JP6809108B2 - Lithium ion secondary battery - Google Patents

Description

The present disclosure relates to a lithium ion secondary battery.

Japanese Patent Application Laid-Open No. 2014-522549 (Patent Document 1) discloses that a porous coating layer is formed on the surface of the electrode active material layer. The porous coating layer contains inorganic oxide particles.

Special Table 2014-522549

Graphite particles are widely used as a negative electrode active material for lithium ion secondary batteries. Silicon oxide (SiO) particles are also known as negative electrode active materials having a higher capacity than graphite particles. However, SiO particles are inferior to graphite particles in terms of cycle life. Therefore, from the viewpoint of the balance between capacity and cycle life, it is conceivable to use graphite particles and SiO particles in combination.

By the way, the porous coating layer of Patent Document 1 is formed by applying a paste containing inorganic oxide particles to the surface of the electrode active material layer. Patent Document 1 states that the stability of the battery is improved by suppressing the penetration of the paste into the electrode active material layer during coating. However, depending on the composition of the electrode active material layer, the output may decrease because the paste does not penetrate into the electrode active material layer.

That is, if a porous coating layer that has not penetrated into the negative electrode active material layer is formed on the surface of the negative electrode active material layer including graphite particles and SiO particles, it becomes difficult for the electrolytic solution to diffuse into the negative electrode active material layer. , Resistance may increase.

Therefore, an object of the present disclosure is to provide a lithium ion secondary battery having a high capacity and a high output.

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 correctness of the mechanism of action should not limit the scope of the invention of the present disclosure.

The lithium ion secondary battery of the present disclosure includes a positive electrode, a negative electrode, and an electrolytic solution. The negative electrode includes a current collector, a first layer and a second layer. The first layer is arranged on the surface of the current collector. The second layer is laminated on the first layer.
The first layer comprises graphite particles and silicon oxide particles. The second layer comprises inorganic oxide particles. Inorganic oxide particles are different from silicon oxide particles.
There is a gap in the surface layer of the first layer. A part of the second layer is interposed in the gap.

In the lithium ion secondary battery of the present disclosure, the first layer corresponds to the negative electrode active material layer (electrode active material layer). The second layer corresponds to a porous coating layer. The first layer comprises graphite particles and SiO particles. The negative electrode active material layer including the graphite particles and the SiO particles can have a higher capacity than the negative electrode active material layer containing the graphite particles alone.

The negative electrode active material layer including graphite particles and SiO particles has a property of having a low reaction resistance and a high diffusion resistance. That is, in this negative electrode active material layer, the diffusion of Li ions tends to be the rate-determining step of the discharge reaction. In such a negative electrode active material layer, a large amount of electrolytic solution is required in order to promote the diffusion of Li ions.

In Patent Document 1, the porous coating layer does not penetrate into the electrode active material layer. Therefore, the gaps between the particles are filled in the surface layer of the electrode active material layer. If the gaps between the particles are filled in the surface layer of the electrode active material layer, it is considered that the electrolytic solution cannot penetrate into the electrode active material layer.

In the lithium ion secondary battery of the present disclosure, there is a gap in the surface layer of the first layer. That is, in the surface layer of the first layer, there are gaps between the particles. A part of the second layer is interposed in the gap between the particles. That is, the inorganic oxide particles are interposed in the gaps between the particles. Therefore, it is considered that the electrolytic solution easily permeates the first layer and the diffusion resistance of Li ions is reduced. Therefore, the lithium ion secondary battery of the present disclosure can be a high capacity and high output lithium ion secondary battery.

FIG. 1 is a schematic cross-sectional view showing an example of the configuration of a lithium ion secondary battery. FIG. 2 is a schematic cross-sectional view showing an example of the configuration of the electrode group according to the embodiment of the present disclosure. FIG. 3 is a schematic cross-sectional view showing the configuration of the negative electrode according to the embodiment of the present disclosure. FIG. 4 is a flowchart showing an outline of a method for manufacturing a lithium ion secondary battery according to the embodiment of the present disclosure.

