JP5930290B2 - Lithium ion secondary battery and electrical equipment including the same - Google Patents

Description

  The present invention relates to a lithium ion secondary battery and an electric device including the same.

  In recent years, development of lithium ion secondary batteries mounted on mobile phone devices and electric vehicles has been promoted. An electrode constituting a lithium ion secondary battery is composed of an active material directly related to electrical energy storage, a conductive agent responsible for a conduction path between the active materials, a binder, and a current collector. The characteristics of the lithium ion secondary battery are largely dependent on the electrodes, and are greatly influenced by the characteristics of each material itself and the way the materials are combined.

  In particular, the binder is required to have a small abundance ratio in the electrode obtained from the composite material containing the active material, the conductive agent and the binder, to have excellent affinity with the electrolytic solution, and to minimize the electric resistance of the electrode. . In addition, stability to withstand high voltage operation is also important.

  This binder is roughly classified into an aqueous type or a non-aqueous type. Examples of the aqueous binder include styrene-butadiene rubber (SBR) aqueous dispersion (eg, Patent Document 1) and carboxymethyl cellulose (CMC) (eg, Patent Documents 2 to 4). Moreover, the combined use of these binders has also been proposed as a prior art (Patent Documents 3, 5, 6, etc.). Since these water-based binders have relatively high adhesion to the active material and the conductive agent, there is an advantage that the content in the composite material is small.

  Examples of the non-aqueous binder include polytetrafluoroethylene (PTFE) (Patent Documents 7 and 8, etc.) and an N-methyl-2-pyrrolidone (NMP) solution of polyvinylidene fluoride (PVdF) (Patent Documents 9, 10 and the like). In particular, it has an advantage in that it works advantageously in a high-pressure operation type device.

  On the other hand, it is disclosed that a polysaccharide-based natural polymer is used as a binder for an electrode for a lithium ion secondary battery, and is applicable as a binder, and for a lithium ion secondary battery using this binder It is described that the cycle durability of the electrode is high (Patent Document 11, Non-Patent Document 1).

Japanese Patent No. 3101775 (issued on October 23, 2000) Japanese Patent No. 3968771 (issued August 29, 2007) Patent No. 4329169 (issued on September 9, 2009) Patent No. 4244401 (issued on March 25, 2009) Japanese Patent No. 3449679 (issued on September 22, 2003) Patent No. 3958781 (issued on August 15, 2007) Japanese Patent No. 3356021 (issued on December 9, 2002) Japanese Patent Laid-Open No. 7-326357 (released on December 12, 1995) Japanese Patent No. 3619711 (issued February 16, 2005) Japanese Patent No. 3619890 (issued February 16, 2005) Japanese Patent No. 3668579 (issued July 6, 2005) I. Kovalenko et al., Science, Vol.75, pp.75-79 (2011)

  However, the conventional binder has the following problems.

  First, in a composite material using SBR, which is an aqueous binder, the active material and the conductive agent become non-uniform and lack uniformity, so the performance reproducibility of the lithium ion secondary battery tends to be low. Further, since CMC has poor adhesion to the active material and the conductive agent, it is necessary to increase the content of CMC in the electrode to 10% by weight or more. As a result, the content of the active material is lowered.

  In order to eliminate these disadvantages, it is necessary to use SBR and CMC in combination in practice. However, since SBR has a double bond in the main chain, when SBR is used for the positive electrode, it is oxidized along with charge / discharge of the device. Deterioration due to. Furthermore, since SBR and CMC expand when they come into contact with the electrolyte, the active material peels off from the current collector and falls off. Therefore, in a lithium ion secondary battery using both SBR and CMC, cycle durability and There is a problem that the output characteristics deteriorate.

  Next, fluorine-based polymers such as PTFE and PVdF that are non-aqueous binders have a low intermolecular force with respect to the active material, and thus there is a tendency that sufficient adhesive force cannot be expressed. In order to compensate for insufficient adhesive strength, it is possible to ensure the strength of the electrode by increasing the content ratio of the binder. In this case, however, the electrical resistance of the electrode increases, and in particular an active material such as activated carbon having a large specific surface area. Loses its active point. Moreover, the content rate of an active material falls and the capacity | capacitance of a lithium ion secondary battery falls. Furthermore, PTFE and PVdF have (1) low affinity for carbon materials such as activated carbon, resulting in low reproducibility of the resulting electrode, and (2) a dispersant required for uniform mixing with the carbon material. There is a problem of becoming.

  In Non-Patent Document 1, alginic acid is used as a binder for silicon electrodes. Although a lithium ion secondary battery provided with this silicon electrode has high cycle durability, high output has not been achieved and further improvement is required.

  The present invention has been made in view of the problems of the above-described lithium ion secondary battery, and its purpose is to focus on the combination of a binder and an electrolytic solution, and a lithium ion secondary having excellent cycle durability and output characteristics. To provide a battery.

  In order to solve the above problems, the lithium ion secondary battery of the present invention includes a positive electrode and a negative electrode, and the lithium ion secondary battery includes an electrolyte between the positive electrode and the negative electrode. The binder in the negative electrode contains alginic acid. In addition, the solvent of the electrolytic solution is an ionic liquid containing a bis (fluorosulfonyl) imide anion.

