JP2019169460A - Lithium-ion secondary battery - Google Patents

Abstract

To improve volumetric energy density in a lithium-ion secondary battery.SOLUTION: A lithium-ion secondary battery includes at least a positive electrode 10 and a negative electrode 20. The negative electrode 20 is porous. The negative electrode 20 includes a carbon fiber aggregate 21 and a polymer film 22. The carbon fiber aggregate 21 is formed of a plurality of carbon fibers 1 bound together in a three-dimensional fashion. The polymer film 22 covers a surface of each of the carbon fibers 1. The positive electrode 10 includes a group 11 of particles and an electrolyte 12. The group 11 of particles is dispersed in the electrolyte 12. The group 11 of particles contains a positive electrode active material and a conductive material. Pores that are present within the negative electrode 20 are filled with the positive electrode 10. The polymer film 22 is swollen with the electrolyte 12.SELECTED DRAWING: Figure 3

Description

  The present disclosure relates to a lithium ion secondary battery.

  Japanese Patent Application Laid-Open No. 2010-244911 forms an electrode cell by coating a carbon fiber nonwoven fabric with an electrolyte membrane, filling a void in the carbon fiber nonwoven fabric with a positive electrode active material and a solvent, and further evaporating the solvent. Is disclosed.

JP 2010-244911 A

  According to Patent Document 1, it is said that the electrode cell having the above configuration can be charged and discharged at high speed. This is probably because the facing area per unit volume is large. The facing area indicates the area of the region where the positive electrode and the negative electrode face each other.

  In Patent Document 1, a particle dispersion is used in the process of forming an electrode cell. The particle dispersion is formed by dispersing a positive electrode active material (particles) in a solvent. The particle dispersion is filled in the voids in the carbon fiber nonwoven fabric, and then the particle dispersion is dried. It is considered that voids are formed in the carbon fiber nonwoven fabric after drying by the volume of the solvent. The presence of voids decreases the volume energy density (energy per unit volume). That is, the electrode cell of Patent Document 1 is considered to have room for improvement in volume energy density.

  An object of the present disclosure is to improve volume energy density.

  The technical configuration and operational effects of the present disclosure will be described below. However, the mechanism of action of the present disclosure includes estimation. The scope of the claims should not be limited by the correctness of the action mechanism.

  [1] The lithium ion secondary battery of the present disclosure includes at least a positive electrode and a negative electrode. The negative electrode is porous. The negative electrode includes a carbon fiber aggregate and a polymer film. The carbon fiber aggregate is formed by three-dimensionally bonding a plurality of carbon fibers. The polymer film covers the surface of each of the plurality of carbon fibers. The positive electrode includes a group of particles and an electrolytic solution. The particle group is dispersed in the electrolytic solution. The particle group includes a positive electrode active material and a conductive material. The gap in the negative electrode is filled with the positive electrode. The polymer film is swollen by the electrolytic solution.

  In the lithium ion secondary battery of the present disclosure (hereinafter may be abbreviated as “battery”), the positive electrode is in a liquid state. That is, the positive electrode includes a particle group (positive electrode active material or the like) and an electrolytic solution. The particle group is dispersed in the electrolytic solution.

  The negative electrode includes a carbon fiber aggregate and a polymer film. The carbon fiber aggregate is composed of a plurality of carbon fibers. In each of the plurality of carbon fibers, an intercalation reaction of lithium (Li) ions can occur. That is, carbon fiber is a negative electrode active material. Gaps are formed between the plurality of carbon fibers. That is, the negative electrode is porous. The gap in the negative electrode is filled with the positive electrode. Since the positive electrode is in a liquid state, it is considered that the voids in the negative electrode can be filled without gaps. Therefore, it is considered that the generation of useless voids in the battery is suppressed. That is, an improvement in volume energy density is expected.

  The polymer film is swollen by the electrolytic solution contained in the positive electrode. The polymer film before swelling can be a thinner film than the polymer film after swelling. It is considered that the void volume when the positive electrode is filled increases because the polymer film before swelling is a thin film. This is expected to facilitate filling of the positive electrode (liquid).

  Furthermore, the volume of the positive electrode decreases due to swelling of the polymer film. However, the content of the positive electrode active material contained in the positive electrode does not change. Therefore, the volume energy density of the positive electrode increases. This is expected to improve the volume energy density.

  In the battery according to the present disclosure, the electrolyte solution serves as an ion conduction path between the positive electrode active material and the negative electrode active material. Therefore, it is considered that the movement of Li ions is smooth. Therefore, the battery of the present disclosure is expected to have high output.

  [2] The electrolytic solution contains at least a solvent and a lithium (Li) salt. The solvent may contain ether.

  In general, carbonates [for example, ethylene carbonate (EC), ethyl methyl carbonate (EMC), etc.] are used as the solvent of the electrolytic solution. The use of ether [for example, 1,2-dimethoxyethane (DME)] as the solvent is expected to improve conductivity. It is considered that ether has a low viscosity and promotes dissociation of the Li salt. Reduction in resistance is expected by improving the conductivity of the electrolytic solution.

