JP7823864B2 - Lithium-ion battery - Google Patents

Description

The present invention relates to a lithium-ion battery.

Due to their high energy density, lithium-ion batteries are widely used in electronic and electrical devices such as smartphones and laptops. Lithium-ion batteries typically use a flammable non-aqueous electrolyte, in which lithium salts are dissolved in a non-aqueous solvent. Lithium-ion batteries can generate heat due to overcharging or short circuits between the positive and negative electrodes. Furthermore, the positive electrode active material can release oxygen from its crystals due to thermal decomposition or overcharging. This puts lithium-ion batteries at risk of abnormal heat generation and fire.
To prevent accidents caused by abnormal heat generation and fire, it has been proposed to use ionic liquids as electrolyte solvents in lithium-ion batteries (see, for example, Patent Documents 1 and 2). Ionic liquids are liquids composed of anions and cations and generally have low vapor pressure and are non-flammable. Therefore, using ionic liquids as electrolyte solvents can improve the safety of lithium-ion batteries.

Japanese Patent Application Laid-Open No. 2017-195129 JP 2018-116840 A

Ionic liquids have lower ionic conductivity than organic solvents. Therefore, in conventional lithium-ion batteries using ionic liquid electrolytes, the amount of lithium ions supplied to the positive and negative electrode active materials during charging and discharging is uneven, resulting in poor charge-discharge performance. Furthermore, in conventional lithium-ion batteries using ionic liquid electrolytes, the supply of lithium ions to the positive and negative electrode active materials can stagnate during high-rate charging and discharging, resulting in poor rate performance.
The present invention has been made in view of the above circumstances, and provides a lithium ion battery having excellent charge/discharge characteristics and excellent rate characteristics.

The present invention provides a lithium-ion battery comprising: a positive electrode having a positive electrode active material layer; a negative electrode having a negative electrode active material layer; a porous insulating layer disposed between the positive electrode and the negative electrode; and an electrolyte solution; the porous insulating layer has a porous structure containing a plurality of electrically insulating particles and is disposed adjacent to the negative electrode active material layer; the median diameter of the plurality of electrically insulating particles is 1 μm or more and 5 μm or less; the thickness of the porous insulating layer is 5 μm or more and 50 μm or less; and the electrolyte solution contains a lithium salt and an ionic liquid.

The lithium-ion battery of the present invention possesses the above-mentioned characteristics, resulting in excellent charge/discharge characteristics and excellent rate characteristics. This was made clear through experiments conducted by the inventors.

1 is a schematic cross-sectional view of a lithium-ion battery according to one embodiment of the present invention. 1 is a schematic cross-sectional view of a lithium-ion battery according to one embodiment of the present invention. FIG. 2 is an enlarged view of the surface of a porous insulating layer. 1 is a SEM photograph of a porous insulating layer. 1 is a SEM photograph of a nonwoven fabric.

The lithium-ion battery of the present invention comprises a positive electrode having a positive electrode active material layer, a negative electrode having a negative electrode active material layer, a porous insulating layer disposed between the positive electrode and the negative electrode, and an electrolyte solution, wherein the porous insulating layer has a porous structure containing a plurality of electrically insulating particles and is disposed adjacent to the negative electrode active material layer, the median diameter of the plurality of electrically insulating particles being 1 μm or more and 5 μm or less, the thickness of the porous insulating layer being 5 μm or more and 50 μm or less, and the electrolyte solution containing a lithium salt and an ionic liquid.

The lithium-ion battery of the present invention preferably includes a separator, which is disposed between the positive electrode and the negative electrode and is a nonwoven fabric or a porous resin film, and the porous insulating layer coats at least one major surface of the separator, and the separator is preferably disposed so that the major surface of the separator coated with the porous insulating layer faces the negative electrode active material layer, thereby improving the charge/discharge characteristics and rate characteristics of the lithium-ion battery.
The porous insulating layer may coat the surface of the negative electrode active material layer.
The concentration of the lithium salt in the electrolyte solution is preferably 1.6 mol/L or more and 3.2 mol/L or less.

Preferably, the ionic liquid contains an anion component and a cation component, the lithium salt is a compound that dissociates into a lithium ion and an anion component in the electrolyte solution, and the anion component of the ionic liquid or the anion component of the lithium salt is a bis(fluorosulfonyl)amide ion.
Preferably, the ionic liquid contains an anion component and a cation component, and the cation component of the ionic liquid is a chain or cyclic quaternary ammonium ion.
The electrically insulating particles are preferably ceramic particles or synthetic resin particles.
It is preferable that the contact angle measured 10 seconds after the electrolytic solution is dropped onto the surface of the porous insulating layer is 60 degrees or less.

