JP7035928B2 - Lithium ion secondary battery - Google Patents

Description

The present disclosure relates to a lithium ion secondary battery.

Japanese Patent Application Laid-Open No. 2010-244911 describes an electrode cell by covering a carbon fiber non-woven fabric with an electrolyte membrane, filling the voids in the carbon fiber non-woven fabric with a positive electrode active material and a solvent, and further evaporating the solvent. Is disclosed.

Japanese Unexamined Patent Publication No. 2010-244911

According to Patent Document 1, 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 liquid 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 dried carbon fiber nonwoven fabric by the volume of the solvent. The presence of voids reduces the volumetric energy density (energy per unit volume). That is, it is considered that the electrode cell of Patent Document 1 has room for improvement in volume energy density.

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

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

[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 contains 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 contains a group of particles and an electrolytic solution. The particle swarms are dispersed in the electrolytic solution. The particle group includes a positive electrode active material and a conductive material. The void in the negative electrode is filled with the positive electrode. The polymer membrane 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 contains a particle group (positive electrode active material, etc.) and an electrolytic solution. The particle swarms are dispersed in the electrolytic solution.

The negative electrode contains a carbon fiber aggregate and a polymer film. The carbon fiber aggregate consists of a plurality of carbon fibers. An intercalation reaction of lithium (Li) ions can occur in each of the plurality of carbon fibers. That is, carbon fiber is a negative electrode active material. A void is formed between the plurality of carbon fibers. That is, the negative electrode is porous. The void 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 unnecessary voids in the battery is suppressed. That is, improvement in volume energy density is expected.

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

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

Further, in the battery of the present disclosure, the electrolytic solution is responsible for the 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 a high output.

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

Generally, a carbonic acid ester [for example, ethylene carbonate (EC), ethyl methyl carbonate (EMC), etc.] is used as the solvent of the electrolytic solution. The use of ether [for example, 1,2-dimethoxyethane (DME), etc.] as the solvent is expected to improve the conductivity. It is considered that the ether has a low viscosity and promotes the dissociation of the Li salt. By improving the conductivity of the electrolytic solution, it is expected that the resistance will be reduced.

In lithium primary cells, ether is used as the solvent for the electrolytic solution. However, in lithium ion secondary batteries, ether is not widely used as a solvent for an electrolytic solution. In lithium-ion secondary batteries, it is thought that ether causes co-insertion. Generally, the negative electrode active material of a lithium ion secondary battery is graphite. Graphite crystals are formed by laminating a plurality of carbon hexagonal mesh planes. In the electrolytic solution, Li ions are solvated with a solvent molecule (here, ether). "Co-insertion" means that the solvent molecule coordinated to the Li ion is inserted into the gap between the carbon hexagonal network surfaces together with the Li ion. When co-insertion occurs, solvent molecules are decomposed in the gaps between the carbon hexagonal mesh surfaces. As a result, the carbon hexagonal net surfaces are separated from each other and the crystal structure is destroyed. It is considered that the storage sites of Li ions are reduced due to the collapse of the crystal structure. It is considered that this reduces the capacity.

In the lithium ion secondary battery of the present disclosure, the negative electrode active material is carbon fiber. According to the new findings of the present disclosure, when the negative electrode active material is carbon fiber, the capacity can be rather increased by using ether as a solvent. That is, 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 fibers is lower than that of graphite, it is considered that the structure is unlikely to collapse even if co-insertion occurs. Further, for example, it is considered that the structure of the carbon fiber surface is appropriately disturbed by the co-insertion of ether at the time of initial charging, so that the diffusion path of Li ions into the inside of the carbon fiber is widened. As a result, the capacity may have increased.

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

Generally, LiPF ₆ is used as the Li salt (supporting electrolyte salt) of the electrolytic solution. Lithium (Li) imide salts are also being studied as Li salts. The Liimide salt is a salt of a Li ion and a fluorine-containing sulfonylimide anion. "Fluorine-containing sulfonylimide anion" indicates a sulfonylimide anion containing a fluorine atom. The electrolytic solution containing the Liimide salt may have high conductivity. It is considered that the Liimide salt may have a high degree of dissociation. Further, since the degree of dissociation of the Liimide salt is high, it is expected that the resistance component (for example, charge transfer resistance) in the positive electrode will be reduced. The synergistic effect of these actions is expected to reduce resistance.

