JP2000228202A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the area of contact between a carbon material and a collector, enhance a collecting effect of the collector, and obtain a lithium ion storage material using carbon material as a negative electrode, which has high efficient discharging characteristic by forming a plated coat made of metal material on one side of the negative electrode as a collector. SOLUTION: As a plated coat 21 formed as a collector on one side of a sheet-like negative electrode 23 having carbon material as a lithium ion storage material, nickel or copper plated coat is preferable because conductivity is good and the collecting effect is large. As plating for the negative electrode formed of a resin such as a binder into a sheet shape, chemical plating is preferable because uneven plating is hardly caused. The proportion of the negative electrode 23 and the plated coat 21 is preferably 60:40-80:20 by atom ratio of carbon to a metal element. In a partial section of the sheet-like negative electrode where the plated coat is formed, the plated coat 21 is formed to closely adhere to the projecting and recessed shape of a carbon layer which is positioned on the surface part of the sheet-like negative electrode 22 and formed of carbon particles 23.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery including a positive electrode, a sheet-like negative electrode having a carbon material as a lithium ion storage material, and a non-aqueous electrolyte.
More specifically, the present invention relates to improvement of a negative electrode current collector for the purpose of improving discharge characteristics when discharging at a high current density, that is, high-rate discharge characteristics (load characteristics).

[0002]

2. Description of the Related Art In recent years,
A lithium secondary battery using a carbon material as the lithium ion storage material for the negative electrode has been put on the market because there is no internal short-circuit due to electrodeposition of dendritic lithium on the negative electrode during charging and good charge / discharge cycle characteristics. ing.

Conventionally, a negative electrode (carbon negative electrode) using a carbon material as a lithium ion storage material is prepared by applying a slurry containing a carbon material and a binder to a metal foil (current collector) such as copper or stainless steel, and drying the slurry. Then, it was produced by forming a carbon layer on a metal foil.

However, in the bonding between the carbon material and the current collector by the binder, the current collecting effect is small because the contact area between the carbon material and the current collector is small. For this reason, the lithium secondary battery using the carbon negative electrode has a problem that high-rate discharge characteristics are not good.

[0005] As a carbon anode having enhanced conductivity, a carbon layer (negative electrode) containing a carbon material mainly composed of graphite, a specific metal element such as gold, silver and copper, and a binder is collected. An article coated and formed on a body has been proposed (JP-A-8-4554).
No. 8). According to this carbon anode, since the carbon layer contains the specific metal element, a decrease in electrical conductivity caused by contact resistance between carbon particles and anisotropy of graphite is prevented. .

However, in the above-described carbon negative electrode, the contact area between the carbon material and the current collector is small because the adhesion between the current collector and the negative electrode depends on the binder as in the conventional case. The effect is small. Therefore, even though the conductivity inside the negative electrode is improved, the current collecting effect of the current collector is small,
It is difficult to sufficiently improve the high rate discharge characteristics. In particular, when a low-crystalline, low-crystallinity, non-graphitizable carbon material disclosed in Japanese Patent Application Laid-Open No. 7-335262 is used as a carbon material in order to increase the capacity, the high-rate discharge characteristics are reduced. It decreases significantly.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a lithium secondary battery having a good high-rate discharge characteristic and using a carbon material as a lithium ion storage material for a negative electrode. This object is achieved by increasing the contact area between the carbon material and the current collector and increasing the current collection effect of the current collector, as described below.

[0008]

A lithium secondary battery (battery of the present invention) according to the present invention includes a positive electrode, a sheet-like negative electrode having a carbon material as a lithium ion storage material, and a non-aqueous electrolyte. A plating film made of a metal material is formed on one surface of the negative electrode as a current collector. In addition, in this specification, a sheet shape is a term including a sheet shape, a film shape, and a foil shape.

[0009] The battery of the present invention has a large contact area between the carbon material and the current collector and a large current collecting effect, and therefore has good high-rate discharge characteristics. The reason why the contact area between the carbon material and the current collector is large is that
This is because the plating film as the current collector is formed in close contact with the uneven shape of the carbon layer on the negative electrode surface.