  In the lithium primary battery, ether is used as a solvent for the electrolytic solution. However, in lithium ion secondary batteries, ether is not widely used as a solvent for the electrolytic solution. This is probably because ether causes co-insertion in lithium ion secondary batteries. Generally, the negative electrode active material of a lithium ion secondary battery is graphite. The graphite crystal is formed by laminating a plurality of carbon hexagonal mesh surfaces. In the electrolytic solution, Li ions are solvated into solvent molecules (here, ether). “Co-insertion” indicates that solvent molecules coordinated to Li ions are inserted into the gaps between the carbon hexagonal networks together with Li ions. When co-insertion occurs, solvent molecules are decomposed in the gaps between the carbon hexagonal mesh surfaces. As a result, the carbon hexagonal network surfaces are separated from each other, and the crystal structure is destroyed. It is considered that the storage site of Li ions decreases due to the collapse of the crystal structure. This is considered to reduce the capacity.

  In the lithium ion secondary battery of the present disclosure, the negative electrode active material is carbon fiber. According to the new knowledge of the present disclosure, when the negative electrode active material is carbon fiber, the capacity can be rather increased by using ether as the solvent. That is, an improvement in volume energy density is expected. The details of this mechanism are not clear at this time. For example, since the crystallinity of carbon fiber is lower than that of graphite, even if co-insertion occurs, it is considered that the collapse of the structure hardly occurs. In addition, for example, during the first charge, it is considered that the diffusion path of Li ions into the carbon fiber is widened by moderately disturbing the structure of the carbon fiber surface by co-insertion of ether. As a result, the capacity may increase.

  [3] In the configuration of [2], the solvent may contain at least one selected from the group consisting of 1,4-dioxane (DX) and 1,2-dimethoxyethane (DME).

  [4] In the configuration of [2] or [3] above, the lithium salt may contain a lithium imide salt. The lithium imide salt is a salt of a lithium ion and a fluorine-containing sulfonylimide anion.

In general, LiPF ₆ is used as the Li salt (supporting electrolyte salt) of the electrolytic solution. Lithium (Li) imide salts have also been studied as Li salts. The Li imide salt is a salt of Li ions and fluorine-containing sulfonylimide anions. The “fluorinated sulfonylimide anion” refers to a sulfonylimide anion containing a fluorine atom. An electrolyte solution containing a Li imide salt can have high electrical conductivity. It is considered that the Li imide salt can have a high degree of dissociation. Further, since the dissociation degree of the Li imide salt is high, it is expected that the resistance component (for example, charge transfer resistance) in the positive electrode is reduced. The synergy of these actions is expected to reduce resistance.

  However, Li imide salts have the disadvantage of corroding aluminum (Al). The positive electrode of a normal lithium ion secondary battery includes a positive electrode current collector. The positive electrode current collector is a conductive electrode substrate. In general, the positive electrode current collector is an Al foil. There is a possibility that the resistance may increase due to corrosion of the positive electrode current collector.

  In the lithium ion secondary battery of the present disclosure, the positive electrode is liquid. The positive electrode of the present disclosure does not require an electrode substrate (ie, an Al foil). Therefore, it is considered that the advantage (reduction in resistance) of the Li imide salt can be enjoyed without causing the disadvantage of the Li imide salt (corrosion of Al).

  [5] In the configuration of the above [4], the lithium salt may contain lithium bis (fluorosulfonyl) imide. That is, the Li imide salt may be lithium bis (fluorosulfonyl) imide. Hereinafter, lithium bis (fluorosulfonyl) imide may be abbreviated as “LiFSI”.

  [6] In the above configurations [2] to [5], the plurality of carbon fibers may include polyacrylonitrile (PAN) -based carbon fibers.

  Although the details of the mechanism are unknown, it is considered that the PAN-based carbon fiber has particularly good compatibility with ether. When the carbon fiber (negative electrode active material) contains the PAN-based carbon fiber, an increase in capacity and a decrease in polarization are expected. This is expected to improve the volume energy density.

FIG. 1 is a conceptual diagram showing an example of the configuration of the lithium ion secondary battery of the present embodiment. FIG. 2 is a conceptual diagram showing an example of the configuration of the negative electrode of the present embodiment. FIG. 3 is a conceptual cross-sectional view showing an example of the electrode arrangement of this embodiment. FIG. 4 is a first schematic diagram showing a laminated electrode group. FIG. 5 is a second schematic diagram showing the laminated electrode group. FIG. 6 is a first graph showing the results of Experiment 2. FIG. 7 is a second graph showing the results of Experiment 2. FIG. 8 is a third graph showing the results of Experiment 2. FIG. 9 is a fourth graph showing the results of Experiment 2.

  Hereinafter, an embodiment of the present disclosure (also referred to as “this embodiment” in the present specification) will be described. However, the following description does not limit the scope of the claims.

<Lithium ion secondary battery>
FIG. 1 is a conceptual diagram showing an example of the configuration of the lithium ion secondary battery of the present embodiment.
The battery 100 is a lithium ion secondary battery. Battery 100 includes a case 90. Case 90 may have any outer shape. The outer shape of the case 90 may be, for example, a cylindrical shape or a square shape. The case 90 may be made of, for example, stainless steel (SUS) or aluminum (Al) alloy. The case 90 may be a pouch made of an aluminum laminated film. However, when the electrolyte solution 12 (described later) contains a Li imide salt, it is desirable that the portion of the case 90 that contacts the electrolyte solution 12 does not contain Al. The case 90 accommodates the positive electrode 10 and the negative electrode 20. That is, the battery 100 includes at least the positive electrode 10 and the negative electrode 20.

