JP5418828B2 - Lithium secondary battery and manufacturing method thereof - Google Patents

Description

  The present invention relates to a lithium secondary battery and a method for manufacturing the same. In detail, it is related with the property of the negative mix layer provided in the negative electrode of a lithium secondary battery.

A lithium secondary battery (typically a lithium ion battery) that is charged and discharged by moving lithium ions back and forth between the positive electrode and the negative electrode is lightweight and has a high energy density. The importance is increasing as what is preferably used for the power supply of a personal computer and a portable terminal.
In a typical negative electrode of a lithium ion battery, a particulate carbon material (carbon particle) containing at least a part of a graphite structure (layered structure) as a material capable of reversibly occluding and releasing lithium ions (negative electrode active material) Is used. For example, a typical configuration of the negative electrode includes a configuration in which a layer mainly composed of carbon particles as a negative electrode active material (negative electrode mixture layer) is held on a negative electrode current collector. As a preferred method for forming the negative electrode mixture layer, a composition prepared by dispersing or dissolving carbon particles in an appropriate solvent to prepare a paste or slurry (hereinafter, the composition is referred to as “negative electrode mixture layer forming paste”). Is applied to the negative electrode current collector, and then the applied product is pressed to obtain an appropriate density and thickness. Patent documents 1-5 are mentioned as conventional technology about this kind of lithium secondary battery. For example, Patent Document 1 describes a battery in which charge / discharge cycle characteristics are improved while maintaining a high energy density by defining a specific surface area of a negative electrode material.

Japanese Patent Laid-Open No. 2002-270160 Japanese Patent Laid-Open No. 10-228896 JP 2006-236753 A JP 2004-296305 A JP 2004-127913 A

By the way, when a lithium secondary battery is used as a power source for driving a vehicle, the battery is required to be charged and discharged at a high rate under relatively severe environmental conditions. For example, when high-rate charging is performed under low-temperature conditions (for example, in an environment of about −15 ° C.), depending on the mode, lithium ions are not occluded between the layers of the negative electrode active material (carbon material) and do not enter the surface of the negative electrode active material. Lithium may precipitate and the capacity may decrease. As a measure to deal with such problems, the specific surface area of the negative electrode active material should be specified in order to reduce lithium deposition by increasing the surface area of the negative electrode active material and increasing the area in contact with the electrolyte interface. It has been known.
However, even if the specific surface area of the negative electrode active material is defined, when the negative electrode active material is kneaded to produce a negative electrode mixture layer forming paste, a binder (binder) adheres to the surface of the negative electrode active material. In addition, when the negative electrode is manufactured, the negative active material may be cracked or cracked by the pressing process, so that the substantial effective reaction area may change.
When the surface area of the negative electrode mixture layer after pressing is too small, an event may occur in which lithium is deposited as a metal on the surface and the capacity decreases when performing high-rate charging (input) at a low temperature. In addition, if the surface area of the negative electrode mixture layer after pressing is too large, the SEI film (Solid Electrolyte Interface) formed by consuming lithium ions increases, resulting in a decrease in lithium ions and a decrease in the SEI film. In order to inhibit the movement of ions, an event may occur in which the volume decreases.

  Therefore, the present invention was created to solve the conventional problems related to lithium secondary batteries, and the object is to define the surface area per unit area of the negative electrode mixture layer after pressing. Another object of the present invention is to provide a lithium secondary battery excellent in cycle characteristics and a method for producing the same, in which the deposition of lithium and the increase in SEI film are suppressed even for high-rate charge / discharge under low temperature conditions. Another object is to provide a vehicle including such a lithium secondary battery.

In order to achieve the above object, the present invention provides a method for producing a lithium secondary battery. The manufacturing method disclosed here includes the following steps: (1). A step of preparing a negative electrode mixture layer forming paste in which a negative electrode mixture containing a carbon material capable of reversibly occluding and releasing lithium ions and a binder is dispersed in a predetermined solvent; (2). Applying the paste to the surface of the negative electrode current collector, (3). (4) a step of pressing the coated material to form a negative electrode mixture layer having a surface area per unit area of the current collector after pressing (based on the BET method) of 180 cm 2 / cm 2 to 240 cm 2 / cm 2; A step of constructing the lithium secondary battery using the produced negative electrode mixture layer. Preferably, the pressing is performed at a pressing pressure of 2 MPa to 10 MPa so that the specific surface area increase rate of the carbon material is at least 20% or more.

In the present specification, the “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by movement of lithium ions between the positive and negative electrodes. A secondary battery generally referred to as a lithium ion battery is a typical example included in the lithium secondary battery in this specification.
Further, in this specification, the “negative electrode active material” means a negative electrode side capable of reversibly occluding and releasing (typically inserting and removing) chemical species (here, lithium ions) as charge carriers in the secondary battery. The active material.