An embodiment of the present invention will be described below using the drawings. The configurations shown in the drawings and the following description are merely examples, and the scope of the present invention is not limited to those shown in the drawings or the following description.

1 and 2 are schematic cross-sectional views of the lithium ion battery of this embodiment, and FIG. 3 is an enlarged view of the surface of the porous insulating layer.
The lithium ion battery 20 of this embodiment includes a positive electrode 2 having a positive electrode active material layer 7, a negative electrode 3 having a negative electrode active material layer 12, a porous insulating layer 8 arranged between the positive electrode 2 and the negative electrode 3, and an electrolyte 5, wherein the porous insulating layer 8 has a porous structure having a plurality of electrically insulating particles 9 and is arranged adjacent to the negative electrode active material layer 12, the plurality of electrically insulating particles 9 having a median diameter of 1 μm or more and 5 μm or less, the porous insulating layer 8 having a thickness of 5 μm or more and 50 μm or less, and the electrolyte 5 containing a lithium salt and an ionic liquid.
The lithium ion battery 20 may be a secondary battery. The lithium ion battery 20 may also include a separator 4 disposed between the positive electrode 2 and the negative electrode 3.

The positive electrode 2 has a positive electrode current collector sheet 6 and a porous positive electrode active material layer 7 provided on the positive electrode current collector sheet 6 .
The positive electrode current collector sheet 6 is a sheet that serves as a base material for providing the positive electrode active material layer 7, and is a conductor that electrically connects the positive electrode battery terminal and the positive electrode active material layer 7. Furthermore, a part of the positive electrode current collector sheet 6 may serve as the positive electrode battery terminal. The positive electrode current collector sheet 6 is, for example, aluminum foil.

The positive electrode active material layer 7 is a porous layer containing a positive electrode active material. The positive electrode active material layer 7 may be provided on one side of the positive electrode current collector sheet 6 or on both sides of the positive electrode current collector sheet 6.
The thickness of the positive electrode active material layer 7 is, for example, not less than 1 μm and not more than 100 μm.
By making the thickness of the positive electrode active material layer 7 1 μm or more, the amount of the positive electrode active material contained in the positive electrode 2 can be increased, thereby increasing the capacity of the lithium ion battery. In addition, the positive electrode active material layer 7 can be easily formed by coating.
By setting the thickness of the positive electrode active material layer 7 to 100 μm or less, it is possible to shorten the migration distance (diffusion distance) of lithium ions between the vicinity of the interface between the positive electrode active material layer 7 and the positive electrode current collector sheet 6 and the vicinity of the surface of the positive electrode active material layer 7, and it is possible to prevent a shortage of lithium ions in the electrolyte 5 in the pores near the interface during discharge and an excess of lithium ions in the electrolyte 5 in the pores near the interface during charge.

The positive electrode active material is a material that is directly involved in the transfer of electrons accompanying charge transfer in the positive electrode. Examples of the positive electrode active material contained in the positive electrode active material layer 7 include _LiCoO2 , _LiNiO2 , LiNi _x Co _{1 -x} O ₂ (x = 0.01 to 0.99), LiMnO2, LiMn ₂ O ₄ , LiCo _x Mn _y Ni _z O ₂ (x + y + z = 1), or olivine-type _LiFePO4 or Li _x Fe _1-y M _{y PO4} (where 0.05 ≤ x ≤ 1.2, 0 ≤ y ≤ 0.8, and M is at least one of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and Nb). The positive electrode active material layer 7 can contain one type of these positive electrode active materials alone or a mixture of two or more types.

The positive electrode active material layer 7 can have a porous structure in which positive electrode active material powder is bonded with a binder. This allows the positive electrode active material layer 7 to have pores between the positive electrode active material particles. These pores are filled with the electrolyte solution 5, and an electrode reaction proceeds on the surfaces of the positive electrode active material particles.
For example, when charging the lithium ion battery 20, the lithium atoms contained in the positive electrode active material particles are released into the electrolyte 5 as lithium ions (Li ⁺ ), and when discharging the lithium ion battery 20, the lithium ions in the electrolyte 5 are inserted into the positive electrode active material particles as lithium atoms.

The positive electrode active material particles contained in the positive electrode active material layer 7 may have a conductive coating on their surfaces. This improves the conductivity of the particle surfaces where the electrode reaction progresses, thereby reducing the internal resistance of the positive electrode 2. The conductive coating may be, for example, a carbon coating.

