JP6057644B2 - Lithium ion battery - Google Patents

Description

  The present invention is applied to a lithium ion battery for use in which a long life is required.

  A storage battery is incorporated into the system for leveling the amount of power generated by natural energy and leveling loads such as peak cuts. To cope with the rapid fluctuations in energy supply and demand. Rapid charge / discharge is required, and lithium ion batteries are considered suitable among storage batteries. Although this lithium-ion battery has a history of about 20 years for consumer use, it has not been able to meet the demand for a life of more than 10 years under such severe conditions. If diverted as it is, sufficient battery life cannot be obtained.

  Therefore, as disclosed in Non-Patent Document 1, Patent Document 1, Patent Document 2, and Patent Document 3, an iron phosphate positive electrode is mainly used for these applications. Since the iron phosphate positive electrode is inferior in energy density to lithium cobaltate and lithium nickelate, it has not been widely used for portable consumer use. However, in the case of stationary use, that is, when used outdoors, the life is more important than the energy density. The long life of the iron phosphate positive electrode is considered to be because the crystal structure is stable, so that the disorder of the crystal structure accompanying lithium insertion / extraction is less likely to occur. The deterioration of properties associated with the cycle of lithium cobaltate and lithium nickelate changes the crystal structure, especially on the outermost surface part in contact with the electrolyte, which becomes a barrier to lithium diffusion and makes rapid charge / discharge difficult. Since the positive electrode does not have this deterioration mode, a longer life can be obtained than these materials. However, just using an iron phosphate positive electrode is still insufficient for practical use.

When a battery is composed of an iron phosphate-graphite system and long-term charge / discharge is performed, the battery capacity decreases. As a result of investigating this cause, the inventors have found that the electrode material is not deteriorated in both the positive electrode and the negative electrode, and that a substantially initial capacity can be obtained when tested with a single electrode. Furthermore, as a result of detailed investigations, when the battery is completely discharged, the positive electrode is partially charged and does not participate in the discharge, and this is the reason that the battery capacity is reduced. Was learned.
The discharge capacity of the battery in such a state is governed by the capacity of the negative electrode. At this time, the reason why the positive electrode is partially charged is considered that a side reaction has progressed on the negative electrode active material at the time of charging, and an amount of electricity corresponding to this has accumulated in the positive electrode. Even small amounts of side reactions, when accumulated over thousands of cycles, result in this.

  As a method for solving this, Non-Patent Document 2 and Patent Document 4 describe a method of pre-doping lithium into the negative electrode. According to this method, in anticipation of side reactions occurring over a long cycle of the negative electrode, the negative electrode is discharged until the positive electrode is completely discharged by increasing the charge level of the negative electrode in advance. Even in the phenomenon, it is possible to prevent the battery capacity from being lowered. However, this method of supporting lithium in advance on the active material requires a process such as contacting a lithium metal foil with an electrode containing the active material, and is effective for a coin type using a relatively thick electrode. In a laminated structure battery or a wound structure battery in which a plurality of thin electrodes are laminated, there are problems in that the process becomes complicated and measures for ensuring safety are required.

JP 2011-28870 A JP 2009-200013 A JP 2011-44320 A JP 2006-156330 A

J. Electrochem. Soc., Vol. 144, No. 4, April 1997 Electrochemistry, 78 (7), 606-610 (2010)

  Recently, the demand for utilization of natural energy, peak cut, and the like has increased, and as a storage battery that contributes to these demands, there is an increasing expectation for a lithium ion battery. Lithium ion batteries have a history of more than 20 years for consumer use, but as described above, these technologies have not been able to meet demands that can be rapidly charged and discharged and require a life of 10 or more years. If diverted as it is, the battery life becomes insufficient. Therefore, in order to obtain a life of 10 years or more, it is necessary to take countermeasures in view of the degradation mode that occurs when the conventional technology is used. Also, it is desirable to avoid methods that complicate the manufacturing process.

  In the lithium ion battery of the present invention, the positive electrode potential is electrochemically oxidized in the positive electrode active material at a potential lower than the charge potential of the positive electrode active material, and the battery is used for charge and discharge after the second cycle. Is characterized by adding a non-reducing substance.