  In the lithium ion secondary battery, alginic acid contained in the binder has a high affinity with the constituent material in the negative electrode, specifically, a high affinity with the negative electrode active material and the conductive agent, and the negative electrode is difficult to decompose. The lithium ion secondary battery is excellent in cycle durability because the negative electrode active material and the conductive agent are hardly peeled from the electric body and the negative electrode is hardly deteriorated. Further, the interface resistance between the negative electrode and the constituent material of the negative electrode is lower than that of the conventional electrode. Therefore, the lithium ion secondary battery has excellent output characteristics.

  Furthermore, the effect of reducing resistance between the negative electrode and the electrolyte interface obtained by using the binder and the ionic liquid containing bis (fluorosulfonyl) imide anion is high, and the cycle durability and output characteristics can be further improved. A high-quality lithium ion secondary battery can be provided.

  In the lithium ion secondary battery of the present invention, the negative electrode preferably includes silicon as a negative electrode active material.

  Usually, a lithium ion secondary battery provided with a negative electrode containing silicon is excellent in discharge capacity, but tends to be inferior in cycle durability. However, since the lithium ion secondary battery of the present invention is less susceptible to deterioration of the negative electrode due to the binder containing alginic acid, even if silicon is included in the negative electrode, the cycle durability is excellent and the discharge capacity is high. The effect of cycle durability is remarkably confirmed when the charge capacity is regulated.

  The electric device of the present invention includes the lithium ion secondary battery.

  The lithium ion secondary battery of the present invention includes a positive electrode and a negative electrode, and the lithium ion secondary battery includes an electrolytic solution between the positive electrode and the negative electrode. The binder in the negative electrode includes alginic acid, and the solvent of the electrolytic solution is , An ionic liquid containing a bis (fluorosulfonyl) imide anion.

  According to the above invention, the alginic acid contained in the binder has high affinity with the constituent material in the negative electrode, the negative electrode is difficult to decompose, and the negative electrode is difficult to deteriorate. Therefore, the lithium ion secondary battery is excellent in cycle durability. Further, the interface resistance between the negative electrode and the constituent material of the negative electrode is lower than that of the conventional electrode. Therefore, the lithium ion secondary battery has excellent output characteristics. Furthermore, the effect of reducing resistance between the negative electrode and the electrolyte interface obtained by using the binder and the ionic liquid containing bis (fluorosulfonyl) imide anion is high, and the cycle durability and output characteristics can be further improved. The high-quality lithium ion secondary battery can be provided.

6 is a graph showing measurement results of discharge rate characteristics in Example 1 and Comparative Example 1. 4 is a graph showing measurement results of cycle durability in Example 1 and Comparative Example 1. It is a graph which shows the cycle durability which did not regulate charge capacity in Example 2 and Comparative Examples 2 and 3. It is a graph which shows the cycle durability which performed charge capacity regulation in Example 2 and comparative example 2. It is a graph which shows the measurement result of the alternating current impedance in Example 2 and Comparative Example 2.

  An embodiment of the present invention will be described as follows, but the present invention is not limited to the embodiment.

[Lithium ion secondary battery]
The lithium ion secondary battery according to the present invention includes a positive electrode and a negative electrode, and the lithium ion secondary battery includes an electrolytic solution between the positive electrode and the negative electrode. The binder in the negative electrode includes alginic acid, and the solvent of the electrolytic solution. Is an ionic liquid containing a bis (fluorosulfonyl) imide anion as an anion component. Hereinafter, each member which comprises a lithium ion secondary battery is demonstrated.

[Positive electrode]
The positive electrode of the present invention is composed of a positive electrode active material, a conductive agent, a binder, and a current collector.

The positive electrode active material is not particularly limited as long as it can insert or desorb lithium ions. For example, transition metal oxides such as CuO, Cu ₂ O, MnO ₂ , MoO ₃ , V ₂ O ₅ , CrO ₃ , MoO ₃ , Fe ₂ O ₃ , Ni ₂ O ₃ , CoO ₃ ; Li _x CoO ₂ , Li _{X NiO 2, Li X Mn 2} O 4, a lithium complex oxide containing lithium and a transition metal of LiFePO ₄ or the _{like; TiS 2, MoS 2, NbSe} 3 , etc. of metal chalcogenides; polyacene, polyparaphenylene, polypyrrole, polyaniline And the like, and the like.

Among them, a composite oxide of lithium and one or more kinds selected from transition metals such as cobalt, nickel, and manganese, which is generally called a high voltage system, is preferable in that lithium ions can be released and a high voltage can be easily obtained. . Specific examples of the complex oxide of cobalt, nickel, manganese and lithium include LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _X Co _(1-X) O ₂ , LiMn _a Ni _b Co _c O. ₂ (a + b + c = 1) and the like.

In addition, these lithium composite oxides doped with a small amount of elements such as fluorine, boron, aluminum, chromium, zirconium, molybdenum, iron, etc., or the surface of lithium composite oxide particles are made of carbon, MgO, Al ₂ O ₃ , those treated with SiO ₂ or the like can also be used.

  The positive electrode active materials may be used alone or in combination of two or more. The amount of the positive electrode active material varies depending on its use and the like, and is not particularly limited.

  As the conductive agent, any electronic conductive material that does not adversely affect the battery performance can be used. Usually, carbon black such as acetylene black and ketchin black is used, but natural graphite (scale-like graphite, scale-like graphite, earth-like graphite, etc.), artificial graphite, carbon whisker, carbon fiber powder, metal (copper, nickel, aluminum) , Silver, gold, etc.) Conductive materials such as powder, metal fibers, and conductive ceramic materials may be used. These may be used alone or as a mixture of two or more. The addition amount of the conductive agent is preferably 1% by weight or more and 20% by weight or less, and more preferably 2% by weight or more and 10% by weight or less with respect to the weight of the positive electrode active material.