The positive electrode active material layer 7 may contain a conductive agent. This can improve the conductivity of the positive electrode active material layer 7 and reduce the internal resistance of the positive electrode 2. The conductive agent is, for example, acetylene black. Alternatively, the conductive agent may be fine particles of coke-based soft carbon, which is easily graphitizable carbon.
The positive electrode active material layer 7 may contain a binder, such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene copolymer (SBR), carboxymethyl cellulose (CMC), acrylonitrile rubber, or an acrylonitrile rubber-PTFE mixture.

For example, a positive electrode active material powder, a conductive agent, and a binder are mixed to prepare a slurry, which is then applied to the positive electrode current collector sheet 6 (for example, by roll-to-roll coating). The applied layer is then dried and pressed to form the positive electrode active material layer 7. Examples of solvents that can be used to prepare the slurry include water, dimethylformamide, N-methylpyrrolidone, isopropanol, and toluene.

The negative electrode 3 is an electrode containing a negative electrode active material. The negative electrode active material is a substance that is directly involved in the transfer of electrons that accompanies charge transfer at the negative electrode. Examples of negative electrode active materials include graphite, partially graphitized carbon, hard carbon, soft carbon, lithium titanate (LTO), and Sn alloys. The negative electrode active material layer 12 can contain one or more of these negative electrode active materials.

The negative electrode 3 can include a negative electrode current collector sheet 11 and a porous negative electrode active material layer 12 provided on the negative electrode current collector sheet 11. The negative electrode current collector sheet 11 is a sheet that serves as a base for providing the negative electrode active material layer 12, and is a conductor that electrically connects the negative electrode battery terminal and the negative electrode active material layer 12. Alternatively, a portion of the negative electrode current collector sheet 11 may serve as the negative electrode battery terminal. The negative electrode current collector sheet 11 is, for example, a copper foil or an aluminum foil.
The negative electrode active material layer 12 is a porous layer containing a negative electrode active material. The negative electrode active material layer 12 may be provided on one side of the negative electrode current collector sheet 11, or on both sides of the negative electrode current collector sheet 11. The negative electrode active material layer 12 may contain, for example, fine particles of a negative electrode active material. As in the lithium-ion battery shown in FIG. 2 , the surface of the negative electrode active material layer 12 on the positive electrode side may be covered with a porous insulating layer 8. In this case, the porous insulating layer 8 can prevent a short circuit between the positive electrode 2 and the negative electrode 3, so the separator 4 can be omitted.
The negative electrode active material layer 12 may contain a binder, such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene copolymer (SBR), acrylonitrile rubber, sodium carboxymethyl cellulose (CMC), acrylonitrile rubber, or an acrylonitrile rubber-PTFE mixture.
The negative electrode active material layer 12 may contain a thickener, such as carboxymethyl cellulose (CMC).

The thickness of the negative electrode active material layer 12 is, for example, 1 μm or more and 100 μm or less. By making the thickness of the negative electrode active material layer 12 1 μm or more, the amount of negative electrode active material contained in the negative electrode 3 can be increased, thereby increasing the capacity of the lithium-ion battery. Furthermore, the negative electrode active material layer 12 can be easily formed by coating. By making the thickness of the negative electrode active material layer 12 100 μm or less, the migration distance (diffusion distance) of lithium ions between the vicinity of the interface between the negative electrode active material layer 12 and the negative electrode current collector sheet 11 and the vicinity of the surface of the negative electrode active material layer 12 can be shortened, thereby preventing a shortage of lithium ions in the electrolyte solution 5 in the pores near the interface during charging and an excess of lithium ions in the electrolyte solution 5 in the pores near the interface during discharge.

The negative electrode active material layer 12 can have a porous structure in which negative electrode active material powder is bonded with a binder. This allows the negative electrode active material layer 12 to have pores between the negative electrode active material particles. These pores are filled with the electrolyte solution 5, and an electrode reaction proceeds on the surfaces of the negative electrode active material particles.
For example, when charging the lithium ion battery 20, the lithium ions in the electrolyte 5 are inserted into the negative electrode active material particles as lithium atoms, and when the lithium ion battery 20 is discharged, the lithium atoms contained in the negative electrode active material particles are released into the electrolyte 5 as lithium ions (Li ⁺ ).

For example, a negative electrode active material powder is mixed with a binder to prepare a slurry, which is then applied to the negative electrode current collector sheet 11. The applied layer is then dried and pressed to form the negative electrode active material layer 12. Examples of solvents used to prepare the slurry include water, dimethylformamide, N-methylpyrrolidone, isopropanol, and toluene.

