JP3349373B2 - Lithium secondary battery - Google Patents

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, and more particularly, to an improvement in a lithium ion storage material used for a negative electrode for the purpose of obtaining a lithium secondary battery having excellent charge / discharge cycle characteristics. .

[0002]

2. Description of the Related Art In recent years,
Unlike alkaline rechargeable batteries, such as nickel-cadmium rechargeable batteries, which use an aqueous alkaline solution as the electrolyte, lithium secondary batteries do not need to consider the decomposition voltage of water. For reasons, attention has been paid.

Conventionally, metallic lithium was initially considered as a negative electrode material for this type of secondary battery. When this negative electrode material is used, lithium is deposited on the surface of the lithium negative electrode during charging, and the lithium deposited during discharging becomes lithium ions and elutes into the electrolyte. However, when metal lithium is used, an internal short circuit may occur due to the growth of dendritic lithium. Therefore, in a practical battery, a carbon material that does not have such a problem and that can electrochemically occlude and release lithium ions during charge and discharge is used.

[0004] Typical carbon materials capable of electrochemically storing and releasing lithium ions include:
There are a coke type and a graphite type. The coke type carbon material has a specific capacity of about 200 mAh / g, and the graphite type carbon material has a specific capacity of about 370 mAh / g.

[0005] Recently, a demand for a longer charging interval for a power supply for a portable device or the like has led to a demand for higher battery capacity. For this reason, tin oxides such as SnO ₂ and SnO, which have a larger specific capacity than the carbon material,
It has been proposed as a lithium ion storage material for a negative electrode (see JP-A-7-122274).

However, a lithium secondary battery using tin oxide as a lithium ion storage material for a negative electrode has a problem that the charge / discharge cycle characteristics are extremely poor because the crystal structure of tin oxide is extremely unstable. was there.

[0007] Tin oxide having excellent charge-discharge cycle characteristics includes tin, lithium, sodium, potassium, magnesium, calcium, titanium, zirconium, vanadium, and tin, which are less likely to undergo a crystal structure collapse due to repeated charge and discharge. Niobium, tantalum, molybdenum, tungsten, manganese, iron, rhodium, iridium, copper, silicon,
A composite oxide with germanium or bismuth has been proposed (see JP-A-6-338325). When this composite oxide is used, the charge / discharge cycle characteristics can be considerably improved as compared with the case where the tin oxide is used.

However, the above-mentioned composite oxide is apt to be overcharged because the potential change at the end of charging is extremely sharp. Overcharging contributes to capacity degradation. Therefore, the present inventors have considered that if the capacity deterioration due to overcharging is suppressed, the charge / discharge cycle characteristics can be further improved.

The present invention has been made based on such findings, and has as its object to provide a lithium secondary battery having extremely excellent charge / discharge cycle characteristics.

[0010]

The lithium secondary battery according to the present invention for achieving the above object (hereinafter referred to as "battery of the present invention") comprises tin, lithium, sodium, potassium, magnesium, Calcium, titanium, zirconium, vanadium, niobium, tantalum, molybdenum, tungsten, manganese, iron, rhodium, iridium,
Possible copper, zinc, boron, aluminum, silicon, phosphorus, and composite oxide of the element M ¹ of at least one selected from the group consisting of germanium and bismuth, be electrochemically occluding and releasing lithium ions A mixture having a weight ratio of 4: 1 to 3: 2 with the carbon material (excluding the composite oxide coated with the carbon material) was used as a lithium ion storage material.

The composite oxide of tin and the specific element M ¹ absorbs lithium ions during charging and releases lithium ions during discharging. As a composite oxide of tin and a specific element M ¹ , a composition formula: Sn _x M ¹ _y O ₂ (where M ¹ is lithium, sodium, potassium, magnesium, calcium, titanium, zirconium, vanadium, niobium, At least one element selected from the group consisting of tantalum, molybdenum, tungsten, manganese, iron, rhodium, iridium, copper, zinc, boron, aluminum, silicon, phosphorus, germanium and bismuth; 0.2 ≦ y / x ≦
5; 0 <x <2).

In the battery of the present invention, a mixture of a composite oxide of tin and a specific element M ¹ and a carbon material in a weight ratio of 4: 1 to 3: 2 (provided that the carbon oxide is coated with the composite oxide) Is used as a lithium ion storage material for the negative electrode. If the amount of the carbon material in the mixture is too small, a rapid drop in the negative electrode potential at the end of charging cannot be sufficiently suppressed, so that the charge / discharge cycle characteristics deteriorate due to overcharging. On the other hand, if the amount of the carbon material in the mixture is too large, a rapid increase in the negative electrode potential at the end of discharge cannot be suppressed, so that the charge / discharge cycle characteristics are reduced due to overdischarge and the specific capacity is large. the amount of the composite oxide of the element M ¹ of to reduce the battery capacity is reduced.

