JP5034261B2 - Metallic lithium secondary battery - Google Patents

Description

  The present invention relates to a metal lithium secondary battery using lithium or a lithium alloy having excellent cycle characteristics as a negative electrode.

  Lithium has the lowest electrochemical potential and at the same time has a very small electrochemical equivalent (capacity per mass). Therefore, a lithium secondary battery using metallic lithium as a negative electrode can be expected to have a large energy density. Development of a lithium secondary battery using metal lithium as a negative electrode (hereinafter referred to as a metal lithium secondary battery) has been made since around 1970, but none has been commercialized so far. A commercially available lithium secondary battery is a lithium ion secondary battery in which carbon is used for the negative electrode and a transition metal oxide lithium compound is used for the positive electrode. The reason why the commercialization of the metal lithium secondary battery is delayed is that it faced a big problem that dendrite (resin-like precipitate of metal lithium) was formed on the negative electrode surface after repeated charge and discharge. Dendrite is a phenomenon in which metallic lithium generated by reduction of lithium ions during charging is locally deposited on the negative electrode surface. As the growth proceeds, the dendrite peels off from the negative electrode, and a portion (dead lithium) that cannot contribute to the discharge is generated. Therefore, when the cycle is repeated, the discharge capacity rapidly decreases. In addition, dendrites may break through the separator, resulting in an internal short circuit between the positive and negative electrodes, resulting in no battery operation. Furthermore, due to an internal short circuit between the positive and negative electrodes, the organic solvent in the battery may ignite, which may cause a serious problem related to safety and reliability, such as a fire accident.

  In order to solve the problem of dendrite in such a metal lithium secondary battery, modification of the lithium electrode of the negative electrode has been conventionally proposed. For example, a lithium battery using a negative electrode provided with a layer made of amorphous lithium or an amorphous lithium alloy as a lithium metal (Patent Document 1), and further a lithium battery excellent in energy density and electromotive force and excellent in cycle life are provided. Therefore, a method of forming a hydrophobic substance layer on the surface of the lithium metal or lithium alloy of the negative electrode (Patent Document 2) and the like can be mentioned. However, even in these, the cycle life is not sufficient for commercialization, and further improvement has been demanded.

  On the other hand, the present inventors have proposed a battery using an oxidation-reduction reaction of an organic radical compound as an active material in order to provide a secondary battery having high energy density, high capacity and excellent stability. For example, Patent Document 3 discloses a battery using a radical compound such as a nitroxide radical compound, an aryloxy radical compound, and a polymer compound having a specific aminotriazine structure as a positive electrode material.

Moreover, in the battery using the oxidation-reduction reaction of the organic radical compound described in Patent Document 4, a positive electrode in which a nitroxyl polymer and carbon (conducting agent) are mixed is used.
JP 7-296812 A JP 2002-15728 A JP 2002-151084 A JP 2002-304996 A

  Even in a battery using lithium or a lithium alloy for the negative electrode and organic radicals for the positive electrode, if charge and discharge are repeated, dendrite on the negative electrode surface may grow and break through the separator. As a result, there is a possibility of causing an internal short circuit between the positive and negative electrodes. An object of the present invention is to provide a battery that has good charge / discharge cycle characteristics even when lithium or a lithium alloy is used for the negative electrode and that does not cause an internal short circuit due to repeated charge / discharge.

  As a result of intensive studies by the present inventors, an oxoammonium cation (nitroxyl cation) partial structure represented by the following chemical formula (I) is taken in the oxidized state, and a nitroxyl radical moiety represented by the following chemical formula (II) in the reduced state. In a metallic lithium secondary battery that contains a nitroxyl polymer having a structure in the positive electrode and uses the reaction represented by the reaction formula (A) in which electrons are transferred between the two states as an electrode reaction of the positive electrode, the positive electrode is electrically conductive By using a structure consisting of a lower layer composed of a conductive material and a nitroxyl polymer and an upper layer mainly composed of a nitroxyl polymer that does not contain a conductive material, no internal short circuit will occur even if lithium or a lithium alloy is used for the negative electrode. I found.