  The binder contained in the positive electrode binds the positive electrode active material and the conductive agent, is mixed so as to cover the positive electrode active material and the conductive agent, and fixes the conductive agent to the positive electrode active material.

  As the binder of the positive electrode, PVdF copolymer such as polyvinylidene fluoride (PVdF); copolymer of PVdF and hexafluoropropylene (HFP), copolymer of perfluoromethyl vinyl ether (PFMV) and tetrafluoroethylene (TFE), etc. Polymer resins; Fluorine resins such as polytetrafluoroethylene (PTFE) and fluororubber; polymers such as styrene-butadiene rubber (SBR), ethylene-propylene rubber (EPDM), styrene-acrylonitrile copolymer, and carboxy Polysaccharides such as methylcellulose (CMC), thermoplastic resins such as polyimide resins, and the like can be used in combination, but the binder of the positive electrode is not limited to these specific examples.

  Moreover, these may be used independently and may mix and use 2 or more types. The addition amount of the positive electrode binder is preferably 1% by weight or more and 10% by weight or less, and more preferably 1% by weight or more and 5% by weight or less with respect to the positive electrode active material.

  The positive electrode used by this invention can be manufactured by apply | coating the coating liquid which consists of the said positive electrode active material, a electrically conductive agent, a binder, etc. to the collector for positive electrodes.

  As the current collector for the positive electrode, an electronic conductor that does not adversely affect the constructed battery can be used. For example, aluminum, titanium, stainless steel, nickel, baked carbon, conductive polymer, conductive glass, and the like can be given. For the purpose of improving adhesion, conductivity, oxidation resistance, etc., a positive electrode current collector in which the surface of aluminum or the like is treated with carbon, nickel, titanium, silver, or the like may be used.

  It is also possible to oxidize the surface of these positive electrode current collectors. In addition to the foil shape, the shape of the positive electrode current collector may be a film shape, a sheet shape, a net shape, a punched or expanded material, a lath body, a porous body, a foamed body or the like. Good. The thickness is not particularly limited, but a thickness of 1 μm or more and 100 μm or less is usually used.

[Negative electrode]
The negative electrode of the present invention is composed of a negative electrode active material, a conductive agent, a binder, and a current collector.

  The negative electrode active material is not particularly limited as long as it can insert or desorb metallic lithium or lithium ions. Examples thereof include carbon materials such as natural graphite, artificial graphite, non-graphitizable carbon, and graphitizable carbon. Moreover, metal materials, such as metallic lithium, an alloy, and a tin compound; lithium transition metal nitride; crystalline metal oxide; amorphous metal oxide; silicon material;

  The negative electrode active material may be used alone or in combination of two or more. The amount of the negative electrode active material varies depending on its use and is not particularly limited, but is usually 80% by weight or more and 100% by weight or less based on the total weight of the negative electrode active material, the conductive agent and the binder.

  In the present invention, a negative electrode containing 90 wt% or more and 100 wt% or less of a carbon material as a negative electrode active material is referred to as a carbon negative electrode, and a negative electrode containing 40 wt% or more of a silicon material as a negative electrode active material and containing an active material other than silicon. This is called a silicon-based composite negative electrode.

  Since a carbon negative electrode has high versatility, it is easy to produce. In addition, a silicon-based composite negative electrode is preferable because a lithium ion secondary battery having excellent discharge capacity can be produced. Usually, a lithium ion secondary battery including a silicon-based composite negative electrode is excellent in charge / discharge capacity, but tends to be inferior in cycle durability. However, the lithium ion secondary battery of the present invention is excellent in cycle durability even if silicon is contained in the negative electrode because the negative electrode is less likely to deteriorate due to the binder containing alginic acid. The effect of cycle durability due to the use of a binder containing alginic acid is remarkably confirmed regardless of whether there is a restriction on the charge capacity.

  As the conductive agent, any electronic conductive material that does not adversely affect the battery performance can be used. Usually, carbon black such as acetylene black and ketchin black is used, but natural graphite (scale-like graphite, scale-like graphite, earth-like graphite, etc.), artificial graphite, carbon whisker, carbon fiber powder, metal (copper, nickel, aluminum) , Silver, gold, etc.) Conductive materials such as powder, metal fibers, and conductive ceramic materials may be used. These may be used alone or as a mixture of two or more. The addition amount of the conductive agent is preferably 1% by weight or more and 20% by weight or less, and more preferably 2% by weight or more and 10% by weight or less with respect to the total weight of the negative electrode.

  The binder contained in the negative electrode connects the negative electrode active material and the conductive agent, and is mixed so as to cover or connect the negative electrode active material, the conductive agent, and the current collector, and fixes them. The binder of the present invention contains alginic acid. Alginic acid is usually derived from brown algae plants such as kombu, wakame and kajime.

  Alginic acid is a polymer, and examples thereof include non-cross-linked alginic acid (hereinafter also referred to as non-algined alginic acid) and cross-linked alginic acid (hereinafter also referred to as alginic acid cross-linked).

  Examples of the non-crosslinked alginic acid include non-ionized free alginic acid or monovalent salt of alginic acid. Examples of the monovalent salt of alginic acid include alginic acid alkali metal salts such as lithium alginate, potassium alginate, and sodium alginate; ammonium alginate and the like.