  The positive electrode 10 is liquid. The positive electrode 10 is in contact with the inner wall of the case 90. When the case 90 is formed of a conductive material, the case 90 can be a positive electrode terminal. The negative terminal 92 is electrically connected to the negative electrode 20. The negative electrode terminal 92 may be made of, for example, nickel (Ni). The insulating material 80 insulates the negative electrode terminal 92 from the positive electrode 10 and the case 90. The insulating material 80 may be, for example, a fluororesin.

<Negative electrode>
FIG. 2 is a conceptual diagram showing an example of the configuration of the negative electrode of the present embodiment.
The negative electrode 20 is porous. The negative electrode 20 forms a three-dimensional matrix. A gap 30 to be filled with the positive electrode 10 is formed in the three-dimensional matrix. The negative electrode 20 includes a carbon fiber aggregate 21 and a polymer film 22.

(Carbon fiber aggregate)
The carbon fiber aggregate 21 is formed by three-dimensionally bonding a plurality of carbon fibers 1. “Three-dimensionally bonded” indicates that the plurality of carbon fibers 1 are bonded to each other and the plurality of carbon fibers 1 form a three-dimensional network structure. The carbon fiber aggregate 21 can have any outer shape. The outer shape of the carbon fiber aggregate 21 may be, for example, a sheet shape, a rod shape, a spherical shape, or the like.

  A space 30 is formed between the plurality of carbon fibers 1. The carbon fiber aggregate 21 may be, for example, a non-woven fabric of carbon fibers 1, a felt of carbon fibers 1, or the like. Nonwoven fabrics and felts have advantages such as easy formation of current collectors. As a modified form, it is also conceivable to use a porous material formed of crystalline carbon instead of the carbon fiber aggregate 21. Examples of the porous material include carbon monolith. Carbon monoliths can also have a three-dimensional network structure.

  The carbon fiber 1 may be, for example, a polyacrylonitrile (PAN) -based carbon fiber, a pitch-based carbon fiber, a cellulose-based carbon fiber, or the like. That is, the plurality of carbon fibers 1 may include PAN-based carbon fibers. The plurality of carbon fibers 1 may consist essentially of PAN-based carbon fibers. The PAN-based carbon fiber indicates a carbon fiber using PAN as a raw material. The pitch-based carbon fiber refers to a carbon fiber made from, for example, petroleum pitch. Cellulosic carbon fiber refers to carbon fiber made of, for example, viscose rayon.

  The plurality of carbon fibers 1 can be bonded by the following method, for example. A mixture is prepared by mixing a plurality of carbon fibers 1 and a binder. By heating the mixture in an inert atmosphere, the carbon fiber 1 and the binder are graphitized (crystallized). Thereby, the plurality of carbon fibers 1 can be bonded to each other. The binder may be, for example, coal tar pitch, phenol resin, epoxy resin or the like.

  The plurality of carbon fibers 1 may have an average diameter of 1 μm to 50 μm, for example. For example, the average diameter may be an average value of 100 or more carbon fibers 1. The carbon fiber 1 is typically a short fiber. The carbon fiber 1 may have a number average fiber length of 1 mm or more and 50 mm or less, for example. The number average fiber length may be an average value of 100 or more carbon fibers 1, for example.

FIG. 3 is a conceptual cross-sectional view showing an example of the electrode arrangement of this embodiment.
The carbon fiber 1 is made of crystalline carbon. The carbon fiber 1 is a negative electrode active material. Li ion intercalation reaction may occur on the surface of the carbon fiber 1. The carbon fiber 1 may include a core portion 1a and a sheath portion 1b. The sheath portion 1b covers the core portion 1a. The crystallinity of the sheath 1b may be lower than the crystallinity of the core 1a. It is expected that the speed of the intercalation reaction on the surface of the carbon fiber 1 is increased when the carbon fiber 1 includes the low crystalline sheath portion 1b. Further, for example, it is expected that side reactions at high temperatures are suppressed.

  The sheath 1b can be formed, for example, by the following method. Carbon fiber to be the core 1a is prepared. The core 1a (carbon fiber) is covered with an amorphous carbon material. The core 1a and the amorphous carbon material are heat treated. The crystallinity of the amorphous carbon material is adjusted by the heat treatment. Thereby, the sheath part 1b can be formed.

  The amorphous carbon material may be, for example, coal tar pitch. The coating method of the core part 1a should not be specifically limited. The coating method may be, for example, a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method. The CVD method may be, for example, a thermal CVD method or a plasma CVD method. The PVD method may be, for example, a vacuum deposition method, an ion plating method, a sputtering method, or the like.

(Polymer film)
The polymer film 22 covers the surface of each of the plurality of carbon fibers 1. The polymer film 22 may have a thickness of 1 μm or more and 50 μm or less, for example. The polymer film 22 separates the carbon fiber 1 (negative electrode active material) and the particle group 11 (positive electrode active material).

  The polymer film 22 is swollen by the electrolytic solution 12. That is, the polymer film 22 and the electrolyte solution 12 form a polymer gel. The polymer gel is considered to have Li ion conductivity and no electron conductivity. Therefore, it is considered that the positive electrode 10 and the negative electrode 20 are connected by ion conduction, and a short circuit (electrical connection) between the positive electrode 10 and the negative electrode 20 is suppressed.