In the method for producing a lithium secondary battery provided by the present invention, the negative electrode mixture layer forming paste is applied to the surface of the negative electrode current collector, and the applied material is pressed (typically after the applied material is dried). press.), the surface area of the collector per unit area after pressing forms a negative electrode mixture layer is ^{180cm 2 / cm 2 ~240cm 2 /} cm 2.
Thus, the surface area per unit area of the current collector (hereinafter simply referred to as “surface area after pressing”) after pressing the negative electrode mixture layer (that is, after pressing after the form is changed as compared with that immediately after coating). In some cases, the surface area value after pressing of the negative electrode mixture layer is too smaller than the above range when charging at a high rate under low temperature conditions (for example, about −15 ° C.). Compared to the case, lithium ions can be prevented from being deposited (dendrite) as metallic lithium on the surface of the negative electrode active material without being inserted (inserted) into the negative electrode active material (typically carbon-based particles). . Furthermore, it is possible to prevent an increase in the SEI film accompanying the consumption of lithium ions on the surface of the negative electrode active material as compared with the case where the surface area value after pressing of the negative electrode mixture layer is too larger than the above range. Thereby, the capacity | capacitance fall of the whole lithium secondary battery accompanying the reduction | decrease of the lithium ion in a lithium secondary battery can be avoided. That is, according to the present invention, it is possible to manufacture a lithium secondary battery excellent in cycle characteristics even for high rate charge / discharge under low temperature conditions.

Preferably, in the paste application step, the coating amount per unit area of the current collector applied to the negative electrode current collector is 7.5 mg / cm ² or less. When the paste is applied to the negative electrode current collector, the post-press negative electrode mixture having a suitable post-press surface area in the above-described range by setting the coating amount of the applied product to 7.5 mg / cm ² or less. Layers can be easily formed.

In a preferred embodiment of the production method disclosed herein, in the negative electrode mixture layer forming step, a surface area per unit area of the current collector after pressing of the negative electrode mixture layer is 200 cm ² / cm ^{2 to} 230 cm. ² / cm ² . Producing a lithium secondary battery with better cycle characteristics for high-rate charge / discharge under low temperature conditions by adjusting the surface area per unit area of the current collector after pressing the negative electrode mixture layer as described above can do.

In a preferred embodiment of the production method disclosed herein, the specific surface area of the carbon material is increased by 20% to 50% (preferably 25% to 40%) by the press.
The negative electrode mixture layer surface area after pressing is ^{180cm 2 / cm 2 ~240cm 2 /} cm 2 by performing a press to produce this degree of specific surface area increase can be easily formed.

Further, the lithium secondary battery provided with the positive electrode, the negative electrode, and the electrolyte provided by the present invention has a negative electrode current collector and reversibly occludes and releases lithium ions formed on the current collector. A negative electrode mixture layer composed of a negative electrode mixture containing a possible carbon material and a binder, and the surface area of the negative electrode mixture layer per unit area of the current collector (based on the BET method) is 180 cm ^2. characterized in that it is a ^{/ cm 2 ~240cm 2 / cm 2} .
Lithium secondary battery provided by the present invention, after pressing surface area of the negative electrode mixture layer satisfies the ^{180cm 2 / cm 2 ~240cm 2 /} cm 2. Thereby, when performing high-rate charging under low temperature conditions, it is possible to prevent metallic lithium from being deposited on the surface of the negative electrode active material and increase of the SEI film accompanying consumption of lithium ions. As a result, it is possible to provide a lithium secondary battery excellent in cycle characteristics even for high-rate charge / discharge under low temperature conditions.

In a preferred embodiment of the lithium secondary battery disclosed herein, the content of the negative electrode mixture per unit area of the current collector is 7.5 mg / cm ² or less. When the content per unit area of the current collector of the negative electrode mixture is 7.5 mg / cm ² or less, the thickness direction of the negative electrode mixture layer (the negative electrode is less than when the content exceeds 7.5 mg / cm ^2. The reaction inhomogeneity between lithium ions and electrons in the depth direction) is greatly reduced, and lithium ions can be prevented from being deposited as metallic lithium on the surface of the negative electrode active material. As a result, it is possible to provide a lithium secondary battery that suppresses lithium deposition and has excellent cycle characteristics.

Further, in one preferred embodiment of the lithium secondary battery disclosed herein, the negative electrode mixture layer, the surface area per collector unit area is ^{200cm 2 / cm 2 ~230cm 2 /} cm 2. By having the surface area after pressing in such a numerical range, it is possible to provide a lithium secondary battery having more excellent cycle characteristics with respect to high-rate charge / discharge under low temperature conditions.

