyrrolidone (NMP) which is a viscosity adjusting solvent. Add 10 parts by mass of polyvinylidene fluoride as an adhesive (manufactured by Kureha Co., Ltd., PVDF (# 9200)), add 90 parts by mass of hard carbon as an electrode active material, mix and stir with a defoaming kneader, Except having produced the electrode mixture slurry, the same operation as Example 2-1 was repeated, and the electrode of this example was obtained. Furthermore, except having used the electrode of this example, operation similar to Example 2-1 was repeated and the negative electrode half cell (sodium secondary battery) of this example was obtained.

<Measurement of water content by Karl Fischer method>
The moisture content of the electrode active material and electrode mixture layer used in Example 2-1, Example 2-2, Comparative Example 2-1 and Comparative Example 2-2 was measured using a Karl Fischer moisture meter under the following conditions. did. The obtained results are shown in Table 2. In addition, the electrode active material measured what was heat-dried at 350 degreeC under vacuum conditions for 12 hours, and what was not so, and the electrode mixture layer measured what was heat-dried at 130 degreeC for 12 hours.

(Measurement condition)
・ Temperature: 300 ℃
・ Time: 20 minutes ・ Nitrogen flow rate: 200 mL / min

[Performance evaluation]
(Charge / discharge test)
The sodium secondary batteries of Example 2-1, Example 2-2, Comparative Example 2-1 and Comparative Example 2-2 are placed in a thermostatic chamber of a charge / discharge tester, and the temperature in the thermostatic bath is 25 ° C. The charge / discharge test was conducted with Specifically, in a charging process (referring to a process of inserting Na into the electrode for evaluation), a constant current / constant voltage mode was set, and charging was performed from 2 V (vs. Na / Na ⁺ ) to 10 mV at 0.1 C. Thereafter, in a discharging process (referring to a Na desorption process from the electrode for evaluation), a constant current mode was set, and discharging was performed from 10 mV to 2 V at 0.1 C. The above charging / discharging cycle was made into 1 cycle, and the charging / discharging test of 2 cycles (2nd cycle and 3rd cycle) was done on the same charging / discharging conditions. In the first cycle, the theoretical capacity of hard carbon was set to 250 mAh / g, and charge / discharge of one cycle was repeated, and each of the obtained charge capacities was set to true 0.1 C (current at which the battery was fully charged in 10 hours). Value). In FIG. 3, the charge / discharge capacity is indicated by a bar graph, and the irreversible capacity is indicated by a line graph. The obtained results are average values of three samples.

  From Table 2 and FIG. 3, it can be seen that excellent initial discharge capacities were not obtained in Comparative Examples 2-1 and 2-2 outside the present invention in which polyvinylidene fluoride was applied as a binder. It can also be seen from the ratio of decrease in irreversible capacity in FIG. 3 that the cycle durability is not excellent. 2 together with the results shown in FIG. 2, in Comparative Example 2-1 and Comparative Example 2-2, even when the water content of the electrode mixture layer is about 300 ppm, fluorination is caused by the reaction between polyvinylidene fluoride and water. This is probably because hydrogen is generated, and decomposition of the electrolytic solution or deposition of decomposition products on the active material surface occurs. Further, from Table 2 and FIG. 3, in Example 2-1 and Example 2-2 of the present invention in which polyacrylic acid is applied as a binder, the water content of the electrode mixture layer may be about 800 to 1100 ppm. It can be seen that an excellent initial discharge capacity is obtained. This is because polyvinylidene fluoride as a binder has water repellency, so that the moisture contained in the electrode mixture layer tends to be relatively present on the surface of the active material itself or the binder. However, the moisture contained in the electrode mixture layer is relatively high because polycarboxylic acid as a binder has many hydrogen bonding sites and water absorption. This is probably because such moisture does not affect the battery characteristics. In addition, it is considered that the initial discharge capacity is excellent because hard carbon is used. Furthermore, it can also be seen that Example 2-1 and Example 2-2, especially Example 2-1 are excellent in cycle durability from the reduction ratio of the irreversible capacity in FIG. In addition, when the moisture content is 1100 ppm or less, it can be seen that the smaller the moisture content, the better the initial discharge capacity and cycle durability than when polyvinylidene fluoride is applied. It turns out that it becomes discharge capacity and cycle durability. The electrode preparation step is desirably performed under anhydrous conditions. For example, the electrode active material powder obtained by heating and drying at 150 ° C. or higher and 550 ° C. or lower for 6 to 12 hours under vacuum conditions is used for the electrode mixture slurry. By using it for preparation, the moisture content of the electrode mixture layer obtained can be 900 ppm.