For a sheet-like negative electrode having a carbon material as a lithium ion storage material, for example, a slurry prepared by kneading a carbon material and a binder solution is applied on a support,
After drying to form a sheet, the sheet can be obtained by peeling the sheet from the support. A typical example of the carbon material is graphite. But,
Graphite has a theoretical capacity of 3 even in the case of graphite with a very high degree of graphitization, in which most of the carbon is converted to C ₆ Li at the end of charging.
Since 72mAh / g and less, in order to obtain a lithium secondary battery of high capacity, spacing of lattice planes (002) plane d ₀₀₂
It is preferable to use a low-crystalline carbon material having a thickness of 3.50 to 4.00 °. Such a low-crystalline carbon material is suitable for increasing the capacity because lithium is occluded not only between the layers but also in the voids between the crystal lattices. However, the low-crystalline carbon material has low conductivity, so there is a concern that the high-rate discharge characteristics may decrease.However, in the case of the battery of the present invention, the current collector has a large current collection effect, and thus has a high capacity. The use of a low-crystalline carbon material provides a lithium secondary battery having good high-rate discharge characteristics.

As the plating film, a nickel plating film or a copper plating film is preferable because of good conductivity and large current collecting effect. Metal plating includes chemical plating (electroless plating) and electroplating. For plating of a negative electrode formed into a sheet shape with a resin such as a binder, chemical plating that is unlikely to cause plating unevenness is preferable.

The ratio between the negative electrode and the plating film is preferably from 60:40 to 80:20 in the atomic ratio between carbon and the metal element. If the plating film becomes too thick, the carbon material content decreases and the negative electrode capacity decreases.On the other hand, if the plating film is too thin, the current collecting effect cannot be sufficiently increased, so that the high-rate discharge characteristics are sufficiently increased. It will be difficult to improve.

A feature of the present invention resides in that a plating film is formed as a current collector on one surface of a sheet-shaped negative electrode having a carbon material as a lithium ion storage material. Therefore, for other battery members such as the positive electrode and the non-aqueous electrolyte, conventionally known materials for lithium secondary batteries can be used.

The active material of the positive electrode is LiCoO_Two, L
iNiO_Two, LiFeO_Two, LiMn_TwoO_FourAnd LiC
o_XNi_YMn_ZO_Two(0 ≦ X <1; 0 ≦ Y <1; 0 ≦
Z <1; X + Y + Z = 1; at least among X, Y and Z
Are also positive). In addition, the solvent of the non-aqueous electrolyte
Are ethylene carbonate, propylene carbonate
G, vinylene carbonate, butylene carbonate, etc.
Cyclic carbonate, cyclic carbonate and dimethyl carb
, Diethyl carbonate, methyl ethyl carbonate
1,2-diethoxyethane, 1,2-dimethoxy
Mixing with low-boiling solvents such as ethane and ethoxymethoxyethane
A solvent is exemplified. As the solute of the non-aqueous electrolyte, Li
PF₆, LiBF_Four, LiClO_Four, LiCF_ThreeS
O_Three, LiSbF₆, LiAsF₆, LiN (C_mF
_{2m + 1}SO_Two) (C_nF_{2n + 1}SO_TwoWhere m and n are
Each independently an integer from 1 to 5; ], LiC (C_pF_{2p + 1}SO
_Two) (C _qF_{2q + 1}SO_Two) (C_rF_{2r + 1}SO_Two)〔formula
Wherein p, q and r are each independently an integer of 1 to 5. ] Is an example
Is done.

[0015]

EXAMPLES The present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples and can be carried out by appropriately changing the scope of the invention without changing its gist. It is something.

(Experiment 1) A battery of the present invention and a comparative battery were manufactured, and high-rate discharge characteristics of each battery were examined.

(Preparation of Battery A1 of the Present Invention) [Preparation of Positive Electrode] LiCoO ₂ as a positive electrode active material and artificial graphite as a conductive agent were mixed at a weight ratio of 18: 1.
A slurry was prepared by kneading 95 parts by weight of the obtained mixture and a solution of 5 parts by weight of polyvinylidene fluoride as a binder in NMP (N-methyl-2-pyrrolidone) to prepare a slurry. An aluminum foil as an electrical conductor was applied to both sides by a doctor blade method, and dried in a vacuum at 150 ° C. for 2 hours to produce a sheet-shaped positive electrode.

[Preparation of Negative Electrode] A petroleum pitch is baked in an argon atmosphere at 700 ° C. for 10 hours and pulverized to prepare a carbon powder having a lattice spacing (002) plane spacing d ₀₀₂ of 3.50 °. did. 90 parts by weight of this carbon powder and a solution of 10 parts by weight of polyvinylidene fluoride dissolved in NMP are kneaded to prepare a slurry, and this slurry is used as a copper foil (support).
Is coated at a density of 0.50 g / cm ^{2 by} a doctor blade method, dried in a vacuum at 150 ° C. for 2 hours, rolled, and peeled off from a copper foil to form a 40 μm thick sheet-shaped negative electrode. Was prepared.