The electrolyte solution 5 is an ion-conducting medium between the positive electrode and the negative electrode, and is an ionic liquid electrolyte containing an ionic liquid (solvent) composed of anions and cations, and a lithium salt dissolved in the solvent. The electrolyte solution 5 is contained in the casing 16. The lithium-ion battery 20 may also contain both the electrolyte solution 5 and a solid electrolyte as ion-conducting media between the positive electrode and the negative electrode. The solid electrolyte can be disposed, for example, between the porous insulating layer 8 and the positive electrode.
Ionic liquids generally have low vapor pressure and are difficult to burn, so by using an ionic liquid electrolyte as the electrolyte solution 5, the safety of the lithium ion battery 20 can be improved.

The cationic component of the ionic liquid contained in the ionic liquid electrolyte 5 is preferably a chain or cyclic quaternary ammonium ion. By using a chain or cyclic quaternary ammonium ion as the cationic component of the ionic liquid, the viscosity of the ionic liquid is reduced, allowing the electrolyte solution 5 to be sufficiently impregnated into the positive electrode 2, the negative electrode 3, and the separator 4. An example of the chain quaternary ammonium ion is an ammonium ion represented by the following chemical formula [1] (where _R1 , _R2 , _R3 , and _R4 are alkyl groups or alkoxyalkyl groups).

Examples of the cyclic quaternary ammonium ion include a pyrrolidinium cation represented by the following chemical formula [2] (where _R1 and _R2 are alkyl or alkoxyalkyl groups) and an imidazolium cation represented by the following chemical formula [3] (where _R1 and _R2 are alkyl or alkoxyalkyl groups, and _R3 , _R4 , and _R5 are alkyl, alkoxyalkyl, or hydrogen atoms). More specifically, examples of the cation component include a methylpropylpyrrolidinium ion (MPP ion) and an ethylmethylimidazolium ion (EMI ion).

The anion component of the ionic liquid is preferably a bis(fluorosulfonyl)amide ion (FSA ion). By using a bis(fluorosulfonyl)amide ion as the anion component of the ionic liquid, the interfaces between the positive electrode active material, negative electrode active material, and electrolyte 5 are stabilized, improving the charge/discharge characteristics and rate performance of the lithium-ion battery 20. Furthermore, an increase in the viscosity of the ionic liquid can be suppressed, allowing electrolyte 5 to sufficiently impregnate the positive electrode, negative electrode, and separator.

The lithium salt contained in the ionic liquid electrolyte 5 (lithium salt dissolved in the ionic liquid) is a compound that dissociates into lithium ions and an anion component in the electrolyte solution 5. This anion component is preferably a bis(fluorosulfonyl)amide ion. Furthermore, the lithium salt is preferably lithium bis(fluorosulfonyl)amide (hereinafter referred to as LiFSA). By using a bis(fluorosulfonyl)amide ion as the anion component of the lithium salt, the interfaces between the positive electrode active material, the negative electrode active material, and the electrolyte solution 5 are stabilized, thereby improving the charge/discharge characteristics and rate performance of the lithium-ion battery 20. However, there is no problem in using lithium salts other than those mentioned above.

The concentration of lithium salt in the electrolyte solution 5 can be 1.6 mol/L or more and 3.2 mol/L or less. By setting the lithium salt concentration to 1.6 mol/L or more, the ionic conductivity of the ionic liquid electrolyte 5 can be increased, improving the charge/discharge characteristics and rate performance of the lithium-ion battery 20. Furthermore, by setting the lithium salt concentration to 3.2 mol/L or less, an increase in the viscosity of the electrolyte solution 5 is suppressed, allowing the electrolyte solution 5 to sufficiently impregnate the positive electrode 2, negative electrode 3, and separator 4.

The viscosity of the electrolyte solution 5 is not particularly limited, but can be set to 30 mPa·s or more at 25° C. This allows the lithium salt concentration in the electrolyte solution 5 to be increased.
The ionic conductivity of the electrolyte solution 5 is not particularly limited, but can be set to 0.01 mS/cm or less at 25° C. This allows the internal resistance of the lithium ion battery 20 to be reduced.

The separator 4 is in the form of a sheet and is disposed between the positive electrode 2 and the negative electrode 3. The separator 4 can also constitute an electrode laminate together with the positive electrode 2 and the negative electrode 3. The provision of the separator 4 can prevent short-circuit current from flowing between the positive electrode 2 and the negative electrode 3. The separator 4 also has a large number of openings, which are filled with the electrolyte 5. This allows lithium ions conducting between the positive electrode 2 and the negative electrode 3 to pass through the separator 4, thereby reducing the internal resistance of the lithium-ion battery 20.
When the surface of the negative electrode active material layer 12 facing the positive electrode 2 is covered with the porous insulating layer 8, the separator 4 can be omitted. In this case, the porous insulating layer 8 prevents a short-circuit current from flowing between the positive electrode 2 and the negative electrode 3.