  Examples of the above-mentioned alginic acid cross-linked product include, for example, alginic acid polyvalent salt, which is a salt of free alginic acid or alginic acid monovalent salt and bivalent or higher metal ion, free alginic acid or monovalent alginic acid salt, etc. Such as things. Examples of the alginic acid polyvalent salt include calcium alginate.

  For convenience of use as a binder, the alginate preferably has a 1% (g / 100 ml) aqueous solution of alginic acid having a viscosity at 20 ° C. of 300 mPa · s or more and 2000 mPa · s or less, 1000 mPa · s. As mentioned above, what is 2000 mPa * s or less is more preferable. The viscosity is a value measured by a rotary viscometer (manufactured by Brookfield) using an RV-1 spindle at 20 ° C. under a rotation speed of 60 rpm and a measurement time of 1 minute.

  The alginic acid is preferably prepared using an aqueous solution of a monovalent salt of alginic acid.

  The alginic acid is preferably prepared using an aqueous solution of 0.5% by weight or more and 5.0% by weight or less of an alginic acid monovalent salt, and is 2.0% by weight or more and 3.0% by weight. It is more preferable that the composition is prepared using an aqueous solution of a monovalent salt of alginic acid not more than%. By being prepared using an aqueous solution of a monovalent salt of alginic acid having an alginic acid content of 0.5 wt% or more and 5.0 wt% or less, more preferably 2.0 wt% or more and 3.0 wt% or less. , The negative electrode active material, the conductive agent and the binder can be easily mixed.

  The content of alginic acid in the binder according to the present invention is preferably 50% by weight or more and 100% by weight or less, more preferably 70% by weight or 100% by weight or less, and 90% by weight or more and 100% by weight or less. The following is particularly preferable, and 100% by weight is most preferable. When the content of alginic acid is less than 100% by weight, examples of the binder component other than alginic acid include styrene-butadiene rubber and carboxymethylcellulose.

  Although the compounding ratio (weight%) of a negative electrode active material, a electrically conductive agent, and a binder is not specifically limited, For example, it is set as negative electrode active material: conductive agent: binder = 80-97: 4-10: 2-15. be able to. The total content ratio of the negative electrode active material, the conductive agent and the binder is 100. That is, the blending ratio of the binder in the negative electrode is preferably 2% by weight or more and 15% by weight or less. More preferably, it is 5 wt% or more and 10 wt% or less. When the content is less than 2% by weight, it becomes difficult to produce a coating liquid in which the negative electrode active material, the conductive agent and the binder are uniformly mixed. When the content exceeds 15% by weight, the blending ratio of the binder increases. This leads to a decrease in the blending ratio.

  You may mix | blend alginic acid in the state of aqueous solution with respect to a negative electrode active material and a electrically conductive agent. Moreover, you may mix | blend water etc. with a negative electrode active material, a electrically conductive agent, and alginic acid for viscosity adjustment. Alginic acid, which is a binder, has a high affinity with both the negative electrode active material and the conductive agent, and is characterized in that a very uniform coating solution can be obtained. Thus, a negative electrode excellent in design can be obtained.

  The negative electrode used in the present invention can be produced by applying a coating liquid comprising the above negative electrode active material, a conductive agent, a binder, and the like to a negative electrode current collector.

  As the current collector for the negative electrode, an electronic conductor that does not have an adverse effect on the constructed battery can be used. For example, copper, stainless steel, nickel, aluminum, titanium, calcined carbon, conductive polymer, conductive glass, Al—Cd alloy, and the like can be given. For the purpose of improving adhesiveness, conductivity, oxidation resistance, etc., a negative electrode current collector in which the surface of copper or the like is treated with carbon, nickel, titanium, silver, or the like may be used.

  It is also possible to oxidize the surface of these negative electrode current collectors. Further, the shape of the negative electrode current collector may be a foil, a film, a sheet, a net, a punched or expanded article, a lath, a porous body, a foam, or the like. Good. The thickness is not particularly limited, but a thickness of 1 μm or more and 100 μm or less is usually used.

[Electrode production method]
An example of obtaining a positive electrode and a negative electrode will be described. The positive electrode coating liquid is applied to the positive electrode current collector, and the negative electrode coating liquid is applied to the negative electrode current collector at a desired thickness. Application methods include applying a coating solution to a current collector and removing excess coating solution with a doctor blade, applying a coating solution to a current collector, and rolling the coating solution with a roller. A known coating method may be mentioned.

  The temperature at which the coating liquid is dried is not particularly limited, and may be appropriately changed depending on the blending ratio of each material in the coating liquid, but is usually 70 ° C. or higher and 90 ° C. or lower. Moreover, what is necessary is just to change suitably the thickness of the obtained positive electrode or negative electrode according to the use of a lithium ion secondary battery.

[Electrolyte]
The electrolytic solution of the present invention is an ionic liquid in which a solvent contains a bis (fluorosulfonyl) imide anion (hereinafter, “bis (fluorosulfonyl) imide” is appropriately abbreviated as “FSI”). Such an electrolytic solution has good compatibility with alginic acid contained in the binder of the negative electrode, and can provide a lithium ion secondary battery excellent in cycle durability and output characteristics. In addition, the lithium ion secondary battery of the present invention has high performance even during high rate charge / discharge, and can be used with high energy density and high voltage. Furthermore, since the electrolyte solution is nonflammable, the obtained lithium secondary battery is excellent in safety.