  Examples of the polymer material that forms the polymer film 22 include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyethylene oxide (PEO), polymethyl methacrylate (PMMA), and polyvinyl alcohol (PVA). There may be. One kind of polymer material may be contained alone in the polymer film 22. Two or more kinds of polymer materials may be contained in the polymer film 22. The polymer film 22 may further contain inorganic particles. The inorganic particles may be, for example, alumina, silica, zirconia or the like. When the polymer film 22 further contains inorganic particles, for example, an improvement in the strength of the polymer film 22 is expected. The particle diameter, content, and the like of the inorganic particles can be appropriately adjusted according to the strength of the polymer film 22 and the like.

  The polymer film 22 can be formed by immersing the carbon fiber aggregate 21 in a solution (polymer solution) in which a polymer material is dissolved. The polymer film 22 may be formed by electrophoretic electrodeposition in a polymer solution.

《Positive electrode》
The positive electrode 10 is liquid. The positive electrode 10 is filled in a gap 30 in the negative electrode 20 (see FIGS. 2 and 3). The positive electrode 10 includes a particle group 11 and an electrolytic solution 12. The particle group 11 is dispersed in the electrolytic solution 12. That is, the positive electrode 10 is a particle dispersion (slurry). The solid content ratio of the positive electrode 10 may be, for example, 40% by mass or more and 60% by mass or less. The solid content ratio indicates the ratio of the solid component (particle group 11) in the positive electrode 10.

(Particle group)
The particle group 11 includes a positive electrode active material and a conductive material. The positive electrode active material is a particle capable of Li ion intercalation reaction. The positive electrode active material may have a D50 of 1 μm or more and 30 μm or less, for example. D50 indicates the particle size at which the cumulative particle volume from the fine particle side is 50% of the total particle volume in the volume-based particle size distribution. D50 can be measured by, for example, a laser diffraction particle size distribution measuring apparatus.

The positive electrode active material should not be particularly limited. The positive electrode active material is, for example, lithium cobaltate, lithium nickelate, spinel type lithium manganate, layered rock salt type lithium manganate, nickel cobalt lithium manganate (eg, LiNi _1/3 Co _1/3 Mn _1/3 O _2, etc.) Nickel cobalt lithium aluminate (for example, LiNi _0.82 Co _0.15 Al _0.03 O ₂ ), lithium iron phosphate, or the like may be used. One kind of positive electrode active material may be contained alone in the particle group 11. The particle group 11 may contain two or more positive electrode active materials.

  The conductive material is a particle that can form an electron conduction path between the positive electrode active materials. The conductive material should not be particularly limited. The conductive material may be, for example, carbon black (for example, acetylene black), vapor grown carbon fiber (VGCF), carbon nanotube (CNT), graphene flake, graphite, or the like. One kind of conductive material may be included in the particle group 11 alone. The particle group 11 may contain two or more kinds of conductive materials. The content of the conductive material may be, for example, 1 part by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the positive electrode active material.

  In the present embodiment, the positive electrode active material (particles) is dispersed in the electrolytic solution. Therefore, the conductive material is required to form a long-distance electron conduction path. A fibrous conductive material (eg, VGCF, CNT, etc.) is considered suitable for forming a long-distance electron conduction path. In the present embodiment, the conductive material that is included in the positive electrode 10 and is fibrous is regarded as particles.

(Electrolyte)
The electrolytic solution 12 includes at least a solvent and a Li salt. Examples of the solvent include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), γ- Butyrolactone (GBL), δ-valerolactone, tetrahydrofuran (THF), 1,3-dioxolane (DOL), 1,4-dioxane (DX), 1,2-dimethoxyethane (DME), methyl formate (MF), It may be methyl acetate (MA), methyl propionate (MP), ionic liquid, or the like. One kind of solvent may be contained in the electrolyte solution 12 alone. Two or more kinds of solvents may be contained in the electrolytic solution 12.

  The solvent may contain ether. When the solvent contains ether, an improvement in volume energy density is expected. In this embodiment, it is considered that the negative electrode active material is carbon fiber. The solvent may contain, for example, 50% by volume or more of ether. The solvent may contain, for example, 80% by volume or more of ether. The solvent may consist essentially of ether, for example. In the solvent, the remainder excluding the ether may be, for example, a carbonate ester.

  The solvent may be a mixture of cyclic ether and chain ether. The cyclic ether and the chain ether may satisfy, for example, the relationship of “cyclic ether: chain ether = 1: 9 to 9: 1 (volume ratio)”. The cyclic ether and the chain ether may satisfy, for example, a relationship of “cyclic ether: chain ether = 3: 7 to 7: 3 (volume ratio)”. The cyclic ether and the chain ether may satisfy, for example, the relationship of “cyclic ether: chain ether = 4: 6 to 6: 4 (volume ratio)”.

  The cyclic ether may be, for example, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), 1,3-dioxolane (DOL), 1,3-dioxane, 1,4-dioxane (DX) and the like. The chain ether may be, for example, 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1,2-dibutoxyethane (DBE), and the like. The solvent may contain one kind of ether alone. The solvent may contain two or more ethers. The solvent may contain, for example, at least one selected from the group consisting of THF, 2-MeTHF, DOL, 1,3-dioxane, DX, DME, DEE, and DBE.

  The solvent may contain at least one selected from the group consisting of DX and DME, for example. The solvent may be composed of at least one selected from the group consisting of DX and DME, for example. The solvent may consist essentially of DX and DME.