That is, the present invention comprises the following technical means.
[1] A positive electrode active material mixture layer containing a positive electrode active material, a binder and a conductive material is electrochemically oxidized as an additive at a lower potential than the positive electrode active material, and used as a battery for charging and discharging. A lithium ion battery comprising a positive electrode to which a substance not reduced at a positive electrode potential is added.
[2] The lithium ion battery according to [1], wherein the additive is a lithium-containing material that releases lithium at a lower potential than the positive electrode active material.
[3] The lithium ion battery according to [2], wherein the lithium-containing substance is a composite oxide of titanium and lithium or a composite oxide of silicon and lithium.
[4] The lithium ion battery according to [3], wherein the composite oxide of titanium and lithium is Li ₇ Ti ₅ O ₁₂ (rock salt type).
[5] The amount of the substance that is electrochemically oxidized at a lower potential than the positive electrode active material and is not reduced at the positive electrode potential used for charging and discharging as a battery is 1 when the combined weight of the positive electrode active material is 1 The lithium ion battery according to any one of [1] to [4], which is 0.5 to 10%.
[6] In any one of [1] to [5], the positive electrode active material is olivine type lithium iron phosphate (LiFePO ₄ ) or spinel type lithium manganate (LiMn ₂ O ₄ ). The lithium ion battery as described.

  When used as a stationary battery, that is, as a storage battery incorporated in a system such as utilization of natural energy or peak cut, a longer life is obtained and the service life is longer than that of a conventional lithium ion battery.

  The lithium ion secondary battery according to the present invention is electrochemically oxidized at a lower potential than the positive electrode active material as an additive in the positive electrode active material mixture layer containing the positive electrode active material, the binder and the conductive material, Further, the battery is characterized by comprising a positive electrode to which a substance not reduced at a positive electrode potential used for charging and discharging as a battery is added.

  The above-mentioned additive, “a substance that is electrochemically oxidized at a lower potential than the positive electrode active material and does not reduce at the positive electrode potential used for charging / discharging as a battery and is not reduced at the positive electrode potential used for charging / discharging” is In the first cycle, it is oxidized, and the product is a substance that does not chemically react after that, and there are lithium-containing substances that release lithium at a lower potential than the positive electrode active material and substances that do not contain lithium. Lithium-containing materials are preferred in terms of ease of handling.

Examples of the lithium-containing material that releases lithium at a lower potential than the positive electrode active material include titanium and lithium composite oxides such as Li ₇ Ti ₅ O ₁₂ and Li ₂ TiO ₂ , and SiLixOy including SiLi ₂ O _3. The described composite oxides of silicon and lithium, lithium aluminum alloy LixAl, and the like correspond thereto.

  Examples of the substance not containing lithium include quinone compounds such as benzoquinone, naphthoquinone, and anthraquinone and derivatives thereof, conductive polymers such as polypyrrole, polyaniline, polyacetylene, polythiophene, and polyparaphenylene, organic disulfide compounds, and carbon. Examples thereof include organic sulfur compounds such as sulfide and elemental sulfur.

The additive is preferably a lithium-containing material that releases lithium at a lower potential than the positive electrode active material, and among the lithium-containing materials, a composite oxide of titanium and lithium is preferably used. Among them, it is more preferable to use Li ₇ Ti ₅ O ₁₂ (rock salt type).

  In addition, the amount of the additive (a substance that is electrochemically oxidized at a lower potential than the positive electrode active material and is not reduced at the positive electrode potential used for charging and discharging as a battery) is the weight combined with the positive electrode active material. When it is 1, it is preferably 0.5 to 10%. If the addition amount is 0.05% or less, the reserve amount of lithium stored in the negative electrode is small, so that a sufficient effect cannot be obtained. If the addition amount is 10% or more, the battery capacity that can be designed decreases.

Furthermore, in the present invention, the positive electrode active material in the positive electrode active material mixture layer to which the additive is added may be any material that changes its oxidation number with charge and discharge reactions. , Olivine type lithium iron phosphate (LiFePO ₄ ), spinel type lithium manganate (LiMn ₂ O ₄ ), lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ) lithium cobaltate, lithium nickelate, cobalt-nickel - such as manganese ternary layered oxide (Li (Co-Ni-Mn ) O 2) can be used. Among them, olivine type lithium iron phosphate (LiFePO ₄ ) or spinel type lithium manganate (LiMn ₂ O ₄ ) is preferable.

Next, a method for producing a positive electrode used in the present invention will be described.
A positive electrode is comprised from a positive electrode active material mixture layer and a collector like the positive electrode of a normal lithium ion battery.
The positive electrode active material mixture (positive electrode active material, additive, binder, conductive material mixture layer) is prepared by the following method.
To the positive electrode active material whose surface is coated with carbon, a substance that is electrochemically oxidized at a lower potential than the positive electrode active material that is the additive and that does not reduce at the positive electrode potential used for charging and discharging as a battery, A conductive material and a binder were added to and mixed with this to prepare a positive electrode active material mixture.

  As the conductive material, carbon black, vapor grown carbon fiber, carbon nanotube, conductive polymer, or the like can be used.