Hereinafter, embodiments of the present disclosure (hereinafter referred to as “the present embodiment”) will be described. However, the following description does not limit the scope of the invention of the present disclosure. Hereinafter, the lithium ion secondary battery may be abbreviated as "battery".

<Lithium-ion secondary battery>
FIG. 1 is a schematic cross-sectional view showing an example of the configuration of a lithium ion secondary battery. The battery 100 includes a housing 90. The housing 90 is square (flat rectangular parallelepiped). However, the housing 90 may be cylindrical. For example, a metal such as an aluminum (Al) alloy constitutes the housing 90. However, as long as it has a predetermined airtightness, a composite material of metal and resin may form the housing 90. Examples of the metal and resin composite material include an aluminum laminated film and the like. The housing 90 may include an external terminal, a liquid injection hole, a gas discharge valve, a current cutoff mechanism (CID), and the like.

The electrode group 50 and the electrolytic solution 60 are housed in the housing 90. That is, the battery 100 includes the electrolytic solution 60. The electrolytic solution 60 is impregnated in the electrode group 50. FIG. 2 is a schematic cross-sectional view showing an example of the configuration of the electrode group according to the present embodiment. The electrode group 50 includes a positive electrode 10, a negative electrode 20, and a separator 30. That is, the battery 100 includes a positive electrode 10 and a negative electrode 20. The electrode group 50 may be a laminated type electrode group or a wound type electrode group. The laminated electrode group is formed by alternately laminating the rectangular positive electrode 10 and the negative electrode 20 with the rectangular separator 30 sandwiched between them. The winding type electrode group is formed by laminating a band-shaped positive electrode 10 and a negative electrode 20 with a band-shaped separator 30 sandwiched between them, and further winding the band-shaped electrode group.

《Negative electrode》
The negative electrode 20 is a strip-shaped or rectangular sheet. FIG. 3 is a schematic cross-sectional view showing an example of the configuration of the negative electrode according to the present embodiment. The negative electrode 20 includes a current collector 23, a first layer 21, and a second layer 22. The first layer 21 is arranged on the surface of the current collector 23. The second layer 22 is laminated on the first layer 21. The first layer 21 includes graphite particles 1 and SiO particles 2. The second layer 22 includes the inorganic oxide particles 3. There is a gap in the surface layer of the first layer 21. A part of the second layer 22 is interposed in the gap.

(Current collector)
The current collector 23 may have a thickness of, for example, 5 to 30 μm. The current collector 23 may be, for example, a copper (Cu) foil. The Cu foil may be a pure Cu foil or a Cu alloy foil.

(1st layer)
The first layer 21 of this embodiment corresponds to the negative electrode active material layer. The first layer 21 may have a thickness of, for example, 10 to 150 μm. The first layer 21 includes graphite particles 1. The first layer 21 includes, for example, 80 to 96% by mass (typically 90 to 96% by mass) of graphite particles 1. The graphite particles 1 function as a negative electrode active material. That is, the graphite particles 1 can occlude and release Li ions. The graphite particles 1 may have an average particle size of, for example, 1 to 20 μm. The average particle size of the present specification indicates a cumulative 50% particle size from the fine particle side in the volume-based particle size distribution measured by the laser diffraction / scattering method.

The graphite particles 1 are particles containing graphite. The graphite may be natural graphite or artificial graphite. The graphite particles 1 may be scaly particles or spherical particles. The graphite particles 1 may be, for example, spherical graphite. The spheroidized graphite refers to scaly graphite that has been spheroidized. The spheroidizing treatment refers to, for example, a treatment in which the outer shape of the reptile graphite is brought closer to a sphere by applying friction and impact to the reptile graphite in an air flow. The graphite particles 1 may further contain amorphous carbon as long as it contains graphite. For example, the graphite particles 1 may be composite particles having spherical graphite and amorphous carbon. In the composite particles, the amorphous carbon may, for example, coat the surface of sphericalized graphite.