It is a perspective view which shows typically the external shape of the lithium secondary battery which concerns on one Embodiment. It is a longitudinal cross-sectional view which follows the II-II line | wire in FIG. It is a schematic diagram which shows the form of the negative electrode active material (after press) contained in the negative mix layer apply | coated to the negative electrode collector which concerns on one Embodiment. It is a side view which shows typically the vehicle (automobile) provided with the battery which concerns on this invention.

  Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings. Note that matters other than the matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of a person skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

  The lithium secondary battery provided by the present invention is characterized by defining the post-press surface area of the negative electrode mixture layer. Hereinafter, although the lithium secondary battery constructed | assembled using the said negative mix layer and the manufacturing method of this secondary battery are demonstrated in detail, it is not intending to limit this invention to this embodiment.

FIG. 1 is a perspective view schematically showing a lithium secondary battery according to an embodiment. FIG. 2 is a longitudinal sectional view taken along line II-II in FIG.
As shown in FIGS. 1 and 2, the lithium secondary battery 10 according to the present embodiment typically has a predetermined battery constituent material (a positive and a negative current collector and a positive and negative electrode current collector, respectively), as in the conventional unit cell. A wound electrode body 30 having a sheet-like electrode, a separator, and the like on which the active material is held, and a flat rectangular parallelepiped shape (that is, a square shape) that accommodates the electrode body 30 and an appropriate liquid electrolyte (electrolyte) Battery case 50.
The case 50 is attached to (for example, welded) the box-shaped case main body 54 in which one of the narrow surfaces in the above-described flat rectangular parallelepiped shape is the opening 52 and the opening 52 is closed. And a lid 56. As a material constituting the case 50, a material similar to that used in a general lithium secondary battery can be appropriately used, and there is no particular limitation. From the viewpoint of heat dissipation and the like, a case 50 made of metal (for example, aluminum, steel, etc.) can be preferably used.
The lid 56 is formed in a rectangular shape that matches the shape of the opening 52 of the case main body 54. Further, the lid 56 is provided with a positive terminal 60 and a negative terminal 70 for external connection, respectively, and a part of these terminals 60, 70 protrudes from the lid 56 toward the outside of the case 50. It is formed to do.

  As shown in FIG. 2, the wound electrode body 30 is accommodated in the case main body 54 in a posture in which the winding shaft is laid down (that is, the direction in which the opening is positioned in the lateral direction with respect to the winding shaft). Yes. The electrode body 30 includes a positive electrode sheet (positive electrode) 36 in which a positive electrode mixture layer 34 is formed on the surface of a long sheet-like positive electrode current collector 32 and a negative electrode composite on the surface of a long sheet-like negative electrode current collector 42. It consists of a negative electrode sheet (negative electrode) 46 on which an agent layer 44 is formed and a long sheet-like separator 20. Then, the positive electrode sheet 36 and the negative electrode sheet 46 are overlapped with the two separators 20 and wound, and the obtained wound electrode body 30 is flattened by being crushed from the side surface direction.

In the wound positive electrode sheet 36, the positive electrode current collector layer 34 is not formed at one end along the longitudinal direction, and the positive electrode current collector 32 is exposed, while the negative electrode is wound. Also in the sheet 46, the negative electrode current collector 42 is exposed at one end portion along the longitudinal direction without forming the negative electrode mixture layer 44. Then, the positive electrode terminal 60 is joined to the exposed end portion of the positive electrode current collector 32, and the negative electrode terminal 70 is joined to the exposed end portion of the negative electrode current collector 42, respectively. The positive electrode sheet 36 or the negative electrode sheet 46 is electrically connected. The positive and negative electrode terminals 60 and 70 and the positive and negative electrode current collectors 32 and 42 can be joined by, for example, ultrasonic welding, resistance welding, or the like.
Similar to the conventional case, a safety valve (not shown) for discharging the gas generated inside the case 50 to the outside of the case 50 is provided on the upper surface side of the battery case 50. The safety valve can be used without any limitation as long as it has a mechanism that opens and discharges gas to the outside of the case 50 when the pressure inside the case 50 rises above a predetermined level.

Hereinafter, each component of the negative electrode of the lithium secondary battery 10 according to the present embodiment will be described. The negative electrode (typically, the negative electrode sheet 46) of the lithium secondary battery disclosed herein has a configuration in which a negative electrode mixture layer 44 is formed on the surface of the negative electrode current collector 42.
As the negative electrode current collector 42, a conductive member made of a highly conductive metal is preferably used. For example, copper or an alloy mainly composed of copper (copper alloy) can be used. The shape of the negative electrode current collector 42 may vary depending on the shape of the lithium secondary battery, and is not particularly limited, and may be various forms such as a rod shape, a plate shape, a sheet shape, a foil shape, and a mesh shape. In the present embodiment, a sheet-like copper negative electrode current collector 42 is used.