However, the Liimide salt has the drawback 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 base material. Generally, the positive electrode current collector is an Al foil. Corrosion of the positive electrode current collector may actually increase resistance.

In the lithium ion secondary battery of the present disclosure, the positive electrode is in a liquid state. The positive electrode of the present disclosure does not require an electrode base material (that is, Al foil). Therefore, it is considered that the advantages of the Liimide salt (reduction of resistance) can be enjoyed without causing the drawbacks of the Liimide salt (corrosion of Al).

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

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

Although the details of the mechanism are unknown, PAN-based carbon fibers are considered to have particularly good compatibility with ether. Since the carbon fiber (negative electrode active material) contains PAN-based carbon fiber, it is expected that the capacity will be increased and the polarization will be reduced. This is expected to improve the volumetric 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 cross-sectional conceptual diagram showing an example of the electrode arrangement of the present embodiment. FIG. 4 is a first schematic view showing a group of laminated electrodes. FIG. 5 is a second schematic view 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, embodiments of the present disclosure (also referred to as “the present embodiment” in the present specification) will be described. However, the following explanation does not limit the scope of 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. The battery 100 includes a case 90. The case 90 may have any outer shape. The outer shape of the case 90 may be, for example, a cylinder or a square. The case 90 may be made of, for example, stainless steel (SUS), aluminum (Al) alloy, or the like. The case 90 may be a pouch made of an aluminum laminated film or the like. However, when the electrolytic solution 12 (described later) contains a Liimide salt, it is desirable that the portion of the case 90 in contact with the electrolytic solution 12 does not contain Al. The case 90 houses the positive electrode 10 and the negative electrode 20. That is, the battery 100 includes at least a positive electrode 10 and a negative electrode 20.

The positive electrode 10 is in a liquid state. The positive electrode 10 is in contact with the inner wall of the case 90. When the case 90 is made of a conductive material, the case 90 can be a positive electrode terminal. The negative electrode 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 or the like.

《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 void 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" means that a plurality of carbon fibers 1 are bonded to each other and a plurality of carbon fibers 1 form a three-dimensional network structure. The carbon fiber assembly 21 may 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 void 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 fiber 1, felt of carbon fiber 1, or the like. Nonwoven fabrics and felts have advantages such as easy formation of current collectors. As a modified form, it is conceivable to use a porous material formed of crystalline carbon instead of the carbon fiber aggregate 21. As the porous material, for example, carbon monolith or the like can be considered. 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 contain PAN-based carbon fibers. The plurality of carbon fibers 1 may be substantially composed of only PAN-based carbon fibers. The PAN-based carbon fiber indicates a carbon fiber made from PAN as a raw material. The pitch-based carbon fiber indicates, for example, carbon fiber made from petroleum pitch or the like. Cellulose-based carbon fiber refers to carbon fiber made from, for example, viscose rayon or the like.

The plurality of carbon fibers 1 can be bonded by, for example, the following method. 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 fibers 1 and the binder are graphitized (crystallized). As a result, a plurality of carbon fibers 1 can be bonded to each other. The binder may be, for example, a coal tar pitch, a phenol resin, an epoxy resin, or the like.

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

FIG. 3 is a cross-sectional conceptual diagram showing an example of the electrode arrangement of the present embodiment.
The carbon fiber 1 is formed of crystalline carbon. The carbon fiber 1 is a negative electrode active material. An intercalation reaction of Li ions can 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 portion 1b may be lower than the crystallinity of the core portion 1a. It is expected that the inclusion of the low crystalline sheath portion 1b in the carbon fiber 1 will increase the rate of the intercalation reaction on the surface of the carbon fiber 1. Further, for example, it is expected that side reactions at high temperatures will be suppressed.