  Although the preparation method of the ionic liquid containing the said FSI anion is not specifically limited, Well-known methods, such as reaction of fluorosulfonic acid and urea, can be used. Since the FSI compound obtained by these methods generally has low purity, in order to obtain a preferable ionic liquid having impurities of 10 ppm or less, it may be appropriately purified with water, an organic solvent or the like. The confirmation of impurities can be analyzed using a plasma emission analyzer (ICP).

Moreover, the anion component contained in the ionic liquid of this invention may contain anions other than this FSI anion in the range which does not leave the objective of this invention. Examples _{^{thereof, BF 4 -, PF 6 -}} , SbF 6 -, NO 3 -, CF 3 SO 3 -, (CF 3 SO 2) 2 N - ( bis (trifluoromethanesulfonyl) imide anion, TFSI denoted below ), (C ₂ F ₅ SO ₂ ) ₂ N ⁻ , (CF ₃ SO ₂ ) ₃ C ⁻ , CF ₃ CO ₂ ⁻ , C ₃ F ₇ CO ₂ ⁻ , CH ₃ CO ₂ ⁻ , (CN) ₂ N ^-, and the like. These anions may contain two or more types.

The content of FSI with respect to the total anions in the ionic liquid is preferably 50 mol% or more and 100 mol% or less, more preferably 70 mol% or more and 100 mol% or less, from the viewpoint of sufficiently exerting the effects of the present invention. . The content of FSI in the ionic liquid is calculated as follows.
FSI content in ionic liquid (%) =
Number of moles of FSI in ionic liquid / total number of moles of anion in ionic liquid × 100
In the ionic liquid of the present invention, the cation combined with the FSI anion is not particularly limited, but a combination with a cation forming an ionic liquid having a melting point of 50 ° C. or lower is preferable. When the melting point exceeds 50 ° C., the viscosity of the electrolytic solution is increased, which may cause a problem in cycle durability of the lithium ion secondary battery or decrease the discharge capacity.

  Examples of the cation include N, P, S, O, C, or Si, or a cation having a chain structure or a cyclic structure such as a 5-membered ring or a 6-membered ring in the structure. Is used.

  Specific examples of cyclic cation such as 5-membered ring and 6-membered ring include furan cation, thiophene cation, pyrrole cation, pyridinium cation, oxazolium cation, isoxazolium cation, thiazolium cation, isothiazolium cation , Furanzan cations, imidazolium cations, pyrazolium cations, pyrazinium cations, pyrimidinium cations, pyridazinium cations, pyrrolidinium cations, piperidinium cations, etc. heteromonocyclic cations; benzofuran cations, iso Examples thereof include condensed heterocyclic cations such as benzofuran cation, indole cation, isoindole cation, indolizinium cation, and carbazole cation.

  Among these cations, a chain or cyclic compound containing a nitrogen element is particularly preferable because it is industrially inexpensive and is chemically and electrochemically stable.

  Specific examples of the cation containing nitrogen element include alkylammonium cations such as triethylammonium cation and tetraalkylammonium cation; 1-propylpyridinium cation, 1-butylpyridinium cation, 1-ethyl-3- (hydroxymethyl) pyridinium cation, Pyridinium cations such as 1-ethyl-3-methylpyridinium cation; 1-ethyl-3-methylimidazolium cation, 1-butyl-3-methylimidazolium cation, 1-ethyl-2,3-dimethylimidazolium cation, 1 -Imyl such as allyl-3-methylimidazolium cation, 1-allyl-3-ethylimidazolium cation, 1-allyl-3-butylimidazolium cation, 1,3-diallylimidazolium cation Tetrazolium cation; - 1-methyl-1-propyl pyrrolidinium cation such as pyrrolidinium cation; piperidinium cations such methylpropyl piperidinium cation.

As the lithium salt dissolved in the ionic liquid as the supporting electrolyte of the electrolytic solution, any lithium salt that is usually used as an electrolyte for non-aqueous electrolytic solution can be used without any particular limitation. Examples of such lithium salts include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF ₃ SO ₂ ) ₂ (LiTFSI), LiN ( _{FSO 2) 2 (LiFSI),} LiBC 4 O 8, LiB (C 6 H 5) 4, etc. LiSO ₃ CH ₃ and the like. Among these, LiFSI and LiTFSI are preferable. These lithium salts may be used alone or in combination of two or more.

  Such a lithium salt is usually 0.1 mol / l (mol / liter) or more and 2.0 mol / l or less, preferably 0.3 mol / l or more and 1.5 mol / l or less in an ionic liquid. It is desirable to be included in.

[Separator]
In the lithium ion secondary battery of the present invention, a separator is provided between them in order to prevent a short circuit between the positive electrode and the negative electrode. A known separator can be used, and is not particularly limited. Specifically, the separator is a microporous film of polyethylene or polypropylene film; a multilayer film of porous polyethylene film and polypropylene; polyester fiber, aramid fiber, glass fiber, or the like. Nonwoven fabrics may be mentioned, and more preferred are separators in which ceramic fine particles such as silica, alumina, titania and the like are attached to the surface thereof.

  The separator preferably has a porosity of 70% or more, more preferably 80% or more and 95% or less. The air permeability obtained by the Gurley test method is preferably 200 seconds / 100 cc or less.