  Ether and PAN-based carbon fiber are considered to be particularly compatible. When the solvent contains ether and the plurality of carbon fibers 1 contain PAN-based carbon fibers, an improvement in volume energy density is expected.

Li salt is dissolved in the solvent. The concentration of the Li salt may be, for example, 0.5 mol / L or more and 5 mol / L or less. The concentration of the Li salt may be, for example, 1 mol / L or more and 2 mol / L or less. The Li salt may be, for example, LiPF ₆ , LiBF ₄ , Li [N (FSO ₂ ) ₂ ] (LiFSI), Li [N (CF ₃ SO ₂ ) ₂ ] (LiTFSI), and the like. One kind of Li salt may be contained alone in the electrolyte solution 12. Two or more types of Li salts may be contained in the electrolyte solution 12.

Li salt may contain Li imide salt. The Li salt may contain, for example, a Li imide salt and LiPF ₆ . The Li salt may consist essentially of, for example, a Li imide salt. The Li imide salt is a salt of Li ions and fluorine-containing sulfonylimide anions. The “fluorinated sulfonylimide anion” refers to a sulfonylimide anion containing a fluorine atom. The above “LiFSI” and “LiTFSI” correspond to Li imide salts.

Examples of the Li imide salt include the following general formula:
Li [N (X ¹ SO ₂ ) (X ² SO ₂ )]
[Wherein, X ¹ and X ² each independently represent a fluorine atom or a fluoroalkyl group having 1 to 6 carbon atoms. ]
May be represented by

  The “fluoroalkyl group” refers to a functional group in which part or all of the hydrogen atoms contained in the alkyl group are substituted with fluorine atoms. The fluoroalkyl group may be, for example, a fluoromethyl group, a difluoromethyl group, a trifluoromethyl group, a fluoroethyl group, a difluoroethyl group, a trifluoroethyl group, or a pentafluoroethyl group.

  Li salt may contain 1 type of Li imide salt independently. Li salt may contain 2 or more types of Li imide salt. The Li salt may include, for example, at least one selected from the group consisting of LiFSI, LiTFSI, and lithium bis (pentafluoroethanesulfonyl) imide (BETI).

  Li imide salts can have a high degree of dissociation. Therefore, when Li salt contains Li imide salt, reduction of resistance is anticipated, for example. However, the Li imide salt has a drawback of corroding the positive electrode current collector (Al foil). There is a possibility that the resistance may increase due to corrosion of the positive electrode current collector. The present embodiment may be configured such that the positive electrode 10 does not require an Al foil (positive electrode current collector). Therefore, in embodiment, it is anticipated that the resistance reduction effect by Li imide salt will become large.

  Li salt may contain LiFSI, for example. That is, the Li imide salt may be LiFSI. The Li salt may consist essentially of LiFSI.

The electrolytic solution may further contain various additives in addition to the solvent and the Li salt. The electrolytic solution may contain, for example, a film forming agent, a gas generating agent (overcharge additive), a flame retardant, and the like. As the film forming agent, for example, Li [B (C ₂ O ₄ ) ₂ ] (LiBOB) can be considered. As the gas generating agent, for example, cyclohexylbenzene (CHB) can be considered. As the flame retardant, for example, phosphazene can be considered.

<< Relationship between void, negative electrode capacity density and positive electrode capacity density >>
In the present embodiment, it is expected that the battery capacity is increased when the gap 30 to be filled with the positive electrode 10, the negative electrode capacity density, and the positive electrode capacity density satisfy a specific relationship. That is, further improvement in volume energy density is expected.

In the present embodiment, the ratio of the volume of the void 30 to the sum of the volume of the negative electrode 20 (true volume) and the volume of the void 30 is described as “porosity (X, unit%)”.
The porosity (X) is represented by the following formula (I):
[100 / {(B / A) +1}] − 10 ≦ X ≦ [100 / {(B / A) +1}]
... (I)
May be satisfied.

In the above formula (I), “A” represents the negative electrode capacity density (unit: mAh / cm ³ ). The negative electrode capacity (unit: mAh) is measured in a half battery (half cell). In the half battery, the counter electrode of the negative electrode 20 is Li metal. The negative electrode capacity density is calculated by dividing the negative electrode capacity by the volume (true volume) of the negative electrode 20. The negative electrode capacity density (A) may be, for example, 250 mAh / cm ³ or more.

In the above formula (I), “B” represents a positive electrode capacity density (unit: mAh / cm ³ ). The positive electrode capacity (unit: mAh) is measured in a half-cell. In the half cell, the counter electrode of the positive electrode 10 is Li metal. The positive electrode capacity density is calculated by dividing the positive electrode capacity by the volume (true volume) of the positive electrode 10. The positive electrode capacity density (B) may be, for example, 146 mAh / cm ³ or more. It is considered that a dense conductive network is easily formed in the positive electrode 10 due to the moderately high positive electrode capacity density.

  By satisfying the relationship of the above formula (I), an improvement in charge / discharge efficiency is also expected. This is probably because the irreversible capacity is suppressed by appropriately increasing the ratio of the negative electrode capacity to the positive electrode capacity.

  Examples of the present disclosure are described below. However, the following description does not limit the scope of the claims.