The sheath portion 1b can be formed, for example, by the following method. The carbon fiber to be the core portion 1a is prepared. The core portion 1a (carbon fiber) is coated with an amorphous carbon material. The core 1a and the amorphous carbon material are heat treated. The heat treatment adjusts the crystallinity of the amorphous carbon material. As a result, the sheath portion 1b can be formed.

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

(Polymer membrane)
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, for example, 1 μm or more and 50 μm or less. 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 electrolytic solution 12 form a polymer gel. It is considered that the polymer gel has Li ion conductivity and does not have electron conductivity. Therefore, it is considered that the positive electrode 10 and the negative electrode 20 are connected by ion conduction, and the short circuit (electrical connection) between the positive electrode 10 and the negative electrode 20 is suppressed.

The polymer material forming the polymer film 22 is, for example, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyethylene oxide (PEO), polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), or the like. There may be. The polymer film 22 may contain one kind of polymer material alone. The polymer film 22 may contain two or more kinds of polymer materials. Inorganic particles may be further contained in the polymer film 22. The inorganic particles may be, for example, alumina, silica, zirconia or the like. Since the polymer film 22 further contains inorganic particles, it is expected that the strength of the polymer film 22 will be improved, for example. The particle size, content, etc. of the inorganic particles can be appropriately adjusted according to the strength and the like of the polymer film 22.

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 in a liquid state. The positive electrode 10 fills the void 30 in the negative electrode 20 (see FIGS. 2 and 3). The positive electrode 10 contains 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 liquid (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 swarm)
The particle group 11 contains a positive electrode active material and a conductive material. The positive electrode active material is a particle capable of intercalating reaction of Li ions. The positive electrode active material may have, for example, D50 of 1 μm or more and 30 μm or less. D50 indicates a particle size in which the integrated 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 type particle size distribution measuring device or the like.

The positive electrode active material should not be particularly limited. The positive electrode active material is, for example, lithium cobalt oxide, lithium nickel oxide, spinel type lithium manganate, layered rock salt type lithium manganate, nickel cobalt manganate (for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ etc.). , Lithium nickel cobalt oxide (eg LiNi _0.82 Co _0.15 Al _0.03 O ₂ , etc.), lithium iron phosphate, etc. may be used. The particle group 11 may contain one kind of positive electrode active material alone. The particle group 11 may contain two or more kinds of 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 or the like), vapor-grown carbon fiber (VGCF), carbon nanotube (CNT), graphene flakes, graphite or the like. The particle group 11 may contain one kind of conductive material 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 this 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. Fibrous conductive materials (eg, VGCF, CNTs, etc.) are considered suitable for forming long-distance electron conduction paths. In this embodiment, the conductive material contained in the positive electrode 10 and in the form of fibers is regarded as particles.

(Electrolytic solution)
The electrolytic solution 12 contains at least a solvent and a Li salt. The solvent is, for example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), γ-. Butyrolactone (GBL), δ-valerolactone, tetrahydrofuran (THF), 1,3-dioxolane (DOL), 1,4-dioxane (DX), 1,2-dimethoxyethane (DME), methylformate (MF), It may be methyl acetate (MA), methyl propionate (MP), ionic liquid or the like. The electrolytic solution 12 may contain one kind of solvent alone. The electrolytic solution 12 may contain two or more kinds of solvents.

The solvent may contain ether. The inclusion of ether in the solvent is expected to improve the volumetric energy density. 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, for example, substantially only ether. In the solvent, the balance excluding the ether may be, for example, a carbonic acid ester or the like.

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, the 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 kinds of ethers. The solvent may contain at least one selected from the group consisting of, for example, THF, 2-MeTHF, DOL, 1,3-dioxane, DX, DME, DEE and DBE.

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

It is considered that ether and PAN-based carbon fiber are particularly compatible with each other. When the solvent contains ether and the plurality of carbon fibers 1 contain PAN-based carbon fibers, it is expected that the volumetric energy density will be improved.

The Li salt is dissolved in the solvent. The concentration of the Li salt may be, for example, 0.5 mL / L or more and 5 mL / 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), or the like. The electrolytic solution 12 may contain one Li salt alone. The electrolytic solution 12 may contain two or more kinds of Li salts.