  By combining the ionic liquid containing the FSI anion with a lithium ion secondary battery including the separator as described above, the liquid impregnation property of the ionic liquid into the separator is significantly improved as compared with the conventional one. Accordingly, the internal resistance of the battery is greatly reduced, and the output characteristics and cycle durability of a lithium ion secondary battery using an ionic liquid as an electrolyte are improved.

Here, the porosity is a value calculated by the following equation from the apparent density of the separator and the true density of the solid content of the constituent material.
Porosity (%) = 100− (apparent density of separator / true density of solid material content) × 100
The Gurley air permeability is an air resistance according to a Gurley tester method defined in JIS P 8117.

  The separator contains 80% by weight or more of glass fibers having an average fiber diameter of 1 μm or less and less than 20% by weight of organic components containing fibrillated organic fibers, and the glass fibers are entangled with the fibrillated organic fibers. A wet papermaking sheet bonded to a porosity of 85% or more is particularly preferably used.

  The fibrillated organic fiber is formed by a device that disaggregates fibers, for example, a double disc refiner, and is subjected to the action of shearing force by beating and the like, and a single fiber is formed by very finely cleaving in the fiber axis direction. It is preferable that at least 50% by weight or more of fibril-containing fibers are fibrillated to a fiber diameter of 1 μm or less, and more preferably 100% by weight is fibrillated to a fiber diameter of 1 μm or less. preferable.

  As the fibrillated organic fiber, polyethylene fiber, polypropylene fiber, polyamide fiber, cellulose fiber, rayon fiber, acrylic fiber and the like can be used.

[Lithium ion secondary battery shape]
The lithium secondary battery of the present invention can be formed into a cylindrical shape, a coin shape, a rectangular shape, or any other shape, and the basic configuration of the battery is the same regardless of the shape, and the design can be changed according to the purpose. Can be implemented. For example, a cylindrical lithium ion secondary battery has a negative electrode produced by applying a coating liquid containing a negative electrode active material to a negative electrode current collector, and a coating liquid containing a positive electrode active material applied to a positive electrode current collector. A wound body obtained by winding the positive electrode produced through the separator is accommodated in a battery can, injected with an electrolytic solution, and sealed in a state where the insulating plates are placed on the upper and lower sides. Also, when producing a coin-type lithium ion secondary battery, a disc-shaped negative electrode, a separator, a disc-shaped positive electrode, and a stainless steel plate are stacked and stored in a coin-type battery can. After the electrolyte is injected in between, the coin-type battery can is sealed.

[Electrical equipment]
The lithium ion secondary battery of the present invention is excellent in cycle durability and output characteristics, and is provided as a power source or an electricity storage device in various electric devices. For this reason, the performance of the electric device provided with the said lithium ion secondary battery is also excellent, and has an advantage compared with the conventional electric device.

  The electrical device of the present invention is not particularly limited as long as it includes the lithium ion secondary battery of the present invention. For example, small electronic devices such as mobile phone devices, laptop computers, personal digital assistants (PDAs), video cameras and digital cameras; mobile devices (vehicles) such as electric bicycles, electric cars and trains; thermal power generation, wind power generation, hydropower Examples include power generation equipment such as power generation, nuclear power generation, and geothermal power generation.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the technical means disclosed in different embodiments can be appropriately combined. Such embodiments are also included in the technical scope of the present invention.

  The present invention will be described more specifically based on Examples and Comparative Examples and FIGS. 1 to 5, but the present invention is not limited thereto. Those skilled in the art can make various changes, modifications, and alterations without departing from the scope of the present invention. Each characteristic of the lithium ion secondary battery produced in the Example and the comparative example was measured as follows.

[Discharge rate characteristics (carbon electrode)]
With respect to the bipolar half cell produced in Example 1 and Comparative Example 1, the discharge capacity value at 1.0 C rate and 1 cycle was set to 1, and charging and discharging were performed 5 cycles at each C rate under the following conditions. The discharge capacity retention rate was measured. The discharge capacity retention rate and cycle durability were measured under the following conditions.
Operating voltage range: 0.005-1.5V VS. Li / Li ⁺
Charge: constant current (0.1, 1.0, 2.0, 3.0C rate) + constant voltage / discharge: constant current (0.1, 1.0, 2.0, 3.0C rate)
[Cycle durability (carbon electrode)]
With respect to the bipolar half cell produced in Example 1 and Comparative Example 1, the discharge capacity retention rate was measured under the following conditions to evaluate cycle durability.
Operating voltage range: 0.005 to 1.5V vs. Li / Li ⁺
Charging: Constant current (1.0C rate) + Constant voltage / Discharging: Constant current (1.0C rate)
[Cycle durability (silicon composite negative electrode)]
(Measurement method 1: No regulation)
With respect to the bipolar half-cells produced in Example 2 and Comparative Examples 2 and 3, the discharge capacity retention rate was measured under the following conditions to evaluate cycle durability. Note that the charge and discharge currents were set to 100 mA g ⁻¹ only in the first cycle.
Operating voltage range: 0.005 to 1.5V vs. Li / Li ⁺
Charge: constant current (500 mAg ⁻¹ ) + constant voltage / discharge: constant current (500 mA g ⁻¹ )
(Measurement method 2: regulated)
The discharge capacity and cycle durability were measured for the bipolar half cells produced in Example 2 and Comparative Example 2 under the following conditions. Note that only in the first cycle, the charging and discharging currents were set to 350 mA g ⁻¹ .
Charging: Constant current (1000 mAg ⁻¹ ) + constant voltage and charging capacity to 1000 mA g ⁻¹ Regulated discharge: Constant current (1000 mAg ⁻¹ )
[AC impedance]
For the bipolar half cell produced in Example 2 and Comparative Example 2, the AC impedance was measured at an open circuit voltage under the conditions of a potential amplitude of 10 mV and an AC frequency range of 20 kHz to 10 mHz. The results are shown in a Nyquist plot (Cole-Cole plot) with the real impedance (Z ′) on the X axis and the imaginary impedance (Z ″) on the Y axis.