Next, materials constituting the negative electrode mixture layer 44 formed on the surface of the negative electrode current collector 42 will be described.
On the surface of the negative electrode current collector 42, a negative electrode mixture prepared by adding a negative electrode active material 43, a binder, and the like to a suitable solvent (water, an organic solvent and a mixed solvent thereof) and dispersing or dissolving them. The negative electrode mixture layer 44 is formed by using a layer forming paste.

  As the negative electrode active material 43 contained in the negative electrode mixture layer 44 used for the negative electrode of the lithium secondary battery 10 disclosed herein, a carbon material (typically particulate) capable of reversibly occluding and releasing lithium. Is mentioned. A carbon material containing a graphite structure (layered structure) at least partially is preferable. Any carbon material of a so-called graphitic material (graphite), a non-graphitizable carbon material (hard carbon), a graphitizable carbon material (soft carbon), or a combination of these materials is preferably used. obtain. In particular, the graphite particles can be a negative electrode active material more suitable for rapid charge / discharge (for example, high-power discharge) because the particle size is small and the surface area per unit volume is large.

As the carbon material, for example, can be the specific surface area measured by the BET method by nitrogen gas adsorption (in accordance with JIS Z 8830) is used preferably those of about ^{2m 2} / ^g~5m 2 / g. Among them, a specific surface area of about 2.5m ² / g~4m ² / g carbon material is preferred. As a method for adjusting the specific surface area of the carbon material in this way, for example, a method of coating the surface of the carbon material with amorphous carbon can be cited. The method of coating the surface of the carbon material with amorphous carbon is such that the carbonization material such as pitch, tar, etc., which becomes a precursor of amorphous carbon adheres to the carbon material and the graphitization of the precursor does not proceed. It is carried out by firing at a temperature of Examples of the method for attaching the precursor to the surface of the carbon material include a dry method and a wet method. The dry method and the wet method are the same as conventionally known methods and do not characterize the present invention at all, so that further explanation is omitted.

  In addition to the negative electrode mixture layer, the negative electrode mixture layer can contain one or two or more materials that can be blended in the negative electrode mixture layer of a general lithium secondary battery, if necessary. Examples of such materials include various polymer materials that can function as a binder. For example, a water-based liquid composition (typically a composition prepared in a paste or slurry form, hereinafter referred to as a negative electrode mixture layer forming paste) is used as the composition for forming the negative electrode mixture layer. For this, a polymer material that dissolves or disperses in water can be preferably used as the binder. Cellulose polymers such as carboxymethylcellulose (CMC), methylcellulose (MC), cellulose acetate phthalate (CAP), hydroxypropylmethylcellulose (HPMC), etc .; polyvinyl alcohol (PVA) And the like are exemplified. Examples of polymer materials that are dispersed in water (water dispersible) include fluorine resins such as polytetrafluoroethylene (PTFE); vinyl acetate copolymers; rubbers such as styrene butadiene rubber (SBR); The Alternatively, when a solvent-based liquid composition is used as the composition for forming the negative electrode mixture layer, a polymer material such as polyvinylidene fluoride (PVDF) or polyvinylidene chloride (PVDC) can be used. In addition, the polymer material illustrated above may be used as a thickener and other additives in the above composition in addition to being used as a binder.

  Here, the “aqueous liquid composition” is a concept indicating a composition using water or a mixed solvent mainly composed of water (aqueous solvent) as a dispersion medium of the active material. As the solvent other than water constituting the mixed solvent, one or more organic solvents (lower alcohol, lower ketone, etc.) that can be uniformly mixed with water can be appropriately selected and used. The “solvent-based liquid composition” is a concept indicating a composition in which a dispersion medium of an active material is mainly an organic solvent (non-aqueous solvent). As the organic solvent, for example, N-methylpyrrolidone (NMP) can be used.