As a carbon material capable of electrochemically occluding and releasing lithium ions, the crystallite size (Lc) in the c-axis direction is 400 ° in order to obtain a lithium secondary battery having a large discharge capacity. As described above, a carbon material having a large specific capacity (a carbon material having a high degree of graphitization) with a lattice spacing (d ₀₀₂ ) of the lattice plane (002) of 3.35 to 3.37 ° is preferable.

A feature of the present invention is that in order to provide a lithium secondary battery having extremely excellent charge / discharge cycle characteristics, a specific mixture having a gradual change in potential at the end of charging is used as a lithium ion storage material for a negative electrode. is there. Therefore, other materials constituting the battery such as the positive electrode material and the electrolyte are not particularly limited, and various materials conventionally used or proposed for lithium secondary batteries can be used. .

As the positive electrode material (positive electrode active material), LiC
_{oO 2, LiNiO 2, LiFeO 2} , LiMn 2 O 4
And the like.

As the electrolyte, ethylene carbonate,
Diethyl carbonate, dimethyl carbonate, 1,2
- dimethoxyethane, sulfolane, or a mixture of these _{solvents, LiPF 6, LiClO 4, LiCF} 3 SO 3
A solution in which 0.7 to 1.5 mol / liter of an electrolyte salt such as described above is dissolved is exemplified.

In the battery of the present invention, as the lithium ion storage material of the negative electrode, a composite oxide of tin and a specific element M ¹ having a sharp change in potential at the end of charging, and a carbon material having a gradual change in potential at the end of charging are used. Is used (except that the carbon material is coated with the composite oxide), so that a mixture of tin and the specific element M ¹ is used alone. In addition, the potential change of the negative electrode at the end of charging is gradual, and overcharging is difficult. For this reason, the battery of the present invention hardly causes a decrease in capacity due to overcharge, and exhibits extremely excellent charge / discharge cycle characteristics.

[0018]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the following Examples, and may be carried out by appropriately changing the scope of the invention without changing its gist. Is possible.

[Preparation of Positive Electrode] LiCoO ₂ as a positive electrode active material and a carbon material as a conductive agent were mixed at a weight ratio of 18: 1.
Parts by weight and 5 parts by weight of polyvinylidene fluoride in NMP (N
-Methyl-2-pyrrolidone) solution to prepare a slurry, and apply the slurry to both surfaces of an aluminum foil as a positive electrode current collector by a doctor blade method.
Vacuum drying was performed at 0 ° C. for 2 hours to produce a positive electrode.

[Preparation of Negative Electrode] Stannic oxide (SnO ₂ ) and lithium oxalate (Li ₂ C)
₂ O ₄ ) in a molar ratio of 1: 1 in air at 90
Calcination was performed at 0 ° C. for 1 hour to obtain a composite oxide. The element M ¹ of this composite oxide is lithium. This composite oxide,
Natural graphite powder (Lc>1000Å; d ₀₀₂ = 3.356)
Å; average particle size 10 μm) or coke powder (Lc = 15)
{; D ₀₀₂ = 3.40}; average particle size of 10 μm) and a weight ratio of 19: 1, 9: 1, 4: 1, 7: 3, 3: 2, and 1:
1,2: 3 to prepare a lithium ion storage material. A slurry was prepared by kneading 90 parts by weight of this negative electrode material and an NMP solution of 10 parts by weight of polyvinylidene fluoride, and this slurry was applied to both surfaces of a copper foil as a negative electrode current collector by a doctor blade method, C was vacuum dried for 2 hours to produce a negative electrode.

[Preparation of Electrolyte Solution] LiP was added to a mixed solvent of ethylene carbonate (EC) and 1,2-dimethoxyethane (DME) at a volume ratio of 1: 1.
F ₆ was dissolved at 1 mol / liter to prepare an electrolytic solution.

[Preparation of Battery] Using the above positive electrode, negative electrode and electrolytic solution, cylindrical lithium secondary batteries A1, A2,..., A7 and A11, A22,
..., A77 was produced. The battery dimensions are an outer diameter of 14 mm and a height of 50 mm. A microporous polypropylene membrane was used as a separator. The batteries A1, A2, ..., A
7 is a battery using natural graphite powder as a carbon material, and batteries A11, A22,..., A77 are batteries using coke powder as a carbon material. That is, a battery code with a one-digit number after an alphabet is a battery using natural graphite powder as a carbon material, and a battery with a zigzag number is a battery using coke powder as a carbon material ( The same applies to the following batteries). Also,
Batteries A5, A6, A7, A55, A66, A77 are the batteries of the present invention, and batteries A1, A2, A3, A4, A11,
A22, A33, and A44 are comparative batteries.