  Examples of the binder include polyvinylidene fluoride powder, polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, a copolymer of vinylidene fluoride and chlorotrifluoroethylene, and vinylidene fluoride and tetra Copolymers of fluoroethylene and hexafluoropropylene or fluoropolymers such as vinylidene fluoride, hexafluoropropylene and chlorotrifluoroethylene, styrene butadiene rubber, polyacrylic acid, polyimide, polyamideimide, polyvinyl alcohol, polyethylene terephthalate , Polyethylene naphthalate, and the like can be used.

Further, a positive electrode active material mixture and a binder such as N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethyl carbonate, acetone, cyclohexanone, N-methyl-2-pyrrolidone, methyl ethyl ketone, and water are dissolved or dispersed. A slurry is prepared by mixing with a possible solution and applied to both surfaces of a current collector such as an aluminum foil by a doctor blade method to form a positive electrode active material mixture layer. Then, it compresses using a compression roller and obtains an electrode.
Other examples of the coating method include, but are not limited to, spray coating, bar coating, and slit coating.

  As the current collector, a commonly used aluminum foil, aluminum mesh, or the like can be used.

Next, a method for producing a negative electrode used in the present invention will be described.
Carbon or the like is used as the negative electrode active material. Examples of the negative electrode active material include graphite, pyrolytic carbons, cokes, glassy carbons, organic polymer compound fired bodies, mesocarbon microbeads, carbon fibers, activated carbon, graphite and other carbon materials, polyacetylene, and polypyrrole. Examples thereof include, but are not limited to, polymers such as SnO ₂ , Li ₄ Ti ₅ O ₁₂ , Si, Al, Sn, or a mixture thereof. An active material and a binder were mixed in a solvent to prepare a slurry, and then applied to both surfaces of a copper foil as a negative electrode current collector. As a coating method, the general method described in the preparation of the positive electrode can be used. Then, after drying the solvent and compressing to a predetermined thickness with a roller, the negative electrode combined with the said positive electrode was produced.

  As the binder and the solvent for producing the negative electrode, the same materials as those used for producing the positive electrode can be used.

Moreover, as the negative electrode current collector, a copper foil, a nickel foil, a copper mesh, a nickel mesh, or the like is used.

Next, the electrolytic solution used in the present invention will be described.
The electrolyte is prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1.5 mol / liter in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in an equal volume ratio.

Examples of the solvent used in the present invention include lactone solvents such as γ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. Carbonate solvents such as diethyl carbonate, vinylene carbonate or vinyl ethylene carbonate, 1,2-dimethoxyethane, 1-
Ether solvents such as ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran or 2-methyltetrahydrofuran, nitrile solvents such as acetonitrile, sulfolane solvents, phosphoric acids, phosphate solvents, pyrrolidones, etc. The nonaqueous solvent of these is mentioned. Any one type of solvent may be used alone, or two or more types may be mixed and used.

Moreover, as an electrolyte, a lithium salt used in a normal battery electrolyte can be used. Specifically, lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), lithium perchlorate (LiClO ₄ ), trifluoromethanesulfone Lithium acid (LiCF ₃ SO ₃ ), bis (trifluoromethanesulfonyl) imido lithium (LiN (SO ₂ CF ₃ ) ₂ ), tris (trifluoromethanesulfonyl) methyl lithium (LiC (SO ₂ CF ₃ ) ₃ ), tetrafluoride Examples thereof include lithium aluminate (LiAlCl ₄ ) and lithium hexafluorosilicate (LiSiF ₆ ).

  Next, the production method of the lithium ion battery of the present invention can be produced by a method usually performed using a positive electrode, a negative electrode, an electrolytic solution, and a separator. One example will be described below.

  A separator made of a microporous film using a plurality of positive and negative plates is laminated in the order of separator / negative electrode / separator / positive electrode / separator / negative electrode, and this electrode group is inserted into an aluminum laminate film. A terminal is provided, an electrolytic solution is injected, heat sealing is performed, a laminate type test cell is created, and a charge / discharge cycle is repeated to measure the life.

  In addition, an electrode group including one positive electrode and a negative electrode may be wound and inserted into a cylindrical battery can.

  The separator means a material that separates and insulates the positive electrode from the negative electrode, and is known as a battery separator such as polyethylene, polypropylene, polyamide, polyimide, polyamideimide, polyethylene terephthalate, cellulose acetate, and polyvinylidene fluoride. A porous film or a non-woven fabric can be used as a separator. In addition, a stack of a plurality of these materials may be used. Furthermore, for the purpose of facilitating the adhesion between the electrode plate and the separator, an adhesive layer that can be bonded by heat may be provided on the surface of the separator.

    Hereinafter, the steps for constituting the lithium ion battery of the present invention and the effects on the cycle life improvement of the battery using the present invention will be described.