  This is because the surface of the positive electrode on the negative electrode side consists only of a layer mainly composed of a nitroxyl polymer that does not contain a conductive substance. Since this layer is an insulator, the metal lithium of the negative electrode forms a separator due to the formation of dendrites. This is because even if it breaks through, an internal short circuit does not occur.

  According to the present invention, in a secondary battery using a nitroxyl radical as a positive electrode active material, even when metallic lithium or a metallic lithium alloy is used for the negative electrode, the possibility of an internal short circuit due to the formation of dendrites is eliminated. As a result, a safe secondary battery is obtained.

  FIG. 1 shows the configuration of an embodiment of the positive electrode of the present invention. The upper layer 1-1 is a layer mainly composed of a nitroxyl polymer not containing a conductive substance, and the lower layer 1-2 is a layer made of a conductive substance and a nitroxyl polymer, and these are formed on the aluminum foil 2. . FIG. 2 shows a configuration of an embodiment of the secondary battery of the present invention. The illustrated battery has a configuration in which the positive electrode 1 and the metal lithium negative electrode 5 are stacked so as to face each other with a separator 6 containing an electrolyte interposed therebetween. Between the stainless steel outer package (negative electrode side) 1 and the stainless steel outer package (positive electrode side) 7, an insulating packing 4 made of an insulating material such as a plastic resin is disposed for the purpose of preventing electrical contact therebetween. In addition, when using solid electrolyte and gel electrolyte, it can replace with a separator and can also be made into the form which interposes these electrolytes between electrodes. Also, the positive and negative electrodes can be directly joined.

[1] Cathode Active Material The electrode active material in the present invention is a substance that directly contributes to electrode reactions such as charge reaction and discharge reaction, and plays a central role in the battery system.

  In the present invention, as a positive electrode active material, a polymer compound having a nitroxide radical represented by the formula (I) in the reduced state and an oxoammonium (nitroxide cation) represented by the formula (II) in the oxidized state as a partial structure in the molecule Can be used.

As such a nitroxyl polymer, the nitroxyl polymers disclosed in Patent Document 3 and Patent Document 4 can be used, but the spin concentration (ESR spectrum) in an equilibrium state is 10 ²⁰ spin / g or more. Those that last more than a second are preferred. Among these, a polymer compound containing a cyclic nitroxyl structure represented by the following general formula (1) or (2) in the reduced state is preferable.

(In the formula, R represents a hydrogen atom or a methyl group, Y represents —COO— or —O—, and n represents the number of repeating units.)

  Typical structures of these compounds are shown below.

  The molecular weight of the polymer compound of the present invention is preferably 500 or more, more preferably 5000 or more. This is because when the molecular weight is 500 or more, it is difficult to dissolve in the battery electrolyte, and when the molecular weight is 5000 or more, it is almost insoluble. The polymer may be in the form of a chain, a branch, or a network. Moreover, the structure which bridge | crosslinked with the crosslinking agent may be sufficient.

  Further, in one electrode of the battery of the present invention, the nitroxide radical represented by the formula (I) in the reduced state which is the positive electrode active material and the oxoammonium (nitroxide cation) represented by the formula (II) in the oxidized state are partially contained. Although the high molecular compound which has in a molecule | numerator as a structure can be used independently, you may use it in combination of 2 or more types. Moreover, you may use in combination with another active material.

  If the thickness of the upper layer of the positive electrode not containing the conductive material is too thin, the dendrite generated at the negative electrode cannot be sufficiently prevented from coming into contact with the lower layer of the positive electrode and may cause a short circuit. Is preferably 10 μm or more. Further, the upper limit is not particularly defined, but if it is too thick, the battery thickness becomes thick and the battery capacity may be lowered. Therefore, it is preferably 200 μm or less, more preferably 100 μm or less.