[Formation of Plating Film] Before plating, the unplated portion of the above sheet-shaped negative electrode was masked. FIG. 1 is a perspective view showing a state of masking. As shown in FIG. 1, a fluororesin sheet 2 is coated on one surface of a sheet-shaped negative electrode 1 so as to cover the entire surface, and a negative electrode 1 is coated on the other surface. , Fluororesin sheet 3 with hollow center
, And pressurized to perform masking. In the experiments described below, the same masking was performed before plating.

The masked sheet-shaped negative electrode was immersed in an aqueous methanol solution for 5 minutes to wash the surface to be plated, and then dried at 60 ° C. for 1 hour. Next, the sheet-shaped negative electrode was immersed in a mixed solution of stannous chloride (SnCl ₂ · 2H ₂ O) and hydrochloric acid for 2 hours, and then palladium chloride (PdC
l ₂ · 2H ₂ O) and then immersed for 2 hours in a mixture of hydrochloric acid to precipitate a plating growth nuclei (activation treatment). Next, after immersion in a methanol aqueous solution for 5 minutes for washing, immersion for 90 minutes in a nickel plating bath (bath temperature: 60 ° C.) of the following composition adjusted to a pH of about 10 with aqueous ammonia for nickel plating, The mask was removed, washed with an aqueous methanol solution, and heat-treated at 300 ° C. for 1 hour in a vacuum to prepare a sheet-shaped negative electrode having a nickel plating film as a current collector formed on one surface.

(Bath composition) Nickel chloride hexahydrate 0.1 mol / L Sodium pyrophosphate decahydrate 0.1 mol / L Water balance

When the cross section of the sheet-shaped negative electrode was observed with a scanning electron microscope, the thickness of the plating film (any 10
Average thickness at the point) was 8.9 μm. FIG.
FIG. 1 is a schematic partial cross-sectional view of a sheet-shaped negative electrode on which a nickel plating film is formed, observed by a scanning electron microscope. As shown in the drawing, a plating film 21 is located on the surface of a sheet-shaped negative electrode 22. Carbon particles 23, 23, ..., 23
Was formed in close contact with the uneven shape of the carbon layer formed. Further, a sheet-shaped negative electrode was punched out, a part was taken out, immersed in hydrochloric acid to dissolve the nickel plating film, and the atomic ratio between carbon and nickel was calculated from the weight of the sample before and after dissolution. there were.

[Preparation of Nonaqueous Electrolyte] A nonaqueous electrolyte was prepared by dissolving 1 mol / l of LiPF ₆ in a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1.

[Preparation of Lithium Secondary Battery] The above two sheet-shaped negative electrodes having a nickel plating film formed on one surface were
The nickel-plated film is superposed on the inner side, the separator, and the positive electrode are laminated in this order, and the obtained laminate is spirally wound to form a cylindrical battery can. And after injecting the above non-aqueous electrolyte,
The lid was closed and sealed to produce a cylindrical lithium secondary battery A1 (battery of the present invention) having an outer diameter of 14 mm and a height of 50 mm. A polypropylene microporous film was used as the separator.

(Preparation of Comparative Battery B1) 90 parts by weight of the same carbon powder as used for preparing the negative electrode of the battery A1 of the present invention and a solution obtained by dissolving 10 parts by weight of polyvinylidene fluoride as a binder in NMP were used. Knead to prepare a slurry,
This slurry was applied on both sides of a copper foil as a current collector at a density of 0.50 g / cm ² per side by a doctor blade method, and dried in a vacuum at 150 ° C. for 2 hours to incorporate a current collector. A sheet-shaped negative electrode was produced. Comparative battery similar to Battery A1 of the present invention except that one sheet-shaped negative electrode containing a current collector was used instead of two sheet-shaped negative electrodes having a nickel plating film formed on one surface. B1
Was prepared.

<High-rate discharge characteristics> At room temperature, the battery was charged to 4.1 V at 0.4 C, and then discharged to 2.75 V at 4 C, and the discharge capacity of each battery was examined. Table 1 shows the results.

[0027]

[Table 1]

As shown in Table 1, the discharge capacity of the battery A1 of the present invention is much larger than that of the comparative battery B1. This indicates that the present invention provides a lithium secondary battery having good high-rate discharge characteristics.