  Hereinafter, in the examples of the present disclosure, an electrolytic solution whose solvent is a carbonate ester is also referred to as an “ester electrolytic solution”. An electrolytic solution whose solvent is ether is also referred to as “ether-based electrolytic solution”.

<Experiment 1>
In experiment 1, the volume energy density was evaluated.

Example 1
1. Preparation of Negative Electrode A carbon fiber nonwoven fabric was prepared as the carbon fiber aggregate 21. Each surface of the carbon fiber (core portion 1a) was coated with an amorphous carbon material by a thermal CVD method. The amorphous carbon material is coal tar pitch. After coating, the carbon fiber assembly 21 was heat-treated at 1000 ° C. This adjusted the crystallinity of the amorphous carbon material. That is, the sheath part 1b was formed.

  A polymer solution was prepared by dissolving PVDF-HFP in NMP. The carbon fiber aggregate 21 was immersed in the polymer solution. The carbon fiber aggregate 21 was pulled up from the polymer solution. The carbon fiber aggregate 21 to which the polymer solution was adhered was dried. Thereby, the polymer film 22 was formed on each surface of the carbon fiber 1.

Thus, the negative electrode 20 was formed. After the formation of the polymer film 22, the porosity (X) is 64.2%. The negative electrode capacity density (A) is 289 mAh / cm ³ .

2. Preparation of positive electrode The following materials were prepared.
Particle group:
Cathode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂
Conductive materials: Acetylene black and VGCF
Electrolyte (ester electrolyte):
Li salt: LiPF ₆ (concentration 1 mol / L)
Solvent: EC + DMC + EMC [EC: DMC: EMC = 3: 4: 3 (volume ratio)]

The positive electrode 10 was prepared by mixing 55.6 parts by mass of the particle group 11 and 44.4 parts by mass of the electrolyte solution 12. The particle group 11 is composed of 50 parts by mass of a positive electrode active material and 5.6 parts by mass of a conductive material. The positive electrode capacity density (B) is 146 mAh / cm ³ .

  In Example 1, the calculation result of the right side of the above formula (I) is 66.4%, and the calculation result of the left side of the above formula (I) is 56.4%. The porosity (X) in Example 1 is 64.2%. Therefore, in Example 1, the relationship of the above formula (I) is satisfied.

3. Assembly Part of the polymer film 22 was peeled off, so that part of the carbon fiber 1 was exposed in the carbon fiber assembly 21. Gold (Au) was deposited on the exposed portion of the carbon fiber 1. As a result, a current collector was formed. A negative electrode terminal 92 (tab lead) was electrically connected to the current collector. The negative terminal 92 is made of Ni.

Case 90 was prepared. Case 90 is made of SUS. The volume of the internal space of the case 90 is 1.4 cm ³ . The negative electrode 20 was stored in the case 90. Further, the positive electrode 10 was poured into the case 90. Thus, it is considered that the positive electrode 10 is filled in the voids 30 in the negative electrode 20. Further, it is considered that the electrolyte solution 12 contained in the positive electrode 10 penetrates the polymer film 22 and the polymer film 22 swells. Case 90 was sealed. Thus, the battery 100 was manufactured.

5. Evaluation Case 90 (positive electrode terminal) and negative electrode terminal 92 were connected to a charge / discharge test apparatus. Charging / discharging was repeated several times in a voltage range of 3 to 4.2 V with a current of 0.2 C. At a current of 0.2 C, the design capacity of the battery 100 is discharged in 5 hours.

  After several charge / discharge cycles, the effective capacity of the battery 100 was measured. The effective capacity is 130 mAh. Therefore, the volume energy density is estimated to be 356 Wh / L. Further, the resistance of the battery 100 was measured.

Example 2
In Example 2, the negative electrode 20 was formed to have a porosity (X) of 68.2%. In Example 2, the negative electrode capacity density (A) and the calculation result of the right side of the formula (I) are 73.2% and the calculation result of the left side of the formula (I) is 63.2%. The positive electrode capacity density (B) was adjusted. Therefore, also in Example 2, the relationship of the above formula (I) is satisfied. Furthermore, in Example 2, the following electrolytic solution (ether-based electrolytic solution) was used. A battery 100 was manufactured in the same manner as in Example 1 except for these. The effective capacity was measured in the same manner as in Example 1, and the volume energy density was calculated. The resistance was measured in the same manner as in Example 1.

Electrolyte (ether electrolyte):
Li salt: LiFSI (concentration 1 mol / L)
Solvent: DX + DME [DX: DME = 5: 5 (volume ratio)]

<< Comparative Example 1 >>
FIG. 4 is a first schematic diagram showing a laminated electrode group.
In Comparative Example 1, a laminated battery including the laminated electrode group 200 was manufactured. The laminated battery has a conventional electrode structure.

FIG. 5 is a second schematic diagram showing the laminated electrode group.
The laminated electrode group 200 is formed by alternately laminating plate positive electrodes 210 and plate negative electrodes 220. A separator 230 is disposed between the plate positive electrode 210 and the plate negative electrode 220. Each member is manufactured by the following procedure.