As mentioned above, although this invention was demonstrated with some embodiment and an Example, this invention is not limited to these, A various deformation | transformation is possible within the range of the summary of this invention.
That is, the sodium secondary battery electrode of the present invention can be used not only as a negative electrode of a sodium ion secondary battery but also as a positive electrode of a sodium metal secondary battery, for example.

  Moreover, the sodium secondary battery of this invention should just be provided with the predetermined electrode mentioned above, and it does not specifically limit regarding other requirements.

For example, the present invention can be applied not only to the laminate type battery and coin (button) type battery described above, but also to conventionally known forms and structures such as a can type battery.
In addition, for example, the present invention can be applied not only to the above-described stacked type (flat type) battery but also to a conventionally known form / structure such as a wound type (cylindrical) battery.
Further, for example, the present invention is not only the above-described normal (internal parallel connection type) battery but also a bipolar type (internal series connection type) when viewed in terms of an electrical connection form (electrode structure) in a sodium ion secondary battery. ) Conventionally known forms and structures such as batteries can also be applied.
A battery element in a bipolar battery generally includes a bipolar electrode in which a negative electrode active material layer is formed on one surface of a current collector and a positive electrode active material layer is formed on the other surface, an electrolyte layer, A plurality of layers.

DESCRIPTION OF SYMBOLS 1 Sodium ion secondary battery 10 Battery element 11 Positive electrode 11A Positive electrode collector 11B Positive electrode mixture layer 12 Negative electrode 12A Negative electrode collector 12B Negative electrode mixture layer 13 Electrolyte layer 14 Single battery layer 21 Positive electrode tab 22 Negative electrode tab 30 Exterior body

Claims (12)

An electrode mixture layer containing an electrode active material and a binder containing a polycarboxylic acid and / or a polycarboxylic acid alkali metal salt;
The sodium secondary battery electrode, wherein a content of the binder in the electrode mixture layer is less than 15% by mass.   The electrode for a sodium secondary battery according to claim 1, wherein the electrode active material contains hard carbon.   The sodium secondary battery electrode according to claim 1 or 2, wherein the amount of water contained in the electrode mixture layer is 1100 ppm or less.   The amount of water contained in the electrode mixture layer is 1000 ppm or less, and the electrode for a sodium secondary battery according to any one of claims 1 to 3.   The polycarboxylic acid and / or alkali metal salt of polycarboxylic acid contains at least one selected from the group consisting of polyacrylic acid, polyacrylic acid alkali metal salt, carboxymethylcellulose and carboxymethylcellulose alkali metal salt. The electrode for sodium secondary batteries as described in any one of Claims 1-4.   The electrode for a sodium secondary battery according to any one of claims 1 to 5, wherein the binder contains polyacrylic acid. A step (1) of heating and drying the electrode active material;
The process of producing the electrode mixture slurry which is implemented after the said process (1) and contains the electrode active material dried by heating, and the binder containing polycarboxylic acid and / or polycarboxylic acid alkali metal salt (2) )When,
And a step (3) of forming an electrode mixture layer using the prepared electrode mixture slurry after the step (2), and a method for producing an electrode for a sodium secondary battery.   The method for manufacturing an electrode for a sodium secondary battery according to claim 7, wherein the electrode active material contains hard carbon.   In the said process (1), it heat-drys at 100 degreeC or more and 600 degrees C or less, The manufacturing method of the electrode for sodium secondary batteries of Claim 7 or 8 characterized by the above-mentioned.   In the said process (1), it heat-drys at 150 degreeC or more and 550 degrees C or less, The manufacturing method of the electrode for sodium secondary batteries as described in any one of Claims 7-9 characterized by the above-mentioned.   In the said process (1), it heat-drys on vacuum conditions, The manufacturing method of the electrode for sodium secondary batteries of any one of Claims 7-10 characterized by the above-mentioned.   A sodium secondary battery comprising the sodium secondary battery electrode according to any one of claims 1 to 6 as at least one of a negative electrode and a positive electrode.

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