The Li salt may contain a Liimide salt. The Li salt may contain, for example, a Liimide salt and LiPF ₆ . The Li salt may consist substantially only of the Liimide salt, for example. The Liimide salt is a salt of a Li ion and a fluorine-containing sulfonylimide anion. "Fluorine-containing sulfonylimide anion" indicates a sulfonylimide anion containing a fluorine atom. The above "LiFSI" and "LiTFSI" correspond to Liimide salts.

The Liimide salt is, for example, the following general formula:
Li [N (X ¹ SO ₂ ) (X ² SO ₂ )]
[However, in the formula, X ¹ and X ² independently represent a fluorine atom or a fluoroalkyl group having 1 to 6 carbon atoms. ]
May be represented by.

A "fluoroalkyl group" refers to a functional group in which some 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, a pentafluoroethyl group or the like.

The Li salt may contain one Liimide salt alone. The Li salt may contain two or more kinds of Liimide salts. The Li salt may contain, for example, at least one selected from the group consisting of LiFSI, LiTFSI and lithium bis (pentafluoroethanesulfonyl) imide (BETI).

Liimide salts can have a high degree of dissociation. Therefore, when the Li salt contains a Liimide salt, for example, it is expected that the resistance will be reduced. However, the Liimide salt has a drawback of corroding the positive electrode current collector (Al foil). Corrosion of the positive electrode current collector may actually increase resistance. In this embodiment, the positive electrode 10 may have a configuration that does not require an Al foil (positive electrode current collector). Therefore, in the embodiment, it is expected that the resistance reducing effect of the Liimide salt will be increased.

The Li salt may contain, for example, LiFSI. That is, the Liimide salt may be LiFSI. The Li salt may consist substantially only 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) and the like can be considered. As the gas generating agent, for example, cyclohexylbenzene (CHB) or the like can be considered. As the flame retardant, for example, phosphazene or the like can be considered.

<< Relationship between voids, negative electrode capacity density and positive electrode capacity density >>
In the present embodiment, it is expected that the battery capacity will be increased by satisfying a specific relationship between the void 30 to be filled with the positive electrode 10, the negative electrode capacity density, and the positive electrode capacity density. 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 total volume (true volume) of the negative electrode 20 and the volume of the void 30 is described as "porosity (X, unit%)".
The porosity (X) is the following formula (I):
[100 / {(B / A) +1}] -10≤X≤ [100 / {(B / A) +1}]
… (I)
The relationship may be satisfied.

In the above formula (I), "A" indicates the negative electrode capacitance density (unit: mAh / cm ³ ). The negative electrode capacity (unit: mAh) is measured in a half cell (half cell). In the half cell, 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 capacitance density (A) may be, for example, 250 mAh / cm ³ or more.

In the above formula (I), "B" indicates the positive electrode capacitance 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 capacitance density (B) may be, for example, 146 mAh / cm ³ or more. It is considered that a dense conductive network is likely to be formed in the positive electrode 10 because the positive electrode capacitance density is moderately high.

By satisfying the relationship of the above formula (I), it is expected that the charging / discharging efficiency will be improved. It is considered that the irreversible capacity is suppressed by appropriately increasing the ratio of the negative electrode capacity to the positive electrode capacity.

Hereinafter, examples of the present disclosure will be described. However, the following explanation does not limit the scope of claims.

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

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

<< Example 1 >>
1. 1. Preparation of Negative Electrode A carbon fiber non-woven 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 the thermal CVD method. The amorphous carbon material is coal tar pitch. After coating, the carbon fiber aggregate 21 was heat-treated at 1000 ° C. This adjusted the crystallinity of the amorphous carbon material. That is, the sheath portion 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 out of the polymer solution. The carbon fiber aggregate 21 to which the polymer solution was attached was dried. As a result, a polymer film 22 was formed on each surface of the carbon fiber 1.

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

2. 2. Preparation of positive electrode The following materials were prepared.
Particle swarm:
Positive electrode active material: LiNi _1/3 Co _1/3 Mn _1/3 O ₂
Conductive material: 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 electrolytic solution 12. The particle group 11 is composed of 50 parts by mass of the positive electrode active material and 5.6 parts by mass of the conductive material. The positive electrode capacitance density (B) is 146 mAh / cm ³ .