Hereinafter, a method for manufacturing the negative electrode (typically, the negative electrode sheet 46) of the lithium secondary battery 10 according to the present embodiment will be described. FIG. 3 is a side view schematically showing the negative electrode sheet 46 according to the embodiment.
The method disclosed here can be produced by forming the negative electrode mixture layer 44 on the surface of the negative electrode current collector 42. In forming the negative electrode mixture layer 44, first, a negative electrode active material (for example, a carbon material obtained by coating natural graphite with amorphous carbon) 43 having a specific surface area, a binder (for example, CMC and SBR), A negative electrode mixture layer-forming paste is prepared by dispersing a negative electrode mixture containing a mixture in a predetermined solvent (for example, water). The adjusted paste is applied to the negative electrode current collector 42. Here, when the paste is applied to the negative electrode current collector 42, the coating amount (content) per unit area of the current collector of the coating material (that is, the negative electrode mixture) is 7.5 mg / cm ² or less. It is preferable. Such coating amount (content) may be any generally 7.5 mg / cm ² or less, but from the viewpoint of the resulting improved properties, as preferable example 5.0~7.5mg / cm ^2, typically Is preferably about 6.0 to 7.0 mg / cm ² . If the coating amount (content) of the negative electrode mixture is too higher than 7.5 mg / cm ² , the reactivity between the lithium ions and the charges in the thickness direction of the negative electrode mixture layer (the negative electrode depth direction) decreases. In other words, lithium ions may precipitate as metallic lithium on the surface of the negative electrode active material, leading to a decrease in charge / discharge capacity of the battery. In addition, as a method of applying the paste to the negative electrode current collector 42, a technique similar to a conventionally known method can be appropriately employed. For example, the paste can be suitably applied to the negative electrode current collector 42 by using an appropriate application device such as a slit coater, a die coater, a gravure coater, or a comma coater.

As shown in FIG. 3, the coating applied to the negative electrode current collector 42 is dried and then pressed (compressed) to form a negative electrode mixture layer 44 (cracks occur in the negative electrode active material 43 due to pressing). The specific surface area of the negative electrode active material 43 increases). Here, the press pressure at the time of pressing the coated material is about 2 MPa to 10 MPa (preferably about 3 MPa to 10 MPa, for example, 5 MPa to 10 MPa). It is appropriate to press the negative electrode active material (carbon material) so that the specific surface area increase rate is at least 20% or more, for example, 20% or more and 50% or less (preferably 25% or more and 40% or less). When forming the negative electrode mixture layer 44 of the coating was pressed, the surface area of the negative electrode current collector 42 per unit area of the negative electrode mixture layer 44 after the pressing in ^{180cm 2 / cm 2 ~240cm 2 /} cm 2 Preferably there is. More preferably from ^{200cm 2 / cm 2 ~230cm 2 /} cm 2. When the surface area per unit area is less than 180 cm ² / cm ² , lithium ions are not occluded (intercalated) into the negative electrode active material at the time of high-rate charging at a low temperature, and are formed on the surface of the negative electrode active material as metallic lithium. There is a risk of precipitation. Further, if the surface area per unit area of the current collector is too larger than 240 cm ² / cm ² , the SEI film that is a reaction product of lithium ions and an electrolytic solution may increase on the surface of the negative electrode active material. . When the SEI film is formed on the surface of the negative electrode active material, a part of the lithium ions in the lithium secondary battery is consumed and irreversibly taken into the SEI film. Since the lithium ions irreversibly taken in do not participate in the subsequent charge / discharge process, the charge / discharge capacity of the battery may be reduced accordingly.
Moreover, as said press (compression) method, conventionally well-known compression methods, such as a roll press method and a flat plate press method, are employable. In adjusting the thickness, the thickness may be measured with a film thickness measuring instrument, and the press pressure may be adjusted to compress a plurality of times until a desired thickness is obtained.

  The other materials and members constituting the lithium secondary battery 10 including the negative electrode (negative electrode sheet) 46 manufactured by the method disclosed herein may be the same as those conventionally provided for the same type of battery, and there are no particular restrictions. Absent. Hereinafter, other components will be described, but the present invention is not intended to be limited to such embodiments.

  For example, the positive electrode (positive electrode sheet) 36 may have a configuration in which a positive electrode mixture layer 34 is formed on a sheet-like positive electrode current collector 32 (for example, an aluminum foil). The positive electrode mixture layer 34 is a paste-like (or slurry-like) composition (hereinafter referred to as a slurry) obtained by mixing a positive electrode active material, a conductive material, a binder, a thickener, a solid polymer electrolyte, and the like in an appropriate solvent. The positive electrode mixture layer forming paste) is applied to the positive electrode current collector 32, and the applied material is dried and compression molded.

As the positive electrode active material contained in the positive electrode mixture layer 34, a granular active material capable of occluding and releasing lithium is used. A layered oxide positive electrode active material known as a positive electrode active material of this type of lithium secondary battery, an oxide positive electrode active material having a spinel structure, or the like can be preferably used. Examples thereof include lithium transition metal composite oxides such as lithium nickel composite oxides, lithium cobalt composite oxides, and lithium manganese composite oxides. An olivine type lithium phosphate represented by the general formula LiMPO ₄ (M is at least one element of Co, Ni, Mn, Fe; for example, LiFePO ₄ , LiMnPO ₄ ) is used as the positive electrode active material. Also good.