[Example 1]
The bipolar half cell for carbon negative electrode evaluation concerning the lithium ion secondary battery of this invention was produced with the following methods. The bipolar half cell includes a carbon electrode. The following materials were used as materials for the bipolar half cell.

Active material: Natural graphite Conductive agent: Scaly graphite Binder: Sodium alginate (Alg)
Electrolyte: FSI ionic liquid (0.45 mol dm ⁻³ : LiTFSI / EMImFSI)
Separator: Polyolefin separator Current collector: Etched aluminum foil Counter electrode: Lithium metal foil
LiTFSI = bis (trifluoromethylsulfonyl) imide
EMImFSI = 1-ethyl-3-methylimidazolium bis (fluorosulfonyl) imide First, a 3 wt% aqueous sodium alginate solution was prepared. Next, the active material, the conductive agent, and the sodium alginate aqueous solution were mixed so that the composition ratio after drying (content ratio in the electrode) was 85: 5: 10 to prepare a slurry coating solution. After adding pure water to this coating liquid and adjusting the slurry viscosity, the coating liquid was applied to the etched aluminum foil with a doctor blade, and the coating liquid was heated on a hot plate at 80 ° C. for about 10 minutes. Thereafter, the coating solution applied to the etched aluminum foil is dried under a temperature atmosphere of 80 ° C. under a reduced pressure of 10 ⁻¹ Pa for 24 hours to obtain a carbon electrode as a target product (configuration corresponding to the negative electrode of the present invention). Obtained). The carbon electrode and the counter electrode were disposed on both sides, a separator was disposed between both electrodes, and an electrolyte was injected to produce a bipolar half cell having a diameter of 12 mm.

  The discharge rate and cycle durability were measured for this bipolar half cell. The results are shown in FIGS.

[Comparative Example 1]
A bipolar half cell was prepared in the same manner as in Example 1 except that the following organic electrolyte was used instead of LiTFSI / EMImFSI as the electrolyte, and the following separator for organic electrolyte was used as the separator instead of the polyolefin-based separator. did. The discharge rate and cycle durability were measured for this bipolar half cell. The results are shown in FIGS.

Electrolyte: Organic electrolyte (1.0 mol dm ⁻³ : LiPF ₆ / EC + DMC)
Separator: Cellulosic separator
LiPF ₆ = lithium hexafluorophosphate
EC + DMC = 1: 1 mixture of ethylene carbonate and dimethyl carbonate [Example 2]
A bipolar half cell for evaluation of a silicon-based composite negative electrode according to the lithium ion secondary battery of the present invention was produced by the following method. The bipolar half cell includes a silicon-based composite electrode. The following materials were used as materials for the bipolar half cell.

Active material: Metallic silicon powder, metallic nickel powder, natural graphite Conductive agent: Scale-like graphite Binder: Sodium alginate (Alg)
Electrolyte: FSI ionic liquid (0.45 mol dm ⁻³ : LiTFSI / EMImFSI)
Separator: Polyolefin separator Current collector: Etched aluminum foil Counter electrode: Lithium metal foil First, a 3 wt% aqueous sodium alginate solution was prepared. Next, in an argon atmosphere, the Si chip was sealed in an alumina sealed container together with alumina media, and subjected to ball milling at a rotational speed of 1000 rpm for 10 minutes using a planetary ball mill apparatus. The obtained Si powder and Ni powder were weighed to a weight ratio of 6: 1, sealed in a tungsten carbide sealed container together with tungsten carbide ground balls, and ball milled at a rotational speed of 300 rpm with a planetary ball mill device. . The processing time was 10 hours. The Ni—Si powder synthesized above and natural graphite were weighed at a weight ratio of 1: 1 in an argon atmosphere, and ball milled at a rotational speed of 250 rpm with a planetary ball mill device. The treatment time was 30 minutes.

The active material, the conductive agent, and the sodium alginate aqueous solution were mixed so that the composition ratio after drying (content ratio in the electrode) was 86: 4: 10 to prepare a slurry coating solution. After adding pure water to this coating liquid and adjusting the slurry viscosity, the coating liquid was applied to the etched aluminum foil with a doctor blade, and the coating liquid was heated on a hot plate at 80 ° C. for about 10 minutes. Thereafter, the coating liquid applied to the etched aluminum foil is dried under a temperature atmosphere of 80 ° C. under a reduced pressure of 10 ⁻¹ Pa for 24 hours, and a silicon-based composite electrode as an object (corresponding to the negative electrode of the present invention). Obtained).
This silicon-based composite electrode and the counter electrode were disposed on both sides, a separator was disposed between both electrodes, and an electrolyte was injected to prepare a bipolar half cell.

  With respect to this bipolar half-cell, cycle durability and AC impedance were measured by two types of measurement methods with and without regulation. The results are shown in FIG. 3, FIG. 4, and FIG.