The first layer 21 includes SiO particles 2. The first layer 21 includes, for example, 1 to 19% by mass (typically 1 to 9% by mass) of SiO particles 2. The SiO particles 2 may have an average particle size of, for example, 0.1 to 20 μm. The SiO particles 2 function as a negative electrode active material. The SiO particles 2 can occlude and release more Li ions than the graphite particles 1. Since the first layer 21 includes graphite particles 1 and SiO particles 2, the battery 100 can have a high capacity. In the first layer 21 including the graphite particles 1 and the SiO particles 2, the diffusion of Li ions tends to be the rate-determining step of the discharge reaction.

The SiO particles 2 include silicon oxide (SiO). SiO should not be limited to silicon monoxide as long as Li ions can be occluded and released. As long as Li ions can be occluded and released, the "x" in the general formula: SiO _x can take any conventionally known value. For example, x may be greater than 0.5 and less than or equal to 1.5, or greater than or equal to 0.5 and less than or equal to 1. The SiO particles 2 may also contain other substances as long as they contain SiO. For example, the SiO particles 2 may be composite particles containing SiO and amorphous carbon. In the composite particle, the amorphous carbon may cover the surface of SiO, for example.

The first layer 21 typically further comprises a binder. The first layer 21 includes, for example, a binder of 1 to 3% by mass. Binders should not be particularly limited. Examples of the binder include carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA) and the like. One type of binder may be used alone, or two or more types may be used in combination.

(2nd layer)
The second layer 22 is laminated on the first layer 21. The second layer 22 includes the inorganic oxide particles 3. The second layer 22 may include, for example, 80 to 99% by mass of the inorganic oxide particles 3. The inorganic oxide particles 3 are different from the silicon oxide particles 2. The inorganic oxide particles 3 do not occlude and release Li ions. That is, the inorganic oxide particles 3 are not the negative electrode active material. It is desirable that the inorganic oxide particles 3 have heat resistance. This is because the thermal stability of the battery 100 is expected to be improved. The inorganic oxide particles 3 may be at least one selected from the group consisting of, for example, alumina particles, boehmite particles, magnesia particles and titania particles. However, the inorganic oxide particles 3 should not be limited to these particles.

A part of the second layer 22 is interposed in the gap between the surface layers of the first layer 21. That is, in the surface layer of the first layer 21, some of the inorganic oxide particles 3 have entered the gaps between the particles. Since the inorganic oxide particles 3 are interposed in the gaps between the particles, the electrolytic solution 60 can penetrate deeply into the first layer 21. It is considered that this reduces the diffusion resistance. It can be confirmed by SEM observation of the cross section in the thickness direction (FIG. 3) of the negative electrode 20 that a part of the second layer 22 is interposed in the gap between the surface layers of the first layer 21.

The inorganic oxide particles 3 may have an average particle size of, for example, 0.1 to 2 μm. The ratio (d2 / d1) of the average particle size (d2) of the inorganic oxide particles 3 to the average particle size (d1) of the graphite particles 1 is preferably 0.05 or more and 0.15 or less. When the ratio (d2 / d1) is 0.15 or less, the inorganic oxide particles 3 easily enter the gaps between the graphite particles 1 in the surface layer of the first layer 21. Further, when the ratio (d2 / d1) is 0.05 or more, it becomes difficult to fill the gap in which the inorganic oxide particles 3 have entered. This is expected to improve the output. The ratio (d2 / d1) is calculated by dividing the average particle size (d2) of the inorganic oxide particles 3 by the average particle size (d1) of the graphite particles 1.

The inorganic oxide particles 3 preferably have a greater crushing strength than the graphite particles 1. As a result, it is considered that it becomes difficult to fill the gap in which the inorganic oxide particles 3 have entered during compression. The crushing strength of the inorganic oxide particles 3 and the graphite particles 1 was based on "JIS R 1639-5: Method for measuring fine ceramics or (condyle) grain characteristics, Part 5: Single or grain crushing strength". Measured by method.