1. Preparation of negative electrode The following materials were prepared.
Negative electrode active material: Spherical natural graphite Binder: Carboxymethylcellulose (CMC) and styrene butadiene rubber (SBR)
Solvent: Water Negative electrode current collector: Copper (Cu) foil

A slurry was prepared by mixing spheronized natural graphite, CMC, SBR and water. The slurry was applied to the surface (both front and back surfaces) of the Cu foil and dried to form a coating film. The basis weight of the coating film (mass per unit area) is 15 mg / cm ² . The coating was compressed. The film density after compression is 1.6 g / cm ³ . Thus, a negative electrode raw material was produced. The negative electrode original fabric was cut into a predetermined dimension, whereby a flat plate negative electrode 220 was manufactured.

2. Preparation of positive electrode The following materials were prepared.
Cathode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂
Conductive material: Acetylene black Binder: Polyvinylidene fluoride (PVDF)
Solvent: N-methyl-2-pyrrolidone (NMP)
Positive electrode current collector: Al foil

A slurry was prepared by mixing LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , acetylene black, PVDF and NMP. The slurry was applied to the surface (both front and back surfaces) of the Al foil and dried to form a coating film. The basis weight of the coating film is 25 mg / cm ² . The coating was compressed. The coating density after compression is 3.2 g / cm ³ . Thus, a positive electrode raw material was manufactured. The plate positive electrode 210 was manufactured by cutting the positive electrode original fabric into predetermined dimensions.

3. Assembly Separator 230 was prepared. The separator 230 is a porous film made of polyethylene. Separator 230 has a thickness of 15 μm. The laminated electrode group 200 was formed by alternately laminating the plate positive electrode 210 and the plate negative electrode 220. A separator 230 was disposed between the plate positive electrode 210 and the plate negative electrode 220.

  An aluminum laminate film pouch was prepared. The laminated electrode group 200 was accommodated in the pouch. The ester electrolyte solution of Example 1 was injected into the pouch. After injection of the electrolyte, the pouch was sealed. Thus, a laminated battery was manufactured. The effective capacity was measured in the same manner as in Example 1, and the volume energy density was calculated. The resistance was measured in the same manner as in Example 1.

<< Comparative Example 2 >>
A laminated battery was produced in the same manner as in Comparative Example 1 except that the ether electrolyte solution of Example 2 was used. The effective capacity was measured in the same manner as in Example 1, and the volume energy density was calculated. The resistance was measured in the same manner as in Example 1.

"Experimental result"
Table 1 below shows the results of Experiment 1. In Table 1 below, “volume energy density” is a relative value when the volume energy density of Comparative Example 1 is 100. In Table 1 below, “resistance” is a relative value when the resistance of Comparative Example 1 is 100.

  In Example 1, the volume energy density is improved as compared with Comparative Example 1. The electrode structure of Example 1 is considered to have a higher active material filling rate than the electrode structure of Comparative Example 1.

  In Example 1, the resistance is reduced as compared with Comparative Example 1. The electrode structure of Comparative Example 1 is a two-dimensional structure. In the two-dimensional structure, the positive electrode and the negative electrode are adjacent in a plane. That is, in the two-dimensional structure, the plate positive electrode 210 and the plate negative electrode 220 are opposed to each other (see FIG. 5). The electrode structure of Example 1 is a three-dimensional structure (see FIG. 2). In the electrode structure of Example 1, the positive electrode 10 and the negative electrode 20 can be three-dimensionally adjacent (see FIGS. 2 and 3). In the three-dimensional structure, the facing area (reaction area) per unit volume is considered to be larger than that in the two-dimensional structure. This is considered to reduce the resistance.

  The volume energy density of Comparative Example 2 is lower than that of Comparative Example 1. It is considered that charge / discharge efficiency is reduced by co-insertion of ether (DX and DME) in the negative electrode active material (graphite).

  In Example 2, the volume energy density is improved as compared with Example 1. In Examples 1 and 2, the negative electrode active material is carbon fiber. Carbon fiber is considered to have lower crystallinity than graphite. In carbon fiber, even if co-insertion occurs, it is considered that the structure is unlikely to collapse. It is considered that the structure is moderately disturbed by the co-insertion of ether, so that the diffusion path of Li ions spreads and the capacity increases. Further, in Example 2, since the porosity (X) of the negative electrode 20 is higher than that in Example 1, it can be considered that the fact that the filling amount of the positive electrode 10 can be increased accordingly contributes to the improvement of the volume energy density. It is done.

  In Comparative Example 2, the resistance is increased as compared with Comparative Example 1. This is probably because the Li salt (LiFSI) corrodes the positive electrode current collector (Al foil).

  In Example 2, the resistance is reduced as compared with Example 1. In Examples 1 and 2, there is no positive electrode current collector (Al foil). Therefore, corrosion of Al foil does not occur. Since the degree of dissociation of LiFSI is high, it is considered that resistance components (for example, charge transfer resistance) in the positive electrode are reduced and input / output characteristics are improved.

  From the above results, it is considered that the battery 100 of the present disclosure may have a high volume energy density.

<Experiment 2>
In Experiment 2, the charge / discharge behavior in a combination of various negative electrode active materials and various electrolytic solutions was investigated.

<< Bipolar Electrochemical Cell >>
A bipolar electrochemical cell having the following configuration was produced.
Working electrode (WE): PAN-based carbon fiber nonwoven fabric, pitch-based carbon fiber nonwoven fabric, or graphite negative electrode Counter electrode (CE): Li metal Electrolytic solution: Ester-based electrolytic solution or ether-based electrolytic solution

The graphite negative electrode is cut from the flat plate negative electrode 220 of Comparative Examples 1 and 2 above.
The ester electrolyte solution is the one used in Example 1 above.
The ether electrolyte solution is the one used in Example 2 above.