In the first embodiment, the calculation result on the right side of the above formula (I) is 66.4%, and the calculation result on 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. 3. Assembling A part of the carbon fiber 1 was exposed in the carbon fiber aggregate 21 by peeling off a part of the polymer film 22. Gold (Au) was vapor-deposited on the exposed portion of the carbon fiber 1. As a result, a current collector was formed. The negative electrode terminal 92 (tab lead) was electrically connected to the current collector. The negative electrode terminal 92 is made of Ni.

Case 90 was prepared. The 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 housed in the case 90. Further, the positive electrode 10 was poured into the case 90. As a result, it is considered that the positive electrode 10 is filled in the void 30 in the negative electrode 20. Further, it is considered that the electrolytic solution 12 contained in the positive electrode 10 permeated the polymer film 22 and the polymer film 22 swelled. The case 90 was sealed. From the above, the battery 100 was manufactured.

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

After several charges and discharges, 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 so as to have a porosity (X) of 68.2%. In Example 2, the negative electrode capacitance density (A) and the negative electrode capacitance density (A) so that the calculation result on the right side of the above formula (I) is 73.2% and the calculation result on the left side of the above formula (I) is 63.2%. The positive electrode capacitance density (B) was adjusted. Therefore, the relationship of the above formula (I) is satisfied in the second embodiment as well. Further, in Example 2, the following electrolytic solution (ether-based electrolytic solution) was used. Except for these, the battery 100 was manufactured in the same manner as in Example 1. The effective capacity was measured and the volumetric energy density was calculated in the same manner as in Example 1. Resistance was measured in the same manner as in Example 1.

Electrolyte (ether-based 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 view showing a group of laminated electrodes.
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 view showing the laminated electrode group.
The laminated electrode group 200 is formed by alternately laminating the flat plate positive electrode 210 and the flat plate negative electrode 220. A separator 230 is arranged between the flat plate positive electrode 210 and the flat plate negative electrode 220. Each member is manufactured by the following procedure.

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

Slurry was prepared by mixing sphericalized 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 (mass per unit area) of the coating film is 15 mg / cm ² . The coating film was compressed. The coating film density after compression is 1.6 g / cm ³ . From the above, the negative electrode raw fabric was manufactured. The flat plate negative electrode 220 was manufactured by cutting the negative electrode raw fabric to a predetermined size.

2. 2. Preparation of positive electrode The following materials were prepared.
Positive electrode 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 film was compressed. The coating film density after compression is 3.2 g / cm ³ . From the above, the positive electrode raw fabric was manufactured. The flat plate positive electrode 210 was manufactured by cutting the positive electrode raw fabric to a predetermined size.

3. 3. The assembly separator 230 was prepared. The separator 230 is a polyethylene porous membrane. The separator 230 has a thickness of 15 μm. The laminated electrode group 200 was formed by alternately laminating the flat plate positive electrode 210 and the flat plate negative electrode 220. A separator 230 was arranged between each of the flat plate positive electrode 210 and the flat plate negative electrode 220.

A pouch made of aluminum laminated film was prepared. The laminated electrode group 200 was housed in a pouch. The ester-based electrolytic solution of Example 1 was injected into the pouch. After injecting the electrolyte, the pouch was sealed. From the above, the laminated battery was manufactured. The effective capacity was measured and the volumetric energy density was calculated in the same manner as in Example 1. Resistance was measured in the same manner as in Example 1.

<< Comparative Example 2 >>
A laminated battery was manufactured in the same manner as in Comparative Example 1 except that the ether-based electrolytic solution of Example 2 was used. The effective capacity was measured and the volumetric energy density was calculated in the same manner as in Example 1. Resistance was measured in the same manner as in Example 1.

"Experimental result"
The results of Experiment 1 are shown in Table 1 below. 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 volumetric energy density is improved as compared with Comparative Example 1. It is considered that the electrode structure of Example 1 has a higher filling rate of the active material 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 planarly adjacent to each other. That is, in the two-dimensional structure, the flat plate positive electrode 210 and the flat plate negative electrode 220 face 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 sterically adjacent to each other (see FIGS. 2 and 3). In the three-dimensional structure, it is considered that the facing area (reaction area) per unit volume is larger than that in the two-dimensional structure. It is considered that this reduces the resistance.