FIG. 1 is a schematic cross-sectional view of a manufactured lithium secondary battery. The illustrated battery BA has a positive electrode 1, a negative electrode 2, a separator 3 separating them from each other, a positive electrode lead 4, a negative electrode lead 5, and a positive external terminal. 6, a negative electrode can 7 and the like. The positive electrode 1 and the negative electrode 2 are housed in a negative electrode can 7 while being spirally wound through a separator 3 into which a non-aqueous electrolyte is injected. The terminal 6 and the negative electrode 2 are connected to a negative electrode can 7 via a negative electrode lead 5, so that chemical energy generated inside the battery can be extracted to the outside as electric energy.

Table 1 shows the weight ratio of the composite oxide used as the lithium ion storage material to the negative electrode of each battery to the carbon material.

[0025]

[Table 1]

<Discharge Capacity and Charge / Discharge Cycle Characteristics of Each Battery> The process of charging each battery to 4.1 V at 200 mA at room temperature and then discharging it to 2.75 V at 200 mA was repeated 200 times for each battery. The capacity deterioration rate per cycle up to the 200th cycle was calculated based on the following equation. Table 1 shows the capacity deterioration rate (% / cycle) of each battery and the discharge capacity (mAh) at the 200th cycle. The number below the battery code is the capacity deterioration rate, and the number further below is the discharge capacity at the 200th cycle.

Capacity deterioration rate = {(discharge capacity at first cycle-discharge capacity at 200th cycle) / discharge capacity at first cycle} charge / discharge cycle (199 cycles) × 100

According to Table 1, when the mixing ratio of the composite oxide and the carbon material is regulated in a range of 4: 1 to 3: 2 by weight,
It can be seen that a battery having an extremely low capacity deterioration rate and extremely excellent charge / discharge cycle characteristics can be obtained. In addition, in order to obtain a battery having not only excellent charge / discharge cycle characteristics but also a large discharge capacity, as a carbon material used in combination with the composite oxide,
It can be seen that it is preferable to use a carbon material having a high degree of graphitization.

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 shape of the battery is not particularly limited, and can be applied to lithium secondary batteries of various shapes such as a flat shape and a square shape.

Although a liquid electrolyte is used in the above embodiment, the present invention is also applicable to a solid electrolyte battery.

[0031]

According to the battery of the present invention, since the potential change of the negative electrode at the end of charging is gradual and it is difficult to overcharge, the capacity is hardly reduced due to overcharging, and the battery exhibits extremely excellent charge / discharge cycle characteristics.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a cylindrical lithium secondary battery manufactured in an example.

[Explanation of symbols]

 BA lithium secondary battery 1 positive electrode 2 negative electrode 3 separator 4 positive electrode lead 5 negative electrode lead 6 positive external terminal 7 negative electrode can

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masahisa Fujimoto 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Toshiyuki Noma 2-5-2 Keihanhondori, Moriguchi-shi, Osaka No. 5 Sanyo Electric Co., Ltd. (72) Inventor Koji Nishio 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (56) References JP-A-6-338325 (JP, A) JP-A-10-21913 (JP, A) JP-A-8-78003 (JP, A) JP-A-8-130036 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01M 4 / 00-4/62 H01M 10/40

Claims (2)

(57) [Claims] 1. Tin and lithium, sodium, potassium,
Magnesium, calcium, titanium, zirconium, vanadium, niobium, tantalum, molybdenum, tungsten, manganese, iron, rhodium, iridium, copper, zinc,
Boron, aluminum, silicon, phosphorus, and a composite oxide of at least one element M ¹ selected from the group consisting of germanium and bismuth, a carbon material capable of electrochemically occluding and releasing lithium ions Weight ratio 4:
A lithium secondary battery in which a mixture of 1 to 3: 2 (except for the case where the carbon material is coated with the composite oxide) is used as a lithium ion storage material for a negative electrode. 2. The carbon material having a crystallite size (Lc) of at least 400 ° in the c-axis direction and a lattice spacing (d ₀₀₂ ) of lattice planes ( ₀₀₂ ) of 3.35 ° to 3.37 °. The lithium secondary battery according to claim 1, wherein