The second layer 22 typically further comprises a binder. The second layer 22 may include, for example, 1 to 20% by mass of a binder. Binders should not be particularly limited. Examples of the binder include CMC, SBR, PAA, and acrylic copolymers. Examples of the acrylic copolymer include copolymers of styrene and acrylic acid esters. One type of binder may be used alone, or two or more types may be used in combination.

The second layer 22 may have a thickness of, for example, 1 to 30 μm. The ratio (T2 / T1) of the thickness (T2) of the second layer 22 to the thickness (T1) of the first layer 21 is preferably 5% or more and 20% or less. When the ratio (T2 / T1) is 5% or more, the electrolytic solution 60 is more likely to permeate into the first layer 21. This is expected to improve the output. In the range where the ratio (T2 / T1) is 5% or more and 20% or less, the higher the ratio (T2 / T1), the more the output tends to improve. The ratio (T2 / T1) may exceed 20%. However, if the ratio (T2 / T1) exceeds 20%, the output improving effect may be saturated. It is conceivable that the capacity decreases as the ratio (T2 / T1) increases. The ratio (T2 / T1) is more preferably 8% or more and 20% or less, even more preferably 10% or more and 20% or less, and most preferably 15% or more and 20% or less.

The ratio (T2 / T1) is calculated as a percentage of the value obtained by dividing the thickness (T2) of the second layer 22 by the thickness (T1) of the first layer 21. When dividing, the numbers after the decimal point are rounded off. The thickness of the first layer 21 (T1) and the thickness of the second layer 22 (T2) are measured in a cross section in the thickness direction of the negative electrode 20 (typically, a cross section SEM image). The cross section in the thickness direction is a cross section perpendicular to the surface of the negative electrode 20. However, the vertical here does not mean only the geometrically perfect vertical. If the angle between the cross section and the surface of the negative electrode 20 is 89 ° or more and 91 ° or less, the cross section is considered to be perpendicular to the surface of the negative electrode 20.

In the present specification, the distance between the graphite particles 1 located farthest from the current collector 23 and the current collector 23 in the thickness direction of the negative electrode 20 is regarded as the thickness (T1) of the first layer 21. Is done. In the thickness direction of the negative electrode 20, the distance between the graphite particles 1 located farthest from the current collector 23 and the inorganic oxide particles 3 located farthest from the current collector 23 is the second layer 22. Is considered to be the thickness of (T2). Thickness measurements are taken at at least 5 locations. The average value of at least 5 measurements is taken as the thickness.

The second layer 22 preferably has a porosity of 30% or more and 60% or less. When the porosity of the second layer 22 is 30% or more, the electrolytic solution 60 easily permeates the second layer 22. This is expected to improve the output. When the negative electrode 20 is compressed in the normal pressure range, the porosity becomes about 60% or less.

Porousness is measured, for example, in a cross-sectional SEM image. For example, a reflected electron image of the cross section of the second layer 22 is imaged. In the backscattered electron image, the portion having dark contrast can be considered as a hole. By binarizing the reflected electron image at an appropriate threshold value, the ratio of the area occupied by the pores in the cross section of the second layer 22 is measured. The ratio of the area is porosity. Porousness is measured at at least 5 locations. The average value of the measurements at at least 5 points is adopted as the porosity.

《Positive electrode》
The positive electrode 10 is a strip-shaped or rectangular sheet. The positive electrode 10 includes a current collector 13 and a positive electrode active material layer 11. The positive electrode active material layer 11 is arranged on the surface of the current collector 13. The current collector 13 may have a thickness of, for example, 5 to 30 μ. The current collector 13 may be, for example, an Al foil or the like. The Al foil may be a pure Al foil or an Al alloy foil.