At least one major surface of the separator 4 (the front or back surface of the separator) may be coated with a porous insulating layer 8. Alternatively, both major surfaces of the separator 4 (the front and back surfaces) may each be coated with a porous insulating layer 8.

Separator 4 is not particularly limited as long as it can prevent the flow of short-circuit current and is permeable to ions that conduct between the positive and negative electrodes. For example, it can be a microporous film (porous resin membrane) made of polyolefin, polyester, polyacrylonitrile, polyethylene terephthalate, polyamide, polyimide, or the like. Separator 4 may also be a nonwoven fabric containing at least one of fibers such as cellulose, polyolefin, polyester, polyacrylonitrile, polyethylene terephthalate, polyamide, or polyimide.

The porous insulating layer 8 is a layer that coats the main surface of the separator 4 (the front or back surface of the substrate sheet) or a layer that coats the surface of the negative electrode active material layer 12 .
1 , when the porous insulating layer 8 coats the separator 4, the porous insulating layer 8 can be provided so as to cover the portion of the main surface of the separator 4 that is adjacent to the negative electrode active material layer 12. Alternatively, the porous insulating layer 8 may be provided so as to cover the entire main surface of the separator 4.
When the porous insulating layer 8 coats the negative electrode active material layer 12 as in the lithium ion battery shown in FIG. 2 , the porous insulating layer 8 can be provided so as to cover the entire main surface of the negative electrode active material layer 12 opposite to the negative electrode current collector sheet 11.

The porous insulating layer 8 has a porous structure containing a plurality of electrically insulating particles. For example, the porous insulating layer 8 can have a porous structure in which a plurality of electrically insulating particles 9 are bonded together with a binder. This allows openings 10 to be formed between the electrically insulating particles 9. The porous insulating layer 8 may also have a porous structure in which a plurality of electrically insulating particles 9 are fixed together without a binder. The openings within the porous structure of the porous insulating layer 8 are filled with the electrolyte solution 5. The porous insulating layer 8 can have a porous structure such as that shown in Figure 3, for example.

The electrically insulating particles 9 contained in the porous insulating layer 8 are not particularly limited as long as they are electrically insulating particles, but examples include ceramic particles and synthetic resin particles. Materials for ceramic particles include magnesium oxide, aluminum oxide, silicon oxide, titanium oxide, and zirconium oxide. Materials for synthetic resin particles include polyester, polyacrylonitrile, polyethylene terephthalate, polyamide, and polyimide.

The median diameter of the electrically insulating particles 9 contained in the porous insulating layer 8 is 1 μm or more and 5 μm or less. A median diameter of 1 μm or more allows the porous insulating layer 8 to have sufficiently large pores, allowing the ionic liquid electrolyte 5 to be rapidly impregnated into the porous insulating layer 8. Furthermore, a median diameter of 5 μm or less allows lithium ions to be uniformly supplied to the positive electrode active material and the negative electrode active material during charging and discharging. Furthermore, lithium ions are rapidly supplied to the positive electrode active material and the negative electrode active material even during high-rate charging and discharging.
For example, the median diameter can be calculated by measuring the particle size distribution (particle diameter distribution) of the powder of the electrically insulating particles 9 before forming the porous insulating layer 8. Alternatively, the particle diameters of the electrically insulating particles 9 may be measured using an SEM photograph of the porous insulating layer 8 to create a particle diameter distribution, and the median diameter may be calculated based on this particle diameter distribution.

The porous insulating layer 8 may contain a binder. Examples of binders include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene copolymer (SBR), sodium carboxymethyl cellulose (CMC), acrylonitrile rubber, or an acrylonitrile rubber-PTFE mixture.

The thickness of the porous insulating layer 8 is 5 μm or more and 50 μm or less. A thickness of 5 μm or more prevents the positive electrode active material and negative electrode active material from penetrating the porous insulating layer 8, preventing short-circuit current from flowing between the positive electrode 2 and the negative electrode 3. Furthermore, a thickness of 50 μm or less suppresses solution resistance between the positive electrode 2 and the negative electrode 3, allowing lithium ions to be supplied quickly to the positive electrode active material and the negative electrode active material even during high-rate charging and discharging.

The porosity of the porous insulating layer 8 is not particularly limited, but is preferably 50% or more and 90% or less. A porosity of 50% or more allows the porous insulating layer 8 to contain a sufficient volume of ionic liquid electrolyte 5. Furthermore, a porosity of 90% or less allows the porous insulating layer 8 to have sufficient mechanical strength.

