JPH08171934A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To obtain a battery having a superior shelf life under a charged state by using a low-viscosity solvent consisting of specific carbonate for a solvent for use in the nonaqueous electrolyte of a battery having positive electrodes and negative electrodes consisting of a carbon material. CONSTITUTION: A battery is provided with positive electrodes 1, negative electrodes 4 using a carbon material having a d value at the lattice plane (002) plane of 3.35-3.39Å and a crystallite size in the c axial direction of at least 50Å as an electrode material, a nonaqueous electrolyte, and separators 3. A solvent for the nonaqueous electrolyte uses a mixed solvent consisting of a low-viscosity solvent 70-30vol.% selected from a group consisting of ethylene carbonate 30-70vol.%, methyl propyl carbonate, methyl isopropyl carbonate, and ethyl propyl carbonate. Thus, less self-discharge is evolved even if the battery is stored under a charged state to depress a decrease in discharge capacity.

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 provided with a negative electrode using a carbon material as an electrode material, and more specifically, to provide a lithium secondary battery having excellent storage characteristics in a charged state. The present invention relates to improvement of the solvent of the electrolytic solution (non-aqueous electrolytic solution).

[0002]

2. Description of the Related Art In recent years,
Carbon materials capable of inserting and extracting lithium ions, such as graphite, coke, amorphous carbon, and non-graphitizable carbon, are different from the metallic lithium that has been used in the past and are responsible for the growth of dendritic electrodeposited lithium. Since it has no fear of internal short circuit, it is attracting attention as a new negative electrode material for lithium secondary batteries.

In a lithium secondary battery (lithium ion battery) using a carbon material such as graphite as a negative electrode material, ethylene carbonate, which is a high dielectric constant solvent, and dimethyl, which is a low viscosity solvent, are used to increase the conductivity of the non-aqueous electrolyte. A mixed solvent with carbonate or diethyl carbonate is used.

However, when a lithium secondary battery using this type of solvent is stored in a charged state, the dimethyl carbonate or diethyl carbonate is decomposed (self-discharge) on the surface of the negative electrode, and therefore the discharge capacity is preserved. After that, it had a problem of being significantly reduced.

The present invention has been made to solve this problem, and its object is to replace a specific acyclic carbonate ester with dimethyl carbonate or diethyl carbonate as a low-viscosity solvent used in combination with ethylene carbonate. Is to provide a lithium secondary battery having excellent storage characteristics in a charged state.

[0006]

The lithium secondary battery (the battery of the present invention) according to the present invention for achieving the above object has a positive electrode and a d value (d ₀₀₂ ) on the lattice plane (002) plane. 3.35 to 3.39Å and a negative electrode using a carbon material having a crystallite size (Lc) in the c-axis direction of 50 Å or more as an electrode material, a non-aqueous electrolytic solution containing a solvent and a solute, and a separator A lithium secondary battery, wherein the solvent is 30 to 70% by volume of ethylene carbonate, and 70 to 30% by volume of a low-viscosity solvent selected from the group consisting of methylpropyl carbonate, methylisopropyl carbonate and ethylpropyl carbonate. It is a mixed solvent consisting of and.

The solvent of the non-aqueous electrolyte in the present invention is 30 to 70% by volume of ethylene carbonate and at least one low-viscosity solvent 70 selected from the group consisting of methylpropyl carbonate, methylisopropyl carbonate and ethylpropyl carbonate. It is a mixed solvent consisting of ˜30% by volume. When the volume ratio of ethylene carbonate to the low-viscosity solvent is out of the above range, the discharge capacity when stored in a charged state significantly decreases. This is because when the proportion of the low-viscosity solvent exceeds 70% by volume, the dielectric constant of the non-aqueous electrolytic solution decreases, while when the proportion is less than 30% by volume, the viscosity of the non-aqueous electrolytic solution increases. It is considered that this is because the conductivity of the non-aqueous electrolytic solution is lowered.

As the solute of the non-aqueous electrolyte in the present invention, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAs are used.
Examples are F ₆ , LiCF ₃ SO ₃ and LiN (CF ₃ SO ₂ ) ₂ . A suitable addition ratio of these solutes to the solvent is 0.5 to 3 mol / liter.

Examples of the positive electrode active material include lithium-containing manganese oxides (LiMnO ₂ , LiMn ₂ O _4, etc.), lithium-containing cobalt oxides (LiCoO _2, etc.), lithium-containing nickel oxides (LiNiO _2, etc.), Lithium-containing nickel-cobalt composite oxide (Li ₂ Ni
CoO _4, etc.), lithium-containing manganese / cobalt composite oxide, and lithium-containing manganese / nickel composite oxide.