  In addition, the conductive material included in the positive electrode mixture layer 34 may be any conductive material conventionally used in this type of lithium secondary battery, and is not limited to a specific conductive material. For example, carbon materials such as carbon powder and carbon fiber can be used. As the carbon powder, various carbon blacks (for example, acetylene black, furnace black, ketjen black), graphite powder, and the like can be used. Among these, you may use together 1 type, or 2 or more types. Moreover, as a binder, the thing similar to the binder which can be used for the negative mix layer mentioned above etc. can be used.

The solid polymer electrolyte is not particularly limited as long as it is a polymer exhibiting ionic conductivity, and examples thereof include polyethylene oxide (PEO), polypropylene oxide (PPO), and copolymers thereof. Such a polyalkylene oxide polymer readily dissolves lithium salts such as LiBF ₄ , LiPF ₆ , LiN (SO ₂ CF ₃ ) ₂ , and LiN (SO ₂ C ₂ F ₅ ) ₂ .

  Moreover, as a suitable sheet-like separator 20 used between positive and negative electrode sheets, one made of a porous polyolefin-based resin can be mentioned. When a solid electrolyte or a gel electrolyte is used as the electrolyte, a general resin separator sheet may not be necessary (that is, in this case, the electrolyte itself may function as a separator).

As the electrolyte accommodated together with the wound electrode body 30 in the battery case 50, the same electrolyte as the non-aqueous electrolyte conventionally used in lithium secondary batteries can be used without any particular limitation. Such an electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. Examples of the non-aqueous solvent include one or two selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like. More than seeds can be used. Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF _One or two or more lithium compounds (lithium salts) selected from ₃ SO ₂ ) ₃ , LiI and the like can be used. In addition, in a conventional lithium secondary battery, an additive such as vinylene carbonate (VC) may be added together with an electrolyte for the purpose of forming a SEI film on the negative electrode surface and improving cycle characteristics. The amount may decrease due to repeated charge and discharge, and battery characteristics may change rapidly. In the lithium secondary battery according to the present embodiment, by specifying the surface area after pressing, high cycle characteristics and stable use can be realized without using an additive.

The produced positive electrode sheet 36 and the negative electrode sheet 46 are stacked and wound together with the two separators 20 and 20, and the obtained wound electrode body 30 is accommodated in the battery case 50 and injected with the electrolytic solution, The lithium secondary battery 10 of this embodiment can be constructed by attaching and sealing the lid 56 to the case opening 52.
In particular, the structure, size, material (for example, made of metal or laminate film) of the battery case 50, and the structure of the electrode body (for example, a wound structure or a laminated structure) having the positive and negative electrodes as main components, etc. There is no limit.

  In order to confirm that a lithium secondary battery excellent in quality stability (cycle characteristics) under low temperature conditions can be constructed by constructing the lithium secondary battery, the following tests were conducted as examples. It should be noted that the present invention is not intended to be limited to those shown in the specific examples.

[Press test of negative electrode active material]
Natural graphite as a negative active material The specific surface area of the (electron microscope (SEM or TEM, etc.) the average particle diameter _{D 50 was} 11μm based on observation) carbon material coated with amorphous carbon (anode active material) of 3.2 m ² Natural graphite was coated with amorphous carbon so as to be / g. The carbon material was pressed at a press pressure of 4 MPa. The specific surface area of the carbon material after pressing was 4.2 m ² / g, and it was confirmed that the increase rate of the specific surface area by pressing was about 30%.

[Construction of Lithium Secondary Battery According to Example]
<Example 1>
First, the negative electrode of the lithium secondary battery according to Example 1 was produced. That is, the specific surface area of natural graphite as a negative electrode active material (electron microscope (SEM or TEM, etc.) the average particle diameter D ₅₀ based on observations 11 [mu] m) carbon material coated with amorphous carbon (anode active material) is 3. Natural graphite was coated with amorphous carbon so as to be 0 m ² / g. Then, the carbon material, SBR, and CMC are weighed so that the mass ratio of these materials is 98: 1: 1, these materials are mixed with ion-exchanged water, and the aqueous negative electrode mixture layer forming paste is prepared. Prepared. The paste coating amount (solid content standard of carbon material, SBR, and CMC) on both surfaces of a negative electrode current collector (thickness 10 μm) composed of a long copper foil was 6.4 mg / cm ² . It applied so that it might become. The coated material was pressed at a press pressure of 4 MPa to form a negative electrode mixture layer having a surface area per unit area of the current collector of the negative electrode mixture layer after the press of 204 cm ² / cm ² . In this way, a sheet-like negative electrode (negative electrode sheet) was produced.
Next, the positive electrode of the lithium secondary battery according to Example 1 was produced. That is, these materials include LiNi _0.8 Co _0.15 Al _0.05 O ₂ as a positive electrode active material, acetylene black, CMC, and PEO on both surfaces of a positive electrode current collector composed of an aluminum foil. A positive electrode mixture layer forming paste was prepared so that the mass ratio of the mixture was 88: 10: 1: 1, a predetermined amount of the paste was applied, and then the positive electrode mixture layer was formed by pressing. Thus, a sheet-like positive electrode (positive electrode sheet) was produced.