[0029] were examined (experiment 2) the relationship of the surface spacing d ₀₀₂ and the high-rate discharge characteristics of the lattice plane (002) plane of the carbon material.

A petroleum-based pitch is placed in an argon atmosphere,
550 ° C, 590 ° C, 700 ° C, 1100 ° C,
It is baked at 1900 ° C. or 2800 ° C. for 10 hours, pulverized, and the lattice distance (002) plane spacing d ₀₀₂ is 3.85 °, 3.70 °, 3.50 °, 3.45, respectively.
Six types of carbon powders of {3.40} and 3.36 ° were produced. A lithium secondary battery A2 similar to the battery A1 of the present invention was prepared in the same manner as the battery A1 of the present invention except that the carbon powder having the lattice spacing (002) plane spacing d ₀₀₂ of 3.50 ° was used in place of the carbon powder in preparing the negative electrode. To A6 (the battery of the present invention). Next, a charge / discharge test was performed under the same conditions as in Experiment 1, and the high-rate discharge characteristics of each battery were examined. Table 2 shows the results. Table 2
Shows the results of the battery A1 of the present invention.

[0031]

[Table 2]

From Table 2, it can be seen that in order to obtain a lithium secondary battery having good high-rate discharge characteristics, the lattice spacing (002) plane distance d
It is understood that it is preferable to use a carbon material having a ₀₀₂ of 3.50 ° or more.

(Experiment 3) The relationship between the type of metal material forming the plating film and the high-rate discharge characteristics was examined.

According to the following plating methods, the battery A1 of the present invention was
A copper plating film, an iron plating film, or a cobalt plating film was formed on one surface of the same sheet-shaped negative electrode as that used in Example 1. The average thickness of each of the plating films at arbitrary 10 points was 9.6 μm, 9.5 μm, and 8.9 μm, respectively. The atomic ratio of carbon to nickel is 7
It was 0:30.

(Copper plating) The negative electrode in the form of a sheet is masked and activated, and then the pH is adjusted to about 11 by adding sodium hydroxide (bath temperature:
(50 ° C) for 90 minutes to perform copper plating. Next, the masking was removed, washed with an aqueous methanol solution, and heat-treated at 300 ° C. for 1 hour in vacuum to prepare a sheet-shaped negative electrode having a copper plating film as a current collector formed on one surface.

(Bath composition) Copper sulfate pentahydrate 0.03 mol / l Sodium potassium tartrate tetrahydrate 0.3 mol / l sodium carbonate monohydrate 0.08 mol / l formalin (37% by weight) 25 ml / liter water balance

(Iron plating) The negative electrode in the form of a sheet was masked and activated, and then the pH was adjusted to about 10 by adding sodium hydroxide to the plating bath having the following composition (bath temperature:
(75 ° C.) for 85 minutes to perform iron plating. Next, the masking was removed, washed with an aqueous methanol solution, and heat-treated at 300 ° C. for 1 hour in a vacuum to produce a sheet-shaped negative electrode having an iron plating film as a current collector formed on one surface.

(Bath composition) Ferrous sulfate heptahydrate 0.1 mol / l Sodium potassium tartrate tetrahydrate 0.2 mol / l sodium hypochlorite monohydrate 0.1 mol / l water Rest

(Cobalt Plating) The negative electrode in the form of a sheet is masked and activated, and the pH is adjusted to about 9 by adding sodium hydroxide (bath temperature: 90 ° C.). For 85 minutes to perform cobalt plating. Next, the masking was removed, washed with an aqueous methanol solution, and subjected to a heat treatment at 300 ° C. for 1 hour in vacuum to prepare a sheet-shaped negative electrode having a cobalt plating film as a current collector formed on one surface.

(Bath composition) Cobalt sulfate heptahydrate 0.07 mol / L Sodium citrate dihydrate 0.2 mol / L Ammonium sulfate 0.6 mol / L Sodium hypochlorite monohydrate 2 mol / l water balance

The battery A of the present invention was the same as the battery A of the present invention except that a sheet-shaped negative electrode having the above-mentioned plating films formed on one surface was used instead of the sheet-shaped negative electrode having a nickel plating film formed on one surface.
Lithium secondary battery A
7 to A9 were prepared. Next, a charge / discharge test was performed under the same conditions as in Experiment 1, and the high-rate discharge characteristics of each battery were examined. Table 3 shows the results. Table 3 also shows the results of the battery A1 of the present invention.