The carbon material as the negative electrode material in the present invention is
The d value (d ₀₀₂ ) on the lattice plane (002) plane is 3.35.
~ 3.39Å, and the crystallite size in the c-axis direction (Lc)
Is over 50Å. A carbon material having a d value and Lc in this range has a high capacity and an advantage of providing a flat discharge potential, but has a drawback of causing decomposition of dimethyl carbonate and diethyl carbonate.

[0011]

[Function] As a low-viscosity solvent used together with ethylene carbonate, even if the battery is stored in a charged state, the negative electrode (carbon material)
Since a specific acyclic carbonic acid ester that is difficult to decompose on the surface of is used, self-discharge is less likely to occur. Therefore, the decrease in discharge capacity when stored in the charged state is suppressed.

[0012]

EXAMPLES The present invention will be described in more detail based on the following examples, but the invention is not intended to be limited to the following examples, and various modifications may be made without departing from the scope of the invention. Is possible.

(Examples 1 to 6) AA size lithium secondary battery (cell of the present invention) A1 (battery size: outer diameter 13.8 mm, height 4) using the following positive electrode, negative electrode and non-aqueous electrolyte.
8.9 mm) was assembled. A polyethylene microporous membrane was used as the separator.

[Positive electrode] To 85 parts by weight of LiCoO ₂ powder,
10 parts by weight of carbon powder (artificial graphite) as a conductive agent was mixed, and the resulting mixture was added to a 5% by weight N-methylpyrrolidone (NMP) solution containing 5 parts by weight of polyvinylidene fluoride (PVdF) as a binder. It is dispersed into a slurry, and this slurry is applied by a doctor blade method to an aluminum foil having a thickness of 20 μm as a positive electrode current collector, and the temperature is 100 °
It was dried at C to prepare a positive electrode.

[Negative electrode] Graphite powder (d ₀₀₂ = 3.35Å,
Lc = 1000 Å or more of natural graphite) 95 parts by weight of polyvinylidene fluoride as a binder 5 parts by weight of 5% by weight N-
A methylpyrrolidone solution was dispersed to form a slurry, and this slurry was applied to a copper foil having a thickness of 18 μm as a negative electrode current collector by a doctor blade method and dried at 100 ° C. to prepare a negative electrode.

[Non-aqueous electrolyte] ethylene carbonate (E
Lithium hexafluorophosphate (LiPF ₆ ) as a solute was dissolved at 1 mol / liter in a mixed solvent of C) and methylpropyl carbonate (MPC) at a volume ratio of 1: 1 to prepare a non-aqueous electrolytic solution.

FIG. 1 is a cross-sectional view of the assembled battery A1 of the present invention. The battery A1 shown in the drawing is a positive electrode 1, a negative electrode 2, a separator 3 for separating these electrodes 1, 2 from each other, a positive electrode lead 4, and a negative electrode lead. 5, a positive electrode external terminal 6, a negative electrode can 7 and the like.

The positive electrode 1 and the negative electrode 2 are housed in the negative electrode can 7 in a state of being spirally wound via the separator 3 impregnated with the non-aqueous electrolyte, and the positive electrode 1 is connected via the positive electrode lead 4. The positive electrode external terminal 6 and the negative electrode 2 are connected to the negative electrode can 7 via the negative electrode lead 5, respectively, so that the chemical energy generated in the battery can be extracted to the outside from the positive electrode external terminal 6 and the negative electrode can 7 as electric energy. It has become.

Example 2 Ethylene carbonate (E
A non-aqueous electrolyte was prepared by dissolving 1 mol / liter of lithium hexafluorophosphate as a solute in a mixed solvent of C: and methyl isopropyl carbonate (MiPC) at a volume ratio of 1: 1. A battery A2 of the invention was assembled in the same manner as in Example 1 except that this nonaqueous electrolytic solution was used as the electrolytic solution.

(Example 3) Ethylene carbonate (E
A non-aqueous electrolyte was prepared by dissolving 1 mol / liter of lithium hexafluorophosphate as a solute in a mixed solvent of C) and ethylpropyl carbonate (EPC) at a volume ratio of 1: 1. A battery A3 of the invention was assembled in the same manner as in Example 1 except that this nonaqueous electrolytic solution was used as the electrolytic solution.

Comparative Example 1 Ethylene carbonate (E
Volume ratio of C) to dimethyl carbonate (DMC) 1:
Lithium hexafluorophosphate as a solute was dissolved in the mixed solvent of 1 at 1 mol / liter to prepare a non-aqueous electrolytic solution. Comparative battery B1 was assembled in the same manner as in Example 1 except that this nonaqueous electrolytic solution was used as the electrolytic solution.