The positive electrode active material layer 11 may have a thickness of, for example, 10 to 150 μm. The positive electrode active material layer 11 includes positive electrode active material particles, a conductive material, a binder, and the like. For example, the positive electrode active material layer 11 includes 80 to 98% by mass of positive electrode active material particles, 1 to 15% by mass of a conductive material, and 1 to 5% by mass of a binder.

Positive electrode active material particles are particles capable of occluding and releasing Li ions. The positive electrode active material particles may have, for example, an average particle size of 1 to 20 μm. The positive electrode active material particles are typically lithium metal composite oxide particles or lithium metal composite phosphate particles. Lithium metal composite oxides include, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , and LiMn ₂ O ₄ , and general formula: LiNi _a Co _b Mn _c O ₂ (0 <a <1, 0 <b <1, Examples thereof include a lithium metal composite oxide represented by 0 <c <1, a + b + c = 1).

General formula: Lithium metal composite oxide represented by LiNi _a Co _b Mn _c O ₂ includes, for example, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ , LiNi Examples thereof include _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.4 Co _0.2 Mn _0.4 O ₂ , LiNi _0.4 Co _0.4 Mn _0.2 O ₂ . Examples of the lithium metal composite phosphate include LiFePO _{4 and the} like. The positive electrode active material particles may be used alone or in combination of two or more.

The conductive material should not be particularly limited. Examples of the conductive material include carbon black such as acetylene black, furnace black and thermal black, vapor-grown carbon fiber (VGCF), graphite and the like. The conductive material may be used alone or in combination of two or more. Binders should not be particularly limited either. Examples of the binder include polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE) and the like. One type of binder may be used alone, or two or more types may be used in combination.

<< Separator >>
The battery 100 may include a separator 30. The separator 30 is a strip-shaped or rectangular porous membrane. The separator 30 is electrically insulating. The separator 30 may have a thickness of, for example, 5 to 30 μm. For example, a resin such as polyethylene (PE) or polypropylene (PP) may form the separator 30. The separator 30 may have a multi-layer structure. For example, the separator 30 may be formed by laminating a porous film of PP, a porous film of PE, and a porous film of PP in this order.

《Electrolytic solution》
The electrolytic solution 60 includes a solvent and a lithium salt. The solvent is aprotic. The solvent may be, for example, a mixture of cyclic carbonate and chain carbonate. The mixing ratio of the cyclic carbonate and the chain carbonate may be, for example, "cyclic carbonate: chain carbonate = 1: 9 to 5: 5 (volume ratio)". Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and the like. Examples of the chain carbonate include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and the like. The cyclic carbonate and the chain carbonate may be used individually by 1 type, or may be used in combination of 2 or more types.

The lithium salt is dispersed and dissolved in the solvent. The lithium salt functions as a supporting electrolyte salt. The lithium salt may be, for example, LiPF ₆ , LiBF ₄ , Li [N (FSO ₂ ) ₂ ] (abbreviated as “LiFSI”) or the like. One type of lithium salt may be used alone, or two or more types may be used in combination. In the electrolytic solution 60, the lithium salt may have a concentration of, for example, 0.5 to 2.0 mol / l. The electrolytic solution 60 may include various additives in addition to the solvent and the lithium salt.

<Manufacturing method of lithium ion secondary battery>
The lithium ion secondary battery of the present embodiment can be manufactured by the following manufacturing method. FIG. 4 is a flowchart showing an outline of a method for manufacturing a lithium ion secondary battery of the present embodiment.

The manufacturing method of this embodiment is:
(A) To prepare a first paste containing graphite particles 1, silicon oxide particles 2, and a first solvent;
(B) To prepare a second paste containing the inorganic oxide particles 3 and the second solvent;
(C) The first layer 21 is formed by applying the first paste to the surface of the current collector 23 and drying it;
(D) The second layer 22 is formed by applying the second paste to the surface of the first layer 21 and drying it;
(E) The negative electrode 20 is manufactured by compressing the first layer 21 and the second layer 22 after the second layer 22 is formed; and (F) the positive electrode 10, the negative electrode 20, and the electrolytic solution 60 are provided. Manufacture of lithium-ion secondary batteries;
including.
The inorganic oxide particles 3 are different from the silicon oxide particles 2. The first layer 21 is formed so that there is a gap in the surface layer. The second layer 22 is formed so that a part of the second layer 22 is interposed in the gap.