The air permeability of the porous insulating layer 8 (air permeability measured by the Gurley tester) is not particularly limited, but is preferably 1 s/100 mL or more and 100 s/100 mL or less. An air permeability of 1 s/100 mL or more prevents the positive electrode active material and negative electrode active material from penetrating the porous insulating layer 8, preventing short-circuit current from flowing between the positive electrode 2 and the negative electrode 3. Furthermore, an air permeability of 100 s/100 mL or less allows the ionic liquid electrolyte 5 to quickly impregnate the porous insulating layer 8.

The contact angle measured 10 seconds after the electrolyte solution 5 is dropped onto the porous insulating layer 8 is 60° or less. The contact angle is a parameter that indicates the affinity between the porous insulating layer 8 and the electrolyte solution 5, and a contact angle of 60° or less allows the ionic liquid electrolyte 5 to quickly impregnate the porous insulating layer 8.

The porous insulating layer 8 is disposed adjacent to the negative electrode active material layer 12. For example, as in the lithium ion battery shown in FIG. 1 , the porous insulating layer 8 coated with the separator 4 may be disposed adjacent to the negative electrode active material layer 12. Alternatively, as in the lithium ion battery shown in FIG. 2 , the porous insulating layer 8 may coat the negative electrode active material layer 12.
Side reactions such as electrolyte decomposition and lithium deposition are likely to occur on the negative electrode active material. By disposing the porous insulating layer 8 adjacent to the negative electrode active material layer 12, the supply of lithium ions to the negative electrode active material during charge and discharge is made uniform, and the occurrence of side reactions can be suppressed. This improves the charge and discharge characteristics and rate characteristics of the lithium-ion battery 20. This was made clear by experiments conducted by the present inventors.
Furthermore, as in the lithium ion battery shown in FIG. 1 , the separator 4 can be disposed so that the main surface of the separator 4 that is not coated with the porous insulating layer 8 is adjacent to the positive electrode active material layer 7 .

Fabrication of Lithium Ion Batteries Lithium ion batteries of Examples 1 and 2 and comparative examples 1 to 7 were fabricated.
[Example 1]
A positive electrode mixture containing 91% by mass of lithium iron phosphate (positive electrode active material), 4% by mass of acetylene black (conductive agent), and 5% by mass of polyvinylidene fluoride (binder) was dispersed in N-methyl-2-pyrrolidone (dispersion medium) to prepare a positive electrode slurry. This positive electrode slurry was then applied to both sides of an aluminum foil (positive electrode current collector sheet), and the dispersion medium was dried to produce a positive electrode having a positive electrode active material layer.

A negative electrode mixture consisting of 94% by mass of soft carbon (negative electrode active material), 5% by mass of styrene-butadiene rubber (binder), and 1% by mass of carboxymethyl cellulose was dispersed in water (dispersion medium) to prepare a negative electrode slurry. This negative electrode slurry was then applied to both sides of copper foil (negative electrode current collector sheet), and the dispersion medium was allowed to dry, producing a negative electrode with a negative electrode active material layer.

Magnesium oxide particles (median diameter: 3 μm), which are electrically insulating particles, and a binder were dispersed in water to prepare an MgO slurry. This MgO slurry was then applied to one of the main surfaces (front or back) of a polyester fiber (fiber diameter: 5 μm to 10 μm) nonwoven fabric (thickness: 15 μm) (separator) and dried to form a porous insulating layer (MgO layer) (thickness: 10 μm), producing a two-layer separator.

An ionic liquid electrolyte was prepared by dissolving lithium bis(fluorosulfonyl)amide (LiFSA), a lithium salt (solute), in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (MPP-FSA), a solvent (ionic liquid), at a concentration of 2.4 mol/L.

Using the above-described positive electrode, negative electrode, separator, and electrolyte, a lithium ion battery (pouch battery) as shown in FIG. 1 was fabricated.
Specifically, one positive electrode was sandwiched between two separators, and this laminate was sandwiched between two negative electrodes to produce an electrode laminate. The separators were arranged so that the porous insulating layers (MgO layers) of the two separators were adjacent to the negative electrode active material layers, and the nonwoven fabrics of the two separators were adjacent to the positive electrode active material layers. The produced electrode laminate was sandwiched between laminate films so that the terminals of the positive electrode current collector sheet and the negative electrode current collector sheet were exposed to the outside, and the peripheral edges of the laminate films were fused, leaving an electrolyte inlet. The prepared ionic liquid electrolyte was then poured into a pouch of the laminate film through the inlet, and the inlet was fused to produce a lithium-ion battery (pouch battery).

[Comparative Example 1]
A lithium ion battery (pouch battery) was produced in the same manner as in Example 1, except that in the electrode laminate, the separators were arranged so that the porous insulating layers (MgO layers) of the two separators were adjacent to the positive electrode active material layers, and the nonwoven fabrics of the two separators were adjacent to the negative electrode active material layers.