(Comparative Example 2) Ethylene carbonate (E
Volume ratio of C) to diethyl carbonate (DEC) 1:
Lithium hexafluorophosphate as a solute was dissolved in the mixed solvent of 1 at 1 mol / liter to prepare a non-aqueous electrolytic solution. Comparative battery B2 was assembled in the same manner as in Example 1 except that this nonaqueous electrolytic solution was used as the electrolytic solution.

[Storage characteristics when stored in a charged state] The batteries A1 to A3 of the present invention and the comparative batteries B1 and B2 were stored at 25 °
In C, after charging to 4.1V at 60mA, 20
The battery was discharged at 0 mA to 2.75 V, and the discharge capacity of each battery without storage was determined.

Then, each battery was charged to 4.1 V at 60 mA and stored in a constant temperature bath at 60 ° C. for 1 month, and then 2
The discharge capacity after storage at the state of charge of each battery was obtained by discharging to 2.75 V at 00 mA. The results are shown in Table 1.

[0025]

[Table 1]

As shown in Table 1, the batteries A1 to A3 of the present invention.
In contrast, the discharge capacity after storage in the charged state hardly decreased, whereas in the comparative batteries B1 and B2, the discharge capacity after stored in the charged state significantly decreased. From this, as a low-viscosity solvent used in combination with ethylene carbonate, instead of conventional dimethyl carbonate, by using methyl propyl carbonate, methyl isopropyl carbonate or ethyl propyl carbonate, graphite which easily decomposes the electrolytic solution as a negative electrode material, etc. It can be seen that even when the above carbon material is used, a lithium secondary battery having excellent storage characteristics in a charged state can be obtained.

[Relationship Between Solvent Ratio of Mixed Solvent and Storage Characteristics in Charge State] (1) Binary Mixed Solvent The volume ratio of ethylene carbonate to methyl propyl carbonate, methyl isopropyl carbonate or ethyl propyl carbonate is 100. : 0, 90: 1
0, 80:20, 70:30, 60:40, 40: 6
0, 30:70, 20:80, 10:90 or 0:10
A solvent two-component non-aqueous electrolytic solution was prepared in the same manner as in Example 1 except that the value was 0.

Separately, the volume ratio of ethylene carbonate to dimethyl carbonate or diethyl carbonate is 100: 0, 90:10, 80:20, 70: 3.
0, 60:40, 40:60, 30:70, 20: 8
A solvent two-component nonaqueous electrolytic solution was prepared in the same manner as in Comparative Example 1 except that the ratio was 0, 10:90, or 0: 100.

Then, a lithium secondary battery was assembled in the same manner as in Example 1 except that each of these nonaqueous electrolytic solutions was used as the electrolytic solution.

A storage characteristic test was conducted on each of these lithium secondary batteries under the same conditions as above, and the lithium secondary battery was charged at 60 ° C.
The discharge capacity after storage for 1 month was calculated. The results are shown in Figure 2.

FIG. 2 shows the relationship between the solvent ratio of the mixed solvent and the storage characteristics in the charged state, and the vertical axis shows the discharge capacity (mA) after storage.
3 is a graph showing h) and the ratio (volume%) of ethylene carbonate in the mixed solvent on the horizontal axis. In addition,
FIG. 2 shows batteries A1 to A3 of the present invention and comparative batteries B1 and B.
The results of No. 2 are also shown in Table 1 after transcription.

From FIG. 2, ethylene carbonate 30 to 7
When a mixed solvent of 0% by volume and 70 to 30% by volume of methylpropyl carbonate, methylisopropyl carbonate or ethylpropyl carbonate is used, a lithium secondary battery having excellent storage characteristics in a charged state can be obtained. I understand.

(2) Three component mixed solvent The volume ratio of ethylene carbonate to methyl propyl carbonate and methyl isopropyl carbonate is 1
00:00, 90:10, 80:20, 70:30, 6
0:40, 50:50, 40:60, 30:70, 2
A solvent three-component nonaqueous electrolytic solution was prepared in the same manner as in Example 1 except that the ratio was 0:80, 10:90, or 0: 100. The volume ratios of methyl propyl carbonate and methyl isopropyl carbonate were all 1: 1.

Then, a lithium secondary battery was assembled in the same manner as in Example 1 except that each of these nonaqueous electrolytic solutions was used as the electrolytic solution.

A storage characteristic test was carried out on each of these lithium secondary batteries under the same conditions as above, and at 60 ° C. in a charged state.
The discharge capacity after storage for 1 month was calculated. The results are shown in Fig. 3.