The negative electrode sheet and the positive electrode sheet prepared above were laminated together with two long separators (here, a porous polypropylene sheet was used), and the laminated sheet was wound in the long direction to form a wound electrode body. Produced. A lithium secondary battery according to this example was constructed by housing this electrode body in a rectangular battery case together with an electrolytic solution. As an electrolytic solution, about 1 mol / L LiPF ₆ was dissolved as a supporting salt in a 3: 3: 4 (mass ratio) mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). Used.

<Example 2>
In this example, a lithium secondary battery was constructed in the same manner as in Example 1 except that a negative electrode mixture layer having a surface area per unit area of the current collector after pressing of 222 cm ² / cm ² was used.

<Example 3>
In this example, a point using a carbon material having a specific surface area of 3.5 m ² / g and a point using a negative electrode mixture layer having a surface area per unit area of the current collector after pressing of 212 cm ² / cm ² are used. Except for the above, a lithium secondary battery was constructed in the same manner as in Example 1.

<Example 4>
In this example, the amount of the mixture applied to the negative electrode current collector was 6.9 mg / cm ² per side, and the surface area per unit area of the current collector after the press was applied. A lithium secondary battery was constructed in the same manner as in Example 1 except that a negative electrode mixture layer having a thickness of 202 cm ² / cm ² was used.

<Example 5>
In this example, the amount of the mixture applied to the negative electrode current collector was 8.0 mg / cm ² per side, and the surface area per unit area of the current collector after the press was applied. A lithium secondary battery was constructed in the same manner as in Example 1 except that a negative electrode mixture layer having a thickness of 228 cm ² / cm ² was used.

<Example 6>
In this example, a point using a carbon material having a specific surface area of 3.3 m ² / g and a point using a negative electrode mixture layer having a surface area per unit area of the current collector after pressing of 181 cm ² / cm ² were used. Except for the above, a lithium secondary battery was constructed in the same manner as in Example 1.

<Example 7>
In this example, a point using a carbon material having a specific surface area of 3.3 m ² / g and a point using a negative electrode mixture layer having a surface area per unit area of the current collector after pressing of 227 cm ² / cm ² Except for the above, a lithium secondary battery was constructed in the same manner as in Example 1.

[Construction of lithium secondary battery according to comparative example]
<Comparative Example 1>
In this example, a lithium secondary battery was constructed in the same manner as in Example 1 except that a negative electrode mixture layer having a surface area per unit area of the current collector after pressing of 161 cm ² / cm ² was used.

<Comparative example 2>
In this example, a point using a carbon material having a specific surface area of 3.5 m ² / g and a point using a negative electrode mixture layer having a surface area per unit area of the current collector after pressing of 168 cm ² / cm ² were used. Except for the above, a lithium secondary battery was constructed in the same manner as in Example 1.

<Comparative Example 3>
In this example, a point using a carbon material having a specific surface area of 3.3 m ² / g and a point using a negative electrode mixture layer having a surface area per unit area of the current collector after pressing of 248 cm ² / cm ² were used. Except for the above, a lithium secondary battery was constructed in the same manner as in Example 1.

[Low temperature high input test]
For each of the lithium secondary batteries of Examples 1 to 5 and Comparative Examples 1 and 2 constructed as described above, after 2000 cycles of charge and discharge, the capacity reduction rate of each lithium secondary battery was measured. did. That is, each battery was charged at a constant current and constant voltage (CC-CV) under a temperature condition of −15 ° C. to adjust the SOC to 40%, and the discharge capacity (initial capacity) at this time was measured. Next, each battery was charged with a constant current of 150 A under a condition of −15 ° C. for 0.1 seconds, rested for 60 seconds, and then discharged with a constant current of 1.5 A for 10 seconds. The cycle of resting for 60 seconds was repeated 2000 times, and the discharge capacity (post-cycle capacity) at 2000th cycle was measured. And the capacity | capacitance reduction rate (%) with respect to the said 2000 charging / discharging cycle was calculated | required by following Formula {(initial capacity)-(capacity after a cycle)} / (initial capacity) x100; The obtained results are shown in Table 1.