[Production of positive electrode]
The iron phosphate having an olivine structure represented by the general formula LiFePO ₄ was prepared by the following method. First, after mixing a divalent iron salt and a lithium aqueous solution containing phosphoric acid and lithium hydroxide to crystallize lithium iron phosphate, kettin black and sucrose are added, heat-treated and baked, and carbon is added. Coated LiFePO ₄ was obtained. The obtained LiFePO ₄ and carbon composite particles were 99% by mass of LiFePO _{4 and} 1% by mass of carbon coated on the surface, and the average particle diameter was measured to be 90 μm.
85 parts by mass of the composite particles and rock salt type Li ₇ Ti ₅ O ₁₂ (average particle size 2 μm) as an additive, 10 parts by mass of ketchin black powder as a conductive agent, polyfluoride as a binder A vinylidene powder was mixed so as to be 5 parts by mass, and this was mixed with an N-methyl-2-pyrrolidone (NMP) solution to prepare a slurry. This slurry was applied to both surfaces of an aluminum current collector (thickness: 18 μm) by a doctor blade method to form a positive electrode active material mixture layer. Then, it compressed using the compression roller and produced the positive electrode used by Examples 1-4 and the comparative example whose length of a short side is 100 mm and whose length of a long side is 150 mm. Four types of weight ratios of LiFePO ₄ and Li ₇ Ti ₅ O ₁₂ of 90:10, 95: 5, 98: 2, and 99.5: 0.5 were prepared.

(Production of negative electrode)
As the negative electrode active material, a powder having an average particle size of 20 μm obtained by coating the surface of natural graphite with amorphous carbon was prepared. After preparing a slurry by mixing 5% by weight of polyvinylidene fluoride as a binder and an NMP solution as a solvent, the welded portion of the negative electrode current collector tab was removed, and the slurry was copper as a negative electrode current collector. It applied to both surfaces of foil (thickness: 16 micrometers). Then, after drying the solvent and compressing to a predetermined thickness with a roller, a negative electrode was prepared so that the length of the short side was 105 mm and the length of the long side was 155 mm.

[Production of test cell]
Using the 10 positive electrode plates and 11 negative electrode plates produced in the above process, a separator made of these positive and negative electrode plates and a polyethylene microporous film was separated into a separator / negative electrode / separator / positive electrode / separator / An electrode group having an outer shape of 115 mm × 160 mm × 4 mm was prepared by laminating in order of the negative electrode. This electrode group was inserted into an aluminum laminate film and provided with a positive electrode / negative electrode terminal and heat sealed. The electrolytic solution was prepared by dissolving lithium hexafluorophosphate (LiPF ₆ ) at a concentration of 1.5 mol / liter in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in this volume ratio. The liquid was injected.

[Effectiveness test]
The above cell was subjected to a charge / discharge cycle test. The charge was maintained at a constant voltage of 3.9 V for 90 minutes until the end voltage reached 3.9 V at 1 C corresponding to 1 C. Discharging was performed at 1C until the end voltage reached 2.7V. Table 1 shows the discharge capacity at the 10th cycle and the discharge capacity at the 1000th cycle in which charging and discharging are stabilized. In addition, the numerical value described in a table | surface is an average value of 3 cells except the thing with the largest deterioration rate and the thing with the smallest deterioration rate among 5 cells produced on the same conditions and tested.

According to the present invention, the capacity retention rate after 1000 cycles is higher than that of the comparative example of the prior art, and the Li ₇ Ti ₅ O ₁₂ addition amount in the active material is in the range of 0.05 to 10%. The capacity of is high. When the addition amount is 0.05% or less, such an effect cannot be obtained because the reserve amount of lithium stored in the negative electrode is small. Further, if it is 10% or more, as can be inferred from the results in Table 1, the battery capacity that can be designed decreases, and therefore the absolute value of the discharge capacity obtained at the 1000th cycle is not added (Comparative Example 1). ).

  The present invention provides a battery having a long life and a long service life compared to a conventional lithium ion battery when used for stationary use, that is, utilization of natural energy, peak cut, etc. Economic value is high.

Claims (3)

In the positive electrode active material mixture layer containing the positive electrode active material, the binder, and the conductive material, the positive electrode potential that is electrochemically oxidized at a lower potential than the positive electrode active material as an additive and is used for charging and discharging as a battery A lithium ion battery comprising a positive electrode to which Li ₇ Ti ₅ O ₁₂ (rock salt type) which is a substance that is not reduced is added. The amount of the substance that is electrochemically oxidized at a lower potential than the positive electrode active material and is not reduced at the positive electrode potential used for charging and discharging as a battery is 0. The lithium ion battery according to claim 1, wherein the content is 5 to 10%. The positive electrode active material, a lithium ion battery according to claim 1 or claim 2, characterized in that it is olivine type lithium iron phosphate (LiFePO ₄₎ or spinel lithium manganate (LiMn ₂ O _4).