From FIG. 3, ethylene carbonate 30 to 7
It can be seen that when a mixed solvent of 0% by volume and 70 to 30% by volume of methyl propyl carbonate and methyl isopropyl carbonate is used, a lithium secondary battery having excellent storage characteristics in a charged state can be obtained. The volume ratio of ethylene carbonate to the mixed solvent of the three-component system of ethylene carbonate, methylpropyl carbonate and ethylpropyl carbonate and the mixed solvent of the three-component system of ethylene carbonate, methylisopropyl carbonate and ethylpropyl carbonate is 30 to 30%. It was confirmed that a lithium secondary battery having excellent storage characteristics in a charged state was obtained when the content was 70% by volume.

(3) 4-component mixed solvent The volume ratio of ethylene carbonate to methyl propyl carbonate, methyl isopropyl carbonate and ethyl propyl carbonate is 100: 0 and 90: 1.
0, 80:20, 70:30, 60:40, 50: 5
0, 40:60, 30:70, 20:80, 10:90
Alternatively, a three-component solvent type non-aqueous electrolytic solution was prepared in the same manner as in Example 1 except that the ratio was 0: 100. The volume ratios of methyl propyl carbonate, methyl isopropyl carbonate and ethyl propyl carbonate were all 1: 1: 1.

Then, a lithium secondary battery was assembled in the same manner as in Example 1 except that each of these nonaqueous electrolytic solutions was used as the electrolytic solution.

A storage characteristic test was carried out on each of these lithium secondary batteries under the same conditions as above, and at 60 ° C. in a charged state.
The discharge capacity after storage for 1 month was calculated. FIG. 4 shows the results.

From FIG. 4, ethylene carbonate 30 to 7
When a mixed solvent of 0% by volume and 70 to 30% by volume of methyl propyl carbonate, methyl isopropyl carbonate and ethyl propyl carbonate is used, a lithium secondary battery having excellent storage characteristics in a charged state can be obtained. I understand.

In the above embodiments, the case where the present invention is applied to a cylindrical battery has been described as an example, but the present invention is not particularly limited in the shape of the battery, and various shapes such as a flat shape and a square shape can be used. It is applicable to a lithium secondary battery.

[0042]

EFFECT OF THE INVENTION As the low-viscosity solvent used in combination with ethylene carbonate, a specific amount of a specific acyclic carbonic acid ester which is hard to decompose on the surface of the carbon material of the negative electrode is used. Excellent in.

[Brief description of drawings]

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

FIG. 2 is a graph showing the relationship between the ratio of ethylene carbonate in a binary solvent mixture and the storage characteristics in a charged state.

FIG. 3 is a graph showing the relationship between the ratio of ethylene carbonate in a ternary mixed solvent and the storage characteristics in a charged state.

FIG. 4 is a graph showing the relationship between the ratio of ethylene carbonate in a four-component mixed solvent and the storage characteristics in a charged state.

[Explanation of symbols]

 A1 Cylindrical lithium secondary battery (cell of the present invention) 1 Positive electrode 2 Negative electrode 3 Separator

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Mikiya Yamazaki 2-5-5 Keihan Hondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Koji Nishio 2-5 Keihan-hondori, Moriguchi-shi, Osaka No. 5 Sanyo Electric Co., Ltd. (72) Inventor Toshihiko Saito 2-5-5 Keihan Hondori, Moriguchi City, Osaka Sanyo Electric Co., Ltd.

Claims (3)

[Claims] 1. A carbon having a positive electrode and a d value (d ₀₀₂ ) of 3.35 to 3.39Å in the lattice plane (002) plane and a crystallite size (Lc) in the c-axis direction of 50 Å or more. A negative electrode using a material as an electrode material, a non-aqueous electrolyte solution containing a solvent and a solute,
In a lithium secondary battery including a separator, the solvent is ethylene carbonate 30 to 70% by volume, and at least one low-viscosity solvent 70 to 30 selected from the group consisting of methylpropyl carbonate, methylisopropyl carbonate and ethylpropyl carbonate. volume%
A lithium secondary battery, which is a mixed solvent consisting of 2. The positive electrode comprises a lithium-containing manganese oxide, a lithium-containing cobalt oxide, a lithium-containing nickel oxide, a lithium-containing manganese-nickel composite oxide, a lithium-containing manganese-cobalt composite oxide or a lithium-containing nickel-cobalt. The lithium secondary battery according to claim 1, wherein a composite oxide is used as an active material. 3. The solute is LiPF ₆ , LiBF ₄ , L
iClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ or Li
The lithium secondary battery according to claim 1, which is N (CF ₃ SO ₂ ) ₂ .