When the first layer 21 is formed, there is a gap in the surface layer of the first layer 21. It is considered that the gap between the surface layers of the first layer 21 is filled when the first layer 21 is compressed. Therefore, in the manufacturing method of the present embodiment, the second paste is applied to the surface of the first layer 21 before the first layer 21 is compressed. As a result, the second paste can penetrate into the gaps in the surface layer of the first layer 21. That is, the second layer 22 is formed so that a part of the second layer 22 is interposed in the gap between the surface layers of the first layer 21. After the second layer 22 is formed, the first layer 21 and the second layer 22 are collectively compressed. A part of the second layer 22 interposed in the gap between the surface layers of the first layer 21 suppresses the gap from being filled by compression.

The first solvent and the second solvent may be an aqueous solvent or an organic solvent. However, when the first solvent is an aqueous solvent, it is desirable that the second solvent is also an aqueous solvent. When the first solvent is an organic solvent, it is desirable that the second solvent is also an organic solvent. It is considered that this makes it easier for the second paste to penetrate into the first layer 21. The aqueous solvent indicates water; or; a mixture of water and an organic solvent that is miscible with water; Examples of the organic solvent miscible with water include ethanol, isopropyl alcohol, acetone, tetrahydrofuran and the like.

The positive electrode 10 can be manufactured by a conventionally known method. The electrolytic solution 60 can be prepared by a conventionally known method. The lithium ion secondary battery manufactured to include the negative electrode 20, the positive electrode 10 and the electrolytic solution 60 manufactured through the above steps (A) to (E) can have a high capacity and a high output.

<Use>
The lithium ion secondary battery of the present embodiment can have a high capacity and a high output. Therefore, the lithium ion secondary battery of the present embodiment is particularly suitable as a power source for a hybrid vehicle (HV), an electric vehicle (EV), a plug-in hybrid vehicle (PHV), or the like. However, the use of the lithium ion secondary battery of this embodiment should not be limited to such in-vehicle use. The lithium ion secondary battery of the present embodiment can be applied to all applications.

Examples will be described below. However, the following examples do not limit the scope of the invention of the present disclosure.

<Manufacturing of lithium-ion secondary batteries>
<< Example 1 >>
(A) Preparation of First Paste Graphite particles (average particle size: 20 μm, crushing strength: 30 MPa) and SiO particles (average particle size: 5 μm) were prepared. The first paste was prepared by mixing graphite particles, SiO particles, CMC, SBR and water. The solid content of the first paste was set to "graphite particles: SiO particles: CMC: SBR = 93.6: 5: 0.7: 0.7 (mass ratio)". The solid content indicates a component other than the solvent in the paste.

(B) Preparation of Second Paste Alumina particles (average particle size: 1 μm, crush strength: 1.8 GPa) were prepared as inorganic oxide particles. A second paste was prepared by mixing alumina particles, an acrylic copolymer and N-methyl-2-pyrrolidone (NMP). The solid content of the second paste was "alumina particles: acrylic copolymer = 97: 3 (mass ratio)".

(C) Formation of First Layer A Cu foil having a thickness of 10 μm was prepared as a current collector. The first paste was applied to the surface (both sides) of the Cu foil by a die coater and dried. As a result, the first layer was formed. A gap was formed in the surface layer of the first layer.

(D) Formation of the second layer The surface of the first layer was coated with the second paste by a gravure coater and dried. At the time of coating, the second paste penetrated into the surface layer of the first layer. As a result, the second layer was laminated on the first layer. The second layer was formed so that a part of the second layer was interposed in the gap between the surface layers of the first layer.