[2] Conductive substance for positive electrode lower layer As the conductive compound used for the lower layer of the positive electrode, metal oxide particles such as copper, iron, gold, platinum, nickel, carbon, conductive polymer, and the like can be used. Examples of the conductive polymer include polyacetylene, polyphenylene, polyaniline, and polypyrrole. In the present invention, these conductive materials can be used alone or in combination. The content of these conductive substances is preferably 5% by mass or more and 80% by mass or less, and more preferably 10% by mass or more and 60% by mass or less.

  The thickness of the lower layer of the positive electrode is not particularly limited, and may be a thickness that can secure a necessary battery capacity. Usually, it may be 30 μm or more and 200 μm or less, preferably 50 μm or more and 150 μm or less.

[3] Negative electrode As the negative electrode, lithium metal or lithium alloy can be used. As a lithium alloy, lithium, Al, Pb, Sn, In, Bi, Ag, Ba, Ca, Hg, Pd, Pt, Sr, Te, etc. are mixed and alloyed to two or three or more. Or what added Si, Cd, Zn, La etc. to these etc. is mentioned. The lithium content in the lithium alloy is preferably 30% by mass or more. From the viewpoint of operating voltage and energy density, it is preferable to use lithium metal. These shapes are not particularly limited. For example, lithium metal is not limited to a thin film shape, but may be a bulk shape, a powdered shape, a fiber shape, a flake shape, or the like. These lithium metals and lithium alloys can be used alone or in combination of a plurality of shapes.

[4] Binder A binder can also be used to strengthen the connection between the constituent materials of the electrode. Examples of such binders include polytetrafluoroethylene, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, styrene / butadiene copolymer rubber, polypropylene, and polyethylene. , Polyimide, partially carboxylated cellulose, various polyurethanes and the like.

[5] Current collector and separator As negative electrode current collector and positive electrode current collector, foil, metal flat plate, mesh shape, etc. made of nickel, aluminum, copper, gold, silver, aluminum alloy, stainless steel, carbon, etc. Can be used. Further, the current collector may have a catalytic effect, or the active material and the current collector may be chemically bonded. These current collectors may also serve as a battery outer package. On the other hand, a separator such as a porous film made of polyethylene, polypropylene, or the like, a cellulose film, or a nonwoven fabric can be used so that the positive electrode and the negative electrode are not in contact with each other.

[6] Electrolyte In the present invention, the electrolyte performs charge carrier transport between the negative electrode and the positive electrode, and generally has an ionic conductivity of 10 ⁻⁵ to 10 ⁻¹ S / cm at 20 ° C. Preferably it is. As the electrolyte, for example, an electrolytic solution in which an electrolyte salt is dissolved in a solvent can be used. Examples of the electrolyte salt include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, Li (C ₂ F ₅ SO ₂ ) ₂ N, Li (CF ₃ SO ₂ ) ₃ C Conventionally known materials such as Li (C ₂ F ₅ SO ₂ ) ₃ C can be used.

  When a solvent is used for the electrolytic solution, examples of the solvent include ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, tetrahydrofuran, dioxolane, sulfolane, dimethylformamide, dimethylacetamide, N-methyl- An organic solvent such as 2-pyrrolidone can be used. These solvents can be used alone or in admixture of two or more.

  Furthermore, in the present invention, a solid electrolyte can be used as the electrolyte. Polymer compounds used in these solid electrolytes include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-monofluoroethylene copolymer, vinylidene fluoride. -Vinylidene fluoride polymers such as trifluoroethylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, and acrylonitrile-methyl methacrylate copolymer Polymer, acrylonitrile-methyl acrylate copolymer, acrylonitrile-ethyl methacrylate copolymer, acrylonitrile-ethyl acrylate copolymer, acrylonitrile-methacrylic acid copolymer, acrylonitrile-acrylic Le acid copolymers, acrylonitrile - vinyl acetate copolymer, acrylonitrile-based polymer, further polyethylene oxide, ethylene oxide - propylene oxide copolymers, and polymers of these acrylates body or methacrylate body thereof. Even if these polymer compounds are made into a gel by adding an electrolytic solution, only the polymer compounds may be used as they are.