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

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

Comparative Example 2 has an increased resistance as compared with Comparative Example 1. It is considered that the Li salt (LiFSI) corrodes the positive electrode current collector (Al foil).

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

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

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

<< Two-pole electrochemical cell >>
A bipolar electrochemical cell having the following configuration was produced.
Working electrode (WE): PAN-based carbon fiber non-woven fabric, pitch-based carbon fiber non-woven fabric, or graphite negative electrode counter electrode (CE): Li metal Electrolyte: ester-based electrolytic solution or ether-based electrolytic solution

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

"Experimental result"
A charge / discharge test was carried out in a two-pole electrochemical cell. The results are shown in FIGS. 6-9.

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

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

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

The reason why the capacity is increased by the combination of carbon fiber and the ether-based electrolytic solution is not clear at this time. For example, it is considered that the structure of the carbon fiber surface is appropriately disturbed by the co-insertion of ether at the time of the first charge, so that the diffusion path of Li ions into the inside of the carbon fiber is widened. As a result, the capacity may have increased.

FIG. 9 is a fourth graph showing the results of Experiment 2.
FIG. 9 shows the relationship between the rate of change in the initial charge / discharge efficiency and the rate of change in the discharge capacity.
The "rate of change in initial charge / discharge efficiency" is the following formula (II):
(Rate of change in initial charge / discharge efficiency) = (Initial charge / discharge efficiency when ether-based electrolytic solution is used) ÷ (Initial charge / discharge efficiency when ester-based electrolytic solution is used) x 100 ... (II)
It is calculated by.

The "rate of change in discharge capacity" is the following formula (III):
(Rate of change in discharge capacity) = (Discharge capacity when ether-based electrolyte is used) ÷ (Discharge capacity when ester-based electrolyte is used) x 100 ... (III)
It is calculated by.

As shown in FIG. 9, in the PAN-based carbon fiber, since the electrolytic solution is changed from the ester-based to the ether-based, 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, it is expected that the volumetric energy density will be further improved by using the PAN-based carbon fiber and the ether-based electrolytic solution in combination.

The embodiments and examples of the present disclosure are exemplary and not restrictive in all respects. The technical scope defined by the description of the scope of claims includes all changes within the meaning and scope equivalent to the description of the scope of claims.

1 carbon fiber, 1a core part, 1b sheath part, 10 positive electrode, 11 particle group, 12 electrolytic solution, 20 negative electrode, 21 carbon fiber aggregate, 22 polymer film, 30 voids, 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 (5)

Including at least positive and negative electrodes,
The negative electrode is porous and
The negative electrode contains a carbon fiber aggregate and a polymer film, and contains 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 contains a group of particles and an electrolytic solution, and contains a group of particles and an electrolytic solution.
The particle group is dispersed in the electrolytic solution, and the particles are dispersed in the electrolytic solution.
The particle swarm contains a positive electrode active material and a conductive material, and includes a positive electrode active material and a conductive material.
The positive electrode is filled in the void in the negative electrode, and the positive electrode is filled.
The polymer membrane is swollen by the electrolytic solution,
The electrolytic solution contains at least a solvent and a lithium salt, and contains at least a solvent and a lithium salt.
The solvent comprises ether,
Lithium-ion secondary battery. The solvent comprises at least one selected from the group consisting of 1,4-dioxane and 1,2-dimethoxyethane.
The lithium ion secondary battery according to claim 1 . The lithium salt contains a lithium imide salt and contains
The lithium imide salt is a salt of lithium ions and a fluorine-containing sulfonylimide anion.
The lithium ion secondary battery according to claim 1 or 2 . The lithium salt contains a lithium bis (fluorosulfonyl) imide,
The lithium ion secondary battery according to claim 3 . The plurality of carbon fibers include polyacrylonitrile-based carbon fibers.
The lithium ion secondary battery according to any one of claims 1 to 4 .

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