(E) Production of Negative Electrode After the second layer was formed, the first layer, the second layer and the current collector were rolled together by a roller press. That is, the first layer and the second layer were compressed. As a result, a negative electrode was manufactured. The negative electrode was cut into strips. As a result, a strip-shaped negative electrode was prepared. This negative electrode has the following dimensions.

Length of first layer (negative electrode active material layer): 5835 mm
Width of first layer: 122 mm
Thickness of first layer: 67 μm (single side), 134 μm (both sides)
Second layer thickness: 2 μm (single side), 4 μm (both sides)
Ratio of second layer thickness to first layer thickness: 3.0%

(F) Production of Lithium Ion Secondary Battery LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (hereinafter abbreviated as “NCM”) was prepared as positive electrode active material particles. A positive electrode paste was prepared by mixing NCM, a conductive material, PVdF and NMP. The solid content of the positive electrode paste was set to "NCM: conductive material: PVdF = 93: 4: 3 (mass ratio)". As a current collector, an Al foil having a thickness of 15 μm was prepared. A positive electrode active material layer was formed by applying a positive electrode paste to the surface (both sides) of the Al foil and drying it. As a result, a positive electrode was manufactured. The positive electrode was rolled and cut into strips. As a result, a strip-shaped positive electrode was prepared. This positive electrode has the following dimensions.

Length of positive electrode active material layer: 5681 mm
Width of positive electrode active material layer: 117 mm
Thickness of positive electrode active material layer: 63.5 μm (single side), 127 μm (both sides)

A separator having a thickness of 20 μm was prepared. This separator is composed of a porous film of PP, a porous film of PE, and a porous film of PP laminated in this order. The positive electrode and the negative electrode were laminated so as to face each other with a separator sandwiched between them, and further wound. As a result, a winding type electrode group was formed. A winding electrode group was formed into a flat shape by a flat plate press. Molding was carried out without heating. The molding pressure was 4 kN / cm ^2, and the molding time was 2 minutes. External terminals were attached to the electrode group. A square housing (made of Al) was prepared. The electrode group was housed in the housing.

An electrolytic solution having the following components was prepared.
Solvent: [EC: DMC: EMC = 3: 4: 3 (volume ratio)]
Lithium salt: LiPF ₆ (1.0 mol / l)

120 ml of electrolyte was injected into the housing. The housing was sealed. From the above, the lithium ion secondary battery according to Example 1 was manufactured. This battery has a rated capacity of 35 Ah.

<< Comparative Example 1 >>
The battery according to Comparative Example 1 was manufactured by the same procedure as in Example 1 except that the second layer was not formed.

<< Examples 2 to 7 >>
As shown in Table 1 below, the batteries according to Examples 2 to 7 were manufactured by the same procedure as in Example 1 except that the thickness of the second layer was changed.

<< Comparative Example 2 >>
In Comparative Example 2, the first layer was compressed before the second layer was formed. After the first layer was compressed, the surface of the first layer was coated with the second paste, so that the second layer was laminated on the first layer. Except for these, the battery according to Comparative Example 2 was manufactured by the same procedure as in Example 1.

In the column of "compression timing" in Table 1 below, the timing at which the first layer is compressed is shown in the example including both the first layer and the second layer. Since Comparative Example 1 does not have a second layer, it is described as "no second layer". Comparative Example 2 corresponds to an example in which the first layer is compressed “before laminating the second layer”. Examples 1 to 7 correspond to an example in which the first layer is compressed "after laminating the second layer".

<Evaluation>
The output of each battery was evaluated by the IV resistance. The SOC (State Of Charge) of the battery was adjusted to 50%. The battery was discharged for 10 seconds by a current of 35 A in an environment of 25 ° C. The amount of voltage drop 10 seconds after discharge was measured. Similarly, the amount of voltage drop when the battery was discharged for 10 seconds by the current of 105 A and the amount of voltage drop when the battery was discharged for 10 seconds by the current of 175 A were measured.