[7] Battery shape In the present invention, the shape of the battery is not particularly limited, and a conventionally known battery can be used. Examples of the battery shape include an electrode laminate or a wound body sealed with a metal case, a resin case, or a laminate film made of a metal foil such as an aluminum foil and a synthetic resin film, etc. A mold, a coin mold, and a sheet mold are used, but the present invention is not limited to these.

[8] Battery Manufacturing Method The battery manufacturing method is not particularly limited, and various methods can be used depending on the material. For example, a nitroxyl polymer as a positive electrode active material and a conductive material are added with a solvent to form a slurry, which is applied to an electrode current collector, and the solvent is volatilized by heating or room temperature to form a lower layer of the positive electrode. Similarly, a solvent is added to a nitroxyl polymer that does not contain substances, and is applied to the lower layer of the positive electrode in the form of a slurry. After forming the upper layer of the positive electrode by heating or volatilizing the solvent at room temperature, the metal lithium or lithium alloy that is the counter electrode If necessary, the negative electrode is laminated or wound with a separator in between and wrapped with an outer package, and an electrolyte is injected and sealed. Solvents for slurrying include ether solvents such as tetrahydrofuran and diethyl ether, amine solvents such as N-methylpyrrolidone, aromatic hydrocarbon solvents such as benzene, toluene and xylene, and aliphatic solvents such as hexane and heptane. Examples thereof include hydrocarbon hydrocarbons and halogen hydrocarbons such as chloroform and dichloromethane.

When manufacturing a battery, the compound is produced by using a polymer itself having a structure represented by the formula (1) or formula (2) in the molecule as an active material, and by an electrode reaction. In some cases, a battery is produced using a compound that changes into a slab. Examples of the compound that changes to the compound by such an electrode reaction include a lithium salt or a sodium salt composed of an anion obtained by reducing the nitroxyl radical compound and an electrolyte cation such as lithium ion or sodium ion, or the nitroxyl. radical compound and a cation body oxidized PF ₆ ^- or BF ₄ ^-, etc. salt comprising the electrolyte anions such like.

  In the present invention, a conventionally known method can be used as a method for manufacturing a battery for other manufacturing conditions such as lead extraction from an electrode and outer packaging.

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

Example 1
1.2 g of a finely divided polyradical compound represented by the following formula (a) (soluble part molecular weight: 1000), 1.2 g of carbon powder as a conductive substance, 100 mg of carboxymethylcellulose (CMC), polytetrafluoroethylene (PTFE) ) 25 mg and 8 ml of water were stirred with a homogenizer to prepare a uniform paste. This slurry was applied onto an aluminum foil (thickness 20 μm) with an electrode production coater, and further dried at 80 ° C. for 3 minutes to form a lower layer having a thickness of 100 μm.

  A uniform paste was prepared by stirring 2.4 g of the microradical compound represented by the following formula (a), 100 mg of carboxymethylcellulose (CMC), 25 mg of polytetrafluoroethylene (PTFE) and 8 ml of water using a homogenizer. . This slurry was applied on an aluminum foil having a lower layer (100 μm) formed thereon with a coater for electrode preparation, and further dried at 80 ° C. for 3 minutes to form a positive electrode having a thickness of 100 μm for the lower layer and 20 μm for the upper layer.

Next, the obtained positive electrode was punched into a circular shape having a diameter of 12 mm, immersed in the electrolytic solution, and impregnated with the electrolytic solution in the voids in the electrode. As the electrolytic solution, an ethylene carbonate / diethyl carbonate mixed solution (mixing ratio 3: 7) containing 1.0 mol / L LiPF ₆ electrolyte salt was used. A polypropylene porous film separator impregnated with the electrolytic solution was laminated on the electrode impregnated with the electrolytic solution. Further, a lithium-bonded copper foil (lithium thickness 30 μm) serving as a negative electrode was laminated, put into a stainless steel coin-type battery outer package, and pressure was applied by a caulking machine to obtain a sealed coin-type battery.