As shown in Table 1, according to Example 1 to Example 5, compared with Comparative Example 1 and Comparative Example 2 in which the surface area of the negative electrode mixture layer after pressing was less than 180 cm ² / cm ² , the temperature was reduced. It can be seen that the rate of capacity reduction during high-rate charging is small. In still from Example 1 to Example 4, it is recognized that the surface area of the negative electrode mixture layer is ^{200cm 2 / cm 2 ~230cm 2 /} cm 2 Further the capacity reduction rate after the press is reduced. Moreover, according to Example 1 to Example 3, compared with Comparative Example 2, even if the specific surface area of the carbon material is defined in advance, the surface area of the negative electrode mixture layer after pressing is less than 180 cm ² / cm ^2. It can be seen that the rate of capacity reduction during high-rate charging under low temperature conditions is large. Furthermore, according to Examples 1, 2, and 4, compared with Example 5 in which the coating amount of the mixture exceeds 7.5 mg / cm ² , the capacity reduction rate in high-rate charging under low temperature conditions is small. It is recognized that

[High temperature storage output test]
For each of the lithium secondary batteries of Examples 6 and 7 and Comparative Example 3 constructed as described above, the rate of increase in internal resistance after storage for 50 days at a temperature of 60 ° C. was measured. That is, each secondary battery was adjusted to a SOC 80% charge state by constant current constant voltage (CC-CV) charging, and the internal resistance (initial internal resistance) at this time was measured. Specifically, each secondary battery is discharged at a constant current of 100 A for 10 seconds, the open circuit voltage before discharge and the voltage after 10 seconds are measured, and the voltage drop that is the difference between the two is obtained and discharged. The initial internal resistance was determined by dividing by the current. Next, each of these secondary batteries is stored in a constant temperature bath at 60 ° C. for 50 days, and each secondary battery after storage is discharged at a constant current of 100 A for 10 seconds, and the open circuit voltage before discharge and 10 seconds after discharge. The voltage was measured to determine the voltage drop, which was the difference between the two, and this was divided by the discharge current to determine the internal resistance (the internal resistance after storage). Then, the resistance increase rate (%) after storage for 50 days was determined by the following formula {(internal resistance after storage) − (initial internal resistance)} / (initial internal resistance) × 100; The obtained results are shown in Table 2.

As shown in Table 2, according to Example 6 and Example 7, the rate of increase in resistance is small as compared with Comparative Example 3 in which the surface area of the negative electrode mixture layer after pressing exceeds 240 cm ² / cm ² . That is, it the surface area of 240 cm ² / cm ² by weight, the reduction capacity of the lithium secondary battery is large is observed.
Summarizing the above measurement results, the surface area of the negative electrode mixture layer after pressing is at ^{180cm 2 / cm 2 ~240cm 2 /} cm 2, and excellent cycle characteristics even for high-rate charge and discharge at low temperature conditioned It was shown that

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible.

  Since the lithium secondary battery (for example, lithium ion battery) according to the present invention is excellent in quality stability and exhibits better battery performance as described above, the power source for a motor (electric motor) mounted on a vehicle such as an automobile is particularly preferred. Can be suitably used. Therefore, as schematically shown in FIG. 4, the present invention provides a vehicle (typically, a battery (typically, an assembled battery in which a plurality of such batteries 10 are connected in series) as a power source. An automobile, in particular an automobile equipped with an electric motor such as a hybrid automobile, an electric automobile, and a fuel cell automobile) 100 is provided.

10 lithium secondary battery 20 separator 30 wound electrode body 32 positive electrode current collector 34 positive electrode mixture layer 36 positive electrode sheet 42 negative electrode current collector 43 negative electrode active material 44 negative electrode mixture layer 46 negative electrode sheet 50 battery case 52 opening 54 case Body 56 Lid 60 Positive terminal 70 Negative terminal 100 Vehicle

Claims (4)

A method for manufacturing a lithium secondary battery, comprising:
A step of preparing a negative electrode mixture layer forming paste in which a negative electrode mixture containing a carbon material capable of reversibly occluding and releasing lithium ions and a binder is dispersed in a predetermined solvent;
Applying the paste to the surface of the negative electrode current collector;
By pressing the coating material, a step of surface area of the collector per unit area after the press to form a negative electrode mixture layer is ^{180cm 2 / cm 2 ~240cm 2 /} cm 2,
Building the lithium secondary battery using the produced negative electrode mixture layer;
It encompasses,
Here, the pressing is performed at a pressing pressure of 2 MPa to 10 MPa so that the specific surface area increase rate of the carbon material is at least 20% or more . ^2. The manufacturing method according to claim 1, wherein in the paste application step, a coating amount per unit area of the current collector applied to the negative electrode current collector is 7.5 mg / cm ² or less. . In the negative electrode mixture layer formation step, according to claim 1 or 2, characterized in that the surface area of the collector per unit area after pressing of the negative electrode mixture layer and ^{200cm 2 / cm 2 ~230cm 2 /} cm 2 The manufacturing method as described in.   The manufacturing method according to claim 1, wherein the pressing increases a specific surface area of the carbon material by 20% or more and 50% or less.

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