"Experimental result"
A charge / discharge test was conducted in a bipolar electrochemical cell. The results are shown in FIGS.

FIG. 6 is a first graph showing the results of Experiment 2.
FIG. 6 shows an initial charge / discharge curve in the graphite negative electrode. In the case of an ether electrolyte, 1 V vs. A minute plateau (potential flat portion) is observed in the vicinity of Li ⁺ / Li. The plateau is believed to be derived from co-insertion of ether. In the graphite negative electrode, when the ether electrolyte is used, the charge / discharge capacity is reduced as compared with the case where the ester electrolyte is used. It is considered that the carbon hexagonal network surfaces are separated from each other by co-insertion of ether, and the graphite crystal structure is destroyed.

FIG. 7 is a second graph showing the results of Experiment 2.
FIG. 7 shows an initial charge / discharge curve in the pitch-based carbon fiber nonwoven fabric. In the case of an ether electrolyte, 1 V vs. A minute plateau is observed in the vicinity of Li ⁺ / Li. The plateau is believed to be derived from co-insertion of ether. In pitch-based carbon fibers, the charge / discharge capacity is increased when an ether electrolyte is used, compared to when an ester electrolyte is used. Furthermore, a reduction in polarization is also observed.

FIG. 8 is a third graph showing the results of Experiment 2.
FIG. 8 shows an initial charge / discharge curve in the PAN-based carbon fiber nonwoven fabric. In the case of an ether electrolyte, 1 V vs. A minute plateau is observed in the vicinity of Li ⁺ / Li. The plateau is believed to be derived from co-insertion of ether. In the PAN-based carbon fiber, the charge / discharge capacity is increased when an ether electrolyte is used, compared to when an ester electrolyte is used. Furthermore, a reduction in polarization is also observed.

  The reason why the capacity is increased by the combination of the carbon fiber and the ether electrolyte is not clear at present. For example, at the time of initial charge, it is considered that the diffusion path of Li ions into the carbon fiber is expanded by moderately disturbing the structure of the carbon fiber surface by co-insertion of ether. As a result, the capacity may increase.

FIG. 9 is a fourth graph showing the results of Experiment 2.
FIG. 9 shows the relationship between the change rate of the initial charge / discharge efficiency and the change rate of the discharge capacity.
“Change rate of initial charge / discharge efficiency” is expressed by the following formula (II):
(Change rate of initial charge / discharge efficiency) = (initial charge / discharge efficiency when ether electrolyte is used) ÷ (initial charge / discharge efficiency when ester electrolyte is used) × 100 (II)
It is calculated by.

“Change rate of discharge capacity” is expressed by the following formula (III):
(Change rate of discharge capacity) = (discharge capacity when ether electrolyte is used) ÷ (discharge capacity when ester electrolyte is used) × 100 (III)
It is calculated by.

  As shown in FIG. 9, in the PAN-based carbon fiber, when the electrolytic solution is changed from the ester system to the ether system, the initial charge / discharge efficiency does not decrease and the discharge capacity increases. Therefore, in the configuration of the battery 100 of the present disclosure, the volume energy density is expected to be further improved by using the PAN-based carbon fiber and the ether-based electrolyte in combination.

  Embodiments and examples of the present disclosure are illustrative in all respects and not restrictive. The technical scope determined by the description of the claims includes all modifications within the meaning and scope equivalent to the description of the claims.

  DESCRIPTION OF SYMBOLS 1 Carbon fiber, 1a core part, 1b sheath part, 10 positive electrode, 11 particle group, 12 electrolyte solution, 20 negative electrode, 21 carbon fiber aggregate, 22 polymer film, 30 space | gap, 80 insulating material, 90 case, 92 negative electrode terminal , 100 battery (lithium ion secondary battery), 200 laminated electrode group, 210 flat plate positive electrode, 220 flat plate negative electrode, 230 separator.

Claims (6)

Including at least a positive electrode and a negative electrode,
The negative electrode is porous;
The negative electrode includes a carbon fiber aggregate and a polymer film,
The carbon fiber aggregate is formed by three-dimensionally bonding a plurality of carbon fibers,
The polymer film covers the surface of each of the plurality of carbon fibers,
The positive electrode includes a particle group and an electrolytic solution,
The particle group is dispersed in the electrolyte solution,
The particle group includes a positive electrode active material and a conductive material,
The positive electrode is filled in the voids in the negative electrode,
The polymer membrane is swollen by the electrolyte;
Lithium ion secondary battery. The electrolyte includes at least a solvent and a lithium salt,
The solvent includes ether;
The lithium ion secondary battery according to claim 1. The solvent includes at least one selected from the group consisting of 1,4-dioxane and 1,2-dimethoxyethane;
The lithium ion secondary battery according to claim 2. The lithium salt includes a lithium imide salt,
The lithium imide salt is a salt of a lithium ion and a fluorine-containing sulfonylimide anion.
The lithium ion secondary battery according to claim 2 or claim 3. The lithium salt includes lithium bis (fluorosulfonyl) imide,
The lithium ion secondary battery according to claim 4. The plurality of carbon fibers include polyacrylonitrile-based carbon fibers.
The lithium ion secondary battery of any one of Claims 2-5.

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