[Comparative Example 2]
A lithium ion battery (pouch battery) was produced in the same manner as in Example 1, except that a nonwoven fabric (thickness: 20 μm) of polyester fiber (fiber diameter: 5 μm to 10 μm) without a porous insulating layer was used as the separator.

[Example 2]
A lithium ion battery (pouch battery) was fabricated in the same manner as in Example 1, except that an ionic liquid electrolyte was used, which was prepared by dissolving a lithium salt (solute) LiFSA in a solvent of 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide (EMI-FSA) at a concentration of 2.7 mol/L.

[Comparative Example 3]
A lithium ion battery (pouch battery) was produced in the same manner as in Example 2, except that in the electrode laminate, the separators were arranged so that the porous insulating layers (MgO layers) of the two separators were adjacent to the positive electrode active material layers, and the nonwoven fabrics of the two separators were adjacent to the negative electrode active material layers.

[Comparative Example 4]
A lithium ion battery (pouch battery) was produced in the same manner as in Example 2, except that a nonwoven fabric (thickness: 20 μm) of polyester fiber (fiber diameter: 5 μm to 10 μm) without a porous insulating layer was used as the separator.

[Comparative Example 5]
A lithium ion battery (pouch battery) was fabricated in the same manner as in Example 1, except that an organic liquid electrolyte was used, which was prepared by dissolving a lithium salt (solute) _LiPF6 in a carbonate-based solvent (EC/DEC/EMC=27.5/5.0/67.5 (containing 0.7% VC and 0.3% FEC as additives)) at a concentration of 1.2 mol/L.

[Comparative Example 6]
A lithium ion battery (pouch battery) was produced in the same manner as in Comparative Example 5, except that in the electrode laminate, the separators were arranged so that the porous insulating layers (MgO layers) of the two separators were adjacent to the positive electrode active material layers, and the nonwoven fabrics of the two separators were adjacent to the negative electrode active material layers.

[Comparative Example 7]
A lithium ion battery (pouch battery) was produced in the same manner as in Comparative Example 5, except that a nonwoven fabric (thickness: 20 μm) of polyester fiber (fiber diameter: 5 μm to 10 μm) without a porous insulating layer was used as the separator.

Measurement of Electrolyte Viscosity The viscosities of the ionic liquid electrolytes prepared in Examples 1 and 2 were measured using a rotational viscometer.
The viscosity of the ionic liquid electrolyte prepared in Example 1 was 170 mPa·s, and the viscosity of the ionic liquid electrolyte prepared in Example 2 was 90 mPa·s.

Measurement of Contact Angle An ionic liquid electrolyte was dropped onto the surface of the prepared separator using a dispenser, and the contact angle was measured 10 seconds after dropping.
The contact angle when the prepared LiFSA-MPPFSA electrolyte was dropped onto the surface of the porous insulating layer of the bilayer separator was 41°, the contact angle when the prepared LiFSA-MPPFSA electrolyte was dropped onto the surface of the nonwoven fabric of the bilayer separator was 30°, the contact angle when the prepared LiFSA-EMIFSA electrolyte was dropped onto the surface of the porous insulating layer of the bilayer separator was 30°, and the contact angle when the prepared LiFSA-EMIFSA electrolyte was dropped onto the surface of the nonwoven fabric of the bilayer separator was 29°.

Charge/Discharge Tests Charge/Discharge tests were conducted using the lithium ion batteries of Examples 1 and 2 and Comparative Examples 1 to 7. Specifically, at 25°C, the lithium ion batteries were charged at a constant current of 0.1 C until the voltage reached 3.6 V, followed by constant voltage charging at 3.6 V until the current reached 0.05 C. After a 5-minute pause, constant current discharge was performed at a current of 0.1 C until the voltage reached 2.0 V. Here, the unit of current value, C, is the current value when the battery capacity is fully charged or discharged in 1 hour. The charge capacity at this time was defined as the initial charge capacity, and the discharge capacity at this time was defined as the initial discharge capacity (0.1 C discharge capacity). The initial charge/discharge efficiency was calculated using the following formula (1):
Initial charge/discharge efficiency (%) = initial discharge capacity ÷ initial charge capacity × 100 (1)

Thereafter, constant current charging was performed at a current value of 1 C until the voltage reached 3.6 V, followed by constant voltage charging at a voltage of 3.6 V until the current value reached 0.05 C. After a 5-minute break, constant current discharging was performed at a current value of 1 C until the voltage reached 2.0 V. The discharge capacity at this time was defined as the 1 C discharge capacity, and the initial discharge capacity was defined as the 0.1 C discharge capacity, and a value serving as an index of rate characteristics was calculated from the following formula (2).
Rate characteristic (%) = 1 C discharge capacity ÷ 0.1 C discharge capacity × 100 (2)

SEM Observation SEM observation was carried out on the porous insulating layer of the two-layer separator and on the polyester fiber nonwoven fabric of the two-layer separator.