The coin-type battery fabricated as described above was charged with a constant current of 0.113 mA (0.1 mA / cm ² per positive electrode area) until the voltage reached 4.2 V, and then a constant current of 0.113 mA. Was discharged to 2.5V. As a result, a flat voltage portion was observed near 3.55 V during discharge. The discharge capacity density per active material was 118 mAh / g. Similarly, charging / discharging was repeated 100 times in the range of 4.2 to 2.5V. As a result, in all of the 100 charging / discharging operations, the voltage became constant at around 3.5 V during discharging, and (300th discharge capacity) / (1st discharge capacity) was 91.5%.

(Examples 2 to 4)
The coin-type battery manufactured in Example 1 was subjected to a charge / discharge test while changing the current amount to 0.565 mA, 1.13 mA, and 3.39 mA.

  The results are summarized in the table below.

(Example 5)
A coin-type battery was produced in the same manner as in Example 1, except that no separator was laminated between the positive and negative electrodes. The coin-type battery was charged with a constant current of 0.113 mA (0.1 mA / cm ² per positive electrode area) until the voltage reached 4.2 V, and then discharged to 2.2 V with a constant current of 0.113 mA. went. As a result, a flat voltage portion was observed around 3.5 V during discharge. The discharge capacity density per active material was 106 mAh / g. Similarly, charging / discharging was repeated 50 times in the range of 4.2 to 2.2V. As a result, in all 50 charge / discharge cycles, the voltage was constant at around 3.5 V during discharge, and (100th discharge capacity) / (1st discharge capacity) was 90.6%.

(Comparative Example 1)
A sealed coin-type battery was produced in the same manner as in Example 1 except that the positive electrode upper layer was not formed. The coin-type battery was charged at a constant current of 0.113 mA (0.1 mA / cm ² per positive electrode area) until the voltage reached 4.2 V, and then discharged to 2.5 V at a constant current of 0.113 mA. went. As a result, a flat voltage portion was observed near 3.55 V during discharge. The discharge capacity density per active material was 112 mAh / g. Similarly, charging / discharging was repeated 300 times in the range of 4.2 to 2.5V. As a result, the operating voltage decreased as charging / discharging was repeated 300 times, and became 3.35 V at the 300th time. Further, the discharge capacity also decreased, and (300th discharge capacity) / (First discharge capacity) was 33.8%.

It is a schematic sectional drawing which shows the structure of one Embodiment of the positive electrode of this invention. It is a conceptual diagram which shows an example of a structure of the battery of this invention.

Explanation of symbols

1-1 Upper layer (nitroxyl polymer)
1-2 Lower layer (nitroxyl polymer / conductive substance mixed layer)
1 Positive electrode 2 Aluminum foil 3 Stainless steel exterior (negative electrode side)
4 Insulation packing 5 Metal lithium negative electrode (disk shape)
6 Separator aluminum foil 7 Stainless steel exterior (positive electrode side)

Claims (4)

Contains in the positive electrode a nitroxyl polymer that takes the nitroxyl radical partial structure represented by chemical formula (I) in the reduced state and the oxoammonium cation (nitroxyl cation) partial structure represented by chemical formula (II) in the oxidized state In the secondary battery using the reaction represented by the following reaction formula (A) for transferring and receiving electrons between the two states as the electrode reaction of the positive electrode, the positive electrode has a lower layer made of a conductive substance and a nitroxyl polymer, A lithium metal secondary battery comprising an upper layer mainly composed of a nitroxyl polymer not containing a functional substance, and a negative electrode comprising lithium or a lithium alloy.

The secondary battery according to claim 1, wherein the nitroxyl polymer is a polymer compound including a cyclic nitroxyl structure represented by the following general formula (1) or (2) in a reduced state.

(In the formula, R represents a hydrogen atom or a methyl group, Y represents —COO— or —O—, and n represents the number of repeating units.)   The secondary battery according to claim 1 or 2, wherein the content of the conductive material in the lower layer in the positive electrode is 80 mass% or less and 5 mass% or more. The secondary battery according to claim 1, wherein the positive electrode and the negative electrode are in direct contact with each other.

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