[0042]

[Table 3]

Table 3 shows that in order to obtain a lithium secondary battery having good high rate discharge characteristics, it is preferable to use nickel or copper as a metal material for forming a plating film.

(Experiment 4) The relationship between the thickness of the plating film and the high rate discharge characteristics was examined.

A slurry was prepared by kneading 90 parts by weight of the same carbon powder as used for producing the negative electrode of the battery A1 of the present invention and an NMP solution containing 10 parts by weight of polyvinylidene fluoride as a binder. 0.02 g / cm of slurry
² , 0.07 g / cm ² , 0.17 g / cm ² , 0.3
3 g / cm ² , 0.85 g / cm ² , 1.21 g / cm
² or 1.94 g / cm ^{2 at} a density of 1 on a copper foil (support) on one side by a doctor blade method.
After drying at 50 ° C. for 2 hours, rolling, and peeling from the copper foil, the thickness was 11 μm, 17 μm, 21 μm, 26 μm in order.
m, 68 μm, 97 μm and 155 μm sheet-shaped negative electrodes were produced. Lithium secondary batteries A10 to A16 (batteries of the present invention) in the same manner as in the method of producing electrode A1 of the present invention, except that the above-described sheet-shaped negative electrodes were used instead of the sheet-shaped negative electrodes of 40 μm in thickness. Was prepared. The atomic ratio of carbon to nickel is, in order, 40:60, 50: 5
0, 55:45, 60:40, 80:20, 85:15
And 90:10. Next, a charge / discharge test was performed under the same conditions as in Experiment 1, and the high-rate discharge characteristics of each battery were examined. Table 4 shows the results. Table 4 also shows the results of the battery A1 of the present invention.

[0046]

[Table 4]

As shown in Table 4, the batteries A1 and A13 to A of the present invention were obtained.
Since the discharge capacity of No. 16 is particularly large, it is understood that the ratio between the negative electrode and the nickel plating film is preferably 60:40 to 80:20 in terms of the atomic ratio of carbon and nickel. When using other metals such as copper, iron, and cobalt as the metal material of the plating film, it is preferable to adjust the thickness of the plating film so that the atomic ratio between carbon and the metal is within the above range. It was confirmed.

In the above embodiment, the case where the present invention is applied to a cylindrical lithium secondary battery has been described as an example.
The present invention is not particularly limited in the shape of the battery, and is applicable to lithium secondary batteries having various shapes such as a flat shape.

Although a non-aqueous electrolyte is used in the above embodiment, the present invention is also applicable to a lithium secondary battery using a gel polymer electrolyte or a solid electrolyte.

[0050]

According to the present invention, there is provided a lithium secondary battery having a high rate discharge characteristic and using a carbon material as a lithium ion storage material for a negative electrode.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a state of masking performed before plating.

FIG. 2 is a schematic partial cross-sectional view of a sheet-shaped negative electrode on which a plating film is formed.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Sheet-shaped negative electrode 2 Fluororesin sheet (masking material) 3 Fluororesin sheet (masking material) hollowed out in the center 21 Plating film 22 Sheet-shaped negative electrode 23 Carbon particles

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toshiyuki Noma 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Koji Nishio 2-chome Keihanhondori, Moriguchi-shi, Osaka No. 5-5 Sanyo Electric Co., Ltd. F-term (reference) 5H003 AA01 BA07 BB02 BB14 BC01 BC05 BC06 BD02 BD03 5H014 AA04 BB11 CC01 EE05 EE07 HH01 HH06 5H017 AA03 AS10 BB16 CC01 EE04 HH01 HH03 5H029 AJ03 AM04 BJ04 BJ12 BJ14 CJ24 DJ17 EJ01 EJ04 HJ02 HJ04 HJ13

Claims (5)

[Claims] 1. A lithium secondary battery comprising a positive electrode, a sheet-shaped negative electrode having a carbon material as a lithium ion storage material, and a non-aqueous electrolyte, wherein a plating film made of a metal material is formed on one surface of the negative electrode. A lithium secondary battery formed as an electric body. 2. A face spacing d ₀₀₂ of the lattice plane (002) plane of the carbon material, a lithium secondary battery according to claim 1 wherein the 3.50～4.00A. 3. The lithium secondary battery according to claim 1, wherein said metal material is nickel or copper. 4. The lithium secondary battery according to claim 1, wherein the ratio between the negative electrode and the plating film is 60:40 to 80:20 in atomic ratio between carbon and a metal element. 5. The lithium secondary battery according to claim 1, wherein said plating film is formed by chemical plating.