The measurement results and the measurement results of the evaluation charge/discharge test are shown in Table 1. Also, Fig. 4 is an SEM image of the porous insulating layer, and Fig. 5 is an SEM image of the polyester fiber nonwoven fabric.

The battery of Example 1 and the batteries of Comparative Examples 1 and 2 have exactly the same configuration except for the separator. However, the initial charge-discharge efficiency and rate characteristics of the battery of Example 1, in which the separator is arranged so that the porous insulating layer (MgO layer) is on the negative electrode side, are higher than those of the batteries of Comparative Examples 1 and 2.
As described above, the battery of Example 2 and the batteries of Comparative Examples 3 and 4 have exactly the same configuration except for the separator. However, the initial charge-discharge efficiency and rate characteristics of the battery of Example 2, in which the separator was arranged so that the porous insulating layer (MgO layer) was on the negative electrode side, exceeded those of the batteries of Comparative Examples 3 and 4.
Therefore, it was found that in lithium-ion batteries using ionic liquid electrolytes, batteries in which a two-layer separator is arranged so that the porous insulating layer composed of electrically insulating particles is on the negative electrode side have excellent charge/discharge characteristics and rate characteristics.

The reason for this is unclear, but by positioning the two-layer separator so that the porous insulating layer is on the negative electrode side, it is thought that the supply of lithium ions to the positive electrode active material and negative electrode active material during charge and discharge is uniform, and that lithium ions are also supplied quickly to the positive electrode active material and negative electrode active material even during high-rate charge and discharge. Therefore, it is thought that the lithium-ion batteries of Examples 1 and 2 have excellent charge and discharge characteristics and rate characteristics.

In addition, the batteries of Comparative Examples 5 to 7 used an organic liquid electrolyte instead of an ionic liquid electrolyte, but the initial charge/discharge efficiency and rate characteristics of these batteries were similar regardless of the arrangement and presence or absence of the porous insulating layer.
Therefore, the effects of the present invention are uniquely manifested only in lithium ion batteries that use ionic liquid electrolytes.

2: Positive electrode 3: Negative electrode 4: Separator 5: Electrolyte (ionic liquid electrolyte) 6: Positive electrode current collector sheet 7: Positive electrode active material layer 8: Porous insulating layer 9: Electrically insulating particles 10: Opening 11: Negative electrode current collector sheet 12: Negative electrode active material layer 16: Casing 20: Lithium-ion battery

Claims (6)

a positive electrode having a positive electrode active material layer, a negative electrode having a negative electrode active material layer , a separator, a porous insulating layer disposed between the positive electrode and the negative electrode, and an electrolyte;
the porous insulating layer has a porous structure containing a plurality of electrically insulating particles, and is disposed adjacent to the negative electrode active material layer;
the median diameter of the plurality of electrically insulating particles is 1 μm or more and 5 μm or less;
the thickness of the porous insulating layer is 5 μm or more and 50 μm or less;
the electrolyte solution contains a lithium salt and an ionic liquid,
the separator is disposed between the positive electrode and the negative electrode and is a nonwoven fabric or a porous resin film;
the porous insulating layer coats one major surface of the separator;
a separator disposed such that a major surface of the separator coated with the porous insulating layer faces the negative electrode active material layer, and a major surface of the separator not coated with the porous insulating layer faces the positive electrode active material layer . 2. The lithium ion battery according to claim 1 , wherein the concentration of the lithium salt in the electrolyte is 1.6 mol/L or more and 3.2 mol/L or less. The ionic liquid contains an anion component and a cation component,
the lithium salt is a compound that dissociates into lithium ions and an anion component in the electrolyte solution,
3. The lithium ion battery according to claim 1 , wherein the anion component of the ionic liquid or the anion component of the lithium salt is a bis(fluorosulfonyl)amide ion. The ionic liquid contains an anion component and a cation component,
4. The lithium ion battery according to claim 1, wherein the cationic component of the ionic liquid is a chain or cyclic quaternary ammonium ion. 5. The lithium ion battery according to claim 1, wherein the electrically insulating particles are ceramic particles or synthetic resin particles. 6. The lithium ion battery according to claim 1 , wherein the contact angle measured 10 seconds after the electrolyte solution is dropped onto the surface of the porous insulating layer is 60 degrees or less.