The results are plotted in two-dimensional coordinates where the vertical axis is the amount of voltage drop and the horizontal axis is the current. The IV resistance was calculated as the slope of a straight line connecting the three points. The IV resistance of each example is shown in Table 1 below. The numerical values shown in the “IV resistance” column of Table 1 below are values obtained by dividing the IV resistance of each example by the IV resistance of Example 2. The smaller the value, the higher the output of the battery.

<Result>
As shown in Table 1 above, the example in which compression is performed after the second layer is laminated on the first layer has a lower IV resistance than the comparative example which does not satisfy the same conditions. Since a part of the second layer (alumina particles) is interposed in the gap between the surface layers of the first layer, it is considered that the electrolytic solution easily permeates into the first layer and the diffusion resistance is reduced.

Comparative Example 1 has a high IV resistance. Comparative Example 1 does not include a second layer. At the time of compression, the gaps between the particles are filled in the surface layer of the first layer (negative electrode active material layer), so that it is considered that the electrolytic solution is difficult to permeate into the first layer.

Comparative Example 2 also has a high IV resistance. Since the first layer is compressed before the second layer is laminated on the first layer, the gaps between the particles are filled in the surface layer of the first layer, and it is difficult for the electrolytic solution to penetrate into the first layer. Conceivable.

From the results of Examples 1 to 7, it can be confirmed that the IV resistance tends to decrease as the ratio (T2 / T1) of the thickness (T2) of the second layer to the thickness (T1) of the first layer increases. It is considered that the higher the ratio (T2 / T1), the more easily it is suppressed that the gaps between the particles are filled in the surface layer of the first layer.

From the results of Examples 1 to 7, the ratio (T2 / T1) is preferably 5% or more, more preferably 8% or more, even more preferably 10% or more, and most preferably 15% or more. Is. However, when the ratio (T2 / T1) exceeds 20%, the resistance reducing effect is saturated. Therefore, the ratio (T2 / T1) is preferably 20% or less.

The embodiments and examples disclosed this time should be considered as exemplary in all respects and not restrictive. The scope of the invention of the present disclosure is indicated by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

1 Graphite particles, 2 Silicon oxide (SiO) particles, 3 Inorganic oxide particles, 10 Positive electrode, 11 Positive electrode active material layer, 13, 23 Current collector, 20 Negative electrode, 21 First layer, 22 Second layer, 30 Separator, 50 electrode group, 60 electrolyte, 90 housing, 100 batteries (lithium ion secondary battery).

Claims (4)

Positive electrode,
Equipped with negative electrode and electrolyte
The negative electrode includes a current collector, a first layer, and a second layer.
The first layer is arranged on the surface of the current collector.
The second layer is laminated on the first layer.
The first layer comprises graphite particles and silicon oxide particles.
The second layer includes inorganic oxide particles and has
The inorganic oxide particles are different from the silicon oxide particles.
There is a gap in the surface layer of the first layer,
A part of the second layer is interposed in the gap.
The second layer has a porosity of 30% or more and 60% or less.
Lithium-ion secondary battery. The second layer is composed of the inorganic oxide particles and a binder.
The lithium ion secondary battery according to claim 1. The inorganic oxide particles have a higher compressive strength than the graphite particles.
The lithium ion secondary battery according to claim 1 or 2. Positive electrode,
Negative electrode and
Electrolyte
With
The negative electrode includes a current collector, a first layer, and a second layer.
The first layer is arranged on the surface of the current collector.
The second layer is laminated on the first layer.
The first layer comprises graphite particles and silicon oxide particles.
The second layer includes inorganic oxide particles and has
The inorganic oxide particles are different from the silicon oxide particles.
There is a gap in the surface layer of the first layer,
A part of the second layer is interposed in the gap.
The ratio of the thickness of the second layer to the thickness of the first layer is 7.5% or more and 20.9% or less.
Lithium-ion secondary battery.

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