[Comparative Example 2]
A bipolar half cell was prepared in the same manner as in Example 2 except that the following organic electrolyte was used instead of LiTFSI / EMImFSI as the electrolyte, and the following separator for organic electrolyte was used instead of the polyolefin-based separator as the separator. did. With respect to this bipolar half-cell, cycle durability and AC impedance were measured by two types of measurement methods with and without regulation. The results are shown in FIG. 3, FIG. 4, and FIG.

Electrolyte: Organic electrolyte (1.0 mol dm ⁻³ : LiPF ₆ / EC + DMC)
Separator: Cellulosic separator [Comparative Example 3]
A bipolar half cell was prepared in the same manner as in Example 2 except that polyvinylidene fluoride was used instead of sodium alginate as the binder. With respect to this bipolar half cell, cycle durability was measured without regulation. The results are shown in FIG.

〔Measurement result〕
FIG. 1 is a graph showing the measurement results of the discharge rate characteristics in Example 1 and Comparative Example 1, where ● corresponds to Example 1 and ◇ corresponds to Comparative Example 1. FIG. As shown in the figure, since Example 1 and Comparative Example 1 use sodium alginate as a binder, the resistance between the electrode and the electrolyte interface decreases, and a high capacity retention rate is shown. Yes. However, in Example 1, by using sodium alginate and the ionic liquid, the effect of reducing the resistance between the electrode and the electrolyte solution interface is high, a higher capacity retention rate is shown, and the discharge rate characteristic is Higher results. This result shows that the bipolar half cell of Example 1 is excellent in output characteristics.

  FIG. 2 is a graph showing the measurement results of cycle durability in Example 1 and Comparative Example 1, where ● corresponds to Example 1 and ◇ corresponds to Comparative Example 1. As shown in the figure, in the case of a general organic electrolyte system according to Comparative Example 1, the capacity retention rate decreases each time the charge / discharge cycle is repeated, but the capacity retention rate of the ionic liquid system of Example 1 is It is stable at about 1.0 and hardly decreases. This is presumably because sodium alginate exists more stably in the ionic liquid of the present invention and the binding effect on the electrode is not lost.

  FIG. 3 is a graph showing the cycle durability without charge capacity restriction in Example 2 and Comparative Examples 2 and 3, where ● is Example 2, ◇ is Comparative Example 2, and ○ is Comparative Example 3 is supported.

  As shown in the figure, when Example 2 (●) and Comparative Example 2 (◇), which use sodium alginate as a binder in common and have different electrolytes, are compared, the discharge capacity of Example 2 is greater. On the other hand, in Comparative Example 2, the discharge capacity rapidly decreases in the vicinity of 20 cycles. That is, the result of Example 2 exceeded the discharge capacity when the charge / discharge cycles were repeated. In other words, the alginic acid binder acts more effectively in the presence of the FSI ionic liquid. Next, when Example 2 (●) and Comparative Example 3 (◯) are compared, Example 2 has a higher discharge capacity in all cycles and is more excellent in cycle durability than Comparative Example 3. Results were obtained.

  FIG. 4 is a graph showing the cycle durability with restrictions in Example 2 and Comparative Example 2, where ● corresponds to Example 2 and ◇ corresponds to Comparative Example 2. FIG.

As shown in the figure, by restricting the charging capacity to 1000 mA g ⁻¹ at the time of charging, the discharge capacity of Example 2 is hardly lowered from 1000 mA g ⁻¹ until 100 cycles, and the cycle durability is very excellent. Could be achieved. On the other hand, when an organic electrolytic solution is used as the electrolytic solution as in Comparative Example 2, the discharge capacity decreases from around 60 cycles, and the discharge capacity becomes 500 mA g ⁻¹ or less at 100 cycles, resulting in a difference in cycle characteristics. Remarkably appeared.

  FIG. 5 is a graph showing the measurement results of AC impedance in Example 2 and Comparative Example 2. FIG. 4 (a) shows the measurement result of Example 2, and FIG. 4 (b) shows the measurement result of Comparative Example 2. Is shown. Both figures show that the resistance value of the bipolar half cell of Example 2 is lower than that of Comparative Example 2. This is due to the high affinity of alginic acid for the silicon material and the carbon material, which confirms that the resistance reduction between the electrode and the electrolyte interface is realized in the bipolar half cell of Example 2.

  As described above, the effects of the present invention are shown by Examples and Comparative Examples, and the binder in the negative electrode contains alginic acid, and the solvent of the electrolytic solution is an ionic liquid containing bis (fluorosulfonyl) imide anion. The half cell has excellent cycle durability and output characteristics.

  The present invention relates to a lithium ion secondary battery, and can be used in the capacitor industry, the automobile industry, the battery industry, the home appliance industry, and the like.

Claims (4)

In a lithium ion secondary battery comprising a positive electrode and a negative electrode and containing an electrolyte between the positive electrode and the negative electrode,
The binder in the negative electrode contains alginic acid,
The lithium ion secondary battery, wherein the solvent of the electrolytic solution is an ionic liquid containing a bis (fluorosulfonyl) imide anion.   The lithium ion secondary battery according to claim 1, wherein the negative electrode contains silicon as a negative electrode active material. The alginic acid is alginate,
3. The lithium ion secondary according to claim 1, wherein the alginate has a viscosity at 20 ° C. of a 1% (w / v) aqueous solution of alginate of 300 mPa · s or more and 2000 mPa · s or less. battery. An electric apparatus provided with the lithium ion secondary battery of any one of Claims 1-3.

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